Enhancement of the acoustic startle response by dopamine agonists after 6-hydroxydopamine lesions of the substantia nigra pars compacta: corresponding changes in c-Fos expression in the caudate–putamen

Abstract

Rats with 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal pathway show enhanced locomotor and stereotyped behaviors when challenged with direct and indirect dopamine (DA) agonists due to the development of postsynaptic supersensitivity. To determine if this phenomenon generalizes to other motor behaviors, we have used this rat model of Parkinson’s disease to examine the effects of the direct dopamine D₁ receptor agonist SKF 82958 and the indirect DA agonist l-3,4-dihydroxyphenylalanine (l-DOPA) on the acoustic startle response. In addition, we used the expression of c-Fos protein as a marker of neuronal activity to assess any corresponding drug-induced changes in the caudate–putamen (CPu) after l-DOPA administration. Male Sprague–Dawley rats received bilateral injections of 6-OHDA into the substantia nigra pars compacta and 1 week later were tested for startle after systemic administration of SKF 82958 (0.05 mg/kg) or l-DOPA (1, 5, 10 mg/kg). SKF 82958 produced a marked enhancement of startle with a rapid onset in 6-OHDA-lesioned but not SHAM animals. l-DOPA produced a dose- and time-dependent enhancement of startle in 6-OHDA-lesioned rats that had no effect in SHAM animals even at the highest dose (10 mg/kg). Furthermore, l-DOPA produced a dramatic induction of c-Fos in the CPu in 6-OHDA-lesioned animals. Consistent with other literature, these data suggest that neurons in the CPu become supersensitive to the effects of DA agonists after 6-OHDA-induced denervation of the nigrostriatal pathway and that supersensitive dopamine D₁ receptors may mediate the enhancement of startle seen in the present study.

Introduction

The acoustic startle reflex in rats is a rapid sensorimotor response elicited by a sudden and intense auditory stimulus [15]. The amplitude of this short-latency response can be easily quantified, is sensitive to various behavioral and pharmacological manipulations, and has been used extensively to study the neurocircuitry and neurochemistry involved in the modulation of reflex/motor behavior [14]. Along these lines, we have found that systemic administration of drugs that increase dopamine (DA) transmission increase the acoustic startle response [16]. In particular, the benzazepine derivative SKF 82958, a selective and high-efficacy dopamine D₁ receptor agonist [53], markedly enhances the acoustic startle response [43] by activation of dopamine D₁ receptors in the basal ganglia [42]. Hence, the enhancement of this reflex by D₁ agonists has been a useful behavior with which to study the neural mechanisms underlying dopaminergic control of motor responses.

Over the past 30 years, this field of research has been aided by the use of the 6-hydroxydopamine (6-OHDA)-lesioned rat model of Parkinson’s disease [71]. In a preparation that models the type of neural degeneration seen in this akinetic disease, DA-containing cells of the nigrostriatal pathway are selectively destroyed by intracerebral injection of the neurotoxin 6-OHDA. Typically, the lesion is unilateral, leaving a denervated and an intact side within the same animal. Subsequently, systemic challenge with DA agonists in this ‘hemi-parkinsonian’ rat produces a circling behavior that is specific to the type of DA agonist used. Indirect DA agonists such as amphetamine induce ipsilateral rotation [73] by releasing DA at the level of the striatum from the intact side. In contrast, direct DA agonists such as apomorphine [22] and SKF 38393 [48] induce contralateral rotation, presumably due to activation of supersensitized DA receptors in the denervated striatum [72]. Interestingly, systemic administration of the anti-Parkinson’s drug l-3,4-dihydroxyphenylalanine (l-DOPA) also induces contralateral rotation [24], [72], despite the fact that 6-OHDA lesions dramatically reduce the activity (>80%) of aromatic l-amino acid decarboxylase (AADC, i.e. the enzyme responsible for the conversion of l-DOPA into DA) [40]. Spared DA neurons [23], serotonergic neurons [3], [25], or striatal interneurons that contain AADC [40], [51] may all be possible sites for the conversion of l-DOPA to DA which can act at sensitized striatonigral DA receptors to induce contralateral rotation in 6-OHDA-lesioned animals.

In the present study, we have used the 6-OHDA lesion preparation to examine the effects of the dopamine D₁ receptor agonist SKF 82958 on startle in animals with destruction of the DA-containing cells of the substantia nigra pars compacta (SNc). Because striatonigral neurons mediate the enhancement of startle by SKF 82958 [42], putative 6-OHDA-induced supersensitivity of D₁ receptors located on these neurons should lead to a potentiated startle response by SKF 82958. In addition, we tested the effects of the indirect DA agonist l-DOPA on startle in 6-OHDA-lesioned animals. Previous studies have reported either no effect [19] or an enhanced startle response only after high doses of l-DOPA [29] in normal animals. Like SKF 82958, we predict that l-DOPA should potentiate the startle response in 6-OHDA-lesioned animals via supersensitive striatonigral DA receptors. An additional goal of the present study was to measure any l-DOPA-induced changes in the activity of caudate–putamen (CPu) cells using c-Fos immunohistochemistry, a well-established marker for neuronal activation [64]. Several studies have reported a dramatic increase in c-Fos expression in the CPu of unilateral 6-OHDA-lesioned animals after l-DOPA treatment [11], [61] that may reflect a supersensitive response by striatal receptors to DA converted from exogenous l-DOPA.