MODULATION OF NEUROMUSCULAR-TRANSMISSION BY CONVENTIONAL AND PEPTIDE TRANSMITTERS RELEASED FROM EXCITATORY AND INHIBITORY MOTOR-NEURONS IN APLYSIA

The anterior portion of intrinsic buccal muscle 3 (I3a) is innervated by two excitatory motor neurons, B3 and B38, and the newly identified inhibitory motor neuron, B47. We show that B47 is cholinergic while B3 and B38 are not. B3 and B38 have previously been shown to express the neuropeptides FMRFamide and the small cardioactive peptides (SCPs) A and B, respectively. We present evidence here that B47 synthesizes the neuropeptide myomodulin A (Mma). When placed in culture, B3, B38, and B47 continued to synthesize their respective peptides. These peptides were released in a stimulation- and Ca2+-dependent manner, suggesting that they are transmitters in these neurons. By using B3-evoked excit atory junction potentials (EJPs) and muscle contractions as assays, we next examined the modulatory effects of superfusion of peptides and s timulation of motor neurons B38 and B47. Superfusing the muscle with l ow concentrations of the SCPs, FMRFamide, or Mma enhanced B3-evoked EJ Ps and contractions. Stimulation of B47 simultaneously with B3 reduced the amplitude of B3-evoked contractions. However, when either B47 or B38 was stimulated in extended bursts designed to release their peptid e transmitters, subsequent B3-evoked EJPs and contractions were enhanc ed. We believe that this modulation is due at least in part to the rel ease of peptides from the terminals of B38 and B47. The SCPs potently increase cAMP levels in I3a muscle fibers. Likewise, stimulation of B3 8 in extended bursts increased cAMP levels in the muscle. This provide s independent evidence that the SCPs are released from B38 terminals i n the muscle. Therefore, we have described a neuromuscular preparation amenable to the study of both excitatory and inhibitory motor neurons that utilize a variety of conventional and peptide transmitters. Our results suggest that these motor neurons can function in two states. W hen stimulated in single brief bursts, they primarily release conventi onal transmitters. When stimulated in a series of prolonged bursts, th ey release both conventional transmitters and peptide cotransmitters. These dual states are most pronounced in the case of B47, which, depen ding on the stimulation paradigm, can act selectively to inhibit or en hance the effects of a second motor neuron innervating the same muscle .