The effects of denervation and chronic haloperidol treatment on neostriatal dopamine receptor density are not additive in the rat

Ex vivo [11C]-(+)-PHNO binding is unchanged in animal models displaying increased high-affinity states of the D2 receptor in vitro

Dopamine (DA) D2 receptor supersensitivity has been linked to an increase in the density of the D2 high-affinity state as measured in vitro. The two- affinity-state model of the D2 receptor predicts that the ex vivo specific binding of [11C]-(+)-PHNO, an agonist radiotracer thought to bind selectively to the high-affinity state in vivo, should be increased in animal models that display in vitro increases in the proportion of receptors in the D2 high-affinity state. Here, we test this hypotheses by comparing the ex vivo SBR of [11C]-(+)-PHNO with that of the antagonist radiotracer [3H]-raclopride in three dopaminergically supersensitive rat models-AMPH-sensitized rats, rats withdrawn from chronic ethanol, and unilaterally 6-OHDA-lesioned rats-using ex vivo dual-radiotracer biodistribution studies. We find that in AMPH-sensitized rats and rats withdrawn from chronic ethanol treatment, models that exhibited approximately 4-fold increases in the D2 high-affinity state in vitro, the SBRs of [11C]-(+)-PHNO and [3H]-raclopride are unchanged relative to control rats. In unilaterally 6-OHDA-lesioned rats, we find that the increase in [11C]-(+)-PHNO SBR is no different than that observed for the antagonist radiotracer [3H]-raclopride (54% +/- 16% and 52% +/- 14%, respectively). In addition, the effect of acute AMPH pretreatment (4 mg/kg, i.v.) on the SBRs of [11C]-(+)-PHNO and [3H]-raclopride is equivalent in AMPH-sensitized (-38% +/- 12% and -36% +/- 8%, respectively) and in control rats (-40% +/- 11% and -38% +/- 7%). These data emphasize a significant discrepancy between in vitro and in vivo measures of D2 agonist binding, indicating that the two-affinity-state model of the D2 receptor may not apply veridically to living systems. The potential implications of this discrepancy are discussed.

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.

Neuroleptics cause stimulation of dopamine D1 receptors and their desensitization after chronic treatment

Recent evidence indicates that the neuroleptic-induced increase of in vivo acetylcholine output in the striatum does not depend on the relief of cholinergic neurons from the inhibitory control by dopamine, but on increased dopamine output onto dopamine D1 receptors. The present microdialysis study was aimed at finding if the neuroleptic-induced increase in striatal acetylcholine release persists after chronic treatment, and how it is correlated with dopamine output. Rats were chronically treated with the dopamine D2 receptor antagonists, haloperidol and (-)-sulpiride (0.5 mg/kg and 50 mg/kg i.p., respectively, daily, for 30 days). The stimulant effect of both neuroleptics on striatal dopamine release persisted unaltered throughout the chronic treatment (by about 100% over basal values). In contrast, the enhancing effects of haloperidol and (-)-sulpiride on striatal acetylcholine release remained unchanged up to day 12 of treatment. Thereafter, tolerance developed, so that both neuroleptics became totally ineffective on day 30 of treatment. Both on day 1 and 30, the neuroleptic-induced dopamine release was reversed by gamma-butyrolactone (gamma-hydroxybutyric acid lactone), suggesting that this effect is mediated by enhanced neuronal activity. On day 1 and day 10, the neuroleptic-induced acetylcholine release was antagonized by the blockade of dopamine D1 receptors with SCH 39166 (trans-(-)-(6aS,13bR)-11-chloro-6,6a,7,8,9,13b- hexahydro-7-methyl-5H-benzo[d]napht[2,1-b]azepine-12-ol, hydrochloride) (0.5 mg/kg i.p.). SKF 38393 (1-phenyl-2,3,4,5-tetrahydro-(1H)-3- benzazepine-7,8-diol hydrochloride) (5 mg/kg i.p.) increased acetylcholine release by about 50% in control rats and in rats treated with (-)-sulpiride or haloperidol for up to 7 days.(ABSTRACT TRUNCATED AT 250 WORDS)

A quantitative estimate of the role of striatal D-2 receptor proliferation in dopaminergic behavioral supersensitivity: the contribution of mesolimbic dopamine to the magnitude of 6-OHDA lesion-induced agonist sensitivity in the rat

Rats with unilateral depletions of neostriatal dopamine display increased sensitivity to dopamine agonists estimated to be 30 to 100 x in the 6-hydroxydopamine (6-OHDA) rotational model. Given that mild striatal dopamine D-2 receptor proliferation occurs (20-40%), it is difficult to explain the extent of behavioral supersensitivity by a simple increase in receptor density. This study was designed to investigate the quantitative aspects of the rotational behavior model utilizing constrained non-linear curve fitting routines. A dose-response curve for the rotational response arising from apomorphine stimulation of the normosensitive striatum was obtained in animals bearing unilateral lesions of striatal efferents (predominantly the striato-nigral pathway as previously described). After the control dose-response experiment, rats received a dopamine- (DA) depleting lesion in the contralateral hemisphere. In one group, 6-OHDA was infused into the medial forebrain bundle (MFB), a placement which is common in the literature and is known to deplete DA in both the striatum and nucleus accumbens. In a second group of rats, 6-OHDA was infused into the globus pallidus at a site which depletes caudate DA, but leaves n. accumbens DA relatively intact. The two experimental groups were tested in identical apomorphine-induced rotation dose-response experiments. The ED50's of the MFB- and caudate-lesioned rats were reduced by 36 and 5.8 fold, respectively, as compared to the control dose-response curve. The MFB and caudate lesions depleted striatal DA and produced a 30 and 36% increase in striatal D-2 binding sites, respectively. Modeling the behavioral and biochemical data with the null model for receptor occlusion indicated that increased striatal D-2 receptor density could account for the magnitude of behavioral supersensitivity in neither the MFB-lesioned group, nor even in the caudate-lesioned group. Thus simple up-regulation or D-2 receptors is unlikely to account for supersensitization as measured in the rotational model. Further, we suggest that quantitative modeling of such hypotheses is a valuable experimental technique for assessing relationships between biochemical and behavioral variables.

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.

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.

Haloperidol treatment increases D2 dopamine receptor protein independently of RNA levels in mice

Haloperidol, administered to mice in their drinking water, produced a 21% increase in striatal D2 dopamine receptor density after seven days of continuous exposure. The steady-state D2 receptor RNA prevalence was unaffected by this treatment, yet the RNA coding for preproenkephalin was elevated, as expected. These data indicate that the homologous up-regulation of dopamine receptor density by antipsychotic drugs proceeds by mechanisms other than changes in RNA abundance.

Chronic eticlopride and dopamine denervation induce equal nonadditive increases in striatal D2 receptor density: autoradiographic evidence against the dual mechanism hypothesis

In an attempt to resolve experimental discrepancies regarding the mode of action of D2 receptor regulation following denervation or chronic receptor blockade, rats with extensive unilateral destruction of the mesotelencephalic dopaminergic projections induced by intracerebral 6-hydroxydopamine were injected daily for 21 days with either saline or the potent, selective D2 antagonist eticlopride (0.5 mg/kg, i.p.). Four days after the last injection of eticlopride or saline, rats were killed, and brain sections through the caudate-putamen and nucleus accumbens septi were incubated with [3H]spiroperidol or (R)-(+)-8-chloro-2,3,4,5-tetrahydro-3-methyl-5- phenyl-1H-3-benzazepin-7-ol ([3H]SCH 23390) to assay D2 and D1 receptors, respectively. Autoradiographic analysis revealed that chronic eticlopride treatment increased the density of D2 sites in the intact hemisphere for all regions examined without further augmenting the already increased density of D2 receptors seen in the dopamine-denervated hemisphere. D2 receptor density was independent of functional sensitivity as evidenced by the fact that rats treated chronically with eticlopride rotated contralateral to the 6-hydroxydopamine lesion following systemic administration of the selective D2 agonist quinpirole during the neuroleptic wash-out period, despite the fact that D2 receptor binding was not significantly different in the left and right hemispheres of these subjects. D1 receptor density was not affected by eticlopride treatment but was significantly reduced reduced in the dopamine-denervated hemisphere. [3H]Mazindol labeling of high-affinity DA uptake sites indicated that the extent of dopamine denervation was greater than 97% in both saline- and eticlopride-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-additivity of D2 receptor proliferation induced by dopamine denervation and chronic selective antagonist administration: evidence from quantitative autoradiography indicates a single mechanism of action

Experiments were conducted to investigate whether chronic dopamine (DA) D2 receptor blockade and DA denervation exert additive effects on striatal D2 receptor density. We employed for the first time chronic treatment with a pure D2 antagonist, metoclopramide, and measured regional striatal DA receptor binding with quantitative receptor autoradiography. Rats with extensive unilateral DA denervation induced by intracerebral 6-hydroxydopamine (6-OHDA) were injected daily for 21 days with either metoclopramide (30 mg/kg i.p.) or saline. Following a 72-h drug wash-out period, rats were sacrificed and brain sections through the caudate-putamen and nucleus accumbens were incubated with [3H]spiroperidol or [3H]SCH 23390 to assay D2 and D1 receptors, respectively. Autoradiographic analysis revealed that chronic metoclopramide treatment increased the density of D2 sites in the intact hemisphere for all regions examined without further augmenting the already increased density of D2 receptors seen in the 6-OHDA-treated hemisphere. In addition, chronic metoclopramide and 6-OHDA treatment by themselves exhibited remarkably parallel anterior-posterior gradients in their effects on D2 receptor density. D1 receptor density was not affected by metoclopramide treatment but was slightly reduced in the DA-denervated hemisphere. [3H]Mazindol labelling of high-affinity DA uptake sites indicated that the extent of DA denervation was greater than 98% in both saline- and metoclopramide-treated rats. These findings are consistent with the view that chronic D2 receptor blockade and DA denervation act via a single, common mechanism to increase D2 receptor density. Work from other laboratories, in which additive effects of denervation and chronic neuroleptic treatment have been purported, may have resulted from incomplete denervation. Experimental discrepancies may also be due to differing means by which the mesotelencephalic dopaminergic neurons are injured.

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.

The absolute density of neurotransmitter receptors in the brain. Example for dopamine receptors

Since the absolute density of dopamine receptors can vary in disease, it is essential to establish the normal values for the absolute densities of D1 and D2 receptors in the brain. Absolute densities are most conveniently reported in units of picomoles per gram of original tissue, readily permitting their comparison to data obtained by positron emission tomography in patients. The density of D1 receptors is approximately 120 pmol/g in the rat striatum and 19 pmol/g in the human striatum. The density of D2 receptors is about 32 pmol/g in the rat striatum and 14 pmol/g in the human striatum, these values being determined by Teflon-glass homogenization and the centrifugation method. The customary Polytron-homogenization procedure results in a loss of about 9% of the D2 receptors in rat tissue and about 28% in human tissues; filtration results in a further loss of about 12%. There is general agreement between the in vitro and in vivo densities, but only if the receptors are measured by the amount of radioisotope specifically displaced.

Relationship between dopamine receptor occupation by spiperone and acetylcholine levels in the rat striatum after long-term haloperidol treatment depends on dopamine innervation

The effect of chronic neuroleptic treatment on the relationship between the blockade of dopamine (DA) receptors by the neuroleptic drug spiperone and the decline in acetylcholine (ACh) levels was determined in the rat striatum in vivo. In rats, a unilateral lesion of the nigrostriatal pathway was produced with 6-hydroxydopamine. The rats were treated for 6 weeks with haloperidol (twice a day at 1 mg kg-1). Partial and complete receptor occupation was determined with radioactive spiperone (a D2 antagonist), given in various doses of different specific activity 2 h before death. ACh, choline, and radioactivity contents were measured in the same striatum. Following long-term haloperidol treatment, an increase in the maximal number of binding sites for spiperone was found. Virtually identical negative (linear) correlations between striatal ACh content and the number of receptors occupied by spiperone were found in saline- or subchronic haloperidol-treated rats when DA innervation was intact. The slope of the line describing the decrease in ACh content per occupied receptor, however, was much lower in haloperidol-treated rats than in saline-treated animals. After lesioning of the dopaminergic pathway, there was no longer a correlation between the receptor occupation and ACh levels in the striatum. These results show that receptor occupation by a neuroleptic correlates highly with function only when dopaminergic innervation is intact. Also, it appears that there is no fixed number of striatal ACh molecules per DA receptor, and, finally, that in vivo receptor detection methods distinguish differences in receptor density (as do in vitro techniques).

Reserpine-induced up-regulation of dopamine D2 receptors in the rat striatum is enhanced by denervation but not by chronic receptor blockade

Compensatory increases in the density of dopamine (DA) D2 receptors in the rat striatum occur following chronic interruption of dopaminergic neurotransmission. Substantia nigra lesions, DA depletion with reserpine and D2 receptor blockade by neuroleptics increase the number of striatal D2 receptors as identified with the D2 ligand, [3H]spiperone [( 3H]SPIP). Chronic administration of haloperidol to substantia nigra-lesioned rats causes an additive increase in binding over levels obtained with one treatment alone. In this study we have found a similar response when lesioned animals are treated with reserpine. However, compensatory increases in the number of [3H]SPIP binding sites found after combined administration of reserpine and haloperidol to intact rats do not exceed levels obtained following administration of either drug alone. The data suggest that up-regulation of striatal D2 binding sites occurring after substantia nigra lesions is unique relative to other forms of up-regulation and may involve the loss of a presynaptic regulatory factor other than DA.

Gamma-aminobutyric acid and benzodiazepine receptor changes induced by unilateral 6-hydroxydopamine lesions of the medial forebrain bundle

Quantitative autoradiography was used to ascertain alterations in [3H]muscimol, [3H]flunitrazepam (FLU), [3H]naloxone, [3H]D-alanine-D-leucine-enkephalin (DADL), and [3H]spiroperidol binding in basal ganglia 1 week, 4 weeks, and 5 months after unilateral 6-hydroxydopamine lesions of the medial forebrain bundle (MFB) in the rat. At 1 and 4 weeks following lesions, [3H]spiroperidol binding increased 33% in striatum. At 5 months, [3H]spiroperidol was only nonsignificantly increased above control. At 1 week, [3H]muscimol binding decreased 39% in ipsilateral globus pallidus (GP), but increased 41% and 11% in entopeduncular nucleus (EPN) and substantia nigra pars reticulata (SNr), respectively. At 4 weeks, [3H]muscimol binding was reduced 19% in striatum and 44% in GP and remained enhanced by 32% in both EPN and SNr. These changes in [3H]muscimol binding persisted at 5 months. [3H]FLU binding was altered in the same direction as [3H]muscimol binding; however, changes were slower in onset and became significant (and remained so) only at 4 weeks after lesions. Decreases in [3H]naloxone and [3H]DADL binding were seen in striatum, GP, EPN, and SNr. Scatchard analyses revealed that only receptor numbers were altered. This study provides biochemical evidence for differential regulation of striatal GABAergic output to GP and EPN/SNr.

Dopamine receptor agonists: mechanisms underlying autoreceptor selectivity. II. Theoretical considerations

In a companion article, we extensively reviewed the pharmacological actions of the enantiomers of the dopamine analogue 3-(3-hydroxyphenyl)-N-n-propylpiperidine, 3-PPP. The profiles of action exhibited by transdihydrolisuride (TDHL) and the trans-fused 7-OH-1,2,3,4,4a,5,6,10-octahydrobenzo(f)quinoline (HW 165) were also described. These latter agents, along with (-)-3-PPP, exert a variety of effects at different DA receptors depending on the anatomical location of these receptor sites and the experimental conditions. In the first part of the present article, it is suggested that the intrinsic activity of these agents in different pharmacological models is dependent on the responsiveness of the relevant DA-receptors which, in turn, is related to the degree of previous agonist occupancy of these sites. In situations where these agents exhibit partial agonist activity, their pharmacological effect is also dependent on the relative concentrations of drug and endogenous DA competing for common receptor sites. A number of theoretical implications will be discussed relevant to the suggestion that DA receptors exist in various adaptational states which can influence drug action. In the second part of this review, we will consider the behavioural profile exhibited by (-)-3-PPP in relation to that observed with classical DA antagonists. In addition, the potential clinical application of (-)-3-PPP and similar-acting agents will be discussed.