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

Pannexin-1 channels in epilepsy

Pannexin-1 (Panx1) expression is raised in several animal seizure models and in resected human epileptic brain tissue, suggesting relevance to epilepsy. Multiple factors that are characteristic of seizures are thought to regulate Panx1 channel opening, including elevated levels of extracellular K⁺. Panx1, when open, 1) releases ATP, glutamate, and other metabolites into the extracellular medium, and 2) may depolarize the membrane due to a channel reversal potential around 0mV. Resultant ATP release from stimulated Panx1 can activate purinergic receptors, including P2X7 receptors. Glutamate and other signaling molecules released by Panx1 opening may have both excitatory and inhibitory actions on seizure generation. This review examines the critical and complex roles of Panx1 channels in epilepsy, which could provide a basis for future therapeutics.

The role of dopamine signaling in epileptogenesis

Clinical and experimental studies implicate most neuromodulatory systems in epileptogenesis. The dopaminergic system has a seizure-modulating effect that crucially depends on the different subtypes of dopamine (DA) receptors involved and the brain regions in which they are activated. Specifically, DA plays a major role in the control of seizures arising in the limbic system. Studies performed in a wide variety of animal models contributed to illustrate the opposite actions of D1-like and D2-like receptor signaling in limbic epileptogenesis. Indeed, signaling from D1-like receptors is generally pro-epileptogenic, whereas D2-like receptor signaling exerts an anti-epileptogenic effect. However, this view might appear quite simplistic as the complex neuromodulatory action of DA in the control of epileptogenesis likely requires a physiological balance in the activation of circuits modulated by these two major DA receptor subtypes, which determines the response to seizure-promoting stimuli. Here we will review recent evidences on the identification of molecules activated by DA transduction pathways in the generation and spread of seizures in the limbic system. We will discuss the intracellular signaling pathways triggered by activation of different DA receptors in relation to their role in limbic epileptogenesis, which lead to the activation of neuronal death/survival cascades. A deep understanding of the signaling pathways involved in epileptogenesis is crucial for the identification of novel targets for the treatment of epilepsy.

Rhes: a GTP-binding protein integral to striatal physiology and pathology

Rhes, the Ras Homolog Enriched in Striatum, is a GTP-binding protein whose gene was discovered during a screen for mRNAs preferentially expressed in rodent striatum. This 266 amino acid protein is intermediate in size between small Ras-like GTP-binding proteins and α-subunits of heterotrimeric G proteins. It is most closely related to another Ras-like GTP-binding protein termed Dexras1 or AGS1. Although subsequent studies have shown that the rhes gene is expressed in other brain areas in addition to striatum, the striatal expression level is relatively high, and Rhes protein is likely to play a vital role in striatal physiology and pathology. Indeed, it has recently been shown to interact with the Huntingtin protein and play a pivotal role in the selective vulnerability of striatum in Huntington's disease (HD). Not surprisingly, Rhes can interact with multiple proteins to affect striatal physiology at multiple levels. Functional studies have indicated that Rhes plays a role in signaling by striatal G protein-coupled receptors (GPCR), although the details of the mechanism remain to be determined. Rhes has been shown to bind to both α- and β-subunits of heterotrimeric G proteins and to affect signaling by both Gi/o- and Gs/olf-coupled receptors. In this context, Rhes can be classified as a member of the family of accessory proteins to GPCR signaling. With documented effects in dopamine- and opioid-mediated behaviors, an interaction with thyroid hormone systems and a role in HD pathology, Rhes is emerging as an important protein in striatal physiology and pathology.

Cell signaling underlying epileptic behavior

Epilepsy is a complex disease, characterized by the repeated occurrence of bursts of electrical activity (seizures) in specific brain areas. The behavioral outcome of seizure events strongly depends on the brain regions that are affected by overactivity. Here we review the intracellular signaling pathways involved in the generation of seizures in epileptogenic areas. Pathways activated by modulatory neurotransmitters (dopamine, norepinephrine, and serotonin), involving the activation of extracellular-regulated kinases and the induction of immediate early genes (IEGs) will be first discussed in relation to the occurrence of acute seizure events. Activation of IEGs has been proposed to lead to long-term molecular and behavioral responses induced by acute seizures. We also review deleterious consequences of seizure activity, focusing on the contribution of apoptosis-associated signaling pathways to the progression of the disease. A deep understanding of signaling pathways involved in both acute- and long-term responses to seizures continues to be crucial to unravel the origins of epileptic behaviors and ultimately identify novel therapeutic targets for the cure of epilepsy.

Mice lacking rhes show altered morphine analgesia, tolerance, and dependence

Rhes, the Ras Homolog Enriched in Striatum, is an intermediate-size GTP binding protein. Although its full functions are not yet known, it has been shown to affect signaling and behaviors mediated by G protein-coupled receptors. Here we have tested whether Rhes affects behaviors mediated by opioid receptors. Wild type and rhes-deficient mice were administered morphine and tested for analgesia in formalin and tail flick tests. Rhes⁻/⁻ mice showed significantly enhanced analgesia in both tests relative to rhes+/+ mice. Furthermore, rhes⁻/⁻ mice did not display tolerance to repeated morphine administration and displayed significantly less withdrawal than rhes+/+ mice. These findings indicate that Rhes is involved in behaviors mediated by mu opioid receptors and in the adaptive response to repeated morphine administration.

Electrophysiological effects of SKF83959 on hippocampal CA1 pyramidal neurons: potential mechanisms for the drug's neuroprotective effects

Although the potent anti-parkinsonian action of the atypical D₁-like receptor agonist SKF83959 has been attributed to the selective activation of phosphoinositol(PI)-linked D₁ receptor, whereas the mechanism underlying its potent neuroprotective effect is not fully understood. In the present study, the actions of SKF83959 on neuronal membrane potential and neuronal excitability were investigated in CA1 pyramidal neurons of rat hippocampal slices. SKF83959 (10–100 µM) caused a concentration-dependent depolarization, associated with a reduction of input resistance in CA1 pyramidal neurons. The depolarization was blocked neither by antagonists for D₁, D₂, 5-HT_2A/2C receptors and α₁-adrenoceptor, nor by intracellular dialysis of GDP-β-S. However, the specific HCN channel blocker ZD7288 (10 µM) antagonized both the depolarization and reduction of input resistance caused by SKF83959. In voltage-clamp experiments, SKF83959 (10–100 µM) caused a concentration-dependent increase of Ih current in CA1 pyramidal neurons, which was independent of D₁ receptor activation. Moreover, SKF83959 (50 µM) caused a 6 mV positive shift in the activation curve of Ih and significantly accelerated the activation of Ih current. In addition, SKF83959 also reduced the neuronal excitability of CA1 pyramidal neurons, which was manifested by the decrease in the number and amplitude of action potentials evoked by depolarizing currents, and by the increase of firing threshold and rhoebase current. The above results suggest that SKF83959 increased Ih current through a D₁ receptor-independent mechanism, which led to the depolarization of hippocampal CA1 pyramidal neurons. These findings provide a novel mechanism for the drug's neuroprotective effects, which may contributes to its therapeutic benefits in Parkinson's disease.

Pharmacology of signaling induced by dopamine D(1)-like receptor activation

Dopamine D(1)-like receptors consisting of D(1) and D(5) subtypes are intimately implicated in dopaminergic regulation of fundamental neurophysiologic processes such as mood, motivation, cognitive function, and motor activity. Upon stimulation, D(1)-like receptors initiate signal transduction cascades that are mediated through adenylyl cyclase or phosphoinositide metabolism, with subsequent enhancement of multiple downstream kinase cascades. The latter actions propagate and further amplify the receptor signals, thus predisposing D(1)-like receptors to multifaceted interactions with various other mediators and receptor systems. The adenylyl cyclase response to dopamine or selective D(1)-like receptor agonists is reliably associated with the D(1) subtype, while emerging evidence indicates that the phosphoinositide responses in native brain tissues may be preferentially mediated through stimulation of the D(5) receptor. Besides classic coupling of each receptor subtype to specific G proteins, additional biophysical models are advanced in attempts to account for differential subcellular distribution, heteromolecular oligomerization, and activity-dependent selectivity of the receptors. It is expected that significant advances in understanding of dopamine neurobiology will emerge from current and anticipated studies directed at uncovering the molecular mechanisms of D(5) coupling to phosphoinositide signaling, the structural features that might enhance pharmacological selectivity for D(5) versus D(1) subtypes, the mechanism by which dopamine may modulate phosphoinositide synthesis, the contributions of the various responsive signal mediators to D(1) or D(5) interactions with D(2)-like receptors, and the spectrum of dopaminergic functions that may be attributed to each receptor subtype and signaling pathway.

Dopamine D(1) receptor activation of adenylyl cyclase, not phospholipase C, in the nucleus accumbens promotes maternal behavior onset in rats

A body of evidence supports the idea that the mesolimbic dopamine (DA) system modulates the natural increase in responsiveness female rats show toward offspring (biological or foster) at birth. In the absence of the full hormonal changes associated with pregnancy and birth, female rats do not show immediate responsiveness toward foster offspring. Activation of the mesolimbic DA system can produce an immediate onset of maternal behavior in these females. For example, female rats that are hysterectomized and ovariectomized on day 15 of pregnancy (15HO) and presented with pups 48 hours later normally show maternal behavior after 2-3 days of pup exposure, but will show maternal behavior on day 0 of testing after microinjection of the DA D(1) receptor agonist, SKF 38393, into the nucleus accumbens (NA) at the time of pup presentation. DA D(1) receptor stimulation is known to activate cAMP intracellular signaling cascades via its stimulation of adenylyl cyclase (AC). However, some DA D(1) receptors are also linked to phospholipase C (PLC) and are capable of activating phosphatidylinositol signaling cascades. SKF 38393 stimulates both types of D(1) receptors. Here we provide evidence that the facilitatory effects of DA D(1) receptor stimulation in the NA on maternal behavior are mediated by AC-linked DA D(1) receptors. By examining the effects of intra-NA application of SKF 83822, a drug which selectively binds DA D(1)-AC receptors, or SKF 83959, a drug which selectively activates D(1)-PLC-linked receptors, we find that only SKF 83822 facilitates maternal behavior onset.

Age-related behavioural phenotype and cellular characterisation of mice with progressive ablation of D1 dopamine receptor-expressing cells

In this study we characterize the behavioural and cellular phenotype of mutant (MUT) mice with progressive loss of D1 dopamine receptor (Drd1a)-expressing cells. Adult [14-19 weeks] MUT mice showed intact working memory in the spontaneous alternation test but evidenced anxiety-like behaviour in the elevated plus maze and the light-dark test. The ethogram of mature adult MUT [average age 22 weeks] was compared with that of young adult MUT mice [average age 12 weeks]. While MUT mice evidenced hyperactivity over initial exploration at both time points, the topography of hyperactivity shifted. Moreover, initial hyperactivity was sustained over habituation at 12 weeks, but not at 22 weeks. Thus, by 22 weeks MUT mice evidenced shifts in, and mitigation of, these early phenotypic effects. However, orofacial behaviours of chewing and sifting were reduced similarly at 12 and 22 weeks. These data support the hypothesis that aspects of the mutant phenotype change with time. Quantitative autoradiography at 20 weeks revealed loss of D1-like dopamine receptor binding in the entire basal ganglia, with upregulated D2-like binding. There appear to be topographically specific interactions between normal maturational processes and compensatory mechanisms evoked subsequent to targeted ablation of D1 dopamine receptor-expressing cells. Understanding the mechanistic bases of mitigation vs persistence of individual phenotypes in relation to neural adaptation consequent to cell loss may lead to novel therapeutic strategies for basal ganglia disorders.

Arylbenzazepines are potent modulators for the delayed rectifier K+ channel: a potential mechanism for their neuroprotective effects

(+/-) SKF83959, like many other arylbenzazepines, elicits powerful neuroprotection in vitro and in vivo. The neuroprotective action of the compound was found to partially depend on its D(1)-like dopamine receptor agonistic activity. The precise mechanism for the (+/-) SKF83959-mediated neuroprotection remains elusive. We report here that (+/-) SKF83959 is a potent blocker for delayed rectifier K(+) channel. (+/-) SKF83959 inhibited the delayed rectifier K(+) current (I(K)) dose-dependently in rat hippocampal neurons. The IC(50) value for inhibition of I(K) was 41.9+/-2.3 microM (Hill coefficient = 1.81+/-0.13, n = 6), whereas that for inhibition of I(A) was 307.9+/-38.5 microM (Hill coefficient = 1.37+/-0.08, n = 6). Thus, (+/-) SKF83959 is 7.3-fold more potent in suppressing I(K) than I(A). Moreover, the inhibition of I(K) by (+/-) SKF83959 was voltage-dependent and not related to dopamine receptors. The rapidly onset of inhibition and recovery suggests that the inhibition resulted from a direct interaction of (+/-) SKF83959 with the K(+) channel. The intracellular application of (+/-) SKF83959 had no effects of on I(K), indicating that the compound most likely acts at the outer mouth of the pore of K(+) channel. We also tested the enantiomers of (+/-) SKF83959, R-(+) SKF83959 (MCL-201), and S-(-) SKF83959 (MCL-202), as well as SKF38393; all these compounds inhibited I(K). However, (+/-) SKF83959, at either 0.1 or 1 mM, exhibited the strongest inhibition on the currents among all tested drug. The present findings not only revealed a new potent blocker of I(K) , but also provided a novel mechanism for the neuroprotective action of arylbenzazepines such as (+/-) SKF83959.

Ontogeny and dopaminergic regulation in brain of Ras homolog enriched in striatum (Rhes)

Rhes is one of several signaling molecules preferentially expressed in the striatum. This GTP-binding protein affects dopamine-mediated signaling and behavior. Denervating the striatum of its dopaminergic inputs in adulthood reduces rhes mRNA expression. Here we show that dopamine depletion in adult rats by 6-hydroxydopamine caused a significant decrease in striatal Rhes protein levels as measured by Western blotting. The role of dopamine input on rhes mRNA induction during ontogeny was also examined. Rhes mRNA was measured on postnatal days 4, 6, 8, 10, 15, and 24 with in situ hybridization to determine its normal ontogeny. Signal in striatum was detectable, but very low, on postnatal day 4 and increased gradually to peak levels at days 15 and 24. Outside of the striatum, rhes mRNA was expressed at high levels in hippocampus and cerebellum during the postnatal period. Hippocampal signal was initially highest in CA3 and dentate gyrus, but shifted to higher expression in CA1 by the late postnatal period. Several other nuclei showed low levels of rhes mRNA during ontogeny. Depletion of dopamine by 6-hydroxydopamine injection on postnatal day 4 did not affect the ontogenetic development of rhes mRNA, such that expression did not differ statistically in lesioned versus vehicle-treated animals tested in adulthood. These findings suggest that although dopamine input is not necessary for the ontogenetic development of rhes mRNA expression, changes in both rhes mRNA and Rhes protein are integral components of the response of the adult striatum to dopamine depletion.

The Ras homolog Rhes affects dopamine D1 and D2 receptor-mediated behavior in mice

Dopamine activates five different receptor subtypes and a complex array of intracellular signaling pathways. Rhes is a striatally expressed guanidine triphosphate-binding protein involved in dopamine signaling. Here we have used mutant mice to test whether Rhes (Ras homolog enriched in striatum) is involved in D1 and D2 dopamine receptor-mediated behaviors. Rhes was not necessary for the expression of normal D1/D2 receptor synergism, as measured by apomorphine-induced stereotypy. The stereotypic responses to D1/D2 costimulation and to D2 stimulation alone were significantly increased in mice lacking Rhes, but D1 receptor-mediated grooming was reduced in these mice. These results suggest that Rhes is normally inhibitory to behaviors induced by D1/D2 receptor costimulation and by D2 receptor stimulation alone. Rhes, however, seems to facilitate the D1-specific behavior of grooming.

Dopamine D1 vs D5 receptor-dependent induction of seizures in relation to DARPP-32, ERK1/2 and GluR1-AMPA signalling

Recent reports have shown that the selective dopamine D(1)-like agonist SKF 83822 [which stimulates adenylate cyclase, but not phospholipase C] induces prominent behavioral seizures in mice, whereas its benzazepine congener SKF 83959 [which stimulates phospholipase C, but not adenylate cyclase] does not. To investigate the relative involvement of D(1) vs D(5) receptors in mediating seizures, ethological behavioral topography and cortical EEGs were recorded in D(1), D(5) and DARPP-32 knockout mice in response to a convulsant dose of SKF 83822. SKF 83822-induced behavioral and EEG seizures were gene dose-dependently abolished in D(1) knockouts. In both heterozygous and homozygous D(5) knockouts, the latency to first seizure was significantly increased and total EEG seizures were reduced relative to wild-types. The majority (60%) of homozygous DARPP-32 knockouts did not have seizures; of those having seizures (40%), the latency to first seizure was significantly increased and the number of high amplitude, high frequency polyspike EEG events was reduced. In addition, immunoblotting was performed to investigate downstream intracellular signalling mechanisms at D(1)-like receptors following challenge with SKF 83822 and SKF 83959. In wild-types administered SKF 83822, levels of ERK1/2 and GluR1 AMPA receptor phosphorylation increased two-fold in both the striatum and hippocampus; in striatal slices DARPP-32 phosphorylation at Thr34 increased five-fold relative to vehicle-treated controls. These findings indicate that D(1), and to a lesser extent D(5), receptor coupling to DARPP-32, ERK1/2 and glutamatergic signalling is involved in mediating the convulsant effects of SKF 83822.

Neuroprotective effects of atypical D1 receptor agonist SKF83959 are mediated via D1 receptor-dependent inhibition of glycogen synthase kinase-3 beta and a receptor-independent anti-oxidative action

3-methyl-6-chloro-7,8-hydroxy-1-(3-methylphenyl)-2,3,4,5-tetrahydro-1H-3-benzazepine (SKF83959), a selective agonist for the putative phosphatidylinositol (PI)-linked dopamine receptor (DAR), has been shown to possess potent anti-Parkinson disease effects but produces less dyskinesia and motor fluctuation that are frequently observed in Parkinson disease drug therapies. The present study was designed to detect the neuroprotection of SKF83959 and its potential mechanism for the effect in cultured rat cortical cells. The presence of SKF83959 with a dose range of 0.1-30 micromol/L improved H2O2-reduced cell viability in a dose-dependent manner. The anti-apoptotic action of SKF83959 was partially abolished by pre-application of the D1 antagonist SCH23390 (30 micromol/L) and the PI 3-kinase (PI 3-K) inhibitor LY294002 but not by the MEK1/2 inhibitor PD98059 (30 micromol/L). Moreover, SKF83959 treatment significantly inhibited H2O2-activated glycogen synthase kinase-3beta (GSK-3beta) which was associated with the drug's neuroprotective effect, but this inhibition was attenuated by SCH23390 and a selective PI 3-K inhibitor. Moreover, the application of either SKF83959 or a pharmacological inhibitor of GSK-3beta attenuated the inhibition by H2O2 on the expression of inducible NO synthase and production of NO. This indicates that D1-like receptor, presumably PI-linked D1 receptor, -mediated alteration of PI 3-K/Akt/GSK-3beta pathway is involved in the neuroprotection by SKF83959. In addition, SKF83959 also effectively decreased the level of the lipid peroxidation and increased the activity of GSH-peroxidase altered by H2O2. These results suggest that SKF83959 exerts its neuroprotective effect through both receptor-dependent and independent mechanisms: Inhibition of GSK-3beta and consequently increasing the expression of inducible NO synthase via putative PI-linked DAR; and its anti-oxidative activity which is independent of DAR.

Stereotypic circling behavior in mice with vestibular dysfunction: asymmetrical effects of intrastriatal microinjection of a dopamine agonist

Bronx Waltzer (bv) mouse, which has been used as a model of hearing and vestibular dysfunction, shows remarkable repetitive circling behavior. This study investigated whether the behavior is caused by the asymmetry of striatal function by observing the behavior of the bv mice following microinjection of dopamine D1 agonist, A68930 into the striatum ipsilaterally and contralaterally to the preferred direction of rotation separately. High dose of the drug induced opposite effects on ipsilateral rotations by the side of injections with statistical significance (p = .0026). These results suggested that the stereotypic circling behavior involves striatum and is based on striatal asymmetry.

Chronic SKF83959 induced less severe dyskinesia and attenuated L-DOPA-induced dyskinesia in 6-OHDA-lesioned rat model of Parkinson's disease

SKF83959, a recently identified selective agonist of putative phosphoinositide-linked (PI-linked) D(1) dopamine (DA) receptor, is found to elicit excellent anti-parkinsonism effects in monkeys and rodents. In the present study, the effects of SKF83959 on L-dihydroxyphenylalanine (L-DOPA)-induced dyskinesia (LID) were assessed in a unilateral 6-hydroxydopamine (6-OHDA) lesioned rat model of Parkinson's disease (PD). The results indicated that chronic L-DOPA (6 mg/kg) induced a progressive dyskinesia-like behavior in PD rats, whereas SKF83959 (0.5 mg/kg) elicited significantly less severe dyskinesia while exerts its anti-parkinsonian action effectively. Application of D(1) receptor, but not D(2), alpha or 5-HT receptor antagonist attenuated SKF83959-induced dyskinesia, indicating that a D(1) receptor-mediated events, assumed via PI-linked D(1) receptor. Interestingly, chronic co-administration of SKF83959 significantly reduced LID at no expanse of reduction in the anti-parkinsonian potency in PD rats. However, this anti-dyskinesia effect was not observed while SKF83959 was acutely administered in rats with established LID. This implies that chronic SKF83959 attenuated the development of dyskinesia. Immediate early gene FosB is previously reported to positively associate with dyskinesia. However, we found that the anti-dyskinesia effect of chronic SKF83959 was independent of FosB since SKF83959 produced stronger FosB expression in the lesioned striatum than that of L-DOPA while exerting its anti-dyskinesia action. The present data demonstrated that SKF83959 reduces LID by attenuating the development of dyskinesia; the underlying signaling pathway for the anti-dyskinesia action of SKF83959 appears not to depend on FosB.

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.

Susceptibility genes for schizophrenia: Characterisation of mutant mouse models at the level of phenotypic behaviour

A wealth of evidence indicates that schizophrenia is heritable. However, the genetic mechanisms involved are poorly understood. Furthermore, it may be that genes conferring susceptibility interact with one another and with non-genetic factors to modulate risk status and/or the expression of symptoms. Genome-wide scanning and the mapping of several regions linked with risk for schizophrenia have led to the identification of several putative susceptibility genes including neuregulin-1 (NRG1), dysbindin (DTNBP1), regulator of G-protein signalling 4 (RGS4), catechol-o-methyltransferase (COMT), proline dehydrogenase (PRODH) and disrupted-in-schizophrenia 1 (DISC1). Genetic animal models involving targeted mutation via gene knockout or transgenesis have the potential to inform on the role of a given susceptibility gene on the development and behaviour of the whole organism and on whether disruption of gene function is associated with schizophrenia-related structural and functional deficits. This review focuses on data regarding the behavioural phenotype of mice mutant for schizophrenia susceptibility genes identified by positional candidate analysis and the study of chromosomal abnormalities. We also consider methodological issues that are likely to influence phenotypic effects, as well as the limitations associated with existing molecular techniques.

Behavioral effects of the R-(+)- and S-(-)-enantiomers of the dopamine D(1)-like partial receptor agonist SKF 83959 in monkeys

Dopamine D(1)-like partial receptor agonists such as SKF 83959 have been proposed as potential candidates for the treatment of cocaine addiction. The present studies were conducted to further characterize SKF 83959 by pharmacologically evaluating effects of its R-(+)- and S-(-)-enantiomers, MCL 202 and MCL 201, respectively, on overt behavior (eye blinking) and schedule-controlled performance in squirrel monkeys. MCL 202, like the D(1) full receptor agonist SKF 82958, produced dose-related increases in eye blinking and decreases in rates of fixed-ratio responding. However, the magnitude of effects of MCL 202 on eye blinking was less than observed with SKF 82958. In contrast to the effects of its R-(+) enantiomer, MCL 201 was relatively devoid of behavioral activity up to doses that were approximately 10-fold greater than MCL 202. Pretreatment with the selective D(1)-like receptor antagonist SCH 39166 dose-dependently antagonized increases in eye blinking produced by MCL 202, confirming the involvement of D(1) mechanisms in its effects. A dose-ratio analysis of the antagonism of effects of MCL 202 by SCH 39166 revealed an apparent pA(2) value of 7.675 with a slope of -0.78+/-0.04. In further studies, pretreatment with MCL 202 antagonized the effects of SKF 82958 on eye blinking and, like SCH 39166, schedule-controlled behavior in a dose-related manner. A dose-ratio analysis of the antagonist effects of MCL 202 on the SKF 82958-induced increases in eye blinking revealed ratios of 2.7, 4.8 and 31.1 for 0.1, 0.3 and 1.0 mg/kg dose of the antagonist, respectively, indicative of a significant change in the potency of SKF 82958. These results suggest that MCL 202, like its parent compound SKF 83959, has both D(1) receptor-mediated agonist and antagonist properties, consistent with its characterization as a partial agonist at the D(1)-like receptor. In addition, the inactivity of MCL 201, the S-(-)-enantiomer, suggests that the behavioral effects of SKF 83959 can be attributed primarily to the activity of its R-(+)-enantiomer.

Ethological resolution of behavioral topography and D2-like vs. D1-like agonist responses in congenic D4 dopamine receptor "knockouts": identification of D4:D1-like interactions

To clarify the involvement of dopamine D4 receptors in behavioral regulation, the phenotypic ethogram of congenic D4 "knockout" mice was studied in terms of (i) course of exploration and habituation, and (ii) topographical responsiveness to the selective D2-like agonist RU 24213 and the selective D1-like agonists A 68930, SK&F 83959 and SK&F 83822. Congenic D4 knockouts were characterized by a small reduction in exploratory sniffing with delayed habituation of sifting. The magnitude and topographical specificity of these effects indicated that any functional role for D4 receptors in exploratory processes is subtle. Induction of stereotyped, ponderous locomotion by RU 24213 was reduced in D4-null mice consistent with an involvement of D4 receptors in the topographical expression of stereotypy. Induction of grooming and, at higher doses, seizures by A 68930, which stimulates both adenylyl cyclase (AC) and phospholipase C (PLC), were unaltered in congenic D4 knockouts. In contrast, induction of grooming by SK&F 83959, which stimulates PLC but not AC and fails to induce seizures, was reduced in D4-null mice; this indicates that D4 receptors interact with PLC-coupled D1-like receptors in regulating D1-like-mediated grooming. Conversely, induction of seizures by SK&F 83822, which stimulates AC but not PLC and fails to induce grooming, was reduced in congenic D4 knockouts; this indicates that D4 receptors interact with AC-coupled D1-like receptors in regulating D1-like-mediated seizures. These studies identify novel functional roles for the D4 receptor that are distinct from those of closely related D2-like family members and involve interactions with their D1-like counterparts.

The atypical dopamine D1 receptor agonist SKF 83959 induces striatal Fos expression in rats

The effects of dopamine D1 receptor agonists are often presumed to result from an activation of adenylyl cyclase, but dopamine D1 receptors may also be linked to other signal transduction cascades and the relative importance of these various pathways is currently unclear. SKF 83959 is an agonist at dopamine D1 receptors linked to phospholipase C, but has been reported to be an antagonist at receptors linked to adenylyl cyclase. The current report demonstrates that SKF 83959 induces pronounced, nonpatchy, expression of the immediate-early gene product Fos in the striatum of intact rats which can be converted to a patchy pattern by pretreatment with the dopamine D2-like receptor agonist quinpirole. In rats with unilateral 6-hydroxydopamine lesions SKF 83959 induces strong behavioral rotation and a greatly potentiated Fos response. All of the responses to SKF 83959, in both intact and dopamine-depleted animals, can be blocked by pretreatment with the dopamine D1 receptor antagonist SCH-23390. In intact subjects, SKF 83959 induced Fos expression less potently than the standard dopamine D1 receptor agonist SKF 82958, but the two drugs were approximately equipotent in deinnervated animals. These results demonstrate for the first time that possession of full efficacy at dopamine D1 receptors linked to adenylyl cyclase is not a necessary requirement for the induction of striatal Fos expression in intact animals and suggest that alternative signal transduction pathways may play a role in dopamine agonist induced Fos expression, especially in dopamine-depleted subjects.

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.

Phenotypic studies on dopamine receptor subtype and associated signal transduction mutants: insights and challenges from 10 years at the psychopharmacology-molecular biology interface

BACKGROUND

Mutants with targeted gene deletion ('knockout') or insertion (transgenic) of D1, D2, D3, D4 and D5 dopamine (DA) receptor subtypes are complemented by an increasing variety of double knockout and transgenic-'knockout' models, together with knockout of critical components of DA receptor signalling cascades such as G alpha(olf)[G gamma7], adenylyl cyclase type 5, PKA [RIIbeta] and DARPP-32. However, it is increasingly recognised that these molecular techniques have a number of inherent limitations. Furthermore, there are poorly understood methodological factors that contribute to inconsistent phenotypic findings between laboratories.

OBJECTIVE

This review seeks to document the impact of DA receptor subtype and related transduction mutants on our understanding of the behavioural roles of these entities, primarily at the level of unconditioned psychomotor behaviour.

METHODS

It includes ethologically based and orofacial movement studies in our own laboratories, since these are the only studies to systematically compare each of the D1, D2, D3, D4 and D5 receptor and DARPP-32 signal transduction 'knockouts'.

DISCUSSION

There is a particular emphasis on identifying methodological factors that might influence phenotypic effects and account for inconsistencies. The findings are offered empirically to (1) specify the extent of phenotypic diversity among individual DA receptor subtypes and transduction components and (2) indicate relationships between D1, D2, D3, D4 and D5 receptor subtype proteins, associated G alpha(i)/G alpha(s)/G alpha(olf)[G gamma7]-adenylyl cyclase type 5-PKA [RIIbeta]-DARPP-32 signalling cascades and behaviour. The findings are also offered heuristically as a base for such phenotypic comparisons at additional levels of behaviour so that a yet more complete phenotypic profile might emerge.

A comparison of the locomotor stimulant effects of D1-like receptor agonists in mice

Efficacy in stimulating adenylyl cyclase (AC) has traditionally been used to distinguish dopamine D1-like receptor agonists from dopamine D2-like receptor agonists. However, there is a limited association between the effects of D1-like agonists in behavioral assays and their effectiveness at stimulating AC. Other second messenger actions might contribute to the behavioral effects of D1-like agonists, as there is evidence for a link to the hydrolysis of phosphoinositide (PI). The present study compared the locomotor stimulant effects of five D1-like receptor agonists having different efficacies in assays of AC and PI activity. All D1-like agonists produced long-lasting biphasic effects on locomotor activity. SKF 38393, the prototypical partial agonist (based on AC activity), produced limited changes in locomotor activity, whereas the partial agonists SKF 75670 and SKF 77434 produced locomotor stimulant effects that were similar to or greater than those of the full efficacy agonists SKF 82958 and SKF 81297. However, there did not appear to be a relationship between maximal behavioral effects and AC stimulation or PI hydrolysis. The results suggest a complex relationship between the behavioral effects of D1-like agonists and their intrinsic efficacies as measured by AC and /or PI stimulation. Although a limited number of compounds were examined, neither second messenger system alone appears to account fully for these behavioral effects. The current classification of D1-like agonists according to their intrinsic efficacies as defined by AC stimulation needs further scrutiny.

Ethological resolution of behavioural topography and D1-like versus D2-like agonist responses in congenic D5 dopamine receptor mutants: identification of D5:D2-like interactions

The phenotypic ethogram of congenic dopamine D(5) receptor "knockout" mice was evaluated. Each individual topography of behaviour within the natural repertoire was assessed over the extended course of initial exploration of and subsequent habituation to the environment, and following challenge with a series of D(1)-like agonists. Over initial exploration, D(5)-null mice evidenced a modest reduction in locomotion and a modest increase in sifting. Subsequent habituation revealed additional phenotypic effects, primarily overall reduction in grooming and delayed habituation of rearing. Among D(1)-like agonists, A 68930 stimulates both adenylyl cyclase and a putative D(1)-like receptor coupled to stimulation of phospholipase C-mediated phosphoinositide hydrolysis; conversely, SK&F 83959 stimulates phosphoinositide hydrolysis but not adenylyl cyclase while SK&F 83822 stimulates adenylyl cyclase but not phosphoinositide hydrolysis. Though programmed grooming syntax and episodic seizure activity induced by A 68930 and SK&F 83822 were unaltered, grooming induced by SK&F 83959 was reduced in D(5) mutants. Stereotyped, ponderous locomotion induced by the D(2)-like agonist RU 24213 was enhanced in D(5) mutants. Phenotypic and pharmacological characterisation of congenic D(5)-null mice at an ethological level identifies novel functional roles for the D(5) receptor in mediating discrete topographies of behaviour relating to exploration, sequential motor coordination, and how these processes change over the course of interaction with and habituation to the environment. Additionally, they indicate the involvement of phosphoinositide hydrolysis and D(5):D(2)-like interactions in regulating these processes.

Phenotypic analysis of dopamine receptor knockout mice; recent insights into the functional specificity of dopamine receptor subtypes

The functional specificity of dopamine receptor subtypes remains incompletely understood, in part due to the absence of highly selective agonists and antagonists. Phenotypic analysis of dopamine receptor knockout mice has been instrumental in identifying the role of dopamine receptor subtypes in mediating dopamine's effects on motor function, cognition, reward, and emotional behaviors. In this article, we provide an update of recent studies in dopamine receptor knockout mice and discuss the limitations and future promise of this approach.

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.

Dopamine Receptor Signaling

The D1-like (D1, D5) and D2-like (D2, D3, D4) classes of dopamine receptors each has shared signaling properties that contribute to the definition of the receptor class, although some differences among subtypes within a class have been identified. D1-like receptor signaling is mediated chiefly by the heterotrimeric G proteins Galphas and Galphaolf, which cause sequential activation of adenylate cyclase, cylic AMP-dependent protein kinase, and the protein phosphatase-1 inhibitor DARPP-32. The increased phosphorylation that results from the combined effects of activating cyclic AMP-dependent protein kinase and inhibiting protein phosphatase 1 regulates the activity of many receptors, enzymes, ion channels, and transcription factors. D1 or a novel D1-like receptor also signals via phospholipase C-dependent and cyclic AMP-independent mobilization of intracellular calcium. D2-like receptor signaling is mediated by the heterotrimeric G proteins Galphai and Galphao. These pertussis toxin-sensitive G proteins regulate some effectors, such as adenylate cyclase, via their Galpha subunits, but regulate many more effectors such as ion channels, phospholipases, protein kinases, and receptor tyrosine kinases as a result of the receptor-induced liberation of Gbetagamma subunits. In addition to interactions between dopamine receptors and G proteins, other protein:protein interactions such as receptor oligomerization or receptor interactions with scaffolding and signal-switching proteins are critical for regulation of dopamine receptor signaling.

Ethologically Based Resolution of D2-Like Dopamine Receptor Agonist-versus Antagonist-Induced Behavioral Topography in Dopamine- and Adenosine 3′,5′-Monophosphate-Regulated Phosphoprotein of 32 kDa “Knockout” Mutants Congenic on the C57BL/6 Genetic Background

Given the critical role of dopamine- and adenosine 3',5'-monophosphate-regulated phosphoprotein of 32 kDa (DARPP-32) in the regulation of dopaminergic function, DARPP-32-null mutant mice congenic on the inbred C57BL/6 strain for 10 generations were examined phenotypically for their ethogram of responsivity to the selective D2-like receptor agonist RU 24213 (N-n-propyl-N-phenylethyl-p-3-hydroxyphenylethylamine) and the selective D2-like receptor antagonist YM 09151-2 (cis-N-[1-benzyl-2-methyl-pyrrolidin-3-yl]-5-chloro-2-methoxy-4-methylaminobenzamide), using procedures that resolve all topographies of behavior in the natural repertoire. After vehicle challenge, levels of sniffing and rearing seated were reduced in DARPP-32 mutants; the injection procedure seems to constitute a "stressor" that reveals phenotypic effects of DARPP-32 deletion not apparent under natural conditions. Topographical effects of 0.3 to 10.0 mg/kg RU 24213, primarily induction of sniffing and ponderous locomotion with accompanying reductions in rearing, grooming, sifting and chewing, were not altered to any material extent in DARPP-32-null mice. However, topographical effects of 0.005 to 0.625 mg/kg YM 09151-2, namely, reduction in sniffing, locomotion, rearing, grooming, and chewing but not sifting, were essentially absent in DARPP-32 mutants. Thus, the D2-like receptor agonist-mediated ethogram was essentially conserved, whereas major elements of the corresponding D2-like receptor antagonist-mediated ethogram were essentially absent in DARPP-32-null mice. This suggests some relationship between 1) extent of tonic dopaminergic activation of DARPP-32 mechanisms and 2) compensatory mechanisms consequent to the developmental absence of DARPP-32, which may emerge to act differentially on individual elements of the DARPP-32 system. Critically, the present data indicate that phenotypic effects of a given gene deletion using an agonist acting on the system disrupted cannot be generalized to a corresponding antagonist, and vice versa.

Topographical Assessment of Ethological and Dopamine Receptor Agonist-Induced Behavioral Phenotype in Mutants with Congenic DARPP-32 'Knockout'

Congenic (10 backcrosses into C57BL/6J) mutants with targeted gene deletion of DARPP-32, a neuronal phosphoprotein regarded as an essential mediator of the biological effects of dopamine (DA), were assessed phenotypically using an ethologically based approach that resolves all topographies of behavior in the mouse repertoire. Over initial exploration, female, but not male, DARPP-32 mutants evidenced increased locomotion and decreased grooming, while a decrease in rearing seated was evident in mutants of both genders; continuing assessment over several hours did not reveal additional phenotypic effects. Following challenge with the nonselective DA receptor agonist apomorphine, low doses were associated with reduced levels of sniffing, grooming, total rearing, and rearing seated in DARPP-32 mutants relative to wildtypes; this would suggest some role for DARPP-32 in mediating the biological effects of presynaptic D(2)-like autoreceptor or inhibitory postsynaptic D(2)-like receptor activation. Following challenge with higher doses, while stereotyped sniffing and locomotion with chewing was largely unaltered, the additional murine response of Straub tail was essentially abolished in DARPP-32 mutants, indicating some specific involvement of DARPP-32 in mediating this topography of behavior; additionally, there were overall reductions in levels of sniffing, total rearing, rearing seated, and grooming in DARPP-32 mutants that were unrelated to the dose of apomorphine administered, indicating broader topographical effects following the stress of the injection procedure relative to more naturalistic conditions. The developmental absence of DARPP-32 following targeted gene deletion appears to be associated with compensatory processes that maintain certain topographies of spontaneous and agonist-induced DAergic function, while other topographies remain impaired.

Neurochemical changes in dopamine D1, D3 and D1/D3 receptor knockout mice

Neurochemical changes were examined in dopamine D1 receptor knockout (D1(-/-)), dopamine D3 receptor knockout (D3(-/-)) and dopamine D1/D3 receptor double knockout (D1(-/-)D3(-/-)) mice. The level of dopamine D1- and D2-like receptors and gamma-aminobutyric acid (GABA(A)) receptor was assessed by ligand autoradiography and dopamine D1- and D2 receptor, enkephalin, dynorphin and substance P transcripts measured by in situ hybridization. D1(-/-) mice had normal GABA(A) receptor levels, reduced dynorphin and substance P, and increased enkephalin mRNA and dopamine D2-like binding. D1(-/-)D3(-/-) mice evidenced decreased dynorphin and substance P but normal enkephalin expression, whereas dopamine D2-like and GABA(A) receptor binding were increased. Major changes occur in substance P and dynorphin expression in D1(-/-) mice and these changes are unaffected by loss of dopamine D3 receptors. Upregulated dopamine D2-like binding and enkephalin in D1(-/-) mice may be due to decreased dopamine turnover. Upregulated enkephalin in D1(-/-) mice is dependent on functional dopamine D3 receptors.

Comparison of the discriminative-stimulus effects of SKF 38393 with those of other dopamine receptor agonists

The dopamine D(1)-like receptor agonists have traditionally been defined molecularly by their efficacy in stimulating adenylyl cyclase. However, evidence correlating the effectiveness of these drugs in behavioral assays and their effectiveness biochemically has not been forthcoming. The present study compared the discriminative-stimulus effects of the D(1)-like partial agonist SKF 38393 with several other D(1)-like agonists, an indirect agonist, cocaine, and a D(2)-like agonist, quinpirole. Rats were trained under a fixed-ratio 30-response schedule to discriminate SKF 38393 (5.6 mg/kg) from vehicle. Under this schedule, 30 consecutive responses on one of two keys were reinforced with food presentation after a pre-session injection of 5.6 mg/kg SKF 38393, and 30 consecutive responses on the alternative key were reinforced after saline injection. When daily performances were stable, substitution patterns for several compounds were assessed during test sessions in which 30 consecutive responses on either key were reinforced. Quinpirole and cocaine each produced saline-appropriate responding. In contrast, the D(1)-like agonists, SKF 75670 and SKF 77434, fully substituted for SKF 38393. Curiously, SKF 82958, which is considered a full agonist based on adenylyl cyclase assays, was less effective in substituting for SKF 38393 (maximum drug-appropriate responding 66%) than was the partial agonist SKF 75670. The present results suggest that second messenger effects other than stimulation of adenylyl cyclase may play an important role in the behavioral effects of dopamine D(1)-like agonists.

Essential conservation of D1 mutant phenotype at the level of individual topographies of behaviour in mice lacking both D1 and D3 dopamine receptors

RATIONALE

In the absence of agonists and antagonists evidencing appropriate selectivities, individual and interactive properties of D(1) and D(3) dopamine receptors would be illuminated most powerfully by their co-deletion.

OBJECTIVES

To define and contrast the behavioural phenotype of D(1)/D(3) double knockout mice in comparison with wild types, and with individual D(1) and D(3 )mutants.

METHODS

Behavioural phenotype was characterised using an ethologically based topographical technique.

RESULTS

On comparison with wild types, D(1)/D(3) double mutants were characterised topographically as follows: increases in sniffing and locomotion, which evidenced delayed habituation; reductions in rearing free, rearing seated, grooming, chewing and stillness. Though the D(1)/D(3) double mutant ethogram comprised elements of both single mutant D(1) and D(3) lines, this phenotype was largely reflective of the D(1) mutant component.

CONCLUSIONS

Distinct patterns of initial exploratory behaviour and of temporal change over subsequent habituation were evident across the three genotypes, with particular conservation of the D(1) phenotype in D(1)/D(3 )double mutants. Under the present conditions, there was little systematic evidence for D(1):D(3) interactions in the regulation of these aspects of behaviour.

SKF83959 selectively regulates phosphatidylinositol-linked D1 dopamine receptors in rat brain

Previously a distinct D1-like dopamine receptor (DAR) that selectively couples to phospholipase C/phosphatidylinositol (PLC/PI) was proposed. However, lack of a selective agonist has limited efforts aimed at characterizing this receptor. We characterized the in vitro and in vivo effects of SKF83959 in regulating PI metabolism. SKF83959 stimulates (EC50, 8 micro m) phosphatidylinositol 4,5-biphosphate hydrolysis in membranes of frontal cortex (FC) but not in membranes from PC12 cells expressing classical D1A DARs. Stimulation of FC PI metabolism was attenuated by the D1 antagonist, SCH23390, indicating that SKF83959 activates a D1-like DAR. The PI-linked DAR is located in hippocampus, cerebellum, striatum and FC. Most significantly, administration of SKF83959 induced accumulations of IP3 in striatum and hippocampus. In contrast to other D1 DAR agonists, SKF83959 did not increase cAMP production in brain or in D1A DAR-expressing PC12 cell membranes. However, SKF83959 inhibited cAMP elevation elicited by the D1A DAR agonist, SKF81297, indicating that the compound is an antagonist of the classical D1A DAR. Lastly, we demonstrated that SKF83959 enhances [35S]guanosine 5'-O-(3-thiotriphosphate) binding to membrane Galphaq and Galphai proteins, suggesting that PI stimulation is mediated by activation of these guanine nucleotide-binding regulatory proteins. Results indicate that SKF83959 is a selective agonist for the PI-linked D1-like DAR, providing a unique tool for investigating the functions of this brain D1 DAR subtype.

Congenic D1A dopamine receptor mutants: ethologically based resolution of behavioural topography indicates genetic background as a determinant of knockout phenotype

D(1A)-null mice were backcrossed over 14 generations into a C57BL/6 background to result in essential elimination (to <0.005%) of any contribution from the 129/Sv component of their initially mixed (129/SvxC57BL/6) background. Their phenotype was assessed using an ethologically based approach that resolves each individual topography of behaviour in the natural repertoire. Habituation of sniffing, locomotion, rearing seated, and rearing to wall in wild types over several hours was profoundly retarded in congenic D(1A) mutants; conversely, rearing free and sifting were essentially abolished. Resultant increases in individual topographies of behaviour were substantially greater in congenic D(1A) mutants than in those on a mixed background. This phenotype was little altered by the selective D(1)-like antagonist SCH 23390 and could not be blocked by the selective D(2)-like antagonist YM 09151-2. The selective D(1)-like agonist SK&F 83959 could not further elevate those behaviours already heightened in congenic D(1A) mutants, while the induction of stereotyped sniffing and plodding locomotion by the selective D(2)-like agonist RU 24213 was disrupted. Genetic background appears to modulate critically the magnitude but not the general nature of the D(1A)-null phenotype, which may involve compensatory processes independent of other D(1)-like or D(2)-like receptors.

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.

Impaired D2 dopamine receptor function in mice lacking type 5 adenylyl cyclase

Dopamine receptor subtypes D1 and D2, and many other seven-transmembrane receptors including adenosine receptor A2A, are colocalized in striatum of brain. These receptors stimulate or inhibit adenylyl cyclases (ACs) to produce distinct physiological and pharmacological responses and interact with each other synergistically or antagonistically at various levels. The identity of the AC isoform that is coupled to each of these receptors, however, remains unknown. To investigate the in vivo role of the type 5 adenylyl cyclase (AC5), which is preferentially expressed in striatum, mice deficient for the AC5 gene were generated. The genetic ablation of the AC5 gene eliminated >80% of forskolin-induced AC activity and 85-90% of AC activity stimulated by either D1 or A2A receptor agonists in striatum. However, D1- or A2A-specific pharmaco-behaviors were basically preserved, whereas the signal cascade from D2 to AC was completely abolished in AC5(-/-), and motor activity of AC5(-/-) was not suppressed by treatment of cataleptic doses of the antipsychotic drugs haloperidol and sulpiride. Interestingly, both haloperidol and clozapine at low doses remarkably increased the locomotion of AC5(-/-) in the open field test that was produced in part by a common mechanism that involved the increased activation of D1 dopamine receptors. Together, these results suggest that AC5 is the principal AC integrating signals from multiple receptors including D1, D2, and A2A in striatum and the cascade involving AC5 among diverse D2 signaling pathways is essential for neuroleptic effects of antipsychotic drugs.

Phenotypic, ethologically based resolution of spontaneous and D(2)-like vs D(1)-like agonist-induced behavioural topography in mice with congenic D(3) dopamine receptor "knockout"

Uncertainty as to the functional role of the D(3) dopamine receptor, due primarily to a paucity of selective agonists or antagonists, is being addressed in mice with targeted gene deletion ("knockout") thereof. This study describes, for the first time, the phenotype of congenic D(3)-null mice. Initially, 129/Sv x C57BL/6 D(3)-null mice were backcrossed 14 times onto C57BL/6; they were then assessed using an ethologically based approach which resolves all topographies of behaviour within the mouse repertoire. The ethogram of D(3)-null mice, on comparison with wildtypes, was characterised by no alteration in any topography of behaviour over an initial period of exploration; subsequent assessment over several hours revealed only increased rearing among females due to delayed habituation. Low doses of the selective D(2)-like agonist RU 24213 (0.016-0.25 mg/kg) inhibited topographies of exploratory behaviour; this effect was diminished in D(3)-null mice only when investigated following prolonged habituation, and then only for certain topographies of behaviour, primarily sniffing and rearing. High doses of RU 24213 (0.1-12.5 mg/kg) induced stereotyped sniffing and "ponderous" locomotion, while the selective D(1)-like agonist SK&F 83959 (0.016-2.0 mg/kg) promoted characteristic grooming syntax; these effects did not differ materially between the genotypes. When examined topographically on an essentially congenic C57BL/6 background (<0.005% 129/Sv), the resultant phenotype indicated essential conservation of the mouse ethogram, high-dose D(2)-like stimulatory effects, and D(1)-like stimulatory effects in the absence of D(3) receptors. A role for D(3) receptors in inhibitory processes appeared topographically circumscribed and only when baseline levels of behaviour were low.

Essential fatty acids given from conception prevent topographies of motor deficit in a transgenic model of Huntington's disease

Transgenic R6/1 mice incorporate a human genomic fragment containing promoter elements exon 1 and a portion of intron 2 of the Huntingtin gene responsible for Huntington's disease. They develop late-onset neurological deficits in a manner similar to the motor abnormalities of the disorder. As essential fatty acids are phospholipid components of cell membranes which may influence cell death and movement disorder phenotype, R6/1 and normal mice were randomised to receive a mixture of essential fatty acids or placebo on alternate days throughout life. Over mid-adulthood, topographical assessment of behaviour revealed R6/1 transgenics to evidence progressive shortening of stride length, with progressive reductions in locomotion, elements of rearing, sniffing, sifting and chewing, and an increase in grooming. These deficits were either not evident or materially diminished in R6/1 transgenics receiving essential fatty acids. R6/1 transgenics also showed reductions in body weight and in brain dopamine D(1)-like and D(2)-like quantitative receptor autoradiography which were unaltered by essential fatty acids.These findings indicate that early and sustained treatment with essential fatty acids are able to protect against motor deficits in R6/1 transgenic mice expressing exon 1 and a portion of intron 2 of the Huntingtin gene, and suggest that essential fatty acids may have therapeutic potential in Huntington's disease.

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.

SKF 83959 is an antagonist of dopamine D1-like receptors in the prefrontal cortex and nucleus accumbens: a key to its antiparkinsonian effect in animals?

SKF 83959 that has a unique antiparkinson profile in animal models of Parkinson's disease is an in vitro dopamine D1 antagonist of receptors coupled to adenylyl cyclase. We hypothesized that SKF 83959, among others, interacts with dopamine D1 receptors coupled to adenylyl cyclase in the nucleus accumbens and the prefrontal cortex. Effects of intra-accumbal injections of SKF 83959 on locomotor activity were compared to effects of the dopamine D1 agonist SKF 81297 and the dopamine D1 antagonist SCH 39166. Similarly to SCH 39166, SKF 83959 did not affect locomotor activity, but counteracted SKF 81297-induced locomotor activity. Effects of unilateral intra-prefrontal injections of SKF 83959 on rotational behaviour were compared to the effects of the dopamine D1 agonist SKF 81297 and the dopamine D1 antagonists SCH 23390 and SCH 39166 in rats selected on basis of their high locomotor response to novelty and pretreated with a subcutaneous injection of 0.75 mg/kg dexamphetamine. Like SCH 39166 and SCH 23390, SKF 83959 induced a bias for contralateral rotating and blocked the SKF 81297-induced bias for ipsilateral rotating. In conclusion, SKF 83959 is an in vivo antagonist of dopamine D1 receptors that are coupled to adenylyl cyclase in the nucleus accumbens and the prefrontal cortex. The role of these receptors in the antiparkinson profile of SKF 83959 is discussed.

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.

Locomotor behavior of dopamine D1 receptor transgenic/D2 receptor deficient hybrid mice

Mice that incorporate the dopamine D1 receptor transgene controlled by the D1 receptor promoter exhibit a marked increase of D1 binding in several extra-striatal brain regions and show a paradoxical hypokinetic response to D1 agonist [Exp. Neurol. 157 (1999) 169]. The agonist-induced locomotor behavior of D1 receptor transgenic mice is similar to baseline locomotor activity manifested by D2 receptor deficient mice [J. Neurosci. 18 (1998) 3470]. The similarity between these two behavioral phenotypes raised the possibility that stimulation of the over-expressed D1 receptors in the transgenic mice could cause a suppression of D2 receptor responses that manifest in hypokinesia. Alternatively, the similar phenotypes could result from altered D1/D2 receptor balance in both animal models. Two different approaches were undertaken to test these alternative hypotheses. (1) The effects of pharmacological blockade of D2 receptors on D1 agonist-stimulated hypokinesia of the D1 over-expressing animals were investigated. (2) The behavioral phenotype of hybrid D1 receptor over-expressing/D2 receptor deficient mice generated by crossbreeding the D2 knockout mice and the D1 transgenic animals was studied. The results of these studies suggested that the hypomotor response of the D1 transgenic mice was not a result of an interaction of the over-expressed D1 receptors with the native D2 receptors and that over-expressed D1 receptors likely mediate hypokinesia in the D1 transgenic animals. Considering the significance of the D1 dopamine receptor as a therapeutic target for Parkinson's disease, this D1 receptor over-expressing model provides an important experimental system to probe the basis for altered behavioral responses following stimulation of transgenetically up-regulated receptors.

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.

Phenotypic characterization of an alpha 4 neuronal nicotinic acetylcholine receptor subunit knock-out mouse

Neuronal nicotinic acetylcholine receptors (nAChR) are present in high abundance in the nervous system (Decker et al., 1995). There are a large number of subunits expressed in the brain that combine to form multimeric functional receptors. We have generated an alpha(4) nAChR subunit knock-out line and focus on defining the behavioral role of this receptor subunit. Homozygous mutant mice (Mt) are normal in size, fertility, and home-cage behavior. Spontaneous unconditioned motor behavior revealed an ethogram characterized by significant increases in several topographies of exploratory behavior in Mt relative to wild-type mice (Wt) over the course of habituation to a novel environment. Furthermore, the behavior of Mt in the elevated plus-maze assay was consistent with increased basal levels of anxiety. In response to nicotine, Wt exhibited early reductions in a number of behavioral topographies, under both unhabituated and habituated conditions; conversely, heightened levels of behavioral topographies in Mt were reduced by nicotine in the late phase of the unhabituated condition. Ligand autoradiography confirmed the lack of high-affinity binding to radiolabeled nicotine, cytisine, and epibatidine in the thalamus, cortex, and caudate putamen, although binding to a number of discrete nuclei remained. The study confirms the pivotal role played by the alpha(4) nAChR subunit in the modulation of a number of constituents of the normal mouse ethogram and in anxiety as assessed using the plus-maze. Furthermore, the response of Mt to nicotine administration suggests that persistent nicotine binding sites in the habenulo-interpeduncular system are sufficient to modulate motor activity in actively exploring mice.

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.