Chronic morphine-induced changes in mu-opioid receptors and G proteins of different subcellular loci in rat brain

Prolonged exposure to opioid agonists can induce adaptive changes resulting in tolerance and dependence. Here, rats were rendered tolerant by subcutaneous injections of increasing doses of morphine from 10 to 60 mg/kg for 3, 5, or 10 consecutive days. Binding parameters of the mu-opioid receptor in subcellular fractions were measured with [(3)H]DAMGO ([D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin). Although the density of surface mu-sites did not change after the 5-day morphine treatment, up-regulation of synaptic plasma membrane binding was detected after the 10-day drug administration. In contrast, the number of mu-binding sites in a light vesicle or microsomal fraction (MI) was elevated by 68 and 30% after 5 and 10 days of morphine exposure, respectively. The up-regulated MI mu-sites displayed enhanced coupling to G proteins compared with those detected in saline-treated controls. Pertussis toxin catalyzed ADP ribosylation, and Western blotting with specific antisera was used to quantitate chronic morphine-induced changes in levels of various G protein alpha-subunits. Morphine treatment of 5 days and longer induced significant increases in levels of Galpha(o), Galpha(i1), and Galpha(i2) in MI fractions that are part of an adaptation process. Up-regulation of intracellular mu-sites may be the result of post-translational changes and in part de novo synthesis. The results provide the first evidence that distinct regulation of intracellular mu-opioid receptor G protein coupling and G protein levels may accompany the development of morphine tolerance.

Receptor binding and G-protein activation by new Met5-enkephalin-Arg6-Phe7 derived peptides

Met5-enkephalin-Arg6-Phe7 (Tyr-Gly-Gly-Phe-Met-Arg-Phe, MERF) is a naturally occurring heptapeptide that binds to opioid and non-opioid recognition sites in the central nervous system. Four synthetic analogs with single or double amino acid substitutions were prepared by solid phase peptide synthesis to achieve proteolytically more stable structures: Tyr-D-Ala-Gly-Phe-Met-Arg-Phe (I), Tyr-D-Ala-Gly-Phe-D-Nle-Arg-Phe (II), Tyr-D-Ala-Gly-Phe-L-Nle-Arg-Phe (III) and Tyr-Gly-Gly-Phe-L-Nle-Arg-Phe (IV). In this study receptor binding characteristics and G-protein activation of MERF and its derivatives were compared in crude membrane fractions of frog and rat brain. Synthetic MERF-derived peptides were potent competitors for [3H]MERF and [3H]naloxone binding sites with the exception of analog (II) which turned to be substantially less active. The presence of 100 mM NaCl or 100 microM 5'-guanylylimidodiphosphate, Gpp(NH)p, decreased the affinity of the peptides in [3H]naloxone binding assays, suggesting that these ligands might act as agonists at the opioid receptors. Some of the compounds were also used to stimulate guanosine-5'-O-(3-[gamma-[35S]thio)triphosphate ([35S]GTPgammaS) binding in rat and frog brain membranes at concentrations of 10(-9)-10(-5) M. The EC50 values of analog (II) were the highest in both tissues. Analog (I) was as effective as MERF in rat brain membranes, but showed lower maximal stimulation in frog brain preparation. Again, analog (II) seemed to be the least efficacious peptide that stimulated [35S]GTPgammaS binding only by 59%. Specificity of the peptides was further investigated by the inhibition of agonist-stimulated [35S]GTPgammaS binding in the presence of selective antagonists for the opioid receptor types. The mu-selective antagonist cyprodime displayed the lowest potency in inhibiting the effects of the peptides, whereas norbinaltorphimine (kappa-selective antagonist) and naltrindole (delta-selective antagonist) were quite potent in both tissues. We concluded that MERF and its derivatives are able to activate G-proteins mainly via kappa- and delta-opioid receptors.

Expression of G-protein subtypes in cultured cerebral endothelial cells

This paper describes Western-blotting evidence for the presence of various guanine nucleotide binding proteins, G-proteins in cultured rat cerebral endothelial cells (CECs) and two immortalized cerebral endothelial cell lines, RBE4 and GP8. By using specific antibodies raised against known sequences of appropriate G-protein types that were previously characterized, we demonstrated the presence of Gsalpha, Gi2alpha, Gi3alpha, Gq/11alpha, Goalpha and Gbeta in cell lysates of primary cultures of CECs, and plasma membranes of RBE4 and GP8 cells. The appearance of Goalpha proteins in CECs might be of special importance, since they were not detected in peripheral endothelial cells in previous studies. Isoproterenol and bradykinin displayed significant, dose-dependent stimulation of [35S]GTPgammaS binding above basal values. This assay, reflecting the GDP-GTP exchange reaction on Galpha-subunits by receptor agonists, suggested that there were functional, G-protein coupled beta-adrenergic and bradykinin receptors in these systems. No significant stimulation of [35S]GTP7gammaS binding was noted with serotonin under our experimental conditions. Since stimulation of [35S]GTPgammaS binding by isoproterenol and bradykinin was additive, it was concluded that different Galpha proteins were activated by these two ligands. In analogy to other systems, activation of Gs is most likely by isoproterenol, while Gi and/or Gq/11 proteins might be activated by bradykinin receptors. The possible significance of the receptors and G-proteins detected is being discussed in the functioning of cerebral endothelium, and thus the blood-brain barrier.