Frequency-dependent inversion of net striatal output by endocannabinoid-dependent plasticity at different synaptic inputs

Abstract

Understanding how striatal neurons integrate glutamatergic and GABAergic inputs is essential for understanding the control of movement and the formation of striatal-based memories. Here we show that GABAergic synapses on striatal medium spiny neurons (MSNs) are more sensitive than glutamatergic synapses on the same cells to endocannabinoid (eCB) signaling, and that protocols that induce short-lasting cannabinoid 1 receptor (CB(1)R)-dependent depression at glutamatergic synapses are sufficient to induce long-term depression (LTD) at GABAergic synapses. We also show that the frequency and duration of glutamatergic input are strong determinants of the net effect of eCB signaling, and key factors in determining if LTD has a net disinhibitory or inhibitory action in striatum. Plastic changes in net output from striatal MSNs are thus a complex function of disinhibitory and inhibitory LTD combined with other forms of synaptic plasticity such as long-term potentiation at excitatory synapses.

Figure 1.

1 Hz stimulation for 1 min induces mGluR-dependent LTD at GABAergic synapses. a, A train of 60 pulses delivered at 1 Hz induced LTD at GABAergic synapses, while EPSC amplitude was only slightly decreased. b, Stimulation-induced depression at GABAergic synapses required activation of group I mGluRs. c, AM251 perfusion prevented depression induced by 1 Hz stimulation, but was insufficient to reverse an established depression. Example traces show IPSC amplitude at baseline (black) and after 15–20 min (gray). Arrows mark time points when 1 Hz stimulation was given. Calibration: 100 pA, 25 ms. EPSC/IPSC amplitude data are mean ± SEM.

Figure 2.

GABAergic synapses are highly sensitive to CB₁R activation and LTD induction. a, Treatment with a low concentration of the CB₁R agonist WIN55,212–2 induced a robust depression at GABAergic but not glutamatergic synapses. b, Depression in response amplitude produced by WIN55,212–2 (1 μm) was reversed by AM251 at glutamatergic synapses, but not at GABAergic synapses. Example traces show IPSC amplitude at baseline (black) and after 20–25 min (gray). Calibration: 100 pA, 25 ms. EPSC/IPSC amplitude data are mean ± SEM.

Figure 3.

Postsynaptic endocannabinoid loading is sufficient to induce LTD at GABAergic synapses. a, AEA-induced depression at GABAergic synapses was not reversed by AM251 even if the postsynaptic cell was held at −70 mV and IPSCs were activated with single pulse stimulation. b, Depression induced by AEA-loading was not inhibited by cycloheximide showing that blockade of protein synthesis does not interfere with eCB release or CB₁R activation. c, AEA-induced depression of sIPSCs was reversed by AM251 treatment in cycloheximide-treated slices (filled circles) suggesting that postsynaptic AEA-loading is sufficient to induce LTD at GABAergic synapses, but not glutamatergic synapses. d, AEA-induced depression at glutamatergic synapses was reversed by AM251, but when combined with a train of 60 pulses delivered at 1 Hz the AEA-induced depression was converted to LTD. Arrows mark time point when 1 Hz stimulation was given. EPSC/IPSC amplitude data are mean values ± SEM.

Figure 4.

Net output from striatal MSNs is a function of disinhibitory and inhibitory LTD and LTP. a, Perfusion of picrotoxin (50 μm) increased population spike (PS) amplitude, showing that GABAergic inhibition regulates striatal output. A train of 60 pulses delivered at 1 Hz did not significantly affect PS amplitude in slices treated with picrotoxin (n = 16). b, A small but consistent potentiation was detected after 1 Hz stimulation in AP-5 (50 μm) treated slices, while HFS (4 trains of 1 s, 100 Hz stimulation, intertrain interval 10 s) induced LTD. Example traces showing PS at baseline, after 60 s, 1 Hz stimulation and following HFS stimulation in an AP-5 treated slice. Calibration: 0.2 mV, 4 ms. c, The 60 s, 1 Hz stimulation protocol did not alter PS amplitude in slices treated with AP-5 and picrotoxin, suggesting that the increase in PS amplitude involves decreased GABAergic inhibition (DLL). d, Neither low nor high frequency stimulation affected PS amplitude in slices treated with AP-5 and AM251, suggesting that all plasticity in AP-5-treated slices is mediated through CB₁R activation. e, 60 s, 5 Hz stimulation induced a robust DLL, while 120 s, 5 Hz had no effect on PS amplitude. In contrast, 60 s, 10 Hz stimulation was insufficient to alter PS amplitude, while 30 s, 10 Hz induced DLL. f, Stimulation for 120 s at 0.5 Hz stimulation or 30 s at 1 Hz, but not 60 s at 0.5 Hz, induced DLL.