Secretoneurin as a hormone regulator in the pituitary

Secretoneurin (SN) is a 33–34 amino acid peptide derived from the most conserved sequence of the secretogranin (SgII) precursor. SgII is a granin protein found in the secretory granules of neuroendocrine tissues. There are two paralogs of teleost SgII that we name here SgIIa and SgIIb. Processing of these proteins would yield SNa and SNb in fish. Secretoneurin immunoreactivity is found within all the major pituitary cell types in mammals. In goldfish, it appears to be mainly expressed in the prolactin cells of the rostral pars distalis. We have investigated the paracrine role of goldfish SN (SNa) to stimulate luteinizing hormone from gonadotrophs in the neighboring proximal pars distalis. Another source of SN is the hypophysiotropic neurons that may deliver SN to target cells by direct pituitary innervation. Little else is known about the neuroendocrine role of SN. We also discuss the evolution, distribution and production of SN in the pituitary.