Brain levels of arginine–vasotocin and isotocin in dominant and subordinate males of a cichlid fish

摘要

The nonapeptides arginine–vasotocin (AVT) and isotocin (IT), which are the teleost homologues of arginine–vasopressin and oxytocin in mammals, have well established peripheral effects on osmoregulation and stressresponse, and central effects on social behavior. However, all studies that have looked so far into the relationshipbetween these nonapeptides and social behavior have used indirectmeasures of AVT/IT activity (i.e. immunohistochemistryof AVT/IT immunoreactive neurons, or AVT/IT or their receptorsmRNA expressionwith in situ hybridizationor qPCR) and therefore directmeasures of peptide levels in relation to social behavior are still lacking. Herewe use a recently developed high-performance liquid chromatography analysis with fluorescence detection(HPLC-FL)method to quantify the levels of both AVT and IT inmacro-dissected brain areas [i.e. olfactory bulbs, telencephalon,diencephalon, optic tectum, cerebellum, and hindbrain (= rhombencephalon minus cerebellum)]and pituitary of dominant and subordinate male cichlid fish (Oreochromis mossambicus). The pituitary showshigher levels of both peptides than any of the brain macroareas, and the olfactory bulbs have the highest AVTamong all brain areas. Except for IT in the telencephalon there is a lack of correlations between central levelsand pituitary peptide levels, suggesting an independent control of hypophysial and CNS nonapeptide secretion.There were also no correlations between AVT and IT levels either for each brain region or for the pituitarygland, suggesting a decoupled activity of the AVT and IT systems at the CNS level. Subordinate AVT pituitary levelsare significantly higher than those of dominants, and dominant hindbrain IT levels are significantly higher thanthose of subordinates, suggesting a potential involvement of AVT in social stress in subordinate fish and of IT inthe regulation of dominant behavior at the level of the hindbrain. Since in this species dominant males useurine to communicate social status and since AVT is known to have an antidiuretic effect,we have also investigatedthe effect of social status on urine storage. As predicted, dominantmales stored significantly more urine than subordinates.Given these results we suggest that AVT/IT play a key role in orchestrating social phenotypes, actingboth as central neuromodulators that promote behavioral plasticity and as peripheral hormones that promote integratedphysiological changes.