Inhibition of fatty acid amide hydrolase unmasks CB1 receptor and TRPV1 channel-mediated modulation of glutamatergic synaptic transmission in midbrain periaqueductal grey.

BACKGROUND AND PURPOSE: While arachidonyl ethanolamine (anandamide) produces pharmacological effects mediated by cannabinoid CB1 receptors, it is also an agonist at the transient receptor potential vanilloid type 1 (TRPV1) ion channel. This study examined the cellular actions of anandamide in the midbrain periaqueductal grey (PAG), a region implicated in the analgesic actions of cannabinoids, and which expresses both CB1 receptors and TRPV1.

EXPERIMENTAL APPROACH: In vitro whole cell patch clamp recordings of glutamatergic excitatory postsynaptic currents (EPSCs) were made from rat and mouse PAG slices.

KEY RESULTS: Capsaicin (1 µM) increased the rate, but not the amplitude of miniature EPSCs in subpopulations of neurons throughout the rat and mouse PAG. Capsaicin had no effect on miniature EPSCs in PAG neurons from TRPV1 knock-out mice. In mouse PAG neurons, anandamide (30 µM) had no effect on the rate of miniature EPSCs alone, or in the presence of either the CB1 antagonist AM251 (3 µM) or the TRPV1 antagonist iodoresiniferatoxin (300 nM). Anandamide produced a decrease in miniature EPSC rate in the presence of the fatty acid amide hydrolase (FAAH) inhibitor URB597 (1 µM). By contrast, anandamide produced an increase in miniature EPSC rate in the presence of both URB597 and AM251, which was absent in TRPV1 knock-out mice.

CONCLUSIONS AND IMPLICATIONS: These results suggest that the actions of anandamide within PAG are limited by enzymatic degradation by FAAH. FAAH blockade unmasks both presynaptic inhibition and excitation of glutamatergic synaptic transmission which are mediated via CB1 receptors and TRPV1 respectively.