Primary structure and neuronal effects of alpha-bag cell peptide, a second candidate neurotransmitter encoded by a single gene in bag cell neurons of Aplysia.

A discharge of impulse activity in a group of neuroendocrine cells, the bag cells, produces several types of prolonged responses in various identified neurons of the abdominal ganglion of Aplysia. Two excitatory responses are almost certainly mediated by egg-laying hormone, but this peptide cannot account for other responses, such as inhibition of left upper quadrant neurons. We report here the isolation from bag cell clusters of three structurally similar peptides, seven, eight, and nine residues long, that are candidate transmitters for mediating bag cell-induced inhibition. They may also serve as autoexcitatory transmitters since the seven-residue peptide produces a slow depolarization of the bag cells similar to that which occurs during bag cell discharge. The amino acid sequence of the largest peptide, termed alpha-bag cell peptide[1-9], is H-Ala-Pro-Arg-Leu-Arg-Phe-Tyr-Ser-Leu-OH. The other two peptides are identical to alpha-BCP[1-9] except that they lack the COOH-terminal Ser-Leu or leucine residues. The three peptides inhibit left upper quadrant neurons at relative potencies of 10:30:1 (seven-, eight-, and nine-residue peptides, respectively). Recent molecular genetic analysis shows that both alpha-BCP[1-9] and egg-laying hormone are encoded by the same bag cell-specific gene. The multiple neuronal effects of bag cells are therefore likely to be mediated by at least two transmitters that are cleaved from a common precursor molecule.