E. Leong Way

beta-Endorphin in vitro inhibition of striatal dopamine release.

THERE has been considerable recent interest in endogenous peptides as possible analgesic agents since the isolation of a pentapeptide with opiate-like activity (methionine-enkephalin) from porcine brain1. Of particular interest is β-endorphin, isolated from camel pituitary glands2, and now synthesised3. We report here the inhibitory effect in vitro of β-endorphin on striatal dopamine release from the central nervous system (CNS).

Morphine Tolerance, Physical Dependence, and Synthesis of Brain 5-Hydroxytryptamine

Tolerance and physical dependence development to morphine in mice can be prevented by concomitant administration of cycloheximide. The fact that the rate of synthesis of brain 5-hydroxytryptamine (5HT) increases with tolerance to morphine suggests that the protein involved may be associated with 5HT synthesis. Inhibition of this synthesis with p-chlorophenylalanine markedly decreases tolerance and physical dependence development to morphine.

Thyrotrophin-releasing hormone and shaking behaviour in rat.

THYROTROPHIN-RELEASING hormone (TRH) produces behavioural effects in experimental animals1 and may have psychoactive properties in man2–5. Winokur and Utiger6 and Brownstein et al.7 have described the distribution of TRH in rat brain, suggesting that TRH has a modifying role in synaptic functions in addition to its effect on the pituitary. The behavioural effects of endogenous TRH release in the brain are not known; however, Prange et al.8 noted that the systemic administration of TRH to pentobarbital-anaesthetised rats resulted in lacrimation, paw tremor and a peculiar shaking movement of the head and trunk. These behavioural effects were also obtained in partially anaesthetised rats following intracisternal injection of 10 µg TRH per animal. The TRH-induced shaking was particularly interesting because we have observed this behaviour as a characteristic sign of morphine abstinence in the anaesthetised rat9. Here, we have studied the central sites of TRH-induced shaking to determine if these sites parallel the endogenous distribution of TRH in the rat brain and also to determine if these sites correspond to brain areas where morphine withdrawal shakes are obtained.

Inhibition of morphine tolerance and physical dependence development and brain serotonin synthesis by cycloheximide

Abstract Mice were rendered tolerant to and physically dependent on morphine by daily subcutaneous (s.c.) injections of increasing doses of morphine for 3 weeks. The degree of tolerance was determined by measuring the increase in the median analgetic dose of morphine in relation to response to thermal stimulus, and dependence was determined by the precipitation of withdrawal jumping with the antagonist, naloxone. The concomitant daily administration of cycloheximide with morphine prevented the development of tolerance and physical dependence. The increase in brain serotonin turnover, which was noted to accompany development of tolerance and physical dependence, was also blocked. The findings suggest that the three responses to morphine may be closely related.

Discrete changes in brain calcium with morphine analgesia, tolerance-dependence, and abstinence

Abstract Subcellular localization of 45 Ca ++ in brain was determined after intracerebroventricular injection of the isotope in mice. Acute morphine injection selectively depleted 45 Ca ++ from synaptic vesicles while chronic morphine treatment increased the 45 Ca ++ in vesicular fractions. The elevated vesicular 45 Ca ++ found in tolerant-dependent animals rapidly declined during naloxone precipitated abstinence. These effects of morphine on brain Ca ++ localization are discussed in terms of their possible relationship to neurotransmitter release and tolerance and dependence development.

Stereospecific binding of narcotics to brain cerebrosides

Abstract Cerebrosides were shown to bind etorphine and naloxone stereo-specifically with high affinity. The relative potency of several narcotic analgesics in preventing the binding of etorphine and naloxone to cerebrosides correlated well with their reported intraventricular analgetic activity. The data indicate similarities between cerebroside sulfate and a purified opiate receptor from mouse brain which has been reported to be a proteolipid. Explanations for the apparent proteo-like behavior of the opiate receptor are provided.

Pharmacological manipulation of gamma-aminobutyric acid (GABA) in morphine analgesia, tolerance and physical dependence

Abstract The findings from our laboratory indicated that pharmacological manipulations of GABA system modified morphine analgesia, tolerance and physical dependence. Elevating brain levels of GABA by slowing its destruction with aminooxyacetic acid not only antagonized the analgesic action of morphine in both non-tolerant and tolerant mice, but also enhanced the development of tolerance and physical dependence. On the other hand, blockade of postsynaptic sites of GABA receptors by bicuculline resulted in an inhibition of tolerance and dependence development. Administration of 2,4-diaminobutyric acid, an inhibitor of GABA uptake in the neurons, antagonized morphine analgesia in both non-tolerant and tolerant mice. However, it did not modify naloxone precipitated withdrawal jumping. On the contrary, β-alanine, an inhibitor of the GABA uptake process in glial cells, potentiated naloxone precipitated withdrawal jumping in morphine dependent mice, but it had no effect on morphine antinociception in both non-tolerant and tolerant mice.

MODIFICATION OF ENDORPHIN/ENKEPHALIN ANALGESIA AND STRESS-INDUCED ANALGESIA BY DIVALENT CATIONS, A CATION CHELATOR AND AN IONOPHORE

1 The possibility that divalent cations may antagonize opiate peptide analgesia and stress‐induced analgesia was examined. 2 Intracerebroventricular injection of low doses of Ca2+, Mn2+ and Mg2+ antagonized β‐endorphin and methionine‐enkephalin analgesia. Ba2+ and Cd2+ were without effect. 3 The ionophore, A23187, significantly antagonized β‐endorphin analgesia and the effect was increased when a low dose of Ca2+ was injected at the same time as the ionophore. 4 Ethylene glycol tetraacetic acid (but not ethylenediamine tetraacetic acid) significantly potentiated endorphin analgesia. 5 Stress‐induced analgesia, as determined by increased tail‐flick latencies following intraperitoneal injection of acetic acid, was effectively antagonized by naloxone, Ca2+ and Mn2+. The frequency of writhing following acetic acid injection was increased by both naloxone and divalent metal ions, again suggesting antagonism of endogenous opiates. 6 These results confirm previous findings indicating that divalent metal ions (and especially Ca2+) may be involved in the actions of opiates.

Cholinergic modification of naloxone-induced jumping in morphine dependent mice

Abstract The effects of several i.p. administered, centrally active cholinergic agonists and antagonists on the naloxone-induced jumping behavior of morphine tolerant and dependent mice were studied in an effort to differentiate muscarinic and nicotinic modification of the withdrawal jumping behavior. Nicotine, tremorine, oxotremorine, arecoline and physostigmine significantly inhibited jumping, whereas atropine, benzotropine, pempidine and mecamylamine significantly potentiated jumping. Quaternary cholinergic drugs did not modify jumping. These drug effects were obtained generally without an alteration of the amount of naloxone or morphine in the brain. Cholinergic drugs also modified the jumping incidence in mice undergoing abrupt morphine abstinence. It is concluded that the inhibition of withdrawal jumping in morphine dependent mice by both muscarinic and nicotinic agonists and its enhancement by muscarinic and nicotinic antagonists reflect cholinergic modifications of the jumping response by a central mechanism(s).

Circling behavior in rats with 6-hydroxydopamine or electrolytic nigral lesions☆

Unilateral destruction of dopaminergic cell bodies in the substantia nigra zona compacta (SNC) was performed in rats using either electrocoagulation or chemical lesioning (6-hydroxydopamine, 6-OHDA). Neostriatal dopamine concentration ipsilateral to an electrolytic lesion was 34% of the contralateral side 2-3 weeks after operation; serotonin and noradrenaline brain levels were not altered. In contrast, dopamine and noradrenaline forebrain concentrations ipsilateral to a 6-OHDA lesion were 20 and 31%, respectively, of the contralateral side. After 6-OHDA, dopamine concentrations in the ipsilateral neostriatum were reduced to levels below the sensitivity of the fluorometric assay; cortical, brainstem and neostriatal serotonin levels, on the other hand, were not altered after 6-OHDA. Ipsilateral circling behavior was elicited by d-amphetamine after electrolytic and chemical lesioning. In contrast, the direction of circling produced after apomorphine differed between the two lesion models: contralateral circling behavior was exhibited by 6-OHDA-lesioned rats, whereas ipsilateral circling was produced in animals with electrolytic lesions. Contralateral circling was induced in both lesion-type models by haloperidol or pimozide. S.c. atropine administration induced ipsilateral circling in rats with 6-OHDA lesions, whereas contralateral circling was observed after arecoline. Animals with electrolytic SNC lesions turn ipsilaterally after s.c. administrations of either arecoline or atropine. The data indicate that the electrolytic and 6-OHDA circling behavior models represent two different neuropharmacological states and it is, therefore, suggested that comparisons of data obtained from models using different methods of lesioning be made with caution.