Hypothalamic afferent connections mediating adrenocortical responses that follow hippocampal stimulation

A common substrate for prefrontal and hippocampal inhibition of the neuroendocrine stress response

A network of interconnected limbic forebrain cell groups, including the medial prefrontal cortex (mPFC) and hippocampal formation (HF), is known to shape adaptive responses to emotionally stressful experiences, including output of the hypothalamo-pituitary-adrenal (HPA) axis. While disruption of limbic HPA-inhibitory systems is implicated in stress-related psychiatric and systemic illnesses, progress in the field has been hampered by a lack of a systems-level understanding of the organization that provides for this regulation. Using rats, we first localized cell groups afferent to the paraventricular hypothalamic nucleus (PVH) (the initiator of HPA responses to stress) whose engagement following acute (30 min) restraint was diminished by excitotoxin lesions of the ventral subiculum, a component of the HF. This identified a candidate relay for imparting HF influences in a circumscribed portion of the anterior bed nucleus of the stria terminalis (aBST), which we previously identified as a GABAergic relay subserving mPFC inhibition of the stress axis. Anatomical tracing experiments then indicated that extrinsic projections from HF and mPFC converge onto regions of aBST that contain neurons that are both stress sensitive and PVH projecting. Two final experiments provided evidence that (1) HPA-inhibitory influences of mPFC and HF are additive and (2) aBST plays a more prominent inhibitory role than ventral subiculum over stress-induced HPA endpoints. These findings support the view that stress-inhibitory influences of mPFC and HF are exerted principally via convergence onto a common relay, as opposed to a serial, parallel, or more complex multisynaptic network.

Decreased anxiety-like behavior in beta3 nicotinic receptor subunit knockout mice

Nicotine, via a family of nicotinic acetylcholine receptors, elicits many physiological responses, including alterations in anxiety. Studies suggest that the effects of nicotine on anxiety may support smoking behaviors. We reported previously that mice lacking the beta3 nicotinic receptor subunit demonstrate increased activity in the open field arena. Open field activity has been shown to be a composite of anxiety and locomotor activity, behaviors that are both altered by nicotine. We therefore sought to differentiate the role(s) of beta3-containing receptors in anxiety and locomotor activity. Anxiety behaviors were examined in the elevated plus maze, the black/white box and the mirrored chamber. Beta3 null mutant mice demonstrated decreased anxiety with more time spent on the open arm of the elevated plus maze than their wildtype littermates. No significant differences were observed with the black/white box or the mirrored chamber. Levels of the stress hormone, corticosterone, were significantly higher in the beta3 null mutant mice at baseline and following exposure to stress. Increased locomotor activity in the Y-maze was also observed for the beta3 null mutant mice, but only following exposure to stress. These findings strongly suggest that beta3-containing nicotinic receptors influence anxiety and may be critical for the continuation of smoking behaviors.

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.

Serotonin and the neuroendocrine regulation of the hypothalamic--pituitary-adrenal axis in health and disease

Serotonin (5-hydroxytryptamine, 5-HT)-containing neurons in the midbrain directly innervate corticotropin-releasing hormone (CRH)-containing cells located in paraventricular nucleus of the hypothalamus. Serotonergic inputs into the paraventricular nucleus mediate the release of CRH, leading to the release of adrenocorticotropin, which triggers glucocorticoid secretion from the adrenal cortex. 5-HT1A and 5-HT2A receptors are the main receptors mediating the serotonergic stimulation of the hypothalamic-pituitary-adrenal axis. In turn, both CRH and glucocorticoids have multiple and complex effects on the serotonergic neurons. Therefore, these two systems are interwoven and communicate closely. The intimate relationship between serotonin and the hypothalamic-pituitary-adrenal axis is of great importance in normal physiology such as circadian rhythm and stress, as well as pathophysiological disorders such as depression, anxiety, eating disorders, and chronic fatigue.

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.

Electrical stimulation of the dorsal hippocampus caused a long lasting inhibition of ACTH and adrenocortical responses to photic stimuli in freely moving rats

The effect of a single train of electrical hippocampal stimulation on ACTH and corticosterone (CS) responses to subsequent photic stimulation was studied in freely moving male rats. The hippocampal stimulation inhibited the stress-induced rise [corrected] in serum CS levels up to 150 h when compared to sham stimulated animals. This effect did not exist at 300 h following stimulation. This sustained hippocampal inhibitory effect on the adrenocortical response, which was not reported previously, was partially abolished by section of the dorsal fornix. The present data demonstrate that dorsal hippocampal stimulation has a long lasting inhibitory effect on pituitary adrenocortical secretion following neural stimuli and this is partially mediated by the dorsal fornix.

The role of limbic structures in the modulation of ACTH responses following adrenalectomy

The role of the HIPP, DF, and the lateral SPT as well as of the central AMG nucleus in ACTH hypersecretion following Adex, was studied in male rats. In animals with bilateral dorsal hippocampectomy, DF section, or SPT lesions there was a much greater increase in ACTH hypersecretion when compared to Adex alone. Implants of CS in the paraventricular nucleus of the hypothalamus prevented the rise in serum ACTH following Adex, and this effect was reversed by hippocampectomy. Bilateral lesions in the AMG prevented the Adex-induced rise of serum ACTH. These results indicate that the dorsal HIPP and its efferent pathways to the hypothalamus have normally an inhibitory effect on Adex-induced ACTH secretion. Their removal permits an elevated ACTH hypersecretion as well as attenuation of the CS feedback effect. The central AMG nucleus, which has a facilitatory effect on the hypothalamopituitary-adrenocortical axis, can also modulate ACTH secretion following Adex.

Limbic pathways and hypothalamic neurotransmitters mediating adrenocortical responses to neural stimuli

One of the major phenomena related to the stress response is the activation of the hypothalamo-pituitary-adrenocortical (HPA) axis. This axis consists of corticotropin releasing factor-41 in the paraventricular nucleus of the hypothalamus (PVN), which in response to a variety of stimuli is released into the portal circulation and stimulates pituitary ACTH secretion and subsequently adrenocortical discharge. The mechanisms involved in the activation are not uniform and the responses to various stimuli are mediated by different neural pathways. Since extrahypothalamic limbic structures play a significant role in the HPA function, it is the purpose of this review to describe the neural pathways between the hippocampus, septum and amygdala and the hypothalamus in relation to adrenocortical activity and the differential role of the medial forebrain bundle as well as the effects of various hypothalamic deafferentation on the transmission of the neural impulses to the hypothalamus. Also, the importance of norepinephrine and serotonin in the activation of the HPA axis will be delineated.

Decreased hippocampal noradrenaline does not affect corticosterone release following electrical stimulation of CA1 pyramidal cells

Bipolar electrodes were implanted into the CA1 pyramidal cells of the dorsal hippocampus and the effect of electrical stimulation of these cells on corticosterone secretion was investigated in freely moving rats. Histology showed that the electrodes were positioned in close proximity to the CA1 pyramidal cells. Rats that were subjected to high intensity electrical stimulation (1, 10, and 100 microA) behaved differently when compared to their sham stimulated controls. They were more active and displayed wet dog shakes. Plasma corticosterone levels increased dose-dependently in rats subjected to different electrical stimulation intensities. Although prior treatment (24 hours) of rats with DSP4 (60 mg/kg, i.p.) significantly reduced hippocampal noradrenaline content by 46%, it did not bring about any behavioural changes. DSP4 treatment also had no effect on electrically stimulated corticosterone release. These data suggested that stimulation of CA1 pyramidal cells may lead to increased corticosterone release and that a decrease in hippocampal noradrenaline concentration was unable to alter this corticosterone response.

The dorsal hippocampus modifies the negative feedback effect of glucocorticoids on the adrenocortical and median eminence CRF-41 responses to photic stimulation

In the present study we have evaluated the role of the dorsal hippocampus on the negative feedback effect of glucocorticoids (GC) following photic stimulation. In hippocampectomized rats the recovery of serum corticosterone (CS) to basal levels following photic stimulation, was significantly attenuated in relation to sham hippocampectomized rats. The inhibitory effect of either systemic dexamethasone administration or CS implanted in the paraventricular nucleus (PVN), on the adrenocortical responses to photic stimulation, was completely prevented in hippocampectomized rats in comparison to sham operated animals. In rats with sham operation, the depletion of median eminence CRF-41 induced by photic stimulation, was prevented by pretreatment with CS PVN implants or systemic dexamethasone. These effects were reversed in rats with dorsal hippocampectomy. The results suggest that the dorsal hippocampus modulates the negative feedback of GC on the adrenocortical response following photic stimulation at the PVN level and this effect is mediated by median eminence CRF-41.

Medial posterior hypothalamic input is involved in adrenocortical activation following forebrain limbic stimulation

Previous experiments have demonstrated that posterior hypothalamic deafferentation, which involved also the medial forebrain bundle, has prevented the rise in serum corticosterone following limbic stimuli. Consequently, the effects of a small medial posterior hypothalamic deafferentation, excluding the medial forebrain bundle, on corticosterone responses following electrical stimulation of the hippocampus, amygdala, septum and reticular formation in the rat were studied. Posterior hypothalamic deafferentation did not change basal corticosterone levels but significantly inhibited the adrenocortical responses following stimulation of the above structures when compared to intact or sham-stimulated rats. Posterior hypothalamic deafferentation did not affect median eminence corticotropin releasing factor-41 content. It is concluded that a medial posterior hypothalamic input is involved in adrenocortical activation following limbic stimulation.

Adrenal responses and neurotransmitters in posterior hypothalamic deafferentation

The effects of a small posterior hypothalamic deafferentation (PHD) on adrenocortical responses to peripheral neural stimuli were investigated in rats. PHD inhibited the rise in plasma corticosterone (CS) following photic and acoustic stimulation, but did not affect the adrenocortical response following sciatic nerve stimulation. PHD did not change the content of norepinephrine in the paraventricular nucleus of the hypothalamus, however, it reduced the serotonin content by about 30%. The possible role of serotonin or of another tonic caudal input into the hypothalamus for the activation of the pituitary-adrenocortical axis, following certain neural stimuli, is discussed.

Catecholaminergic projections to tuberoinfundibular neurones of the paraventricular nucleus: I. Effects of stimulation of A1, A2, A6 and C2 cell groups

Extracellular electrical activity was recorded from 203 paraventricular nucleus (PVN) neurones antidromically identified as projecting to the median eminence. Spontaneous activity and the effects of stimulation of the A1, A2, A6 and C2 catecholaminergic cell groups upon the PVN neurones were examined. Cells were located at a mean height 2.29 +/- 0.03 mm above the base of the brain, corresponding with the corticotropin-releasing factor (CRF) rich component of the nucleus. The mean firing rate was 3.2 +/- 0.3 Hz and antidromic invasion latency was 9.9 +/- 0.3 msec. Seventy-six % of cells tested were activated by painful somatosensory stimuli. Electrical stimulation of the A1 or A2 region evoked excitatory responses from the majority of cells tested (76% and 85%, respectively), whilst stimulation of the A6 and C2 regions evoked more inhibitory responses (43% and 59%, respectively). Most responses (56%), whether excitatory or inhibitory, were not clearly defined in terms of latency, and were only observed following delivery of 5-10 single shocks at 0.5 Hz. Excitation recorded following A1 and A2 stimulation suggests a facilitatory role for noradrenaline in the regulation of PVN activity. Inhibitory responses following C2 stimulation indicate that adrenaline may serve to inhibit such activity, whilst the more mixed responses following A6 stimulation suggest that the projections of this region differ in some way from those of the A1 and A2 cells. Response reversals were observed, after delivery of higher frequency stimulation, for a substantial proportion (20%) of the cells tested.

Hypothalamic norepinephrine mediates limbic effects on adrenocortical secretion

The purpose of this study was to elucidate the role of norepinephrine (NE) in the mediation of adrenocortical responses following limbic stimuli. The effects of stimulation of the dorsal and ventral hippocampus and the midbrain reticular formation on the plasma corticosterone (CS) levels was studied in rats with vehicle or 6-hydroxydopamine (6-OHDA) injected bilaterally into the paraventricular nucleus of the hypothalamus (PVN). The injection of 6-OHDA caused a very significant reduction in the concentration of PVN NE and blocked the rise in plasma CS following the stimulation of the above three limbic structures. The basal CS level and the response to ether stress were not affected. The present study supports previous observations on the stimulatory role of NE on CS secretion and that the modulatory effects of extrahypothalamic limbic structures on the adrenocortical activity depend on the presence of NE in the PVN.