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

Abstract

Prior evidence has suggested that meal satiation in the marine mollusk Aplysia is associated with stretch of the crop. The current data, however, suggest that under some conditions, bulk in the crop can be dissociated from the propensity to feed. The crop was hyper-distended 6 h after a satiating meal of rehydrated seaweed; that is, the crop took in water and therefore contained 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 to additional experiments designed to study possible orosensory decrement during presentation of food. Orosensory input was assessed by recording from the metacerebral cell (MCC) in free-moving animals. The MCC receives excitatory input in response to chemosensory stimulation of the lips, and exhibited a slow decrement during the course of a meal or during repeated lip stimulation without ingestion. Lesions of the cerebro-buccal connectives abolished the long-term MCC response decrement to lip stimulation. This result suggests that the MCC long-term response decrement to lip stimulation is a product 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 factor that determines satiation is a stretch stimulus of the posterior esophagus/anterior crop. This stretch stimulus may subside over several hours as the crop contents are redistributed or as receptors slowly adapt.