Effects of voluntary exercise and sex on multiply-triggered heroin reinstatement in male and female rats

BACKGROUND

The rise in heroin addiction has heightened the need for novel and effective treatments. Physical exercise has been shown as an effective treatment for stimulant abuse in clinical and pre-clinical research. However, this treatment has not yet been tested on opioid addiction. This study examined the effects of physical activity (wheel running) on heroin-seeking in rats within a reinstatement paradigm (i.e., heroin relapse model).

METHODS

Female and male rats were trained to self-administer intravenous heroin (0.015 mg/kg). Once trained, rats were placed into extinction (i.e., heroin abstinence) for 21 days with continuous access to a locked or unlocked running wheel. After extinction, rats were tested for drug- (heroin, caffeine, and yohimbine) and cue-primed reinstatement of heroin-seeking.

RESULTS

Females completed more wheel revolutions than males across all study phases. Access to an unlocked running wheel reduced extinction and reinstatement of heroin-seeking, with greater reductions in females than males across several reinstatement conditions. In the locked wheel group, female rats showed greater reinstatement of heroin-seeking than males across several priming conditions.

CONCLUSIONS

Wheel running reduced heroin-seeking in male and female rats, with females showing a more robust effect during reinstatement. The locked wheel group allowed an examination of sex differences in heroin reinstatement, which revealed that females showed greater vulnerability to heroin reinstatement than males, but with no other sex differences observed in maintenance or extinction. Overall, the results indicate that voluntary physical exercise may be an effective treatment for heroin dependence in humans.

Cocaine self-administration and reinstatement in female rats selectively bred for high and low voluntary running

BACKGROUND

Previous research has found that rats behaviorally screened for high (vs. low) wheel running were more vulnerable to cocaine abuse. To assess the extent to which a genetic component is involved in this drug-abuse vulnerability, rats selectively bred for high or low voluntary running (HVR or LVR, respectively) were examined for differences in cocaine seeking in the present study.

METHODS

Female rats were trained to lever press for food and then were assessed for differences in acquisition of cocaine (0.4mg/kg; i.v.) self-administration across 10 sessions. Once acquired, rats self-administered cocaine for a 14-day maintenance phase, followed by a 14-day extinction phase when cocaine was no longer available. Subsequently, reinstatement of cocaine seeking was examined with priming injections of cocaine (5, 10 & 15mg/kg), caffeine (30mg/kg), yohimbine (2.5mg/kg) and cocaine-paired cues.

RESULTS

A greater percentage of LVR rats met the acquisition criteria for cocaine self-administration and in fewer sessions than HVR rats. No differences in responding for cocaine were observed between phenotypes during maintenance. However, during extinction LVR rats initially responded at higher rates and persisted in cocaine seeking for a greater number of sessions. No phenotype differences were observed following drug and cue-primed reinstatement of cocaine seeking.

CONCLUSIONS

In general, LVR rats were more sensitive to the reinforcing effects of cocaine than HVR rats during periods of transition into and out of cocaine self-administration. Thus, LVR rats sometimes showed a greater vulnerability cocaine seeking than HVR rats.

Ketoconazole suppresses food restriction-induced increases in heroin self-administration in rats: sex differences

The effects of ketoconazole, an inhibitor of corticosterone synthesis, were examined during food satiation and food restriction in male and female rats to determine whether increases in heroin self-administration prompted by food restriction were due to a stress response. Females self-administered more heroin infusions than males under both feeding conditions. Food restriction increased heroin infusions by an average of 96% for both females and males. Ketoconazole suppressed the increase due to food restriction in females but not in males. Corticosterone reversed the effect of ketoconazole in a group of 8 females, suggesting an interaction between feeding conditions, sex, and the stress response in rats.

Ketoconazole suppresses food restriction-induced increases in heroin self-administration in rats: sex differences

The effects of ketoconazole, an inhibitor of corticosterone synthesis, were examined during food satiation and food restriction in male and female rats to determine whether increases in heroin self-administration prompted by food restriction were due to a stress response. Females self-administered more heroin infusions than males under both feeding conditions. Food restriction increased heroin infusions by an average of 96% for both females and males. Ketoconazole suppressed the increase due to food restriction in females but not in males. Corticosterone reversed the effect of ketoconazole in a group of 8 females, suggesting an interaction between feeding conditions, sex, and the stress response in rats.

Regulation of drug intake

Regulation of drug intake refers to the maintenance of relatively constant levels of drug over a specified time period. An understanding of regulation of drug intake may be critical in determining how drugs function as reinforcers and how their reinforcing effects may be modified. However, little is known about regulation of drug intake, and the mechanisms underlying it are poorly understood. Three mechanisms that have proposed to account for findings of regulation of drug intake were discussed to determine their relevance for drug-reinforced responding. These mechanisms include aversive effects, direct effects, and satiation. Although a greater role for satiation was supported in this review, drugs may vary on the degree to which they can produce satiation and whether satiation acts in concert with either the aversive effects or the direct effects of drugs is unclear.

Role of estrogen in the acquisition of intravenously self-administered cocaine in female rats

Previous work from this laboratory has revealed that female rats acquired cocaine self-administration at a faster rate than male rats and that a greater percentage of females acquired self-administration [Psychopharmacology 144 (1999) 77.]. It has been suggested that sex differences in stimulant self-administration may be related to ovarian hormones, particularly estrogen. To investigate this possibility, we compared four groups (n = 10) of female rats: ovariectomized (OVX) treated with either estradiol benzoate (EB) or vehicle (VEH), and sham-operated intact (SH) females treated with either the antiestrogen tamoxifen (TAM) or VEH. An autoshaping procedure was used to train rats to lever press for intravenous infusions of cocaine (0.2 mg/kg). The criterion for cocaine acquisition was a mean of 100 self-administered infusions over five consecutive 6-h sessions. Results revealed that 70% of the OVX + EB group and 80% of the SH + VEH group acquired self-administration, while only 30% of the OVX + VEH group and 50% of the SH + TAM group met the acquisition criterion. Rats that had estrogen chemically or surgically blocked exhibited significantly less responding for cocaine over the acquisition testing period, and fewer of these rats met the acquisition criterion compared to intact rats and to OVX rats with estrogen (EB) replacement. The percentages for females with estrogen (70% and 80%) vs. those without (OVX, 30%) were similar to those reported for intact females (70%) and males (30%) in the previous study [Psychopharmacology (2000)]. Taken together, these results suggest that estrogen is a key factor influencing drug-seeking behavior in female rats, and it may underlie sex differences in drug-reinforced responding.

Effects of ketoconazole on the acquisition of intravenous cocaine self-administration under different feeding conditions in rats

RATIONALE

Ketoconazole, an inhibitor of corticosterone synthesis, has been reported to decrease the self-administration of low doses of cocaine and prevent stress-induced reinstatement of cocaine-reinforced behavior in rats.

OBJECTIVES

The effects of ketoconazole were extended to the acquisition of i.v. cocaine self-administration during food restriction, a form of stress. Food restriction accelerates the acquisition of cocaine self-administration, and the purpose of this experiment was to determine whether ketoconazole would block the food-restriction effect. As control conditions, the effects of ketoconazole on the acquisition of cocaine self-administration in food-satiated rats and acquisition of food-reinforced responding were also evaluated.

METHODS

Six groups of rats (groups 1-6) were trained to self-administer i.v. cocaine (0.2 mg/kg; groups 1-4) or food pellets (45 mg; groups 5 and 6) under a fixed-ratio 1 (FR 1) schedule. Food availability was restricted to 20 g per day in groups 1, 2, 5, and 6, while groups 3 and 4 were fed ad libitum. Daily sessions included a 6-h autoshaping component followed by a 6-h self-administration component. During autoshaping, 10 infusions or food pellets were delivered each h under a random interval 15-s schedule after extension and retraction of a lever. During self-administration, the lever remained extended and infusions or food pellets were available under an FR 1 schedule. The criterion for acquisition was a 5-day period during which a mean of 100 cocaine infusions or 150 food pellets was obtained during the self-administration component. Rats were given 30 days to reach this criterion. They were pretreated with ketoconazole (25 mg/kg, i.p.; groups 1, 3, and 5) or vehicle (i.p.; groups 2, 4, and 6) 30 min prior to the autoshaping and self-administration components.

RESULTS

Pretreatment with ketoconazole decreased both the rate of acquisition of cocaine self-administration and the percentage of rats meeting the acquisition criterion but only under food-restricted conditions. Ketoconazole had no effect on the acquisition of food-reinforced responding.

CONCLUSIONS

These results extended previous findings of the suppressant effects of ketoconazole on cocaine-reinforced responding in rats to the acquisition of cocaine self-administration using food restriction as a stressor.

Deconstructing relative reinforcing efficacy and situating the measures of pharmacological reinforcement with behavioral economics: a theoretical proposal

BACKGROUND

Relative reinforcing efficacy has been assumed to be a homogeneous phenomenon referring to the behavior-strengthening or behavior-maintaining effects of a drug reinforcer. However, a variety of studies suggest that relative reinforcing efficacy may be heterogeneous.

OBJECTIVES

The purpose of this theoretical proposal is to examine the difficulties associated with this conception of reinforcing efficacy and to explore whether relative reinforcing efficacy is a homogenous concept or whether it is composed of several functionally related heterogeneous phenomena. In examining this issue, we explore whether behavioral economic theory may address some of the challenges to the current conception of relative reinforcing efficacy and use this theory to suggest how the differing measures of reinforcing efficacy may relate to one another.

RESULTS

Results indicate that peak-response rate and breakpoint are related to the economic measure of maximal output and elasticity of demand, respectively. Preference is related to and predicted by the relative location of the demand curves obtained under single schedule conditions. This behavioral economic analysis may provide a theoretical understanding of reinforcement that can reconcile results of studies that both support and fail to support the notion of reinforcing efficacy as a homogenous phenomenon.

CONCLUSIONS

If this theoretical proposal is validated by additional studies, then like other natural phenomena found to be heterogeneous, the study of drug reinforcers may require the adoption of several new scientific terms, such as those used in behavioral economics, each of which has analytical precision and refers to homogeneous phenomena.

Effects of sex and the estrous cycle on regulation of intravenously self-administered cocaine in rats

RATIONALE

Previous research with both humans and animals suggests that there are sex differences in cocaine self-administration; in rodents, ovarian hormones may underlie these differences.

OBJECTIVES

A two-lever drug self-administration procedure was used to compare regulation of intravenously self-administered cocaine in male and female rats and among females in different phases of the estrous cycle.

METHODS

Eleven female and seven male age-matched Wistar rats were trained to self-administer nine doses of cocaine (0.0-2.4 mg/kg) during daily 5-h sessions. Experimental test chambers were equipped with two levers and associated stimulus lights. A response on the lever with stimuli signaling an increase in cocaine dose increased the infusion duration by 3 s, and a response on the other lever decreased the infusion duration by 3 s.

RESULTS

After responding for cocaine stabilized, regulation was disrupted more in females than in males (r2=78.9, r2=92.6, respectively) with the greatest disruption observed in females during the estrus phase (r2=48.5). Mean dose size varied considerably for males and for females in the metestrus/diestrus and proestrus phases; however, estrus females responded almost exclusively on the lever associated with an increase in cocaine dose.

CONCLUSIONS

These findings indicate sex differences in the regulation of cocaine self-administration, and they suggest that ovarian hormones may be responsible for the observed sex differences.

Quantification of reinforcing effectiveness: comment on Meisch (2000)

R. A. Meisch (2000) introduces an innovative method for quantifying the reinforcing effectiveness of drugs and other substances. Advantages are that it models the persistence of drug use in humans and its persistence ratios are the same whether responding or consumption is measured. Data indicate that the method is sensitive to factors that affect drug self-administration. The measure's core feature is "the systematic variation of both reinforcer magnitude and schedule size" (Meisch, 2000, p. 347); however, the method may not be widely used with such extensive parametric analysis. Variation of only reinforcer magnitude (or dose/concentration) seems to produce similar results, and this abbreviated analysis is similar to the behavioral economic analysis of demand. When responding is plotted as a function of unit price (responses/milligram), the peak of the curve (where maximum responding occurs) is a measure of persistence. Further work will determine how these and other measures of reinforcing effectiveness agree.

Acquisition of oral phencyclidine self-administration in rhesus monkeys: effect of sex

RATIONALE

There are increasing reports of sex differences in the etiology of drug abuse in humans. A nonhuman primate model is useful for examining sex as a variable in drug abuse.

OBJECTIVES

To determine whether there are sex differences in the acquisition of oral phencyclidine (PCP) self-administration and to compare the effect of altered feeding conditions on drug self-administration in male and female monkeys.

METHODS

Acquisition of orally delivered PCP was studied using 7 female and 11 male adult rhesus monkeys. Initially, the monkeys were not food restricted, and they were given access to water under concurrent fixed-ratio (FR) 1 schedules during daily 3-h sessions. Each lip-contact response on a drinking spout resulted in a 0.3 ml liquid delivery. After baseline levels of water intake were obtained for 5 days, water was replaced with PCP (0.125 mg/ml) at both drinking spouts. Body weights were then reduced to 85% of free-feeding weights, and the monkeys were fed 30 min before the session began. The FR value was increased from 1 to 2, 4, and 8, at both drinking spouts. As a final step in the procedure, water and PCP were concurrently available at the two spouts under FR 8 schedules. Acquisition of PCP-reinforced behavior was considered to have occurred if PCP intake was consistently greater than water intake.

RESULTS

Lip-contact responses and liquid deliveries were not significantly different between the females and males throughout the acquisition period, but there was a significant increase in responding and decrease in liquid intake as FR increased, and a significant increase in PCP consumption due to food restriction that did not differ in males and females. On a milligram per kilogram basis, female monkeys consumed nearly twice as much PCP as the males; however, this effect was not significant. The females showed significantly higher PCP than water intake while the males consumed approximately equal amounts of PCP and water. Of the seven females, 100% met the acquisition criterion of significantly greater PCP than water intake, while only 36.4% of the males met the criterion.

CONCLUSION

These results concur with previous rat studies and indicate that female monkeys are more likely than males to acquire drug-reinforced behavior.

Reductions in ethanol, phencyclidine, and food-maintained behavior by naltrexone pretreatment in monkeys is enhanced by open economic conditions

RATIONALE

Previous results suggested that naltrexone had opioid-selective effects on behavior reinforced by orally delivered drugs and food. The present study explores the possibility that previous results were due to presentation of the reinforcers in a closed economy (all reinforcers earned by subjects) and that naltrexone's effect may be nonselective under conditions of an open economy (earned reinforcers supplemented by unearned).

OBJECTIVES

The purpose of this experiment was to compare the effect of naltrexone on ethanol-, phencyclidine-(PCP), and food-reinforced responding under several open-economy conditions and compare them with those from a previous report using a closed economy.

METHODS

Ethanol (8% w/v), PCP (0.25 mg/ml), and food were available under independent, concurrent, progressive-ratio (PR) schedules with water, and side positions were alternated daily. Three naltrexone doses (0.1, 0.3, and 1.0 mg/kg) were administered in a nonsystematic order for 5 days, with the 5-day pretreatment period serving as a control. Four economic conditions were compared: data for the closed economy were taken from a recently completed experiment using the same monkeys. The open-economy condition was varied by allowing free access to 1, 2 or 3 times the amount of drug or food that was earned during session later during a postsession time-out period.

RESULTS

In an open economy, naltrexone nonselectively suppressed ethanol, PCP, and food-maintained behavior, in contrast to results in the previous study which used a closed economy, wherein naltrexone did not significantly suppress food- or PCP self-administration. As the economy for PCP, ethanol, and food became more open, the suppressant effects of naltrexone increased and, in many animals, behavioral measures decreased to zero.

CONCLUSION

These results indicated that the economy under which drugs and food are self-administered is an important determinant of the effectiveness of naltrexone's ability to suppress drug- and food-reinforced behavior. The results also suggest that testing medications for drug abuse using a food control condition under a closed economy can bias the results toward a conclusion of selectivity of the treatment medication for drug-reinforced behavior.

Reinstatement of cocaine self-administration in rats: sex differences

RATIONALE

Results obtained with both humans and animals suggest that rates of relapse, or levels of reinstatement responding, may differ between males and females. However, the results obtained with humans are equivocal, and few studies have compared male and female animals on reinstatement responding.

OBJECTIVES

The present experiment was designed to compare male (n=8) and female (n=8) rats on reinstatement of extinguished cocaine-reinforced responding.

METHODS

Reinstatement of responding was examined using a priming model in which lever pressing for cocaine (0.2 mg/kg) was extinguished by replacing cocaine infusions (2 h) with saline infusions (5 h). After responding extinguished during hour 3, reinstatement of responding was tested by administering one of several priming injections of cocaine (0.32, 1.0 and 3.2 mg/kg) or an equal volume of saline.

RESULTS

Although males and females did not differ in the number of saline infusions self-administered after either saline or 0.32 mg/kg cocaine-priming injections, female rats self-administered significantly more saline infusions than males after 1.0 mg/kg and 3.2 mg/kg cocaine-priming injections. Additionally, the effects of 0.32 mg/kg cocaine-priming injections were significantly different from those of saline-priming injections for female, but not male, rats. There was no significant difference between males and females in total cocaine self-administered during hours 1 and 2.

CONCLUSIONS

These findings indicate that female rats are more sensitive than males during the reinstatement phase of drug abuse.

The effects of restraint stress on voluntary ethanol consumption in rats

Sixty Wistar rats (Rattus norvegicus) were assigned to 4 groups of 15 rats each: ethanol stress (ES), ethanol no-stress (EN), isocaloric stress (IS) and isocaloric no-stress (IN). The effect of restraint stress on daily intake of ethanol and a 0.72% solution of glucose was examined in an ABA design (stress-no stress-stress). During the stress phases, 2 groups were subjected to daily 15-min restraint stress, whereas 2 groups were placed in different cages for 15 min as a control. All 4 groups were then given 6-hr access to their assigned liquid alone for 4 days followed by a choice between their assigned liquid and water on the 5th day. The ES group significantly increased their ethanol intake (g/kg) compared to the EN group on choice days but not on forced days. Percentage preference for ethanol was significantly greater and increased at a faster rate over the 75-day testing period compared with the EN group. However, total ethanol consumption (g/kg) and percentage preference did not vary as a function of phase. It is notable that the effects of restraint stress on ethanol self-administration persisted even after the stress schedule was removed.

Reduction of drug self-administration by an alternative non-drug reinforcer in rhesus monkeys: magnitude and temporal effects

RATIONALE

Recent studies have shown that non-drug alternative reinforcers reduce drug self-administration. A goal of the present study was to explore factors such as magnitude of the alternative reinforcer and inter-session access to the alternative to identify conditions that lead to optimal reductions in drug intake.

OBJECTIVES

To evaluate the effects of increasing the volume/delivery (v/d) of saccharin on oral phencyclidine (PCP) self-administration in rhesus monkeys given continuous access to PCP and saccharin during daily sessions using a behavioral economic analysis. The effects of availability of a saccharin solution during the inter-session period on session PCP consumption in drug-experienced monkeys was also investigated.

METHODS

Subjects had access to PCP (0.25 mg/ml) and either water or saccharin (0.03%) from two drinking spouts under concurrent and independent fixed-ratio (FR) schedules during daily 3-h sessions. The FR requirements for both available liquids were simultaneously increased (FR4-64). The v/d of saccharin or water was increased (from 0.3 ml to 1.2 ml), while the v/d of PCP remained constant (0.6 ml). In a second experiment, subjects had access to water or saccharin and water during the inter-session period (17.5 h) under an FR1 schedule. PCP and water were available during daily 3-h sessions under concurrent FR schedules. The FR for both liquids was increased (FR16-128).

RESULTS

PCP intake was reduced at all FRs and magnitude conditions when saccharin (versus water) was concurrently available. Varying the v/d of saccharin only had a modest effect on the extent to which PCP intake was decreased at the higher FR values. Inter-session saccharin availability (versus water) reduced session PCP intake and the magnitude of this effect was also greater at the higher FR values.

CONCLUSIONS

The magnitude of the saccharin delivery had an effect on PCP consumption at higher FRs, suggesting that economic factors such as high drug cost (FR) and low cost (responses/ml) of the alternative reinforcer (saccharin) interact to produce a maximum suppression of drug intake. Between-session availability of saccharin also effectively reduced drug intake, and it had a greater effect on the maintenance levels of drug self-administration when the unit price of drug was high.

Regulation of intravenously self-administered nicotine in rats

Ten male Wistar rats had access to 9 doses of nicotine (0.01-0.10 mg/kg i.v.) during daily 5-hr sessions. Once responding for nicotine stabilized, nicotine infusions were replaced with either cocaine infusions (0.0-2.4 mg/kg) or saline infusions. Saline substitution results indicate that nicotine functioned as a reinforcer. Regulation of nicotine intake was compared with that of cocaine by obtaining the correlation between mean interdose interval and preceding dose size. Results reveal that although this correlation was significant for both nicotine and cocaine self-administration, nicotine self-administration was less precisely regulated than cocaine self-administration. This procedure suggests that there are differences in regulation among self-administered drugs and that it may serve as a useful baseline for studying differences in vulnerability to drug abuse and potential treatment strategies.

Effects of dopamine receptor antagonists (D1 and D2) on the demand for smoked cocaine base in rhesus monkeys

RATIONALE

Previous studies suggest that dopamine antagonists may reduce the reinforcing effects of cocaine. However, the effects of these antagonists on the demand for smoked cocaine base have not been quantified.

OBJECTIVES

To evaluate the effects of selective D1 (SCH 23390) and D2 (raclopride) dopamine receptor antagonists on the demand for smoked cocaine base in rhesus monkeys using a behavioral economic analysis.

METHODS

Six rhesus monkeys were trained to self-administer smoked cocaine base (1.0 mg/kg/delivery) under chained fixed-ratio (FR) schedules (FR64, 128, 256, 512, 1024 or 2048 for lever presses and FR5 for inhalations) during daily 4-h sessions. A maximum of ten smoke deliveries were available. After 5 days of stable behavior at each FR, SCH 23390 (0.01 and 0.056 mg/kg) or raclopride (0.03 and 0.056 mg/kg) were injected intramuscularly, before each session, for 3 consecutive days.

RESULTS

Pretreatment with both antagonists dose-dependently reduced cocaine intake across most FR values tested; however, the decrease in consumption was greater at the higher unit prices than at the lower unit prices. A statistical estimate of the price (FR) at which maximum responding occurred (Pmax) was decreased during drug pretreatment, indicating weakened reinforcing effectiveness of cocaine.

CONCLUSIONS

These data suggest that both antagonists reduce the reinforcing effectiveness of smoked cocaine base, and they have a greater effect on cocaine consumption at higher FR values.

Sex differences in the acquisition of intravenously self-administered cocaine and heroin in rats

RATIONALE

Despite numerous reports that male and female animals differ in behavioral responses to drugs, few studies have investigated sex differences in drug-reinforced behavior.

OBJECTIVES

Acquisition of IV cocaine and heroin self-administration was compared in 20 female and 22 male Wistar rats.

METHODS

An autoshaping procedure was used to train rats to press a lever that resulted in either a 0.2 mg/kg infusion of cocaine or a 0.015 mg/kg infusion of heroin under a fixed-ratio 1 (FR 1) schedule. Daily sessions consisted of six 1-h autoshaping components followed by a 6-h self-administration component. During each autoshaping component, a retractable lever briefly (15 s) extended into the test chamber on a random interval schedule with a mean of either 90 s (cocaine groups) or 480 s (heroin groups) and either ten (cocaine groups) or five (heroin groups) computer-automated infusions were delivered each hour. During each 6-h self-administration component, the lever remained extended and each response on the lever resulted in an infusion of either cocaine (0.2 mg/kg) or heroin (0.015 mg/kg). The criterion for acquisition of cocaine self-administration was a mean of at least 100 infusions and the criterion for heroin self-administration was a mean of at least 20 infusions during the self-administration component over five consecutive sessions.

RESULTS

Female rats acquired both cocaine and heroin self-administration more rapidly than males. Acquisition of cocaine self-administration occurred in a greater percentage of female rats compared to males. Female rats self-administered more cocaine than males after acquisition criteria had been met.

CONCLUSIONS

These findings indicate that female rats were more vulnerable than males to the acquisition of cocaine and heroin self-administration under the conditions of the present experiment.

Concurrent progressive-ratio schedules to compare reinforcing effectiveness of different phencyclidine (PCP) concentrations in rhesus monkeys

RATIONALE

Progressive ratio (PR) schedules have become well accepted for testing the reinforcing effectiveness of drugs. This study extends the methods to concurrent PR schedules with different concentrations of orally delivered phencyclidine (PCP).

OBJECTIVE

The sensitivity of the procedure is tested by presenting different PCP concentrations with independently-operating PR schedules.

METHOD

PCP self-administration was investigated in seven rhesus monkeys. Six different PCP concentrations (0.03-1.0 mg/ml) and water were randomly paired (21 pairings). Liquid delivery (24 ml) was contingent upon lip-contact responses on solenoid-operated drinking spouts; whereby, the response requirement or fixed-ratio (FR) increased (from 8 to 16, 32, 64, 128... to 4096) after each successful completion of a previous FR and subsequent liquid delivery. Monkeys self administered PCP during daily 3-h sessions, and each pair of concentrations was held constant until behavior had stabilized for at least 4 days.

RESULTS

The higher of the two PCP concentrations always maintained greater responding, PR break point (BP), or the last ratio completed, and liquid deliveries than did the lower concentration. However, the monkeys did not exclusively respond on the drinking spout that yielded the higher drug concentration. When examined across all drug pairings, the percentage of total available deliveries of the higher concentration was significantly greater than those of the lower concentration. The monkeys maximized the amount (mg) consumed for the response output. Responding, BPs and liquid deliveries maintained by 0.12 and 0.25 mg/ml PCP were significantly greater than other PCP concentrations; however, drug intake (mg) increased directly with PCP concentration.

CONCLUSION

These results indicate that concurrent PR schedules using oral drug self-administration and a concurrent choice paradigm reliably provide an estimation of relative reinforcing strength, and behavior maintained by these schedules is sensitive to small changes in PCP concentration.

Effects of baclofen on maintenance and reinstatement of intravenous cocaine self-administration in rats

RATIONALE

Recent studies suggest that the GABA(B) receptor agonist, baclofen, may be a useful pharmacotherapy for cocaine abuse.

OBJECTIVES

To investigate further the effects of baclofen on maintenance and reinstatement of cocaine-reinforced behavior in rats.

METHODS

Two groups of rats were trained to self-administer IV cocaine (0.2 or 0.4 mg/kg per infusion) during daily 7-h sessions under a fixed-ratio 1 schedule. Rats were pretreated with baclofen (1.25, 2.5 or 5 mg/kg IP) or saline before the session for 5 consecutive days. An additional group of rats was trained to self-administer IV cocaine (0.4 mg/kg per infusion) during the first 2 h of daily 7-h sessions. Cocaine was replaced by saline for the remaining 5 h of the session. Once behavior had stabilized over the 7-h period, priming injections of saline (IV), cocaine (3.2 mg/kg IV) or baclofen (1.25 or 2.5 mg/kg IP) were administered prior to hour 4. Injections of baclofen (1.25 or 2.5 mg/kg IP) or saline were also given before priming injections of cocaine.

RESULTS

Pretreatment with the two higher doses of baclofen (2.5 and 5 mg/kg) decreased the number of cocaine infusions in both maintenance groups (0.2 and 0.4 mg/kg) over the 5-day treatment period. Baclofen had a greater suppressant effect on responding maintained by the lower dose of cocaine. Priming injections of baclofen (1.25 and 2.5 mg/kg) or saline did not reinstate responding. However, these same doses of baclofen dose-dependently reduced the reinstatement of responding produced by priming injections of cocaine.

CONCLUSIONS

1) The magnitude of the suppressant effects of baclofen on maintenance of cocaine self-administration depends upon the maintenance dose, 2) baclofen may be useful in preventing reinstatement of cocaine-seeking behavior, and 3) compared to maintenance, reinstatement of responding is more sensitive to the suppressant effects of baclofen.

Ethanol consumption in rats when dose size is under subject control

Rats orally self-administered ethanol when they could control dose size as follows: A lever press initiated a trial; a press on 1 lever increased the previous trial duration by 30% whereas a press on a 2nd lever decreased the previous trial duration by 30%. During a trial, rats could drink either water, 8%, or 16% (wt/vol) ethanol (ETOH) from a lick-sensitive fountain. Mean per session intake of 8% and 16% ETOH was 0.8 g/kg and 1.3 g/kg, respectively. Water intake was negligible. Rats did not show a tendency to maintain a particular trial duration, nor did they adjust intertrial intervals to previous dose size (number of licks). When trials were collected into bouts, a positive relationship was found between bout size and interbout interval; this relationship was statistically significant for the 8% but not the 16% ETOH condition.

Naltrexone pretreatment decreases the reinforcing effectiveness of ethanol and saccharin but not PCP or food under concurrent progressive-ratio schedules in rhesus monkeys

The purpose of this experiment was to determine whether attenuation of ethanol consumption by naltrexone is the result of selective changes in the reinforcing effectiveness of drug and non-drug reinforcers. A range of naltrexone doses (0.1-1.0 mg/kg) was administered for 5 days, and the effects on the reinforcing effects of orally delivered 8% (w/v) ethanol, 0.25 mg/ml phencyclidine (PCP), 0.03% (w/v) saccharin and food were studied in eight rhesus monkeys. Food and liquids were available under independent and concurrent progressive-ratio (PR) schedules (ratio range 8-4096) during daily 3-h sessions. Ethanol-maintained responding was attenuated by 0.3 and 1.0 mg/kg doses of naltrexone, while saccharin-maintained responding was decreased at the 1.0 mg/kg dose. Furthermore, there was a significant linear trend that consumption of available ethanol and saccharin was attenuated dose-dependently by naltrexone. Following 5 days of naltrexone pretreatment, ethanol- and saccharin-maintained responding immediately returned to or exceeded baseline levels. Food- and PCP-maintained responding and intake were not significantly affected by any of the naltrexone doses examined. The decreased break point (BP) values for ethanol and saccharin suggest that their reinforcing effects are mediated through opioid reinforcement mechanisms. The lack of naltrexone attenuation of PCP- and food-maintained responding suggests that these reinforcers: 1) are not sensitive to naltrexone antagonism at the doses examined, 2) are mediated by non-opioid reinforcement mechanisms, and/or 3) have less intrinsic palatability.

Effects of amphetamine, butorphanol, and morphine pretreatment on the maintenance and reinstatement of cocaine-reinforced responding

Three groups of rats (N = 23) self-administered intravenous cocaine (0.4 mg/kg/infusion) during the initial 2 hr of daily 7-hr sessions. During the remaining 5 hr of these sessions, responding produced saline infusions. On test days, each rat was pretreated with an assigned dose of either butorphanol, morphine, amphetamine, or saline before the self-administration session followed by a priming injection of cocaine (3.2 mg/kg i.v.) or saline at the beginning of the 4th hr. Results showed that both amphetamine (0.5, 1.0, and 2.0 mg/kg) and morphine (0.3, 1.0, and 2.0 mg/kg) pretreatment dose-dependently reduced cocaine self-administration whereas only amphetamine affected (increased) reinstatement responding. Butorphanol pretreatment (8 mg/kg) suppressed cocaine self-administration and blocked the effect of the cocaine priming injections on pretreatment testing days. These findings indicate that (a) pretreatment drugs with similar stimulus properties potentiate reinstatement of responding and (b) pretreatment drugs can differentially affect maintenance and reinstatement of responding.

Caffeine withdrawal in normal school-age children

OBJECTIVE

Caffeine is widely consumed by children around the world. The purpose of this study was to determine whether children manifest withdrawal effects after cessation of caffeine intake.

METHOD

Thirty normal children completed the single-blind, within-subjects, repeated-measures study with weekly sessions. Subjects were tested four times: (1) baseline (on regular caffeine diet); (2) on caffeine (approximately 120 to 145 mg/day); (3) during withdrawal (24 hours after discontinuation of caffeine taken for 13 consecutive days); and (4) at return to baseline. Subjects were evaluated with self-report measures of symptoms and objective measures of attention, motor performance, processing speed, and memory.

RESULTS

During caffeine withdrawal, there was a significant deterioration on response time of a visual continuous performance test of attention. This finding is consistent with caffeine withdrawal. The deterioration in response time appeared to persist for 1 week.

CONCLUSIONS

Twenty-four hours after children discontinued caffeine, there was a decrease in performance on reaction time of a task requiring sustained attention. Further work is indicated to determine whether children manifest caffeine withdrawal effects after cessation of caffeine intake.

A novel paradigm to investigate regulation of drug intake in rats self-administering cocaine or heroin intravenously

The purpose of this experiment was to investigate the regulation of drug intake in rats (n = 20) self-administering heroin or cocaine during daily 5-hr sessions. Operant chambers were equipped with 2 levers and associated stimulus lights. A response on the lever with stimuli signaling an increase in dose size increased the infusion duration by 3 s, and a response on the lever with stimuli signaling a decrease in dose size decreased the infusion duration by 3 s. Results showed that daily and hourly drug intake for cocaine and heroin groups were relatively constant. Significant correlation coefficients were obtained for heroin and cocaine groups for the relationship between interdose interval (IDI) and infusion duration (dose size). These findings indicate that subjects regulated their drug intake by adjusting IDI throughout drug self-administration sessions.

Acquisition of oral phencyclidine (PCP) self-administration in rhesus monkeys: effects of dose and an alternative non-drug reinforcer

The effects of drug dose and a non-drug alternative reinforcer on acquisition of oral PCP self-administration in rhesus monkeys were examined. Acquisition was studied using three groups of monkeys (seven subjects per group). One group received a low PCP dose (0.0375 mg/delivery) and the other two received a high PCP dose (0.15 mg/delivery). One of the high dose groups had concurrent access to a saccharin solution (0.03% w/v) and water during the intersession (17.5-h) period. Food non-restricted monkeys were initially given access to water under a fixed-ratio (FR) 1 schedule during daily 3-h sessions. Water was then replaced with PCP during the session. The monkeys were then reduced to 85% of their free-feeding body weights and fed before the session, and the FR value was increased from 1 to 2, 4 and 8. Subsequently, food was given post-session and water and PCP were available under concurrent FR 8 schedules. At this final step of the procedure, acquisition of PCP self-administration was considered to occur if PCP intake consistently exceeded water intake. When all three groups were given concurrent access to PCP and water, PCP intake was greater than water intake only in the group of monkeys receiving the high PCP dose. PCP intake increased when water replaced saccharin during intersession in the high PCP dose group. Within-group data revealed that 85.7% of monkeys acquired PCP reinforcement in the group given access to the high PCP dose while only 42.8% acquired in the other two groups. These data suggest that drug dose and presence of alternative non-drug reinforcers affect acquisition of drug self administration in non-human primates.

Dietary additives and the acquisition of cocaine self-administration in rats

The effects of dietary caffeine and the amount and palatability of food on the acquisition of cocaine (0.2 mg/kg) self-administration were examined. Using an autoshaping procedure, seven groups of 13 rats each were trained to press a lever resulting in a cocaine (0.2 mg/kg infusion under a fixed-ratio 1 (FR 1) schedule. One group had ad libitum access to caffeine- (0.2% w/w) admixed food. Three groups had access to 10 g, 20 g or ad lib food each day. Another three groups had the same three amounts of ground food with powdered saccharin (0.2% w/w) added. During daily 6-h autoshaping sessions, ten infusions were delivered each hour under a random-time 90-s schedule after a brief (15 s) extension of a retractable lever. These were followed by 6-h self-administration sessions, when the lever remained extended and cocaine infusions were available under an FR 1 schedule. The acquisition criterion was self-administration of a mean of 100 infusions over 5 days. Cocaine self-administration was accelerated in the caffeine group compared to the regular chow group. However, by 30 days nearly the same percentage of rats in the caffeine and regular food groups met the acquisition criterion. In the other six groups, as the amount of food increased, the rate of acquisition and percentage of rats per group meeting the acquisition criterion decreased. In the ad lib group, acquisition was further reduced when saccharin was added to food. In summary, dietary caffeine accelerated acquisition and a greater amount and increased palatability of food independently interfered with acquisition of cocaine self-administration in rats.

Smoked heroin and cocaine base (speedball) combinations in rhesus monkeys

The purpose of this investigation was to compare the self-administration of heroin and cocaine base, alone and in combination, in rhesus monkeys (Macaca mulatta) self-administering a combination of heroin (0.1 mg/kg/delivery) and cocaine base (1.0 mg/kg/delivery) via the smoking route. Smoke deliveries were contingent on completion of a chained fixed ratio (FR; lever press), FR 5 (inhalation) schedule. The lever press FR values (64, 128, 256, 512, and 1024) represented increasing drug price. Demand functions (Consumption X price) were obtained for the heroin and cocaine combination and compared with previously determined demand functions for smoked heroin and cocaine alone. As the FR increased and the number of responses emitted increased, the number of drug deliveries decreased. The demand functions were not different for heroin versus cocaine alone or for the cocaine alone versus the cocaine-heroin combination. However, the demand for heroin alone was significantly less than the demand for the cocaine-heroin combination, suggesting that smoked cocaine base enhances the behavioral effects of smoked heroin.

A comparison of progressive ratio schedules versus behavioral economic measures: effect of an alternative reinforcer on the reinforcing efficacy of phencyclidine

Alternative non-drug reinforcers have been demonstrated to decrease drug-reinforced behavior by both decreasing relative reinforcing efficacy and substituting for the drug reinforcer. The effect of saccharin on responding maintained by orally delivered phencyclidine (PCP) was examined in this study using concurrent progressive-ratio (PR) schedules of reinforcement and a behavioral economic analysis of demand. Seven adult male rhesus monkeys self-administered PCP (0.06, 0.12, 0.25, 0.50 and 1.0 mg/ml) and either concurrent water or saccharin (0.03% wt/vol) from two drinking spouts under concurrent independent PR schedules. During daily sessions the response requirements (lip contacts on automatic drinking spouts) increased across 15 levels, from 8 to 4096. Each successful ratio completion resulted in the availability of 40 liquid deliveries under an FR 1 schedule and a subsequent increment in the PR. Concentrations of PCP were presented in a non-systematic order and presentation of the concurrent liquid, saccharin or water, was counterbalanced across subjects. All behaviors maintained by PCP were significantly greater than those maintained by water. Replacement of water with saccharin served to significantly decrease PCP-maintained responding and break points (BP) across the range of PCP concentrations; however, saccharin did not significantly decrease deliveries of PCP. Saccharin maintained significantly greater responding, BPs and deliveries compared to either PCP or water, across all PCP concentrations. The use of BP as a measure of reinforcing efficacy suggests that saccharin decreased the relative reinforcing efficacy of PCP. Furthermore, behavioral economic analyses suggested that saccharin decreased maximal PCP-maintained responding (Pmax) in a similar fashion, suggesting that BP and Pmax may be analogous measures of reinforcing efficacy.

Smoked heroin and cocaine base (speedball) combinations in rhesus monkeys

The purpose of this investigation was to compare the self-administration of heroin and cocaine base, alone and in combination, in rhesus monkeys (Macaca mulatta) self-administering a combination of heroin (0.1 mg/kg/delivery) and cocaine base (1.0 mg/kg/delivery) via the smoking route. Smoke deliveries were contingent on completion of a chained fixed ratio (FR; lever press), FR 5 (inhalation) schedule. The lever press FR values (64, 128, 256, 512, and 1024) represented increasing drug price. Demand functions (Consumption X price) were obtained for the heroin and cocaine combination and compared with previously determined demand functions for smoked heroin and cocaine alone. As the FR increased and the number of responses emitted increased, the number of drug deliveries decreased. The demand functions were not different for heroin versus cocaine alone or for the cocaine alone versus the cocaine-heroin combination. However, the demand for heroin alone was significantly less than the demand for the cocaine-heroin combination, suggesting that smoked cocaine base enhances the behavioral effects of smoked heroin.

Effects of buprenorphine and an alternative nondrug reinforcer, alone and in combination on smoked cocaine self-administration in monkeys

The abuse of smoked cocaine base, also known as 'crack', continues to be a major public health problem and to date the success of pharmacological or behavioral interventions has been limited. The purpose of this study was to evaluate the efficacy of a behavioral (alternative reinforcer-saccharin) and pharmacological (0.01 mg/kg buprenorphine) treatment alone and in combination. Five adult male rhesus monkeys self-administered cocaine base (1.0 mg/kg/delivery) via the smoking/inhalation route. Each day ten smoke deliveries were available contingent upon completion of a chained FR (lever press), FR (inhalation response) response schedule during 4 hr sessions. The data were analyzed using a behavioral economic framework in which the lever press response requirements were varied from 64 to 1024 to generate a demand function (consumption x FR) for cocaine under the following conditions: (1) buprenorphine pretreatment alone (0.01 mg/kg, i.m., 30 min presession); (2) concurrent access to saccharin alone (0.03% wt/vol); and (3) buprenorphine pretreatment in the presence of concurrent access to saccharin. Under all conditions, increases in the lever FR resulted in significant decreases in smoked cocaine base deliveries. Neither buprenorphine pretreatment alone nor concurrent saccharin alone produced significant decreases in smoked cocaine deliveries; however, the combination of buprenorphine pretreatment and concurrent saccharin significantly decreased the mean number of smoked cocaine deliveries from the no treatment baseline and from the buprenorphine alone condition. These data suggest that the combination of pharmacotherapy and alternative reinforcers may be an effective treatment strategy to alter smoked cocaine self-administration.

Acquisition of i.v. amphetamine and cocaine self-administration in rats as a function of dose

The effect of dose on the acquisition of i.v. amphetamine and cocaine self-administration was examined. Three unit doses of amphetamine (0.03, 0.06 and 0.12 mg/kg) and three unit doses of cocaine (0.05, 0.2 and 0.8 mg/kg) were tested in separate groups of ten (amphetamine) or 13 (cocaine) rats. Autoshaping methods were used to train rats to press a lever that resulted in drug infusion under a fixed-ratio (FR) 1 schedule. A daily 6-h autoshaping component non-contingently delivered 60 infusions according to a 60-s random time schedule with ten infusions delivered during the first half of each h. Each day autoshaping sessions were followed by a 6-h self-administration session. The criterion for acquisition was a 5-day period during which a daily mean of 100, 50 or 25 infusions for the three amphetamine doses and 400, 100 or 25 infusions were earned during the 6-h self-administration period for the three cocaine doses, respectively. As dose increased, more rats per group acquired drug self-administration and the mean number of days to meet the acquisition criterion decreased. The percentage of rats acquiring amphetamine self-administration increased with dose and ranged from 80 to 100%. Only one rat at the lowest cocaine dose met the acquisition criterion, but 100 percent of the rats at the two higher doses acquired. During the last 2 days of acquisition, mean infusions decreased and mean drug intake (mg/kg) increased as dose increased. On the last day of acquisition, the time course of infusions during the 6-h self-administration component was characterized by a steady rate of infusions per hour, and number of infusions was inversely related to dose. These findings indicate that the initial available dose of a drug is an important determinant of the rate and probability that successful acquisition will occur.

Progressive ratio and behavioral economic evaluation of the reinforcing efficacy of orally delivered phencyclidine and ethanol in monkeys: effects of feeding conditions

The effect of feeding conditions on the reinforcing efficacy of orally-delivered drugs was evaluated using a progressive-ratio (PR) paradigm and a behavioral economic analysis of demand. Seven monkeys self-administered phencyclidine (PCP) (0.06, 0.12, 0.25, 0.5, and 1.0 mg/ml) or ethanol (2, 4, 8, 16, and 32% wt/vol) and concurrent water from two drinking spouts under concurrent PR schedules. The ratios increased from 8 to 4096, and 40 liquid deliveries were available after completion of each ratio schedule. The entire range of drug concentrations was presented in nonsystematic order under two feeding conditions, food restriction and food satiation. Drug maintained responses, deliveries and break points were significantly greater than those maintained by water. Food restriction significantly increased the rate of PCP-maintained responses, deliveries and PR break points over the food satiation baseline. There was also a significant interaction between feeding condition and drug concentration. Although ethanol-maintained responses, liquid deliveries and break points consistently increased in five of seven monkeys during food restriction, only drug concentration produced significant differences in these measures. Using break point as a measure of reinforcing efficacy, food restriction increased the reinforcing efficacy of PCP and had a more pronounced effect at higher drug unit prices.

Oral caffeine pretreatment produced modest increases in smoked cocaine self-administration in rhesus monkeys

Several recent studies have shown that caffeine potentiates the reinforcing, discriminative stimulus, and motor activating effects of cocaine in rats. The present study was designed to determine whether oral caffeine pretreatment would enhance the reinforcing effects of cocaine in rhesus monkeys trained to self-administer smoked cocaine base. The effects of oral caffeine pre-treatment (0, 100, or 200 mg) and fixed-ratio (FR) value on cocaine-base smoking were evaluated in four male rhesus monkeys. Monkeys responded on a lever under a fixed-ratio (FR) schedule (FR 128, 256, 512, 1024, 2048, or 4096) and then made five inhalations on a smoking spout to gain access to volatilized cocaine base (0.25 or 1.0 mg/kg per delivery) during daily experimental sessions. Twenty pellets [20 non-caffeinated (0 mg caffeine), ten non-caffeinated+ten caffeinated (100 mg caffeine), or 20 caffeinated (200 mg caffeine) pellets] were administered 30 min prior to experimental sessions. The lever FR value was held constant within each experimental session, but was increased after 3 consecutive days of stable responding. Although the number of smoke deliveries that was self-administered significantly decreased from FR 128 to FR 4096, it did not change as a function of cocaine dose across the range of FR values tested. However, the interaction between cocaine dose and caffeine pretreatment was statistically significant. Compared to 0 mg caffeine, three of four monkeys pretreated with 200 mg caffeine responded for a greater number of smoke deliveries when they were maintained on a cocaine dose of 1.0 mg/kg per delivery, but not 0.25 mg/kg per delivery. Thus, caffeine pretreatment can produce small, but statistically significant increases in smoked cocaine self-administration in rhesus monkeys.

Smoked heroin self-administration in rhesus monkeys

The purpose of the present study was to evaluate behavioral and pharmacological determinants of smoked heroin self-administration. Eight rhesus monkeys were trained to self-administer smoked heroin under a chained fixed-ratio, (FR, 64-1024) for lever presses, FR 5 for inhalations schedule during daily experimental sessions. Demand for heroin was determined by plotting consumption (smoke deliveries) as a function of price which was varied by increasing the FR lever press requirement from 64 to 1024. The heroin demand curve was compared to that obtained with smoked cocaine base. Dose-effect determinations were obtained by varying the unit dose of heroin from 0.025 to 1.6 mg/kg per delivery. Pretreatment with naloxone (0.01-1.0 mg/kg IM, 10 min presession) and substitution tests with the peripherally acting opioid loperamide (0.1 mg/kg per delivery) were also conducted. Deliveries of smoked heroin decreased, but lever responding per delivery increased as the FR increased. Demand for heroin was elastic and comparable to demand for smoked cocaine base. Varying the dose of heroin available for self-administration resulted in an asymptotic dose-effect curve. Naloxone pretreatment produced dose-dependent decreases in heroin self-administration. Substitution of loperamide for heroin produced extinction-like responding within one or two sessions, with the total smoke deliveries decreasing by 80% of heroin levels within 8-15 days. Reinstatement of heroin resulted in a rapid return to baseline levels of self-administration. These data suggest that rhesus monkeys will readily and reliably self-administer heroin via the inhalation route, and behavioral and pharmacological manipulations indicate that smoked heroin functioned as a positive reinforcer.

Combined effects of buprenorphine and a nondrug alternative reinforcer on i.v. cocaine self-administration in rats maintained under FR schedules

Although previous studies have shown that pharmacological agents, such as buprenorphine, and alternative nondrug reinforcers, such as money or sweetened solutions, reduce cocaine self-administration, few studies have examined the combined effects of these two approaches. The purpose of the present study was to evaluate the effects of the opioid partial against buprenorphine (0.1 mg/kg) and concurrent access to either water or a glucose plus saccharin solution (G+S, 3% and 0.125% wt/vol) in rats self-administering intravenous (IV) cocaine (0.4 mg/kg per infusion) under fixed-ratio schedules (FR2, 8 or 32). One group had concurrent access to water and another group had concurrent access to G+S. After 3 consecutive days of stable cocaine self-administration, a single buprenorphine injection (0.1 mg/kg IV) was administered 30 min before the start of the experimental session for 3 consecutive days. To summarize the results, (1) the presence of an alternative non-drug reinforcer significantly reduced cocaine self-administration, (2) buprenorphine selectively decreased cocaine, but not water or G+S, self-administration; (3) the decrease in cocaine infusions by buprenorphine was greatest on the first day of buprenorphine administration; and (4) expressed as a percentage of baseline conditions, the combination of buprenorphine and G+S produced a greater decrease in cocaine self-administration than either buprenorphine or G+S alone. These results indicate that combined treatment with buprenorphine and concurrent access to a sweetened solution is a more effective strategy for reducing cocaine self-administration than either strategy alone.

The relationships among saccharin consumption, oral ethanol, and i.v. cocaine self-administration

The purpose of the present experiment was to replicate previously reported observations of a relationship between saccharin consumption and oral ethanol self-administration in rats using operant measures (2,8) and to determine whether saccharin intake was related to the rate of acquisition of IV cocaine self-administration. Groups of Wistar rats selected for high and low saccharin (0.1% wt/vol) intake were tested for rate of acquisition of IV cocaine (0.2 mg/kg/infusion) self-administration using an autoshaping procedure. They were subsequently tested for self-administration of oral ethanol (8% wt/vol) under ascending fixed-ratio (FR) schedules (FR 1, 2, 4, and 8). Finally, ethanol deliveries were compared under food-deprivation and food-satiation conditions under an FR 8 schedule. Saccharin intake was redetermined after each phase of the experiment. No significant differences between high and low saccharin groups were found in rate of acquisition of IV cocaine self-administration, and there was not a significant correlation between saccharin and cocaine consumption. However, the high saccharin group drank significantly more ethanol than the low saccharin group during the FR 8 food satiation component. A significant correlation between saccharin and ethanol consumption was also found. For high and low saccharin groups, responding for ethanol increased proportionally with increases in FR such that consumption of ethanol remained relatively constant as FR increased. Ethanol consumption was significantly increased under food deprivation relative to food satiation conditions for both saccharin groups. A significant correlation between ethanol consumption and cocaine consumption was also found. Significant increases in saccharin consumption across successive saccharin consumption tests were found for both groups, although relative intake for the high and low saccharin groups remained stable throughout the experiment. These results indicate that higher ethanol intake is predicted by higher saccharin intake, but saccharin intake did not predict the rate of acquisition of IV cocaine self-administration.

Food deprivation affects extinction and reinstatement of responding in rats

Food deprivation has been shown to increase the self-administration of a wide variety of drugs in a number of different species. However, the effects of food deprivation on other phases of drug taking have not been established. The purpose of the present study was to evaluate the effects of food deprivation on reinstatement of responding for cocaine. Rats trained to self-administer 0.2, 0.4, or 1.0 mg/kg cocaine intravenously (IV) under a fixed-ratio 1 schedule for the first 2 h during daily 7-h sessions were fed either before or after the experimental session. During hours 3-7, rats self-administered saline. Saline replaced cocaine in the infusion pumps at the beginning of hour 3 and a priming injection of either saline or cocaine (0.32, 1.0, or 3.2 mg/kg IV) was administered at the beginning of hour 4. The number of infusions that was self-administered was measured throughout the 7-h session. During hours 1 and 2 when cocaine was available, the number of infusions was inversely related to cocaine dose. During hour 3, rats typically self-administered several infusions of saline, which gradually decreased to near-zero levels by hours 4-7 (extinction responding). A priming injection of cocaine administered at the beginning of hour 4 reinstated responding in a dose-related manner. The magnitude of extinction responding during hour 3 and reinstatement of responding during hour 4 were similar regardless of cocaine maintenance dose.(ABSTRACT TRUNCATED AT 250 WORDS)

Concurrent self-administration of ethanol and an alternative nondrug reinforcer in monkeys: effects of income (session length) on demand for drug

Eight rhesus monkeys (Macaca mulata) were trained to self-administer orally delivered ethanol (8%) and saccharin (0.03 or 0.3% wt/vol) or water under concurrent fixed-ratio (FR) schedules. The FR requirement for saccharin was fixed at 32, while the FR for ethanol was varied (4, 8, 16, 32, 64 and 128) in a non-systematic order to assess demand for drug. Demand was defined as consumption plotted as a function of price (FR). Income was defined as the duration of access to available resources. Income was varied by allowing access to the concurrently available liquids 20, 60 or 180 min per day. Order of testing was counter-balanced across monkeys. Saccharin deliveries were much higher than ethanol deliveries under the 180-min income condition; however, they were lower than ethanol deliveries when income was reduced to 20 min and the ethanol FR was 4, 8 or 16. Thus, when the price of drug was relatively low, consumption of drug exceeded that of the nondrug reinforcer, and that relationship was reversed as income decreased. Saccharin deliveries sustained a proportionally greater reduction due to decreased income compared to ethanol deliveries. As income decreased from 180 to 20 min, saccharin deliveries were reduced by an average of 79.1% (across ethanol FR conditions) while ethanol deliveries were reduced by an average of 41.2 and 40.8% when concurrent saccharin or water were available, respectively; thus, drug self-administration was more resistant to income changes than saccharin. The demand for ethanol was shifted downward in a parallel fashion as income decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of food deprivation on cocaine base smoking in rhesus monkeys

Studies have shown that both food deprivation and response cost have important influences on the magnitude of self-administration of a wide variety of psychoactive drugs. In an attempt to extend these findings to the smoked route of drug self-administration, the effects of food allotment and fixed-ratio (FR) value were evaluated in four male rhesus monkeys trained to smoke cocaine base. In the first phase of the experiment, monkeys were trained to self-administer experiment, monkeys were trained to self-administer smoked cocaine base under a chained progressive-ratio (PR), fixed-ratio (FR) schedule during daily experimental sessions. Monkeys were required to make 20 lever-press responses and then five inhalations on a smoking spout to obtain the first smoke delivery. The lever ratio than increased to 60, 140, 300, 620, 1260, 2540, and 4940 for each successive smoke delivery. The initial lever ratio value was reset to 20 at the beginning of each daily session. The body weights of three monkeys were determined under free-feeding conditions. Monkeys were then restricted to 100 g food and, when body weights had stabilized, the daily food allotment was increased to 150 g, approximately 210 g, or greater than 400 g (satiation). As the daily food allotment and body weight increased, the mean number of smoke deliveries decreased in two of three monkeys. In the second phase of the experiment, three monkeys were maintained under either food-satiated or food-restricted conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

Choline acetyltransferase activity is reduced in rat nucleus accumbens after unlimited access to self-administration of cocaine

Choline acetyltransferase (ChAT) activity was measured in discrete areas of rat brain after chronic, unlimited access to self-administration of cocaine. Mean activity of ChAT was reduced by approximately 30% in the nucleus accumbens, both on the last day of cocaine access and after 3 weeks cocaine withdrawal. These data suggest that chronic cocaine exposure might inhibit nucleus accumbens cholinergic neurones which could underlie some of the behavioral effects of cocaine.

Food deprivation history and cocaine self-administration: an animal model of binge eating

In this two-part study, an animal model of binge eating was first produced, then the rate of acquisition of cocaine self-administration was assessed. Initially, 16 female weanling rats were food deprived (DEPR) at 25, 95, and 143 days of age. Another group of 16 age-matched controls was allowed ad lib access to food. Each time the DEPR group was food deprived, they were allowed to recover to normal weight. They were then injected with butorphanol tartrate (BUTR), an opioid that stimulates feeding, and food intake was measured for 4 h. All rats given BUTR consumed significantly more food than those given saline. Animals with DEPR history consumed food over a longer period of time, and at h 4 after BUTR injection, they consumed significantly more food than controls. In the second part of the experiment, an autoshaping procedure was used to quantitatively evaluate the rate of acquisition of cocaine self-administration. By day 30, 86% of the DEPR and 69% of the control groups had acquired cocaine self-administration.

Phencyclidine (PCP) self-administration and withdrawal in rhesus monkeys: effects of buprenorphine and dizocilpine (MK-801) pretreatment

The effects of dizocilpine and buprenorphine pretreatment on behavior reinforced by orally delivered phencyclidine (PCP) and saccharin, and on PCP withdrawal-induced disruptions in food-maintained responding were examined. Sixteen male rhesus monkeys were used in six different experimental protocols. Two groups of monkeys (N = 4-5) self-administered PCP (0.25 mg/ml) and water under concurrent FR 16 schedules, and were pretreated with IM injections of saline, and dizocilpine (0.001-0.1 mg/kg), or buprenorphine (0.003-0.8 mg/kg) 30 min before the 3-h sessions for 5 days. Two other groups (N = 5) were treated similarly except they had access to saccharin (0.03% or 0.3% w/v) and water under concurrent FR 16 schedules. In two other groups (N = 3), the effects of saline, dizocilpine (0.005-0.1 mg/kg), or buprenorphine (0.2 and 0.8 mg/kg) pretreatment were studied on PCP (0.25 mg/ml) withdrawal-induced disruptions in food-maintained responding. Dizocilpine and buprenorphine reduced both PCP (0.25 mg/ml) and saccharin (0.03% or 0.3% w/v) self-administration, especially at the 0.1-mg/kg dizocilpine dose and 0.2-mg/kg buprenorphine dose. Dizocilpine attenuated the PCP withdrawal effect, but buprenorphine had no effect on behavioral disruptions induced by PCP withdrawal. When dizocilpine was administered 2 days after PCP withdrawal began, the withdrawal effects were almost completely reversed. These results suggest that although drugs from the same and different pharmacological classes can suppress self-administration of drug and nondrug reinforcers, the same doses may produce an opposite effect or no effect on food-maintained behavior during PCP withdrawal.

Effects of concurrent saccharin availability and buprenorphine pretreatment on demand for smoked cocaine base in rhesus monkeys

The effects of saccharin and the opioid partial agonist buprenorphine on cocaine base smoking were evaluated in five male rhesus monkeys. Monkeys completed a sequence of responding consisting of lever-press responses maintained under a fixed-ratio (FR) schedule followed by inhalation responses (FR5) on a smoking spout to gain access to a single delivery of volatilized cocaine base (1.0 mg/kg per delivery). Monkeys could receive a maximum of ten smoke deliveries per session. In the first experiment, either saccharin (0.03% wt/vol) or water was concurrently available under an FR1 schedule through a lip-operated drinking device. As lever FR values increased from 128 to 256, 512, 1024 and 2048, the number of cocaine smoke deliveries decreased. Cocaine intake was not statistically different when water versus saccharin was concurrently available. However, as cocaine consumption decreased, saccharin intake increased demonstrating that under these conditions, saccharin was substituting for cocaine as a reinforcer. On the first day that lidocaine replaced cocaine, all of the monkeys received the maximum number of smoke deliveries (ten) and saccharin intake increased. Lever-press responding gradually extinguished over days when lidocaine (1.0 mg/kg per delivery) was available with concurrent saccharin. In the second experiment, water was concurrently available with cocaine and buprenorphine (0.01 or 0.1 mg/kg) was administered intramuscularly (IM) 30 min before the start of the session. Although pretreatment with the lower dose of buprenorphine (0.01 mg/kg) had little effect on cocaine intake overall, individual differences in cocaine intake occurred.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterogeneous subregional binding patterns of 3H-WIN 35,428 and 3H-GBR 12,935 are differentially regulated by chronic cocaine self-administration

We examined the influence of chronic cocaine exposure, in an unlimited access self-administration paradigm, on density of the dopamine transporter (3H-WIN 35,428 and 3H-GBR 12,935 binding) and concentration of monoamine (dopamine, serotonin, noradrenaline and metabolites) neurotransmitters in rat brain. In normal rodent striatum 3H-WIN 35,428 and 3H-GBR 12,935 binding to the dopamine transporter, although generally similar, showed different subregional rostrocaudal and mediolateral gradients, suggesting that the two ligands might bind to different subtypes or states of the dopamine transporter. Following chronic, unlimited access cocaine self-administration, binding of 3H-WIN 35,428 was significantly elevated in whole nucleus accumbens (+69%, p < 0.001) and striatum (+65%, p < 0.001) on the last day of cocaine exposure ("on-cocaine group"); whereas in the 3 week withdrawn animals ("cocaine-withdrawn group"), levels were either normal (striatum) or reduced (-30%, p < 0.05, nucleus accumbens). Although similar changes in 3H-GBR 12,935 binding were observed, this dopamine transporter ligand showed a smaller and highly subregionally dependent increase in binding in striatal subdivision of the on-cocaine group, but a more marked binding reduction in the cocaine-withdrawn animals. As compared with the controls, mean dopamine levels were reduced in striatum (-15%, p < 0.05) of the on-cocaine group and in nucleus accumbens (-40%, p < 0.05) of the cocaine-withdrawn group. These data provide additional support to the hypothesis that some of the long-term effects of cocaine exposure (drug craving, depression) could be consequent to reduced nucleus accumbens dopamine function. Our data also suggest that dopamine transporter concentration, and perhaps function, might undergo up- or downregulation, either as a direct effect of cocaine, or indirectly as part of a homeostatic response to altered synaptic dopamine levels, and therefore might participate in the neuronal events underlying cocaine-induced behavioral changes.

Caffeine effects on learning, performance, and anxiety in normal school-age children

OBJECTIVE

The purpose of this investigation was to study the acute effects of caffeine on learning, performance, and anxiety in normal prepubertal children.

METHOD

Twenty-one children were evaluated in a double-blind, placebo-controlled crossover design. Subjects were studied during four sessions, 1 week apart, under the following conditions: baseline, placebo, 2.5 mg/kg caffeine, and 5.0 mg/kg caffeine. Subjects were randomized to order of placebo and the two dosages of caffeine. Dependent measures included tests of attention, manual dexterity, short-term memory, and processing speed. Anxiety rating scales were also administered. Saliva samples were analyzed for caffeine levels.

RESULTS

Caffeine improved performance on two of four measures of the Test of Variables of Attention and on a test of manual dexterity in the dominant hand. There was a trend toward increased current level of self-reported anxiety after caffeine on a visual analogue measure of anxiety. Children reported feeling significantly less "sluggish" after caffeine ingestion than after placebo ingestion.

CONCLUSIONS

In a small sample size, there was indication that caffeine enhanced performance on a test of attention and on a motor task. Children also reported feeling less "sluggish" but somewhat more anxious. Because caffeine is so widely available and frequently consumed by children, these results are important and need replication.

Effects of buprenorphine and naltrexone on reinstatement of cocaine-reinforced responding in rats

Reinstatement of responding previously maintained by cocaine was measured after noncontingent "priming" injections of cocaine, the opioid partial agonist buprenorphine, the opioid antagonist naltrexone and the opioid agonist etonitazene. The effects of pretreatment with buprenorphine, naltrexone or etonitazene on the reinstatement of responding produced by a priming injection of cocaine were also evaluated. The rats were trained to respond on a lever under a fixed-ratio 1 schedule to receive i.v. infusions of cocaine (1.0 mg kg-1 infusion-1) for the initial 2 hr during daily 7-hr sessions. Saline replaced cocaine at the beginning of hour 3, which resulted in an extinction of responding during the third hour and low levels of responding during the subsequent 4 hr of the session. Priming i.v. injections of cocaine (0.4-3.2 mg/kg), but not buprenorphine (0.025-0.4 mg/kg), naltrexone (1.6 and 3.2 mg/kg) or etonitazene (2.5 and 5.0 micrograms/kg), administered at the beginning of hour 4 of the session (i.e., during the extinction period), produced a dose-related reinstatement of responding. Pretreatment with either buprenorphine (0.025-0.4 mg/kg) or etonitazene (2.5 and 5.0 micrograms/kg), but not naltrexone (1.6 and 3.2 mg/kg), produced a dose-related suppression of the reinstatement of responding produced by 3.2 mg/kg of cocaine. These results indicate that 1) buprenorphine and naltrexone have little potential for producing reinstatement of responding in cocaine-maintained rats and 2) buprenorphine's effectiveness in preventing a reinstatement of responding produced by a cocaine priming injection may be related to its opioid agonist actions.

The economic context of drug and non-drug reinforcers affects acquisition and maintenance of drug-reinforced behavior and withdrawal effects

The focus of this review is to examine the effect of non-drug alternative reinforcers on drug-reinforced behavior. An increasing number of animal laboratory as well as human clinical studies have demonstrated the effectiveness of non-drug reinforcers in reducing steady-state levels of drug self-administration. One goal of this review was to determine what behavioral economic conditions are optimal for reducing drug-reinforced behavior. Variables such as price of the drug and non-drug reinforcer have been manipulated by changing fixed-ratio (FR) value of these commodities. Income has been changed by limiting the amount of access to the commodities or by changing session length. Substitution was evaluated by determining whether decreased demand for a drug (due to increased price) was related to increased demand for a non-drug reinforcer. A second goal of this review was to investigate transition states in the drug addiction process with respect to the role of alternative non-drug reinforcers. Animal models of acquisition and withdrawal were examined to identify behavioral economic conditions under which acquisition may be prevented or withdrawal effects (and potential for relapse) may be alleviated.

Autoshaping i.v. cocaine self-administration in rats: effects of nondrug alternative reinforcers on acquisition

The purpose of this experiment was to examine the effects of a nondrug alternative reinforcer and feeding conditions on the acquisition of cocaine self-administration. Rats were autoshaped to press a lever that resulted in a 0.2 mg/kg i.v. cocaine infusion. Responses on the lever were monitored during six consecutive autoshaping sessions that occurred each day. A retractable lever was inserted into the operant chamber on a random time 60 s schedule 10 times per session for six sessions that began each hour. Each day the six autoshaping sessions were followed by a 6-h cocaine self-administration session. During self-administration the lever remained extended, and each response on the lever resulted in a cocaine infusion (0.2 mg/kg). The criterion for acquisition of cocaine-reinforced behavior was met when there were 5 consecutive days during which the mean number of infusions during the 6-h self-administration session was at least 100. This procedure was repeated daily until the criterion was met or 30 days elapsed. The rats were also trained to respond on lick-operated automatic drinking devices that delivered 0.05 ml water or a glucose and saccharin solution (G + S) contingent upon each lick response. Five groups of 12-14 rats were compared. The first four groups constituted a 2 x 2 factorial design whereby either G + S or water was available in the home cage for 3 weeks before autoshaping began and G + S or water was available in the operant chamber during autoshaping. These groups were limited to 20 g food per day and all had free access to water.(ABSTRACT TRUNCATED AT 250 WORDS)