Effects of inhibitory neurotransmitters on the mudpuppy (Necturus maculatus) locomotor pattern in vitro

Effects of inhibitory neurotransmitters bn the locomotor rhythm and pattern generation were investigated using an in vitro preparation isolated from t he mudpuppy (Necturus maculatus). The preparation consisted of the first fi ve segments of the spinal cord and the right forelimb attached by the brach ial nerves. During N-methyl-D-aspartate (NMDA)-induced locomotion, the rhyt hmic motor output (EMG) was recorded unilaterally from elbow flexor and ext ensor muscles. While neither glycine nor gamma-aminobutyric acid (CABA)-rel ated substances induced locomotion in the absence of NMDA, they modulated N MDA-induced locomotion. Bath application of glycine and GABA suppressed the rhythmic motor pattern induced by NMDA. Addition of glycine receptor: anta gonist strychnine or CABA, receptor antagonist bicuculline disrupted the ph ase relationship between antagonistic motor pools during ongoing locomotion , thereby changing the normal alternating pattern into synchronous EMG burs ts. Both the GABAA receptor agonist muscimol and GABA(B) receptor agonist b aclofen mimicked the effects of GABA as they either slowed down or stopped locomotion, Nipecotic acid, a GABA uptake blocker, bad a similar effect. Th is suggested that an endogenous release of GABA modulated the locomotor rhy thm. The endogenous release was antagonized by the GABAA and CABA, receptor antagonists bicuculline and CGP-35348, respectively. Immunocytochemistry r evealed that glycine and CABA-positive neurons and fibers were present in m udpuppy spinal cord. Although the GABAergic neurons were more numerous than glycinergic neurons, both cell types contributed processes directed toward s the white matter and occasionally towards the ependymal lining of the cen tral canal. Our results suggest that inhibitory neurotransmitters exert pow erful actions upon the neuronal network governing forelimb locomotion in th e mudpuppy. The effects we observed may be mediated by a network of segment ally distributed glycinergic and GABAergic spinal neurons.