Diencephalo-telencephalic changes of tyrosine hydroxylase in rats and common frogs (Rana temporaria) after sleep deprivation

Based on sleep deprivation-produced changes of electrographic parameters of the wakefulness--sleep cycle (WSC) in rats and common frogs, dynamics of activity of tyrosine hydroxylase, the key enzyme of dopamine synthesis, was studied immunohistochemically in substantia nigra and nigrostriatal pathway in rats and in striatum, paraventricular organ, and extrahypothalamic pathways in frogs. There are revealed changes in dynamics of tyrosine hydroxylase in rats and in common frogs after the 6-h sleep deprivation and after 2 h of postdeprivation sleep. This allows determining the degree of participation of corticostriatal neuroregulatory and hypothalamo-pituitary neurosecretory systems and their role in regulation of WSC. Possible evolutionary peculiarities of morphofunctional differences in homoiothermal and poikilothermal animals are discussed.     

Effect of apomorphine on the wakefulness--sleep cycle of the common frog Rana temporaria

This work considers effects of introduction into spinal lymphatic sac of dopamine agonist--apomorphine-(APO) at doses of 0.1, 1.0, 2.0 and 4.0 mg/kg body weight on the common frog wakefulness-sleep cycle (WSC). Usually the frog WSC is represented by wakefulness and three types of passive-protective behavior: by immobility states of the type of catalepsy, catatonia, and cataplexy that are characterized by high thresholds of arousal and by different (corresponding to the name) skeletal musculature tones. These immoboloty forms are considered as homologues of mammalian stress-reaction, hibernation, and sleep. Low apomorphine doses produced in WSC a marked decrease of portion of wakefulness and an increase of the immoboloty state of the catalepsy; high doses, on the contrary, initially promoted in CNS an increase of wakefulness and the state of catalepsy by demonstrating thereby its stressogenic action; after this, in WSC these increased the portion of the sleep-like immobility state of the catalepsy type that is considered as a functional homologue of sleep of homoiotherms. In spectra of electrograms of the flog telencephalon the representation of waves of the delta diapason rose. Taking into account that the states of catalepsy and cataplexy in frogs are under control of the anterior hypothalamus, it can be suggested that manifestations of cataplexy (sleep) in frog are due to the low level of dopaminergic activity, whereas manifestations of catalepsy (the homologue of stress reaction) are due to the high dopamine content in the anterioi hypothalamic structures. Comparative analysis of changes in WSC of amphibians and mammals in response to administration of dopamine and its agonists allows thinking that the role of the dopaminergic neurotransmitter system in regulation of the vertebrate WSC is unanimous: the low level of activity of this system facilitates development of sleep (catalepsy), whereas the high level provides reaction of arousal and is actively included in the system providing stress-reaction.     

Effect of apomorphine on the wakefulness-sleep cycle of the common frog <Emphasis Type="Italic">Rana temporaria</Emphasis>

This work considers effects of introduction into spinal lymphatic sac of dopamine agonist-apomorphine (APO)-at doses of 0.1, 1.0, 2.0, and 4.0 mg/kg body weight on the common frog wakefulness-sleep cycle (WSC). Usually the frog WSC is represented by wakefulness and three types of passive-protective behavior: the immobility states of the type of catalepsy, catatonia, and cataplexy that are characterized by high thresholds of arousal and by different (corresponding to the name) skeletal muscle tones. These immobility forms are considered as homologues of mammalian stressreaction, hibernation, and sleep. Low apomorphine doses produced in WSC a marked decrease of portion of wakefulness and an increase of the immobility state of the catalepsy type; high doses, on the contrary, initially promoted in CNS an increase of wakefulness and the state of catalepsy by demonstrating thereby its stressogenic action; after this, in WSC there increased the portion of the sleep-like immobility state of the catalepsy type that is considered a functional homologue of sleep of homoiothermal animals. In spectra of electrograms of the frog telencephalon the representation of waves of the delta diapason rose. Taking into account that the states of catalepsy and cataplexy in frogs are under control of anterior hypothalamus, it can be suggested that manifestations of cataplexy (sleep) in frog are due to the low level of dopaminergic activity, whereas manifestations of catalepsy (the homologue of stress reaction) are due to the high dopamine content in the anterior hypothalamic structures. Comparative analysis of changes in WSC of amphibians and mammals in response to administration of dopamine and its agonists allows thinking that the role of the dopaminergic neurotransmitter system in regulation of the vertebrate WSC certainly consists in that the low level of activity of this system facilitates development of sleep (catalepsy), whereas the high level provides reaction of arousal and is actively included in the system providing stress-reaction.     

Morphofunctional interaction of CART peptide and dopaminergic brain neurons

In Wistar rats, after 6 h of sleep deprivation and subsequent 2 h postdeprivation sleep, we found significant changes in optical density of CART peptide in neurons of nucleus accumbens and hypothalamic nucleus arcuatus as well as in processes coming into substantia nigra from nucleus accumbens. The obtained data revealed unidirectional changes of optical density of CART and tyrosine hydroxylase in the studied structures: a decrease after sleep deprivation (p < 0.05) and, on the contrary, an increase after postdeprivation sleep (p < 0.05). Confocal laser microscopy showed morphological connections of CART and dopaminergic neurons and possible colocalization of these both substances in the same neuron at the postdeprivation sleep. In experiments in vitro, after 1 h of incubation of surviving brain sections from the substantia nigra area in the medium with CART peptide there was revealed a rise of optical density of tyrosine hydroxylase in the substantia nigra pars compacta by 55% (p < 0.05). The obtained data indicate an activating effect of CART peptide on brain dopaminergic neurons and its role as a modulator of their functional activity.     

The effect of dopaminergic nigrostriatal system on sleep deprivation in rats

The analysis of the electrophysiological features of sleep-wakefulness cycle in Wistar rats for 9h after a 6h sleep deprivation was carried out. The delay of sleep rebound (since 2.5-3 h after deprivation) was found in the form of moderate increasing of slow-wave sleep and fast-wave sleep phases. According to these sleep-wakefulness cycle changes, a quantitative immunohistochemical study of tyrosine hydroxylase: a key enzyme of dopamine synthesis--and D1 and D2 receptors in nigro-striatal projections has been performed. After sleep, an elevation of D1 receptors immunoreactivity in caudate nucleus and reduction of tyrosine hydroxylase immunoreactivity in compact part of substancia nigra was found. After postdeprivation sleep, a decrease of D1 receptors immunoreactivity and increase of D2 receptors immunoreactivity in caudate nucleus together some increase of tyrosine hydroxilase immunoreactivity in substancia nigra compacta has been observed. These data can testify about active role of dopaminergic nigrostriatal system which provide at the same time with another neurotransmitters of the central nervous system the telencephalo-diencephalic interaction in sleep-wakefulness-sleep cycle.     

Tyrosine hydroxylase expression and activity in the rat brain: differential regulation after long-term intermittent or sustained hypoxia.

Tyrosine hydroxylase, a hypoxia-regulated gene, may be involved in tissue adaptation to hypoxia. Intermittent hypoxia, a characteristic feature of sleep apnea, leads to significant memory deficits, as well as to cortex and hippocampal apoptosis that are absent after sustained hypoxia. To examine the hypothesis that sustained and intermittent hypoxia induce different catecholaminergic responses, changes in tyrosine hydroxylase mRNA, protein expression, and activity were compared in various brain regions of male rats exposed for 6 h, 1 day, 3 days, and 7 days to sustained hypoxia (10% O(2)), intermittent hypoxia (alternating room air and 10% O(2)), or normoxia. Tyrosine hydroxylase activity, measured at 7 days, increased in the cortex as follows: sustained > intermittent > normoxia. Furthermore, activity decreased in the brain stem and was unchanged in other brain regions of sustained hypoxia-exposed rats, as well as in all regions from animals exposed to intermittent hypoxia, suggesting stimulus-specific and heterotopic catecholamine regulation. In the cortex, tyrosine hydroxylase mRNA expression was increased, whereas protein expression remained unchanged. In addition, significant differences in the time course of cortical Ser(40) tyrosine hydroxylase phosphorylation were present in the cortex, suggesting that intermittent and sustained hypoxia-induced enzymatic activity differences are related to different phosphorylation patterns. We conclude that long-term hypoxia induces site-specific changes in tyrosine hydroxylase activity and that intermittent hypoxia elicits reduced tyrosine hydroxylase recruitment and phosphorylation compared with sustained hypoxia. Such changes may not only account for differences in enzyme activity but also suggest that, with differential regional brain susceptibility to hypoxia, recruitment of different mechanisms in response to hypoxia will elicit region-specific modulation of catecholamine response.     

Participation of agouti-related peptide in the regulation of the wakefulness-sleep cycle

Agouti-related peptide is expressed in the hypothalamic neurons in humans and animals. Immunohistochemical studies in Wistar rats shows significant changes in the optical density of agouti-related peptide in the neurons of the arcuate hypothalamic nucleus, as well as in their processes in the hypothalamus and nucleus accumbens after 6 h of sleep deprivation (an increase) and after 2 h of post-deprivative sleep (a decrease). Comparison of these findings with the earlier results shows the opposite trends in the changes in the optical density of agouti-related peptide and the speed of the limiting enzyme of dopamine synthesis, tyrosine hydroxylase, in the hypothalamus and in the striatonigral system. An increase in the agouti-related peptide level was accompanied by a decrease in tyrosine hydroxylase, while a decrease in agouti-related peptide, on the contrary, was accompanied by an increase in the tyrosine hydroxylase activity. Our data show the role played by agouti-related peptide as a modulator of the functional activity of the dopaminergic brain neurons. The interrelation between various functions of the body, such as food behavior, sleep, and stress, is considered to be mediated by the participation of the same neurotransmitter systems in their regulation.     

Morphofunctional Analysis of Effect of Short-Term Sleep Deprivation on the Wakefulness–Sleep Cycle in Young and Adult Rats

The work presents comparative data on changes of neurophysiological, time characteristics of the wakefulness–sleep cycle (WSC) and morphofunctional state of neurosecretory cells in supraoptic nucleus of the hypothalamus, which develop under influence of a 6-h long sleep deprivation in adult and one-month old rats. It is shown that the rebound of sleep develops in adult animals with a delay, after the 3rd hour and is characterized by a moderate increase of portions of slow-wave (SSP) and fast-wave (FSP) sleep phases in WSC and by a decrease of the wakefulness portion. Morphological analysis of the hypothalamus nonapeptidergic system has revealed a rise of content of neurosecretory material in fibers of supraoptic nucleus cells an in area of supraoptic-pituitary tract, as well as marked hyperemia that indicates activation of processes of secretion of neurohormones into the general blood flow; these reactions are similar to reactions of this system to stress. In rat pups the sleep rebound develops in 0.5 h after the end of the deprivation procedure and is characterized by more pronounced, statistically significant changes in WSC. Individual WSC become very short and almost all of them are completed with episodes of FSP. A statistically significant rise of power of the -wave band in electrogram spectra of hippocampus and somatosensory cortex in SSP, whereas peak of the activity in FSP is shifted to -waves. Ratios of SSP and FSP to wakefulness in individual WSC in mature animals increase after the deprivation 1.53 and 1.85 times, while they are elevated in one-month old animals 5.25 and 6.75 times, respectively. The obtained morphofunctional data allow believing that deprivation is the stress factor of low intensity for adult animals, whereas it may be considered as the stress action of intermediate and even high intensity for rat pups, which changes essentially the interrelations in WSC. Participation of central mechanisms of regulation of sleep and vigilance, which provide processes of compensation of damaging action of deprivation on WSC in the maturing animals, is discussed.     

Decreased tyrosine hydroxylase activity in the adrenals of spontaneously hypertensive rats

Higher activity of the peripheral sympathetic nervous system, accompanied by higher tyrosine hydroxylase activity is frequently and consistently reported in human essential hypertension as well as in animal models of hypertension. However, results obtained in the adrenals, particularly in young animals before the development of hypertension, are scarce and controversial. In the present study tyrosine hydroxylase activity and catecholamine content in the adrenals of spontaneously hypertensive rats and of age-matched control Wistar Kyoto rats were evaluated before, during and after the development of hypertension (5, 12 and 22-week-old animals). Results show that both tyrosine hydroxylase activity and total amine content in the adrenals of spontaneously hypertensive rats were significantly reduced (35% reduction) at all studied ages. Determination of the kinetic parameters for tyrosine hydroxylase in the adrenals of 5 week-old spontaneously hypertensive rats revealed a 38% reduction in V(max) values (13.4 versus 21.3 nmol L-DOPA/mg prot/h in age-matched controls) accompanied by lower levels of expression of both tyrosine hydroxylase total protein and phosphoSer40 observed by Western-Blot. In contrast, norepinephrine content in both plasma and tail artery were significantly higher in the spontaneously hypertensive strain. In conclusion, contrary to the higher peripheral sympathetic activity, tyrosine hydroxylase activity and catecholamine content in the adrenals of spontaneously hypertensive rats are markedly reduced before, during and after the development of hypertension. End product, long-term feedback inhibition by the high norepinephrine plasma levels could be responsible for this reduction, establishing yet another regulatory mechanism of tyrosine hydroxylase operating in adrenal cromaffin cells.     

The effects of reserpine and haloperidol on tyrosine hydroxylase activity in the brains of aged rats

Many neurotransmitter systems appear to be altered with aging. The effects of aging on the regulation of tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of catecholamines in the brain has been examined. The endogenous basal activity of tyrosine hydroxylase was lower in the hypothalamus of 24 month old Fisher 344 rats than in the hypothalamus of 3 month old or 6 month old animals. There was no difference in the basal activity of tyrosine hydroxylase in the locus ceruleus, frontal cortex, hippocampus, substantia nigra, or the striatum of rats of ages 3 months, 6 months and 24 months. Tyrosine hydroxylase activity was increased in the striatum of 3 month old (60%) and 6 month old (28%) rats after treatment with haloperidol or reserpine, whereas no change in enzyme activity followed administration of these drugs to 24 month old animals. In conclusion, increases in tyrosine hydroxylase activity in the brain that normally occur in the striatum of 3 month old rats after haloperidol or reserpine treatment are significantly decreased in 6 month old rats and not apparent in 24 month old rats.     

Deficits in the Activation and Phosphorylation of Hippocampal Tyrosine Hydroxylase in the Aged Fischer 344 Rat Following Intraventricular Administration of 6-Hydroxydopamine

Abstract: Tyrosine hydroxylase activity was measured under optimal and suboptimal assay conditions in hippocampal extracts from young (2 month), mature (12 month), and old (24 month) Fischer 344 male rats 72 h after the infusion of 200 µg of the neurotoxin 6-hydroxydopamine or vehicle into the lateral ventricle. The lesion resulted in a 45–55% decrease of tyrosine hydroxylase activity measured under optimal conditions (pH 6.1, 3.0 m M 6-methyl-5,6,7,8-tetrahydropterin) and an ∼35% decrease in the relative concentration of immunoreactive tyrosine hydroxylase. When measured under suboptimal conditions (pH 6.6, 0.7 m M 6-methyl-5,6,7,8-tetrahydropterin), tyrosine hydroxylase activity in 2- and 12-month-old lesioned animals was twice that measured in vehicle-treated animals. However, in the old lesioned animals, tyrosine hydroxylase activity measured under suboptimal conditions was not different from that measured in age-matched vehicle-treated animals. Isoforms of tyrosine hydroxylase were identified on immunoblots after two-dimensional gel electrophoresis using enhanced chemiluminescence. The relative proportion of lower pl isoforms of tyrosine hydroxylase in the 2-month-old lesioned animals was greater than that observed in vehicle-treated controls. In contrast, no difference was seen in the relative proportion of tyrosine hydroxylase isoforms in the 24-month-old lesioned versus control animals. These data indicate that the ability of locus ceruleus neurons to rapidly respond to and compensate for insult is attenuated in 24-month-old Fischer 344 rats due to a deficit in stimulus-evoked enzyme phosphorylation.     

Adrenal tyrosine hydroxylase activation in the developing rat: influence of the thyroid status

Adrenal tyrosine hydroxylase activation was elicited in developing control, hypo- and hyperthyroid rats by insulin-hypoglycaemia. Rats were deeply anaesthetized with chloroform at a low concentration, since intrinsic tyrosine hydroxylase activation was very low with this technique, as compared to Ketamine injection or chloroform at a high concentration. The study of time-course of tyrosine hydroxylase activation showed that the maximum value was observed 2 h after insulin administration. In control animals, tyrosine hydroxylase activation increased between 4 and 20 days, and then decreased. Hypothyroidism is associated with a decreased tyrosine hydroxylase activation between 4 and 50 days, as compared to controls and hyperthyroidism with an increased activation between 6 and 30 days. While tyrosine hydroxylase from saline-treated rats exhibits two different forms (with two apparent Km values for the cofactor), enzyme from insulin-treated animals was present in a single form with a Km corresponding to the low Km value of the saline-injected rats. At 6 and 14 days, hypothyroidism increases tyrosine hydroxylase Km values as compared to euthyroid animals.     

Tyrosine hydroxylase activity in rat brain following "REM" sleep deprivation

I n R ecent years biogenic amines have been implicated in the control mechanism for induction and maintenance of sleep processes (J ouvet , 1969). Investigators have looked for changes in the rate of synthesis of cerebral norepinephrine from [³H]tyrosine after REM sleep deprivation and reported increased rates of synthesis during REM sleep deprivation (M ark , H einer , M andel and G odin , 1969) and REM sleep rebound following 91 h of deprivation (P ujol , M ouret and G lowinski , 1968). Because tyrosine is thought to be the rate-limiting enzyme (U denfriend , 1966) in the synthetic pathways for norepinephrine and since the above-mentioned studies are suggestive of changes in the activity of the enzyme, we decided to measure tyrosine hydroxylase activity following REM sleep deprivation.     

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.     

Inhibition of phenylalanine hydroxylase activity by alpha-methyl tyrosine, a potent inhibitor of tyrosine hydroxylase.

Inhibitors of phenylalanine hydroxylase and tyrosine hydroxylase were used in the assay of phenylalanine hydroxylase in liver and kidney of rats and mice. Parachlorophenylalanine (PCPA), methyl tyrosine methyl ester and dimethyl tyrosine methyl ester showed 5–15% inhibition while α-methyl tyrosine seemed to inhibit phenylalanine hydroxylase to the extent of 95–98% at concentrations of 5 × 10 ⁻⁵M –1 × 10 ⁻⁴M. After a phenylketonuric diet (0.12% PCPA + 3% excess phenylalanine), the liver showed 60% phenylalanine hydroxylase activity and kidney 82% that present in pair-fed normals. Hepatic activity was normal after 8 days refeeding normal diet whereas kidney showed 63% of normal activity. The PCPA-fed animals showed 34% in liver and 38% in kidney as compared to normals; in both cases normal activity was noticed after refeeding. The phenylalanine-fed animals showed activity similar to that seen in phenylketonuric animals. The temporary inducement of phenylketonuria in these animals may be due to a slight change in conformation of the phenylalanine hydroxylase molecule; once the normal diet is resumed, the enzyme reverts back to its active form. This paper also suggests that α-methyl tyrosine when fed in conjunction with the phenylketonuric diet may suppress phenylalanine hydroxylase activity completely in the experimental animals thus yielding normal tyrosine levels as seen in human phenylketonurics.     

Tyrosine Hydroxylase Content of Residual Striatal Dopamine Nerve Terminals Following 6-Hydroxydopamine Administration: A Flow Cytometric Study

Fluorescence-activated cell sorting based on immunolabeling with a monoclonal antibody to tyrosine hydroxylase and a fluorescein-conjugated secondary antibody was used to identify striatal synaptosomes derived from nigrostriatal dopamine nerve terminals. The amount of tyrosine hydroxylase immunoreactivity in dopaminergic striatal synaptosomes prepared from control rats was compared to the amount in dopaminergic synaptosomes prepared from rats that had received intraventricular injections of 6-hydroxydopamine. Although the absolute number of dopaminergic synaptosomes was decreased in lesioned animals, those residual dopamine terminals present contained more tyrosine hydroxylase than did dopamine terminals from control rats. Both the decrease in the absolute number of dopamine terminals and the increase in tyrosine hydroxylase immunoreactivity in residual terminals were proportional to the extent of the lesion, as determined by measurement of striatal dopamine levels. These results suggest that an increase in the amount of tyrosine hydroxylase protein in residual terminals may represent one compensatory mechanism by which residual dopamine neurons maintain normal striatal function after partial destruction of the nigrostriatal dopamine projection.     

Role of endothelin receptor type A on catecholamine regulation in the olfactory bulb of DOCA-salt hypertensive rats: Hemodynamic implications

Increasing evidence shows that the olfactory bulb is involved in blood pressure regulation in health and disease. Enhanced noradrenergic transmission in the olfactory bulb was reported in hypertension. Given that endothelins modulate catecholamines and are involved in the pathogenesis of hypertension, in the present study we sought to establish the role of the endothelin receptor type A on tyrosine hydroxylase, the rate limiting enzyme in catecholamine biosynthesis, in the olfactory bulb of DOCA-salt hypertensive rats.Sprague-Dawley male rats, randomly divided into Control and DOCA-Salt hypertensive groups, were used to assess endothelin receptors by Western blot and confocal microscopy, and their co-localization with tyrosine hydroxylase in the olfactory bulb. Blood pressure and heart rate as well as tyrosine hydroxylase expression and activity were assessed following BQ610 (ET_A antagonist) applied to the brain. DOCA-Salt hypertensive rats showed enhanced ET_A and decreased ET_B expression. ET_A co-localized with tyrosine hydroxylase positive neurons. Acute ET_A blockade reduced blood pressure and heart rate and decreased the expression of total tyrosine hydroxylase and its phosphorylated forms. Furthermore, it also diminished mRNA tyrosine hydroxylase expression and accelerated the enzyme degradation through the proteasome pathway as shown by pretreatment with MG132, (20s proteasome inhibitor) intracerebroventricularly applied.Present findings support that the brain endothelinergic system plays a major role through ET_A activation in the increase of catecholaminergic activity in the olfactory bulb of DOCA-Salt hypertensive rats. They provide rationale evidence that this telencephalic structure contributes in a direct or indirect way to the hemodynamic regulation in salt dependent hypertension.     

Functional cerebral activity of an analogue of serotonin formed in situ

Reduction of the serotonin content of the brain of rats (specifically in the medial raphe nucleus) by various means results in spontaneous increase of adrenal tyrosine hydroxylase activity. This neurally mediated induction is attenuated by appropriate administration of the serotonin precursor 5-hydroxytryptophan to the animals, along with carbidopa (Quik and Sourkes, J. Neurochem.28, 137, 1977). In the present work adrenal tyrosine hydroxylase was induced by giving rats either the neurotoxin 5,7-dihydroxytryptamine (injected into the cerebral ventricles) or the monoamine depletor reserpine (given intraperitoneally). Other rats received alpha-methyltryptophan. This amino acid causes a marked decline of the serotonin content of the brain, but gives rise to relatively large amounts of alpha-methylserotonin in that organ (Roberge et al., Neuropharmacology11, 197, 1972). Alpha-methyltryptophan had no effect on adrenal tyrosine hydroxylase activity but, when it was given with dihydroxytryptamine or reserpine, it prevented the induction of adrenal tyrosine hydroxylase that otherwise occurred. The results are discussed in relation to the effect of alpha-methyltryptophan on the content of indoles (tryptophan, serotonin, 5-hydroxyindoleacetic acid, alpha-methyltryptophan, alpha-methylserotonin) in the plasma and brain, as detected by HPLC. It is concluded that alpha-methylserotonin can functionally replace cerebral serotonin, at least in relation to the transneuronal regulation of adrenal tyrosine hydroxylase activity.     

Effect of tyrosine administration on dopa accumulation in light- and dark-adapted retinas from normal and diabetic rats

The interaction of tyrosine concentration and lighting on in vivo dihydroxyphenylalanine (dopa) accumulation rate was studied in retinas of normal and diabetic rats. In both groups of rats, dopa accumulation and in vitro hydroxylase activity were higher in retinas exposed to light than in those adapted to darkness. In light-adapted diabetic rats, though, retinal tyrosine level, dopa accumulation, and in vitro tyrosine hydroxylase activity were all below normal. In both normal and diabetic rats exposed to light, tyrosine injection raised retinal tyrosine concentrations and stimulated dopa accumulation. Injection of tyrosine into dark-adapted rats raised retinal tyrosine level but did not enhance dopa accumulation. Together, these results suggest that in vivo retinal amacrine cells will vary their dopa accumulation rate as a function of substrate supply, but only in the light, when tyrosine hydroxylase is activated. They further indicate that dopa accumulation rate remains sensitive to tyrosine supply in the light-activated diabetic retina.     

Liver phenylalanine hydroxylase activity in relation to blood concentrations of tyrosine and phenylalanine in the rat

The plasma concentration of phenylalanine and tyrosine decreases in normal rats during the first few postnatal days; subsequently, the concentration of phenylalanine remains more or less constant, whereas that of tyrosine exhibits a high peak on day 13. The basal concentrations of the two amino acids were not altered by injections of thyroxine or cortisol, except in 13-day-old rats, when an injection of cortisol decreased the concentration of tyrosine. In young rats (13-15 days old), treatment with cortisol increased the activity of phenylalanine hydroxylase in the liver (measured in vitro) and accelerated the metabolism of administered phenylalanine: the rate constant of the disappearance of phenylalanine from plasma and the initial increase in tyrosine in plasma correlated quantitatively with the activity of phenylalanine hydroxylase in the liver. In adult rats, the inhibition of this enzyme (attested by assay in vitro) by p-chlorophenylalanine resulted in a proportionate decrease in tyrosine formation from an injection of phenylalanine. However, the quantitative relationship between liver phenylalanine hydroxylase activity and phenylalanine metabolism within the group of young rats was different from that observed among adult rats.