EFFECTS OF EXOGENOUS ARGININE VASOTOCIN ON CIRCULATING LEVELS OF THYROID-HORMONES AND MELATONIN IN THE PIGEON, COLUMBIA-LIVIA

The effects of intravenous injections of two different preparations of synthetic arginine vasotocin (AVT), deamino-dicarba-arginine vasotoci n and arginine vasotocin diacetate tetrahydrate on serum levels of thy roxine (T-4), triiodothyronine (T-3) and melatonin were studied in the resting pigeon. Although variations depending on factors such as the type of AVT preparation and the dose used and also the duration of tre atment were apparent, both preparations of AVT produced an increase in serum levels of T-4 and a decrease in that of T-3. The T-3:T-4 ratio was also significantly lower in response to AVT administration, Howeve r, no statistically significant changes in serum levels of melatonin w ere discernible with the administration of either of the two AVT prepa rations, even though melatonin levels in most AVT-treated groups showe d a trend (P > 0.05) to be higher than that in the controls. The rise in T-4 levels is indicative of increased release of T-4 and/or the inh ibition of its conversion to the active hormone, T-3. The drop in T-3 map account for the AVT-induced reduction in the metabolic rate of the pigeon observed in an earlier study. The absence of a significant inc rease in melatonin indicates that AVT could not produce such a change in the daytime in resting pigeons, in contrast to that in flown homing pigeons, suggesting that the modulating effect of AVT on melatonin le vels is dependent on the physiological state of the animal.