THE NEUROHYPOPHYSIAL HORMONES OF BONY FISHES AND CYCLOSTOMES

Seahorse Male Pregnancy as a Model System to Study Pregnancy, Immune Adaptations, and Environmental Effects

Seahorses, together with sea dragons and pipefishes, belong to the Syngnathidae family of teleost fishes. Seahorses and other Syngnathidae species have a very peculiar feature: male pregnancy. Among different species, there is a gradation of paternal involvement in carrying for the offspring, from a simple attachment of the eggs to the skin surface, through various degrees of egg coverage by skin flaps, to the internal pregnancy within a brood pouch, which resembles mammalian uterus with the placenta. Because of the gradation of parental involvement and similarities to mammalian pregnancy, seahorses are a great model to study the evolution of pregnancy and the immunologic, metabolic, cellular, and molecular processes of pregnancy and embryo development. Seahorses are also very useful for studying the effects of pollutants and environmental changes on pregnancy, embryo development, and offspring fitness. We describe here the characteristics of seahorse male pregnancy, its regulatory mechanisms, the development of immune tolerance of the parent toward the allogeneic embryos, and the effects of environmental pollutants on pregnancy and embryo development.

Special evolution of neurohypophysial hormones in cartilaginous fishes: asvatocin and phasvatocin, two oxytocin-like peptides isolated from the spotted dogfish (Scyliorhinus caniculus)

In contrast to most vertebrate species that possess one oxytocin-like hormone and one vasopressin-like hormone, a few groups, such as marsupials or cartilaginous fishes, are endowed with two peptides of either or both types, suggesting possible gene duplications. We have now isolated two oxytocin-like hormones from the pituitary of the spotted dogfish Scyliorhinus caniculus (suborder Galeoidei). Microsequencing as well as chromatographic and pharmacological comparisons with synthetic peptides show that these peptides are [Asn4,Val8]oxytocin (asvatocin) and [Phe3,Asn4,Val8]-oxytocin (phasvatocin). Asvatocin and phasvatocin display oxytocic activity on rat uterus, about 80 and 5 milliunits per nmol, respectively, and virtually no pressor activity on anesthetized rats. They occur in roughly equal molar amounts in the gland; vasotocin is also present in a proportional amount that is lower by about a factor of 20. In addition to the duality, conservative amino acid substitutions are observed in the two oxytocic peptides in positions 4 (Gln-4-->Asn) and 8 (Leu-8-->Val), when compared with oxytocin. Furthermore, replacement of the isoleucine residue found in position 3 of all other oxytocin-like hormones by phenylalanine in phasvatocin is exceptional; it determines a dramatic decrease of the oxytocic activity. Preservation of the C-terminal-amidated nonapeptide pattern in the 12 vertebrate neurohypophysial hormones known to date suggests that both precursors and processing enzymes have coevolved tightly. On the other hand, whereas the great evolutionary stability of the mature hormones (generally observed in vertebrates) suggests a strict messenger-receptor coevolution, the exceptional diversity found in cartilaginous fishes (six oxytocin-like peptides identified out of eight known) might be due to a looseness of selective constraints, perhaps in relationship with their specific urea osmoregulation.

Arginine vasotocin from the pituitary gland of the lamprey (Petromyzon marinus): isolation and amino acid sequence

Arginine vasotocin (AVT) was isolated from extracts of sea lamprey pituitary glands (Petromyzon marinus). The amino acid sequence Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Arg-Gly-NH2 is identical to the molecule isolated from teleosts and tetrapods. The total yield was estimated to be 9.6 pmol per gland. No evidence could be found for the existence of a second neurohypophyseal nonapeptide in the lamprey pituitary.

The nonmammalian-mammalian transition through neurohypophysial peptides

Neurohypophysial hormones are particularly proper evolutionary tracers. Whereas Eutherian mammals have oxytocin and virtually always arginine vasopressin, nonmammalian tetrapods possess instead mesotocin and vasotocin. The transitions of mesotocin-oxytocin and vasotocin-vasopressin involved in the passage of reptiles-mammals seem to have occurred independently. Australian marsupials are endowed with mesotocin but American marsupials have either oxytocin (South-American opossums) or both oxytocin and mesotocin (North-American opossum). These results suggest that Australian Metatherians have preserved reptilian mesotocin and used it for milk-ejecting function whereas the change mesotocin-oxytocin appeared only in the American line. All marsupials have either arginine vasopressin or lysipressin and phenypressin (Australians) or lysipressin and arginine vasopressin (Americans). It is assumed that the change of vasotocin into arginine vasopressin occurred very early, perhaps in mammal-like reptiles, and duplication of the gene with subsequent mutations has led to the presence of two vasopressin-like peptides in most Metatherians.

The reticular formation of lampreys (Petromyzonidae)--a target area for exohypothalamic vasotocinergic fibres

The octapeptide vasotocin, which is formed in the classical neurosecretory nuclei of lampreys (Petromyzonidae), is transported, bound to the carrier protein neurophysin, not only to the neurohypophysis but also to various other regions of the brain via exohypothalamic fibres. A target area of this exohypothalamic vasotocinergic system is, in the brook lamprey (Lampetra planeri Bloch), a relatively well circumscribed area in the isthmus region of the rhombencephalic tegmentum motoricum, which is called area lateralis tegmenti. In this area, which belongs to the reticular formation, the vasotocinergic fibres form synaptic contacts with nerve cell perikarya and processes. The vesicles contained in the fibres were identified, ultrahistochemically, as neurophysin vesicles. They correspond to the neurophysin vesicles observed in the neurohypophysis of the same species. The functional significance of the vasotocinergic supply to portions of the reticular formation in lampreys is open to discussion.

Immunocytochemical demonstration of thehypothalamo-hypophysial vasotocinergic system of Lampetra fluviatilis

With the use of the unlabeled antibody peroxidase-antiperoxidase (PAP) technique at the light microscopic level, it was shown that the preoptico-hypophysial neurosecretory system of the adult migrating Lampetra fluviatilis is a vasotocinergic system. It does not synthesize vasopressin. The results are entirely consistent with earlier chromatographic and pharmacological indications thatit produces little or no oxytocin-like peptide hormone. In the adenohypophysis, immunoreactive neurohypophysial peptidergic fibres are absent.

Ultrastructure of pars nervosa and pars intermedia of the Lamprey, Lampetra tridentata

The pars intermedia of the adult lamprey is separated by perivascular spaces and a capillary plexus from the pars nervosa. No penetration of nerve fibers into the pars intermedia was found. The pars nervosa, which constitutes the posterior wall of the infundibulum, consists of an ependymal layer and a fuchsinophilic fiber layer; the latter contains at least four different types of axonal endings. The pars intermedia is avascular and is composed of a small proportion of non-secretory cells and a large proportion of secretory cells. The secretory granules in the cells of the pars intermedia seem to be discharged toward the capillaries that separate the pars intermedia from the pars nervosa. Although no direct nervous or vascular connections were found between the pars nervosa and pars intermedia, a mechanism of control of secretory activity in the pars intermedia cells by the central nervous system appears likely.