Influence of dopaminergic mechanisms on 24-hour secretory patterns of prolactin, luteinizing hormone and testosterone in recumbent men.

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This study examines the influence of bromocriptine (2.5 mg tid x 4 days) on recumbent 24 hour prolactin (PRL), luteinizing hormone (LH) and testosterone concentrations in 4 males ages 42 to 64 years. Before bromocriptine a clear circadian rhythm of PRL was present. An increase in PRL occurred between 60 to 90 min after sleep onset and this was followed by several larger secretory pulses resulting in progressively higher levels during the night with peak values occurring near the later stages of sleep. During the hour after awakening a fall in PRL began and lowest levels were reached at 11:00 h. Bromocriptine markedly lowered 24-h PRL levels and eliminated the PRL circadian rhythm. LH varied episodically throughout the 24 h, without displaying a circadian rhythm. Following bromocriptine treatment LH was still secreted in an episodic fashion but the amplitude of the LH pulses was decreased. Bromocriptine decreased (p < 0.05) the mean 24 h LH from 12.4 ± 0.7 mlU/ml to 10.5 ± 0.7 mlU/ml. Unlike previous observations in younger men our subjects failed to demonstrate a testosterone circadian rhythm. Rather, testosterone concentrations fluctuated throughout the 24 h. Maximal correlations between testosterone and LH pulses were observed when rises of testosterone were preceded by LH, with a lead time of 30 to 60 min. This temporal relationship between LH and testosterone was eliminated by bromocriptine treatment. There was no relationship between PRL rises and testosterone fluctuations before or after bromocriptine. The 24-h secretory pattern for testosterone was not affected by bromocriptine. These observations suggest the following: i) circadian variations in PRL secretion appear to be modulated by dopaminergic mechanisms; ii) dopamine-induced inhibition of LH secretion in men is primarily associated with a reduction in the amplitude of LH secretory pulses; iii) the 24-h testosterone secretory pattern is not related to PRL secretion or dopaminergic mechanisms.