Hypothalamic regulation of circadian noradrenergic input to the chick pineal gland

Summary While the avian pineal gland contains circadian oscillators and photoreceptors capable of producing circadian rhythms of the hormone melatonin, it is extensively innervated by post-ganglionic fibers of the superior cervical ganglia which release norepinephrine (NE) rhythmically. Norepinephrine turnover is higher during subjective day than during subjective night. In mammals, this rhythmic input, which is higher in subjective night than subjective day, derives from the hypothalamic suprachiasmatic nuclei (SCN) and is essential for rhythmic melatonin production. The present study was designed to determine whether one of two candidates for the avian homologue of the mammalian SCN is necessary for rhythmic NE turnover in the chick pineal gland. Either electrolytic lesions or sham lesions were delivered to the periventricular preoptic nuclei (PPN) or to the visual suprachiasmatic nucleus (vSCN). After recovery, the rates of decline in [NE] were determined following pretreatment with -methyl-p-tyrosine, a tyrosine hydroxylase inhibitor, at mid-subjective day or at mid-subjective night. Birds receiving sham surgeries in either PPN or vSCN and birds receiving lesions of the PPN exhibited rhythmicity in NE turnover. No rhythm of NE turnover could be determined in birds with ablated vSCN.Abbreviations AMPT -methyl-p-tyrosine - DS supraoptic decussation - EBZ ear bar zero (see Methods) - GLv ventral lateral geniculate body - NE norepinephrine - PPN periventricular preoptic nuclei - RH retinohypothalamic projection - SCN suprachiasmatic nuclei - vSCN visual suprachiasmatic nucleus     

Effect of high- and low-protein diets on the regulation of rat liver tyrosine aminotransferase and tryptophan oxygenase activities by l-tyrosine and l-tryptophan

Induction of rat liver tyrosine aminotransferase by l-tyrosine and tryptophan oxygenase by l-tryptophan was studied in groups of rats fed on diets containing 18 or 5% protein. The basal activity of hepatic tyrosine aminotransferase of rats receiving 5% protein gradually increased with the age of the animals but that of rats receiving 18% protein did not. l-Tyrosine induced hepatic tyrosine aminotransferase in rats receiving 18% protein when tested at ages from 4 to 20 weeks. When induction by l-tyrosine was carried out in rats receiving the 5% protein diet, significant induction of tyrosine aminotransferase occurred only in 4- or 6-week-old rats. Induction by l-tryptophan of tryptophan oxygenase in liver or the basal activity of this enzyme in liver did not differ between the groups fed on 5 and 18% protein. On changing the diet from 0 to 18% protein, the above-mentioned effects on the induction of hepatic tyrosine aminotransferase were reversed.     

The effect of phencyclidine on dopamine metabolism in the mouse brain

The administration of phencyclidine (PCP) to mice resulted in no change in brain levels of tyrosine, dopamine (DA), norepinephrine (NE), or homovanillic acid (HVA). Although PCP reduced plasma tyrosine levels, no effect of PCP on the utilization of DA of NE after blockade of synthesis with α-methyl-p-tyrosine (AMPT) was observed. In addition, PCP did not affect the probenecid-induced accumulation of HVA. However, PCP was observed to potentiate the haloperidol-induced increase in HVA concentration, and the haloperidol-induced decline in DA levels after AMPT. The former effect was blocked by baclofen, suggesting that PCP mobilizes DA for impulse-dependent release. This effect could not be attributed to an antagonism of presynaptic DA receptors. These effects are similar to those of the “non-amphetamine” stimulant class of drugs.     

Plasma gonadotropin,prolactin levels and hypothalamic tyrosine hydroxylase activity in rats during estrous cycle,after overiectomy and after blockade of catecholamine biosynthesis

Plasma gonadotropin, prolactin levels and hypothalamic tyrosine hydroxylase activity were evaluated at 0900, 1200 and 1700 h during diestrus, proestrus and estrus, ovariectomized and after systemic administration of reserpine or α-methyl p-tyrosine, which interfere with catecholamine biosynthesis, in rats. Gonadotropin and prolactin levels showed peak values during the afternoon of proestrus, while hypothalamic tyrosine hydroxylase activity was markedly lowered at 1200 on proestrus. Gonadotropin levels were slightly lowered whereas prolactin concentrations and hypothalamic tyrosine hydroxylase activity were significantly increased by reserpine. Depletion of hypothalamic dopamine by reserpine apparently resulted in significant elevation of prolactin levels which inturn induce tyrosine hydroxylase. Gonadotropin levels and hypothalamic tyrosine hydroxylase activity were significantly suppressed after the administration of α-methyl p-tyrosine. Prolactin levels, however, were elevated significantly. These results indicate that catecholamines are involved in the control of gonadotropin and prolactin release during estrous cycle and inhibition of catecholamines biosynthesis by α-methyl p-tyrosine could result in suppression of gonadotropin levels, whereas removal of tonic inhibition of hypothalamic dopamine by α-methyl-p-tyrosine elevate prolactin levels.     

Plasma catecholamine responses to tyrosine hydroxylase inhibition and cold exposure

The acute effect of α-methyl-p-tyrosine (αMPT), a tyrosine hydroxylase inhibitor, on plasma levels of norepinephrine (NE), epinephrine (E), dopamine (DA), and adrenal cholesterol content in male rats at room temperature (24°C) and during acute cold exposure (4°C) was evaluated. Compared to saline-treated controls, αMPT: 1) significantly reduced plasma NE and DA in both normal and cold stress conditions, 2) significantly increased plasma E in both environments, and 3) stimulated the adrenal cortex. These findings suggest that tyrosine hydroxylase inhibition and consequent catecholamine synthesis blockade disrupts the homeokinesis of adrenergic processes and may present a significant stress to the intact animal.     

Increase in hepatic tyrosine aminotransferase mRNA during enzyme induction by N6,O2'-dibutyryl cyclic AMP.

Tyrosine aminotransferase mRNA was quantitated by translation in a cell-free system derived from wheat germ followed by specific immunoprecipitation of the newly synthesized enzyme subunit. Hepatic poly(A)-containg RNA prepared from rats treated for 4 h with N6, O2'-dibutyryl cyclic AMP and theophylline was approximately 5.6 times more active in directing the synthesis of the tyrosine aminotransferase subunit relative to untreated controls. The overall template activity of the RNA prepared from control and cyclic AMP-treated animals was virtually identical, demonstrating that the cyclic nucleotide effect was specific for the tyrosine aminotransferase mRNA. At all times, after a single injection of dibutyryl cyclic AMP and theophylline, the increase in hepatic enzyme activity was accompanied by corresponding induction in the level of functional tyrosine aminotransferase mRNA. Other inducers of tyrosine aminotransferase, such as glucagon and hydrocortisone, also increased the level of tyrosine aminotransferase mRNA in proportion to their effect on enzyme activity. The RNA polymerase II inhibitor, alpha-amanitin, completely blocked the dibutyryl cyclic AMP-mediated increase in tyrosine aminotransferase mRNA activity. These studies demonstrate that, in intact animals, the induction of tyrosine aminotransferase activity by dibutyryl cyclic AMP can be completely accounted for by a corresponding increase in the level of functional mRNA coding for the enzyme.     

Circadian Variations in the Activity of Tyrosine Hydroxylase, Tyrosine Aminotransferase, and Tryptophan Hydroxylase: Relationship to Catecholamine Metabolism

Abstract— Circadian variations in the activity of tyrosine hydroxylase, tyrosine aminotransferase, and tryptophan hydroxylase were observed in the rat brain stem. Tyrosine hydroxylase exhibited a bimodal pattern with peaks occurring during both the light and dark phases of the circadian cycle. Tyrosine aminotransferase had one daily peak of activity occurring late in the light phase, whereas tryptophan hydroxylase activity was maximal late in the dark phase. Circadian fluctuations in tyrosine hydroxylase activity did not correlate well with circadian variations in the turnover rates of norepinephrine or dopamine nor with levels of these catecholamines. This supports the idea that although tyrosine hydroxylase is the rate-limiting enzyme in the synthesis of catecholamines, other factors must also be involved in the in vivo regulation of this process. Administration of α -methyl- p -tyrosine (AMT) methyl ester HC1 (100 mg/kg) had no effect on the activity of tryptophan hydroxylase, but effectively eliminated the peak of tyrosine hydroxylase activity that occurred during the light phase. AMT also lowered levels of tyrosine aminotransferase, but only at times near the daily light to dark transition. These chronotypic effects of AMT emphasize the importance of "time of day" as a factor that must be taken into account in evaluating the biochemical as well as the pharmacological and toxicological effects of drugs.     

Regulation of hepatic tyrosine aminotransferase in genetically obese rats

The activities of hepatic tyrosine aminotransferase, tryptophan oxygenase and serine dehydratase were increased in obese rats shortly after weaning. Immunotitration experiments showed that the increase in tyrosine aminotransferase activity resulted from an increase in enzyme protein in obese rats. No increase in hepatic tyrosine aminotransferase was observed in suckling pre-obese rats. The post-weaning increase in hepatic tyrosine aminotransferase of obese rats was only observed during the light phase of the diurnal cycle, but was prevented by pair-feeding and by starvation. Tryptophan increased hepatic tyrosine aminotransferase of lean rats to obese levels but had no effect in obese rats until tyrosine aminotransferase levels were reduced by starvation or adrenalectomy. Adrenalectomy abolished the increase in hepatic tyrosine aminotransferase activity in obese rats although serum corticosterone was normal in these animals. Hepatic and brain tyrosine concentrations were decreased in obese rats but normalized after adrenalectomy. The results suggest that the corticosteroid-dependent increase in food and tryptophan intake may be the primary cause of the increased hepatic amino acid catabolism of obese rats.     

The induction of tyrosine aminotransferase activity and its use as an indirect assay for endotoxin in mice infected with Plasmodium vinckei petteri

Gauci R., Bennett D., Clark I. A. and Bryant C. 1982. The induction of tyrosine aminotransferase activity and its use as an indirect assay for endotoxin in mice infected with Plasmodium vinckei petteri. International Journal for Parasitology12: 279–284. It has been suggested that the malaria parasite contains an endotoxin-like substance which, by activating the reticuloendothelial system, causes much of the pathology of malaria when it is released into the host bloodstream during schizogony. In this study, an in vivo assay was developed, based on the determination of hepatic tyrosine aminotransferase activity in infected mice, to measure substances which act like endotoxin. Tyrosine aminotransferase is important in gluconeogenesis and is induced by endotoxin. Mice infected with Plasmodium vinckei petteri become sensitised to bacterial endotoxin as small amounts of endotoxin, without effect in uninfected mice, elevate tyrosine aminotransferase activity. The increase in sensitivity is gradual and progressive and is detectable by day 2 of the 9 day infection. Tyrosine aminotransferase activity is first lowered and then raised markedly during the course of the disease. A cell-free preparation of malaria parasites increased tyrosine aminotransferase activity when injected into mice sensitised with Coxiella antigen. These results are consistent with the hypothesis that parasitised red cells contain an endotoxin-like substance which directly or indirectly may be responsible for producing some of the symptoms of malaria in mice.     

The influence of reserpine on nitrogen metabolizing enzymes in chick liver

Reserpine, a Rauwolfia alkaloid, was shown to increase activity of the hepatic nitrogen metabolizing enzymes xanthine dehydrogenase, purine nucleoside phosphorylase, and tyrosine aminotransferase, when administered orally to young chicks. Using immunochemical techniques, this increase in xanthine dehydrogenase was shown to result from an enhanced de novo enzyme synthesis. The response pattern of the three enzymes to reserpine follows the same pattern to induction by high dietary protein suggesting that a common mode of action may be involved in the regulation of these enzymes. α-Adrenergic blockers, phentolamine and phenoxybenzamine, effectively prevented the increased enzyme activities caused by administration of reserpine.     

Effects on induction of tyrosine aminotransferase in fetal mouse liver in vitro of prednisolone,insulin and thyroxine

The hormonal requirements for formation of tyrosine aminotransferase (EC 2.6.1.5) in fetal mouse liver were investigated in organ culture using chemically defined medium. The hormones tested were insulin, thyroxine and prednisolone. Prednisolone alone resulted in a two-fold increase in tyrosine amino-transferase activity in explanted liver in hormone-free medium on day 6, and its effect was dose dependent, but neither insulin nor thyroxine alone induced the enzyme. Addition of prednisolone plus thyroxine and prednisolone plus insulin increased the enzyme activity 1.4- and 1.3-fold, respectively, over that of explants with prednisolone alone. These three hormones together had the greatest effect, causing induction of 1.5-fold more activity than that with prednisolone plus insulin or plus thyroxine. The three hormones were not all needed continuously during the culture period: prednisolone and insulin were required during the early part of cultivation and thyroxine during the later part. The effects of these hormones were blocked by actinomycin D or puromycin, suggesting that these hormones increase de novo synthesis of tyrosine aminotransferase. Phase-contrast microscopy showed that prednisolone stimulated liver epithelial cell outgrowth, probably acting with insulin.     

Free‐running rhythmicity in the life spans of hydranths of the marine cnidarian,Campanularia flexuosa

Abstract

The adrenergic agonists and antagonists, norepinephrine, phenylephrine, iso‐proterenol, phentolamine, and propranolol, were administered to rats in different phases of the diurnal cycle, and their effects on tyrosine aminotransferase activity were studied. All substances tested, regardless whether being typically α‐ or ß ‐adrenergic or whether being agonists or antagonists, elevated tyrosine aminotransferase during the minimum of enzyme activity. The effects at the enzyme maximum, however, were generally more or less depressive. The extent of the depressions by norepinephrine and by propranolol highly depended on the duration of treatment. The hepatic concentration of cyclic AMP did not exhibit a significant rhythmicity. The results do not favour the idea of an adrenergic control of the diurnal rhythm in tyrosine aminotransferase activity.     

Immunological study of carbon tetrachloride-mediated induction of tyrosine aminotransferase in rat liver

Administration of CCl₄ (1.0 ml/kg) to rats resulted in a rise of liver tyrosine aminotransferase (l-tyrosine:2-oxoglutarate aminotransferase, EC 2.6.1.5) activity to a maximum of about 3.6 times the normal level 6 hr later. An immunological titration study proved that the phenomenon was due to increased enzyme content. Using an isotopic-immunochemical procedure the half-life of liver tyrosine aminotransferase at 3.5 hr after CCl₄ administration was shown to be 11.9 hr in contrast to 2.1 hr in the normal liver. Immunochemical analysis revealed that enzyme synthesis was decreased by CCl₄. Thus, in the early stage of CCl₄ poisoning, enzyme synthesis proceeded at a moderate rate while degradation was markedly impaired, resulting in the rise of tyrosine aminotransferase in the liver tissue.Several hours after administration of hydrocortisone to adrenalectomized rats, induced tyrosine aminotransferase reached its peak activity and then subsided to the basal level. At any time following hydrocortisone administration, administration of CCl₄ consistently caused an elevation of the enzyme activity above the level in controls not treated with CCl₄. Actinomycin D (5 mg/kg) also increased the enzyme at an early period of induction cycle but failed to do so at a later period.The CCl₄-mediated “superinduction” of hormonally preinduced tyrosine aminotransferase, like the induction of this enzyme by CCl₄ at a basal level, was found to be caused by the differential inhibitory effect of CCl₄ on the synthesis and degradation of this enzyme.     

Correlation between tyrosine hydroxylase activity and catecholamine concentration or turnover in brain regions.

Tyrosine hydroxylase activity correlated significantly with norepinephrine concentration and turnover, when results from regions containing predominantly noradrenergic terminals were compared, and with dopamine concentration and turnover when results from regions containing predominantly dopaminergic terminals were compared. Regions containing dopamine or norepinephrine cell bodies were characterized by higher tyrosine hydroxylase activities as compared to regions containing mostly nerve terminals. Higher levels of tyrosine hydroxylase activity and transmitter turnover were observed in regions containing dopaminergic terminals than in regions containing norepinephrine terminals. These findings are consistent with the view that tyrosine hydroxylase activity is linked to rates of catecholamine utilization by neurons in the CNS.     

Relationships Between Behavioral Rhythms,Plasma Corticosterone and Hypothalamic Circadian Rhythms

Circadian rhythms in physiological processes and behaviors were compared with hypothalamic circadian rhythms in norepinephrine (NE) metabolites, adrenergic transmitter receptors, cAMP, cGMP and suprachiasmatic nucleus (SCN) arginine vasopressin (AVP) in a single population of rats under D: D conditions. Eating, drinking and locomotor activity were high during the subjective night (the time when lights were out in L: D) and low during the subjective day (the time when lights were on in L: D). Plasma corticosterone concentration rose at subjective dusk and remained high until subjective dawn. Binding to hypothalamic α₁- and β-adrenergic receptors also peaked during the subjective night. Cyclic cGMP concentration was elevated throughout the 24-hr period except for a trough at dusk, whereas DHPG concentration peaked at dawn. Arginine vasopressin levels in the suprachiasmatic nucleus peaked in the middle of the day. No rhythm was found either in binding to the α₂-adrenergic receptor, or in MHPG or cAMP concentration. Behavioral and corticosterone rhythms, therefore, are parallel to rhythms in hypothalamic α₁-and β-receptor binding and NE-release. Cyclic GMP falls only at dusk, suggesting the possibility that cGMP inhibits activity much of the day and that at dusk the inhibition of nocturnal activity is removed. SCN AVP, on the other hand, peaking at 1400 hr, may play a role in the pacemaking function of the SCN that drives these other rhythms.     

The role of monoamines in female puberty

The estradiol positive feedback mechanism appears to become mature between days 10 and 20 after birth. Rising serum prolactin levels between day 20 after birth and puberty are correlated with high hypothalamic norepinephrine turnover. High prolactin levels stimulate hypothalamic dopamine (DA) turnover, which may actively inhibit hypothalamic luteinizing hormone-releasing hormone (LHRH) release. Hypothalamic DNA receptor sensitivity is high in 10- to 20-day-old rats and gradually decreases between day 20 after birth and puberty. The reason for this desensitization may be the high hypothalamic DA turnover. This may result in a less strong inhibition of LHRH release allowing the positive feedback action of estradiol to elicit the first preovulatory luteinizing hormone (LH) surge initiating puberty.     

Studies on the mechanism of the stimulation of tyrosine aminotransferase activity in vivo by pyrimidine analogs: the role of enzyme synthesis and degradation

The pyrimidine analogs, 5-fluoroorotate and 5-azacytidine, have been shown to stimulate the basal level as well as the cortisone, tryptophan, and casein hydrolysate-induced levels of the rat liver enzyme, tyrosine aminotransferase. This stimulation was most marked in the case of dietary and hormonal induction when the analog was given 4–6 hr prior to the administration of the inducer. When tryptophan induced tyrosine aminotransferase, maximal stimulation by the analog occurred if it were given 2 hr prior to the administration of the amino acid. The optimal stimulatory dose of 5-azacytidine was 5 mg/kg body weight whereas 5-fluoroorotate gave its highest stimulation at a dose of 60 mg/kg. Of several orotic acid analogs tested, only the chloro-analog had an effect similar to the fluoro-congener.Utilizing quantitative immunochemical precipitation and pulse labeling in vivo, it was demonstrated that the administration of 5-fluoroorotate or 5-azacytidine at doses of 60 and 5 mg per kg, respectively, while causing a stimulation in the basal level of tyrosine aminotransferase, did not result in any change in the rate of enzyme synthesis. Furthermore, after cortisone induction of the enzyme, the delayed administration of these analogs caused either a further stimulation in the level of the enzyme or the maintenance of a high level while the enzyme activity decayed in animals not given the analogs. The rates of synthesis either showed no change or a decrease while the amount of enzyme was increasing. Prelabeling of the enzyme in vivo after induction with cortisone and followed by the administration of 5-fluoroorotate resulted in a marked decrease in the t$\text{1}\text{2}$ of the decay rate of the enzyme measured either by loss of radioactivity or by loss of enzyme activity. These studies suggest that these analogs act in some manner to prevent enzyme turnover by an inhibition of enzyme degradation.     

Glucocorticoid regulation of hepatic cytosolic glucocorticoid receptors in vivo and its relationship to induction of tyrosine aminotransferase

Regulation of rat hepatic cytosolic glucocorticoid receptors was studied using our newly developed exchange assay. Injecting 1 mg of dexamethasone or corticosterone into 150-250 g adrenalectomized rats caused a rapid decline in glucocorticoid receptor binding. Glucocorticoid receptor levels were depressed 80-90% in less than 15 min after hormone treatment, and remained low for about 24-48 h after glucocorticoid administration. 80-90% of glucocorticoid receptor binding was regenerated by 48 h, and complete binding was recovered by 72 h. Regenerated glucocorticoid receptor binding (48-72 h after first hormone injection) could be re-depressed by a second injection of the hormone. Similar results were obtained using normal (intact) rats. Optimum induction of tyrosine aminotransferase activity was obtained within 2 h following the first hormonal injection. Induction of tyrosine aminotransferase activity (measured 2 h after a second injection of the glucocorticoid) correlated with glucocorticoid receptor levels. Thus, 1 mg of dexamethasone or corticosterone greatly enhanced the liver tyrosine aminotransferase activity in the adrenalectomized rats (not previously hormone treated) and in adrenalectomized rats previously injected (48-72 h) with 1 mg of the glucocorticoid hormone. Enhancement of tyrosine aminotransferase activity was lowest 16-24 h after the first hormone injection (when receptor levels were extremely low). These results indicate that the induction of liver tyrosine aminotransferase activity by glucocorticoid hormones is correlated with cytosolic glucocorticoid receptor levels.