The biological, social and clinical bases of drug addiction: commentary and debate

The role of stress in drug self-administration

Environmental experiences have an important effect on the sensitivity of an individual to drugs of abuse. Studies of drug self-administration in laboratory animals have shown that both physical and psychological stressors facilitate the acquisition of drug self-administration, probably by increasing the reinforcing efficacy of drugs of abuse. Stressors also facilitate the reinstatement of drug taking even after prolonged periods of withdrawal. The adrenal hormones, glucocorticoids, which increase the sensitivity of mesencephalic dopaminergic neurones to drugs, seem to be one of the biological substrates of the effects of stress on the propensity to develop drug intake. In this review, Pier Vincenzo Piazza and Michel Le Moal discuss theories of drug abuse, the influence of different stressful experiences on drug self-administration and their possible mechanisms of action.

An initial, three-day-long treatment with alcohol induces a long-lasting phenomenon of selective tolerance in the activity of the rat hypothalamic-pituitary-adrenal axis

We determined whether an initial alcohol challenge induced long-lasting changes in the activity of the hypothalamic-pituitary-adrenal (HPA) axis. Adult male rats received intragastric injections of the vehicle or a moderately intoxicating dose of alcohol (3.0 gm/kg) daily for 3 d. When animals were acutely challenged with alcohol 3-12 d later, their ACTH and corticosterone responses were significantly blunted, compared with that of vehicle-pretreated rats. In contrast, exposure to mild electric foot shocks induced a pattern of ACTH secretion that was comparable in animals administered alcohol or the vehicle previously, indicating a lack of cross-tolerance. No significant differences were observed in pituitary responsiveness to corticotropin-releasing factor or vasopressin in rats pretreated with the vehicle or alcohol. The influence of the initial drug treatment was not mimicked by exposure to foot shocks, nor was it prevented by administering a potent corticotropin-releasing factor antagonist to block the elevations in plasma ACTH and corticosterone induced by this initial treatment. Finally, we found that rats injected initially with the vehicle and challenged subsequently with alcohol exhibited the expected increased neuronal activation (measured by the upregulation of steady-state mRNA and protein levels of immediate early genes) in the paraventricular nucleus of their hypothalamus. In contrast, this response was markedly decreased in animals exposed previously to the drug. To our knowledge, this is the first report that exposure to a stress (i.e., alcohol), although not immediately altering the response of the HPA axis to this particular signal, induces a selective tolerance that is both slow to develop and long-lasting. The primary mechanism mediating the ability of an initial drug treatment to decrease subsequent responses of the HPA axis to a second drug challenge seems to be the inability of hypothalamic neurons to respond adequately to this second challenge.

Functional magnetic resonance imaging of the acute effect of intravenous heroin administration on visual activation in long-term heroin addicts: results from a feasibility study

This preliminary report is the first demonstration of the acute effects of diacetylmorphine (heroin) administration on functional activation in the human brain using functional magnetic resonance imaging (fMRI). Four opiate addicts who received regular prescriptions for heroin, underwent fMRI using a visual activation paradigm before and after a dose of 30 mg heroin. All four showed a decrease after the heroin dose in the extent of significant activation. This method shows promise for sequential scanning to determine brain activity in response to different drugs and routes of drug administration.

Etorphines: mu-opioid receptor-selective antinociception and low physical dependence capacity

Comparative analgesic studies revealed that dihydroetorphine was more potent than etorphine in the tail-flick and hot-plate tests, respectively and nearly equipotent in the phenylquinone assay. Both compounds were short acting. Studies with selective opioid receptor antagonists beta-funaltrexamine, nor-binaltorphimine and naltrindole revealed that both etorphines were mu-selective agonists. Presumptive evidence for competitive antagonism of these compounds with naloxone was provided by Schild regressions with slopes of near unity. In a suppression test in rhesus monkeys maximally dependent on morphine, dihydroetorphine and etorphine dose-dependently replaced morphine. Drug-naive simians chronically exposed to frequent, intermittent and escalating doses of dihydroetorphine for 42 days showed few withdrawal signs when challenged with large doses of naloxone or were abruptly withdrawn from this drug. The results suggest that these atypical opioids may be useful in the clinical treatment of pain and opiate drug abuse.

Pathological mutations of nicotinic receptors and nicotine-based therapies for brain disorders

Nicotinic acetylcholine receptors are allosteric ligand-gated ion channels present in muscle and brain. Recent studies suggest that mutations altering their functional properties may produce congenital myasthenia and familial frontal lobe epilepsy. Current research also indicates that although nicotinic ligands often possess addictive properties, they could serve as therapeutic agents for Alzheimer's disease and Tourette's syndrome, as well as for schizophrenia.

Comparative effects of alcohol and marijuana on mood, memory, and performance

This study compared subjective and behavioral effect profiles of alcohol and smoked marijuana using technology that controlled puffing and inhalation parameters. Male volunteers (n = 5) with histories of moderate alcohol and marijuana use were administered three doses of alcohol (0.25, 0.5, or 1.0 g/kg), three doses of marijuana [4.8, or 16 puffs of 3.55% delta 9-tetrahydrocannabinol (THC)], and placebo in random order under double blind conditions in seven separate sessions. Blood alcohol concentration (10-90 mg/dl) and THC levels (63-188 ng/ml) indicated that active drug was delivered to subjects dose dependently. Alcohol and marijuana produced dose-related changes in subjective measures of drug effect. Ratings of perceived impairment were identical for the high doses of alcohol and marijuana. Both drugs produced comparable impairment in digit-symbol substitution and word recall tests, but had no effect in time perception and reaction time tests. Alcohol, but not marijuana, slightly impaired performance in a number recognition test. These data are useful for understanding the relative performance impairment produced by alcohol and marijuana at the delivered doses and the relationship between their subjective and behavioral effects.

Evidence for the involvement of dopamine receptors in ethanol-induced hyperactivity in mice

Based on the hypothesis that drugs of abuse increase locomotor activity through mechanisms related to reinforcement, i.e. the mesolimbic dopamine (DA) system, ethanol-induced hyperactivity might provide a screening model to investigate the effect of ethanol on reward pathways. In the present study, ethanol had bidirectional effects on locomotion in mice: hyperactivity at low doses (2-3 g/kg) and sedation at high doses (4-5 g/kg). Such high doses induced a loss of righting reflex (LRR). The stimulant effect of ethanol was blocked by the D2/D3 antagonists, haloperidol (0.2 mg/kg) and tiapride (30-60 mg/kg), and by the D1 antagonist, SCH 23390 (0.03 mg/kg) whereas the non selective DA antagonist, clozapine decreased ethanol-induced hyperactivity at a dose (1 mg/kg) which also decreased activity in control animals. Unlike haloperidol and clozapine which potentiated LRR induced by ethanol, the selective DA antagonists, tiapride and SCH 23390, had no effect. Pretreatment with the D2/D3 agonist, quinpirole (0.1-0.3 mg/kg), reduced hyperactivity induced by ethanol presumably by stimulation of pre-synaptic receptors but did not change LRR. The D1 full agonist, SKF 81297 which produced hyperactivity by itself and the D1 partial agonist, SKF 38393, did not specifically affect ethanol-induced activities. The results indicate that activation of D1 and D2/D3 DA receptors is implicated in ethanol-induced hyperactivity whereas other mechanisms might mediate the sedative effects of ethanol. Tiapride and haloperidol, both used in the management of alcohol dependence, might exert beneficial effects by counteracting the reinforcing effects of ethanol. Tiapride's lack of interaction with the depressant effects of ethanol may account for its better tolerance in alcoholic patients.

Cocaine-induced increase in cortical acetylcholine release: interaction with the hypothalamo-pituitary-adrenal axis

An influence on drug-taking behaviours of the stress-related hypothalamo-pituitary-adrenal (HPA) axis and its final hormonal mediator, corticosterone, has previously been demonstrated. A role for cortically projecting cholinergic neurons in these behaviours can also be proposed. The experiments presented here examine the effect of the drug of abuse cocaine (15 mg/kg) on the release of acetylcholine (ACh) in the cortex of freely moving rats, using the technique of in vivo microdialysis. To assess a possible modulatory influence of the HPA axis via its final hormonal mediator corticosterone, the cocaine-induced effect on cortical ACh release in intact rats was compared to that in adrenalectomized (ADX) rats, which thus lacked their endogenous source of corticosterone, and in ADX rats in which the cocaine-induced corticosterone peak and/or the basal circadian concentrations of serum corticosterone were simulated by replacement treatments. The results reported here demonstrate that cortical ACh release is greatly increased by cocaine in intact rats; ADX prolongs the return to basal levels of cortical ACh, and the chronic replacement of circadian levels of corticosterone normalizes this effect. In contrast, during the plateau period of cocaine-induced increased cortical ACh release, where no effect of ADX is evident, rats with chronic replacement of corticosterone show an attenuated cocaine-induced cortical ACh release, and the acute replacement of the cocaine-induced corticosterone secretion further attenuates this response. These results demonstrate that cocaine stimulates cortically projecting cholinergic neurons, and that the HPA hormone corticosterone modulates this interaction in a complex manner which merits further investigation.