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.

Progesterone attenuates cocaine-induced responses.

In this review, we summarize literature focused on how progesterone alters cocaine-induced psychomotor, reinforcement, and physiological responses. Clinical studies suggest that progesterone attenuates the subjective effects of cocaine. Similarly, preclinical studies have demonstrated that cocaine-induced reward and psychomotor responses are attenuated after progesterone administration. In rats progesterone also reduces the reinforcement effects of cocaine attenuates acquisition, escalation, reinstatement of cocaine self-administration, and cocaine-seeking behaviors. Progesterone also counteracts the facilitatory effects of estrogen on cocaine self-administration and psychomotor activation. These findings suggest that progesterone has a potential in clinical applications as a treatment for cocaine addiction. Constantly changing progesterone serum levels in female humans and rats affect the female's reinforcement responses to cocaine and may in part contribute to the known sex differences in cocaine responses.

Cocaine-induced sex differences in D1 dopamine receptor mRNA levels after acute cocaine administration.

INTRODUCTION: Although it is known that female rats have a more robust behavioral response to acute cocaine administration than male rats, the neurobiological mechanisms underlying these differences remain unclear. The purpose of the present study was to determine if there are sex differences in cocaine's regulation of dopamine D1 and D2 receptor mRNA levels. METHODS: Male and female Fischer rats received acute cocaine (20 mg/kg, intraperitoneal) or saline. Ambulatory activity was recorded one hour post drug treatment. Rats were then sacrificed either 1 or 24 hours post drug treatment and D1/D2 DA receptor mRNA levels were measured via solution hybridization assay. RESULTS: Cocaine-induced ambulatory activity was greater in female than male rats. There were no sex differences in baseline levels of D1 and D2 receptor mRNA in the caudate putamen (CPu) or the nucleus accumbens (NAc). Cocaine administration reduced levels of D1 mRNA in the NAc only in male rats. CONCLUSION: Our findings suggest that the regulation of striatal D1 mRNA levels after acute cocaine administration is a sexually dimorphic process. We also hypothesize that the D1 receptor may be an important substrate in the regulation of sex differences in cocaine-induced locomotor activity.

Acute and chronic estradiol replacements differentially alter corticosterone and COX-mediated responses to an inflammatory stimulus in female rats.

INTRODUCTION: This study aimed to determine whether the previously reported differential effects of estradiol on inflammation-induced behavioral responses are in part explained through differential activation of the corticosterone-cyclooxygenase (CORT-COX) regulatory pathway. METHODS: Prostaglandin E2 (PGE2), COX, and CORT levels were analyzed before and after a formalin administration (1% vs. 5%, representing different intensities of inflammatory stimuli). RESULTS: In vehicle-treated rats, chronic estradiol administration increased corticosterone, and decreased COX and PGE2. After acute estradiol administration, although corticosterone serum levels were increased, COX protein levels were unchanged. In rats treated with formalin, PGE2 serum levels were higher in rats administered 5% formalin than vehicle- and 1%-treated rats. Significant correlations were observed between PGE2 serum levels, CORT serum levels, and COX protein levels. CONCLUSIONS: Our results suggest that the administration of exogenous estradiol may mediate inflammatory responses by regulating the levels of PGE2 and/or CORT release, thereby mediating the nociceptive response to an inflammatory stimulus.

Endogenous gonadal hormones regulate females' behavioral responses to formalin through prostaglandin E2 release.

INTRODUCTION: This study aimed to determine if endogenous gonadal hormones affect the intracellular mechanisms in the spinal cord that control inflammatory pain responses. METHODS: We analyzed behavioral responses to, and changes in, serum levels of prostaglandin E2, estradiol, progesterone, and corticosterone after administration of 5% formalin in intact and ovariectomized (OVX) female rats. RESULTS: OVX females displayed significantly more flinching than did intact females during Phase I, and after formalin administration their corticosterone levels were significantly lower. No differences were seen across COX-1 and COX-2 protein expression in the spinal cord of either naive or formalin-treated rats. However, subsequent to formalin a main effect of gonadectomy was seen in prostaglandin E2 levels; OVX animals had significantly lower prostaglandin E2 levels than intact animals. CONCLUSIONS: These results indicate that in female rats nociceptive responses to formalin are regulated through the levels of prostaglandin E2, an important mediator in inflammation, whereas protein levels of COX-1 and COX-2 play a more limited role.

Sex differences in dopamine D2-like receptor-mediated G-protein activation in the medial prefrontal cortex after cocaine.

INTRODUCTION: Sexually dimorphic behavioral responses to cocaine have been linked to a difference in activation of dopamine receptors. Our study was conducted to determine whether dopamine D2-like receptor-activated G-protein contributes to sex differences in response to cocaine in the medial prefrontal cortex (mPFC). METHOD: In vitro functional autoradiography was performed using dopamine receptor D2 agonist (quinpirole, 100 microM) to stimulate [35S]GTPgammaS binding in brain tissue sections from male and female Fischer rats treated with saline (1 mL/kg) or cocaine (20 mg/kg; i.p.). RESULTS: Overall, quinpirole increased G-protein activation in the caudate-putamen, nucleus accumbens, and frontal cortex in both sexes. Although saline-treated male rats had higher [35S]GTPyS binding in the mPFC than their female counterparts, cocaine-treated females had higher [35S]GTPgammaS binding in the mPFC than cocaine-treated males. CONCLUSIONS: These data suggest that both intrinsic and activational effects of dopamine D2-like receptor-mediated G-protein activation in the mPFC may contribute to the differences between males and females in their response to acute cocaine administration.

Acute progesterone treatment impairs spatial working memory in intact male and female rats.

INTRODUCTION: The aim of this study was to determine if progesterone affects spatial and non-spatial working memory in intact male and female rats. METHODS: Rats received subcutaneous injections of progesterone (500 microg) or vehicle (sesame oil). Four hours after hormone treatments, spatial and non-spatial memories were tested using novel object recognition and spatial object recognition tasks. RESULTS: Vehicle-treated female rats had higher progesterone serum levels than males, but progesterone treatment produced equivalent progesterone serum levels in both sexes. In the object recognition task--a non-spatial memory task-females showed better performance than males, and progesterone had no effect on either sex. However, in the object replacement task--a spatial memory task-progesterone significantly impaired the retention in both male and female rats as compared with vehicle-treated groups. CONCLUSION: These results suggest that acute progesterone treatment interferes with spatial working memory consolidation, but not recognition (non-spatial) working memory. As such, the observed sexual incongruities in progesterone's effects on working memory suggest that progesterone-based hormone therapies have a negative impact on cognition.

Progesterone does not affect cocaine-induced conditioned place preference or locomotor activity in male rats.

INTRODUCTION: The present study aimed to determine if, as occurs in female rats, progesterone attenuates cocaine-induced reward and psychomotor responses in male rats. METHODS: The role of progesterone in the acquisition and/or expression of cocaine-induced conditioned place preference (CPP) and locomotor responses of intact male rats was studied. For chronic progesterone treatment, rats received Silastic capsules with either progesterone (100%) or vehicle 1 week prior to conditioning. For acute progesterone treatment, rats received subcutaneous injections of progesterone (500 microg) or vehicle (sesame oil) 4 hours before intraperitoneal injections of saline or cocaine administration (20 mg/kg) on conditioning days (acquisition phase-formation of reward associations) or before testing (expression phase-recall of reward associations). RESULTS: Both progesterone-treatment paradigms produced equivalent progesterone serum levels. Progesterone administered chronically or acutely during the acquisition and expression phases of cocaine conditioning did not block cocaine-induced CPP. Nor did progesterone affect ambulatory or rearing behaviors after cocaine administration. CONCLUSION: These results suggest that, unlike the findings with female rats (in which similar treatment paradigms inhibited the formation and recall of cocaine-induced CPP), progesterone plays a limited role in the cocaine-induced reward or psychomotor responses of male rats.

Basal and cocaine-induced sex differences in the DARPP-32-mediated signaling pathway.

RATIONALE: Behavioral and dopamine responses to cocaine are sexually dimorphic: Female rats exhibit higher levels of locomotor and reward-associated behaviors after cocaine administration and dopamine release than do males. Activation of the dopamine- and cAMP-regulated phosphoprotein of Mr 32 kDa (DARPP-32) intracellular cascade mediates responses to cocaine. OBJECTIVE: To examine the possibility that acute cocaine administration alters the DARPP-32 cascade in a sexually dimorphic pattern. MATERIALS AND METHODS: Male and female rats received either saline or cocaine (30 mg/kg). Protein levels of DARPP-32, phosphorylation of DARPP-32 at the Thr34 site (P-Thr34-DARPP-32), protein phosphatase 1 (PP-1), and protein phosphatase 2B (PP-2B) in nucleus accumbens were measured via Western blot analysis. RESULTS: Females had higher protein levels of DARPP-32, P-Thr34-DARPP-32, calcineurin A (CaN-A; catalytic subunit of PP-2B), and calcineurin B (CaN-B; regulatory subunit of PP-2B) than males 5 min after saline treatment. In females, CaN-A protein levels were also higher at 15 min and PP-1 protein levels were higher 30 min after saline administration than males. In male rats, cocaine significantly increased CaN-A protein levels at 30 min and CaN-B protein levels at 15 min. In females, cocaine administration significantly decreased protein levels of DARPP-32, P-Thr34-DARPP-32, and CaN-A at 45 min but increased PP-1 protein levels at 30 min. Overall, males had higher activation of the DARPP-32 pathway after cocaine administration than did females. CONCLUSION: These novel results show that basal and cocaine-induced sex differences in the DARPP-32/PP-1 cascade may be responsible for the sexual dimorphism in acute cocaine-induced behavioral responses.