Restraint inhibits luteinizing hormone and testosterone secretion in intact male rhesus macaques: Effects of concurrent naloxone administration

The present study investigates how restraint affects the hypothalamo-hypophysial adrenocortical axis and the hypothalamo-hypophysial gonadal axis in intact, adult male rhesus macaques. Restraint was chosen because it is not physically painful or harmful to the animal, but rather serves both as a physical and psychological stressor. Blood samples were collected from a remote site at 15-min intervals beginning at 07.00 h from tethered adult male rhesus macaques. Each of 4 animals was subjected to 6 h of chair restraint after a 3-hour control period in the animals’ home cage. Samples were collected for an additional 6 h at the end of the restraint period when the animal was returned to its home cage. Brief anesthesia with ketamine (administered through the indwelling catheter) facilitated transfer of the animals to and from the chair. Blood samples were collected from 4 undisturbed males to document LH and testosterone secretion throughout the day. Plasma ACTH and cortisol, measured as indexes of stress, were elevated within 15 min after initiation of restraint and remained elevated for most of the restraint period. Conversely, LH and testosterone began to fall immediately after restraint and remained suppressed for several hours after the animals were removed from restraint and returned to their home cage. Testosterone levels were more consistently inhibited than were LH levels, a reflection of the fact that in some animals, testosterone remained low after the return of pulsatile LH secretion. In studies with naloxone (Nx), the opiate receptor antagonist (5 mg bolus plus 5 mg/h) was given beginning either at the initiation of restraint (n = 2) or 2 h thereafter (n = 2), and continued until the end of the restraint period. With Nx treatment of the restrained animals, both ACTH and cortisol were elevated as in the controls and LH and testosterone secretion were significantly increased within 1-2 h. However, after the Nx treatment was terminated and the animals were returned to their home cages, plasma levels of LH and testosterone were not different from levels in restrained animals and were significantly less than levels in untreated animals. These data show that restraint is a potent stimulus for activation of the HPAC axis and inhibits both LH and testosterone release. The pathway through which restraint inhibits LH release probably includes endogenous opiate suppression of hypothalamic GnRH release since Nx partially blocks the effect of stress.