Further studies of bulk and orosensory decrement in producing satiation offeeding in Aplysia

Prior evidence has suggested that meal satiation in the marine mollusk Aply sia is associated with stretch of the crop. The current data, however, sugg est that under some conditions, bulk in the crop can be dissociated from th e propensity to feed. The crop was hyper-distended 6 h after a satiating me al of rehydrated seaweed; that is, the crop took in water and therefore con tained a greater volume than it had contained immediately after satiation. Animals presented with food 6 h after an initial satiating meal consumed a new meal despite the fact that their crop was distended beyond the level at which they had previously terminated feeding. This unexpected result led t o additional experiments designed to study possible orosensory decrement du ring presentation of food. Orosensory input was assessed by recording from the metacerebral cell (MCC) in free-moving animals, The MCC receives excita tory input in response to chemosensory stimulation of the lips, and exhibit ed a slow decrement during the course of a meal or during repeated lip stim ulation without ingestion. Lesions of the cerebro-buccal connectives abolis hed the long-term MCC response decrement to lip stimulation. This result su ggests that the MCC long-term response decrement to lip stimulation is a pr oduct of buccal-ganglion feedback and may not reflect sensory decrement of chemosensory pathways. Therefore, satiation may not produce a change in lip sensitivity to chemosensory input. Our data suggest that one important fac tor that determines satiation is a stretch stimulus of the posterior esopha gus/anterior crop, This stretch stimulus may subside over several hours as the crop contents are redistributed or as receptors slowly adapt. (C) 2001 Elsevier Science B.V. All rights reserved.