In using opiate receptor blockades, which decrease the level of beta-endorphins, the monkeys responded with an increased desire to be groomed. In contrast, when the monkeys were given morphine, the desire to be groomed dropped significantly. Beta- endorphins have been difficult to measure in animal species, differently from oxytocin which can be measured by sampling cerebrospinal fluid, and therefore have not been linked as strongly with social behaviors. Glucocorticoids receptors Glucocorticoids are steroid hormones that are synthesized in the adrenal cortex and are a part of the group of corticosteroids. Glucocorticoids are involved in immune function, and are a part of the feedback system that reduces inflammation. Further, glucocorticoids are involved in glucose metabolism. Studies in macaques have shown that increased social stress results in glucocorticoid resistance, further inhibiting immune function. Macaques who participated in social grooming showed decreased levels of viral load, which points toward decreased levels of social stress resulting in increased immune function and glucocorticoid sensitivity. Additionally, an article published in 1997 concluded that an increase in maternal grooming resulted in a proportionate increase in Glucocorticoid receptors on target tissue in the neonatal rat. In the study on neonatal rats, it was found that the receptor number was altered because of a change in both serotonin and thyroid-stimulating hormone concentrations. An increase in the number of receptors might influence the amount of negative feedback on corticosteroid secretion and prevent the undesirable side effects of an abnormal physiologic stress response. Social grooming can change the number of glucocorticoid receptors, which can result in increased immune function. Studies have also shown that male baboons who participate more in social grooming show lower basal cortisol concentrations. Faecal glucocorticoid (fGCs) is a hormone metabolite associated with stress that is seen to be present in lower levels in female baboons with stronger, well-established grooming networks. When potentially infanticidal male baboons immigrate into a group, the females' fGC levels are seen to rise, indicative of higher stress; however, females with reliable and well-established grooming partners have less of a fGC rise than those with weaker grooming networks. Hence, the social support received from a 'friendship' aids baboons in stress management. Similarly, fGC levels are also seen to rise in females when a close 'friend' dies; however, these rising fGC levels are seen to decrease in females that form new grooming partners, replacing their deceased friends. Opioids Endogenous opioids are chemical molecules produced in