Temperature-dependent peptidergic feedback: Potential role in seasonal egg laying in aplysia

Abstract

α-Bag cell peptide (α-BCP), one of the secretory products of the neuroendocrine bag cells in Aplysia, has been reported by various investigators to have either excitatory or inhibitory feedback effects. Though conflicting, these results may be explained by the difference in temperature at which the experiments were performed. Because egg laying in this animal is temperature dependent, the alteration in function of this peptide by temperature may offer a possible molecular basis for the seasonal regulation of egg laying. This hypothesis was investigated by assessing the feedback actions of α-BCP at various temperatures. At 15°C, α-BCP hyperpolarized bag cells, shortened the duration of synaptically evoked bag cell discharges, and reduced the number of action potentials per discharge. However, at 20°C, the peptide depolarized bag cells, lengthened discharges, and increased the number of action potentials per discharge. A temperature-dependent influence on bag cell cAMP levels may underlie these effects, because α-BCP reduced basal cAMP levels in intact bag cells at temperatures of 15°C and below, while at 17-22°C it increased these levels. However, the inhibitory effects of α-BCP on stimulated adenylate cyclase activity in bag cell homogenates were not temperature dependent. Moreover, a low-Ca2+/high-Mg2+ solution abolished α-BCP's ability to increase bag cell cAMP levels at 20°C. This suggests that the peptide may evoke the secretion of an excitatory modulator at the higher temperature. These results imply that α-BCP is autoinhibitory at typical winter temperatures, but becomes autoexcitatory as ocean temperature rises in the summer. Thus, the peptide may function in coordination with other factors to regulate egg laying in response to seasonal temperature variations. Copyright © 1991 Society for Neuroscience.