The role of catecholamines, 5-hydroxytryptamine and m-tyramine in the behavioural effects of m-tyrosine in the rat

The behavioural and neurochemical effects of m-tyrosine and a Monoamine Oxidase Inhibitor in the rat are described. Systemic injections of m-tyrosine (50-150 mg/kg) 30 min after the administration of pargyline (75 mg/kg) produced intense behavioural stimulation which was not evident after injection of either compound alone. The behavioural syndrome induced consisted of forepaw padding, headweaving, backward walking, splayed hindlimbs, wet dog shakes, hyperactivity and hyperreactivity. m-Tyrosine alone or in combination with pargyline caused a significant increase in brain m-tyramine levels and a significant depletion of catecholamines. 5-Hydroxytryptamine (5-HT) levels, however, were unaffected by the administration of m-tyrosine at most of the times studied. The increase in levels of m-tyramine produced by m-tyrosine plus pargyline was 10 times greater than that produced by m-tyrosine alone, whereas the depletion in levels of the more abundant amines was not potentiated by pargyline pretreatment. The biochemical results suggest that an increased formation of m-tyramine may have been responsible for the observed behavioural stimulation and that a threshold level of m-tyramine in the brain appears to be necessary to produce an overt behavioural effect. The behavioural components observed indicate that m-tyramine could act by releasing newly synthesized catecholamines or 5-HT. Alternatively, m-tyrosine may function as a direct agonist at 5-HT or dopamine receptors, although an action on a specific tyraminergic receptor cannot be ruled out at present.