Chronic opiate receptor activation in vivo alters the level of G-protein subunits in guinea-pig myenteric plexus

Post-opioid receptor adaptations to chronic morphine; Altered functionality and associations of signaling molecules

Opioid desensitization/tolerance mechanisms have largely focused on adaptations that occur on the level of the mu-opioid receptor (MOR) itself. These include opioid receptor phosphorylation and ensuing trafficking events. Recent research, however, has revealed additional adaptations that occur downstream from the opioid receptor, which involve covalent modification of signaling molecules and altered associations among them. These include augmented isoform-specific synthesis of adenylyl cyclase (AC) and their phosphorylation as well as augmented phosphorylation of the G(beta) subunit of G(beta gamma). The aggregate effect of these changes is to shift mu-opioid receptor-coupled signaling from predominantly G(i alpha) inhibitory to (G(i)-derived) G(beta gamma) stimulatory AC signaling. Most recently, chronic morphine has been shown to enhance the association (interaction) between MOR and G(s), which should provide an additional avenue for offsetting inhibitory MOR signaling sequelae. The unfolding complexity of chronic morphine-induced sequelae demands an evolving broader and more encompassing perspective on opioid tolerance-producing mechanisms. This should facilitate understanding tolerance within the context of physiological plasticity that is activated by chronic exposure to drugs of abuse. Additional research is required to integrate the various tolerance-producing adaptations that have been elucidated to date. Specifically, the relative contribution to opioid tolerance of identified adaptations is still unknown as is the extent to which they vary among different regions of the central nervous system.

Changes of the level of G protein alpha-subunit mRNA by tolerance to and withdrawal from butorphanol

Butorphanol was infused continuously into cerebral ventricle at a constant rate of 26 nmol/microl/h for 3 days, and the withdrawal from opioid was rendered 7 h after the cessation of infusion. The G-protein alpha-subunit has been implicated in opioid tolerance and withdrawal. The effects of continuous infusion of butorphanol on the modulation of G protein alpha-subunit mRNA were investigated by using in situ hybridization techniques. In situ hybridization showed marked changes in the levels of Galpha s during butorphanol tolerance and withdrawal. Specifically, the level of Galpha s mRNA was significantly decreased in almost all areas of brain except hippocampus during the butorphanol withdrawal. It was also decreased in the septum and cerebellar granule layer in butorphanol tolerant rats. The level of Galpha i mRNA was significantly decreased only in the cerebral cortex of butorphanol tolerant rat. However, no such change was noted during the withdrawal from butorphanol. The level of Galpha o mRNA was not changed either in butorphanol tolerant or in the butorphanol withdrawal rats. No alterations were noted in the level of [3H]forskolin binding to adenylyl cyclase in butorphanol tolerant as well as withdrawing rats. The levels of pCREB were significantly elevated in the hippocampus in the butorphanol withdrawal rats. These results suggest that region-specific changes of G protein alpha-subunit mRNA and pCREB without marked changes in the level of adenylyl cyclase may underlie the tolerance to and withdrawal from butorphanol.

Morphine tolerance and reward but not expression of morphine dependence are inhibited by the selective glutamate carboxypeptidase II (GCP II, NAALADase) inhibitor, 2-PMPA

Inhibition of glutamate carboxypeptidase II (GCP II; NAALADase) produces a variety of effects on glutamatergic neurotransmission. The aim of this study was to investigate effects of GCP II inhibition with the selective inhibitor, 2-PMPA, on: (a) development of tolerance to the antinociceptive effects, (b) withdrawal, and (c) conditioned reward produced by morphine in C57/Bl mice. The degree of tolerance was assessed using the tail-flick test before and after 6 days of twice daily (b.i.d.) administration of 2-PMPA and 10 mg/kg of morphine. Opioid withdrawal was measured 3 days after twice daily morphine (30 or 10 mg/kg) administration, followed by naloxone challenge. Conditioned morphine reward was investigated using conditioned place preference with a single morphine dose (10 mg/kg). High doses of 2-PMPA inhibited the development of morphine tolerance (resembling the effect of 7.5 mg/kg of the NMDA receptor antagonist, memantine) while not affecting the severity of withdrawal. A high dose of 2-PMPA (100 mg/kg) also significantly potentiated morphine withdrawal, but inhibited both acquisition and expression of morphine-induced conditioned place preference. Memantine inhibited the intensity of morphine withdrawal as well as acquisition and expression of morphine-induced conditioned place preference. In addition, 2-PMPA did not affect learning or memory retrieval in a simple two-trial test, nor did it produce withdrawal symptoms in morphine-dependent, placebo-challenged mice. Results suggest involvement of GCP II (NAALADase) in phenomena related to opioid addiction.

Receptor-G protein gamma specificity: gamma11 shows unique potency for A(1) adenosine and 5-HT(1A) receptors

G protein coupled receptors activate signal transducing guanine nucleotide-binding proteins (G proteins), which consist of an alpha subunit and a betagamma dimer. Whole cell studies have reported that receptors signal through specific betagamma subtypes. Membrane reconstitution studies with the adenosine A(1) and alpha(2A) adrenergic receptors have reached a similar conclusion. We aimed to test the generality of this finding by comparing the gamma subtype specificity for four G(i)-coupled receptors: alpha(2A) adrenergic; A1 adenosine (A(1)-R); 5-hydroxytryptamine(1A) (5-HT(1A)-R); mu opioid. Membranes were reconstituted with Galpha(i)(1) and five gamma subtypes (dimerized to beta1). Using a sensitive alpha-betagamma binding assay, we show that all recombinant betagamma (except beta1gamma1) had comparable affinity for alpha(i)(1). Using high affinity agonist binding as a measure of receptor-G protein coupling, betagamma-containing gamma11 was the most potent for A(1)-R and 5-HT(1A)-R (p < 0.05, one way ANOVA) while gamma7 was most potent for the other two receptors. gamma11 was 3-8-fold more potent for the A(1)-R than were the other gamma subtypes. Also, gamma11 was 2-8-fold more potent for A(1)-R than at the other receptors, suggesting a unique coupling specificity of the A(1)-R for gamma11. In contrast, the discrimination by receptors for the other betagamma subtypes (beta1 and gamma1, gamma2, gamma7, and gamma10) was limited (2-3-fold). Thus the exquisite betagamma specificity of individual receptors reported in whole cell studies may depend on in vivo mechanisms beyond direct receptor recognition of betagamma subtypes.

Morphine induces short-lived changes in G-protein gene expression in rat prefrontal cortex

We have utilized a reverse transcriptase-polymerase chain reaction (RT-PCR) methodology followed by enzymatic restriction analysis to detect changes in G-protein mRNA levels in morphine-treated rats. The relative distribution of mRNA levels for Galpha(o) Galpha(i1), Galpha(i2), Gbeta(1) and Gbeta(2) in the nucleus accumbens, striatum, locus coeruleus and prefrontal cortex was found to be similar to that previously estimated with other techniques. Morphine-induced changes of G-protein mRNA levels were detected only in the prefrontal cortex. Acute treatments (30 mg/kg, intraperitoneally) resulted in a significant increase of Galpha(o) mRNA and significant decreases of Galpha(i1) and Galpha(i2) mRNAs. Chronic morphine administration (10-50 mg/kg over 14 days, intraperitoneally) increased Gbeta(1) and Galpha(i1) and Galpha(i2) mRNAs levels to 148%, 410% and 451% of control, respectively. G-protein mRNA returned to control levels within 48 h of termination of the chronic treatments. The morphine-induced changes in G-protein mRNA levels may reflect changes in gene expression and could result in changes in G-protein levels affecting signal transduction pathways in chronically treated animals.

Opioid tolerance and the emergence of new opioid receptor-coupled signaling

Multiple cellular adaptations are elicited by chronic exposure to opioids. These include diminution of spare opioid receptors, decreased opioid receptor density, and G-protein content and coupling thereof. All imply that opioid tolefance is a manifestation of a loss of opioid function, i.e., desensitization. Recent observations challenge the exclusiveness of this formulation and indicate that opioid tolerance also results from qualitative changes in opioid signaling. In this article, Gintzler and Chakrabarti discuss the evidence that suggests that opioid tolerance results not only from impaired opioid receptor functionality, but also from altered consequences of coupling. Underlying the latter are fundamental changes in the nature of effectors that are coupled to the opioid receptor/G-protein signaling pathway. These molecular changes include the upregulation of adenylyl cyclase isoforms of the type II family as well as a substantial increase in their phosphorylation state. As a result, there is a shift in opioid receptor/G-protein signaling from predominantly Gialpha inhibitory to Gbetagamma stimulatory following chronic in vivo morphine exposure. These adaptations to chronic morphine indicate the plasticity of opioid-signal transduction mechanisms and the ability of chronic morphine to augment new signaling strategies.

Differential alteration of adenylyl cyclase subtypes I, II, and V/VI in postmortem human brains of heroin addicts

In animal and culture cell experiments, the upregulation of cAMP-related signal transduction after chronic opioid administration has been hypothesized to be an adaptive change of the molecular mechanism to maintain homeostasis in intracellular signals downstream from opioid receptors. Herein, we have examined the quantitative changes of three adenylyl cyclase (AC) subtypes (I, II, and V/VI) in temporal cortex membranes from brains of heroin addicts and age-matched controls by immunoblotting. The immunoreactivity of AC-I decreased significantly (p < 0.05) in heroin addicts, compared with controls; whereas those of AC-II and AC-V/VI were not changed. The present findings indicate that differential regulation of AC subtypes occurs and that AC-I may play an important role in the signal transduction for opiate-induced tolerance and dependence mechanisms in human brain cortex.

Translocation of phosducin in living neuroblastoma x glioma hybrid cells (NG 108-15) monitored by red-shifted green fluorescent protein

Activation of G protein-coupled receptors triggers translocation of certain proteins from cytoplasm to cell membrane located targets. One of these cytosolic proteins is phosducin (Phd) which has been described to compete with G protein-coupled receptor kinases for Gbetagamma dimers attached to the cell membrane, thereby attenuating desensitization of activated receptors. These features of protein redistribution prompted us to examine whether stimulation of membrane associated E-prostaglandin receptors coupled to Gs causes Phd to migrate towards the plasma membrane. We made use of enhanced green fluorescence protein (EGFP), a reporter protein, to follow redistribution of Phd both by means of confocal microscopy and biochemical techniques in living neuronal NG 108-15 hybrid cells challenged with prostaglandin E1 (PGE1). The cells were transiently transfected to express Phd fused to the C-terminus of EGFP, or to express EGFP only. Overexpression of the proteins is implied by FACS analysis as well as by western blot technique, and the functional integrity of EGFP-tagged Phd was confirmed by its ability to elevate cAMP accumulation. Time-lapse imaging of single living cells by means of confocal microscopy revealed that exposure to prostaglandin causes EGFP/Phd, which is evenly spread throughout the cell, to relocate towards the membrane within few minutes. Fluorescence associated with the cell nucleus displayed little rearrangement. The principle finding that prostaglandin triggers translocation of Phd from cytosol to the cell periphery was verified with membranes prepared from EGFP/Phd expressing cells. We found maximal concentrations of membrane associated fluorescent material 5 to 7 min upon prostaglandin exposure. The present study reports for living NG 108-15 hybrid cells that PGE1 stimulation causes cytosolic Phd to translocate towards the membrane, where it is believed to bind to G protein subunits such as Gbetagamma and Galphas.

Retinoic acid-induced differentiation of human neuroblastoma SH-SY5Y cells is associated with changes in the abundance of G proteins

Western blot analysis, using subtype-specific anti-G protein antibodies, revealed the presence of the following G protein subunits in human neuroblastoma SH-SY5Y cells: Gs alpha, Gi alpha 1, Gi alpha 2, Go alpha, Gz alpha, and G beta. Differentiation of the cells by all-trans-retinoic acid (RA) treatment (10 mumol/L; 6 days) caused substantial alterations in the abundance of distinct G protein subunits. Concomitant with an enhanced expression of mu-opioid binding sites, the levels of the inhibitory G proteins Gi alpha 1 and Gi alpha 2 were found to be significantly increased. This coordinate up-regulation is accompanied by functional changes in mu-opioid receptor-stimulated low-Km GTPase, mu-receptor-mediated adenylate cyclase inhibition, and receptor-independent guanosine 5'-(beta gamma-imido)triphosphate [Gpp(NH)p; 10 nmol/L]-mediated attenuation of adenylate cyclase activity. In contrast, increased levels of inhibitory G proteins had no effect on muscarinic cholinergic receptor-mediated adenylate cyclase inhibition. With respect to stimulatory receptor systems, a reciprocal regulation was observed for prostaglandin E1 (PGE1) receptors and Gs alpha, the G protein subunit activating adenylate cyclase. RA treatment of SH-SY5Y cells increases both the number of PGE1 binding sites and PGE1-stimulated adenylate cyclase activity, but significantly reduced amounts of Gs alpha were found. This down-regulation is paralleled by a decrease in the stimulatory activity of Gs alpha as assessed in S49 cyc- reconstitution assays.(ABSTRACT TRUNCATED AT 250 WORDS)

Antibodies directed against alpha subunits of Gi, Gx/z, GO and GS transducer proteins reduced the morphine withdrawal syndrome in mice

The effect of intracerebroventricular (i.c.v.) injection of antibodies directed against alpha subunits of Gi, Gx/z, GO and GS regulatory proteins on morphine dependence was analyzed in mice. Animals were rendered tolerant-dependent by subcutaneous (s.c.) implantation of an oily suspension (10 ml/kg) containing 0.1 g/ml of morphine. After 72 h of chronic morphine, 1 mg/kg s.c. naloxone precipitated the withdrawal syndrome. The anti Gi2 alpha, Gx/z alpha, GO1/2 alpha and GS alpha antibodies given 24 h before starting the chronic morphine treatment, reduced the number of jumps recorded. An effect also produced by pertussis toxin, agent impairing the function of Gi/GO transducer proteins. The antibodies injected 24 h before the naloxone challenge reduced the number of animals presenting the jumping behavior, as well as the average number of jumps. This was observed for antibodies against alpha subunits of Gi, Gx/z and GO1/2 proteins. Thus, i.c.v. injection of anti G alpha antibodies by reducing the function of opioid and non-opioid receptors alleviated the morphine withdrawal syndrome.

Alterations in the expression of G-proteins and regulation of adenylate cyclase in human neuroblastoma SH-SY5Y cells chronically exposed to low-efficacy mu-opioids

Western-blot analysis of human neuroblastoma SH-SY5Y cells (mu- and delta-receptors) revealed the presence of the following G-protein subunits: Gi alpha 1, Gi alpha 2, Gs alpha, G(o) alpha, Gz alpha, and G beta, a pattern resembling that observed in central nervous tissue. Chronic treatment of differentiated [all-trans-retinoic acid (10 microM; 6 days)] SH-SY5Y cells with D(-)-morphine (10 microM; 3 days) significantly increased the abundance of all G-protein subunits identified. Co-incubation of morphine-exposed cells together with naloxone (10 microM; 3 days) or the mu-selective opioid antagonist CTOP (10 microM; 3 days), but not with the delta-selective antagonist ICI-174,864 (10 microM; 3 days), completely abolished this effect, suggesting that the increase in G-protein abundance is specifically mediated by mu-receptors. Moreover, the biologically inactive enantiomer L(+)-morphine (10 microM; 3 days) failed to produce a similar effect. G-protein up-regulation developed in a time- and dose-dependent manner and is most likely due to enhanced protein synthesis de novo, since concomitant treatment of the cells with cycloheximide (100 micrograms/ml; 3 days) prevented this effect. Chronic treatment with the low-efficacy mu-selective opioid peptide morphiceptin (10 microM; 3 days), but not with the highly potent mu-agonist DAGO (0.1 microM; 3 days) produced a comparable increase in G-protein abundance. Coincident with quantitative effects on G-protein levels in morphine-tolerant/dependent SH-SY5Y cells, we found elevated levels of basal, forskolin (1 microM)- and prostaglandin-E1 (1 microM)-stimulated adenylate cyclase activities. Reconstitution experiments using S49 cyc- lymphoma-cell membranes suggest that this increase is most likely due to elevated levels of functionally intact Gs. Chronic treatment with both morphine and DAGO induces high degrees of tolerance in this cell line. However, the intrinsic activity of G1 was unchanged, as assessed in functional studies with low-nanomolar concentrations of guanosine 5'-[beta gamma- imido]triphosphate. Our data demonstrate that chronic treatment of SH-SY5Y cells with low-efficacy mu-opioids increases G-protein abundance, a phenomenon which might contribute to the biochemical mechanisms underlying opioid tolerance/dependence.

In situ hybridization reveals specific increases in G alpha s and G alpha o mRNA in discrete brain regions of morphine-tolerant rats

In situ hybridization histochemistry has been used to detect the basal distribution of mRNA encoding the alpha subunit of Gs, Go and Gi2 proteins throughout the rat brain. Based on these data we investigated the effect of chronic morphine on the content of these G protein alpha subunits mRNA. We observed an increase in the expression of alpha s and alpha o messages of chronically morphine-treated animals, while no changes were seen in alpha i2 mRNA. Specifically a 30% increase in expression for alpha s was seen only in the paraventricular nucleus of hypothalamus and a 20% elevation for alpha o was detected in the claustrum and endopiriform nucleus. Immunoblotting analysis was used to correlate the changes in alpha s and alpha o messages with equivalent changes in protein levels. Chronic morphine significantly increased alpha s amounts in the hypothalamus (70%), and produced a minor elevation (30%) in G alpha o levels in the olfactory area. Our results indicate that in discrete brain regions altered G protein expression is part of the adaptive changes underlying opiate tolerance.

Distribution of low Km GTPase activity in mouse CNS: effect of chronic morphine

The low Km GTPase displayed an apparent Km value of 0.2-0.4 microM in P2 fractions from whole mouse brain. The activity of this enzyme ranged from 102 (pmols of GTP hydrolysed per micrograms of protein per min) in the striatum to 39 in the pons-medulla oblongata. Intermediate values were found in other structures, 74-62 in thalamus, hypothalamus, periaqueductal gray matter (PAG), rest of mesencephalon, cortex and spinal cord. The Km also varied throughout the mouse CNS: the spinal cord, striatum and PAG exhibited Km values (0.308-0.271 microM) higher than cortex, thalamus, pons-medulla, hypothalamus and remaining mesencephalon (0.239-0.193 microM). Chronic morphine (3 days) decreased the low Km GTPase activity of PAG (42), whereas it increased the one of thalamus (99). After chronic exposure to the opioid the Km values of the enzyme in striatum (0.193), PAG (0.192) and spinal cord (0.201) diminished, and the ones of hypothalamus (0.357) and rest of mesencephalon (0.287) augmented. The herein reported diversity of low GTPase activity might be due to the presence of different ratios of G alpha types/subtypes in the neural structures studied. As a result of chronic morphine the ratio and/or the functionality of G proteins would be altered in particular areas of mouse CNS.

(D-Ala, D-Leu) enkephalin reduces the binding of GTP in hippocampal membranes

The effect of (D-Ala, D-Leu) enkephalin (DADLE) on the binding of GTP in hippocampal preparations was studied. It was observed that treatment of hippocampal slices with 10(-5) -5 x 10(-5) M DADLE followed by the preparation of membrane fractions reduced the binding of 35S-GTP-gamma-S. There was no change in the affinity of the binding. This decrease of 35S-GTP-gamma-S binding was reversed when 5 x 10(-5) M naltrindole was included. The effect was not observed when the membrane fractions were incubated with DADLE. Photoaffinity labeling with the use of 32P P3-(4-azidoanilido)-P1 5'-GTP followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography revealed the incorporation of radioactivity into molecular mass of the 43 kDa and 33-34 kDa proteins. 32P Photolabeling of both the 43 kDa and 33-34 kDa bands decreased following treatment of hippocampal slices with 10(-4) M DADLE. These results suggested that DADLE reduces the GDP-GTP exchange in hippocampal membranes.

Opioid-controlled adenylate cyclase in the guinea-pig myenteric plexus is confined to nerve somata

Previous studies with the electrically stimulated longitudinal muscle-myenteric plexus preparation of the guinea-pig ileum suggested that opioid control of adenylate cyclase is confined to nerve somata. No indication was found for an opioid effect on the enzyme at nerve terminals of the neuro-muscular junction. The aim of the present investigation was to directly study the effect of opioids on cAMP generation in nerve fragments associated with somata or terminals of the myenteric plexus. Employing the ultracentrifugation technique an enrichment of cell organelles in fractions relating to either somata or terminals was achieved. Opioid binding studies revealed specific mu-receptors which in both fractions were regulated by GTP. Challenge of these fractions with forskolin and prostaglandin E1, respectively, resulted in an increased production of cAMP regardless of their neuroanatomical assignment. Examining the response of neuronal material to the selective mu-opioid DAMGO ([D-Ala2, MePhe4, Gly-ol5]enkephalin) revealed an inhibitory action on cAMP synthesis in somata-enriched fractions. No effect of DAMGO was observed in material linked to nerve terminals, although the presence of mu-opioid receptors and adenylate cyclase has been demonstrated. We conclude that opioid control of adenylate cyclase in the myenteric plexus of the guinea pig is confined to nerve somata.

Prolonged activation of inhibitory somatostatin receptors increases adenylate cyclase activity in wild-type and Gs alpha-deficient (cyc-) S49 mouse lymphoma cells

Many cells develop enhanced adenylate cyclase activity after prolonged exposure to drugs that acutely inhibit the enzyme and it has been suggested that this adaptation may be due to an increase in Gs alpha. We have treated wild-type and Gs alpha-deficient cyc- S49 mouse lymphoma cells with a stable analogue (SMS 201-995) of the inhibitory agonist somatostatin. After incubation with SMS for 24 h, the forskolin-stimulated cAMP synthetic rate in intact cyc- cells was increased by 76%, similar to the increase found in the wild-type cells. Forskolin-stimulated adenylate cyclase activity in the presence of Mn2+ was also increased in membranes prepared from SMS-treated cyc- cells; however, guanine nucleotide-mediated inhibition of adenylate cyclase activity was not changed despite a small decrease in inhibitory Gi alpha subunits detected by immunoblotting. Pretreatment of cyc- cells with pertussis toxin prevented SMS from inducing the enhancement of forskolin-stimulated cAMP accumulation in intact cells. After chronic incubation of cyc- cells with SMS, exposure to N-ethylmaleimide, which abolished receptor-mediated inhibition of cAMP accumulation, did not attenuate the enhanced rate of forskolin-stimulated cAMP synthesis compared to N-ethylmaleimide-treated controls. These results with cyc- cells demonstrate that an adaptive increase in adenylate cyclase activity induced by chronic treatment with an inhibitory drug can occur in the absence of expression of Gs alpha.

Intracerebroventricular injection of antibodies directed against Gs alpha enhances the supraspinal antinociception induced by morphine, beta-endorphin and clonidine in mice

The intracerebroventricular (i.c.v.) injection of antisera directed against different sequences of Gs alpha to mice enhanced the antinociceptive potency of the opioids morphine, beta h-endorphin-(1-31) and of the alpha 2-agonist clonidine when studied 24 h later in the tail-flick test. The activity of DAGO, DADLE, DPDPE and [D-Ala2]-Deltorphin II remained unchanged after that treatment. Cholera toxin (0.5 microgram/mouse, i.c.v.), agent that impairs the receptor regulation of Gs transducer proteins promoted comparable changes in the supraspinal analgesia induced by these substances. Six days after a single i.c.v. injection (0.5 microgram/mouse) of pertussis toxin the antinociceptive activity of all the opioids and clonidine appeared diminished. It is concluded that opioids and clonidine promote analgesia after binding to receptors functionally coupled to Gi/G(o) proteins, moreover, the activity of morphine, beta-endorphin and clonidine in this test seems to be counteracted by a process involving activation of Gs alpha transducer proteins.

Cholera toxin and pertussis toxin on opioid- and alpha 2-mediated supraspinal analgesia in mice

Cholera toxin, an agent that impairs the function of Gs transducer proteins, was injected (0.5 microgram/mouse, icv) and the antinociceptive activity of opioids and clonidine was studied 24h later in the tail-flick test. In these animals, an enhancement of the analgesic potency of morphine, beta-endorphin and clonidine could be observed. Cholera toxin did not modify the antinociception evoked by the enkephalin derivatives DAGO and DADLE. Pertussis toxin that catalyses the ADP ribosylation of alpha subunits of Gi/Go regulatory proteins was given icv (0.5 microgram/mouse). This treatment reduced the analgesic effect of opioids and clonidine. However, while the analgesia elicited by DAGO, DADLE and clonidine was greatly decreased, the effect of morphine and beta-endorphin was reduced to a moderate extent. It is concluded that Gi/Go regulatory proteins functionally coupled to opioid and alpha 2 receptors are implicated in the efficacy displayed by opioids and clonidine to produce supraspinal analgesia. Moreover, these two receptors are susceptible to regulation by a process that might involve a Gs protein.