Neural mechanisms of emesis

Emesis is a reflex, developed to different degrees in different species, that allows an animal to rid itself of ingested toxins or poisons. The reflex can be elicited either by direct neuronal connections from visceral afferent fibers, especially those from the gastrointestinal tract, or from humoral factors. Emesis from humoral factors depends on the integrity of the area postrema; neurons in the area postrema have excitatory receptors for emetic agents. Emesis from gastrointestinal afferents does not depend on the area postrema, but probably the reflex is triggered by projections to some part of the nucleus tractus solitarius. As with a variety of other complex motor functions regulated by the brain stem, it is likely that the sequence of muscle excitation and inhibition is controlled by a central pattern generator located in the nucleus tractus solitarius, and that information from humoral factors via the area postrema and visceral afferents via the vagus nerve converge at this point. This central pattern generator, like those for motor functions such as swallowing, presumably projects to the various motor nuclei, perhaps through interneuronal pathways, to elicit the sequential excitation and inhibition that controls the reflex.     

刺激大鼠穹窿下器官诱发饮水和脑内c—fos表达的胆碱能机制

实验以饮水行为脑内c-fos表达为指标,,观察刺激大鼠穹窿下器官（SFO）的效应,结果显示,刺激SFO能诱发明显的饮水行为,与此同时,前脑8个部位（终板血管器官,正中视前核,室旁核,视上核,下丘脑外侧区,穹窿周核背侧区,丘脑联合核和无名质）和后脑3个部位（最后区,孤束核和壁旁外侧核）的Fos蛋白表达明显增强,免疫组化双重染色结果显示,刺激SFO能诱导视上核和室旁核中部分神经元呈Fos蛋白和加压素共同表达。脑室注射阿托品能部分阻断刺激SFO诱发的饮水行为,脑内上述各部位所诱导的Fos蛋白表达也明显减弱,以上结果提示,M胆碱能机制参与 刺激SFO诱发的饮水行为和脑内Fos蛋白的表达。     

Neuronal organization of the avian paraventricular nucleus: intrinsic, afferent, and efferent connections

The heterogeneous paraventricular nucleus (PVN) of birds offers favorable conditions for the analysis of intrinsic, afferent, and efferent connections of neuroendocrine systems. Paraventricular neurons are successfully impregnated with the Golgi-technique. The findings indicate a direct influence of the cerebrospinal fluid (CSF) on the magnocellular neurons that, via their axon terminals in the neural lobe of the pituitary, are also exposed to the hemal milieu. The magnocellular neurons are intermingled with parvocellular elements which may represent local interneurons. A group of parvocellular nerve cells is identified as CSF-contacting neurons. This type of cell forms a basic morphologic component of the avian neuroendocrine apparatus. Immunocytochemical and ultrastructural studies further support the concept of neuronal interactions between parvocellular and magnocellular elements. Moreover, these findings speak in favor of the existence of recurrent collaterals of the magnocellular neurons. Nerve cells giving rise to afferent connections to the PVN are located in the limbic system and autonomic areas of the upper and lower brainstem. Further afferents may originate from the subfornical organ, the organon vasculosum laminae terminalis, the ventral tegmentum, and the area postrema. Via efferent projections, the PVN is connected to the nucleus accumbens, lateral septum, several hypothalamic nuclei, the neural lobe of the pituitary, the organon vasculosum laminae terminalis, the subfornical organ, the pineal organ, the area postrema, the lateral habenular complex, and various autonomic areas of the reticular formation in the upper and lower brainstem and the spinal cord. In conclusion, the PVN may be regarded as an integral component of the neuroendocrine apparatus reciprocally coupled to the limbic system, several circumventricular organs, and various autonomic centers of the brain.     

Gastric distension-induced release of 5-HT stimulates c-fos expression in specific brain nuclei via 5-HT3 receptors in conscious rats

We examined c-fos expression in specific brain nuclei in response to gastric distension and investigated whether 5-HT released from enterochromaffin (EC) cells was involved in this response. The role of 5-HT3 receptors in this mechanism was also addressed. Release of 5-HT was examined in an ex vivo-perfused stomach model, whereas c-fos expression in brain nuclei induced by gastric distension was examined in a freely moving conscious rat model. Physiological levels of gastric distension stimulated the vascular release of 5-HT more than luminal release of 5-HT, and induced c-fos expression in the nucleus of the solitary tract (NTS), area postrema (AP), paraventricular nucleus (PVN), and supraoptic nucleus (SON). The c-fos expression in all these brain nuclei was blocked by truncal vagotomy as well as by perivagal capsaicin treatment, suggesting that vagal afferent pathways may mediate this response. Intravenous injection of 5-HT3 receptor antagonist granisetron blocked c-fos expression in all brain nuclei examined, although intracerebroventricular injection of granisetron had no effect, suggesting that 5-HT released from the stomach may activate 5-HT3 receptors located in the peripheral vagal afferent nerve terminals and then induce brain c-fos expression. c-fos Positive cells in the NTS were labeled with retrograde tracer fluorogold injected in the PVN, suggesting that neurons in the NTS activated by gastric distension project axons to the PVN. The present results suggest that gastric distension stimulates 5-HT release from the EC cells and the released 5-HT may activate 5-HT3 receptors located on the vagal afferent nerve terminals in the gastric wall leading to neuron activation in the NTS and AP and subsequent activation of neurons in the PVN and SON.     

The subfornical organ's neural connections and their role in water balance

The subfornical organ (SFO) has projections to specific sets of nuclei within the preoptic area and hypothalamus which enable it to influence behavioral and physiological controls of water balance. It projects to the nuclei of the anteroventral third ventricular area, to vasopressinergic (heavily) and oxytocinergic (moderately) magnocellular neurons of the supraoptic and paraventricular nucleus. It also projects to the parvocellular areas of the paraventricular nucleus which project to the median eminence and to all the motor nuclei of the autonomic nervous system. In addition the SFO projects to regions of the lateral preoptic area, lateral hypothalamus and the dorsal perifornical region. Cutting the efferent projections from the SFO causes disturbances in behavioral and physiological controls of water balance. There is moderate polyuria and a concentrating defect in urine osmolality. The rats do not drink to intravenous angiotensin II but retain their ability to drink to angiotensin II given intracerebroventricularly. They appear to drink normally to overnight water deprivation but remain in negative water balance because of excessive urinary water loss during the deprivation period.     

Regulation of angiotensin II binding sites in the subfornical organ and other rat brain nuclei after water deprivation

1. Binding sites for angiotensin II have been localized in forebrain and brain-stem areas of water-deprived and control Sprague-Dawley rats, employing autoradiography with computerized microdensitometry. 2. Angiotensin II receptor sites were identified in the organum vasculosum of the lamina terminalis, subfornical organ, paraventricular nucleus, median preoptic nucleus, area postrema, nucleus of the solitary tract, and inferior olive. 3. After dehydration a significant increases in the concentration of angiotensin II receptors was detected only in the subfornical organ. Although there was an increased concentration of angiotensin II binding sites in the organum vasculosum of the lamina terminalis, the median preoptic nucleus, and the paraventricular nucleus after dehydration, these changes did not reach statistical significance. Other brain nuclei investigated did not show differences in angiotensin II binding sites in the dehydrated rats compared to controls. 4. These results indicate that angiotensin II receptors in the subfornical organ may play an important role in fluid homeostasis during dehydration.     

Cholecystokinin-octapeptide like immunoreactivity in the area postrema of the rat and cat

The distribution of cholecystokinin-8 (CCK-8)-like immunoreactivity in the area postrema of the rat and cat was visualized using the peroxidase, antiperoxidase technique. In the rat the greatest amount of immunostaining occurred in peripheral regions of the area postrema at intermediate and rostral levels. Caudally, scattered immunoreactivity predominated. After colchicine treatment, numerous immunoreactive somata were observed throughout the area postrema. The cat area postrema had a different and more complex pattern of immunostaining than the rat. Moderate to dense accumulations of immunostaining occurred in the ventromedial region of the area postrema bordering the solitary tract and dorsal vagal nuclei. The central region of the area postrema possessed scattered amounts of immunoreactivity at rostral levels. Following colchicine treatment, no visible CCK-8-like immunoreactive cell bodies were observed in the cat area postrema. Results of the present investigation provide morphological evidence for the role of CCK-8 in cardiovascular regulation and satiety. The difference in the distribution of CCK-8 in the rat and cat suggest a possible role in the emetic reflex.     

Coexistence of NADPH-diaphorase with vasopressin and oxytocin in the hypothalamic magnocellular neurosecretory nuclei of the rat

Coexistence of NADPH-diaphorase with vasopressin and oxytocin was studied in the magnocellular neurosecretory nuclei of the rat hypothalamus by use of sequential histochemical and immunocytochemical techniques in the same sections. Coexistence was found in all the nuclei examined (supraoptic, paraventricular, circular, fornical, and in some isolated neurons located in the hypothalamic area between the paraventricular and supraoptic nuclei). The ratios of neurons expressing both markers (NADPH-diaphorase and vasopressin, NADPH-diaphorase and oxytocin) in each of the nuclei were very similar. Although further studies must be carried out, the partial coexistence found in all nuclei suggests that NADPH-diaphorase is probably not related to general mechanisms involving vasopressin and oxytocin, but rather in specific functions shared by certain hypothalamic neuronal cell populations.     

C-fos expression in rat brain during heat stress

Rats, under urethane anesthesia, 0, 20, 40 or 80 min after the start of heat stress (42°C) were sacrificed for determination of c-fos expression in different brain regions. In situ hybridization and immunocytochemistry methods were used, respectively, for determination of c-fos mRNA and protein, respectively. In general, either colon temperature (T_CO), mean arterial pressure (MAP), local cerebral blood flow (CBF) or c-fos expression in different brain regions (including the preoptic area, supraoptic nuclei, paraventricular nuclei, thalamus, amygdala, nucleus tract solitarii, area postrema and ventrolateral medulla) increased at 20–40 min after the start of heat exposure. However, the heatstroke, which appears as profound decreases in both MAP and local CBF and increases in T_CO, was produced 80 min after heat stress. The c-fos expression was heavily induced in all these brain regions after the onset of heatstroke. The data suggest that c-fos expression in rat brain during heatstroke is associated with hyperthermia, arterial hypotension or cerebral ischemia.     

Ventricular, paraventricular and circumventricular structures involved in peptide-induced satiety

Cholecystokinin, bombesin or gastrin (2 microliter of 50 ng/microliter) was injected stereotaxically into the paraventricular nucleus of the hypothalamus, the arcuate/ventromedial area, the subfornical organ, the area postrema and the cerebral aqueduct of Sprague-Dawley rats and the effects of these injections on food and water intake were studied. While the injection of cholecystokinin reduced food intake when it was injected into both hypothalamic loci, food and water intake were most severely affected by the injection of this peptide into the cerebral aqueduct. Bombesin reduced food intake after its injection into all areas except the subfornical organ and reliable reductions in water intake were seen after injection of this peptide into all areas except the paraventricular nucleus. Minor reductions in food intake were seen following gastrin injection into the paraventricular nucleus while increased water consumption was observed after this peptide was injected into the paraventricular nucleus and cerebral aqueduct. In a second study 6-hydroxydopamine injections (2 microliter of 8 micrograms/microliter were made into the five areas studied 10 days before animals were injected with 100 micrograms/kg of cholecystokinin (i.p.). All 6-hydroxydopamine-injected animals reduced their food and water intake in response to the cholecystokinin challenge as did intact controls. These results indicate that while the changes in food and water intake produced by the central injection of cholecystokinin, bombesin or gastrin may involve central catecholamine systems, those occurring after its systemic administration do not. Therefore, if the release of gastrointestinal peptides during natural feeding is part of a homeostatic mechanism regulating hunger and satiety, this mechanism may operate without directly involving central catecholamine systems.     

Autoradiographic visualization of CNS receptors for vasoactive intestinal peptide

Receptors for VIP were characterized in the rat CNS. 125I-VIP bound with high affinity to rat brain slices. Binding was time dependent and specific. Pharmacology studies indicated that specific 125I-VIP binding was inhibited with high affinity by VIP and low affinity by secretin and PHI. Using in vitro autoradiographic techniques high grain densities were present in the dentate gyrus, pineal gland, supraoptic and suprachiasmatic nuclei, superficial gray layer of the superior colliculus and the area postrema. Moderate grain densities were present in the olfactory bulb and tubercle, cerebral cortex, nucleus accumbens, caudate putamen, interstitial nucleus of the stria terminalis, paraventricular thalamic nucleus, medial amygdaloid nucleus, subiculum and the medial geniculate nucleus. Grains were absent in the corpus callosum and controls treated with 1 microM unlabeled VIP. The discrete regional distribution of VIP receptors suggest that it may function as an important modulator of neural activity in the CNS.     

Phospholipid composition of some parts of rat hypothalamus, and its relation with some histochemical observations

—In continuation of a histochemical investigation of phospholipids in rat brain, the content and composition of these lipids in some regions of rat hypothalamus were examined using ultra-micromethods. The supraoptic and paraventricular nuclei, the anterior area of the hypothalamus, and the adjacent optic chiasma were chosen for this study. The phospholipid content of all of these regions was found to be almost equally high. Proportionally higher amounts were found for phosphatidylethanolamine in the optic chiasma and for phosphatidylcholine in the neurosecretory nuclei. In all regions investigated the sphingomyelin content was nearly the same. From these results it was concluded that the large differences in staining properties of myelinated and unmyelinated areas of the tissue when applying Baker's acid haematein test could not be ascribed to large differences in the content of the Baker-positive, choline-containing, phospholipids. Treatment of fixed tissue with cold acetone in order to extract non-polar lipids that may mask phospholipids, did not abolish the staining differences. The possible involvement of cerebrosides and proteolipids mainly found in myelinated tissue in the Baker reaction is discussed. It is suggested that the differences in staining properties might be ascribed to differences in structure of myelinated and unmyelinated tissue, rather than to differences in the content of Baker-positive lipids.     

c-Fos expression in hypothalamic nuclei of food-entrained rats

The present study aimed to identify the hypothalamic nuclei involved with food entrainment by using c-Fos-like immunoreactivity (c-Fos-IR) as a marker of functional activation. We studied rats entrained 3 wk to restricted feeding schedules (RF), their ad libitum (AL) controls, and the persistence of c-Fos-IR temporal patterns in entrained-fasted rats. In addition, we included 22-h fasting and 22-h fasting-refeeding groups as controls of fasting and refeeding acute effects. Diurnal patterns of c-Fos-IR were observed in the tuberomammilar nucleus (TM) and suprachiasmatic nucleus (SCN) in AL rats. In all nuclei, except the SCN and ventromedial nucleus (VMH), restricted feeding schedules imposed a temporal pattern of increased c-Fos-IR around mealtime. An increase in c-Fos-IR before and after meal time was observed in dorsomedial nucleus (DMH), lateral nucleus (LH), perifornical area (PeF), and TM, and a marked increase was observed in the paraventricular nucleus (PVN) after feeding. Food-entrained c-Fos-IR patterns persisted after 3 days in fasting in DMH, LH, and PeF. Present data suggest that FEO might not rely on a single nucleus and rather may be a distributed system constituted of interacting nuclei in which the PVN is mainly involved with the response to signals elicited by food ingestion and, therefore, with the entraining pathway. We can suggest that the PeF and TM may be involved with the arousal state during food anticipation and the DMH and LH with the time-keeping mechanism of FEO or its output.     

Norepinephrine uptake modifications in circumventricular organs, pons and myelencephalic areas and nuclei in prehepatic portal hypertensive rats.

Peripheral noradrenergic activity is enhanced in portal hypertension and correlates with the progression of the disease. However, little is known about the status of central norepinephrine (NE) in portal hypertension. The aim of the present work was to study the uptake of NE in several areas rich in NE in experimental prehepatic portal hypertension. The experiments were performed in vitro in several encephalic areas and nuclei, obtained according to the 'punch-out technique'. Results showed that in portal hypertensive rats NE uptake enhanced in all areas and nuclei studies (subfornical organ, organum vasculosum lamina terminalis, area postrema, locus coeruleus and nucleus tractus solitarius). The present results suggest that these encephalic areas and brainstem nuclei may be related to the development and/or maintenance of portal hypertension in this animal model.     

Central actions of angiotensin in cardiovascular control: multiple roles for a single peptide.

Angiotensin II (ANG II) acts peripherally as a hormone, with actions on the vasculature, adrenals, and kidney. In addition, certain actions of ANG II in the central nervous system are directed toward cardiovascular control and fluid volume homeostasis. Dense binding sites for ANG II are found at circumventricular organs, which apparently have the ability to relay information to cardiovascular centers via neural circuitry. Microinjection of ANG II into the subfornical organ (SFO) or area postrema (AP) produces site-specific increases in blood pressure. In addition, electrophysiological studies demonstrate profound effects of ANG II, acting at the SFO, on activity of neurohypophysial neurons and release of oxytocin and vasopressin, which can be antagonized by ANG II blockers or attenuated by SFO lesions. Evidence from microinjection, electrophysiological, and lesion studies indicate a complex interaction between central sites involved in mechanisms of cardiovascular control: the SFO, AP, organum vasculosum of the lamina terminalis, and paraventricular and supraoptic nuclei of the hypothalamus. Not only is ANG II a humoral messenger in this central scenario, but evidence suggests it acts as a neurotransmitter or neuroendocrine substance within specific CNS pathways, suggesting multiple roles for this peptide in central cardiovascular control.     

Conditioned taste aversion and motion sickness in cats and squirrel monkeys

The relationship between vomiting and conditioned taste aversion was studied in intact cats and squirrel monkeys and in cats and squirrel monkeys in which the area postrema was ablated by thermal cautery. In cats conditioned 7-12 months after ablation of the area postrema, three successive treatments with xylazine failed to produce either vomiting or conditioned taste aversion to a novel fluid. Intact cats, however, vomited and formed a conditioned aversion. In squirrel monkeys conditioned 6 months after ablation of the area postrema, three treatments with lithium chloride failed to produce conditioned taste aversion. Intact monkeys did condition with these treatments. Neither intact nor ablated monkeys vomited or evidenced other signs of illness when injected with lithium chloride. When the same ablated cats and monkeys were exposed to a form of motion that produced vomiting prior to surgery, conditioned taste aversion was produced and some animals vomited. These findings confirm other studies indicating motion can produce vomiting in animals with the area postrema destroyed and demonstrate that motion-induced conditioned taste aversion can be produced after ablation of the area postrema. The utility of conditioned taste aversion as a measure of subemetic motion sickness is discussed by examining agreement and disagreement between identifications of motion sickness by conditioned taste aversion and vomiting. It is suggested that a convincing demonstration of the utility of conditioned taste aversion as a measure of nausea requires the identification of physiological correlates of nausea, and caution should be exercised when attempting to interpret conditioned taste aversion as a measure of nausea.     

The penetration of sodium into the brain following a cisternal injection of sodium chloride with particular emphasis on the area postrema

Summary Following an intracisternal injection of sodium chloride, sodium has been localized in paraventricular and subpial tissues of the posterior fossa by means of the pyroantimonate histochemical technique, with the use of a buffered pyroantimonate medium. The electron dense deposit is present in these tissues within 4 minutes after injection and is found only extracellularly except in the area postrema. This finding supports the contention that sodium is chiefly an extracellular ion and that the cerebrospinal fluid and the extracellular fluid are in equilibrium at these sites. In the area postrema, an intracellular precipitate is noted in the vesicular structures of the atypical astrocytes of this structure and in pinocytic vesicles of the large blood vessels. The intraglial localization of sodium in the area postrema is discussed in relation to a possible function of this structure as a regulator of cerebrospinal fluid ionic content.This work was supported by grant number NB-08549-02 from the National Institute of Neurological Disease and Stroke, Bethesda, Maryland.     

Differential effects of intravenous hyperosmotic solutes on drinking latency and c-Fos expression in the circumventricular organs and hypothalamus of the rat

Hyperosmotic intravenous infusions of NaCl are more potent for inducing drinking and vasopressin (AVP) secretion than equally osmotic solutions of glucose or urea. The fact that all three solutes increased cerebrospinal fluid osmolality and sodium concentration led the investigators to conclude that critical sodium receptors or osmoreceptors for stimulating drinking and AVP secretion were outside the blood-brain barrier (BBB) in the circumventricular organs (CVOs). We tested an obvious prediction of this hypothesis: that all three solutes should increase c-Fos-like immunoreactivity (Fos-ir) inside the BBB, but that only NaCl should increase Fos-ir in the CVOs. We gave intravenous infusions of 3.0 Osm/l NaCl, glucose, or urea to rats for 11 or 22 min at 0.14 ml/min and perfused the rats for assay of Fos-ir at 90 min. Controls received isotonic NaCl at the same volume. Drinking latency was measured, but water was then removed. Drinking consistently occurred with short latency during hyperosmotic NaCl infusions only. Fos-ir in the forebrain CVOs, the subfornical organ, and organum vasculosum laminae terminalis was consistently elevated only by hyperosmotic NaCl. However, all three hyperosmotic solutes potently stimulated Fos-ir in the supraoptic and paraventricular nuclei of the hypothalamus inside the BBB. Hyperosmotic NaCl greatly elevated Fos-ir in the area postrema, but even glucose and urea caused moderate elevations that may be related to volume expansion rather than osmolality. The data provide strong support for the conclusion that the osmoreceptors controlling drinking are located in the CVOs.     

Immunocytochemical localization of somatostatin-containing neurons in the rat hypothalamus

Summary The rat hypothalamus was studied at the light microscopic level with the use of single and double immunocytochemical staining methods. It was shown that the rat supraoptic and paraventricular hypothalamic nuclei, and their accessory neurosecretory nuclei, do not contain magnocellular somatostatin neurons. The distribution of the hypothalamic parvocellular somatostatin cells is described. The parvocellular component of the rat hypothalamic paraventricular nucleus is, at least partly, composed of somatostatin cells: they form a fairly well circumscribed periventricular cell mass. The rat suprachiasmatic nuclei contain separate somatostatin neurons and vasopressin neurons. Scattered somatostatin cells are present in the entire arcuate nucleus. In addition to the periventricular somatostatin cells located in the preopticanterior hypothalamic area and in the arcuate nucleus, the rat hypothalamus also contains numerous scattered somatostatin cells located distant from the third ventricle.This investigation was supported by a grant from the Belgian Nationaal Fonds voor Geneeskundig Wetenschappelijk Onderzoek     

The localisation of the extraneuronal monoamine transporter (EMT) in rat brain

The extraneuronal monoamine transporter plays an important role in the inactivation of monoamine transmitters. A basal extraneuronal tissue expression of this transporter has been reported, but it is also expressed in CNS glia. As little is known about the expression pattern and the function of the extraneuronal monoamine transporter in the brain, we performed a detailed investigation. Firstly, a northern blot analysis of different rat organs revealed that the transporter is strongly expressed in placenta, lung and heart and less prominently in the whole brain, brain stem, intestine, testis, epididymis, stomach, kidney and skeletal muscle. It was not expressed in cerebellum, liver and embryo. Using an in situ hybridization to the rat brain, we detected a marked and highly confined expression of the extraneuronal monoamine transporter in the area postrema, but in no other brain areas. These findings were confirmed by polyclonal antibodies against rat extraneuronal monoamine transporter showing an intensive signal in the area postrema, although a few cells in the cerebellum and the brain stem also showed a signal. Additionally, a partly overlapping expression pattern of the monoamine oxidase-B was detected. Summarizing, we firstly describe a marked and highly confined expression of the extraneuronal monoamine transporter in the rat area postrema by in situ hybridisation which may play a role in physiological functions of this circumventricular organ such as emesis, food intake and the regulation of cardiovascular functions.