Pregnancy increases excitability of mechanosensitive afferents innervating the uterine cervix

Oxytocin augmentation and neurotransmitters in prolonged delivery: An experimental appraisal

The uterus is a highly innervated organ, and during labor, this innervation is at its highest level. Oxytocinergic fibers play an important role in labor and delivery and, in particular, the Lower Uterine Segment, cervix, and fundus are all controlled by motor neurofibers. Oxytocin is a neurohormone that acts on receptors located on the membrane of the smooth cells of the myometrium. During the stages of labor and delivery, its binding causes myofibers to contract, which enables the fundus of the uterus to act as a mediator. The aim of this study was to investigate the presence of oxytocinergic fibers in prolonged and non-prolonged dystocic delivery in a cohort of 90 patients, evaluated during the first and second stages of labor. Myometrial tissue samples were collected and evaluated by electron microscopy, in order to quantify differences in neurofibers concentrations between the investigated and control cohorts of patients. The authors of this experiment showed that the concentration of oxytocinergic fibers differs between non-prolonged and prolonged dystocic delivery. In particular, in prolonged dystocic delivery, compared to non-prolonged dystocic delivery, there is a lower amount of oxytocin fiber. The increase in oxytocin appeared to be ineffective in patients who experienced prolonged dystocic delivery, since the dystocic labor ended as a result of the altered presence of oxytocinergic fibers detected in this group of patients.

Non-Invasive Ring Electrode With a Wireless Electrical Recording and Stimulating System for Monitoring Preterm Labor

Preterm labor and birth are the primary causes of neonatal morbidities and mortalities. The early detection and treatment of preterm uterine muscular contraction are crucial for the management of preterm labor. In this work, a ring electrode with a wireless electrical recording and stimulating (RE-WERS) system was designed, fabricated, and investigated for the non-invasive monitoring of uterine contraction/relaxation as a diagnostic and therapeutic tool for preterm labor. By using an organ bath system, we confirmed that the uterine contraction force in mice can be decreased by the application of electrical stimulation. Then, the RE-WERS system was inserted non-invasively through the vagina to the cervix of a pregnant minipig, and it successfully recorded the uterine contraction and reflect signals when various electrical stimulating conditions were applied. The difference in the uterine signals before and after the injection of a labor induction drug, such as oxytocin and prostaglandin [Formula: see text], was recorded, and the difference was remarkable. In addition, the uterine signal that was recorded was well matched with the signal of the electromyography (EMG) kit during open abdominal surgery. It seemed that the continuous and various electrical stimulations affected the delay or inhibition of childbirth in the pregnant minipig.

Pharmacological modulation of voltage-gated sodium (NaV) channels alters nociception arising from the female reproductive tract

Dyspareunia, also known as vaginal hyperalgesia, is a prevalent and debilitating symptom of gynaecological disorders such as endometriosis and vulvodynia. Despite this, the sensory pathways transmitting nociceptive information from female reproductive organs remain poorly characterised. As such, the development of specific treatments for pain associated with dyspareunia is currently lacking. Here, we examined, for the first time, (1) the mechanosensory properties of pelvic afferent nerves innervating the mouse vagina; (2) the expression profile of voltage-gated sodium (NaV) channels within these afferents; and (3) how pharmacological modulation of these channels alters vaginal nociceptive signalling ex vivo, in vitro, and in vivo. We developed a novel afferent recording preparation and characterised responses of pelvic afferents innervating the mouse vagina to different mechanical stimuli. Single-cell reverse transcription-polymerase chain reaction determined mRNA expression of NaV channels within vagina-innervating dorsal root ganglia neurons. Vagina-innervating dorsal root ganglia neuroexcitability was measured using whole-cell patch-clamp electrophysiology. Nociception evoked by vaginal distension was assessed by dorsal horn neuron activation within the spinal cord and quantification of visceromotor responses. We found that pelvic afferents innervating the vagina are tuned to detect various mechanical stimuli, with NaV channels abundantly expressed within these neurons. Pharmacological modulation of NaV channels (with veratridine or tetrodotoxin) correspondingly alters the excitability and mechanosensitivity of vagina-innervating afferents, as well as dorsal horn neuron activation and visceromotor responses evoked by vaginal distension. This study identifies potential molecular targets that can be used to modulate vaginal nociceptive signalling and aid in the development of approaches to manage endometriosis and vulvodynia-related dyspareunia.

Spinal Cord Stimulation in Pregnant Patients: Current Perspectives of Indications, Complications, and Results in Pain Control: A Systematic Review

Spinal cord stimulation (SCS) has been described as a valuable neuromodulator procedure in the management of chronic medically untreated neuropathic pain. Although the use of this technique has been published in many papers, a question still remains regarding its applicability in pregnant patients. The goal of this paper is to discuss the risks, complications, and results as well as the prognosis of SCS in pregnant patients. We performed a systematic review from 1967 to 2018 using the databases MEDLINE, LILACS, SciELO, PubMed, and BIREME, utilizing language as selection criteria. Eighteen studies that met our criteria were found and tabulated. SCS is a reversible and adjustable surgical procedure, which results in patients that demonstrated a significant effect in the reduction of pain intensity in pregnant patients. The etiologies most frequent were complex regional pain and failed back pain syndromes, which together represented 94% of analyzed cases. The technical complications most frequent were lead migration (3%, n = 1). Regarding the risks, the authors did not show significative factors among the categorical variables that can suggest a teratogenicity, while the maternal risks have been associated to the consequences of technical complications due to, among other factors, improvement of abdominal pressure during pregnancy and delivery. Finally, although there are not significative cohorts of pregnant patients, the procedure is still an effective surgical approach of neuropathic pain associated to lower rates of complications and significative improvement in the quality of life of patients during pregnancy.

Preemptive Caffeine Administration Blocks the Increase in Postoperative Pain Caused by Previous Sleep Loss in the Rat: A Potential Role for Preoptic Adenosine A2A Receptors in Sleep-Pain Interactions

Sleep and pain are reciprocally related, but the precise mechanisms underlying this relationship are poorly understood. This study used a rat model of surgical pain to examine the effect of previous sleep loss on postoperative pain and tested the hypothesis that preoptic adenosinergic mechanisms regulate sleep-pain interactions. Relative to ad libitum sleep, 6 hours of total sleep deprivation prior to a surgical incision significantly enhanced postoperative mechanical hypersensitivity in the affected paw and prolonged the time to recovery from surgery. There were no sex-specific differences in these measures. There were also no changes in adrenocorticotropic hormone and corticosterone levels after sleep deprivation, suggesting that this effect was not mediated by the stress associated with the sleep perturbation. Systemic administration of the nonselective adenosine receptor antagonist caffeine at the onset of sleep deprivation prevented the sleep deprivation-induced increase in postoperative hypersensitivity. Microinjection of the adenosine A2A receptor antagonist ZM 241385 into the median preoptic nucleus (MnPO) blocked the increase in surgical pain levels and duration caused by prior sleep deprivation and eliminated the thermal hyperalgesia induced by sleep deprivation in a group of nonoperated (i.e., without surgical incision) rats. These data show that even a brief sleep disturbance prior to surgery worsens postoperative pain and are consistent with our hypothesis that adenosine A2A receptors in the MnPO contribute to regulate these sleep-pain interactions.

Polymorphism in the ADRB2 gene explains a small portion of intersubject variability in pain relative to cervical dilation in the first stage of labor

BACKGROUND

Variability in labor pain has been associated with demographic, clinical, and psychological factors. Polymorphisms of the β2-adrenergic receptor gene (ADRB2) influence sensitivity to experimental pain in humans and are a risk factor for chronic pain. The authors hypothesized that polymorphisms in ADRB2 may influence labor pain.

METHODS

After Institutional Review Board approval and written informed consent, the authors prospectively obtained hourly pain reports from 233 nulliparous parturients during the first stage of labor, of which 199 were included in the current analysis. DNA from blood samples was genotyped at polymorphisms in the genes for the β2-adrenergic receptor, the μ opioid receptor subtype 1, catechol-O-methyltransferase, fatty acid amide hydrolase, and the oxytocin receptor. Labor pain as a function of cervical dilation was modeled with previously described methods. Patient covariates, ADRB2 genotype, and obstetrical and anesthesia treatment were evaluated as covariates in the model.

RESULTS

Labor pain more rapidly became severe in parturients heterozygous or homozygous for the G allele at rs1042714 in the ADRB2 gene. Labor pain increased more rapidly after artificial rupture of membranes, augmentation with oxytocin, and in younger women. Inclusion of covariates explained approximately 10% of the variability between subjects. ADRB2 genotype explained less than 1% of the intersubject variability.

CONCLUSION

ADRB2 genotype correlates with labor pain but explained less than 1% of the intersubject variance in the model.

Sex differences and hormonal modulation of deep tissue pain

Women disproportionately suffer from many deep tissue pain conditions. Experimental studies show that women have lower pain thresholds, higher pain ratings and less tolerance to a range of painful stimuli. Most clinical and epidemiological reports suggest female gonadal hormones modulate pain for some, but not all, conditions. Similarly, animal studies support greater nociceptive sensitivity in females in many deep tissue pain models. Gonadal hormones modulate responses in primary afferents, dorsal horn neurons and supraspinal sites, but the direction of modulation is variable. This review will examine sex differences in deep tissue pain in humans and animals focusing on the role of gonadal hormones (mainly estradiol) as an underlying component of the modulation of pain sensitivity.

Role of Capsaicin in a Murine Model of Labor and Delivery

Background:

The objectives of this study were to develop a murine model of labor and delivery and to use this model to examine whether capsaicin diminishes labor pain and expedites delivery.

Methods:

To develop a murine model of labor pain, the authors identified and compared the incidence of four proposed pain behaviors in 46 mice: (1) No analgesia in labor and the postpartum period, and (2) increasing doses of an analgesic, morphine. The model was then used to examine the impact of topical cervical capsaicin on: (1) labor pain behaviors and (2) labor progress by examining its impact on the time from treatment to delivery of the first pup and on the duration of delivery per pup. The treatment was randomly allocated and the behavioral observation was blinded.

Results:

In the absence of analgesia, there was a statistically significant decrease in all four proposed pain behaviors in the postpartum period compared with labor (cumulative 55.0 ± 16.1/h vs. 16.1 ± 8.7/h; P < 0.0001). Additionally, morphine reduced their incidence during labor in a dose-dependent manner (cumulative 55.0 ± 16.1.7/h control, 46.4 ± 15.8 morphine 0.1 mg/kg/h, 34.6 ± 5.6/h, morphine 0.5 mg/kg/h; P = 0.1988, 0.0014). In addition, the incidence of identified pain behaviors was reduced by pericervical capsaicin (cumulative 55.0 ± 16.1.7/h control, 38.9 ± 15.4 capsaicin, P = 0.02).

Conclusions:

In this pilot study, the authors developed a novel mouse model of labor and delivery. Pericervical capsaicin applied days before delivery reduces labor pain behaviors.

Resolution of pain after childbirth

BACKGROUND

Chronic pain after surgery occurs in 10-40% of individuals, including 5-20% of women after cesarean delivery in previous reports. Pain and depression 2 months after childbirth are independently associated with more severe acute post-delivery pain. Here we examine other predictors of pain at 2 months and determine the incidence of pain at 6 and 12 months after childbirth.

METHODS

Following Institutional Review Board approval, 1228 women were interviewed within 36 h of delivery. Of these, 937 (76%) were successfully contacted by telephone at 2 months, and, if they had pain, at 6 and 12 months after delivery. The primary outcome measure was presence of pain which began at the time of delivery. We also generated a model of severity of acute post-delivery pain and 2 month pain and depression.

RESULTS

Pain which began at the time of delivery was remarkably rare 6 and 12 months later (1.8% and 0.3% [upper 95% confidence limit, 1.2%], respectively). Past history of pain and degree of tissue damage at delivery accounted for 7.0% and 16.7%, respectively, of one aspect in the variability in acute post-delivery pain. Neither of these factors was associated with incidence of pain 2 months later.

CONCLUSIONS

Using a definition of new onset pain from delivery, we show a remarkably low incidence of pain 1 yr after childbirth, including those with surgical delivery. Additionally, degree of tissue trauma and history of chronic pain, risk factors for pain 2 months after other surgery, were unimportant to pain 2 months after cesarean or vaginal delivery.

Transection of the pelvic or vagus nerve forestalls ripening of the cervix and delays birth in rats

Innervation of the cervix is important for normal timing of birth because transection of the pelvic nerve forestalls birth and causes dystocia. To discover whether transection of the parasympathetic innervation of the cervix affects cervical ripening in the process of parturition was the objective of the present study. Rats on Day 16 of pregnancy had the pelvic nerve (PnX) or the vagus nerve (VnX) or both pathways (PnX+VnX) transected, sham-operated (Sham) or nonpregnant rats served as controls. Sections of fixed peripartum cervix were stained for collagen or processed by immunohistochemistry to identify macrophages and nerve fibers. All Sham controls delivered by the morning of Day 22 postbreeding, while births were delayed in more than 75% of neurectomized rats by more than 12 h. Dystocia was evident in more than 25% of the PnX and PnX+VnX rats. Moreover, on prepartum Day 21, serum progesterone was increased severalfold in neurectomized versus Sham rats. Assessments of cell nuclei counts indicated that the cervix of neurectomized rats and Sham controls had become equally hypertrophied compared to the unripe cervix in nonpregnant rats. Collagen content and structure were reduced in the cervix of all pregnant rats, whether neurectomized or Shams, versus that in nonpregnant rats. Stereological analysis of cervix sections found reduced numbers of resident macrophages in prepartum PnX and PnX+VnX rats on Day 21 postbreeding, as well as in VnX rats on Day 22 postbreeding compared to that in Sham controls. Finally, nerve transections blocked the prepartum increase in innervation that occurred in Sham rats on Day 21 postbreeding. These findings indicate that parasympathetic innervation of the cervix mediates local inflammatory processes, withdrawal of progesterone in circulation, and the normal timing of birth. Therefore, pelvic and vagal nerves regulate macrophage immigration and nerve fiber density but may not be involved in final remodeling of the extracellular matrix in the prepartum cervix. These findings support the contention that immigration of immune cells and enhanced innervation are involved in processes that remodel the cervix and time parturition.

Retrograde tracing of spinal cord connections to the cervix with pregnancy in mice

In contrast to the uterus, the cervix is well innervated during pregnancy and the density of nerve fibers increases before birth. To assess neural connections between the cervix and the spinal cord, the cervix of pregnant mice was injected with the trans-synaptic retrograde neural tract tracer pseudorabies virus (PRV). After 5 days, the virus was present in nerve cells and fibers in specific areas of the sensory, autonomic, and motor subdivisions of the thoracolumbar spinal cord. In nonpregnant controls, the virus was predominantly distributed in laminae I-III in the dorsal gray sensory areas with the heaviest label in the substantia gelatinosa compared with the autonomic or motor areas. Labeled cells and processes were sparse in other regions, except for a prominent cluster in the intermediolateral column (lamina VII). Photomicrographs of spinal cord sections were digitized, and the total area with the virus was estimated. Compared with nonpregnant controls, the area with PRV was significantly decreased in all the spinal cord subdivisions in pregnant mice except in the intermediolateral column. However, areas with the virus were equivalent in mice injected with PRV at 4 days or 1 day before birth. These findings suggest that the predominant innervation of the murine cervix is from the sensory regions of the thoracolumbar spinal cord, and that these connections diminish with pregnancy. The results raise the possibility that the remaining connections from sensory and autonomic subdivisions, particularly the intermediolateral column, of the thoracolumbar spinal cord may be important for increased density of nerve fibers in the cervix as pregnancy nears term.