Circadian variations of the response of plasma prolactin to brief exposure to ether in the male rat.

Male adult rats, supplied with an indwelling chronic intrajugular cannula, were exposed to ether for 1 min at various times of the day. Blood samples were taken before and at regular intervals during 1 h following the ether exposure. Basal plasma prolactin showed a circadian variation. The highest level occurred at 1.00 h, and the lowest values were measured at 10.00 and 13.00 h, while a significant increase was observed at 16.00 h compared to the level at 10.00 and 13.00 h. At all times of the day studied exposure for 1 min to ether increased plasma prolactin for at least 30 min and the magnitude of this response to ether varied with the time of day. At 1.00 h the greatest response was observed, while at 10.00 and 13.00 h the most moderate response was measured. The data indicate that there is either a circadian rhythm in basal prolactin, which may cause a significant circadian variation in the magnitude of the stress response, or there is over the 24 h period a significant variation in stress-sensitivity of the hormone, which may contribute to the circadian variation of 'basal' prolactin.