Changes in and dorsal to the rat suprachiasmatic nucleus during early pregnancy

Abstract

Circadian rhythms in behavior and physiology change as female mammals transition from one reproductive state to another. The mechanisms responsible for this plasticity are poorly understood. The suprachiasmatic nucleus (SCN) of the hypothalamus contains the primary circadian pacemaker in mammals, and a large portion of its efferent projections terminate in the ventral subparaventricular zone (vSPZ), which also plays important roles in rhythm regulation. To determine whether these regions might mediate changes in overt rhythms during early pregnancy, we first compared rhythms in Fos and Per2 protein expression in the SCN and vSPZ of diestrous and early pregnant rats maintained in a 12:12-h light/dark (LD) cycle. No differences in the Fos rhythm were seen in the SCN core, but in the SCN shell, elevated Fos expression was maintained throughout the light phase in pregnant, but not diestrous, rats. In the vSPZ, the Fos rhythm was bimodal in diestrous rats, but this rhythm was lost in pregnant rats. Peak Per2 expression was phase-advanced by 4 h in the SCN of pregnant rats, and some differences in Per2 expression were found in the vSPZ as well. To determine whether differences in Fos expression were due to altered responsivity to light, we next characterized light-induced Fos expression in the SCN and vSPZ of pregnant and diestrous rats in the mid-subjective day and night. We found that the SCN core of the two groups responded in the same way at each time of day, whereas the rhythm of Fos responsivity in the SCN shell and vSPZ differed between diestrous and pregnant rats. These results indicate that the SCN and vSPZ are functionally re-organized during early pregnancy, particularly in how they respond to the photic environment. These changes may contribute to changes in overt behavioral and physiological rhythms that occur at this time.

Fig. 1

Photomicrograph of the SCN and vSPZ of a female lab rat stained for AVP-immunoreactivity (-ir). The middle SCN core and AVP-rich SCN shell are outlined, and the sampling box used for vSPZ counts is placed above the SCN. ocx, optic chiasm; 3v, third ventricle. Scale bar = 200 μm.

Fig. 2

Representative photomicrographs of protein expression of Fos at ZT 2, 6, and 18 (A) and Per2 at ZT 2, 14, and 18 (B) in the middle SCN and vSPZ of diestrous (left) and pregnant rats (right) kept in a 12:12 LD cycle. Per2 is only expressed in the cell nucleus. Larger cells in (B) that are stained in the cytoplasm are TH-ir cells and are primarily located around the vSPZ and 3v (see METHODS). ocx, optic chiasm; 3v, third ventricle. Scale bars = 100 μm.

Fig. 3

Expression of Fos (left column) and Per2 (right column) in the SCN core (A,B), SCN shell (C,D), and vSPZ (E,F) of diestrous and pregnant rats kept in a 12:12 LD cycle. Expression is measured as the percentage of the maximum individual value for each protein in each region. *indicates a time point where expression is significantly elevated relative to at least two other time points within the same reproductive state (P<0.05, post-hoc LSD (A,B,E,F) or Mann-Whitney U (C,D) tests), and **indicates a time point at which expression significantly differs between reproductive states (P<0.05, post-hoc LSD (A,B,E,F), or P<0.017, Mann-Whitney U (C) tests).

Fig. 4

Representative photomicrographs of protein expression of Fos in the middle SCN and vSPZ of control and light-pulsed diestrous (left) and pregnant rats (right) at CT 6 (top row) and CT 18 (bottom row). ocx, optic chiasm; 3v, third ventricle. Scale bars = 100 μm.

Fig. 5

Expression of Fos in the SCN core (A), SCN shell (B), and vSPZ (C) of control and light-pulsed diestrous and pregnant rats kept in DD for two days. Expression is measured as the percentage of the maximum individual Fos-positive cell count within the region of interest. * indicates significantly higher Fos expression in light-pulsed than control females (P<0.01, Mann-Whitney U (A,B) tests, or P<0.05, post-hoc LSD (C) tests).