QUANTIFICATION OF AXOTOMY-INDUCED ALTERATION OF NEUROPEPTIDE MESSENGER-RNAS IN DORSAL-ROOT GANGLION NEURONS WITH SPECIAL REFERENCE TO NEUROPEPTIDE-Y MESSENGER-RNA AND THE EFFECTS OF NEONATAL CAPSAICIN TREATMENT

Alteration in mRNA expression in dorsal root ganglia (DRG) neurons enc oding 5 neuropeptides was quantitatively compared in normal rats and i n those neonatally treated with capsaicin, a selective neurotoxin whic h destroys a subpopulation of DRG neurons with unmyelinated axons. Adu lt rats received a unilateral transection of the sciatic nerve and wer e killed 7 days later. Oligonucleotide probes specific for the genes e ncoding neuropeptide Y (NPY), vasoactive intestinal polypeptide (VIP), galanin (GAL), somatostatin (SOM), and calcitonin gene-related peptid e (CGRP) were used for in situ hybridization and RNA blot analysis. Fo llowing the nerve cut, RNA blot analysis demonstrated a dramatic induc tion of NPY, VIP, and GAL mRNA levels from the undetectable constituti ve level of expression. Conversely, CGRP and SOM mRNAs, which are cons titutively expressed, were reduced 55% and 70%, respectively, followin g the nerve cut. A unimodal size distribution for neurons expressing N PY mRNA was determined, with a mean cross-sectional area of 1700 mum2 representing 24.4% of DRG neurons ipsilateral to the nerve cut. Neuron s expressing VIP mRNA were mainly small sized, with a cross-sectional area of approximately 700 mum2, while those expressing GAL mRNA were b oth small (approximately 700 mum2) and medium (approximately 1,300 mum 2) sized. The percentages of neurons expressing VIP or GAL mRNA were 1 9.9% and 33.7%, respectively. In neonatal capsaicin-treated rats, ther e was a 10% reduction in neurons expressing NPY mRNA, a 37% reduction for VIP, and a 27% for GAL mRNA compared to vehicle-treated rats after nerve cut. Capsaicin-sensitive neurons comprised 37% of CGRP neurons and 83% of SOM neurons. These observations suggest that NPY is primari ly induced in myelinated primary afferent neurons, while VIP and GAL m RNA induction occurs in a mixed population, a sizeable percentage of w hich has unmyelinated axons. Additionally, SOM mRNA expression is asso ciated mainly with unmyelinated primary afferents.