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.

Calcineurin is required for TRPV1-induced long-term depression of hippocampal interneurons.

Transient receptor potential vanilloid 1 (TRPV1) mediates a novel form of presynaptic long-term depression (LTD) in hippocampal interneurons. To date, while TRPV1 is currently being heavily studied in the PNS for its anti-nociceptive and anti-inflammatory properties, much less is known regarding TRPV1 signaling and function in the CNS, including the mechanism mediating hippocampal interneuron LTD. Here we performed whole-cell voltage clamp electrophysiology experiments on CA1 hippocampal interneurons from Sprague-Dawley male rats to identify this signaling mechanism. Because calcineurin is linked to multiple synaptic plasticity types, we investigated whether TRPV1 activates presynaptic calcineurin, which in turn induces LTD. To do so we employed calcineurin inhibitors cyclosporin A or FK-506. To determine the location of the calcineurin involved we either bath applied calcineurin antagonists, blocking calcineurin activity ubiquitously in the slice, presynaptically and postsynaptically, or applied antagonists to the internal solution to block calcineurin postsynaptically. Both calcineurin antagonists applied to the bath blocked TRPV1-dependent LTD, indicating calcineurin involvement in LTD. Because calcineurin antagonist applied to the internal solution did not block TRPV1-LTD, it suggests presynaptic calcineurin is required for LTD. Finally, because high frequency stimulus used to induce LTD could potentially activate receptors besides TRPV1, we confirmed that bath, but not intracellularly applied cyclosporin A, also blocked TRPV1 agonist-induced depression of CA1 interneurons. In conclusion, these data illustrate that presynaptic calcineurin activity is required for both TRPV1-induced LTD and TRPV1 agonist-induced depression. This finding is the first to demonstrate the TRPV1-induced signaling mechanism in CA1 hippocampus.

Transient receptor potential vanilloid 1 agonists modulate hippocampal CA1 LTP via the GABAergic system.

Transient receptor potential vanilloid 1 (TRPV1) was shown to modulate hippocampal CA1 pyramidal cell synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD). Synaptic plasticity is the cellular mechanism thought to mediate declarative learning and memory in the hippocampus. Although TRPV1 is involved in modulating hippocampal plasticity, it has yet to be determined how TRPV1 mediates its effects. Using field electrophysiology in hippocampal CA1 stratum radiatum we investigated how TRPV1 agonists modulate LTP, low frequency stimulation-induced LTD, and (RS)-3,5-dihydroxyphenylglycine (DHPG)-induced LTD. First we confirmed that TRPV1 agonists induce enhancement of CA1 pyramidal cell LTP in the absence the GABA(A) receptor antagonist picrotoxin. Because it was recently determined that TRPV1 mediates a novel form of LTD in CA1 inhibitory GABAergic interneurons, which can disinhibit CA1 pyramidal cells, we used picrotoxin to block the effect of the GABAergic circuitry on CA1 LTP. When using picrotoxin, the TRPV1 agonist-induced enhancement of CA1 LTP was eliminated suggesting that the GABAergic circuitry is required for TRPV1 agonist mediated increases. Regarding LTD, in contrast to previously reported data, we did not see TRPV1 agonist-mediated effect on low frequency-induced stimulus LTD. However, during DHPG-induced LTD, TRPV1 was involved in the acute, but not the long-term depression phase of this plasticity. In summary, our findings support TRPV1 agonist involvement in hippocampal synaptic plasticity, including its enhancement of CA1 LTP. We demonstrate that the enhancement mediated by TRPV1 agonists requires GABA input to pyramidal cells thus providing a mechanism for how TRPV1 agonists modulate hippocampal synaptic plasticity.