Brain-mediated responses to vaginocervical stimulation in spinal cord-transected rats: role of the vagus nerves

Serotonin neurons in mating female mice are activated by male ejaculation

Sexual stimulation triggers changes in female physiology and behavior, including sexual satiety and preparing the uterus for pregnancy. Serotonin (5-HT) is an important regulator of reproductive physiology and sexual receptivity, but the relationship between sexual stimulation and 5-HT neural activity in females is poorly understood. Here, we investigated dorsal raphe 5-HT neural activity in female mice during sexual behavior. We found that 5-HT neural activity in mating females peaked specifically upon male ejaculation and remained elevated above baseline until disengagement. Artificial intravaginal mechanical stimulation was sufficient to elicit increased 5-HT neural activity but the delivery of ejaculatory fluids was not. Distal penis expansion ("penile cupping") at ejaculation and forceful expulsion of ejaculatory fluid each provided sufficient mechanical stimulation to elicit 5-HT neuron activation. Our study identifies a female ejaculation-specific signal in a major neuromodulatory system and shows that intravaginal mechanosensory stimulation is necessary and sufficient to drive this signal.

Serotonin neurons in mating female mice are activated by male ejaculation

Sexual stimulation triggers changes in female physiology and behavior, including sexual satiety and preparing the uterus for pregnancy. Serotonin is an important regulator of reproductive physiology and sexual receptivity, but the relationship between sexual stimulation and serotonin neural activity in females is poorly understood. Here, we investigated dorsal raphe serotonin neural activity in females during sexual behavior. We found that serotonin neural activity in mating females peaked specifically upon male ejaculation, and remained elevated above baseline until disengagement. Artificial intravaginal mechanical stimulation was sufficient to elicit increased 5-HT neural activity but the delivery of ejaculatory fluids was not. Distal penis erectile enlargement ("penile cupping") at ejaculation and forceful expulsion of ejaculatory fluid each provided sufficient mechanical stimulation to elicit serotonin neuron activation. Our study identifies a female ejaculation-specific signal in a major neuromodulatory system and shows that intravaginal mechanosensory stimulation is necessary and sufficient to drive this signal.

Vagus nerve afferent stimulation: Projection into the brain, reflexive physiological, perceptual, and behavioral responses, and clinical relevance

The afferent vagus nerves project to diverse neural networks within the brainstem and forebrain, based on neuroanatomical, neurophysiological, and functional (fMRI) brain imaging evidence. In response to afferent vagal stimulation, multiple homeostatic visceral reflexes are elicited. Physiological stimuli and both invasive and non-invasive electrical stimulation that activate the afferent vagus elicit perceptual and behavioral responses that are of physiological and clinical significance. In the present review, we address these multiple roles of the afferent vagus under normal and pathological conditions, based on both animal and human evidence.

Effect of Osteopathic Obstetrical Management on the Duration of Labor in the Inpatient Setting: A Prospective Study and Literature Review

Context

Several studies have investigated the effects of osteopathic manipulative treatment (OMT) on labor duration, but the outcomes remain ambiguous. Confounding the relationship between OMT and labor duration is the lack of standardization between treatment settings, gestational ages at the time of treatment, OMT techniques, and overall obstetrical management principles from foundational and modern osteopathic approaches.

Objective

To evaluate the effect of OMT on labor duration when applied in tandem with standard obstetrical management in the inpatient setting.

Methods

This pilot prospective observational study was conducted from June 2017 through September 2017. All patients who received OMT as part of their labor management were included. These patients were matched with controls who did not receive OMT. The OMT protocol involved once-daily administration of suboccipital decompression, thoracic inlet release, rib raising, paraspinal inhibition, and sacral inhibition. Obstetrical decisions regarding labor management were made by 1 senior attending osteopathic obstetrician. Labor management as well as OMT was carried out by osteopathic obstetricians in the OMT group, whereas allopathic obstetricians carried out labor management in the control group.

Results

A total of 100 patients were enrolled. Fifty patients who underwent adjunctive OMT in addition to standard labor management were matched to controls who received standard labor management only. Each group was represented by an ethnically diverse population. The mean (SD) labor duration for patients receiving OMT was significantly shorter than the labor duration for controls (11.34 [6.62] hours [range, 1.1-27.0 hours] vs 16.57 [4.39] [range, 1.0-58.8 hours], respectively; P=.03). All other measures studied did not achieve statistical significance.

Conclusion

Pregnancy and labor present many musculoskeletal and neurovisceral challenges to obstetrical patients and, to the authors' knowledge, this is the first study to present an effective, efficient, and feasible approach to intrapartum osteopathic obstetrical management in the inpatient setting to reduce labor duration.

Copulation-induced antinociception in female rats is blocked by atosiban, an oxytocin receptor antagonist

AIMS

We hypothesized that copulation-induced temporary anti-nociception in female rats is mediated by the activation of central and/or peripheral oxytocin receptors. To test this hypothesis, we assessed the effects of intraperitoneal (ip), intrathecal (it), and intra-cerebroventricular (icv) administration of an oxytocin receptor antagonist (atosiban), on copulation-induced temporary anti-nociception in estrous rats.

MAIN METHODS

The treatment groups were ovariectomized rats pre-treated subcutaneously (sc) with 10 μg of estradiol benzoate (EB) followed 24 h later by an sc injection of 5 μg EB, and 4 h later, by an sc injection of 2 mg progesterone (P4). Rats were then administered saline vehicle (ip, it, or icv: control groups) or atosiban (500 μg/kg ip; 500 ng it; or 500 ng icv: experimental groups). Thirty minutes after drug or saline administration, their sexual behavior was tested by pairing with a sexually-experienced male rat. Brief pulse trains of 50 Hz, 300 ms duration, supra-threshold tail electrical shocks (STS) were delivered before and during copulatory activity i.e., while the female was receiving mounts, intromissions, or ejaculations, and we recorded whether vocalization occurred in response to each STS.

KEY FINDINGS

Replicating our previous findings, the vocalization response to STS in control rats was significantly attenuated during intromissions and ejaculations, compared to their baseline (pre-mating) response, indicative of anti-nociception. By contrast, rats pre-treated with atosiban (each route of administration) failed to show an attenuation of the vocalization response to shock.

SIGNIFICANCE

These findings provide evidence that the temporary anti-nociceptive effect of copulation in female rats is mediated by copulation-induced release of endogenous oxytocin in brain, spinal cord and periphery.

Measuring sperm backflow following female orgasm: a new method

BACKGROUND

Human female orgasm is a vexed question in the field while there is credible evidence of cryptic female choice that has many hallmarks of orgasm in other species. Our initial goal was to produce a proof of concept for allowing females to study an aspect of infertility in a home setting, specifically by aligning the study of human infertility and increased fertility with the study of other mammalian fertility. In the latter case - the realm of oxytocin-mediated sperm retention mechanisms seems to be at work in terms of ultimate function (differential sperm retention) while the proximate function (rapid transport or cervical tenting) remains unresolved.

METHOD

A repeated measures design using an easily taught technique in a natural setting was used. Participants were a small (n=6), non-representative sample of females. The introduction of a sperm-simulant combined with an orgasm-producing technique using a vibrator/home massager and other easily supplied materials.

RESULTS

The sperm flowback (simulated) was measured using a technique that can be used in a home setting. There was a significant difference in simulant retention between the orgasm (M=4.08, SD=0.17) and non-orgasm (M=3.30, SD=0.22) conditions; t (5)=7.02, p=0.001. Cohen's d=3.97, effect size r=0.89. This indicates a medium to small effect size.

CONCLUSIONS

This method could allow females to test an aspect of sexual response that has been linked to lowered fertility in a home setting with minimal training. It needs to be replicated with a larger sample size.

‘Birthgasm’

Childbirth is a fundamental component of a woman’s sexual cycle. The sexuality of childbirth is not well recognized in Western society despite research showing that some women experience orgasm(s) during labor and childbirth. Current thinking supports the view that labor and childbirth are perceived to be physically painful events, and more women are relying on medical interventions for pain relief in labor. This review explores the potential of orgasm as a mode of pain relief in childbirth and outlines the physiological explanations for its occurrence. Potential barriers to sexual expression during childbirth and labor, including the influence of deeply held cultural beliefs about sexuality, the importance of privacy and intimacy in facilitating orgasmic birth experiences, and the value of including prospective fathers in the birthing experience, are discussed. The role of midwives and their perceptions of the use of complementary and alternative therapies for pain relief in labor are examined. While there are indications of widespread use of complementary and alternative therapies such as hydrotherapy, herbal remedies, and breathing techniques for pain relief in childbirth, orgasm was not among those mentioned. Lack of recognition of the sexuality of childbirth, despite findings that orgasm can attenuate the effects of labor pain, suggests the need for greater awareness among expectant parents, educators, and health professionals of the potential of orgasm as a means of pain relief in childbirth.

Non-invasive Access to the Vagus Nerve Central Projections via Electrical Stimulation of the External Ear: fMRI Evidence in Humans

BACKGROUND

Tract-tracing studies in cats and rats demonstrated that the auricular branch of the vagus nerve (ABVN) projects to the nucleus tractus solitarii (NTS); it has remained unclear as to whether or not the ABVN projects to the NTS in humans.

OBJECTIVE

To ascertain whether non-invasive electrical stimulation of the cymba conchae, a region of the external ear exclusively innervated by the ABVN, activates the NTS and the "classical" central vagal projections in humans.

METHODS

Twelve healthy adults underwent two fMRI scans in the same session. Electrical stimulation (continuous 0.25ms pulses, 25Hz) was applied to the earlobe (control, scan #1) and left cymba conchae (scan #2). Statistical analyses were performed with FSL. Two region-of-interest analyses were performed to test the effects of cymba conchae stimulation (compared to baseline and control, earlobe, stimulation) on the central vagal projections (corrected; brainstem P < 0.01, forebrain P < 0.05), followed by a whole-brain analysis (corrected, P < 0.05).

RESULTS

Cymba conchae stimulation, compared to earlobe (control) stimulation, produced significant activation of the "classical" central vagal projections, e.g., widespread activity in the ipsilateral NTS, bilateral spinal trigeminal nucleus, dorsal raphe, locus coeruleus, and contralateral parabrachial area, amygdala, and nucleus accumbens. Bilateral activation of the paracentral lobule was also observed. Deactivations were observed bilaterally in the hippocampus and hypothalamus.

CONCLUSION

These findings provide evidence in humans that the central projections of the ABVN are consistent with the "classical" central vagal projections and can be accessed non-invasively via the external ear.

Maternal prenatal stress in rats influences c-fos expression in the spinal cord of the offspring

Previous studies in humans have reported a link between maternal stress and disturbed infant physiological behavior. The objective of our study was to examine in experimental rats how maternal prenatal stress induced by a forced swim test affects offspring afferent spinal responses mediated by stimulation of vaginocervical receptors. The activation of spinal cord neurons showing c-fos expression was examined following vaginocervical mechanical stimulation in adult rats, which were the offspring of dams exposed to gestational stress from E10 until delivery. Vaginocervical stimulation of both prenatal-stressed and non-prenatal-stressed rats induced an increase in immunoreactive protein in the spinal cord ranging from T12 to S1 segmental levels. However, a significantly higher (40%) increase in the expression of Fos-immunoreactive neurons was observed in vaginocervical stimulated prenatally stressed rats than in non-stimulated prenatally stressed ones. This increase was higher in L5-S1 levels than in T12-L4. When the regional distribution was examined, results showed that up to 80% of activated neurons were located in the dorsal horn in both non-stimulated prenatally stressed and stimulated prenatally stressed groups, with a significantly higher density in the latter. Our results demonstrate that maternal prenatal stress can have consequences on vaginocervical responses conveyed to the spinal cord. The increase in Fos labeled neurons in T12-S1 in prenatally stressed rats induced by vaginocervical stimulation suggests the hypersensitivity of the genital tract associated with activation of spinal circuits spanning multiple segments.

Doing it … wild? On the role of the cerebral cortex in human sexual activity

BACKGROUND

We like to think about sexual activity as something fixed, basic and primal. However, this does not seem to fully capture reality. Even when we relish sex, we may be capable of mentalizing, talking, voluntarily postponing orgasm, and much more. This might indicate that the central control mechanisms of sexual activity are quite flexible and susceptible to learning mechanisms, and that cortical brain areas play a critical part.

OBJECTIVE

This study aimed to identify those cortical areas and mechanisms most consistently implicated in sexual activity.

DESIGN

A comprehensive review of the human functional neuroimaging literature on sexual activity, i.e. genital stimulation and orgasm, is made.

RESULTS

Genital stimulation recruits the classical somatosensory matrix, but also areas far beyond that. The posterior insula may be particularly important for processing input from the engorged penis and coordinating penile responses. Extrastriate visual cortex tracks sexual arousal and responds to genital stimulation even when subjects have their eyes closed. The ventromedial prefrontal cortex is also tightly coupled to sexual arousal, but low activity in this area predicts high sexual arousal.

CONCLUSION

This review has indicated cortical sites where activity is moderated by tactile genital inflow and high sexual arousal. Behavioral implications are discussed and where possible the relevance for learning mechanisms is indicated. Overall, it is clear that the cerebral cortex has something to say about sexual activity.

Are there different types of female orgasm?

In attempt to identify and validate different types of orgasms which females have during sex with a partner, data collected by Mah and Binik (2002) on the dimensional phenomenology of female orgasm were subjected to a typological analysis. A total of 503 women provided adjectival descriptions of orgasms experienced either with a partner (n = 276) or while alone (n = 227). Latent-class analysis revealed four orgasm types which varied systematically in terms of pleasure and sensations engendered. Two types, collectively labelled "good-sex orgasms," received higher pleasure and sensation ratings than solitary-masturbatory ones, whereas two other types, collectively labelled "not-as-good-sex orgasms," received lower ratings. These two higher-order groupings differed on a number of psychological, physical and relationship factors examined for purposes of validating the typology. Evolutionary thinking regarding the function of female orgasm informed discussion of the findings. Future research directions were outlined, especially the need to examine whether the same individual experiences different types of orgasms with partners with different characteristics, as evolutionary theorizing predicts should be the case.

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.

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.

Noradrenergic nuclei that receive sensory input during mating and project to the ventromedial hypothalamus play a role in mating-induced pseudopregnancy in the female rat

In female rats, vaginal-cervical stimulation (VCS) received during mating induces bicircadian prolactin surges that are required for the maintenance of pregnancy or pseudopregnancy (PSP). The neural circuits that transmit VCS inputs to the brain have not been fully described, although mating stimulation is known to activate medullary noradrenergic cell groups that project to the forebrain. In response to VCS, these neurones release noradrenaline within the ventrolateral division of the ventromedial hypothalamus (VMHvl) and the posterodorsal medial amygdala (MePD), two forebrain sites that are implicated in the initiation of PSP. Noradrenaline receptor activation within the VMHvl is both necessary and sufficient for PSP induction, suggesting that noradrenaline acting within the VMHvl is particularly important in mediating the effects of VCS towards the establishment of PSP. We therefore investigated whether or not endogenous, VCS-induced noradrenaline release within the VMHvl is involved in PSP induction in the rat. Before the receipt of sufficient mating stimulation to induce PSP, a retrograde neurotoxin, dopamine-β-hydroxylase-saporin (DBH-SAP), was infused bilaterally into the either the VMHvl or the MePD to selectively destroy afferent noradrenergic nuclei in the brainstem. DBH-SAP infusions into the VMHvl lesioned mating-responsive noradrenergic neurones in A1 and A2 medullary nuclei and reduced the incidence of PSP by 50%. Infusions of DBH-SAP into the MePD had no effect on the subsequent induction of PSP. These results suggest that VCS is conveyed to mating-responsive forebrain areas by brainstem noradrenergic neurones, and that the activity of noradrenergic cells projecting to the VMHvl is involved in the induction of PSP.

Pregnancy-related changes in connections from the cervix to forebrain and hypothalamus in mice

The transneuronal tracer pseudorabies virus was used to test the hypothesis that connections from the cervix to the forebrain and hypothalamus are maintained with pregnancy. The virus was injected into the cervix of nonpregnant or pregnant mice, and, after 5 days, virus-labeled cells and fibers were found in specific forebrain regions and, most prominently, in portions of the hypothalamic paraventricular nucleus. With pregnancy, fewer neurons and fibers were evident in most brain regions compared to that in nonpregnant mice. In particular, little or no virus was found in the medial and ventral parvocellular subdivisions, anteroventral periventricular nucleus, or motor cortex in pregnant mice. By contrast, labeling of virus was sustained in the dorsal hypothalamus and suprachiasmatic nucleus in all groups. Based upon image analysis of digitized photomicrographs, the area with label in the rostral and medial parvocellular paraventricular nucleus and magnocellular subdivisions was significantly reduced in mice whose cervix was injected with virus during pregnancy than in nonpregnant mice. The findings indicate that connections from the cervix to brain regions that are involved in sensory input and integrative autonomic functions are reduced during pregnancy. The findings raise the possibility that remaining pathways from the cervix to the forebrain and hypothalamus may be important for control of pituitary neuroendocrine secretion, as well as for effector functions in the cervix as pregnancy nears term.

Conditioned place preference for mating is preserved in rats with pelvic nerve transection

Female rats exhibit a conditioned place preference (CPP) for a context paired with mating. The present experiment tested the hypothesis that the activation of the pelvic nerve mediates the reinforcing effects of mating for female rats. Rats underwent bilateral pelvic nerve or sham transection and then received paced mating, nonpaced mating, or the control treatment during a CPP procedure. Pelvic nerve transection did not affect the CPP for paced or nonpaced mating. In tests of paced mating behavior, contact-return latencies following intromissions were significantly shorter in rats with pelvic nerve transection than they were in rats with sham transections. These results show that the pathway conveying the reinforcing effects of mating stimulation does not depend on the integrity of the pelvic nerve, but that activation of the pelvic nerve contributes to the display of paced mating behavior.

Cervix remodeling and parturition in the rat: lack of a role for hypogastric innervation

The hypogastric nerve is a major pathway innervating the uterine cervix, yet its contribution to the processes of cervical ripening and parturition is not known. The main objective of this study was to determine the effect of hypogastric nerve transection on remodeling of the cervix and timing of birth. As an initial goal, processes associated with remodeling of the peripartum cervix were studied. The cervix was obtained from time-dated pregnant rats on days 15, 19, 21, and 21.5 of pregnancy, and post partum on the day of birth (day 22). The cervix was excised, post-fixed overnight, and sections stained to evaluate collagen content and structure or processed by immunohistochemistry to identify macrophages or nerve fibers. The census of macrophages and density of nerve fibers in the cervix peaked on day 21, the day before birth, and then declined post partum. These results replicate in time course and magnitude previous studies in mice. To address the main objective, the hypogastric nerve was bilaterally transected on day 15 post-breeding; sham-operated rats served as controls. Pups were born in both groups at normal term. Transection of the hypogastric nerves did not affect remodeling of collagen or the census of macrophages or the density of nerve fibers in the cervix. These findings support the contention that enhanced innervation and immigration of immune cells are associated with remodeling of the cervix and parturition, but that a neural pathway other than the hypogastric nerve may participate in the process of cervical ripening.

The sympathetic postganglionic and sensory innervation of oviducal magnum in hen: a choleratoxin subunit B-conjugated horseradish peroxidase study

The anatomy of the extrinsic innervation of the avian magnum has not been accurately demonstrated previously. In the present study, choleratoxin subunit B-conjugated horseradish peroxidase (CB-HRP) was used as a retrograde tracer to determine the sympathetic postganglionic and sensory innervation of the magnum of hens. With regard to the sympathetic postganglionic innervation, following CB-HRP injections under the serosa of the magnum, CB-HRP-positive neurons were found bilaterally in the C12-LS13 ganglia of the sympathetic chain, splanchnic ganglia and adrenal ganglia. The number of labelled neurons in the left ganglia of the sympathetic chain and splanchnic ganglia was approximately 2.1 times that in the right ganglia. This suggests that the unilateral magnum is bilaterally innervated with sympathetic postganglionic nerves, the left nerves being predominant. With regard to the sensory innervation, following tracer injections, CB-HRP-positive neurons were found bilaterally in the spinal ganglia C13-LS12, jugular ganglia and nodose ganglia. The number of positive cells in the left ganglia was about 2.2 times that in the right ganglia. In the spinal ganglia, 85.6% of the labelled neurons were in the T5-LS2 and LS8-LS11 ganglia. These results suggest that the sensory nerve fibres of the magnum reach the central nervous system principally via two groups of spinal ganglia and vagus nerves, and that the innervation is bilateral although the left-hand route predominates. Moreover, 45.7% of all the CB-HRP-labelled neurons were found in the rectal region of the intestinal nerve of Remak (INR), which suggests that the INR plays a very important role in the functional regulation of the magnum.

Effects of pelvic, pudendal, or hypogastric nerve cuts on Fos induction in the rat brain following vaginocervical stimulation

In the female rat, genitosensory input is conveyed to the central nervous system predominantly through the pelvic, pudendal, and hypogastric nerves. The present study examined the relative contribution of those three nerves in the expression of Fos immunoreactivity within brain regions previously shown to be activated by vaginocervical stimulation (VCS). Bilateral transection of those nerves, or sham neurectomy, was conducted in separate groups of ovariectomized, sexually-experienced females. After recovery, females were primed with estrogen and progesterone and given either 50 manual VCSs with a lubricated glass rod over the course of 1 h. VCS increased the number of neurons expressing Fos immunoreactivity in the medial preoptic area, lateral septum, bed nucleus of the stria terminalis, ventromedial hypothalamus, and medial amygdala of sham neurectomized females. Transection of the pelvic nerve reduced Fos immunoreactivity in the medial preoptic area, bed nucleus of the stria terminalis, ventromedial hypothalamus, and medial amygdala, whereas transection of the pudendal nerve had no effect. In contrast, transection of the hypogastric nerve increased Fos immunoreactivity in the medial preoptic area and lateral septum, whereas transaction of the pelvic nerve increased Fos immunoreactivity in the lateral septum, following VCS. All females given VCS, except those with pelvic neurectomy, displayed a characteristic immobility during each application. These data confirm that the pelvic nerve is largely responsible for the neural and behavioral effects of VCS, and support a separate function for the hypogastric nerve.

Physiology of female sexual function: animal models

INTRODUCTION

Data concerning the physiology of desire, arousal, and orgasm in women are limited because of ethical constraints. Aim. To gain knowledge of physiology of female sexual function through animal models.

METHODS

To provide state-of-the-art knowledge concerning female sexual function in animal models, representing the opinions of seven experts from five countries developed in a consensus process over a 2-year period.

MAIN OUTCOME MEASURE

Expert opinion was based on the grading of evidence-based medical literature, widespread internal committee discussion, public presentation, and debate.

RESULTS

Sexual desire may be considered as the presence of desire for, and fantasy about, sexual activity. Desire in animals can be inferred from certain appetitive behaviors that occur during copulation and from certain unconditioned copulatory measures. Proceptive behaviors are dependent in part on estrogen, progesterone, and drugs that bind to D1 dopamine receptors, adrenergic receptors, oxytocin receptors, opioid receptors, or gamma-amino butyric acid receptors. Peripheral arousal states are dependent on regulation of genital smooth muscle tone. Multiple neurotransmitters/mediators are involved including adrenergic, and nonadrenergic, noncholinergic agents such as vasoactive intestinal polypeptide, nitric oxide, neuropeptide Y, calcitonin gene-related peptide, and substance P. Sex steroid hormones, estrogens and androgens, are critical for structure and function of genital tissues including modulation of genital blood flow, lubrication, neurotransmitter function, smooth muscle contractility, mucification, and sex steroid receptor expression in genital tissues. Orgasm may be investigated by urethrogenital (UG) reflex, in which genital stimulation results in rhythmic contractions of striated perineal muscles and contractions of vagina, anus, and uterine smooth muscle. The UG reflex is generated by a multisegmental spinal pattern generator involving the coordination of sympathetic, parasympathetic, and somatic efferents innervating the genital organs. Serotonin and dopamine may modulate UG reflex activity.

CONCLUSIONS

More research is needed in animal models in the physiology of female sexual function.

Ascending spinal pathways from sexual organs: effects of chronic spinal lesions

A recent survey of paraplegics indicates that regaining sexual function is of the highest priority for both males and females (Anderson, K.D. (2004) Targeting recovery: priorities of the spinal cord-injured population J. Newrotrauma, 21: 1371-1383). Our understanding of the neural pathways and mechanisms underlying sexual behavior and function is limited at the present time. More studies are obviously needed to direct experiments geared toward developing effective therapeutic interventions. In this chapter, a review of studies on the processing of sensory inputs from the male and female reproductive organs is presented with a review of what is known about the location of ascending spinal pathways conveying this information. The effect of spinal cord injury on sexual function and the problems that ensue are discussed.

Female sexual function after spinal cord injury

Over the past 10 years, studies of the impact of spinal cord injuries on female sexuality have expanded from questionnaire studies in small populations with unknown levels and degrees of injury to laboratory-based analyses of women with known injury patterns. These studies have provided detailed information on how specific injury patterns affect specific aspects of the female sexual response. Research findings have supported the hypothesis that the sympathetic nervous system is regulatory for psychogenic genital vasocongestion and that orgasm is a reflex response of the autonomic nervous system. Based on these results, a new system for the classification of sexual function in women with spinal cord injury (SCI) is proposed. Moreover, studies related to the treatment of sexual dysfunction in women with cord injury are reviewed.

Brain activation during vaginocervical self-stimulation and orgasm in women with complete spinal cord injury: fMRI evidence of mediation by the vagus nerves

Women diagnosed with complete spinal cord injury (SCI) at T10 or above report vaginal-cervical perceptual awareness. To test whether the Vagus nerves, which bypass the spinal cord, provide the afferent pathway for this response, we hypothesized that the Nucleus Tractus Solitarii (NTS) region of the medulla oblongata, to which the Vagus nerves project, is activated by vaginal-cervical self-stimulation (CSS) in such women, as visualized by functional magnetic resonance imaging (fMRI). Regional blood oxygen level-dependent (BOLD) signal intensity was imaged during CSS and other motor and sensory procedures, using statistical parametric mapping (SPM) analysis with head motion artifact correction. Physiatric examination and MRI established the location and extent of spinal cord injury. In order to demarcate the NTS, a gustatory stimulus and hand movement were used to activate the superior region of the NTS and the Nucleus Cuneatus adjacent to the inferior region of the NTS, respectively. Each of four women with interruption, or "complete" injury, of the spinal cord (ASIA criteria), and one woman with significant, but "incomplete" SCI, all at or above T10, showed activation of the inferior region of the NTS during CSS. Each woman showed analgesia, measured at the fingers, during CSS, confirming previous findings. Three women experienced orgasm during the CSS. The brain regions that showed activation during the orgasms included hypothalamic paraventricular nucleus, medial amygdala, anterior cingulate, frontal, parietal, and insular cortices, and cerebellum. We conclude that the Vagus nerves provide a spinal cord-bypass pathway for vaginal-cervical sensibility in women with complete spinal cord injury above the level of entry into spinal cord of the known genitospinal nerves.

Cholinergic facilitation of erection and ejaculation in spinal cord-transected rats

Penile reflexes (PRs) were monitored in chronic spinal cord-transected rats by identifying them visually, and at the same time they were recorded as the electromyographic activity of bulbospongiosus muscles. Intraperitoneal injection of the agonist muscarine (10 microg) produced a facilitation of PRs. A decrease in the latency, an increase in the number of clusters and often an increase in the duration of cups were found after muscarine. In addition, 66% (six out of nine) of the animals ejaculated after muscarine. These results suggest that cholinergic receptor stimulation may be involved in erectile and ejaculatory mechanisms mediated by the spinal cord.

Brain stem convergence of pelvic viscerosomatic inputs via spinal and vagal afferents

Single medullary reticular formation (MRF) neurons receive ascending spinal inputs from multiple somatic and pelvic visceral territories. MRF neurons were examined for responses to both pelvic (PN) and vagus (abdominal branches: VAG-abd) nerve stimulation, which dually innervate certain pelvic viscera. Recordings in 12 urethane-anesthetized male rats were performed. Of 121 PN-responsive MRF neurons, 50% responded to VAG-abd. Twenty-seven (22%) responded to colonic distention. All 121 neurons responded to noxious stimulation of somatic territories, including many areas outside the perigenital region (including the hindpaws, ears, face). These data demonstrate input originating from different spinal and cranial nerves. The functional significance of this viscerosomatic convergence to MRF is unknown, but could relate to sensory/autonomic integration for coordinating multiple bodily functions, including reproductive and eliminative events.

Acute vagus nerve stimulation using different pulse widths produces varying brain effects

BACKGROUND

Vagus nerve stimulation (VNS) is an approved treatment for epilepsy and has been investigated in clinical trials of depression. Little is known about the relationship of VNS parameters to brain function. Using the interleaved VNS /functional magnetic resonance imaging (fMRI) technique, we tested whether variations of VNS pulse width (PW) would produce different immediate brain activation in a manner consistent with single neuron PW studies.

METHODS

Twelve adult patients with major depression, treated with VNS, underwent three consecutive VNS/fMRI scans, each randomly using one of three PWs (130 micros, 250 micros, or 500 micros). The data were analyzed with SPM2.

RESULTS

Global activations induced by PWs 250 and 500 were both significantly greater than that induced by PW 130 but not significantly different from each other. For global deactivation, PWs 130 and 250 were both significantly greater than PW 500 but not significantly different from each other. Regional similarities and differences were also seen with the various PWs.

CONCLUSIONS

The data confirm our hypothesis that VNS at PW 500 globally produces no more activation than does PW 250, and PW 130 is insufficient for activation of some regions. These data suggest that PW is an important variable in producing VNS brain effects.

The effects of pregnancy and estrogen on the expression of calcitonin gene-related peptide (CGRP) in the uterine cervix, dorsal root ganglia and spinal cord

Before parturition the uterine cervix undergoes a ripening process ("softens" and dilates) to allow passage of the fetus at term. The exact mechanism(s) responsible for cervical ripening are unknown, though a role for peptidergic sensory neurons is emerging. Previous work demonstrated that administration of substance P (SP) to ovariectomized rats caused events associated with cervical ripening, that production of SP in cervix-related dorsal root ganglion (DRG) is estrogen responsive, and that release of SP from neurons terminating in the cervix and spinal cord peaks prior to parturition. The present study was designed to test the hypothesis that calcitonin gene-related peptide (CGRP), a neuropeptide co-stored with SP in many sensory neurons, undergoes changes with pregnancy and hormonal environment. Immunohistochemistry, in situ hybridization, reverse transcriptase-polymerase chain reaction (RT-PCR) and radioimmunoassay (RIA) were used to investigate CGRP in L6-S1 DRG, spinal cord and cervix during pregnancy and the role of estrogen in CGRP synthesis. CGRP-immunoreactive primary sensory neurons expressed estrogen receptors (ER-alpha and ER-beta). In the cervix, CGRP concentrations decreased, but in the L6-S1 DRG and the spinal cord segments, CGRP levels increased, with peak effects observed at day 20 of gestation. CGRP mRNA synthesis increased in DRG over pregnancy. Sensory neurons of ovariectomized rats treated with estrogen showed increased CGRP mRNA synthesis in a dose-related manner, an effect blocked by the ER antagonist ICI 182 780. From these results, we postulate that synthesis of CGRP in L6-S1 DRG and utilization in the cervix increase over pregnancy and this synthesis is the under influence of the estrogen-ER system. Collectively, these data are consistent with the hypothesis that CGRP plays a role in cervical ripening and, consequently in the birth process.

Neural pathways mediating vaginal function: the vagus nerves and spinal cord oxytocin

The initial observations, made in our laboratory with Knut Larsson, of the ability of vaginocervical stimulation (VCS) to block withdrawal responses to foot pinch in rats has led to findings of multiple behavioral, autonomic, and neuroendocrine effects of this potent stimulus in rats and also in women. It has led to an understanding of: (1) the neuroanatomical and neurochemical basis of a novel and potent pain-blocking mechanism; (2) likely neuroanatomical pathways mediating both the Ferguson reflex and a specific autonomic response - the pupil-dilating effect of VCS; (3) a role for oxytocin as a putative central nervous system neurotransmitter that stimulates autonomic sympathetic preganglionic neurons within the spinal cord; and (4) a novel pathway that can convey sensory activity from the cervix, adequate to induce orgasm, via the vagus nerves. This latter pathway bypasses the spinal cord and projects directly to the medulla oblongata, and thus can convey genital afferent activity despite complete spinal cord injury at any level.

Estrogen Receptors in the Spinal Cord, Sensory Ganglia, and Pelvic Autonomic Ganglia

Until relatively recently, most studies of the effects of estradiol in the nervous system focused on hypothalamic, limbic, and other brain centers involved in reproductive hormone output, feedback, and behaviors. Almost no studies addressed estradiol effects at the spinal cord or peripheral nervous system level. Prior to the mid-1960s-1970s, few studies examined neural components of reproductive endocrine organs (e.g., ovary or testis) or the genital organs (e.g., uterus or penis) because available data supported endocrine regulation of these structures. Over the last two decades interest in and studies on the innervation of the genital organs have burgeoned. Because of the responsiveness of genital organs to sex steroid hormones, these neural studies seeded interest in whether or not autonomic and sensory neurons that innervate these organs, along with their attendant spinal cord circuits, also are responsive to sex hormones. From the mid-1980s there has been a steady growth of interest in, and studies of the neuroanatomy, neurochemistry, neural connectivity, and neural functional aspects in reproductive organs and the response of these parameters to sex steroids. Thus, with the growth of probes and techniques, has come studies of anatomy, neurochemistry, and circuitry of sex hormone-responsive neurons and circuits in the spinal cord and peripheral nervous system. This review focuses on estrogen receptors in sensory, autonomic, and spinal cord neurons in locales that are associated with innervation of female reproductive organs.

Effect of kappa opioid agonists on visceral nociception induced by uterine cervical distension in rats

Although uterine distension in rats results in an escape reflex, there exists no model of uterine cervical distension (UCD), the pain stimulus during the first stage of labor. The aims of this study were to develop such a model in virgin rats and to test whether peripherally restricted kappa opioid receptor (KOR) agonists (ADL 10-0101, ADL 10-0102, ADL 10-0116) inhibit responses to UCD. Under intravenous (i.v.) pentobarbital and alpha-chloralose anesthesia, fine metal rods were inserted in both uterine cervical osses through a small midline laparotomy. UCD was performed by manual separation of the rods (25-100 g). Single-unit afferent responses in hypogastric nerve or reflex rectus abdominis electromyographic (EMG) activity were determined before and after i.v. KOR agonists. UCD resulted in a stimulus-dependent increase in single-unit afferent activity. Units could be characterized as low threshold (mean threshold 6.6+/-2.7 g), or high threshold (mean threshold 55+/-8.8 g); all were C fibers, all responded to topical bradykinin. ADL 10-0116 (10 mg/kg) reduced the afferent response to UCD. Reflex EMG response occurred over a distension force range of 25-100 g, unaffected by i.v. saline. All three KOR agonists produced a dose-dependent, naloxone-reversible inhibition of the EMG response with a potency relationship of ADL 10-0102 (ED50 0.04 mg/kg)>ADL 10-0101 (ED50 0.65 mg/kg)=ADL 10-0116 (ED50 0.60 mg/kg). These data support the use of acute UCD as a noxious stimulus, inducing afferent and reflex activity. Like other visceral stimuli, UCD is sensitive to inhibition by KOR agonists.

Influence of endometriosis on pain behaviors and muscle hyperalgesia induced by a ureteral calculosis in female rats

Endometriosis and urinary calculosis can co-occur. Clinical studies have shown that both painful and non-painful endometriosis in women are associated with enhanced pain and referred muscle hyperalgesia from urinary calculosis, but the mechanisms underlying this phenomenon are still poorly understood. The aim of this study was to develop an animal model adequate to explore this viscero-visceral interaction in standardized conditions. Using a model of endometriosis previously developed to study reduced fertility and vaginal hyperalgesia, endometriosis (endo) or sham-endometriosis (sham-endo) was induced in rats by autotransplantation of small pieces of uterus (or, for sham-endo, fat) on cascade mesenteric arteries, ovary, and abdominal wall. After the endometrial, but not the fat autografts had produced fluid-filled cysts (3 weeks), urinary calculosis was induced by implanting an artificial stone into one ureter. Pain behaviors were monitored by continuous 24-h videotape recordings before and after stone implantation. Referred muscle hyperalgesia was assessed by measuring vocalization thresholds to electrical stimulation of the oblique musculature (L1 dermatome). The data were compared with previously reported data from rats that had received only the stone. Neither endo nor sham-endo alone induced pain behaviors. Following stone implantation, in endo rats compared to sham-endo and stone-only rats, pain behaviors specifically associated with urinary calculosis were significantly increased and new pain behaviors specifically associated with uterine pathology became evident. Muscle hyperalgesia was also significantly increased. To explore the relationship between the amount of endometriosis and that of ureteral pain behavior, two separate groups of endo rats were treated with either a standard non-steroidal anti-inflammatory drugs (ketoprofen) or placebo from the 12th to the 18th day after endometriosis induction. The stone was implanted on the 21st day. Ketoprofen treatment compared to placebo significantly reduced the size of the cysts and both ureteral and uterine pain behaviors post-stone implantation. The size of the cysts showed a significant linear correlation with the post-stone ureteral pain behaviors. In conclusion, endo increased pain crises and muscle hyperalgesia typically induced by a ureteral calculosis, and the ureteral calculosis revealed additional pain behaviors typically induced by uterine pathophysiology; and this enhancement was a function of the degree of endometriosis. This result closely reproduces the condition observed in humans and could be due to a phenomenon of 'viscero-visceral' hyperalgesia, in which increased input from the cyst implantation sites to common spinal cord segments (T10-L1) facilitates the central effect of input from the urinary tract.

c-Fos expression as endogenous marker of lumbosacral spinal neuron activity in response to vaginocervical-stimulation

The focus of this paper is to describe a method for the simultaneous stimulation of the vagina and uterine cervix and recording of vaginal contractions in the female rat. The influence of the estrous cycle on vaginocervical sensitivity was also investigated. The use of a latex balloon, inflated with water via a syringe and connected to a pressure transducer allowed us to record vaginal contractions, the intensity of which is an index of vaginal sensitivity, and to stimulate those spinal neurons involved in the reflex arc; at the end of the stimulation the deflated balloon was used as a probe to perform a vaginal smear in order to determine the stage of the estrous cycle at the moment of the experiment. Activated neurons were identified by Fos-immunocytochemistry. Light microscope counting of Fos-immunoreactive neurons at different stages of the estrous cycle permitted us to quantify the response to vaginocervical stimulation and to demonstrate that vaginocervical sensitivity changes significantly throughout the estrous cycle. This finding confirms the important role of sex hormones in the modulation and control of the sensitivity of the vagina and cervix, a part of the female genital tract which is implicated in a variety of neuroendocrine, behavioral and neural changes.

Brain (PET) responses to vaginal-cervical self-stimulation in women with complete spinal cord injury: preliminary findings

Our recent research provides evidence that women with complete spinal cord injury (SCI) at the midthoracic level show perceptual responses to vaginal and/or cervical self-stimulation (for example, pain suppression and sexual response, including orgasm). On the basis of studies in laboratory rats, we hypothesized that the vagus nerves provide a sensory pathway from the vagina, cervix, and uterus directly to the brain in women. To test this hypothesis, we performed a PET-MRI study on two women with complete SCI and 1 woman with no injuries. Whereas control foot stimulation of the women with SCI did not activate the somatosensory thalamus, cervical self-stimulation increased activity in the region of the nucleus of the solitary tract, which is the brainstem nucleus to which the vagus nerves project. These preliminary findings suggest that the vagus nerves can convey genital sensory input directly to the brain in women, completely bypassing SCI at any level.

Sensory nerves and neuropeptides in uterine cervical ripening

At the time of parturition (fetal delivery) the uterine cervix must "ripen," becoming soft, pliable, and dilated to accommodate the fetus' delivery. The fundamental processes underlying cervical ripening remain poorly understood. Knowledge that abundant autonomic and sensory nerves supply the uterine cervix, that transection of afferent nerves supplying the cervix blocks parturition, and that some of the changes in the cervix resemble those seen in inflammatory reactions suggests nerves may have a role in the cervical ripening changes. The present study utilized immunohistochemistry, plasma extravasation, and solution hybridization-nuclease protection assay to elucidate the complement of primary afferent nerves and some receptors in the rat cervix during pregnancy, and to determine if they may have roles in the ripening process at term. This study revealed an abundance of nerves associated with the cervical vasculature and myometrial smooth muscle containing immunoreactivity for substance P, calcitonin gene-related peptide, secretoneurin, and nitric oxide synthase throughout pregnancy. Many of these are small unmyelinated capsaicin-sensitive C-fibers. Substance P- (NK1-) and calcitonin gene-related peptide receptors were apparent on uterine cervix vasculature from pregnant, parturient, and postpartum rats. NK1 receptor mRNA was maximal at 20 days of pregnancy. Plasma extravasation of i.v. administered Evans Blue or Monastral Blue was most pronounced at parturition (shortly after NK1 mRNA is maximal); this was similar to plasma extravasation evoked by i.v. administration of substance P or capsaicin-treatment. This study revealed new data about the nervous system of the rat uterine cervix and that these nerves and their transmitters could very well be part of a neurogenic inflammatory process involved in cervical ripening.

Neurogenic female sexual dysfunction: a review

The effect of various spinal lesions on female sexual response has recently been investigated in detail. Studies of women with neurologic disabilities and studies of animal models have provided substantial information regarding the spinal control of sexual responses. In this report, the authors explore findings regarding the neurologic pathways underlying the spinal control of sexual arousal and orgasm. Information available about the effects of multiple sclerosis and various cerebral disorders on female sexual function will also be reviewed, with special attention to areas where further research is needed. Lastly, the current status and techniques available to improve the sexual functioning of women with neurologic disabilities affecting the central nervous system will be reviewed.

Vaginocervical stimulation-induced release of classical neurotransmitters and nitric oxide in the nucleus of the solitary tract varies as a function of the oestrus cycle

The effects of vaginocervical stimulation (VCS) on glutamate (GLU), aspartate (ASP), gamma-aminobutyric acid (GABA), noradrenaline (NA), arginine (ARG) and nitric oxide (NO) (citrulline) release in the nucleus of the solitary tract (nTS) were measured in anaesthetised female rats as a function of the oestrus cycle. During pro-oestrus/oestrus (P/E), but not during met-oestrus/di-oestrus (M/D), VCS significantly increased concentrations of NA, ASP, GLU, NO (citrulline) and GABA, but not ARG. Basal NA concentrations were also increased in P/E. These effects were prevented by bilateral section of either the vagus nerve or pelvic and hypogastric nerves. Vagotomy also significantly decreased basal NO concentrations in M/D and P/E while pelvic and hypogastric nerve section significantly increased GABA concentrations. Our results therefore confirm that the nTS is a relay structure for the visceral afferents sending information from the uterus into the central nervous system. The ability of VCS to trigger classical transmitter release and NO in the female is influenced by the stage of the oestrous cycle and is routed both via the vagus and pelvic/hypogastric nerves.

Evidence that oxytocin is an endogenous stimulator of autonomic sympathetic preganglionics: the pupillary dilatation response to vaginocervical stimulation in the rat

Vaginocervical mechanostimulation (VS) was shown previously to release oxytocin within the spinal cord and to induce pupillary dilatation. In the present study, (a) injection of oxytocin directly to the spinal cord (10 or 25 microg intrathecally [i.t.] in 5 microl saline) induced pupillary dilatation when observed 1 min after the end of the injection and (b) injection of an oxytocin receptor antagonist ([d(CH2)5-Tyr (Me)2-Orn8]-Vasotocin [OTA]; 25 microg i.t. in 5 microl saline) significantly attenuated the pupillary dilatation response to VS, when VS was applied 3 min after the end of the injection. Since activation of autonomic sympathetic preganglionic neurons in the thoracic spinal cord produces pupillary dilatation, we propose that oxytocin is a central nervous system neurotransmitter that stimulates these neurons directly, or perhaps indirectly, and thus is a mediator of VS-produced pupillary dilatation.

The estrous cycle affects pseudorabies virus (PRV) infection of the CNS

Previous work had suggested that mucosal immunity may be affected by the stage of the estrous cycle. Here, susceptibility to a neurotropic virus infection at different stages of the estrous cycle was assessed in a rodent model after direct injection of the virus into visceral organs. In the first two experiments, female Sprague-Dawley rats were infected with pseudorabies virus (PRV, Bartha's K-strain) by injection into either the cervix or the kidney after monitoring their estrous cycle. After either 4- or 5-day survival period post-infection, the rats were euthanized by transcardially perfusion and peripheral and central nervous system tissues were removed for immunocytochemical staining. The number of infected neurons was counted in various regions. Statistical analysis revealed that: (1) the number of infected cells in the sympathetic or parasympathetic ganglion, or the dorsal root ganglia was not affected regardless of the stage of the estrous cycle after cervix injection with PRV; (2) in contrast, the number of infected neurons in the spinal cord was affected significantly by the stage of the estrous cycle during viral infection of the cervix; (3) after kidney infection, the number of infected neurons found within the spinal cord or dorsal root ganglia varied significantly across the estrous cycle. In both cases, animals infected in proestrus or estrus had fewer infected neurons than animals infected in diestrus I or diestrus II (proestrous and estrous animals had less than 20% of infected cells found in diestrus I or diestrus II rats). In the third experiment, older, persistent estrous or persistent diestrous rats were infected by kidney injection and given a 4-day survival period, prior to virus isolation from lower thoracic spinal cord. Animals in persistent estrous had significantly less virus per gram of tissue than the persistent diestrous rats. These data suggest that the CNS of animals in proestrus or estrus is less susceptible to PRV infection compared to animals in either diestrus I or diestrus II. Because estrogen replacement therapy is known to restore some immune functions during reproductive ageing, it is speculated that plasma estrogen levels modulate the central nervous system's susceptibility to viral infections.

Influence of estrous cycle on vaginocervical sensitivity: a fos-immunohistochemical study of lumbosacral spinal cord

Expression of c-fos in L(5)-S(1) spinal segments in response to mechanical vaginocervical stimulation was investigated in both cycling and ovariectomized females. The aim of this paper was to verify the influence of estrous cycle on females genital tract sensitivity using immunodetection of a neural activity endogenous marker. The results indicate that lumbosacral spinal Fos-labeling was highly increased in vaginocervical stimulated rats relative to control, and labeled neurons were present more intensively in the dorsal horn in comparison to other spinal areas. Significant differences in Fos-labeling were observed according to the estrous cycle stage at which the stimulation was applied. In estrous females, the response was greater than that obtained at diestrous and much greater than the response of proestrous females. The spinal Fos-labeling of ovariectomized females is equivalent to that of diestrous females. These results give evidence that the vaginocervical induced expression of c-fos is modulated by cyclic changes in circulating sex hormones, whereas results observed in ovariectomized females indicate the likely involvement of other mechanisms independent of ovarian hormones.

Sex-related hormonal influences on pain and analgesic responses

Considerable evidence indicates sex-related differences in pain responses and in the effectiveness of various analgesic agents. Specifically, females are at greater risk for experiencing many forms of clinical pain and are more sensitive to experimentally induced pain relative to males. Regarding analgesic responses, nonhuman animal studies indicate greater opioid analgesia for males, while a limited human literature suggests the opposite. Though the mechanisms underlying these effects remain unclear, the influence of gonadal hormones on nociceptive processing represents one plausible pathway whereby such sex differences could emerge. The present article reviews the complex literature concerning sex steroid effects on pain responses and analgesia. First, nonhuman animal research related to hormonal effects on nociceptive sensitivity and analgesic responses is presented. Next, human studies regarding gonadal hormonal influences on experimental pain responses are reviewed. Several potential mechanisms underlying hormonal effects on nociceptive processing are discussed, including hormonal effects to both peripheral and central nervous system pathways involved in pain transmission. Finally, based on these findings we draw several conclusions and make specific recommendations that will guide future research as it attempts to elucidate the magnitude and importance of sex-related hormonal effects on the experience of pain.

Neuronal activity in female rat preoptic area associated with sexually motivated behavior

Single unit activities were recorded from 31 neurons in the preoptic area (POA) of female rats engaging in sexual interactions. Concurrent videotape recordings were used to establish a relationship between neuronal activity and particular behavioral events. In 14 of the 31 neurons, the firing rate changed in association with bouts of sexual activity. The remaining 17 fired with more variability regardless of episodes of sexual interactions. Peri-event histograms identified four types of neurons: type 1 (n=4) increased their firing rate when the female rats initiated proceptive behavior; type 2 (n=4) showed a brief activation when the male mounted; type 3 (n=4) fired in response to intromission, and type 4 (n=2) were inhibited prior to and throughout the display of lordosis reflex. Type 1 neurons fired at significantly higher rates during the solicitatory period, from the initiation of solicitatory locomotion to the male mounts. Their activity was suppressed when the males mounted successfully with intromission. Types 1-3 neurons were recorded from the transitional region between the medial and lateral POAs. Type 4 neurons were located more medially in the medial POA. Systemic injection of pimozide, a dopamine receptor blocker, diminished firing in type 1 neurons and abolished proceptivity. The firing pattern in type 1 neurons appeared to embody the motivational state of the animal with an implication for a consummatory value of penile intromission. Visceral or somatosensory inputs may be responsible for short bursts in types 2 and 3 neurons. Type 4 neurons behaved exactly as if they inhibit the execution of the lordosis reflex. The results showed separate sets of POA neurons each specifically associated with proceptive and receptive components of female rat sexual behavior.