Localization of striatal dopamine receptor function by central injection of an irreversible receptor antagonist

The stereotypic head-down sniffing response to systemically administered apomorphine (0.65 mumol/kg) was assessed in rats 48 h after the bilateral injection of 0.2-0.5 microliters of the irreversible receptor antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (60 micrograms/microliters) into the caudate-putamen and nucleus accumbens. This response was significantly attenuated in animals that had received injections of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline into the anterior/ventral part of the caudate-putamen but not in those that received injections into regions more dorsal/posterior. Animals were killed after apomorphine challenge and the region of dopamine D1 or D2 receptor reduction due to N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline mapped and quantified. This analysis revealed that the dopamine receptors involved in the apomorphine-induced stereotyped head-down sniffing response were located in a discrete region of the ventrolateral caudate-putamen and the dorsolateral nucleus accumbens. Animals that were pretreated with the selective dopamine D2 receptor antagonist raclopride (0 20 mumol/kg, i.p.) 20 min prior to central injection of N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline into this area showed a dose-dependent protection of the stereotyped sniffing response to systemic apomorphine 48 h later. This combination of techniques constitutes a novel way to investigate striatal function and the results obtained support the concept of a functional subdivision of both the caudate-putamen and the nucleus accumbens.     

Effects of flunarizine on dopamine dependent behaviours in rats

1. Radio-ligand binding study has demonstrated that flunarizine has a high affinity for the rat striatal D 2 dopamine (DA) receptors. 2. In the present behavioural study conducted in rats it was observed that flunarizine, unlike the postsynaptic striatal D 2 DA receptor agonist apomorphine, did not induce stereotyped behaviour (SB) in rats. This indicates that flunarizine does not act as an agonist at the postsynaptic striatal D 2 DA receptors. 3. Flunarizine however, like the postsynaptic striatal D 2 DA receptor antagonist haloperiodal, inhibited the conditioned avoidance response, induced catalepsy and antagonized the SB induced by the DA agonists apomorphine and methamphetamine. 4. Our findings indicate that flunarizine acts as a postsynaptic striatal D 2 DA receptor antagonist.     

Chronic ascorbate potentiates the effects of chronic haloperidol on behavioral supersensitivity but not D2 dopamine receptor binding.

Ample behavioral evidence suggests that ascorbate parallels the action of haloperidol, a widely used neuroleptic. To determine the extent to which this parallel extends to chronic treatment, 21 days of exposure to ascorbate (100 or 500 mg/kg) alone or combined with haloperidol (0.5 mg/kg) were assessed on stereotyped behavior and neostriatal D2 dopamine receptor binding in rats. Our results indicate that when challenged with the dopamine agonist, apomorphine (0.5 mg/kg), animals chronically treated with haloperidol or high-dose ascorbate alone display a supersensitive sniffing response relative to controls, while animals chronically treated with the combination of haloperidol and high-dose ascorbate display a further potentiation of sniffing relative to the haloperidol groups. In addition, [3H]spiperone saturation studies showed, as expected, an up-regulation of striatal D2 dopamine receptors in rats treated with haloperidol as reflected by a change in receptor density (Bmax) but not affinity (KD). Ascorbate treatment, however, had no effect on D2 receptor density or the distribution of [3H]apomorphine in whole brain. Even though chronic treatment with the haloperidol-high-dose-ascorbate combination produced an up-regulation of striatal D2 dopamine receptors, this treatment did not cause a further up-regulation relative to haloperidol alone nor did it have any effect on [3H]apomorphine distribution. Taken together, these findings indicate that although chronic ascorbate produces behavioral supersensitivity to apomorphine through central mechanisms, they appear to differ from those induced by chronic haloperidol.     

Effects of inactivation of D1 dopamine receptors on stereotypic and thermic responses to quinpirole (LY 171555)

The purpose of the present study was to investigate reports that the expression of dopamine agonist-induced behaviours is dependent upon the ratio of D1 to D2 receptor activation. Selective inactivation of the D1 dopamine receptor was achieved using the irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) while D2 receptors were maintained at control levels by the use of a D2 antagonist, raclopride. Stereotypic and hypothermic responses to quinpirole (LY 171555) were assessed and related to striatal D1 and D2 receptor concentrations. Results showed that the incidence of stereotyped sniffing behaviour induced by LY 171555 was reduced in parallel with D1 receptor loss and sniffing behaviour was abolished at low D1 receptor (less than 40% of control) concentrations. Hypothermic responses to LY 171555 were unaffected by D1 receptor loss. These findings suggest that activation of D1 receptors is a critical component of stereotypic, but not hypothermic, responses to LY 171555, the magnitude of the sniffing response being positively associated with D1 receptor concentration.     

Prior D1 dopamine receptor stimulation is required to prime D2-mediated striatal Fos expression in 6-hydroxydopamine-lesioned rats

Repeated dopamine agonist administration to rats with unilateral 6-hydroxydopamine lesions of the nigrostriatal pathway potentiates behavioral and neuronal activation in response to subsequent dopamine agonist treatment. This response sensitization has been termed "priming" or "reverse-tolerance". Our prior work has shown that three pretreatment injections of the mixed D1/D2 agonist apomorphine (0.5 mg/kg) into 6-hydroxydopamine-lesioned rats permits a previously inactive dose of the D2 agonist quinpirole (0.25 mg/kg) to induce robust contralateral rotation and striatal Fos expression in striatoentopeduncular "direct" pathway neurons. These striatal neurons typically express D1 but not D2 receptors. Because apomorphine acts as an agonist at both D1 and D2 receptors, the present study sought to determine whether D1, D2, or concomitant D1/D2 receptor stimulation was required to prime D2-mediated contralateral rotation and striatal Fos expression. Twenty-one days following unilateral stereotaxic injection of 6-hydroxydopamine into the medial forebrain bundle, rats received three pretreatment injections, at three- to six-day intervals, with either: the mixed D1/D2 agonist apomorphine, the D1 agonist SKF38393, the D2 agonist quinpirole, or a combination of SKF38393 + quinpirole. Ten days following the third pretreatment injection, 6-hydroxydopamine-lesioned rats were challenged with the D2 agonist quinpirole (0.25 mg/kg). Pretreatment with SKF38393 (10 mg/kg), quinpirole (1 mg/kg) or SKF38393 (1 mg/kg) + quinpirole (0.25 mg/kg) permitted an otherwise inactive dose of quinpirole (0.25 mg/kg) to induce robust contralateral rotation which was similar in magnitude to that observed following apomorphine priming. However, only pretreatment with SKF38393 (10 mg/kg) or SKF38393 (1 mg/kg) + quinpirole (0.25 mg/kg) permitted the same dose of quinpirole (0.25 mg/kg) to induce striatal Fos expression. These results demonstrate that while prior stimulation of D1, D2 or D1/D2 receptors can effectively prime D2-mediated contralateral rotation, prior stimulation of D1 receptors is required to prime D2-mediated striatal Fos expression. This study demonstrates that priming of 6-hydroxydopamine-lesioned rats with a D1 agonist permits a subsequent challenge with a D2 agonist to produce robust rotational behavior that is accompanied by induction of immediate-early gene expression in neurons that comprise the "direct" striatal output pathway. These responses are equivalent to the changes observed in apomorphine-primed 6-hydroxydopamine-lesioned rats challenged with D2 agonist. In contrast, D2 agonist priming was not associated with D2-mediated induction of striatal immediate-early gene expression even though priming of D2-mediated rotational behavior was not different from that observed following priming with apomorphine or D1 agonist. Therefore, while priming-induced alterations in D2-mediated immediate early gene expression in the "direct" striatal output pathway may contribute to the enhanced motor behavior observed, such changes in striatal gene expression do not appear to be required for this potentiated motor response in dopamine-depleted rats.     

Role of the substantia nigra in the expression of dopamine D1 receptor-mediated and D2 receptor-mediated behaviours

This study examines the proposal that striatonigral pathways support circling mediated by dopamine D1 receptors, but not D2 receptors, in unilaterally 6-hydroxydopamine-treated rats. In this model the D1/D2 agonist apomorphine, the D1 agonist 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine hydrochloride and the D2 agonists N-n-propyl-N-phenylethyl-P-(3-hydroxyphenyl) ethylamine hydrochloride, trans-(-)-4aR,4,4a,5,6,7,8,8a,9-octahydro-5-propyl-1H-pyrazolo-(3, 4-g) quinolino monohydrochloride and lisuride evoked a characteristic spectrum of motor responses when administered systemically. In addition apomorphine, 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine hydrochloride and lisuride replicated their systemic effects following stereotaxic injection into the supersensitive caudate nucleus. Three months after injecting the pars reticulata of the dopamine-denervated nigra with kainic acid (1 microgram in 1 microliter), all motor responses to intracaudate dopamine agonists were reduced or abolished. Systemic responses were modified differentially, often as early as one day post-kainate. Contraversive circling and posturing were reduced, or even reversed (apomorphine only), grooming was attenuated (all drugs) and 2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine hydrochloride-induced forepaw nibbling and dyskinesia were abolished. By contrast, sniffing, movements of the head and locomotion were either unaffected, or significantly potentiated, suggesting these components of behaviour arose from dopamine receptors outside the denervated striatum. These behavioural changes showed no signs of recovery three months after kainate, and were not produced by partial lesions of the reticulata (1 microgram kainate in 0.2 microliter). Contrary to earlier opinion our results indicate that the structural integrity of the substantia nigra pars reticulata is essential for the development of all forms of dopamine behaviour mediated by striatal D1 and D2 receptors, though not necessarily by dopamine receptors present at other locations.     

Experimental hemiparkinsonism in the rat following chronic unilateral infusion of MPP+ into the nigrostriatal dopamine pathway--III. Reversal by embryonic nigral dopamine grafts

In a previous study conducted over six months, we demonstrated that 1-methyl-4-phenylpyridinium ion (MPP+) the active metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, chronically infused (10 micrograms/24 h for seven days) into one median forebrain bundle of the rat can cause long-lasting damage to the nigrostriatal dopamine system. The present study was carried out in animals 18-19 months after MPP+ infusion to determine firstly, if the lesion was indeed permanent and secondly, if embryonic nigral dopamine suspension grafts implanted into the dopamine-denervated neostriatum can reverse the neurochemical and behavioural deficits induced by MPP+. All the animals within the MPP(+)-lesioned group showed robust contralateral and ipsilateral turning in response to apomorphine (0.05 mg/kg) and methamphetamine (2.5 mg/kg), respectively, at each time point of testing. In the grafted animals there was a progressive significant reduction in the number of rotations in response to both apomorphine and methamphetamine over the three-month test period. Autoradiographic analysis of [125I]sulpiride binding to striatal sections showed a 27% increase in dopamine D2 receptor density in the ipsilateral striatum of MPP(+)-lesioned animals. This increase in D2 receptor density was completely abolished by the dopamine grafts so that the D2 receptor density in the grafted striatum was similar to the contralateral striatum of MPP(+)-lesioned animals. This increase in D2 receptor density was completely abolished by the dopamine grafts so that the D2 receptor density in the grafted striatum was similar to the contralateral striatum of the grafted animals or the ipsilateral striatum of control non-lesioned animals. In all the animals of the lesioned and grafted groups there was a complete loss of dopamine neurons in the ipsilateral substantia nigra as demonstrated by tyrosine hydroxylase-immunohistochemistry and in-situ hybridization histochemistry. In all the animals that received nigral dopamine grafts, numerous cells were localized within the grafts which contained tyrosine hydroxylase immunoreactivity and tyrosine hydroxylase mRNA. Moreover, immunohistochemical staining showed a dense network of tyrosine hydroxylase-positive fibres within the grafted striatum. The results of the present study are important in two respects. Firstly, they demonstrate that MPP+ infusions into the rat nigrostriatal dopamine pathway can produce a permanent degeneration of nigral dopamine neurons. Thus, in animals assessed 18-19 months after the initial MPP(+)-lesion there was no significant behavioural or neurochemical compensation with time. Secondly, the results clearly show that embryonic nigral dopamine grafts implanted into the dopamine-denervated striatum can reverse the behavioural and neurochemical deficits induced by MPP+.     

Age-related changes in the regulation of behavior by D-1:D-2 dopamine receptor interactions.

Functional interactions between D-1 and D-2 dopamine receptor systems appear important in the regulation of psychomotor behavior, and may alter with aging. Male Sprague-Dawley rats of 5 and 20-24 months were challenged with the selective D-2 agonist LY 163502 alone or following pretreatment with the selective D-1 antagonist SCH 23390. Typical sniffing and locomotor responses to LY 163502 alone were significantly reduced in aged animals. Pretreatment with SCH 23390 blocked these typical responses in both young and aged animals, consistent with their regulation by cooperative D-1:D-2 interactions; however, SCH 23390 released a significant excess of atypical limb/body jerking to LY 163502 in aged animals, a response which appears to have its basis in oppositional D-1:D-2 interactions. These results suggest that the net effect of aging on dopaminergic transmission is to reduce tonic activity through D-1 receptors to a greater extent than that occurring through D-2 receptors. As the present aged animals showed a selective loss of striatal D-2 but not of D-1 receptors in radioligand binding studies, such a reduction of D-1-mediated transmission with aging would seem to involve loss of presynaptic function or of postsynaptic mechanisms beyond the D-1 recognition site.     

Opposite tonic modulation of dopamine and adenosine on c-fos gene expression in striatopallidal neurons

The impulse flow-dependent dopamine release in the striatum was acutely blocked by unilateral lesion of the medial forebrain bundle with 6-hydroxydopamine. Within 45 min this disruption reduced the striatal extracellular dopamine levels by 80% as determined by in vivo voltammetry. A strong induction of c-fos messenger RNA was detected in the ipsilateral dorsolateral striatum 75 min after 6-hydroxydopamine injection by in situ hybridization. Double labelling demonstrates that this induction was confined to neurons expressing the dopamine D2 receptor messenger RNA. At this time-point, there were no changes in the striatal levels of either tyrosine hydroxylase immunoreactivity or dopamine D2 receptor messenger RNA. The c-fos messenger RNA expression induced by acute 6-hydroxydopamine injection was abolished by intraperitoneal pretreatment with the dopamine D2 receptor agonist, quinelorane (2 mg/kg) and strongly reduced by administration of the selective adenosine A2A receptor antagonist SCH-58261 (5 mg/kg). The results reported here show, by using a novel methodological approach, that an acute decrease of dopamine release causes an induction of c-fos messenger RNA in dopamine D2 receptor-containing striatopallidal neurons. This, together with previous findings, demonstrates that the c-fos gene expression is tonically inhibited by the impulse flow-dependent dopamine release via D2 receptors. In addition, this study provides evidence that endogenous adenosine, acting via adenosine A2A receptors, induces striatal c-fos messenger RNA when extracellular dopamine levels are strongly reduced. Thus endogenous dopamine and adenosine exert opposite effects on the activity of the D2-containing striatopallidal neurons.     

Stimulation of D1 dopamine receptors reveals direct effects of the preferential dopamine autoreceptor agonist B-HT 920 on postsynaptic dopamine receptors

Possible postsynaptic effects of the preferential dopamine autoreceptor agonist B-HT 920 were studied by means of the mouse motor activity. In reserpine-treated mice, B-HT 920 did not cause any motor activity by itself but it markedly potentiated the slight stimulating effect of the D1 dopamine agonist SKF 38 393. The effect was blocked by either the D2-receptor antagonist sulpiride or the D1-receptor antagonist SCH 23 390, indicating that motor activity is dependent on simultaneous activation of both dopamine receptor types. The hyperactivity produced by 0.1 mg kg-1 B-HT 920 in combination with SKF 38 393 in reserpine-treated mice was at least as great as that following a maximal dose of apomorphine, indicating that B-HT 920 is a full agonist at postsynaptic D2 receptors. The effect of 0.1 mg kg-1 B-HT 920 peaked earlier than those of 1 mg kg-1 and particularly, 10 mg kg-1 suggesting additional effects of the later two doses. B-HT 920 stimulates dopamine autoreceptors almost maximally following 0.1 or 1 mg kg-1 but only the latter dose (with or without SKF 38 393) caused hyperactivity of mice not treated with reserpine. This finding indicates that the postsynaptic D2 receptors are less sensitive to B-HT 920 than the D2 dopamine autoreceptors.     

Electrophysiological study of the effects of D1 and D2 dopamine antagonists on the interaction of converging inputs from the sensory-motor cortex and substantia nigra neurons in the rat

The interaction of stimulation of the cerebral cortex and of the substantia nigra on the activity of neostriatal neurons was investigated in urethane-anesthetized rats. Neurons of the dorsal striatum were activated by single pulse stimulation of the sensory-motor cortex. The effects of nigral conditioning stimulation on this excitatory response of striatal neurons to cortical stimulation were studied in a series of parametric experiments in which the length of the train of pulses and the intensity of the nigral stimulation were varied. One and five pulses of nigral conditioning stimulation had little or no effect. Ten pulses of nigral conditioning stimulation reduced the excitatory response, the magnitude of the reduction being greater with higher current intensities. In another series of experiments, the effects of dopaminergic receptor antagonists on the interaction of cortical and nigral inputs to striatal neurons were studied. Sulpiride, a D2 antagonist, reversed the attenuating effects of nigral conditioning stimulation on the excitatory response of striatal neurons to cortical stimulation, whereas SCH 23390 a D1 antagonist, had no effect. The present findings support the hypothesis that the nigrostriatal dopaminergic pathway modulates the excitatory response of striatal neurons to cortical stimulation by means of dopamine D2 receptors.     

Topography of dopamine behaviours mediated by D1 and D2 receptors revealed by intrastriatal injection of SKF 38393, lisuride and apomorphine in rats with a unilateral 6-hydroxydopamine-induced lesion

Stereotaxic injections of a dopamine D1 receptor agonist (SKF 38393) into different regions of the supersensitive striatum of rats with a unilateral 6-hydroxydopamine-induced lesion duplicated the systemic effects of the drug in a topographical manner. Although there was considerable overlap, it was possible to recognize discrete active zones or "hot-spots" giving rise to prominent sniffing, head movements and contralaterally directed circling, posture and grooming, both in the coronal plane and along the rostro-caudal axis. Two behaviours peculiar to D1 stimulation included contralateral forepaw myoclonus and forepaw nibbling, which paradoxically was directed mainly ipsilaterally. Each of the behavioural elements occurred independently of the others and after an inexplicably long latency. They were inhibited by the D1 antagonist SCH 23390, but not by the D2 blocking drug metoclopramide. Comparable circling responses were evoked by a D2 agonist (lisuride) injected into the neostriatum after a short delay, and instantaneously by apomorphine (D1/D2 agonist). Both drug behaviours originated diffusely from all parts of the denervated striatum with no obvious "hot-spots", except for circling which exhibited a bimodal distribution rostro-caudally. The actions of lisuride were blocked by systemic metoclopramide, but not by SCH 23390, while the actions of apomorphine were inhibited by both antagonists. Topographies of D2 receptor-mediated events were quite different from those encountered for D1 receptor stimulation by SKF 38393, though neither corresponded to the autoradiographic distribution of D1 and D2 binding sites in the intact striatum. These results reiterate the importance of D1 receptors in motor control and provide a basis for future investigations of the output pathways subserving D1-mediated behaviours.     

Methamphetamine-induced reduction in D1 and D2 dopamine receptors as evidenced by autoradiography: comparison with tyrosine hydroxylase activity

As determined by autoradiographic techniques, multiple high doses of methamphetamine elicited a reduction in dopamine receptor population (both D1 and D2) in several areas of the rat central nervous system. D1 receptors were labeled with the D1-selective antagonist, [3H]SCH 23390, and D2 receptors were labeled with the D2-selective neuroleptic, [3H]sulpiride. Scatchard analysis, obtained from saturation data in caudate-putamen, indicated that the receptor alterations were due to a decrease in the number of receptors (Bmax) without an apparent change in affinity (Kd). A time course demonstrated that five doses of methamphetamine were required to elicit significant changes in receptors in most brain areas examined. The onset of the receptor alterations in various brain regions correlated with the development of methamphetamine-induced depression of striatal tyrosine hydroxylase activity. In most brain areas, the dopamine receptors returned to normal within 7 days following methamphetamine.     

The dopamine-γ aminobutyric acid interaction in the striatum of the rat is differently regulated by dopamine D-1 and D-2 types of receptor: evidence obtained with rotational behavioural experiments

The effects of GABA antagonists on apomorphine-and pergolide-induced rotational behaviour were studied with models combining intracerebral and systemic pharmacological treatments. Whether given systemically or intrastriatally to 6-hydroxydopaminelesioned rats, the GABA antagonist picrotoxin inhibited the rotational responses produced by s. c. administration of the dopamine (DA) D-1/D-2 agonist apomorphine while it enhanced the rotational behaviour produced by the DA D-2 agonist pergolide. Following unilateral injection of picrotoxin or bicuculline into the striatum of naive rats, apomorphine produced ispsilateral rotation, while pergolide produced contralateral rotation. These contrasting effects are compared to the behavioural responses produced by intracerebral administration of GABAergic drugs alone. Intrapallidal injection of picrotoxin produced contralateral rotational behaviour which was independent of pallido-nigral pathways. Contralateral rotation was also produced by GABA agonists, but only following intranigral injections. The results are discussed in terms of differences in the localization of DA D-1 and DA D-2 receptors on striatal GABAergic neurons. The DA D-2 receptor agonist pergolide may induce inhibition of striato-pallidal GABAergic neurons, as well as of a local GABAergic circuit exerting inhibition on a striato-pallidal enkephalinergic pathway. However, the DA D-1/D-2 receptor agonist apomorphine may inhibit striatal interneurons exerting inhibition on a striato-nigral GABAergic projection. Such a neuronal arrangement may explain that striatal DA stimulation increases GABA release from the striato-nigral terminals.     

Treadmill running induces striatal Fos expression via NMDA glutamate and dopamine receptors

 Several non-physiological stimuli (i.e. pharmacological or electrical stimuli) have been shown to induce Fos expression in striatal neurons. In this work, striatal Fos (i.e. Fos-like) expression was studied after physiological stimulation, i.e. motor activity (treadmill running at 36 m/min for 20 min). In rats killed 2 h after the treadmill session, Fos expression was observed in the medial region of the rostral and central striatum, and in the dorsal region of the caudal striatum. Fos expression was prevented by pretreatment with the non-competitive N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK-801 (0.1 mg/kg) or the D₁ dopamine receptor antagonist SCH-23390 (0.1 mg/kg), but not by pretreatment with the D₂ receptor antagonist eticlopride (0.5 mg/kg). Thirty-six hours after 6-hydroxydopamine lesion, a considerable reduction in treadmill-induced Fos expression was observed in both sides; however, Fos expression in the lesioned striatum was higher than in the contralateral intact striatum. Several weeks after unilateral 6-hydroxydopamine lesion of the nigrostriatal system, treadmill-induced Fos expression was significantly, but not totally, reduced in the lesioned striatum. Corticostriatal deafferentation also led to considerable reduction in treadmill-induced Fos expression. The present results indicate that exercise induces striatal Fos expression and that, under physiological stimulation, concurrent activation of D₁ and NMDA receptors is necessary for such expression to occur. Reduction of Fos expression is practically absolute after acute blockage of these receptors, but not after lesions, possibly due partially to compensatory changes. Received: 5 November 1996 / Accepted: 17 January 1997     

The dopamine-gamma aminobutyric acid interaction in the striatum of the rat is differently regulated by dopamine D-1 and D-2 types of receptor: evidence obtained with rotational behavioural experiments

The effects of GABA antagonists on apomorphine- and pergolide-induced rotational behaviour were studied with models combining intracerebral and systemic pharmacological treatments. Whether given systemically or intrastriatally to 6-hydroxydopamine-lesioned rats, the GABA antagonist picrotoxin inhibited the rotational responses produced by s.c. administration of the dopamine (DA) D-1/D-2 agonist apomorphine while it enhanced the rotational behaviour produced by the DA D-2 agonist pergolide. Following unilateral injection of picrotoxin or bicuculline into the striatum of naive rats, apomorphine produced ispsilateral rotation, while pergolide produced contralateral rotation. These contrasting effects are compared to the behavioural responses produced by intracerebral administration of GABAergic drugs alone. Intrapallidal injection of picrotoxin produced contralateral rotational behaviour which was independent of pallido-nigral pathways. Contralateral rotation was also produced by GABA agonists, but only following intranigral injections. The results are discussed in terms of differences in the localization of DA D-1 and DA D-2 receptors on striatal GABAergic neurons. The DA D-2 receptor agonist pergolide may induce inhibition of striato-pallidal GABAergic neurons, as well as of a local GABAergic circuit exerting inhibition on a striato--pallidal enkephalinergic pathway. However, the DA D-1/D-2 receptor agonist apomorphine may inhibit striatal interneurons exerting inhibition on a striato-nigral GABAergic projection. Such a neuronal arrangement may explain that striatal DA stimulation increases GABA release from the striato-nigral terminals.     

Experimental hemiparkinsonism in the rat following chronic unilateral infusion of MPP+ into the nigrostriatal dopamine pathway--II. Differential localization of dopamine and cholecystokinin receptors

The autoradiographical localization of dopamine D1, D2 and cholecystokinin receptors has been investigated in rat brain 6 months following unilateral infusion of 1-methyl-4-phenyl pyridinium ion (MPP+) (10 micrograms/day for 7 days) into the nigrostriatal dopamine pathway. Treatment with 1-methyl-4-phenyl pyridinium ion produced a marked depletion of dopamine cell bodies in the substantia nigra together with greater than 95% loss of tyrosine hydroxylase immunoreactivity in the striatum. Measurement of specific [3H]spiperone binding to D2 receptors indicated a 38% increase (P less than 0.01) in the maximal binding capacity of [3H]spiperone to striatal membrane homogenates and a 13% increase (P less than 0.05) in specific [3H]spiperone binding to striatal tissue sections, verifying striatal D2 receptor denervation supersensitivity. In contrast, MPP+ lesion of the nigrostriatal tract had no effect on the autoradiographical localization of striatal D1 or cholecystokinin receptors. In addition, there was a 38% loss (P less than 0.05) of D2 receptor binding sites in the substantia nigra pars compacta, whilst D1 receptors remained unchanged. Similar changes in dopamine and cholecystokinin receptor number were found following 6-hydroxydopamine lesion of the nigrostriatal dopamine pathway. These results provide further evidence that 1-methyl-4-phenyl pyridinium ion treatment in rats produces extensive destruction of the dopaminergic nigrostriatal tract and supports the differential anatomical localization of striatal and nigral D1, D2 and cholecystokinin receptors.     

Chronic treatment with 1-dopa plus carbidopa in hemitransected rats: preferential effects at intact dopamine synapses leading to behavioural signs of dopamine receptor supersensitivity

Chronic i.p. treatment with 1-dopa-carbidopa for 3 weeks in hemitransected male rats leads on one hand to tolerance to 1-dopa induced turning behaviour and on the other hand to behavioural and biochemical signs of dopamine (DA receptor supersensitivity on the intact side. Thus, the apomorphine induced ipsilateral turning behaviour is enhanced and the KD values of the 3H-spiperone binding sites linked to DA receptors of the D2 type on the intact, but not on the denervated side are reduced by 40%. However, the number of 3H-spiperone binding sites is reduced by 20% in striatal membranes on the intact side after this type of treatment. Therefore, chronic treatment with a catecholamine (CA) precursor leads to selective adaptive changes at intact striatal DA synapses with certain signs of the expected development of DA receptor subsensitivity, but above all with signs of paradoxical development of DA receptor supersensitivity. A hypothesis is introduced to explain these results.     

Autoradiographic study of striatal dopamine re-uptake sites and dopamine D1 and D2 receptors in a 6-hydroxydopamine and quinolinic acid double-lesion rat model of striatonigral degeneration (multiple system atrophy) and effects of embryonic ventral mesencephalic, striatal or co-grafts

The influence of embryonic mesencephalic, striatal and mesencephalic/striatal co-grafts on amphetamine- and apomorphine-induced rotation behaviour was assessed in a rat model of multiple system atrophy/striatonigral degeneration type using dopamine D1 ([3H]SCH23390) and D2 ([3H]spiperone) receptor and dopamine re-uptake ([3H]mazindol) autoradiography. Male Wistar rats subjected to a sequential unilateral 6-hydroxydopamine lesion of the medial forebrain bundle followed by a quinolinic acid lesion of the ipsilateral striatum were divided into four treatment groups, receiving either mesencephalic, striatal, mesencephalic/striatal co-grafts or sham grafts. Amphetamine- and apomorphine-induced rotation behaviour was recorded prior to and up to 10 weeks following transplantation. 6-Hydroxydopamine-lesioned animals showed ipsiversive amphetamine-induced and contraversive apomorphine-induced rotation behaviour. Amphetamine-induced rotation rates persisted after the subsequent quinolinic acid lesion, whereas rotation induced by apomorphine was decreased. In 11 of 14 animals receiving mesencephalic or mesencephalic/striatal co-grafts, amphetamine-induced rotation scores were decreased by >50% at the 10-week post-grafting time-point. In contrast, only one of 12 animals receiving non-mesencephalic (striatal or sham) grafts exhibited diminished rotation rates at this time-point. Apomorphine-induced rotation rates were significantly increased following transplantation of mesencephalic, striatal or sham grafts. The largest increase of apomorphine-induced rotation rates approaching post-6-hydroxydopamine levels were observed in animals with striatal grafts. In contrast, in the co-graft group, there was no significant increase of apomorphine-induced rotation compared to the post-quinolinic acid time-point. Morphometric analysis revealed a 63-74% reduction of striatal surface areas across the treatment groups. Striatal [3H]mazindol binding on the lesioned side (excluding the demarcated graft area) revealed a marked loss of dopamine re-uptake sites across all treatment groups, indicating missing graft-induced dopaminergic re-innervation of the host. In eight (73%) of the 11 animals with mesencephalic grafts and reduced amphetamine-induced circling, discrete areas of [3H]mazindol binding ("hot spots") were observed, indicating graft survival. Dopamine D1 and D2 receptor binding was preserved in the remaining lesioned striatum irrespective of treatment assignment, except for a significant reduction of D2 receptor binding in animals receiving mesencephalic grafts. "Hot spots" of dopamine D1 and D2 receptor binding were observed in 10 (83%) and nine (75%) of 12 animals receiving striatal grafts or co-grafts, consistent with survival of embryonic primordial striatum grafted into a severely denervated and lesioned striatum. Our study confirms that functional improvement may be obtained from embryonic neuronal grafts in a double-lesion rat model of multiple system atrophy/striatonigral degeneration type. Co-grafts appear to be required for reversal of both amphetamine- and apomorphine-induced rotation behaviour in this model. We propose that the partial reversal of amphetamine-induced rotation asymmetry in double-lesioned rats receiving mesencephalic or mesencephalic/striatal co-grafts reflects non-synaptic graft-derived dopamine release. The changes of apomorphine-induced rotation following transplantation are likely to reflect a complex interaction of graft- and host-derived striatal projection pathways and basal ganglia output nuclei. Further studies in a larger number of animals are required to determine whether morphological parameters and behavioural improvement in the neurotransplantation multiple system atrophy rat model correlate.     

Neonatal dopamine depletion reveals a synergistic mechanism of mRNA regulation that is mediated by dopamine(D1) and serotonin(2) receptors and is targeted to tachykinin neurons of the dorsomedial striatum

It has been hypothesized that dopamine(D1) and serotonin(2) receptors become sensitized to agonist-mediated regulation of gene expression following loss of dopaminergic innervation to the striatum. We have previously demonstrated that the combined administration of dopamine(D1) and serotonin(2) receptor agonists to dopamine-depleted adult rats induced preprotachykinin mRNA expression within the periventricular rostral striatum to levels which were significantly different than what could be elicited by either agonist alone. In the present study, we have determined that this phenomenon is revealed only after dopamine depletion. In addition, it is targeted primarily to tachykinin producing neurons of the dorsomedial striatum and is dependent on both dopamine(D1) and serotonin(2) receptor activation. Preprotachykinin mRNA levels in the intact striatum were unaltered 4 h following an i.p. injection of either SKF-38393 (1 mg/kg, dopamine(D1) partial agonist) or (+/-)-1-(4-Iodo-2,5-dimethoxyphenyl)-2-aminopropane (DOI 1 mg/kg, serotonin(2) agonist). However, the combined application of both agonists increased (+44%) preprotachykinin message levels, but these changes were restricted to the dorsomedial striatum. In adult animals depleted of dopamine as neonates, striatal preprotachykinin mRNA expression was reduced by approximately 50%. From this lowered level of basal expression, DOI or SKF-38393 raised preprotachykinin mRNA levels within the dorsomedial, but not the dorsolateral striatum. Furthermore, co-stimulation of dopamine(D1) and serotonin(2) receptors produced a nearly four-fold induction of preprotachykinin message levels in the dorsomedial striatum that was significantly greater than either agonist alone. Application of both agonists also elevated preprotachykinin mRNA expression within the dorsolateral striatum, but to a lesser extent. All increases in preprotachykinin mRNA resulting from co-application of SKF-38393 and DOI were prevented by pretreatment with either SCH-23390 (1 mg/kg, dopamine(D1) antagonist) or ritanserin (1 mg/kg, serotonin(2) antagonist). Alternately, preproenkephalin mRNA expression was unaffected by dopamine(D1) receptor stimulation, but was slightly elevated by DOI or both agonists together (42-58%) in intact animals. However, neither agonist treatment in this experiment significantly altered preproenkephalin mRNA expression in the dopamine-depleted striatum which was elevated in response to dopamine lesion alone.Dopamine depletion appears to promote a synergistic interaction between dopamine(D1) and serotonin(2) receptors that leads to enhanced expression of striatal preprotachykinin mRNA levels. The localization of this phenomenon to tachykinin neurons of the direct striatonigral pathway specifically within the dorsomedial regions of the rostral striatum may be relevant to the problem of dyskinetic behaviors which arise during the pharmacological treatment of movement disorders.