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.     

Neural pathways that mediate the effects of afferent stimuli on paraventricular nucleus multiunit activity in freely moving rats

The direct involvement of the hypothalamic paraventricular nucleus (PVN) in the control of adrenocortical secretion is now generally accepted. In order to contribute to our understanding of the electrical activity of cells in this region during adrenocortical activation, we have recorded multiunit electrical activity (MUA) in response to acute neural stimuli in freely moving male rats and have examined the pathways involved. Photic, acoustic, olfactory, and sciatic nerve stimulation all increased PVN MUA by between 130% and 250%. These responses were selectively blocked, according to the stimulus modality tested, by radiofrequency lesions of central neural structures. Thus PVN responses to photic stimulation were blocked by lesions of the suprachiasmatic nuclei and reduced by mammillary peduncle lesions but were unaffected by lesions of the bed nuclei of the stria terminalis. Responses to acoustic stimulation were blocked by lesions of the mammillary peduncles but not by those placed in the suprachiasmatic nuclei, the septum, or the bed nuclei of the stria terminalis. Lesions of the septum blocked the response to sciatic nerve stimulation but did not affect the response to olfactory stimulation with amyl acetate fumes, which was blocked by lesions of the bed nuclei of the stria terminalis. The data confirm those obtained in endocrine studies concerning the neural pathways involved in the transmission of neural stimuli that produce adrenocortical activation.     

Changes in single cell responsiveness in the hypothalamus in cats following cortisol administration

The effects of cortisol on the spontaneous activity of single cells in the anteriortuberal and posterior hypothalamus and their responsiveness to photic, acoustic and sciatic stimulation were studied in cats under pentobarbital anesthesia. It was found that while before hormone administration the sensory stimuli caused a predominant increase in cell firing in the anterior-tuberal hypothalamus, after the hormone administration the effect was mainly inhibitory. On the other hand, in the posterior hypothalamus the hormone caused a further increase in cell firing following sensory stimulation. These results are discussed in view of the role played by the anteriortuberal hypothalamus in adrenocortical regulation and the negative feedback exerted by the glucocorticoids at hypothalamic levels. It is suggested that the present data may possibly demonstrate some electrophysiological correlates of the neuroendocrine effects of the adrenocortical hormones, exerted on the hypothalamus.     

Differential adrenocortical responses to neural stimuli following intracerebroventricular injection of nicotinic acetylcholine receptor antibodies in rats

The aim of the present study was to investigate the involvement of central nervous system nicotinic receptors for acetylcholine in the adrenocortical responses to neural stimuli. Adult male rats received a daily intraventricular injection of partially purified nicotinic acetylcholine receptor antibodies (anti-AchR), obtained from a patient with myasthenia gravis, for 5 consecutive days. Control animals were treated similarly with immunoglobulins obtained from normal human serum. Blood samples were obtained under basal conditions or following photic, sciatic nerve stimulation, and acoustic stress. Treatment with anti-AchR increased basal ACTH and corticosterone levels (by approximately twofold) but inhibited the response to acoustic stress. The response to photic and sciatic nerve stimulation stress was not affected. This study demonstrated that nicotinic acetylcholine receptors may be differentially involved in the mediation of adrenocortical responses to neural stimuli.     

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.     

Paraventricular nucleus serotonin mediates neurally stimulated adrenocortical secretion

The purpose of this study was to further elucidate the role of serotonin (5-HT) in adrenocortical regulation. The effects of stimulating the frontal cortex and extrahypothalamic limbic structures, on plasma corticosterone (CS) responses, were studied in rats with vehicle or 5,7-dihydroxytryptamine (5,7-DHT) injection into the midbrain raphe nuclei. In another group of rats the neurotoxin was injected locally into the paraventricular nucleus (PVN) in view of its importance in adrenocortical regulation, and the effects of photic and dorsal hippocampal stimulation on plasma CS were studied. 5,7-DHT caused a significant depletion of hypothalamic 5-HT and blocked the rise in plasma CS following the stimulation of the above neural modalities. These studies suggest that the PVN 5-HT mediates the adrenocortical responses following afferent neural stimuli.     

Norepinephrine depletion in the amygdala inhibits CRF-41, ACTH, and corticosterone responses following photic simulation

The effects of amygdaloid norepinephrine depletion by 6-hydroxydopamine on changes in corticotropin releasing factor-41 (CRF-41) and serum adrenocorticotropic hormone (ACTH) and corticosterone levels, following neural stimuli were investigated. In intact animals, photic or acoustic stimulation caused CRF-41 depletion from the median eminence and a rise in serum ACTH and corticosterone levels. In rats with amygdalar norepinephrine depletion there were no changes in basal CRF-41, ACTH, or corticosterone levels. However, the above responses of the hypothalamo-pituitary adrenocortical axis were blocked following photic, but not acoustic, stimulation. These results indicate that the facilitatory role of the amygdala on the above responses following photic stimulation depends on the presence of norepinephrine in this region.     

Hypothalamic afferent connections mediating adrenocortical responses that follow hippocampal stimulation

This study identified some neural pathways which mediate the adrenocortical responses that follow hippocampal stimulation. The increase in plasma corticosterone following dorsal hippocampus stimulation, in rats with electrodes chronically implanted under pentobarbital anesthesia, was blocked by dorsal fornix and lateral septal lesions and by small posterior hypothalamic deafferentation. Fimbria transection, lateral septal lesions, and posterior hypothalamic deafferentation, but not midbrain reticular formation lesions, also blocked the adrenocortical responses to ventral hippocampus stimulation. Our present and previous studies indicate that the dorsal and ventral hippocampal effects on the hypothalamus, which increase plasma corticosterone concentrations, are mediated by the dorsal fornix and fimbria, respectively, as well as by the lateral septum. A posterior hypothalamic input, which does not involve the medial forebrain bundle or the midbrain reticular formation is also essential for the activation of this response.     

Corticosterone implants in the paraventricular nucleus inhibit ACTH and corticosterone responses and the release of corticotropin-releasing factor following neural stimuli.

Experiments were conducted on the possible role of corticosterone (CS), at the level of the paraventricular nucleus of the hypothalamus (PVN) and dorsal hippocampus (D.HIPP), in the negative feedback effects following neural stimuli. In rats with bilateral PVN cholesterol (CHO) implants, acoustic and photic stimuli caused a significant rise in serum CS and ACTH and depletion of median eminence (ME) CRF-41 content. CS PVN implants have prevented the rise in serum CS, ACTH and ME CRF-41 depletion. Bilateral CHO or CS implants in the D.HIPP did not modify the responses of the hypothalamo-pituitary-adrenal axis to the above neural stimuli. In PVN CS-implanted rats, i.v. injection of CRF-41 increased serum CS similar to that observed in PVN CHO-implanted animals. These data are discussed in view of previous experiments on corticosteroid implants in the brain. It is concluded that CS PVN implants, by acting possibly via type II hypothalamic receptors, have prevented the release of ME CRF-41 following neural stimuli and consequently the secretion of ACTH and CS.     

Neural pathways mediating the prolactin secretory response to acute neurogenic stress in the male rat

Adult male rats, intact (N) or bearing complete, anterior or posterior hypothalamic deafferentations (CHD, AHD or PHD, respectively), or bilateral medial forebrain bundle (MFB) lesions, were acutely exposed to visual or audiogenic stimulation. At 2, 4,10 or 30 min following stress onset the animals were decapitated and trunk blood was collected for prolactin (PRL) determinations. Basal serum PRL levels were found to be similar in all groups. In N animals, exposure to both modalities resulted in rapid and marked PRL secretory responses. These responses were totally abolished in the CDH group. In AHD rats, no significant elevation in serum PRL concentration was found upon stress exposure. In PHD animals, the PRL secretory responses were only slightly attenuated when compared with the N group. In MFB-lesioned rats, a marked elevation in serum PRL concentrations was recorded following visual stimulation; contrary thereto, the PRL secretory response following audiogenic stress was markedly attenuated. These data (1) describe the temporal aspects of the PRL secretory response to acute exposure to neurogenic stresses in the male rat, and (2) demonstrate that these PRL responses are elicited via a neural pathway impinging upon the medial basal hypothalamus from the rostral direction.     

Adrenal demedullation blocks and brain norepinephrine depletion potentiates the hyperglycemic response to a variety of stressors

The role of adrenal hormones in mediating the increase in blood glucose levels following several stressful stimuli (environmental and pharmacological) was studied. The role for brain norepinephrine systems in the initiation of the BG response to these challenges was investigated as well. There is disagreement as to whether stress-induced increases in blood glucose levels are mediated primarily by hormonal or neural stimulation of the liver. A stressful stimulus probably causes increases in blood glucose levels by activating neural connections from the brain to both the liver and the adrenal medulla. The relative contribution that each of these pathways makes to the overall blood glucose response may be dependent on certain factors, such as the type of preparation used (awake or anesthetized, fasted or fed) and the intensity of the stimulus used to induce hyperglycemia. In the experiments reported here, which were performed in awake male rats, we found that increases in blood glucose levels following brief footshock stress, injection of 2-deoxy-D-glucose, exposure to the odor of a predator, and electrical stimulation of the hypothalamus were almost entirely eliminated by removal of the adrenal medullae, a procedure that does not damage hypothalamic norepinephrine systems or the multi-synaptic neural pathways from the hypothalamus to the liver. Furthermore, rather than having impaired blood glucose responses, rats that were depleted of brain norepinephrine showed normal responses to the injection of adrenergic agonists (including epinephrine), and potentiated responses to stressful stimuli compared to non-depleted controls. We conclude that: (1) rapid changes in blood glucose levels that occur following the stressful stimuli used here are mediated mainly by the release of epinephrine from the adrenal medullae and (2) intact brain norepinephrine systems are not required for these increases in blood glucose to occur.     

Fos-like immunoreactive neurons following electrical stimulation of the dorsal periaqueductal gray at freezing and escape thresholds

Electrical stimulation of the dorsal regions of the periaqueductal gray (PAG) leads to defensive reactions characterized as freezing and escape responses. Until recently it was thought that this freezing behavior could be due to the recruitment of neural circuits in the ventrolateral periaqueductal gray (vlPAG), while escape would be mediated by other pathways. Nowadays, this view has been changing mainly because of evidence that freezing and escape behaviors thus elicited are not altered after lesions of the vlPAG. It has been suggested that there are at least two pathways for periaqueductal gray-mediated defensive responses, one involving the hypothalamus and the cuneiform nucleus (CnF) which mediates responses to immediate danger and another one involving the amygdala and vlPAG which mediates cue-elicited responses, either learned or innate. To examine this issue further we measured Fos protein expression in brain areas activated by electrical stimulation of the dorsolateral PAG (dlPAG) at the freezing and escape thresholds. The data obtained showed that freezing-provoking stimulation caused increases in Fos expression in the dorsomedial PAG (dmPAG), while escape-provoking stimulation led to increases at both dmPAG and dlPAG. Surprisingly, neither escape- nor freezing-provoking stimulations altered Fos expression in the central nucleus of amygdala (CeA). Escape-provoking stimulation caused increased Fos expression in the ventromedial hypothalamus (VMH), dorsal premammilary nucleus (PMd) and in the cuneiform nucleus. Significant increases in Fos labeling were found in the dmPAG and PMd following freezing-provoking stimulation. Therefore, the present data support the notion of a neural segregation for defensive behaviors in the dorsal columns of PAG, with increased Fos expression in the dmPAG following freezing, while dlPAG is affected by both freezing and escape responses. dlPAG, CnF, VMH and PMd are part of a brain aversion network activated by fear unconditioned stimuli. The present data also suggests that the defensive responses generated at the dlPAG level do not recruit the neural circuits of the vlPAG and CeA usually activated by conditioned fear stimuli.     

Neurogenic stimuli alter preoptic area and amygdala unit activity: central effects of olfactory projections on paraventricular nucleus units

Unit responses were recorded in the preoptic area and amygdala of conscious male rats during exposure to stressful neurogenic stimuli. Olfactory stimulation elicited increases in preoptic area activity on all occasions and also increased activity in the intercalating, medial, and basomedial nuclei of the amygdala, but not in other regions. Acoustic stimulation had less specific effects, even inhibiting unit activity in the central amygdala. A separate series of experiments using urethane-anesthetized rats was carried out to examine the effects of electrical stimulation of the medial amygdala and olfactory tubercle on single-unit activity within the hypothalamic paraventricular nucleus. Inhibition was the predominant response following olfactory tubercle stimulation while excitatory responses predominated following stimulation of the medial amygdala. This was the case particularly for those paraventricular nucleus units identified as projecting to the median eminence (P less than 0.005 vs unidentified cells). The results obtained may be related to the neural regulation of adrenocortical activity as well as higher central nervous activity and have been discussed within these contexts.     

Corticotrophin and corticosterone secretory patterns following acute neurogenic stress, in intact and in variously hypothalamic deafferented male rats

Adult male rats, intact (N) or bearing complete (CHD), anterior (AHD), or posterior (PHD) hypothalamic deafferentations, were acutely exposed to either visual or audiogenic stimulation. At 2, 4, 10 or 30 min following the onset of stress exposure the animals were decapitated and trunk blood was collected for ACTH (RIA) and corticosterone (CS, CBG) determinations. Basal serum concentrations of both hormones were elevated in CHD and AHD, but not in PHD animals as compared to N animals. In N rats, exposure to both stresses resulted in elevated serum ACTH and CS concentrations, with the ACTH response to audiogenic but not visual stimulation being biphasic. In CHD animals, serum ACTH concentrations decreased, and those of CS were unchanged following stress exposure. While audiogenic stimulation caused elevation in serum levels of both hormones in AHD rats, the normal ACTH and CS response to visual stimulation were completely abolished by anterior hypothalamic deafferentation. In PHD animals, no ACTH response to either of the stress exposure was apparent; in spite of this, partial CS responses were elicited. These data thus describe the temporal aspects of the ACTH and CS secretory responses to different neurogenic stresses, and provide insight into the neural pathways mediating these responses.     

Depletion of hypothalamic norepinephrine and serotonin enhances the dexamethasone negative feedback effect on adrenocortical secretion.

The role of norepinephrine (NE) and serotonin (5-HT) in the negative feedback effect of dexamethasone (DEX) on the adrenocortical response to ether stress was investigated. Injection of the catecholamine neurotoxin, 6-hydroxydopamine, into the ventral noradrenergic bundle or the paraventricular nucleus of the hypothalamus (PVN) which produced a very significant depletion in hypothalamic NE content enhanced the negative feedback effect of DEX. Injection of the 5-HT neurotoxin, 5,7-dihydroxytryptamine, into the raphé nuclei or PVN, which caused a depletion of hypothalamic 5-HT, produced a similar effect on the adrenocortical response to DEX. The degree of negative feedback may be viewed as a balance of neural stimulatory and glucocorticoid influences of the hypothalamus. Thus the removal of the stimulatory effects of NE and 5-HT on adrenocortical secretion, by the neurotoxic lesions, enhanced the inhibitory influence of DEX.     

Stimulation of the paraventricular nucleus area of the hypothalamus elevates urinary 6-hydroxymelatonin during daytime

The paraventricular nucleus of the hypothalamus (PVN) is thought to be a part of the neural circuit comprising the melatonin rhythm generating system (MRGS). Electrical stimulation of the PVN during the early lights-on period significantly elevated urinary 6-hydroxymelatonin content to nearly 50% of night levels; stimulation during the lights-off period did not produce significant changes. In contrast to the effects of PVN stimulation, stimulation of surrounding brain areas was without effect at either time. This observation confirms the participation of the PVN in the MRGS.     

Hypothalamic control of liver glycogen metabolism in adult and aged rats

The regulatory function of the hypothalamus in the metabolism of liver glycogen was investigated by analyzing the changes in the activities of the rate-limiting enzymes implicated in glycogen breakdown (glycogen phosphorylase) and synthesis (glycogen synthetase), after electrical stimulation of the ventromedial hypothalamic nucleus (VMH) and the lateral hypothalamic nucleus (LH) of rats. Electrical stimulation of the VMH induced rapid and marked increase in the content of the active form of phosphorylase in the liver, but it did not affect the synthetase. Stimulation of the LH, on the other hand, caused an increase in the level of the active form of synthetase, but produced little change in the phosphorylase activity. The concentration of insulin in the portal blood decreased significantly on stimulation of the VMH, but did not change appreciably on stimulation of the LH. The ability of the hypothalamus to control liver glycogen metabolism was found to be impaired in 2-year-old rats: the magnitude and duration of the early increase in active phosphorylase in response to VMH stimulation were reduced, and the response of synthetase to LH stimulation was slow.The results are discussed from the viewpoint that the VMH and LH are the centers for controlling irect neural and neural-hormonal regulations of liver glycogen metabolism. It is concluded that the VMH-splanchnic nerve pathway is an important neural component for controlling glycogenolysis and glucose output by the liver, and that the LH-vagal pathway is an opposing neural component for controlling glycogenesis in the liver.     

Central Amygdala Projections to Lateral Hypothalamus Mediate Avoidance Behavior in Rats

Persistent avoidance of stress-related stimuli following acute stress exposure predicts negative outcomes such as substance abuse and traumatic stress disorders. Previous work using a rat model showed that the central amygdala (CeA) plays an important role in avoidance of a predator odor stress-paired context. Here, we show that CeA projections to the lateral hypothalamus (LH) are preferentially activated in male rats that show avoidance of a predator odor-paired context (termed Avoider rats), that chemogenetic inhibition of CeA-LH projections attenuates avoidance in male Avoider rats, that chemogenetic stimulation of the CeA-LH circuit produces conditioned place avoidance (CPA) in otherwise naive male rats, and that avoidance behavior is associated with intrinsic properties of LH-projecting CeA cells. Collectively, these data show that CeA-LH projections are important for persistent avoidance of stress-related stimuli following acute stress exposure.SIGNIFICANCE STATEMENT This study in rats shows that a specific circuit in the brain [i.e., neurons that project from the central amygdala (CeA) to the lateral hypothalamus (LH)] mediates avoidance of stress-associated stimuli. In addition, this study shows that intrinsic physiological properties of cells in this brain circuit are associated with avoidance of stress-associated stimuli. Further characterization of the CeA-LH circuit may improve our understanding of the neural mechanisms underlying specific aspects of stress-related disorders in humans.     

A temporal study of post-adrenalectomy increase in ACTH secretion in the rat: effect of various hypothalamic deafferentations

Adult male rats, intact (N) or with complete (CHD), anterior (AHD), or posterior (PHD) hypothalamic deafferentations were bilaterally adrenalectomized. At 3, 6, 12 and 20 days post-adrenalectomy they were decapitated and trunk blood was collected for ACTH determinations. In N rats, ACTH markedly elevated up to 850 pg/ml. A similar ACTH response was found in PHD rats but the values were lower by approximately 20%. In contrast, in either AHD or CHD rats, ACTH responses were markedly attenuated and reached a plateau of about 350 pg/ml. These data suggest that: (1) neural inputs entering the medio-basal hypothalamus (MBH) from both the caudal and rostral directions are important for obtaining maximal ACTH responses following adrenalectomy; (2) at least part of this ACTH response is mediated by sites inside the MBH or in the pituitary.     

Catecholaminergic projections to tuberoinfundibular neurones of the paraventricular nucleus: II. Effects of stimulation of the ventral noradrenergic ascending bundle: evidence for cotransmission

In order to further elucidate the neural mechanisms underlying the control of adrenocortical secretion, responses of paraventricular nucleus (PVN) tuberoinfundibular neurones were examined following stimulation of the ventral noradrenergic ascending bundle (VNAB). Stimulation at low frequencies (0.5/5 Hz) excited the majority (52/64, 81%) of cells but only 15 showed a clear-cut, stimulus-locked, activation with onset latency of 44.5 +/- 10.0 msec and offset at 71.9 +/- 11.3 msec: the remaining 37 excited cells showed overall increases in firing after delivery of 5-10 stimuli. High frequency (50 Hz) trains of stimuli reversed the direction of response to inhibition for 14/52 of the excited cells. Inhibition of (nor)adrenaline synthesis by alpha-methylparatyrosine was without effect upon the firing of cells examined or the distribution and latencies of their responses following low frequency stimulation; high frequency trains reversed the response direction of only 4/35 cells, (p less than 0.05 vs. control rats; chi 2-test). Intracerebroventricular administration of 6-hydroxydopamine, a catecholaminergic neurotoxin, reduced the proportion of cells excited by the stimulation (10/47; p less than 0.005; chi 2-test). Unit responses to painful somatosensory stimuli were recorded from the majority of the cells tested (74%), except following 6-hydroxydopamine treatment, when only 38% were excited (p less than 0.005; chi 2-test). The results demonstrate that the VNAB provides an excitatory input to the PVN and that noradrenaline is probably responsible for this effect but a cotransmitter (neuropeptide Y?) may also be responsible for the observed excitatory responses. Inhibitory responses following high frequency stimulation were probably also mediated by (nor)adrenaline.(ABSTRACT TRUNCATED AT 250 WORDS)