Received for publication October 30, 2006.
Revised January 29, 2007.
Accepted for publication January 29, 2007.

EFFECTS OF CANNABINOIDS ON SYNAPTIC TRANSMISSION IN THE FROG NEUROMUSCULAR JUNCTION

Abstract

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 WIN and ACPA and the cannabinoid antagonists AM281 and AM630. Adding WIN to the external medium decreased the frequency and amplitude of the MEPPs; the WIN EC₅₀ value was 5.8 ± 1.0 µM. Application of ACPA, a selective agonist of 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. Pre-treatment with the Pertussis toxin inhibited the effects of WIN and ACPA, suggesting that their effects are mediated through G_i/o-protein activation. The N-type Ca²⁺-channel blocker -CgTX diminished the frequency of the MEPPs, being the -CgTX; EC₅₀ value of 2.5 ± 0.40 µM. Blocking the N-type Ca²⁺-channels with -CgTX (5 µM) before addition of ACPA to the bath had no additional inhibitory effect on the MEPPs, while in the presence of -CgTX (1 µM), ACPA whether had additional inhibition effect. These results suggest that cannabinoids modulate transmitter release in the endplate of the frog neuromuscular junction by activating CB₁ cannabinoid receptors in the nerve ending.

Key words: agonist ACPA, agonist WIN, cannabinoids receptor, neuromuscular junction, skeletal muscle, synaptic transmission