An immunocytochemical study using the PAP method for tyrosine hydroxylase and serotonin in alternate sections, and in situ hybridization to detect tryptophan hydroxylase mRNA in the rat's locus ceruleus.

In situ hybridization (LARRSON and HOUGAARD 1990), using the APAAP complex, shows that many small and medium-sized neurons with signals of tryptophan hydroxylase mRNA are uniformly scattered throughout the rat's locus ceruleus, and that a few extrinsic neurons just ventro-medial to it also shows the hybridization signals. The specificity of this technique has been established by Northern blot hybridization. Synthesis of serotonin in intrinsic neurons is indicated not only by the results obtained from in situ hybridization, but also by the fact that, in distribution, masked serotonin cells, immunocytochemically revealed after pargyline and 5-hydroxytryptophan loading, correspond to the neurons showing mRNA hybridization signals. Identification of the same neurons in adjacent cryostat sections, immunostained alternately for serotonin or tyrosine hydroxylase after loading, provides evidence for the coexistence of serotonin and noradrenaline in a single neuron of this center. A few extrinsic, non-specific indoleamine cells located just ventro-medial to the center may be related to the lateralization of the raphe's neurons. The expression of tryptophan hydroxylase in CNS appears to be restricted to the specific regions such as the locus ceruleus and raphe's nuclei.