Animal models for studying penile hemodynamics

Hiroya Mizusawa¹, Osamu Ishizuka², Osamu Nishizawa²

Asian J Androl 2002 Sep; 4: 225-228 

Keywords: erectile dysfunction; impotence; intracavernous pressure; sexual dysfunction; corpus cavernosum; electrophysiology

Abstract 

Animal models for the study of erectile function monitoring the changes in intracavernous pressure (ICP) during penile erection was reviewed. The development of new models using small commercially-available experimental animals, rats and mice, in the last decade facilitated in vivo investigation of erectile physiology. The technique enabled to evaluate even subtle erectile responses by analyzing ICP and systemic blood pressure. Moreover, the method has been well improved and studies using conscious animal models without the influence of any drug or anesthesia are more appropriate in exploring the precise physiological and pharmacological mechanisms in erection. Also, more natural and physiological sexual arousal instead of electrical or pharmacological stimulation is desirable in most of the studies.This article reviewed the development of ICP studies in rats and mice.

1 Introduction

Penile erection is a hemodynamic event in the penis. It is the end result of relaxation of the cavernous tissues that involves both the central nervous system and the local factors. The contemporary basic concepts of erectile physiology were obtained greatly from in vivo and in vitro investigations mostly with animals [1-3].

There are various in vivo animal models for the study of penile erection. Eckhard reported relevant involvement of the pelvic nerve on erection in dogs in the latter half of the 19th century [4]. Historical advances have enabled to measure intracavernous pressure (ICP), which was performed in the bull by Lewis et al in 1968 [5]. Later, to investigate the erectile function, several animal models, including monkeys, dogs and rabbits were developed and used. The technique for ICP measurement using commercially available small animals, i.e., rats and mice, was established in the last decade [6-17]. The use of transgenic animals widens the physiological and pharmacological understanding of erection. The neuroanatomy and physiology of the penis in rats and mice was also gradually elucidated [6, 18], and contemporary knowledge and technique on animal experiments have been formed.

ICP measurement represents a direct investigation of erectile function. It made the objective evaluation on even subtle erectile responses possible, but also facilitated the physiologic approach and obtained new understandings on penile erction.

The present review is an attempt to update the information on the hemodynamics of the penis and to discuss the significance of monitoring ICP.

In the flaccid status, the cavernous tissues are contracted by a dominant sympathetic control to the arterioles and the cavernous smooth muscles [1,2]. After receiving sexual stimulation, parasympathetic nerve activity dominates and influences the local factors and there is an increasing blood flow through the cavernous artery. During erection, mainly with the parasympathetic effect, the cavernous tissues are relaxed and the influx of a huge amount of blood induces a rapid increase in ICP. It may reach a value 10-20 mmHg below the systolic blood pressure. At this phase, the voluntary or reflexogenic contraction of the ischiocavernous and bulbocavernous muscles produces the burst peak pressure, which reaches to well above the arterial blood pressure and gives the maximal rigidity to the penis. During the detumscence phase, the cavernous tissues become less relaxed because the attenuated sexual stimulation decreases the parasympathetic dominance and leads to a relative enhancement of adrenergic regulation. The smooth muscle tone in the penis depends on the balance between the contractile and the relaxant factors.

Steers et al reported in 1988 [7] that electrophysiological techniques were used to examine the neural activity of the penile nerve in the rat. Later, other group also described the ICP monitoring in rats of a different strain upon electrostimulation on the lumbosacral roots. In the rat model, erectile responses were produced by electrical stimulation following laparotomy under general anesthesia [8].

Chen et al [9] modified an anesthetized rat model for the investigation of erectile physiology, monitoring the hemodynamics in the corpus cavernosum and femoral artery. Penile erection was induced by intracavernous administration of drugs and the maximal intracavernous pressure and the duration of increase in ICP were evalu-ated. The blood pressure and heart rate were also assessed simultaneously.

The first mouse model monitoring the ICP was reported by Sezen and Burnett [10] in 2000. Both electrical stimulation and intracavernous drug administration produced reproducible ICP increases in an anesthetized animal. Several other authors also reported the techniques for measuring ICP.

One of the basic methods is described as follows [11]. Male rats were lightly anesthetized with intraperitoneal pentobarbital, so that the rats breathed spontaneously during the experiment. For continuous systemic blood pressure measurements, a heparinized polyethylene catheter was introduced into the femoral artery. With a midline perineal incision, followed by blunt dissection of the overlying striated muscles, entrance to the tunica albuginea of the crus corpus cavernosum was achieved. A fine needle attached to a heparinized polyethylene catheter was inserted into the crus corpus cavernosum and the ICP was registered by means of a pressure transducer. For pharmacological stimulation giving intracavernously, another fine needle should be placed at the other crus for drug injection.

In mice, the basic procedure was similar to that in rats except a smaller needle for monitoring ICP will be used and the catheter for the femoral artery needs to be slightly stretched to reduce the size, otherwise the internal carotid artery should be selected. To maintain a good general condition, the use of a blanket or heating mat is advisable.

In general, electrical stimulation is performed via the cavernous nerve following laparotomy under general anesthesia. A very important point is to use submaximal stimulation, both electrically and pharmacologically, in order to correctly appraise the increase or decrease in ICP [12-14].

When using electrical stimulation, the basal ICP (BICP), the peak ICP (PICP), the ratio of PICP/blood pressure and the slopes of tumescence and detumscence were evaluated (Figure 1). In pharmacological stimulation, the time to the first response, the number of response, the duration, the BICP, the PICP, the ratio of PICP/blood pressure and the area under curve were analyzed [6, 13, 14].

Figure 1. Typical curve showing increase in ICP induced by nerve stimulation. During stimulation, the time for ICP to reach 80% of maximal increase (peak ICP - basal ICP) was recorded (T80). At this point, the increase per second (rT80) was evaluated. After stimulation, the time for and the rate by which a decrease to 20 % of maximal pressure occurred (D20 and rD20) were determined. A bar indicates stimulation period.

5 ICP curves in sexually stimulated rats and mice

Figure 2 showed the curves of changes in ICP, induced by the administration of intrathecal or intracere-broventricular oxytocin and a-melanocyte-stimulating hormone in an anesthetized rat [11]. The drug produced reproducible responses of ICP increases. The typical ICP response exhibited a fast increase in pressure up to the peak ICP. In anesthetized animals, it was approximately 60%-80% of the mean blood pressure with pharmacological or electrical stimulation. At this pressure, frequent rapid changes in the pressure were observed and then the peak ICP decreased sharply to the previous basal ICP.

Figure 2. Tracings showing ICP changes induced by icv alpha-MSH and oxytocin and intrathecal oxytocin in anesthetized rats.

Andersson et al [15] described the effect of sildenafil on the apomorphine-evoked increase in ICP in the conscious rat model. A fine catheter was inserted into the corpus cavernoum and the tubing was brought out subcutaneously to the back. It showed that the systemic administration of apomorphine induced reproducible ICP increase with the burst peak pressure by the action of the skeletal muscles. It was recently reported that apomorphine also elicited the erectile responses in spinal rat model. Even in anesthetized rats, systemic apomorphine administration induced increasing ICP in response to the dose for a conscious rat (Figure 3) [13, 14].

Figure 3. Tracing showing the erectile responses induced by sc administration of apomorphine.
A) Pretreatment with vehicle
B) Pretreatment with a1D adrenoceptor antagonist, A-119637

6 In vivo animal models in the future

If a method of ICP measurement in transgenic mice is employed, investigations on the mechanism of penile erection at the molecular level will be expanded. Even without the monitoring of ICP, some good papers have been published in this field [19, 20].

Mizusawa et al have reported morphological and functional in vitro and in vivo characterization of the mouse corpus (Figure 4) [6]. It has been suggested that there are many similarities in peripheral structure between the humans and rats [18]. Central innervations and regulation on penile erection in mice should be studied.

Figure 4. ICP changes in mouse corpus cavernosum in response to cavernous nerve stimulation. After pretreatment with ip NO synthase inhibitor (L-NAME), erectile response was abolished. Increase in ICP induced by forskolin, an adenylyl cyclase activator, was unaffected by L-NAME. Bars indicate stimulation period.

Recently, with the advances in the anatomy of the central nervous system in laboratory animals and in the experimental techniques, drug stimulation was performed not only intracavernously, but also intrathecally or intracerebroventricularly. In addition, electrical and pharmacological stimulation began o focus on narrower areas in the brain such as the medial preoptic area and the paraventricular nucleus of hypothalamus [21, 22]. It facilitates researches on physiological and pharmacological involvement in the brain and the spinal cord.

Most experiments have been performed under general anesthesia or sedation, while a conscious model is much preferable to evaluate the physiological status without drug intervention. Bernabe et al [23] reported a new integrative approach based on telemetric recording in the rats. As mentioned above, a conscious model without a special device is more desirable.

To investigate the sexual physiology, another point for consideration is the natural stimulation by a female animal. If it is possible to monitor ICP and blood pressure simultaneously by non-contact erection, this could be an ideal solution.

7 Conclusion

We reviewed the in vivo animal models for the study of erectile function from a viewpoint of monitoring changes in ICP. The technique enabled to evaluate even subtle erectile responses by analyzing ICP and systemic blood pressure. The development of the new models using small commercially-available animals such as rats and mice facilitated in vivo investigation. Future development in employing conscious animal models without any drug intervention seems to be advisable. Also, natural and physiological sexual arousal instead of electrical or pharmacological stimulation is more desirable.

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Correspondence to: Osamu Ishizuka, M.D., Ph.D., Department of Urology, Shinshu University School of Medicine, 3-1-1 Asahi, Matsumoto 390-8621, Japan.
Tel: +81-263-37 2661, Fax: +81-263-37 3082
E-mail: ishizuk@hsp.md.shinshu-u.ac.jp
Received 2002-08-21      Accepted 2002-09-05