Daily Fluctuations in the Activity of the Tuberoinfundibular GABAergic System and Plasma Prolactin Levels

In view of the role exerted by gamma-aminobutyric acid (GABA) in the control of prolactin (PRL) secretion the circadian periodicity of hypothalamo-pituitary GABAergic activity and PRL secretion was evaluated in adult male rats to ascertain if a meaningful correlation between biochemical and endocrine indices may be evidenced. Anterior pituitary and median eminence GABA concentrations peaked during the late afternoon hours (18.00 h), while the glutamic acid decarboxylase activity reached higher concentrations at 15.00 h. Plasma PRL presented two circadian surges at 15.00 h and at midnight. Although the changes of GABA in the hypothalamo-pituitary complex and the PRL surges did not show a close temporal relation, the possibility may be considered that GABA circadian changes occurred as a delayed response to PRL circadian surges. However, it cannot be excluded that changes in the biochemical indices could be related to other neuroendocrine events.