Naloxone antagonizes behavioural effects of d-amphetamine in mice and rats

Maintenance on naltrexone+amphetamine decreases cocaine-vs.-food choice in male rhesus monkeys

BACKGROUND

Cocaine use disorder remains a significant public health issue for which there are no FDA-approved pharmacotherapies. Amphetamine maintenance reduces cocaine use in preclinical and clinical studies, but the mechanism of this effect is unknown. Previous studies indicate a role for endogenous opioid release and subsequent opioid receptor activation in some amphetamine effects; therefore, the current study examined the role of mu-opioid receptor activation in d-amphetamine treatment effects in an assay of cocaine-vs-food choice.

METHODS

Adult male rhesus monkeys with double-lumen intravenous catheters responded for concurrently available food pellets and cocaine injections (0-0.1mg/kg/injection) during daily sessions. Cocaine choice and overall reinforcement rates were evaluated during 7-day treatments with saline or test drugs.

RESULTS

During saline treatment, cocaine maintained a dose-dependent increase in cocaine-vs.-food choice. The mu-opioid receptor agonist morphine (0.032-0.32mg/kg/h) dose-dependently increased cocaine choice and decreased rates of reinforcement. A dose of the mu-selective opioid receptor antagonist naltrexone (0.0032mg/kg/h) that completely blocked morphine effects had no effect on cocaine choice when it was administered alone, but it enhanced the effectiveness of a threshold dose of 0.032mg/kg/h amphetamine to decrease cocaine choice without also enhancing nonselective behavioral disruption by this dose of amphetamine. Conversely, the kappa-selective opioid antagonist norbinalorphimine did not enhance amphetamine effects on cocaine choice.

CONCLUSIONS

These results suggest that amphetamine maintenance produces mu opioid-receptor mediated effects that oppose its anti-cocaine effects. Co-administration of naltrexone may selectively enhance amphetamine potency to decrease cocaine choice without increasing amphetamine potency to produce general behavioral disruption.

Separate and Combined Effects of Naltrexone and Extended-Release Alprazolam on the Reinforcing, Subject-Rated, and Cardiovascular Effects of Methamphetamine

Opioid antagonists (eg, naltrexone) and positive modulators of γ-aminobutyric acid type A receptors (eg, alprazolam) each modestly attenuate the abuse-related effects of stimulants. A previous study demonstrated that acute pretreatment with the combination of naltrexone and alprazolam attenuated a greater number of the subject-rated effects of D-amphetamine than the constituent drugs alone. This study tested the hypothesis that maintenance on the combination of naltrexone and alprazolam XR would attenuate the reinforcing and "positive" subject-rated effects of methamphetamine to a greater extent than the constituent drugs alone.Eight non-treatment-seeking, stimulant-using individuals completed a placebo-controlled, crossover, double-blind inpatient protocol. Participants were maintained on naltrexone (0 and 50 mg), alprazolam XR (0 and 1 mg), and the combination of naltrexone and alprazolam XR (50 mg and 1 mg, respectively) for 6 to 7 days. Under each maintenance condition, participants sampled intranasal doses of methamphetamine (0, 10, and 30 mg), and were then offered the opportunity to work for the sampled dose on a modified progressive-ratio procedure. Subject-rated drug effect questionnaires, psychomotor, and physiology assessments were collected.Intranasal methamphetamine functioned as a reinforcer and produced prototypical stimulant-like "positive" subject-rated and physiological effects. Maintenance on naltrexone significantly decreased the reinforcing, but not subject-rated drug effects of 10-mg methamphetamine. Alprazolam XR and the combination of naltrexone and alprazolam XR did not impact methamphetamine self-administration or subject-rated drug effects. The results support the continued evaluation of naltrexone for methamphetamine dependence, as well as the identification of other drugs that enhance its ability to reduce drug-taking behavior.

Separate and combined impact of acute naltrexone and alprazolam on subjective and physiological effects of oral d-amphetamine in stimulant users

RATIONALE

Opioid antagonists (e.g., naltrexone) and positive modulators of γ-aminobutyric-acidA (GABAA) receptors (e.g., alprazolam) modestly attenuate the abuse-related effects of stimulants like amphetamine. The use of higher doses to achieve greater efficacy is precluded by side effects. Combining naltrexone and alprazolam might safely maximize efficacy while avoiding the untoward effects of the constituent compounds.

OBJECTIVES

The present pilot study tested the hypothesis that acute pretreatment with the combination of naltrexone and alprazolam would not produce clinically problematic physiological effects or negative subjective effects and would reduce the positive subjective effects of d-amphetamine to a greater extent than the constituent drugs alone.

METHODS

Eight nontreatment-seeking, stimulant-using individuals completed an outpatient experiment in which oral d-amphetamine (0, 15, and 30 mg) was administered following acute pretreatment with naltrexone (0 and 50 mg) and alprazolam (0 and 0.5 mg). Subjective effects, psychomotor task performance, and physiological measures were collected.

RESULTS

Oral d-amphetamine produced prototypical physiological and stimulant-like positive subjective effects (e.g., VAS ratings of Active/Alert/Energetic, Good Effect, and High). Pretreatment with naltrexone, alprazolam, and their combination did not produce clinically problematic acute physiological effects or negative subjective effects. Naltrexone and alprazolam each significantly attenuated some of the subjective effects of d-amphetamine. The combination attenuated a greater number of subjective effects than the constituent drugs alone.

CONCLUSIONS

The present results support the continued evaluation of an opioid receptor antagonist combined with a GABAA-positive modulator using more clinically relevant experimental conditions like examining the effect of chronic dosing with these drugs on methamphetamine self-administration.

Naltrexone attenuates amphetamine-induced locomotor sensitization in the rat

Amphetamine, and other stimulants, readily induces behavioral sensitization, an effect hypothesized to reflect neurobiological changes that may underlie certain aspects of drug addiction. Apart from the effects on the dopamine system, previous studies have also shown that amphetamine interacts with other neurotransmitters, including the endogenous opioid system. The unselective opioid receptor antagonist naltrexone (NTX) modulates amphetamine-induced effects in both laboratory animals and humans. To further examine this interaction, the aim of the present study was to investigate the effect of NTX on the expression of locomotor sensitization and conditioned locomotor response in animals previously conditioned with amphetamine. Sensitization was induced by repeated administration of amphetamine (2 mg/kg) for 10 consecutive days. After a 10-day drug-free period, the rats were administered NTX (3 mg/kg) 30 minutes prior to the administration of a challenge dose of either amphetamine (0.5 mg/kg) (test for drug-induced sensitization) or saline (test for conditioned locomotor response). NTX had no effect on acute amphetamine-induced locomotor activity or on general locomotor activity in animals without a history of amphetamine conditioning. However, animals previously conditioned with amphetamine showed a sensitized locomotor response to the amphetamine challenge following the 10-day drug-free period. This sensitized response was significantly inhibited by NTX pre-treatment. In addition, NTX pre-treatment blocked the conditioned locomotor response when the amphetamine-conditioned animals were placed in the previously amphetamine-paired context. This study showed that NTX attenuates drug- and cue-induced locomotor behavior in amphetamine-conditioned animals, supporting recent clinical findings that indicated a potential role of NTX as a treatment for amphetamine dependence.

The effect of naltrexone on amphetamine-induced conditioned place preference and locomotor behaviour in the rat

Whereas amphetamine and other psychostimulants primarily act on the dopamine system, there is also evidence that other neurotransmitter systems, such as the endogenous opioid system, modulate psychostimulant-induced effects. Several studies have investigated the role of opioid antagonists on cocaine-induced conditioned place preference (CPP), but there is limited information about the interaction with amphetamines. The aim of the present study was to investigate the effect of the opioid receptor antagonist, naltrexone (NTX) on the conditioning, expression and reinstatement of amphetamine-induced place preference. In addition, the effect of NTX on locomotor behaviour was measured during all sessions. During training, animals were conditioned with amphetamine (2 mg/kg) to induce place preference. In order to extinguish the conditioned behaviour, animals received saline for 12 days. Reinstatement of CPP was induced by a priming dose of amphetamine (0.5 mg/kg). The interaction of NTX and amphetamine was evaluated using three paradigms of CPP: with NTX (vehicle, 0.3, 1.0 and 3.0 mg/kg) administered either 30 minutes prior to amphetamine conditioning, or 30 minutes before the expression, or 30 minutes before the amphetamine priming to induce reinstatement. Naltrexone had no effect on the conditioning, the expression or the reinstatement induced by a priming dose of amphetamine. Further, NTX by itself did not induce place preference or place aversion. In contrast, NTX significantly attenuated the locomotor response to a priming dose of amphetamine without affecting general locomotor behaviour. The results suggest differences in opioid modulation of amphetamine-induced behaviours in the rat.

Naloxone does not alter amphetamine-induced rotational behavior or striatal dopamine levels of nigrally-lesioned rats

Previous studies on rats have shown that the opioid antagonist naloxone attenuates amphetamine-induced stimulation of locomotor activity and increases in extracellular dopamine in the brain. However, in this study, naloxone did not attenuate amphetamine-induced rotational behavior or increases of extracellular dopamine in the intact striatum of nigrally-lesioned rats. These results suggests differences in the way in which endogenous opioids contribute to the behavioral and neurochemical effects of amphetamine in nigrally-lesioned compared to intact rats.

The effect of naloxone on spontaneous and evoked dopamine release in the central and peripheral nervous systems

A number of studies have reported that the opiate antagonist naloxone (NX) inhibits behaviors dependent upon central dopamine (DA) release. However, equally compelling evidence from other studies suggests that NX excites a facilitatory effect. The present review was undertaken to resolve the issue by critically evaluating the effects of NX on DA release; the substrate subserving these behaviors. Included are studies reporting an effect of NX on spontaneous as well as drug altered DA release in various central regions. In the preponderant majority of these studies, NX was found to significantly enhance DA release in the virtually every major DA pathway, irrespective of whether DA release was initially stimulated or inhibited by various agents. It is concluded that NX most probably enhances behaviors induced by DA release, especially when administered in low, specific doses. Studies finding an inhibitory effect of NX on such behaviors may inadvertently produce conditions which mask the stimulatory effects of NX on DA release-dependent behaviors.

Reductions in locomotor activity following central opioid receptor subtype antagonists in rats

Opioid agonists produce biphasic (decreases then increases) effects upon activity in rats. General opioid antagonists typically suppress activity. Selective opioid antagonists reduce weight and food intake. However, the latter effects cannot fully account for the former effects. To assess the possibility that selective opioid antagonists might decrease weight by increasing activity, the present study examined whether central administration of either mu (beta-funaltrexamine: 20 micrograms), mu1 (naloxonazine: 50 micrograms), delta1 ([D-Ala2,Leu5,Cys6]enkephalin: 40 micrograms), delta2 (naltrindole isothiocyanate: 20 micrograms), or kappa1 (nor-binaltorphamine: 20 micrograms) opioid antagonists altered total, ambulatory, or stereotypic activity. Each of the antagonists significantly reduced total (mu: 18%, mu1: 31%, delta1: 42%, delta2: 37%, kappa1: 31%), ambulatory (mu: 17%, mu1: 27%, delta1: 34%, delta2: 37%, kappa1: 31%), and stereotypic (mu: 19%, mu1: 34%, delta1: 49%, delta2: 37%, kappa1: 31%) activity on the first day. All three activity measures were reduced by delta1 and delta2 antagonism on the second day, whereas mu antagonism reduced total and stereotypic activity on the second day. The activity reductions induced by selective opioid receptor subtype antagonists parallel effects induced by general opioid antagonism, and suggest that antagonist-induced weight loss effects independent of intake reductions are not due to antagonist-induced hyperactivity.

Naloxone reduces the neurochemical and behavioral effects of amphetamine but not those of cocaine

The specific opioid receptor antagonist naloxone modifies the effects of amphetamine in a wide variety of behavioral paradigms. Naloxone also attenuates the amphetamine-induced increase in extracellular dopamine in the brain of rats. Therefore, these experiments were designed to replicate the neurochemical and behavioral interactions between naloxone and amphetamine, and to extend these observations to interactions between naloxone and cocaine. Microdialysis was performed on adult male rats of Sprague-Dawley descent. Rats were pretreated with a subcutaneous injection of 5.0 mg/kg naloxone or vehicle, which was followed 30 min later by cumulative doses of subcutaneous d-amphetamine (0.0, 0.1, 0.4, 1.6, 6.4 mg/kg) or intraperitoneal cocaine (0, 3, 10, 30, 56 mg/kg) at 30 min intervals. The microdialysis probes were perfused at a flow rate of 0.6 microliter/min with artificial cerebrospinal fluid. Dialysate samples were collected every 10 min from either the nucleus accumbens or striatum and analyzed for dopamine content by high-performance liquid chromatography (HPLC). Locomotor activity (photobeam breaks) was monitored simultaneously with the collection of dialysate samples. Amphetamine and cocaine dose-dependently increased extracellular dopamine in both the nucleus accumbens and striatum. Naloxone pretreatment significantly reduced the amphetamine-induced increase in extracellular dopamine in both brain regions and also attenuated the increase in locomotor activity elicited by amphetamine. Naloxone pretreatment had no effect, however, on the cocaine-induced increase in extracellular dopamine or locomotor activity. These findings suggest that endogenous opioid systems play a role in mediating the neurochemical and behavioral effects of amphetamine, but not those of cocaine.

Effects of chronic naloxone administration on vacuous chewing movements and catalepsy in rats treated with long-term haloperidol decanoate

Most antipsychotic medications produce motoric side effects, including parkinsonism and tardive dyskinesia (TD). Correlates of these behaviors in rats (catalepsy and vacuous chewing movements, respectively) were used as a model to assess the usefulness of chronic naloxone administration in symptom reduction. Previous studies have suggested that increased neurotransmission in the endogenous opioid system modulates neuroleptic-induced motoric side effects. Rats were treated with haloperidol decanoate or vehicle for 27 weeks, and withdrawn for 30 weeks. Subsequently, naloxone (0.5 to 2.0 mg/kg SC twice daily) was given for 5 weeks. Long-term haloperidol treatment produced a syndrome of vacuous chewing movements (VCMs) that persisted during the drug withdrawal period. Catalepsy developed rapidly and also persisted. Naloxone treatment had little effect on VCMs but increased catalepsy scores in both haloperidol and vehicle treated groups. Naloxone reduced rearing and grooming in haloperidol rats while increasing these measures in vehicle treated rats. The results indicate that neuroleptic-induced motoric side effects are not reversed by naloxone in rats. Furthermore, they suggest that increased opioid neurotransmission may not underlie the expression of VCMs. This does not rule out the possibility that endogenous opioid system may be involved in the development of VCMs. To the extent that this animal model is valid, naloxone may not be effective in treating TD and neuroleptic-induced parkinsonism in humans.

Influence of naloxone upon motor activity induced by psychomotor stimulant drugs

Naloxone, an opioid receptor antagonist, attenuates a wide range of behavioral effects of d-amphetamine, such as the stimulation of motor activity. To investigate the pharmacological selectivity of the naloxone/amphetamine interaction, we assessed the effects of naloxone (5.0 mg/kg SC) upon motor activity induced in rats by a range of psychomotor stimulant drugs with a mechanism of action either similar to or different from that of d-amphetamine. Each of the drugs tested caused dose-dependent increases in both gross and fine activity. Naloxone attenuated the gross but not the fine activity response to d- and l-amphetamine, but had no influence upon the other catecholamine-releasing drugs, methamphetamine and phendimetrazine. In contrast, naloxone increased the gross but not the fine activity response to the catecholamine uptake inhibitors cocaine and mazindol, but had no effects upon the motor response to methylphenidate. The responses to other stimulant drugs (apomorphine, caffeine, scopolamine) were unaffected by naloxone pretreatment. The present findings extend the range of conditions under which naloxone and, by inference, endogenous opioid systems, modulate the behavioral response to psychomotor stimulants. However, the differential effects of naloxone upon the motor response to individual stimulant drugs support previous suggestions of fundamental differences in mechanisms of action among these compounds.

Delta-opioid receptor antagonists attenuate motor activity induced by amphetamine but not cocaine

Naloxone and naltrindole attenuate the locomotor response to amphetamine, implicating delta-opioid receptors in the opioid-antagonist/amphetamine interaction. To determine the role of delta-opioid receptor subtypes in this phenomenon, rats were pretreated with the following selective antagonists administered intracisternally: naltrindole, [D-Ala2,Leu5,Cys6]enkephalin (DALCE, delta 1 receptor selective), naltrindole-5'-isothiocyanate (delta 2 receptor selective). Cumulative dose-response curves to amphetamine were constructed (saline, 0.1, 0.4, 1.6 and 6.4 mg/kg s.c.), with injections every 30 min. Naltrindole was also tested against cumulative doses of cocaine (saline, 3.0, 10, 30 and 56 mg/kg i.p.). Gross and fine motor activity were recorded for 20 min, commencing 10 min postinjection. Amphetamine and cocaine dose dependently increased both gross and fine movements. Naltrindole (10 micrograms) attenuated the gross but not fine activity response to amphetamine, but 10 or 30 micrograms failed to influence the response to cocaine. Naltrindole-5'-isothiocyanate (30 micrograms) attenuated slightly but significantly the gross activity response to amphetamine, whereas DALCE (30 micrograms) was without effect. However, a combination of 10 micrograms each of DALCE and naltrindole-5'-isothiocyanate markedly attenuated the amphetamine-induced increases in gross movements without altering fine activity. These data provide further evidence for the involvement of delta-opioid receptors in the modulation of behavioral effects of amphetamine; both delta 1- and delta 2-opioid receptors appear to play a role. The differential effects of opioid antagonists on locomotor activity stimulated by amphetamine and cocaine suggests differences in the mechanism of action of these drugs not previously appreciated.

Naloxone reduces amphetamine-induced stimulation of locomotor activity and in vivo dopamine release in the striatum and nucleus accumbens

This study tested the possibility that naloxone (NX), an opioid antagonist, reduces the behavioral effects of amphetamine (AMPH) in rats by attenuating the dopaminergic response to AMPH. In the first experiment, adult, male rats were injected SC with either NX (5.0 mg/kg) or saline and 30 min later received doses of AMPH (0.0, 0.1, 0.4, 1.6, and 6.4 mg/kg) cumulatively at 30-min intervals. Gross locomotor counts following AMPH administration were significantly lower for rats pretreated with NX than for rats pretreated with saline. In the second experiment, the same drug treatments were given while performing microdialysis in either the striatum (STR) or nucleus accumbens (NACC). STR rats treated with vehicle showed a larger percentage increase in DA levels following AMPH treatment than did NACC rats treated with vehicle. NX pretreatment did not affect dopamine concentrations in either brain region. However, compared to pretreatment with saline pretreatment with NX significantly decreased the dopaminergic response to AMPH in the STR. There was no difference between the two groups in the peak dopaminergic response to AMPH in the NACC, but there was a significant AMPH x treatment x time interaction due to differences between the groups during the later portion of the response to 6.4 mg/kg AMPH. There was also a difference in locomotor activity following AMPH treatment between NX- and saline-treated subjects during dialysis. These findings suggest that a decrease in the dopaminergic response to AMPH is the mechanism by which NX attenuates behavioral stimulant effects of AMPH. In addition, there is a difference between the STR and NACC in dopaminergic responsiveness to AMPH.

Naloxone blockade of amphetamine place preference conditioning

Amphetamine and naloxone were examined in place conditioning, in order to study possible interactions between endogenous opioids and catecholamines in reinforcement. After initial preferences were determined, animals were conditioned with amphetamine alone (1.0 mg/kg SC), naloxone alone (0.02, 0.2 or 2.0 mg/kg SC) or combinations of amphetamine plus naloxone. A reliable, long-lasting preference for the compartment associated with amphetamine was observed, reflecting the reinforcing properties of this drug. No preference or aversion was observed in animals that received saline in both compartments. Naloxone (0.02, 0.2 and 2.0 mg/kg) produced a dose-dependent place aversion; while the lowest dose had effects similar to saline, the higher doses produced significant place aversions. Naloxone, at all three doses examined, prevented the ability of amphetamine to produce a place preference. Thus, the lowest dose of naloxone, having no effects alone in place conditioning was still able to block the reinforcing effects of amphetamine. These results suggest that the reinforcing effects of amphetamine are dependent on activation of opiate receptors, and provide further evidence that interactions between endogenous opioids and catecholamines may be important in reinforcement.

Regulation of striatonigral prodynorphin peptides by dopaminergic agents

The primary purpose of this study was to examine the regulation of prodynorphin peptides by dopaminergic agents in the central nervous system. The indirectly acting catecholamine agonist D-amphetamine sulfate (AMPH) and the dopamine receptor antagonist haloperidol (HAL) were administered to rats across a variety of treatment schedules and drug doses. The striatum, substantia nigra and hippocampus were dissected and examined by radioimmunoassay for 5 different prodynorphin peptides, covering all 3 opioid domains in the prodynorphin precursor: dynorphin A(1-8) and dynorphin A(1-17) of the dynorphin A domain, dynorphin B(1-13) of the dynorphin B domain, and alpha-neo-endorphin and beta-neo-endorphin of the neo-endorphin domain. In addition, the proenkephalin peptide Met-enkephalin-arg6-gly7-leu8 (MERGL) was examined in the striatum. AMPH administered one hour prior to sacrifice caused a dose-dependent depletion of prodynorphin peptides in both the striatum and substantia nigra. In animals treated with AMPH once each day for 7 days and sacrificed 24 h later, a dramatic dose-dependent increase in prodynorphin peptides was observed in these brain regions. Animals treated with AMPH once each day for 7 days and sacrificed one hour after the final injection showed no changes in prodynorphin peptides. In addition to changes in individual prodynorphin peptides, AMPH treatment caused alterations in the relationships between intermediate peptides (dynorphin A(1-17) and alpha-neo-endorphin) and their immediate products (dynorphin A(1-8) and beta-neo-endorphin). AMPH caused no consistent changes in prodynorphin peptides in the hippocampus, or in MERGL in the striatum. Taken together these data suggest that acute dopaminergic activation causes depletion of dynorphins from striatonigral prodynorphin neurons, presumably due to dopamine-dependent release of these peptides; repeated activation causes repeated release, with a rebound increase in biosynthesis. HAL, in contrast to AMPH caused relatively subtle changes in striatonigral prodynorphin peptides. Although no significant changes in individual prodynorphin peptides were observed, HAL treatment caused a change in the relationship between dynorphin A(1-17) and dynorphin A(1-8), a change opposite in direction to that observed with AMPH treatment. As has been previously reported, repeated HAL administration caused a dose-dependent increase in the proenkephalin peptide MERGL. The relatively subtle effects of HAL on prodynorphin peptides suggests that tonic dopamine activity is not important in the regulation of striatonigral prodynorphin neurons. The potential functional and behavioral significance of the present results are discussed.

Effects of naltrexone, and d-amphetamine, and their interaction on the stimulus control of choice behavior of rats

The hypothesis that endogenous opioid peptides modulate attentional processes was tested. The effects of the opioid antagonist naltrexone (NALT), d-amphetamine (AMP), and their interaction were investigated in rats trained in a two-choice task in which the position of a short-duration light served as a cue for food-reinforced responses. NALT (0.25, 1.0, 5.0, and 10.0 mg/kg) produced no significant changes in performance (accuracy, choice latency, and food retrieval time). As predicted, AMP induced dose-dependent biphasic effects. Low doses of AMP (0.25 and 0.5 mg/kg) significantly enhanced accuracy, decreased choice latency, and lengthened food retrieval time; 1.25 mg/kg AMP disrupted accuracy, increased choice latency, and further lengthened food retrieval time. The combination of NALT (0.25, 1.0, and 10.0 mg/kg) and subthreshold doses of AMP (0.07 and 0.1 mg/kg) had no effect on performance except for an increase in food retrieval time with 10.0 mg/kg NALT, whereas the combination of NALT and moderate doses of AMP (0.5 and 1.0 mg/kg) disrupted accuracy, increased choice latencies, and lengthened food retrieval time. These results do not support the hypothesis that endogenous opioid peptides play a vital role in attentional processes or that opioid antagonists may be useful in the treatment of attentional deficit disorders.

The interaction of d-amphetamine and naloxone differs for rats trained on separate fixed-interval or fixed-ratio schedules of reinforcement

The effects of d-amphetamine and naloxone were investigated using two groups of rats trained on either an FR30 or F12 schedule of reinforcement. Amphetamine (0.1-1.0 mg/kg), and naloxone (1.0 and 10 mg/kg) administered separately reduced responding on the FR procedure in a dose-dependent manner. The combined administration of naloxone with amphetamine had an additive suppressive effect on responding. The same doses of amphetamine and naloxone, when given separately, did not significantly depress responding in the FI procedures. However, naloxone/amphetamine combinations produced a marked inhibition of lever-pressing. Naloxone did not alter the characteristic pattern of responding engendered by amphetamine in this schedule, as measured by the quarter-life and Index of Curvature. It appears that the type of procedure used is a critical factor in demonstrating the effects of naloxone on behavior, and the nature of naloxone/amphetamine interactions.

Differential effects of amphetamine and related compounds on locomotor activity and metabolic rate in mice

Locomotor activity was measured by photobeam interruptions, and metabolic rate by the production of CO2 (as minute volume expired CO2, or VECO2) in mice. d-Amphetamine (0.3 to 10 mg/kg IP) increased locomotor activity in a dose-dependent manner while suppressing VECO2 over the same 72-min test period, compared to saline-injected controls. This phenomenon of divergent effects on locomotor activity and metabolic rate required central stimulation, as neither ammonium sulfate nor p-hydroxyamphetamine suppressed VECO2. Oxygen consumption was also suppressed by d-amphetamine, indicating that the suppression of VECO2 involved more than a change in respiratory quotient. When baseline activity rates were increased with running wheels, VECO2 and activity were both suppressed by d-amphetamine; VECO2 was suppressed by d-amphetamine more in exercising mice than in sedentary mice. Anorexigenic agents phenmetrazine, aminoxaphen, and fenfluramine, when administered in doses equimolar to maximally effective doses of d-amphetamine, did not consistently affect activity or VECO2. Evidence for mediation of the VECO2 response by corticosterone and endogenous opioid peptides was negative. Further work, with other mediators of the stress response, or with more complete dose-effect studies with anorexigenic compounds, may be necessary to explicate the mechanism of this counter-intuitive divergence of two measures of activity in mice.

Different pattern of association of beta-endorphin and cortisol responses to dextroamphetamine in postmenopausal women and young men

A negative correlation between plasma beta-endorphin and cortisol responses to 0.15 mg/kg dextroamphetamine i.v. was found in a group of seven normal postmenopausal women, while the responses of the two hormones were positively correlated in nine normal young men. These results suggest that even though the adrenocorticotropic hormone (ACTH)-cortisol and beta-endorphin are usually regulated by the same mechanism (both ACTH and beta-endorphin are derived from proopiocortin), there are situations in which these systems can be associated in a different pattern. The elucidation of these situations may contribute to the understanding of regulatory mechanisms of the two systems.

Behavioral response to apomorphine and its interaction with opiates in domestic pigeons

Domestic pigeons received peripheral injections of saline or the dopamine agonist apomorphine (AM) at doses of 0.025, 0.05, 0.1, 0.25, 0.5 or 1 mg and their behavior was studied for 30 min after these treatments. Given at a dose of 0.025 mg, AM decreased pecking, whereas doses ranging from 0.1 to 1 mg strongly stimulated this behavior. The frequency of headshaking was enhanced by the administration of each dose of AM; at the 3 higher doses, the drug also attenuated the frequency of preening. In another experiment, AM was administered 40 min after the injection of either naloxone (0.5, 1 or 4 mg), the opiate agonist levorphanol (0.25, 0.5 or 1 mg) or its dextroisomer, dextrorphan (0.25, 0.5 or 1 mg), while the birds were observed as before. No interaction between AM and either naloxone or dextrorphan was detected. By contrast, injection of each dose of levorphanol attenuated preening, and completely antagonized the stimulating effect of AM treatment on headshaking. At a dose of 1 mg, levorphanol also slightly decreased the frequency and increased the latency of occurrence of pecking. It is concluded that in pigeons, opiates modulate the behavioral response to apomorphine in a complex fashion.

Effects of naloxone and buprenorphine on intravenous acetaldehyde self-injection in rats

Rats can be induced to self-inject acetaldehyde under an appropriate operant conditioning schedule. The narcotic antagonist naloxone (30 mg/kg) is shown to produce a decrease in schedule-induced acetaldehyde self-injection, but was without effect on both barpress responding and spontaneous activity in rats tested individually for fine, gross and total activity. On the other hand buprenorphine (0.3 and 3 mg/kg), the mixed agonist-antagonist derived from the opium alkaloid thebaine, also produced a significant decrease in acetaldehyde self-injection. However, a significant effect of buprenorphine on barpressing in otherwise drug naive rats indicated that this finding should not be dissociated from a possible involvement of buprenorphine on motor responding. While the findings are consistent with the hypothesis of opiate involvement in acetaldehyde self-administration, caution must be exercised when drawing conclusions about the participation of endogenous opiates in acetaldehyde-mediated behavior.

Potentiation of total horizontal activity and ambulation in rats treated with combinations of beta-phenylethylamine and naloxone

The effects of beta-phenylethylamine (PEA) (6.25-50 mg kg-1) and of naloxone (1-10 mg kg-1), administered alone and in combination, were studied in the rat. Adult, male rats were individually tested in a novel enclosure over a 30 min period, after administration of drugs. Measures of total horizontal activity, ambulation and rearing activity were counted automatically on the basis of interruptions of photobeams. In addition, observational data were collected, using behavioural categories which have previously been used to characterize the behavioural stereotypy syndrome induced by phenylethylamine in large doses. The results showed that certain combinations of phenylethylamine and naloxone produced striking increases in total horizontal activity and ambulation scores, but not in vertical activity scores. Furthermore, naloxone antagonized hyperreactivity and splayed-hindlimb components of the stereotypy syndrome induced by phenylethylamine, but not the head-weaving component. The behavioural specificity of the potentiation of total horizontal activity and ambulation by phenylethylamine in combination with naloxone is emphasized, and the enhancement of activity is contrasted with previous reports of the effects of naloxone administered in conjunction with d-amphetamine.

Effects of naloxone on dopamine release in the nigrostriatal system

Simultaneously with a systemic injection of naloxone (NAL), the effects of the nigral application of the d(+)-amphetamine (AMPH) or of right forepaw stimulation on the release of [3H]dopamine [( 3H]DA) in the two caudate nuclei (CN) and the two substantia nigrae (SN) were examined in halothane-anesthetized cats. These experiments were carried out using four push-pull cannulae implanted bilaterally in the CN and SN and continuously supplied with [3H]tyrosine [( 3H]Tyr), the metabolic precursor of dopamine. Nigral AMPH application (1 muM) produced a local increase of the [3H]DA in spite of the NAL injection (5 mg/kg) but the presence of this drug prevented the expected effect of AMPH in the three other structures. Furthermore, the effects of electrical forepaw stimulation (EPS) were abolished by injection of NAL. NAL alone had no effect on the spontaneous release of [3H]DA. It is concluded that antagonism between NAL and AMPH could be due: (1) to enkephalinergic control of dopamine regulated nigral efferents, (2) to an action on the thalamic structures responsible for the reciprocal control of the two nigrostriatal dopaminergic pathways.

Effect of naloxone and amphetamine on acquisition and memory consolidation of active avoidance responses in rats

Pretraining IP injection of naloxone (0.3 mg/kg) or amphetamine (2 mg/kg) enhanced performance during acquisition, but did not improve retention of active avoidance responses in rats. Naloxone (0.1 or 3 mg/kg) had no effect on acquisition or on retention. The combination of naloxone (0.3 mg/kg) plus amphetamine (2 mg/kg) did not produce the facilitation observed when each of the two drugs was administered alone. Pretreatment with the higher dose of naloxone (3 mg/kg) blocked the facilitative effect of amphetamine on acquisition. Post-training administration of naloxone (0.3 mg/kg) or amphetamine (2 mg/kg) improved retention. Naloxone (0.1 or 3 mg/kg) had no effect. When naloxone and amphetamine were combined, at respective doses of 0.3 mg/kg and 2 mg/kg, the improvement did not occur, i.e., the higher dose of naloxone prevented the facilitative effect of amphetamine. In addition, an ineffective dose of amphetamine (0.5 mg/kg), given either pre- or post-training together with the lower dose of naloxone (0.1 mg/kg), produced a significant enhancement of acquisition or consolidation, respectively. The results are consistent with the possibility that naloxone might exert its facilitative action on acquisition and memory consolidation through the release of catecholaminergic systems from inhibitory influences of opioids.

Effect of midbrain stimulation on amphetamine-induced stereotypy in rats

Behavioral changes were induced in rats by administration of high doses of amphetamine (5 mg/kg): among these was the development of rapid sniffing. The effect of electrical stimulation of the midbrain periaqueductal gray was then examined. This stimulation is known to release an enkephalin-like substance into the ventricular spinal fluid and to induce analgesia. Stimulation blocked amphetamine-induced sniffing. This effect was blocked in turn by pretreatment with naloxone, a specific opiate antagonist. We discuss this finding in the context of opiate-catecholamine antagonism in the CNS.

Effects of naloxone on d-amphetamine- and apomorphine-induced behavior

The effects of acute naloxone administration on d-amphetamine- and apomorphine-induced behavior were studied. Naloxone, in doses of 0.3-10 mg/kg (s.c.), antagonized the increase in ambulation and rearing induced by 1 mg/kg of d-amphetamine. When the dose of d-amphetamine was increased to 3 mg/kg, naloxone (3 mg/kg) antagonized only the increase in rearing activity. No dose (0.3-10 mg/kg, s.c.) of naloxone significantly affected d-amphetamine- or apomorphine-induced stereotyped activity. Naloxone (3 mg/kg) significantly augmented the apomorphine (1 mg/kg, s.c.)-induced increase in ambulation but attenuated the apomorphine (0.3 mg/kg)-induced increase in rearing activity. Naloxone (3 mg/kg) or apomorphine (0.03 mg/kg) significantly decreased the ambulation and rearing induced by a novel environment. In combination and in these doses, naloxone and apomorphine produced an additive effect on these behaviors. The neurochemical mechanisms by which naloxone affects d-amphetamine- and apomorphine-induced behavior were investigated. Naloxone (10(-6) M) had no significant effect on [3H]spiroperidol binding in either the caudate nucleus or nucleus accumbens except for a modest inhibition (24%) of both the Km and Bmax in the accumbens microsomal fraction. Similarly, naloxone (10(-6) M) had no significant effect on [3H]dopamine(DA) uptake into either brain region nor did naloxone alter the d-amphetamine-inhibition of uptake. Using perfused tissue slices, naloxone (10(-6)-10(-5) M) significantly attenuated the increase in [3H]DA release induced by d-amphetamine (10(-5) M) in both brain regions. Naloxone (1 mg/kg) had no significant effect on DA or dihydroxyphenyl-acetic acid (DOPAC) levels or on the DA/DOPAC ratio in the caudate nucleus or nucleus accumbens. However, naloxone did reverse the marked increases in the DA-DOPAC ratio induced by d-amphetamine (1 mg/kg) in both brain regions.

Peptides and thermoregulation

Several peptides are now known to affect thermoregulation. These include beta-endorphin, bombesin, MIF-I, alpha-MSH, neurotensin, TRH, and DSIP. Some of these have been found to interact with the thermal effects of d-amphetamine, a drug with well established actions on thermoregulation. The effects of morphine on body temperature provide some notable comparisons with beta-endorphin, as do the similarities between the effects of naloxone and MIF-I. In general, it seems that two of the major variables which interact and modify the thermal effects of peptides are ambient temperature and route of administration.

Lack of effect of choline and narcotic antagonists upon apomorphine discrimination

Rats were trained to discriminate between the stimulus properties of intraperitoneal 0.16 mg/kg apomorphine and saline in a two-lever, food-motivated operant task. Subjects were then injected with either 79 mg choline chloride, 0.02 mg/kg haloperidol, 10 mg/kg naloxone or 10 mg/kg naltrexone prior to apomorphine or saline discriminative testing. Only haloperidol pretreatment was observed to significantly alter the rats' ability to discriminate apomorphine. The results are discussed in relation to the possible mechanism(s) of action of narcotic antagonist and cholinomimetic drug effects upon dopaminergically mediated behaviors.

Interaction of amphetamine and naloxone in feeding behavior in guinea pigs

Chronic naloxone treatment of schedule-fed guinea pigs caused an initial anorexia followed by a resumption of food and water intake, despite the continued presence of the narcotic antagonist. During this state of chronic naloxone treatment, amphetamine exerted a similar degree of anorexia to that in animals not pretreated with naloxone. In contrast, amphetamine tolerant guinea pigs exhibited supersensitivity to naloxone with respect to feeding behavior. Since chronic amphetamine exposure results in an increased level of beta-endorphin in the hypothalamus it is suggested that an enhanced endorphinergic activity compensates for the effect of amphetamine on feeding mechanisms.

D-Amphetamine-induced hypothermia and hypermotility in rats: changes after systemic administration of beta-endorphin

Systemically administered beta-endorphin was tested in rats for its ability to modify the hypothermia and hypermotility induced by d-amphetamine. Colonic temperature and motor activity were measured in a cold (4 degrees C) ambient temperature in animals given IP injections of beta-endorphin (0.1, 1.0 or 3.0 mg/kg), naloxone (10 mg/kg), or morphine (30 mg/kg). The same measurements were taken in animals given beta-endorphin (1.0 mg/kg) in combination with naloxone or saline pretreatment and d-amphetamine (15 mg/kg) or saline post-treatment. Morphine alone had a biphasic effect on thermoregulation, but did not affect d-amphetamine-induced hypothermia. Activity scores were decreased by morphine, in both d-amphetamine and saline treated animals. The thermal response of rats to beta-endorphin alone was variable, depending on dosage, but all 3 dosages partially blocked the hypothermic effect of d-amphetamine. Naloxone blocked the thermal effects of both beta-endorphin and d-amphetamine. Motor activity tended to be decreased by naloxone, regardless of amphetamine treatment, but beta-endorphin tended to increase activity in amphetamine-treated animals and reduce it in saline-treated controls. In their action on both thermoregulation and activity, naloxone and beta-endorphin appeared to interact independently with d-amphetamine, often producing effects in the same direction, but in combination, they tended to be mutually inhibitory.

Effects of d-amphetamine and naloxone on brain stimulation reward

Self-stimulation thresholds were determined in rats by means of a modification of the psychophysical method of limits. Reinforcement values were determined after the administration of d-amphetamine alone, naloxone alone, and naloxone administered concurrently with d-amphetamine. d-Amphetamine yielded dose-related decreases in the threshold (0.25--2.00 mg/kg IP), while naloxone alone (2.0--16 mg/kg IP) caused no consistent changes. For each animal, a dose of d-amphetamine that substantially lowered the threshold was then selected to be administered with varying doses of naloxone. The threshold-lowering effect of d-amphetamine was blocked by naloxone at doses as low as 2.0 or 4.0 mg/kg. This finding suggests the possible involvement of an opiate receptor in the mediation of the enhancement by d-amphetamine of brain stimulation reward.