This review is a summary of our current knowledge of the localization of the neurotransmitters excitatory and inhibitory amino acids, monoamines, neuropeptides and nitric oxide in the mesencephalic trigeminal nucleus (MTN) of the cat. Particular emphasis is given to identifying the possible physiological involvement of MTN inputs in the transmission of proprioceptive information at the first synaptic relay. It is shown that the large MTN neuron population contains only glutamate that is a strong candidate for a major neurotransmitter in this brain region. However, certain small MTN neurons, most likely interneurons, are found to be gamma-aminobutyric acid (GABA)-ergic. Furthermore, nitric oxide synthase immunoreactivity can be detected in the caudal as well as the mesencephalic-pontine junction parts of the MTN and this suggests a mediatory role for nitric oxide in some aspects of synaptic transmission in the MTN. No immunoreactivity to any of the neuropeptides examined is observed in the cell bodies of MTN neurons and only fibers and their terminals show peptide-immunolabelling. Most of the labeled peptidergic fibers have immunopositive varicosities that form pericellular basket-like arborizations around unlabelled MTN perikarya. Together with earlier studies from other laboratories, the present results give strong evidence for the occurrence of both excitatory (glutamatergic) and inhibitory (GABAergic) transmission in the cat MTN. In addition, MTN receives synaptic inputs from peptidergic fibers and these will play a significant role in the trigeminal proprioceptive afferent transmission. Potential factors and mechanisms regulating neurochemical events in the MTN neurons are discussed.
Biomedical Reviews 1997; 8: 1-20.
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