THE CALCITONIN-GENE-RELATED PEPTIDE (CGRP) PHENOTYPE IS EXPRESSED EARLY AND UP-REGULATED BY RESINIFERATOXIN (RTX) IN MOUSE SENSORY NEURONS

Calcitonin gene-related peptide (CGRP) immunoreactivity was detected a t day 2 in vitro (embryonic day 15) in developing mouse dorsal root ga nglion (DRG) neurons in primary culture. During 2 weeks of culture the proportion of CGRP-immunoreactive (CGRP-IR) neurons remained around 6 5-70%, much higher than usually found in adult animals (45-50%). Treat ment of cultures with the capsaicin analog resiniferatoxin (RTX; 0.3-3 0 nM) significantly augmented CGRP immunoreactivity per neuron at all ages investigated without increasing the number of CGRP-immunoreactive cells. The increased CGRP immunoreactivity was observed both in the a xonal varicosities and in the perinuclear region of cell bodies. This RTX-induced increase in CGRP immunoreactivity was completely blocked b y Ruthenium red (RR). Treatment with the non-esterified form of RTX (r esiniferol 9,13,14 orthophenylacetate, ROPA) produced no increase. The se results suggest that: (1) early expression of the CGRP phenotype is regulated in a cell-autonomous way in developing mouse DRG neurons in vitro; and (2) the RTX-induced increase in CGRP biosynthesis is most likely the result of activating the capsaicin/RTX receptor rather than directly activating the protein kinase C (PKC) pathway in vitro. The results may also reflect qualitative and quantitative differences in c apsaicin/RTX sensitivity of sensory neurons between embryonal and adul t ages.