The effect of the dopamine receptor blocking drug pimozide on the stimulant and anorectic actions of dextroamphetamine in man

Animal models for bipolar disorder: from bedside to the cage

Bipolar disorder is characterized by recurrent manic and depressive episodes. Patients suffering from this disorder experience dramatic mood swings with a wide variety of typical behavioral facets, affecting overall activity, energy, sexual behavior, sense of self, self-esteem, circadian rhythm, cognition, and increased risk for suicide. Effective treatment options are limited and diagnosis can be complicated. To overcome these obstacles, a better understanding of the neurobiology underlying bipolar disorder is needed. Animal models can be useful tools in understanding brain mechanisms associated with certain behavior. The following review discusses several pathological aspects of humans suffering from bipolar disorder and compares these findings with insights obtained from several animal models mimicking diverse facets of its symptomatology. Various sections of the review concentrate on specific topics that are relevant in human patients, namely circadian rhythms, neurotransmitters, focusing on the dopaminergic system, stressful environment, and the immune system. We then explain how these areas have been manipulated to create animal models for the disorder. Even though several approaches have been conducted, there is still a lack of adequate animal models for bipolar disorder. Specifically, most animal models mimic only mania or depression and only a few include the cyclical nature of the human condition. Future studies could therefore focus on modeling both episodes in the same animal model to also have the possibility to investigate the switch from mania-like behavior to depressive-like behavior and vice versa. The use of viral tools and a focus on circadian rhythms and the immune system might make the creation of such animal models possible.

The catecholaminergic-cholinergic balance hypothesis of bipolar disorder revisited

Bipolar disorder is a unique illness characterized by fluctuations between mood states of depression and mania. Originally, an adrenergic-cholinergic balance hypothesis was postulated to underlie these different affective states. In this review, we update this hypothesis with recent findings from human and animal studies, suggesting that a catecholaminergic-cholinergic hypothesis may be more relevant. Evidence from neuroimaging studies, neuropharmacological interventions, and genetic associations support the notion that increased cholinergic functioning underlies depression, whereas increased activations of the catecholamines (dopamine and norepinephrine) underlie mania. Elevated functional acetylcholine during depression may affect both muscarinic and nicotinic acetylcholine receptors in a compensatory fashion. Increased functional dopamine and norepinephrine during mania on the other hand may affect receptor expression and functioning of dopamine reuptake transporters. Despite increasing evidence supporting this hypothesis, a relationship between these two neurotransmitter systems that could explain cycling between states of depression and mania is missing. Future studies should focus on the influence of environmental stimuli and genetic susceptibilities that may affect the catecholaminergic-cholinergic balance underlying cycling between the affective states. Overall, observations from recent studies add important data to this revised balance theory of bipolar disorder, renewing interest in this field of research.

The relationship between oxidative stress and post-translational modification of the dopamine transporter in bipolar disorder

Bipolar disorder (BD) has been consistently associated with altered levels of oxidative stress markers, although the cause and consequences of these alterations remain to be elucidated. One of the main hypotheses regarding the pathogenesis of mania involves increased dopaminergic transmission. In this review, the authors aim to discuss a potential mechanism by which increased oxidative stress inhibits the uptake of dopamine through the post-translational modification of the dopamine transporter and its implications for BD. Within the next 5 years, the authors believe that the mechanisms of dopamine transporter oxidation and its impact on the pathophysiology of BD will be elucidated, which may open avenues for the development of more specific interventions for the treatment of this debilitating illness.

The role of dopamine in bipolar disorder

OBJECTIVE

Despite effective pharmacological treatments for bipolar disorder, we still lack a comprehensive pathophysiological model of the illness. Recent neurobiological research has implicated a number of key brain regions and neuronal components in the behavioural and cognitive manifestations of bipolar disorder. Dopamine has previously been investigated in some depth in bipolar disorder, but of late has not been a primary focus of attention. This article examines the role of dopamine in bipolar disorder, incorporating recent advances into established models where possible.

METHODS

A critical evaluation of the literature was undertaken, including a review of behavioural, neurochemical, receptor, and imaging studies, as well as genetic studies focusing on dopamine receptors and related metabolic pathways. In addition, pharmacologic manipulation of the central dopaminergic pathways and comparisons with other disease states such as schizophrenia were considered, principally as a means of exploring the hypothesised models.

RESULTS

Multiple lines of evidence, including data from pharmacological interventions and structural and functional magnetic resonance imaging studies, suggest that the dopaminergic system may play a central role in bipolar disorder.

CONCLUSION

Future research into the pathophysiological mechanisms of bipolar disorder and the development of new treatments for bipolar disorder should focus on the dopaminergic system.

Valproate attenuates dextroamphetamine-induced subjective changes more than lithium

Dextroamphetamine administration in healthy controls produces a range of subjective and physiological effects, which have been likened to those occurring during mania. However, it is uncertain if these can be attenuated by lithium since conflicting results have been reported. To date there have been no previous studies examining the effects of valproate on dextroamphetamine-induced mood and physiological changes. The current study was a double-blind, placebo-controlled, study in which volunteers received either 1000 mg sodium valproate (n=12), 900 mg lithium (n=9), or placebo (n=12) pre-treatment for 14 days. Subjective and physiological measures were then obtained prior to administration of a 25 mg dose of dextroamphetamine, and at two time points after administration. Differences in the response to dextroamphetamine were assessed between the three treatment groups. The results of this study show that pre-treatment with lithium only significantly attenuated dextroamphetamine-induced change in happiness, while valproate pre-treatment significantly attenuated the effects of dextroamphetamine on happiness, energy, alertness and on the diastolic blood pressure. These results suggest that lithium and valproate do not have the same mechanism of action on dextroamphetamine-induced changes, and this finding may relate to differences in their mechanism of action in mood disorders.

Clinical neuropharmacology of drugs of abuse: a comparison of drug-discrimination and subject-report measures

Advances in molecular pharmacology and behavioral science have helped elucidate the structure and function of the central nervous system and its relationship to behavior and has sparked the development of pharmacological agents that have increasingly selective and potent effects with fewer adverse side effects. The sensitivity and predictive validity of the two most commonly used methodologies for assessing the neuropharmacological effects of centrally active drugs, subject report of drug effects and drug discrimination, were examined. The sensitivity of the measures was comparable across stimulant, sedative, and opioid drugs. Results with drug-discrimination methodologies were generally consistent with hypothesized neuropharmacological mechanisms across all drug classes, whereas subject reports conformed under more limited testing conditions. Firm conclusions regarding the relative utility of drug-discrimination and subject-report measures for clinical studies of neuropharmacological mechanisms are limited by the small number of studies in which the two methodologies have been tested using identical pharmacological pretreatment manipulations.

Relationship of plasma amphetamine levels to physiological, subjective, cognitive and biochemical measures in healthy volunteers

Acute administration of the stimulant dextro-amphetamine produces multiple physiological, subjective cognitive and biochemical changes. These effects are similar to those seen in mania, and may be a useful model for mania. The aim of the present study was more fully to determine the multiple effects of dextro-amphetamine and to relate these to changes in plasma levels of the drug. In a double-blind, placebo-controlled crossover study in 25 healthy volunteers (ages 18-45), the effects of 25 mg of oral dextro-amphetamine were examined. Physiological, subjective, cognitive changes, concentrations of amino acids and metabolites of biogenic amines period were related to changes in plasma amphetamine concentrations over 500 min. Peak concentrations of dextro-amphetamine occurred at 2.5-3.5 h post-administration and levels decreased to 75% of peak value after 500 min. The results from the present study indicate that the subjective psychological, cognitive and blood pressure changes frequently did not mirror the time course of plasma levels of the drug. Thus, there was no clear-cut relationship between plasma levels and effects. In addition, dextro-amphetamine caused no significant changes in amino acids or amino metabolite concentrations. In conclusion, while dextro-amphetamine administration definitely causes several changes which are seen in mania, there remain some physiological and metabolic differences between these two conditions.

Pharmacokinetic and pharmacodynamic drug interactions in the treatment of attention-deficit hyperactivity disorder

The psychostimulants methylphenidate, amphetamine and pemoline are among the most common medications used today in child and adolescent psychiatry for the treatment of patients with attention-deficit hyperactivity disorder. Frequently, these medications are used in combination with other medications on a short or long term basis. The present review examines psychostimulant pharmacology, summarises reported drug-drug interactions and explores underlying pharmacokinetic and pharmacodynamic considerations for interactions. A computerised search was undertaken using Medline (1966 to 2000) and Current Contents to provide the literature base for reports of drug-drug interactions involving psychostimulants. These leads were further cross-referenced for completeness of the survey. Methylphenidate appears to be more often implicated in pharmacokinetic interactions suggestive of possible metabolic inhibition, although the mechanisms still remain unclear. Amphetamine was more often involved in apparent pharmacodynamic interactions and could potentially be influenced by medications affecting cytochrome P450 (CYP) 2D6. No published reports of drug interactions involving pemoline were found. The alpha2-adrenergic agonists clonidine and guanfacine have been implicated in several interactions. Perhaps best documented is their antagonism by tricyclic antidepressants and phenothiazines. In additional, concurrent beta-blocker use, or abrupt discontinuation, can lead to hypertensive response. Although there are few published well-controlled interaction studies with psychostimulants and alpha2-adrenergic agonists, it appears that these agents may be safely coadministered. The interactions of monoamine oxidase inhibitors with psychostimulants represent one of the few strict contraindications.

Effects of isradipine, a dihydropyridine-class calcium channel antagonist, on d-methamphetamine-induced reduction in hunger

1. The authors studied the effects of isradipine, a dihydropyridine-class calcium channel antagonist, on d-methamphetamine-induced changes in somatic and psychological perceptions of hunger state using a placebo-controlled, double-blind, Latin Square, cross-over design in 18 healthy volunteers. 2. D-methamphetamine significantly decreased these subjective ratings of hunger, presumably by increasing monoaminergic turnover. 3. Effects on hunger are hypothesized to be mediated by norepinephrine primarily, while dopamine plays only a modest role. Isradipine alone, an inhibitor of dopamine release, had no significant effect on the hunger measures. Additionally, isradipine pretreatment did not significantly alter d-methamphetamine's anorexic effects. 4. Isradipine may, therefore, not significantly modify the control of hunger in humans.

Body weight gain induced by antipsychotic drugs: mechanisms and management

Long-term administration of typical and atypical antipsychotic drugs (AP) induces excessive weight gain which afflicts up to 50% of patients, impairs health and interferes with treatment compliance. Basic and clinical research has shown that AP may affect body weight through diverse mechanisms. Increased appetite is probably related to the interaction of AP with neuronal receptors to dopamine, serotonin and histamine. Additional metabolic-endocrine disruption of weight regulation may be related to the effects of AP-induced hyperprolactinaemia on gonadal-adrenal steroids and insulin sensitivity. In humans, programmed physical activity, dietary restriction, anorectic agents, and drugs that counteract hyperprolactinaemia have been shown to be successful in a limited number of studies. Two novel strategies could expand the available therapeutic options. First, in preclinical experiments in female rats the estradiol antagonist/agonist drug tamoxifen or estradiol itself have been shown to completely prevent the obesity provoked by the AP sulpiride, and to induce an endocrine-metabolic milieu that seems to counteract AP-induced obesity. Secondly, it has also been shown that oral antihyperglycaemic agents such as metformin may decrease body weight and counteract insulin resistance and hyperinsulinaemia which is correlated with several metabolic abnormalities in obese subjects. Lastly, estradiol replacement, tamoxifen and/or antihyperglycaemic agents are not devoid of significant side-effects, and these drugs have not been tested in obese psychiatric patients. Therefore, further research is needed before their clinical use may be recommended.

Lithium does not attenuate the effects of D-amphetamine in healthy volunteers

It has previously been suggested that an acute dose of 20 mg D-amphetamine can be a good model of mania. In the present study we attempted to validate this model by determining if subacute administration of lithium (for 7 days) would attenuate the effects of 20 mg D-amphetamine in human volunteers. Sixteen healthy volunteers were enrolled in this double-blind, placebo-controlled study. Subjects received either oral lithium at night for 7 nights (1200 mg) (n = 10) or matching placebo (n = 6). On day 8, subjects were assessed at baseline and then once each hour for 3 h following an oral dose of D-amphetamine (20 mg). Subjective states were measured with visual analog scales, and pulse, systolic and diastolic blood pressure were also repeatedly assessed. The results showed that D-amphetamine alone produced a number of subjective and cardiovascular changes, as expected. However, lithium did not attenuate any of these subjective or cardiovascular changes, as would be predicted if D-amphetamine were a valid model of mania. It is suggested that whilst D-amphetamine may produce effects similar to mania, its mechanism of action is different from that which occurs in mania, and therefore the usefulness of the amphetamine model of mania is called into question.

Subjective responses to d-amphetamine alone and after pimozide pretreatment in normal, healthy volunteers

The role of dopamine in d-amphetamine-induced euphoria has not been systematically examined in normal volunteers. Therefore, we examined the effects of the dopamine antagonist, pimozide, on responses to d-amphetamine in healthy volunteers, using a within-subjects, double-blind design. Ten subjects received single oral doses of d-amphetamine (0, 10, 20 mg) 2 hours following pretreatment with pimozide (0, 1, 2 mg). Subjective, behavioral, and physiological effects were assessed predrug and for 3 hours after d-amphetamine administration. d-Amphetamine alone produced prototypic effects on a variety of measures, including euphoria and drug liking. Pimozide did not produce any effects when administered alone and produced inconsistent effects on responses to d-amphetamine. Although higher doses of pimozide may be needed to antagonize the euphorigenic effects of d-amphetamine, these results raise the possibility that the role of dopamine in the subjective effects of stimulants may be more complex than initially appreciated.

Menstrual cycle effects on the action of buspirone on food intake in healthy female volunteers

We studied the effect of single doses of the 5-HT(1A) agonist buspirone (10 mg and 20 mg) on food intake during the menstrual cycle of nine healthy women, none of whom were on a contraceptive pill. In a double-blind placebo-controlled trial, subjects were tested over three cycles, receiving one treatment per cycle. Within each cycle, they were tested at two time points; mid-follicular and late luteal. Food intake following 20 mg buspirone was significantly higher in the luteal phase, suggesting the possibility of pre-menstrually enhanced 5-HT receptor sensitivity.

Does ondansetron attenuate amphetamine-induced behaviour in human volunteers?

The effect of the 5-HT3 receptor antagonist, ondansetron, on the decrease in hunger produced by amphetamine was assessed in nine male volunteers using a double-blind cross-over design. Amphetamine (15 mg orally) produced a significant decrease in self-ratings of hunger 2.5 h after administration. This effect was significantly attenuated by pre-treatment with ondansetron (12 mg orally over 24 h). These findings in humans are consistent with data from animal studies suggesting that ondansetron can attenuate certain catecholamins-mediated behaviours produced by amphetamine. However, explanations founded on pharmacokinetic factors cannot be presently excluded.

Evidence for 5-HT2 receptor mediation in quipazine anorexia

Doses of d-amphetamine (3.2 mg/kg), fenfluramine (10 mg/kg) and quipazine (8 mg/kg) cause a significant reduction in food intake during a 30-min daily feeding session in food-deprived rats. Pirenperone and ritanserin, 5-HT2 receptor antagonists, significantly blocked the anorectic effect of quipazine, while d-amphetamine and fenfluramine effects were not modified. Metergoline, a non-specific blocker of 5-HT receptors, significantly blocked the anorectic effects of fenfluramine and quipazine, but not the d-amphetamine effect. Pretreatment with alpha- and beta-adrenergic receptor antagonists (prazosin, propranolol and pindolol), dopamine receptor antagonists (haloperidol and pimozide), the catecholamine synthesis inhibitor alpha-methyl-para-tyrosine, and the opioid receptor antagonist naloxone failed to modify the anorectic effects of all three agents, with the exception that quipazine-induced anorexia was significantly reduced by pimozide. These results suggest that the quipazine anorexia is largely mediating through 5-HT2 receptors, although the effect of pimozide remains to be explained. Consistent with previous studies, the fenfluramine effect appears to be mediated through 5-HT1B receptors. Receptors involved in the anorectic effect of higher doses of d-amphetamine are still unidentified by this analysis. Further investigation is required to define the mechanisms by which quipazine and larger doses of d-amphetamine bring about a reduced appetite for food.

Prevalence of obesity in patients receiving depot antipsychotics

Antipsychotic drugs have long been noted to cause pronounced weight gain, and drug-induced obesity can assume major clinical importance in long-term medication in the management of chronic schizophrenia. Obesity is associated with increased morbidity and may reduce compliance, leading to a return of psychotic symptoms. In a survey of 226 patients attending depot neuroleptic clinics in one inner London borough, it was found that the prevalence of clinically relevant obesity was four times that in the general population.

Receptor blocking drugs and amphetamine anorexia in human subjects

The interaction between the receptor antagonist thymoxamine (THYM), propranolol (PPL) and metergoline (MTG) with dexamphetamine (d-Amp)-induced anorexia was examined in a series of studies in normal female volunteers. Visual analogue scale (VAS) ratings of hunger were made and food intake was measured using an automated solid food dispenser (AFD). d-Amp (10 mg) significantly depressed hunger ratings compared to placebo in two of the three studies and its effect was countered by the addition of MTG (4 mg). d-Amp significantly reduced food intake compared to placebo in two studies. In all trials the reduction in food intake following d-Amp was significantly greater than would have been predicted from its effect on hunger. THYM (160 mg) and PPL (40 mg) were associated with no changes in food intake when given alone or with d-Amp, MTG increased food intake (but not significantly) and the combined effects of MTG and d-Amp was the algebraic sum of the effect of each; but there appeared to be no true pharmacological interaction between blocker and anorectic. The results indicated that there may be some dissociation between the effect of d-Amp on hunger and food intake but have failed to produce evidence that noradrenergic pathways are involved. The results are consistent with the theories that d-Amp anorexia does not involve the release of 5-hydroxytryptamine (5-HT) but that 5-HT pathways are involved in the feeding process.

Effect of mazindol, d-amphetamine and diethylpropion on purine nucleotide binding to brown adipose tissue

Amphetamine, diethylpropion and mazindol were administered to rats in both acute and chronic experiments to measure the changes in purine nucleotide (GDP) binding to the mitochondria from interscapular brown adipose tissue. There was a dose-dependent response to acute treatment with mazindol, but no such effect with diethylpropion. The effects of mazindol and amphetamine were present as early as 3 hours after treatment, and persisted for at least 48 hours, when compared to vehicle-injected rats when all rats were fasted from the time of injection until study. There was no effect when these drugs were added in vitro to mitochondria from brown adipose tissue. Diethylpropion had no effect on GDP binding either in vivo or in vitro at any of the times tested. Following 11 days of treatment with diethylpropion, amphetamine or mazindol, there was a significant increase in purine nucleotide (GDP) binding to mitochondria only in the amphetamine-treated animals. There was no difference in body weight or food intake with any of the three drugs after the third day of chronic treatment. The differences between the effects of these three drugs and those of fenfluramine are discussed in terms of their different central mechanisms of action.

Dextroamphetamine-induced arousal in human subjects as a model for mania

Because of the practical difficulties which arise in studying manic patients, a reproducible model for mania using human subjects would be a valuable adjunct to research in this condition. Dextroamphetamine, given as a single oral 20 mg dose, fulfils the criteria for such a model in that there are very close similarities between the changes which occur after dextroamphetamine and those which have been observed in mania in terms of subjective experience, physiological and endocrine changes, and response to pharmacological agents.

Dopamine in manic depressive illness. A pharmacological synthesis

If the clinical symptoms of mania are a consequence of increased activity in central dopaminergic (DA) pathways in predisposed individuals, then drugs increasing DA neurotransmission should precipitate or exacerbate mania in such people, whereas drugs which reduce DA neurotransmission should ameliorate manic symptoms. Of the drugs which enhance DA neurotransmission, those which increase synthesis of DA (levodopa), those which promote DA release (amphetamine), and those which act directly as agonists or DA receptors (bromocriptine) have all been shown to precipitate mania. Conversely, drugs which reduce DA neurotransmission by inhibiting synthesis (alpha-methylparatyrosine) or by blocking DA receptors (pimozide) are effective in reducing manic symptoms. DA systems are not working in isolation; evidence is presented showing an influence on manic illness of central cholinergic and GABA-ergic processes. It is suggested that there is an interacting set of neurotransmitter pathways linking the limbic system and the ventral tegmental (A10) area involving DA, acetylcholine and GABA upon which drugs can act to influence the course of a manic illness.

Differential dose-response effects of dexamphetamine sulphate on hunger, arousal and mood in human volunteers

The effect of dexamphetamine (d-Amp) and placebo on visual analogue scale (VAS) ratings of hunger, arousal and mood in nine male volunteers was observed. d-Amp (10 mg) significantly depressed hunger ratings but did not significantly affect arousal and mood ratings. d-Amp (20 mg) had a significant effect on all three ratings. There was a difference in the pattern of dose-response effects. Whereas 20 mg d-Amp produced greater changes than 10 mg in ratings of mood and arousal, there was no significant difference on ratings of hunger. These differences in dose-response relationships may reflect differences in the underlying neurochemical mechanisms mediating the stimulant and anorectic effects of d-Amp.

The effect of d-amphetamine and haloperidol alone and in combination on milk drinking in rats

The effects of d-amphetamine (0.5-4.0 mg/kg IP) and haloperidol (0.015-1.0 mg/kg IP) alone and in combination on intake of a sweetened milk solution were determined in rats. Experimental sessions consisted of a 15-min access to the milk solution once a day, 7 days a week. d-Amphetamine was administered 15 min prior to the session and haloperidol was administered 60 min prior to the session. The interaction of these compounds was determined by repeated determination of the dose-response function for d-amphetamine in combination with different doses of haloperidol. When given alone, each drug produced a dose-dependent decrease in milk intake. In combination with haloperidol, the decrement caused by d-amphetamine was shifted to lower doses in a dose-dependent manner. Isobolographic analysis of the interaction indicates that the intermediate doses (0.03-0.06 mg/kg) of haloperidol were additive and the highest doses (0.12-0.25 mg/kg) of haloperidol were infraadditive with d-amphetamine. Dose-dependent neuropharmacological actions of these compounds may account for dose-dependent interactions.