Effects of cannabinoids on synaptic transmission in the frog neuromuscular junction.

This study aimed to investigate the function of the cannabinoid receptor in the neuromuscular junction of the frog (Rana pipiens). Miniature end-plate potentials were recorded using the intracellular electrode recording technique in the cutaneous pectoris muscle in the presence of the cannabinoid agonists WIN55212-2 (WIN; R-(+)-[2,3-dihydro-5-methyl-3-[(morpholinyl)]-pyrolol[1,2,3de]-1,4-benzoxazinyl]-(1-naphthalenyl)methanone) and arachidonylcyclopropylamide [ACPA; N-(2-cyclopropyl)-5Z,8Z,11Z,147-eicosatetraenamide] and the cannabinoid antagonists 1-(2,4-dichlorophenyl)-5-(4-iodophenyl)-4-methyl-N-4-morpholinyl-1H-pyrazole-3-carboxamide (AM281) and 6-iodo-2-methyl-1-[2-(4-morpholinyl)ethyl]-1H-indol-3-yl](4-methoxyphenyl)methanone (AM630). Adding WIN to the external medium decreased the frequency and amplitude of the miniature end-plate potentials (MEPPs); the WIN EC50 value was 5.8+/-1.0 microM. Application of ACPA, a selective agonist of cannabinoid receptor CB1, also decreased the frequency of the MEPPs; the ACPA EC50 value was 115.5+/-6.5 nM. The CB2 antagonist AM630 did not inhibit the effects of WIN, indicating that its action is not mediated through the CB2 receptor. However, the CB1 antagonist AM281 inhibited the effects of WIN and ACPA, suggesting that their actions are mediated through the CB1 receptor. Pretreatment with the pertussis toxin inhibited the effects of WIN and ACPA, suggesting that their effects are mediated through Gi/o protein activation. The N-type Ca2+ channel blocker omega-conotoxin GVIA (omega-CgTX) diminished the frequency of the MEPPs, with an omega-CgTX EC50 value of 2.5+/-0.40 microM. Blocking the N-type Ca2+ channels with 5 microM omega-CgTX before addition of ACPA to the bath had no additional inhibitory effect on the MEPPs, whereas in the presence of 1 microM omega-CgTX, ACPA had an additional inhibition effect. These results suggest that cannabinoids modulate transmitter release in the end-plate of the frog neuromuscular junction by activating CB1 cannabinoid receptors in the nerve ending.