Early Life Stress Alters Expression of Glucocorticoid Stress Response Genes and Trophic Factor Transcripts in the Rodent Basal Ganglia

Abstract

Early life stress shapes the developing brain and increases risk for psychotic disorders. Yet, it is not fully understood how early life stress impacts brain regions in dopaminergic pathways whose dysfunction can contribute to psychosis. Therefore, we investigated gene expression following early life stress in adult brain regions containing dopamine neuron cell bodies (substantia nigra, ventral tegmental area (VTA)) and terminals (dorsal/ventral striatum). Sprague-Dawley rats (14F, 10M) were separated from their mothers from postnatal days (PND) 2-14 for 3 h/day to induce stress, while control rats (12F, 10M) were separated for 15 min/day over the same period. In adulthood (PND98), brain regions were dissected, RNA was isolated and five glucocorticoid signalling-related and six brain-derived neurotrophic factor (Bdnf) mRNAs were assayed by qPCR in four brain regions. In the VTA, levels of glucocorticoid signalling-related transcripts differed in maternally separated rodents compared to controls, with the Fkbp5 transcript significantly lower and Ptges3 transcript significantly higher in stressed offspring. In the VTA and substantia nigra, maternally separated rodents had significantly higher Bdnf IIA and III mRNA levels than controls. By contrast, in the ventral striatum, maternally separated rodents had significantly lower expression of Bdnf I, IIA, IIC, IV and VI transcripts. Sex differences in Nr3c1, Bag1 and Fkbp5 expression in the VTA and substantia nigra were also detected. Our results suggest that early life stress has long-lasting impacts on brain regions involved in dopamine neurotransmission, changing the trophic environment and potentially altering responsiveness to subsequent stressful events in a sex-specific pattern.

Keywords: BDNF; FKBP5; brain-derived neurotrophic factor; dopamine; early life stress; glucocorticoid receptor.

Figure 1

Simplified diagram of the glucocorticoid receptor (GR) stress signalling pathway. GR (unfolded) is unable to bind cortisol when bound to Bag1, and release of Bag1 allows GR to bind cortisol with low affinity, whereas Fkbp51 (encoded by Fkbp5) and p23 (encoded by Ptges3) are involved in increasing GR affinity to cortisol by stabilizing the GR heterocomplex into a high affinity state. Furthermore, Fkbp52 (encoded by Fkbp4) dislocates Fkbp51 and facilitates nuclear translocation of the cortisol-bound GR heterocomplex into the nucleus to activate or repress target genes. Thus, higher Fkbp51 would promote GR retention in the cytoplasm, rendering target genes less responsive to stress [16,17,18,19]. GR = glucocorticoid receptor; Hsp = heat shock protein.

Figure 2

Effects of early life stress on stress-related transcript levels. (A,C,D) There were no main effects of early life stress in the substantia nigra, dorsal striatum or ventral striatum; (B) maternally separated rats of both sexes had lower Fkbp5 and higher Ptges3 mRNA in the VTA than control rats. Data are expressed as a percentage of the control group mean. Each data point represents a single animal. Filled triangles and hollow circles represent control and early life stress animals, respectively. Horizontal lines depict group means. * p < 0.05.

Figure 3

Differences between females and males in stress-related transcript levels and effects of early life stress. (A) Females had significantly higher Fkbp4 mRNA levels in the substantia nigra and dorsal striatum than males; (B) female control rats had higher Fkbp5 mRNA levels in the substantia nigra than male control rats. They also had higher Fkbp5 mRNA levels than maternally separated female rats; (C) maternally separated male rats had lower Nr3c1 (GR) mRNA levels in the VTA than male control rats; (D) maternally separated male rats had lower Bag1 mRNA levels in the VTA than male control rats. Each data point represents a single animal. Filled squares and hollow diamonds represent female and male animals, respectively. Horizontal lines depict group means. * p < 0.05, ** p < 0.005, *** p < 0.0005.

Figure 4

Effects of early life stress on BDNF transcript levels. (A,B) Maternally separated rats had higher levels of Bdnf IIA and III mRNAs in the substantia nigra and VTA than control rats; (C) there were no main effects of early life stress on Bdnf transcripts in the dorsal striatum; (D) maternally separated rats had lower levels of Bdnf I, IIA, IIC, IV and VI mRNAs in the ventral striatum than control rats. Data are expressed as a percentage of the control group mean. Each data point represents a single animal. Filled triangles and hollow circles represent control and early life stress animals, respectively. Horizontal lines depict group means. * p < 0.05, ** p < 0.005.

Figure 5

Differences between females and males in Bdnf transcript levels. (A–C) Females had significantly higher levels of Bdnf I, IIC and VI in the substantia nigra than males; (D,E) females had significantly higher levels of Bdnf I and IIC in the VTA than males. Each data point represents a single animal. Filled squares and hollow diamonds represent female and male animals, respectively. Horizontal lines depict group means. * p < 0.05, ** p < 0.005.