Striatal D1 dopamine receptor distribution following chemical lesion of the nigrostriatal pathway

The Effects of Eating a High Fat Diet on Sensitivity of Male and Female Rats to Methamphetamine and Dopamine D1 Receptor Agonist SKF 82958

Rats eating high fat chow are more sensitive to the behavioral effects of dopaminergic drugs, including methamphetamine and the dopamine D2/D3 receptor agonist quinpirole, than rats eating standard chow. However, limited work has explored possible sex differences regarding the impact of diet on drug sensitivity. It is also unknown whether eating high fat chow enhances sensitivity of rats to other dopamine (e.g., D1) receptor agonists. To explore these possibilities, male and female Sprague-Dawley rats eating standard laboratory chow (17% kcal from fat) or high fat chow (60% kcal from fat) were tested once per week for 6 weeks with dopamine D1 receptor agonist SKF 82958 (0.01-3.2 mg/kg) or methamphetamine (0.1-3.2 mg/kg) using cumulative dosing procedures. Eating high fat chow increased sensitivity of male and female rats to methamphetamine-induced locomotion; however, only female rats eating high fat chow were more sensitive to SKF 82958-induced locomotion. SKF 82958-induced eye blinking was also marginally, although not significantly, enhanced among female rats eating high fat chow, but not males. Further, although dopamine D2 receptor expression was significantly increased for SKF 82958-treated rats eating high fat chow regardless of sex, no differences were observed in dopamine D1 receptor expression. Taken together, the present study suggests that although eating high fat chow enhances sensitivity of both sexes to dopaminergic drugs, the mechanism driving this effect might be different for males versus females. These data further demonstrate the importance of studying both sexes simultaneously when investigating factors that influence drug sensitivity. SIGNIFICANCE STATEMENT: Although it is known that diet can impact sensitivity to some dopaminergic drugs, sex differences regarding this effect are not well characterized. This report demonstrates that eating a high fat diet enhances sensitivity to methamphetamine, regardless of sex; however, sensitivity to dopamine D1 receptor agonist SKF 82958 is increased only among females eating high fat chow, but not males. This suggests that the mechanism(s) driving diet-induced changes in drug sensitivity might be different between sexes.

Dopamine, Noradrenaline and Serotonin Receptor Densities in the Striatum of Hemiparkinsonian Rats following Botulinum Neurotoxin-A Injection

Parkinson's disease (PD) is characterized by a degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNpc) that causes a dopamine (DA) deficit in the caudate-putamen (CPu) accompanied by compensatory changes in other neurotransmitter systems. These changes result in severe motor and non-motor symptoms. To disclose the role of various receptor binding sites for DA, noradrenaline, and serotonin in the hemiparkinsonian (hemi-PD) rat model induced by unilateral 6-hydroxydopamine (6-OHDA) injection, the densities of D₁, D₂/D₃, α₁, α₂, and 5HT_2A receptors were longitudinally visualized and measured in the CPu of hemi-PD rats by quantitative in vitro receptor autoradiography. We found a moderate increase in D₁ receptor density 3 weeks post lesion that decreased during longer survival times, a significant increase of D₂/D₃ receptor density, and 50% reduction in 5HT_2A receptor density. α₁ receptor density remained unaltered in hemi-PD and α₂ receptors demonstrated a slight right-left difference increasing with post lesion survival. In a second step, the possible role of receptors on the known reduction of apomorphine-induced rotations in hemi-PD rats by intrastriatally injected Botulinum neurotoxin-A (BoNT-A) was analyzed by measuring the receptor densities after BoNT-A injection. The application of this neurotoxin reduced D₂/D₃ receptor density, whereas the other receptors mainly remained unaltered. Our results provide novel data for an understanding of the postlesional plasticity of dopaminergic, noradrenergic and serotonergic receptors in the hemi-PD rat model. The results further suggest a therapeutic effect of BoNT-A on the impaired motor behavior of hemi-PD rats by reducing the interhemispheric imbalance in D₂/D₃ receptor density.

Increased sensitivity in the interaction of the dopaminergic/adenosinergic system at the level of the adenylate cyclase activity in the striatum of the "weaver" mouse

The specific antagonistic interaction between dopamine D1 and adenosine A1 receptors (D1/A1), as well as between dopamine D2 and adenosine A2a receptors (D2/A2a) exist not only at the receptor/receptor level, but also at the level of the secondary messengers. In this study, we examined the possible changes in these interactions at the level of cAMP formation in membrane preparation from "weaver" mouse striatum (a genetic model of Parkinson disease), by using specific agonists of these receptors. We also examined in the striatum of the "weaver" mouse the interaction between D1 and D2 dopamine receptors. Our results showed that in the striatum of "weaver" mice: a) the cAMP synthesis induced by D1 receptor activation (SKF 38393), was significantly reduced compared to control mice, while A1 receptor activation (L-PIA) leaded to a more intense inhibition of the D1-induced cAMP-formation compared to the controls, b) the cAMP synthesis which was induced by A2a receptor activation (CGS 21680), was significantly increased compared to the control mice. The specific D2 receptor agonist Quinpirole, added in low concentrations, caused a significant reduction of the A2a-induced cAMP formation, which was not observed in the control mouse. Furthermore, the D1 receptor induced cAMP synthesis was significantly higher in control compared to "weaver" striatum, which was more efficiently downregulated by D2 receptor agonist Quinpirole. These results suggest that the sensitivity to D1 and A2a receptor agonists is altered and that the interaction between D1/A1 and D2/A2a receptors is enhanced in the striatum of the "weaver" mutation, while an uncoupling between D1 and D2 receptors was observed. Since the adenylate cyclase basal activity did not differ between "weaver" and control striatum, the above-mentioned changes seem to be due to alterations in the function of the adenosine/dopamine receptors and their coupling to the G-proteins.

Asymmetrical changes of dopamine receptors in the striatum after unilateral dopamine depletion

Dopamine plays an important role in modulating synaptic transmission in the striatum and has great influence on the function of the basal ganglia. Degeneration of dopamine neurons in the substantia nigra (SN) is the major cause of many neurological disorders, and the reduction of dopamine innervation results in alterations of dopamine receptors in the striatum. It has been shown that the nigrostriatal dopamine system has functional and neurochemical asymmetry. To investigate the lateralization of dopamine receptors in the striatum after dopamine denervation, the present study used quantitative autoradiography to compare the changes in dopamine receptor binding in the left and right striatum in rats after unilateral dopamine depletion. In comparison to control levels, dopamine D1)-like receptor binding, labeled with [3H]-SCH23390, in the dorsal striatum was reduced 2 weeks after unilateral lesions of the SN with 6-hydroxydopamine. D1-like receptor binding was decreased in the ipsilateral striatum following unilateral lesions of either the left or right SN. The left and right striatum responded similarly to unilateral SN lesions, as there were no significant differences in the percent decrease in D1-like binding in the two striata. In contrast, D2-like receptor binding, labeled with [3H]-spiroperidol, was significantly increased in the dorsal striatum following an ipsilateral SN lesion. Furthermore, the up-regulation of D2-like receptors in the right striatum was significantly greater than that in the left striatum after an ipsilateral lesion. The asymmetrical up-regulation of striatal D2 receptors after extensive dopamine depletion might contribute to the lateralization of the nigrostriatal system observed in some pathological conditions.

Partial bilateral mesencephalic lesions affect D1 but not D2 binding in both the striatum and cortex

The substantia nigra pars compacta (SNc) and the ventral tegmental area (VTA) are the two major mesencephalic dopaminergic systems. Mesencephalic dopamine denervation is followed by long-term modifications in striatum and cortex that preserve dopamine functions. Here, we have studied the impact of isolated bilateral 6-hydroxydopamine lesioning of the SNc or the VTA on D(1) and D(2) dopamine receptor binding in striatal and cortical areas of rat. Neither SNc nor VTA bilateral partial lesioning changed D(2) binding at the striatal or cortical level. Intriguingly, only VTA lesioning increased D(1) binding in the cortex, whereas both bilateral partial lesioning of the SNc or the VTA increased striatal D(1) binding. This suggests that increased cortical D(1) binding could be an indicator of VTA lesioning. Further behavioural experiments may explain the pathophysiological meaning of increased cortical D(1) binding, and determine whether this observation is involved in compensatory mechanisms.

Induction of stereotypy in dopamine-deficient mice requires striatal D1 receptor activation

Motor stereotypies are abnormally repetitive behaviors that can develop with excessive dopaminergic stimulation and are features of some neurologic disorders. To investigate the mechanisms required for the induction of stereotypy, we examined the responses of dopamine-deficient (DD) mice to increasing doses of the dopamine precursor L-DOPA. DD mice lack the ability to synthesize dopamine (DA) specifically in dopaminergic neurons yet exhibit robust hyperlocomotion relative to wild-type (WT) mice when treated with L-DOPA, which restores striatal DA tissue content to approximately 10% of WT levels. To further elevate brain DA content in DD mice, we administered the peripheral L-amino acid decarboxylase inhibitor carbidopa along with L-DOPA (C/l-DOPA). When striatal DA levels reached >50% of WT levels, a transition from hyperlocomotion to intense, focused stereotypy was observed that was correlated with an induction of c-fos mRNA in the ventrolateral and central striatum as well as the somatosensory cortex. WT mice were unaffected by C/L-DOPA treatments. A D1, but not a D2, receptor antagonist attenuated both the C/L-DOPA-induced stereotypy and the c-fos induction. Consistent with these results, stereotypy could be induced in DD mice by a D1, but not by a D2, receptor agonist, with neither agonist inducing stereotypy in WT mice. Intrastriatal injection of a D1 receptor antagonist ameliorated the stereotypy and c-fos induction by C/L-DOPA. These results indicate that activation of D1 receptors on a specific population of striatal neurons is required for the induction of stereotypy in DD mice.

Age-dependent neurobehavioral plasticity following forebrain dopamine depletions

The differential neurobehavioral effects of forebrain dopamine (DA) depletions in neonatal and adult rats are reviewed. In contrast to the severe and long-lasting parkinsonian sensorimotor deficits seen in rats sustaining large DA depletions as adults, rats comparably depleted as neonates are spared from these gross behavioral deficits. While DA released from residual striatal DA terminals remains necessary for the gradual recovery of sensorimotor function in rats lesioned as adults and the sparing from deficits in rats lesioned as neonates, the specific roles of D1- and D2-like receptors differ between the two age groups. Coactivation of striatal D1 and D2 receptors by residual DA is necessary for the expression of sensorimotor behavior in rats depleted of DA as adults (and in intact rats) whereas activation of either D1 or D2 receptors is sufficient for these behaviors in rats depleted of DA as neonates. We discuss the D1/D2 modulation of several important markers for striatal transmission (acetylcholine release from interneurons, induction of c-fos, and the expression of GAD65 mRNA in striatal efferents) as potential mechanisms underlying this striking age-dependent plasticity following forebrain DA depletions.

Compensatory mechanisms in experimental and human parkinsonism: towards a dynamic approach

This paper provides an overview of the compensatory mechanisms which come into action during experimental and human parkinsonism. The intrinsic properties of the dopaminergic neurones of the substantia nigra pars compacta (SNc) which degenerate during Parkinson's disease are described in detail. It is generally considered that the nigrostriatal pathway is principally responsible for the compensatory preservation of dopaminergic function. It is also becoming clear that the morphological characteristics of dopaminergic neurones and the dual character, synaptic and asynaptic, of striatal dopaminergic innervation engender two modes of transmission, wiring and volume, and that both these modes play a role in the preservation of dopaminergic function. The plasticity of the dopamine neurones, extrinsic or intrinsic to the striatum, can thus be regarded as another compensatory mechanism. Recent anatomical and electrophysiological studies have shown that the SNc receives both glutamatergic and cholinergic inputs. The dynamic role this innervation plays in compensatory mechanisms in the course of the disease is explained and discussed. Recent developments in the field of compensatory mechanisms speak for the urgence to develop a valid chronic model of Parkinson's disease, integrating all the clinical features, even resting tremor, and illustrating the gradual evolution of nigral degeneration observed in human Parkinson's disease. Only a dynamic approach to the physiopathological study of compensatory mechanisms in the basal ganglia will be capable of elucidating these complex questions.

D1 receptor activation enhances evoked discharge in neostriatal medium spiny neurons by modulating an L-type Ca2+ conductance

Most in vitro studies of D1 dopaminergic modulation of excitability in neostriatal medium spiny neurons have revealed inhibitory effects. Yet studies made in more intact preparations have shown that D1 receptors can enhance or inhibit the responses to excitatory stimuli. One explanation for these differences is that the effects of D1 receptors on excitability are dependent on changes in the membrane potential occurring in response to cortical inputs that are seen only in intact preparations. To test this hypothesis, we obtained voltage recordings from medium spiny neurons in slices and examined the impact of D1 receptor stimulation at depolarized and hyperpolarized membrane potentials. As previously reported, evoked discharge was inhibited by D1 agonists when holding at negative membrane potentials (approximately -80 mV). However, at more depolarized potentials (approximately -55 mV), D1 agonists enhanced evoked activity. At these potentials, D1 agonists or cAMP analogs prolonged or induced slow subthreshold depolarizations and increased the duration of barium- or TEA-induced Ca2+-dependent action potentials. Both effects were blocked by L-type Ca2+ channel antagonists (nicardipine, calciseptine) and were occluded by the L-type channel agonist BayK 8644-arguing that the D1 receptor-mediated effects on evoked activity at depolarized membrane potential were mediated by enhancement of L-type Ca2+ currents. These results reconcile previous in vitro and in vivo studies by showing that D1 dopamine receptor activation can either inhibit or enhance evoked activity, depending on the level of membrane depolarization.

DA uptake sites, D1 and D2 receptors, D2 and preproenkephalin mRNAs and Fos immunoreactivity in rat striatal subregions after partial dopaminergic degeneration

Stereotaxic injection of a limited amount of 6-hydroxydopamine in the lateral part of the rat substantia nigra induces a partial degeneration of the nigrostriatal dopaminergic system. This animal model in which the destruction of the dopaminergic nigral cell population reaches approximately 50% could be considered as a preclinical Parkinson's model. Autoradiography of dopaminergic uptake sites performed with a specific marker ([3H]GBR 12935) allowed the precise determination of dopaminergic denervated and non-denervated areas in the striatum 1 month after partial lesion of the substantia nigra pars compacta. In both striatal areas, dopaminergic D1 and D2 receptor densities and dopaminergic D2 and preproenkephalin mRNAs levels were measured by autoradiography and in situ hybridization coupled to an image analysis system. Our results show that in the denervated striatal subregion, none of the dopaminergic targets were modified, contrary to the observations made after complete lesion of the nigrostriatal DA system at the same post-lesion delay. However, striatal Fos activation induced by amphetamine (5 mg/kg i.p., 2 h before killing) revealed that the number of Fos-positive cells detected in the denervated striatal subregion was lower than that observed in the non-denervated one. These data argue in favour of the existence of compensatory mechanisms different from the up-regulation of DA receptor densities, thereby allowing the maintenance of striatal dopaminergic transmission. Such mechanisms could contribute to the delay of the appearance of neurological symptoms (which are reported to be clinically apparent only when depletion of striatal dopamine levels reaches near 80%) in Parkinsonian patients.

Characteristic upregulation of dopamine D1-receptor in rat striatum after 6-hydroxydopamine treatment

We measured the number of D1-receptors (Bmax) in the striatum whose dopaminergic terminals were destroyed to various degrees by 6-hydroxydopamine (6-OHDA) to clarify the relationship between the degree of denervation and the change in Bmax for D1-receptor. Rats were sacrificed 28 days after 6-OHDA treatment, and [3H]SCH23390 (R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4, 5-tetrahydro-1H-3-benzazepine) binding capacity and monoamine content were measured in striatal homogenate. Bmax was significantly upregulated when dopamine content was reduced to less than 5% of that in the vehicle-treated striatum. In contrast, it was significantly decreased when dopamine content was 5-25% of that in the vehicle-treated striatum. The D1-receptors showed either upregulation or downregulation depending on the degree of denervation.

Unilateral 6-hydroxydopamine lesions of meso-striatal dopamine neurons and their physiological sequelae

One of the primary approaches in experimental brain research is to investigate the effects of specific destruction of its parts. Here, several neurotoxins are available which can be used to eliminate neurons of a certain neurochemical type or family. With respect to the study of dopamine neurons in the brain, especially within the basal ganglia, the neurotoxin 6-hydroxydopamine (6-OHDA) provides an important tool. The most common version of lesion induced with this toxin is the unilateral lesion placed in the area of mesencephalic dopamine somata or their ascending fibers, which leads to a lateralized loss of striatal dopamine. This approach has contributed to neuroscientific knowledge at the basic and clinical levels, since it has been used to clarify the neuroanatomy, neurochemistry, and electrophysiology of mesencephalic dopamine neurons and their relationships with the basal ganglia. Furthermore, unilateral 6-OHDA lesions have been used to investigate the role of these dopamine neurons with respect to behavior, and to examine the brain's capacity to recover from or compensate for specific neurochemical depletions. Finally, in clinically-oriented research, the lesion has been used to model aspects of Parkinson's disease, a human neurodegenerative disease which is neuronally characterized by a severe loss of the meso-striatal dopamine neurons. In the present review, which is the first of two, the lesion's effects on physiological parameters are being dealt with, including histological manifestations, effects on dopaminergic measures, other neurotransmitters (e.g. GABA, acetylcholine, glutamate), neuromodulators (e.g. neuropeptides, neurotrophins), electrophysiological activity, and measures of energy consumption. The findings are being discussed especially in relation to time after lesion and in relation to lesion severeness, that is, the differential role of total versus partial depletions of dopamine and the possible mechanisms of compensation. Finally, the advantages and possible drawbacks of such a lateralized lesion model are discussed.

Functional and molecular differentiation of the dopamine system induced by neonatal denervation

The administration of the neurotoxin 6-hydroxydopamine (6-OHDA) to damage the mesostriatal dopamine (DA) system in the neonate results in different neurochemical and behavioral consequences as compared to lesions made in adulthood. There have been few direct data to support the conclusion that the behavioral changes following neonatal 6-OHDA lesions reflect plasticity of the DA system. It is our hypothesis that the plasticity of the developing DA system is fundamentally different from that of the adult. Responses to 6-OHDA lesions can only be understood within the context of the status of the mesostriatal DA system at the time of the lesion. There are stages of development in the early postnatal period when certain components of the mesostriatal DA system are differentially sensitive to 6-OHDA lesions. These "windows" of vulnerability can be predicted from an analysis of the developmental expression of DA receptors and the maturation of the subpopulation of the mesostriatal DA system that innervates them. We review the differences in the behavioral plasticity of the adult and neonate sustaining 6-OHDA lesions to the mesostriatal DA system, the mechanisms responsible for the behavioral plasticity in the adult, and our conceptualization of which mechanisms are affected in the neonate.

D1-class dopamine receptor involvement in the behavioral recovery from prefrontal cortical injury

Following unilateral aspiration of the left medial agranular cortex (AGm) region of prefrontal cortex, rats demonstrate contralateral neglect, characterized by a failure to orient to visual, tactile and auditory stimuli presented on the contralateral body side. While dopamine (DA) has been implicated in cortical neglect and its recovery, this study specifically examined D1-class DA receptors for their involvement in spontaneous recovery from neglect caused by AGm ablation. In the first experiment, left AGm-ablated rats demonstrated severe neglect of contralateral stimuli of each modality which spontaneously recovered over a period of several weeks. Recovered rats were given 7.0 micrograms/kg (s.c.) of the D1-selective antagonist SCH 23390. SCH 23390 reinstated severe neglect of contralateral stimuli, yet had no effect on orientation to ipsilateral stimuli. The same dose had no effect on the orientation behavior of controls. In a second experiment, D1 receptor characteristics were quantified via binding of [3H]SCH 23390 to tissue homogenates of the caudate-putamen of recovered AGm-ablated rats. Numbers and affinities of striatal D1 receptors of rats with unilateral AGm ablations did not differ between hemispheres or from values obtained from lesioned controls. Considered together, these findings indicate that recovery from neglect produced by cortical injury is associated with an increased dependence on D1-class receptor-mediated events, and that this increased dependence is unlikely to be mediated through changes in D1-class receptor numbers or affinities within caudate-putamen.

Stimulation of D1- or D2-receptors in drug-naive rats with different degrees of unilateral nigro-striatal dopamine lesions

We had previously found that in animals with moderate nigro-striatal dopamine (DA) lesions (i.e. 45-65% residual neostriatal DA) the mixed D1/D2-agonist apomorphine induced ipsiversive rather than the usual contraversive turning found after more radical DA lesions. Since this result promised to provide a behavioral animal model for pre-clinical Parkinson's disease, we hoped to delineate the responsible receptor by challenging with selective D1- and D2-agonists. Thus, in the present study, the behavioral effects of the D1-agonist SKF38393 (5.0 mg/kg) and the D2-agonist LY171555 (0.5 mg/kg) were tested in drug-naive rats with unilateral 6-hydroxydopamine lesions of the nigro-striatal DA system. This analysis was performed dependent on the degree of the lesion, classified post-mortem with respect to the level of residual DA in the neostriatum: < 20%, 20-45%, 45-65%, and > 65% (as percentage of the intact hemisphere). The measures of turning, thigmotactic scanning and locomotion did not yield differences between animals treated with the D1-agonist and vehicle-treated rats. For example, animals with severe lesions (residual DA < 20%) showed ipsiversive asymmetries in turning and scanning, which were similar after vehicle or the D1-agonist, both with respect to degree and time-course. However, the analysis of grooming behavior, which was performed in a subset of animals with moderate lesions yielded differences between vehicle and the D1-agonist, since the duration of grooming was increased after SKF38393. In contrast to the D1-agonist, behavioral effects after the D2-agonist LY17155 were evident in all behavioral measures. The general response to this agonist could be characterized by a rapid decrease of behavioral activity including turning, scanning, locomotion and grooming. Although we failed to find significant behavioral asymmetries with either agonist, a micro-analysis showed evidence for selective effects after the D2-agonist, since a contraversive asymmetry in turning (and scanning) became apparent between 45 and 60 min after injection in animals with severe lesions (residual DA of about 10% or less), and since there was a weak ipsiversive turning asymmetry in animals with residual DA levels of 45-65%. Such asymmetries were not observed after vehicle or the D1-agonist. The possible physiological mechanisms of these effects, i.e. DA receptor mechanisms and DA availability, are discussed in the context of results from previous experiments using lesioned or intact animals.

D2 dopamine receptor messenger RNA is altered to a greater extent by blockade of glutamate receptors than by blockade of dopamine receptors

To study further the molecular mechanisms by which glutamate and dopamine interact to regulate the functions of the basal ganglia, the effects of persistently inhibiting dopamine receptors and glutamate N-methyl-D-aspartate receptors on the density of D1 and D2 dopamine receptors and on the level of their transcripts were examined in mouse brain. To block dopamine receptors, mice were treated with N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline once daily for two and six days, or were treated with fluphenazine-N-mustard once daily for five days. To block N-methyl-D-aspartate receptors, mice were treated with dizocilpine by continuous infusion with osmotic mini-pumps for two and six days. The density of D1 and D2 dopamine receptors was measured by receptor autoradiography, and the level of D1 and D2 dopamine receptor messenger RNA was measured by in situ hybridization histochemistry. The results showed that N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline blocked about 90% of both D1 and D2 dopamine receptors, but had no significant effect on the level of either D1 or D2 dopamine receptor messenger RNA. Fluphenazine-N-mustard, which was as effective as N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline in blocking D2 dopamine receptors but had little effect on D1 dopamine receptors, also had no significant effect on the level of D1 and D2 dopamine receptor messenger RNAs. By contrast, continuously infusing dizocilpine significantly decreased the levels of D2 dopamine receptor messenger RNA in striatum, nucleus accumbens and olfactory tubercle. Dizocilpine also caused small decreases in the density of D2 dopamine receptors, but only in posterior striatum was this decrease statistically significant. Dizocilpine slightly and transiently decreased the levels of D1 dopamine receptor messenger RNA in striatum but had no significant effect on the density of D1 dopamine receptors in any region examined. This study demonstrates that persistent blockade of D1 and D2 dopamine receptors has relatively little effect on the levels of D1 and D2 dopamine receptor messenger RNA, but that blockade of N-methyl-D-aspartate receptors produces a rapid and profound decrease in the levels of D2 dopamine receptor messenger RNA and a smaller decrease in the density of D2 dopamine receptors. These results suggest that N-methyl-D-aspartate receptors play an important role in the expression of D2 dopamine receptors in basal ganglia.

Effect of M1- and M2-muscarinic drugs on striatal dopamine release and metabolism: an in vivo microdialysis study comparing normal and 6-hydroxydopamine-lesioned rats

Microdialysis was used to study the effect of M1 and M2 selective agonists and antagonists on striatal dopamine release and metabolism. Microdialysis probes were implanted, under anesthesia, in the left and the right striatum of the normal rats and in the normal and denervated striatum of the nigral 6-hydroxydopamine-lesioned rats. Dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) were determined by liquid chromatography and electrochemical detection. The different drugs were infused through the dialysis probe during 40 min. Pirenzepine (5 microM), a selective M1 antagonist, produced a significant decrease in DA release in the normal and the 6-hydroxydopamine-lesioned rats, with no significant difference between both groups. Methoctramine, a selective M2 antagonist, produced a dose-dependent increase in DA release between 20 and 200 microM in the normal rats, with no significant effect on DOPAC and HVA. Infusing 75 microM methoctramine produced a significant increase in DA release with a more pronounced effect in the intact animals compared to the 6-hydroxydopamine-lesioned animals. The non-selective agonist carbachol produced a decrease in dopamine release after infusion of 50 microM (M2 effect) and an increase in dopamine release after infusion of 50 mM (M1 effect) in the normal rats. Infusing 50 microM carbachol in the denervated striatum, produced a slight increase in DA release. Our data suggest that presynaptic M1-muscarinic receptors enhance and M2-muscarinic receptors inhibit DA release in the striatum of the rat; and that 3 weeks after 6-hydroxydopamine lesioning there may be a normalisation of the number of M1-receptors with a loss of M2-receptors at the denervated side.

Dopamine receptors status after unilateral nigral 6-OHDA lesion. Autoradiographic and in situ hybridization study in the rat brain

The physiological effects of dopamine (DA) are mediated by several distinct receptor subtypes. The effects of unilateral nigral 6-hydroxydopamine (6-OHDA) lesions on DA receptors were investigated by receptor autoradiography using the D1 selective ligand [3H]SCH 23390 as well as the D2 ligand [3H]spiroperidol. mRNA distribution was studied by in situ hybridization. Lesioned rats were sacrificed at different time intervals. Receptor binding studies were performed on tissue sections using selective ligands. [35S]UTP labeled RNA probes were prepared from the different cDNA (D1, D2, D3) and used for in situ hybridization. A specific loss of receptor binding sites and mRNA hybridization was found in the lesioned substantia nigra pars compacta (SNc) at all times examined. Receptor binding studies revealed a different time-dependent increase in both D1 and D2 receptors. In situ hybridization showed that only D2 receptor mRNA increased in the caudate-putamen (CPu) of the lesioned side 15 d after 6-OHDA. No changes were observed in D1 and D3 receptor mRNA during the entire time-course.

Dopamine supersensitivity and D1/D2 synergism are unrelated to changes in striatal receptor density

Experiments were conducted to elucidate the relationships among striatal dopamine receptor density, behavioral manifestations of D1/D2 synergism (i.e., the requirement of concomitant stimulation of D1 and D2 receptors for the expression of stereotyped sniffing, licking and gnawing), and behavioral supersensitivity to dopamine agonists. The state of D1/D2 synergism was found to be independent of striatal D1 or D2 receptor density in rats as: (1) increasing striatal D1 and/or D2 receptor density (as confirmed by quantitative receptor autoradiography) by chronic treatment with SCH 23390 (0.5 mg/kg/day for 21 days) and/or haloperidol (0.5 mg/kg/day for 21 days) did not alter the normal pattern of D1/D2 synergism as determined by behavioral responsiveness to agonist stimulation of D1 or D2 receptors, and (2) 5 days of reserpine treatment (1 mg/kg/day), although not significantly changing striatal D1 or D2 receptor density, induced a breakdown in D1/D2 synergism (i.e., behavior was elicited by independent stimulation of D1 or D2 receptors). In addition, the density of striatal D2 binding sites was not indicative of behavioral sensitivity to D2 agonists. Chronic haloperidol treatment increased behavioral sensitivity to the D2 agonist quinpirole by a factor of 2. When tested 96 h after bilateral 6-hydroxy-dopamine injections or after 5 daily reserpine injections, supersensitivity to quinpirole was at least double that following chronic haloperidol, without accompanying increases in striatal D2 density. This enhanced sensitivity to quinpirole was no greater than that observed in neurologically intact rats treated concomitantly with a maximally stimulating dose of SKF 38393. Furthermore, rats with unilateral 6-hydroxydopamine lesions that were treated chronically with eticlopride continued to rotate contralateral to the lesion in response to quinpirole despite having hemispheric symmetry of striatal D2 receptor binding. By contrast, when rats with unilateral 6-hydroxydopamine lesions were given 5 daily reserpine injections, rotation was abolished, having been replaced by intense stereotyped sniffing, indicative of bilateral supersensitivity. The results support the hypothesis that two distinct types of dopamine supersensitivity exist: a modest one associated with increased D2 density, and a more profound one associated with a breakdown in D1/D2 synergism and independent of D2 density.

Dissociation of the striatal D-2 dopamine receptor from adenylyl cyclase following 6-hydroxydopamine-induced denervation

Intracellular cyclic AMP accumulation following exposure to dopamine (DA) agonists and and antagonists was measured in striatal slices from rats with a unilateral 6-hydroxydopamine (6-OHDA) lesion of the nigrostriatal pathway and which showed contralateral circling to apomorphine. Both DA (10-320 microM) and the D-1 agonist SKF 38393 (0.1-32 microM) increased cyclic AMP accumulation in striatal slices from the lesioned and intact hemispheres. The EC50 for DA to increase cyclic AMP accumulation in slices was greater in the 6-OHDA-lesioned striata compared to the intact striatum, but the EC50 for SKF 38393 was not affected. The D-1 antagonist SCH 23390 (10 microM) completely inhibited the ability of DA and SKF 38393 to increase cyclic AMP accumulation in striatal slices from both denervated and intact sides of the brain. In slices from the intact hemisphere the increase in DA-induced cyclic AMP accumulation was enhanced by the D-2 antagonist (+/-)-sulpiride (50 microM) but (+/-)-sulpiride had no effect on the DA response in slices from the lesioned side. Similarly, the ability of SKF 38393 to enhance cyclic AMP accumulation was blocked by the D-2 agonist quinpirole (10 microM) in striatal slices from the intact hemisphere but not in tissue from the lesioned side. The density of striatal D-1 and D-2 receptors assessed by [3H]SCH 23390 and [3H]spiperone binding did not differ between the hemispheres although there was an increase in the affinity of D-1 receptors for [3H]SCH 23390 in the lesioned striatum. After striatal deafferentiation there appears to be an uncoupling of the "inhibitory" D-2 receptor from the D-1 receptor-associated adenylyl cyclase.

Autoradiographic study of striatal D1 and D2 dopamine receptors in 6-OHDA-lesioned rats receiving foetal ventral mesencephalic grafts and chronic treatment with L-dopa and carbidopa

Foetal dopamine cell suspensions or sham preparations were implanted into the denervated striatum of rats with a unilateral 6-hydroxy-dopamine (6-OHDA) lesion of the medial forebrain bundle. Some animals were also treated with L-DOPA (200 mg/kg/24 h) and carbidopa (25 mg/kg/24 h) in the drinking water for 5 weeks, followed by a 3-week drug-free period. Rotational responses to apomorphine and (+)-amphetamine were assessed, and the density of D1 and D2 dopamine receptors was evaluated autoradiographically in striatal slices exposed to [3H]SCH 23390 or [3H]spiperone. Foetal grafts reduces apomorphine-induced contralateral rotation and prevented the development of apomorphine-induced stereotypy. Foetal grafts abolished (+)-amphetamine-induced ipsilateral rotation. These effects of the grafts were not altered by treatment with L-DOPA. A unilateral 6-OHDA lesion of the nigrostriatal pathway resulted in an ipsilateral increase in D2 receptor density most marked in the lateral and dorsomedial quadrants of the striatum compared with the contralateral side. Foetal ventral mesencephalic grafts implanted into the lesioned striatum decreased D2 receptor density to levels found in the contralateral intact striatum. Chronic L-DOPA and carbidopa treatment did not alter the effect of the grafts. A 6-OHDA lesion resulted in a reduction of D1 receptor density in the lateral areas of the lesioned striatum at Level 2. The presence of a foetal ventral mesencephalic graft either alone or together with L-DOPA treatment did not alter the lesion-induced changes in D1 binding density.

Enhancement of rotational behavior induced by repeated administration of SKF38393 in rats with unilateral nigrostriatal 6-OHDA lesions

To clarify if the enhancement of rotational behavior induced by repeated administration of SKF38393 is mediated by upregulation of D1 and/or D2 receptors in the striatum, we investigated effects of SCH23390 and sulpiride on SKF38393-induced rotational behavior and the changes in striatal dopamine receptors in rats with unilateral nigrostriatal 6-hydroxydopamine lesions (1). Repeated weekly administration of SKF38393 markedly enhanced the number of rotations and shortened the latency of rotational behavior depending on the number of SKF38393 administrations 1 or 6 weeks after the treatment with 6-OHDA (2). A selective D1 antagonist, SCH23390, but not a selective D2 antagonist, sulpiride, suppressed SKF38393-induced rotation and inhibited the enhancement by the repeated administration (3). Repeated administration of SKF38393 did not modify the density and the affinity of either the striatal D1 or D2 receptors in the striatum. These results suggest that the enhancement of SKF38393-induced rotational behavior by the repeated administration is not associated with the upregulation of striatal D1 and D2 receptors.

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.

Behavioral quantification of striatal dopaminergic supersensitivity after bilateral 6-hydroxydopamine lesions in the mouse

Quantitative studies using dopamine (DA) agonist-induced rotational behavior after denervation have found that the behavioral sensitivity is much greater than would be predicted on the basis of striatal DA receptor upregulation alone. The sensitivity to DA agonists after chronic treatment with neuroleptics, which elicits striatal receptor alterations equal to denervation, displays increases more consistent with alterations in striatal receptor density. Since the behavioral paradigms used to assess agonist supersensitivity after denervation are different than that for chronic neuroleptic treatment (rotational vs. stereotypic behavior), we measured the behavioral supersensitivity after bilateral denervation using stereotypic behavior. The increase in sensitivity to apomorphine after bilateral nigrostriatal 6-hydroxydopamine lesions was consistent with the increases measured previously with rotational behavior. These data suggest that the quantitative difference observed in behavioral supersensitivity resulting from the different preparations lies with the biological consequences of denervation rather than with the behavioral paradigm.

Intrastriatal dopamine-rich implants reverse the changes in dopamine D2 receptor densities caused by 6-hydroxydopamine lesion of the nigrostriatal pathway in rats: an autoradiographic study

The aim of the present study was to test whether intrastriatal implants of embryonic dopaminergic neurons are able to normalize the lesion-induced hypersensitivity of striatal dopaminergic receptors. The ascending dopaminergic pathway of adult rats was unilaterally lesioned using 6-hydroxydopamine. Three weeks later a cell suspension obtained from the mesencephali of ED 14 rat embryos was implanted into the denervated striatum. Rotational responses to dopaminergic agonists were tested five months after implantation. One month later animals were killed and striatal dopaminergic receptor densities were quantified using autoradiography, the dopaminergic reinnervation of the host striatum being visualized with [3H]GBR 12935, a ligand labelling dopamine uptake sites. The lesion induced a behavioural hypersensitivity to dopaminergic agonists and lesioned animals displayed a strong rotation contralateral to the lesion in response to a test dose of the D1 agonist compound SKF 38393 (2.5 mg/kg) or of the D2 agonist LY 171555 (0.15 mg/kg). These responses were completely abolished by the graft. The normal distribution of D1 and D2 dopaminergic receptors in the rat striatum was similar to that described previously. Seven months after the lesion of the nigrostriatal dopaminergic pathway, the density of D1 receptors was not significantly affected while the density of D2 receptors was increased by about 25-50%. The implantation of embryonic dopaminergic neurons into the denervated striatum led to a slight decrease of D1 receptor densities and to a reversal of the lesion-induced increase of striatal dopaminergic D2 receptors six months later. Moreover, this reversal concerned not only the reinnervated striatal region but also extended into non-reinnervated areas of the striatum. It is concluded that grafts of embryonic dopaminergic neurons can normalize the density of dopaminergic D2 receptors.

Effect of unilateral perinatal hypoxic-ischemic brain injury in the rat on striatal muscarinic cholinergic receptors and high-affinity choline uptake sites: a quantitative autoradiographic study

The binding characteristics and distribution of M1 and M2 muscarinic cholinergic receptors and high-affinity choline uptake sites were studied in the striatum of the rat at 3-4 and 9-12 weeks of age after exposure to unilateral perinatal hypoxic-ischemic brain injury. High-affinity choline uptake sites were labeled with [3H]hemicholinium-3, M1 receptors with [3H]pirenzepine, and M2 receptors with [3H]AF-DX 116. Saturation experiments revealed a significant decrease in the maximal binding capacity (Bmax) for [3H]pirenzepine-labeled M1 receptors in the lesioned caudate/putamen complex in immature rats with moderate brain injury, in comparison with controls. In contrast, the Bmax value for [3H]hemicholinium-3-labeled high-affinity choline uptake sites was significantly increased. No changes in dissociation constants (KD) were observed. These changes were most pronounced in the dorsolateral region of striatum. Striatal regional distribution of [3H]AF-DX 116 was not affected. In mature rats, binding of [3H]pirenzepine returned to control values, whereas [3H]hemicholinium binding showed a persistent increase (23%). The increase in [3H]hemicholinium-3 binding, as a specific marker of cholinergic nerve terminals, is consistent with our prior morphologic studies demonstrating relative preservation of cholinergic neurons and neuropil, and supports the concept that striatal cholinergic systems are resistant to hypoxic-ischemic injury.

Differential response of striatal dopamine and muscarinic cholinergic receptor subtypes to the loss of dopamine. I. Effects of intranigral or intracerebroventricular 6-hydroxydopamine lesions of the mesostriatal dopamine system

Quantitative autoradiography was utilized to examine the response of the dopamine (DA) and muscarinic cholinergic system within the striatum to lesions of the mesostriatal DA system following intranigral 6-hydroxydopamine (6-OHDA) injections. In addition, the response of DA system was examined in the striatum of animals treated with low, medium, or high doses of 6-OHDA made intracerebroventricularly (icv). Three weeks following removal of the mesostriatal DA fibers with intranigral 6-OHDA, there was an almost complete depletion of DA and [3H]mazindol binding throughout the striatum. The resulting increase in D2 receptors labeled with [3H]spiroperidol (27%) was most evident in the lateral striatum and topographically correlated with an increase in choline uptake sites labeled with [3H]hemicholinium-3 (20%). There was a smaller but significant decrease in D1 receptors labeled with [3H]SCH 23390 (15-18%) that was not topographically related to changes in [3H]spiroperidol or [3H]hemicholinium-3 binding. All doses of icv 6-OHDA produced a significant loss of DA and of [3H]mazindol binding as compared to vehicle injections that was more pronounced in the medial than in the lateral striatum. No increase in D1 receptors was observed with any dose of 6-OHDA and greater than 90% loss of DA and [3H]mazindol resulted in an increase in D2 receptors in the lateral striatum and a reduction in D1 receptors in the dorsal striatum. These data are consistent with the evidence that there is independent regulation of the two subtypes of the DA receptor. Moreover, the distribution and regulation of the subtypes of the muscarinic receptor were independent. Muscarinic M2 receptors ([3H]N-methylscopolamine in presence of excess pirenzepine) showed a lateral to medial gradient (highest laterally) that was related to the pattern of choline uptake sites and D2 receptors. Loss of DA resulted in a reduction in M2 receptors (24-30%) that was correlated with the increase in choline uptake sites. In contrast, M1 ([3H]pirenzepine) receptors showed a reverse gradient from the M2 receptor and a smaller reduction following loss of DA.

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.

Multiple fluorescent ligands for dopamine receptors. I. Pharmacological characterization and receptor selectivity

We report the synthesis and pharmacological characterization of novel fluorescently labeled ligands with high affinity and specificity for D1 and D2 dopamine receptors. D1-selective antagonist probes have been synthesized using (R,S)-5-(4'-aminophenyl)-8-chloro-2,3,4,5-tetrahydro-3-methyl-[1H]-3- benzazepin-7-ol, the 4'-amino derivative of the high affinity D1-selective antagonist, SCH-23390, while D2-selective antagonist probes were synthesized using the high affinity, D2-selective agonist, N-(p-aminophenethyl)spiperone (NAPS). In addition, we have synthesized fluorescent probes using an amino-derivative of the high affinity, D2-selective agonist, 2-(N-phenethyl-N-propyl)amino-5-hydroxytetralin (PPHT or N-0434). These ligands were coupled to the fluorescent moieties, fluorescein, rhodamine, coumarin, Texas red, Cascade blue, or Bodipy. This resulted in a wide variety of dopaminergic ligands which fluoresce at different wavelengths: Cascade blue and coumarin are blue fluorophores, fluorescein and Bodipy, are yellow-green, and Texas red and rhodamine are red. The interaction of these fluorescent ligands with dopamine and serotonin receptors was evaluated by examining their ability to compete for radioligand binding to D1 and D2 dopamine receptors and 5-HT1A, 5-HT1C and 5-HT2 serotonin receptors. We report here that these novel fluorescent ligands exhibit high affinity and, in general, selectivity for either D1 or D2 dopamine receptors. In addition, we demonstrate that the fluorescent derivatives of PPHT retain the full agonist efficacy exhibited by the parent compound.(ABSTRACT TRUNCATED AT 250 WORDS)

Striatal D1 dopamine receptor morphochemistry following continuous or intermittent L-dopa replacement therapy

Striatal dopamine deafferentation has previously been found to diminish D1 dopamine receptor clustering in association with striatal cyclic AMP-immunoreactive neurons. The administration of the dopamine precursor levodopa (L-DOPA) to animals with unilaterally placed 6-hydroxydopamine nigrostriatal tract lesions now appears to partially restore D1 dopamine receptor morphochemical organization in the deafferented striatum. Differences in the mode of levodopa delivery produced dissimilar D1 recovery patterns. The prodrug, L-DOPA methyl ester, was administered in combination with the peripheral aromatic amino acid decarboxylase inhibitor, benserazide, to achieve consistent plasma levels of the dopamine precursor. Continuous levodopa infusion (100 mg/kg/day, ip) led to a slight dorsomedial reassociation of D1 receptor binding sites with the postsynaptic cyclic AMP transduction system on the deafferented side. In contrast, intermittent levodopa therapy (50 mg/kg, ip, bid) produced a noticeable down regulation of the dopamine receptor system and also contributed to some region-specific recovery of the morphochemical pattern of D1 receptor binding site reaggregation with the postsynaptic cyclic AMP second messenger transduction system. These results suggest that exogenous levodopa replacement therapy desensitizes striatal D1 dopamine receptors. This was substantiated using image analysis of densitometric histograms. The down regulation of D1 receptors is dependent on the levodopa treatment regimen employed. Our findings provide a potential morphological basis for the behavioral desensitization shown previously in response to chronic, intermittent levodopa administration.

Characterization of anti-peptide antibodies for the localization of D2 dopamine receptors in rat striatum

Seven different peptides of 14-23 residues in length based on the predicted amino acid sequence of the cloned rat D2 receptor cDNA were used as immunogens to develop antibodies in rabbits. Two of these peptides were derived from the amino terminus and four were from the third cytoplasmic loop, including one to the splice variant insertion sequence and one to the carboxyl terminus of the receptor protein. These peptides were conjugated to bovine thyroglobulin prior to rabbit immunization. Antibody production was monitored by a solid-phase ELISA. With the exception of the carboxyl-terminal peptide, all of the peptide immunogens produced antiserum of high titer ranging from 1:10(4) to 1:10(6) on ELISA. Specificity of the reaction was demonstrated by the absence of a response in the preimmune serum and by the absence of cross-reactivity between the various antisera and the nonimmunization peptides. Moreover, preincubation of the antiserum with the immunization peptide, but not other peptides, blocked the subsequent ELISA reactions. Some of the antisera were additionally characterized by immunodot assays using solubilized rat striatal membranes blotted onto nitrocellulose. Positive reactions with antiserum dilutions of 1:500 were observed that were dependent on the presence and concentration of membrane protein and were not observed using preimmune serum. Additionally, immunofluorescent staining by the D2 receptor antiserum was observed on cells that had been transfected with the D2 receptor cDNA but not on untransfected cells. Immunoprecipitation of the photoaffinity-labelled and solubilized D2 receptor also suggested that the antisera were able to directly recognize the native receptor protein. Immunohistochemical localization of the D2 receptor in slices of fresh frozen and perfusion-fixed rat brain was performed using these antisera. Within the striatum, about 50% of the medium-sized neurons were labeled as well as large, putatively cholinergic interneurons.

Retrograde degeneration of nigrostriatal neurons induced by intrastriatal 6-hydroxydopamine injection in rats

Quantitative receptor autoradiography was used to assess the effects of unilateral intrastriatal injections of 6-hydroxydopamine (6-OHDA) on the distribution of D1 and D2 dopamine (DA) receptors and of DA uptake sites in the mesostriatal pathway. [3H]Mazindol-labeled DA uptake sites were reduced both in the striatum (-97%) and in the substantia nigra pars compacta (SNpc) (-88%) on the injected side. There were also significant decreases of dopamine uptake sites in the nucleus accumbens (NAc) (-73%) and in the ventral tegmental area (VTA) (-70%). Changes in [3H]mazindol binding were also found within the contralateral VTA (-30%) and SNpc (-13%) but not in the contralateral-striatum. [3H]SCH23390-labeled D1 receptors were significantly reduced in the dorsomedial (-18%) and ventromedial (-14%) aspects of the striatum ipsilateral to the side of the lesions. In contrast, the concentration of [3H]spiperone-labeled D2 receptors was not altered. There were also significant decreases in D1 (-18%) and of D2 (-27%) receptors in the SNpc and of D1 (-10%) in the SN pars reticulata (SNpr). These results suggest that oxyradical-induced damage in striatal DA terminals could lead to retrograde changes in the SNpc. In addition, the data indicate that unilateral striatal damage can result in bilateral changes in the SNpc, thus confirming the interdependence of the two nigrostriatal pathways in rats.

A light and electron microscopical study of enkephalin-immunoreactive structures in the rat neostriatum after removal of the nigrostriatal dopaminergic pathway

The pattern of enkephalin immunoreactivity was examined in the adult rat neostriatum, at various times after unilateral removal of the nigrostriatal dopamine input by 6-hydroxydopamine injection into the medial forebrain bundle. Animals were examined 12 days, 26 days or 13 months after the lesion. Enkephalin-immunoreactive synaptic boutons (n = 1018) in the control and the dopamine-depleted neostriatum were analysed in the electron microscope. The area of enkephalin-immunoreactive synaptic bouton profiles was significantly larger in the dopamine-depleted neostriatum and this increase was maximal in rats in which the lesion had been made 26 days or 13 months previously (50% increase). The synaptic specializations of these enkephalin-immunoreactive boutons were significantly longer in the neostriatum from the injected side. Dendritic shafts were the principal postsynaptic target of these boutons (67%) but dendritic spines (18%), perikarya (6.5%) and unidentifiable small dendrites or spines (8.5%) were also contacted. The proportions of enkephalin-immunoreactive boutons on the different postsynaptic targets were not altered by the 6-hydroxydopamine lesion. The increase in enkephalin immunoreactivity observed in the dopamine-depleted neostriatum in previous studies may be explained by the increase in the size of enkephalin-immunoreactive synaptic boutons found in the present ultrastructural investigation. The observations do not rule out the possibility that there is also an increase in the number of immunoreactive synaptic boutons, due to, for example, sprouting of the existing enkephalin-containing fibres.

Selective localization of striatal D1 receptors to striatonigral neurons

A new technique for producing anatomically selective lesions within the brain was used to investigate the cellular localization of the D1 and D2 receptor. The cytotoxic lectin, volkensin, is taken up by nerve terminals and retrogradely transported, killing those neurons projecting to the site of injection. Comparison of D1 and D2 binding following a unilateral volkensin injection into the substantia nigra has demonstrated that striatal D1 binding sites are selectively localized to striatonigral projection neurons.

Age-related decline in rat striatal dopamine metabolism is regionally homogeneous

Previous research has established that the age-related decrease in rat striatal D2 sites occurs predominantly in the posterior ventral caudate-putamen, and the present work was undertaken to determine whether a corresponding preferential reduction in dopamine, its metabolites, or its synthesis rate occurs in this region. Male F344 rats 4-8 or 25-27 months old were used for regional HPLC electrochemical determinations of 1) dopamine, homovanillic acid (HVA), or dihydroxyphenylacetic acid (DOPAC) obtained from striatal micropunch samples, or 2) 3,4-dihydroxyphenylalanine (DOPA) concentrations in these same micropunch regions 30 minutes after treatment with the aromatic amino decarboxylase inhibitor, NSD-1015 (100 mg/kg, IP). Aged rats had significantly less dopamine, HVA, and DOPAC in their striatal samples than did young adult controls, as well as having less DOPA accumulation after NSD-1015. However, for none of these measures was the age x region interaction significant, suggesting that the decline in these markers of presynaptic dopaminergic function occurs uniformly throughout the striatum. The results provide evidence that the effects of aging on striatal dopamine receptors are dissociable from the influences on the dopaminergic innervation of this structure, suggesting independent control of pre- and postsynaptic elements of these synapses during the lifespan.

Direct visualization and cellular localization of D1 and D2 dopamine receptors in rat forebrain by use of fluorescent ligands

The regional and cellular localization of the two subtypes of dopamine receptors, D1 and D2, have been ascertained in rat forebrain by use of fluorescent dopaminergic antagonist ligands. (R,S)-5-(4'-aminophenyl)-8-chloro-2,3,4,5-tetrahydro-3- methyl-[1H]-3-benzazepin-7-ol, the 4'-amino derivative of the high-affinity D1-specific antagonist SCH 23390, and the D2 selective antagonist N-(p-aminophenethyl)spiperone were chemically derivatized using the fluorescent compound tetramethylrhodamine. The modification of these antagonist ligands has allowed the specific, cellular resolution of the D1 and D2 receptor binding sites in intact, highly organized regions of forebrain slices in a very rapid experimental time frame. The regional localization of receptors labeled by the fluorescent probes is in agreement with previous receptor autoradiography studies. Moreover, the specific cellular binding patterns for both receptors can now be compared and contrasted to one another in the same tissue by using these fluorescent ligands. D1 receptor sites are most evident within the striatum and exhibit regions of intense "patch" fluorescence corresponding to receptor reactivity in cells and their processes. The distribution of D1 receptor binding is highly analogous to the pattern of dopamine terminal histofluorescence in the caudate nucleus. D2 receptor sites are less prevalent overall and may be localized to a subpopulation of the D1 fluorescent neurons in the caudate nucleus and nucleus accumbens regions.

Decreased striatal D1 binding density following mesotelencephalic 6-hydroxydopamine injections: an autoradiographic analysis

Autoradiographic experiments performed on rats with unilateral mesotelencephalic 6-hydroxydopamine (6-OHDA) injections revealed reduced binding of [3H]SCH23390 to D1 receptors in the striatum ipsilateral to the neurotoxin as well as increased binding of [3H]spiroperidol to D2 receptors in that hemisphere. These opposite influences of injury on the dopamine receptor subtypes occurred in rats sacrificed at 2 weeks or 11 months postoperatively, but neither change was evident at 4 days postoperatively. Equilibrium saturation analysis performed on rats sacrificed at 8 weeks postoperatively indicated that D1 and D2 receptor changes reflected altered Bmax values without KD modifications. Topographic analysis of the D1 decline by quantitative autoradiography revealed that the D1 decrease was greater in dorsal striatum than ventrally. Those striatal regions that showed greater declines in D1 density correspondingly had the greater losses of [3H]mazindol binding after the denervation, suggesting that the decline of D1 binding is a postsynaptic consequence of the reduced mesostriatal dopaminergic innervation. The findings indicate opposite influences of injury on D2 and D1 receptor levels and raise important questions concerning the mechanism by which 6-OHDA injection affects the D1 sites.

Characterization of novel fluorescent ligands with high affinity for D1 and D2 dopaminergic receptors

We have synthesized and characterized a series of novel fluorescently labeled ligands with high affinity and specificity for D1 and D2 dopamine receptors. D1-selective probes were synthesized using (R,S)-5-(4'-aminophenyl)-8-chloro-2,3,4,5-tetrahydro-3-methyl- [1H]-3-benzazepin-7-ol, the 4'-amino derivative of the high-affinity, D1-selective antagonist SCH-23390, whereas D2-selective probes were synthesized using the high-affinity, D2-selective antagonist N-(p-aminophenethyl)spiperone (NAPS). These ligands were coupled via spacer arms of various lengths to the fluorophores fluorescein and bodipy, which fluoresce in the yellow-green region, and to tetramethylrhodamine, which is a red fluorophore. The interaction of these fluorescent ligands with dopamine receptors was evaluated by examining their ability to compete for the binding of the radiolabeled antagonists [3H]SCH-23390 or [3H]methylspiperone to rat striatal D1 or D2 dopamine receptors, respectively. We report here that these novel fluorescent ligands exhibit very high affinity and specificity for either D1 or D2 dopamine receptors. The availability of various fluorescent ligands with different emission maxima and with high affinity and specificity for D1 and D2 dopamine receptors will now permit investigations involving the visualization and localization of these receptor subtypes at the single cell and intracellular levels in the CNS and on intact cells in culture.

Dopamine D1 binding sites in the striatum of the mutant mouse weaver

In the weaver mouse there is a major abnormality in the dopamine-containing innervation of the striatum. Dopamine islands from during development, along with some innervation of the non-islandic matrix; but during the first postnatal month much of the islandic innervation degenerates and there is a failure of the normal postnatal development of the diffuse nigrostriatal innervation. In the experiments reported here we analysed the distribution of D1 dopamine receptor-related binding sites in the weaver striatum in an effort to test the relationship between the dopamine-containing innervation of the striatum and the synthesis and distribution of dopamine receptors there. Dopamine D1 receptor binding sites labeled by the D1 specific antagonist [3H]SCH 23390 were studied in the striatum of 7-day and adult homozygous weaver (wv/wv) and homozygous control (+/+) mice. Saturation analysis of [3H]SCH 23390 binding in adult animals suggested that the dissociation constants of the binding sites are similar in mutants and controls. The Bmax values in the striatum of weavers were 16% higher than in the controls when the data were expressed as fmoles/mg protein. The protein content of the adult weaver's striatum was decreased by 15 to 30%, however, so that when values were expressed as fmoles/section, no significant difference between values in weavers and homozygous controls were found. Quantitative autoradiography supported the results of saturation analysis. We conclude that the apparent increase of [3H]SCH23390 binding sites in the mutants occurred as the result of shrinkage of the weaver's caudoputamen and that dopamine D1 receptor binding sites in the caudoputamen, as assessed with [3H]SCH 23390, are normal. The studies of regional distribution of [3H]SCH 23390 binding sites in 7-day and adult mice indicated that the characteristic postnatal transition of the [3H]SCH 23390 binding pattern from islandic to a diffuse distribution occurred normally in the weaver's caudoputamen. Thus, in spite of the degeneration and failure of development of the nigrostriatal innervation in weaver mice, D1 binding in the weaver's striatum undergoes the elaborate change in distribution of these sites that is a hallmark of normal striatal development.

Long-term changes in striatal D1 dopamine receptor distribution after dopaminergic deafferentation

The morphochemical disposition of the adenylate cyclase-linked dopamine receptor (D1 type) in the rat striatum has been assessed at various time points after a neurotoxic lesion of the dopaminergic afferent pathway to the caudate nucleus. D1 receptor binding sites in the caudate nucleus were determined by in vitro autoradiography of the substituted benzazepine D1 antagonists, [3H]SCH 23390 or [125I]SCH 23982, and contrasted to the pattern of striatal immunohistochemical reactivity of the second messenger compound, cyclic 3',5'-adenosine monophosphate. The results demonstrate that the specific association of this dopamine receptor type with cyclic 3',5'-adenosine monophosphate-stained neurons is abolished at 7 days following chemical interruption of the nigrostriatal pathway, and the receptor disruption is persistent for durations as long as 20 weeks. This investigation suggests that once the postsynaptic receptor pathology is produced by deafferentation, it does not recover the selective morphochemical relationship normally established with the target cell containing the second messenger. This is in contrast to modest biochemical recuperation in D1 dopamine receptor binding seen using this experimental paradigm. This change in D1 dopamine receptor morphochemistry is discussed in relation to the neurochemical deficits produced by dopaminergic denervation and in Parkinson's disease.