Cocaine euphoria, dysphoria, and tolerance assessed using drug-induced changes in brain-stimulation reward

Brain dysfunctions and neurotoxicity induced by psychostimulants in experimental models and humans: an overview of recent findings

Preclinical and clinical studies indicate that psychostimulants, in addition to having abuse potential, may elicit brain dysfunctions and/or neurotoxic effects. Central toxicity induced by psychostimulants may pose serious health risks since the recreational use of these substances is on the rise among young people and adults. The present review provides an overview of recent research, conducted between 2018 and 2023, focusing on brain dysfunctions and neurotoxic effects elicited in experimental models and humans by amphetamine, cocaine, methamphetamine, 3,4-methylenedioxymethamphetamine, methylphenidate, caffeine, and nicotine. Detailed elucidation of factors and mechanisms that underlie psychostimulant-induced brain dysfunction and neurotoxicity is crucial for understanding the acute and enduring noxious brain effects that may occur in individuals who use psychostimulants for recreational and/or therapeutic purposes.

Dysthymia and depressive disorders: dopamine hypothesis

This paper reviews the recent literature supporting the hypothesis that reduced neurotransmission in the mesolimbic dopamine (DA) system may sustain some of the symptoms of depressive conditions including dysthymia. Experimental evidences indicate that mesolimbic DA plays a crucial role in controlling incentive, motivation and reward. Additionally, in different models of depression, a reduced DA activity in the limbic system, reversed by chronic antidepressant treatment, is observed. Finally, different antidepressants, irrespective of their acute action on the uptake of norepinephrine or serotonin, have the common property when given chronically to potentiate behavioural responses to DA agonists. The DA hypothesis of depression offers an explanation for the antidepressive effect of drugs such as sulpiride and amisulpride given at low doses, that preferentially block DA autoreceptors and thereby increase DA output.

Alcohol Interactions with Psychostimulants: An Overview of Animal and Human Studies

Alcohol consumption with psychostimulants is very common among drug addicts. There is little known about the possible pharmacological interactions between alcohol and psychostimulants. Among most commonly co-abused psychostimulants with alcohol are methamphetamine, cocaine, 3,4-methylenedioxymethamphetaminen, and nicotine. Co-abuse of alcohol with psychostimulants can lead to several neurophysiological dysfunctions such as decrease in brain antioxidant enzymes, disruption of learning and memory processes, cerebral hypo-perfusion, neurotransmitters depletion as well as potentiation of drug seeking behaviour. Moreover, co-abuse of alcohol and psychostimulants can lead to increase in heart rate, blood pressure, myocardial oxygen consumption and cellular stress, and the risk of developing different types of cancer. Co-abuse of alcohol with psychostimulants during pregnancy can lead to fetal brain abnormalities. Further studies are needed to investigate the pharmacokinetics, pharmacodynamics, and neurochemical changes on co-abuse of alcohol and psychostimulants.

Tolerance to repeated rewarding electrical stimulation of the insular cortex

The insular cortex (IC) has been related to various reinforcing behavioral processes. This study examined the effect of electrical stimulation of the posterior agranular IC on concurrent place preferences. Two groups of animals and their respective controls underwent rewarding brain stimulation every day or on alternate days. While the rats stimulated every other day maintained their preference for the place associated with brain stimulation, those stimulated every day evidenced a reduction in their place preference, suggesting tolerance to the stimulation's rewarding effect. A 15% increase in the current intensity produced a recovery of the preferences of the daily-stimulated rats but had no effect on those stimulated on alternate days. These results are discussed in terms of the rewarding effects induced by different electrical and chemical rewarding agents.

Selective activation of the trace amine-associated receptor 1 decreases cocaine's reinforcing efficacy and prevents cocaine-induced changes in brain reward thresholds

The newly discovered trace amine-associated receptor 1 (TAAR1) has emerged as a promising target for medication development in stimulant addiction due to its ability to regulate dopamine (DA) function and modulate stimulants' effects. Recent findings indicate that TAAR1 activation blocks some of the abuse-related physiological and behavioral effects of cocaine. However, findings from existing self-administration studies are inconclusive due to the very limited range of cocaine unit doses tested. Here, in order to shed light on the influence of TAAR1 on cocaine's reward and reinforcement, we studied the effects of partial and full activation of TAAR1on (1) the dose-response curve for cocaine self-administration and (2) cocaine-induced changes in intracranial self-stimulation (ICSS). In the first experiment, we examined the effects of the selective full and partial TAAR1 agonists, RO5256390 and RO5203648, on self-administration of five unit-injection doses of cocaine (0.03, 0.1, 0.2, 0.45, and 1mg/kg/infusion). Both agonists induced dose-dependent downward shifts in the cocaine dose-response curve, indicating that both partial and full TAAR1 activation decrease cocaine, reinforcing efficacy. In the second experiment, RO5256390 and the partial agonist, RO5263397, dose-dependently prevented cocaine-induced lowering of ICSS thresholds. Taken together, these data demonstrated that TAAR1 stimulation effectively suppresses the rewarding and reinforcing effects of cocaine in self-administration and ICSS models, supporting the candidacy of TAAR1 as a drug discovery target for cocaine addiction.

Tolerance to cocaine in brain stimulation reward following continuous cocaine infusions

This study examined tolerance to cocaine's threshold-lowering effect in brain stimulation reward (BSR) following continuous cocaine infusions and secondly, used the nitric oxide synthase inhibitor Nω-nitro-L-arginine methyl ester (L-NAME) to determine NO's involvement in the development of cocaine tolerance. Animals were continuously infused with saline or cocaine (30 mg/kg per day) via osmotic minipump for 14 days and injected daily with saline or L-NAME (30 mg/kg, i.p.) following BSR testing. Saline-treated animals continuously infused with saline showed stable BSR thresholds across the 14-day infusion period. Saline-treated animals continuously infused with cocaine showed markedly lowered BSR thresholds on Day 1 followed by a progressive increase in BSR thresholds across the infusion period - indicating the development of tolerance. L-NAME-treated animals continuously infused with cocaine showed stimulation thresholds that were not significantly different from saline-treated animals continuously infused with cocaine. A cocaine challenge injection (10 mg/kg, i.p.) administered 3 and again at 10 days following minipump removal revealed that saline-treated animals continuously infused with saline showed lowered BSR thresholds. Saline-treated animals continuously infused with cocaine displayed lowered BSR thresholds that were not significantly different from saline-infused animals. L-NAME treated animals continuously infused with cocaine showed higher BSR thresholds to a challenge 3 days following pump removal. However, stimulation thresholds for this group failed to reach statistical significance on both days (i.e., Days 3 and 10) following pump removal. Results showed that animals continuously infused with cocaine develop robust tolerance to cocaine's threshold-lowering effect during the 14-day infusion period. Tolerance to cocaine's threshold-lowering effect was short-lived and dissipated soon after minipump removal. L-NAME treatment failed to significantly alter the development of tolerance to cocaine's threshold-lowering suggesting that NO does not have a primary role in the development of cocaine tolerance.

Cocaine tolerance in honey bees

Increasingly invertebrates are being used to investigate the molecular and cellular effects of drugs of abuse to explore basic mechanisms of addiction. However, in mammals the principle factors contributing to addiction are long-term adaptive responses to repeated drug use. Here we examined whether adaptive responses to cocaine are also seen in invertebrates using the honey bee model system. Repeated topical treatment with a low dose of cocaine rendered bees resistant to the deleterious motor effects of a higher cocaine dose, indicating the development of physiological tolerance to cocaine in bees. Cocaine inhibits biogenic amine reuptake transporters, but neither acute nor repeated cocaine treatments caused measurable changes in levels of biogenic amines measured in whole bee brains. Our data show clear short and long-term behavioural responses of bees to cocaine administration, but caution that, despite the small size of the bee brain, measures of biogenic amines conducted at the whole-brain level may not reveal neurochemical effects of the drug.

Dopaminergic modulation of incentive motivation in adolescence: age-related changes in signaling, individual differences, and implications for the development of self-regulation

Behavioral activation that is associated with incentive-reward motivation increases in adolescence relative to childhood and adulthood. This quadratic developmental pattern is generally supported by behavioral and experimental neuroscience findings. It is suggested that a focus on changes in dopamine neurotransmission is informative in understanding the mechanism for this adolescent increase in reward-related behavioral activation and subsequent decline into adulthood. Evidence is presented to indicate that incentive-reward motivation is modulated by mesoaccumbens dopamine, and that it increases in adolescence before declining into adulthood because of normative developmental changes at the molecular level. Potential mechanisms of variation in functional mesoaccumbens dopamine transmission are discussed with a focus on the interplay between tonic and phasic modes of dopamine transmission in modulating both general incentive-motivational biases and the efficacy of reward learning during exposure to novel reward experiences. Interactions between individual difference factors and these age-related trends are discussed.

Cocaine enhances resistance to extinction of responding for brain-stimulation reward in adult prenatally stressed rats

The present experiment assessed whether prenatal stress (PS) can alter the ability of acute and chronic cocaine administration to increase and decrease the rewarding effectiveness of the medial forebrain bundle (MFB) using intracranial self-stimulation (ICSS), and also whether PS can affect the extinction of the MFB stimulation response. Adult male offspring of female rats that received PS or no PS (nPS) were implanted with MFB stimulating electrodes, and were then tested in ICSS paradigms. In both nPS and PS offspring, acute cocaine injection decreased ICSS thresholds dose-dependently. However, the threshold-lowering effects at any dose were not significantly different between groups. There was also no group-difference in the threshold-elevating effects of chronic cocaine administration. Nevertheless, chronically drug-administered PS rats exhibited a resistance to the extinguishing of the response for brain-stimulation reward when acutely treated with cocaine, as compared to extinction without cocaine treatment. The results suggest that PS may weaken the ability for response inhibition under cocaine loading in male adult offspring.

Neural substrates of psychostimulant withdrawal-induced anhedonia

Psychostimulant drugs have powerful reinforcing and hedonic properties and are frequently abused. Cessation of psychostimulant administration results in a withdrawal syndrome characterized by anhedonia (i.e., an inability to experience pleasure). In humans, psychostimulant withdrawal-induced anhedonia can be debilitating and has been hypothesized to play an important role in relapse to drug use. Hence, understanding the neural substrates involved in psychostimulant withdrawal-induced anhedonia is essential. In this review, we first summarize the theoretical perspectives of psychostimulant withdrawal-induced anhedonia. Experimental procedures and measures used to assess anhedonia in experimental animals are also discussed. The review then focuses on neural substrates hypothesized to play an important role in anhedonia experienced after termination of psychostimulant administration, such as with cocaine, amphetamine-like drugs, and nicotine. Both neural substrates that have been extensively investigated and some that need further evaluation with respect to psychostimulant withdrawal-induced anhedonia are reviewed. In the context of reviewing the various neurosubstrates of psychostimulant withdrawal, we also discuss pharmacological medications that have been used to treat psychostimulant withdrawal in humans. This literature review indicates that great progress has been made in understanding the neural substrates of anhedonia associated with psychostimulant withdrawal. These advances in our understanding of the neurobiology of anhedonia may also shed light on the neurobiology of nondrug-induced anhedonia, such as that seen as a core symptom of depression and a negative symptom of schizophrenia.

Potentiation of intracranial self-stimulation during prolonged subcutaneous infusion of cocaine

Subcutaneous administration of cocaine yields a longer duration of action than administration via the intraperitoneal or intravenous routes. However, cocaine is a powerful vasoconstrictor, and thus injection of this drug at a single subcutaneous locus entails significant risk of necrotic skin lesions. This paper introduces a new method for subcutaneous administration of cocaine that reduces the probability of dermonecrosis by dispersing the drug under a large area of skin. Two experiments were conducted to evaluate the new method. In the first, changes in dopamine tone in the nucleus accumbens were measured by means of microdialysis during prolonged subcutaneous infusions of cocaine. The dopamine concentration attained a fairly stable, elevated level, suggesting that absorption, distribution, and excretion of the drug approached steady state. In a second experiment, performance for rewarding electrical stimulation was measured during repeated prolonged infusions of cocaine. The pulse frequency required to sustain responding was decreased by the drug, in a manner that was stable both within and across test sessions. Thus, the new method is appropriate for studies requiring stable neurochemical and behavioral conditions during repeated long test sessions, high rates of drug delivery and alternation between administration of the drug and the vehicle.

Animal models and treatments for addiction and depression co-morbidity

The high rates of co-morbidity of drug addiction with depression may be attributable to shared neurobiology. Here, we discuss shared neurobiological substrates in drug withdrawal and depression, with an emphasis on changes in brain reward circuitry that may underlie anhedonia, a core symptom of depression and drug withdrawal. We explored experimentally whether clinical antidepressant medications or other treatments would reverse the anhedonia observed in rats undergoing spontaneous nicotine or amphetamine withdrawal, defined operationally as elevated brain reward thresholds. The co-administration of selective serotonin reuptake inhibitors with a serotonin-1A receptor antagonist, or the tricyclic antidepressant desipramine, or the atypical antidepressant bupropion ameliorated nicotine or amphetamine withdrawal in rats. Thus, increases in monoaminergic neurotransmission, or neuroadaptations induced by increased monoaminergic neurotransmission, ameliorated depression-like aspects of drug withdrawal. Further, chronic pretreatment with the atypical antipsychotic clozapine, that has some efficacy in the treatment of the depression-like symptoms of schizophrenia, attenuated nicotine and amphetamine withdrawal. Finally, a metabotropic glutamate 2/3 receptor antagonist reversed threshold elevations associated with nicotine withdrawal. The effects of these pharmacological manipulations are consistent with the altered neurobiology observed in drug withdrawal and depression. Thus, these data support the hypothesis of common substrates mediating the depressive symptoms of drug withdrawal and those seen in psychiatric patients. Accordingly, the anhedonic state associated with drug withdrawal can be used to study the neurobiology of anhedonia, and thus contribute to the identification of novel targets for the treatment of depression-like symptoms seen in various psychiatric and neurological disorders.

The basolateral complex of the amygdala mediates the modulation of intracranial self-stimulation threshold by drug-associated cues

Learning and memory appear to be critical aspects of drug abuse; presumably playing an especially important role in craving and relapse. Thus, understanding the interaction of learning- and memory-related brain areas with the classical reward circuitry is of importance. Toward this goal, the effect of drug-associated contextual cues on intracranial self-stimulation (ICSS) behaviour was assessed in rats. We used a method that allows the establishment of baseline behaviour, the pairing of drug exposure with unique cues, and testing the effect of cue exposure within the same apparatus. ICSS thresholds were decreased by morphine (5 mg/kg, i.p.) or cocaine (10 mg/kg, i.p.) during five days of paired drug-cue training sessions. Subsequent presentation of the drug-associated cues decreased thresholds in the absence of drug. Cues associated with saline had no effect. These results suggest a Pavlovian conditioning phenomenon in which the functioning of brain reward circuitry is modulated by drug-associated cues. In a second experiment, we tested the hypothesis that the mechanism by which conditioning affects ICSS thresholds may include the basolateral complex of the amygdala (BLC) due to its known role in conditioning and anatomical linkage with classical reward circuitry. Lesions of the BLC abolished the ability of cocaine-associated cues to lower ICSS threshold. Lesions did not alter response capability or the unconditioned effect of cocaine. We conclude that the BLC is necessary for cues associated with previous drug exposure to modulate activity within or downstream from the classical reward circuitry of the medial forebrain bundle.

Toxic cocaine- and convulsant-induced modification of forced swimming behaviors and their interaction with ethanol: comparison with immobilization stress

BACKGROUND

Swimming behaviors in the forced swimming test have been reported to be depressed by stressors. Since toxic convulsion-inducing drugs related to dopamine [cocaine (COC)], benzodiazepine [methyl 6,7-dimethoxy-4-ethyl-beta-carboline-carboxylate (DMCM)], gamma-aminobutyric acid (GABA) [bicuculline (BIC)], and glutamate [N-methyl-D-aspartate (NMDA)] receptors can function as stressors, the present study compared their effects on the forced swimming behaviors with the effects of immobilization stress (IM) in rats. Their interactions with ethanol (EtOH), the most frequently coabused drug with COC which also induces convulsions as withdrawal symptoms but interferes with the convulsions caused by other drugs, were also investigated.

RESULTS

Similar to the IM (10 min) group, depressed swimming behaviors (attenuated time until immobility and activity counts) were observed in the BIC (5 mg/kg IP) and DMCM (10 mg/kg IP) groups at the 5 h time point, after which no toxic behavioral symptoms were observed. However, they were normalized to the control levels at the 12 h point, with or without EtOH (1.5 g/kg IP). In the COC (60 mg/kg IP) and NMDA (200 mg/kg IP) groups, the depression occurred late (12 h point), and was normalized by the EtOH cotreatment. At the 5 h point, the COC treatment enhanced the swimming behaviors above the control level.

CONCLUSIONS

Although the physiological stress (IM), BIC, and DMCM also depressed the swimming behaviors, a delayed occurrence and EtOH-induced recovery of depressed swimming were observed only in the COC and NMDA groups. This might be correlated with the previously-reported delayed responses of DA and NMDA neurons rather than direct effects of the drugs, which could be suppressed by EtOH. Furthermore, the characteristic psychostimulant effects of COC seemed to be correlated with an early enhancement of swimming behaviors.

Neuroadaptations to chronic exposure to drugs of abuse: relevance to depressive symptomatology seen across psychiatric diagnostic categories

Depressive symptomatology is expressed across a wide spectrum of psychiatric disorders including major depression and schizophrenia. Further, depressive symptomatology is also observed in individuals undergoing withdrawal from chronic exposure to various drugs of abuse including cocaine, amphetamine and nicotine. The negative affective state associated with drug withdrawal is phenomenonologically similar to that observed in depressed and schizophrenia patients suggesting that common underlying pathophysiological deficits may be involved in the depressive symptomatology seen across these different psychiatric disorders. The aim of the present review is to examine clinical and preclinical evidence in support of a common neurobiological substrate mediating the negative affect associated with different psychiatric illnesses. First, clinical and epidemiological data are presented demonstrating the high comorbidity between nicotine and psychostimulant dependence, and depression or schizophrenia. It is hypothesized that drug-use may represent an attempt to self-medicate an underlying negative affective state present in depressed and schizophrenia patients. Second, preclinical findings are presented that demonstrate common neurochemical deficits in drug withdrawal and depression. Taken together, these clinical and preclinical data support the hypothesis that common neurobiological substrates may mediate the depressive state observed across psychiatric diagnostic categories. Therefore, it is proposed that the study of drug-induced depressions in laboratory animals may have heuristic value in identifying the mechanisms underlying the depressive symptomatology associated not only with drug withdrawal but also major depression and schizophrenia.

Recognition and treatment of dysthymia in elderly patients

This review focuses on recent literature concerning dysthymia in the elderly population. Epidemiological data and clinical picture, diagnostic and therapeutic issues are evaluated and discussed. Although depressive syndromes are common in older patients, prevalence rates of dysthymia in the elderly are lower than in younger adults. This finding may be the consequence of the diagnostic criteria provided by the Diagnostic and Statistical Manual of Mental Disorders (DSM) which are not specific for older adults. Other factors that complicate making diagnoses of dysthymia in older individuals are comorbid general conditions, cognitive deterioration and disorders, and frequent adverse life events (e.g. bereavement). The effects of these factors should be better defined to clarify whether elderly dysthymia is underestimated and if modified diagnostic criteria should be provided. A few researchers have identified a series of clinical features that are clearly different in the elderly and in young adult patients with dysthymia. These features are particularly related to the late onset and to the peculiar comorbidity of this disorder and suggest that dysthymia is a different disorder in the elderly. Drug treatment of depressive conditions in the elderly is currently based on new antidepressants [selective serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitors, norepinephrine (noradrenaline) reuptake inhibitors, benzamides]. These agents have an improved adverse effect profile compared with some of the older agents. Moreover, very few systematic studies have been performed using these drugs in samples of older patients with dysthymia and available data do not allow conclusions on drug choice and dosage. Besides, no specific data are available concerning the psychotherapy of dysthymia in this age group. All these topics need to be further investigated in studies comparing the elderly with control groups of younger patients with dysthymia.

Effects of withdrawal from an escalating dose schedule of d-amphetamine on sexual behavior in the male rat

The present study sought to determine the effect of withdrawal from an escalating dose schedule of d-amphetamine on sexual behavior in male rats. Tests were conducted every 5 days until stable levels of sexual behavior were obtained. With repeated testing, male rats displayed an increase in their exploration of the testing chambers prior to the introduction of an estrous female. Half of the male rats were then subjected to a 4-day escalating dose schedule of d-amphetamine administration (1-12 mg/kg), while half received vehicle. Twelve hours after the final drug injection, subjects were tested for sexual behavior. Withdrawal from the drug was associated with decrements in several motivational components of sexual behavior, including decreased anticipatory locomotor and increased postejaculatory intervals, while consummatory measures remained largely unaffected. This pattern of sexual deficits resembles those seen in human depressive disorders, and therefore, provides additional support for the use of psychostimulant withdrawal as a rodent model of depression.

Dopaminergic agents for the treatment of cocaine abuse

Cocaine is a major drug of abuse whose devastating effects have captured the attention of health officials and policy makers. Based upon the alarming health and crime-related costs associated with the use of this powerful reinforcing drug, immediate therapies are needed for the treatment of cocaine addiction. In this review, some of the small-molecule-based approaches that have been pursued in the search for such medications are highlighted. Because the pharmacological actions of cocaine stem laargely from its ability to block the dopamine transporter, many intervention strategies have focused on the dopaminergic pathway.

Effects of D1 and D2 dopamine receptor antagonists on cocaine-induced self-stimulation and locomotor activity in rats

To clarify the involvement of D1 and D2 dopamine systems in intracranial self-stimulation (ICSS) and locomotor activity in rats, we studied the acute effects of cocaine and the interaction between cocaine and dopamine antagonists with respect to these behaviors. Although cocaine (5.0, 10.0, or 20.0 mg/kg) dose-dependently increased locomotor activity, it augmented the rate of ICSS only at 5.0 mg/kg. The failure of high doses of cocaine to augment purpose-oriented behavior such as ICSS may result from its induction of a manic-like state. The D1 dopamine receptor antagonist SCH23390 (0.02, 0.1, or 0.5 mg/kg) or the D2 antagonist nemonapride (0.04, 0.2, or 1.0 mg/kg) significantly decreased cocaine augmentation of ICSS. The higher two doses of either antagonist also produced a significant decrease in cocaine-induced locomotor activity. We therefore suspect that cocaine's augmentative effect on those behaviors, especially ICSS, requires activation of both D1 and D2 dopamine receptors.

Neural mechanisms of tolerance to the effects of cocaine

Chronic use of cocaine in high doses can produce tolerance as assessed by various behavioral, neurochemical, cellular and molecular measures in specific brain regions. Tolerance to cocaine is indicated by drug discrimination and intracranial self-stimulation models, which show the development of tolerance after approximately 1 week of frequent cocaine treatment, with recovery after a similar period of cocaine abstinence. Tolerance to the reinforcing properties of cocaine depends on dose, duration and frequency of cocaine self-administered by experimental animal or human subjects. The mechanism underlying this effect may involve an absolute or relative attenuation of dopamine response to cocaine challenge after frequent or repeated treatment in the nucleus accumbens (NAc). Similarly, afferent and efferent NAc circuits exhibit reduced metabolic activity, which lasts throughout the early period of withdrawal following repeated treatment. Attenuation of immediate early gene response also occurs, which might be related to a functional desensitization of dopamine D1-like receptors. Furthermore, intracellular adaptive responses to chronic cocaine exposure induce striatal dynorphin expression decreasing the behavioral potency of subsequent drug treatment. Thus, a combination of various pharmacodynamic mechanisms and the attenuation of dopamine response induced by sufficient dose, duration and frequency of cocaine exposure ultimately invoke the transient development of tolerance to the reinforcing effects of cocaine.

Tolerance-like attenuation to contingent and noncontingent cocaine-induced elevation of extracellular dopamine in the ventral striatum following 7 days of withdrawal from chronic treatment

Time-dependent changes in mesolimbic dopamine (DA) function are believed to play a role in behavioral sensitization and drug craving experienced during withdrawal from chronic cocaine administration. The present study utilized intravenous (IV) cocaine self-administration coupled with intracranial microdialysis in rats to investigate time dependent changes during withdrawal from chronic cocaine exposure. Following 2 weeks of IV cocaine self-administration, rats were allowed contingent access to cocaine at 1 and 7 days of withdrawal while extracellular levels of DA were measured from the ventral striatum. A second group of animals received yoked, noncontingent cocaine for 2 weeks and were then administered noncontingent cocaine on days 1 and 7 of withdrawal. In addition, a third group of animals received 2 weeks of yoked saline followed by noncontingent cocaine 1 day after withdrawal. There were no significant differences between groups for the overall cocaine dosage or temporal pattern of infusions on days 1 and 7 of withdrawal. Basal extracellular DA concentrations did not differ between any treatment groups at either withdrawal time. Extracellular DA levels were increased throughout the session on both days; however, the increases at day 7 were significantly less than day 1 for both contingent and noncontingent conditions. DA overflow on day 1 did not differ between animals receiving chronic yoked cocaine or saline. These results suggest that tolerance-like attenuation to the DA-elevating effects of cocaine is not apparent early in withdrawal, but does develop by later time points.(ABSTRACT TRUNCATED AT 250 WORDS)

Withdrawal from chronic amphetamine elevates baseline intracranial self-stimulation thresholds

Intracranial self-stimulation was assessed before, within, and after a chronic amphetamine treatment regimen. Amphetamine was given twice daily 5 days per week for 6 weeks at dosages escalating from 1 to 10 mg/kg per injection. Lateral hypothalamic self-stimulation rate-frequency functions were taken 36 h after the last injection in each weekly series and weekly for 3 weeks following the last injection. Frequency thresholds increased and maximal response rates decreased progressively as a function of amphetamine withdrawal during treatment; each returned to near normal levels within 2 weeks of the last injection. When subsequently tested under amphetamine, animals previously receiving the 6-week amphetamine treatment regimen had self-stimulation thresholds and maximal response rates that did not differ significantly from those of saline-treated control animals. These data confirm that chronic amphetamine treatment results in a dependence syndrome characterized in part by a phasic depression in the brain mechanism mediating the reinforcing effects of lateral hypothalamic electrical stimulation.

A simple computer-based method for performing and analyzing intracranial self-stimulation experiments in rats

Intracranial self-stimulation (ICSS) in the rat is a useful tool for studying the importance of various brain monoamines in positive reinforcement. The effects of compounds interacting with dopaminergic neurotransmission is measurable by studying the changes of reward thresholds. By computerisation of the analysis of these thresholds, standardisation and reproducibility is greatly enhanced. The use of an object-oriented programming language simplifies the programming of a specific application and it provides scientists without formal training in computer programming the means to create their own software. A system for the acquisition, execution, analysis and storage of ICSS experiments is described. The hardware is based on Apple Macintosh computers, interfaced to the test chambers and physiological stimulators using a plug-in card supporting A/D, D/A, digital I/O and timer functions. The software written in G (LabVIEW) provides the user with a graphically based 'Virtual Instrument' performing all aspect of the ICSS experiment. The software performs threshold analysis immediately after completion of the ICSS experiment, thereby greatly reducing the total time previously needed to evaluate these experiments. The graphical approach used in LabVIEW allows the programmer to make fast and simple alterations to suit different experimental problems.