Dopaminergic innervation of Aplysia gill muscle.

1. The fluorescence histochemical method of Hillarp and Falck was employed to investigate the distribution of dopamine in the gill of Aplysia. 2. Dopamine-containing nerve fibers and varicosities were found in close association with the lateral and medial external pinnule muscles, the circular and longitudinal muscles of the afferent vessel, and the circular muscles of the efferent vessel. 3. Other large, dopamine-containing, intensely fluorescent processes were observed lining the vascular sinuses of the gill and within the muscle bundles themselves. 4. Our findings support the hypothesis that dopamine is a neuromuscular transmitter in the gill. However, the data also suggest that dopamine may play a hormonal role as well.