Differential regulation of the release of the same peptide transmitters from individual identified motor neurons in culture.

Aplysia motor neurons B1, B2, and B15 synthesize the small cardioactive peptides A and B (SCPs). In previous studies using semi-intact preparations we have demonstrated that the SCPs are released from B15. Significant peptide release only occurred when B15 was stimulated at a high frequency or at lower frequencies with a long burst duration. In the behaving animal B15 fires in patterns expected to release the SCPs. In contrast, in the behaving animal, neurons B1 and B2 fire at much lower frequencies. We therefore examined whether similar aspects of the stimulation pattern governed the release of the SCPs from B1 and B2. To monitor peptide release, all three neurons were individually cultured and newly synthesized peptides labeled with 35S-methionine. Release of labeled SCPs was detected by HPLC of extracts of superfusates. By keeping spike number constant and varying the stimulation pattern, the release of the SCPs from B1 and B2 was found to be pattern insensitive. That is, regardless of the stimulation paradigm, each action potential released a similar amount of peptide. By severing the primary neurite, peptide release was found to occur mainly from the regenerated neurites and most likely from the intensely immunoreactive varicosites. Calcium- and stimulation-dependent release of the SCPs and a third neuropeptide termed buccalin A from motor neuron B15 was also observed in culture. The release of these peptides from B15 was found to be pattern sensitive as was observed in the semi-intact preparations.(ABSTRACT TRUNCATED AT 250 WORDS)