Responses of substantia nigra pars reticulata neurons to intrastriatal D1 and D2 dopaminergic agonist injections in the rat

Extracellular neuronal recordings were performed in the substantia nigra pars reticulata (SNr) of normal and 6-hydroxydopamine (6-OHDA)-lesioned rats during intrastriatal D1 and D2 dopaminergic-agonist injections. Three types of responses were observed, inhibition, excitation and/or regularization of neuronal discharge patterns. In normal rats, variable responses were observed after D2 injections and a predominance of inhibition after D1 injections. In lesioned rats, the percentage of neurons inhibited and that of neurons regularized increased following D2 injections. During sequential intrastriatal D1 and D2 agonist injections, a response to both dopaminergic agents was observed in 22.5% and 55.5% of SNr neurons in normal and lesioned rats, respectively. Our data suggests that the physiological relevance of direct and indirect pathways convergence upon a given SNr target neuron depends on their respective synaptic weight, the influence of surrounding SNr neurons and the stale of dopamine depletion.