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

The neuropathology of Self-Injurious Behavior: Studies using animal models

Self-injurious behavior is a debilitating characteristic that is highly prevalent in autism and other neurodevelopmental disorders. In these populations, self-injury has typically been interpreted in relation to behavioral reinforcement and/or sensory stimulation. However, self-injury is also commonly exhibited by people with a variety of neuropsychiatric disorders, where it is typically described in relation to emotional regulation and the presence or absence of suicidal ideation. Interestingly, self-injury has also been documented in many non-human animal species, especially when exposed to early environmental deprivation, isolation, and distress. Despite the propensity of animals to self-injure under adverse conditions, animal models of self-injury have not been the focus of much research, and translation of the data from these models has largely been limited to autism and neurodevelopmental disorders. This review summarizes evidence that common biological and environmental mechanisms may contribute to vulnerability for self-injury in neurodevelopmental disorders, psychiatric disorders, and distressed animals, and that investigations using animal models may be highly beneficial when considering self-injury as a behavioral phenotype that exists across diagnostic categories. Investigations using animal models have revealed that individual differences in stress responses and anxiety-related behavior contribute to vulnerability for self-injury. Animal models have implicated dysregulation of monoaminergic, glutamatergic, and other neurotransmitter systems in expression of self-injury, and these models have suggested neural targets for pharmacotherapy that have potential relevance for diverse clinical populations.

Animal Models of Self-Injurious Behavior: An Update

Although self-injurious behavior is a common comorbid behavior problem among individuals with neurodevelopmental disorders, little is known about its etiology and underlying neurobiology. Interestingly, it shows up in various forms across patient groups with distinct genetic errors and diagnostic categories. This suggests that there may be shared neuropathology that confers vulnerability in these disparate groups. Convergent evidence from clinical pharmacotherapy, brain imaging studies, postmortem neurochemical analyses, and animal models indicates that dopaminergic insufficiency is a key contributing factor. This chapter provides an overview of studies in which animal models have been used to investigate the biochemical basis of self-injury and highlights the convergence in findings between these models and expression of self-injury in humans.

Loss of Striatonigral GABAergic Presynaptic Inhibition Enables Motor Sensitization in Parkinsonian Mice

Degeneration of dopamine (DA) neurons in Parkinson's disease (PD) causes hypokinesia, but DA replacement therapy can elicit exaggerated voluntary and involuntary behaviors that have been attributed to enhanced DA receptor sensitivity in striatal projection neurons. Here we reveal that in hemiparkinsonian mice, striatal D1 receptor-expressing medium spiny neurons (MSNs) directly projecting to the substantia nigra reticulata (SNr) lose tonic presynaptic inhibition by GABAB receptors. The absence of presynaptic GABAB response potentiates evoked GABA release from MSN efferents to the SNr and drives motor sensitization. This alternative mechanism of sensitization suggests a synaptic target for PD pharmacotherapy.

Brain regions and genes affecting myoclonus in animals

Myoclonus is defined as large-amplitude rhythmic movements. Brain regions underlying myoclonic jerks include brainstem, cerebellum, and cortex. Gamma-aminobutyric acid (GABA) appears to be the main neurotransmitter involved in myoclonus, possibly interacting with biogenic amines, opiates, acetylcholine, and glycine. Myoclonic jumping is a specific subtype seen in rodents, comprising rearing and hopping continuously against a wall. Myoclonic jumping can be seen in normal mouse strains, possibly as a result of simply being put inside a cage. Like other types, it is also triggered by changes in GABA, 5HT, and dopamine neurotransmission. Implicated brain regions include hippocampus and dorsal striatum, possibly with respect to D(1) dopamine, NMDA, and δ opioid receptors. There is reason to suspect that myoclonic jumping is underreported due to insufficient observations into mouse cages.

Animal models of self-injurious behaviour: an overview

Self-injurious behaviour is highly prevalent in neurodevelopmental disorders. Interestingly, it is not restricted to any individual diagnostic group. Rather, it is exhibited in various forms across patient groups with distinct genetic defects and classifications of disorders. This suggests that there may be shared neuropathology that confers vulnerability. Convergent evidence from clinical pharmacotherapy, brain imaging studies, postmortem neurochemical analyses, and animal models indicates that dopaminergic insufficiency is a key culprit. This chapter provides an overview of studies in which animal models have been used to investigate the biochemical basis of self-injury, and highlights the convergence in findings between these models and expression of self-injury in humans.

Persistent alterations in dendrites, spines, and dynorphinergic synapses in the nucleus accumbens shell of rats with neuroleptic-induced dyskinesias

Chronic treatment of humans or experimental animals with classical neuroleptic drugs can lead to abnormal, tardive movements that persist long after the drugs are withdrawn. A role in these neuroleptic-induced dyskinesias may be played by a structural change in the shell of the nucleus accumbens where the opioid peptide dynorphin is upregulated in treated rats that show vacuous chewing movements (VCMs). The shell of the nucleus accumbens normally contains a dense plexus of dynorphinergic fibers especially in its caudomedial part. After 27 weeks of haloperidol administration and 18 weeks of withdrawal, the immunoreactive labeling of this plexus is intensified when compared with that after vehicle treatment. In addition, medium spiny neurons here show a significant increase in spine density, dendritic branching, and numbers of terminal segments. In the VCM-positive animals, the dendritic surface area is reduced, and dynorphin-positive terminals contact more spines and form more asymmetrical specializations than do those in animals without the syndrome (VCM-negative and vehicle-treated groups). Persistent, neuroleptic-induced oral dyskinesias could therefore be caused by incontrovertible alterations, involving terminal remodeling or sprouting, to the synaptic connectivity of the accumbal shell.

Dopamine D(1A) receptor function in a rodent model of tardive dyskinesia

Tardive dyskinesia develops as a common complication of long-term neuroleptic use. The emergence of such dyskinesias may reflect a shift in the balance of dopamine D(1) and D(2) receptor-mediated activity, with a relative increase in activity in the D(1) receptor-regulated direct striatonigral pathway. In rats, chronic treatment with the antipsychotic fluphenazine triggers a syndrome of vacuous chewing movements, which are attenuated by dopamine D(1) receptor antagonists. A similar syndrome can be seen in drug-naive animals following acute administration of selective dopamine D(1) receptor agonists. However, not all dopamine D(1) receptor agonists elicit these mouth movements. Thus, some investigators have suggested the existence of novel subtypes of the dopamine D(1) receptor. In these studies, we sought to clarify the role of the dopamine D(1A) receptor in vacuous chewing movements induced both by the selective dopamine D(1) receptor agonist SKF 38393, as well as by chronic neuroleptic administration, using in vivo oligonucleotide antisense to dopamine D(1A) receptor messenger RNA. Intrastriatal antisense treatment significantly and selectively attenuated striatal dopamine D(1) receptor binding, accompanied by reductions in SKF 38393- and chronic fluphenazine-induced vacuous chewing movements. These findings suggest that the dopamine D(1A) receptor plays an important role in the expression of vacuous chewing movements in a rodent model of tardive dyskinesia and may contribute to the pathogenesis of the human disorder. This may have important implications for the treatment of tardive dyskinesia in humans.

Super-stereotypy II: enhancement of a complex movement sequence by intraventricular dopamine D1 agonists

This study compared the effect of intraventricular administration of dopamine D1 or D2 agonists or of ACTH on the sequential stereotypy of a serial pattern of grooming movements ("syntactic chain"). In a previous study, we showed that peripheral administration of D1 agonists increased the probability of occurrence and enhanced the stereotypy of the already-stereotyped movement pattern. Here we made microinjections of either SKF 38393 (a partial D1 agonist; 5, 10, 15, 20, 40 microg), SKF 82958 (a full D1 agonist; 5, 10, 20 microg), quinpirole (a D2 agonist; 5, 10, 20 microg), or ACTH-(1-24) (2, 5, 10 microg) into the lateral ventricles of rats. We measured the amount of grooming, the relative probability that the complex sequence pattern would occur, and the degree to which the syntactic pattern was completed faithfully. The total amount of grooming behavior was increased by intraventricular SKF 82958 and by ACTH, but was not changed by SKF 38393 and was decreased by quinpirole. Super-stereotypy of the sequential pattern was produced only by dopamine D1 agonists. The relative probability of initiating the syntactical sequence was increased by both SKF 38393 and SKF 82958, but was reduced by quinpirole and ACTH. The full D1 agonist, SKF 82958, also increased the likelihood that the pattern would be completed, thus causing sequential super-stereotypy in the strongest sense. Our results highlight a role for dopamine D1 receptors, probably within the basal ganglia, in the production of sequential super-stereotypy of complex behavioral patterns.

Action sequencing is impaired in D1A-deficient mutant mice

The role of dopamine in the production of behaviour is multifarious in that it can influence different aspects of movement (e.g. movement initiation, sensorimotor integration, and movement sequencing). A characteristic of the dopamine system which seems to be critical for the expression of this diverse influence is its varied receptor population. Previous studies have shown that specific receptor subtype activation leads to specific behavioural responses or alterations of selective aspects of movement. It is known that one of the important influences of dopamine includes sequential co-ordination of 'syntactic' patterns of grooming movements because moderate loss of the dopaminergic nigrostriatal projections specifically disrupts these patterns without affecting grooming actions in a general fashion (Berridge, K.C. Psychobiology, 15, 336, 1989). The specific receptors of the dopamine family which play a key part in this co-ordination of movement sequences is not known. In the present study, we examined the serial order of particular syntactic sequences or chains of grooming actions in mice lacking D1A receptors to explore the relationship between this receptor subtype and movement sequencing. Mutant mice had shorter grooming bouts and a disruption of the organization of sequential patterns compared with wild-type littermate controls. Sequential disruption was reflected in the failure of D1A mutants to follow the syntactic pattern of grooming to completion. This sequential disruption deficit appeared to be specific, as mutant mice initiated more syntactic chains than wild-type controls even though they were less likely to complete them. These results support the hypothesis that D1A receptor activation plays a part in the sequencing of natural action. This conclusion has important implications for the understanding of the functional heterogeneity of dopamine receptor subtypes and of the aetiology of symptoms observed in patients with basal ganglia disease.

Behavioral effects of clozapine and dopamine receptor subtypes

The atypical neuroleptic clozapine (CLZ) is an extremely effective antipsychotic that produces relatively few motoric side effects. However, CLZ displays limited antagonism at the dopamine (DA) D2 receptor, the receptor commonly thought to mediate the antipsychotic activity of neuroleptics. The mechanism of action behind the efficacy of CLZ remains to be determined. Miller, Wickens and Beninger [Progr. Neurobiol., 34, 143-184 (1990)] propose a "D1 hypothesis of antipsychotic action" that may explain the antipsychotic effects of CLZ. This hypothesis is built on the interactions between D2, cholinergic and D1 mechanisms in the striatum. These authors assert that although typical neuroleptics block D2 receptors, it is through an indirect action on D1 receptors that their antipsychotic action is manifest. The extra-pyramidal side effects produced by typical neuroleptics are hypothesized to be due to an indirect action on cholinergic receptors. It is argued that the anticholinergic properties of CLZ negate the D2 (motor side effects) action of CLZ, allowing CLZ to diminish psychotic symptoms through a direct action on D1 receptors. Thus, CLZ may function as a D1 receptor antagonist in behavioral paradigms. The current paper reviews and compares the behavioral profile of CLZ to those produced by D2- and D1-selective antagonists with specific reference to unconditioned and conditioned behaviors in order to more fully evaluate the "D1 hypothesis of CLZ action". Although the actions of CLZ remain unique, they do share some striking similarities with D1 receptor antagonists especially in tests of unconditioned behavior, possibly implicating the D1 receptor in the action of this antipsychotic drug.

Intrastriatal AP5 differentially affects behaviors induced by local infusions of D1 vs. D2 dopamine agonists

Using bilateral infusions into the rat striatum, the effects of the competitive NMDA receptor antagonist 2-amino-5-phosphopentanoic acid (AP5) on behaviors induced by the dopamine (DA) D1 receptor agonist SKF 82526 (fenoldopam) or the D2 receptor agonist quinpirole were determined. These effects were tested in DA-replete (intact) rats and in rats that were receiving injections of the monoamine-depleting drug reserpine. In both intact and reserpinized rats, fenoldopam induced significant sniffing. This effect was attenuated by simultaneous co-infusion of AP5 in the reserpinized rats. Quinpirole induced locomotion, sniffing, and oral behaviors, all of which were attenuated by AP5 co-infusion in the intact rats. In contrast, AP5 enhanced the quinpirole-induced sniffing of reserpinized rats. These findings suggest that distinct D1/glutamate and D2/glutamate relationships exist in the striatum, and that the nature of the latter is influenced by DA tone.

Enhanced oral activity responses to intrastriatal SKF 38393 and m-CPP are attenuated by intrastriatal mianserin in neonatal 6-OHDA-lesioned rats

Enhanced oral activity is induced in neonatal 6-hydroxydopamine- (6-OHDA-) lesioned rats by systemic administration of the dopamine (DA) D1 receptor agonist SKF 38393 and serotonin (5-HT) 5-HT2A,2C agonist m-chlorophenylpiperazine (m-CPP). The DA D1 receptor antagonist SCH 23390 effectively attenuates the effect of SKF 38393 but not m-CPP. The 5-HT2 antagonist mianserin attenuates the effects of both m-CPP and SKF 38393, suggesting that DA agonist effects are mediated by 5-HT neurochemical systems. To test whether DA and 5-HT agonist effects and interactions might occur within the neostriatum, rats were implanted with permanent injection cannulae, with tips in the ventral striatum. One group of rats was lesioned at 3 days after birth with 6-OHDA HBr (100 micrograms salt form, in each lateral ventricle; desipramine HCl pretreatment, 20 mg/kg IP, base form, 1 h), while controls received the vehicle in place of 6-OHDA. Cannulae were implanted when rats weighed 200-250 g. During a 1-h observation session SKF 38393 (5 nmol per side) produced 74.3 +/- 19.2 oral movements in intact rats and 310.7 +/- 97.0 oral movements in 6-OHDA-lesioned rats. m-CPP (10 nmol per side) produced 72.6 +/- 15.1 and 274.5 +/- 65.0 oral movements in these respective groups. These responses were several-fold greater than the 25.3 +/- 7.3 and 41.8 +/- 9.5 oral movements in the same groups after saline (0.5 microliter per side) (P < 0.05). Mianserin (6 nmol per side) alone had no effect on oral activity but attenuated responses to both SKF 38393 and m-CPP in intact and 6-OHDA-lesioned rats.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Anatomical localization of SKF-38393-induced behaviors in rats using the irreversible monoamine receptor antagonist EEDQ

This study was designed to localize the population of dopamine D1-like receptors involved in grooming and oral movements elicited by systemic administration of the D1-selective agonist SKF-38393. Receptors in specific dopamine terminal regions were inactivated by intracranial injection of the nonselective irreversible antagonist N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ). The effect of these injections on behaviors induced by systemic administration of SKF-38393 (10 mg/kg) was measured 48 hours later. The specific populations of D1-like receptors inactivated by the EEDQ injections were identified as a loss of 3H-SCH-23390 binding in a given region using quantitative autoradiography. EEDQ (1.5 micrograms/microliters/side) injected into the nucleus accumbens (NAc) did not alter SKF-38393-induced behaviors. Similarly, injection of EEDQ into the medial caudate-putamen (CPu) failed to alter these behaviors. In contrast, EEDQ (0.15-1.5 micrograms/microliters/side) injected into the lateral CPu decreased both SKF-38393-induced grooming and oral movements, with complete blockade of grooming observed at the highest dose. To determine whether this effect of EEDQ was due to inactivation of D1-like receptors, separate groups of animals were pretreated with SCH-23390 (3 mg/kg, S.C.) 15 min prior to injection with EEDQ. Pretreatment with SCH-23390 prevented the disruption of SKF-38393-induced behaviors, as well as the loss of 3H-SCH-23390-labeled binding sites observed after injection of EEDQ into the lateral CPu. EEDQ injections that produced disruption of SKF-38393-induced behaviors were associated with a greater loss of binding in the lateral CPu relative to other regions examined including the NAc, medial CPu, and globus pallidus. Furthermore, EEDQ injections that produced the greatest loss of 3H-SCH-23390 binding in the latter three regions did not disrupt SKF-38393-induced behavior. These results demonstrate that stimulation of D1-like receptors in the lateral CPu is necessary for behaviors induced by systemic administration of SKF-38393. The results also demonstrate the utility of this "receptor lesion" technique to localize receptor-mediated behaviors.

Alterations in mRNA levels of D2 receptors and neuropeptides in striatonigral and striatopallidal neurons of rats with neuroleptic-induced dyskinesias

Chronic neuroleptic treatment in rat produces vacuous chewing movements (VCMs), analogous to TD in humans. We hypothesized that these hyperkinetic movements were due to alterations in striatonigral and striatopallidal GABAergic spiny II neurons. Rats were treated for 36 weeks with haloperidol decanoate and withdrawn for 28 weeks. Striatonigral and striatopallidal neurons were assessed using in situ hybridization histochemistry for mRNA levels of D1 and D2 dopamine receptors, preproenkephalin (ENK), prodynorphin (DYN), protachykinin (substance P), and glutamate decarboxylase (GAD67) in the dorsolateral and ventromedial striatum as well as the nucleus accumbens. Rats that did not develop VCMs (-VCM) had increased D2 receptor and DYN mRNA, and reduced substance P mRNA in the dorsolateral striatum. Rats with persistent VCMs (+VCM) had increased D2 receptor, ENK, and DYN mRNA in both striatal regions, and increased ENK and DYN mRNA in the nucleus accumbens, compared with controls. Relative to -VCM rats, however, +VCM rats only had increased ENK mRNA in the nucleus accumbens. Considering the overall pattern of mRNA changes, the data suggest that alterations in both the D1-mediated striatonigral and the D2-mediated striatopallidal pathways play a role in the expression of the VCM syndrome. To the extent that gene expression parallels changes in neuronal activity, this implies that the VCM syndrome is associated with increased activity in both pathways.

Intracellularly recorded response of rat striatal neurons in vitro to fenoldopam and SKF 38393 following lesions of midbrain dopamine cells

The effect of long-term (6-19 weeks) 6-hydroxydopamine-induced (6-OHDA) lesions of midbrain dopamine cells on dopamine D1-like agonist-induced changes in the excitability of rat striatal neurons was investigated in vitro using tissue slices and intracellular recording techniques. Fenoldopam and (+/-)-SKF 38393 predominantly decreased excitability in control preparations including striatal neurons located contralateral to 6-OHDA injection sites and neurons obtained from rats receiving sham injections or no treatment. Fenoldopam also inhibited neurons ipsilateral to lesions of midbrain dopamine cells. (+/-)-SKF 38393, unlike fenoldopam, produced predominantly increases in the excitability of ipsilateral striatal neurons. Superfusion of the D1 receptor antagonist, SCH 23390, blocked fenoldopam-induced decreases in excitability but not the (+/-)-SKF 38393-induced excitation of neurons ipsilateral to the lesion. Sequential application of fenoldopam and quinpirole, a D2/D3 receptor agonist, produced responses to both drugs in a majority of neurons. The results demonstrate that inhibitory responses to fenoldopam are mediated by D1 receptors, while excitatory effects of (+/-)-SKF 38393 in the striatum ipsilateral to the lesion are apparently not dependent on D1 receptor activation. These findings also suggest that dopamine D1 and D2/D3 receptors are able to concurrently influence the excitability of striatal neurons in the dopamine deafferentated striatum. Similar regulation of striatal neurons in vivo may contribute to dopaminergic regulation of basal ganglia output and the ability of dopaminomimetic agents to ameliorate symptoms of dopaminergic deficiency in Parkinson's disease.

Dopamine D1 receptor agonist-induced grooming is blocked by the opioid receptor antagonist naloxone

The effects of the opioid receptor antagonist naloxone on behavioural responses to the dopamine D1 receptor agonist SKF 38393 ((+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrochloride) were assessed in the rat. SKF 38393 (5 mg/kg s.c.) induced grooming and vacuous chewing mouth movements. SKF 38393-induced grooming was dose-dependently attenuated by naloxone (0.375-1.5 mg/kg s.c), while vacuous chewing movements were unaffected. These findings suggest that dopamine D1 receptor agonist-induced grooming is dependent upon opioid systems, while vacuous chewing movements are likely to be mediated via different pathways.

Dopamine receptors: molecular biology, biochemistry and behavioural aspects

The description of new dopamine (DA) receptor subtypes, D1-(D1 and D5) and D2-like (D2A, D2B, D3, D4), has given an impetus to DA research. While selective agonists and antagonists are not generally available yet, the receptor distribution in the brain suggests that they could be new targets for drug development. Binding characteristics and second messenger coupling has been explored in cell lines expressing the new cloned receptors. The absence of selective ligands has meant that in vivo studies have lagged behind. However, progress has been made in understanding the function of DA-containing discrete brain nuclei and the functional consequence of the DA's interaction with other neurotransmitters. This review explores some of the latest advances in these various areas.

Grafts in the treatment of Parkinson's disease: animal models

Long-term treatment of parkinsonian patients with L-DOPA leads to a loss of efficacy over time and the appearance of important side effects such as dyskinesias. Grafts of chromaffin cells of the adrenal medulla or fetal ventral mesencephalic neurons bring behavioral improvement in animal models of Parkinson's disease. These improvements are likely to be related to the secretion of dopamine by the grafted cells and/or to the reinnervation of the host tissue. In addition, a leak in the blood-brain barrier may allow peripheral catecholamines to gain access to the brain. Lack of clear effects of grafts in parkinsonian patients may be due to their poor survival in the human brain. Improvement of grafting techniques as well as the addition of neurotrophic factors to grafts may help increase their survival and improve behavioral effects. Recently, genetic techniques have allowed the creation of genetically modified cell lines which can produce L-DOPA and these cells may be grafted in the brain. Interestingly, these cell lines may be encapsulated in permselective membranes which can protect them from immunological rejection and avoid the uncontrolled cell growth of these mitotically active cells. Grafting techniques seem to be an interesting alternative to treat parkinsonian patients. Improvement of grafting procedures may help increase survival of grafts and thus enhance behavioral improvements. Moreover, genetic modification of well-known tumor cell lines or patient's own cells such as astrocytes may help avoid the low availability as well as ethical and immunological problems linked to the use of fetal human tissue.

Putative Neurotoxicity of SKF 38393 and Other D1Dopaminergic Drugs Investigated in Rat Striatum

The recently alleged neurotoxicity of the D1 receptor agonist, SKF 38393, was investigated in rat striatum by measuring the enzymes acetylcholinesterase (AChE) and glutamate decarboxylase (GAD). First, unilateral intrastriatal microinjection of the excitotoxin kainic acid (2 micrograms in 1 microliter) was shown to evoke vigorous contraversive circling, followed 1 or 2 weeks later by profound decreases in striatal AChE (24 and 54%), GAD (51 and 75%), and protein (36 and 47%), as well as loss of GAD (45% at 2 weeks) in the ipsilateral substantia nigra. Similar striatal treatments with SKF 38393 (30 micrograms in 0.5-1 microliter), the related benzazepines SKF 82526 (D1 agonist, 30 micrograms in 1 microliter) and SCH 23390 (D1 antagonist, 5 micrograms in 1 microliter), or the phenanthridine D1 agonist CY 208-243 (5 micrograms in 1 microliter) failed to affect the rats' behaviour or their striatal levels of AChE, GAD, and protein. Intrastriatal SKF 38393 (30 micrograms in 0.5 microliter) also had no influence on these enzymes in the substantia nigra. It is concluded that none of the D1 dopaminergic compounds examined here was neurotoxic toward the many different cell groups that contain AChE and/or GAD in the striatum.

Differential modulation of (+)-amphetamine-induced rotation in unilateral substantia nigra-lesioned rats by alpha 1 as compared to alpha 2 agonists and antagonists

In rats sustaining unilateral 6-hydroxy-dopamine lesions of the substantia nigra (SN), the indirect dopaminergic agonist, (+)-amphetamine (AMPH), dose-dependently induced robust, ipsilateral rotation: this could be dose-dependently abolished by the dopamine (D2/D1) antagonist, haloperidol. The selective alpha 1 antagonist, prazosin, dose-dependently attenuated the action of AMPH though rotation was not completely abolished. In the presence of a constant dose of prazosin, the dose-response curve for induction of rotation by AMPH was shifted to the right. The action of prazosin was mimicked by a further alpha 1 antagonist, corynanthine. In contrast, the selective alpha 1 agonist, ST 587, potentiated the rotation evoked by AMPH. The selective alpha 2 antagonist, idazoxan, dose-dependently potentiated the action of AMPH and, in the presence of a constant dose of idazoxan, the dose-response curve for AMPH was shifted to the left. This effect of idazoxan was mimicked by a further alpha 2 antagonist, yohimbine. In distinction, the selective alpha 2 agonist, UK 14,304, dose-dependently attenuated the action of AMPH, an action mimicked by the alpha 2 partial agonist, clonidine. Upon administration alone, the above mentioned drugs did not induce rotation. The data indicate that activation and antagonism of alpha 1 receptors enhance and inhibit rotation, respectively, whereas activation and antagonism of alpha 2 receptors inhibit and enhance rotation, respectively. These findings demonstrate an opposite alpha 1 and alpha 2 receptor-mediated control of rotation in this model. They suggest that an increase and decrease in noradrenergic tone, respectively, facilitate and inhibit locomotor activity controlled via the nigro-striatal dopaminergic pathway. The possible relevance of these findings to Parkinson's disease is discussed.

Regional changes of striatal dopamine receptors following denervation by 6-hydroxydopamine and fetal mesencephalic grafts in the rat

Young adult female rats received a 6-hydroxydopamine lesion in the left substantia nigra and, 3 weeks later, some of them were grafted with a cell suspension from the ventral mesencephalon of rat embryos (14-15 days old). Six months after transplantation, some grafted rats, following injection of amphetamine, had switched to turning only toward the intact side (type 1), whereas others turned toward the intact side only during the first half of the test (type 2). Levels of dopamine, dihydroxyphenylacetic acid and homovanillic acid were, respectively, 2%, 15% and 35% of the intact side in the denervated striatum of 6-hydroxydopamine rats. Dopamine concentrations were restored to 13% and 10% of the intact side in the grafted striatum of type 1 and type 2 animals, respectively. Levels of homovanillic acid were unchanged following grafts whereas those of dihydroxyphenylacetic acid increased by 209% and 247% in the grafted striatum of type 1 and type 2 animals, respectively. The ratios of dihydroxyphenylacetic acid/dopamine as well as homovanillic acid/dopamine were low in the intact striatum whereas they increased in the denervated striatum with or without graft. The tyrosine hydroxylase immunoreactivity decreased by about 80% in the denervated striatum of 6-hydroxydopamine rats. In type 1 rats, tyrosine hydroxylase immunoreactivity revealed that the graft was localized in the dorsomedial part of the denervated striatum, whereas in type 2 animals, it was also in the medial striatum but it overlapped the dorsal and ventral parts of it equally. D1 as well as D2 dopamine receptors were measured throughout the striatum (9.0-7.6 rostral-caudal coordinates), by autoradiography, using [3H]SCH 23390 (D1 antagonist) and [3H]spiperone (D2 antagonist) binding. Supersensitive D2 receptors were normalized in the dorso- and ventromedial parts of the grafted striatum. D2 receptor density was higher in type 2 than in type 1 rats, more specifically at 8.6-8.2 rostral-caudal coordinates, where the graft was. D1 receptor supersensitivity was modest compared to D2 receptors in the striatum of 6-hydroxydopamine rats and decreased following grafts. DA receptors changes in the striatum, following fetal mesencephalic grafts, may explain the behavioral recovery seen in grafted rats.

Localized intracaudate dopamine D2 receptor activation during the post-training period improves memory for visual or olfactory conditioned emotional responses in rats

Rats with cannulas aimed at the posteroventral (PV) or ventrolateral (VL) areas of the caudate nucleus were trained on a conditioned emotional response (CER) task. Post-training microinjections of the indirect catecholamine agonist, d-amphetamine (5 micrograms), or of the dopamine D2 receptor agonist, LY171555 (1 microgram), into the PV area improved retention of a CER with a visual CS, but had no effect on a CER with an olfactory CS. Post-training injections of the same two drugs into the VL area improved retention of a CER with an olfactory CS, but had no effect on a CER with a visual CS. Post-training injections of the dopamine D1 receptor agonist, SKF38393 (0.5, 1.0, 2.0 micrograms), into either site had no effects on either CER. These findings suggest that different areas of the caudate nucleus mediate acquisition of CERs with different CSs, possibly implicating the topographically organized corticostriatal innervation in the acquisition of certain types of memories in the caudate nucleus. The findings also suggest that dopamine D2 receptors in the caudate nucleus are involved in the acquisition of these CERs.

Dopaminergic grafts in the striatum reduce D1 but not D2 receptor-mediated rotation in 6-OHDA-lesioned rats

Rats received fetal dopaminergic neuronal grafts in the striatum and/or substantia nigra ipsilateral to a 6-hydroxydopamine (6-OHDA) lesion of the medial forebrain bundle (MFB). Dopaminergic grafts in the striatum substantially and significantly reduced turning elicited by the selective D1 agonist SKF 38393, but did not reduce turning elicited by the selective D2 agonist LY 171555. Thus, reduced turning in such grafted animals in response to non-selective dopaminergic agonists may be the result of diminished D1 supersensitivity. Fetal dopaminergic grafts in the ipsilateral substantia nigra (SN) did not augment the decreases in turning produced by concomitant ipsilateral dopaminergic grafts in the striatum in response to SKF 38393. LY 171555, D-amphetamine or L-DOPA. Dopaminergic grafts in the SN increased, while dopaminergic grafts in the striatum or in striatum and SN decreased, the facilitatory effect of D-amphetamine on rotation elicited by subsequent injection of dopamine agonists.

Neurotoxicity induced by the D-1 agonist SKF 38393 following microinjection into rat brain

Microinjection of the D-1 agonist R-SKF 38393 into rat striatum resulted in extensive neurotoxic damage as revealed by the presence of large lesions upon histological analysis. These lesions were observed following a single injection of the drug into the ventrolateral striatum (30 micrograms/0.5 microliters) or nucleus accumbens (3.0 or 30 micrograms/0.5 microliters). This neurotoxic damage was also observed following microinjection of the inactive isomer S-SKF 38393. The mechanisms underlying these effects are presently unknown. The use of this compound for intracerebral microinjection studies should be re-evaluated. In view of the usefulness of central microinjections in studying the role of D-1 and D-2 receptors in behavior, it is suggested that alternative D-1 agonists, or ways of minimizing the toxicity of SKF 38393, should be considered.

Nigral D1 and striatal D2 receptors mediate the behavioral effects of dopamine agonists

The mediation of behavior by nigral and striatal dopamine (DA) D1 and D2 receptors was investigated in rats that had sustained extensive unilateral 6-hydroxydopamine-induced injury to ascending DA neurons. Selective D1 and D2 agonists and antagonists were injected directly into the DA-denervated substantia nigra pars reticula or the caudate-putamen via a chronically indwelling cannula. Contralateral rotation resulting from unilateral stimulation of supersensitive DA receptors was quantified over 46 min. Intrastriatal apomorphine (5 micrograms) or the selective D2 agonist quinpirole (5 micrograms), but not the selective D1 agonist (+/-)-SKF 38393 (15 micrograms), induced vigorous rotation. The rotation induced by intrastriatal quinpirole was greatly diminished by systemic administration of the selective D2 antagonist eticlopride (0.5 mg/kg, i.p.) and could not be enhanced by additional injection of intrastriatal (+/-)-SKF 38393. Intranigral administration of apomorphine or (+/-)-SKF 38393, but not quinpirole (same doses as above), elicited vigorous rotation. However, the rotation induced by intranigral (+/-)-SKF 38393 could not be blocked by systemic administration of the selective D1 antagonist SCH 23390 (0.5 mg/kg, s.c.), and was mimicked by intranigral (-)-SKF 38393 (15 micrograms), which exhibits 100-fold less activity than the dextrorotatory enantiomer at the D1 receptor. In order to circumvent the problem of this drug's apparent non-D1-mediated action when injected intranigrally, rotation was induced by systemic (+/-)-SKF 38393 (2.0 mg/kg, i.p.) 10 min after intranigral administration of selective antagonists. Intranigral SCH 23390 (10 micrograms), but not eticlopride (10 micrograms), powerfully antagonized the rotation induced by systemic (+/-)-SKF 38393. Conversely, rotation induced by systemic quinpirole (0.5 mg/kg, i.p.) was potently blocked by intrastriatal eticlopride but not SCH 23390. Rotation induced by systemic apomorphine (0.25 mg/kg, i.p.) was not attenuated by either antagonist alone, regardless of intracerebral injection site. The results indicate that both nigral D1 and striatal D2 receptors mediate the behavioral effects of DA agonists. These data may be useful in elucidating the mechanism(s) underlying the D1/D2 synergism observed in neurologically intact animals, as well as in understanding the action of drugs used in the treatment of Parkinson's disease.

Topography of dopamine D-1 and D-2 receptor-mediated rotation after intrastriatal injections of dopamine-related drugs in normosensitive rats

Naive rats were challenged systematically with apomorphine after receiving unilateral dorsal or ventral intracaudate injections of the dopamine D-1 receptor antagonist, SCH23390, or the D-2 receptor antagonist, sulpiride. Sulpiride injections into both the dorsal and ventral striatum induced a robust ipsilateral rotation, while SCH23390 elicited a weaker ipsilateral rotation only on injection into the ventral striatum. Both drugs were ineffective in saline-treated rats, although sulpiride injections into the ventral striatum after systemic saline elicited a small ipsilateral preference. The results from this rotational model mediated by normosensitive receptors indicate that only dopamine D-1 receptors in the ventral striatum mediate rotation while D-2 receptors in both striatal regions mediate rotation. A functional dichotomy between these two neostriatal regions is thus proposed.

Differential modulation of striatonigral dynorphin and enkephalin by dopamine receptor subtypes

Previous studies indicate that dopaminergic transmission inhibits the biosynthesis of enkephalin and stimulates that of dynorphin in the striatonigral pathway of intact rat. The purpose of this study was to determine which dopamine (DA) receptor subtype(s) mediate the modulatory actions of DA. We measured striatal and nigral levels of enkephalin and dynorphin in: (1) intact rats repeatedly injected with D1 (SKF-38393, 5 mg/kg, i.p.) or D2 (LY-171555, 1 mg/kg, i.p.) agonists, alone or in combination, (2) 6-hydroxydopamine (6-OHDA)-lesioned rats repeatedly injected with the same D1 or D2 agonists, and (3) intact rats repeatedly injected with D1 (SCH-23390, 0.05 mg/kg, s.c.) or D2 (sulpiride, 100 mg/kg, s.c.) antagonists, given alone or in combination with the mixed D1/D2 agonist apomorphine (5 mg/kg, i.p.). Repeated injections of the D1 agonist to intact rats (twice daily for 7 days) produced a small but not statistically significant increase in striatal levels of dynorphin; similar treatment with the D2 agonist did not affect dynorphin levels at all. Combined treatments with D1 and D2 agonists did not potentiate the effect of the D1 agonist. 6-OHDA lesions of the nigrostriatal DA pathway alone decreased the level of dynorphin in both the striatum and substantia nigra. However, repeated D1 agonist, but not D2, injections not only reversed the decrease in dynorphin levels, but caused a significant increase above control levels. In intact rats, repeated injections of the D1 or D2 antagonist alone failed to alter the levels of dynorphin, but the D1 antagonist, not the D2 antagonist, attenuated the apomorphine-induced increase in striatal dynorphin.(ABSTRACT TRUNCATED AT 250 WORDS)

Concentration-dependent actions of stimulated dopamine release on neuronal activity in rat striatum

Voltammetric analysis was combined with single unit recording to measure the effects of endogenous dopamine, released by electrical stimulation of the median forebrain bundle, on neuronal activity in the rat striatum in vivo. Fast differential ramp voltammetry, a more sensitive form of fast cyclic voltammetry, was used to measure extracellular dopamine levels during a 50-ms scan epoch every 500 ms. Using the same carbon fibre microelectrode, neuronal activity was recorded in between the electrochemical epochs. A steady-state electrochemical signal equivalent to about 100 nM dopamine was seen in the unstimulated striatum. The responses of 122 striatal units to stimulated dopamine release were recorded in 37 acute experiments. Ninety-one units which displayed a large spike amplitude (greater than or equal to 50 microV) were recorded during stimulated release of dopamine initially to levels of between 100 and 500 nM. The majority (49) showed a profound excitation, 23 showed inhibition, and nine units gave complex responses. Only 10 units were unresponsive. All the responses of these large units outlasted the transient increase in dopamine levels, often for more than 1 min. In contrast, all the 31 units which displayed a small spike amplitude (less than 50 microV) were powerfully activated by dopamine release within this range. Administration of alpha-methyl-para-tyrosine (250 mg/kg i.p.) abolished both dopamine release and the response of the five large units and four small units examined, indicating that the neuronal response was directly attributable to dopamine. Dopamine release was increased by increasing the stimulus duration over the range 0.25-10 s. With increasing levels of dopamine release the excitatory response of large units rose to a maximum and then decreased until it was eventually transformed entirely into an inhibition at dopamine levels above 1 microM. In contrast, the excitatory response of small units always increased in magnitude with increasing dopamine release to levels greater than 1 microM. The large units that showed inhibition at low levels of dopamine were also inhibited at high levels. Tail-pinch stimuli excited 21/23 large units and all seven small units tested, although this stimulus did not evoke a detectable rise in dopamine levels. We suggest that the fundamental action of dopamine in the striatum is excitation, whether involving D1 or D2 receptors. The small units described here could be inhibitory interneurons which convert the excitatory response of large units into inhibition. Dopamine may regulate striatal function by enhancing particular input-output pathways while also activating lateral inhibitory mechanisms serving to "gate-out" alternative outputs.

The role of D1 and D2 dopamine receptors in oral stereotypy induced by dopaminergic stimulation of the ventrolateral striatum

Microinjection of amphetamine into the ventrolateral region of the striatum results in compulsive and intense oral stereotypies in the rat. Although these stereotyped behaviors are known to be a direct result of excessive stimulation of the striatal dopamine neurons, the relative roles of the D1 and D2 receptors in oral stereotypies are not clearly understood. It is reported here that microinjection of the selective D1 agonist, SKF 38393 (0, 0.3, 3.0, 30.0 micrograms in 0.5 microliters vehicle) into the ventrolateral striatum resulted in no observable changes in behavior during the 30-min test period. However, it was observed that intense self-biting emerged 3-4 h following injection. Examination of histology from these animals revealed extensive tissue damage and the delayed onset of biting was hypothesized to result from a neurotoxic effect of SKF 38393. Infusion of quinpirole (0, 0.3, 3.0, 30.0 micrograms in 0.5 microliter vehicle), a selective D2 agonist, resulted in a dose-dependent increase in orofacial behaviors such as licking, wood-chip eating, head-down sniffing and mouth movements. Intense oral stereotypies such as biting or gnawing were not observed following treatment with quinpirole. Infusion of the mixed agonist dopamine (0, 2.0, 10.0, 20.0 micrograms in 0.5 microliter vehicle) into the ventrolateral striatum was found to elicit intense oral stereotypy. This behavior consisted almost exclusively of self-biting similar to that observed following amphetamine microinjection into this region. Haloperidol, when given as either a systemic (0.2 mg/kg) or intra-ventrolateral striatum (2.5 micrograms/0.5 microliter) pretreatment, effectively blocked oral stereotypies induced by amphetamine microinjection into the ventrolateral striatum. Pretreatment with either the D1 antagonist SCH 23390 (0, 0.01, 0.1 mg/kg, i.p.) or the D2 antagonist raclopride (0, 0.05, 0.50, 1.0 mg/kg, i.p.) antagonized amphetamine-induced oral stereotypy in a dose-dependent manner. These findings demonstrate that within the striatal site specifically implicated in oral behavior, concurrent stimulation of both receptor subtypes is necessary for the expression of intense oral stereotypies.

Enhanced behavioral stereotypies elicited by intrastriatal injection D1 and D2 dopamine agonists in intact rats

Five components of behavior elicited by dopamine (DA) agonists (locomotor hyperactivity, sniffing, oral activity, grooming and paw nibbling) were evaluated after bilateral infusion of the selective D1 agonist fenoldopam (SKF 82526; 2.5-10 micrograms), the selective D2 agonist quinpirole (LY 171555; 5-40 micrograms) and the muscarinic cholinergic antagonist scopolamine (5-20 micrograms) into the ventral striatum of awake, unrestrained rats. Simultaneous bilateral infusion of various dose combinations of fenoldopam (2.5-10 micrograms) and quinpirole (5-20 micrograms) elicited dramatic increases in stereotyped behaviors relative to the effects produced by corresponding doses of each drug alone. Stereotyped sniffing and paw nibbling (self-directed oral activity) were markedly enhanced, whereas conventional oral behaviors (licking, chewing and/or biting) were either slightly or not at all increased. These potentiated responses were reduced or blocked by concomitant infusion of either the selective D1 antagonist SCH 23390 (1 and 5 micrograms) or the selective D2 antagonist sulpiride (0.15 microgram). Scopolamine (10 micrograms) only slightly increased the effects of quinpirole (5 micrograms) on both sniffing and oral behaviors, whereas it dramatically potentiated the effects of fenoldopam (2.5 micrograms) on oral activity; sniffing was only slightly increased. The effects of both drug combinations were almost completely antagonized by infusion of either SCH 23390 (1 microgram) or sulpiride (0.1 microgram). The results demonstrate that the synergistic effects of co-activation of D1 and D2 receptors observed after systemic administration are mediated at least in part by an interaction at the level of the striatum. Differences and similarities between the behaviors expressed after various treatments are discussed.

Heterogeneity of stimulated dopamine overflow within rat striatum as observed with in vivo voltammetry

Overflow of dopamine has been measured in the striatum of anesthetized rats with 60-Hz, 300-microA electrical stimulations of the medial forebrain bundle. Electrodes were placed in the region of the caudate-putamen or the nucleus accumbens. The elliptical electrodes were fabricated from carbon fibers and had a major radius of 35 microns. Overflow was detected with fast-scan voltammetry repeated at 100-ms intervals. In both brain regions the detected substance had the voltammetric properties of dopamine. Measurements were made at different vertical electrode positions, varied by 100-microns intervals. Significant differences in stimulated overflow were observed between each position in both brain regions. The dimensions over which overflow was observed compare to that of previous descriptions of the patch and matrix topology of the striatum. When the electrode was moved in 10-microns intervals the variance was considerably less. The observed rate of overflow during stimulation exactly correlated with the maximal amount of dopamine observed during a stimulation. In addition, the overflow rate and the disappearance rate also correlated. This suggests that in both brain regions uptake and release sites are anatomically close to one another. The major difference between overflow curves measured in the two different regions was the appearance of an apparent mass transfer barrier to the electrode in the caudate-putamen.

Further evidence for two directions of D-1:D-2 dopamine receptor interaction revealed concurrently in distinct elements of typical and atypical behaviour: studies with the new enantioselective D-2 agonist LY 163502

The new, extremely potent and enantioselective D-2 agonist LY 163502 failed to induce compulsive stereotyped behaviour. Very low doses (3-6 micrograms/kg) inhibited spontaneous sniffing and locomotion, while higher doses (12-50 micrograms/kg) induced episodes of non-stereotyped sniffing and chewing; these actions showed complete enantioselectivity. Up to 200-fold higher doses modestly induced only locomotion. Responsivity to LY 163502 was enantioselectively blocked by the selective D-2 antagonist R-piquindone. This responsivity was also enantioselectively blocked by the selective D-1 antagonist R-SK&F 83566 but, additionally, episodes of atypical limb/body jerking behaviour were released; thus, LY 163502 induced such jerking only when tonic D-1 activity was suppressed. These data extend our notion that there may be at least two forms of functional interaction between D-1 and D-2 receptor systems: one cooperative, as in the regulation of typical sniffing, and another oppositional, as in the regulation of atypical jerking.

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

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

Differential effects of pallidal lesions on the behavioural responses to SKF 38393, LY 171555 and apomorphine in the rat

The purpose of this study was to determine the role of the globus pallidus in the expression of dopamine D1- and D2-receptor mediated motor events. Rats were first injected stereotaxically with 6-hydroxydopamine in one medial forebrain bundle to denervate the ascending dopamine pathways in that hemisphere. Apomorphine and selective D1 and D2 agonists were then administered, at two dose levels, to establish characteristic response patterns. Subsequently the animals were given a secondary lesion by injecting kainic acid (0.2-1 microgram) into the ipsilateral globus pallidus and retested with the dopamine agonists over a period of two months. The kainate treatment itself caused spontaneous motor asymmetries, followed by aphagia, adipsia and hypersensitivity to touch. Contraversive circling, contralateral posture and grooming induced by systemic apomorphine were all abolished by the kainate treatment, whilst sniffing and head movements were facilitated. All activities induced by D1 stimulation were abolished or severely reduced under these conditions. By contrast, the contralateral posture and grooming elicited by D2 stimulation were spared, and only D2-dependent contraversive rotation, sniffing and head movements were reduced. All behavioural deficits were temporary and recovered partially or completely during the course of the experiment, but could not be overcome by increasing the dose of dopamine agonist. Post mortem histology revealed a consistent loss of pallidal neurons, together with more variable damage to extrapyramidal structures and the thalamus. The results show that all the D1-mediated, and certain of the D2-mediated motor responses depend on the integrity of the pallidum for their expression in the unilaterally 6-hydroxydopamine-treated rat.

Protein kinase C and dopamine release--II. Effect of dopamine acting drugs in vivo

The hypothesis that protein kinase C (PKC) plays a role in the release of dopamine (DA) in the nigrostriatal pathway was examined. It was found that injections of apomorphine, SKF 38393 (D1 agonist), LY 171555 (D2 agonist) or gamma-butyrolactone (GBL) (which decreases impulse-induced release of DA) resulted in a decrease in particulate, and an increase in soluble, PKC activity. Injections of fluphenazine, haloperidol, SCH 23390 (D1 antagonist), sulpiride (D2 antagonist) or picrotoxin (gamma-aminobutyric acid antagonist which increases DA release transneuronally) had the opposite effect of increasing particulate and decreasing soluble PKC activity. The total activity was not changed. These effects were receptor mediated since the effect of each agonist could be reversed by its specific antagonist. These drugs influenced PKC in the striatum in a dose-dependent manner. In contrast, no effects were seen in the cerebellum, a region with sparse dopaminergic innervations. The change in PKC activity was mediated via a change in the Km for calcium, while the Vmax was unchanged. The phosphorylation of endogenous substrate proteins by PKC was also altered by injections of these drugs. Besides affecting PKC, these DA acting drugs also affected the calmodulin-dependent protein kinase activity, but the direction of change was opposite to that for PKC. In a synaptosomal preparation, PKC acting drugs also affected the depolarization-induced release of DA. Adriamycin and melittin decreased the potassium-induced release of DA, whereas tetradecanoyl-phorbol-13-acetate (TPA) enhanced this release. These results showed that there was a good correlation between the ability of drugs to alter the impulse-induced release of DA in vivo and their ability to affect changes in particulate and soluble PKC activity. They lend support to the hypothesis that PKC, together with calmodulin, plays a key role in the release of DA in the nigrostriatal pathway.

D1 and D2-type dopamine receptors in patients with Parkinson's disease and progressive supranuclear palsy

The densities of D1- and D2-type dopamine receptors were measured with [3H]SCH23390 and [3H]spiperone, in the caudate nucleus and putamen of a large series of patients with Parkinson's disease or progressive supranuclear palsy, in relation to markers of dopaminergic and cholinergic innervation of the striatum ([3H]dihydrotetrabenazine binding and choline acetyltransferase activity). Correlations were sought between these parameters and clinical characteristics of the patients (abnormal involuntary movements, dementia, confusional syndrome or treatment). In Parkinson's disease, the densities of both types of receptors were unchanged, whereas in PSP, the density of D2, but not D1-type dopamine receptors, was decreased in the caudate nucleus and the putamen. No correlations between the biochemical and clinical data were found.

Alkylation of striatal dopamine receptors abolishes stereotyped behavior but has no effect on dopamine stimulated adenylate cyclase activity

The role of dopamine stimulated adenylate cyclase (AC) activity in behaviours elicited by apomorphine stimulation of striatal dopamine receptors was investigated. Rats were treated with the irreversible dopamine receptor alkylating agent N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) alone or after pretreatment with either a D1 or D2 receptor antagonist and subsequently challenged with apomorphine. Animals that received no antagonist pretreatment showed significantly decreased striatal concentrations of D1 and D2 receptors and an abolition of apomorphine induced sniffing behaviour despite showing no change in striatal AC activity. In the groups which received antagonist pretreatment the reduction in the sniffing response paralleled the reduction in D2 receptor concentration whereas the incidence of vacuous oral movements was inversely related. In no case were the behavioural responses associated with changes in AC activity. We conclude that these behavioural effects observed in response to dopamine stimulation by apomorphine may be mediated through another second messenger system.

Intracerebral SCH 23390 and catalepsy in the rat

Stereotaxic injection of SCH 23390 (D-1 antagonist), but not water or ritanserin (5-HT antagonist), into all parts of the caudate-putamen, elicited an immediate and often long-lasting catalepsy in 79% of rats. Similar results were obtained with SCH 23390 delivered into the nucleus accumbens and globus pallidus, but not into a variety of other brain sites. The results support the idea that SCH 23390-induced catalepsy is related to the occlusion of dopamine D-1 receptors in the forebrain, particularly the striatum, although the topography of the catalepsy evoked by intrastriatal SCH 23390 did not match the distribution of D-1 sites labelled in autoradiographic studies.

Heterogeneous rotational responsiveness in 6-hydroxydopamine-denervated rats: pharmacological and neurochemical characterization

Qualitative differences in pharmacological responsiveness to various types of dopamine agonists have been reported in rats that have undergone unilateral 6-hydroxydopamine (6-OHDA)-induced denervation of the nigro-striatal pathway. The present experiments further characterize these differences, pharmacologically and neurochemically. Rats were classified as having high rotational sensitivity (0.03 mg/kg SC apomorphine sufficient to induce more than 100 rotations/20 min) or low sensitivity (0.3 mg/kg SC apomorphine required to meet this criterion). High sensitivity rats showed marked contralateral rotational behavior (approximately 150 rotations/20 min) in response to apomorphine (ED50 = 0.08 mg/kg IP), CGS 15855A (ED50 = 0.07 mg/kg), CGS 15873A (ED50 = 0.43 mg/kg), (+)-3-PPP (ED50 = 2.3 mg/kg), (-)-3-PPP (ED50 = 0.87 mg/kg) and quinpirole (peak effective dose, 0.03 mg/kg). In low sensitivity rats, 3- to 10-fold higher doses of apomorphine induced a maximal rate of rotational behavior, but only partial effects were produced by quinpirole, CGS 15855A, CGS 15873A, (+)-3-PPP, and (-)-3-PPP (40-80 rotations/20 min). Because apomorphine is a nonselective D1 and D2 agonist, it is proposed that activation of either D1 or D2 receptors suffices to induce high rates of rotation in high sensitivity rats, whereas in low sensitivity rats, D1 or D2 agonism alone induces submaximal rotation rates. The ipsilateral rotational behavior induced by d-amphetamine was more pronounced and occurred at lower doses in the high-sensitivity rats. Striatal dopamine depletion on the lesioned side did not differ between the groups, but low sensitivity rats showed two-fold higher DOPAC/DA ratios on the lesioned side than did high-sensitivity rats.(ABSTRACT TRUNCATED AT 250 WORDS)

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

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

Behavioural evidence for the functionality of D-2 but not D-1 dopamine receptors at multiple brain sites in the 6-hydroxydopamine-lesioned rat

Lisuride (D-2 agonist) and SKF 38393 (D-1 agonist) evoked characteristic circling and stereotyped orofacial movements when administered systemically or stereotaxically into the supersensitive caudate nucleus of unilaterally 6-hydroxydopamine-lesioned rats, which were respectively blocked by D-2 and D-1 antagonists. Lisuride induced similar behaviours from the accumbens, frontal cortex, pallidum, ventromedial thalamus, substantia nigra and periaqueductal grey, while SKF 38393 did not. Multiple brain sites may mediate D-2-induced (not D-1) behaviours following treatment with 6-hydroxydopamine.

D1 dopamine receptor--the search for a function: a critical evaluation of the D1/D2 dopamine receptor classification and its functional implications

The present review focuses on the hypothesized D1/D2 dopamine (DA) receptor classification, originally based on the form of receptor coupling to adenylate cyclase activity. The pharmacological effects of compounds exhibiting putative selective agonist or antagonist profiles at those DA receptors positively coupled to adenylate cyclase activity (D1 DA receptors) are extensively reviewed. Comparisons are made with the effects of putative selective D2 DA receptor agonists and antagonists, and on the basis of this work, the DA receptor classification is critically evaluated. A variety of biochemical, behavioral, and electrophysiological evidence is presented which supports the view that D1 and D2 DA receptors can interact in both an opposing and synergistic fashion. Particular attention is focused on the possibility that D1 receptor stimulation is required to enable the expression of certain D2 receptor-mediated effects, and the functional consequences of this form of interaction are considered. A hypothetical model is presented which considers how both the opposing and enabling forms of interaction between D1 and D2 DA receptors can control behavioral expression. Finally, the clinical relevance of this work is discussed and the potential use of selective D1 receptor agonists and antagonists in the treatment of psychotic states and Parkinson's disease is considered.