Effects of repeated prenatal glucocorticoid exposure on long-term potentiation in the juvenile guinea-pig hippocampus

Abstract

Synthetic glucocorticoids (sGCs) are routinely used to treat women at risk of preterm labour to promote fetal lung maturation. There is now strong evidence that exposure to excess glucocorticoid during periods of rapid brain development has permanent consequences for endocrine function and behaviour in the offspring. Prenatal exposure to sGC alters the expression of N-methyl-D-aspartate receptor (NMDA-R) subunits in the fetal and neonatal hippocampus. Given the integral role of the NMDA-R in synaptic plasticity, we hypothesized that prenatal sGC exposure will have effects on hippocampal long-term potentiation (LTP) after birth. Further, this may occur in either the presence or absence of elevated cortisol concentrations, in vitro. Pregnant guinea-pigs were injected with betamethasone (Beta, 1 mg kg(-1)) or vehicle on gestational days (gd) 40, 41, 50, 51, 60 and 61 (term approximately 70 days), a regimen comparable to that given to pregnant women. On postnatal day 21, LTP was examined at Schaffer collateral synapses in the CA1 region of hippocampal slices prepared from juvenile animals exposed to betamethasone or vehicle, in utero. Subsequently, the acute glucocorticoid receptor (GR)- and mineralocorticoid receptor (MR)-dependent effects of cortisol (0.1-10 microM; bath applied 30 min before LTP induction) were examined. There was no effect of prenatal sGC treatment on LTP under basal conditions. The application of 10 microM cortisol depressed excitatory synaptic transmission in all treatment groups regardless of sex. Similarly, LTP was depressed by 10 microM cortisol in all groups, with the exception of Beta-exposed females, in which LTP was unaltered. Hippocampal MR and GR protein levels were increased in Beta-exposed females, but not in any other prenatal treatment group. This study reveals sex-specific effects of prenatal exposure to sGC on LTP in the presence of elevated cortisol, a situation that would occur in vivo during stress.

Figure 1

Control (no cortisol) LTP in (A) female control (○; n = 16) and female Beta (•; n = 13) and (B) male control (▵; n = 10) and male Beta (▴; n = 7) Data are expressed as means ± s.e.m. Inset shows representative traces.

Figure 2

Cortisol effects on baseline fEPSPs and LTP during and after 30 min perfusion with cortisol A and B, 0.1 μm cortisol. A, female control (○; n = 9) and female Beta (•; n = 8). B, male control (▵; n = 8) and male Beta (▴; n = 10). C and D, 1.0 μm cortisol. C, female control (○; n = 10) and female Beta (•; n = 10). D, male control (▵; n = 7) and male Beta (▴; n = 11). E and F, 10 μm cortisol. E, female control (○; n = 9) and female Beta (•; n = 9) offspring. F, male control (▵; n = 8) and male Beta (▴; n = 11). Data are expressed as means ± s.e.m.* Significant effect (P < 0.05) of prenatal betamethasone treatment on STP and LTP after cortisol pretreatment. † Significant effect (P < 0.05) of cortisol over entire 30 min perfusion in control offspring. ‡ Significant effect (P < 0.05) of cortisol over entire 30 min perfusion in Beta offspring.

Figure 3

Hippocampal protein expression of GR (A) and MR (B) expressed as a ratio to α-tubulin Data are means ± s.e.m. relative optical density (ROD) for female and male offspring treated prenatally with either betamethasone (filled bars) or saline (open bars). * Significant effect (P < 0.05) of prenatal betamethasone treatment within sex. † Significant effect (P < 0.05) of sex in control offspring. Representative blots are bands from two different subjects per treatment group from one experiment.

Figure 4

Hippocampal expression of CaMKII (A) and P-CaMKII (B) expressed as a ratio to α-tubulin Data are means ± s.e.m. relative optical density (ROD) for female and male offspring treated prenatally with either betamethasone (filled bars) or saline (open bars). * Significant effect (P < 0.05) of prenatal betamethasone treatment within sex.