Properties of monoamine oxidases in sympathetic nerve and pineal gland

Generation of the melatonin endocrine message in mammals: a review of the complex regulation of melatonin synthesis by norepinephrine, peptides, and other pineal transmitters

Melatonin, the major hormone produced by the pineal gland, displays characteristic daily and seasonal patterns of secretion. These robust and predictable rhythms in circulating melatonin are strong synchronizers for the expression of numerous physiological processes in photoperiodic species. In mammals, the nighttime production of melatonin is mainly driven by the circadian clock, situated in the suprachiasmatic nucleus of the hypothalamus, which controls the release of norepinephrine from the dense pineal sympathetic afferents. The pivotal role of norepinephrine in the nocturnal stimulation of melatonin synthesis has been extensively dissected at the cellular and molecular levels. Besides the noradrenergic input, the presence of numerous other transmitters originating from various sources has been reported in the pineal gland. Many of these are neuropeptides and appear to contribute to the regulation of melatonin synthesis by modulating the effects of norepinephrine on pineal biochemistry. The aim of this review is firstly to update our knowledge of the cellular and molecular events underlying the noradrenergic control of melatonin synthesis; and secondly to gather together early and recent data on the effects of the nonadrenergic transmitters on modulation of melatonin synthesis. This information reveals the variety of inputs that can be integrated by the pineal gland; what elements are crucial to deliver the very precise timing information to the organism. This also clarifies the role of these various inputs in the seasonal variation of melatonin synthesis and their subsequent physiological function.

Evidence for a regulatory role of melatonin on serotonin release and uptake in the pineal gland

Melatonin has been proposed to exert some regulatory actions within the pineal gland itself. The present study examined the effect of melatonin on the release of serotonin (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) from rat pineal glands by using an in vitro perifusion system. Melatonin induced a concentration-dependent stimulatory effect on 5-HT secretion from 10(-6) M to 10(-3) M. Maximal effects were obtained with melatonin 10(-3) M and concentrations lower than 10(-6) M were without effect. The secretion of 5-HIAA was inhibited by melatonin 10(-3) and 10(-4) M, but it was increased when pineals were incubated with 10(-5) and 10(-6) M of melatonin. The indoleamine secretion was also studied on peripherally denervated rat pineal glands. Basal output of 5-HT from these glands was increased when compared with those from control rats. In contrast, the secretion of 5-HIAA was strongly reduced after removal of the sympathetic input to the pineal gland. Melatonin 10(-3) M failed to stimulate 5-HT release from denervated pineal glands, although it inhibited 5-HIAA secretion. In contrast, melatonin 10(-5) M enhanced 5-HT release without altering 5-HIAA output. Fluoxetine, a 5-HT uptake inhibitor, produced similar effects than mM concentrations of melatonin on the indoleamine secretion from control pineal glands, but it had no effect on glands taken from peripherally denervated rats. These data suggest that mM concentrations of the pineal hormone are able to stimulate 5-HT release from the pinealocyte, while mM concentrations of melatonin increase extracellular 5-HT by inhibiting its reuptake in the adrenergic nerve endings. These findings are discussed in relation to the possible role of melatonin regulating the intra- and extracellular availability of 5-HT in the pineal gland and its significance as an autocrine factor.

Identification of serotonin 5HT2 receptors in bovine pineal gland

The concentration of serotonin in the pineal gland is extremely high, which prompted speculation that in addition to serving as a precursor of melatonin, serotonin may have an independent function of its own. By using [3H]-spiperone as a ligand, and ketanserine as a selective serotonin 5HT2 receptor antagonist, we have identified 5HT2 receptor in the bovine pineal gland, revealing a single population of binding sites with a dissociation equilibrium constant (Kd) value of 1.26 +/- 0.41 nM and a receptor density (Bmax) value of 193 +/- 38.85 fmol/mg protein. In displacement experiments, the concentrations of the drugs required to inhibit 50% of the specific binding of [3H]-spiperone in descending order of potency were methysergide greater than ritanserin greater than pirenperone greater than pipamperone greater than ketanserin greater than cyproheptadine greater than M-trifluoromethylphenyl-piperazine greater than prazosin greater than 5-methoxy-N-N-dimethyltryptamine hydrogen oxalate greater than 1-(3-chlorophenol) piperazine greater than serotonin. In the rat pineal gland, [3H]-spiperone revealed a low affinity serotonin binding site with a Kd value of 25.77 +/- 10.7 nM and a Bmax value of 1244 +/- 472 fmol/mg protein. The results of these studies are interpreted to indicate that the bovine pineal gland possess serotonin 5HT2 receptor. However, the rat pineal gland possess a serotoninergic binding site of unknown nature.

Histochemical detection of monoamine oxidase and alcohol dehydrogenase activities in the Syrian hamster Harderian glands: existence of a sexual dimorphism

Monoamine oxidase (MAO) and alcohol dehydrogenase (AD) activities were studied histochemically in the Syrian hamster Harderian gland using tryptamine as substrate and Nitroblue Tetrazolium as the final electron acceptor. No dark: light-related changes were observed. Male type I secretory cells showed an intense MAO reaction. Female type I cells exhibited a moderate MAO activity. Both male and female glands showed a moderate/intense AD-positive reaction. Male type II cells were lacking MAO and AD activities. MAO activity found in the hamster Harderian glands corresponded mainly to MAO type A since treatment with chlorgyline (0.01, 0.1 and 0.5 mM) totally inhibited it. The possible role of these two enzymes in Harderian gland indolalkylamine metabolism is discussed.

Predominant cytosolic distribution of serotonin in rat pineal gland in contrast to biogenic monoamine localization in midbrain and adrenal gland

The subcellular distribution of serotonin and norepinephrine in the rat pineal gland was studied by tissue fractionation and compared with that of biogenic monoamines in the adrenal gland and midbrain. Homogenized tissues were fractionated by ultracentrifugation or by filtration through cellulose ester membranes. Most of the epinephrine (70-80%) and norepinephrine (62-82%) present in the adrenal glands was detected in the particulate fraction. The same distribution was found for serotonin (68.5%) and norepinephrine (59%) in the midbrain and for norepinephrine (62.5%) in the pineal gland. However, most of the serotonin in the pineal was found in the soluble fraction (89.5-98%). This suggests that the great majority of serotonin in the rat pinealocytes is cytosolic and thus is not stored in subcellular vesicles, in contrast to the biogenic monoamines in the midbrain or adrenal gland.

Pineal indoles: significance and measurement

Despite intensive investigation, particularly over the past fifteen years, many aspects of pineal function with respect to mammalian physiology remain obscure. Much of this work is reviewed and particular attention focussed on indole metabolism within the pineal gland. Emphasis is placed on the development of new analytical techniques with special reference to high performance liquid chromatography coupled with electrochemical detection. The growth in knowledge regarding pineal indole synthesis which can be attributed to the use of this technique is discussed. The possibility that pineal indoles other than melatonin may function as hormones or neuromodulators is considered. A functional role for 5-hydroxytryptophol as a neuromodulator, possibly associated with diffuse neuroendocrine function (amine precursor, uptake and decarboxylation, APUD) is suggested.

Does maximal serotonin N-acetyltransferase activity necessarily reflect maximal melatonin production in the rat pineal gland?

p-Chlorophenylalanine (PCPA) treatment reduced pineal 5-hydroxytryptamine (5-HT) levels by 68.9% of non-PCPA treated levels in intact rat pineal glands and by 95.9% of non-PCPA treated levels in adrenergically denervated rat pineal glands. Although isoproterenol stimulation resulted in increased activity of serotonin N-acetyltransferase (NAT) activity in all experimental groups, pineal melatonin production was maximal only in non-PCPA treated rats, suggesting that 5-HT concentrations may exert considerable influence on the level of melatonin production in the rat pineal gland, independent of the level of NAT activity.

Monoamine oxidase type A: differences in selectivity towards l-norepinephrine compared to serotonin

l-Norepinephrine and serotonin have been regarded as preferential substrates for monoamine oxidase (MAO) type A. A close comparative examination of a number of tissues from different species, however, indicated the following differences. Serotonin was a more selective substrate for MAO-A, being inhibited by low concentrations (less than 10(-7) M) of the irreversible MAO-A inhibitor, clorgyline, more consistently and to a greater extent (80-100%) than was l-norepinephrine (30-85%). These serotonin-norepinephrine differences were greater in humans and other primates than in rodents. Serotonin also had a 2- to 4-fold smaller apparent Km for MAO-A than l-norepinephrine and was deaminated 2- to 5-fold more readily by MAO in vitro in most tissues. In contrast, the MAO-B in human platelets deaminated l-norepinephrine more readily than serotonin. Thus, l-norepinephrine, like dopamine, should be regarded as a substrate for both MAO-A and MAO-B in vitro. The prominent role of MAO-B in norepinephrine degradation in primates may need to be considered in interpreting laboratory and clinical studies of clorgyline and other selective MAO-inhibiting drugs.

Regulation of rat pineal melatonin synthesis: effect of monoamine oxidase inhibition

Inhibition of pineal monoamine oxidase (MAO) activity either by harmine or pargyline in adult male Sprague-Dawley rats housed in a 12 : 12 LD cycle resulted in increase pineal N-acetyltransferase (NAT) activity. Pineal MAO inhibition also increased pineal melatonin content, presumably as a result of the increased NAT activity. Conjunct treatment with propranolol, a beta-adrenergic receptor antagonist, nullified these effects, regardless of the MAO inhibitor (harmine, pargyline or both) used or the inhibitor dose given. MAO inhibition during continuous light resulted in increased NAT activity greater than that observed following MAO inhibition during a 12 : 12 LD cycle. On the other hand, the increase in melatonin content following MAO inhibition during continuous light was not significantly different from that following MAO inhibition during a 12 : 12 LD cycle. Conjunct propranolol administration negated the effects of MAO inhibition on both the level of NAT activity and melatonin content, regardless of the lighting conditions. The level of pineal NAT activity is apparently regulated by the level of pineal beta-adrenergic receptor stimulation. While melatonin production appears to be dependent on increases in NAT activity, biosynthesis of this methoxyindole may also be regulated, in part, by other factors or processes in metabolic pathway.

Direct effect of phenylethylamine upon isolated rat aortic strip

Phenylethylamine (PEA) has been implicated in a number of central and peripheral nervous system disorders. Its possible mechanisms of action include stimulation via catecholamine release and direct stimulation by PEA. We have examined the effects of PEA on isolated vascular smooth muscle (VSM) to further explore the mechanism by which PEA produces contraction in this tissue. Helical strips of rat aorta were suspended in a muscle bath. Smooth muscle contractions were recorded via force transducer. PEA elicited a concentration dependent contraction from these strips with a threshold near 10(-6) M and a maximum response at 5 X 10(-3) M. Pretreatment of rats with reserpine dramatically reduced the norepinephrine (NE) content of kidney, heart and spleen of these animals but did not prevent the action of PEA on VSM. The presence of phentolamine (10(-4) M) completely blocked the strip response to PEA. The presence of propranolol (10(-7) or 10(-4) M) altered but did not block the VSM response to PEA. These results argue that the effects of PEA upon the aortic strip preparation involve a direct action of this amine upon VSM.

Effect of phospholipid depletion by phospholipases on the properties and formation of the multiple monoamine oxidase forms in the rat liver

The effect of phospholipid depletion by phospholipases on the properties and formation of monoamine oxidase A and B been investigated. The enzyme was solubilized, partially purified, treated with phospholipases and subjected to get filtration to reduce the amount of enzyme-associated phospholipids. Phospholipase A treatment of the purified monoamine oxidase fraction had no effect on the deprenil inhibition pattern or the observed transition temperatures in the Arrhenius plots. However, the rate of enzyme inactivation by heat and trypsin were greatly increased but differences in rates of inactivation of monoamine oxidase A and B were still observed. Phospholipase C treatment of the enzyme fraction had no effect on the deprenil inhibition pattern, Arrhenius plots, heat stability or trypsin digestibility. The inhibition pattern of membrane-bound monoamine oxidase and the phospholipase-treated fractions by propargylamine showed a reduced substrate specificity compared to deprenil suggesting a hydrophobic region in the enzyme is a factor involved in the structural differences of monoamine oxidase A and B.

Effect of dexamethasone on monoamine oxidase inhibiton by iproniazid in rat brain

Chronic (6 days) dexamethasone administration caused a slight decrease of rat brain MAO enzyme activity which was reflected by lower levels of 14C-homovanillic acid (HVA) and increased levels of 14C-3-methoxytyramine (3MT) following intracisternal injections of 14C-dopamine (DA). Opposite results with dexamethasone were obtained in iproniazid (MAO-inhibited)-treated rats. In these animals, brain MAO enzyme activity was significantly increased by dexamethasone. This effect increased with the duration of dexamethasone treatment and appeared to be dose dependent. In the brain areas tested (hypothalamus, midbrain, cerebellum, pons and medulla, olfactory, rest of brain) increases of MAO enzyme activity were also indicated by lower levels of 14C-3MT and increased levels of 14C-HVA formed from intracisternally injected radiolabeled DA. Treatment with other glucocorticoids (16alpha-methyldichlorisone, 16beta-methylprednisone and prednisolone) had a similar effect on 14C-DA metabolism. On the other hand, desoxycorticosterone, progestone, estradiol and testosterone, did not exhibit this property. The data indicate that chronic glucocorticoid treatment may have a slight inhibitory effect on brain MAO and also has the ability to partially reverse or antagonize the inhibition of MAO caused by iproniazid.

Alpha-methyl derivatives of biogenic amines as inhibitors of monoamin oxidase

The values of Km app and Vmax for three natural substrates of monoamine oxidase have been determined at various stages in the isolation of the enzyme from rat liver tissue. The results are consistent with the presence in the enzyme preparation of at least two distinct molecular forms of the enzyme. Using the alpha-methyl derivatives of the natural substrates as inhibitors of the enzyme, the substrate dependence of Ki further substantiates this view. In addition, the kinetics of the inhibition suggest that the value of Km app may not for all substrates, necessarily be a measure of the affinity of the substrate for the enzyme.

The inhibition of A and B forms of MAO in the production of a characteristic behabioural syndrome in rats after 1-tryptophan loading

1-Tryptophan was administered to rats pretreated with selective inhibitors of the A and B forms of MAO deprenyl, a selective inhibitor of MAO-B, produced minor changes in behaviour and in the concentrations of apparent 5-HT and 5-HAA in brain. High doses of clorgyline, a selective inhibitor MAO-A, produced a characteristic stereotyped syndrome of hypermotility and tremor as well as an increase in apparent 5-HT and a decrease in apparent 5-HIAA in brain. Small doses of deprenyl and clorgyline in combination, but not singly, produced maximal effects on behaviour as well as on the concentrations of apparent 5-HT and 5-HIAA in brain. Maximum behavioural and biors before the other. It is concluded that the syndrome may be dependent on the formation of an N-substituted derivative of 5-HT which is at least partly deaminated by MAO-B. Alternatively, the syndrome may be dependent on a sufficiently high concentration of 5-HT in a special compartment where it is partly deaminated by MAO-B.

The reaction pathway of membrane-bound rat liver mitochondrial monoamine oxidase

1. A preparation of a partly purified mitochondrial outer-membrane fraction suitable for kinetic investigations of monoamine oxidase is described. 2. An apparatus suitable for varying the O(2) concentration in a spectrophotometer cuvette is described. 3. The reaction catalysed by the membrane-bound enzyme is shown to proceed by a double-displacement (Ping Pong) mechanism, and a formal mechanism is proposed. 4. KCN, NaN(3), benzyl cyanide and 4-cyanophenol are shown to be reversible inhibitors of the enzyme. 5. The non-linear reciprocal plot obtained with impure preparations of benzylamine, which is typical of high substrate inhibition, is shown to be due to aldehyde contamination of the substrate.

Monoamine oxidase in rat arteries: evidence for different forms and selective localization

1. Two forms of monoamine oxidase activity were differentiated in rat mesenteric and femoral artery by means of substrate and inhibitor specificities: one form deaminated tyramine, 5-hydroxytryptamine and noradrenaline and was highly sensitive to pargyline and clorgyline but resistant towards carbonyl reagents. This form resembled type A monoamine oxidase previously described. The other deaminated tyramine but not 5-hydroxytryptamine or noradrenaline and was inhibited by carbonyl reagents but not by clorgyline or pargyline.2. About one third of the total monoamine oxidase in homogenates of rat mesenteric artery was recovered in a 10(5)g supernatant. Both forms were partially soluble, but relatively less of the type A activity was recovered in the soluble fraction.3. Chemical sympathectomy with 6-hydroxydopamine resulted in a loss of 59% of monoamine oxidase activity in the mesenteric artery. There was a selective loss of type A activity, as revealed by the 70% decrease in 5-hydroxytryptamine deaminating ability and by the marked decrease in clorgyline sensitivity. The second monoamine oxidase species was resistant to 6-hydroxydopamine. The soluble activity was not affected by chemical sympathectomy. Most of the transmitter-specific monoamine oxidase of the arterial wall was localized within the adrenergic nerve endings. Our observations are consistent with the hypothesis that extraneuronal monoamine oxidase plays only a minor role in metabolizing noradrenaline in sympathetically innervated tissues.4. Plasma amine oxidase might originate from the arterial wall since it has similar characteristics to that found in the mesenteric artery.