Avian locomotion activated by brainstem infusion of neurotransmitter agonists and antagonists. I. Acetylcholine excitatory amino acids and substance P.

Previous studies have demonstrated that focal electrical stimulation of regions within the brainstem of a decerebrate bird will elicit all the normal patterns of avian locomotion. However, electrical stimulation can activate a variety of neuronal elements within the radius of effective current spread, including axons of passage traversing the stimulation point. To restrict activation to neuronal cell bodies within the immediate vicinity, we have utilized direct intracerebral injection of neurotransmitters, their agonists and antagonists, into identified brainstem locomotor regions. To undertake these studies, birds (geese or ducks) were placed in a stereotaxic frame and decerebrated under halothane anesthesia. After completion of surgery, several discrete locomotor regions were first identified with electrical microstimulation. Acetylcholine (ACh) and excitatory amino acid (EAA) agonists and antagonists, as well as Substance P were then slowly infused into each brainstem region. Any change in locomotor behavior was recorded by electromyographic techniques. When injected into a variety of sites, carbachol (an ACh nicotinic (AChN) and muscarinic (AChM) agonist) and pilocarpine (an AChM agonist) evoked locomotion, whereas atropine (an AChM antagonist) blocked locomotion. N-methyl-D-aspartate NMDA), but not glutamate, also elicited locomotion or reduced the current intensity threshold for electrically-evoked locomotion. The NMDA-induced locomotion evoked locomotion. The NMDA-induced locomotion could be blocked by the injection of glutamic acid diethyl ester (GDEE, an EAA antagonist) or D-2-amino-5-phosphonopentanoic acid (AP5) into the same site. Finally. Substance P also evoked locomotion. The above observations strongly suggest that brainstem electrically-stimulated locomotion in decerebrate birds is not due to activation of fibers traversing a brainstem locomotor region, but instead, is due to the activation of receptors located on neuronal cell bodies, dendrites or presynaptic terminals in the immediate vicinity of the micropipette tip. After correlating our findings with similar lamprey and mammalian studies, the comparable discoveries serve to underscore the suggestion that the neuroanatomical substrates underlying the brainstem control of locomotion appear to be highly conserved in all vertebrates.