Estrogen alters baseline and inflammatory-induced cytokine levels independent from hypothalamic-pituitary-adrenal axis activity.

Although estrogen reduces inflammatory-mediated pain responses, the mechanisms behind its effects are unclear. This study investigated if estrogen modulates inflammatory signaling by reducing baseline or inflammation-induced cytokine levels in the injury-site, serum, dorsal root ganglia (DRG) and/or spinal cord. We further tested whether estrogen effects on cytokine levels are in part mediated through hypothalamic-pituitary-adrenal (HPA) axis activation. Lumbar DRG, spinal cord, serum, and hind paw tissue were analyzed for cytokine levels in 17ß-estradiol-(20%) or vehicle-(100% cholesterol) treated female rats following ovariectomy/sham adrenalectomy (OVX), adrenalectomy/sham ovariectomy (ADX) or ADX+OVX operation at baseline and post formalin injection. Formalin significantly increased pro-inflammatory interleukin (IL)-6 levels in the paw, as well as pro- and anti-inflammatory cytokine levels in the DRG, spinal cord and serum in comparison to naïve conditions. Estrogen replacement significantly increased anti-inflammatory IL-10 levels in the DRG. Centrally, estradiol significantly decreased pro-inflammatory tumor necrosis factor (TNF)-α and IL-1ß levels, as well as IL-10 levels, in the spinal cord in comparison to cholesterol treatment. At both sites, most estradiol modulatory effects occurred irrespective of pain or surgical condition. Estradiol alone had no influence on cytokine release in the paw or serum, indicating that estrogen effects were site-specific. Although cytokine levels were altered between surgical conditions at baseline and following formalin administration, ADX operation did not significantly reverse estradiol's modulation of cytokine levels. These results suggest that estrogen directly regulates cytokines independent of HPA axis activity in vivo, in part by reducing cytokine levels in the spinal cord.

Estradiol-induced antinociceptive responses on formalin-induced nociception are independent of COX and HPA activation.

Estrogen modulates pain perception but how it does so is not fully understood. The aim of this study was to determine if estradiol reduces nociceptive responses in part via hypothalamic-pituitary-adrenal (HPA) axis regulation of cyclooxygenase (COX)-1/COX-2 activity. The first study examined the effects of estradiol (20%) or vehicle with concurrent injection nonsteroidal antiinflammatory drugs (NSAIDs) on formalin-induced nociceptive responding (flinching) in ovariectomized (OVX) rats. The drugs were ibuprofen (COX-1 and COX-2 inhibitor), SC560 (COX-1 inhibitor), or NS398 (COX-2 inhibitor). In a second study, estradiol's effects on formalin-induced nociception were tested in adrenalectomized (ADX), OVX, and ADX+OVX rats. Serum levels of prostaglandins (PG) PGE(2) and corticosterone were measured. Estradiol significantly decreased nociceptive responses in OVX rats with effects during both the first and the second phase of the formalin test. The nonsteroidal antiinflammatory drugs (NSAIDs) did not alter nociception at the doses used here. Adrenalectomy neither altered flinching responses in female rats nor reversed estradiol-induced antinociceptive responses. Estradiol alone had no effect on corticosterone (CORT) or prostaglandin levels after the formalin test, dissociating the effects of estradiol on behavior and these serum markers. Ibuprofen and NS398 significantly reduced PGE2 levels. CORT was not decreased by OVX surgery or by estradiol below that of ADX. Only IBU significantly increased corticosterone levels. Taken together, our results suggest that estradiol-induced antinociception in female rats is independent of COX activity and HPA axis activation.

Effect of dopamine D1 and D2 receptor antagonism in the lateral hypothalamus on the expression and acquisition of fructose-conditioned flavor preference in rats.

The attraction to sugar-rich foods is influenced by conditioned flavor preferences (CFP) produced by the sweet taste of sugar (flavor-flavor learning) and the sugar's post-oral actions (flavor-nutrient) learning. Brain dopamine (DA) circuits are involved in both types of flavor learning, but to different degrees. This study investigated the role of DA receptors in the lateral hypothalamus (LH) on the flavor-flavor learning produced the sweet taste of fructose. In an acquisition study, food-restricted rats received bilateral LH injections of a DA D1 receptor antagonist (SCH23390), a D2 antagonist (RAC, raclopride) or vehicle prior to 1-bottle training sessions with a flavored 8% fructose+0.2% saccharin solution (CS+/F) and a less-preferred flavored 0.2% saccharin solution (CS-). Drug-free 2-bottle tests were then conducted with the CS+ and CS- flavors presented in saccharin. The fructose-CFP did not differ among groups given vehicle (76%), 12 nmol SCH (78%), 24 nmol (82%) or 24 nmol RAC (90%) during training. In an expression study with rats trained drug-free, LH injections of 12 or 24 nmol SCH or 12-48 nmol RAC prior to 2-bottle tests did not alter CS+ preferences (77-90%) relative to vehicle injection (86%). Only a 48 nmol SCH dose suppressed the CS+ preference (61%). The minimal effect of LH DA receptor antagonism upon fructose flavor-flavor conditioning differs with the ability of LH SCH injections to block the acquisition of glucose flavor-nutrient learning.

Interactions of estradiol and NSAIDS on carrageenan-induced hyperalgesia.

How exogenous estrogen affects inflammatory responses is poorly understood despite the large numbers of women receiving estrogen-alone hormone therapy. The aim of this study was to determine if estradiol alters injury- or inflammation-induced nociceptive responses after carrageenan administration in females and whether its effects are mediated through cyclo-oxygenase (COX) and prostaglandins (PG). To this end, paw withdrawal latencies and serum levels of PGE2 and PGD2 were measured in rats treated with estradiol (0, 10, 20, and 30%) and/or SC560 (COX-1 inhibitor) or NS398 (COX-2 inhibitor) after intraplantar carrageenan administration. Estradiol significantly increased withdrawal latencies before (baseline condition) and after carrageenan administration to one hindpaw. NS398 was anti-nociceptive only in carrageenan treated animals. SC560 increased withdrawal latencies in both paws at 1 and 5hours after carrageenan administration. Co-administration of estradiol and NS398, but not SC560, was additive except for a prolonged anti-nociceptive effects of estradiol combined with NS398. The anti-nociceptive effect extended beyond that observed with either drug or estradiol alone at the 5-hour time point. Estradiol had no significant effect on PGE2 serum levels, but both COX antagonists decreased them. Although neither estradiol nor the COX inhibitors alone had an effect on PGD2 serum levels, co-administration of NS398 and estradiol significantly elevated PGD2 levels. Taken together, our results suggest that estradiol is anti-nociceptive in the thermal test and reduces carrageenan-induced hyperalgesia. These effects are minimally altered through PG-mediated mechanisms.