Arginine vasotocin gene expression during osmotic challenge in the chicken

The avian hypothalamic nonapeptide arginine vasotocin (AVT) is released from axon terminals in the neural lobe upon the application of osmotic stimuli. We have investigated whether, and to what extent, hormone secretion from the neurohypophysis is related to gene expression in the hypothalamus. Results from hybridization experiments with an AVT-specific cDNA probe indicate that in adult chickens stimulated by water deprivation or by hypertonic saline (2% w/v) drinking water, an upregulation of the AVT mRNA pool takes place, since consistently higher AVT mRNA levels compared to controls were monitored in osmotically challenged birds. This stimulatory effect was even visible at the transcriptional level after 19 h of water deprivation when osmolality was still near the basal value. In hens osmotically challenged by hypertonic saline drinking water for 5 days, a dissociation between osmolality and AVT plasma concentration was visible: extremely high plasma osmolality was accompanied by only moderately increased plasma AVT concentration. This might be caused either by exhaustion of stored hormone, or by downregulation of the system after chronic challenge. The latter suggestion is supported by the fact that the AVT mRNA concentration after 5 days of hypertonic saline challenge was well below the AVT mRNA levels of the groups with the more short-term stimuli of water deprivation for 19 or 48 h. In 30-day-old chicks the hypothalamic AVT mRNA concentration hardly reached 70% of the adult value, although AVT plasma concentrations were similar to those in the mature bird. We conclude that osmotic challenge of the hypothalamo-neurohypophysial system not only causes secretion of AVT from stores in the neural lobe but is accompanied by upregulation of AVT gene expression. Upregulation already occurs after marginal increase in plasma osmolality, as seen after 19 h of water deprivation in hens. In 30-day-old chicks gene expression is only slightly upregulated after short-term water deprivation while increase in plasma AVT is even greater compared to hens.