Noradrenaline synthesis from L-DOPA in rodents and its relationship to motor activity

Abstract

Evidence has been obtained for an increase in noradrenaline (NA) turnover after administration of L0DOPA to rodents. Normal mice, and those pre-treated with either reserpine or alpha-methyl-p-tyrosine (AMPT) were given L-DOPA (200 mg/kg) plus MK 486 (alpha-methyldopahydrazine; 25 mg/kg). In all cases L-DOPA produced a rise in cerebral dopamine (DA) levels. Cerebral NA levels were increased by L-DOPA in reserpinised and AMPT-treated mice. The same dose of L-DOPA produced no change in NA in normal mice, although pre-treatment with the monoamine oxidase inhibitor pargyline (200 mg/kg) resulted in a greater rise in NA 1 hr after L-DOPA compared to animals receiving pargyline alone. This evidence suggests that NA is synthesized from- L-DOPA in all these situations. But whole brain 3-methoxy-4-hydroxyphenylglocol sulphate (MOPEG-SO4), a major metabolite of NA, measured after administration of the same dose of L-DOPA plus MK 486, was unaltered in normal and AMPT-treated rats, and was significantly decreased in reserpinised rats. However, an elevation of whole brain MOPEG-SO4 was found in reserpinised and AMPT-treated rats after a lower dose of L-DOPA (50 mg/kg). This discrepancy may be explained by high doses of L-DOPA causing inhibiton of catechol-O-methyl transferase (COMT), which is suggested by the observation that the forebrain homovanillic acid (HVA): 3,4-dihydroxyphenylacetic acid (DOPAC) ratio was significantly lower after the high dose of L-DOPA than in untreated mice. Such an inhibition would prevent formation of MOPEG-SO4. Pretreatment with dopamine-beta-hydroxylase inhibitor FLA (63(bis-(1-methyl-4-monopiperazinyl-thiocarbonyl)disulphide) prevented the increase in NA And MOPEG-SO4 formation observed following L-DOPA induced motor activity in these groups of animals suggesting the involvement of NA in the production of such behaviour.