Sexually dimorphic gastrin releasing peptide system in the spinal cord controls male reproductive functions

Abstract

Neurons in the upper lumbar spinal cord project axons containing gastrin-releasing peptide (GRP) to innervate lower lumbar regions controlling erection and ejaculation. This system is vestigial in female rats and in males with genetic dysfunction of androgen receptors, but in male rats, pharmacological stimulation of spinal GRP receptors restores penile reflexes and ejaculation after castration. GRP offers new avenues for understanding potential therapeutic approaches to masculine reproductive dysfunction.GRP, a member of the bombesin-like peptide family 1 , is distributed widely in the central nervous system and gastrointestinal tract of mammals 2 , 3 . GRP and neuromedin B (NMB), the mammalian counterpart of bombesin, play a role in many physiological processes, including itch 4 , circadian rhythms 5 , food intake 6 and fear memory consolidation 7 , 8 . In mammals, bombesin-like peptides act through a family of at least three G protein-coupled receptors: GRP-preferring receptor (GRP-R), NMB-preferring receptor (NMB-R) and bombesin receptor subtype-3 (BRS-3) 9 .Using immunocytochemistry (ICC) directed at GRP, we found a group of neurons within a region previously dubbed the 'spinal ejaculation generator' because toxins that selectively lesion galanin-containing neurons there virtually eliminate ejaculation in rats 10 . These galaninergic neurons project to the thalamus 10 , but it had been unclear whether there are also direct connections between this center and the lower spinal cord regions that directly trigger ejaculation 11 . The separate, GRP-containing neurons that we found within the center projected axons to more caudal spinal regions and were much more prominent in wild-type (WT) males than in WT females ( Fig. 1a,b; Supplementary Fig. 1 online) (n = 5, F 2,12 = 299.9, P < 0.001). Semiquantitative reverse transcription (RT)-PCR confirmed more pre-pro NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptGrp transcripts in this region of males than of females ( Supplementary Fig. 2 online). To test whether androgen receptors direct sexual dimorphism of these neurons, we examined genetically male (XY) Long-Evans rats carrying the testicular feminization mutation (Tfm) of the androgen receptor gene Ar. These rats develop testes embryologically and secrete testosterone pre-and postnatally but, because their androgen receptor protein is dysfunctional, develop a wholly feminine exterior, including a clitoris rather than a penis. The spinal cord of Tfm rats was hyperfeminine, having even fewer GRP-positive neurons in this region than did WT females (P <0.001) (Fig. 1c,d). In normal males, GRP-expressing neurons also expressed androgen receptor (96.1 ± 1.7%; n = 4 WT males), but not estrogen receptor alpha (ERα) (Fig. 1e-h). Because androgens such as testosterone augment ejaculation in male rats and humans 12 , the presence of androgen receptor in the GRPpositive neurons of the ejaculation center offers a locus for androgenic modulation of ejaculation and other sexual reflexes.ICC r...

Autonomic Consequences of Spinal Cord Injury

“…Following ejaculation, these neurons express the immediate early gene c-fos (340). (iv) GRPexpressing neurons located at the L3-L4 spinal levels exhibit a clear male-dominant sexual dimorphism (291). The neurons project to the SNB and parasympathetic nuclei in the more caudal segments of lumbosacral spinal cord.…”

“…The neurons project to the SNB and parasympathetic nuclei in the more caudal segments of lumbosacral spinal cord. GRP is a crucial excitatory molecule in ejaculation and penile reflexes (291). (v) Primary sensory afferents show sex differences that are dependent on effects of testosterone (231).…”

Pharmacology for the Treatment of Premature Ejaculation

“…A great majority of LSt neurons contain galanin and cholecystokinin (Ju et al, 1987;Truitt et al, 2003), and express the preferential receptor for substance P (SP) neurokinin-1 receptor (NK1; Truitt and Coolen, 2002). In addition, numerous LSt neurons have been found to contain enkephalin and gastrin-releasing peptide (Nicholas et al, 1999;Sakamoto et al, 2008). Neuroanatomical tracing studies were performed to describe the spinal ejaculation circuitry.…”

“…A population of neurons within the SGE in rats has been reported to contain GRP and to send projections to the sacral parasympathetic nucleus and Onuf's nucleus, where GRP receptors (GRP-preferring subtype) were identified (Sakamoto et al, 2008). Pharmacological manipulations using selective agonist and antagonist of this GRP receptor subtype showed that the GRP spinal pathway controls ejaculation but also erection in rats (Sakamoto et al, 2008).…”

The gastrin-releasing peptide system in the spinal cord mediates masculine sexual function

“…We recently found evidence that neurons within the ejaculation generator (Truitt and Coolen 2002) in the lumbar spinal cord (lumbar segments 3 and 4; L3-L4 level) project axons containing GRP to the lower lumbar spinal cord (L5-L6 level), innervating the somatic and autonomic regions that are also known to control erection and ejaculation ( Fig. 1) (Sakamoto andKawata 2006, 2009;Sakamoto et al 2008Sakamoto et al , 2010. All these target regions express specific receptors for GRP (GRP-R) .…”