Changes in Levels of Dopamine and Tyramine in the Rat Caudate Nucleus Following Alterations of Impulse Flow in the Nigrostriatal Pathway

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Abstract
Following electrical stimulation of the substantia nigra for 1 h, there was a substantial increase in dopamine (DA) turnover in the rat caudate nucleus evidenced by an increase in its acid metabolite homovanillic acid (HVA). Concurrently, there was an increase in striatal m-tyramine (mTA) and a substantial decrease in p-tyramine (pTA). Lesioning the substantia nigra to decrease impulse flow resulted in a buildup of striatal DA and mTA, but again a decrease in pTA. Following pretreatment with a tyrosine hydroxylase inhibitor, the effects of stimulation of the nigra on mTA were reversed, there being a significant decrease in this amine. The decrease of pTA in response was partially prevented by tyrosine hydroxylase inhibition. The effects of stimulation or substantia nigra lesions on pTA levels were reversed by tyrosine hydroxylase inhibition, a significant increase in this amine being recorded. mTA and DA levels were largely unaffected by a combination of lesion and tyrosine hydroxylase inhibition. The results provide insight into the possible biosynthetic interrelationships between DA and the tyramine isomers in the rat caudate nucleus.