Circadian rhythms of neuroendocrine dopaminergic neuronal activity in ovariectomized rats

Prolactin (PRL) secretion is inhibited by dopamine (DA) released from hypothalamic neuroendocrine neurons designated tuberoinfundibular (TIDA), tuberohypophyseal (THDA) and periventricular hypophyseal (PHDA) dopaminergic (DAergic) neurons. Since PRL is secreted in many physiological states with a circadian rhythm, the purpose of these experiments was to determine if patterns of neuroendocrine DAergic neuronal activity in rats are also circadian. The activity of neuroendocrine DAergic neurons, defined as DA turnover rate in nerve terminals and quantitated as the ratio of DOPAC (a primary DA metabolite) to DA content, was measured by high-performance liquid chromatography with electrochemical detection (HPLC-EC) in these populations of DA neurons of OVX rats. TIDA neurons exhibit a rhythm of activity in a light:dark cycle which free-runs in constant dark (DD) and is entrained by light, indicating that TIDA neuronal activity is circadian. THDA and PHDA neurons also display daily rhythms entrained to a photoperiod and PHDA neuronal activity free-runs in DD with a period of approximately 24 h. However, a significant rhythm of THDA neuronal activity was not detected under DD. In the OVX rat, the activities of TIDA and PHDA neurons, but not THDA neurons, describe all the characteristics of a circadian rhythm as they are both entrained by light, but only TIDA and PHDA neurons maintain a significant rhythm of activity under DD.