SEARCH

SEARCH BY CITATION

Keywords:

  • Ejaculation;
  • erection;
  • gastrin-releasing peptide;
  • lumbar spinal cord;
  • posttraumatic stress disorder;
  • testosterone

Abstract

  1. Top of page
  2. Abstract
  3. Local Neuronal Circuits in the Lumbar Spinal Cord Controlling Erection and Ejaculation
  4. A Recently Identified Spinal GRP System
  5. The Effect of Androgens and Stress on Sexual Functions and on the Spinal Cord
  6. Conclusion
  7. Acknowledgment
  8. References

ABSTRACT: We recently reported a previously unknown peptidergic system within the lumbosacral spinal cord that uses gastrin-releasing peptide (GRP) to trigger erection and ejaculation in male rats. Many men suffering from stress, including posttraumatic stress disorder (PTSD) and major depressive disorder, report sexual dysfunction. Sexual dysfunction in men suffering from stress and major depressive disorder is traditionally treated via psychological counseling. To determine whether acute severe stress could alter the male-specific GRP system, we used single prolonged stress (SPS) exposure in a putative rat model for PTSD. Exposure of male rats to SPS decreases the local content and the axonal distribution of GRP in the lower lumbar spinal cord and results in an attenuation of penile reflexes in vivo. Pharmacological stimulation of GRP receptors remarkably restores penile reflexes in SPS-exposed male rats and in castrated male rats. The administration of a GRP agonist to these animal models interestingly induces spontaneous ejaculation in a dose-dependent manner. Furthermore, although the circulating level of androgens is normal 1 week after SPS exposure, there is a significant decrease in the expression of androgen receptor protein in lumbar segments 3 and 4 of the spinal cord. This might make the spinal center less responsive to androgens. In this report, I review findings on a recently identified spinal GRP system that could be vulnerable to stress and that controls male reproductive function. This system provides new insights into the clinical treatment of psychogenic erectile dysfunction triggered by stress and psychiatric disorders.

Sexual behaviors differ in males and females, suggesting that the mechanisms underlying these behaviors also differ between the sexes. Male sexual behavior is complex and depends on intrinsic and extrinsic factors, including olfactory, somatosensory, and visceral cues (Rosen and Sachs, 2000; Coolen, 2005). The lumbar spinal cord is of particular interest because it is sexually dimorphic and contains several neural circuits that, in association with neural circuits of the thoracic and sacral spinal cord, are important in eliciting male sexual responses (ie, erection and ejaculation) (Breedlove, 1985; Morris et al, 2004; Matsuda et al, 2008). For example, one spinal center that mediates male sexual reflexes is the spinal nucleus of the bulbocavernosus (SNB), which innervates striated muscles attached to the base of the penis (Breedlove and Arnold, 1981; Sengelaub and Forger, 2008). These muscles are responsible for penile rigidity in various animal species during reflexive erections and play a role in flips of the penis (Sachs, 1982). These reactions are eliminated by castration, but return in response to exogenous treatment with androgens such as testosterone (T) or the nonaromatizable dihydrotestosterone (DHT); they do not return in response to treatment with estrogens (Hart, 1973, 1979). The SNB system likewise responds to DHT during development (Breedlove and Arnold, 1983a,b) and in adulthood (Forger et al, 1992). Furthermore, there is also evidence that androgens act directly on the spinal cord to activate masculine copulatory behaviors (Hart and Haugen, 1968). Little is known about the spinal neurons—other than the SNB—that might respond to androgen and thereby modulate the sexual functions underlying these behaviors. We recently identified a gastrin-releasing peptide (GRP) system in the lumbar spinal cord that could be a primary mediator of male reproductive function (Sakamoto and Kawata, 2006, 2009; Sakamoto et al, 2008). On the other hand, the neural mechanisms within the spinal reproductive centers that are involved in stress-related male sexual dysfunction have never been investigated until recently. We have more recently demonstrated that acute severe stress can alter the male-specific GRP system. In a putative rat model of posttraumatic stress disorder (PTSD) (Liberzon et al, 1997), exposure to single prolonged stress (SPS) decreases (at the L3–L4 level) the lumbar spinal content of GRP and the axonal distribution of GRP in the lower lumbar spinal cord (at the L5–L6 level), and reduces the capacity of the system to respond to androgens. This results in an attenuation of penile reflexes in vivo more than 1 week later (Sakamoto et al, 2009, Sakamoto et al, 2009). In this report, I review current findings on the recently identified spinal GRP system controlling male reproductive function, which could be vulnerable to stress and thus provide new insights into the clinical treatment of erectile dysfunction (ED) triggered by stress and by psychiatric disorders.

Local Neuronal Circuits in the Lumbar Spinal Cord Controlling Erection and Ejaculation

  1. Top of page
  2. Abstract
  3. Local Neuronal Circuits in the Lumbar Spinal Cord Controlling Erection and Ejaculation
  4. A Recently Identified Spinal GRP System
  5. The Effect of Androgens and Stress on Sexual Functions and on the Spinal Cord
  6. Conclusion
  7. Acknowledgment
  8. References

Onuf's nucleus, located in the ventral horn of the sacral spinal cord of many mammals, including humans, is a sexually dimorphic nucleus that innervates the perineal muscles involved in copulatory behavior. The number of neurons in Onuf's nucleus is greater in males than in females (Onufrowicz, 1899; Sato et al, 1978; Nakagawa, 1980; Forger and Breedlove, 1986), and the distribution of serotonergic fibers and terminals in this nucleus also differs between the sexes (Kojima et al, 1983, 1984; Kojima and Sano, 1984). On the other hand, the SNB of rats in the fifth and sixth lumbar segments is homologous to Onuf's nucleus, in that it innervates the striated perineal muscles attached to the base of the penis (Breedlove and Arnold, 1980; Forger and Breedlove, 1986; Sengelaub and Forger, 2008). Male rats have larger and a greater number of SNB motoneurons than do females; this dimorphism results from differences in perinatal androgen signaling via a mechanism mediated by the androgen receptor (AR) (Breedlove and Arnold, 1980). Furthermore, a number of studies have revealed another group of neurons exhibiting sexual dimorphism in the lumbar spinal cord. This population of neurons express galanin (Newton, 1992), cholecystokinin (Phan and Newton, 1999), and enkephalin (Nicholas et al, 1999). These neurons are dorsolateral to the central canal in lamina X within the third and fourth lumbar spinal cord and project to the thalamic region of the brain. These so-called lumbar spinothalamic (LSt) neurons (Ju et al, 1987; Truitt et al, 2003) are also sexually dimorphic, with males possessing a greater number of neurons than females. Fos induction in LSt neurons is specifically associated with ejaculation, but not with mounting or intromission. This indicates that a specific subpopulation of LSt neurons conveys information associated with ejaculation (Truitt et al, 2003). Truitt and Coolen (2002) reported that a population of LSt neurons in lumbar segments 3 and 4 of the spinal cord acts as a “spinal ejaculation generator” because toxin treatments that selectively create lesions of the galanin-expressing neurons in this region completely eliminate ejaculation in rats, whereas other aspects of male sexual behavior remain unaffected. It is clear that these galaninergic neurons project to the thalamus (Ju et al, 1987; Truitt et al, 2003). Connections have been confirmed between LSt neurons and autonomic neurons projecting to the prostate or the seminal vesicle and the bulbocavernosus muscle at the thoraco-lumbar level and at the lower lumbar level (Xu et al, 2005, 2006; Sun et al, 2009, Sun et al, 2009). In terms of neuropeptides, however, scientists do not fully understand the molecular mechanisms in the center that directly trigger ejaculation or erection.

A Recently Identified Spinal GRP System

  1. Top of page
  2. Abstract
  3. Local Neuronal Circuits in the Lumbar Spinal Cord Controlling Erection and Ejaculation
  4. A Recently Identified Spinal GRP System
  5. The Effect of Androgens and Stress on Sexual Functions and on the Spinal Cord
  6. Conclusion
  7. Acknowledgment
  8. References

Bombesin-Like Peptides and Their Specific Receptors—

Bombesin was first discovered in the skin of the European frog Bombina bombina (Anastasi et al, 1971; McDonald et al, 1979). It has potent bioactivity in the mammalian nervous system. The mammalian bombesin-like GRPs (McDonald et al, 1979) and neuromedin B (NMB) (Minamino et al, 1983) were subsequently isolated. Considerable evidence indicates that they play a role in many physiological processes, including food intake (Ladenheim et al, 1996; Yamada et al, 2000; Ladenheim et al, 2009), circadian rhythms (Shinohara et al, 1993; Okamura and Ibata, 1994; Karatsoreos et al, 2006), itch (Sun and Chen, 2007; Sun et al, 2009, Sun et al, 2009), and fear memory consolidation (Shumyatsky et al, 2002; Roesler et al, 2004; Merali et al, 2006). In mammals, bombesin-like peptides act through a family of at least 3 G protein–coupled receptors—namely, the GRP-preferring receptor (GRP-R) (Battey et al, 1991), the NMB-preferring receptor (Wada et al, 1991), and the bombesin receptor subtype-3 (which is considered an orphan receptor) (Fathi et al, 1993). Thus, we focused on the function of the GRP/GRP-R system in the reproductive system because earlier studies have shown that the bombesin-like peptides play a significant role in the central regulation of the neuroendocrinological or autonomic systems or both (Panula et al, 1988; Battey and Wada, 1991) that are generally relevant to sexual behaviors in mammals.

Identification and Characterization of the GRP System in the Lumbar Spinal Cord—

Using immunocytochemistry directed against GRP, we found a group of GRP-containing neurons in the lumbar spinal region at the L3–L4 level that shows an obvious male-dominant sexual dimorphism (Sakamoto and Kawata, 2006; Sakamoto et al, 2008). These GRP-expressing neurons project axons to the more caudal lumbar spinal cord at the L5–L6 level (such as the sacral parasympathetic nucleus and the SNB) and are much more prominent in males than in females (Sakamoto and Kawata, 2006; Sakamoto et al, 2008, 2010) (Figure 1). To test whether the androgen signaling system directly influences the generation of sexual dimorphism of the GRP neurons, we examined genetically male rats (XY) carrying the testicular feminization mutation (Tfm) of the Ar gene (Bardin et al, 1971; Yarbrough et al, 1990). These rats develop testes embryologically and secrete T prenatally; however, because their AR protein is dysfunctional, they develop a wholly feminine exterior phenotype (Bardin et al, 1971; Yarbrough et al, 1990), including developing a clitoris rather than a penis. The spinal cord of genetically XY male rats carrying the Tfm allele of the Ar gene was hyperfeminine in terms of the GRP system. The spinal cord of these male rats had fewer GRP-immunoreactive neurons in the lumbar region, compared with that of the wild-type females (Sakamoto et al, 2008). These GRP-expressing neurons in the lumbar spinal cord of male rats express AR, but do not express the estrogen receptor alpha subtype. Because androgens such as T play a significant role in the generation of ejaculation in male rats (Hull and Dominguez, 2007) and humans (Meston and Frohlich, 2000), the presence of the AR in the GRP-expressing neurons of the ejaculation center offers a locus for androgenic modulation of ejaculation and other sexual reflexes. The hyperfeminine appearance in Tfm male rats demonstrates the dependence of this GRP-positive system on the AR. This further argues for the sex steroidal sensitivity of this system and its role in masculine behaviors. Because this system specifically relies on the peptide GRP, it might offer an avenue for pharmacological interventions to relieve psychogenic ED. Thus, it can be concluded that treatment with a specific GRP compound could be used to improve ejaculation in castrated rats. This represents the first identified neural pathway to reinstate ejaculation in a mammal without androgen (Figure 1).

image

Figure 1. . A schematic drawing summarizing the gastrin-releasing peptide (GRP) system, which controls male sexual functions in the lumbar spinal cord. A sexually dimorphic spinal cord system of GRP-containing neurons in the lumbar spinal cord (at the L3–L4 level)—“the ejaculation center”—projects axons to the autonomic center (eg, sacral parasympathetic nucleus [SPN]) and to the somatic center (eg, spinal nucleus of the bulbocavernosus [SNB]) in the lower lumbar spinal cord. These centers mediate penile reflexes and trigger ejaculation. AR indicates androgen receptor; GRP-R, GRP receptor.

Download figure to PowerPoint

The Effect of Androgens and Stress on Sexual Functions and on the Spinal Cord

  1. Top of page
  2. Abstract
  3. Local Neuronal Circuits in the Lumbar Spinal Cord Controlling Erection and Ejaculation
  4. A Recently Identified Spinal GRP System
  5. The Effect of Androgens and Stress on Sexual Functions and on the Spinal Cord
  6. Conclusion
  7. Acknowledgment
  8. References

The Correlation Between Psychogenic ED and Circulating Androgens—

It is widely accepted that PTSD is a psychiatric disorder involving long-lasting symptoms that can occur after exposure to a life-threatening traumatic event. It is characterized by intrusive memories (ie, flashbacks), a hyperarousal state, and avoidance of stimuli associated with the trauma (Pitman, 1997; Yehuda, 2002). Clinical data have indicated increased rates of sexual dysfunction, including erection and ejaculation difficulties, in men with PTSD (Kaplan, 1988, 1989; Cosgrove et al, 2002). Most combat veterans with PTSD experience clinically relevant sexual difficulties, and 69% of veterans have ED (Letourneau et al, 1997). Erections are clearly androgen dependent, as evidenced by a marked reduction in the frequency, amplitude, and rigidity of erections in men with hypogonadism (Rajfer, 2000). However, little is known about the role that androgen-dependent neuropathy within the central nervous system plays in the development of psychogenic ED. Previous studies on the interaction between stress and the hypothalamic-pituitary-gonadal axis indicate that the circulating total T level fluctuates in response to physical and psychological stress (Kreuz et al, 1972; Mason et al, 1988; Retana-Marquez et al, 2003). Because the comorbidity between ED and major depressive disorder (MDD) is high, it is possible that a reduced circulating total T level in patients with MDD contributes to the development of ED (Seidman and Roose, 2001). In contrast to patients with MDD, there is evidence that plasma or serum total T levels do not change in patients with combat-related PTSD (Mulchahey et al, 2001) or in refugees suffering from PTSD (Bauer et al, 1994). Therefore, any sexual difficulties accompanying PTSD appear to be unrelated to the circulating levels of androgens.

Androgenic Regulation of the GRP and SNB Systems in Adult Males and Females—

Orchiectomy of adult male rats significantly reduces the intensity of GRP-expressing fibers in the lumbar spinal cord 28 days later, but long-term androgen replacement averts this reduction (Sakamoto et al, 2008, 2009, 2009). It has also been demonstrated that castration of adult male rats leads to the shrinkage of SNB motoneuronal somata and dendrites, but this process can be prevented by treating castrates with exogenous T (Breedlove and Arnold, 1981; Kurz et al, 1986; Goldstein et al, 1990; Yang et al, 2004). Treating ovariectomized adult female rats with exogenous T increases the SNB motoneuronal cell size, but not to the level seen in males (Breedlove and Arnold, 1981; Sengelaub and Forger, 2008). Sakamoto et al (2008) have similarly demonstrated that long-term (28 days) treatment of castrated adult female rats with exogenous T does not fully masculinize their spinal cord in terms of GRP immunoreactivity. This suggests that androgens have organizational effects on the GRP neuronal cell number itself or on the ability of the neurons in adulthood to respond to androgens with increased GRP expression. On the other hand, chronic restraint stress exposure reportedly significantly suppresses plasma total T levels and also results in a substantial reduction in the synaptic density of SNB motoneuronal dendrites. T replacement in male rats can completely reverse this reduction (Matsumoto, 2005). These results taken together consequently indicate that T and the AR, as an androgen signaling system, are both necessary to maintain both the GRP system and the SNB system in the lumbar spinal cord (Sengelaub and Forger, 2008; Forger, 2009; Sakamoto and Kawata, 2009, Sakamoto and Kawata, 2009). These 2 systems could interact directly within the lumbar spinal cord. GRP axonal projections within the lower lumbar spinal cord (at the L5–L6 level) provide synaptic input onto the dendrites of SNB motoneurons (Sakamoto et al, 2010) or provide input via interneurons located between the GRP and SNB neurons. Gap junctions or paracrine communication (Zoli and Agnati, 1996) between GRP and SNB neurons within the lower lumbar spinal cord are more speculative avenues for the interaction between the GRP and SNB neurons. In any case, the activation or functional synchronization of both systems in these spinal centers is required for normal ejaculation.

The Relationship Between Circulating Androgens, Sexual Function, and Stress—

There are reports of a lowered plasma total T level in men with MDD and a negative correlation between total T levels and the severity of MDD (Yesavage et al, 1985; Mason et al, 1988). Exposing rats to chronic restraint stress (as a model of human depression) also reportedly decreases plasma total T levels (Retana-Marquez et al, 2003; Matsumoto, 2005). On the contrary, we did not find a significant difference between the SPS-exposed and control rats in plasma total T levels (Sakamoto et al, 2009, Sakamoto et al, 2009). This suggests that some mechanism underlies male reproductive dysfunction. To draw a firm conclusion, future attention should therefore be focused on a paradigm of the effects of chronic stress (as an MDD model) on the spinal GRP system. Our results, which were obtained with an animal model, are consistent with those of human subjects in previous reports that found normal plasma total T levels in patients with combat-related PTSD (Mulchahey et al, 2001) and in refugees suffering from PTSD (Bauer et al, 1994). However, cerebrospinal fluid (CSF) T levels are lower in patients with combat-related PTSD compared with normal controls (Mulchahey et al, 2001). The plasma level of total T in rats was normal 7 days after SPS exposure; however, we found a significant decrease in the expression of AR proteins in the lumbar spinal cord, but no change in estrogen receptor alpha proteins (Sakamoto et al, 2009, Sakamoto et al, 2009). We also previously reported that nearly every GRP-expressing neuron in the lumbar spinal cord (at the L3–L4 level) normally contains AR, but these neurons do not contain any estrogen receptor alpha (Sakamoto et al, 2008; Sakamoto and Kawata, 2009). On the other hand, there are no reports concerning the expression of estrogen receptor beta in lumbar spinal GRP neurons. These results taken together suggest that, after SPS exposure, the decline of AR expression in the lumbar spinal cord (at the L3–L4 level) reduces the expression level of GRP at the mRNA and protein levels in the lumbar spinal cord. This consequently contributes to sexual dysfunction, including erection and ejaculation difficulties (Figure 2).

image

Figure 2. . Possible mechanisms for the attenuation of the spinal gastrin-releasing peptide (GRP) system after a traumatic stress exposure. Acute severe stress significantly affects the spinal GRP system (which regulates male reproductive function) by reducing the expression of GRP and by reducing androgen receptor (AR)–mediated testosterone effects on the system. This interferes with male reproductive functions in vivo. GRP-R indicates GRP receptor; SNB, spinal nucleus of the bulbocavernosus; SPN, sacral parasympathetic nucleus.

Download figure to PowerPoint

Reduced Libido in Patients With Stress Disorders—

Psychiatric disorders such as PTSD and MDD affect emotional and social functioning and reduce aggression and libido (Pitman, 1997; Gruden and Gruden, 2000; Newport and Nemeroff, 2000; Seidman and Roose, 2001; Williams and Reynolds, 2006; Kennedy and Rizvi, 2009). Reduced libido is a concern in both men and women and is likely due to a higher brain dysfunction (Green, 2003; Williams and Reynolds, 2006). Several findings have clinically focused on the limbic system. Neuroimaging studies of patients with PTSD suggest that pathology in the hippocampus (Gilbertson et al, 2002; Villarreal et al, 2002), amygdala (Matsuoka et al, 2003), and anterior cingulate cortex (Rauch et al, 2003; Yamasue et al, 2003) plays an important role in the disorder. Thus, severe psychological stress might influence these brain regions and undermine the higher brain functions that regulate libido. Abnormalities in corticolimbic structure and function are also apparent in patients with MDD. These include dysfunction in the neural regions central to emotional processing such as the amygdala, ventral striatum, hippocampus, anterior cingulate cortex, and the ventromedial prefrontal cortex (Mayberg et al, 2000; Chen et al, 2007). Hence, decreased libido and the sexual centers in the lumbar spinal cord (as observed previously) (Sakamoto et al, 2009, Sakamoto et al, 2009) could act synergistically to impair sexual function. Further studies that focus on the pathogenesis of loss of libido in the brain and sexual dysfunctions in the spinal cord (including a focus on the GRP and SNB systems) should clarify the neural basis of the different symptom clusters in patients with stress disorders.

Conclusion

  1. Top of page
  2. Abstract
  3. Local Neuronal Circuits in the Lumbar Spinal Cord Controlling Erection and Ejaculation
  4. A Recently Identified Spinal GRP System
  5. The Effect of Androgens and Stress on Sexual Functions and on the Spinal Cord
  6. Conclusion
  7. Acknowledgment
  8. References

We recently reported a previously unknown peptidergic system within the lumbosacral spinal cord that uses GRP to trigger erection and ejaculation in male rats. We consequently demonstrated in rats that traumatic stress significantly decreases the expression level of GRP, which regulates male reproductive functions in the lumbar spinal cord. Traumatic stress reduces the expression of GRP and reduces the AR-mediated T effects on the system. This interferes with male sexual function. Thus, the GRP system in the lumbar spinal cord appears to be a stress-vulnerable center for male reproductive functions and can be morphologically altered by exposure to a traumatic stress. This suggests a hitherto unrecognized clinical target for the treatment of psychogenic ED triggered by stress and by psychiatric disorders.

Acknowledgment

  1. Top of page
  2. Abstract
  3. Local Neuronal Circuits in the Lumbar Spinal Cord Controlling Erection and Ejaculation
  4. A Recently Identified Spinal GRP System
  5. The Effect of Androgens and Stress on Sexual Functions and on the Spinal Cord
  6. Conclusion
  7. Acknowledgment
  8. References

The author is grateful to Drs Mitsuhiro Kawata, Ken-Ichi Matsuda, Keiko Takanami, Hisayuki Hongu, Nobuko Nishiura, Etsuko Wada, Keiji Wada, Tatsuo Arii, Tatsuya Sakamoto, Damian G. Zuloaga, Cynthia L. Jordan, and S. Marc Breedlove for their valuable discussions and collaborations. All experimental procedures for the author's research project cited in the present review have been authorized by the Committee for Animal Research, Kyoto Prefectural University of Medicine, and Okayama University, Japan, or the Institutional Animal Care and Use Committee of Michigan State University, East Lansing, Michigan, or both.

References

  1. Top of page
  2. Abstract
  3. Local Neuronal Circuits in the Lumbar Spinal Cord Controlling Erection and Ejaculation
  4. A Recently Identified Spinal GRP System
  5. The Effect of Androgens and Stress on Sexual Functions and on the Spinal Cord
  6. Conclusion
  7. Acknowledgment
  8. References
Footnotes
  1. This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports, Culture, and Technology, Japan; The Naito Memorial Grant for Natural Science Researches, Japan; The Uehara Memorial Foundation, Japan; The Inamori Foundation, Japan; Narishige Neuroscience Research Foundation, Japan; and Cooperative Study by High-Voltage Electron Microscopy (H-1250M) from the National Institute for Physiological Sciences, Okazaki, Japan.