Gestational and ovarian sex steroid antinociception: relevance of uterine afferent and spinal alpha(2)-noradrenergic activity

The Programmed Cell Death Ligand-1/Programmed Cell Death-1 Pathway Mediates Pregnancy-Induced Analgesia via Regulating Spinal Inflammatory Cytokines

The maternal pain threshold gradually increases during pregnancy, especially in late pregnancy. A series of mechanisms underlying pregnancy-induced analgesia have been reported. However, these mechanisms are still not completely clear, and the underlying molecular mechanisms need further investigation. We examined the relationship between the antinociceptive effect and the expression level of programmed cell death ligand-1 (PD-L1) during pregnancy and further observed the changes in pain thresholds and expression levels of cytokines in late-pregnant mice before and after blockade of PD-L1 or programmed cell death-1 (PD-1).

T-Cell Mediation of Pregnancy Analgesia Affecting Chronic Pain in Mice

It has been reported consistently that many female chronic pain sufferers have an attenuation of symptoms during pregnancy. Rats display increased pain tolerance during pregnancy due to an increase in opioid receptors in the spinal cord. Past studies did not consider the role of non-neuronal cells, which are now known to play an important role in chronic pain processing. Using an inflammatory (complete Freund's adjuvant) or neuropathic (spared nerve injury) model of persistent pain, we observed that young adult female mice in early pregnancy switch from a microglia-independent to a microglia-dependent pain hypersensitivity mechanism. During late pregnancy, female mice show no evidence of chronic pain whatsoever. This pregnancy-related analgesia is reversible by intrathecal administration of naloxone, suggesting an opioid-mediated mechanism; pharmacological and genetic data suggest the importance of δ-opioid receptors. We also observe that T-cell-deficient (nude and Rag1-null mutant) pregnant mice do not exhibit pregnancy analgesia, which can be rescued with the adoptive transfer of CD4⁺ or CD8⁺ T cells from late-pregnant wild-type mice. These results suggest that T cells are a mediator of the opioid analgesia exhibited during pregnancy.SIGNIFICANCE STATEMENT Chronic pain symptoms often subside during pregnancy. This pregnancy-related analgesia has been demonstrated for acute pain in rats. Here, we show that pregnancy analgesia can produce a complete cessation of chronic pain behaviors in mice. We show that the phenomenon is dependent on pregnancy hormones (estrogen and progesterone), δ-opioid receptors, and T cells of the adaptive immune system. These findings add to the recent but growing evidence of sex-specific T-cell involvement in chronic pain processing.

Perinatal 17α-ethinylestradiol exposure affects formalin-induced responses in middle-aged male (but not female) rats

17α-Ethinylestradiol (EE), the main component of the contraceptive pill, is a synthetic estrogen found in rivers of the United States and Europe as an environmental contaminant. It is one of the most studied xenoestrogens due to its possible effect on the reproductive system. In the present study we evaluated the modulation of pain responses induced by formalin injection (licking, flexing, paw-jerk) in 8-month-old male and female offspring of female rats treated with two different doses of EE (4ng/kg/day or 400ng/kg/day) during pregnancy and lactation. Spontaneous behaviors and gonadal hormone levels were also determined. Both concentrations of EE induced an increase of pain behaviors in males only, i.e. higher flexing and licking of the formalin-injected paw than in OIL-exposed rats, during the second, inflammatory, phase of the formalin test. Grooming duration was increased by EE exposure in both males and females. Prenatal EE exposure (both concentrations) decreased estradiol plasma levels in the formalin-injected females but not in the males. These results underline the possibility that exposure to an environmental contaminant during the critical period of development can affect neural processes (such as those involved in pain modulation) during adulthood, indicating long-term changes in brain circuitry. However, such changes may be different in males and females.

Estrogenic influences in pain processing

Gonadal hormones not only play a pivotal role in reproductive behavior and sexual differentiation, they also contribute to thermoregulation, feeding, memory, neuronal survival, and the perception of somatosensory stimuli. Numerous studies on both animals and human subjects have also demonstrated the potential effects of gonadal hormones, such as estrogens, on pain transmission. These effects most likely involve multiple neuroanatomical circuits as well as diverse neurochemical systems and they therefore need to be evaluated specifically to determine the localization and intrinsic characteristics of the neurons engaged. The aim of this review is to summarize the morphological as well as biochemical evidence in support for gonadal hormone modulation of nociceptive processing, with particular focus on estrogens and spinal cord mechanisms.

Analgesic effects of intrathecal tramadol in patients undergoing caesarean section: a randomised, double-blind study

BACKGROUND

Intrathecal tramadol combined with local anaesthetics has been used for postoperative analgesia following lower abdominal and perineal surgery. The present study evaluated the effect of intrathecal tramadol on spinal block characteristics and neonatal outcome after elective caesarean section.

METHODS

Eighty full-term parturients scheduled for elective caesarean section were randomly divided into two groups. In the fentanyl group, patients received intrathecal 0.5% bupivacaine 10 mg with fentanyl 10 μg; in the tramadol group, patients were given the same dose of bupivacaine with tramadol 10 mg. Sensory and motor block characteristics, duration of postoperative analgesia, maternal side effects, and neonatal outcome were compared.

RESULTS

One patient in the tramadol group and two patients in the fentanyl group were excluded from data analysis. Median [interquartile range] duration of postoperative analgesia in the tramadol and the fentanyl groups was 300 [240-360] min and 260 [233-300] min respectively (P = 0.02). The incidence of shivering was lower in patients who received tramadol (5%) than those who had fentanyl (32%) (P = 0.003). Apgar scores, umbilical cord acid-base measurement and neurologic and adaptive capacity scores were comparable between the two groups.

CONCLUSION

Compared to intrathecal fentanyl 10 μg, tramadol 10 mg, as an adjunct to bupivacaine for subarachnoid block for caesarean section, showed a longer duration of analgesia with a reduced incidence of shivering.

Sex, pain, and opioids: interdependent influences of sex and pain modality on dynorphin-mediated antinociception in rats

The role of dynorphin A (1-17; Dyn) and its associated kappa opioid receptor (KOR) in nociception represents a longstanding scientific conundrum: Dyn and KOR (Dyn/KOR) have variously been reported to inhibit, facilitate, or have no effect on pain. We investigated whether interactions between sex and pain type (which are usually ignored) influenced Dyn/KOR-mediated antinociception. Blockade of the spinal α(2)-noradrenergic receptor (α(2)-NAR) using yohimbine elicited comparable spinal Dyn release in females and males. Nevertheless, the yohimbine-induced antinociception exhibited sexual dimorphism that depended on the pain test used: in the intraperitoneal acetic acid-induced writhing test, yohimbine produced antinociception only in females, whereas in the intraplantar formalin-induced paw flinch test, antinociception was observed only in males. In females and males, both intrathecal Dyn antibodies and spinal KOR blockade eliminated the yohimbine-induced antinociception, indicating that Dyn/KOR mediated it. However, despite the conditional nature of spinal Dyn/KOR-mediated yohimbine antinociception, both intraplantar formalin and intraperitoneal acetic acid activated spinal Dyn neurons that expressed α(2)-NARs. Moreover, Dyn terminals apposed KOR-expressing spinal nociceptive neurons in both sexes. This similar organization suggests that the sexually dimorphic interdependent effects of sex and pain type may result from the presence of nonfunctional (silent) KORs on nociceptive spinal neurons that are responsive to intraplantar formalin (in females) versus intraperitoneal acetic acid (in males). Our findings that spinal Dyn/KOR-mediated antinociception depends on interactions between sex and pain type underscore the importance of using both sexes and multiple pain models when investigating Dyn/KOR antinociception.

Importance of sex to pain and its amelioration; relevance of spinal estrogens and its membrane receptors

Estrogens have a multitude of effects on opioid systems and are thought to play a key role in sexually dimorphic nociception and opioid antinociception. Heretofore, classical genomic actions of estrogens are largely thought to be responsible for the effects of these steroids on nociception and opioid antinociception. The recent discovery that estrogens can also activate estrogen receptors that are located in the plasma membrane, the effects of which are manifest in seconds to minutes instead of hours to days has revolutionized our thinking concerning the ways in which estrogens are likely to modulate pain responsiveness and the dynamic nature of that modulation. This review summarizes parameters of opioid functionality and nociception that are subject to modulation by estrogens, underscoring the added dimensions of such modulation that accrues from rapid membrane estrogen receptor signaling. Implications of this mode of signaling regarding putative sources of estrogens and its degradation are also discussed.

Effect of amniotic-fluid ingestion on vaginal-cervical-stimulation-induced Fos expression in female rats during estrus

Placental Opioid-Enhancing Factor (POEF) is a substance found in amniotic fluid (AF) that, when ingested, potentiates opioid-mediated, but not non-opioid-mediated, hypoalgesia. Vaginal-cervical stimulation (VCS) produces a stimulus-bound, partially opioid-mediated hypoalgesia that previous research has shown to be potentiated by AF ingestion. To understand the mechanism of opioid enhancement by POEF we investigated the pattern of neural activation after a bout of VCS that produced hypoalgesia, with and without co-administration of AF. Specifically, virgin Long-Evans rats showing vaginal estrus were handled briefly (control) or received VCS (75g pressure, 1 min), in a pattern that approximated early parturition rather than copulation, using a spring-loaded glass-rod probe. Rats were given an orogastric infusion (0.25 ml) of either AF or 0.9% saline resulting in four groups (VCS or handling; AF or saline). Rats were perfused 90 min after treatment and tissue was processed by immunohistochemistry for Fos. The number of Fos-immunoreactive cells was counted in structures previously shown to express Fos in response to VCS (the medial preoptic area, MPOA; the ventrolateral portion of the ventromedial hypothalamic nucleus, vlVMH; the arcuate nucleus, ARC). We found that this pattern of VCS did not produce a significant increase in Fos expression in the MPOA and vlVMH unless it was paired with AF. VCS produced a significant increase in Fos in the ARC. The interaction of AF and VCS on Fos expression in the MPOA suggests that POEF may enhance vaginal-cervical sensory input at parturition to facilitate sensitization of the MPOA, and presumably facilitate maternal-behavior onset.

Sex-specific modulation of spinal nociception by alpha2-adrenoceptors: differential regulation by estrogen and testosterone

Sex-related differences in antinociception produced by the activation of alpha(2)-adrenoceptors (alpha(2)-ARs) have been reported, however, the precise role of gonadal steroids is still unknown. Hence, we hypothesized that estrogen and testosterone modulate antinociceptive effects of clonidine (an alpha(2)-AR agonist) on N-methyl-D-aspartate- (NMDA) and heat-induced spinal nociception. We also investigated whether estrogen or testosterone alters the expression of alpha(2A)-adrenoceptors in the spinal cord. Sprague-Dawley (SD) rats were implanted with PE10 cannulae in the intrathecal space of the lumbosacral spinal cord and divided into male, proestrous and diestrous female, ovariectomized (OVX), estradiol-treated OVX (OVX+E), castrated male (GDX), testosterone (GDX+T) and estradiol-treated castrated male (GDX+E) groups. Clonidine dose-dependently inhibited NMDA-induced scratching behavior in the male and OVX groups but to a significantly lesser extent in the OVX+E group. It also increased the tail withdrawal latency in the male, OVX, diestrous and GDX+T groups but not in the OVX+E, proestrous, GDX and GDX+E groups. Levels of alpha(2A)-AR mRNA were significantly higher in the OVX, estradiol-treated OVX, GDX and GDX+E animals. In contrast, alpha(2A)-AR protein levels were higher in estradiol-treated OVX, GDX, GDX+T and GDX+E animals as compared with the male. Indeed, no correlations were observed between changes in the mRNA or protein levels of alpha(2A)-AR and behavioral observations. These results support our hypothesis that sex-related differences in alpha(2)-AR-mediated modulation of spinal nociception are gonadal hormone-dependent: estrogen attenuates antinociceptive effects in females whereas testosterone is required for the expression of antinociception in males. In addition, results also revealed that the mechanism of action of gonadal hormones may not involve a global alternation in expression of alpha(2A)-AR in the spinal cord. Estrogen-induced attenuation of alpha(2)-AR-mediated inhibition of nociception could contribute to the higher prevalence of pain syndromes in women.

Sexually Dimorphic Recruitment of Spinal Opioid Analgesic Pathways by the Spinal Application of Morphine

Current evidence for sex-based nociception and antinociception, largely confined to behavioral measures of pain sensitivity, chronic pain syndromes, and analgesic efficacy, provides little mechanistic insights into biological substrates causally associated with sexual dimorphic pain experience. Spinal cord has been shown to be a central nervous system region in which regulation of opioid antinociceptive substrates manifest sexual dimorphism. This site was therefore chosen to explore whether or not differential mechanisms underlie comparable spinal opioid antinociception in male and female rodents. Intrathecal (i.t.) application of morphine to male and female rats produces a thermal antinociception equivalent in magnitude and temporal profile. Nevertheless, it results from the sex-based differential recruitment of spinal analgesic components. As expected, the spinal micro-opioid receptor is critical for i.t. morphine antinociception in both sexes. However, in females, but not males, activation by i.t. morphine of spinal kappa-opioid receptors is a prerequisite for spinal morphine antinociception. Furthermore, in females, but not males, i.t. application of antidynorphin antibodies substantially attenuates the antinociception produced by i.t. morphine. This indicates that the antinociception that results from the i.t. application of morphine in females requires the functional recruitment of spinal dynorphin. Female-specific recruitment by i.t. morphine of a spinal dynorphin/kappa-opioid receptor pathway results from organizational consequences of ovarian sex steroids and not the absence of testicular hormones. These observations suggest that sexual dimorphic pain and analgesic mechanisms might be far more pervasive than commonly thought and underscore the imperative for including female as well as male subjects in all studies of pain and antinociception.

Estrogen synthesis in the spinal dorsal horn: a new central mechanism for the hormonal regulation of pain

The data summarized here suggest the existence of a new central pathway for the hormonal regulation of pain. These data mainly collected in quail, a useful model in neuroendocrinology, demonstrate that numerous neurons in the superficial laminae of the spinal cord express aromatase (estrogen-synthase). Chronic and systemic blockade of this enzyme in quail alters nociception within days, indicating that the slow genomic effects of sex steroids on nociception classically observed in mammals also occur in birds and require aromatization of androgens into estrogens. However, by contrast with these slow effects, acute intrathecal inhibition of aromatase in restricted spinal cord segments reveals that estrogens can also control nociception much faster, within 1 min, presumably through the activation of a nongenomic pathway and in a manner that depends on an immediate response to fast activation/deactivation of local aromatase activity. This emergent central and rapid paracrine mechanism might permit instantaneous and segment-specific changes in pain sensitivity; it draws new interesting perspectives for the study of the estrogenic control of pain, thus far limited to the classical view of slow genomic changes in pain, depending on peripheral estrogens. The expression of aromatase in the spinal cord in other species and in other central nociception-related areas is also briefly discussed.

Activation of alpha2-adrenoceptors in the trigeminal region produces sex-specific modulation of nociception in the rat

Sex-related differences in the sensitivity to pain and in the response to analgesics have been reported including higher perceptual responses to experimentally induced pain and the higher prevalence of many pain syndromes in women compared with men. This study examines whether alpha2-adrenoceptor-mediated antinociceptive effects are reduced by estrogen which could account for the sex-related differences in pain perception and modulation. Clonidine, an alpha2-adrenoceptor agonist, has been shown to inhibit noxious stimulus-evoked nociceptive behavior as well as the responses of nociceptive neurons in the medullary dorsal horn. Intracisternal microinjection of clonidine (7 microg/5 microl) through the implanted PE-10 cannulae dorsal to the trigeminal region in male, ovariectomized (OVX), and diestrous (DiE) Sprague-Dawley rats produced a strong antinociceptive effect on N-methyl-D-aspartic acid (NMDA)-induced nociceptive scratching behavior and heat-induced face withdrawal nociceptive tests. However, it failed to produce any inhibition in the estradiol-treated ovariectomized (OVX+E) group regardless of the dose of estradiol (1, 10 or 100 microg/100 microl sesame oil) or in the proestrous (ProE) group. Further, clonidine produced dose-dependent effects in male and OVX groups but not in the OVX+E group on the NMDA-induced nociceptive behavior. Finally, the effect of clonidine was reversed by yohimbine, an alpha2-adrenoceptor antagonist, in male and OVX groups on thermal nociceptive test. These results lead us to conclude that activation of alpha2-adrenoceptors produces sex-specific, estrogen dependent modulation of nociception in the trigeminal region of the rat. A decreased alpha2-adrenoceptor-mediated inhibition could be one of the factors responsible for the higher prevalence of pain syndromes in females.

Does regular massage from late pregnancy to birth decrease maternal pain perception during labour and birth?—A feasibility study to investigate a programme of massage, controlled breathing and visualization, from 36 weeks of pregnancy until birth

The present study was undertaken to produce a detailed specification of a programme of massage, controlled breathing and visualization performed regularly by birth partners, from 36 weeks gestation and assisted by a trained professional, following hospital admission during labour and birth. As current research on massage interventions for pain relief in labour is poorly characterized, we began by undertaking a feasibility study on an established massage programme [Goldstone LA. Massage as an orthodox medical treatment past and future. Complementary Therapies in Nursing & Midwifery. 2000;6:169-75]. The intervention was designed in light of experimental findings that repeated massage sessions over 14 days increases pain threshold, by an interaction between oxytocin and opioid neurons [Lund I, Yu L-C, Uvnas-Moberg K, Wang J, Yu C, Kurosawa M, et al. Repeated massage-like stimulation induces long-term effects on nociception: contribution of oxytocinergic mechanisms. European Journal of Neuroscience 2002;16:330-8]. A 4 week time-frame was selected to coincide with a physiological increase in maternal pain threshold [Cogan R, Spinnato JA. Pain and Discomfort Thresholds in Late Pregnancy. Pain 1986;27:63-8, Whipple B, Josimovich JB, Komisaruk BR. Sensory thresholds during the antepartum, intrapartum, and postpartum periods. International Journal of Nursing Studies 1990;27(3):213-21, Gintzler AR, Komisaruk BR. Analgesia is produced by uterocervical mechano-stimulation in rats: roles of afferent nerves and implications for analgesia of pregnancy and parturition. Brain Research 1991;566:299-302, Gintzler AR, Liu N-J. The maternal spinal cord: biochemical and physiological correlates of steroid-activated antinociceptive processes. In: Russell JA, Douglas AJ, Windle RJ, Ingram CD, editors., Progress in Brain Research. Volume 133. The Maternal Brain. Neurobiological and Neuroendocrine adaptation and disorders in pregnancy and postpartum. Amsterdam: Elsevier Science, 2001. p. 83-97]. The main objective was to measure the effects of the programme on maternal pain perception during labour and birth. To detect any effect of massage during labour, on maternal cortisol and catecholamines, cord venous blood was taken to measure plasma concentrations following birth. Twenty-five nulliparous (N) and 10 multiparous (M) women participated in the study. Cortisol values were similar to published studies following labour without massage but pain scores on a Visual Analogue Scale (VAS), at 90min following birth were significantly lower than scores recorded 2 days postpartum [Capogna G, Alahuhta S, Celleno D, De Vlieger H, Moreira J, Morgan B, et al. Maternal expectations and experiences of labour pain and analgesia: a multi-centre study of nulliparous women. International Journal of Obstetric Anaesthesia 1996;5:229-35]. The mean score was 6.6. Previous studies suggest that a reduction from 8.5 to 7.5 would significantly reduce pharmacological analgesia in labour [Capogna G, Alahuhta S, Celleno D, De Vlieger H, Moreira J, Morgan B, et al. Maternal expectations and experiences of labour pain and analgesia: a multi-centre study of nulliparous women. International Journal of Obstetric Anaesthesia 1996;5:229-35].

Organizational manipulation of gonadal hormones and systemic morphine analgesia in female rats: effects of adult ovariectomy and estradiol replacement

Previous research has indicated the importance of sex in mediating the larger magnitude of mu-opioid receptor agonist-induced analgesia in male relative to female rodents. Whereas manipulations involving the adult activational effects of gonadal hormones minimally alter these analgesic sex differences, manipulations involving neonatal organizational effects of gonadal hormones have previously been shown to profoundly affect morphine analgesia. Thus, adult male rats neonatally castrated on the first day after birth displayed reductions in morphine analgesia relative to sham-operated males, and adult female rats neonatally treated with testosterone propionate on the first day after birth displayed enhancements in morphine analgesia relative to vehicle-treated females. Because neonatal androgenization in female rats produces an anovulatory syndrome that could change their adult hormonal milieu, the present study examined whether adult ovariectomy altered the magnitude of systemic morphine analgesia (1-5 mg/kg) in neonatal androgenized female rats relative to neonatal vehicle-treated female rats as well as gonadal steroid hormone replacement with estradiol benzoate. Intact male rats displayed significantly greater magnitudes and potencies (2- to 2.3-fold leftward shift) of systemic morphine analgesia than female rats treated neonatally with either vehicle (1-5 mg/kg) or testosterone (1.7-5 mg/kg). In turn, neonatal androgenized female rats displayed significantly greater magnitudes of systemic morphine (1, 5 mg/kg) analgesia than vehicle-treated female rats accompanied by a smaller 20% leftward shift in potency. Adult ovariectomy minimally affected morphine analgesia in neonatal vehicle-treated females, while significantly reducing the magnitude (1 mg/kg), but not the potency of morphine analgesia in neonatal androgenized female rats. Estradiol replacement therapy significantly increased the magnitude of morphine analgesia in both groups at some doses, but only changed the potency (20-30%) in females treated neonatally with vehicle. Taken together, these data suggest a limited organizational-activational gonadal hormone interaction in the mediation of systemic morphine analgesia in female rats.

Estrogen receptor-alpha and -beta immunoreactive neurons in the brainstem and spinal cord of male and female mice: relationships to monoaminergic, cholinergic, and spinal projection systems

For many populations of estrogen-sensitive neurons it remains unknown how they are associated with central nervous system circuitries that mediate estrogen-induced modulation of behavioral components. With the use of double-labeling immunohistochemistry and tracing techniques, the relationships of estrogen receptor (ER)-alpha- and ER-beta-immunoreactive (IR) neurons in the mouse brainstem and spinal cord to monoaminergic, cholinergic, and spinal projection systems are explored. Similar distributions of ER-IR neurons were present in females and males, with differences in labeling intensity of ER-alpha immunoreactivity among males and estrogen-, and oil-treated females. Barrington's nucleus, the ventrolateral medulla, and the nucleus of the solitary tract contained spinal-projecting ER-alpha-IR neurons, whereas ER-alpha-IR neurons in the periaqueductal gray, parabrachial nucleus, and catecholaminergic A1 cell group received spinal input. Numerous tyrosine hydroxylase (TH)-IR ER-alpha-IR neurons were present in the ventral periaqueductal gray, nucleus of the solitary tract, A1 cell group, and lumbosacral cord. The dorsal raphe nucleus contained ER-alpha-IR and ER-beta-IR neurons that colocalized with serotonin (5HT), and the reticulotegmental nucleus contained 5HT-IR ER-alpha-IR neurons. Fibers IR for vesicular acetylcholine transporter (VAChT), TH, and 5HT were located among ER-alpha-IR neurons in the dorsal horn and spinal autonomic regions. Robust staining for TH and VAChT, but not 5HT, was present among ER-alpha-IR neurons in the lumbosacral lateral collateral pathway. Possible modulatory actions of estrogen on each of these ER-IR populations are discussed in the context of their specific function, including micturition, sexual behavior, ejaculation, cardiovascular and respiratory control, tactile and nociceptive sensory processing, anti-nociception, endocrine regulation, and feeding.

Sex differences in pain and analgesia: the role of gonadal hormones

There is now strong evidence for sex differences in pain and analgesia. These differences imply that gonadal steroid hormones such as estradiol and testosterone modulate sensitivity to pain and analgesia. The goal of this review is to present an overview of gonadal steroid modulation of pain and analgesia in animals and humans, and to describe mechanisms by which males' and females' biology may differentially predispose them to pain and to analgesic effects of drugs and stress. Evidence is presented to demonstrate that sex differences in pain and analgesia may be both quantitative and qualitative in nature. Current research suggests that sex-specific management of clinical pain will be a reality in the not-so-distant future.

Chronic morphine-induced loss of the facilitative interaction between vasoactive intestinal polypeptide and delta-opioid: involvement of protein kinase C and phospholipase Cbetas

This laboratory recently demonstrated a multiplicative interaction between the pelvic visceral afferent transmitter vasoactive intestinal polypeptide (VIP) and the delta-opioid receptor (DOR)-selective agonist [D-Pen2,5] enkephalin (DPDPE) to regulate cAMP levels in spinal cord [Brain Res. 959 (2003) 103]. Although DOR activation is required for the manifestation of the VIP-DPDPE facilitative interaction, its relevance to opioid antinociception remains unclear. The current study investigates whether or not the VIP-DPDPE facilitation of cAMP formation is subject to tolerance formation, a hallmark characteristic of opioid antinociception. Chronic morphine exposure abolishes the VIP-DPDPE facilitative interaction, consistent with its relevance to DOR antinociception. However, acute in vitro inhibition of protein kinase C (PKC) reinstates the VIP-DPDPE multiplicative interaction characteristic of opioid naïve spinal tissue. This suggests that its chronic morphine-induced loss requires a PKC phosphorylation. PKC phosphorylation negatively modulates phospholipase C (PLC)beta, enzymes intimately associated with phosphoinositide turnover and calcium trafficking. These are essential determinants of acute and chronic opioid effects. Accordingly, the effect of chronic morphine on their state of phosphorylation was also investigated. Central nervous system opioid tolerance is associated with the reciprocal phosphorylation (regulation) of two PLCbeta isoforms, PLCbeta1 and PLCbeta3. However, although chelerythrine reinstates the chronic morphine-induced loss of the multiplicative VIP-DPDPE interaction, it does not alter the associated changes in PLCbeta phosphorylation, possibly indicating different time courses of restitution of function and/or involvement of different kinases for different components of tolerance. These results could provide a mechanistic rubric for understanding positive modulation of opioid antinociception by afferent transmission.

Spinal anaesthesia for obstetrics

For a long time, epidural anaesthesia has been considered the method of choice for Caesarean delivery. The increased incidence of hypotension by the rapid onset of sympathetic blockade under spinal anaesthesia has been associated with a decline in uteroplacental blood flow and significant fetal acidosis, which may compromise neonatal well-being. Nevertheless, a decrease in fetal pH has not been shown to reduce neonatal Apgar or neurobehavioural assessment scores. Maternal blood pressure can be preserved with little side effects with low doses of vasopressors. On the other hand, spinal anaesthesia conveys significant advantages over epidural anaesthesia such as the simplicity of its use and the speed of onset, which allows neuraxial anaesthesia in urgent Caesarean sections and thus reduces the necessity for general anaesthesia. The small doses of local anaesthetics required to perform spinal anaesthesia reduce the risks of systemic toxicity to zero. Spinal anaesthesia is now considered the method of choice for urgent Caesarean section. The use of intrathecal opioids has profoundly changed the quality of spinal anaesthesia, with improved analgesia, a reduction in local anaesthetic requirements and shorter duration of motor blockade. Preliminary studies indicate that spinal anaesthesia may be safely performed in patients with severe pre-eclampsia, in whom spinal anaesthesia was previously considered contraindicated.

Influence of ovarian sex steroids on spinal methionine-enkephalin release: comparison with dynorphin reveals asymmetrical regulation

The concomitant activation of spinal kappa- and delta-opioid systems is a prerequisite for the antinociception of gestation and its hormonal simulation [via 17 beta-estradiol and progesterone administration; hormone-simulated pregnancy (HSP)]. However, it is not known whether the release of kappa- and delta-opioids is also concomitantly regulated. This study investigates whether the release of methionine-enkephalin and modulation thereof is altered during HSP, as has been reported for dynorphin. K+-stimulated release of spinal methionine-enkephalin from lumbar spinal tissue obtained from control animals is negatively modulated by nociceptin (orphanin FQ; N/OFQ) in a dose-dependent manner, but not by opioids. Conversely, selective blockade of spinal N/OFQ, but not opioid receptors, augments the K+-induced increase in methionine-enkephalin release, indicating that endogenous N/OFQ also functions as a negative modulator of methionine-enkephalin release. The magnitude of K+-evoked methionine-enkephalin release from spinal tissue obtained from ovarian steroid-treated animals remains unchanged, consistent with the insensitivity of its modulation by N/OFQ to the ovarian sex steroid milieu. These characteristics of methionine-enkephalin release stand in sharp contrast to those previously reported for the evoked release of spinal dynorphin. Dynorphin release is subject to negative modulation by opioid (predominantly delta) as well as N/OFQ, both of which are offset during HSP, resulting in an approximately 2-fold increase in the magnitude of its release. These observations reveal that regulation of spinal dynorphin/kappa- and methionine-enkephalin/delta-spinal opioid antinociceptive systems is independent, divergent, and not symmetrical and support the formulation that spinal methionine-enkephalin/delta-opioid tone acts in a permissive/facilitative capacity to accentuate spinal dynorphin/kappa-activity.

Facilitative interactions between vasoactive intestinal polypeptide and receptor type-selective opioids: implications for sensory afferent regulation of spinal opioid action

Afferent tone is known to influence spinal opioid antinociception but the underlying neurochemical events are not well defined. This study investigates the consequence on cAMP formation of the coincident activation of signal transduction sequelae initiated by an afferent transmitter and opioid using dissociated spinal cord tissue. Afferent transmission was simulated via the addition of vasoactive intestinal polypeptide (VIP), a pelvic visceral afferent transmitter. Individually, mu, delta-, or kappa-selective opioids (1 microM each) did not alter basal spinal content of cAMP. However, VIP (1 microM) and the delta-opioid selective agonist, [D-Pen(2,5)] enkephalin (DPDPE; 1 microM), in combination, manifest a striking facilitative interaction to augment spinal levels of cAMP. Facilitative interactions between VIP and kappa- or mu-opioids were of a reduced magnitude or not observed, respectively. Blockade of delta-opioid or VIP receptors using naltrindole or VIP6-28, respectively antagonized the VIP-DPDPE facilitative interaction, as did pertussis toxin treatment. The VIP-DPDPE facilitative interaction was also eliminated by phospholipase Cbeta inhibition and inositol trisphosphate receptor blockade. This suggests that modulation of Ca(2+) trafficking by VIP and delta-opioid agonists is a point of convergence of their respective signal transduction cascades, the concomitant action at which achieves cytosolic Ca(2+) concentrations that are now sufficient for the activation of signaling molecules, e.g. Ca(2+)/calmodulin-stimulated adenylyl cyclase isoforms. These data underscore the plasticity of spinal delta-opioid neurochemical sequelae and their dependence on concomitant afferent transmitter-initiated neurochemical events.

The current perception thresholds in normal pregnancy

The purpose of this study was to evaluate a quantitative analysis of the nociceptive threshold, using the current perception threshold (CPT), in women with normal pregnancies and to assess the relationship between nociceptive thresholds and ovarian sex steroids. The subjects consisted of 10 women with singleton pregnancies and 14 age-matched healthy female volunteers. The CPTs (5,250, and 2,000 Hz) of the dominant ankle section were determined with a Neurometer CPT/C (Neurotron, Baltimore, MD). Blood samples were collected after these examinations, and the total 17beta-estradiol (E(2)) and progesterone concentrations in sera were measured. The present findings clearly indicated that the CPTs at 2,000 Hz in women at term in normal pregnancies were significantly higher than those in nonpregnant women (p<0.05). At 5 and 250 Hz, there was no significant difference between pregnant and nonpregnant women. While there was also no significant correlation between CPT and E(2), and progesterone, there was significant correlation between CPT and the ratio of 17 beta-estradiol/progesterone (E(2)/P) at 2,000 Hz (p<0.05, r=0.67). We suggest from these data that changes in pressure sensitivity occur at term in pregnancy, and that other factors, possibly stimulated by both E(2) and progesterone, may play an important role in this change.

Reversal of sex differences in morphine analgesia elicited from the ventrolateral periaqueductal gray in rats by neonatal hormone manipulations

Male rats display significantly greater analgesic responses than female rats following systemic, ventricular and intracerebral morphine administration into either the ventrolateral periaqueductal gray (vlPAG) or the rostral ventromedial medulla, and following beta-endorphin administration into the vlPAG. Although adult gonadectomy severely reduces nonopioid forms of swim stress-induced analgesia, the marked sex differences in morphine analgesia were minimally affected by either male or female adult gonadectomy. Since very little is known about neonatal effects of gonadal hormones upon sex differences in morphine analgesia elicited from the vlPAG, the present study evaluated the effects of neonatal (within 1 day of birth) castration in male rat pups relative to sham-operated controls, and systemic androgenization with testosterone propionate in female rat pups relative to vehicle-injected controls upon baseline nociceptive thresholds and morphine analgesia elicited from the vlPAG in rats tested as adults. Significant sex differences in morphine analgesia elicited from the vlPAG were observed with adult males receiving neonatal sham surgeries displaying significantly greater morphine analgesia on two nociceptive measures than adult females tested during the estrous phase and receiving neonatal vehicle injections. Neonatal gonadectomy essentially reversed the pattern of sex difference effects upon morphine analgesia elicited from the vlPAG. Neonatally-castrated male rats tested in adulthood displayed dramatic reductions in morphine analgesia elicited from the vlPAG on both the tail-flick (approximately 15-fold rightward shift) and jump (6-fold rightward shift) tests relative to sham-operated males, and essentially mirrored those of vehicle-treated females. Conversely, neonatally-androgenized female rats tested in adulthood displayed dramatic increases in morphine analgesia elicited from the vlPAG on the tail-flick (5-fold leftward shift) and jump (12-fold leftward shift) tests relative to vehicle-treated females, and approximated those observed in sham-operated males. The potent differences between neonatally-castrated and sham-operated male rats and between neonatally-androgenized and vehicle-treated female rats suggest a possible 'organizational' role of gonadal hormones in mediating sex differences in morphine analgesia elicited from the vlPAG.

Estrogen receptor-alpha immunoreactive neurons in the ventrolateral periaqueductal gray receive monosynaptic input from the lumbosacral cord in the rhesus monkey

Estrogen affects female sexual behavior, analgesia, and micturition in mammals. One of the possible sites at which estrogen might exert its effect on these functions is the periaqueductal gray (PAG). The PAG is involved in each of these functions, it receives sensory input relevant to these functions from the lumbosacral cord, and contains estrogen receptor-alpha immunoreactive (ER-alpha IR) neurons. The present light (LM) and electron microscopic (EM) study seeks to determine whether there are monosynaptic projections from the lumbosacral cord to ER-alpha IR neurons in the PAG of the female rhesus monkey. Tracer was injected into the lumbosacral cord to visualize the lumbosacral-PAG projection, and the distribution of ER-alpha IR neurons in the PAG was studied immunohistochemically. The medial part of the ventrolateral caudal PAG received the densest projection from the lumbosacral cord. Another prominent projection was found in the lateral PAG at the intercollicular level. Although ER-alpha IR neurons were widely distributed throughout the PAG, approximately 40% of ER-alpha IR PAG neurons were located as a distinct cluster in the medial portion of the ventrolateral, caudal PAG. Double labeling experiments showed that the location of this cluster precisely overlapped with the densest lumbosacral-PAG projection. EM revealed that axons from the lumbosacral cord made asymmetrical synaptic contacts with unlabeled dendrites and ER-alpha IR neuronal somata in the ventrolateral PAG. It is concluded that there exists a specific, monosynaptic pathway from lumbosacral neurons to ER-alpha expressing PAG neurons in the rhesus monkey. This pathway might be involved in the mechanisms of analgesia, blood pressure, mating behavior, and micturition.

The maternal spinal cord: biochemical and physiological correlates of steroid-activated antinociceptive processes

Physiological gestation, as well as the simulation of the associated changes in estrogen and progesterone, is associated with significant elevations in nociceptive response thresholds. This is mediated by spinal cord kappa- and delta-opIoid systems. The predominant spinal mu-opioid system does not appear to participate. One hallmark of pregnancy- and hormonally-induced antinociception is the multiplicative interaction among its components. Approximately 40% results from spinal kappa/delta analgesic synergy on which is superimposed an additional increment (approximately 60%) of synergy that results from the interaction between descending spinal alpha 2-noradrenergic and spinal kappa/delta activities. An intact hypogastric nerve is required for the spinal alpha 2-noradrenergic component. This would explain the requirement for an intact hypogastric nerve in order for the antinociception of pregnancy and its hormonal simulation to be fully manifest. The predominant means by which spinal dynorphin-containing neurons adjust to increased demand is increased post-translational processing of dynorphin precursor intermediates which are present at approximately 10x the concentration of mature dynorphin peptides (1-17 and 1-8). This is indicated by the concomitant decline (approximately 50%) in the spinal cord content of dynorphin precursors and increase (approximately 87%) in the content of prohormone convertase 2, a processing enzyme sufficient to generate mature dynorphin peptides from prodynorphin. The presence of 'high gain' multiplicative spinal opioid antinociceptive pathways that can be activated by estrogen and progesterone has hyperalgesic implications as well, i.e. it could result in disproportionately increased pain responsiveness. This might explain, in part, findings that women are more prone to recurrent pain and pain of greater duration and intensity than men. The underlying mechanisms of gestational antinociception could point the way to pain pharmacotherapies that are gender-based.

Brain preparations for maternity--adaptive changes in behavioral and neuroendocrine systems during pregnancy and lactation. An overview

Pregnancy, parturition and lactation comprise a continuum of adaptive changes necessary for the development and maintenance of the offspring. The endocrine changes that are driven by the conceptus and are essential for the maintenance of pregnancy and are involved in the preparations for motherhood are outlined. These changes include large increases in the secretion of sex steroid hormones, and the secretion of peptide hormones that are unique to pregnancy. The ability of these pregnancy hormones to alter several aspects of brain function in pregnancy is considered, and the adaptive importance of some of these changes is discussed, for example in metabolic and body fluid adjustments, and the induction of maternal behavior. The importance of sex steroids in determining the timing of the various adaptive changes in preparing for parturition and maternal behavior is emphasized, and the concept that the actions of prolactin and oxytocin, quintessential mammalian motherhood neuropeptides, can serve to coordinate a spectrum of adaptive changes is discussed. The part played by oxytocin neurons and their regulatory mechanisms is reviewed to illustrate how neural systems involved in maternity are prepared in pregnancy via changes in phenotype, synaptic organization and in the relative importance of their different inputs, to function optimally when needed. For oxytocin neurons secreting from the posterior pituitary, important in parturition and essential in lactation, these changes include mechanisms to restrain their premature activation, and adaptations to support synchronized burst firing for pulsatile oxytocin secretion in response to stimulation via afferents from the birth canal, olfactory system or suckled nipples. Within the brain, expression of oxytocin receptors permits centrally released oxytocin to facilitate the expression of maternal behavior. Changes in other neuroendocrine systems are similarly extensive, leading to lactation, suppression of ovulation, reduced stress responses and increased appetite; these changes in lactation are driven by the suckling stimulus. The possible link between these adaptations and changes in cognition and mood in pregnancy and post partum are considered, as well as the dysfunctions that lead to common problems of depression and puerperal psychoses.

Localization and controls of aromatase in the quail spinal cord

In adult male and female Japanese quail, aromatase-immunoreactive cells were identified in the spinal dorsal horns from the upper cervical segments to the lower caudal area. These immunoreactive cells are located mostly in laminae I-III, with additional sparse cells being present in the medial part of lamina V and, at the cervical level exclusively, in lamina X around the central canal. Radioenzyme assays based on the measurement of tritiated water release confirmed the presence of substantial levels of aromatase activity throughout the rostrocaudal extent of the spinal cord. Contrary to what is observed in the brain, this enzyme activity and the number of aromatase-immunoreactive cells in five representative segments of the spinal cord are not different in sexually mature males or females and are not influenced in males by castration with or without testosterone treatment. The aromatase activity and the numbers of aromatase-immunoreactive cells per section are higher at the brachial and thoracic levels than in the cervical and lumbar segments. These experiments demonstrate for the first time the presence of local estrogen production in the spinal cord of a higher vertebrate. This production was localized in the sensory fields of the dorsal horn, where estrogen receptors have been identified previously in several avian and mammalian species, suggesting an implication of aromatase in the modulation of sensory (particularly nociceptive) processes.

Prolonged ovarian sex steroid treatment of male rats produces antinociception: identification of sex-based divergent analgesic mechanisms

Simulation of the pregnancy blood concentration profile of 17beta-estradiol (E(2)) and progesterone (P) in nonpregnant ovariectomized rats has been shown to result in a significant elevation of nociceptive response thresholds. The present report demonstrates that spinal opioid antinociceptive responsiveness to these ovarian steroids is not sex-specific. Treatment of orchidectomized sexually mature males with an analogous regimen of E(2) and P also elicits an antinociception, the robustness and temporal profile of which is comparable with that previously observed in females. Neither E(2) nor P, alone, is sufficient to produce antinociception in male rats, as was previously demonstrated in females. Neurobiological substrates and antinociceptive mechanisms underlying ovarian sex steroid antinociception do, however, exhibit sex specificity. In males, the analgesia resulting from ovarian steroid treatment derives from the independent contributions of spinal kappa and mu, not delta, opioid receptor pathways that are additive, not synergistic. Spinal alpha(2)-noradrenergic receptor activity and its attendant analgesic synergy with spinal opioid systems do not contribute to ovarian sex steroid analgesia in males. This is in contrast to the previous demonstrations that ovarian sex steroid-induced antinociception in females results from antinociceptive synergy between activated spinal kappa/delta opioid as well as alpha(2)-noradrenergic receptor systems. The current data reveal that ovarian steroid-activated multiplicative spinal antinociceptive pathways that had been demonstrated in female rats are not manifest in their male counterparts.