Selective Mu Research Articles

Centrally mediated opioid induced depression of hepatic glutathione: Effects of intracerebroventricular administration of mu kappa, sigma and delta agonists

It has recently been demonstrated that morphine produces a loss of hepatocellular glutathione in mice by virtue of its action within the central nervous system. The ability of opioid receptor antagonists to abolish morphine's effect glutatione suggests that this action is opioid-receptor mediated. The involvement of opioid receptors in this phenomenon is confirmed in the present study in mice by the ability of naltrexone, 100 μg administered intracerebroventricularly (i.c.v),to completely block the decrease in hepatic glutathione induced by an i.c.v. injection of 100 μg of morphine. Intracerebroventricular administration of the selective mu (μ) opioid receptor agonist, (D-Ala2, N-MePhe4, Gly-ol5)enkephalin (DAGO; 25–50 μg), or the selective delta (δ) opioid agonist, [D-Pen2, D-Pen5]enkephalin (DPDPE; 3–50 μg), like morphine, produced significant decreases in hepatic glutathione 3 h after administration. The selective kappa (κ) opioid receptor agonists, ethylketocyclazocine (1–30 μg) and trans-(±)3,4-dichloro-N-methyl- N-[2-(1-pyrrolidinyl)-cyclohexyl]benzenacetamide-methane sufonate (U50 488; 10–300 μg), as well as the selective sigma (gs) opioid agonists, phencyclidine (PCP; 50–300 μx) and N-allylnormetazocine (SKF 10 047; 1–30 μg), had no effect on the concentrations of glutathione ion the liver. It appears from these data that stimulation of μ- or δ-, but not κ- or σ-opioid receptors within the] central nervous system in a loss of hepatocelluclar glutatione

Opioid mu and kappa receptors on axons of cholinergic excitatory motor neurons supplying the circular muscle of guinea-pig ileum.

In preparations of guinea-pig ileum comprising the circular muscle and the axonal processes of myenteric neurons, electrical stimulation evoked contractions of the circular muscle which were abolished by tetrodotoxin and by hyoscine, indicating that they resulted from action potential-mediated release of acetylcholine. The selective mu opioid agonist, (D-Ala2-N-Me-Phe4-Gly5-ol)-enkephalin (DAGO), and the selective kappa opioid agonist, trans-(+/-)-3,4-dichloro-N-(2-(1-pyrrolidinyl) cyclohexyl) benzeneacetamide, U-50488H, caused concentration-dependent and naloxone-reversible inhibitions of nerve-mediated contractions. The experiments indicate that opioid mu and kappa receptors are present on the axonal processes of cholinergic excitatory motor neurons supplying the circular muscle of the guinea-pig ileum.

Mu and delta opiate receptors coupled negatively to adenylate cyclase on embryonic neurons from the mouse striatum in primary cultures

Primary cultures of pure populations of neuronal or glial cells from the striatum, the cerebral cortex, and the mesencephalon of the mouse embryo were used to look for the presence of opiate receptors coupled to adenylate cyclase. Leu-enkephalin inhibited cAMP production in membranes of embryonic striatal neurons but not in those of other cell types examined. Mu and delta opiate receptors seemed to be coupled negatively to adenylate cyclase in embryonic striatal neurons. It was found that DTLET (a selective delta agonist), as well as DAGO (a selective mu agonist), inhibited cAMP production on these cells. DTLET but not, however, DAGO produced a similar effect on homogenates from the adult rat striatum and on membranes from the neuroblastoma x glioma hybrid cell line NG 108-15, two preparations known to possess only delta receptors negatively coupled to adenylate cyclase. The selective kappa agonist U 50.488 was ineffective on all types of membrane preparations used. The inhibitory effects of both DTLET and DAGO on basal adenylate cyclase activity in striatal neurons were reversed by naloxone with a similar efficacy. Two other selective mu agonists, trimu 5 and morphiceptin, inhibited cAMP production in membranes of striatal neurons as well. The nonadditivity of the inhibitory effects of DTLET and DAGO on basal or forskolin-induced activation of adenylate cyclase suggested that mu and delta receptors were colocalized on a similar subpopulation of striatal cells in primary culture. These cells possess dopaminergic receptors of the D1 subtype as well since the amplitude of the inhibitory effects of DTLET and DAGO on cAMP production was increased in the presence of dopamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Anticonvulsant effects of mu (DAGO) and delta (DPDPE) enkephalins in rats.

The effects of highly selective mu and delta opioid peptide agonists were determined in two rat models of experimentally-induced convulsions, the flurothyl threshold test and the maximal electroshock test. Intracerebroventricular injections of the mu selective enkephalin DAGO (0.3-2.2 nmol) resulted in a dose-related protection in both seizure models. Pretreatment with a low dose of naloxone (29 nmol) or the irreversible mu antagonist beta-FNA (21 nmol), but not the delta opioid antagonist ICI 154,129 (50 nmol), antagonized the anticonvulsant actions of DAGO. Intracerebroventricular injections of the delta selective enkephalin DPDPE (70-140 nmol) also resulted in seizure protection. These effects were selectively antagonized by the delta antagonist ICI 174,864 (2.8 nmol), but not by pretreatment with beta-FNA. Thus, using agonists and antagonists highly selective for mu and delta opioid receptors, anticonvulsant actions of enkephalin have been described against chemically- and electrically-induced convulsions in rats.

MU but not delta opioid receptor stimulation intensifies kainic acid-induced neurotoxicity in rat hippocampus

In the present study, we compared the effects of the selective mu agonist, [D-Ala 2-N-methyl-pHe 4-Gly-ol]-enkephalon (DAGO), and the selective delta agonist, [D-Pen 2,5]-enkephalin (DPDPE), on kainic acid-induced neurotoxicity in rats. Infusion of kainic acid ( 0.5 ug 1.5 ul , i.c.v. ) alone caused pyramidal cell loss predominantly in hippocampal field CA3 with minimal involvement of the CA1 field. Coadministration of DAGO plus kainic acid into the lateral ventricle intensified the extent of degeneration of hippocampal pyramidal cells in the CA1 field. The potentiating effect of DAGO was completely blocked by naltrexone. In contrast, DPDPE had no significant effect on kainic acid-induced neurotoxicity. Thus, activation of mu but not delta receptors intensifies the neurotoxic effects of kainic acid in the hippocampus.

Opiate binding differentially associated with oxytocin and vasopressin nerve endings from porcine neurohypophyses.

We examined opioid binding in fractions with disconnected nerve endings (secretosomes) which were prepared from porcine neurohypophyses by centrifugation in a discontinuous Percoll gradient. Specific (= displaceable) binding was observed with 3H-etorphine and with 3H-diprenorphine, two ligands with low selectivity for distinct opiate receptor subclasses. No displaceable binding was found with the prototypic mu- and delta-ligands 3H-dihydromorphine and 3H-(D-Ala, D-Leu) enkephalin. Displacement of 3H-diprenorphine binding was almost absent with the selective mu- and delta-ligands morphiceptin and ICI-174864. Partial displacement occurred with the selective kappa-ligand U-50488 and with dynorphin. Binding of 3H-etorphine was stereo-specific. 3H-diprenorphine binding was saturable with a KD between 2 and 4 nM. Maximum of opiate binding activity was detected in the fractions with accumulated secretosomes. By autoradiography specific 3H-diprenorphine binding is shown to be mainly associated with secretosomes. In imunocytochemical preparations an oxytocin antibody was immunoreactive in 85% of the secretosomes in the fraction with highest opiate binding. These fractions in radioimmunoassays exhibited the largest contents in oxytocin and low vasopressin levels. The data therefore suggest that in the porcine neurohypophysis opioid binding sites of the kappa-type occur in secretory endings presumably of the oxytocin type.

Involvement of μ opioid receptors in the amygdala in the control of feeding

The intake of food by rats was measured after unilateral injections of opioid peptides into or near the central nucleus of the amygdala. The selective mu receptor agonist Tyr d-Ala-Gly-(Me)Phe-Gly-ol (DAGO) caused an increase in intake of food at doses of 1 and 3 nmol. Injections into the amygdala of [ d-Ser 2,Leu 5]enkephalin-Thr 6 (DSLET), a selective delta agonist, and dynorphin A, a selective kappa agonist, were ineffective at doses up to 3 nmol. However, dynorphin (2 nmol) did increase intake when injected into the medial hypothalamus. Bilateral injections of DAGO into the amygdala were no more effective than unilateral injections. The effect of DAGO was blocked by injections into the amygdala of naloxone or β-chlornaltrexamine, an ultralong-lasting opioid receptor antagonist. These studies suggest that mu opioid receptors in the amygdala contribute to the regulation of intake of food. A role for kappa and delta receptors was not established but cannot be ruled out without further testing.

Loss of striatal mu 1 opiate binding by substantia nigra lesions in the rat

Opiate receptors have been identified within the striatum and some have been localized presynaptically to nigrostriatal neurons. Using unilateral ablative lesions of the substantia nigra, we examined binding in the ipsilateral and contralateral striata. Lesions significantly lowered both 3H[D-Ala 2, MePhe 4, Gly(ol) 5] enkephalin (DAGO) and 3H[D-Ala 2, Leu 5] enkephalin (DADL) binding. The inclusion of competitors in these assays revealed a decrease in both mu 1 and mu 2 receptors. Mu 1 binding was slightly more sensitive to the lesioning than mu 2 binding. Selective mu 1 and mu 2 binding assays supported these observations. No change in delta binding was observed in the lesioned striata. These studies raise the possibility that both mu 1 and mu 2, but not delta, receptors are localized presynaptically on nigrostriatal neurons.

Quantitative autoradiography of [ 3H]CTOP binding to mu opioid receptors in rat brain

[ 3H]-HDPhe tsqbCysTyrDTrpOrnThrPen ThrNH 2 ([ 3H]CTOP), a potent and highly selective mu opioid antagonist, was used to localize the mu receptors in rat brain by light microscopic autoradiography. Radioligand binding studies with [ 3H]CTOP using slide-mounted tissue sections of rat brain produced a Kd value of 1.1nM with a Bmax value of 79.1 fmol/mg protein. Mu opioid agonists and antagonists inhibited [ 3H]CTOP binding with high affinity (IC 50 values of 0.2–2.4nM), while the delta agonist DPDPE, delta antagonist ICI 174,864, and kappa agonist U 69,593 were very weak inhibitors of [ 3H]CTOP binding (IC 50 values of 234–3631nM). Light microscopic autoradiography of [ 3H]CTOP binding sites revealed regions of high density (nucleus of the solitary tract, clusters in the caudate-putamen, interpeduncular nucleus, superior and inferior colliculus, subiculum, substantia nigra zona reticulata, medial geniculate, locus coeruleus and dorsal motor nucleus of the vagus) and regions of moderate labeling (areas outside of clusters in the caudate-putamen, cingulate cortex, claustrum and nucleus accumbens). The cerebral cortex (parietal) showed a low density of [ 3H]CTOP binding.

Autoradiographic distribution of multiple classes of opioid receptor binding sites in human forebrain

Receptor binding parameters and autoradiographic distribution of various opioid receptor sites have been investigated in normal human brain, post-mortem. [ 3H]DAGO, a highly selective mu ligand, binds to a single class of high affinity ( K d = l. l nM), low capacity ( B max = 160 fmol/mg protein) sites in membrane preparations of frontal cortex. These sites show a ligand selectivity profile that resembles that of the mu opioid receptor. On the other hand, [ 3H]bremazocine, in presence of saturating concentrations of mu and delta blockers, appears to selectively bind to a single population of kappa opioid sites ( K d =0.13 nM; B max =93.0 fmol/mg protein) in human frontal cortex. Whole hemisphere in vitro receptor autoradiography reveals that [ 3H]DAGO-mu, [ 3H]DSLET-delta and [ 3H]bremazocine (plus blockers)-kappa binding sites are discretely and differentially distributed in human forebrain. In the cortex, mu sites are concentrated in laminae I and IV, delta sites in laminae I and II while kappa sites are found in deeper layers (laminae V and VI). In subcortical nuclei, high densities of mu and delta sites are seen in the caudate and putamen while high amounts of kappa sites are present in the claustrum and amygdala. The nucleus basalis of Meynert is enriched in all three classes of sites while the globus pallidus only contains moderate densities of kappa sites. Thus, the possible alterations of these various classes of opioid receptors in neurological and psychiatric diseases certainly deserve further investigation.

Subcutaneous formalin-induced activity of dorsal horn neurones in the rat: differential response to an intrathecal opiate administered pre or post formalin

Many studies of pain and nociception use short-lasting acute stimuli which may have limited relevance to prolonged or chronic pain states. Using extracellular single-unit recording in the dorsal horn of the rat lumbar spinal cord the present study examines the response of neurones to a long-lasting nociceptive stimulus i.e., 50 μl 5% formalin injected into the corresponding receptive field in the ipsilateral hind paw, and modulation of this response by an opioid. Formalin produced a distinct biphasic excitatory response in all convergent neurones tested; an immediate acute or phasic peak of neuronal firing (mean maximum 22 spikes/sec) 0–10 min post injection, and a second more prolonged tonic excitatory response (mean maximum 12 spikes/sec) over a period 20–65 min after formalin. Cells only activated by innocuous stimuli were not excited by formalin indicating the involvement of C fibre afferents in the excitatory response of convergent neurones to formalin. Both the biphasic nature and the time course of the neuronal response are similar to those observed in behavioural studies. Intrathecal DAGO (Tyr- d-AlaGlyMePheGly-ol), a potent and selective mu opioid receptor agonist, applied 20 min prior to formalin completely inhibited both peaks of excitation. Co-administration of intrathecal naloxone with the agonist restored the biphasic response. By contrast, when the administration of naloxone was delayed to 2 min post formalin so that inhibition of the first peak by DAGO pretreatment occurred, there was no subsequent second peak of activity although antagonism of the opioid would have occurred. When DAGO was applied 2 min post formalin so the initial acute response occurred, the inhibitory effect of the agonist on the second peak was far less. Thus the relative ability of DAGO to modulate the biphasic excitatory response of cells to formalin depends on whether the agonist is administered prior to or after the formalin and the appearance of the second peak may depend on the presence of the first. These results are discussed in light of the role of these neurones in nociception, opioid effects and changes in neural systems following peripheral stimuli

Appraising the instantaneous secretory rates of luteinizing hormone and testosterone in response to selective mu opiate receptor blockade in late pubertal boys.

The pulsatile properties of gonadotropin and testosterone release were examined before and after chronic mu opiate receptor blockade with naltrexone, 50 mg every other day, in four normal boys in late puberty (ages 14 8/12 to 15 1/12 years). The nature of spontaneous secretory events was appraised for immunoactive LH and testosterone in blood withdrawn every 20 minutes for 24 hours, using a novel, discrete deconvolution algorithm to estimate apparent instantaneous secretory rates. The application of this methodology revealed that the frequency of discrete LH instantaneous secretory rates increased after mu opiate receptor blockade (P = 0.011). More strikingly, all parameters of testosterone secretory events responded significantly to mu opiate receptor blockade, including increases in mean estimated secretory rate (+47%, P = 0.02), testosterone pulse frequency (+ 64%, P less than 0.001) and amplitude (+ 20%, P = 0.027). Correspondingly, decreases in testosterone interpulse secretory intervals (-35%, P = 0.001), secretory pulse duration (-19%, P = 0.042) and interpulse valley duration (-35%, P = 0.006) also were noted. There was a prominent diurnal rhythm in testosterone secretion with maximal values in the morning and late evening, and marked reductions in the afternoon, sometimes to prepubertal levels. This variation in the testosterone secretory profile paralleled that of LH. In response to naltrexone, the FSH concentration series showed a significant increase in the mean FSH concentration (+ 18%) P = 0.003) and mean peak amplitude (+ 15%, P = 0.002). These data provide indirect evidence of functional coupling of the opiate system with the hypothalamic GnRH pulse generator.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of a novel opioid peptide antagonist on rat bladder motility in vivo

The agonist, and opioid antagonist, effects of intracerebroventricularly (ICV) given D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Pen-Thr-NH 2 (CTP), a cyclic analogue of somatostatin octapeptide, were evaluated using the micturition reflex of the anesthetized rat as the endpoint. Antagonist effects were evaluated against equieffective doses of selective mu [D-Ala 2,NMPhe 4,Gly-ol]enkephalin (DAGO) and delta [D-Pen 2,D-Pen 5] enkephalin (DPDPE) opioid agonists. At low ICV doses, CTP preferentially antagonized DPDPE rather than DAGO; increasing the dose of CTP further effectively antagonized both mu and delta agonists, while even higher doses showed an agonist effect alone which was not blocked by adrenergic, cholinergic or opioid antagonists. Selective opioid antagonist doses of CTP failed to block the inhibition of the micturition reflex produced by pentobarbital. Possible residual somatostatin like properties of CTP were tested by using somatostatin as a possible antagonist of equieffective doses of DPDPE and DAGO; somatostatin did not antagonize these agonists. Repeated exposure to CTP resulted in the development of acute tolerance to the agonist effect, and also prevented the inhibition of the reflex by high doses of somatostatin, with the converse experiment showing a similar pattern; thus, repeated somatostatin resulted in tolerance and subsequent cross-tolerance to the agonist effects of CTP. In animals tolerant to somatostatin, CTP nevertheless behaved as an opioid antagonist. The present results indicate that CTP possesses opioid antagonist properties in vivo which are pharmacological in nature but nevertheless retains residual somatostatin-like activity at higher doses.

ChemInform Abstract: Synthesis and Biological Activities of Dynorphin A Analogues with Opioid Antagonist Properties.

Dynorphin A, which displays a wide variety of physiological effects, binds to opioid receptors preferentially at the kappa receptor type. kappa-selective antagonists would be very useful as pharmacological and biochemical probes to study and better understand the action of dynorphin A at its preferred receptor. However, the development of such molecules has been elusive, and very few are known at this time. Taking these features into account, we have synthesized by the solid-phase procedure several analogues of dynorphin A containing various D-amino acid substitutions. The binding properties of the peptides have been examined at three main opioid binding sites (mu, delta, and kappa) and their kappa selectivity determined. Their biological activities have been tested in three specific pharmacological assays for agonist and/or antagonist properties. Introduction of D-Trp substitution leads to analogues, in particular [D- Trp2,8,D-Pro10]-, [D-Trp5,8,D-Pro10]-, and [D-Trp2,4,8,D-Pro10]dynorphin(1-11), showing antagonist properties in the isolated rabbit vas deferens preparation, a kappa specific bioassay. The antagonism against dynorphin A is weak, as indicated by the observed Ke values (433, 199, and 293 nM, respectively), and not very selective (kappa vs. mu). Such peptide analogues derived from the endogenous ligand and endowed with antagonist properties are the first ones reported to date and could open a promising way in designing more potent and selective kappa opioid antagonists.

Estimation of the affinity of naloxone at supraspinal and spinal opioid receptors in vivo: Studies with receptor selective agonists

The apparent affinity of naloxone at cerebral and spinal sites was estimated using selective mu [D-Ala 2, Gly-ol 5]-enkephalin (DAGO) and delta [D-Pen 2, D-Pen 5]enkephalin] (DPDPE) opioid agonists in the mouse warm water tail-withdrawal test in vivo; the mu agonist morphine was employed as a reference compound. The approach was to determine the naloxone pA 2 using a time-dependent method with both agonist and antagonist given intracerebroventricularly (i.c.v.) or intrathecally (i.th.); naloxone was always given 5 min before the agonist. Complete time-response curves were determined for each agonist at each site in the absence, and in the presence, of a single, fixed i.c.v. or i.th. dose of naloxone. From these i.c.v. or i.th. pairs of time-response curves, pairs of dose-response lines were constructed at various times; these lines showed decreasing displacement with time, indicative of the disappearance of naloxone. The graph of log (dose ratio - 1) vs. time was linear with negative slope, in agreement with the time-dependent form of the equation for competitive antagonism. From this plot, the apparent pA 2 and naloxone half-life was calculated at each site and against each agonist. The affinity of naloxone was not significantly different when compared between agonists after i.c.v. administration. A small difference was seen between the affinity of i.th. naloxone against DPDPE and DAGO; the i.th. naloxone pA 2 against morphine, however, was not different than that for DPDPE and DAGO. The naloxone half-life varied between 6.6 and 16.9 min, values close to those previously reported for this compound. These results suggest that the agonists studied may produce their i.c.v. analgesic effects at the same receptor type or that alternatively, the naloxone pA 2 may be fortuitously similar for mu and delta receptors in vivo. Additionally, while the affinity of naloxone appears different for the receptors activated by i.th. DAGO and DPDPE, further work may be necessary before firm conclusions regarding the nature of the spinal analgesic receptor(s) can be drawn.

Cyclic somatostatin octapeptide analogues with high affinity and selectivity toward mu opioid receptors

A series of cyclic conformationally restricted penicillamine containing somatostatin octapeptide analogues have been prepared by standard solid phase synthetic techniques and tested for their ability to inhibit specific [ 125I]CGP 23,996 (des-Ala 1-,Gly 2-[desamino-Cys 3Tyr 11]-dicarba 3,14- somatostatin), [ 3H]naloxone or [ 3H]DPDPE ([D-Pen 2-D-Pen 5]enkephalin) binding in rat brain membrane preparations. We now report structure-activity relationship studies with the syntheis of our most potent and selective mu opioid receptor compound DPheCysTyrDTrpOrnThrPenThrNH 2, which we refer to as Cys 2Tyr 3Orn 5Pen 7-amide. While this octapeptide exhibited high affinity (IC 50 = 2.80 nM) for an apparently single population of binding sites (n H = 0.89 ± 0.1) and exceptional selectivity for mu opioid receptos with an IC 51(DPDPE)/IC 50 (naloxone) ratio of 4,829, it also displayed very low affinity for somatostatin receptors (IC 50 = 22,700 nM). Thus, Cys 2Tyr 3Orn 5 Pen 7-amide may be the ligand of choice for further characterization of mu opioid receptors and for examining the physiological role of this class of receptors.

Subclass specificity of anti-idiotypic anti-opiate receptor antibodies in rat brain, guinea pig cerebellum, & neuroblastoma x glioma (NG 108-15)

Receptor subclass recognition properties of affinity-purified rabbit polyclonal anti-idiotypic anti-opiate receptor antibodies in various membrane preparations have been determined. The anti-receptor immunoglobulins significantly decrease binding of 3H-[D-Ala 2,-MePhe 4,Gly-ol 5]enkephalin, a highly selective mu agonist, in rat neural membrane. In the presence of a concentration of the unlabeled ligand sufficient to block existing mu sites, the antibodies compete, to a lesser degree with 3H-[D-Ala 2,D-Leu 5]enkephalin for delta site occupancy in both rat neural membrane, and neuroblastoma x glioma membrane preparations. The antibodies do not displace 3H-ethylketocyclazocine from rat brain or guinea pig cerebellum.

Differences in binding properties of mu and delta opioid receptor subtypes from rat brain: kinetic analysis and effects of ions and nucleotides.

Differences in binding properties of mu and delta opioid receptors were investigated using DAGO (Tyr-D-Ala-Gly-MePhe-Gly-ol) and DTLET (Tyr-D-Thr-Gly-Phe-Leu-Thr), which occur, respectively, as the most selective mu and delta radioligands available. At high concentration, each agonist is able to interact with its nonspecific sites. Competition experiments indicated that a two-site competitive model was adequate to explain the interactions of DAGO and DTLET with [3H]DTLET and [3H]DAGO binding sites, respectively. The weak cross-reactivity (congruent to 10%) of DTLET for mu sites was taken into account in these experiments. On the other hand, DAGO and DTLET exhibit differential binding kinetics. Thus, at 35 degrees C, the lifetime of DTLET within its receptor site is about 14 times longer than that of the mu agonist. Sodium and manganese ions decrease the maximal number of high affinity mu and delta sites, but the sensitivity of mu receptors is three times higher towards Na+ and 20-fold higher towards Mn2+ than that of delta receptors. GTP reduces similarly the mu and delta binding whereas only the DAGO binding was modified by the nonhydrolyzable analogue guanylylimidodiphosphate [GMP-P(NH)P]. However, in the presence of Na+ ions, GMP-P(NH)P inhibits the DTLET binding in a concentration-dependent manner. The effects of Na+ and GMP-P(NH)P could be explained by a sequential transformation of delta receptors to low-affinity states. This model predicts that Na+, by lowering the affinity of a fraction of sites, produces a decrease in the maximal number of high-affinity delta receptors and that GMP-P(NH)P enhances the Na+ effect. Moreover, the binding kinetic to this high-affinity state was also modified by Na+ and nucleotides. All of these data support the existence of two independent mu and delta binding sites, the properties of which are differentially regulated by these endogenous effectors.

The analgesic and respiratory depressant actions of metorphamide in mice and rabbits

Metorphamide (MET) elicited a potent, dose-dependent analgesia and respiratory depression in mice and rabbits. MET induced-analgesia was naloxone reversible and potentiated by bestatin. Naloxonazine, a relatively selective mu 1 blocker, at certain dosage (50 μg per rabbit, icv), could abolish the analgesia but not the respiratory inhibition produced by MET. Our result indicates that mu 1 receptors mediate the MET induced-analgesia but not its respiratory effect. Since MET is a mu- and kappa-ligand with very low delta activity, the MET induced respiratory depression may be mediated by mu 2 or kappa binding sites.

Visualization of opiate receptor upregulation by light microscopy autoradiography.

Light microscopy autoradiography has been used to visualize neuroanatomical patterns of brain opiate receptor upregulation in response to chronic naltrexone administration. Slide-mounted brain sections of frozen rat brain were labeled in vitro with dihydro[3H]morphine, a relatively selective mu opioid ligand. The greatest relative increases in opiate receptor density were observed in the nucleus accumbens, the amygdala, striatal patches, nuclei of the thalamus and hypothalamus, layers I and III of neocortex, substantia nigra compacta, midbrain periaqueductal gray regions, and the parabrachial nuclei of the brainstem. The substantia nigra reticulata, surrounding areas of striatal patches, and the locus ceruleus, were not affected by this drug treatment. These findings demonstrate that chronically administered naltrexone differentially regulates opiate receptors throughout the brain. In particular, three brain systems appear to be target areas of receptor upregulation : (i) the dopamine A9/A10 systems, (ii) the limbic system, and (iii) structures that receive input from afferent sensory pathways. Two possible mechanisms to account for this finding are (i) that the drug does not have uniform effects throughout the brain or (ii) that the receptors themselves may be associated with different functional systems. Receptor density changes are paralleled by increases in methionine-enkephalin content in the striatum, nucleus accumbens, periaqueductal gray, and hypothalamic areas of chronic naltrexone-treated rats relative to control rats. Thus opiate receptors and opioid peptides appear to be subject to regulatory mechanisms similar to those that modulate other neurotransmitters and their receptors. These results document in a visual manner brain patterns of opiate receptor upregulation .