Exposure to prenatal PCBs shifts the timing of neurogenesis in the hypothalamus of developing rats

Abstract

The developing brain is highly sensitive to the hormonal milieu, with gonadal steroid hormones involved in neurogenesis, neural survival, and brain organization. Limited available evidence suggests that endocrine-disrupting chemicals (EDCs) may perturb these developmental processes. In this study, we tested the hypothesis that prenatal exposure to a mixture of polychlorinated biphenyls (PCBs), Aroclor 1221, would disrupt the normal timing of neurogenesis in two hypothalamic regions: the ventromedial nucleus (VMN) and the preoptic area (POA). These regions were selected because of their important roles in the control of sociosexual behaviors that are perturbed in adulthood by prenatal EDC exposure. Pregnant Sprague-Dawley rats were exposed to PCBs from Embryonic Day 8 (E8) to E18, encompassing the period of neurogenesis of all hypothalamic neurons. To determine the birth dates of neurons, bromo-2-deoxy-5-uridine (BrdU) was administered to dams on E12, E14, or E16. On the day after birth, male and female pups were perfused, brains immunolabeled for BrdU, and numbers of cells counted. In the VMN, exposure to PCBs significantly advanced the timing of neurogenesis compared to vehicle-treated pups, without changing the total number of BrdU+ cells. In the POA, PCBs did not change the timing of neurogenesis nor the total number of cells born. This is the first study to show that PCBs can shift the timing of neurogenesis in the hypothalamus, specifically in the VMN but not the POA. This result has implications for functions controlled by the VMN, especially sociosexual behaviors, as well as for sexual selection more generally.

Keywords: Aroclor 1221; endocrine-disrupting chemical; hypothalamus; neurogenesis; polychlorinated biphenyl; preoptic area; ventromedial nucleus.

FIGURE 1

(a) The region of the ventromedial nucleus (labeled here as VMH) used for the analysis is shown, with the region of interest indicated with a green outline. (b) A representative image of BrdU nuclear labeling in the VMN of a DMSO female pup whose dam was injected with BrdU on E14. The region of interest outlined in green (a) is shown at ×20 magnification in (b). White scale bar = 25 μm. The brain map shown in (a) was modified from Paxinos (1991). BrdU, bromo-2-deoxy-5-uridine; DMSO, dimethyl sulfoxide; E14, Embryonic Day 14; VMH, ventromedial nucleus of the hypothalamus; VMN, ventromedial nucleus

FIGURE 2

The number of BrdU+ cells in the VMN of P1 pups is shown in relationship to day of injection of the dams with BrdU. (a) Bars with different letters are significantly different. The numbers of pups per group are shown within bars except for E12 DMSO (n = 12). (b) The same data are shown with curves fitted to the timecourse of neurogenesis (black for DMSO and red for PCB). PCB treatment shifted the timing of neurogenesis, with the peak occurring about a day earlier (~E14) than the DMSO group (~E15). BrdU, bromo-2-deoxy-5-uridine; DMSO, dimethyl sulfoxide; E12, Embryonic Day 12; P1, Postnatal Day 1; PCB, polychlorinated biphenyl; VMN, ventromedial nucleus

FIGURE 3

(a) The region of the preoptic area (labeled here as MPO) used for the analysis is shown, with the region of interest indicated with a green outline. (b) A representative image of BrdU nuclear labeling in the POA of a DMSO female pup whose dam was injected with BrdU on E14. The region of interest outlined in green (a) is shown at ×20 magnification in (b). White scale bar = 25 μm. The brain map shown in (a) was modified from Paxinos (1991). BrdU, bromo-2-deoxy-5-uridine; DMSO, dimethyl sulfoxide; E14, Embryonic Day 14; MPO, medial preoptic area; POA, preoptic area

FIGURE 4

The number of BrdU+ cells in the POA of P1 pups is shown in relationship to day of injection of the dams with BrdU. (a) Bars with different letters are significantly different. Numbers of pups per group are shown within bars. (b) The same data are shown with curves fitted to the timecourse of neurogenesis (black for DMSO and red for PCB). The curves are entirely superimposed, with the peak for both PCB and DMSO occurring at ~E14. BrdU, bromo-2-deoxy-5-uridine; DMSO, dimethyl sulfoxide; E14, Embryonic Day 14; P1, Postnatal Day 1; PCB, polychlorinated biphenyl; POA, preoptic area

FIGURE 5

Examples of the bootstrapping process in the VMN (a) and POA (b). The fitted curves of 500 bootstrapped replicates (DMSO in gray and PCB in red) against fits to the original dataset (black lines; original means are open circles for DMSO and open squares for PCB). The distributions of peak loci over 10,000 replicates for each group are inset at the bottom; DMSO in white and PCB in red. Ninety-five percent confidence intervals for the distributions are shown as dashed vertical lines. BrdU, bromo-2-deoxy-5-uridine; DMSO, dimethyl sulfoxide; PCB, polychlorinated biphenyl; POA, preoptic area; VMN, ventromedial nucleus