Erectile dysfunction occurs following substantia nigra lesions in the rat

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1 (2001) 13, ß 2001 Nature Publishing Group All rights reserved /01 $ Erectile dysfunction occurs following substantia nigra lesions in the rat AR Zahran 1, N Simmerman 1, S Carrier 1 and P Vachon 1,2 * 1 Lady Davis Institute of the Jewish General Hospital, Department of Medicine, McGill University, Montreal, Quebec, Canada; and 2 MDS Pharma Services, Department of Metabolism and Animal Pharmacokinetics, Montreal, Quebec, Canada Erectile function was assessed 6 weeks following uni- and bilateral injections of 6-hydroxydopamine in the substantia nigra nucleus of the brain. Behavioral apomorphine-induced penile erections were reduced (5=8) and increased (3=8) in uni- and bilateral lesioned animals. Intracavernous pressures, following electrical stimulation of the cavernous nerve, decreased in lesioned animals. Lesions of the substantia nigra were confirmed by histology. Concentration of dopamine and its metabolites were decreased in the striatum of substantia nigra lesioned rats. Lesions of the substantia nigra are therefore associated with erectile dysfunction in rats and may serve as a model to study erectile dysfunction in Parkinson s disease. International Journal of Impotence Research (2001) 13, Keywords: rats; penile erection; apomorphine; intracavernous pressure; substantia nigra. Introduction James Parkinson first described idiopathic Parkinson s disease in 1817 as a neurodegenerative disorder with clinical signs such as bradykinesia, resting tremor, rigidity and postural imbalance. 1 The disease leads to progressive destruction of dopaminergic neurons that transport dopamine (DA) from the substantia nigra (SN) via the nigro-stiatal tract to the striatum and clinical signs of the disease appears when striatial DA is severely reduced. 2 Erectile dysfunction is a common problem in men with Parkinson s disease Pharmacological agents activating dopaminergic neurons via the D2 receptors have been shown to induce sexual responses and penile erections in animals and humans No animal model has been validated to study the pathophysiology of erectile dysfunction in Parkinson s disease and offer a better understanding of the effects of medications to treat Parkinson s disease on erectile function. The objective of the present study is to evaluate erectile function following lesions of the SN using injections of 6-hydroxydopamine (6-OH-DA) which mimic Parkinson s disease by destroying dopaminergic SN cells. 6-OH-DA was the first agent that had *Correspondence: P Vachon, Metabolism and Animal Pharmacokinetics, MDS Pharma Services, Ville St-Laurent, Montreal, Canada, H4R 2N6. Received 13 November 2000; accepted 4 May 2001 specific neurotoxic properties in the cathecolaminergic nervous system. 15 Injections of 6-OH-DA in the posterior hypothalamic regions cause severe neurobehavioral disturbances 16,17 producting prominent hypokinesia and tremor in rats. Specificity is achieved by using known stereotaxic coordinates targeting 6-OH-DA to the SN, the ventral tegmental area, the nigro-striatial tract, or the striatum. 18 Intrahypothalamic lesions provide the closest resemblance to the human disease in terms of behavioral manifestations and pharmacological responsiveness. To test our hypothesis, 6-OH-DA (experimental rats) and saline (control rats) were injected in the SN vicinity. Six weeks following surgery, erectile function was evaluated by behavioral apomorphineinduced penile erections and electrical stimulation of the cavernous nerve. Histological preparations, using the Klüver-Barrera staining method, were done to confirm lesions in the SN. Contents of DA and its metabolites, homovanillic acid (HVA) and dihydroxyphenylacetic acid (DOPAC), were evaluated in the cerebral hemispheres of all rats by high performance liquid chromatography (HPLC). Materials and methods Eighteen male Sprague Dawley rats (Charles River, Canada) weighing between 250 and 300 g were used in this study. They were kept in a standard laboratory animal environment (15 air changes=h,

2 256 temperature: 21 3 C, humidity: 40 60%, and light dark cycle: 12:12 h). They were housed in groups of two or three, in polycarbonate cages with treated pine shavings as bedding. Rats received fresh tap water daily and commercial laboratory rat food (Charles River, 5075 Rodent chow) ad libitum. Rats were kept in the animal facility of the Lady Davis Institute for Medical Research, Montreal Jewish General Hospital, which is accredited by the Canadian Council of Animal Care. The Institutional Animal Care and Use Committee prior to animal use approved the experimental protocol. Rats were anesthetized with an intraperitoneal dose of pentobarbital (50 mg=kg) (Somnotol, MTC Pharmaceuticals, Cambridge, Ontario, Canada) and positioned in a stereotaxic apparatus for 6-OH-DA injections. A midline incision of the skin of the skull was performed to expose bregma. A 1 mm trepanation was performed 5 mm posterior to bregma, 2 mm lateral to the midline and 8 mm ventral to the dura mater according to a previously established method. 15 A 6-OH-DA hydrobromide (Sigma, St Louis, MO, USA) solution (6.5 mg in3ml 0.9% saline containing 0.04% ascorbic acid; ph ) was injected using a 10 ml Hamilton syringe (Hamilton, Reno, NV, USA) immediately following preparation to minimize oxidation. Total injection time was 5 min, the needle was left in place for another 5 min. Bilateral 6-OH-DA animals were first injected in the vicinity of the right SN, immediately followed by the left SN. Unilateral 6-OH-DA animals were injected in the vicinity of the right SN only. Total surgical time, following anesthesia induction, did not exceed 30 min for both unilateral and bilateral cerebral hemisphere injected animals. Buprenorphine (0.01 mg=kg s.c.), a long acting opioid analgesic, was administered immediately prior to surgical procedures and 12 h post-surgery. Controls (n ¼ 5) consisted of sham operated agematched Sprague Dawley rats with bilateral stereotaxic injections of 0.9% saline containing 0.04% ascorbic acid in the same brain coordinates as experimental animals. The same surgical and postoperative procedures used in experimental rats were performed in the control rats. Six weeks after surgery, behavioral apomorphine-induced erections and cavernous nerve stimulation as well as HPLC assays, for brain DA and its metabolites, were performed. Six weeks following surgery, rats were acclimatized one-by-one to a custom built Plexiglas box in a semi-light room on three consecutive days for a period of 30 min. On the fourth day, they received 80 mg=kg of apomorphine subcutaneously (apomorphine hydrochloride, Sigma, St Louis, MO, USA) and filmed for 30 min with a home video camera using a mirror angled at 45 localized underneath the rats according to a previously described protocol. 19 Videos were subsequently viewed and the number of erections following apomorphine injections were counted. No other animal or personnel was present in the room for the 3 days of acclimation and for the erectile induction with apomorphine. Intracavernous pressure (ICP) measurements were done as previously published. 20 An initial intraperitoneal dose of 50 mg=kg pentobarbital (Somnotol, MTC Pharmaceuticals, Cambridge, Ontario, Canada) and subsequent quarter doses were given upon awakening to maintain the level of anesthesia throughout the experiments. Following a lower abdominal midline incision, the cavernous nerve and the crus of the corpus cavernosum were exposed and identified. A 23G needle coupled to PE-50 tubing was inserted in the corpus cavernosum. Cannulations were connected through a heparin-saline solution (250 IU=ml; Leo Laboratories, Ajax, Ontario, Canada) to pressure transducers (Abbott, Chicago, IL, USA). The signal was amplified (transducer amplifier model eth-200, AD Instruments, Castle Hill, Australia) and processed through MacLab 8S (AD Instruments, Castle Hill, Australia). Electrostimulation of the cavernous nerve was performed with stainless-steel bipolar hook electrodes. Monophasic rectangular pulses were delivered by a signal generator with constant current amplifier (MacLab Electrostimulator). Stimulus parameters were 2.0 ma, 20 Hz frequency, 0.2 ms pulse width and a duration of 1 min. At the end of each experiment and under deep anesthesia, animals were perfused with a saline solution (0.9% NaCl). Brains were rapidly removed from the cranium and immersed in 750 C solution. Sections were made at the injection site for histological controls. Sections (30 mm) were stained with the method of Klüver-Barrera to evaluate neuron loss in the SN. The striatum was removed and DA as well as major metabolite (HVA and DOPAC) concentrations were measured as previously described. 21 DA and its metabolites were analyzed using a Waters 600- MS HPLC (Waters, Canada) equipped with a mm Novapak C 18 column (Waters, Canada). The striatum from each cerebral hemisphere were weighed and analyzed for all rats. DA and its metabolites are reported as pmol=mg wet weight of striatal tissue. Standard concentration curves were established for DA and its metabolites prior to brain tissue evaluations. Statistical analysis was performed using a Student t-test. Statistical significance level was set a priori at P < Data in text and tables is expressed as mean s.d. Results Results showing the number of erections following subcutaneous apomorphine are shown in Table 1. Two types of behavior were clearly seen in uni-

3 Table 1 Sexual and motor behaviors following subcutaneous apomorphine injections 257 Groups No. animals WC a Average number of erections b (no. animals) Bilateral 6-OH-DA injections & (5 and 3) Unilateral 6-OH-DA injections & (3 and 2) Saline injection controls (5) a Number of animals presenting whole body circling (WC) following subcutaneous apomorphine injections. b Reported as sexual activity measured as the average number of apomorphine-induced erections with the corresponding number of animals in each category in parentheses. lateral and bilateral 6-OH-DA injected animals. Following apomorphine some animals (5=8) in both unilateral and bilateral injected 6-OH-DA rats had a decrease in the number of erections compared to normal rats (P < 0.01). Whereas in other animals (3=8), the average number of erections were increased following 6-OH-DA injections (P < 0.01). The number of erections in control rats following apomorphine injections was used as a basis of comparison. In both uni- and bilateral injected animals approximately 50% of the rats presented whole body circling following subcutaneous apomorphine. This is not surprising since clinical signs appear when striatal DA is reduced by 80% 2 and that although all animals had a significant decrease in DA levels this decrease was less than 80% for some animals. Mean peak intracavernous pressure was cm H 2 O in control rats following cavernous nerve stimulation whereas increases of and cm H 2 O(P < 0.001) were recorded in unilateral and bilateral lesioned rats, respectively. An example of SN cell loss following an injection of 6-OH-DA is shown in Figure 1. The destruction of the nigro-striatal pathway responsible for the Parkinson s syndrome occurs when the pars compacta of the SN is affected. Figure 1 shows a massive destruction of pars compacta and a less important loss of cells in the pars reticulata of the SN. Table 2 shows the concentrations of DA and its metabolites, DOPAC and HVA, from both right and left cerebral hemispheres. In unilateral 6-OH-DA injected rats only the right cerebral hemisphere shows a significant decrease in DA and its metabolites compared to controls. In bilateral 6-OH-DA injected rats, both hemispheres show a decrease of DA and its metabolites, however the left side of the brain is less affected than the right side. Discussion Results show that penile function is altered in rats following a decrease in DA content of the striatum with unilateral and bilateral 6-OH-DA intracerebral injections in the SN. Following 6-OH-DA injections, the number of apomorphine-induced erections were Figure 1 Microphotographs of a sagittal section of a unilateral 6- OH-DA injected rat brain showing a decrease in substantia nigra (SN) cells. (A) Normal SN from the contralateral side of the 6-OH- DA injection. (B) SN from the 6-OH-DA injected side. Note the almost complete absence of SN pars compacta (PC) neurons and the important accumulation of microglia at the injection site (arrowhead). A lesser number of neurons in the SN pars reticulata (PR) are also observed in the immediate vicinity of the injection site (Klüver-Barrera stain, magnification 6 100). decreased in 62% of the animals (8=13) and increased for 38% (5=13) of the animals. Penile intracavernous pressure was decreased in unilateral and bilateral lesioned animals following electrical stimulation of the cavernous nerve. Therefore central and peripheral effects on erectile mechanisms

4 258 Table 2 Left and right cerebral striatum concentrations of dopamine and dopamine metabolites measured by HPLC Left cerebral striatum concentrations (pmol=mg) Right cerebral striatum concentrations (pmol=mg) Dopamine DOPAC a HVA b Dopamine DOPAC a HVA b Control rats (n ¼ 5) c Bilateral injected rats (n ¼ 6) d ** * * *** *** ** Unilateral injected rats (n ¼ 3) d *** ** ** a Dihydroxyphenylacetic acid; b Homovanilic acid; c Stereotaxic saline injections in substantia nigra coordinates; d Stereotaxic injections of 6-OH-DA in substantia nigra coordinates. ***P < 0.001; **P < 0.01; *P < were observed following 6-OH-DA injections. Animals for which apomorphine-induced erections were increased could be partly explained by hypersensitivity of central receptor observed following denervation lesions, which may compensate for the loss of DA neurons. This could be accomplished by a decrease of DA autoreceptors and=or an increase in post-synaptic DA receptors. 22 The erectile hypersensitivity response could also be responsible for hypersexuality that has been observed following the administration of dopaminergic drugs in some Parkinson s patients. 23 However no study has clearly established the effects of dopaminergic-like drugs on the penile erection with respect to the duration of the disease or the extent of the SN lesion. The 6-OH-DA lesions and subsequent destruction of cells in the SN was confirmed by histology and the measurement of DA, and its metabolites in the striatum in corresponding lesioned hemispheres. The unilateral and bilateral lesions were performed since clinical signs in Parkinson s disease patients are mainly unilateral in the first stages of the disease and become bilateral in the later stages. 24 It is also recognized that the right hemisphere is dominant for sexual function EEG studies in normal men show that there is greater activation of the right cerebral hemisphere during periods of nocturnal penile tumescence. Stroke and lesions in the limbic system are associated with sexual dysfunction when the right cerebral hemisphere is affected. 28,29 Therefore, the right cerebral hemisphere seems dominant for penile erection. Our results show that the left hemisphere was less affected than the right hemisphere in bilateral injected 6-OH-DA animals. Since 6-OH-DA was first injected in the right hemisphere and the fact that 6-OH-DA is easily oxidized, it is likely that a delay of injection in the left hemisphere could explain the lower number of SN neurons being affected. However, compared to control animals, the DA content in the left striatum was 54% of controls and is still an appreciable decrease. Central dopaminergic mechanisms are involved in male sexual behavior. Apomorphine may restore erectile function in Parkinson s patients and in men suffering from psychogenic impotence. 30 Administration of DA agonists causes an increase in penile erections in rats. DA agonists injected in the medial preoptic area of the hypothalamus 31 and in the paraventricular nucleus (PVN) 11 regulates penile erection. The PVN could play a key role in penile erection since it receives projections from dopaminergic midbrain neurons and that efferent PVN projections mediate erection via oxytocin. 32 The PVN is also known to be important for yawning behavior and DA also seems to play a critical role since lesions of the PVN inhibit apomorphineinduced yawning in rats. 31 Interestingly SN lesions cause a decrease in apomorphine-induced yawning and these results are correlated with a decrease in DA and its metabolites in the striatum. 33 We also observed a decrease in ICP following electrical stimulation of the cavernous nerve. These results strongly suggest that a decrease in excitation from higher brain centers had an effect on peripheral mechanisms controlling penile erection. In this respect, although some SN lesioned rats had a greater number of erections following apomorphine administration, our results suggest that the quality of these erections may have been greatly diminished. A possible explanation for the observed peripheral effects could be that a decrease of DA afferent activity to the PVN would in turn decrease excitatory effects of oxytocin originating from the PVN onto the lumbar nuclei responsible for the motor control of erection. 34 A decrease of neuronal excitability onto lumbar neurons would also unbalance the inhibitory and facilitatory supraspinal descending controls causing inhibition of the lumbar nuclei responsible for erections. Both of these mechanisms could in turn down regulate the activity of NO fibers needed for erection and would be responsible for the observed decreased ICP response following cavernous nerve stimulations. SN is known to play a role in arterial blood pressure, however Turgut et al 35 have reported increases, and decreases, in mean arterial blood pressure ( 5mm Hg) in rats following 6-OH-DA lesions. Therefore, the change in arterial pressure following SN lesions is not sufficient to explain the decreased ICP

5 observed following cavernous nerve stimulation in the SN lesioned rats. The 6-OH-DA rat model of Parkinson s disease has been useful for the understanding of the pathophysiology and related pharmacology of the disease, however most studies show that it does not mimic the slow progressive dopaminergic neural degeneration seen in human patients. 36 However, Schwarting and Huston (1996) have shown that degeneration may be progressive in rats after 6-OH- DA administration. 37 Our results suggest that this model may be useful in understanding erectile dysfunction present in Parkinson s disease. Further work is needed to establish if the degenerative process evolves over time causing an erectile dysfunction in all animals injected with 6-OH-DA in the SN. Intracerebral injections of 6-OH-DA in the SN in rats caused erectile dysfunction. Behavioral apomorphine-induced penile erections were reduced in most uni- and bilateral 6-OH-DA lesioned animals. Intracavernous pressures following electrical stimulation of the cavernous nerve were decreased in these animals. Contents of DA and its metabolites, HVA and DOPAC, were decreased in the right cerebral striatum and the right and left cerebral striatum of the unilateral and bilateral 6-OH-DA lesioned rats, respectively. Measured parameters of penile dysfunction may therefore be related to a diminished DA brain content. Some animals showed an increase in the number of erections following apomorphine administration while intracavernous pressures were decreased and SN lesions were present. In these animals, striatal DA and its metabolites were also decreased. It is possible that receptor hypersensitivity in the central nervous system could be responsible for the increased sexual excitability following apomorphine injections. Acknowledgements This research project was supported by the Jewish General Foundation of the Montreal Jewish General Hospital. We greatly acknowledge Mme Jeanne Lavoie for histological preparations. We also wish to thank Dr Jane Montgomery and Ms Line Ste-Marie from the University of Montreal Hospital Research Center for the analysis of DA and metabolites by HPLC. References 1 Parkinson J. An essay on the shaking palsy. Whittingham and Roland: London, Tolwani RJ, et al. Experimental models of Parkinson s disease: insights from many models. Lab Anim Sci 1999; 49: Brown E, Brown GN, Kofman O, Quarrington B. Sexual function and affect in Parkinsonian men treated with L-dopa. Am J Psychiat 1978; 135: Koller WC et al. Sexual dysfunction in Parkinson s disease. Clin Neuropharmacol 1990; 13: Lipe H, Longstretch WT, Bird TD, Linde M. Sexuality and Parkinson s disease. Neurology 1989; 3: O Brien CP, Digiacomo JN, Fahn S, Schwarz GA. Mental effects of high-dosage Levodopa. Arch Gen Psychiat 1971; 24: Singer C, Sanchez-Ramos J, Weiner WJ, Ackerman M. Sexual dysfunction in Parkinsonian males. Neurology 1989; 3: Welsh M, Hung L, Waters CH. Sexuality in women with Parkinson s disease. Mov Dis 1997; 12: Wermuth L, Stenager E. Sexual aspects of Parkinson s disease. Seminars Neurol 1992; 12: Wermuth L, Stenager E. Sexual problems in young patients with Parkinson s disease. Acta Neurol Scand 1995; 91: Benassi-Benelli A, Ferrari F, Quarantotti BP. Penile erection induced by apomorphine and N-n-propyl-norapomorphine in rats. Arch Int Pharmacodyn Ther 1979; 242: Chen K-K, Chan JYH, Chang LK. Dopaminergic neurotransmission at the paraventricular nucleus of the hypothalamus in the central regulation of penile erection in the rat. J Urol 1999; 162: Heaton JP et al. Recovery of erectile function by the oral administration of apomorphine. Urology 1995; 45: Melis MR, Mauri A, Argiolas A. Apomorphine- and oxytocininduced penile erection and yawning in intact and castrated male rats: effect of sexual steroids. Neuroendocrinology 1994; 59: Ungerstedt U. 6-hydroxydopamine induced degeneration of central monoamine neurons. Eur J Pharmacol 1968; 5: Butterworth RF, Belanger F, Barbeau A. Hypokinesia produced by anterolateral hypothalamic 6-hydroxydopamine lesions and its reversal by some antiparkinson drugs. Pharmacol Biochem Behav 1978; 8: Rondeau DB, Jolicoeur FB, Belanger F, Barbeau A. Differential behavioral activities from anterior and posterior hypothalamic lesions in the rat. Pharmacol Biochem Behav 1998; 9: Perese DA et al. A 6-hydroxydopamine-induced selective parkinsonian rat model. Brain Res 1989; 494: Carrier S et al. Age decreases nitric oxide synthase-containing nerve fibers in the rat penis. J Urol 1997; 157: Quinlan DM et al. The rat as a model for the study of penile erection. J Urol 1989; 141: Ste-Marie L, Boismenu D, Vachon L, Montgomery J. Evaluation of sodium 4-hydroxybenzoate as an hydroxyl radical trap using gas chromatography-mass spectrometry and high performance liquid chromatography with electrochemical detection. Anal Biochem 1996; 241: Kelly PH, Seviour PW, Iversen D. Amphetamine and apomorphine responses in the rat following 6-OHDA lesions of the nucleus accumbens septi and corpus striatum. Brain Res 1975; 94: Stern G, Lees A. Parkinson s disease. The Facts. Oxford University Press: Oxford, Cambier J, Masson M, Dehlen H. Syndromes parkinsoniens. In Masson SA (ed). Neurologie. Masson: Paris, 1985, pp Weinstein E. Effects of brain damage on sexual behavior. Med Asp Hum Sex 1981; 15: Coslett HB, Heilman KM. Male sexual function. Impairment after right cerebral hemisphere stoke. Arch Neurol 1986; 43: Hisrshkowitz M, Ware JC, Turner D. EEG amplitude asymmetry during sleep. Sleep Res 1979; 8: Coslett HB, Heilman KM. Male sexual function. Impairment after right cerebral hemisphere stroke. 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6 O Sullivan JD, Hughes AJ. Apomorphine-induced penile erections in Parkinson s disease. Mov Disord 1998; 13: Giuliano F et al. Stimulation of the medical preoptic area of the hypothalamus in the rat elicits increases in intracavernous pressure. Neurosoi Lett 1996; 209: Argiolas A, Melis MR, Mauri A, Gessa GL. Paraventricular nucleus lesion prevents yawning and penile erection induced by apomorphine and oxytocin by not by ACTH in rats. Brain Res 1987; 421: Laping NJ, Ramirez VD. Prolactin-induced yawning behavior requires an intact nigro-striatal dopamine system. Pharmacol Biochem Behav 1988; 29: Veronneau-Longueville F et al. Oxytocinergic innervation of autonomic nuclei controlling penile erection in the rat. Neuroscience 1999; 93: Turgut N et al. Asymetry of sympathetic activity in a rat model of Parkinson s disease induced by 6-hydroxydopamine: heamodynamic, electrocardiographic and biochemical changes. Res Exp Med 1998; 197: Bernheimer H, Birkmayer W, Hornykiewicz O. Brain dopamine and the syndromes of Parkinson and Huntington. Clinical, morphological and neurochemical correlations. J Neurol Sci 1973; 20: Schwarting RK, Huston JP. Unilateral 6-hydroxydopamine lesions of the meso-striatal dopamine neurons and their physiological sequelea. Prog Neurobiol 1996; 49: Editorial Comment This manuscript tried to demonstrate an interesting animal model for deprivation of central dopamine. This animal model has the potential for advancing knowledge of the central mechanism of penile erection. However, this model may have a couple of questionable aspects. First, unfortunately, the results did not clearly show the relationship between the deprivation of dopamine and the number of erections following apomorphine injection. The authors divided the animals in the groups with bilateral and unilateral 6- OH-DA injections into two subgroups based on the average number of erections in the control group. Eight animals and five animals demonstrated higher and lower numbers of apomorphine-induced erections, respectively. There seems to be no correlation between the number of erections and deprivation of central dopamine. In addition, I do not think this method of data analysis is reasonable. Instead of such analysis, the average numbers of erections of all animals in each treatment group (ie bilateral or unilateral 6-OH-DA injections) are appropriate parameters to prove the authors hypothesis. The average values of the erections should then be compared with that of the control group statistically. If there is a significant difference, there is clear evidence that supports the authors hypothesis. Secondly, the animals injected with 6-OH-DA demonstrated a significantly lower ICP response following cavernous nerve stimulation. However, electrical stimulation of the cavernous nerve may not be a suitable stimulation method for the purpose of the authors, because the ICP response due to the cavernous nerve stimulation provides information about erectile conditions in peripheral systems (ie the peripheral nervous system and corporal smooth muscle, and peripheral vascular system). In other words, cavernous nerve stimulation can induce a normal ICP response, if the peripheral conditions are normal, even if disorders in the CNS exist. Therefore, for the authors purpose, measurement of ICP during electrical stimulation of the CNS (ie the MPOA, 1,2 or apomorphine-induced erectile response 3 are more suitable. To this end, this animal model that mimics Parkinson s disease may have potential, but more work and better analyses are needed for it to become a reliable animal model. References YSato 1 Giuliano F, Rampin O, Brown K, Courtois F, Benoit G, Jardin A. Stimulation of the medial preoptic area of the hypothalamus in the rat elicits increase in intracavernous pressure. Neurosci Lett 1996; 209: Sato Y, Christ JG. Differential ICP response elicited by electrical stimulation of the medial preoptic area. Am J Physiol 2000; 278: H964 H Anderson K-E, Gemalmaz H, Waldeck K, Chapman TN, Tuttle JB, Steers WD. The effect of sildenafil on apomorphine-evoked increases in intracavernous pressure in the awake rat. J Urol 1999; 161: