Immunoreactive Calcitonin Gene‐Related Peptide, Vasoactive Intestinal Polypeptide, and Somatostatin in Developing Chicken Spinal Cord Motoneurons: Distribution and role in regulation of cAMP in cultured muscle cells

The distribution of calcitonin gene‐related peptide (CGRP)‐, vasoactive intestinal polypeptide (VIP)‐, and somatostatin (SOM)‐like immunoreactivities (‐LI) in neurons of the spinal cord of developing chickens was characterized by use of the indirect immunofluorescence technique, and the findings related to a possible role for these peptides in the development of muscles and motor endplates. CGRP‐LI in presumptive motoneurons of the ventral horn was first observed at embryonic day 6. During the following days the number of CGRP‐immunoreactive (IR) cells increased reaching high numbers between days 12 and 18 of incubation, and thereafter decreasing in numbers until hatching. SOM‐LI was first observed on embryonic day 7, while VIP‐LI appeared on days 12–13. The number of SOM‐ and VIP‐IR cells was considerably lower than that observed for CGRP‐LI, but they also exhibited an initial increase followed by a decrease towards hatching. Intrathecal administration of colchicine increased the number of CGRP‐IR motoneurons at days 7 and 30 after hatching and of VIP‐IR ones at day 7, while at day 30 no expression of VIP‐LI was found. Colchicine treatment did not cause any significant change in the number of SOM‐IR motoneurons after hatching. The effect of VIP, SOM, and CGRP on cAMP accumulation in primary cultures of chick muscle cells was determined after labelling of the cells by (2‐³H) adenine and by radioimmunoassay. All three peptides stimulated the accumulation of cAMP. However, the development of the pharmacological response of each of the peptides followed a different time course during in vitro differentiation of the primary cultures. The response of CGRP was the latest to develop and did not significantly decrease after the maximal response had been reached around day 3. The data are discussed in terms of ‘trophic’ effects of these neuropeptides upon muscle and spinal cord differentiation and synaptogenesis.