Opposite effect of harmaline on serotonin and on dopamine and its metabolites, homovanillic acid and norepinephrine, in the brain of the cat

Abstract

A comparison between the effects of a single injection and of several injections (24 h) of harmaline on the concentrations of serotonin, dopamine, homovanillic acid (HVA), and norepinephrine in different structures of the brains of normal cats gave the following results. Treatment during a 24-h period caused a somewhat higher increase of serotonin in the thalamus and in the striatum than after a single injection but neither treatment affected the concentration of norepinephrine in the hypothalamus. Under the same conditions, a single injection of harmaline produced a slight decrease of HVA but not of dopamine in the striatum whereas the 24-h treatment caused a significant decrease of dopamine and almost complete depletion of HVA in the same structure. A single dose of harmaline (15 mg/kg) injected before L-DOPA (30 mg/kg) prevented the marked increase of dopamine and HVA that normally occurs in the striatum after the injection of L-DOPA alone. Harmaline injected before 5-HTP did not prevent the marked increase of serotonin in the thalamus and in the striatum. Harmaline appears to have a trapping effect on dopamine in the striatum by inhibiting its conversion to homovanillic acid and by simultaneously preventing its formation from L-DOPA This peculiar effect of harmaline is considered in relation to its effect on extrapyramidal syndromes which are known to be associated with a metabolic imbalance of the monoamines in extrapyramidal structures.