Christopher L. Coe

Mother infant attachment in the squirrel monkey adrenal response to separation

The pituitary-adrenal response following separation was evaluated in mother and infant squirrel monkeys. Four mother-infant pairs and a pregnant female, living in a social group, were the subjects of this experiment. The plasma cortisol levels of the mothers and infants were determined after the following conditions: (1) basal levels at 1100 hr, (2) 30 min after momentary separation and reunion, (3) 30 min after infant removal from the group, and (4) 30 min after mother removal from the group. The levels of plasma cortisol were significantly elevated in both mothers and infants following separation and the response was not reduced by the presence of familiar animals (e.g., separated infants which were “aunted” by the pregnant female). Separation followed by immediate reunion did not result in elevated values. These data indicate that a specific attachment relationship develops between mother and infant, and that the agitation following separation is reduced only by reunion with the object of attachment.

Psychoneuroendocrinology of Stress: A Psychobiological Perspective

Publisher Summary This chapter discusses specific psychological variables that are involved in the regulation of the Pituitary–Adrenal (P–A) activity and specific aspects of the psychological variables that can selectively affect and regulate the secretion of gonadal hormones. The influence of psychological factors on P–A hormones is bidirectional. The psychological stimuli not only participate in activating this system but also effectively inhibit it. This inhibition is manifested either by reduced elevations of plasma corticoids during aversive stimulation or by an actual decrease in circulating levels of corticoids. This effect is particularly pronounced when the predictable or reinforcing stimulus involves consummatory events. Apart from induced stress, feedback is another factor involved in this process. Feedback refers to stimuli or information occurring after a behavioral response has been made in reaction to an event. These stimuli may be used to convey information to the responding organism indicating that it has made the correct response to a noxious event. The field of neuroendocrinology has made tremendous advances in the past few decades in identifying many chemical substances that qualify as hormones not only in the pituitary and peripheral organs but also in the central nervous system.

Social status constrains the stress response in the squirrel monkey.

Abstract The influence of dominance on the pituitary-adrenal and gonadal systems was evaluated in male squirrel monkeys. Basal and stress levels of plasma cortisol and testosterone were determined in eight male pairs across a 5-week period. The data indicated that squirrel monkeys have unusually high levels of steroid hormones in comparison to other species. Dominant males had higher levels of cortisol and testosterone and showed a smaller stress response than did subordinate males.

Hormonal responses accompanying fear and agitation in the squirrel monkey

The adrenocortical and gonadal responses of 14 male monkeys were evaluated during four experimental conditions in order to evaluate the influence of social interactions on endocrine responsiveness. Plasma hormone levels were determined during the establishment of social relations, after 60-min exposures to a novel environment, after 60-min exposures to a snake, and 60 min after ACTH administration. Both adrenal and gonadal secretion changed significantly during the first day after social relations were established, although only dominant males showed increases in testosterone, whereas cortisol levels rose in all subjects. Increases in cortisol, but not testosterone, were also observed following exposure to novelty or a snake. The presence of a social partner reduced signs of behavioral disturbance during these test conditions, although the adrenal responses were equivalent or greater than when tested alone. This finding qualifies earlier research which indicated that social support was beneficial for reducing stress when squirrel monkeys were tested in larger groups in their home environment.

Behavioral, but not physiological, adaptation to repeated separation in mother and infant primates

Mother and infant squirrel monkeys were subjected to a series of brief separations in order to evaluate how behavioral and physiological responses change following multiple exposures to stress. Beginning when the infants reached three months of age, their behavioral and hormonal responses were assessed during six 1-hr separations; and additional five dyads served as controls for the effect of repeated disturbance. The separated infants showed a marked and progressive decrease in distress calling across time, but no change was observed in the high levels of agitated activity or the plasma cortisol response to separation. This finding questions the traditional use of distress vocalizations as a measure of stress and indicates that certain types of behavior can change independently of physiological arousal responses, which may continue to occur even after repeated exposures to stress.

Hormonal Response to Stress in the Squirrel Monkey (Saimiri sciureus)

The pituitary-adrenal and gonadal responses following stress were evaluated in the squirrel monkey. Plasma levels of cortisol (CS), ACTH and testosterone (T) were determined during a 4-h period following the combined stress of capture and ether anesthesia. The results indicated that the squirrel monkey manifests higher basal levels of steroids than typically found in other mammals. The endocrine response following stress was biphasic, involving an initial elevation and subsequent decline in hormone levels. Males manifested significantly higher plasma levels of CS and T and lower plasma levels of ACTH than did females.

The Endocrine System of the Squirrel Monkey

In 1968, when the first treatise on the squirrel monkey appeared, it was already apparent that the physiology of New World monkeys differed from that of Old World primates, but few investigators were aware of the extensive differences in hormone metabolism and secretion (Rosenblum and Cooper, 1968). Since that time, it has become increasingly evident that the endocrine system of New World monkeys is extremely divergent from the general primate pattern in terms of both absolute hormone levels and the synthetic pathways utilized. We now know that most of the Callithricidae and smaller Cebidae have unusually high levels of adrenal and gonadal hormones, although typical values are difficult to describe because of the remarkable diversity in their reproductive physiology [for a species comparison, see Hearn (1983)]. In this chapter we will review a series of recent studies on the endocrine system of the squirrel monkey, which has some of the highest hormone levels observed so far. The level and magnitude of the hormone responses in the squirrel monkey have also provided a unique opportunity for assessing the influence of psychological and environmental variables, which may exert less overt effects in species with lower hormone levels. We will discuss the influence of seasonal and diurnal rhythms and the effect of genetic and maturational variables, and finally consider several of the important psychological processes that affect endocrine function.

Prolonged cortisol elevation in the infant squirrel monkey after reunion with mother.

Abstract Pituitary-adrenal response in mother and infant squirrel monkeys was assessed following either 30 min separation or 5, 15 and 30 min after a brief separation and reunion of mother and infant. Basal (undisturbed) samples were also obtained. The results revealed that there was an increment (28%) in plasma cortisol levels in the separation-reunion condition. This change was slight however when compared to the changes in plasma cortisol following separation (120%). A second experiment assessed plasma cortisol levels in mothers and infants that were reunited following a 30 min separation. Blood samples obtained 30 min after reunion indicated that the plasma cortisol levels in mothers had returned to basal levels, whereas the infants remained highly elevated during this period. Thus the effects of reunion following a very brief separation show the operation of an inhibitory process which occurs as a result of the interaction between mother and infant. Reunion following a longer separation appears to be more effective in reducing the pituitary-adrenal response in the mother than in the infant.

Behavioral and adrenocorticoid responsiveness of squirrel monkeys to a live snake: Is flight necessarily stressful?

Bolivian and Guyanese squirrel monkeys were assessed for responses to a live boa constrictor snake and to the novelty/disturbance associated with this presentation. Wild-born animals from adult groups which contained males, females, and pregnant females were tested during both individual and group conditions. The latter condition involved placing the empty or snake-occupied wire-mesh stimulus box on top of the groups home cage. In the individual conditions, an animal was removed from the social group cage and placed alone in a vertically oriented test cage with the stimulus box on top. Spatial and behavioral measures were recorded during these 30-min experimental sessions, and blood samples for plasma cortisol analysis were obtained at the end of each weekly session. Blood samples were also taken during undisturbed conditions 1 week before and after the experimental period. All monkeys revealed agitated behavior in the presence of the snake. In contrast, the cortisol levels in response to the snake and empty box were not different for any group of animals. However, the mean of the two individual tests was greater than the mean of the group tests for males, females, or pregnant females. These data support previous studies which indicate that wild-born squirrel monkeys clearly respond with behavioral distress to a live snake. Although cortisol elevations were not specifically produced by exposure to the snake, the responsiveness of the squirrel monkey pituitary—adrenal system to novelty was demonstrated, as all types of monkeys showed significant elevations to the disturbance/novelty of the individual tests.

Adrenal responses to reinforcement and extinction: Role of expectancy versus instrumental responding.

Plasma corticosterone levels were evaluated during operant conditioning in order to determine the effect of reinforced and nonreinforced responding (extinction) on adrenal activation. The influence of instrumental responding was assessed by comparing trained rats with yoked subjects that received a matched reward schedule in the absence of an operant task. Reinforcement sessions resulted in a significant decrease in adrenal secretion at 20 min, but not at 5 min, whereas extinction caused a rapid increase in corticosterone levels at 5 min and an even greater elevation by 20 min. Comparison of the operant and yoked subjects showed that this effect of reinforcement and extinction was not dependent on instrumental responding, but rather on the receipt or withdrawal of the expected reward.