Presynaptic auto- and allelo-receptor regulation of hypothalamic opioid peptide release

Recent studies have shown that inhibitory feedback mechanisms regulate the release of the endogenous opioid peptides beta-endorphin (acting predominantly at mu opioid receptors in the brain), dynorphin (a kappa opioid receptor ligand) and [Met]enkephalin (a delta opioid receptor ligand) from the rat hypothalamus. By using specific antagonists of the various opioid receptor types, it is shown that the release of these peptides from hypothalamic slices in vitro is not only controlled by homologous (auto)-receptors, but that cross-regulation between the three neuronal opioid receptor types also occurs; thus, the delta receptor antagonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu increases the release of all three peptides, the mu receptor antagonist D-tetrahydroisoquinoline-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 increases that of beta-endorphin and dynorphin, and the kappa receptor antagonist nor-binaltorphimine increases that of dynorphin; all these effects occur in the presence of tetrodotoxin, indicating a presynaptic site of action. We propose the term "allelo-receptors" to describe this particular form of neuronal regulation in which an endogenous ligand, acting via its own specific receptor, also regulates the release of related peptides which activate different classes of opioid receptors.     

The distribution of opioid binding subtypes in the bovine adrenal medulla

Autoradiography has been used to examine the distribution of opioid binding subtypes in the bovine adrenal gland. Specific opioid binding sites were restricted to the adrenal medulla. Kappa sites, labelled with [3H]bremazocine (in the presence of excess unlabelled mu and delta ligands), were highly concentrated over nerve tracts. These nerve tract associated binding sites were sensitive to competition by the endogenous opioid, dynorphin (1-13). Specific [3H]bremazocine binding sites were also found over the adrenal medullary chromaffin tissue. These binding sites were concentrated over the peripheral, adrenaline-containing region of the medulla and were sensitive to competition by diprenorphine but not dynorphin (1-13). Delta opioid sites, labelled with [3H][D-Ala2,D-Leu5] enkephalin (in the presence of excess unlabelled mu ligand) were selectively localized to the central, noradrenaline-containing region of the adrenal medulla. Mu opioid sites, labelled with [3H][D-Ala2, NMePhe4,Gly-ol5]enkephalin, were low in number and distributed throughout the adrenal medulla. These studies demonstrate that mu, delta and two distinct kappa opioid binding sites are differently distributed within the bovine adrenal medulla and suggest possible new sites of action for the adrenal medullary opioid peptides.     

Gene expression and localization of opioid peptides in immune cells of inflamed tissue: functional role in antinociception.

Our previous studies indicate that endogenous opioids (primarily beta-endorphin) released during stressful stimuli can interact with peripheral opioid receptors to inhibit nociception in inflamed tissue of rats. This study sought to localize opioid precursor mRNAs and opioid peptides deriving therefrom in inflamed tissue, identify opioid containing cells and demonstrate their functional significance in the inhibition of nociception. In rats with Freund's adjuvant-induced unilateral hindpaw inflammation we show that: (i) pro-opiomelanocortin and proenkephalin-mRNAs (but not prodynorphin mRNA) are abundant in cells of inflamed, but absent in non-inflamed tissue; (ii) numerous cells infiltrating the inflamed subcutaneous tissue are stained intensely with beta-endorphin and [Met]enkephalin (but only few scattered cells with dynorphin) antibodies; (iii) beta-endorphin is present in T- and B-lymphocytes, monocytes and macrophages; and (iv) whole-body irradiation suppresses stress-induced antinociception in the inflamed paw. Taken together, these data suggest that endogenous opioid peptides are synthesized and processed within various types of immune cells at the site of inflammation. Immunosuppression abolishes the intrinsic antinociception in inflammatory tissue confirming the functional significance of these cells.     

Ontogeny and regional distribution of proenkephalin- and prodynorphin-derived peptides and opioid receptors in rat hippocampus

Levels of prodynorphin- and proenkephalin-derived peptides were determined in whole hippocampus of prenatal and early postnatal rats and in five regions of the hippocampus of the adult rat. Using autoradiography, opioid receptor subtypes were localized in coronal sections of adult hippocampus. The opioid peptides are present in very low concentrations in prenatal hippocampus, with only dynorphin B and alpha-neo-endorphin being present in significant amounts. The main increase in concentrations of the opioid peptides occur between day 7 and 14 postnatally, when dynorphin A, dynorphin A-(1-8), dynorphin B and alpha-neo-endorphin reach their adult levels. beta-Neo-endorphin and [Met]enkephalyl-Arg-Gly-Leu do not reach their maximal level until later in development. There is a distinct differential distribution of the opioid peptides in the subregions of the hippocampus; the subiculum and CA1 are relatively poor in prodynorphin-derived peptides but do contain significant amounts of [Met]enkephalin and [Leu]enkephalin. Very high concentrations of dynorphin B and alpha-neo-endorphin are present in region CA4. Dynorphin A-(1-8) and [Met]enkephalin have their highest concentrations in the dentate gyrus. There is a 5-fold higher concentration of [Met]enkephalin in the ventral hippocampus compared to the dorsal hippocampus. A similar trend is seen with dynorphin A-(1-8) but not with the other opioid peptides. The most abundant opioid receptor population in the hippocampus is of the mu type and it is densest in and around stratum pyramidale of the region CA3. There are relatively few kappa opioid receptors in the rat hippocampus. These results indicate the presence of at least two independent opioid neuronal systems (enkephalin and dynorphin) in rat hippocampus and the presence of mu-, delta- and kappa-opioid receptor subtypes.     

Different motivational effects induced by the endogenous mu-opioid receptor ligands endomorphin-1 and -2 in the mouse

The present study was designed to investigate the motivational effects of the newly discovered endogenous mu-opioid receptor ligands, endomorphin-1 and endomorphin-2, using the conditioned place preference paradigm in mice. The binding properties of these peptides were first examined using an opioid binding assay. In membranes obtained from the mouse whole brain, the binding of [3H][D-Ala2, NMePhe4, Gly(ol)5]enkephalin (DAMGO; mu), but not of [3H][D-Phe2, D-Phe5]enkephalin (DPDPE; delta) or [3H]U69593 (kappa) selectively and concentration-dependently competed with that of endomorphin-1 and endomorphin-2, indicating that both endomorphin-1 and endomorphin-2 are specific ligands for mu-opioid receptors in the brain. Endomorphin-1 (1-30 nmol/mouse) given i.c.v. produced a dose-related place preference. This effect was abolished by pre-treatment with the mu-opioid receptor antagonist beta-funaltrexamine but not the delta-opioid receptor antagonist naltrindole or the kappa-opioid receptor antagonist nor-binaltorphimine. In contrast, endomorphin-2 (5.6 nmol/mouse) produced place aversion. This aversive effect was inhibited by nor-binaltorphimine as well as beta-funaltrexamine, but not by naltrindole. The place aversion produced by endomorphin-2 was also attenuated by pre-treatment with antiserum against the endogenous kappa-opioid receptor ligand dynorphin A (1-17). These findings indicate that endomorphin-1 may produce its rewarding effect via mu-opioid receptors. On the other hand, the aversive effect induced by endomorphin-2 may be associated with the stimulation of endomorphin-1-insensitive mu-opioid receptors and the activation of dynorphinergic systems in the mouse brain.     

Differential involvement of mu(1)-opioid receptors in endomorphin- and beta-endorphin-induced G-protein activation in the mouse pons/medulla

Several genetic mouse models of differential sensitivity to opioids have been used to investigate the mechanisms underlying individual variation in responses to opioids. The CXBK mice are inbred recombinant mice which have a lower level of mu(1)-opioid receptors than their parental strain. Endomorphin-1 and endomorphin-2 are endogenous opioid peptides that are highly selective for mu-opioid receptors, while beta-endorphin, which is also an endogenous opioid peptide, is non-selective for mu-, delta- and putative epsilon-opioid receptors. The present study was designed to investigate the effects of these endogenous opioid peptides on G-protein activation by monitoring guanosine-5'-o-(3-[35S]thio)triphosphate binding to pons/medulla membranes of CXBK mice and their parental strain C57BL/6 ByJ mice. Endomorphin-1 (0.1-10 microM), endomorphin-2 (0.1-10 microM) and beta-endorphin (0.1-10 microM) increased guanosine-5'-o-(3-[35S]thio)triphosphate binding to the pons/medulla membranes from C57BL/6 ByJ and CXBK mice in a concentration-dependent manner. However, the increases of guanosine-5'-o-(3-[35S]thio)triphosphate binding induced by either endomorphin-1 or endomorphin-2 in CXBK mice were significantly much lower than those in C57BL/6ByJ mice. However, no significant difference was found in the increases of the guanosine-5'-o-(3-[35S]thio)triphosphate binding induced by beta-endorphin in C57BL/6 ByJ and CXBK mice. Moreover, whereas the increase of guanosine-5'-o-(3-[35S]thio)triphosphate binding induced by 10 microM endomorphin-1 or endomorphin-2 were almost completely blocked by a mu-opioid receptor antagonist beta-funaltrexamine (10 microM) in both strains, the increase of guanosine-5'-o-(3-[35S]thio)triphosphate binding induced by 10 microM beta-endorphin was attenuated to approximately 70% of stimulation by co-incubation with 10 microM beta-funaltrexamine in both strains. The residual stimulation of [35S]guanosine-5'-o-(3-thio)triphosphate binding by 10 microM beta-endorphin in the presence of 10 microM beta-funaltrexamine was further attenuated by the addition of putative epsilon-opioid receptor partial agonist beta-endorphin (1-27) (1 microM) in both strains. Like the endomorphins, the synthetic mu-opioid receptor agonist [D-Ala(2),N-MePhe(4), Gly-ol(5)]enkephalin at 10 microM showed lower increases of guanosine-5'-o-(3-[35S]thio)triphosphate binding in CXBK mice than those in C57BL/6ByJ mice. However, there was no strain difference in the stimulation of guanosine-5'-o-(3-[35S]thio)triphosphate binding induced by 10 microM of the selective delta(1)-opioid receptor agonist [D-Pen(2,5)]enkephalin, delta(2)-opioid receptor agonist [D-Ala(2)]deltorphin II or kappa-opioid receptor agonist U50,488H. The results indicate that the G-protein activation by endomorphin-1 and endomorphin-2 in the mouse pons/medulla is mediated by both mu(1)- and mu(2)-opioid receptors. Moreover, beta-endorphin-induced G-protein activation in the mouse pons/medulla is, in part, mediated by mu(2)- and putative epsilon-, but not by mu(1)-opioid receptors.     

Influence of opioid peptides on learning and memory processes in the chick

Several experiments were conducted to examine the effects of intracranial injection of opioid peptides and antagonists on learning and memory in the chick. Pretraining injection of [leu5]enkephalin and the selective delta receptor agonist [D-Pen2,L-Pen5]enkephalin (DPLPE) into the intermediate medial hyperstriatum ventrale (IMHV) produced impairment. ICI 174,864, a delta-selective antagonist, reversed the impairment produced by either [leu5]enkephalin or DPLE, results indicating that delta receptors may play a role in learning in the chick and suggesting that the impairment produced by [leu5]enkephalin is mediated through delta opioid receptors. beta-endorphin produced a naloxone-reversible impairment in performance, which suggests that this impairment is mediated by opioid receptors. Bilateral injection of beta-endorphin into the IMHV produced impairment, as did unilateral injection into the right, but not left, IMHV. Only bilateral injections into IMHV of [leu5]enkephalin were effective. These results suggest that the effects of beta-endorphin are centrally mediated whereas the effects of [leu5]enkephalin may be localized to other brain regions or are peripherally mediated. These initial results suggest that opioids are associated with learning and memory in the chick.     

Acupuncture and endorphins

Acupuncture and electroacupuncture (EA) as complementary and alternative medicine have been accepted worldwide mainly for the treatment of acute and chronic pain. Studies on the mechanisms of action have revealed that endogenous opioid peptides in the central nervous system play an essential role in mediating the analgesic effect of EA. Further studies have shown that different kinds of neuropeptides are released by EA with different frequencies. For example, EA of 2 Hz accelerates the release of enkephalin, beta-endorphin and endomorphin, while that of 100 Hz selectively increases the release of dynorphin. A combination of the two frequencies produces a simultaneous release of all four opioid peptides, resulting in a maximal therapeutic effect. This finding has been verified in clinical studies in patients with various kinds of chronic pain including low back pain and diabetic neuropathic pain.     

Increases in opioid-mediated swim antinociception following endopeptidase 24.15 inhibition.

The duration of action and potency of endogenous opioid peptides are limited by proteolytic enzymes such as endopeptidases 24.11 and 24.15. Whereas endopeptidase 24.11 cleaves enkephalin pentapeptides, endopeptidase 24.15 degrades longer-chained opioids including dynorphin A1-8 and met-enkephalin-Arg6-Gly7-Leu8 (MERGL). Inhibitors of endopeptidase 24.11 and 24.15 both increase basal nociceptive thresholds and respective forms of opioid antinociception. Acute exposure to certain environmental stressors can produce antinociception which is opioid mediated; inhibitors of endopeptidase 24.11 potentiate this effect. The present study evaluated whether central administration of a selective inhibitor of endopeptidase 24.15, N-[1-(RS)-carboxy-3-phenylpropyl]-Ala-Ala-Phe-p-aminobenzoate (cFP-AAF-pAB) increased antinociception following intermittent cold-water swims (ICWS) in rats. cFP-AAF-pAB (0.25-25 nmol, ICV) dose-dependently increased ICWS antinociception on the tail-flick and jump tests without affecting basal nociceptive thresholds. The opioid mediation of ICWS antinociception was confirmed by significant reductions in this response following naloxone. These data indicate that longer-chained endogenous opioid peptides participate in the antinociception induced by ICWS.     

Dose- and strain-dependent effects of dermorphin and [D-Ala2-D-Leu5]enkephalin on passive avoidance behavior in mice

Dermorphin and [D-Ala2-D-Leu5]enkephalin, administered intracerebroventricularly (icv) immediately after training in a passive avoidance test, exerted dose- and strain-dependent effects in DBA/2 (DBA) and C57BL/6 (C57) mice. In fact, the peptides impaired, at a low dose (5 ng), the retention performances of both strains; a higher dose (50 ng) impaired retention in DBA but improved it in C57 mice. A dose of naloxone (0.3 microgram, icv), which was ineffective when administered alone, antagonized the effects observed. The results are discussed in terms of strain differences in central mechanisms.     

Field-specific changes in hippocampal opioid mRNA, peptides, and receptors due to prenatal morphine exposure in adult male rats

Alterations in the opioid system in the hippocampal formation and some of the possible functional consequences were investigated in adult male rats that were prenatally exposed to either saline or morphine (10 mg/kg twice daily on gestational days 11-18). In situ hybridization and Northern blots were used to measure proenkephalin and prodynorphin mRNA, and radioimmunoassays quantified proenkephalin- and prodynorphin-derived peptide levels in the dentate gyrus, CA3, and CA1 subfields of the hippocampal formation. Prenatal morphine exposure in male rats decreases proenkephalin and increases prodynorphin mRNA selectively in the granule cell layer of the dentate gyrus. Similarly, met-enkephalin peptide levels are decreased and dynorphin B peptide levels are increased in the dentate gyrus but not CA3 or CA1 of prenatally morphine-exposed males. In addition, there are decreases in dynorphin-derived peptides in the CA3 subfield. Receptor autoradiography revealed increases in the density of micro but not delta receptor labeling in discrete strata of specific hippocampal subfields in morphine-exposed males. Because alterations in the hippocampal opioid system suggest possible alterations in the excitability of the hippocampal formation, changes in opioid regulation of seizures were examined. Morphine exposure, however, does not alter the latency to onset or number of episodes of wet dog shakes or clonic seizures induced by infusion of 10 nmol [D-Ala2, MePhe4, Gly-ol5]enkephalin into the ventral hippocampal formation. Interestingly, a naloxone (5 mg/kg) injection 30 min before bicuculline administration reverses the increased latency to onset of clonic and tonic-clonic seizures in morphine-exposed males. Thus, the present study suggests that exposure of rats to morphine during early development alters the hippocampal opioid system, suggesting possible consequences for hippocampal-mediated functions.     

Stimulation of hypothalamic opioid peptide release by lithium is mediated by opioid autoreceptors: evidence from a combined in vitro, ex vivo study

The effect of both chronic and acute lithium treatment on hypothalamic opioid peptides was investigated. Acute treatment with lithium was found to stimulate the release of beta-endorphin, dynorphin and Met-enkephalin from perfused rat hypothalamic slices. Application of tetrodotoxin was found to have no effect upon the stimulation indicating it to be mediated at the nerve terminal level. The release of hypothalamic opioid peptides is known to be under the chronic control of a system of inhibitory autoreceptors. Blockade of these autoreceptors with, for example, the opioid receptor antagonist naloxone causes a release of all three opioid peptides. Simultaneous addition of naloxone and lithium was found to have no additive effect on the release of any opioid, suggesting lithium acts via an inhibition of the inhibitory autoreceptor. Preincubation with pertussis toxin prevented the lithium stimulation of dynorphin and Met-enkephalin, but not beta-endorphin, release, indicating lithium interacts with a G-protein to affect the autoreceptor controlling the release of dynorphin and Met-enkephalin. Chronic treatment with lithium in vivo (10 days) had no effect on the basal release or hypothalamic content of any of the opioids, but prevented the naloxone-stimulated release of dynorphin and Met-enkephalin in vitro. Long-term treatment with lithium would thus appear to inactivate the autoreceptor(s) controlling their release. These data demonstrate a lithium-stimulated release of hypothalamic beta-endorphin, Met-enkephalin and dynorphin, apparently mediated via an inhibition of the autoreceptors controlling their release. Chronic treatment with lithium permanently inactivated the autoreceptor(s) controlling the release of dynorphin and Met-enkephalin but not beta-endorphin. Lithium would appear to mediate its effects upon Met-enkephalin and dynorphin release via an interaction with a pertussis toxin-sensitive G-protein. The mechanisms underlying its release of beta-endorphin are at present uncertain.     

Effects of morphine and D-Ala, D-Leu enkephalin on the electrical activity of supraoptic neurosecretory cells in vitro

Experiments were performed to investigate the effects of morphine and [D-Ala2, D-Leu5]enkephalin on supraoptic cells in hypothalamic slices in vitro. To ensure the presence of a steady background activity, the cells were recorded with glutamate-filled glass microelectrodes and the level of activity was controlled by selecting a suitable retaining current (0.1-9.8 nA). Under these conditions, supraoptic cells showed either the non-phasic (65%) or phasic (35%) firing pattern previously associated with oxytocin or vasopressin cells, respectively. During perifusion of the slice with morphine (10 microM), 10 out of 17 non-phasic supraoptic cells were profoundly inhibited, five cells showed no response and the remaining 2 cells were excited. Similarly with [D-Ala2, D-Leu5]enkephalin (10 microM), 11 out of 15 non-phasic cells were inhibited, 3 cells showed no response and 1 cell was excited. The inhibition produced by morphine or [D-Ala2, D-Leu5]enkephalin could be reversed by concomitant application of naloxone (10 microM). In contrast to the profound effects seen in the non-phasic cells, only 1 out of 13 phasic cells tested with either morphine or [D-Ala2, D-Leu5]enkephalin was inhibited. The remaining 12 phasic cells showed no change in either their overall firing rate or pattern of activity during opiate perifusion. These results provide further evidence that, in addition to their inhibitory effects within the posterior pituitary, opiates can directly suppress the electrical activity of magnocellular neurosecretory cells at the level of the hypothalamus. However, the absence of an opiate effect on the phasic cells might suggest that the action of opioid peptides within the hypothalamus would be exerted predominantly on the oxytocin, rather than the vasopressin cells.     

Disparate spinal and supraspinal opioid antinociceptive responses in beta-endorphin-deficient mutant mice

The role of endogenous opioid systems in the analgesic response to exogenous opiates remains controversial. We previously reported that mice lacking the peptide neurotransmitter beta-endorphin, although unable to produce opioid-mediated stress-induced antinociception, nevertheless displayed intact antinociception after systemic administration of the exogenous opiate morphine. Morphine administered by a peripheral route can activate opioid receptors in both the spinal cord and brain. However, beta-endorphin neuronal projections are confined predominantly to supraspinal nociceptive nuclei. Therefore, we questioned whether the absence of beta-endorphin would differentially affect antinociceptive responses depending on the route of opiate administration. Time- and dose-response curves were obtained in beta-endorphin-deficient and matched wild-type C57BL/6 congenic control mice using the tail-immersion/withdrawal assay. Null mutant mice were found to be more sensitive to supraspinal (i.c.v.) injection of the micro-opioid receptor-selective agonists, morphine and D-Ala(2)-MePhe(4)-Gly-ol(5) enkephalin. In contrast, the mutant mice were less sensitive to spinal (i.t.) injection of these same drugs. Quantitative receptor autoradiography revealed no differences between genotypes in the density of mu, delta, or kappa opioid receptor binding sites in either the spinal cord or pain-relevant supraspinal areas.Thus we report that the absence of a putative endogenous ligand for the mu-opioid receptor results in opposite changes in morphine sensitivity between discrete areas of the nervous system, which are not simply caused by changes in opioid receptor expression.     

Role of dynorphin and enkephalin in the regulation of striatal output pathways and behavior

Projection neurons in the striatum give rise to two output systems, the “direct” and “indirect” pathways, which antagonistically regulate basal ganglia output. While all striatal projection neurons utilize GABA as their principal neurotransmitter, they express different opioid peptide co-transmitters and also different dopamine receptor subtypes. Neurons of the direct pathway express the peptide dynorphin and the D1 dopamine receptor, whereas indirect pathway neurons express the peptide enkephalin and the D2 receptor. In the present review, we summarize our findings on the function of dynorphin and enkephalin in these striatal output pathways. In these studies, we used D1- or D2-receptor-mediated induction of immediate-early genes as a cellular response in direct or indirect projection neurons, respectively, to investigate the role of these opioid peptides. Our results suggest that the specific function of dynorphin and enkephalin is to dampen excessive activation of these neurons by dopamine and other neurotransmitters. Levels of these opioid peptides are elevated by repeated, excessive activation of these pathways, which appears to be an adaptive or compensatory response. Behavioral consequences of increased opioid peptide function in striatal output pathways may include behavioral sensitization (dynorphin) and recovery of motor function (enkephalin).     

Codeine-induced mast cell degranulation in human skin: effect of calcium channel blockers

Codeine and other opiates can induce immediate type wheal and flare skin reactions. Calcium channel blockers including nifedipine have been shown to inhibit mast cell degranulation in different systems. The oral administration of nifedipine (10 mg) did not affect the size of codeine-induced skin reactions in ten normal volunteers. Mean wheal over flare sizes were 11.7 mm/29.2 mm before nifedipine and 11.5 mm/31.0 mm at peak nifedipine blood levels. Similar observations were made when codeine was injected locally with or without 20 micrograms nifedipine (12.1/29.2 mm and 13.2/29.2 mm, respectively). These data suggest that codeine-induced mast cell degranulation may be mediated by a calcium-independent mechanism. Alternatively, mast cells in the human skin may differ in their reactions to secretagogues when compared with basophils and mast cells from other human tissues or other species.     

Deprivation of REM sleep in the rat and the opioid peptides beta-endorphin and dynorphin

Effects of 'rapid eye movement' sleep deprivation (REMd) on two opioid peptides, beta-endorphin and dynorphin, were studied in rats. Both peptides were measured by radioimmunoassay techniques. The level of beta-endorphin was estimated in the hypothalamus, in the anterior lobe of the pituitary and in the blood. The amount of dynorphin was estimated in the hypothalamus. REMd was induced for 72 h and achieved by two different methods, the platform technique and the pendulum technique. Three control groups were additionally run. As a consequence of REMd, an increase in beta-endorphin level was discovered in the blood plasma, while a small decrease was found in the hypothalamus. No changes could be detected for beta-endorphin levels in the pituitary or for hypothalamic dynorphin concentration. The deprivation effects are interpreted as belonging to a group of changes, all of which point to a small increase in tonic arousal as a result of REMd.     

The contribution of endogenous opioids to food reward is dependent on sex and background strain

Complex behaviors such as those associated with reward to unconditioned positive reinforcers are polygenic processes. In studies using genetically modified mice specific for the endogenous opioid systems an observed phenotype in a complex behavior is likely to be dependent on interacting genes which, in inbred mouse lines, influence that phenotype. To address this issue we examined operant responding for palatable food reinforcers in mice lacking the expression of beta-endorphin, enkephalin or both peptides congenic to two different genetic backgrounds; C57BL/6J and DBA/2J. These two inbred strains were chosen because their endogenous opioid states differ and they respond differently to exogenous opioids in many behavioral assays. We found that wildtype and mutant C57BL/6J mice acquired operant responding for food reinforcers faster than DBA/2J mice, regardless of their opioid genotype. Although wildtype DBA/2J mice had a significant deficit in acquisition of bar-pressing behavior to reach a pre-established performance criterion, no subsequent deficit was observed under two different schedules of reinforcement. Additionally, we found that mice lacking enkephalin had decreased motivation to bar press for palatable food reinforcers under a progressive ratio regardless of sex or background strain. In contrast, the only subset of beta-endorphin-deficient mice that had decreased motivation to bar press under a progressive ratio was males on the C57BL/6J background. Of the two classical endogenous opioid peptides with preferential activation of the mu opioid receptor, the knockout models would suggest that enkephalins play a more consistent role than beta-endorphin in mediating the motivation for food reward when tested under a progressive ratio.     

Effect of terbutaline and bambuterol on immediate-type allergic skin responses and mediator release in human skin

Background: Beta-2 agonists are potent inhibitors of mast cell degranulation in vitro. Intradermally injected they also inhibit mast cell activation in human skin in vivo. To what extent orally administered ₂-agonists inhibit mast cell degranulation and allergic skin responses in vivo in daily recommended doses remains unclear.Purpose: The main purpose was to study the effects of oral administered terbutaline and bambuterol on allergen- and codeine-induced histamine release and skin responses in intact human skin in vivo. In addition, control studies were carried out with intradermally injected terbutaline.Methods: Ten allergic subjects were randomized to receive bambuterol (10 mg tablets twice daily), terbutaline (7.5 mg controlled release tablets twice daily) and corresponding placebo for 5 days with a washout phase of 3 days between treatments in a double-blind, double-dummy, cross-over trial. The patients were studied at the fifth day of each regimen, i.e. at day 5, 13, and 21. Allergen- and codeine-induced histamine release was measured by microdialysis technique. Wheal and flare reactions to allergen, codeine, and histamine were measured planimetrically. Measurements were performed in the morning on day 5 on each regimen before medication and for additional 5 h after administration of the morning dose. In a separate series of experiments in another 10 allergic patients, 1–1,000 nM (0.05–50 pmoles) of terbutaline was injected intradermally for measurement of histamine release, prostaglandin D2 (PGD2) synthesis and skin responses.Results: Neither orally administered terbutaline nor bambuterol significantly reduced allergen- or codeine-induced histamine release. Flare reactions to allergen, codeine and histamine remained unaffected which was also the case for the majority of the wheal reactions. In comparison, intradermally injected terbutaline significantly reduced allergen-induced histamine release, PGD₂ synthesis, and skin reactions. Codeine-induced histamine release remained unaffected. Terbutaline significantly reduced flare reactions to codeine and histamine with no effect on wheal reactions.Conclusions: Terbutaline, in micromolar concentrations, was a potent inhibitor of immediate allergic skin reactions primarily due to inhibition of mast cell degranulation. However orally administered terbutaline, as the active drug itself or released from its pro-drug bambuterol, did not inhibit mast cell activation or allergic skin responses. Received 28 January 2003; returned for revision 7 March 2003; accepted by M. Parnham 29 April 2003     

Dopamine inhibition of histamine-mediated cutaneous responses

Several patients receiving dopamine for hypotension were skin tested for possible penicillin sensitivity. Not only were the penicillin skin tests negative but also the histamine control. On the possibility that dopamine might affect cutaneous histamine responses, we examined the effect of dopamine on histamine, antigen, morphine, and compound 48/80 skin responses. Both intradermal and intravenous dopamine selectively inhibited histamine but not antigen, morphine, or compound 48/80 skin responses, and the inhibition was in a dose-related fashion. This observation indicates that histamine should not be used to demonstrate dermal reactivity in patients receiving dopamine. The results of this study also suggest that histamine may not be the sole mast cell-derived mediator involved in the wheal-and-flare reaction characteristic of immediate-type skin tests since dopamine did not affect skin reactions caused by endogenous mast cell degranulation. Finally, the possible use of dopaminergic drugs in diseases with histamine-associated symptoms is discussed.