THE ELECTRIC LOBES OF THE ELECTRIC RAY (TORPEDO-MARMORATA) ARE INNERVATED BY GABAERGIC FIBERS - IMMUNOCYTOCHEMICAL EVIDENCE FOR DUAL INNERVATION OF ELECTROMOTONEURONS

It is currently thought that the electric lobes of electric rays are i nnervated by a single neuronal system, the oval nucleus system. In the work reported here, the innervation of the electric lobes was studied with silver staining methods, acetylcholinesterase histochemistry and gamma-aminobutyric acid (GABA) immunocytochemistry. Two types of axon were observed in the lobes: thick GABA-immunonegative fibres, which o riginated from the oval nucleus, and thin GABAergic fibres of unknown origin, here reported for the first time. Electromotoneurons were stro ngly acetylcholinesterase-positive. Non-GABAergic and non-cholinergic neurons were observed in the oval nucleus, which is innervated by GABA -immunoreactive fibres. These results suggest that GABA may modulate e lectric discharge both directly, by GABAergic fibres that project to t he lobes, and indirectly, by GABAergic fibres that project to the oval nucleus.