Three neurons in the abdominal ganglion of elicit gill movements similar to those observed when dopamine is added to a gill perfusate. One, which we designate L, produces contractions of all medial and lateral external pinnule muscles, the circular and longitudinal muscles of the afferent vessel, and the circular muscles of the efferent vessel. The others produce identical contractions in anterior portions (L) or posterior (L) portions of the gill. These contractions are similar to, but less pronounced than, those produced by the motoneuron, L. 2. Clear population excitatory junction potentials (EJPs) can be recorded extracellularly from the gill when other gill motoneurons, L and LDG, are activated. However, EJPs are observed only occasionally upon activation of the L neurons. Nonetheless, with averaging techniques L EJPs are measurable and their latencies, times to peak, and durations are similar to those of L and LDG. These observations suggest that the L cells are true gill motoneurons. 3. The contrations induced by activation of the L cells, but not those induced by L or LDG, are blocked by ergometrine and fluphenazine. Since these drugs are thought to act exclusively on dopamine receptors and since L-induced movements are similar to dopamine-induced movements, it is suggested that the L neurons are dopaminergic motoneurons. 4. Dopamine is known to modulate muscle contractions induced by activation of various gill motoneurons. However, the L neurons do not appear to produce such modulation directly. Even though an increased response of other motoneurons was occasionally observed after L stimulation, it was always preceded by a small additional contraction, which may reflect release of dopamine from an independent, peripheral storage site. 5. While considerable dopamine is present in ganglia and nerve trunks to the gill, no identified neuronal cell bodies, including L and other gill motoneurons, contained detectable dopamine. Because our electrophysiological, pharmacological, and histochemical data are all consistent with the L cells being dopaminergic, we suggest that dopamine may be synthesized in the axons and nerve terminals of L cells but not in their cell bodies.
