Release of exqenous glycine in the pigeon optic tectum during stimulation of a midbrain nucleus.

A pathway having an affinity for glycine has been investigated in the pigeon optic lobe; it originates in the nucleus isthmi pars parvocellularis (Ipc) and terminates in the tectum, In an attempt to obtain evidence that glycine plays a role as a transmitter in this system, the effect of electrical stimulation on release of labeled substances previously injected in the tectum was tested. By perfusing the upper strata of the optic tectum with a push-pull cannula the release of radioactive glycine was shown to be markedly increased by electrical stimulation of Ipc, but not by stimulation of other sites. Ipc stimulation did not affect the efflux of exogenous leucine or urea, whereas a GABA release was observed. With K+ (40 mM) stimulation all amino acids tested were released. It is suggested that the Ipc neuron terminals in the tectum take up glycine and release it upon stimulation of the Ipc nucleus.