Autonomic innervation of the mammalian penis: a histochemical and physiological study

Regional differences in the extracellular matrix of the human spongy urethra as evidenced by the composition of glycosaminoglycans

PURPOSE

Despite the concept that the spongy urethra is a unique entity clinical evidence suggests the existence of segmental structural differences. The spongy urethra has a vascular nature, its cells may express different phenotypes and the extracellular matrix that they synthesize should reflect these differences. Glycosaminoglycans are components of the extracellular matrix that have key roles in the normal physiology and pathology of several tissues. Although total collagen content of the urethra was determined, we also analyzed urethral glycosaminoglycans (GAGs).

MATERIALS AND METHODS

Fresh, macroscopically normal cadaveric urethral samples were obtained from 15 men who died at a mean age of 25.4 years. The urethra was divided into glanular, penile and bulbar segments, which were then analyzed separately. Total GAG concentration was assessed by hexuronic acid assay and expressed as microg. hexuronic acid per mg. dry tissue, while the proportions of sulfated GAGs were determined by agarose gel electrophoresis. Hyaluronan concentration was determined by ion exchange chromatography and total tissue collagen was estimated as hydroxyproline content.

RESULTS

Total GAG concentration was heterogeneous along the spongy urethra (p <0.001). Mean values plus or minus standard deviation in the glanular, penile and bulbar segments were 2.53 +/- 0.42, 2.11 +/- 0.47 and 1.47 +/- 0.4 microg./mg., respectively. The most predominant GAG was hyaluronan and its highest mean concentration of 50.1% +/- 3.7% was found in the glanular urethra. The most predominant sulfated GAG in the male urethra was dermatan sulfate, followed by chondroitin sulfate and heparan sulfate. Total collagen content and the GAG-to-collagen ratio varied along the spongy urethra and were lowest in the bulbar segment.

CONCLUSIONS

The extracellular matrix of the human spongy urethra shows regional differences, as evidenced by biochemical analysis of GAG and collagen. This heterogeneity implies functional adaptations in the various segments and may affect the physiology and segmental incidence of urethral diseases.

The pathophysiological role of prostaglandins in penile erection

Penile erection is a complex haemodynamic event and relaxation of the vascular smooth muscle of the penis is fundamental to this process. This is primarily mediated by nitric oxide (NO) produced from non-adrenergic non-cholinergic (NANC) nerves and the endothelium surrounding the corporal sinusoids. However, corporal tissue has been shown to produce a number of other vasoactive factors, including prostaglandins (PGs), which appear to not only have diverse physiological effects, but are also able to modulate the effects of other neurotransmitters, including NO.

The role of nitric oxide in penile erection

The functional state of the penis, flaccid or erect is governed by smooth muscle tone. Sympathetic contractile factors maintain flaccidity whilst parasympathetic factors induce smooth muscle relaxation and erection. It is generally accepted that nitric oxide (NO) is the principal agent responsible for relaxation of penile smooth muscle. NO is derived from two principal sources: directly from non-adrenergic non-cholinergic parasympathetic nerves and indirectly from the endothelium lining cavernosal sinusoids and blood vessels in response to cholinergic stimulation. The generation of NO from L-arginine is catalysed by nitric oxide synthase (NOS). There has been controversy over the relative prevalence of endothelial or neuronal NOS within the penis of different animal species. This review examines the role of NO in the penis in detail. Established and new treatments for erectile dysfunction whose effects are mediated via manipulation of the NO pathway are also described.

The role of prostaglandins in penile erection

The balance of penile smooth muscle tone is finely controlled, with contractile factors acting in opposition to relaxant factors. The principal agents in this process are undoubtedly noradrenaline and nitric oxide. Prostaglandins probably have a crucial role in the 'fine tuning' of corporal smooth muscle tone. Their effects on control mechanisms in the healthy penis are more likely to be modulatory rather than direct.

Possible involvement of nitric oxide in pilocarpine induced seminal emission in rats

Intraperitoneal injection of pilocarpine (0.75-3.0 mg/kg) caused a dose-related seminal emission in adult male rats. The seminal emission response to 3 mg/kg of pilocarpine was greatly reduced in atropinized (5 and 10 mg/kg, SC) animals, suggesting a cholinomimetic effect. Nw-nitro-L-arginine methyl ester (5, 10, and 20 mg/kg, SC), a nitric oxide synthesis inhibitor, also inhibited the pilocarpine-induced seminal emission, which was reversed by L-arginine (600 mg/kg, SC) or by coinjection of sodium nitroprusside (0.5 mg/kg, SC). Urine analysis for levels of nitric oxide metabolites, nitrate/nitrite (NO3-/NO2-), showed marked alterations in accordance with the drug treatments. The results suggest that nitric oxide mediates the inhibitory neurotransmission responsible for seminal emission in pilocarpine stimulated rats.

Effects of SSRIs on sexual function: a critical review

Sexual problems are highly prevalent in both men and women and are affected by, among other factors, mood state, interpersonal functioning, and psychotropic medications. The incidence of antidepressant-induced sexual dysfunction is difficult to estimate because of the potentially confounding effects of the illness itself, social and interpersonal comorbidities, medication effects, and design and assessment problems in most studies. Estimates of sexual dysfunction vary from a small percentage to more than 80%. This article reviews current evidence regarding sexual side effects of selective serotonin reuptake inhibitors (SSRIs). Among the sexual side effects most commonly associated with SSRIs are delayed ejaculation and absent or delayed orgasm. Sexual desire (libido) and arousal difficulties are also frequently reported, although the specific association of these disorders to SSRI use has not been consistently shown. The effects of SSRIs on sexual functioning seem strongly dose-related and may vary among the group according to serotonin and dopamine reuptake mechanisms, induction of prolactin release, anticholinergic effects, inhibition of nitric oxide synthetase, and propensity for accumulation over time. A variety of strategies have been reported in the management of SSRI-induced sexual dysfunction, including waiting for tolerance to develop, dosage reduction, drug holidays, substitution of another antidepressant drug, and various augmentation strategies with 5-hydroxytryptamine-2 (5-HT2), 5-HT3, and alpha2 adrenergic receptor antagonists, 5-HT1A and dopamine receptor agonists, and phosphodiesterase (PDE5) enzyme inhibitors. Sexual side effects of SSRIs should not be viewed as entirely negative; some studies have shown improved control of premature ejaculation in men. The impacts of sexual side effects of SSRIs on treatment compliance and on patients' quality of life are important clinical considerations.

Erectile response to hypothalamic stimulation in rats: role of peripheral nerves

The role of peripheral parasympathetic and sympathetic pathways was explored in erectile responses elicited by hypothalamic medial preoptic area (MPOA) stimulation in adult male anesthetized rats. Under control conditions, MPOA stimulation reliably elicited erectile responses evidenced by an increase of the intracavernous pressure-to-blood pressure ratio. The erectile response was abolished by 1) acute bilateral section of cavernous or pelvic nerves or cauda equina and 2) chronic lesions of pelvic nerves or cauda equina. Acute section of the hypogastric nerve did not significantly decrease the erectile response. The erectile response was significantly depressed after acute or chronic sections of the paravertebral sympathetic chain at the L4-L5 level or chemical sympathectomy with 6-hydroxydopamine. The decrease due to acute sympathetic chain lesion was reversed by bilateral ligation of the external iliac arteries. Accordingly MPOA stimulation elicits erectile responses via 1) activation of the parasympathetic outflow conveyed by the pelvic and cavernous nerves and 2) activation of neural fibers conveyed by the sympathetic pathways. We propose that sympathetic fibers running in the paravertebral sympathetic chain are responsible for vasoconstriction of nonpenile areas to divert blood to the penis, allowing the dramatic increase of penile arterial inflow required for erection.

Renin angiotensin system in rabbit corpus cavernosum: functional characterization of angiotensin II receptors

PURPOSE

The regulation of the corporal smooth muscle tone is important in the process of penile erection. Although specific angiotensin (ANG) II binding to and effects of ANG II on some reproductive structures have been studied, the presence of the renin-angiotensin system has not yet been defined in the corpus cavernosum. ANG II is formed from ANG I by angiotensin I-converting enzyme (ACE). ANG II and ANG I produce contractions in vascular smooth muscles. Two subtypes of ANG II receptors (AT1 and AT2) have been characterized. The purpose of the present experiments was to determine whether the renin-angiotensin system regulates rabbit corpus cavernosum smooth muscle tone.

MATERIALS AND METHODS

A strip of rabbit corpus cavernosum was mounted in an organ chamber to measure the isometric tension. The specific binding for 125I-ANG II was characterized by in vitro autoradiography.

RESULTS

ANG II and ANG I, precursor of ANG II, contracted corpus cavernosum smooth muscle dose-dependently, but the response of smooth muscle to ANG I was 10-fold less than that to ANG II. Contractile responses of smooth muscle to ANG II and ANG I were blocked by Dup 753, a specific inhibitor of ANG II type 1 receptor, but not by PD 123,319, a specific inhibitor of ANG II type 2 receptor. The effect of ANG I was attenuated by captopril, an inhibitor of ACE. Specific binding sites for 125I-ANG II were found in the corpus cavernosum. The dissociation constant (Kd) was 5.32 +/- 1.65 nM. and maximum binding capacity (Bmax) was 305.72 +/- 85.24 amol/mm. Specific binding of 125I-ANG II was displaced by Dup 753 (10(-6) M) but not by PD 123,319 (10(-5) M). The inhibitory constant (Ki) for Dup 753 was 8.09 +/- 2.51 nM.

CONCLUSION

The present results suggest that the renin-angiotensin system is involved in the regulation of corpus cavernosum smooth muscle tone of rabbit and the ANG II receptor subtype AT1 is important in the regulation of penile erection.

A critical review of selective serotonin reuptake inhibitor-related sexual dysfunction; incidence, possible aetiology and implications for management

There is a high incidence of sexual dysfunction in the general population and sexual dysfunction is often an integral symptom of a depressive disorder. In addition, all antidepressants have effects on sexual functioning, as the result of side-effects of these medications and as a reflection of therapeutic success. The selective serotonin reuptake inhibitors (SSRIs) are clearly associated with delayed ejaculation, inability to ejaculate and absent or delayed orgasm. Furthermore, the incidence of sexual dysfunction obtained by patient self-report does not appear to reflect the true incidence of sexual dysfunction associated with antidepressant therapy and systematic inquiry is needed as sexual dysfunction may be an unrecognized cause of noncompliance. The SSRIs may have advantageous effects on sexual functioning and these may also be underreported due to the same factors resulting in an under-reporting of sexual side-effects in general. In addition, studies have suggested a role for the SSRIs in the management of premature ejaculation. The effects of SSRIs on sexual functioning are clearly dose-related and may vary amongst the group due to their relative effects on the serotonin and dopamine systems and the extent to which plasma levels of these drugs accumulate in the body over time. A variety of strategies have been found useful in the management of SSRI-induced sexual dysfunction including waiting for tolerance to develop, dosage reduction, drug holidays, switching to a different antidepressant and various augmentation strategies with 5-HT2, alpha2 adrenergic receptor antagonists and dopamine receptor agonists.

Effect of magnesium on the function of the rabbit corpus cavernosum

Contraction and relaxation of the smooth muscle, including the corpus cavernosum, are mediated by changes in the intracellular concentration of calcium. Since magnesium modulates the movement of calcium, it can modify the function of the erectile tissue. We designed this study to investigate the effects of magnesium in doses ranging from 5 to 30 mM on the function of the rabbit corpus cavernosum in vitro. The resting tension of tissue strips was significantly reduced by exposure to a solution high in magnesium (5-30 mM). The contractile response to field stimulation under resting conditions, and the contraction to phenylephrine, were significantly decreased by magnesium (5-30 mM). There were no differences in the contractile strength of the corpus cavernosum to KCl. Although the relaxation induced by field stimulation under preincubation with 200 microM phenylephrine was abolished in the presence of 30 mM magnesium, there were no differences at a concentration of 5 mM or of 10 mM magnesium. The relaxation induced by sodium nitroprusside under precontraction with 200 microM phenylephrine was further increased by magnesium dose dependently. A high concentration of magnesium (30 mM) enhanced both bethanechol-induced and ATP-induced relaxations under precontraction with phenylephrine. Our study demonstrated that magnesium reduced the receptor-mediated contraction of the rabbit corpus cavernosum and enhanced the relaxation of this tissue induced by sodium nitroprusside, bethanechol, and ATP.

Involvement of nitric oxide in the non-adrenergic non-cholinergic neurotransmission of horse deep penile arteries: role of charybdotoxin-sensitive K(+)-channels

1. The involvement of nitric oxide (NO) and the signal transduction mechanisms mediating neurogenic relaxations were investigated in deep intracavernous penile arteries with an internal lumen diameter of 600-900 microns, isolated from the corpus cavernosum of young horses. 2. The presence of nitric oxide synthase (NOS)-positive nerves was examined in cross and longitudinal sections of isolated penile arteries processed for NADPH-diaphorase (NADPH-d) histochemistry. NADPH-d-positive nerve fibres were observed in the adventitia-media junction of deep penile arteries and in relation to the trabecular smooth muscle. 3. Electrical field stimulation (EFS) evoked frequency-dependent relaxations of both endothelium-intact and denuded arterial preparations treated with guanethidine (10(-5) M) and atropine (10(-7) M), and contracted with 10(-6) M phenylephrine. These EFS-induced relaxations were tetrodotoxin-sensitive indicating their non-adrenergic non-cholinergic (NANC) neurogenic origin. 4. EFS-evoked relaxations were abolished at the lowest frequency (0.5-2 Hz) and attenuated at higher frequencies (4-32 Hz) by the NOS inhibitor, NG-nitro-L-arginine (L-NOARG, 3 x 10(-3) M). This inhibitory effect was antagonized by the NO precursor, L-arginine (3 x 10(-3) M). NG-nitro-D-arginine (10(-4) M) did not affect the relaxations to EFS. 5. Incubation with either the NO scavenger, oxyhaemoglobin (10(-5) M), or methylene blue (10(-5) M), an inhibitor of guanylate cyclase activation by NO, caused significant inhibitions of the EFS-evoked relaxations, and while oxyhaemoglobin abolished the relaxations to exogenously added NO (acidified sodium nitrite, 10(-6) - 10(-3) M), there still persisted a relaxation to NO of 24.4 +/- 5.1% (n = 6) in the presence of methylene blue. 6. Glibenclamide (3 x 10(-6) M), an inhibitor of ATP-activated K(+)-channels, did not alter the relaxations to either EFS-stimulation or NO, while the blocker of Ca(2+)-activated K(+)-channels, charybdotoxin (3 x 10(-8) M), caused a significant inhibition of both the electrically-induced relaxations and the relaxations to exogenously added NO. Furthermore, charybdotoxin blocked relaxations induced by the cell permeable analogue of cyclic GMP, 8-bromo cyclic GMP (8 Br-cyclic GMP). 7. These results suggest that relaxations of horse deep penile arteries induced by NANC nerve stimulation involve mainly NO or a NO-like substance from nitrergic nerves. NO would stimulate the accumulation of cyclic GMP followed by increases in the open probability of Ca(2+)-activated K(+)-channels and hyperpolarization leading to relaxation of horse penile arteries.

Isolation and perfusion of the pudendal vasculature in male rats

PURPOSE

The present study outlines a novel in situ technique to assess the regulation of vascular resistance in the penile vascular bed of the Wistar rat.

MATERIALS AND METHODS

The isolation and perfusion of the pudendal artery were achieved by ligating all branches of the external iliac artery not directly connected to the internal pudendal artery.

RESULTS

A linear flow-perfusion pressure curve was generated to ensure a viable preparation. A cumulative concentration-vascular response curve to the alpha 1-adrenoceptor agonist methoxamine (0.5-64 micrograms./ml.) was obtained.

CONCLUSIONS

This novel methodology will allow reproducible and consistent quantitative assessments of the numerous factors (both neural and hormonal) that are proposed to govern the flow of blood in the penile vascular bed.

Calcitonin gene-related peptide: possibly neurotransmitter contributes to penile erection in monkeys

The distribution of calcitonin gene-related peptide (CGRP) immunoreactivity in the cavernous tissue and the erectile response to intracavernous injection of CGRP was investigated in 7 monkeys. Intracavernous CGRP increased cavernous arterial flow and induced cavernous smooth muscle relaxation and venous outflow occlusion. Intracavernous injection of CGRP antibody did not significantly change the erectile response to cavernous nerve stimulation. Histologic staining for CGRP immunoreactivity showed nerve fiber-like staining within the cavernous arterial wall and the cavernous smooth muscles. These data suggest that CGRP may contribute to penile erection in monkeys.

Sympathetic inhibition of papaverine induced erection

We studied the effect of neurostimulation of the lumbar sympathetic trunks on papaverine-induced penile erection in dogs and monkeys. The mean of 124 cm. H2O intracavernous pressure after papaverine injection was reduced under sympathetic trunk stimulation to 40.6 cm. H2O within a mean of 39 sec. In addition the flow of the internal pudendal artery was reduced indicating a decrease of the inflow to the penis. After the sympathetic stimulation was terminated, the intracavernous pressure increased again to the same level as before. This pressure recovery time was delayed after several sympathetic stimulations from 134 sec. (5 min. after papaverine injection) to at least 362 sec. (45 minutes after papaverine injection). Stimulation of the inferior hypogastric plexus had no effect on the intracavernous pressure. The neurostimulation range (3-4V; 20 Hz) was most effective in reducing the papaverine action on penile smooth muscles. The effect of sympathetic trunk stimulation seems to antagonize the relaxing action of papaverine on the cavernous smooth muscles and the cavernous arterioles. Our results imply that high sympathetic tone in some patients with psychogenic impotence may explain their poor response to intracavernous injection of papaverine.

The sympathetic role as an antagonist of erection

The effects of the lumbar and pelvic sympathetic system on penile erection were studied in a canine model. Erection was induced by cavernous nerve stimulation and detumescence by sympathetic trunk stimulation. Erection induced by cavernous nerve stimulation normally subsides slowly. After discontinuation of electrical stimulation the intracavernous pressure drops within a mean of 291 s to 50% and after a mean of 372 s to 10% of the highest level. However, stimulation of the sympathetic trunk at the level of L4-S1 applied directly after discontinuation of cavernous nerve stimulation accelerated this drop of intracavernous pressure significantly: to 50% after a mean of 19 s and to 10% after a mean of 36 s. If stimulation of the sympathetic trunk was initiated 20 s before cavernous nerve stimulation, the pressure rise was aborted completely. Neurostimulation of the hypogastric nerves alone or in combination with cavernous nerve stimulation did not change the intracavernous pressure. These results were not altered after neurotomy of the pudendal or hypogastric nerves. The main pathway of the fibers from the sympathetic trunk to the penis seems to run via the pelvic plexus. The stimulation voltage and frequency to induce erection or detumescence were equivalent. Our results suggest that an elevated central sympathetic tone may be one of the causes of psychogenic impotence.

Fine structure of the human corpus cavernosum

This paper demonstrates the ultrastructure of the human corpus cavernosum from eight male transexuals (aged 20 to 30 years) undergoing penectomy. The presence of collagen, smooth muscle, endothelial cells lining cavernous spaces, mast cells, and different types of nerve terminals, including those of a nonadrenergic and noncholinergic type, are illustrated.

K(+)-channel openers for relaxation of isolated penile erectile tissue from rabbit

The effects of the K(+)-channel openers (KCOs) cromakalim (BRL 34915) and pinacidil were investigated and compared with those of papaverine on isolated corpus cavernosum from rabbit. Preparations were mounted in organ baths and isometric tension was recorded. Spontaneous contractile activity was effectively abolished by the KCOs tested, cromakalim being the most potent of them. The KCOs concentration-dependently and effectively depressed electrically induced contractions and also contractions induced by exogenously applied noradrenaline and by low (less than or equal to 20 mM) concentrations of K+. Cromakalim was three to four times more potent than pinacidil. Pinacidil and cromakalim were shown to cause increases in the efflux of 86Rb from preloaded cavernous tissue. Papaverine also effectively depressed spontaneous contractile activity, and contractions evoked by electrical stimulation and noradrenaline. It had a potency 19 to 36 times lower than that of cromakalim. However, papaverine did not increase 86Rb efflux from preloaded tissue. The results show that cromakalim and pinacidil effectively relax penile erectile tissue, probably by the opening of K(+)-channels and subsequent hyperpolarization. Further investigations on human material seems motivated in order to elucidate if the principle of K(+)-channel opening offers any therapeutic advantages to other drugs in the diagnosis and treatment of penile erectile dysfunction.

Actions of endothelin on isolated corpus cavernosum from rabbit and man

The effects of endothelin, a vasoconstrictor peptide produced by vascular endothelial cells, were investigated in isolated rabbit and human corpus cavernosum (CC). Preparations from both rabbit and man were potently contracted by endothelin in a concentration-dependent manner. The contractions developed slowly, could not be reversed despite frequent washings, and were only partly inhibited by the Ca2+ channel blocker nimodipine. Even in Ca2(+)-free medium containing the chelator EGTA a small contractile component persisted. In rabbit CC, the contractions in Ca2(+)-free medium were not affected by nimodipine, the Ca2(-)-channel agonist BAY K 86(44), or by depletion of intracellular Ca2(+) stores sensitive to noradrenaline (NA) and caffeine, but were almost abolished by the protein kinase C inhibitor H7. In both rabbit and man, carbachol and vasoactive intestinal polypeptide concentration-dependently relaxed preparations contracted by endothelin. The relaxations induced by carbachol were antagonized by atropine. Endothelin enhanced concentration-dependently the contractions induced by exogenously applied NA in rabbit CC. The enhancement was more pronounced at low concentrations of NA. This study shows that endothelin potently contracts isolated penile erectile tissue. The contraction seems to be mediated mainly by influx of Ca2+ through the cell membrane, which partly occurs through a pathway other than voltage-operated calcium channels. However, involvement of other mechanisms cannot be excluded. The results suggest that endothelin can play a role in penile erectile mechanisms.

A possible role for calcitonin-gene-related peptide in the regulation of the smooth muscle tone of the bladder and penis

We investigated the effect of calcitonin-gene-related peptide (CGRP) on bladder contractions and penile erection in 12 dogs. In a system in which the arteries were tied bilaterally to ensure delivery of high drug levels to the bladder, arterial injections of CGRP significantly reduced the peak intravesical pressure of bladder contractions induced by pelvic nerve stimulation or arterial injection of carbachol. When given intravenously, CGRP had no effect on bladder contractions consequent to neural stimulation. Intravesical instillation of CGRP, however, reduced the bladder contractions significantly. Histologic staining showed CGRP-immunoreactive nerve fibers within the smooth muscle layers of the bladder wall. Intracavernous CGRP increased cavernous arterial flow and induced cavernous smooth muscle relaxation and venous outflow occlusion. Muscarinic blockade had no effect on the canine intracavernous pressure response to intracavernous injection of CGRP. Histologic staining for CGRP-immunoreactivity showed nerve-fiber-like staining within the cavernous arterial wall, the nerves running near the cavernous arteries, and the cavernous smooth muscles. Our results suggest a possible role for CGRP in the regulation of the smooth muscle tone of the bladder and penis.

Central effect of yohimbine on sexual behavior in the rat

A large range of doses of yohimbine (Y) was administered intracerebroventricularly (ICV) (5-100 micrograms/rat) or intraperitoneally (IP) (0.35-10 mg/kg) to male rats and the effects on sexual, locomotor and general behavior were evaluated. For both routes there was a clear-cut inverted-U effect (stimulating/depressing), calculable as parabolic regressions on the log of administered doses. The maximal stimulating doses (15 micrograms/rat ICV and 1 mg/kg IP) significantly shortened mount, intromission and ejaculation latencies and the mean interintromission interval. These data indicate the importance of CNS mechanisms in the sexual effect of Y.

Cholinergic constriction in the general circulation and its role in coronary artery spasm

The release of acetylcholine from autonomic nerves in those tissues that receive a cholinergic innervation is widely believed to dilate blood vessels. Exogenously administered acetylcholine in vivo does dilate vascular beds and produce hypotension; however, this latter effect is indirect and probably the result of liberation of endothelium-derived relaxing factor (EDRF) from endothelial cells. Some blood vessels contain a substantial population of medial constrictor receptors for acetylcholine, and the implications of this presence for vascular control systems has been largely ignored, although it needs to be considered. A survey of the evolution of vasomotor control systems indicates that acetylcholine serves principally as an excitatory transmitter to blood vessels. Neurally mediated cholinergic constriction and not dilation is found in fish, amphibians, reptiles, and birds, with responses initiated by medial muscarinic receptors. Acetylcholine constricts many vascular preparations from these lower animals, but some vessels relax, reflecting the emergence of an EDRF responsive to acetylcholine. An examination of cholinergic responses in mammalian vessels reveals that cholinergic (neurogenic) dilation is limited to a very few vascular beds and to only a few species. Both experimental evidence and evolutionary considerations support the likelihood that cholinergic (neural) constriction operates in some vascular regions in mammals and, in particular, in the coronary circulation of some species, including humans. In fact, constriction, and not dilation, may be the dominant vascular response to activation of the cholinergic axis in most mammals, including humans. The complications and contradictions introduced by the simultaneous presence of both EDRF and a cholinergic constrictor innervation involving medial muscarinic receptors are discussed. A variety of evidence is also presented that implicates cholinergic constriction in at least some instances of coronary artery spasm and sudden death.

Acetylcholine as a possible neurotransmitter in penile erection

We investigated the erectile response to intracavernous injection of increasing doses of acetylcholine (0.5 to 500 micrograms.) in 10 monkeys. To differentiate between nicotinic (ganglionic) and muscarinic (parasympathetic postganglionic) effects, acetylcholine was likewise administered after 1.6 mg. trimethaphan camsylate and 0.1 mg. atropine, alone or sequentially. Erections were induced by cavernous nerve stimulation before and after atropine. Acetylcholine induced a dose-dependent, triphasic erectile response: a first tumescence phase followed by contraction and a subsequent second phase of tumescence. Atropine reduced but did not abolish the erectile response to acetylcholine: attainment of maximal intracavernous pressure after neurostimulation was both delayed and reduced (mean 25 cm. H2O). Only after combined nicotinic and muscarinic blockade was the erectile response to acetylcholine completely abolished. Histologic staining for acetylcholinesterase in five additional monkeys that had not received acetylcholine showed dense staining within the cavernous erectile tissue and around the cavernous arteries. Our data suggest that acetylcholine is a possible neurotransmitter for penile erection in monkeys.

Responses of erectile tissue from impotent men to pharmacological agents

Erectile tissue was removed from the corpora cavernosa of 25 impotent men undergoing surgery for insertion of penile prostheses. Strips, set up in an organ bath, were contracted by the alpha-adrenergic agonist phenylephrine. There was no significant difference between tissue taken from men with diabetes, alcoholism, Peyronie's disease or men with no obvious condition causing the impotence. The sensitivity of tissues from hypertensive patients was significantly reduced but this was probably due to drugs being taken for hypertension. Precontracted tissues could be relaxed by acetylcholine or isoprenaline. The responses, however, were inconsistent, so that no difference between the different groups of patients was apparent.

The rat as a model for the study of penile erection

A model has been developed for the study of penile erection in the Sprague-Dawley rat. Anatomical dissections demonstrate a bilateral ganglion lateral to the prostate called the major pelvic ganglion. This ganglion receives input from the pelvic and hypogastric nerves and innervates the pelvic viscera. A large fiber from the major pelvic ganglion courses along the urethra and innervates the corpus cavernosum, the cavernous nerve. In 40 animals, electrical stimulation of either the cavernous nerve or the pelvic nerve resulted in reproducible repetitive tumescence of the corpora cavernosum. Following ablation of the cavernous nerve, electrical stimulation failed to produce erections. Standard mating behavior tests of mounting, intromission and ejaculation in 38 rats showed that surgical ablation of the cavernous nerve resulted in a decrease in the rate of intromissions and ejaculations compared with sham operated controls. Present models for the study of erection have been limited to the dog, monkey and cat. The rat model presented here offers several advantages over these existing models: 1) the cavernous nerve is easily identified, 2) electrical stimulation is easily accomplished and reproducible, 3) behavioral and neurophysiological studies are possible, and 4) animal purchase, housing, and maintenance costs are low. These advantages make this model a uniquely useful tool in the further study of penile erection.

Autoradiographic evidence of muscarinic cholinergic receptors in the corpora cavernosa penis of the rat

Recent studies have questioned the role of acetylcholine in the physiology of penile erectile tissue. The responsiveness of penile erectile tissue to acetylcholine would depend, in part, on the presence of cholinergic receptors on the smooth muscle. The specific binding of [3H]quinuclidinyl benzilate (QNB) to cholinergic receptors in sections of penile crura of the rat was analyzed by in vitro neurotransmitter autoradiography. Silver grain density measurements indicated that muscarinic cholinergic receptor binding sites are located almost entirely over the corpora cavernosa penis. Virtually no specific [3H]QNB binding was present in the tunica albuginea or adjacent skeletal muscle tissue. Within the erectile tissue, specific binding occurred both over the columns of intrinsic smooth muscle which form the walls of the cavernous spaces and around the more distal branches of the penile arteries. The high concentration of muscarinic cholinergic receptors in the corpora cavernosa penis is consistent with the suggestion that acetylcholine has an important, albeit undefined role in the function of penile erectile tissue.

Vascular changes during penile erection in the dog

1. The vascular effects of pelvic nerve stimulation on the penis were studied in dogs anaesthetized with sodium pentobarbitone and halothane. Changes in pressure and blood flow were measured through scalp vein needles inserted into the erectile bodies. 2. The penis contains two types of erectile body, which responded independently during erection induced by pelvic nerve stimulation. Pressure in the corpus spongiosum increased immediately upon stimulation, but only reached one-third of the more delayed pressure response of the corpora cavernosa. 3. At rest, arterial inflow resistance into the corpora cavernosa was high, whereas venous outflow resistance was low. Pelvic nerve stimulation (10-50 V, 10-16 Hz, 1 ms) caused an immediate increase in arterial flow, an increase in corpus cavernosal pressure (CCP), and a decrease in venous outflow. Saline infusion experiments showed there was active venous occlusion. Upon cessation of stimulation, these parameters returned to pre-stimulation levels. 4. The time taken to reach 50% of maximum change in arterial inflow was significantly less than for CCP, which was significantly less than for venous outflow. Occlusion of the aorta 1 min after cessation of stimulation decreased the pressure in the arterial tree supplying the corpora cavernosa, but CCP remained elevated, indicating that both inflow and outflow resistances were high. Thus, inflow resistance had returned to its pre-stimulation state before outflow resistance. 5. Direct measurements of blood flow through the corpus cavernosum were made with a hydrogen probe. There was a transient increase in blood inflow as CCP increased during pelvic nerve stimulation. There was some blood flow while CCP was elevated, indicating that the venous occlusion was not complete. 6. Sympathetic chain stimulation caused an increase in arterial resistance, and a decrease in CCP and venous resistance. 7. Infusions of acetylcholine (330 micrograms min-1) and vasoactive intestinal polypeptide (1-3.3 micrograms min-1) decreased arterial resistance and increased CCP and venous resistance. 8. This study suggests that during pelvic nerve-induced erection, arterial flow into the corpus cavernosum increases, followed by an increase in CCP and an actively controlled decrease in venous outflow.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of smooth muscle strips from penile erectile tissue to drugs and transmural nerve stimulation

1. The mechanical response to drugs and to electrical stimulation of nerves was investigated in isolated strips of intrinsic smooth muscle from the corpora cavernosa penis of the rat. 2. Noradrenaline caused muscle strips to contract in a dose-dependent manner. Contractions could be blocked by pretreatment with the alpha-adrenoreceptor antagonist, phentolamine. 3. Acetylcholine and carbachol had no effect on the baseline tension of muscle strips. Both drugs were relatively ineffective in relaxing noradrenaline-contracted strips. 4. Field stimulation of isolated muscle strips elicited contractions which were blocked by tetrodotoxin and greatly attenuated with phentolamine or reserpine pretreatment. Acetylcholine inhibited the excitatory response to field stimulation. This inhibitory effect of acetylcholine could be blocked with atropine. 5. Field stimulation of noradrenaline-contracted muscle strips caused relaxation. This inhibitory effect, due to nerves which arise from the pelvic plexus, is unaffected by substances which act on cholinergic systems. 6. The results suggest that the erectile muscle of the rat is similar to that of man in that it receives an excitatory noradrenergic innervation and an inhibitory innervation which may have a non-cholinergic component. Although acetylcholine may have a role in penile physiology of the rat, it is unlikely that it has a postsynaptic action.

The anatomy of erection

Erection is due to a neurovascular mechanism: an inflow of arterial blood and an obstruction of venous return. The direction of this mechanism is of neurologic origin: (1) control of the smooth muscle tissue of the corpora cavernosa by the autonomic system controlled by reflex centers in the cord or cortical centers accessible to multiple (visual) stimuli (2) control of the perineal muscles by the somatic system. Tumescence is due to a reduction in the alpha-sympathetic tonus of the cavernous tissue permitting influx of arterial blood, and to decreased venous flow from compression of the subalbugineal venous network against the tunica albuginea of the corpus cavernosum. Once this obstruction to the venous return has been achieved, the arterial flow in the corpora cavernosa decreases but persists; the arterial blood in the corpora cavernosa is renewed and not shut off. Rigidity is due to an increase in intracavernous arterial pressure simultaneous with contraction of the perineal muscles (ischiocavernosus) under the somatic control of the pudendal nerve.

Changes in the VIPergic, cholinergic and adrenergic innervation of human penile tissue in diabetic and non-diabetic impotent males

Penile tissue (consisting of corpus cavernosum and tunica albuginea) was obtained from 19 patients undergoing surgery for the implantation of penile prostheses. The tissue was examined for vasoactive intestinal polypeptide-like immunoreactivity in nerves, acetylcholinesterase-positive staining in nerves and noradrenaline content. Impotence was due to a variety of causes; 11 patients were classified as a 'non-neuropathic' group on the basis of their clinical history which included Peyronie's disease, vascular disease, hypertension and psychogenic impotence. Vasoactive intestinal polypeptide-like immunoreactive and acetylcholinesterase-positive nerves were present and the pattern and distribution were similar in each patient in this group. The noradrenaline content of the tunica albuginea was significantly lower than the corpus cavernosum (p less than 0.02), although there was a linear relationship between the noradrenaline contents of the two regions (r = 0.95, p less than 0.01). By comparison, a complete absence of vasoactive intestinal polypeptide-like immunoreactivity in nerves was observed in a patient with a cauda equina lesion. Five out of six diabetic patients studied revealed a marked reduction in vasoactive intestinal polypeptide-like immunoreactivity in nerves associated with the cavernous smooth muscle, while acetylcholinesterase-positive staining was reduced in three out of five diabetic patients studied. The noradrenaline content of the corpus cavernosum from diabetic patients was significantly lower (p less than 0.02) than that of the 'non-neuropathic' group. The noradrenaline content of the tunica albuginea, however, was similar in both groups. The results provide evidence that VIPergic, cholinergic and adrenergic nerves in the penis are affected in diabetes mellitus and thus may contribute to the development of impotence in diabetic patients.

Penile electrodiagnosis. Value of bulbocavernosus reflex latency versus nerve conduction velocity of the dorsal nerve of the penis in diagnosis of diabetic impotence

The nerve conduction velocity of the dorsal nerve of the penis was evaluated with a direct measuring technique in impotent men with and without diabetes mellitus. The average nerve conduction velocity was 37 M. per second in impotent diabetics and 45 M. per second in nondiabetics. The latency of the bulbocavernosus reflex showed no significant difference between the groups and was within normal limits. The measurement of the nerve conduction velocity of the dorsal nerve of the penis is a valuable test for assessment of impotence in patients with diabetes mellitus.

Origin and central projections of rat dorsal penile nerve: possible direct projection to autonomic and somatic neurons by primary afferents of nonmuscle origin

Cell number, size, and somatotopic arrangement within the spinal ganglia of the cells of origin of the rat dorsal penile nerve (DPN), and their spinal cord projections, were studied by loading the proximal stump of the severed DPN with horseradish peroxidase (HRP). The DPN sensory cells were located entirely in the sixth lumbar (L6) dorsal root ganglia (DRG), in which a mean of 468 +/- 78 cells per side were observed, measuring 26.7 +/- 0.8 microns in their longest axis (range 10-65 microns) and distributed apparently randomly within the ganglia. Within the spinal cord, no retrograde label was found, i.e., no motoneurons were labeled, indicating that in the rat the DPN is formed exclusively of sensory nerve fibers. Although labeled fibers entered the cord only through L6, transganglionically transported HRP was evident in all spinal segments examined, i.e., T13-S2. Labeled fibers projected along the inner edge of the dorsal horn (medial pathway) throughout their extensive craniosacral distribution. However, laminar distribution varied with spinal segment. In the dorsal horn, terminals or preterminal axons were found in the dorsal horn marginal zone (lamina I), the substantia gelatinosa (lamina II), the nucleus proprius (laminae III and IV--the most consistent projection), Clarke's column (lamina VI), and the dorsal gray commissure. In the ventral horn, terminals were found in lamina VII and lamina IX. Label apposed to cell somas and dendrites in lamina VII may represent direct primary afferent projections onto sympathetic autonomic neurons. In lamina IX, labeled terminals delineated the somas and dendrites of cells that appeared to be motoneurons. This is the first description of an apparently monosynaptic contact onto motoneurons by a primary afferent of nonmuscle origin.

Pharmacology of the erectile tissue of the canine penis

To assess the role of different neuroeffector systems in penile erection, pharmacological agents were tested in vitro using strips of corpus cavernosum tissue from the dog. Alpha-adrenoceptor agonists, K+ or histamine caused concentration-dependent contraction. Tyramine caused a poorly sustained contraction, as did nicotine in some cases. In precontracted preparations, relaxation was caused by beta2-adrenoceptor or muscarinic cholinoceptor agonists, or by vasoactive intestinal peptide. The results suggest that a noradrenergic alpha-adrenoceptor system maintains penile flaccidity in the dog. Penile erection may result from the activation of inhibitory beta2-adrenergic, muscarinic and/or peptidergic systems.

Characterization of muscarinic cholinergic receptor binding in the vas deferens, bladder, prostate and penis of the rabbit

Radioligand receptor binding techniques were used to characterize the muscarinic cholinergic receptor in the vas deferens, bladder, prostate and penis of the rabbit. This study represents the first comparative investigation of a neurotransmitter receptor in the genitourinary tract using radioligand receptor binding methods. A single high affinity muscarinic binding site was identified in the vas deferens (Kd = 0.16 nM), bladder (Kd = 0.15 nM) and prostate (Kd = 0.17 nM), using [3H]N-methylscopolamine ([3H]NMS), a muscarinic antagonist. Two high affinity [3H]NMS binding sites (Kd1 = 0.08 nM; Kd2 = 1.39 nM) were found in the penis. The pharmacology of the NMS binding sites in the vas deferens, bladder and prostate was characterized by competitive binding experiments with [3H]NMS and several unlabelled muscarinic and nonmuscarinic drugs. Noncholinergic drugs, as expected, were weak inhibitors of [3H]NMS binding. The values of the IC50's for the muscarinic drugs atropine, pirenzepine and oxotremorine in the genitourinary tissues of the rabbit were similar to values reported in nongenitourinary tissues demonstrating homogeneity of muscarinic receptors. The mean Hill coefficients for the muscarinic antagonist (atropine 0.81 to 0.878) were significantly different than for the muscarinic agonist (oxotremorine 0.39 to 0.44) in all genitourinary tissues, a binding property of muscarinic receptors identified in other tissues. The requirements for the characterization of cholinergic muscarinic receptors have been fulfilled for several genitourinary tissues of the rabbit. Radioligand receptor binding methods can now be applied to investigating the relationship between genitourinary dysfunction and alterations in the muscarinic cholinergic receptors.

Peptidergic innervation of the human male genital tract

Four peptides--vasoactive intestinal polypeptide, substance P, somatostatin and a peptide-like avian pancreatic polypeptide--have been found in nerves of the human male genitalia using highly sensitive and specific methods of immunocytochemistry and radioimmunoassay. Five other peptides (met-enkephalin, leu-enkephalin, neurotensin, bombesin and cholecystokinin-8) were absent. Vasoactive intestinal polypeptide was the most abundant peptide, its highest concentration being in the proximal corpus cavernosum. Immunoelectron microscopy localized this peptide to large (97 +/- 20 nm), round, electron-dense granules of p-type nerve terminals. Vasoactive intestinal polypeptide-immunoreactive neuronal cell bodies were found in the prostate gland and the root of the corpus cavernosum. Substance P immunoreactive material was present in smaller concentration and was mainly localized in nerves around the corpuscular receptors of the glans penis. Somatostatin immunoreactive nerves were associated mainly with the smooth muscle of the seminal vesicle and the vas deferens. When antiserum to avian pancreatic polypeptide was applied, certain nerves were stained, particularly in the vas deferens, the prostate gland and the seminal vesicle. However, chromatography detected no pure avian pancreatic polypeptide suggesting the presence of a structurally related substance, possibly neuropeptide Y, which cross-reacts with the avian pancreatic polypeptide antiserum. Similar distributions between vasoactive intestinal polypeptide-immunoreactive and acetylcholinesterase-positive nerves and between avian pancreatic polypeptide-immunoreactive and adrenergic nerves were observed. A general neuronal marker, neuron-specific enolase, was used to investigate the general pattern of the organ's innervation. The abundance and distribution patterns of these peptide-immunoreactive nerves indicate that they may play important roles in the male sexual physiology.

Cholinoreceptors in the corpus cavernosum muscle of the human penis

Acetylcholine produced contraction, relaxation or a biphasic effect on corpus cavernosum muscle strips of human penis. These effects were antagonized by atropine but were modified by hexamethonium or tetrodotoxin suggesting the presence of excitatory and inhibitory muscarinic receptors in the human penis. Nicotine and DMPP also produced contraction or relaxation of the corpus cavernosum muscle strips. These effects were presumably indirect as they were abolished by hexamethonium and tetrodotoxin. Contractions produced by nicotine or DMPP were abolished by atropine whereas the relaxations produced by these agents were only partially blocked by atropine. Hence, the receptors mediating these relaxations are unlikely to be entirely muscarinic.

Noradrenergic innervation of the penis in control and streptozotocin-diabetic rats: evidence of autonomic neuropathy

The noradrenergic sympathetic innervation of the penis of control and 4-month streptozotocin-diabetic rats was examined with the glyoxylic acid histofluorescence method. Noradrenergic varicosities were found in the corpora cavernosa in a dense subtunical plexus and in the perisinusoidal and trabecular regions of the erectile tissue, in the corpus spongiosum in perisinusoidal tissue, around large arteries and veins, and around small tortuous arterioles and small draining veins of the corpora cavernosa and spongiosum. Noradrenergic varicosities were diminished in number and fluorescent intensity in all regions of the penis of diabetic rats compared with controls. The subtunical plexus was absent, perisinusoidal and trabecular varicosities were sparse, and only occasional intermittent, discontinuous, dull fluorescent fibers or plexuses were found around the vessels. Quantitation with high-performance liquid chromatography revealed a significant reduction of norepinephrine in the penis of diabetic rats compared with controls. The present study suggests that long-term streptozotocin diabetes in the rat is accompanied by sympathetic autonomic neuropathy of the penis that seems to parallel changes in the noradrenergic content of penile corpora of men with diabetes and erectile impotence. The streptozotocin-diabetic rat merits further study to explore the relationship between noradrenergic innervation of the penis and erectile tissue.

High pressure within corpus cavernosum in man during erection. Its probable mechanism

Corpus cavernosum pressure in man during erection has not been measured previously with a noninvasive technique. Using such a technique it has been shown that during erection in young men intracavernosal pressure has peaks of at least ten times higher than the systemic systolic pressure. These findings indicate that during erection the intracavernosal circulation is contained, at least partially, within a closed system, separated from the general circulation. Also erection cannot be the result of circulatory function only.

The vasculature of the human penis: a reexamination of the morphological basis for the polster theory of erection

The morphology of human penile blood vessels has been studied to characterize columns of smooth muscle found in the intima. Although previous workers (Kiss, 1921; Conti, 1952) termed these intimal pads or polsters and concluded that they were actively responsible for the production of erection, more recent work has questioned these conclusions (Benson et al., 1980; Newman and Tchertkoff, 1980). Serial sections were taken throughout the length of the penis from 11 cadavers and stained with various histopathological stains. Numerous polster-like structures were found, but all could be identified as either branch points in vessels, intimal cushions located at branch points, or arteriosclerotic lesions at various stages of development. None of these structures seem capable of functioning in the manner attributed to them by Conti (1952) and others. Furthermore, comparison of the structures found in the present study to illustrations of polsters in Conti's paper suggest that these are identical. The present study refutes the polster theory of erection and leaves open to further question the mechanisms involved in the control of blood flow into and within the penis.

Autonomic innervation of the urogenital system: adrenergic and cholinergic elements

The major organs of the male urogenital (UG) system have been examined in various mammals, including man, using light and electron microscopic (EM) histochemical methods. For the light microscopic study, the urinary bladder, the vas deferens and the penis (corpora cavernosa and corpus spongiosum) were studied in the rat, cat, dog, monkey and man using a glyoxylic acid (GA) method modified for peripheral adrenergic nerve fibers, and a thiocholine method for acetylcholinesterase (AChE). Fine structural analysis was done on the vasa of rat, cat, monkey and man, and on the bladder and penis of cat, dog, monkey and man. Tissue was fixed in glutaraldehyde (GMO) as a control or in glutaraldehyde-dichromate (GDC) for the specific localization of norepinephrine (NE). All organs studied demonstrated numerous adrenergic nerve fibers throughout the muscular layers, in the connective tissue, and in the adventitia of most blood vessels. These fibers had a brilliant fluorescence when visualized with the GA method, and demonstrated many varicosities with small (400-600 A) and/or large (800-1200 A) granular vesicles in both control and GDC-fixed tissue examined with the EM. Evaluation of the vesicles with the analytical electron microscope (AEM) verified that those in the GDC-fixed tissue were chrome-positive, and, therefore, NE-containing. In the vas and penis, acetylcholinesterase(AChE)-positive nerve fibers were encountered less frequently at the light microscopic level than adrenergic fibers, and few typical cholinergic varicosities were seen in these organs with the EM. In the bladder, cholinergic nerves were seen with about the same frequency as adrenergic fibers in both light microscopic and EM preparations. Also observed frequently in each of the viscera were varicosities with large to very large (800-2000 A) granular vesicles of the kind presently hypothesized to be peptidergic or purinergic. Few varicosities of the type considered sensory, with large (800-1200 A) clear vesicles and numerous mitochondria, were observed in this tissue. Evidence from this study suggests that mammalian UG organs are innervated extensively by adrenergic nerves, and, excepting the bladder, have a limited cholinergic innervation; in the bladder, numerous fibers of each type can be found. In addition, another type of nerve fiber, perhaps peptidergic or purinergic, is found in large numbers in each of the organs studied and thus may represent a significant effector of autonomic regulation.

Electron microscopic cytochemical correlates of histofluorescence

The biogenic amines, norepinephrine and dopamine, have been localized electron microscopically in the substantia nigra, pineal body, adrenal medulla and stellate ganglion of squirrel monkeys. The method of localization has been by the use of a specific cytochemical technique, utilizing a chromium (Cr) complex with an isoquiniline derivative formed by the reaction of the unsubstituted biogenic amine with glutaraldehyde. The final reaction product, i.e., amine-glutaraldehyde-Cr, has been identified using energy dispersive X-ray analysis and the positive Cr reactive sites have been correlated with light microscopy conducted by using glyoxylic acid histofluorescence. All tissues were taken from squirrel monkeys which had either been perfused with the fixative or had been perfused with an F-12 buffer prior to fixation and/or histochemical treatment. The best results were obtained from the F-12 perfusion followed by tissue being taken through the various histochemical-cytochemical procedures. The electron microscopic localization of deposits shows significantly large structures containing the Cr positive amine material. These large deposits have not heretofore been described and correlate well in size and location with what is seen with histofluorescence. This electron microscopic technique should lead to further advances in the study of biogenic amines in nervous tissue when fine structural localization, plus well preserved morphology, are essential.