Reinforcing properties of pups versus cocaine for fathers and associated central expression of Fos and tyrosine hydroxylase in mandarin voles (Microtus mandarinus)

The reciprocal interaction of pups and cocaine on reward effects in rodent mothers is known. However,it remains unclear whether such effects are apparent in father-offspring bonding. The mandarin vole (Microtus mandarinus) is a monogamous rodent with a high level of paternal care. We investigated the reinforcing properties of pups on vole fathers using a conditioned place preference paradigm across the postpartum period and looked for interactions and differences between the reinforcing effects of pups and cocaine. We also measured neuronal Fos and tyrosine hydroxylase (TH) expression underlying the preferences of fathers for pups or cocaine. Our data showed that fathers developed strong preferences for pups at various times (postnatal day 5–9, 13–17 and 19–23) without cocaine conditioning. Fathers showed a reduced preference for pups following simultaneous conditioning with cocaine. Although they preferred cocaine over postnatal day (PND) 5–9 pups, this preference was not detected for PND 13–17 pups. Fathers preferring cocaine exhibited an increase in Fos-immunoreactive neurons in the accumbens,medial nucleus of the amygdala, cingulate cortex, medial preoptic area and ventral tegmental area and had more TH-IR neurons in the ventral tegmental area compared to fathers preferring PND 5–9 pups. These results showed that similar to cocaine, mandarin vole pups elicit significant reward value to their fathers, but that paternal motivation is impaired by cocaine. A preference for cocaine over pups arose from the release of more dopamine and activation of a greater number of neurons within specific reward-associated neuronal subsets.     

Postnatal separation prevents the development of prenatal stress‐induced anxiety in association with changes in oestrogen receptor and oxytocin immunoreactivity in female mandarin vole (Microtus mandarinus) offspring

Oestrogen has both anxiogenic and anxiolytic effects because of variation in opposing action on alpha (ERα) and beta (ERβ) estrogen receptors in the medial preoptic area (mPOA), bed nucleus of the stria terminalis (BNST) and medial amygdala (MeA). Oxytocin (OT) reverses some of the anxiogenic effects of oestrogen in the hypothalamic paraventricular nucleus (PVN) and supraoptic nucleus (SON). Because anxiety disorders are twice as common in women as in men, and oestrogen and OT are more important in females, we examined interactions between prenatal restraint stress (GS) and postnatal early short‐term maternal separation (MS) and female mandarin vole behaviour, estrogen receptors and OT. The results show that adult female offspring from GS/noMS mothers showed increased anxiety in open‐field and elevated plus‐maze tests and had lower serum 17‐beta‐oestradiol (E₂) levels than female offspring from GS/MS, noGS/MS and noGS/noMS mothers. GS/noMS females had more immunoreactive neurons for ERα in several brain regions and less ERβ‐ and OT‐immunoreactive neurons in brain areas compared to GS/MS, noGS/MS and noGS/noMS offspring. Interestingly, noGS/MS and GS/MS offspring were similar to noGS/noMS offspring in that they did not develop anxiety as adults. We propose that MS alters the serum concentration of E₂ and that the ERβ/ERα ratio and OT level in the brain may be responsible for the decrease in anxiety‐like behaviour in adult female offspring initially exposed to anxiety‐inducing conditions via an adverse foetal environment.     

Involvement of GABAA receptors in the regulation of social preference and emotional behaviors by oxytocin in the central amygdala of female mandarin voles

The central nucleus of the amygdala (CeA) is the main output of the amygdala and plays an important role in behavioral and neuroendocrine responses to stress. Receptors for the neuropeptide oxytocin (OT) and GABA_A are found in high concentration in the CeA. The mechanisms underlying regulation of CeA OT in emotional and social behavior remain unclear. In this study we evaluated the effects of intra-CeA OT administration of different doses (0.1, 1 and 10 ng/side), OT receptor antagonist (OTR-A) (1, 10 and 100 ng/side) and OT plus OTR-A on social and emotional behavior using a social preference paradigm, open field test and elevated plus maze test in female monogamous mandarin voles (Microtus mandarinus). We then examined whether different doses of the GABA_A receptor antagonist bicuculline (5, 10 and 100 ng/side) affected the behavioral changes induced by intra-CeA microinjection of OT (1 ng/side). We found that administration of OT to the CeA increased social preference, central area investigation times in the open field test, and visits, transitions and time spent in the open arms in the elevated plus maze test; all responses were dose-dependent. Administration of OT plus OTR-A to the CeA produced no effects. Administration of bicuculline in combination with OT to the CeA decreased social preference, central area investigation times in the open field test, and visits, transitions and time spent in the open arms of the elevated plus maze test. These data suggest that OT in the CeA facilitates sociality and reduces levels of anxiety by interacting with local GABA_A receptors.     

NOS在棕色田鼠胃肠道各段肌间神经丛分布的比较

目的：解释棕色田鼠(Microtus mandarinus)消化道对其生活史特征的适应性调节机制。方法：用NAD-PH-黄递酶组织化学法对NOS在棕色田鼠胃肠肌间神经丛的分布进行比较研究。结果：棕色田鼠胃肠肌间神经丛NOS阳性神经元形态各异，大小悬殊数倍；神经节和阳性神经纤维构成网络结构，阳性神经元可呈“串珠”状和“U”形排列。NOS阳性神经元分布密度在胃肠道各段差异较大：结肠密度最高，回肠次之，胃和盲肠比回肠低，比十二指肠和空肠高。结论：棕色田鼠NOS阳性神经元在胃肠道各节段肌间神经丛的分布特点可能是与其食性和生理机能相适应的。     

NOS在棕色田鼠胃肠道各段肌间神经丛分布的比较

目的:解释棕色田鼠(Microtus mandarinus)消化道对其生活史特征的适应性调节机制.方法:用NAD-PH-黄递酶组织化学法对NOS在棕色田鼠胃肠肌间神经丛的分布进行比较研究.结果:棕色田鼠胃肠肌间神经丛NOS阳性神经元形态各异,大小悬殊数倍;神经节和阳性神经纤维构成网络结构,阳性神经元可呈"串珠"状和"U"形排列.NOS阳性神经元分布密度在胃肠道各段差异较大:结肠密度最高,回肠次之,胃和盲肠比回肠低.比十二指肠和空肠高.结论:棕色田鼠NOS阳性神经元在胃肠道各节段肌间神经丛的分布特点可能是与其食性和生理机能相适应的.     

Organization of sensory neocortex in prairie voles (Microtus ochrogaster)

In the current investigation, the functional organization of visual, auditory, and somatosensory cortex was examined in prairie voles (Microtus ochrogaster) by using electrophysiological recording techniques. Functional boundaries of cortical fields were directly related to myeloarchitectonic boundaries. Our results demonstrated that most of the neocortex is occupied by the visual, auditory, and somatosensory areas. Specifically, a small area 17, or primary visual area (V1), was located on the caudomedial pole of the neocortex; a large auditory cortex (AC), which contains the primary auditory area (A1) and other auditory fields, encompassed almost the entire temporal pole; and a large area 3b, or primary somatosensory area (S1), contained a complete representation of the contralateral body surface. Furthermore, these areas were coextensive with distinct myeloarchitectonic appearances. We also observed that the AC appeared to be disproportionately large in the prairie vole compared with other rodents. In addition, we found that both primary and nonprimary areas contained neurons that responded to auditory stimulation. Finally, we observed within S1 a disproportionate amount of cortex that was devoted to representing the perioral hairs and the snout and also that neurons within this representation had very small receptive fields. We discuss the expanded auditory domain and the enlarged representation of perioral hairs as they relate to the specialized life style of the prairie vole.     

Urocortin II increases spontaneous parental behavior in prairie voles (Microtus ochrogaster)

Stress and anxiety play a role in many psychological processes including social behavior. The present study examines the effects of urocortin II (UCN II) on spontaneous parental behavior in adult prairie voles (Microtus ochrogaster). UCN II was found to increase passive parental behavior in voles while not affecting any stress-related measures. Delineating the mechanism of this change will aid in our understanding of the regulation of parenting.     

The involvement of oxytocin in the effects of chronic social defeat stress on emotional behaviours in adult female mandarin voles

Chronic social defeat stress (CSDS) can induce anxiety and depression in male rodents, but the prevalence of anxiety and depression is much higher in females, and effects of CSDS on adult females and its underlying mechanism remain unclear. Oxytocin is a stress‐buffering hormone in the brain that modulates the physiological effects of stress. Strikingly, research regarding the effect of oxytocin on emotional changes caused by CSDS is still lacking in females. Thus, we focused on the involvement of the oxytocin system in changes in emotional regulation induced by CSDS in female voles. Seventy‐day‐old female mandarin voles (Microtus mandarinus) were exposed to aggressive adult females for 14 days, and the effects of CSDS on emotion and regulation of oxytocin system were characterized. In addition, we injected vehicle, oxytocin and oxytocin receptor antagonist into the nucleus accumbens (Nacc) of female voles to investigate the involvement of Nacc oxytocin in the effect of CSDS on emotion. Herein, we reported that CSDS increased anxiety and depression‐like behaviour and the circulating level of corticosterone, but decreased the number of oxytocin projections and the protein and mRNA expression levels of oxytocin receptor in the Nacc. Injection of oxytocin into the Nacc reversed the effects of CSDS on anxiety‐like and depressive‐like behaviour, whereas combined injections of oxytocin and oxytocin receptor antagonist eliminated these effects. In conclusion, CSDS increases the levels of anxiety and depression possibly via a reduction in oxytocin projections and the oxytocin receptor level in the Nacc. Nacc oxytocin may be involved in the effects of CSDS on emotional behaviours.     

Reproductive activation of pine voles (Microtus pinetorum): examination of physiological markers

We tested the hypothesis that the presence of an opposite-sex conspecific will result in time-related changes in measures of reproductive activation. We housed male-female pairs of pine voles together for 0, 2, 6, 12, or 24 h before collecting blood, reproductive organs and brains for immunocytochemical analysis of LHRH and c-fos. Control animals were never exposed to an opposite-sex conspecific. Following exposure to a male, there was a significant increase in uterine weight but not in LH levels. In males, there were no changes in peripheral indices of activation, i.e. LH levels, testes and seminal vesicle weights were not altered. Consistent with no change in circulating levels of LH, there was no change in LHRH immunoreactivity at any time. However, c-fos immunoreactivity was significantly greater in both males and females in the cingulate cortex and rostral bed nucleus of the stria terminalis (BNST) at 2 h, and in the caudal BNST at 2, 6 and 12 h. Similarly, c-fos immunoreactivity was increased in the rostral MPOA in both males and females at 2 and 6 h. However, in the caudal MPOA, there was a significant interaction between sex and time due to increased c-fos immunoreactivity in females only at 6 h. These results indicate that, in both male and female pine voles, exposure to an opposite-sex conspecific is sufficient to produce rapid, neural activation in brain areas known to be involved in reproductive activation and sexual behavior. This early activation did not occur in LHRH neurons. It is not known if this activation, particularly at early times, is due to reproductive activation or to the formation of pair bonds.     

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.     

Copulatory behavior and the initiation of pregnancy in California voles (Microtus californicus).

The copulatory behavior of Microtus californicus and its effect on pregnancy initiation were examined in two experiments. In experiment 1 18 males and 18 females which had received exogenous hormones participated in 54 tests of copulatory behavior, each continued to a satiety criterion of 30 min with no copulations. The basic pattern involved no lock, intravaginal thrusting, ejaculation possible on a single insertion and multiple ejaculation. Ejaculation frequency ranged from 1 to 5, with a mean of 2.2. In experiment 2, it was found that all females receiving satiety tests of copulatory behavior while in male-induced estrus ovulated and became pregnant. Whereas all 10 females receiving one ejaculation ovulated, only 60% became pregnant. Thus, it appears that copulation beyond one ejaculation functions in increasing the likelihood of pregnancy. By comparing different species of Microtus it is proposed that copulatory patterns in which males persist for many thrusts and ejaculations may have evolved in conjunction with ornate penile morphology, large litter sizes, and high stimulus requirements for the initiation of ovulation and a funtional luteal phase.     

Food restriction affects the gonadotropin releasing hormone neuronal system of male prairie voles (Microtus ochrogaster)

Individuals of species inhabiting temperate and boreal latitudes optimize the timing of energetically costly processes by curtailing nonessential energetically demanding processes when environmental conditions are not favourable. One proximate environmental variable used to fine-tune moment-to-moment changes in reproductive physiology and behaviour is food intake. The neuroendocrine mechanisms by which food restriction leads to the cessation of reproduction in seasonally breeding rodent species remain largely unspecified. The present study sought to determine the effects of extended food restriction on the gonadotropin releasing hormone (GnRH) neuronal system. Male prairie voles (Microtus ochrogaster) were either fed ad libitum or were exposed to either 1, 2 or 3 weeks of moderate (70% of daily mean) food restriction. In accordance with previous studies of food restriction, gross reproductive organ masses and body mass were unaffected by food deprivation. Although 1 week of food restriction did not result in alterations in the GnRH neuronal system, food restriction for 2 weeks was associated with increased GnRH-immunoreactive (GnRH-ir) neurone soma size. Three weeks of food restriction resulted in a pronounced increase in GnRH-ir neurone numbers, as well as an increase in fibre intensity in the main fibre pathway to the median eminence. Taken together, these findings suggest that extended food restriction leads to modifications in the GnRH neuronal system, providing a means for temporary cessation of reproduction without gross alterations in reproductive physiology. This transient change in the hypothalmo-pituitary-gonadal axis, without pronounced changes in reproductive organ morphology, likely provides a mechanism for the rapid reinitiation of breeding in nature when local conditions provide adequate food availability.     

Influence of gonadal hormones on the development of parental behavior in adult virgin prairie voles (Microtus ochrogaster)

Prairie voles (Microtus ochrogaster) are a socially monogamous species and both sexes are parental after the birth of pups. In contrast, sexually inexperienced adult prairie voles differ in their behavior towards pups such that virgin males are paternal whereas virgin females are often infanticidal. To test whether there exists a discrete perinatal 'sensitive period' during which gonadal hormones influence this behavior, and to distinguish between the relative contributions of estrogenic and androgenic mechanisms to this influence, prairie voles were exposed to testosterone propionate (TP), the anti-androgen flutamide, or the aromatase inhibitor 1,4,6-androstatriene-3,17-doine (ATD) either prenatally via their pregnant dam for the last 15-19 days of the 22-day gestational period or postnatally on days 1-7. None of the treatments altered the high paternal responsiveness of males or the high infanticide rate in females when compared with controls. Females exposed prenatally to ATD, however, had levels of parental behavior that were significantly higher than the lowest levels observed in prenatally TP-treated females. These results suggest that sex differences in the parental behavior of adult virgin prairie voles are not generated exclusively by androgenic or estrogenic mechanisms during a restricted prenatal or early postnatal 'sensitive period' and that the parental behavior of virgin females may be more susceptible to any influence of gonadal hormones during development than males.     

Ventral striatopallidal oxytocin and vasopressin V1a receptors in the monogamous prairie vole (Microtus ochrogaster)

Oxytocin receptors (OTR) and vasopressin V1a receptors (V1aR) in the ventral forebrain play critical roles in the formation of pair bonds in the monogamous prairie vole. Previous reports have been inconsistent in the identification of the specific brain regions in the ventral forebrain that express these receptors. To delineate more clearly the neuroanatomical boundaries of the OTR and V1aR fields in this species, we compared OTR and V1aR binding in adjacent brain sections and also with markers that delineate neuroanatomical boundaries in the ventral forebrain. OTR binding displayed an overlapping distribution with substance P mRNA and preproenkephalin mRNA, both markers for the shell and core of the nucleus accumbens. V1aR binding was nonoverlapping with each of these markers but colocalized with iron accumulation as shown by Perls' iron stain as well as leucine-enkephalin immunoreactivity, both markers for the ventral pallidum. OTR and V1aR mRNA were also restricted within the nucleus accumbens and ventral pallidum, respectively. Furthermore, destruction of ventral striatal dopaminergic terminals with 6-hydroxydopamine infusions into the nucleus accumbens did not alter OTR binding. Immunocytochemical analysis of oxytocin and vasopressin in the ventral forebrain demonstrated the presence of oxytocin-immunoreactive fibers in the nucleus accumbens and vasopressin-immunoreactive fibers in the ventral pallidum, with males showing a greater density of vasopressin fibers than females, but there was no such sex difference in the oxytocin system. Based on these results, we discuss potential neural mechanisms by which receptors in these brain regions mediate pair bond formation in this monogamous species. J. Comp. Neurol. 468:555-570, 2004.     

Control of ovulation, vaginal estrus, and behavioral receptivity in voles (Microtus)

Rodents of the genus Microtus provide an excellent focus for the study of the effects of environmental and behavioral influences on reproductive physiology. Despite some early reports, there is little definitive evidence of truly spontaneous ovulation in Microtus. Stimuli from copulation with males appear capable of triggering ovulation in all species: the requisite amount of stimulation for ovulation and a functional luteal phase varies with the species. In addition, however, a certain percentage of the females of some, but not all, species ovulate when placed in proximity to males when copulation is prevented. There are few reports of regular cycles of cell types in the vaginal smears of Microtus females; the relative preponderance of cornified, nucleated, and leukocytic cells varies with species and conditions. Behavioral receptivity is not invariably correlated with smear type. The complexity of environment-behavior relationships in different species suggests the need for a more comprehensive classification schema for systems of female reproductive physiology and a study of their adaptive significance.     

Early postnatal gene expression in the developing neocortex of prairie voles (Microtus ochrogaster) is related to parental rearing style

The earliest and most prevalent sensory experience includes tactile, thermal, and olfactory stimulation delivered to the young via contact with the mother, and in some mammals, the father. Prairie voles (Microtus ochrogaster), like humans, are biparental and serve as a model for understanding the impact of parent/offspring interactions on the developing brain. Prairie voles also exhibit natural variation in the level of tactile stimulation delivered by the parents to the offspring, and this has been well documented and quantified. Previous studies revealed that adult prairie vole offspring who received either high (HC) or low (LC) tactile contact from their parents have differences in the size of cortical fields and the connections of somatosensory cortex. In the current investigation, we examined gene expression, intraneocortical connectivity, and cortical thickness in newborn voles to appreciate when differences in HC and LC offspring begin to emerge. We observed differences in developmentally regulated genes, as well as variation in prelimbic and anterior cingulate cortical thickness at postnatal Day 1 (P1) in HC and LC voles. Results from this study suggest that parenting styles, such as those involving high or low physical contact, impact the developing neocortex via very early sensory experience as well as differences in epigenetic modifications that may emerge in HC and LC voles.     

Neonatal manipulation of oxytocin prevents lipopolysaccharide-induced decrease in gene expression of growth factors in two developmental stages of the female rat

Oxytocin production and secretion is important for early development of the brain. Long-term consequences of manipulation of oxytocin system might include changes in markers of brain plasticity – cytoskeletal proteins and neurotrophins. The aim of the present study was (1) to determine whether neonatal oxytocin administration affects gene expression of nestin, microtubule-associated protein-2 (MAP-2), brain derived neurotrophic factor (BDNF) and nerve growth factor (NGF) in the brain of two developmental stages of rat and (2) to evaluate whether neonatal oxytocin administration protects against lipopolysaccharide (LPS) induced inflammation. Neonatal oxytocin did not prevent a decrease of body weight in the LPS treated animals. Oxytocin significantly increased gene expression of BDNF in the right hippocampus in 21-day and 2-month old rats of both sexes. Gene expression of NGF and MAP-2 significantly increased in males treated with oxytocin. Both, growth factors and intermediate filament-nestin mRNA levels, were reduced in females exposed to LPS. Oxytocin treatment prevented a decrease in the gene expression of only growth factors. In conclusion, neonatal manipulation of oxytocin has developmental and sex-dependent effect on markers of brain plasticity. These results also indicate, that oxytocin may be protective against inflammation particularly in females.