Immunoprecipitation of high-affinity, guanine nucleotide-sensitive, solubilized mu-opioid receptors from rat brain: coimmunoprecipitation of the G proteins G(alpha o), G(alpha i1), and G(alpha i3).

Antibodies directed against the C-terminal and the N-terminal regions of the mu-opioid receptor were generated to identify the G proteins that coimmunoprecipitate with the mu receptor. Two fusion proteins were constructed: One contained the 50 C-terminal amino acids of the mu receptor, and the other contained 61 amino acids near the N terminus of the receptor. Antisera directed against both fusion proteins were capable of immunoprecipitating approximately 70% of solubilized rat brain mu receptors as determined by [3H][D-Ala2,N-Me-Phe4,Gly-ol5]-enkephalin ([3H]DAMGO) saturation binding. The material immunoprecipitated with both of the antisera was recognized as a broad band with a molecular mass between 60 and 75 kDa when screened in a western blot. Guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS) had an EC50 of 0.4 nM in diminishing [3H]DAMGO binding to the immunoprecipitated pellet. The ratio of G proteins to mu receptors in the immunoprecipitated material was 1:1. When the material immunoprecipitated with affinity-purified antibody was screened for the presence of G protein a subunits, it was determined that G(alpha)o, G(alpha)i1, G(alpha)i3, and to a lesser extent G(alpha)i2, but not G(alpha)s or G(alpha)q11, were coimmunoprecipitated with the mu receptor. Inclusion of GTPgammaS during the immunoprecipitation process abolished the coimmunoprecipitation of G proteins.

Solubilization of high-affinity, guanine nucleotide-sensitive mu-opioid receptors from rat brain membranes.

High-affinity mu-opioid receptors have been solubilized from rat brain membranes. In most experiments, rats were treated for 14 days with naltrexone to increase the density of opioid receptors in brain membranes. Occupancy of the membrane-associated receptors with morphine during solubilization in the detergent 3-[(3-cholamidopropyl)dimethyl]-1-propane sulfonate appeared to stabilize the mu-opioid receptor. After removal of free morphine by Sephadex G50 chromatography and adjustment of the 3-[(3-cholamidopropyl)dimethyl]-1-propane sulfonate concentration to 3 mM, the solubilized opioid receptor bound [3H][D-Ala2,N-Me-Phe4,Gly-Dl5]-enkephalin([3H]DAMGO), a mu-selective opioid agonist, with high affinity (KD = 1.90 +/- 0.93 nM; Bmax = 629 +/- 162 fmol/mg of protein). Of the membrane-associated [3H]-DAMGO binding sites, 29 +/- 7% were recovered in the solubilized fraction. Specific [3H]DAMGO binding was completely abolished in the presence of 10 microM guanosine 5'-O-(3-thiotriphosphate). The solubilized receptor also bound [3H]diprenorphine, a nonselective opioid antagonist, with high affinity (KD = 1.4 +/- 0.39 nM, Bmax = 920 +/- 154 fmol/mg of protein). Guanosine 5'-O-(3-thiotriphosphate) did not diminish [3H]diprenorphine binding. DAMGO at concentrations between 1 nM and 1 microM competed with [3H]diprenorphine for the solubilized binding sites; in contrast, [D-Pen2, D-Pen5]-enkephalin, a delta-selective opioid agonist, and U50488H, a kappa-selective opioid agonist, failed to compete with [3H]diprenorphine for the solubilized binding sites at concentrations of < 1 microM. In the absence of guanine nucleotides, the DAMGO displacement curve for [3H]diprenorphine binding sites better fit a two-site than a one-site model with KDhigh = 2.17 +/- 1.5 nM, Bmax = 648 +/- 110 fmol/mg of protein and KDlow = 468 +/- 63 nM, Bmax = 253 +/- 84 fmol/mg of protein. In the presence of 10 microM guanosine 5'-O-(3-thiotriphosphate), the DAMGO displacement curve better fit a one- than a two-site model with KD = 815 +/- 33 nM, Bmax = 965 +/- 124 fmol/mg of protein.

Antisense oligodeoxynucleotide to the Gi2 protein alpha subunit sequence inhibits an opioid-induced increase in the intracellular free calcium concentration in ND8-47 neuroblastoma x dorsal root ganglion hybrid cells.

In ND8-47 cells, a neuroblastoma x dorsal root ganglion hybrid cell line, activation of delta-opioid receptors induced an increase in the intracellular free calcium concentration ([Ca2+]i) through dihydropyridine-sensitive calcium channels. This effect was mediated by pertussis toxin-sensitive G proteins. The G protein alpha subunits alpha i2, alpha i3, alpha q, and alpha s were detected using Western blots, whereas alpha o and alpha i1 were not found in ND8-47 cell membranes. To identify the specific G protein alpha subunit(s) responsible for the increase in [Ca2+]i, we treated ND8-47 cells with antisense oligodeoxynucleotides (AS) complementary to the mRNA for each G protein alpha subunit (alpha i2, alpha i3, or alpha s), at a concentration of 10 microM, for up to 6 days and examined their effects on opioid-induced increases in [Ca2+]i and on the levels of G protein alpha subunits. [Ca2+]i was measured in adherent cells using the fluorescent dye fura-2. Treatment of cells with alpha i2-AS (10 microM, for 6 days) resulted in a 73% inhibition of the [D-Ser2,Leu5]-enkephalin-Thr-induced increase in [Ca2+]i. In contrast, pretreatment of cells with alpha i3-AS (10 microM, for 6 days) or alpha s-AS (10 microM, for 6 days) had no effect on the [D-Ser2,Leu5]-enkephalin-Thr-induced responses. Western blots indicated that the levels of alpha i2 were decreased when cells were exposed to alpha i2-AS (10 microM) for 6 days, whereas the levels of alpha i3, alpha s, and alpha q were not affected by this treatment. Treatment of the cells with alpha i3-AS or alpha s-AS for 6 days significantly reduced alpha i3 or alpha s levels, respectively. These results indicate that the opioid-induced increase in [Ca2+]i in ND8-47 cells is mediated by G alpha i2.

Inhibition of adenylyl cyclase activity by a homogeneous population of dopamine receptors: selective blockade by antisera directed against Gi1 and/or Gi2.

The 7315c pituitary tumor cell expresses a homogeneous population of dopamine receptors that are functionally similar to brain dopamine D2 receptors. [3H]-Sulpiride binding to 7315c cell homogenates was specific and saturable, and Ki values for compounds to compete for these sites were highly correlated with values for the same compounds at D2 receptors in brain. Dopamine maximally inhibited approximately 65% of forskolin-stimulated cyclase activity in cell membranes. Some D2 agonists had lower efficacies, suggesting that some compounds are partial agonists at this receptor. Removal of GTP from the assay buffer or pretreatment of the tissue with pertussis toxin abolished the inhibition of adenylyl cyclase by dopamine. Immunodetection of most of the known G alpha subunits revealed that Gi1, Gi2, Gi3, Go, Gq, and Gs are present in the 7315c membrane. Pretreatment with the AS antibody (which recognizes the C-terminal regions of G alpha i and G alpha i2) significantly attenuated the inhibition of adenylyl cyclase activity by dopamine, whereas antibodies to C-terminal regions of the other G alpha subunits had no effect. These findings suggest that the dopamine D2 receptor regulates cyclase inhibition predominantly via Gi1 and/or Gi2 and that the 7315c tumor cells provide a useful model for studying naturally expressed dopamine D2 receptors in the absence of other dopamine receptor subtypes.

Naltrexone-induced upregulation of mu opioid receptors on 7315c cell and brain membranes: enhancement of opioid efficacy in inhibiting adenylyl cyclase.

The effect of chronic naltrexone administration on the expression of mu opioid receptors on 7315c tumor cells was examined. Osmotic minipumps containing either saline or naltrexone were subcutaneously implanted into Buffalo rats that had been injected intraperitoneally with 7315c cells. Fourteen days after the pumps were implanted, 7315c tissue and brain tissue were removed and examined for their ability to bind [3H]DAMGO and to respond to morphine (or DAMGO) and guanosine 5'-O-(3-thiotriphosphate) in an adenylyl cyclase assay. Naltrexone treatment caused a doubling in the density of [3H]DAMGO binding sites in both whole brain membranes and the 7315c cell membranes. Naltrexone treatment may have slightly diminished the affinity of mu opioid receptors for [3H]DAMGO (by 1.5- to 2-fold), but the precision of the assay was inadequate to determine whether this difference was significant. Naltrexone treatment also had no effect on the potency or efficacy of guanosine 5'-O-(3-thiotriphosphate) in diminishing [3H]DAMGO binding to either whole brain or 7315c cell membranes. The influence of naltrexone treatment on opioid inhibition of adenylyl cyclase activity was also investigated in both tissues. In 7315c membranes, naltrexone treatment caused a 40% increase in the efficacy (maximal effect) of morphine but had no effect on the potency (IC50) of morphine in inhibiting forskolin-stimulated adenylyl cyclase activity. In whole brain membranes from control rats, DAMGO failed to affect significantly forskolin-stimulated adenylyl cyclase. However, in whole brain membranes from naltrexone-treated rats, DAMGO caused a 30% inhibition of forskolin-stimulated adenylyl cyclase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic repeated cocaine administration alters basal and opioid-regulated adenylyl cyclase activity.

Repeated daily cocaine injections have been shown to alter mu-opioid receptor densities in the caudate putamen and nucleus accumbens of rat brain (Unterwald et al., 1991, 1992). Adenylyl cyclase activity was measured in rat rostral caudate putamen and nucleus accumbens following repeated cocaine administration to determine the functional consequences of cocaine-induced opioid receptor changes. Male Fischer rats were injected daily for 14 days with saline or cocaine HCl (30 or 45 mg/kg/day, i.p.) in three equal doses at 1-hr intervals. Basal adenylyl cyclase activity and the effects of the selective mu- and delta-opioid agonists [D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin (DAMGO) and [D-penicillamine2,D-Penicillamine5]enkephalin (DPDPE), respectively, on adenylyl cyclase activity were examined 30 min after the last injection using a cAMP radioligand binding assay in crude membrane preparations. Basal adenylyl cyclase activity was 49% and 34% lower in the caudate putamen of animals treated with 30 and 45 mg/kg/day of cocaine, respectively, as compared to those receiving saline injections. Basal adenylyl cyclase activity was unchanged in the nucleus accumbens following cocaine treatment. DAMGO and DPDPE each maximally inhibited approximately 25% and 30%, respectively, of basal adenylyl cyclase in the caudate putamen and nucleus accumbens of saline-injected animals. Administration of cocaine attenuated the ability of DPDPE to inhibit adenylyl cyclase in both brain regions, but had no effect on the efficacy or potency of DAMGO for inhibiting adenylyl cyclase activity. These results suggest that chronic, repeated cocaine administration results in a selective impairment of delta-opioid receptor-mediated effector function in the caudate putamen and nucleus accumbens.

Solubilization of high-affinity, guanine nucleotide-sensitive mu-opioid receptors from 7315c cell membranes.

High-affinity mu-opioid receptors have been solubilized from 7315c cell membranes. Occupancy of the membrane-associated receptors with morphine before their solubilization in the detergent 3-[(3-cholamidopropyl) dimethyl]-1-propane sulfonate was critical for stabilization of the receptor. The solubilized opioid receptor bound [3H]-etorphine with high affinity (KD = 0.304 +/- 0.06 nM; Bmax = 154 +/- 33 fmol/mg of protein). Of the membrane-associated [3H]etorphine binding sites, 40 +/- 5% were recovered in the solubilized fraction. Both mu-selective and non-selective enkephalins competed with [3H]etorphine for the solubilized binding sites; in contrast, delta- and kappa-opioid enkephalins failed to compete with [3H]etorphine for the solubilized binding sites at concentrations of < 1 microM. The mu-selective ligand [3H][D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin also bound with high affinity (KD = 0.79 nM; Bmax = 108 +/- 17 fmol/mg of protein) to the solubilized material. Of the membrane-associated [3H][D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin binding sites, 43 +/- 3% were recovered in the solubilized material. Guanosine 5'-O-(3-thiotriphosphate), GTP, and guanosine 5'-O-(2-thiodiphosphate), but not adenylylimidodiphosphate, diminished [3H][D-Ala2,N-Me-Phe4,Gly5-ol] enkephalin binding in a concentration-dependent manner. Finally, mu-opioid receptors from rat brain membranes were also solubilized in a high-affinity, guanine nucleotide-sensitive state if membrane-associated receptors were occupied with morphine before and during their solubilization with the detergent 3-[(3-cholamidopropyl)dimethyl]-1-propane sulfonate.

Muscarinic depression of synaptic transmission and blockade of norepinephrine-induced long-lasting potentiation in the dentate gyrus.

Bath application of the muscarinic receptor agonist, muscarine, produced a concentration-dependent depression of synaptic activity in the dentate gyrus of hippocampal slices. A concentration of 10 microM muscarine produced a reversible depression that could be competitively antagonized by the muscarinic receptor antagonist pirenzepine. However, other muscarinic receptor subtype (M1-M3) antagonists could also block the effects of muscarine. The rank order of antagonist potency was: 4-diphenylacetoxy-N-methyl-piperidine methiodide (M3/M1 antagonist) > pirenzepine (M1) > AFDX-116 (M2). The depression produced by 10 microM muscarine was not affected by in vivo pretreatment with pertussis toxin, and therefore was not mediated by a pertussis toxin-sensitive G-protein. In addition, high concentrations of muscarine did not affect either basal or isoproterenol-stimulated accumulation of cyclic AMP from slices of dentate gyrus. Muscarine also produced a concentration-dependent blockade of the induction of norepinephrine-induced long-lasting potentiation in the dentate gyrus. Norepinephrine-induced long-lasting potentiation is a form of long-lasting plasticity induced in medial perforant path synapses by beta-adrenergic agonists such as isoproterenol. The muscarinic blockade of norepinephrine-induced long-lasting potentiation was also prevented by pretreatment with pirenzepine. Based on these pharmacological data, we conclude that muscarinic depression of evoked responses, as well as blockade of norepinephrine-induced long-lasting potentiation, involves activation of either M3 or M1, but not M2, muscarinic receptors. These data also demonstrate that in addition to modulating normal synaptic transmission, muscarinic receptors may also play an important role in modulating synaptic plasticity.

Angiotensin II receptor recognized by DuP753 regulates two distinct guanine nucleotide-binding protein signaling pathways.

The 7315c cell, derived from a rat anterior pituitary tumor, expresses an angiotensin II (AII) receptor. [3H]AII binds to 7315c membranes specifically and saturably (Kd = 2.1 +/- 0.6 x 10(-6) M, Bmax = 282 +/- 33 fmol/mg of protein). GTP diminished the affinity of the membranes for [3H]AII (Kd = 4.1 +/- 0.4 x 10(-9) M, Bmax = 210 +/- 26 fmol/mg of protein). [3H]AII binding was displaced by AII (Ki = 1.3 +/- 0.6 x 10(-9) M), angiotensin III (AIII) (Ki = 0.9 +/- 0.4 x 10(-9) M), and the nonpeptide AII antagonist DuP753 (Ki = 1.4 +/- 0.6 x 10(-8) M). In contrast, a second nonpeptide AII ligand, PD123177, did not compete for [3H]AII binding sites. In intact cells, AII and AIII stimulated inositol trisphosphate (IP3) production (EC50 = 1.1 +/- 0.6 x 10(-8) M and 1.1 +/- 0.5 x 10(-8) M, respectively); this response to AII was antagonized by DuP753 (Ki = 1.7 +/- 0.3 x 10(-7) M). Pertussis toxin treatment failed to affect the ability of AII to stimulate IP3 production. In a crude membrane preparation, GTP was required for maximal AII-induced IP3 stimulation; guanosine thio-diphosphate abolished the agonist-GTP stimulation of IP3 production, in a concentration-dependent fashion. AII and AIII also inhibited adenylyl cyclase (EC50 = 2.9 +/- 1.1 x 10(-8) M and 6.0 +/- 1.0 x 10(-8) M, respectively). DuP753 antagonized the inhibition by AII of adenylyl cyclase (Ki = 2.8 +/- 0.4 x 10(-8) M). PD123177 failed to antagonize AII-induced cyclase inhibition. Pertussis toxin treatment abolished the AII and AIII inhibition of adenylyl cyclase. GTP was required for AII-induced inhibition of adenylyl cyclase. These data suggest that, in 7315c cells, a single subtype of AII receptor, identified by DuP753, is capable of regulating two different guanine nucleotide-binding protein (G protein) signalling pathways; one G protein, which is insensitive to pertussis toxin, stimulates IP3 production and the other G protein, which is sensitive to pertussis toxin, inhibits adenylyl cyclase.

Reconstitution of the solubilized mu-opioid receptor coupled to a GTP-binding protein.

A mu-opioid receptor-GTP binding protein (mu-opioid receptor-G-protein) complex from the 7315c cell was solubilized with CHAPS (3-[(3-cholamidopropyl)dimethylammonio]-1-propane sulfonate) and reconstituted into phospholipid vesicles. Pretreatment of the tissue with either [3H]etorphine or morphine greatly improved recovery of the receptor and maintained it in a GTP-sensitive state. GTP sensitivity was consistent with the hypothesis that a receptor-G-protein complex had been obtained. Other evidence consistent with this hypothesis was that recovery of the solubilized, prelabelled receptor was decreased by approximately 70% by pretreatment of 7315c cells with pertussis toxin. The reconstituted receptor was mu-selective: DAGO (Tyr-D-Ala-Gly-Met-Phe- NH(CH2)2OH), but not ICI 174864 or U50488-H, displaced [3H]etorphine binding with high affinity. The affinity of the reconstituted receptor for [3H]etorphine (1.25 +/- 0.20 nM) was similar to that observed for the membrane-associated receptor (0.53 +/- 0.25 nM). GTP gamma S decreased this affinity 3-fold without changing the number of binding sites. The potencies of GTP gamma S and GTP in diminishing [3H]etorphine binding were similar in the membrane and vesicle preparations, but were 10-fold lower than the potencies observed in diminishing binding to the solubilized receptor. The ability to reconstitute a functional mu-opioid receptor-G-protein complex will facilitate further study of the structure and function of the receptor and the specific identification of the associated GTP-binding protein(s).

Site of pertussis toxin-induced ADP-ribosylation on Gi is critical for receptor modulation of GDP interaction with Gi.

In this study, the influence of the inhibitory mu-opioid receptor on the potencies of 5'-guanosine alpha-thiotriphosphate (GTP gamma S) and GDP at the inhibitory GTP-binding protein (Gi) were investigated in an adenylyl cyclase system. It was hoped that a receptor-mediated change in the potency of either GTP gamma S or GDP in affecting adenylyl cyclase activity may elucidate how a receptor alters cyclase activity via its G-protein. In an adenylyl cyclase system employing 5'-adenylyl imidodiphosphate as substrate, GTP gamma S, a nonhydrolyzable analog of GTP, inhibited forskolin-stimulated adenylyl cyclase activity in the absence of morphine; morphine failed to significantly affect the apparent potency of GTP gamma S. GDP blocked the GTP gamma S-induced inhibition of adenylyl cyclase; morphine profoundly diminished the ability of GDP to block the inhibitory effect of GTP gamma S. The IC50 values of GTP gamma S were 0.02 +/- 0.01, 0.18 +/- 0.04, and 2.2 +/- 0.5 microM in the absence of other drugs, in the presence of a combination of 100 microM GDP and morphine, and in the presence of 100 microM GDP, respectively. GDP blocked the inhibitory effect of GTP gamma S (0.3 microM) in a concentration-dependent manner; the EC50 for GDP was 16 +/- 2.6 microM in the absence of morphine and 170 +/- 32 microM in the presence of morphine. Exposure of 7315c cells to pertussis toxin for 3 h resulted in a small decrease in the potency of GTP gamma S in inhibiting cyclase. However, the relative potency of GDP in blocking the GTP gamma S-mediated inhibition of cyclase was increased: the EC50 values of GDP were 11 +/- 4 and 0.81 +/- 0.2 microM in untreated and pertussis toxin-treated membranes, respectively. In untreated membranes, there was a brief lag in the GTP gamma S-induced inhibition of adenylyl cyclase; morphine diminished this lag. In membranes treated with pertussis toxin, there was an exaggerated lag in the onset of GTP gamma S inhibition of adenylyl cyclase activity; morphine could no longer affect this lag. Thus, uncoupling the mu-opioid receptor from Gi appeared to increase the affinity of Gi for GDP. These data suggest that the effect of an inhibitory receptor is to decrease the affinity of Gi for GDP by virtue of its interaction with the carboxy-terminal region of Gi alpha.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of thyrotropin-releasing hormone receptor binding and phospholipase C activation by a single GTP-binding protein.

In a crude membrane preparation of rat 7315c cells, GTP was found to enhance thyrotropin-releasing hormone- (TRH) stimulated inositol triphosphate (IP3) formation with a potency of 0.97 +/- 0.1 microM. TRH stimulation of IP3 formation was inhibited by high GDP concentrations. Neither nucleotide had any effect in the absence of TRH. 5'-Guanosine gamma-thiotriphosphate (GTP gamma S) stimulated IP3 formation in the absence of TRH; the apparent affinity of GTP gamma S was 0.16 +/- 0.05 microM. GTP blocked GTP gamma S stimulation of IP3 formation in a concentration-dependent manner. The apparent affinity of GTP for the site of action shared by GTP gamma S was calculated to be 0.98 +/- 0.3 microM. TRH was able to reverse inhibition of GTP gamma S-stimulated IP3 formation by GTP but could not reverse inhibition by GDP. A lag in the rate of IP3 formation in response to GTP gamma S was abolished by addition of TRH. These data support the proposal that activation of the TRH receptor enhances turnover of guanine nucleotides at the binding protein coupling the receptor to phospholipase C. In addition, GTP gamma S diminished high affinity [3H]Me-TRH binding. The potency of GTP gamma S at decreasing [3H]Me-TRH binding was 0.092 +/- 0.03 microM. GTP gamma S (0.1 microM) decreased the affinity of the TRH receptor for [3H]Me-TRH from 2 to 100 nM. Maximally effective concentrations of GTP gamma S, Gpp(NH)p, GTP, and GDP decreased specific [3H]Me-TRH binding by 80%. Pretreatment of cells with pertussis toxin (30 ng/ml for 24 h) failed to affect TRH receptor affinity or the potency or efficacy of GTP gamma S in diminishing [3H]Me-TRH binding, supporting the identification of Gp (a GTP-binding protein associated with phospholipase C and Ca2+-mobilizing receptors) as distinct from Gi (an inhibitory GTP-binding protein). In contrast to its lack of effect on TRH receptor binding, 3-h pertussis toxin treatment decreased agonist affinity of the mu-opiate receptor and abolished the ability of GTP gamma S to shift the affinity of the mu-opiate receptor for its agonist. The affinities calculated for GTP, GDP, GTP gamma S, and Gpp (NH)p for the G-protein regulating receptor affinity and IP3 formation are nearly identical for each guanine nucleotide tested, suggesting the same G-protein regulates both activities.

Sodium regulation of agonist binding at opioid receptors. I. Effects of sodium replacement on binding at mu- and delta-type receptors in 7315c and NG108-15 cells and cell membranes.

The effects of varying the sodium concentration (at constant ionic strength) on opioid binding at mu- and delta-opioid receptors in 7315c and NG108-15 cells has been examined. The binding of [3H]etorphine to mu-receptors on 7315c cells was increased by replacing the sodium in the incubation medium with potassium or N-methyl-D-glucamine. This effect was shown to be attributable to an increase in affinity, with no change in the maximum number of binding sites, both in cell membrane suspensions and in intact 7315c cells. Replacement of sodium with potassium or N-methyl-D-glucamine in NG108-15 membrane or intact cell suspensions also resulted in an increase in [3H]etorphine binding, but in these cells the effect was associated with an increase in the number of binding sites measurable under these experimental conditions. The effects of sodium on opioid inhibition of adenylate cyclase in membrane preparations from 7315c and NG108-15 cells also differed. Sodium reduced apparent agonist affinity in 7315c membranes. In NG108-15 cell membranes, sodium was essential for the demonstration of opioid inhibition of cyclase activity. Increasing the sodium concentration above 0.5 mM resulted in an increase in the fraction of total enzyme activity inhibited by opioid, but the opioid IC50 did not change. In the companion paper, it is shown that the effects of sodium removal on mu- and delta-receptor binding in guinea pig brain neural membranes were similar to those observed in the cell preparations. An increase in intracellular sodium concentration without change in extracellular concentration was effected by incubation of 7315c and NG108-15 cells with the sodium-selective ionophore, monensin. When sodium was present in the extracellular medium, monensin reduced [3H]etorphine binding by 50% or more, both at mu-receptors in 7315c cells and at delta-receptors in NG108-15 cells. In the absence of sodium, however, monensin treatment produced only a small inhibition of binding. These results suggest that sodium acts at an intracellular site to regulate opioid agonist binding at both mu- and delta-receptors, but that the mode of regulation is not identical at each site. Since a reduction in intracellular sodium concentration by removal of extracellular sodium increases agonist binding, and an increase in intracellular sodium following monensin treatment reduces agonist binding, it is probable that the intracellular sodium concentration is a critical regulator of opioid agonist binding in intact cells.

Coupling of the thyrotropin-releasing hormone receptor to phospholipase C by a GTP-binding protein distinct from the inhibitory or stimulatory GTP-binding protein.

Thyrotropin-releasing hormone (TRH) stimulated a rapid rise in inositol trisphosphate (IP3) formation and prolactin release from 7315c tumor cells. The potencies (half-maximal) of TRH in stimulating IP3 formation and prolactin release were 100 +/- 30 and 140 +/- 30 mM, respectively. Pretreatment of the cells with pertussis toxin (for up to 24 h) had no effect on either process. Pretreatment of the cells with cholera toxin (30 nM for 24 h) also failed to affect basal or TRH-stimulated IP3 formation. TRH was also able to stimulate IP3 formation with a half-maximal potency of 118 +/- 10 nM in a lysed cell preparation of 7315c cells; the TRH-stimulated formation of IP3 was enhanced by GTP. 5'-Guanosine gamma-thiotriphosphate (GTP gamma S) and 5'-guanylyl imidodiphosphate (Gpp(NH)p), nonhydrolyzable analogs of GTP, stimulated IP3 formation in the absence of TRH with half-maximal potencies of 162 +/- 50 and 7500 +/- 4300 nM, respectively. In contrast to the lack of effect of pertussis toxin on the TRH receptor system, treatment of 7315c cells with pertussis toxin for 3 h or longer completely abolished the ability of morphine, an opiate agonist, to inhibit either adenylate cyclase activity or prolactin release. During this 3-h treatment, pertussis toxin was estimated to induce the endogenous ADP ribosylation of more than 70% of Ni, the inhibitory GTP-binding protein. GTP gamma S and Gpp(NH)p inhibited cholera toxin-stimulated adenylate cyclase activity (presumably by acting at Ni) with half-maximal potencies of 25 +/- 9 and 240 +/- 87 nM, respectively. Finally, Gpp(NH)p was also able to inhibit the [32P]ADP ribosylation of Ni with a half-maximal potency of 300 nM. These results suggest that a novel GTP-binding protein, distinct from Ni, couples the TRH receptor to the formation of IP3.

D-2 dopamine receptor-mediated inhibition of pro-opiomelanocortin synthesis in rat intermediate lobe. Abolition by pertussis toxin or activators of adenylate cyclase.

Stimulation of the D-2 dopamine receptor inhibits pro-opiomelanocortin (POMC) synthesis in isolated rat intermediate lobe tissue. Intermediate lobe tissue was incubated in the absence or presence of various dopaminergic compounds, and then its capacity to incorporate [3H]tyrosine into POMC was tested. D-2 dopaminergic agonists caused a dose-dependent inhibition of POMC synthesis; the maximal inhibitory effect was approximately a 50% reduction in the amount of POMC synthesized. D-2 dopaminergic antagonists blocked the inhibitory effect of each agonist. Pretreatment of the tissue with pertussis toxin abolished the D-2 dopaminergic inhibition of POMC synthesis. The potency of pertussis toxin in abolishing the dopaminergic inhibition of POMC synthesis corresponded to its potency in abolishing the D-2 dopaminergic inhibition of adenylate cyclase activity. Cholera toxin, forskolin, and 8-bromo-cAMP, compounds that activate the cAMP pathway, enhanced the capacity of intermediate lobe tissue to synthesize POMC and counteracted the dopaminergic inhibition of POMC synthesis. Incubation of intermediate lobe tissue for 24 h with bromocriptine, a D-2 dopaminergic agonist, decreased the POMC mRNA content by 46% as determined by hybridization of RNA to a 32P-labeled probe. Incubation of intermediate lobe tissue with forskolin increased the level of POMC mRNA; incubation of the tissue with a combination of bromocriptine and forskolin also resulted in an increase in the level of POMC mRNA. It is proposed that Ni, the inhibitory guanyl nucleotide binding protein, and possibly adenylate cyclase mediate the dopaminergic inhibition of POMC synthesis.

Altered activity of the inhibitory guanyl nucleotide-binding component (Ni) induced by pertussis toxin. Uncoupling of Ni from receptor with continued coupling of Ni to the catalytic unit.

The D-2 dopamine receptor mediates inhibition of adenylate cyclase in rat intermediate lobe; this receptor is linked to cyclase by the inhibitory guanyl nucleotide-binding protein (Ni). The functioning of components in the inhibitory system was compared in control and pertussis toxin-treated tissues. (-)-N-n-Propylnorapomorphine ((-)-NPA), a dopamine agonist, and 5'-guanylyl imidodiphosphate (Gpp(NH)p), a nonhydrolyzable GTP analog, caused a dose-dependent inhibition of adenylate cyclase in control tissue. Pertussis toxin abolished dopamine receptor-mediated inhibition of adenylate cyclase but did not alter Gpp(NH)p-induced inhibition of cyclase. In control tissue, GTP blocked Gpp(NH)p inhibition of cyclase in the absence, but not in the presence of (-)-NPA. Following pertussis toxin treatment, GTP blocked the inhibitory effect of Gpp(NH)p either in the absence or in the presence of (-)-NPA. Pertussis toxin did not alter the number of dopamine receptors or the affinity of the receptor for [3H]spiroperidol, a dopamine antagonist. However, pertussis toxin decreased the potency of (-)-NPA in the binding assay and abolished the ability of GTP to affect agonist binding. Furthermore, pertussis toxin abolished the dopamine receptor-mediated inhibition of immunoreactive alpha-melanocyte-stimulating hormone release, and induced the ADP-ribosylation of the Mr = 41,000 subunit of Ni.

Bromocriptine-induced changes in the biochemistry, physiology, and histology of the intermediate lobe of the rat pituitary gland.

Treatment of rats with 2-Br-alpha-ergocryptine (bromocriptine; CB 154) elicits biochemical, physiological, and histological changes in the intermediate lobe (IL) of the rat pituitary gland, suggesting a decrease in activity in the IL. CB 154 treatment decreases 1) the capacity of the IL to synthesize proopiomelanocortin (POMC), 2) the content of mRNA directing the synthesis of POMC, and 3) the capacity of the IL to synthesize the peptides desacetyl alpha MSH, N,O-diacetyl alpha MSH, and alpha MSH. CB 154 treatment also causes a 40% loss of IL protein and an atrophy of the IL. CB 154 treatment has a biphasic effect upon the IL content of alpha MSH-like peptides; the drug first increases and then diminishes the content of these molecules. Control experiments using CB 154-treated IL tissue suggest that these effects of CB 154 are not a toxic effect of CB 154 on the IL. Spiroperidol reverses the effects of CB 154 on POMC synthesis and POMC mRNA content; by itself, spiroperidol increases the IL synthesis of POMC, the IL content of POMC mRNA, and the capacity of the IL to synthesize immunoreactive alpha MSH. Stalk section of rat pituitary gland also results in an increase in the capacity of the IL to synthesize POMC. These results suggest that a D-2 dopamine receptor mediates a tonic inhibition of the function of the IL.

Evidence that the D-2 dopamine receptor in the intermediate lobe of the rat pituitary gland is associated with an inhibitory guanyl nucleotide component.

Stimulation of the D-2 dopamine receptor in the intermediate lobe (IL) of the rat pituitary gland diminishes both basal and isoproterenol-stimulated adenylate cyclase activity. Cholera toxin increases IL adenylate cyclase activity and reduces the ability of beta-adrenergic agonists to further enhance enzyme activity but does not alter the functioning of the D-2 dopamine receptor. Indeed, cholera toxin-treated IL tissue provides a useful experimental system to investigate the involvement of guanyl nucleotides in the functioning of the IL D-2 dopamine receptor. GTP is obligatory for dopaminergic agonists to inhibit adenylate cyclase activity of cholera toxin-treated IL tissue. Furthermore, 5'-guanylyl imidodiphosphate (Gpp[NH]p), a nonhydrolyzable analog of GTP, inhibits adenylate cyclase activity in the absence of a dopaminergic agonist. GTP reverses the Gpp(NH)p-induced inhibition of adenylate cyclase activity; apomorphine, a dopaminergic agonist, abolishes this effect of GTP. It is hypothesized that the D-2 dopamine receptor in the IL interacts with an inhibitory guanyl nucleotide component (Ni); stimulation of the D-2 dopamine receptor alters the properties of Ni so that Ni can interact with GTP and inhibit adenylate cyclase activity.

Desensitization of the D-2 dopamine receptor and the ?(2)-adrenoceptor in the intermediate lobe of the rat pituitary gland.

A D-2 dopamine receptor and a ?(2)-adrenoceptor occur in the intermediate lobe of the rat pituitary gland (IL). Exposure of intact IL tissue to a D-2 agonist diminished the ability of dopaminergic agonists [but not 5?-guanylyl imidodiphosphate (Gpp(NH)p)] to inhibit adenylate cyclase activity. Conversely, exposure of intact IL tissue to a ?-adrenergic agonist diminished the ability of a ?-adrenergic agonist (but not forskolin) to stimulate adenylate cyclase activity. Treatment of ovariectomized rats with 17?-estradiol desensitizes the ?(2)-adrenoceptor but not the D-2 receptor. Desensitization of the IL catecholamine receptors is discussed within the framework of a previously published "working model" of these receptors.