In Vivo Voltammetric Monitoring of Catecholamine Metabolism in the A1 and A2 Regions of the Rat Medulla Oblongata

Catecholaminergic metabolism was estimated in A1 and A2 noradrenergic nuclei of the rat medulla oblongata using differential normal pulse voltammetry combined with electrochemically treated carbon fiber microelectrodes. In both areas an oxidation peak appearing at +50 mV was recorded. Electrochemical data and pharmacological experiments indicated that 3,4-dihydroxyphenylacetic acid (DOPAC) synthesized by noradrenergic neurons was the major contributor to this signal. Indeed, alpha-methyl-p-tyrosine, by inhibiting tyrosine hydroxylase, and pargyline, by inhibiting monoamine oxidase, both totally suppressed the peak appearing at +50 mV in A1 and A2 areas. Conversely, FLA-63, an inhibitor of dopamine-beta-hydroxylase, increased it. Moreover, a local and unilateral injection of catecholaminergic neurotoxin (6-hydroxydopamine) in the vicinity of A1 induced a 60% decrease in the peak height. This effect was prevented by pretreatment with desipramine, an inhibitor of noradrenaline reuptake, which is known to protect noradrenergic neurons against the action of 6-hydroxydopamine. Finally, specific drugs acting on alpha-2-noradrenergic receptors (clonidine and piperoxane) modulated the peak height recorded from both structures. Thus, as previously shown in the locus ceruleus, the variations in the extracellular DOPAC levels reflect the metabolic activity of A1 and A2 noradrenergic neurons.