Photoperiodic and temporal influences on chemosensory induction of brain fos expression in female prairie voles

Prairie voles (Microtus ochrogaster) typically stop breeding during winter. Male prairie voles respond to winter day lengths with gonadal regression, whereas female voles are relatively unresponsive to photoperiod. Unlike commonly studied laboratory rodents, female prairie voles do not exhibit spontaneous oestrous cycles. Instead, females are induced into oestrus by chemosensory cues from conspecific male urine. The present study investigated the interaction among day length, chemosensory cues and the initial brain responses during oestrus induction in female voles. A single drop of male conspecific urine, saline or skimmed milk was applied to the nares of female prairie voles housed for 9 weeks in either long (LD 16 : 8 h) or short (LD 8 : 16 h) days. Animals were killed 0.5, 1, 2 or 24 h after chemosensory treatment and their brains were processed for Fos immunocytochemistry. Body mass and ovarian fat pad mass were higher, but uterine and ovarian mass were lower, in short-day compared to long-day females. Regardless of photoperiod, Fos- immunoreactivity increased in the granule layer of the accessory olfactory bulb (AOB), the supraoptic nucleus and bed nucleus of the stria terminalis (BNST) (anterior medial) in females treated with male urine compared to the two control groups. Fos staining intensified in the AOB, medial and posterocortical medial amygdala and BNST (posterior ventral), 1 h and 2 h after urine treatment. In the medial preoptic area, anterior and lateral hypothalamus, and ventromedial nucleus of the hypothalamus, Fos-immunoreactivity was elevated in females 2 h after receiving urine. Overall, long-day females displayed higher Fos expression in response to urine than females maintained in short days. These results identify a putative neural circuitry of oestrus induction in this species, and provide an approximate time line of activation in the brain circuit responsible for oestrus induction in prairie voles.     

Lesions of the vomeronasal organ disrupt mating-induced pair bonding in female prairie voles (Microtus ochrogaster)

The prairie vole (Microtus ochrogaster) is a highly social, monogamous species and displays pair bonding that can be assessed by the presence of selective affiliation with the familiar partner versus a conspecific stranger. In female prairie voles, exposure to a male or to male sensory cues is essential for estrus induction, and the subsequent mating facilitates pair bond formation. In the present study, we examined the role of the vomeronasal organ (VNO) in estrus induction and pair bonding in female prairie voles. VNO lesions did not alter olfaction mediated by the main olfactory system, but did prevent male-induced estrus induction. We by-passed the necessity of the VNO for estrus induction by estrogen priming the females. Despite the fact that all subjects displayed similar levels of mating, social contact and locomotor activities, VNO lesioned females failed to show mating-induced pair bonding whereas intact and sham-lesioned females displayed a robust preference for the familiar partner. Our data not only support previous findings that the VNO is important for estrus induction but also indicate that this structure is crucial for mating-induced pair bonding, suggesting an important role for the VNO in reproductive success in prairie voles.     

Oxytocin Receptor Binding in Female Prairie Voles: Endogenous and Exogenous Oestradiol Stimulation

Previous studies have demonstrated that oxytocin receptors in specific nuclei of rat forebrain are regulated by gonadal steroids. The current study used in vitro receptor autoradiography to investigate the distribution and regulation of oxytocin receptors in the forebrain of the female prairie vole (Microtus ochrogaster). In contrast to rats, in female prairie voles gonadal steroid secretion and oestrus behaviour result from male chemosignal stimulation and ovulation is induced by mating. Thus, the prairie vole brain provides an opportunity for investigating links between environmental stimuli, gonadal steroids and oxytocin receptors. Using a selective oxytocin receptor ligand [¹²⁵l]d(CH²)⁵[Tyr(Me)²,Tyr-NH₂⁹]ornithine vasotocin ([¹²⁵I]OTA), specific binding was found in several regions including the anterior olfactory nucleus, the ventromedial nucleus of the hypothalamus, the bed nucleus of the stria terminalis, the amygdala and several cortical areas. Following ovariectomy, oestradiol benzoate (10 μg) administration increased oxytocin receptor binding 100% in the anterior olfactory nucleus, but did not affect receptors in other regions. Gonadallyintact females, exposed to male chemosignals, had significant increases in both endogenous oestradiol levels and anterior olfactory nucleus oxytocin receptor binding relative to gonadally-intact females unexposed to male chemosignals. Following prolonged exposure to males with ad libitum mating, [¹²⁵I]OTA receptor binding decreased to the levels found in unstimulated females. These results demonstrate that increases in oestrogen levels, of either exogenous or endogenous origin, can modulate oxytocin receptors in the brains of female prairie voles. In contrast to rats, oestrogen in female prairie voles appears to affect receptors in the anterior olfactory nucleus rather than the hypothalamus. We suggest that the species differences in oxytocin receptor distribution and gonadal steroid responsiveness reflect variations in reproductive physiology and possibly behaviour.     

Vomeronasal organ-mediated induction of fos in the central accessory olfactory pathways in repetitively mated female rats

Removal of the VNO significantly reduced the enhancement of lordosis and the induction of fos immunoreactivity in luteinizing hormone-releasing hormone (LHRH) neurons in ovariectomized estrogen-primed rats. There was a significant positive correlation between the two variables. In the accessory olfactory bulb (AOB) of the repetitively mated rats, the number of fos-positive cells in the granule (G) cell layer was significantly lower in the VNO-removed rats, whereas that in the mitral (M) cell layer was not significantly different between VNO-removed and VNO-sham females. The G/M ratio (calculated by dividing the mean number of fos-positive cells in the G cell layer by that in the M cell layer), taken as an estimate of the output of the AOB, was relatively larger in the VN-sham as compared with the VNO-removed rats. There were significant positive correlations between G/M ratio and the increase in LQ and between the G/M ratio and the percentage of fos-positive LHRH cells. The positive correlation between the number of fos-positive cells in the posterodorsal medial amygdala (PDMA) and the increase in LQ and that between the number of fos-positive cells in the PDMA and the percentage of fos-positive LHRH cells were significant, supporting the role of the medial nucleus of amygdala in lordosis. However, the correlation between G/M ratio and the number of fos-positive cells in the PDMA was not significant, indicating that fos immunoreactivity in the PDMA is not directly related to that in the AOB. In conclusion, the results support the involvement of the accessory olfactory system in mediating the facilitatory effects of repeated mating on lordosis in female rats and suggest that the influence of the accessory olfactory system is mediated likely through the LHRH neuronal system. Integration and filtering of sensory information may take place at various levels of the brain, such as the AOB and the medial amygdala, before being transmitted to higher brain centers controlling lordosis behavior in female rats.     

Male chemosignals inhibit the neural responses of male mice to female chemosignals

Reproductive function in mice is regulated by reproductively-stimulating and reproductively-inhibiting primer pheromones released by conspecifics. When experienced simultaneously, their responses to reproductively-inhibiting chemosignals take precedence over their responses to reproductively-stimulating chemosignals. For example, while female urine induces luteinizing hormone (LH) release in males, this response is blocked when male urine is presented in conjunction with female urine. In the present study, we examined the neural correlates of these responses to male and female urine. Sexually experienced, male CF1 mice were exposed to water, female urine, or a mixture of male and female urine. The resulting patterns of Fos-immunoreactivity (Fos-ir) were then compared between groups. Female urine induced significantly more Fos-ir within the main and accessory olfactory systems (MOS and AOS, respectively) than did water, male urine or mixed urine. Notably, within the main and accessory olfactory bulbs, male urine attenuated the responses of mitral cells, but not granule cells, to female urine. Overall, the results indicate that exposure to male urine inhibited the responses of cells within the MOS and AOS to female urine. The specific pattern of Fos-ir in the olfactory bulbs suggests that this may be due to an inhibition in the responses of mitral cells to female urine.     

Chemosensory cues affect amygdaloid neurogenesis and alter behaviors in the socially monogamous prairie vole

The current study examined the effects of pheromonal exposure on adult neurogenesis and revealed the role of the olfactory pathways on adult neurogenesis and behavior in the socially monogamous prairie vole (Microtus ochrogaster). Subjects were injected with a cell proliferation marker [5‐bromo‐2′‐deoxyuridine (BrdU)] and then exposed to their own soiled bedding or bedding soiled by a same‐ or opposite‐sex conspecific. Exposure to opposite‐sex bedding increased BrdU labeling in the amygdala (AMY), but not the dentate gyrus (DG), of female, but not male, voles, indicating a sex‐, stimulus‐, and brain region‐specific effect. The removal of the main olfactory bulbs or lesioning of the vomeronasal organ (VNOX) in females reduced BrdU labeling in the AMY and DG, and inhibited the male bedding‐induced BrdU labeling in the AMY, revealing the importance of an intact olfactory pathway for amygdaloid neurogenesis. VNOX increased anxiety‐like behavior and altered social preference, but it did not affect social recognition memory in female voles. VNOX also reduced the percentage of BrdU‐labeled cells that co‐expressed the neuronal marker TuJ1 in the AMY, but not the DG. Together, our data indicate the importance of the olfactory pathway in mediating brain plasticity in the limbic system as well as its role in behavior.     

Sexual and social experience is associated with different patterns of behavior and neural activation in male prairie voles

Monogamous prairie voles (Microtus ochrogaster) show mating-induced aggression towards conspecific strangers. This behavior is both selective and enduring. The present study was designed to investigate the behavioral conditions for the emergence of selective aggression (by varying prior experience with a female and identity of intruders) and the limbic activation in response to an intruder (by mapping regional staining for c-fos) in male prairie voles. In a first experiment, males that mated with a female for 24 h exhibited aggression towards a male intruder and had more Fos-immunoreactive (Fos-ir) cells in the medial amygdala (AMYGme) and medial preoptic area (MPO) relative to males that cohabited with a female without mating or that had no prior exposure to a female. Cohabited males did not become aggressive. However, these males along with mated males had an increased number of Fos-ir cells in the lateral septum (LS) and the bed nucleus of the stria terminalis (BST) relative to males without prior exposure to a female. In a second experiment, mated males exhibited more offensive aggression to a male intruder but more defensive aggression to a female intruder. Both types of aggression, however, induced an increase in the number of Fos-ir cells in the AMYGme. In addition, Fos-ir staining in the BST was induced selectively in response to a male intruder and a similar trend was found in the LS. Exposure to a male or female intruder did not increase Fos-ir staining in the MPO. Taken together, our data suggest the neural substrates activated by social/sexual activity and involved in response to intruders. The AMYGme was involved in processing intruder-related cues and/or in the regulation of aggressive response to both male and female intruders. The BST and LS were modulated by social experience with a female (mating or cohabitation) and were responsive to male-related cues even in the absence of aggression. Finally, the MPO was activated at different magnitudes by social or sexual experience but did not respond to intruder-related cues as measured by Fos-ir.     

Lesions of the vomeronasal organ disrupt mating-induced pair bonding in female prairie voles (Microtus ochrogaster)

The prairie vole (Microtus ochrogaster) is a highly social, monogamous species and displays pair bonding that can be assessed by the presence of selective affiliation with the familiar partner versus a conspecific stranger. In female prairie voles, exposure to a male or to male sensory cues is essential for estrus induction, and the subsequent mating facilitates pair bond formation. In the present study, we examined the role of the vomeronasal organ (VNO) in estrus induction and pair bonding in female prairie voles. VNO lesions did not alter olfaction mediated by the main olfactory system, but did prevent male-induced estrus induction. We by-passed the necessity of the VNO for estrus induction by estrogen priming the females. Despite the fact that all subjects displayed similar levels of mating, social contact and locomotor activities, VNO lesioned females failed to show mating-induced pair bonding whereas intact and sham-lesioned females displayed a robust preference for the familiar partner. Our data not only support previous findings that the VNO is important for estrus induction but also indicate that this structure is crucial for mating-induced pair bonding, suggesting an important role for the VNO in reproductive success in prairie voles.     

Combined c-fos and C-2-deoxyglucose method to differentiate site-specific excitation from disinhibition: analysis of maternal behavior in the rat

On the basis of evidence that ¹⁴C-2-deoxyglucose (2-DG) autoradiography indicates activity at axonal terminals, whereas c-fos immunocytochemistry indicates activity of neuronal cell bodies, we combined these techniques in adjacent histological brain sections to assess excitatory and disinhibitory synaptic relations in selected sites in female rats in which maternal behavior was elicited by natural parturition, sensitization (7- to 10-day cohabitation with foster pups), or hysterectomy. All individuals in these three groups expressed maternal behavior immediately before 2-DG injection. Controls were non-maternal virgins. Parturient and Hysterectomized groups: elevation (compared with controls) in both 2-DG and c-fos activity in medial preoptic area (MPOA) indicated an increase in its input and output activity, i.e., an excitatory interaction; the MPOA was previously shown to be critical for maternal behavior. Sensitized group: a decrease in 2-DG activity of vomeronasal nuclei (bed nucleus of the accessory olfactory tract, BAOT, and medial amygdala, ME, replicating our previous study) and an elevation in c-fos activity, jointly indicate disinhibition of these nuclei, that were previously shown to modulate pup-chemostimulation-induced sensitization. All other sites showed evidence of excitatory input–output relationships (i.e., joint increase in both 2-DG and c-fos activity), e.g., bed nucleus of the stria terminalis (BNST), lateral habenula (LHAB), central gray (CG), thalamus (THAL), septum (SEPT), and ventral tegmental area (VTA). The present study demonstrates the feasibility of measuring 2-DG and c-fos activity jointly in adjacent sections of the same brain, thereby providing evidence to distinguish between localized excitation and disinhibition.     

Outstanding issues surrounding vomeronasal mechanisms of pregnancy block and individual recognition in mice

Male mouse urine contains a complex mixture of chemosignals, which signal the presence of a reproductively active male. These have powerful effects as primer pheromones on female reproductive state, including the ability to block pregnancy. Male mouse urine also contains individuality chemosignals that enable the female to recognise her mate and prevent his pheromones from eliciting the pregnancy block effect. A range of neurochemical and electrophysiological evidence suggests that memory formation to the mating male’s pheromones involves synaptic changes in the accessory olfactory bulb, at the first stage of the vomeronasal pathway. This results in a selective inhibition of the mate’s pheromonal signal, preventing it from activating neural circuits in the corticomedial amygdala and hypothalamus that mediate the endocrine changes responsible for pregnancy block. This article reviews the current state of knowledge regarding the neurobiological basis of this mate recognition, highlighting important gaps in our current understanding. Despite recent findings of the ability of peptides associated with the major histocompatibility complex to influence mate recognition, the pregnancy blocking and individuality chemosignals remain to be identified. Recent research has also shed doubt on our understanding of the mechanism by which noradrenaline imprints the male pheromonal signal. Finally, the effect of learning on the transmission of information in the vomeronasal system and its relationship to chemosensory information processed by the main olfactory system remain to be elucidated.     

Low molecular weight constituents of male mouse urine mediate the pregnancy block effect and convey information about the identity of the mating male

Mouse urine contains a complex mixture of chemosignals including a variety of small volatile molecules that are bound to major urinary proteins. In addition to signalling maleness, male urine also conveys information about individuality, which allows recently mated female mice to distinguish the urinary chemosignals of the mating male from those of an unfamiliar male. The highly polymorphic nature of the major urinary proteins makes them a likely candidate for conveying individuality information in the context of the pregnancy block effect. This was investigated by comparing the pregnancy-blocking effectiveness of a high molecular weight urinary fraction, containing major urinary proteins, with that of a low molecular weight fraction containing volatile ligands. Not only was the high molecular weight fraction ineffective in blocking pregnancy, but it also appeared to be less important in signalling individuality than the low molecular fraction. The high molecular weight fraction was ineffective in inducing expression of the immediate early gene product egr-1 in the accessory olfactory bulb. In contrast, the low molecular weight fraction induced egr-1 expression in the mitral/tufted neurons in the anterior subregion of the accessory olfactory bulb, suggesting that they activate the V1R class of vomeronasal receptor neuron.     

Changes in electrophysiological activity in the accessory olfactory bulb and medial amygdala associated with mate recognition in mice

The ability of female mice to recognize their mate's pheromonal identity is critical for the maintenance of their pregnancy and is hypothesized to involve increases in the inhibitory control of mitral/tufted projection neurons in the accessory olfactory bulb. Local field potential recordings from this region of freely behaving female mice showed oscillating neural activity over a wide range of frequencies, which was affected by chemosensory input and prior experience. Mating caused lasting increases in the baseline neural activity in the accessory olfactory bulb, with large increases in the amplitude of local field potential oscillations across a range of frequencies. Exposure to the mate's urinary cues remained effective in increasing the power of these oscillations following mating, but urinary cues from an unfamiliar male were ineffective. A differential response to the familiar and unfamiliar chemosignals was also observed at the level of the amygdala following mating. Individual neurons in the medial amygdala responded more strongly to urine from an unfamiliar male than from the mating male. These findings are consistent with the selective enhancement of inhibition of the familiar pheromonal signal at the level of the accessory olfactory bulb, which is proposed to underlie recognition of the mating male.     

Temperature and photoperiod interact to affect reproduction and GnRH synthesis in male prairie voles

Prairie voles (Microtus ochrogaster), like most rodent species, exhibit a phenotypic polymorphism in reproductive response to winter conditions or to short day lengths in the laboratory. Laboratory studies on seasonally breeding species have traditionally focused on the role of photoperiod in modulating reproduction and other seasonal adaptions. However, because animals use proximate environmental factors in addition to photoperiod to phase seasonal adaptions with the appropriate time of year, the present study investigated the interaction of photoperiod and temperature on reproductive function and the gonadotropin-releasing hormone (GnRH) neuronal system. Male prairie voles were housed in either long (LD 16:8) or short (LD 8:16) photoperiods. Voles in each photoperiodic condition were also exposed to either mild (20 degrees C) or low (8 degrees C) temperatures. After 10 weeks, voles were killed and their brains were processed using in situ hybridization for mRNA for proGnRH. The results suggest that GnRH synthesis is not affected by exposure to a single inhibitory proximate factor (i.e. short days or low temperatures alone), even when reproduction is inhibited, whereas a combination of inhibitory proximate factors leads to a decrease in GnRH synthesis (i.e. fewer neurones staining for mRNA for proGnRH). These data suggest that the neuroendocrine mechanisms regulating seasonal alterations in reproductive function are likely to differ between harsh and mild winters.     

Influence of photoperiod and sex on locomotor behavior of meadow voles (Microtus pennsylvanicus) in an automated light-dark 'anxiety' test

This study examined the influence of photoperiod on affective behavior (anxiety) of adult male and female meadow voles (Microtus pennsylvanicus), maintained in either a long or short day photoperiod, when tested in an automated (VersaMax) light-dark test. The light-dark test is based on an innate aversion of rodents to novel, brightly illuminated spaces and has been used with laboratory raised species, such as mice, to assess anxiety and/or fear related behaviors. Male and female meadow voles, housed either in a long day (LD: 16 h light) or short day (SD: 8 h light) photoperiod, were tested in the light-dark apparatus for 30 min on 3 consecutive days. All animals spent significantly (p < 0.001) less time in the brightly lit chamber (900 lux) than in the dark chamber. LD voles, especially females, spent significantly less time in the brightly lit area than did SD voles. Both horizontal and vertical movements occurred less frequently per unit time in the dark area relative to the light, but only in the LD voles. LD female voles were the least active group in the dark area on the first test day but the most active group in the light area, despite spending the least amount of time in this area on the second and third test days. The present results show that LD voles exhibit more anxiety related behaviors in this test situation than do SD voles. LD females avoided the brightly lit area the most, particularly when the apparatus was novel. Thus, both photoperiod and sex influence situation-based anxiety in this species. These findings suggest that meadow voles are an excellent animal model in which to examine the role of gonadal hormones, and their modulation of defence related neural systems, in the induction of anxiety.     

The ventromedial hypothalamic circuitry and male alloparental behaviour in a socially monogamous rodent species

As prairie voles (Microtus ochrogaster) display spontaneous biparental care, and the ventromedial hypothalamus (VMH) has been implicated in reproductive behaviour, we conducted experiments to test the hypothesis that the VMH neurochemical circuitry is involved in alloparental behaviours in male prairie voles. We compared alloparental behaviours of adult, sexually naïve male and female voles—both displayed licking/grooming, huddling and retrieving behaviours towards conspecific pups. We also stained for the immediate‐early gene encoded early growth protein Egr‐1 in the vole brain. The pup‐exposed animals showed levels of Egr‐1 staining that was higher in the VMH but lower in the amygdala compared to animals exposed to a pup‐sized piece of plastic (control). A retrograde tracer, Fluoro‐Gold (FG), was injected into the VMH of male voles that were subsequently tested in the pup exposure or control condition. More FG/Egr‐1 cells were detected for glutamatergic (GLU) staining in the ventral bed nucleus of the stria terminalis (BNSTv) and medial amygdala (MeA), whereas less FG/Egr‐1 cells were stained for gamma‐aminobutyric acid (GABA) in the MeA of the pup‐exposed group compared to the control group. Further, the ratio of GLU:GABA expression in FG/Egr‐1 projection neurons from both the BNSTv and MeA to the VMH was increased following pup exposure. Finally, pharmacological blockade of either dopamine D1 receptor or oxytocin receptor in the VMH impaired the onset of male alloparental behaviour. Together, these data suggest that the VMH may be involved in the onset of alloparental care and play a role in regulating social approach in male prairie voles.     

Sex and species differences in the effects of cohabitation on vasopressin messenger RNA expression in the bed nucleus of the stria terminalis in prairie voles (Microtus ochrogaster) and meadow voles (Microtus pennsylvanicus)

Three days of male and female cohabitation dramatically reduces the density of vasopressin-immunoreactive (AVP-ir) fibers in the lateral septum and lateral habenular nucleus of male, but not of female prairie voles. Here we tested whether this reduction is associated with changes in AVP messenger RNA (mRNA) expression in the bed nucleus of the stria terminalis (BST), the presumed source of these fibers, and with changes in testosterone levels, which may influence AVP biosynthesis in the BST. In addition, we tested whether similar changes can be found in meadow voles, which unlike prairie voles do not dramatically change their social behaviors after mating. In both species, males showed more AVP mRNA-labeled cells in the BST and more grains per labeled cell than females. In prairie vole males, cohabitation increased the number of AVP mRNA labeled BST cells and the density of grains per labeled cells. It also raised plasma testosterone levels. No changes were found in prairie vole females nor in meadow voles of either sex. The changes in prairie vole males suggest that cohabitation stimulates AVP biosynthesis. The previously observed decrease in AVP-ir fiber density in the lateral septum and lateral habenular nucleus may therefore reflect increased synaptic release of AVP, which may contribute to mating-induced changes in social behaviors in prairie vole males.     

Component-dependent urine responses in the rat accessory olfactory bulb

To investigate how pheromonal information is processed in the rat accessory olfactory bulb, we optically imaged intrinsic signals to obtain high-resolution maps of activation induced by urinary stimulation. Application of volatile components in male urine mainly induced activation in the anterior accessory olfactory bulb, irrespective of the sex, whereas volatile female urine elicited activation not only in the anterior but also to some extent in the caudal part of the posterior accessory olfactory bulb of male, but not female, rats. Nonvolatile components of both male and female urine induced activation mainly in the rostral part of the posterior and to a lesser extent in the anterior accessory olfactory bulb, irrespective of the sex. These results indicate that volatile and nonvolatile urinary components activate the anterior and posterior subdivisions of the accessory olfactory bulb, respectively.     

Refining the dual olfactory hypothesis: pheromone reward and odour experience

In rodents, sexual advertisement and gender recognition are mostly (if not exclusively) mediated by chemosignals. Specifically, there is ample evidence indicating that female mice are ‘innately’ attracted by male sexual pheromones that have critical non-volatile components and are detected by the vomeronasal organ. These pheromones can only get access to the vomeronasal organ by active pumping mechanisms that require close contact with the source of the stimulus (e.g. urine marks) during chemoinvestigation.We have hypothesised that male sexual pheromones are rewarding to female mice. Indeed, male-soiled bedding can be used as a reinforcer to induce conditioned place preference, provided contact with the bedding is allowed. The neural mechanisms of pheromone reward seem, however, different from those employed by other natural reinforcers, such as the sweetness or postingestive effects of sucrose.In contrast to vomeronasal-detected male sexual pheromones, male-derived olfactory stimuli (volatiles) are not intrinsically attractive to female mice. However, after repeated exposure to male-soiled bedding, intact female mice develop an acquired preference for male odours. On the contrary, in females whose accessory olfactory bulbs have been lesioned, exposure to male-soiled bedding induces aversion to male odorants. These considerations, together with data on the different properties of olfactory and vomeronasal receptors, lead us to make a proposal for the complementary roles that the olfactory and vomeronasal systems play in intersexual attraction and in other forms of intra- or inter-species communication.     

Experience facilitates vomeronasal and olfactory influence on Fos expression in medial preoptic area during pheromone exposure or mating in male hamsters

Chemosensory stimuli are essential for mating in male hamsters but either main olfactory or vomeronasal input is sufficient in sexually experienced males. Activation in central chemosensory pathways and medial preoptic area, after stimulation with female chemosignals or after mating, was estimated by counting neurons expressing Fos protein in experienced and naive males, with or without vomeronasal organ lesions. Regions counted included main and accessory olfactory bulbs, corticomedial amygdala, bed nucleus stria terminalis and medial preoptic area. Chemosensory stimulation was more effective in activating medial preoptic area in experienced than in naive males. In experienced males with vomeronasal organs removed, main olfactory input was as effective in activating medial preoptic area as was the combination of main and accessory input available to intact animals. As previously reported, the main olfactory input remaining after vomeronasal lesions in naive males was poorly effective in activating medial preoptic area, and these animals had impaired mating behavior. The change in access of chemosensory input to medial preoptic area after experience suggests that an experience-dependent synaptic modulation in this pathway, perhaps in the amygdala, may underlie some changes in mating behavior with experience.