Increases in Clitoral and Vaginal Blood ¯Ow Following Clitoral and Pelvic Plexus Nerve Stimulations in the Female Rat

Home , Vaginal lubrication, Vasocongestion

Increases in clitoral and vaginal blood ¯ow following clitoral and pelvic plexus nerve stimulations in the female rat

P Vachon1, N Simmerman1, AR Zahran1 and S Carrier1

1Lady Davis Institute of the Jewish General Hospital, Department of Medicine, McGill University, Montreal, Quebec, Canada H3T 1E2

The objective of this present study is to establish a model in the rat for the study of female clitoral and vaginal vascular changes during sexual excitation. A laser Doppler was used to measure blood ¯ow changes following clitoral and pelvic plexus nerve stimulations. Results show an increase in clitoral blood ¯ow following clitoral nerve (df1 12, df2 108, F 21.4, P < 0.001) and pelvic plexus nerve stimulations (n 3). A vaginal blood ¯ow increase is also observed following pelvic plexus nerve stimulations (df1 12, df2 108, F 4.75, P < 0.001). The female rat can therefore be used as a model for the study of the physiology, pharmacology and sexual dysfunction relating to blood ¯ow in clitoral and vaginal tissue. International Journal of Impotence Research (2000) 12, 53±57.

Keywords: rat; female; blood ¯ow; laser Doppler; nerve stimulation

Introduction Materials and methods

Arousability in female sexuality is de®ned as the Ten female Sprague-Dawley rats (Charles River, ability to attain and maintain a lubrication-swelling Canada) weighing between 250±300 g were used in response during sexual activity.1 Sexual vascular this study. Under general anesthesia with pentobar- responses in females include erection of the clitoris bitol (60 mg=kg, MTC Pharmaceuticals, Cambridge, and congestion of the lower third of the vagina with Ontario) the dorsal clitoral nerves were isolated transudation of ¯uids on the vaginal surface.2 following a mid-line incision of the skin immedi- Defects in neurovascular mechanisms that accom- ately cranial to the clitoris. A laser Doppler probe pany arousability will lead to sexual dysfunction (diameter 0.5 mm, Transonic Systems, Ithaca, New and consequently, to somatic complications and York) was placed on the surface of the clitoral glans psychosocial burden in women. to measure capillary blood ¯ow. The re¯ected laser The objective of this present study is to establish light is registered on a photodetector and is propor- a model in the rat for the study of female clitoral and tional to red blood cell velocity.3 The probe was vaginal vascular changes during sexual excitation. A attached to a ¯owmeter (ALF-21, Transonic Sys- laser Doppler was used to measure blood ¯ow tems, Ithaca, New York). The laser Doppler ¯ow- changes following clitoral and pelvic plexus nerve meter was calibrated to an internal standard. A drop stimulations. Data gathered from these experiments of mineral oil was placed at the tip of the probe for will establish baseline values for future physiologi- proper blood ¯ow readings in units of ml=min=100 g cal and pharmacological studies of sexual arousal as of tissue. Stimulations of the dorsal clitoral nerve as well as pathological changes occurring in female well as the ventral aspect of the neuro-vascular sexual dysfunctions. bundle (Figure 1) leading to the clitoris, were done while recording capillary blood ¯ow changes with the laser doppler. Electrostimulation was performed with a stainless-steel bipolar hook electrode. Rec- tangular monophasic pulses were delivered by a signal generator (MacLab 8S stimulus isolator, Correspondence: Dr P Vachon, Lady Davis Institute for ADinstruments, Australia). Stimulus parameters Medical Research, Sir Mortimer B. DavisÐJewish General were a current of 2 mA, frequency 20 Hz, pulse Hospital, Pascal Vachon, 3755, ch. CoÃte Ste-Catherine, Montreal, Quebec H3T 1E2, Canada. width 0.2 ms and a duration of 5 s. Received April 23 1999; revised May 24 1999; accepted 30 Following the recordings from clitoral tissue, the July 1999 abdomen was incised and the symphysis pubis was Increases in clitoral and vaginal blood ¯ow P Vachon et al 54 fashion starting with the nerves innervating the external sphincter of the bladder. Upon stimulation, a contraction of the sphincter and the bladder could be clearly observed. Responses were recorded when nerve stimulations resulted in an increase in vaginal blood ¯ow. Electrostimulation parameters were iden- tical to those of the dorsal clitoral nerve stimulation. Baseline recordings were taken every 5 s for 30 s prior to electrostimulation. Nerve stimulations lasted 5 s and blood ¯ow was recorded every second. Following electrostimulation, blood ¯ow was recorded every 5 s for a duration of 30 s. Statistical analysis was performed to compare baseline to electrostimulation blood ¯ow values. For statistical analysis, a repeated measured design was used to calculate an F ratio. Figure 1 Photograph of the clitoris (small arrow) and neurovascular bundle (large arrow) of a female rat. Results cut to gain access to the full length of the vagina and to visualize the pelvic plexus nerves. For peritoneal In the ten female rats, clitoral blood ¯ow changes vaginal wall recordings, a plastic tube with an following stimulation of the dorsal clitoral nerve are external diameter of 0.5 mm, was placed within the shown in Figure 3. Results demonstrate an immedi- vagina to distend it. Vaginal recordings were also ate increase in blood ¯ow following stimulations taken while the laser doppler probe was placed (df1 12, df2 108, F 21.4, P < 0.001). Average base- directly in the vagina following the removal of the line and electrostimulation blood ¯ow values are plastic tube. 9.44 Æ 3.6 and 16.35 Æ 5.8 ml=min=100 g respec- Using a dissecting microscope (6Â), the pelvic tively. In three rats, clitoral blood ¯ow increased plexus nerves were isolated for electrostimulation from 8.97 Æ 1.5 to 13.7 Æ 1.94 ml=min=100 g follow- (Figure 2). Nerves innervating the vagina are derived ing pelvic plexus nerve stimulation. A decrease in from the pelvic ganglion and are located in the blood ¯ow from 8.4 Æ 3.15 to 4.3 Æ 1.35ml=min= mesometrium along with blood vessels.4,5 Nerves 100 g was observed following stimulation of the were isolated and stimulated in a cranial to caudal ventral aspect of the neuro-vascular bundle.

Figure 2 Schematic drawing showing pelvic plexus nerve innervation to the vagina and bladder of a female rat. The stimulating electrode (E) is placed on post-ganglionic ®bers dissected from the mesometrium. V vagina, N nerves, U ureter, B bladder, H uterine horn.

International Journal of Impotence Research Increases in clitoral and vaginal blood ¯ow P Vachon et al 55 of the pelvic plexus nerves also leads to increase clitoral blood ¯ow, however this was only demonstrated in three animals. To lend support to the methodology used and the results obtained in this study, decreases in clitoral blood ¯ow were recorded following stimulation of the ventral aspect of neurovascular bundle leading to the clitoris. This can be explained either by a vasospasm in the clitoral artery or stimulation of sympathetic ®bres. The parasympathetic component of the auto- nomic nervous system is responsible for the clitoral swelling during sexual excitation as well as the lubrication, vasocongestion and lengthening of the 2,6 Figure 3 Clitoral blood ¯ow changes recorded before and after vagina. These physiological changes are asso- electrical stimulations of the dorsal clitoral nerves. The laser ciated with an increase in blood ¯ow and would Doppler probe placed directly on the clitoris (Stim. duration of therefore be mediated by the parasympathetic electrical stimulation). Blood ¯ow during electrical stimulation system suggesting that pelvic nerve stimulations are statistically different from baseline values (P < 0.001). were performed on parasympathetic ®bres. Previous studies have demonstrated the impor- tance of the male rat model in erectile function and dysfunction. In males, stimulation of the cavernous nerve or the dorsal penile nerve7±12 have shown an arterial blood ¯ow increases in the corpora cavernosa, and consequently an erection is obtained. This increase in blood ¯ow is triggered by smooth muscle relaxation. After sexual stimulation, women experi- ence increases in vaginal lubrication and clitoral erection that are triggered by an increase in blood ¯ow in pelvic organs. Our model has shown the same physiological response and will enable us to study sexual dysfunction. Different neuromodulators have been identi®ed that play a role in clitoral and penile erection as well Figure 4 Vaginal blood ¯ow changes recorded before and after as lubrication of the vagina during arousal. Nitric electrical stimulations of the pelvic plexus nerves. The laser oxide has been thought of as the main neuromodu- Doppler probe placed on the peritoneal wall of the vagina. lator implicated in the initiation and maintenance of (Stim. duration of electrical stimulation). Blood ¯ow during 13 electrical stimulation are statistically different from baseline penile erection. Since there are many homologies values (P < 0.001). between male and female sexual tissues.14 it is possible that nitric oxide is an important neuro- transmitter in smooth muscle relaxation in the Vaginal blood ¯ow recorded from the laser probe clitoris and vagina. In humans, nitric oxide synthase placed on the peritoneal wall of the vagina are isoforms are present in clitoral15 and vaginal16 shown in Figure 4. Results show an imme- tissue and therefore nitric oxide donors may play a diate increase in vaginal blood ¯ow following role in controlling blood ¯ow in these tissues. pelvic plexus nerve stimulations (df1 12, df2 108, Studies in laboratory animals17±20 and farm ani- F 4.75, P < 0.001). Average baseline and electro- mals21 also show that nitric oxide may play a role in stimulation values are 6.3 Æ 4.7 and 25.8 Æ female genital organs such as neurogenic vasodila- 9.5 ml=min=100 g respectively. In three rats where tion and relaxation of smooth muscles. Other the laser Doppler probe was placed in the vagina to vasoactive peptides with relaxant properties, such record blood ¯ow changes following pelvic plexus as vasoactive intestinal polypeptide (VIP), have nerve stimulation, increases in blood ¯ow from been shown to be present in the vaginal tissue of 3.35 Æ 0.22 to 8.35 Æ 0.59 were recorded. animals22,23 and humans24,25. In humans, VIP increases blood ¯ow in the vagina and provokes vaginal lubrication.26,27 No comparative study has Discussion been done in the same animal species to localise different vasoactive neuropeptides that are important in the regulation of vascular and smooth muscle Results show that clitoral and vaginal blood ¯ow control of the vagina and clitoris of female rats. increase during the stimulation of the dorsal clitoral Increases in blood ¯ow of clitoral and vaginal and pelvic plexus nerves respectively. Stimulation tissue during nerve stimulations has been shown in

International Journal of Impotence Research Increases in clitoral and vaginal blood ¯ow P Vachon et al 56 rabbits and dogs.28,29 Using laser Doppler ¯owmetry Ackowledgements in rabbits, clitoral and vaginal blood ¯ow increases are 100% and 160% respectively following pelvic This research project was supported by the Jewish nerve stimulation. In female dogs, clitoral pressure General Foundation of the Montreal Jewish General measured directly with a needle placed in the Hospital and the department of Urology of McGill corpus cavernosal tissue show a 62% increase University. We greatly acknowledge Dr J Montgom- following cavernous nerve stimulation. Our results ery and Mme L Ste-Marie for lending us the laser show that clitoral and vaginal blood ¯ows increase Doppler for these experiments, and Mme M-T Parent by 37% and 309% respectively following stimula- for the preparation of the illustrations. tions of the clitoral and pelvic plexus nerve stimulations. 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International Journal of Impotence Research