The parasympathetic responsiveness in young and aged rats:Involvement of aminergic and vasopressinergic mechanisms

The studies presented in this thesis were designed to investigate whether the decline in parasympathetic responsiveness to stress in aged rats can be generalized for various (environmental) challenges. In addition, the contribution of this reduced vagal responsiveness in aged animals to autonomic dysbalance was determined. Finally, considerable attention was paid to the role of arninergic and vasopressinergic systems in the age-related reduction of vagal responses. The results show that in aged rats parasympathetic responsiveness to a variety of challenges is reduced. However, sympathetic responsiveness is reduced as well. Therefore, autonomic output remains in balance during mild emotional stress situations. Enhancement of the aminergic “state” reinstates the vagal responsiveness to emotional but not to metabolic challenges in aged rats. Systemic administration of vasopressin improves and prolongs both the behavioral and vagal emotional stress responses in rats but only until late-adult age (14months) is reached

The parasympathetic responsiveness in young and aged rats:Involvement of aminergic and vasopressinergic mechanisms

The studies presented in this thesis were designed to investigate whether the decline in parasympathetic responsiveness to stress in aged rats can be generalized for various (environmental) challenges. In addition, the contribution of this reduced vagal responsiveness in aged animals to autonomic dysbalance was determined. Finally, considerable attention was paid to the role of arninergic and vasopressinergic systems in the age-related reduction of vagal responses. The results show that in aged rats parasympathetic responsiveness to a variety of challenges is reduced. However, sympathetic responsiveness is reduced as well. Therefore, autonomic output remains in balance during mild emotional stress situations. Enhancement of the aminergic “state” reinstates the vagal responsiveness to emotional but not to metabolic challenges in aged rats. Systemic administration of vasopressin improves and prolongs both the behavioral and vagal emotional stress responses in rats but only until late-adult age (14months) is reached

Rodent models of social stress and neuronal plasticity:Relevance to depressive-like disorders

Exposure to severe or persistent social stress may lead to the development of psychiatric disorders such as anxiety and depression. These mood disorders are associated with structural alterations of neural architecture in limbic brain regions that control emotion, mood and cognition. Structural remodeling may either be a sign of successful adaptation, or of failure to do so. In neuropsychiatric disorders like depression structural remodeling involves apoptosis, reduced neurogenesis, and structural remodeling of neuronal dendrites which most likely reflects the latter. Here we review key findings from animal models of psychosocial stress that have been used to gain insights into the relation between stress-related behavioral disorders like depression and structural plasticity. Specifically, we focus on models having a high face validity like social defeat stress in the resident-intruder paradigm and chronic stress of social subordination in social housing conditions. Moderate to severe social stress appears to stimulate plasticity and neuronal growth in regions of the amygdala, whereas the effects in the hippocampus and prefrontal cortex tend to be opposite. A major focus of the current review is to characterize social stress induced structural changes in these brain regions, aiming to provide insight in pathways and factors that underlie behavioral effects of stress and depression.</p

Repeated victorious and defeat experiences induce similar apical dendritic spine remodeling in CA1 hippocampus of rats

In this study, apical dendritic spine density of neurons in hippocampal, amygdalar and prefrontal cortical areas was compared in rats that were repeatedly winning or losing social conflicts. Territorial male wild-type Groningen (WTG) rats were allowed multiple daily attacks (>20 times) on intruder males in the resident-intruder paradigm. Frequent winning experiences are known to facilitate uncontrolled aggressive behavior reflected in aggressive attacks on anesthetized males which was also observed in the winners in this study. Both winners and losers were socially housed during the experiments; winners with females to stimulate territorial behavior, and losers with two other losing male rats. Twenty-four hours after the last social encounter, brains from experienced residential winners and repeatedly defeated intruder rats were collected and neuronal morphology in selected brain regions was studied via Golgi-Cox staining. Results indicate that spine density in the apical dendrites of the hippocampal CA1 reduced similarly in both winners and losers. In addition, winners showed increased spine densities at the proximal segments (20-30 µm) of the basolateral amygdala neurons and losers tended to show a decreased spine density at the more proximal segments of the infralimbic region of prefrontal cortex neurons. No effect of winning and losing was observed in the medial amygdala. The atrophic effect of repeated defeats in hippocampal and prefrontal regions was anticipated despite the fact that social housing of the repeatedly losing intruder males may have played a protective role. The reduction of hippocampal spine density in the winners seems surprising but supports previous findings in hierarchical dominant males in rat colonies. The dominants showed even greater shrinkage of the apical dendritic arbors of hippocampal CA3 pyramidal neurons compared to the stressed subordinates

Temporal and spatial dynamics of corticosteroid receptor down-regulation in rat brain following social defeat

The experiments explored the nature and time course of changes in glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) binding in homogenates of various brain regions and pituitary of male Wistar rats following social defeat stress. One week after defeat, the binding capacity of GRs was decreased in the hippocampus and the hypothalamus while no changes were observed in the parietal cortex and the pituitary. The number of MRs remained at the same level as in undefeated rats. Three weeks postdefeat, the initially down-regulated GR returned to baseline level in the hippocampus and the hypothalamus. However, GR binding was now decreased in the parietal cortex. Severe down-regulation of MRs was detected in the hippocampal and septal tissue. The results show that brief but intense stress like social defeat induces a long-lasting down-regulation of corticosteroid receptors and that the temporal dynamics of these changes are not only differential for GRs and MRs but also for brain sites.

Decreased dendritic spine density in posterodorsal medial amygdala neurons of proactive coping rats

There are large individual differences in the way animals, including humans, behaviorally and physiologically cope with environmental challenges and opportunities. Rodents with either a proactive or reactive coping style not only differ in their capacity to adapt successfully to environmental conditions, but also have a differential susceptibility to develop stress-related (psycho)pathologies when coping fails. In this study, we explored if there are structural neuronal differences in spine density in brain regions important for the regulation of stress coping styles. For this, the individual coping styles of wild-type Groningen (WTG) rats were determined using their level of offensive aggressiveness assessed in the resident-intruder paradigm. Subsequently, brains from proactive (high-aggressive) and reactive (low-aggressive) rats were Golgi-cox stained for spine quantification. The results reveal that dendritic spine densities in the dorsal hippocampal CA1 region and basolateral amygdala are similar in rats with proactive and reactive coping styles. Interestingly, however, dendritic spine density in the medial amygdala (MeA) is strikingly reduced in the proactive coping rats. This brain region is reported to be strongly involved in rivalry aggression which is the criterion by which the coping styles in our study are dissociated. The possibility that structural differences in spine density in the MeA are involved in other behavioral traits of distinct coping styles needs further investigation

Enhanced sensitivity of postsynaptic serotonin-1A receptors in rats and mice with high trait aggression

Individual differences in aggressive behaviour have been linked to variability in central serotonergic activity, both in humans and animals. A previous experiment in mice, selectively bred for high or low levels of aggression, showed an up-regulation of postsynaptic serotonin-1A (5-HT1A) receptors, both in receptor binding and in mRNA levels, in the aggressive line. The aim of this experiment was to study whether similar differences in 5-HT1A receptors exist in individuals from a random-bred rat strain, varying in aggressiveness. In addition, because little is known about the functional consequences of these receptor differences, a response mediated via postsynaptic 5-HT1A receptors (i.e., hypothermia) was studied both in the selection lines of mice and in the randomly bred rats. The difference in receptor binding, as demonstrated in mice previously, could not be shown in rats. However, both in rats and mice, the hypothermic response to the 5-HT1A agonist alnespirone was larger in aggressive individuals. So, in the rat strain as well as in the mouse lines, there is, to a greater or lesser extent, an enhanced sensitivity of postsynaptic 5-HT1A receptors in aggressive individuals. This could be a compensatory up-regulation induced by a lower basal 5-HT neurotransmission, which is in agreement with the serotonin deficiency hypothesis of aggression.

Az agy öregedése és a neurodegeneráció: A neuroendokrin szabályozás szerepe = The role of neuroendocrine regulation in brain aging and neurodegeneration

A kutatás célja a hypophysis-mellékvesekéreg (HPA) hormonok és az ösztrogének szerepének vizsgálata volt az agy öregedésére és a neurodegenerációra patkányokban. Vizsgálták az öregkorban adott kortikoszteron (CORT) hatását a szerotoninerg és a kolinerg rostok degenerációjára az öregedés alatt. Vizsgálták továbbá az újszülöttkorban adott ACTH 4-9 analóg peptid hatását a HPA rendszer működésére és az adaptációs magatartásra felnőtt- és öregkorban. Tanulmányozták az agy öregedésével együtt járó szerotoninerg neurodegenerációt félmajmokon (Tupaia belangeri). Az öregkori CORT túladagolás krónikus körülmények között fokozta a szerotoninerg idegrostok degenerációját és csökkentette a tanulási teljesítményt. Immuncitokémiai módszerrel részletes leírást adtak az alfa és a béta típusú ösztrogén receptorok jellegzetességeiről öreg nőstény patkányok hippocampusában. Kimutatták, hogy az újszülöttkori ACTH peptid kezelés mellett az öregkorra jellemző napi kortikoszteron szint-ingadozás beszűkülése jelentős mértékben elmarad. Közölték, hogy az öregedés során a szerotoninerg rostok sűrűsége a hippocampus egyes régióiban szignifikánsan csökken félmajmokon. Végül jelentős előrelépés történt az in vivo állatkísérletes demencia modellek kidolgozásában patkányokon. Az eredmények azt mutatják, hogy az öregedés alatt jelentkező HPA és gonád hormon változások szerepet játszhatnak az öregedésre jellemző neuronális involúcióban és különösen a neurodegeneráció mechanizmusában. | The role of pituitary-adrenocortical hormones (HPA hormones) and that of estrogens on brain aging and neurodegeneration has been studied in rats. During aging the spontaneous degeneration of serotonergic and cholinergic fibers were studied after prolonged corticosterone (CORT) exposure. The immediate and long-term effects of neonatal administration of ACTH4-9 peptide analogue were investigated on the development and age-related functioning of HPA axis and adaptive behaviour. Furthermore, the age-dependent degeneration of serotonergic fibers was also followed in the hippocampus of tree shrews (Tupaia belangeri). Chronic CORT exposure in aged rats resulted in enhanced serotonergic fiber degeneration and inhibited learning performance. A detailed description of qualitative and quantitative aspects of alpha and beta estrogen receptors has been provided in the hippocampus of aging female rats. Neonatal treatment with ACTH4-9 peptide analogue prevented the age-specific attenuation of amplitude of diurnal CORT rhythm in the circulation. Aging has been coupled with a decrement of serotonergic fiber density in some subregions of hippocampus in old tree shrews. A considerable progress has also been achieved in the improvement of in vivo animal models studying brain neurodegeneration. The results show that the disturbed HPA and gonadal functions during aging might be contributing factors to neuronal decline in the advanced age and especially during neurodegeneration