CANNABINOID RECEPTOR AGONISTS INHIBIT GLUTAMATERGIC SYNAPTIC TRANSMISSION IN RAT HIPPOCAMPAL CULTURES

Activation of cannabinoid receptors inhibits voltage-gated Ca2+ channe ls and activates K+ channels, reminiscent of other G-protein-coupled s ignaling pathways that produce presynaptic inhibition. We tested canna binoid receptor agonists for effects on excitatory neurotransmission b etween cultured rat hippocampal neurons. Reducing the extracellular Mg 2+ concentration to 0.1 mM elicited repetitive, transient increases in intracellular Ca2+ concentration ([Ca2+](i) spikes) that resulted fro m bursts of action potentials, as measured by combined whole-cell curr ent clamp and indo-1-based microfluorimetry. Pharmacological character ization indicated that the [Ca2+](i) spikes required glutamatergic syn aptic transmission. Cannabinoid receptor ligands inhibited stereoselec tively the frequency of [Ca2+](i) spiking in the rank order of potency : CP 54,939 > CP 55,940 > Win 55,212-2 > anandamide, with EC(50) value s of 0.36, 1.2, 2.7, and 71 nM, respectively. CP 55,940 was potent, bu t not efficacious, and reversed the inhibition produced by Win55,212-2 , indicating that it is a partial agonist. Inhibition of [Ca2+](i) spi king by Win 55,212-2 was prevented by treatment of cultures with activ e, but not heat-treated, pertussis toxin. Win 55,212-2 (100 nM) inhibi ted stereoselectively CNQX-sensitive excitatory postsynaptic currents (EPSCs) elicited by presynaptic stimulation with an extracellular elec trode, but did not affect the presynaptic action potential or currents elicited by direct application of kainate. Consistent with a presynap tic site of action, Win 55,212-2 increased both the number of response failures and the coefficient of variation of the evoked EPSCs. In con trast, cannabimimetics did not affect bicuculline-sensitive inhibitory postsynaptic currents. Thus, activation of cannabinoid receptors inhi bits the presynaptic release of glutamate via an inhibitory G-protein.