Rats chirp with their mouth full: During an experimental meal, adult male Wistar rats emitted flat ultrasonic vocalisations upon feeding

Rats produce ultrasonic vocalisation (USVs) that are classified into different types, based on their average frequency. In pups 40 kHz USVs are produced upon social isolation, and in adults USVs can be associated with affective states and specific behavioural patterns (i.e., appetitive 50 kHz vocalisations of frequency range 30-100 kHz, or aversive 20 kHz vocalisations of frequency range 18-30 kHz). Generally, USVs of frequency around 50 kHz are linked to activation of brain reward pathways, during anticipation or experience of rewarding stimuli. Previous studies have described several subtypes of 50 kHz USVs, according to their acoustic properties. We asked whether USV production might be relevant to feeding behaviour. We recorded USVs from 14-week old adult rats during the satisfaction of a physiological need: refeeding following mild food deprivation (17 h overnight fast). We analysed a 10 min consummatory phase, preceded by a 10 min anticipatory phase, as a control for the experimental meal. Following identification of USV subtypes, we applied frequentist and Bayesian (Monte Carlo shuffling) statistical analyses to investigate the relationship between USV emission and rat behaviour. We found that it was not total USV quantity that varied in response to food consumption, but the subtype of USV produced. Most importantly we found that rats who feed tend to produce flat USVs of a frequency around 40 kHz. Beyond the previous reports of circumstantial association feeding-flat USVs, our observation directly correlate vocalisation and ingestive behaviour. Our study highlights that, in addition to quantification of the production rate, study of USV subtypes might inform us further on rat consummatory behaviour. Since this vocalisation behaviour can have a communicative purpose, those findings also illustrate nutrition studies might benefit from considering the possible social dimension of feeding behaviour.

Diet Protein Content and Individual Phenotype Affect Food Intake and Protein Appetence in Rats

BACKGROUND

A low-protein diet can induce compensatory intake of excess energy. This must be better evaluated to anticipate the obesogenic risk that may result from the dietary recommendations for reducing animal protein consumption.

OBJECTIVES

We aimed to further characterize the behavioral and physiological responses to a reduction in dietary protein and to identify the determinants of protein appetite.

METHODS

Thirty-two male Wistar rats [4 wk old, (mean ± SEM) 135 ± 32 g body weight] were fed a low-protein (LP; 6% energy value) or normal-protein (NP; 20%) diet for 8 wk. Food intake and body mass were measured during the entire intervention. During self-selection sessions after 4 wk of experimental diets, we evaluated rat food preference between LP, NP, or high-protein (HP; 55%) pellets. At the end of the experiment, we assessed their hedonic response [ultrasonic vocalizations (USVs)] and c-Fos neuronal activation in the olfactory tubercle and nucleus accumbens (NAcc) associated with an LP or HP meal.

RESULTS

Rats fed an LP diet had greater food intake (24%), body weight (5%), and visceral adiposity (30%) than NP rats. All LP rats and half of the NP rats showed a nearly exclusive preference for HP pellets during self-selection sessions, whereas the other half of the NP rats showed no preference. This suggests that the appetite for proteins is driven not only by a low protein status but also by individual traits in NP rats. LP or HP meal induced similar USV emission and similar neuronal activation in the NAcc in feed-deprived LP and NP rats, showing no specific response linked to protein appetite.

CONCLUSIONS

Protein appetite in rats is driven by low protein status or individual preferences in rats receiving adequate protein amounts. This must be considered and further analyzed, in the context of current recommendations for protein intake reduction.

Massive transient damage of the olfactory epithelium associated with infection of sustentacular cells by SARS-CoV-2 in golden Syrian hamsters

Anosmia is one of the most prevalent symptoms of SARS-CoV-2 infection during the COVID-19 pandemic. However, the cellular mechanism behind the sudden loss of smell has not yet been investigated. The initial step of odour detection takes place in the pseudostratified olfactory epithelium (OE) mainly composed of olfactory sensory neurons surrounded by supporting cells known as sustentacular cells. The olfactory neurons project their axons to the olfactory bulb in the central nervous system offering a potential pathway for pathogens to enter the central nervous system by bypassing the blood brain barrier. In the present study, we explored the impact of SARS-CoV-2 infection on the olfactory system in golden Syrian hamsters. We observed massive damage of the OE as early as 2 days post nasal instillation of SARS-CoV-2, resulting in a major loss of cilia necessary for odour detection. These damages were associated with infection of a large proportion of sustentacular cells but not of olfactory neurons, and we did not detect any presence of the virus in the olfactory bulbs. We observed massive infiltration of immune cells in the OE and lamina propria of infected animals, which may contribute to the desquamation of the OE. The OE was partially restored 14 days post infection. Anosmia observed in COVID-19 patient is therefore likely to be linked to a massive and fast desquamation of the OE following sustentacular cells infection with SARS-CoV-2 and subsequent recruitment of immune cells in the OE and lamina propria.

Behavioral responses to odors from other species: introducing a complementary model of allelochemics involving vertebrates

It has long been known that the behavior of an animal can be affected by odors from another species. Such interspecific effects of odorous compounds (allelochemics) are usually characterized according to who benefits (emitter, receiver, or both) and the odors categorized accordingly (allomones, kairomones, and synomones, respectively), which has its origin in the definition of pheromones, i.e., intraspecific communication via volatile compounds. When considering vertebrates, however, interspecific odor-based effects exist which do not fit well in this paradigm. Three aspects in particular do not encompass all interspecific semiochemical effects: one relates to the innateness of the behavioral response, another to the origin of the odor, and the third to the intent of the message. In this review we focus on vertebrates, and present examples of behavioral responses of animals to odors from other species with specific reference to these three aspects. Searching for a more useful classification of allelochemical effects we examine the relationship between the valence of odors (attractive through to aversive), and the relative contributions of learned and unconditioned (innate) behavioral responses to odors from other species. We propose that these two factors (odor valence and learning) may offer an alternative way to describe the nature of interspecific olfactory effects involving vertebrates compared to the current focus on who benefits.

Electrophysiological responses of rat olfactory tubercle neurons to biologically relevant odours

Biologically relevant odours were used to stimulate olfactory tubercle neurons in anaesthetized male rats. Among 120 recorded neurons, 118 showed spontaneous activity (mean firing rate, 15.0 ± 1.4 spikes/s). Ninety-eight neurons were exposed to at least one of the four following odour sources: an empty vial, or a vial containing food pellets (familiar odour), a sample of oestrous rat faeces (conspecific sexual odour), or a sample of male fox faeces (predator odour). The proportion of neurons responding with a change in activity was significantly linked to the odour applied. Repetition of the stimulation with the same odour elicited the same activity change. Between 50 and 70% of neuronal activity changes were not accompanied by respiration changes. Fifty-six neurons were exposed successively to all four odours, and 38 of them showed an activity change in response to at least one. The response of a neuron to an odour was not affected by its response to the previous one, and no neuron responded in the same manner to all odours. Conversely, no odour elicited a unique response in this population of neurons. However, the proportions of excited, inhibited and insensitive neurons depended significantly on the odour applied, suggesting that the recruitment of olfactory tubercle neurons is directly dependent on the biological significance of the odour.

A mixture of odorant molecules potentially indicating oestrus in mammals elicits penile erections in male rats

A common set of odorous molecules may indicate female receptiveness across species, as male rats display sexual arousal when exposed to the odour of oestrous faeces from rats, vixens and mares. More than 900 different compounds were identified by GC-MS analyses performed on faeces samples from di-oestrous and oestrous females and from males of the three species. Five carboxylic acids were found in lower concentrations in faeces from all oestrous females. We subjected 12 sexually trained male rats to a 30 min exposure to different dilutions of a mixture of these five molecules in the same proportions as found in female oestrous faeces. The behavioural responses of the rats were compared to those displayed when exposed to water (negative control) and faeces from oestrous female rats (positive control). Frequency of penile erections were found to be significantly dependent on mixture dilution, with two intermediate dilutions eliciting frequencies of penile erections that did not differ from those obtained during exposure to oestrous female rat faeces. Higher and lower dilutions did not elicit more penile erections than observed with water. These results support our hypothesis that a small set of odorous molecules may indicate sexual receptiveness in mammalian females.

Three-dimensional macronutrient-associated Fos expression patterns in the mouse brainstem

Background

The caudal brainstem plays an important role in short-term satiation and in the control of meal termination. Meal-related stimuli sensed by the gastrointestinal (GI) tract are transmitted to the area postrema (AP) via the bloodstream, or to the nucleus tractus solitarii (NTS) via the vagus nerve. Little is known about the encoding of macronutrient-specific signals in the caudal brainstem. We hypothesized that sucrose and casein peptone activate spatially distinct sub-populations of NTS neurons and thus characterized the latter using statistical three-dimensional modeling.

Methodology/Principal Findings

Using immunolabeling of the proto-oncogene Fos as a marker of neuronal activity, in combination with a statistical three-dimensional modeling approach, we have shown that NTS neurons activated by sucrose or peptone gavage occupy distinct, although partially overlapping, positions. Specifically, when compared to their homologues in peptone-treated mice, three-dimensional models calculated from neuronal density maps following sucrose gavage showed that Fos-positive neurons occupy a more lateral position at the rostral end of the NTS, and a more dorsal position at the caudal end.

Conclusion/Significance

To our knowledge, this is the first time that subpopulations of NTS neurons have be distinguished according to the spatial organization of their functional response. Such neuronal activity patterns may be of particular relevance to understanding the mechanisms that support the central encoding of signals related to the presence of macronutrients in the GI tract during digestion. Finally, this finding also illustrates the usefulness of statistical three-dimensional modeling to functional neuroanatomical studies.

Three-dimensional statistical modeling of neuronal populations: illustration with spatial localization of supernumerary neurons in the locus coeruleus of quaking mutant mice

An algorithm for the three-dimensional statistical representation of neuronal populations was designed and implemented. Using this algorithm a series of 3D models, calculated from repeated histological experiments, can be combined to provide a synthetic vision of a population of neurons taking into account biological and experimental variability. Based on the point process theory, our algorithm allows computation of neuronal density maps from which isodensity surfaces can be readily extracted and visualized as surface models revealing the statistical organization of the neuronal population under study. This algorithm was applied to the spatial distribution of locus coeruleus (LC) neurons of 30- and 90-day-old control and quaking mice. By combining 12 3D models of the LC, a region of the nucleus in which a subpopulation of neurons loses its noradrenergic phenotype between 30 and 90 days postnatally was demonstrated in control mice but not in quaking mice, leading to the hyperplasia previously reported in adult mutants. Altogether, this algorithm allows computation of 3D statistical and graphical models of neuronal populations, providing a contribution to quantitative 3D neuroanatomical modeling.

Study of hypothalamic leptin receptor expression in low-birth-weight piglets and effects of leptin supplementation on neonatal growth and development

Low birth weight resulting from intrauterine growth retardation (IUGR) is a risk factor for further development of metabolic diseases. The pig appears to reproduce nearly all of the phenotypic pathological consequences of human IUGR and is likely to be more relevant than rodents in studies of neonatal development. In the present work, we characterized the model of low-birth-weight piglets with particular attention to the hypothalamic leptin-sensitive system, and we tested whether postnatal leptin supplementation can reverse the precocious signs of adverse metabolic programming. Our results demonstrated that 1) IUGR piglets present altered postnatal growth and increased adiposity; 2) IUGR piglets exhibit abnormal hypothalamic distribution of leptin receptors that may be linked to further disturbance in food-intake behavior; and 3) postnatal leptin administration can partially reverse the IUGR phenotype by correcting growth rate, body composition, and development of several organs involved in metabolic regulation. We conclude that IUGR may be characterized by altered leptin receptor distribution within the hypothalamic structures involved in metabolic regulation and that leptin supplementation can partially reverse the IUGR phenotype. These results open interesting therapeutic perspectives in physiopathology for the correction of defects observed in IUGR.

Are oestrus odours species specific?

Adult male rats were exposed to faeces odours of three animal species (rat, fox and horse). They displayed erections in the presence of faeces from oestrous females (whatever the species). In addition, fox faeces (whatever the gender or hormonal status) elicited an expected freezing reaction. It is suggested that oestrous female faeces of these three species share common odorants which depend on the hormonal status and characterize female receptivity.

Anatomical evidence for glutamatergic transmission in primary sensory neurons and onto postganglionic neurons controlling penile erection in rats: an ultrastructural study with neuronal tracing and immunocytochemistry

In male rats, the dorsal penile nerve (DPN) conveys sensory information from the genitals to the lumbosacral spinal segments of the spinal cord. DPN is the afferent limb of a reflex loop that supports reflexive erections, and that includes a network of spinal interneurons and autonomic and somatic motoneurons to the penis and perineal striated muscles. Autonomic efferent pathways to the penis relay in the major pelvic ganglion (MPG). Glutamate (Glu) is a likely candidate as a neurotransmitter of reflexive erections. Both AMPA and NMDA glutamatergic receptor subunits are present in the lumbosacral spinal cord, and AMPA and NMDA receptor antagonists block reflexive erections. In the present study, we used tract-tracing experiments combined with immunohistochemical and immunocytochemical techniques to ascertain the presence of Glu at two different levels of the network controlling reflexive erections. DPN afferents were localized in the dorsal horn of the lumbosacral cord and displayed the characteristics of either C-fibers or Adelta fibers. DPN terminals (some of them glutamatergic) were mainly distributed in the medial edge of the dorsal horn in the L6 spinal segment. GluR1 subunits were present in some DPN afferents, suggesting that they could be autoreceptors. DPN fibers were also present in the MPG, as were Glu terminals and GluR4 subunits. The results reveal the presence of Glu in DPN fibers and terminals and suggest that both the spinal cord and the MPG use glutamatergic transmission to control reflexive erections.

Possible neural mediation of the central effects of oxytocin on uterine motility

The central nervous system contains the nuclei at the origin of autonomic and neuroendocrine pathways to the uterus. Although the anatomical basis of these pathways is known, the conditions of their recruitment and their interactions in the context of copulation remain to be explored. We tested the hypothesis that some central mechanisms could simultaneously recruit both pathways to the uterus. In this aim, we recorded intrauterine pressure changes in anesthetized female rats at the estrus stage after intracerebroventricular (ICV) administration of oxytocin (OT). Doses of 0.3–300 ng elicited increases of frequency and amplitude of uterine contractions. These effects were partly mimicked by the OT agonist [Thr4,Gly7]OT but not by arginine vasopressin. They were blocked by the OT receptor antagonist atosiban delivered either ICV or intravenously. The latter suggests that ICV OT activated the systemic release of OT. The effects of OT were also blocked by hexamethonium, a ganglionic blocking agent, by atropine, a muscarinic receptor antagonist, and by Nω-nitro-l-arginine methyl ester, an inhibitor of nitric oxide synthesis. The results reveal that ICV OT recruits autonomic efferent pathways to the uterus. These results support our hypothesis that the activation of central nuclei can promote uterine contractility, and that OT may be a central coordinator of autonomic and neuroendocrine pathways. The hypothalamus, the source of direct OT-ergic projections to the pituitary, the brain stem, and the spinal cord, may be a target of central OT.

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.

Spinal effects of oxytocin on uterine motility in anesthetized rats

The rat uterus receives an innervation from the lumbosacral and thoracolumbar segments of the spinal cord. These segments receive descending oxytocinergic projections from the paraventricular nucleus of the hypothalamus. We tested the hypothesis that oxytocin regulates uterine motility through a spinal site of action. Oxytocin was administered in anesthetized female rats either intrathecally at the lumbosacral or thoracolumbar spinal cord levels or intravenously. Uterine activity was revealed by measuring changes of intrauterine pressure using an indwelling balloon placed in one caudal uterine horn. The uterus displayed a spontaneous activity characterized by intrauterine pressure rises, the frequency, amplitude, and duration of which were dependent on the stage of the estrous cycle. Oxytocin delivered at the lumbosacral level affected the frequency (during proestrus, estrus, and diestrus) and amplitude (during proestrus and estrus) of uterine activity. During estrus, oxytocin delivered at the thoracolumbar level affected the frequency, amplitude, and duration of the intrauterine pressure rises. Intravenous oxytocin not only affected intrauterine pressure rises (namely amplitude during proestrus and estrus and frequency and duration during estrus) but also increased the basal tone during estrus. The effects of lumbosacral oxytocin were partly mimicked by the oxytocin agonist [Thr4,Gly7]-oxytocin blocked by the oxytocin receptor antagonist atosiban and by hexamethonium. Arginine vasopressin delivered at the lumbosacral level had no effect. These results support our hypothesis that oxytocin released by descending paraventriculo-spinal pathways and acting on spinal oxytocin receptors modulates the activity of the uterus. This regulation is cycle dependent.

Spinal control of erection by glutamate in rats

The lumbosacral spinal network controlling penile erection is activated by information from peripheral and supraspinal origins. We tested the hypothesis that glutamate, released by sensory afferents from the genitals, activates this proerectile network. In anesthetized intact and T8 spinalized (i.e., freed from supraspinal inhibition) male rats, the parameters of electrical stimulation of the dorsal penile nerve (DPN) that elicited intracavernous pressure (ICP) rises were determined. In T8 spinalized rats, DPN stimulations were applied in the presence of d(-)-2-amino-5-phosphonopentanoic acid (d-AP5), a competitive NMDA receptor antagonist, or of 2,3-dioxo-6-nitro-1,2,3,4-tetrahydrobenzo[f]quinoxaline-7-sulphonamide (NBQX), an AMPA-kainate receptor antagonist, injected intrathecally at the lumbosacral level. Both antagonists, alone or in combination, dose dependently decreased the ICP rise and increased its latency. In conscious rats, reflexive erections were depressed by d-AP5 and NBQX, as revealed by an increased latency of the first erection and by decreases of the number of rats displaying erections, of the number of erection clusters and of the number of erections per cluster. In anesthetized rats, the combined administration of the glutamatergic agonists NMDA and AMPA elicited ICP rises in the absence of DPN stimulation. In contrast, both agonists moderately decreased the ICP rise elicited by DPN stimulation but did not affect its latency. These results support our hypothesis that glutamate, released on stimulation of the genitals and acting at AMPA and NMDA receptors, is a potent activator of the spinal proerectile network.

[Neural control of erection]

Activation of sacral parasympathetic pathways elicits penile erection through the release of vasorelaxant neurotransmitters that increase blood flow to the penis and relax the penile erectile tissue. Sympathetic pathways are antierectile. The pudendal pathway, responsible for the contraction of the perineal striated muscles, enhances an already present erection. All pathways originate in the spinal cord, but at various levels and areas. The convergence of information from peripheral and supra-spinal origins onto spinal neurones is very likely activating more specifically the spinal pro-erectile network. Peripheral information is the afferent limb of reflexive erections, impinges onto spinal interneurones and is able to activate or regulate the activity of sympathetic, parasympathetic and somatic nuclei. Supra-spinal information impinges onto either the same or a different spinal network. Premotor neurones located in supra-spinal structures, that project directly onto spinal sympathetic, parasympathetic or pudendal motoneurones, are present in the medulla, pons and diencephalon. Several of these premotor neurones may in turn be activated by sensory information from the genitals. Descending pathways release a variety of aminergic and peptidergic neurotransmitters in the vicinity of spinal neurones, thereby exerting complex effects on the spinal pro-erectile network. Brainstem and hypothalamic nuclei (among the latter, the paraventricular nucleus and the medial preoptic area) may not reach directly the spinal pro-erectile network. They are prone to regulate penile erection in more integrated and coordinated responses of the body, as those occurring during sexual behaviour. The pro-erectile central and spinal effects of neuropeptides such as oxytocin, melanocortins and endorphins have only recently been analyzed. Such compounds may represent therapeutic strategies to treat erectile dysfunction through a central site of action.

[Physiology and pharmacology of ejaculation]

Ejaculation requires an interplay of peripheral actors comprising, among others, smooth and skeletal fibers, glandular and endothelial cells. These actors are driven by vegetative and somatic innervations, deriving essentially from the spinal cord, in turn controlled by cerebral structures and endocrine factors, mostly steroids; These controls require sensitive afferences and command two steps, emission under autonomic control, and ejaculation per se which further involves somatic motoneurons. This review first describes the peripheral innervation of the part of the genital tract concerned in ejaculation, in which the sympathetic component is predominant and releases noradrenalin and neuropeptides; however parasympathetic and somatic components also play a role. At the spinal level, control circuits are organized into networks influenced by spinal structures, which have been discovered through selective lesions or stimulations, as well as by retrograde trans-synaptic tracing with neurotropic viruses. Among these structures, the median preoptic area and the hypothalamic paraventricular nucleus are major regulation sites. On the other hand, serotoninergic and also dopaminergic and adrenergic systems are implicated as well in the command of ejaculation; the latter constitute priviledged targets for a pharmacological treatment of dysfunctions.

Total subdiaphragmatic vagotomy does not suppress high protein diet-induced food intake depression in rats

This study was undertaken to determine whether the subdiaphragmatic vagus nerve is involved in the depression of food intake induced by the ingestion of a high protein diet (P50) in rats. After total subdiaphragmatic vagotomy (Vago group) or sham surgery (Sham group), rats consumed the control diet for a 2-wk recovery period and then both groups consumed the high protein diet for 16 d. Daily food intake, meal pattern analysis and behavioral satiety sequence were measured. Total subdiaphragmatic vagotomy did not modify the daily intake of the control diet or suppress the dramatic depression in food intake produced by acute transition to a high protein diet. However, the daily intake of a high protein diet was slightly reduced under acute conditions or even after adaptation (P < 0.005). Analysis of meal parameters and the behavioral satiety sequence after adaptation indicated no major metabolic distress. In conclusion, these results suggest that the subdiaphragmatic vagus nerve does not constitute an obligatory pathway for the transfer of information to the brain, resulting in a depression of high protein diet intake. In contrast, a defect in this visceral regulating system could reinforce the metabolic-associated food intake depression signal.

Pharmacokinetics and feeding responses to muramyl dipeptide in rats

N-acetyl-muramyl-L-alanine-D-isoglutamine or muramyl dipeptide (MDP) is the minimally active subunit of bacterial peptidoglycan. During a systemic infection, the involvement of MDP has been demonstrated in food intake depression by the macrophage hydrolysis of Gram-positive bacteria. Under normal conditions, mammals are constantly exposed to the release of endogenous MDP from degraded gut flora and that of exogenous MDP from the diet. However, MDP digestion and absorption in the gastrointestinal tract are not fully understood, and their physiological significance needs to be clarified. After gavage (1.5 mg/kg), very low levels of MDP were found in the systemic circulation of rats and feeding patterns were not altered. In contrast, after the intraperitoneal injection of a similar dose, a depression in food intake was observed. The rats reduced their meal frequency and constant feeding rate, showing signs of satiety. The behavioral satiety sequence (BSS) was modified by behavioral changes, similar to those which appear during sickness, such as an increase in resting and a reduction in grooming. Our data suggest that the hypophagic effect of MDP may result from satiety and sickness behavior.

Proerectile effects of apomorphine in mice

Dopaminergic pathways play a key role in the central control of sexual behavior. Stimulation of central dopaminergic receptors elicits penile erection in a variety of species and has been proposed as a treatment option for erectile dysfunction in humans. The present study investigated the proerectile effects of apomorphine in mice. In this species, subcutaneous injection of apomorphine (range: 0.11-110 microg/kg sc) elicited three different behavioral responses: erection, erection-like responses and genital grooming. Proerectile effects of apomorphine were dose-dependent. More than 50% of mice displayed erections after administration of 1.1-11 microg/kg of apomorphine sc. Proerectile effects of apomorphine were blocked by haloperidol, a central D2 antagonist, but not by domperidone, a peripherally active dopaminergic antagonist. We conclude that apomorphine elicits erection in mice. This effect is dose-dependent and due to activation of central D2 dopaminergic receptors. The mouse model may be useful for pharmacological approaches designed to provide a better understanding of the central mechanisms of penile erection and sexual behavior.

Spatial segregation within the sacral parasympathetic nucleus of neurons innervating the bladder or the penis of the rat as revealed by three-dimensional reconstruction

The purpose of the present investigations was (1) to examine the spatial organization of preganglionic neurons of the sacral parasympathetic nucleus in the lumbosacral spinal cord of male adult rats and (2) to search, in this nucleus, for a possible segregation of sub-populations of neurons innervating the penis or the bladder, respectively. To estimate their spatial organization, neurons of the sacral parasympathetic nucleus were retrogradely labeled by wheat germ agglutinin coupled to horseradish peroxidase applied to the central end of the sectioned pelvic nerve. The sub-populations of lumbosacral neurons innervating the corpus cavernosum of the penis or the dome of the bladder were identified using transsynaptic retrograde labeling by pseudorabies virus injected into these organs in different rats. In both wheat germ agglutinin-labeled and pseudorabies virus-labeled rats, serial coronal sections were cut through the spinal L5-S1 segments. Labeled neurons were revealed by histochemistry (peroxidase experiments) or immunohistochemistry (pseudorabies virus experiments). By means of a three-dimensional reconstruction software developed in our laboratory, three-dimensional models were calculated from each spinal section image series. They revealed the spatial organization of (i) preganglionic neurons and (ii) neurons innervating the bladder or the penis. The different three-dimensional models were subsequently merged into a single one which revealed the segregation, within the sacral parasympathetic nucleus, of the sub-populations of neurons. Neurons labeled by virus injected into the penis extended predominantly from the rostral part of the L6 segment to the rostral part of the S1 segment while those labeled by bladder injections were distributed predominantly from the caudal part of the L6 segment to the caudal part of the S1 segment. These results support the hypothesis of a viscerotopic organization of sacral neurons providing the spinal control of pelvic organs.

alpha(2a) and alpha(2c) adrenoceptors on spinal neurons controlling penile erection

The thoracolumbar and lumbosacral spinal cord contain respectively sympathetic and parasympathetic preganglionic neurons that supply the organs of the pelvis including the penis. These neurons are influenced by supraspinal information and receive aminergic projections from the brainstem. The presence of the alpha(1)- and alpha(2)-adrenoceptor subtypes has been demonstrated in the rat spinal cord. In this species, we looked for the presence of alpha(2a)- and alpha(2c)-adrenoceptor subtypes in the sympathetic and parasympathetic preganglionic neurons controlling erection. In adult male rats, transsynaptic axonal transport of pseudorabies virus injected into the penis was combined with immunohistochemistry against alpha(2a)- and alpha(2c)-adrenoceptor subtypes. At 4 days survival time, neurons infected with the pseudorabies virus were solely found in the intermediolateral cell column and dorsal gray commissure of segment T12-L2 and in the intermediolateral cell column of segment L6-S1. Neurons and fibers immunoreactive for alpha(2a)- and alpha(2c)-adrenoceptor subtypes were mainly present in the intermediolateral cell column, the dorsal gray commissure and the ventral horn of the T12-L2 and L5-S1 spinal cord, the dorsal horn displayed only immunoreactive fibers. Pseudorabies virus-infected neurons in the autonomic nuclei were both immunoreactive for alpha(2a)- and alpha(2c)-adrenoceptor subtypes and closely apposed by alpha(2a)- and alpha(2c)-immunoreactive fibers. The results suggest an intraspinal modulation of the noradrenergic and adrenergic control of the autonomic outflow to the penis by pre- and postsynaptic alpha(2) adrenoceptors.

[Central nervous control of erection]

Penile erection is caused by a change of the activity of efferent autonomic pathways to the erectile tissues and of somatic pathways to the perineal striated muscles. The sympathic outflow is mainly antierectile, the sacral parasympathic outflow is proerectile and the pudendal outflow, through contraction of the perineal striated muscles, enhances an erection already present. Spinal neurones controlling erection are activated by afferents from the genitals. It is likely that these primary afferents do not directly stimulate the spinal sympathetic, parasympathetic and somatic nuclei, but do through spinal interneurones. This spinal network is able of integrating information from periphery to elicit reflexive erections. It also receives supraspinal descending pathways from pons and hypothalamic nuclei, among the latter, the paraventricular nucleus and the medial preoptic area. These structures are likely involved to regulate penile erection in more integrated and coordinated responses occurring during sexual behavior. By receiving ascending projections from the spinal level which convoy informations from genitals, they could reinforce penile erection. Role of putative neuromediators or regulatory peptides is evoked.

Evidence for a direct projection from the paraventricular nucleus of the hypothalamus to putative serotoninergic neurons of the nucleus paragigantocellularis involved in the control of erection in rats

In the male rat, serotoninergic neurons of the ventrolateral medulla send direct projections onto spinal preganglionic neurons that innervate the penis. The role of the paraventricular nucleus of the hypothalamus in the control of penile erection is well recognized. Our aim was to demonstrate anatomical relation between paraventricular neurons and medullary serotoninergic neurons innervating the penis. In adult male rats, stereotaxic iontophoretic injections of Phaseolus vulgaris leuco-agglutinin were performed in the paraventricular nucleus. Neurons in the ventrolateral medulla were retrogradely labelled using transneuronal retrograde transport of pseudorabies virus injected in the corpus cavernosum. Sections of the ventro-lateral medulla were processed for double immunofluorescence to reveal both Phaseolus vulgaris leuco-agglutinin and pseudorabies virus using specific antibodies. Sections were also processed for the simultaneous detection of pseudorabies virus and serotonin. Pseudorabies virus-infected neurons in the ventrolateral medulla were present in the nucleus paragigantocellularis, reticular formation of the medulla, raphe pallidus and raphe magnus. In the nucleus paragigantocellularis, all pseudorabies virus-infected-neurons were immunoreactive for serotonin. Some of them received Phaseolus vulgaris leuco-agglutinin-labelled varicose fibres that ran along the soma of pseudorabies virus-infected neurons. Confocal microscopy suggested the presence of several close appositions between them, which were demonstrated using three-dimensional reconstruction of serial optical sections. Our results show that paraventricular neurons send direct projections in the nucleus paragigantocellularis onto neurons that innervate the penis. They suggest a possible role of the paraventricular nucleus in penile erection through the control of descending serotoninergic raphe-spinal neurons. The neurotransmitter used in this pathway remains to be determined.

Catecholaminergic projections onto spinal neurons destined to the pelvis including the penis in rat

In rats, the spinal cord contains proerectile autonomic motoneurons destined to the penile tissue and its vasculature, and somatic motoneurons destined to the perineal striated muscles. It receives dense catecholaminergic projections issued from the medulla and pons. In adult male rats, we evidenced the catecholaminergic innervation of spinal neurons controlling lower urogenital tissues and regulating penile erection. We combined retrograde tracing techniques and immunohistochemistry against synthetic enzymes of noradrenaline and adrenaline. Both sympathetic and parasympathetic preganglionic neurons, labeled from the major pelvic ganglion or from the corpus cavernosum, were apposed by catecholaminergic immunoreactive fibers. Motoneurons, retrogradely labeled from the striated muscles, were also apposed by catecholaminergic immunoreactive fibers. Synapses between these motoneurons and fibers were suggested by confocal microscopy and confirmed by electron microscopy in some cases. The results reinforce the hypothesis of a catecholaminergic control of autonomic and somatic motoneurons regulating penile erection at the spinal level.

Presence of the N-methyl-D-aspartic acid R1 glutamatergic receptor subunit in the lumbosacral spinal cord of male rats

The lumbosacral spinal cord contains neurones that control the lower urogenital and digestive tracts. Spinal neurones respond to activation from the periphery and supraspinal nuclei. Glutamate, acting through a variety of receptors, is an established transmitter of excitatory pathways to the spinal cord. Using immunohistochemical methods, we reveal the presence of the N-methyl-D-aspartic acid R1 (NMDAR1) glutamatergic receptor subunit in the lumbosacral spinal network that controls urogenital and digestive functions: the dorsal horn; the area around the central canal including the dorsal grey commissure; the sacral parasympathetic nucleus; and pudendal motoneurones. A complete thoracic spinal section did not alter labelling. Using retrograde labelling techniques, we identify sacral preganglionic neurones and pudendal neurones that are NMDAR1 immunoreactive. Glutamate, acting at NMDA receptors, can therefore co-ordinate the activity of the autonomic and somatic outflows to the pelvic organs.

AMPA glutamatergic receptor-immunoreactive subunits are expressed in lumbosacral neurons of the spinal cord and neurons of the dorsal root and pelvic ganglia controlling pelvic functions in the rat

Sacral preganglionic neurons innervate the pelvic organs via a relay in the major pelvic ganglion. Pudendal motoneurons innervate striated muscles and sphincters of the lower urinary, genital and digestive tracts. The activity of these spinal neurons is regulated by sensory afferents of visceral and somatic origins. Glutamate is released by sensory afferents in the spinal cord, and interacts with a variety of receptor subtypes. The aim of the present study was to investigated the presence of AMPA glutamate receptor subunits (GluR1-GluR4) in the neural network controlling the lower urogenital and digestive tracts of male rats. We performed double-immunohistochemistry directed against a neuronal tracer, the cholera toxin beta subunit (Ctbeta) and each of the four receptor subunits. GluR1, GluR2 and GluR3 subunits were present in many sacral preganglionic neurons retrogradely labelled with Ctbeta applied to the pelvic nerve, and in some dorsolateral and dorsomedian motoneurons retrogradely labelled with Ctbeta injected in ischiocavernosus and bulbospongiosus muscles. The four subunits were detected in postganglionic neurons of the major pelvic ganglion retrogradely labelled with Ctbeta injected in the corpus cavernosum, and in some somata of sensory afferents of the L6 dorsal root ganglion labelled with Ctbeta applied to the dorsal penile nerve or injected in corpus cavernosum. The results provide a detailed knowledge of the neural targets expressing the various AMPA receptor subunits and suggest that part of the neural network that controls pelvic organs, including sensory afferents and postganglionic neurons, is sensitive to glutamate through the whole family of AMPA subunits.

c-Fos expression as endogenous marker of lumbosacral spinal neuron activity in response to vaginocervical-stimulation

The focus of this paper is to describe a method for the simultaneous stimulation of the vagina and uterine cervix and recording of vaginal contractions in the female rat. The influence of the estrous cycle on vaginocervical sensitivity was also investigated. The use of a latex balloon, inflated with water via a syringe and connected to a pressure transducer allowed us to record vaginal contractions, the intensity of which is an index of vaginal sensitivity, and to stimulate those spinal neurons involved in the reflex arc; at the end of the stimulation the deflated balloon was used as a probe to perform a vaginal smear in order to determine the stage of the estrous cycle at the moment of the experiment. Activated neurons were identified by Fos-immunocytochemistry. Light microscope counting of Fos-immunoreactive neurons at different stages of the estrous cycle permitted us to quantify the response to vaginocervical stimulation and to demonstrate that vaginocervical sensitivity changes significantly throughout the estrous cycle. This finding confirms the important role of sex hormones in the modulation and control of the sensitivity of the vagina and cervix, a part of the female genital tract which is implicated in a variety of neuroendocrine, behavioral and neural changes.

Pro-erectile effect of systemic apomorphine: existence of a spinal site of action

PURPOSE

Apomorphine exerts pro-erectile effects by acting on neurons in the paraventricular nucleus of the hypothalamus. In spinal cord injured rats we assessed whether apomorphine also directly activates the spinal autonomic and somatic neurons controlling penile erection

MATERIALS AND METHODS

Intracavernous and blood pressure was monitored in groups of 10 anesthetized rats to quantify intracavernous pressure increases elicited after intravenous apomorphine. We determined the number and duration of increases, percent of maximum intracavernous pressure/mean diastolic blood pressure using the formula, maximum intracavernous pressure/diastolic blood pressure x 100, area under the intracavernous pressure curve/diastolic blood pressure and sum of the area under the curve/diastolic blood pressure.

RESULTS

Of 2, 10, 50 and 250 microg./kg. intravenous apomorphine 50 microg./kg. induced significant pro-erectile effects and was subsequently used. In spinal cord injured rats 50 microg./kg. intravenous apomorphine significantly increased median maximum intracavernous pressure/diastolic blood pressure x 100 compared with vehicle injection (56 versus 27 seconds, p <0.001), area under the curve/diastolic blood pressure (21 versus 12 seconds, p = 0.07) and the sum of area under the curve/diastolic blood pressure (132 versus 32 seconds, p = 0.01). These pro-erectile effects of apomorphine were prevented by 50 mg./kg. hexamethonium intravenously or bilateral transection of the pelvic nerves. They were not affected by 3 mg./kg. of the peripheral D1/D2 antagonist domperidone intraperitoneally. In spinal cord injured rats subcutaneous pretreatment with 0.2 mg./kg. of the D1 antagonist SCH23390 significantly enhanced apomorphine induced erections, as indicated by an area under the curve/diastolic blood pressure of 23 to 30 seconds (p = 0.003), whereas they were not changed by 25 mg./kg. of the D2 antagonist sulpiride intraperitoneally. Under the same conditions 1 mg./kg. of the central D1/D2 antagonist haloperidol intraperitoneally only reduced the number of responding rats to 5 versus 10 of 10.

CONCLUSIONS

In spinal cord injured rats systemic apomorphine elicits erection by acting at the spinal cord level. This finding suggests that systemic apomorphine elicits penile erections via spinal and supraspinal targets.

Neurophysiology and pharmacology of female genital sexual response

Vaginal sexual arousal is a vasocongestive and neuromuscular event controlled by facilitatory parasympathetic and inhibitory sympathetic inputs. Autonomic preganglionic parasympathetic and inhibitory sympathetic fibers to the vagina and clitoris originate in the spinal cord in the sacral parasympathetic nucleus at the sacral level and in the dorsal gray commissure and the intermediolateral cell column at the thoracolumbar level, respectively. Parasympathetic fibers are conveyed by the pelvic nerve, and sympathetic fibers are conveyed by the hypogastric nerve and the paravertebral sympathetic chain. The activity of these spinal nuclei is controlled by descending projections from the brain and sensory afferens (conveyed in the pudendal, hypogastric, pelvic, and vagus nerves) from the genitalia. A key but unresolved issue concerns the neurotransmitters involved in the control of genital sexual arousal. At the peripheral level, acetylcholine plays a minor role in the regulation of vaginal blood flow, however, recent data suggests that it may be involved in the control of vaginal smooth muscle contractions. Vasoactive intestinal peptide and nitric oxide may be responsible for the increase in vaginal blood flow during sexual arousal, whereas noradrenaline is likely inhibitory. Within the central nervous system, serotoninergic projections from the brain to the spinal cord likely inhibit the induction of genital arousal by peripheral informations (spinal reflex). Although some neurotransmitters regulating the display of sexual behavior have been identified (for example, dopamine), their involvement in the control of genital sexual arousal has not been invested. Anatomical and electrophysiological data point to a contribution of the paraventricular nucleus of he hypothalamus and the median preoptic area, respectively, as key elements in the control of genital arousal. The recent development of models allowing the assessment of vaginal sexual arousal in anesthetized female rats should assist in deciphering the neurochemical pathways controlling vaginal sexual arousal and the development of suitable pharmacological treatment for female sexual dysfunctions.

Sexual behavior of stallions during in-hand natural service and semen collection: an observation in French studs

The sexual behavior of 42 stallions from French national and private studs was examined in two contexts: semen collection for artificial insemination (AI) and in-hand natural service (NS). Each stallion was observed twice in the same context. Erection and ejaculation latencies, the number of mounts leading to ejaculation, dismount latency and total breeding time were measured and compared between AI and NS. Mount without erection was rare (6/83 observations). Erection latency was 89+/-11s, and was not different between NS (62+/-22s) and AI (100+/-13s, P=0.128). Stallions ejaculated after either one mount (62/83 observations), or two (11/83 observations) or three mounts (10/83 observations). Ejaculation latency was 85+/-15s (84+/-19 in AI and 86+/-28 in NS). If 1st mount did not lead to ejaculation, then ejaculation latency increased several fold following the 2nd mount during both AI and NS. The results provide reference measures for semen collection in French studs. Difference in erection latency between AI and NS, although not statistically significant, may reflect different contributions of excitatory inputs from the brain and the genital area to the activation of spinal networks controlling erection. In contrast, lack of difference in ejaculation latency between AI and NS suggests that the spinal network that controls ejaculation follows a more rigid motor pattern.

Spinal proerectile effect of oxytocin in anesthetized rats

The spinal cord contains the neural network that controls penile erection. This network is activated by information from peripheral and supraspinal origin. We tested the hypothesis that oxytocin (OT), released at the lumbosacral spinal cord level by descending projections from the paraventricular nucleus, regulated penile erection. In anesthetized male rats, blood pressure and intracavernous pressure (ICP) were monitored. Intrathecal (it) injection of cumulative doses of OT and the selective OT agonist [Thr4,Gly7]OT at the lumbosacral level elicited ICP rises whose number, amplitude, and area were dose dependent. Thirty nanograms of OT and one-hundred nanograms of the agonist displayed the greatest proerectile effects. Single injections of OT also elicited ICP rises. Preliminary injection of a specific OT-receptor antagonist, hexamethonium, or bilateral pelvic nerve section impaired the effects of OT injected it. NaCl and vasopressin injected it at the lumbosacral level and OT injected it at the thoracolumbar level or intravenously had no effect on ICP. The results demonstrate that OT, acting at the lumbosacral spinal cord, elicits ICP rises in anesthetized rats. They suggest that OT, released on physiological activation of the PVN in a sexually relevant context, is a potent activator of spinal proerectile neurons.

Spinal proerectile effect of apomorphine in the anesthetized rat

Considering the presence of dopaminergic receptors in the lumbosacral spinal cord, we tested whether apomorphine could exert a proerectile effect by acting at the spinal level. Intracavernous (ICP) and blood pressures (BP) were measured in anesthetized rats. ICP rises were quantified (duration, percentage of ICPmaximum/meanBP (ICPmax/BPx100), area under ICP curve (AUC/BP) and sum of AUC/BP after intravenous (i.v.) and intrathecal (i.t.) injections of apomorphine alone or in presence of i.t. oxytocin (10 ng). Both 10 and 30 microg i.v. apomorphine dosings elicited erectile events evidenced by ICP rises. Upon the 30 microg i.v. injection, duration of ICP rises were increased from 25+/-10 to 69+/-18 s (P<0.001), ICPmax/BPx100 from 21+/-3 to 50+/-14% (P=0.001), AUC/BP from 3+/-1 to 14+/-6 s (P=0.002) and sum of AUC/BP from 5+/-7 to 34+/-35 s (P=0.021). Upon 30 microg i.t. injections of apomorphine at the lumbosacral level, the number of ICP rises was increased from 0.2+/-0.4 to 3.0+/-1.5, ICPmax/BPx100 from 16+/-9 to 43+/-12 and sum of AUC/BP from 1+/-3 to 31+/-15 s compared to vehicle injection (P<0.05 for all parameters). Injection of 30 microg i.v. or i.t. apomorphine non-significantly enhanced the number and amplitude of the ICP rises induced by 10 ng i.t. oxytocin. However, the enhancement of the amplitude of the ICP rises elicited by i.t. oxytocin was more pronounced with i.t. apomorphine than with i.v. apomorphine. These results suggest the existence of a spinal site of action for apomorphine which may (1) participate to generation of erection and (2) exerts a facilitator effect on erection of supraspinal origin.

Brain control of penile erection

The spinal cord contains a network that controls erection. This network can be activated by information from the periphery and by supraspinal nuclei. Besides anatomical studies that have detailed central pathways putatively involved in the central process of proerectile information, functional approaches have focused on pharmacological manipulations of specific systems, e.g. central dopaminergic pathways, leading to clinical perspectives in the treatment of erectile dysfunction. The present review focuses on some aspects of the recent research in the field.

Central control of erection and its pharmacological modification

Advances in our understanding of the local mechanisms of penile erection have paralleled the use of pharmacological treatments of erectile dysfunction. In contrast, the spinal and supraspinal mechanisms that control penile erection are less well understood. Although the role of hypothalamic areas (medial preoptic area, paraventricular nucleus) and brainstem nuclei (raphe nuclei) in penile erection has been evaluated, as has the role of an association between neuromediators and receptors (serotonin, dopamine, noradrenalin, glutamate, gamma-aminobutyric acid, nitric oxide), an integrative view of the central mechanisms of penile erection is lacking. New strategies to treat erectile dysfunction employ oral agents, some of which target central brain nuclei. The future of such treatments largely depends on a better understanding of the central mechanisms of penile erection.

Influence of estrous cycle on vaginocervical sensitivity: a fos-immunohistochemical study of lumbosacral spinal cord

Expression of c-fos in L(5)-S(1) spinal segments in response to mechanical vaginocervical stimulation was investigated in both cycling and ovariectomized females. The aim of this paper was to verify the influence of estrous cycle on females genital tract sensitivity using immunodetection of a neural activity endogenous marker. The results indicate that lumbosacral spinal Fos-labeling was highly increased in vaginocervical stimulated rats relative to control, and labeled neurons were present more intensively in the dorsal horn in comparison to other spinal areas. Significant differences in Fos-labeling were observed according to the estrous cycle stage at which the stimulation was applied. In estrous females, the response was greater than that obtained at diestrous and much greater than the response of proestrous females. The spinal Fos-labeling of ovariectomized females is equivalent to that of diestrous females. These results give evidence that the vaginocervical induced expression of c-fos is modulated by cyclic changes in circulating sex hormones, whereas results observed in ovariectomized females indicate the likely involvement of other mechanisms independent of ovarian hormones.

Central noradrenergic control of penile erection

Penile erection is completely dependent on commands from the central nervous system. Spinal centers controlling penile erection are located in the thoracolumbar and lumbosacral spinal cord. These centers are activated by information from the periphery and supraspinal nuclei so as to elicit penile erection in a variety of physiological contexts. A small number of nuclei including the locus coeruleus located in the pons sends noradrenergic fibers to the forebrain and spinal cord, including those areas controlling penile erection. Recent morphological techniques such as in situ hybridization and autoradiography using radioligand binding permit investigation of the brain and spinal pathways utilizing alpha adrenoceptor subtypes. Furthermore, pharmacological experiments suggest a modulatory role for noradrenaline in the control of penile erection either in the brain or in the spinal cord. The most robust evidence is that central inhibition of alpha-2 adrenoceptors facilitates sexual function. Taken together, the data propose new directions in the physiological exploration of penile erection and the therapeutic approach of erectile dysfunction.

Central control of the cardiovascular and erection systems: possible mechanisms and interactions

Sexual activity is accompanied by vascular changes mediated by parasympathetic and sympathetic outflow to the peripheral organs. The brain stem and spinal cord contain the neurons that innervate the cardiovascular system and the penis. Heart rate and blood pressure increase, suggesting a decrease of the cranial parasympathetic outflow and an increase of the activity of sympathetic efferent pathways. In contrast, penile erection occurs in response to increased activity of the sacral parasympathetic innervation and a decreased activity of sympathetic pathways. A modulation of the balance between sympathetic and parasympathetic activities may result from an adaptation of an intraspinal network that (1) would be the recipient of peripheral and supraspinal information; and (2) would coordinate the activity of the different efferent pathways. A variety of nuclei in the medulla, pons, and hypothalamus contain premotor neurons that exert an influence on brain stem and spinal autonomic motoneurons. These descending pathways release amines (noradrenaline, adrenaline, serotonin, dopamine) and peptides. A fine tuning of brain stem and spinal activity is made possible by the great variety of receptor subtypes through which these neuromediators act. More recently, the role of nitric oxide, synthesized and released by different cell populations, has been evaluated in the brain and spinal control of the cardiovascular system and penile erection. Depending on its central neural target, nitric oxide may either activate or inhibit the cardiovascular system. In contrast, its role on the central control of penile erection is only excitatory.

Central neural regulation of penile erection

Penile erection is caused by a change of the activity of efferent autonomic pathways to the erectile tissues and of somatic pathways to the perineal striated muscles. The spinal cord contains the cell bodies of autonomic and somatic motoneurons that innervate the peripheral targets. The sympathetic outflow is mainly antierectile, the sacral parasympathetic outflow is proerectile, and the pudendal outflow, through contraction of the perineal striated muscles, enhances an erection already present. The shift from flaccidity to erection suggests relations among these neuronal populations in response to a variety of informations. Spinal neurons controlling erection are activated by information from peripheral and supraspinal origin. Both peripheral and supraspinal information is capable of eliciting erection, or modulating or inhibiting an erection already present. One can hypothesize a spinal network consisting of primary afferents from the genitals, spinal interneurons and sympathetic, parasympathetic and somatic nuclei. This system is capable of integrating information from the periphery and eliciting reflexive erections. The same spinal network, eventually including different populations of spinal interneurons, would be the recipient of supraspinal information. Premotor neurons that project directly onto spinal sympathetic, parasympathetic or somatic motoneurons, are present in the medulla, pons and diencephalon. Several of these premotor neurons may in turn be activated by sensory information from the genitals. Aminergic and peptidergic descending pathways in the vicinity of spinal neurons, exert complex effects on the spinal network that control penile erection. This is caused by the potential interaction of a great variety of receptors and receptor subtypes present in the spinal cord. Brainstem and hypothalamic nuclei (among the latter, the paraventricular nucleus and the medial preoptic area) may not necessarily reach spinal neurons directly. However they are prone to regulate penile erection in more integrated and coordinated responses of the body, such as those occurring during sexual behavior. Finally, the central and spinal role of regulatory peptides (oxytocin, melanocortins, endorphins) has only recently been elucidated.