Effects of developmental alcohol exposure vs. intubation stress on BDNF and TrkB expression in the hippocampus and frontal cortex of neonatal rats

Abstract

Third trimester-equivalent alcohol exposure causes significant deficits in hippocampal and cortical neuroplasticity, resulting in alterations to dendritic arborization, hippocampal adult neurogenesis, and performance on learning tasks. The current study investigated the impact of neonatal alcohol exposure (postnatal days 4-9, 5.25 g/kg/day) on expression of brain-derived neurotrophic factor (BDNF) and the tropomyosin-related kinase B (TrkB) receptor in the hippocampal and frontal cortex of infant Long-Evans rats. Levels of BDNF protein were increased in the hippocampus, but not frontal cortex, of alcohol-exposed rats 24h after the last dose, when compared with undisturbed (but not sham-intubated) control animals. BDNF protein levels showed a trend toward increase in hippocampus of sham-intubated animals as well, suggesting an effect of the intubation procedure. TrkB protein was increased in the hippocampus of alcohol-exposed animals compared to sham-intubated pups, indicating an alcohol-specific effect on receptor expression. In addition, expression of bdnf total mRNA in alcohol-exposed and sham-intubated pups was enhanced in the hippocampus; however, there was a differential effect of alcohol and intubation stress on exon I- and IV-specific mRNA transcripts. Further, plasma corticosterone was found to be increased in both alcohol-exposed and sham-intubated pups compared to undisturbed animals. Upregulation of BDNF could potentially represent a neuroprotective mechanism activated following alcohol exposure or stress. The results suggest that alcohol exposure and stress have both overlapping and unique effects on BDNF, and highlight the need for the stress of intubation to be taken into consideration in studies that implement this route of drug delivery.

Keywords: Cortex; Corticosterone; FASD; Neurotrophin; Plasticity.

Figure 1

Experimental timeline and body weights. A) On PD 4-9, rats were exposed to 5.25 g/kg/day ethanol via intragastric intubation and sacrificed on PD10. AE: Alcohol-exposed; SI: Sham-intubated; SC: Suckle control. B) Average body weight ± SEM are shown for each day during the neonatal treatment period (PD4-9) as well as the day of sacrifice for alcohol-exposed (AE), sham-intubated (SI), and suckle controls (SC). AE animals weighed significantly less than SI and SC animals on PD5–10. * p < 0.05.

Figure 2

BDNF and TrkB protein in the hippocampus and cortex on PD10. A) Alcohol-exposed (AE) animals had significantly elevated BDNF protein compared to suckle control (SC) animals. Sham-intubated (SI) animals showed a trending increase in BDNF protein compared to the SC group (p = 0.074) but were not significantly different from AE pups. B) No effect of neonatal condition was found in frontal cortex. C) AE animals had increased TrkB protein in the hippocampus relative to SI animals, but not SC pups. No change was found in frontal cortex (D). * p < 0.05. Data is shown as mean percent of control ± SEM.

Figure 3

BDNF gene expression in the PD10 hippocampus. A) Alcohol-exposed (AE) and sham-intubated (SI) animals exhibited an increase in total bdnf gene expression in the hippocampus. B) and C) Exon I- and IV-specific transcripts were significantly elevated in AE animals. A trending increase was also found for exon IV-specific transcript gene expression in SI pups relative to controls (p = 0.09, indicated as ^ on graph). * p < 0.05. Data is expressed as a fold change from the suckle control (SC) group (shown as 1 on graphs).

Figure 4

Plasma corticosterone on PD10. Both alcohol-exposed (AE) and sham-intubated (SI) animals showed an increase in corticosterone compared to the suckle control (SC) group. * p < 0.05; ** p < 0.01. SI and AE did not statistically differ from one another. Data is shown as mean percent of control ± SEM.

Figure 5

Plasma corticosterone and time of sacrifice correlations. Significant correlations between time of day (minutes since lights on at 9 AM) and corticosterone levels were observed in suckle control (SC) and sham-intubated (SI) pups, but there was no correlation observed in alcohol-exposed (AE) animals. Pearson’s r value given for each correlation. SC and SI: p < 0.01; AE: p = n.s.