Target-dependent control of synaptic inhibition by endocannabinoids in the thalamus

Inhibitory neurons in the thalamic reticular nucleus (TRN) play a critical role in controlling information transfer between thalamus and neocortex. GABAergic synapses formed by TRN neurons contact both thalamic relay cells and neurons within TRN. These two types of synapses are thought to have distinct roles for the generation of thalamic network activity, but their selective regulation is poorly understood. In many areas throughout the brain, retrograde signaling mediated by endocannabinoids acts to dynamically regulate synaptic strength over both short and long time scales. However, retrograde signaling has never been demonstrated in the thalamus. Here, we show that depolarization-induced suppression of inhibition (DSI) is prominent at inhibitory synapses interconnecting TRN neurons. DSI is completely abolished in the presence of a cannabinoid receptor 1 (CB1R) antagonist and in mice lacking CB1Rs. DSI is prevented by DAG lipase inhibitors and prolonged by blocking the 2-arachidonoylglycerol (2-AG) degradation enzyme monoacylglycerol lipase, indicating that it is mediated by the release of 2-AG from TRN neurons. By contrast, DSI is not observed at TRN synapses targeting thalamic relay neurons. A combination of pharmacological and immunohistochemical data indicate that the differences in endocannabinoid signaling at the two synapses are mediated by a synapse-specific targeting of CB1Rs, as well as differences in endocannabinoid release between the two target neurons. Together, our results show that endocannabinoids control transmitter release at specific thalamic synapses, and could dynamically regulate sensory information processing and thalamus-mediated synchronous oscillations.