Tachykinin-1 gene products in porcine ocular tissues: evidence for transcriptional and post-translational regulation.

The tachykinin-1 gene in mammals produces structurally-related regulatory peptides, substance P (SP), neurokinin A (NKA), neuropeptide K (NPK) and neuropeptide-gamma. The production of these peptides is regulated by both differential mRNA transcription and post-translational precursor processing. Such processes are known to be highly tissue- and species-specific. In this study, we have examined tachykinin-1 gene expression and precursor processing in porcine ocular tissues by employing specific tachykinin radioimmunoassays coupled with reverse phase HPLC characterization. Optic nerve, cornea, iris, ciliary body, retina, choroid and sclera were micro-dissected from freshly enucleated porcine eyes (n = 10). Following acidified ethanol extraction of tissues, dried extracts were reconstituted and subjected to two radioimmunoassays, one of which is highly specific for intact SP, the other for NKA, NKB, NPK and neuropeptide-gamma. In all tissue extracts except the retina, the molar concentration of SP immunoreactivity was significantly greater than that of NKA. These data would imply expression of both alpha- and beta-preprotachykinin-1 in these ocular tissues. Reverse phase HPLC analysis confirmed the presence of authentic SP and NKA in all tissue extracts. However, in extracts of the retina, NKA immunoreactivity co-eluted with synthetic NPK standard. These chromatographic data suggest differential processing of the beta-preprotachykinin-1 precursor in the retina compared with the other ocular tissues. Thus differential mRNA transcription of the tachykinin-1 gene coupled with differential precursor processing appears to occur in porcine ocular tissues and may be a process of functional significance in the regulation of visual physiology.