Responding for rewarding brain stimulation: cocaine and isradipine plus naltrexone

Interactions between opioids and stimulants: Behavioral pharmacology of abuse-related effects

Opioid abuse continues to be a significant public health challenge, with rates of opioid-related overdose deaths increasing continuously over the last two decades. There also has been a sharp increase in overdose deaths involving stimulant drugs, primarily cocaine and methamphetamine. Recent estimates indicate a high prevalence of co-use of opioids and stimulants, which is a particularly complex problem. Behavioral pharmacology research over the last few decades has characterized interactions between opioids and stimulants as well as evaluated potential treatments. This chapter describes interactions between opioids and stimulants, with a focus on pre-clinical studies of abuse-related behavioral effects using self-administration, reinstatement, drug discrimination, place conditioning, and intracranial self-stimulation paradigms in laboratory animals. In general, the literature provides substantial evidence of mutual enhancement between opioids and stimulants for abuse-related effects, although such results are not ubiquitous. Enhanced abuse-related effects could manifest in many ways including engaging in drug seeking and taking behaviors with greater persistence, effort, and motivation and/or increased likelihood of relapse. Moreover, studies on opioid/stimulant combinations set the stage for evaluating potential treatments for polysubstance use. Behavioral pharmacology research has proven invaluable for elucidating these relationships using rigorous experimental designs and quantitative analyses of pharmacological and behavioral data.

Naltrexone maintenance fails to alter amphetamine effects on intracranial self-stimulation in rats

Pharmacotherapy to treat stimulant use disorders continues to be an unmet medical need. Some evidence supports both the role of opioids in mediating abuse-related amphetamine effects and the potential utility of opioid antagonists as therapeutic candidates for treating amphetamine abuse. This study used intracranial self-stimulation (ICSS) to evaluate effects of exposure to and termination of naltrexone maintenance on rewarding amphetamine effects in an ICSS procedure in rats. Morphine and cocaine were included as positive and negative controls, respectively. Male Sprague-Dawley rats (N = 40) were trained to lever press for electrical brain stimulation to the medial forebrain bundle via an implanted electrode. Rats were then implanted with osmotic pumps delivering naltrexone (0.001 mg/kg/h, SC, 0.01 mg/kg/h, SC, or 0.1 mg/kg/h, SC) or saline for 14 days. Cumulative dose-effect curves were determined for amphetamine (0.032 mg/kg to 0.32 mg/kg), cocaine (1 mg/kg to 10 mg/kg), and morphine (1 mg/kg to 10 mg/kg) during the 2nd week of naltrexone maintenance. Additionally, dose-effect curves for morphine and amphetamine were determined again 24 hr after pump removal. Our results suggest that (a) exposure to and termination of naltrexone maintenance do not affect baseline ICSS responding, (b) naltrexone doses sufficient to antagonize morphine did not alter amphetamine or cocaine effects, and (c) termination of naltrexone treatment produced weak evidence for increased morphine sensitivity but no change in amphetamine effects. Our results do not support naltrexone as a pharmacotherapy for amphetamine and cocaine abuse and also suggest that termination from chronic naltrexone does not increase sensitivity to abuse-related morphine or amphetamine effects in ICSS. (PsycINFO Database Record

Augmented behavioral response and enhanced synaptosomal calcium transport induced by repeated cocaine administration are decreased by calcium channel blockers

Recent studies suggest that calcium influx via L-type calcium channels is necessary for psychostimulant-induced behavioral sensitization. In addition, chronic amphetamine upregulates subtype Cav1.2-containing L-type calcium channels. In the present studies, we assessed the effect of calcium channel blockers (CCBs) on cocaine-induced behavioral sensitization and determined whether the functional activity of L-type calcium channels is altered after repeated cocaine administration. Rats were administered daily intraperitoneal injections of either flunarizine (40 mg/kg), diltiazem (40 mg/kg) or cocaine (20 mg/kg) and the combination of the CCBs and cocaine for 30 days. Motor activities were monitored on Day 1, and every 6th day during the 30-day treatment period. Daily cocaine administration produced increased locomotor activity. Maximal augmentation of behavioral response to repeated cocaine administration was observed on Day 18. Flunarizine pretreatment abolished the augmented behavioral response to repeated cocaine administration while diltiazem was less effective. Measurement of tissue monoamine levels on Day 18 revealed cocaine-induced increases in DA and 5-HT in the nucleus accumbens. By contrast to behavioral response, diltiazem was more effective in attenuating increases in monoamine levels than flunarizine. Cocaine administration for 18 days produced increases in calcium uptake in synaptosomes prepared from the nucleus accumbens and frontal cortex. Increases in calcium uptake were abolished by flunarizine and diltiazem pretreatment. Taken together, the augmented cocaine-induced behavioral response on Day 18 may be due to increased calcium uptake in the nucleus accumbens leading to increased dopamine (DA) and serotonin (5-HT) release. Flunarizine and diltiazem attenuated the behavioral response by decreasing calcium uptake and decreasing neurochemical release.

Effects of naltrexone and isradipine, alone or in combination, on cocaine responses in humans

Preclinical studies suggested that combination of naltrexone and isradipine may be useful for the treatment of cocaine addiction. This study examined whether naltrexone and isradipine, alone or in combination, would attenuate the subjective and physiological effects of cocaine in humans. Seven cocaine users participated in a randomized, double-blind, placebo-controlled inpatient study. Before each of the seven experimental sessions, subjects were treated orally with naltrexone (50 mg or placebo), isradipine (10 mg or placebo), or naltrexone plus isradipine. Subjects then received a single dose of intranasal cocaine (4 mg or 100 mg/70 kg). Isradipine alone attenuated the systolic blood pressure response to cocaine. In contrast, isradipine plus naltrexone treatment attenuated both the systolic and diastolic blood pressure responses. Naltrexone alone did not affect the blood pressure response to cocaine. For subjective response to cocaine, isradipine, alone or in combination with naltrexone, did not have significant effects. Naltrexone treatment alone attenuated the rating of "good effects" from cocaine without affecting other subjective responses. These results suggest that isradipine alone or in combination with naltrexone attenuates some of the physiological effects of cocaine.

Nifedipine potentiates the toxic effects of cocaine in mice

The calcium channel blockers (CCBs) have been shown to be effective in attenuating the behavioral effects of cocaine in rodents and subjective effects in cocaine-using volunteers. There have been reports indicating that, in the presence of toxic doses of cocaine, the CCBs could actually potentiate cocaine toxicity in rats. The present study was undertaken to make toxicological assessment of the potentiating effect of CCBs in mice. Nifedipine and nimodipine dose-dependently increased the lethalities produced by 80 mg/kg cocaine. In the presence of 40 mg/kg nifedipine, the LD50 of cocaine was decreased from 80.7 to 66.3 mg/kg. Nifedipine potentiated cocaine toxicities in both ICR and Swiss-Webster mice. The increased toxicity was not accompanied by alterations in blood electrolytes. The mechanism of increased cocaine toxicity by CCBs remains to be determined. However, our results corroborate previous findings in rats and suggest that the possibility of an antidote exacerbating the toxic effects of cocaine has to be taken into consideration when screening for therapeutic agents.

Research with rats germane to medication for alcoholism: consequences of noncompliance

Results of prior work indicate that (a) rats take stable, toxic levels of ethanol when they receive a daily regimen of limited opportunities to take both water and sweetened ethanol solution and (b) the combination of isradipine plus naltrexone persistently reduces those intakes. What are the effects of periodically missing doses of isradipine, naltrexone, or both? That is, what are the effects of differing levels of compliance? To get relevant information, rats were placed on a daily regimen, leading them to take, by choice, large amounts of ethanol (>2.0 g of ethanol per kilogram of body weight during 2 h a day). After being on this regimen for more than 60 days and after 28 days of no opportunity to take ethanol, 55 rats were divided into five groups. The opportunity to drink was then reinstated. One group received placebos, and another group received the combination of isradipine plus naltrexone daily. The other three groups received doses periodically, thereby conforming to good, moderate, and poor compliance. After abstinence, the intakes for rats receiving placebos rapidly returned to high levels. Intakes for rats receiving daily isradipine plus naltrexone did not return to high levels. The intakes for the other three groups were intermediate to intakes of the reference groups, corresponding to frequency of medication. When medication was not given, intakes approached placebo control levels, but the combination of isradipine plus naltrexone was effective when given subsequently. Daily dosing clearly is effective in reducing intakes, and suspension of dosing leads to higher intakes. A missed day of dosing, however, has limited consequences, provided that administration of medication is resumed.

Endogenous opiates: 1998

This paper is the twenty-first installment of our annual review of research concerning the opiate system. It summarizes papers published during 1998 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.

Amlodipine, a calcium channel inhibitor, and cocaine and ethanol's reinforcing effects

The effects of amlodipine (from 0.1 to 3.0 mg/kg) on rats' pressing for rewarding brain stimulation, with and without cocaine administration, were assessed. None of the doses reliably modified the effects of cocaine. Also, amlodipine was given to two groups of rats taking alcohol: one group that was regularly taking a sweetened alcoholic beverage and the other taking an unsweetened alcoholic beverage. The only discernible effects of amlodipine on alcohol intake were associated with the highest dose and only with rats taking the sweetened beverage. The effects of this high dose could easily be attributable to behavioral toxicity elicited by the dose. In contrast, and confirming previous work, isradipine, another calcium channel inhibitor, produced reliable reductions on both cocaine's and alcohol's reinforcing effects. Despite the similarity of isradipine and amlodipine, isradipine apparently has some unique features with respect to cocaine and alcohol.