Influence of hypophysectomy on dopamine receptors and dopaminergic behaviors

Hypophysectomy (Hypox) has been proposed to alter the behavioral and biochemical indices of striatal dopamine (DA) function. Since the regulation of striatal DA receptors by hormones may involve the pituitary, it was relevant to reevaluate the effects of Hypox in male and female rats. Behaviorally, dopaminergic agonists exerted enhanced activity in Hypox male and female rats. It has been suggested that these changes are due to altered metabolism since no increase in the DA receptor populations was observed. Dopaminergic antagonists showed equivalent behavioral actions in male and female rats, whether intact or Hypox. Biochemically, neither the density nor the affinity of the striatal DA receptors is altered by Hypox in female rats for 1-2 weeks or male rats for 5-6 weeks. However, in female rats at 5-6 weeks after Hypox there is a significant decrease in receptor number. This decrease in density is not reflected in behavioral changes to either DA receptor agonists or antagonists. Therefore, all dopaminergic behavioral changes do not result from alterations in DA receptors and changes in DA receptors do not necessarily dictate altered behavioral responses to dopaminergic agents.