The bag cells of Aplysia as a multitransmitter system: identification of alpha bag cell peptide as a second neurotransmitter.
The bag cell neurons of the marine mollusk, Aplysia, are a putative multitransmitter system that utilizes two or more peptide transmitters derived from a common precursor protein. Two putative transmitters are egg-laying hormone (ELH), a 36 amino acid peptide that induces egg laying and mediates bag cell-induced excitatory effects on certain abdominal ganglion neurons, and alpha-bag cell peptide (alpha BCP), which mimics bag cell-induced inhibition of the left upper quadrant (LUQ) neurons and the depolarization of the bag cells that occurs during the bag cell burst discharge. Alpha BCP was previously purified from bag cell extracts in three neuroactive forms: alpha BCP(1-9), a nine amino acid peptide encoded on the ELH/BCP precursor protein, and two NH2-terminal fragments, alpha BCP(1-8) and alpha BCP(1-7). Analyzing bag cell-induced inhibition of LUQ neurons, we report here that alpha BCP fulfills the main criteria for transmitter identification: stimulation of individual bag cells produces inhibition of the neurons; inhibitory activity is present in releasate collected following an elicited bag cell burst discharge in the presence of protease inhibitors; alpha BCP(1-9) and alpha BCP(1-8) are detected in the releasate in the presence of protease inhibitors; alpha BCP is rapidly inactivated after release, as indicated by the lack of detectable alpha BCP or inhibitory activity in the releasate in the absence of protease inhibitors, and by the increase in potency of the arterially perfused peptide in the presence of protease inhibitors; alpha BCP and the endogenously released transmitter produce apparently identical changes in membrane conductance; bag cell-induced inhibition is reduced or abolished following desensitization of the inhibitory response by long-term application of high concentrations of alpha BCP. The results provide additional evidence that the bag cells are a multitransmitter system and also suggest that many of the physiological properties of alpha BCP-mediated neurotransmission differ from those of ELH. First, unlike ELH, alpha BCP is rapidly inactivated after release. Second, alpha BCP(1-9) may be activated by carboxypeptidase cleavage since alpha BCP(1-8) and alpha BCP(1-7) are 30 and 10X as potent, respectively, as alpha BCP(1-9). Third, the inhibitory action of alpha BCP on its targets has a more rapid onset and a shorter time course than the excitatory actions of ELH. Thus, alpha BCP may diffuse to less distant targets than ELH and serve to regulate the more rapidly occurring neural events underlying egg-laying behavior.