Neurotransmitter receptors on gill muscle fibers and the gill peripheral nerve plexus in Aplysia

Isolated pinnules of the gill of Aplysia contract when dopamine (DA) is perfused through the bath. The contraction is not blocked by high-Mg²⁺ seawater, and reflects excitatory receptors for DA on the smooth muscle cells of the gill. The pinnule often shows irregular, spontaneous contractions which are blocked by high-Mg²⁺ seawater and 30mM CoCl₂. These contractions reflect spontaneous activity of a peripheral nerve plexus. No other transmitter was found to be directly excitatory on the muscle fibers, although there are inhibitory receptors for serotonin (5-HT). Tactile stimulation of the pinnule evoked a two-component contractile reflex contraction due to activation of the peripheral nerve plexus. Acetylcholine, octopamine, and 5-HT but not several other transmitters depressed these responses, presumably due to inhibitory receptors on the neurons of the peripheral plexus.