It was shown recently that Delta9-tetrahydrocannabinol, like several other drugs eliciting euphoria, stimulates dopaminergic neurons projecting from the ventral tegmental area (VTA) to the nucleus accumbens. The aim of the present work was to clarify the mechanism of this stimulatory effect. Our hypothesis was that cannabinoids depress the GABAergic inhibition of dopaminergic neurons in the VTA. Electrophysiological properties of VTA neurons in rat coronal midbrain slices were studied with the patch-clamp technique. GABA(A) receptor-mediated inhibitory postsynaptic currents (IPSCs) were evoked by electrical stimulation in the vicinity of the recorded neurons. The amplitude of IPSCs was depressed by the synthetic mixed CB1/CB2 cannabinoid receptor agonist WIN55212-2 (10(-6) and 10(-5) m). The CB1 cannabinoid receptor antagonist SR141716A (10(-6) m) prevented the inhibition produced by WIN55212-2 (10(-5) m). Two observations showed that IPSCs were depressed with a presynaptic mechanism. WIN55212-2 (10(-5) m) did not change the amplitude of miniature IPSCs recorded in the presence of tetrodotoxin. Currents evoked by pressure ejection of muscimol from a pipette were also not changed by WIN55212-2 (10(-5) m). The results indicate that activation of CB1 cannabinoid receptors inhibits GABAergic neurotransmission in the VTA with a presynaptic mechanism. Depression of the GABAergic inhibitory input of dopaminergic neurons would increase their firing rate in vivo. Accordingly, dopamine release in the projection region of VTA neurons, the nucleus accumbens, would also increase.

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The hypothesis of the present work was that activation of CB 1 cannabinoid receptors inhibits GABAergic neurotransmission between basket and Purkinje cells in the cerebellar cortex. The aim was to test this hypothesis under near-physiological conditions. Action potentials of basket cells and spontaneous inhibitory postsynaptic currents (sIPSCs) in synaptically coupled Purkinje cells were recorded simultaneously in rat brain slices. The canna-decreased the amplitude of sIPSCs occurring simultaneously with basket cell action potentials and lowered the success rate of synaptic transmission. These effects were prevented by the CB 1 receptor antagonist N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichloro-phenyl)-4-methyl-3-pyrazolecarboxamide (SR141716). Depolarization of Purkinje cells also led to suppression of neurotransmission; prevention of this suppression by CP55940 and SR141716 indicates that endocannabinoids released from Purkinje cells were involved. WIN 55212-2 lowered the amplitude of autoreceptor currents recorded in basket cells (autoreceptor currents are due to the action of GABA released from axon terminals on GABA A autoreceptors of the same axon terminals); this is novel proof of the presynaptic action of cannabinoids. Autoreceptor current experiments also indicated that endogenous cannabinoids are not released by basket cell axon terminals. A presynaptic action is additionally supported by the observation that WIN 55212-2 lowered the frequency of miniature IPSCs recorded in the presence of tetrodotoxin and the calcium ionophore ionomycin. In conclusion, activation of CB 1 receptors by exogenous cannabinoids and by endogenous cannabinoids released by Purkinje cells presynaptically inhibits GABAergic neurotransmission between basket and Purkinje cells. This was demonstrated under near-physiological conditions: transmitter release was elicited by action potentials generated by spontaneously firing intact presynaptic neurons.Mammals possess two G i/o protein-coupled cannabinoid receptors, CB 1 and CB 2 receptors (Howlett et al., 2002). They are the primary targets of the natural cannabinoid agonist ⌬ 9 -tetrahydrocannabinol and of the endogenous cannabinoids (endocannabinoids) anandamide and 2-arachidonylglycerol. The CB 2 receptor is localized in peripheral non-neuronal tissues. The CB 1 receptor is widely distributed in the central and the peripheral nervous system (Mailleux and Vanderhaeghen, 1992;Matsuda et al., 1993;Tsou et al., 1998). High concentrations are found in the cerebral cortex, hippocampus, caudate-putamen, globus pallidus, substantia nigra pars reticulata, and cerebellum.Most of the CB 1 receptors in the cerebellum are found in the cortex, and their exact neuronal localization can be deduced by comparing the localization of CB 1 receptor mRNA (in situ hybridization; Mailleux and Vanderhaeghen, 1992;Matsuda et al., 1993) and CB 1 receptor protein (receptor autoradiography and immunohistochemistry; Mailleux and Vanderhaeghen, 1992;Tsou et al., 1998;Diana et al., 2002). The output neur...

The CB(1) cannabinoid receptor is widely distributed in the central nervous system. The substantia nigra pars reticulata (SNR) belongs to the brain regions with the highest density of CB(1) receptors. According to anatomical studies, most of the CB(1) receptors in the SNR are localized on terminals of striatonigral GABAergic neurons. The aim of the present study was to clarify the function of these receptors.Electrophysiological properties of SNR neurons were studied in brain slices with the patch-clamp technique. Inhibitory postsynaptic currents (IPSCs) were elicited in parasagittal slices by electrical stimulation in the internal capsule. The mixed CB(1)/CB(2) cannabinoid receptor agonist WIN55212-2 (1 microM and 10 microM) concentration dependently decreased the amplitude of IPSCs. CP55940, another mixed CB(1)/CB(2) cannabinoid receptor agonist, also lowered IPSC amplitude. Superfused alone, the CB(1)-selective antagonist SR141716A (1 microM) increased the amplitude of IPSCs. In interaction experiments, SR141716A (1 microM) prevented the inhibition produced by WIN55212-2 (1 microM). WIN55212-2 (1 microM) had no effect on GABAergic currents elicited by ejection of muscimol (1 mM) to the surface of the slices. WIN55212-2 (10 microM) did not influence the frequency and amplitude of spontaneously occurring IPSCs (sIPSCs) and the firing rate of SNR neurons. The results show that activation of CB(1) cannabinoid receptors inhibits GABAergic neurotransmission in the SNR. The likely mechanism is presynaptic inhibition of GABA release, since cannabinoids had no effects on currents evoked by direct stimulation of GABA(A) receptors by muscimol and on the amplitude of sIPSCs. The enhancement of IPSCs by the cannabinoid antagonist probably reflects continuous inhibition of GABAergic neurotransmission by an endogenous cannabinoid. SNR neurons receive GABAergic input from three sources: from the corpus striatum, the globus pallidus and from neighbouring SNR neurons. The observed inhibition of GABAergic neurotransmission was probably due to depression of the transmission between striatonigral axons and SNR neurons. No direct actions of cannabinoids on SNR neurons were observed in addition to this synaptic effect.