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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptNIH Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
J Neurosci. Author manuscript; available in PMC Oct 14, 2010.
Published in final edited form as:
PMCID: PMC2860606
NIHMSID: NIHMS195661
RIM1α and Interacting Proteins Involved in Presynaptic Plasticity Mediate Prepulse Inhibition and Additional Behaviors Linked to Schizophrenia
Jacqueline Blundell,1* Pascal S. Kaeser,2 Thomas C. Südhof,2 and Craig M. Powell1
1 Departments of Neurology and Psychiatry, The University of Texas Southwestern Medical Center, Dallas, Texas, 75390-8813, USA
2 Department of Molecular and Cellular Physiology, and Howard Hughes Medical Institute, Stanford University, Palo Alto, CA 94304-5543, USA
Correspondence and requests for materials should be addressed to C.M.P. (5323 Harry Hines Blvd. Dallas, TX 75390-8813, craig.powell/at/utsouthwestern.edu)
*Present Address: Dept. of Psychology, Memorial University of Newfoundland, St. John’s, Newfoundland, Canada.
Several presynaptic proteins involved in neurotransmitter release in the central nervous system have been implicated in schizophrenia in human clinical genetic studies, in post-mortem studies, and in studies of putative animal models of schizophrenia. The presynaptic protein RIM1α mediates presynaptic plasticity and cognitive function. We now demonstrate that mice deficient in RIM1α exhibit abnormalities in multiple schizophrenia-relevant behavioral tasks including prepulse inhibition, response to psychotomimetic drugs, and social interaction. These schizophrenia-relevant behavioral findings are relatively selective to RIM1α deficient mice, as mice bearing mutations in the RIM1α binding partners Rab3A or synaptotagmin 1 only show decreased prepulse inhibition. In addition to RIM1α’s involvement in multiple behavioural abnormalities, these data suggest that alterations in presynaptic forms of short-term plasticity are linked to alterations in prepulse inhibition, a measure of sensorimotor gating.
Keywords: schizophrenia, RIM1α, Rab3A, synaptotagmin 1, presynaptic protein, prepulse inhibition, paired pulse facilitation, short-term plasticity
Impaired glutamatergic synapse function has been implicated in schizophrenia (Carlsson et al., 1999; Lipska and Weinberger, 2000; Mechri et al., 2001; Coyle et al., 2003; Moghaddam and Jackson, 2003; Carlsson et al., 2004; Weinberger, 2005; Coyle, 2006; Javitt, 2007; Stone et al., 2007; Paz et al., 2008; Sodhi et al., 2008). In particular, pharmacologic blockade of NMDA receptors with phencyclidine (PCP), ketamine, or MK-801 can cause or exacerbate psychotic symptoms in humans (Moghaddam, 1994; Lahti et al., 1995; Braff et al., 2001; Gunduz-Bruce, 2009). Furthermore, such blockade leads to behavioral abnormalities in rodents that are often touted as relevant to human schizophrenia including social isolation, decreased cognitive function, and decreased prepulse inhibition (Javitt and Zukin, 1991; Powell and Miyakawa, 2006; Large, 2007; Bubenikova-Valesova et al., 2008; Gunduz-Bruce, 2009; Noda et al., 2009).
Several studies in human post-mortem tissues have identified altered expression of some presynaptic proteins in schizophrenia (Gabriel et al., 1997; Glantz and Lewis, 1997a, b; Blennow et al., 1999; Karson et al., 1999; Eastwood et al., 2001; Sawada et al., 2002; Vawter et al., 2002; Harrison et al., 2003; Eastwood and Harrison, 2005; Sawada et al., 2005; Scarr et al., 2006; Bowden et al., 2008; Mudge et al., 2008; Roberts et al., 2008; Shen et al., 2009). Moreover, clinical genetic studies have identified genes encoding presynaptic proteins as risk factors for schizophrenia (Karson et al., 1999; Mirnics et al., 2000; Tachikawa et al., 2001; Chen et al., 2004; Lee et al., 2005; Muller et al., 2005; Verma et al., 2005; Kirov et al., 2008; Sudhof, 2008; Walsh et al., 2008; Rujescu et al., 2009). Consistent with these findings, mutation of genes encoding presynaptic proteins has resulted in mouse models relevant to schizophrenia (Drew et al., 2007; Dyck et al., 2007, 2009; Etherton et al., 2009). Thus, we hypothesized that deletion or mutation of the presynaptic protein RIM1α, and its binding partners Rab3A and synaptotagmin, would lead to schizophrenia-related behavioural abnormalities (Powell and Miyakawa, 2006).
Deficits in prepulse inhibition, a measure of sensorimotor gating, are associated with human schizophrenia and other neuropsychiatric disorders (Braff and Geyer, 1990; Braff et al., 2001; Ludewig et al., 2003; Geyer, 2006b). While the anatomy and pharmacology of the startle response in the brainstem and the many higher brain regions that modify prepulse inhibition of the startle response have been well described (Braff and Geyer, 1990; Dulawa and Geyer, 1996; Geyer, 1998; Swerdlow et al., 2001; Geyer et al., 2002), the neuronal and synaptic mechanisms responsible for prepulse inhibition are less well characterized.
In the prepulse inhibition measure of sensorimotor gating, the interval between a prepulse and the startle pulse is on the order of 50–200 msec, roughly the same interval required for short-term, presynaptic plasticity (Zucker and Regehr, 2002; Abbott and Regehr, 2004). We therefore hypothesized that alterations in presynaptic short-term plasticity at central synapses may play a role in prepulse inhibition of startle. To test this hypothesis, we examined three presynaptic protein mutant mouse lines exhibiting increased PPF as well as one presynaptic mutant mouse line with no change in PPF.
Behavioral Overview
RIM1α−/−, Rab3A−/−, Syt1R233Q KI, and RIM1αS413A KI mice were generated as previously described (Geppert et al., 1994a; Fernandez-Chacon et al., 2001; Schoch et al., 2002; Kaeser et al., 2008b). All mice were generated using SM1 129S6/SvEvTac or R1 129X1/SvJ embryonic stem cell clones, and the resulting chimeric mice were bred with C57BL/6J mice to obtain F1 heterozygous mice. Each strain was backcrossed to C57BL/6J for at least 3–4 generations with the exception of RIM1αS413A KI mice which were backcrossed only once. All mice used were sex-matched, littermate products of heterozygous matings largely tested between 4–12 months of age. Only male mice were used in all studies except for the Syt1R233QKI mice in which roughly equal numbers of males and females were used. No sex differences were observed in the Syt1R233QKI mice, so data were pooled from both sexes.
Social Interaction
Interaction with a novel juvenile target mouse was performed essentially as described (Kwon et al., 2006; Tabuchi et al., 2007; Blundell et al., 2009).
Prepulse Inhibition (sensorimotor gating)
We used a variation on the protocol of Dulawa and Geyer (Dulawa and Geyer, 1996, 2000). Startle chambers (San Diego Instruments) modified for mice were mounted atop a piezoelectric accelerometer that detects and transduces animal movements. Acoustic stimuli were delivered by high-frequency speakers mounted 33 cm above the cylinders. Animal movements were digitized and stored using computer software supplied by SD Instruments. From onset of startle stimuli, 65, 1 msec readings were recorded and the amplitude of the startle response was obtained in arbitrary units. Chambers were calibrated prior to each set of mice and sound levels monitored using a sound meter (Tandy).
Mice were subjected to five trial types in a 22 min session: Pulse alone (40 ms, 120 dB, white noise pulse), 3 different Prepulse/Pulse trials (20 ms prepulse of 4, 8, or 16 dB above background noise level of 70 dB precedes the 120 dB pulse by 100 ms (onset to onset), and No Stimulus. All trials were presented pseudorandomly with an average of 15 s (7–23 s) between the 62 trials. Testing began with a 5 minute acclimation to the cylinders followed by 4 blocks of test trials. The first and last blocks consisted of 6 Pulse Alone trials. Blocks 2 and 3 contained six Pulse Alone trials, five of each level of Prepulse/Pulse trials and five no stimulus trials. Data were analyzed for baseline startle amplitude (initial Pulse Alone Trials) and prepulse inhibition (% decrease in startle amplitude for Prepulse/Pulse Trials compared to Pulse Alone Trials).
Locomotor Activity
Mice were placed in a clean mouse cage with minimal bedding for 3 hours. Horizontal activity was monitored using photobeams linked to computer data acquisition software (San Diego Instruments). Twelve mice were tested simultaneously under low-light conditions.
Locomotor Response to Psychotomimetics
The apparatus described above was used. Mice were given a saline injection immediately prior to placement into the apparatus for 3 hours. After the end of the first and the second hours, an i.p. injection of MK-801 (0.1 then 0.2 mg/kg) was given and mice remained in the chambers for two additional hours. MK-801 was prepared in sterile normal saline and given in volumes of approximately 0.2 +/− 0.1 cc. Normal saline injection volume was matched to that of the drug for each individual mouse. In RIM1α−/− mice, 4 days of locomotor activity with 3 saline injections was performed prior to the “drug day” with either MK-801. This was to eliminate baseline locomotor differences on the testing day so that locomotor responses to psychotomimetics could be accurately measured. Similar 4-day habituation periods were given to Syt1R233QKI mice, but not to Rab3A−/− or RIM1αS413AKI mice.
Statistics
For social interaction data, Student’s t-test was used. PPI data were analysed with 2-way mixed ANOVAs with Genotype as the between-subjects factor and PPI level as the within-subjects factor. For all locomotor activity data (with and without psychotomimetics), 2-way mixed ANOVAs were conducted with Genotype as a between-subjects factor and Time as a within-subjects factor. Each hour of locomotor activity (i.e. the phase of locomotor activity after each i.p. injection) was analyzed separately. For all non-reported ANOVA main effects and interactions, p > 0.05.
Prepulse Inhibition in Mutant Mice with Altered Presynaptic Short-term Plasticity
Consistent with the hypothesis that presynaptic short-term plasticity plays a role in prepulse inhibition, mice deficient in RIM1α, known to exhibit increased PPF and short-term presynaptic facilitation at excitatory synapses of area CA1 of the hippocampus (Schoch et al., 2002), were significantly impaired in prepulse inhibition of startle (PPI) (Fig. 1a). In fact, RIM1α deficient mice exhibit significantly decreased PPI at all three levels of prepulse tested (Fig. 1a) with no significant alteration in initial startle amplitude (2-way mixed ANOVA, main effect of genotype, F1,26=14.35, p < 0.001, main effect of prepulse level, F2,52=35.81, p < 0.001, no significant interaction; p values for post-hoc planned comparisons at each prepulse level shown above bars in all panels, N = 14). Because RIM1α deficient mice also exhibit increased locomotor activation in response to novel environments (Fig. 3a & b), we additionally tested PPI following four days of habituation to the startle chambers. Even following habituation to the chambers and background noise for four days, RIM1α−/− mice still exhibited the same significantly decreased in PPI at all levels of prepulse (Fig. 1b; 2-way Mixed ANOVA, main effect of genotype, F1,26=12.45, p < 0.01, main effect of prepulse level, F2,52=49.56, p < 0.001). Thus, this decreased PPI phenotype in RIM1α−/− mice was not due to an altered response to novelty and is robust and repeatable.
Figure 1
Figure 1
Prepulse inhibition (PPI) deficits in presynaptic mutant mice with abnormal short-term, presynaptic facilitation (RIM1α−/−, Rab3A−/−, Syt1R233QKI, but not RIM1αS413A KI)
Figure 3
Figure 3
Exaggerated response to the psychotomimetic drug MK-801 selectively in RIM1α−/− mice
Mice deficient in the presynaptic protein, Rab3A, also exhibit a significant decrease in PPI (Fig. 1c) concomitant with a known increase in PPF in area CA1 of hippocampus (Geppert et al., 1997; Schoch et al., 2002). Again, while Rab3A knockout mice showed no significant change in initial startle amplitude (2-way Mixed ANOVA, main effect of genotype, F1,26=13.08, p < 0.01, main effect of prepulse level, F2,52=43.55, p < 0.001, no significant interaction, N = 15). PPI was reduced at all three levels of prepulse (Fig. 1c).
Similarly, synaptotagmin 1 R233Q point mutant knockin mice (Syt1R233Q KI) mice have increased PPF (Fernandez-Chacon et al., 2001) and demonstrated significantly decreased PPI (Fig. 1d). The decrease in PPI in Syt1R233QKI knockin mice was significant at a single prepulse level (8 dB above background). No significant change in initial startle amplitude was observed in the Syt1R233Q KI mice (2-way Mixed ANOVA revealed no significant main effect of genotype, F1,52=3.45, p = 0.07, main effect of prepulse level, F1,52=28.38, p < 0.0001, no significant interaction, p < 0.05, N = 14).
In an effort to demonstrate selectivity of the deficits in PPI, we examined another presynaptic protein mutant, the RIM1αS413A point mutant KI mice (RIM1αS413A KI), that do not exhibit any alteration in PPF or short-term facilitation (Kaeser et al., 2008b). The RIM1αS413A KI mice did not display a deficit in PPI at any level of prepulse tested (Fig. 1e), nor did they exhibit a change in initial startle response (2-way Mixed ANOVA, no main effect of genotype, F1,21=0.01, p = 0.94, main effect of prepulse level, F2,42=73.0, p < 0.001, no significant interaction, N = 11).
Social Interaction Deficits Selective to RIM1α−/− Mice
It is possible that alterations in presynaptic short-term plasticity or in presynaptic proteins in general lead to broad behavioural abnormalities across multiple domains. To examine the selectivity of the PPI deficit and to determine whether additional schizophrenia-relevant behavioural abnormalities are present in these presynaptic mutant mice, we tested direct social interaction with a juvenile target mouse in each line.
Consistent with previously observed schizophrenia-related cognitive deficits (Powell et al., 2004) and decreased PPI (Fig. 1a–c) in RIM1α knockout mice, these mice exhibited a dramatic decrease in social interaction with a juvenile (Fig. 2a). This finding indicates that RIM1α knockout mice may exhibit a selective pattern of schizophrenia-relevant behavioural abnormalities (Powell and Miyakawa, 2006).
Figure 2
Figure 2
Selective deficit in social interaction in RIM1α−/− mice
To examine the selectivity of this social deficit for RIM1α knockout mice versus other presynaptic protein mutants with increased PPF, we tested social interaction in each of the other three presynaptic mutant lines. Rab3A−/−, Syt1R233QKI, and RIM1αS413AKI mice all demonstrated normal direct juvenile social interaction (Fig. 2b–d). These findings are consistent with a selective pattern of schizophrenia-related behavioural abnormalities in RIM1α knockout mice compared to the three other presynaptic protein mutants.
Altered Response to MK-801 in RIM1α−/− Mice
In addition to cognitive deficits (Powell et al., 2004), decreased PPI, and decreased social interaction, RIM1α knockout mice demonstrated an increased locomotor response to novelty (Fig. 3a & b) and significantly enhanced locomotor activity after administration of the psychotomimetic, non-competitive NMDA receptor antagonist MK-801 (Fig. 3b). To examine the RIM1α locomotor response to MK-801, we first habituated RIM1α knockout mice to the locomotor testing apparatus for four days until there was no significant difference in locomotor activity in response to the chamber (Fig. 3a, Day 4, final injection, 2-way ANOVA with repeated measures, main effect of genotype, F(1,22)=2.92, p > 0.05). These data demonstrate that the RIM1α deletion-associated increase in locomotor response to novelty habituates over time as the novel context becomes familiar. On day 5, the initial locomotor response to saline injection was not significantly different in RIM1α knockout mice (1st saline injection, 2-way ANOVA with repeated measures, main effect of genotype, F(1,22)=4.12, p > 0.05, main effect of time, F(11,242)=4.28, p < 0.05, interaction, F(11,242)=1.31, p > 0.05). The locomotor response to 0.1 and 0.2 mg/kg MK-801, however, was significantly increased compared to wild-type littermate controls, as indicated by a significant interaction between genotype and time for both the 0.1 mg/kg injection (main effect of genotype, F(1,22)=3.33, p = 0.08; main effect of time, F(11,242)=4.28, p < 0.05; interaction, F(11,242)=14.26, p < 0.05) and the 0.2 mg/kg injection (main effect of genotype, F(1,22)=1.97, p >0.05; main effect of time, F(11,242)=14.26, p < 0.05; interaction, F(11,242)=3.16, p < 0.05). Thus, RIM1α knockout mice also exhibit increased locomotor response to novelty and increased sensitivity to the locomotor activating effects of the psychotomimetic drug MK-801.
Once again we tested whether the enhanced locomotor response to MK-801 in RIM1α knockout mice might be a more general effect due to presynaptic protein mutation or altered presynaptic short-term plasticity. Rab3A knockout mice showed no significant alteration in their response to MK-801 at either dose (Fig. 3c). Similarly, Syt1R233QKI mice did not exhibit any significant alteration in locomotor response to these doses of MK-801 (Fig. 3d). Because the locomotor activation to 0.1 and 0.2 mg/kg of MK-801 appeared to be somewhat less robust in Syt1R233Q KI mice than in the other three mouse lines, we repeated this experiment using two higher doses of MK-801 (0.4 and 0.6 mg/kg) which produced a much more robust locomotor response in both wild-type littermate controls and Syt1R233Q KI mice, but no significant difference between the two groups (data not shown). Finally, RIM1αS413AKI mice also exhibited no significant difference in locomotor response to MK-801 at either dose (Fig. 3e). These data, along with previously published findings in these presynaptic mutants (Powell et al., 2004; Kaeser et al., 2008b), suggest a pattern of selective, schizophrenia-relevant behavioural abnormalities in RIM1α knockout mice compared to each of the three other presynaptic mutant mouse models, with the notable exception of PPI deficits in the mouse lines with altered presynaptic short-term plasticity.
Based on the correlation between the timescale of PPI and the timescale of the presynaptic plasticity phenotypes observed in the genetic mouse mutants, we suggest a model where presynaptic short-term plasticity may play a central role in mediating sensorimotor gating (Fig. 4). In our experiments, all three genetic manipulations that alter the magnitude of PPF, a common form of presynaptic short-term plasticity (Zucker and Regehr, 2002), changed PPI, a measure used to assess sensorimotor gating (Swerdlow et al., 2000), whereas one genetic manipulation that has no effect on presynaptic function does not change PPI. Furthermore, deletion of RIM1α also diminished social interactions and increased the locomotor response to the non-competitive NMDA-receptor antagonist MK-801. These schizophrenia-related behaviors likely operate at a much slower time scale, and are not affected in the genetic deletion of Rab3A and in the Syt1R233Q point mutant mice, suggesting that these deficits depend on RIM1α-mediated synaptic mechanisms that are not directly related to presynaptic short-term plasticity.
Figure 4
Figure 4
Suggested mechanisms responsible for behavioral abnormalities
Prepulse inhibition of acoustic startle experimentally defines the phenomenon of sensorimotor gating, an evolutionarily conserved neuronal mechanism whereby an initial, lower amplitude noise pulse can decrease the response to a louder acoustic startle pulse with as little as a 50–200 msec delay between the two pulses (Braff and Geyer, 1990; Swerdlow et al., 1994; Dulawa and Geyer, 1996; Swerdlow et al., 2000; Swerdlow et al., 2001; Geyer et al., 2002). The activity of several brain regions is known to modify PPI including limbic cortex, striatum, pallidum or pontine tegmentum (Swerdlow et al., 2001). Indeed, several neuropsychiatric disorders, among them schizophrenia, bipolar mania, obsessive/compulsive disorder, and panic disorder and are associated with abnormal prepulse inhibition of startle in humans. While much is known regarding the brain regions involved in modification of PPI, and a neuropharmacology has evolved around the alteration of PPI in a variety of models (Dulawa and Geyer, 1996; Brody et al., 2003; Brody et al., 2004; Swerdlow et al., 2005; Geyer, 2006a), the neurophysiological and synaptic basis of sensorimotor gating remains a mystery.
This manuscript implicates short-term presynaptic plasticity in PPI of acoustic startle and sensorimotor gating in general. Our data provide support for the hypothesis that short-term presynaptic plasticity that occurs on the same time scale as PPI plays a role in, or modulates, sensorimotor gating. Three different presynaptic protein mutant mouse lines with altered central nervous system short-term synaptic plasticity exhibit significant decreases in PPI. In the case of RIM1α−/− and Syt1R233QKI mice, the short-term plasticity changes (increased PPF) are also associated with a decrease in initial probability of neurotransmitter release (Fernandez-Chacon et al., 2001; Schoch et al., 2002). In the case of Rab3A−/− mice, however, the increased PPF is not associated with altered release probability (Schoch et al., 2002). As an additional control, RIM1αS413A KI mice that do not show increased PPF also do not show alterations in PPI, indicating some degree of specificity for mouse lines with altered short-term plasticity. The RIM1αS413A KI mice were backcrossed to C57BL/6J only once prior to testing, though the other lines were backcrossed to C57BL/6J for 3–4 generations. This presents only a minor caveat to interpretation of this additional control mouse line; it is possible that RIM1αS413Amutant mice may lead to PPI changes following further backcrossed to C57BL/6J, but these changes are somehow masked by additional 129Sv background remaining in this line. In spite of this caveat, the correlation between PPI and PPF in all backgrounds, comparing mutant mice to their littermate controls of the same background, suggest that increased PPF can lead to decreased PPI of acoustic startle responses.
The alterations in PPI observed were not associated with significant alterations in initial startle response amplitude. In fact, in the Syt1R233QKI and Rab3A−/− mice exhibit opposite trends toward alterations in initial startle response, increased in Rab3A−/− and decreased in Syt1R233QKI mice. In spite of these trends toward opposing effects on initial startle responses, both mice exhibit significant decreases in PPI, making initial startle changes an unlikely cause of the observed PPI deficits in these mouse lines.
Alteration of presynaptic genes linked to schizophrenia has resulted in additional mouse models relevant to the disorder and to PPI alterations. For example, neurexin 1 has been linked in many studies to both schizophrenia (Kirov et al., 2008; Sudhof, 2008; Walsh et al., 2008; Rujescu et al., 2009) and autism (Sebat et al., 2007; Szatmari et al., 2007; Kim et al., 2008; Marshall et al., 2008; Yan et al., 2008; Zahir et al., 2008; Bucan et al., 2009; Glessner et al., 2009). We recently demonstrated altered PPI in neurexin-1αKO mice, along with increased repetitive grooming behaviour and impaired nest building. These behavioral changes were associated with decreased glutamatergic synaptic transmission, but not alterations in PPF (Etherton et al., 2009). A recent synapsin II mouse model relevant to schizophrenia found several putative behavioural abnormalities relevant to schizophrenia including PPI (Dyck et al., 2009). Synapsin II KO mice, however, do not show altered PPF, though they do have enhanced post-tetanic potentiation, a form of plasticity with a longer time course on the order of minutes rather than tens to hundreds of milliseconds (Rosahl et al., 1995; Silva et al., 1996). Thus, alterations in PPF are not the only mechanism whereby presynaptic protein mutations can lead to altered PPI.
We suggest that short-term plasticity of synaptic responses in the area of the brainstem startle circuitry or in modulatory regions plays a role in the effect of the prepulse on the startle pulse presented 50–200 msec later. Of course, it may be that alterations in presynaptic proteins and short-term plasticity lead to developmental circuit alterations in the acoustic startle areas or in any one of the many brain regions known to modify PPI. Future studies will focus on conditional regional or post-developmental deletions and mutations of presynaptic proteins in an effort to localize the synaptic plasticity deficits contributing to this alteration.
RIM1α appears to be particularly important for a wide variety of classic schizophrenia-relevant behavioural abnormalities (Powell et al., 2004; Powell, 2006; Powell and Miyakawa, 2006). Among the behaviours classically ascribed as schizophrenia-relevant in RIM1α−/− mice, but not in RIM1αS413AKI, Rab3A−/−, or Syt1R233QKI mice, are hippocampus-dependent cognitive function (Powell et al., 2004), locomotor response to novelty stress (Fig. 3 and Powell et al, 2004), social interaction (Fig. 2), locomotor response to psychotomimetics (Fig. 3), and PPI (Fig. 1). Several other behaviours are completely normal in the RIM1α−/− mice including motor coordination and motor learning on the rotarod, locomotor habituation, anxiety in the elevated plus maze and dark/light apparati, and nociception (Powell et al., 2004).
It is important to note that deletion of RIM1α in mice results in a relatively wide synaptic deficit in the hippocampus, with distinct phenotypes in different synapses (Castillo et al., 2002; Schoch et al., 2002; Kaeser et al., 2008a), probably because of overlapping functions of multiple RIM isoforms (Schoch et al., 2006; Kaeser et al., 2008a). These phenotypes include a reduction in synaptic strength at excitatory and inhibitory synapses in area CA1 (Schoch et al., 2002; Calakos et al., 2004; Kaeser et al., 2008a), multiple changes in short-term plasticity at excitatoryand inhibitory synapses in area CA1, but not in mossy fiber terminal in area CA3 (Schoch et al., 2002; Calakos et al., 2004; Kaeser et al., 2008a), a reduction in the readily releasable pool of vesicles at excitatory hippocampal synapses (Calakos et al., 2004), and a lack of many forms of presynaptic long-term plasticity at multiple synapses in the hippocampus and in other brain areas (Castillo et al., 2002; Huang et al., 2005; Fourcaudot et al., 2006; Chevaleyre et al., 2007). Thus, it is difficult to attribute certain behavioral parameters to a specific from of synaptic plasticity at a defined synapse at this time. We consider the PPI abnormality likely to be mediated by paired-pulse ratios, because 1) the time scale of the behaviour matches well with the time scale of the electrophysiological phenomenon, 2) because changes in paired-pulse ratios are the only electrophysiological deficit that is shared in all 3 lines (Geppert et al., 1994b; Fernandez-Chacon et al., 2001; Schoch et al., 2002), and 3) a related mouse line lacking altered PPF does not show altered PPI. For the other behavioural parameters measured, however, it is premature to draw more precise conclusions beyond the observation that they depend on RIM1α.
A recent study calls into question the relevance of increased locomotor response to novelty for schizophrenia (Perry et al., 2009). Based on findings from human clinical studies, it is possible that this behavioural abnormality may be more akin to bipolar mania in humans and not schizophrenia (Perry et al., 2009). Although assigning a specific neuropsychiatric disorder to a set of rodent behavioural abnormalities is fraught with difficulty, we feel the entire constellation of behavioural abnormalities in the RIM1α KO mice is more consistent with schizophrenia than bipolar mania or other disorders, given the cognitive abnormalities, response to psychotomimetic drugs, and social interaction deficits that are not typical of bipolar mania. That said, it is possible that the PPI alterations and locomotor response to novelty could be interpreted as relevant to other neuropsychiatric disorders such as bipolar mania. It will be of interest to see how these behavioral abnormalities respond to pharmacologic treatments used in humans for both schizophrenia and bipolar mania.
Overall, our data suggest that loss of RIM1α function can lead to selective abnormalities in behaviours typical of many other preclinical mouse models of schizophrenia, though this may also be interpreted as relevant for other neuropsychiatric disorders. More than one presynaptic protein has been implicated in schizophrenia, and the present results suggest the RIM gene family may embody potential candidate genes for human schizophrenia or other neuropsychiatric disorders. The subtleties of how some presynaptic protein abnormalities, but not others, lead to synaptic and circuit-level dysfunction resulting in selective behavioral symptoms will be critical to determine in future studies.
Acknowledgments
T.C.S. and P.S.K. supplied the mice; C.M.P. conceived and designed the experiments; C.M.P. & J.B. carried them out; C.M.P. performed statistical analysis; and C.M.P. wrote the paper with input from J.B., P.S.K., & T.C.S. We thank current members of the Powell lab for critical reading of the manuscript. Supported by grants from NARSAD (C.M.P., Constance & Stephen Lieber Investigator, P.S.K.), NIMH (C.M.P. & T.C.S.), NICHD (C.M.P.), HHMI (T.C.S.), & The Hartwell Foundation (C.M.P.).
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