Morphological and neurochemical plasticity of rat mesencephalic trigeminal neurons

The mesencephalic trigeminal nucleus (Me5) is a unique structure in the central nervous system (CNS), made up of pseudounipolar sensory neurons. It is also a suitable paradigm for studying the plastic alterations in neurons. It is known that the Me5 neurons utilize various neurotransmitters under normal conditions, though little information is available about the morphological and chemical events taking place in the nucleus after injury. This review provides concise description of the structural adaptive changes in Me5 neurons following peripheral axotomy of the masseteric nerve. Furthermore, it validates NADPH-diaphorase activity in them, and using immunohistochemistry for glutamate (Glu), substance P (SP), calcitonin-gene related protein (CGRP), neuropeptide tyrosine (NPY) and galanin (GAL), it deals with the altered neurochemical phenotype of the injured neurons. Our results distinctly show that the Me5 neurons in the rat are extremely sensitive to peripheral injury and we demonstrate their distinct structural and neurochemical plasticity. The adaptive morphological alterations comprise of both qualitative and quantitative alterations in the axotomized Me5 population which are statistically significant when compared with the number and phenotype of the neurons on the contralateral intact side. Besides, the axotomy-induced alterations in the neurochemical character of Me5 are best signified by the down-regulation of the classical neurotransmitters under normal conditions, and the up-regulation of nitric oxide synthase and de novo synthesis of certain neuroactive substances such as NPY, SP, GAL and VIP. It can be inferred that the described phenomena only occur in the nucleus in cases of injury and changes in the environmental cues, and serve as adaptive mechanisms and powerful trophic factors for the neuronal survival in the Me5. There is, undoubtedly, still a long way to go in order to clarify the dynamic and plastic alterations occurring in the CNS in health and disease, and also explain their role in such important functions as pain, perception, learning, cognition and memory.