Action of Thymoxamine on Mydriasis induced by Levodopa and Dopamine

Eye-drops of levodopa and dopamine induce pupillary dilatation which is inhibited by thymoxamine, an alpha-adrenergic blocking drug. This indicates that the mydriatic action of levodopa and dopamine involves excitation of alpha-adrenergic receptors of the dilator pupillae muscle. Such a conclusion is in accord with the previously expressed suggestion that levodopa is rapidly converted to dopamine, which displaces noradrenaline from adrenergic nerve endings.The findings that dopamine exerts alpha-adrenergic effects at the periphery may be construed as evidence in support of the view that the hypotensive action of levodopa is mediated via the central nervous system.     

An Improved Method for the Purification of "Light" Noradrenaline Vesicles from Rat Vas Deferens: Some Biochemical Characteristics

"Light" noradrenaline storage vesicles from nerve endings have been isolated by differential centrifugation and differential gradient centrifugation. They have been further purified by isopycnic sucrose/D2O centrifugation. By using these centrifugation techniques, we obtained an isopycnic gradient fraction in which noradrenaline was enriched about 41 times versus a total homogenate. This factor could be raised to 61 by using seminal ducts of castrated rats. Comparison of the distribution patterns in sucrose/D2O isopycnic gradients indicated that light noradrenaline vesicles of nerve endings contain Mg2+-stimulated ATPase and ATP, but that only a minor part of the dopamine beta-hydroxylase can be associated with these vesicles.     

Effect of prostacyclin on myogenic activity and adrenergic neuroeffector interaction in canine isolated veins.

In isolated strips of canine mesenteric vein prostacyclin (PGI2) causes a dose-dependent depression of the amplitude of the spontaneous rhythmic contractions without influencing their frequency. This suggests that prostacyclin affects the events leading from the depolarization of the smooth muscle cells to their contractions, rather than the induction of the myogenic activity itself. Furthermore, prostacyclin reduces the noradrenaline-induced contraction of the canine saphenous vein without affecting the electrically induced responses, suggesting a possible dual effect of the drug: at the smooth muscle it causes depression of the responsiveness to noradrenaline whereas at the adrenergic nerve endings it enhances the evoked release of the adrenergic transmitter.     

Effect of prostacyclin on myogenic activity and adrenergic neuroeffector interaction in canine isolated veins

In isolated strips of canine mesenteric vein prostacyclin (PGI₂) causes a dose-dependent depression of the amplitude of the spontaneous rhythmic contractions without influencing their frequency. This suggests that prostacyclin affects the events leading from the depolarization of the smoole muscle cells to their contractions, rather than the induction of the myogenic activity itself. Furthermore, prostacyclin reduces the noradrenaline-induced contraction of the canine saphenous vein without affecting the electrically induced responses, suggesting a possible dual effect of the drug: at the smooth muscle it causes depression of the responsiveness to noradrenaline whereas at the adrenergic nerve endings it enhances the evoked release of the adrenergic transmitter.     

The effect of MPTP on the neuronal uptake of monoamines]

The results of kinetic analysis of synaptosomal uptake of dopamine, noradrenaline, adrenaline and serotonin showed the presence of their own carrier systems with high or low affinity for each monoamine. The low affinity system of the uptake of monoamines by nerve endings differs from extraneuronal one by higher affinity. MPTP noncompetitively inhibits the system of highly effective uptake of the studied monoamines by nerve endings, competitively inhibiting synaptosomal uptake with low affinity of noradrenaline, adrenaline and noncompetitively serotonin and dopamine. The constant values of inhibition showed that MPTP most strongly blocks the system of synaptosomal uptake of low affinity serotonin and approximately 2-times weaker affects its system of high affinity. Carrier systems of high affinity of dopamine, adrenaline and noradrenaline block MPTP 150-500 times weaker than that of serotonin, and as for low affinity--in 2000-4000 times. It may be supposed that synaptosomal uptake of low affinity serotonin is most perceptible to the effect of MPTP and is of a particular importance in the development of Parkinson's disease symptoms.     

Functional Significance of the Effects of Neurotransmitters on the Na+,K+-ATPase System

The aim of the present experiments was to study the effects of the neurotransmitters acetylcholine, noradrenaline, 5-hydroxytryptamine, and dopamine on the Na+,K+-ATPase of rat brain synaptosomal fractions. It is shown that dopamine at low concentrations specifically inhibits the Na+,K+-ATPase of synaptic membranes from the brain regions rich in dopaminergic endings, but has no effect on the synaptosomal Na+,K+-ATPase from the other parts of brain. Acetylcholine and noradrenaline have similar specific effects on Na+,K+-ATPase from cholinergic and adrenergic synaptosomes. The Na+,K+-ATPase of synaptic membranes from the different brain regions, characterised by different distributions of cholinergic, adrenergic, and 5-hydroxytryptaminergic endings, show different reactions with neurotransmitters. These data indicate a functional significance of the effects of the neurotransmitters on the synaptosomal Na+,K+-ATPase.     

RESOLUTION OF THREE DISTINCT POPULATIONS OF NERVE ENDINGS FROM RAT BRAIN HOMOGENATES BY ZONAL ISOPYCNIC CENTRIFUGATION

Conditions have been established for the fractionation of subcellular components of rat forebrain homogenates by zonal isopycnic equilibration in continuous sucrose density gradients using a B-XIV rotor. The fractions were analyzed biochemically and by ultra-structural morphometry. Starting from postnuclear supernates of forebrain homogenates, it has been possible to resolve three distinct populations of nerve endings from one another, as well as from free mitochondria and myelin fragments. The three types of nerve endings differ in their apparent specific gravity, their biochemical properties, and their ability selectively to accumulate exogenous transmitter substances in vitro. These three particle populations are likely to represent, in order of increasing modal equilibrium density, (a) cholinergic nerve endings, characterized by their high content of acetylcholine, (b) γ-amino butyric acid (GABA)-containing nerve endings with high glutamate decarboxylase activity and the ability to accumulate exogenous GABA, (c) adrenergic nerve endings that accumulate exogenous dopamine and noradrenaline and exhibit high monoamine oxidase activity.     

The calcium-magnesium interaction in the process of release of noradrenaline by nicotine

The calcium-magnesium (Ca2+-Mg2+) interaction in the process of nicotine-induced release of [3H]noradrenaline ([3H]NA) from rat isolated vas deferens was studied. Increasing extracellular concentrations of Mg2+ caused a dose-dependent depression of release of [3H]NA by nicotine, and this inhibitory effect of Mg2+ was overcome by raising the concentration of CA2+. It is concluded that Mg2+ antagonizes the nicotine-induced increase in the Ca2+ influx into the adrenergic nerve terminals, and that nicotine acts on adrenergic neuronal membrane rather than intraneuronally to cause release of NA.     

Postnatal development of chronotropic responses to chemical and electrical stimulation of adrenergic nerve endings in the sinoatrial node of the heart of the albino rat

The positive chronotropic response to stimulation of adrenergic nerve endings in the sinoatrial node was studied in isolated atria from the hearts of rats of different ages. Dimethylphenylpiperazinium (DMPP) was used for chemical stimulation and transmural stimulation of the sinoatrial node region as electrical stimulation; in both cases noradrenaline is released from the nerve endings. With both stimulation methods, postnatal development was recorded in two phases. In the first phase, positive chronotropic responses are markedly increased and attained the maximum at the age of 14 days on using DMPP and of 24 days on using electrical stimulation. In the second phase, positive chronotropic responses diminish and at the age of about 45 days, with both stimulation methods, they become reduced to adult level. The first developmental phase can be attributed to an increase in the noradrenaline content of the nerve endings and the release of a larger amount of the transmitter during stimulation, together with an increase in the noradrenaline sensitivity of the cells of the sinoatrial node. It is not clear why positive chronotropic responses decrease in the second phase, when the noradrenaline content of the myocardial tissue continues to rise and pacemaker sensitivity to noradrenaline is not reduced.     

EFFECTS OF 6-HYDROXYDOPAMINE ON CATECHOLAMINE CONTAINING NEURONES IN THE RAT BRAIN

After the intraventricular injection of 6-hydroxydopamine (6-OHDA), there was a long lasting reduction in the brain concentrations of noradrenaline (NA) and dopamine (DA). The brain concentration of NA was affected by lower doses of 6-OHDA than were required to deplete DA. A high dose of 6-OHDA which depleted the brain of NA and DA by 81 per cent and 66 per cent respectively, had no significant effect on brain concentrations of 5-hydroxytryptamine (5-HT) or γ-aminobutyric acid (GABA). The fall in catecholamines was accompanied by a long lasting reduction in the activities of tyrosine hydroxylase and DOPA decarboxylase in the hypothalamus and striatum, areas in the brain which are rich in catecholamine containing nerve endings. There was, however, no consistent effect on catechol-O-methyl transferase or monamine oxidase activity in these brain regions. The initial accumulation of [³H]NA into slices of the hypothalamus and striatum was markedly reduced 22–30 days after 6-OHDA treatment. These results are consistent with the evidence in the peripheral sympathetic nervous system that 6-OHDA causes a selective destruction of adrenergic nerve endings and suggest that this compound may have a similar destructive effect on catecholamine neurones in the CNS.     

Localization of putative neurotransmitters in the mantle and siphuncle of the mollusc Nautilus pompilius L. (Cephalopoda)

The neurotransmitter supply in the nerve endings of the mantle and the siphuncle, i.e. in organs that are responsible for the shell formation in the ectocholeate Nautilus pompilius, were investigated with electron microscopical, fluorescence-, immuno- and enzyme histochemical methods as well as with high pressure liquid chromatography (HPLC). Using antibodies against serotonin and the tetrapeptide FMRF-amide, positive reactions were demonstrated immunohistochemically within the terminal nerve fibres of the mantle and the vessels of the siphuncle. Enzyme histochemical proof of the presence of specific acetylcholinesterase yielded positive results in the muscle fibres of the mantle and siphuncle. Additionally, in the mantle, glyoxylic acid-induced fluorescence was shown within the nerve endings indicating catecholamines as neurotransmitters, whereas in the siphuncle such fluorescence did not appear. However, the HPLC-analyses showed that in the mantle and also in the siphuncle the content of dopamine is higher than that of noradrenaline whereas only traces of adrenaline occur in both organs suggesting dopamine as a putative neurotransmitter. Transmission electron microscopical examination of the nerve endings of both organs revealed that different types of vesicles were distinguished that could be considered as cholinergic, aminergic and peptidergic structures.     

The effect of cocaine, reserpine, and 6-hydroxydopamine on the response of the perfused central artery of the rabbit ear to sympathomimetic amines.

The perfused central artery of the rabbit ear was less sensitive to extraluminal than to intraluminal noradrenaline, but the reverse was true for metaraminol, methoxamine, metanephrine, and isoproterenol. No difference was noted between the extraluminal and intraluminal potency of phenylephrine. Cocaine potentiated the effect of extraluminal and intraluminal noradrenaline, but decreased that of intraluminal phenylephrine. Irrespective of the route of administration, the constrictor potencies of the sympathomimetic amines were not affected by cocaine. Arteries of reserpine-treated rabbits were supersensitive to extraluminally and intraluminally applied noradrenaline and phenylephrine, but they were not supersensitive to metaraminol. 6-Hydroxydopamine effectively destroyed adrenergic nerve endings of the central ear artery and increased its responses to both extraluminal and intraluminal noradrenaline and phenylephrine. However, only the constrictor potencies of intraluminally applied metaraminol and methoxamine were enhanced by 6-hydroxydopamine. The apparent discrepancies between the results obtained by various procedures that eliminate or impair the nerve uptake process suggest that the difference in the constrictor potency of extraluminal and intraluminal sympathomimetic amines is probably unrelated to their uptake by nerves located in the adventitio-medial junction of the artery.     

Endogenous Noradrenaline and Dopamine in Nerve Terminals of the Hippocampus: Differences in Levels and Release Kinetics

The presence and release of endogenous catecholamines in rat and guinea pig hippocampal nerve terminals was studied by fluorimetric HPLC analysis. In isolated nerve terminals (synaptosomes) the levels and breakdown of endogenous catecholamines were determined and the release process was characterized with respect to its kinetics and Ca2+ and ATP dependence. Endogenous noradrenaline and dopamine, but not adrenaline, were detected in isolated hippocampal nerve terminals. For dopamine both the levels and the amounts released were more than 100-fold lower than those for noradrenaline. In suspension, released endogenous catecholamines were rapidly broken down. This could effectively be blocked by monoamine oxidase inhibitors, Ca(2+)-free conditions, and glutathione. The release of both noradrenaline and dopamine was highly Ca2+ and ATP dependent. Marked differences were observed in the kinetics of release between the two catecholamines. Noradrenaline showed an initial burst of release within 10 s after K+ depolarization. The release of noradrenaline was terminated after approximately 3 min of K+ depolarization. In contrast, dopamine release was more gradual, without an initial burst and without clear termination of release within 5 min. It is concluded that both catecholamines are present in nerve terminals in the rat hippocampus and that their release from (isolated) nerve terminals is exocytotic. The characteristics of noradrenaline release show several similarities with those of other classical transmitters, whereas dopamine release characteristics resemble those of neuropeptide release in the hippocampus but not those of dopamine release in other brain areas. It is hypothesized that in the hippocampus dopamine is released from large, dense-cored vesicles, probably colocalized with neuropeptides.     

Plasma catecholamines in man are not influenced by the inhibition of prostaglandin synthesis.

Indomethacin has been reported to potentiate the release of noradrenaline from sympathetic nerve endings in vitro and to increase urinary noradrenaline excretion in rats. We have studied the influence of indomethacin on plasma catecholamine levels in 10 normal men, using measurement of plasma renin activity (PRA) as an index of the pharmacodynamic effect of indomethacin. Both in the supine and standing positions indomethacin failed to alter the plasma concentrations of noradrenaline, adrenaline or dopamine, while PRA was markedly suppressed. It is concluded that in the intact human indomethacin does not influence catecholamine concentrations.     

Hypotension Caused by L-Dopa

In 20 patients with idiopathic Parkinsonism maximum tolerated doses of L-dopa were found to induce a mean reduction in blood pressure (erect systolic) of 19·3 mm.Hg, without any significant change in pulse rate. This hypotension may be due to dopamine, acting on adrenergic nerve endings or on the central nervous system itself.     

Activities of prostaglandins and prostaglandin endoperoxides at adrenergic neuroeffector junctions.

The results presented in this paper indicate that: 1. The prostaglandin synthesis inhibitor, indomethacin, increases noradrenaline turnover in a variety of rat organs. This observation increases the probability that prostaglandins are involved in the control of adrenergic neurotransmission in vivo. 2. Administration of endoperoxides inhibits the release of noradrenaline from adrenergic nerve terminals. The effect can be explained, however, at least in part, by formation of degradation products, presumably mainly prostaglandin E2. 3. Prostaglandin F2 alpha enhances smooth muscle responses to adrenergic nerve stimulation in rabbit heart and guinea pig vas deferens. These actions must be considered prostjunctional, since the release of noradrenaline is unchanged or depressed.     

Monoaminergic mechanisms in the nervous system of Lumbricus terrestris (L.)

Summary The localization and intraneuronal distribution of the monoaminergic transmitters in the nervous system of the earthworm, Lumbricus terrestris, have been investigated in detail with the aid of the histochemical fluorescence method of Falck and Hillarp.In the ventral nerve cord, many yellow fluorescent, 5-hydroxytryptamine containing neurons are found, but only few green fluorescent noradrenaline containing cell bodies, which, however, are numerous in the peripheral nervous system. There is an abundance of both fibre types in the neuropile.The 5-hydroxytryptaminergic neurons probably have a motor (possibly inhibitor) function; the adrenergic neurons in the body segments are supposed to have a receptor (exteroceptive and possibly proprioceptive) function.In the cerebral ganglion, both 5-hydroxytryptamine and noradrenaline containing neurons are found in large numbers, and there are closely packed numerous fibres of both types in the neuropile. Their function is more obscure, though an associative function can be presumed for some adrenergic neurons; smaller 5-hydroxytryptaminergic neurons might have a motor (perhaps inhibitor) function.Adrenergic sensory cells are found in the body integument, most frequently in the clitellum segments, in the prostomium, and in the roof of the buccal cavity. These cells give off varicose fibres that form a basi-epithelial network which is in communication with the green fluorescent sensory fascicles in the ventral nerve cord via the epidermal nerves, the ring nerves, and the segmental nerves. No direct adrenergic sensory-effector innervation of either circular and/or longitudinal musculature or gland cells seems to exist. No adrenergic free nerve endings in the body integument have been observed. Instead, there must be a synaptic contact with the motoneurons, either directly in the neuropile or via an interjacent neuron.No synaptic contacts have been observed in the ventral nerve cord between adrenergic or 5-hydroxytryptaminergic fibres and either the giant fibres or fluorescent or nonfluorescent perikarya.An adrenergic innervation of the pharynx musculature has been found, and sensory cells of a different type are present in and below the epithelium; here, a direct senso-motoric innervation of the pharyngeal musculature cannot be excluded. It is established that the adrenergic neurons in the stomatogastric nervous system have an exciting function on the pharynx, whereas a direct monoaminergic influence of the muscular movements of the intestine probably does not exist.Abbreviations Used A adrenaline - CA catecholamine - DA dopamine - 5-HT 5-hydroxytryptamine - MA monoamine - NA noradrenaline The research reported in this document has been sponsored by the Air Force Office of Scientific Research under Grant AF EOAR 67-15 through the European Office of Aerospace Research (OAR), United States Air Force, by the Swedish Natural Science Research Council (99-34, 6627), and by the Swedish Medical Research Council (B67-12X-712-02A).     

Plasma catecholamines in man are not influenced by the inhibition of prostaglandin synthesis

Indomethacin has been reported to potentiate the release of noradrenaline from sympathetic nerve endings $\text{in vitro}$ and to increase urinary noradrenaline excretion in rats. We have studied the influence of indomethacin on plasma catecholamine levels in 10 normal men, using measurement of plasma renin activity (PRA) as an index of the pharmacodynamic effect of indomethacin. Both in the supine and standing positions indomethacin failed to alter the plasma concentrations of noradrenaline, adrenaline or dopamine, while PRA was markedly suppressed. It is concluded that in the intact human indomethacin does not influence catecholamine concentrations.     

The monoamines in molluscs. II. Dopamine and neurotransmission. Cardiac dopaminergic innervation in Helix pomatia (author's transl)]

In the molluscs, dopamine is very probably a chemical transmitter at the level of both the central nervous system and certain peripheral structures. The heart of Helix pomatia does not have any intrinsic innervation, but it receives extrinsic innervation from fibres coming from the visceral nerve. Formaldehyde fluorescence histochemistry localizes the cardiac catecholamines in some of these fibres and in their endings. However, dopamine, which dominates, does not seem to be a transmitter involved in cardioregulation in the same way as 5-hydroxytryptamine. The quantities of active dopamine (stimulants) cannot be compared with those required for a neurotransmitter. This is also true for noradrenaline. Dopamine more certainly plays a role at the metabolic and tropic level by acting within a more or less short period as a regulator of cellular activity and contractility. The Helix heart is a suitable model for future research in this field.     

Chronic losartan treatment decreases angiotensin II-mediated facilitation of noradrenaline release in the caudal artery of spontaneously hypertensive rats

Sympathetic activity is modulated by angiotensin II (AII), both at pre- and postsynaptic level in the rat caudal artery. In the spontaneously hypertensive rat (SHR), this artery receives more dense sympathetic innervation than blood vessels of normotensive strains. This fact seems to be linked to the enhanced pressor responses elicited by noradrenaline in SHR. In this work we describe, in the SHR, the effect of a chronic treatment with the angiotensin II AT1-receptor antagonist, losartan, in modulating noradrenergic mechanisms involved in caudal artery contraction. The effect of losartan is compared to that of captopril, given at doses leading to a similar decrease of both arterial blood pressure and left ventricular hypertrophy. The contractile response of caudal artery rings induced by endogenous noradrenaline released by low frequency transmural nerve stimulation (TNS) has been studied. Under our conditions, TNS (0.5-1 Hz) induced higher contractile responses in SHR treated with losartan than in the control and captopril-treated groups. This difference seems to be due to an increase of the postsynaptic effect of noradrenaline (NA) rather than to an increase of noradrenaline release from sympathetic endings, since i) DE50 value for NA was lower in losartan-treated SHR than in the other groups, and ii) AII induced a dose-dependent increase of TNS-evoked release of radioactivity from caudal artery segments loaded with [3H]-NA, in both control and captopril-treated groups but had no effect in the losartan-treated group. These results show that chronic treatment with losartan, although slightly enhancing the pressor effect of NA at postsynaptic level, fully supresses the facilitatory role of AII on NA release.