The relationship of efferent projections from the area postrema to vagal motor and brain stem catecholamine-containing cell groups: an axonal transport and immunohistochemical study in the rat

Dual separate pathways for sensory and hedonic aspects of taste

It is proposed that in the gustatory system there exist separate sensory and hedonic (reward-aversion) representations in each of the primary structures in which processing of gustatory stimuli occurs. Anatomical and physiological data are used to determine putative separate sensory and hedonic representations in the nucleus of the solitary tract, parabrachial complex, gustatory thalamus, and cortical gustatory areas. In the nucleus of the solitary tract, the sensory representation is located in the rostralmost part of the nucleus, and the hedonic representation most probably in the intermediate parts. In the parabrachial complex, the sensory representation is located in the central medial and ventral lateral subnuclei, and in the waist area, and the hedonic representation in the inner division of the external lateral subnucleus and in the external medial subnucleus. In the rodent gustatory thalamic relay, the sensory representation occurs in the dorsal lateral parts of the nucleus, and the hedonic representation in the ventromedial parts. In rodent gustatory insular cortex, the sensory representation is found in anterior parts of the gustatory area, and the hedonic representation caudal to the sensory representation. The function of the separate sensory and hedonic representations is discussed in relation to the conditioned taste aversion paradigm.

Immunocytochemical localization of extracellular signal-regulated kinases 1 and 2 phosphorylated neurons in the brainstem of rat following visceral noxious stimulation

The aim of the present study was to determine the activation of the extracellular signal-regulated kinases (ERKs) 1 and 2 in brainstem neurons following noxious visceral stimulation. Ether and urethane anaesthetized rats received an intraperitoneal injection of acetic acid (ENS, UNS) or were left untreated (ECT, UCT). Paraffin embedded brain sections immunoreacted with an antibody specific for phosphorylated ERKs. In noxious stimulated rats ERKs activated neuron profiles in the periaqueductal gray matter, parabrachial, dorsal raphe, solitary tract nucleus, area postrema and superior colliculus suggest that ERKs activation takes place mainly in brainstem nuclei in which nociception and visceral activities interact. The comparison between ENS and UNS rats shows that the long acting anaesthetic urethane attenuates the number of the ERKs activated neurons compared to the short acting ether.

Intraperitoneal corticotropin-releasing factor and urocortin induce Fos expression in brain and spinal autonomic nuclei and long lasting stimulation of colonic motility in rats

CRF injected intraperitoneally (i.p.) stimulates colonic motor function and induces Fos expression in colonic myenteric neurons. We investigated central and spinal Fos expression and changes in colonic motility in response to i.p. injection of CRF and urocortin. Ovine CRF(9-33) that is devoid of intrinsic activity at the CRF receptors, was used as control peptide. Myoelectrical activity was monitored for 1 h before and after peptide injection (10 microg/kg, i.p.) in conscious non fasted rats with chronically implanted intraparietal electrodes in the cecum and proximal colon. Brain and lumbosacral spinal cord were processed for Fos immunohistochemistry at 1 h postinjection. CRF and urocortin elicited defecation and a new pattern of ceco-colonic clustered spike bursts that peaked within 15 min and lasted for the 1 h experimental period while CRF(9-33) did not modify baseline myoelectrical activity and defecation. CRF increased significantly Fos expression in the central nucleus of the amygdala (lateral part), parabrachial nucleus (external lateral subnucleus), area postrema, nucleus tractus solitarius, locus coeruleus, paraventricular nucleus of the hypothalamus, the intermediolateral column and area I-VII, X at the L6-S1 level of the spinal cord by 11-, 6.5-, 5.3-, 5.0-, 4.7-, 2.7- and 1.4-fold, respectively compared with i.p. CRF(9-33) injected rats that had little Fos expression. Urocortin induced a similar pattern of Fos response in the brain and the spinal cord. These results indicate that i.p. CRF and urocortin induce a peptide specific activation of brain nuclei receiving viscerosensory inputs and involved in autonomic circuitries whose effector limbs may impact on visceral function.

Glucagon-like peptide-1-responsive catecholamine neurons in the area postrema link peripheral glucagon-like peptide-1 with central autonomic control sites

Glucagon-like peptide-1 (GLP-1) released from the gut is an incretin that stimulates insulin secretion. GLP-1 is also a brain neuropeptide that has diverse central actions, including inhibition of food and water intake, gastric emptying, and stimulation of neuroendocrine responses characteristic of visceral illness. Both intravenous and intracerebroventricular administration of GLP-1 receptor (GLP-1R) agonists increase blood pressure and heart rate and induce Fos-like immunoreactivity (Fos-IR) in autonomic regulatory sites in the rat brain. The area postrema (AP) is a circumventricular organ and has been implicated in processing visceral sensory information. GLP-1Rs are densely expressed in the AP, and peripheral GLP-1R agonists induce Fos-IR in AP neurons to a greater degree than intracerebroventricular administration. Because the AP lacks a blood-brain barrier, we hypothesized that the AP is a key site for peripheral GLP-1 to activate central autonomic regulatory sites. In this study, we found that many tyrosine hydroxylase (TH)-containing neurons in the AP expressed GLP-1Rs and Fos-IR after intravenous GLP-1R agonists. Furthermore, intravenous but not intracerebroventricular GLP-1R agonists induced TH transcription in the AP in vivo. In addition, GLP-1R agonists directly activated TH transcription in an in vitro cell system. Finally, we found that GLP-1-responsive TH neurons in the AP innervate autonomic control sites, including the parabrachial nucleus, nucleus of solitary tract, and ventrolateral medulla. These findings suggest that catecholamine neurons in the AP link peripheral GLP-1 and central autonomic control sites that mediate the diverse neuroendocrine and autonomic actions of peripheral GLP-1.

Fos expression in non-catecholaminergic neurons in medullary and pontine nuclei after volume depletion induced by polyethylene glycol

Fos immunocytochemistry was combined with immunolabeling for dopamine-beta-hydroxylase (DBH) to examine neuronal activation in the medulla and pons after administration of polyethelene glycol (PEG), which produces volume depletion without altering arterial blood pressure. Increased Fos immunoreactivity was observed in the area postrema, nucleus of the solitary tract, rostral ventrolateral medulla, lateral parabrachial nucleus, and the dorsomedial pons at the level of the locus coeruleus. Fos immunolabeling in the caudal nucleus of the solitary tract, rostral ventrolateral medulla, and the dorsomedial pons occurred primarily in neurons that did not contain DBH. Thus, PEG activates non-catecholaminergic neurons in medullary and pontine areas associated with cardiovascular and body fluid regulation.

Involvement of the arcuate nucleus of the hypothalamus in interleukin-1-induced anorexia

Cytokine-mediated anorexia is a component of "sickness behavior" and presents a significant obstacle in the treatment of chronic illnesses. We hypothesized an involvement of the hypothalamic arcuate nucleus (ARH) in mediating the anorexic effects of a systemic interleukin-1 (IL-1) challenge based on its content of peptidergic neurons involved in feeding, its expression of IL-1 receptors and its sensitivity to systemic IL-1. IL-1 (6 microg/kg, i.v.) was found to induce Fos expression in both pro-opiomelanocortin- and neuropeptide Y-expressing neurons in and around the ARH. Contrary to expectations, rats that had sustained lesions of the arcuate nucleus, produced by neonatal monosodium glutamate treatment, displayed a more pronounced suppression (by 25%) of food intake than nonlesioned controls when treated with IL-1 after a 20 hr fast. To confirm and further characterize this unexpected result, a second ablation method was used in a similar paradigm. Animals bearing knife cuts designed to sever major ARH projections displayed an even more accentuated loss of appetite (by 60%, relative to controls) in response to systemic IL-1. This effect exhibited at least some degree of specificity, because the knife cuts did not alter either IL-1 effects on another centrally mediated acute phase response (fever) or the anorexia produced by an alternate agent, fenfluramine. These results fail to support the hypothesized ARH mediation of IL-1-induced anorexia and may suggest rather that the net output of this cell group may serve normally to restrain cytokine-induced reductions in food intake.

Vesicular glutamate transporter DNPI/VGLUT2 mRNA is present in C1 and several other groups of brainstem catecholaminergic neurons

The mouse glutamate vesicular transporter VGLUT2 has recently been characterized. The rat homolog of VGLUT2, differentiation-associated Na(+)/P(i) cotransporter (DNPI), was examined using a digoxigenin-labeled DNPI/VGLUT2 cRNA probe in the present study to determine which, if any, of the various groups of pontine or medullary monoaminergic neurons express DNPI/VGLUT2 mRNA and, thus, are potentially glutamatergic. DNPI/VGLUT2 mRNA was widely distributed within the brainstem and seemed exclusively neuronal. By using a double in situ hybridization method, the presence of the mRNA for DNPI/VGLUT2 and glutamic acid decarboxylase (GAD)-67 was mutually exclusive. By combining DNPI/VGLUT2 mRNA detection and conventional immunohistochemistry, DNPI/VGLUT2 mRNA was undetectable in lower brainstem cholinergic and serotonergic cells, but it was present in several tyrosine hydroxylase-immunoreactive (TH-ir) cell groups. DNPI/VGLUT2 mRNA was detected in most of the adrenergic neurons of the C1, C2, and C3 groups (75-80% of TH-ir neurons), in the A2 noradrenergic group (80%), and in vast numbers of area postrema cells. Within the A1 region, many fewer TH-ir cells contained DNPI/VGLUT2 (16%). Finally, DNPI/VGLUT2 mRNA was undetectable in the pontine noradrenergic cell groups (A5 and A6/locus coeruleus). In conclusion, the general pattern of DNPI/VGLUT2 expression and its exclusion from GABAergic, cholinergic, and serotonergic neurons supports the notion that DNPI/VGLUT2 mRNA identifies a subset of glutamatergic neurons in the lower brainstem. Within this region several catecholaminergic cell groups appear to be glutamatergic, including but not limited to the adrenergic cell groups C1-C3. Based on the present evidence, the noradrenergic cell groups of the pons (A5 and A6) do not contain either known vesicular glutamate transporter and are most likely not glutamatergic.

Characterization of the central nervous system innervation of the rat spleen using viral transneuronal tracing

Splenic immune function is modulated by sympathetic innervation, which in turn is controlled by inputs from supraspinal regions. In the present study, the characterization of central circuits involved in the control of splenic function was accomplished by injecting pseudorabies virus (PRV), a retrograde transynaptic tracer, into the spleen and conducting a temporal analysis of the progression of the infection from 60 hours to 110 hours postinoculation. In addition, central noradrenergic cell groups involved in splenic innervation were characterized by dual immunohistochemical detection of dopamine-beta-hydroxylase and PRV. Infection in the CNS first appeared in the spinal cord. Splenic sympathetic preganglionic neurons, identified in rats injected with Fluoro-Gold i.p. prior to PRV inoculation of the spleen, were located in T(3)-T(12) bilaterally; numerous infected interneurons were also found in the thoracic spinal cord (T(1)-T(13)). Infected neurons in the brain were first observed in the A5 region, ventromedial medulla, rostral ventrolateral medulla, paraventricular hypothalamic nucleus, Barrington's nucleus, and caudal raphe. At intermediate survival times, the number of infected cells increased in previously infected areas, and infected neurons also appeared in lateral hypothalamus, A7 region, locus coeruleus, subcoeruleus region, nucleus of the solitary tract, and C3 cell group. At longer postinoculation intervals, infected neurons were found in additional hypothalamic areas, Edinger-Westphal nucleus, periaqueductal gray, pedunculopontine tegmental nucleus, caudal ventrolateral medulla, and area postrema. These results demonstrate that the sympathetic outflow to the spleen is controlled by a complex multisynaptic pathway that involves several brainstem and forebrain nuclei.

Stressor specificity of central neuroendocrine responses: implications for stress-related disorders

Despite the fact that many research articles have been written about stress and stress-related diseases, no scientifically accepted definition of stress exists. Selye introduced and popularized stress as a medical and scientific idea. He did not deny the existence of stressor-specific response patterns; however, he emphasized that such responses did not constitute stress, only the shared nonspecific component. In this review we focus mainly on the similarities and differences between the neuroendocrine responses (especially the sympathoadrenal and the sympathoneuronal systems and the hypothalamo-pituitary-adrenocortical axis) among various stressors and a strategy for testing Selye's doctrine of nonspecificity. In our experiments, we used five different stressors: immobilization, hemorrhage, cold exposure, pain, or hypoglycemia. With the exception of immobilization stress, these stressors also differed in their intensities. Our results showed marked heterogeneity of neuroendocrine responses to various stressors and that each stressor has a neurochemical "signature." By examining changes of Fos immunoreactivity in various brain regions upon exposure to different stressors, we also attempted to map central stressor-specific neuroendocrine pathways. We believe the existence of stressor-specific pathways and circuits is a clear step forward in the study of the pathogenesis of stress-related disorders and their proper treatment. Finally, we define stress as a state of threatened homeostasis (physical or perceived treat to homeostasis). During stress, an adaptive compensatory specific response of the organism is activated to sustain homeostasis. The adaptive response reflects the activation of specific central circuits and is genetically and constitutionally programmed and constantly modulated by environmental factors.

Lateral parabrachial lesions impair taste aversion learning induced by blood-borne visceral stimuli

The lateral parabrachial area (LPB), main relay from the area postrema (AP), plays a role in processing visceral information and is thus of potential importance in taste aversion learning (TAL). This study used a lesion approach to address whether LPB functional relevance depends upon the features of toxins that serves as visceral stimuli in TAL. In addition, we explored whether LPB involvement in TAL is restricted to those toxic events detected by the AP or whether it has a more general role. Results showed that LPB-lesioned animals were disrupted in acquiring a TAL induced by blood-borne AP-dependent aversive stimuli (intraperitoneal methylscopolamine) and by AP-independent stimulus (intraperitoneal ethanol), but still, clearly developed strong aversions when intragastric hypertonic sodium chloride, a vagally processed aversive stimulus, served as the aversive stimulus. These findings suggest that the LPB plays a critical role in TAL induced by blood-borne toxins, such as methylscopolamine or ethanol, but is not necessary for vagally mediated stimulus, such as sodium chloride. The present results are discussed in the context of the hypothesis holding separable and independent neural systems underlying TAL.

Cardiovascular responses to subseptic doses of endotoxin contribute to differential neuronal activation in rat brain

The contribution of cardiovascular activity in the early central responses to systemic inflammation was assessed in rats following intravenous administration of subseptic doses of lipopolysaccharide (LPS). LPS at 12.5 microg/kg increased heart rate (HR) but did not alter mean arterial pressure (MAP), and induced interleukin-1 beta (IL-1 beta) gene expression at 1 h in circumventricular organs (CVOs), choroid plexus, meninges, blood vessels, and pituitary gland. IL-1 beta mRNA levels were attenuated at 2 h in most regions studied. LPS at 50 microg/kg caused a biphasic change in MAP, increased HR, increased levels of arginine vasopressin heteronuclear RNA in the hypothalamic paraventricular nucleus (PVN), and induced IL-1 beta gene expression in the nucleus of the solitary tract (NTS) at 1 h. LPS (both doses) induced Fos-like immunoreactivity (FLI) in the area postrema, organum vasculosum of the lamina terminalis, NTS, preoptic area, supraoptic nucleus, and PVN at 1 h. In the PVN, neurons with FLI were found primarily in the dorsal and dorsal medial parvocellular divisions after 12.5 microg/kg of LPS whereas neurons with FLI were found throughout the PVN after 50 microg/kg of LPS. After 2 h, FLI was widespread throughout the brain. Plasma ACTH levels were elevated at 1 and 2 h in response to both doses of LPS, and levels of CRF mRNA were increased after 2 h in the parvocellular PVN. Our results reveal that central responses to increasing doses of LPS show different patterns which are related to activation of distinct immune and viscerosensory pathways, and that cardiovascular responses contribute to early neuronal activation as LPS concentrations are increased.

Lateral parabrachial lesions impair intraperitoneal but not intraventricular methylscopolamine-induced taste aversion learning

The role of the lateral parabrachial area (lPB) in the acquisition of a delayed taste aversion learning task (TAL) was examined by delivering the peripherally acting aversive compound, methylscopolamine (MSP), through two different routes, intraperitoneal and intraventricular. Consistent with previous anatomical, behavioral and molecular work, electrolytic lesions centered at the lPB did impair TAL when the MSP was injected intraperitoneally. However, lPB-lesioned animals exhibited intact learning capacities when MSP was administered intraventricularly. These results are interpreted in terms of the lPB as a critical anatomical relay involved in bottom-up visceral processing of aversive stimuli and also in relation to the relevance of forebrain structures in TAL.

Specificity of amygdalostriatal interactions in the involvement of mesencephalic dopaminergic neurons in affective perception

We have recently shown that dopaminergic responses to an attractive or an aversive stimulus were respectively increased and decreased in the core part of the nucleus accumbens and the ventromedial dorsal striatum. By contrast, increases in dopaminergic responses were obtained in the shell part of the nucleus accumbens with stimuli of both affective values. In addition, the involvement of the basolateral amygdala in affective processes has been reported by several authors. Anatomo-functional relationships between the basolateral amygdala and striatal structures have also been described. Thus, in the present work we studied the regulation by the basolateral amygdala of affective dopaminergic responses in the two parts of the nucleus accumbens (core and shell) and the ventromedial dorsal striatum. More precisely, variations in extracellular levels of dopamine induced by an attractive or an aversive olfactory stimulus were studied using in vivo voltammetry in freely moving rats. Changes in dopamine levels in the three left striatal regions were measured after functional blockade of the ipsilateral basolateral amygdala with tetrodotoxin. Changes in place attraction or aversion toward the stimulus were studied in parallel to dopamine variations. The results obtained suggest a specific regulation of affective dopaminergic responses in the two parts of the nucleus accumbens by the basolateral amygdala and a lack of influence of the basolateral amygdala on the ventromedial dorsal striatum. The results suggest that attraction or aversion toward a stimulus are correlated with dopamine variations in the core of the nucleus accumbens and that the basolateral amygdala controls affective behavioural responses. These data may provide new insights into the pathophysiology of schizophrenic psychoses.

Fos induction in selective hypothalamic neuroendocrine and medullary nuclei by intravenous injection of urocortin and corticotropin-releasing factor in rats

CRF and urocortin, administrated systemically, exert peripheral biological actions which may be mediated by brain pathways. We identified brain neuronal activation induced by intravenous (i.v.) injection of CRF and urocortin in conscious rats by monitoring Fos expression 60 min later. Both peptides (850 pmol/kg, i.v.) increased the number of Fos immunoreactive cells in the paraventricular nucleus of the hypothalamus, supraoptic nucleus, central amygdala, nucleus tractus solitarius and area postrema compared with vehicle injection. Urocortin induced a 4-fold increase in the number of Fos-positive cells in the supraoptic nucleus and a 3.4-fold increase in the lateral magnocellular part of the paraventricular nucleus compared with CRF. Urocortin also elicited Fos expression in the accessory hypothalamic neurosecretory nuclei, ependyma lining the ventricles and choroid plexus which was not observed after CRF. The intensity and pattern of the Fos response were dose-related (85, 255 and 850 pmol/kg, i.v.) and urocortin was more potent than CRF. Neither CRF nor urocortin induced Fos expression in the lateral septal nucleus, Edinger-Westphal nucleus, dorsal raphe nucleus, locus coeruleus, or hypoglossal nucleus. These results show that urocortin, and less potently CRF, injected into the circulation at picomolar doses activate selective brain nuclei involved in the modulation of autonomic/endocrine function; in addition, urocortin induces a distinct activation of hypothalamic neuroendocrine neurons.

The area postrema mediates insulin hypoglycaemia-induced suppression of pulsatile LH secretion in the female rat

The caudal brainstem has been implicated in mediating the suppressive effect of glucoprivation on the reproductive neuroendocrine axis, specifically inhibition of pulsatile gonadotrophin-releasing hormone (GnRH)/luteinising hormone (LH) release in the rat. In the present study, removal of the area postrema completely prevented the profound inhibitory effect of insulin-induced hypoglycaemic stress on pulsatile LH release. These results suggest a pivotal role for this brainstem structure in mediating hypoglycaemic stress-induced suppression of the hypothalamic GnRH pulse generator.

Fos expression in brain stem nuclei of pregnant rats after hydralazine-induced hypotension

Fos and dopamine beta-hydroxylase immunoreactivity were evaluated in the brain stems of 21-day pregnant and virgin female rats injected with either hydralazine (HDZ; 10 mg/kg iv) or vehicle. HDZ produced significant hypotension in both groups, although baseline blood pressure was lower in pregnant rats (96 +/- 2.5 mmHg) than in virgin female rats (121 +/- 2.8 mmHg). There were no differences in Fos immunoreactivity in the brain stems of pregnant and virgin female rats after vehicle treatment. HDZ-induced hypotension significantly increased Fos expression in both groups; however, the magnitude of the increases differed in the caudal ventrolateral medulla (CVL), the area postrema (AP), and the rostral ventrolateral medulla (RVL). Fos expression after HDZ in pregnant rats was augmented in noncatecholaminergic neurons of the CVL but was attenuated in the AP and in noncatecholaminergic neurons in the RVL. These results are consistent with differences in the sympathetic response to hypotension between pregnant and virgin female rats and indicate that the central response to hypotension may be different in pregnant rats.

Distribution, regulation and colocalization of the genes encoding the EP2- and EP4-PGE2 receptors in the rat brain and neuronal responses to systemic inflammation

It is currently believed that prostaglandin (PG) of E2 type plays a crucial role in transferring the information received from circulating immune factors to brain parenchymal cells. Although PGE2 is synthesized quite essentially by cells of the blood-brain barrier, the organization and regulation of its receptor subtypes within neuronal elements remain unknown. In this study, intravenous (i.v.) injection of the endotoxin lipopolysaccharide (LPS) or recombinant rat interleukin-1beta (IL-1beta), and intramuscular (i.m.) injection of turpentine were used as different models of systemic immune stimuli. Rats were perfused at various times after the insults (30 min to 24 h), their brains cut and hybridized with full-length rat cRNA probes. Double-labelling procedures were accomplished to determine the cellular phenotype and activity. A very distinct distribution of both EP2 and EP4 receptors was found across the brain under basal conditions; the hybridization signal for the type 2 was detected in the bed nucleus of the stria terminalis (BNST), lateral septum, subfornical organ (SFO), ventromedial hypothalamic nucleus (VMH), central nucleus of the amygdala (CeA), locus coeruleus (LC) and the area postrema (AP), whereas the ventral septal/anterior preoptic area, the magnocellular paraventricular nucleus (PVN), supraoptic nucleus, parabrachial nucleus, LC, the nucleus of the solitary tract (NTS) and the ventrolateral medulla (VLM) exhibited moderate to strong levels for the EP4 mRNA under basal conditions. Upregulation of the genes encoding EP2 and EP4 receptors was detected in selective regions and neuronal populations during systemic inflammatory challenges. The most dramatic one being the robust transcriptional activation of the EP4 subtype within corticotropin-releasing factor (CRF) neurons of the parvocellular PVN following i.v. LPS and IL-1beta injection, and the localized i.m. aggression. These neurons of the endocrine hypothalamus as well as those of numerous autonomic-related nuclei were activated by the proinflammatory cytokine, as they were immunoreactive (ir) to Fos nuclear protein. The EP4 transcript was also present in activated catecholaminergic neurons of the LC, NTS and VLM, although only the A1 cell group exhibited an increase in EP4 transcription in response to circulating IL-1beta. Moreover, the systemic immunogenic insults caused a significant increase in the EP2 mRNA levels in the CeA, SFO, AP and the leptomeninges. These data provide a distinct pattern of EP2 and EP4 expression throughout the rat brain under both basal and immune-challenged conditions, and underlie the possible role of the EP4 subtype in mediating the effects of PGE2 on different autonomic and neuroendocrine functions. The presence of Fos-ir nuclei in various populations of EP4 neurons of IL-1beta-treated animals clearly supports this concept and suggests that the selectivity of the neuronal response during systemic inflammation may depend on the expression of specific PGE2 receptors in key structures of the brain.

Impaired osmoregulatory responses in rats with area postrema lesions

Area postrema lesions (APX) in adult male rats produced a robust spontaneous intake of 0.5 M NaCl, as reported previously. The largest NaCl intakes (up to 108 ml/day) were observed when there was little incidental damage in the medial subnucleus of the nucleus of the solitary tract adjacent to the caudal and middle portions of the area postrema. Rats with discrete APX also drank substantial amounts of 0.5 M NaCl when access to saline was restricted to 7 h/day (up to 30 ml in 1 h, 48 ml in 7 h). Such large NaCl intakes stimulated considerable water ingestion and renal sodium excretion, but together these responses usually were insufficient for osmoregulation during the 7-h test period. After systemic administration of hypertonic NaCl solution, rats with APX excreted less Na(+) in urine and secreted less vasopressin and oxytocin than control rats did. The prominent salt appetite, insufficient thirst and natriuresis in response to an ingested NaCl load, and blunted natriuresis and neurohypophysial hormone secretion in response to an injected NaCl load, all indicate that osmoregulatory responses are impaired in rats after APX.

The extended amygdala and salt appetite

Both chemo- and mechanosensitive receptors are involved in detecting changes in the signals that reflect the status of body fluids and of blood pressure. These receptors are located in the systemic circulatory system and in the sensory circumventricular organs of the brain. Under conditions of body fluid deficit or of marked changes in fluid distribution, multiple inputs derived from these humoral and neural receptors converge on key areas of the brain where the information is integrated. The result of this central processing is the mobilization of homeostatic behaviors (thirst and salt appetite), hormone release, autonomic changes, and cardiovascular adjustments. This review discusses the current understanding of the nature and role of the central and systemic receptors involved in the facilitation and inhibition of thirst and salt appetite and on particular components of the central neural network that receive and process input derived from fluid- and cardiovascular-related sensory systems. Special attention is paid to the structures of the lamina terminalis, the area postrema, the lateral parabrachial nucleus, and their association with the central nucleus of the amygdala and the bed nucleus of the stria terminalis in controlling the behaviors that participate in maintaining body fluid and cardiovascular homeostasis.

Distribution of neurons projecting to the rostral ventrolateral medullary pressor region that are activated by sustained hypotension

Hypotension produces a reflex increase in the activity of sympathetic vasomotor and cardiac nerves. It is believed that the reflex sympathoexcitation is due largely to disinhibition of sympathoexcitatory neurons in the rostral ventrolateral medulla, but it is possible that it may also be mediated by excitatory inputs from interneurons that are activated by a fall in blood pressure. The aim of this study in conscious rabbits was to identify and map neurons with properties that are characteristic of interneurons conveying excitatory inputs to the rostral ventrolateral medullary pressor region in response to hypotension. In a preliminary operation, a retrogradely-transported tracer, fluorescent-labelled microspheres, was injected into the functionally-identified pressor region in the rostral ventrolateral medulla. After a waiting period of at least one week, a moderate hypotension (decrease in arterial pressure of approximately 20 mmHg) was induced in conscious rabbits for 60 min by the continuous infusion of sodium nitroprusside. In confirmation of a previous study from our laboratory, [Li and Dampney (1994) Neuroscience 61, 613634] hypotension resulted in the expression of Fos (the protein product of c-fos, a marker of neuronal activation) in many neurons in several distinct regions in the brainstem and hypothalamus. Some of these regions (nucleus tractus solitarius, area postrema, caudal and intermediate ventrolateral medulla, parabrachial complex in the pons, and paraventricular nucleus in the hypothalamus) also contained large numbers of retrogradely-labelled cells. Approximately 10% of the Fos-positive neurons in the nucleus tractus solitarius, and 15-20% of Fos-positive neurons in the caudal and intermediate ventrolateral medulla were also retrogradely-labelled from the rostral ventrolateral medullary pressor region. In other brain regions, very few double-labelled neurons were found. In previous studies from our laboratory, we have determined the distribution of neurons in the brainstem that project to the rostral ventrolateral medullary pressor region and that are also activated by hypertension [Polson et al. (1995) Neuroscience 67, 107-123] or by hypoxia. [Hirooka et al. (1997) Neuroscience 80, 1209-1224] Comparison of the present results with those from these previous studies indicate that although hypotension and hypoxia both elicit powerful reflex sympathoexcitatory responses, the central pathways subserving these effects in conscious animals are fundamentally different. Hypoxia activates rostral ventrolateral medullary sympathoexcitatory neurons mainly via a major direct excitatory projection from the nucleus tractus solitarius, as well as from the Kölliker-Fuse nucleus in the pons, while in contrast the activation of these neurons in response to hypotension appears to be due mainly to disinhibition, mediated via inhibitory interneurons. In addition, however, inputs originating from excitatory interneurons in the nucleus tractus solitarius and caudal and intermediate parts of the ventrolateral medulla appear to contribute to the hypotension-evoked activation of sympathoexcitatory neurons in the rostral ventrolateral medulla.

The area postrema of newborn swine is activated by hypercapnia: relevance to sudden infant death syndrome?

This study was performed to investigate a role of the neonatal area postrema (AP) in the chemoreceptor response to hypercapnia which is defective in sudden infant death syndrome (SIDS). AP responses to CO2 inhalation were monitored in 1 to 5 week old piglets by mapping neurons that were induced to express the c-fos gene product, Fos--a marker of functional activation. Interpretive confounds were minimized by controlling for hypoxia, the effects of surgical procedures and ambient environmental stressors on neuronal activity (c-fos expression). The AP demonstrated a powerful and reproducible response in neonatal swine breathing 10% CO2 for 1 h. Intensely immunolabeled nuclei were detected throughout the longitudinal extent of the circumventricular organ, and were especially heavily concentrated at rostral levels proximal to obex. Quantitative analysis verified statistically significant increases in numbers of cells that were induced to express Fos-like immunoreactivity (FLI) in the AP of CO2- stimulated piglets as compared to control groups. No detectable age-related differences were observed in AP response patterns. Conclusions. The AP responds to hypercapnic stress in the newborn piglet. A mature circumventricular organ response in the neonate may be crucial in defending against common environmental stressors, such as nicotine exposure--an emetic agent acting via the AP and a major risk factor in SIDS. Hence, a defect of the AP or its network may underlie a loss of state-dependent controls over cardiopulmonary reflex function in SIDS.

Cardiovascular responses to microinjections of excitatory amino acids into the area postrema of the rat

Although, area postrema (AP) as been implicated in the regulation of cardiovascular function, there is no consensus regarding the type of responses elicited by stimulation of this brain structure. Microinjections (50 nl) of smaller concentrations of excitatory amino acid receptor agonists (e.g., NMDA, KA and trans-ACPD, 10 microM each) into the AP elicited pressor and tachycardic responses in unanesthetized decerebrate as well as urethane-anesthetized rats. Microinjections of higher concentrations (e.g., 50 microM NMDA) of excitatory amino acids (EAAs) into the AP elicited an initial pressor and tachycardic response which was followed by a depressor and bradycardic response; when high concentrations of NMDA were microinjected into the AP, enough concentration may have reached the nucleus tractus solitarius (nTS) to elicit depressor and bradycardic responses. Alternatively, high concentrations of NMDA may excite known projections from AP to the nTS.

Temporal and spatial relationships between lipopolysaccharide-induced expression of Fos, interleukin-1beta and inducible nitric oxide synthase in rat brain

Interleukin-1beta plays an important role in mediating central components of the host response to peripheral infection such as fever and neuroendocrine activation by acting in the brain. The present study assessed whether interleukin-1beta produced in the brain is relevant to neuronal activation and the fever response induced by intraperitoneal injection of bacterial lipopolysaccharide. The distributions of Fos protein, interleukin-1beta protein and inducible nitric oxide synthase messenger RNA, used as an anatomical indicator of interleukin-1beta bioactivity, were compared in brains of animals killed 2, 4 or 8 h after lipopolysaccharide (250 microg/kg) or saline injection. Saline did not induce interleukin-1beta or Fos immunoreactivity in the brain. Interleukin-1beta positive cells were found 2 h after lipopolysaccharide injection in circumventricular organs. Fos immunoreactivity at this time-point was not found in circumventricular organs, but in parenchymal structures such as the nucleus of the solitary tract, paraventricular hypothalamus and ventromedial preoptic area. Fos expression did occur in circumventricular organs only 8 h after lipopolysaccharide injection. This late pattern of Fos expression coincided with the rise in body temperature and the induction of inducible nitric oxide synthase messenger RNA. These data show that after peripheral lipopolysaccharide administration interleukin-1beta is synthesized and bioactive in circumventricular organs. Interleukin-1beta may activate local neurons that induce fever and neuroendocrine activation via projections to the ventromedial preoptic area and the nucleus of the solitary tract.

Picrotoxin-, kainic acid- and seizure-induced Fos in brainstem, with special reference to catecholamine cell groups

One of the original views about motor convulsions with bilateral synchronous EEG discharges was that ascending projections from deep midline brainstem neurons subserve discharge synchronisation. We examined this in kainic acid- and picrotoxin-induced seizures using the distribution of Fos protein expression in the brainstem as a possible marker of brainstem neuronal activation. Before seizure, Fos was present in parabrachial nucleus, central grey and A1/C1 cell groups and, to a lesser extent, in A2/C2 cell groups and A6 neurons. Seizure correlated with further induction of Fos in these cell groups as well as in A6 neurons. There was no Fos in other cell groups with bilateral cerebro-cortical projections nor in cell groups likely to participate in the expression of seizures, reflecting a limitation of the Fos method. This study provides evidence of locus coeruleus involvement in these two models of seizure.

Induction of pro-inflammatory cytokine mRNAs in the brain after peripheral injection of subseptic doses of lipopolysaccharide in the rat

Although it is generally accepted that pro-inflammatory cytokines produced by cells of the central nervous system play important roles in the communication between the central nervous system and the immune system during sepsis, it is not clear whether these cytokines are produced in the brain under subseptic conditions. In this study, we used in situ hybridization to examine the mRNA expression of the pro-inflammatory cytokines IL-1beta and TNFalpha in the brains of rats 2 and 12 h after they were challenged by peripheral injections of lipopolysaccharide (LPS) ranging from 0.01 to 1000 microg/kg. Unlike septic doses of LPS (> 500 microg/kg), which induce global expression of pro-inflammatory cytokines in the brain, subseptic doses of LPS (0.01-10 microg/kg) induced IL-1beta and TNFalpha mRNA expression only in the choroid plexus, the circumventricular organs, and meninges. The expression of the cytokine-responsive immediate early gene I kappaB alpha was induced in the brain after doses of LPS as low as 0.1 microg/kg. I kappaB alpha mRNA expression was confined to sites where IL-1beta and TNFalpha were expressed. These results indicate that the induction and action of pro-inflammatory cytokines during subseptic infection occur at the blood-brain barrier and at circumventricular organs, which may be sites for elaboration of signal molecules that communicate peripheral immune status to the brain.

N-(6-18F-fluorohexyl)-N-methylpropargylamine: a fluorine-18-labeled monoamine oxidase B inhibitor for potential use in PET studies

We have synthesized N-(6-18F-fluorohexyl)-N-methylpropargylamine (18F-FHMP) as a positron emission tomography (PET) radiotracer for monoamine oxidase B (MAO-B). The radiosynthesis was carried out by a fluorine-for-bromine substitution in 30-40% radiochemical yield in specific activities of 1-2 Ci/micromol. Selectivity for MAO-B was demonstrated by the high affinity of (R)-deprenyl (IC50 = 6.8 nM) and lower affinity of clorgyline (IC50 = 1.2 microM) for the inhibition of 18F-FHMP binding in vitro in rat brain homogenates. In vitro autoradiographic studies in rat brain slices showed localization of 18F-FHMP in regions such as the ependyma of the lateral ventricle, dorsal raphe, area postrema, and other regions such as the cerebellum. The specific binding observed in the autoradiograms was displaced by preincubation with (R)-deprenyl. In in vivo experiments, the uptake of 18F-FHMP in the rat brains was high (0.10-0.20% injected dose/g). The binding of 18F-FHMP in the rat brain correlated with the general distribution of MAO-B and was displaced completely by preadministration of 10 microM (R)-deprenyl. These results suggest that 18F-FHMP is a potential PET radiotracer for MAO-B for use in in vitro and in vivo experiments.

Severe pathological changes of parabrachial nucleus in Alzheimer's disease

In the first of a series of studies aimed at mapping brain stem pathological changes in patients with Alzheimer's disease (AD), we report a new finding regarding the parabrachial nucleus (PBN), a unit of paramount importance in the relay and integration of visceral and nociceptive information as well as in homeostatic control. The brains of 20 patients with AD were surveyed. The PBN contained pervasive neuropathological changes in 100% of the brains from those with early-onset dementia and in 80% from those with late-onset dementia. These changes were entirely absent in all 10 normal controls. The pathological changes of PBN, would cause autonomic dysfunction in patients with AD and perhaps contribute to the disproportionate mortality encountered in these patients.

Autonomic brainstem projections to the pancreas: a retrograde transneuronal viral tracing study in the rat

The present study describes brainstem nuclei that participate in the autonomic innervation of the pancreas, using a retrograde viral transneuronal tracing technique. It aimed at identifying the neuronal architecture of the parasympathetic, gustatory-induced insulin release by the endocrine pancreas (preabsorptive insulin response, PIR). Autonomic pathways organized for reflex adjustments of the end organ, as it happens in the PIR, involve relatively simple circuits. This implies a short brainstem circuit from the rostral gustatory nucleus of the solitary tract to the dorsal motor nucleus of the vagus. The present findings confirm projections to the pancreas, originating from preganglionic neurons in the dorsal motor nucleus of the vagus. Transneuronal labeling was detected in the medial, and to a lesser extent in the lateral nucleus of the solitary tract mainly at caudal and intermediate levels. Furthermore, infected neurons were seen in the brainstem in the dorsal and ventral part of the medullary reticular formation, in the area postrema and in the raphe nuclei. Sparse labeling was found in the gustatory zone of the nucleus tractus solitarius. These results indicate that a direct connection between the rostral nucleus tractus solitarius and the medial dorsal motor nucleus of the vagus is very unlikely, so that one or more intermediate stations may be involved. Candidates to complete this pathway are the intermediate or caudal nucleus tractus solitarius, the medullary reticular formation or the parabrachial nucleus.

Cholecystokinin actions in the parabrachial nucleus: effects on thirst and salt appetite

The present study investigated the effects of bilateral injections of the nonselective CCK receptor antagonist proglumide or CCK-8 into the lateral parabrachial nuclei (LPBN) on the ingestion of 0.3 M NaCl and water induced by intracerebroventricular injection of ANG II or by a combined treatment with subcutaneous furosemide (Furo) + captopril (Cap). Compared with the injection of saline (vehicle), bilateral LPBN injections of proglumide (50 micrograms . 200 nl-1 . site-1) increased the intake of 0.3 M NaCl induced by intracerebroventricular ANG II (50 ng/1 microliter). Bilateral injections of proglumide into the LPBN also increased ANG II-induced water intake when NaCl was simultaneously available, but not when only water was present. Similarly, the ingestion of 0.3 M NaCl and water induced by the treatment with Furo (10 mg/kg) + Cap (5 mg/kg) was increased by bilateral LPBN proglumide pretreatment. Bilateral CCK-8 (0.5 microgram . 200 nl-1 . site-1) injections into the LPBN did not change Furo + Cap-induced 0.3 M NaCl intake but reduced water consumption. When only water was available after intracerebroventricular ANG II, bilateral LPBN injections of proglumide or CCK-8 had no effect or significantly reduced water intake compared with LPBN vehicle-treated rats. Taken together, these results suggest that CCK actions in the LPBN play a modulatory role on the control of NaCl and water intake induced by experimental treatments that induce hypovolemia and/or hypotension or that mimic those states.

Dorsomedial hindbrain participation in glucoprivic feeding response to 2DG but not 2DG-induced hyperglycemia or activation of the HPA axis

2-Deoxy-d-glucose (2DG) is a glucose analogue that inhibits intracellular utilization of glucose and produces a characteristic behavioral response known as glucoprivic feeding. The area postrema (AP) is a caudal hindbrain structure shown previously to be involved in 2DG-induced glucoprivic feeding. In addition, peripheral administration of 2DG is known to elicit activation of both the hypothalamic-pituitary-adrenal (HPA) axis and the sympathoadrenomedullary system. The neural substrates for these neuroendocrine and neural responses to 2DG are not known although they may also involve the AP. The possible role of the AP in 2DG-induced feeding, activation of the HPA axis and hyperglycemia was investigated in Sprague-Dawley rats with lesions centered on the area postrema (APX) and sham-operated (SHM) rats administered 2DG (200 mg/kg) or physiological saline (1 ml/kg). Peripheral administration of 2DG evoked a feeding response in SHM rats that was abolished in APX animals. Interestingly, 2DG administered at this dose produced a significant increase in plasma corticosterone and plasma glucose in both SHM and APX rats for up to 4 h after drug treatment. Collectively, these findings suggest that the AP is involved in the behavioral (feeding) response to peripheral administration of 2DG, but does not appear to be a common neural substrate for the neuroendocrine (HPA axis) and sympathoadrenal (hyperglycemic) responses to this agent.

Efferent projection from the rostral ventrolateral medulla to the area postrema in rats

The rostral ventrolateral medulla (RVLM) is a region of the brain primarily involved in cardiovascular control. It receives information from several areas of the brainstem, among which the area postrema (AP) and the nucleus of the solitary tract (NTS). The medial subnuclei of the solitary tract (TS) project towards the RVLM, providing cardiopulmonary information, and the AP serves information about circulatory hormones. Although the efferent pathways are well known, it is not the case for the connections from the RVLM towards the AP and the NTS. The present study was designed to examine the efferent connections from the RVLM onto the dorsal structures of the medulla: quantitatively by means of anatomical techniques, and functionally by means of electrophysiological techniques. Morphologically, Biocytin or Biotinylated dextran amine microinjections into the RVLM were followed by labelling of many fibres running towards the bulbar dorsomedial structures, with some pathways lying in the AP itself, or located in its caudal vicinity. Conversely, when microinjections of Fast Blue (FB) were made into the AP, FB-labelled cells could be observed within the RVLM. Electrophysiologically, single shock stimulation carried on AP allowed identification of axonal fibres issuing from somata located into the cardiovascular neuronal pool in the RVLM. From these results, we can assume: (1) the existence of dense efferent projection from RVLM to aspects of the dorsal vagal complex, including the AP and, among this dense projection, (2) the existence of some fibres terminating in, or crossing through the AP, and identified as conveying baroreceptor-related information, in the rat.

Area postrema removal abolishes stimulatory effects of intravenous interleukin-1beta 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-1beta (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 microg/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.

Endogenous ANG II supports lumbar sympathetic activity in conscious sodium-deprived rats: role of area postrema

This study tests the hypothesis that the area postrema (AP) is necessary for endogenous ANG II to chronically maintain lumbar sympathetic nerve activity (LSNA) and heart rate (HR) in conscious sodium-deprived rats. The effect of the ANG II type 1-receptor antagonist, losartan, on LSNA and HR was determined in rats that were either AP lesioned (APX) or sham lesioned. The sham rats were divided into groups, with (SFR) or without (SAL) food restriction, to control for the decreased food intake of APX rats. Before losartan, basal mean arterial pressure (MAP), HR, and baroreflex control of LSNA and HR were similar between groups, with the exception of lower maximal reflex LSNA and higher maximal gain of the HR-MAP curve in APX rats. In all groups, losartan similarly shifted (P < 0.01) the LSNA-MAP curve to the left without altering maximal gain. Losartan also decreased (P < 0.05) minimal LSNA in all groups, and suppressed (P < 0.01) maximal LSNA (% of control) in SFR (240 +/- 13 to 205 +/- 15) and SAL (231 +/- 21 to 197 +/- 26) but not APX (193 +/- 10 to 185 +/- 8) rats. In general, losartan similarly shifted the HR-MAP curve to a lower MAP in all groups. The results suggest that the AP is not necessary for endogenous ANG II to chronically support LSNA and HR at basal and elevated MAP levels in sodium-deprived rats. However, the AP is required for endogenous ANG II to increase maximal reflex LSNA at low MAP levels.

Central action of adrenomedullin to prevent ethanol-induced gastric injury through vagal pathways in rats

Adrenomedullin (AM), belongs to the calcitonin gene-related peptide (CGRP) family and interacts with AM and CGRP1 receptors. Specific AM receptors and immunoreactivity are present in the rat brain. The effect of intracisternal injection of rat AM on ethanol-induced gastric lesions was studied in conscious Wistar rats. The peptide was injected intracisternally or intravenously under short anesthesia 20 min before intragastric injection of 70% ethanol. Corpus lesions were determined 1 h after ethanol. Intracisternal AM (75, 150, and 300 pmol) dose-dependently inhibited ethanol-induced gastric lesions by 40-72% and rat alpha-CGRP (150 pmol ic) by 76%. Intravenous AM (300 pmol) had no effect. The CGRP1 receptor antagonist CGRP-(8-37) (9.6-19.2 nmol ic) dose-dependently inhibited the protective effect of intracisternal alpha-CGRP but not that of AM. Subdiaphragmatic vagotomy and peripheral injection of atropine, indomethacin, or NG-nitro-L-arginine methyl ester (L-NAME) prevented AM protective action. L-Arginine but not D-arginine blocked L-NAME action. These data suggest that both AM and CGRP act in the brain to prevent ethanol-induced gastric lesions through interaction with their specific receptors. AM action may involve vagal cholinergic-dependent modulation of prostaglandins and nitric oxide protective mechanisms.

Regulation of vasopressin synthesis and release by area postrema in rats

There is evidence indicating that the area postrema (AP), the most caudal circumventricular organ located on the dorsal surface of the medulla, is involved in several physiological regulations. In this study, we investigated the role of AP in the regulation of arginine vasopressin (AVP) synthesis and release, using rats of which the AP was lesioned 6 weeks previously. The level of plasma AVP in the AP lesioned (APX) group was significantly lower than in the sham operated (Sham) group in the basal state. AVP release induced by either hyperosmolality or hypovolemia was significantly attenuated by APX. To clarify the role of AP in AVP synthesis in the hypothalamus, we examined the AVP gene expression using in situ hybridization. AVP messenger RNA levels in paraventricular (PVN) and supraoptic nuclei (SON) in the APX group were significantly lower than in the Sham group in the basal state. Moreover, the AVP messenger RNA levels in PVN and SON in the APX group were also significantly lower than in the Sham group after water deprivation for 3 days. These results suggest that AVP synthesis and release are tonically stimulated by AP in the basal state and that AVP synthesis and release in stimulated states are also regulated, at least partially, by AP.

Functional circuitry in the brain of immune-challenged rats: partial involvement of prostaglandins

This study investigated the role of prostaglandins (PGs) on the neuronal activity and the transcription of corticotropin-releasing factor (CRF) in the brain of conscious immune-challenged rats. Intravenous (i.v.) administration of indomethacin, an inhibitor of PG synthesis, was performed prior to and after the intraperitoneal (i.p.) injection of different doses [250 microg, 25 microg, and 2.5 microg/100 g body weight (b.w.)] of the immune activator lipopolysaccharide (LPS). Systemic administration of the high and middle doses of LPS caused a robust and widespread induction of both immediate-early genes (IEGs), c-fos and nerve growth factor-inducible gene B (NGFI-B) mRNAs, whereas injection of the low dose selectively triggered c-fos expression within the sensorial circumventricular organs. Pretreatment with indomethacin did not prevent c-fos transcription in the rat brains challenged with the high dose of LPS at 3 hours postinjection. Inhibition of PG formation was more effective for interruption of the neuronal activation in animals injected with 25 microg LPS/100 g b.w., although the influence depended on the structures and the groups of activated cells. Indeed, PG inhibition significantly altered LPS-induced c-fos mRNA expression in the medial preoptic area/organum vasculosum of the lamina terminalis, the periventricular nucleus, the paraventricular nucleus of the hypothalamus (PVN), and the ventrolateral medulla (VLM) but not in many other regions, including the subfornical organ, the central nucleus of the amygdala, the arcuate nucleus/median eminence, the parabrachial nucleus, the choroid plexus, and the nucleus of the solitary tract (NTS). In the hypothalamic PVN, inhibition of both c-fos and NGFI-B transcripts by indomethacin was also associated to an abolished influence of the endotoxin on the transcription of neuroendocrine CRF; induction of CRF primary transcript by the middle dose of LPS was selective to the PVN and was completely blocked by pretreatment with indomethacin. Moreover, a large number of tyrosine hydroxylase (TH)-immunoreactive neurons of the VLM (A1/C1) and the NTS (A2/C2) were positive for c-fos mRNA in immune-challenged rats, an effect that was largely prevented by indomethacin in the VLM but not in the NTS. These results indicate that the role of PGs in mediating the stimulatory influence of the acute-phase response depends on the severity of the systemic stressful situation, the brain regions, and the cell groups as well as the activated target genes.

Evidence for an intramedullary prostaglandin-dependent mechanism in the activation of stress-related neuroendocrine circuitry by intravenous interleukin-1

We have provided evidence that the stimulatory effects of intravenous interleukin-1 (IL-1) on neurosecretory neurons in the paraventricular nucleus (PVH) that express corticotropin-releasing factor (CRF) depend specifically on the integrity of catecholaminergic projections originating in caudal medulla. Here we report on experiments designed to test alternative means by which circulating IL-1 might access medullary aminergic neurons, including mechanisms involving sensory components of the vagus, the area postrema, or perivascular cells bearing IL-1 receptors. Neither abdominal vagotomy nor area postrema lesions reliably altered Fos expression induced in the medulla or PVH in response to a moderately suprathreshold dose of IL-1beta. Cytokine-stimulated increases in CRF mRNA in the PVH were also unaffected by either ablation. By contrast, systemic administration of the cyclooxygenase inhibitor indomethacin resulted in parallel dose-related attenuations of IL-1 effects in hypothalamus and medulla. Microinjections of prostaglandin E2 (PGE2; >/=10 ng) in rostral ventrolateral medulla, the principal seat of IL-1-sensitive neurons that project to the PVH, provoked discrete patterns of cellular activation in hypothalamus and medulla that mimicked those seen in response to intravenous IL-1. We interpret these findings as supporting the hypothesis that paracrine effects of PGE2 released from perivascular cells in the medulla as a consequence of IL-1 stimulation and, acting through prostanoid receptors on or near local aminergic neurons that project to the PVH, contribute to the stimulatory effects of increased circulating IL-1 on neurons constituting the central limb of the hypothalamo-pituitary-adrenal axis.

Central action of adrenomedullin to inhibit gastric emptying in rats

The central action of human adrenomedullin (AM) to influence gastric emptying and the peripheral mechanisms involved were studied in conscious rats. The 20-min rate of gastric emptying of a methylcellulose solution was assessed after intracisternal (i.c.) injection of AM or rat alpha-calcitonin gene-related peptide (alphaCGRP). AM and alphaCGRP dose-dependently inhibited gastric emptying with i.c. ED50 values of 120 and 100 pmol, respectively. Human proadrenomedullin N-terminal 20 peptide (150-600 pmol, i.c.) and AM (150 pmol, i.v.) had no effect. The inhibitory actions of AM and alphaCGRP (150 pmol, i.c.) were completely blocked by the CGRP antagonist, human CGRP-(8-37) injected i.c. at 30 microg, but not at 15 microg. The CRF antagonist, [D-Phe12,Nle(21,38),C(alpha)MeLeu37]CRF-(12-41) (10 microg/rat) injected i.c. prevented i.c. rat/human CRF (150 pmol)-induced 53% inhibition of gastric emptying while not modifying the effect of AM. The action of AM (150 pmol, i.c.) was abolished by bilateral adrenalectomy or the beta-adrenergic blocker, propranolol (1 mg/kg, i.p.), but was not altered by indomethacin (5 mg/kg, i.p.) or subdiaphragmatic vagotomy. These results indicate that i.c. AM and alphaCGRP equipotently inhibit gastric emptying through mechanisms similarly antagonized by a high dose of CGRP-(8-37). The central AM action is mediated through adrenal-dependent, beta-adrenergic pathways independently from activation of central CRF receptors.

CNS sites involved in sympathetic and parasympathetic control of the pancreas: a viral tracing study

The viral transneuronal tracing method was used to identify the CNS cell groups that regulate the parasympathetic and sympathetic outflow systems of the pancreas. Pseudorabies virus (PRV) was injected into the pancreas of vagotomized rats and after 6 days survival, the pattern of transneuronal labeling in the CNS sympathetic regulatory regions was determined. The converse experiment was performed in order to elucidate the central parasympathetic cell groups that regulate the pancreas. Immunohistochemical methods were used to identify putative neuropeptide- and catecholamine-containing CNS neurons involved in these regulatory circuits. The major finding of this study indicates that five brain regions, viz., paraventricular hypothalamic nucleus, perifornical hypothalamic region, A5 catecholamine cell group, rostral ventrolateral medulla, and lateral paragigantocellular reticular nucleus, contain a considerable amount of overlap in cell body labeling. In addition, the ventrolateral part of the periaqueductal gray matter and gigantocellular reticular nucleus, ventral part also showed a similar overlap, but the numbers of neurons found in these areas were considerably lower than the five major regions. These data suggest that these brain regions may provide parallel and possibly redundant, autonomic pathways affecting glucagon and adrenaline release.

Central c-Fos expression in neonatal and adult rats after subcutaneous injection of hypertonic saline

Centrally-mediated responses to plasma hyperosmolality include compensatory drinking and pituitary secretion of vasopressin and oxytocin in both adult and neonatal rats. However, the anorexia that is produced by plasma hyperosmolality in adult rats is not evident in neonates, perhaps due to functional immaturity of osmoresponsive hindbrain circuits. To examine this possibility, the present study compared treatment-induced brain expression of the immediate-early gene product c-Fos as a marker of neural activation in adult and two-day-old rats after subcutaneous injection of 2 M NaCl (0.1 ml/10 g body weight). This treatment produced marked hypernatremia in adult and two-day-old rats without altering plasma volume. Several brain regions (including components of the lamina terminalis, the paraventricular and supraoptic nuclei of the hypothalamus, and the area postrema) were activated to express c-Fos similarly in adult and two-day-old rats after 2 M NaCl injection, consistent with previous reports implicating a subset of these regions in osmotically-stimulated drinking and neurohypophyseal secretion. In contrast, other areas of the brain that were activated to express c-Fos in adult rats after 2 M NaCl injection were not activated in neonates: these areas included the central nucleus of the amygdala, the parabrachial nucleus and catecholamine cell groups within the caudal medulla. This study demonstrates that certain brain regions that are osmoresponsive in adult rats (as defined by induced c-Fos expression) are not osmoresponsive in two-day-old rats. When considered in the context of known differences between the osmoregulatory capacities of adult and neonatal rats, our results are consistent with the idea that osmoresponsive forebrain centres are primarily involved in osmotically-stimulated compensatory drinking and neurohypophyseal secretion, whereas osmoresponsive regions of the hindbrain are important for concomitant inhibition of feeding and gastric emptying.

The neuropeptide Y (Y4) receptor is highly expressed in neurones of the rat dorsal vagal complex

Recently, the cDNA encoding the Y4 neuropeptide Y (NPY) receptor cDNA was cloned from a rat genomic library. The Y4 receptor is characterized by having a high affinity for pancreatic polypeptide (PP) and peptide YY (PYY). By using in situ hybridization histochemistry with 35S-labelled riboprobes, we have visualized the cellular expression of mRNA encoding the Y4 receptor protein in the rat dorsal vagal complex at the light microscopical level. High densities of silver grains were observed over neurones of the dorsal vagal motor nucleus, and over neurones of a subregion of the nucleus of the solitary tract known as the subnucleus gelatinosus. Furthermore, cells within the ventral margin of the area postrema expressed high levels of Y4 mRNA. These observations indicate that circulating PP and/or NPY/PYY via the blood-brain barrier-free area postrema and subpostremal area could influence neurones of the dorsal vagal complex with profound influence on numerous homeostatic mechanisms governed by this nuclear complex.

The neuroendocrinology of thirst and salt appetite: visceral sensory signals and mechanisms of central integration

This review examines recent advances in the study of the behavioral responses to deficits of body water and body sodium that in humans are accompanied by the sensations of thirst and salt appetite. Thirst and salt appetite are satisfied by ingesting water and salty substances. These behavioral responses to losses of body fluids, together with reflex endocrine and neural responses, are critical for reestablishing homeostasis. Like their endocrine and neural counterparts, these behaviors are under the control of both excitatory and inhibitory influences arising from changes in osmolality, endocrine factors such as angiotensin and aldosterone, and neural signals from low and high pressure baroreceptors. The excitatory and inhibitory influences reaching the brain require the integrative capacity of a neural network which includes the structures of the lamina terminalis, the amygdala, the perifornical area, and the paraventricular nucleus in the forebrain, and the lateral parabrachial nucleus (LPBN), the nucleus tractus solitarius (NTS), and the area postrema in the hindbrain. These regions are discussed in terms of their roles in receiving afferent sensory input and in processing information related to hydromineral balance. Osmoreceptors controlling thirst are located in systemic viscera and in central structures that lack the blood-brain barrier. Angiotensin and aldosterone act on and through structures of the lamina terminalis and the amygdala to stimulate thirst and sodium appetite under conditions of hypovolemia. The NTS and LPBN receive neural signals from baroreceptors and are responsible for inhibiting the ingestion of fluids under conditions of increased volume and pressure and for stimulating thirst under conditions of hypovolemia and hypotension. The interplay of multiple facilitory influences within the brain may take the form of interactions between descending angiotensinergic systems originating in the forebrain and ascending adrenergic systems emanating from the hindbrain. Oxytocin and serotonin are additional candidate neurochemicals with postulated inhibitory central actions and with essential roles in the overall integration of sensory input within the neural network devoted to maintaining hydromineral balance.

Effects of sinoaortic denervation on Fos expression in the brain evoked by hypertension and hypotension in conscious rabbits

We have previously shown [Li and Dampney (1994) Neuroscience 61, 613-634] that periods of sustained hypertension and hypotension each induces a distinctive and reproducible pattern of neuronal expression of Fos (a marker of neuronal activation) in specific regions of the brainstem and forebrain of conscious rabbits. The aim of this study was to determine the contribution of afferent inputs from arterial baroreceptors to the activation of neurons in these various brain regions that is caused by a sustained change in arterial pressure. Experiments were carried out on rabbits in which the carotid sinus and aortic depressor nerves were cut in a preliminary operation. Following a recovery period of seven to 10 days, a moderate hypertension or hypotension (increase or decrease in arterial pressure of 20-30 mmHg) was induced in conscious barodenervated rabbits for 60 min by the continuous infusion of phenylephrine or sodium nitroprusside, respectively. In control experiments, barodenervated rabbits were subjected to the identical procedures except that they were infused with the vehicle solution alone. Compared with the effects seen in barointact rabbits, [Li and Dampney (1994) Neuroscience 61, 613-634] the number of neurons that expressed Fos in response to hypertension was reduced by approximately 90% in the nucleus of the solitary tract and in the caudal and intermediate parts of the ventrolateral medulla. In supramedullary regions, baroreceptor denervation resulted in a reduction of approximately 60% in hypertension-induced Fos expression in the central nucleus of the amygdala and in the bed nucleus of the stria terminalis, but no significant reduction in the parabrachial complex in the pons. Following hypotension, the number of neurons that expressed Fos in barodenervated rabbits, compared with barointact rabbits, [Li and Dampney (1994) Neuroscience 61, 613-634] was reduced by approximately 90% in the nucleus of the solitary tract, area postrema, and caudal, intermediate and rostral parts of the ventrolateral medulla. Baroreceptor denervation also resulted in a similar large reduction in hypotension-induced Fos expression in many supramedullary regions (locus coeruleus, midbrain periaqueductal grey, hypothalamic paraventricular nucleus, and in the central nucleus of the amygdala and the bed nucleus of the stria terminalis in the basal forebrain). In the supraoptic nucleus, hypotension-induced Fos expression in barodenervated rabbits was reduced by 75% compared to barointact animals, but was still significantly greater than in control animals. There was also a high level of Fos expression, much greater than in control animals, in the circumventricular organs surrounding the third ventricle (subfornical organ and organum vasculosum lamina terminalis). The results indicate that in conscious rabbits the activation of neurons that occurs in several discrete regions at all levels of the brain following a sustained change in arterial pressure is largely dependent upon inputs from arterial baroreceptors, with the exception of neurons in the circumventricular organs surrounding the third ventricle that are activated by sustained hypotension. The latter group of neurons are known to project to vasopressin-secreting neurons in the supraoptic nucleus, and may therefore via this pathway trigger the hypotension-induced release of vasopressin that occurs in the absence of baroreceptor inputs.

Adrenergic responses in silent and putative inhibitory pacemaker-like neurons of the rat rostral ventrolateral medulla in vitro

Noradrenaline and adrenergic agonists were tested on pacemaker-like and silent neurons of the rat rostral ventrolateral medulla using intracellular recording in coronal brainstem slices as well as in punches containing only the rostral ventrolateral medullary region. Noradrenaline (1-100 microM) depolarized or increased the frequency of discharge of all cells tested in a dose-dependent manner. The noradrenaline-induced depolarization was associated with an apparent increase in cell input resistance at low concentrations and a decrease or no significant change at higher concentrations. Moreover, it was voltage dependent and its amplitude decreased with membrane potential hyperpolarization. Noradrenaline caused a dose-related increase in the frequency and amplitude of spontaneous inhibitory postsynaptic potentials. The alpha 1-adrenoceptor antagonist prazosin (0.5 microM) abolished the noradrenaline depolarizing response as well as-the noradrenaline-evoked increase in synaptic activity and unmasked an underlying noradrenaline dose-dependent hyperpolarizing response associated with a decrease in cell input resistance and sensitive to the alpha 2-adrenoceptor/antagonist yohimbine (0.5 microM). The alpha 1-adrenoceptor agonist phenylephrine (10 microM) mimicked the noradrenaline depolarizing response associated with an increase in membrane resistance as well as the noradrenaline-induced increase in synaptic activity. The alpha 2-adrenoceptor agonists UK-14,304 (1-3 microM) and clonidine (10-30 microM) produced only a small hyperpolarizing response, whereas the beta-adrenoceptor agonist isoproterenol (10-30 microM) had no effect. Baseline spontaneous postsynaptic potentials were abolished by strychnine (1 microM), bicuculline (30 microM) or both. However, only the strychnine-sensitive postsynaptic potentials had their frequency increased by noradrenaline or phenylephrine and they usually occurred with a regular pattern. Tetrodotoxin (1 microM) eliminated 80-95% of baseline spontaneous postsynaptic potentials and prevented the increase in synaptic activity evoked by noradrenaline and phenylephrine. Similar results were obtained in rostral ventrolateral medulla neurons impaled in both coronal slices and punches of the rostral ventrolateral medulla. It is concluded that noradrenaline could play an important inhibitory role in the rostral ventrolateral medulla via at least two mechanisms: an alpha 2-adrenoceptor-mediated hyperpolarization and an enhancement of inhibitory synaptic transmission through activation of alpha 1-adrenoceptors located on the somatic membrane of glycinergic interneurons. Some of these interneurons exhibit a regular discharge similar to the pacemaker-like neurons and might, at least in part, constitute a central inhibitory link in the baroreceptor-vasomotor reflex pathway.

C-fos mRNA pattern and corticotropin-releasing factor neuronal activity throughout the brain of rats injected centrally with a prostaglandin of E2 type

The present study investigated the effect of central administration of the prostaglandin of E2 type (PGE2) on the distribution of the immediate early gene (IEG) c-fos mRNA and the transcriptional activity of corticotropin-releasing factor (CRF) and its type 1 receptor in the brain of conscious rats. Adult male rats were sacrificed 30 min and 2 h after a single infusion of PGE2 into the right lateral ventricle (2 micrograms/10 microliters) and their brains cut from the olfactory bulb to the end of the medulla in 30 micrometer coronal sections. mRNAs encoding the IEG c-fos and CRF1 receptor were assayed by in situ hybridization histochemistry using 35S-labeled exonic riboprobes whereas the primary transcript (heteronuclear (hn)RNA) for CRF was detected using intronic probe technology as an index of CRF transcriptional activity. Colocalization of c-fos mRNA within CRF, vasopressin (AVP), and oxytocin (OT) neurons was determined by means of a combination of immunocytochemistry and in situ hybridization techniques on the same brain sections. Thirty min after PGE2 injection, a moderate to strong positive signal for c-fos mRNA was detected in multiple structures of the brain such as the medial preoptic area/organum vasculosum of the lamina terminalis, supraoptic nucleus (SON), parvocellular and magnocellular divisions of the paraventricular nucleus (PVN) of the hypothalamus, central nucleus of the amygdala, nucleus of the solitary tract, dorsal motor nucleus of the vagus, area postrema, dorsal division of the ambiguus nucleus, and throughout the choroid plexus and leptomeninges. A smaller but significant c-fos expression was observed in various structures including the subfornical organ, bed nucleus of the stria terminalis, arcuate nucleus, and periventricular nucleus of the hypothalamus. Two h after treatment with the PG, the signal for c-fos mRNA in most of these brain nuclei vanished. In the parvocellular nucleus of the PVN, c-fos was expressed in CRF-immunoreactive (ir) and OT-ir neurons, whereas in the magnocellular part of that nucleus and in the SON, this transcript was essentially colocalized in OT-ir neurons. Activation of CRF neuroendocrine cells was also associated with an increase in CRF transcription as revealed by the selective presence of CRF primary transcript (hnRNA), which was stimulated only in the PVN but not in any other nuclei in the brains of PGE2-treated rats. Central administration of PGE2 also induced expression of the CRF type 1 receptor in the parvocellular PVN. Taken together, these results provide clear anatomical evidence that central PGE2 injection causes specific and selective expression of c-fos in several brain structures recognized to be activated in the brains of endotoxin-challenged rats. It is therefore possible that PG of E2 type plays a crucial role within the CNS in the interface between the immune and nervous systems to modulate neuroendocrine responses, such as the hypothalamic-pituitary-adrenal axis.

Mediation of dehydration-induced peptidergic gene expression in the rat lateral hypothalamic area by forebrain afferent projections

We have previously shown in dehydrated rats that cellular levels of the mRNAs encoding the precursor peptides for corticotropin-releasing hormone and neurotensin/neuromedin N significantly increase in a restricted region of the lateral hypothalamic area (Watts, 1992, Brain Res. 581:208-216). The experiments reported here address the role that forebrain osmosensitive cells groups or regions associated with autonomic regulation play in developing this mRNA response. The first experiment showed that unilateral knife cuts placed between the rostral forebrain and the lateral hypothalamic area (LHA) will unilaterally attenuate the mRNA response in the LHA to dehydration. In a second experiment, small injections of the retrograde tracer Fluorogold into the region of the LHA containing these mRNAs revealed a direct input from the osmosensitive median preoptic nucleus and subfornical organ and from the fusiform nucleus of the bed nuclei of the stria terminalis, which is part of a complex of cell groups associated with autonomic regulation. We found that at least 30% of the neurons in the median preoptic nucleus and subfornical organ and 14% of the neurons in the fusiform nucleus of the bed nuclei of the stria terminalis that project to the LHA responded to a rapid increase in plasma osmolality with increased c-fos mRNA levels. In the final experiment, injections of Fluorogold into the LHA were made simultaneously with ipsilateral rostral knife cuts. Here the numbers of neurons accumulating Fluorogold in the median preoptic nucleus, subfornical organ, and the fusiform nucleus were all significantly decreased concomitantly with attenuated mRNA responses in the LHA to dehydration. We conclude that the LHA receives direct and functional projections from the median preoptic nucleus, subfornical organ, and the fusiform nucleus. These projections appear capable of mediating a substantial part of the response of peptidergic mRNAs in the LHA to dehydration.