Old rats are unresponsive to the behavioral effects of adrenocorticotropin

Abstract In 28 month-old male rats, the i.c.v. injection of adrenocorticotropin [ACTH-(1–24)] (4 μg/rat) did not induce the typical behavioral syndrome (excessive grooming, stretching, yawning, penile erections). This indicates that the behavioral effects of melanocortins are age-dependent, suggesting either an aging-linked impairment of the nervous circuitries involved or a reduction of the number (or affinity, or both) of the brain melanocortin receptors in the elderly.