Fig. 11. Schematic illustrating sex differences in the hippocampal opioid system in Sal- and Oxy-CIS rats.

Arrows indicate predicted effects (upward = increased; downward = decreased) of DOR and MOR trafficking changes on excitation (plus signs) and inhibition (minus signs) in the CA3 and dentate gyrus (DG). #1–4 indicate points where there are differences in the distribution of DORs and MORs in Sal- and Oxy-CIS rats. gc, granule cell; hil, hilus; LPP, lateral perforant path; MF, mossy fiber; pc, pyramidal cell; slu, stratum lucidum; sr, stratum radiatum. Blue color = MORs; Green color = LEnk levels; orange color = DORs. Circles = plasmalemmal receptors; squares = near-plasmalemmal receptors; crosses = cytoplasmic receptors.

1. Consistent with previous studies (Mazid et al., 2016; Pierce et al., 2014), the levels of LEnk in mossy fibers are comparable in Sal-CIS females and Sal-CIS males. Sal-CIS females compared to Sal-CIS males have fewer cytoplasmic and total DORs in CA3b pyramidal cell dendrites. However, Sal-CIS females compared to Sal-CIS males have more DORs in mossy fiber – CA3 synapses suggesting mechanisms for promoting opioid-mediated LTP are still in place in females following CIS. Prior studies have shown that CIS results in the retraction of CA3 dendrites in male rats (McEwen, 1999).

2. CA3b mossy fiber LEnk levels in both Oxy-CIS females and Oxy-CIS males are unaltered compared to Sal-CIS rats. In Oxy-CIS females, the density of cytoplasmic and total DORs in CA3 dendrites increases so that they are comparable to that seen in Sal- and Oxy-CIS males. Moreover, the density of DORs in mossy fiber – CA3 synapses remains elevated in Oxy-CIS females and remains low in Oxy-CIS males. These findings indicate that the redistribution of DORs in CA3 pyramidal cells in CIS females is similar to that reported previously in unstressed females (Ryan et al., 2018).

3. Sal-CIS females compared to Sal-CIS males have elevated cytoplasmic and total DORs in the dendrites of GABAergic-labeled interneurons previously shown to colocalize SOM/NPY (Commons and Milner, 1996; Williams and Milner, 2011). These findings suggest a greater capacity for synthesis or storage of DORs in GABAergic interneurons. Sal-females compared to Sal-males have no differences in the density of MORs in any compartment in the dendrites of PARV-containing interneurons. However, prior studies have shown that the number of PARV-labeled interneurons is reduced by about 30% in males, but not females, following CIS (Czeh et al., 2005; Hu et al., 2004; Milner et al., 2013), suggesting that the redistribution of MORs occurs in a subset of PARV interneurons.

4. Cytoplasmic and total DORs in GABA-labeled dendrites remain elevated in Oxy-CIS females compared to Oxy-CIS males. The number of dual labeled MOR/PARV cells is decreased in Oxy-CIS females compared to Sal-CIS females. However, the cytoplasmic and total density of MORs is elevated in Oxy-CIS females compared to Sal-CIS females, suggesting a greater capacity for synthesis or storage of MORs in a subset of PARV interneurons. In Oxy-CIS males compared to Sal-CIS males, plasmalemmal MORs are elevated in large PARV-labeled dendrites, suggesting that a subset of PARV neurons could have greater disinhibitory responses to MOR agonists.

Together, these results demonstrate that CIS females, but not males, acquire CPP to oxycodone and CIS “primes” the hippocampal opioid system in females for oxycodone-associated learning. They also suggest that low levels of DORs in mossy fiber – CA3 synapses and hilar GABAergic interneurons may contribute to the lack of displayed oxycodone CPP in CIS males.