Differential involvement of the suprachiasmatic nucleus in lipopolysaccharide-induced plasma glucose and corticosterone responses

The hypothalamic suprachiasmatic nucleus (SCN) is an essential component of the circadian timing system, and an important determinant of neuroendocrine and metabolic regulation. Recent data indicate a modulatory role for the immune system on the circadian timing system. The authors investigated how the circadian timing system affects the hypothalamo-pituitary-adrenal (HPA) axis and glucose regulatory responses evoked by an immune challenge induced by lipopolysaccharide (LPS). LPS-induced increases in corticosterone were minimal during the trough of the daily corticosterone rhythm; in contrast, LPS effects on glucose, glucagon, and insulin did not vary across time-of-day. Complete ablation of the SCN resulted in increased corticosterone responses but did not affect LPS-induced hyperglycemia. The paraventricular nucleus (PVN) of the hypothalamus is an important neuroendocrine and autonomic output pathway for hypothalamic information, as well as one of the main target areas of the SCN. Silencing the neuronal activity in the PVN did not affect the LPS-induced corticosterone surge and only slightly delayed the LPS-induced plasma glucose and glucagon responses. Finally, surgical interruption of the neuronal connection between hypothalamus and liver did not affect the corticosterone response but slightly delayed the LPS-induced glucose response. Together, these data support the previously proposed circadian modulation of LPS-induced neuroendocrine responses, but they are at variance with the suggested major role for the hypothalamic pacemaker on the autonomic output of the hypothalamus, as reflected by the effects of LPS on glucose homeostasis. The latter effects are more likely due to direct interactions of LPS with peripheral tissues, such as the liver.     

Modulation of mammalian circadian rhythms by tumor necrosis factor-α

Systemic low doses of the endotoxin lipopolysaccharide (LPS, 100?µg/kg) administered during the early night induce phase-delays of locomotor activity rhythms in mice. Our aim was to evaluate the role of tumor necrosis factor (Tnf)-alpha and its receptor 1/p55 (Tnfr1) in the modulation of LPS-induced circadian effects on the suprachiasmatic nucleus (SCN). We observed that Tnfr1-defective mice (Tnfr1 KO), although exhibiting similar circadian behavior and light response to that of control mice, did not show LPS-induced phase-delays of locomotor activity rhythms, nor LPS-induced cFos and Per2 expression in the SCN and Per1 expression in the paraventricular hypothalamic nucleus (PVN) as compared to wild-type (WT) mice. We also analyzed Tnfr1 expression in the SCN of WT mice, peaking during the early night, when LPS has a circadian effect. Peripheral inoculation of LPS induced an increase in cytokine/chemokine levels (Tnf, Il-6 and Ccl2) in the SCN and in the PVN. In conclusion, in this study, we show that LPS-induced circadian responses are mediated by Tnf. Our results also suggest that this cytokine stimulates the SCN after LPS peripheral inoculation; and the time-related effect of LPS (i.e. phase shifts elicited only at early night) might depend on the increased levels of Tnfr1 expression. We also confirmed that LPS modulates clock gene expression in the SCN and PVN in WT but not in Tnfr1 KO mice.

Highlights: We demonstrate a fundamental role for Tnf and its receptor in circadian modulation by immune stimuli at the level of the SCN biological clock.     

DIURNAL,AGE, AND IMMUNE REGULATION OF INTERLEUKIN-1β AND INTERLEUKIN-1 TYPE 1 RECEPTOR IN THE MOUSE SUPRACHIASMATIC NUCLEUS

Circadian clocks serve to impose a near-24-h temporal architecture on an organism's physiology, metabolism, and behavior. In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus functions as the master circadian pacemaker. There is growing evidence that immunomodulators, such as cytokines, may impinge on circadian timekeeping. We examined whether there is endogenous expression of the proinflammatory cytokine interleukin-1β (IL-1β) and its signaling receptor IL-1R1 in the SCN of young and older mice across the diurnal cycle. We found expression of both IL-1β and IL-1R1 in the young SCN, although only IL-1R1 displayed temporal regulation. In the older SCN, levels of IL-1β were expressed at lower levels than in the young SCN, and IL-1R1 did not vary across the 24?h. We also report age-related day-night variation of IL-1β and IL-1R1 in the paraventricular nucleus (PVN) of the hypothalamus. Further, we examined the effect of peripheral immune challenge on IL-1β and IL-1R1 in the SCN. We found that IL-1β immunoreactivity was not altered 6 or 24?h after a septic dose of lipopolysaccharide (LPS; 5?mg/kg), whereas IL-1R1 was significantly up-regulated in the SCN both 6 and 24?h after LPS. We also demonstrate cellular activation in the SCN 24?h following LPS treatment, as evidenced by increased c-Fos and p65-NF-κB (nuclear factor kappa B) expression. Our results indicate that IL-1β and its associated signaling system may play a role in mediating the response of the circadian timing system to immune challenge as well as potentially contributing to the basal functioning of the SCN clock. (Author correspondence: andrew.coogan@nuim.ie)     

Expressions of <Emphasis Type="Italic">per1</Emphasis> Clock Gene and Genes of Signaling Peptides Vasopressin,Vasoactive Intestinal Peptide,and Oxytocin in the Suprachiasmatic and Paraventricular Nuclei of Hypertensive TGR[mREN2]27 Rats

Hypertensive rats with multiple extra copies of the renin gene (TGR) exert an inverted circadian blood pressure (BP) profile. We investigated whether circadian oscillations in the hypothalamic suprachiasmatic nucleus (SCN), a main circadian oscillator, and the paraventricular nucleus (PVN), involved in BP control, are influenced in TGR rats. The expression of the clock gene per1, a marker of circadian timing, was measured in the SCN and PVN. Moreover, the expression of genes encoding vasopressin (AVP), vasoactive intestinal peptide (VIP) in the SCN, and AVP and oxytocin (OXT) in the PVN were studied by in situ hybridization. Expression of the per1 gene showed a distinct circadian rhythm in both the SCN and PVN with no differences observed between the TGR and control Sprague–Dawley (SD) rats. The expression of avp in the SCN was rhythmic in both strains and moderately higher in TGR than in SD rats while no significant changes were found in the PVN. The expression of vip in the SCN and oxt in the PVN did not differ between both strains. Our results may indicate that changes occurring downstream to the SCN are responsible for the development of the inverted BP rhythm in TGR hypertensive rats.     

Role of Proinflammatory Cytokines on Lipopolysaccharide-Induced Phase Shifts in Locomotor Activity Circadian Rhythm

We previously reported that early night peripheral bacterial lipopolysaccharide (LPS) injection produces phase delays in the circadian rhythm of locomotor activity in mice. We now assess the effects of proinflammatory cytokines on circadian physiology, including their role in LPS-induced phase shifts. First, we investigated whether differential systemic induction of classic proinflammatory cytokines could explain the time-specific behavioral effects of peripheral LPS. Induction levels for plasma interleukin (IL)-1α, IL-1β, IL-6, or tumor necrosis factor (TNF)-α did not differ between animals receiving a LPS challenge in the early day or early night. We next tested the in vivo effects of central proinflammatory cytokines on circadian physiology. We found that intracerebroventricular (i.c.v.) delivery of TNF-α or interleukin IL-1β induced phase delays on wheel-running activity rhythms. Furthermore, we analyzed if these cytokines mediate the LPS-induced phase shifts and found that i.c.v. administration of soluble TNF-α receptor (but not an IL-1β antagonistic) prior to LPS stimulation inhibited the phase delays. Our work suggests that the suprachiasmatic nucleus (SCN) responds to central proinflammatory cytokines in vivo, producing phase shifts in locomotor activity rhythms. Moreover, we show that the LPS-induced phase delays are mediated through the action of TNF-α at the central level, and that systemic induction of proinflammatory cytokines might be necessary, but not sufficient, for this behavioral outcome. (Author correspondence: dgolombek@unq.edu.ar)     

Inactivation of the PVN during hypoglycemia partially simulates hypoglycemia-associated autonomic failure

The anatomic connections of the paraventricular nucleus of the hypothalamus (PVN) are such that it is ideally situated to modulate and/or control autonomic responses to a variety of stressors, including hypoglycemia. In our experimental model of hypoglycemia-associated autonomic failure (HAAF), a syndrome in which the counterregulatory response to hypoglycemia is partially compromised via unknown mechanisms, activation of the PVN is blunted (15). We hypothesized that this blunted PVN activation during HAAF may be sufficient to cause the impaired counterregulatory response. To test this hypothesis, we anesthetized the PVN with lidocaine during insulin-induced hypoglycemia in rats and measured counterregulatory hormone levels. PVN inactivation decreased indexes of the sympathoadrenal response (plasma epinephrine and norepinephrine) and the hypothalamic-pituitary axis response (ACTH). Inactivation decreased the peak epinephrine response to hypoglycemia by almost half (-42 +/- 6% from control; P = 0.04) and the peak norepinephrine response by 34 +/- 5% (P = 0.01). The peak plasma ACTH levels attained were suppressed by 35 +/- 6% (P = 0.02). Adrenal corticosterone and pancreatic glucagon responses were not impaired. This pattern of neuroendocrine response is unlike that previously seen with our HAAF model. Control infusions of lidocaine >or=1 mm anterior or posterior to the PVN did not simulate this neuroendocrine pattern. Thus it appears that decreased PVN activation, as occurs with HAAF, may be involved in specific components of HAAF (i.e., blunting the sympathoadrenal and hypothalamic-pituitary-adrenocortical axis response), but not in others (i.e., blunting the glucagon response).     

Vagotomy Affects Lipopolysaccharide-Induced Changes of Urocortin 2 Gene Expression in the Brain and on the Periphery

The corticotropin-releasing hormone family of peptides is involved in regulating the neuroendocrine stress response. Also, the vagus nerve plays an important role in the transmission of immune system-related signals to brain structures, thereby orchestrating the neuroendocrine stress response. Therefore, we investigated gene expression of urocortin 2 (Ucn2) and c-fos, a markers of neuronal activity, within the hypothalamic paraventricular nucleus (PVN), a brain structure involved in neuroendocrine and neuroimmune responses, as well as in the adrenal medulla and spleen in vagotomized rats exposed to immune challenge. In addition, markers of neuroendocrine stress response activity were investigated in the adrenal medulla, spleen, and plasma. Intraperitoneal administration of lipopolysaccharide (LPS) induced a significant increase of c-fos and Ucn2 gene expression in the PVN, and adrenal medulla as well as increases of plasma corticosterone levels. In addition, LPS administration induced a significant increase in the gene expression of tyrosine hydroxylase (TH) and phenylethanolamine N-methyltransferase (PNMT) in the adrenal medulla. In the spleen, LPS administration increased gene expression of c-fos, while gene expression of TH and PNMT was significantly reduced, and gene expression of Ucn2 was not affected. Subdiaphragmatic vagotomy significantly attenuated the LPS-induced increases of gene expression of c-fos and Ucn2 in the PVN and Ucn2 in the adrenal medulla. Our data has shown that Ucn2 may be involved in regulation of the HPA axis in response to immune challenge. In addition, our findings indicate that the effect of immune challenge on gene expression of Ucn2 is mediated by vagal pathways.

     

Circadian control of the daily plasma glucose rhythm: an interplay of GABA and glutamate

The mammalian biological clock, located in the hypothalamic suprachiasmatic nuclei (SCN), imposes its temporal structure on the organism via neural and endocrine outputs. To further investigate SCN control of the autonomic nervous system we focused in the present study on the daily rhythm in plasma glucose concentrations. The hypothalamic paraventricular nucleus (PVN) is an important target area of biological clock output and harbors the pre-autonomic neurons that control peripheral sympathetic and parasympathetic activity. Using local administration of GABA and glutamate receptor (ant)agonists in the PVN at different times of the light/dark-cycle we investigated whether daily changes in the activity of autonomic nervous system contribute to the control of plasma glucose and plasma insulin concentrations. Activation of neuronal activity in the PVN of non-feeding animals, either by administering a glutamatergic agonist or a GABAergic antagonist, induced hyperglycemia. The effect of the GABA-antagonist was time dependent, causing increased plasma glucose concentrations only when administered during the light period. The absence of a hyperglycemic effect of the GABA-antagonist in SCN-ablated animals provided further evidence for a daily change in GABAergic input from the SCN to the PVN. On the other hand, feeding-induced plasma glucose and insulin responses were suppressed by inhibition of PVN neuronal activity only during the dark period. These results indicate that the pre-autonomic neurons in the PVN are controlled by an interplay of inhibitory and excitatory inputs. Liver-dedicated sympathetic pre-autonomic neurons (responsible for hepatic glucose production) and pancreas-dedicated pre-autonomic parasympathetic neurons (responsible for insulin release) are controlled by inhibitory GABAergic contacts that are mainly active during the light period. Both sympathetic and parasympathetic pre-autonomic PVN neurons also receive excitatory inputs, either from the biological clock (sympathetic pre-autonomic neurons) or from non-clock areas (para-sympathetic pre-autonomic neurons), but the timing information is mainly provided by the GABAergic outputs of the biological clock.     

EARLY PROGRAMMING OF ASTROCYTE ORGANIZATION IN THE MOUSE SUPRACHIASMATIC NUCLEI BY LIGHT

The principal pacemaker in mammals, controlling physiology and behavior, is located in the suprachiasmatic nuclei (SCN) of the hypothalamus. Early photic experience has long-term effects on the animal's rhythmic behavior, as indicated by alterations in the phase shift induced by a light pulse, and in the expression of the circadian rhythm of locomotor activity under light-dark (LD), constant light (LL), and constant darkness (DD) environments. However, the brain substrates targeted by early light have not yet been identified. Possible candidates are astrocytes, as they develop postnatally in parallel to the circadian system, and are involved in SCN function by modulating intercellular communication and mediating photic input. Here, we reared three groups of mice under different light environments (LD, LL, and DD) during the suckling period. Later on, all mice were entrained to LD, and we determined associated astrocytic modifications by examining the expression of glial fibrillary acidic protein (GFAP) in the SCN. We observed that although LL-reared mice showed lowest GFAP expression in the SCN, as determined by quantification of immunostaining levels, the number of GFAP-positive cells was highest in this group, suggesting structural remodelling of SCN astrocytes by early light experience. These results indicate the postnatal light environment has long-term effects on the astrocytic population of the SCN. We argue that these neurochemical and structural alterations may affect clock function, which may in turn modify animal behavior (Author correspondence: maria.canal@manchester.ac.uk, Jose.Rodriguez-arellano@manchester.ac.uk).     

DIURNAL VARIATION IN ENDOTOXIN-INDUCED MORTALITY IN MICE: CORRELATION WITH PROINFLAMMATORY FACTORS

Many immune parameters exhibit daily and circadian oscillations, including the number of circulating cells and levels of cytokines in the blood. Mice also have a differential susceptibility to lipopolysaccharide (LPS or endotoxin)-induced endotoxic shock, depending on the administration time in the 24?h light-dark (LD) cycle. We replicated these results in LD, but we did not find temporal differences in LPS-induced mortality in constant darkness (DD). Animals challenged with LPS showed only transient effects on their wheel locomotor activity rhythm without modification of circadian period and phase. Levels of several key factors involved in the pathology of sepsis and septic shock were tested in LD. We found that LPS-induced levels of interleukin (IL)-1β, IL-6, JE (MCP-1), and MIP1α were significantly higher at zeitgeber time (ZT) 11 (time of increased mortality) than at ZT19 (ZT12?=?time of lights-off in the animal quarters for the 12L:12D condition). Our results indicate that the differences found in mortality that are dependent on the time of LPS-challenge are not directly related to an endogenous circadian clock, and that some relevant immune factors in the development of sepsis are highly induced at ZT11, the time of higher LPS-induced mortality, compared to ZT19. (Author correspondence: dgolombek@unq.edu.ar)     

PVN activation is suppressed by repeated hypoglycemia but not antecedent corticosterone in the rat

The mechanism(s) underlying hypoglycemia-associated autonomic failure (HAAF) are unknown. To test the hypothesis that the activation of brain regions involved in the counterregulatory response to hypoglycemia is blunted with HAAF, rats were studied in a 2-day protocol. Neuroendocrine responses and brain activation (c-Fos immunoreactivity) were measured during day 2 insulin-induced hypoglycemia (0.5 U insulin x 100 g body x wt(-1) x h(-1) iv for 2 h) after day 1 hypoglycemia (Hypo-Hypo) or vehicle. Hypo-Hypo animals demonstrated HAAF with blunted epinephrine, glucagon, and corticosterone (Cort) responses and decreased activation of the medial hypothalamus [the paraventricular (PVN), dorsomedial (DMH), and arcuate (Arc) nuclei]. To evaluate whether increases in day 1 Cort were responsible for the decreased hypothalamic activation, Cort was infused intracerebroventricularly (72 microg) on day 1 and the response to day 2 hypoglycemia was measured. Intracerebroventricular Cort infusion failed to alter the neuroendocrine response to day 2 hypoglycemia, despite elevating both central nervous system and peripheral Cort levels. However, day 1 Cort blunted responses in two of the same hypothalamic regions as Hypo-Hypo (the DMH and Arc) but not in the PVN. These results suggest that decreased activation of the PVN may be important in the development of HAAF and that antecedent exposure to elevated levels of Cort is not always sufficient to produce HAAF.     

DLK1 is a somato-dendritic protein expressed in hypothalamic arginine-vasopressin and oxytocin neurons

Delta-Like 1 Homolog, Dlk1, is a paternally imprinted gene encoding a transmembrane protein involved in the differentiation of several cell types. After birth, Dlk1 expression decreases substantially in all tissues except endocrine glands. Dlk1 deletion in mice results in pre-natal and post-natal growth deficiency, mild obesity, facial abnormalities, and abnormal skeletal development, suggesting involvement of Dlk1 in perinatal survival, normal growth and homeostasis of fat deposition. A neuroendocrine function has also been suggested for DLK1 but never characterised. To evaluate the neuroendocrine function of DLK1, we first characterised Dlk1 expression in mouse hypothalamus and then studied post-natal variations of the hypothalamic expression. Western Blot analysis of adult mouse hypothalamus protein extracts showed that Dlk1 was expressed almost exclusively as a soluble protein produced by cleavage of the extracellular domain. Immunohistochemistry showed neuronal DLK1 expression in the suprachiasmatic (SCN), supraoptic (SON), paraventricular (PVN), arcuate (ARC), dorsomedial (DMN) and lateral hypothalamic (LH) nuclei. DLK1 was expressed in the dendrites and perikarya of arginine-vasopressin neurons in PVN, SCN and SON and in oxytocin neurons in PVN and SON. These findings suggest a role for DLK1 in the post-natal development of hypothalamic functions, most notably those regulated by the arginine-vasopressin and oxytocin systems.     

PLASMA LEVELS OF INTERFERON-γ CORRELATE WITH AGE-RELATED DISTURBANCES OF CIRCADIAN RHYTHMS AND SURVIVAL IN A NON-HUMAN PRIMATE

Aging can be associated with changes in circadian rhythms and reduction in adaptive immune responses accompanied by expansion of memory T cells and elevated levels of pro-inflammatory cytokines. Recent findings suggest the cytokine interferon-γ (IFN-γ) can affect the function of the hypothalamic suprachiasmatic nucleus (SCN), the master mammalian circadian pacemaker, both in vitro and in vivo. We studied the correlation of plasma levels of IFN-γ and changes in circadian rhythms in a non-human primate species, the nocturnal mouse lemur (Microcebus murinus). Plasma IFN-γ and dehydroepiandrosterone sulfate (DHEA-S), a known biomarker of aging, were determined in middle- to old-age animals by immunoenzymoassay. Daily rhythms of locomotor activity and body temperature as well as survival time of the lemurs were recorded. With aging, mean levels of DHEA-S decreased whereas IFN-γ increased. Aged animals showed biological rhythm alterations characterized by a high percentage of diurnal activity, anticipation of the activity onset relative to lights-off, short free-running period, and delayed occurrence of minimal body temperature. The magnitude of these disturbances was correlated with the plasma level of IFN-γ but not DHEA-S. Most remarkably, in contrast to DHEA-S, increased levels of IFN-γ correlated with duration of the lifetime of the lemurs. These results show the degree of circadian rhythm alterations in an individual is correlated with plasma IFN-γ level during aging, and that plasma IFN-γ level may predict survival, at least in this non-human primate. (Author correspondence: fabienne.aujard@wanadoo.fr)     

Effects of central and systemic administration of leptin on neurotransmitter concentrations in specific areas of the hypothalamus

Leptin, a hormone produced by adipocytes, has been shown to affect a number of central functions, such as regulation of the hypothalamo-pituitary-adrenal axis, feeding, and body weight regulation. Because hypothalamic monoamines are intricately involved in the regulation of these functions, we hypothesized that leptin may produce its effects by altering the activity of these neurotransmitters. To test this hypothesis, male rats received peripheral (0, 100, or 500 microg ip), or central (0 or 5 microg icv) injections of leptin. The animals were killed 5 h later, and their brains were removed, frozen, and sectioned. Serum was collected to measure leptin and corticosterone by RIA. The paraventricular nucleus (PVN), arcuate nucleus (AN), ventromedial hypothalamus (VMH), dorsomedial dorsal nucleus (DMD), median eminence (ME), and medial preoptic area (MPA) were obtained using Palkovits' microdissection technique, and monoamine concentrations in these areas were determined using HPLC-EC. Intraperitoneal administration of leptin increased serum leptin concentrations in a dose-dependent manner (P < 0.05). Both intraperitoneal and intracerebroventricular administration of leptin decreased serum corticosterone significantly (P < 0.05). Norepinephrine (NE) concentration decreased significantly in the PVN, AN, and VMH after both intraperitoneal and intracerebroventricular administration of leptin (P < 0.05). NE concentrations decreased significantly in the DMN after intracerebroventricular administration of leptin (P < 0.05). Leptin treatment (both ip and icv) decreased dopamine concentrations significantly in the PVN. Serotonin (5-HT) concentration decreased significantly in the PVN after both intraperitoneal and intracerebroventricular injections of leptin and decreased in the VMH only with intracerebroventricular treatment of leptin. Leptin did not affect any of the monoamines in the ME and MPA. These results indicate that both central and systemic administration of leptin can affect hypothalamic monoamines in a region-specific manner, which, in turn, could mediate many of leptin's central and neuroendocrine effects.     

Adrenalectomy enhances endotoxemia-induced hypophagia: higher activation of corticotrophin-releasing-factor and proopiomelanocortin hypothalamic neurons

Inflammatory and infectious processes evoke neuroendocrine and behavioral changes known as acute-phase response that includes activation of the hypothalamo-pituitary-adrenal (HPA) axis and reduction of food intake. Besides its action as the most important ACTH secretagogue, corticotrophin-releasing factor (CRF), synthesized in the paraventricular nucleus (PVN), is also involved in the control of food intake. Alpha-melanocyte stimulating hormone (α-MSH) in the arcuate nucleus also plays a role in the energy homeostasis, possessing anorexigenic effects. To investigate the participation of neuropeptides involved in the regulation of food intake during endotoxemia, we administrated lipopolysaccharide (LPS) in sham-operated and adrenalectomized (ADX) male Wistar rats to evaluate food intake, hormone responses and Fos-CRF and Fos-α-MSH immunoreactivity in the PVN and arcuate nucleus, as well as CRF and POMC mRNA expression in these hypothalamic nuclei. In sham-operated rats, treatment with LPS (100 µg/kg) showed lower food intake, higher plasma ACTH and corticosterone levels, as well as an increase in Fos-CRF double labeled neurons and CRF mRNA expression in the PVN, with no changes in Fos-α-MSH immunoreactivity and POMC mRNA expression in the arcuate nucleus, compared to saline treated rats. After LPS treatment, ADX rats showed further increase in plasma ACTH levels, marked decrease of food intake, higher Fos-CRF immunoreactive neurons in the PVN and CRF mRNA expression, as well as an increase in Fos-α-MSH immunoreactivity and POMC mRNA expression in the arcuate nucleus, compared to sham-operated rats treated with LPS. In conclusion, the present data indicate that the marked hypophagia during endotoxemia following ADX is associated with an increased activation of CRF and POMC neurons in the hypothalamus and an increased mRNA expression of these neuropeptides.     

Actions of neuropeptide W in paraventricular hypothalamus: implications for the control of stress hormone secretion

Neuropeptide W (NPW) is produced in neurons located in hypothalamus and brain stem, and its receptors are present in the hypothalamus, in particular in the paraventricular nucleus (PVN). Intracerebroventricular (ICV) administration of NPW activated, in a dose-related fashion, the hypothalamic-pituitary-adrenal axis, as determined by plasma corticosterone levels in conscious rats but, at those same doses, did not stimulate the release of oxytocin or vasopressin into the peripheral circulation or alter blood pressure or heart rate. The ability of ICV-administered NPW to stimulate the hypothalamic-pituitary-adrenal axis in conscious male rats was blocked by intravenous pretreatment with a corticotropin-releasing hormone antagonist. This suggested an action of NPW in the parvocellular division of the PVN. Indeed, in hypothalamic slice preparations (whole cell patch recording), bath application of NPW depolarized and increased the spike frequency of the majority of electrophysiologically identified putative neuroendocrine PVN neurons. Effects on membrane potential were maintained in the presence of TTX, suggesting them to be direct postsynaptic actions on these neuroendocrine cells. Our data suggest that endogenous NPW, produced in brain, may play a physiologically relevant role in the neuroendocrine response to stress.     

PHOTIC INDUCTION OF Fos IN THE SUPRACHIASMATIC NUCLEUS OF AFRICAN MOLE-RATS: RESPONSES TO INCREASING IRRADIANCE

African mole-rats (family Bathyergidae) are strictly subterranean rodent species that are rarely exposed to environmental light. Morphological and physiological adaptations to the underground environment include a severely reduced eye size and regressed visual system. Responses of the circadian system to light, however, appear to be intact, since mole-rats are able to entrain their circadian activity rhythms to the light-dark cycle and light induces Fos expression in the suprachiasmatic nucleus (SCN). Social organization varies from solitary species to highly elaborated eusocial structures, characterized by a distinct division of labor and in which one reproductive female regulates the behavior and reproductive physiology of other individuals in the colony. The authors studied light-induced Fos expression in the SCN to increasing light intensities in four mole-rat species, ranging from strictly solitary to highly social. In the solitary Cape mole-rat, light induces significant Fos expression in the SCN, and the number of Fos-immunopositive cells increases with increasing light intensity. In contrast, Fos induction in the SCN of social species was slightly greater than, but not statistically different from, the dark-control animals as is typical of most rodents. One species showed a trend for an increase in expression with increased light, whereas a second species showed no trend in expression. In the naked mole-rat, Fos expression appeared higher in the dark-controls than in the animals exposed to light, although the differences in Fos expression were not significant. These results suggest a gradient in the sensitivity of the circadian system to light in mole-rats, with a higher percentage of individuals that are unresponsive to light in correlation with the degree of sociality. In highly social species, such as the naked mole-rat that live in a relatively stable subterranean milieu in terms of food availability, temperature, constant darkness, and devoid of 24-h cyclic environmental cues, the temporal coordination of rest-wake activities may be dependent on social interactions and social status rather than on photic regulation of the circadian timing system. (Author correspondence: moosthuizen@zoology.up.ac.za)     

Medical History of Optic Chiasm Compression in Patients With Pituitary Insufficiency Affects Skin Temperature and Its Relation to Sleep

The hypothalamus is crucially involved in the circadian timing of the sleep-wake rhythm, yet also accommodates the most important thermoregulatory neuronal network. We have shown before that adults with pituitary insufficiency and history of chiasm compression due to a tumor with suprasellar extension fall asleep later and sleep shorter than those without such history and presumed hypothalamic involvement. To solidify the hypothesized link between vigilance and thermoregulation by the hypothalamus, we aimed to test the hypothesis that the presumed hypothalamic impairment in these patients also affects skin temperature and its association with sleep onset. In a case-control study of 50 patients (54.7?±?14.5 yrs of age, 30 males) with pituitary insufficiency, 33 of whom had a history of chiasm compression, ambulatory distal and proximal skin temperatures were assessed continuously for 24?h. Sleep parameters were assessed via questionnaire. Group differences in mean skin temperature, calculated over the wake and sleep periods separately, and group differences in the strength of association between pre-sleep skin temperature and sleep onset latency were compared. Results showed that patients with a medical history of chiasm compression had lower proximal skin temperature during the day (34.1°C?±?.7°C vs. 34.6°C?±?.7°C, p?=?.045). Additionally, the typical association between sleep onset latency and pre-sleep distal-to-proximal skin temperature gradient was absent in these patients (r?=??.01, p?=?.96), whereas it was unimpaired in those without chiasm compression (r?=??.61, p?=?.02). Thus, patients with history of chiasm compression show impaired skin temperature regulation in association with disturbed sleep. The findings support the hypothesis that a medical history of chiasm compression affects hypothalamic regulation of both vigilance and temperature, possibly by chronically affecting relevant nuclei, including the ventrolateral preoptic area and anterior hypothalamic preoptic area. (Corresponding Author: n.romeijn@nin.knaw.nl)