Central Nomega-nitro-L-arginine methyl ester does not influence lithium-induced c-Fos and conditioned taste aversion

LiCl at doses sufficient to induce conditioned taste aversion (CTA) causes c-Fos expression in the brain regions implicated in CTA formation. It has been reported that nitric oxide (NO) may play a role in CTA learning and LiCl increases both the synthesis and activity of NO synthase (NOS) in the brain. In this study, we examined the effect of central Nomega-nitro-L- arginine methyl ester (L-NAME) on the brain c-Fos expression and CTA learning induced by lithium in rats. In the results, intracerebroventricular L-NAME given prior to lithium did not change either the lithium-induced CTA or c-Fos in the relevant brain regions. This suggests that the brain NO system may not be involved in the neuronal activation during lithium-induced CTA formation.     

Area postrema lesions attenuate LiCl-induced c-Fos expression correlated with conditioned taste aversion learning

Lesions of the area postrema (AP) block many of the behavioral and physiological effects of lithium chloride (LiCl) in rats, including formation of conditioned taste aversions (CTAs). Systemic administration of LiCl induces c-Fos immunoreactivity in several brain regions, including the AP, nucleus of the solitary tract (NTS), lateral parabrachial nucleus (latPBN), supraoptic nucleus (SON), paraventricular nucleus (PVN), and central nucleus of the amygdala (CeA). To determine which of these brain regions may be activated in parallel with the acquisition of LiCl-induced CTAs, we disrupted CTA learning in rats by ablating the AP and then quantified c-Fos-positive cells in these brain regions in sham- and AP-lesioned rats 1 h following LiCl or saline injection. Significant c-Fos induction after LiCl was observed in the CeA and SON of AP-lesioned rats, demonstrating activation independent of an intact AP. LiCl-induced c-Fos was significantly attenuated in the NTS, latPBN, PVN and CeA of AP-lesioned rats, suggesting that these regions are dependent on AP activation. Almost all of the lesioned rats showed some damage to the subpostremal NTS, and some rats also had damage to the dorsal motor nucleus of the vagus; this collateral damage in the brainstem may have contributed to the deficits in c-Fos response. Because c-Fos induction in several regions was correlated with magnitude of CTA acquisition, these regions are implicated in the central mediation of lithium effects during CTA learning.     

The nociceptin receptor antagonist [Nphe1,Arg14,Lys15]nociceptin/orphanin FQ-NH2 blocks the stimulatory effects of nociceptin/orphanin FQ on the HPA axis in rats

Nociceptin/orphanin FQ (N/OFQ) is an opioid-related peptide that stimulates corticosterone release after i.c.v. administration in non-stressed rats. We employed in situ hybridization histochemistry to investigate N/OFQ-stimulated activation of the HPA axis at the hypothalamic and pituitary level. We have demonstrated that N/OFQ-induced activation of the HPA axis is mediated via the central N/OFQ peptide receptor (NOP) using the recently described selective NOP antagonist [Nphe(1),Arg(14),Lys(15)]nociceptin/orphanin FQ-NH(2) (UFP-101).We found that, at 30 min post-i.c.v. injection, N/OFQ dose-dependently increased plasma adrenocorticotrophin hormone and corticosterone compared with the vehicle-injected controls. N/OFQ (1.0 microg) significantly increased CRF mRNA but not AVP mRNA within the parvocellular hypothalamic paraventricular nucleus compared with the control group, and significantly increased pro-opiomelanocortin (POMC) mRNA in the anterior pituitary. While UFP-101 (1.0 microg) alone had no significant effect on plasma corticosterone concentration it blocked the effect of N/OFQ (1.0 microg) on plasma corticosterone levels when compared with N/OFQ administered alone. UFP-101 also blocked the N/OFQ-induced increase in CRF mRNA and POMC mRNA. These results demonstrate that centrally administered N/OFQ activates the HPA axis via up-regulation of CRF and POMC mRNA and stimulation of corticosterone release in rats. Further, we have demonstrated for the first time that the selective NOP receptor antagonist UFP-101 blocks these effects indicating that N/OFQ-induced HPA axis activation is mediated via central NOP receptors.     

Does the amygdala modulate adaptation to repeated stress?

Exposure of the rat to restraint results in activation of the hypothalamic-pituitary-adrenal (HPA) axis, a characteristic pattern of c-fos expression in the brain and increased cardiovascular function. These responses adapt with repeated exposure of an individual to the same stress. Corticosterone secretion habituates, and c-fos mRNA expression in the paraventricular nucleus of the hypothalamus (PVN) decreases. The increased expression of corticotropin releasing hormone mRNA in the PVN also becomes less prominent, whereas vasopressin mRNA progressively increases. The neural mechanisms responsible for this adaptation remain obscure. Because of its role in conditioned learning, we have hypothesised that the amygdala might be involved in this adaptive process. Here we show that large neurotoxic lesions of the amygdala in male rats do not prevent acute stress activation of the HPA axis following 30 min restraint, whilst more discrete lesions of the central nucleus actually exacerbate the acute response. Rats with large amygdala lesions demonstrate delayed habituation of corticosterone and c-fos to repeated restraint, an affect not apparent with central nucleus lesions. Furthermore we show that neither type of lesion significantly reduced tachycardiac responses to single or repeated restraint as measured by telemetry. We conclude that the amygdala and the central nucleus are not necessary for HPA and cardiovascular activation in response to stress (though the central nucleus may modulate it), and that adaptation to repeated stress is only modestly dependent upon the amygdala.     

Forebrain structures specifically activated by conditioned taste aversion

This study investigates which forebrain structures show Fos protein expression during conditioned taste aversion (CTA) acquisition and whether Fos expression depends on the aversion strength. A novel taste paired with an intraperitoneal injection of a low dose of the malaise-inducing agent lithium chloride (LiCl) induced a weak CTA, whereas associating this novel taste with a high dose of LiCl induced a strong CTA. Increasing the strength of the gastric malaise alone enhanced Fos expression in central, basal, and lateral amygdala nuclei and decreased Fos expression in the nucleus accumbens core. Taste-malaise association induced specific Fos activation in the insular cortex (with both the low and the high doses of LiCl) and the nucleus accumbens shell (with the high LiCl dose only). No significant variation of Fos expression was measured in the perirhinal cortex. Several forebrain areas may be sites of taste-malaise convergence during CTA acquisition depending on the strength of the aversion.     

The involvement of dopamine and nitric oxide in the endocrine and behavioural action of endomorphin-1

Previous publications have demonstrated a prominent central and corticotropin releasing hormone-mediated action of the endomorphins (EMs) on both open-field behaviour and the hypothalamo-pituitary-adrenal (HPA) axis. In the present experiments, the direct action of endomorphin-1 (EM1) on pituitary adrenocorticotropic hormone (ACTH) release, adrenal corticosterone secretion and the roles of nitric oxide (NO) and dopamine (DA) in the HPA and behavioural responses elicited by EM1 were investigated in mice. In vitro perifusion studies indicated that the action of EM1 on the HPA system appears to be confined to the hypothalamus, as EM1 did not influence the corticosterone secretion from adrenal slices and moderately attenuated the ACTH release from anterior pituitary slices. In in vivo experiments, NG-nitro-L-arginine (L-NNArg) pretreatment brought about a profound inhibition of both the endocrine and the behavioural responses. On the other hand, haloperidol completely abolished the increases in square crossing and rearing, without affecting corticosterone release. The direct action of EM1 on striatal DA release was therefore also investigated in an in vitro superfusion system. Although EM1 did not influence the basal release of tritiated DA, it significantly enhanced the transmitter release evoked by electric impulses and pretreatment with L-NNArg resulted in a considerable inhibition of the release elicited by EM1. In conclusion, our endocrine studies suggest an important role of NO in the mediation of the EM1-evoked corticosterone secretion. They also indicate that EM1 activates the HPA axis at a hypothalamic level and dopamine is not involved in this process. In contrast, the behavioural experiments reflect that the locomotor activation induced by EM1 is mediated by NO and dopamine, and the superfusion studies demonstrate that NO transmits the dopamine release enhancing effect of EM1.     

Noradrenergic inputs to the paraventricular hypothalamus contribute to hypothalamic-pituitary-adrenal axis and central Fos activation in rats after acute systemic endotoxin exposure

Noradrenergic (NA) neurons within the nucleus of the solitary tract (NST) and caudal ventrolateral medulla (VLM) innervate the hypothalamic paraventricular nucleus (PVN) to initiate and modulate hypothalamic-pituitary-adrenal (HPA) axis responses to interoceptive stress. Systemic endotoxin (i.e. bacterial lipopolysaccharide, LPS) activates NA neurons within the NST and VLM that project to the PVN and other brain regions that receive interoceptive signals. The present study examined whether NA neurons with axonal inputs to the PVN are necessary for LPS to activate Fos expression within the PVN and other interoceptive-related brain regions, and to increase plasma corticosterone. Male Sprague-Dawley rats received bilateral stereotaxic microinjections of DSAP (saporin toxin conjugated to an antibody against dopamine-beta-hydroxylase, DbH) into the PVN to destroy NA inputs. Control rats were microinjected with vehicle into the PVN or received no PVN injections. Two weeks later, DSAP and control rats were injected i.p. with LPS (200 microg/kg BW) or saline vehicle, and perfused with fixative 2.5-3 h later. Brain tissue sections were processed to reveal nuclear Fos protein and cytoplasmic DbH immunolabeling. DSAP lesions depleted NA terminals in the PVN and bed nucleus of the stria terminalis, reduced the number of NA cell bodies in the NST and VLM, attenuated PVN Fos activation after LPS, and attenuated LPS-induced increases in plasma corticosterone. These findings support the view that NA projections from hindbrain to hypothalamus are necessary for a full HPA axis response to systemic immune challenge.     

Vascular endothelial growth factor-B is neuroprotective in an in vivo rat model of Parkinson's disease

Occlusal disharmony induced by placing an acryl cap on the lower incisors of rats is perceived as chronic stress. This chronic stress activates corticotropin-releasing hormone (CRH) neurons in the hypothalamic paraventricular nucleus (PVN), resulting in stimulation of the hypothalamic-pituitary-adrenal (HPA) axis. The ventral ascending noradrenergic bundles (V-NAB) from the brainstem innervate the PVN. To investigate the relationship between the response of the HPA axis and the V-NAB, we examined changes in extracellular noradrenaline (NA) in the PVN and plasma corticosterone, the final output of the HPA axis, following occlusal disharmony in rats injected with 6-hydroxydopamine (6-OHDA), a specific catecholamine neurotoxin. 6-OHDA microinjection into the V-NAB reduced the magnitude of the responses of extracellular NA in the PVN and the plasma corticosterone to occlusal disharmony. Our results suggest that V-NAB to the PVN are involved in occlusal disharmony-induced activation of the HPA axis.     

Chronic lithium chloride injection increases glucocorticoid receptor but not mineralocorticoid receptor mRNA expression in rat brain

Lithium has been used clinically for the treatment of bipolar disorders. However, the brain mechanisms, by which lithium acts, are still unclear. An impaired hypothalamic-pituitary-adrenal (HPA) axis has been implicated in the pathogenesis of mood disorders. In this study, we investigated the effects of chronic lithium on the corticosteroid receptors in the brain. Male Wistar rats were injected with LiCl (1.5 mEq/kg) or saline intraperitoneally (i.p.) once a day for 14 days. Twenty-four hours after the last injection, the expressions of mRNA for glucocorticoid receptor (GR) and mineralocorticoid receptor (MR) in the brain were determined by non-radioactive in situ hybridization. Chronic administration of LiCl increased the expression of GR mRNA in the hippocampus and paraventricular nucleus of the hypothalamus (PVN). However, no significant changes were observed in the expression of either MR mRNA in the hippocampus or GR mRNA in the locus ceruleus. Since the hippocampus and PVN mediate negative feedback regulation of the HPA axis, an increased expression of GR mRNA in these regions may normalize HPA axis activity in mood disorders. Thus, the effect of chronic lithium on GR function may be involved in its antimanic and/or prophylactic activity in bipolar disorders.     

Cortical substrates of taste aversion learning: involvement of dorsolateral amygdaloid nuclei and temporal neocortex in taste aversion learning

The amygdaloid complex is functionally implicated in conditioned taste aversion (CTA) learning. Results of previous neurobehavioral studies have provided equivocal evidence concerning the involvement of specific amygdaloid nuclei in CTA learning. The present study was conducted to examine the involvement of the central (CE), lateral (LA), and basolateral (BL) amygdaloid nuclei and the temporal neocortices (area 20) in CTA learning. To that end, distinct groups of rats received bilateral electrolytic lesion placements in the CE, LA, BL, or the temporal neocortices. Control animals received scalp and meningeal incisions only. Following recovery, animals were habituated to a restricted drinking schedule with distilled water. Animals then received CTA conditioning, with LiCl used both as the conditioned stimulus and as the unconditioned stimulus. Anterograde degeneration histologies were performed on all brain tissue to evaluate relations between CTA learning deficits and axonal pathology induced by lesion placements. Results of behavioral manipulations indicated that destruction of the CE, LA, or temporal neocortex impaired CTA acquisition, but damage induced to the basolateral amygdaloid nucleus did not. Anatomical observations indicated that degeneration of amygdalofugal and/or corticofugal projections to the convolutions of the olfactory tubercle (medial), subthalamic nucleus, and the parabrachial complex is correlated with CTA learning deficits. These results indicate that destruction of the dorsolateral amygdaloid nuclei and/or the temporal neocortices may produce CTA learning deficits by affecting olfactory, gustatory, and/or gastrointestinal processing in various portions of the forebrain.     

Threshold-like pattern of neuronal activation in the hypothalamus during treadmill running: establishment of a minimum running stress (MRS) rat model

Despite the indication that the hypothalamo-pituitary-adrenal (HPA) axis is activated during treadmill running, there have not been any studies focusing on the relationship between exercise intensity and region-specific neural activities in hypothalamus. To address this, rats were subjected to 30 min of running, either at middle (supra-LT, 25 m min(-1)) or low speeds (sub-LT, 15 m min(-1)), and c-Fos-(+) cells were counted and compared with control rats. Significant increases in blood glucose and lactate levels, and plasma ACTH and osmolality levels were observed during supra-LT running. Only supra-LT running significantly increased c-Fos induction in various hypothalamic regions, namely, the medial preoptic area (MPO), periventricular nucleus (Pe), suprachiasmatic nucleus (SCN), supraoptic nucleus (SON), parvocellular division of the paraventricular nucleus (pPVN), anterior hypothalamic area (AH), arcuate nucleus (ARC) and posterior hypothalamic nucleus (PH). However, sub-LT caused no effect on c-Fos accumulation. This indicates that the hypothalamus responds uniquely to running in a threshold-like pattern distinct from the speed-dependent pattern previously reported for the medulla oblongata [Ohiwa et al., 2006a,b]. In addition, these results showed a physiologic basis for mild exercise useful for establishing our minimum running stress (MRS) rat model, or the running conditions that minimize the activation of the HPA axis.     

Pharmacological antagonism of tyrosine kinases and MAP kinase in brainstem blocks taste aversion learning in mice

Although c-Fos induction in the brainstem is a reliable correlate of taste aversion learning and appears necessary for the encoding of the unconditioned stimulus, little is known about the intracellular signaling pathways in the brainstem that regulate c-Fos expression during taste aversion learning. Infusion of the tyrosine kinase inhibitor genistein and the MAP kinase kinase (MEK) inhibitor PD98059 into the fourth ventricle of mice potently blocks acquisition of a learned taste aversion. The unconditioned stimulus LiCl produces a rapid and robust phosphorylation of MAP kinase, as revealed by immunohistochemistry with an antibody specific to the dually phosphorylated active form of MAP kinase. This immunoreactivity is localized to the same region of the intermediate nucleus tractus solitarius in which we have shown large increases in c-Fos immunoreactive cells, suggesting that in at least a subset of these cells, MAP kinase activation may lead to c-fos induction.     

Effects of proadrenomedullin N-terminal 20 peptide on the rat pituitary-adrenocortical axis under basal and stressful conditions

Proadrenomedullin N-terminal 20 peptide (PAMP) derives, along with adrenomedullin (AM), from prepro-AM. AM has been reported to modulate the activity of the hypothalamic-pituitary-adrenal (HPA) axis, and this study aimed at ascertaining whether PAMP exerts similar effects. PAMP was subcutaneously administered to non-stressed and stressed rats, and the plasma concentrations of ACTH and corticosterone were measured by radioimmune assay. In non-stressed rats, PAMP raised ACTH and corticosterone blood levels at 60 min, and ACTH plasma concentration at 120 min. Ether and cold stresses increased the plasma levels of both ACTH and corticosterone, and PAMP dampened HPA axis response to cold stress, without affecting that to ether stress. The conclusion is drawn that PAMP i) stimulates rat HPA axis, through a mechanism similar to that of ether stress; and ii) interferes with the neural pathways involved in the cold stress-induced activation of HPA axis.     

The brain mapping of the retrieval of conditioned taste aversion memory using manganese-enhanced magnetic resonance imaging in rats

Manganese-enhanced MRI (MEMRI) is a newly developed noninvasive imaging technique of brain activities. The signal intensity of MEMRI reflects cumulative activities of the neurons. To validate the use of MEMRI technique to investigate the neural mechanisms of learning and memory, we tried to map brain areas involved in the retrieval of conditioned taste aversion (CTA) memory. CTAs were established to saccharin (conditioned stimulus: CS) by pairing its ingestion with an i.p. injection of LiCl (unconditioned stimulus: US). LiCl solutions (as a robust aversion chemical) of 0.15 M were injected i.p. 15 min after drinking the saccharine solution (CS). After the two times conditionings, these rats showed a robust aversion to the saccharine solution (CS). Rats of the control group were injected saline i.p. instead of LiCl solutions. The MRI signal intensities at the gustatory cortex (GC), the core subregion of the nucleus accumbens (NAcC), the shell subregion of the nucleus accumbens (NAcSh), the ventral pallidum (VP), the central nucleus of amygdala (CeA), the lateral hypothalamus (LH), and the basolateral nucleus of amygdala (BLA) of the conditioned group were higher than those of the control group. There were no significant differences between the conditioned and the control groups in the intensities for other regions, such as the striatum area, motor cortex, cingulate cortex, interstitial nucleus of the posterior limb of the anterior commissure and hippocampus. These indicate that the GC, NAcC, NAcSh, VP, CeA, LH and BLA have important roles in the memory retrieval of CTA.     

Differential involvement of medial prefrontal cortex and basolateral amygdala extracellular signal-regulated kinase in extinction of conditioned taste aversion is dependent on different intervals of extinction following conditioning

Extinction reflects a decrease in the conditioned response (CR) following non-reinforcement of a conditioned stimulus. Behavioral evidence indicates that extinction involves an inhibitory learning mechanism in which the extinguished CR reappears with presentation of an unconditioned stimulus. However, recent studies on fear conditioning suggest that extinction erases the original conditioning if the time interval between fear acquisition and extinction is short. The present study examined the effects of different intervals between acquisition and extinction of the original memory in conditioned taste aversion (CTA). Male Long-Evans rats acquired CTA by associating a 0.2% sucrose solution with malaise induced by i.p. injection of 4 ml/kg 0.15 M LiCl. Two different time intervals, 5 and 24 h, between CTA acquisition and extinction were used. Five or 24 h after CTA acquisition, extinction trials were performed, in which a bottle containing 20 ml of a 0.2% sucrose solution was provided for 10 min without subsequent LiCl injection. If sucrose consumption during the extinction trials was greater than the average water consumption, then rats were considered to have reached CTA extinction. Rats subjected to extinction trials lasting 24 h, but not 5 h, after acquisition re-exhibited the extinguished CR following injection of 0.15 M LiCl alone 7 days after acquisition. Extracellular signal-regulated kinase (ERK) in the medial prefrontal cortex (mPFC) and basolateral nucleus of the amygdala (BLA) was examined by Western blot after the first extinction trial. ERK activation in the mPFC was induced after the extinction trial beginning 5 h after acquisition, whereas the extinction trial performed 24 h after acquisition induced ERK activation in the BLA. These data suggest that the original conditioning can be inhibited or retained by CTA extinction depending on the time interval between acquisition and extinction and that the ERK transduction pathway in the mPFC and BLA is differentially involved in these processes.     

Systemic blockade of complement C5a receptors reduces lipopolysacharride-induced responses in the paraventricular nucleus and the central amygdala

The complement anaphylatoxin C5a is a potent mediator of the innate immune response to infection. Recent evidence also reveals that C5a contributes to central nervous system effects in addition to its well-known peripheral functions. However, it is not known if C5a has a role in the activation of the hypothalamic–pituitary–adrenal (HPA) axis; a critical cascade that exemplifies neuroimmune interactions between the periphery and the brain. In the present study we examined if systemic pre-treatment with a C5a receptor antagonist, PMX53, can affect lipopolysaccharide-induced (LPS; 1 mg/kg, i.p.) activation of the HPA axis in the rat. Using Fos protein as a marker of neuronal activation, we found that systemic administration of PMX53 reduced the LPS-induced activation of paraventricular corticotropin-releasing factor (PVN CRF) and central amygdala cells. However, PMX53 did not alter LPS-induced responses in the bed nucleus of the stria terminalis, nucleus tractus solitarius and ventrolateral medulla. Our findings demonstrate that C5a may have a role in the activation of the HPA axis in response to systemic LPS.     

Behavioral, neuroendocrine and thermoregulatory actions of apelin-13

As the distribution of apelinergic neurons in the brain suggests an important role of apelin-13 in the regulation of neuroendocrine processes, in the present experiments the effects of this recently identified neuropeptide on the open-field activity, the hypothalamo-pituitary-adrenal (HPA) system and the body temperature were investigated. I.c.v. administration of apelin-13 (1-10 microg) to rats caused significant increases in square crossing, rearing, plasma corticosterone release and core temperature, whereas it did not influence the spontaneous motor activity during telemetric observation. To determine the mediation of the actions of apelin, a corticotropin-releasing hormone (CRH) antagonist, the nonselective dopamine antagonist haloperidol, the selective dopamine D1 receptor antagonist SCH-23390 and the nitric oxide synthase inhibitor Nomega-nitro-L-arginine-methyl ester (L-NAME) were administered to the rats. The apelin-evoked HPA activation was diminished by preadministration of the CRH antagonist, while the dopamine antagonist and L-NAME attenuated only the square crossing and rearing induced by apelin-13. To characterize the transmission of the thermoregulatory action of apelin, animals were pretreated either with L-NAME, the CRH antagonist or with the cyclooxygenase inhibitor noraminophenazone. L-NAME and the CRH antagonist did not cause significant inhibition of the apelin-evoked increase in core temperature, while the cyclooxygenase inhibitor, applied 30 min before peptide treatment, did not prove effective in preventing the apelin-evoked thermoregulatory response, whereas when it was administered 2 h after the peptide treatment, it transiently and significantly reduced the hyperthermic response. The present data suggest that apelin-13 plays an important role in the regulation of behavioral, endocrine and homeostatic responses in the CNS, and dopamine, nitric oxide and prostaglandins seem to take part in the mediation of its effects. Since the corticosterone response could be blocked by the CRH antagonist, it is likely to be mediated through the activation of the CRH neurons.     

Stimulation of the prelimbic cortex differentially modulates neuroendocrine responses to psychogenic and systemic stressors

The medial prefrontal cortex is important for normal regulation of stress responses, and is implicated in stress-related affective disease states (e.g. depression). In the current study, we investigated the role of the prelimbic division of the prefrontal cortex in control of responses to psychogenic and systemic stressors (restraint and hypoxia, respectively). Acute stimulation of the prelimbic cortical region with bicuculline methiodide (BMI) caused significant reduction of ACTH and corticosterone responses to restraint and reduced Fos activation of paraventricular nucleus neurons, consistent with a role in central inhibition of acute psychogenic stress responses. In contrast, BMI enhanced corticosterone (but not ACTH) responses to hypoxia via a mechanism suggestive of central PVN drive and enhanced adrenal sensitivity. Acute BMI increased restraint stress-induced Fos activation in known downstream targets of the prelimbic cortex (e.g., the basolateral amygdala and central amygdaloid nuclei), suggesting a connection between modulation of amygdalar signaling and stress inhibition. In contrast, hypoxia caused robust Fos activation in the basolateral and central amygdala, which was not affected by prelimbic BMI injection. The data suggest that the prelimbic cortex stimulation is sufficient to trigger inhibition of the HPA axis to psychogenic stress, but may play a very different role in enhancing HPA responsiveness to physical threats.     

Acute nicotine activates c-fos and activity-regulated cytoskeletal associated protein mRNA expression in limbic brain areas involved in the central stress-response in rat pups during a period of hypo-responsiveness to stress

In adult rats, acute nicotine, the major psychoactive ingredient in tobacco smoke, stimulates the hypothalamic-pituitary-adrenal axis (HPA), resulting in activation of brain areas involved in stress and anxiety-linked behavior. However, in rat pups the first two postnatal weeks are characterized by hypo-responsiveness to stress, also called the 'stress non-responsive period' (SNRP). Therefore, we wanted to address the question if acute nicotine stimulates areas involved in the stress response during SNRP. To determine neuronal activation, the expression of the immediate-early genes c-fos and activity-regulated cytoskeletal associated protein (Arc) was studied in the central nucleus of the amygdala (CeA), bed nucleus stria terminalis (BST) and paraventricular hypothalamic nucleus (PVN), which are areas involved in the neuroendocrine and central stress response. Rat pups received nicotine tartrate (2 mg/kg) or saline by i.p. injection at postnatal days (P) 5, 7 and 10 and their brains were removed after 30 min. We used semi-quantitative radioactive in situ hybridization with gene specific antisense cRNA probes in coronal sections. In control pups, c-fos expression was low in most brain regions, but robust Arc hybridization was found in several areas including cingulate cortex, hippocampus and caudate. Acute nicotine resulted in significant induction of c-fos expression in the PVN and CeA at P5, P7 and P10, and in the BST at P7 and P10. Acute nicotine significantly induced expression of Arc in CeA at P5, P7 and P10, and in the BST at P10. In conclusion, acute nicotine age dependently activated different brain areas of the HPA axis during the SNRP. After P7, the response was more pronounced and included the BST, suggesting differential maturation of the HPA axis in response to nicotine.     

Physiological and behavioral responses to interleukin-1beta and LPS in vagotomized mice

It is well established that peripheral administration of interleukin-1 (IL-1) and lipopolysaccharide (LPS) can activate the hypothalamo-pituitary-adrenocortical (HPA) axis, alter brain catecholamine and indoleamine metabolism, and affect behavior. However, the mechanisms of these effects are not fully understood. Stimulation of afferents of the vagus nerve has been implicated in the induction of Fos in the brain, changes in body temperature, brain norepinephrine, and some behavioral responses. In the present study, the IL-1beta- and LPS-induced changes in certain behaviors, HPA axis activation, and catecholamine and indoleamine metabolism were studied in mice following subdiaphragmatic vagotomy. IL-1beta and LPS induced the expected decreases in sweetened milk, food intake, and locomotor activity, and the responses to IL-1beta, but not LPS, were slightly attenuated in vagotomized mice. Subdiaphragmatic vagotomy also attenuated the IL-1beta- and LPS-induced increases in plasma ACTH and corticosterone, but the attenuations of the responses to IL-1beta were only marginally significant. There were also slight reductions in the responses in catecholamine and serotonin metabolism, and the increases in brain tryptophan in several brain regions. These results indicate that the vagus nerve is not the major pathway by which abdominal IL-1beta and LPS effect behavioral, HPA and brain catecholamine and indoleamine responses in the mouse. These results resemble those we observed in subdiaphragmatically vagotomized rats, but in that species the subdiaphragmatic vagotomy markedly attenuated the ACTH and corticosterone responses, and prevented the hypothalamic noradrenergic activation, as well as the fever. Overall the results indicate that the various responses to peripheral IL-1 and LPS involve multiple mechanisms including vagal afferents, and that there are species differences in the relative importance of the various mechanisms.