Endocannabinoid- and mGluR5-dependent Short-Term Synaptic Depression in an Isolated Neuron/Bouton Preparation from the Hippocampal CA1 Region Running title: Endocannabinoid- and mGluR5-dependent STD

Endocannabinoids released from the postsynaptic neuronal membrane can activate presynaptic CB1 receptors and inhibit neurotransmitter release. In hippocampal slices, depolarization of the CA1 pyramidal neurons elicits an endocannabinoid-mediated inhibition of GABA release known as depolarization-induced suppression of inhibition (DSI). Utilizing the highly reduced neuron/synaptic bouton preparation from the CA1 region of hippocampus, we have begun to examine endocannabinoid-dependent shortterm depression (STD) of inhibitory synaptic transmission under well controlled physiological and pharmacological conditions in an environment free of other cells. Application of the CB1 synthetic agonist WIN55212-2 and endogenous cannabinoids 2AG and anandamide (AEA) produced a decrease in sIPSC frequency and amplitude, indicating the presence of CB1 receptors at synapses in this preparation. Endocannabinoid-dependent STD is different from DSI found in hippocampal slices and the neuron/bouton preparation from basolateral amygdala (BLA) since depolarization alone was not sufficient to induce suppression of spontaneous inhibitory postsynaptic currents (sIPSCs). However, concurrent application of the mGluR agonist DHPG and postsynaptic depolarization resulted in a transient (30 – 50 s) decrease in sIPSC frequency and amplitude. Application of DHPG alone had no effect on sIPSCs. The depolarization/DHPG-induced STD was blocked by the CB1 antagonist SR141716A and the mGluR5 antagonist MPEP, and was sensitive to intracellular calcium concentration. Comparing the present findings with earlier work in hippocampal slices and BLA, it appears that endocannabinoid release is less robust in isolated hippocampal neurons.