Loss of interneuron LTD and attenuated pyramidal cell LTP in Trpv1 and Trpv3 KO mice

Abstract

TRPV (transient receptor potential, vanilloid) channels are a family of nonselective cation channels that are activated by a wide variety of chemical and physical stimuli. TRPV1 channels are highly expressed in sensory neurons in the peripheral nervous system. However, a number of studies have also reported TRPV channels in the brain, though their functions are less well understood. In the hippocampus, the TRPV1 channel is a novel mediator of long-term depression (LTD) at excitatory synapses on interneurons. Here we tested the role of other TRPV channels in hippocampal synaptic plasticity, using hippocampal slices from Trpv1, Trpv3 and Trpv4 knockout (KO) mice. LTD at excitatory synapses on s. radiatum hippocampal interneurons was attenuated in slices from Trpv3 KO mice (as well as in Trpv1 KO mice as previously reported), but not in slices from Trpv4 KO mice. A previous study found that in hippocampal area CA1, slices from Trpv1 KO mice have reduced tetanus-induced long-term potentiation (LTP) following high-frequency stimulation; here we confirmed this and found a similar reduction in Trpv3 KO mice. We hypothesized that the loss of LTD at the excitatory synapses on local inhibitory interneurons caused the attenuated LTP in the mutants. Consistent with this idea, blocking GABAergic inhibition rescued LTP in slices from Trpv1 KO and Trpv3 KO mice. Our findings suggest a novel role for TRPV3 channels in synaptic plasticity and provide a possible mechanism by which TRPV1 and TRPV3 channels modulate hippocampal output.

Keywords: TRPV channel; TRPV1; TRPV3; synaptic plasticity.

Figure 1. Slices from Trpv1 KO and Trpv3 KO Mice Lack Interneuron LTD

(A) A single experiment illustrating that interneurons from Trpv1 KO mice lack HFS-induced LTD of excitatory synapses onto CA1 stratum radiatum interneurons. HFS was delivered at the arrow in all figures. (B) Averaged experiments from interneurons from WT (closed circles) and Trpv1 KO mice (open squares). Insets: ten averaged EPSCs from a single experiment taken at the times indicated before (1, black) and 15-20 min after HFS (2, red). Calibration: 50 pA, 5 ms. (C) Scatter plot of individual experiments illustrating LTD observed in each cell. (D) Single experiment showing that interneurons from Trpv3 KO mice also lack LTD. (E) Averaged experiments from WT (closed circles) and Trpv3 KO mice (open squares). (F) Scatter plot of individual experiments illustrate EPSC amplitudes from interneurons in WT and Trpv3 KO mice.

Figure 2. Slices from Trpv4 KO Mice Express Normal Interneuron LTD

(A) Individual experiment showing LTD in an interneuron from a Trpv4 KO mouse. (B) Averaged experiments from WT (closed circles) and Trpv4 KO mice (open squares) showing that that interneurons from Trpv4 KO mice express HFS-induced LTD comparable to those from WT mice. Insets: ten averaged EPSCs from a single experiment taken at the times indicated before (1, black) and 15-20 min after HFS (2, red). Calibration: 50 pA, 5 ms. (C) Scatter plot of individual experiments illustrating LTD magnitude in each experiment.

Figure 3. Extracellular Recordings from Trpv1 KO and Trpv3 KO Mice have Attenuated Pyramidal Cell LTP that is Rescued by a GABA_AR Antagonist

(A) Hippocampal slices from Trpv1 KO mice have significantly less LTP at CA3-CA1 pyramidal cell synapses. Time courses of averaged experiments from WT (closed circles) and Trpv1 KO mice (open squares). (B) CA3-CA1 pyramidal cell synapses from Trpv3 KO mice also have significantly less LTP (WT, closed circles; Trpv3 KO, open squares). (C) CA3-CA1 pyramidal cell synapses from Trpv1 KO (V1, black) and Trpv3 KO (V3, stripes) mice have significantly less LTP than those in WT mice (WT, white)(15 to 20 min post-HFS,* indicates p<0.05). Inset: averaged fEPSPs before (black) and 15-20 min post-HFS (red). Calibration, WT: 0.5 mV, 5 ms. Calibration, Trpv1 KO and Trpv3 KO mice: 0.4 mV, 5 ms.

Figure 4. Blocking GABA_A Receptors Rescues Pyramidal Cell LTP in Slices from Trpv1 KO and Trpv3 KO Mice

(A) When HFS is delivered in the presence of the GABA_AR antagonist, picrotoxin (100 μM), LTP is rescued to WT levels in slices from Trpv1 KO mice (WT, closed circles; Trpv1 KO + picrotoxin, open squares). (B) Similarly, when HFS is delivered in the presence of picrotoxin, LTP in Trpv3 KO mice is rescued to WT levels (WT, closed circles; Trpv3 KO + picrotoxin, open squares). (C) Picrotoxin rescues LTP in slices from Trpv1 KO and Trpv3 KO mice (15 to 20 min post-HFS). Picrotoxin has no significant effect on WT fEPSPs (WT, n=27; WT + picrotoxin, 97± 2% of WT, n=4), while LTP from Trpv1 KO and Trpv3 KO mice is not significantly different from that in WT slices. Top inset: averaged fEPSPs before (black) and 15-20 min after HFS (red). Calibration for WT, WT+PTX, Trpv3 KO + PTX: 0.5 mV, 5 ms. Calibration for Trpv1 KO + PTX 0.35 mV, 5 ms. (D) Picrotoxin does not significantly increase fEPSPs in untetanized slices from Trpv1 KO (open squares, n=4) and Trpv3 KO mice (closed squares, n=4) compared to slices from WT controls (closed circles, n=5).

Figure 5. Diagram of Local Circuit effects of TRPV channels

Glutamate release onto GABAergic interneurons (green) after HFS undergoes TRPV1/TRPV3-dependent LTD. The GABAergic depression may cause feedforward disinhibition, promoting NMDAR-dependent LTP in the excitatory glutamatergic (blue) synapses on CA1 pyramidal cells. A consequence of knocking out TRPV1 and TRPV3 channels is that excitatory synapses onto GABAergic interneurons no longer undergo LTD. As a result, inhibition of CA1 neurons is relatively greater during HFS, and the magnitude of LTP of the pyramidal cells is reduced.