Dopamine Autoreceptors Modulate the Phosphorylation of Tyrosine Hydroxylase in Rat Striatal Slices

The hypothesis that dopamine (DA) autoreceptors modulate the phosphorylation of tyrosine hydroxylase (TH; EC 1.14.16.2) was investigated in rat striatal slices. Tissue was prelabeled with 32P inorganic phosphate, and TH recovered by immunoprecipitation with anti-TH rabbit serum. The TH monomer was resolved on sodium dodecyl sulfate polyacrylamide gels, and the extent of phosphorylation was determined by scanning densitometry of autoradiographs. Depolarization of striatal slices with 55 mM K+ markedly increased the incorporation of 32P into several proteins, including the TH monomer (Mr = 60,000). A similar increase in TH phosphorylation occurred in response to the adenylate cyclase activator forskolin and the cyclic AMP analog dibutyryl cyclic AMP. An increase in TH phosphorylation was not observed in response to the D1-selective agonist SKF 38393. The D2-selective DA autoreceptor agonist pergolide decreased the phosphorylation of TH below basal levels and blocked the increase in phosphorylation elicited by 55 mM K+. The inhibitory effect of pergolide was antagonized by the D2-selective antagonist eticlopride. Changes observed in the phosphorylation of TH were mirrored by changes in tyrosine hydroxylation in situ. These observations support the hypothesis that a reduction in TH phosphorylation is the mechanism by which DA autoreceptors modulate tyrosine hydroxylation in nigrostriatal nerve terminals.     

Reduction of Dopamine Synthesis Inhibition by Dopamine Autoreceptor Activation in Striatal Synaptosomes with In Vivo Reserpine Administration

Abstract: In an attempt to clarify the mechanisms by which dopamine (DA) autoreceptor activation inhibits DA synthesis, the efficacy and potency of the D₂ DA agonists bromocriptine, lisuride, and pergolide, and the D₁,-D₂ DA agonist apomorphine were studied in rat striatal synapto- somes, in which the rate of DA synthesis (formation of ¹⁴CO₂ from l -[1–¹⁴C]tyrosine) was increased 103% by treating the animals from which the synaptosomes were obtained with reserpine (5 mg/kg i.p. twice, 24 and 2 h before they were killed), using the striatal total homogenate as the standard synaptosomal preparation. The increase in DA synthesis evoked by reserpine was additive with that produced by treatment of the synaptosomes with dibutyryl cyclic AMP, suggesting that, not a cyclic AMP-dependent, but possibly a Ca²⁺-dependent mechanism was involved. The DA agonists showed a concentration-dependent inhibition of DA synthesis in the control synaptosomes, which was antagonized by the selective D₂ DA antagonist (-)-sulpiride. In the synaptosomes with increased rate of DA synthesis obtained from the rats treated with reserpine, the concentration-response curves of DA synthesis inhibition for the other DA agonists were shifted to the right, and the effect of bromocriptine was completely eliminated, whereas bromocriptine antagonized the effect of apomorphine. The increased rate of DA synthesis was not preserved in the striatal P₁+ P₂ fraction obtained from the reserpine-treated rats, but the effects of the DA agonists were still reduced to the same degree as those in the total homogenate. (-)-Sulpiride did not enhance DA synthesis in synaptosomes from the reserpine- treated rats. The results presented indicate that the reduced effect of the DA agonists in synaptosomes from the reserpine-treated rats was not due to endogenous DA occupying the DA autoreceptors. Because it is known from the literature that reserpine in vivo increases impulse activity in DA neurons and, as a result, increases the Ca²⁺ concentration, these results suggest that the effect of DA agonists was reduced because DA autoreceptors may normally control DA synthesis by decreasing the free intraneuronal Ca²⁺ concentration, and consequently, the Ca²⁺-dependent phosphorylation of tyrosine hydroxylase.     

Glutamatergic Control of Dopamine Release in the Rat Striatum: Evidence for Presynaptic N-Methyl-D-Aspartate Receptors on Dopaminergic Nerve Terminals

The N-methyl-D-aspartate (NMDA) receptor-mediated regulation of the release of newly synthesized [3H]dopamine [( 3H]DA) was studied in vitro, both on rat striatal slices using a new microsuperfusion device and on rat striatal synaptosomes. Under Mg2(+)-free medium conditions, the NMDA (5 X 10(-5) M)-evoked release of [3H]DA from slices was found to be partly insensitive to tetrodotoxin (TTX). This TTX-resistant stimulatory effect of NMDA was blocked by either Mg2+ (10(-3) M) or the noncompetitive antagonist MK-801 (10(-6) M). In addition, the TTX-resistant NMDA-evoked response could be potentiated by glycine (10(-6) M) in the presence of strychnine (10(-6) M). The coapplication of NMDA (5 X 10(-5) M) and glycine (10(-6) M) stimulated the release of [3H]DA from striatal synaptosomes. This effect was blocked by Mg2+ (10(-3) M) or MK-801 (10(-5) M). These results indicate that some of the NMDA receptors involved in the facilitation of DA release are located on DA nerve terminals. These presynaptic receptors exhibit pharmacological properties similar to those described in electrophysiological studies for postsynaptic NMDA receptors.     

Monovalent Cations and Striatal Tyrosine Hydroxylase

Abstract: The kinetic properties of soluble tyrosine hydroxylase from rat striatum and the activation of the enzyme by the polyanion heparin were assessed as a function of the monovalent cations K⁺, Na⁺, tetramethylammonium (TMA⁺), and Tris. Substitution of K⁺ or Na⁺ for TMA⁺ or Tris can alter the kinetic properties of tyrosine hydroxylase in the absence of heparin, the nature of the interaction of the enzyme with heparin and also the kinetic properties of the heparin-activated enzyme. The data suggest that monovalent cations can support unique conformational states of the enzyme.     

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.     

Evidence for Protein Kinase C Involvement in the Short-Term Activation by Prolactin of Tyrosine Hydroxylase in Tuberoinfundibular Dopaminergic Neurons

Abstract: The mechanism of the short-term activation by prolactin (PRL) of tyrosine hydroxylase (TH) in tuberoinfundibular dopaminergic neurons was examined in vitro on hypothalamic slices from ovariectomized rats. TH activity (determined by 3,4-dihydroxyphenylalanine accumulation in the median eminence after blockade of decarboxylase with NSD 1055) showed a dose-dependent increase within 2 h of incubation of the hypothalamic slices with PRL. To determine whether a phosphorylation process was involved in this increase in TH activity, we studied the sensitivity of the enzyme to dopamine (DA) feedback inhibition. In control median eminences, two kinetically different forms of TH coexisted, one exhibiting a K _1(DA) value of 29.92 ± 0.49 μ M , the other being × 15-fold more sensitive to DA inhibition with a K _1(DA) of 1.96 ± 0.09 μ M , likely corresponding to a phosphorylated and active form and to a nonphosphorylated and less active form, respectively. After PRL treatment, the TH form of low K _1(DA) remained unaffected, whereas the K _1(DA) of the purported active form of TH increased to 62.6 ± 0.8 μ M , suggesting an increase in the enzyme phosphorylation. This increase in the K _I(DA) of TH was selectively prevented by GF 109203X, a potent and selective inhibitor of protein kinase C, but not by a specific inhibitor of protein kinase A or calmodulin. Finally, this action of PRL could be mimicked by 12- O -tetradecan-oylphorbol 13-acetate (a direct activator of protein kinase C). These results suggest that PRL, at the median eminence level, activates TH by increasing the enzyme phosphorylation and that this action may involve an activation of protein kinase C.     

Effects of Phospholipases on the Kinetic Properties of Rat Striatal Membrane-Bound Tyrosine Hydroxylase

Abstract: Rat striatal tyrosine hydroxylase can be isolated in both a soluble and a synaptic membrane-bound form. The membrane-bound enzyme, which exhibits lower K _ms for both tyrosine (7 μ M ) and reduced pterin cofactor (110 μ M ) relative to the soluble enzyme (47 μ M and 940 μ M , respectively), can be released from the membrane fraction with mild detergent, and concomitantly its kinetic properties revert to those of the soluble enzyme. Treatment of membrane-bound tyrosine hydroxylase with C. perfringens phospholipase C increased the K _m of the enzyme for tyrosine to 27 μ M and the V _max by 60% without changing the K _m for cofactor. In contrast, treatment of membrane-bound tyrosine hydroxylase with V. russelli phospholipase A₂ increased the K _m for tyrosine to 48 μ M increased the V _max and increased the K _m for cofactor to 560 μ M . The enzyme remained bound to the membrane fraction following both phospholipase treatments. Addition of phospholipids to treated enzyme could partially reverse the effects of phospholipase A₂ treatment, but not the effects of phospholipase C treatment. The kinetic properties of phospholipase-treated, detergent-solubilized tyrosine hydroxylase were identical to those of the control solubilized enzyme. Tyrosine hydroxylase appears to interact with synaptic membrane components to produce at least two separately determined consequences for the kinetic properties of the enzyme.     

Tyrosine Hydroxylase Activation and Inactivation by Protein Phosphorylation Conditions

Tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, catalyzes the conversion of tyrosine to DOPA, Cyclic AMP-dependent protein phosphorylation conditions alter tyrosine hydroxylase activity in rat striatal homogenates. In agreement with other laboratories, we find that short-term pre-incubation (3 min) of extracts under phosphorylating conditions (Mg . ATP, cAMP) increases enzyme activity two- to tenfold over control as measured during a subsequent 15-min assay. We now report that preincubation under phosphorylating conditions for longer periods (30 min) results in a loss of activity to levels equal to or below that of the control enzyme. Addition of purified bovine brain protein kinase catalytic subunit and Mg . ATP enhances activation and increases the rate of inactivation. To demonstrate that inactivation is not associated with proteolytic degradation or irreversible denaturation, the inactivated form of the enzyme can be reactivated. The protein kinase inhibitor protein decreases the activation process and prevents inactivation of the enzyme to below control values. The sedimentation coefficient is not changed by phosphorylation conditions (S = 8.8 +/- 0.1). Although the apparent Km of the enzyme for the 6-methyltetrahydropterine (6-MPH4) cofactor is reduced (0.86 mM, control; 0.32 mM, activated), it is also reduced in the inactivated form (0.38 mM). The Ki for dopamine is increased from 4.5 microM for the control to 28 microM for the activated enzyme, whereas the inactivated form of the enzyme exhibits a Ki of 10 microM. Removal of catecholamines by gel filtration fails to alter activity and the apparent cofactor Km. Moreover, both the activated and the inactivated states persist following gel filtration. It therefore appears that the activation-inactivation process is not mediated solely by the modulation of enzyme feedback inhibition or changes in the Km for 6-MPH4. We also describe a coupled decarboxylase assay in which labeled dopamine is resolved from the precursors tyrosine and DOPA by low-voltage paper electrophoresis.     

Influence of Calcium on Dopamine Synthesis and Tyrosine Hydroxylase Activity in Rat Striatum

Abstract: We have investigated three aspects of the relationship between calcium and tyrosine hydroxylase activity in rat striatum. In the first series of experiments, we examined the hypothesis that the rise in dopamine synthesis during increased impulse flow results from a calcium-induced activation of tyrosine hydroxylase. Calcium (12.5–200 μ M ) had no effect when added to crude enzyme or enzyme partially purified by gel filtration. Moreover, incubation of synaptosomes with excess calcium (up to 3.5 m M ) had little or no effect on dopamine synthesis. Incubation with the depolarizing alkaloid veratridine (75 μ M ) did increase dopamine synthesis, but did not alter the activity of tyrosine hydroxylase subsequently prepared from the synaptosomes, despite the presumed rise in intracellular calcium. In the second series we examined the hypothesis that increased dopamine synthesis after axotomy results from activation of tyrosine hydroxylase owing to a decrease in intracellular calcium. Addition of the calcium chelator EGTA (100 μ M ) to crude or partially purified enzyme was without effect, whereas incubation of synaptosomes with EGTA (500 μM ) decreased cell-free enzyme activity. In the third experimental series we examined the relationship between calcium and activation of tyrosine hydroxylase by dibutyryl cyclic AMP. EGTA failed to alter the increase in the activity of tyrosine hydroxylase prepared from synaptosomes incubated with dibutyryl cyclic AMP. However, it blocked the increase in synaptosomal dopamine synthesis and dopamine content normally produced by the cyclic AMP analogue. Thus, tyrosine hydroxylase does not appear to be activated by either increases or decreases in calcium availability. However, calcium may be important for the maintenance of basal tyrosine hydroxylase activity, and may play an indirect role in the expression of tyrosine hydroxylase activation produced by other means.     

Determination of Some Molecular Parameters of Tyrosine Hydroxylase From Rat Adrenal, Rat Striatum, and Human Pheochromocytoma

The molecular parameters of tyrosine hydroxylase (EC 1.14.16.2) from rat adrenal, rat striatum, and human pheochromocytoma were determined by combined gel filtration and sucrose gradient ultracentrifugation. The enzyme from rat adrenal has a calculated molecular weight of 228,000, a Stokes radius of 60.9 A, a sedimentation coefficient of 9.10S, and a frictional ratio of 1.39. The enzyme from rat striatum has a calculated molecular weight of 210,000, a Stokes radius of 54.3 A, a sedimentation coefficient of 9.38S, and a frictional ratio of 1.28. Tyrosine hydroxylase from human pheochromocytoma tissue has a calculated molecular weight of 255,000, a Stokes radius of 68.2 A, a sedimentation coefficient of 9.08S, and a frictional ratio of 1.50. These results indicate that the tyrosine hydroxylases from central and peripheral tissue in the rat are quite similar although the human enzyme appears to be significantly larger.     

Long-Term Induction of Tyrosine Hydroxylase Expression: Compensatory Response to Partial Degeneration of the Dopaminergic Nigrostriatal System in the Rat Brain

Abstract: The present study was undertaken to examine the adaptive changes occurring 1 and 6 months after moderate or severe unilateral 6-hydroxydopamine-induced lesions confined to the lateral part of the rat substantia nigra pars compacta (SNC). The expression of tyrosine hydroxylase (TH) enzyme was analyzed in the remaining dopaminergic nigral cell bodies and in the corresponding striatal nerve endings. In the cell bodies of the lesioned SNC, TH mRNA content was increased (+20 to +30%) 6 months after the lesion without changes in cellular TH protein amounts. The depletion of TH protein in the nerve terminal area was less severe than the percentage of cell loss observed in the SNC at 1- and 6-month postlesion intervals. Moreover, the decrease in TH protein in the ipsilateral striatum was less pronounced 6 months after lesion than 1 month after. That no corresponding change in TH protein content was observed in the cell bodies at a time when TH increased in nerve terminals suggests that the newly synthesized protein is probably rapidly transported to the striatal fibers. These results suggest the existence of a sequence of changes in TH expression between cell bodies and fibers, occurring spontaneously after partial denervation of the nigrostriatal pathway.     

Involvement of Intracellular or Extracellular Calcium in Activation of Tyrosine Hydroxylase Gene Expression in PC12 Cells

Abstract: The relationship between elevations in intracellular free Ca²⁺ concentration ([Ca²⁺]_i) by different mechanisms and tyrosine hydroxylase (TH) gene expression was examined. Depolarization by an elevated K⁺ concentration triggered rapid and sustained increases in [Ca²⁺]_i from a basal level of ～50 to 110–150 nM and three- to fourfold elevations in TH mRNA levels, requiring extracellular calcium but not inositol 1,4,5-trisphosphate (IP₃). On the other hand, bradykinin or thapsigargin, both of which induce release of intracellular calcium stores via IP₃ or inhibition of Ca²⁺-ATPase, rapidly elevated [Ca²⁺]_i to >200 nM and increased TH gene expression (three-to fivefold). Confocal imaging showed that the elevations in [Ca²⁺]_i in each case occurred throughout the cyto- and nucleoplasm. The initial rise in [Ca²⁺]_i due to either bradykinin or thapsigargin, which did not require extracellular calcium, was sufficient to initiate the events leading to increased TH expression. Consistent with this, the effects of bradykinin on TH expression were inhibited by 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid or 3,4,5-trimethoxybenzoic acid 8-(diethylamino)-octyl ester which chelates or inhibits the release of intracellular calcium, respectively. Bradykinin required a rise in [Ca²⁺]_i for <10 min, as opposed to 10–30 min for depolarization to increase TH mRNA levels. These results demonstrate that although each of these treatments increased TH gene expression by raising [Ca²⁺]_i, there are important differences among them in terms of the magnitude of elevated [Ca²⁺]_i, requirements for extracellular calcium or release of intracellular calcium stores, and duration of elevated [Ca²⁺]_i, indicating the involvement of different calcium signaling pathways leading to regulation of TH gene expression.     

Short-Term Inhibitory Effect of Estradiol on Tyrosine Hydroxylase Activity in Tuberoinfundibular Dopaminergic Neurons In Vitro

Abstract: The short-term inhibition by estradiol of tyrosine hydroxylase (TH) in tuberoinfundibular dopaminergic neurons was examined in vitro on hypothalamic slices from ovariectomized rats. TH activity (determined by L-3,4-di-hydroxyphenylalanine accumulation in the median eminence after blockade of decarboxylase with NSD 1055) showed a 30–40% decrease within 1 h of incubation with estradiol. To determine whether a dephosphorylation process was involved in this decline in TH activity, we studied the sensitivity of the enzyme to dopamine (DA) feedback inhibition: In controls, we observed that two kinetically different forms of TH coexisted, with one exhibiting a K_l(DA) of 26.4 ± 2 μM the other being ∼ 10-fold more sensitive to DA inhibition, with a [k_1{DA)] of 2.56 ± 0.17 μM. likely corresponding to a phosphorylated and active form and to a non-phosphorylated and poorly active form, respectively. Conversely. after estradiol treatment all TH molecules exhibited the same K_1(DA) of 2.5 ± 0.3 μM. This effect was stereospecific, because 17α-estradiol could not promote it. whereas with 17β-estradiol. it could be observed at only 10⁻¹¹M and after a short delay (30 min). Finally, this decrease in the K_1(DA) of the purported active form of TH could be prevented by okadaic acid (an inhibitor of protein phosphatases). These results suggest that estradiol can act directly on the mediobasal hypothalamus to trigger a rapid decline in TH activity and that this action may involve a decrease in TH phosphorylation.     

Activation and Multiple-Site Phosphorylation of Tyrosine Hydroxylase in Perfused Rat Adrenal Glands

Tryptic digestion of tyrosine hydroxylase (TH) isolated from rat adrenal glands labeled with 32Pi produced five phosphopeptides. Based on the correspondence of these phosphopeptides with those identified in TH from rat pheochromocytoma cells, four phosphorylation sites (Ser8, Ser19, Ser31, and Ser40) were inferred. Field stimulation of the splanchnic nerves at either 1 or 10 Hz (300 pulses) increased 32P incorporation into TH. At 10 Hz, the phosphorylation of Ser19 and Ser40 was increased, whereas at 1 Hz, Ser19, Ser31, and Ser40 phosphorylation was increased. Stimulation at either 1 or 10 Hz also increased the catalytic activity of TH, as measured in vitro (pH 7.2) at either 30 or 300 microM tetrahydrobiopterin. Nicotine (3 microM, 3 min) increased Ser19 phosphorylation, vasoactive intestinal polypeptide (10 microM, 3 min) increased Ser40 phosphorylation, and muscarine (100 microM, 3 min) increased TH phosphorylation primarily at Ser19 and Ser31. Vasoactive intestinal polypeptide, but not nicotine or muscarine, mimicked the effects of field stimulation on TH activity. Thus, the regulation of rat adrenal medullary TH phosphorylation by nerve impulses is mediated by multiple first and second messenger systems, as previously shown for catecholamine secretion. However, different sets of second messengers are involved in the two processes. The action of vasoactive intestinal polypeptide as a secretagogue involves the mobilization of intracellular calcium, whereas its effects on TH phosphorylation are mediated by cyclic AMP. This latter effect of vasoactive intestinal polypeptide and the consequent increase in Ser40 phosphorylation appear to be responsible for the rapid activation of TH by splanchnic nerve stimulation.     

Tyrosine Hydroxylase Inactivation Following cAMP-Dependent Phosphorylation Activation

Tyrosine hydroxylase, the rate-limiting enzyme in catecholamine biosynthesis, is activated following phosphorylation by the cAMP-dependent protein kinase (largely by decreasing the K^m of the enzyme for its pterin co-substrate). Following its phosphorylation activation in rat striatal homogenates, we find that tyrosine hydroxylase is inactivated by two distinct processes. Because cAMP is hydrolyzed in crude extracts by a phospho-diesterase, cAMP-dependent protein kinase activity declines following a single addition of cAMP. When tyrosine hydroxylase is activated under these transient phosphorylation conditions, inactivation is accompanied by a reversion of the activated kinetic form (low apparent K^m for pterin co-substrate, ≤0.2 mM) to the kinetic form characteristic of the untreated enzyme (high apparent K^m, ≥1.0 mM). This inactivation is readily reversed by the subsequent addition of cAMP. When striatal tyrosine hydroxylase is activated under constant phosphorylation conditions (incubated with purified cAMP-dependent protein kinase catalytic subunit), however, it is also inactivated. This second inactivation process is irreversible and is characterized kinetically by a decreasing apparent V^max with no change in the low apparent K^m for pterin co-substrate (0.2 mM). The latter inactivation process is greatly attenuated by gel filtration which resolves a low-molecular-weight inactivating factor(s) from the tyrosine hydroxylase. These results are consistent with a regulatory mechanism for tyrosine hydroxylase involving two processes: in the first case, reversible phosphorylaton and dephos-phorylation and, in the second case, an irreversible loss of activity of the phosphorylated form of tyrosine hydroxylase.     

Tyrosine Hydroxylase Activation in Mesocortical 3,4-Dihydroxyphenylethylamine Neurons Following Footshock

Mild electric footshock resulted in activation of tyrosine hydroxylase (TH) in prefrontal cortex of mice and rats. In mice, the activation was also observed following restraint. Shock-evoked activation of prefrontal cortex TH was characterized by a decrease of apparent Km for the pterin cofactor 6-methyl-5,6,7,8-tetrahydropterin and an increase of Vmax. Activation of prefrontal cortical TH was also demonstrated in vitro following preincubation under conditions that activate cyclic AMP-dependent protein kinase. Treatment of mice with the noradrenergic neurotoxin N-2-chloroethyl-N-ethyl-2-bromobenzylamine (DSP-4) caused a 70% decrease in prefrontal cortex norepinephrine levels but had no significant effect on the activity of TH in that brain region. Footshock resulted in the activation of prefrontal cortex TH of DSP-4-treated mice, suggesting that shock-evoked activation of the enzyme occurs in terminals of mesocortical 3,4-dihydroxyphenylethylamine neurons.     

Activation of Retinal Tyrosine Hydroxylase In Vitro by Cyclic AMP-Dependent Protein Kinase: Characterization and Comparison to Activation In Vivo by Photic Stimulation

Abstract: Tyrosine hydroxylase in rat retina is activated in vivo as a consequence of photic stimulation. Tyrosine hydroxylase in crude extracts of dark-adapted retinas is activated in vitro by incubation under conditions that stimulate protein phosphorylation by cyclic AMP-dependent protein kinase. Comparison of the activations of the enzyme by photic stimulation in vivo and protein phosphorylation in vitro demonstrated several similarities. Both treatments decreased the apparent K _m of the enzyme for the synthetic pterin cofactor 6MPH₄. Both treatments also produced the same change in the relationships of tyrosine hydroxylase activity to assay pH. When retinal extracts containing tyrosine hydroxylase activated either in vivo by photic stimulation or in vitro by protein phosphorylation were incubated at 25°C, the enzyme was inactivated in a time-dependent manner. The inactivation of the enzyme following both activation in vivo and activation in vitro was partially inhibited by sodium pyrophosphate, an inhibitor of phosphoprotein phosphatase. In addition to these similarities, the activation of tyrosine hydroxylase in vivo by photic stimulation was not additive to the activation in vitro by protein phosphorylation. These data indicate that the mechanism for the activation of tyrosine hydroxylase that occurs as a consequence of light-induced increases of neuronal activity is similar to the mechanism for activation of the enzyme in vitro by protein phosphorylation. This observation suggests that the activation of retinal tyrosine hydroxylase in vivo may be mediated by phosphorylation of tyrosine hydroxylase or some effector molecule associated with the enzyme.     

Is Dopamine-Induced Inhibition of Adenylate Cyclase Involved in the Autoreceptor-Mediated Negative Control of Tyrosine Hydroxylase in Striatal Dopaminergic Terminals?

Abstract The mechanism of the negative control of tyrosine hydroxylase (TH) activity induced by the stimulation of presynaptic 3,4-dihydroxyphenylethylamine (dopamine, DA) autoreceptors was investigated using rat striatal slices and synaptosomes incubated under control ([K⁺] = 4.8 mM) or depolarizing ([K⁺] = 60 mM) conditions. The stimulation of DA autoreceptors by 7-hydroxy-2-(di-n-propylamino) tetralin (1 μM 7-OH-DPAT) produced a significant decrease in TH activity extracted from striatal slices maintained under control conditions. This effect was associated with the complete conversion of TH into an enzyme form with a low affinity for its pterin cofactor (K_m～0.80 mM). Furthermore, compared to TH extracted from control tissues, that from 7-OH-DPAT-exposed striatal slices was more sensitive to the stimulatdry effects of exogenous heparin and cyclic AMP-dependent phosphorylation. Such changes were opposite to those induced by incubating striatal slices with the adenylate cyclase activator forskolin. Indeed, forskolin treatment completely converted TH into an enzyme form with a high affinity for its pterin cofactor (K_m～0.16 mM). Such conversion was associated with a shift in the optimal pH for TH activity from 5.8 (control) to 7.2 (forskolin). Under depolarizing conditions, the blockade by (—)-sulpiride of the stimulation of DA autoreceptors by endogenous DA was associated with a marked activation of TH. Modifications of enzymatic characteristics triggered by (—)-sulpiride were then similar to those induced by forskolin treatment. These data suggest that presynaptic DA autoreceptors modulate the activity of TH by controlling the degree of cyclic AMP-dependent phosphorylation of the enzyme. The blockade by Pertussis toxin of the 7-OH-DPAT-induced inhibition of TH activity is coherent with a possible negative coupling of presynaptic DA autoreceptors (closely related to the D₂ type) with adenylate cyclase. Such negative coupling would account for the reduction of TH activity when presynaptic DA autoreceptors are stimulated.     

Rapid Activation of Tyrosine Hydroxylase in Response to Nerve Growth Factor

Abstract: Nerve growth factor protein (NGF) was found to rapidly promote the activation of tyrosine hydroxylase in cultured rat PC 12 pheochromocytoma cells. PC 12 cultures were exposed to NGF for periods of less than 1 h and the soluble contents of homogenates prepared from the cells were assayed for tyrosine hydroxylase activity. Under these conditions, the specific enzymatic activity was increased by 60 ± 10% (n = 13) in comparison with that in untreated sister cultures. The increase was half maximal by 2–5 min of exposure and at NGF concentrations of about 10 ng/ml (0.36 n M ). Antiserum against NGF blocked the effect. Tyrosine hydroxylase activity could also be rapidly increased by NGF in cultures of PC12 cells that had been treated with the factor for several weeks in order to produce a neuron-like phenotype. This was achieved by withdrawing NGF for about 4 h and then readding it for 30 min. The NGF-induced increase of tyrosine hydroxylase activity in PC12 cultures was not affected by inhibition of protein synthesis and therefore appeared to be due to activation of the enzyme. Kinetic experiments revealed that NGF brought about no change in the apparent K_m of the enzyme for tyrosine or for co-factor (6-methyltetrahydropteridine), but that it did significantly increase the apparent maximum specific activity of the enzyme. These observations suggest that NGF (perhaps released by target organs) could promote a rapid and local enhancement of noradrenergic transmission in the sympathetic nervous system.