c-fos reduces corticosterone-mediated effects on neurotrophic factor expression in the rat hippocampal CA1 region

Abstract

The transcription of neurotrophic factors, i.e., basic fibroblast growth factor (bFGF) and brain-derived neurotrophic factor (BDNF) is regulated by glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) activation despite the lack of a classical glucocorticoid response element in their promoter region. A time course for corticosterone (10 mg/kg, s.c.) in adrenalectomized rats revealed a peak hormone effect at the 4 hr time interval for bFGF (110-204% increase), BDNF (53-67% decrease), GR (53-64% decrease), and MR (34-56% decrease) mRNA levels in all hippocampal subregions using in situ hybridization. c-fos mRNA levels were affected exclusively in the dentate gyrus after 50 min to 2 hr (38-46% decrease). Furthermore, it was evaluated whether corticosterone regulation of these genes depends on interactions with the transcription factor complex activator protein-1. c-fos antisense oligodeoxynucleotides were injected into the dorsal hippocampus of adrenalectomized rats. Corticosterone was given 2 hr later, and the effects on gene expression were measured 4 hr later. In CA1, antisense treatment significantly and selectively enhanced the hormone action on the expression of bFGF (44% enhanced increase) and BDNF (38% enhanced decrease) versus control oligodeoxynucleotide treatment. In addition, an upregulation of c-fos expression (89% increase) was found. There were no effects of c-fos antisense on hippocampal GR and MR expression. Thus it seems that a tonic c-fos mechanism exists within CA1, which reduces GR- and MR-mediated effects on expression of bFGF and BDNF.

Figure 3.

a, Schematic representation to show the position of the cannula guide and of the sampled areas for densitometric evaluation in a coronal section through the dorsal hippocampus (bregma level -3.3 mm). CA1–CA3, Cornus Ammon areas; DG, dentate gyrus. b, Distribution of ³⁵S-end-labeled antisense c-fos ODN 1 hr after intrahippocampal injection as shown in a cresyl violet-counterstained coronal section (bregma level = -3 to -4 mm). c, Dark-field microphotographs from in situ hybridization of autoradiograms of bFGF and BDNF mRNA levels in response to CORT (10 mg/kg, s.c.) after randomized (left panel) and antisense c-fos ODN (right panel) treatment. Arrowheads show mRNA signals in the CA1 region (bregma levels = -3 to -4.5 mm). Scale bar, 1 mm. d, Densitometric evaluation of in situ hybridization autoradiograms. Values are given as percentage of random oligo-treated group (mean ± SEM; n = 6). Statistical analysis was performed by one-way ANOVA (treatment) followed by Bonferroni's correction. ^*p < 0.05, ^**p < 0.01 versus random oligo group. CA1, Cornus Ammon area. For details on treatment, see Materials and Methods.

Figure 4.

Bright-field photomicrographs showing a strongly labeled nerve cell enriched in silver grains, representing c-fos transcripts, and a GFAP-ir (brown color) astroglial cell lacking silver grains in the same cresyl violet counterstained section in the CA1 region of corticosterone-treated (10 mg/kg, s.c.; 4 hr) ADX rat. CA1, Cornus Ammon area. For details on treatment, see Materials and Methods. Scale bar, 15 μm. Bregma level = -3 to -4.5 mm.

Figure 1.

Densitometric evaluation of in situ hybridization from autoradiograms. Values are given in percentage of the ADX group (mean ± SEM; n = 5) versus vehicle-treated control ADX rats. Statistical analysis was performed by two-way ANOVA (treatment × time) followed by Fisher's PLSD test and Bonferroni's correction. ^*p < 0.05, ^**p < 0.01, ^***p < 0.001 versus ADX group. CA1, CA3, Cornus Ammon areas; DG, dentate gyrus. For details on treatment, see Materials and Methods.

Figure 2.

Dark-field microphotographs from autoradiograms of in situ hybridization of bFGF, BDNF, GR, and MR mRNA levels 4 hr after injection and of c-fos mRNA levels 50 min after subcutaneous injection in ADX + vehicle-treated (left panel) and ADX + CORT-treated (10 mg/kg, right panel) rats. Arrowheads show mRNA signals in the dentate gyrus. Bregma levels were -3 to -4 mm. Scale bar, 1 mm.

Figure 5.

Fluorescence photomicrographs involving double immunolabeling procedures showing the colocalization (yellow color) of c-fos-ir (Texas Red) with NeuN-ir (FITC, green, left) or with GR-ir (FITC, green, right), but not with GFAP-ir (FITC, middle) in the CA1 region of corticosterone-treated (10 mg/kg, s.c.; 1.5 hr) ADX rats. CA1, Cornus Ammon area. For details on treatment, see Materials and Methods. Scale bar, 40 μm. Bregma level = -3 to -4.5 mm.