Tyrosine hydroxylase activity in the locus coeruleus of conscious rats: differences with the anesthetized rats

Microdialysis was used to estimate in vivo tyrosine hydroxylase (TH) activity in locus coeruleus (LC) of conscious rats. An aromatic amino acid decarboxylase inhibitor, difluoromethyl-DOPA (DFMD), was perfused through the dialysis probe and the DOPA accumulating in dialysates was measured. A 3 h perfusion of a 6 x 10(-4) mol/l DFMD solution was needed to obtain measurable levels of DOPA in LC dialysates: a linear increase in DOPA concentrations was first observed and, then, a steady state. DOPA disappeared completely after inhibition of TH by alpha-methyl-p-tyrosine, showing that it reflects TH activity. The DFMD perfusion needed in this study was six times higher in concentration and three times longer in duration than the one required in a previous study we performed in LC of anesthetized rats. Furthermore, TH activity was found lower than the one we previously reported. We conclude that these differences could be explained by the effects of halothane anesthesia on DFMD and DOPA degradation and/or on LC neurons TH activity itself.