Localization of tyrosine hydroxylase and its messenger RNA in the brain of rainbow trout by immunocytochemistry and in situ hybridization.

This report describes the distribution of tyrosine hydroxylase (TH)-expressing structures in the brain of rainbow trout (Oncorhynchus mykiss). TH neurons have been localized by the use of two complementary techniques, immunocytochemistry and in situ hybridization of TH messenger RNA. Results obtained from in situ hybridization and immunocytochemistry were in agreement. TH cells were observed in many areas of the brain, with a higher density at the level of the olfactory bulbs where TH-positive neurons are abundant in the internal cell layer. In the telencephalon, two populations of TH neurons can be distinguished: one group is located in the area ventralis telencephali pars dorsalis, and the other group is located in the area ventralis telencephali pars ventralis and extends laterally in the area ventralis telencephali pars lateralis. Many labeled neurons are also seen in the preoptic area as well as in the hypothalamus, where several clusters of TH-positive cells are observed. Some of these neurons located in the paraventricular organ grow a short cytoplasmic extension directed to the ventricular wall and are known to be cerebrospinal fluid-contacting cells. The most caudal TH neurons are observed at the level of the locus caeruleus. At the level of the pituitary, TH-positive fibers are observed in the neurohypophysis. The TH-immunoreactive innervation at the level of the pituitary provides a neuroanatomic basis for the effects of dopamine and/or norepinephrine on the release of pituitary hormones in fish.