GTPgammaS incorporation in the rat brain: a study on mu-opioid receptors and CXCR4

Chemokine and opioid receptors are G-protein-coupled receptors that play important roles in both the central nervous system and the immune system. The long-term goal of our research is to establish whether opioids regulate the activity of the chemokine receptor CXCR4 (one of the major HIV co-receptors) in the brain. In this research, we studied the anatomical distribution of functional receptors in young and adult animals by using the [(35)S]GTPgammaS "binding" assay as an indication of G-protein activation by CXCL12 (the natural CXCR4 ligand) or by mu-opioid agonists. Brain slices or homogenates from Holtzmann rats of different ages (from 2 to 21 days old and adult animals) were treated with CXCL12 (0.001-100 nM), D: -ala2,MePhe4,gly-ol5]enkephalin (DAMGO; 0.0003-10 microM) or morphine (0.0003-10 microM) and then processed for the assay. Our results show stimulation of both mu-OR and CXCR4 in several brain areas, including cortex and hippocampus (p < 0.001); this effect is dose and age dependent, and the magnitude of response varies among different brain regions. Furthermore, AMD3100 (100 ng/ml), a specific CXCR4 antagonist, abolished CXCL12 stimulation in all the brain regions analyzed (p < 0.001). Our findings suggest a similar pattern of expression for mu-OR and CXCR4 in the brain, supporting the possibility of an interaction between the two G-protein-coupled receptors in vivo. This might be relevant to the role of opiates in HIV neuropathogenesis.