Weaning-induced development of delta-opioid receptors in rat brain: differential effects of guanine nucleotides and sodium upon ligand-receptor recognition

Desensitization of δ-opioid receptors in nucleus accumbens during nicotine withdrawal

RATIONALE

The synthesis and release of met-enkephalin and β-endorphin, endogenous ligands for δ-opioid peptide receptors (DOPrs), are altered following nicotine administration and may play a role in nicotine addiction.

OBJECTIVES

To investigate the consequences of altered opioidergic activity on DOPr expression, coupling, and function in striatum during early nicotine withdrawal.

METHODS

Mice received nicotine-free base, 2 mg/kg, or saline, subcutaneously (s.c.), four times daily for 14 days and experiments performed at 24, 48, and 72 h after drug discontinuation. DOPr binding and mRNA were evaluated by [³H]naltrindole autoradiography and in situ hybridization. DOPr coupling and function were investigated by agonist pCl-DPDPE-stimulated [³⁵S]GTPγS binding autoradiography and inhibition of adenylyl cyclase activity.

RESULTS

During nicotine withdrawal DOPr binding was unaltered in caudate/putamen (CPu) and nucleus accumbens (NAc) shell and core. Receptor mRNA was slightly increased in the shell at 72 h, but significant elevations were observed in prefrontal cortex and hippocampus. pCl-DPDPE-stimulated [³⁵S]GTPγS binding was attenuated in NAc, but not CPu. In the shell, binding was decreased by 48 h and remained decreased over 72 h; while in the core, significant reduction was seen at 72 h. Basal adenylyl cyclase activity was suppressed in striatum at 24 h, but recovered by 48 h. DOPr stimulation with pCl-DPDPE failed to inhibit adenylyl cyclase activity at 24 h and produced attenuated responses at 48 and 72 h.

CONCLUSIONS

These observations suggest that DOPr coupling and function are impaired in the NAc during nicotine withdrawal. DOPr desensitization might be involved in the affective component of nicotine withdrawal.

Evidence for the functional expression and pharmacological characterization of adenine receptors in native cells and tissues

An orphan G protein-coupled receptor from rat has recently been discovered to be activated by the nucleobase adenine (Proc Natl Acad Sci USA 99:8573-8578, 2002). In the present study, we show for the first time that the adenine receptor is expressed in membrane preparations of native tissues and cell lines in high density, including rat brain cortex, rat brain striatum, and the mouse neuroblastoma x rat glioma hybrid cell line NG108-15. Saturation analysis with [3H]adenine at rat brain cortical membranes exhibited a single high-affinity binding site with a KD value of 27.2 nM, and a binding capacity of 2.28 pmol/mg of protein. Kinetic studies revealed unusual binding kinetics of [3H]adenine with rapid association and slow dissociation. A series of compounds were investigated in [3H]adenine competition experiments at rat brain cortex. Only minor substitution of the adenine structure was tolerated, the most potent compounds of the present series being 2-fluoroadenine (Ki value of 620 nM), 8-thioadenine (Ki value of 2.77 microM), N6-methyladenine (Ki value of 3.64 microM), and 7-methyladenine (Ki value of 4.13 microM), all of which were partial agonists (40-60% intrinsic activity). Adenine dose dependently inhibited forskolin-stimulated adenylate cyclase in membrane preparations of NG108-15 cells as well as in intact cells, showing that the receptor is functional in NG108-15 cells. Reverse transcriptase-polymerase chain reaction experiments followed by sequencing indicate that the NG108-15 cells express the murine ortholog of the adenine receptor. Moreover, preliminary radioligand binding studies with [3H]adenine at membranes of human astrocytoma 1321N1 cells suggest that a human ortholog of the rat adenine receptor exists.

G-protein coupling of delta-opioid receptors in brains of mu-opioid receptor knockout mice

Mu-opioid receptor knockout mice have been reported to show loss of some delta-opioid receptor function. We hypothesised that this is due to some delta-opioid receptors being uncoupled from G-proteins in the absence of the mu-opioid receptor. To address this possibility, we have used quantitative autoradiography to determine the binding of three delta-opioid receptor agonist ligands ([3H]deltorphin I, [3H] [R-Atc(3), Ile(5,6)]deltorphin II, [3H] 4-[(alpaR)-alpha-((2S,5R)-4-propyl-2,5-dimethyl-1-piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC-121)) and the delta-opioid receptor antagonist, [3H]naltrindole in the presence and absence of a GTP analogue, guanylylimidodiphosphate (GMPPNP) in the brains of mice lacking the mu-opioid receptor gene. Guanylylimidodiphosphate caused a decrease in the binding of the agonist ligands (to differing extents) and an increase in binding for the antagonist in wild-type controls. Overall, there were no major differences in the effects of guanylylimidodiphosphate for the agonist ligands in mu-knockout mice although a few structures showed a smaller sensitivity to the effects of this GTP analogue most notably for [3H]naltrindole. These findings suggest that in the majority of brain regions, G-protein coupling is unaltered in mu-opioid receptor knockout mice.

Opioid system diversity in developing neurons, astroglia, and oligodendroglia in the subventricular zone and striatum: impact on gliogenesis in vivo

Accumulating evidence, obtained largely in vitro, indicates that opioids regulate the genesis of neurons and glia and their precursors in the nervous system. Despite this evidence, few studies have assessed opioid receptor expression in identified cells within germinal zones or examined opioid effects on gliogenesis in vivo. To address this question, the role of opioids was explored in the subventricular zone (SVZ) and/or striatum of 2-5-day-old and/or adult ICR mice. The results showed that subpopulations of neurons, astrocytes, and oligodendrocytes in the SVZ and striatum differentially express mu-, delta-, and/or kappa-receptor immunoreactivity in a cell type-specific and developmentally regulated manner. In addition, DNA synthesis was assessed by examining 5-bromo-2'-deoxyuridine (BrdU) incorporation into glial and nonglial precursors. Morphine (a preferential mu-agonist) significantly decreased the number of BrdU-labeled GFAP(+) cells compared with controls or mice co-treated with naltrexone plus morphine. Alternatively, in S100beta(+) cells, morphine did not significantly decrease BrdU incorporation; however, significant differences were noted between mice treated with morphine and those treated with morphine plus naltrexone. Most cells were GFAP(-)/S100beta(-). When BrdU incorporation was assessed within the total population (glia and nonglia), morphine had no net effect, but naltrexone alone markedly increased BrdU incorporation. This finding suggests that DNA synthesis in GFAP(-)/S100beta(-) cells is tonically suppressed by endogenous opioids. Assuming that S100beta and GFAP, respectively, distinguish among younger and older astroglia, this implies that astroglial replication becomes increasingly sensitive to morphine during maturation, and suggests that opioids differentially regulate the development of distinct subpopulations of glia and glial precursors.

Endogenous opiates: 1998

This paper is the twenty-first installment of our annual review of research concerning the opiate system. It summarizes papers published during 1998 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.