Mißbräuchlich verwendete Pharmaka und Drogen beeinflussen nachhaltig Stimmung und Antrieb. Beim Menschen bewirken sie Wohlbefinden (Euphorie), im Tierversuch führen sie zu „drug seeking behaviour“ und damit zu psychischer Abhängigkeit, die am Beginn des Suchtgeschehens steht. Bei fortgesetzter Drogeneinnahme kommt es bei gewissen Substanzgruppen (Opioide und vorwiegend dämpfend wirkende Pharmaka, z.B. Benzodiazepine) zur Ausbildung von Toleranz und körperlicher Abhängigkeit. Obwohl diese Phänomene bei der Aufrechterhaltung der Sucht beteiligt sein können, spielen sie bei der Entwicklung des süchtigen Verhaltens keine ursächliche Rolle. Vielmehr kann heute davon ausgegangen werden, daß die zu psychischer Abhängigkeit führenden Substanzen bestimmte Hirnstrukturen („reward pathways“) aktivieren, wodurch das Suchtgeschehen in Gang gesetzt wird. Das vom Mittelhirn zum Zwischen- und Endhirn aufsteigende mesolimbische Dopaminsystem spielt hierbei eine zentrale Rolle (Wise 1978; Herz u. Shippenberg 1989). Die wirksamen Mechanismen sollen hier am Beispiel des Prototyps der Suchtmittel, der Opioide, beschrieben und diskutiert werden. Die Identifizierung verschiedener Typen von Opioidrezeptoren sowie endogener und exogener Liganden (einschließlich spezifischer Antagonisten) der verschiedenen Rezeptortypen ermöglicht in dieser Substanzgruppe detaillierte Einblicke in die zugrundeliegenden komplexen Vorgänge. Schließlich soll gezeigt werden, daß Modulation des dopaminergen Reward-Systems durch Opioide nicht für diese Pharmaka spezifisch ist, sondern ganz ähnliche Mechanismen auch bei anderen Klassen von Suchtstoffen wirksam sind.

In the present study we used in vivo microdialysis to examine the influence of beta-endorphin-(1-27) (beta-EP-(1-27) upon beta-endorphin (beta-EP)-induced dopamine (DA) release in the nucleus accumbens of anesthetized rats. Microdialysis probes were inserted into the nucleus accumbens and perfusates were analyzed for DA and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), using a reversed-phase HPLC system with electrochemical detection. Intracerebroventricular (i.c.v.) administration of beta-EP-(1-27) (5-20 micrograms) resulted in a dose-dependent increase in DA release which was smaller than the beta-EP-induced DA release, whereas metabolite levels were not altered. Pretreatment with beta-EP-(1-27) (5-20 micrograms) significantly altered the beta-EP (5 micrograms)-induced increase in DA release. These results indicate that beta-EP-(1-27) antagonizes the beta-EP-induced release of DA in the nucleus accumbens. In addition to its antagonistic properties at the beta-endorphin binding site, beta-EP-(1-27) appears to be a partial agonist, inducing increased DA release. These findings suggest a regulatory function for this naturally occurring beta-EP fragment within the mesolimbic system.

General aspects opioid receptor biochemistry and signal transduction CNS opioidergic systems distribution and modulation functional aspects opioid tolerance and dependence.

In vivo microdialysis was used to compare the effects of β-endorphin upon dopamine (DA) release in the nucleus accumbens (NAC) of anesthetized versus freely moving rats, and to examine the role of the the mesolimbic DA system in mediating both the motoric and secondary reinforcing effects of this peptide. Microdialysis probes were inserted into the NAC and perfusates were analyzed for DA and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), using a reversed phase HPLC system with electrochemical detection for separation and quantification. Intracerebroventricular (ICV) administration of β-endorphin (2.5 and 5.0 µg) increased DA release and metabolites in both freely moving and anesthetized rats. This effect was of greater magnitude and duration in freely moving rats and was accompanied by stimulation of locomotor activity. The 5 µg dose also functioned as a secondary reinforcer in a conditioned place preference paradigm. A higher dose of β-endorphin (7.5 µg) stimulated DA release and metabolites in anesthetized rats but failed to affect these parameters in freely moving rats. At this dose, catalepsy and a loss of the reinforcing effects of this peptide were observed. These data demonstrate marked differences in the effects of β-endorphin upon DA release in the awake versus anesthetized rat. Further, the finding that the reinforcing and locomotor stimulating effects of β-endorphin only occur at those doses which stimulate DA release suggest that this action is critical for the expression of both behavioral effects.

In the present study we used in vivo microdialysis to examine the influence of β-endorphin on dopamine (DA) release in the nucleus accumbens of anesthetized rats and to identify the opioid receptor types mediating its effects. Microdialysis probes were inserted into the nucleus accumbens and perfusates were analysed for DA and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), using a reversed phase HPLC system with electrochemical detection for separation and quantification. Intracerebroventricular (i.c.v.) administration of β-endorphin resulted in a dose-dependent increase in DA and its metabolites. Pretreatment with the selective δ-antagonist ICI 174,864 significantly attenuated the β-endorphin-induced increase in DA release and metabolism whereas pretreatment with the selective μ-antagonist CTOP resulted abolition of the β-endorphin effect. These data demonstrate that the blockade of either μ- or δ-opioid receptors is sufficient to antagonize the stimulatory effects of β-endorphin on DA release and metabolism. As such these findings suggest that the concomitant activation of both μ-and δ-receptors underlies the effects of β-endorphin on DA release in the nucleus accumbens.

In situ hybridization was used to study the macroscopic distribution and regulatory control of proenkephalin mRNA and prodynorphin mRNA in rat striatum. While proenkephalin mRNA was widely distributed throughout the striatum, levels of prodynorphin mRNA were highest in the medial and ventral portions of the striatum. Furthermore, in contrast to the results for proenkephalin mRNA, the levels of prodynorphin mRNA appeared higher in the nucleus accumbens than in the striatum. The mesostriatal dopaminergic pathway was destroyed by discrete, unilateral injection of 6-hydroxy-dopamine (6-OHDA) into either the substantia nigra or the neighboring ventral tegmental area (VTA). Lesions of the substantia nigra caused a dramatic ipsilateral increase in the hybridization signal for proenkephalin mRNA, but no change was observed in the hybridization signal for prodynorphin mRNA. Similar effects were seen with VTA lesions. Since destruction of the mesostriatal dopamine system elevates the levels of proenkephalin mRNA, but not of prodynorphin mRNA, in the striatal target neurons, it appears that the mesostriatal pathway exerts a tonic and selective suppression of striatal proenkephalin gene expression at the mRNA level.

Bovine adrenal medullary chromaffin cells, prelabeled with [ 3 H]norepinephrine, released a large proportion of cellular 3 H-labeled catecholamines (CAs) when stimulated with nicotine, K + , histamine, γ-aminobutyric acid (GABA) and several peptidic hormones [bradykinin, angiotensin II, thyrotropin releasing hormone (TRH) and neurotensin]. The histamine-induced response was dose dependent and occurred through H 1 histaminergic receptors. Quantitatively and temporally the histamine- and nicotine-induced responses differed. Nicotine, during the first minutes, induced a large increase of [ 3 H]CAs, but this response was desensitized rapidly. In contrast, histamine initially provoked a smaller release of [ 3 H]CAs than nicotine but, with prolonged exposure (hours), a much greater response was found with histamine. Moreover, little desensitization was observed with histamine even during extended stimulation. External Ca 2+ was obligatory for the histamine response, and both inorganic (Co 2+ and Ni 2+ ) and organic (verapamil, nifedipine and D-600) Ca 2+ channel blockers significantly reduced release of [ 3 H]CAs. These studies suggest that histamine as well as certain other neuroactive substances could play an important role in the physiology and biochemistry of adrenal medullary chromaffin cells.

factor 268 Atropine 303 AtT-20 169 AtT-20/D-16v cells 32 Autoradiography 398, 533 -, in vitro 473 Basal hypothalamus 83 Bed nucleus, stria terminalis 225 Behaviour 109 Beta-adrenergic receptors 259 Biphasic changes 149 Blood flow 567 Body weight 263 Brain 50, 398 -cell cultures 50 Bromocriptine 159,236 Ca" 95 Calcium 20 -channel 169 -current 169 cAMP 95 Carbachol 303

AbstractEmotions, experienced subjectively, appear enigmatic and inaccessible to scientific analysis, but affect many concrete psychological phenomena, such as pain sensitivity and reactions to every-day experiences. Recent evidence suggests that opioid peptides, in particular β-endorphin, may influence motivational processes by affecting emotions. Opioid peptides are found in the nervous system and peripheral tissues and produce effects similar to those of opioid drugs. β-Endorphin, whose action has a long duration, may function as a hormone and as a neuromodulator, rather than as a classical neurotransmitter. It is released by stress and causes hyperthermia and an increase in threshold pain. Endogenous opioids also stimulate food and water intake. Biochemical and behavioural experiments indicate that endogenous opioids, in particular β-endorphin, are released in purely rewarding situations. β-Endorphin may stimulate intake by supporting positive emotional reactions; in addition, it reduces negative reactions to aversive stimuli. This is consistent with the fact that β-endorphin produces positive emotional changes and that the aversiveness of antagonists, which reflects opioid blockade, is eliminated by destruction of the β-endorphin system in the brain. Such an emotional effect would also explain behavioural changes, for example, a decreased sensitivity to noxious stimulation and an increase in exploration tendencies. In nature, such changes might aid in foraging and in the formation of positive associations between environment and reward. Opioids lower the threshold for rewarding electrical stimulation to the brain. They also alter the response to sensory input and modify the initiation of motor activity. Naturally released opioids may, through effects on sensory processing, enable sensory input to excite more easily reward systems. Consistent with such a general effect are reports of opioid influences on several forms of motivated behaviour other than food and water intake. Some behaviours which are stimulated by aversive stimuli, such as shock-induced fighting and the need for closeness in social animals, are inhibited by opioids, whereas inter-male aggression is potentiated. Current results suggest that opioids suppress sexuality, perhaps through an interaction with hormones. Opioid effects on learning and memory can probably be explained in terms of their emotional actions.

Opioid-receptor binding and the opioid-mediated stimulation of low Km GTPase and inhibition of adenylate cyclase were studied in membranes derived from NG 108-15 cells pretreated with either the opioid peptide [d-Ala2, d-Leu5]enkephalin (DADLE) or morphine. Pretreatment with DADLE resulted in a concentration-dependent loss of responsiveness of GTPase to the peptide; this effect was entirely accounted for by a reduction in the maximal stimulation produced acutely by DADLE, without changes in the ec50, of the peptide, indicating a non-competitive type of desensitization. The degree of desensitization of GTPase was similar after one and 24 hr of pretreatment with DADLE, indicating that the process occurs rapidly. In contrast, morphine, which was 70–80% as potent as DADLE in stimulating GTPase and inhibiting adenylate cyclase in acute conditions, induced only a minimal desensitization of the opioid-GTPase system and, in contrast to DADLE, did not desensitize adenylate cyclase. Pretreatment with DADLE for one hour led to a decrease in opioid receptor density which was quantitatively similar to the degree of desensitization of GTPase: both these effects of DADLE were antagonized to a similar extent when morphine was also present in the pretreatment. Thus, desensitization of the opioid-stimulated GTPase appears to be correlated with down-regulation of the opioid receptor. Moreover, these findings suggest that partial agonists cannot induce this process.

Equilibrium binding isotherms of [3H]diprenorphine in membranes from NG 108-15 cells are consistent with a homogenous population of binding sites. Upon addition of Na+, Mg2+ and GTP, only a 2-fold reduction in affinity with a minor decrease in the number of sites is observed. Dissociation curves of [3H]diprenorphine, however, are clearly biphasic: in the absence of Na+, Mg2+ and GTP, 80% of the bound ligand dissociates slowly with at1/2 of 100 min, and only 20% rapidly (t1/2 4.5 min). In the presence of Mg2+, nearly all the binding is found in the slowly dissociating form. Upon the addition of either Na+ or GTP, 20–30% of the binding dissociates more rapidly. The rate constant of the rapidly dissociating form generated by Na+, however, is 2.5 times greater than that induced by the presence of GTP. Thus, the addition of both, Na+ and GTP, converts about 80% of the receptor into a very fast dissociating form (t1/2 1.7 min).

Using taste and place preference conditioning, the present study examined the motivational properties produced in adult rats by systemic administration of (−) and (+) morphine, levorphanol, and dextrorphan. Conditioned place preference was stereospecific; it was only produced by the opioid receptor active isomers, levorphanol and (−) morphine. Similarly, a conditioned taste preference produced by a low dose of morphine was only seen with the active isomer. Conditioned taste aversion, however, was produced in a comparable dose range by both the active and the inactive isomers. In addition injections of inactive isomers also produced tolerance to the taste aversion produced by (−) morphine.

The present study examines the influence of destruction of the medio-basal arcuate hypothalamus (MBH), the primary site of synthesis of central pools of β-endorphin (β-EP), upon the aversive properties of naloxone in a conditioned place preference paradigm. Bilateral radiofrequency lesions of the MBH resulted in a pronounced fall in levels of immunoreactive β-EP in the brain. Lesioned rats, in contrast to non-operated animals, showed a clear reduction in the conditioned place aversion produced by naloxone. However, they showed no loss of the conditioned preference produced by the mu-selective opioid receptor agonist, morphine, or the conditioned aversion produced by the kappaselective agonist, U50-488. In contrast to the effect of the lesions, suppression of circulating β-EP by dexamethasone treatment failed to influence conditioning produced by naloxone. Thus, the data indicate that the aversive properties of naloxone are attenuated by disruption of central (but not peripheral) β-EP activity. We suggest that these properties of naloxone reflect an antagonism of β-EP activity in the brain. In addition, the data indicate that differing mechanisms underlie the aversive actions of naloxone as compared to U50-488.

Rats were trained in a two-lever food-reinforced procedure to discriminate between the effects of saline and the opioid kappa receptor agonist ethylketocyclazocine. After acquisition of this discrimination, generalization tests with opioid peptides such as β-endorphin, α-neoendorphin, dynorphin A and some dynorphin-derived peptides were conducted. The rats dose-dependently generalized the effects of intracerebroventricularly injected ethylketocyclazocine but not β-endorphin, α-neoendorphin, dynorphin A1–8, dynorphin A1–13, D-Cys2-L-Cys5-dynorphin A1–13 or dynorphin A.

This chapter discusses the organization, characteristics, release, and modulation of hypophyseal and other endocrinologically relevant pools of opioid peptides and their possible target sites and functions. The nature and role of opioid mechanisms in the control of the endocrine secretion of various tissues are considered in the chapter. Many conditions—for example, stress, nutritional status, degree of hydration, and phase of circadian or estrus cyclicity—have biochemically been shown to be (1) associated with alterations in the activity of endocrine-like opioid systems and (2) related to opioid-mediated changes in behavioral, physiological, and endocrinological measures. It is the unification of these complementary sources of information and the assignation of particular functions to particular endocrine-like pools of opioids that are of importance. The chapter also discusses a particular aspect of the function of opioid mechanisms, including endocrine-like opioid peptide systems—that is, of their modulation of the secretion of hormones into the systemic circulation. The ability of opioids to modify these endocrine parameters is among their most striking properties and, especially in the case of antagonists, no less impressive than their role in antinociceptive processes. There are multifarious sites of attack and mechanisms of action by which opioid mechanisms may modulate endocrine secretion. Similarly, a diversity of opioid ligands and receptor types is implicated in particular cases. An elucidation of the roles of individual ligands and receptor types at specific loci and their relationships to other control systems under specific conditions has not been achieved.

The reinforcing properties of various opioid agonists acting preferentially on the kappa and mu opioid receptors were assessed using taste and place preference conditioning procedures.

Lesions of the hypothalamic paraventricular nucleus depleted immunoreactive (ir)-vasopressin (VP) and -oxytocin (OT) from rat spinal cord but failed to modify nociceptive thresholds. Further, intrathecal introduction of VP and OT into the cord failed either to influence nociceptive thresholds or to modify the antinociceptive action of morphine. However, doses of VP as low as 20 ng caused, in contrast to OT, a hind-limb muscular flaccidity and respiratory disturbances. Rats suffering from chronic arthritis did not, finally, reveal any alterations in levels or ir-VP or ir-OT in the spinal cord.

The concentration of immunoreactive (ir) beta-endorphin (beta-END) in the neurointermediate pituitary lobe was 15-fold higher in adult than in newborn rats; in contrast, that of ir-beta-END in the anterior lobe was twice as high in newborn as in adult animals. Ir-beta-END in the neurointermediate lobe of newborn rats consisted exclusively of beta-END-sized peptides, indicating that at birth rats are capable of processing the opioid peptide precursor proopiomelanocortin (POMC) to beta-END. Moreover, beta-END-related peptides in the neurointermediate lobe of newborn rats were found to be predominantly alpha-N-acetylated and, therefore, inactivated with respect to their opiate-like properties. Further analysis of these alpha-N-acetylated forms on high performance liquid chromatography indicated that newborn rats predominantly contained alpha-N-acetyl-(Ac-)beta-END-(1-31), whereas the major forms in adult rats were Ac-beta-END-(1-27) and -(1-26). Thus, the C-terminal processing of Ac-beta-END-(1-31) to -(1-27) and -(1-26) may not yet be fully active at birth, in contrast to the processing of POMC to beta-END. In the anterior lobe of newborn rats, however, the ratio of beta-lipotropin/beta-END resembled that of adults, and more than 80% of beta-END-sized ir-material was found to consist of nonacetylated (and therefore opiate-active) beta-END-(1-31), as in adults, suggesting that the enzymatic system responsible for processing of POMC to beta-lipotropin and beta-END is already mature at birth. The high concentrations of beta-END in the anterior lobe of newborn rats suggest a possible role of this opioid peptide in perinatal development and/or parturition.