Intranigral injections of SCH 23390 inhibit SKF 82958-induced rotational behavior

Ultra-high-field pharmacological functional MRI of dopamine D1 receptor-related interventions in anesthetized rats

The dopamine D1 receptor (D1R) is associated with schizophrenia, Parkinson's disease, and attention deficit hyperactivity disorder. Although the receptor is considered a therapeutic target for these diseases, its neurophysiological function has not been fully elucidated. Pharmacological functional MRI (phfMRI) has been used to evaluate regional brain hemodynamic changes induced by neurovascular coupling resulting from pharmacological interventions, thus phfMRI studies can be used to help understand the neurophysiological function of specific receptors. Herein, the blood oxygenation level-dependent (BOLD) signal changes associated with D1R action in anesthetized rats was investigated by using a preclinical ultra-high-field 11.7-T MRI scanner. PhfMRI was performed before and after administration of the D1-like receptor agonist (SKF82958), antagonist (SCH39166), or physiological saline subcutaneously. Compared to saline, the D1-agonist induced a BOLD signal increase in the striatum, thalamus, prefrontal cortex, and cerebellum. At the same time, the D1-antagonist reduced the BOLD signal in the striatum, thalamus, and cerebellum by evaluating temporal profiles. PhfMRI detected D1R-related BOLD signal changes in the brain regions associated with high expression of D1R. We also measured the early expression of c-fos at the mRNA level to evaluate the effects of SKF82958 and isoflurane anesthesia on neuronal activity. Regardless of the presence of isoflurane anesthesia, c-fos expression level was increased in the region where positive BOLD responses were observed with administration of SKF82958. These findings demonstrated that phfMRI could be used to identify the effects of direct D1 blockade on physiological brain functions and also for neurophysiological assessment of dopamine receptor functions in living animals.

Locomotor response to L-DOPA in reserpine-treated rats following central inhibition of aromatic L-amino acid decarboxylase: further evidence for non-dopaminergic actions of L-DOPA and its metabolites

L-DOPA is the most widely used treatment for Parkinson's disease. The anti-parkinsonian and pro-dyskinetic actions of L-DOPA are widely attributed to its conversion, by the enzyme aromatic L-amino acid decarboxylase (AADC), to dopamine. We investigated the hypothesis that exogenous L-DOPA can induce behavioural effects without being converted to dopamine in the reserpine-treated rat-model of Parkinson's disease. A parkinsonian state was induced with reserpine (3 mg/kg s.c.). Eighteen hours later, the rats were administered L-DOPA plus the peripherally acting AADC inhibitor benserazide (25 mg/kg), with or without the centrally acting AADC inhibitor NSD1015 (100 mg/kg). L-DOPA/benserazide alone reversed reserpine-induced akinesia (4158+/-1125 activity counts/6 h, cf vehicle 1327+/-227). Addition of NSD1015 elicited hyperactive behaviour that was approximately 7-fold higher than L-DOPA/benserazide (35755+/-5226, P<0.001). The hyperactivity induced by L-DOPA and NSD1015 was reduced by the alpha(2C) antagonist rauwolscine (1 mg/kg) and the 5-HT(2C) agonist MK212 (5 mg/kg), but not by the D2 dopamine receptor antagonist remoxipride (3 mg/kg) or the D1 dopamine receptor antagonist SCH23390 (1 mg/kg). These data suggest that L-DOPA, or metabolites produced via routes not involving AADC, might be responsible for the generation of at least some L-DOPA actions in reserpine-treated rats.

Somatodendritic dopamine release in rat substantia nigra influences motor performance on the accelerating rod

The physiological role of somatodendritic dopamine release in the rat substantia nigra was evaluated with a combination of dual probe microdialysis and simultaneous motor performance tests on an accelerating rod. Three main findings support a modulating influence of somatodendritic dopamine release on motor coordination. (1) The rod performance tests were associated with an increase in extracellular dopamine but not 5-hydroxytryptamine concentrations in substantia nigra and with increases in both dopamine and 5-hydroxytryptamine concentrations in the striatum. (2) Nigral application of dopamine antagonists without intrinsic activity resulted in changed performances on the accelerating rod. The response to nigral perfusion with low concentrations (0.1, 1.0 microM) of the D(2)/D(3)-antagonist raclopride consisted of an impairment in rod performance to 63% of the pre-perfusion performance. Higher concentrations (10, 100 microM), however, were not associated with impaired rod performance, but with increased striatal dopamine concentrations. Perfusion of the substantia nigra with 1, 10 and 100 microM of the D(1)/D(5)-antagonist SCH 23390 dose-dependently impaired rod performance. SCH 23390 consistently increased dopamine and 5-hydroxytryptamine concentrations in substantia nigra but did not change the dialysate in the striatum. (3) In unilaterally 6-hydroxydopamine-lesioned rats, a dose-dependent improvement in rod performance was observed during perfusion of the substantia nigra with the non-selective dopamine agonist apomorphine.

Effects of acute administration of DA agonists on locomotor activity: MPTP versus neonatal intracerebroventricular 6-OHDA treatment

The effects of several dopamine (DA) receptor agonists upon locomotor activity on adult MPTP-treated mice and postnatal 6-hydroxydopamine- (6-OHDA-) treated rats were assessed in ten experiments. C57 BL/6 mice were administered 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP, 2 x 40 mg/kg, s.c., 24-hr interval between injections) at 5-months-age, while 1-day-old male Wistar rat pups were given intracisternal 6-OHDA (50 mg, once following desipramine, 25 mg/kg). MPTP-treated mice were tested 4-5 weeks following MPTP injections whereas neonatal 6-OHDA rats were tested at 3-months-age. Locomotor activity was measured in respective activity test chambers following acute administration of DA receptor agonists. In MPTP-treated mice, apomorphine failed to elevate locomotor activity but instead further exacerbated (1.0 and 3.0 mg/kg, s.c.) the hypokinesia of these animals while inducing marked increases in control mice. Cabergoline (0.3 mg/kg, s.c.) and bromocriptine (3.0 mg/kg, s.c.) caused dose-specific elevations of locomotion in MPTP and control mice but suppressed activity at the highest doses. Quinpirole (0.2 mg/kg) and 7-hydroxydipropylaminotetralin (7-OH-DPAT; 300 nmole/kg) increased locomotion in hypokinesic MPTP-treated mice; in control mice, activity was elevated by quinpirole (0.2 and 0.7 mg/kg) and 7-OH-DPAT (100 and 300 nmole/kg), while higher doses suppressed activity. Neither SKF 38393 (1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine-7,8-diol) nor FCE 23884 [4-(9,10-didehydro-6-methylergolin-8 beta-yl) methyl-piperazine-2,6-dione] affected locomotor activity. Apomorphine (0.3, 1.0 and 2.0 mg/kg), bromocriptine (3.0 mg/kg) and cabergoline (1.0 mg/kg) stimulated locomotion in sham-operated rats, and to a greater extent in the 6-OHDA-treated rats. Higher dose cabergoline (3.0 mg/kg) induced increased activity of similar extent in sham controls and 6-OHDA treated rats. Activity-enhancing effects of quinpirole (0.2, 0.7 and 2.1 mg/kg) in sham rats were attenuated in 6-OHDA treated rats. Both SKF 38393 (10 mg/kg) and FCE 23884 (0.3 and 1.0 mg/kg) induced locomotor activity increases in 6-OHDA, but not sham, rats. Finally, 7-OH-DPAT (1200 mg/kg) enhanced activity in 6-OHDA rats vs. shams. The effects of the DA agonists are discussed with regard to the putative antihypokinesic effects in MPTP mice and DA-receptor supersensitivity effects in neonatal 6-OHDA rats, pertaining to their more-or-less selective subreceptor profiles.

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 agonist-mediated rotation in rats with unilateral nigrostriatal lesions is not dependent on net inhibitions of rate in basal ganglia output nuclei

Current models of basal ganglia function predict that dopamine agonist-induced motor activation is mediated by decreases in basal ganglia output. This study examines the relationship between dopamine agonist effects on firing rate in basal ganglia output nuclei and rotational behavior in rats with nigrostriatal lesions. Extracellular single-unit activity ipsilateral to the lesion was recorded in awake, locally-anesthetized rats. Separate rats were used for behavioral experiments. Low i.v. doses of D1 agonists (SKF 38393, SKF 81297, SKF 82958) were effective in producing rotation, yet did not change average firing rate in the substantia nigra pars reticulata or entopeduncular nucleus. At these doses, firing rate effects differed from neuron to neuron, and included increases, decreases, and no change. Higher i.v. doses of D1 agonists were effective in causing both rotation and a net decrease in rate of substantia nigra pars reticulata neurons. A low s.c. dose of the D1/D2 agonist apomorphine (0.05 mg/kg) produced both rotation and a robust average decrease in firing rate in the substantia nigra pars reticulata, yet the onset of the net firing rate decrease (at 13-16 min) was greatly delayed compared to the onset of rotation (at 3 min). Immunostaining for the immediate-early gene Fos indicated that a low i.v. dose of SKF 38393 (that produced rotation but not a net decrease in firing rate in basal ganglia output nuclei) induced Fos-like immunoreactivity in the striatum and subthalamic nucleus, suggesting an activation of both inhibitory and excitatory afferents to the substantia nigra and entopeduncular nucleus. In addition, D1 agonist-induced Fos expression in the striatum and subthalamic nucleus was equivalent in freely-moving and awake, locally-anesthetized rats. The results show that decreases in firing rate in basal ganglia output nuclei are not necessary for dopamine agonist-induced motor activation. Motor-activating actions of dopamine agonists may be mediated by firing rate decreases in a small subpopulation of output nucleus neurons, or may be mediated by other features of firing activity besides rate in these nuclei such as oscillatory firing pattern or interneuronal firing synchrony. Also, the results suggest that dopamine receptors in both the striatum and at extrastriatal sites (especially the subthalamic nucleus) are likely to be involved in dopamine agonist influences on firing rates in the substantia nigra pars reticulata and entopeduncular nucleus.

Reevaluation of the striatal role in the expression of turning behavior in the rat model of Parkinson's disease

A traditional view holds the belief that the behavioral effects of l-dihydroxyphenilalanine (l-DOPA) in Parkinsonian patients are achieved through the action of the newly produced dopamine (DA) on striatal DA receptors. In contrast to this view, recent studies in the rat model of Parkinson's disease point to the substantia nigra pars reticulata as an important target for the behavioral effects of l-DOPA. In the present study, we tested the contribution of the substantia nigra vs. that of the striatum, in the expression of contralateral turning induced by l-DOPA in rats with unilateral dopaminergic depletion. Rats turned contralaterally to the lesion in response to either intrastriatal or systemic l-DOPA administration. Injections of lidocaine into the denervated striatum substantially decreased, and occasionally completely abolished, the contralateral turning after systemic l-DOPA. These findings indicate that activation of the DA depleted striatum is both sufficient and essential for the expression of behavioral response after systemic administration of l-DOPA. The contribution of the substantia nigra to this behavioral response seems to depend to a great extent on an active striatal outflow.

Effects of SCH-23390 on dopamine D1 receptor occupancy and locomotion produced by intraaccumbens cocaine infusion

This study examined the effects of both systemic and intraaccumbens administration of SCH-23390 in rats on dopamine D1 receptor occupancy and on locomotor activity produced by intraaccumbens infusion of cocaine. In experiment 1, rats received SCH-23390 (0-1 mg/kg, i.p.) 15 minutes prior to intraaccumbens infusion of cocaine (0 or 100 microg/side). In experiment 2, rats received coinfusion of SCH-23390 (0-1 microg/side) and cocaine (0 or 100 microg/side) into the nucleus accumbens (NAc). After behavioral testing, receptors occupied by SCH-23390 were quantified by injecting animals with their respective dose of SCH-23390, followed by a systemic injection of the irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Receptors occupied by SCH-23390, and therefore protected from EEDQ-induced inactivation, were quantified from autoradiograms of sections labeled with 3H-SCH-23390. Systemic administration of SCH-23390 dose-dependently (0.1-1.0 mg/kg) reversed cocaine-induced locomotion and occupied 72-100% of D1-like receptors in the anterior NAc. D1 receptor occupancy following systemic administration of SCH-23390 was evident as an inverted U-shaped, dose-dependent change, with the greatest occupancy observed at the intermediate dose of 0.3 mg/kg. Intraaccumbens infusion of SCH-23390 did not alter cocaine-induced locomotor activity despite occupying 40-60% of D1-like receptors in the anterior NAc core and shell. The findings that systemic, but not intraaccumbens, administration of SCH-23390 potently reversed locomotion produced by intraaccumbens cocaine infusion suggest that stimulation of D1 receptors in regions other than the NAc is involved in locomotion produced by intraaccumbens infusion of cocaine, and that stimulation of D1 receptors in the NAc is not necessary for this behavior.

The regulation of motor control: an evaluation of the role of dopamine receptors in the substantia nigra

The importance of the nigrostriatal dopaminergic pathway in motor control is widely accepted and it is generally believed that the motor symptoms of Parkinson's disease result solely from reduced release of dopamine from terminals in the striatum. Over recent years there has been a growing body of evidence which suggests that dendritic dopamine release in the substantia nigra is of importance in the regulation of neuronal activity and behaviour. This evidence is reviewed together with a description of our recent findings that show nigral dopamine receptors are essential for the maintenance of normal muscle tone. It is concluded that current views of the basal ganglia circuitry involved in motor control need to be re-evaluated to take into account these recent reports. A scheme is suggested to explain how dopamine mechanisms in the substantia nigra regulate motor activity.

(-)-Stepholidine acts as a D1 partial agonist on firing activity of substantia nigra pars reticulata neurons in 6-hydroxydopamine-lesioned rats

(-)-Stepholidine (SPD), a novel dopamine (DA) D1 and/or D2 receptor antagonist in normosensitive animals, shows agonistic effects on D1 receptors in rotational behavior of 6-hydroxydopamine (6-OHDA)-lesioned rats. To further characterize the pharmacological properties of SPD, we investigated the effects of SPD on firing activity of substantia nigra pars reticulata (SNR) neurons in different sensitive models. In control rats, the selective D1 agonist SKF38393 (4 mg/kg, i.v.) induced inconsistent changes (i.e. increase, decrease or no change) in firing of SNR neurons. These effects were completely antagonized by SPD (i.v.), regardless of the changes induced by SKF38393. SPD (4 mg/kg), per se, increased firing by 30.9 +/- 14.4%. In reserpinized rats, SKF38393 also induced SPD-reversible inconsistent changes as in control rats. Nevertheless, SPD per se produced no alteration in firing of SNR neurons. In 6-OHDA-lesioned rats, 5/6 SNR neurons were inhibited by SKF38393. The inhibition was completely abolished by Sch23390, a selective D1 antagonist (0.5-2 mg/kg), but partially reversed by SPD (1-16 mg/kg). Moreover, SPD (4 mg/kg) itself caused SNR increased or decreased neuron firing, and these effects were completely reversed by Sch23390 (0.5-2 mg/kg) in 8/12 neurons recorded. These results suggest that SPD acts as a partial agonist to D1 receptors in 6-OHDA-lesioned rats, but as an antagonist to D1 receptors in normal and reserpinized rats.

Independent mediation of unconditioned motor behavior by striatal D1 and D2 receptors in rats depleted of dopamine as neonates

The effects of systemic administration of DA receptor antagonists suggest that unconditioned motor behavior in rats depleted of DA as neonates continues to be dependent upon dopaminergic transmission, yet the specific contribution of D1 and D2 receptors to these behaviors has been altered. The purpose of the present study was to determine whether these depletion-induced receptor changes are occurring at the level of striatal DA terminals and their targets. The ability of bilateral intrastriatal injections (0.5 microliter) of DA receptor antagonists to induce motoric deficits was determined in adult rats treated with vehicle or 6-OHDA (100 micrograms, intraventricular) on postnatal day 3. Administration of the D1-like antagonist SCH 23390 (0.5-2.0 micrograms) or the D2-like antagonist clebopride (1.0-4.0 micrograms) induced dose-dependent akinesia, catalepsy, and somatosensory neglect in vehicle-treated controls. In contrast, neither antagonist produced deficits in rats depleted of forebrain DA as neonates. However, combined administration of SCH 23390 + clebopride induced similar akinesia, catalepsy, and somatosensory neglect in both controls and DA depleted animals. Animals depleted of DA were more sensitive than controls to the low doses of this combined D1 + D2 antagonism. These results demonstrate that activation of striatal DA receptors remains necessary for unconditioned motor behavior in rats depleted of DA as neonates. However, the specific contributions of D1- and D2-like receptors to these behaviors differ between intact animals and those depleted of DA as neonates. The ability of endogenous DA acting at either D1 or D2 receptors to support spontaneous motor behavior in rats depleted of DA as neonates may contribute to their relative sparing from parkinsonian deficits.

Expression of stereotyped behaviour requires stimulation of nigral D1 dopamine receptors

Dopamine receptors in the pars reticulata of the substantia nigra were inactivated following bilateral injections of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). Loss of D1, but not D2, receptors was associated with a significant decrease in the incidence of stereotyped head-down sniffing elicited by the mixed D1/D2 receptor agonist, apomorphine. The results support the hypothesis that D1 receptors in the substantia nigra are necessary for the expression of dopamine mediated stereotyped behaviour.

Localization of dopamine receptor subtypes occupied by intra-accumbens antagonists that reverse cocaine-induced locomotion

The purpose of this study was to examine whether blockade of either dopamine D1-like or D2-like receptors by selective antagonist administration into the nucleus accumbens (NAc) is sufficient to reverse cocaine-induced locomotion, and to develop a new technique that enables the population of receptors occupied by the antagonists to be quantified. Locomotor activity was assessed in rats that had received bilateral intra-accumbens injections of the D1-selective antagonist SCH-23390 (0-3.0 micrograms/0.5 microliters/side) or the D2/D3-selective antagonist sulpiride (0-1.0 micrograms/0.5 microliters/side), followed 15 min later by injections of saline or cocaine (15 mg/kg, i.p.). To assess receptor occupancy by the antagonists, 105 min prior to sacrifice the rats received intra-accumbens injections of the antagonist, followed 15 min later by an injection of the non-selective irreversible antagonist, N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ; 10 mg/kg, i.p.). Receptors were labeled with [3H]SCH-23390 or [3H]sulpiride in sections containing the NAc, and the autoradiograms allowed quantitation of receptors occupied (i.e. protected from EEDQ) by the antagonist given in vivo. Only a dose of 3 micrograms/side SCH-23390 reversed cocaine-induced locomotion, whereas a dose of 0.5 microgram/side did not alter cocaine-induced locomotion despite occupying the same amount of [3H]SCH-23390 binding sites in the NAc. Intermediate doses of 0.1 and 0.3 microgram/side sulpiride reversed cocaine-induced locomotion, and also occupied the greatest number of [3H]sulpiride binding sites in the NAc. The results suggest that blockade of D2-like, but not D1-like, receptors in the NAc is sufficient to reverse cocaine-induced locomotion, and also demonstrate the importance of quantifying receptors occupied by drugs administered intracranially.