Alteration of tyrosine hydroxylase activity in the locus coeruleus after administration of p-chlorophenylalanine.

The time course of the variations in tyrosine hydroxylase activity (THA) and serotonin (5-HT) content were measured in the rat locus coeruleus after parachlorophenylalanine (PCPA) administration. Highly significant decreases in the 5-HT content in LC were found 24-48 h after PCPA treatment (300 mg/kg daily). An increase in THA (in the LC) was found to be significant 4 days after 2 successive injections of PCPA and after 4 successive injections of the drug THA gradually increased, reaching a maximum around 4 days after the last injection. This maximum increase in THA was greatly reduced when 5-HTP was simultaneously administered with PCPA. These results join others which suggest that a serotonin-mediated mechanism could be one of the processes controlling noradrenaline metabolism in the locus coeruleus.