An analysis of DOPA-induced locomotor stimulation in mice with inhibited extracerebral decarboxylase

Effects of dietary tyrosine on L-dopa- and amphetamine-induced changes in locomotor activity and neurochemistry in mice

Recent findings suggest that intraperitoneal injections of L-tyrosine at high doses (100 mg/kg) alters amphetamine-induced changes in behavior by restoring amphetamine-induced decreases in whole brain norepinephrine (NE). The present study examined the motor effects of L-dihydroxyphenylalanine (L-dopa) and d-amphetamine sulfate in mice after treatment with a basal casein diet supplemented with L-tyrosine. The basal diet supplemented with 1-4% L-tyrosine, or 1-4% L-phenylalanine, produced no changes in motor activity in otherwise untreated mice. Whereas L-dopa (25-100 mg/kg) following inhibition of extracerebral decarboxylase by Ro 4-4602 (25 mg/kg) slightly decreased activity in diet control (casein) animals, this drug treatment enhanced motor activity in a dose-related fashion when L-tyrosine was added to the diet. Increases in motor activity following low doses of amphetamine (0.75-1.5 mg/kg) in casein control mice were antagonized by dietary L-tyrosine, but a higher dose of d-amphetamine (3 mg/kg) interacted with the addition of L-tyrosine producing an increase in motor activity. Neurochemical changes observed in brain concentrations of tyrosine, dopamine (DA), norepinephrine (NE), dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA), tryptophan, serotonin (5-HT) and 5-hydroxy-indoleacetic acid (5-HIAA) following drug and diet treatments suggest that 5-HT systems, in addition to catecholamine systems, may be involved in mediating these effects.

Studies on the activity of L-threo-3,4-dihydroxyphenylserine (L-DOPS) as a catecholamine precursor in the brain. Comparison with that of L-dopa

L-Threo-3,4-dihydroxyphenylserine (L-DOPS) was compared with L-3,4-dihydroxyphenylalanine (L-DOPA) with respect to their activities as central amine precursors. The apparent Km value (the substrate affinity) of L-DOPS for aromatic L-amino acid decarboxylase was nearly equal to that of L-DOPA, whereas the vmax value (the rate of decarboxylation) of L-DOPS was much smaller than that of L-DOPA, the penetration of L-DOPS into the brain through the blood-brain barrier was found to be smaller (about one-fourth) than that of L-DOPA but, for an amine precursor, it was still substantial. Unlike L-DOPA, L-DOPS did not cause a marked accumulation of norepinephrine (NE), the corresponding catecholamine in the brain, but nialamide, a monoamine oxidase inhibitor significantly enhanced the L-DOPS-induced rise of NE. Moreover, the brain concentration of 3-methoxy-4-hydroxy-phenylethyleneglycol (MHPG), the principal end metabolite of NE, was increased markedly by L-DOPS. These results suggest that L-DOPS may act as an NE precursor in the brain and activate NE neurons by increasing the turnover rate of NE.

Studies on the central action of L-threo-3,4-dihydroxyphenyl-serine (L-threo-DOPS) in FLA-63-treated mice

In order to clarify the central action of L-threo-DOPS, the effect of benserazide on behavioral and biochemical changes by L-threo-DOPS in FLA-63-treated mice was studied. L-threo-DOPS in combination with nialamide markedly increased both the locomotor activity and the concentrations of the brain, heart and kidney norepinephrine (NE) in the FLA-63-treated mice. Benserazide at low doses did not alter either the rise of the brain NE level or the increase in locomotor activity, whereas it significantly inhibited the rise of the heart and kidney NE levels. Benserazide at a high dose significantly inhibited all of them. These results suggested that the increase in locomotor activity might be mediated via activation of the central noradrenergic neurons system by L-threo-DOPS.

Effect of brain monoamine precursors on stress-induced behavioral and neurochemical changes in aged mice

Male CF-1 mice aged 22 months showed approximately the same level of motor activity and aggressive behavior as 3-month-old mice under control (no stress) conditions, or 45 min following cold swim stress. Increasing brain catecholamine activity by dietary L-tyrosine treatment had no effect on these two age groups either under control conditions or after stress. In contrast, 30-month-old mice showed lower motor activity under control conditions which was raised significantly by supplementation of the diet with L-tyrosine. However, marked reductions in activity and aggression following stress were observed in the 30-month-old animals and these deficits were not reversed by L-tyrosine treatment prior to stress. Reduction in motor activity was greatest in stressed, 30-month-old mice on L-tyrosine supplemented diets. Compared to 3-month-old mice, the 30-month-old animals had lower brain tyrosine following dietary L-tyrosine treatment, lower brain tryptophan, norepinephrine (NE), dopamine (DA) and DOPAC, but higher HVA, serotonin (5-HT) and 5-HIAA levels. Under both control (no stress) and stress conditions, L-tyrosine pretreatment decreased brain 5-HT in the young animals, but increased 5-HT in the old mice. After stress the 30-month-old animals evidenced only slight increases in levels of blood corticosterone. Brain tyrosine was reduced by stress in the young animals but increased by stress in the old animals. Stress-induced decreases in brain NE and increases in serotonin and 5-HIAA levels were observed in both age groups. These results are consistent with hypotheses concerning age-related alterations in brain monoamine functions and adrenocortical control mechanisms.

Effects of drug-induced changes in brain monoamines on aggression and motor behavior in mice

Mice maintained on a basal casein diet supplemented with 4% L-tyrosine potentiated L-DOPA effects on aggression. At low doses (12.5-25 mg/kg) L-DOPA increased aggression whereas at high doses (50-100 mg/kg) it decreased aggression. 5-HTP (50-200 mg/kg) produced a dose-dependent decrease in aggression and motor activity which was antagonized by pretreatment with dietary L-tyrosine (4%) or L-DOPA (50 mg/kg). L-DOPA induced reductions in motor activity were, in turn, antagonized by 5-HTP. Increases in motor activity following d-amphetamine (3 mg/kg) were sharply reduced by 5-HTP (50-100 mg/kg), but 5-HTP potentiated reductions in aggression following d-amphetamine. The concentration in brain of tyrosine, DOPA, dopamine (DA), noradrenaline (NA), DOPAC, HVA, tryptophan, serotonin (5-HT), and 5-HIAA were obtained following drug and diet treatments. The changes observed, particularly in DA and 5-HT metabolites, provide further evidence for an inhibitory role of brain 5-HT systems in the mediation of the behavioral effects of d-amphetamine and the catecholamine precursors, L-tyrosine and L-DOPA.

Time and dose influences on the behavioral effects of L-DOPA and 5-hydroxytryptophan after inhibition of extracerebral decarboxylase

The influence of time and dose factors on the locomotor activity and gross behavioral effects of l-3,4-dihydroxyphenylalanine (L-DOPA) and 5-hydroxytryptophan (5-HTP) were assessed a albino mice pretreated with the peripheral decarboxylase inhibitor, MK-486, L-DOPA caused stereotyped behaviors and a decrease in orienting responses and locomotor activity, followed at the highest dose by increased locomotor activity. 5-HTP caused a dose-dependent increase in orienting responses and locomotor activity followed after 90 min by a dose-dependent decrease in locomotor activity. Administration of MK-486 and L-DOPA followed in the same animal by 5-HTP resulted in a diminution of the effects seen after either drug alone. This apparent antagonism may result from competition between the two amino acids at sites of membrane transport in the brain.