Regulatory Mechanism of Synthesis of Peptides and Receptors in Motoneurons : in Situ Hybridization Study

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Acetylcholine (Ach) has been a sole candidate of neurotransmitter in motoneurons for a long time. Recent advance of molecular biology and applied morphological techniques have revealed the presence of neuropeptides in moto-neurons. These peptides are considered to modulate the activity of moto-neurons and muscles, or exert neurotrophic effects on them. In addition to peptides, neurotransmitter receptors are also synthesized in moto-neurons indicating that various kinds of neurotransmitters regulate the activity of motoneurons. Moreover, motoneurons are under the control of neurotrophic factors, such as nerve growth factor (NGF). It has been demonstrated that both NGF-receptor mRNA and NGF-receptor protein are expressed in developing motoneurons, and also in mature motoneurons under some experimental conditions. In the present study, we employed in situ hybridization histochemistry to demonstrate how the production of these peptides and receptors are regulated in injured motoーneurons.It … More is well documented that calcitonin gene-related peptide (CGRP)-like immunoreactivity in motoneurons of the rat spinal cord is increased after peripheral axotomy. In these neurons, axotomy caused the increase of alpha -CGRP mRNA, but not beta -CGRP mRNA, indicating that alpha -CGRP and beta -CGRP are regulated independently and have different roles in motoneurons. We have also shown that alpha - and beta -subunits constructing neuronal nicotinic Ach receptor are also differentially regulated in motoneurons. alpha 3 and beta 2 subunit mRNAs were expressed in 12% and 40% of facial motoneurons of the rat. alpha 3 subunit mRNA signals disappeared in response to axotomy, whereas the beta 2 subunit mRNA signal was remarkably enhanced.Some 38%, 55% and 7% of the facial motoneurons of the rat expressed alpha -CGRP, beta -CGRP and CCK mRNAs, respectively. No galanin mRNA containing motoneurons were observed. The levels of mRNA for alpha -CGRP, CCK and galanin were increased while beta -CGRP mRNA level was decreased after axotomy. The levels of mRNAs for these peptides returned to the control values by 2-4 weeks after injury, nerve transection had greater effects on beta -CGRP, CCK and galanin mRNAs than did nerve crush. These changing patterns of neuropeptides in motoneurons do not coincide with that of NGF-receptor mRNA after the same procedures ; the increase of NGF-receptor mRNA with crushing was more pronounced than with transection from the 3rd to the 14th day after the injury. These findings suggest that the production of these peptides in motoneurons is not regulated by NGF. Less