A multiple schedule model of limited access drinking in the cynomolgus macaque

Punishment of ethanol choice in rhesus monkeys

A defining characteristic of individuals diagnosed with alcohol use disorder (AUD) is that negative outcomes related to drinking do not lead them to reduce their alcohol use. In rodent models of AUD, this characteristic has been studied by adding the bitter tastant quinine to an ethanol solution. In this study, we extended this approach to a nonhuman primate model in which the ability of quinine to decrease the choice of a 4% ethanol solution vs. water was measured. Five adult female rhesus monkeys with 7.3 years of experience drinking ethanol were given access to a 4% ethanol solution and water for 3 h per day. When ethanol choice was stable, a single quinine concentration (0.03-5.6 g /L) was added to the ethanol solution for 1 day until a quinine concentration-effect curve was generated. After determining the quinine concentration that reduced ethanol choice by half (the quinine EC 50 ), the relative reinforcing strength of ethanol was manipulated by adding quinine or sucrose to the water alternative depending on the monkey's baseline choice. Adding quinine to ethanol produced a concentration-dependent decrease in ethanol choice and intake. Importantly, water intake increased, indicating an effect on response allocation rather than simply a decrease in fluid consumption. Consistent with this conclusion, the addition of quinine or sucrose to the water alternative resulted in predictable increases and decreases, respectively, in ethanol choice. These studies establish a model of punishment of ethanol choice in nonhuman primates that can be used to understand the contextual, biologic and pharmacologic factors that influence sensitivity to the punishment of alcohol drinking.

Diurnal pituitary-adrenal activity during schedule-induced polydipsia of water and ethanol in cynomolgus monkeys (Macaca fascicularis)

RATIONALE

Intermittent delivery of an important commodity (e.g., food pellets) generates excessive behaviors as an adjunct to the schedule of reinforcement (adjunctive behaviors) that are hypothesized to be due to conflict between engaging and escaping a situation where reinforcement is delivered, but at suboptimal rates.

OBJECTIVES

This study characterized the endocrine correlates during schedule-induced polydipsia of water and ethanol using a longitudinal approach in non-human primates.

METHODS

Plasma adrenocorticotropic hormone (ACTH) and cortisol were measured in samples from awake cynomolgus monkeys (Macaca fascicularis, 11 adult males) obtained at the onset, mid-day, and offset of their 12-h light cycle. The monkeys were induced to drink water and ethanol (4 % w/v, in water) using a fixed time (FT) 300-s interval schedule of pellet delivery. The induction fluid changed every 30 sessions in the following order: water, 0.5 g/kg ethanol, 1.0 g/kg ethanol, and 1.5 g/kg ethanol. Following induction, ethanol and water were concurrently available for 22 h/day.

RESULTS

The FT 300-s schedule gradually increased ACTH, but not cortisol, during water induction to a plateau sustained throughout ethanol induction in every monkey. Upon termination of the schedule, ACTH decreased to baseline and cortisol below baseline. Diurnal ACTH and cortisol were unrelated to the dose of ethanol, but ACTH rhythm flattened at 0.5 g/kg/day and remained flattened.

CONCLUSIONS

The coincidence of elevated ACTH with the initial experience of drinking to intoxication may have altered the mechanisms involved in the transition to heavy drinking.

Synaptic and morphological neuroadaptations in the putamen associated with long-term, relapsing alcohol drinking in primates

Alcoholism and alcohol use disorders are characterized by several months to decades of heavy and problematic drinking, interspersed with periods of abstinence and relapse to heavy drinking. This alcohol-drinking phenotype was modeled using macaque monkeys to explore neuronal adaptations in the striatum, a brain region controlling habitual behaviors. Prolonged drinking with repeated abstinence narrowed the variability in daily intake, increased the amount of ethanol consumed in bouts, and led to higher blood ethanol concentrations more than twice the legal intoxication limit. After the final abstinence period of this extensive drinking protocol, we found a selective increase in dendritic spine density and enhanced glutamatergic transmission in the putamen, but not in the caudate nucleus. Intrinsic excitability of medium-sized spiny neurons was also enhanced in the putamen of alcohol-drinking monkeys in comparison with non-drinkers, and GABAeric transmission was selectively suppressed in the putamen of heavy drinkers. These morphological and physiological changes indicate a shift in the balance of inhibitory/excitatory transmission that biases the circuit toward an enduring increase in synaptic activation of putamen output as a consequence of prolonged heavy drinking/relapse. The resultant potential for increased putamen activation may underlie an alcohol-drinking phenotype of regulated drinking and sustained intoxication.

Drinking typography established by scheduled induction predicts chronic heavy drinking in a monkey model of ethanol self-administration

BACKGROUND

We have developed an animal model of alcohol self-administration that initially employs schedule-induced polydipsia (SIP) to establish reliable ethanol consumption under open access (22 h/d) conditions with food and water concurrently available. SIP is an adjunctive behavior that is generated by constraining access to an important commodity (e.g., flavored food). The induction schedule and ethanol polydipsia generated under these conditions affords the opportunity to investigate the development of drinking typologies that lead to chronic, excessive alcohol consumption.

METHODS

Adult male cynomolgus monkeys (Macaca fascicularis) were induced to drink water and 4% (w/v in water) ethanol by a Fixed-Time 300 seconds (FT-300 seconds) schedule of banana-flavored pellet delivery. The FT-300 seconds schedule was in effect for 120 consecutive sessions, with daily induction doses increasing from 0.0 to 0.5 g/kg to 1.0 g/kg to 1.5 g/kg every 30 days. Following induction, the monkeys were allowed concurrent access to 4% (w/v) ethanol and water for 22 h/day for 12 months.

RESULTS

Drinking typographies during the induction of drinking 1.5 g/kg ethanol emerged that were highly predictive of the daily ethanol intake over the next 12 months. Specifically, the frequency in which monkeys ingested 1.5 g/kg ethanol without a 5-minute lapse in drinking (defined as a bout of drinking) during induction strongly predicted (correlation 0.91) subsequent ethanol intake over the next 12 months of open access to ethanol. Blood ethanol during induction were highly correlated with intake and with drinking typography and ranged from 100 to 160 mg% when the monkeys drank their 1.5 g/kg dose in a single bout. Forty percent of the population became heavy drinkers (mean daily intakes >3.0 g/kg for 12 months) characterized by frequent "spree" drinking (intakes >4.0 g/kg/d).

CONCLUSION

This model of ethanol self-administration identifies early alcohol drinking typographies (gulping the equivalent of 6 drinks) that evolve into chronic heavy alcohol consumption in primates (drinking the equivalent of 16 to 20 drinks per day). The model may aid in identifying biological risks for establishing harmful alcohol drinking.