Deep brain stimulation of the subthalamic nucleus in the 6-hydroxydopamine rat model of Parkinson's disease: effects on sensorimotor gating

Deep brain stimulation (DBS) of the subthalamic nucleus (STN) is effectively used to treat motor symptoms in Parkinson's disease (PD). Recently more attention has been paid to behavioral disturbances caused by PD itself and by STN DBS. In the 6-hydroxydopamine (6-OHDA) PD rat model we investigated the effect of STN DBS on deficient prepulse inhibition (PPI) induced by the dopamine (DA) receptor agonist apomorphine, which is an operative measure for disturbed sensorimotor gating seen in certain neuropsychiatric disturbances. Male Sprague Dawley rats with bilateral lesions of the nigrostriatal DA system (striatal injection of 6-OHDA or vehicle for sham-lesion) were bilaterally implanted with electrodes for DBS into the STN. After determination of individual thresholds rats were stimulated (130Hz, 80μs pulse width) or sham-stimulated for epochs of six days. On the sixth day of each epoch rats were tested for PPI of the acoustic startle response after apomorphine or vehicle injection in a within randomized cross-over design. Stimulation of the STN improved PPI in vehicle-treated (control) rats, but deteriorated PPI after apomorphine treatment. This effect was more pronounced in sham-lesioned rats. Furthermore, in lesioned rats the startle reaction was marginally enhanced without effect of stimulation or apomorphine treatment. These data suggest that STN DBS interacts with dopaminergic action. With respect to functional neurosurgery, STN DBS alone may improve certain aspects of psychiatric disturbances, but may have a different impact when combined with dopaminergic medication.     

Effects of haloperidol and SCH 23390 on acoustic startle and prepulse inhibition under basal and stimulated conditions

Schwarzkopf, Steven B., John P. Bruno, and Tanmoy Mitra: Differential Effects of Haloperidol and SCH 23390 on Acoustic Startle and Prepulse Inhibition Under Basal and Stimulated Conditions. Prog. Neuro-Psychopharmacol. & Biol. Psychiat. 1993, 17(6): 1023–1036.

1. 1. Adult Sprague-Dawley rats underwent startle testing for assessment of baseline startle amplitude and prepulse inhibition (PPI) of the startle reflex.

2. 2. Animals were tested after administration of either: saline, a selective D1 dopamine (DA) receptor antagonist, a relatively selective D2 DA antagonist, or combined low dose D1 and D2 antagonists.

3. 3. Changes due to antagonists were assessed with and without administration of the D1/D2agonist apomorphine.

4. 4. Testing without apomorphine stimulation showed that both D1 and D2 antagonists reduce baseline startle and enhance PPI. Further, the two antagonists exhibited a synergistic interaction.

5. 5. Testing with apomorphine showed that D1 and D2 antagonists reduce apomorphineinduced startle enhancement. Again, the two exhibited a synergistic interaction.

6. 6. For PPI, the D2 but not D1 antagonist reduced the apomorphine effect. However, the D1 antagonist potentiated the effect of the D2 antagonist.

Opiate-dopamine interactions in the neural substrates of acoustic startle gating in the rat

1. The acoustic startle reflex (ASR) was measured in adult male Dawley rats using startling acoustic stimuli presented either alone or 60-500 msec after a weak acoustic prepulse. 2. The inhibition of the ASR by the prepulse, termed "prepulse inhibition" (PPI), was blocked in animals treated either with the indirect dopamine (DA) agonist d-amphetamine (AMPH) or with the direct DA receptor agonist apomorphine (APO). 3. Pretreatment with the opiate receptor antagonist naloxone (NAL) prevented the AMPH-induced loss of PPI, but did not diminish the APO-induced loss of PPI. 4. The opiate heroin had no significant effect on PPI. 5. Dopaminergic mechanisms that modulate PPI in the rat may be regulated by opiate systems that act presynaptic to the DA receptor; brain opiate receptors may not have direct effects on startle gating independent of this opiate-DA interaction.     

Rat strain differences in the ability to disrupt sensorimotor gating are limited to the dopaminergic system, specific to prepulse inhibition, and unrelated to changes in startle amplitude or nucleus accumbens dopamine receptor sensitivity.

Previous studies indicate that a variety of pharmacological agents interfere with the prepulse inhibition of the acoustic startle (PPI) response including phencyclidine (PCP), 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), amphetamine, and apomorphine. Strain differences have been observed in the ability of apomorphine to disrupt PPI, although the degree to which these strain differences occur after administration of nondopaminergic drugs or the degree to which differences can be observed in other models of dopamine (DA) receptor activation has not been elucidated. The present study tested the effects of apomorphine, amphetamine, 8-OH-DPAT, and PCP on PPI in the Sprague Dawley and Wistar rat strains. Because apomorphine disrupts PPI via activation of DA receptors in the nucleus accumbens, apomorphine-induced hyperlocomotion, also a behavioral model of nucleus accumbens DA receptor activation, was measured in both rat strains. Administration of PCP or 8-OH-DPAT attenuated PPI in both strains, whereas apomorphine and amphetamine only attenuated PPI in Wistar rats. The ability of apomorphine to increase motor activity in the absence of a startle-eliciting stimulus was similar in the two strains, as was apomorphine-induced hyperlocomotion. A time course analysis of the effects of apomorphine on startle response in Sprague Dawley rats found that changes in the magnitude of PPI followed changes in basic startle amplitude. Similarly, no apomorphine-induced attenuation of PPI was observed in Sprague Dawley rats after 6-OHDA-induced DA receptor supersensitivity in the nucleus accumbens. These data suggest a dissociation between the effects of DA receptor agonists in PPI and other behavioral models of DA receptor activation.     

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

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(1) 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 D(1) receptors may mediate the enhancement of startle seen in the present study.     

Dopamine depletion in the medial prefrontal cortex induces sensitized-like behavioral and neurochemical responses to cocaine

It has been postulated that behavioral sensitization to cocaine is associated with an attenuation of cocaine-induced dopamine (DA) transmission in the medial prefrontal cortex (mPFC). Hence, experiments were designed to examine the effects of chemically-induced cortical DA depletion on the acute behavioral and neurochemical responses to cocaine. One week following two bilateral 6-hydroxydopamine (6-OHDA) injections into the mPFC, animals received injections of cocaine (7.5, 15 or 30 mg/kg, i.p.) or saline (1 ml/kg, i.p.) in a randomized fashion with a minimum 3 day intertrial interval. Cocaine produced a dose-dependent increase in motor activity which was significantly enhanced in animals depleted (mean of 76%) of dopamine in the mPFC. Likewise, 6-OHDA lesions of the mPFC produced a significant enhancement of cocaine-induced DA transmission in the nucleus accumbens (NAC) as estimated by in vivo microdialysis. These data indicate a permissive involvement of cortical DA in mediating behavioral and neurochemical responses to cocaine, as well as confirm the ability of the mPFC to influence subcortical structures in response to an acute injection of cocaine. Collectively, the present findings suggest that alterations in cortical DA transmission may be a neural substrate mediating the development of sensitization to cocaine, and thus, may contribute to the addictive properties of cocaine.     

Neonatal cytomegalovirus exposure decreases prepulse inhibition in adult rats: implications for schizophrenia.

The goal of these studies was to determine whether neonatal viral exposure leads to a deficit in information processing in adulthood. To accomplish this, rats were infected neonatally with rat cytomegalovirus, and acoustic startle responses were measured when rats were 120 days old. Acoustic startle was elicited by using a 118-decibel (dB) white noise alone or after a prepulse 10 dB above background (65 dB); responses were measured after an injection of saline or the dopamine agonist apomorphine. Response amplitudes after the pulse alone were not significantly altered by either viral exposure or apomorphine. Responses of animals exposed to the prepulse before the pulse were approximately 10% of that after the pulse alone and did not differ between control or virus-exposed animals injected with saline. Animals injected with apomorphine exhibited a greater startle response than animals injected with saline, and control and virus-exposed rats injected with apomorphine differed in the magnitude of their responses. Apomorphine attenuated responses after the prepulse, and virus-exposed animals exhibited more than twice the attenuation than non-virus-exposed animals. Analysis of prepulse inhibition, calculated from the acoustic startle data, indicated that although viral exposure alone did not significantly affect information processing, when virus-injected rats were exposed to apomorphine, a significant 38% decrease in prepulse inhibition was apparent. Findings demonstrate that rats infected neonatally with rat cytomegalovirus exhibit a deficit in sensorimotor gating upon dopamine stimulation, supporting a possible link between viral infection and schizophrenia.     

Effects of locally applied D1 and D2 agonists on striatal neurons with 6-OHDA and pertussis toxin lesions.

Electrophysiological recordings were performed on caudate neurons in rats with dopamine (DA) depleted striatum in combination with pertussis toxin (PT) lesions. Pertussis toxin inactivates the G protein coupled to D2 receptors. DA depletions were performed by unilateral injections of 6-hydroxydopamine (6-OHDA). After the 6-OHDA lesion, rats were challenged with low doses of apomorphine. When a double peak rotational pattern was stable over repeated rotational tests, PT was injected into striatum ipsilateral to the DA depleted side. Two days after the PT injections extracellular recordings with local applications of the D1 agonist SKF 38393 and the D2 agonist N-0437 were performed. Spontaneous firing rates, measured before drug application, were elevated in animals with both 6-OHDA and 6-OHDA/PT combination of lesions. In rats with only 6-OHDA lesions, a supersensitivity to N-0437 was observed, while no significant change in response to the D1 agonist was detected. Recordings from caudate neurons in rats with a combination of 6-OHDA and PT resulted in no response to the D2 agonist. However, a subsensitivity to the D1 agonist was detected and only 60% of neurons were inhibited by SKF 38393. Taken together, these data suggest an interaction between the D1 and D2 receptors, which is revealed only after an upregulation of the D2 receptors and subsequent blockade of D2 mediated effects.     

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

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 -3,4-dihydroxyphenylalanine ( -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 -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 -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. -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, -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.     

Failure of haloperidol to block the effects of phencyclidine and dizocilpine on prepulse inhibition of startle.

Prepulse inhibition of acoustic or tactile startle (PPI), a form of sensorimotor gating, occurs when a weak prestimulus precedes a startling stimulus and inhibits the startle response. Studies of PPI have revealed that schizophrenic patients exhibit a deficit in this form of sensorimotor gating. In rats, PPI is blocked by dopamine agonists such as apomorphine or quinpirole, effects that are antagonized by haloperidol. Phencyclidine (PCP) has been suggested as a possible model psychotogen and produces a deficit in PPI that is similar to what is observed in schizophrenic patients. Dizocilpine is an anticonvulsant drug that, like PCP, is a noncompetitive antagonist of N-methyl-D-aspartate (NMDA)-induced excitations in brain and also disrupts PPI. In the present study, PPI of acoustic and tactile startle was measured in male Sprague-Dawley rats after injections of 5.0 mg/kg PCP with or without pretreatment with 0.02 or 0.1 mg/kg haloperidol, or with 0.5 mg/kg dizocilpine with or without pretreatment with 0.1 mg/kg haloperidol. The 0.1 mg/kg dose of haloperidol blocks the effects of apomorphine or quinpirole on PPI in rats. Startle was elicited by noise bursts at 105 or 120 dB or by air-puffs (tactile) and was inhibited by 75 or 85 dB prepulse stimuli presented 100 msec before the startle stimuli. The different eliciting stimuli produced different levels of startle in both control and drug-treated animals. Both NMDA antagonists significantly reduced the amount of PPI induced by the 75 dB prestimulus, independently of the level of startle responses elicited by the startle stimuli. Haloperidol did not block the disruption of PPI induced by either PCP or dizocilpine. In addition, PCP was unable to block PPI when the 85 rather than the 75 dB prepulse was used to inhibit either acoustic or tactile startle. These results confirm that putative NMDA antagonists inhibit sensorimotor gating in rats and suggest that these effects are not mediated by the activation of central dopamine systems.     

Male rat copulation following 6-OHDA lesions of the medial preoptic area: resistance to repeated administration and rapid behavioral recovery

Dopamine (DA) in the medial preoptic and (MPOA) has been shown to facilitate male rat sexual behavior. However, injections of the catecholamine (CA) neurotoxin 6-OHDA into the MPOA did not impair copulation in tests 3 days after injection. In the present study, three weekly (serial) injections produced no copulatory deficits compared to animals that received a single injection or to preinjection copulatory behavior scores. However, blocking CA synthesis, which did not impair control rats, produced deficits in both single and serial lesion animals, with significantly fewer serial than single lesion animals initiating copulation. Biochemical analysis of tissue punches showed no difference in MPOA concentrations of dopamine, norepinephrine, epinephrine, or the dopamine metabolite DOPAC between the two groups. Additional animals were tested at earlier intervals after 6-OHDA injections into the MPOA. Tests conducted 30 min after an MPOA injection 6-OHDA revealed that all measures of copulation were impaired, relative to scores 24 h later. However, these scores were not significantly different from animals tested 30 min after a vehicle injection. A final group, tested 4 h after injection, showed impairment of all measures of copulation compared to vehicle injections and to tests 24 h later. Furthermore, in the tests 24 h later, 6-OHDA animals were not different from vehicle animals. Results from all experiments show that 6-OHDA injections into the MPOA impair copulation for at least 4 h, but that behavioral recovery is complete 24 h later. However, deficits can be reinstated by inhibiting DA synthesis, suggesting that increased synthesis in undamages terminals contributed to behavioral recovery.     

Repeated D1 dopamine receptor agonist administration prevents the development of both D1 and D2 striatal receptor supersensitivity following denervation.

Following 6-hydroxydopamine (6-OHDA) lesions of the nigrostriatal dopamine (DA) pathway, rat caudate-putamen (CPu) neurons are supersensitive to the inhibitory effects of both D1 and D2 dopamine (DA) receptor selective agonists. In addition, both the necessity of D1 receptor stimulation for D2 agonist-induced inhibition and the synergistic inhibitory effects of D1 and D2 agonists are abolished by denervation. The present study attempted to determine the relative roles of D1 and D2 DA receptors in the development of denervation supersensitivity to DA agonists and the "uncoupling" of functional interactions between the receptors following 6-OHDA lesions of the nigrostriatal DA pathway. Beginning on the day after an intraventricular 6-OHDA (or vehicle) injection, groups of rats received daily injections of either the selective D1 receptor agonist SKF 38393 (8.0 mg/kg, s.c.), the D2 agonist quinpirole (0.5 mg/kg, s.c.), or saline for 7 days. On the day following the last agonist injection, rats were anesthetized and prepared for extracellular single cell recording with iontophoretic drug administration. Daily administration of quinpirole selectively prevented the development of D2 receptor supersensitivity, whereas daily administration of SKF 38393 prevented the development of both D1 and D2 receptor supersensitivity. In addition, D1, but not D2, agonist treatment prevented the loss of synergistic inhibitory responses typically produced by 6-OHDA lesions. Behavioral observations revealed similar effects; daily injections of SKF 38393, but not quinpirole, prevented contralateral rotational responses to the mixed D1/D2 agonist apomorphine (1.0 mg/kg, s.c.) in rats with unilateral 6-OHDA lesions of the nigrostriatal pathway. After a 4-week withdrawal from repeated D1 agonist treatment, both supersensitive inhibitory responses of CPu neurons and contralateral rotations to apomorphine were evident, indicating that the preventative effects on DA receptor supersensitivity were not permanent. These findings indicate that continued agonist occupation of striatal D1 DA receptors following DA denervation not only prevents the development of D1 DA receptor supersensitivity but also exerts a similar regulation of D2 receptor sensitivity.     

The effects of intra-accumbens neurotensin on sensorimotor gating

Neurotensin is a neuropeptide which coexists with mesolimbic dopamine. Previous studies have shown that centrally administered neurotensin can modulate the activity of mesolimbic dopamine with a profile similar to neuroleptics. For example, infusions of neurotensin into the nucleus accumbens inhibit amphetamine-induced hyperlocomotion. Prepulse inhibition (PPI) occurs when a weak prestimulus (‘prepulse') inhibits the amplitude of the startle response to an intense stimulus (‘pulse'). PPI is an operational measure of sensorimotor gating which is strongly regulated by mesolimbic dopamine. This study examined the effects of various doses of neurotensin infused into the nucleus accumbens of rats on the prepulse inhibition (PPI) of their acoustic startle reflex. Neurotensin (0.25–5.0 μg) was infused into the nucleus accumbens of rats. Animals then received subcutaneous injections of amphetamine (2 mg/kg) or saline and were placed in startle chambers where measures of startle amplitude and PPI were obtained. Neurotensin increased baseline PPI and blocked amphetamine-induced disruption of PPI in a dose-dependent fashion. The lowest dose of neurotensin tested (0.25 μg) significantly increased baseline PPI and both 0.25 and 1.0 μg neurotensin blocked amphetamine-induced decreases in PPI. The 5.0 μg dose of neurotensin had no significant effect on prepulse inhibition. Neurotensin had no effect on the amplitude of the acoustic startle reflex in amphetamine- or saline-treated rats. The results suggest that intra-accumbens neurotensin has a significant, dose-dependent effect on sensorimotor gating in which lower doses (0.25–1.0 μg) exhibit a neuroleptic-like action.     

Hyperactivity and hypoactivity produced by lesions to the mesolimbic dopamine system

Spontaneous locomotor activity and the locomotor response to amphetamine and apomorphine were studied in rats subjected to either radiofrequency (RF), 6-hydroxydopamine (6-OHDA) or both RF and 6-OHDA lesions of the mesolimbic dopamine (DA) system. Large 6-OHDA lesions of the ventral tegmental area (VTA) or of the nucleus accumbens (N.Acc.) produced hypo-activity in the open field, a complete blockade of the locomotor stimulating effects of D-amphetamine and a profound supersensitive response to apomorphine as measured by a significant increase in locomotor activity as compared to sham-operated animals. In contrast, smaller 6-OHDA lesions of the VTA produced significant increases in spontaneous daytime and nocturnal activity with the biggest effect occurring at the lowest dose. RF lesions to the VTA produced even greater hyperactivity which was blocked by the addition of a 6-OHDA lesion to the N.Acc. The rats with RF lesions to VTA alone that were spontaneously hyperactive remained hyperactive after injection of amphetamine, whereas apomorphine produced a significant decrease in this hyperactivity. In contrast, the rats with the combined RF lesion and N.Acc. 6-OHDA lesion showed a blockade of the locomotor stimulating effects of D-amphetamine and a potentiated response to apomorphine identical to that observed with a N.Acc. lesion alone. All lesion groups revealed massive depletion of DA in the N.Acc. and anterior striatum with significantly greater depletions in those groups showing hypoactivity and hypo-responsiveness to amphetamine. All groups except the N.Acc. 6-OHDA alone group showed significant depletions of DA in the posterior striatum. Thus, limited destruction of the mesolimbic DA system can produce hyperactivity, but more extensive destruction of this system in the region of the N.Acc. and anterior striatum can reverse this hyperactivity and produce a hypo-responsiveness to the locomotor stimulating effects of amphetamine. These results suggest an essential role for dopamine in the expression of spontaneous and stimulant-induced activity. Furthermore, the much larger increase in spontaneous activity in the RF-VTA lesion group as compared to the VTA-6-OHDA groups suggests the presence of an, as yet unidentified, powerful inhibitory influence to the mesolimbic DA system within the midbrain tegmentum.     

Influence of nigrostriatal dopaminergic tone on the biosynthesis of dynorphin and enkephalin in rat striatum

The purpose of this study was to obtain direct evidence that the nigrostriatal dopamine (DA) pathway modulates the metabolism of striatal dynorphin and [Met5]-enkephalin. This was achieved by repeated injections of apomorphine (APO) or D-amphetamine (AMP) in unilateral nigral 6-hydroxydopamine (6-OHDA)-lesioned rats. Three weeks after a 6-OHDA lesion, dynorphin A(1-8)-like immunoreactivity (DN-LI) and the level of mRNA encoding prodynorphin in the striatum on the lesioned side were decreased compared with the contralateral control side. Activation of DA receptors by 7 daily injections of APO (5 mg/kg, Bid, s.c.), however, caused a large increase (3- to 4-fold of saline control) in striatal levels of DN-LI and prodynorphin mRNA on the 6-OHDA lesioned side, which is far greater than the increase on the contralateral side (2-fold of saline control). Presumably, the potentiated effect of APO in 6-OHDA lesioned rats is due to hypersensitivity of DA receptors resulting from DA denervation. Seven daily injections of AMP (5 mg/kg, Bid, s.c.), a DA-releasing agent, increased striatal DN-LI (187% of saline control) on the non-lesioned side, but not on the 6-OHDA-lesioned side. Taken together, the data indicate that the nigrostriatal pathway exerts a tonic excitatory influence over the biosynthesis of dynorphin and that this influence is not maximal since an additional increase in dopaminergic tone further increases the expression of dynorphin. In contrast, [Met5]-enkephalin-like immunoreactivity (ME-LI) in the striatum was increased by a 6-OHDA-lesion (145% of contralateral control), which was blocked by repeated administration of APO but not AMP.(ABSTRACT TRUNCATED AT 250 WORDS)     

Regional depletion of central nervous system catecholamines: Effects on blood pressure and drinking behavior

The purpose of the present study was to identify which catecholamine-containing neurons (norepinephrine (NE) or dopamine (DA)) and which central nervous system (CNS) region(s) innervated by them might participate in the pressor and drinking responses produced by central drug stimulation. Forebrain NE was reduced in rats by injecting 4 micrograms of 6-hydroxydopamine (6-OHDA) into the ascending noradrenergic bundles. Spinal cord NE was depleted by intracisternal injection of 50 micrograms 6-OHDA. Depletion of forebrain DA was produced by bilateral injection of 4 micrograms 6-OHDA into the substantia nigra of desipramine-pretreated rats. Pressor responses to various doses of angiotensin II (AII), carbachol or hyperosmolar NaCl injected into the lateral ventricles (LVT); and drinking responses to LVT AII and carbachol were examined. Injection of 6-OHDA into the noradrenergic bundles reduced telencephalic and hypothalamic NE by more than 80% without significantly affecting brain DA or spinal cord NE. Intracisternal 6-OHDA depleted spinal cord NE by 80% and forebrain NE by 20-25% without reducing brain DA. Injection of 6-OHDA into the substantia nigra reduced telencephalic DA by 86% and NE by 29% without significantly affecting NE in other CNS regions. Substantia nigra 6-OHDA injected animals evidenced attenuated drinking to both LVT AII and carbachol. Pressor responses to LVT AII, carbachol and hypertonic saline were largely unaffected. Almost complete depletion of brain and/or spinal cord NE failed to alter centrally mediated drinking or pressor responses. These data indicate that the integrity of brain DA neurons is required for the behavioral but not hypertensive responses produced by central drug stimulation.     

Intrastriatal quinolinic acid injections protect against 6-hydroxydopamine-induced lesions of the dopaminergic nigrostriatal system

We tested the effect of intrastriatal quinolinic acid (QA) injections 2 weeks before subsequent intrastriatal injections of 6-hydroxydopamine (6-OHDA). Levels of DA and its metabolites were measured 2 days and 21 days after lesioning the dopaminergic nigrostriatal system with 6-OHDA. Intrastriatal 6-OHDA injections in the absence of prior treatment of QA significantly decreased dopamine (DA) and its metabolite levels in striatum but not in substantia nigra at day 2, and in striatum and substantia nigra at day 21, a clear indication of a time-dependent retrograde axonal degeneration of substantia nigra cell bodies. Intrastriatal QA injections 2 weeks before subsequent intrastriatal injection of 6-OHDA partially prevented the 6-OHDA-depleting effect on DA and its metabolite levels in both striatum and substantia nigra 21 days after 6-OHDA injection. However, no statistically significant differences were found between QA + 6-OHDA- and 6-OHDA-treated animals at day 2. Our results suggest that intrastriatal QA injections partially prevent the naturally-occurring retrograde axonal degeneration of substantia nigra cell bodies caused by 6-OHDA, and illustrate a target-derived interaction between dopaminergic nerve endings and cell bodies. We suggest that the protective effect found in the QA-injected animals against the neurotoxic action of 6-OHDA is mediated by neurotrophic agents released by activated astroglia.     

Regulation of male rat copulatory behavior by preoptic incertohypothalamic dopamine neurons

The role of dopaminergic terminals in the medial preoptic area (MPO) in the regulation of male rat copulatory behavior was investigated. A 6-hydroxydopamine (6-OHDA) injection into the MPO of animals pretreated with desipramine resulted in a small (23%) depletion of DA, and no impairment of copulatory activity. Further depletion of catecholamines with alpha-methyl p-tyrosine (AMPT) produced several deficits in the copulatory behavior of 6-OHDA-treated males, at a dose of AMPT that did not adversely affect copulation prior to 6-OHDA administration. The dose-related effects of intracranial apomorphine (APO) injections were also altered by 6-OHDA injections into the MPO. The inhibition previously found with 0.2 microgram of APO into the lateral ventricle of normal males was abolished by 6-OHDA treatment. A facilitation of copulatory behavior was observed following the injection of 0.2 microgram of APO into the MPO of 6-OHDA-treated animals, whereas this treatment did not affect the copulatory behavior of intact animals. Finally, inhibitory effects observed following an injection of 0.1 microgram of APO into the MPO of normal males were blocked by 6-OHDA administration. The relative roles of presynaptic autoreceptors and postsynaptic DA receptors in the MPO in mediating the dose-related effects of APO on copulatory behavior are discussed.     

Central noradrenergic involvement in yohimbine excitation of acoustic startle: Effects of DSP4 and 6-OHDA

It was previously shown that i.p. administration of the alpha 2-adrenergic antagonist yohimbine increased the magnitude of the acoustic startle response in rats. The purpose of the present study was to determine possible central noradrenergic involvement in yohimbine's effect on startle. Pretreatment with N-(2-chloroethyl)-N-ethyl-2-bromo-benzylamine (DSP4; 50 mg/kg, i.p.; 1-2 days before testing) completely blocked the excitatory effect of yohimbine on startle. DSP4 reduced forebrain and spinal cord NE levels by 47% and 56%, respectively, without affecting forebrain or spinal serotonin (5-HT), or forebrain dopamine (DA). Pretreatment with the NE reuptake blocker desmethylimipramine (DMI; 20 mg/kg, i.p.; 30 min before DSP4) prevented the ability of DSP4 to block the yohimbine effect. DMI partially reversed the NE-depleting effects of DSP4. Neither bilateral adrenalectomy nor intravenously administered 6-hydroxydopamine (6-OHDA; 20 mg/kg; 1-2 days before testing) altered the excitatory effect of yohimbine, indicating that peripheral NE is not involved. 6-OHDA (2 X 200 micrograms) injected into the lateral ventricles blocked yohimbine's effect, and depleted NE by 95% (spinal cord) and 86% (forebrain), without affecting 5-HT in either region. 6-OHDA also depleted forebrain DA levels by 49%. Finally, intrathecal administration of 6-OHDA (20 micrograms; 14 days before testing) into the subarachnoid space of the lumbar spinal cord blocked the excitatory effect of yohimbine, and produced an extensive (94%) depletion of spinal cord NE. Intrathecal 6-OHDA did not alter spinal levels of 5-HT or forebrain levels of NE, 5-HT or DA. In summary, these data indicate that central descending NE neurons are necessary for yohimbine's excitatory effect on startle.     

Differential response of striatal dopamine and muscarinic cholinergic receptor subtypes to the loss of dopamine. II. Effects of 6-hydroxydopamine or colchicine microinjections into the VTA or reserpine treatment.

In the previous paper it was demonstrated that striatal dopamine (DA) D1 and D2 receptor subtypes and muscarinic M1 and M2 receptor subtypes show differing responses to lesions of the mesostriatal DA system. To examine this differential regulation further rats were given unilateral injections of 6-hydroxydopamine (6-OHDA) or colchicine into the ventral tegmental area (VTA), or treated chronically with reserpine or saline. Two weeks later the animals were tested for their behavioral response to a subthreshold dose of apomorphine and 24 h later their brains were removed and processed for quantitative autoradiography or for analysis of DA levels by high-performance liquid chromatography. The 6-OHDA-lesioned animals showed a supersensitive rotational response to apomorphine. The loss of DA, loss of DA uptake sites, regulation of DA D1 and D2 receptors and regulation of the muscarinic cholinergic system was similar to the previous paper. Injection of colchicine in the VTA resulted in incomplete loss of striatal DA (50%), [3H]mazindol binding (50%), and no behavioral supersensitivity to apomorphine. There was a small loss of presynaptically located D2 receptors (13%). Similar to the 6-OHDA lesions there was a loss of D1 (12%) and M1 receptors. Reserpine treatment produced an 86% decrease in DA levels, an enhanced stereotyped responsiveness to apomorphine, and an increase of both D2 (28%) and D1 receptors (26%). There was a loss of muscarinic M1 but not M2 receptors. Thus removal of DA terminals or blockade of transport of proteins in the mesostriatal axons can lead to a reduction in D1 receptor density in the striatum. In contrast, loss of DA without removal of DA terminals leads to a significant up-regulation of the D1 receptor. D2 receptors show increases following removal of DA or of DA terminals. Alteration in the muscarinic cholinergic system following damage to the mesostriatal DA system is a complex response not mimicked by either reserpine or colchicine treatment.