Introduction tion of drugs that increase dopamine DA transmission

Brain Research 879 2000 93–104 www.elsevier.com locate bres Research report 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 Edward G. Meloni , Michael Davis The Department of Psychiatry and Behavioral Sciences , Emory University, Atlanta, GA 30322, USA Accepted 25 July 2000 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 D receptor agonist SKF 82958 and the indirect DA agonist L -3,4-dihydroxyphenylalanine L -DOPA on the acoustic 1 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 D receptors may mediate the enhancement of startle seen in the present study.  2000 Elsevier Science B.V. All rights 1 reserved. Theme : Neural basis of behavior Topic : Monoamines and behavior Keywords : SKF 82958; L -DOPA; D1 receptor; Startle; 6-OHDA

1. Introduction tion of drugs that increase dopamine DA transmission

increase the acoustic startle response [16]. In particular, the The acoustic startle reflex in rats is a rapid sensorimotor benzazepine derivative SKF 82958, a selective and high- response elicited by a sudden and intense auditory stimulus efficacy dopamine D receptor agonist [53], markedly 1 [15]. The amplitude of this short-latency response can be enhances the acoustic startle response [43] by activation of easily quantified, is sensitive to various behavioral and dopamine D receptors in the basal ganglia [42]. Hence, 1 pharmacological manipulations, and has been used exten- the enhancement of this reflex by D agonists has been a 1 sively to study the neurocircuitry and neurochemistry useful behavior with which to study the neural mechanisms involved in the modulation of reflex motor behavior [14]. underlying dopaminergic control of motor responses. Along these lines, we have found that systemic administra- 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 Corresponding author. Fax: 11-404-727-3436. E-mail address : emeloniemory.edu E.G. Meloni. preparation that models the type of neural degeneration 0006-8993 00 – see front matter  2000 Elsevier Science B.V. All rights reserved. P I I : S 0 0 0 6 - 8 9 9 3 0 0 0 2 7 5 3 - 0 94 E seen in this akinetic disease, DA-containing cells of the River, Raleigh-Durham, NC housed in group cages of nigrostriatal pathway are selectively destroyed by in- four rats each until the time of surgery when they were tracerebral injection of the neurotoxin 6-OHDA. Typically, singly housed. Animals were maintained on a 12-h light the lesion is unilateral, leaving a denervated and an intact dark cycle lights on at 07.00 h with food and water side within the same animal. Subsequently, systemic continuously available. challenge with DA agonists in this ‘hemi-parkinsonian’ rat produces a circling behavior that is specific to the type of 2.2. Startle apparatus DA agonist used. Indirect DA agonists such as amphet- amine induce ipsilateral rotation [73] by releasing DA at Startle testing of animals was conducted using four the level of the striatum from the intact side. In contrast, identical stabilimeter devices located in separate, custom- direct DA agonists such as apomorphine [22] and SKF designed 90370370 cm sound-attenuating chambers. Ven- 38393 [48] induce contralateral rotation, presumably due tilation for each chamber was provided by the ventilation to activation of supersensitized DA receptors in the unit from an Industrial Acoustics Minibooth Industrial denervated striatum [72]. Interestingly, systemic adminis- Acoustic Company, Bronx, New York. Each stabilimeter tration of the anti-Parkinson’s drug L -3,4-dihydroxyphenyl- consisted of a 9315315 cm Plexiglas and wire-mesh cage alanine L -DOPA also induces contralateral rotation suspended between compression springs within a steel [24,72], despite the fact that 6-OHDA lesions dramatically frame. The floor of each stabilimeter consisted of four 6.0 reduce the activity .80 of aromatic L -amino acid mm diameter stainless steel bars spaced 18 mm apart. Cage decarboxylase AADC, i.e. the enzyme responsible for the movement resulted in displacement of an accelerometer conversion of L -DOPA into DA [40]. Spared DA neurons PCB Piezotronics, Depew, NY where the resultant [23], serotonergic neurons [3,25], or striatal interneurons voltage was proportional to the velocity of the cage that contain AADC [40,51] may all be possible sites for displacement. The analog output of the accelerometer was the conversion of L -DOPA to DA which can act at amplified PCB Piezotronics, Model 483B21 and digitized sensitized striatonigral DA receptors to induce contralateral on a scale of 0–2500 units by an InstruNET device GW rotation in 6-OHDA-lesioned animals. Instruments, Model 100B; Somerville, MA interfaced to a In the present study, we have used the 6-OHDA lesion Macintosh G3 computer. Startle amplitude was defined as preparation to examine the effects of the dopamine D the peak accelerometer voltage that occurred during the 1 receptor agonist SKF 82958 on startle in animals with first 200 ms after onset of the startle stimulus. A sur- destruction of the DA-containing cells of the substantia veillance camera Burle, Model TC651B with a TC9907a nigra pars compacta SNc. Because striatonigral neurons lens, Operational Security Systems, Atlanta, GA was mediate the enhancement of startle by SKF 82958 [42], positioned behind each stabilimeter and connected to a TV putative 6-OHDA-induced supersensitivity of D receptors monitor located outside the isolation chamber. A red light 1 located on these neurons should lead to a potentiated bulb 7.5 W was located on the ceiling of the startle box startle response by SKF 82958. In addition, we tested the to provide the illumination for the cameras in the otherwise effects of the indirect DA agonist L -DOPA on startle in completely dark box. 6-OHDA-lesioned animals. Previous studies have reported Constant wide band background noise 60 dB was either no effect [19] or an enhanced startle response only produced by a General Radio noise generator Type 1390- after high doses of L -DOPA [29] in normal animals. Like B and delivered through high frequency speakers Radio SKF 82958, we predict that L -DOPA should potentiate the Shack Supertweeters, range 5–40 kHz located 5 cm from startle response in 6-OHDA-lesioned animals via superse- the front of each cage. Startle responses were evoked by 50 nsitive striatonigral DA receptors. An additional goal of ms bursts of white-noise 5 ms rise-decay generated by the present study was to measure any L -DOPA-induced the Macintosh G3 computer 0–22 kHz, amplified by a changes in the activity of caudate–putamen CPu cells Radio Shack amplifier 100 W; Model MPA-200, and using c-Fos immunohistochemistry, a well-established delivered through the same speakers. Sound level measure- ¨ marker for neuronal activation [64]. Several studies have ments SPL were made with a Bruel and Kjaer model reported a dramatic increase in c-Fos expression in the 2235 sound-level meter A scale; random input with the CPu of unilateral 6-OHDA-lesioned animals after L -DOPA microphone Type 4176 located 10 cm from the center of treatment [11,61] that may reflect a supersensitive response the speaker, which approximates the distance of the rat’s by striatal receptors to DA converted from exogenous ear from the speaker during testing. The presentation and L -DOPA. sequencing of all stimuli were under the control of the Macintosh G3 computer using specially designed software The Experimenter, Glassbeads Inc., Newton, CT.

2. Materials and methods