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NEUROPHARMACOLOGY
Department of Biochemistry, State University of New York, Downstate Medical Center, Brooklyn, New York (M.S., A.R.G.); Institute of Biochemistry, Biological Research Center of the Hungarian Academy of Sciences, Szeged, Hungary (M.S.); and Department of Medical Informatics, University of Szeged, Szeged, Hungary (K.B.)
Received for publication
February 10, 2004
Accepted
March 2, 2004.
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Abstract |
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-linked signaling is critical to the chronic morphine-induced enhanced facilitative action of CTAP. Interestingly, the (augmented) CTAP facilitation of FSK-stimulated AC activity that is observed in opioid tolerant (but not in naive) membranes is also sensitive to PTX. This can best be explained by postulating the involvement of Gi-derived G
, which would stimulate type 2 ACs, conditional on the presence of activated Gs. The emergence of a G dimension of AC stimulation by CTAP after chronic morphine could explain its ability to augment the stimulatory action of CTAP on AC. These results support putative MOR coupling to Gs and underscore the multifaceted nature and plasticity of MOR G protein coupling.
; Barker et al., 1994; Bond et al., 1995; Gether et al., 1995; Cetani et al., 1996; Smit et al., 1996). Inverse agonists are compounds that can attenuate the ability of GPCRs to activate their cognate G protein in the absence of agonist ligand and thereby produce effects that are opposite to those that usually result from receptor activation. Although the concept and existence of such compounds has become very well established and widely accepted, the underlying molecular mechanisms remain unclear.
The discovery of inverse agonists has forced reconsideration of the dynamics that influence receptor coupling to G proteins. Specifically, it has fostered renewed interest in multistate receptor activation models. In these formulations, receptors can exist in equilibrium between an inactive conformation, unable to couple to G proteins and an active conformation that is compatible with G protein coupling, which is spontaneously achieved in the absence of agonist (Leff, 1995a,b).
All opioid receptors belong to the superfamily of GPCRs. Most research to date has confirmed coupling predominantly with Gi/Go, the subunits of which are thought to be the predominant mediators of opioid action. This notwithstanding, there is also pharmacological evidence consistent with opioid receptor coupling to Gs (Shen and Crain, 1990; Gintzler and Xu, 1991; Wang and Gintzler, 1997), but this remains controversial. More recently, the importance of the G subunit of G proteins to opioid receptor-coupled signaling has become apparent (Chakrabarti et al., 1998a).
The plethora of G protein-coupled signaling strategies and pathways used by opioid receptors makes them ideal substrates for the study of inverse agonists (Chiu et al., 1996; Merkouris et al., 1997; Liu et al., 2001). Conversely, inverse agonists would seem to be a useful tool to probe opioid receptor functionality and G protein subunit mediators thereof.
Thus far, the study of opioid inverse agonists has been largely confined to interactions with the 
-opioid receptor. The present study investigates inverse agonist properties of a commonly used µ-opioid receptor (MOR) antagonist, D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP). Although previously characterized as a neutral antagonist in vivo (Bonner et al., 1997; Sterious and Walker, 2003) and in GH3 cells (Liu and Prather, 2001), the present study reveals that it manifests inverse agonism in Chinese hamster ovary (CHO) cells stably transfected with the µ-opioid receptor (MOR-CHO), underscoring the plasticity of inverse agonist responsiveness and its dependence on local cellular factors. Additionally, use of two bacterial toxins commonly used to assess receptor G protein coupling, pertussis toxin (PTX) and cholera toxin (CTX), provides further pharmacological evidence for µ-opioid receptor coupling to Gs and augmented µ-opioid receptor-coupled signaling via the G subunit after chronic morphine. The relevance of chronic morphine-induced adaptations to inverse agonist actions is discussed.
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Materials and Methods |
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-32P]ATP and [3H]cAMP were purchased from New PerkinElmer Life and Analytical Sciences (Boston, MA). Morphine sulfate, [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO), CTAP, (+)-naloxone-HCl, and (±)-naloxone-HCl were supplied by the National Institute on Drug Abuse (Rockville, MD). ATP, cAMP, GTP, phosphocreatine, creatine phosphokinase from rabbit muscle, MgCl2, NaCl, 3-isobutyl-1-methylxanthine, forskolin (FSK), bacitracin, benzamidine, aprotinin, leupeptin, trypsin-chymotrypsin inhibitor from soybean, HEPES, Na2HPO4, KH2PO4, and EDTA were from Sigma-Aldrich (St. Louis, MO). Cell Culture and Treatment. MOR-CHO cells (3.63 ± 0.32 pmol of MOR/mg protein assessed with [3H]naloxone) were maintained in Dulbecco's modified Eagle's medium high glucose with L-glutamine (Invitrogen, Carlsbad, CA) supplemented with 10% fetal calf serum (Nova-Tech Inc., Grand Island, NE), 1% penicillin/streptomycin (Invitrogen), and 3.2 ng/ml geneticin (Invitrogen). Cells were grown at 37°C in humidified atmosphere of 10% CO2, 90% air. For chronic morphine treatment, MOR-CHO cells at 50% confluence were treated with morphine (1 µM). Morphine was replenished 24 h later along with fresh media and cultured for an additional 24 h before harvest. For toxin treatments, cells were treated with either 100 ng/ml pertussis toxin, PTX (List Biological Labs., Inc. Campbell, CA) and/or 1 µg/ml cholera toxin, CTX (List Biological Laboratories Inc., Campbell, CA) for the last 24 h in culture unless otherwise stated. At the end of morphine and/or toxin exposure, cells were washed twice with ice-cold phosphate-buffered saline in the absence or presence of 1 µM morphine for opioid naive and tolerant preparations, respectively. Cells were harvested with phosphate-buffered saline containing 1 mM EDTA in the absence or presence of 1 µM morphine for opioid-naive and -tolerant preparations, respectively. Cell suspension was spun at 2500 rpm for 5 min, after which preparation of cell membranes commenced.
Membrane Preparation. Freshly collected cell pellets were homogenized with a Wheaton Teflon-glass homogenizer in 10 volumes (v/w) of ice-cold homogenization buffer, pH 7.4, composed of 25 mM HEPES, 1 mM EDTA, 0.5 mg/l aprotinin, 1 mM benzamidine, 100 mg/l bacitracin, 3.2 mg/l leupeptin, 3.2 mg/l soybean trypsin inhibitor, and 10% sucrose in the absence or presence of 1 µM morphine for opioid-naive and -tolerant preparations, respectively. Homogenates were spun at 1000g for 10 min at 4°C, and supernatant was collected. Pellets were suspended in one-half of the original volume of the homogenization buffer and centrifuged as described above. Combined supernatants from the two low-speed centrifugations were spun at 20,000g for 30 min. Opioid naive and morphine-treated cell pellets were taken up in appropriate volumes of homogenization buffer in the absence or presence of 100 nM morphine, respectively. Aliquots were stored at -80°C until use. Protein content was determined using bovine serum albumin as a standard with the Bradford assay (Bradford, 1976).
Adenylyl Cyclase (AC) Assay. AC activity was determined by measuring the synthesis of [-32P]cAMP from [-32P]ATP as published previously (Chakrabarti et al., 1998a, and references therein) with some modifications. Assay mixture contained 1 mM ATP, 1 µCi/tube [-32P]ATP, 10 µM GTP, 0.1 mM cAMP, 10 mM MgCl2, 100 mM NaCl, 5 mM creatine phosphate, 30 µg/tube creatine kinase, 1 U/tube myokinase, and 1 mM 3-isobutyl-1-methylxanthine in 50 mM HEPES buffer, pH 7.4. DAMGO, CTAP, naloxone, and FSK (1 µM) were also included where indicated. Reaction was initiated by the addition of appropriate MOR-CHO membrane preparations (50–100 µg protein/tube), incubated for 15 min at 30°C, and terminated by the addition of 0.1 N HCl. After boiling for 2 min, the samples were spun at 5000 rpm for seconds in an Eppendorf centrifuge and loaded onto columns containing alumina (WN-6 neutral; Sigma-Aldrich). Columns were eluted with 0.1 N ammonium acetate (Sigma-Aldrich), and radioactivity was determined in UltimaGold XR (PerkinElmer Life and Analytical Sciences) scintillation cocktail in an LKB Rack-beta (Amersham Biosciences Inc., Piscataway, NJ) liquid scintillation counter. [3H]cAMP (0.005 µCi/tube) was used as internal standard to correct for column recovery of cAMP.
Data Analysis. Unless otherwise stated, AC activity values assessed in the presence of 1 µM FSK, but in the absence of opioid ligands, are defined as 100% for each tissue treatment. Data reported are percentage of FSK and represent the mean ± S.E.M. for at least three experiments that were each performed in triplicate. When two groups were compared, significant differences between the means were determined by Student's t test. Statistical significance is set at p 
0.05. DAMGO dose-response curves were analyzed by nonlinear regression with the use of the GraphPad Prism program (GraphPad Software Inc., San Diego, CA) to obtain maximal inhibition and EC50 (the concentration of the ligand that elicits half-maximal effect) of FSK-stimulated AC activity.
Effects of CTAP doses (0, 0.1, 1, and 10 µM), condition (naive and tolerant), bacterial toxins (CTX and CTX + PTX), and interactions thereof were assessed by a three-way repeated measures analysis of variance (ANOVA) using SAS software (SAS Institute, Cary, NC). A mixed model ANOVA was performed supposing first-order autoregressive covariance structure between dose levels to compensate for unequal sample sizes. Post hoc comparisons were performed for condition and toxin effects based on estimated least-squares means.
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Results |
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PTX Reverses DAMGO Inhibition to a Facilitation of FSK-Stimulated AC Activity. PTX treatment of cells slightly enhanced the magnitude of FSK-stimulated AC activity from 36.6 ± 7.3 to 51.0 ± 4.1 pmol/mg protein/min. As expected, PTX treatment of MOR-CHO cells abolished the 
40 and 20% inhibitory effect of 1 µM DAMGO on FSK stimulation of AC activity in membranes obtained from opioid-naive or chronic morphine-treated MOR-CHO cells, respectively (Fig. 1B). Unexpectedly, however, in membranes obtained from opioid-naive cells, PTX not only blocked the DAMGO inhibition but also unmasked its ability to facilitate FSK activation of AC (128.9 ± 8.5%; p = 0.01; n = 8). In membranes obtained from chronic morphine-treated MOR-CHO cells, the DAMGO enhancement of FSK stimulation of AC activity that was unmasked by PTX was much more modest (111.5 ± 5.7%) and did not achieve statistical significance (p = 0.08; n = 8) (Fig. 1B). Importantly, the DAMGO stimulation of cAMP formation that was unmasked after treatment of opioid naive cells with PTX was no longer apparent in membranes obtained from cells that had been treated with a combination of PTX and CTX (Fig. 1C).
Effect of CTAP on FSK-Stimulated AC Activity. As expected, CTAP markedly reduced the DAMGO inhibition of FSK stimulated cAMP formation in opioid-naive membranes (Fig. 2, column A). Interestingly, however, in membranes obtained from chronic morphine-treated MOR-CHO cells, CTAP not only blocked the DAMGO inhibition of AC but also significantly enhanced the magnitude of FSK stimulation of AC activity by 134.4 ± 6.1% (p < 0.05; n = 10; Fig. 2, column C). In fact, even in the absence of opioid agonist, CTAP increased FSK-stimulated AC activity by 134.8 ± 7.3 and 185 ± 6.8% in opioid-naive and opioid-tolerant membranes, respectively (Fig. 2, columns B and D). These observations suggest that CTAP can behave as an inverse agonist in MOR-CHO cells.
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To probe the contribution of Gs-coupled signaling to the CTAP facilitation of AC activity and its augmentation by chronic morphine, dose responsiveness, and effects of CTX were assessed. As expected, after a 16-h incubation with CTX, FSK-stimulated AC activity was substantially enhanced (34.8 ± 4.9 pmol/mg protein/min, n = 10 versus 430 ± 7.2 pmol/mg protein/min, n = 5 before and after CTX treatment, respectively). ANOVA revealed significant main effects for dose of CTAP (F3,145 = 28.17; p < 0.001), condition (F1,73 = 3.86; p = 0.05), and toxin (F2,75 = 47.09; p < 0.05). The ANOVA also revealed significant dose by condition (F3,145 = 2.81; p < 0.05) and dose by toxin (F6,146 = 14.97; p < 0.0001) interactions. This indicates that CTAP dose responsiveness differs among membranes obtained from opioid-naive versus chronic morphine-treated cells and among membranes obtained from cells grown in the absence versus the presence of toxin (CTX versus CTX + PTX), respectively. Furthermore, the significant toxin by condition by dose interaction (F6,146 = 4.15; p = 0.0007) indicates that toxin treatment (CTX or CTX + PTX) alters the interaction between dose and condition.
Post hoc comparisons revealed that in the absence of toxin (Fig. 3A), the effect of CTAP differs between naive and tolerant membranes at the higher concentrations (1 and 10 µM). In contrast, after treatment with either CTX or CTX + PTX, differences in CTAP stimulatory responsiveness between naive versus chronic morphine-treated membranes were no longer discernible (Fig. 3, B and C). In opioid-naive membranes, CTX significantly (p 0.05) attenuated the inverse agonist effect of 1 and 10 µM CTAP. In contrast, this effect of CTX treatment was manifested at all CTAP concentrations in the tolerant membranes (p < 0.05). Pretreatment of MOR-CHO cells concomitantly with PTX as well as CTX produced an additional diminution in CTAP facilitative responsiveness to 1 and 10 µM CTAP in opioid-tolerant but not opioid-naive preparations.
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PTX Abolishes CTAP Facilitation of AC in Opioid Tolerant but Not Opioid Naive Preparations. Because the experiments described in Fig. 3 indicate that 1 µM CTAP was optimal for the manifestation of inverse agonism, this concentration was used to probe the contribution of Gi-coupled µ-opioid receptors to the CTAP facilitation of cAMP formation. PTX treatment did not significantly influence the stimulatory effect of 1 µM CTAP in the naive membranes (Fig. 4, first column). In contrast, however, treatment of chronic morphine-exposed membranes with PTX abolished the significant facilitative effect of CTAP on FSK-stimulated AC activity (without PTX: 185 ± 6.8, p < 0.001; with PTX: 111.5 ± 2.4%, p > 0.05; n = 8 for both) (Fig. 4, second column). This implies a role of G derived from Gi/Go G proteins in CTAP stimulation of AC activity in the tolerant, but not naive tissue.
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To explore the generality of the effect of chronic morphine exposure on the manifestation of inverse agonism, the prototypic general opioid receptor antagonist naloxone was also assessed for inverse agonist actions. (±)-naloxone, but not its pharmacologically inactive stereoisomer (+)-naloxone, enhanced AC stimulation by FSK. The stereoisomeric specificity of the observed effect underscores its mediation via opioid receptors (Fig. 5). In contrast to CTAP, however, statistically significant inverse agonism of (±)-naloxone was only manifested after chronic morphine treatment, consistent with a previous report (Wang et al., 2001).
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Discussion |
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/Go subunit of G proteins. This notwithstanding, CTX-sensitive, presumably Gs-mediated, excitatory opioid modulation of transmitter (methionine-enkephalin) release and cAMP formation have been reported in the guinea pig ileum myenteric plexus preparation (Gintzler and Xu, 1991; Wang and Gintzler, 1997) as has opioid modulation of cAMP formation in F-11 (neuroblastoma-dorsal root ganglion neuron) hybrid cells (Cruciani et al., 1993). The present findings are consonant with these reports. The observation that chronic morphine exposure alters Gs levels in a time-dependent manner in subcellular fractions of rat brain further supports the putative relevance of Gs to opioid receptor-coupled signaling (Fábián et al., 2002). Present results are also reminiscent of the PTX-induced reversal of opioid inhibition to a CTX-sensitive facilitation of electrically stimulated cAMP formation that has been reported in the guinea pig myenteric plexus (Wang and Gintzler, 1997). Notably, in the present study, abolition by CTX of facilitative effects of DAMGO on cAMP formation that are unmasked by PTX would eliminate putative contributions of Gq-coupled µ-opioid receptors to facilitative actions of DAMGO. Collectively, these data reinforce putative coupling of µ-opioid receptors to Gs as well as Gi. Thus, responses to DAMGO in the absence of PTX represent the net effect of two opposing forces, the predominant effect of which is inhibition and whose elimination is required for stimulatory effects to be manifest. The failure to observe AC stimulation by DAMGO in membranes obtained from MOR-CHO cells treated chronically with morphine and PTX indicates that tolerance develops to stimulatory as well as inhibitory actions of DAMGO.
The other notable finding of the present work is that CTAP, previously considered to be a neutral µ-opioid receptor antagonist based on in vivo and in vitro observations using other systems (Bonner et al., 1997; Liu and Prather, 2001; Sterious and Walker, 2003), enhanced FSK-stimulated AC activity (Figs. 2, 3, 4). Thus, in MOR-CHO cells, CTAP behaves not only as a µ-opioid receptor antagonist but also exhibits properties characteristic of inverse agonists as well. Notably, consistent with these findings, CTAP has been reported to possess a pharmacology that significantly differs from that of traditional opioid antagonists (Hawkins et al., 1989; Sterious and Walker, 2003). Inverse agonistic effects of CTAP on stimulated accumulation of cAMP is analogous to observations made for ICI174864 in -opioid receptor expressing rat-1 fibroblasts (Merkouris et al., 1997) and human embryonic kidney cells stably expressing the -opioid receptor (Chiu et al., 1996).
Sensitivity to CTX, a bacterial toxin that is frequently used to distinguish G proteins involved in receptor-second messenger coupling, was used to reflect involvement of Gs in CTAP inverse agonism signal transduction. CTX catalyzes the ADP ribosylation of the subunit of the Gs protein on arginine. As a consequence, the receptor-coupled activation/deactivation of the Gs protein cycle is interrupted. Of particular relevance to the current study, pretreatment with CTX has been associated with altered Gs-mediated signal transduction, i.e., decreased ability of guanine nucleotides to induce a decrease in receptor affinity for agonist via the destabilization of the high-affinity agonist-receptor ternary complex. For example, in frog erythrocyte membranes, CTX treatment substantially decreased (>10-fold) the potency of GTP or 5'-guanylylimidodiphosphate to regulate the affinity of the -adrenergic receptor for agonists (Stadel and Lefkowitz, 1981). In contrast, in the same membranes, CTX increased to a far lesser extent the ability of GTP to activate AC activity, consistent with observations made in turkey erythrocyte membranes (Cassell and Selinger, 1977) and illuminated bovine rod outer segment membranes (Wieland et al., 1994). Collectively, these observations support the use of CTX sensitivity to reflect Gs mediation. CTX attenuated the ability of CTAP to enhance FSK-stimulated cAMP formation in opioid-naive cells. Moreover, it eliminated the chronic morphine-induced increment in the inverse agonistic action of CTAP (Fig. 3B). This suggests that facilitative effects of CTAP on cAMP accumulation require Gs signaling and furthermore, that augmentation of Gs-linked events underlies the enhancement of the facilitative effects of CTAP after opioid tolerance formation.
In contrast to CTX, PTX did not modify the stimulatory effect of 1 µM CTAP in opioid-naive MOR-CHO membranes. It did, however, abolish the inverse agonistic actions of CTAP in membranes obtained from cells that had been chronically pretreated with morphine (Fig. 4). Thus, in opioid-naive cells, facilitative effects of CTAP on cAMP formation are mediated via Gs, whereas an additional component is induced by chronic morphine. PTX sensitivity of a process resulting in the enhancement of cAMP formation can best be explained postulating the involvement of Gi-derived G. This would stimulate ACs of the type 2 family (II, IV, and VII), conditional on the presence of activated Gs (Tang and Gilman, 1991). Importantly, the mRNAs encoding AC II, IV, and VII are present in MOR-CHO cells (A. Regec and A. R. Gintzler, unpublished observations) and have been shown to be up-regulated in the longitudinal muscle myenteric plexus preparation after chronic morphine (Chakrabarti et al., 1998a; Rivera and Gintzler, 1998). The chronic morphine-induced emergence of a (PTX-sensitive) G component of CTAP inverse agonistic effects (PTX does not alter CTAP inverse agonism in naive membranes), which would enhance Gs stimulation of AC (Chakrabarti et al., 1998a,b; Rivera and Gintzler, 1998; Chakrabarti and Gintzler, 2003), could, in fact, explain the ability of chronic morphine to augment the inverse agonist actions of CTAP and unmask inverse agonist actions of (±)-naloxone (Fig. 5), as was initially observed in human embryonic kidney 293 cells (Wang et al., 2001). This formulation could also account for the apparent limited concentration range of CTAP that manifests augmented inverse agonism after chronic morphine because AC stimulation by G occurs at relatively high concentrations of this G protein subunit (Birnbaumer, 1992; Tang and Gilman, 1992). Moreover, the dose-response relationship for CTAP facilitation of AC activity in tolerant cells would thus be multifactorial, depending not only on the dose relationship for CTAP-induced generation of Gi-derived G but also on its dose relationship for stimulating AC. The ability of chronic morphine to amplify the inverse agonist effects of naloxone, a general opioid receptor antagonist, as well as CTAP underscores the generality of this sequela of opioid tolerance.
PTX has previously been reported to prevent the inverse agonistic effects of the -opioid antagonist ICI174864 on cAMP accumulation in both -opioid receptor expressing rat-1 fibroblasts and human embryonic kidney cells expressing either MOR or -opioid receptors (Chiu et al., 1996; Merkouris et al., 1997). The current finding in MOR-CHO cells that pretreatment with CTX or a combination of CTX and PTX substantially attenuates and abolishes, respectively, the ability of CTAP to enhance FSK-stimulated cAMP accumulation underscores the paradoxical prerequisite for functional receptor G protein coupling for inverse opioid agonistic actions. The predominant effect of CTX on CTAP inverse agonism in µ-opioid receptor-expressing CHO cells (indicative of Gs mediation) versus PTX on the inverse agonist effects of the -opioid receptor antagonist ICI174864 in rat-1 fibroblasts and MOR-HEK cells (indicative of coupling to Gi/Go proteins) could reflect the plasticity and pleiotropic coupling of opioid receptors in different overexpressing cellular milieus (Chiu et al., 1996; Merkouris et al., 1997) and/or differences in coupling of overexpressed - versus µ-opioid receptors. In this regard, it should be noted that level of receptor expression has been reported to influence specificity of receptor/G protein coupling (Kenakin, 1995) as well as the manifestation of inverse agonistic action (Merkouris et al., 1997). Variations among cell lines in levels of opioid receptor expression, relative abundance of specific G proteins, and/or AC isoforms, etc., could underlie behavior of CTAP as a neutral antagonist in GH3 cells (Liu and Prather, 2001) versus the present demonstration of an inverse agonist in MOR-CHO cells.
Dependence of inverse agonism on the integrity of opioid receptor G protein coupling remains an enigma that confounds conceptual formulations of opioid inverse agonists. Actions of inverse agonists are usually explained by postulating a two-state model in which there is equilibrium between an inactive conformation of a receptor and a spontaneously active one that can couple to G proteins in the absence of agonists. In this scenario, inverse agonists are thought to abolish spontaneous, agonist-independent coupling to Gi/Go by stabilizing the inactive receptor conformation, thereby reduce the likelihood of assuming the active, Gi/Go-coupled state. However, this simplistic model would also predict that the effects of inverse agonists and PTX would be qualitatively similar, which has not proven to be the case (Chiu et al., 1996; Merkouris et al., 1997). In attempts to account for the paradoxical dependence of opioid inverse agonistic effects on functional opioid receptor Gi/Go coupling, more complicated models have been proposed, which unlike the two-state model, embody a role of G proteins in the induction and/or stabilization of various conformational states of the receptor (Samama et al., 1993). In this scenario, receptors would exist in an inactive G protein-coupled state, formation of which would be limited by PTX that transduces inverse agonistic activity, e.g., ICI174864, via their stabilization (Chiu et al., 1996).
Putative µ-opioid receptor coupling to Gs in MOR-CHO cells as discussed above is consistent with the independent demonstration of PTX reversal of DAMGO inhibition to facilitation of FSK-stimulated cAMP production in the same preparation. The current findings are consistent with a model in which antagonists such as CTAP, in the absence of agonist, can induce a receptor conformation that predisposes coupling to an otherwise less favored G protein, e.g., Gs, that regulates downstream signaling molecules in a manner opposite that of the G protein to which coupling is induced by agonists at that receptor (Gi/Go). Differences among antagonists in their ability to induce such receptor conformations could underlie differences in their respective potency as inverse agonists.
The physiological relevance of the current findings remains to be established. The absolute expression levels of GPCRs and the stoichiometry of their associated G proteins can influence the level of constitutive activity and its detection. Thus, overexpression systems are more likely to manifest constitutive activity, i.e., spontaneous coupling of the receptor to G proteins, than their in vivo counterparts. This notwithstanding, many naturally occurring GPCR systems do manifest constitutive activity. These include the -adrenoreceptor (Bond et al., 1995; Gether et al., 1995), the 5-hydroxytryptamine2C receptor (Barker et al., 1994), the histamine H2 receptor (Smit et al., 1996), the -opioid receptor (Costa and Herz, 1989), and the thyrotropin receptor (Cetani et al., 1996). These examples underscore the putative relevance of current findings to naturally occurring in vivo opioid systems.
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Acknowledgements |
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Footnotes |
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ABBREVIATIONS: GPCR, G protein-coupled receptor; MOR, µ-opioid receptor; CTAP, D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2; ICI174864, N,N-diallyl-Tyr-Aib-Aib-Phe-Leu; CHO, Chinese hamster ovary; MOR-CHO, Chinese hamster ovary cells stably transfected with µ-opioid receptors; PTX, pertussis toxin; CTX, cholera toxin; DAMGO, [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin; FSK, forskolin; AC, adenylyl cyclase; ANOVA, analysis of variance.
Address correspondence to: Dr. Alan Gintzler, Box 8, Department of Biochemistry, SUNY Downstate Medical Center, 450 Clarkson Ave., Brooklyn, NY 11203. E-mail: agintzler{at}netmail.hscbklyn.edu
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) and chronic morphine-treated (
) MOR-CHO, n = 4. B, membranes were obtained from opioid-naive or chronic morphine-treated cells that had been grown in either the absence (open columns) or presence (cross-hatched columns) of 100 ng/ml PTX, which was added during the last 24 h in culture. * denotes a significant DAMGO effect on FSK-stimulated AC activity (inhibition or stimulation), # denotes a significant effect of PTX on DAMGO responsiveness; 
denotes significant difference in DAMGO responsiveness in naive versus chronic morphine-treated membranes; n = 7 to 11. C, varying concentrations of DAMGO (10-8–10-5 M) were incubated for 15 min at 30°C with membranes obtained from MOR-CHO cells that had been treated for 24 h with 1 µM CTX and 100 ng/ml PTX, n = 3. All curves were fitted with GraphPad Prism. In each experiment, FSK-stimulated AC activity measured in the absence of opioids, but in the presence of PTX ± CTX where appropriate, was defined as 100%. The ability of DAMGO to facilitate AC activation by FSK is unmasked by PTX treatment of opioid-naive cells.
) and tolerant (
) MOR-CHO cells that were untreated (A), pretreated with 1 µM CTX FOR 16 h (B), or pretreated with 1 µM CTX and 100 ng/ml PTX for the last 24 h in culture (C). Tolerant membranes were maintained in the presence of 1 µM morphine for 48 h. The effect of CTAP (0.1–10 µM) was evaluated on FSK-stimulated AC activities and expressed as percentage of FSK. 
5. Facilitation of FSK stimulation of AC activity by CTAP was augmented after chronic morphine treatment, which could be markedly attenuated by CTX and abolished by the combined pretreatment with CTX and PTX.
