Circadian rhythms in plasma prolactin, luteinizing hormone and hypophyseal prolactin levels in lactating rats.

During a 24 h period lactating rats were decapitated at 2 h intervals and the trunk blood and pituitaries were collected. Suckling activity was registered at the time of sacrifice. Plasma prolactin (PRL), LH and hypophyseal (HYP) PRL levels were measured by RIA. The trough of the plasma PRL curve was at 09.30 h and was followed by about 4.5-fold elevation by 13.30 (the lights were on from 03.30 to 17.30). The plasma PRL levels showed 2.5-fold decrease by 19.30 h. The HYP PRL concentration exhibited 3.5-fold elevation from 01.30 to 05.30 h. After this peak concentration, the HYP PRL showed a 4-fold decrease at 11.30 h. A statistically non-significant rhythm was found in the plasma LH levels. Our data suggest that there are circadian rhythms in the plasma and HYP PRL levels of lactating rats in the early lactation period. The two rhythms are in an 8 h phase shift. Neither these rhythms nor the LH levels depended on the actual suckling activity of freely behaving lactating rats.