Influence of fentanyl,levorphanol and pethidine on the release of noradrenaline from rat brain cortex slices

Summary In slices of rat brain cortex preincubated with (–)-³H-noradrenaline, the influence of fentanyl, levorphanol and pethidine on the efflux of tritium was investigated. The spontaneous outflow of tritium was not changed by low, and was accelerated by high concentrations of the drugs. The overflow of tritium evoked by electrical stimulation at 3 Hz was diminished by 10^–8–10^–7 M fentanyl and by 10^–7–10^–6 M levorphanol, but was augmented by 10^–5 M levorphanol. Naloxone prevented the inhibitory effect of fentanyl and levorphanol. In contrast to fentanyl and levorphanol, pethidine did not decrease, but at concentrations of 10^–6–10^–5 M greatly increased the stimulation-induced overflow of tritium. However, the increase was abolished, and the stimulation-evoked overflow slightly reduced, after the re-uptake of noradrenaline had been blocked by cocaine. It is concluded that fentanyl, levorphanol and pethidine share with morphine the ability to inhibit the release of transmitter from cerebrocortical noradrenaline neurones evoked by nerve impulses.     

Influence of morphine and naloxone on the release of noradrenaline from rat brain cortex slices

Summary In slices of rat brain cortex preincubated with (–)-³H-noradrenaline, the influence of morphine and naloxone on the efflux of tritium was investigated. The spontaneous outflow of tritium was not changed by 10^–7–10^–5 M morphine and by 10^–6–10^–4 M naloxone, but was accelerated by 10^–4 M morphine. Electrical field stimulation augmented tritium outflow. The overflow evoked per ppulse decreased as the frequency of stimulation was increased from 0.3 to 3 Hz, but remained approximately constant when it was further increased to 10 Hz. At frequencies of 0.3, 1, and 3 Hz, but not at 10 Hz, morphine in concentrations of 10^–7–10^–5 M depressed the stimulation-induced overflow of tritium. 10^–4 M morphine did not influence the overflow induced by stimulation at 0.3 and 1 Hz and increased that evoked by stimulation at 10 Hz. Naloxone (10^–6–10^–4 M) did not change the response to stimulation. In the presence of 10^–4 M naloxone, 10^–6 M morphine did not diminish, and 10^–5 M morphine even enhanced the stimulation-induced overflow of tritium. The inhibitory effect of 10^–6 M morphine was not reduced, after tyrosine hydroxylase had been blocked by -methyltyrosine-methylester. It is concluded that morphine through an action on specific opiate receptors inhibits the release of transmitter from cerebrocortical noradrenergic neurones evoked by nerve impulses. By an action unrelated to opiate receptors, morphine at high concentrations increases the stimulation-induced overflow of noradrenaline, presumably by inhibiting its re-uptake into nerve endings.     

Influence of morphine and naloxone on the release of noradrenaline from rat cerebellar cortex slices

Summary In slices of rat cerebellar cortex preincubated with (-)-³H-noradrenaline, the influence of morphine and naloxone on the efflux of tritium was investigated. The spontaneous outflow was not changed by 10^–5 M of either morphine or naloxone. On the other hand, morphine caused a concentration-dependent decrease of the overflow, of tritium evoked by electrical field stimulation. Naloxone did not change the stimulation-induced overflow, but prevented its inhibition by morphine. It is concluded that morphine, through an action on opiate receptors located on cerebellar noradrenergic neurones, inhibits the secretion of the transmitter in response to nerve impulses.     

Alpha-receptor-mediated modulation of transmitter release from central noradrenergic neurones

Summary Slices of rat cerebral cortex were preincubated with 10^–7 M (-)-³H-noradrenaline, and the outflow of tritium was determined. Oxymetazoline, phentolamine and cocaine did not change the spontaneous efflux. The overflow of tritium evoked by electrical field stimulation was decreased by oxymetazoline, and enhanced by phentolamine, phenoxybenzamine, and cocaine. Oxymetazoline did not counteract the increase of the stimulation-induced overflow caused by cocaine, but strongly antagonized the increase caused by phentolamine and phenoxybenzamine. When the stimulation-induced overflow was large under control conditions (high frequency of stimulation, addition of cocaine), the inhibitory effect of oxymetazoline was diminished. The results indicate that an -receptor-mediated feed-back control of noradrenaline release, previously demonstrated in postganglionic sympathetic nerves, also operates in central noradrenergic neurones.     

Indirect adrenergic effect of histamine in cat cerebral arteries

Summary Histamine (10(^–4 M) induced an increase in the tritium outflow from cat cerebral arteries preloaded with ³H-noradrenaline. Pretreatment with reserpine (3 mg/kg, i.p., total dose) or removal of both superior cervical ganglia two weeks before the experiment abolished that increase. The presence of cocaine or diphenhydramine also prevented the rise in tritium efflux induced by histamine.Histamine (10(^–8 M to 10(^–3 M) elicited dose-dependent contractions in the isolated posterior communicating artery of the cat which were reduced in the presence of diphenhydramine at all doses except the highest three. The addition of phentolamine to the bath decreased the contractile responses at the doses lower than 10(^–6 M. Pretreatment with reserpine or removal of both superior cervical ganglia also diminished the responses at doses of histamine below 10(^–6 M and 10(^–5 M, respectively. When cocaine was added to the bath there was a decrease in the contraction elicited at all doses except the last one.These results suggest the existence of an indirect adrenergic mechanism in the contractile response to histamine in cat cerebral arteries.     

Involvement of alpha-receptors in clonidine-induced inhibition of transmitter release from central monoamine neurones

Slices of rat cerebral cortex preincubated with (?)-³H-noradrenaline or ³H-5-hydroxytryptamine were stimulated by an electrical field, and the stimulation-induced overflow of tritium was determined. (1) Clonidine diminished the stimulation-evoked tritium overflow from slices preincubated with ³H-noradrenaline. The degree of this inhibition was greater at a low than at a high frequency of stimulation. (2) A high concentration of clonidine (10^?5 M) did not antagonize the increase of the stimulation-induced overflow caused by 10^?6 M or 10^?5 M cocaine, but abolished the increase caused by 10^?7 M of phentolamine or phenoxybenzamine. In the presence of cocaine, the inhibitory effect of clonidine was reduced. (3) 10^?5 M clonidine diminished the stimulation-evoked overflow of tritium from slices preincubated with ³H-5-hydroxytryptamine. (4) It is concluded that clonidine decreases, and phentolamine and phenoxybenzamine increase, the stimulation-induced release of noradrenaline from cerebral neurones by an activation and a blockade of α-receptors, respectively. A variety of secretory cells (secreting catecholamines, acetylcholine, 5-hydroxytryptamine, insulin or renin) seem to be endowed with structures similar to α-adrenergic receptors, which can modulate the secretion process.     

Effect of extracellular dopamine on the release of dopamine in the rabbit caudate nucleus: Evidence for a dopaminergic feedback inhibition

Summary Slices of the head of the rabbit caudate nucleus were preincubated with 10^–7 M ³H-dopamine and then superfused, and the effect of unlabeled dopamine on the outflow of tritium was investigated. In most experiments, nomifensine was added throughout superfusion in order to block uptake of the unlabeled amine. Nomifensine was a potent inhibitor of the uptake of ³H-dopamine into rabbit caudate synaptosomes, with an IC₅₀ of 5·10^–8 M at a ³H-dopamine concentration of 4·10^–8 M.In the absence of nomifensine, unlabeled dopamine (10^–7 M and higher concentrations) accelerated the basal outflow of tritium from preincubated slices. 10^–5 M nomifensine strongly counteracted the acceleration. In the presence of nomifensine, unlabeled dopamine (10^–7 to 10^–6 M) caused a concentrationdependent decrease of the overflow of tritium evoked by electrical stimulation at 0.1 Hz. Chlorpromazine and haloperidol (in the presence of nomifensine) increased the stimulation evoked overflow and antagonized the inhibitory effect of dopamine.It is concluded that extracellular dopamine shares with other dopaminergic agonists the ability to inhibit action potential-evoked release of intraneuronal dopamine. The inhibition is mediated by specific receptors. The results support the hypothesis that previously released dopamine, by an action on these receptors, can inhibit further release of dopamine.     

Cardiac electrophysiology of four neuroleptics: Melperone,haloperidol, thioridazine and chlorpromazine

Summary Effects of dopamine receptor agonists and antagonists on the release of dopamine were studied in the caudate nucleus of the rabbit. The nucleus contained 6.7 g/g of dopamine, but negligible levels of noradrenaline and dopamine--hydroxylase. No formation of ³H-noradrenaline was detected in caudate slices preincubated with ³H-dopamine, and more than 95% of the tritium content of the tissue consisted of ³H-dopamine.When caudate slices were preincubated with ³H-dopamine and then superfused with amine-free medium, there was a basal outflow of tritium that was not or only slightly changed by tetrodotoxin (10^–7 and 10^–6 M), apomorphine (up to 10^–5 M), bromocriptine (up to 10^–6 M), chlorpromazine (up to 10^–6 M), haloperidol (up to 10^–7 M), or omission of calcium. Electrical stimulation (3 Hz, 24 mA, 2 ms pulse duration, 2-min periods) greatly increased the outflow of tritium. The stimulation-evoked overflow was abolished by tetrodotoxin (10^–7 and 10^–6 M) and in calcium-free medium. Apomorphine (10^–8–10^–5 M) and bromocriptine (10^–8–10^–6 M) reduced, whereas chlorpromazine (10^–7 and 10^–6 M) and haloperidol (10^–8 and 10^–7 M) enhanced the evoked overflow. The inhibitory effect of apomorphine and bromocriptine was antagonized by chlorpromazine and haloperidol, but not by phentolamine.Silicone tubings that had been in contact with ³H-haloperidol retained tritiated material that was slowly eluted during perfusion with water or physiological salt solution. The material was identified as ³H-haloperidol. When silicone tubings pretreated with unlabelled haloperidol were used in subsequent dopamine release experiments, the inhibitory effect of apomorphine was not reproduced.It is concluded that, in the caudate nucleus of the rabbit, apomorphine and bromocriptine depress, whereas chlorpromazine and haloperidol facilitate action potential-evoked release of dopamine. The effects are mediated by specific receptors which may be located on the dopaminergic nerve terminals. The receptors appear to be normally activated by released dopamine itself, which thus inhibits its own further release. Part of the discrepancies in the literature concerning dopaminergic modulation of dopamine release may be due to retention of neuroleptic drugs in superfusion assemblies, followed by slow elution and interference with subsequent experiments.     

Metabolism of 3H-noradrenaline released from isolated rat hypothalamus by extracts of black widow spider glands

Summary In the isolated rat hypothalamus preincubated with ³H-noradrenaline, an extract from the venom glands of Latrodectus mactans elicited an overflow of tritium which consisted of 70–71% ³H-noradrenaline and 18% ³H-3,4-dihydroxyphenylglycol. Exposure to cocaine or pargyline diminished the formation of ³H-3,4-dihydroxyphenylglycol, suggesting that the glycol is formed presynaptically after neuronal reuptake of the transmitter released by the venom.Our data suggest that the venom depolarizes the noradrenergic neurones of rat hypothalamus, and that the depolarization leads to calcium-dependent release of ³H-noradrenaline.This work was supported by Grant, 5727/E/78 and 6638 from CONICET, ArgentinaSenior Investigator of the CONICET, Argentina. Deceased June 1979     

The extent of neuronal re-uptake of 3H-noradrenaline in isolated vasa deferentia and atria of the rat

Summary After pretreatment of rats with reserpine and pargyline (to inhibit vesicular uptake and monoamine oxidase, respectively) and after inhibition of catechol-O-methyl transferase (by U-0521) and in calcium-free solution, the adrenergic neurones of isolated vasa deferentia and atria were loaded with ³H-noradrenaline. The spontaneous efflux of ³H-noradrenaline and 3H-dihydroxyphenylglycol was determined, as well as the steady-state effect of two concentrations of desipramine.On the basis of a mathematical model of the adrenergic nerve ending, fractional rates (FR = rate of flux divided by tissue tritium content) were calculated for unidirectional outward diffusion, for outward transport and for neuronal re-uptake (all for 3H-noradrenaline). Although the density of adrenergic innervation is lower in atria than in vasa deferentia, neuronal re-uptake amounted to about 90% of the spontaneous efflux of ³H-noradrenaline in both tissues.While the FR for unidirectional outward diffusion was virtually the same in both tissues, the FR for outward transport of ³H-noradrenaline was more than three times higher in atria than in vasa deferentia. There is, as yet, no explanation for this pronounced difference.Abbreviations COMT catechol-O-methyl transferase - DOMA dihydroxymandelic acid - DOPEG dihydroxyphenylglycol - FR fractional rate (= rate of flux divided by simultaneously determined tritium content of tissue) - FRL fractional rate of loss (= rate of net efflux divided by simultaneously determined tritium content of tissue) - MAO monoamine oxidase     

Metabolism of endogenous and exogenous noradrenaline in guinea-pig atria

Summary The outflow of noradrenaline, 3,4-dihydroxyphenylglycol (DOPEG) and 3,4-dihydroxymandelic acid (DOMA) from guinea-pig isolated atria was studied by chromatography on alumina followed by high pressure liquid chromatography with electrochemical detection. In the absence of drugs, the outflow of endogenous noradrenaline over a period of 3 h averaged 1.6 pmol×g^–1×min^–1 and the outflow of DOPEG 17 pmol×g^–1×min^–1. The outflow of DOMA was below the detection limit (<0.31 pmol×g^–1×min^–1). Tyramine greatly increased the outflow of noradrenaline and DOPEG, and the reserpine-like compound Ro 4-1284 selectively increased the outflow of DOPEG; DOMA remained below the detection limit. When atria were exposed to (–)-noradrenaline 1.7 or 17 M, the subsequent outflow of noradrenaline and DOPEG was enhanced. Moreover, substantial amounts of DOMA were now found. This outflow of DOMA was prevented when atria were exposed to (–)-noradrenaline in the presence of cocaine or after an initial incubation with amezinium. Exposure to (–)-noradrenaline 1.7 M mainly enhanced the formation of DOPGE, while exposure to (+)-noradrenaline 1.7 M mainly enhanced the formation of DOMA.Our experiments confirm some and qualify other conclusions drawn from studies in which exogenous ³H-noradrenaline had been used to examine the metabolism of noradrenaline in guinea-pig atria. In agreement with the isotope studies, DOPEG is a major metabolite of endogenous noradrenaline. In contrast to what the isotope studies had suggested, however, endogenous DOMA is a very minor product, at least as long as the neurones are at rest. DOMA is only formed when the tissue is exposed to high concentrations of exogenous noradrenaline. In further contrast to previous conclusions, DOMA is then formed intra- and not extraneuronally.     

Patch clamp studies on the kinetics and selectivity of N-methyl-d-aspartate receptor antagonism by memantine (1-amino-3,5-dimethyladamantan)

Memantine (1-amino-3,5-dimethyladamantan) was tested as an antagonist of N-methyl-d-aspartate (NMDA) receptors on cultured superior collicular and hippocampal neurones using the patch clamp technique and its actions were compared to those of Mg²⁺ ions, ketamine, dextrorphan, dextromethorphan, phencyclidine and dizocilpine (MK-801). Memantine (2–33 μM) concentration-dependently antagonized responses to NMDA 100 μM with an IC₅₀ of 2.92 ± 0.05 μM. In contrast, current responses to (S)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (l-AMPA 50–100 μM) and γ-amino butyric acid (GABA 10 μM) were unaffected by Memantine 8 μM. Memantine 8 μM caused a non-parallel shift of the NMDA concentration-response curve to the right in a manner indicative of uncompetitive open channel block. The effects of memantine were similar to ketamine in that both antagonists were weakly use- and strongly voltage-dependent. In contrast, MK-801, phencyclidine and dextrorphan showed much slower kinetics that was reflected in their marked use- and weaker voltage-dependency. The antagonistic effects of memantine were not reversed by increasing concentrations of glycine (0.1–100 μM) ruling out the possibility of an interaction of memantine with the strychnine-insensitive glycine modulatory site associated with the NMDA receptor-channel complex. Memantine (1–100 μM) also selectively antagonized responses to NMDA (40 μM) in the cortical wedge preparation with IC₅₀ of 12.9 ± 1.5 μM.     

Inhibition of 3H-noradrenaline accumulation by dopexamine hydrochloride in the isolated aorta of the rabbit

Summary The aim of the present work was to study the ability of dopexamine hydrochloride to interfere with the neuronal and extraneuronal uptake mechanisms by investigating the effect of dopexamine hydrochloride on ³H-noradrenaline accumulation by rabbit isolated aorta. Dopexamine hydrochloride (3 × 10^–9 – 10^–5 mol/l) reduced the accumulation of tritium by aorta incubated with ³H-noradrenaline (10^–8 mol/l). The effect of dopexamine was compared to cocaine, dopamine, dobutamine, ADTN [(+-)-2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene], ouabain and isoprenaline. Dopexamine hydrochloride (3 × 10^–9 – 10^–7 mol/l) caused the same degree of inhibition irrespective of whether corticosterone (4 × 10^–5 mol/l) was present or not. The order of inhibitory potency was: desipramine > dopexamine hydrochloride > dopamine > ADTN cocaine > dobutamine > ouabain > isoprenaline. In the presence of desipramine (10^–6 mol/l), corticosterone (10^–6 – 10^–4 mol/l), but not dopexamine hydrochloride (10^–6 – 10^–4 mol/l), reduced the ³H-accumulation. It is concluded that dopexamine hydrochloride is a potent inhibitor of uptake-1 in rabbit isolated aorta. Dopexamine hydrochloride has no affinity for the uptake-2 mechanism in this tissue. Send offprint requests to O. A. Nedergaard at the above address     

Pharmacological characterization of the inhibitory effects of neurotensin on the rabbit ileum myenteric plexus preparation.

1 [³H]-amezinium is taken up selectively into noradrenergic axons and their transmitter-storing vesicles and is released from these axons by action potentials. We used it as a non-α-adrenergic marker in order to study the α-adrenergic autoinhibition of noradrenaline release.

2 Rat occipitocortical slices were preincubated with [³H]-amezinium 0.03 μM and then superfused and stimulated electrically (3 Hz for 3 min). The stimulation-evoked overflow of tritium was measured in six groups of slices: from saline-pretreated rats; from saline-pretreated rats, the slices being exposed to exogenous noradrenaline before preincubation with [³H]-amezinium; from saline-treated rats, slices from which were exposed simultaneously to noradrenaline and cocaine before preincubation with [³H]-amezinium; from rats in which noradrenaline stores had been depleted by pretreatment with α-methyltyrosine (α-MT); from α-MT-treated rats, the slices being exposed to noradrenaline before preincubation with [³H]-amezinium; and from α-MT-treated rats, slices from which were exposed to noradrenaline plus cocaine before preincubation with [³H]-amezinium.

3 The stimulation-evoked overflow of tritium, expressed as a percentage of the tritium content of the tissue, was 1.15% in slices from saline-pretreated rats, and was similar in slices from saline-pretreated rats after exposure to noradrenaline or noradrenaline plus cocaine. It was 2.56% in slices from α-MT-treated rats, 1.20% from α-MT-treated rats after exposure to noradrenaline, and 2.88% from α-MT-treated rats after exposure to noradrenaline plus cocaine.

4 Yohimbine 0.1 and 1 μM increased the stimulation-evoked overflow of tritium in slices from all groups of saline-pretreated rats and in those slices from α-MT rats that had been in contact with exogenous noradrenaline. Yohimbine did not change the evoked overflow in slices from α-MT rats that had not been exposed to noradrenaline, or had been exposed to noradrenaline plus cocaine.

5 Clonidine 0.01-1 μM decreased the stimulation-evoked overflow of tritium moderately in slices from saline-pretreated rats, markedly in slices from α-MT-treated rats, and moderately again when the latter slices had been exposed to noradrenaline.

6 It is concluded that the action potential-evoked release of [³H]-amezinium as well as the modulation of this release by yohimbine and clonidine depend on the presence or absence of α-adrenergic autoinhibition caused by the co-secretion of noradrenaline. When there is co-secretion of noradrenaline, the evoked release of [³H]-amezinium is relatively small, yohimbine increases the release, and clonidine can cause only moderate inhibition. When there is no or very little co-secretion of noradrenaline, the evoked release of [³H]-amezinium is at least doubled, yohimbine causes no further increase and clonidine produces strong inhibition.

     

Effect of phentolamine on noradrenaline uptake and release

Summary The influence of phentolamine on the uptake of exogenous noradrenaline infused into the aortic cannula and on the overflow of endogenous noradrenaline caused by sympathetic nerve stimulation was investigated in the isolated perfused rabbit heart. 10^–6 M phentolamine doubled the overflow of endogenous noradrenaline, but did not change noradrenaline uptake. 10^–5 M phentolamine increased the stimulation-induced overflow of noradrenaline 4-fold and inhibited amine uptake by about 50%. 10^–4 M phentolamine elevated the overflow of noradrenaline less than 10^–5 and 3×10^–5 M did. The augmentation of transmitter overflow was only partly reversed by 13 min perfusion with drug-free medium.Pretreatment of hearts with 1.5×10^–5 M cocaine or with 10^–7 or 10^–6 M desipramine did not change the effect of phentolamine on the overflow of noradrenaline evoked by nerve impulses. Pretreatment of hearts with 10^–5 M, but not with 10^–6 M, phentolamine prevented the increase of transmitter overflow by cocaine.It is concluded that low concentrations of phentolamine potentiate the overflow of noradrenaline during nerve stimulation by a mechanism different from that of cocaine, i.e. different from inhibition of neuronal re-uptake. The nature of this mechanism is discussed.This work was supported by the Deutsche Forsehungsgemeinschaft. We have the pleasure to thank Mrs. Ch. Arts, Miss B. Piel and Mr. E. Hagelskamp for skilful technical assistance.     

Presynaptic serotonin receptors regulate the release of 3H-serotonin in hypothalamic slices of the rabbit

Summary Hypothalamic slices of the rabbit brain were incubated with 10^–7 M of ³H-serotonin (³H-5HT). After the incubation and an initial washout period, a nearly constant basal efflux of tritium was detected. This basal efflux was not significantly altered by Ca²⁺-free solution or by the 5HT-antagonist metitepin (10^–5 M), but was augmented by chlorimipramine (10^–5 M) and by unlabelled 5HT (10^–6 M); the acceleration caused by unlabelled 5HT was absent in presence of chlorimipramine (10^–5 M). Both electrical stimulation (4 Hz, 50 mA, 2 min) and high K⁺ (50 mM) induced an overflow of ³H. This overflow was nearly abolished in Ca²⁺-free solution. In presence of chlorimipramine (10^–5 M) both the tritium overflow evoked by electrical stimulation and that evoked by high K⁺ were augmented by metitepin (10^–5 M) and decreased in a concentration dependent manner by unlabelled serotonin (10^–8–10^–6 M); the latter effect was antagonized by metitepin (10^–6 M and 10^–5 M). These experiments suggest that in rabbit hypothalamic slices, the release of ³H-5HT is controlled by a negative feedback mechanism acting via presynaptic serotonin receptors.     

Cocaine-like effect of ketamine on vascular adrenergic neurones

The influence of ketamine on adrenergic and cholinergic responses was studied in the rabbit pulmonary artery in vitro. Ketamine and cocaine potentiated smooth muscle contractions elicited by nonadrenaline, adrenaline and transmural electrical stimulation of adrenergic neurone terminals. Arterial contractions elicited by tyramine were enhanced by ketamine, while cocaine blocked them. Ketamine moved the concentration-response curve of carbachol slightly to the right. Nicotine-induced contractions were blocked by ketamine and cocaine. The adrenergic neurone blocking effect of bretylium and dimethylphenylpiperazinium on the response to transmural stimulation was prevented by ketamine. The uptake of ³H-noradrenaline by rabbit isolated aorta was inhibited by ketamine and cocaine. It is concluded that ketamine inhibits the membrane amine pump in the terminals of postganglionic adrenergic neurones in a cocaine-like manner.     

Noradrenaline release from slices of the thalamus of normal and morphine-dependent rats

Summary The effect of morphine on potassium-induced stimulation of (³H)-noradrenaline release from slices of the rat thalamus was investigated. The in vitro addition of morphine (10^–6 M) significantly depressed potassium-induced tritium overflow by 42% and this was prevented by the prior addition of naloxone (3×10^–6 M) to the medium. The stimulation-evoked overflow of tritium from slices of the thalamus of morphine-dependent rats was not significantly different from normal controls. Addition of naloxone (10^–5 M) 10 min before exposure of the tissues to 20 mM K⁺ significantly enhanced noradrenaline release from dependent slices. The results suggest that the basic release mechanism may have adapted to the continuous presynaptic inhibition of release by morphine.     

Effects of flow-stop on the metabolism of noradrenaline released by nerve stimulation in the perfused spleen

Summary The metabolic pathway of ³H-noradrenaline released spontaneously and by nerve stimulation was studied in the isolated perfused spleen of the cat. The deaminated glycol, DOPEG, (3,4 dihydroxyphenylglycol) was the main metabolite in spontaneous outflow, accounting for 62.5±1.6% of the total radioactivity (n=13). Of the total increase in radioactive products elicited by nerve stimulation at 5 Hz or 10 Hz around 30% was accounted for by the noradrenaline metabolites, particularly DOPEG and the O-methylated fraction. In the presence of 2.9×10^–6 M of cocaine the total overflow of radioactivity induced by stimulation was unchanged but DOPEG formation from released noradrenaline was abolished. These findings indicate that DOPEG formation results from the recapture of the released transmitter by adrenergic nerve endings and subsequent intraneuronal deamination. The total overflow of noradrenaline was reduced by flow-stop while the metabolism of the released transmitter was increased significantly. Cocaine, 2.9×10^–6 M, prevented the increase in DOPEG when stimulation was applied under flow-stop conditions. The decrease in noradrenaline overflow induced by flow-stop is partly due to the increase in the metabolism of the released transmitter.     

β-Endorphin-(1-27) is a naturally occurring antagonist of the reinforcing effects of opioids

Summary The influence of inhibitors of metabolism and uptake of noradrenaline on the ³H-noradrenaline removal from the perfusion fluid by the isolated rat liver was studied. Livers were perfused with 3 nmol/l ³H-noradrenaline and ³H-noradrenaline and ³H-metabolites were determined in effluent, liver and bile. After the perfusion with 14,900 ±920 dpm · g^–1 · min^–1 during 90 min, cumulative removal of tritium was 323,574 ± 63,103 dpm/g. ³H-metabolites recovered from the liver after 90 min perfusion represented 71.1 ± 9.0% of total metabolite formation. Only the OMDA-fraction appeared in the perfusate; its approach to steady state of efflux was slow. The inhibition either of MAO or COMT changed neither the total removal of tritium nor the ³H-metabolites recovered from the liver. Cocaine (10 mol/l) reduced the accumulation of ³H-noradrenaline in the liver. The uptake₂ inhibitor corticosterone (30 mol/l) diminished total removal of tritium and the ³H-metabolites recovered from the liver without changing the accumulation of ³H-noradrenaline. The hypothesis of two different compartments, one responsible for the metabolism and the other for the accumulation of the amine is discussed.Abbreviations NA noradrenaline - NMN normetanephrine - OMDA O-methylated deaminated metabolites - MAO monoamine oxidase - COMT catechol-O-methyl transferase Send offprint requests to M. C. Rubio