Effects of dopamine and dibutyryl cyclic adenosine monophosphate on delayed release of transmitter at the rat neuromuscular junction

The effects of dopamine and dibutyryl cyclic adenosine monophosphate (db-cAMP) on delayed release of transnutter were studied in vitro in the phrenic nervediaphragm muscle preparation of the rat using intracellular recording techniques. Dopamine at 1 × 10−4 mol 1−1 prevented the initial facilitation of delayed release of transmitter. This inhibitory phase was transformed into a transient facilitation of delayed release. We observed that dopamine hyperpolarized muscle fibres by about 9%. Thus motor nerve terminals may also have been hyperpolarized by dopamine; however, it is unlikely that this hyperpolarization explains the observed effects on delayed release of transmitter. Db-cAMP at 1 × 10−3 mol 1−1 predominantly augmented delayed release of acetylcholine. These effects of dopamine and db-cAMP on delayed release of transmitter are discussed in terms of a modulation of calcium fluxes in the presynaptic nerve terminal.