The spinal and peripheral innervation of the clitoris and vagina are fairly well understood. However, little is known regarding supraspinal control of these pelvic structures. The multisynaptic tracer pseudorabies virus (PRV) was used to map the brain neurons that innervate the clitoris and vagina. To delineate forebrain input on PRV-labeled cells, the anterograde tracer biotinylated dextran amine was injected in the medial preoptic area (MPO), ventromedial nucleus of the hypothalamus (VMN), or the midbrain periaqueductal gray (PAG) 10 days before viral injections. These brain regions have been intimately linked to various aspects of female reproductive behavior. After viral injections (4 days) in the vagina and clitoris, PRV-labeled cells were observed in the paraventricular nucleus (PVN), Barrington’s nucleus, the A5 region, and the nucleus paragigantocellularis (nPGi). At 5 days postviral administration, additional PRV-labeled cells were observed within the preoptic region, VMN, PAG, and lateral hypothalamus. Anterograde labeling from the MPO terminated among PRV-positive cells primarily within the dorsal PVN of the hypothalamus, ventrolateral VMN (VMNvl), caudal PAG, and nPGi. Anterograde labeling from the VMN terminated among PRV-positive cells in the MPO and lateral/ventrolateral PAG. Anterograde labeling from the PAG terminated among PRV-positive cells in the PVN, ventral hypothalamus, and nPGi. Transynaptically labeled cells in the lateral hypothalamus, Barrington’s nucleus, and ventromedial medulla received innervation from all three sources. These studies, together, identify several central nervous system (CNS) sites participating in the neural control of female sexual responses. They also provide the first data demonstrating a link between the MPO, VMNvl, and PAG and CNS regions innervating the clitoris and vagina, providing support that these areas play a major role in female genital responses.
- medial preoptic area
- ventromedial hypothalamus
- periaqueductal gray
- clitoris vagina
- pseudorabies virus
numerous anatomic, physiological, and behavioral studies have been conducted to delineate the essential neural substrates that mediate female reproductive behavior (13, 15, 56, 58, 60, 70–72, 83, 84). These studies, conducted primarily in rodents, have identified several central nervous system (CNS) regions involved in the sensory, autonomic, and/or motor aspects of the lordosis reflex, a receptive behavior essential for vaginal penetration and pregnancy (e.g., see Ref. 70). More recent studies examining proceptive or solicitation behavior are being used to identify neural circuits involved in pacing behavior (21, 25). Together, these studies have identified at least three supraspinal regions essential to various aspects of female reproductive behavior, including the ventromedial nucleus of the hypothalamus (VMN), the medial preoptic area (MPO), and the midbrain periaqueductal gray (PAG). The VMN is considered an integral component of the lordosis reflex; stimulation of the VMN in estrogen-primed animals facilitates the display of the lordosis reflex, and lesions of this region significantly disrupt it (56, 68–70). In contrast to the VMN, bilateral lesions of MPO increase the occurrence of lordosis (e.g., 31, 64, 79), whereas stimulation of the MPO attenuates the display of the posture (57, 70). The MPO is also identified as a crucial area for the mediation of female pacing behavior (25, 102), and activation of the MPO results in increased blood flow to the vagina and increases vaginal wall tension (24). Both the VMN and MPO send descending projections to the PAG (26, 41, 52, 63, 69), and the PAG itself has been shown to play a facilitatory role in female reproductive behavior (13, 19, 46, 58, 84, 85, 89). The PAG sends dense projections to the nucleus paragigantocellularis (nPGi), which subsequently projects to motoneurons in the thoracolumbar spinal region that innervate the axial musculature, as well as to lumbosacral neurons that innervate the pelvic organs (17, 18, 27, 29, 51, 61). The PAG also receives direct inputs from the lumbosacral spinal cord (34, 36, 37, 62, 96) and may function to integrate ascending and descending information from multiple brain regions related to sexual behavior.
Although the MPO, VMN, and PAG have been implicated in some aspects of female sexual behavior, the role of these regions in regulating genital arousal and sexual climactic-like responses are unknown. Vaginal vasocongestion, muscle contractions, and clitoral engorgement occur during genital arousal and sexual climax (5, 24, 35, 43). However, the CNS pathways that mediate these responses remain unknown. The MPO, VMN, and PAG do not project directly to spinal regions that innervate spinal motor and preganglionic neurons involved in sexual responses. Therefore, modulation of genital responses by these regions must involve a multisynaptic pathway that relays in the brain. To delineate the anatomic pathways linking regions implicated in female sexual function (MPO, VMN, and PAG) with the descending circuits that innervate the vagina and clitoris, the transneuronal tracer pseudorabies virus (PRV) was injected in the vagina and clitoris in combination with injections of the anterograde tracer biotin dextran amine in the MPO, VMN, or PAG. The results of these studies identified several supraspinal-spinal circuits that may provide coordinated sensory, autonomic, and hormonal modulation over female sexual responses.
METHODS AND MATERIALS
Viral and Anterograde Tracer Injections
All methods were performed in strict compliance with the Institutional Animal Care and Use Committee at the University of Maryland, Baltimore. Female Sprague-Dawley rats (250–350 g; Zivic Miller) were deeply anesthetized with chloral hydrate (4% wt/vol ip) and placed in a stereotaxic apparatus. The skull was adjusted so that bregma and lambda were on a horizontal plane. A small craniotomy was made, and a glass micropipette (25–50 μm) filled with biotinylated dextran amine (BDA; 10% solution, 10,000 mol wt; Sigma Chemicals) was lowered in one of the following regions: MPO (bregma −0.15 mm, 0.1 mm lateral, and 7.1 mm ventral; n = 8), VMN (bregma −3.5 mm, 0.75 mm lateral, and 9 mm ventral; n = 5), or PAG (lambda +1.20, 1.20 lateral, and −3.5 mm ventral; n = 8). For the PAG injections, the manipulator was placed at a 7.5° angle to avoid the sagittal sinus. After BDA injection (100 nl), the glass pipette remained in place for 10 min before removal to prevent backflow up the injection tract.
Later (10 days), animals were reanesthetized with chloral hydrate, and PRV [2–3 × 107 plaque-forming units/ml (74), a gift from Dr. L. Enquist] was injected in the clitoris and vagina using a 25-gauge Hamilton syringe. Each female received a single injection in the clitoris (0.5–1 μl) and two injections in the ventral region of the vagina (∼1 cm from the vaginal orifice, 1 μl each). The needle remained in place for 1 min postinjection. After withdrawal of the needle, pressure was applied to the injection site using a cotton tip applicator to prevent leakage of the virus to the surrounding muscle. Both organs were injected to label the majority of CNS neurons that are engaged during genital arousal and climactic-like responses (5, 26, 36, 45).
At the end of the survival period, animals were given an overdose of pentobarbital sodium and perfused transcardially (descending aorta clamped) with 250 ml of 0.9% sodium chloride containing 2% sodium nitrite solution followed by 300 ml of 4% paraformaldehyde in 0.1 M phosphate buffer containing 2% acrolein (Polyscience). A final rinse with the sodium chloride/sodium nitrite solution was used to remove any residual acrolein from the animal. Brains were removed and placed in 30% sucrose solution until sectioned. Sections were cut using a freezing microtome at 25 μm, collected in cryoprotectant-antifreeze solution, and stored at −20°C until immunocytochemical processing.
A 1:6 series through the rostrocaudal axis of the brain was processed for BDA and PRV immunoreactivity. BDA was always visualized first as a black reaction product using nickel-enhanced diaminobenzidine (DAB); PRV was always visualized second as a brown reaction product using nonenhanced DAB (see below). Spinal cord sections (1:4 series) through the L6/S1 level were processed for PRV immunoreactivity.
Sections were removed from the cryoprotectant-antifreeze solution, rinsed extensively in potassium PBS (0.1 M KPBS, pH 7.4), and then reacted for 15 min in 1% sodium borohydride to remove excess aldehydes. Sections were incubated in goat anti-biotin antibody (1:60,000 dilution; Vector Laboratories, Burlington, CA) in KPBS containing 0.4% Triton X for 1 h at room temperature followed by 48 h at 4°C. After being rinsed, the tissue was incubated for 1 h in rabbit anti-goat antibody (1:600 dilution; Vector Laboratories), followed by a 1-h incubation in avidin-biotin peroxidase complex (1:10; ABC Elite Kit; Vector Laboratories). After rinsing, BDA was visualized as a blue-black reaction product using a nickel-sulfate intensified 3,3′-DAB solution containing 0.08% hydrogen peroxide in 0.175 M sodium acetate buffer. The reaction product was terminated after 7–25 min by rinsing in sodium acetate buffer.
Cells containing the PRV were identified using a rabbit anti-PRV antibody (1:200,000 dilution, kindly donated by Dr. Lynn Enquist). Sections were rinsed several times in KPBS and then incubated for 1 h in primary antibody directed against PRV at room temperature followed by 48 h at 4°C. Secondary and ABC reactions were as stated above. PRV was visualized as a brown reaction product using a 3,3′-DAB solution containing 0.08% hydrogen peroxide in Tris buffer. Sections were mounted on gelatin-subbed slides and covered with a cover slip using DPX.
The distribution of PRV neurons was plotted using a Nikon Drawing Tube attached to a Nikon Optiphot microscope. Plots were imported in the computer using the Waccom Drawing Tablet and Adobe Illustrator 10.0 software. Color photomicrographs were generated using a Synsys camera attached to a Nikon Eclipse E800 microscope. Images were captured using Adobe Photoshop 7.0. Alterations to images were strictly limited to enhancement of brightness/contrast.
Distribution of PRV-Labeled Neurons in the Spinal Cord
After a 4-day survival period, spinal PRV labeling was confined to the region containing the sacral parasympathetic nucleus and dorsal gray commissure with an occasional PRV-positive cell in the dorsal horn and intermediate gray (Fig. 1). After 5 days survival, PRV-positive neurons were widely distributed in the spinal cord (Fig. 1). Regions containing PRV labeling included the superficial (laminae I and II), medial, and lateral regions of the dorsal horn, lateral gray, including the sacral parasympathetic nucleus, the dorsal gray commissure and intermediomedial nucleus, the intermediate gray (laminae V, VI, and VII), and ventral horn.
Distribution of PRV-Labeled Neurons in the Brain
Injections of PRV in the clitoris and vagina resulted in viral positive cells in selective regions of the brain. After a 4-day survival period, PRV-containing cells (Fig. 1) were primarily observed in the dorsolateral region of the paraventricular nucleus (PVN; Fig. 2A), Barrington’s nucleus, A5 (Fig. 2B), rostral ventromedial medulla (RVM), including the raphe magnus and the ventral gigantocellular reticular formation, and the ventral lateral medulla primarily in the nPGi (Fig. 2C). In addition, PRV-positive cells were occasionally localized in the lateral hypothalamus (LH; dorsal and lateral to the fornix), caudal ventrolateral PAG, raphe pallidus, and raphe obscurus (Fig. 1).
After 5 days’ survival, more PRV-containing cells were observed in the areas described above and additional brain regions (Fig. 1). In the forebrain, numerous PRV-positive cells were noted throughout the LH, including PRV cells that were clustered around the fornix and in the parvocellular PVN. In addition, PRV-positive cells were present in the MPO, including the medial preoptic nucleus (MPN), retrochiasmatic area, and ventrolateral VMN (VMNvl). Fewer PRV cells were observed scattered in the ventral portion of the bed nucleus of the stria terminalis, zona inercta, caudal medial amygdala, and dorsal and ventral hypothalamic area. In the midbrain and pons, numerous PRV-positive cells were found in the dorsal, lateral, and ventrolateral PAG, Barrington’s nucleus, A7/Kolliker-Fuse region, lateral lemniscus, and the pontine reticular formation, including the area containing the subcoeruleus. In the medulla, PRV-containing neurons were located in A5, RVM, nPGi, and nucleus tractus solitarii (NTS). PRV labeling in the locus coeruleus (LC) was always inconsistent.
MPO Output on PRV-Labeled Cells
MPO injection sites spread from 0.3 to 1.4 mm caudal to bregma and did not spread in the lateral preoptic area. The data are described from analysis of three cases that had overlapping injection sites that were centered 0.8 mm caudal to bregma (Fig. 3A). BDA injections resulted in dense anterograde fiber labeling similar to that previously described (52, 63, 77, 78, 90, 91). Briefly, ascending BDA fibers ran dorsal to the MPO and innervated the bed nucleus of the stria terminalis. BDA fibers in the ventral hypothalamus coursed dorsal to the optic tract and innervated the caudal medial amygdala. A few BDA fibers and varicosities were found in the dorsal and medial PVN. A dense projection of BDA fibers was observed in the VMNvl. Descending fibers coursed through the posterior hypothalamus and periventricular fiber system. In the brain stem, BDA fibers were localized in the ventral tegmental region and dorsal to the substantia nigra. Numerous fibers were found running vertically through the medial and central PAG, and BDA fibers and terminals innervated the caudal dorsal, lateral, and ventrolateral PAG. A dense projection from the MPO was observed in Barrington’s nucleus. A moderate projection from the MPO was noted in the dorsal raphe and RVM. A few scattered fibers were always present in the ventral part of the LC, subcoeruleus, A5, nPGi, and NTS.
MPO output on PRV-labeled cells.
In the hypothalamus, numerous BDA fibers were intermingled with PRV-positive cells in the dorsal, lateral, and posterior hypothalamic areas. A close association between BDA fibers and varicosities and PRV-labeled neurons was noted in the perifornical region of the LH, dorsal PVN, and VMNvl (Fig. 3B). In the rostral PAG, BDA fibers were located primarily medial to the PRV-labeled cells. More caudally in the PAG, MPO projections preferentially terminated among PRV-labeled cells in the lateral and ventrolateral PAG, in Barrington’s nucleus, and the subcoeruleus region. Although fewer BDA fibers projected to the A5, nPGi (Fig. 3C), and RVM (Fig. 3D), BDA fibers were consistently found in close apposition to PRV cells in these regions. No association of BDA fibers and PRV-labeled cells was noted in the LC, raphe obscurus, or raphe pallidus.
VMNvl Output on PRV-Labeled Cells
Projections from the VMN were mapped in two animals that had similar injection sites in the ventral part of the VMN (∼2.4–3.8 mm caudal to bregma) that did not diffuse in the more dorsal VMN (Fig. 4A). BDA-labeled ascending fibers terminated primarily in the bed nucleus of stria terminalis and MPO, including the MPN and the ventral preoptic area. BDA fibers were also found in the medial amygdala, LH, retrochiasmatic area, and paraventricular thalamic nuclei. Only a few BDA fibers were found in the ventral PVN near the third ventricle. These studies confirm previous mapping of VMN outputs (45, 87). The descending fiber system was not as pronounced as the ascending fiber system. Fibers traveled close to the third ventricle in the periventricular fiber system. A few fibers were also noted in the ventral/posterior hypothalamus. In the midbrain, BDA-labeled fibers terminated densely in the dorsal, lateral, and ventrolateral PAG. More caudally, BDA labeling was found in the lateral dorsal tegmental region and the ventral tegmental area. Occasional BDA fibers were found in Barrington’s nucleus, RVM, and nPGi.
VMNvl output on PRV-labeled cells.
BDA fibers and PRV-positive cells consistently overlapped in the MPO (Fig. 4C), including the MPN, and the LH. A moderate overlap was observed in the ventral preoptic area. In the midbrain, VMNvl fibers terminated heavily among PRV-positive cells in the lateral and ventrolateral PAG and Barrington’s nucleus (Fig. 4B). A minor overlap was also present in the retrochiasmatic area and RVM, including the raphe magnus and parapyramidal region of the medulla. No overlap of BDA fibers and PRV-labeled cells was found in other brain regions, including the PVN, arcuate nucleus, amygdala, A5, and nPGi.
PAG Output on PRV Cells
In three animals, the BDA spread through the dorsal, lateral, and ventral PAG but was confined to one side; in two animals the BDA injection site was more localized to the dorsal and dorsolateral PAG and did not spread to the ventrolateral and lateral portions of the PAG. All injection sites mapped were centered ∼6.0–7.6 mm caudal to bregma (Fig. 5A) and resulted in anterograde labeling within the lateral preoptic area, ventral hypothalamic area, LH, and zona incerta. Fibers were also noted in the anterior and dorsal hypothalamus, and many BDA fibers ran dorsal to the optic tract coursing toward the amygdala. A small innervation of BDA fibers was present in the MPO, including the MPN and the dorsal PVN. In the brainstem, heavy anterograde labeling was observed in Barrington’s nucleus, subcoeruleus, A5, ventrolateral medulla, including the nPGi, and RVM. A small projection to LC and NTS was also observed.
Injections that were confined to the caudal dorsal/dorsolateral PAG (Fig. 5A) produced a similar labeling pattern to that described above; however, there were some differences. BDA fibers were primarily localized in the lateral preoptic area, LH, dorsal hypothalamus, zona incerta, dorsal to the optic tract, and anterior hypothalamic area. Very few fibers were present in the MPO and PVN. BDA fibers were also observed in the ventral tegmental area and dorsal to the substantia nigra and geniculate nucleus. Similar to that noted above, descending BDA fibers were present primarily in the A5, RVM, and nPGi. Projections from the dorsal/dorsolateral PAG did not result in a large innervation of the sexual dimorphic MPO, subcoeruleus region, or Barrington’s nucleus, suggesting that these areas receive input primarily from the ventrolateral PAG. These studies confirm previous neuroanatomic studies reporting specific topographically defined PAG inputs and outputs (1, 2, 3, 6).
PAG output on PRV cells.
Anterograde labeling from the PAG terminated among PRV-labeled cells in the lateral and ventral hypothalamic area. Although few PRV cells were found in the lateral preoptic area, they were always closely apposed to BDA fibers (Fig. 5, B and C). A minor overlap of BDA fibers and PRV-positive cells was observed in the caudal PVN. The major overlap in the brain stem was found in Barrington’s nucleus, subcoeruleus, A5, RVM (Fig. 5D), and nPGi (Fig. 5D). In some animals, a few PRV-labeled cells and BDA fibers were observed in the LC.
Injections in the caudal dorsal/dorsolateral PAG resulted in a large overlap of BDA fibers and PRV-positive cells in the LH and the dorsal and posterior hypothalamus. A consistent overlap was also found within the lateral preoptic area with a smaller overlap in the caudal PVN. In the brainstem, a large population of PAG output fibers terminated heavily among PRV neurons in the RVM, nPGi, and A5. However, anterogradely labeled PAG fibers primarily ran medial to PRV-positive cells in Barrington’s nucleus.
Transneuronal tracing with PRV was used to identify supraspinal regions involved in the central regulation of the clitoris and vagina that may be associated with sexual responses. Anterograde tracing from the MPO, VMNvl, and PAG, in combination with PRV, was used to determine whether neurons transynaptically labeled from the sexual organs receive preferential inputs from brain regions previously shown to mediate various aspects of female reproductive behavior. The present studies provide the first data demonstrating a direct link between the MPO, VMNvl, and PAG with CNS regions innervating the clitoris and/or vagina, providing support that these areas play a major role in female genital responses. The present results also demonstrate that several brain sites, including the LH, PVN, Barrington’s nucleus, A5, RVM, and nPGi, provide input to the clitoris and/or vagina and receive input from more than one brain site involved in sexual behavior. These brain regions may participate in the integration of hormonal and sensory signals related to female arousal and muscle responses that accompany sexual climactic-like reflexes.
PRV Labeling in the Brain
The retrograde transynaptic tracer (PRV) was used to identify CNS regions that innervate the clitoris and/or vagina (7–9, 74, 79). These data confirm and extend previous studies describing CNS pathways that innervate female pelvic organs. Brain areas labeled with PRV after a 4-day survival time included the dorsolateral PVN, Barrington’s nucleus, A5, raphe magnus, and ventrolateral medulla, suggesting that these regions make direct contact with spinal efferent pathways mediating genital organ function. These results confirm anatomic tracing studies using conventional tracers showing that these regions provide direct input to parasympathetic and sympathetic preganglionic regions, motoneurons, and/or interneurons in the lumbosacral spinal cord that are part of the circuit regulating sexual reflexes (12, 18, 27, 28, 44, 45, 51, 52, 59, 100). Brain regions that were only labeled after a 5-day survival included the bed nucleus of the stria terminalis, MPO, medial amygdala, retrochiasmatic area, VMNvl, dorsal and lateral PAG, and A7 and most likely represent sites projecting to the brain regions that innervate spinal circuits that regulate the clitoris and/or vagina.
The majority of transynaptically labeled neurons was found in brain regions previously shown to be involved in the descending innervation of the back muscles, uterus, clitoris, and cervix (17, 42, 51, 67). However, some differences were found in the present study compared with previous viral tracing studies of the reproductive organs. Transynaptically labeled neurons were not observed in the VMNvl after PRV injections in the clitoris (50), suggesting that the PRV-positive neurons in the VMNvl in the present study may have been labeled from the vagina, although further studies examining vaginal projections are required to confirm this hypothesis. Injections of PRV in the corpus cavernosum, prostate, or perineal muscles in the male rat did not result in PRV-labeled neurons in the VMNvl, even when many forebrain structures were labeled (54, 55, 65). Therefore, PRV labeling of the VMNvl appears to be specific to females. The dorsal motor nucleus of the vagus was labeled after injections of PRV in uterine cervix but not labeled in the present study (42, 66, 67); labeling of this structure after uterine, but not clitoral and/or vaginal injections, supports the anatomic and physiological studies demonstrating vagal input to the uterus (32, 66).
Several brain regions labeled with PRV in the present study have been shown previously to be activated during female sexual behavior. Vaginocervical stimulation, induced by artificial mechanical stimuli or during mating, resulted in c-fos expression in the bed nucleus of the stria terminalis, medial amygdala, MPO, PVN, LH, PAG, and nPGi (21, 72, 73, 75, 83, 94, 97). Further studies examining the specific role of each of the brain sites are required to understand their contribution to specific aspects of sexual function.
Brain Areas Containing PRV Neurons that Receive Multiple Inputs
In the forebrain, PRV neurons were consistently found in the LH in close association with outputs from the MPO, VMNvl, and the PAG. The densest overlap between anterograde labeling and PRV-positive cells was within the caudal LH, surrounding the fornix. The LH is involved in visceral sensory information, somatomotor control, feeding behavior, and wakefulness (11, 22, 38, 76). The LH may also be important for some aspects of male sexual behavior. For example, serotonin released in the LH during ejaculation appears to regulate the postejaculatory refractory period, and injection of selective serotonin reuptake inhibitors in the LH delays copulation and ejaculation (33, 47, 48). In both males and females, neurons in the LH are activated with sexual behavior (21, 72, 73, 75, 83, 94). No direct evidence for a role of the LH in female sexual behavior has been documented to date. However, we propose that the LH may integrate hormonal, sensory, and autonomic inputs to modulate the arousal level of the animal during reproductive behavior. This hypothesis is based on several factors: 1) the dense PRV labeling in the LH from the clitoris and/or vagina, 2) inputs from the MPO, VMNvl, and PAG that terminated in close apposition to the PRV-labeled cells in the LH, and 3) the role of the LH in sleep/wakefulness.
Transynaptically labeled PRV neurons that received inputs from both the MPO and the VMNvl were primarily localized to the lateral and ventrolateral PAG, confirming previous reports that the MPO and the VMNvl project to specific regions of the PAG (41, 53, 63, 77, 87). The lateral and ventrolateral PAG have long been implicated in lordosis (14, 59, 84, 85), and previous studies using PRV injected in the clitoris, uterus, or back muscles reported similar labeling of ventrolateral PAG neurons at early survival times (17, 51, 67). Very few neurons in the lateral PAG project directly to the spinal cord (52, 61); therefore, the majority of PRV-labeled neurons in the PAG were most likely transynaptically labeled from the ventral medulla, including the nPGi and raphe magnus, regions that show a dense direct spinal projection using conventional anatomic tracing techniques (27, 28, 44, 45, 52, 63, 95, and Loyd D and Murphy AZ, unpublished observation). Interestingly, anterograde labeling from the MPO and VMNvl was localized predominantly in PAG regions containing PRV-positive cells, suggesting a direct MPO/VMN-PAG-nPGi-spinal cord circuit that modulates female sexual responses. Some PAG areas containing PRV-positive cells also receive direct inputs from the lumbosacral spinal cord (34, 62, 96), suggesting a direct feedback circuit for the coordination of somatosensory input with descending modulatory input from the forebrain.
PRV-positive cells in the PVN, particularly the parvocellular region, received moderate inputs from the MPO and the PAG. PVN neurons are consistently labeled after injections of PRV in the reproductive organs (42, 50, 55, 67) and are also labeled after injection of PRV in the heart and kidney, etc. (e.g., 88, 92). The parvocellular neurons have long been linked to cardiovascular regulation, and these neurons have also been shown to regulate other autonomic functions. PVN neurons are activated during sex (e.g., 21, 72, 73, 75) and may be involved in the increased circulating levels of oxytocin reported during sexual behavior (10, 22). The PVN is also reportedly activated with orgasm in women with spinal cord injury (37). The firing rate of PVN neurons increases with uterine distension, which may account for the release of circulating hormones associated with pregnancy and parturition. Oxytocin PVN neurons project directly to the lumbosacral spinal cord (86, 98, 99, 100) and are transneuronally labeled after injections of PRV in the vagina, further suggesting the PVN may be directly involved in the regulation of genital organ function.
Barrington’s nucleus also received innervation from all three sources. The primary role of Barrington’s nucleus is to regulate continence and voiding (4, 93). Because this basic function would interfere with the process of sexual behavior, it is likely that overlapping pathways function to coordinate micturition and sexual behavior. Further studies are required to confirm this observation and to address the role of Barrington’s nucleus in female sexual responses. Other areas that project to the clitoris/vagina and receive inputs from the MPO, VMNvl, and PAG include the ventromedial and ventrolateral medulla. Neurons in the ventrolateral medulla, including the nPGi, mediate contractions of the skeletal muscles of the back, respond to genital stimulation, and regulate the descending inhibition of the urethrogenital reflex (15, 30, 59, 70, 80, 81, 103). The raphe magnus, and its descending projections to the dorsal horn of the spinal cord, constitute the endogenous descending analgesia circuit; therefore, forebrain and PAG input to the raphe magnus may serve to increase somatosensory thresholds during reproductive behavior (16, 39, 41, 101).
Transneuronal tracing with PRV was used to identify brain regions involved in the regulation of clitoral and vaginal responses during sexual arousal and climactic-like responses. The results of these studies are the first to link input from the MPO, VMNvl, and PAG with neurons transynaptically labeled from the clitoris and vagina and establish a potential circuit for the elaboration of female sexual responses. Input from the MPO, VMN, and PAG may also allow for the coordination of hormonal input with sensory, autonomic, and motor output. Further studies examining the specific contribution of these brain circuits and their neurotransmitters in regulating female sexual responses are required.
This work was supported by National Institutes of Health Grants NS-39166 (to L. Marson) and MH-59197 and DA-16272 (to A. Z. Murphy).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
- Copyright © 2006 the American Physiological Society