A hepatoportal osmoreceptive afferent projection from nucleus tractus solitarius to caudal ventrolateral medulla

This study was performed to examine projection of nucleus tractus solitarii (NTS) neurons to the caudal ventrolateral medulla (cVLM) in rats. One hundred and seven neurons that responded antidromically to electrical stimulation of the cVLM were recorded within the NTS. Electrical stimulation of the hepatic branch of the vagus nerve (hepatic vagus) elicited facilitation on 62 neurons (facilitatory neurons) and suppression on 6 neurons (suppressed neurons). Effect of portal infusion of hypertonic saline was examined on 44 facilitatory and 4 suppressed neurons. Twelve facilitatory and 2 suppressed neurons showed a decrease in the discharge rate. One suppressed neuron showed an increase in the discharge rate. It is concluded that hepatoportal osmoreceptive signals are conveyed from the NTS to the cVLM. The responses are mostly characterized by the decrease in the discharge rate by portal infusion of hypertonic saline.     

Choline acetyltransferase activity in the nucleus tractus solitarius: Regulation by the afferent vagus nerve

The influence of nodose ganglionectomy or transection of the peripheral branches of the afferent vagus nerve on choline acetyltransferase (ChAT) activity in the nucleus tractus solitarius (NTS) was studied. ChAT activity was reduced in the microdissected caudal and intermediate portions of the NTS in vagotomized as well as ganglionectomized rats. However, only the ganglionectomy resulted in the degeneration of medullary nerve fibers. These results suggest that the changes in ChAT activity in the NTS are independent of neuronal degeneration and may be due to transynaptic modulation of ChAT activity by afferent vagal impulses. The presence of ChAT in the sensory nodose projection to the NTS, however, cannot be ruled out.     

Gustatory functions of the nucleus tractus solitarius in the rabbit

Multiunit analysis revealed a rostral region of NTS containing cells responsive to taste stimulation of rostral tongue. Using representative stimuli for the 4 basic types of taste, maximal incidence and magnitude of response was found to NaCl, followed by HCl, sucrose and QHC1. Further analysis of temporal patterns of response to the tastants revealed differences among stimuli in latency and time course. A principal components analysis indicated that time course, apart from magnitude of response, could contribute to neural differentiation of tastants. Information was also derived on neural intensity functions for these basic types of tastants. Additional observations with sodium saccharin revealed minimal neuronal responsivity despite reported evidence of behavioral preference for this tastant by rabbits.     

Nitric oxide inhibits excitatory vagal afferent input to nucleus tractus solitarius neurons in anaesthetized rats

Objective

Endogenous nitric oxide (NO) has been implicated in the regulation of neuronal activity which mediates cardiovascular reflexes. However, there is controversy concerning the role of NO in the nucleus tractus solitarius (NTS). The present study aims to elucidate the possible physiological role of endogenous NO in modulating the excitatory vagal afferent input to NTS neurons.

Methods

All the experiments in the rat were conducted under anaesthetic conditions. Ionophoresis method was used for the application of NO donor or nitric oxide synthase (NOS) inhibitor, and single unit recording method was employed to detect the effects of these applications on vagal afferent- or cardio-pulmonary C-fibre reflex-evoked neuronal excitation in NTS.

Results

Ionophoresis applications of L-arginine (L-Arg), a substrate of NOS, and sodium nitroprusside (SNP), a NO donor, both attenuated the vagal afferent-evoked discharge by (51.5±7.6)% (n = 17) and (68.3±7.1)% (n = 9), respectively. In contrast, application of D-Arg at the same current exerted no overall effect on this input. Also, both L-Arg and SNP inhibited spontaneous firing of most of the recorded neurons. In contrast, ionophoresis application of NG-nitro-Larginine methyl ester (L-NAME) enhanced vagal afferent-evoked excitation by (66.3±11.4)% (n = 7). In addition, ionophoresis application of L-Arg and SNP significantly attenuated cardio-pulmonary C-fibre reflex-induced excitation in the tested NTS neurons.

Conclusion

Activation of local NO pathway in the NTS could suppress vagal afferent-evoked excitation, suggesting that NO is an important neuromodulator of visceral sensory input in the NTS.     

Catecholamine inputs to the nucleus tractus solitarius

The projections of brainstem catecholaminergic (CA) cell groups to the rat nucleus tractus solitarius (NTS) were examined using 6-hydroxydopamine (6-OHDA) injections and glass microknife cuts. 6-OHDA (4 micrograms) was injected into the intermediate NTS, and this resulted in depletion of CA fluorescent varicosities from the NTS at this rostrocaudal level, except for varicosities along the periventricular edge of the NTS. In addition, a band of swollen fluorescent axons extended between the CA A1 cell group of the ventrolateral medulla and the lateral NTS. Microknife cuts were used to interrupt the projections of the CA A1 and A2 cell groups (located in the caudal NTS) and tissues were examined for changes in CA varicosity density within the intermediate NTS. Following transverse knife cuts of the intermediate NTS, rostral to the A2 cell group, fluorescent varicosities rostral to the cut virtually disappeared, and the fluorescence intensity of the ipsilateral A2 neurons caudal to the cut was increased. These cuts also eliminated the 6-OHDA-resistant varicosities along the periventricular NTS. After microknife cuts lateral to the intermediate NTS, the fluorescent varicosity density in the NTS was unchanged. These results indicate that the major CA projection to the NTS arises from the ipsilateral A2 cell group. The 6-OHDA-resistant varicosities arising from neurons caudal to the knife cut probably arise from the adrenergic C2 cell group.     

Distribution of neurotensin-immunoreactivity within baroreceptive portions of the nucleus of the tractus solitarius and the dorsal vagal nucleus of the rat

We have examined the distribution of neurotensin immunoreactivity within subnuclear regions of the nucleus of the tractus solitarius (NTS) and the dorsal motor nucleus of the vagus nerve (DVN) in the rat. In order to determine which regions of the NTS were involved in the regulation of baroreceptor reflexes, we mapped the central distribution of the aortic branch of the vagus nerve using transganglionic transport of horseradish peroxidase. Comparison of the pattern of aortic nerve innervation with that of the distribution of neurotensin-immunoreactive cells and fibers shows the dorsomedial nucleus of the NTS both to be the primary site of aortic baroreceptor termination and to contain the highest concentration of neurotensin-immunoreactive elements within the NTS. Neurotensin-immunoreactive fibers are also present in medial regions of the NTS adjacent to the area postrema where they may be involved in the modulation of vagal gastric afferents. Double-label experiments, in which, on the same tissue sections, neurotensin immunohistochemistry was combined with retrograde horseradish peroxidase labeling of DVN neurons, reveal a topographic innervation of vagal preganglionic motoneurons by neurotensin-immunoreactive fibers. The heaviest innervation is of lateral portions of the DVN and adjacent ventral portions of the NTS at the level of the obex, an area which may contain cardiac motoneurons. In this region neurotensin-immunoreactive fibers can be observed in close proximity to retrogradely labeled cells. The concentration of neurotensin elements in a region of the NTS which is involved in the control of baroreceptor reflexes provides a morphological basis for the cardiovascular effects produced by central administration of the peptide. Additional control may be exerted at the level of the motoneuron, as evidenced by apparent neurotensin fiber innervation of presumptive cardiac preganglionic neurons. Similarly, the distribution of neurotensin fibers suggests that the peptide may be acting in gastric regulatory areas of the NTS or on vagal secretomotor neurons to regulate gastric acid secretion.     

Arginine-vasopressin in nucleus of the tractus solitarius induces hyperglycemia and brain glucose retention

Hypothalamic arginine-vasopressin (AVP) plays an important role both as a neurotransmitter and hormone in the regulation of blood glucose and feeding behavior. AVP-containing axons from the parvocellular subdivision of paraventricular nucleus of the hypothalamus terminate in the nucleus of the tractus solitarius (NTS), but the function of this projection is not known. Interestingly, the NTS also receives afferent information from the carotid body and other peripheral receptors involved in glucose homeostasis. We have previously reported that stimulation of the carotid body receptors initiates a hyperglycemic reflex and increases brain glucose retention. Here we show that direct administration of micro-doses of AVP into the NTS of anesthetized or awake rats rapidly increased the levels of blood glucose concentration and brain arterio-venous (A-V) glucose difference. This effect was not observed when the same doses of AVP were injected in the brainstem outside NTS. Arginine-vasopressin antagonist microinjections alone produced a small but significant reduction in brain A-V glucose. Pre-administered VP1-receptor antagonist [beta-mercapto-beta,beta-cyclopentamethylene-propionyl(1),O-Me-Tyr(2),Arg(8)]vasopressin blocked the effects of AVP. These results indicate that AVP acting on its receptors locally within the NTS participates in glucose homeostasis, increasing both blood glucose concentration and brain A-V glucose differences. Hypothalamic AVP may facilitate hyperglycemic responses initiated by peripheral signals processed at the level of the NTS.     

Effect of a high-salt diet on gamma-aminobutyric acid-mediated responses in the nucleus tractus solitarius of Sprague-Dawley rats

Previous study using an indirect gamma-aminobutyric acid (GABA) agonist indicated that high salt intake enhances sensitivity of nucleus tractus solitarius (NTS) projecting inhibitory input to rostral ventrolateral medulla sympathoexcitatory neurons. We further investigated the relationship between salt intake and the GABA system in NTS. Sprague-Dawley (S-D) rats consuming high dietary salt (8%) or low dietary salt (0.3%) for 3 weeks were used. Under chloralose-anesthesia, baseline arterial pressure (AP) and heart rate (HR) were similar in both groups. Bilateral injection into NTS of nipecotic acid, GABA(A) receptor agonist (muscimol), or GABA(B) receptor agonist (baclofen) elicited greater pressor responses in high-salt group. GABA(A) receptor antagonist, bicuculline and GABA(B) receptor antagonist, CGP-35348 elicited greater depressor responses. Phenylephrine or nitroprusside (i.v.) elicited similar respective increases or decreases in AP in both groups. Baroreflex sensitivity was similar. Thus, high-salt intake enhances both GABA(A) receptor- and GABA(B) receptor-mediated responses within NTS, thereby inhibiting elevation of AP.     

Substance P release in the feline nucleus tractus solitarius during ergoreceptor but not baroreceptor afferent signaling

Substance P (SP) is associated with metabo- and mechanoreceptor afferent fibers ('ergoreceptors') in skeletal muscle as well as the afferent fibers from carotid sinus baroreceptors. Afferent activity from each of these are at least partially integrated in the nucleus tractus solitarius (NTS). The purpose of this study was to determine whether SP was released from the NTS during acute reflex-induced changes in blood pressure caused by stimulating these receptors. Both the muscle pressor response and the baroreflex were studied in adult cats anaesthetized with alpha-chloralose. SP antibody-coated microprobes were used to measure the possible release of SP from the NTS. The muscle pressor response caused a release of immunoreactive SP-like substances (irSP) from the rostral medial NTS, as well as the dorsal motor nucleus (DMV) and lateral tegmental field (FTL). This release was not dependent on intact afferent input from the carotid sinus nerve, but was a function of activation of muscle ergoreceptors, since no irSP was released in response to stimulation of the motor nerves after the muscle was paralyzed. There was no detectable release of irSP from the mNTS during carotid artery occlusions (baroreceptor unloading). Baroreceptor activation, induced by the i.v. injection of the vasoconstrictor, phenylephrine, did not cause the release of irSP from the mNTS above resting baseline levels. These data suggest that SP is involved with the mediation of the afferent signal from muscle ergoreceptor fibers in the medial NTS. SP is not involved with the mediation of baroreceptor afferent signaling in the medial NTS. The release of SP in response to ergoreceptors activation may function to excite an inhibitory pathway which inhibits baroreflex signals that would tend to reduce the blood pressure and heart rate during the muscle pressor response.     

Modulation of brown adipose tissue-mediated thermogenesis by lesions to the nucleus tractus solitarius in the rat

Given that relatively little is known regarding the central control of brown adipose tissue (BAT)-mediated thermogenesis the present study assessed whether the direct pharmacological stimulation of β- or α-adrenergic receptors located on the brown adipocytes would result in a typical thermogenic response following electrolytic lesions to the nucleus tractus solitarius (NTS). Bilateral electrolytic lesions to the NTS in the rat effectively disrupted the baroreceptor reflex arc. It was observed that the metabolic and temperature responses to either norepinephrine (1, 5, or 25 μg/kg/min) or to the β-agonist isoproterenol (0.5 μg/kg/min) were significantly attenuated in the NTS-lesioned rats relative to the control animals with an intact baroreflex. Conversely, the cardiovascular effects of norepinephrine or of the α-agonist phenylephrine (10 μg/kg/min) were enhanced in the NTS-lesioned animals. The results suggest that the functional capacity of the brown adipocytes was reduced following NTS lesions and points to an alteration in the ability of β-receptors to respond to pharmacological stimulation with a typical thermogenic response.     

Dopaminergic mechanisms in the nucleus tractus solitarius and effects on blood pressure

The actions of dopamine (DA) agonists and antagonists upon spontaneously active neurons in the nucleus tractus solitarius (NTS), dorsal motor nucleus of the vagus (nX), and nucleus nervi hypoglossi (nXII) were studied. DA was applied microiontophoretically to 42 neurons within the NTS and nX. Sixteen of these cells were stimulated by DA and 9 neurons were depressed. In neurons localized in the nXII nucleus, DA inhibited 19 cells out of 23. Noradrenaline (NA) excited 11 out of 30 cells in the NTS and nX. In the area of nXII nucleus, NA inhibited 6 out of 19 cells and did not modify 11 cells. Acetylcholine (ACh) stimulated the firing of 18 out of 22 neurons. The effects on NA and DA were directly compared on 21 neurons in the NTS. On 16 cells of this group the responses to DA were quantitatively or qualitatively different from those to NA. The DA receptor blockers sulpiride and fluphenazine antagonized the effects of DA but not those due to NA or ACh. Bilateral microinjections of DA (50 nmole) in the area of the NTS induced an increase in systemic blood pressure and heart rate. ACh induced similar effects. NA in the NTS decreased blood pressure and produced bradycardia. The effects of DA on heart rate and blood pressure were blocked by sulpiride or fluphenazine. The cardiovascular role played by DA in the NTS is discussed.     

Dual effects of neurokinin on calcium channel currents and signal pathways in neonatal rat nucleus tractus solitarius

Neurokinins, such as substance P (SP), modulate the reflex regulation of cardiovascular and respiratory function in the CNS, particularly in the nucleus tractus solitarius (NTS). There is considerable evidence of the action of SP in the NTS, but the precise effects have not yet been determined. Voltage-dependent Ca2+ channels (VDCCs) serve as crucial mediators of membrane excitability and Ca2+ -dependent functions such as neurotransmitter release, enzyme activity and gene expression. The purpose of this study was to investigate the effects of neurokinins on VDCCs currents (ICa) in the NTS using patch-clamp recording methods. In 142 of 282 neurons, an application of [Sar(9), Met(O(2)11]-substance P (SSP, NK(1) receptor agonist) caused facilitation of L-type I(Ba). Intracellular dialysis of the Galpha(q/11)-protein antibody attenuated the SSP-induced facilitation of I(Ba). In addition, phospholipase C (PLC) inhibitor, protein kinase C (PKC) inhibitor and PKC activator attenuated the SSP-induced the facilitation of I(Ba). In contrast, in 115 of 282 neurons, an application of SSP caused inhibition of N- and P/Q-types I(Ba). Intracellular dialysis of the Gbetagamma-protein antibody attenuated the SSP-induced inhibition of I(Ba). These results indicate that NK(1) receptor facilitates L-type VDCCs via Galpha(q/11)-protein involving PKC in NTS. On the other hand, NK(1) receptor inhibits N- and P/Q-types VDCCs via Galpha(q/11)-protein betagamma subunits in NTS.     

Multimodal responses of taste neurons in the frog nucleus tractus solitarius

The responses of 216 neurons in the nucleus tractus solitarius (NTS) of the American bullfrog were recorded following taste, temperature, and tactile stimulation. Cells were classified on the basis of their responses to 5 taste stimuli: 0.5 M NaCl, 0.0005 M quinine-HCl (QHCl), 0.01 M acetic acid, 0.5 M sucrose, and deionized water (water). Neurons showing excitatory responses to 1, 2, 3, or 4 of the 5 kinds of taste stimuli were named Type I, II, III, or IV, respectively. Cells whose spontaneous rate was inhibited by taste and/or tactile stimulation of the tongue were termed Type V. Type VI neurons were excited by tactile stimulation alone. Of the 216 cells, 115 were excited or inhibited by taste stimuli (Types I-V), with 35 being Type I, 34 Type II, 40 Type III, 2 Type IV and 4 Type V. The remaining 101 cells were responsive only to tactile stimulation (Type VI). Of those 111 cells excited by taste stimulation (Types I-IV), 106 (95%) responded to NaCl, 66 (59%) to acetic acid, 44 (40%) to QHCl, 10 (9%) to water, and 9 (8%) to warming. No cells responded to sucrose. Of the 111 cells of Types I-IV, 76 (68%) were also sensitive to mechanical stimulation of the tongue. There was some differential distribution of these neuron types within the NTS, with more narrowly tuned cells (Type I) being located more dorsally in the nucleus than the more broadly tuned (Type III) neurons. Cells responding exclusively to touch (Type VI) were also more dorsally situated than those responding to two or more taste stimuli (Types II and III).     

Collateral input to the paraventricular and supraoptic nuclei in rat. II. Afferents from the ventral lateral medulla and nucleus tractus solitarius

In the rat, medullary afferents to the hypothalamic magnocellular nuclei mediate the baroreceptor reflexes of vasopressinergic neurons and the cholecystokinin- or gastric distention-induced excitation of oxytocinergic neurons. One strategy that reflexes such as these may use to coordinate the activity of magnocellular neuroendocrine neurons is collateral branching of input. Previous work has shown that the distributions of medullary neurons projecting to the paraventricular and the supraoptic nuclei overlap and that their axons branch. Thus, we hypothesized that single neurons in the ventral lateral medulla and/or the nucleus tractus solitarius would project to both the paraventricular and supraoptic nuclei via collateral branches of their axons. Medullary afferent neurons were retrogradely labeled after injection into the paraventricular and the supraoptic nucleus on one side of the brain with two different fluorescent tracers: Fluoro-Gold or rhodamine-labeled latex microspheres. The topographic distribution of labeled cells in the medulla containing either a single fluorescent tracer or both tracers were plotted. Of these labeled neurons, a small percentage (7%) contained both dyes, suggesting that they send collateral branches to both of the magnocellular neuroendocrine nuclei injected. Single labeled cells were both ipsi- and contralateral to the injected side (53% ipsilateral), but most double-labeled cells were ipsilateral (84%). In rats, areas that project to both the paraventricular and the supraoptic nuclei may act upon both nuclei together. Thus, afferent inputs, in conjunction with the known inter- and intracellular changes that take place within the magnocellular nuclei, may be involved with the coordinated responses throughout magnocellular neuroendocrine system during medullary reflexes, i.e., the baroreceptor-mediated reflexes or the gastric distention reflexes.     

Effects of vasoactive intestinal polypeptide microinjected into the nucleus tractus solitarius on jejunal glucose absorption in rats

It has been shown that vasoactive intestinal polypeptide (VIP) injected into the nucleus tractus solitarius and into the dorsal motor nucleus of the vagus inhibits alanine absorption across the jejunum. The aim of the present study was to investigate the effects of VIP injection into the nucleus tractus solitarius on jejunal absorption of glucose in the rat. Forty Wistar rats were submitted to midline laparotomy to expose and isolate 20 cm of jejunal loop and to perform a subdiaphragmatic troncular vagotomy. Saline or VIP (10 pg 100 nl(-1)) was injected into the rostral nucleus tractus solitarius using a stereotaxic instrument. Tyrode solution, pH 8, containing twice glucose, sodium, and potassium concentrations was infused (0.5 ml min(-1)) into the jejunal loop. Samples were taken at 10-min intervals during the 40-min experiment. Injection of VIP into the nucleus tractus solitarius associated with vagotomy resulted in inhibition of jejunal glucose absorption by VIP alone at 10 and 40 min after perfusion (2.75+/-0.19 vs. 3.53+/-0.29 mg). The vagal outflow tract maintained jejunal glucose absorption even when VIP was microinjected into the nucleus tractus solitarius.     

Gastric distention increases [C]2-deoxyglucose uptake in the rat nucleus tractus solitarius

Stomach balloons were inflated with 20 ml of warm water in anesthetized rats who had a left cervical vagotomy. This increased [¹⁴C]2-deoxyglucose (2-DG) uptake in the commissural and medial portions of the right nucleus solitarius. This effect was not present in controls which received 2 ml of water in their stomach balloons.     

Cortical projections to the nucleus of the tractus solitarius: An HRP study in the cat

Recent evidence suggests that autonomic reflexes involving sensations such as olfaction and gustation may be cortically mediated via centripetal pathways to brainstem autonomic centers. A study was therefore undertaken to elucidate one of these pathways in greater detail. Lectin conjugated horseradish peroxidase was injected into the nucleus tractus solitarius. Following standard light microscopic histochemical procedures to reveal horseradish peroxidase activity, the distribution of retrogradely labeled neurons in the cortex was recorded. Retrogradely labeled somata were seen bilaterally in layer five of the orbital gyrus, anterior insular cortex and infralimbic cortex. In other cats, the same tracer was injected into the orbital gyrus or anterior insular cortex. Bilateral anterograde labeling was seen in various subnuclei throughout the rostrocaudal extent of the nucleus tractus solitarius, but was heaviest in rostral regions of the nucleus. Labeling was also seen bilaterally in the spinal trigeminal nucleus. The projection to the nucleus tractus solitarius could allow for cortical modulation of gustatory and visceral information which is conveyed to the brainstem via the facial, glossopharyngeal and vagus nerves.     

Choline acetyltransferase and glutamate uptake in the nucleus tractus solitarius and dorsal motor nucleus of the vagus: effect of nodose ganglionectomy

Unilateral removal of the nodose ganglion resulted in a significant decrease in choline acetyltransferase activity in the ipsilateral dorsal motor nucleus of the vagus but was without effect on enzyme activity in the nucleus of the solitary tract. High affinity glutamate uptake in the dorsal motor nucleus of the vagus and along the rostrocaudal extent of the nucleus of the solitary tract was not affected by nodose ganglionectomy.     

Abolition of the behavioral effects of cholecystokinin following bilateral radiofrequency lesions of the parvocellular subdivision of the nucleus tractus solitarius

Cholecystokinin (CCK) has been implicated as a signal for the syndrome of satiety in a variety of species. Several lines of evidence point to a peripheral site of action for the behavioral effects of CCK. Peripheral CCK receptors appear to activate a gut-brain pathway involving the sensory fibers of the vagus nerve. To investigate the central anatomical substrate of this visceral-behavioral control system, the terminal regions of the sensory tract of the vagus were lesioned. Selective destruction of the parvocellular subdivisions of the nucleus tractus solitarius (NTS) blocked the effects of acute doses of CCK on exploratory behaviors. Sham lesions and lesions destroying only the remaining regions of the NTS or the vagal motor nuclei had no effect on baseline exploratory behaviors and did not influence the ability of CCK to decrease spontaneous exploratory behaviors. These findings delineate the first central site along the ascending sensory pathway which appears to mediate the satiety-related behavioral effects of CCK.     

Functional analysis of GABAA receptors in nucleus tractus solitarius neurons from neonatal rats

To gain insight into specific GABA_A receptor configurations functionally expressed in the nucleus tractus solitarius (NTS), we conducted several physiological and pharmacological assessments. NTS neurons were characterized in thin brain slices from 1–14 day old rats using whole-cell patch clamp recordings. GABA_A− receptor-mediated currents were detected in all neurons tested, with an average EC₅₀ of 22.2 μM. GABA currents were consistently stimulated by diazepam (EC₅₀=63 nM), zolpidem (EC₅₀=85 nM), loreclezole (EC₅₀=10.1 μM) and the neurosteroid 5α-pregnan-3α-hydroxy-20-one (3α-OH-DHP). In contrast, GABA-gated currents of the NTS were inhibited by the divalent cation Zn²⁺ (IC₅₀=33.6 μM) picrotoxin (IC₅₀=2.4 μM) and blockade of endogenous protein tyrosine kinase. GABA-activated currents were insensitive to furosemide (10–1000 μM) in all NTS neurons tested. Collectively, the data suggest that in neonatal rats, the predominant α subunit isoform present in GABA_A receptors of the NTS appears to be the α1 and/or α2 subunit. β2 and/or β3 subunits are the major β isoform, while the predominant γ subunit is likely γ2. Our data suggest the contribution to NTS GABA currents by α3–α6, β1, γ1 and δ subunits, if present, is minor by comparison.