Stimulation Of Tyrosine Hydroxylase Activity Research Articles

Interleukin-1β stimulates tyrosine hydroxylase activity in the median eminence

The effects of intracerebroventricularly (i.c.v.) injected interleukin-1β (IL-1β) on tyrosine hydroxylase (TH) activity in the median eminence and on serum adrenocorticotropin hormone (ACTH) and prolactin (PRL) levels were studied in freely moving male rats chronically implanted with i.c.v. cannulas. IL-1β stimulated TH activity of the median eminence in a dose-dependent manner. Intracerebroventricular injections of 100 ng and 200 ng IL-1β significantly increased TH activity in the median eminence by 40.6% and 74.7% respectively over the salineinjected control group. The stimulation of ACTH secretion was statistically significant for all doses used, however i.c.v. injections of IL-1β failed to elicit significant changes in the serum PRL concentrations. These results suggest that IL-1β increases dopaminergic activity of the medial basal hypothalamus to produce its neuroendocrine effects.

Interleukin-1β stimulates tyrosine hydroxylase activity in the median eminence

The effects of intracerebroventricularly (i.c.v.) injected interleukin-1 beta (IL-1 beta) on tyrosine hydroxylase (TH) activity in the median eminence and on serum adrenocorticotropin hormone (ACTH) and prolactin (PRL) levels were studied in freely moving male rats chronically implanted with i.c.v. cannulas. IL-1 beta stimulated TH activity of the median eminence in a dose-dependent manner. Intracerebroventricular injections of 100 ng and 200 ng IL-1 beta significantly increased TH activity in the median eminence by 40.6% and 74.7% respectively over the saline-injected control group. The stimulation of ACTH secretion was statistically significant for all doses used, however i.c.v. injections of IL-1 beta failed to elicit significant changes in the serum PRL concentrations. These results suggest that IL-1 beta increases dopaminergic activity of the medial basal hypothalamus to produce its neuroendocrine effects.

Synthesis of [N‐C3H3]‐trans‐(1R,3S)‐(−)‐1‐Phenyl‐3‐N,N‐dimethylamino‐1,2,3,4‐tetrahydronaphthalene (H2‐PAT)

AbstractSubsequent to the discovery that the (+)‐benzomorphan sigma receptor ligands, (+)‐pentazocine and (+)‐N‐allylnormetazocine, stimulated tyrosine hydroxylase activity and dopamine synthesis in rat striatum in vitro, we reported a similar effect on a structurally similar series of 1‐phenyl‐3‐aminotetrahydronaphthalenes (phenylaminotetralins, PAT's). Both racemic 1‐phenyl‐3‐dimethylamino‐6‐chloro‐7‐hydroxytetralin (CI,OH‐PAT) and racemic 1‐phenyl‐3‐dimethylaminotetralin (H2‐PAT) stimulated tyrosine hydroxylase with an EC50 of approximately 0.1 μM. The former was also found to have a non‐specific dopamine releasing effect while the latter was devoid of such activity affording it the less complicated pharmacological profile of the two analogs. We previously reported the synthesis of tritium labeled Cl,OH‐PAT to be used in radioreceptor and autoradiography studies and found that it labeled a sigma‐like site in guinea pig brain with an apparent Kd of ∼50 pM and with a pharmacological profile unique from other known CNS receptors. Here we report the synthesis of high specific activity tritium labeled trans‐(1R,3S)‐(−)‐H2‐PAT as this enantiomer was found to be more active in the tyrosine hydroxylase assay and possessed approximately 45 fold greater affinity for the novel neuromodulatory sigma‐like receptor.

Pituitary adenylate cyclase activating polypeptide (PACAP) potently enhances tyrosine hydroxylase (TH) expression in adrenal chromaffin cells

In primary cultured bovine adrenal chromaffin cells (BACC), pituitary adenylate cyclase activating polypeptide 1–38 (PACAP) produced a dose related increase in tyrosine hydroxylase (TH) Vmax when measured 48 hours after the beginning of the treatment; a significant increase was observed with 0.5 nM and the maximal induction of close to 2.5-fold was found with 0.1 μM PACAP. The potency of PACAP was nearly 3 orders of magnitude greater than forskolin and VIP in inducing TH activity. These effects were preceded by an increase in TH mRNA levels, that started 2 hours after treatment and peaked 12 hours later. The presence of the phosphodiesterase inhibitor HL 725 further increased the stimulation of TH activity by PACAP, indicating that this activation was mediated via a cascade of events initiated by cAMP. Nicotine (1 μM) failed to increase TH activity significantly, however, when added in association with PACAP, a statistically significant increase of TH was elicited with peptide concentrations 5 times lower (0.1 nM) than the threshold dose of the peptide. The stimulation of nicotinic receptors facilitates the TH induction elicited by PACAP.

Prenatal cocaine administration stimulates fetal brain tyrosine hydroxylase activity

Functional effects of prenatal cocaine exposure may be mediated in part by changes in catecholaminergic development. The present study examined whether cocaine administration influenced fetal brain activity of tyrosine hydroxylase (TH), the rate-limiting enzyme in catecholamine biosynthesis. Subcutaneous (s.c.) injection of pregnant rats with 40 mg/kg of cocaine HCl daily from gestational day (GD) 8 to GD20 resulted in an 8.7% stimulation of fetal whole-brain TH activity compared to controls. We then switched to a s.c. implantation procedure involving Silastic capsules filled with 80 mg of cocaine base dissolved in polyethylene glycol (PEG). Implantation of 2 such capsules on GD18 produced a 28% increase in fetal TH activity measured only 3 days later on GD21. Subsequent experiments demonstrated that GD14 implantation was equally effective in stimulating fetal TH activity on GD17, but that the enzyme was unaffected in the brains of the treated dams. When cocaine-containing capsules were implanted on GD18, removed on GD21, and the females were allowed to deliver normally, offspring TH activity was still elevated on postnatal day 10 but not later. Finally, the presence of cocaine implants from GD18 to GD21 had no influence on fetal brain neurotransmitter and metabolite concentrations, however, the treated dams exhibited significant reductions in dopamine (DA) and the serotonin metabolite, 5-hydroxyindoleacetic acid. We conclude that maternal cocaine implants rapidly but transiently stimulate TH activity in the fetal brain, and that such stimulation prevented the DA depletion observed in the dams.

Is forskolin stimulation of rat striatal tyrosine hydroxylase dependent on adenylate cyclase activation?

Because some responses to forskolin are adenosine 3′,5′-cyclic phosphate (cyclic AMP) independent, we investigated the involvement of adenylate cyclase in the stimulatory effect of forskolin on synaptosomal tyrosine hydroxylase (TH) activity of rat striatum. The forskolin analogue, 1,9-dideoxyforskolin, which mimics the cyclic AMP-independent effects of forskolin, was a very weak activator of both striatal adenylate cyclase and tyrosine hydroxylase activities, whereas forskolin stimulated the two enzymes effectively and with similar potencies. Moreover, exposure of synaptosomes to the specific cyclic AMP antagonist Rp-adenosine 3′,5′-cyclic phosphorothioate reduced basal TH activity and counteracted the stimulatory effect on the enzyme activity by submaximal concentrations of forskolin. These results provide circumstantial evidence that in striatal dopaminergic nerve terminals a presynaptic adenylate cyclase mediates the stimulation of TH activity by forskolin.

GABA B receptor activation partially inhibits N-methyl-D-aspartate-mediated tyrosine hydroxylase stimulation in rat striatal slices

The effect of the GABA B agonist, (±)-baclofen, on the N-methyl-D-aspartate (NMDA)-mediated stimulation of tyrosine hydroxylase activity was investigated in slices of rat striatum. Tyrosine hydroxylase activity was stimulated by NMDA in a concentration-dependent manner (EC 50, 1.3 ± 0.3 μM; maximum stimulation 194 ± 7% of basal activity). The action of NMDA was reversed by the NMDA antagonist, 2-amino-5-phosphonovalerate (AP-5). (±-Baclofen (100 μM) decreased the maximum effect of NMDA by 24 ± 2% without significantly modifying its EC 50. The IC 50 for the inhibitory action of (±)-baclofen was 4.2 ± 1.2 μM. These results show that GABA B receptor activation modulates NMDA-stimulated tyrosine hydroxylase activity, further supporting the possibility of a role of GABA in the regulation of striatal dopaminergic neurotransmission.

γ-Aminobutyric acid (GABA B) receptor-mediated inhibition of tyrosine hydroxylase activity in the striatum of rat

The effects of GABA and GABA agonists on calcium and depolarization-dependent stimulation of tyrosine hydroxylase activity in striatal slices were studied. γ-Aminobutyric acid and (−)baclofen inhibited, while muscimol and (+)baclofen did not affect, the stimulation of tyrosine hydroxylase. The inhibitory effect ofGABA and (−)baclofen was blocked by the GABA bB antagonist, phaclofen but not by the GABA A antagonist, picrotoxin. The data suggest that the activity of tyrosine hydroxylase on dopaminergic nigrostriatal terminals may be modulated by GABA via GABA B receptors.

Insulin‐Like Growth Factor‐I Enhances Tyrosine Hydroxylase Activation in Bovine Chromaffin Cells

Previous studies have shown that insulin-like growth factor-I (IGF-I) enhances secretagogue-stimulated Ca2+ uptake and catecholamine release in bovine chromaffin cells. This report describes the effect of IGF-I on the activity of tyrosine hydroxylase (tyrosine 3-monooxygenase, EC 1.14.16.2), the major regulatory enzyme in the pathway of catecholamine biosynthesis. Tyrosine hydroxylase activity was assayed by measuring 3,4-dihydroxyphenylalanine (Dopa) accumulation in the presence of brocresine, an inhibitor of Dopa decarboxylase. Chromaffin cells cultured in serum-free medium produced approximately 40% less Dopa when stimulated by 55 mM K+ than did cells that had been cultured in the presence of serum. Incubation of cells for 3 days in serum-free medium containing 10 nM IGF-I restored high K(+)-stimulated Dopa accumulation to a level comparable to that seen in cells cultured continuously in serum-containing medium. In eight experiments, IGF-I increased high K(+)-stimulated Dopa accumulation (expressed as picomoles per minute per milligram of protein) by 96 +/- 13%. IGF-I increased the protein content of chromaffin cells by approximately 30%; consequently, its effect on tyrosine hydroxylase activity was even greater when Dopa synthesis was expressed as picomoles per minute per 10(7) cells. IGF-I also enhanced the rate of Dopa accumulation in cells stimulated by dimethylphenylpiperazinium, 8-bromo-cyclic AMP, phorbol 12,13-dibutyrate, or Ba2+. The effect of IGF-I on high K(+)-stimulated tyrosine hydroxylase activity was measurable when enzyme activity was assayed in vitro, suggesting that this effect was due to a stable modification of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Testicular Inhibition of Tyrosine Hydroxylase Expression in Tuberoinfundibular and Nigrostriatal Dopaminergic Neurons*

We have investigated, in young and aged rats, tuberoinfundibular dopaminergic (TIDA) and nigrostriatal neurons of the brain, and the role of the testes and anterior pituitary (AP) on 1) the in situ activity of tyrosine hydroxylase (TH), 2) translation of TH mRNA, as reflected by the mass of TH, and 3) transcription of the TH gene, as revealed by the mass of TH mRNA. The median eminence (ME) and corpus striatum (CS) were used as the sources of proximal neurites of the TIDA neurons and nigrostriatal neurons, respectively. The arcuate-periventricular nuclei of the ventral hypothalamus were used as the source of perikarya of the TIDA neurons, and substantia nigra (SN) nuclei of the midbrain were used as the source of perikarya of nigrostriatal neurons. The in situ activity of TH was calculated using the rate of accumulation of dihydroxyphenylalanine after pharmacological inhibition of dihydroxyphenylalanine decarboxylase activity. TH mass and TH mRNA were measured using an immunoblot assay and an S1 nuclease protection assay, respectively. Compared to intact animals, orchidectomized young and aged male rats had significantly (P less than 0.001) increased in situ activity of TH in the ME and CS, but not the SN. Orchidectomy also caused a significant (P less than 0.01) increase in the quantity of TH in the ME and a 2- to 3-fold increase in TH in the CS and SN. In contrast to castration, AP grafts, implanted in the lateral ventricles of the brain, caused a significant (P less than 0.001) increase only in TH activity in the ME. No effect of AP grafts was seen on TH activity of the CS or SN. AP grafts had no effect on the amount of TH in the ME, CS, or SN. In the TIDA neurons of young males and in the nigrostriatal neurons of young and aged castrates, the amount of TH mRNA was not different from that in intact males. AP grafts had no effect on TH mRNA in these dopaminergic neurons. These studies show that TIDA neurons and nigrostriatal neurons share some common aspects in their regulation and are dissimilar in others. AP grafts stimulate TH activity in young as well as aged TIDA neurons, but not in nigrostriatal neurons. Castration leads to an increase in TH mass in both TIDA and nigrostriatal neurons. The increase in TH mass is not associated with an increase in the amount of TH mRNA. It is suggested that castration results in increased translation of TH mRNA, but not transcription of the TH gene.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of nitroglycerin on release, synthesis and metabolism of norepinephrine and activation of tyrosine hydroxylase in guinea-pigs

Nitroglycerin produced concentration-dependent increases in outflow of norepinephrine (NE) in perfused guinea-pig hearts. These effects of nitroglycerin were inhibited by guanethidine, but were not blocked by desmethylimipraminc or rauwolscine. Nitroglycerin-induced increases in NE outflow correlated with its positive chronotropic responses. Higher concentrations of nitroglycerin also increased outflow of 3,4-dihydroxyphenylglycol (DOPEG) which were inhibited by desmethylimipramine. Nitroglycerin-induced maximal outflow of NE occurred during the first 2 min, but maximal outflow of DOPEG was 2–4 min. In addition, the 3,4-dihydroxyphenylalanine formation in atrial and striatal slices were enhanced by nitroglycerin in the presence of m-hydroxybenzylhydrazine dihydrochloride (an inhibitor of aromatic L-amino acid decarboxylase). The results suggest that nitroglycerin increases release of NE which correlates with cardiac stimulation by the drug. The drug also stimulates tyrosine hydroxylase activity resulting in increased synthesis of NE.

Effects of Peptides of the Secretin‐Glucagon Family and Cyclic Nucleotides on Tyrosine Hydroxylase Activity in Sympathetic Nerve Endings

Previous studies have shown that certain peptides of the secretin-glucagon family stimulate tyrosine hydroxylase activity in sympathetic neurons of the superior cervical ganglion and three of its end organs, i.e., the iris, pineal gland, and submaxillary gland. To determine whether a similar regulation occurs in other sympathetic neurons, the effects of two of these peptides, secretin and vasoactive intestinal peptide, were examined in the right cardiac ventricle of the rat, a tissue innervated primarily by the middle and inferior cervical ganglia. Both peptides stimulated tyrosine hydroxylase activity, measured in situ, in this tissue. In addition, several second messenger systems were investigated as possible mediators of this peptidergic stimulation of tyrosine hydroxylase activity in autonomic end organs. 8-Bromoadenosine 3',5'-cyclic monophosphate and forskolin elevated tyrosine hydroxylase activity in slices of both the right ventricle and the submaxillary gland. 8-Bromoguanosine 3',5'-cyclic monophosphate also stimulated tyrosine hydroxylase activity in both tissues, whereas nitroprusside stimulated activity only in the submaxillary slices. Furthermore, the phosphodiesterase inhibitors 3-isobutyl-1-methylxanthine and/or Ro 20-1724 potentiated the stimulation by secretin, as well as the stimulations by forskolin and nitroprusside. Phorbol 12,13-dibutyrate also stimulated tyrosine hydroxylase activity in cardiac and submaxillary slices; however, no potentiation of these effects was seen following addition of either phosphodiesterase inhibitor. These data, taken together with those of previous studies, suggest a role for a cyclic nucleotide, probably adenosine 3',5'-cyclic monophosphate, in the peptidergic stimulation of tyrosine hydroxylase activity in sympathetic nerve terminals.

Light onset stimulates tyrosine hydroxylase activity in isolated teleost retinas

The action of tyrosine hydroxylase is the rate-limiting step in the synthesis of dopamine, the most abundant catecholamine in vertebrate retinas. I have examined the activation and regulation of this enzyme in isolated retinas of green sunfish, Lepomis cyanellus. Exposing previously dark-adapted retinas to constant illumination for a period of 10 min increased enzymatic activity 2.2-fold over that present in retinas incubated in darkness. Thus, light onset activates tyrosine hydroxylase in teleost retinas. Stimulation of the activity of tyrosine hydroxylase under these conditions was associated with a decrease in the apparent K m of the enzyme for its pteridine cofactor without a change in the apparent V max of the reaction. This result suggests that short-term exposure to light increases dopamine synthesis by enhancing the affinity of the enzyme for its naturally occurring cofactor. These findings are consistent with the idea that light activates dopaminergic neurons in teleost retinas.

Adenosine A 2 stimulation of tyrosine hydroxylase in rat striatal minces is reversed by dopamine D 2 autoreceptor activation

The adenosine agonist, 2-chloroadenosine, stimulated tyrosine hydroxylase activity in rat striatal minces; this effect was attenuated by activation of dopamine (DA) D 2 autoreceptors with N-n-propylnorapomorphine and antagonized by theopylline. Forskolin and 8-bromo-cAMP also increased tyrosine hydroxylase activity and their effects were not altered by 2-chloroadenosine. D 1, α, β and 5-HT agonists did not affect tyrosine hydroxylase activity. Evidently, A 2 receptors on DA nerve terminals stimulate striatal DA synthesis and this effect is negatively modulated by D 2 autoreceptors, probably via changes in intracellular cAMP levels.

Blockers of Voltage‐Sensitive Ca2+ Channels Inhibit K+ Stimulation of Tyrosine Hydroxylase Activity in Sympathetic Neurons

Blockers of Voltage‐Sensitive Ca²⁺ Channels Inhibit K⁺ Stimulation of Tyrosine Hydroxylase Activity in Sympathetic Neurons

Effects of subchronic nicotine administration on central dopaminergic mechanisms in the rat

Nicotine was administered acutely and subchronically (14 days) to determine whether various synaptic mechanisms are selectively altered in the nigrostriatal and mesolimbic dopaminergic systems in the rat. When added to tissue preparations in vitro, nicotine had no effects on tyrosine hydroxylase, synaptosomal uptake of [3H]dopamine or binding of [3H]spiperone to D2 receptors in either system. However, acute treatment in vivo stimulated tyrosine hydroxylase activity in the nucleus accumbens. This effect was prevented by pretreatment with a nicotinic antagonist, suggesting that it was mediated by nicotinic receptors. Since subchronic exposure to nicotine had no effect on tyrosine hydroxylase, it appears that tolerance develops to this action. In vivo treatment with nicotine did not alter dopamine uptake or receptor binding. The results suggest that, in doses which result in moderate plasma levels, nicotine has selective stimulant actions on nerve terminals of the mesolimbic system.

Secretin and vasoactive intestinal peptide activate tyrosine hydroxylase in sympathetic nerve endings

Secretin and vasoactive intestinal peptide (VIP) are known to stimulate tyrosine hydroxylase (TH) activity acutely in the rat superior cervical ganglion (SCG). Because TH-containing neurons in the SCG innervate the iris, submaxillary gland, and pineal gland, we examined the effects of secretin and VIP in these 3 autonomic end organs in vitro. Both peptides stimulated TH activity in each tissue. These stimulations resembled those in the SCG in that (1) secretin displayed a higher potency than VIP in all 3 end organs, (2) the peptide effects were unchanged when calcium was excluded from the incubation medium, and (3) they were mimicked by activators of the cyclic adenosine monophosphate (cAMP) pathway. These findings indicate that secretin and VIP can regulate transmitter metabolism in both the cell bodies and axon terminals of neurons originating in the SCG. Furthermore, the data raise the possibility that catecholamine synthesis in sympathetic nerve terminals is modulated by peptides released by other, nearby nerve endings.

Regulation of tryptophan hydroxylase activity by a cyclic AMP-dependent mechanism in rat striatum

Evidence is presented indicating that a cAMP-dependent mechanism activates tryptophan hydroxylase (TrpH), the rate-limiting enzyme for serotonin (5-HT) biosynthesis. Forskolin, a selective activator of adenylate cyclase, stimulated 5-HT formation in synaptoneurosome preparations of rat striatum, substantia nigra, hypothalamus, and amygdala. Further studies of striatum revealed that the forskolin-induced activation of serotonin synthesis is readily reversible. Also, it may be self-limited by a mechanism of desensitization, since after an initial exposure to forskolin followed by removal, a re-exposure of synaptoneurosomes to forskolin was no longer stimulatory. In contrast to these results for 5-HT synthesis, forskolin-induced stimulation of dopamine synthesis persisted following removal of forskolin; hence the response was not rapidly reversible or desensitized. In soluble extracts of striatum, 8-thiomethyl-cyclic AMP enhanced TrpH activity, supporting a direct role of cyclic AMP and cyclic AMP-dependent protein kinase in regulating TrpH. In agreement with previous reports, 8-thiomethyl cyclic AMP also stimulated tyrosine hydroxylase activity in soluble striatal extracts. We conclude that cyclic AMP is an important regulator of TrpH, in addition to its known effects on tyrosine hydroxylase.

Characteristics of Tyrosine Hydroxylase Activation by K+‐Induced Depolarization and/or Forskolin in Rat Striatal Slices

The mechanisms of tyrosine hydroxylase (TH) activation by depolarization or exposure of dopaminergic terminals to cyclic AMP have been compared using rat striatal slices. Tissues were incubated with veratridine or 60 mM K+ (depolarizing conditions), on the one hand, and forskolin or dibutyryl cyclic AMP, on the other. K+-(or veratridine-)induced depolarization triggered an activation of TH (+75%) that persisted in soluble extracts of incubated tissues. This effect disappeared when drugs (EGTA, N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide, Gallopamil) preventing Ca2+- and calmodulin-dependent processes were included in the incubating medium. In contrast, prior in vivo reserpine treatment or in vitro addition of benztropine did not affect the depolarization-induced activation of TH. In vitro studies of soluble TH extracted from depolarized tissues indicated that activation was associated with a marked increase in the enzyme Vmax but with no change in its apparent affinity for the pteridin cofactor 6-methyl-5,6,7,8-tetrahydropterin (6-MPH4) or tyrosine. Furthermore, the activated enzyme from depolarized tissues exhibited the same optimal pH (5.8) as native TH extracted from control striatal slices. In contrast, TH activation resulting from tissue incubation in the presence of forskolin or dibutyryl cyclic AMP was associated with a selective increase in the apparent affinity for 6-MPH4 and a shift in the optimal pH from 5.8 to 7.0-7.2. Clear distinction between the two activating processes was further confirmed by the facts that heparin- and cyclic AMP-dependent phosphorylation stimulated TH activity from K+-exposed (and control) tissues but not that from striatal slices incubated with forskolin (or dibutyryl cyclic AMP). In contrast, the latter enzyme but not that from depolarized tissues could be activated by Ca2+-dependent phosphorylation. These data strongly support the concept that Ca2+- but not cyclic AMP-dependent phosphorylation is responsible for TH activation in depolarized dopaminergic terminals.

Substance P inhibits the acute stimulation of ganglionic tyrosine hydroxylase activity by a nicotinic agonist

Incubation of rat superior cervical ganglia with dimethylphenylpiperazinium (30μM) for 30 min resulted in a two-fold increase in tyrosine hydroxylase activity. This effect was completely inhibited by substance P (30 μM) but not by substance P-free acid, kassinin or physalaemin. Neither of these four peptides alone produced any change in the activity of tyrosine hydroxylase. The IC 50 for the inhibitory effect of substance P was approximately 3μM. Substance P did not inhibit the stimulatory effects of bethanechol or vasoactive intestinal peptide on this enzyme activity. Thus substance P, acting at a site which has a different pharmacology than previously characterized substance P receptors, selectively inhibits nicotinic stimulation of tyrosine hydroxylase activity. These data raise the possibility that substance P may modulate the nicotinic regulation of catecholamine synthesis in sympathetic ganglia in vivo.