Chronic ethanol modulates delta and mu-opioid receptor expression in rat CNS: immunohistochemical analysis with quantitiative confocal microscopy

Ethanol consumption affects levels of endogenous opioids as well as opioid receptors in both animals and humans. We studied the expression of delta (δ) and mu (μ) opioid receptors (ORs) in brain sections of adult male Sprague-Dawley rats after 2 weeks of consuming ethanol in a liquid diet, with comparisons to sections from pair-fed control animals. Immunohistochemical staining for the ORs, using selective antibodies, and quantitation of confocal images, revealed increased expression of δ-ORs in hippocampal CA1 of the chronic ethanol-treated rats. In contrast, μ-ORs decreased in their expression after ethanol treatment in multiple brain areas, including cortex, hippocampus, midbrain colliculi, striatum and nucleus accumbens. The alterations in immunoreactive OR expression may be related to reduced functional coupling of the ORs to G-proteins, as found in prior studies in several brain regions, using the same chronic ethanol diet protocol. Changes in OR expression and functional coupling in the CNS may be factors in ongoing ethanol consumption and tolerance.