The hormonal data in the current study suggested that individual housing was less stressful than quadrant social housing in young adult male cynomolgus monkeys. However, it should be emphasized that the quadrant caging is relatively restricted space and does not allow animals to leave the presence of any other member of the group. Ethanol self-administration was then assessed during individual housing or daily social grouping. In contrast to the tension reduction hypothesis, continuous individual housing, which was less stressful, was associated with greater ethanol consumption. Greater ethanol self-administration was previously reported during social isolation compared to social housing in rats (Parker and Radow 1974
; Wolffgramm and Heyne 1991
; Ehlers et al. 2007
) and monkeys (Kraemer and McKinney 1985
; McKenzie-Quirk and Miczek 2008
; Higley et al. 1991
). From a behavioral economics viewpoint, ethanol self-administration may have increased during individual housing due to the absence of alternative sources of reinforcement, including physical interaction with conspecifics (Crowley 1983
). This seems unlikely, however, because in the current study, ethanol access from the panel occurred when all monkeys were individually housed, after the daily social grouping (i.e., barriers prevented direct physical interaction as a source of reinforcement). The decreased ethanol drinking among dominant monkeys was more likely due to associative processes related to the daily reminder of social rank, rather than the proximal environment. Dominant monkeys may avoid ethanol intoxication during social housing because it is incompatible with behaviors that maintain dominance (Winslow and Miczek 1985
The current study showed that protection against heavy ethanol drinking by dominant social status was unrelated to diurnal pituitary–adrenal activity, which was similar during daily social grouping among the ranks. The lower ethanol intake of dominant compared to nondominant monkeys is similar to previous studies showing lower self-administration of cocaine (Morgan et al. 2002
; but see Czoty et al. 2004
) and ethanol (McKenzie-Quirk and Miczek 2008
). Apart from an involvement of pituitary–adrenal activity, differences in dopamine activity between social ranks (e.g., female cynomolgus monkeys; Grant et al. 1998
) could explain lower self-administration of drugs among dominant monkeys. Lower synaptic dopamine and dopamine D2
receptors in the striatum of subordinates (Grant et al. 1998
) could enhance drug reinforcement or behavioral processes involved in the acquisition of operant self-administration. Indeed, once self-administration was acquired, male cynomolgus monkeys of different social ranks did not differ in self-administration of cocaine (Czoty et al. 2004
). Additional studies are needed to determine the extent to which striatal dopamine activity provides a biological basis for protection against consumption of excessive quantities of ethanol by dominant social status.
Two hours of social housing in the morning had a lasting effect on pituitary activity, as indicated by increased ACTH in evening samples. Indeed, ACTH remained elevated throughout social housing prior to ethanol exposure. The average concentrations of ACTH in the current study are lower than concentrations after an acute stressor (presentation of “catch gloves,” >100 pg/ml; female cynomolgus monkeys; Herod et al. 2011
). Overall, the levels of ACTH found in the present study appear to be indicative of the vigilance associated with living in a social setting, rather than a “fight or flight” response. In addition, average cortisol measured across all social ranks was <40 μg/dl, consistent with normal, healthy macaques trained to participate in venipuncture (Wilson et al. 2005
). On the first day of social housing, cortisol was transiently elevated for monkeys of all social ranks (maximum, 60 μg/dl). Similar to Czoty et al. (2008)
, cortisol concentrations returned to normal within days after a switch to social housing. In another study, when socially housed squirrel monkeys were subsequently housed alone, cortisol increased nearly twofold in the first hour, peaking after 1 day, and remained elevated above baseline for almost a week (Lyons et al. 1999
). The current study showed that transiently elevated cortisol was related to housing condition, rather than just a change of housing, because rather than increasing, cortisol decreased slightly when the monkeys were switched from social to individual housing.
The maintenance of cortisol rhythms despite fluctuating ACTH is not surprising as adrenocortical rhythms are generated in part independently of pituitary ACTH (Meier 1976
) and adrenocortical sensitivity to ACTH is adaptable (Oster et al. 2006
). Indeed, adrenal sensitivity to ACTH has been reported to vary with social status in monkeys. In females (Shively 1998
) and males (Czoty et al. 2008
), socially housed subordinate cynomolgus monkeys had greater plasma cortisol following the administration of ACTH compared to dominant and intermediate-ranked monkeys. In the current study, similar to past studies (e.g., Czoty et al. 2008
), diurnal adrenocortical rhythms only slightly differed between the social ranks. We found that subordinate and intermediate-ranked monkeys had more similar cortisol (peak and nadir) before ethanol access, but dominant and intermediate-ranked monkeys had more similar cortisol during ethanol access before social housing. This appeared to be due to the maintenance of cortisol peaks and nadirs among intermediate-ranked monkeys during ethanol access.
Intermediate-ranked monkeys may have a distinct temperament. For example, previous studies showed that intermediate-ranked monkeys initiated grooming more frequently than dominant or subordinate monkeys (Morgan et al. 2000
) and had lower adrenal cortisol response to ACTH during individual housing (Czoty et al. 2008
). The data in the current study could indicate that adrenocortical rhythms are more dynamic among individuals at the extremes of social rank. Insofar as social rank is an outcome of trait-like behaviors, endocrine activity and social behaviors should be consistent even if a monkey changes rank. Social upheaval due to the death of the dominant monkey (90) in rack 3 resulted in a prolonged, but reversible, increase in ACTH in the next highest-ranking monkey (96), suggesting that social rank is a trait. Physical contact was not possible during the upheaval, so the transient increase in ACTH could have been related to increased vocal aggression. While monkey 90 was alive and dominant, monkey 96 was an intermediate-ranked low drinker, and he remained a low drinker after becoming dominant. The data suggest that low ethanol consumption may be a behavioral trait of dominant nonhuman primates.
Despite a constant light–dark cycle, the current study showed that peak cortisol was more likely in the morning during ethanol access. This result contrasts with flattened diurnal corticosterone in mice that consumed an ethanol liquid diet up to 100 mg/dl (Kakihana and Moore 1976
). The shift in daily peak cortisol levels from a mix of morning/afternoon to predominantly morning samples during ethanol self-administration is an interesting effect. Diurnal hormone rhythms can be entrained to food access, and therefore, possibly ethanol access. For example, in food- and water-restricted rats, corticosteroids peaked immediately before food and water access (Krieger 1974
). However, in the current study, ethanol access began 4 h after (11:00 AM) the start of the light cycle. The light cycle began at the same time as the blood draw for morning cortisol (7:00 AM). Thus, if diurnal cortisol was entrained to the first meal of the day (and to ethanol access), then peak cortisol would be expected to coincide with the noon sample, not the 7:00 AM sample. However, the data suggest that the increased consistency of cortisol rhythms during ethanol access reflects greater entrainment to the light cycle rather than the feeding schedule.
Although the endogenous rhythm of the adrenal gland became quite consistent during ethanol self-administration, the absolute concentration of cortisol was suppressed, especially at morning and noon. Lower cortisol levels associated with chronic ethanol self-administration is consistent with our previous results in male cynomolgus monkeys (Cuzon Carlson et al. 2011
). In social drinkers, self-reported heavy alcohol drinking (men, >3 drinks/day and women, >2 drinks/day) was associated with greater cortisol at awakening and in the evening compared to moderate drinkers (men, ≤3 drinks/day and women, ≤2 drinks/day; Boschloo et al. 2011
). From this study, it would appear that the results in humans are at odds with those reported here in the monkey. However, morning cortisol can reflect neural control over diurnal rhythm variation or accelerated production of cortisol that follows awakening and reflects morning activities (Wilhelm et al. 2007
). This is an important distinction because different neural mechanisms mediate the awakening response (e.g., brain stem, thalamus; Balkin et al. 2002
) versus morning concentrations due to diurnal variation (e.g., suprachiasmatic nucleus; Krieger et al. 1977
), and the different neural circuitry could be differentially affected by ethanol.
Although this study did not directly investigate hypothalamic signals for adrenal activation, the function of the pituitary was assessed by measuring ACTH. In short, ethanol self-administration was accompanied by blunted diurnal pituitary rhythms that were not correlated with the quantity of ethanol consumed or housing conditions. These data suggest a very low threshold for ethanol disruption of diurnal pituitary activity. Few studies have examined the threshold for ethanol modulation of circadian processes, and to our knowledge, none in humans. Rats consuming 8 g/kg ethanol/day (estimated maximum BEC, 50–100 mg/dl) had decreased entrainment of activity rhythms to light compared to controls (Rosenwasser et al. 2005
). Two weeks of an ethanol liquid diet (35 % of daily calories) in rats flattened the circadian mRNA expression of pro-opiomelanocortin (POMC) and period genes in the arcuate nucleus of the hypothalamus (Chen et al. 2004
). Lastly, 6 weeks of exposure to 11.5 g/kg/day, but not 5.5 g/kg/day, ethanol in a liquid diet disrupted sleep patterns in rats (Mukherjee et al. 2008
). Compared to the studies above, the current results appear to indicate an even lower threshold for disruption of diurnal rhythms of pituitary ACTH in cynomolgus monkeys. In men, Ekman et al. (1994)
reported that 1.0 g/kg, but not 0.5 g/kg, ethanol stimulated POMC activity in the afternoon, although the cleavage products β-endorphin and ACTH were not elevated. In contrast, excessive quantities of alcohol, e.g., during alcoholism, are associated with decreased expression of clock genes in peripheral blood mononuclear cells (Huang et al. 2010
). During severe ethanol withdrawal in humans, cortisol was elevated throughout the day, obliterating its diurnal rhythm in one study (Risher-Flowers et al. 1988
) or delaying peak cortisol in another study in which the severity of withdrawal was not described (Iranmanesh et al. 1989
). We have also reported increased morning cortisol in cynomolgus monkeys during abstinence from ethanol self-administration (Cuzon Carlson et al. 2011
In summary, the main findings of this study are that social housing in quadrant cages resulted in a state-dependent increase in ACTH across all social ranks, whereas cortisol was transiently increased, then returned to baseline. After ethanol self-administration was established, dominant monkeys were at low risk for chronic heavy consumption. Ethanol self-administration resulted in a dampening of cortisol and ACTH concentrations and ACTH circadian rhythm. Finally, limited daily social housing resulted in a significant decrease in ethanol intake compared to individual housing, but interestingly, BEC was lower only among dominant monkeys. The results suggest that social dominance is a protective feature, particularly in a social setting, against chronic heavy ethanol drinking, even though pituitary–adrenal activity appears to be similarly affected by ethanol self-administration across housing conditions and social ranks.