Working memory impairment in calcineurin knock-out mice is associated with alterations in synaptic vesicle cycling and disruption of high-frequency synaptic and network activity in prefrontal cortex.

Working memory is an essential component of higher cognitive function, and its impairment is a core symptom of multiple CNS disorders, including schizophrenia. Neuronal mechanisms supporting working memory under normal conditions have been described and include persistent, high-frequency activity of prefrontal cortical neurons. However, little is known about the molecular and cellular basis of working memory dysfunction in the context of neuropsychiatric disorders. To elucidate synaptic and neuronal mechanisms of working memory dysfunction, we have performed a comprehensive analysis of a mouse model of schizophrenia, the forebrain-specific calcineurin knock-out mouse. Biochemical analyses of cortical tissue from these mice revealed a pronounced hyperphosphorylation of synaptic vesicle cycling proteins known to be necessary for high-frequency synaptic transmission. Examination of the synaptic vesicle cycle in calcineurin-deficient neurons demonstrated an impairment of vesicle release enhancement during periods of intense stimulation. Moreover, brain slice and in vivo electrophysiological analyses showed that loss of calcineurin leads to a gene dose-dependent disruption of high-frequency synaptic transmission and network activity in the PFC, correlating with selective working memory impairment. Finally, we showed that levels of dynamin I, a key presynaptic protein and calcineurin substrate, are significantly reduced in prefrontal cortical samples from schizophrenia patients, extending the disease relevance of our findings. Our data provide support for a model in which impaired synaptic vesicle cycling represents a critical node for disease pathologies underlying the cognitive deficits in schizophrenia.

A novel non-CB1/TRPV1 endocannabinoid-mediated mechanism depresses excitatory synapses on hippocampal CA1 interneurons.

Endocannabinoids (eCBs) mediate various forms of synaptic plasticity at excitatory and inhibitory synapses in the brain. The eCB anandamide binds to several receptors including the transient receptor potential vanilloid 1 (TRPV1) and cannabinoid receptor 1 (CB1). We recently identified that TRPV1 is required for long-term depression at excitatory synapses on CA1 hippocampal stratum radiatum interneurons. Here we performed whole-cell patch clamp recordings from CA1 stratum radiatum interneurons in rat brain slices to investigate the effect of the eCB anandamide on excitatory synapses as well as the involvement of Group I metabotropic glutamate receptors (mGluRs), which have been reported to produce eCBs endogenously. Application of the nonhydrolysable anandamide analog R-methanandamide depressed excitatory transmission to CA1 stratum radiatum interneurons by ∼50%. The Group I mGluR agonist DHPG also depressed excitatory glutamatergic transmission onto interneurons to a similar degree, and this depression was blocked by the mGluR5 antagonist MPEP (10 µM) but not by the mGluR1 antagonist CPCCOEt (50 µM). Interestingly, however, neither DHPG-mediated nor R-methanandamide-mediated depression was blocked by the TRPV1 antagonist capsazepine (10 µM), the CB1 antagonist AM-251 (2 µM) or a combination of both, suggesting the presence of a novel eCB receptor or anandamide target at excitatory hippocampal synapses. DHPG also occluded R-methanandamide depression, suggesting the possibility that the two drugs elicit synaptic depression via a shared signaling mechanism. Collectively, this study illustrates a novel CB1/TRPV1-independent eCB pathway present in the hippocampus that mediates depression at excitatory synapses on CA1 stratum radiatum interneurons.

Clorgyline-mediated reversal of neurological deficits in a Complexin 2 knockout mouse.

Complexin 2 is a protein modulator of neurotransmitter release that is downregulated in humans suffering from depression, animal models of depression and neurological disorders such as Huntington's disease in which depression is a major symptom. Although complexin 2 knockout (Cplx2-/-) mice are overtly normal, they show significant abnormalities in cognitive function and synaptic plasticity. Here we show that Cplx2-/- mice also have disturbances in emotional behaviours that include abnormal social interactions and depressive-like behaviour. Since neurotransmitter deficiencies are thought to underlie depression, we examined neurotransmitter levels in Cplx2-/- mice and found a significant decrease in levels of noradrenaline and the serotonin metabolite 5-hydroxyindoleacetic acid in the hippocampus. Chronic treatment with clorgyline, an irreversible inhibitor of monoamine oxidase A, restored hippocampal noradrenaline to normal levels (from 60 to 97% of vehicle-treated Cplx2+/+ mice, P<0.001), and reversed the behavioural deficits seen in Cplx2-/- mice. For example, clorgyline-treated Cplx2-/- mice spent significantly more time interacting with a novel visitor mouse compared with vehicle-treated Cplx2-/- mice in the social recognition test (34 compared with 13%, P<0.01). We were also able to reverse the selective deficit seen in mossy fibre-long-term potentiation (MF-LTP) in Cplx2-/- mice using the noradrenergic agonist isoprenaline. Pre-treatment with isoprenaline in vitro increased MF-LTP by 125% (P<0.001), thus restoring it to control levels. Our data strongly support the idea that complexin 2 is a key player in normal neurological function, and that downregulation of complexin 2 could lead to changes in neurotransmitter release sufficient to cause significant behavioural abnormalities such as depression.

Hot flash: TRPV channels in the brain.

TRPV1 (transient receptor potential, vanilloid) channels belong to a family of ligand-gated ion channels gated not only by the binding of certain lipophilic molecules but also by extracellular protons and physical stimuli such as heat or osmotic pressure changes. These nonselective cation channels are permeable to Na(+) and K(+) and are also very Ca(2+) permeable; in fact, TRPV1 is as Ca(2+) permeable as the NMDA receptor channel and can, thus, act as a trigger for Ca(2+)-mediated cell signaling. Although these channels are highly expressed in primary sensory afferents, accumulating evidence indicates that TRPV family channels are also present in the brain. Here, we review evidence that TRPV channels in the central nervous system might contribute to many basic neuronal functions including resting membrane potential, neurotransmitter release and synaptic plasticity.

TRPV1 channels mediate long-term depression at synapses on hippocampal interneurons.

TRPV1 receptors have classically been defined as heat-sensitive, ligand-gated, nonselective cation channels that integrate nociceptive stimuli in sensory neurons. TRPV1 receptors have also been identified in the brain, but their physiological role is poorly understood. Here we report that TRPV1 channel activation is necessary and sufficient to trigger long-term synaptic depression (LTD). Excitatory synapses onto hippocampal interneurons were depressed by either capsaicin, a potent TRPV1 channel activator, or the endogenously released eicosanoid, 12-(S)-HPETE, whereas neighboring excitatory synapses onto CA1 pyramidal cells were unaffected. TRPV1 receptor antagonists also prevented interneuron LTD. In brain slices from TRPV1-/- mice, LTD was absent, and neither capsaicin nor 12-(S)-HPETE elicited synaptic depression. Our results suggest that, in the hippocampus, TRPV1 receptor activation selectively modifies synapses onto interneurons. Like other forms of hippocampal synaptic plasticity, TRPV1-mediated LTD may have a role in long-term changes in physiological and pathological circuit behavior during learning and epileptic activity.

A similar impairment in CA3 mossy fibre LTP in the R6/2 mouse model of Huntington's disease and in the complexin II knockout mouse.

Complexin II is reduced in Huntington's disease (HD) patients and in the R6/2 mouse model of HD. Mice lacking complexin II (Cplx2-/- mice) show selective cognitive deficits that reflect those seen in R6/2 mice. To determine whether or not there is a common mechanism that might underlie the cognitive deficits, long-term potentiation (LTP) was examined in the CA3 region of hippocampal slices from R6/2 mice and Cplx2-/- mice. While associational/commissural (A/C) LTP was not significantly different, mossy fibre (MF) LTP was significantly reduced in slices from R6/2 mice and Cplx2-/- mice compared with wild-type (WT) and Cplx2+/+ control mice. MF field excitatory postsynaptic potentials (fEPSPs) in response to paired stimuli were not significantly different between control mice and R6/2 or Cplx2-/- mice, suggesting that MF basal glutamate release is unaffected. Forskolin (30 microm) caused an increase in glutamate release at MF synapses in slices from R6/2 mice and from Cplx2-/- mice that was not significantly different from that seen in control mice, indicating that the capacity for increased glutamate release is not diminished. Thus, R6/2 mice and Cplx2-/- mice have a common selective impairment of MF LTP in the CA3 region. Together, these data suggest that complexin II is required for MF LTP, and that depletion of complexin II causes a selective impairment in MF LTP in the CA3 region. This impairment in MF LTP could contribute to spatial learning deficits observed in R6/2 and Cplx2-/- mice.