Modulatory role of estradiol in nicotinic antinociception in adult female rats

Effect of chronic pretreatment with 17β-estradiol and/or progesterone on the nociceptive response to uterine cervical distension in a rat model

It is widely known that visceral pain is more prevalent in women than in men, and this phenomenon is interpreted as a consequence of the gonadal hormone modulation of pain perception and transduction. Uterine cervical distension might cause obstetric and gynecologic pain with clinical relevance to visceral pain. In this study, we focused on the roles of 17β-estradiol and progesterone in visceral nociception with the use of a rat model of uterine cervical distension. Female ovariectomized rats were injected with 17β-estradiol (E₂) or progesterone (P₄) for 21 days, after which visceral pain-induced spinal c-fos expression and visceromotor reflex changes were compared between ovariectomized and hormone-substituted groups. We found that uterine cervical distension induced a drastic increase in spinal c-fos expression and visceromotor reflex activity, and ovariectomy inhibited the increase in c-fos expression induced by visceral pain; this inhibition was reversed by estrogen but not progesterone replacement. This study demonstrates that estrogen is involved in uterine cervical nociception, while progesterone plays less of a significant role.

Inhibition of substance P signaling aggravates the bone loss in ovariectomy-induced osteoporosis

INTRODUCTION

Substance P signaling regulates the functions of both osteoblast and osteoclast. Available reports on the effects of substance P on bone mass are contradictory. The objective of this study was to determine the change of substance P expression in the osteoporotic bone of OVX mice. The effects of substance P signaling blockade by using its specific receptor antagonist L-703606 on bone remodeling in sham-operated mice and OVX mice were also investigated.

METHODS

Forty-eight nine-week-old female C57BL/6J mice were evenly distributed into three groups with sham surgery, OVX or OVX with estrogen replacement. Substance P expression in the bones of each group of mice was evaluated by immunohistochemistry and enzyme immunoassay. Another thirty-two nine-week-old female C57BL/6J mice were divided into a SHAM group (sham surgery followed by vehicle treatment with DMSO), a SHAM + L group (sham surgery followed by 15 mg/kg/d L-703606 repeated intraperitoneal injections), an OVX group (ovariectomy with the same vehicle treatment) and an OVX + L group (ovariectomy with the same L-703606 injections), with 8 mice in each group. Treatment started 3 weeks after surgery and last for 3 weeks. A 2 × 2 factorial experimental design was used to detect the effects of substance P signaling blockade on bone remodeling in sham-operated mice and OVX mice. Techniques including micro-computed tomography, biomechanical testing, histomorphometric analysis, enzyme immunoassay, and real-time PCR were employed.

RESULTS

Immunohistochemistry and enzyme immunoassay revealed that substance P expression significantly decreased in the bones of OVX mice both at 3 weeks and 6 weeks after surgery. Micro-CT tomography demonstrated that application of L-703606 led to bone loss in sham-operated mice, and aggravated the micro-structural deterioration of bones in OVX mice. This was shown by reduced BV/TV (Mean bone volume fraction), Tb.N (Mean trabecular number) and Tb.Th (Mean trabecular thickness), and increased Tb.Sp (Mean trabecular separation). Biomechanical analysis demonstrated that blockade of substance P signaling reduced the maximum stress and maximum load of L3 vertebrae and tibiae. Inhibited recruitment of bone mesenchymal stem cells (BMSCs) to bone remodeling sites, which was evidenced by increased number of osteoclasts, decreased number of osteoblasts and increased osteoid volume in the secondary spongiosa, was observed in the mice treated with L-703606. A significant decrease of OPG/RANKL ratio was also found in the bones of mice treated with L-703606. Body weight, uterine weight and serum estradiol level were not significantly different between the mice treated with L-703606 and those treated with vehicle.

CONCLUSION

The results demonstrated that blocking substance P signaling led to bone loss in sham-operated mice, and exacerbated the bone loss in OVX mice. Substance P signaling had an important role in the maintenance of bone mass.

Blockade of substance P receptor attenuates osteoporotic pain, but not bone loss, in ovariectomized mice

OBJECTIVE

The aim of this study was to investigate the effect of a substance P (SP) receptor (NK1 receptor [NK1-R]) antagonist on hyperalgesia and bone metabolism in ovariectomized mice.

METHODS

Thirty-six 9-week-old mice were subjected to either bilateral ovariectomy or sham surgery. Three weeks after the operation, the mice were treated with either a single-dose injection or 2-week repeated daily administration of L-703606, an NK1-R antagonist. Behavioral tests were performed for pain assessment; tibiae and the third lumbar vertebrae were dissected and assessed for microarchitectural or biomechanical properties. The expressions of SP and NK1-R in the dorsal root ganglia and spinal cord were also evaluated.

RESULTS

Both single-dose injection and 2-week repeated injections of L-703606 led to a significant increase in nociceptive threshold in ovariectomized mice. However, the antihyperalgesic effect faded at 2 hours and almost disappeared at 5 hours after a single-dose injection. With the 14-day repeated treatment of ovariectomized mice, the effect was not detectable at 24 hours after the first injection but was obvious at 24 hours after 1-week and 2-week administrations and still existed at 48 hours after the last injection. Ovariectomized mice at the hyperalgesic state had enhanced SP immunoreactivity in the dorsal root ganglia and up-regulated SP and NK1-R expressions in the spinal cord. However, no significant change in serum SP level was detected. Two-week treatment with L-703606 could down-regulate these expressions but failed to salvage the deteriorated trabecular microstructure and reduced compressive strength in ovariectomized mice.

CONCLUSIONS

Estrogen deficiency-induced hyperalgesia is achieved through up-regulation of SP and NK1-R expressions. Blockade of SP receptor can alleviate pain but cannot ameliorate bone loss. NK1-R antagonist is not recommended for the treatment of estrogen deficiency osteoporosis.

Sex-specific differences in GABA(A) -benzodiazepine receptor availability: relationship with sensitivity to pain and tobacco smoking craving

Sex differences exist in tobacco smoking behaviors. Nicotine, the primary addictive ingredient in tobacco smoke, indirectly affects γ-amino butyric acid (GABA) function. Previous studies reported sex-by-smoking interactions in brain GABA levels. The goal of the present study was to evaluate if there is a sex-by-smoking interaction at the GABA(A)-benzodiazepine receptors (GABA(A)-BZRs), as well as relationships between GABA(A)-BZR availability and behavioral variables before and after 1 week of smoking cessation. Twenty-six women (8 non-smokers, age 36.0 ± 13.4 years; 19 smokers, age 34.6 ± 8.9 years) and 25 men (8 non-smokers, age 37.9 ± 13.8 years; 17 smokers, 34.1 ± 12.4 years) were imaged using [123I]iomazenil and single-photon emission computed tomography. Smokers were imaged at baseline 7 hours after the last cigarette. A significantly great number of men were able to abstain from smoking for 1 week (P = 0.003). There were no significant differences in nicotine dependence and cigarette craving, mood or pain sensitivity between male and female smokers. There was a significant effect of gender across all brain regions (frontal, parietal, anterior cingulate, temporal and occipital cortices, and cerebellum; P < 0.05), with all women (smokers and non-smokers combined) having a higher GABA(A)-BZR availability than all men. There was a negative correlation between GABA(A)-BZR availability and craving (P ≤ 0.02) and pain sensitivity (P = 0.04) in female smokers but not male smokers. This study provides further evidence of a sex-specific regulation of GABA(A)-BZR availability in humans and demonstrates the potential for GABA(A)-BZRs to mediate tobacco smoking craving and pain symptoms differentially in female and male smokers.

The effect of intra-locus coeruleus injection of 17beta-estradiol on inflammatory pain modulation in male rat

Estradiol is a neuroactive steroid found in several brain areas such as locus coeruleus (LC). It modulates nociception by binding to its receptors and also by allosteric interaction with other membrane-bound receptors like glutamate and GABA(A) receptors. LC is involved in noradrenergic descending pain modulation. In order to study the effect of 17beta-estradiol on both acute and persistent pain modulation and its mechanisms, formalin was injected into the male rat's hind paw. Formalin-induced responses including licking, flexing duration and paw jerking frequency were recorded for 60 min after injection of 50 microl of 2% formalin. The results of the current study showed that intra-locus coeruleus injection of 17beta-estradiol attenuated the second phase, but not the acute phase of formalin-induced pain (P<0.05). AMPA receptor antagonists CNQX had no effect on pain-modulatory effect of 17beta-estradiol. Estrogen and GABA(A) receptor antagonists (ICI 182,780 and bicuculline, respectively) could not reverse the antinociceptive effect of 17beta-estradiol. However, NMDA receptor antagonist APV significantly antagonized the analgesic effect of 17beta-estradiol on flexing behaviour (P<0.05). It may be concluded that the analgesic effect of 17beta-estradiol in formalin-induced inflammatory pain is mediated through interaction with membrane-bound receptors, probably the NMDA receptors.