Relationship between Norepinephrine Release in the Hypothalamic Paraventricular Nucleus and Circulating Prolactin Levels in the Siberian Hamster: Role of Photoperiod and the Pineal Gland

Abstract

The impact of norepinephrine (NE) and its metabolite, 3-methoxy-4-hydroxyphenylglycol (MHPG), on circulating prolactin (PRL) was evaluated in the paraventricular region of the hypothalamus as a function of photoperiod and integrity of the pineal gland. In Experiment 1, whole tissue content of NE and MHPG was assessed in male and female hamsters that had been pinealectomized or sham-pinealectomized and exposed to long or short photoperiods for 5 weeks. The results revealed a marginal effect of photoperiod in males, but no overall effects of surgery. Because analysis of whole tissue content can be complicated by concurrent changes in synthesis and storage rates, Experiment 2 was conducted using microdialysis to assess extracellular levels of NE and MHPG in female hamsters. Pinealectomy completely prevented the short-day-induced suppression of luteinizing hormone, but it only partially prevented the effects of short days on PRL. Furthermore, both NE and MHPGlevels were significantly elevated in short-day-exposed pinealectomized and shamoperated animals. These results suggest that NErelease within the paraventricular nucleus inhibits the circulating PRL levels and is one mechanism by which direct photic information can influence the neuroendocrine axis independently of the pineal melatonin signal.