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Ovine prolactin has been shown to initiate porphyropsin synthesis in “second metamorphosis” of the eastern red-spotted newt ( Notophthalmus viridescens ). Prolactin activity is demonstrable in both terrestrial juvenile efts and aquatic adults. In adults, thyroxine initiates molting, antagonizes prolactin-induced water-drive behavior, and yet does not appear to affect visual pigment synthesis. Adaptation of adults to aquatic and terrestrial conditions does not alter visual pigment composition; prolactin initiates porphyropsin synthesis in animals maintained in both experimental environments.

An immunocytochemical unlabeled antibody enzyme technique has been applied to localize mammalian-like immunoreactive luteinizing hormone-releasing hormone (ir-LH-RH) in central nervous tissue of Pacific lamprey (Entosphenus tridentata; anadromous spawning migrants) and of Pacific hagfish (Eptatretus stouti, sexually undifferentiated and mature). Specific immunocytochemical staining was located over perikarya in the preoptic nucleus of lampreys; beaded axons containing herring body-like dilatations were stained and appeared to pass ventro-posteriorly into the infundibulum where presumed nerve endings were observed in the neurohypophysis. Cell bodies exhibiting ir-LH-RH were distinct from those, presumably vasotocinergic ones, which stained with aldehyde fuchsin. Specific staining was absent in other regions of the lamprey brain, in control preparations, and in any part of the hagfish central nervous tissues. The possible hypophysiotropic actions of ir-LH-RH in the lamprey preoptico-neurohypophysial system are unclear because no anatomical relationship is evident which might provide for transfer of hypothalamic influences to the pars distalis. However, detection of mammalian-like ir-LH-RH in brain of this primitive agnathan fish indicates that this peptide may be of great evolutionary antiquity.

Hagfish ( Eptatretus stouti ) thyroid tissue co-cultured with pituitary tissue increased thyroxine (T 4 ) concentration in medium relative to thyroids incubated alone. Synthetic thyrotropin releasing hormone and arginine vasotocin failed (a) to augment medium T 4 in co-cultures and (b) to retard the decline in pituitary thyrotropic activity observed over time.