Involvement of serotonergic pathways in mediating the neuronal activity and genetic transcription of neuroendocrine corticotropin-releasing factor in the brain of systemically endotoxin-challenged rats

Therapeutic potential of serotonin 4 receptor for chronic depression and its associated comorbidity in the gut

The latest estimates from world health organization suggest that more than 450 million people are suffering from depression and other psychiatric conditions. Of these, 50-60% have been reported to have progression of gut diseases. In the last two decades, researchers introduced incipient physiological roles for serotonin (5-HT) receptors (5-HTRs), suggesting their importance as a potential pharmacological target in various psychiatric and gut diseases. A growing body of evidence suggests that 5-HT systems affect the brain-gut axis in depressive patients, which leads to gut comorbidity. Recently, preclinical trials of 5-HT4R agonists and antagonists were promising as antipsychotic and prokinetic agents. In the current review, we address the possible pharmacological role and contribution of 5-HT4R in the pathophysiology of chronic depression and associated gut abnormalities. Physiologically, during depression episodes, centers of the sympathetic and parasympathetic nervous system couple together with neuroendocrine systems to alter the function of hypothalamic-pituitary-adrenal (HPA) axis and enteric nervous system (ENS), which in turn leads to onset of gastrointestinal tract (GIT) disorders. Consecutively, the ENS governs a broad spectrum of physiological activities of gut, such as visceral pain and motility. During the stages of emotional stress, hyperactivity of the HPA axis alters the ENS response to physiological and noxious stimuli. Consecutively, stress-induced flare, swelling, hyperalgesia and altered reflexes in gut eventually lead to GIT disorders. In summary, the current review provides prospective information about the role and mechanism of 5-HT4R-based therapeutics for the treatment of depressive disorder and possible consequences for the gut via brain-gut axis interactions. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Central serotonin prevents hypotension and hypothermia and reduces plasma and spleen cytokine levels during systemic inflammation

An exceptionally high mortality rate is observed in sepsis and septic shock. Systemic administration of lipopolysaccharide (LPS) has been used as an experimental model for sepsis resulting in an exacerbated immune response, brain neurochemistry adjustments, hypotension, and hypothermia followed by fever. Central serotonergic pathways not only modulate systemic inflammation (SI) but also are affected by SI, including in the anteroventral region of the hypothalamus (AVPO), which is the hierarchically most important region for body temperature (Tb) control. In this study, we sought to determine if central serotonin (5-HT) plays a role in SI induced by intravenous administration of LPS (1.5 mg/kg) in male Wistar rats (280-350 g) by assessing 5-HT levels in the AVPO, mean arterial pressure, heart rate, and Tb up to 300 min after LPS administration, as well as assessing plasma and spleen cytokine levels, nitric oxide (NO) plasma levels, and prostaglandin (PG) E₂ levels in the AVPO at 75 min and 300 min after LPS administration. We observed reduced AVPO 5-HT levels, hypotension, tachycardia, hypothermia followed by fever, as well as observing increased plasma NO, plasma and spleen cytokines and AVPO PGE₂ levels in SI. Intracerebroventricular (icv) administration of 5-HT 30 min before LPS administration prevented hypotension and hypothermia, which were accompanied by reduced plasma NO, as well as plasma TNF-α, IL-1β, IL-6, and IL-10 and spleen TNF-α and IL-10 levels. We suggest that SI reduced 5-HT levels in the AVPO favor an increased pro-inflammatory status both centrally and peripherally that converge to hypotension and hypothermia. Moreover, our results are consistent with the notion that exogenous 5-HT given icv prevents hypotension and hypothermia probably activating the splenic anti-inflammatory pathway.

mCSF-Induced Microglial Activation Prevents Myelin Loss and Promotes Its Repair in a Mouse Model of Multiple Sclerosis

A pathological hallmark of multiple sclerosis (MS) is myelin loss in brain white matter accompanied by compromised remyelination. Demyelinated lesions are deeply associated with oligodendrocyte apoptosis and a robust inflammatory response. Although various studies point towards a noxious role of inflammation in MS, others emphasize a positive role for the innate immune cells in disease progression. A cytokine well-known to stimulate cell survival, proliferation and differentiation of myeloid cells, macrophage colony-stimulating factor (mCSF), was administered to mice during a 5 week-long cuprizone diet. Treated mice exhibited reduced myelin loss during the demyelination phase, together with an increased number of microglia and oligodendrocyte precursor cells in lesion sites. Tamoxifen-induced conditional deletion of the mCSF receptor in microglia from cuprizone-fed mice caused aberrant myelin debris accumulation in the corpus callosum and reduced microglial phagocytic response. mCSF therefore plays a key role in stimulating myelin clearance by the brain innate immune cells, which is a prerequisite for proper remyelination and myelin repair processes.

Intermittent severe hypoxia induces plasticity within serotonergic and catecholaminergic neurons in the neonatal rat ventrolateral medulla

5-HT neurons contribute to autoresuscitation and survival during intermittent severe hypoxia (IsH). In adults, catecholaminergic neurons in the ventrolateral medulla (VLM) contribute to the autonomic response to hypoxia. We hypothesized that 1) catecholaminergic neurons in the neonatal VLM are activated following IsH, 2) this activation is compromised following an acute loss of brain stem 5-HT, and 3) IsH induces cellular and/or transcriptomic plasticity within catecholaminergic and serotonergic neurons that are within or project to the VLM, respectively. To test these hypotheses, we treated rat pups with 6-fluorotryptophan, a tryptophan hydroxylase (TPH) inhibitor, and then exposed treated and vehicle controls to IsH or air. Along with immunohistochemistry to detect tyrosine hydroxylase (TH)- or Fos-positive neurons, we used RNA sequencing to resolve the effects of IsH and 5-HT deficiency on the expression of serotonergic and catecholaminergic system genes in the VLM. 5-HT deficiency compromised autoresuscitation and survival. IsH significantly increased the number of identifiable TH-positive VLM neurons, an effect enhanced by 5-HT deficiency (P = 0.003). Contrary to our hypothesis, 5-HT-deficient pups had significantly more Fos-positive neurons following IsH (P = 0.008) and more activated TH-positive neurons following IsH or air (P = 0.04). In both groups the expression of the 5-HT transporter and TPH2 was increased following IsH. In 5-HT-deficient pups, the expression of the inhibitory 5-HT1A receptor was decreased following IsH, while the expression of DOPA decarboxylase was increased. These data show that the serotonergic and catecholaminergic systems in the VLM of the neonatal rat are dynamically upregulated by IsH, potentially adapting cardiorespiratory responses to severe hypoxia.

Effects of citalopram on serotonin and CRF systems in the midbrain of primates with differences in stress sensitivity

This chapter reviews the neurobiological effects of stress sensitivity and s-citalpram (CIT) treatment observed in our nonhuman primate model of functional hypothalamic amenorrhea (FHA). This type of infertility, also known as stress-induced amenorrhea, is exhibited by cynomolgus macaques. In small populations, some individuals are stress-sensitive (SS) and others are highly stress-resilient (HSR). The SS macaques have suboptimal secretion of estrogen and progesterone during normal menstrual cycles. SS monkeys also have decreased serotonin gene expression and increased CRF expression compared to HSR monkeys. Recently, we found that CIT treatment improved ovarian steroid secretion in SS monkeys, but had no effect in HSR monkeys. Examination of the serotonin system revealed that SS monkeys had significantly lower Fev (fifth Ewing variant, rodent Pet1), TPH2 (tryptophan hydroxylase 2), 5HT1A autoreceptor and SERT (serotonin reuptake transporter) expression in the dorsal raphe than SR monkeys. However, CIT did not alter the expression of either Fev, TPH2, SERT or 5HT1A mRNAs. In contrast, SS monkeys tended to have a higher density of CRF fiber innervation of the dorsal raphe than HSR monkeys, and CIT significantly decreased the CRF fiber density in SS animals. In addition, CIT increased CRF-R2 gene expression in the dorsal raphe. We speculate that in a 15-week time frame, the therapeutic effect of S-citalopram may be achieved through a mechanism involving extracellular serotonin inhibition of CRF and stimulation of CRF-R2, rather than alteration of serotonin-related gene expression.

Evidence that PGE2 in the dorsal and median raphe nuclei is involved in LPS-induced anorexia in rats

Anorexia is an element of the acute-phase immune response. Its mechanisms remain poorly understood. Activation of inducible cyclooxygenase-2 (COX-2) in blood-brain-barrier endothelial cells and subsequent release of prostaglandins (e.g., prostaglandin E2, PGE2) may be involved. Therefore, we sought to relate the effects of prostaglandins on the anorexia following gram-negative bacterial lipopolysaccharide treatment (LPS) to neural activity in the dorsal and median raphe nuclei (DRN and MnR) in rats. COX-2 antagonist (NS-398, 10mg/kg; IP) administration prior to LPS (100μg/kg; IP) prevented anorexia and reduced c-Fos expression the DRN, MnR, nucleus tractus solitarii and several related forebrain areas. These data indicate that COX-2-mediated prostaglandin synthesis is necessary for LPS anorexia and much of the initial LPS-induced neural activation. Injection of NS-398 into the DRN and MnR (1ng/site) attenuated LPS-induced anorexia to nearly the same extent as IP NS-398, suggesting that prostaglandin signaling in these areas is necessary for LPS anorexia. Because the DRN and MnR are sources of major serotonergic projections to the forebrain, these data suggest that serotonergic neurons originating in the midbrain raphe play an important role in acute-phase response anorexia.

5-HT receptors mediate lineage-dependent effects of serotonin on adult neurogenesis in Procambarus clarkii

Background

Serotonin (5-HT) is a potent regulator of adult neurogenesis in the crustacean brain, as in the vertebrate brain. However, there are relatively few data regarding the mechanisms of serotonin's action and which precursor cells are targeted. Therefore, we exploited the spatial separation of the neuronal precursor lineage that generates adult-born neurons in the crayfish (Procambarus clarkii) brain to determine which generation(s) is influenced by serotonin, and to identify and localize serotonin receptor subtypes underlying these effects.

Results

RT-PCR shows that mRNAs of serotonin receptors homologous to mammalian subtypes 1A and 2B are expressed in P. clarkii brain (referred to here as 5-HT_1α and 5-HT_2β). In situ hybridization with antisense riboprobes reveals strong expression of these mRNAs in several brain regions, including cell clusters 9 and 10 where adult-born neurons reside. Antibodies generated against the crustacean forms of these receptors do not bind to the primary neuronal precursors (stem cells) in the neurogenic niche or their daughters as they migrate, but do label these second-generation precursors as they approach the proliferation zones of cell clusters 9 and 10. Like serotonin, administration of the P. clarkii 5-HT_1α-specific agonist quipazine maleate salt (QMS) increases the number of bromodeoxyuridine (BrdU)-labeled cells in cluster 10; the P. clarkii 5-HT_2β-specific antagonist methiothepin mesylate salt (MMS) suppresses neurogenesis in this region. However, serotonin, QMS and MMS do not alter the rate of BrdU incorporation into niche precursors or their migratory daughters.

Conclusion

Our results demonstrate that the influences of serotonin on adult neurogenesis in the crayfish brain are confined to the late second-generation precursors and their descendants. Further, the distribution of 5-HT_1α and 5-HT_2β mRNAs and proteins indicate that these serotonergic effects are exerted directly on specific generations of neuronal precursors. Taken together, these results suggest that the influence of serotonin on adult neurogenesis in the crustacean brain is lineage dependent, and that 5-HT_1α and 5-HT_2β receptors underlie these effects.

Stress sensitive female macaques have decreased fifth Ewing variant (Fev) and serotonin-related gene expression that is not reversed by citalopram

Female cynomolgus monkeys exhibit different degrees of reproductive dysfunction with moderate metabolic and psychosocial stress. When stressed with a paradigm of relocation and diet for 60 days or two menstrual cycles, highly stress resilient monkeys (HSR) continued to ovulate during the stress cycles whereas stress sensitive monkeys (SS) did not. After cessation of stress, monkeys characterized as HSR or SS were administered placebo (PL) or S-citalopram (CIT) for 15 weeks at doses that normalized ovarian steroid secretion in the SS animals and that maintained blood CIT levels in a therapeutic range. After euthanasia, the brain was perfused with 4% paraformaldehyde. The pontine midbrain was blocked and sectioned at 25 microm. The expression of four genes pivotal to serotonin neural function was assessed in the four groups of monkeys (n=4/group). Fev (fifth Ewing variant) ETS transcription factor, tryptophan hydroxylase 2 (TPH2), the serotonin reuptake transporter (SERT), and the 5HT1A autoreceptor were determined at 7-8 levels of the dorsal raphe nucleus with in situ hybridization (ISH) using radiolabeled- and digoxygenin-incorporated riboprobes. Positive pixel area and cell number were measured with Slidebook 4.2 in the digoxigenin assay for Fev. Optical density (OD) and positive pixel area were measured with NIH Image software in the radiolabeled assays for TPH2, SERT and 5HT1A. All data were analyzed with two-way ANOVA. SS monkeys had significantly fewer Fev-positive cells and lower Fev-positive pixel area in the dorsal raphe than HSR monkeys. SS monkeys also had significantly lower levels of TPH2, SERT and 5HT1A mRNAs in the dorsal raphe nucleus than HSR monkeys. However, CIT did not alter the expression of either Fev, TPH2, SERT or 5HT1A mRNAs. These data suggest that SS monkeys have fewer serotonin (5-HT) neurons than HSR monkeys, and that they have deficient Fev expression, which in turn, leads to deficient TPH2, SERT and 5HT1A expression. In addition, the therapeutic effect of CIT is probably achieved through mechanisms other than alteration of 5-HT-related gene expression.

The role of the arginine vasopressin Avp1b receptor in the acute neuroendocrine action of antidepressants

In times of stress the hypothalamic-pituitary-adrenal (HPA) axis is activated and releases two neurohormones, corticotropin-releasing hormone (Crh) and arginine vasopressin (Avp), to synergistically stimulate the secretion of adrenocorticotropin hormone (ACTH) from the anterior pituitary, culminating in a rise in circulating glucocorticoids. Avp mediates its actions at the Avp V1b receptor (Avpr1b) present on pituitary corticotropes. Dysregulation of the stress response is associated with the pathophysiology of depression and a major treatment involves increasing the availability of monamines at the synaptic cleft. Acute administration of selective serotonin reuptake inhibitors (SSRI) and tricyclic antidepressants (TCA) has previously been shown to activate the HPA axis. The present study was undertaken to evaluate the involvement of the Avpr1b in the HPA axis response to acute SC administration of an SSRI (fluoxetine 10mg/kg) and a TCA (desipramine 10mg/kg). We measured plasma ACTH and corticosterone (CORT) levels and neuropeptide mRNA expression in the hypothalamic paraventricular nucleus (PVN) of Avpr1b knockout (KO) mice and wild-type controls. Fluoxetine and desipramine administration significantly attenuated plasma ACTH and CORT levels in male and female Avpr1b KO mice when compared to their wild-type counterparts. Avp, oxytocin (Oxt) and Crh mRNA expression in the PVN did not change in fluoxetine-treated male Avpr1b KO or wild-type mice. In contrast, fluoxetine treatment increased PVN Avp mRNA levels in female Avpr1b wild type but not KO animals. PVN Oxt mRNA levels increased in fluoxetine-treated female mice of both genotypes. The data suggests that the Avpr1b is required to drive the HPA axis response to acute antidepressant treatment and provides further evidence of a sexual dichotomy in the regulation of PVN Avp/Oxt gene expression following antidepressant administration.

The dorsomedial hypothalamus: a new player in thermoregulation

Neurons in the dorsomedial hypothalamus (DMH) play key roles in physiological responses to exteroceptive (“emotional”) stress in rats, including tachycardia. Tachycardia evoked from the DMH or seen in experimental stress in rats is blocked by microinjection of the GABAA receptor agonist muscimol into the rostral raphe pallidus (rRP), an important thermoregulatory site in the brain stem, where disinhibition elicits sympathetically mediated activation of brown adipose tissue (BAT) and cutaneous vasoconstriction in the tail. Disinhibition of neurons in the DMH also elevates core temperature in conscious rats and sympathetic activity to least significant difference interscapular BAT (IBAT) and IBAT temperature in anesthetized preparations. The latter effects are blocked by microinjection of muscimol into the rRP, while microinjection of muscimol into either the rRP or DMH suppresses increases in sympathetic nerve activity to IBAT, IBAT temperature, and core body temperature elicited either by microinjection of PGE2 into the preoptic area (an experimental model for fever), or central administration of fentanyl. Neurons concentrated in the dorsal region of the DMH project directly to the rRP, a location corresponding to that of neurons transsynaptically labeled from IBAT. Thus these neurons control nonshivering thermogenesis in rats, and their activation signals its recruitment in diverse experimental paradigms. Evidence also points to a role for neurons in the DMH in thermoregulatory cutaneous vasoconstriction, shivering, and endocrine adjustments. These directions provide intriguing avenues for future exploration that may expand our understanding of the DMH as an important hypothalamic site for the integration of autonomic, endocrine, and behavioral responses to diverse challenges.

Lipopolysaccharide has indomethacin-sensitive actions on Fos expression in topographically organized subpopulations of serotonergic neurons

Peripheral immune activation results in physiological and behavioral responses including changes in the level of behavioral arousal. One mechanism through which immune activation can influence these responses is via actions on brainstem neuromodulatory systems, including serotonergic systems. To investigate the effects of peripheral immune activation on serotonergic systems and behavior, and the potential role of prostanoids in mediating these effects, we compared the effects of intraperitoneal injections of lipopolysaccharide (LPS), in the presence or absence of the cyclooxygenase inhibitor indomethacin, on total plasma L-tryptophan concentrations, Fos expression in subdivisions of the brainstem raphe complex, and home cage behaviors. Peripheral LPS administration had no effect on total plasma L-tryptophan concentrations but increased Fos expression in serotonergic neurons selectively within the interfascicular (DRI) and ventrolateral (DRVL) subdivisions of the dorsal raphe nucleus 4 h following treatment; pretreatment with indomethacin blocked the LPS-induced increases in Fos expression within the DRI and DRVL. Peripheral LPS administration decreased measures of behavioral arousal including locomotion, rearing, climbing, and self-grooming; LPS administration had no effect on these behaviors in mice pretreated with indomethacin. The indomethacin-sensitive effects of LPS on Fos expression in the DRI may be due to selective activation of Type II serotonergic neurons which are largely restricted to the DRI region and have unique afferent regulatory mechanisms and behavioral correlates. Further studies of the effects of peripheral immune activation on DRI serotonergic systems may lead to a better understanding of the relationships among immune function, serotonergic systems, and behavior.

Mapping the areas sensitive to long-term endotoxin tolerance in the rat brain: a c-fos mRNA study

We have recently found that a single endotoxin administration to rats reduced the hypothalamic-pituitary-adrenal response to another endotoxin administration 4 weeks later, which may be an example of the well-known phenomenon of endotoxin tolerance. However, the time elapsed between the two doses of endotoxin was long enough to consider the above results as an example of late tolerance, whose mechanisms are poorly characterized. To know if the brain plays a role in this phenomenon and to characterize the putative areas involved, we compared the c-fos mRNA response after a final dose of endotoxin in animals given vehicle or endotoxin 4 weeks before. Endotoxin caused a widespread induction of c-fos mRNA in the brain, similar to that previously reported by other laboratories. Whereas most of the brain areas were not sensitive to the previous experience with endotoxin, a few showed a reduced response in endotoxin-pretreated rats: the parvocellular and magnocellular regions of the paraventricular hypothalamic nucleus, the central amygdala, the lateral division of the bed nucleus and the locus coeruleus. We hypothesize that late tolerance to endotoxin may involve plastic changes in the brain, likely to be located in the central amygdala. The reduced activation of the central amygdala in rats previously treated with endotoxin may, in turn, reduce the activation of other brain areas, including the hypothalamic paraventicular nucleus.

Comparison between the influence of shocks and endotoxemia on the activation of brain cells that contain nitric oxide

We sought to identify the brain circuitry that underlies the stimulatory role of nitric oxide (NO) role on the hypothalamic-pituitary-adrenal (HPA) axis. Specifically, we determined whether electrofootshocks (60 min) or the intravenous administration of lipopolysaccharide (LPS, 100 microg/kg)-activated neurocircuitries that express either neuronal NO synthase (nNOS), a constitutive enzyme responsible for NO formation, or L-citrulline, an amino acid that is produced in equimolar amounts with NO. Shocks significantly increased the number of cells showing Fos immunoreactivity (ir) in the paraventricular nucleus (PVN) of the hypothalamus, the lateral hypothalamus (LH), amygdaloid complex (AD) and thalamus (TH), and to a lesser extent, in the hippocampus (HP), caudate putamen (CP) and frontal cortex (FC). However, shocks did not alter the number of nNOS-positive cells nor increased citrulline signals in these brain regions. LPS significantly upregulated the number of cells with fos-like ir in the PVN, LH, AD, TH, HP, CP and FC, but only increased the number of cells positive for citrulline in the PVN, 87% of which co-expressed Fos. Thus, while shocks did not alter nNOS gene expression or citrulline levels in the brain regions studied, LPS significantly increased the number of PVN cells expressing citrulline without concomitant changes in other brain areas. Endotoxemia also upregulated significantly more PVN cells that co-expressed Fos and nNOS, compared to shocks. As NO stimulates the PVN circuitries that participate in shocks- and LPS-induced ACTH release, the lack of changes in nNOS or citrulline levels due to shocks suggests that, in this model, constitutively formed NO may modulate HPA axis activity in the absence of changes in its synthesis.

Central mechanisms of stress integration: hierarchical circuitry controlling hypothalamo-pituitary-adrenocortical responsiveness

Appropriate regulatory control of the hypothalamo-pituitary-adrenocortical stress axis is essential to health and survival. The following review documents the principle extrinsic and intrinsic mechanisms responsible for regulating stress-responsive CRH neurons of the hypothalamic paraventricular nucleus, which summate excitatory and inhibitory inputs into a net secretory signal at the pituitary gland. Regions that directly innervate these neurons are primed to relay sensory information, including visceral afferents, nociceptors and circumventricular organs, thereby promoting 'reactive' corticosteroid responses to emergent homeostatic challenges. Indirect inputs from the limbic-associated structures are capable of activating these same cells in the absence of frank physiological challenges; such 'anticipatory' signals regulate glucocorticoid release under conditions in which physical challenges may be predicted, either by innate programs or conditioned stimuli. Importantly, 'anticipatory' circuits are integrated with neural pathways subserving 'reactive' responses at multiple levels. The resultant hierarchical organization of stress-responsive neurocircuitries is capable of comparing information from multiple limbic sources with internally generated and peripherally sensed information, thereby tuning the relative activity of the adrenal cortex. Imbalances among these limbic pathways and homeostatic sensors are likely to underlie hypothalamo-pituitary-adrenocortical dysfunction associated with numerous disease processes.

Angiotensin II reduces serotonin release in the rat subfornical organ area

In the present study we used intracerebral microdialysis techniques to examine whether angiotensin II (ANG II) modulates the release of serotonin (5-hydroxytryptamine, 5-HT) in the subfornical organ (SFO) in freely moving rats. Extracellular concentrations of 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA) in the region of the SFO were significantly decreased by microinjection of ANG II (10 pmol, 50 nl), but not by vehicle, into the dialysis site. No significant changes in the 5-HT and 5-HIAA levels caused by ANG II were observed in the sites away from the SFO. Water ingestion significantly enhanced the amount of the decrease in the 5-HT and 5-HIAA concentrations in the SFO area elicited by the ANG II injection. These results show that ANG II may reduce the release of 5-HT in the SFO area, and imply that the 5-HT receptor mechanism in the SFO area may participate in the elicitation of the drinking behavior to ANG II.

Serotonin in the developing stomatogastric system of the lobster, Homarus americanus

We studied the development of the serotonergic modulation of the stomatogastric nervous system of the lobster, Homarus americanus. Although the stomatogastric ganglion (STG) is present early in embryonic development, serotonin immunoreactivity is not visible in the STG until the second larval stage. However, incubation of the STG with exogenous serotonin showed that a serotonin transporter is present in embryonic and early larval stages. Serotonin uptake was blocked by paroxetine and 0% Na(+) saline. The presence of a serotonin transporter in the embryonic STG suggests that hormonally liberated serotonin could be taken up by the STG, and potentially released as a "borrowed transmitter". Consistent with a potential hormonal role, serotonin is found in the pericardial organs, a major neurosecretory structure, by midembryonic development. The rhythmic motor patterns produced by embryonic and larval STGs were decreased in frequency by serotonin. Lateral Pyloric (LP) neuron-evoked excitatory junctional potentials (EJPs) in the embryos and the first larval stage (LI) were larger, slower, and more variable than those in the adult. The amplitude of adult LP neuron-evoked EJPs was increased more than twofold in serotonin, but in embryos and LI preparations this effect was negligible. In embryos and LI preparations, serotonin increased the occurrence of muscle fiber action potentials and altered the EJP wave-form. These data demonstrate that serotonin receptors are present in the stomatogastric nervous system early in development, and suggest that the role of serotonin changes from modulation of muscle fiber excitability early in development to enhancement of neurally evoked EJPs in the adult.

Chronic treatment with the antidepressant tianeptine attenuates lipopolysaccharide-induced Fos expression in the rat paraventricular nucleus and HPA axis activation

The antidepressant tianeptine has been shown to decrease the response of the hypothalamic-pituitary-adrenal (HPA) axis to stress and to attenuate the behavioral effects of the cytokine inducer, lipopolysaccharide (LPS). Since LPS also activates the HPA axis, the objective of this study was to assess the effects of tianeptine on the HPA axis activation and Fos expression induced by intraperitoneal (i.p.) administration of LPS (30 and 250 microg/kg respectively). Chronic, but not acute, tianeptine treatment (10 mg/kg twice a day for 15 days, i.p.) attenuated LPS-induced increase of plasma ACTH and corticosterone in rats bearing an indwelling catheter in the jugular vein and Fos immunoreactivity in the paraventricular nucleus (PVN). These results open new vistas on the pharmacological activity of tianeptine and provide further insights on the action mechanisms of antidepressants in clinics.

Potentiation of tumor metastasis in adulthood by neonatal endotoxin exposure: sex differences

Previous research in rodents has demonstrated that neonatal exposure to bacterial endotoxin alters the hypothalamic-pituitary-adrenal (HPA) axis resulting in hypersecretion of corticosterone in response to stress in adulthood. Given the known interactions between glucocorticoids and the immune system we tested the hypothesis that such alterations may impact on immune outcomes. Male and female Fischer 344 neonate rats were treated with endotoxin (0.05 mg/kg lipopolysaccaride from Salmonella enteritidis) or vehicle on days 1, 3, 5 and 7 postpartum. In adulthood, animals were subjected to chronic stress and the effect on resistance to tumor colonization (Exp. 1), natural killer (NK) cell activity (Exp. 2), and HPA reactivity (Exp. 3) was assessed. Neonatal endotoxin treatment was found to significantly impair NK cell activity and decrease resistance to tumor colonization in male but not female rats (P<0.05). Neonatal endotoxin exposure did not affect corticosterone responses to chronic stress in male or female rats, but the corticosterone response to acute stress was potentiated by endotoxin exposure, most notably in females. In conclusion, neonatal endotoxin exposure was found to be associated with a sexually differentiated impairment in tumor colonization and NK activity and long-term alterations in corticosterone responses to stress. The effect on tumor colonization and NK activity was not, however, critically mediated by corticosterone levels. These findings suggest that neonatal bacterial infections may have long-term health implications, specifically in terms of resistance to cancer spread in adulthood.

Evidence for a mutual interaction between noradrenergic and serotonergic agonists in stimulation of ACTH and corticosterone secretion in the rat

We investigated the mutual interactions between hypothalamic norepinephrine (NE) and serotonin (5-HT) in mediating the ACTH and corticosterone responses to direct stimulation of the paraventricular nucleus (PVN) with adrenergic and serotonergic agonists. The hormone responses to the intrahypothalamic injection of the alpha1-adrenergic agonist phenylephrine (20 nmol/2 microl) were significantly reduced by prior depletion of hypothalamic 5-HT with intra-PVN injection of the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), but not after depletion of hypothalamic NE by intra-PVN injection of the noradrenergic neurotoxin 6-hydroxydopamine (6-OHDA). The ACTH and corticosterone responses to intrahypothalamic injection of the 5-HT(1A) receptor agonist 8-OH-DPAT (20 n mol/2 microl) were significantly reduced by depletion of hypothalamic NE with 6-OHDA, but not after depletion of hypothalamic 5-HT with 5,7-DHT. These mutual interactions between the NE and 5-HT neuronal systems, which innervate the PVN, may explain previous findings of equivalent reductions in the hypothalamic-pituitary-adrenal axis responses to neural stimulation after neurotoxic lesioning of either the NE or 5-HT systems.

Diverse actions of ovarian steroids in the serotonin neural system

All of the serotonin-producing neurons of the mammalian brain are located in 10 nuclei in the mid- and hindbrain regions. The cells of the rostal nuclei project to almost every area of the forebrain and regulate diverse neural processes from higher order functions in the prefrontal cortex such as integrative cognition and memory, to limbic system control of arousal and mood, to diencephalic functions such as pituitary hormone secretion, satiety, and sexual behavior. The more caudal serotonin neurons project to the spinal cord and interact with numerous autonomic and sensory systems. All of these neural functions are sensitive to the presence or absence of the ovarian hormones, estrogen and progesterone. We have shown that serotonin neurons in nonhuman primates contain estrogen receptor beta and progestin receptors. Thus, they are targets for ovarian steroids which in turn modify gene expression. Any change in serotoninergic neural function could be manifested by a change in any of the projection target systems and in this manner, serotonin neurons integrate steroid hormone information and partially transduce their action in the CNS. This article reviews the work conducted in this laboratory on the actions of estrogens and progestins in the serotonin neural system of nonhuman primates. Comparisons to results obtained in other laboratory animal models are made when available and limited clinical data are referenced. The ability of estrogens and progestins to alter the function of the serotonin neural system at various levels provides a cellular mechanism whereby ovarian hormones can impact cognition, mood or arousal, hormone secretion, pain, and other neural circuits.

Transient uptake of serotonin by newborn olfactory projection neurons

A life-long turnover of sensory and interneuronal populations has been documented in the olfactory pathways of both vertebrates and invertebrates, creating a situation where the axons of new afferent and interneuronal populations must insert into a highly specialized glomerular neuropil. A dense serotonergic innervation of the primary olfactory processing areas where these neurons synapse also is a consistent feature across species. Prior studies in lobsters have shown that serotonin promotes the branching of olfactory projection neurons. This paper presents evidence that serotonin also regulates the proliferation and survival of projection neurons in lobsters, and that the serotonergic effects are associated with a transient uptake of serotonin into newborn neurons.

Corticotropin-releasing hormone and its receptors; an evaluation at the transcription level in vivo

Activation of the hypothalamic-pituitary-adrenal (HPA) axis is the main defining feature of the stress response. The primary mediator of this response is corticotropin-releasing hormone (CRH), a 41-residue peptide acknowledged as the principal hypophysiotropic factor driving stress-induced adrenocorticotropic hormone (ACTH) secretion. Although CRH is widely distributed within the central nervous system (CNS), the paraventricular nucleus (PVN) of the hypothalamus is the principal site of the parvocellular neurosecretory neurons responsible for delivering CRH to the hypophyseal portal system, an event that initiates the activity of the pituitary-adrenal axis. Stress-induced transcriptional activation of CRH takes place quite uniquely in this hypothalamic nucleus, despite the robust constitutive hybridization signal for CRH mRNA across the brain. The fact that CRH itself is capable of mimicking these effects and that de novo but transient expression of its type one receptor occurs in the PVN are data that make this hypothalamic region of great interest to study the mechanisms that lead to such specific transcriptional activity. This review will present evidence of such phenomenon by stressors of different categories as well as the possible neuromediators involved.

Corticotropin-releasing hormone messenger RNA distribution and stress-induced activation in the thalamus

Corticotropin-releasing hormone plays a critical role in mediating the stress response. Brain circuits hypothesized to mediate stress include the thalamus, which plays a pivotal role in distributing sensory information to cortical and subcortical structures. In situ hybridization revealed neurons containing corticotropin-releasing hormone messenger RNA in the posterior thalamic nuclear group and the central medial nucleus of the thalamus, which interfaces with the ventral posteromedial nucleus (parvicellular part). These regions are of interest because they process somatosensory and visceral information. In the first experiment, the effect of acute stress on thalamic corticotropin-releasing hormone messenger RNA levels was assessed. Rats restrained for 1 h and killed 1 h later were found to have increased corticotropin-releasing hormone messenger RNA in the posterior thalamic nuclear group. The time course of these changes was examined in a second experiment in which rats were killed immediately or 3 h after restraint. While no changes occurred in the thalamus immediately after restraint, 3 h after restraint, increases in corticotropin-releasing hormone messenger RNA occurred in both the posterior thalamic nuclear group and the central medial-ventral posteromedial nucleus (parvicellular part) of the thalamus. A different pattern of activation was observed in the paraventricular nucleus of the hypothalamus with increased corticotropin-releasing hormone messenger RNA immediately after restraint, but not 1 or 3 h later. In addition to the stress-induced changes, a prominent decrease in baseline thalamic corticotropin-releasing hormone messenger RNA was observed from 1000 to 1300 h. These results show that the thalamus contains corticotropin-releasing hormone messenger RNA that increases after restraint stress, indicating a role for thalamic corticotropin-releasing hormone systems in the stress response. Stress-induced changes in thalamic corticotropin-releasing hormone messenger RNA expression appears to be regulated differently than that in the paraventricular nucleus of the hypothalamus, and may be influenced by diurnal mechanisms.

Corticotropin-releasing hormone and animal models of anxiety: gene-environment interactions

The study of the neural substrates underlying stress and anxiety has in recent years been enriched by a burgeoning pool of genetic information gathered from rodent studies. Two general approaches have been used to characterize the interaction of genetic and environmental factors in stress regulation: the evaluation of stress-related behavioral and endocrine responses in animals with targeted deletion or overexpression of specific genes and the evaluation of changes in central nervous system gene expression in response to environmental perturbations. We review recent studies that have used molecular biology and genetic engineering techniques such as in situ hybridization, transgenic animal, and antisense oligonucleotide gene-targeting methodologies to characterize the function of corticotropin-releasing hormone (CRH) system genes in stress. The effects of genetic manipulations of each element of the CRH system (CRH, its two receptors, and its binding protein) on stress-related responses are summarized. In addition, the effects of stress (acute, repeated, or developmental) on CRH system gene expression are described. The results from these studies indicate that experimentally engineered or stress-induced dysregulation of gene expression within the CRH system is associated with aberrant responses to environmental contingencies. These results are discussed in the context of how CRH system dysfunction might contribute to stress-related psychopathology and are presented in conjunction with clinical findings of CRH system dysregulation in psychiatric illness. Finally, future research strategies (i.e., high-throughput gene screening and novel gene-targeting methodologies) that may be used to gain a fuller understanding of how CRH system gene expression affects stress-related functioning are discussed.

Corticotropin releasing hormone and proopiomelanocortin involvement in the cutaneous response to stress

The skin is a known target organ for the proopiomelanocortin (POMC)-derived neuropeptides alpha-melanocyte stimulating hormone (alpha-MSH), beta-endorphin, and ACTH and also a source of these peptides. Skin expression levels of the POMC gene and POMC/corticotropin releasing hormone (CRH) peptides are not static but are determined by such factors as the physiological changes associated with hair cycle (highest in anagen phase), ultraviolet radiation (UVR) exposure, immune cytokine release, or the presence of cutaneous pathology. Among the cytokines, the proinflammatory interleukin-1 produces important upregulation of cutaneous levels of POMC mRNA, POMC peptides, and MSH receptors; UVR also stimulates expression of all the components of the CRH/POMC system including expression of the corresponding receptors. Molecular characterization of the cutaneous POMC gene shows mRNA forms similar to those found in the pituitary, which are expressed together with shorter variants. The receptors for POMC peptides expressed in the skin are functional and include MC1, MC5 and mu-opiate, although most predominant are those of the MC1 class recognizing MSH and ACTH. Receptors for CRH are also present in the skin. Because expression of, for example, the MC1 receptor is stimulated in a similar dose-dependent manner by UVR, cytokines, MSH peptides or melanin precursors, actions of the ligand peptides represent a stochastic (predictable) nonspecific response to environmental/endogenous stresses. The powerful effects of POMC peptides and probably CRH on the skin pigmentary, immune, and adnexal systems are consistent with stress-neutralizing activity addressed at maintaining skin integrity to restrict disruptions of internal homeostasis. Hence, cutaneous expression of the CRH/POMC system is highly organized, encoding mediators and receptors similar to the hypothalamic-pituitary-adrenal (HPA) axis. This CRH/POMC skin system appears to generate a function analogous to the HPA axis, that in the skin is expressed as a highly localized response which neutralizes noxious stimuli and attendant immune reactions.

Lipopolysaccharide effects on neuronal activity in rat basal forebrain and hypothalamus during sleep and waking

Peripheral administration of lipopolysaccharide (LPS) is associated with alterations in sleep and the electroencephalogram. To evaluate potential neuronal mechanisms for the somnogenic effects of LPS administration, we used unanesthetized rats to survey the firing patterns of neurons in various regions of rat basal forebrain (BF) and hypothalamus during spontaneous sleep and waking and during the epochs of sleep and waking that occurred after the intraperitoneal administration of LPS. In the brain regions studied, LPS administration was associated with altered firing rates in 39% of the neurons examined. A larger proportion of LPS-responsive units showed vigilance-related alterations in firing rates compared with nonresponsive units. Approximately equal proportions of LPS-responsive neurons showed increased and decreased firing rates after LPS administration, with some units in the lateral preoptic area of the hypothalamus showing particularly robust increases. These findings are consistent with other studies showing vigilance-related changes in neuronal activity in various regions of BF and hypothalamus and further demonstrate that peripheral LPS administration alters neuronal firing rates in these structures during both sleep and waking.

How the blood talks to the brain parenchyma and the paraventricular nucleus of the hypothalamus during systemic inflammatory and infectious stimuli

There are exciting new developments regarding the molecular mechanisms involved in the influence of circulating proinflammatory molecules within cells of the blood-brain barrier (BBB) during systemic immune challenges. These molecules, when present in the circulation, have the ability to trigger a series of events in cascade, leading to either the mitogen-activated protein (MAP) kinases/nuclear factor kappa B (NF-kappaB) or the janus kinase (JAK)/signal transducer and activator of transcription (STAT) transduction pathways in vascular-associated cells of the central nervous system (CNS). The brain blood vessels exhibit both constitutive and induced expression of receptors for different proinflammatory ligands that have the ability to stimulate these signaling molecules. Depending on the challenges and the cytokines involved, the transduction signal(s) solicited in cells of the BBB may orient the neuronal activity in a very specific manner in activating the transcription and production of soluble factors, such as prostaglandins (PGs). It is interesting to note that cytokines as well as systemic localized inflammation stimulate the cells of the BBB in a nonselective manner (i.e., within both large blood vessels and small capillaries across the brain). This nonselectivity raises several questions with regard to the localized neuronal activation induced by different experimental models of inflammation and cytokines. It is possible that the selectivity of the neuronal response is a consequence of the fine interaction between nonparenchymal synthesis of soluble mediators and expression of specific receptors for these ligands within parenchymal elements of different brain nuclei. This review will present the recent developments on this concept and the mechanisms that take place in cells of the BBB, which lead to the neuronal circuits involved in restoring the body's homeostasis during systemic immunogenic challenges. The induction of fever, the hypothalamic-pituitary adrenal (HPA) axis, and other autonomic functions are among the physiological outcomes necessary for the protection of the mammalian organism in the presence of foreign material.