The induction of grooming and vacuous chewing by a series of selective D-1 dopamine receptor agonists: two directions of D-1:D-2 interaction

Norepinephrine and dopamine contribute to distinct repetitive behaviors induced by novel odorant stress in male and female mice

Exposure to unfamiliar odorants induces an array of repetitive defensive and non-defensive behaviors in rodents which likely reflect adaptive stress responses to the uncertain valence of novel stimuli. Mice genetically deficient for dopamine β-hydroxylase (Dbh-/-) lack the enzyme required to convert dopamine (DA) into norepinephrine (NE), resulting in globally undetectable NE and supranormal DA levels. Because catecholamines modulate novelty detection and reactivity, we investigated the effects of novel plant-derived odorants on repetitive behaviors in Dbh-/- mice and Dbh+/- littermate controls, which have catecholamine levels comparable to wild-type mice. Unlike Dbh+/- controls, which exhibited vigorous digging in response to novel odorants, Dbh-/- mice displayed excessive grooming. Drugs that block NE synthesis or neurotransmission suppressed odorant-induced digging in Dbh+/- mice, while a DA receptor antagonist attenuated grooming in Dbh-/- mice. The testing paradigm elicited high circulating levels of corticosterone regardless of Dbh genotype, indicating that NE is dispensable for this systemic stress response. Odorant exposure increased NE and DA abundance in the prefrontal cortex (PFC) of Dbh+/- mice, while Dbh-/- animals lacked NE and had elevated PFC DA levels that were unaffected by novel smells. Together, these findings suggest that novel odorant-induced increases in central NE tone contribute to repetitive digging and reflect psychological stress, while central DA signaling contributes to repetitive grooming. Further, we have established a simple method for repeated assessment of stress-induced repetitive behaviors in mice, which may be relevant for modeling neuropsychiatric disorders like Tourette syndrome or obsessive-compulsive disorder that are characterized by stress-induced exacerbation of compulsive symptoms.

Preclinical profile of a dopamine D1 potentiator suggests therapeutic utility in neurological and psychiatric disorders

DETQ, an allosteric potentiator of the dopamine D1 receptor, was tested in therapeutic models that were known to respond to D1 agonists. Because of a species difference in affinity for DETQ, all rodent experiments used transgenic mice expressing the human D1 receptor (hD1 mice). When given alone, DETQ reversed the locomotor depression caused by a low dose of reserpine. DETQ also acted synergistically with L-DOPA to reverse the strong hypokinesia seen with a higher dose of reserpine. These results indicate potential as both monotherapy and adjunct treatment in Parkinson's disease. DETQ markedly increased release of both acetylcholine and histamine in the prefrontal cortex, and increased levels of histamine metabolites in the striatum. In the hippocampus, the combination of DETQ and the cholinesterase inhibitor rivastigmine increased ACh to a greater degree than either agent alone. DETQ also increased phosphorylation of the AMPA receptor (GluR1) and the transcription factor CREB in the striatum, consistent with enhanced synaptic plasticity. In the Y-maze, DETQ increased arm entries but (unlike a D1 agonist) did not reduce spontaneous alternation between arms at high doses. DETQ enhanced wakefulness in EEG studies in hD1 mice and decreased immobility in the forced-swim test, a model for antidepressant-like activity. In rhesus monkeys, DETQ increased spontaneous eye-blink rate, a measure that is known to be depressed in Parkinson's disease. Together, these results provide support for potential utility of D1 potentiators in the treatment of several neuropsychiatric disorders, including Parkinson's disease, Alzheimer's disease, cognitive impairment in schizophrenia, and major depressive disorder.

Lifelong Rodent Model of Tardive Dyskinesia-Persistence After Antipsychotic Drug Withdrawal

Tardive dyskinesia (TD), first appearing in humans after introduction of the phenothiazine class of antipsychotics in the 1950s, is now recognized as an abnormality resulting predominately by long-term block of dopamine (DA) D₂ receptors (R). TD is thus reproduced in primates and rodents by chronic administration of D₂-R antagonists. Through a series of studies predominately since the 1980s, it has been shown in rodent modeling of TD that when haloperidol or other D₂-R antagonist is added to drinking water, rats develop spontaneous oral dyskinesias, vacuous chewing movements (VCMs), after ~3 months, and this TD is associated with an increase in the number of striatal D₂-R. This TD persists for the duration of haloperidol administration and another ~2 months after haloperidol withdrawal. By neonatally lesioning dopaminergic nerves in brain in neonatal rats with 6-hydroxydopamine (6-OHDA), it has been found that TD develops sooner, at ~2 months, and also is accompanied by a much higher number of VCMs in these haloperidol-treated lesioned rats, and the TD persists lifelong after haloperidol withdrawal, but is not associated with an increased D₂-R number in the haloperidol-withdrawn phase. TD apparently is related in part to supersensitization of both D₁-R and serotoninergic 5-HT₂-R, which is also a typical outcome of neonatal 6-OHDA (n6-OHDA) lesioning. Testing during the haloperidol-withdrawn phase in n6-OHDA rats displaying TD reveals that receptor agonists and antagonists of a host of neuronal phenotypic classes have virtually no effect on spontaneous VCM number, except for 5-HT₂-R antagonists which acutely abate the incidence of VCMs in part. Extrapolating to human TD, it appears that (1) 5-HT₂-R supersensitization is the crucial alteration accounting for persistence of TD, (2) dopaminergic-perhaps age-related partial denervation-is a risk factor for the development of TD, and (3) 5-HT₂-R antagonists have the therapeutic potential to alleviate TD, particularly if/when an antipsychotic D₂-R blocker is withdrawn.

Effects of 6-hydroxydopamine lesioning of the medial prefrontal cortex on social interactions in adolescent and adult rats

Bilateral depletion of dopamine (DA) in the medial prefrontal cortex (mPFC) following local infusions of 6-hydroxydopamine (6-OHDA) was reported to affect mesolimbic DA neurotransmission and augment spontaneous and amphetamine-induced locomotion. However, the effects of 6-OHDA lesioning of the mPFC of adolescent rats have never been investigated. Given that dopaminergic neurons reach the peak of maturation during adolescence, we hypothesized that 6-OHDA lesioning of the mPFC during adolescence would have greater impact on subsequent behavioral parameters than would such lesioning during adulthood. The aim of this study was to investigate the effects of 6-OHDA lesioning of the mPFC on the open-field activities and novel investigative and socially interactive behaviors of adolescent and adult rats. Using a stereotaxic apparatus, 6-OHDA (8.0 microg) was injected bilaterally into the mPFC of adolescent and adult rats. After a 1-week recovery period, rats were placed in an open-field chamber, and spontaneous locomotion and other behaviors were monitored. Next, a novel toy was place in the center and behavioral responses were observed. One day later, socially interactive behaviors were measured by placing the lesioned rats into a cage with four unfamiliar rats matched for age. The tests of locomotor activity and novel investigative behaviors revealed no significant differences between the lesioned and sham groups of adolescent or adult rats. Grooming and socially interactive behaviors were significantly lower in the adolescent and adult lesioned groups than in each sham group. Interestingly, we observed more extensive impairment in socially interactive behaviors among the adolescent lesioned rats compared to the adult lesioned rats. The present study indicates that DA depletion in the mPFC causes significantly reduced grooming and socially interactive behaviors; this phenomenon may be comparable to the negative symptoms observed in schizophrenia. Further research is warranted to investigate the mechanisms underpinning the detrimental effects of 6-OHDA lesioning of the mPFC on social behaviors.

Modeling tardive dyskinesia: Predictive 5-HT2C receptor antagonist treatment

Tardive dyskinesia (TD), a movement disorder produced by long-term treatment with a classical antipsychotic drug, is generally considered to be a disorder of dopamine (DA) systems, since classical antipsychotics are potent DA D(2) receptor blockers. Also, acute DA D(1) agonist treatment of rats is known to produce vacuous chewing movements (VCMs), a behavioral feature resembling the oral dyskinesia that is so prominent in most instances of TD. In this paper we outline a series of studies in a new animal model of TD in which DA D(1) receptor supersensitivity was produced by neonatal 6-hydroxydopamine (6-OHDA) -induced destruction of nigrostriatal DA fibers. In rats so-lesioned 5-HT receptor supersensitivity is additionally produced, and in fact 5-HT receptor antagonists attenuate enhanced DA D(1) induction of VCMs. Moreover, in 6-OHDA-lesioned rats treated with haloperidol for one year, there a 2-fold increase in numbers of VCMs (vs intact rats treated with haloperidol); and this high frequency of VCMs persists for more than 6 months after discontinuing haloperidol treatment. During this stage, 5-HT(2) receptor antagonists, but not DA D(1) receptor antagonists, attenuate the incidence of VCMs. This series of findings implicates the 5-HT neuronal phenotype in TD, and promotes 5-HT(2) receptor antagonists, more specifically 5-HT(2C) receptor antagonists, as a rational treatment approach for TD in humans.

Effects of dopamine D1-like receptor agonists on food-maintained operant behavior in rats

The effects on learned operant behavior of agonist actions at dopamine D1-like receptors have not been fully characterized. We compared three D1-like receptor agonists (SKF 38393, SKF 77434 and SKF 82958), both alone and in combination with the D1-like receptor antagonist, SCH 23390. Binding affinities for the agonists at dopamine D1 receptors from rat striatum membranes were determined and compared with effects on behavior. Lever pressing was maintained by food reinforcement under a fixed-ratio 30-response schedule (each 30th response produced reinforcement), and the effects of the three agonists were assessed by cumulative dosing. Each drug produced dose-related reductions in response rates, with an order of potency (SKF 82958>SKF 77434>SKF 38393) that agreed with rank order of binding affinities. Antagonism of these behavioral effects by SCH 23390 was only significant for SKF 82958; surprisingly, SCH 23390 enhanced the effects of SKF 38393. For SKF 82958, the antagonism was receptor subtype-specific, as the D2-like receptor antagonist spiperone was ineffective. The nonselective serotonergic antagonist metergoline produced a significant rightward shift of the SKF 38393 dose-response function, indicating effective antagonism, although the degree of antagonism was not dose-related. These results support the view that the behavioral effects of D1-like receptor agonists differ in their susceptibility to antagonism by D1-like receptor antagonists, and that some effects of SKF 38393 may be mediated by serotonergic activity rather than by activity at D1-like receptors.

Disruption of orofacial movement topographies in congenic mutants with dopamine D5 but not D4 receptor or DARPP-32 transduction 'knockout'

The role of D(1)-like [D(1), D(5)] and D(2)-like [D(2), D(3), D(4)] dopamine receptors and dopamine transduction via DARPP-32 in topographies of orofacial movement was assessed in restrained mice with congenic D(4) vs. D(5) receptor vs. DARPP-32 'knockout'. D(4) and DARPP-32 mutants evidenced no material phenotype; also, there were no alterations in topographical responsivity to either the selective D(2)-like agonist RU 24213 or the selective D(1)-like agonist SK and F 83959. In contrast, D(5) mutants evidenced an increase in spontaneous vertical jaw movements, which habituated more slowly than in wildtypes, and a decrease in horizontal jaw movements; topographical responsivity to SK and F 83959 and RU 24213 was unaltered. D(5) receptors regulate distinct topographies of vertical and horizontal jaw movement in an opposite manner. In assuming that the well-recognised role of the D(1)-like family in regulating orofacial movements involves primarily D(1) receptors, a role for their D(5) counterparts may have been overlooked.

Topographical resolution of jaw movements mediated by cyclase- vs. non-cyclase-coupled dopamine D1-like receptors: Studies with SK&F 83822

This study examined the effects on orofacial movement topography of SK&F 83822 ([R/S]-6-chloro-7,8-dihydroxy-3-allyl-1-[3-methylphenyl]-2,3,4,5-tetrahydro-1H-3-benzazepine), which stimulates dopamine D(1)-like receptors coupled to stimulation of adenylyl cyclase (AC) but not phosphoinositide (PI) hydrolysis, in comparison with SK&F 83959 ([R/S]-3-methyl-6-chloro-7,8-dihydroxy-1-[3-methyl-phenyl]-2,3,4,5-tetrahydro-1H-3-benzazepine), which stimulates PI hydrolysis but not AC. SK&F 83822 alone induced chattering, while SK&F 83959 alone exerted little effect. SK&F 83822 and SK&F 83959 each in combination with the dopamine D(2)-like agonist quinpirole resulted in synergistic induction of non-chattering movements with tongue protrusions. These effects were blocked by the dopamine D(1)-like receptor antagonist SCH 23390 ([R]-3-methyl-7-chloro-8-hydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine). However, the dopamine D(2)-like receptor antagonist YM 09151-2 (cis-N-[1-benzyl-2-methyl-pyrrolidin-3-yl]-5-chloro-2-methoxy-4-methylaminobenzamide) exerted a biphasic effect on synergism with SK&F 83822: chattering was initially released but antagonised thereafter. Only antagonism was seen for synergism with SK&F 83959. While both AC- and PI-coupled dopamine D(1)-like receptors participate in synergistic dopamine D(1)-like:D(2)-like receptor interactions, topographically specific synergistic and oppositional dopamine D(1)-like:D(2)-like interactions evident with SK&F 83822 reflect the involvement primarily of D(1)-like receptors coupled to AC rather than PI.

The effect of chronic administration of sarizotan, 5-HT1A agonist/D3/D4 ligand, on haloperidol-induced repetitive jaw movements in rat model of tardive dyskinesia

Dyskinesia is the most troublesome side effect in long-term treatment of both Parkinson's disease (PD) and schizophrenia. The 5-HT1A agonist and D3/D4 ligand sarizotan [Bartoszyk, G.D., van Amsterdam, C., Greiner, H.E., Rautenberg, W., Russ, H., Seyfried, C.A., 2004. Sarizotan, a serotonin 5-HT1A receptor agonist and dopamine receptor ligand. 1. Neurochemical profile. J. Neural Transm. 111, 113-126.] is in clinical development for the treatment of PD-associated dyskinesia. Because 5-HT1A agonists are known to counteract antipsychotic-induced motor side effects, sarizotan was investigated for its effects in two rat models of tardive dyskinesia (TD). The acute administration of sarizotan (0.17-13.5 mg/kg i.p.) reduced episodes of SKF 38393-induced repetitive jaw movements (RJM) in rats with a maximal effect at 1.5 mg/kg. In a chronic study, sarizotan (0.04-9 mg/kg/day), administered in the drinking water for 7 weeks during withdrawal from chronic haloperidol treatment (1.5 mg/kg/day), dose-dependently reversed haloperidol-induced RJM, significant at the doses of 1.5 and 9 mg/kg. Agonism at 5-HT1A receptors may be mediating the inhibitory effect of sarizotan on RJM in rat models of tardive dyskinesia.

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.

Prenatal ethanol preferentially enhances reactivity of the dopamine D1 but not D2 or D3 receptors in offspring

Reports of prenatal ethanol (ETOH) effects on the dopamine system are inconsistent. In an attempt to clarify this issue, dams were given 35% ethanol-derived calories as the sole nutrient source in a liquid diet from the 10th through the 20th day of gestation (ETOH). Controls were pair-fed (PF) an isocaloric liquid diet or given ad libitum access to laboratory chow (LC). Prenatal exposure to both liquid diets reduced body weight of offspring relative to LC controls, more so for ETOH than for PF exposure. Prenatal ETOH also decreased litter size and viability, relative to both LC and PF control groups. On postnatal days 21-23, male and female offspring were given an injection of saline vehicle or one of eight specific dopamine receptor agonists or antagonists. Immediately after injection subjects were placed in individual observation cages, and over the following 30 min, eight behaviors (square entries, grooming, rearing, circling, sniffing, yawning, head and oral movements) were observed and quantified. No prenatal treatment effects on drug-induced behaviors were observed for dopamine D2 (Apomorphine, DPAT or Quinpirole) or D3 (PD 152255, Nafadotride, Apo or Quin effects on yawning) receptor agonists or antagonists, or for the vehicle control. In contrast, prenatal treatment effects were seen with drugs affecting the dopamine D1 receptor. Both D1 agonists (SKF 38393) and antagonists (SCH 23390 and high doses of spiperone) altered behaviors, especially oral and sniffing behaviors, in a manner which suggested enhanced dopamine D1 drug sensitivity in both ETOH and PF offspring relative to LC controls. These results suggest that at this age, both sexes experience a prenatal undernutrition-linked increase in the behavioral response to dopamine D1 agonists and antagonists, which can be intensified by gestational exposure to alcohol.

Effect of lead exposure on dopaminergic transmission in the rat brain

Lead is a neurotoxicant with known behavioral and neurochemical effects. In this study we attempted to relate the behavioral effects of lead to neurotransmission. Oral administration of 1000 ppm of lead acetate to young rats for 30 days caused a reduction in locomotor activity and stereotypic exploratory behavior during a 20 min testing period. This locomotor hypoactivity induced by lead was accompanied by a reduction in stereotypic behavior (sniffing, lickings, biting and grooming). These outcomes suggested that lead might interfere with catecholaminergic and particularly dopaminergic neurotransmission. Therefore, we examined the effect of the lead acetate on the uptake of dopamine in striatal synaptosomal preparations. The collected data showed a clear inhibition of the uptake of 3H-DA with an IC50 of 3.5 x 10(-5)M. This inhibition of the uptake of dopamine suggests that the behavioral effects of lead may be involved in dopaminergic neurotransmission.

SK&F 83822 distinguishes adenylyl cyclase from phospholipase C-coupled dopamine D1-like receptors: behavioural topography

Effects of SK&F 83822 [3-allyl-6-chloro-7,8-dihydroxy-1-(3-methylphenyl)-2,3,4,5-tetrahydro-1H-3-benzazepine], an agonist at dopamine D1-like receptors which stimulate adenylyl cyclase but not phosphoinositide hydrolysis, were studied topographically so as to clarify differences between these receptors in the regulation of behaviour. Using cloned receptors, SK&F 83822 showed high, selective affinity for dopamine D1 and D5 over D2, D3, D4 and several non-dopamine receptors. SK&F 83822 induced little intense grooming, but readily induced sniffing, locomotion and rearing; seizures were evident at higher doses, characterised by tonic convulsions, forepaw myoclonus and explosive hyperlocomotion. The dopamine D1-like receptor antagonist SCH 23390 [R(+)-3-methyl-7-chloro-8-hydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine] readily antagonised these responses to SK&F 83822, particularly seizure activity. The dopamine D2-like receptor antagonist YM 09151-2 [cis-N-(1-benzyl-2-methyl-pyrrolidin-3-yl)-5-chloro-2-methoxy-4-methylaminobenzamide] did not alleviate seizures induced by SK&F 83822; YM 09151-02 did, however, attenuate SK&F 83822-induced sniffing, locomotion and rearing, and released vacuous chewing. These findings indicate that dopamine D1-like receptors linked to adenylyl cyclase can be differentiated from those not linked to adenylyl cyclase in terms of their roles in the topographical regulation of behaviour. For example, the seizure and vacuous chewing responses appear to involve dopamine D1-like receptors that stimulate adenylyl cyclase, while intense grooming involves those which do not.

Topographical effects of D1-like dopamine receptor agonists on orofacial movements in mice and their differential regulation via oppositional versus synergistic D1-like: D2-like interactions: cautionary observations on SK&F 82958 as an anomalous agent

Using a novel procedure, the regulation of individual topographies of orofacial movement in the mouse by oppositional versus cooperative/synergistic D1-like: D2-like dopamine receptor interactions was studied. The D1-like agonists SK&F 38393 and SK&F 83959 each induced vertical, but not horizontal, jaw movements, together with tongue protrusions and incisor chattering; however, SK&F 82958 induced a different profile which, consistent with other neurochemical and neurophysiological studies, suggests that this agent shows anomalous properties relative to other D1-like agonists. When given alone, the D2-like agonist quinpirole reduced horizontal jaw movements and incisor chattering. On coadministration, both SK&F 38393- and SK&F 83959-induced vertical jaw movements and tongue protrusions were inhibited by quinpirole, while SK&F 82958 again showed an anomalous profile. These findings indicate that, in the mouse, vertical jaw movements and tongue protrusions are regulated by oppositional D1-like: D2-like interactions, and appear to involve a D1-like receptor that is not coupled to adenylyl cyclase, whereas horizontal jaw movements are inhibited by D2-like receptors. Additionally, results obtained using SK&F 82958 as a probe for D1-like mechanisms should be treated with considerable caution until they are confirmed using other D1-like agonists.

Effects of adenosine A1, dopamine D1 and metabotropic glutamate 5 receptors-modulating agents on locomotion of the reserpinised rats

The pathophysiology of Parkinson's disease and l-3,4-dihydroxyphenylalanine (l-DOPA)-induced dyskinesia are characterised by an imbalance between activity of the direct and indirect pathways regulated by dopamine D1 and D2 receptors, respectively. In this study, we investigated the effects of treatments combining adenosine A(1) and metabotropic glutamate 5 (mGlu5) receptors modulators on locomotion induced by dopamine D1 receptor activation in the reserpine-treated rats. Administration of the adenosine A(1) receptor agonist and mGlu5 receptor antagonist resulted in the significant reduction of dopamine D1 receptor agonist-induced locomotion. The combination of adenosine A(1) receptor agonist with mGlu5 receptor antagonist had no greater effect than these compounds alone. However, the adenosine A(1) receptor antagonist attenuated the inhibitory effect of mGlu5 receptor antagonist. The data suggest that the effect of mGlu5 receptor blockade on locomotion elicited by dopamine D1 receptor stimulation involves activation of adenosine A(1) receptors. This interaction can improve our understanding of pathophysiology of L-DOPA-induced dyskinesia.

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.

Dopaminergic inhibition of secretin-stimulated choleresis by increased PKC-gamma expression and decrease of PKA activity

To determine the role and mechanisms of action by which dopaminergic innervation modulates ductal secretion in bile duct-ligated rats, we determined the expression of D1, D2, and D3 dopaminergic receptors in cholangiocytes. We evaluated whether D1, D2 (quinelorane), or D3 dopaminergic receptor agonists influence basal and secretin-stimulated choleresis and lumen expansion in intrahepatic bile duct units (IBDU) and cAMP levels in cholangiocytes in the absence or presence of BAPTA-AM, chelerythrine, 1-(5-isoquinolinylsulfonyl)-2-methyl piperazine (H7), or rottlerin. We evaluated whether 1) quinelorane effects on ductal secretion were associated with increased expression of Ca(2+)-dependent PKC isoforms and 2) increased expression of PKC causes inhibition of PKA activity. Quinelorane inhibited secretin-stimulated choleresis in vivo and IBDU lumen space, cAMP levels, and PKA activity in cholangiocytes. The inhibitory effects of quinelorane on secretin-stimulated ductal secretion and PKA activity were blocked by BAPTA-AM, chelerythrine, and H7. Quinelorane effects on ductal secretion were associated with activation of the Ca(2+)-dependent PKC-gamma but not other PKC isoforms. The dopaminergic nervous system counterregulates secretin-stimulated ductal secretion in experimental cholestasis.

Link between dopamine D1 and D2 receptors in rat and human striatal tissues

Many psychomotor behaviors depend on an interaction between dopamine D(1) and D(2) receptors. This study tested the hypothesis that agonist stimulation of dopamine D(1) receptors leads to the conversion of D(2) receptors from a state of high affinity for dopamine into a state of low affinity for dopamine. To test this hypothesis, dopamine was competed against [(3)H]raclopride for binding to rat and human striatal homogenates. Although the detection of high-affinity states of the dopamine D(2) receptor in rat or postmortem human striatum is normally difficult because the proportions of such sites are very low in the presence of physiological concentrations of sodium ions, the present work found that in the presence of 100 nM SCH 23390 to block D(1) receptors, a significant proportion of D(2) high-affinity sites were unmasked and readily revealed to be 10-35% of the D(2) population, illustrating the presence of a strong D(1)-D(2) link in both rat and human striata.

Comparative phenotypic resolution of spontaneous, D2-like and D1-like agonist-induced orofacial movement topographies in congenic mutants with dopamine D2 vs. D3 receptor ?knockout?

Using a novel system, the role of D2-like dopamine receptors in distinct topographies of orofacial movement was assessed in mutant mice with congenic D2 vs. D3 receptor knockout, and compared with findings in D1A mutants. Under spontaneous conditions, D2 mutants evidenced increased vertical jaw movements and unaltered horizontal jaw movements, with reductions in tongue protrusions and incisor chattering; in D3 mutants, only incisor chattering was reduced. Given previous evidence that D1A mutants show reduced horizontal but not vertical jaw movements, this indicates that apparent oppositional D1-like:D2-like interactions in the regulation of composited jaw movements may in fact reflect the independent actions of D2 receptors to inhibit vertical jaw movements and of D1A receptors to facilitate horizontal jaw movements. Effects of the D2-like agonist RU 24213 to exert greater reduction in horizontal than in vertical jaw movements were not altered prominently in either D2 or D3 mutants. The D1-like agonists A 68930 and SK&F 83959 induced vertical jaw movements, tongue protrusions, and incisor chattering; induction of tongue protrusions by A 68930 was reduced in D2 mutants. D2 receptors exert topographically specific regulation of orofacial movements in a manner distinct from their D1A counterparts, while D3 receptors exert only minor regulation of such movements.

Phenotypic resolution of spontaneous and D1-like agonist-induced orofacial movement topographies in congenic dopamine D1A receptor 'knockout' mice

A novel system was used to assess the role of D(1)-like dopamine receptors in distinct topographies of orofacial movements in mice with congenic D(1A) receptor knockout. Under spontaneous conditions, vertical jaw movements in wild-types declined with time at a rate that was reduced in D(1A) mutants, while horizontal jaw movements emerged progressively in wild-types but not in D(1A) mutants; tongue protrusions were absent in D(1A) mutants, while incisor chattering was initially reduced in D(1A) mutants but rose subsequently to reach the level of wild-types. D(1A) receptors exert a topographically specific role in regulating individual spontaneous orofacial movements, and these involve interactions with psychomotor processes which 'sculpt' behavioural change over time. The anomalous D(1)-like agonist SK&F 83959, which fails to stimulate, and indeed inhibits the stimulation of adenylyl cyclase induced by dopamine, readily stimulated vertical jaw movements, tongue protrusions and incisor chattering, and these response topographies were absent in D(1A) mutants. These results suggest that D(1A) receptors may exert some form of permissive role over orofacial topographies initiated via a novel, putative D(1)-like site not linked to adenylyl cyclase, or that some D(1A) receptors might be coupled to a transduction system other than adenylyl cyclase.

Dopaminergic transmission in the rat striatum in vivo in conditions of pharmacological modulation

The effects of pharmacological modulation of striatal dopaminergic neurotransmission were studied in freely mobile rats by intracerebral microdialysis and HPLC to assay dopamine and dopamine metabolite levels and the rate of dopamine synthesis, in combination with observations of stereotypical behavior. Inhibition of catechol O-methyltransferase (COMT) with tolcapone led to increases in extracellular dopamine levels only when the baseline dopamine level was elevated by administration of L-3,4-dihydroxyphenylalanine in combination with the decarboxylation inhibitor carbidopa. Increases in dopamine levels in striatal dialysates by blockade of reuptake were enhanced by inhibition of metabolic degradation of dopamine by tolcapone, a selective catechol O-methyltransferase inhibitor. GBR-12909, a blocker of the dopamine transporter, increased extracellular dopamine and induced motor stereotypy. Both of these effects were potentiated by tolcapone. The rate of dopamine biosynthesis decreased when reuptake was inhibited. These data provide evidence for the key role of the dopamine transporter in maintaining neurochemical homeostasis at the synaptic level.

Effects of individual and concurrent stimulation of striatal D1 and D2 dopamine receptors on electrophysiological and behavioral output from rat basal ganglia

Bilateral infusions of d-amphetamine into the rat ventral-lateral striatum (VLS) were previously shown to cause a robust behavioral activation that was correlated temporally with a net increase in firing of substantia nigra pars reticulata (SNpr) neurons, a response opposite predictions of the basal ganglia model. The current studies assessed the individual and cooperative contributions of striatal D1 and D2 dopamine receptors to these responses. Bilateral infusions into VLS of the D1/D2 agonist apomorphine (10 microg/microl/side) caused intense oral movements and sniffing, and an overall increase in SNpr cell firing to 133% of basal rates, similar to effects of d-amphetamine. However, when striatal D2 receptors were stimulated selectively by infusions of quinpirole (30 microg/microl/side) + the D1 antagonist R-(+)-7-chloro-8-hydroxy-3-methyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine (SCH 23390; 10 microg/microl/side), no behavioral response and only modest and variable changes in SNpr cell firing were observed. Selective stimulation of striatal D1 receptors by (+/-) 6-chloro-APB hydrobromide (SKF 82958; 10 microg/microl/side) + the D2 antagonist cis-N-(1-benzyl-2-methyl-pyrrolidin-3-yl)-5-chloro-2-methoxy-4-methyl-aminobenzamide (YM 09151-2; 2 microg/microl/side) caused a weak but sustained increase in oral movements and modestly increased SNpr cell firing, but neither response was of the magnitude observed with apomorphine. When the two agonists were infused concurrently, however, robust oral movements and sniffing again occurred over the same time period that a majority of SNpr cells exhibited marked, sometimes extreme and fluctuating, changes in firing (net increase, 117% of basal rates). These data confirm that concurrent striatal D1/D2 receptor stimulation elicits a strong motor activation that is correlated temporally with a net excitation rather than inhibition of SNpr firing, and reveal that D1 and D2 receptors interact synergistically within the striatum to stimulate both forms of output.

SKF-82958 is a subtype-selective estrogen receptor-alpha (ERalpha ) agonist that induces functional interactions between ERalpha and AP-1

The transcriptional activity of estrogen receptors (ERs) can be regulated by ligands as well as agents such as dopamine, which stimulate intracellular signaling pathways able to communicate with these receptors. We examined the ability of SKF-82958 (SKF), a previously characterized full dopamine D1 receptor agonist, to stimulate the transcriptional activity of ERalpha and ERbeta. Treatment of HeLa cells with SKF-82958 stimulated robust ERalpha-dependent transcription from an estrogen-response element-E1b-CAT reporter in the absence of estrogen, and this was accompanied by increased receptor phosphorylation. However, induction of ERbeta-directed gene expression under the same conditions was negligible. In our cell model, SKF treatment did not elevate cAMP levels nor enhance transcription from a cAMP-response element-linked reporter. Control studies revealed that SKF-82958, but not dopamine, competes with 17beta-estradiol for binding to ERalpha or ERbeta with comparable relative binding affinities. Therefore, SKF-82958 is an ERalpha-selective agonist. Transcriptional activation of ERalpha by SKF was more potent than expected from its relative binding activity, and further examination revealed that this synthetic compound induced expression of an AP-1 target gene in a tetradecanoylphorbol-13-acetate-response element (TRE)-dependent manner. A putative TRE site upstream of the estrogen-response element and the amino-terminal domain of the receptor contributed to, but were not required for, SKF-induced expression of an ERalpha-dependent reporter gene. Overexpression of the AP-1 protein c-Jun, but not c-Fos, strongly enhanced SKF-induced ERalpha target gene expression but only when the TRE was present. These studies provide information on the ability of a ligand that weakly stimulates ERalpha to yield strong stimulation of ERalpha-dependent gene expression through cross-talk with other intracellular signaling pathways producing a robust combinatorial response within the cell.

SKF83959 exhibits biochemical agonism by stimulating [(35)S]GTP gamma S binding and phosphoinositide hydrolysis in rat and monkey brain

SKF83959, a benzazepine with high affinity for aminergic receptors, elicits behaviors such as grooming and vacuous chewing that are characteristic of dopamine D(1)-like receptor stimulation in rodents. Unlike classical D(1) agonists, however, SKF83959 does not stimulate adenylyl cyclase. Knowing that some D(1)-like receptors are coupled to phospholipase C-mediated signaling cascades in the brain, the present study aimed to determine whether SKF83959 exhibits an agonistic action at the biochemical level and also whether this benzazepine can modulate phosphoinositide hydrolysis in a manner that would be consistent with the behavioral effects of the drug. Similar to dopamine and the selective D(1)-like agonist SKF38393, SKF83959 competitively displaced the receptor binding of [(3)H]dopamine in an agonist-like manner, significantly stimulated [(35)S]guanosine-5'-O-(3-thio)triphosphate binding, and potently enhanced phospholipase C-mediated phosphoinositide hydrolysis in rat and monkey brain tissues. SKF83959 was generally more potent than SKF38393, whereas SKF38393 consistently exhibited greater pharmacological efficacy. These findings may implicate a role for the phospholipase C signaling cascade in the agonistic behavioral and antiparkinsonian activity of SKF83959. Dopamine-sensitive phospholipase C signaling should probably be considered in subsequent formulations of mechanisms and models of dopaminergic function in the normal or diseased brain.

The psychopharmacology-molecular biology interface: exploring the behavioural roles of dopamine receptor subtypes using targeted gene deletion ('knockout')

In the absence of selective agonists and antagonists able to discriminate between individual members of the D1-like and D2-like families of dopamine receptor subtypes, functional parcellation has remained problematic. 'Knockout' of these subtypes by targeted gene deletion offers a new approach to evaluating their roles in the regulation of behaviour. Like any new technique, 'knockout' has associated with it a number of methodological limitations that are now being addressed in a systematic manner. Studies on the phenotype of D1(A/1), D(1B/5), D2, D3 and D4 'knockouts' at the level of spontaneous and agonist/antagonist-induced behaviour are reviewed, in terms of methodological issues, neuronal implications and potential clinical relevance. Dopamine receptor subtype 'knockout' is a nascent technology that is now beginning to fulfil its potential. It is being complemented by more systematic phenotypic characterisation at the level of behaviour and additional, molecular biologically-based approaches.

Topographical assessment and pharmacological characterization of orofacial movements in mice: dopamine D(1)-like vs. D(2)-like receptor regulation

A novel procedure for the assessment of orofacial movement topographies in mice was used to study, for the first time, the individual and interactive involvement of dopamine D(1)-like vs. D(2)-like receptors in their regulation. The dopamine D(1)-like receptor agonists A 68930 ([1R,3S]-1-aminomethyl-5,6-dihydroxy-3-phenyl-isochroman) and SK&F 83959 (3-methyl-6-chloro-7,8-dihydroxy-1-[3-methyl-phenyl]-2,3,4,5-tetrahydro-1H-3-benzazepine) each induced vertical jaw movements with tongue protrusions and incisor chattering. The dopamine D(1)-like receptor antagonists SCH 23390 ([R]-3-methyl-7-chloro-8-hydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine) and BW 737C ([S]-6-chloro-1-[2,5-dimethoxy-4-propylbenzyl]-7-hydroxy-2-methyl-1,2,3,4-tetrahydroisoquinoline) antagonised these responses, while the dopamine D(2)-like receptor antagonist YM 09151-2 (cis-N-[1-benzyl-2-methyl-pyrrolidin-3-yl]-5-chloro-2-methoxy-4-methylaminobenzamide) attenuated those to SK&F 83959 and released horizontal jaw movements. These findings suggest some role for a dopamine D(1)-like receptor that is coupled to a transduction system other than/additional to adenylyl cyclase, and for dopamine D(1)-like:D(2)-like receptor interactions, in the regulation of individual orofacial movement topographies in the mouse. This methodology will allow the use of knockout mice to clarify the roles of individual dopamine receptor subtypes in their regulation.

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.

Comparative effects of scopolamine and quinpirole on the striatal fos expression induced by stimulation of D(1) dopamine receptors in the rat

Treatment of intact rats with the full D(1) dopamine agonist A-77636 induced Fos-like immunoreactivity in the medial and, to a lesser extent, the lateral portions of the striatum. Pretreatment with the muscarinic antagonist scopolamine hydrobromide (1.5-6 mg/kg) potentiated the response to A-77636 and eliminated the mediolateral staining gradient seen after A-77636 alone. Similar effects were not produced by scopolamine methylbromide, which fails to cross the blood-brain barrier, demonstrating that the actions of scopolamine were centrally mediated. The effects of scopolamine were further compared to those of the D(2)-like dopamine agonist quinpirole using a factorial design in which subjects were pretreated with either scopolamine, quinpirole, or a combination of the two drugs before receiving A-77636. Pretreatment with either scopolamine or quinpirole increased staining in the lateral striatum, but the combination of the two drugs was no more effective than was quinpirole alone. Pretreatment with quinpirole, but not scopolamine, resulted in a markedly "patchy" pattern of staining and actually suppressed staining in the region between patches in the medial striatum. These findings demonstrate that there are both differences and similarities between the effects of scopolamine and quinpirole on D(1) agonist-induced Fos expression and suggest that although inhibition of cholinergic neurons may be one of the mechanisms through which the effects of quinpirole are produced, other factors must also contribute.

Mammal-like striatal functions in Anolis. II. Distribution of dopamine D(1) and D(2) receptors, and a laminar pattern of basal ganglia sub-systems

We used in situ autoradiographic ligand binding methods to determine the occurrence and distribution of dopamine D(1) and D(2) receptor sub-types in the anole lizard, Anolis carolinensis. Both were present and exhibited pharmacological specificity characteristics similar to those described for mammals. However, unlike in mammals where in the neostriatum [outside the nucleus accumbens/olfactory tubercle complex (NA/OT)] these receptors exhibit only slight dorsolateral (D(2) high, D(1) low) to ventromedial (D(1 )high, D(2) low) gradients that co mingle extensively, in the anole striatum outside the NA/OT there was a striking laminar pattern, with little if any overlap between D(2) (high in a dorsal band) and D(1) (high ventral to the D(2) band) distributions. As D(1) receptors are related to the direct and D(2) to the indirect basal ganglia (BG) subsystems in mammals, we also determined anole striatal distributions of pre-proenkephalin mRNA, a marker for striatal efferents to the indirect BG subsystem in mammals. Here, too, there was a striking laminar pattern, with pre-proenkephalin mRNA in a band similar to that seen for D(2) receptors. The crisp neuroanatomical separation between these classic BG subsystem markers in Anolis striatum make this species attractive for the study of such systems' functions during behavior.

In vivo NMDA/dopamine interaction resulting in Fos production in the limbic system and basal ganglia of the mouse brain

Glutamatergic and dopaminergic effects on molecular processes have been extensively investigated in the basal ganglia. It has been demonstrated that NMDA and dopamine D(1) and D(2) receptors interact in the regulation of signal transduction and induction of transcription factors. In the present experiments, NMDA/dopamine interactions were investigated in the normosensitive caudate nucleus, hippocampus and amygdala by monitoring Fos production. We demonstrated that NMDA and the D(1) receptor agonist SKF 38393 triggered Fos levels in a distinct, non-overlapping and region-specific pattern. NMDA injected intraperitoneally (i.p.) elevated Fos levels in all hippocampal subfields and the central amygdala, whereas SKF 38393 triggered Fos production in basomedial, cortical, medial amygdala and caudate nucleus. The NMDA receptor antagonist CGS 19755 prevented NMDA- and SKF 38393-triggered Fos production in all investigated brain areas. Similarly, the D(1) receptor antagonist SCH 23390 inhibited the effects produced by SKF 38393 or NMDA. The D(2) receptor antagonist sulpiride exerted synergistic and antagonistic effects on NMDA- and SKF 38393-triggered Fos production, in a region specific manner. These data suggest that NMDA and dopamine receptors regulate Fos production within the limbic system and basal ganglia through regionally differentiated but interdependent actions.

Neurochemical changes in the entopeduncular nucleus and increased oral behavior in rats treated subchronically with clozapine or haloperidol

The purpose of the present experiment was to test the possibility that atypical antipsychotics and classical antipsychotics differentially regulate specific neurochemical processes within the entopeduncular nucleus. For these experiments, rats were administered clozapine (25 mg/kg), haloperidol (1 mg/kg), or Tween-80 (control) daily for 21 days. Dopamine D(1)-receptor binding was assessed with in vitro receptor autoradiographic methods and the mRNAs corresponding to the two forms of glutamate decarboxylase (glutamate decarboxylase-65 and glutamate decarboxylase-67) were analyzed using in situ hybridization histochemical methods. In addition, vacuous chewing movements (VCM) were measured throughout the drug administration period as a functional indicator of drug action and changes in striatal dopamine D(2)-receptor binding were measured as a positive control for D(2)-receptor antagonist properties of haloperidol and clozapine. In agreement with previous reports, haloperidol increased D(2)-receptor binding throughout the striatum while clozapine had a more limited impact on D(2)-receptors. Behavioral analysis revealed that both haloperidol and clozapine enhanced the display of vacuous chewing movements to a similar extent but with a different postinjection latency. In the entopeduncular nucleus, clozapine increased D(1)-receptor binding compared to controls while haloperidol was without effect. With respect to the regulation of GAD mRNAs, haloperidol increased glutamate decarboxylase-65 and glutamate decarboxylase-67 mRNA levels throughout the entopeduncular nucleus. The effects of clozapine were restricted to increases in glutamate decarboxylase-65 mRNA. These studies show that clozapine and haloperidol, both of which increase the occurrence of VCM, differentially modulate the neurochemistry of the entopeduncular nucleus.

Differential behavioural effect of quinpirole in neuroleptic-pretreated rats - role of alpha(1)-adrenoceptor

This paper presents the effect of 14-day intraperitoneal (i.p.) neuroleptic treatment on the behavioural response of Wistar rats to (-)-quinpirole hydrochloride (3 mg/kg, i.p.) administered 24 h after the last neuroleptic dose. Chlorpromazine hydrochloride (10 mg/kg), haloperidol (2 mg/kg) or (+/-)-sulpiride (100 mg/kg) increased the effect of quinpirole; however, there were qualitative and quantitative differences between the neuroleptics. Chlorpromazine and haloperidol, but not sulpiride, pretreatment enhanced quinpirole-induced locomotor hyperactivity. Prazosin (0.5 mg/kg, i.p. ) given to chlorpromazine-treated rats 1 h before quinpirole attenuated the quinpirole-induced hyperlocomotion. In chlorpromazine-pretreated rats, quinpirole elicited defensive aggressive behaviour with vocalization, copulatory attempts, intense rearing and head-down sniffing. When prazosin was given before quinpirole, head-down sniffing and object-directed oral activity were mainly observed. In haloperidol-pretreated rats, quinpirole induced intense head-down sniffing, rearing, grooming and object-directed oral activity. In sulpiride-pretreated rats, quinpirole induced intense head-down sniffing, grooming and object-directed oral activity. The results of the study suggest that differences in the behavioural expression of dopamine D(2) receptor supersensitivity induced by neuroleptics may be, at least in part, caused by concurrent stimulation of alpha(1)-adrenoceptors.

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.

Timing: A critical determinant of the functional consequences of neonatal 6-OHDA lesions

Previous data have indicated that intrastriatal (IS) lesions of the dopamine (DA) system early in development result in a selective effect on D1 receptor expression and sensitivity, which is not seen with adult lesions or lesions made later in development. The purpose of the present study was to test the hypothesis that the timing of the lesion is a critical determinant of the consequences of DA depletion during development. Rats received IS injections of 6-hydroxydopamine (6-OHDA) on day of birth/postnatal day 1 (P0/1) or P7, which resulted in similar decreases in the number of DA uptake sites (> or =70% loss), a measure of DA terminal density. As adults, lesioned rats were challenged with DA receptor agonists to examine the functional sensitivity of D1 and D2 receptors. In adulthood, P0/1-lesioned rats exhibited increases in oral dyskinesias and rearing behavior following treatment with the partial D1 receptor agonists, SKF38393 and SKF77434, whereas rats lesioned on P7 exhibited increases in grooming. P7-lesioned rats also exhibited increases in gnawing, explosive jumping, and self-biting behavior following treatment with the full D1 receptor agonist SKF82958, which were not observed in the other groups. The results support the hypothesis that the timing of DA denervation is of paramount importance for governing the functional consequences of neonatal lesions, as measured by the incidence of DA agonist-induced behaviors in adulthood.

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.

Alpha1-adrenergic, D1, and D2 receptors interactions in the prefrontal cortex: implications for the modality of action of different types of neuroleptics

The activation of rat mesocortical dopaminergic (DA) neurons evoked by the electrical stimulation of the ventral tegmental area (VTA) induces a marked inhibition of the spontaneous activity of prefrontocortical cells. In the present study, it was first shown that systemic administration of either clozapine (a mixed antagonist of D1, D2, and alpha1-adrenergic receptors) (3-5 mg/kg, i.v.), prazosin (an alpha1-adrenergic antagonist) (0.2 mg/kg, i.v.), or sulpiride (a D2 antagonist) (30 mg/kg, i.v.), but not SCH 23390 (a D1 antagonist) (0.2 mg/kg, i.v.), reversed this cortical inhibition. Second, it was found that following the systemic administration of prazosin, the VTA-induced cortical inhibition reappeared when either SCH 23390 or sulpiride was applied by iontophoresis into the prefrontal cortex. Third, it was seen that, whereas haloperidol (0.2 mg/kg, i.v.), a D2 antagonist which also blocks alpha1-adrenergic receptors, failed to reverse the VTA-induced inhibition, the systemic administration of haloperidol plus SCH 23390 (0.2 mg/kg, i.v.) blocked this inhibition. Finally, it was verified that the cortical inhibitions obtained following treatments with either "prazosin plus sulpiride" or "prazosin plus SCH 23390" were blocked by a superimposed administration of either SCH 23390 or sulpiride, respectively. These data indicate that complex interactions between cortical D2, D1, and alpha1-adrenergic receptors are involved in the regulation of the activity of prefrontocortical cells innervated by the VTA neurons. They confirm that the physiological stimulation of cortical alpha1-adrenergic receptors hampers the functional activity of cortical D1 receptors and suggest that the stimulations of cortical D1 and D2 receptors exert mutual inhibition on each other's transmission.

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.

Place conditioning with the dopamine D1-like receptor agonist SKF 82958 but not SKF 81297 or SKF 77434

While self-administration and place conditioning studies have shown that dopamine D2-like receptor agonists produce reward-related learning, the effects of dopamine D1-like receptor agonists remain equivocal. The present study tested three dopamine D1-like receptor agonists for their ability to induce a place preference. Like control rats treated with amphetamine (2.0 mg/kg i.p.), rats treated with SKF 82958 (+/- -6-chloro-7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1-phenyl-1H- 3-benzazepine hydrobromide; 0.05 but not 0.01, 0.025, 0.075, or 0.10 mg/kg s.c. and/or i.p.) during conditioning showed a significant increase in the amount of time spent on the drug-paired side during the drug free test. Neither SKF 81297 (+/- -6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide; 0.25, 0.50, 1.0, 2.0, and 4.0 mg/kg i.p.) nor SKF 77434 (+/- -7,8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrochloride; 0.20, 1.0, 5.0, and 10.0 mg/kg i.p.) produced place conditioning. Significant increases in locomotion were seen at some doses of all drugs. Results show for the first time that systemic administration of a dopamine D1-like receptor agonist produces a place preference and are consistent with previous findings showing that dopamine D1-like receptor activation produces reward-related learning.

Dopamine depresses excitatory and inhibitory synaptic transmission by distinct mechanisms in the nucleus accumbens

The release of dopamine (DA) in the nucleus accumbens (NAc) is thought to be critical for mediating natural rewards as well as for the reinforcing actions of drugs of abuse. DA and amphetamine depress both excitatory and inhibitory synaptic transmission in the NAc by a presynaptic D1-like DA receptor. However, the mechanisms of depression of excitatory and inhibitory synaptic transmission appear to be different. DA depressed the frequency of spontaneous miniature EPSCs, but the frequency of miniature IPSCs was depressed only when spontaneous release was made dependent on Ca2+ influx through voltage-dependent Ca2+ channels. Furthermore, the K+ channel blocker Ba2+ attenuated the effects of DA on evoked IPSPs, but not on EPSPs. Thus, DA appears to depress inhibitory synaptic transmission in the NAc by reducing Ca2+ influx into the presynaptic terminal, but depresses excitatory transmission by a distinct mechanism that is independent of the entry of Ca2+.

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.

Alcohol self-administration: further examination of the role of dopamine receptors in the nucleus accumbens

One of the functions of the mesolimbic dopamine (DA) system is to regulate the process of reinforcement, a process that is thought to influence drug self-administration. This study tested the effects of centrally administered DA receptor ligands on ethanol self-administration behavior. Long-Evans rats were trained to lever press on a fixed-ratio 4 schedule of ethanol (10% v/v) reinforcement. DA agonists and antagonists were then bilaterally microinjected (0.5 microliter/side) into the nucleus accumbens (N Acc) 10-min before sessions to test for effects on the onset, maintenance, and termination of ethanol self-administration. Infusions of the D1-like agonist SKF 38393 (0.03 to 3.0 micrograms) produced no effect on ethanol self-administration. The D1-like antagonist SCH 23390 (0.5 to 2.0 micrograms) reduced total responding by decreasing the time course of self-administration without altering response rate. The D2-like agonist quinpirole produced a biphasic effect on self-administration. Quinpirole (1.0 microgram) increased total responses and response rate, whereas higher doses (4.0 to 10.0 micrograms) decreased total responding as a result of early termination. The D2-like antagonist raclopride (0.1 to 1.0 microgram) reduced total responding by decreasing time course and response rate. Co-administration of either SKF 38393 or SCH 23390 with quinpirole prevented the behavioral effects observed with the low doses of quinpirole. Thus, in the N Acc either increased activation of D1-like receptors or their blockade can affect the expression of the behavioral effects of the D2-like agonist. This suggests that some intermediate level of D1 activation is required to observe the D2 effect. The decreases in total responding produced by raclopride were enhanced by co-administration of SKF 38393, but not altered by SCH 23390, thus suggesting that D1-like and D2-like receptors in the N Acc interact in the regulation of ethanol self-administration in a manner similar to their interactive regulation of other behaviors.

Conditioned grooming induced by the dopamine D1-like receptor agonist SKF 38393 in rats

Stimulation of dopamine D1-like receptors reliably increases grooming in rats and mice. The study examined whether the grooming response elicited by the prototypical D1-like agonist SKF 38393 (8 mg/kg s.c.) could be conditioned to the specific environment in which it occurred. Rats in one group (Paired) received SKF 38393 and rats in another group (Unpaired) received saline in observation boxes outside of their housing room; the rats were then scored for duration and frequency of grooming bouts over 25 min. The ordering of injections was reversed the next day in the rats, housing room. The procedure was repeated twice, with at least one intervening drug-free day, to give three conditioning trials. The D1-like agonist significantly increased grooming on each of the three conditioning trials, without obvious tolerance or sensitization, and the effect tended to persist for the duration of each trial. On the test trial for conditioned grooming, mean grooming duration was significantly greater in the Paired than the Unpaired group, suggesting that SKF 38393-induced grooming had been conditioned to the test environment. This is the first time that drug-elicited grooming has been conditioned to environmental cues.

Effects of perospirone (SM-9018), a potential atypical neuroleptic, on dopamine D1 receptor-mediated vacuous chewing movement in rats: a role of 5-HT2 receptor blocking activity

We compared the acute and subacute effects of perospirone (SM-9018), a novel neuroleptic with potent 5-HT2 and D2 blocking actions, and of haloperidol (HAL) on dopamine D1 receptor-mediated vacuous chewing movement (VCM) in rats. A selective D1 agonist, SKF 38393 (SKF), markedly increased the incidence of VCM, which was blocked by SCH 23390 (a D1 antagonist) but not by sulpiride (a D2 antagonist). Perospirone and HAL inhibited the SKF-induced VCM in a dose-dependent manner. The potency of the inhibitory actions of perospirone was considerably weaker (about 30 times) than that of HAL despite their similar affinities for D1 receptors. Subacute treatment with perospirone for 2 weeks failed to affect the behavioral sensitivity of rats to SKF. However, the HAL treatment markedly enhanced the incidence of the SKF-induced VCM. On the other hand, the selective 5-HT2 antagonists ritanserin and ketanserin significantly reduced the inhibitory actions of HAL and SCH 23390 on the SKF-induced VCM. In addition, combined treatment of ritanserin with HAL for 2 weeks abolished the enhancement of SKF-induced VCM by HAL treatment. These findings suggest that perospirone is weaker than HAL in altering the behavioral sensitivity of D1 receptor-mediated VCM under repeated administration, which may be related to the 5-HT2 blocking activity of perospirone.

Single restraint stress sensitizes acute chewing movements induced by haloperidol, but not if the 5-HT1A agonist 8-OH-DPAT is given prior to stress

The objective of this study was two-fold: (i) to analyze behavioral sensitization to haloperidol 2 weeks after single restraint stress, and (ii) to establish the effects of 8-OH-DPAT treatment prior to stress on sensitized behavioral responses. Overall behavior was analyzed and not only catalepsy, but also sedation (immobility), grooming, exploration and vacuous chewing movements were evaluated. Results indicated that single restraint stress induced a long-lasting sensitization of acute vacuous chewing movements induced by haloperidol (0.25, 0.5 mg/kg i.p.). Interestingly, this behavioral sensitization was prevented by 8-OH-DPAT (0.35 mg/kg s.c.) prior to stress. Finally, haloperidol-induced sedation was not disrupted by either restraint stress or 8-OH-DPAT treatment.

Talipexole or pramipexole combinations with chloro-APB (SKF 82958) in MPTP-induced hemiparkinsonian monkeys

The effects of two predominant dopamine D2-like receptor agonists, talipexole (6-allyl-2-amino-5,6,7,8-tetrahydro-4H-thiazolo [4,5-d]-azepine dihydrochloride, B-HT 920 CL2) and pramipexole (S(-)2-amino-4,5,6,7-tetrahydro-6-propyl-aminobenzothiazole dihydrochloride, SND 919 CL2Y), were studied alone and in combination with the selective dopamine D1-like receptor agonist chloro-APB ((+/-)6-chloro-7-8-dihydroxy-3-allyl-1-phenyl-2,3,4,5-tetrahydro-1H-benz azepine hydrobromide, SKF 82958) in five chronic 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) lesioned hemiparkinsonian Macaca nemestrina monkeys. Talipexole induced contraversive rotation in a dose-dependent manner up to 32 microg/kg, i.m. Talipexole was more potent than pramipexole (10 vs. 32 microg/kg, i.m.), but pramipexole was more efficacious in producing contraversive rotational behavior and significant hand movements in the afflicted limb. Larger doses of chloro-APB also produced contraversive rotation. Combinations of each dopamine D2-like receptor agonist in a median effective dose with chloro-APB (23.4 and 74.8 microg/kg, i.m.) had synergistic effects, producing either addition or potentiation, depending upon the dose used. The effects noted with these combinations were less than the effect of a large dose (100 microg/kg) of pramipexole. Talipexole, in the largest dose studied (100 microg/kg, i.m.), produced sedation which was not seen with the same dose of pramipexole. No significant extrapyramidal side effects were noted with either agent.