IMMUNOCYTOCHEMICAL LOCALIZATION OF TYROSINE-HYDROXYLASE AND GAMMA-AMINOBUTYRIC-ACID IN THE MESENCEPHALIC TRIGEMINAL NUCLEUS OF THE CAT - A LIGHT AND ELECTRON-MICROSCOPIC STUDY

Background: Recent studies conducted on the rat have demonstrated that the mesencephalic trigeminal nucleus (MTN) neurons, involved in the p roprioceptive transmission contain some neuroactive substances, includ ing classical and amino acid neurotransmitters. In addition, there is evidence that serotonin could not act as a neurotransmitter at the fir st synaptic relay in the cat MTN. In the present study, we aimed to ex amine two other possible neurotransmitter systems, i.e., catecholamine s and gamma-aminobutyric acid (GABA), and the relationships between GA BA-immunoreactive (IR) neurons and tyrosine hydroxylase (TH)-IR axonal varicosities in the MTN of the cat. Methods: To ensure the localizati on of immunoreactive structures, the experiments were carried out at t he light and electron microscopic level using single immunostaining fo r TH and GABA alone. The correlation between GABA-IR cell bodies and T H-IR fibers was investigated by means of double-labeling immunogold an d peroxidase technique for GABA and TH. Results: Light microscopically , a few GABA-IR neurons were observed in the cat MTN. These small-size , labeled cells, most likely interneurons, were apposed to unstained l arge mesencephalic trigeminal cells. Most of the large nonreactive MTN neurons were closely surrounded by fine TH-IR varicose or nonvaricose fibers and dot-like structures, presumably nerve terminals. Under the electron microscope, TH-IR fibers were not seen in synaptic contact a nd only rarely appeared to be in close proximity to neuronal profiles of small GABAergic cells, which contained gold particles. Conclusions: Taken together with earlier studies from other laboratories, the pres ent findings suggest that GABAergic system might play an indirect role in the proprioceptive information processing in the cat MTN by intera ctions of GABA-immunoreactive neurons with the systems that control th e transmission of selected sensory information. In contrast, the prese nce of TH-IR fibers in direct apposition to the majority of MTN neuron s provide further evidence that presumed GABAergic interneurons have e xtensive interactions with catecholamine varicosities and raise the po ssibility that catecholamines could modify the transmission on these n eurons. (C) 1995 Wiley-Liss, Inc.