The effects of age at the onset of drinking to intoxication and chronic ethanol self-administration in male rhesus macaques

Rationale

Consumption of alcohol begins during late adolescence in a majority of humans, and the greatest drinking occurs at 18–25 years then decreases with age.

Objectives

The present study measured the differences in ethanol intake in relation to age at the onset of ethanol access among nonhuman primates to control for self-selection in humans and isolate age effects on heavy drinking.

Methods

Male rhesus macaques were assigned first access to ethanol during late adolescence (n?=?8), young adulthood (n?=?8), or early middle age (n?=?11). The monkeys were induced to drink ethanol (4 %?w/v in water) in increasing doses (water then 0.5, 1.0, 1.5 g/kg ethanol) using a fixed-time (FT) 300-s schedule of food delivery, followed by 22 h/day concurrent access to ethanol and water for 12 months. Age-matched controls consumed isocaloric maltose–dextrin solution yoked to the late adolescents expected to be rapidly maturing (n?=?4).

Results

Young adult monkeys had the greatest daily ethanol intake and blood-ethanol concentration (BEC). Only late adolescents escalated their intake (ethanol, not water) during the second compared to the first 6 months of access. On average, plasma testosterone level was consistent with age differences in maturation and tended to increase throughout the experiment more for control than ethanol-drinking adolescent monkeys.

Conclusions

Young adulthood in nonhuman primates strongly disposes toward heavy drinking, which is independent of sociocultural factors present in humans. Ethanol drinking to intoxication during the critical period of late adolescence is associated with escalation to heavy drinking.     

Naltrexone decreases D-amphetamine and ethanol self-administration in rhesus monkeys

Amphetamines are the second most highly abused illicit drugs worldwide, yet there is no pharmacological treatment for amphetamine abuse and dependence. Preclinical studies and, more recently, human studies, suggest that the opioid receptor antagonist, naltrexone, might be useful in the treatment of amphetamine abuse. Naltrexone, an opioid receptor antagonist, is currently used for the treatment of alcohol dependence. The aim of this study was to explore the ability of naltrexone to modify self-administration of amphetamine or ethanol in rhesus monkeys. Monkeys were trained to respond to intravenous injections of either D-amphetamine (0.003 mg/kg/injection) or ethanol (0.05 g/kg/injection) on a fixed ratio 30 schedule. Naltrexone (0.01-1 mg/kg) was administered intramuscularly 30 min before the start of treatment test sessions. Naltrexone dose-dependently decreased both amphetamine and ethanol self-administration. These findings support the potential use of naltrexone as therapy for amphetamine and polydrug abuse.     

The effects of cocaine on gonadal steroid hormones and LH in male and female rhesus monkeys.

Cocaine stimulates significant increases in estradiol, testosterone (T), and luteinizing hormone (LH) in rhesus monkeys, but the temporal interactions between the gonadal steroid hormones and LH have not been determined. The effects of i.v. cocaine (0.8 mg/kg) or saline placebo administration on estradiol, T, and LH were compared in follicular phase female and male rhesus monkeys. Samples for hormone analysis were collected at 2-min intervals for 20 min, then at 10-min intervals for 50 min. Peak plasma cocaine levels were detected at 4 min and pharmacokinetic analyses showed no significant gender differences. Baseline hormone levels were equivalent before saline and cocaine administration, and saline did not alter LH or estradiol levels. In females, when baseline estradiol levels were low (< 100 pg/ml), LH increased significantly within 8 min after cocaine administration (P < 0.05), but when baseline estradiol levels were high (> 100 pg/ml), LH levels did not change significantly after cocaine administration. Estradiol and T increased significantly after LH, within 16 min after cocaine administration (P < 0.01-0.001). In males, significant LH increases were detected at 16 min after cocaine administration (P < 0.05-0.001), but estradiol and T did not change significantly. Thus, cocaine may stimulate significant increases in estradiol and T in females but not in males. These rapid hormonal changes may contribute to cocaine's abuse-related effects, as well as to disruptions of the menstrual cycle during chronic cocaine administration.     

Increased aggression after ethanol self-administration in male resident rats

In order to study experimental alcohol intake that leads to heightened aggression, we established ethanol self-administration in aggressive rats. The focus was on low doses of self-administered ethanol and to assess their effects on aggressive behavior in resident rats, using a limited access paradigm followed by a 5-min confrontation with an intruder. In the first phase of the experiment, rats were established as “residents”, and their consistent aggressive behavior in confrontations with an intruder was verified. In the second phase, these resident rats were trained to self-administer alcohol, using a sucrose-fading technique. In the third phase, alcohol self-administration was followed by intruder confrontations in order to study the effect of alcohol on aggression. Confrontations after ethanol consumption leading to low (5–20 mg/dl) and moderate (20–50 mg/dl) blood alcohol concentration (BAC) were compared to confrontations without alcohol, each animal serving as its own control. On average, the group showed no change in aggressive behavior after low or moderate ethanol intake. However, six out of 16 individuals significantly increased the number of attack bites and the duration of aggressive behavior by up to 90% after alcohol self-administration. When these rats were assigned post-hoc to an alcohol heightened aggression group, the group was characterized by a 40% increase in number of attack bites and a 90% increase in aggressive posture over control (BAC 0 mg/dl), whereas the alcohol non-heightened aggression group showed no significant changes. These results extend previous observations of increased aggression in a subpopulation of animals after experimenter-administered ethanol in mice, rats and monkeys to self-administered alcohol. Using this animal model, individuals showing enhanced or reduced aggression after oral alcohol self-administration can be characterized behaviorally, physiologically, and neurochemically. Received: 17 September 1996/Final version: 9 December 1996     

Interaction of morphine and naltrexone on oral ethanol self-administration in rhesus monkeys.

Opioid antagonists, such as naltrexone (NTX), reduce ethanol consumption and opioid agonists increase or decrease ethanol consumption in rats depending upon the dose. If the opioid antagonist and agonist effects on ethanol consumption are mediated by mu-opioid receptors, then NTX doses that reduce ethanol consumption should be similar to the doses necessary to antagonize the effects of opioid agonists on ethanol consumption. The purpose of these experiments was: (1) to determine whether morphine increases ethanol consumption in rhesus monkeys as it does in rodents; (2) to determine if the mu-receptor mediates the effects of morphine on ethanol consumption by conducting a pK(B) analysis using NTX; and (3) to determine if the mu-receptor also mediates the NTX-induced decreases in ethanol consumption by making comparisons between the NTX doses that affect ethanol consumption and the NTX doses that block the effects of morphine on ethanol consumption. Three male rhesus monkeys responded for 2% ethanol and water for 2 h/day on a fixed-ratio 4 schedule of reinforcement. Morphine doses as low as 0.0032mg/kg failed to increase ethanol fluid deliveries, whereas higher doses produced a dose-related decrease in ethanol fluid deliveries. Although 0.01 mg/kg NTX alone had no effect on ethanol fluid deliveries, it reduced the suppressant effects of morphine with a mu-receptor pK(B) of 8.21 (8.08-8.34). When given alone, 0.1 mg/kg NTX decreased ethanol fluid deliveries but failed to reverse the suppression caused by 1 mg/kg morphine. Therefore, monkeys may differ from rats in their response to morphine when ethanol consumption is the dependent variable. Furthermore, because the NTX dose that reduced the effects of morphine on responding for ethanol was smaller than the NTX doses that suppressed ethanol-reinforced responding when given alone, NTX may exert these two effects through different mechanisms.     

Seasonal variation in CSF 5-HIAA concentrations in male rhesus macaques.

Seasonal changes in cerebrospinal fluid (CSF) 5-hydroxyindoleacetic acid (5-HIAA) concentrations were assessed on multiple occasions in 103 free-ranging male rhesus macaques (Macaca mulatta). At the time of sampling subjects ranged between the ages of 2 and 6 years. CSF samples were collected between the hours of 0900 and 1600 throughout the Fall, Winter, and Spring from 1990 through 1994. Data were analyzed in a general linear mixed model with random intercepts. Results indicated that CSF 5-HIAA concentrations decreased with age. CSF 5-HIAA concentrations were significantly increased in the Fall (October and November), which is the height of the breeding season, with no evidence of differences between Winter and Spring. There was also some evidence that the seasonal variation in CSF 5-HIAA concentrations was blunted in younger, more immature subjects.     

Increase in free choice oral ethanol self-administration in catechol-o-methyltransferase gene-disrupted male mice

The effect of catechol-O-methyltransferase (Comt) gene disruption on the voluntary oral consumption of water, ethanol (2.5-20%, v/v) and cocaine (0.1-0.8 mg/ml) was studied in the free-choice, two-bottle paradigm in male and female mice. Solutions containing ethanol or cocaine, or tap water were available ad libitum from drinking burettes for 4 weeks. Catechol-O-methyltransferase-deficient male mice consumed significantly more ethanol than their wild-type male littermates. In contrast, female mice did not show genotype differences in the consumption of ethanol solutions. During the cocaine experiment, male mice developed either a side preference or an aversion that obscured cocaine consumption. This pattern of drinking was not dependent on Comt genotype. In female mice, Comt genotype was not associated with cocaine consumption. In conclusion, disruption of Comt gene influenced ethanol consumption in a gender-dependent manner in mice, supporting the hypothesis that low catechol-O-methyltransferase activity is one of the predisposing factors for high alcohol consumption in males.     

Oral self-administration of ethanol, phencyclidine, methadone, pentobarbital and quinine in rhesus monkeys

Rationale: Simultaneous and sequential drug use among clinical populations is the norm, whereas the pattern of self-administration of multiple drugs among non-human primate populations has not been thoroughly explored. Objectives: To determine the relationship between the preferences and intakes of a large group of rhesus monkeys exposed to various orally available solutions. Methods: Thirteen male and eleven female young adult rhesus monkeys (Macaca mulatta) were exposed to orally available drug solutions using a concurrent choice (drug and water) procedure, where fluid delivery was made contingent upon single spout contacts (fixed ratio one). Results: Ethanol (0.25–16% w:v) produced biphasic effects on the number of fluid deliveries obtained, with peak ethanol preferences over water demonstrated at the 1–2% w:v concentrations. No preferences for the N-methyl-d-aspartate receptor antagonist phencyclidine or water were demonstrated at lower concentrations (0.0078125–0.125 mg/ml) and, at higher concentrations (0.25, 0.5 mg/ml), a preference for water was demonstrated. The μ opioid receptor agonist methadone (0.001–0.3 mg/ml) and the prototypic bitter substance quinine (0.001–0.3 mg/ml) failed to produce preferences for drug or water. A large preference for water over the barbiturate pentobarbital (0.01–3 mg/ml) was also demonstrated. After rank-ordering the subjects based on their drug preferences or intakes, modest to no correlations across drugs were demonstrated. Conclusions: These results reveal that a robust ethanol preference is not predictive of a preference for drugs of abuse from other classes and suggests that fluid intakes were correlated, irrespective of the presence or absence of drug in the solution. Received: 6 October 1998 / Final version: 27 May 1999     

Mazindol self-administration in the rhesus monkey

The ability of the intravenous administration of mazindol (SaH 42–548) to act as a reinforcer in monkeys previously conditioned to self-administer cocaine was ascertained. Unit dosages i.e. dosage per injection, of 50 and 100 μg/kg resulted in self-administration rates significantly greater than that which occurred with saline. An inverse relationship existed between unit dosage and frequency of self-administration over the unit dosage range 50–200 μg/kg. The total mazindol dosage self-administration per session was however independent of unit dosage. Approximately 2–3 mg/kg was self-administered by each animal during a 4 hr session at each of the 3 unit dosages. This tends to indicate that the 200 μg/kg unit dosage was also reinforcing even though the self-administration rate was similar to that of saline. This study indicates that mazindol can serve as a reinforcer and that the relationship between total session intake, unit dosage, and self-administration frequency of mazindol are similar to these seen with other reinforcing psychomotor stimulant drugs.     

Naltrexone effects on pituitary and gonadal hormones in male and female rhesus monkeys

The long-acting opioid antagonist, naltrexone, stimulates LH and FSH in women during the early follicular phase of the menstrual cycle and is a new provocative test of hypothalamic-pituitary function (42,63). The acute effects of naltrexone (0.25, 0.50 and 1.0 mg/kg IV) on anterior pituitary (LH, FSH, PRL) and gonadal steroid (T or E2) hormones were studied in 7 female and 4 male rhesus monkeys (Macaca mulatta). Integrated plasma samples were collected at 20 min intervals for 60 min before and for 300 min after intravenous infusion of naltrexone over 10 min. In females studied during the early follicular phase (cycle days 1-3), naltrexone did not stimulate LH and significantly suppressed E2 (p less than 0.0003-0.0001) and FSH (p less than 0.006-0.0001). Naltrexone (0.50 and 1.0 mg/kg) also did not stimulate LH release in late follicular phase females (cycle days 10-12) when estradiol levels were in the peri-ovulatory range. FSH and E2 were significantly suppressed (p less than 0.01-0.05) after 1.0 mg/kg naltrexone, but not after 0.5 mg/kg naltrexone. However, in males all doses of naltrexone significantly stimulated LH (p less than 0.003-0.0001) and T (p less than 0.001-0.0001) but not FSH. LH increased significantly above baseline within 20 to 40 min and T increased significantly within 60 min. These gender differences in naltrexone's effects on pituitary gonadotropins and gonadal steroid hormones were unanticipated. These data are not concordant with clinical studies which report significant naltrexone stimulation of LH in men and in women during the early follicular phase.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

Phencyclidine self-administration in the rhesus monkey

Two experiments were performed in which rhesus monkeys self-administered phencyclidine through indwelling venous catheters. In the first experiment, monkeys trained to lever press for cocaine injections, maintained higher response rates as compared to saline control rates when phencyclidine at unit doses from1.5?25.0 μg/kg were substituted for the cocaine baseline. In the second experiment, experimentally naive monkeys spontaneously initiated lever-pressing for injections of 50 μg/kg/inj phencyclidine. In both studies the animals self-administered enough drug to produce behavioral effects resembling general anesthesia.     

Genetic and environmental factors in ethanol self-administration

Findings presented in this paper from pharmacogenetic studies of oral ethanol self-administration suggest a correlation between ethanol preference and self-administration and indicate that there are important genetic as well as environmental determinants of ethanol reinforced behavior. AA (alcohol accepting) and ANA (alcohol nonaccepting) rats, animals bred selectively for differential ethanol preference, showed large differences in operant responding for ethanol. AA rats drank significantly more ethanol than water, and their intake varied as a function of ethanol concentration. Intake of water and ethanol solutions did not differ in the ANA rats. In two inbred strains of rats, F344 and LEWIS, ethanol maintained higher response rates and was consumed in larger volumes than the water vehicle. In a third series of studies, C57BL/6J mice, which exhibit high ethanol preference and low sensitivity, readily self-administered ethanol in an operant situation. Conversely, BALB/cJ mice, which exhibit low preference and high sensitivity, were not positively reinforced by ethanol. The results demonstrate the experimental control possible by the utilization of genetically defined animals, even when complex learned behavioral sequences are being measured, and indicate that genotype and environment interact in a complex but definable way to determine the degree to which ethanol comes to function as a positive reinforcer.     

Differential effects of bremazocine on oral phencyclidine (PCP) self-administration in male and female rhesus monkeys

Sex differences exist in many phases of drug abuse, but few studies have focused on sex differences in drug abuse treatment. In this study, the effects of bremazocine, a kappa-opioid receptor agonist, were compared in age-matched male and female rhesus monkeys self-administering orally delivered phencyclidine (PCP). Bremazocine (0.00032. 0.001, and 0.0025 mg/kg, intramuscular) was administered for 5 consecutive days. 15 min prior to daily 3-hr sessions when PCP (0.25 mg/ml) and water were available under concurrent fixed-ratio schedules. Bremazocine dose-dependently decreased PCP-maintained responding and consumption (mg/kg) in males and females, and these measures were suppressed at a lower bremazocine dose in females than in males. The percentage reduction in PCP-maintained responding and intake (mg/kg) was significantly greater in females than it was in males at the low and middle doses of bremazocine, suggesting that females may be more responsive to kappa agonist treatment than males.     

Effects of sweetened ethanol solutions on ethanol self-administration and blood ethanol levels

The enhancement of voluntary self-administration of ethanol by sucrose or saccharin was tested in conjunction with measurements of blood ethanol levels. Adult male rats were given access to both tap water and one of five solutions: 0.125% saccharin, 10% sucrose, ethanol, saccharin+ethanol, or sucrose+ethanol. The rats receiving the sucrose+ethanol solution drank consistently more ethanol (>5 g/kg/day) than did the rats receiving the saccharin+ethanol solution (<3 g/kg/day) or ethanol only (<2 g/kg/day). Both sweetened solutions produced higher ethanol consumption during these periods than ethanol alone. However, no significant differences in blood ethanol levels were found between the sucrose+ethanol and saccharin+ethanol conditions, when tested at different intervals on Day 44 or Day 45 of ethanol consumption. Following 45 days of consumption, no change in the bicuculline seizure threshold was observed in the ethanol-consuming rats compared to the controls. In a separate study using 90 naive rats, rats were gavaged with ethanol (1, 2, or 3 g/kg) containing either 10% sucrose (n=10 for each dose of ethanol), 0.125% saccharin (n=10 for each dose of ethanol), or ethanol alone (n=10 for each dose of ethanol), and blood was collected from the tip of the tail 30, 60, 180, 300, and 540 min later and analyzed for ethanol concentrations. Sucrose significantly decreased the resultant blood ethanol levels at several time points following gavage. These results indicate that sucrose can significantly alter blood ethanol levels and that chronic self-administration of a sweetened ethanol solution for 6 weeks does not produce ethanol dependence.     

Evaluation of nefazodone self-administration in rhesus monkeys.

Intravenous self-administration of nefazodone, a potential new antidepressant medication, was evaluated using a substitution procedure in rhesus monkeys. Subjects had established stable rates of responding for cocaine (0.033 mg/kg per infusion) under a fixed-ratio 10 schedule during 60-min sessions each day. Various does of nefazodone hydrochloride (0.03-0.3 mg/kg per infusion) were substituted for cocaine for four consecutive daily self-administration sessions. Compared to rates of responding obtained during saline substitutions, nefazodone failed to function as a reinforcer when substituted for cocaine. In only one monkey, at one dose, did the rate of responding exceed the range of saline responding; an effect not observed in two subsequent replications in that subject. In all three monkeys, the total number of infusions tended to decrease during the 4-day nefazodone substitution and the majority of nefazodone infusions occurred during the first quarter of each session, with few infusions occurring in the latter three-quarters. Overall intake of nefazodone increased as a function of dose per infusion. Such a result is expected when response rates do not vary with dose, which is more likely to occur when a test drug is not a reinforcer. In summary, the present results provide no evidence of reinforcing effects with nefazodone and suggest that it would possess little or no abuse liability.