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

Dose-Dependent Regulation on Prefrontal Neuronal Working Memory by Dopamine D1 Agonists: Evidence of Receptor Functional Selectivity-Related Mechanisms

Low doses of dopamine D1 agonists improve working memory-related behavior, but high doses eliminate the improvement, thus yielding an ‘inverted-U’ dose-response curve. This dose-dependency also occurs at the single neuron level in the prefrontal cortex where the cellular basis of working memory is represented. Because signaling mechanisms are unclear, we examined this process at the neuron population level. Two D1 agonists (2-methyldihydrexidine and CY208,243) having different signaling bias were tested in rats performing a spatial working memory-related T-maze task. 2-Methyldihydrexidine is slightly bias toward D1-mediated β-arrestin-related signaling as it is a full agonist at adenylate cyclase and a super-agonist at β-arrestin recruitment, whereas CY208,243 is slightly bias toward D1-mediated cAMP signaling as it has relatively high intrinsic activity at adenylate cyclase, but is a partial agonist at β-arrestin recruitment. Both compounds had the expected inverted U dose-dependency in modulating prefrontal neuronal activities, albeit with important differences. Although CY208,243 was superior in improving the strength of neuronal outcome sensitivity to the working memory-related choice behavior in the T-maze, 2-methyldihydrexidine better reduced neuron-to-neuron variation. Interestingly, at the neuron population level, both drugs affected the percentage, uniformity, and ensemble strength of neuronal sensitivity in a complicated dose-dependent fashion, but the overall effect suggested higher efficiency and potency of 2-methyldihydrexidine compared to CY208,243. The differences between 2-methyldihydrexidine and CY208,243 may be related to their specific D1 signaling. These results suggest that D1-related dose-dependent regulation of working memory can be modified differentially by functionally selective ligands, theoretically increasing the balance between desired and undesired effects.

D1 dopamine receptors intrinsic activity and functional selectivity affect working memory in prefrontal cortex

Dopamine D₁ agonists enhance cognition, but the role of different signaling pathways (e.g., cAMP or β-arrestin) is unclear. The current study compared 2-methyldihydrexidine and CY208,243, drugs with different degrees of both D₁ intrinsic activity and functional selectivity. 2-Methyldihydrexidine is a full agonist at adenylate cyclase and a super-agonist at β-arrestin recruitment, whereas CY208,243 has relatively high intrinsic activity at adenylate cyclase, but much lower at β-arrestin recruitment. Both drugs decreased, albeit in dissimilar ways, the firing rate of neurons in prefrontal cortex sensitive to outcome-related aspects of a working memory task. 2-Methyldihydrexidine was superior to CY208,243 in prospectively enhancing similarity and retrospectively distinguishing differences between correct and error outcomes based on firing rates, enhancing the micro-network measured by oscillations of spikes and local field potentials, and improving behavioral performance. This study is the first to examine how ligand signaling bias affects both behavioral and neurophysiological endpoints in the intact animal. The data show that maximal enhancement of cognition via D₁ activation occurred with a pattern of signaling that involved full unbiased intrinsic activity, or agonists with high β-arrestin activity.

Selective activation of D1 dopamine receptors exerts antidepressant-like activity in rats

BACKGROUND

Major depressive disorder is a common illness that severely decreases psychosocial functioning. Due to the major limitations of current treatments including response failure, it is crucial to develop better therapy strategies. Evidence suggests that dopamine dysregulation might play a major role in major depressive disorder physiopathology.

AIMS

This study investigates whether the dopamine D1 receptor agonist A77636 modulates antidepressant-like activity in rats.

METHODS

Rats were injected with an acute single dose of A77636 (0.75, 1.5 or 3 mg/kg), a potent and selective dopamine D1-like receptor agonist. Their locomotor activity, social interactions and behavioural response to the forced swim test were analysed 30 min after the injection.

RESULTS

During the forced swim test, the D1 agonist dose dependently reduced the immobility while the time of bursting was increased. Social interactions were significantly increased in the animals exposed to 3 mg/kg of A77636 whereas no significant changes were measured in general motor activity.

CONCLUSIONS

The present results provide evidence that pharmacological modulation of D1 receptor by the selective agonist A77636 induces antidepressant-like effects in rats, which encourages further studies regarding D1-specific modulation in major depressive disorder treatment.

Pridopidine: Overview of Pharmacology and Rationale for its Use in Huntington's Disease

Despite advances in understanding the pathophysiology of Huntington’s disease (HD), there are currently no effective pharmacological agents available to treat core symptoms or to stop or prevent the progression of this hereditary neurodegenerative disorder. Pridopidine, a novel small molecule compound, has demonstrated potential for both symptomatic treatment and disease modifying effects in HD. While pridopidine failed to achieve its primary efficacy outcomes (Modified motor score) in two trials (MermaiHD and HART) there were consistent effects on secondary outcomes (TMS). In the most recent study (PrideHD) pridiopidine did not differ from placebo on TMS, possibly due to a large enduring placebo effect.

This review describes the process, based on in vivo systems response profiling, by which pridopidine was discovered and discusses its pharmacological profile, aiming to provide a model for the system-level effects, and a rationale for the use of pridopidine in patients affected by HD. Considering the effects on brain neurochemistry, gene expression and behaviour in vivo, pridopidine displays a unique effect profile. A hallmark feature in the behavioural pharmacology of pridopidine is its state-dependent inhibition or activation of dopamine-dependent psychomotor functions. Such effects are paralleled by strengthening of synaptic connectivity in cortico-striatal pathways suggesting pridopidine has potential to modify phenotypic expression as well as progression of HD. The preclinical pharmacological profile is discussed with respect to the clinical results for pridopidine, and proposals are made for further investigation, including preclinical and clinical studies addressing disease progression and effects at different stages of HD.

Time-dependent role of prefrontal cortex and hippocampus on cognitive improvement by aripiprazole in olfactory bulbectomized mice

Dopamine (DA) modulates cognitive functions in the prefrontal cortex (PFC) and hippocampus. Olfactory bulbectomy (OBX) in mice induces cognitive dysfunctions. Recently, we reported that aripiprazole (ARI) normalizes the behavioral hyper-responsivity to DA agonists in OBX mice. However, it remains unclear whether ARI affects OBX-induced cognitive dysfunctions. To address this question we evaluated ARI-treated and untreated OBX mice in a passive avoidance test. Then, we investigated the effects of ARI on cell proliferation in the hippocampal dentate gyrus by immunohistochemistry, and on c-fos levels in the PFC and hippocampus, as well as nerve growth factor (NGF) levels in the hippocampus by western blotting. On the 14th day after surgery OBX mice showed an alteration in passive avoidance and decreases in both cell proliferation and levels of p-ERK, p-CREB and NGF in the hippocampus. The cognitive dysfunctions in OBX mice improved 30min to 24h after the administration of ARI (0.01mg/kg). C-fos levels in the PFC but not in the hippocampus was increased 30min after the administration (early response). This early response was inhibited by the selective D₁ receptor antagonist SCH23390. Cell proliferation and NGF levels in the hippocampus increased 24h after ARI administration (late response), and these effects were also inhibited by SCH23390. The MEK1/2 inhibitor U0126 prevented ARI from improving the behavioral impairment as well as enhancing NGF levels in OBX mice. These findings revealed the potential of ARI to improve cognitive dysfunctions via D₁ receptors with the PFC and hippocampus being affected sequentially.

Synthesis and Pharmacological Evaluation of Novel 1-(1,4-Alkylaryldisubstituted-4,5-dihydro-1H-imidazo)-3-substituted Urea Derivatives

Novel 1-(1,4-alkylaryldisubstituted-4,5-dihydro-1H-imidazo)-3-substituted urea derivatives have been synthesized and evaluated for their central nervous system activity. Compounds 3a–m were prepared in the reaction between the respective 1-alkyl-4-aryl-4,5-dihydro-1H-imidazol-2-amines 1a–c and appropriate isocyanates 2 in dichloromethane. The compounds were subjected to in silico ADMET studies in order to select best candidates for in vivo experiments. The effects of the compounds on the spontaneous locomotor activity and amphetamine-evoked hyperactivity were estimated. Analgesic activity, without or in the presence of naloxone, was assessed in the writhing test. The tendency to change the HTR, evoked by l-5-HTP and the involvement in alteration in body temperature in mice was studied. Additionally, to check possible occurrence of drug-induced changes in the muscle relaxant activity of mice, which may have contributed to their behaviour in other tests, the rota-rod and chimney tests were performed. The new urea derivatives exerted significant activities in the performed pharmacological tests, although the presented results show a preliminary estimation, and thus, need to be extended for identification and understanding the complete pharmacological profile of the examined compounds.

Effects of the D1 dopamine receptor agonist dihydrexidine (DAR-0100A) on working memory in schizotypal personality disorder

Pharmacological enhancement of prefrontal D1 dopamine receptor function remains a promising therapeutic approach to ameliorate schizophrenia-spectrum working memory deficits, but has yet to be rigorously evaluated clinically. This proof-of-principle study sought to determine whether the active enantiomer of the selective and full D1 receptor agonist dihydrexidine (DAR-0100A) could attenuate working memory impairments in unmedicated patients with schizotypal personality disorder (SPD). We performed a randomized, double-blind, placebo-controlled trial of DAR-0100A (15 mg/150 ml of normal saline administered intravenously over 30 min) in medication-free patients with SPD (n=16) who met the criteria for cognitive impairment (ie, scoring below the 25th percentile on tests of working memory). We employed two measures of verbal working memory that are salient to schizophrenia-spectrum cognitive deficits, and that clinical data implicate as being associated with prefrontal D1 availability: (1) the Paced Auditory Serial Addition Test (PASAT); and (2) the N-back test (ratio of 2-back:0-back scores). Study procedures occurred over four consecutive days, with working memory testing on Days 1 and 4, and DAR-0100A/placebo administration on Days 2-4. Treatment with DAR-0100A was associated with significantly improved PASAT performance relative to placebo, with a very large effect size (Cohen's d=1.14). Performance on the N-back ratio was also significantly improved; however, this effect rested on both a non-significant enhancement and diminution of 2-back and 0-back performance, respectively; therefore interpretation of this finding is more complicated. DAR-0100A was generally well tolerated, with no serious medical or psychiatric adverse events; common side effects were mild to moderate and transient, consisting mainly of sedation, lightheadedness, tachycardia, and hypotension; however, we were able to minimize these effects, without altering the dose, with supportive measures, eg, co-administered normal saline. Although preliminary, these findings lend further clinical support to the potential of D1 receptor agonists to treat schizophrenia-spectrum working memory impairments. These data suggest a need for further studies with larger group sizes, serum DAR-0100A levels, and a more comprehensive neuropsychological battery.

Update on the Mechanism of Action of Aripiprazole: Translational Insights into Antipsychotic Strategies Beyond Dopamine Receptor Antagonism

Dopamine partial agonism and functional selectivity have been innovative strategies in the pharmacological treatment of schizophrenia and mood disorders and have shifted the concept of dopamine modulation beyond the established approach of dopamine D2 receptor (D2R) antagonism. Despite the fact that aripiprazole was introduced in therapy more than 12 years ago, many questions are still unresolved regarding the complexity of the effects of this agent on signal transduction and intracellular pathways, in part linked to its pleiotropic receptor profile. The complexity of the mechanism of action has progressively shifted the conceptualization of this agent from partial agonism to functional selectivity. From the induction of early genes to modulation of scaffolding proteins and activation of transcription factors, aripiprazole has been shown to affect multiple cellular pathways and several cortical and subcortical neurotransmitter circuitries. Growing evidence shows that, beyond the consequences of D2R occupancy, aripiprazole has a unique neurobiology among available antipsychotics. The effect of chronic administration of aripiprazole on D2R affinity state and number has been especially highlighted, with relevant translational implications for long-term treatment of psychosis. The hypothesized effects of aripiprazole on cell-protective mechanisms and neurite growth, as well as the differential effects on intracellular pathways [i.e. extracellular signal-regulated kinase (ERK)] compared with full D2R antagonists, suggest further exploration of these targets by novel and future biased ligand compounds. This review aims to recapitulate the main neurobiological effects of aripiprazole and discuss the potential implications for upcoming improvements in schizophrenia therapy based on dopamine modulation beyond D2R antagonism.

Stress-mediated modulations in dopaminergic system and their subsequent impact on behavioral and oxidative alterations: an update

CONTEXT

Stress-induced changes in the dopaminergic system and subsequent enhancement of oxidative load and behavior are associated with a wide range of central and peripheral nervous disorders. Dopamine acts as a key neurotransmitter in the brain plays an important role in the regulation of motor and limbic functions.

OBJECTIVE

This article reviews the effect of stress on central dopaminergic system and its subsequent impact on the alterations in behavior and oxidative stress.

METHODS

A literature survey in PubMed (Bethesda, MD), Scopus (Philadelphia, PA), SciFinder (Columbus, OH) and Google Scholar (PMV, CA) was performed to gather information regarding the role of stress on central dopaminergic system and its associated behavioral and oxidative alterations.

RESULTS

Our collective data on behavioral studies and oxidative distress in stressful conditions show the functional reduction in dopaminergic neuronal system that could be one of the factors for the development of stress-induced motor suppression. Collectively, stress caused significant behavioral and oxidative alterations via suppression of neuronal functions of the central dopaminergic system.

CONCLUSIONS

This study provides an insight into the overall pathophysiological alterations in neuronal functions of the central dopaminergic system caused by acute and chronic unpredictable stress that, in our opinion, represent optimal utility as future therapeutic targets for neurodegenerative disorders.

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

RATIONALE

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSION

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

Role of medial prefrontal cortex dopamine in age differences in response to amphetamine in rats: locomotor activity after intra-mPFC injections of dopaminergic ligands

Changes in medial prefrontal cortex (mPFC) dopamine receptor expression and in mPFC projections to the nucleus accumbens in adolescence suggest that there may be age differences in the regulation of drug-related behavior by the mPFC. The age-specific role of prelimbic D1 dopamine receptors on amphetamine-induced locomotor activity was investigated. In experiment 1, rats aged postnatal day 30 (P30), P45, and P75, corresponding to early and late adolescence and adulthood, were given an injection of D1 and D2 antagonists into the prelimbic mPFC before a systemic injection of 1.5 mg/kg of amphetamine and locomotor activity was recorded. In experiment 2, effects of intra-prelimbic injections of a D1 agonist and antagonist on locomotor activity produced by a lower dose (0.5 mg/kg) of amphetamine were investigated. D2 receptor antagonist did not alter amphetamine-induced activity, whereas the D1 receptor antagonist reduced activity produced by 1.5 mg/kg of amphetamine more in P30 than in P45 and P75 rats. In addition, D1 agonist enhanced the locomotor activating effects of 0.5 mg/kg of amphetamine in adolescent rats and decreased activity in adult rats. These results suggest that insufficient activation of mPFC D1 receptors may underlie the reduced activity at the low dose of amphetamine in early adolescent compared to adult rats.

Adolescent social defeat alters markers of adult dopaminergic function

Stressful experiences during adolescence can alter the trajectory of neural development and contribute to psychiatric disorders in adulthood. We previously demonstrated that adolescent male rats exposed to repeated social defeat stress show changes in mesocorticolimbic dopamine content both at baseline and in response to amphetamine when tested in adulthood. In the present study we examined whether markers of adult dopamine function are also compromised by adolescent experience of social defeat. Given that the dopamine transporter as well as dopamine D1 receptors act as regulators of psychostimulant action, are stress sensitive and undergo changes during adolescence, quantitative autoradiography was used to measure [(3)H]-GBR12935 binding to the dopamine transporter and [(3)H]-SCH23390 binding to dopamine D1 receptors, respectively. Our results indicate that social defeat during adolescence led to higher dopamine transporter binding in the infralimbic region of the medial prefrontal cortex and higher dopamine D1 receptor binding in the caudate putamen, while other brain regions analyzed were comparable to controls. Thus it appears that social defeat during adolescence causes specific changes to the adult dopamine system, which may contribute to behavioral alterations and increased drug seeking.

New approaches to the pharmacological treatment of obesity: can they break through the efficacy barrier?

In this review we assess the range of centrally active anorectics that are either in human clinical trials, or are likely to be so in the near future. We describe their weight loss efficacy, mode of action at both pharmacological and behavioural levels, where understood, together with the range of side effects that might be expected in clinical use. We have however evaluated these compounds against the considerably more rigorous criteria that are now being used by the Federal Drugs Agency and European Medicines Agency to decide approvals and market withdrawals. Several trends are evident. Recent advances in the understanding of energy balance control have resulted in the exploitation of a number of new targets, some of which have yielded promising data in clinical trials for weight loss. A second major trend is derived from the hypothesis that improved weight loss efficacy over current therapy is most likely to emerge from treatments targeting multiple mechanisms of energy balance control. This reasoning has led to the development of a number of new treatments for obesity where multiple mechanisms are targeted, either by a single molecule, such as tesofensine, or through drug combinations such as qnexa, contrave, empatic, and pramlintide+metreleptin. Many of these approaches also utilise advances in formulation technology to widen safety margins. Finally, the practicality of peptide therapies for obesity has become better validated in recent studies and this may allow more rapid exploitation of novel targets, rather than awaiting the development of orally available small molecules. We conclude that novel, more efficacious and better tolerated treatments for obesity may become available in the near future.

Blockade of D1 dopamine receptors in the medial prefrontal cortex attenuates amphetamine- and methamphetamine-induced locomotor activity in the rat

The medial prefrontal cortex (mPFC) is a component of the mesolimbic dopamine (DA) system involved in psychostimulant-induced hyperactivity and previous studies have shown that altering DA transmission or D2 receptors within the mPFC can decrease this stimulant effect. The goal of this study was to investigate a potential modulatory role for D1 receptors in the mPFC in amphetamine (AMPH)- and methamphetamine (METH)-induced hyperactivity. Locomotor activity in an open-field arena was measured in male, Sprague-Dawley rats given an intra-mPFC infusion of vehicle or the D1 receptor antagonist SCH 23390 (0.25 or 1.0 microg) prior to systemic (i.p.) injection of saline, AMPH (1 mg/kg), or METH (1 mg/kg). We found that SCH 23390 produced a dose-dependent decrease in AMPH- and METH-induced locomotion and rearing but had no significant effect on spontaneous behavior that occurred following systemic saline injections. Because SCH 23390 has been shown to have agonist-like properties at 5-HT(2C) receptors, a follow-up experiment was performed to determine if this contributed to the attenuation of METH-induced activity that we observed. Rats were given intra-mPFC infusions of both SCH 23390 (1.0 microg) and the 5-HT(2C) antagonist RS 102221 (0.25 microg) prior to METH (1 mg/kg, i.p.). The addition of the 5-HT(2C) antagonist failed to alter SCH 23390-induced decreases in METH-induced locomotion and rearing; infusion of RS 102221 alone had no significant effects on locomotion and produced a non-significant decrease in rearing. The results of these studies suggest that D1 activation in the mPFC plays a significant role in AMPH- and METH-induced hyperactivity.

D1 receptor antagonist-induced long-term depression in the medial prefrontal cortex of rat, in vivo: an animal model of psychiatric hypofrontality

The objective of the following experiment was to induce a pathogenic hypofrontal condition by administering a dopamine-1 receptor (D(1)R) antagonist to rats. The pathophysiological effect of this manipulation upon glutamate-based long-term potentiation (LTP) in the medial prefrontal cortex (mPFC) was examined in vivo. Subjects were surgically implanted with stimulating electrodes into the corpus callosum and recording electrodes into the mPFC. High-frequency stimulation (HFS) was combined with the administration of the selective D(1)R family agonist A68930 hydrochloride (0.4 mg/kg/mL) and the selective D(1)R family antagonist SKF 83566 (0.15 mg/kg/mL). The administration of SKF 83566 hydrobromide prevented mPFC LTP, and resulted in HFS-induced long-term depression. This indicates that D(1)R activation is necessary for the induction of mPFC glutamate-based LTP. This is supported by our finding that the administration of A68930 hydrochloride combined with HFS induced LTP comparable with saline control levels, suggesting that D(1)R activation is necessary for the induction of baseline levels of mPFC LTP. Given that the mPFC governs executive behaviours that are subserved by LTP, such as working memory, these findings are relevant for the study of psychopathological conditions in which hypodopaminergic conditions exist in the mPFC and are correlated with psychiatric symptomotology, such as drug addiction and schizophrenia.

Performance- and task-dependent effects of the dopamine D1/D5 receptor agonist SKF 38393 on learning and memory in the rat

Dopamine D(1)/D(5) receptor agonists may enhance cognition by mimicking dopamine's neurophysiological actions on the processes underlying learning and memory. The present study examined the task- and performance- dependence of the cognitive effects of a partial agonist at dopamine D(1)/D(5) receptors, SKF 38393 [(+/-)-1-phenyl-2,3,4,5-tetrahydro-(1H)-3-benzazepine-7,8-diol hydrobromide], in rats. Spatial working memory was assessed in a T-maze, spatial reference memory in a water maze and habituation learning in a novel environment, a hole board. The muscarinic acetylcholine receptor antagonist scopolamine (1.5 mg/kg, i.p.) was used to cause an impairment of performance of these learning tasks. Administration of SKF 38393 (6 mg/kg, i.p.) alone had no significant effect on spontaneous alternation in the T-maze, latency to escape to a hidden platform in the water maze or the habituation of spontaneous behaviour in the hole board. In contrast, in scopolamine-treated rats, whereas SKF 38393 prevented the scopolamine-induced deficit in the T-maze, it exacerbated the impairment in the water maze and did not significantly alter the disruption of habituation. These results suggest that dopamine D(1)/D(5) receptor activation has performance- and task-dependent effects on cognitive function.

Motor inhibitory role of dopamine D1 receptors: implications for ADHD

Dysregulation of dopamine (DA) neurotransmission in frontal-striatal circuitry has been hypothesized to underlie several neurodevelopmental disorders, including attention-deficit/hyperactivity disorder (ADHD). The actions of DA are mediated by five distinct receptor subtypes that belong to the G-protein-coupled receptor super-family and are divided into two major classes, D1-like (D1 and D5) and D2-like (D2, D3, and D4). Accumulating evidence implicates the D1 receptor subtype (D1R) in the regulation of motor and cognitive processes. It is generally assumed that D1R is linked to motor activity in a stimulatory fashion. However, recent findings in rodents suggest a potential role of D1R on motor inhibition, which emerges during late postnatal development. Several lines of evidence indicate that the locus of the inhibitory effects involve subregions of the prefrontal cortex (PFC). These results may be relevant for understanding the neurobiology of ADHD.

Differential effects of a selective dopamine D1-like receptor agonist on motor activity and c-fos expression in the frontal-striatal circuitry of SHR and Wistar-Kyoto rats

BACKGROUND

Molecular genetic studies suggest the dopamine D1 receptor (D1R) may be implicated in attention-deficit/hyperactivity disorder (ADHD). As little is known about the potential motor role of D1R in ADHD, animal models may provide important insights into this issue.

METHODS

We investigated the effects of a full and selective D1R agonist, SKF-81297 (0.3, 3 and 10 mg/kg), on motor behaviour and expression of the plasticity-associated gene, c-fos, in habituated young adult male Spontaneously Hypertensive Rats (SHR), the most commonly used animal model of ADHD, and Wistar-Kyoto (WKY; the strain from which SHR were derived).

RESULTS

SHR rats were more behaviourally active than WKY rats after injection with vehicle. The 0.3 mg/kg dose of SKF-81297 increased motor behaviour (locomotion, sifting, rearing, and sniffing) in both SHR and WKY rats. Total grooming was also stimulated, but only in WKY rats. The same dose increased c-fos mRNA expression in the piriform cortex of both strains. The 3 mg/kg dose increased sifting and sniffing in both strains. Locomotion was also stimulated towards the end of the testing period. The intermediate dose decreased total rearing in both strains, and produced a significant increase in c-fos mRNA in the striatum, nucleus accumbens, olfactory tuberculum, and in the cingulate, agranular insular and piriform cortices. The 10 mg/kg dose of SKF-81297 produced a biphasic effect on locomotion, which was characterized by an initial decrease followed by later stimulation. The latter stimulatory effect was more pronounced in SHR than in WKY rats when compared to their respective vehicle-injected groups. The 10 mg/kg dose also stimulated sifting and sniffing in both strains. Both the 3 and 10 mg/kg doses had no effect on total grooming. The 10 mg/kg dose induced significantly higher levels of c-fos mRNA expression in the nucleus accumbens and adjacent cortical regions (but not striatum) of SHR when compared to WKY rats.

CONCLUSION

The present results suggest a potential alteration in D1R neurotransmission within the frontal-striatal circuitry of SHR involved in motor control. These findings extend our understanding of the molecular alterations in SHR, a heuristically useful model of ADHD.

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.

Tolerance to the procholinergic action of the D1 receptor full agonist dihydrexidine

RATIONALE

It has been well established that dopamine D1 receptor agonists increase acetylcholine (ACh) release in the brain, an effect that has been suggested to contribute to their procognitive properties.

OBJECTIVES

We sought to examine whether the ACh-releasing action of the D1 receptor full agonist dihydrexidine HCl (DHX) is altered after repeated administration.

RESULTS

DHX dose-dependently (3, 9, 18, and 36 mg/kg, i.p.) increased ACh efflux in the hippocampus of freely moving rats through D1 receptor stimulation, as the D1 receptor antagonist SCH 23390 (0.3 mg/kg, i.p.) abolished the effect of DHX (18 mg/kg, i.p.). Daily injections of DHX (18 mg/kg, i.p.) over 14 days did not significantly affect basal hippocampal ACh concentrations, but they significantly curtailed the stimulatory action of a challenge injection of DHX (18 mg/kg, i.p.) on ACh efflux.

CONCLUSIONS

Tolerance to the procholinergic action of DHX develops with repeated administration.

Deletion of dopamine D1 and D3 receptors differentially affects spontaneous behaviour and cocaine-induced locomotor activity, reward and CREB phosphorylation

Co-localization of dopamine D1 and D3 receptors in striatal neurons suggests that these two receptors interact at a cellular level in mediating dopaminergic function including psychostimulant-induced behaviour. To study D1 and D3 receptor interactions in cocaine-mediated effects, cocaine-induced locomotion and reward in mice lacking either D1, D3 or both receptors were analysed. Spontaneous locomotor activity was increased in D1-/- and D1-/-D3-/- mice and D1-/-D3-/- mice did not exhibit habituation of spontaneous rearing activity. Cocaine (20 mg/kg) increased locomotor activity in wild-type and D3-/- mice, failed to stimulate activity in D1-/- mice and reduced activity in D1-/-D3-/- mice. In the conditioned place preference, all groups exhibited reward at 5, 10 and 20 mg/kg of cocaine. D1-/-D3-/- mice did not demonstrate preference at 2.5 mg/kg of cocaine although preference was observed in wild-type, D1-/- and D3-/- mice. The transcription factor cAMP-responsive element binding protein (CREB) is activated by phosphorylation in striatal regions following dopamine receptor activation. Striatal pCREB levels following acute cocaine were increased in wild-type and D3-/- mice and decreased in D1-/- and D1-/-D3-/- mice. After repeated administration of 2.5 mg/kg of cocaine, D1-/- mice had lower pCREB levels in caudate-putamen and nucleus accumbens. Our findings suggest that, although spontaneous and cocaine-induced horizontal activity depended mainly on the presence of the D1 receptor, there may be crosstalk between D1 and D3 receptors in rearing habituation and the perception of cocaine reward at low doses of the drug. Furthermore, alterations in pCREB levels were associated with changes in cocaine-induced locomotor activity but not reward.

Dihydrexidine--the first full dopamine D1 receptor agonist

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

Disruption of neurogenesis on gestational day 17 in the rat causes behavioral changes relevant to positive and negative schizophrenia symptoms and alters amphetamine-induced dopamine release in nucleus accumbens

Gestational disruption of neurodevelopment has been proposed to lead to pathophysiological changes similar to those underlying schizophrenia. We induced such disruption by treating pregnant rat dams with methylazoxymethanol acetate (MAM) on gestational day 17 (GD17). Total brain size and that of the prefrontal cortex and hippocampus were reduced in adult rats exposed prenatally to MAM. When locomotor activity was assessed in an open field, MAM-exposed rats were hyper-responsive to a mild stress and to amphetamine (2 mg/kg, s.c.). They also engaged in less social interaction than controls. We studied, by microdialysis, the effect of amphetamine on extracellular dopamine in the nucleus accumbens and the medial prefrontal cortex of freely moving control and MAM-exposed rats. Amphetamine (2 mg/kg, s.c.) induced an increase in dopamine release that was larger in the nucleus accumbens of MAM-exposed rats than in controls, whereas no difference was seen in the medial prefrontal cortex. In controls, amphetamine infused into the medial prefrontal cortex (50 microM) led to a slight decrease in extracellular dopamine in the nucleus accumbens. This effect was absent in MAM-exposed rats, where a transient increase in nucleus accumbens dopamine levels was seen after amphetamine infusion. These results show that the late gestational disruption of neurogenesis in the rat leads to behavioral changes that mimic positive and negative schizophrenia symptoms, and also to a dysregulation of subcortical dopamine neurotransmission. This study contributes to the evaluation of the validity of the prenatal MAM GD17 treatment in rats as an animal model for schizophrenia.

Effect of methylphenidate on executive functioning in adults with attention-deficit/hyperactivity disorder: normalization of behavior but not related brain activity

BACKGROUND

We examined the effect of prolonged methylphenidate (MPH) treatment on the functional neuroanatomy of executive functioning in adult men with attention-deficit/hyperactivity disorder (ADHD).

METHODS

Positron emission tomography with [(15)O] water measured alterations of regional cerebral blood flow (rCBF) during the Paced Auditory Serial Addition Task and a control task in 10 ADHD and 11 normal control men. Attention-deficit/hyperactivity disorder men were imaged unmedicated and after a clinically optimal dose of MPH for 3 weeks.

RESULTS

Methylphenidate improved ADHD task performance, reduced rCBF in the prefrontal cortex (PFC), and increased rCBF in the right thalamus and precentral gyrus. Comparisons between the ADHD and normal control groups showed that normal control participants exhibited greater anterior cingulate cortex and temporal gyrus rCBF than ADHD participants under both conditions. Executive functioning was associated with greater subcortical (basal ganglia and cerebellar vermis) activation in the ADHD than normal control group under both conditions.

CONCLUSIONS

Methylphenidate does not normalize task-related activity in ADHD. Task-related rCBF decreases in the PFC may be due to improved filtering out of task-irrelevant stimuli by way of MPH-mediated dopamine release in the PFC.