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

Peritraumatic 17β-estradiol levels influence chronic posttraumatic pain outcomes

ABSTRACT

Biologic factors that predict risk for and mediate the development of common outcomes of trauma exposure such as chronic posttraumatic pain (CPTP) are poorly understood. In the current study, we examined whether peritraumatic circulating 17β-estradiol (E2) levels influence CPTP trajectories. 17β-estradiol levels were measured in plasma samples (n = 254) collected in the immediate aftermath of trauma exposure from 3 multiethnic longitudinal cohorts of men and women trauma survivors. Chronic posttraumatic pain severity was evaluated 6 weeks, 6 months, and 1 year after traumatic stress exposure. Repeated measures mixed models were used to test the relationship between peritraumatic E2 levels and prospective CPTP. Secondary analyses in a nested cohort assessed the influence of participant body mass index on the E2-CPTP relationship. In women, a statistically significant inverse relationship between peritraumatic E2 and CPTP was observed (β = -0.280, P = 0.043) such that higher E2 levels predicted lower CPTP severity over time. Secondary analyses identified an E2 * body mass index interaction in men from the motor vehicle collision cohort such that obese men with higher E2 levels were at greater risk of developing CPTP. In nonobese men from the motor vehicle collision cohort and in men from the major thermal burn injury cohort, no statistically significant relationship was identified. In conclusion, peritraumatic circulating E2 levels predict CPTP vulnerability in women trauma survivors. In addition, these data suggest that peritraumatic administration of E2 might improve CPTP outcomes for women; further research is needed to test this possibility.

Involvement of glutamate receptors of the paragigantocellularis lateralis nucleus in the pain modulatory effect of 17β-estradiol in male rats

The pain modulatory role of the paragigantocellularis lateralis nucleus (LPGi) and the 17β-estradiol has thoroughly been probed. This study investigates the contribution of ionotropic glutamate receptors in pain modulatory effect of intra-LPGi injection of 17β-estradiol. For this purpose, the LPGi nucleus cannulation was performed and drugs were injected into this nucleus, 15 min prior to the formalin test. The duration of formalin-induced flexing and licking behaviors was recorded for 60 min immediately after formalin injection. The results showed that the flexing behavior is significantly decreased by intra-LPGi injection of 0.8 µmol 17β-estradiol duringboth phases of formalin test (P < 0.001). However, 17β-estradiol attenuated the licking duration only in the second phase (P < 0.001). Interestingly, NMDA and AMPA/kainate receptor antagonists (AP5 and CNQX, respectively) significantly counteracted the analgesic effect of intra-LPGi injection of 17β-estradiol in both phases of the formalin test (P < 0.001). Consequently, the revealing results showed that the analgesic effect of intra-LPGi injection of 17β-estradiol on acute inflammatory pain might be mediated via the activation of ionotropic glutamate receptors.

Histamine H4 receptor stimulation in the locus coeruleus attenuates neuropathic pain by promoting the coeruleospinal noradrenergic inhibitory pathway

The locus coeruleus (LC) adrenergic nuclei constitute a pain-control inhibitory system nucleus implicated in descending modulation of pain through the action on spinal α₂-adrenoceptors. Histaminergic innervation from the tuberomammillary nucleus of the LC increases firing of noradrenergic neurons and might contribute to pain control. Here we evaluated the contribution of LC histaminergic innervation in descending modulation of neuropathic hypersensitivity, by investigating the role of the histamine H₄ receptor subtype in a mouse model of neuropathic pain. Intra LC administration of the H₄ agonist VUF 8430 attenuated mechanical and thermal allodynia of mice that underwent spared nerve injury (SNI). Similarly, histamine in the LC showed mechanical and thermal anti-hypersensitivity. Pretreatment of LC with JNJ 10191584 (H₄ antagonist) prevented the beneficial effect of VUF 8430 and histamine on nociceptive behaviour. Comparable results were obtained after intrathecal administration of drugs. The intrathecal administration of the α₂-adrenoceptor agonist clonidine ameliorated mechanical and thermal allodynia in SNI mice. The clonidine-induced anti-hypersensitivity effect was prevented by intra LC pretreatment with JNJ 10191584. In addition, clonidine failed to suppress neuropathic pain in H₄ deficient mice. LC H₄ receptors showed a ubiquitous distribution within LC, a neuronal localization and H₄ immunostaining was detected on noradrenergic neurons expressing phosphorylated cAMP response element-binding protein (CREB), a marker of neuronal activation. Under pain pathological conditions H₄ stimulation might promote the activation of the coeruleospinal noradrenergic neurons that exert an inhibitory control over spinal dorsal horn neuronal excitability. Thus, histamine H₄ receptor stimulation may represent a perspective for neuropathic pain management.

Sex differences in pain and opioid mediated antinociception: Modulatory role of gonadal hormones

Sex-related differences in pain and opioids has been the focus of many researches. It is demonstrated that women experience greater clinical pain, lower pain threshold and tolerance, more sensitivity and distress to experimentally induced pain compared to men. Sex differences in response to opioid treatment revealed inconsistent results. However, the etiology of these disparities is not fully elucidated. It is, therefore, conceivable now that this literature merits to be revisited comprehensively. Possible multifaceted factors seem to be associated. These include neuroanatomical, hormonal, neuroimmunological, psychological, social and cultural aspects and comorbidities. This review aims at providing an overview of the substantial literature documenting the sex differences in pain and analgesic response to opioids from animal and human studies within the context of the modulatory effects of the aforementioned factors. A detailed and critical discussion of the cellular and molecular signaling pathways underlying the modulatory actions of gonadal hormones in the sexual dimorphism in pain processing and opioid analgesia is extensively presented. It is indicated that sexual dimorphic activation of certain brain regions contributes to differential pain sensitivity between females and males. Plausible crosstalk between sex hormones and neuroimmunological signaling pertinent to toll-like and purinergic receptors is uncovered as causal cues underlying sexually dimorphic pain and opioid analgesia. Conceivably, a thorough understanding of these factors may aid in sex-related advancement in pain therapeutic management.

Analgesic Effect of 17β-Estradiol on Nucleus Paragigantocellularis Lateralis of Male Rats Mediated Via GABAA Receptors

Introduction:

Beside its autonomic functions, the nucleus paragigantocellularis lateralis (LPGi) is involved in the descending pain modulation. 17β-Estradiol is a neuroactive steroid found in several brain areas such as LPGi. Intra-LPGi microinjection of 17β-estradiol can elicit the analgesic responses. 17β-Estradiol modulates nociception by binding to estrogenic receptors as well as allosteric interaction with other membrane-bound receptors like GABA_A receptors. This study aimed to examine the role of GABA_A receptors in the pain modulating effect of intra-LPGi injection of 17β-estradiol.

Methods:

To study the antinociceptive effects of 17β-estradiol, cannulation into the LPGi nucleus of male Wistar rats was performed. About 500 nL of drug was administered 15 minutes prior to formalin injection (50 μL of 4%). Then, formalin-induced flexing and licking behaviors were recorded for 60 minutes. For evaluating the role of GABA_A receptors in the estradiol-induced pain modulation, 17β-estradiol was administered into the LPGi nucleus 15 minutes after the injection of 25 ng/μL bicuculline (the GABA_A receptor antagonist). Then, the formalin-induced responses were recorded.

Results:

The results of the current study showed that intra-LPGi injection of 17β-estradiol decreased the flexing duration in both phases of formalin test (P<0.001); but it only attenuated the second phase of licking behavior (P<0.001). 17β-estradiol attenuated the second phase of formalin test of both behaviors (P<0.001). Bicuculline prevented the antinociceptive effect of intra-LPGi 17β-estradiol in both first and second phases of formalin-induced responses (P<0.001).

Conclusion:

According to the results of this study, the analgesic effect of intra-LPGi 17β-estradiol on the formalin-induced inflammatory pain might be mediated via GABA_A receptors.

A role for the locus coeruleus in the analgesic efficacy of N-acetylaspartylglutamate peptidase (GCPII) inhibitors ZJ43 and 2-PMPA

N-acetylaspartylglutamate (NAAG) is the third most prevalent and widely distributed neurotransmitter in the mammalian nervous system. NAAG activates a group II metabotropic glutamate receptor (mGluR3) and is inactivated by an extracellular enzyme, glutamate carboxypeptidase II (GCPII) in vivo. Inhibitors of this enzyme are analgesic in animal models of inflammatory, neuropathic and bone cancer pain. NAAG and GCPII are present in the locus coeruleus, a center for the descending noradrenergic inhibitory pain system. In the formalin footpad model, systemic treatment with GCPII inhibitors reduces both phases of the inflammatory pain response and increases release of spinal noradrenaline. This analgesic efficacy is blocked by systemic injection of a group II mGluR antagonist, by intrathecal (spinal) injection of an alpha 2 adrenergic receptor antagonist and by microinjection of an α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptor antagonist directly into the contralateral locus coeruleus. Footpad inflammation increases release of glutamate in the contralateral locus coeruleus and systemic treatment with a GCPII inhibitor blocks this increase. Direct injection of GCPII inhibitors into the contralateral or ipsilateral locus coeruleus reduces both phases of the inflammatory pain response in a dose-dependent manner and the contralateral effect also is blocked by intrathecal injection of an alpha 2 adrenergic receptor antagonist. These data support the hypothesis that the analgesic efficacy of systemically administered GCPII inhibitors is mediated, at least in part, by the contralateral locus coeruleus via group II mGluR, AMPA and alpha 2 adrenergic receptors.

Noradrenergic Locus Coeruleus pathways in pain modulation

The noradrenergic system is crucial for several activities in the body, including the modulation of pain. As the major producer of noradrenaline (NA) in the central nervous system (CNS), the Locus Coeruleus (LC) is a nucleus that has been studied in several pain conditions, mostly due to its strategic location. Indeed, apart from a well-known descending LC-spinal pathway that is important for pain control, an ascending pathway passing through this nucleus may be responsible for the noradrenergic inputs to higher centers of the pain processing, such as the limbic system and frontal cortices. Thus, the noradrenergic system appears to modulate different components of the pain experience and accordingly, its manipulation has distinct behavioral outcomes. The main goal of this review is to bring together the data available regarding the noradrenergic system in relation to pain, particularly focusing on the ascending and descending LC projections in different conditions. How such findings influence our understanding of these conditions is also discussed.

17β-Estradiol alters the response of subfornical organ neurons that project to supraoptic nucleus to plasma angiotensin II and hypernatremia

This study was done in urethane anesthetized, ovariectomized (OVX) female rats that were either implanted or not implanted with silastic capsules containing17β-estradiol (E2) to investigate the effect of systemic changes in E2 on the discharge rate of subfornical organ (SFO) neurons that projected to supraoptic nucleus (SON) and responded to changes in plasma levels of angiotensin II (ANG II) or hypernatremia. Extracellular single unit recordings were made from 146 histologically verified single units in SFO. Intra-carotid infusions of ANG II excited ~57% of these neurons, whereas ~23% were excited by hypertonic NaCl. Basal discharge rate of neurons excited by ANG II or hypertonic NaCl was significantly lower in OVX+E2 rats compared to OVX only animals. The response of SFO neurons antidromically activated by SON stimulation to intra-carotid injections of ANG II or hypertonic NaCl was greater in the OVX only compared to the OVX+E2 rats. Intra-carotid injections of E2 in either group attenuated not only the basal discharge of these neurons, but also their response to ANG II or hypertonic NaCl. In all cases this inhibitory effect of E2 was blocked by an intra-carotid injection of the E2 receptor antagonist ICI-182780, although ICI-182780 did not alter the neuron's response to ANG II or hypertonic NaCl. Additionally, ICI-182780 in the OVX+E2 animals significantly raised the basal discharge of SFO neurons and their response to ANG II or hypertonic NaCl. These data indicate that E2 alters the response of SFO neurons to ANG II or NaCl that project to SON, and suggest that E2 functions in the female to regulate neurohypophyseal function in response to circulating ANG II and plasma hypernatremia.

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.