Neural basis for fasting activation of the hypothalamic-pituitary-adrenal axis.

Fasting initiates a multitude of adaptations to allow survival. Activation of the hypothalamic-pituitary-adrenal (HPA) axis and subsequent release of glucocorticoid hormones is a key response that mobilizes fuel stores to meet energy demands1-5. Despite the importance of the HPA axis response, the neural mechanisms that drive its activation during energy deficit are unknown. Here, we show that fasting-activated hypothalamic agouti-related peptide (AgRP)-expressing neurons trigger and are essential for fasting-induced HPA axis activation. AgRP neurons do so through projections to the paraventricular hypothalamus (PVH), where, in a mechanism not previously described for AgRP neurons, they presynaptically inhibit the terminals of tonically active GABAergic afferents from the bed nucleus of the stria terminalis (BNST) that otherwise restrain activity of corticotrophin-releasing hormone (CRH)-expressing neurons. This disinhibition of PVHCrh neurons requires γ-aminobutyric acid (GABA)/GABA-B receptor signalling and potently activates the HPA axis. Notably, stimulation of the HPA axis by AgRP neurons is independent of their induction of hunger, showing that these canonical 'hunger neurons' drive many distinctly different adaptations to the fasted state. Together, our findings identify the neural basis for fasting-induced HPA axis activation and uncover a unique means by which AgRP neurons activate downstream neurons: through presynaptic inhibition of GABAergic afferents. Given the potency of this disinhibition of tonically active BNST afferents, other activators of the HPA axis, such as psychological stress, may also work by reducing BNST inhibitory tone onto PVHCrh neurons.

Deafferentation of the hypothalamic paraventricular nucleus (PVN) exaggerates the sympathoadrenal system activity in stressed rats.

OBJECTIVE: The hypothalamic paraventricular nucleus is a key structure in the regulation of the autonomic and neuroendocrine systems response to acute and chronic stress challenges. In this study, we examined the effect of a mechanical posterolateral deafferentation of the PVN on the activity of sympathoadrenal system (SAS) and hypothalamo-pituitary-adrenal (HPA) axis by measuring plasma concentrations of epinephrine (EPI), norepinephrine (NE), and corticosterone (CORT) in rats exposed to acute immobilization (IMO) stress. METHODS: The surgical posterolateral deafferentation of the PVN (PVN-deaf) was performed by Halasz knife, in brain of the adult male Sprague Dawley rats, according to coordinates of a stereotaxic atlas. Sham-operated (SHAM) animals underwent a craniotomy only. The animals were allowed to recover 14 days. Thereafter, the tail artery was cannulated and the animals exposed to acute IMO for 2 h. The blood samples were collected via cannula at the time points of 0, 5, 30, 60, and 120 min of the IMO. Concentrations of plasma EPI, NE, and CORT were determined by radioimmunoassay. RESULTS: The IMO-induced elevation of plasma EPI concentrations in the PVN-deaf rats reached statistical significance at 60 min of the IMO, when compared to SHAM rats. Similarly, the stress-induced elevation of the NE plasma levels in the PVN-deaf rats was significantly exaggerated at all time intervals of IMO in comparison with SHAM rats, whereas plasma CORT levels were significantly reduced. CONCLUSIONS: In contrast to the traditional view of excitatory role of the PVN in response to stress, our data indicate that some projections from the PVN to caudally localized hypothalamic structures, the brainstem or the spinal cord, exert inhibitory effect on the SAS system activity during acute IMO stress. The data indicate that stress-induced activation of the HPA axis is partially dependent on inputs from the brainstem to the PVN.

The role of DNA methylation in stress-related psychiatric disorders.

Epigenetic modifications in response to traumatic experience and stress are emerging as important factors in the long-term biological trajectories leading to stress-related psychiatric disorders, reflecting both environmental influences as well as individual genetic predisposition. In particular, recent evidence on DNA methylation changes within distinct genes and pathways but also on a genome-wide level provides new insights into the pathophysiology of stress related psychiatric disorders. This review summarizes current findings and concepts on DNA methylation changes in stress-related disorders with a focus on major depressive disorder and posttraumatic stress disorder (PTSD). We highlight studies of DNA methylation in animals and humans pertinent to these disorders, both focusing on candidate loci as well as genome-wide studies. We describe molecular mechanisms of how exposure to stress can induce long lasting changes in DNA methylation and how these may relate to the pathophysiology of depression and PTSD. We discuss data suggesting that DNA methylation, even in peripheral tissues, appears to be an informative reflection of environmental exposures on the genome and may have potential as a biomarker for the early prevention of stress-related disorders.

Acupuncture blocks cold stress-induced increases in the hypothalamus-pituitary-adrenal axis in the rat.

Electroacupuncture (EA) is used to treat chronic stress; however, its mechanism(s) of action in allaying stress remains unclear. The interplay of stress hormones of the hypothalamus-pituitary-adrenal axis (HPA) and the sympathetic nervous system (SNS) is critical in the stress response. Our objective was to determine whether EA at acupoint, stomach 36 (EA St36) is effective in preventing chronic cold stress-induced increased hormone levels in the rat by examining four groups of animals, three of which were exposed to cold and one of which was a non-treatment control group. Before exposure to the cold, two groups were treated with either EA St36, or Sham-EA, before 10 days of cold stress. The EA St36 animals demonstrated a significant decrease in peripheral HP hormones (ACTH and CORT) compared with stress animals (P<0.05). These effects were specific; rats receiving Sham-EA had elevation of these hormones, similar to the stress-only animals. These effects were mirrored centrally in the brain; CRH levels were significantly (P<0.05) reduced in EA St36 animals compared with the other animals. Finally, EA effect on peripheral and adrenal SNS hormones (norepinephrine (NE) and neuropeptide Y (NPY) respectively) was examined, with no significant difference noted in adrenal tyrosine hydroxylase or circulating NE in any of the groups. However, EA St36 was effective in preventing stress-induced elevation is adrenal Npy mRNA. These results indicate that EA St36 blocks the chronic stress-induced elevations in the HPA and the sympathetic NPY pathway, which may be a mechanism for its specific stress-allaying effects.

C1 neurons excite locus coeruleus and A5 noradrenergic neurons along with sympathetic outflow in rats.

C1 neurons activate sympathetic tone and stimulate the hypothalamic­pituitary­adrenal axis in circumstances such as pain, hypoxia or hypotension. They also innervate pontine noradrenergic cell groups, including the locus coeruleus (LC) and A5. Activation of C1 neurons reportedly inhibits LC neurons; however, because these neurons are glutamatergic and have excitatory effects elsewhere, we re-examined the effect of C1 activation on pontine noradrenergic neurons (LC and A5) using a more selective method. Using a lentivirus that expresses channelrhodopsin2 (ChR2) under the control of the artificial promoter PRSx8, we restricted ChR2 expression to C1 neurons (67%), retrotrapezoid nucleus neurons (20%) and cholinergic neurons (13%). The LC contained ChR2-positive terminals that formed asymmetric synapses and were immunoreactive for vesicular glutamate transporter type 2. Low-frequency photostimulation of ChR2-expressing neurons activated LC (38 of 65; 58%) and A5 neurons (11 of 16; 69%) and sympathetic nerve discharge. Locus coeruleus and A5 inhibition was not seen unless preceded by excitation. Locus coeruleus activation was eliminated by intracerebroventricular kynurenic acid. Stimulation of ChR2-expressing neurons at 20 Hz produced modest increases in LC and A5 neuronal discharge. In additional rats, the retrotrapezoid nucleus region was destroyed with substance P­saporin prior to lentivirus injection into the rostral ventrolateral medulla, increasing the proportion of C1 ChR2-expressing neurons (83%). Photostimulation in these rats activated the same proportion of LC and A5 neurons as in control rats but produced no effect on sympathetic nerve discharge owing to the destruction of bulbospinal C1 neurons. In conclusion, low-frequency stimulation of C1 neurons activates pontine noradrenergic neurons and sympathetic nerve discharge, possibly via the release of glutamate from monosynaptic C1 inputs.

Orexins/hypocretins act in the posterior paraventricular thalamic nucleus during repeated stress to regulate facilitation to novel stress.

Orexins/hypocretins heavily innervate the posterior division of the paraventricular nucleus of the thalamus (pPVT), which expresses both orexin receptor types. The pPVT is important for adaptations to repeated stress, particularly the ability to facilitate to novel stress after repeated stress exposure. Here, we examined how orexins acting in the pPVT regulate facilitation of hypothalamic-pituitary-adrenal (HPA) responses to novel restraint after 4 d of repeated swim stress. Blockade of orexin receptors in the pPVT with SB334867 before novel restraint did not change the facilitated HPA response. However, blockade of orexin receptors before each of four daily swim exposures prevented the facilitated ACTH and facilitated hypothalamic c-Fos response to restraint as well as the repeated swim stress-induced increase in CRH mRNA in the paraventricular hypothalamus. These results suggest that orexin actions in the pPVT during the 4 d of swim, but not during restraint, are necessary for the facilitated HPA response to heterotypic restraint. Exposure to the fourth swim produced a shift in orexin1 receptors from membrane to cytosolic fractions. OrexinA also changed the firing patterns of pPVT cells to be more responsive in repeatedly swim stressed rats compared with nonstressed rats. Together, the results suggest that orexin actions in the pPVT, mediated by orexin1 receptors, are important for the ability to adapt to repeated stress.

The hypothalamic-pituitary-adrenal-immune axis. A critical assessment.

The hypothalamic-pituitary-adrenal (HPA) system has been a model for neuroendocrine control of responses of organisms to stressors since the turn of the century. Despite this, the pathways by which infectious insults interact with the HPA system remained poorly defined. Recently, evidence has been presented suggesting that humoral mediators released by inflammatory cells (cytokines) may participate in two-way communication between the site of inflammation and the central nervous system. In this review, we detail the current understanding of the responses of the HPA system to the classic physiologic stimuli of hypovolemia and pain, with an emphasis on the cellular mechanisms and mediators discovered in recent years. We also examine the data substantiating a role of interleukin 1, interleukin 6, and tumor necrosis factor in the direct humoral activation of the HPA system and consider the evidence favoring a physiologic negative feedback relationship between the HPA and the immune systems. Such as interaction is an exciting concept with broad clinical implications. However, we believe that the temporal and quantitative aspects of experiments designed to evaluate this interaction must be carefully evaluated to assure that true physiologic stimuli are studied and that the responses observed are not due to pharmacologic effects of inflammatory mediators acting through "classic" neuroendocrine pathways.

Interactions between hippocampal serotonin and the pituitary-adrenal axis in the septal driving of hippocampal theta-rhythm.

Stimulation of the septal area at a frequency between 6 and 10 Hz is able to drive hippocampal theta. In freely moving male rats, the minimum threshold current for driving theta occurs at 7.7 Hz. Disruption of the pituitary-adrenal axis by injection of corticosterone to normal rats or by bilateral adrenalectomy (ADX) causes a shift of the minimum theta-driving threshold to 6.9 Hz. Corticosterone injection to ADX rats returns the minimum to 7.7 Hz. Specific and localized removal of hippocampal serotonergic fibers by intracerebral injections of 5,7-dihydroxytryptamine (5,7-DHT) produces the same shift to a 6.9-Hz minimum threshold as does corticosterone or ADX. We further report that these effects of manipulating the adrenocortical and serotonergic systems act through related (or common) mechanisms since: (1) 5,7-DHT shift to 6.9 Hz can be reversed to 7.7 Hz by injection of corticosterone; (2) 5,7-DHT lesions in an ADX rat produce a normal theta threshold minimum at 7.7 Hz, and (3) in a combined ADX- and 5,7-DHT-lesioned rat, corticosterone again produces a 6.9-Hz minimum as this hormone does in normal rats. These results suggest that the serotonergic inputs to the hippocampus interact with the same neurons which concentrate corticosterone.

In vitro neurophysiology of identified rat hypothalamic 'neuroendocrine' neurons.

Stable synaptic and antidromic electrophysiological recordings were obtained for 10-15 h from identified neurohypophyseal and tuberoinfundibular neurons using an acute explant of rat hypothalamus maintained in vitro through perfusion of the carotid artery with oxygenated artificial media. Synaptic potentials from preoptic and median eminence stimulation were blocked during perfusion with 12 m M Mg++-containing solutions. Antidromic and spontaneous action potentials recorded in neurohypophyseal neurons were followed by a transient after-hyperpolarization of 3-10 mV, due primarily to a brief increase in potassium conductance. Evidence favoring the existence of a synaptic recurrent inhibitory pathway was present in records obtained from tuberoinfundibular neurons, but not from neurohypophyseal cells.