Acute and chronic morphine treatments and morphine withdrawal differentially regulate GRK2 and GRK5 gene expression in rat brain

Opioid agonist stimulates activation of G protein-coupled receptor kinase (GRK) and causes desensitization of opioid signaling, which plays an important role in opioid tolerance. The current study investigated the potential regulatory effects of acute and chronic morphine administration and withdrawal on GRK2 and GRK5 gene expression in rat brain. Our results showed that the initial morphine treatment (10 mg/kg) significantly increased GRK mRNA levels in cerebral cortex, hippocampus, and lateral thalamic nuclei. A significant decrease in GRK5 mRNA levels was observed in periaqueductal gray. In strong contrast, repeated administration of morphine for 9 days failed to cause any significant increase in GRK5 mRNA in any of these brain regions. Chronic morphine treatment resulted in 30-70% down-regulation of GRK2 expression in cerebral cortex, hippocampus, thalamus, and locus coeruleus, opposite to what observed with the single morphine administration. Moreover, spontaneous and naloxone-precipitated morphine withdrawal resulted in aberrant increases in GRK2 and GRK5 mRNA levels in these brain regions. Taken together, our study suggests that opioid not only induces rapid negative feedback regulation on opioid signals through activation of GRK but also exerts its impact, via controlling levels of GRK gene expression, on the regulatory machinery itself over a longer period of time in brain.