The pathophysiology of self -injurious behavior in rhesus macaques

Some individually housed laboratory monkeys spontaneously develop self-injurious behavior (SIB). In rhesus macaques, this pathology usually takes the form of intense self-directed biting that, on occasion, can result in wounding severe enough to require veterinary treatment. Little is known about the underlying pathophysiology of SIB in socially reared monkeys. Most investigations of SIB have focused on monkeys that were reared in total social isolation, a form of deprivation that results in numerous abnormalities besides SIB. The objective of this dissertation was to characterize and identify the underlying pathophysiology of SIB in a group of socially reared, individually housed rhesus monkeys. Central monoamine systems, hypothalamic-pituitary-adrenal (HPA) and gonadal HPG) axes, and sympatho-adrenomedullary system (SAMS) activity were compared in monkeys with a veterinary record of self-inflicted wounding (SIB group) to that of control monkeys with similar rearing histories and ages (Control group). In addition to our focus on wounding, we examined the pathophysiology of SIB as it relates to the frequency of self-directed biting. Monkeys with SIB did not differ in their basal central monoamine activity from controls. Likewise, no group differences in basal gonadal or SAMS activity were found. However, monkeys with SIB did show persistently lower plasma cortisol levels following the stress of restraint and venipuncture, but not in 24-h urine samples collected under stress-free conditions. It was concluded that monkeys with SIB are hyporesponsive to the mild stress of blood sampling. Hypersensitivity in HPA negative feedback or reduced adrenocortical sensitivity were assessed as possible mechanisms for the observed attenuated stress response. Monkeys with SIB did not differ from controls in their response to a low dose of dexamethasone or to pharmacological challenge with ACTH. In contrast, when the relationship between HPA dysregulation and self-directed biting was assessed, significant negative correlations between self-biting and cortisol were found in post-stress blood samples and in response to ACTH, but not in 24-h urine samples. These data suggest that monkeys with high rates of self-biting show a blunted HPA response to stress. The role of early psychological and/or physical trauma as possible factors in the development of SUB and HPA dysregulation are discussed