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 inc rease during pregnancy and with steroid hormones, and nerve growth factor ( NGF) stimulates calcitonin/CGRP promoter and CGRP synthesis in DRG. We prev iously 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 hypot hesized that the female sex steroid hormones upregulate NGF receptors, trkA and p75(NTR), in DRG. We examined the effects of 17 beta -estradiol (E-2) and progesterone (P-4) on NGF receptors in DRG obtained from ovariectomized (ovx) rats. Groups of 4 ovx rats were injected s.c. with 5 mug E-2, 4 mg P -4, or 5 mug E-2 + 4 mg P-4 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 injecte d s.c. with varying doses (0.2, 1.0, 5.0, 25 mug) of E-2 (0.5, 1.5, 4, 10 m g) P-4, and (5 mug) E-2 + (0.5, 1.5, 4.0, 10 mg) P-4 in 0.2 ml sesame oil, or vehicle, and killed at 6 (for E-2) or 24 (for P-4 and E-2 + P-4) h. Furt hermore, groups of ovx rats were also killed at 12 and 24 h; 3 and 7 days; 2, 4, and 6 wk after ovariectomy. The DRGs were collected from all groups a nd then processed for Western immunoblotting to examine both trkA and p75(N TR) levels. Estradiol increased trkA at 6 h but not p75(NTR). Progesterone caused upregulation of trkA and p75(NTR) at 6 and 24 h. 17 beta -Estradiol + P-4 increased trkA at 6 and 24 h and p75(NTR) at all time points examined . One microgram of E-2 increased trkA but did not affect p75NTR levels. Pro gesterone at 4 and 10 mg upregulated trkA but only 10 mg P-4 increased p75( NTR). Five micrograms of E-2 coinjected with P-4 at 1.5 and 4 mg increased trkA, while p75(NTR) receptor was upregulated when coinjected with P-4 at 1 .5 to 10 mg. The ovariectomy caused a decrease in trkA receptors compared t o proestrus rats, and these decreases were significant by 6 wk, but surpris ingly p75(NTR) increased at 2 wk after ovariectomy. 17 beta -Estradiol incr eased trkA but not p75(NTR) receptors in DRG, whereas P-4 caused increases in both trkA and p75(NTR) in DRG. In addition, the combination of these ste roid hormones had more effect on both receptors than either hormone alone. Thus, we concluded that high levels of female steroid hormones such as thos e due to pregnancy or hormonal replacement therapy could increase NGF recep tor 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.