Gamma-aminobutyric acid synthesised in the olfactory nerve.

Abstract

WE have found the candidate neurotransmitter γ-aminobutyric acid (GABA) and its biosynthetic enzyme in the olfactory nerve of two fish. In the light of these findings current concepts of GABA as an exclusively inhibitory transmitter may have to be reconsidered. The olfactory nerve is a component of the rhinencephalon, a phylogenetically primitive part of the central nervous system (CNS). In addition to its receptive sensory function, the olfactory nerve conducts impulses. Fish have either short olfactory nerves and long olfactory tracts, as do most vertebrates, or long nerves and short tracts, as found in the gar fish (Lepisosteus osseus) and the pike (Esox estor)1. The olfactory cilia and nerve cell body are in the olfactory mucosa, from where the nerve action potentials are propagated to the olfactory bulb, 3 cm (pike) to 15 cm (gar) away, depending on the size of the animal. The primary synapse occurs within the olfactory bulb with the mitral cells. The olfactory nerve in the gar and the pike contains no motor nor other known sensory fibres2,3. Furthermore, it is neurally homogeneous, sensory, unmyelinated and is of a reasonable size for biochemical study (50–150 mg per nerve per fish).