Sex differences in delta opioid receptor immunoreactivity in rat medial amygdala.

Although gonadal hormones can modulate the opioid system in several limbic structures, it remains unclear if there are sex differences in amygdalar opioid systems. This study compared enkephalin immunoreactivity, delta opioid receptor immunoreactivity (DORir), and mu opioid receptor immunoreactivity in the amygdala of male and proestrous female rats. A striking sex difference in DORir was observed in the posterodorsal region of the medial nucleus of the amygdala, with males showing much greater DORir than females. No other sex differences were seen in amygdalar regions. Although the functional significance of this sexually dimorphic staining for DORir in medial amygdala is unknown, it may contribute to sex differences in reproductive functions, social behaviors, and/or stress responses.