Activation by cyclic 3':5'-adenosine monophosphate of tyrosine hydroxylase in the rat brain

L-Tyrosine availability affects basal and stimulated catecholamine indices in prefrontal cortex and striatum of the rat

We previously found that L-tyrosine (L-TYR) but not D-TYR administered by reverse dialysis elevated catecholamine synthesis in vivo in medial prefrontal cortex (MPFC) and striatum of the rat (Brodnik et al., 2012). We now report L-TYR effects on extracellular levels of catecholamines and their metabolites. In MPFC, reverse dialysis of L-TYR elevated in vivo levels of dihydroxyphenylacetic acid (DOPAC) (L-TYR 250-1000 μM), homovanillic acid (HVA) (L-TYR 1000 μM) and 3-methoxy-4-hydroxyphenylglycol (MHPG) (L-TYR 500-1000 μM). In striatum L-TYR 250 μM elevated DOPAC. We also examined L-TYR effects on extracellular dopamine (DA) and norepinephrine (NE) levels during two 30 min pulses (P2 and P1) of K+ (37.5 mM) separated by t = 2.0 h. L-TYR significantly elevated the ratio P2/P1 for DA (L-TYR 125 μM) and NE (L-TYR 125-250 μM) in MPFC but lowered P2/P1 for DA (L-TYR 250 μM) in striatum. Finally, we measured DA levels in brain slices using ex-vivo voltammetry. Perfusion with L-TYR (12.5-50 μM) dose-dependently elevated stimulated DA levels in striatum. In all the above studies, D-TYR had no effect. We conclude that acute increases within the physiological range of L-TYR levels can increase catecholamine metabolism and efflux in MPFC and striatum. Chronically, such repeated increases in L-TYR availability could induce adaptive changes in catecholamine transmission while amplifying the metabolic cost of catecholamine synthesis and degradation. This has implications for neuropsychiatric conditions in which neurotoxicity and/or disordered L-TYR transport have been implicated.

Calmodulin and a cyclic nucleotide-dependent protein kinase facilitate the prolactin-induced increase in tyrosine hydroxylase activity in tuberoinfundibular dopaminergic neurons

Many aspects of tuberoinfundibular dopaminergic neuronal function are increased by elevated prolactin (PRL) levels, including the activity of tyrosine hydroxylase, the rate-limiting enzyme in the biosynthesis of dopamine. This study evaluated the roles of calmodulin, cyclic nucleotide-dependent protein kinase, and calcium/calmodulin-dependent protein kinase II in the PRL-induced increase in tyrosine hydroxylase activity. Ovariectomized rats were treated with haloperidol or ovine PRL (oPRL) for 20-30 h before the experiment, respectively. Treatment with haloperidol increased circulating PRL levels 8-fold and tyrosine hydroxylase activity in the stalk-median eminence 1.8-fold. Treatment with oPRL increased tyrosine hydroxylase activity 1.9-fold. W-7, a calmodulin antagonist, reversed both the haloperidol- and oPRL-induced increase in tyrosine hydroxylase activity to control levels. H-8, a cyclic nucleotide-dependent protein kinase inhibitor, also reversed the haloperidol induced increase in tyrosine hydroxylase activity. KN62, a selective calcium/calmodulin-dependent protein kinase II inhibitor, attenuated the haloperidol-induced increase in tyrosine hydroxylase activity, but KNO4, a structurally related control compound, had no effect. By contrast, the oPRL- and haloperidol-induced increases in tyrosine hydroxylase activity were not altered by KN93, a selective calcium/calmodulin-dependent protein kinase II inhibitor. These data indicate that calmodulin and a cyclic nucleotide-dependent protein kinase contribute to the PRL-induced increase in tyrosine hydroxylase activity, but the role of calcium/calmodulin-dependent protein kinase II is still unclear.

Parallel modulation of striatal dopamine synthetic enzymes by second messenger pathways

The activity of tyrosine hydroxylase and aromatic L-amino acid decarboxylase in the striatum and their mRNA content in the midbrain were assayed in mice following the intracerebroventricular injection of forskolin or phorbol-12,13-myristic acid (PMA). Control and 1-methyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned animals were studied. Both forskolin and PMA induced a rapid and transient increase of tyrosine hydroxylase and aromatic L-amino acid decarboxylase activity in the striatum that lasted less than 45 and 60 min, respectively. A second belated increase of striatal tyrosine hydroxylase and aromatic L-amino acid decarboxylase activities was seen only after forskolin, and it was accompanied by a rise of tyrosine hydroxylase and aromatic L-amino acid decarboxylase mRNA in the midbrain. In the MPTP-lesioned mouse, the rise of tyrosine hydroxylase and aromatic L-amino acid decarboxylase following forskolin appeared exaggerated, while the response to PMA was not. These studies suggest that tyrosine hydroxylase and aromatic L-amino acid decarboxylase of striatum can be modulated in parallel by protein kinase A and protein kinase C, and that exaggerated responsiveness to protein kinase A is observed in the partially denervated striatum.

Activation of striatal tyrosine hydroxylase by in vivo electrical stimulation: comparison with cyclic AMP-mediated activation

These studies were carried out to characterize the activation of rat striatal tyroxine hydroxylase produced by depolarization of the medial forebrain bundle and to evaluate the possible role of cyclic AMP as a mediator of this activation. The enzymatic properties of tyrosine hydroxylase following in vivo depolarization were compared to those produced by treatment of striatal synaptosomes with dibutyryl cyclic AMP (dbcAMP). Similar effects were observed with regard to enzyme distribution, altered sensitivity to dopamine-induced inhibition, and activity as a function of tyrosine concentration. However, differences between the two treatments were also apparent. First, treatment with dbcAMP shifted the pH optimum from 6.2 to 7.0. In contrast, electrical stimulation decreased the rate of decline in activity as the pH was increased above the optimum, but did not shift the pH optimum. Second, plots of tyrosine hydroxylase activity versus cofactor concentration revealed two enzyme forms for both control and electrically stimulated preparations. However, dbcAMP treatment converted the enzyme to a single high affinity form. These results can be explained by one of the following: (1) cyclic AMP is the sole mediator of enzyme activation, but does not produce a maximally activated enzyme following in vivo depolarization, (2) cyclic AMP is only one of several mediators involved or (3) cyclic AMP is not involved in depolarization-induced activation, with activation occurring via the mediation of other intracellular messengers, such as calcium.

The effect of cyclic AMP on the growth of Toxoplasma gondii in vitro

To assess the role of cAMP on the growth and proliferation of Toxoplasma in HL-60 cells we tested the effect of exogenous cAMP and cAMP analogues to the co-culture system of Toxoplasma and HL-60 cells. cAMP, dbcAMP, and br-cAMP stimulated the growth of Toxoplasma at a specific concentration, i.e., 10(0) mM, 10(0) mM, and 10(-1) mM, respectively. There were differences in growth induction kinetics and in the rate of promotion. These results were further verified by treating the co-culture with adenylate cyclase activator, pNHppG, cAMP phosphodiesterase activators, imidazole and A23187, and cAMP phosphodiesterase inhibitors, IBMX, compound 48/80, and theophylline, separately. When the cytosolic cAMP levels increased by the reagents mentioned above, Toxoplasma in the cytoplasm of HL-60 cells stimulated to proliferate more rapidly with concentration-dependent modes compared to the control, and vice versa. It is suggested that some mechanisms are activated by the high levels of cAMP in the cytoplasm, which result in the stimulation of Toxoplasma proliferation.

Influence of neonatal hypothyroidism on adrenal tyrosine hydroxylase activation in the young rat

1. Adrenal TH activation was elicited in young rats (aged 4, 6 and 14 days) by insulin hypoglycaemia. In the control rats, TH activation varied between 125 and 147% above basal values. 2. Neonatal hypothyroidism induced by PTU treatment impaired TH activation. Compensatory treatment with T3 to the PTU-treated young rats led to a return to control activation.

Dopamine release in rat striatum: physiological coupling to tyrosine supply

Intracerebral microdialysis was used to monitor dopamine release in rat striatal extracellular fluid following the intraperitoneal administration of dopamine's precursor amino acid, L-tyrosine. Dopamine concentrations in dialysates increased transiently after tyrosine (50-100 mg/kg) administration. Pretreatment with haloperidol or the partial lesioning of nigrostriatal neurons enhanced the effect of tyrosine on dopamine release, and haloperidol also prolonged this effect. These data suggest that nigrostriatal dopaminergic neurons are responsive to changes in precursor availability under basal conditions, but that receptor-mediated feedback mechanisms limit the magnitude and duration of this effect.

Evidence for the involvement of a cyclic AMP-independent protein kinase in the activation of soluble tyrosine hydroxylase from rat striatum

Activation of rat striatal tyrosine hydroxylase [TyrOHase; tyrosine monooxygenase; L-tyrosine, tetrahydropteridine:oxygen oxidoreductase (3-hydroxylating), EC 1.14.16.2] by ATP/Mg2+ and endogenous protein kinase can be produced without the addition of cAMP. This activation is not due to endogenous free catalytic subunit derived from cAMP-dependent protein kinase. In the presence of amounts of protein kinase inhibitor sufficient for complete inhibition of striatal cAMP-dependent protein kinase and the cAMP-mediated activation of TyrOHase, addition of ATP/Mg2+ results in an enhancement of TyrOHase activity. Enzyme activation does not occur when the nonhydrolyzable form of ATP, adenylyl imidodiphosphate, is substituted for ATP. When TyrOHase is assayed in the presence of ATP/Mg2+ and different concentrations of either tyrosine or 6-methyltetrahydropterin co-factor, a 2-fold increase in enzyme Vmax is demonstrable, with no change in the Km for either substrate or cofactor. In contrast, in the presence of cAMP and ATP/Mg2+, both an increase in Vmax and an enhanced affinity for pterin cofactor are demonstrable. In the latter circumstance, the 2-fold increase in Vmax can be attributed entirely to the action of cAMP-independent protein kinase. The addition of either EGTA or CaCl2 does not modify the effect seen in the presence of ATP, suggesting that the effect of ATP/Mg2+ is not mediated by a Ca2+-dependent protein kinase. These data support the existence of a cAMP-independent striatal protein kinase that can catalyze the activation of TyrOHase.

Tyrosine hydroxylase activity in rat brain synaptosomes: direct measurement using high performance liquid chromatography

By use of high performance liquid chromatography with electrochemical detection to measure dopamine production, tyrosine hydroxylase (EC 1.14.16.2) activity has been measured in rat brain synaptosomes from striatum and forebrain. Normal specific activities three- to fivefold higher than previously reported in the literature for radiochemical methods of assay were found. It is suggested that synaptosomes contain a significant amount of endogenous substrate for tyrosine hydroxylase, which causes dilution of the added labelled tyrosine and hence underestimation of the activity of this enzyme when radiochemical methods are used.

Short-term regulation of hydroxylase cofactor activity in rat brain

Differential drug effects on hydroxylase cofactor activities were observed in the corpus striatum and the locus coeruleus when conditions of sacrifice were controlled. A conformational stability-dependent variable degree of stoichiometric coupling between quinonoid dihydropteridine reductase and tyrosine hydroxylase is proposed as a short-latency influence on hydroxylase cofactor levels.

L-Glutamate decarboxylase

Much progress has been made in recent years regarding enzymological aspects of mammalian brain GAD, such as its purification and characterization, but some uncertainty still remains concerning its molecular weight and forms, and its subunit structure. The availability of antibodies to this enzyme has allowed immunocytochemical studies which have provided important information on the intrinsic organization of GABA-ergic neurones in the CNS, particularly in the cerebellum and nigrostriatal pathway. With the increased understanding of the enzymology of GAD and the distribution of central GABA-ergic neurones, it is becoming feasible to study the regulatory biochemistry of GAD in terms of control and adaptive mechanisms at the cellular level. In our own laboratory, as well as in others, initial approaches have already begun. Obviously, cellular regulation of this phenotypic enzyme is an important issue for the understanding of GABA-ergic neurones and their functions.

Modification of the tyrosine hydroxylase assay. Increased enzyme activity in the presence of ascorbic acid

A modification of the tyrosine hydroxylase assay is described in which ascorbate, rather than 2-mercaptoethanol or dihydropteridine reductase with NADPH, is used as the reductant. Enzyme activity is 3-4 times higher with ascorbate than with the other reducing agents. Low blanks are obtained with the ascorbate system provided that catalase is also included. The tissue distribution and kinetic activation of the enzyme have been studied with the ascorbate assay. The results obtained are consistent with the biological and regulatory properties of the enzyme which have been determined with the other reducing systems.

Effects of db cAMP on tyrosine hydroxylase activity of ganglia and nerve endings

Preincubation of intact superior cervical ganglia or nictitating membrane for 2 h with dibutyryl cyclic AMP (db cAMP) increased the hydroxylation of tyrosine. This effect was not blocked by the protein synthesis inhibitor, cycloheximide. The Km of tyrosine hydroxylase for the substrate, tyrosine, and for the cofactor, reduced pteridine, were decreased by db cAMP. There were no changes in the Vmax of the enzyme. The inhibitory potency of noradrenaline on the hydroxylation of tyrosine was also decreased. Thus an inductive effect may be ruled out. The activation of the enzyme was only observed when the tissues were preincubated with the db cAMP and not when the cyclic nucleotide was added to the isolated enzyme. Preincubation of cervical ganglia for 4 h with db cAMP increased activity of decarboxylase and monoamine oxidase in tissue homogenates without changing the tyrosine hydroxylase activity.

Haloperidol: effect of long-term treatment on rat striatal dopamine synthesis and turnover

The short- and long-term effects of neuroleptic drugs differ both clinically and biochemically. Short-term treatment with such a drug causes a kinetic activation of striatal tyrosine hydroxylase. Long-term treatment causes a prompt activation of the enzyme which is followed by a delayed, compensatory deactivation below control levels. Tolerance also develops to the stimulating effect of haloperidol on striatal dopamine turnover.

Catecholamine-stimulated cyclic GMP accumulation in the rat pineal: apparent presynaptic site of action

Guanosine 3':5'-cyclic monophosphate (cGMP) increased 7-fold in rat pineal glands incubated in the presence of l-norepinephrine. This response consisted of two components-one was stereospecific and inhibited by alpha-adrenergic antagonists while the other was not stereospecific and not readily inhibited by antagonists. Although l-isoproterenol was more potent than l-norepinephrine it had less intrinsic activity and its action was not stereospecifc. The increase in cGMP caused by these catecholamines, unlike that of adenosine 3':5'-cyclic monophosphate (cAMP), was dependent upon extracellular calcium. Ouabain and high levels of potassium produced a marked, calcium-dependent increase in pineal cGMP, without affecting cAMP. There was no effect of cholinergic agonists on cGMP. Surgical denervation markedly reduced the cGMP response to stimulation by l-norepinephrine, potassium, or ouabain. This was in contrast to the enhanced response of cAMP in denervated glands. The nonspecific increase in cGMP caused by l-isoproterenol, however, was not affected by denervation. These data demonstrate the existence of a calcium-dependent presynaptic mechanism for the generation of cGMP which may be mediated by an alpha-adrenergic-like receptor. In addition, the mechanisms regulating pineal cGMP appear to be physiologically distinct from those regulating cAMP.

Tyrosine hydroxylase: allosteric activation induced by stimulation of central noradrenergic neurons

Electrical stimulation of the rat locus coeruleus cases about a 300% increase in the activity of the tyrosine hydroxylase prepared from the hippocampus on the stimulated side and assayed in the presence of subsaturating concentrations of tyrosine and pteridine cofactor. Addition of calcium or cAMP to soluble preparations of tyrosine hydroxylase isolated from the hippocampus produces a similar activation of tyrosine hydroxylase. The activation of tyrosine hydroxylase produced by calcium is reversed by addition of the calcium chelator, EGTA, while the activation produced by cAMP addition or by electrical stimulation of the locus coeruleus is unaffected by addition of EGTA to the assay medium. The activation of tyrosine hydroxylase produced by electrical stimulation or by addition of calcium or cAMP to the assay medium appears to be mediated in part by alterations in the kinetic properties of the enzyme. All treatment causes the enzyme to have an increased affinity for substrate and pteridine cofactor and a decreased affinity for the endproduct inhibitor, norepinephrine. These results are suggestive that the activation of tyrosine hydroxylase which occurs during periods of increased impulse flow in noradrenergic neurons may be initiated by alterations in calcium fluxes or by changes in the steady state levels of cAMP which accompany neuronal depolarization.