Daily limited access to sweetened drink attenuates hypothalamic-pituitary-adrenocortical axis stress responses.

Stress can promote palatable food intake, and consumption of palatable foods may dampen psychological and physiological responses to stress. Here we develop a rat model of daily limited sweetened drink intake to further examine the linkage between consumption of preferred foods and hypothalamic-pituitary-adrenocortical axis responses to acute and chronic stress. Adult male rats with free access to water were given additional twice-daily access to 4 ml sucrose (30%), saccharin (0.1%; a noncaloric sweetener), or water. After 14 d of training, rats readily learned to drink sucrose and saccharin solutions. Half the rats were then given chronic variable stress (CVS) for 14 d immediately after each drink exposure; the remaining rats (nonhandled controls) consumed their appropriate drinking solution at the same time. On the morning after CVS, responses to a novel restraint stress were assessed in all rats. Multiple indices of chronic stress adaptation were effectively altered by CVS. Sucrose consumption decreased the plasma corticosterone response to restraint stress in CVS rats and nonhandled controls; these reductions were less pronounced in rats drinking saccharin. Sucrose or saccharin consumption decreased CRH mRNA expression in the paraventricular nucleus of the hypothalamus. Moreover, sucrose attenuated restraint-induced c-fos mRNA expression in the basolateral amygdala, infralimbic cortex, and claustrum. These data suggest that limited consumption of sweetened drink attenuates hypothalamic-pituitary-adrenocortical axis stress responses, and calories contribute but are not necessary for this effect. Collectively the results support the hypothesis that the intake of palatable substances represents an endogenous mechanism to dampen physiological stress responses.

Regulation of forebrain GABAergic stress circuits following lesion of the ventral subiculum.

Ventral subiculum (vSUB) lesions enhance corticosterone responses to psychogenic stressors via trans-synaptic influences on paraventricular nucleus (PVN) neurons. Synaptic relays likely occur in GABA-rich regions interconnecting the vSUB and PVN. The current study examines whether vSUB lesions compromise stress-induced c-fos induction and GABA biosynthetic capacity in putative limbic-hypothalamic stress relays. Male Sprague-Dawley rats received bilateral ibotenate or sham lesions of the vSUB. Animals were divided into two groups, with one group receiving exposure to novelty stress and the other left unstressed. Exposure to novelty stress increased c-fos mRNA expression in the PVN to a greater degree in vSUB lesion relative to shams, consistent with an inhibitory role for the vSUB in the HPA stress response. However, c-fos induction was not affected in other forebrain GABAergic stress pathways, such as the lateral septum, medial preoptic area or dorsomedial hypothalamus. vSUB lesions increased GAD65 or GAD67 mRNA levels in several efferent targets, including anterior and posterior subnuclei of the bed nucleus of the stria terminalis and lateral septum. Lesions did not effect stress-induced increases in GAD65 expression in principal output nuclei of the amygdala. The current data suggest that loss of vSUB innervations produces a compensatory increase in GAD expression in subcortical targets; however, this up-regulation is insufficient to block lesion-induced stress hyperresponsiveness, perhaps driven by amygdalar disinhibition of the PVN.

Role of the ventral subiculum in stress integration.

The mammalian subiculum plays a prominent role in inhibition of the hypothalamo-pituitary-adrenocortical (HPA) axis. Lesion and stimulation studies indicate that the hippocampus, acting via output neurons of the ventral subiculum, acts to attenuate stress-induced glucocorticoid release. Lesions of the ventral subiculum enhance glucocorticoid secretion following psychogenic, but not systemic stressors, indicating that the influence of this structure on the HPA system is stressor-specific. Anatomical analyses fail to demonstrate direct interactions of the subiculum with principal stress-effector neurons in the paraventricular hypothalamus, consistent with a trans-synaptic mechanism of action. Accordingly, tracing data indicate that glutamatergic ventral subiculum neurons innervate GABAergic neurons in several paraventricular nucleus-projecting neurons in the hypothalamus and basal forebrain, suggesting that inhibition is mediated by glutamate-GABA relays. The subiculum also innervates several limbic forebrain structures that in turn have bisynaptic projections to paraventricular neurons, such as the prefrontal cortex, amygdala and lateral septum, suggesting that the subiculum may have a generalized up-stream influence on limbic stress integration. Finally, recent information suggests that the subiculum may also be stress excitatory under some circumstances, and that there may be substantial strain or individual differences in the net contribution of the subiculum, to stress integration. Overall, the present state of knowledge indicates that the role of the subiculum in stress integration is complex, and likely involves interactions of stress-relevant subicular output with limbic-hypothalamic stress-integrative circuits.

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.

Activity-dependent modulation of neurotransmitter innervation to vasopressin neurons of the supraoptic nucleus.

Confocal microscopy was used to assess activity-dependent neuroplasticity in neurotransmitter innervation of vasopressin immunoreactive magnocellular neurons in the supraoptic nucleus (SON). Vesicular glutamate transporter 2, glutamic acid decarboxylase, and dopamine beta-hydroxylase (DBH) synaptic boutons were visualized in apposition to vasopressin neurons in the SON. A decrease in DBH synaptic boutons per cell was seen upon salt loading, indicating diminished noradrenergic/adrenergic innervation. Loss of DBH appositions to vasopressin neurons was associated with a general loss of DBH immunoreactivity in the SON. In contrast, the number of vesicular glutamate transporter 2 synaptic boutons per neuron increased with salt loading, consistent with increased glutamatergic drive of magnocellular SON neurons. Salt loading also caused an increase in the total number of glutamic acid decarboxylase synaptic boutons on vasopressinergic neurons, suggesting enhanced inhibitory innervation as well. These studies indicate that synaptic plasticity compensates for increased secretory demand and may indeed underlie increased secretion, perhaps via neurotransmitter-specific, activity-related changes in synaptic contacts on vasopressinergic magnocellular neurons in the SON.

Role of GABA and glutamate circuitry in hypothalamo-pituitary-adrenocortical stress integration.

GABA and glutamate play a major role in central integration of hypothalamo-pituitary-adrenocortical (HPA) stress responses. Recent work in our group has focused on mechanisms whereby GABAergic and glutamatergic circuits interact with parvocellular paraventricular nucleus (PVN) neurons controlling the HPA axis. GABAergic neurons in the bed nucleus of the stria terminalis, preoptic area, and hypothalamus can directly inhibit PVN outflow and thereby reduce ACTH secretion. In contrast, glutamate activates the HPA axis, presumably by way of hypothalamic and brainstem projections to the PVN. These inhibitory and excitatory PVN-projecting neurons are controlled by descending information from limbic forebrain structures, including glutamatergic neurons of the ventral subiculum, prefrontal cortex, and GABAergic cells from the amygdala and perhaps septum. Lesion studies indicate that the ventral subiculum and prefrontal cortex are involved in inhibition of HPA axis responses to psychogenic stimuli, whereas the amygdala is positioned to enhance hormone secretion by way of GABA-GABA disinhibitory connections. Thus, it seems the psychogenic responses to stress are gated by discrete sets of GABAergic neurons in the basal forebrain and hypothalamus. As such, these neurons are positioned to summate limbic inputs into net inhibitory tone on the PVN and may thus play a major role in HPA dysfunction seen in affective disease states and aging.

Stressor-selective role of the ventral subiculum in regulation of neuroendocrine stress responses.

The ventral subiculum (vSUB) confers inhibitory effects of the hippocampus on hypothalamo-pituitary-adrenocortical (HPA) axis responses to novelty and restraint. The current study was designed to evaluate the role of the vSUB in regulating HPA axis responses to stressors of diverse modalities. Male Sprague Dawley rats received bilateral ibotenic acid or saline injections into the region of the vSUB. Corticosterone secretion was assessed after exposure to hypoxia and elevated plus maze, with the two stress exposures occurring 5 d apart. Peak corticosterone responses to hypoxia were reduced in vSUB-lesion animals, indicating an attenuation of HPA axis responsiveness. A subsequent study revealed that hyporesponsivity to hypoxia was evident in chamber-naive as well as chamber-adapted animals, verifying that this effect was independent of previous experience in the testing environment. In contrast, the effects of vSUB lesions on corticosterone responses to the elevated plus maze exposure were substantially more circumspect, being limited to a slight increase in secretion at the 2-h poststress time point. The limited vSUB lesion-induced increase in the plasma corticosterone response to elevated plus maze exposure occurred despite an increased open-arm time in the maze, suggesting that lesions reduced anxiety-like behavior. In combination with previous studies, these data suggest that the vSUB has excitatory as well as inhibitory input into HPA axis responsivity, depending on the nature of the stressful stimulus, and suggest that behavioral and neuroendocrine responses to stressful or anxiogenic stimuli may be dissociable.

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.