Localization of biogenic amines in the foregut of Aplysia californica: catecholaminergic and serotonergic innervation.

This study examined the catecholaminergic and serotonergic innervation of the foregut of Aplysia californica, a model system in which the control of feeding behaviors can be investigated at the cellular level. Similar numbers (15-25) of serotonin-like-immunoreactive (5HTli) and tyrosine hydroxylase-like-immunoreactive (THli) fibers were present in each (bilateral) esophageal nerve (En), the major source of pregastric neural innervation in this system. The majority of En 5HTli and THli fibers originated from the anterior branch (En(2)), which innervates the pharynx and the anterior esophagus. Fewer fibers were present in the posterior branch (En(1)), which innervates the majority of the esophagus and the crop. Backfills of the two En branches toward the central nervous system (CNS) labeled a single, centrifugally projecting serotonergic fiber, originating from the metacerebral cell (MCC). The MCC fiber projected only to En(2). No central THli neurons were found to project to the En. Surveys of the pharynx and esophagus revealed major differences between their patterns of catecholaminergic (CA) and serotonergic innervation. Whereas THli fibers and cell bodies were distributed throughout the foregut, 5HTli fibers were present in restricted plexi, and no 5HTli somata were detected. Double-labeling experiments in the periphery revealed THli neurons projecting toward the buccal ganglion via En(2). Other afferents received dense perisomatic serotonergic innervation. Finally, qualitative and quantitative differences were observed between the buccal motor programs (BMPs) produced by stimulation of the two En branches. These observations increase our understanding of aminergic contributions to the pregastric regulation of Aplysia feeding behaviors.