The influences of reproductive status and acute stress on the levels of phosphorylated delta opioid receptor immunoreactivity in rat hippocampus

In the hippocampus, ovarian hormones and sex can alter the trafficking of delta opioid receptors (DORs) and the proportion of DORs that colocalize with the stress hormone, corticotropin releasing factor. Here, we assessed the effects of acute immobilization stress (AIS) and sex on the phosphorylation of DORs in the rat hippocampus. We first localized an antibody to phosphorylated DOR (pDOR) at the SER363 carboxy-terminal residue, and demonstrated its response to an opioid agonist. By light microscopy, pDOR-immunoreactivity (ir) was located predominantly in CA2/CA3a pyramidal cell apical dendrites and in interneurons in CA1-3 stratum oriens and the dentate hilus. By electron microscopy, pDOR-ir primarily was located in somata and dendrites, associated with endomembranes, or in dendritic spines. pDOR-ir was less frequently found in mossy fibers terminals. Quantitative light microscopy revealed a significant increase in pDOR-ir in the CA2/CA3a region of male rats 1 h following an injection of the opioid agonist morphine (20 mg/kg, I.P). To look at the effects of stress on pDOR, we compared pDOR-ir in males and cycling females after AIS. The level of pDOR-ir in stratum radiatum of CA2/CA3a was increased in control estrus (elevated estrogen and progesterone) females compared to proestrus and diestrus females and males. However, immediately following 30 min of AIS, no significant differences in pDOR levels were seen across estrous cycle phase or sex. These findings suggest that hippocampal levels of phosphorylated DORs vary with estrous cycle phase and that acute stress may dampen the differential effects of hormones on DOR activation in females.