Indomethacin Attenuates Oxytocin and Hypothalamic-Pituitary-Adrenal Axis Responses to Systemic Interleukin-1β

Systemic administration of the cytokine IL-1β produces a significant release of ACTH into the plasma and activation of hypothalamic oxytocin (OT) and corticotropin releasing factor (CRF) cells. However, the mechanism(s) by which systemic IL-1β induces these responses is not clear. In the present study, we have investigated the proposal that catecholamine cells of the ventrolateral medulla (VLM) and nucleus of the solitary tract (NTS) can relay circulating IL-1 signals via a prostaglandin-dependent mechanism to effect the HPA axis responses in the rat. Intra-arterial administration of IL-1β (1 pg/kg) to otherwise untreated animals produced a prominent release of ACTH into the plasma, substantial c-fos expression in paraventricular medial parvocellular (mPVN) corticotropin releasing factor (CRF) cells, supraoptic (SON) and paraventricular nucleus (PVN) OT cells, area postrema cells, NTS and VLM catecholamine cells and cells of the central amygdala. Pretreatment with the prostaglandin synthesis inhibitor, indomethacin (10 mg/kg body weight ia) 15 min before IL-1β administration (1 pg/kg ia) significantly reduced plasma ACTH release and c-fos expression in PVN and SON OT cells and MPVN CRF cells. In addition, the area postrema, A1 and C1 catecholamine cell groups of the VLM and A2 and C2 catecholamine cell groups of the NTS, all exhibited concomitant reductions in c-fos expression. Conversely indomethacin administration did not alter the IL1β-induced expression of c-fos in the central amygdala. These data suggest that central pathways involved in the IL-1β-induced activation of the HPA axis and OT cells are, at least in part, dependent upon prostaglandin synthesis. It is proposed that neurons in the area postrema, NTS and VLM might mediate this IL-1β-induced activation of hypothalamic CRF and OT cells and release of ACTH into the plasma.     

Effects of intracerebroventricular dizocilpine (MK801) on dehydration-induced dipsogenic responses, plasma vasopressin and c-fos expression in the rat forebrain

This study determined the interaction between glutamate receptors and dehydration-induced drinking, vasopressin (AVP) release, plasma osmolality and c-fos expression in the brain of conscious rats. The NMDA receptor antagonist dizocilpine (100 nmol infused into the cerebral ventricles) suppressed drinking following either 22 h water deprivation or intragastric injection of hypertonic saline (1.5 M), attenuated the increased plasma vasopressin induced by dehydration, but had no effects on peripheral hyperosmolality caused by either water deprivation or injections of hypertonic saline. Dizocilpine had no inhibitory effects on feeding after 24 h food deprivation. Dizocilpine also suppressed c-fos expression induced by dehydration in the median preoptic nucleus (MPN), the supraoptic and paraventricular nuclei (SON and PVN), but did not influence c-fos expression in the subfornical organ (SFO). The non-NMDA receptor antagonists CNQX (400 nmol) or DNQX (60 nmol) affected neither the animals' drinking nor c-fos expression induced by dehydration. Double staining showed that suppression of c-fos expression following dizocilpine occurred in the NMDA R1 receptor containing neurons in the hypothalamus. These results suggest that the NMDA-type glutamate receptors may be involved in dehydration induced dipsogenic and neuroendocrinological responses. They complement our earlier findings that dizocilpine also attenuated drinking and c-fos expression following intraventricular infusions of angiotensin II.     

Effects of unilateral or bilateral lesions within the anteroventral third ventricular region on c-fos expression induced by dehydration or angiotensin II in the supraoptic and paraventricular nuclei of the hypothalamus

This paper reports the effects of AV3V lesions on the pattern of c-fos induced by 24 h dehydration. As expected, bilateral electrolytic lesions within the AV3V region (the ventral median preoptic nucleus) suppressed water intake following 24 h water deprivation. C-fos expression was also suppressed in the supraoptic (SON) and (less completely) in the paraventricular (PVN) nuclei, but not in the subfornical organ (SFO). Unilateral lesions of the AV3V region suppressed c fos expression in the ipsilateral SON, but this selective ipsilateral effect was less in the PVN. The SFO was again unaffected. Unilateral lesions also suppressed c-fos expression in the ipsilateral SON and PVN (to a lesser degree) following intraventricular infusions of angiotensin 11 (250 pmol). These results suggest that the cellular response of supraoptic neurons to osmotic stimuli require inputs from the AV3V region, but that this is less absolute for the PVN; that the projection from the ventral AV3V area to the SON is ipsilateral, but that to the PVN may be less lateralised. Activation of the SFO by dehydration is not dependent upon the integrity of the ventral AV3V region. These results are closely comparable to the effects of similar lesions on c-fos expression following intraventricular infusions of angiotensin 11, and suggest that the effect of dehydration on forebrain c-fos expression may be related to the central actions of angiotensin II.     

Increased Fos Expression in Oxytocin Neurons Following Masculine Sexual Behavior

Induction of the c-fos protein product (Fos) was used to immunocytochemically identify oxytocin (OT) neurons that may be activated during copulatory interactions. Fos induction was quantified in sexually-experienced male rats after either (a) exposure to a testing arena recently vacated by an estrous female, (b) copulatory interactions such as mounting and intromission without ejaculation, or (c) mounting and intromissions culminating in ejaculation. In the parvocellular regions of the paraventricular nucleus of the hypothalamus (PVN), the number of neurons expressing Fos increased following either intromission (53%) or ejaculation (124%). Significant, but less striking, increases in the number of cells expressing Fos were noted in magnocellular regions of the PVN where intromission resulted in a 13% increase and ejaculation in a 49% increase in Fos. The number of perikarya immunoreactive for OT and AVP did not differ as a function of increasing sexual contacts. In control (novel arena) males, 33–73% of the Fos labeling occurred in OT cells. Sexual interactions did not enhance the number of double-labeled cells in most parvocellular regions. However, in lateral parvocellular regions located in the most caudal aspects of the PVN, 31% of the Fos-positive cells occurred in OT neurons in ejaculated males, while in control males none of the OT cells were double-labeled. This PVN subdivision is known to consist of neurons that project to the brain stem and spinal cord at lumbar levels which contain motor neurons that regulate penile reflexes. The present data suggest a possible neurochemical circuit which incorporates oxytocinergic neurons in the mediation of masculine sexual responses.     

Role of arginine vasopressin and corticotropin-releasing factor in mediating alcohol-induced adrenocorticotropin and vasopressin secretion in male rats bearing lesions of the paraventricular nuclei

In male rats, lesions of the paraventricular nucleus (PVN) of the hypothalamus attenuate, but do not abolish, adrenocorticotropin (ACTH) secretion in response to acute alcohol injection. As the PVN is the major source of corticotropin-releasing factor (CRF) in the median eminence, this observation suggests that extra-PVN brain regions, and/or ACTH secretagogues other than CRF (e.g. arginine vasopressin (AVP)), mediate ACTH stimulation by alcohol. This hypothesis was tested by examining the effect of AVP immunoneutralization in PVN-lesioned (PVNx) rats. Removal of endogenous AVP diminished alcohol-evoked ACTH secretion in both sham-operated and PVNx animals, indicating that AVP from outside the PVN partially mediates the hypothalamic-pituitary-adrenal (HPA) axis response to alcohol. This led us to determine whether alcohol might also regulate AVP steady-state gene expression in the supraoptic nucleus (SON) and PVN, and/or CRF mRNA in the PVN and the central nucleus of the amygdala (AMY). In the magnocellular portion of the PVN, sham-operated animals showed significantly increased PVN levels of both CRF and AVP mRNAs 3 h after alcohol. In the SON, alcohol administration tended to decrease AVP gene expression in PVNx rats, while the drug increased AVP mRNA levels in the SON of sham-operated rats. AMY levels of CRF mRNA were unaffected by these manipulations. Finally, since the regulation of alcohol-induced AVP mRNA levels in the SON appeared to depend on the presence of the PVN, we measured peripheral levels of AVP in both sham-operated and PVNx animals after injection of vehicle or alcohol. Although AVP decreased in all groups, alcohol depressed AVP secretion to a greater extent in PVNx animals, suggesting that AVP systems are more sensitive to inhibition in the absence of the PVN. Our results demonstrate that although AVP of PVN origin may participate in regulating the stimulatory effect to AVP on ACTH secretion, AVP from areas other than the PVN also plays a role. Additionally, regulation of both AVP gene expression in the SON and secretion in the systemic circulation are altered in rats bearing lesions of the PVN.     

Inhibition of Luteinizing Hormone Secretion and Expression of c-fos and Corticotrophin-Releasing Factor Genes in the Paraventricular Nucleus During Insulin-Induced Hypoglycaemia in Sheep

Insulin can act within the brain to stimulate ovine luteinizing hormone (LH) secretion, but insulin-induced hypoglycaemia inhibits LH via unknown brain sites, possibly involving corticotrophin-releasing factor (CRF). Castrate male sheep, with (E+) or without (E?) subcutaneous oestradiol implants, were blood sampled every 12 min for 8 h. Insulin (0.25 or 0.5 IU/kg) was injected at 4 h via the carotid artery or jugular vein. All treatments reduced LH output with no differences between dose rate nor route of administration, but sensitivity was greater in E+ than E?sheep. There was no evidence for an effect of insulin on LH 0–1 h postinjection; however, 1–3 h after insulin, when hypoglycaemia was established, LH pulses were inhibited in both E+ and E? sheep (P<0.001). Additional intravenous (i.v.) glucose injections given 1 h (20 mmol) and 2 h (10 mmol) after insulin (0.5 IU/kg) were each followed by an LH pulse within 30 min (75% response in both E+ and E? sheep). In a separate experiment, sheep were killed 2 h after i.v. insulin (0.5 IU/kg) or saline. In-situ hybridization revealed c-fos mRNA in the paraventricular nucleus (PVN), but not in any other hypothalamic nuclei nor in the hindbrain; and this was linked with increased CRF gene expression in the PVN. Similar c-fos and CRF gene expression was seen in insulin-treated sheep given additional i.v. glucose (20 and 10 mmol, respectively, 40 and 20 min ante mortem), but not in saline-treated controls. Therefore, insulin-induced hypoglycaemia inhibited LH secretion, with oestradiol potentiating the effect, and was associated with gonadal steroid-independent c-fos gene expression and increased CRF gene expression in the PVN. The ovine PVN may be involved in mediating insulin-induced hypoglycaemic inhibition of LH by a mechanism which might involve CRF.     

Chronic electroconvulsive shock treatment elicits up-regulation of CRF and AVP mRNA in select populations of neuroendocrine neurons

The effects of repeated electroconvulsive seizures (ECS) on expression of mRNAs coding for corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) in neuroendocrine neurons of the hypothalamo-pituitary-adrenocortical (HPA) axis and hypothalamo-neurohypophysial system (HNS) were assessed via semi-quantitative in situ hybridization histochemical analysis. Measures of mRNA content were accompanied by measurement of peptide- and hormone-expression in the relevant neuroendocrine systems. Following 7 daily ECS treatments, CRF mRNA was significantly increased in the medial parvocellular paraventricular nucleus (PVN) of treated rats relative to controls. CRF peptide content of whole PVN homogenates was decreased to 50% of control levels. Changes in CRF message and peptide levels were accompanied by increases in pituitary ACTH content and by elevated plasma corticosterone, suggesting ECS elicits long-term up-regulation of the HPA axis. AVP mRNA in the medial parvocellular PVN, which is known to up-regulate in response to HPA challenge by adrenalectomy, was not increased by ECS. Chronic ECS causes a clear up-regulation of HNS neurons of the supraoptic nucleus, characterized by increased AVP mRNA content, decreased AVP peptide content, and depletion of neurohypophysial AVP. However, no changes were observed in magnocellular vasopressinergic neurons of the PVN, indicating that magnocellular SON and PVN neurons respond differentially to stimulation by ECS. The data indicate that ECS is a potent stimulus for activation of select components of both the HPA axis and the HNS. As such, ECS provides a useful tool for examining mechanism underlying neuroendocrine processes.     

Sequential Exposure to Estrogen and Testosterone (T) and Subsequent Withdrawal of T Increases the Level of Arginine Vasopressin Messenger Ribonucleic Acid in the Hypothalamic Paraventricular Nucleus of the Female Rat

The hypothalamic peptides arginine vasopressin (AVP) and oxytocin (OT) have been implicated as mediators of socio-sexual behaviors in addition to their roles in osmolar homeostasis (AVP), milk ejection and uterine contractility (OT). Within 24  h of parturition, OT and AVP messenger ribonucleic acid (mRNA) levels increase in the hypothalamic paraventricular, and to a lesser degree, the supraoptic nucleus (PVN and SON) of the rat. We previously reported that the prepartum increase in OT mRNA is related to the spontaneous decline in progesterone levels prior to parturition. We also reported that increases in PVN and SON OT mRNA can be induced by exposing the ovariectomized rat to a steroid regimen that mimics the steroid milieu of pregnancy, namely sequential estrogen and progesterone and subsequent progesterone withdrawal. Levels of PVN and SON AVP mRNAs were not affected by progesterone withdrawal in late pregnant rats or the steroid regimen that increased OT mRNA in ovariectomized rats. These observations suggest that other factors, perhaps hormonal, may influence AVP mRNA levels. A decline in testosterone coincident with waning progesterone levels also occurs prepartum. Since peak levels of AVP mRNA prepartum coincide with the prepartum decline in testosterone, we questioned whether declining testosterone levels are important for the increase in AVP mRNA levels.     

Intracerebroventricular administration of β-endorphin increases the expression of c-fos and of corticotropin-releasing factor messenger ribonucleic acid in the paraventricular nucleus of the rat

We evaluated the effects of intracerebroventricular (i.c.v.) administration of β-endorphin and naloxone, an opioid antagonist, on the induction of c-fos and corticotropin-releasing factor (CRF) mRNA to clarify the effects of β-endorphin on cellular activity and CRF gene expression in the paraventricular nucleus (PVN) of the rat using in situ hybridization. A significant induction of c-fos mRNA was noted in the PVN after i.c.v. injection of β-endorphin, compared to control. This induction was inhibited by the administration of naloxone. A significant increase in CRF mRNA levels in the PVN was observed 120 min after the i.c.v. injection of β-endorphin. This increase was partially, but significantly, inhibited by naloxone administration. In addition, i.c.v. administration of β-endorphin increased plasma ACTH concentration in freely moving rats, which was inhibited by intravenous injection of CRF antiserum. These results suggest that the i.c.v. injection of β-endorphin increases the neuronal activity and the biosynthesis of CRF in the PVN, and stimulates the secretion of ACTH by increasing CRF secretion. This effect on the PVN was mediated, at least in part, via the opioid receptor.     

Stress-induced activation of CRF and c-fos mRNAs in the paraventricular nucleus are not affected by serotonin depletion

The role of serotonin in regulating the stress response is controversial. We have investigated the effects of serotonin depletion byp-chlorophenylalanine (PCPA) on corticotrophin-releasing factor (CRF) mRNA and c-fos mRNA responses in the paraventricular nucleus (PVN) together with circulating levels of ACTH and corticosterone to both physical and psychological stressors in the rat. PCPA pretreatment, which resulted in a 95% depletion in hypothalamic serotonin, had no effect on basal levels of ACTH or the increase in response to the physical stress of hypertonic saline. Plasma ACTH concentrations were also not affected by serotonin depletion in response to the predominantly psychological stress of restraint. Both basal and restraint stress-induced circulating corticosterone levels were however further stimulated in the PCPA-pretreated rats suggesting a possible inhibitory serotoninergic tone at the adrenal level. C-fos mRNA was undetectable in control animals. Activation of c-fos mRNA in response to stress was unaffected by serotonin depletion and the activation of magnocellular PVN and supraoptic nucleus cells was demonstrated to be stressor dependent. Basal and stress-induced levels of CRF mRNA were unaffected by PCPA pretreatment. It appears therefore that under these experimental conditions there is little if any involvement of serotonin in either basal levels or the stress-induced activation of the hypothalamo-pituitary-adrenal axis in vivo.     

Differential effects of central and peripheral injection of interleukin-1β on brain c-fos expression and neuroendocrine functions

Cytokines such as interleukin-1β (IL-1β) alter the activity of the hypothalamic-pituitary-adrenal (HPA) and hypothalamic-pituitary-gonadal (HPG) axes in the rat. However, the brain sites at which IL-1β exerts these effects have not been well identified. The present study sought to identify some of these sites, using c-fos protein expression as an index of cellular activation. We also attempted to determine possible differences between the effects of peripheral and central injection of IL-1β on the activation of specific brain areas. Castrated male rats received intravenous (i.v.) or intracerebroventricular (i.c.v.) injections of IL-1β through a jugular catheter or a permanent cannula implanted in the right lateral ventricle, respectively. Blood samples were taken before, as well as 30 and 120 min after i.v. or i.c.v. IL-1β infusion in order to measure plasma ACTH and LH levels. Immediately thereafter, the rats were anesthetized with pentobarbital, then perfused. Their brains were removed and postfixed for one hour. Thirty-μm frozen sections were cut and approximately every fourth tissue section was processed for c-fos expression by an avidin-biotin-peroxidase method. Both i.v. (1 μg) and i.c.v. (100 ng) injection of IL-1β significantly increased plasma ACTH levels, but only i.c.v. treatment measurably inhibited LH secretion. I.c.v. infusion of the cytokine markedly augmented c-fos expression in the paraventricular nucleus (PVN) and the arcuate nucleus (ARC) of the hypothalamus. A large amount of CRF cells in the PVN contained labelled c-fos protein (as measured by a double labelling technique), which indicates that CRF perikarya in this hypothalamic region are activated by the central administration of IL-1β. In contrast, i.v. injection of IL-1β did not significantly alter c-fos expression in the PVN or the ARC of the hypothalamus. These results suggest that the increased HPA axis activity which follows the peripheral IL-1β administration, a phenomenon previously shown to depend on endogenous CRF, does not require immediate activation of hypothalamic CRF perikarya. Thus our results indicate that the stimulatory effect of blood-born cytokine may be exerted at the level of nerve terminals in the median eminence. In contrast, i.c.v.-injected IL-1β appears to activate the HPA axis through a stimulation of CRF neurons within the parvocellular part of PVN. Finally, we postulate that the increase in cellular activity observed in the ARC of the hypothalamus may be involved in the decrease in LH secretion observed after i.c.v. infusion of IL-1β.     

LPS-induced Fos expression in oxytocin and vasopressin neurons of the rat hypothalamus

The aim of this study was to examine the involvement of the hypothalamic oxytocin (OXT) and vasopressin (AVP) neurons in acute phase reaction using quantitative dual-labeled immunostaining with Fos and either OXT and AVP in several hypothalamic regions. Administration of low dose (5 μg/kg) and high dose (125 μg/kg) of LPS induced intense nuclear Fos immunoreactivity in many OXT and AVP neurons in all the observed hypothalamic regions. The percentage of Fos-positive nuclei in OXT magnocellular neurons was higher than that of AVP magnocellular neurons in the supraoptic nucleus (SON), the magnocellular neurons in the paraventricular nucleus (magPVN), rostral SON (rSON), and nucleus circularis (NC), whose axons terminate at the posterior pituitary for peripheral release. The percentage of Fos-positive nuclei in AVP parvocellular neurons in the paraventricular nucleus (parPVN) was higher than that of OXT parvocellular neurons, whose axons terminate within the brain for central release. Moreover, the percentage of Fos-positive nuclei in AVP magnocellular neurons of the SON and rSON was significantly higher than that of the magPVN and NC when animals were given LPS via intraperitoneal (i.p.)-injection. This regional heterogeneity was not observed in OXT magnocellular neurons of i.p.-injected rats or in either OXT or AVP magnocellular neurons of intravenous (i.v.)-injected rats. The present data suggest that LPS-induced peripheral release of AVP and OXT is due to the activation of the magnocellular neurons in the SON, magPVN, NC, and rSON, and the central release of those hormones is in part derived from the activation of parvocellular neurons in the PVN. It is also suggested that the activation of AVP magnocellular neurons is heterogeneous among the four hypothalamic regions, but that of OXT magnocellular neurons is homogenous among these brain regions in response to LPS administration.     

Corticotropin-releasing factor type-1 receptor mRNA is not induced in mouse hypothalamus by either stress or osmotic stimulation

In rats, acute stress substantially increases corticotropin-releasing factor (CRF) type 1 receptor (CRFR-1) mRNA expression in the paraventricular nucleus (PVN) and osmotic stimulation induces both CRF and CRFR-1 mRNA in magnocellular PVN and supraoptic nucleus (SON). However, these phenomena have not been analysed in other species. We compared CRF and CRFR-1 expression in rat and mouse hypothalamus. Male C57BL/6 mice and Wistar rats were exposed to acute restraint stress for 3 h, or to hypertonic saline ingestion for 7 days. Restraint stress increased CRF and c-fos mRNA expression in both rat and mouse PVN. CRFR-1 mRNA was barely detectable in controls, whereas restraint stress substantially increased CRFR-1 mRNA in rat PVN, but not in mouse. Hypertonic saline ingestion induced CRF mRNA in magnocellular PVN and SON of the rat, but did not alter CRF mRNA levels in mouse hypothalamus. CRFR-1 mRNA was also induced in magnocellular PVN and SON of the rat in response to osmotic stimulation, but not in mouse. Immunohistochemistry demonstrated that CRFR-1-like immunoreactivity (ir) was distributed within parvocellular and magnocellular PVN of mouse and rat. CRFR-1-ir in rat PVN was increased by acute stress and osmotic stimulation. By contrast, these treatments did not alter CRFR-1-ir in mouse PVN. Combined immunohistochemistry and in situ hybridization revealed that CRFR-1-ir was most frequently colocalized to CRF in mouse PVN, whereas only a small percentage of oxytocin and vasopressin-producing cells coexpressed CRFR-1-ir. These results indicate that (i) by contrast to rats, neither acute stress nor osmotic stimulation induces CRFR-1 mRNA expression in the mouse PVN; (ii) osmotic stimulation does not alter CRF mRNA expression in parvocellular and magnocellular neurones of mouse PVN; and (iii) acute stress increases c-fos and CRF mRNA to a similar degree in mouse and rat PVN. Thus, differences may exist between mouse and rat in the regulation of CRF and CRFR-1 gene expression in hypothalamus following stress and osmotic stimulation.     

Expression of Fos immunoreactivity in rat brain during dehydration: effect of duration and timing of water deprivation

Water deprivation induces expression of the immediate early gene c-fos in specific brain regions, most likely as a result of the activation of cells that are responsive to changes in osmolality and/or blood volume. We hypothesized that the magnitude of c-fos expression would be a function of both the duration of water deprivation and the time of day at which the deprivation started. This study was designed to examine the pattern of Fos-like immunoreactivity (FLI) following water deprivation in rats under normal light/dark conditions (nLD) and reverse light/dark conditions (rLD). Rats were deprived of water but not food either for 0, 5, 16, 24 or 48 h. As expected, hematocrit ratio (HCT), osmolality (OSM), plasma renin activity (PRA) and weight loss increased as a function of duration of water deprivation. In non-deprived rats (0 h), very little FLI was observed in most brain regions. The number of cells showing FLI increased with duration of water deprivation in the supraoptic nucleus (SON), paraventricular nucleus (PVN), organum vasculosum laminae terminalis (OVLT), median preoptic nucleus (MnPO) and subfornical organ (SFO) in both nLD and rLD conditions. However, the pattern of FLI differed between nLD and rLD conditions. Compared to corresponding nLD groups after 5 or 24-h water deprivation, rLD groups had significantly more FLI in SON and PVN, and higher PRA and HCT. Also, weight loss and FLI in the MnPO were greater after 5 h, and FLI in the SFO was greater after 24 h under rLD compared to nLD conditions. Our findings indicate that the magnitude of c-fos expression, and change in weight and plasma parameters were a function of both the duration of water deprivation and the time of day at which the deprivation started. This may result from ingestion of food early in the deprivation periods during the rLD tests, thus producing greater change in osmolality and blood volume.     

Area postrema removal abolishes stimulatory effects of intravenous interleukin-1β on hypothalamic-pituitary-adrenal axis activity and c-fos mRNA in the hypothalamic paraventricular nucleus

This study examined the role of the area postrema (AP) in transducing peripheral immune signals, represented by intravenous (i.v.) interleukin-1β (IL-1), into neuroendocrine responses. The AP, a circumventricular organ with a leaky blood–brain barrier, lies adjacent to the nucleus of the solitary tract (NTS) in the medulla. The AP was removed by aspiration, and 2 weeks later, AP-lesioned or sham-lesioned rats were injected i.v. with 0.5 μg/kg IL-1 or sterile saline. After 30 min, brains were removed and analyzed for c-fos mRNA levels in various structures implicated in the hypothalamic-pituitary-adrenal axis response to peripheral cytokine challenge. The sham-lesioned animals responded to IL-1 with large elevations in adrenocorticotropic hormone (ACTH) and corticosterone levels in the plasma and c-fos mRNA levels in cells of the AP, NTS, central nucleus of the amygdala, bed nucleus of the stria terminalis, hypothalamic paraventricular nucleus (PVN), and meninges. Prior AP removal abolished the IL-1-induced increases in ACTH and corticosterone in the plasma and c-fos mRNA levels in the NTS and PVN. However, AP removal had no effect on IL-1-induced increases in c-fos mRNA levels in the other areas examined. The selective AP lesion effects suggest that the AP and adjacent NTS play a pivotal role in transducing a circulating IL-1 signal into hypothalamic-pituitary-adrenal axis activation by a pathway that may be comprised of known anatomical links between the AP, NTS, and corticotropin-releasing hormone neurons of the PVN.