Methionine enkephalin increases CNS suppressive control exerted over gill reflex behaviours and associated neural activity in Aplysia california.

Suprafusion of methionine (Met) enkephalin (10(-9) to 10(-12) M) over the abdominal ganglion of Aplysia californica suppressed the amplitude of the siphon-evoked gill withdrawal reflex (GWR), increased the rate at which the GWR habituated and decreased the number of action potentials evoked in identified central gill motor neurons. The suppressive effects of Met-enkephalin could be blocked by pretreatment with naloxone; in addition, the presentation of a sensitizing stimulus immediately but transiently reversed the effects induced by the peptide. Met-Enkephalin did not affect the passive membrane properties of the gill motor neurons and thus it may act by affecting the activity of neurons which exert suppressive control over both gill reflex behaviours and afferent activity to the gill motor neurons.