Regulation of respiratory pumping activity in the marine gastropod mollusc Aplysia californica

Abstract

Gill ventilation in Aplysia californica is achieved by a stereotypic behavior pattern referred to as Respiratory Pumping (RP). The behavior consists of a rapidly successive contraction of the gill, siphon, mantle and parapodia. This forces hemolymph, contained within the gill, into the heart and hemoceol while water surrounding the gill is displaced through the siphon. Upon relaxation, hemolymph refills the gill and fresh seawater returns to the mantle cavity. RP activity in Aplysia californica has been described as an irregularly-occurring behavior which is responsive to extrinsic stimuli (mechanical, photic and chemical). There are, however, three forms of RP activity, (1) simple RP activity, that which responds strictly to metabolic and respiratory requirements, (2) elicited RP activity, that which is triggered by sensory input unrelated to respiratory demands, and (3) RP seizures a previously unrecognized form of RP behavior. The irregularity of RP is derived from the imposition of RP seizures and elicited RP into the regularly-occurring simple RP activity. All RP behavior is mediated by neuronal activity arising in two clusters of autogenously firing nerve cells which rhythmically generate bursts of activity (8.5 bursts/hour in vitro), and produce simple RP behavior at a low steady rate in vivo (4.01 contractions/hour). Other forms of RP behavior (i.e., elicited RP and RP seizures) are produced by sensory modulation of the "pacemaker" cell's endogenous rhythm. It was determined that D-lactate, a product of anaerobic glycolysis, profoundly increases the frequency of bursting from these cell clusters and concomitantly increases the rate of the simple RP contractions. D-lactate concentrations in Aplysia hemolymph may be an important rate determining factor for simple RP activity during short-term exposure to hypoxic environmental conditions. The initiating mechanisms for the newly discovered behavior pattern, RP seizure, have not been identified. Some evidence suggests that the stimulus is blood-borne but not a product of anaerobiosis. The function of the RP seizure is also unknown but the fact that it is produced by special modulation of the general neuronal mechanisms which mediate simple RP behavior, suggests that the two behaviors may have parallel physiological roles.