Cell type-specific regulation of inhibition via cannabinoid type 1 receptors in rat neocortex

Endogenous cannabinoid type 1 (CB1) receptors demonstrate a cell type-specific expression and are potent modulators of synaptic transmission within the central nervous system. We aimed to investigate whether two classes of multipolar interneuron in the neocortex displayed a form of short-term synaptic plasticity, depolarization-induced suppression of inhibition (DSI), and whether the DSI was mediated by a common receptor. Paired whole cell recordings combined with biocytin labeling were performed between pyramidal cells and either multipolar adapting or multipolar nonadapting interneurons in layers II-IV of male Wistar rat (postnatal day 17-22) somatosensory cortex. Inhibitory postsynaptic potentials elicited by multipolar adapting interneurons were sensitive to DSI, which was blocked by the CB1 receptor antagonist AM-251 (8 microM), indicating that the suppression of inhibition was mediated by CB1 receptors. Two subpopulations of multipolar nonadapting interneuron-to-pyramidal cell connections were discovered on the basis of their susceptibility to DSI. Whereas 50% were insensitive to DSI, the remaining half were sensitive to DSI, which could not be prevented by AM-251. DSI at these connections was also insensitive to the group I (mGluRIa) and III metabotropic glutamate receptor antagonists (RS)-1-aminoindan-1,5-dicarboxylic acid (100 microM) and (RS)-α-cyclopropyl-4-phosphonophenylglycine (100 microM) and the group III agonist l-2-amino-4-phosphonobutanoate (50 microM). However, multipolar nonadapting interneuron-to-pyramidal cell connections were sensitive to the endocannabinoid anandamide (9 microM), mimicking the effects of DSI, which also could not be prevented by AM-251, implying a CB1 receptor-independent suppression of inhibition. These results reveal an interneuron type-specific modulation of synaptic transmission via CB receptors in the neocortex.