Effects of THC and endogenous cannabinoids on synaptic transmission between hippocampal neurons in primary culture

The CB1 receptor is expressed in the hippocampus at presynaptic axon terminals. Cannabinoids activate this receptor to inhibit excitatory synaptic transmission between hippocampal neurons. Delta9-Tetrahydrocannabinol (THC), the principal component in marijuana, is a partial agonist at these synapses. When THC is applied in combination with the full agonist WIN55212-2, it partially reversed the effects this potent and highly efficacious cannabinoid.; How THC might interact with endocannabinoid signaling is addressed here. Reducing the extracellular Mg2+ concentration elicited a pattern of glutamatergic synaptic activity that produced intracellular Ca2+ concentration spikes as measured by microfluorimetry. The endocannabinoid 2-arachidonyl glycerol (2-AG) produced a concentration-dependent and complete inhibition of [Ca2+]i spiking activity that was blocked by the CB1 antagonist SR141716. THC antagonized the actions of 2-AG producing a parallel shift in the concentration response relationship for 2-AG. The attenuation of 2-AG-mediated inhibition of synaptic activity by THC was concentration-dependent. These results demonstrate that THC can antagonize endocannabinoid signaling and suggest a novel mechanism for the effects of marijuana.; Endogenous cannabinoids are removed from the synapse by facilitated diffusion, a process inhibited by N-(hydroxyphenyl)-arachidonamide (AM404). The effects of AM404 on glutamatergic synaptic transmission were examined using the [Ca 2+]i spiking assay. AM404 inhibited [Ca2+] i spiking at low micromolar concentrations. The CB1 antagonist SR141716A, the VR1 antagonist capsazapine, and treatment with pertussis toxin failed to block the inhibition produced by AM404. AM404 inhibited action potential-evoked Ca2+ influx but failed to affect calcium influx evoked by depolarization with 30 mM K+. This observation suggested that the inhibition of electrically evoked [Ca2+]i increases and [Ca 2+]i spiking was due to inhibition of Na + channels. Palmitoyl ethanolamide, capsaicin, and VDM11, compounds structurally similar to AM404, also inhibited [Ca2+]i spiking. Thus, AM404 and structural analogs directly inhibit action potential-induced Ca2+ influx, suggesting caution when using these agents as pharmacological probes to study synaptic transmission.; This investigation has addressed the interactions between the active component of marijuana and endocannabinoid-mediated effects on synaptic transmission. Further, agents thought to modulate the regulation of endocannabinoids were found to mimic the effects of endocannabinoids by acting directly on ion channels.