Vasotocin is an oligopeptide homologous to oxytocin and vasopressin found in all non-mammalian vertebrates and possibly in mammals during the fetal stage of development. Arginine vasotocin, a hormone produced by neurosecretory cells within the posterior pituitary gland of the brain, is a major endocrine regulator of water balance and osmotic homoeostasis and is involved in social and sexual behavior in non-mammalian vertebrates. In mammals, it appears to have biological properties similar to those of oxytocin and vasopressin. It has been found to have effects on the regulation of REM sleep. Evidence for the existence of endogenous vasotocin in mammals is limited and no mammalian gene encoding vasotocin has been confirmed. AVT, which occurs in the lamprey, represents the ancestral form in the phylogeny of the vertebrate neurohypophysial hormones. Gene duplication and point mutation have produced two distinct lineages, one involved in reproduction and the other in osmoregulation. These hormones have remained highly conserved throughout evolution. Each is a peptide of nine amino acids derived from a preprohormone precursor by proteolytic cleavage, with an intramolecular disulfide bridge between the cysteine residues; the C-terminal glycine residue is amidated. Six of the residues have been found to be invariant in homologous peptides from numerous species of vertebrates. The vasopressin-like peptides, which differ in positions 3 and/or 8, include AVT and the mammalian hormones arginine vasopressin and lysine vasopressin. The oxytocin-like peptides, which differ in positions 4 and/or 8, include oxytocin, mesotocin, and isotocin ; they differ from the vasopressin-like peptides in having a neutral amino acid in place of a basic amino acid at position 8. Oxytocin occurs in placental mammals; mesotocin occurs in amphibians, reptiles, and birds, and isotocin occurs in fishes.
Biosynthesis
AVT is synthesized as a preprohormone that includes a second peptide, neurophysin VT ; intracellular proteolytic processing generates the mature peptides. In the chicken, the 161-amino acid vasotocin-neurophysin VT preprohormone is encoded by the gene AVP, which is considered homologous to the mammalian genes encoding arginine vasopressin. Removal of the 19-amino acid N-terminal signal peptide generates the prohormone, which is hydrolysed to AVT and neurophysin VT. The existence of two AVT preprohormones with different sequences in fishes is evidence for gene duplication.
Behavioral Effects
Several animal studies have been conducted that explore the behavioral effects of AVT. The main findings of these studies have revealed that AVT plays an integral role in the pair bonding behavior and social hierarchy in non-mammalian vertebrates. In a study conducted with zebra finches, increased levels of AVT were linked to an increase in aggressive, competitive behavior in non-paired male finches, but were subsequently related to an increase in defensive behavior after the finches had been paired. However, this study also found that blocking AVT receptors did not directly affect pair bonding ability. The shift in behaviors were explained by the location of the release of AVT in the brain. Competitive aggressive behavior was found to be linked with AVT release in the BSTm, whereas defensive, nest-protecting behavior was linked with AVT release in the neurons of the Hypothalamus and Paraventricular Nucleus. In a study conducted with male Japanese quail, AVT was found to have an effect on later social interactions amongst the species. Immediately after injection with AVT, the quails displayed less aggressive behavior. However, the next day, the quail that were injected with AVT displayed more dominant behavior towards familiar birds, but not unfamiliar birds. This study shows that AVT may play a role in establishing social hierarchy. A study that investigated the role of social construction and AVT compared territorial and non-territorial species of tropical coral reef fish. Experimenters administered Manning compound, an AVT agonist to the fish and found that, after treatment, non-territorial species displayed more territorial behavior whereas territorial species displayed less territorial behavior. Research suggests that the effects of AVT on aggression may be influenced by the social construction of the species. For example, in a study done with Rainbow Trout, increased levels of AVT were associated with more subordinate behavior. It is currently hypothesized that the contrasting effects of AVT are related to the distinction between territorial versus colonial social systems. In a territorial species, such as Rainbow Trout, AVT is linked to less dominant behavior. This may be due to the differences in the distribution of AVT receptors in territorial and colonial species.
