Motoneurons innervating guinea pig perineal muscles are sexually dimorphic in size but not number

Sex difference in Onuf's nucleus homologue in the Asian musk shrew

Perineal muscles essential for copulatory functioning are innervated by Onuf's nucleus in humans and the spinal nucleus of the bulbocavernosus (SNB) and dorsolateral nucleus (DLN) in rats. These structures sexually differentiate as a result of developmental androgen exposure in most species examined. The homologous structure in the Asian musk shrew (Suncus murinus) is a single cluster in the lateral DLN/Onuf's position in the ventral horn of the spinal cord; these motoneurons innervate both the bulbocavernosus and ischiocavernosus muscles of the musk shrew. We found the expected sex difference in motoneuron number in the shrew DLN, but not in two neighboring motoneuron clusters, the retrodorsolateral nucleus (RDLN) and ventrolateral nucleus (VLN). Male musk shrews also have significantly larger soma areas in the VLN and DLN than females, and male DLN motoneurons have significantly larger nuclei than female. The sex difference in DLN motoneuron number was evident both in raw counts and after accounting for split nuclei error.

Breeding status affects motoneuron number and muscle size in naked mole-rats: recruitment of perineal motoneurons?

Naked mole-rats live in large colonies and exhibit a strict reproductive hierarchy. Each colony has one breeding female and one to three breeding males; all other individuals are nonreproductive subordinates. Subordinates show a remarkable lack of sex differences in behavior and anatomy, but can become reproductive if removed from the colony. We recently reported that the striated perineal muscles and their innervating motoneurons, which are sexually dimorphic in all other mammals examined to date, are not dimorphic in subordinate naked mole-rats. Here we asked whether sexual differentiation of this neuromuscular system occurs when a subordinate becomes a breeder. The size and number of cells within Onuf's nucleus (homologue of the rat spinal nucleus of the bulbocavernosus) as well as perineal muscle volume were examined in subordinate and breeding naked mole-rats of both sexes. Sex differences in perineal motoneurons were not observed, regardless of social status. To our surprise, however, counts of motoneurons in Onuf's nucleus were increased approximately 30% in breeders of both sexes. This was accompanied by a reciprocal decrease in cells in Onuf's nucleus that were characterized by small soma size, and lacked a clear nucleus or nucleolus. Although not exhibiting typical motoneuron morphology, some of these small cells were positive for the motoneuron marker, SMI-32. The neuronal changes correlate with increased perineal muscle volumes in breeders. We propose that small, relatively undifferentiated cells are recruited to the pool of large Onuf's nucleus motoneurons when subordinate naked mole-rats become breeders.

Participation of estradiol and progesterone in the retrograde labeling of pubococcygeus motoneurons of the female rat

Retrograde labeling with horseradish peroxidase conjugated to wheat germ agglutinin showed that the pubococcygeus muscles of the female rat are innervated by a population of motoneurons located in a column approximately 2 mm in length in the central region of lamina IX of the sixth lumbar-first sacral spinal cord segments. These neurons have a dendritic distribution that projects to the lateral, medial and ventral regions of the gray matter. Values for soma size, primary dendrite length and arborization area obtained from intact animals that were in diestrous-2, were significantly reduced following ovariectomy. After hormone priming of the ovariectomized animals with estradiol benzoate and progesterone, an additional injection of estradiol benzoate alone or followed by progesterone increased the labeled length of the primary dendrites distributed to the lateral, but not to the medial or ventral regions of the gray matter in the spinal cord. However, dendritic labeling was not significantly increased when only progesterone was additionally injected. It therefore seems that pubococcygeus muscle motoneurons of the female rat are sensitive to the effects of gonadal hormones.

Location of bladder and urethral sphincter motoneurons in the male guinea pig (Cavia porcellus)

Although the guinea pig is used widely in experimental medical research, including in studies on micturition control, the spinal origin of preganglionic parasympathetic bladder and somatic external urethral sphincter motoneurons is not known. In the male guinea pig using wheat germ agglutinin-conjugated horseradish peroxidase and dextran Alexa Fluor 488/568 tracers, preganglionic parasympathetic bladder motoneurons were observed in the ventrolateral part of the intermediolateral cell group of the first sacral segment. The external urethral sphincter motoneurons were found to be located in the ventral horn of the first sacral segment, in a cell group corresponding with the nucleus of Onuf in cat and human.

Normal ontogeny of perineal muscles and testosterone levels in Mongolian gerbils; Response to testosterone in developing females

The spinal nucleus of the bulbocavernosus (SNB) of Mongolian gerbils (Meriones unguiculatus) becomes sexually dimorphic during postnatal life, rather than prenatally as in rats. We therefore examined the early postnatal ontogeny of Mongolian gerbils, focusing on growth, serum testosterone (T) levels, and the sexually dimorphic perineal musculature innervated by the SNB. Serum T levels were higher in males than in females from birth through adulthood, with several early postnatal peaks and a large increase in T occurring during puberty in males. The SNB target muscles-the bulbocavernosus (BC) and levator ani (LA)-were present in both sexes on postnatal day 1 (PND1). Cross-sectional areas of BC fibers in males increased with age, and concurrently the myofibers of the BC became more fully developed and organized. In PND10 female pups, the BC muscle was virtually absent, while the LA muscle remained (although it was reduced in size). Postnatal treatment of female gerbils with androgen caused the BC muscle to remain and the LA muscle to become larger by PND10. Sexual dimorphism of the SNB develops differently in gerbils compared to other species, although its target muscles appear to respond to androgen in a manner similar to that in rats.

Early postnatal response of the spinal nucleus of the bulbocavernosus and target muscles to testosterone in male gerbils

This study examined the response of the spinal nucleus of the bulbocavernosus (SNB) and the bulbocavernosus (BC) muscle, to testosterone in male Mongolian gerbils (Meriones unguiculatus) during the early postnatal period. Male gerbil pups were given testosterone propionate (TP) or vehicle for 2 days, then perfused on postnatal day (PND) 3, 5, 10 or 15. The BC and levator ani (LA) muscles were removed, weighed, and sectioned. Cross-sections of BC muscle fibers were measured and muscle fiber morphology examined. Spinal cords were removed and coronally sectioned in order to count and measure the SNB motoneurons. Following TP treatment, male pups of all ages had significantly heavier BC-LA muscles and larger fibers in the BC muscle compared to age-matched controls. The increase in muscle weight following TP treatment was greatest at PND10, while fiber size increased to a similar degree at all ages suggesting that hyperplasia as well as hypertrophy was responsible for the increase in muscle mass at this time. SNB motoneurons increased significantly in number and size with age and TP treatment. We hypothesize that the increase in SNB motoneuron number during normal ontogeny that can be augmented by TP treatment and represents an unusual means of establishing sexual dimorphism in the nervous system of a mammal through cell recruitment to the motor pool of a postnatal animal.

Perineal muscles and motoneurons are sexually monomorphic in the naked mole-rat (Heterocephalus glaber)

Naked mole-rats are eusocial mammals that live in colonies with a single breeding female and one to three breeding males. All other members of the colony, known as subordinates, are nonreproductive and exhibit few sex differences in behavior or genital anatomy. This raises questions about the degree of sexual differentiation in subordinate naked mole-rats. The striated perineal muscles associated with the phallus [the bulbocavernosus (BC), ischiocavernosus (IC), and levator ani (LA) muscles], and their innervating motoneurons, are sexually dimorphic in all rodents examined to date. We therefore asked whether perineal muscles and motoneurons were also sexually dimorphic in subordinate naked mole-rats. Muscles similar to the LA and IC of other rodents were found in naked mole-rats of both sexes. No clear BC muscle was identified, although a large striated muscle associated with the urethra in male and female naked mole-rats may be homologous to the BC of other rodents. There were no sex differences in the volumes of the LA, IC, or the urethral muscles. Motoneurons innervating the perineal muscles were identified by retrograde labeling with cholera-toxin-conjugated horseradish peroxidase. All perineal motoneurons were found in a single cluster in the ventrolateral lateral horn, in a position similar to that of Onuf's nucleus of carnivores and primates. There was no sex difference in the size or number of motoneurons in Onuf's nucleus of naked mole-rats. Thus, unlike findings in any other mammal, neither the perineal muscles nor the perineal motoneurons appear to be sexually differentiated in subordinate naked mole-rats.

Sexual dimorphisms in a copulatory neuromuscular system in the green anole lizard

Sexual dimorphisms in neuromuscular systems have been investigated in several vertebrate groups, but data on reptiles are limited. The present studies were designed to establish the copulatory neuromuscular system of the green anole lizard (Anolis carolinensis) as an appropriate model. Like mammals, male reptiles have copulatory organs. However, each individual has two "hemipenes" that are controlled by bilateral sets of muscles. First, the anatomy of the hemipenes and associated muscles was described in males and the same anatomical region examined in females. Second, spinal motoneurons innervating one of these muscles, the transversus penis (TPN), were localized by using the retrograde tracer biocytin. They were detected in the last trunk and first sacral segments (T17-S1). Third, motoneuron number and soma size were assessed in Nissl-stained sections of spinal cord segments T17-S1 of breeding males and females. Male-biased sexual dimorphisms were detected in both measures, but the motoneurons innervating a tail muscle, the caudifemoralis (CF), are also located in the same region of cord. Therefore, in the last study, the CF was injected with biocytin in both sexes to eliminate its motoneurons from the analysis and gain a more accurate representation of the TPN motoneuron pool. An equivalent number of CF motoneurons were labeled in both sexes, and the results from the previous study were replicated. Thus, similar to other vertebrate models, parallels between morphology and function exist in the green anole copulatory system. Future investigations will broaden the comparative perspective on mechanisms regulating sexual dimorphisms relating to reproductive behaviors in vertebrates.

Perineal musculature and its innervation by spinal motoneurons in the male rabbit: effects of testosterone

Striated muscles in the perineum, which include the ischiocavernosus (IC), bulbocavernosus (BC), and levator ani (LA), were identified in an attempt to understand motor regulation of penile erection in the male rabbit. The IC surrounded the corpus spongiosum of the penis whereas the BC attached to the dorsum of the penis at the midline. The LA encircled the rectum and attached to the base of the penis. This anatomy suggested that the IC plays the primary role in the erection in the rabbit, whereas the BC may cause flips of the erected penis. Spinal motoneurons that innervate the IC were identified by retrograde labeling by horseradish peroxidase (HRP). As in other mammals, spinal labels from the IC appeared ipsilaterally in the ventral horn that encompassed the sixth lumbar (L6) and the first sacral segments. HRP injections into the BC labeled a small number of cells bilaterally at the same spinal levels. Rabbits are peculiar in having the IC motoneurons scattered among other motoneurons, unlike the rat and other rodents that have their IC motoneurons aggregated to form a spinal nucleus. Castration caused significant decreases in both the wet weight of IC muscles and the size of IC motoneurons within 2 weeks. Testosterone supplement following castration maintained the IC muscle weight and the neuronal size. Neither castration nor testosterone supplement induced changes in the number of IC motoneurons.

Comparison of the response of motoneurons innervating perineal and hind limb muscles to spaceflight and recovery

The succinate dehydrogenase (SDH) activities and cell body sizes of motoneurons in the dorsomedial (DM) region of the ventral horn at the lower portion of the L5 and the L6 segmental levels of the rat spinal cord were determined following 14 days of spaceflight and after 9 days of recovery on Earth and compared with those in the retrodorsolateral (RDL) region of the ventral horn at the same segmental levels. No changes in the mean SDH activity of motoneurons in the DM region were observed following spaceflight or after recovery. However, a decrease in the mean SDH activity of motoneurons with cell body sizes between 500 and 900 microm(2) in the RDL region was observed following spaceflight and after recovery. These data indicate that moderate-sized motoneurons in the RDL region, which are most likely associated with the hind limb musculature, were responsive to the microgravity environment. In contrast, the motoneurons in the DM region associated with the perineal muscles (associated with predominantly fast, low-oxidative muscles which are recruited for relatively brief periods at high activation levels and have no load-bearing function at 1G) were not affected by microgravity.

Sexual dimorphisms in a neuromuscular system regulating courtship in the green anole lizard: effects of season and androgen treatment

During the breeding season, male green anoles (Anolis carolinensis) court females by extending a red throat fan called a dewlap. Motoneurons controlling this sexually dimorphic behavior are located in two portions of the brain stem: (a) the vagal portion of nucleus ambiguus (AmbX), and (b) the region containing the glossopharyngeal portion of nucleus ambiguus and the ventral motor nucleus of the facial nerve (AmbIX/VIImv). These motoneurons project to the ceratohyoideus muscle via the ramus pharyngo-laryngeus IX+X. To investigate the effects of season on and androgen regulation of neural and peripheral structures controlling dewlap extension, two experiments were conducted: (a) During the breeding and nonbreeding seasons, motoneuron number, soma size, and nucleus size were investigated in intact males and females and in castrated males treated with a testosterone propionate (TP) or a blank Silastic capsule. (b) Cross-sectional area of the nerve and muscle fiber size, number, and density were investigated in the four treatment groups during the breeding season only. No significant differences were found in motoneuron number. In the breeding season, subtle male-biased sex differences existed in both AmbX and AmbIX/VIImv soma size. Nerve cross-sectional area and muscle fiber size and number were substantially larger in males than females. Muscle fiber density was higher in females. No consistent effects due to season or androgen treatment were detected, although characteristics of motoneurons were in some cases slightly larger in the nonbreeding season. These results suggest that, while parallels to behavior exist between the sexes, morphological changes in adulthood in the dewlap motoneurons and muscle do not normally regulate courtship behavior in the male green anole.

Testosterone has potent, selective effects on the morphology of pelvic autonomic neurons which control the bladder, lower bowel and internal reproductive organs of the male rat

Although gonadal steroids are important determinants of the development and activity of various neuronal circuits in the brain and spinal cord, their function has been relatively poorly studied in the peripheral nervous system. In the present work, the effects of pre- and postpubertal castration were examined on the morphology of autonomic neurons that supply pelvic visceral organs in male rats. These neurons were identified by peripheral injection of fluorescent retrograde tracers and, in the major pelvic ganglion, were further classified as sympathetic or parasympathetic by means of immunostaining for tyrosine hydroxylase. Sizes of ganglion cell somata were indicated by areas of nucleated profiles in cryosections. The results show that, irrespective of whether castration was carried out at two or seven weeks-of-age, noradrenergic pelvic neurons that supply the vas deferens, prostate gland, urinary bladder or colon achieved only approximately 60% of the size of those in unoperated controls. In contrast, cholinergic pelvic neurons were unaffected by castration unless they supplied reproductive targets. Pre- and paravertebral sympathetic neurons that supplied the pelvic viscera were only slightly smaller following castration or were unchanged, depending on their target. All effects of castration were prevented by testosterone replacement following surgery. Androgen receptor-immunoreactivity was particularly prevalent in the nuclei of some pelvic ganglion neurons. The studies suggest that circulating androgens are essential for the maturation and maintenance of the structure of select groups of autonomic neurons that supply the viscera. The presence of androgen receptor immunoreactivity in many of these neurons indicates that direct neuronal effects of androgens are possible. However this does not exclude other less direct mechanisms of steroid action on neurons, such as by an effect on target organs, neurotrophic factor release or peripheral vascular supply. These studies point to the androgenic steroids as potentially important determinants of autonomic reflex function.