Exogenous and endogenous cannabinoids suppress inhibitory neurotransmission in the human neocortex.

Activation of CB(1) receptors on axon terminals by exogenous cannabinoids (eg, Δ(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 cannabinoid-sensitive 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.

Purine receptor-mediated endocannabinoid production and retrograde synaptic signalling in the cerebellar cortex.

BACKGROUND AND PURPOSE: Presynaptic CB1 cannabinoid receptors can be activated by endogenous cannabinoids (endocannabinoids) synthesized by postsynaptic neurones. The hypothesis of the present work was that activation of calcium-permeable transmitter-gated ion channels in postsynaptic neurones, specifically of P2X purine receptors, can lead to endocannabinoid production and retrograde synaptic signalling. EXPERIMENTAL APPROACH: GABAergic inhibitory postsynaptic currents (IPSCs) were recorded with patch-clamp techniques in Purkinje cells in mouse cerebellar slices. Purine receptors on Purkinje cells were activated by pressure ejection of ATP from a pipette. KEY RESULTS: ATP evoked an inward current in Purkinje cells, most likely due to P2X receptor activation. The ATP-evoked currents were accompanied by currents via voltage-gated calcium channels. ATP suppressed electrical stimulation-evoked IPSCs and miniature IPSCs (mIPSCs) recorded in the presence of tetrodotoxin, and these effects were prevented by the CB1 antagonist rimonabant and the calcium chelator BAPTA (applied into the Purkinje cell). ATP also suppressed mIPSCs when voltage-gated calcium channels were blocked by cadmium, and intracellular calcium stores were depleted by thapsigargin. However, ATP failed to suppress mIPSCs when the extracellular calcium concentration was zero. CONCLUSIONS AND IMPLICATIONS: ATP elicits CB1 receptor-dependent retrograde synaptic suppression, which is probably mediated by an endocannabinod released by the postsynaptic neurone. An increase in intracellular calcium concentration in the postsynaptic neurone is necessary for this retrograde signalling. We propose that ATP increases the calcium concentration by two mechanisms: calcium enters into the neurone via the P2X receptor ion channel and the ATP-evoked depolarization triggers voltage-gated calcium channels.

Depolarizing GABAergic synaptic input triggers endocannabinoid-mediated retrograde synaptic signaling.

Endocannabinoids released by postsynaptic neurons inhibit neurotransmitter release from presynaptic axon terminals. One typical stimulus of endocannabinoid production is an increase of calcium concentration in postsynaptic neurons. The aim of the present study was to clarify whether depolarizing GABAergic synaptic input, by increasing calcium concentration in postsynaptic neurons, can trigger endocannabinoid production. Spontaneous GABAergic inhibitory postsynaptic currents (sIPSCs) were recorded in Purkinje cells in mouse cerebellar slices with patch-clamp pipettes containing 151 mM chloride (a usual recording mode). sIPSCs were depolarizing inward currents under this condition. Combined electrophysiological and fluorometric calcium imaging experiments indicated that sIPSCs frequently triggered calcium spikes. After the calcium spikes, a short-term suppression of sIPSCs occurred. This suppression was prevented by the CB(1) cannabinoid receptor antagonist rimonabant and the diacylglycerol lipase inhibitor orlistat, but not changed by URB597, an inhibitor of anandamide degradation. It is, therefore, likely that CB(1) receptors and 2-arachidonoylglycerol were involved. For testing the physiological significance of the above observation, we carried out experiments on brains of 3- to 5-day-old mice. The gramicidin-induced perforated patch-clamp mode was used for preserving the physiological intracellular chloride concentration of the neurons. Depolarizing GABAergic sIPSCs occurred under this condition, but at a very low rate. Rimonabant did not change the frequency of these sIPSCs, arguing against the persistence of an endocannabinoid tone. The results point to a new kind of trigger of endocannabinoid production: depolarizing GABAergic synaptic input can elicit endocannabinoid production in postsynaptic neurons by activating calcium channels. The produced endocannabinoid suppresses GABA release from presynaptic axon terminals.