Endocannabinoids acting on CB 1 cannabinoid receptors are involved in short-and long-term depression of synaptic transmission. The aim of the present study was to determine which endocannabinoid, anandamide or 2-arachidonoylglycerol (2-AG), is involved in depolarization-induced suppression of inhibition (DSI) in the cerebellar cortex, which is the most widely studied form of short-term depression. Depolarization of Purkinje cells in the mouse cerebellum led to an increase in intracellular calcium concentration and to suppression of the inhibitory input to these neurons (i.e. DSI occurred). Orlistat and RHC80267, two blockers of sn-1-diacylglycerol lipase, the enzyme catalysing 2-AG formation, abolished DSI by acting downstream of calcium influx. In contrast, DSI occurred also in the presence of a phospholipase C inhibitor. Intact operation of the calcium-dependent messengers calmodulin and Ca 2+ -calmodulin-dependent protein kinase II were necessary for DSI. DSI was potentiated by an inhibitor of the main 2-AG-degrading enzyme, monoacylglycerol lipase. Interference with the anandamide metabolizing enzyme, fatty acid amide hydrolase, did not modify DSI. Thus, three kinds of observations identified 2-AG as the endocannabinoid involved in DSI in the mouse cerebellum: DSI was abolished by diacylglycerol lipase inhibitors; DSI was potentiated by a monoglyceride lipase inhibitor; and DSI was not changed by an inhibitor of fatty acid amide hydrolase. Further experiments indicated that 2-AG is the endocannabinoid mediating short-term retrograde signalling also at other synapses: orlistat abolished DSI in the rat cerebellum, DSI in the mouse substantia nigra pars reticulata and depolarization-induced suppression of excitation in the mouse cerebellum.

CB 1 cannabinoid receptors are expressed in many neurons in the caudate-putamen. However, it is not known how the activation of these receptors influences synaptic transmission between different neuron classes. The aim was to establish a method for studying identified synaptic connections in the caudate-putamen, and to determine the effects of cannabinoids on these connections. Brain slices were prepared from transgenic mice expressing enhanced green fluorescent protein (EGFP) in parvalbumin-positive fast spiking interneurons (PV-FSNs). PV-FSNs were identified based on their fluorescence. Non-fluorescent medium-sized neurons were considered to be medium spiny neurons (MSNs). Synaptic transmission was studied by simultaneous patch-clamp recording from identified neuron pairs. In the case of PV-FSN → MSN neurotransmission, the synthetic cannabinoid receptor agonist WIN55212-2 lowered the success rate of transmission and the amplitude of successful postsynaptic events. Analysis of miniature inhibitory postsynaptic currents indicated that WIN55212-2 inhibited synaptic transmission presynaptically. WIN55212-2 did not elicit somatodendritic effects in PV-FSNs: membrane potential, membrane current and evoked firing were not changed. WIN55212-2 also depressed the MSN → MSN neurotransmission. The inhibitory synaptic input to MSNs was only weakly suppressed by endocannabinoids released by depolarized postsynaptic MSNs. The results show that the combined use of transgenic animals and paired-recording techniques allows the study of synaptic connections between rare neurons. Using these techniques, we showed that activation of CB 1 receptors on axon terminals of (i) PV-FSNs and (ii) MSNs leads to presynaptic inhibition of GABAergic synaptic transmission between these axons and their postsynaptic targets, the MSNs. The cannabinoids acted preferentially on axon terminals without effects on the somatodendritic region of the neurons.

Globus pallidus neurons receive GABAergic input from the caudate-putamen via the striatopallidal pathway. Anatomical studies indicate that many CB 1 cannabinoid receptors are localized on terminals of striatopallidal axons. Accordingly, the hypothesis of the present work was that activation of CB 1 receptors presynaptically inhibits neurotransmission between striatopallidal axons and globus pallidus neurons. In sagittal mouse brain slices, striatopallidal axons were electrically stimulated in the caudate-putamen, and the resulting GABAergic inhibitory postsynaptic currents (IPSCs) were recorded in globus pallidus neurons. The synthetic cannabinoid receptor agonists R(ϩ)- [2,3-dihydro-5-methyl-3-[(morpholinyl) Prevention of DSI by rimonabant indicates that endocannabinoids released from the postsynaptic neurons acted on CB 1 receptors to suppress synaptic transmission. WIN55212-2 did not modify currents in globus pallidus neurons elicited by GABA released from its chemically bound ("caged") form by a flash pulse, suggesting that WIN55212-2 depressed neurotransmission presynaptically. For studying the mechanism of the inhibition of GABA release, terminals of striatopallidal axons were labeled with a calcium-sensitive fluorescent dye. WIN55212-2 depressed the action potentialevoked increase in axon terminal calcium concentration. The results show that activation of CB 1 receptors by exogenous and endogenous cannabinoids leads to presynaptic inhibition of neurotransmission between striatopallidal axons and globus pallidus neurons. Depression of the action potential-evoked calcium influx into axon terminals is the probable mechanism of this inhibition.The G␣ i/o protein-coupled CB 1 cannabinoid receptor is the primary neuronal target of the phytocannabinoid ⌬ 9 -tetrahydrocannabinol and of the endogenous cannabinoids (endocannabinoids) anandamide and 2-arachidonylglycerol (Howlett et al., 2002;Abood, 2005). The CB 1 receptor is widely distributed in the central and the peripheral nervous system (Herkenham et al., 1991b;Mailleux and Vanderhaeghen, 1992;Tsou et al., 1998). Activation of CB 1 receptors leads to presynaptic inhibition of synaptic transmission in many regions of the central and peripheral nervous system (Freund et al., 2003;Szabo and Schlicker, 2005).The present work focuses on the neuronal connection between the caudate putamen and the globus pallidus (also called external or lateral globus pallidus). Globus pallidus neurons receive strong GABAergic input from medium spiny neurons of the caudate-putamen (striatopallidal projection neurons; Gerfen, 2004). The concentration of CB 1 receptor protein in the globus pallidus is very high (Herkenham et al., 1991b;Mailleux and Vanderhaeghen, 1992;Tsou et al., 1998). Two kinds of observations support the idea that the majority of CB 1 receptors in the globus pallidus is localized This work was supported by the Deutsche Forschungsgemeinschaft (Sz 72/5-1).Article, publication date, and citation information can be found at

Activation of CB 1 receptors on axon terminals by exogenous cannabinoids (eg, D 9 -tetrahydrocannabinol) and by endogenous cannabinoids (endocannabinoids) released by postsynaptic neurons leads to presynaptic inhibition of neurotransmission. The aim of this study was to characterize the effect of cannabinoids on GABAergic synaptic transmission in the human neocortex. Brain slices were prepared from neocortical tissues surgically removed to eliminate epileptogenic foci. Spontaneous GABAergic inhibitory postsynaptic currents (sIPSCs) were recorded in putative pyramidal neurons using patch-clamp techniques. To enhance the activity of cannabinoidsensitive presynaptic axons, muscarinic receptors were continuously stimulated by carbachol. The synthetic cannabinoid receptor agonist WIN55212-2 decreased the cumulative amplitude of sIPSCs. The CB 1 antagonist rimonabant prevented this effect, verifying the involvement of CB 1 receptors. WIN55212-2 decreased the frequency of miniature IPSCs (mIPSCs) recorded in the presence of tetrodotoxin, but did not change their amplitude, indicating that the neurotransmission was inhibited presynaptically. Depolarization of postsynaptic pyramidal neurons induced a suppression of sIPSCs. As rimonabant prevented this suppression, it is very likely that it was due to endocannabinods acting on CB 1 receptors. This is the first demonstration that an exogenous cannabinoid inhibits synaptic transmission in the human neocortex and that endocannabinoids released by postsynaptic neurons suppress synaptic transmission in the human brain. Interferences of cannabinoid agonists and antagonists with synaptic transmission in the cortex may explain the cognitive and memory deficits elicited by these drugs.

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