Loyd DR, Wang X, Murphy AZ (2008) Sex differences in micro-opioid receptor expression in the rat midbrain periaqueductal gray are essential for eliciting sex differences in morphine analgesia. J Neurosci 28:14007-14017. doi: 10.1523/JNEUROSCI.4123-08.2008
Summary: The authors test whether the periaqueductal gray (PAG), that contains a dense population of µ-opioid receptor (MOR)-expressing neurons, is sexually dimorphic. Rats were injected with 3 pmol of Dermorphin-SAP (Cat. #IT-12) into the PAG. Blank-SAP (Cat. #IT-21) was used as a control. Both behavioral and immunohistochemical evidence suggest that differential expression of MOR-expressing neurons in the PAG between male and female rats accounts for the difference in response to morphine.