Sexual Behavior Increases Cell Proliferation in the Rostral Migratory Stream and Promotes the Differentiation of the New Cells into Neurons in the Accessory Olfactory Bulb of Female Rats

Prolactin promotes the recruitment of main olfactory bulb cells and enhances the behavioral exploration toward a socio-sexual stimulus in female mice

Olfactory communication is triggered by pheromones that profoundly influence neuroendocrine responses to drive social interactions. Two principal olfactory systems process pheromones: the main and the vomeronasal or accessory system. Prolactin receptors are expressed in both systems suggesting a participation in the processing of olfactory information. We previously reported that prolactin participates in the sexual and olfactory bulb maturation of females. Therefore, we explored the expression of prolactin receptors within the olfactory bulb during sexual maturation and the direct responses of prolactin upon pheromonal exposure. Additionally, we assessed the behavioral response of adult females exposed to male sawdust after prolactin administration and the consequent activation of main and accessory olfactory bulb and their first central relays, the piriform cortex and the medial amygdala. Last, we investigated the intracellular pathway activated by prolactin within the olfactory bulb. Here, prolactin receptor expression remained constant during all maturation stages within the main olfactory bulb but decreased in adulthood in the accessory olfactory bulb. Behaviorally, females that received prolactin actively explored the male stimulus. An increased cFos activation in the amygdala and in the glomerular cells of the whole olfactory bulb was observed, but an augmented response in the mitral cells was only found within the main olfactory bulb after prolactin administration and the exposure to male stimulus. Interestingly, the ERK pathway was upregulated in the main olfactory bulb after exposure to a male stimulus. Overall, our results suggest that, in female mice, prolactin participates in the processing of chemosignals and behavioral responses by activating the main olfactory system and diminishing the classical vomeronasal response to pheromones.

Effects of crocin on the enhancement of in vitro neurogenesis: Involvement of Notch and CREB/BDNF signaling pathways

Objectives

Adult neurogenesis, the process of generating new neurons, continues throughout life. Unfortunately, this process is insufficient in pathological conditions and needs to be promoted. Crocin, the active component of saffron, affects neurogenesis in vivo and in vitro. We aimed to investigate the enhancing effects of crocin on the neurogenesis of adipose-derived mesenchymal stem cells in the presence of retinoic acid, as well as the molecular pathways involved.

Materials and Methods

Differentiation capacities and stemness potential of harvested ADSCs were evaluated by differentiating into osteocytes and adipocytes, and expression of mesenchymal CD markers by flow cytometry. The optimum dose of crocin was assessed with an MTT assay. Crocin, retinoic acid, CREB/BDNF, and Notch inhibitors and their combination were added to the culture medium. Jag1, Hes1, Notch, and BDNF gene expression were analyzed by RT-PCR on days 7, 14, and 21, while CREB, DCX, SOX2, and NeuN expression were analyzed by immunofluorescence.

Results

Expression of mesenchymal CD markers as well as adipogenic and osteogenic differentiation confirmed the origin and properties of ADSCs. The optimal dose of crocin was 1 mM. Crocin significantly (P<0.05) increased, while inhibitors (DATP&Naphthol) significantly (P<0.05) decreased Jag1, Hes1, Notch, and BDNF expression. Immunofluorescent assessments showed that expression of DCX, BDNF, NeuN, and Sox2 proteins increased significantly (P<0.05) after crocin administration and decreased significantly (P<0.05) after inhibitor administration.

Conclusion

Crocin can be used as an enhancer for neural differentiation of MSCs in vitro in the presence of retinoic acid. The mechanism is proposed through Notch and CREB/BDNF signaling pathways.

Pregnancy-responsive pools of adult neural stem cells for transient neurogenesis in mothers

Adult neural stem cells (NSCs) contribute to lifelong brain plasticity. In the adult mouse ventricular-subventricular zone, NSCs are heterogeneous and, depending on their location in the niche, give rise to different subtypes of olfactory bulb (OB) interneurons. Here, we show that multiple regionally distinct NSCs, including domains that are usually quiescent, are recruited on different gestation days during pregnancy. Synchronized activation of these adult NSC pools generates transient waves of short-lived OB interneurons, especially in layers with less neurogenesis under homeostasis. Using spatial transcriptomics, we identified molecular markers of pregnancy-associated interneurons and showed that some subsets are temporarily needed for own pup recognition. Thus, pregnancy triggers transient yet behaviorally relevant neurogenesis, highlighting the physiological relevance of adult stem cell heterogeneity.

Increased proliferation and neuronal fate in prairie vole brain progenitor cells cultured in vitro: effects by social exposure and sexual dimorphism

BACKGROUND

The prairie vole (Microtus ochrogaster) is a socially monogamous rodent that establishes an enduring pair bond after cohabitation, with (6 h) or without (24 h) mating. Previously, we reported that social interaction and mating increased cell proliferation and differentiation to neuronal fate in neurogenic niches in male voles. We hypothesized that neurogenesis may be a neural plasticity mechanism involved in mating-induced pair bond formation. Here, we evaluated the differentiation potential of neural progenitor cells (NPCs) isolated from the subventricular zone (SVZ) of both female and male adult voles as a function of sociosexual experience. Animals were assigned to one of the following groups: (1) control (Co), sexually naive female and male voles that had no contact with another vole of the opposite sex; (2) social exposure (SE), males and females exposed to olfactory, auditory, and visual stimuli from a vole of the opposite sex, but without physical contact; and (3) social cohabitation with mating (SCM), male and female voles copulating to induce pair bonding formation. Subsequently, the NPCs were isolated from the SVZ, maintained, and supplemented with growth factors to form neurospheres in vitro.

RESULTS

Notably, we detected in SE and SCM voles, a higher proliferation of neurosphere-derived Nestin + cells, as well as an increase in mature neurons (MAP2 +) and a decrease in glial (GFAP +) differentiated cells with some sex differences. These data suggest that when voles are exposed to sociosexual experiences that induce pair bonding, undifferentiated cells of the SVZ acquire a commitment to a neuronal lineage, and the determined potential of the neurosphere is conserved despite adaptations under in vitro conditions. Finally, we repeated the culture to obtain neurospheres under treatments with different hormones and factors (brain-derived neurotrophic factor, estradiol, prolactin, oxytocin, and progesterone); the ability of SVZ-isolated cells to generate neurospheres and differentiate in vitro into neurons or glial lineages in response to hormones or factors is also dependent on sex and sociosexual context.

CONCLUSION

Social interactions that promote pair bonding in voles change the properties of cells isolated from the SVZ. Thus, SE or SCM induces a bias in the differentiation potential in both sexes, while SE is sufficient to promote proliferation in SVZ-isolated cells from male brains. In females, proliferation increases when mating is performed. The next question is whether the rise in proliferation and neurogenesis of cells from the SVZ are plastic processes essential for establishing, enhancing, maintaining, or accelerating pair bond formation. Highlights 1. Sociosexual experiences that promote pair bonding (social exposure and social cohabitation with mating) induce changes in the properties of neural stem/progenitor cells isolated from the SVZ in adult prairie voles. 2. Social interactions lead to increased proliferation and induce a bias in the differentiation potential of SVZ-isolated cells in both male and female voles. 3. The differentiation potential of SVZ-isolated cells is conserved under in vitro conditions, suggesting a commitment to a neuronal lineage under a sociosexual context. 4. Hormonal and growth factors treatments (brain-derived neurotrophic factor, estradiol, prolactin, oxytocin, and progesterone) affect the generation and differentiation of neurospheres, with dependencies on sex and sociosexual context. 5. Proliferation and neurogenesis in the SVZ may play a crucial role in establishing, enhancing, maintaining, or accelerating pair bond formation.

Behavioral evidence of the functional interaction between the main and accessory olfactory system suggests a large olfactory system with a high plastic capability

Olfaction is fundamental in many species of mammals. In rodents, the integrity of this system is required for the expression of parental and sexual behavior, mate recognition, identification of predators, and finding food. Different anatomical and physiological evidence initially indicated the existence of two anatomically distinct chemosensory systems: The main olfactory system (MOS) and the accessory olfactory system (AOS). It was originally conceived that the MOS detected volatile odorants related to food, giving the animal information about the environment. The AOS, on the other hand, detected non-volatile sexually relevant olfactory cues that influence reproductive behaviors and neuroendocrine functions such as intermale aggression, sexual preference, maternal aggression, pregnancy block (Bruce effect), puberty acceleration (Vandenbergh effect), induction of estrous (Whitten effect) and sexual behavior. Over the last decade, several lines of evidence have demonstrated that although these systems could be anatomically separated, there are neuronal areas in which they are interconnected. Moreover, it is now clear that both the MOS and the AOS process both volatile and no-volatile odorants, indicating that they are also functionally interconnected. In the first part of the review, we will describe the behavioral evidence. In the second part, we will summarize data from our laboratory and other research groups demonstrating that sexual behavior in male and female rodents induces the formation of new neurons that reach the main and accessory olfactory bulbs from the subventricular zone. Three factors are essential for the neurons to reach the AOS and the MOS: The stimulation frequency, the stimulus's temporal presentation, and the release of opioids induced by sexual behavior. We propose that the AOS and the MOS are part of a large olfactory system with a high plastic capability, which favors the adaptation of species to different environmental signals.

Mating-induced prolactin surge is not required for subsequent neurogenesis in male mice

Parenting involves major behavioral transitions that are supported by coordinated neuroendocrine and physiological changes to promote the onset of novel offspring-directed behaviors. In comparison to maternal care, however, the mechanisms underlying the transition to paternal care are less understood. Male laboratory mice are predominantly infanticidal as virgins but show paternal responses 2 weeks after mating. Interestingly, males show a mating-induced surge of prolactin, which we hypothesized may be involved in initiating this behavioral transition. During pregnancy, prolactin stimulates olfactory bulb neurogenesis, which is essential for maternal behavior. Mating induces olfactory bulb neurogenesis in males, but it is unknown whether this is driven by prolactin or is important for subsequent paternal care. New olfactory neurons are generated from cells in the subventricular zone (SVZ) and take about 2 weeks to migrate to the olfactory bulb, which may account for the delayed behavioral change in mated males. We investigated whether mating increases cell proliferation at the SVZ. Males were either mated, exposed to receptive female cues, or left alone (control) and injected with Bromodeoxyuridine (BrdU, a marker of cell division). Contrary to our hypothesis, we found that mating decreased cell proliferation in the caudal lateral portion of the SVZ. Next, we tested whether prolactin itself mediates cell proliferation in the SVZ and/or new cell survival in the olfactory bulb by administering bromocriptine (prolactin inhibitor), vehicle, or bromocriptine + prolactin prior to mating. While suppressing prolactin had no effect on cell proliferation in the SVZ, administering exogenous prolactin resulted in significantly higher BrdU-labeled cells in mated but not virgin male mice. No effects of prolactin were observed on new olfactory cell survival. Taken together, prolactin may have context-dependent effects on new cell division in the SVZ, while other unknown mechanisms may be driving the effects on new olfactory cell survival following mating.

Pair-bonding and social experience modulate new neurons survival in adult male and female prairie voles (Microtus ochrogaster)

Prairie voles are a socially monogamous species that, after cohabitation with mating, form enduring pair bonds. The plastic mechanisms involved in this social behavior are not well-understood. Neurogenesis in adult rodents is a plastic neural process induced in specific brain areas like the olfactory bulbs (OB) and dentate gyrus (DG) of the hippocampus. However, it is unknown how cell survival is modulated by social or sexual experience in prairie voles. This study aimed to evaluate if cohabitation with mating and/or social exposure to a vole of the opposite sex increased the survival of the new cells in the main and accessory OB and DG. To identify the new cells and evaluate their survival, voles were injected with the DNA synthesis marker 5-bromo-2’-deoxyuridine (BrdU) and were randomly distributed into one of the following groups: (A) Control (C), voles that did not receive any sexual stimulation and were placed alone during the behavioral test. (B) Social exposure (SE), voles were individually placed in a cage equally divided into two compartments by an acrylic screen with small holes. One male and one female were placed in opposite compartments. (C) Social cohabitation with mating (SCM), animals mated freely. Our findings demonstrated that SCM females had increases in the number of new cells (BrdU-positive cells) in the main olfactory bulb and new mature neurons (BrdU/NeuN-positive cells) in the glomerular layer (GlL). In contrast, these new cells decrease in males in the SE and SCM conditions. In the granular cell layer (GrL), SCM females had more new cells and neurons than the SE group. In the accessory olfactory bulb, in the anterior GlL, SCM decreased the number of new cells and neurons in females. On the other hand, in the DG, SCM and SE increase the number of new cells in the suprapyramidal blade in female voles. Males from SCM express more new cells and neurons in the infrapyramidal blade compared with SE group. Comparison between male and females showed that new cells/neurons survival was sex dependent. These results suggest that social interaction and sexual behavior modulate cell survival and influence the neuronal fate in a sex-dependent manner, in the OB and DG. This study will contribute to understand neural mechanisms of complex social and pair bond behaviors in the prairie voles; supporting adult neurogenesis as a plastic mechanism potentially involved in social monogamous strategy.

Brain circuits activated by female sexual behavior evaluated by manganese enhanced magnetic resonance imaging

Magnetic resonance imaging (MRI) allows obtaining anatomical and functional information of the brain in the same subject at different times. Manganese-enhanced MRI (MEMRI) uses manganese ions to identify brain activity, although in high doses it might produce neurotoxic effects. Our aims were to identify a manganese dose that does not affect motivated behaviors such as sexual behavior, running wheel and the rotarod test. The second goal was to determine the optimal dose of chloride manganese (MnCl₂) that will allow us to evaluate activation of brain regions after females mated controlling (pacing) the sexual interaction. To achieve that, two experiments were performed. In experiment 1 we evaluated the effects of two doses of MnCl₂, 8 and 16 mg/kg. Subjects were injected with one of the doses of MnCl₂ 24 hours before the test on sessions 1, 5 and 10 and immediately thereafter scanned. Female sexual behavior, running wheel and the rotarod were evaluated once a week for 10 weeks. In experiment 2 we followed a similar procedure, but females paced the sexual interaction once a week for 10 weeks and were injected with one of the doses of MnCl₂ 24 hours before the test and immediately thereafter scanned on sessions 1, 5 and 10. The results of experiment 1 show that neither dose of MnCl₂ induces alterations on sexual behavior, running wheel and rotarod. Experiment 2 demonstrated that MEMRI allow us to detect activation of different brain regions after sexual behavior, including the olfactory bulb (OB), the bed nucleus of the stria terminalis (BNST), the amygdala (AMG), the medial preoptic area (MPOA), the ventromedial hypothalamus (VMH), the nucleus accumbens (NAcc), the striatum (STR) and the hippocampus (Hipp) allowing the identification of changes in brain circuits activated by sexual behavior. The socio sexual circuit showed a higher signal intensity on session 5 than the reward circuit and the control groups indicating that even with sexual experience the activation of the reward circuit requires the activation of the socio sexual circuit. Our study demonstrates that MEMRI can be used repeatedly in the same subject to evaluate the activation of brain circuits after motivated behaviors and how can this activation change with experience.

Effects of Mating and Social Exposure on Cell Proliferation in the Adult Male Prairie Vole (Microtus ochrogaster)

Microtus ochrogaster is a rodent with a monogamous reproductive strategy characterized by strong pair bond formation after 6 h of mating. Here, we determine whether mating-induced pair bonding increases cell proliferation in the subventricular zone (SVZ), rostral migratory stream (RMS), and dentate gyrus (DG) of the hippocampus in male voles. Males were assigned to one of the four groups: (1) control: males were placed alone in a clean cage; (2) social exposure to a female (SE m/f): males that could see, hear, and smell a sexually receptive female but where physical contact was not possible, because the animals were separated by an acrylic screen with small holes; (3) social exposure to a male (SE m/m): same as group 2 but males were exposed to another male without physical contact; and (4) social cohabitation with mating (SCM): males that mated freely with a receptive female for 6 h. This procedure leads to pair bond formation. Groups 2 and 3 were controls for social interaction. Male prairie voles were injected with 5-bromo-2′-deoxyuridine (BrdU) during the behavioral tests and were sacrificed 48 h later. Brains were processed to identify the new cells (BrdU-positive) and neuron precursor cells (neuroblasts). Our principal findings are that in the dorsal region of the SVZ, SCM and SE m/f and m/m increase the percentage of neuron precursor cells. In the anterior region of the RMS, SE m/f decreases the percentage of neuron precursor cells, and in the medial region SE m/f and m/m decrease the number of new cells and neuron precursor cells. In the infrapyramidal blade of the subgranular zone of the DG, SE m/m and SCM increase the number of new neuron precursor cells and SE m/m increases the percentage of these neurons. Our data suggests that social interaction, as well as sexual stimulation, leads to pair bonding in male voles modulating cell proliferation and differentiation to neuronal precursor cells at the SVZ, RMS, and DG.

Repeated Paced Mating Increases the Survival of New Neurons in the Accessory Olfactory Bulb

In female rats, the first sexual experience under paced mating conditions increases the number of newborn cells that migrate into the granular layer of the accessory olfactory bulb (AOB). Repeated paced mating has a potentiating effect on the number of new neurons that migrate to the AOB compared with a single session 15 days after paced mating. On the other hand, one paced mating session does no increases the survival of new cells 45 days after mating. In the present study, we evaluated if four paced mating sessions could increase the survival of new neurons in the AOB and main olfactory bulb (MOB) 45 days after females mated. Sexually naive female rats were ovariectomized, hormonally supplemented and randomly assigned to one of five groups: (1) Control, no sexual contact (C); (2) Four sessions in which females were exposed, without mating, to a sexually experience male rat (SE); (3) One session of paced mating (PM1); (4) Four sessions of paced mating (PM4); and (5) Four sessions of non-paced mating (NPM4). In the first behavioral test, females received the DNA synthesis marker 5-bromo-2'deoxyuridine and were euthanized 45 days later. Our data showed that the number of new cells that survived in the mitral cell layer of the AOB decreased when females were exposed to a sexually active male, in comparison to females that mated once pacing the sexual interaction. Repeated sexual behavior in pacing conditions did not increase the survival of new cells in other layers of the MOB and AOB. However, a significant increase in the percentage of new neurons in the granular and glomerular layers of the AOB and granular layer of the MOB was observed in females that mated in four sessions pacing the sexual interaction. In the group that paced the sexual interaction for one session, a significant increase in the percentage of neurons was observed in the glomerular layer of the AOB. Our data suggest that repeated paced mating increases the percentage of new neurons that survive in the olfactory bulb of female rats.

Sexual behavior in rodents: Where do we go from here?

Hormones and Behavior was first published 50 years ago including some articles related to the hormonal regulation of sexual behavior in different species. Since then, this research field has produced outstanding discoveries that have contributed to our understanding of the control of sexual behavior. The refinement of classical techniques and the development of new experimental tools has opened the door to a new era of research that will allow us to understand different aspects of sexual behavior. It would also expand the possible extrapolation from animal models to understand human sexuality and its dysfunctions. In this review, we summarize some of the most recent findings about sexual behavior in both sexes including the refinement of classical methods of study with new approaches and questions as well as the development of new methods trying to explain mechanisms of action on motivational and consummatory elements of mating behavior. We also reviewed other aspects that modulate sexual behavior such as attractivity, olfactory signals and learning which model mate selection. Additionally, we described studies demonstrating that sexual behavior induces permanent brain modifications in neuronal circuits. Finally, we briefly describe recent contributions on animal models of human sexuality dysfunctions which, although with their own limitations, are under continuous refining.

Sexual Behavior and Synaptic Plasticity

Although sex drive is present in many animal species, sexual behavior is not static and, like many other behaviors, can be modified by experience. This modification relies on synaptic plasticity, a sophisticated mechanism through which neurons change how they process a given stimulus, and the neurophysiological basis of learning. This review addresses the main plastic effects of steroid sex hormones in the central nervous system (CNS) and the effects of sexual experience on the CNS, including effects on neurogenesis, intracellular signaling, gene expression, and changes in dendritic spines, as well as behavioral changes.

In vivo serial MRI of age-dependent neural progenitor cell migration in the rat brain

The subventricular zone (SVZ) is a neurogenic niche in the mammalian brain, giving rise to migratory neural progenitor cells (NPC). In rodents, it is well-established that neurogenesis decreases with aging. MRI-based cell tracking has been used to measure various aspects of neurogenesis and NPC migration in rodents, yet it has not yet been validated in the context of age-related decrease in neurogenesis. This validation is critical to using these MRI techniques to study changes in neurogenesis that arise in diseases prevalent in aging populations and their combination with advanced cellular therapeutic approaches aiming to combat neurodegeneration. As such, in this work we used MRI-based cell tracking to measure endogenous neurogenesis and cell migration from the SVZ along the rostral migratory stream to the olfactory bulb, for 12 days duration, in rats aged 9 weeks to 2 years old. To enable the specific detection of NPCs by MRI, we injected micron sized particles of iron oxide (MPIOs) into the lateral ventricle to endogenously label cells within the SVZ, which then appeared as hypo-intensive spots within MR images. In vivo MRI data showed that the rate of NPC migration was significantly different between all ages examined, with decreases in the distance traveled and migration rate as age progressed. The total number of MPIO-labeled cells within the olfactory bulb on day 12, was significantly decreased when compared across ages in ex vivo high-resolution scans. We also demonstrate for the first-time, provocative preliminary data suggesting age-dependent MPIO uptake within the dentate gyrus (DG) as well. Histology to identify doublecortin-positive NPCs, verified the decrease in cell labeling as a function of aging, for both regions. The dramatic reduction of NPC labeling within the SVZ observed with MRI, validates the sensitivity of MRI-based cell tracking to neurogenic potential and demonstrates the importance of understanding the impact of age on the relationship of NPC and disease.

Adult mammalian neurogenesis and motivated behaviors

Adult neurogenesis continues to captivate the curiosity of the scientific community; and researchers seem to have a particular interest in identifying the functional implications of such plasticity. While the majority of research focuses on the association between adult neurogenesis and learning and memory (including spatial learning associated with hippocampal neurogenesis and olfactory discrimination associated with neurogenesis in the olfactory system), the following review will explore the link to motivated behaviors. In particular, goal-directed behaviors such as sociosexual, parental, aggressive, as well as depression- and anxiety-like behaviors and their reciprocal association to adult neurogenesis will be evaluated. The review will detail research in humans and other mammalian species. Furthermore, the potential mechanisms underlying these neurogenic alterations will be highlighted. Lastly, the review will conclude with a discussion on the functional significance of these newly generated cells in mediating goal-directed behaviors.

The First Mating Experience Induces New Neurons in the Olfactory Bulb in Male Mice

In rodents, neurogenesis in the olfactory bulbs (OBs) is enhanced by exposure to olfactory enriched environments including sexually relevant odors. In the present study we evaluated whether sexual stimulation in male mice increases the number of newly generated cells that reach the OB and whether these cells differentiate into neurons. To this end, we used sexually naive male C57BL mice randomly assigned to one of three groups: (1) control, in which animals were left alone in their home cages; (2) exposure, in which animals were exposed to a receptive female precluding any physical contact; and (3) mating, in which males copulated with females. Males were given three injections of the DNA synthesis marker 5-bromo-2'-deoxyuridine (BrdU) 2 h before, at the end and 2 h after the test. Fifteen days after BrdU administration, brains were removed and processed to identify new cells and evaluate if they had differentiated into neurons in the granular (GR), mitral (MI) and glomerular (GL) cell layers of the main and accessory OB (MOB and AOB, respectively). We found an increase in the percentage of new cells that differentiate into neurons in the GL cell layer of the MOB of males from the mating group compared with those from the exposure and control groups. No differences were found in the number of new cells or percentage of new neurons in the rest of the analyzed regions. In male mice, the first sexual experience increases the percentage of new cells that differentiate into neurons in the GL cell layer of the MOB.

Neurogenesis and sexual behavior

Different conditions induce proliferation, migration and integration of new neurons in the adult brain. This process of neurogenesis is a clear example of long lasting plastic changes in the brain of different species. Sexual behavior is a motivated behavior that is crucial for the survival of the species, but an individual can spend all his life without displaying sexual behavior. In the present review, we briefly describe some of the effects of pheromones on neurogenesis. We review in detail studies describing the effects of sexual behavior in both males and females on proliferation, migration and integration of new cells and neurons. It will become evident that most of the studies have been done in rodents, assessing the effects of this behavior on neurogenesis within the dentate gyrus of the hippocampus and in the subventricular zone - rostral migratory stream - olfactory bulb system.

Incorporation of new neurons in the olfactory bulb after paced mating in the female rat

One of the regions that constantly produces neurogenesis in the adult brain is the subventricular zone (SVZ), whose new cells migrate to the olfactory bulbs (OB). When the females regulate the copulatory events (paced mating) the number of new cells in the SVZ increases, as well as those observed in the OB 15 days later. However, no changes were observed in the number of cells 45 days after the females paced the sexual interaction. Constant sensory stimulation is an important promoter of cell survival in the OB circuit. Hence, we increased the number of mating sessions in this study to cover the period where stimulation of the new cells is critical for their incorporation into pre-existing circuits in the OB. Ovariectomized female Wistar rats, were injected with the mitotic marker 5-bromo-2'-deoxyuridine (BrdU, 100 mg/kg, per injection) before, at the end and one hour after mating. Sexual behavior was recorded for 1 h in 10 weekly sessions. After the last mating session, brain sections were processed to determine BrdU immunoreactivity. Our results indicate that females that paced the sexual interaction for 10 sessions had a higher number of cells in the glomerular layer (GL) of the accessory olfactory bulb (AOB) and a higher number of neurons in the granular layer (GrL) of the main olfactory bulb (MOB) in comparison to the control group. These results indicate that continued sexual interaction contributes to the integration of new cells and neurons, induced in the first sexual experience, into pre-exiting circuits of the OB.

Neurogenesis in the olfactory bulb induced by paced mating in the female rat is opioid dependent

The possibility to control the rate of sexual stimulation that the female rat receives during a mating encounter (pacing) increases the number of newborn neurons that reach the granular layer of the accessory olfactory bulb (AOB). If females mate repeatedly, the increase in the number of neurons is observed in other regions of the AOB and in the main olfactory bulb (MOB). It has also been shown that paced mating induces a reward state mediated by opioids. There is also evidence that opioids modulate neurogenesis. In the present study, we evaluated whether the opioid receptor antagonist naloxone (NX) could reduce the increase in neurogenesis in the AOB induced by paced mating. Ovariectomized female rats were randomly divided in 5 different groups: 1) Control (not mated) treated with saline, 2) control (not mated) treated with naloxone, 3) females that mated without controlling the sexual interaction (no-pacing), 4) females injected with saline before pacing the sexual interaction and 5) females injected with NX before a paced mating session. We found, as previously described, that paced mating induced a higher number of new cells in the granular layer of the AOB. The administration of NX before paced mating, blocked the increase in the number of newborn cells and prevented these cells from differentiating into neurons. These data suggest that opioid peptides play a fundamental role in the neurogenesis induced by paced mating in female rats.

Olfactory bulb plasticity ensures proper olfaction after severe impairment in postnatal neurogenesis

The olfactory bulb (OB) neurons establish a complex network that ensures the correct processing of the olfactory inputs. Moreover, the OB presents a lifelong addition of new neurons into its existing circuitry. This neurogenesis is considered essential for the OB function. However, its functional impact on physiology and behavior is still unclear. Here, we investigate the mechanisms of OB plasticity that underlie bulbar physiology in relation to severe damage of neurogenesis. The neurogenesis of young mice was altered by ionizing radiation. Afterwards, both multi-channel olfactometry and electrophysiological studies were performed. Furthermore, neurogenesis and differentiation of the newly formed cells were assessed using bromodeoxyuridine labeling combined with a wide battery of neuronal markers. Our results demonstrate a reduction in both neurogenesis and volume of the OB in irradiated animals. The number of neuroblasts reaching the OB was reduced and their differentiation rate into interneurons selectively changed; some populations were noticeably affected whereas others remained preserved. Surprisingly, both olfactory detection and discrimination as well as electrophysiology presented almost no alterations in irradiated mice. Our findings suggest that after damaging postnatal neurogenesis, the neurochemical fate of some interneurons changes within a new biological scenario, while maintaining homeostasis and olfaction.