Electrophysiological characterization of substantia nigra dopaminergic neurons in partially lesioned rats: Effects of subthalamotomy and levodopa treatment

Abstract

Progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta is the main histopathological characteristic of Parkinson's disease. We studied the electrophysiological characteristics of the spontaneous activity of substantia nigra pars compacta dopaminergic neurons in rats with a partial, unilateral, 6-hydroxydopamine lesion of the nigrostriatal pathway. In addition, the effects of subthalamotomy and prolonged levodopa treatment on the activity of dopaminergic neurons were investigated. As a result of the lesion (∼50% neuronal loss), the number of spontaneously active neurons was significantly reduced. Basal firing rate, burst firing and responsiveness to intravenously administered apomorphine remained unchanged. In contrast, the variation coefficient, a measure of interspike interval regularity, was significantly increased. Ibotenic acid (10 μg) lesion of the ipsilateral subthalamic nucleus in lesioned rats did not modify the electrophysiological parameters. However, prolonged levodopa treatment (100 mg/kg/day + benserazide 25 mg/kg/day, 14 days) reversed the irregularity observed in cells from lesioned rats, while it induced an irregular firing pattern in cells from intact rats. Our results using an experimental model of moderate Parkinson's disease indicate that surviving substantia nigra pars compacta dopaminergic neurons fire irregularly. In this model, subthalamotomy does not modify the firing pattern while levodopa treatment efficiently restores normal firing of SNpc neurons and does not appear to be toxic to them.

Introduction

The midbrain dopaminergic system has been extensively studied due in part to its proposed involvement in the etiology of multiple neuropsychological disorders. Among them, Parkinson's disease is probably the most extensively studied. It is characterized by a progressive loss of the substantia nigra pars compacta (SNpc) dopaminergic population and a secondary nigrostriatal degeneration. The fact that motor disabilities in parkinsonian patients are not evident until approximately 80% of striatal dopamine is lost indicates that compensatory mechanisms occur, supporting basic motor function despite extensive damage (Bezard et al., 2003, Zigmond et al., 2002). Multiple compensatory changes that can contribute to this recovery have been reported. Classically, an increase in extracellular dopamine (increased synthesis and decreased uptake of dopamine in remaining dopaminergic neurons) has been reported (Agid et al., 1973). Recent studies have failed to confirm these findings (Bergstrom and Garris, 2003, Dentresangle et al., 2001). Alternative mechanisms such as diffusion of the neurotransmitter from remaining cells to target in denervated regions, or a proportional decrease of release and uptake to maintain extracellular dopamine levels, have been proposed (Bergstrom and Garris, 2003). Another compensatory mechanism may involve dopaminergic D₁ and D₂ receptor regulation. However, D₁ receptor binding modifications have not been found in human parkinsonian or in animal models with nigrostriatal degeneration (Bezard et al., 2001b, Bokobza et al., 1984, Chritin et al., 1996). In contrast, a postsynaptic D₂ upregulation is a well-recognized characteristic of the denervated striatum; however, its pathophysiological relevance in presymptomatic stages of the disease is thought to be of minor importance (Bezard et al., 2001b, Chritin et al., 1996). Finally, a compensatory role of structures such as the subthalamic nucleus, (Benazzouz et al., 1996, Bergman et al., 1994, Bezard et al., 1999), the thalamus or the cerebral cortex (Bezard et al., 2001a) before the onset of symptoms has been reported. Taken together, all these compensatory changes are likely to affect SNpc dopaminergic neurons since these are influenced by all the mentioned receptors and brain nuclei.

Nowadays, levodopa remains the single, most effective medication for Parkinson's disease (Tintner and Jankovic, 2002). Motor complications, however, appear in patients with more advanced disease (Olanow, 2004). In these cases and in general when pharmacological agents fail to improve parkinsonian symptoms, surgical intervention is indicated. Direct lesion or deep brain stimulation of the subthalamic nucleus is considered the best option (Alvarez et al., 2005, Nilsson et al., 2005).

In the present study, we assessed the electrophysiological characteristics of the remaining SNpc dopaminergic neurons in partially lesioned rats which may represent a presymptomatic model of parkinsonism in which restorative and compensatory changes are likely to be present. The influence of subthalamotomy and levodopa treatment on the same parameters was also studied.