Regulation of catecholamine synthesis: Multiple mechanisms and their significance

The Role of the Brain in the Regulation of Peripheral Organs-Noradrenaline Sources in Neonatal Rats: Noradrenaline Synthesis Enzyme Activity

This work is aimed at studying the mechanisms of reciprocal humoral regulation of noradrenaline-producing organs in rats in the perinatal period of development. The activity of noradrenaline synthesis enzymes tyrosine hydroxylase and dopamine-beta-hydroxylase was measured in the brain and adrenal glands 48 and 72 h after the injection of immunotoxin (anti-dopamine-beta-hydroxylase-saporin) into the rat brain ventricles. It was shown that, 48 h after the immunotoxin injection into the brain, the activity of tyrosine hydroxylase in the brain decreased; however, 72 h after the injection it reached the control levels. This fact indicates that noradrenaline synthesis in the survived neurons increases. In the adrenal glands, 72 h after the immunotoxin injection into the brain, the activity of dopamine-beta-hydroxylase increased. This points to a compensatory increase in the rate of noradrenaline synthesis in the adrenal glands when the synthesis of noradrenaline in the brain is inhibited.

Effects of chronic activation of dopamine D-2 receptors in cultures of rat fetal dopaminergic neurons: indications for alterations in functional activity

In Parkinsonian patients, previously subjected to neuronal grafting therapy, the survival and functional status of dopaminergic grafts might be impaired by the concurrent pharmacotherapy with L-DOPA and/or dopamine (DA) D-2 receptor agonists. To test this hypothesis in vitro, we studied the effects of chronic DA D-2 receptor activation on the functional capacity of cultured fetal rat mesencephalic DA neurons, using the activity of tyrosine hydroxylase (TH) and the intracellular dopamine content as neurochemical parameters. In cellular extracts prepared from our cultures, TH activity (as determined by the release of 3H2O from 3H-[3,5] tyrosine) appeared to be tetrahydrobiopterin-, Fe2+, and temperature sensitive, while in intact cells, the catalytic activity of TH could be induced by K(+)-evoked depolarization in a Ca(2+)-dependent way. In contrast, no acute DA D-2 receptor mediated inhibitory effects could be demonstrated in intact cells, either when tested under basal or depolarizing conditions. Nevertheless, after chronic exposure to DA D-2 receptor agonists for 14 days clear differences were observed in the functional status of cultured fetal dopaminergic neurons. Thus, whereas the overall survival and basal TH activity of cultured fetal dopaminergic neurons remained virtually unaltered, the depolarization induced activation of TH was enhanced in agonist-treated cultures. Moreover, after long-term treatment for 14 or 21 consecutive days, the intracellular DA content of agonist treated cultures appeared to be higher, as compared to untreated controls. It is concluded that chronic activation of DA D-2 receptors may induce adaptive alterations in the functional activity of cultured fetal dopaminergic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for protein kinase C involvement in the short-term activation by prolactin of tyrosine hydroxylase in tuberoinfundibular dopaminergic neurons

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 Ki(DA) value of 29.92 +/- 0.49 microM, the other being approximately 15-fold more sensitive to DA inhibition with a Ki(DA) of 1.96 +/- 0.09 microM, 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 Ki(DA) remained unaffected, whereas the Ki(DA) of the purported active form of TH increased to 62.6 +/- 0.8 microM, suggesting an increase in the enzyme phosphorylation. This increase in the Ki(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-tetradecanoylphorbol 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.

Relationship between tyrosine hydroxylase content and noradrenergic cell reactivity to piperoxane: an in vivo voltammetric approach in the rat locus coeruleus

A previous electrochemical study showed that the increase in the tyrosine hydroxylase (TH) content of the locus coeruleus (LC) produced by RU24722 administration was associated with a relative decrease in the catecholaminergic metabolic reactivity of this nucleus to a hypotensive stimulus. Since alpha 2 receptors participate in the regulation of the activity of both LC neurons and TH, the aim of the present work was to evaluate the possible involvement of the autoinhibition mediated by alpha 2 autoreceptors in the inverse relationship between the reactivity of the LC and its TH content. Our study was divided into two successive steps: (i) the electrochemical measurement of the in vivo metabolic activation of LC cells in response to alpha 2-adrenergic receptor blockade, and (ii) the evaluation of the quantity of TH every 100 microns along the caudorostral axis in each recorded LC. The capacity of TH protein to be activated was evaluated by the measurement, using differential normal pulse voltammetry, of the in vivo variations of the extracellular 3,4-dihydroxyphenylacetic acid concentrations in response to six cumulated doses of the alpha 2-antagonist piperoxane. The corresponding dose-response curves, determined in control- and RU24722-treated rats, were expressed as a function of the quantity of TH contained either in the whole recorded LC or in the 100 microns-wide coronal interval surrounding the recording site. It was established that the slopes of the dose-response curves were significantly (P < 0.01) and inversely related to the quantity of TH at the level of the recording site. This result suggests that the negative control of the catecholaminergic metabolic reactivity in a restricted area of the LC could be directly or indirectly dependent on the level of expression of TH protein in this particular area.

Neurochemical characterization of the alterations in the noradrenergic afferents to the cerebellum of adult rats exposed to X-irradiation at birth

A single dose of x-irradiation was applied on the cephalic end of newborn rats, and the alterations in the noradrenergic afferents to the cerebellum were studied 180 days later. A net increase in the noradrenaline content of cerebellum was found (122% of nonirradiated controls). The response of noradrenaline content to reserpine injection (0.9 mg/kg, i.p.) was similar in exposed and control rats. Likewise, the 3H release induced by Ro 4-1284 from cerebellar cortex slices labeled with [3H]noradrenaline was unmodified by x-rays, although a mild increase in the spontaneous efflux of 3H was found. The retention of 3H by the slices was reduced in exposed animals (58% of controls). Both the in vitro activity of tyrosine hydroxylase and the accumulation of L-3,4-dihydroxyphenylalanine (L-DOPA) were not significantly different between x-treated rats and controls. In contrast, monoamine oxidase activity was markedly reduced in x-irradiated cerebellum (38% of controls). The x-ray-induced decrease in cerebellar weight (-60%) resulted in marked increases in noradrenaline concentration (223%), tyrosine hydroxylase activity per milligram of protein (206%), and 3H retention (50%). The accumulation of L-DOPA per gram of tissue was also increased at every time considered. These data indicate that x-irradiation at birth produces a cerebellar loss not completely shared by the noradrenergic afferents, and a permanent imbalance between the noradrenergic afferent input and its target cells might eventually result.(ABSTRACT TRUNCATED AT 250 WORDS)

Short-term inhibitory effect of estradiol on tyrosine hydroxylase activity in tuberoinfundibular dopaminergic neurons in vitro

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-dihydroxyphenylalanine 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 Ki(DA) of 26.4 +/- 2 microM and the other being approximately 10-fold more sensitive to DA inhibition, with a Ki(DA) of 2.56 +/- 0.17 microM, likely corresponding to a phosphorylated and active form and to a nonphosphorylated and poorly active form, respectively. Conversely, after estradiol treatment all TH molecules exhibited the same Ki(DA) of 2.5 +/- 0.3 microM. This effect was stereospecific, because 17 alpha-estradiol could not promote it, whereas with 17 beta-estradiol, it could be observed at only 10(-11) M and after a short delay (30 min). Finally, this decrease in the Ki(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.

Increased cardiac production of dihydroxyphenylalanine (DOPA) during sympathetic stimulation in anaesthetized dogs

Entry of dihydroxyphenylalanine (DOPA) into plasma from specific organs may reflect regional activity of tyrosine hydroxylase, the enzyme responsible for the immediate synthesis of DOPA and rate-limiting for subsequent formation of catecholamines. Therefore, cardiac spillovers of DOPA, noradrenaline and the intraneuronal metabolite of noradrenaline, dihydroxyphenylglycol (DHPG), were examined during two periods of graded electrical stimulation of the sympathetic nerves to the heart in anesthetized dogs. Responses were examined before and after neuronal uptake blockade with desipramine. Cardiac spillover of DOPA increased by 1.8- and 4.4-fold during sympathetic stimulation before desipramine and by 1.6- and 3.3-fold after desipramine. Fold increases in cardiac spillover of DOPA were much lower than but positively related with fold increases in noradrenaline spillover (5.9- and 13.8-fold increases before and 9.0- and 15.8-fold increases after desipramine). Increases in cardiac spillover of DHPG (1.5- and 2.3-fold increases) were blocked by desipramine so that fold changes in spillover of DOPA were greater than and poorly related to changes in spillover of DHPG. Fold increases in cardiac spillover of DOPA showed a close one-to-one positive relationship with fold increases in the sum of cardiac spillovers of noradrenaline and dihydroxyphenylglycol before and after desipramine. For a given fold increase in noradrenaline release, transmitter turnover is increased fractionally and noradrenaline synthesis need also only increase fractionally to maintain transmitter stores constant. The close relationship between fold increases in cardiac spillover of DOPA and combined spillovers of noradrenaline and DHPG is consistent with regulation of tyrosine hydroxylase activity to match changes in noradrenaline synthesis with changes in noradrenaline turnover. Changes in cardiac spillover of DOPA appear to reflect local changes in tyrosine hydroxylase activity.

Studies on the origin of rat hippocampal dihydroxyphenylacetic acid using microdialysis

With a dialysis probe implanted in the rat ventral hippocampus we have monitored effects on noradrenaline (NA) and dihydroxyphenylacetic acid (DOPAC) of the infusion through the dialysis probe of drugs acting at dopaminergic and noradrenergic presynaptic autoreceptors; clonidine and idazoxan produced changes in the extracellular concentration of both NA and DOPAC, whereas LY171555 and sulpiride had no effect on NA but produced changes in the concentration of DOPAC. The combined effect of clonidine and LY171555 on DOPAC was additive. Hippocampal DOPAC is therefore derived from both the noradrenergic and the dopaminergic projection to the hippocampus.

Enhanced Tyrosine Hydroxylation in Hippocampus of Chronically Stressed Rats upon Exposure to a Novel Stressor

We have used microdialysis to measure the in vivo level of tyrosine hydroxylation in hippocampus of the freely moving rat. An inhibitor of aromatic amino acid decarboxylase, NSD-1015, was administered through the dialysis probe and the resulting accumulation of 3,4-dihydroxyphenylalanine (DOPA) in extracellular fluid of hippocampus was quantified. Administration of the tyrosine hydroxylase inhibitor, alpha-methyl-p-tyrosine, decreased extracellular DOPA to undetectable level. In addition, both systemic and local application of clonidine, an alpha 2-adrenergic agonist, produced a decrease in extracellular DOPA. In response to acute tail shock, a significant increase in extracellular DOPA was observed. Thus, it appears that in vivo accumulation of DOPA after local administration of NSD-1015 provides a reliable index of hippocampal tyrosine hydroxylation. We have used this technique to investigate whether prior exposure to chronic stress alters the in vivo level of tyrosine hydroxylation in hippocampus under basal conditions as well as in response to a novel stressor. In rats previously exposed to chronic cold stress, the basal accumulation of extracellular DOPA did not differ from naive controls. Acute tail shock, however, produced a significantly greater and more prolonged elevation in extracellular DOPA of chronically stressed rats. These data suggest that enhanced biosynthetic capacity of noradrenergic terminals may be one mechanism underlying adaptation to chronic stress.