A comparison of the effects of opiate antagonists on operant and ingestive behavior

Acute behavioural effects of bupropion and naltrexone, alone and in combination, in non-deprived male rats presented with palatable mash

RATIONALE

In appetite research, drugs frequently progress to clinical trials on the basis of outcome (reduced food intake/body weight gain) with insufficient attention to process (behavioural analysis). Although bupropion and naltrexone (alone and in combination) reduce food consumption in rodents and humans, their effects on behaviour during feeding tests have not been thoroughly investigated.

OBJECTIVES

This study aimed to assess the behavioural specificity of anorectic responses to bupropion, naltrexone and their combination.

METHODS

Video analysis was employed to characterise the behavioural effects of acute systemic treatment with bupropion (10.0-40.0 mg/kg), naltrexone (0.1-3.0 mg/kg) and combined bupropion (20 mg/kg) plus naltrexone (0.1-1.0 mg/kg) in non-deprived male rats exposed for 1 h to palatable mash. Particular attention was paid to the behavioural satiety sequence (BSS).

RESULTS

In experiment 1, the anorectic response to 40 mg/kg bupropion was associated with significant psychomotor stimulation and a complete disruption of the BSS. In experiment 2, the anorectic response to 3 mg/kg naltrexone was associated with an accelerated but otherwise normal BSS. In experiment 3, the co-administration of 20 mg/kg bupropion and naltrexone (0.1 and 1.0 mg/kg) not only produced an additive anorectic profile (including a reduced rate of eating), but the addition of the opioid receptor antagonist also concurrently attenuated the psychomotor stimulant response to the atypical antidepressant.

CONCLUSIONS

Low-dose co-treatment with naltrexone and bupropion produces a stronger suppression of appetite than that seen with either agent alone and has the additional advantage of reducing some of the unwanted effects of bupropion.

Effect of naltrexone during extinction of alcohol-reinforced responding and during repeated cue-conditioned reinstatement sessions in a cue exposure style treatment

The ability of alcohol-related cues to promote craving can be attenuated independently by giving the opioid antagonist naltrexone (NTX) or by subjecting alcohol-dependent patients to a cue exposure treatment. The effects of cue exposure treatment may be enhanced if conducted in the presence of NTX. The purpose of these experiments was to determine if NTX given during extinction of responding for alcohol in rats would alter cue-conditioned reinstatement of responding and to determine if NTX, paired with repeated cue-conditioned reinstatement, would reduce subsequent cue-conditioned reinstatement or reacquisition of self-administration in the absence of NTX. Rats lever pressed for alcohol in the presence of an olfactory cue. Visual and auditory stimuli were presented during alcohol delivery. In the first experiment, rats were injected with saline or 3mg/kg NTX prior to extinction sessions followed by cue-conditioned reinstatement tests. In the second experiment, extinction was followed by cue-conditioned reinstatement sessions presented twice per week. The rats received saline or NTX (3 and 10mg/kg) prior to several sessions. All rats received reinstatement tests with and without a pretreatment of NTX followed by reacquisition of alcohol self-administration. NTX had no effect on responding during extinction or on subsequent cue-conditioned reinstatement. Only 10mg/kg NTX reduced responding during the twice weekly reinstatement sessions. The twice weekly NTX treatment had no effect on subsequent cue-conditioned reinstatement in the absence of NTX. Reacquisition of responding for alcohol was reduced in the group receiving saline during repeated reinstatement sessions, whereas this effect was blocked in the NTX group. These findings support the notion that NTX given during a brief abstinence period (i.e., extinction) has minimal effects on future sensitivity to alcohol cues and alcohol consumption. NTX given during the repeated alcohol cue exposure does not alter the subsequent incentive value of alcohol cues in the absence of NTX or enhance the beneficial effects of cue exposure treatment.

Delta-opioid and 5-HT3 receptor antagonist effects on ethanol reward and discrimination in C57BL/6 mice

The effects of the receptor antagonists MDL 72222 (MDL, 5-HT3) and naltrindole (delta-opioid) on ethanol reward and its discrimination were examined in ethanol-preferring C57BL/6 (C57) mice. MDL attenuated lever responding for 12% ethanol delivered on a fixed-ratio 8 reinforcement schedule at a dose that did not influence responding for water reward, thus confirming a previous report that ICS 205-930 reduced ethanol reward for Long-Evans rats. Our study in combination with the reduced ethanol consumption reported for C57 mice injected with odansetron indicates that 5-HT3 receptor systems are involved in mediating behavior directed toward obtaining ethanol as well as its consumption. By attenuating the rewarding effects of ethanol or of ethanol conditioned cues (e.g., the operant environment), 5-HT3 antagonists may be useful in the treatment of alcohol abuse. The 5-HT3 antagonist effects in this study are comparable with the effects of naltrexone on ethanol reward in C57 mice, although higher doses were required to reduce operant responding for ethanol reward. In contrast to the 5-HT3 antagonist and naltrexone effects, naltrindole, an antagonist with greater specificity for the delta-opioid receptor, was without effect on ethanol reward. This result and recent reports for rats and monkeys suggests that the general antagonists might be more efficacious in attenuating ethanol reward. Both MDL and naltrindole produced only slight reductions in the ethanol discriminative cue, suggesting that the rewarding and discriminative effects of ethanol are not likely mediated by identical neural mechanisms as previously suggested.

Naloxone: lack of effect of a very low dose on fluid intake and activity in rats

Opiate antagonists, such as naloxone, have been employed to indicate the possible involvement of endogenous opioids in a variety of behaviours including the pathogenesis of schizophrenia. This paper describes two experiments which were performed to determine the effects of naloxone on fluid intake and activity in rats. In experiment 1, the administration of 1mg/kg naloxone significantly (p<0.001) reduced water intake. 10mg/kg naloxone considerably reduced water intake, although this result was not significant. This influence was transient, since water intake was restored to control levels at the end of the 4 hours test period, and not dose related. A low dose of naloxone 0.01mg/kg produced no effect. In experiment 2, doses of 1 and 10mg/kg did not influence locomotor activity, rearing or grooming in the open field. These results suggest that naloxone may exert a primary antidipsogenic action that does not depend upon any suppression of concomitant activity. Furthermore, mechanisms controlling water intake and activity appear to be dissociated. A speculative role for the endogenous opioid system is discussed.

Free-choice responding for ethanol versus water in alcohol preferring (P) and unselected Wistar rats is differentially modified by naloxone, bromocriptine, and methysergide

The role of opioids, dopamine and serotonin in ethanol (EtOH) reward and preference was investigated in non-deprived, Alcohol-Preferring (P), and genetically heterogenous Wistar rats. Operant responding for ethanol was initiated using sweet-solution substitution procedures. The rats were then trained in 30-min daily sessions to respond for ethanol (10% v/v) versus water under a two-lever, free-choice contingency. All testing was conducted in the absence of water and food deprivation or addition of sweeteners to the ethanol drinking solution. Rats of both strains developed stable preferences in responding for ethanol over water and consumed ethanol at quantities sufficient to produce pharmacologically relevant mean blood alcohol concentrations (P-Rats: 98 +/- 19.6 mg%; unselected Wistars: 41.7 +/- 8.5 mg%). In P-rats, systemic naloxone (NAL; 0.125, 0.25 and 0.5 mg/kg) pretreatments resulted in a dose-dependent suppression in responding for both ethanol and water, but did not alter ethanol preference (expressed as percent ethanol of total intake). In contrast, bromocriptine (BRO; 1.0, 2.0 and 4.0 mg/kg) produced a significant, dose-dependent shift in preference from ethanol toward water by inhibiting responding for ethanol while enhancing water consumption. In unselected Wistar rats, NAL and BRO treatments produced changes in ethanol preference patterns similar to those observed in P-rats. However, compared to P-rats, these changes were smaller and not consistently dose dependent. No changes in ethanol preference and water or ethanol intake were observed with methysergide (MET; 2.5, 5.0, 10.0 mg/kg) in either strain of rat. Together, the results suggest a possible involvement of dopaminergic mechanisms in the reinforcing properties of ethanol.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of naltrexone on food preference and concurrent behavioral responses in food-deprived rats

Naltrexone (0.05-5.0 mg/kg, SC) was administered to food-deprived rats prior to a 15-min food-preference test. Total food intake and feeding duration was reduced following administration of the opiate antagonist. However, while naltrexone reduced the consumption of the initially-preferred chocolate-coated cookies, the ingestion of the nonpreferred standard laboratory chow pellets was significantly enhanced. These data cannot be explained in terms of a general anorexic effect and nonspecific suppression of feeding responses. Instead, they indicate that naltrexone reduced preference for the highly palatable cookies, so that a feeding response to the chow pellets emerged. Under the conditions of test-familiarity, naltrexone did not reduce grooming, locomotion or rearing duration. An increase in locomotion may have been secondary to the reduction in feeding. The results agree with previous data from animal and human studies in suggesting that endogenous opioid peptide activity is involved in the palatability of preferred foods.

Profile of the selective dopamine D-2 receptor agonist N-0437: its effects on palatability- and deprivation-induced feeding, and operant responding for food

N-0437 is a potent and highly selective dopamine D-2 receptor agonist, which has been used in the present series of experiments to investigate its potential anorectic properties. In doses of 0.3-3.0 mg/kg (IP), N-0437 significantly reduced consumption of a sweetened palatable mash in nondeprived mice (minimal effective dose, 0.3 mg/kg) and rats (minimal effective dose, 0.56 mg/kg). Reduction in food intake were also produced in rats by the less potent, but selective, D-2 agonist RU 24213 (effective at 10.0 mg/kg), and by d-amphetamine (1.0 and 3.0 mg/kg). The anorectic effect of N-0437 (1.0 mg/kg) was completely antagonized by the selective D-2 antagonist, YM-09151-2 (0.01 mg/kg). Over a series of 10 injections, N-0437 (1.0 mg/kg) maintained its effect to reduce palatable food intake. In food-deprived rats, N-0437 (0.3-3.0 mg/kg, IP) also reduced consumption of standard laboratory food, and dose-dependently reduced operant responding for food under a FR8 schedule of reinforcement. The results of the experiments are discussed in terms of a possible direct effect to reduce feeding responses resulting from stimulation of postsynaptic dopamine D-2 receptors.

Ethanol-induced analgesia

The effect of ethanol (ET) on nociceptive sensitivity was evaluated using a new tail deflection response (TDR) method. The IP injection of ET (0.5-1.5 g/kg) produced rapid dose-dependent analgesia. Near maximal effect (97% decrease in TDR) was produced with the 1.5 g/kg dose of ET ten minutes after injection. At ninety minutes post-injection there was still significant analgesia. Depression of ET-induced nociceptive sensitivity was partially reversed by a 1 mg/kg dose of naloxone. On the other hand, morphine (0.5 or 5.0 mg/kg IP) did not modify ET-induced analgesia, while 3.0 minutes of cold water swim (known to produce non-opioid mediated analgesia) potentiated ET-induced analgesic effect. The 0.5 g/kg dose of ET by itself did not depress motor activity in an open field test, but prevented partially the depression in motor activity produced by cold water swim (CWS). Thus the potentiation by ET of the depression of the TDR produced by CWS cannot be ascribed to the depressant effects of ET on motor activity.

Effects of naloxone and diprenorphine on amphetamine-stimulated behavior in guinea pigs and rats

Amphetamine (0.1-10 mg/kg), naloxone (0.1-10 mg/kg) and diprenorphine (0.03-10 mg/kg) were studied for their ability to modulate locomotor behavior in the guinea pig. Naloxone, administered alone, caused a non-significant decrease in locomotor activity and had a similar non-significant effect on amphetamine-stimulated activity. Diprenorphine induced a significant suppression of locomotor activity, the magnitude of which was inversely related to dose: smaller doses of diprenorphine caused a greater suppression of locomotor activity than larger doses. Two doses of diprenorphine (0.1 and 1.0 mg/kg) were tested in combination with amphetamine in the guinea pig. They significantly reduced amphetamine-stimulated behavior and were equipotent in this regard. In contrast, diprenorphine had no effect on amphetamine-stimulated activity in rats. However, in keeping with other reports, naloxone (10 mg/kg) significantly reduced amphetamine-stimulated behavior. The differences in the actions of diprenorphine and naloxone on the behavior of guinea pigs and rats may reflect a different underlying distribution of subtypes of opioid receptor in the two species.

High-dose naltrexone therapy and dietary counseling for obesity

There is considerable evidence that antagonism of the endogenous opioids will suppress food intake in a variety of animal species. The authors report a double-blind, placebo-controlled trial of the long-acting, orally active narcotic antagonist naltrexone in the promotion of weight loss in obese male subjects who were also undergoing dietary counseling for weight reduction. Subjects received medication (naltrexone, 300 mg/day or placebo) for 8 weeks following an initial 2-week single-blind placebo phase. The results failed to demonstrate an advantage for the active drug. However, the naltrexone was associated with hepatotoxicity when used at this dosage in this population.

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.

The role of reward pathways in the development of drug dependence

In commenting on the discovery of "opiate" receptors, Goldstein (1976) said: "It seemed unlikely, a priori, that such highly stereospecific receptors should have been developed by nature to interact with alkaloids from the opium poppy" (p. 1081). Endogenous opioid peptides and opioid receptor systems have now been identified in invertebrates that are unlikely to have had ancestors exposed to opium poppies (Kavaliers et al., 1983; Kream et al., 1980; Leung and Stefano, 1984; Stefano et al., 1980). Moreover, endogenous opioids play a role in stress-induced feeding in the slug (Kavaliers and Hirst, 1986) just as they play a role in stress-induced feeding in rodents (Lowy et al., 1980; Morley and Levine, 1980). If we are to understand the actions of opiates and other drugs of abuse we must understand them in terms of their abilities to interact with neural systems that evolved in the service of primitive biological functions, long before any serious incidence of addiction itself. The most primitive axes of the biological substrates of behavior are the axes of approach and withdrawal. Addictive drugs appear to be able to activate the mechanisms of approach, which is termed "positive reinforcement" and to inhibit the mechanisms of withdrawal, which is termed "negative reinforcement." Anatomically distinct sets of pathways have evolved to serve these two forms of reward. Activation of the medial forebrain bundle and associated structures serves positive reinforcement and induces forward locomotion. Approach and forward locomotion are the unconditioned responses to positive reinforcing stimuli such as food and sex partners, and approach to environmental objects and positive reinforcement is induced by electrical stimulation of this structure. The locomotor stimulating effects and the positive reinforcing effects of opiates and psychomotor stimulants result from their activation of this mechanism; stimulants activate the mechanism at the level of dopaminergic synapses of the nucleus accumbens, frontal cortex, and perhaps other forebrain structures, while opiates activate the system at two points: at the level of the dopaminergic synapse and at the level of the afferents to the dopaminergic cell bodies. Ethanol, nicotine, caffeine and phencyclidine stimulate both locomotor activity and dopamine turnover, but their sites of interaction with reward pathways have not yet been identified. Benzodiazepines and barbiturates stimulate locomotor activity without stimulating dopamine turnover; they may interact with reward pathways at a synapse efferent to the dopaminergic link in the pathways.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of pimozide and naloxone on latency for hypothalamically induced eating

Latency to feed in response to lateral hypothalamic electrical stimulation was assessed at a variety of stimulation frequencies using moderate stimulation intensity; shorter latencies accompanied higher stimulation frequencies. Pimozide (0.0625, 0.125, 0.25 and 0.5 mg/kg, i.p.) and naloxone (0.5, 1.0, 2.0 and 4.0 mg/kg, i.p.) increased latencies in a dose-dependent manner. At 0.5 mg/kg (4 X the lowest effective dose), pimozide blocked eating completely for most trials and in all animals. Naloxone attenuated eating at 1.0 mg/kg, but 4.0 mg/kg was not more effective than 2.0 mg/kg and did not block the behavior completely in any animal. With each drug, latencies were normal or near-normal at the beginning of each session and anorexic effects developed with repeated testing. Thus neither drug caused a simple overall impairment in performance capability. In both cases it is inferred that the proximal food cues lost the ability to maintain eating well before the distal food cues lost the ability to elicit eating. The similarity between the effects of pimozide and naloxone raises the possibility that opioid-containing and dopamine-containing neurons interact to influence feeding through common brain circuitry.

Opiate antagonism reduces placentophagia and pup cleaning by parturient rats

Since endogenous opiate mechanisms are activated during parturition, the present study examined in rats the effects of opiate antagonism on maternal care during and shortly after parturition. Endogenous opiate mechanisms were blocked in late pregnant rats by (1) naltrexone pellet implants (Experiment 1); (2) acute naloxone injections of 10 mg/kg (Experiment 2) or 0.1 mg/kg (Experiment 7); or (3) induction of opiate tolerance (Experiment 3). All methods resulted in a significant decrease in placentophagia and/or in cleaning pups of umbilical cords and birth fluids (Experiment 6). Other aspects of maternal care appeared relatively unaffected and 24 hr pup survival rats were lowered only by induction of morphine tolerance (probably via its effects on the young). In nonpregnant females, naloxone produced a small but significant decrease in placentophagia (Experiment 4) whereas morphine-tolerant nonpregnant females consumed placentas as readily as controls (Experiment 5). Thus the inhibition of placentophagia produced by opiate antagonism may be specific to conditions associated with parturition. These findings suggest that endogenous opiates support placenta eating and pup cleaning during and immediately after birth. Mediation may be via opiate effects on ingestive behavior, and/or via a reduction in the stress of parturition which otherwise can interfere with the female's ability to perform these tasks.

Sugar, opioids and binge eating

There is evidence that endogenous opiates are involved in the control of feeding in experimental animals. Several types of experimental obesity are associated with increased opiate production and/or increased numbers and sensitivity of opiate receptors. Research with experimental animals suggests that nutrients, particularly sugar, have an effect on feeding behavior that is mediated by opiates. For instance, the obesity-producing effect of a palatable diet in rodents is blocked by opiate antagonists. Stress induced feeding in rodents leads to preferential sucrose ingestion and is blocked by opiate antagonists and beta-endorphin. The effect of nutrients on the endogenous opiate system of humans is less clear. Clinical experience suggest that carbohydrates (sugar in particular) play a role in binge eating and obesity. Many binge eaters preferentially eat sweets during a binge. Many obese individuals consume more than half of their total daily calories as carbohydrates. Sweet snacking is a frequent behavior at times of stress. Recent evidence suggests that sugar can lead to increased beta-endorphin production in obese subjects.

Comparison of anorexia and motor disruption by cyclazocine and quipazine

The mixed narcotic agonist-antagonist cyclazocine and the 5-HT agonist quipazine disrupt food-rewarded fixed ratio-40 (FR-40) operant behavior in rats as a dose-dependent decrease in the number of reinforcers obtained and a reciprocal increase in the number of 10-second intervals between responding ("pausing"). This disruption has been shown to result in part from interaction with 5-HT neuronal systems, and may be a consequence of: (1) disruption of cognitive processes, (2) motivational impairment, or (3) motor deficits. To identify which of these components is (are) involved in the disruption of operant responding, female Sprague-Dawley rats were tested for food consumption, spontaneous locomotor activity, or rotarod performance following intraperitoneal injection of cyclazocine, quipazine, or both. Cyclazocine decreased food consumption at doses larger than those required to disrupt operant behavior, while quipazine decreased consumption at doses disruptive to operant responding. Little effect was exerted by either drug on spontaneous locomotor activity, while rotarod performance was disrupted only by very large doses of either drug relative to effects of FR-40 behavior. These data indicate that neither drug appears to disrupt operant behavior by causing gross motor deficits. Thus, cyclazocine may disrupt operant responding by impairing cognition, while quipazine may act through food satiation mechanisms.

Oral ethanol self-administration in the rat: effect of naloxone

Rats responding on a two lever concurrent for ethanol and water, were injected with 5, 10, or 20 mg/kg naloxone hydrochloride 30 min prior to a 30 min session. Only the 20 mg/kg dose had any effect, a decrease in responding for ethanol of up to 50% compared to saline control injection sessions. There were no systematic effects upon water responding. An additional study using sucrose and water as the fluid concurrently available failed to find any effects of naloxone on sucrose responding at the same doses. The effect upon ethanol responding was found not to resemble a pattern of extinction, but rather was best described as a general overall reduction in responding. The relation of these findings to the direct involvement of the endogenous opiate system in ethanol reinforcement is discussed.

Effects of ethanol and naltrexone on free-operant avoidance behavior in rats

Several recent studies have suggested that some effects of ethanol may be mediated by the opioid receptor systems. The present study examined the possibility of a common link between ethanol and opiates by determining whether the effects of ethanol on rat's free operant avoidance behavior could be reversed by the opiate antagonist naltrexone. In Experiment 1 ethanol produced a dose-dependent impairment of avoidance performance (characterized by a decrease in response rate and/or an increase in shock rate) in all three subjects. Naltrexone (3 mg/kg) alone suppressed avoidance rates, but failed to reverse or reduce the effects of ethanol. In contrast to the expectations of the "common-link" hypothesis, the greatest impairment of performance was observed after high doses of ethanol in combination with naltrexone. In Experiment 2 the effects of chronic naltrexone administration were examined. Since previous research has suggested that chronic treatment with opiate antagonists produces a heightened sensitivity to opiate agonists and antagonists, Experiment 2 addressed the issue of whether sensitization to naltrexone would transfer to ethanol. Although increased sensitivity to the rate-decreasing effects of naltrexone was observed, there was no evidence of heightened sensitivity to ethanol. Taken together the results provided little support for the hypothesis that the effects of ethanol on avoidance performance are mediated by the opiate receptor system.

Interactions of naloxone and haloperidol with phencyclidine: effects on milk intake

Phencyclidine (PCP), naloxone and haloperidol were administered alone and in combination to rats trained to drink sweetened-condensed milk during a 20 min daily session. PCP (1.0-16.0 mg/kg) produced a dose-dependent decrease in milk intake. All doses of naloxone (0.1-16.0 mg/kg) produced approximately a 30% decrease in milk intake. Haloperidol (0.125 mg/kg) had virtually no effect on milk intake. When a dose of naloxone which reduced milk intake by approximately 30% (8.0 mg/kg) was administered as a pretreatment to the PCP, the PCP curve was shifted to the left (lowered) to that degree. When haloperidol (0.125 mg/kg) was administered as a pretreatment to the PCP, the PCP dose-response curve was shifted 1.5 fold to the right. These interactions are similar to those observed in other behavioral paradigms and are discussed in reference to PCP's actions as an indirect dopaminergic agonist.

Repeated administration of naltrexone and diprenorphine decreases food intake and body weight in squirrel monkeys

Although chronic administration of naloxone has been reported to reduce food intake and body weight in rats, there have been no comparable investigations using a nonhuman primate. We examined the effects of repeated injections of two long acting opiate antagonists - naltrexone and diprenorphine - on the ad libitum intake of a nutritional complete liquid diet and on body weight in squirrel monkeys. Naltrexone binds with highest affinity to the mu opioid receptor whereas diprenorphine binds with equally high affinity to several subtypes of opioid receptor. Diprenorphine (ED50 = 0.01 mg/kg) was 22 times more potent than naltrexone (ED50 = 0.22 mg/kg) in decreasing 2 h food intake, suggesting that more than one opioid receptor subtype may be involved in the anorectic effects of opiate antagonists. A 1.0 mg/kg dose of drug reduced 24 h food intake by 50% and was associated with a weekly reduction in body weight of 4 and 5% for naltrexone and diprenorphine, respectively. Thus, in contrast with shorter time intervals, 24 h food intakes were similar for the two drugs, and this was associated with comparable body weight profiles. The decreases in food intake and body weight remained constant over the period of drug administration. Some monkeys showed profuse salivation and "wet dog shakes" after 4 days of treatment with the 1.0 mg/kg dose but not after 1 day. Therefore, opiate antagonists given chronically to monkeys reduced food intake and body weight in a dose-dependent manner with no evidence of tolerance to these effects.

Effects of phencyclidine, haloperidol, and naloxone on fixed-interval performance in rats

Phencyclidine (PCP), haloperidol, and naloxone were administered alone and in combination to rats responding under a fixed-interval schedule for water presentation. Lower doses of PCP (0.25-2.0 mg/kg) and naloxone (0.001-0.1 mg/kg) produced increases while higher doses produced dose-dependent decreases in response rate. Haloperidol (0.0625-0.5 mg/kg) produced a monotonic dose-dependent decrease in responding. When a dose of naloxone (8.0 mg/kg) that did not alter responding was administered prior to the PCP, the PCP dose-response curve was shifted to 6.5-fold lower doses of PCP. When a dose of haloperidol (0.0625 mg/kg) that did not alter responding was administered prior to the PCP, the PCP dose-response curve was shifted to 1.5-fold higher dose of PCP. These observations are discussed in relation to current views of the mechanism of PCP action.

Effect of naltrexone on food intake and hoarding in white-footed mice (Peromyscus)

Increases in food consumption and hoarding in mammals have been shown to be immediate and preparative adjustments to the energetic stresses of temperate winters. The sensitivity of these behaviors to the opiate antagonist naltrexone was tested in non-deprived white-footed mice (Peromyscus leucopus). Mice received naltrexone hydrochloride intraperitoneally (10 mg/kg in saline vehicle) once daily for four consecutive days subsequent to an equivalent period of injection with saline as control. Daily food intake was significantly (p less than 0.05) lower after naltrexone treatment. Hoarding, as assessed by logarithmic hoarding scores and the weight of cached food, was not affected by administration of this drug at this dosage. These results suggest that hoarding, a complex behavioral pattern that does not immediately affect internal energy stores, may not fall within the opiate regulatory scheme.

The effect of naloxone on intragastric ethanol self-administration

The acute effect of 1.25 and 2.50 mg/kg naloxone was tested in a group of male Wistar rats readily self-administering 10% w/v ethanol intragastrically following 12 days of forced ethanol intoxication. Compared to saline pretreatment, naloxone did not alter 24 hr intakes of food, water or ethanol. However, both does strongly and significantly inhibited lever pressing for ethanol during 2 hr following pretreatment. The results indicate that naloxones's inhibition of ethanol intake does have a transient postabsorptive component, although this component is unlikely to be specific to ethanol.

N-allylnormetazocine (SKF-10,047): the induction of feeding by a putative sigma agonist

Several distinct classes of opiate receptors have been postulated. It has been suggested that two of these, the kappa and sigma, may play a role in the initiation of feeding. The putative sigma receptor agonist N-allylnormetazocine increased food intake at doses of 0.1 and 1 mg/kg, whereas higher doses caused a decreased intake under some conditions. This stimulatory effect increased after repeated injections and was naloxone reversible. After repeated injections of N-allylnormetazocine, the feeding response to ketocyclazocine, but not morphine, appeared at an earlier point than in naive rats. These experiments support the suggestion that the sigma receptor may play some role in the initiation of feeding.

Endogenous opiates: 1982

This article is the fifth installment in an annual series of reviews of successive year's research dealing with the endogenous opiate peptides. Due to the continuing massive increase in the number of studies in this field, it has become impossible to continue comprehensive reviews of all aspects of this work. As a result we have decided that beginning this year the coverage will be abbreviated to emphasize non-analgesic and behavioral work. The specific areas discussed include stress, tolerance and dependence, consummatory responses, alcohol consumption, schizophrenia and emotional disorders, learning and memory, cardiovascular responses, respiratory effects, thermoregulatory effects, neurological deficits and other disorders, activity, and other, miscellaneous behaviors. As in previous years, we have attempted a relatively comprehensive review of the subjects covered only for the previous year and have not made an attempt to evaluate their contributions relative to those of past years.

Morphine and acceptability of putative reinforcers

Rats were given the opportunity to take one of five concentrations of saccharin solutions. Intake across concentrations generated a preference-aversion curve. Morphine, 2 mg/kg, increased intake of saccharin solutions when rats were 12-hr water deprived or were not deprived. These effects with morphine are opposite to those of naloxone and strengthen the idea that there is opioid involvement in incentive motivation.

Minireview. Benzodiazepine-opiate antagonist interactions in relation to feeding and drinking behavior

Benzodiazepines reliably produce overconsumption of food and fluids. Opiate antagonists, naloxone and naltrexone, block the benzodiazepine-induced hyperphagia and hyperdipsia at low doses. Hence, activation of endogenous opioid mechanisms may be closely involved in the benzodiazepine facilitatory effects on ingestional behavior. Evidence is reviewed that opiate antagonists diminish feeding and drinking responses, and may enhance satiety processes in feeding and drinking, in addition to selectively diminishing the palatability of attractive foods and fluids. It is proposed that a single mechanism of action of the opiate antagonists would be sufficient to account for both effects on feeding and drinking. Biochemical data confirm that acute benzodiazepine treatment in vivo is associated with a naloxone-reversible release of striatal enkephalin. It is possible therefore that there is a close association between the behavioral and biochemical data, which both show that acute benzodiazepine effects are reversed by opiate antagonists. The implied relationship between benzodiazepine and endogenous opioid mechanisms may be relevant to the question of concurrent opiate-benzodiazepine abuse.