Michael Wüster

Acute and chronic ethanol treatment changes endorphin levels in brain and pituitary

Acute ethanol administration increased methionine-enkephalin (met-enkephalin) and β-endorphin levels in distinct areas of the rat brain, whereas chronically supplied ethanol caused a depression of met-enkephalin and β-endorphin levels in most of the brain areas investigated. The β-endorphin content of the intermediate/posterior lobe of the pituitary of rats and guinea pigs decreased by 70%. Withdrawal of ethanol resulted in a complete recovery of endorphin levels in brain and pituitary within two weeks. Whether the observed alterations in endorphin concentrations are causally related to the primary mechanisms under-lying alcohol dependence is uncertain, since no obvious signs of physical dependence were observed in treated animals.

Specificity of opioids towards the μ-, δ- and ϵ-opiate receptors

Abstract The potency of a number of opioid agonists has been determined on three different bioassays, the isolated guinea-pig ileum (GPI), the mouse vas deferens (MVD) and the rat vas deferens (RVD). With the exception of β-endorphin, compounds with opioid activity on the GPI and the MVD exhibited considerably less or no activity on the RVD; the opiate receptor in this latter tissue being thus characterized as β-endorphin selective (‘ϵ-receptor’). A system of classification of opioids according to their relative affinities for particular opiate receptor types is given.

Selective receptors for β-endorphin on the rat vas deferens

Abstract The isolated rat vas deferens, being insensitive to morphine, contains selective binding sites for β-end-orphin. A half-maximal inhibition of twitch tension evoked by electrical stimulation is established with 100 nM β-endorphin, while fragments of β-endorphin, that is, methionine-enkephalin, α- and γ-endorphin, are almost ineffective. The opiate alkaloid etorphine, a powerful inhibitor of guinea-pig ileum and mouse vas deferens, is 100-fold less potent on the rat vas deferens. The unique β-endorphin activity suggests very specific binding sites for this peptide, which cannot be related to the μ- or δ-receptors so far described for opiods on isolated preparations.

The preference of putative pro-enkephalins for different types of opiate receptors

Abstract Certain endogenous opioid peptides, which structurally resemble potential enkephalin precursors (pro-enkephalins), were tested for their interaction with μ-, δ- and κ-type of opiate receptors. These studies employed the technique of the development of tolerance towards selective opioid agonists on the isolated guinea-pig ileum and mouse vas deferens preparations. Additionally, binding studies in rat brain homogenate were performed to determine the affinity of these compounds towards different receptor sites in the CNS. These investigations revealed that dynorphin 1–13 and α-neo-endorphin 1–8 may preferentially interact with the κ-type of opiate receptors, dynorphin 1–9 exhibits both δ- and κ-agonistic activity and met-enkephalin-Arg 6 -Phe 7 is a ligand with selectivity for δ-receptors.

Centrally and peripherally mediated inhibition of intestinal motility by opioids

SummaryIntracerebroventricularly injected morphine is 50-fold more potent in arresting intestinal peristalsis in rats, mice or guinea pigs than morphine administered systemically. Using quaternary naloxone as narcotic antagonist, it has been demonstrated that the peripheral pathway of the centrally mediated constipatory effect of morphine does not involve opioid peptidergic mechanisms. Further, this effect is not due to the release of opioid peptides from the pituitary, since hypophysectomy fails to affect the antipropulsive activity of morphine. On the other hand, the intestinal motility can be affected directly by activation of opiate receptors located in the gut. This was best demonstrated with loperamide, which exhibits predominantly a peripheral site of action. Thus, two mechanisms of the action of morphine on gastrointestinal propulsive activity have been demonstrated. One arises in the central nervous system (CNS) and is mediated peripherally not by opioid peptidergic pathways, whereas the other is due to a direct action of morphine on the gut.

Supersensitivity to opioids following the chronic blockade of endorphin action by naloxone.

SummaryChronic treatment of guinea pigs with the narcotic antagonist naloxone (pellet implantation technique) caused an increased sensitivity to opioids of the electrically stimulated longitudinal muscle-myenteric plexus preparation of the ileum. This supersensitivity is associated with an elevation of the total number of opiate receptor binding sites in both the peripheral and the central nervous system.

The direction of opioid agonists towards μ-, δ- and ε-receptors in the vas deferens of the mouse and the rat

Abstract Chronic treatment of mice with specific opioids results in the development of tolerance of particular opiate receptors in the mouse vas deferens (MVD). Accordingly, the infusion of animals with the specific δ-receptor ligand [D-Ala 2 ,D-Leu 5 ]-enkephalin (DADL) or the potent μ-agonist sufentanyl (SUF) produces MVD highly tolerant to δ- and μ-agonists, respectively. Investigating a series of opioids in these preparations provides unequivocal evidence for the simultaneous existence of δ- and μ-receptors in the MVD. Thus, the possibility exists to obtain vasa deferentia, which almost exclusively contain either μ- or δ-opiate receptors. In combination with the rat vas deferens (RVD), a supposedly selective e-receptor preparation, useful tools are provided for the classification of opioids according to their preference for the μ-, δ- and e-type of the opiate receptors.

Opiate agonist action of antidiarrheal agents in vitro and in vivo--findings in support for selective action.

SummarySynthetic antidiarrheal agents like diphenoxylate and loperamide are characterized by strong opiate-like constipating activity with an almost complete lack of central morphinomimetic effects. The present investigation examines the pharmacological mechanisms underlying the action of such compounds.1.In the radioreceptorassay, antidiarrheal agents competitively inhibited the stereospecific binding of opiates, having affinities comparable to those of strong narcotic nnalgesics. Binding was increased in the absence of sodium. No differences in the affinities of these compounds to central and peripheral opiate receptors, respectively, were found.2.Antidiarrheal agents inhibited the electrically induced contractions of the isolated longitudinal muscle-myenteric plexus preparation of the guinea-pig ileum. This effect was completely antagonized by the narcotic antagonist naloxone; however, it was impossible to reverse drug action by washing the preparation. To obtain pharmacological effects on the isolated organ, up to 70-fold lower concentrations were necessary in case of the antidiarrheals than was calculated from their affinities in the radioreceptorassay.3.The pharmacokinetic behaviour of 3H-loperamide was tested in mice after i.v. injection. Compared to morphine, loperamide was rapidly eliminated from the general circulation and only small concentrations reached the CNS. This phenomenon provides an explanation for the fact, that opiate-like antinociception could only be obtained with antidiarrheal agents after application of subtoxic or toxic dose levels.4.Up to 2 h after i.v. administration of 3H-loperamide, an increase in radioactivity in the wall of the small intestine was found. Accumulation in the gastrointestinal tract might be related to distinct physicochemical properties of the compound. The surface tension lowering effect is a common characteristic by which all antidiarrheals differed from narcotic analgesics and could be related to the peculiar behaviour of these substances in vitro and in vivo.

A selective distribution pattern of different opiate receptors in certain areas of rat brain as revealed by in vitro autoradiography

A differential localization of binding sites in the rat brain for [3H]D-Ala2,Leu5-enkephalin ([3H]DADL) and [3H]etorphine ([3H]Eto) was been demonstrated by an in vitro autoradiography technique. Labeling with [3H]DADL was found predominantly in septum and paraventricular nuclei of hypothalamus, whereas [3H]Eto labeling was primarily observed in the periaqueductal gray and several thalamic nuclei. The distinctive patterns of labeling by the two ligands in certain central nervous system structures suggests the existence of specific neuronal pathways which are preferentially labeled with either [3H]DADL or [3H]Eto.

Rapid changes in enkephalin levels in rat striatum and hypothalamus induced by diazepam

SummaryThe acute treatment of rats with diazepam induces pronounced changes in brain enkephalin concentrations, as was estimated for methionine(met)-enkephalin and in some representative experiments for leucine(leu)-enkephalin, employing highly specific radioimmunoassays. Diazepam selectively increased the enkephalin concentrations in the hypothalamus by about 35%, and lowered it in the corpus striatum by roughly 25%; no changes could be detected in the medulla oblongata/pons or midbrain. The drug-induced changes displayed a rapid onset. Peak effects were reached by 2 to 5 min after injection. Changes observed in the hypothalamus were only short lasting and were apparently paralleled by diazepam concentrations in the brain, whereas the decrease in the striatum was of markedly longer duration. Presently, the mechanism underlying all these changes in unknown. Whereas an increase in enkephalin concentrations in the hypothalamus may be discussed in terms of the anti-stress effect of benzodiazepines, the observed drop in striatal enkephalin is not obviously to be correlated to behavioural changes induced by these drugs.