The Perfusion of the Endogenous Neuropeptide, Phe-Met-Arg-Phe-NH2 (FMRFamide) through the Gill of Aplysia Potentiates the Gill Withdrawal Reflex Evoked by Siphon Stimulation and Prevents its Habituation

Perfusion of Phe-Met-Arg-Phe-NH₂ (FMRFamide), an endogenous neuropeptide, through the gill of Aplysia significantly potentiates the amplitude of the gill withdrawal reflex evoked by tactile stimulation of the siphon with or without the central nervous system present. The potentiating effect of FMRFamide is reversible with washout. FMRFamide perfusion of the gill also prevented the gill withdrawal reflex from undergoing habituation when the tactile stimulus was presented repeatedly. Rather than habituate, the gill withdrawal reflex increased in amplitude. FMRFamide continued to prevent habituation in the absence of the parietal-visceral ganglion. We hypothesize that FMRFamide plays a role in the mediation of behavioural state primarily by affecting activity in the peripheral nervous system in the gill.