The ontogeny of the onset of drinking and plasma osmotic pressure regulation

Pertinent behavioral and physiological variables related to the development of the onset of drinking were measured in albino rats of both sexes between 14 and 100 days of age. Under ad libitum conditions all variables studied were found to increase as a function of age. Following a subcutaneous 16% NaCl injection mean latency to drink was found to range from 4.6-9.5 min across ages. Plasma osmolality levels at the initiation of drinking were found to increase as a function of age and were found to be elevated over ad libitum levels by a relatively constant magnitude. The onset of drinking across ages was associated with 2.8-6.6% increase in plasma osmolality. These data suggest that although absolute levels of ad libitum osmolality increase from birth to maturity, the threshold increase in osmolality required for the initiation of drinking remains quite constant.