Rate Of Tyrosine Hydroxylation Research Articles

Opiate-receptor mediated changes in monoamine synthesis in rat brain.

Abstract The effects of morphine, β-endorphin, naloxone and naltrexone on the rate of tyrosine and tryptophan hydroxylation were investigated in vivo by measuring the accumulation of dopa and 5-hydroxytryptophan (5-HTP) in different brain regions of rats after inhibition of the aromatic L-amino acid decarboxylase. The cerebral concentrations of tyrosine and tryptophan were also measured. Morphine (3–30 mg kg−1) increased the accumulation of dopa dose-dependently (25–50%) in the dopamine-rich areas (limbic forebrain and corpus striatum). In the noradrenaline-predominant parts of the brain (containing hemispheres, diencephalon and lower brain stem) only the highest dose of morphine (30 mg kg−1) significantly increased dopa formation (47%). Similarly to morphine, intracerebroventricularly injected β-endorphin (5–10 βg per rat) increased the formation of dopa. This increase was doubled in limbic forebrain, corpus striatum and cerebral hemispheres. Doses of 10 to 20 μg of β-endorphin were needed to increase dopa accumulation in the diencephalon and the lower brain stem. Naloxone antagonized the β-endorphin-induced increases in dopa. But naloxone and naltrexone (10–100 mg kg−1) decreased the dopa formation in the dopamine-rich areas (about 20–25 %) but not in the noradrenaline-predominant areas. Morphine (30 mg kg−1) and β-endorphin (5 μg per rat) increased the accumulation of 5-HTP whereas naloxone and naltrexone (10 mg kg−1) tended to decrease its formation. Morphine and β-endorphine increased the concentrations of tyrosine and tryptophan, and naloxone decreased the cerebral tryptophan concentration. These results show that the effects of a narcotic agonist (morphine) and of pure narcotic antagonists (naloxone and naltrexone) on the synthesis of dopamine and 5-HT are opposite to each other. Furthermore, the effects of β-endorphine on brain monoamine synthesis are remarkably similar to those of morphine. Thus, it is probable that opiate receptors and their endogenous ligands are involved in the regulation of dopamine and 5-HT synthesis.

In vivo estimation of tyrosine hydroxylation in the dopaminergic terminals of the rat neostriatum.

Abstract An approach has been developed to distinguish dopamine synthesis from its release and utilization in dopamine terminals of the rat neostriatum (NCP). It enables a direct in vivo estimation of tyrosine hydroxylation, a rate limiting step in dopamine synthesis. Locally injected l-3,5[3H]tyrosine is converted to [3H]dopamine in the NCP. The conversion of the labelled tyrosine into [3H]dopa leads to local accumulation of [3H]water (3H-H2O). The initial accumulation (3 min) of the 3H-H2O levels in tissues gives a good reflection of the rate of tyrosine hydroxylation occurring in the dopamine terminals of the nigro-striatal pathway. In effect, 3H-H2O formation (1) disappeared after elective degeneration of the nigro-neostriatal dopamine neurons, (2) was inhibited by α-methyl-p-tyrosine, (3) was not affected by intraperitoneal injection of urea, which is known to reduce brain water content, (4) moreover, 3H-H2O accumulation was enhanced by various neuroleptic drugs and inhibited by (+)-amphetamine. Finally, Ro 4–4602 an inhibitor of dopa-decarboxylase reduced 3H-H2O formation 30 min after its injection.

Improving Effects of Long-Term Growth Hormone Treatment on Monoaminergic Neurotransmission and Related Behavioral Tests in Aged Rats

An age-related decline in cognitive functions and physical performance has been associated with reductions in growth hormone (GH) secretion and brain neurotransmitter function. In vivo experiments were performed to study the long-term effects of exogenously administered GH on the central monoaminergic neurotransmitters serotonin, dopamine, and noradrenaline and behavioral tests in old Wistar rats. The accumulation of 5-hydroxytryptophan (5-HTP) and L-3,4-dihydroxyphenylalanine (DOPA) after decarboxylase inhibition was used as a measure of the rate of tryptophan and tyrosine hydroxylation in vivo. Also, the content of the neurotransmitters serotonin, dopamine, and noradrenaline and some metabolites was measured by high-pressure liquid chromatography (HPLC) in the hippocampus and striatum, brain regions involved in adult memory processing and motor coordination. The age-related decline observed in all the neurochemical parameters in control rats was significantly reversed after repeated subcutaneous administration of GH (2 mg/kg per day, 4 weeks). Thus, GH treatment exerted a long-term effect on serotonin, dopamine, and noradrenaline neurotransmission by enhancing neurotransmitter synthesis and metabolism in aged rats. The results obtained after examining working memory tasks in the eight-radial maze and motor ability in the Rotarod treadmill in aged rats were consistent with these neurochemical data; both tests were significantly improved after chronic GH treatment. Overall, these in vivo findings suggest that the positive effects induced by GH on serotonin, dopamine, and noradrenaline neurotransmitters might explain, at least in part, the effects of chronic GH treatment in improving cognitive and motor ability in aged rats, and could aid in preventing or delaying deficits in monoamines associated with learning or motor disabilities.

Chronic melatonin treatment and its precursor L-tryptophan improve the monoaminergic neurotransmission and related behavior in the aged rat brain

Melatonin has an important role in the aging process as a potential drug to relieve oxidative damage, a likely cause of age-associated brain dysfunction. As age advances, the nocturnal production of melatonin decreases potentially causing physiological alterations. The present experiments were performed to study in vivo the effects of exogenously administered melatonin chronically on monoaminergic central neurotransmitters serotonin (5-HT), dopamine (DA) and norepinephrine (NE) and behavioral tests in old rats. The accumulation of 5-hydroxy-tryptophan (5-HTP) and L-3,4-dihydroxyphenylalanine (DOPA) after decarboxylase inhibition was used as a measure of the rate of tryptophan and tyrosine hydroxylation in rat brain. Also neurotransmitters 5-HT, DA and NE and some metabolites were quantified by HPLC. In control rats, an age-related decline was observed in neurochemical parameters. However, chronic administration of melatonin (1 mg/kg/day, diluted in drinking water, 4 wk) significantly reversed the age-induced deficits in all the monoaminergic neurotransmitters studied. Also, neurochemical parameters were analyzed after administration of melatonin biosynthesis precursor L-tryptophan (240 mg/kg/day, i.p., at night for 4 wk) revealing similar improvement effects to those induced by melatonin. Behavioral data corresponded well with the neurochemical findings since spatial memory test in radial-maze and motor coordination in rota-rod were significantly improved after chronic melatonin treatment. In conclusion, these in vivo findings suggest that melatonin and L-tryptophan treatments exert a long-term effect on the 5-HT, DA and NE neurotransmission by enhancing monoamine synthesis in aged rats, which might improve the age-dependent deficits in cognition and motor coordination.

Synthesis, protein levels, activity, and phosphorylation state of tyrosine hydroxylase in mesoaccumbens and nigrostriatal dopamine pathways of chronically food-restricted rats

Chronic food restriction (FR) enhances the rewarding and motor-activating effects of abused drugs, and is accompanied by changes in dopamine (DA) dynamics and increased D-1 DA receptor-mediated cell signaling and transcriptional responses in nucleus accumbens (NAc). However, little is known about effects of FR on DA synthetic activity in the mesoaccumbens and nigrostriatal pathways. In Experiment 1 of the present study, tyrosine hydroxylase (TH) gene expression was measured in ventral tegmental area and substantia nigra, using real-time RT-PCR and in situ hybridization; no differences were observed between FR and ad libitum fed (AL) rats. In Experiment 2, TH protein levels, determined by Western blot, were found to be elevated in NAc and caudate–putamen (CPu) of FR relative to AL rats. In the absence of increased transcription, this may reflect a slowing of TH degradation. In Experiments 3 and 4, DA synthetic activity was assessed by Western blot measurement of TH phosphorylation at Ser40, and HPLC measurement of in vivo tyrosine hydroxylation rate, as reflected by DOPA accumulation following administration of a decarboxylase inhibitor (NSD-1015; 100 mg/kg, i.p.). Basal phospho-(Ser40)-TH levels did not differ between groups but DOPA accumulation was decreased by FR. Decreased DOPA synthesis, despite increased levels of TH protein, may reflect the inhibitory effect of increased DA binding to TH protein or decreased concentrations of cofactor tetrahydrobiopterin. Finally, in response to d-amphetamine (0.5 and 5.0 mg/kg, i.p.), phospho-(Ser40)-TH was selectively decreased in NAc of FR rats. This suggests increased feedback inhibition of DA synthesis—a possible consequence of postsynaptic receptor hypersensitivity, or increased extracellular DA concentration. These results indicate that FR increases TH protein levels, but may decrease the capacity for DA synthesis by decreasing TH activity. According to this scheme, the previously observed upregulation of striatal cell signaling and transcriptional responses to DA receptor agonist administration may include compensatory neuroadaptations.

Differential effects of acute cannabinoid drug treatment, mediated by CB1 receptors, on the in vivo activity of tyrosine and tryptophan hydroxylase in the rat brain.

The acute effects of cannabinoid drugs on the synthesis of noradrenaline, dopamine, and serotonin (5-HT) were assessed, simultaneously, using the accumulation of 3,4-dihydroxyphenylalanine (dopa) and 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition as a measure of the rate of tyrosine and tryptophan hydroxylation in the rat brain in vivo. Treatment (1 h, i.p.) with Delta(9)-tetrahydrocannabinol (THC, 5, 10, and 20 mg/kg) and the cannabinoid receptor agonist WIN 55,212-2 (WIN, 2 and 4 mg/kg) increased dopa/noradrenaline synthesis (40-70%) in various brain regions enriched in this neurotransmitter (e.g., cerebral cortex, hippocampus, hypothalamus). In most brain regions, the content of noradrenaline was reduced by cannabinoid drugs (27-66%). For the effects of WIN (2 and 4 mg/kg), an inverse correlation ( r=-0.61, P=0.036) was obtained between the accumulation of dopa and the content of noradrenaline in the hypothalamus. The stimulatory effect on dopa accumulation induced by THC was antagonized by the selective CB(1) receptor antagonists SR141716A and AM 281 (10 mg/kg). In contrast, THC and WIN decreased the synthesis of dopa/dopamine in the corpus striatum (16-37%) and that of 5-HTP/5-HT (20-35%) in brain regions enriched in 5-HT (e.g., cerebral cortex and hippocampus). These inhibitory effects of THC and WIN were also antagonized by AM 281 and/or SR141716A. THC did not alter the content of 5-HT or dopamine in the brain. The effects may be related to the activation of presynaptic inhibitory cannabinoid CB(1) receptors located on the neurones themselves (serotonin) and on facilitatory (dopamine) and inhibitory interneurones (noradrenaline).

Supersensitivity of 5-HT1A autoreceptors and alpha2-adrenoceptors regulating monoamine synthesis in the brain of morphine-dependent rats.

The sensitivity of 5-HT1A serotonin receptors and alpha2-adrenoceptors (autoreceptors and heteroreceptors) modulating brain monoamine synthesis was investigated in rats during morphine treatment and after naloxone-precipitated withdrawal. The accumulation of 5-hydroxytryptophan (5-HTP) and 3,4-dihydroxyphenylalanine (DOPA) after decarboxylase inhibition was used as a measure of the rate of tryptophan and tyrosine hydroxylation in vivo. Acute morphine (3-100 mg/kg, 1 h) increased the synthesis of 5-HTP/5-HT in various brain regions (15%-35%) and that of DOPA/dopamine (DA) in striatum (28%-63%), but decreased the synthesis of DOPA/noradrenaline (NA) in hippocampus and cortex (20%-33%). Naloxone (2-60 mg/kg, 1 h) did not alter the synthesis of 5-HTP or DOPA in brain. Tolerance to the inhibitory effect of morphine on DOPA/NA synthesis and a sensitization to its stimulatory effects on DOPA/DA and 5-HTP/5-HT synthesis were observed after chronic morphine and/or in morphine-withdrawn rats. In morphine-dependent rats (tolerant and withdrawn states) the inhibitory effects of the 5-HT1A agonists 8-OH-DPAT and buspirone (0.1 mg/kg, 1 h), and that of the alpha2-adrenoceptor agonist clonidine (0.1 mg/kg, 1 h), on the synthesis of 5-HTP/5-HT were potentiated (25%-50%). Moreover, the effect of 8-OH-DPAT was antagonized by WAY 100135, a selective 5-HT1A antagonist. In morphine-dependent rats (tolerant state), the inhibitory effects of clonidine on the synthesis of DOPA/NA (hippocampus, hypothalamus) and DOPA/DA (striatum) also were potentiated (35%-55%). In summary, we conclude that morphine addiction is associated with supersensitivity of 5-HT1A serotonin receptors and alpha2-adrenoceptors (autoreceptors and heteroreceptors) that modulate the synthesis of monoamines in brain.

Activation and desensitization by cyclic antidepressant drugs of alpha2-autoreceptors, alpha2-heteroreceptors and 5-HT1A-autoreceptors regulating monamine synthesis in the rat brain in vivo.

The effects of antidepressant drugs on the synthesis of noradrenaline and serotonin (5-HT) were assessed using the accumulation of 3,4-dihydroxyphenylalanine (dopa) and 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition as a measure of the rate of tyrosine and tryptophan hydroxylation in the rat brain in vivo. Three inhibitory synthesis-modulating receptors were investigated simultaneously: the alpha2C-autoreceptor modulating dopa/noradrenaline synthesis, and the alpha2A-heteroreceptor and 5-HT1A-autoreceptor modulating 5-HTP/5-HT synthesis. Acute treatment (2 h, i.p.) with desipramine (1-10 mg/kg), protriptyline (0.3-10 mg/kg) and nisoxetine (3-10 mg/kg), selective NA reuptake blockers, dose-dependently decreased dopa synthesis in cortex (15%-40%) and hippocampus (20%-53%). Fluoxetine (1-10 mg/kg) and zimelidine (1-10 mg/kg), selective 5-HT reuptake blockers, did not alter dopa synthesis. Fluoxetine and zimelidine dose-dependently decreased 5-HTP synthesis in cortex (14%-43%) and hippocampus (27%-54%). Desipramine and protryptyline did not alter 5-HTP synthesis in cortex but in hippocampus it was decreased (36%). Repeated desipramine (10 mg/kg for 1-21 days) or fluoxetine (3 mg/kg for 3-21 days) treatment resulted in a time-dependent loss in their ability to decrease dopa or 5-HTP synthesis. Desipramine (1-21 days) did not alter 5-HTP synthesis in cortex, but in hippocampus it was decreased (21%-37%, days 1-14) followed by recovery to control values (day 21). Fluoxetine (3-21 days) did not alter brain dopa synthesis. To further assess the desensitization of alpha2C-autoreceptors, alpha2A-heteroreceptors and 5-HT1A autoreceptors regulating the synthesis of dopa/NA or 5-HTP/5-HT after chronic desipramine and fluoxetine, the effects of clonidine (agonist at alpha2-auto/heteroreceptors) and 8-OH-DPAT (agonist at 5-HT1A-autoreceptors) were tested. In saline-treated rats, clonidine (1 mg/kg, 1 h) decreased dopa and 5-HTP synthesis in cortex (58% and 54%) and hippocampus (54% and 42%). In desipramine-treated rats (10 mg/kg, 21 days), but not in fluoxetine-treated ones (3 mg/kg, 14 days), the effect of clonidine was attenuated in cortex (12% and 18%) and only for dopa synthesis in hippocampus (31%). In saline-treated rats, 8-OH-DPAT (1 mg/kg, 1 h) decreased 5-HTP synthesis in cortex (63%) and hippocampus (75%). In fluoxetine-treated rats, but not in desipramine-treated ones, this inhibitory effect was markedly attenuated in cortex (26%) and hippocampus (9%). These findings indicate that acute treatment with cyclic antidepressant drugs results in activation of inhibitory alpha2C-autoreceptors, alpha2A-heteroreceptors and/or 5-HT1A-autoreceptors regulating the synthesis of dopa/NA and/or 5-HTP/5-HT in brain, whereas chronic treatment with these drugs is followed by desensitization of these presynaptic receptors.

Effects of imidazoline receptor ligands on monoamine synthesis in the rat brain in vivo.

This study was designed to assess the effects of imidazoline drugs on putative presynaptic imidazoline receptors modulating brain monoamine synthesis in vivo. The accumulation of 3,4-dihydroxyphenylalanine (dopa) and 5-hydroxytryptophan (5-HTP) after decarboxylase inhibition was used as a measure of the rate of tyrosine and tryptophan hydroxylation in various brain regions of naive rats and after irreversible alpha2-adrenoceptor inactivation with EEDQ (1.6 mg/kg, i.p., 6 h). Clonidine (1-3 mg/kg), moxonidine (1-10 mg/kg) and rilmenidine (10 mg/kg) (mixed I1/alpha2 agonists) decreased dopa and 5-HTP synthesis in the cerebral cortex (14%-81%), hippocampus (27%-84%) and/or striatum (29%-56%), but these inhibitory effects were abolished in N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ)-treated rats. Similarly, the stimulatory effect of efaroxan (mixed I1/alpha2 antagonist; 10 mg/kg) on dopa synthesis in the cortex (77%) and hippocampus (57%) was abolished by EEDQ. The selective I1-ligand 2-endo-amino-3-exoisopropylbicyclo-heptane (AGN-192403; 5-10 mg/kg) did not modify dopa or 5-HTP synthesis in any brain region in naive or EEDQ-treated rats. Idazoxan (mixed I2/alpha2 antagonist; 20 mg/kg) increased dopa synthesis in the cortex (111%) and hippocampus (87%), but the stimulatory effects were abolished by EEDQ. Moreover, idazoxan and efaroxan decreased 5-HTP synthesis in the cortex (12%-34%) and hippocampus (30%-34%) in a manner sensitive to blockade by the 5-HT1A receptor antagonist WAY 100135. The selective I2-igands 2-(2-benzofuranyl)-2-imidazoline (2-BFI; 20 mg/kg) and 2-styryl-2-imidazoline (LSL 61122; 10 mg/kg) did not alter the synthesis of dopa or 5-HTP in the cortex or hippocampus. In striatum, 2-BFI (1-20 mg/kg) dose-dependently decreased dopa synthesis (ED50: 5.9 mg/kg), reduced dopamine levels (6%-36%) and increased those of its metabolites DOPAC (15%-95%) and HVA (24%-74%). The inhibitory effect of 2-BFI on dopa/dopamine synthesis in striatum remained unchanged after alkylation of imidazoline receptors with isothiocyanatobenzyl imidazoline (IBI; 60 mg/kg, 6 h) or blockade of these receptors with 2-(2-ethyl 2,3-dihydro-2-benzofuranyl)-2-imidazole (KU-14R; 7-20 mg/kg). Therefore, most imidazoline drugs modulated the synthesis of brain monoamines through interaction with alpha2-adrenoceptors or 5-HT1A receptors. The results do not provide functional evidence for the existence of presynaptic imidazoline receptors regulating the synthesis of monoamines in the rat brain.

The α2-adrenoceptor antagonist idazoxan is an agonist at 5-HT 1A autoreceptors modulating serotonin synthesis in the rat brain in vivo

The in vivo effects of the α 2-adrenoceptor antagonists idazoxan, rauwolscine and phentolamine on α 2-auto/heteroreceptors and 5-HT 1A autoreceptors modulating the synthesis of dopa/noradrenaline and 5-HTP/serotonin were assessed in rats, using the accumulation of dopa and 5-HTP after decarboxylase inhibition as a measure of the rate of tyrosine and tryptophan hydroxylation. The acute administration of idazoxan (0.1–40 mg/kg) induced a pronounced dose-dependent increase in the synthesis of dopa in the cerebral cortex (22–86%) and hippocampus (8–80%), as a consequence of the powerful blockade of α 2-autoreceptors. However, idazoxan did not increase the synthesis of 5-HTP in these brain regions, as it would have been expected by the concurrent blockade of α 2-heteroreceptors on serotonergic terminals. Instead, idazoxan decreased the synthesis of 5-HTP in the cerebral cortex (13–33%) and hippocampus (25–48%), suggesting that these inhibitory effects were mediated through activation of 5-HT 1A autoreceptors. Similar results were obtained for rauwolscine. Pre-treatment of rats with the selective 5-HT 1A receptor antagonist WAY100135 (10 mg/kg) fully antagonized the inhibitory effects of idazoxan (10 mg/kg) on 5-HTP synthesis, but it did not prevent the stimulatory effects of idazoxan on dopa synthesis. The results indicate that idazoxan is a potent and specific agonist at 5-HT 1A autoreceptors modulating brain serotonin synthesis in vivo.

Tyrosine enhances behavioral and mesocorticolimbic dopaminergic responses to aversive conditioning.

Tyrosine is a precursor in the biosynthesis of catecholamines and, when administered systemically, has been shown to enhance the in vivo rate of tyrosine hydroxylation in the medial prefrontal cortex. Additionally, exogenous tyrosine has been demonstrated to enhance the pharmacologically-induced increase in dopamine metabolism seen following administration of haloperidol or the anxiogenic B-carboline, FG-7142. In this report, we examine the effect of a physiologically relevant dose of tyrosine (25 mg/kg) on biochemical and behavioral consequences of aversive conditioning. Rats were conditioned to fear a tone by pairing it with footshock, so that when challenged with the tone alone, rats responded with immobility, defecation, and elevated dopamine metabolism in the medial prefrontal cortex and nucleus accumbens. When tyrosine was administered on the test day (tones alone), the rats displayed an even greater elevation of dopamine metabolism in the nucleus accumbens and prolonged immobility to the tone, compared to the saline/conditioned controls. Tyrosine did not alter mobility or dopamine utilization in the nucleus accumbens in nonconditioned controls. However, dopamine metabolism in the medial prefrontal cortex of nonconditioned rats treated with tyrosine was increased to levels similar to those in the conditioned groups. This may be accounted for by handling and by exposure to an unfamiliar environment necessary for nonconditioned controls. We conclude that exogenous tyrosine is able to 1) elevate stress-induced dopamine metabolism in the nucleus accumbens, 2) alter dopamine utilization in the medial prefrontal cortex of handled, nonconditioned controls, and 3) enhance fear-induced immobilization. These data suggest a role for dietary tyrosine in biochemical and behavioral responses to aversive stimuli.

Acute tyrosine depletion reduces tyrosine hydroxylation rate in rat central nervous system

An amino acid cocktail was devised that would rapidly reduce central nervous system (CNS) tyrosine levels in rats following gastric intubation. The effect of this treatment on in vivo tyrosine hydroxylation rate was examined. Serum tyrosine (TYR) levels, the serum ratio of TYR to the sum of its transport competitors, and CNS TYR concentrations fell substantially within 60 minutes of intubation and remained low for at least 3 hr. In vivo tyrosine hydroxylation rate, evaluated in hypothalamus and retina 2 hr after amino acid intubation, also declined significantly. The results suggest that an amino acid mixture can be devised that will cause an acute reduction in TYR levels and hydroxylation rate in rat CNS. This procedure may ultimately prove applicable to humans to examine functional consequences in particular CNS regions of reducing neuronal catecholamine synthesis.

Delayed increase of tyrosine hydroxylation in the rat A2 medullary neurons upon long-term hypoxia

In vivo and in vitro activity of tyrosine hydroxylase (TH) was estimated in the catecholaminergic A2 cell group of the nucleus tractus solitarius (NTS) in rats exposed to normobaric hypoxia (10% O 2 in nitrogen) for 2 h, 3, 7, 14 or 21 days. The A2 cell group was subdivided into two subgroups. In the caudal A2 subgroup located caudal to the calamus scriptorius, long-term but not acute hypoxia elicited an increase of in vivo tyrosine hydroxylation rate after 7 days of exposure (+ 60% above normoxic controls). The increase of in vivo TH activity was maintained at the same level at the end of hypoxic exposure. In vitro TH activity was increased transiently after 7 days of hypoxia (+ 92% above normoxic controls). In the rostral A2 subgroup, hypoxia elicited a significant increase of in vivo tyrosine hydroxylation at 7 days (+ 38%) but did not alter in vitro TH activity throughout the whole exposure. Hypoxia produced no detectable change in TH activity in other noradrenergic cell groups of the brain stem (locus coeruleus, A5) except for a transient inhibition of in vivo TH activity in A5 after 2 h. Immunocytochemical analyses confirmed that the catecholaminergic neurons in the caudal A2 area are only of a noradrenergic nature. These neurons were located in the commissural subnucleus of the NTS. On the other hand, the rostral A2 area contains noradrenergic neurons intermingled with a small number of adrenergic cell bodies. The data show that long-term hypoxia exerts a delayed stimulatory influence on the caudal A2 area, a discrete region which has been recognized as the main site of projection for chemosensory nerve fibers coming from peripheral arterial chemoreceptors. The delayed activation of TH suggests that noradrenergic neurons of the caudal A2 cell group may be involved in the central chemoreceptor pathway during long-term but not acute hypoxia.

Effect of chronic protein ingestion on rat central nervous system tyrosine levels and in vivo tyrosine hydroxylation rate

Groups of young adult male rats ingested ad libitum for two weeks diets containing 2%, 5%, 10% or 20% protein. They were then killed early in the daily dark period, 30 min after receiving m-hydroxybenzylhydrazine (100 mg/kg i.p., to allow measurement of in vivo tyrosine hydroxylation rate). Tyrosine levels in retina, hypothalamus and cerebral cortex were lowest in rats ingesting 2% protein, and rose progressively to a plateau in rats ingesting 10% protein. No consistent increase occurred between 10% and 20% protein. Tyrosine hydroxylation rate in retina and hypothalamus, but not prefrontal cortex rose in parallel with the increments in tyrosine level between 2% and 10% protein, showing no further increase between 10% and 20% protein. These changes were found to be related to the level of protein, not caloric intake. And, the low rates of hydroxylation observed in rats consuming low protein (2%) were found not to be attributable to low endogenous tyrosine hydroxylase activity. Together, the results indicate that the differences in tyrosine levels in some regions of the central nervous system (retina, hypothalamus) produced by chronic variations in protein intake may influence directly the rate of tyrosine hydroxylation, and thus perhaps the overall rate of catecholamine synthesis. This relationship might provide the hypothalamus (a region important in food intake control) with a signal for monitoring and ultimately modulating the chronic level of protein intake.

Chronic protein ingestion by rats influences central nervous system (CNS) tyrosine concentrations and hydroxylation rates: J. D. FERNSTROM and M. H. FERNSTROM. Department of Psychiatry, University of Pittsburgh Medical School, Pittsburgh, PA 15213, U.S.A.

Chronic protein ingestion by rats influences central nervous system (CNS) tyrosine concentrations and hydroxylation rates: J. D. FERNSTROM and M. H. FERNSTROM. Department of Psychiatry, University of Pittsburgh Medical School, Pittsburgh, PA 15213, U.S.A.

Comparative effects of electroconvulsive shock and haloperidol on in vivo tyrosine hydroxylation and tetrahydrobiopterin in the brain of rats with 6-hydroxydopamine lesions

We have evaluated the effects of electroconvulsive shock (ECS) and haloperidol treatment on the in vivo tyrosine hydroxylation rate and the tetrahydrobiopterin levels in the nigrostriatal system of 6-OHDA-lesioned rats. The rate of DOPA accumulation was significantly decreased by 96% in the ipsilateral striatum and by 50% in substantia nigra of the 6-OHDA-lesioned rats compared to the control activity of contralateral non-lesioned striatum and substantia nigra. The loss of total biopterin was found to be 75% and 50% in the ipsilateral striatum and substantia nigra, respectively. Following administration of haloperidol, the rate of DOPA accumulation increased significantly in the striatum and substantia nigra on the lesioned side compared to that in the vehicle treatment group. Application of ECS also significantly increased the rate of DOPA accumulation in the ipsilateral striatum and substantia nigra compared to that obtained in the non-shocked rats. The biopterin levels in the nigrostriatal system of 6-OHDA-lesioned rats were elevated significantly in the striatum after haloperidol treatment; in contrast the biopterin levels were unchanged in response to ECS. Our results show that both haloperidol and ECS significantly enhanced the rate of in vivo tyrosine hydroxylation in the striatum and substantia nigra of rats with greater than 90% lesions. These results suggest that the nigrostriatal system, although up-regulated following 6-OHDA lesions, still maintains the potential for further up-regulation of dopaminergic function in response to haloperidol and ECS treatment.

Alpha 2-autoreceptor-mediated modulation of tyrosine hydroxylase activity in noradrenergic regions of the rat brain in vivo.

The physiological importance of brain alpha 2-adrenoceptors in controlling the activity of tyrosine hydroxylase in noradrenergic regions was investigated using the accumulation of 3,4-dihydroxyphenylalanine (DOPA) after decarboxylase inhibition as a measure of the rate of tyrosine hydroxylation (and synthesis of noradrenaline) in vivo. In the hypothalamus and cerebral cortex, clonidine (0.025-1 mg/kg, i.p.) decreased (18%-43%) and idazoxan (0.1-80 mg/kg, i.p.) increased (20%-73%) the synthesis of DOPA in a dose-dependent manner. Moreover, pretreatment with idazoxan (0.1 mg/kg) antagonized the effect of clonidine (0.1 mg/kg) in the hypothalamus. After treatment with reserpine (5 mg/kg, s.c., 18 h before decapitation) and depletion of noradrenaline, clonidine (0.5 mg/kg) continued to decrease (50%-55%) but idazoxan (20 mg/kg) failed to increase the synthesis of DOPA, which suggested the involvement of an alpha-auto-receptor mechanism. Acute treatments of rats (not exposed to reserpine) with a wide variety of adrenoceptor agonists such as guanfacine 6, B-HT920, xylazine, bromoxidine (1 mg/kg) and antagonists such as yohimbine, phentolamine, prazosin (10 or 20 mg/kg) resulted in significant decreases (15%-55%) or increases (21%-99%) in the synthesis of DOPA in both brain regions. However, other agonists (oxymetazoline, azepexole, tramazoline, methoxamine) and antagonists (tolazoline, dihydroergotamine, phenoxybenzamine, propranolol) did not modify the synthesis of DOPA. In the hypothalamus and cerebral cortex the effects of the drugs were consistent with the selectivity of alpha-adrenoceptor agonists and antagonists (except prazosin) for an alpha 2-adrenoceptor. The results also suggest that the alpha 2-autoreceptor that modulates the synthesis of noradrenaline in the rat brain appears to belong to the prazosin-sensitive alpha 2B-subtype.

Differential time course activation of the brain stem catecholaminergic groups following chronic adrenalectomy.

The activity of the brain stem catecholaminergic (CA) cell groups of the ventrolateral (A1C1) and dorsomedial (A2C2) medulla that are known to contain primarily nonadrenergic neurones (A1 and A2) and a smaller proportion of adrenergic cells (C1 and C2) as well as the noradrenergic group locus ceruleus (LC) in the dorsal pons was determined at various times up to 16 days following surgical adrenalectomy. The activity of the CA cell groups was estimated by the rate of tyrosine hydroxylation in vivo that was assessed by measuring the 3.4-dihydroxyphenylalanine (DOPA) accumulated 20 min following administration of DOPA decarboxylase inhibitor NSD 1015. In the medullary nuclei noradrenaline content was found around 40- up to 70-fold the adrenaline content. This result was taken as evidence that the noradrenergic cells are likely to provide the main contribution to the tyrosine hydroxylation rate that we measured. Endogenous DOPA content represented between 2 and 10% of the noradrenaline content. NSD 1015 induced an accumulation of DOPA that was linear for at least 20 min and reached at this time more than 10-fold the endogenous level. While no modification of the in vivo tyrosine hydroxylation rate was observed in the LC, a significant increase was found in both medullary groups following adrenalectomy. In the A1C1 group it was detected 8 days after surgery and was then maintained with a maximum that represented up to a 60% increase over the basal value. In the A2C2 group the activation was slightly delayed and less marked. Increase in ACTH level occurred much earlier: it was about 70% of the maximal level already 4 days following adrenalectomy.(ABSTRACT TRUNCATED AT 250 WORDS)

Prior exposure to chronic stress results in enhanced synthesis and release of hippocampal norepinephrine in response to a novel stressor

The release and synthesis of norepinephrine (NE) in hippocampus were measured in naive and chronically cold-stressed rats in response to acute tail-shock stress. Using in vivo microdialysis, it was determined that the basal extracellular concentrations of NE and 3,4-dihydroxyphenylacetic acid (DOPAC) in hippocampus were the same in the two groups. However, 30 min of intermittent tail shock produced a greater elevation of extracellular NE and 3,4-dihydroxyphenylacetic acid in the chronically cold-stressed rats than in the native controls. In hippocampus, the extracellular concentration of DOPAC may reflect NE biosynthesis, and thus the enhanced DOPAC response in the chronically stressed rats suggests an increase in NE synthesis. In order to investigate this possibility, two further methods of assessing NE biosynthesis were employed. Tyrosine hydroxylase (TH) activity was assayed in vitro in the presence of saturating concentrations of cofactor. No change in maximal TH activity could be detected in hippocampus of chronically cold-stressed rats. In addition, the in vivo rate of tyrosine hydroxylation in cold-stressed rats was measured by the accumulation of 3,4-dihydroxyphenylalanine in tissue following inhibition of aromatic amino acid decarboxylase. It was found that, whereas basal synthesis was the same in both groups of rats, synthesis accompanying a novel stressor was increased to a greater extent in the chronically stressed rats.

Parasite‐induced enhancement of hemolymph tyrosinase activity in a selected immune reactive strain of Drosophila melanogaster

Larval hemolymph tyrosinase activity in Drosophila melanogaster was detected with high performance liquid chromatography with electrochemical detection. The enzyme hydroxylated L-tyrosine, and oxidized the diphenol substrates L-dopa and dopamine. In larvae of a selected immune-reactive strain the rates of tyrosine hydroxylation, dopa oxidation, and dopamine oxidation were markedly increased during the early stages of melanotic encapsulation of the eggs of the parasitic wasp Leptopilina boulardi. Tyrosinase activity was not modified in parasitized larvae of a selected susceptible strain of D. melanogaster, in which hosts the parasitoids developed unmolested. During the same period of parasitization, the amount of free tyrosine in immune reactive larvae was approximately three times higher than in susceptible hosts. These data indicate that the tyrosinase system of the immune reactive strain is activated during parasitization, and this results in the synthesis of some precursors which ultimately produce a melanotic and sclerotic capsule around the eggs of the parasite. Based on known genetic information of the enzyme system in Drosophila, it appears that at least two genes may be involved in the activation process, one associated with the proenzyme for monophenol oxidase activity, and the second with the proenzyme for diphenol oxidase activity.