Lesions of the hippocampal efferent pathway (fimbria-fornix) do not alter sensitivity of adrenocorticotropin to feedback inhibition by corticosterone in rats

Brain mechanisms of HPA axis regulation: neurocircuitry and feedback in context Richard Kvetnansky lecture

Regulation of stress reactivity is a fundamental priority of all organisms. Stress responses are critical for survival, yet can also cause physical and psychological damage. This review provides a synopsis of brain mechanisms designed to control physiological responses to stress, focusing primarily on glucocorticoid secretion via the hypothalamo-pituitary-adrenocortical (HPA) axis. The literature provides strong support for multi-faceted control of HPA axis responses, involving both direct and indirect actions at paraventricular nucleus (PVN) corticotropin releasing hormone neurons driving the secretory cascade. The PVN is directly excited by afferents from brainstem and hypothalamic circuits, likely relaying information on homeostatic challenge. Amygdala subnuclei drive HPA axis responses indirectly via disinhibition, mediated by GABAergic relays onto PVN-projecting neurons in the hypothalamus and bed nucleus of the stria terminalis (BST). Inhibition of stressor-evoked HPA axis responses is mediated by an elaborate network of glucocorticoid receptor (GR)-containing circuits, providing a distributed negative feedback signal that inhibits PVN neurons. Prefrontal and hippocampal neurons play a major role in HPA axis inhibition, again mediated by hypothalamic and BST GABAergic relays to the PVN. The complexity of the regulatory process suggests that information on stressors is integrated across functional disparate brain circuits prior to accessing the PVN, with regions such as the BST in prime position to relay contextual information provided by these sources into appropriate HPA activation. Dysregulation of the HPA in disease is likely a product of inappropriate checks and balances between excitatory and inhibitory inputs ultimately impacting PVN output.

Hypothalamic-Pituitary--Adrenal Axis-Feedback Control

The hypothalamo-pituitary-adrenal axis (HPA) is responsible for stimulation of adrenal corticosteroids in response to stress. Negative feedback control by corticosteroids limits pituitary secretion of corticotropin, ACTH, and hypothalamic secretion of corticotropin-releasing hormone, CRH, and vasopressin, AVP, resulting in regulation of both basal and stress-induced ACTH secretion. The negative feedback effect of corticosteroids occurs by action of corticosteroids at mineralocorticoid receptors (MR) and/or glucocorticoid receptors (GRs) located in multiple sites in the brain and in the pituitary. The mechanisms of negative feedback vary according to the receptor type and location within the brain-hypothalmo-pituitary axis. A very rapid nongenomic action has been demonstrated for GR action on CRH neurons in the hypothalamus, and somewhat slower nongenomic effects are observed in the pituitary or other brain sites mediated by GR and/or MR. Corticosteroids also have genomic actions, including repression of the pro-opiomelanocortin (POMC) gene in the pituitary and CRH and AVP genes in the hypothalamus. The rapid effect inhibits stimulated secretion, but requires a rapidly rising corticosteroid concentration. The more delayed inhibitory effect on stimulated secretion is dependent on the intensity of the stimulus and the magnitude of the corticosteroid feedback signal, but also the neuroanatomical pathways responsible for activating the HPA. The pathways for activation of some stressors may partially bypass hypothalamic feedback sites at the CRH neuron, whereas others may not involve forebrain sites; therefore, some physiological stressors may override or bypass negative feedback, and other psychological stressors may facilitate responses to subsequent stress.

Embedded synaptic feedback in the neuroendocrine stress axis

Neural regulation of blood glucocorticoid levels is critical for defence of homeostasis during physiological or psychoemotional challenges. In mammals, this function is carried out by the neuroendocrine stress axis, coordinated by parvocellular neuroendocrine cells (PNCs) of the paraventricular hypothalamic nucleus. Feedback regulation of PNCs by glucocorticoids provides complex experience-dependent shaping of neuroendocrine responses. We review recent evidence for metaplastic actions of glucocorticoids as 'circuit breakers' at synapses directly regulating PNC excitability and explore how such mechanisms may serve as substrates for stress adaptation.

Hypothalamo-pituitary-adrenocortical axis, glucocorticoids, and neurologic disease

Neurologic diseases are often accompanied by significant life stress and consequent increases in stress hormone levels. Glucocorticoid stress hormones are known to have deleterious interactions with neurodegenerative processes, and are hypersecreted in neurologic disorders as well as in comorbid psychiatric conditions, such as depression. This article highlights the state of our knowledge on mechanisms controlling activation and inhibition of glucocorticoid secretion, outlines signaling mechanisms used by these hormones in neural tissue, and describes how endogenous glucocorticoids can mediate neuronal damage in various models of neurologic disease. The article highlights the importance of controlling stress and consequent stress hormone secretion in the context of neurologic disease states.

Shaping the stress response: interplay of palatable food choices, glucocorticoids, insulin and abdominal obesity

Activity of the hypothalamo-pituitary-adrenal (HPA) axis is regulated by a negative feedback loop that dampens central drive of the axis via the actions of the secreted glucocorticoids. Conversely, under conditions of chronic stress, glucocorticoids delivered centrally increase hypothalamic paraventricular nucleus (PVN) corticotrophin-releasing factor (CRF) expression and the response to restraint. However, HPA axis activity and PVN CRF mRNA expression under chronic stress conditions are often reduced, implying other indirect peripheral or extra-hypothalamic glucocorticoid actions. Glucocorticoids chronically increase palatable food intake, which increases abdominal fat depots and circulating insulin levels, both of which negatively correlate with PVN CRF mRNA expression and may in turn dampen the response to stress. Such an effect is dependent on food choices, rather than total calories ingested. Considering stress is omnipresent in the workplace, palatable food ingestion may represent a means to combat the feeling of stress which is ultimately maladaptive when unresolved.

Functional role of local GABAergic influences on the HPA axis

Neuronatomical and pharmacological studies have established GABA-mediated inhibition of the HPA axis at the level of the PVN. The origin of this innervation is a series of local hypothalamic and adjacent forebrain regions that project to stress-integrative hypophysiotropic CRH neurons. While a role in tonic inhibition of the stress axis is likely, this system of inhibitory loci is also capable of producing a dynamic braking capacity in the context of the neuroendocrine stress response. The latter function is mediated in large part by glutamatergic forebrain afferents that increase GABA release at the level of the PVN. In addition, this local GABA system can be inhibited by upstream GABAergic projection neurons, producing activation of the HPA axis via removal of GABAergic tone. This PVN projecting GABA network interfaces with a wide range of homeostatic mechanisms, and is capable of biochemical plasticity in response to chronic stress. Collectively, the elements of this system provide for exquisite control of neuroendocrine activation in the face of stressful stimuli, and loss of this regulatory capacity may underlie many stress-related disorders.

Fetal programming of hypothalamic-pituitary-adrenal (HPA) axis function and behavior by synthetic glucocorticoids

Reduced fetal growth has been closely associated with an increased risk for the development of chronic disease in later life. Accumulating evidence indicates that fetal exposure to excess glucocorticoids represents a critical mechanism underlying this association. Approximately 7% of pregnant women are at risk of preterm delivery and these women are routinely treated with synthetic glucocorticoids (sGC) between 24 and 34 of weeks gestation to improve neonatal outcome. Animal studies have demonstrated that maternally administered sGC crosses the placenta, affecting fetal hypothalamic-pituitary-adrenal (HPA) development, resulting in changes in HPA axis function that persist throughout life. These changes appear to be modulated at the level of glucocorticoid receptors (GR) and mineralocorticoid receptors (MR) in the brain and pituitary. As the HPA axis interacts with many other physiological pathways, the changes in endocrine function are also sex-specific and age-dependent. Alterations in behavior, particularly locomotion, in animals exposed to sGC in utero have also been demonstrated. Consistent with the finding in animal models, emerging human data are indicating attention deficit-hyperactivity disorder (ADHD)-like symptoms in children exposed to repeated courses of sGC in utero. This behavioral phenotype is likely linked to alterations in dopamine (DA) signaling, suggesting that sGC are able to permanently modify or 'program' this system. Finally, it is emerging that changes in HPA axis function and behavior following antenatal exposure to sGC are transgenerational and likely involve epigenetic mechanisms. A comprehensive understanding of the acute and long-term impact of sGC exposure in utero is necessary to begin to develop recommendations and treatment options for pregnant women at risk of preterm delivery.

Changes in the expression of corticotrophin-releasing hormone, mineralocorticoid receptor and glucocorticoid receptor mRNAs in the hypothalamic paraventricular nucleus induced by fornix transection and adrenalectomy

The paraventricular nucleus (PVN) in the hypothalamus receives inputs from the hippocampus The present study explored the influence of the hippocampus on genes mediating glucocorticoid feedback in the PVN. Accordingly, the expression of mRNAs for corticotrophin-releasing hormone (CRH), the mineralocorticoid receptor (MR) and the glucocorticoid receptor (GR) in the PVN was examined by in situ hybridisation in rats subjected to transection of the fornix. Significant increases in CRH, MR and GR mRNAs were observed in the parvocellular PVN after fornix transection (FT). FT-animals subjected to adrenalectomy also showed an increase in the number of cells positive for CRH and GR mRNAs. CRH, MR and GR mRNA expression was also increased by bilateral adrenalectomy, and GR mRNA expression was further enhanced in the parvocellular PVN of the FT transected animals. However, no such changes were evident in the magnocellular PVN. These results suggest that the input from the hippocampus to the PVN, particularly to its parvocellular region, has distinct and differential inhibitory effects on the expression of MR,GR and CRH mRNAs that may operate independently from the feedback actions of corticosterone.

Glucocorticoids, chronic stress, and obesity

Glucocorticoids either inhibit or sensitize stress-induced activity in the hypothalamo-pituitary-adrenal (HPA) axis, depending on time after their administration, the concentration of the steroids, and whether there is a concurrent stressor input. When there are high glucocorticoids together with a chronic stressor, the steroids act in brain in a feed-forward fashion to recruit a stress-response network that biases ongoing autonomic, neuroendocrine, and behavioral outflow as well as responses to novel stressors. We review evidence for the role of glucocorticoids in activating the central stress-response network, and for mediation of this network by corticotropin-releasing factor (CRF). We briefly review the effects of CRF and its receptor antagonists on motor outflows in rodents, and examine the effects of glucocorticoids and CRF on monoaminergic neurons in brain. Corticosteroids stimulate behaviors that are mediated by dopaminergic mesolimbic "reward" pathways, and increase palatable feeding in rats. Moreover, in the absence of corticosteroids, the typical deficits in adrenalectomized rats are normalized by providing sucrose solutions to drink, suggesting that there is, in addition to the feed-forward action of glucocorticoids on brain, also a feedback action that is based on metabolic well being. Finally, we briefly discuss the problems with this network that normally serves to aid in responses to chronic stress, in our current overindulged, and underexercised society.

From Malthus to motive: how the HPA axis engineers the phenotype, yoking needs to wants

The hypothalamo-pituitary-adrenal (HPA) axis is the critical mediator of the vertebrate stress response system, responding to environmental stressors by maintaining internal homeostasis and coupling the needs of the body to the wants of the mind. The HPA axis has numerous complex drivers and highly flexible operating characterisitics. Major drivers include two circadian drivers, two extra-hypothalamic networks controlling top-down (psychogenic) and bottom-up (systemic) threats, and two intra-hypothalamic networks coordinating behavioral, autonomic, and neuroendocrine outflows. These various networks jointly and flexibly control HPA axis output of periodic (oscillatory) functions and a range of adventitious systemic or psychological threats, including predictable daily cycles of energy flow, actual metabolic deficits over many time scales, predicted metabolic deficits, and the state-dependent management of post-prandial responses to feeding. Evidence is provided that reparation of metabolic derangement by either food or glucocorticoids results in a metabolic signal that inhibits HPA activity. In short, the HPA axis is intimately involved in managing and remodeling peripheral energy fluxes, which appear to provide an unidentified metabolic inhibitory feedback signal to the HPA axis via glucocorticoids. In a complementary and perhaps a less appreciated role, adrenocortical hormones also act on brain to provide not only feedback, but feedforward control over the HPA axis itself and its various drivers, as well as coordinating behavioral and autonomic outflows, and mounting central incentive and memorial networks that are adaptive in both appetitive and aversive motivational modes. By centrally remodeling the phenotype, the HPA axis provides ballistic and predictive control over motor outflows relevant to the type of stressor. Evidence is examined concerning the global hypothesis that the HPA axis comprehensively induces integrative phenotypic plasticity, thus remodeling the body and its governor, the brain, to yoke the needs of the body to the wants of the mind. Adverse side effects of this yoking under conditions of glucocorticoid excess are discussed.

Administration of antisense oligonucleotide against pro-opiomelanocortin prevents enduring hormonal alterations induced by neonatal handling in male mice

Early life events have been implicated in the programming of adult chronic diseases. Several investigations suggest that the role of early environment in influencing development mainly involves the hypothalamic-pituitary-adrenal axis. Therefore, we examined whether 1) daily neonatal handling, applied from birth to weaning induces HPA hormones alterations in mice lasting up to the adult age; and 2) if the administration of an antisense oligodeoxynucleotide versus pro-opiomelanocortin (As-POMC) prevents hormonal alterations observed in previously handled mice (Handled). In the adult phase (90 days), Handled are overweight and have higher basal plasma immuno-reactive (ir)-corticosterone and adrenocorticotropin (ir-ACTH), and higher pituitary ir-ACTH; while they have lower hypothalamic ir-ACTH and corticotropin-releasing hormone (ir-CRH) in comparison with the non-handled mice. As-POMC (0.05-0.1 nmol/g body weight per day) administered during the same period dose-dependently prevents the increase in body weight, in plasma ir-corticosterone, ir-ACTH, and pituitary ir-ACTH, also preventing the decrease in hypothalamic ir-CRH and ir-ACTH; while the mismatch oligonucleotide is nearly inactive. This data indicates that pharmacological treatment in neonatal life may have enduring effects, reducing the alterations in hormonal homeostatic programming mechanisms induced by early repeated handling.

Fetal fornix transection and gestation length in sheep

Experiments in several species indicate that the hippocampus influences hypothalamo-pituitary-adrenal (HPA) axis function. In fetal sheep, simultaneous ACTH and cortisol rises over the last 30 days of gestation peak at term and are necessary for birth. We hypothesized that if the fetal hippocampal formation is functional in late gestation, loss of hippocampal input to the HPA axis following fetal fornix transection would change gestation length in comparison to controls. At 118-121 days of gestation (dG), stereotaxic technique was used in fetal sheep to sham transect (SHAM; n = 8) or transect (FXTX; n = 6) the dorsal fornix at the level of the hippocampal commissure. No differences were found between SHAM and FXTX fetuses in daily hormone profiles over the last week of gestation or in gestation length (148.0 +/- 1.2 vs. 149.0 +/- 0.4 dG, respectively). We conclude that the fetal hippocampus is immature in late gestation and we speculate that an immature hippocampus is necessary for the loss of negative feedback control that gives rise to the long term, simultaneous increases in ACTH and cortisol that are indispensable for labor and delivery at term in sheep.

Hypothalamic-Pituitary-Adrenal Axis, Glucocorticoids, and Neurologic Disease

Neurologic diseases often are accompanied by significant life stress and consequent increases in stress hormone levels. Glucocorticoid stress hormones are known to have deleterious interactions with neurodegenerative processes and are hypersecreted in neurologic disorders and comorbid psychiatric conditions. This review highlights the current state of knowledge of mechanisms controlling activation and inhibition of glucocorticoid secretion, outlines signalling mechanisms used by these hormones in neural tissue, and describes how endogenous glucocorticoids can mitigate neuronal damage in models of neurologic disease. This review highlights the importance of controlling stress and consequent stress hormone secretion in the context of neurologic disease states.

Immediate-early gene induction in hippocampus and cortex as a result of novel experience is not directly related to the stressfulness of that experience

The stressful quality of an experience, as perceived by rats, is believed to be largely represented by the magnitude of a hypothalamic-pituitary-adrenal (HPA) axis response. The hippocampus may be especially important for assessing the stressfulness of psychological stressors such as novel experience. If such is the case then experience-dependent immediate-early gene expression levels within the hippocampus may parallel relative levels of HPA axis activity. We examined this prospect in rats that were placed in four different novel environments (empty housing tub, circular arena, elevated pedestal or restraint tube). Restraint and pedestal produced the largest magnitude of increased ACTH and corticosterone secretion, arena an intermediate level (Experiment 2) and tub the least magnitude of increase. We saw a very similar experience-dependent pattern of relative Fos protein, c-fos mRNA and zif268 mRNA expression in the paraventricular nucleus of the hypothalamus. However, in hippocampus (and select regions of cortex), immediate-early gene expression was associated with the exploratory potential of the novel experience rather than level of HPA axis activity; pedestal and arena elicited the greatest immediate-early gene expression, tub an intermediate level and restraint the least amount of expression. We conclude that the stressfulness of psychological stressors is not represented by the amount of immediate-early gene induction elicited in hippocampus and cortex, nor does there appear to be a general enhancing or depressive influence of acute stress on immediate-early gene induction in those brain regions.

Programming the offspring of the pig by prenatal social stress: neuroendocrine activity and behaviour

This study provides evidence in the pig that stress experienced during gestation has long-lasting effects on offspring daughters, including their maternal behaviour. Thirty-six primiparous sows were divided into control and two groups that were stressed (by social mixing) during either the second (Mix 2) or third (Mix 3) trimester of pregnancy. We found detrimental effects of mixing on the mothers' growth, body lesions, and cortisol secretion, but did not observe any significant effects on reproductive parameters including birth weight. At 60 days of age, 48 daughters were randomly selected from the three treatments: half were challenged using a restraint and isolation test. Then, all were culled and brain tissue was collected. In situ hybridisation measurements showed increased expression of CRH mRNA in the PVN in unrestrained Mix 2 and in the amygdala of Mix 2 and Mix 3 daughters. At 67 days, 24 further daughters were mixed to measure their responses to this social stress. All showed increased salivary cortisol secretion, but Mix 2 and Mix 3 daughters showed a greater and longer response than controls. Finally, all 24 were inseminated and at parturition maternal behaviour was measured. Mix 2 and Mix 3 daughters were more restless and more responsive to piglets that approached the head of the sow, traits which previously have been shown to be a component of abnormal maternal behaviour. Indeed, Mix 2 and Mix 3 daughters also tended to bite at their piglets more than control daughters.

Limbic system mechanisms of stress regulation: hypothalamo-pituitary-adrenocortical axis

Limbic dysfunction and hypothalamo-pituitary-adrenocortical (HPA) axis dysregulation are key features of affective disorders. The following review summarizes our current understanding of the relationship between limbic structures and control of ACTH and glucocorticoid release, focusing on the hippocampus, medial prefrontal cortex and amygdala. In general, the hippocampus and anterior cingulate/prelimbic cortex inhibit stress-induced HPA activation, whereas the amygdala and perhaps the infralimbic cortex may enhance glucocorticoid secretion. Several characteristics of limbic-HPA interaction are notable: first, in all cases, the role of given limbic structures is both region- and stimulus-specific. Second, limbic sites have minimal direct projections to HPA effector neurons of the paraventricular nucleus (PVN); hippocampal, cortical and amygdalar efferents apparently relay with neurons in the bed nucleus of the stria terminalis, hypothalamus and brainstem to access corticotropin releasing hormone neurons. Third, hippocampal, cortical and amygdalar projection pathways show extensive overlap in regions such as the bed nucleus of the stria terminalis, hypothalamus and perhaps brainstem, implying that limbic information may be integrated at subcortical relay sites prior to accessing the PVN. Fourth, these limbic sites also show divergent projections, with the various structures having distinct subcortical targets. Finally, all regions express both glucocorticoid and mineralocorticoid receptors, allowing for glucocorticoid modulation of limbic signaling patterns. Overall, the influence of the limbic system on the HPA axis is likely the end result of the overall patterning of responses to given stimuli and glucocorticoids, with the magnitude of the secretory response determined with respect to the relative contributions of the various structures.

The selfish brain: competition for energy resources

The brain occupies a special hierarchical position in the organism. It is separated from the general circulation by the blood-brain barrier, has high energy consumption and a low energy storage capacity, uses only specific substrates, and it can record information from the peripheral organs and control them. Here we present a new paradigm for the regulation of energy supply within the organism. The brain gives priority to regulating its own adenosine triphosphate (ATP) concentration. In that postulate, the peripheral energy supply is only of secondary importance. The brain has two possibilities to ensure its energy supply: allocation or intake of nutrients. The term 'allocation' refers to the allocation of energy resources between the brain and the periphery. Neocortex and the limbic-hypothalamus-pituitary-adrenal (LHPA) system control the allocation and intake. In order to keep the energy concentrations constant, the following mechanisms are available to the brain: (1) high and low-affinity ATP-sensitive potassium channels measure the ATP concentration in neurons of the neocortex and generate a 'glutamate command' signal. This signal affects the brain ATP concentration by locally (via astrocytes) stimulating glucose uptake across the blood-brain barrier and by systemically (via the LHPA system) inhibiting glucose uptake into the muscular and adipose tissue. (2) High-affinity mineralocorticoid and low-affinity glucocorticoid receptors determine the state of balance, i.e. the setpoint, of the LHPA system. This setpoint can permanently and pathologically be displaced by extreme stress situations (chronic metabolic and psychological stress, traumatization, etc.), by starvation, exercise, infectious diseases, hormones, drugs, substances of abuse, or chemicals disrupting the endocrine system. Disorders in the 'energy on demand' process or the LHPA-system can influence the allocation of energy and in so doing alter the body mass of the organism. In summary, the presented model includes a newly discovered 'principle of balance' of how pairs of high and low-affinity receptors can originate setpoints in biological systems. In this 'Selfish Brain Theory', the neocortex and limbic system play a central role in the pathogenesis of diseases such as anorexia nervosa and obesity.

Reinterpretation of Basal Glucocorticoid Feedback: Implications to Behavioral and Metabolic Disease

A number of metabolic (e.g., abdominal obesity) and psychological (e.g., depression) pathologies commonly present together and have been associated with dysregulation in the hypothalamo-pituitary-adrenal (HPA) axis. Glucocorticoid hormones represent the final product of this classic neuroendocrine axis, and these steroids modulate neuroendocrine, metabolic, and behavioral function. A primary characteristic of the HPA axis is a negative feedback loop, and glucocorticoids act through the brain to inhibit drive to this neuroendocrine system. Slight and chronic perturbations in glucocorticoid levels, below or above normal, throughout the body lead to metabolic (e.g., abdominal obesity) and behavioral (e.g., depression) pathology. Appropriate feedback in the HPA axis is, therefore, critical, and determining how and where glucocorticoids act to impart their feedback effects have been the focus of many laboratories. However, the answer to these questions remain, in part, elusive. In this chapter, I review findings that have led me to reinterpret glucocorticoid feedback in the HPA axis. I propose that, under basal (nonstress) conditions, glucocorticoid feedback is a consequence of the metabolic actions of the adrenal steroid, not a direct effect on brain. This new perspective may provide insight into the etiology of diseases such as major depression and the metabolic syndrome X, and might explain the commonly observed coexistence of affective and metabolic disturbances.

Central mechanisms of stress integration: hierarchical circuitry controlling hypothalamo-pituitary-adrenocortical responsiveness

Appropriate regulatory control of the hypothalamo-pituitary-adrenocortical stress axis is essential to health and survival. The following review documents the principle extrinsic and intrinsic mechanisms responsible for regulating stress-responsive CRH neurons of the hypothalamic paraventricular nucleus, which summate excitatory and inhibitory inputs into a net secretory signal at the pituitary gland. Regions that directly innervate these neurons are primed to relay sensory information, including visceral afferents, nociceptors and circumventricular organs, thereby promoting 'reactive' corticosteroid responses to emergent homeostatic challenges. Indirect inputs from the limbic-associated structures are capable of activating these same cells in the absence of frank physiological challenges; such 'anticipatory' signals regulate glucocorticoid release under conditions in which physical challenges may be predicted, either by innate programs or conditioned stimuli. Importantly, 'anticipatory' circuits are integrated with neural pathways subserving 'reactive' responses at multiple levels. The resultant hierarchical organization of stress-responsive neurocircuitries is capable of comparing information from multiple limbic sources with internally generated and peripherally sensed information, thereby tuning the relative activity of the adrenal cortex. Imbalances among these limbic pathways and homeostatic sensors are likely to underlie hypothalamo-pituitary-adrenocortical dysfunction associated with numerous disease processes.

Selective hippocampal lesions do not increase adrenocortical activity

It has been proposed that the hippocampus exerts a tonic inhibitory influence on the hypothalamic-pituitary-adrenal (HPA) stress axis. This claim rests, in particular, on the upregulation of corticosterone secretion and other measures of HPA activity after nonselective lesions of the hippocampal formation. We measured plasma corticosterone concentrations after selective neurotoxic damage to the hippocampus and the subiculum in rats. Concentrations were estimated during rest in the rat's home cage and at several time points after varying degrees of stress. Lesions of the hippocampus did not increase the concentration of corticosterone relative to control rats in any condition. Temporary inactivation of the hippocampus or the ventral subiculum by infusion of the GABAA receptor agonist muscimol also failed to induce hypersecretion, although hippocampal infusions did impair spatial memory. These results suggest that the hippocampus is not necessary for tonic inhibition of adrenocortical activity and imply that the HPA axis receives efficient negative feedback inhibition from other brain systems too.

Corticosterone infused intracerebroventricularly inhibits energy storage and stimulates the hypothalamo-pituitary axis in adrenalectomized rats drinking sucrose

When allowed to drink sucrose, bilaterally adrenalectomized (ADX) rats exhibit normal weight gain, food intake, sympathetic neural activity, and ACTH compared with sham-ADX rats. Furthermore, ADX rats drinking sucrose have normal corticotropin-releasing factor (CRF) mRNA throughout brain. In ADX rats without sucrose, all of these variables are abnormal. Systemic corticosterone (B) replacement also restores these variables in ADX rats to normal. To test whether B acts centrally, we infused B or saline intracerebroventricularly into ADX rats under basal conditions and after repeated restraint. Rats were exposed to no stress or 3 h/d restraint for 3 d. Body weights and food and fluid intakes were measured. Brains were analyzed using immunocytochemistry against glucocorticoid receptors (GR) and CRF. Intracerebroventricular B blocked the positive effects of sucrose on metabolism, increased basal ACTH concentrations, and augmented ACTH responses to restraint on d 3. B-infused rats exhibited nuclear GR staining in perirhinal cortex, hippocampus, and hypothalamic paraventricular nuclei, showing that infused B spreads effectively. CRF staining in the paraventricular nucleus of the hypothalamus was higher in B- than in saline-infused rats. We conclude that under basal conditions B acts systemically, but not in the brain, to restore metabolism and neuropeptides after adrenalectomy. By contrast, tonic GR occupancy in brain initiates metabolic and ACTH responses characteristic of stress.

Functional subsets of serotonergic neurones: implications for control of the hypothalamic-pituitary-adrenal axis

Serotonergic systems play an important role in the regulation of behavioural, autonomic and endocrine responses to stressful stimuli. This includes modulation of both the hypothalamic-pituitary-adrenal (HPA) axis and the hypothalamic-spinal-adrenal (HSA) axis, which converge at the level of the adrenal cortex to regulate glucocorticoid secretion. Paradoxically, serotonin can either facilitate or inhibit HPA axis activity and stress-related physiological or behavioural responses. A detailed analysis of the brainstem raphé complex and its ascending projections reveals that facilitatory and inhibitory effects of serotonergic systems on glucocorticoid secretion may be due to influences of topographically organized and functionally diverse serotonergic systems. (i) A serotonergic system arising from the middle and caudal dorsal raphé nucleus and projecting to a distributed central autonomic control system and a lateral 'emotional motor system'. Evidence suggests that serotonin can sensitize this subcortical circuit associated with autonomic arousal, anxiety and conditioned fear. (ii) A serotonergic system arising from the median raphé nucleus and projecting extensively and selectively to a ventral subiculum projection system. Evidence suggests that serotonin facilitates this limbic circuit associated with inhibition of ultradian, circadian and stress-induced activity of both the HPA axis and the HSA axis. These new perspectives, based on functional anatomical considerations, provide a hypothetical framework for investigating the role of serotonergic systems in the modulation of ultradian, circadian and stress-induced neuroendocrine function.

Local circuit regulation of paraventricular nucleus stress integration: glutamate-GABA connections

Limbic neurocircuits play a central role in regulation of the hypothalamic-pituitary-adrenocortical (HPA) axis. Limbic influences on adrenocortical hormone secretion are mediated by transynaptic activation or inhibition of hypophysiotrophic neurons in the medial parvocellular paraventricular nucleus (PVN). Projections from the ventral subiculum, prefrontal cortex, medial amygdala, lateral septum, paraventricular thalamus and suprachiasmatic nucleus (SN) terminate in the immediate surround of the PVN, an area heavily populated by GABAergic interneurons. As such, these regions are positioned to modulate paraventricular output via excitation or inhibition of interneuronal projections into the PVN. In addition, the same limbic and diencephalic regions have projections to local PVN-projecting hypothalamic and basal telencephalic nuclei, including the dorsomedial and medial preoptic nuclei and the bed nucleus of the stria terminalis. These regions are involved in both inhibitory and excitatory regulation of the stress axis, indicating that they contain heterogeneous neuronal populations whose relative impact on the PVN is determined by the nature of afferent stimuli. Thus, limbic modulation of the pituitary-adrenocortical system appears to be a multisynaptic process integrated at the level of local PVN-projecting neurocircuits. Local circuits are likely the primary integrators of anticipatory stress responses, and may indeed be the focus of HPA dysfunction seen with aging or affective disease.

Prefrontal cortical regulation of hypothalamic-pituitary-adrenal function in the rat and implications for psychopathology: side matters

In recent years, dysfunction of hypothalamic-pituitary-adrenal (HPA) axis function has been implicated in a wide variety of psychiatric conditions. The importance of this system in responding to and coping with stress is well documented, and the integrity of such systems is of obvious significance to good mental health. The prefrontal cortex (PFC) is also heavily implicated in numerous psychopathological conditions. There is thus a growing interest in the potential role the PFC might play in regulating HPA function, and whether abnormalities of these systems are linked. The present paper reviews a number of recent animal studies which have attempted to elucidate the role of the PFC in regulation of HPA axis function, and how these systems may interact. It is concluded that the PFC is involved both in activating HPA responses to stress and in the negative feedback regulation of this system. Cerebral laterality is an important feature of this regulation, with the right PFC being most directly linked to stress-regulatory systems. On this basis, a number of parallels are drawn to the human literature, where asymmetrical disturbances in PFC activity may help explain associated patterns of HPA dysfunction.

Stress and the aging hippocampus

The "glucocorticoid cascade hypothesis" of hippocampal aging has stimulated a great deal of research into the neuroendocrine aspects of aging and the role of glucocorticoids, in particular. Besides strengthening the methods for investigating the aging brain, this research has revealed that the interactions between glucocorticoids and hippocampal neurons are far more complicated than originally envisioned and involve the participation of neurotransmitter systems, particularly the excitatory amino acids, as well as calcium ions and neurotrophins. New information has provided insights into the role of early experience in determining individual differences in brain and body aging by setting the reactivity of the hypothalamopituitary-adrenal axis and the autonomic nervous system. As a result of this research and advances in neuroscience and the study of aging, we now have a far more sophisticated view of the interactions among genes, early development, and environmental influences, as well as a greater appreciation of events at the cellular and molecular levels which protect neurons, and a greater appreciation of pathways of neuronal damage and destruction. While documenting the ultimate vulnerability of the brain to stressful challenges and to the aging process, the net result of this research has highlighted the resilience of the brain and offered new hope for treatment strategies for promoting the health of the aging brain.

Maintenance of hippocampal cell numbers in young and aged rats submitted to chronic unpredictable stress. Comparison with the effects of corticosterone treatment

Exposure of rats to sustained stress has been associated with behavioural impairments, the degree of impairment being greater with increasing age of the subject. Although the behavioural deficits have been frequently attributed to stress-induced neuronal loss in the hippocampus, the validity of that view may be disputed since it is based on data collected using conventional morphometric methods which are subject to bias. The question of whether stress per se does indeed induce hippocampal cell losses was therefore re-examined using unbiased stereological tools in the present work. Specifically, we used the optical fractionator and the Cavalieri principle, to respectively estimate the total number of neurons and volumes of the main divisions of the hippocampal formation of young and old rats which had been exposed for 1 month to an unpredictable stress paradigm. The efficacy of the treatment was confirmed by elevated serum corticosterone levels measured at various intervals during the experimental period. In order to evaluate whether any deleterious effects might have occurred merely due to the stress-induced elevations in corticosterone secretion, we conducted a parallel study on animals that were injected with corticosterone over a similar duration. Neither stress nor treatment with corticosterone was found to result in significant cell losses in any division of the hippocampal formation; likewise, neither treatment produced significant volumetric differences. Further, these results were not influenced by age of the experimental subjects. The present findings therefore call for a reappraisal of the hypothesis that hippocampal cell loss accounts for the behavioural impairments observed by others following prolonged stress and/or chronic elevation of serum corticosterone levels.

Ventral subiculum regulates hypothalamo-pituitary-adrenocortical and behavioural responses to cognitive stressors

The hippocampus plays an important role in central stress integration. The present study tests the hypothesis that the ventral subiculum, as a principal source of hippocampal efferents, is involved in co-ordination of hypothalamo-pituitary-adrenocortical and behavioural responses to cognitively-processed information. Basal hypothalamo-pituitary-adrenocortical activation appears to be normal in ventral subiculum lesion rats, as basal corticosterone and adrenocorticotropic hormone secretion, anterior pituitary pro-opiomelanocortin and type 1 corticotropin-releasing hormone receptor messenger RNA expression, adrenal and thymus weight, and splenic mitogen activity are not affected by lesion. Lesions of the ventral subiculum induce glucocorticoid hypersecretion following restraint stress or open field exposure, whereas responses to ether inhalation are unaffected. Interestingly, ventral subiculum lesion does not affect fast glucocorticoid negative feedback inhibition of restraint-induced adrenocorticotropic hormone release. Corticotropin-releasing hormone immunoreactivity is increased in the hypothalamic paraventricular nucleus of ventral subiculum lesion rats, and is differentially depleted by acute stress exposure (relative to sham-lesion rats). However, ventral subiculum lesion does not affect basal and stress-induced corticotropin-releasing hormone, arginine vasopressin and cFOS messenger RNA expression in paraventricular nucleus neurons. Behavioural analysis reveals that ventral subiculum lesion rats are hyper-responsive to open field exposure, showing decreased total ambulation and reduced incidence of central square entry. The results suggest that the ventral subiculum plays a specific role in integrating cognitively-processed stimuli (e.g., restraint and open field exposure) into appropriate neuroendocrine and behavioural responses to stress. Enhanced stress-induced glucocorticoid secretion and increased corticotropin-releasing hormone biosynthesis are likely due to removal of oligosynaptic inhibitory input to the paraventricular nucleus subsequent to ventral subiculum lesion.

Dysregulation of diurnal rhythms of serotonin 5-HT2C and corticosteroid receptor gene expression in the hippocampus with food restriction and glucocorticoids

Both serotonergic dysfunction and glucocorticoid hypersecretion are implicated in affective and eating disorders. The adverse effects of serotonergic (5-HT)2C receptor activation on mood and food intake, the antidepressant efficacy of 5-HT2 receptor antagonists, and the hyperphagia observed in 5-HT2C receptor knockout mice all suggest a key role for increased 5-HT2C receptor-mediated neurotransmission. Glucocorticoids, however, downregulate 5-HT2C receptor mRNA in the hippocampus, and it is unclear how increased 5-HT2C receptor sensitivity is achieved in the presence of elevated glucocorticoid levels in depression. Here we show a monophasic diurnal rhythm of 5-HT2C receptor mRNA expression in the rat hippocampus that parallels time-dependent variations in 5-HT2C receptor agonist-induced behaviors in open field tests. Rats entrained to chronic food restriction show marked but intermittent corticosterone hypersecretion and maintain an unaltered 5-HT2C receptor mRNA rhythm. The 5-HT2C receptor mRNA rhythm, however, is suppressed by even modest constant elevations of corticosterone (adrenalectomy + pellet) or with elevated corticosterone during the daytime (8 A.M.), whereas a normal rhythm exists in animals that have the same dose of corticosterone in the evening (6 P.M.). Thus, animals showing even a transient daytime corticosterone nadir exhibit normal hippocampal 5-HT2C receptor mRNA rhythms, even in the presence of overt corticosterone hypersecretion. Chronic food restriction also abolishes the normal diurnal variation in hippocampal glucocorticoid receptor (GR) and mineralocorticoid receptor mRNAs and produces, unusually, both elevated corticosterone and increased GR. The mismatch between elevated glucocorticoids and maintained 5-HT2C receptor and increased GR gene expression in the hippocampus provides a new model to dissect mechanisms that may underlie affective and eating disorders.

Neurocircuitry of stress: central control of the hypothalamo-pituitary-adrenocortical axis

Integration of the hypothalamo-pituitary-adrenal stress response occurs by way of interactions between stress-sensitive brain circuitry and neuroendocrine neurons of the hypothalamic paraventricular nucleus (PVN). Stressors involving an immediate physiologic threat ('systemic' stressors) are relayed directly to the PVN, probably via brainstem catecholaminergic projections. By contrast, stressors requiring interpretation by higher brain structures ('processive' stressors) appear to be channeled through limbic forebrain circuits. Forebrain limbic sites connect with the PVN via interactions with GABA-containing neurons in the bed nucleus of the stria terminalis, preoptic area and hypothalamus. Thus, final elaboration of processive stress responses is likely to involve modulation of PVN GABAergic tone. The functional and neuroanatomical data obtained suggest that disease processes involving inappropriate stress control involve dysfunction of processive stress pathways.

Contribution of the ventral subiculum to inhibitory regulation of the hypothalamo-pituitary-adrenocortical axis

Anatomical studies indicate that the ventral subiculum is in a prime position to mediate hippocampal inhibition of the hypothalamo-pituitary-adrenocortical (HPA) axis. The present study evaluated this hypothesis by assessing HPA function following ibotenic acid lesion of the ventral subiculum region. Rats with lesions of the ventral subiculum (vSUB) or ventral hippocampus (vHIPPO) did not show changes in basal corticosterone (CORT) secretion at either circadian peak or nadir time points when compared to sham-lesion rats (SHAM) or unoperated controls. However, rats with vSUB lesions exhibited a prolonged glucocorticoid stress response relative to all other groups. Baseline CRH mRNA levels were significantly increased in the medial parvocellular paraventricular nucleus (PVN) of the vSUB group relative to controls. CRH mRNA differences were particularly pronounced at caudal levels of the nucleus, suggesting topographic organization of vSUB interactions with PVN neurons. Notably, the vHIPPO group, which received large lesions of ventral CA1, CA3 and dentate gyrus without significant subicular damage, showed no change in stress-induced CORT secretion, suggesting that the ventral subiculum proper is principally responsible for ventral hippocampal actions on the HPA stress response. No differences in medial parvocellular PVN AVP mRNA expression were seen in either the vSUB or vHIPPO groups. The results indicate a specific inhibitory action of the ventral subiculum on HPA activation. The increase in CRH biosynthesis and stress-induced CORT secretion in the absence of changes in baseline CORT secretion or AVP mRNA expression suggests that the inhibitory actions of ventral subicular neurons affect the response capacity of the HPA axis.