RESUMO
Previous work has demonstrated that activation of muscarinic acetylcholine receptors at the lizard neuromuscular junction (NMJ) induces a biphasic modulation of evoked neurotransmitter release: an initial depression followed by a delayed enhancement. The depression is mediated by the release of the endocannabinoid 2-arachidonylglycerol (2-AG) from the muscle and its binding to cannabinoid type 1 receptors on the motor nerve terminal. The work presented here suggests that the delayed enhancement of neurotransmitter release is mediated by cyclooxygenase-2 (COX-2) as it converts 2-AG to the glycerol ester of prostaglandin E2 (PGE2-G). Using immunofluorescence, COX-2 was detected in the perisynaptic Schwann cells (PSCs) surrounding the NMJ. Pretreatment with either of the selective COX-2 inhibitors, nimesulide or DuP 697, prevents the delayed increase in endplate potential (EPP) amplitude normally produced by muscarine. In keeping with its putative role as a mediator of the delayed muscarinic effect, PGE2-G enhances evoked neurotransmitter release. Specifically, PGE2-G increases the amplitude of EPPs without altering that of spontaneous miniature EPPs. As shown previously for the muscarinic effect, the enhancement of evoked neurotransmitter release by PGE2-G depends on nitric oxide (NO) as the response is abolished by application of either N(G)-nitro-l-arginine methyl ester (l-NAME), an inhibitor of NO synthesis, or carboxy-PTIO, a chelator of NO. Intriguingly, the enhancement is not prevented by AH6809, a prostaglandin receptor antagonist, but is blocked by capsazepine, a TRPV1 and TRPM8 receptor antagonist. Taken together, these results suggest that the conversion of 2-AG to PGE2-G by COX-2 underlies the muscarine-induced enhancement of neurotransmitter release at the vertebrate NMJ.