Hybridohistochemical demonstration of alternative splicing of the CALC-I gene.

Precursor RNA is processed to mature mRNA by excision of the noncoding introns. This process is called splicing and usually is studied by in vitro transcription experiments and by Northern blotting. The pre-mRNA of the first calcitonin gene (CALC-I) can be spliced alternatively into two different mRNAs encoding either for calcitonin or for calcitonin gene-related peptide (alpha-CGRP). The expression of CALC-I was studied by in situ hybridization in the nervous system of the rat with calcitonin-specific and CGRP-specific probes. We found evidence for alternative splicing of the pre-mRNA of CALC-I in the motor neurons of the spinal cord. alpha-CGRP mRNA was found in the nuclei and the cytoplasm of the spinal motor neurons, whereas hybridization with a calcitonin-specific probe was located strictly in the nuclei. Because no calcitonin RNA and protein was detected in the cytoplasm of the cells, we concluded that the hybridization obtained in the nucleus was the pre-mRNA encoding for alpha-CGRP.