INTERACTIONS AMONG AGING, GENDER, AND GONADECTOMY EFFECTS UPON NALOXONE HYPOPHAGIA IN RATS

The present study examined the dose-dependent (0.25-5 mg/kg) effects o f systemic naloxone upon deprivation-induced intake and high-fat intak e as functions of age (4, 8, 14, and 20 months), gender, and gonadecto my in rats. Significant increases in body weight were observed as func tions of age and gonadectomy. Whereas aging significantly reduced basa l deprivation-induced intake, it generally failed to alter basal high- fat intake. Whereas age, gender, and gonadectomy failed to alter the d ecreases in deprivation-induced intake following low (0.25-2.5 mg/kg) naloxone doses, sham males displayed significantly greater age-related and gender-related inhibition following the 5 mg/kg dose of naloxone. Young gonadectomized rats displayed significant increases in naloxone 's inhibition of deprivation-induced intake as well. More dramatic cha nges occurred in naloxone's inhibition of high-fat intake. Naloxone's potency increased in sham female rats as a function of age, and decrea sed in sham males and ovariectomized females as a function of age. Whe reas sham males and ovariectomized females were most sensitive to nalo xone's inhibition of high-fat intake at young ages, sham females were most sensitive at older ages. These data indicate that effects of age, gender, and gonadectomy upon naloxone-induced hypophagia dissociate a s a function of the type of intake. Because selective opioid antagonis t studies demonstrate that deprivation-induced intake is mediated by t he mu1 receptor and high-fat intake is mediated by kappa and mu2 recep tors, it is postulated that the differential effects of aging, gender, and gonadectomy variables upon opioid mediation of the two forms of i ntake may reflect their interaction with different opioid receptor sub types.