LOCALIZATION OF GTP CYCLOHYDROLASE IN MONOAMINERGIC BUT NOT NITRIC OXIDE-PRODUCING CELLS

The first and rate-limiting enzyme in tetrahydrobiopterin (BH4) biosyn thesis is GTP cyclohydrolase (GTPCH). BH4 serves as the essential cofa ctor for aromatic L-amino acid hydroxylases, such as tyrosine hydroxyl ase (TH) and tryptophan hydroxylase (TPH), as well as for nitric oxide synthase (NOS). We hypothesized that to provide access to the cofacto r, a close association exists between BH4-synthesizing and BH4-depende nt enzymes, and we determined the relationship among GTPCH, neuronal N OS (nNOS), and TH in rat brain and adrenal gland using immunohistochem istry and in situ hybridization. Analyses of adjacent sections reveale d specific localization of GTPCH in TH-containing cells of the substan tia nigra, ventral tegmental area, hypothalamus, locus ceruleus, and a drenal medulla, and also in TPH-containing cells of the dorsal raphe n ucleus and pineal gland. Thus, BH4 can be synthesized in all monoamine rgic cells and is readily available for the enzymes requiring it. In c ontrast, analysis of adjacent sections showed that nNOS was not coloca lized with GTPCH. Scattered nNOS-positive cells were found in the cort ex, striatum, cerebellum, and olfactory bulb, all areas that receive m onoaminergic innervation. The absence of GTPCH in nNOS cells suggests that nitric oxide-producing cells may either obtain biopterin from mon oamine-containing processes which terminate in close proximity, or tak e up biopterin released into the blood. Double labelling of the same s ection for TH and nNOS revealed the TH nerve terminals connecting with the nNOS-positive cell bodies, suggesting the possibility that the BH 4-containing nerve terminals may directly donate this cofactor to the nNOS-containing cells. (C) 1998 Wiley-Liss, Inc.