The effects of opioids such as morphine are mediated
The effects of opioids such as morphine are mediated through the opioid receptors. Opioid receptors are widely expressed both peripherally and in the CNS (Chen et al., 1993, Evans et al., 1992). The opioid agonist morphine has been shown to amplify the effect of the tubular cell-gp120 interaction on the proliferation of kidney fibroblasts (Singhal et al., 1998) and to potentiate gp120-induced neuronal apoptosis (Hu et al., 2005). Morphine significantly downregulated the gene expression of the beta chemokine, MIP-1 beta, while reciprocally upregulating the gene expression of its specific receptors, CCR3 and CCR5, suggesting that mu opioids can increase HIV-1 coreceptor (Mahajan et al., 2005). HIV gp120 and morphine alter mu opiate receptor expression in human vascular endothelium (Cadet et al., 2001). Long-term exposure of human blood vessels to HIV gp120, morphine, and anandamide increases endothelial adhesion of monocytes, and decreases nitric oxide release (Stefano et al., 1998).
The PAG, a critical sglt area in processing pain signals, is a primary site of action of many analgesic compounds (Basbaum and Fields, 1984, Behbehani and Fields, 1979, Vaughan et al., 2003, Xin et al., 1997). CXCR4 immunoreactivity (Banisadr et al., 2003) and colocalization between CXCR4 is expressed in 23 regions of the brain, including hypothalamus, by a semiquantitativeimmunohistochemical analysis (Van der Meer et al., 2000) CXCR4 and mu opioid receptor (Heinisch et al., 2011) have been observed in the PAG. In both in vitro and in vivo experiments in rats, it was found that in PAG, binding at receptors for selected chemokines can desensitize opioid receptors, resulting in a decrease in the in vivo analgesic responses in rodents to morphine (Chen et al., 2007, Adler and Rogers, 2005, Zhang and Oppenheim, 2005, Zhang et al., 2004). This occurs by a process of heterologous desensitization in which activation of the chemokine receptor blocks the signaling of the opioid receptor. Indeed, we have shown that the direct infusion of CCL5 or CXCL12 in the PAG can block analgesic actions of morphine (Chen et al., 2007, Adler and Rogers, 2005). Through the process of heterologous desensitization, chemokine ligands for CXCR4 and CCR5 can apparently inactivate the signaling pathway involved in reducing the sensation of pain. Because gp120 binds and activates these receptors, the possibility exists that gp120, via CXCR4 activation, desensitizes the opioid receptors, resulting in a reduction of morphine’s effect. In addition to diminishing analgesia, the chemokine receptors may also decrease unwanted side effects of opioids through chemokine desensitization of selected opioid pathways.
Although no data are available regarding the in vivo effect of gp120 on the physiological function of morphine, our recent studies showed the capacity of chemokines, particularly CXCL12 or CCL5, to attenuate mu-opioid receptor function and diminish the analgesic effect of morphine (Adler et al., 2006, Heinisch et al., 2011). As CXCL12 and HIV-1 coat protein gp120 can bind and activate the CXCR4, we hypothesized that the presence of gp120 in the brain would diminish the analgesic activities of morphine. On the basis of the above studies showing the relationship among gp120, chemokines and opioids, investigations into whether gp120 itself has an effect and whether gp120 microinjected into the PAG interferes with the antinociception induced by morphine were conducted in the cold-water tail-flick test and in the hot-plate test. Parallel electrophysiology studies were also conducted in PAG slices in order to investigate gp120-morphine interactions at the single-cell level.
Materials and methods
Discussion Gp120 is implicated in the pathogenesis of neurological disorders associated with HIV and is capable of initiating neurotoxic cascades via an interaction with the CXCR4 and/or CCR5 chemokine receptors. It can mimic many of the behavioral and physiological dysfunctions that are characteristic of HIV infection, including pain (Milligan et al., 2000). It has been linked to genesis of neuropathic pain (Oh et al., 2001) and is capable of producing thermal hyperalgesia and mechanical allodynia (Milligan et al., 2000). Many studies have revealed a relationship between the opioids and the HIV type 1 coat protein, gp120. While there are many mechanisms by which opioids could affect HIV replication, there is also the possibility, heretofore only minimally explored, that HIV and its products also affect the physiological functions of opioids. To our knowledge, no one has previously examined the in vivo physiological consequences of the presence of gp120 in the brain on the effect of morphine, particularly in terms of pain. In the present studies, we investigated the effect of gp120 on morphine-induced analgesia. First, our data showed that gp120 itself microinjected directly into the PAG does not alter antinociceptive thresholds in the CWT test. Other investigators have reported that intrathecal administration of gp120 activates astrocytes and microglia to release products that induce thermal hyperalgesia (Milligan et al., 2001, Milligan et al., 2000). Those studies utilized the radiant heat tail-flick and Hargreaves tests, intrathecal injection and higher doses of gp120. However, although by itself it did not affect antinociceptive thresholds, gp120 given directly into the PAG was able to attenuate significantly morphine-induced antinociception (given either peripherally or directly into the PAG). Our present data show that gp120 diminished the morphine-induced analgesia in both the cold-water (−3°C) tail-flick test and the hot-plate (+54°C) test.