GABA, Not Glutamate, Controls the Activity of Substantia Nigra Reticulata Neurons in Awake, Unrestrained Rats

Substantia nigra pars reticulata (SNr) receives both GABAergic and glutamatergic (GLU) inputs that are believed to act together to regulate neuronal activity in this structure. To examine the role of these inputs, single-unit recording was coupled with iontophoresis of GLU and GABA in rats under two conditions: awake, unrestrained and under chloral hydrate anesthesia. Although GABA potently inhibited SNr cells in both conditions, freely moving rats showed lower sensitivity than anesthetized animals. Likewise, GLU effectively induced excitations in most SNr neurons in anesthetized animals but was much less effective in awake, unrestrained animals in terms of both the number of sensitive cells and the magnitude of GLU-induced excitation. These findings, along with consistent excitations induced by bicuculline in awake, unrestrained rats, suggest that modulation of GABA inhibitory input, not the opposing actions of GLU and GABA, is the primary factor that regulates the activity state of SNr neurons.