Cannabinoids depress excitatory neurotransmission between the subthalamic nucleus and the globus pallidus

The globus pallidus receives its major glutamatergic input from the subthalamic nucleus and subthalamic nucleus neurons synthesize CB 1 cannabinoid receptors. The hypothesis of the present work was that CB 1 receptors are localized in terminals of subthalamo-pallidal glutamatergic axons and that their activation leads to presynaptic modulation of neurotransmission between these axons and globus pallidus neurons. Patch-clamp studies were carried out on oblique-sagittal mouse brain slices. The subthalamic nucleus was stimulated electrically and the resulting excitatory postsynaptic currents (EPSCs) were recorded in globus pallidus neurons. The mixed CB 1/CB 2 receptor agonist R(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)methyl]pyrrolo[1,2,3-de]-1,4-benzoxazin-yl]-(1-naphthalenyl)methanone mesylate (WIN55212-2; 3×10 −7 M) had no effect on EPSCs. WIN55212-2 (10 −5 M) decreased the amplitude of EPSCs by 44±8%. The inhibition by WIN55212-2 (10 −5 M) was prevented by the CB 1 antagonist N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methyl-3-pyrazolecarboxamide (10 −6 M). WIN55212-2 (10 −5 M) did not change the amplitude of spontaneous EPSCs (sEPSCs) recorded in globus pallidus neurons but lowered their frequency. Moreover, WIN55212-2 (10 −5 M) had no effect on currents elicited by direct activation of postsynaptic receptors on globus pallidus neurons by glutamate (10 −3 M) ejected from a pipette. In a final series of experiments, the firing of subthalamic nucleus neurons was recorded; WIN55212-2 (10 −5 M) did not change the firing of these neurons. The results show that activation of CB 1 receptors inhibits glutamatergic neurotransmission between the subthalamic nucleus and the globus pallidus. Lack of effect of cannabinoids on the amplitude of sEPSCs and on currents evoked by direct stimulation of postsynaptic glutamate receptors indicates that the mechanism is presynaptic inhibition of glutamate release from axon terminals. Cannabinoids seem to act preferentially presynaptically: in contrast to their action on axon terminals, they have no effect on somadendritic receptors regulating firing rate. Cannabinoids elicit catalepsy in vivo. The observed inhibition of glutamatergic neurotransmission in the globus pallidus would favor catalepsy.