Electrophysiological and morphological characteristics of nucleus tractus solitarii neurons projecting to the ventrolateral medulla

Fibroblast Growth Factor 19 Increases the Excitability of Pre-Motor Glutamatergic Dorsal Vagal Complex Neurons From Hyperglycemic Mice

Intracerebroventricular administration of the protein hormone fibroblast growth factor 19 (FGF19) to the hindbrain produces potent antidiabetic effects in hyperglycemic mice that are likely mediated through a vagal parasympathetic mechanism. FGF19 increases the synaptic excitability of parasympathetic motor neurons in the dorsal motor nucleus of the vagus (DMV) from hyperglycemic, but not normoglycemic, mice but the source of this synaptic input is unknown. Neurons in the area postrema (AP) and nucleus tractus solitarius (NTS) express high levels of FGF receptors and exert glutamatergic control over the DMV. This study tested the hypothesis that FGF19 increases glutamate release in the DMV by increasing the activity of glutamatergic AP and NTS neurons in hyperglycemic mice. Glutamate photoactivation experiments confirmed that FGF19 increases synaptic glutamate release from AP and NTS neurons that connect to the DMV in hyperglycemic, but not normoglycemic mice. Contrary to expectations, FGF19 produced a mixed effect on intrinsic membrane properties in the NTS with a trend towards inhibition, suggesting that another mechanism was responsible for the observed effects on glutamate release in the DMV. Consistent with the hypothesis, FGF19 increased action potential-dependent glutamate release in the NTS in hyperglycemic mice only. Finally, glutamate photoactivation experiments confirmed that FGF19 increases the activity of glutamatergic AP neurons that project to the NTS in hyperglycemic mice. Together, these results support the hypothesis that FGF19 increases glutamate release from AP and NTS neurons that project to the DMV in hyperglycemic mice. FGF19 therefore modifies the local vago-vagal reflex circuitry at several points. Additionally, since the AP and NTS communicate with several other metabolic regulatory nuclei in the brain, FGF19 in the hindbrain may alter neuroendocrine and behavioral aspects of metabolism, in addition to changes in parasympathetic output.

Gastric distension causes changes in heart rate and arterial blood pressure by affecting the crosstalk between vagal and splanchnic systems in anesthetised rats

Various hindbrain nuclei have been demonstrated to be involved in the control of the cardiovascular reflexes elicited by both non-noxious and noxious gastric distension, through parasympathetic and sympathetic activation. The different role played by the branches of autonomic nervous system in exerting these effects and their crosstalk in relation to low-/high-pressure distension rate has not been examined yet. Therefore, in the present work, monolateral and bilateral vagotomy and splanchnicotomy were performed in anesthetised rats to analyse the involvement of hindbrain nuclei in haemodynamic changes caused by gastric distension at high (80 mmHg) and low (15 mmHg) pressure. The analysis of c-Fos expression in neuronal areas involved in cardiovascular control allowed us to examine their recruitment in response to various patterns of gastric distension and the crosstalk between vagal and splanchnic systems. The results obtained show that the low-pressure (non-noxious) gastric distension increases both heart rate and arterial blood pressure. In addition, the vagus nerve and hindbrain nuclei, such as nucleus ambiguous, ventrolateral medulla and lateral reticular nucleus, appear to be primarily involved in observed responses. In particular, we have found that although vagus nerve plays a central role in exerting those cardiovascular reflex changes at low gastric distension, for its functional expression an intact splanchnic system is mandatory. Hence, the absence of splanchnic input attenuates pressor responses or turns them into depressor responses. Instead at high-pressure (noxious) gastric distension, the splanchnic nerve represents the primary component in regulating the reflex cardiovascular effects.

Hypoxia-induced increases in serotonin-immunoreactive nerve fibers in the medulla oblongata of the rat

Hypoxia induces respiratory responses in mammals and serotonergic neurons in the medulla oblongata participate in respiratory control. However, the morphological changes in serotonergic neurons induced by hypoxia have not yet been examined and respiratory controls of serotonergic neurons have not been clarified. We herein investigated the distribution of immunoreactivity for serotonin (5-hydroxytryptamine; 5-HT) in the medulla oblongata of control rats and rats exposed to 1-6h of hypoxia (10% O₂). We also examined the medulla oblongata by multiple immunofluorescence labeling for 5-HT, neurokinin 1 receptors (NK1R), a marker for some respiratory neurons in the pre-Bötzinger complex (PBC), and dopamine β-hydroxylase (DBH), a marker for catecholaminergic neurons. The number of 5-HT-immunoreactive nerve cell bodies in the raphe nuclei was higher in rats exposed to hypoxia than in control rats. The number of 5-HT-immunoreactive nerve fibers significantly increased in the rostral ventrolateral medulla of rats exposed to 1-6h of hypoxia, caudal ventrolateral medulla of rats exposed to 2-6h of hypoxia, and lateral part of the nucleus of the solitary tract and dorsal motor nucleus of the vagus nerve of rats exposed to 1-2h of hypoxia. Multiple immunofluorescence labeling showed that 5-HT-immunoreactive nerve fibers were close to NK1R-immunoreactive neurons in ventrolateral medulla and to DBH-immunoreactive neurons in the medulla. These results suggest that serotonergic neurons partly regulate respiratory control under hypoxic conditions by modulating the activity of NK1R-expressing and catecholaminergic neurons.

Low-fidelity GABA transmission within a dense excitatory network of the solitary tract nucleus

Visceral primary afferents enter the CNS at the caudal solitary tract nucleus (NTS), and activate central pathways key to autonomic and homeostatic regulation. Excitatory transmission from primary solitary tract (ST)-afferents consists of multiple contacts originating from single axons that offer a remarkably high probability of glutamate release and high safety factor for ST afferent excitation. ST afferent activation sometimes triggers polysynaptic GABAergic circuits, which feedback onto second-order NTS neurons. Although inhibitory transmission is observed at second-order neurons, much less is known about the organization and mechanisms regulating GABA transmission. Here, we used a focal pipette to deliver minimal stimulus shocks near second-order NTS neurons in rat brainstem slices and directly activated single GABAergic axons. Most minimal focal shocks activated low jitter EPSCs from single axons with characteristics resembling ST afferents. Much less commonly (9% of sites), minimal focal shocks activated monosynaptic IPSCs at fixed latency (low jitter) that often failed (30%) and had no frequency-dependent facilitation or depression. These GABA release characteristics contrasted markedly to the unfailing, large amplitudes for glutamate released during ST-EPCSs recorded from the same neurons. Surprisingly, unitary GABAergic IPSCs were only weakly calcium dependent. In some neurons, strong focal shocks evoked compound IPSCs indicating convergent summation of multiple inhibitory axons. Our studies demonstrate that second-order NTS neurons receive GABAergic transmission from a diffuse network of inhibitory axons that rely on an intrinsically less reliable and substantially weaker release apparatus than ST excitation. Effective inhibition depends on co-activation of convergent inputs to blunt excitatory drive.

Quantitative and immunohistochemical analysis of neuronal types in the mouse caudal nucleus tractus solitarius: focus on GABAergic neurons

gamma-Aminobutyric acid-ergic (GABAergic) neurons are major inhibitory interneurons that are widely distributed in the central nervous system. The caudal nucleus tractus solitarius (cNTS), which plays a key role in respiratory, cardiovascular, and gastrointestinal function, contains GABAergic neurons for regulation of neuronal firing. In the present study, GABAergic neuronal organization was analyzed in relation to the location of subnuclei in the mouse cNTS. According to the differential expression of glutamate decarboxylase 67 (GAD67), vesicular glutamate transporter 2 (VGLUT2), calbindin, and tyrosine hydroxylase (TH) mRNAs, the cNTS was divided into four subnuclei: the subpostrema, dorsomedial, commissural, and medial subnuclei. The numerical density and size of soma in the four subnuclei were then quantified by an unbiased dissector analysis. Calbindin-positive cells constituted subpopulations of small non-GABAergic neurons preferentially localized in the subpostrema subnucleus. TH-positive cells constituted large neurons preferentially localized in the medial subnucleus. GABAergic neurons constituted a subpopulation of small neurons, preferentially localized in the commissural and medial subnuclei, which represented > or =50% of small cells in these subnuclei. Thus, the GABAergic small neurons were located around TH-positive large cells in the ventrolateral portion of the cNTS. This finding, in combination with results of previous studies in the rat cNTS showing that large cells originate efferents from the cNTS, suggests that GABAergic small neurons in the commissural and medial subnuclei might regulate output from the cNTS.

Glial coverage of the small cell somata in the rat nucleus of tractus solitarius during postnatal development

Astrocytes are thought to be active participants in synaptic plasticity in the developing nervous system. Previous studies suggested that axosomatic synapses decreased in number on the small cells of the rat caudal nucleus of tractus solitarius (cNTS) toward the end of the first postnatal week. Astrocytes might be involved in this phenomenon. We examined the morphological development of astrocytic processes around the small cell soma in the rat cNTS using light and electron microscopy. Glial fibrillary acidic protein (GFAP), glutamate-aspartate transporter (GLAST), and glutamate transporter-1 (GLT-1)-positive structures within the cNTS became more intensely stained as development proceeded. GLAST-positive structures encompassed calbindin-positive small cell somata after postnatal day 10. Electron microscopic observations indicated that astrocytic processes encompass the small cell soma, while the number of axosomatic synapses decreases as development proceeds. The timing for glial coverage of the small cell soma appears to be consistent with the decrease in axosomatic synapses on the small cells. These observations imply that astrocytes may participate actively in regulating the decrease of axosomatic synapses on small cells in the cNTS during postnatal development.

Activity-dependent reorganization of local circuitry in the developing visceral sensory system

Neural activity during critical periods could fine-tune functional synaptic connections. N-methyl-d-aspartate (NMDA) receptor activation is critically implicated in this process and blockade leads to disruption of normal circuit formation. This phenomenon has been well investigated in several neural systems including the somatosensory system, but not yet evidenced in the visceral sensory system. Ultrastructural analysis of GABAergic synapses and electrophysiological analysis of inhibitory and excitatory postsynaptic currents of the rat caudal nucleus tractus solitarii (NTS) cells revealed that developmental changes in the synaptic organizations were blocked by MK-801, an NMDA receptor antagonist, when administered at postnatal days 5-8, a presumed critical period for the visceral sensory system. Normal synapse reorganization during postnatal development dictates undifferentiated neonatal caudal NTS neurons in terms of synaptic input patterns measured by electron microscopy and electrophysiology into two cell groups: small and large cells under far stronger excitatory and inhibitory influence, respectively. Blockade by MK-801 during the critical period might leave adult neurons wired in the undifferentiated synaptic networks, possibly preventing synapse elimination and subsequent stabilization of the proper wiring.

Postnatal development of GABAergic axon terminals in the rat nucleus of tractus solitarius

The proper function of the brain depends on a precise arrangement of excitatory and inhibitory synapses. Although the caudal nucleus of tractus solitarius (cNTS) plays a pivotal role in cardiorespiratory reflexes, we know little about the formation of the local neural network in the cNTS. In the present study, we have focused on GABAergic axon terminals and investigated postnatal changes in GABAergic synaptic organizations in the rat cNTS immunocytochemically at both light and electron microscopic levels. Counting synaptic and non-synaptic GABAergic axon terminals revealed that GABAergic axon terminal number in the cNTS seemed constant until the second postnatal week and that GABAergic axon terminals were reorganized around postnatal day 10 (P10). Electron microscopic observation revealed that more than 20% GABAergic axon terminals formed axosomatic synapses at P2 to P4, but the number of GABAergic axosomatic synapse on neurons with smaller soma (smaller neurons) decreased considerably after P8. Orphan GABAergic boutons were present around somata of smaller neurons at P10, and axodendritic synapse number on thicker dendrites decreased gradually during postnatal development. These results show that GABAergic axon terminals detach from somata of smaller neurons at the second postnatal week. Such morphologic changes in axon terminals could cause changes in electrophysiological activity and might contribute to reorganization of the local network within the cNTS from neonatal to adult type. These postnatal changes in the cNTS local network might be prerequisite for the cardiorespiratory reflexes of the adult type.

Pattern differentiation of excitatory and inhibitory synaptic inputs on distinct neuronal types in the rat caudal nucleus of the tractus solitarius

Region- and size-specific neuronal organizations of the caudal nucleus of the tractus solitarius (cNTS) were investigated, followed by analyses of excitatory and inhibitory synaptic input patterns onto specific cell types by patch clamp recordings and immunoelectron microscopy. Cell size distribution and numerical density of cNTS neurons were examined in subregions at levels of the area postrema. In the subpostremal and dorsomedial subnuclei, characterized by the presence of dense glutamatergic and sparse GABAergic somata, small calbindin neurons constituted 42% of the total cells. The medial subnucleus contained large numbers of glutamatergic, GABAergic, and catecholaminergic somata and large tyrosine hydroxylase-containing cells constituted 13% in this region. In total, small neurons (<150 microm2) represented about 80% of the cell population in the cNTS. Predominant excitatory postsynaptic currents were observed in the adult small neurons, while inhibitory postsynaptic currents were more evident in larger neurons, irrespective of subnuclear location. This distinct differentiation of postsynaptic current patterns was not evident in neonates. GABAergic synapses were more frequently associated with dendrites of large catecholaminergic cells (73%) than with those of small calbindin-containing cells (10%) in adults. These results indicate that differential synaptic input patterns were developmentally established in distinct small and large neurons.

Rapid inhibition of neural excitability in the nucleus tractus solitarii by leptin: implications for ingestive behaviour

The fat-derived peptide leptin regulates cellular activity in areas of the CNS related to feeding, and application of leptin to the fourth ventricle or the nucleus tractus solitarii (NTS) inhibits food intake and weight gain. The hypothesis that leptin modulates cellular activity in the NTS was tested using whole-cell patch-clamp recordings in brainstem slices. Leptin caused a rapid membrane hyperpolarization in 58% of rat NTS neurones, including neurones receiving tractus solitarius input (i.e. viscerosensory) and those involved in regulating output to the stomach, identified after gastric inoculation with a transneuronal retrograde viral label. The hyperpolarization was accompanied by a decrease in input resistance and cellular responsiveness, reversed near the K(+) equilibrium potential, and was prevented by intracellular Cs(+). Perfusion of tolbutamide (200 microm) or wortmannin (100-200 nm) prevented the hyperpolarization, indicating activation of an ATP-sensitive K(+) channel via a PI3 kinase-dependent mechanism. Constant latency tractus solitarius-evoked EPSCs were decreased in amplitude by leptin, and the paired-pulse ratio was increased, suggesting effects on evoked EPSCs involved activation of receptors on vagal afferent terminals. Leptin reduced the frequency of spontaneous and miniature EPSCs, whereas IPSCs were largely unaffected. Leptin's effects were observed in neurones from lean, but not obese, Zucker rats. Neurones that expressed enhanced green fluorescent protein (EGFP) in a subpopulation of putative GABAergic neurones in transgenic mice did not respond to leptin, whereas unlabelled murine neurones responded similarly to rat neurones. Leptin therefore directly and rapidly suppresses activity of excitatory NTS neurones likely to be involved in viscerosensory integration and/or premotor control of the stomach.

Local axonal arborization patterns of distinct neuronal types in the caudal nucleus of the tractus solitarius

Neurons in the caudal nucleus of the tractus solitarius (cNTS) are quite heterogeneous in cell size (50 to 450 microm(2) in somal area) and other morphologic characteristics. For a more objective classification of cNTS neurons, their morphologic features were analyzed quantitatively based on reconstructed biocytin-filled cells after whole-cell patch-clamp recordings. According to the patterns of axonal branching behaviors, cNTS cells could be classified into two groups: smaller cells (94.1 microm(2) in mean somal area, range 62-120 microm(2), n = 22) and larger cells (245 microm(2) in mean somal area, range 142-411 microm(2), n = 23). Extensive axonal arborization with numerous possible synaptic boutons was specifically associated with smaller neurons, while larger cells possessed no or few axon collaterals, suggesting their distinct roles as local circuit neurons (or interneurons) and projection neurons, respectively. With regard to somatodendritic characteristics, the following correlations with cell size were found: smaller cells had larger form factors than larger cells (P < 0.05). Larger neurons had more extensive dendritic arborization, expressed by total dendritic length (P < 0.01) and number of dendritic branching points (P < 0.01), than smaller cells. It was suggested that small cNTS neurons contribute specifically to an integration of input information generated in the local circuits, while large neurons convey the integrated information to other autonomic brain regions.

Characterization of neurons of the nucleus tractus solitarius pars centralis

Esophageal sensory afferent inputs terminate principally in the central subnucleus of the tractus solitarius (cNTS). Neurons of the cNTS comprise two major neurochemical subpopulations. One contains neurons that are nitric oxide synthase (NOS) immunoreactive (-IR) while the other comprises neurons that are tyrosine hydroxylase (TH)-IR. We have shown recently that TH-IR neurons are involved in esophageal-distention induced gastric relaxation. We used whole cell patch clamp techniques in rat brainstem slices combined with immunohistochemical and morphological reconstructions to characterize cNTS neurons. Postrecording reconstruction of cNTS neurons revealed two morphological neuronal subtypes; one group of cells (41 out of 131 neurons, i.e., 31%) had a multipolar soma, while the other group (87 out of 131 neurons, i.e., 66%) had a bipolar soma. Of the 43 cells in which we conducted a neurochemical examination, 15 displayed TH-IR (9 with bipolar morphology, 6 with multipolar morphology) while the remaining 28 neurons did not display TH-IR (18 with bipolar morphology, 10 with multipolar morphology). Even though the range of electrophysiological properties varied significantly, morphological or neurochemical distinctions did not reveal characteristics peculiar to the subgroups. Spontaneous excitatory postsynaptic currents (sEPSC) recorded in cNTS neurons had a frequency of 1.5 +/- 0.15 events s(-1) and an amplitude of 27 +/- 1.2 pA (Vh = -50 mV) and were abolished by pretreatment with 30 muM AP-5 and 10 muM CNQX, indicating the involvement of both NMDA and non-NMDA receptors. Some cNTS neurons also received a GABAergic input that was abolished by perfusion with 30-50 muM bicuculline. In conclusion, our data show that despite the heterogeneity of morphological and neurochemical membrane properties, the electrophysiological characteristics of cNTS neurons are not a distinguishing feature.

Plasticity of central mechanisms for cough

Cough is associated with plasticity of putative cough afferent fibres, but whether plasticity in the brainstem network contributes is less well understood. A key site in the CNS network is the nucleus tractus solitarius (NTS), the first synaptic contact of the primary afferent fibres. We sought to develop a conscious guinea pig model to detect enhanced cough, to focus on the NTS as a potential site for plasticity, and to test a role for substance P in the NTS since the neuropeptide has been implicated in plasticity of the vagal afferent fibres. Guinea pigs were exposed to second-hand tobacco smoke (SHS) or filtered air (FA) from 1-6 weeks of age. At 5 weeks, cannulae were implanted in the NTS. At 6 weeks, either vehicle or a neurokinin 1 (NK-1) receptor antagonist was injected into the NTS of the conscious guinea pigs who were then exposed to citric acid aerosol. SHS exposure significantly enhanced citric acid-induced cough (56%, P<0.05), an effect attenuated by NTS NK-1 receptor blockade (P<0.05). The findings suggest that one possible mechanism for plasticity in cough is related to substance P effects in the NTS. Future studies will be required to investigate the possible mechanisms underlying the role of substance P as well as other mechanisms in generating SHS-induced cough.

The pattern of c-Fos immunoreactivity in the hindbrain of the rat following stomach distension

It has been previously shown that the walls of the stomach contain vagal and splanchnic afferents, connected to low and high threshold (LT and HT) gastric receptors, that convey physiological and noxious information to areas of the hindbrain involved mainly in the control of gastrointestinal function. Because distension of the stomach also reflexly increases the sympathetic drive to the cardiovascular system, the present study was planned to examine the pattern of activation of all nuclei encountered throughout the hindbrain in response to gastric distension. In anaesthetized rats, the stimulus was controlled by employing different transmural pressures and frequencies of distension, and c-Fos immunohistochemistry was used to characterize neuronal activation. Low intensity stimulation induced c-Fos expression in the cranial part of nucleus of solitary tract (NTS), the nucleus ambiguus (NA), the lateral reticular area (LRt) and the ventrolateral medulla (RVL/CVL). At low frequency of stimulation c-Fos positive nuclei (p.n.) were found in NTS only. At high frequency of stimulation an increase in c-Fos immunoreactivity was found. High intensity stimulation induced c-Fos expression in area postrema (AP), the lateral vestibular nucleus (LVe) and the caudal part of the NTS. At low frequency, only the number of c-Fos p.n. was increased. Increasing the frequency of stimulation induced c-Fos expression in further nuclei such as the parabrachial nucleus (PBN), the inferior olive subnuclei (IOn), the oral part of spinal trigeminal nucleus (Sp5O) and locus coeruleus (LC). At higher frequencies c-Fos immunoreactivity decreased in NTS and LRt, disappeared in VLM and increased in NA. Thus stomach distension activated several neuronal excitatory and inhibitory circuits that are involved in the control of gastrointestinal function as well as in cardiovascular, respiratory and pain regulation. The differences in c-Fos immunoreactivity induced by changing the distension patterns suggested interactions between groups of vagal and splanchnic afferents.

Branching projections of catecholaminergic ventrolateral reticular neurons to the fastigial nucleus and superior colliculus in the rat: triple labelling procedure

In this study, we employed triple fluorescent-labelling to reveal the distribution of the catecholaminergic neurons within rostral ventrolateral reticular nucleus which supply branching collateral input to the superior colliculus (SC) and to the cerebellar fastigial nucleus (FN). The catecholaminergic identity of the neurons was revealed by immunocytochemical detection of the biosynthetic enzyme, tyrosine hydroxylase. The projections were defined by injections of two retrograde tracers: rhodamine and fluoro gold in the SC and FN, respectively.