Sex and chronic stress differentially alter phosphorylated mu and delta opioid receptor levels in the rat hippocampus following oxycodone conditioned place preference

Abstract

Following oxycodone conditioned place preference (CPP) in naïve female and male Sprague Dawley rats, delta- and mu-opioid receptors (DORs and MORs) redistribute in hippocampal CA3 pyramidal cells and GABAergic interneurons in a manner that would promote opioid-associative learning processes, particularly in females. MORs and DORs similarly redistribute in CA3 and hilar neurons following chronic immobilization stress (CIS) in females, but not males, essentially "priming" the opioid system for oxycodone-associative learning. Following CIS, only females acquire oxycodone CPP. The present study determined whether sex and CIS differentially affect the levels of phosphorylated MORs and DORs (pMORs and pDORs) in the hippocampus following oxycodone CPP as phosphorylation is important for opioid receptor internationalization and trafficking. In naïve oxycodone-injected (Oxy) female rats, the density of pMOR-immunoreactivity (ir) was increased in CA1 stratum oriens and CA3a,b strata lucidum and radiatum compared to saline-injected (Sal)-females. Additionally, the density of pDOR-ir increased in the pyramidal cell layer and stratum radiatum of CA2/3a in Oxy-males compared to Sal-males. In CIS females that acquire CPP, pDOR-ir levels were increased in the CA2/3a. These findings indicate only rats that acquire oxycodone CPP have activated MORs and DORs in the hippocampus but that the subregion containing activated opioid receptors differs in females and males. These results are consistent with previously observed sex differences in the hippocampal opioid system following Oxy-CPP.

Keywords: Drug addiction; Hippocampus; Learning; Opioid receptors.

Fig 1:. pMOR levels are increased in naïve Oxy-females compared to Sal-females in CA1 and CA3b.

A. pMOR-ir is most dense in the SLu of CA3a, b and c and less dense in the SO and SR of the CA1 and CA3. Sparse diffuse pMOR-ir is seen in the central hilus of the dentate gyrus (DG). B,C. Representative images show pMOR-ir in the SO, PCL, SR, and SLM of CA1 in a Sal-female (B) and an Oxy-female (C) rat. D. The density of pMOR-ir increases in the SO of CA1 in Oxy-females compared to Sal-females. E,F. Representative images of pMOR-ir in the SR, SLu, PCL, and SO of CA3b in a Sal-female (E) and an Oxy-female (F) rat. G. The density of pMOR-ir increases in the SLu and SR of CA3b in Oxy-females compared to Sal-females. Scale bar A = 250 μm; B,C,E,F = 100 μm; *p < 0.05.

Fig 2:. pDOR levels are increased in naïve Oxy-males compared to Sal-males and in CIS Oxy-females compared to CIS Sal-females in CA2/3a.

A. pDOR-ir is most dense in the pyramidal cell layer and SR of CA2/3a. B,C. Representative images show pDOR-ir in the SR, PCL, and SO of CA2/3a in a Sal-male (B) and an Oxy-male (C) rat. D. The density of pDOR-ir increases in the PCL and SO of CA2/3a in Oxy-males compared to Sal-males. E,F. Representative images of pDOR-ir in the SR, PCL, and SO of CA2/3a in a CIS Sal-female (E) and a CIS Oxy-female (F) rat. G. The density of pDOR-ir increases in the SR of CA2/3a in CIS Oxy-females compared to CIS Sal-females. Scale bar A = 250 μm; B,C,E,F = 100 μm; *p < 0.05; **p < 0.01.