Preliminary study of the effects of morphine treatment on opioid receptor gene expression in brain structures of the female rat

Opioid receptors play an important role in female physiology. They modulate directly and indirectly neuroendocrine phenomena that influence pregnancy maintenance, pain threshold, parturition, lactation, maternal behavior, rewarding and addiction. Thus understanding the gene expression levels of the three major opioid receptors, μ, δ and κ in different brain regions is essential for investigating dynamic mechanisms of opioidergic transmission. Adult virgin female rats were treated acutely with morphine sulfate (3.5 mg/kg or 20 mg/kg s.c.) or chronically for 5 days (3.5 mg/kg). Rats were killed 1 h after the last injection. In the acute treatment, expression levels for the encoded μ-opioid receptor Oprm1, as detected by reverse transcription–polymerase chain reaction, were significantly decreased in the periaqueductal gray. In chronic treatment, both Oprk1 and Oprm1 expression levels, that encoded κ and μ-opioid receptor respectively, showed significant decreases in the periaqueductal gray and striatum. Regional changes in opioid receptor gene expression levels might reflect highly specialized roles for these receptors with possible functional meaning for the plasticity of the opioidergic transmission.