Leptin Modulates Spike Coding in the Rat Suprachiasmatic Nucleus

The mammalian circadian pacemaker, the suprachiasmatic nucleus (SCN), contains receptors to the adipose tissue hormone leptin. In the present study, the effects of leptin on the electrophysiological activity of the SCN cells were characterised in vitro in rat brain slices. During extracellular recording, application of 20 n m leptin (n = 36) decreased mean spike frequency (Wilcoxon signed rank test, z = −3.390, P < 0.001) and increased the irregularity of firing measured by the entropy of the log interspike interval distribution (Student's paired t-test, t = 2.377, P = 0.023), but had no consistent effect on spike patterning as measured by the mutual information between adjacent log interspike intervals (z = 0.745, P = 0.456). Intracellular current-clamp recordings (n = 25) revealed a hyperpolarising effect of 20 n m leptin on SCN neurones (z = −2.290, P = 0.022). The hyperpolarisation largely resulted from the effect of leptin on the subgroup of cells (n = 13) that generated 'rebound' spikes upon termination of a hyperpolarising current pulse (z = −2.697, P = 0.007). Leptin application also increased the group mean duration of the afterhyperpolarisation (n = 25, t = 2.512, P = 0.023). The effects of leptin on extracellularly recorded spike activity were consistent with the changes in membrane potential and spike shape. They suggest that leptin can directly modulate the electrical properties of SCN neurones and, in this way, contribute to the mechanism by which metabolic processes influence the circadian clock.     

Serotonin Reinnervation of the Suprachiasmatic Nucleus by Intrahypothalamic Fetal Raphe Transplants,with Special Reference to Possible Influences of the Target

Survival and development of fetal serotonin (5-HT) neurons grafted to various brain areas in adult mammals have been suggested to be under host influences. The aim of this study was to determine whether the suprachiasmatic nucleus of the hypothalamus (SCN), a region receiving a 5-HT input which is one of the densest and the most heavily synaptic in the brain, can actually support the development of transplanted 5-HT neurons. The time course and extent of 5-HT reinnervation were therefore investigated with 5-HT immunocytochemistry in adult rats subjected to intraventricular injection of 5,7-dihydroxytryptamine and subsequent grafting of fetal cell suspension of mesencephalic raphe neurons. The ultrastructural features of the newly formed 5-HT terminal plexa were also examined. Serotonin reinnervation of the SCN remained partial up to 4 months post-transplantation, with no apparent predilection of the reinnervating fibres for any particular portion of the nucleus, thus differing from the normal 5-HT innervation of the SCN both quantitatively and qualitatively. This was in sharp contrast to the 5-HT hyperinnervation observed in neighbouring areas such as the supraoptic nucleus, a structure normally provided with only few 5-HT fibres, and the ventral wall of the third ventricle. The graft-derived 5-HT axons, however, displayed ultrastructural features that did not appear different from those of their normal counterparts; in particular they re-established defined synaptic contacts with the host population. These results may indicate that the mature SCN specifically lacks a trophic factor necessary for the ingrowth of graft-derived 5-HT fibres, or that it represents an inhibitory environment for such an ingrowth. The limited ability of regrowing 5-HT axons to restore a normal density of 5-HT innervation could also be related to the fact that these neurons normally establish a relatively high number of synaptic contacts in the target region.     

Ontogeny of a Photic Response in the Suprachiasmatic Nucleus in the Siberian Hamster (Phodopus sungorus)

The ontogeny of photic responsiveness in the suprachiasmatic nucleus of the Siberian hamster (Phodopus sungorus) was studied using the enhanced expression of the immediate early gene c-fos as a marker of neuronal activation. c-fos expression was assessed by immunocytochemical localization of its protein product. Hamsters were kept on a 16 h light:8 h dark photocycle. The adult Siberian hamster showed a marked increase in the number of c-fos-immunoreactive (c-fos-ir) cells within the suprachiasmatic nuclei (SCN) in response to a 1 h light pulse delivered 1–3 h after lights off, in comparison to controls kept in the dark. This is consistent with previous studies in the Syrian hamster and rat. The development of the photic response was examined. The first study investigated the effects of a light pulse on c-fos induction in pups at 5, 9, 12 and 24 postnatal days of age (PD). The suprachiasmatic region was identified by immunocytochemical localization of peptide-histidine-isoleucine in adjacent sections, a peptide expressed early in the development of the rodent SCN. The distribution of c-fos-ir cells was also compared with the location of retinal efferents, as determined by intraocular injection of the tract tracer cholera toxin B subunit 24 h previously. At PD 9, 12 and 24, significant increases in the number of c-fos-ir cells occurred in the light pulsed animals in comparison to age-matched control animals which were moved within the non-illuminated room to provide a ‘dark’ pulse. Induction of c-fos mainly occurred in the ventrolateral region of the medial and caudal SCN, the region which receives the greatest density of retinal innervation. The second study focused on the earliest age at which light could induce c-fos expression in the SCN by investigating the effects of light on c-fos-ir cells in pups at PD 2–4. Animals at PD 3 and 4 showed enhanced c-fos expression in the ventrolateral region, whilst PD 2 animals showed no response. The number of c-fos-ir cells in the SCN region in light-pulsed animals increased from PD 3 to adulthood. These results indicate that retinal input can activate cells in the SCN even before the eyelids open on PD 11, and the SCN can potentially be entrained by photic inputs as early as day 3 after birth.     

Lesions of the Suprachiasmatic Nucleus Indicate the Presence of a Direct Vasoactive Intestinal Polypeptide-Containing Projection to Gonadotrophin-Releasing Hormone Neurons in the Female Rat

In non-seasonal breeders like the rat, the influence of the suprachiasmatic nucleus (SCN) on reproduction is most clearly expressed in the female. Complete lesions of the SCN induce persistent oestrus (anovulation) in intact female rats, whereas oestrogen implantation in ovariectomized rats results in daily luteinizing hormone surges. Vasoactive intestinal polypeptide (VIP), a peptide synthesized in cell bodies of the SCN, inhibits the increase in pulsatile luteinizing hormone release observed in ovariectomized female rats. In search of the anatomical basis for these observations, the present study employs an immunocytochemical double staining for VIP and gonadotrophin-releasing hormone (GnRH) at the light microscopical level. It was demonstrated that approximately 45% of the GnRH positive neurons in the diagonal band of Broca, the preoptic and anterior hypothalamic area of female rats are innervated by VIP-containing processes. To investigate whether these VIP-containing fibres represent a direct projection of the SCN to the GnRH system, unilateral thermic SCN lesions were made. Lesions that unilaterally destroyed the majority of the VIP synthesizing cells in the SCN resulted in at least a 50% decrease of the VIP innervation of GnRH cell bodies at the lesioned side compared to the intact side. Lesions not affecting the VIP synthesizing cell population in the SCN did not change the percentage of GnRH neurons innervated by VIP-containing fibres, while partial lesions resulted in intermediate effects. These results indicate that the majority of the light microscopical VIP-containing input on GnRH neurons in the hypothalamus is derived from the SCN. It is suggested that the reported effects of VIP on luteinizing hormone release may, at least in part, be induced via a direct effect of VIP on GnRH cell bodies. This direct SCN-GnRH pathway provides an anatomical basis for diurnal influences on the regulation of the female reproductive cycle.     

Oestrogen Receptor-α-Immunoreactive Neurones Project to the Suprachiasmatic Nucleus of the Female Syrian Hamster

Ovarian steroid hormones regulate circadian period and phase, but classical receptors for these hormones are absent in the circadian pacemaker localized in the suprachiasmatic nucleus of the hypothalamus (SCN). In order to determine whether effects of oestrogen may be exerted through steroid-binding systems afferent to the SCN we have performed double label immunocytochemistry for oestrogen receptor-alpha(ER-alpha) and the retrograde tracer cholera toxin B subunit (CtB) after its application to the SCN. Most of the areas that contain ER-alpha-immunoreactive (ERalpha-ir) cells also contained cells afferent to the SCN. The percentage of neurones afferent to the SCN which show ERalpha-immunoreactivity varies between areas. As many as one-third of the neurones afferent to the SCN in some parts of the preoptic area and the corticomedial amygdala are ERalpha-ir. Very few of the afferent neurones from the septum and the central grey are ERalpha-ir, whereas an intermediate proportion of afferents from the bed nucleus of the stria terminalis and the arcuate nucleus are ERalpha-ir. Our retrograde tracing results were compared with results of anterograde tracing from some of the sites containing SCN afferents. Using a combined retrograde and anterograde tracing technique we tested the possibility that single ERalpha-ir neurones afferent to the SCN could receive reciprocal innervation by SCN efferents. Although we found SCN input to some SCN afferent neurones, we found no evidence of reciprocity between single ERalpha-ir cells and the SCN. Our results indicate the existence of oestrogen binding systems afferent to the SCN. These neuroanatomical pathways may mediate effects of gonadal steroid hormones on circadian rhythms.     

Suprachiasmatic neurons in tissue slices from ovariectomized rats: Electrophysiological and neuropharmacological characterization and the effects of estrogen treatment

Single-unit activity and unit responses to putative neurotransmitters were recorded from suprachiasmatic nucleus (SCN) neurons in brain tissue slices from ovariectomized rats either treated or untreated with estrogen. Altogether, 204 units were studied from estrogen-treated and untreated preparations, and at the resting state, 37% of these units fired regularly, 57% fired irregularly, and 6% were silent but evokable by electrical stimulation. Most of the irregular units fired continuously (n = 100), while the rest fired intermittently (n = 12) or phasically (n = 4). Neurons with different types of firing patterns also varied significantly in resting firing rate and in responsiveness to transmitters and to estrogen treatment. The average resting firing rate decreased significantly from regular, irregular and continuous, intermittent, to silent units. Acetylcholine (ACh) and/or serotonin (5-HT) injected directly into the perfusion chamber evoked responses from more irregular (69% of 61 units) than regular units (20% of 46 units). None of the 5 silent units tested was activated by ACh or 5-HT. Responses to ACh (predominantly inhibitory) and 5-HT (predominantly excitatory) seen here in vitro were opposite to those observed in vivo with iontophoretic application, and were not reversed or abolished by the blockade of synaptic transmission. Comparisons of data between the two types of preparations showed that only the responsiveness of the irregular units to ACh and to 5-HT were significantly different: both types of responsiveness were higher in estrogen-treated than in untreated preparations. No significant difference was found in the responsiveness of regular units, or in firing patterns or firing rate. Thus, the present in vitro studies have demonstrated that SCN contains a heterogeneous population of neurons distinguishable by their electrophysiological and neuropharmacological characteristics, and that estrogen has a specific action on specific types of SCN neurons.     

Hypophysiotropic Role and Hypothalamic Gene Expression of Corticotropin-Releasing Factor and Vasopressin in Rats Injected with Interleukin-1β Systemically or into the Brain Ventricles

Intact adult male rats were injected intravenously (i.v., 400 ng/kg), intraperitoneally (i.p., 400 ng/kg) or intracerebroventricularly (i.c.v., 100 ng/kg) with interleukin-1β (IL-1β) or its vehicle. In comparison with vehicle-treated animals, IL-1β induced significant (P<0.01) increases in plasma ACTH levels measured 30 min later regardless of the route of cytokine administration. These changes were markedly blunted in rats administered specific antibodies directed against corticotropin-releasing factor (CRF). In contrast, vasopressin (VP) antibodies significantly blunted ACTH released by the i.c.v. injection of IL-1β, but only modestly altered the effect of the systemic injection of the cytokine. We then used semi-quantitative in situ hybridization analysis to measure changes in steady-state mRNA levels, as they might occur in response to these same doses of IL-1β. Following administration of the vehicle, measurement of gene expression in the paraventricular (PVN) portion of the hypothalamus indicated a measurable amount of hybridization signals for both CRF and VP. No detectable changes in either CRF or VP gene expression were observed in rats injected with IL-1β i.v. or i.p. 5 h earlier. In contrast, the i.c.v. administration of the cytokine significantly (P<0.01) increased both CRF and VP mRNA levels measured 5 h later. These results suggest that while endogenous CRF modulates the response of the corticotrophs to this cytokine regardless of the route of administration, the role of VP is more important in rats injected centrally than in those injected peripherally. The observation that at the dose tested and over the time-course studied, systemic injection of IL-1β failed to alter CRF or VP gene expression, supports our earlier hypothesis that blood-born IL-1β acts primarily at the level of nerve terminals in the median eminence.     

Vasoactive Intestinal Polypeptide in the Suprachiasmatic Nucleus of the Mink (Mustela vison) Could Play a Key Role in Photic Induction

The present study was conducted to visualize neuropeptides in the SCN of a mustelid, the American mink in which seasonal cycles of reproduction rely totally on the annual changes in day length. At this time, data in mustelids are lacking. Results were obtained with in situ hybridization (ISH) using synthetic oligonucleotide vasopressin (AVP) and somatostatin (SOM) and with single and dual immunohistochemistry (IHC) performed with antisera against AVP, SOM, vasoactive intestinal polypeptide (VIP), gastrin releasing peptide (GRP) and met-enkephalin (Met-ENK) in untreated (AVP and VIP) or colchicine (SOM, Met-ENK and GRP) treated adult male and female mink. The most striking result, evidenced by ISH as well as IHC was the lack of AVP, SOM and Met-ENK immunoreactive (ir)-neurons in the SCN. In contrast, strongly VIP ir-perikarya were widely distributed within the SCN and gave rise to a dense network of fibres extending within the periventricular (peVA) and subparaventricular (subPVA) areas. Weakly GRP ir-perikarya were also observed in the median part of the SCN. Dual IHC revealed that the magnocellular neurons located just dorsal to the SCN, in the peVA and subPVA co-stored AVP with VIP, SOM or Met-ENK. The lack of SCN AVP and SOM ir-neurons, reported for the first time in a mammalian species, raises the question of their implication in the functions of the circadian pacemaker and its entrainment by the light/dark cycle in other species. The significance of the large neurons co-storing peptides in the terminal field of VlPergic fibres originating in the SCN has also to be determined. These results suggest that VIP could be of major importance in processing photic information mediating circadian entrainment and consequently annual rhythms.     

IL1β- and LPS-induced serotonin secretion is increased in EC cells derived from Crohn's disease

Abstract Gut mucosal enterochromaffin (EC) cells are regarded as key regulators of intestinal motility and fluid secretion via secretion of serotonin (5HT), are increased in numbers in mucosal inflammation and located in close proximity to immune cells. We examined whether interleukin (IL)1β and Escherichia coli lipopolysaccharide (LPS) induced EC cell 5HT release through Toll‐like/IL‐1 (TIL) receptor activation, nuclear factor kappa B (NFκB) and mitogen‐activated protein kinase (MAPK) phosphorylation and evaluated whether somatostatin could inhibit this phenomenon. Pure (>98%) human intestinal EC cells were isolated by fluorescent activated cell sorting from preparations of normal (n = 5) and Crohn’s colitis (n = 6) mucosa. 5HT release was measured (ELISA), and NFκB and ERK phosphorylation quantitated (ELISA) in response to IL1β and LPS. 5HT secretion was increased by both E. coli LPS (EC₅₀ = 5 ng mL^?1) and IL1β (EC₅₀ = 0.05 pmol L^?1) >2‐fold (P < 0.05) in Crohn’s EC cells compared with normal EC cells. Secretion was reversible by the TLR4 antagonist, E. coli K12 LPS (IC₅₀ = 12 ng mL^?1) and the IL1β receptor antagonist (ILRA; IC₅₀ = 3.4 ng mL^?1). IL1β caused significant (P < 0.05) NFκB and MAPK phosphorylation (40–55%). The somatostatin analogue, lanreotide inhibited IL1β‐stimulated secretion in Crohn’s (IC₅₀ = 0.61 nmol L^?1) and normal EC cells (IC₅₀ = 1.8 nmol L^?1). Interleukins (IL1β) and bacterial products (E. coli LPS) stimulated 5HT secretion from Crohn’s EC cells via TIL receptor activation (TLR4 and IL1β). Immune‐mediated alterations in EC cell secretion of 5HT may represent a component of the pathogenesis of abnormal bowel function in Crohn’s disease. Inhibition of EC cell‐mediated 5HT secretion may be an alternative therapeutic strategy in the amelioration of inflammatory bowel disease symptomatology.     

Suppressed oxytocin neuron responses to immune challenge in late pregnant rats: a role for endogenous opioids

Cytokine challenge (mimicking infection) with systemic interleukin-1beta (IL-1beta) stimulates oxytocin neurons via a noradrenergic brainstem pathway similar to that involved in parturition. As the responses of oxytocin neurons to several stimuli are reduced in late pregnancy, we have investigated whether responses to IL-1beta are also suppressed. In virgin Sprague-Dawley rats, IL-1beta (500 ng/kg i.v.) rapidly increased oxytocin secretion (3.2-fold), via a central action as the firing rate of oxytocin neurons in the supraoptic nucleus was increased. In contrast, IL-1beta had no significant effect on the electrical or secretory activity of oxytocin neurons in late pregnant rats. In pregnancy activation of a central inhibitory opioid mechanism restrains oxytocin neuron responses to various stimuli. Accordingly, we tested the effects of the opioid antagonist, naloxone, on oxytocin neuron responses to IL-1beta in pregnancy. Naloxone (5 mg/kg i.v.) did not affect the oxytocin secretory response to IL-1beta in virgin rats, whereas in late pregnant rats naloxone revealed a greater oxytocin secretory response to IL-1beta (3.5-fold) than in virgin rats. In virgin rats, naloxone decreased oxytocin neuron firing rate after IL-1beta, however, in pregnant rats naloxone increased the firing rate response to IL-1beta to the level seen in virgin rats. Thus, systemic IL-1beta acts centrally to increase oxytocin secretion. In pregnancy this response is suppressed by endogenous opioids, thus preserving neurohypophysial oxytocin stores for parturition and minimizing the risk of preterm labour. The exaggerated oxytocin secretory response to IL-1beta in pregnancy after naloxone reflects increased oxytocin stores and/or increased efficiency of excitation-secretion coupling at the posterior pituitary.     

Inhibitory Function of the Dorsomedial Hypothalamic Nucleus on the Hypothalamic–Pituitary–Adrenal Axis Response to an Emotional Stressor but not Immune Challenge

Accumulating evidence implicates the dorsomedial hypothalamic nucleus (DMH) in the regulation of autonomic and neuroendocrine stress responses. However, although projections from the DMH to the paraventricular hypothalamic nucleus (PVN), which is the critical site of the neuroendocrine stress axis, have been described, the impact of DMH neurones in the modulation of hypothalamic‐pituitary‐adrenal (HPA) axis activation during stress is not fully understood. The present study aimed to investigate the role of the DMH in HPA axis responses to different types of stimuli. Male Sprague–Dawley rats fitted with a chronic jugular venous catheter were exposed to either an emotional stressor (elevated platform‐exposure) or immune challenge (systemic interleukin‐1β administration). Bilateral electrolytic lesions of the DMH disinhibited HPA axis responses to the emotional stressor, as indicated by higher plasma adrenocorticotrophic hormone levels during and after elevated platform exposure in lesioned animals compared to sham‐lesioned controls. Moreover, DMH‐lesioned animals showed increased neuronal activation in the PVN, as indicated by a higher c‐Fos expression after elevated‐platform exposure compared to controls. By contrast, DMH‐lesions had no effects on HPA axis responses to immune challenge. Taken together, our data suggest an inhibitory role of DMH neurones on stress‐induced HPA axis activation that is dependent upon the nature of the stimulus being important in response to an emotional stressor but not to immune challenge.     

Endogenous alpha-MSH modulates the hypothalamic-pituitary-adrenal response to the cytokine interleukin-1beta

We and others have previously shown that exogenous alpha-MSH antagonizes the stimulatory effects of the cytokine interleukin (IL)-1 on the hypothalamic-pituitary-adrenal (HPA) axis. It is currently unknown, however, if endogenous alpha-MSH plays a physiological role in regulating the HPA response to IL-1. We have therefore examined the HPA response to IL-1beta in rats pretreated with an affinity purified alpha-MSH antiserum (AS) infused intracerebroventricularly to neutralize endogenous alpha-MSH within the brain. alpha-MSH AS or a similarly purified fraction of normal rabbit serum (NRS) was injected intracerebroventricularly at 16 h and at 1 h prior to the i.c.v. injection of IL-1beta (2 ng or 20 ng) and blood samples were collected through an indwelling atrial catheter. After 2 ng IL-1beta, the adrenocorticotropic hormone (ACTH) response was significantly greater in the alpha-MSH AS treated rats (n = 7) compared to the NRS treated rats (n = 7) (P <0.01); the mean ACTH level rose to a peak of 594+208 pg/ml in the alpha-MSH AS treated rats vs 274+/-122 pg/ml in the NRS treated rats. The area under the ACTH response curve in the alpha-MSH AS treated animals was 181% of that in the NRS treated animals (P<0.05). A significant effect of alpha-MSH AS on the corticosterone response to i.c.v. IL-1beta was also noted during the first 3 h of the study (P<0.05). The mean area under the corticosterone response curve for the first 3 h in the alpha-MSH AS treated animals was 144% of that in the NRS treated animals (P <0.05). After 20 ng IL-1beta, the ACTH response over time was again significantly greater in the alpha-MSH AS treated rats (n=8) compared to the NRS treated rats (n=9) (P<0.02); the mean ACTH level rose to a peak of 673+/-190 pg/ml after alpha-MSH AS vs 490+/-115 pg/ml after NRS. Corticosterone levels rose to a peak of 42+/-3.9 microg/dl in the alpha-MSH AS treated rats vs 37+/-4.6 microg/dl in the NRS treated rats; this difference was not significant. We conclude that the IL-1beta induced stimulation of ACTH is significantly enhanced by antagonizing the activity of alpha-MSH. These results support a physiological role for endogenous alpha-MSH in limiting the HPA response to this inflammatory cytokine.     

Involvement of Medullary Catecholamine Cells in Neuroendocrine Responses to Systemic Cholecystokinin

Systemic administration of cholecystokinin (CCK) stimulates neurosecretory oxytocin (OT) and tuberoinfundibular corticotrophin releasing factor (CRF) cells of the hypothalamus. Data from previous studies suggest that A2 noradrenergic neurons of the dorsomedial medulla contribute to the OT  cell response, but the role of other medullary catecholamine cells remains unclear. Using c- fos expression as a marker for cellular activity, we have found that CCK (100  μg/kg, ip) activates substantial populations of tyrosine hydroxylase and phenyl-N-methyl-transferase immunoreactive cells in the medulla, consistent with recruitment of overlapped noradrenergic and adrenergic cell populations in both the ventrolateral and dorsomedial medulla. In the ventrolateral medulla there was a particularly prominent activation of C1 adrenergic neurons at the level of the obex. To directly test the contribution of VLM catecholamine cells to hypothalamic neuroendocrine cell responses to CCK, animals were prepared with unilateral VLM lesions corresponding to those areas that had displayed the most marked response to CCK. VLM lesioned animals treated with CCK displayed a significant although small reduction in paraventricular nucleus (PVN) OT  cell c- fos expression ipsilateral to the lesion, but no change in the responses of supraoptic nucleus OT  cells or in cells of the medial parvocellular PVN, many of which are CRF cells. These findings indicate that VLM catecholamine cells make little contribution to hypothalamic neuroendocrine cell responses to CCK and thus serve to further highlight the role of dorsomedial catecholamine cells. However, it is now apparent that, in addition to A2 noradrenergic cells, CCK treatment also recruits C2 adrenergic cells of the dorsomedial medulla, many of which have previously been shown to project to the PVN.     

IL-1beta inhibits IK and increases [Ca2+]i in the carotid body glomus cells and increases carotid sinus nerve firings in the rat

Increasing evidence indicates that there exists a reciprocal communication between the immune system and the brain. Interleukin 1beta (IL-1beta), a proinflammatory cytokine produced during immune challenge, is believed to be one of the mediators of immune-to-brain communication, but how it gets into the brain is unknown because of its large molecular weight and difficulty in crossing the blood-brain barrier. Our previous work has demonstrated that IL-1 receptor type I is strongly expressed in the glomus cells of rat carotid body (CB), a well characterized polymodal chemoreceptive organ which serves not only for the detection of hypoxia, hypercapnia and acidity, but also for low temperature and blood glucose. The present study was designed to test whether IL-1beta could stimulate the CB glomus cells and alter the discharge properties in the carotid sinus nerve, the afferent nerve innervating the organ. The results from whole-cell patch-clamp recordings and calcium imaging showed that extracellular application of IL-1beta significantly decreased the outward potassium current and triggered a transient rise in [Ca(2+)](i) in the cultured glomus cells of rat CB. Furthermore, by using extracellular recordings and pharmacological intervention, it was found that IL-1beta stimulation of the CB in the anaesthetized rat in vivo significantly increased the discharge rate in the carotid sinus nerve, most probably mediated by ATP release. This experiment provides evidence that the CB responds to cytokine stimulation and proposes the possibility that the CB might play a role in immune-to-brain communication.     

Neonatal Programming by Neuroimmune Challenge: Effects on Responses and Tolerance to Septic Doses of Lipopolysaccharide in Adult Male and Female Rats

A mild immune challenge experienced during the neonatal period leads to attenuated febrile responses to a similar challenge experienced later in life. However, the immune response to an endotoxin differs depending upon the severity of the challenge and it is not clear whether a neonatal immune challenge will also affect responses to a severe, potentially life‐threatening stimulus, such as sepsis. In the present study, we examined the effects of a neonatal immune challenge with lipopolysaccharide (LPS) on adult sickness responses, as well as the development of endotoxin tolerance, to a septic dose (1 or 3 mg/kg) of the same LPS in male and female rats. We demonstrate significant differences, particularly in males, in the fever profiles of neonatally LPS‐treated rats compared to neonatally saline‐treated controls. Specifically, male rats treated neonatally with LPS have reduced hypothermic and enhanced hyperthermic responses to both septic doses of LPS in adulthood. A somewhat different profile is seen in females, with neonatally LPS‐treated females having reduced hypothermia and enhanced hyperthermia compared to controls with 1 mg/kg but no differences with 3 mg/kg LPS. The results obtained demonstrate that alterations in innate immune responses previously reported for low doses of LPS can, for the most part, also be observed after severe immune challenge in later life.     

Differential Dependence of ACTH Secretion Induced by Various Cytokines on the Integrity of the Paraventricular Nucleus

Effect of different cytokines, human recombinant interleukin-la and fl (IL-1 a, 1L-1 j), interleukin-6 and tumor necrosis factor-a (TNF) on adrenocorticotropin (ACTH) secretion was compared in sham-operated rats and those with lesions of the hypothalamic paraventricular nucleus. IL-1 a was less active than IL-1 b in stimulating ACTH in sham-operated rats. Intravenous injection of IL-1 fl in sham-operated animals resulted in a rapid elevation of ACTH secretion. Five days after surgical lesion of the paraventricular nucleus, the main hypothalamic source of hypophysiotropic corticotropin-releasing factor-41, the response to IL-1 /I was attenuated but not abolished. This suggests involvement of extra-paraventricular releasing factors in mediation of ACTH-releasing activity of IL-1 P, altered responsiveness of pituitary to CRFs, and/or direct action of IL-1 j on the corticotrope cells. TNF resulted in a biphasic stimulation of ACTH concentration, with peaks at 15 min and 90 min. In paraventricular-lesioned, TNF injected rats both of these ACTH peaks disappeared, suggesting that CRFs from the paraventricular origin mediates ACTH-inducing activity of TNF. IL-6 elevated ACTH secretion much later than the other intravenously injected cytokines, the peak was at 1 h in sham-lesioned rats. Paraventricular lesion completely prevented the increase of ACTH plasma levels after IL-6 injection. These data suggest that: (1) Effect of TNF and IL-6 on hypothalamo-pituitary-adrenal axis is mediated through the hypothalamic paraventricular nucleus and (2) IL-lB is able to release ACTH even in the absence of hypothalamic drive.     

Hypothalamic-Pituitary-Adrenal Axis Abnormalities in Response to Deletion of 11β-HSD1 is Strain-Dependent

Inter‐individual differences in hypothalamic‐pituitary‐adrenal (HPA) axis activity underlie differential vulnerability to neuropsychiatric and metabolic disorders, although the basis of this variation is poorly understood. 11β‐Hydroxysteroid dehydrogenase type 1 (11β‐HSD1) has previously been shown to influence HPA axis activity. 129/MF1 mice null for 11β‐HSD1 (129/MF1 HSD1^?/?) have greatly increased adrenal gland size and altered HPA activity, consistent with reduced glucocorticoid negative feedback. On this background, concentrations of plasma corticosterone and adrenocorticotrophic hormone (ACTH) were elevated in unstressed mice, and showed a delayed return to baseline after stress in HSD1‐null mice with reduced sensitivity to exogenous glucocorticoid feedback compared to same‐background genetic controls. In the present study, we report that the genetic background can dramatically alter this pattern. By contrast to HSD1^?/? mice on a 129/MF1 background, HSD1^?/? mice congenic on a C57Bl/6J background have normal basal plasma corticosterone and ACTH concentrations and exhibit normal return to baseline of plasma corticosterone and ACTH concentrations after stress. Furthermore, in contrast to 129/MF1 HSD1^?/? mice, C57Bl/6J HSD1^?/? mice have increased glucocorticoid receptor expression in areas of the brain involved in glucocorticoid negative feedback (hippocampus and paraventricular nucleus), suggesting this may be a compensatory response to normalise feedback control of the HPA axis. In support of this hypothesis, C57Bl/6J HSD1^?/? mice show increased sensitivity to dexamethasone‐mediated suppression of peak corticosterone. Thus, although 11β‐HSD1 appears to contribute to regulation of the HPA axis, the genetic background is crucial in governing the response to (and hence the consequences of) its loss. Similar variations in plasticity may underpin inter‐individual differences in vulnerability to disorders associated with HPA axis dysregulation. They also indicate that 11β‐HSD1 inhibition does not inevitably activate the HPA axis.