FUNCTION AND METABOLISM OF CATECHOLAMINES IN THE BRAIN

Long-term regulation of catecholamine formation by ouabain in cultured bovine adrenal chromaffin cells

The long-term effects of ouabain, an inhibitor of Na+/K+ -ATPase, on catecholamine formation in cultured bovine adrenal chromaffin cells were examined. The increase in [14C]catecholamine formation from [14C]tyrosine induced by ouabain was dependent on incubation time, and its maximal effect was observed after incubation for 8 h. The stimulatory effect of ouabain was concentration dependent (10-300 nM), causing maximal stimulation at 300 nM. The formation of [14C]catecholamines induced by ouabain was not increased by incubation with [14C]DOPA instead of [14C]tyrosine. Ouabain-induced [14C]catecholamine formation was influenced by decreases in extracellular Ca2+ concentration, but not by the presence of cycloheximide or actinomycin D. These results suggested that ouabain stimulates continuous activation of hydroxylation of tyrosine through a Ca2+ -dependent mechanism in cultured bovine adrenal chromaffin cells.

Stimulatory effect of pituitary adenylate cyclase-activating polypeptide on catecholamine synthesis in cultured bovine adrenal chromaffin cells: involvements of tyrosine hydroxylase phosphorylation caused by Ca2+ influx and cAMP

In cultured bovine adrenal chromaffin cells, pituitary adenylate cylase-activating polypeptide (PACAP) stimulated [14C]catecholamine synthesis from [14C]tyrosine (but not from [14C]DOPA) in a concentration-dependent manner, causing maximal stimulation at 10(-7) M. The stimulatory action of PACAP was not affected by staurosporine (an inhibitor of protein kinase C) or in the cells in which protein kinase C was down-regulated by prolonged exposure to TPA (an activator of protein kinase C), whereas it was partially attenuated in Ca(2+)-free medium. PACAP (10(-7) M) increased the formation of [3H]inositol phosphates, [Ca2+]i and 45Ca2+ uptake as well as cAMP. The peptide also stimulated the phosphorylation of tyrosine hydroxylase, the enzyme catalyzing the rate-limiting step in catecholamine synthesis. Catecholamine synthesis and tyrosine hydroxylase phosphorylation stimulated by the maximal effective concentration of dibutyryl cAMP or high K+, which activates Ca2+ uptake, were further enhanced by PACAP, suggesting that both cAMP- and Ca(2+)-dependent protein kinases may be involved in the stimulation of tyrosine hydroxylase phosphorylation and catecholamine synthesis caused by PACAP.

Stimulation by vasoactive intestinal polypeptide of catecholamine synthesis in isolated bovine adrenal chromaffin cells. Possible involvement of protein kinase C

In isolated bovine adrenal medullary cells, vasoactive intestinal polypeptide (VIP) stimulated 14C-catecholamine synthesis from 14C-tyrosine, but not from 14C-DOPA. This stimulatory effect of VIP on 14C-catecholamine synthesis was not dependent upon extracellular Ca2+. VIP did not affect the intracellular cyclic AMP (cAMP) level. The stimulatory effect of VIP on 14C-catecholamine synthesis was additive with that of carbamylcholine, which was dependent upon extracellular Ca2+, but not with that of phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA), an activator of protein kinase C. Moreover, 1-(isoquinolinyl-sulfonyl)-2-methylpiperazine (H-7), an inhibitor of protein kinase C, inhibited not only TPA-stimulated, but also VIP-stimulated 14C-catecholamine synthesis from 14C-tyrosine. These results suggested that VIP stimulated catecholamine synthesis by activation of tyrosine hydroxylase and that protein kinase C was involved in this stimulatory mechanism.

Phorbol ester stimulates catecholamine synthesis in isolated bovine adrenal medullary cells

In isolated bovine adrenal medullary cells, the phorbol ester 12-O-tetradecanoyl phorbol 13-acetate (TPA), an activator of protein kinase C, stimulated [14C]catecholamine synthesis from [14C]tyrosine, but not from [14C]DOPA. This stimulatory effect of TPA on [14C]catecholamine synthesis was not dependent upon extracellular Ca2+, and TPA did not affect the uptake of 45Ca2+ or the release of catecholamine by the cells. TPA also did not affect the intracellular cyclic AMP (cAMP) level. 4 alpha-Phorbol 12, 13-didecanoate, which is not an activator of protein kinase C, did not stimulate the synthesis of [14C]catecholamine from [14C]tyrosine. The stimulatory effect of TPA on [14C]catecholamine synthesis was additive with that of carbamylcholine, but not with that of dibutyryl cAMP (DB-cAMP). From these results, it was suggested that protein kinase C is involved in the regulation of tyrosine hydroxylase activity and that this regulatory mechanism might be similar to that involving cAMP.

Effect of tetrabenazine and alpha-methyl-m-tyrosine on exploratory activity and brain catecholamines in rats

1. Spontaneous exploratory locomotor activity of Wistar rats was measured in photocell activity cages, and brain noradrenaline (NA) and dopamine (DA) were determined fluorometrically after ion exchange purification.2. Tetrabenazine (TBZ) (10 mg/kg) produced a fall in NA and DA concentrations in rat brain stems which was correlated with the fall in activity in female Wistar rats.3. alpha-Methyl-m-tyrosine (alphaMMT) reduced the concentration of rat brain NA without affecting DA concentration or activity.4. Pretreatment with alphaMMT did not stop TBZ from producing a marked reduction in activity and NA concentration, but partially protected DA from the depleting action of TBZ.5. These results support a role for catecholamines in the control of motor activity, but they do not implicate NA more than DA and they emphasize that the mechanism by which drugs affect the concentrations of catecholamines may be more important than the gross concentrations attained.

Effect of drugs used in psychoses on cerebral dopamine metabolism

1. Chlorpromazine 15 mg/kg, given daily to cats for 2 weeks, produced a rise in homovanillic acid (HVA) content of the caudate nucleus, whereas the same dose of thioridazine lacked this effect. Of these two drugs, only chlorpromazine causes a high incidence of drug-induced Parkinsonism in man.2. In the mouse, chlorpromazine, thioridazine and haloperidol increased striatal concentrations of HVA and accelerated the disappearance of dopamine (DA) after inhibition of catecholamine synthesis with alpha-methyltyrosine. Low doses of the three compounds increased, whereas high doses reduced, the concentration of DA in the striatum. In their effects on the DA metabolism of the mouse, chlorpromazine and thioridazine had the same potency, but haloperidol was between 10 and 100 times more active than the other two drugs. In producing hypothermia and sedation, the three compounds were equiactive.3. Oxypertine, another drug apt to produce Parkinsonism in man, caused a severe reduction in striatal DA and hypothalamic noradrenaline (NA). Though the clinical signs produced in the mouse were indistinguishable from those seen after the same dose of chlorpromazine, the biochemical changes in the brain were thus quite different.4. Though all the drugs used caused temporary motor disabilities in animals, these bore no resemblance to human Parkinsonism, even when treatment was continued for 7 weeks or more as it was in cats and monkeys. The latter were treated with chlorpromazine 7.5 mg/kg daily, a dose chosen to avoid loss of weight and which may have been too small to produce toxic side-effects. It caused no changes in striatal DA turnover.5. Even at the high dose of 50 mg/kg, phenoxybenzamine did not increase DA turnover in mouse brain, but it sedated the mice as did the tranquillizers.6. Atropine sulphate, 25 mg/kg, reduced the HVA content of mouse striatum and partially antagonized the rise in HVA produced by phenothiazines. The effect was surmountable. Possible modes of action of atropine are discussed.7. At present we know of two types of biochemical changes which may occur in the brain of animals after treatment with drugs apt to cause Parkinsonism in man: a loss of cerebral catecholamines, as seen after reserpine or oxypertine, or an increase in turnover of DA as after phenothiazines and butyrophenones.