In the present investigation, the effects of inhibitory amino acids and their antagonists were tested on isthmo-tectal projection in the pigeon. The majority of superficial tectal cells are inhibited following stimulation of the parvocellular division of nucleus isthmi. A smaller portion of tectal cells showed excitatory responses followed by an inhibitory period. Both inhibitory mechanisms are blocked by the specific γ-aminobutyric acid (GABA)-antagonist bicuculline but not by strychnine. Our results support the idea that GABA acts as an inhibitory neurotransmitter in the pigeon isthmo-tectal pathways.
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