Semicircadian Rhythms of c-Fos Expression in Several Hypothalamic Areas during Pregnancy in the Rat: Relationship to Prolactin Secretion

Abstract

Prolactin (PRL) serves an important luteotrophic function in the rat during early pregnancy, expressed as a nocturnal surge in the early morning and a diurnal surge in the late afternoon. Several areas of the hypothalamus, including the preoptic area (POA), the suprachiasmatic nucleus (SCN) and the ventromedial and dorsomedial nuclei (VM-DM) have been implicated in PRL surges. We investigated the temporal relationship between neuronal activity as measured by c-Fos immunocytochemistry in these areas and PRL secretion during early and late pregnancy. Brains were collected at nine time points (24:00, 02:00, 04:00, 06:00, 10:00, 14:00, 16:00, 18:00 and 20:00 h) on days 6–7 and three time points (02:00, 14:00 and 18:00 h) on days 14–15 of pregnancy. Plasma PRL levels determined by radioimmunoassay revealed two surges with peaks at 02:00 and 18:00 h and a trough at 14:00 h on days 6–7, which were absent on days 14–15 of pregnancy. The number of neurons expressing c-Fos in the anterior medial preoptic nucleus, the medial preoptic area and the medial preoptic nucleus, but not the anteroventral preoptic nucleus of the POA, and the VM-DM, showed a semicircadian rhythm which was maximal at 02:00 h or/and 04:00 and 18:00 h and reached the lowest value at 14:00 h, in parallel with the PRL surges in early pregnancy. However, the temporal pattern of c-Fos in these areas was reversed during late pregnancy, with a peak at 14:00 h and low levels at 02:00 and 18:00 h. PRL surges were absent and levels were uniformly low during these times. Neuronal activity in the SCN did not show any correlation with PRL surges. The dorsomedial subdivision of the SCN showed high neuronal activity during the daytime in both stages of pregnancy. Neuronal activity in the ventrolateral subdivision of the SCN was high during the nighttime in early pregnancy, however it exhibited high levels during the daytime in late pregnancy. These results suggest that the two daily surges of PRL secretion during the first half of pregnancy might be related to the temporal rhythm of neuronal activity in the POA and the VM-DM, and a major change in the pattern of neuronal activity in these hypothalamic areas might result in termination of the PRL surges at midpregnancy.

Fos Expression in the Female Rat Brain during the Proestrous Prolactin Surge and following Mating

“…The amygdala has been shown to alter the incidence of induction of pseudopregnancy [36, 37]and the termination of the nocturnal surge [38]. In addition, our previous work showed that Fos expression in the mPOA and VMH exhibited activated neuronal activity during the mating-induced nocturnal and diurnal PRL surges [23]. Mating likely triggers the activation of VCS-responsive neurons in these brain areas that, in turn, are involved in the initiation of the PRL surges.…”

“…Immunocytochemistry was performed as previously described [23]. Briefly, sections were rinsed with KPBS followed by an incubation with 0.6% hydrogen peroxide in methanol to quench endogenous peroxide activity and with 1% normal goat serum in KPBS to reduce nonspecific binding.…”

“…Recently we reported that neuronal activity assessed by Fos expression in the preoptice area, VMH and DMH showed a semicircadian rhythm in parallel with the twice-daily PRL surges of early pregnancy [23]. The present study was undertaken to examine patterns of Fos expression in six brain areas, the mPOA, paraventricular nucleus (PVN), arcuate nucleus (ARC), VMH, DMH and mAMYG, during diestrus, the proestrous PRL surge, and following mating, thus enabling a comparison of neuronal activities in these brain areas during diestrus, proestrus and proestrous rats that received VCS.…”

c-<i>fos</i> Expression in the Forebrain after Mating in the Female Rat Is Altered by Adrenalectomy

“…Lastly, the labeling which we observed throughout the ARC did not differ between mated and nonmated groups in the Sham animals. Dopaminergic cells expressing FOS or FOS-related antigen within the dorsomedial ARC show a semicircadian rhythm [45, 46], and our behavioral testing occurred at a time when dopamine activity is high, potentially masking an effect of VCS in this area.…”

Fos Immunoreactivity in the Suprachiasmatic Nuclei of Golden Hamsters Bearing an Ectopic Pituitary Graft

“…On the other hand, exposure of rats to continuous light inhibited the prolactin surges [24,25], further supporting the influence of the circadian pacemaker on prolactin secretion. Under physiological hyperprolactinemia caused by pregnancy in rats, Lee et al [26] demonstrated a differential pattern of Fos expression in the two regions of the SCN (VL and DM). The aim of the present study was to establish whether hypoprolactinemia could modify Fos-ir in the SCN as a response to a light pulse at night.…”