Comparison of the effects of alpha-methyl-p-tyrosine and a tyrosine-free amino acid load on extracellular noradrenaline in the rat hippocampus in vivo

Peripheral administration of an amino acid load lacking tyrosine and its pr ecursor, phenylalanine, causes a lowering of central tyrosine levels. The a im of the present study was to examine the effects of tyrosine depletion on extracellular noradrenaline using microdialysis. Extracellular noradrenali ne was measured in hippocampus of the anaesthetized rat under both baseline conditions (with reuptake inhibitor, desipramine, in the perfusion medium) and following administration of the as-adrenoreceptor antagonist, idazoxan . The tyrosine free amino acid load did not alter either baseline noradrena line or the twofold rise in noradrenaline evoked by idazoxan compared with saline controls. In contrast, the catecholamine synthesis inhibitor, alpha- methyl-p-tyrosine, caused a marked reduction in baseline extracellular nora drenaline and abolished the rise induced by idazoxan. In conclusion, the pr esent data indicate that under the conditions used, a tyrosine-free amino a cid mixture may not be an effective means to interfere with central noradre naline function. This contrasts with recent findings demonstrating that the tyrosine-depletion approach can be used to decrease presynaptic dopamine f unction.