Electrophysiological alterations in subthalamic neurons after unilateral dopamine depletion in the rat

The present study examined the effects of unilateral 6-hydroxydopamine (6-OHDA) lesions of the substantia nigra pars compacta (SNc) on electrophysiological properties of subthalamic neurons (STN) in adult rats. Most neurons displayed regular spontaneous tonic firing patterns in both control and lesioned animals; however, the percentage of neurons with spontaneous burst firing at hyperpolarized membrane potentials was increased significantly in lesioned animals compared with controls (45% vs. 14% respectively). In the presence of bicuculline, a gamma-aminobutyric acid type A (GABAA) receptor antagonist, electrical stimulation of the internal capsule produced monosynaptic excitatory postsynaptic potentials (EPSPs) in almost all recorded neurons. DA (50 microM) increased the amplitude and/or duration of the EPSPs in neurons from both groups, whereas the DA D1 receptor agonist SKF 81297 (10 microM) produced a significant increase in amplitude and/or duration of EPSPs in neurons from the lesioned group only. This latter increase was blocked by pretreatment with the DA D1 antagonist SCH 23390 (10 microM). These data suggest that unilateral degeneration of DA neurons in the SNc changes firing properties and enhances electrophysiological responsiveness of STN neurons to activation of DA D1 receptors.