Nocturnal prolactin pulses in relation to luteinizing hormone and thyrotropin

The two hypothalamic releasing factors, luteinizing hormone releasing hormone (LHRH) and thyrotropin releasing hormone (TRH), have been shown to stimulate pituitary prolactin (PRL) release as well as their respective pituitary hormones, luteinizing hormone (LH) and thyrotropin (TSH). In this study the influence of LH and TSH regulatory mechanisms on nocturnal PRL secretion was investigated by evaluating whether the coincidence of PRL with LH and TSH pulses occurred more frequently than would be expected if the hormone generators were not coupled. Thirty night studies were conducted in twelve healthy male subjects. Six subjects underwent 3 studies and 6 subjects 2 studies. Blood was collected into aliquots at 10 min intervals throughout the night and plasma concentrations of PRL, TSH, and LH were determined. From the plasma profiles, hormone secretory rates were calculated using a method of deconvolution. Significant plasma and secretory hormone pulses were identified by a peak detection computer program. For statistical analysis the night studies of each subject were concatenated. Concomitance between the plasma pulses of both TSH and LH with PRL was insufficient to reject the null hypothesis of random coincidence. An increase in the number of subjects demonstrating significant coincidence between the hormone pulses was obtained when secretory pulses were analysed. Seven of the 12 and 10 of the 12 subjects showed significant concomitance between PRL and respectively TSH and LH. This proportion was sufficient to confirm copulsatility between PRL and LH. These results suggest that LH regulatory mechanisms are involved in the generation of the nocturnal pulsatile PRL profile, TRH may also play a role in the secretion of PRL at a central level, but was not reflected in the plasma or secretory profiles because of other overriding regulatory factors.