Characterization of haloperidol-mediated effects on rat striatal tyrosine hydroxylase

[3H]DOPA formed from [3H]tyrosine in living rat brain is not committed to dopamine synthesis

Tyrosine hydroxylase of catecholamine neurons catalyzes the synthesis of 3,4-dihydroxphenylalanine (DOPA), which is subsequently metabolized to dopamine by DOPA decarboxylase (DDC). However, DOPA is not committed to decarboxylation in vivo because export of DOPA from brain and metabolism of DOPA other than decarboxylation are possible. To estimate the relative magnitudes of the several fates of DOPA, the kinetics of the uptake and metabolism of L-[3H]tyrosine ([3H]Tyr, intravenous infusion) was measured in brain of rats pretreated with NSD 1015, an inhibitor of DDC. Some rats were pretreated with haloperidol before the blockade of DDC. The [3H]Tyr was incorporated into brain protein at a rate constant of 0.03 min(-1). The relative tyrosine hydroxylase activity in striatum was 0.005 min(-1) at 30 minutes after NSD 1015, 0.011 min(-1) 3 hours later, and 0.020 min(-1) after haloperidol treatment. The rate constant for the clearance of DOPA from brain (0.06 min(-1)) and earlier estimates of the rate constant of DDC activity in striatum (0.26 min(-1)) together predict that 80% of DOPA formed in normal rat striatum normally is available for dopamine synthesis. It follows that modulation of DDC activity can influence the rate of DA synthesis by affecting the relative magnitude of the several fates of DOPA in living brain.

Evidence for a specific effect of BHT 920, an azepine derivative, on tyrosine hydroxylase in the dopaminergic system of the rat

The effect of BHT 920, a putative presynaptic dopamine receptor agonist, on tyrosine hydroxylase was investigated in rats. The activity of the high affinity (BH4) form of striatal tyrosine hydroxylase was investigated dose-dependent manner in rats treated with BHT 920. This effect was pronounced in the dopaminergic system and was not observed to the same extent in the adrenal medulla. In vitro, BHT 920 had no effect upon striatal tyrosine hydroxylase activity. BHT 920 also did not affect either striatal adenylate cyclase activity or the extent of its stimulation by dopamine. The results concerning tyrosine hydroxylase were complemented by measurements of dopamine and DOPA in the striatal and the limbic system. The reduction in DOPA accumulation and in the high affinity form of tyrosine hydroxylase activity elicited by BHT 920 could be blocked by haloperidol, suggesting that BHT 920 may interact with the D2 dopamine receptor although a functional antagonism could not be ruled out. The present results suggest that BHT 920 may exert a specific effect upon tyrosine hydroxylase in dopaminergic nervous tissue which is not mediated by alpha 2-adrenoceptors.

Apomorphine enantiomers' effects on dopamine metabolism: receptor and non-receptor related actions

The enantiomers of apomorphine (APO) inhibited dopamine synthesis in rat striatal synaptosomes, with R(-)-APO being about twice as potent as S(+)-APO. Sulpiride, a DA receptor antagonist, partially antagonized the inhibitory effects of only (-)-APO, suggesting that (-)-APO's, but not (+)-APO's, effects on dopamine synthesis may be at least partially receptor-mediated. The addition of 6-methyl-5,6,7,8-tetrahydrobiopterine (6-MPH4), an artificial cofactor for tyrosine hydroxylase, partially antagonized the inhibitory effects of both enantiomers, being considerably more effective against the (+)enantiomer. These data suggest that the APO enantiomers may directly inhibit enzymes within the synaptosome which regulate dopamine synthesis. Furthermore, investigations measuring DA synthesis rates in synaptosomes that had been pre-incubated with (-)-APO and then washed to remove the (-)-APO in the medium, indicate that (-)-APO may be retained by synaptosomes. Preliminary studies measuring the accumulation of [3H](-)-APO by synaptosomes also suggest that synaptosomes can accumulate APO. Although both APO enantiomers suppressed DA synthesis in vitro, only (-)-APO reduced striatal DA metabolite concentrations in vivo, and this reduction was prevented by haloperidol, a DA receptor antagonist. In addition, 6-MPH4 prevented the decrease in the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) produced by (-)-APO but not the decrease in the DA metabolite homovanillic acid (HVA).

Calcium, ATP, and magnesium activate soluble tyrosine hydroxylase from rat striatum

Soluble tyrosine hydroxylase (TH) prepared from rat striatum by sonication, centrifugation, and gel filtration on Sephadex G-25 was activated by preincubation with Ca2+, ATP, and Mg2+. Activation occurred with micromolar concentrations of Ca2+ and required the presence of both ATP and Mg2+. The activation was reversible and was characterized by a large decrease of apparent Km for the pteridine cofactor and a small increase of Vmax. Ca2+-induced activation was small when TH activity was assayed at pH values near the optimum, but the magnitude of the activation increased with increasing assay pH. The activation apparently did not involve Ca2+-activated protease because it was not affected by the protease inhibitor leupeptin. Nor did it involve cyclic AMP-dependent protein kinase, as evidenced by the failure of the heat-stable inhibitor of this kinase to decrease Ca2+-induced TH activation. Furthermore, the activation of TH evoked by Ca2+ and that produced by cyclic AMP was additive. These experiments indicate that striatal TH can be activated in vitro by an endogenous Ca2+-dependent mechanism. The similarity of the Ca2+-induced activation of TH to that elicited by increased neuronal activity and terminal depolarization is discussed.

Effect of dexamethasone on biopterin levels and tyrosine hydroxylase activity in PC-12 cells

Culture of PC-12 cells in 1 microM dexamethasone for 24, 48 or 72 hr did not alter significantly PC-12 cell total biopterin levels, although tyrosine hydroxylase activity in extracts of cell homogenates was increased 2- to 3-fold. Increasing the concentration of dexamethasone to 10 microM did not change biopterin levels or result in further increases in tyrosine hydroxylase activity measured after 72 hr. Culture of cells in dexamethasone markedly decreased the ratio of biopterin concentration to tyrosine hydroxylase activity. The molar ratio of tyrosine hydroxylase subunits to biopterin in control cells can be estimated to be approximately one. Therefore, following induction of the enzyme by dexamethasone, there appears to be an excess of enzyme molecules relative to cofactor molecules in these cells.

Short-term regulation of tyrosine hydroxylase in tonically-active and in tonically-inactive dopamine neurons: effects of haloperidol and protein phosphorylation

Dopamine (DA)-containing neurons of retina were employed as an experimental model for studying the short-term regulation of tyrosine hydroxylase (TH) in tonically-active and tonically-inactive neurons. These DA-containing neurons are trans-synaptically activated by light. Two mechanisms have been observed in this system for regulation of TH activity. A short-term activation of TH that is characterized by a decreased apparent Km for pteridine cofactors occurs in response to rapid increases of neuronal activity. A second mechanism occurs in response to prolonged, tonic changes of neuronal activity and is characterized by changes of Vmax. Both the Km changes and Vmax changes represent changes of specific activity of TH rather than enzyme induction. To determine the effects of short-term increases of neuronal activity on TH in tonically-active and tonically-inactive neurons, the effects of acute administration of haloperidol were examined in rats that were continuously light-exposed or light-deprived for 4 days. Haloperidol increased TH activity in both light-exposed and light-deprived retinas. The drug elicited the same percent stimulation in both experimental conditions. However, because the basal activity of TH was higher in the light-exposed than the light-deprived retinas, the absolute increase of TH specific activity was greater in the light-exposed samples. The effect of protein phosphorylation on TH activity in extracts of chronically light-exposed or light-deprived retinas was also examined to determine if the differences in the response to haloperidol might be due to a difference in the amount of TH available for short-term activation. Phosphorylation by endogenous cyclic AMP-dependent protein kinase (APK) or by purified catalytic subunit of APK resulted in larger increases of TH specific activity in extracts of light-exposed retinas than in those of light-deprived retinas. As was observed for haloperidol-induced activation, the percent stimulation elicited by phosphorylation was similar in extracts of light-exposed and light-deprived retinas. These observations suggest that more enzyme is available for short-term activation in tonically-active neurons than in those that are tonically inactive. A hypothetical model is proposed in which TH exists in active and inactive forms, the ratio of which depends on the tonic level of neuronal activity.

Adaptation to stress: tyrosine hydroxylase activity and catecholamine release

The effects of adaptation to stress and of genetic differences on levels of in vitro tyrosine hydroxylase (TH) activity and in vivo catecholamine (CA) release are reviewed. It is shown that adaptation of animals to a wide variety of stressors including immobilization, electroconvulsive shock, footshock, hemorrhage, exercise and cold exposure results in a reduced CA response in the plasma, brainstem and heart to subsequent exposure to the same stress. Adaptation to many of the latter stressors also produces increased in vitro levels of TH activity. A similar inverse relation between in vitro TH activity and in vivo CA release is described for two inbred rat strains which differ in emotionality (Brown-Norway and Wistar Kyoto). The inverse relationship between TH activity and CA release may reflect different processes of biochemical adaptation utilized either for acclimation to stress, for preparation for emergency reactions or for changes in the metabolic costs of transmitter release. The similarity between environmental and genetic effects on these variables suggests that the above changes have a common adaptive function.

Tyrosine hydroxylase activation. Comparison of in vitro phosphorylation and in vivo administration of haloperidol

Rat striatal tyrosine hydroxylase (TH) was assayed 2 hr following treatment with 1 mg/kg haloperidol. TH activity in striata from haloperidol-treated rats (haloperidol TH) was increased significantly relative to control when assayed at pH 7.0, but not at pH 6.0, in the presence of 175 muM tetrahydrobiopterin (BH4). TH was also phosphorylated in vitro, catalyzed by sufficient quantities of the catalytic subunit of bovine heart protein kinase to cause greater than 90% activation after 10 min. TH was activated by phosphorylation at both pH 6.0 and pH 7.0, but the activation was greater at pH 7.0. Haloperidol TH, activated relative to control TH at pH 7.0, was activated by phosphorylation, but there was no difference between haloperidol TH and control TH activity at either pH 6.0 or 7.0 following phosphorylation. Comparison of Lineweaver-Burk plots of nonphosphorylated and phosphorylated TH indicated that activation by phosphorylation was due to a 5-fold change in Km for BH4 and a 2-fold change in Vmax at pH 7.0. Haloperidol TH kinetics were intermediate between those of nonphosphorylated and phosphorylated TH at pH 7.0. Analysis by Lineweaver-Burk, Hanes-Woolf, and Eadie-Scatchard plots suggested that the haloperidol TH kinetic data were the result of a mixture of two forms of the enzyme, with different affinities for cofactor. Theoretical calculations of TH activity of mixtures of nonphosphorylated and phosphorylated TH suggested that the haloperidol data could be explained by postulating a mixture of 25-35% phosphorylated TH molecules with 65-75% nonphosphorylated TH molecules. An hypothesis of the role of TH phosphorylation during conditions of increased neuronal firing rate, such as may occur with haloperidol treatment, is presented.

Tetrahydrobiopterin in striatum: localization in dopamine nerve terminals and role in catecholamine synthesis

The hydroxylase cofactor, tetrahydrobiopterin, and its biosynthetic system are localized in dopaminergic nerve terminals in the striatum. This conclusion is based on the nearly equivalent loss of tyrosine hydroxylase and tetrahydrobiopterin and its initial biosynthetic enzyme, guanosine triphosphate cyclohydrolase, after injection of 6-hydroxydopamine into the substantia nigra. The role of the hydroxylase cofactor in the regulation of dopamine synthesis is reassessed.