Functional reconstruction of purified Gi and Go with mu-opioid receptors in guinea pig striatal membranes pretreated with micromolar concentrations of N-ethylmaleimide

Review of Kyotorphin Research: A Mysterious Opioid Analgesic Dipeptide and Its Molecular, Physiological, and Pharmacological Characteristics

Tyrosine-arginine (kyotorphin), an opioid analgesic dipeptide, was discovered more than 40 years ago in 1979. The evidence accumulated during this period has established the physiological significance of kyotorphin as a neuromodulating peptide, and pharmacological applications. Some of the following important findings have been discussed in this review: (1) kyotorphin is unevenly distributed in the brain; it is found in high concentrations in the pain pathway, which involves the regions associated with morphine analgesia; (2) kyotorphin is subcellularly localized in the synaptosome fraction or nerve-ending particles; (3) a specific synthetase generates kyotorphin from tyrosine and arginine; (4) kyotorphin may be also processed from calpastatin by a novel calcium-activated neutral protease or calpain; (5) kyotorphin preloaded into the synaptosome is released by high K⁺ depolarization in a Ca²⁺-dependent manner; (6) kyotorphin has a specific G protein coupled receptor, which mediates the activation of phospholipase C (PLC) and inhibition of adenylyl cyclase through G_i; (7) leucine-arginine works as a specific kyotorphin receptor antagonist; 8) membrane-bound aminopeptidase or excretion through a peptide transporter, PEPT2, may contribute to the inactivation of kyotorphin; and (9) kyotorphin causes increased Met-enkephalin release from brain and spinal slices. It is also known that the opening of plasma membrane Ca²⁺ channels through a conformational coupling of the InsP₃ receptor with the transient receptor potential C1, which is downstream of the kyotorphin receptor-mediated activation of G_i and PLC, could be a potential underlying mechanism of Met-enkephalin release. Considering these findings, translational research is an exciting domain that can be explored in the future. As kyotorphin is a small molecule, we could design function-added kyotorphin derivatives. These studies would include not only the brain-permeable kyotorphin derivatives but also hybrid kyotorphin derivatives conjugated with small compounds that have additional pharmacological actions. Further, since there are reports of kyotorphin being involved in either the etiology or treatment of Alzheimer's disease, epilepsy, inflammation, and chronic pain, studies on the beneficial effects of kyotorphin derivatives should also be expected in the future.

Nitric oxide inhibits nociceptive transmission by differentially regulating glutamate and glycine release to spinal dorsal horn neurons

Nitric oxide (NO) is involved in many physiological functions, but its role in pain signaling remains uncertain. Surprisingly, little is known about how endogenous NO affects excitatory and inhibitory synaptic transmission at the spinal level. Here we determined how NO affects excitatory and inhibitory synaptic inputs to dorsal horn neurons using whole-cell recordings in rat spinal cord slices. The NO precursor L-arginine or the NO donor SNAP significantly increased the frequency of glycinergic spontaneous and miniature inhibitory postsynaptic currents (IPSCs) of lamina II neurons. However, neither L-arginine nor SNAP had any effect on GABAergic IPSCs. L-arginine and SNAP significantly reduced the amplitude of monosynaptic excitatory postsynaptic currents (EPSCs) evoked from the dorsal root with an increase in paired-pulse ratio. Inhibition of the soluble guanylyl cyclase abolished the effect of L-arginine on glycinergic IPSCs but not on evoked monosynaptic EPSCs. Also, inhibition of protein kinase G blocked the increase in glycinergic sIPSCs by the cGMP analog 8-bromo-cGMP. The inhibitory effects of L-arginine on evoked EPSCs and high voltage-activated Ca(2+) channels expressed in HEK293 cells and dorsal root ganglion neurons were abolished by blocking the S-nitrosylation reaction with N-ethylmaleimide. Intrathecal injection of L-arginine and SNAP significantly increased mechanical nociceptive thresholds. Our findings suggest that spinal endogenous NO enhances inhibitory glycinergic input to dorsal horn neurons through sGC-cGMP-protein kinase G. Furthermore, NO reduces glutamate release from primary afferent terminals through S-nitrosylation of voltage-activated Ca(2+) channels. Both of these actions probably contribute to inhibition of nociceptive transmission by NO at the spinal level.

Voltage-dependent kappa-opioid modulation of action potential waveform-elicited calcium currents in neurohypophysial terminals

Release of neurotransmitter is activated by the influx of calcium. Inhibition of Ca(2+) channels results in less calcium influx into the terminals and presumably a reduction in transmitter release. In the neurohypophysis (NH), Ca(2+) channel kinetics, and the associated Ca(2+) influx, is primarily controlled by membrane voltage and can be modulated, in a voltage-dependent manner, by G-protein subunits interacting with voltage-gated calcium channels (VGCCs). In this series of experiments we test whether the kappa- and micro-opioid inhibition of Ca(2+) currents in NH terminals is voltage-dependent. Voltage-dependent relief of G-protein inhibition of VGCC can be achieved with either a depolarizing square pre-pulse or by action potential waveforms. Both protocols were tested in the presence and absence of opioid agonists targeting the kappa- and micro-receptors in neurohypophysial terminals. The kappa-opioid VGCC inhibition is relieved by such pre-pulses, suggesting that this receptor is involved in a voltage-dependent membrane delimited pathway. In contrast, micro-opioid inhibition of VGCC is not relieved by such pre-pulses, indicating a voltage-independent diffusible second-messenger signaling pathway. Furthermore, relief of kappa-opioid inhibition during a physiologic action potential (AP) burst stimulation indicates the possibility of activity-dependent modulation in vivo. Differences in the facilitation of Ca(2+) channels due to specific G-protein modulation during a burst of APs may contribute to the fine-tuning of Ca(2+)-dependent neuropeptide release in other CNS terminals, as well.

Gi/Go protein-dependent presynaptic mechanisms are involved in clozapine-induced down-regulation of tyrosine hydroxylase in PC12 cells

Although the clinical effects of antipsychotics have been extensively studied, the molecular mechanisms underlying their antipsychotic activity are unclear. Chronic clozapine has been reported to reduce significantly the expression of tyrosine hydroxylase (TH) in the mesolimbic system. To characterize the mechanisms of action of clozapine on TH expression, PC12 cells turned out to be a useful model, being by far less complex than the entire brain. Both the quantity of TH protein and the amount of TH mRNA in PC12 cells were found to be decreased during incubation of the cells in the presence of clozapine. This decline was followed by a decrease in the enzymatic activity of TH. The effect of clozapine was blocked by preincubation with N-ethylmaleimide, a sulphydryl-alkylating reagent that interferes in Gi/o protein-mediated second messenger pathways. Clozapine may thus decrease TH expression by interacting with Gi/o protein-coupled receptors, such as D2 and 5HT1A. Knowledge of the molecular mechanisms underlying the clinical effects of established antipsychotics will promote the development of new and more efficient antipsychotic drugs.

Role of presynaptic 5-HT1A and 5-HT3 receptors in modulation of synaptic GABA transmission in dissociated rat basolateral amygdala neurons

Serotonin (5-HT) is considered to play a significant role in anxiety-related behaviors in animals through actions on the amygdaloid complex. To evaluate this role from the point of neurotransmitter release regulation, nystatin-perforated patch recording was employed on mechanically dissociated basolateral amygdala neurons containing functional synaptic boutons. GABAAergic miniature inhibitory postsynaptic currents (mIPSCs) were pharmacologically separated. In subsets of neurons, 8-OH-DPAT (1 microM), a specific 5-HT1A agonist, continuously inhibited mIPSC frequency without effects on mIPSC amplitude. By comparison, mCPBG (1 microM), a specific 5-HT3 agonist, transiently facilitated mIPSC frequency without effects on mIPSC amplitude. Together these results suggest the presynaptic existence of both 5-HT receptor subtypes. In these neurons, application of 8-OH-DPAT and its subsequent removal still suppressed mCPBG-induced responses on mIPSCs. This suppression was not caused by a reduction of presynaptic 5-HT3 receptor affinities to mCPBG and was completely eliminated by pretreatment with N-ethylmaleimide, a pertussis toxin sensitive GTP-binding protein inhibitor. In the neurons exhibiting presynaptic modulation with mCPBG but not 8-OH-DPAT, such suppression by exposure to 8-OH-DPAT was not observed. In conclusion, activation of presynaptic 5-HT1A receptors inhibited mIPSC frequency and at the same time suppressed, via a G-protein-mediated mechanism, the transient facilitation of mIPSC frequency produced by activation of presynaptic 5-HT3 receptors.

Quantitative stoichiometry of G-proteins activated by mu-opioid receptors in postmortem human brain

Paradoxically, the potencies (EC(50)) of agonists stimulating [35S]GTPgammaS binding are several orders of magnitude lower than their affinities in receptor binding assays. We have investigated the quantitative stoichiometry of mu-opioid receptor-G-protein coupling in postmortem human brain. [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]enkephalin (DAMGO) displaced [3H]naloxone binding in a biphasic pattern. The ratio between K(i-low) and EC(50) of DAMGO stimulating [35S]GTPgammaS binding was lower than one. The K(A) of DAMGO was calculated following mu-opioid receptor alkylation by beta-funaltrexamine from [35S]GTPgammaS binding data using the "nested hyperbolic method", yielding K(A)/EC(50)>1. Thus, only 1.2 +/- 0.2% of mu-opioid receptors was needed to be occupied to achieve the half-maximal effect of DAMGO. The estimated ratio between the G-proteins activated by 10 microM DAMGO (determined by isotopic dilution curves) and the occupied-mu-opioid receptors was 1304. In conclusion, we have determined the stoichiometric and the kinetic parameters in the mu-opioid receptor-G-protein system.

Modulation of Chelidonii herba on GABA activated chloride current in rat PAG neurons

Modulation of Chelidonii herba on gamma-aminobutyric acid (GABA) activated chloride current in the acutely dissociated periaqueductal gray (PAG) neuron was studied by nystatin-perforated patch-clamp technique. High concentrations of Chelidonii herba elicited ion current, that was blocked by bicuculline. Low concentrations reduced the GABA activated current in PAG. Two types of inhibitory action of Chelidonii herba on GABA activated current have been implicated in PAG. One is the inhibitory action of Chelidonii herba on GABA was abolished by naltrexone and the other is that of Chelidonii herba was potentiated by naltrexone. In addition, all of two types of action of Chelidonii herba are linked to pertussis toxin-sensitive GTP-binding proteins. These results suggest that the inhibitory modulation of Chelidonii herba on GABA activated current via G-proteins in PAG neuron is an important analgesic mechanism.

Pharmacological characterization of metabotropic glutamate receptor-mediated high-affinity GTPase activity in rat cerebral cortical membranes

Activation of heterotrimeric guanine nucleotide-binding regulatory proteins (G-proteins) functionally coupled to metabotropic glutamate receptors (mGluRs) was assessed by agonist-induced high-affinity GTPase (EC3.6.1.-) activity in rat cerebral cortical membranes. L-Glutamate (1 mM) stimulated high-affinity GTPase activity to the same extent throughout the incubation period up to 20 min, in a Mg(2+)-dependent manner. The addition of 1 mM L-glutamate augmented V(max) of the enzyme activity (1670 to 3850 pmol mg(-1) protein 15 min(-1)) with slight increase in K(M) value (0.26 to 0.63 microM). The high-affinity GTPase activity was stimulated by the following compounds with a rank order of potency of (2S,2'R,3'R)-2-(2', 3'-dicarboxycyclopropyl) glycine (DCG-IV) > (2S,1'S, 2'S)-2-(carboxycyclopyropyl)glycine (L-CCG-I) > L-glutamate > or = 2R, 4R-4-aminopyrrolidine-2,4-dicarboxylate [(2R,4R)-APDC] > 1S, 3R-1-aminocyclopentane-1,3-dicarboxylate [(1S,3R)-ACPD] > (S)-4-carboxy-3-hydroxyphenylglycine [(S)-4C3HPG] > (S)-3-carboxy-4-hydroxyphenylglycine [(S)-3C4HPG] > ibotenate, but not by L-(+)-2-amino-4-phosphonobutyrate (L-AP4), (RS)-3, 5-dihydroxyphenylglycine [(RS)-3,5-DHPG], quisqualate, or L-serine-O-phosphate (L-SOP), indicative of involvement of group II mGluRs, in particular mGluR2. (2S)-alpha-Ethylglutamate (EGLU), a presumably selective antagonist against group II mGluRs, inhibited DCG-IV-stimulated high-affinity GTPase activity in a competitive manner with an apparent K(B) of 220 microM. L-Glutamate-stimulated activity was eliminated by pretreatment of the membranes with sulfhydryl alkylating agent N-ethylmaleimide (NEM) at 30-50 microM, indicating that G-proteins of the G(i) family are involved. These results indicate that mGluR agonist-induced high-affinity GTPase activity in rat cerebral cortical membranes may be used to detect the functional interaction between group II mGluRs, in particular mGluR2, and NEM-sensitive G(i) proteins.

Autoradiography of receptor-activated G-proteins in post mortem human brain

The agonist-stimulated guanosine 5'-(gamma-[(35)S]thio)triphosphate binding assay was used to anatomically localize receptor-activated G-proteins by autoradiography in post mortem human brain. The optimal conditions for guanosine 5'-(gamma-[(35)S]thio)triphosphate binding to human brain sections were established in post mortem samples of the prefrontal cortex, hippocampus, basal ganglia, brainstem and cerebellar cortex. An excess of GDP (2mM) was required to decrease basal activity and obtain effective stimulation by specific agonists. guanosine 5'-(gamma-[(35)S]Thio)triphosphate binding was increased after stimulation with specific agonists of different G-protein-coupled receptors. They include cannabinoid (WIN55212-2), mu-opioid ([D-Ala(2),N-Me-Phe(4), Gly(5)-ol]enkephalin), serotonin-1A [(+/-)-8-hydroxy-2-(di-n-propylamino)tetralin] and serotonin-1B/1D (sumatriptan), cholinergic muscarinic receptors (carbachol) and alpha(2)-adrenoceptors (UK14304). Such stimulation reached 1458%, 440%, 188%, 219%, 61% and 339%, respectively, over the basal levels. In tissue sections, the use of the above-mentioned agonists (10(-4)M) showed patterns of anatomical distribution similar to those already described by receptor autoradiography, with high densities over the hippocampus (serotonin-1A receptors), cortex (alpha(2)-adrenoceptors) and striatum (mu-opioid receptors). The highest binding levels were reached with the cannabinoid receptor agonist in most of the analysed brain regions. Carbachol produced only moderate stimulation of those same regions. The blockage of agonist-stimulated guanosine 5'-(gamma-[(35)S]thio)triphosphate binding by selective antagonists verified that the effect was receptor mediated. This technique provides a method to identify modifications of the receptor-mediated activation of G-proteins in post mortem human brain with anatomical resolution. It also provides valuable information on the level of drug efficacy in the human species.

Anatomical distribution of sodium-dependent [(3)H]naloxone binding sites in rat brain

The sulfhydryl alkylating reagent N-ethylmaleimide (NEM) blocks opioid receptor binding and receptor/G-protein coupling. Sodium partially restores [(3)H]naloxone binding after inhibition by NEM to reveal sodium-dependent [(3)H]naloxone sites, defined as binding in the presence of 50-100 mM NaCl after treatment of membranes or sections with 750 microM NEM. In the present study, receptor autoradiography of [(3)H]naloxone binding in control and NEM-treated tissue was used to examine the anatomical distribution of sodium-dependent [(3)H]naloxone sites in rat brain. In brain membranes, the pharmacology of sodium-dependent [(3)H]naloxone sites was consistent with that of mu opioid receptors. Relatively high IC(50) values for agonists and lack of effect of Gpp(NH)p on DAMGO displacement of [(3)H]naloxone binding in NEM-treated membranes indicated that the sodium-dependent sites were low affinity sites, presumably uncoupled from G-proteins. Autoradiograms revealed that NEM treatment dramatically reduced [(3)H]naloxone binding in all brain regions. However, [(3)H]naloxone binding was increased in specific regions in NEM-treated sections in the presence of sodium, including bed nucleus of the stria terminalis, interpeduncular nucleus, periaqueductal gray, parabrachial nucleus, locus coeruleus, and commissural nucleus tractus solitarius. Sodium-dependent [(3)H]naloxone binding sites were not found in other areas that exhibited [(3)H]naloxone binding in control tissue, including the striatum and thalamus. These studies revealed the presence of a subpopulation of [(3)H]naloxone binding sites which are sodium-dependent and have a unique regional distribution in the rat brain.

Characterization of receptor-mediated [35S]GTPgammaS binding to cortical membranes from postmortem human brain

The [35S]GTPgammaS binding assay represents a functional approach to assess the coupling between receptors and G-proteins. The optimal conditions for [35S]GTPgammaS binding to human brain homogenates were established in postmortem samples of prefrontal cortex. The influence of protein content, incubation time, GDP, Mg(2+), and NaCl concentrations on the [35S]GTPgammaS binding were assessed in the absence and presence of the alpha(2)-adrenoceptor agonist UK14304 5-bromo-N-(4,5-dihydro-1H-imidazol-2-yl)-6-quinoxalinamine). In conditions of 50 microM GDP and 100 mM NaCl, UK14304 increased the apparent affinity of the specific [35S]GTPgammaS binding without changing the apparent density. Concentration-response curves to agonists of alpha(2)-adrenoceptors, mu-opioid, 5-HT(1A), cholinergic muscarinic, and GABA(B) receptors displayed, in the presence of NaCl, maximal stimulations between 24% and 61% with EC(50) values in the micromolar range. Selective antagonists shifted to the right the agonist-induced stimulation curves. The G(i)/G(o)-protein alkylating agent N-ethylmaleimide decreased basal [35S]GTPgammaS binding in a concentration-dependent manner and inhibited the stimulation induced by the different agonists. In cortical sections, [35S]GTPgammaS binding to gray matter was stimulated by the agonist UK14304. The present study demonstrates that functional studies of the receptor coupling to G(i)/G(o)-proteins can be performed in postmortem human brain samples.

Selective coupling of mouse brain metabotropic sigma receptor with recombinant Gi1

Various sigma (sigma) ligands including (+)-pentazocine stimulated [35S]GTPgammaS binding in synaptic membranes from the mouse cerebellum. The (+)-pentazocine-stimulated [35S]GTPgammaS binding was blocked by the treatment of membranes with pertussis toxin (PTX), but completely recovered by the reconstitution of PTX-treated membranes with recombinant Gi1, but not with GoA. These findings suggest that metabotropic sigma receptors are selectively coupled to Gi1 protein.

Mu-opioid agonist-induced activation of G-protein-coupled inwardly rectifying potassium current in rat periaqueductal gray neurons

The characteristics of the inwardly rectifying K+ current activated by a mu-type opioid agonist, D-Ala2,N-MePhe4,Gly5-ol-enkephalin (DAMGO), were examined in the acutely dissociated rat periaqueductal gray neurons using the nystatin-perforated and the conventional whole-cell recording modes under voltage-clamp conditions. DAMGO activated inward currents in a concentration- and voltage-dependent manner. The DAMGO-induced current was an inwardly rectifying K+ current (I(DAMGO)) which was sensitive to K+ channel blockers, quinine and Ba2+ but insensitive to Cs+ and tetraethylammonium. In the conventional whole-cell clamp mode, guanosine 5'-O-(2-thiodiphosphate) trilithium salt (GDPbetas, 0.4 mM) inhibited the amplitude of I(DAMGO) to 28% of that of the initial current. After the intracellular perfusion with guanosine 5'-O-(3-thiotriphosphate) tetralithium salt (GTPgammas, 0.4 mM) for 1 min, the first application of DAMGO irreversibly activated I(DAMGO). By the extracellular application of N-ethylmaleimide at a concentration of 50 microM for 2 min, I(DAMGO) was completely abolished. When a conventional whole-cell patch was made with a patch-pipette containing 1 microg/ml of pertussis toxin together with 1 mM of beta-nicotinamide adenine dinucleotide, I(DAMGO) gradually declined to about 41% of its initial amplitude. The extracellular application of second messenger modulators including protein kinase inhibitor (staurosporin), protein kinase A activators (forskolin, 3-isobutyl-l-methyl-xanthine and dibutyryladenosine 3'5'-cyclic monophosphate) and protein kinase C activators (phorbol-12-myristate-13-acetate and 1-oleoyl-2-acetyl-sn-glycerol) had no effect on I(DAMGO). These results suggest that (i) DAMGO-activated inwardly rectifying K+ current is mediated by pertussis toxin-sensitive guanine nucleotide binding proteins (G-proteins); (ii) the types of G protein involved in I(DAMGO) are Gi and/or Go; and (iii) the G-proteins exert their roles in I(DAMGO) without any mediation of the second messenger systems.

Mechanism of endothelial nitric oxide-dependent vasorelaxation induced by wine polyphenols in rat thoracic aorta

The mechanisms by which red wine polyphenolic compounds (RWPCs) induced endothelium-dependent relaxation were investigated in rat thoracic aorta rings with endothelium. RWPCs produced relaxation that was prevented by the nitric oxide (NO) synthase inhibitor, N(omega)-nitro-L-arginine-methyl-ester. This relaxation was abolished in the absence of extracellular calcium in the medium or in the presence of the Ca2+ entry blocker, La3+, but it was not affected by the nonselective K+ channels blocker, tetrabutylammonium. N-Ethyl-maleimide (NEM), a sulfhydryl alkylating agent, abolished vasorelaxation produced by RWPCs and acetylcholine but not that produced either by the sarcoendoplasmic reticulum Ca2+-adenosine triphosphatase (ATPase) pump inhibitor, cyclopyazonic acid (CPA) or the calcium ionophore, ionomycin. Neither pertussis toxin (PTX) nor cholera toxin (CTX) inhibited the vasorelaxant effect of RWPC. The effect of RWPC was not affected by the phospholipase C (PLC) blocker, L-alpha-glycerophospho-D-myo-inositol 4-monophosphate (Gro-pip), and the phospholipase A2 pathway blockers, quinacrine and ONO-RS-082. Finally, the protein kinase C (PKC) inhibitor, GF 109203X, and tyrosine kinase inhibitors, tyrphostin A-23 and genistein, did not impair the response to RWPCs. These results suggest that RWPCs produce endothelium-NO-derived vasorelaxation through an extracellular Ca2+-dependent mechanism via an NEM-sensitive pathway. They also show that PTX- or CTX-sensitive G proteins, activation of PLC or PLA2 pathways, PKC, or tyrosine kinase may not be involved.

Non-specific action of methoxamine on Ito, and the cloned channels hKv 1.5 and Kv 4.2

The alpha1-adrenoceptor agonist methoxamine acted independently of receptor activation to reduce Ito and the sustained outward current in rat ventricular myocytes, and hKv 1.5 and Kv 4.2 cloned K+ channel currents. Two hundred microM methoxamine reduced Ito by 36% in the presence of 2 microM prazosin, and by 37 and 38% after preincubation of myocytes with either N-ethylmaleimide or phenoxybenzamine (n=6). The EC50 values at +60 mV for direct reduction of Ito, hKv 1.5, and Kv 4.2 by methoxamine were 239, 276, and 363 microM, respectively, with Hill coefficients of 0.87-1.5. Methoxamine accelerated Ito and Kv 4.2 current inactivation in a concentration- and voltage-dependent manner. Apparent rate constants for methoxamine binding and unbinding gave Kd values in agreement with EC50 values measured from dose-response relations. The voltage-dependence of block supported charged methoxamine binding to a putative intracellular site that sensed approximately 20% of the transmembrane electrical field. In the presence of methoxamine, deactivating Kv 4.2 tail currents displayed a distinct rising phase, and were slowed relative to control, such that tail current crossover was observed. These observations support a dominant mechanism of open channel block, although closed channel block could not be ruled out. Single-channel data from hKv 1.5 patches revealed increased closed times with blank sweeps and decreased burst duration in the presence of drug, and a reduction of mean channel open time from 1.8 ms in control to 0.4 ms in 500 microM methoxamine. For this channel, therefore, both open and closed channel block appeared to be important mechanisms for the action of methoxamine.

Activation of various subtypes of G-protein alpha subunits by partial agonists of the adenosine A1 receptor

The activation of different G protein subtypes by the rat adenosine A1 receptor initiated by stimulation with the full agonist 2-chloro-N6-cyclopentyladenosine (CCPA) and by six structurally distinct partial agonists of this receptor was investigated. Endogenous G protein alpha subunits in rat cortical membranes were inactivated by N-ethylmaleimide (NEM). Activation of rat recombinant myristoylated alpha(o), alpha(i1), alpha(i2) and alpha(i3) by partial agonists in comparison to the full agonist was assessed by guanosine-5'-(gamma-[35S]thio)triphosphate ([35S]GTPgammaS) binding after reconstitution of G protein alpha subunits with the adenosine A1 receptor in N-ethylmaleimide-treated membranes. 2-Chloro-N6-cyclopentyladenosine and 3' -deoxy-N6-cyclopentyladenosine (3'-d-CPA), the partial agonist with the highest intrinsic activity, were significantly more potent in activation of alpha(i) subtypes than alpha(o). In contrast, 5'-methylthioadenosine (MeSA), 2'-deoxy-2-chloroadenosine (cladribine), 2'-deoxy-N6-cyclopentyladenosine (2'-d-CPA), 2-phenylaminoadenosine (CV 1808) and C8-aminopropyl-N6-cyclopentyladenosine (C8-aminopropyl-CPA) did not exhibit higher potency for Go or any Gi subtype. All partial agonists, although carrying structurally different modifications, showed higher relative intrinsic activities in activation of Gi than of Go, indicating that Gi-coupled pathways may be activated selectively via the A1 receptor by partial agonists, but not Go-mediated responses.

Influence of Gz and Gi2 transducer proteins in the affinity of opioid agonists to mu receptors

The affinity displayed by different opioids to mu receptors (ORs) was determined in mouse brain membranes incubated with antibodies directed to Galpha subunits of the guanine nucleotide-binding proteins Gi2 and Gz. Assays were conducted with 10 pm 125I-Tyr27-beta-endorphin in the presence of 300 nm N, N-diallyl-Tyr-(alpha-aminoisobutyric acid)2-Phe-Leu-OH (ICI-174 864), which prevented the binding of the iodinated neuropeptide to delta-ORs. Gpp(NH)p or the preincubation of mouse brain membranes with IgGs to Gi2alpha or Gzalpha subunits, promoted reductions in the affinity exhibited by the labelled probe. The potencies of beta-endorphin, [D-Ala2,N-MePhe4,Gly-ol5]-enkephalin (DAMGO) and [D-Pen2,5]enkephalin (DPDPE) were reduced after impairing the coupling of mu-ORs to Gi2 or Gz proteins. Morphine showed a loss of affinity towards the mu-OR after preincubation of membranes with IgGs to Gzalpha subunits. However, it retained its potency after treatment with the anti-Gi2alpha IgGs. Conversely, [D-Ala2, D-Leu5]enkephalin (DADLE) and [D-Ser2, Leu5] enkephalin-Thr6 (DSLET) showed decreased affinity to mu-ORs after treatment with anti-Gi2alpha IgGs, with no noticeable change following the use of IgGs to Gzalpha subunits. The affinity exhibited by the opioid antagonists naloxone, naltrexone, naloxonazine and [Cys2,Tyr3,Orn5, Pen7 amide]somatostatin analogue (CTOP) remained unchanged after either treatment. Therefore, the affinity exhibited by opioid agonists of mu-ORs, but not antagonists, depends on the nature of the G-protein coupled to these receptors.

Tandospirone-induced K+ current in acutely dissociated rat dorsal raphe neurones

1. The effects of tandospirone (TDS) on dissociated rat dorsal raphe neurones were investigated using the patch-clamp method. 2. Under current-clamp conditions, TDS hyperpolarized the cell membrane, resulting in the reduction of firing rates. 3. Under voltage-clamp conditions, TDS induced an inward rectifying K+ current in a concentration-dependent manner. 4. The TDS-induced K+ currents (I(TDS)) were mimicked by 8-OH-DPAT, a 5-HT1A agonist. The I(TDS) was blocked by spiperone, a 5-HT1A receptor antagonist, in a concentration-dependent manner. 5. N-Ethylmaleimide, an agent which uncouples between the receptor and the G-protein, irreversibly blocked the I(TDS). 6. In neurones perfused intracellularly with a pipette-solution containing GTP using the conventional whole-cell patch recording, the I(TDS) showed a gradual rundown. When the neurones were perfused with GTPgammaS, TDS activated the inwardly rectifying K+ current in an irreversible manner. 7. In the inside-out patch recording mode, TDS-activated single K+ channel currents (i(TDS)) which also showed an inward rectification. When the GDP in cytosolic side was completely replaced with GTP, the open probability of i(TDS) significantly increased. 8. These results indicate that the activation of 5-HT1A receptors by TDS directly opens the inward rectifying K+ channels via a G-protein mediated process.

Lack of interfering effects of lithium on receptor/G protein coupling in human platelet and rat brain membranes

To verify the theory that lithium exerts its multiple effects by altering the receptor-mediated G protein's activation, in vitro effects of lithium on agonist-induced guanosine triphosphate (GTP) hydrolysis were examined. The basal GTP hydrolyzing activity in human platelet membranes was decreased nonselectively by either LiCl or NaCl at millimolar concentrations, whereas (-)-epinephrine-stimulated increase in the activity (an index of alpha (2A)-adrenoceptor coupled Gi2 function) was unaltered. Furthermore, the stimulation of high-affinity GTPase activity induced by dopamine, carbachol, and R-N(6)-phenylisopropyladenosine in rat brain membranes (indices of the functional coupling between dopamine D2-like, pirenzepine-insensitive muscarinic, and adenosine A1 receptors and their respective Gi proteins) was substantially unaltered regardless of whether 0.5 mmol/L adenosine 5'-(beta, gamma-imido)triphosphate (i.e., 1.75 mmol/L lithium) was included in the assay mixture or not. These results indicate that lithium does not affect in vitro the receptor-mediated activational process of G proteins, at least not of Gi associated with adenylate cyclase inhibition.

Sigma ligands stimulate GTPase activity in mouse prefrontal membranes: evidence for the existence of metabotropic sigma receptor

We studied effects of various sigma ligands on GTPase activity in mouse prefrontal membranes. Some representative sigma agonists, such as (+)-pentazocine, SA4503 and (+)-3-PPP, stimulated the GTPase activity in a concentration-dependent manner in ranges of 10 nM to 10 microM. Maximal effect was almost 10% increase to the control without treatment of drugs. However, another representative agonist, (+)-SKF10,047 showed only a partial activity with maximal effect 5% at 1 microM. NE-100, a representative antagonist, showed no effect at concentrations not more than 100 nM, while it did stimulate GTPase activity at 1 and 10 microM. Furthermore, these stimulative effects of both (+)-pentazocine and SA4503 on GTPase activity were significantly antagonized by NE-100 at 100 nM, suggesting that NE-100 possesses agonist-antagonist property. These findings suggest the possibility that there exist metabotropic sigma receptors.

Mu and delta opioids but not kappa opioid inhibit voltage-activated Ba2+ currents in neuronal F-11 cell

Whole-cell patch-clamp recordings were used to study Ba2+ currents through voltage-dependent Ca2+ channels in dorsal root ganglion x mouse neuroblastoma hybrid (F-11) cells. Opioid agonists selective for either mu (Tyr-D-Ala-Gly-Mephe-Gly-ol; DAMGO) or delta (Tyr-D-Pen-Gly-Phe-D-Pen-OH; DPDPE) receptors inhibited high-threshold Ba2+ currents. The inhibition was reversible, naloxone-sensitive, and dose-dependent. The inhibitory effects of both DAMGO and DPDPE were blocked by pretreatment of the cells with pertussis toxin (PTX) as well as by brief exposure to the sulfhydryl alkylating agent, N-ethylmaleimide (NEM). The N-type Ca2+ channel antagonist omega-conotoxin GVIA (omega-CTX GVIA) irreversibly inhibited high threshold Ba2+ currents by 66% and blocked the inhibitory effect of DAMGO or DPDPE. In contrast, the L-type Ca2+ channel blocker nifedipine inhibited high threshold Ba2+ currents by 15% and failed to block the inhibitory effect of DAMGO or DPDPE. These results demonstrate that mu and delta opioid receptors are negatively coupled to N-type Ca2+ channels via PTX- and NEM-sensitive GTP-binding proteins in F-11 cells.

Aripiprazole, a novel antipsychotic drug, inhibits quinpirole-evoked GTPase activity but does not up-regulate dopamine D2 receptor following repeated treatment in the rat striatum

Aripiprazole, a quinolinone derivative, is a new dopaminergic agent which has been recently developed and demonstrated to be clinically useful as an antipsychotic drug with reduced extrapyramidal motor side effects. Here, we found that aripiprazole competed [3H]spiperone binding with a 100-fold higher affinity than [3H]SCH23390 binding, and inhibited the quinpirole-induced facilitation of high-affinity GTPase activity in rat striatal membranes. The effects of chronic administration of aripiprazole and haloperidol on dopamine D2 receptor binding and mRNA level in rat striata were examined by a [3H]spiperone binding assay and a ribonuclease protection assay. Haloperidol induced a significant rise in Bmax of [3H]spiperone binding at 1 mg/kg and in the level of dopamine D2L receptor mRNA at 4 mg/kg. A high dose of aripiprazole (100 mg/kg) only tended to increase the Bmax of [3H]spiperone binding non-significantly, and had no effect on the level of dopamine D2L receptor mRNA. These results indicated that aripiprazole had an antagonistic activity to dopamine D2 receptors with a high affinity, but that the potency of aripiprazole to up-regulate dopamine D2 receptors in the striatum was much smaller than that of haloperidol. This small up-regulation may be related to the ability to aripiprazole to act without side effects including tardive dyskinesia.

Characterization of the K+ current mediated by 5-HT1A receptor in the acutely dissociated rat dorsal raphe neurons

The action of 5-hydroxytryptamine (5-HT) via the 5-HT1A receptor on dissociated rat dorsal raphe neurons was characterized under the whole-cell mode by using the nystatin-perforated patch-clamp technique. Under voltage-clamp conditions, 5-HT induced an inwardly rectifying K+ current (I5-HT) in a concentration-dependent manner. I5-HT was mimicked by 8-OH-DPAT and buspirone, which are both 5-HT1A receptor agonists. I5-HT was reversibly blocked by such 5-HT1A receptor antagonists as (S)-UH-301 a 5-HT4 receptor antagonist. I5-HT was antagonized concentration-dependently by such K+ channel blockers as quinine, Ba2+ and 4-aminopyridine but was relatively insensitive to both CS+ and tetraethylammonium. When the neurons were loaded with guanosine 5'-O-3-thiotriphosphate through a patch pipette, the K+ current induced by 5-HT became irreversible. N-ethylmaleimide (NEM), a sulfhydryl alkylating agent, irreversibly blocked I5-HT. The intracellular perfusion with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA), a Ca2+ chelator, or neomycine, a phospholipase C inhibitor, never significantly affected the 5-HT-induced response. 12-Myristate 13-acetate diester (PMA), a protein kinase C (PKC) activator, had only a weak inhibitory effect on I5-HT, and staurosporine, a PKC inhibitor, failed to significantly occlude I5-HT. Therefore, the K+ conductance activated via the 5-HT1a receptor of dorsal raphe neurons was thus characterized by the sensitivity to such K+ channel blockers as quinine, Ba2+ and 4-aminopyridine. Moreover, G protein which is NEM-sensitive and can couple to the 5-HT1A receptor, is thus considered to activate the inwardly rectifying K+ conductance without being mediated by such second messengers as Ca2+ and PKC.

Protein kinase C inhibitor potentiates the agonist-induced GTPase activity in COS cell membranes expressing delta-opioid receptor

In COS-7 cell membranes expressing cloned delta-opioid receptor, [D-Ser2, Leu5]enkephalin-Thr6, an opioid delta-agonist, showed no significant stimulation of high-affinity GTPase, while this agonist binding showed a guanine nucleotide sensitivity. Significant stimulation of GTPase activity by this agonist was observed only when the cells were pretreated with 0.1 microM calphostin C, a protein kinase C inhibitor, and when this inhibitor was further added to the reaction mixture at 1 microM. These findings suggest that protein kinase C is involved in the heterologous desensitization of delta-opioid receptor in the cells.

Purification of chlorpromazine-sensitive GTPase from rat cerebral cortex

The chlorpromazine-sensitive GTPase from the cell membrane of rat cerebral cortex was purified to homogenity by using DEAE Bio-Gel A agarose, hydroxyapatite and heparin agarose chromatography. The purified chlorpromazine-sensitive GTPase was purified 370-fold to obtain a final specific activity of 40 mumol GTP hydrolyzed2min/mg protein. The purified enzyme was inhibited by chlorpromazine but not by compound 48/80. Magnesium was required for its activity instead of calcium. The purified enzyme had an apparent pH optimum of 8.0, and molecular weight was estimated to be 58,000.

Functional coupling of dopamine D2 and muscarinic cholinergic receptors to their respective G proteins assessed by agonist-induced activation of high-affinity GTPase activity in rat striatal membranes

Agonist-induced high-affinity GTPase activity was investigated using the crude membrane preparation from rat striatum. High-affinity GTPase activity was stimulated by dopamine and carbachol in a Mg(2+)-dependent manner and with possible optimum NaCl concentrations of 50-100 mM to detect the percent increase induced by each agonist. Dopamine and selective (as well as non-selective) D2 receptor agonists, but not selective D1 receptor agonists, stimulated activity in a concentration-dependent manner, with affinities which were significantly correlated with those for adenylate cyclase inhibition as previously reported in the literature. Maximal percent stimulation above basal high-affinity GTPase activity was 9.8 +/- 0.6% and 4.4-7.6% for dopamine and other synthetic dopamine D2 receptor agonists, respectively. Dopamine-stimulated activity was inhibited by several dopamine receptor antagonists with the following rank order of potency: (+)-butaclamol > spiperone > raclopride > S(-)-sulpiride; but not by (-)-butaclamol or SCH 23390. High-affinity GTPase activity was also stimulated by carbachol and acetylcholine through the pirenzepine-insensitive muscarinic receptors. Preincubation of the membranes with AS/7, a specific antiserum to Gi1 and Gi2, appeared to attenuate dopamine-sensitive activity, suggesting that Gi1 and/or Gi2 may be at least partially involved. These results indicate that high-affinity GTPase activity in rat striatal membranes is activated through dopamine D2-like receptors and pirenzepine-insensitive muscarinic receptors, both of which are negatively coupled to adenylate cyclase via Gi proteins.

A subtype of opioid kappa-receptor is coupled to inhibition of Gi1-mediated phospholipase C activity in the guinea pig cerebellum

PLC activity was stimulated either by 1-100 microM of GTP or by 100-3,000 microM Ca2+ in lysed synaptosomal membranes of the guinea pig cerebellum. The kappa-opioid receptor agonist selectively inhibited the PLC activity stimulated by 100 microM GTP, but not by 100-3,000 microM Ca2+. Pretreatment of membranes with PTX abolished such a kappa-agonist-induced inhibition of PLC activity. The reconstitution of Gi1, but not of Go purified from porcine brains with PTX-treated membranes showed a complete recovery of the kappa-agonist-inhibition of PLC activity. These findings suggest that a novel subtype kappa-receptor mediates inhibition of PLC through inhibiting the intrinsic activity of PTX-substrate G-proteins.

Subgroups among mu-opioid receptor agonists distinguished by ATP-sensitive K+ channel-acting drugs

1. We evaluated the effects of the i.c.v. administration of different K+ channel blockers (gliquidone, 4-aminopyridine and tetraethylammonium) and an opener of K+ channels (cromakalim) on the antinociception induced by several mu-opioid receptor agonists in a tail flick test in mice. 2. The s.c. administration of all agonists of mu-opioid receptors tested (morphine, 1-16 mg kg-1; metadone, 1-6 mg kg-1; buprenorphine, 0.04-0.64 mg kg-1; fentanyl, 0.02-0.32 mg kg-1 and levorphanol, 0.2-3.2 mg kg-1) elicited a dose-dependent antinociceptive effect. 3. The ATP-sensitive K+ channel blocker, gliquidone (0.06-16 micrograms per mouse, i.c.v.) antagonized the antinociception induced by buprenorphine, morphine and metadone. In contrast, gliquidone (0.25-160 micrograms per mouse) did not modify the antinociceptive effects of fentanyl and levorphanol. 4. Cromakalim (4-64 micrograms per mouse, i.c.v.), an opener of ATP-sensitive K+ channels, enhanced the antinociception produced by buprenorphine, morphine, and methadone, and did not significantly modify the antinociceptive effects of fentanyl and levorphanol. 5. The i.c.v. administration of the K+ channel blockers tetraethylammonium (10 micrograms per mouse) or 4-aminopyridine (25 ng per mouse) did not significantly modify the antinociception induced by any mu-opioid receptor agonist tested. 6. These results suggest that the opening of ATP-sensitive K+ channels is involved in the antinociceptive effect of morphine, buprenorphine and methadone, but not in that of fentanyl or levorphanol. Consequently, we suggest that at least two subgroups can be distinguished among mu-opioid receptor agonists, each inducing antinociception through different effector mechanisms.

Supersensitization of neurochemical responses by L-DOPA and dopamine receptor agonists in the striatum of experimental Parkinson's disease model rats

We studied the mechanisms of dopamine receptor agonist- and L-DOPA-mediated supersensitization in experimental Parkinson's disease model rats, by measuring in vivo acetylcholine (ACh) release, GTPase activities, and mRNA expression in the striatum of 6-hydroxydopamine-treated rats. D1 agonist (SKF38393) and D2/D3 agonists (bromocriptine and quinpirole) showed more potent stimulation or inhibitions on ACh release in the model rat than in the control. However, quinpirole-evoked stimulation of GTPase activity was enhance in the model rats, compared to the control, while there was no significant enhancement of the bromocriptine-evoked stimulation. On the other hand, L-DOPA at 0.3-10 pM showed a biphasic action including significant inhibition on the GTPase activity in the lesioned striatal membranes, but not in the control. In the RNAase protection assay, neither D1, D2, Gi1 alpha, GoA alpha nor Gs alpha mRNA expression in the model was significantly different from the control. These findings suggest that there is supersensitization of D1 and D2/D3 receptors in the experimental Parkinson's disease model, while the upregulation of their receptors or GTP-binding proteins (G-proteins) to be coupled to their receptors is unlikely involved in major parts of such mechanisms. In addition, the present report provides the first evidence that L-DOPA mediates neurochemical responses in the plasma membranes, possibly through its receptor.

Prejunctional actions of N-ethyl-maleimide and phenoxybenzamine in rat vas deferens

In studies of electrically evoked isometric contractions of rat vas deferens, N-ethyl-maleimide (30 microM) pretreatment significantly reduced the prejunctional inhibitory potencies of xylazine and 5-hydroxytryptamine but failed to affect the potency of the alpha 1-adrenoceptor agonist amidephrine. Phenoxybenzamine (1 microM) or N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) (10 microM) produced significant shifts in the potency of xylazine and significantly reduced the maximum inhibition, but the combination of phenoxybenzamine or EEDQ and N-ethyl-maleimide (30 microM) produced no further alteration in the effects of xylazine. In displacement studies, N-ethyl-maleimide displaced the binding of [3H]MK 912 ((2S,12bS)1',3'-dimethylspiro- (1,3,4,5',6,6',7,12b-octahydro-2H-benzo[b]furo[2,3-a]quinazoline)- 2,4'- pyrimidin-2'one) to rat renal cortex membranes with a Ki of 466 +/- 133 microM (n = 5), and so does not bind to alpha 2-adrenoceptors in the concentration range in which it affects prejunctional receptor mediated responses. This may suggest that N-ethyl-maleimide has actions other than inactivation of G-proteins or that the irreversible alpha 2-adrenoceptor antagonists phenoxybenzamine and EEDQ inactivate G-proteins sensitive to N-ethyl-maleimide in concentrations at which they bind to alpha 2-adrenoceptors.

Species and age-dependent differences of functional coupling between opioid delta-receptor and G-proteins and possible involvement of protein kinase C in striatal membranes

[D-Ser2,Leu5]-enkephalin-Thr6 (DSLET) and [D-Pen2,5]-enkephalin (DPDPE), both delta-agonists, stimulated the high affinity GTPase in the rat striatal membranes in a naltrindole-reversible manner. Similar stimulation was also observed in the striatal membrane preparations of 16-week-old guinea pigs, while not in those preparations of 4-week-old ones. When calphostin C, a protein kinase C inhibitor, was added to the reaction mixture, DSLET showed a marked stimulation in this activity in 4-week guinea pig striatal membranes. There was no effect of KT5720, a cyclic AMP-dependent protein kinase inhibitor, on such delta-opioid-mediated responses. These findings suggest that protein kinase C is locally involved in the functional uncoupling of delta-receptors to G-proteins in the striatum of young guinea pigs.

Supersensitivity of quinpirole-evoked GTPase activation without changes in gene expression of D2 and Gi protein in the striatum of hemi-dopaminergic lesioned rats

Quinpirole with D2 agonist activity showed a two-fold increase in the GTPase activity in the 6-hydroxydopamine-lesioned side of the striatum, compared with that in the control side. However, neither qualitative nor quantitative changes were observed in the gene expression of two alternative splicing products derived from dopamine D2 receptor gene or of Gil alpha, in the RNAase protection assay. These results suggest that D2 receptor-mediated supersensitization does not come from the D2 receptor upregulation.

Effect of delta-opioid antagonists on the functional coupling between opioid receptors and G-proteins in rat brain membranes

It is currently accepted that occupancy of opioid receptors by agonists, but not antagonists, promotes the association of the receptors to guanine nucleotide binding proteins (G-proteins) and stimulates a high affinity GTPase as part of the mechanism that links the receptor-ligand complex to adenylate cyclase inhibition. In this work we report that in rat brain membranes selective delta-opioid antagonists, the peptides N,N-Diallyl-Tyr-D-Leu-Gly-Tyr-Leu-OH (Diallyl-G) and N-N-Diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI174,864), inhibit the low Km GTPase activity in a concentration dependent way. On the other hand the delta-opioid agonists D-Ala2-D-Leu5-enkephalin (DADLE) and D-Ser2-Leu5-Thr6-enkephalin stimulate dose-dependently the low Km GTPase activity in rat brain membranes. This stimulation was blocked in the presence of Diallyl-G, and reciprocally the inhibition induced by Diallyl-G was reversed by DADLE. The inhibitory effect of Diallyl-G as well as the stimulation induced by DADLE were abolished when membranes were exposed to low concentrations of N-ethylmaleimide or by ADP ribosylation with pertussis toxin which interferes with the ability of the receptor to couple to G-proteins. These observations indicate that the inhibitory effect of Diallyl-G on GTPase requires a functional G-protein and suggest that certain delta-opioid antagonists exhibit negative intrinsic activity and may have the ability to inhibit the receptor-mediated activation of G-proteins.

A pertussis toxin-sensitive G protein mediates inhibition by morphine of spontaneous electrical activity of oxytocin neurones in anaesthetized rats

We investigated the effects of intracerebroventricular (i.c.v.) pertussis toxin upon the sensitivity of supraoptic oxytocin neurones to intravenous morphine (1-5000 micrograms/kg) in urethane-anaesthetized rats. The maximal inhibitory capacity of morphine was diminished by prior administration of pertussis toxin. Some cells were tested with both morphine and with the kappa-opioid agonist U50,488 (1-5000 micrograms/kg): U50,488-induced inhibition of firing rate was apparently unimpaired by pertussis toxin pre-treatment. The opioid inhibition of firing rate seen in the absence of and after pertussis toxin pre-treatment was naloxone-reversible. Thus a pertussis toxin-sensitive G protein may mediate the inhibitory action of morphine upon supraoptic putative oxytocin neurones or inputs to them.

Go mediates the coupling of the mu opioid receptor to adenylyl cyclase in cloned neural cells and brain

In membranes from SH-SY5Y human neuroblastoma cells differentiated with retinoic acid, the mu-selective agonist Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol (DAMGO) inhibited cAMP formation with an IC50 of 26 nM. Two separate antibodies raised against distinct regions of the Go alpha sequence attenuated the effect of DAMGO by 50-60%, whereas antibodies to Gi alpha 1,2 or Gi alpha 3 reduced the mu-opioid signal insignificantly or to a lesser extent. In contrast, inhibition of adenylyl cyclase by the delta-opioid agonist Tyr-D-Pen-Gly-Phe-D-Pen-OH (DPDPE; Pen = penicillamine) was very sensitive to the Gi alpha 1,2 antibody. In membranes from rat brain striatum, coupling of the mu opioid receptor to adenylyl cyclase was also maximally blocked by antibodies to Go alpha. After long-term treatment of the cells with DAMGO, the content of Go alpha was reduced by 26%, whereas the levels of Gi alpha 1,2, Gi alpha 3, and Gs alpha were unaltered. Addition of Go, purified from bovine brain, to membranes from pertussis toxin-treated SH-SY5Y cells restored the inhibition of adenylyl cyclase by DAMGO to 70% of that in toxin-untreated cells. To comparably restore the effect of DPDPE, much higher concentrations of Go were required. By demonstrating mediation of cAMP-dependent signal transduction by Go, these results describe (i) an additional role for this G protein present at a high concentration in brain, (ii) preferential, although not exclusive, interaction of mu and delta opioid receptors with different G protein subtypes in coupling to adenylyl cyclase, and (iii) reduced levels of Go following chronic opioid treatment of SH-SY5Y cells with mu opioids.

Opioid peptides activate phospholipase D and protein kinase C-epsilon in chicken embryo neuron cultures

The mu-opioid peptide morphiceptin stimulated a Ca(2+)-independent protein kinase C (PKC-epsilon) that is expressed both in embryonic day 6 chicken telencephalon and in derived neuronal cultures. This activation was seen as a 2-fold increase in the activity and level of cytosolic PKC-epsilon and as a transient increase in membrane-associated PKC-epsilon following morphiceptin treatment. Morphiceptin did not activate phospholipase C-mediated phosphatidylinositol hydrolysis but did transiently activate (2- to 3-fold) phospholipase D (PLD), as measured by phosphatidylethanol formation in neuron cultures derived from embryonic day 6 or day 7 cerebral hemispheres. This PLD activation could provide an alternative source of diacylglycerol for the activation of PKC-epsilon and was naloxone-reversible and at least partially blocked by the tyrosine kinase inhibitor herbimycin A. Addition of phorbol 12-myristate 13-acetate stimulated both PLD and PKC-epsilon activities to a greater extent than opioids. The phorbol ester and insulin stimulation of PLD was also blocked by herbimycin. Both morphiceptin (in a naloxone-reversible manner) and phorbol ester increased phosphorylation of similar cytosolic proteins in intact cells, demonstrating a functional role for the PKC-epsilon activation by opioids. This is evidence that opioid receptors are transiently coupled to tyrosine kinase, PLD and PKC-epsilon activation and, by implication, to neuronal cell growth during brain morphogenesis.

Effects of chronic low-level lead exposure on mRNA expression, ADP-ribosylation and photoaffinity labeling with [alpha-32P]guanine triphosphate-gamma-azidoanilide of GTP-binding proteins in neurons isolated from the brain of neonatal and adult rats

The effects of chronic lead exposure on mRNA expression, ADP-ribosylation and photoaffinity labeling with [alpha-32P]guanine triphosphate-gamma-azidoanilide ([32P]GTP-A) of alpha i or alpha s subunit of G protein were investigated in neurons isolated from the brain of neonatal and adult rats exposed to lead acetate or sodium acetate (for control). Rats were exposed by oral feeding for 10 days or 20 weeks to a low level of lead acetate or sodium acetate. The exposure started either prenatally or at an adult age. The expression of alpha i-mRNA in neurons obtained from the brain of control neonatal rats was significantly higher than that of the expression in samples obtained from the brain of control adult rats or the brain of rats exposed to lead at an adult age. The expression of alpha i-mRNA in neurons obtained from the brain of control neonatal rats, lead-exposed neonatal rats and adult rats prenatally exposed to lead did not differ significantly. Chronic lead exposure did not affect the expression of alpha s-mRNA in neurons obtained from the brain of neonatal and adult rats. The ADP-ribosylation or the photoaffinity labeling with [32P]GTP-A of alpha i or alpha s subunits reflected the developmental pattern of the expression of alpha i or alpha s-mRNA. The incorporation of radioactivity in alpha i-subunit obtained from the brain of control neonatal rats, lead-exposed neonatal rats and rats prenatally exposed to lead was greater than the incorporation in alpha i-subunit obtained from the brain of control adult rats or rats exposed to lead at an adult age. The incorporation of radioactivity did not differ significantly in alpha s-subunits obtained from control or lead-exposed neonatal and adult rats. These observations indicate that (1) the mRNA expression, ADP-ribosylation and photoaffinity labeling with [32P]GTP-A of alpha i-subunit decrease, whereas the mRNA expression, ADP-ribosylation and photoaffinity labeling with [32P]GTP-A of alpha s-subunit do not change as animals age after postnatal day 10, (2) chronic prenatal lead exposure delays the age-dependent decrease in mRNA expression, ADP-ribosylation and photoaffinity labeling of alpha i subunit, and (3) chronic adult exposure does not cause these changes.

Prejunctional alpha 2-adrenoceptors in mouse atria function through G-proteins which are sensitive to N-ethylmaleimide, but not pertussis toxin

1. The identity of the G-proteins involved in prejunctional alpha 2-adrenoceptor signal transduction in mouse atria was examined by use of the G-protein inactivators N-ethylmaleimide and pertussis toxin. 2. The alpha 2-adrenoceptor partial agonist clonidine (0.03 microM) inhibited the electrical stimulation-induced (S-I) outflow of radioactivity from mouse atria which were incubated with [3H]-noradrenaline and stimulated at 5 Hz. The partial alpha 2-adrenoceptor agonist St 363 (10 microM) inhibited the S-I outflow of radioactivity at the lower stimulation frequency of 2.5 Hz. The inhibitory effects of these compounds were not altered in mice pretreated with pertussis toxin (1.5 micrograms, i.v.). 3. The alpha 2-adrenoceptor antagonist, idazoxan (0.1 microM), increased the S-I outflow of radioactivity from mouse atria stimulated at 5 Hz, and this effect was not altered in atria from mice pretreated with pertussis toxin. 4. The inhibitory effects of clonidine and St 363 and the facilitatory effect of idazoxan on the S-I outflow of radioactivity from mouse atria were significantly less in atria incubated with N-ethylmaleimide (NEM, 3 microM) for 60 min before the [3H]-noradrenaline incubation. 5. The results suggest that prejunctional alpha 2-adrenoceptors in mouse atria function through G-proteins which are NEM-sensitive, but pertussis toxin insensitive.

Affinity cross-linked delta-opioid receptor in NG108-15 cells is low molecular weight (25 kDa) and coupled to GTP-binding proteins

The affinity cross-linking of the delta-opioid receptor in neuroblastoma x glioma NG108-15 cells was undertaken using (3-[125I]iodotyrosyl27)human-beta-endorphin ([125I]beta-endorphin) and disuccinimidyl suberate (DSS) or bis(sulfosuccinimidyl) suberate (BS3) in order to estimate molecular size. Following sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis, two radioactive bands were observed. Labeling of a major band of 29 kDa diminished in the presence of unlabeled selective delta-opioid agonist, [D-Pen2,D-Pen5]enkephalin (DPDPE), in a concentration-dependent manner, while labeling of a minor band of 58 kDa was hardly affected. The labeling intensity of the 29 kDa band decreased by addition of guanosine 5'-(3-o-thio)triphosphate (GTP gamma S) or by pretreatment of cells with pertussis toxin. These results, taking the molecular weight of covalently bound beta-endorphin (3.6 kDa) into consideration, suggest that the delta-opioid receptor in NG108-15 cell membrane is a 25 kDa protein which is coupled to pertussis toxin-sensitive guanosine triphosphate-binding proteins (G-proteins).

The aspartic acid in deltorphin I and dermenkephalin promotes targeting to delta-opioid receptor independently of receptor binding

Recent studies on the highly potent and selective delta-opioid agonists demenkephalin (Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2) and deltorphin I (Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2) suggested that key structural features necessary for specific targetting to the delta-opioid receptor are located within the C-terminal halves of these naturally occurring heptapeptides. To investigate the contribution of aspartic acid 4 residue in deltorphin I and aspartic acid 7 residue in dermenkephalin to the delta-addressing ability of the C-terminal ends, fourteen analogs were synthesized and assessed for their ability to bind to mu and delta-opioid receptors in rat brain membrane homogenates. Results showed that i/ although the tetrapeptide C-terminus of dermenkephalin and deltorphin I differ in amino acid composition, they play a similar role in specifying correct addressing of these peptides to the delta-receptor, ii/ the negatively charged side chain of aspartic acid 4 residue in deltorphin I and aspartic acid 7 residue in dermenkephalin is not involved in binding contact at the delta-receptor site, nor in maintaining a delta-bioactive folding of the peptides, iii/ these side chains are, in contrast, functionally or structurally required to confer high delta-selectivity by preventing mu-site recognition and/or binding.

D1 dopamine receptors can interact with both stimulatory and inhibitory guanine nucleotide binding proteins

Pretreatment of striatal membranes with N-ethylmaleimide in the presence of a D1-specific agonist inactivated endogenous guanine nucleotide binding proteins (G proteins), but not D1 dopamine receptors, resulting in a loss of high-affinity agonist binding sites. Such D1 receptors were solubilized, mixed with exogenous G proteins from cells not containing D1 receptors, and reconstituted into phospholipid vesicles. These reconstituted receptors were able to couple to the exogenous G proteins, and the proportion of agonist high-affinity sites of the receptor (40-57%) was similar to levels obtained with naive receptors coupling to endogenous G proteins (40%) upon solubilization and reconstitution. These hybrid high-affinity sites were fully modulated by guanine nucleotides. Pretreatment of cells with pertussis toxin prior to extraction of G proteins resulted in a 50% decrease in the proportion of high-affinity sites; these sites remained sensitive to guanine nucleotides. When D1 receptors were reconstituted with extracts of cyc- cells, which lack stimulatory G proteins, the proportion of high-affinity sites was reduced to 31% of the total. Pertussis toxin treatment of the cyc- cells completely abolished the formation of high-affinity sites. These results demonstrate that D1-dopaminergic receptors are able to couple to not only stimulatory G proteins (Gs), but also to inhibitory G proteins (Gi).

Dermenkephalin and deltorphin I reveal similarities within ligand-binding domains of mu- and delta-opioid receptors and an additional address subsite on the delta-receptor

Dermorphin (Tyr-D-Ala-Phe-Gly-Tyr-Pro-Ser-NH2), dermenkephalin (Tyr-D-Met-Phe-His-Leu-Met-Asp-NH2) and deltorphin I (Tyr-D-Ala-Phe-Asp-Val-Val-Gly-NH2) are the first naturally occurring peptides highly potent for and almost specific to the mu- and delta-opioid receptors, respectively. The amino-terminal domains Tyr-D-X-Phe (where X is either Ala or Met) of these peptides behave as selective and potent mu-receptor ligands. Routing of Tyr-D-X-Phe to the delta- or the mu- receptor is associated with the presence or the absence at the C-terminus of an additional hydrophobic and negatively charged tetrapeptide by-passing the mu-addressing ability of the amino-terminal moiety. A study of 20 Tyr-D-X-Phe-Y-NH2 analogs with substitution of X and Y by neutral, hydrophobic, aromatic amino acids as well as by charged amino acid residues shows that tetrapeptides maintain high binding affinity and selectivity for the mu-opioid receptor. Although residue in position 4 serves a delta-address function, the tripeptide motif at the C-terminus of dermenkephalin and deltorphin I are critical components for high selectivity at delta-opioid receptor. Results demonstrate that mu- and delta-opioid receptors share topologically equivalent ligand-binding domains, or ligand-binding sequences similarities, that recognized Tyr-D-X-Phe as a consensus message-binding sequence. The delta-receptor additionally contains a unique address subsite at or near the conserved binding domain that accommodates the C-terminal tetrapeptide motif of dermenkephalin and deltorphin I.

Antagonism of the morphine-induced locomotor activation of mice by fructose: comparison with other opiates and sugars, and sugar effects on brain morphine

The mouse locomotor activation test of opiate action in a 2+2 dose parallel line assay was used in a repeated testing paradigm to determine the test, opiate and hexose specificities of a previously reported antagonism of morphine-induced antinocociception by hyperglycemia. In opiate specificity studies, fructose (5 g/kg, i.p.) significantly reduced the potency ratio for morphine and methadone, but not for levorphanol, meperidine or phenazocine when intragroup comparisons were made. In intergroup comparisons, fructose significantly reduced the potencies of levorphanol and phenazocine, but not methadone or meperidine. In hexose/polyol specificity studies, tagatose and fructose significantly reduced the potency ratio for morphine, whereas glucose, galactose, mannose and the polyols, sorbitol and xylitol, caused no significant decrease in potency. Fructose, tagatose, glucose and mannose (5 g/kg, i.p.) were tested for effects on brain morphine levels 30 min after morphine (60 min after sugar), and all four sugars significantly increased brain morphine relative to saline-pretreated controls. It is concluded that the antagonism of morphine by acute sugar administration shows specificity for certain sugars and occurs despite sugar-induced increases in the distribution of morphine to the brain. Furthermore, the effects of fructose show an opiate specificity similar to that of glucose on antinociception observed previously in our laboratory, except that methadone was also significantly inhibited in the present study, when a repeated-testing experimental design was used.

Opioid receptor-coupled second messenger systems

Although pharmacological data provide strong evidence for different types of opioid receptors (e.g., mu, delta, and kappa), they share many common properties in their ability to couple to second messenger systems. All opioid receptor types are coupled to G-proteins, since agonist binding is diminished by guanine nucleotides and agonist-stimulated GTPase activity has been identified in several preparations. Moreover, all three types inhibit adenylyl cyclase. This second messenger system has been identified for opioid receptors in both isolated brain membranes and in transformed cell culture. Studies with chronic treatment with opioid agonists suggest that the coupling of receptors with G-proteins and second messenger effectors may play important roles in development of opioid tolerance.

Evidence for receptor-mediated inhibition of intrinsic activity of GTP-binding protein, Gi1 and Gi2, but not G0 in reconstitution experiments

The receptor-mediated inhibition of intrinsic activities of GTP-binding proteins (G-proteins) was studied. Pertussis toxin (IAP)-substrate G-protein, Gi1, Gi2 or G0, was prelabeled with [alpha-32P]GDP and reconstituted with synaptic membranes of the guinea pig cerebellum in the presence of 0.02% of Chaps. Intrinsic activities of G-proteins were evaluated by the release of [alpha-32P]GDP in exchange for added GppNHp or GDP in reconstituted preparations. U-50,488H (1 nM-10 microM), a specific kappa-subtype of opioid receptor agonist, inhibited the [alpha-32P]GDP release in exchange for added 1 microM GppNHp in Gi1-reconstituted preparations in a concentration-dependent manner. On the other hand, the kappa-opioid agonist at 10 microM increases the Km values of GppNHp, but not GDP in exchange for [alpha-32P]GDP release in preparations reconstituted with Gi1 or Gi2, but not with G0. These findings indicate that kappa-opioid receptor is coupled to inhibition of intrinsic activities of Gi1 and Gi2, but not G0, in guinea pig cerebellar membranes. In addition, it was revealed that the mode of action is mediated by a decrease in affinity of GTP (or its analog) for G proteins, but not by a change in affinity of GDP.