Odorant/receptor binding and initial olfactory information processing occurs in olfactory receptor neurons (ORNs) within the olfactory epithelium. Subsequent information coding involves high-frequency spike synchronization of paired mitral/tufted cell dendrites within olfactory bulb (OB) glomeruli via positive feedback between glutamate receptors and closely-associated gap junctions. With mRNA for connexins Cx36, Cx43 and Cx45 detected within ORN somata and Cx36 and Cx43 proteins reported in ORN somata and axons, abundant gap junctions were proposed to couple ORNs. We used freeze-fracture replica immunogold labeling (FRIL) and confocal immunofluorescence microscopy to examine Cx36, Cx43 and Cx45 protein in gap junctions in olfactory mucosa, olfactory nerve and OB in adult rats and mice and early postnatal rats. In olfactory mucosa, Cx43 was detected in gap junctions between virtually all intrinsic cell types except ORNs and basal cells; whereas Cx45 was restricted to gap junctions in sustentacular cells. ORN axons contained neither gap junctions nor any of the three connexins. In OB, Cx43 was detected in homologous gap junctions between almost all cell types except neurons and oligodendrocytes. Cx36 and, less abundantly, Cx45 were present in neuronal gap junctions, primarily at “mixed” glutamatergic/electrical synapses between presumptive mitral/tufted cell dendrites. Genomic analysis revealed multiple miRNA (micro interfering RNA) binding sequences in 3′-untranslated regions of Cx36, Cx43 and Cx45 genes, consistent with cell-type-specific post-transcriptional regulation of connexin synthesis. Our data confirm absence of gap junctions between ORNs, and support Cx36- and Cx45-containing gap junctions at glutamatergic mixed synapses between mitral/tufted cells as contributing to higher-order information coding within OB glomeruli.
Auditory afferents terminating as “large myelinated club endings” on goldfish Mauthner cells are identifiable “mixed” (electrical and chemical) synaptic terminals that offer the unique opportunity to correlate physiological properties with biochemical composition and specific ultrastructural features of individual synapses. By combining confocal microscopy and freeze-fracture replica immunogold labeling (FRIL), we demonstrate that gap junctions at these synapses contain connexin35 (Cx35). This connexin is the fish ortholog of the neuron-specific human and mouse connexin36 that is reported to be widely distributed in mammalian brain and to be responsible for electrical coupling between many types of neurons. Similarly, connexin35 was found at gap junctions between neurons in other brain regions, suggesting that connexin35-mediated electrical transmission is common in goldfish brain. Conductance of gap junction channels at large myelinated club endings is known to be dynamically modulated by the activity of their colocalized glutamatergic synapses. We show evidence by confocal microscopy for the presence of the NR1 subunit of the NMDA glutamate receptor subtype, proposed to be a key regulatory element, at these large endings. Furthermore, we also show evidence by FRIL double-immunogold labeling that the NR1 subunit of the NMDA glutamate receptor is present at postsynaptic densities closely associated with gap junction plaques containing Cx35 at mixed synapses across the goldfish hindbrain. Given the widespread distribution of electrical synapses and glutamate receptors, our results suggest that the plastic properties observed at these identifiable junctions may apply to other electrical synapses, including those in mammalian brain.
gap junction; connexin36; electrical synapse; NMDA; synaptic plasticity; electrical coupling; auditory
Auditory afferents terminating as mixed, electrical, and chemical, synapses on the goldfish Mauthner cells constitute an ideal experimental model to study the properties of gap junctions in the nervous system as well as to explore possible functional interactions with the other major form of interneuronal communication—chemically mediated synapses. By combining confocal microscopy and freeze-fracture replica immunogold labeling (FRIL), we found that gap junctions at these synapses contain connexin35 (Cx35), the fish ortholog of the neuron-specific human and mouse connexin36 (Cx36). Conductance of gap junction channels at these endings is known to be dynamically modulated by the activity of their co-localized chemically mediated glutamatergic synapses. By using simultaneous pre- and postsynaptic recordings at these single terminals, we demonstrate that such functional interaction takes place in the same ending, within a few micrometers. Accordingly, we also found evidence by confocal and FRIL double-immunogold labeling that the NR1 subunit of the NMDA glutamate receptor, proposed to be a key regulatory element, is present at postsynaptic densities closely associated with gap junction plaques containing Cx35. Given the widespread distribution of Cx35- and Cx36-mediated electrical synapses and glutamatergic synapses, our data suggest that the local functional interactions observed at these identifiable junctions may also apply to other electrical synapses, including those in mammalian brain.
Auditory; connexin36; electrical coupling; electrical synapse; gap junction; NMDA; synaptic plasticity
“Dye-coupling”, whole-mount immunohistochemistry for gap junction channel protein connexin 35 (Cx35), and freeze-fracture replica immunogold labeling (FRIL) reveal an abundance of electrical synapses/gap junctions at glutamatergic mixed synapses in the 14th spinal segment that innervates the adult male gonopodium of Western Mosquitofish, Gambusia affinis (Mosquitofish). To study gap junctions’ role in fast motor behavior, we used a minimally-invasive neural-tract-tracing technique to introduce gap junction-permeant or -impermeant dyes into deep muscles controlling the gonopodium of the adult male Mosquitofish, a teleost fish that rapidly transfers (complete in <20 mS) spermatozeugmata into the female reproductive tract. Dye-coupling in the 14th spinal segment controlling the gonopodium reveals coupling between motor neurons and a commissural primary ascending interneuron (CoPA IN) and shows that the 14th segment has an extensive and elaborate dendritic arbor and more gap junctions than do other segments. Whole-mount immunohistochemistry for Cx35 results confirm dye-coupling and show it occurs via gap junctions. Finally, FRIL shows that gap junctions are at mixed synapses and reveals that >50 of the 62 gap junctions at mixed synapses are in the 14th spinal segment. Our results support and extend studies showing gap junctions at mixed synapses in spinal cord segments involved in control of genital reflexes in rodents, and they suggest a link between mixed synapses and fast motor behavior. The findings provide a basis for studies of specific roles of spinal neurons in the generation/regulation of sex-specific behavior and for studies of gap junctions’ role in regulating fast motor behavior. Finally, the CoPA IN provides a novel candidate neuron for future studies of gap junctions and neural control of fast motor behaviors.
connexin 35/36; connexins; dye-coupling; freeze-fracture replica immunogold labeling; gap junctions; mixed synapses; neurons; spinal cord
Electrical synaptic transmission via gap junctions has become an accepted feature of neuronal communication in the mammalian brain, and occurs often between dendrites of interneurons in major brain structures, including the hippocampus. Electrical and dye-coupling has also been reported to occur between pyramidal cells in the hippocampus, but ultrastructurally-identified gap junctions between these cells have so far eluded detection. Gap junctions can be formed by nerve terminals, where they contribute the electrical component of mixed chemical/electrical synaptic transmission, but mixed synapses have only rarely been described in mammalian CNS. Here, we used immunofluorescence localization of the major gap junction forming protein connexin36 to examine its possible association with hippocampal pyramidal cells. In addition to labelling associated with gap junctions between dendrites of parvalbumin-positive interneurons, a high density of fine, punctate immunolabelling for Cx36, non-overlapping with parvalbumin, was found in subregions of the stratum lucidum in the ventral hippocampus of rat brain. A high percentage of Cx36-positive puncta in the stratum lucidum was localized to mossy fiber terminals, as indicated by co-localization of Cx36-puncta with the mossy terminal marker vesicular glutamate transporter-1, as well as with other proteins that are highly concentrated in, and diagnostic markers of, these terminals. These results suggest that mossy fiber terminals abundantly form mixed chemical/electrical synapses with pyramidal cells, where they may serve as intermediaries for the reported electrical and dye-coupling between ensembles of these principal cells.
granule cells; gap junctions; mixed electrical-chemical synapses; pyramidal cells
Axon terminals forming mixed chemical/electrical synapses in the lateral vestibular nucleus of rat were described over forty years ago. Because gap junctions formed by connexins are the morphological correlate of electrical synapses, and with demonstrations of widespread expression of the gap junction protein connexin36 (Cx36) in neurons, we investigated the distribution and cellular localization of electrical synapses in the adult and developing rodent vestibular nuclear complex, using immunofluorescence detection of Cx36 as a marker for these synapses. In addition, we examined Cx36 localization in relation to that of the nerve terminal marker vesicular glutamate transporter-1 (vglut-1). An abundance of immunolabelling for Cx36 in the form of Cx36-puncta was found in each of the four major vestibular nuclei of adult rat and mouse. Immunolabelling was associated with somata and initial dendrites of medium and large neurons, and was absent in vestibular nuclei of Cx36 knockout mice. Cx36-puncta were seen either dispersed or aggregated into clusters on the surface of neurons, and were never found to occur intracellularly. Nearly all Cx36-puncta were localized to large nerve terminals immunolabelled for vglut-1. These terminals and their associated Cx36-puncta were substantially depleted after labyrinthectomy. Developmentally, labelling for Cx36 was already present in the vestibular nuclei at postnatal day 5, where it was only partially co-localized with vglut-1, and did not become fully associated with vglut-1-positive terminals until postnatal day 20 to 25. The results show that vglut-1-positive primary afferent nerve terminals form mixed synapses throughout the vestibular nuclear complex, that the gap junction component of these synapses contain Cx36, that multiple Cx36-containing gap junctions are associated with individual vglut-1 terminals and that the development of these mixed synapses is protracted over several postnatal weeks.
Electrical synapses; neuronal gap junctions; vesicular glutamate transporter-1
The hippocampal mossy fibers (MFs), the axons of the granule cells (GCs) of the dentate gyrus, innervate mossy cells and interneurons in the hilus on their way to CA3 where they innervate interneurons and pyramidal cells. Synapses on each target cell have distinct anatomical and functional characteristics. In recent years, the paradigmatic view of the MF synapses being only glutamatergic and, thus, excitatory has been questioned. Several laboratories have provided data supporting the hypothesis that the MFs can transiently release GABA during development and, in the adult, after periods of enhanced excitability. This transient glutamate-GABA co-transmission coincides with the transient up-regulation of the machinery for the synthesis and release of GABA in the glutamatergic GCs. Although some investigators have deemed this evidence controversial, new data has appeared with direct evidence of co-release of glutamate and GABA from single, identified MF boutons. However, this must still be confirmed by other groups and with other methodologies. A second, intriguing observation is that MF activation produced fast spikelets followed by excitatory postsynaptic potentials in a number of pyramidal cells, which, unlike the spikelets, underwent frequency potentiation and were strongly depressed by activation of metabotropic glutamate receptors. The spikelets persisted during blockade of chemical transmission and were suppressed by the gap junction blocker carbenoxolone. These data are consistent with the hypothesis of mixed electrical-chemical synapses between MFs and some pyramidal cells. Dye coupling between these types of principal cells and ultrastructural studies showing the co-existence of AMPA receptors and connexin 36 in this synapse corroborate their presence. A deeper consideration of mixed neurotransmission taking place in this synapse may expand our search and understanding of communication channels between different regions of the mammalian CNS.
mossy fibers; dentate gyrus; GABA; co-release; gap junctions; CA3
Combined confocal microscopy and freeze-fracture replica immunogold labeling (FRIL) were used to examine the connexin identity at electrical synapses in goldfish brain and rat retina, and to test for “co-localization” vs. “close proximity” of connexins to other functionally interacting proteins in synapses of goldfish and mouse brain and rat retina. In goldfish brain, confocal microscopy revealed immunofluorescence for connexin35 (Cx35) and NMDA-R1 (NR1) glutamate receptor protein in Mauthner Cell/Club Ending synapses. By FRIL double labeling, NR1 glutamate receptors were found in clusters of intramembrane particles in the postsynaptic membrane extraplasmic leaflets, and these distinctive postsynaptic densities were in close proximity (0.1–0.3 μ m) to neuronal gap junctions labeled for Cx35, which is the fish ortholog of connexin36 (Cx36) found at neuronal gap junctions in mammals. Immunogold labeling for Cx36 in adult rat retina revealed abundant gap junctions, including several previously unrecognized morphological types. As in goldfish hindbrain, immunogold double labeling revealed NR1-containing postsynaptic densities localized near Cx36-labeled gap junction in rat inferior olive. Confocal immunofluorescence microscopy revealed widespread co-localization of Cx36 and ZO-1, particularly in the reticular thalamic nucleus and amygdala of mouse brain. By FRIL, ZO-1 immunoreactivity was co-localized with Cx36 at individual gap junction plaques in rat retinal neurons. As cytoplasmic accessory proteins, ZO-1 and possibly related members of the membrane-associated guanylate kinase (MAGUK) family represent scaffolding proteins that may bind to and regulate the activity of many neuronal gap junctions. These data document the power of combining immunofluorescence confocal microscopy with FRIL ultrastructural imaging and immunogold labeling to determine the relative proximities of proteins that are involved in short- vs. intermediate-range molecular interactions in the complex membrane appositions at synapses between neurons.
Electrical coupling between some subclasses of interneurons is thought to promote coordinated firing that generates rhythmic synchronous activity in cortical regions. Synaptic activity of cholecystokinin (CCK) interneurons which co-express cannabinoid type-1 (CB1) receptors are powerful modulators of network activity via the actions of endocannabinoids. We investigated the modulatory actions of endocannabinoids between chemically and electrically connected synapses of CCK cells using paired whole-cell recordings combined with biocytin and double immunofluorescence labeling in acute slices of rat hippocampus at P18–20 days. CA1 stratum radiatum CCK Schaffer collateral-associated cells were coupled electrically with each other as well as CCK basket cells and CCK cells with axonal projections expanding to dentate gyrus. Approximately 50% of electrically coupled cells received facilitating, asynchronously released inhibitory postsynaptic potential (IPSPs) that curtailed the steady-state coupling coefficient by 57%. Tonic CB1 receptor activity which reduces inhibition enhanced electrical coupling between cells that were connected via chemical and electrical synapses. Blocking CB1 receptors with antagonist, AM-251 (5 μM) resulted in the synchronized release of larger IPSPs and this enhanced inhibition further reduced the steady-state coupling coefficient by 85%. Depolarization induced suppression of inhibition (DSI), maintained the asynchronicity of IPSP latency, but reduced IPSP amplitudes by 95% and enhanced the steady-state coupling coefficient by 104% and IPSP duration by 200%. However, DSI did not did not enhance electrical coupling at purely electrical synapses. These data suggest that different morphological subclasses of CCK interneurons are interconnected via gap junctions. The synergy between the chemical and electrical coupling between CCK cells probably plays a role in activity-dependent endocannabinoid modulation of rhythmic synchronization.
CA1; CB1; CCK; DSI; electrically coupled; endocannabinoids; interneurons
Suprachiasmatic nucleus (SCN) neurons generate circadian rhythms, and these neurons normally exhibit loosely-synchronized action potentials. Although electrotonic coupling has long been proposed to mediate this neuronal synchrony, ultrastructural studies have failed to detect gap junctions between SCN neurons. Nevertheless, it has been proposed that neuronal gap junctions exist in the SCN; that they consist of connexin32 or, alternatively, connexin36; and that connexin36 knockout eliminates neuronal coupling between SCN neurons and disrupts circadian rhythms. We used confocal immunofluorescence microscopy and freeze-fracture replica immunogold labeling to examine the distributions of connexin30, connexin32, connexin36, and connexin43 in rat and mouse SCN and used whole-cell recordings to re-assess electrotonic and tracer coupling. Connexin32-immunofluorescent puncta were essentially absent in SCN but connexin36 was relatively abundant. Fifteen neuronal gap junctions were identified ultrastructurally, all of which contained connexin36 but not connexin32, whereas nearby oligodendrocyte gap junctions contained connexin32. In adult SCN, one neuronal gap junction was >600 connexons, whereas 75% were smaller than 50 connexons, which may be below the limit of detectability by fluorescence microscopy and thin-section electron microscopy. Whole-cell recordings in hypothalamic slices revealed tracer coupling with Neurobiotin in <5% of SCN neurons, and paired recordings (>40 pairs) did not reveal obvious electrotonic coupling or synchronized action potentials, consistent with few neurons possessing large gap junctions. However, most neurons had partial spikes or spikelets (often <1 mV), which remained after QX-314 had blocked sodium-mediated action potentials within the recorded neuron, consistent with spikelet transmission via small gap junctions. Thus, a few “miniature” gap junctions on most SCN neurons appear to mediate weak electrotonic coupling between limited numbers of neuron pairs, thus accounting for frequent detection of partial spikes and hypothetically providing the basis for “loose” electrical or metabolic synchronization of electrical activity commonly observed in SCN neuronal populations during circadian rhythms.
immunocytochemistry; electrical synapse; freeze-fracture replica immunogold labeling; spikelet; metabolic coupling; synchrony
Among the 20 members in the connexin family of gap junction proteins, only connexin36 (Cx36) is firmly established to be expressed in neurons and to form electrical synapses at widely distributed interneuronal gap junctions in mammalian brain. Several connexins have recently been reported to interact with the PDZ domain-containing protein zonula occludens-1 (ZO-1), which was originally considered to be associated only with tight junctions, but has recently been reported to associate with other structures including gap junctions in various cell types. Based on the presence of sequence corresponding to a putative PDZ binding motif in Cx36, we investigated anatomical relationships and molecular association of Cx36 with ZO-1. By immunofluorescence, punctate Cx36/ZO-1 colocalization was observed throughout the central nervous system of wild-type mice, whereas labelling for Cx36 was absent in Cx36 knockout mice, confirming the specificity of the anti-Cx36 antibodies employed. By freeze-fracture replica immunogold labelling, Cx36 and ZO-1 in brain were found colocalized within individual ultrastructurally identified gap junction plaques, although some plaques contained only Cx36 whereas others contained only ZO-1. Cx36 from mouse brain and Cx36-transfected HeLa cells was found to coimmunoprecipitate with ZO-1. Unlike other connexins that bind the second of the three PDZ domains in ZO-1, glutathione S-transferase-PDZ pull-down and mutational analyses indicated Cx36 interaction with the first PDZ domain of ZO-1, which required at most the presence of the four c-terminus amino acids of Cx36. These results demonstrating a Cx36/ZO-1 association suggest a regulatory and/or scaffolding role of ZO-1 at gap junctions that form electrical synapses between neurons in mammalian brain.
connexins; electrical synapses; gap junction; neurons; retina
In contrast to chemical transmission, few proteins have been shown associated with gap junction-mediated electrical synapses. Mixed (electrical and glutamatergic) synaptic terminals on the teleost Mauthner cell known as “Club endings” constitute because of their unusual large size and presence of connexin 35 (Cx35), ortholog of the widespread mammalian Cx36, a valuable model for the study of electrical transmission. Remarkably, both components of their mixed synaptic response undergo activity-dependent potentiation. Changes in electrical transmission result from interactions with co-localized glutamatergic synapses, the activity of which leads to the activation of Ca++/calmodulin-dependent kinase II (CaM-KII), required for the induction of changes in both forms of transmission. However, the distribution of this kinase and potential localization to electrical synapses remains undetermined. Taking advantage of the unparalleled experimental accessibility of Club endings, we explored the presence and intraterminal distribution of CaM-KII within these terminals. Here we show: 1) unlike other proteins, both CaM-KII labeling and distribution were highly variable between contiguous contacts, and 2) CaM-KII was not restricted to the periphery of the terminals, where glutamatergic synapses are located, but also was present at the center where gap junctions predominate. Accordingly, double-immunolabeling indicated that Cx35 and CaM-KII were co-localized and biochemical analysis showed that these proteins associate. Because CaM-KII characteristically undergoes activity-dependent translocation, the observed variability of labeling likely reflects physiological differences between electrical synapses of contiguous Club endings, which remarkably co-exist with differing degrees of conductance. Taken together, our results indicate that CaM-KII should be considered a component of electrical synapses although its association is non-obligatory and likely driven by activity.
Electrical synapse; LTP; connexin 36; gap junction; synaptic plasticity; auditory afferent
Corticotropin-releasing hormone (CRH) excites hippocampal neurons and induces death of selected CA3 pyramidal cells in immature rats. These actions of CRH require activation of specific receptors that are abundant in CA3 during early postnatal development. Given the dramatic effects of CRH on hippocampal neurons and the absence of CRH-containing afferents to this region, we hypothesized that a significant population of CRHergic neurons exists in developing rat hippocampus. This study defined and characterized hippocampal CRH-containing cells by using immunocytochemistry, ultrastructural examination, and colocalization with gamma-aminobutyric acid (GABA)-synthesizing enzyme and calcium-binding proteins. Numerous, large CRH-immunoreactive (ir) neurons were demonstrated in CA3 strata pyramidale and oriens, fewer were observed in the corresponding layers of CA1, and smaller CRH-ir cells were found in stratum lacunosum-moleculare of Ammon's horn. In the dentate gyrus, CRH-ir somata resided in the granule cell layer and hilus. Ultrastructurally, CRH-ir neurons had aspiny dendrites and were postsynaptic to both asymmetric and symmetric synapses. CRH-ir axon terminals formed axosomatic and axodendritic symmetric synapses with pyramidal and granule cells. Other CRH-ir terminals synapsed on axon initial segments of principal neurons. Most CRH-ir neurons were coimmunolabeled for glutamate decarboxylase (GAD)-65 and GAD-67 and the majority also contained parvalbumin, but none were labeled for calbindin. These results confirm the identity of hippocampal CRH-ir cells as GABAergic interneurons. Further, a subpopulation of neurons immunoreactive for both CRH and parvalbumin and located within and adjacent to the principal cell layers consists of basket and chandelier cells. Thus, axon terminals of CRH-ir interneurons are strategically positioned to influence the excitability of the principal hippocampal neurons via release of both CRH and GABA.
hippocampus; interneurons; neuropeptides; parvalbumin; development
Electrical synapses play an important role in signaling between neurons and the synaptic connections between many neurons possess both electrical and chemical components. Although modulation of electrical synapses is frequently observed, the cellular processes that mediate such changes have not been studied as thoroughly as plasticity in chemical synapses. In the leech (Hirudo sp), the competitive AMPA receptor antagonist CNQX inhibited transmission at the rectifying electrical synapse of a mixed glutamatergic/electrical synaptic connection. This CNQX-mediated inhibition of the electrical synapse was blocked by concanavalin A (Con A) and dynamin inhibitory peptide (DIP), both of which are known to inhibit endocytosis of neurotransmitter receptors. CNQX-mediated inhibition was also blocked by pep2-SVKI (SVKI), a synthetic peptide that prevents internalization of AMPA-type glutamate receptor. AMPA itself also inhibited electrical synaptic transmission and this AMPA-mediated inhibition was partially blocked by Con A, DIP and SVKI. Low frequency stimulation induced long-term depression (LTD) in both the electrical and chemical components of these synapses and this LTD was blocked by SVKI. GYKI 52466, a selective non-competitive antagonist of AMPA receptors, did not affect the electrical EPSP, although it did block the chemical component of these synapses. CNQX did not affect non-rectifying electrical synapses in two different pairs of neurons. These results suggest an interaction between AMPA-type glutamate receptors and the gap junction proteins that mediate electrical synaptic transmission. This putative interaction between glutamate receptors and gap junction proteins represents a novel mechanism for regulating the strength of synaptic transmission.
Gap junction; innexin; leech; synaptic plasticity; glutamate receptor
Locus coeruleus neurons are strongly coupled during early postnatal development, and it has been proposed that these neurons are linked by extraordinarily abundant gap junctions consisting of connexin32 and connexin26, and that those same connexins abundantly link neurons to astrocytes. Based on the controversial nature of those claims, immunofluorescence imaging and freeze-fracture replica immunogold labeling were used to re-investigate the abundance and connexin composition of neuronal and glial gap junctions in developing and adult rat and mouse locus coeruleus. In early postnatal development, connexin36 and Cx43 immunofluorescent puncta were densely distributed in the locus coeruleus, whereas connexin32 and connexin26 were not detected. By freeze-fracture replica immunogold labeling, connexin36 was found in ultrastructurally-defined neuronal gap junctions, whereas connexin32 and connexin26 were not detected in neurons and only rarely detected in glia. In 28-day postnatal (adult) rat locus coeruleus, immunofluorescence labeling for connexin26 was always co-localized with the glial gap junction marker connexin43; connexin32 was associated with the oligodendrocyte marker CNPase; and connexin36 was never co-localized with connexin26, Cx32 or connexin43. Ultrastructurally, connexin36 was localized to gap junctions between neurons, whereas connexin32 was detected only in oligodendrocyte gap junctions; and Cx26 was found only rarely in astrocyte junctions but abundantly in pia mater. Thus, in developing and adult locus coeruleus, neuronal gap junctions contain connexin36 but do not contain detectable connexin32 or connexin26, suggesting that the locus coeruleus has the same cell-type specificity of connexin expression as observed ultrastructurally in other regions of the central nervous system. Moreover, in both developing and adult locus coeruleus, no evidence was found for gap junctions or connexins linking neurons with astrocytes or oligodendrocytes, indicating that neurons in this nucleus are not linked to the pan-glial syncytium by connexin32- or connexin26-containing gap junctions or by abundant free connexons composed of those connexins.
astrocytes; connexin26; connexin32; connexin43; oligodendrocytes
The subcellular distributions and co-associations of the gap junction-forming proteins connexin47 and connexin32 were investigated in oligodendrocytes of adult mouse and rat CNS. By confocal immunofluorescence light microscopy, abundant connexin47 was co-localized with astrocytic connexin43 on oligodendrocyte somata, and along myelinated fibers, whereas connexin32 without connexin47 was co-localized with contactin-associated protein (caspr) in paranodes. By thin-section transmission electron microscopy, connexin47 immunolabeling was on the oligodendrocyte side of gap junctions between oligodendrocyte somata and astrocytes. By freeze-fracture replica immunogold labeling, large gap junctions between oligodendrocyte somata and astrocyte processes contained much more connexin47 than connexin32. Along surfaces of internodal myelin, connexin47 was several times as abundant as connexin32, and in the smallest gap junctions, often occurred without connexin32. In contrast, connexin32 was localized without connexin47 in newly-described autologous gap junctions in Schmidt-Lanterman incisures and between paranodal loops bordering nodes of Ranvier. Thus, connexin47 in adult rodent CNS is the most abundant connexin in most heterologous oligodendrocyte-to-astrocyte gap junctions, whereas connexin32 is the predominant if not sole connexin in autologous (“reflexive”) oligodendrocyte gap junctions. These results clarify the locations and connexin compositions of heterologous and autologous oligodendrocyte gap junctions, identify autologous gap junctions at paranodes as potential sites for modulating paranodal electrical properties, and reveal connexin47-containing and connexin32-containing gap junctions as conduits for long-distance intracellular and intercellular movement of ions and associated osmotic water. The autologous gap junctions may regulate paranodal electrical properties during saltatory conduction. Acting in series and in parallel, autologous and heterologous oligodendrocyte gap junctions provide essential pathways for intra- and intercellular ionic homeostasis.
confocal microscopy; freeze fracture; immunofluorescence; immunogold labeling; rodent; A/A, astrocyte-to-astrocyte; AQP4, aquaporin4; caspr, contactin-associated protein; Cx, connexin, designated according to molecular weight in kiloDaltons; Cx26, connexin26; Cx29, connexin29; Cx30, connexin30; Cx32, connexin32; Cx43, connexin43; Cx47, connexin47; DAB, diaminobenzidine; E-face, extraplasmic leaflet; FRIL, freeze-fracture replica immunogold labeling; GFAP, glial acidic fibrillary acidic protein; IMP, intramembrane particle/intramembrane protein; KO, knockout; LE, labeling efficiency; LM, light microscopy; O/A, oligodendrocyte-to-astrocyte; O/O, oligodendrocyte-to-oligodendrocyte; PB, phosphate buffer; P-face, protoplasmic leaflet; PNS, peripheral nervous system; TBST, Tris-HCl-buffered saline with Triton X-100; TEM, transmission electron microscopy
Morphological and electrophysiological studies have shown that granule cell axons, the mossy fibers (MFs), establish gap junctions and, therefore, electrical communication among them. That granule cells express gap junctional proteins in their axons suggests the possibility that their terminals express them as well. If this were to be the case, mixed electrical-chemical communication could be supported, as MF terminals normally use glutamate for fast communication with their target cells. Here we present electrophysiological and modeling studies consistent with this hypothesis. We show that MF activation produced fast spikelets followed by excitatory postsynaptic potentials in pyramidal cells (PCs), which, unlike the spikelets, underwent frequency potentiation and were strongly depressed by activation of metabotropic glutamate receptors, as expected from transmission of MF origin. The spikelets, which persisted during blockade of chemical transmission, wee potentiated by dopamine and suppressed by the gap junction blocker carbenoxolone. The various waveforms evoked by MF stimulation were replicated in a multi-compartment model of a PC by brief current pulse injections into the proximal apical dendritic compartment, where MFs are known to contact PCs. Mixed electrical and glutamatergic communication between granule cells and some PCs in CA3 may ensure the activation of sets of PCs, bypassing the strong action of concurrent feed-forward inhibition that granule cells activate. Importantly, MF-to-PC electrical coupling may allow bidirectional, possibly graded communication that can be faster than chemical synapses and subject to different forms of modulation.
CA3; mossy fibers; spikelets; mixed transmission; electrical communication
A system's wiring constrains its dynamics, yet modelling of neural structures often overlooks the specific networks formed by their neurons. We developed an approach for constructing anatomically realistic networks and reconstructed the GABAergic microcircuit formed by the medium spiny neurons (MSNs) and fast-spiking interneurons (FSIs) of the adult rat striatum. We grew dendrite and axon models for these neurons and extracted probabilities for the presence of these neurites as a function of distance from the soma. From these, we found the probabilities of intersection between the neurites of two neurons given their inter-somatic distance, and used these to construct three-dimensional striatal networks. The MSN dendrite models predicted that half of all dendritic spines are within 100µm of the soma. The constructed networks predict distributions of gap junctions between FSI dendrites, synaptic contacts between MSNs, and synaptic inputs from FSIs to MSNs that are consistent with current estimates. The models predict that to achieve this, FSIs should be at most 1% of the striatal population. They also show that the striatum is sparsely connected: FSI-MSN and MSN-MSN contacts respectively form 7% and 1.7% of all possible connections. The models predict two striking network properties: the dominant GABAergic input to a MSN arises from neurons with somas at the edge of its dendritic field; and FSIs are inter-connected on two different spatial scales: locally by gap junctions and distally by synapses. We show that both properties influence striatal dynamics: the most potent inhibition of a MSN arises from a region of striatum at the edge of its dendritic field; and the combination of local gap junction and distal synaptic networks between FSIs sets a robust input-output regime for the MSN population. Our models thus intimately link striatal micro-anatomy to its dynamics, providing a biologically grounded platform for further study.
The brain has an immensely complex wiring diagram, but few computational models of brain regions attempt accurate renditions of the wiring between neurons. Consequently, these models' dynamics may not accurately reflect those of the region. Key barriers here are the difficulty of reconstructing such networks and the paucity of critical data on neuron morphology. We demonstrate an approach that gets around these problems by using the available data to construct prototype neuron morphologies, and uses these to estimate how the probability of a connection between two neurons changes as we change the distance between them. With these in hand, we constructed artificial three-dimensional networks of the rat striatum and find that the connection distributions agree well with current estimates from anatomical studies. Our networks show features and dynamical implications of striatal wiring that would be difficult to intuit: the dominant input to the striatal projection neuron arises from other neurons just at the edge of its dendrites, and the main inhibitory interneurons are coupled locally by electrical connections and more distally by chemical synapses. Together, these properties set a unique state for the input-output computations of the striatum.
GABA (gamma-aminobutyric-acid), the main inhibitory neurotransmitter in the adult brain, exerts depolarizing (excitatory) actions during development and this GABAergic depolarization cooperates with NMDARs (N-methyl-D-aspartate receptors) to drive spontaneous synchronous activity (SSA) that is fundamentally important for developing neuronal networks. Although GABAergic depolarization is known to assist in the activation of NMDARs during development, the subcellular localization of NMDARs relative to GABAergic synapses is still unknown. Here, we investigated the subcellular distribution of NMDARs in association with GABAergic synapses at the developmental stage when SSA is most prominent in mice. Using multiple immunofluorescent labeling and confocal laser-scanning microscopy in the developing mouse hippocampus, we found that NMDARs were associated with both glutamatergic and GABAergic synapses at postnatal day 6–7 and we observed a direct colocalization of GABAA- and NMDA-receptor labeling in GABAergic synapses. Electron microscopy of pre-embedding immunogold-immunoperoxidase reactions confirmed that GluN1, GluN2A and GluN2B NMDAR subunits were all expressed in glutamatergic and GABAergic synapses postsynaptically. Finally, quantitative post-embedding immunogold labeling revealed that the density of NMDARs was 3 times higher in glutamatergic than in GABAergic synapses. Since GABAergic synapses were larger, there was little difference in the total number of NMDA receptors in the two types of synapses. In addition, receptor density in synapses was substantially higher than extrasynaptically. These data can provide the neuroanatomical basis of a new interpretation of previous physiological data regarding the GABAAR-NMDAR cooperation during early development. We suggest that during SSA, synaptic GABAAR-mediated depolarization assists NMDAR activation right inside GABAergic synapses and this effective spatial cooperation of receptors and local change of membrane potential will reach developing glutamatergic synapses with a higher probability and efficiency even further away on the dendrites. This additional level of cooperation that operates within the depolarizing GABAergic synapse, may also allow its own modification triggered by Ca2+-influx through the NMDA receptors.
Synchronous firing is commonly observed in the brain, but its underlying mechanisms and neurobiological meaning remain debated. Most commonly, synchrony is attributed either to electrical coupling by gap junctions or to shared excitatory inputs. In the cerebral cortex and hippocampus, fast-spiking (FS) or somatostatin–containing (SOM) inhibitory interneurons are electrically coupled to same-type neighbors, and each subtype-specific network tends to fire in synchrony. Electrical coupling across subtypes is weak or absent, but SOM-FS and FS-FS pairs are often connected by inhibitory synapses. Theoretical studies suggest that purely inhibitory coupling can also promote synchrony; however, this has not been confirmed experimentally. We recorded from 74 pairs of electrically non-coupled layer 4 interneurons in mouse somatosensory cortex in vitro, and found that tonically depolarized FS-FS and SOM-FS pairs connected by uni- or bidirectional inhibitory synapses often fired within one millisecond of each other. Using a novel, jitter-based measure of synchrony, we found that synchrony correlated with inhibitory coupling strength. Importantly, synchrony was resistant to ionotropic glutamate receptors antagonists but was strongly reduced when GABAA receptors were blocked, confirming that in our experimental system IPSPs were both necessary and sufficient for synchrony. Submillisecond firing lags emerged in a computer simulation of pairs of spiking neurons, in which the only assumed interaction between neurons was by inhibitory synapses. We conclude that cortical interneurons are capable of synchronizing both within and across subtypes, and that submillisecond coordination of firing can arise by mutual synaptic inhibition alone, with neither shared inputs nor electrical coupling.
Coupling of neurons by electrical synapses (gap junctions) transiently increases in the mammalian CNS during development. We report here that the developmental increase in neuronal gap junction coupling and expression of connexin 36 (Cx36; neuronal gap junction protein) are regulated by an interplay between the activity of group II metabotropic glutamate receptors (mGluR) and GABAA receptors. Specifically, using dye coupling, electrotonic coupling, western blots and siRNA in the rat and mouse hypothalamus and cortex in vivo and in vitro, we demonstrate that activation of group II mGluRs augments, and inactivation prevents, the developmental increase in neuronal gap junction coupling and Cx36 expression. However, changes in GABAA receptor activity have the opposite effects. The regulation by group II mGluRs is via cAMP/PKA-dependent signaling and by GABAA receptors is via Ca2+/PKC-dependent signaling. Further, the receptor-mediated up-regulation of Cx36 requires a neuron-restrictive silencer element in the Cx36 gene promoter and the down-regulation involves the 3′UTR of the Cx36 mRNA, as shown using RT-qPCR and luciferase reporter activity analysis. In addition, the MTT analysis indicates that mechanisms for the developmental increase in neuronal gap junction coupling directly control the death/survival mechanisms in developing neurons. Altogether, the results suggest a multi-tiered strategy for chemical synapses in developmental regulation of electrical synapses.
gap junctions; connexin 36; metabotropic glutamate receptors; GABAA receptors; neuronal death; electrical synapses; development
Neuronal gap junctions are abundant in both outer and inner plexiform layers of the mammalian retina. In the inner plexiform layer (IPL), ultrastructurally-identified gap junctions were reported primarily in the functionally-defined and anatomically-distinct ON sublamina, with few reported in the OFF sublamina. We used freeze-fracture replica immunogold labeling and confocal microscopy to quantitatively analyze the morphologies and distributions of neuronal gap junctions in the IPL of adult rat and mouse retina. Under “baseline” conditions (photopic illumination/general anesthesia), 649 neuronal gap junctions immunogold-labeled for connexin36 were identified in rat IPL, of which 375 were photomapped to OFF vs. ON sublaminae. In contrast to previous reports, the volume-density of gap junctions was equally abundant in both sublaminae. Five distinctive morphologies of gap junctions were identified: conventional crystalline and non-crystalline “plaques” (71% and 3%), plus unusual “string” (14%), “ribbon” (7%) and “reticular” (2%) forms. Plaque and reticular gap junctions were distributed throughout the IPL. However, string and ribbon gap junctions were restricted to the OFF sublamina, where they represented 48% of gap junctions in that layer. In string and ribbon junctions, curvilinear strands of connexons were dispersed over 5 to 20 times the area of conventional plaques having equal numbers of connexons. To define morphologies of gap junctions under different light-adaptation conditions, we examined an additional 1150 gap junctions from rats and mice prepared after 30 min of photopic, mesopic and scotopic illumination, with and without general anesthesia. Under these conditions, string and ribbon gap junctions remained abundant in the OFF sublamina and absent in the ON sublamina. Abundant gap junctions in the OFF sublamina of these two rodents with rod-dominant retinas revealed previously-undescribed but extensive pathways for inter-neuronal communication; and the wide dispersion of connexons in string and ribbon gap junctions suggests unique structural features of gap junctional coupling in the OFF vs. ON sublamina.
connexin36; Cx36; freeze-fracture replica immunogold labeling; FRIL; confocal immunocytochemistry
Tight coupling between GABA synthesis and vesicle filling suggests that the presynaptic supply of precursor glutamate could dynamically regulate inhibitory synapses. Although the neuronal glutamate transporter Excitatory Amino Acid Transporter 3 (EAAT3) has been proposed to mediate such a metabolic role, highly efficient astrocytic uptake of synaptically released glutamate normally maintains low extracellular glutamate levels. We examined whether axodendritic inhibitory synapses in stratum radiatum of hippocampal area CA1, which are closely positioned among excitatory glutamatergic synapses, are regulated by synaptic glutamate release via presynaptic uptake. Under conditions of spatially and temporally coordinated release of glutamate and GABA within pyramidal cell dendrites, blocking glial glutamate uptake enhanced quantal release of GABA in a transporter-dependent manner. These physiological findings correlated with immunohistochemical studies revealing expression of EAAT3 by interneurons and uptake of D-asparate into putative axodendritic inhibitory terminals only when glial uptake was blocked. These results indicate that spillover of glutamate between adjacent excitatory and inhibitory synapses can occur under conditions when glial uptake incompletely clears synaptically released glutamate. Our anatomical studies also suggest that perisomatic inhibitory synapses, unlike synapses within dendritic layers of hippocampus, are not capable of glutamate uptake and therefore transporter-mediated dynamic regulation of inhibition is a unique feature of axodendritic synapses that may play a role in maintaining a homeostatic balance of inhibition and excitation.
The transmembrane connexin proteins of gap junctions link extracellularly to form channels for cell-to-cell exchange of ions and small molecules. Two primary hypotheses of gap junction coupling in the CNS are the following: (1) generalized coupling occurs between neurons and glia, with some connexins expressed in both neurons and glia, and (2) intercellular junctional coupling is restricted to specific coupling partners, with different connexins expressed in each cell type. There is consensus that gap junctions link neurons to neurons and astrocytes to oligodendrocytes, ependymocytes, and other astrocytes. However, unresolved are the existence and degree to which gap junctions occur between oligodendrocytes, between oligodendrocytes and neurons, and between astrocytes and neurons. Using light microscopic immunocytochemistry and freeze–fracture replica immunogold labeling of adult rat CNS, we investigated whether four of the best-characterized CNS connexins are each present in one or more cell types, whether oligodendrocytes also share gap junctions with other oligodendrocytes or with neurons, and whether astrocytes share gap junctions with neurons. Connexin32 (Cx32) was found only in gap junctions of oligodendrocyte plasma membranes, Cx30 and Cx43 were found only in astrocyte membranes, and Cx36 was only in neurons. Oligodendrocytes shared intercellular gap junctions only with astrocytes, with each oligodendrocyte isolated from other oligodendrocytes except via astrocyte intermediaries. Finally, neurons shared gap junctions only with other neurons and not with glial cells. Thus, the different cell types of the CNS express different connexins, which define separate pathways for neuronal versus glial gap junctional communication.
astrocyte; connexin; connexon; gap junction; neuron; oligodendrocyte
Functionally distinct subsets of hippocampal inhibitory neurons exhibit large differences in the frequency, pattern and short-term plasticity of GABA release from their terminals. Heterogeneity is also evident in the ultrastructural features of GABAergic axon terminals examined in the electron microscope, but it is not known if or how this corresponds to interneuron subtypes. We investigated the feasibility of separating morphologically distinct clusters of terminal types, using the approach of measuring several ultrastructural parameters of GABAergic terminals in the CA1 area of the rat hippocampus. Septo-hippocampal axon terminals were anterogradely labeled by biotinylated dextran amine and visualized by preembedding immunogold staining to delineate one homogeneous terminal population. Long series (100-150) of ultrathin sections were cut from stratum oriens and stratum radiatum of the CA1 area, and GABAergic terminals were identified by post-embedding immunogold staining. Stereologically unbiased samples of the total GABAergic axon terminal population and a random sample of the septal axon terminals were reconstructed in 3D, and several of their parameters were measured (e.g. bouton volume, synapse surface, volume occupied by vesicles, mitochondria volume).
Septal terminals demonstrated significantly larger mean values for most parameters than the total population of local GABAergic terminals. There was no significant difference between terminals reconstructed in the basal and apical dendritic regions of pyramidal cells, neither for the septal nor for the local population.
Importantly, almost all parameters were highly correlated, precluding the possibility of clustering the local terminals into non-overlapping subsets. Factor and cluster analysis confirmed these findings. Our results suggest that similarly to excitatory terminals, inhibitory terminals follow an “ultrastructural size principle”, and that the terminals of different interneuron subtypes cannot be distinguished by ultrastructure alone.
interneuron; electron microscopy; 3D reconstruction; factor analysis; cluster analysis