Not glutamate but endocannabinoids mediate retrograde suppression of cerebellar parallel fiber to Purkinje cell synaptic transmission in young adult rodents

In the cerebellum of juvenile mice or rats, endocannabinoids are shown to mediate depolarization-induced suppression of excitation (DSE) and retrograde suppression induced by activation of type 1 metabotropic glutamate receptor (mGluR1) at parallel fiber (PF) to Purkinje cell (PC) synapses. However, recent studies showed that glutamate also mediated retrograde signaling through presynaptic kainate receptors in the cerebellum of young adult mice and rats. We reexamined this possibility in C57BL/6 mice at postnatal day 20–35 (P20–P35) and in Sprague–Dawley rats at P18–P24. We found that DSE at PF–PC synapses was abolished by AM251, a cannabinoid receptor antagonist, and by tetrahydrolipstatin (THL), a blocker of diacylglycerol lipase (DGL) that produces an endocannabinoid, 2-arachidonoylglycerol (2-AG). AM251 and THL did not affect depolarization-induced Ca2+ transients in PCs, and THL did not suppress cannabinoid sensitivity of PFs. Moreover, DSE at PF–PC synapses was absent in CB1 knockout mice. AM251 also eliminated transient suppression of PF–PC synaptic transmission following a brief burst of PF stimulation, a phenomenon known to be mediated by mGluR1. These results suggest that DSE and mGluR1-mediated suppression in young adult PCs are mediated by endocannabinoids, and that glutamate, if any, has little contribution.