Chemocytoarchitecture of the rat locus ceruleus.

I shall elaborate on the cytochemical classifications of the rat locus ceruleus (LC) and state how each of these could be identified. In the LC, 80% (1,309/1,642) of the noradrenergic (NA) neurons are also GABAergic. This is found by demonstrating that two immunoreactivities coexist in adjacent sections alternately immunostained by anti-TH or anti-GABA antibody. Pharmacological manipulations with pargyline (75 mg/kg, i.p., 2 h prior to perfusion) and L-tryptophan (150 mg/kg, 1.5 h prior to perfusion) revealed 5-HT-like immunoreactivity (5-HT-LI) in most LC cells (masked 5-HT cells) that produce 5-HT but not other indoleamines. If 5-HTP is applied instead of tryptophan, 5-HT-LI is shown by the raphe nuclei and a few LC cells (masked indoleamine cells) in the marginal zone. Anti-GAD antibody reveals GAD-LI in 32% of GABA neurons predominantly in the dorsal division. In situ hybridization studies detected tryptophan hydroxylase mRNA and GAD mRNA in many small- and medium-sized neurons. It is concluded that the LC consists mostly of an NA population that is possibly synthesizing multiple transmitters, such as GABA, GAD and 5-HT in single neurons so that the system enables the LC simultaneously to innervate the entire CNS.