Organization of excitatory and inhibitory local networks in the caudal nucleus of tractus solitarius of rats revealed in in vitro slice preparation

Modulation of synaptic transmission in the rat nucleus of the solitary tract by endomorphin-1

Activation of opioid receptors in the periphery and centrally in the brain results in inhibition of gastric and other vagally mediated functions. The aim of this study was to examine the role of the endogenous opioid agonist endomorphin 1 (EM-1) in regulating synaptic transmission within the nucleus tractus solitarius (NTS), an integration site for autonomic functions. We performed whole cell patch-clamp recordings from coronal brain slices of the rat medulla. A subset of the neurons studied was prelabeled with a stomach injection of the transsynaptic retrograde virus expressing EGFP, PRV-152. Solitary tract stimulation resulted in constant latency excitatory postsynaptic currents (EPSCs) that were decreased in amplitude by EM-1 (0.01-10 microM). The paired-pulse ratio was increased with little change in input resistance, suggesting a presynaptic mechanism. Spontaneous EPSCs were decreased in both frequency and amplitude by EM-1, and miniature EPSCs were reduced in frequency but not amplitude, suggesting a presynaptic mechanism for the effect. Spontaneous inhibitory postsynaptic currents (IPSCs) were also reduced in frequency by EM-1, but the effect was blocked by TTX, suggesting activity at receptors on the somata of local inhibitory neurons. Synaptic input arising from local NTS neurons, which were activated by focal photolysis of caged glutamate, was inhibited by EM-1. The actions of EM-1 were similar to those of D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) and were blocked by naltrexone, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), or D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP). These results suggest that EM-1 acts at mu-opioid receptors to modulate viscerosensory input and specific components of local synaptic circuitry in the NTS.

Localization and function of the Kv3.1b subunit in the rat medulla oblongata: focus on the nucleus tractus solitarii

The voltage-gated potassium channel subunit Kv3.1 confers fast firing characteristics to neurones. Kv3.1b subunit immunoreactivity (Kv3.1b-IR) was widespread throughout the medulla oblongata, with labelled neurones in the gracile, cuneate and spinal trigeminal nuclei. In the nucleus of the solitary tract (NTS), Kv3.1b-IR neurones were predominantly located close to the tractus solitarius (TS) and could be GABAergic or glutamatergic. Ultrastructurally, Kv3.1b-IR was detected in NTS terminals, some of which were vagal afferents. Whole-cell current-clamp recordings from neurones near the TS revealed electrophysiological characteristics consistent with the presence of Kv3.1b subunits: short duration action potentials (4.2 +/- 1.4 ms) and high firing frequencies (68.9 +/- 5.3 Hz), both sensitive to application of TEA (0.5 mm) and 4-aminopyridine (4-AP; 30 mum). Intracellular dialysis of an anti-Kv3.1b antibody mimicked and occluded the effects of TEA and 4-AP in NTS and dorsal column nuclei neurones, but not in dorsal vagal nucleus or cerebellar Purkinje cells (which express other Kv3 subunits, but not Kv3.1b). Voltage-clamp recordings from outside-out patches from NTS neurones revealed an outward K(+) current with the basic characteristics of that carried by Kv3 channels. In NTS neurones, electrical stimulation of the TS evoked EPSPs and IPSPs, and TEA and 4-AP increased the average amplitude and decreased the paired pulse ratio, consistent with a presynaptic site of action. Synaptic inputs evoked by stimulation of a region lacking Kv3.1b-IR neurones were not affected, correlating the presence of Kv3.1b in the TS with the pharmacological effects.

Action potential-independent release of glutamate by Ca2+ entry through presynaptic P2X receptors elicits postsynaptic firing in the brainstem autonomic network

P2X receptors are ATP-gated channels permeable to cations including Ca(2+). In acute slices containing the nucleus of the solitary tract, in which neuronal ATP release and ATP-elicited physiological responses are demonstrated in vivo, we recorded spontaneous action potential-independent EPSCs [miniature EPSCs (mEPSCs)]. Activation of presynaptic P2X receptors with alpha,beta-methylene ATP (alphabetamATP) triggered Ca(2+)-dependent glutamate release that was resistant to blockade of voltage-dependent calcium channels but abolished by P2X receptor antagonists. mEPSCs elicited with alphabetamATP were of larger amplitude than basal mEPSCs and resulted in postsynaptic firing caused by temporal summation of miniature events. The large-amplitude mEPSCs provoked by alphabetamATP were likely to result from highly synchronized multivesicular release of glutamate at single release sites. Neither alphabetamATP nor ATP facilitated GABA release. We conclude that this facilitated release and consequent postsynaptic firing underlie the profound autonomic responses to activation of P2X receptors observed in vivo.

Neurokinin-1 receptors in the rat nucleus tractus solitarius: pre- and postsynaptic modulation of glutamate and GABA release

Neurokinins such as substance P and neurokinin A have long been thought to act as neurotransmitters or modulators in the nucleus tractus solitarius. However, the role and location of the receptors for these peptides have remained unclear. We examined the consequences of activation of the neurokinin-1 (NK1) receptor subtype in the rat nucleus tractus solitarius using whole-cell patch clamp recordings in brain slices. Application of delta-Ala-Phe-Phe-Pro-MeLeu-D-Pro[spiro-gamma-lactam]-Leu-Trp-NH2 (a specific NK1 agonist) or neurokinin A resulted in depolarization, evident as a slow inward current, mediated by direct postsynaptic NK1 receptor activation. The effect was conserved in the presence of tetrodotoxin, and protein kinase C-dependent since it was blocked by 2-[1-(3-dimethylaminopropyl)indol-3-yl]-3-(indol-3-yl)maleimide, a specific protein kinase C inhibitor. In addition, an increase in the frequency and amplitude of spontaneous excitatory postsynaptic currents was observed, reflecting increased glutamate release induced by NK1 receptor activation. This effect was abolished by tetrodotoxin, suggesting that it resulted from increased firing in afferent neurons, subsequent to somatodendritic excitation via NK1 receptors. Furthermore, spontaneous inhibitory postsynaptic currents were increased in frequency and amplitude showing that GABA release was promoted by NK1 receptor activation. However, amplitude of miniature inhibitory postsynaptic currents was unaltered by NK1 receptor activation, but the increase in frequency persisted. These findings suggest that NK1 receptors are located on presynaptic terminals as well as at somatodendritic sites of GABAergic neurons. The increase in GABA release was also shown to be protein kinase C-dependent. The data presented here show NK1 receptors in the rat nucleus tractus solitarius are present both excitatory and inhibitory neurons. Activation of these receptors can result in increases in release of both GABA and glutamate, suggesting a crucial modulatory role for NK1 receptors in the rat nucleus tractus solitarius.

Synaptic and morphologic properties in vitro of premotor rat nucleus tractus solitarius neurons labeled transneuronally from the stomach

Neurons in the rat nucleus tractus solitarius (NTS) possess morphologic characteristics that have been correlated with the type of synaptic information they receive. These features have been described for viscerosensory neurons but not for premotor NTS neurons. The morphologic and synaptic features of neurons in the rat caudal NTS were assessed using whole-cell patch-clamp recordings and biocytin labeling in brainstem slices. Gastric-related premotor NTS neurons were identified for recording after inoculation of the stomach wall with a transneuronal retrograde viral label that reports enhanced green fluorescent protein. Three morphologic groups of NTS neurons were identified based on quantitative aspects of soma area and proximal dendritic arborization, measures that were consistent across slice recordings. The most common type of cell (group I) had relatively small somata and one to three sparsely branching dendrites, whereas the other groups had larger somata and more than three dendrites, which branched predominantly close to (group II) or distant from (group III) the soma. Voltage-clamp recordings revealed spontaneous excitatory and inhibitory postsynaptic currents in all neurons, regardless of morphology. Gastric-related premotor NTS neurons composed two of the three morphologic types (i.e., groups I and II). Compared with unlabeled neurons, these cells were less likely to receive constant-latency synaptic input from the tractus solitarius. These results refute the hypothesis that general patterns of synaptic input to NTS neurons depend on morphology. Gastric premotor neurons comprise a subset of NTS morphologic types, the organization of the viscerosensory input to which has yet to be defined.

Contraction-sensitive skeletal muscle afferents inhibit arterial baroreceptor signalling in the nucleus of the solitary tract: role of intrinsic GABA interneurons

Arterial baroreceptor and skeletal muscle receptor afferents relay sensory information to the nucleus of the solitary tract (NTS) during exercise. Previous studies have suggested that skeletal muscle afferent input inhibits baroreflex function; however, detailed information on the role of muscle afferents and GABAergic mechanisms in the NTS is limited. Furthermore, identification of specific afferent modalities that activate GABAergic neurons in the NTS remains unknown. In the present study, we examined the neuroanatomical and physiological interactions between spinal dorsal horn cells that transmit contraction-sensitive input from skeletal muscle and GABAergic interneurons in the NTS. Biotinylated dextran amine (BDA, 10%, 25-100 nL) microinjection into dorsal horn of the cervical spinal cord was combined with glutamate decarboxylase (GAD) immunohistochemistry to visualize the nature of the relationship of BDA-labeled fibers in the NTS with GAD immunoreactivity (GAD-ir). BDA-labeled axons and terminal processes were localized in the medial, commissural, dorsomedial and dorsolateral subdivisions of the caudal NTS. Moreover, BDA-labeled fibers were observed in close proximity to GAD-ir structures throughout these regions of the NTS. The physiological interaction between skeletal muscle receptor and arterial baroreceptor afferents was investigated using an arterially perfused, decerebrate rat preparation. Activation of skeletal muscle afferents by electrically evoked twitch contraction of the forelimb attenuated baroreflex responsiveness (BR, calculated as the ratio of changes in heart rate to systemic pressure) from -1.5+/-0.3 bpm.mm Hg(-1) to -0.1+/-0.1 bpm.mm Hg(-1) (control versus contraction, P<0.05, n=15). However, forelimb contraction failed to inhibit the reflex bradycardia evoked by activation of peripheral chemoreceptor afferents, indicating a reflex-specific action. Bilateral microinjection of bicuculline methiodide (BIC, 10 microM, 40-60 nL) into the caudal NTS restored baroreflex responsiveness during contraction (-1.6+/-0.2 versus -0.1+/-0.1 versus -1.5+/-0.2 bpm.mmHg(-1), control versus contraction versus contraction+BIC P<0.05, n=8). We conclude that activation of ascending spinal neurons from the cervical dorsal horn by contraction-sensitive skeletal muscle afferents selectively inhibits arterial baroreceptor signaling in the NTS via activation of a GABAergic mechanism.

Fos expression by glutamatergic neurons of the solitary tract nucleus after phenylephrine-induced hypertension in rats

The baroreflex pathway might include a glutamatergic connection between the nucleus of the solitary tract (NTS) and a segment of the ventrolateral medulla (VLM) called the caudal ventrolateral medulla. The main goal of this study was to seek direct evidence for such a connection. Awake rats were subjected to phenylephrine- (PE-) induced hypertension (N=5) or received saline (N=5). Neuronal activation was gauged by the presence of Fos-immunoreactive (Fos-ir) nuclei. Fos-ir neurons that contained vesicular glutamate transporter 2 mRNA (glutamatergic neurons) or glutamic acid decarboxylase mRNA (GABAergic neurons) were mapped throughout the medulla oblongata. Saline-treated rats had very few Fos-ir neurons. In PE-treated rats, Fos-ir neurons were detected in both NTS and VLM. In NTS, 72% of Fos-ir neurons were glutamatergic and 26% were GABAergic. In the VLM, 41% of Fos-ir neurons were glutamatergic and 56% were GABAergic. In VLM, Fos-ir glutamatergic neurons were evenly distributed and were often catecholaminergic, whereas Fos-ir GABAergic cells were clustered around Bregma -13.0 mm. This region of the VLM was injected with Fluoro-Gold (FG) in eight rats, four of which received PE and the rest saline. Fos-ir NTS neurons retrogradely labeled with FG were detected only in PE-treated rats. These cells were exclusively glutamatergic and were concentrated within the NTS subnuclei that receive the densest inputs from arterial baroreceptors. In conclusion, PE, presumably via baroreceptor stimulation, induces Fos in glutamatergic and GABAergic neurons in both NTS and VLM. At least 29% of the Fos-ir glutamatergic neurons of NTS project to the vicinity of the VLM GABAergic interneurons that are presumed to mediate the sympathetic baroreflex.

Musings on the wanderer: what's new in our understanding of vago-vagal reflexes? III. Activity-dependent plasticity in vago-vagal reflexes controlling the stomach

Vago-vagal reflex circuits modulate digestive functions from the oral cavity to the transverse colon. Previous articles in this series have described events at the level of the sensory receptors encoding the peripheral stimuli, the transmission of information in the afferent vagus, and the conversion of this data within the dorsal vagal complex (DVC) to impulses in the preganglionic efferents. The control by vagal efferents of the postganglionic neurons impinging on the glands and smooth muscles of the target organs has also been illustrated. Here we focus on some of the mechanisms by which these apparently static reflex circuits can be made quite plastic as a consequence of the action of modulatory inputs from other central nervous system sources. A large body of evidence has shown that the neuronal elements that constitute these brain stem circuits have nonuniform properties and function differently according to status of their target organs and the level of activity in critical modulatory inputs. We propose that DVC circuits undergo a certain amount of short-term plasticity that allows the brain stem neuronal elements to act in harmony with neural systems that control behavioral and physiological homeostasis.

GABAergic modulation of primary gustatory afferent synaptic efficacy

Modulation of synaptic transmission at the primary sensory afferent synapse is well documented for the somatosensory and olfactory systems. The present study was undertaken to test whether GABA impacts on transmission of gustatory information at the primary afferent synapse. In goldfish, the vagal gustatory input terminates in a laminated structure, the vagal lobes, whose sensory layers are homologous to the mammalian nucleus of the solitary tract. We relied on immunoreactivity for the GABA-transporter, GAT-1, to determine the distribution of GABAergic synapses in the vagal lobe. Immunocytochemistry showed dense, punctate GAT-1 immunoreactivity coincident with the layers of termination of primary afferent fibers. The laminar nature and polarized dendritic structure of the vagal lobe make it amenable to an in vitro slice preparation to study early synaptic events in the transmission of gustatory input. Electrical stimulation of the gustatory nerves in vitro produces synaptic field potentials (fEPSPs) predominantly mediated by ionotropic glutamate receptors. Bath application of either the GABA(A) receptor agonist muscimol or the GABA(B) receptor agonist baclofen caused a nearly complete suppression of the primary fEPSP. Coapplication of the appropriate GABA(A) or GABA(B) receptor antagonist bicuculline or CGP-55845 significantly reversed the effects of the agonists. These data indicate that GABAergic terminals situated in proximity to primary gustatory afferent terminals can modulate primary afferent input via both GABA(A) and GABA(B) receptors. The mechanism of action of GABA(B) receptors suggests a presynaptic locus of action for that receptor.

Nociceptive afferents selectively modulate the cardiac component of the peripheral chemoreceptor reflex via actions within the solitary tract nucleus

Our previous findings showed that the nucleus of the solitary tract (NTS) mediated part of the tachycardia evoked during somatic noxious stimulation. Here, we investigated the interaction between somatic nociceptor- and peripheral chemoreceptor-evoked cardiac changes. We sought to determine whether this interaction occurred within the NTS, the primary site of termination of chemoreceptor afferents. In a working heart-brainstem preparation of rat, mechanical noxious activation of a forelimb evoked a tachycardia of 17.5+/-3 (mean+/-S.E.M.) b.p.m., whereas sodium cyanide (7-30 microg) stimulation of peripheral chemoreceptors produced a sub-maximal bradycardia of -140+/-15 b.p.m. During nociceptor stimulation the sodium cyanide-evoked bradycardia was attenuated to -42.6+/-12 b.p.m. but could be prevented by a multiple bilateral NTS microinjection of bicuculline (i.e. -173+/-18 b.p.m.). Furthermore, the activity of NTS neurones responding to peripheral chemoreceptor stimulation increased from 2.8+/-1.3 to 9.4+/-1.9 Hz during sodium cyanide injection (n=7; P<0.01). The latter response was attenuated reversibly to 2.9+/-0.9 Hz during simultaneous stimulation of the brachial nerve. Pressure ejection of bicuculline abolished this inhibitory action of brachial-nerve stimulation on the chemoreceptor-evoked excitatory synaptic response. We conclude that somatic noxious stimulation attenuates the chemoreceptor reflex-evoked bradycardia via a GABA(A)ergic mechanism in the NTS.

Axonal projections of pulmonary slowly adapting receptor relay neurons in the rat

We elucidated efferent projections of second-order relay neurons (P-cells) activated by afferents originating from slowly adapting pulmonary receptors (SARs) to determine the central pathway of the SAR-evoked reflexes. Special attention was paid to visualizing the P-cell projections within the nucleus tractus solitarii (NTS), which may correspond to the inhibitory pathway from P-cells to second-order relay neurons (RAR-cells) of rapidly adapting pulmonary receptors. P-cells were recorded from the NTS in Nembutal-anesthetized, paralyzed, and artificially ventilated rats. First, we used electrophysiological methods of antidromic mapping and showed that the majority of the P-cells examined projected their axons to the caudal NTS and to the dorsolateral pons corresponding to the parabrachial complex. Second, a mixture of HRP and Neurobiotin was injected intracellularly or juxtramembranously into P-cells. (1) Stained P-cells (n = 7) were located laterally to the solitary tract and had dendrites extending characteristically along the lateral border of the solitary tract. (2) All P-cells had stem axons projecting to the ipsilateral medulla. Of these, the axons from five P-cells projected to the nucleus ambiguus and its vicinity with distributing boutons. Some of these axons further ascended in the ventrolateral medulla, and distributed boutons in the areas ventral or ventrolateral to the nucleus ambiguus. (3) All the P-cells had axonal branches with boutons in the NTS area. In particular, axons from three P-cells projected bilaterally to the medial NTS caudal to the obex, i.e., to the area of RAR-cells. These results show anatomic substrates for the connections implicated in the P-cell inhibition of RAR-cells as well as the SAR-induced respiratory reflexes.

Neural control of tongue movement with respect to respiration and swallowing

The tongue must move with remarkable speed and precision between multiple orofacial motor behaviors that are executed virtually simultaneously. Our present understanding of these highly integrated relationships has been limited by their complexity. Recent research indicates that the tongue s contribution to complex orofacial movements is much greater than previously thought. The purpose of this paper is to review the neural control of tongue movement and relate it to complex orofacial behaviors. Particular attention will be given to the interaction of tongue movement with respiration and swallowing, because the morbidity and mortality associated with these relationships make this a primary focus of many current investigations. This review will begin with a discussion of peripheral tongue muscle and nerve physiology that will include new data on tongue contractile properties. Other relevant peripheral oral cavity and oropharyngeal neurophysiology will also be discussed. Much of the review will focus on brainstem control of tongue movement and modulation by neurons that control swallowing and respiration, because it is in the brainstem that orofacial motor behaviors sort themselves out from their common peripheral structures. There is abundant evidence indicating that the neural control of protrusive tongue movement by motoneurons in the ventral hypoglossal nucleus is modulated by respiratory neurons that control inspiratory drive. Yet, little is known of hypoglossal motoneuron modulation by neurons controlling swallowing or other complex movements. There is evidence, however, suggesting that functional segregation of respiration and swallowing within the brainstem is reflected in somatotopy within the hypoglossal nucleus. Also, subtle changes in the neural control of tongue movement may signal the transition between respiration and swallowing. The final section of this review will focus on the cortical integration of tongue movement with complex orofacial movements. This section will conclude with a discussion of the functional and clinical significance of cortical control with respect to recent advances in our understanding of the peripheral and brainstem physiology of tongue movement.

Postnatal differentiation of local networks in the nucleus of the tractus solitarius

Whole-cell voltage-clamp recordings from rat brain slice preparation were used to investigate a possible developmental change in the patterns of synaptic interactions among the nucleus tractus solitarii neurons by analysing spontaneous postsynaptic current activity. Three types of patterns of spontaneous postsynaptic current activity were distinguished in the nucleus tractus solitarii neurons which showed high activities in terms of current frequency and amplitude. The first type was characterized by the presence in an individual cell of high frequencies and large amplitudes of both spontaneous glutamatergic and GABAergic postsynaptic currents, and observed exclusively in postnatal day 0-7 rats. The second and third types of cells showed predominant either inhibitory or excitatory postsynaptic currents, respectively. After postnatal day 5, nucleus tractus solitarii neurons with high background activity were shown to differentiate into either the second or the third type, with the latter of about 70% in the adult caudal/intermediate nucleus tractus solitarii. Axon collaterals of some medium to large neurons seemed to be decreased by pruning during postnatal development. The early postnatal differentiation of background synaptic activity observed in the nucleus tractus solitarii presumably reflects the local network reorganization and may be related to maturational changes in cardiovascular and respiratory functions.

Distinct modulation of evoked and spontaneous EPSCs by purinoceptors in the nucleus tractus solitarii of the rat

Whole-cell transmembrane currents of second-order neurones in the caudal part of the nucleus tractus solitarii (cNTS) of brainstem slices of the rat were recorded to analyse the effects of adenosine 5'-triphosphate (ATP) on: (1) EPSCs evoked by the solitary tract stimulation (eEPSCs) and (2) spontaneous EPSCs (sEPSCs). ATP (10-6 to 10-4 m) significantly reduced the amplitude of eEPSCs to 46.6 +/- 7.4 % and increased the frequency of sEPSCs to 268.0 +/- 71.5 % of the control without significant changes in sEPSC amplitude. These opposite effects of ATP on eEPSCs and sEPSCs were concurrently observed in about 80 % of cNTS neurones recorded. The reduction of eEPSC amplitude by ATP was similarly observed with the addition of an equimolar solution of adenosine but not with alpha,beta-methylene ATP and was suppressed by 8-cyclopentyltheophylline (CPT) and 8-cyclopentyl-1,3-dipropylxanthine (DPCPX). Addition of pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) did not affect the reduction of eEPSC amplitude by ATP. The increase in sEPSC frequency by ATP remained under tetrodotoxin addition but was abolished in the presence of PPADS. It is suggested that ATP activates: (1) presynaptic adenosine A1 receptors, after being hydrolysed to adenosine, reducing evoked release of glutamate from the primary afferent terminals and (2) presynaptic P2X receptors on the axon terminals of intrinsic excitatory cNTS neurones facilitating spontaneous release of glutamate. This is the first evidence that ATP modulates excitatory synaptic inputs arising from distinct origins and converging on a single postsynaptic neurone in diametrically opposite directions through activation of distinct presynaptic purinoceptors.

Anatomical substrate for separate processing of ascending and descending visceral information in the nucleus of the solitary tract of the rat

We examined the possible existence of divergent visceral pathways arising from the nucleus of the solitary tract, by co-injecting axonal tracers into the parabrachial nucleus and into the ventrolateral medulla. We found that around 5% of NTS neurons projected to both sites, and that neurons projecting to VLM were larger. This parallel organization allows a differential control of the ascending versus descending visceral pathways at an early stage of processing.

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

Electrophysiological and morphological properties of a direct projection from the nucleus of the tractus solitarius (NTS) to the ventrolateral medulla (VLM) were investigated. NTS neurons projecting to the VLM exhibit a monosynaptic excitatory response followed by an inhibitory one after the tractus solitarius stimulation. These neurons show spontaneous inhibitory postsynaptic currents, and have medium to large soma (14-26 microm in diameter). It is concluded that the projection from the NTS to the VLM is mediated mostly by medium to large neurons that are inhibited locally by GABAergic interneurons within the NTS.

Somatosympathetic reflex in a working heart-brainstem preparation of the rat

The purpose of the present study was to examine the cardiorespiratory responses (CR) evoked by a somatosympathetic reflex (SSR) in the working heart-brainstem preparation (WHBP). Sprague-Dawley rats (75-100 g) were anesthetized with halothane, bisected sub-diaphramatically and decerebrated pre-collicularly (n = 15). The preparation was transferred to a recording chamber and perfused via the thoracic aorta with Ringer's solution containing an oncotic agent (Ficoll, 1.25%). SSR was activated by electrical stimulation (5 s) of the brachial nerve (0.5-40 Hz, 1-20 V, 0.1 ms) or the forelimb (0.5-40 Hz, 5-60 V, 2 ms). Stimulation at 40 Hz significantly increased heart rate (HR, 366 +/- 10 to 374 +/- 9 beats/min), systemic perfusion pressure (PP, 83 +/- 5 to 89 +/- 6 mmHg) and phrenic nerve discharge (PND, 0.4 +/- 0.1 to 1.4 +/- 0.3 Hz). Ganglionic blockade with hexamethonium (300 microM) eliminated the tachycardia and pressor response but did not alter the tachypnea to forelimb stimulation (n = 3). Transection of the brachial nerve plexus abolished the increase in PP and PND (n = 4). This indicates that a neural reflex mediated these responses. Spinal transection (C1-C2) completely abolished all responses indicating that they were mediated via a supraspinal pathway (n = 2). Based upon these findings, we conclude that activation of somatosensory afferent fibers in the WHBP evokes a programmed pattern of autonomic responses altering the activity-state of both the cardiovascular and respiratory systems. The WHBP provides a unique opportunity to investigate the medullary circuits and neuronal mechanisms that may be involved in coupling cardiorespiratory and somatomotor activity during locomotion/exercise.

Wheat germ agglutinin conjugated to TRITC: a novel approach for labeling primary projection neurons of peripheral afferent nerves

Wheat germ agglutinin conjugated to tetramethylrhodamine isothiocyanate-dextran (WGA-TRITC) was studied as a novel tracer of primary projection neurons of pharyngeal (PhN) and superior laryngeal (SLN) branches of the vagus nerve. The SLN and PhN were dissected from rat cervical tissues and the proximal end of the nerves were bathed in tracer for 60-90 min. The animals were sacrificed 42-72 h later. The tissue was fixed, sliced, mounted on slides and viewed under epifluorescence. The clarity of the fluorescent label in projection neurons was confounded in some regions of the brainstem by autofluorescence. A computer image analysis method was developed to quantify fluorescence intensity for definitive identification of labeled neurons. Brainstem neurons labeled by afferent projections of the SLN and PhN were localized to the nucleus tractus solitarius. Efferents were identified in the nucleus ambiguus. WGA-TRITC labeled cells were observed in the ipsilateral brainstem at intensities significantly different from the fluorescence observed in controls (P<0.01). The distribution and density of labeling is in agreement with results of previous investigations, suggesting that WGA-TRITC is a useful alternative for tracing SLN and PhN projections to brainstem nuclei.

A presynaptic mechanism contributes to depression of autonomic signal transmission in NTS

With increasing frequencies of autonomic afferent input to the nucleus tractus solitarii (NTS), postsynaptic responses are depressed. To test the hypothesis that a presynaptic mechanism contributes to this frequency-dependent depression, we used whole cell, voltage-clamp recordings in an NTS slice. First, we determined whether solitary tract stimulation (0.4-24 Hz) resulted in frequency-dependent depression of excitatory postsynaptic currents (EPSCs) in second-order neurons. Second, because decreases in presynaptic glutamate release result in a parallel depression of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) and N-methyl-D-aspartic acid (NMDA) receptor-mediated components of EPSCs, we determined whether the magnitude, time course, and recovery from the depression were the same in both EPSC components. Third, to determine whether AMPA receptor desensitization contributed, we examined the depression during cyclothiazide. EPSCs decreased in a frequency-dependent manner by up to 76% in second- and 92% in higher-order neurons. AMPA and NMDA EPSC components were depressed with the same magnitude (by 83% and 83%) and time constant (113 and 103 ms). The time constant for the recovery was also not different (1.2 and 0.8 s). Cyclothiazide did not affect synaptic depression at >/=3 Hz. The data suggest that presynaptic mechanism(s) at the first NTS synapse mediate frequency-dependent synaptic depression.

Postnatal development of rat nucleus tractus solitarius neurons: morphological and electrophysiological evidence

Postnatal development of neurons in the caudal nucleus tractus solitarii of rats was studied using the Golgi-Cox technique and whole-cell recordings. Two cell classes were defined on the basis of somatic and dendritic morphology. Elongated neurons have two thick primary dendrites originating from the long axis of the soma. The primary dendrites, tapering distally, give rise to one to four secondary dendrites. Multipolar neurons have pyramidal somas. Extending from each apex of the cell body was a long primary dendrite, which gave rise to a variable number of secondary dendrites. The relative proportion of the two classes was rather constant from birth to adulthood. During the first two postnatal weeks, dendritic length and area of influence increase, but neuronal geometry is not altered in either class. Dendritic appendages appear by postnatal day 5, reach a peak at postnatal day P12 and then almost disappear in adult neurons. Combined intracellular injection of neurobiotin and whole-cell recordings indicate that morphological alteration of caudal nucleus tractus solitarius neurons occurs in parallel with changes in passive properties and spike characteristics. However, the firing pattern of discharge is not correlated with morphology. The physiological significance of these results is discussed.

Late vagal inhibition in neurons of the ventrolateral medulla oblongata in the rat

Stimulation of cervical vagal afferents elicits long-lasting inhibitory effects in a variety of neuronal populations, although little is known concerning the cellular mechanisms that are involved in these effects. In the present study, the electrophysiological characteristics of responses elicited by cumulative activation of vagal afferents were examined in neurons of the rostral ventrolateral medulla oblongata, which play an important role in the coordination of cardiovascular and other visceral activities. The study has focused on the late-onset, slow inhibitory component of vagal responses, which is likely to affect the temporal modulation of postsynaptic effects. Vagal stimulation elicited four distinct response patterns in intracellularly penetrated neurons (n = 78): excitation, inhibition, excitation-inhibition and inhibition-inhibition. The late inhibitory component was encountered in 43 (55%) of the cells, including five putative medullospinal neurons. It was due to a postsynaptic hyperpolarization which reversed at potentials more negative than -83 mV. The voltage dependency, as well as the average onset latency (93+/-3.0 ms), duration (270+/-16.5 ms) and amplitude (1.3+/-0.2 mV as measured at resting membrane potentials), of late inhibition were clearly different from those of the short-latency inhibitory response. The differences in the voltage dependency and time-course of the short-latency responses and the late inhibition indicate that they are mediated by different central relays. In the majority of neurons, late inhibition could be elicited by stimulating only myelinated vagal afferents. The magnitude of the response was, however, significantly enhanced in 63% of the examined cells when the intensity of stimulation was raised to recruit further myelinated and non-myelinated fibres. This indicates that late vagal inhibition is often elicited by a cumulative activation of convergent afferent inputs. The intracellularly labelled vagally responsive neurons were present at all rostrocaudal levels of the rostral ventrolateral medulla, with an accumulation in the region of the lateral paragigantocellular nucleus. Neurons that exhibited late vagal inhibition were dominant in the juxtafacial region of this nucleus. Due to its slow time-course, late vagal inhibition may contribute to a tonic modulation of the activity of neurons in the rostral ventrolateral medulla oblongata. It is proposed that late vagal inhibition plays an important role in the temporal integration of sensory inputs in neurons of the rostral ventrolateral medulla oblongata. The time-course and strength of this modulatory effect are related to the level of activity in those visceral sensory inputs that converge onto the inhibitory interneurons that mediate late inhibition to rostral ventrolateral medulla oblongata neurons.

Electrophysiological and morphological characterization of cytochemically-defined neurons in the caudal nucleus of tractus solitarius of the rat

Morphological and electrophysiological properties of calbindin D-28k-, GABA- and dopamine-beta-hydroxylase-immunopositive neurons were investigated in the caudal nucleus of tractus solitarius of rats, using a patch-clamp whole-cell recording combined with intracellular staining and immunocytochemistry. Calbindin D-28K- and GABA-positive neurons had a small cell body (10.9+/-0.3 microm in diameter) and were distributed throughout the caudal nucleus of tractus solitarius. Double fluorescence immunocytochemistry revealed that calbindin- and GABA-positive neurons formed distinct subpopulations. Calbindin- and GABA-positive neurons double stained for biocytin showed extensive axon collaterals within the nucleus of tractus solitarius and some calbindin-positive, but not GABA-positive neurons, had also projection axons leaving the nucleus of tractus solitarius. Dopamine-beta-hydroxylase-immunopositive neurons had a small (10.8+/-0.3 microm) or large (17.2+/-0.4 microm) cell body. Neurons with a small cell body were observed in the dorsomedial nucleus at the level of the area postrema, and in the area postrema, while neurons with a large cell body were observed in the medial nucleus throughout the caudal nucleus of tractus solitarius. Double fluorescence immunocytochemistry revealed that almost all small dopamine-beta-hydroxylase-positive neurons were also immunoreactive for calbindin, while large dopamine-beta-hydroxylase-positive neurons were not. Double staining for dopamine-beta-hydroxylase and biocytin showed that neurons with a small cell body had moderate axon collaterals. On the contrary, neurons with a large cell body had few, if any, axon collaterals and a projection axon which could leave the nucleus of tractus solitarius. Following stimulation of the tractus solitarius, all neurons with a small cell body exhibited a polysynaptic excitatory response (type I neurons), while dopamine-beta-hydroxylase-immunopositive neurons with a large cell body exhibited a monosynaptic excitatory response (type II neurons) or an excitatory followed by an inhibitory response (type III neurons). Spontaneous and evoked excitatory postsynaptic currents of (type I neurons) calbindin- or GABA-positive neurons were reversibly blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. Spontaneous and evoked inhibitory postsynaptic currents of type III neurons were reversibly blocked by bicuculline. Type II neurons showed no spontaneous excitatory nor inhibitory postsynaptic currents. It was concluded that the three kinds of chemically-defined neurons formed distinct neuronal subpopulations in the caudal nucleus of tractus solitarius in terms of synaptic responses and morphological characteristics such as cell size and axonal trajectory.

Differential roles of NMDA and non-NMDA receptors in synaptic responses of neurons in nucleus tractus solitarii of the rat

The relative role of N-methyl-D-aspartate (NMDA) and non-NMDA receptors in synaptic responses of neurons in caudal nucleus tractus solitarii (cNTS) was delineated by immunohistochemical and electrophysiologic experiments in rats. Double immunohistochemical staining in in vivo experiments revealed that approximately 80% of cNTS neurons that showed Fos-like immunoreactivity induced by baroreceptor activation were generally also immunoreactive to non-NMDA receptor subunits GluR1 or GluR2. On the other hand, only 20% of Fos-labeled cNTS neurons showed immunoreactivity to NMDA receptor subunits NMDAR1 or NMDAR2. Stimulation of the ipsilateral solitary tract at suprathreshold intensity in slice preparations induced Fos expression in the cNTS and evoked either a single action potential or a complex synaptic response consisting of an initial action potential followed by a secondary slow depolarization. In a majority (70%) of cNTS neurons that exhibited the complex synaptic response, both the initial and secondary components were eliminated reversibly by 6-cyano-7-nitroquinoxaline-2,3-dione (20 microM). This non-NMDA antagonist also inhibited the single action potential manifested by the other population of cNTS neurons. On the other hand, only the secondary slow depolarization was blocked by D(-)-2-amino-5-phosphonopentanoic acid (250 microM) or potentiated by NMDA (1.7 microM). Our results suggested that NMDA and non-NMDA receptors are involved differentially in the synaptic responses of cNTS neurons. Non-NMDA receptors may be distributed predominantly on a majority of the second-order cNTS neurons that may receive primary baroreceptor afferent inputs. On the other hand, NMDA receptors are located primarily on higher-order neurons, which may be connected reciprocally with the second-order cNTS neurons.

Vagally evoked synaptic currents in the immature rat nucleus tractus solitarii in an intact in vitro preparation

1. Whole-cell voltage-clamp recordings in an in vitro brainstem-cranial nerve explant preparation were used to assess the local circuitry activated by vagal input to nucleus tractus solitarii (NTS) neurones in immature rats. 2. All neurones that responded to vagal stimulation displayed EPSCs of relatively constant latency. Approximately 50 % of these also demonstrated variable-latency IPSCs, and approximately 31 % also displayed variable-latency EPSCs to vagal stimulation. All neurones also had spontaneous EPSCs and IPSCs. 3. Evoked and spontaneous EPSCs reversed near 0 mV and were blocked by the glutamate AMPA/kainate receptor antagonists 6,7-nitroquinoxaline-2,3-dione (DNQX) or 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) at rest. Evoked EPSCs had rapid rise times (< 1 s) and decayed monoexponentially (tau = 2. 04 +/- 0.03 ms) at potentials near rest. 4. At holding potentials positive to approximately -50 mV, a slow EPSC could be evoked in the presence of DNQX or CNQX. This current peaked at holding potentials near -25 mV and was blocked by the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (AP5). It was therefore probably due to activation of NMDA receptors by vagal afferent fibres. 5. Fast IPSCs reversed near -70 mV and were blocked by the GABAA receptor antagonist bicuculline. In addition, bicuculline enhanced excitatory responses to vagal stimulation and increased spontaneous EPSC frequency. Antagonists to AMPA/kainate receptors reversibly blocked stimulus-associated IPSCs and also decreased the frequency of spontaneous IPSCs. 6. These findings suggest that glutamate mediates synaptic transmission from the vagus nerve to neurones in the immature NTS by acting at non-NMDA and NMDA receptors. NTS neurones may also receive glutamatergic and GABAergic synaptic input from local neurones that can be activated by vagal input and/or regulated by amino acid inputs from other brainstem neurones.1. Whole-cell voltage-clamp recordings in an in vitro brainstem-cranial nerve explant preparation were used to assess the local circuitry activated by vagal input to nucleus tractus solitarii (NTS) neurones in immature rats.