In vivo release of glutamate and aspartate following optic nerve stimulation

Abstract

Glutamate (Glu) and aspartate (Asp) are considered to be major excitatory neurotransmitters in the central nervous system¹, but since the Jasper and Koyama report², the release of endogenous Glu or Asp on stimulation of an appropriate pathway in vivo has received little attention. They have been proposed as transmitters in the optic nerve of the pigeon (Columbalivia)^3–7. We now report that, using a push–pull cannula technique, the concentrations of Glu, Asp, glycine (Gly) and γ-aminobutyric acid (GABA) have been determined by mass fragmentography in perfusates from the optic tectum on electrical stimulation of the optic nerve or of the midbrain nucleus isthmi, pars parvocellularis (Ipc). Optic nerve stimulation markedly increased the content of Glu and Asp in the perfusate whereas stimulation of the nucleus Ipc was ineffective. The amount of GABA collected was increased only during Ipc stimulation. We conclude that Glu, Asp or related substances are involved in the transmission of the pigeon optic nerve terminals.