Release and receptor stimulating properties of p-tyramine in rat brain

Radiolabeled p-tyramine could be released from rat striatal slices, which were previously allowed to accumulate this amine, upon depolarization with media containing 40 mM potassium. This depolarization-induced release did not occur in the absence of calcium ions in the medium and over 75% of the labeled material released was identified chromatographically as tyramine. The possible existence of a direct receptor-stimulating activity of p-tyramine was examined by injecting this amine intraventricularly into rats, after pretreatment with various drugs, which inhibit catecholaminergic function, and measuring their motor activity. If injected into rats, which were pretreated with reserpine or alpha-methyl-p-tyrosine, p-tyramine induced a marked increase in the motor activity, which was not accompanied by a significant decrease in brain catecholamines, ruling out the possibility of indirect receptor stimulation. Since the presence of endogenous p-tyramine in the brain has been reported, these data may indicate a specific synaptic role for this amine, perhaps as a neurotransmitter.