Detlef Bieger

Chapter 5 Vagal efferent projections: viscerotopy, neurochemistry and effects of vagotomy

Publisher Summary This chapter focuses on the viscerotopic organization of the cells of origin of the vagus nerve with emphasis on subnuclear organization in the vagus nerve (DMV) and nucleus ambiguus (AMB). Aspects of the neurochemistry and afferent inputs of the DMV and AMB are discussed in terms of the identified viscerotopic organization. The effects of vagus nerve lesions will be assessed in terms of subnuclear differences as they relate to the connectivity connectivity and neurochemistry of the DMV and AMB. The chapter shows that the viscerotopic representation in the two nuclei comports with topographic patterns of specific neurochemicals and that vagal lesions cause differential responses in chemically and functionally distinct groups of neurons in the DMV and AMB. Neurochemical changes following vagal lesions are selective for specific viscera and are not uniform among functional subdivisions in the two nuclei. The viscerotopic organization of vagal motoneurons, thus, may also be reflected in differences in trophic support and pathophysiology.

Nicotinic cholinoceptor-mediated excitatory postsynaptic potentials in rat nucleus ambiguus

In rat brainstem slice preparations, intracellular recording from neurons (n = 39) in the compact formation of the nucleus ambiguus (AMBc) revealed spontaneous and miniature excitatory postsynaptic potentials (EPSPs; n = 11) that, along with acetylcholine-induced depolarization, were enhanced by physostigmine (10 μM; n = 2) and blocked by dihydro-β-erythroidine 1–5 pmol (n = 4). Retrograde neuronal tracing combined with choline acetyltransferase immunocytochemistry demonstrated that the AMBc receives a projection from a subpopulation of cholinergic neurons in the zona intermedialis reticularis parvicellularis. Electrical stimulation of this region in slices evoked fast EPSPs in AMBc neurons (n = 23) that were inhibited by dihydro-β-erythroidine 2–5 pmol (n = 8), but not by methscopolamine 1 pmol (n = 2). The present findings strongly support the existence of a cholinergic nicotinic synapse mediating fast transmission in brainstem vagal motoneurons.

Neuropharmacologic correlates of deglutition: Lessons from fictive swallowing

Pharmacologic investigations into the transmission processes underlying fictive swallowing in the rat have disclosed the potential diversity of chemical signals used in central deglutitive pathways. Monoaminergic mechanisms appear to serve as links between subcortical structures and the medullary pattern generator of swallowing (PGS), and may play a critical role in maintaining internal facilitatory drive, required by the PGS for optimal responsivity to peripheral sensory input. Cholinergic bulbar interneurons form an integral component of the PGS subnetwork controlling esophageal peristalsis. Local GABA neurons exert a tonic inhibition of the buccopharyngeal stage, may regulate bucco-pharyngeal-esophageal coupling, and may contribute to peristaltic rhythmic generation at both the premotoneuronal and motoneuronal level. Receptor subtypes for excitatory amino acids (glutamate, aspartate) are differentially associated with deglutitive premotoneurons for both the buccopharyngeal and esophageal stage, as well as with ambiguus motoneurons. Preliminary evidence suggests the existence of excitatory peptidergic mechanisms involving thyrotropin-releasing hormone, vasopressin, oxytocin, and somatostatin, a probable candidate for excitatory transmitter in the solitarioambigual internuncial projection to motoneurons innervating esophageal striated musculature. Further validation of this experimental model may ultimately help to establish a framework for the clinical recognition, management, and exploitation of drug actions on central deglutitive neuroeffectors.

The brainstem esophagomotor network pattern generator: a rodent model.

The evidence reviewed in this essay supports the following working model of the central function generator for esophageal peristalsis in the rat: solitarial subnucleus centralis (NTSc) neurons operate in a dual capacity as esophagomotor reflex interneurons and as command neurons programming respective outputs from nucleus ambiguus compact formation (AMBc) motoneurons during secondary and primary peristalsis. In both conditions, there is a critical requirement for cholinergic input which enables NTSc neurons to generate the timed sequence of AMBc motoneuronal activity. In primary peristalsis, the cholinergic coupling mechanism is activated centrally, probably via projections from deglutitive premotor neurons to the parvicellular reticular formation and thence to the NTS. In reflex (or secondary) peristalsis, the cholinergic input could in part be generated by cholinergic vagal viscerosensory fibers innervating the esophagus. Postulated connections between NTS deglutitive neurons and the parvicellular cholinergic neurons of the intermediate reticular formation have yet to be demonstrated. Premotor input from NTSc to AMBc is generated by somatostatinergic and excitatory aminoacidergic neurons. Coactivation of both inputs by cholinergic afferents is necessary to generate esophagomotor output from AMBc neurons. The model under study is derived from investigations into central mechanisms governing striated muscle peristaltic activity. Whether the basic operational principles revealed thus far apply to peristaltic pattern generation in species with a smooth muscle esophagus, requires further investigation.

Activation of NMDA receptors in necessary for fast information transfer at brainstem vagal motoneurons

The involvement of N-methyl-D-aspartate (NMDA) excitatory amino acid subtype receptors in synaptically driven excitatory responses of ambigual motoneurons was investigated in vivo and in vitro. In urethane-anaesthetized rats, fictive oesophageal peristalsis evoked by topical application of muscarine (0.05-0.5 nmol) to the dorsal surface of the solitarial complex (NTS) was reversibly blocked by ipsilateral intraambigual injection of DL-2-amino-7-phosphonoheptanoic acid (AP-7, 0.5-1.5 nM) and (+-)-3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; 0.5-1.5 nM). In brainstem sagittal slices, post-synaptic potentials were recorded from neurons of the compact formation of the nucleus ambiguus (AMBc). Stimulation of presumptive NTS afferents elicited a complex excitatory postsynaptic potential (EPSP) which usually consisted of both a high-threshold fast (HTF) and a low-threshold slow (LTS) component. Bath perfusion with AP-7 (30-50 microM) and CPP (50 microM) selectively blocked the HTF without affecting the LTS component, while kynurenate (1 mM) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 5-10 microM) nonselectively suppressed both components. With sufficient stimulus strength, the EPSP generated a single spike arising from the HTF component. AP-7 (50 microM) either blocked the spike or increased the firing threshold. Furthermore, at the resting membrane potential, bath-applied NMDA induced a net inward current (269 +/- 189 pA) which had a negative slope in the range of -95 to -35 mV. In conclusion, NMDA receptors participate in solitario-ambigual synaptic transmission under physiological conditions and activation of these receptors is necessary for functional information transfer in this pathway.

Excitatory action of 5-HT on deglutitive substrates in the rat solitary complex

The excitatory effect of serotonin (5-HT) on the pharyngeal stage of swallowing was investigated in urethane anaesthetised rats with respect to the involvement of neural substrates located in the central and intermediolateral regions of the nucleus tractus solitarii (NTS). Micropneumophoretic ejection of 5-HT 5-50 pmol either produced deglutitory responses or selectively facilitated the S-glutamate-evoked pharyngeal responses when applied in 1-10 pmol prepulses. The excitatory/facilitatory effect of 5-HT was enhanced by intravenous threshold doses of the 5-HT-mimetic, quipazine (0.3-1 mumol/kg) and reversibly blocked by the 5-HT2-receptor antagonists, methysergide, metergoline and ketanserin. 5-HT doses exceeding 10-60 pmol gave rise to a non-selective reversible inhibition of glutamate- and acetylcholine (ACh)-evoked pharyngeal or oesophageal responses which was not prevented or reversed by 5-HT2-receptor antagonists, but was readily overcome by increasing the amount of glutamate or ACh ejected. Non-selective deglutitive inhibition after high doses of 5-HT could, therefore, result from neuronal desensitization secondary to excessive stimulation or activation of a different type of 5-HT receptor. These results corroborate an excitatory role of 5-HT in both reflex and automatic swallowing and demonstrate that the NTS is a major site of serotoninergic facilitation of swallowing.

Nicotinic cholinoceptor-mediated excitation in ambigual motoneurons of the rat

The purpose of this study was to determine if ambigual oesophageal motoneurons of the rat possess functional nicotinic cholinoceptors. In urethane anaesthetized rats, acetylcholine (20-50 pmol) delivered micropneumophoretically from multibarrelled pipettes to the compact formation of the nucleus ambiguus produced either synchronous or propulsive oesophageal contractions which were fully and reversibly blocked by dihydro-beta-erythroidine (8-10 pmol) but were resistant to D-tubocurarine and hexamethonium (10-20 pmol). 1,1-Dimethyl-4-phenyl-piperazinium but not muscarine (8 pmol) exerted an analogous agonist action. Ejection of glutamate at the same sites produced similar oesophageal responses which were, however, resistant to dihydro-beta-erythroidine. Acetylcholine applied 5-15 s prior to glutamate transiently facilitated the glutamate-evoked response. The facilitatory effect of acetylcholine was replicated by 1,1-dimethyl-4-phenyl-piperazinium but not muscarine and inhibited by dihydro-beta-erythroidine. Physostigmine, applied either intra-ambigually (10-20 pmol) or by intravenous injection (0.15-0.3 mumol/kg), enhanced both acetylcholine and glutamate-evoked responses. In brainstem transverse slices, application of acetylcholine and glutamate to quiescent ambigual neurons of the compact formation resulted in a rapid membrane depolarization associated with an increased membrane conductance and spiking. Under voltage clamp, both acetylcholine and glutamate elicited a net inward current. The depolarizing response of these neurons to acetylcholine was blocked by dihydro-beta-erythroidine (0.5-2 pmol), hexamethonium (0.2 mM) and D-tubocurarine (10 microM) and persisted in the presence of tetrodotoxin (10(-6) M) or Mn2+ (5 mM) in the bathing medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Somatostatin regulates excitatory amino acid Letterreceptor-mediated fast excitatory postsynaptic potential components in vagal motoneurons

Somatostatin is considered to be a brain neurotransmitter/neuromodulator; however, there is little concrete information on how this peptide contributes to generation of synaptic potentials in the mammalian central nervous tissue. Recently, a well-defined somatostatin-containing pathway has been traced from the subnucleus centralis of the solitarial complex to the compact formation of the nucleus ambiguus. Moreover, we have demonstrated both in vivo and in vitro that somatostatin enhances glutamate but inhibits acetylcholine excitation of ambigual motoneurons, suggesting involvement of this peptide in central oesophagomotor transmission. The availability of a brainstem slice containing this pathway has allowed us to characterize an excitatory amino acid receptor-mediated excitatory postsynaptic potential in compact formation neurons. This excitatory postsynaptic potential is unusual because its rising phase involves activation of N-methyl-D-aspartate receptors. Here we report that somatostatin participates in ambigual excitatory postsynaptic potential generation by permitting expression of the N-methyl-D-aspartate receptor-mediated component, thereby regulating fast information transfer in this pathway.

Distal and deglutitive inhibition in the rat esophagus: Role of inhibitory neurotransmission in the nucleus tractus solitarii ☆ ☆☆

BACKGROUND & AIMS This study aimed to show the presence of deglutitive and distal inhibition in the rat esophagus and to differentiate the underlying neural mechanisms. METHODS Under urethane anesthesia, the pharyngoesophageal tract was fitted with water-filled balloons for luminal distention and pressure recording. Neural activity was recorded in the nucleus tractus solitarii subnucleus centralis and rostral nucleus ambiguous. RESULTS Distal esophageal distention evoked both rhythmic local contractions and burst discharges of ambiguous neurons that were simultaneously inhibited by a swallow or proximal esophageal distention. In subnucleus centralis interneurons, type I rhythmic burst discharges correlated with distal esophageal pressure waves and were suppressed by midthoracic esophageal distention; type II non-rhythmic excitatory responses, like type III inhibitory responses, were evoked by distention of either the thoracic or distal esophagus. When applied to the surface of the solitarius complex, bicuculline and, less effectively, strychnine suppressed distal inhibition, and 2-(OH)-saclofen and 3-aminopropylphosphonic acid were ineffective. None of the drugs tested, including systemic picrotoxin, affected deglutitive inhibition. CONCLUSIONS Distal and deglutitive inhibition are present in the rat esophagus. The former, unlike the latter, depends on activation of ligand-gated chloride channels associated with subnucleus centralis premotor neurons. Inhibitory aminoacidergic local interneurons are a probable source of type II responses.

Central nervous system control mechanisms of swallowing: A neuropharmacological perspective

Neuropharmacologicalin vivo andin vitro investigations are beginning to provide insight into chemical signaling processes within brainstem networks controlling the individual stages of swallowing. Different subtypes of excitatory amino acid (EAA) receptors operate at the level of solitarial interneurons programming the buccopharyngeal and esophageal stage, as well as motoneurons innervating esophageal striated musculature. Muscarinic cholinoceptors (MAChRs), probably activated via a propriobulbar input, are critically involved in generating output from solitarial neurons to esophageal motoneurons. Inhibition to tonically active GABAA-receptor mediated afferents to solitarial premotor neurons results in rhythmic deglutitive output, reflecting disinhibition of EAA and MACK receptor activity. Motoneuronal EAA receptors may be regulated by a somatostatinergic input arising from solitarial premotoneurons. The available evidence is consistent with a transmitter heterogeneity in esophageal premotor neurons that may operate to provide chemical coding of afferents to the motor output stage of the pattern generator for esophageal peristalsis.