Dopamine and methionine-enkephalin in human brain

The contents of dopamine (determined radioenzymatically) and methionine-enkephalin (assayed by a radioimmunoassay) were measured in several areas of the human brain. The peptide was principally localized in dopamine-rich structures. In patients with Parkinson's disease, in contrast to the general dopamine deficiency, the reduction in methionine-enkephalin was restricted to the mesencephalon, putamen and lateral pallidum.     

Morphofunctional study of mammotropic cells following intraventricular administration of met-enkephalin

Summary An ultrastructural and morphometric study was carried out on the adenohypophyseal mammotropic cells of rats treated intraventricularly with an acute dose (150 g) of Met-enkephalin. In the female rats, clear features of cellular hyperactivity appeared after opioid administration. The changes affected the Golgi complex, the rough endoplasmic reticulum, the mature and immature secretory granules and the images of exocytosis. Such changes did not appear when naloxone was administered before the opioid, and naloxone induced an increase in the numerical density of lysosomal dense bodies with lipoid inclusions. In the male animals, administration of an identical dose of Metenkephalin caused only a few significant changes, similar to those observed in the controls. It is concluded that Metenkephalin administered intraventricularly causes evident modifications in the mammotropic cells of female rats whereas such changes in the male animals are not significant.     

纳洛酮改善慢性酒精中毒大鼠学习记忆障碍的阿片机制研究

目的研究慢性酒精中毒大鼠空间学习记忆障碍的阿片机制。方法3周龄雄性SD大鼠分为4组:给A、B组大鼠饮用乙醇水溶液8周,建立慢性酒精中毒模型,8周后给B、D组大鼠连续10d腹腔注射纳洛酮,A、C组腹腔注射生理盐水。然后分组用Morris水迷宫训练方法,比较4个组大鼠的空间学习记忆能力的差异;采用放射免疫法测定下丘脑、海马、纹状体和前额皮质每克组织中甲脑啡肽(MENK)和亮脑啡肽(LENK)的含量。结果大鼠慢性酒精摄入后下丘脑和海马内的MENK水平[分别为(362.01±172.18)pg/g,(27.92±7.38)pg/g];而海马内LENK水平为(69.74±17.03)pg/g,较正常对照组均出现不同程度的升高(P<0.05或P<0.01),且相关分析显示脑啡肽水平的变化与水迷宫成绩损害相关。而纳洛酮能部分或完全逆转海马内MENK和LENK含量的变化并改善大鼠学习记忆的行为学成绩。结论酒精损害了大鼠的学习记忆能力,其机制可能与学习记忆相关脑区内的MENK和LENK水平的变化有关。     

A study of 24-hour profiles of plasma met-enkephalin in man

Met-enkephalin has recently been demonstrated to circulate in human plasma and using this highly specific extracted radioimmunoassay the fluctuations of plasma Met-enkephalin in man were studied over 24 h. The subjects were 6 healthy volunteers. Following a 24 h adaptation period in the metabolic ward and sleep laboratory, an i.v. catheter was inserted. Blood samples were taken at hourly intervals through the day and at 30 min intervals between 23.00 h and 07.00 h. Sleep was monitored polygraphically. There was no regular rhythm discernible in plasma Met-enkephalin levels throughout the 24 h, nor was there any relationship with sleep or food intake. In a further 3 subjects beta-LPH and beta-endorphin levels as estimated by N- and C-terminal beta-LPH radioimmunoassay were elevated on waking compared with 01.00 h, suggesting a nyctohemeral rhythm. In contrast to the correlated circadian fluctuations in beta-LPH, ACTH and beta-endorphin levels therefore, the lack of circadian rhythmicity and dissociation of plasma Met-enkephalin from plasma levels of the former group of peptides suggests control mechanisms for the secretion of Met-enkephalin are quite different and adds support to the concept of separate Met-enkephalin precursors.     

Enkephalin-induced inhibition of hypothalamic paraventricular neurons

The Met-enkephalin analogues FK 33-824 and FW 34-569 were bath-applied to coronal slices from adult rat hypothalamus. At 5·10⁻⁸ M, these substances caused a 50% reduction of the firing of paraventricular (PVN) neurons, which was antagonized by 10⁻⁶ M naloxone. Morphine at 10⁻⁶−10⁻⁴ M also inhibited PVN neurons. The results, which indicate that opiates produce a potent and apparently specific reduction in PVN firing, suggest that enkephalins may regulate posterior pituitary secretion by acting at hypothalamicand neurohypophysial levels.     

In vivo modulations by GABA-related drugs of met-enkephalin release in basal ganglia of the cat brain

The influence of the intrapallidal application of GABA-related compounds on the release of Met-enkephalin in the globus pallidus and the caudate nucleus in the two hemispheres was investigated in vivo in the cat. For this purpose, the 4 structures were continously superfused with an artificial CSF through implanted push-pull cannulae and Met-enkephalin released in superfusates was determined using a specific radioimmunoassay.GABA (10–500 μM) reduced the local release of Met-enkephalin during its application but once the amino acid was removed from the superfusing fluid, an increase in the peptide release was observed. Diazepam (10 μM) induced only an inhibitory effect whereas muscimol (1 μM) stimulated Met-enkephalin release. Opposite changes in Met-enkephalin release were also seen with the GABA antagonists, bicuculline methiodide (1 μM) and picrotoxin (10 μM), suggesting that the local regulation of Met-enkephalin released by GABA related compounds ay be mediated by at least two types of GABA receptors.In several cases, the unilateral pallidal application of GABA agonists and antagonists induced significant changes in Met-enkephalin release at distant structures. The most striking effect was observed with diazepam which markedly reduced the peptide release in both caudate nuclei and pallida. These data suggest that GABAergic systems can contribute to some bilateral regulation of striato-pallidal enkephalinergic neurones.     

Delta9-tetrahydrocannabinol releases and facilitates the effects of endogenous enkephalins: reduction in morphine withdrawal syndrome without change in rewarding effect

Recent studies have suggested that cannabinoids might initiate the consumption of other highly addictive substances, such as opiates. In this work, we show that acute administration of Delta9-tetrahydrocannabinol in mice facilitates the antinociceptive and antidepressant-like responses elicited by the endogenous enkephalins protected from their degradation by RB 101, a complete inhibitor of enkephalin catabolism. This emphasizes the existence of a physiological interaction between endogenous opioid and cannabinoid systems. Accordingly, Delta9-tetrahydrocannabinol increased the release of Met-enkephalin-like material in the nucleus accumbens of awake and freely moving rats measured by microdialysis. In addition, this cannabinoid agonist displaced the in vivo [3H]diprenorphine binding to opioid receptors in total mouse brain. The repetitive pretreatment during 3 weeks of Delta9-tetrahydrocannabinol in mice treated chronically with morphine significantly reduces the naloxone-induced withdrawal syndrome. However, this repetitive administration of Delta9-tetrahydrocannabinol did not modify or even decrease the rewarding responses produced by morphine in the place preference paradigm. Taken together, these behavioural and biochemical results demonstrate the existence of a direct link between endogenous opioid and cannabinoid systems. However, chronic use of high doses of cannabinoids does not seem to potentiate the psychic dependence to opioids.     

Interaction of Opiate Peptides with Dopamine Effects on Prolactin Secretion and Membrane Electrical Properties in Anterior Pituitary Cells from Lactating Rats

Met-enkephalin and β-endorphin induced a partial reversion of the dopamine inhibition of prolactin release from pituitary cells of lactating rats in primary culture. This effect of opiate peptides was dose-dependent with an EC50 of 40 ± 8 nM and 45 ± 7 nM and maximal blockade of dopamine inhibition of 60% and 68% for Met-enkephalin and β-endorphin, respectively. Naloxone antagonized the effect of Met-enkephalin with an EC50 of 22 ± 12 nM. Furthermore, this Met-enkephalin effect on dopamine inhibition of prolactin secretion appeared non-competitive since it reduced maximal inhibition without affecting the apparent affinity of dopamine. Finally, it should be noted that the two opiate peptides had no effect on spontaneous prolactin release. In electrophysiological experiments, local ejection of dopamine on tested cells induced an hyperpolarization concomitant with an increase of the membrane conductance. Ejection of Met-enkephalin or β-endorphin alone did not modify the electrical properties of the cells (resting potential, membrane conductance and excitability). In contrast, both peptides blocked in a reversible manner the dopamine-induced electrical responses. These effects were antagonized by naloxone. However, this interaction of opiatepeptides with dopamine electrical response was not observed on all cells tested. We conclude that the blocking effect of opiates on dopamine-induced hyperpolarization may account, at least in part, for the ability of these peptides to interact with dopamine inhibition of prolactin release.     

Long-term depression of a sympathetic ganglionic response to opioids by prolonged synaptic activity and by phorbol esters

We studied the effect of the adenylate cyclase activator forskolin, of protein kinase C-activating phorbol esters and of prolonged preganglionic input activation on the inhibitory response of the perfused superior cervical ganglion of the cat to exogenous met-enkephalin (Met-ENK). Met-ENK inhibited, in a concentration-dependent manner, the postganglionic compound action potential evoked by cervical sympathetic trunk stimulation. The inhibition was reversible, was blocked by naloxone as well as by pertussis toxin and showed no homologous desensitization in the concentration range 0.01–10 μM. Pretreatment of the ganglion with 4β-phorbol 12,13-dibutyrate or 4β-phorbol 12,13-diacetate depressed the Met-ENK response for several hours, while pretreatment with forskolin had no effect. This action of phorbol esters was prevented by the protein kinase inhibitor H-7 but not by the calmodulin antagonist W-7 or the protein kinase A inhibitor HA 1004 and was calcium-dependent. Recovery of the response from the depression produced by phorbol esters was not affected by a protein synthesis inhibitor. A 40 Hz 20 min stimulus train to the cervical sympathetic trunk mimicked the effect of phorbol esters, depressing for several hours the inhibition produced by Met-ENK. Stimulus trains of duration shorter than 5 min or frequency lower than 5 Hz were ineffective. This effect of prolonged preganglionic stimulation occurred even when the stimulus train was delivered during complete block of nicotinic and muscarinic ganglionic transmission but was lost when the stimulus train was delivered during perfusion with calcium-free Krebs. The protein kinase inhibitor H-7 prevented the depression of the Met-ENK response by the train, while W-7 and HA 1004 had no effect. These findings suggest that, in the superior cervical ganglion of the cat, a kinase, activated by phorbol esters and inhibited by H-7, exerts a long-term control of the ganglion cell responsiveness to opiate receptor activation. A similar mechanism can be synaptically activated by a non-cholinergic transmitter, released by the preganglionic axons during prolonged, high frequency, activity.