Psychopharmacological distinction between novel full-efficacy "D1-like" dopamine receptor agonists

Influence of the dose of ketamine used on schizophrenia-like symptoms in mice: A correlation study with TH, GAD67, and PPAR-γ

Schizophrenia is a chronic, debilitating mental illness that has not yet been completely understood. In this study, we aimed to investigate the effects of different doses of ketamine, a non-competitive NMDA receptor antagonist, on the positive- and negative-like symptoms of schizophrenia. We also explored whether these effects are related to changes in the immunoreactivity of GAD67, TH, and PPAR-γ in brain structures. To conduct the study, male mice received ketamine (20-40 mg/kg) or its vehicle (0.9 % NaCl) intraperitoneally for 14 consecutive days. We quantified stereotyped behavior, the time of immobility in the forced swimming test (FST), and locomotor activity after 7 or 14 days. In addition, we performed ex vivo analysis of the immunoreactivity of GAD, TH, and PPAR-γ, in brain tissues after 14 days. The results showed that ketamine administration for 14 days increased the grooming time in the nose region at all tested doses. It also increased immobility in the FST at 30 mg/kg doses and decreased the number of rearing cycles during stereotyped behavior at 40 mg/kg. These behavioral effects were not associated with changes in locomotor activity. We did not observe any significant alterations regarding the immunoreactivity of brain proteins. However, we found that GAD and TH were positively correlated with the number of rearing during the stereotyped behavior at doses of 20 and 30 mg/kg ketamine, respectively. GAD was positively correlated with the number of rearing in the open field test at a dose of 20 mg/kg. TH was inversely correlated with immobility time in the FST at a dose of 30 mg/kg. PPAR-γ was inversely correlated with the number of bouts of stereotyped behavior at a dose of 40 mg/kg of ketamine. In conclusion, the behavioral alterations induced by ketamine in positive-like symptoms were reproduced with all doses tested and appear to depend on the modulatory effects of TH, GAD, and PPAR-γ. Conversely, negative-like symptoms were associated with a specific dose of ketamine.

Mouse models of neurodevelopmental disease of the basal ganglia and associated circuits

This chapter focuses on neurodevelopmental diseases that are tightly linked to abnormal function of the striatum and connected structures. We begin with an overview of three representative diseases in which striatal dysfunction plays a key role--Tourette syndrome and obsessive-compulsive disorder, Rett's syndrome, and primary dystonia. These diseases highlight distinct etiologies that disrupt striatal integrity and function during development, and showcase the varied clinical manifestations of striatal dysfunction. We then review striatal organization and function, including evidence for striatal roles in online motor control/action selection, reinforcement learning, habit formation, and action sequencing. A key barrier to progress has been the relative lack of animal models of these diseases, though recently there has been considerable progress. We review these efforts, including their relative merits providing insight into disease pathogenesis, disease symptomatology, and basal ganglia function.

Inhibition of 50-kHz ultrasonic vocalizations by dopamine receptor subtype-selective agonists and antagonists in adult rats

RATIONALE

Adult rats emit ultrasonic calls at around 22 and 50 kHz, which are often elicited by aversive and rewarding stimuli, respectively. Dopamine (DA) plays a role in aspects of both reward and aversion.

OBJECTIVE

The purpose of this study is to investigate the effects of DA receptor subtype-selective agonists on 22- and 50-kHz call rates.

METHODS

Ultrasonic calls were recorded in adult male rats that were initially screened with amphetamine to eliminate low 50-kHz callers. The remaining subjects were tested after acute intraperitoneal or subcutaneous injection of the following DA receptor-selective agonists and antagonists: A68930 (D1-like agonist), quinpirole (D2-like agonist), PD 128907 (D3 agonist), PD 168077 (D4 agonist), SCH 39166 (D1-like antagonist), L-741,626 (D2 antagonist), NGB 2904 (D3 antagonist), and L-745,870 (D4 antagonist). The indirect DA/noradrenaline agonist amphetamine served as a positive control.

RESULTS

As expected, amphetamine strongly increased 50-kHz call rates. In contrast, D1-, D2-, and D3-selective DA receptor agonists, when given alone, inhibited calling; combinations of D1- and D2-like agonists also decreased call rate. Given alone, the D1-like and D3 antagonists significantly decreased call rate, with a similar trend for the D2 antagonist. Agonist-antagonist combinations also decreased calling. The D4 agonist and antagonist did not significantly affect 50-kHz call rates. Twenty-two-kilohertz calls occurred infrequently under all drug conditions.

CONCLUSION

Following systemic drug administration, tonic pharmacological activation of D1-like or D2-like DA receptors, either alone or in combination, does not appear sufficient to induce 50-kHz calls. Dopaminergic transmission through D1, D2, and D3 receptors appears necessary for spontaneous calling.

Behavioural and pharmacological examinations in a transgenic mouse model of early-onset torsion dystonia

Early-onset torsion dystonia is an autosomal dominant movement disorder associated with the DYT1 gene (TOR1A) defect which results in a deletion of a glutamic acid residue in the protein torsinA. The pathophysiology of dystonia is poorly understood. Well characterized animal models can help to give insights into the underlying mechanisms and thereby to develop new therapeutics. In the present study, we further characterized transgenic DYT1 mice, which were initially described to exhibit "dystonia-like" postures. In the present study, several behavioural tests in untreated animals did not show strong differences between transgenic and control mice, but nearly all transgenic mice showed "dystonia-like" postures. However, these movements, also observed in control mice, have to be regarded as a clasping reflex. Since dystonia is thought to be related to dopaminergic dysfunctions, pharmacological investigations have been performed to clarify if dopaminergic substances alter motor behaviour in transgenic mice. Chronic treatment with L-DOPA (combined with carbidopa) enhanced the hindlimb claspings only in transgenic mice, while acute applications of drugs, which exert more selective effects on the dopaminergic system, caused similar reactions in transgenic mice and control mice. Therefore, these data do not provide clear evidence for dysfunctions of the dopaminergic system in this mouse model.

Behavioral synergism between D(1) and D(2) dopamine receptors in mice does not depend on gap junctions

Activation of the D(1) and D(2) classes of dopamine receptor in the striatum synergistically promotes motor stereotypy. The mechanism of D(1)/D(2) receptor interaction remains unclear. To investigate the involvement of electrical synaptic transmission in this phenomenon, genetic inactivation of the neuronal gap junction (GJ) protein connexin 36 and pharmacological blockade of GJs were utilized. Stereotyped motor behavior was quantified after selective activation of D(1) receptors, D(2) receptors, or both receptors. These patterns of activation were produced by injection of the agonist apomorphine (3.0 mg/kg) 30 min after either the D(2) antagonist eticlopride (0.3 mg/kg), the D(1) antagonist SCH 23390 (0.1 mg/kg) or vehicle, respectively. Mixed background C57/BL6-129SvEv mice homozygous or heterozygous for the connexin 36 "knockout" allele displayed potent synergistic interaction between D(1) and D(2) receptor activation, and did not differ significantly from wild-type mice on any measure. All genotypes demonstrated long-lasting stereotypic sniffing, chewing, and/or licking after simultaneous activation of D(1) and D(2) receptors, effects that were absent following selective D(1) or D(2) activation. Swiss-Webster mice treated with the GJ blockers carbenoxolone (35 mg/kg), octanol (350 mg/kg) or mefloquine (50 mg/kg) also demonstrated the normal synergistic interaction between D(1) and D(2) receptors, although these drugs did block the grooming stimulated by selective D(1) receptor activation, independently of D(2) receptors. While D(1) receptor-stimulated grooming depends on GJs composed of connexins or possibly pannexins, the synergistic interaction of D(1) and D(2) receptors in control of stereotypy does not involve GJs.

The debate over dopamine's role in reward: the case for incentive salience

INTRODUCTION

Debate continues over the precise causal contribution made by mesolimbic dopamine systems to reward. There are three competing explanatory categories: 'liking', learning, and 'wanting'. Does dopamine mostly mediate the hedonic impact of reward ('liking')? Does it instead mediate learned predictions of future reward, prediction error teaching signals and stamp in associative links (learning)? Or does dopamine motivate the pursuit of rewards by attributing incentive salience to reward-related stimuli ('wanting')? Each hypothesis is evaluated here, and it is suggested that the incentive salience or 'wanting' hypothesis of dopamine function may be consistent with more evidence than either learning or 'liking'. In brief, recent evidence indicates that dopamine is neither necessary nor sufficient to mediate changes in hedonic 'liking' for sensory pleasures. Other recent evidence indicates that dopamine is not needed for new learning, and not sufficient to directly mediate learning by causing teaching or prediction signals. By contrast, growing evidence indicates that dopamine does contribute causally to incentive salience. Dopamine appears necessary for normal 'wanting', and dopamine activation can be sufficient to enhance cue-triggered incentive salience. Drugs of abuse that promote dopamine signals short circuit and sensitize dynamic mesolimbic mechanisms that evolved to attribute incentive salience to rewards. Such drugs interact with incentive salience integrations of Pavlovian associative information with physiological state signals. That interaction sets the stage to cause compulsive 'wanting' in addiction, but also provides opportunities for experiments to disentangle 'wanting', 'liking', and learning hypotheses. Results from studies that exploited those opportunities are described here.

CONCLUSION

In short, dopamine's contribution appears to be chiefly to cause 'wanting' for hedonic rewards, more than 'liking' or learning for those rewards.

Impaired expression of postsynaptic density proteins in the hippocampal CA1 region of rats following perinatal hypoxia

Perinatal hypoxia is an important cause of brain injury amongst the newborn, such injury often resulting in an increased risk of impaired performance as regards learning and memory in later life for the affected individual. The postsynaptic density 95 (PSD-95) protein is a cytoskeletal specialization involved in the anchoring of N-methyl-d-aspartate (NMDA) receptors in postsynaptic neurons and has been reported to serve several important functions (e.g., synaptogenesis, synaptic plasticity and learning and memory performance) for the mammalian brain. Herein we investigated the long-term effects of perinatal hypoxia upon the complex of PSD-95 with NMDAR subunits by means of downstream signalling cAMP response element binding protein (CREB) phosphorylation at the Serine-133 locus (CREB(Ser-133) phosphorylation) within the hippocampal CA1 area (an essential integration area for mammalian learning and memory) within test-rat brains, as well as the effects upon afflicted-individual long-term learning and memory performance. We also assessed the therapeutic efficacy of dopamine D1/D5 receptor (D1/D5R) activation for such study animals. Perinatal hypoxia on postnatal day ten (P10) led to impaired performance as regards long-term spatial learning and memory (as determined on P45) associated with decreases in the level of CREB(Ser-133) phosphorylation and decreases in the expression of the complex of PSD-95 with NMDAR subunits (NR1, NR2A, and NR2B). In addition, activation of the D1/D5R via A68930 (a selective, CNS-permeable agonist of D1/D5Rs) administration (2 mg/kg/day, P17-23 inclusively) markedly attenuated the hypoxia-induced deleterious effects, suggesting an effective therapeutic efficacy for A68930. Our results demonstrate the long-term effects of perinatal hypoxia upon the developing brain and provide additional insights into the relative vulnerability of postsynaptic density (PSD) proteins to such insult, as well as the impairment of downstream transcription signalling CREB(Ser-133) phosphorylation following perinatal hypoxia. More importantly, D1/D5R activation following perinatal hypoxia may be an alternative therapeutic strategy to that which is currently available and may offer significant clinical potential for hypoxia sufferers.

Dopamine D1 activation shortens the duration of phases in stereotyped grooming sequences

Rats frequently emit grooming actions in a highly stereotyped, syntactic chain in which three distinct phases of facially directed forearm movements are sequentially emitted in a rule-governed pattern and followed by body-directed licking. The present study evaluated the effects of the full dopamine D1 agonist, SKF 81297, and the partial dopamine D1 agonist, SKF 38393, on the duration of individual phases of stereotyped grooming chains. We found that systemic administration of SKF 81297 significantly shortened grooming chain duration. An examination of the fine temporal structure of syntactic grooming chain actions showed that duration changes were correlated with decreased numbers of actions in phases I and IV of the chain. Phases II and III were not changed in duration, although there were some structural distortions introduced. The partial D1 agonist, SKF 38393, had no effect on duration or number of component actions in the grooming chain. Based on these results, we hypothesize that the timing of syntactic grooming phase transitions may involve a D1-mediated internal clock process that is altered by full D1 agonist activation. By this model, SKF 81297 increases the speed of the clock used for the temporal control of grooming actions, and thus shortens phase durations.

Differences between A 68930 and SKF 82958 could suggest synergistic roles of D1 and D5 receptors

The isochroman A 68930 and the benzazepine SKF 82958 are two full dopamine D1 receptor agonists. Responses to these compounds are different in several important aspects. When given to rats in a novel environment, A 68930 caused a dose-dependent (0.019-4.9 mg/kg) suppression of locomotion. SKF 82958 had no such effect at any dose studied (0.051-3.3 mg/kg). In animals habituated to the environment, A 68930 had no effect but SKF 82958 increased locomotor activity. Both A 68930 and SKF 82958 caused a decrease in core temperature at early time points. Both agonists increased c-fos and NGFI-A expression in caudate putamen but only SKF 82958 did so in the accumbens nucleus (at 1.6 mg/kg). Quantitative receptor autoradiography showed that A 68930 is 9-13 times more potent than SKF 82958 at displacing the selective dopamine D1 antagonist [3H]SCH 23390. This difference agrees with the difference observed when the agonists were used to stimulate cAMP formation in cells transfected with the D1 receptor. In contrast, SKF 82958 was 5 times more potent than A 68930 in cells transfected with the D5 receptor. We suggest that the balance between signaling via dopamine D1 and D5 receptors determines the functional effects of agonists at D1/D5 receptors.

Comparative behavioral changes in postpubertal rats after neonatal excitotoxic lesions of the ventral hippocampus and the prefrontal cortex

The neonatal ventral hippocampal (nVH) and the neonatal prefrontal cortex (nPFC) lesions in rats have been used as models to test the hypothesis that early neurodevelopmental abnormalities lead to behavioral changes putatively linked to schizophrenia. We investigated the role of the nVH and the nPFC lesions on behavioral characteristics related to locomotor behaviors, social interaction, and grooming. Bilateral ibotenic acid lesions of the VH, the PFC, or both were made in neonatal Sprague-Dawley rats (postnatal day 7, P7) and their behaviors studied at P35 and P60. No significant differences in any of the behaviors were observed between sham animals and rats with ibotenic acid lesions at P35. Postpubertally (at P60), the spontaneous locomotor activity of nVH-lesioned rats was significantly enhanced compared to the sham controls; however, this hyperactivity was reversed by nVH and nPFC double lesions. Neonatal PFC lesion alone did not alter spontaneous activity, although a trend of increased activity was observed. The duration of grooming was significantly decreased in rats with neonatal lesions of the VH. Similar to the data on locomotion, nVH plus nPFC lesion normalized the grooming behavior. Lesion of the PFC alone was without any significant effect on grooming behavior. Neonatal VH-lesioned animals spent less time in active social interaction, and this effect persisted even in nVH plus nPFC-lesioned animals. By itself, nPFC lesion did not alter social behavior. These data suggest that subtle developmental aberrations within PFC caused by nVH lesions, rather than the lesion of PFC itself, may contribute to some of the behavioral changes seen in the nVH-lesioned rats.

Sequential super-stereotypy of an instinctive fixed action pattern in hyper-dopaminergic mutant mice: a model of obsessive compulsive disorder and Tourette's

BACKGROUND

Excessive sequential stereotypy of behavioral patterns (sequential super-stereotypy) in Tourette's syndrome and obsessive compulsive disorder (OCD) is thought to involve dysfunction in nigrostriatal dopamine systems. In sequential super-stereotypy, patients become trapped in overly rigid sequential patterns of action, language, or thought. Some instinctive behavioral patterns of animals, such as the syntactic grooming chain pattern of rodents, have sufficiently complex and stereotyped serial structure to detect potential production of overly-rigid sequential patterns. A syntactic grooming chain is a fixed action pattern that serially links up to 25 grooming movements into 4 predictable phases that follow 1 syntactic rule. New mutant mouse models allow gene-based manipulation of brain function relevant to sequential patterns, but no current animal model of spontaneous OCD-like behaviors has so far been reported to exhibit sequential super-stereotypy in the sense of a whole complex serial pattern that becomes stronger and excessively rigid. Here we used a hyper-dopaminergic mutant mouse to examine whether an OCD-like behavioral sequence in animals shows sequential super-stereotypy. Knockdown mutation of the dopamine transporter gene (DAT) causes extracellular dopamine levels in the neostriatum of these adult mutant mice to rise to 170% of wild-type control levels.

RESULTS

We found that the serial pattern of this instinctive behavioral sequence becomes strengthened as an entire entity in hyper-dopaminergic mutants, and more resistant to interruption. Hyper-dopaminergic mutant mice have stronger and more rigid syntactic grooming chain patterns than wild-type control mice. Mutants showed sequential super-stereotypy in the sense of having more stereotyped and predictable syntactic grooming sequences, and were also more likely to resist disruption of the pattern en route, by returning after a disruption to complete the pattern from the appropriate point in the sequence. By contrast, wild-type mice exhibited weaker forms of the fixed action pattern, and often failed to complete the full sequence.

CONCLUSIONS

Sequential super-stereotypy occurs in the complex fixed action patterns of hyper-dopaminergic mutant mice. Elucidation of the basis for sequential super-stereotypy of instinctive behavior in DAT knockdown mutant mice may offer insights into neural mechanisms of overly-rigid sequences of action or thought in human patients with disorders such as Tourette's or OCD.

Dihydrexidine--the first full dopamine D1 receptor agonist

The functional role of dopamine D(1) receptors is still controversial. One reason for this controversy is that for a long time the only available agonists for in vivo characterization of dopamine D(1) receptors were benzazepines. Among them was the prototype dopamine D(1) receptor partial agonist, SKF 38393. The lack of a selective and fully efficacious dopamine D(1) receptor agonist hampered basic research on dopamine D(1) receptors and left the potential clinical utility of dopamine D(1) receptor agonists elusive. The research situation improved when the first potent full dopamine D(1) receptor agonist dihydrexidine, a phenanthridine, was introduced in the late 1980s. In contrast to SKF 38393, dihydrexidine was shown to stimulate cyclic AMP synthesis just as well or better than dopamine, and potently displaced [(3)H]SCH 23390 from rat and monkey striatal membranes. Also, dihydrexidine was the first dopamine D(1) receptor agonist that had potent antiparkinsonian activity in a primate model of Parkinson's disease. This finding suggested clinical utility for dopamine D(1) receptor agonists in Parkinson's disease and that this utility might be critically dependent on the intrinsic efficacy of the drug. Clinical utility for dopamine D(1) receptor agonists in other central nervous disorders might also be dependent on the intrinsic efficacy of the drug. However, even though studies with dihydrexidine as a pharmacological tool have pointed to the clinical use for dopamine D(1) receptor agonists, dihydrexidine's unfavorable pharmacokinetic profile and various adverse effects are likely to restrict or even preclude its use in humans. This review article provides an updated overview of the pharmacology of dihydrexidine and discusses possible clinical utility of dopamine D(1) receptor agonists in various central nervous system disorders.

A 68930 and dihydrexidine inhibit locomotor activity and d-amphetamine-induced hyperactivity in rats: a role of inhibitory dopamine d1/5 receptors in the prefrontal cortex?

The behavioral and biochemical effects of the full dopamine D(1/5) receptor agonists, dihydrexidine and (1R,3S)-1-aminomethyl-5,6-dihydroxy-3-phenylisochroman HCl (A 68930), were examined in rats. Both A 68930 (0-4.6 mg kg(-1), s.c.) and dihydrexidine (0-8.0 mg kg(-1), s.c.) caused a dose-dependent suppression of locomotor activity, as assessed in an open-field. This locomotor suppression was dose-dependently antagonized by the selective dopamine D(1/5) receptor antagonist R(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine HCl (SCH 23390; 0-5.0 microg kg(-1), s.c.), but not by the selective dopamine D(2/3) receptor antagonist raclopride (0-25.0 microg kg(-1), s.c.). Furthermore, A 68930 and dihydrexidine did not cause any locomotor activity in habituated rats that displayed a very low base-line activity. Neither did A 68930 nor dihydrexidine produce any excessive stereotypies that could possibly interfere with and mask ambulatory activity. In fact, both A 68930 and dihydrexidine potently blocked hyperactivity produced by d-amphetamine (0-4.0 mg kg(-1), s.c.). Such findings traditionally would be interpreted as a sign of potential antipsychotic properties of A 68930 and dihydrexidine. Examination of neuronal activation, as indexed by the immediate early gene c-fos, showed that A 68930 and dihydrexidine caused a highly significant expression of c-fos in the medial prefrontal cortex. This c-fos expression was sensitive to treatment with SCH 23390, but not with raclopride. The effects of A 68930 and dihydrexidine on c-fos expression in caudate putamen or nucleus accumbens were less marked, or undetectable. The results indicate that stimulation of dopamine D(1/5) receptors, possibly in the medial prefrontal cortex, is associated with inhibitory actions on locomotor activity and d-amphetamine-induced hyperactivity. Assuming an important role of prefrontal dopamine D(1/5) receptors in schizophrenia, such inhibitory actions of dopamine D(1/5) receptor stimulation on psychomotor activation may have interesting clinical implications in the treatment of schizophrenia.

Behavioral response profiles following drug challenge with dopamine receptor subtype agonists and antagonists in developing rat

As part of an investigation into the effects of gestational ethanol (ETOH) exposure on the developing dopamine (DA) system, pregnant Sprague-Dawley rats were exposed to one of three conditions: ETOH, pair-fed (PF) to the ETOH group, or ad libitum lab chow controls (LC). In this paper we report behavioral drug challenge effects for offspring of the two control groups (PF and LC). Male and female pups between postnatal days (PNDs) 21 and 23 in age were exposed to one of three intraperitoneal/subcutaneous doses of one of eight drugs chosen to assess the functional status of the DA D(1), D(2), and D(3) receptor subtype, or a saline control. Agonists were SKF 38393, apomorphine (APO), quinpirole (QUIN), and 7-hydroxy-N,N-di-n-propyl-2-amino-tetralin [7-OH-DPAT (DPAT)]; antagonists were spiperone (SPIP), SCH 23390, and two recently developed D(3) antagonists nafadotride (NAF) and PD 152255. Immediately following drug injection, pups were placed in observation cages, where eight behaviors (square entries, grooming, circling, rearing, sniffing, head and oral movements, and yawning) were scored at 3-min intervals for 30 min. Classic behavioral profiles were generally obtained for the high-dose mixed agonists APO, DPAT, and QUIN, which potently increased square entries, rearing, and sniffing, while reducing grooming and head movements. However, low-dose APO had no effect on behavior. The D(1) agonist, SKF 38393, had a strikingly different behavioral profile; it had no effect on square entries at any dose, while increasing grooming and sniffing at the medium dose. The D(1) antagonist, SCH 23390, profoundly decreased all behaviors except oral and head movements, especially at high doses. In contrast, the effects of the D(2) antagonist, SPIP, were limited to increasing sniffing at the medium dose. The two putative D(3) antagonists, NAF and PD 152255, presented strikingly different profiles. NAF induced a pattern of behavioral suppression that resembled the profile of high-dose SCH, while high-dose PD 152255 stimulated behavior. The failure of low-dose APO to have any effect on behavior suggests that the D(2) autoreceptor is not functional in preweanling rats. This hypothesis is further supported by the lack of behavioral suppression seen with low-dose QUIN and DPAT. Failure of NAF to produce behavioral activation at low doses and the stimulatory effects seen with PD 152255 suggests that either the D(3) autoreceptor, the postsynaptic D(3) receptor, or both are not fully functional at this age as well.

No change in dopamine D1 receptor in vivo binding in rats after sub-chronic haloperidol treatment

A frequent side effect in the long-term treatment of schizophrenia with the dopamine D2 antagonist haloperidol (HAL) is the appearance of tardive dyskinesia or, in animals, of repetitive involuntary vacuous chewing movements (VCMs). In rats, chronic HAL-induced or D1 receptor-stimulated VCMs are suppressed by D1 antagonists, suggesting that this behavioral supersensitivity is mediated by D1 receptors. The goal of this study was to investigate in vivo the possible relationship between D1 receptor binding and D1-mediated behavioral supersensitivity, after subchronic HAL treatments. D1 agonist R-SKF 82957 and antagonist SCH 23390, both labeled with carbon-11, were used to assess in vivo D1 receptor binding. Rats were treated with HAL (1.5 mg/kg, i.p.) or vehicle for 21 days, followed by a 4 day washout period. No significant difference was found in the regional brain binding of either radioligand. D1 receptor-mediated behaviors including VCMs, grooming, and rearing were measured in control or HAL-treated rats. VCMs were significantly increased in HAL-treated rats, suggesting D1 receptor stimulation and possibly receptor supersensitivity. This study failed to link the purported D1 receptor-mediated behaviors with in vivo receptor binding measures of R-[11C]SKF 82957 or [11C]SCH 23390 in rat brain regions.

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 I: enhancement of a complex movement sequence by systemic dopamine D1 agonists

Peripheral administration of D1 dopamine agonists elicits grooming behavior from rodents. The present study examined grooming behavior and the relative probability and stereotypy of a natural sequence of grooming movements (called a syntactic grooming chain) that follows a predictable fixed pattern of serial order. We compared the amount of grooming behavior vs. the stereotypy of sequential patterns after peripheral administration of either a partial D1 agonist (SKF 38393; 2.5, 5.0, 10, 20 mg/kg), a full D1 agonist (SKF 82958; 0.1, 0.2, 0.5, 1.0 mg/kg; i.p.), a D2 agonist (quinpirole; 5.0, 10 mg/kg), or ACTH (2.0, 5.0 mg/kg). There was a dissociation between the elicited grooming amount, the pattern frequency, and the pattern completion or sequential stereotypy after these drugs. Quinpirole and ACTH both reduced the likelihood that the sequential pattern would be completed in the normal pattern (and reduced the overall amount of grooming). Administration of either SKF 38393 or SKF 82958 increased the tendency to engage in complex stereotyped sequential patterns of grooming (even though only the partial D1 agonist increased the total amount of grooming). In addition, SKF 38393 increased the sequential stereotypy of the already-stereotyped pattern itself (as measured by the probability of completing the stereotyped sequence once it began). Thus, dopamine D1 receptor activation appears to contribute to a kind of sequential super-stereotypy in which a complex, stereotyped behavioral sequence is initiated more frequently and more often goes to completion.

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.

Effects of oligonucleotide antisense to dopamine D(1A) receptor messenger RNA in a rodent model of levodopa-induced dyskinesia

Dyskinesias are abnormal involuntary movements which develop as a side-effect of long-term treatment with levodopa in patients with Parkinson's disease. The pathophysiology underlying these dyskinesias remains unclear, although, it has been suggested that heightened activity of dopamine D(1) receptor-bearing striatonigral neurons may play a key role. Chronic pulsatile levodopa administration to hemiparkinsonian rats results in sensitization of rotational responses to apomorphine. This sensitization is thought to be analogous to levodopa-induced dyskinesias in humans. In these studies, we further clarify the role of the dopamine D(1A) receptor in this rodent model of levodopa-induced dyskinesias using an in vivo oligonucleotide antisense approach. Hemiparkinsonian rats received twice daily injections of levodopa for three weeks followed by intrastriatal infusion of dopamine D(1A) receptor antisense (7nmol/day, three days), a scrambled missense control sequence, or saline. Those animals treated with antisense displayed significantly fewer apomorphine-induced rotations than saline- or missense-treated controls.By reducing dopamine D(1A) receptor expression, we were able to attenuate sensitization of the response to apomorphine resulting from chronic pulsatile levodopa treatment. Thus, the dopamine D(1A) receptor appears to play a significant role in levodopa-induced dyskinesias and warrants further examination. These findings may have important implications for the development of selective treatment strategies designed to alleviate parkinsonian symptoms, while minimizing motor complications.

Conservation of behavioural topography to dopamine D1-like receptor agonists in mutant mice lacking the D1A receptor implicates a D1-like receptor not coupled to adenylyl cyclase

Though D1-like dopamine receptors [D1A/B] are defined in terms of linkage to the stimulation of adenylyl cyclase, with D1A assumed to be the functionally prepotent subtype, evidence suggests the existence of another, novel D1-like receptor without such coupling. To investigate these issues we challenged mutant mice having targeted gene deletion of the D1A receptor with selective agonists and used an ethologically-based assessment technique to resolve resultant behavioural topography. D1-like-dependent behaviour was substantially conserved in D1A-null mice relative to wild-types following challenge with each of two selective D1-like agents: A 68930 (0.068-2.0 mg/kg s.c.) which exhibits full efficacy to stimulate adenylyl cyclase, and SKF 83959 (0.016-2.0 mg/kg s.c.) which fails to stimulate adenylyl cyclase, and indeed inhibits the stimulation of adenylyl cyclase induced by dopamine. Furthermore, responsivity to the selective D2-like agonist RU 24213 (0.1-12.5 mg/kg s.c.) was conserved in D1A-null mice, indicating the integrity of D1-like:D2-like interactions at the level of behaviour. These data are consistent with behavioural primacy of a D1-like receptor other than D1A [or D1B] that is coupled to a transduction system other than/additional to adenylyl cyclase.

SK&F 83959 and non-cyclase-coupled dopamine D1-like receptors in jaw movements via dopamine D1-like/D2-like receptor synergism

This study compared the effects of the dopamine D1-like receptor agents SK&F 83959 (3-methyl-6-chloro-7,8-dihydroxy-1-[3-methyl-phenyl]-2,3,4,5-tetrahydro- 1 H-3-benzazepine), which inhibits the stimulation of adenylyl cyclase, and A 68930 ([1R,3S]-1-aminomethyl-5,6-dihydroxy-3-phenyl-isochroman), a full efficacy agonist, in regulating jaw movements in the rat by synergism with dopamine D2-like receptor agonism. When SK&F 83959 and A 68930 were given in combination with quinpirole, there was a synergistic induction of jaw movements. Responsivity to SK&F 83959 + quinpirole was antagonised by the dopamine D1-like receptor antagonists SCH 23390 ([R]-3-methyl-7-chloro-8-hydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-ben zaz epine) and BW 737C ([S]-6-chloro-1-[2,5-dimethoxy-4-propylbenzyl]-7-hydroxy-2-methyl- 1,2,3,4-tetrahydroisoquinoline); synergism was antagonised also by the dopamine D2-like receptor antagonist YM 09151-2 (cis-N-[1-benzyl-2-methyl-pyrrolidin-3-yl]-5-chloro-2-methoxy-4-++ +methyl-aminobenzamide). Responsivity to A 68930 + quinpirole was enhanced by low doses of SCH 23390, BW 737C and YM 09151-2, and antagonised by higher doses of SCH 23390 and YM 09151-2. These results implicate a novel, dopamine D1-like receptor that is coupled to a transduction system other than/additional to adenylyl cyclase, and suggest that its functional role extends to the regulation of jaw movements by synergistic interactions with dopamine D2-like receptors.

Topographical evaluation of the phenotype of spontaneous behaviour in mice with targeted gene deletion of the D1A dopamine receptor: paradoxical elevation of grooming syntax

The phenotype of spontaneous behaviour in mice with targeted gene deletion of the DIA dopamine receptor was investigated topographically. Via direct visual observation, individual elements of behaviour were resolved and quantified using an ethologically-based, rapid time-sampling behavioural check-list procedure. Relative to wildtypes (D1A+/+), D1A-null (-/-) mice evidenced over initial exploration significant reductions in rearing free, sifting and chewing, but significant increases in locomotion, grooming and intense grooming. Sniffing and rearing to a wall habituated less readily in D1A-null mice such that these behaviours occurred subsequently to significant excess: increases in locomotion were persistent. The ethogram of spontaneous behaviour in D1A-null mice was characterised by neither 'hypoactivity' or 'hyperactivity' but, rather, by prominent topographical shifts between individual elements of behaviour that could not be encapsulated by either term. Given the substantial body of evidence that grooming and particularly intense grooming constitute the most widely accepted behavioural index of D1-like receptor function, the elevation of such behaviour in D1A-null mice was paradoxical; it may reflect (over)compensatory processes subsequent to developmental absence of D1A receptors and/or the involvement of a D1-like receptor other than/additional to the D1A subtype.