Nerve Fibers in the Rat Posterior Pituitary Lobe Contain Prosomatostatin (1–64)

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The neurohormone and neurotransmitter somatostatin arises from the processing of a larger precursor, prosomatostatin (proSS). An immunohistochemical investigation in the rat, using a well-characterized antiserum raised against a synthetic peptide identical to the 20-36 residues of the proSS molecule, revealed the presence of immunoreactive nerve fibers and nerve terminals in the median eminence, infundibulum, infundibular stalk and posterior pituitary lobe. The largest number of immunoreactive nerve fibers and nerve terminals was observed in apposition to the portal vessels, whereas a moderate number of proSS-immunoreactive fibers was identified in the infundibular stalk and in the proximal part of the posterior pituitary lobe. The proSS-immunoreactive nerves entered the posterior pituitary lobe from the infundibular and pituitary stalks and were followed to rostral and ventral aspects of the organ. In contrast, positive fibers were rarely identified in caudal and posterior parts. Extracts of rat posterior pituitaries subjected to gel chromatography and reversed-phase high-pressure liquid chromatography (HPLC) analysis showed the presence of a single proSS-immunoreactive molecule corresponding to the size of proSS(1-64). The functional significance of the proSS(1-64) in the hypothalamus and pituitary is at present unknown, but its location in the hypothalamo-hypophyseal system suggests that this end product of the processing of proSS is released into the portal and perhaps also the general circulation.