Evidence for inhibition of dopamine-beta-hydroxylase in vivo after sub-acute pyrazole treatment in rats.

Pyrazole, a widely used inhibitor of alcohol dehydrogenase, has been shown to cause a decrease of brain and heart noradrenaline (NA). An attempt to explain the mechanism of this effect is now described. L-DOPA (50-200 mg/kg, s.c.) was unable to restore brain or heart noradrenaline levels in pyrazole pre-treated rats. After monoamine oxidase inhibition with tranylcypromine or pargyline there was a slight increase in brain NA in these rats but no further increase was observed in response to L-DOPA (30 mg/kg). Brain dopamine levels were relatively higher in pyrazole pre-treated rats. This difference was particularly clear in the hypothalamus but not present at all in striatum. It was impossible to duplicate the above results using nialamide as the monoamine oxidase inhibitor. After depletion of monoamine stores by reserpine (2 x 2 mg/kg) or oxypertine (75 mg/kg) and treatment with tranylcypromine and L-DOPA it is possible to get an indication of the maximal rate of synthesis of NA. In pyrazole treated rats synthesis of NA in brain was 70% reduced and about 50% reduced in heart. Synthesis of dopamine from L-DOPA was unimpaired. Dopamine-beta-hydroxylase activity in the hypothalamus of rats treated for four days with pyrazole (100 mg/kg i.p.) was more than 40% reduced. This inhibition could not be obtained by addition of pyrazole to samples of purified dopamine-beta-hydroxylase. The results strongly suggest that the reason for the decrease in brain and peripheral NA seen after pyrazole administration in rats is due to inhibition of dopamine-beta-hydroxylase.