Norepinephrine-deficient mice lack responses to antidepressant drugs, including selective serotonin reuptake inhibitors

Mice unable to synthesize norepinephrine (NE) and epinephrine due to targeted disruption of the dopamine beta-hydroxylase gene, Dbh, were used to critically test roles for NE in mediating acute behavioral changes elicited by different classes of antidepressants. To this end, we used the tail suspension test, one of the most widely used paradigms for assessing antidepressant activity and depression-related behaviors in normal and genetically modified mice. Dbh(-/-) mice failed to respond to the behavioral effects of various antidepressants, including the NE reuptake inhibitors desipramine and reboxetine, the monoamine oxidase inhibitor pargyline, and the atypical antidepressant bupropion, even though they did not differ in baseline immobility from Dbh(+/-) mice, which have normal levels of NE. Surprisingly, the effects of the selective serotonin reuptake inhibitors (SSRIs) fluoxetine, sertraline, and paroxetine were also absent or severely attenuated in the Dbh(-/-) mice. In contrast, citalopram (the most selective SSRI) was equally effective at reducing immobility in mice with and without NE. Restoration of NE by using L-threo-3,4-dihydroxyphenylserine reinstated the behavioral effects of both desipramine and paroxetine in Dbh(-/-) mice, thus demonstrating that the reduced sensitivity to antidepressants is related to NE function, as opposed to developmental abnormalities resulting from chronic NE deficiency. Microdialysis studies demonstrated that the ability of fluoxetine to increase hippocampal serotonin was blocked in Dbh(-/-) mice, whereas citalopram's effect was only partially attenuated. These data show that NE plays an important role in mediating acute behavioral and neurochemical actions of many antidepressants, including most SSRIs.     

Monoamine metabolism and behavioral responses to ethanol in mitochondrial aldehyde dehydrogenase knockout mice

BACKGROUND: It is widely accepted that, in addition to removing acetaldehyde produced during the metabolism of ethanol, mitochondrial aldehyde dehydrogenase (ALDH2) functions in the pathway by which aldehyde metabolites of the monoamines dopamine (DA) and serotonin (5-HT) are converted to their acidic metabolites. Moreover, studies of ALDH2 inhibitors used for treating alcoholism suggest that their antidipsotropic effects may be related to inhibition of monoamine metabolism. Therefore, we examined the hypothesis that altered brain monoamine metabolism is related to the influence of ALDH2 on behavioral responses to ethanol. METHODS: Mice were generated with a gene-trap mutation of the ALDH2 gene. ALDH2 mRNA was absent in ALDH2-/- mice. Western blot analysis of liver mitochondria confirmed the absence of ALDH2 protein in the ALDH2-/- mice. Wild-type and ALDH2-deficient mice were tested for the effects of different doses of ethanol on locomotor activity, ataxia, and a 2-bottle ethanol-water preference test. RESULTS: Wild-type and ALDH2+/- mice preferred ethanol to water. However, ALDH2-/- mice drank significantly less ethanol than wild-type or ALDH2+/- mice. Locomotor activity and ataxia were significantly more affected by ethanol in ALDH2-/- mice than in wild-type or ALDH2+/- mice. There was no effect of genotype on levels of 5-HT, DA, or their precursors or metabolites in several brain regions, as measured by HPLCec. CONCLUSIONS: The results indicate that: (1) the effect of the mutant genotype on behavioral responses to ethanol is unrelated to altered brain monoamine metabolism and (2) ALDH2 is not required for the metabolism of brain monoamines in vivo.     

Monoamines in the pancreatic islets of the mouse

Summary The functional significance of 5-hydroxytryptamine (5-HT) storage in the pancreatic B cells for insulin secreting mechanisms was studied in normal micein vivo. Pretreatment of the animals withL-5-hydroxytryptophan (L-5-HTP) markedly decreased the insulin releasing capacity after sulphonylurea stimulation. This inhibition of insulin release could be abolished by previous administration of an inhibitor of aromatic amino acid decarboxylation. On the other hand, pretreatment with the monoamine oxidase inhibitor nialamide alone, decreased sulphonylurea-induced insulin release. The combined treatment with nialamide andL-5-HTP did not further decrease the insulin response. Insulin release induced byL-isopropylnoradrenaline (L-IPNA) was also found to diminish after previous administration ofL-5-HTP or nialamide; but, unlike the insulin response to sulphonylurea, insulin release induced by IPNA could be totally suppressed by the combined treatment of nialamide or pargyline andL-5-HTP. Insulin release induced by glucose was not significantly influenced with any of the above treatments. Basal levels of plasma insulin were not affected byL-5-HTP injection, and were not consistently diminished by the combined treatment with monoamine oxidase inhibitor andL-5-HTP. The combined treatment with monoamine oxidase inhibitors andL-5-HTP was found to elicit a profound hypoglycaemia in both normal and alloxan-diabetic mice. The hypoglycaemic condition was accompanied by exhaustion of liver and muscle glycogen. The hypoglycaemia could be abolished by previous treatment with an inhibitor of aromatic amino acid decarboxylation. Combined treatment with pargyline and 5-HT brought about a marked hyperglycaemia. It is concluded that: 1. intracellular levels of 5-HT in the pancreatic B cells possess the ability to modify insulin secreting mechanisms; and 2. the hypoglycaemic action of monoamine oxidase inhibitors is brought about by raised intracellular levels of 5-HT, which is accompanied by a markedly increased glucose utilization by the tissues.This work was supported by the Medical Faculty, University of Lund, Sweden. The skilful technical assistance of Mrs. Lena Kvist, Miss Anita Åkesson and Miss Ann-Christin Helander is gratefully acknowledged.     

Monoamine oxidase: an important intracellular regulator of gastrin release in the rat

The role of monoamine oxidase (MAO) in the meal-induced or amino acid-induced release of gastrin was investigated. Rats that were pretreated with the nonspecific MAO inhibitor nialamide (200 mg/kg) showed a greater rise in meal-induced serum gastrin than did untreated controls. In vitro experiments demonstrated that gastrin secretion from dispersed antral G cells in response to a stimulatory dose of phenylalanine or methylbenzylamine (10 mM) was markedly enhanced if the cells were treated with nialamide. Studies with the more specific MAO inhibitors clorgyline and deprenyl indicated that antral mucosa contained predominantly type A activity. Inhibition of MAO type A with clorgyline, both in vivo and in vitro, resulted in a greater release of gastrin after stimulation by a meal or phenylalanine. It is concluded that MAO may play an important role in the regulation of gastrin release from the G cell by partially controlling the level of amines within the cell.     

Effect of increased pancreatic islet norepinephrine,dopamine and serotonin concentration on insulin secretion in the Golden hamster

Summary The purpose of this study was to determine if increased concentrations of pancreatic islet norepinephrine, dopamine, or serotonin alter insulin secretion. Golden hamsters received intraperitoneal injections of the norepinephrine precursor DL-threo-dihydroxyphenylserine, the dopamine precursor L-3,4-dihydroxyphenylalanine, or the serotonin precursor 5-hydroxytryptophan with and without pretreatment of the hamsters with the monoamine oxidase inhibitor tranylcypromine. Administration of the monoamine precursors to animals pretreated with tranylcypromine resulted in a mean increase in plasma glucose of 192% and a mean decrease in plasma insulin of 58%. Using a collagenase isolation technique, islets from control and treated animals were evaluated for monoamine content and insulin secretory capacity. The monoamine concentrations in control islets, in mol/kg wet weight, were: norepinephrine 42±8; dopamine 8±2; and serotonin 26±9. Administration of the appropriate precursor to control hamsters resulted in a 1.9-fold (norepinephrine), 6-fold (dopamine), and 22-fold (serotonin) increase in monoamines. There was no alteration in the glucose (16.3 mmol/l)-stimulated in vitro insulin secretion from islets obtained from these hamsters. Administration of the precursors to hamsters pretreated with tranylcypromine resulted in a 3.5-fold (norepinephrine), 22-fold (dopamine), and 59-fold (serotonin) increase in monoamines. Glucose-stimulated in vitro insulin secretion from islets obtained from these hamsters was completely blocked. This study suggests that high concentrations of norepinephrine, dopamine, and serotonin in the pancreatic islets can decrease glucose-stimulated insulin secretion.     

Serotonergic stimulation of pituitary-adrenal activity in the mouse.

Female CF1 mice were given L-5-hydroxytryptophan (5-HTP), quipazine, or 6-methoxy-1,2,3,4-beta-carboline (6-MeO-THBC) in conjunction with various serotonergic drugs to determine if the pituitary-adrenal stimulation produced by the former compounds is serotonergically mediated. Corticosterone (CS) responses to 5-HTP were uninfluenced by pretreatment with a tryptophan hydroxylase inhibitor, p-chlorophenylalanine (PCPA), but were significantly potentiated by a serotonin (5-HT) reuptake inhibitor (Lilly 110140), attenuated by two 5-HT receptor blockers, cyproheptadine and methergoline, and almost completely abolished by 2 extracerebral aromatic L-amino acid decarboxylase inhibitors, MK 486 and a low dose of Ro 4-4602. It was also established that L-tryptophan could stimulate pituitary-adrenal activity in animals pretreated with the monoamine oxidase (MAO) inhibitor pargyline. Serotonergic drugs were generally not as effective in modulating the responses to quipazine and 6-MeO-THBC. It is concluded that (1) 5-HTP stimulates pituitary-adrenal activity in mice by being converted to 5-HT and acting on 1 or more groups of serotonergic receptors; (2) these receptors are located either in an area of the brain outside of the blood-brain barrier such as the median eminence or in a peripheral tissue(s); (3) tryptophan-derived 5-HT can stimulate these receptors, but only if allowed to accumulate by inhibiting its catabolism; and (4) it is not yet clear whether pituitary-adrenal responses to quipazine and 6-MeO-THBC are mediated by a serotonergic mechanism.     

Ether stress stimulates noradrenaline release in the hypothalamic paraventricular nucleus

Differential normal-pulse voltammetry was combined with treated carbon fibre electrodes for monitoring in vivo extracellular catechols synthesized by noradrenergic terminals innervating the paraventricular hypothalamic nucleus. From urethane-anaesthetized rats, pretreated with a monoamine oxidase inhibitor, pargyline, we were able to monitor a catechol signal which unequivocally corresponded to extracellular noradrenaline, and we observed that ether inhalation for 2 min induced an immediate increase in this signal. Electrical stimulation of the ventral noradrenergic pathway (10 Hz for 40 s) induced a similar effect. On the other hand, from freely moving rats which were not treated with pargyline, we recorded a catechol peak which mainly corresponded to 3,4-dihydroxyphenylacetic acid which was synthesized by noradrenergic terminals. However, electrochemical and biochemical evidence strongly suggested that the increase in this signal induced by a 2-min ether stress does not correspond to 3,4-dihydroxyphenylacetic acid, but to an increase in the extracellular noradrenaline concentration. In both experimental situations the time course of the effects was identical: ether stress induced an immediate and pronounced increase in norepinephrine release, and this effect lasted as long as the stimulus duration. This effect appeared specific for noradrenergic terminals, since no effect on dopamine release was observed when recorded from the striatum or behind the paraventricular hypothalamic nucleus from the A13 dopaminergic group. In conclusion, our data are consistent with those which suggest a facilitatory action of norepinephrine on neurosecretory neurons whose cell bodies are located in the paraventricular hypothalamic nucleus and which play a major role in the hormonal response to stress.     

Dopamine turnover and glutathione oxidation: implications for Parkinson disease.

Parkinson disease is characterized by a major loss (approximately 80% or more) of dopaminergic nigrostriatal neurons and by an increased turnover of neurotransmitter by surviving neurons of the nigrostriatal tract. In theory, increased turnover of dopamine should be associated with an oxidative stress derived from increased production of hydrogen peroxide. The peroxide is formed during the oxidative deamination of dopamine by monoamine oxidase. In experiments with mice, increased presynaptic turnover of dopamine was evoked by injection of reserpine, which interferes with the storage of dopamine in synaptic vesicles. Loss of dopamine and formation of deaminated metabolites were accompanied by a significant rise (87.8%) in the level of oxidized glutathione in brain. This change was observed in the striatum, which is richly innervated by dopamine terminals, but not in the frontal cortex, which receives a much sparser innervation by catecholamine nerve terminals. The rise in oxidized glutathione was seen even though dopamine terminals constitute only 1% or less of the mass of the striatum. Clorgyline, an inhibitor of monoamine oxidase type A, blocked the formation of oxidized glutathione. These observations confirm that a selective increase in neurotransmitter turnover within nigrostriatal nerve terminals can evoke a change in cellular redox status. We suggest that an oxidative stress may play a role in the natural history of Parkinson disease.     

Locomotor activity induced by noncompetitive NMDA receptor antagonists versus dopamine transporter inhibitors: opposite strain differences in inbred long-sleep and short-sleep mice

Background: The actions of ethanol in the brain involve multiple neuroreceptor systems, including glutamatergic N‐methyl‐D‐aspartate receptor (NMDAR) channels. In a novel environment, both ethanol and the noncompetitive NMDAR antagonist MK‐801 stimulate locomotor activity to a lesser extent in inbred long‐sleep (ILS) mice compared with inbred short‐sleep (ISS) mice. The behaviorally activating effects of noncompetitive NMDAR antagonists are thought to involve increased monoamine neurotransmission. Thus, in this study, we sought to determine whether: (1) habituation to the behavioral environment alters the differential locomotor‐stimulant effects of noncompetitive NMDAR antagonists in ILS and ISS mice and (2) the differential behavioral sensitivity of ILS and ISS mice to noncompetitive NMDAR antagonists is mediated through direct inhibition of the dopamine transporter (DAT). Methods: Open field locomotor activity was measured following acute systemic injection of saline or drug. [³H]DA uptake parameters were determined in striatal synaptosomes prepared from drug‐naïve mice. Results: Habituation to the testing environment abolished the strain differences in saline‐induced locomotor activity. However, ethanol‐ as well as MK‐801‐treated ILS mice still exhibited reduced locomotor activity compared with ISS mice, suggesting that a drug‐environment interaction is not the primary explanation for the strain differences. The noncompetitive NMDAR antagonists phencyclidine and ketamine also induced significantly lower locomotor activity in ILS than in ISS mice. In contrast, the DAT inhibitors cocaine and GBR 12909 and the DA releaser amphetamine induced greater locomotor activity in ILS than in ISS mice, a strain difference opposite that of the noncompetitive NMDAR antagonists. Furthermore, the differential behavioral effect found with DAT inhibitors was not mediated by differences in the affinity nor number of striatal DATs between ILS and ISS mice. Conclusions: Our results support the conclusion that the differential locomotor‐stimulant effects of ethanol and noncompetitive NMDAR antagonists in ILS and ISS mice are not mediated through direct inhibition of DAT.     

Effects of cyclic hydrostatic pressure on the brain biogenic amines concentrations in the flounder, Platichthys flesus

The present study evaluated the effects of cyclic variations of hydrostatic pressure (HP) on neurotransmitters in the whole brain of flounder. The concentrations of the biogenic amines L-3,4-dihydroxyphenylalanine (L-DOPA), dopamine (DA), norepinephrine (NE), epinephrine (E), 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), 3-methoxytyramine (3-MT), 5-hydroxytryptamine (5-HT) and 5-hydroxyindolacetic acid (5-HIAA) were measured. Fish were subjected to HP cyclic variations which mimic naturally occurring conditions for a period of 14 days. DA, NE and 5-HT concentrations were significantly smaller by 21, 24 and 36%, respectively, compared to control fish. The concentrations of monoamine metabolites HVA, 3-MT and 5-HIAA were also smaller than those in control fish. These results suggest that central monoaminergic systems were influenced during long exposure to cyclic HP. The decreases of central neurotransmitters content might be involved in the physiological and behavioral responses to intermittent HP in fish.     

Catecholaminergic control of alpha-melanocyte-stimulating hormone (alpha MSH) release by frog neurointermediate lobe in vitro: evidence for direct stimulation of alpha MSH release by thyrotropin-releasing hormone

The role of dopaminergic and adrenergic innervation of the intermediate lobe of amphibian pituitary in the release of alpha MSH has been studied in vitro. Neurointermediate lobes of frog (Rana ridibunda Pallas) have been perifused in amphibian culture medium (ACM) for 5-7 h. alpha MSH released in the effluent perifusate was measured by means of a sensitive and specific RIA. No significant morphological alteration of neurointermediate lobe cells was observed during the perifusion experiment, even at the electron microscopic level. The existence of dopaminergic receptors, responsible for an inhibition of frog melanotrophs, was shown using the dopaminergic agonists apomorphine (10(-6) M) and bromo-2-ergocryptine (10(-8) and 10(-7) M), which initiated a marked reduction of alpha MSH secretion. The effect of apomorphine was obliterated by the dopaminergic antagonist haloperidol. Haloperidol itself induced a dose-related stimulation, and the monoamine oxidase inhibitor nialamide (4 x 10(-3) M) inhibited alpha MSH secretion. In addition, haloperidol led to a complete reversal of the inhibitory effect of nialamide on alpha MSH secretion. These results demonstrate the existence, in the parenchyme of the intermediate lobe, of dopaminergic nerve fibers that are functionally active. The beta-adrenergic agonist isoproterenol was responsible for a dose-related stimulation of alpha MSH secretion; the stimulatory effect was reversed by the beta-adrenergic antagonist propranolol. TRH is a potent stimulator of alpha MSH secretion in amphibians. Since haloperidol and propranolol did not abolish the stimulation of alpha MSH release induced by TRH, it appeared that TRH action was not mediated via an inhibition of dopamine release or via a stimulation of adrenergic nerve fibers.     

Catechol concentrations in the hemolymph of the scallop, Placopecten magellanicus

Catecholamines have previously been detected in numerous tissues and are thought to control a wide variety of physiological functions in bivalve molluscs. In the present study, alumina extraction and high-performance liquid chromatography reveal the presence of significant concentrations of 3,4-dihydroxyphenylalanine (DOPA), dopamine, and 3,4-dihydroxyphenylacetic acid (DOPAC) in the hemolymph of the sea scallop, Placopecten magellanicus. The concentration of dopamine in the hemolymph averaged 223.8 ng/ml, (+/-48.4, SEM), equivalent to 10(-7) to 10(-6) M. Neither epinephrine nor norepinephrine was reliably detected in significant quantities. Previous studies have demonstrated physiological responses to dopamine with thresholds of 10(-9) to 10(-6) M, thus suggesting that this catecholamine may have an endocrine function. Furthermore, monitoring hemolymph concentrations of catechols might provide a sensitive measure of the physiological status of bivalves. For example, drugs known to affect catechol concentrations in other tissues also effect hemolymph levels. Administration of monoamine oxidase inhibitors such as pargyline, deprenyl, and clorgyline at 10(-4) M for 1 day of incubation followed by a 2-day wash resulted in decreased hemolymph concentrations of DOPAC and increased concentrations of its precursors, DOPA and dopamine. Incubation in 10(-4) M 3,5-dinitrocatechol, a catecholamine-O-methyl transferase blocker, for 1 day followed by a 2-day wash significantly increased the concentration of dopamine and DOPAC in the hemolymph. Scallops incubated in 10(-5) M alpha-methyl-p-tyrosine, a blocker of tyrosine hydroxylase, for 1 day followed by a 3-day wash in artificial seawater had significantly reduced concentrations of DOPA, dopamine, and DOPAC in the hemolymph. In addition to responding to pharmacological agents, dopamine levels also decreased significantly following thermal induction of spawning, thus suggesting that hemolymph concentrations of catechols might provide indices of reproductive activity and/or stress.     

Parkinson disease: A new link between monoamine oxidase and mitochondrial electron flow

Two factors that contribute to the progression of Parkinson disease are a brain defect in mitochondrial respiration and the generation of hydrogen peroxide (H₂O₂) by monoamine oxidase (MAO). Here we show that the two are linked. Metabolism of the neurotransmitter dopamine, or other monoamines (benzylamine, tyramine), by intact rat brain mitochondria suppresses pyruvate- and succinate-dependent electron transport. MAO inhibitors prevent this action. Mitochondrial damage is also reversed during electron flow. A probable explanation is that MAO-generated H₂O₂ oxidizes glutathione to glutathione disulfide (GSSG), which undergoes thiol-disulfide interchange to form protein mixed disulfides, thereby interfering reversibly with thiol-dependent enzymatic function. In agreement with this premise, direct addition of GSSG to mitochondria resulted in similar reversible inhibition of electron transport. In addition, the monoamines induced an elevation in protein mixed disulfides within mitochondria. These observations imply that (i) heightened activity and metabolism of neurotransmitter by monoamine neurons may affect neuronal function, and (ii) apparent defects in mitochondrial respiration associated with Parkinson disease may reflect, in part, an established increase in dopamine turnover. The experimental results also target mitochondrial repair mechanisms for further investigation and may, in time, lead to newer forms of therapy.     

骨化三醇对MPTP诱导慢性帕金森病小鼠模型脑内神经递质的影响

目的探讨骨化三醇对1-甲基-4-苯基-1,2,3,6-四氢吡啶（MPTP）诱导慢性帕金森病小鼠模型脑内氨基酸类和单胺类神经递质的影响。方法C57BL／6小鼠随机分为对照组、模型组、骨化三醇高剂量给药组和低剂量给药组,构建MPTP慢性模型,应用高效液相色谱法测定中脑及纹状体内氨基酸类神经递质,包括天冬氨酸（Asp）、谷氨酸（Glu）、甘氨酸（Gly）、牛磺酸（Tau）、1,-氨基丁酸（GABA）和单胺类神经递质,包括多巴胺（DA）及其代谢产物3,4-双羟苯乙酸（DOPAC）的含量。结果与对照组相比,模型组小鼠氨基酸类神经递质除中脑内GABA外含量增加,纹状体内DA和DOPAC含量降低,代谢率增加俨〈O．01或P〈0．05）。骨化三醇给药组与模型组相比,高剂量可使中脑GIu,Gly,Tau及纹状体内五种氨基酸含量降低,DA和DOPAC含量增加,代谢率降低俨〈O．01或P〈0．05）;低剂量则仅可减缓中脑Tau和纹状体除Tau以外其他4种氨基酸含量增长状况。结论骨化三醇能改善MPTP所致慢性PD小鼠模型脑内神经递质含量异常,直接或间接保护DA能神经元,对其机制的深入研究有助于新型神经保护药物的开发。     

Hydrazide antidepressants possess novel antileishmanial activity in vitro and in vivo

The hydrazide monoamine oxidase inhibitor antidepressants possess a novel antiparasitic action against visceral and cutaneous strains of Leishmania. In vitro, phenelzine was the most active compound tested, while in vivo, nialamide was more potent, and was also effective when applied topically to cutaneous lesions. Despite the coincidence of tricyclic antidepressants also possessing antileishmanial activity, the evidence suggests that the antiparasitic action of the hydrazides is unrelated to either monoamine oxidase inhibition or CNS effects.     

Nialamide in the treatment of essential hypertension

The antihypertensive effectiveness of the monoamine oxidase (MAO) inhibitor, nialamide, was investigated. When nialamide was administered in relatively high dosage (225 mg. daily), 44 per cent of the subjects obtained a significant postural reduction of blood pressure; however, when the drug was used in combination with hydrochlorothiazide, 65 per cent of the group had a significant fall of blood pressure. In the supine position, a significant reduction of blood pressure was noted less often, in 20 and 18 per cent of the two groups, respectively. The incidence of side effects encountered was low and there were no instances of clinical hepatotoxicity.

The mechanism of hypotensive action of the MAO inhibitors is still undetermined. The various speculative mechanisms are discussed. It is suggested that these drugs may ultimately be particularly indicated in the hypertensive subject with complicating coronary artery disease.     

Effect of L-dopa administration on islet monoamine oxidase activity and glucose-induced insulin release in the mouse

Previous studies have shown that the amine precursor L-3,4-dihydroxyphenylalanine (L-DOPA) is rapidly converted to its corresponding amine, dopamine, in islet beta-cells. In the present investigation, we studied the effect of acute L-DOPA administration on islet monoamine oxidase (MAO) activity and on glucose-induced insulin secretory response in mice. It was observed that at 2 min after intravenous L-DOPA administration, there was a marked increase (+35%) in islet MAO activity, with serotonin as substrate. At 7 min, MAO activity towards dopamine was enhanced by 32% and that towards serotonin and phenylethylamine (PEA) was decreased by 23 and 25%, respectively. The inhibitor of L-aromatic amino acid decarboxylase, benserazide, abolished L-DOPA-induced changes of MAO activity, suggesting that the formed dopamine, and not L-DOPA itself, was responsible for the observed effects. At 60 min, no effect by L-DOPA administration on islet MAO activity was noticed. L-DOPA (125 or 250 mumol/kg), given together with glucose, induced a decrease in glucose-induced insulin response. L-DOPA (125 mumol/kg), given 7 min before glucose, totally suppressed glucose-induced insulin response. This inhibition was eliminated through pretreatment with benserazide. Enhancement of glucose-stimulated insulin response, after deposition of horseradish peroxidase (HRP) in beta-cell vacuolar system, was suppressed by L-DOPA. We conclude that acute L-DOPA-induced dopamine accumulation in pancreatic islets is accompanied by rapid changes in MAO activity, concomitant with an inhibitory effect on glucose-stimulated insulin response. Increased hydrogen peroxide production, following increased MAO activity, may possibly augment the inhibitory effect of dopamine accumulation on insulin release.     

Changing the Actions of Neuroactive Drugs by Changing Brain Protein Synthesis

Diminution of cerebral protein synthesis diminished the cerebral responses of mice to some neuroactive drugs, while an increase in synthesis increased the responses. Protein synthesis in whole brains (tested in vitro) was diminished by giving living mice different inhibitors by different routes. The inhibitors tested (chloramphenicol, cycloheximide, and puromycin) diminished the behavioral responses of the mice to levodopa without affecting either its cerebral uptake or its conversion to dopamine. A diminution of the reactions of dopaminergic receptors was suggested by the diminished responses to the dopaminergic drug, apomorphine, while participation of cholinergic ones was suggested by experiments with oxotremorine. Proof that receptors had been specifically involved was secured on homogenized caudate nuclei from chloramphenicol-treated mice, in which the dopamine-activated production of cyclic AMP was markedly diminished. A stimulator of cerebral protein synthesis, the artificial double-stranded RNA, poly(I).poly(C), increased the behavioral responses to these three drugs while it increased the dopamine-activated production of cyclic AMP. Since all these experimental increases or decreases in the responses to drugs required the lapse of only a few hours, proteins with rapid turnover rates must be critical in the activation of several kinds of cerebral receptors.     

Hypothalamic neurochemistry and feeding behavioral responses to clonidine, an alpha-2-agonist, and to trifluoromethylphenylpiperazine, a putative 5-hydroxytryptamine-1B agonist, in genetically obese Zucker rats

Genetically obese Zucker rats are hyperphagic, hyperinsulinemic and hyperlipemic. In order to investigate pathophysiological mechanisms underlying hyperphagia in these animals, monoamine metabolism and turnover were studied in discrete hypothalamic nuclei known to participate in the control of feeding behavior. Neurochemical studies in genetically obese Zucker rats and in their lean littermate controls were complemented by investigating feeding behavioral responses to the acute administration of clonidine (15 and 30 micrograms/kg i.p.), an alpha 2-adrenoceptor agonist, and to trifluoromethylphenylpiperazine (TFMPP; 1, 2 and 5 mg/kg s.c.), a putative serotonergic 5-hydroxytryptamine-1B receptor agonist. Obese Zucker rats had significantly lower concentrations of 5-hydroxyindoleacetic acid, the main deaminated metabolite of 5-hydroxytryptamine, in the nucleus paraventricularis (PVN) and in the nucleus ventromedialis (VMN), when compared to their lean littermate controls. The rate of accumulation of 5-hydroxytryptophan after decarboxylase inhibition was reduced in the PVN, nucleus supraopticus, nucleus periventricularis and nucleus suprachiasmaticus of the obese rats. No differences were observed in basal concentrations of norepinephrine, dopamine or 3,4-dihydroxyphenylacetic acid between obese and lean Zucker rats in the brain areas studied. However, the rate of accumulation of 3,4-dihydroxyphenylalanine was lower in the VMN and in the median eminence of the obese rats. The feeding behavioral tests showed significantly augmented hyperphagic responses to clonidine in obese Zucker rats. The anorexic effect of TFMPP was similar in both phenotypes. It is concluded that serotonergic activity is reduced in obese Zucker rats, particularly in the PVN, which plays a key role in the control of feeding behavior. The reduced serotonergic activity may be associated with enhanced alpha 2-adrenoceptor-mediated feeding responses in obese Zucker rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Stimulation of the hypothalamo-pituitary-adrenocortical axis by the central serotonergic pathway: involvement of endogenous corticotropin-releasing hormone but not vasopressin.

Many reports indicate that serotonin plays a role in the regulation of the hypothalamo-pituitary-adrenocortical axis. The present study was designed to elucidate whether the activation of the central serotonergic pathway enhances adrenocorticotropin and corticosterone secretion, and if so, whether the CRH and vasopressin neuronal systems could be mediating this effect. Intraperitoneal administration of a low dose of L-5-hydroxytryptophan (an aromatic L-amino acid precursor of serotonin synthesis; 20 mg/kg bw, 30 minutes before the sacrifice) in rats pretreated with pargyline (a brain monoamine oxidase inhibitor, which enhances monoamine activity; 75 mg/Kg bw, 16 hours before the sacrifice) and carbidopa (a peripheral active inhibitor of the decarboxylation of aromatic L-amino acids, which would permit more monoamine precursor to be available to the brain; 50 mg/Kg bw, 90 minutes before the sacrifice) increased ACTH and corticosterone secretion in plasma. Such an effect was partially blocked by metergoline (a serotonin type-1 and-2 receptor blocker; 1 mg/Kg bw, 90 minutes before the sacrifice), but not by spiperone (a serotonin type-2 and dopamine receptor antagonist; 0.5 mg/Kg bw. 90 minutes before the sacrifice). The activation of the central serotonergic system enhanced the CRH content in the median eminence, whereas it decreased the content of this neuropeptide in the medial basal hypothalamus. These effects were fully abolished by metergoline, but not by spiperone pretreatment. The activation of the serotonergic pathway did not influence the vasopressinergic neuronal system. In vitro experiments using hypothalamic-median eminence fragments incubated with serotonin solutions indicate that this monoamine possesses a CRH releasing effect at concentrations of 1 microM or more.(ABSTRACT TRUNCATED AT 250 WORDS)