Mechanisms for synapse specificity during striatal long-term depression

Abstract

Endocannabinoid (eCB)-mediated forms of long-term synaptic plasticity occur in several brain regions, but much remains unknown about their basic properties and underlying mechanisms. Here, we present evidence that eCB-mediated long-term depression (eCB-LTD) at excitatory synapses on medium spiny neurons in the striatum requires presynaptic activity coincident with CB1 receptor activation. This dual requirement for CB1 activation and presynaptic activity is a mechanism by which eCB-LTD may be made synapse specific.

Figure 1.

Activity dependence of WIN-induced LTD. A, B, Example (A) and summary (B; n = 5) of two-pathway experiments in which WIN (1 μm) was applied for 10 min and followed by the CB1 receptor antagonist AM251 (5 μm). During WIN application, synaptic stimulation was stopped in one path while the other path received stimulation at 1 Hz for 5 min. C, D, Example (C) and summary (D; n = 6) of two-pathway experiments in which stimulation was stopped in one path during 20 min application of WIN while baseline stimulation at 0.05 Hz was maintained in the other path. E, Summary graph (n = 5) of experiments in which afferents were stimulated at 1 Hz for 5 min. F, Summary graph (n = 6) of experiments in which afferent stimulation was stopped for 25–35 min. The experiments summarized in E and F were done on two separate sets of cells. A, C, Sample EPSCs are shown superimposed above the graphs. Thick traces are averages taken during the 10 min baselines. Thin traces are averages taken at 35–40 min (A) and 50–55 min (C). Calibration: 10 ms, 100 pA. stim, Stimulation.

Figure 2.

Activity dependence of eCB-LTD. A, B, Example (A) and summary (B; n = 6) of experiments in which both pathways received tetanic stimulation (100 Hz/1 s given 4 times at arrows) to elicit eCB production and release. Baseline stimulation (0.05 Hz) was then stopped in one pathway for 10 min. Thick traces of sample EPSCs are averages taken during the 10 min baselines. Thin traces are averages taken at 40–45 min. Calibration: 10 ms, 100 pA.

Figure 3.

WIN does not affect mEPSC frequency or amplitude. A, Sample mEPSCs (recorded continuously in 1 μm TTX) before, during, and after 20 min application of WIN (1 μm) followed by application of AM251 (5 μm). Each panel shows consecutively recorded traces. B, Summary graph (n = 5) showing normalized mEPSC frequency (left) and amplitude (right) during the course of the experiment. Calibration: 500 ms, 10 pA.

Figure 4.

LTD requires presynaptic Ca²⁺. A, Summary graph (n = 4) of experiments in which WIN was applied in the absence of external Ca²⁺. Sample traces (5 min averages) were taken at the indicated times. Calibration: 20 ms, 100 pA. B, Summary graph (n = 6) of experiments in which 10–20 mm BAPTA was included in the postsynaptic recording pipette. C, Summary graph of experiments in which baseline synaptic responses were recorded and then 3 mm kynurenate and 100 μm LY341495 were applied. In one set of cells, WIN and AM251 were then applied (open circles, n = 5), whereas in the other set of cells, WIN and AM251 were not applied (filled circles; n = 7). Calibration: 20 ms, 100 pA. D, Graph from C with expanded axes showing that application of WIN induces LTD even when EPSCs were reduced by ∼95% to 5–7 pA. Sample traces were taken at the indicated times. Calibration: top, 10 ms, 10 pA; bottom, 10 ms, 5 pA. Traces in D are from the same cells as traces in C.