Analysis of tyrosine hydroxylase and insulin transcripts in human neuroendocrine tissues.
We have previously shown that the mRNA for human tyrosine hydroxylase (TH) exists in one of 4 forms as a result of alternative splicing of intron 1. In order to determine the tissue-specific expression of the multiple human transcripts we have utilized specifically primed polymerase chain reactions (PCR) in combination with reverse-transcribed RNA. Using PCR analysis we determined that many human neuronal tissues express all 4 forms of human TH (hTH-1-4); however, hTH-3 and -4 are generally expressed at less than 1% of the other two. Of all tissues examined, only pancreatic beta-islet cells expressed a single form of human TH mRNA (hTH-2). The mRNA for rat TH is present in only one form, the equivalent of hTH-1. Since we have shown that the human genes for TH and insulin are only 2.7 kb apart and several groups have reported insulin-like expression in neuronal tissues, we looked for insulin transcription in the same central and peripheral nervous system samples. Insulin mRNA was not present within the limits of detection.