Our report identifies the developmental dynamics of scgn expression including the migratory routes and final positions of subpallial neurons expressing this CBP in rodent, primate and human fetal brain. Distinct temporal expression patterns of the three ‘classical’ CBPs – CB, CR and PV – have generated broad interest because of the importance of GABAergic neurons in refining the physiological output of neuronal networks (Freund & Buzsaki, 1996
; Klausberger & Somogyi, 2008
). Here, we show that scgn is expressed earlier than CB, CR or PV in pioneer neurons exiting the pallidal differentiation zone by E11 in mouse. Histochemically noticeable scgn expression is restricted to postmitotic neurons because scgn+
cells lack the expression of RC2 and nestin, radial glia and neural stem/progenitor cell markers (Carleton et al., 2003
), respectively. The majority of scgn+
cells we identified migrate towards the prospective EA, and selectively inhabit its subpallial domain by forming a continuum of scgn+
neurons extending from the anterior tip of the VP towards the CA/MA. The lack of Brn-1, a POU homeodomain protein specifying neocortical pyramidal cells (Sugitani et al., 2002
), supports that scgn+
cell contingents are destined towards subpallial territories.
neurons commute in at least two major migratory streams along the palliosubpallial boundary and clearly avoid venturing into neocortical territories during forebrain development. Scgn+
neurons populating the OB travel in the anterior direction and upon reaching the olfactory granular layer frequently (>20%) acquire GAD67+
phenotypes. In contrast, scgn+
neurons travelling caudally to colonize the EA exhibit a substantially lower percentage (7–9%) of co-localization with GAD67 en route
to their final positions. The diversity of neuronal contingents destined to the EA is first evidenced by the bifurcation of their migratory stream at the level of the IPAC: small-to-medium sized scgn+
neurons, many of which are GABAergic (), invade the CA and MA. Whilst we show that scgn can developmentally co-exist with GAD, our prior (Mulder et al., 2009b
) and present analysis in adult mouse and primate forebrain reveal a limited likelihood of co-expression of scgn with the other known neuron-specific CBPs, particularly CR and CB. Alternatively, scgn+
neurons can co-express ChAT, a ubiquitous cholinergic marker (Riedel et al., 2002
), upon populating the SI.
Intracellular Ca2+ signalling underpins the responsiveness of developing neurons to extracellular guidance cues. We unexpectedly find that scgn is already enriched in subsets of neurons engaged in long-distance migration with histochemically-detectable levels of this CBP maintained throughout neuronal morphogenesis. This notion may pinpoint that the scgn-mediated control of intracellular Ca2+ signalling can play a role in generating adequate cellular responses to microenvironmental stimuli that are specifically present at the palliosubpallial boundary. Otherwise, scgn may be one of the early molecular determinants required for amygdala neurons to integrate into neuronal networks and to acquire specialized functions therein.
In the postnatal nervous system, CBPs are important cytosolic modulators of Ca2+
signalling in neurons whose presence has been associated with the maintenance of distinct electric discharge patterns. In the basolateral amygdala, PV+
interneurons form a primary local modulatory neuronal subnetwork affecting the integration of polymodal sensory information by excitatory principal cells (Woodruff & Sah, 2007a
; Woodruff & Sah, 2007b
). Our discovery that scgn+
neurons are only present in circumcised clusters in the EA present a number of intriguing possibilities both at the single cell and neuronal network levels: secretagogin is an EF-hand CBP capable of simultaneously binding 4 Ca2+
ions at physiological intracellular Ca2+
levels (Rogstam et al., 2007
), with an affinity similar to those of the classical neuronal CBPs. Therefore, when scgn is present in neurons otherwise lacking PV, CR or CR, this CBP may contribute to the refinement of intracellular Ca2+
signalling with an as yet unknown impact on cellular excitability, and integration of afferent inputs. When scgn is co-expressed with CR or CB, it could account for a substantially enhanced Ca2+
-buffering capacity thus sub-diversifying the responsiveness and network contribution of a particular neuron. However, we also entertain the possibility that scgn identifies a hitherto unknown, neurochemically distinct class of GABAergic neurons in the CA. Therefore, subsequent studies aimed to elucidate scgn’s functional significance will undoubtedly advance our understanding of the neurobiological principles that govern the organization and function of amygdaloid neuronal circuitries.
Scgn expression exhibits robust phylogenetic differences across mammalian species. Scant scgn expression is found in the SI in rodent brain. However, virtually all cholinergic basal forebrain projection neurons are scgn+
in primate brain. This difference suggests that cholinergic lineage commitment associates with a selective upregulation of scgn expression in higher-order mammals. This evolutionary transitions can be significant in explaining the differential sensitivity of rodent and primate cholinergic neurons to both physiological and noxious stimuli, and might impact cholinergic neurotransmission both at the presynaptic (neurotransmitter release) and postsynaptic (second messenger signalling) levels. Such changes may be required to accommodate the increased complexity and diversity of information processed upon expansion of isocortical areas, the primary targets of cholinergic basal forebrain afferents (Mesulam et al., 1983
). A critical difference between scgn expression in prosimian primate and human brain is the unique scgn expression in pyramidal neurons of the human hippocampus (Attems et al., 2007
; Attems et al., 2008
). Our in situ
hybridization data in mid-gestational human embryos corroborate and extend these findings by demonstrating scgn mRNA expression in the neocortex (cortical plate), hippocampus, and prospective amygdala.
In conclusion, our present report establishes scgn as a fourth developmentally-regulated, neuron-specific CBP, whose phylogenetic preservation and selective association with neurochemically distinct subsets of neurons suggest novel dimensions of functional modalities within the extended amygdala circuitry.