Peripheral neuronal mechanisms for control of the gill movements in Aplysia

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L7 is a gill motor neuron which drives gill longitudinal shortening in Aplysia californica, A. kurodai, and A. juliana. We have identified a neuron in the right hemisphere of the abdominal ganglion, referred to as Arti-L7, which opposes L7-driven longitudinal shortening in the species. Anti-L7 did not block the generation of L7 impulses within the abdominal ganglion but apparently opposed L7-driven longitudinal shortening at the periphery of the gill. The spontaneous firing rate of Anti-L7 impulses was in a range below about 5 Hz. The suppression of L7-driven longitudinal shortening was dependent on the firing rate of Anti-L7. The threshold firing rate of Anti-L7 impulses for the suppressive effect was less than 0.5 Hz. In response to electrical stimulus pulses applied to the pleuro-abdominal connective, the impulse firing of Anti-L7 was suppressed, while a long-lasting hyperpolarizing potential occurred in the neuron. The suppression continued for several minutes even after the stimulation ceased. During the suppression, the L7-driven longitudinal shortening of the gill and gill movements induced by stimulation of the siphon nerve were potentiated. The results may show that sensitization of the gill withdrawal reflex is, at least in part, attributable to suppression of Anti-L7 activity.Immunocytochemical analysis revealed that Anti-L7 shows serotonin-immunoreactivity. The location, the serotonin immunoreactivity and the electrophysiological properties of Anti-L7 indicate that it is identical to a member of the RB neuron cluster known in A. californica. In the gill, serotonin-immunoreactive neural processes were observed for muscle cells, branchial-ganglionic neurons and plexus neurons. These sites may be involved as loci of mechanisms for antagonistic action of Anti-L7 on L7-induced gill movements.