Female steroid hormones modulate receptors for nerve growth factor in rat dorsal root ganglia

Calcitonin gene-related peptide (CGRP) is a vasodilatory peptide, and it is primarily synthesized in dorsal root ganglia (DRG). Plasma CGRP levels increase during pregnancy and with steroid hormones, and nerve growth factor (NGF) stimulates calcitonin/CGRP promoter and CGRP synthesis in DRG. We previously showed that CGRP levels in DRG were stimulated with steroid hormone treatments in vivo but not in vitro. Thus, the stimulation of CGRP by these hormones may be indirect through the upregulation of NGF effects. We hypothesized that the female sex steroid hormones upregulate NGF receptors, trkA and p75^NTR, in DRG. We examined the effects of 17β-estradiol (E₂) and progesterone (P₄) on NGF receptors in DRG obtained from ovariectomized (ovx) rats. Groups of 4 ovx rats were injected s.c. with 5 μg E₂, 4 mg P₄, or 5 μg E₂ + 4 mg P₄ in 0.2 ml sesame oil or injected with oil only and were killed at 6, 24, and 48 h. In addition, ovx rats were also injected s.c. with varying doses (0.2, 1.0, 5.0, 25 μg) of E₂ (0.5, 1.5, 4, 10 mg) P₄, and (5 μg) E₂ + (0.5, 1.5, 4.0, 10 mg) P₄ in 0.2 ml sesame oil, or vehicle, and killed at 6 (for E₂) or 24 (for P₄ and E₂ + P₄) h. Furthermore, groups of ovx rats were also killed at 12 and 24 h; 3 and 7 days; 2, 4, and 6 wk after ovariectonay. The DRGs were collected from all groups and then processed for Western immunoblotting to examine both trkA and p75^NTR levels. Estradiol increased trkA at 6 h but not p75^NTR. Progesterone caused upregulation of trkA and p75^NTR at 6 and 24 h. 17β-Estradiol + P₄ increased trkA at 6 and 24 h and p75^NTR at all time points examined. One microgram of E₂ increased trkA but did not affect p75^NTR levels. Progesterone at 4 and 10 mg upregulated trkA but only 10 mg P₄ increased p75^NTR. Five micrograms of E₂ coinjected with P₄ at 1.5 and 4 mg increased trkA, while p75^NTR receptor was upregulated when coinjected with P₄ at 1.5 to 10 mg. The ovariectomy caused a decrease in trkA receptors compared to proestrus rats, and these decreases were significant by 6 wk, but surprisingly p75^NTR increased at 2 wk after ovariectomy. 17β-Estradiol increased trkA but not p75^NTR receptors in DRG, whereas P₄ caused increases in both trkA and p75^NTR in DRG. In addition, the combination of these steroid hormones had more effect on both receptors than either hormone alone. Thus, we concluded that high levels of female steroid hormones such as those due to pregnancy or hormonal replacement therapy could increase NGF receptor expression in DRG that carry more NGF to elevate the CGRP synthesis in these groups. We suggested that the regulation of NGF receptors by ovarian steroids may underlie steroidal regulation of other factors such as CGRP.