Sex-dependent effects of neonatal inflammation on adult inflammatory markers and behavior

Abstract

Inflammatory molecules, such as cyclooxygenase (COX), a prostaglandin synthetic enzyme, have been identified as a marker of depressive symptomology. Previously, we have observed elevated basal COX-2 expression in the hypothalamus of adult male rats treated neonatally with lipopolysaccharide (LPS), which might suggest a phenotype for disrupted hedonic behavior, a symptom of depression. However, COX-2 and its contribution to the expression of anhedonic behavior has not been investigated in these males or in female rats across the estrous cycle, which is the purpose of the current work. Here, we examine the effects of a neonatal LPS challenge or saline on the sucrose preference test as a measure of anhedonia, and hypothalamic COX-2 expression, in adult male and freely cycling female rats. Our data indicate a sex difference in that neonatal LPS at postnatal d 14 causes elevated basal expression of hypothalamic COX-2 in male, but not in female, rats. Additionally, baseline sucrose preference in male and female rats was unaltered as a function of neonatal LPS treatment or estrous cycle stage. In both male and female animals, 50 microg/kg LPS in adulthood caused elevated plasma IL-6 and hypothalamic COX-2 expression in neonatally saline-treated rats but significantly less so in neonatally LPS-treated rats of both sexes; this neonatal programming was not evident for sucrose preference or for total fluid intake (even after much higher doses of LPS). Our data are suggestive of a dissociation between inflammation and anhedonic behavior and a differential effect of neonatal inflammation in males and females.

FIG. 1

Flow chart of experimental procedures for the sucrose preference test administered to males during the day (light phase) or overnight (dark phase).

FIG. 2

Flow chart of experimental procedures for the sucrose preference test administered to females overnight across their respective estrous cycle group assignments (diestrus, proestrus, or estrus).

FIG. 3

Graphs plot percentage sucrose preference (mean ± SEM) and total fluid intake (mean ± SEM) over 3 wk of baseline and after adult LPS or saline challenge in male animals treated as neonates with LPS or saline (SAL) for the 8-h light (A and C) and 12-h dark (B and D) sucrose preference test (n = 5– 6 for graphs A and C and n = 6 –7 for graphs B and D).

FIG. 4

Graphs plot 8 h percentage sucrose preference (mean ± SEM) and total fluid intake (mean ± SEM) over 3 wk of baseline and for 3 subsequent days after adult LPS or saline challenge in male (A and B) and female (C and D) animals treated as neonates with LPS or saline (n = 5– 6).

FIG. 5

Graphs represent densitometric ratio (mean ± SEM) of hypothalamic COX-2/actin (A) and plasma IL-6 (mean ± SEM) concentrations expressed as pg/ml (B) after adult LPS or adult saline challenge in male animals treated as neonates with LPS or saline (*, P < 0.01; n = 5–6 per group).

FIG. 6

Graphs plot 12-h sucrose preference (mean ± SEM) (left) and total fluid intake (mean ± SEM) (right) over 3 wk of baseline and after adult LPS or adult saline challenge in female animals treated as neonates with LPS or saline (SAL). The graphs represent female animals at either diestrus (A and E), proestrus (B and F), estrus (C and G), or estrous (D and H) cycle collapsed (all animals, irrespective of estrous status), from top to bottom, respectively (n = 6 –7 per group).

FIG. 7

Densitometric ratio (mean ± SEM) of hypothalamic COX-2/actin after adult LPS or adult saline challenge in female animals treated as neonates with LPS or saline. Graphs represents female rats in diestrus (A), proestrus (B), estrus (C), or estrous (D) cycle collapsed, from top to bottom, respectively (*, P < 0.01; n = 5– 6 per group).

FIG. 8

Mean ± SEM plasma concentrations of IL-6 (pg/ml) after adult LPS or saline challenge in animals treated as neonates with LPS or saline. Graphs represents female rats in diestrus (A), proestrus (B), estrus (C), or estrous (D) cycle collapsed, from top to bottom, respectively (*, P < 0.01; n = 5– 6 per group).