IMMUNOHISTOCHEMICAL STUDIES ON LOCALIZATION AND DISTRIBUTION OF MONOAMINE NEURON SYSTEMS IN RAT-BRAIN .1. TYROSINE-HYDROXYLASE IN MESENCEPHALON AND DIENCEPHALON

Abstract

The localization and distribution of tyrosine hydroxylase (TH), the 1st enzyme in catecholamine synthesis, in the mes- and diencephalon was studied with the indirect immunofluorescence technique of Coons and collaborators. Principally, TH was present in neuron systems with a distribution similar to known dopamine, noradrenaline and adrenaline systems. All parts of most central dopamine neurons, i.e., cell body, dendrites, axon and nerve terminals, appeared strongly fluorescent. The adrenaline neurons also appeared strongly fluorescent, except for their axons, which only exhibited a weak fluorescence. Only cell bodies of noradrenaline neurons were strongly fluorescent, whereas the nerve terminals and axons showed a weak or moderate fluorescence intensity. The fine noradrenaline nerve terminals in some areas, such as the thalamus, were invisible or, under favorable conditions weakly fluorescent. The dopamine neurons of the upper brain stem were most extensively studied. There was a widespread occurrence of TH-positive occurrence of TH-positive neuron systems in the mes- and diencephalon. The different mesencephalic dopamine systems and their ascending projections were visualized. Numerous TH-positive cell bodies were present along the ventricle system extending from the aqueductus cerebri to the most cranial periventricular parts of the 3rd ventricle. The caudal part of these neurons, consisting of very small cell bodies, belong to the dorsal periventricular system described by Lindvall and Bjorklund. Several TH-positive cell bodies were also observed in the inferior colliculus of young animals. In the superficial layers of the inferior colliculus TH positive nerve terminals were seen. At the hypothalamic level the A11 to A14 cell groups and some additional cell bodies and extensive nerve terminal plexuses appeared strongly fluorescent. The differences in the intensity of the TH-related immunofluorescence between various brain regions and between various neuron systems may well reflect differences in enzyme levels between the various catecholamine systems rather than different types of TH.