Relaxant responses to calcium channel antagonists and potassium channel opener in human saphenous vein.

1.-- As shown in a parallel study the magnitude of depolarization induced in human saphenous vein by raising external potassium ([K(+)](e)) falls markedly below the theoretical values predicted by the Goldman-Hodgkin-Katz equations. This anomaly prompted us to re-examine the relaxant actions of L-type (nifedipine) and T-type (mibefradil) Ca(2+) channel antagonists, and relaxant and electrophysiological effects of the K(+) channel opener, pinacidil, on saphenous veins contracted by the elevation of [K(+)](e). 2.-- Nifedipine produced concentration-dependent relaxations in tissues contracted at various high [K(+)](e). In tissues contracted with 20 mm [K(+)](e), the pIC(50) for nifedipine was significantly (8.20 +/- 0.05; n = 6; mean +/- SEM; P < 0.05) greater than in tissues contracted with > or =40 mm [K(+)](e). 3.-- Tissues contracted with 20 mm [K(+)](e) also relaxed in response to mibefradil (pIC(50) = 6.1 +/- 0.14) and pinacidil (pIC(50) = 6.45 +/- 0.08), the latter being almost completely reversed (93.4 +/- 9.9%) by addition of glibenclamide (10 microm). 4.-- The resting E(m) of smooth muscle cells of saphenous vein was -77.0 +/- 0.7 mV (n = 52), and 20 mm [K(+)](e) produced a modest but significant depolarization to -73.0 +/- 0.7 mV (n = 52). Incubation with pinacidil plus 20 mm [K(+)](e) resulted in a significant hyperpolarization of the E(m) to -82 +/- 0.6 mV (n = 52). 5.-- N(omega)-nitro-L-arginine methyl ester did not impede the relaxant responses of nifedipine, mibefradil or pinacidil. 6.-- In conclusion, the relaxant effects of nifedipine and pinacidil (i) occurred at an E(m) distinctly below the presumed threshold for the opening of the classic (Ca(V)1.3alpha(1)) L-type Ca(2+) channels, and (ii) did not depend on generation of nitric oxide.

Anomalous response to potassium in vascular smooth muscle cells of human saphenous vein.

1.--We have examined the relationship between the resting membrane potential (E(m)) and the concentration of the external ions, K(+), Cl(-) and Ca(2+), as well as the effects of K(+) on active force generation in human saphenous veins. 2.--As measured with sharp glass microelectrodes, the E(m) of vascular muscle cells was -76.0 +/- 7.0 mV (mean +/- SD; n = 328). Raising the concentration of external potassium ([K(+)](e)) from 4.2 to 20, 40, 80, 120 or 150 mm produced an incremental depolarization, revealing a maximal slope factor of 15 mV per 10-fold increase. 3.--Oubain (1.0 microm) did not have any effect on E(m) (-79.0 +/- 8.0 mV; n = 80). Replacement of external Cl(-) with propionate resulted in significant (P < 0.05) depolarization (E(m): -65.5 +/- 7.5 mV; n = 40). In Cl(-)-free buffer containing 80 mm K(+), E(m) depolarized to -52.0 +/- 6.7 mV (n = 45) compared with -64.7 +/- 6.5 mV (n = 55) (P < 0.05) measured in buffer containing 80 mm [K(+)](e) and Cl(-) 138.7 mm. Removal of Ca(2+) did not significantly modify the depolarizing response to K(+) 80 mm: E(m), -68.2 +/- 4.9 mV (n = 42) vs.-64.7 +/- 6.5 mV (n = 55) in the presence of Ca(2+). 4.--Despite their small size, changes in E(m) correlated closely with force generation in buffer containing high K(+), approximately 3.62 mN force being generated per mV of change in E(m). 5.--These data demonstrate that, in human saphenous smooth muscle cells, (i) the magnitude of depolarization induced by raising [K(+)](e) deviates considerably from the theoretical values predicted by the Goldman-Hodgkin-Katz equations, (ii) Cl(-) appears to contribute to the maintenance of E(m), and (iii) electromechanical coupling has a low threshold.

Rhombencephalic pathways and neurotransmitters controlling deglutition.

Information from neuronal pathway tracing and pharmacologic microstimulation studies, in conjunction with electrophysiological data, has begun to coalesce into a coherent, if still incomplete, picture of the brain stem circuitry responsible for generating the motor patterns underlying deglutition and esophageal peristalsis in the rat. The intermediate, interstitial, and ventral subnuclei of the solitarius complex appear to play a pivotal part, as evidenced by their viscerosensory inputs and extensive projections to the parvicellular intermediate reticular formation of the medulla, to the brain stem deglutitive motor neuron pools, and to general visceral efferent preganglionic neurons controlling the upper alimentary tract striated and smooth musculature, respectively. The dense projections of the solitarial central subnucleus form a separate subcircuit controlling esophageal, and also some aspects of gastric, motility. Although not extensive, direct connections between the latter subnucleus and interneurons coordinating the buccopharyngeal stage of swallowing appear to exist. In both subcircuits, fast information transfer uses excitatory amino acidergic transmission by means of several glutamate-receptor subtypes. Release from tonic GABAergic inhibition exerted by local solitarial interneurons may provide a mechanism for triggering deglutitive premotoneuronal activity. Local or reticular cholinergic neurons are implicated in pharyngoesophageal coupling and the generation of propulsive esophagomotor output. The solitary interneurons under investigation engage in complex local dendritic and axonal projections within the solitarius complex. Further analysis of these local circuits and their transmitters should yield essential clues regarding the mechanisms underlying deglutitive motor pattern generation.

Vagal afferent input determines the volume dependence of rat esophageal motility patterns.

The volume dependence of balloon distension-evoked esophageal rhythmic motor responses and their neural correlates was investigated in 72 urethane-anesthetized rats. With increasing balloon volume (75--200 microl), distal esophageal rhythmic contractions decreased in rate and became tonic in the range of 150--250 microl. This change in motor pattern involved only the striated musculature of the esophageal body and persisted after acute transection of the spinal cord at C(2). Impulse frequency in single vagal afferents of the distal esophagus increased with intraluminal pressure over the entire range of balloon volumes tested (50--300 microl). Distension-responsive neurons in the nucleus tractus solitarii showed rhythmic burst activity (type I), tonic excitation (type II), or inhibition followed by off bursts (type III). Increasing strength of stimulation changed type I responses to nonrhythmic but intensified type II and III responses. We conclude that load-dependent changes in distal esophagus motility pattern are encoded by vagal afferents alone and do not involve a spinal afferent input even at near-noxious stimulus strengths.

Effects of vagal cooling on esophageal cardiovascular reflex responses in the rat.

Distension of the distal esophagus in the anesthetized rat causes a vagally-mediated arterial pressor and tachycardia response. To investigate the nature of viscerosensory fibers in the afferent limb of this reflex, the present study was carried out in urethane-anesthetized rats that were subjected to graded cooling of both cervical vagal trunks in situ. Distal esophageal distension was applied for 20 s by means of a water-filled high compliance balloon. Vagal cooling to 9 degrees C abolished pressor responses and unmasked a depressor component during maximal distension. Cooling to 7.5 degrees C blocked this inhibitory component, well above the temperature known to block C-fibers. We conclude that the cardiovascular response to esophageal distension is triggered via at least two subpopulations of A(delta) type vagal afferents that project to brain stem nuclei regulating central vasomotor tone.

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.

Vagal afferent transmission in the NTS mediating reflex responses of the rat esophagus.

In urethan-anesthetized rats, esophageal distension evoked volume-dependent reflex contractions with phase-locked multiunit discharges in the central subnucleus of the solitary tract complex (NTSC) and the nucleus ambiguus. During blockade of solitarial, but not peripheral, muscarinic cholinoceptors, the volume-response relationship of reflex contractions was shifted rightward with a depression in pressure wave amplitude. Concurrently, premotor NTSC responses were attenuated and nucleus ambiguus activity was abolished during esophagomotor inhibition. Both NTSC discharges and reflex responses were eliminated, or strongly inhibited, during blockade of excitatory amino acid receptors (EAARs) with 6-cyano-7-nitroquinoxaline-2,3-dione, gamma-glutamylglycine or 2-amino-7-phosphonoheptanoate. In brain stem slice preparations, whole cell recordings in the NTSC region revealed fast excitatory postsynaptic potentials (EPSPS) with spikes in response to electrical stimulation of the solitary tract. Although spiking was facilitated by muscarine, EPSPS were resistant to cholinoceptor antagonists but sensitive to EAAR blockers. We conclude that esophageal vagal afferents excite ipsilateral NTSC interneurons via activation of glutamate receptors of the DL-alpha-amino-3-hydroxy-5-methylisoxazole-propionic acid and N-methyl-D-aspartate subtypes. Cholinergic input to the NTSC probably derives from propriobulbar sources and serves to modulate the responsiveness of reflex interneurons.

Vagovagal reflex motility patterns of the rat esophagus.

Esophageal reflex motility and its neural correlates were investigated in 94 urethan-anesthetized adult male albino rats. When distended by means of a stationary balloon, the cervical and thoracic esophageal portion responded with a single pressure wave (type I response), whereas the diaphragmatic (intercrural) segment exhibited rhythmic contractions (type II response). Balloon deflation resulted in an off response aboral to the balloon. Bilateral cervical vagotomy or systemic D-tubocurarine abolished all types of reflex responses. Both type I and type II responses were associated with multiunit discharges in the central subnucleus of the solitary tract complex (NTSC) and the compact formation of the nucleus ambiguus (AMBC). Type I discharges, consisting of single bursts, and type II discharges, consisting of rhythmic 0.6-Hz bursts, preceded intraesophageal pressure waves in a fixed phase relationship, persisted after contralateral vagotomy, and were eliminated by ipsilateral vagotomy. During neuromuscular paralysis, peak intraburst discharge rates were reduced in both the NTSC and AMBC, with a concomitant decrease in rhythmicity. It is concluded that bolusevoked peristalsis of the rat esophagus is 1) segmentally organized; 2) effected by a bilateral uncrossed reflex arc consisting of vagal viscerosensory, NTSC premotor, and AMBC motoneurons innervating the striated muscle tunic and 3) strongly facilitated by reafferent feedback.

Characterization of an esophagocardiovascular reflex in the rat.

A cardiovascular reflex evoked by esophageal distension (ECR) in urethan-anesthetized male Sprague-Dawley rats was studied to 1) determine whether the relevant sensory input from the esophagus is conveyed by vagal and/or spinal afferents and 2) evaluate the effects and sites of action of antinociceptive agents. Esophageal distension evoked a rise in arterial blood pressure and heart rate that increased linearly with the log of inflation pressure (25-150 mmHg). Distension (100 mmHg for 20 s) of the lower esophagus was a more effective stimulus than distension of the upper esophagus. The ECR was attenuated by unilateral and abolished by bilateral cervical vagotomy and dose dependently inhibited by morphine (1.0-4.0 mg/kg iv) or by intrathecal (T4-T5) administration of dexmedetomidine (DX, 0.05-0.5 microgram), but not by intrathecal (T4-T5) morphine (4-16 micrograms) or intrathecal (L1-L2) or intravenous DX (0.05-0.5 microgram). The ECR was also inhibited by capsaicin and by the topical administration of DX or morphine to the solitary complex. The pressor response persisted after intravenous pancuronium, scopolamine, and methscopolamine. The ECR circuit appears to consist of vagal afferents, efferent sympathetic preganglionic pathways originating in the thoracic spinal cord, and bulbospinal neurons yet to be identified. This reflex fulfills some criteria of a nociceptive event, but this interpretation requires further investigation.

Fictive oesophageal peristalsis evoked by activation of muscarinic acetylcholine receptors in rat nucleus tractus solitarii.

The aim of the present study was to determine if muscarinic acetylcholine receptor-mediated peristaltic rhythmogenesis in the rat oesophagus is a central motor program that can be generated without peripheral sensory support. In anaesthetized male Sprague-Dawley rats, pressure-ejection of glutamate (10-20 pmol) and muscarine (5-10 pmol) in the sub-nucleus centralis of the nucleus tractus solitarii (NTSC) evoked monophasic pressure waves and rhythmic oesophageal peristalsis, respectively, but did not change mean arterial blood pressure or respiration. Application of muscarine (50-100 pmol) to the NTS extraventricular surface evoked rhythmic multi-unit burst discharges in the compact formation of the nucleus ambiguus (AMBC) that led to oesophageal peristalsis in a phase-locked manner. Evoked rhythmic AMBC activity persisted during neuromuscular blockade with curare, although the peak frequency of individual bursts was decreased. In a brainstem slice preparation, intracellular and whole cell patch recordings from AMBC neurones during focal stimulation of the NTSC region with muscarine revealed rhythmic depolarizing waves that showed a pattern similar to that of rhythmic oesophageal peristalsis. The present findings support the concept that medullary circuits comprising premotor neurones of the NTSC are intrinsically capable of generating rhythmic oesophagomotor output, but are subject to a powerful modulation by peripheral sensory feedback.

Inhibition of nicotinic cholinoceptor mediated current in vagal motor neurons by local anesthetics.

The effects of local anesthetics on ligand-gated cation channel currents were examined in rat brainstem vagal motoneurons. Etidocaine (0.1-20 microM) blocked nicotinic cholinoceptor gated currents in cells voltage-clamped at -60 mV in a concentration-dependent manner, but at concentrations up to 100 microM did not inhibit glutamate receptor currents induced by (+/-)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA), N-methyl-D-aspartate (NMDA), or glutamate. Relative to etidocaine, procaine displayed about 10-fold lower potency in antagonizing acetylcholine and its inhibitory effect, unlike that of etidocaine, was rapidly reversed by washout. Ketamine (10 microM) caused a 2-fold larger decrease in NMDA current than acetylcholine current, but did not affect AMPA current. In conclusion, (i) etidocaine and procaine possess a moderately potent blocking activity at nicotinic cholinoceptor gated channels in brainstem vagal motoneurons and (ii) in contrast with ketamine, both agents showed similar selectivity in that neither inhibited glutamate receptor gated channels at concentrations up to 0.1 mM.

Nitric oxide, a possible mediator of 1,4-dihydropyridine-induced photorelaxation of vascular smooth muscle.

1. In rat aortic tissues pre-contracted with phenylephrine, certain 1,4-dihydropyridines (DHPs) such as Bay K 8644 (0.1 microM), PN 202791 (1 microM), RK 30 (1 microM), NI 104 (1 microM) and NI 105 (1 microM) enhanced photoactivated relaxations (photorelaxation or PR) whereas NI 72, NI 85, NI 99, NI 102, amlodipine, felodipine, nifedipine and nimodipine were inactive. 2. The PR inducing effects of Bay K 8644 were mimicked by the diabetogenic agent, streptozotocin (STZ). 3. Solutions of Bay K 8644 which had been irradiated for various periods of time initiated light independent transient relaxations followed by contractile responses in aortic tissue partially contracted with phenylephrine. With exposure times to light of 30 to 120 min, the intensity of the relaxation response to irradiated Bay K 8644 increased from 26 +/- 3.3 to 71 +/- 3.7% of the maximum contractile response to phenylephrine (n = 5). Conversely the contractile responses decreased, from 84.2 +/- 4.1 to 19.8 +/- 10.4% of the maximum contractile response to phenylephrine (n = 5). 4. Superoxide ions, generated by incubation of xanthine (2mM) plus xanthine oxidase (10 mu ml-1) in physiological saline solution (PSS) NaCl 118, KCl 4.7, CaCl2 2.5, KH2PO4 1.2, MgSO4 1.2, NaHCO3 12.5 and glucose 11.1 (mM) for 1 h. reduced the PR induced by DHPs, STZ, and also NO-induced relaxations of rat aortic preparations. 5. Direct measurements of NO indicate that, following exposure to a polychromatic light source, equimolar concentrations (0.1 mM) of the DHP compounds that enhance PR, as well as STZ, photodegrade to release NO (25 +/- 2-40.3 +/- 5.9 nmol min-1, n = 6). 6. Structure-activity studies indicate that a nitro group at the -3 position of the dihydropyridine ring is essential for DHPs to support PR. 7. These data suggest that the photodegradation of DHPs and STZ leading to the release of NO provides the primary cellular process underlying the PR response.

GABA receptor-mediated inhibition of reflex deglutition in the cat.

In anesthetized cats, swallowing elicited by electrical stimulation of the superior laryngeal nerves (SLNs) was inhibited by the GABA-mimetic muscimol and by diazepam, an action that was reversed by picrotoxin and bicuculline. This inhibition supports the involvement of GABA receptors, specifically those of the GABAA subtype which both antagonists have been shown to block in various areas of the central nervous system. The inhibition of reflex swallowing and its reversal were unaltered by a transection of the brainstem at a midcollicular level. Stimulation of the SLNs also caused a bradycardia that was inhibited by both muscimol and diazepam and was restored by both GABA antagonists. Data from these experiments provide suggestive evidence for a role of GABA-ergic transmission in the central control of the deglutitory reflex.

Photosensitization of oesophageal smooth muscle by 3-NO2-1, 4-dihydropyridines: evidence for two cyclic GMP-dependent effector pathways.

1. Photoactivated mechanical responses that resulted from exposure to 3-NO2-1,4-dihydropyridines (3-NO2-DHP5) or NO-donors were examined in rat isolated oesophageal smooth muscle with a view to determining the role of calcium and cyclic GMP. 2. Isometric contractile force was recorded in preparations bathed in normal Tyrode or 110 mM K(+)-depolarizing solution. Exposure to (+)-PN 202791, (+/-)-Bay K 8644 and (-)-PN 2020791 or the photodegradable NO-donors, sodium nitroprusside (SNP), streptozotocin (STZ) and sodium nitrite photosensitized precontracted tunica muscularis mucosae preparations in a concentration-dependent fashion. Photosensitizing potency followed the order: (+/-)-PN 202791 > (+/-)-Bay K 8644 > (-)-PN 202791 > SNP > STZ > NaNO2. 3. A low amplitude, slow photorelaxation (slope: 1 mg s-1) was obtained with the L-channel antagonists (-)-PN 202791 and (+)-Bay K 4407. Photosensitization by the agonist enantiomers (+)-PN 202 791 and (-)-Bay K 5407, as well as racemic Bay K 8644, was mimicked by NO donors and showed at least three different components, consisting of (i) a fast relaxation (slope: 140 mg s-1), (ii) a fast "off-contraction', and (iii) a delayed slow relaxation. The fast components, but not the delayed slow relaxation, were abolished by blockade of L-type voltage-operated calcium channels, chelation of extracellular calcium and skinning of the plasmalemma, suggesting their mediation by a process linked to calcium entry through L-channels. 4. Both cyclopiazonic acid (3-30 microM) and ryanodine (30 microM) inhibited the fast response. This inhibition was accelerated in the presence of extracellular calcium and resembled that seen in tissues exposed to the calcium ionophore A 23187 (1 microM). In calcium depleted tissues, cyclopiazonic acid (3 microM) prevented restoration of the cis-dioxolane-induced contraction following re-exposure to a calcium containing high K+ buffer, but failed to inhibit the photoresponse. 5. Both the fast and slow relaxations were potentiated by zaprinast (10 microM) and inhibited by LY B3583 (10 microM). However, in calcium-depleted, calyculin A-precontracted preparations only the slow relaxation was evident. 6. The present results support the conclusion that: (i) functional L-channels are required for the expression of the fast components of the 3-NO2-DHP- or NO-donor-induced photoresponse, (ii) NO photorelease followed by activation of soluble guanylyl cyclase is responsible for the photosensitizing activity of 3-NO2-DHPs and (iii) regulation of the contractile proteins via cyclic GMP-dependent phosphorylation may underlie the slow relaxation.

[Morphine blocked the exitatory amino acid mediated membrane current in ambigual motoneurons of the rat].

Excitatory postsynaptic potentials (EPSPs) and responses of neurons in the compact formation of the neucleus ambiguus (AMBc) to pressure-ejected or bath-applied test substances were recorded intracellularly from sagittal slices of Sprague-Dawley rat medulla containing subnucleus centralis of solitary complex (NTSc), AMBc and solitarioambigual pathway. In 5 cells, recorded spontaneous EPSPs could be blocked by morphine (3-5 pmol) to AMBc. Electrical stimulation of NTSc evoked EPSPs in AMBc neurons, the amplitude of which were decreased to 71.1 +/- 6.2% (P < 0.001) with the addition to morphine. The morphine effect could be abolished by bath-applied naloxone (50 nmol/L). The amplitude of membrane depolarization induced by pressure-ejected N-methyl-D-aspartate (NMDA, 0.5-1.0 pmol), acetylcholine (3 pmol) and quisqualate (0.1-0.5 pmol) to NTSc could also be decreased by bath-applied morphine (10 mumol/L) respectively to 38.1 +/- 5.7% (P < 0.001), 32.8 +/- 5.5% (P < 0.01) and 29.6 +/- 7.1% (P < 0.05). The above results suggest that morphine is capable of block by NMDA, ACh and non-NMDA receptors, increaseing the M-current and decreasing the permeability of Na+ and Ca2+.

Somatostatin regulates excitatory amino acid receptor-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.

Central nervous system control mechanisms of swallowing: a neuropharmacological perspective.

Neuropharmacological in vivo and in 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 MACh 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.

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 microM; n = 2) and blocked by dihydro-beta-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-beta-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.