A. Herz

Opposing tonically active endogenous opioid systems modulate the mesolimbic dopaminergic pathway

The mesolimbic dopaminergic system has been implicated in mediating the motivational effects of opioids and other drugs of abuse. The site of action of opioids within this system and the role of endogenous opioid peptides in modulating dopamine activity therein remain unknown. Employing the technique of in vivo microdialysis and the administration of highly selective opioid ligands, the present study demonstrates the existence of tonically active and functionally opposing mu and kappa opioid systems that regulate dopamine release in the nucleus accumbens, the major terminal area of A10 dopaminergic neurons. Thus, stimulation of mu-type receptors in the ventral tegmental area, the site of origin of A10 dopaminergic neurons, increases dopamine release whereas the selective blockade of this opioid receptor type results in a significant decrease in basal dopamine release. In contrast, stimulation of kappa-type receptors within the nucleus accumbens decreases dopamine release whereas their selective blockade markedly increases basal dopamine release. These data show that tonic activation of mu and kappa receptors is required for the maintenance of basal dopamine release in the nucleus accumbens. In view of the postulated role of the mesolimbic system in the mediation of drug-induced alterations in mood and affect, such findings may have implications for the treatment of opiate dependence and affective disorders.

Psychotomimesis mediated by kappa opiate receptors

The kappa opioid agonists are analgesics that seem to be free of undesired morphine-like effects. Their dysphoric actions observed with the kappa agonist cyclazocine are thought to be mediated by an action at sigma-phencyclidine receptors. The benzomorphan kappa agonist MR 2033 is inactive at sigma-phencyclidine receptors. In male subjects, the opiate-active (-)-isomer, but not the (+)-isomer, elicited dose-dependent dysphoric and psychotomimetic effects that were antagonized by naloxone. Thus, kappa opiate receptors seem to mediate psychotomimetic effects. In view of the euphorigenic properties of mu agonists, our results imply the existence of opposed opioid systems affecting emotional and perceptual experiences.

Endogenous opioid systems and alcohol addiction

Abstract Alcohol exerts numerous pharmacological effects through its interaction with various neurotransmitters and neuromodulators. Among the latter, the endogenous opioids play a key role in the rewarding (addictive) properties of ethanol. Three types of opioid receptors (μ, δ and κ) represent the respective targets of the major opioid peptides (β-endorphin, enkephalins and dynorphins, respectively). The rewarding (reinforcing) properties of μ- and δ-receptor ligands are brought about by activation of the mesolimbic dopamine system which ascends from the ventral tegmentum of the midbrain (VTA) to rostral structures; of these, the nucleus accumbens (NAC) is of particular importance in drug addiction. In contrast, dysphoria results from activation of κ-receptors. The neurochemical manifestations of these opposing effects are, respectively, increases and decreases in dopamine release in the NAC. Several lines of evidence indicate that alcohol interferes with endogenous opioid mechanisms which are closely linked with dopamine transmission in the mesolimbic pathway. The view that condensation products of dopamine and alcohol-derived aldehyde (tetrahydroisoquinolines) play a role remains controversial. There is, however, much information on the direct (acute and chronic) effects of alcohol on the binding properties of opioid receptors, as well as modulation of opioid peptide synthesis and secretion (e.g. a suggested increase in β-endorphin release). In view of the reinforcing properties of alcohol, it is relevant to consider behavioural studies involving alcohol self-administration in rodents and primates. Low doses of morphine have been found to increase, and higher doses of the opiate to decrease, alcohol consumption. Conversely, opioid antagonists such as naloxone and naltrexone (which bind to non-selectively opioid receptors) have been shown to decrease alcohol consumption under various experimental conditions. Similar results have been reported when selective μ- or δ-receptor antagonists are administered. Results obtained in genetic models of high preference for alcohol also support the view that alcohol intake depends on the activity of the endogenous opioid reward system and that alcohol consumption may serve to compensate for inherent deficits in this system. One hypothetical model proposes that reward results from activation of μ-opioid receptors in the VTA and/or δ-receptors in the NAC; both these nuclei are targets of endogenous β-endorphin. It is suggested that alcohol interferes with this reward pathway either directly or indirectly. The available experimental data accord well with those obtained from clinical studies in which opioid antagonists have been used to prevent relapse in alcoholics. Conceptual considerations concerning communalities between various forms of addictions are also discussed in this review.

Development of physical dependence on morphine in respect to time and dosage and quantification of the precipitated withdrawal syndrome in rats

In rats implanted subcutaneously with morphine containing pellets different degrees of dependence were induced by varying the dosage, frequency of implantation and duration of exposure to morphine. Withdrawal was precipitated by intraperitoneal injection of morphine antagonists, mostly levallorphan. The absorption of morphine from the subcutaneous depots was estimated chemically.When withdrawal was precipitated with a constant dose of antagonist the frequency of occurrence of various counted signs and the presence of some checked signs were studied in respect to varying degrees of dependence. The results were compared to those obtained after administration of increasing doses of antagonist in groups of animals that had developed a constant degree of dependence.In both types of experiments the results were rather similar. Some signs became progressively more pronounced when dependence got stronger or the dose of the antagonist was increased. In contrast, other signs showed a maximal frequency at the lower degrees of dependence or after administration of the lower doses of antagonist and decreased or even disappeared when the degree of dependence was higher or the dose of antagonist further increased. Obviously, in withdrawal the intensity of “recessive” signs like writhing and wet dog shaking declines when “dominant” signs like jumping, flying (a vigorous kind of jumping) and teeth chattering increase. An inverse relationship between the occurrence of various signs could also be shown within the 30 min observation period. Changes in the integrative mechanisms controlling behaviour during withdrawal are supposed to be the reason for this shift of signs.In other experiments in which the interval between each morphine implantation was prolonged the frequency of some signs like jumping and teeth chattering tented to plateau. This finding seems to be correlated to some kind of steady state on resorption of morphine from the subcutaneous depots, as was found in chemical analysis.

The Effects of Opioid Peptides on Dopamine Release in the Nucleus Accumbens: An In Vivo Microdialysis Study

Abstract: An involvement of the mesolimbic dopamine (DA) system in mediating the motivational effects of opioids has been suggested. Accordingly, the present study employed the technique of in vivo microdialysis to examine the effects of selective μ‐, δ‐, and k‐ opioids on DA release in the nucleus accumbens (NAC) of anesthetized rats. Microdialysis probes were inserted into the NAC and perfusates were analyzed for DA and its metabolites, dihydroxyphenylacetic acid (DO‐PAC) and homovanillic acid (HVA), using a reverse‐phase HPLC system with electrochemical detection for separation and quantification. Intracerebroventricular (i.c.v.) administration of selective μ‐opioid [D‐Ala2, N‐methyl‐Phe4, Gly5‐ol]‐enkephalin (DAMGO) or δ‐opioid [d‐Pen2, d‐Pen5]‐en‐kephalin (DPDPE) agonists, at doses that function as positive reinforcers in rats, resulted in an immediate and significant increase in extracellular DA. DOPAC and HVA levels were also significantly increased. The effects of DAMGO were blocked by the selective μ‐antagonist d‐Pen‐Cys‐Tyr‐d‐Trp‐Orn‐Thr‐Pen‐Thr‐NH2 (CTOP) whereas those of DPDPE were blocked by the δ‐antagonist allyl2‐Tyr‐Aib‐Aib‐Phe‐Leu‐OH (ICI 174,864). In contrast to μ‐ and δ‐agonists, the k‐agonist N‐CH3‐Tyr‐Gly‐Gly‐Phe‐Leu‐Arg‐N‐CH3‐Arg‐d‐Leu‐NHC2H5 (E‐2078). a dynorphin analog that produces aversive states, decreased DA release in a biphasic manner. Norbin‐altorphimine, a selective k‐antagonist, could block this effect. These results demonstrate that μ‐, δ‐, and k‐opioid agonists differentially affect DA release in the NAC and this action is centrally mediated.

Motivational properties of kappa and mu opioid receptor agonists studied with place and taste preference conditioning

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.Kappa receptor agonists produced conditioned aversions. Taste aversions were produced by all of the drugs used, including racemic mixtures of ethylketazocine, tifluadom, and U50-488, and active isomers (+)-tifluadom, (-)-bremazocine, and Mr 2034; corresponding inactive isomers either produced no effect of were less potent. Place aversions were produced by U50-488 and (-)-bremazocine, but not (+)-bremazocine or any of the other kappa receptor agonists tested with the taste procedure. The mu agonists produced predominantly conditioned preferences. Place preferences were produced by morphine, fentanyl and sufentanil. Taste preferences were produced by low doses of these substances; at higher doses the taste preferences were absent or replaced by aversions. Finally, with naloxone and lithium chloride it was shown that the taste procedure was more sensitive to punishing effects than the place procedure.It is concluded that kappa and mu opioid receptor agonists are effective unconditioned stimuli. From the lower portions of the dose response curves it is further concluded that activation of kappa opioid receptors has aversive properties and activation of mu receptors appetitive reinforcing properties. The findings are also discussed with regard to the prevailing notions of taste conditioning with opiates, and the reinforcing properties of activity of the endogenous opioid peptide systems.

Inflammation of the rat paw enhances axonal transport of opioid receptors in the sciatic nerve and increases their density in the inflamed tissue

The effect of inflammation, induced by unilateral intraplantar injection of Freunds adjuvant, on opioid receptors transported in the sciatic nerve and on opioid receptors present in the paw of the rat was studied by means of in vitro receptor autoradiography using [125I]beta-endorphin (human) as ligand. In the absence of inflammation, human beta-endorphin binding sites accumulated proximally and distally to a ligature placed on the sciatic nerve in a time-dependent manner, indicating bidirectional axonal transport. Some human beta-endorphin binding was also visible in non-inflamed paw tissue. Inflammation of the paw tissue massively increased human beta-endorphin binding on both sides of the sciatic nerve ligature and in the ipsilateral paw tissue. In inflamed paw tissue, beta-endorphin binding accumulated in the cutaneous nerve fibers as well as in the immune cells infiltrating the surrounding tissue. In the sciatic nerve and paw tissue, beta-endorphin binding was displaced by (D-Ala2, N-methyl-Phe4, Gly-ol5)enkephalin and (D-Pen2, D-Pen5)enkephalin, selective mu- and delta-opioid receptor agonists, respectively, and by the universal opioid antagonist naloxone, but not by U-50,488H, a k-selective receptor agonist. Taken together, these data provide neuroanatomical evidence for local inflammation-induced enhanced axonal transport of opioid receptors in rat sciatic nerve and accumulation in paw tissue.

On the central sites for the antinociceptive action of morphine and fentanyl

The inhibition by morphine and fentanyl of a nociceptive response, the licking reaction elicited by electrical stimulation of the tooth-pulp, was studied in rabbits. Both drugs were injected into the entire ventricular system, and into its separate parts and morphine was applied to definite brain structures. Spread of the applied drug was examined autoradiographically, using 14C-labelled morphine. Intraventricular injection of morphine was 500–1000 times more effective than intravenous injection. The onset of action was much slower than after systemic application; this latency is interpreted as being caused by the slow permeation of morphine to the receptor areas in the periventricular structures. Injection into separate parts of the ventricular system did not inhibit the nociceptive reaction when the spread of substance was restricted to one or both lateral ventricles. Inclusion of the third ventricle in the area of distribution resulted in only a slight increase in effectiveness. Application of the drugs into the aqueduct and into the fourth ventricle induced a complete inhibition of the nociceptive reaction. When morphine spread into the surrounding of the cisterna cerebellomedullaris (with exclusion of ventricular system) no effect was observed. Uni- or bilateral microinjection of morphine into various diencephalic and mesencephalic periventricular structures induced a pronounced inhibition of the nociceptive reaction. In at least one part of these experiments, however, morphine may have acted indirectly, by spreading to caudal parts of the ventricular system. The most effective sites of the antinociceptive action of morphine-like substances were situated in the fossa rhomboides and structures near it. Autoradiographical studies showed the sites for the antinociceptive action of morphine to be located in the ventricular wall of these regions at a depth of about 1–2 mm. Diencephalic sites of action were of minor importance, at least when the licking reaction was used as test. The functional structures involved in the drug effects are discussed.

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 Freunds 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.

Opiate receptor binding sites in human brain

Subclasses of opiate receptor binding sites in human brain membranes were investigated by means of competitive binding techniques. The experimental data were analyzed by use of a computerized non-linear regression curve fitting program. mu-, delta-and chi-types of opiate binding were found in 5 different regions of the brain. A more extensive analysis of the regional distribution of subclasses of opiate binding sites was performed using a simple sequential inhibition technique. This method was shown to yield results which are comparable to those obtained by computer analysis of multiple tracer displacement curves. Chi-and mu-sites represented the major component of binding in most brain areas whereas delta-sites were fewer in number. The 3 types of binding showed different distribution patterns, suggesting that they are independent from each other. The distribution pattern observed in human brain resembled the one observed in rat brain, although chi-sites appear to represent a more important, and delta-sites appear to represent a less important, fraction of binding in human as compared to rat brain.