Avian locomotion activated by brainstem infusion of neurotransmitter agonists and antagonists. II. gamma-Aminobutyric acid.

The companion article (Sholomenko et al. 1991) described the brainstem locomoter regions in the bird where direct intracerebral injection of a number of putative excitatory neurochemicals, including cholinergic agonists, N-methyl-d-aspartate (NMDA) and Substance P, evoke locomotion. Using the same experimental protocol, this study focuses on the locomotor effects following discrete brainstem injections of the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA), and some of GABA agonists and antagonists. Brainstem regions that were electrically and chemically stimulated included the ventromedial medullary reticular formation, the pontobulbar locomotor strip of the dorsolateral pons and medulla, the pontine reticular formation, and the mesencephalic reticular formation. Locomotion was evoked after the injection of the GABA antagonists picrotoxin (a GABAA receptor antagonist) and bicuculline (GABAA antagonist) into several brainstem locomoter regions. Brainstem stimulated locomotion (both chemically and electrically induced) could be transiently blocked by intracerebral infusion of GABA and irreversibly blocked by muscimol (GABAA agonist). Our avian results are similar to those described for mammals and provide support for the suggestion that motor circuitry, at least at brainstem levels, is similar in all vertebrates.