Evidence that androgen acts through NMDA receptors to affect motoneurons in the rat spinal nucleus of the bulbocavernosus

Sex-specific effects of different types of prenatal stress on foetal testosterone levels and NMDA expression in mice

Prenatal stress is a critical life event often resulting in mental illnesses in the offspring. The critical developmental processes, which might trigger a cascade of molecular events resulting in mental disorders in adulthood, are still to be elucidated. Here we proposed that sex hormones, particularly testosterone, might determine the "developmental programming" of long-term consequences of prenatal stress in foetuses of both sexes. We observed that severe prenatal stress in the model of repeated corticosterone injections enhanced brain levels of corticosterone and testosterone in male foetuses. The expression of GluN1 and GluN2A, but not GluN2B NMDA receptor subunits were significantly reduced in the brain of stressed male foetuses. However, female foetuses were protected against stress effects on the brain corticosterone and testosterone levels. More moderate types of stress, such as repeated restraint stress and chronic unpredictable stress, did not induce an increase in brain corticosterone in dams and testosterone concentrations in foetuses of both sexes. Moreover, chronic unpredictable stress reduced brain testosterone concentration in male foetuses. Altogether, changes in brain testosterone level might be one of the crucial mechanisms determining the development of long-term consequences of severe prenatal stress in male, but not in female foetuses. Targeting this mechanism might allow to develop principally new prediction and therapeutic approaches for prenatal stress-associated psychiatric disorders.

Effects of uterine and lactational exposure to di-(2-ethylhexyl) phthalate on spatial memory and NMDA receptor of hippocampus in mice

Di-(2-ethylhexyl) phthalate (DEHP) is an environmental endocrine disrupter. Currently, little is known about neurodevelopmental toxicity of DEHP in wildlife and humans. The present study investigated the effects of DEHP, focusing on the changes in the behavior of offspring mice at the ages of 6 and 12w, respectively, following utero and lactational exposure to DEHP (10, 50, and 200mg/kg/d) from gestation day 7 through postnatal day 21. The results of open field tasks showed that DEHP increased the grooming of males at age 6w and females at age 12w but decreased the frequency of rearing of 6-w-old females and the number of grid crossings of 12-w-old females. In the Morris water maze task, 50 and 200mg/kg/d DEHP significantly prolonged the time of searching the hidden platform in water maze and reduced the time staying in the target quadrant during a probe trial of 6-w-old male mice, but not of 6-w-old females nor 12-w-old mice of both sexes, suggesting an impaired spatial learning and memory among younger males after perinatal exposure to DEHP. Western blot analyses further showed that DEHP at 50 and 200mg/kg/d decreased the levels of the N-methyl-d-aspartic acid (NMDA) receptor subunits NR1 and NR2B in the hippocampus of 6-w-old males. These results suggest that uterine and lactational exposure to low doses of DEHP sex-specifically impacted behaviors, including locomotion activity and spatial memory, via the concomitant inhibition of the NMDA receptor of the hippocampus in offspring mice.

Sexually dimorphic nuclei in the spinal cord control male sexual functions

Lower spinal cord injuries frequently cause sexual dysfunction in men, including erectile dysfunction and an ejaculation disorder. This indicates that the important neural centers for male sexual function are located within the lower spinal cord. It is interesting that the lumbar spinal segments contain several neural circuits, showing a clear sexually dimorphism that, in association with neural circuits of the thoracic and sacral spinal cord, are critical in expressing penile reflexes during sexual behavior. To date, many sex differences in the spinal cord have been discovered. Interestingly, most of these are male dominant. Substantial evidence of sexually dimorphic neural circuits in the spinal cord have been reported in many animal models, but major issues remain unknown. For example, it is not known how the different circuits cooperatively function during male sexual behavior. In this review, therefore, the anatomical and functional significance of the sexually dimorphic nuclei in the spinal cord corresponding to the expression of male sexual behavior is discussed.

Differential expression and regulation of brain-derived neurotrophic factor (BDNF) mRNA isoforms in androgen-sensitive motoneurons of the rat lumbar spinal cord

Castration of adult male rats causes dendrites of the spinal nucleus of the bulbocavernosus (SNB) to retract. The neurotrophin brain-derived neurotrophic factor (BDNF) is implicated in mediating these androgenic effects on SNB dendrites. We previously found that castration decreases BDNF mRNA in SNB somata and BDNF protein in proximal SNB dendrites, effects not observed in nearby retrodorsolateral (RDLN) motoneurons. Given that different 5' non-coding exons of BDNF dictate specific subcellular targeting of BDNF mRNA, we set out to identify the specific BDNF transcripts regulated by androgens in SNB motoneurons. We used in situ hybridization to monitor the expression pattern of BDNF transcripts containing non-coding exons I, II, IV, and VI in SNB and RDLN motoneurons in gonadally intact and castrated male rats. While androgen-insensitive RDLN motoneurons expressed all four isoforms, SNB motoneurons contained low levels of BDNF exon IV and little, if any, BDNF exon I. Expression of BDNF isoforms containing exon II and VI was comparable in the two groups of motoneurons. Two weeks after castration, BDNF isoforms containing exon VI were significantly decreased in SNB motoneurons in an androgen-dependent manner, but unaffected in RDLN motoneurons. Because exon VI promotes dendritic localization of BDNF mRNA in other systems, androgens may regulate the dendrites of SNB motoneurons by altering expression of BDNF isoforms, thereby impairing targeting of BDNF protein to dendrites to regulate local synaptic signaling and dendritic structure.

Progesterone: Therapeutic opportunities for neuroprotection and myelin repair

Progesterone and its metabolites promote the viability of neurons in the brain and spinal cord. Their neuroprotective effects have been documented in different lesion models, including traumatic brain injury (TBI), experimentally induced ischemia, spinal cord lesions and a genetic model of motoneuron disease. Progesterone plays an important role in developmental myelination and in myelin repair, and the aging nervous system appears to remain sensitive to some of progesterone's beneficial effects. Thus, the hormone may promote neuroregeneration by several different actions by reducing inflammation, swelling and apoptosis, thereby increasing the survival of neurons, and by promoting the formation of new myelin sheaths. Recognition of the important pleiotropic effects of progesterone opens novel perspectives for the treatment of brain lesions and diseases of the nervous system. Over the last decade, there have been a growing number of studies showing that exogenous administration of progesterone or some of its metabolites can be successfully used to treat traumatic brain and spinal cord injury, as well as ischemic stroke. Progesterone can also be synthesized by neurons and by glial cells within the nervous system. This finding opens the way for a promising therapeutic strategy, the use of pharmacological agents, such as ligands of the translocator protein (18 kDa) (TSPO; the former peripheral benzodiazepine receptor or PBR), to locally increase the synthesis of steroids with neuroprotective and neuroregenerative properties. A concept is emerging that progesterone may exert different actions and use different signaling mechanisms in normal and injured neural tissue.

Androgen-dependent regulation of brain-derived neurotrophic factor and tyrosine kinase B in the sexually dimorphic spinal nucleus of the bulbocavernosus

Castration of adult male rats causes the dendrites of androgen-sensitive motoneurons of the spinal nucleus of the bulbocavernosus (SNB) to retract. Brain-derived neurotrophic factor (BDNF), via activation of tyrosine receptor kinase B (trkB), has been implicated in mediating androgen effects on SNB dendrites. We used in situ hybridization to demonstrate that SNB motoneurons in gonadally intact adult male rats contain mRNA for both BDNF and trkB. Two weeks after gonadectomy, both transcripts were significantly decreased in SNB motoneurons but not in the non-androgen-responsive motoneurons of the adjacent retrodorsolateral nucleus (RDLN). In a second experiment, target perineal and foot muscles of SNB and RDLN motoneurons, respectively, were injected with the retrograde tracer Fluorogold, and then immunocytochemistry was performed to examine the distribution of BDNF and trkB proteins in SNB and RDLN motoneurons and their glutamatergic afferents. Confocal analysis revealed that gonadectomy induces a loss of BDNF protein in SNB dendrites but not in RDLN dendrites. Testosterone treatment of castrates prevented the loss of BDNF from SNB dendrites. Confocal analysis also revealed trkB protein in SNB and RDLN dendrites and in their glutamatergic afferents. Gonadectomy had no discernable effect on trkB protein in SNB or RDLN motoneurons or in their glutamatergic afferents. These results suggest that androgen maintains a BDNF-signaling pathway in SNB motoneurons that may underlie the maintenance of dendritic structure and synaptic signaling.

Androgen-sensitivity of somata and dendrites of spinal nucleus of the bulbocavernosus (SNB) motoneurons in male C57BL6J mice

In rats, androgens in adulthood regulate the morphology of motoneurons in the spinal nucleus of the bulbocavernosus (SNB), including the size of their somata and the length of their dendrites. There are conflicting reports about whether androgens exert similar influences on SNB motoneurons in mice. We castrated or sham-operated C57BL6J mice at 90 days of age and, thirty days later, injected cholera toxin conjugated horseradish peroxidase into the bulbocavernosus muscle (to label SNB motoneurons) on one side, and into intrinsic foot muscles contralaterally (to label motoneurons of the retrodorsolateral nucleus (RDLN)). Castrated mice had significantly smaller SNB somas compared to sham-operated mice while there were no differences in soma size of RDLN motoneurons. Dendritic length in C57BL6J mice, estimated in 3-dimensions, also decreased significantly after adult castration. In rats, androgens act directly through androgen receptors (AR) in SNB motoneurons to control soma size and nearly all SNB motoneurons contain AR. Since SNB somata in C57BL6J mice shrank after adult castration, we used immunocytochemistry to characterize AR expression in SNB cells as well as motoneurons in the RDLN and dorsolateral nucleus (DLN). A pattern of labeling matched that seen previously in rats: the highest percentage of AR-immunoreactive motoneurons are in the SNB (98%), the lowest in the RDLN (25%) and an intermediate number in the DLN (78%). This pattern of AR labeling is consistent with the possibility that androgens also act directly on SNB motoneurons in mice to regulate soma size in mice.

Organizational and activational effects of testosterone on carrageenan-induced inflammatory pain and morphine analgesia

Men are typically reported to have higher pain thresholds than women. Gonadal hormones, particularly testosterone for males, may contribute to this effect. This study tested whether changes in the male hormonal milieu early or late in development alter the inflammatory pain induced by carrageenan (CARR, 3%, intraarticular). Male rats were either gonadectomized or sham gonadectomized neonatally. Once adults, the same rats underwent either sham surgery or gonadectomy, and received a testosterone or oil implant. After baseline testing two weeks later, animals were tested in response to mechanical and thermal stimulation following CARR-induced inflammation and injection of saline, 1 mg/kg, or 5 mg/kg morphine. Hormone alterations did not affect baseline responses, but CARR induced mechanical and thermal hyperalgesia in the injured limb. Gonadectomy in adult males injected with saline enhanced inflammation-induced sensitivity to mechanical but not thermal stimulation and the effect was reversed by testosterone. After inflammation, saline-treated males gonadectomized neonatally displayed lower mechanical threshold than control sham animals but this effect was not reversed by testosterone. Both doses of morphine increased mechanical and thermal thresholds. However, compared with the control group, 1 mg/kg morphine was equally effective in reducing mechanical hyperalgesia among groups of animals gonadectomized as adults, but less effective in males gonadectomized neonatally. The results suggest that in males: 1. the antihyperalgesic effect of testosterone (or its metabolites) in CARR-induced inflammation is established during development and maintained by circulating levels of testosterone in adulthood; 2. the nociception-related interaction between the opioid and gonadal systems influences the sensitivity to mechanical stimuli and is likely established during the period of sexual differentiation.

NMDA receptor binding declines differentially in three spinal motor nuclei during postnatal development

The NMDA subtype of glutamate receptors mediates a variety of neuronal processes involved in the development of dendritic morphology. For example, NMDA receptor antagonism during the early postnatal period attenuates dendritic growth in spinal motoneurons. NMDA receptors are present in high levels in the spinal cord early in the postnatal period and decline during development, a period of extensive dendritic plasticity in the spinal cord. Previous studies have suggested that an adult pattern of distribution of NMDA receptors is established as early as postnatal day (P)21 (day of birth = P1). However, given that dendritic growth in spinal motoneurons is not complete by this age and that NMDA receptor activation is necessary for dendritic growth, we assessed NMDA receptor binding in specific spinal motor nuclei during normal development. NMDA receptors were labeled with [3H]MK-801 at P7, P14, P28, P49, and in adult male rats. Receptor binding in the spinal nucleus of the bulbocavernosus (SNB), dorsolateral nucleus (DLN) and retrodorsolateral nucleus (RDLN) was measured using in vitro quantitative autoradiography. NMDA receptor binding over the SNB, DLN and RDLN in intact males was initially high, and declined to adult levels. However, the time course of the decline differed across nuclei. The local decline in NMDA receptor binding observed in the SNB and DLN is coincident with the periods of dendritic growth in these nuclei, further supporting a role for NMDA receptors in the development of motoneuron dendritic morphology.

Expression of nuclear receptor coactivators in androgen-responsive and -unresponsive motoneurons

Adult rat lumbar motoneurons in the spinal nucleus of the bulbocavernosus (SNB) respond to androgens with an increase in soma size. This response is mediated by the androgen receptor (AR) in these motoneurons. Interestingly, other lumbar motoneurons in the rat possess the AR, yet do not respond to androgens in this fashion. This paradox suggests the existence and participation of nuclear receptor coregulators in conferring direct androgen-responsiveness to select motoneurons in the adult rat spinal cord. Nuclear receptor coregulators have received much attention recently for their proposed role in enhancing or repressing the transcriptional activity of steroid hormone receptors. The present study used immunocytochemistry to identify a number of nuclear receptor coactivators that are expressed by adult lumbar motoneurons: SRC-1, SRC-2, CBP, p300, and cJUN. Results of this study indicate that all five of these coactivators are abundantly expressed in the androgen-responsive SNB, and in two adjacent motor pools, the androgen-responsive dorsolateral nucleus (DLN), and the androgen-unresponsive retrodorsolateral nucleus (RDLN). While we detected significant regional differences for only SRC-1 and cJUN, the SNB consistently contained the highest percentage of immunoreactive motoneurons for all five cofactors examined. Our results indicate five different putative cofactors have the potential to participate in motoneuronal responses to androgens, since their distribution overlaps well with the distribution of ARs in these motoneurons.

Neural control of erection

Penile erection is a vascular event controlled by the autonomic nervous system. The spinal cord contains the autonomic preganglionic neurons that innervate the penile erectile tissue and the pudendal motoneurons that innervate the perineal striated muscles. Sympathetic pathways are anti-erectile, sacral parasympathetic pathways are pro-erectile, and contraction of the perineal striated muscles upon activity of the pudendal nerves improves penile rigidity. Spinal neurons controlling erection are activated by information from peripheral and supraspinal origin. Both peripheral and supraspinal information is capable of either eliciting erection or modulating or inhibiting an erection already present. Sensory information from the genitals is a potent activator of pro-erectile spinal neurons and elicits reflexive erections. Some pre-motor neurons of the medulla, pons and diencephalon project directly onto spinal sympathetic, parasympathetic and pudendal motoneurons. They receive in turn sensory information from the genitals. These spinal projecting pathways release a variety of neurotransmitters, including biogenic amines (serotonin, dopamine, noradrenaline, and adrenaline) and peptides that, through interactions with many receptor subtypes, exert complex effects on the spinal network that controls penile erection. Some supraspinal structures (e.g. the paraventricular nucleus and the medial preoptic area of the hypothalamus, the medial amygdala), whose roles in erection have been demonstrated in animal models, may not project directly onto spinal pro-erectile neurons. They are nevertheless prone to regulate penile erection in more integrated and coordinated responses of the body, as those occurring during sexual behavior. The application of basic and clinical research data to treatment options for erectile dysfunction has recently proved successful. Pro-erectile effects of phosphodiesterase type 5 inhibitors, acting in the penis, and of melanocortin agonists, acting in the brain, illustrate these recent developments.

Androgen receptor immunoreactivity in skeletal muscle: enrichment at the neuromuscular junction

Potential cellular targets of androgen action within skeletal muscle of the rat were determined by comparing the cellular distribution of androgen receptor (AR)-positive nuclei in the highly androgen-responsive levator ani (LA) muscle with that of the relatively androgen-unresponsive extensor digitorum longus (EDL) muscle. We found that androgen responsiveness correlates with AR expression in muscle fibers and not in fibroblasts. Results indicate that a much higher percentage of myonuclei in the LA are AR(+) than in the EDL (74% vs. 7%), correlating with differences in androgen responsiveness. Both muscles contain an equivalent proportion of AR(+) fibroblasts (approximately 62%). AR(+) nuclei were not observed in terminal Schwann cells in either muscle. These results suggest that ARs within LA muscle fibers mediate the androgen-dependent survival and growth of the LA muscle and its motoneurons. We also observed an unexpected enrichment of AR(+) myonuclei and fibroblasts proximate to neuromuscular junctions, suggesting that ARs at muscle synapses may selectively regulate synapse-specific genes important for the survival and growth of motoneurons. Although castration reduced the proportion of AR(+) fibroblasts in both muscles, the proportion of AR(+) myonuclei was reduced only in the LA. As expected, testosterone treatment prevented these effects of castration but, unexpectedly, increased the proportion of AR(+) myonuclei in the EDL to above normal. These results suggest that how AR expression in skeletal muscle is influenced by androgens depends not only on the particular muscle but on the particular cell type within that muscle.