Age-related changes in kappa opioid receptors in the guinea-pig brain: a quantitative autoradiographic study

Investigation into the effect of aging on kappa opioid receptors in the brains of guinea-pigs was carried out in animals aged one, six, 24 and 36 months. Quantitative autoradiography was used to monitor the concentration of kappa receptors in various anatomic regions at five rostrocaudal levels in each age-group. Areas of high binding were found in the deep layers (laminae V, VI) of the neocortex and in the internal band of the periallocortical, dorsal agranular insular cortex. Among non-cortical areas examined, the nucleus accumbens and the substantia nigra possessed kappa binding levels equal to those seen in the deep neocortical layers. In all cases where an age-related change in the level of kappa receptors was detected, the direction of the change was one of decreased binding with advancing age. Statistical analysis of the binding data revealed that the one-month-old animal possessed the highest levels of kappa binding among all age groups examined. The vast majority of age-related changes in kappa binding levels occurred in laminae V and VI of neocortical regions. The per cent decreases (18-42%), as well as their age of onset (six to 36 months) varied in different anatomical regions. Possible mechanisms to explain the age-related decreases in kappa opioid binding are presented. The majority of the age-related decreases in kappa opioid binding are found in areas of the neocortex which are characterized by their motor, sensory and associative functions. It is within these three areas of function that diminutions in performance are most apparent in senescence.