The mesencephalic trigeminal nucleus in the cat.

This review is a concise summary of our current knowledge about the MTN neuroanatomy which in turn is necessary to understand the neurochemistry of this nucleus in the cat. In order to solve the puzzle of neurotransmitter related changes in the synaptic and functional organization of the MTN, we provide a comprehensive description of the neurotransmitter content of MTN neurons. 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 under normal circumstances the large MTN neuron subpopulation contains only Glu 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 GABAergic. Furthermore, NOS 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 NO in some aspects of synaptic transmission in the MTN. The divergent neurochemical content of the cells in the nucleus, should it exist, is likely to be linked with different neuronal functions. Remarkably, 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-immunolabeling. Most of the labeled peptidergic fibers have immunopositive varicosities that form pericellular basket-like arborizations around unlabeled MTN perikarya. It is predicted that under normal conditions the pericellular arborizations can function as an intranuclear key communication medium between immunopositive projections and immunonegative MTN neurons in the proprioceptive information processing. The levels of transmitter substances in MTN neurons may vary in case of marked changes in the environmental conditions. Axotomy-induced alterations include a long-lasting decrease in the content of CaBPs produced in MTN neurons and/or de novo synthesis of GAL, NPY and CGRP, thus implying the interactive nature and a previously unsuspected neurochemical plasticity of MTN neurons. The newly synthesized neuropeptides can enhance neuronal survival and neurite regeneration. Our results support the assumption that a peptide involvement in the proprioceptive function develops mainly in abnormal conditions. Taken together with the existing neuroanatomical and electrophysiological data, the present results give strong evidence for the occurrence of both excitatory (Gluergic) and inhibitory (GABAergic) transmission in the cat MTN. In addition, evidence is also provided that the MTN receives synaptic inputs from peptidergic and catecholaminergic fibers and these possibly play a significant role in the integration and transmission of trigeminal proprioceptive information. These findings have confirmed the existence of a large number of synaptic contacts in the cat MTN with specific morphological features of their boutons and with presumably different neurotransmitter release from the synaptic vesicles. In this way, knowledge of the origin and neurotransmitter nature of the fibers providing the synapses would facilitate the understanding of the important role of MTN neurons responsible for proprioception in this region.