ABC, avidin-biotin complex; AMPA receptors; BSA, bovine serum albumin; CIS, chronic immobilization stress; CPP, conditioned place preference; DAB, diaminobenzidine; DG, dentate gyrus; DOR, delta opioid receptor; Drug associative-learning; Electron microscopy; GABA, Gamma-amino butyric acid; GluA1, AMPA glutamate receptor subunit 1; GluN1, NMDA, glutamate receptor subunit 1; LTP, long-term potentiation; MOR, mu opioid receptor; NMDA receptors; NMDA, N-methyl-D-aspartate; NPY, neuropeptide Y; Oxy, oxycodone; PARV, parvalbumin; PB, phosphate buffer; PFA, paraformaldehyde; PM, plasma membrane; Pyramidal cells; ROI, region of interest; SLM, stratum lacunosum-moleculare; SLu, stratum lucidum; SO, stratum oriens; SOM, somatostatin; SR, stratum radiatum; Sal, saline; TS, tris-buffered saline; ir, immunoreactivity