Presynaptic effects of muscarine on ACh release at the frog neuromuscular junction

1. Presynaptic effects of muscarine on neurotransmitter release were studie d at the frog neuromuscular junction, using focal depolarization of the pre synaptic terminal to different levels. 2. Muscarine (10 mu M) had a dual effect on ACh release: concomitant inhibi tion and enhancement of release at the same patch of presynaptic membrane. 3. These two effects were maximal at low depolarizing pulses and diminished as depolarization increased. 4. At low depolarizing pulses, atropine (1 mu M) enhanced release, suggesti ng that ACh in the synaptic deft causes a net tonic inhibition of ACh relea se. 5. In the presence of the M-2 antagonist methoctramine (1 mu M), muscarine (10 mu M) enhanced ACh release. 6. In the presence of the M-1 antagonist pirenzepine (10 mu M), muscarine ( 10 mu M) produced stronger inhibition. 7. These results show that the M-2 receptor is responsible for inhibition o f ACh release, while the M-1 receptor is responsible for its enhancement. 8. The inhibitory effect of muscarine did not depend on extracellular [Ca2]. Enhancement of release was abolished at low extracellular [Ca2+]. 9. The muscarine inhibitory effect was not associated with a reduction of C a2+ current, while release enhancement was associated with an increase of C a2+ current.