The hypothalamic paraventricular nucleus in two types of Wistar rats with different stress responses. II. Differential Fos-expression

Paternal responsiveness is associated with, but not mediated by reduced neophobia in male California mice (Peromyscus californicus)

Hormones associated with pregnancy and parturition have been implicated in facilitating the onset of maternal behavior via reductions in neophobia, anxiety, and stress responsiveness. To determine whether the onset of paternal behavior has similar associations in biparental male California mice (Peromyscus californicus), we compared paternal responsiveness, neophobia (novel-object test), and anxiety-like behavior (elevated plus maze, EPM) in isolated virgins (housed alone), paired virgins (housed with another male), expectant fathers (housed with pregnant pairmate), and new fathers (housed with pairmate and pups). Corticotropin-releasing hormone (CRH) and Fos immunoreactivity (IR) were quantified in brain tissues following exposure to a predator-odor stressor or under baseline conditions. New fathers showed lower anxiety-like behavior than expectant fathers and isolated virgins in EPM tests. In all housing conditions, stress elevated Fos-IR in the hypothalamic paraventricular nucleus (PVN). Social isolation reduced overall (baseline and stress-induced) Fos- and colocalized Fos/CRH-IR, and increased overall CRH-IR, in the PVN. In the central nucleus of the amygdala, social isolation increased stress-induced CRH-IR and decreased stress-induced activation of CRH neurons. Across all housing conditions, paternally behaving males displayed more anxiety-related behavior than nonpaternal males in the EPM, but showed no differences in CRH- or Fos-IR. Finally, the latency to engage in paternal behavior was positively correlated with the latency to approach a novel object. These results suggest that being a new father does not reduce anxiety, neophobia, or neural stress responsiveness. Low levels of neophobia, however, were associated with, but not necessary for paternal responsiveness.

Chronic psychosocial stress affects corticotropin-releasing factor in the paraventricular nucleus and central extended amygdala as well as urocortin 1 in the non-preganglionic Edinger-Westphal nucleus of the tree shrew

Stressful stimuli evoke neuronal and neuroendocrine responses helping an organism to adapt to changed environmental conditions. Chronic stressors may induce maladaptive responses leading to psychiatric diseases, such as anxiety and major depression. A suitable animal model to unravel mechanisms involved in the control of adaptation to chronic stress is the psychological subordination stress in the male tree shrew. Subordinate male tree shrews exhibit chronic hypothalamo-pituitary-adrenal (HPA) activation as reflected in continuously elevated cortisol secretion, and structural changes in the hippocampal formation. Corticotropin-releasing factor (CRF) is the major peptide released upon activation of the HPA axis in response to stress. Recent evidence suggests that besides CRF, urocortin 1 (Ucn1) also plays a role in stress adaptation. We have tested the significance of CRF and Ucn1 in adaptation to chronic psychosocial stress in male tree shrews exposed for 35 days to daily psychosocial conflict, by performing semi-quantitative immunocytochemistry for CRF in the parvocellular hypothalamic paraventricular nucleus (pPVN), extended amygdala, viz. central extended amygdala (CeA) and dorsolateral nucleus of the bed nucleus of the stria terminalis (BNSTdl) as well as that for Ucn1 in the non-preganglionic Edinger-Westphal nucleus (npEW). Compared to unstressed animals, psychosocial stress resulted in an immediate and sustained activation of the HPA axis and sympathetic tone as well as reduced testosterone concentration and decreased body and testis weights vs. non-stressed tree shrews. In the pPVN, the number of CRF-immunoreactive neurons and the specific signal density of CRF-immunoreactive fiber terminals in the CeA were strongly reduced (-300 and -40%, respectively; P<0.05), whereas no significant difference in CRF fiber density was found in BNSTdl. The npEW revealed 4 times less Ucn1-immunoreactive neurons (P<0.05). These clear effects on both Ucn1- and CRF-neuropeptide contents may reflect a crucial mechanism enabling the animal to adapt successfully to the stressors, and point to the significance of the pPVN, CeA and npEW in stress-induced brain diseases.

Chronic fluoxetine treatment attenuates stressor-induced changes in temperature, heart rate, and neuronal activation in the olfactory bulbectomized rat

The olfactory bulbectomized (OB) rat is a well-characterized animal model that exhibits a number of behavioral and neurochemical changes that have relevance to clinical depression. Hyperactivity in the open field is the most widely used parameter assessed in this model and is reversed following chronic, but not acute, antidepressant treatment. This study investigated OB-induced alterations in heart rate, body temperature, and neuronal activation following open-field exposure and the impact of chronic treatment with fluoxetine on these parameters. Upon placement in the open field, OB rats exhibited a characteristic hyperactivity response. Heart rate and body temperature were increased in sham-operated rats following open-field exposure, a predictable response to stress, which was significantly reduced in OB rats. Moreover bulbectomy reduced open field-induced cFOS expression in the basal nucleus of the stria terminalis while concurrently increasing expression in the hippocampus, amygdala, paraventricular nucleus of the thalamus, and dorsal raphe nucleus. Chronic fluoxetine treatment (10 mg/kg subcutaneous once daily for 5 weeks) attenuated all of these OB-associated changes. In conclusion, OB rats exhibit alterations in behavior, body temperature, heart rate, and neuronal activation in response to open-field exposure, which are reversed following chronic fluoxetine administration. These results identify stress-sensitive regions within the brain which are altered following bulbectomy and which may underlie the abnormal behavioral and physiological changes observed in this rodent model of depression.

Corticotropin-releasing factor, urocortin 1, and their receptors in the mouse spinal cord

Corticotropin-releasing factor (CRF) and urocortin 1 (Ucn1) are involved in stress adaptation. CRF receptor 1 (CRF1) binds CRF and Ucn1 with similar high affinity, but CRF receptor 2 (CRF2) binds Ucn1 with higher affinity than CRF. We tested the hypothesis that in the spinal cord CRF and Ucn1 control peripheral components of the stress response, by assessing the distribution of CRF- and Ucn1-containing fibers, CRF1 and CRF2 mRNAs, and CRF receptor protein (CRFR) in the mouse spinal cord, by using immunofluorescence and in situ hybridization. CRF, Ucn1, and CRFR occurred throughout the spinal cord. CRF fibers predominated in laminae I, V-VII, and X of Rexed. Ucn1 fibers occurred mainly in laminae VII and X and occasionally in lamina IX. Both CRFR mRNAs occurred in all laminae except the superficial laminae of the dorsal horn, but they exhibited different distributions, CRF2 mRNA having a wider occurrence (laminae III-X) than CRF1 mRNA (laminae III-VIII). Double immunofluorescence indicated that CRF and Ucn1 fibers contacted CRFR-containing neurons, mainly in laminae VII and X. The strongest co-distribution of CRF1 and CRF2 mRNAs with CRF and Ucn1 fibers appeared in lamina VII. CRF2 mRNA predominated in lamina IX together with Ucn1, whereas CRF2 mRNA predominated in lamina X, where it had similar distributions with each ligand. In view of the lamina-specific and similar distributions of the two CRF receptor mRNAs with their ligands, we suggest that CRF1 and CRF2 are involved in peripheral stress adaptation processes, such as modulation of stress-induced analgesia and the mediation of visceral nociceptive information by CRF2.

Gene dosage effect on gamma-secretase component Aph-1b in a rat model for neurodevelopmental disorders

A combination of genetic factors and early life events is thought to determine the vulnerability of an individual to develop a complex neurodevelopmental disorder like schizophrenia. Pharmacogenetically selected, apomorphine-susceptible Wistar rats (APO-SUS) display a number of behavioral and pathophysiological features reminiscent of such disorders. Here, we report microarray analyses revealing in APO-SUS rats, relative to their counterpart APO-UNSUS rats, a reduced expression of Aph-1b, a component of the gamma-secretase enzyme complex that is involved in multiple (neuro)developmental signaling pathways. The reduced expression is due to a duplicon-based genomic rearrangement event resulting in an Aph-1b dosage imbalance. The expression levels of the other gamma-secretase components were not affected. However, gamma-secretase cleavage activity was significantly changed, and the APO-SUS/-UNSUS Aph-1b genotypes segregated with a number of behavioral phenotypes. Thus, a subtle imbalance in the expression of a single, developmentally important protein may be sufficient to cause a complex phenotype.

Neurobiological correlates of high (HAB) versus low anxiety-related behavior (LAB): differential Fos expression in HAB and LAB rats

BACKGROUND

Two Wistar rat lines selectively bred for either high (HAB) or low (LAB) anxiety-related behavior were used to identify neurobiological correlates of trait anxiety.

METHODS

We used Fos expression for mapping of neuronal activation patterns in response to mild anxiety-provoking challenges.

RESULTS

In both lines, exposure to an open field (OF) or the open arm (OA) of an elevated plus-maze induced Fos expression in several brain areas of the anxiety/fear circuitry. Rats of the HAB type, which showed signs of a hyperanxious phenotype and a hyperreactive hypothalamic-pituitary-adrenal axis compared with LAB rats, exhibited a higher number of Fos-positive cells in the paraventricular nucleus of the hypothalamus, the lateral and anterior hypothalamic area, and the medial preoptic area in response to both OA and OF. Less Fos expression was induced in the cingulate cortex in HAB than in LAB rats. Differential Fos expression in response to either OA or OF was observed in few brain regions, including the thalamus and hippocampus.

CONCLUSIONS

The present data indicate that the divergent anxiety-related behavioral response of HAB versus LAB rats to OF and OA exposures is associated with differential neuronal activation in restricted parts of the anxiety/fear circuitry. Distinct hypothalamic regions displayed hyperexcitability, and the cingulate cortex showed hypoexcitability, which suggests that they are main candidate mediators of dysfunctional brain activation in pathologic anxiety.

Hypophysiotropic neurons of the paraventricular nucleus respond in spatially, temporally, and phenotypically differentiated manners to acute vs. repeated restraint stress: rapid publication

Hypothalamic-pituitary-adrenal (HPA) responses to stress are initiated by parvicellular neurosecretory neurons in the medial parvicellular (mp) part of the paraventricular hypothalamic nucleus (PVH), which express corticotropin-releasing factor (CRF), among other neuropeptides. We have used an approach guided by patterns of stress-induced Fos expression to explore the manner in which anatomically and phenotypically defined components of the mpPVH respond to acute vs. repeated restraint stress. Hormonal indices of HPA activation in animals exposed to the last of 14 daily repeated restraint sessions were significantly lower than those in rats receiving a single restraint episode. Although this habituation was paralleled by global decrements in activation patterns across all PVH compartments, clear spatial-temporal differences in recruitment profiles were noted between dorsal and ventral aspects of the mpPVH. Thus, acute restraint provoked a biphasic Fos induction, which occurred first within the mpPVH and in an adjoining population of somatostatinergic cells in the periventricular region and only later within other aspects of the PVH. By contrast, Fos responses of habituated animals were monophasic and focused decisively within a discrete ventral aspect of the mpPVH. The ventral population was identified as comprising neurons that express CRF and/or enkephalin and, to a lesser extent, growth hormone-releasing factor. These results indicate a lack of homogeneity among stress-responsive parvicellular neurosecretory neurons and suggest that distinct complements of CRF cells may be preferentially involved in initiating HPA responses to acute stress and sustaining them in the repeated condition.

Rat sex and strain differences in responses to stress

Sensitivity to stress has been linked to the development of a variety of physical and psychological disorders. Studies to-date have focused on extreme stress phenotypes, have studied mostly male responses, have used limited dependent variables, and have included a limited number of measurement time points. The present experiment was designed to address these limitations. Feeding, body weight, open-field activity, acoustic startle reflex (ASR), and prepulse inhibition (PPI) responses of adult male and female Sprague-Dawley and Long-Evans rats to daily immobilization stress (20 min/day) were evaluated for 3 weeks. Stress significantly decreased feeding and body weight of males but generally not of females. Effects were greatest in Long-Evans males. Stress decreased 15-min activity levels for males on Stress Day 1, but not on other days. Stress did not affect 15-min activity levels of Long-Evans females but decreased 15-min activity levels of Sprague-Dawley females on every measurement day. ASR responses to stress differed based on rat strain; percent PPI responses differed based on rat strain and sex. Stress increased startle responses of Sprague-Dawley males and females but not of Long-Evans males and females. Stress reduced PPI of Long-Evans females on every measurement day but not of other groups. These findings indicate that strain and sex of rat is important to consider in evaluating behavioral and physiological responses to stress.

Equal sensitivity to cocaine reward in addiction-prone and addiction-resistant rat genotypes

Rat genotypes tentatively identified as addiction-prone or addiction-resistant on the basis of alcohol preference and locomotor responsiveness to novelty--Lewis versus Fischer strains and Nijmegen high versus low responder lines--differed in time to develop intravenous cocaine self-administration habits, but did not differ in sensitivity to the ability of cocaine reward to summate with lateral hypothalamic brain stimulation reward. Moreover, rats from the Nijmegen low-responder line that initiated self-administration came to do so compulsively and to the same degree as did the Nijmegen high-responder rats. Thus the differences between both sets of genotypes appeared to reflect differences in reactions to the testing situation more than differences in reaction to the reinforcing drug per se.

Distribution of dopamine D1 receptors in the nucleus paraventricularis of the hypothalamus in rats: an immunohistochemical study

The present study investigated the distribution of dopamine D1 receptor protein in the nucleus paraventricularis of the hypothalamus. It was found that the nucleus paraventricularis of the hypothalamus contains a relatively large number of cells which are positive for presence of dopamine D1 receptor protein. The vast majority of dopamine D1 receptor-positive neurons was found in the magnocellular part, but they were also present in considerable quantity in the parvocellular part of this subregion of the hypothalamus. When measured by the Western blot technique, the quantity of D1 receptor protein found in the paraventricular nucleus of the hypothalamus was at the level found in the prefrontal cortex. It was also found that dopamine D1 receptor protein was present in neurons constitutively displaying phosphorylated CREB protein, i.e. neurons which are, as might be speculated, under the tonic influence of neurotransmitters whose receptors operate via cAMP and pCREB as second or third messengers. The presence of dopamine D1 receptors in the nucleus paraventricularis of the hypothalamus may suggest, at an anatomical level, that these receptors are involved in controlling the release of hormones, as well as their synthesis at the level of transcription, which is regulated by phosphorylation of CREB protein. Finally, the present immunocytochemical findings offer an anatomical substrate for the role of dopamine and its receptors of D1 subtype in the regulation of the activity of paraventricular neurons seen in the functional studies.

Variation in hippocampal dynorphin b-immunoreactive mossy fiber terminal fields of apomorphine-(un)susceptible rats

The size of distinct hippocampal sub-fields were measured in the apomorphine-susceptible and apomorphine-unsusceptible rat lines. Mossy fiber terminal fields were delineated using dynorphin B immunoreactivity and area measurements were taken from (1) the supra-pyramidal mossy fiber terminal field; (2) the intra- and infra-pyramidal mossy fiber terminal field; (3) the hilus of the fascia dentata (4) the non dynorphin B immunoreactive area of the regio inferior and fascia dentata and (5) the total area of regio inferior and fascia dentata. The data indicate that statistically significant differences in the morphometry of the hippocampal subfields of the apomorphine susceptible and unsusceptible rats are confined to the intra- and infra terminal field: the relative size of the left and right intra- and infra terminal field of apomorphine unsusceptible rats are significantly larger than those of the apomorphine susceptible rats. These data explain at least in part the differential response of these rats to novelty.

Strain differences in Fos expression following airpuff startle in Spontaneously Hypertensive and Wistar Kyoto rats

The airpuff startle stimulus elicits both a behavioral and a concurrent sympathetic and parasympathetic activation, which have been shown to differ between inbred normotensive Wistar Kyoto and Spontaneously Hypertensive rat strains. Neither the brain sites responsible for the cardiovascular and motor responses, nor the origins of the strain differential responses, have yet been elucidated. The goals of the present study were (i) to define the neuronal pattern of immunoreactive Fos expression to the airpuff stimulus, and (ii) to determine whether this pattern of expression differed between the two contrasting inbred rat strains, thereby relating to observed differences in response. The airpuff stimulus induced Fos protein expression in discrete nuclei within the hypothalamus, thalamus, midbrain, pons and medulla of both strains, with strain-dependent differences evident in the hypothalamus (lateral, ventromedial and dorsomedial), pons (locus coeruleus) and medulla (rostroventrolateral medulla and solitary tract nuclei). To remove Fos expression arising from test chamber novelty, which was observed in both strains, a subset of animals was habituated to the test chamber for four days prior to testing. Habituation reduced Fos expression in several brain regions in the Wistar Kyoto, but failed to do so in the Spontaneously Hypertensive rat. The present results are the first to identify a set of brain regions likely to be responsible for the mediation of the cardiovascular and motor responses associated with the airpuff startle stimulus. Several of the identified areas contain neurotransmitters implicated by prior pharmacological studies. Further, these data identify differences in the degree of activation of specific neuronal structures that probably underlie strain differences in the cardiovascular response to the airpuff. Additionally, the results provide a cellular correlate to reported deficits in behavioral habituation by the Spontaneously Hypertensive rat and suggest a potentially profound difference between the ability of these two strains to adapt to repeated mild stress stimuli.

Nijmegen high and low responders to novelty: a new tool in the search after the neurobiology of drug abuse liability

Knowledge about the differences in structure, function, and reactivity of the brain and body between Nijmegen high responders to novelty and Nijmegen low responders to novelty may help us to understand which factors give rise to the vulnerability and/or susceptibility to drugs of abuse. For that purpose, this contribution provides a short overview of the outcome of the available studies on Nijmegen high responders to novelty and Nijmegen low responders to novelty. These animals can be selected using three major behavioral paradigms: (a) the open-field test (which allows the separation of high and low responders to novelty); (n) the intruder test (which allows the separation of fleeing and nonfleeing rats); (c) the apomorphine test (which allows the separation of apomorphine-susceptible and apomorphine-unsusceptible rats). Data to date suggest that the same traits have been selected by all three paradigms, and point to the hypothesis that the neurochemical state of the nucleus accumbens directs the sensitivity to drugs of abuse. In addition, recent evidence suggests that the sensitivity to the psychostimulant and/or reinforcing effects of dexamphetamine and ethanol is smaller in HR than in LR under certain experimental conditions, whereas the reverse is found when different experimental conditions are chosen. The data all together lay the foundation for the overall hypothesis that there are three factors ultimately determining the individual-specific sensitivity to drug of abuse: (a) the genetic background that predisposes an individual to become a HR or a LR, (b) early postnatal factors that direct the phenotypic expression of a particular genotype at adult age, and (c) the degree of stress during exposure to the drug of abuse. Further testing of this hypothesis may provide important information about the factors that contribute to individual differences in vulnerability to drugs of abuse.

Adolescent development alters stressor-induced Fos immunoreactivity in rat brain

Adult-typical behavioural responses to environmental challenges as well as the stressor responsiveness of several neural systems emerge over adolescent development. The present study was undertaken to determine whether stressors might activate different neural populations in adult vs juvenile male rats. Fos-immunoreactivity was determined in various forebrain nuclei following 15 min or 2 h of restraint in 28- and 60-day-old male rats (representing late juvenile and young adult ages, respectively) and compared to non-restrained control animals at each age. Few Fos-positive cells were identified in unrestrained controls at either age. Restraint, however, induced the production of Fos in several areas. Fos immunoreactivity was marked in parvocellular regions of the paraventricular nucleus of the hypothalamus following both restraint periods and at both ages, an observation consistent with previous observations that restraint increases plasma corticosterone at both ages. And at both ages, Fos immunoreactivity was evident in magnocellular regions of the hypothalamus only following the longer restraint period. Fos immunoreactivity, however, clearly varied as a function of adolescent age in several regions. Moderate to intense Fos immunoreactivity was observed in adults in all divisions of the anterior olfactory nucleus, cortical and medial amygdaloid nuclei, pyriform cortex and tenia tecta. In contrast to the adult, only a few Fos positive cells were observed in any of these regions in juveniles. Exposure to the same stressor induced Fos in a broader spectrum of neurons in young adult than in juvenile male rats. The lack of Fos-positive cells in specific areas of juveniles may relate to maturation in specific amygdaloid nuclei, which project to many of the other regions that showed age-related differences in Fos production. The emergence over adolescence of Fos-positive cells in specific areas in response to stressors may underlie the emergence of adult-typical behavioural and neural stressor-responsiveness.

Plasticity in the stress-regulating circuit: decreased input from the bed nucleus of the stria terminalis to the hypothalamic paraventricular nucleus in Wistar rats following adrenalectomy

The bed nucleus of the stria terminalis is involved in the stress-regulating circuit by funnelling limbic information to the hypothalamic paraventricular nucleus. Since adrenalectomy influences both limbic structures (by inducing cell death in the hippocampus) and the hypothalamic paraventricular nucleus (by increased corticotrophin-releasing hormone synthesis), we investigated whether the bed nucleus of the stria terminalis is also influenced by adrenalectomy. For this purpose, we analysed and compared the projections from the bed nucleus of the stria terminalis to the hypothalamic paraventricular nucleus in normal and adrenalectomized rats by anterograde tracer injections in the bed nucleus of the stria terminalis. Quantitative analysis of the fibre pattern in the hypothalamic paraventricular nucleus of normal rats revealed a homogeneous distribution of fibres of the bed nucleus of the stria terminalis over the different subdivisions of the hypothalamic paraventricular nucleus. In adrenalectomized rats, the absolute fibre density was significantly lower in the whole hypothalamic paraventricular nucleus (1.17 +/- 0.27 10(-3) microm/microm3 in adrenalectomized rats versus 2.59 +/- 0.24 10(-3) microm/microm3 in normal rats; P < 0.01) and all its subdivisions. The largest decrease of fibre density was found in the corticotrophin-releasing hormone-rich part of the hypothalamic paraventricular nucleus (relative fibre density; adrenalectomized rats: 0.602 +/- 0.106, versus 1.095 +/- 0.019 in normal rats, P < 0.01). These results show a loss of input from the bed nucleus of the stria terminalis to the hypothalamic paraventricular nucleus, and particularly to the corticotrophin-releasing hormone neurons, following adrenalectomy. The data suggest that this pathway within the stress-regulating circuit is functionally affected by corticosteroids in adult rats and may imply that human disorders associated with corticosteroid imbalance are allied to a changed circuitry in the brain.

The hypothalamic paraventricular nucleus in two types of Wistar rats with different stress responses. I. Morphometric comparison

The present study evaluates the role of the hypothalamic paraventricular nucleus (PVH) in stress regulation by a morphometric comparison of the vascular, neuronal and synaptic properties of this nucleus in two lines of Wistar rats. It has been previously reported that these two lines of rats, indicated as APO-SUS (apomorphine-susceptible) and APO-UNSUS (apomorphine-unsusceptible) rats on the basis of their reactivity to a subcutaneous injection of apomorphine, display a variety of pharmacological and behavioral differences, including differences in their stress-coping mechanisms (Cools et al., Neuropsychobiology, 28 (1993) 100-105). The results show a similar vascular and neuronal organization of the PVH in both lines, but distinct synaptic differences. The PVH (0.12 mm3 volume with about 15,000 neurons on one side) has an overall vascular density of 5.6%, with significant differences between subdivisions (parvocellular central part: 8.3%, parvocellular dorsal/ventral/posterior part: 4.6-5.3%), which means that vascularity is a useful tool to delineate subdivisions in the parvocellular PVH. The neuronal density of 132 x 10(3)/mm3 as found in the present study is two times higher than reported in a previous study Possible reasons for this discrepancy are extensively discussed. The most significant finding of the present study is the observation that APO-SUS rats have a significantly higher synaptic density (158 x 10(6)/mm3) in the PVH than APO-UNSUS rats (108 x 10(6)/mm3). It is discussed in which way this synaptic difference may be correlated with the different activity of the hypothalamo-pituitary-adrenal axis in both lines of Wistar rats.