Central dopaminergic regulation of adrenomedullary ornithine decarboxylase activity

The administration of the two dopamine receptor agonists apomorphine (APM) and piribedil (PBD) to rats leads to an increase in ornithine decarboxylase (ODC) activity in the adrenal medulla. In this work, we have tried to elucidate the neural pathways involved in the regulation of this enzyme. The treatments used are: unilateral splanchnicotomy, spinal cord section, intraventricular injection of the neurotoxin 6-hydroxydopamine and section of the brain at various levels. Unilateral splanchnicotomy reduces very significantly the induction of ODC produced by either APM or PBD. Spinal cord section at either of two different levels (T(5) or T(2)) also lowers the response to APM. Intracerebroventricular injection of 6-hydroxydopamine, on the other hand, elevates the mean response to APM, although not to a statistically significant extent. Section of the mesencephalon well below the periaqueductal gray does not alter the response of adrenomedullary ODC to APM. Transection of the diencephalon almost prevents it whereas hypothalamic deafferentation and incomplete diencephalic transection potentiate the effect of this drug. These observations strongly suggest that adrenomedullary ODC activity is predominantly regulated by a central system, originating mainly in the diencephalon-telencephalon and including a facilitatory dopaminergic component.