Endogenous prolactin generated during peripheral inflammation contributes to thermal hyperalgesia

Prolactin and pain of endometriosis

Women experience chronic pain more often than men with some pain conditions being specific to women while others are more prevalent in women. Prolactin, a neuropeptide hormone with higher serum levels in women, has recently been demonstrated in preclinical studies to sensitize nociceptive sensory neurons in a sexually dimorphic manner. Dysregulation of prolactin and prolactin receptors may be responsible for increased pain especially in female predominant conditions such as migraine, fibromyalgia, and pelvic pain. In this review, we focus on the role of prolactin in endometriosis, a condition characterized by pelvic pain and infertility that affects a large proportion of women during their reproductive age. We discuss the symptoms and pathology of endometriosis and discuss how different sources of prolactin secretion may contribute to this disease. We highlight our current understanding of prolactin-mediated mechanisms of nociceptor sensitization in females and how this mechanism may apply to endometriosis. Lastly, we report the results of a systematic review of clinical studies conducted by searching the PubMed and EMBASE databases to identify association between endometriosis and blood levels of prolactin. The results of this search strongly indicate that serum prolactin levels are increased in patients with endometriosis and support the possibility that high levels of prolactin may promote pelvic pain in these patients and increase vulnerability to other comorbid pain conditions likely by dysregulating prolactin receptor expression. Targeting of prolactin and prolactin receptors may improve management of pain associated with endometriosis.

Prolactin and oxytocin: potential targets for migraine treatment

Migraine is a severe neurovascular disorder of which the pathophysiology is not yet fully understood. Besides the role of inflammatory mediators that interact with the trigeminovascular system, cyclic fluctuations in sex steroid hormones are involved in the sex dimorphism of migraine attacks. In addition, the pituitary-derived hormone prolactin and the hypothalamic neuropeptide oxytocin have been reported to play a modulating role in migraine and contribute to its sex-dependent differences. The current narrative review explores the relationship between these two hormones and the pathophysiology of migraine. We describe the physiological role of prolactin and oxytocin, its relationship to migraine and pain, and potential therapies targeting these hormones or their receptors.In summary, oxytocin and prolactin are involved in nociception in opposite ways. Both operate at peripheral and central levels, however, prolactin has a pronociceptive effect, while oxytocin appears to have an antinociceptive effect. Therefore, migraine treatment targeting prolactin should aim to block its effects using prolactin receptor antagonists or monoclonal antibodies specifically acting at migraine-pain related structures. This action should be local in order to avoid a decrease in prolactin levels throughout the body and associated adverse effects. In contrast, treatment targeting oxytocin should enhance its signalling and antinociceptive effects, for example using intranasal administration of oxytocin, or possibly other oxytocin receptor agonists. Interestingly, the prolactin receptor and oxytocin receptor are co-localized with estrogen receptors as well as calcitonin gene-related peptide and its receptor, providing a positive perspective on the possibilities for an adequate pharmacological treatment of these nociceptive pathways. Nevertheless, many questions remain to be answered. More particularly, there is insufficient data on the role of sex hormones in men and the correct dosing according to sex differences, hormonal changes and comorbidities. The above remains a major challenge for future development.

Discovery and characterization of prolactin neutralizing monoclonal antibodies for the treatment of female-prevalent pain disorders

Prolactin (PRL) has recently been demonstrated to elicit female-selective nociceptor sensitization and increase pain-like behaviors in female animals. Here we report the discovery and characterization of first-in-class, humanized PRL neutralizing monoclonal antibodies (PRL mAbs). We obtained two potent and selective PRL mAbs, PL 200,031 and PL 200,039. PL 200,031 was engineered as human IgG1 whereas PL 200,039 was reformatted as human IgG4. Both mAbs have sub-nanomolar affinity for human PRL (hPRL) and produce concentration-dependent and complete inhibition of hPRL signaling at the hPRL receptor (hPRLR). These two PRL mAbs are selective for hPRL as they do not inhibit other hPRLR agonists such as human growth hormone or placental lactogen. They also cross-react with non-human primate PRL but not with rodent PRL. Further, both mAbs show long clearance half-lives after intravenous administration in FcRn-humanized mice. Consistent with their isotypes, these mAbs only differ in binding affinities to Fcγ receptors, as expected by design. Finally, PL 200,019, the murine parental mAb of PL 200,031 and PL 200,039, fully blocked stress-induced and PRL-dependent pain behaviors in female PRL-humanized mice, thereby providing in vivo preclinical proof-of-efficacy for PRL mAbs in mechanisms relevant to pain in females.

Sex-dependent pain trajectories induced by prolactin require an inflammatory response for pain resolution

Pain development and resolution patterns in many diseases are sex-dependent. This study aimed to develop pain models with sex-dependent resolution trajectories, and identify factors linked to resolution of pain in females and males. Using different intra-plantar (i.pl.) treatment protocols with prolactin (PRL), we established models with distinct, sex-dependent patterns for development and resolution of pain. An acute PRL-evoked pain trajectory, in which hypersensitivity is fully resolved within 1 day, showed substantial transcriptional changes after pain-resolution in female and male hindpaws and in the dorsal root ganglia (DRG). This finding supports the notion that pain resolution is an active process. Prolonged treatment with PRL high dose (1 μg) evoked mechanical hypersensitivity that resolved within 5-7 days in mice of both sexes and exhibited a pro-inflammatory transcriptional response in the hindpaw, but not DRG, at the time point preceding resolution. Flow cytometry analysis linked pro-inflammatory responses in female hindpaws to macrophages/monocytes, especially CD11b⁺/CD64⁺/MHCII⁺ cell accumulation. Prolonged low dose PRL (0.1 μg) treatment caused non-resolving mechanical hypersensitivity only in females. This effect was independent of sensory neuronal PRLR and was associated with a lack of immune response in the hindpaw, although many genes underlying tissue damage were affected. We conclude that different i.pl. PRL treatment protocols generates distinct, sex-specific pain hypersensitivity resolution patterns. PRL-induced pain resolution is preceded by a pro-inflammatory macrophage/monocyte-associated response in the hindpaws of mice of both sexes. On the other hand, the absence of a peripheral inflammatory response creates a permissive condition for PRL-induced pain persistency in females.

A link between migraine and prolactin: the way forward

Migraine is an incapacitating neurological disorder that predominantly affects women. Sex and other hormones (e.g., oxytocin, and prolactin) may play a role in sexual dimorphic features of migraine. Initially, prolactin was recognized for its modulatory action in milk production and secretion; later, its roles in the regulation of the endocrine, immune and nervous systems were discovered. Higher prolactin levels in individuals with migraine were found in earlier studies, with a female sex-dominant trend. Studies that are more recent have identified that the expression of prolactin receptor in response to neuronal excitability and stress depends on sex with a dominant role in females. These findings have opened up potentials for explanation of sex-related pathophysiology of migraine, but have left some unanswered questions. This focused review examines the past and present of the link between prolactin and migraine, and presents open questions and directions for future experimental and clinical efforts.

Sex hormones (e.g., estrogen and progesterone) have been investigated to explain the sex-related manifestation of migraine, which is predominant in females. Prolactin is known for promoting lactation, but accumulating evidence supports that it can promote pain in females. An increasing number of studies have shown that the expression of a prolactin receptor in female nociceptors and their responses to external stimuli such as stress are different, which can help explain the female sex-dominant feature of migraine. In this focused review, the current knowledge is presented and the directions where prolactin research in migraine may evolve are proposed. The ultimate goal is to shape an overview toward considering sex-based treatments for migraine with highlighting the role of prolactin.

Prolactin, metabolic and immune parameters in naïve subjects with a first episode of psychosis

BACKGROUND

Prolactin (Prl) is a pleiotropic hormone initially described for its regulation of lactation in mammals but later associated with metabolic and immune homeostasis, stress, inflammatory response and human behavior. Its regulation through dopamine receptors highlights its importance in psychiatry mostly because hyperprolactinemia is a common secondary side effect of dopamine antagonists. Despite its undeciphered patho-physiological mechanisms, hyperprolactinemia in naïve psychosis patients has been widely described. Its consequences might underlie the increased morbidity and early mortality found in naïve subjects as described in the general population where prolactin values have been correlated with inflammatory, immune and metabolic parameters.

METHODS

We aimed to evaluate the correlation between prolactin values and other biochemical parameters (C-reactive Protein-CrP, blood cell count, lipid and hepatic profile, fasting glucose) in a cohort of first episode psychosis naïve subjects (N = 491) stratified by sex. Regression analyses with confounders were performed to evaluate the association.

FINDINGS

Prl displayed significant correlations with C-Reactive Protein (CrP), Low-Density Lipoprotein (LDL), Aspartate Transaminase (AST) for females and High-Density Lipoprotein (HDL) and eosinophil count for males. However, and despite previous specific sex correlations, significant associations were described for CrP, HDL, LDL, AST and ALT without sex interaction and despite confounders such as age, Body Mass Index or smoking status.

CONCLUSIONS

Our results show a specific relation of Prl with immune and metabolic parameters describing a heterogeneous pattern. Our results suggest that prolactin might underlie the excess of morbidity and early mortality in naïve patients through a specific pathway.

Sensory Neurons, Neuroimmunity, and Pain Modulation by Sex Hormones

The inclusion of women in preclinical pain studies has become more commonplace in the last decade as the National Institutes of Health (NIH) released its "Sex as a Biological Variable" mandate. Presumably, basic researchers have not had a comprehensive understanding about neuroimmune interactions in half of the population and how hormones play a role in this. To date, we have learned that sex hormones contribute to sexual differentiation of the nervous system and sex differences in behavior throughout the lifespan; however, the cycling of sex hormones does not always explain these differences. Here, we highlight recent advances in our understanding of sex differences and how hormones and immune interactions influence sensory neuron activity to contribute to physiology and pain. Neuroimmune mechanisms may be mediated by different cell types in each sex, as the actions of immune cells are sexually dimorphic. Unfortunately, the majority of studies assessing neuronal contributions to immune function have been limited to males, so it is unclear if the mechanisms are similar in females. Finally, pathways that control cellular metabolism, like nuclear receptors, have been shown to play a regulatory role both in pain and inflammation. Overall, communication between the neuroimmune and endocrine systems modulate pain signaling in a sex-dependent manner, but more research is needed to reveal nuances of these mechanisms.

Meningeal CGRP-Prolactin Interaction Evokes Female-Specific Migraine Behavior

OBJECTIVE

Migraine is three times more common in women. CGRP plays a critical role in migraine pathology and causes female-specific behavioral responses upon meningeal application. These effects are likely mediated through interactions of CGRP with signaling systems specific to females. Prolactin (PRL) levels have been correlated with migraine attacks. Here, we explore a potential interaction between CGRP and PRL in the meninges.

METHODS

Prolactin, CGRP, and receptor antagonists CGRP8-37 or Δ1-9-G129R-hPRL were administered onto the dura of rodents followed by behavioral testing. Immunohistochemistry was used to examine PRL, CGRP and Prolactin receptor (Prlr) expression within the dura. Electrophysiology on cultured and back-labeled trigeminal ganglia (TG) neurons was used to assess PRL-induced excitability. Finally, the effects of PRL on evoked CGRP release from ex vivo dura were measured.

RESULTS

We found that dural PRL produced sustained and long-lasting migraine-like behavior in cycling and ovariectomized female, but not male rodents. Prlr was expressed on dural afferent nerves in females with little-to-no presence in males. Consistent with this, PRL increased excitability only in female TG neurons innervating the dura and selectively sensitized CGRP release from female ex vivo dura. We demonstrate crosstalk between PRL and CGRP systems as CGRP8-37 decreases migraine-like responses to dural PRL. Reciprocally, Δ1-9-G129R-hPRL attenuates dural CGRP-induced migraine behaviors. Similarly, Prlr deletion from sensory neurons significantly reduced migraine-like responses to dural CGRP.

INTERPRETATION

This CGRP-PRL interaction in the meninges is a mechanism by which these peptides could produce female-selective responses and increase the prevalence of migraine in women. ANN NEUROL 2021;89:1129-1144.

Neuroendocrine Mechanisms Governing Sex Differences in Hyperalgesic Priming Involve Prolactin Receptor Sensory Neuron Signaling

Many clinical and preclinical studies report higher prevalence and severity of chronic pain in females. We used hyperalgesic priming with interleukin 6 (IL-6) priming and PGE₂ as a second stimulus as a model for pain chronicity. Intraplantar IL-6 induced hypersensitivity was similar in magnitude and duration in both males and females, while both paw and intrathecal PGE₂ hypersensitivity was more persistent in females. This difference in PGE₂ response was dependent on both circulating estrogen and translation regulation signaling in the spinal cord. In males, the duration of hypersensitivity was regulated by testosterone. Since the prolactin receptor (Prlr) is regulated by reproductive hormones and is female-selectively activated in sensory neurons, we evaluated whether Prlr signaling contributes to hyperalgesic priming. Using ΔPRL, a competitive Prlr antagonist, and a mouse line with ablated Prlr in the Nav1.8 sensory neuronal population, we show that Prlr in sensory neurons is necessary for the development of hyperalgesic priming in female, but not male, mice. Overall, sex-specific mechanisms in the initiation and maintenance of chronic pain are regulated by the neuroendocrine system and, specifically, sensory neuronal Prlr signaling.SIGNIFICANCE STATEMENT Females are more likely to experience chronic pain than males, but the mechanisms that underlie this sex difference are not completely understood. Here, we demonstrate that the duration of mechanical hypersensitivity is dependent on circulating sex hormones in mice, where estrogen caused an extension of sensitivity and testosterone was responsible for a decrease in the duration of the hyperalgesic priming model of chronic pain. Additionally, we demonstrated that prolactin receptor expression in Nav1.8⁺ neurons was necessary for hyperalgesic priming in female, but not male, mice. Our work demonstrates a female-specific mechanism for the promotion of chronic pain involving the neuroendrocrine system and mediated by sensory neuronal prolactin receptor.

An Emerging Role for Prolactin in Female-Selective Pain

Women experience many pain conditions more frequently when compared with men, but the biological mechanisms underlying sex differences in pain remain poorly understood. In particular, little is known about possible sex differences in peripheral nociceptors, the fundamental building blocks of pain transmission. Emerging evidence reveals that prolactin (PRL) signaling at its cognate prolactin receptor (PRLR) in primary afferents promotes nociceptor sensitization and pain in a female-selective fashion. In this review, we summarize recent progress in understanding the female-selective role of PRL/PRLR in nociceptor sensitization and in pathological pain conditions, including postoperative, inflammatory, neuropathic, and migraine pain, as well as opioid-induced hyperalgesia. The clinical implications of the peripheral PRL/PRLR system for the discovery of new therapies for pain control in women are also discussed.

Prolactin Regulates Pain Responses via a Female-Selective Nociceptor-Specific Mechanism

Many clinical and preclinical studies report an increased prevalence and severity of chronic pain among females. Here, we identify a sex-hormone-controlled target and mechanism that regulates dimorphic pain responses. Prolactin (PRL), which is involved in many physiologic functions, induces female-specific hyperalgesia. A PRL receptor (Prlr) antagonist in the hind paw or spinal cord substantially reduced hyperalgesia in inflammatory models. This effect was mimicked by sensory neuronal ablation of Prlr. Although Prlr mRNA is expressed equally in female and male peptidergic nociceptors and central terminals, Prlr protein was found only in females and PRL-induced excitability was detected only in female DRG neurons. PRL-induced excitability was reproduced in male Prlr⁺ neurons after prolonged treatment with estradiol but was prevented with addition of a translation inhibitor. We propose a novel mechanism for female-selective regulation of pain responses, which is mediated by Prlr signaling in sensory neurons via sex-dependent control of Prlr mRNA translation.

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Local or spinal PRL injection induces hyperalgesia in a female-selective manner

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Sensory neuron Prlr regulates tissue injury-induced pain only in females

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PRL regulates excitability in Prlr⁺ neurons depending on sex and estrogen

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Regulation of Prlr translation defines female-selective neuronal excitability

Pituitary Hormones and Orofacial Pain

Clinical and basic research on regulation of pituitary hormones, extra-pituitary release of these hormones, distribution of their receptors and cell signaling pathways recruited upon receptor binding suggests that pituitary hormones can regulate mechanisms of nociceptive transmission in multiple orofacial pain conditions. Moreover, many pituitary hormones either regulate glands that produce gonadal hormones (GnH) or are regulated by GnH. This implies that pituitary hormones may be involved in sex-dependent mechanisms of orofacial pain and could help explain why certain orofacial pain conditions are more prevalent in women than men. Overall, regulation of nociception by pituitary hormones is a relatively new and emerging area of pain research. The aims of this review article are to: (1) present an overview of clinical conditions leading to orofacial pain that are associated with alterations of serum pituitary hormone levels; (2) discuss proposed mechanisms of how pituitary hormones could regulate nociceptive transmission; and (3) outline how pituitary hormones could regulate nociception in a sex-specific fashion. Pituitary hormones are routinely used for hormonal replacement therapy, while both receptor antagonists and agonists are used to manage certain pathological conditions related to hormonal imbalance. Administration of these hormones may also have a place in the treatment of pain, including orofacial pain. Hence, understanding the involvement of pituitary hormones in orofacial pain, especially sex-dependent aspects of such pain, is essential to both optimize current therapies as well as provide novel and sex-specific pharmacology for a diversity of associated conditions.

Involvement of prolactin in the meloxicam-dependent inflammatory response of the gonadotropic axis to prolonged lipopolysaccharide treatment in anoestrous ewes

An immune challenge can affect the reproductive process in females. Peripheral administration of bacterial endotoxin (lipopolysaccharide; LPS) decreases LH secretion and disrupts ovarian cyclicity. The aim of the present study was to determine the effects of a cyclo-oxygenase (COX)-2 inhibitor (meloxicam) on gonadotropin-releasing hormone (GnRH) and LH secretion in anoestrous ewes during systemic inflammation induced by LPS. LPS (400ngkg^-1 per day) suppressed LH release. In three individuals, meloxicam (500μgkg^-1, i.v.) abolished LPS-induced LH suppression. In another three ewes LH was ineffective. Similar changes were observed in hypothalamic GnRH expression. The effect of meloxicam depended on the circulating level of prolactin: meloxicam abolished inflammatory-dependent suppression of GnRH and LH secretion when plasma prolactin levels were similar to those in untreated animals, but was ineffective in those with elevated levels of prolactin. We conclude that COX-2 inhibitors minimise the negative effect of inflammation on the reproductive system but that this effect may be antagonised by prolactin.

Ovariectomy-Induced Mitochondrial Oxidative Stress, Apoptosis, and Calcium Ion Influx Through TRPA1, TRPM2, and TRPV1 Are Prevented by 17β-Estradiol, Tamoxifen, and Raloxifene in the Hippocampus and Dorsal Root Ganglion of Rats

Relative 17β-estradiol (E2) deprivation and excessive production of mitochondrial oxygen free radicals (OFRs) with a high amount of Ca²⁺ influx TRPA1, TRPM2, and TRPV1 activity is one of the main causes of neurodegenerative disease in postmenopausal women. In addition to the roles of tamoxifen (TMX) and raloxifene (RLX) in cancer and bone loss treatments, regulator roles in Ca²⁺ influx and mitochondrial oxidative stress in neurons have not been reported. The aim of this study was to evaluate whether TMX and RLX interactions with TRPA1, TRPM2, and TRPV1 in primary hippocampal (HPC) and dorsal root ganglion (DRG) neuron cultures of ovariectomized (OVX) rats. Forty female rats were divided into five groups: a control group, an OVX group, an OVX+E2 group, an OVX+TMX group, and an OVX+RLX group. The OVX+E2, OVX+TMX, and OVX+RLX groups received E2, TMX, and RLX, respectively, for 14 days after the ovariectomy. E2, ovariectomy-induced TRPA1, TRPM2, and TRPV1 current densities, as well as accumulation of cytosolic free Ca²⁺ in the neurons, were returned to the control levels by E2, TMX, and RLX treatments. In addition, E2, TMX, and RLX via modulation of TRPM2 and TRPV1 activity reduced ovariectomy-induced mitochondrial membrane depolarization, apoptosis, and cytosolic OFR production. TRPM2, TRPV1, PARP, and caspase-3 and caspase-9 expressions were also decreased in the neurons by the E2, TMX, and RLX treatments. In conclusion, we first reported the molecular effects of E2, TMX, and RLX on TRPA1, TRPM2, and TRPV1 channel activation in the OVX rats. In addition, we observed neuroprotective effects of E2, RLX, and TMX on oxidative and apoptotic injuries of the hippocampus and peripheral pain sensory neurons (DRGs) in the OVX rats. Graphical Abstract Possible molecular pathways of involvement of DEX in cerebral ischemia-induced apoptosis, oxidative stress, and calcium accumulation through TRPA1, TRPM2 and TRPV1 in the hippocampus and DRG neurons of rats. The N domain of the TRPM2 contains ADP-ribose (ADPR) pyrophosphate enzyme, which is separately activated by ADPR and oxidative stress, although the channel is reversibly inhibited by N-(p-amylcinnamoyl) anthranilic acid (ACA). The TRPV1 is also activated by mitochondrial oxidative stress and capsaicin, and it is blocked by capsazepine (CPZ). TRPA1 is also activated by oxidative stress it is inhibited by AP18. Increased cytosolic Ca²⁺ concentration through TRPA1, TRPM2 and TRPV1 in ovariectomized (OVX) rats may lead to neuronal toxicity, reactive oxygen species (ROS) processes, and eventual cell death. 17β-Estradiol (E2), tamoxifen (TMX), and raloxifene (RLX) reduced oxidative stress, apoptosis (including caspase-3 and caspase-9), mitochondrial membrane depolarization, and Ca²⁺ influx through the inhibition of TRPA1, TRPM2 and TRPV1 activation.

Influence of hypophysectomy, ovariectomy and gonadectomy on postoperative hypersensitivity in rats

Surgical procedures lead to profound and sustained (up to 1–2 weeks) activation of the pituitary gland, resulting in changes in endocrine function. Questions remain on whether activation of the pituitary influences the threshold and development time-course of postoperative pain. To address these questions, we evaluated postoperative hypersensitivity in female and male rats with ablated pituitary and gonadal hormone productions via hypophysectomy, ovariectomy and gonadectomy, respectively. Plantar incision, a model of acute postoperative pain, or sham operation was performed on rat hind paws. Hypophysectomy, ovariectomy and gonadectomy were achieved by surgical disconnection of pituitary, ovaries and testicles, respectively. Postoperative thermal and mechanical hypersensitivity were monitored for 7 days post incision. Hypophysectomy on female and male rats produced statistically similar thermal and mechanical postoperative hypersensitivity thresholds and time-courses as compared to intact estrous female and male rats. Moreover, ovariectomy and gonadectomy did not significantly change postoperative hypersensitivity observed in control female and male animals. Our experiments demonstrate that hypophysectomy, ovariectomy and gonadectomy do not significantly impact postoperative hypersensitivity observed in normal female and male animals. These data suggest that surgery-induced changes in the endocrine system via activation of pituitary and subsequently gonadal tissues have little impact on the threshold and development of postoperative pain in female and male rats.

Prolactin potentiates the activity of acid-sensing ion channels in female rat primary sensory neurons

Prolactin (PRL) is a polypeptide hormone produced and released from the pituitary and extrapituitary tissues. It regulates activity of nociceptors and causes hyperalgesia in pain conditions, but little is known the molecular mechanism. We report here that PRL can exert a potentiating effect on the functional activity of acid-sensing ion channels (ASICs), key sensors for extracellular protons. First, PRL dose-dependently increased the amplitude of ASIC currents with an EC50 of (5.89 ± 0.28) × 10(-8) M. PRL potentiation of ASIC currents was also pH dependent. Second, PRL potentiation of ASIC currents was blocked by Δ1-9-G129R-hPRL, a PRL receptor antagonist, and removed by intracellular dialysis of either protein kinase C inhibitor GF109203X, protein interacting with C-kinase 1(PICK1) inhibitor FSC-231, or PI3K inhibitor AS605240. Third, PRL altered acidosis-evoked membrane excitability of DRG neurons and caused a significant increase in the amplitude of the depolarization and the number of spikes induced by acid stimuli. Four, PRL exacerbated nociceptive responses to injection of acetic acid in female rats. Finally, PRL displayed a stronger effect on ASIC mediated-currents and nociceptive behavior in intact female rats than OVX female and male rats and thus modulation of PRL may be gender-dependent. These results suggest that PRL up-regulates the activity of ASICs and enhances ASIC mediated nociceptive responses in female rats, which reveal a novel peripheral mechanism underlying PRL involvement in hyperalgesia.

Prolactin receptor in regulation of neuronal excitability and channels

Prolactin (PRL) activates PRL receptor isoforms to exert regulation of specific neuronal circuitries, and to control numerous physiological and clinically-relevant functions including; maternal behavior, energy balance and food intake, stress and trauma responses, anxiety, neurogenesis, migraine and pain. PRL controls these critical functions by regulating receptor potential thresholds, neuronal excitability and/or neurotransmission efficiency. PRL also influences neuronal functions via activation of certain neurons, resulting in Ca(2+) influx and/or electrical firing with subsequent release of neurotransmitters. Although PRL was identified almost a century ago, very little specific information is known about how PRL regulates neuronal functions. Nevertheless, important initial steps have recently been made including the identification of PRL-induced transient signaling pathways in neurons and the modulation of neuronal transient receptor potential (TRP) and Ca(2+) -dependent K(+) channels by PRL. In this review, we summarize current knowledge and recent progress in understanding the regulation of neuronal excitability and channels by PRL.

The prolactin receptor as a therapeutic target in human diseases: browsing new potential indications

INTRODUCTION

Prolactin (PRL) signaling has emerged as a relevant target in breast and prostate cancers. This has encouraged various laboratories to develop compounds targeting the PRL receptor (PRLR). As the latter is widely distributed, it is timely to address whether other conditions could also benefit from such inhibitors.

AREAS COVERED

The authors briefly overview the two classes of PRLR blockers, which involve: i) PRL-core based analogs that have been validated as competitive antagonists in various preclinical models, and ii) anti-PRLR neutralizing antibodies that are currently in clinical Phase I for advanced breast and prostate cancers. The main purpose of this review is to discuss the multiple organs/diseases that may be considered as potential targets/indications for such inhibitors. This is done in light of reports suggesting that PRLR expression/signaling is increased in disease, and/or that systemic or locally elevated PRL levels correlate with (or promote) organ pathogenesis.

EXPERT OPINION

The two immediate challenges in the field are i) to provide the scientific community with potent anti-prolactin receptor antibodies to map prolactin receptor expression in target organs, and ii) to take advantage of the availability of functionally validated PRLR blockers to establish the relevance of these potential indications in humans.

Mechanisms of transient signaling via short and long prolactin receptor isoforms in female and male sensory neurons

Prolactin (PRL) regulates activity of nociceptors and causes hyperalgesia in pain conditions. PRL enhances nociceptive responses by rapidly modulating channels in nociceptors. The molecular mechanisms underlying PRL-induced transient signaling in neurons are not well understood. Here we use a variety of cell biology and pharmacological approaches to show that PRL transiently enhanced capsaicin-evoked responses involve protein kinase C ε (PKCε) or phosphatidylinositol 3-kinase (PI3K) pathways in female rat trigeminal (TG) neurons. We next reconstituted PRL-induced signaling in a heterologous expression system and TG neurons from PRL receptor (PRLR)-null mutant mice by expressing rat PRLR-long isoform (PRLR-L), PRLR-short isoform (PRLR-S), or a mix of both. Results show that PRLR-S, but not PRLR-L, is capable of mediating PRL-induced transient enhancement of capsaicin responses in both male and female TG neurons. However, co-expression of PRLR-L with PRLR-S (1:1 ratio) leads to the inhibition of the transient PRL actions. Co-expression of PRLR-L deletion mutants with PRLR-S indicated that the cytoplasmic site adjacent to the trans-membrane domain of PRLR-L was responsible for inhibitory effects of PRLR-L. Furthermore, in situ hybridization and immunohistochemistry data indicate that in normal conditions, PRLR-L is expressed mainly in glia with little expression in rat sensory neurons (3-5%) and human nerves. The predominant PRLR form in TG neurons/nerves from rats and humans is PRLR-S. Altogether, PRL-induced transient signaling in sensory neurons is governed by PI3K or PKCε, mediated via the PRLR-S isoform, and transient effects mediated by PRLR-S are inhibited by presence of PRLR-L in these cells.

Prolactin regulates TRPV1, TRPA1, and TRPM8 in sensory neurons in a sex-dependent manner: Contribution of prolactin receptor to inflammatory pain

Prolactin (PRL) is a hormone produced in the anterior pituitary but also synthesized extrapituitary where it can influence diverse cellular processes, including inflammatory responses. Females experience greater pain in certain inflammatory conditions, but the contribution of the PRL system to sex-dependent inflammatory pain is unknown. We found that PRL regulates transient receptor potential (TRP) channels in a sex-dependent manner in sensory neurons. At >20 ng/ml, PRL sensitizes TRPV1 in female, but not male, neurons. This effect is mediated by PRL receptor (PRL-R). Likewise, TRPA1 and TRPM8 were sensitized by 100 ng/ml PRL only in female neurons. We showed that complete Freund adjuvant (CFA) upregulated PRL levels in the inflamed paw of both male and female rats, but levels were higher in females. In contrast, CFA did not change mRNA levels of long and short PRL-R in the dorsal root ganglion or spinal cord. Analysis of PRL and PRL-R knockout (KO) mice demonstrated that basal responses to cold stimuli were only altered in females, and with no significant effects on heat and mechanical responses in both sexes. CFA-induced heat and cold hyperalgesia were not changed in PRL and PRL-R KO compared with wild-type (WT) males, whereas significant reduction of heat and cold post-CFA hyperalgesia was detected in PRL and PRL-R KO females. Attenuation of CFA-induced mechanical allodynia was observed in both PRL and PRL-R KO females and males. Thermal hyperalgesia in PRL KO females was restored by administration of PRL into hindpaws. Overall, we demonstrate a sex-dependent regulation of peripheral inflammatory hyperalgesia by the PRL system.

Sex-dependent roles of prolactin and prolactin receptor in postoperative pain and hyperalgesia in mice

Although surgical trauma activates the anterior pituitary gland and elicits an increase in prolactin (PRL) serum levels that can modulate nociceptive responses, the role of PRL and the PRL-receptor (PRL-R) in thermal and mechanical hyperalgesia in postoperative pain is unknown. Acute postoperative pain condition was generated with the use of the hindpaw plantar incision model. Results showed endogenous PRL levels were significantly increased in serum, operated hindpaw and spinal cords of male and female rats 24h after incision. These alterations were especially pronounced in females. We then examined the role of the PRL system in thermal and mechanical hyperalgesia in male and female mice 3-168 h after plantar incision with the use of knock-out (KO) mice with PRL or PRL-R gene ablations and in wild-type (WT) mice. WT mice showed postoperative cold hyperalgesia in a sex-dependent manner (only in females), but with no effect on heat hyperalgesia or mechanical allodynia in either sex. Studies in KO mice showed no effect of PRL and PRL-R gene ablation on heat and cold hyperalgesia in male mice, while heat hyperlgesia were reduced 3-72 h post-surgery in female PRL and PRL-R KO mice. In contrast, PRL and PRL-R ablations significantly attenuated mechanical allodynia 3-72 h post-surgery in both male and female mice. Overall, we found elevated PRL levels in serum, hindpaws and spinal cords after incision, and identify a contributory role for the PRL system in postoperative pain responses to thermal stimuli in females and to mechanical stimuli in both males and females.

Prolactin fractions from lactating rats elicit effects upon sensory spinal cord cells of male rats

Recently it has been suggested that the neurohormone prolactin (PRL) could act on the afferent nociceptive neurons. Indeed, PRL sensitizes transient receptor potential vanilloid 1 (TRPV1) channels present in nociceptive C-fibers and consequently reduces the pain threshold in a model of inflammatory pain. Accordingly, high plasma PRL levels in non-lactating females have been associated with several painful conditions (e.g. migraine). Paradoxically, an increase of PRL secretion during lactation induced a reduction in pain sensitivity. This difference could be attributed to the fact that PRL secreted from the adenopituitary (AP) is transformed into several molecular variants by the suckling stimulation. In order to test this hypothesis, the present study set out to investigate whether PRL from AP of suckled (S) or non-suckled (NS) lactating rats affects the activity of the male Wistar wide dynamic range (WDR) neurons. The WDR neurons are located in the dorsal horn of the spinal cord and receive input from the first-order neurons (Ab-, Ad- and C-fibers). Spinal administration of prolactin variant from NS rats (NS-PRL) or prolactin variant from S rats (S-PRL) had no effect on the neuronal activity of non-nociceptive Ab-fibers. However, the activities of nociceptive Ad-fibers and C-fibers were: (i) increased by NS-PRL and (ii) diminished by S-PRL. Either NS-PRL or S-PRL enhanced the post-discharge activity. Taken together, these results suggest that PRL from S or NS lactating rats could either facilitate or depress the nociceptive responses of spinal dorsal horn cells, depending on the physiological state of the rats.

Use of prolactin receptor antagonist to better understand prolactin regulation of pituitary homeostasis

The anterior pituitary is permanently regulated by processes of apoptosis and proliferation in order to maintain tissue homeostasis. Several factors have been implicated in this regulation and lately, prolactin (PRL) has been included into that list. However, since PRL is secreted by anterior pituitary lactotropes, the actual outcome of its autocrine/paracrine actions on pituitary cells has remained difficult to assess. The availability of the pure PRL receptor antagonist Del1-9-G129R-hPRL has been helpful to circumvent this problem. While PRL has been traditionally associated with increased cell proliferation, recent studies revealed that this hormone actually induces apoptosis and decreases proliferation of anterior pituitary cells, by mechanisms involving the PRL receptor. The aim of this short review is to overview our current understanding of the regulation of pituitary homeostasis by PRL. Moreover, studies involving Del1-9-G129R-hPRL have helped anticipate to what extent future treatments involving PRL receptor inhibitors may interfere with processes regulated by PRL at the central level.