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.

Interactions of gastric vagal and peripheral nerves on single neurons of lateral hypothalamus in the cat.

Extracellularly recorded unitary responses in the lateral hypothalamus were evaluated in anesthetized cats during electrical stimulation of the gastric branches of the ventral and dorsal vagal trunks, the T9 intercostal nerve, and the common peroneal nerve (L6-S2). These nerves were stimulated with single or paired (10-ms interval) pulses of 300-500 microA for 0.3 ms at a frequency of 0.5 Hz. The latency of the evoked lateral hypothalamic responses after stimulation of the gastric vagal branches (373 +/- 39.8 ms; means +/- SD) was significantly longer than the latencies after intercostal nerve (62 +/- 17.0 ms) or common peroneal nerve (109 +/- 29.3 ms) stimulation. Convergence of gastric vagal input from the proximal stomach and peripheral nerves (PNs) on single neurons in the lateral hypothalamus was observed. Ninety-two percent of the lateral hypothalamic neurons tested that responded to gastric vagal stimulation also received inputs from the T9 intercostal nerve. Seventy-seven percent of the lateral hypothalamic gastric vagally evoked unitary responses received convergent inputs from the intercostal nerve and the common peroneal nerve. A condition-test paradigm was applied to determine the time course of convergent gastric and PN input on single lateral hypothalamic neurons. The test revealed that stimulation of the T9 intercostal nerve had a more pronounced effect than common peroneal nerve stimulation on the lateral hypothalamic neurons that receive gastric vagal input. The results demonstrated that gastric vagal afferent and PN inputs converge onto single lateral hypothalamic neurons and suggested that the central processing of visceral input from the stomach can be substantially affected by peripheral nerve stimulation.

Superconducting open-configuration MR imaging system for image-guided therapy.

PURPOSE: To develop a superconducting magnetic resonance (MR) imager that provides direct access to the patient and permits interactive MR-guided interventional procedures. MATERIALS AND METHODS: A 0.5-T superconducting magnet that allows a region of vertical access to the patient was designed and constructed. This magnet was integrated with newly designed shielded gradient coils, flexible surface coils, and nonmagnetic displays and with position-monitoring probes and device-tracking instrumentation. RESULTS: The magnet homogeneity was 12.3 ppm, and the gradient field was linear to within 1% over an imaging region 30 cm in diameter. The signal-to-noise ratio was 10% higher than in a comparable 0.5-T superconducting imager. Images were obtained in several anatomic regions with use of routine pulse sequences. Interactive image plane selection and near real-time imaging, with use of fast gradient-recalled echo sequences, were demonstrated at a rate of one image every 1.5 seconds. CONCLUSION: MR-guided interventional procedures can be performed with full patient access with use of an open-configuration, superconducting MR magnet with near real-time imaging and interactive image plane control.

In vitro brainstem-gastric preparation with intact vagi for study of primary visceral afferent input to dorsal vagal complex in caudal medulla.

An in vitro neonatal rat preparation, consisting of the isolated caudal brainstem and stomach joined by the intact vagi, was developed using Sprague-Dawley rats. The animals were 0 to 4 days of age. This preparation provided an opportunity to investigate the extracellular and intracellular responses of neurons in the nucleus tractus solitarius (NTS) of the brainstem to electrical stimulation of subdiaphragmatic vagal fibers. The dorsal and ventral vagal branches were electrically stimulated at the point of the common subdiaphragmatic vagal trunk. The isolated preparation was superfused in a recording chamber at 28 degrees C with a modified Krebs solution, equilibrated with 95% O2 and 5% CO2. Suction microelectrodes, for electrical stimulation, were positioned on the common vagal trunk just below the diaphragm to evaluate extracellular and intracellular evoked responses in NTS. A total of 204 subdiaphragmatic vagally-evoked (SDVe) brainstem unitary responses in the NTS were recorded. The mean latency of the extracellular SDVe brainstem responses was 89 +/- 12.9 ms (mean +/- SD). The peripheral gastric effects of CCK-8 on SDVe unitary responses in NTS neurons were evaluated. The peptide caused a significant increase in the excitability of these NTS neurons which was blocked by the CCKA receptor antagonist L-364,718. Neurons in the NTS and the dorsal motor nucleus of the vagus which showed excitatory responses to vagal stimulation were filled with Lucifer Yellow to evaluate their morphology.

Gastric vagal-evoked and greater splanchnic-evoked unitary responses in the hypothalamus.

Gastric vagal and greater splanchnic nerve fibers were electrically stimulated to localize and characterize neuronal interactions in the hypothalamus of anesthetized cats. Extracellular recordings from 635 hypothalamic units were identified that responded to electrical stimulation of the left greater splanchnic nerve or gastric vagal fibers serving the proximal stomach. A total of 504 hypothalamic units in this group received input from both gastric vagal and greater splanchnic nerves. The gastric vagal-evoked hypothalamic (GVeH) and greater splanchnic-evoked hypothalamic (SeH) responses were widely distributed in the medial, paraventricular, and dorsomedial nuclei and lateral hypothalamus. The conduction velocity of the SeH response was significantly greater than the GVeH response. The latency of the SeH response showed two peaks [58 +/- 15.7 (SD) ms and 136 +/- 18.3 (SD) ms] indicating that the splanchnic input terminated on two different groups or populations of hypothalamic neurons. It also suggested that different pathways or fiber diameters in the pathway may be involved in the transmission of splanchnic input to the hypothalamus. The majority of the GVeH and SeH unitary responses were multiple spikes or short trains of action potentials. Excitatory and inhibitory responses were observed in tonically active hypothalamic units that responded to gastric vagal or greater splanchnic input. The duration of decreased excitability to gastric vagal or greater splanchnic input was significantly greater than the period of increased excitability. The condition-test paradigm was used to determine the time course of convergent gastric vagal-greater splanchnic input on single hypothalamic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Area postrema: gastric vagal input from proximal stomach and interactions with nucleus tractus solitarius in cat.

The responses of neurons in the area postrema (AP) during electrical stimulation of the gastric vagal branches that serve the proximal stomach were evaluated in halothane-anesthetized cats. The evoked orthodromic responses were localized bilaterally in the AP and had a mean latency of 272 ms (SD +/- 53.1). Convergence of gastric vagal input on single AP units from afferents in the ventral and dorsal vagal trunks serving the proximal stomach was observed on 13% of gastric vagally evoked neurons. Reciprocal connections between neuronal populations in the nucleus tractus solitarius and the AP were identified electrophysiologically by direct microstimulation of the former structure. Spontaneously AP discharging units showed an increase in frequency after NTS microstimulation. Our study provided evidence that the AP neurons received vagal input from the proximal stomach and suggested a potential role of the AP in the regulation of food intake.

Hypothalamic unitary responses to gastric vagal input from the proximal stomach.

Gastric vagally evoked extracellular unitary responses were recorded in the hypothalamus of anesthetized cats. The evoked unitary responses were localized in the paraventricular dorsomedial region, ventromedial nucleus, and lateral hypothalamus. The mean latency of the gastric vagally evoked hypothalamic neuronal responses in these three areas ranged from 368 +/- 39.8 to 371 +/- 45.2 (SD) ms. The majority (82%) of the gastric vagally evoked hypothalamic responses consisted of one to five spikes, while the remaining 18% were tonically active units. The vagal effect was inhibitory in 78% of the tonically active hypothalamic units responding to gastric vagal input. Convergence of gastric vagal input on single hypothalamic units from afferents in the dorsal and ventral vagal trunks was observed. Units were identified in the hypothalamus that responded to activation of mechanoreceptors in the proximal stomach by an intragastric balloon. This study provided new direct evidence of the density, localization, and characteristics of neuronal processing of gastric vagal input from the proximal stomach in the hypothalamus. The reciprocal connections between these areas of the hypothalamus and nucleus tractus solitarius in the caudal brain stem suggest that the hypothalamus may serve an important role in modulating the input of primary vagal afferent input from the proximal stomach.

Parabrachial nucleus: neuronal evoked responses to gastric vagal and greater splanchnic nerve stimulation.

Unitary responses were recorded extracellularly in the parabrachial nucleus (PBN) in anesthetized cats during electrical stimulation of the 1) gastric branches of the ventral and dorsal vagal trunks which serve the proximal stomach, and 2) left greater splanchnic nerve. The gastric vagally evoked parabrachial responses consisted of phasic single and multiple spike orthodromic discharges, which were bilaterally distributed, with a mean latency of 349 ms (S.D. +/- 38.5). The parabrachial-evoked splanchnic unitary responses had a much shorter latency with a bimodal distribution (mean latencies, 53 and 128 ms, respectively). Convergence of gastric vagal input from the proximal stomach and the left greater splanchnic nerve upon single neurons in the PBN was electrophysiologically demonstrated in 132 units. Eighty-seven percent of the gastric vagally evoked parabrachial unitary responses were inhibited by simultaneous electrical stimulation of the splanchnic nerve. The condition-test paradigm was used to evaluate the time course of the splanchnic inhibition of the gastric vagally evoked parabrachial response. Reciprocal connections between neuronal populations in the nucleus tractus solitarius (NTS) which received gastric vagal input and the PBN were also identified electrophysiologically by direct microstimulation of the former structure. The density and characteristics of the gastric vagal and greater splanchnic input to the PBN suggested that this nucleus receives and processes a substantial amount of visceral afferent input. The PBN may serve as an important site for integrating visceral information governing the proximal stomach and ingestive processes.

Units in tegmental nuclei responding to stimulation of gastric vagal and greater splanchnic fibers in the cat.

The response of neurons in the ventral and dorsal tegmental nuclei during electrical stimulation of the gastric vagal fibers which serve the proximal stomach and the left greater splanchnic fibers were evaluated in chloralose-anesthetized cats. The mean latency of 181 gastric vagally evoked unitary responses recorded in the tegmental nuclei was 352.2 ms, whereas the latency of the left greater splanchnic-evoked tegmental response was significantly less (63.2 ms). The unitary responses to the gastric vagal and greater splanchnic fibers stimulation were bilaterally distributed in the ventral and dorsal tegmental nuclei. Convergence of the gastric vagal input from the proximal stomach and the left greater splanchnic input was observed in 151 units (83 percent). Stimulation of the greater splanchnic nerve usually resulted in a short latency excitation followed by an inhibitory effect on gastric vagally evoked responses. The results suggested that some convergent splanchnic inhibition of gastric vagally evoked responses was mediated via an interneuron. Projections from the nucleus tractus solitarius and the parabrachial nucleus to the tegmental nuclei were also identified electrophysiologically by direct microstimulation of the two former areas. The significant number of gastric vagal and splanchnic evoked unitary responses recorded in the ventral and dorsal tegmental nuclei suggested that they may serve as an important pontine site for processing of visceral information between the nucleus tractus solitarius and forebrain sites.

Brainstem evoked response to dorsal vagal gastric input from the proximal stomach.

Unitary responses to electrical stimulation of gastric vagal fibers, that serve the proximal stomach and join the dorsal vagal trunk, were bilaterally distributed in nucleus tractus solitarus (NTS) in the brainstem of the cat. The vagally activated brainstem responses consisted of single or multiple spikes with a mean latency of 281 (SD +/- 43 ms) which translated into a conduction velocity of less than 1 m/s. Ninety-eight percent of the responses were orthodromic in character. Twenty percent of the gastric vagally-evoked brainstem unitary responses received input from both the dorsal and ventral vagal trunks. Excitatory and inhibitory convergent interactions were observed in the brainstem units receiving gastric input from both the dorsal and ventral vagal trunks.

Vagal interactions on brain stem neurons receiving input from the proximal stomach in cats.

Gastric vagal fibers on the proximal stomach that join the dorsal and ventral vagal trunks were electrically stimulated to localize and evaluate brain stem neuronal interactions in anesthetized cats. The brain stem responses were located in nucleus tractus solitarius in the dorsomedial, caudal region of the medulla oblongata. There was no significant difference in the mean latency of the gastric vagally evoked brain stem response between the dorsal and ventral vagal trunks. The responses consisted of single or multiple spikes with a mean latency of approximately 290 +/- 50 (SD) ms. Forty-one percent, or 168 unitary responses of the 406 total responses recorded, showed convergence of proximal gastric vagal input from both the dorsal and ventral vagal trunks on the same recording site or on the same cell. Of those unitary responses that received convergent proximal gastric vagal input, 95 unitary responses (57%) showed convergence of input to the same area, on different cells at the same recording site during a single trial. Seventy-three single units (43%) received convergent input from proximal gastric vagal afferent fibers in both the dorsal and ventral trunks. Fifty-two, or 71%, of the single unit convergent responses were excitatory in nature, whereas the remaining 29% were inhibitory. These data demonstrated that proximal gastric vagal afferent fibers that join the dorsal and ventral trunks converged on a significant number of single neurons in the brain stem. The convergent response was synaptically secure and exerted an identifiable biasing effect on the response of the brain stem neuron.(ABSTRACT TRUNCATED AT 250 WORDS)

Brain stem responses to electrical stimulation of ventral vagal gastric fibers.

The brain stem neuronal responses to electrical stimulation of gastric branches of the ventral vagal trunk serving the proximal stomach were localized and evaluated in anesthetized cats. The responses were equally distributed bilaterally in the region of nucleus solitarius in the caudal brain stem. The mean latency of the response was 289 +/- 46 (SD) ms, which translated into a conduction velocity of less than 1 m/s based on the distance between the stimulating and recording electrodes. The responses consisted of single and multiple spikes that showed slight variability in the latency, indicating orthodromic activation via a synapse in approximately 98% of the responses recorded. Forty two percent of the units tested showed evidence of convergence of input from vagal afferent fibers in different branches of the ventral vagal trunk that served the proximal stomach. The resultant activity pattern of the unitary response appeared to be the product of 1) the gastric sensory input or modality conveyed by the afferent source and 2) the time of arrival and diversity of modalities served by other gastric afferents impinging on the unit. This provides a mechanism capable of responding on the basis of specific sensory modalities that dynamically reflect ongoing events monitored and conveyed by other gastric afferents in the region.

Gastric vagal-splanchnic interactions in the brainstem of the cat.

Brainstem unitary responses to gastric vagal-splanchnic nerve interactions were evaluated in anesthetized cats during electrical stimulation of the nerves. The gastric branches of the dorsal and ventral vagal trunks which serve the proximal stomach were electrically stimulated while recording in nucleus solitarius in the brainstem to identify evoked unitary responses. The vagally evoked brainstem responses were orthodromic in nature as evidenced by a slight variability in latency, reduction in the number of spikes upon decreasing stimulus strength and failure to follow higher stimulus frequencies. Interactions between gastric vagal input from the proximal stomach and left greater splanchnic nerve were evaluated by simultaneously stimulating the nerves electrically. Forty-three (16%) of the 265 gastric vagally evoked brainstem responses recorded were inhibited by simultaneously activating the left greater splanchnic nerve. The gastric vagally evoked responses inhibited by splanchnic input were nearly equally divided between the dorsal and ventral vagal trunks. Eighty-eight percent of those vagally evoked brainstem units inhibited by splanchnic input showed complete inhibition suggesting secure synaptic coupling. In addition, 3 gastric vagally activated brainstem units were identified which were also activated by greater splanchnic input. The latency of the splanchnic evoked brainstem response in these 3 units ranged from 55 to 89 ms compared to a mean latency of 290 ms (S.D. +/- 50 ms) for the vagally evoked brainstem response. Splanchnic electrical stimulation usually produced an inhibitory effect upon the vagally evoked brainstem response which persisted for approximately 30 s following termination of splanchnic stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

The response of brainstem neurons to chemical activation of gastric sensory receptors.

Sensory receptors have been identified in the proximal stomach of the cat that respond to mechanical and chemical stimuli. The neurotransmitters or neuromodulators that underlie the brainstem excitatory responses initiated by gastric mechanical and chemical stimuli may provide a very important link in the control of obesity.

Brain-gut interactions: brain stem neuronal response to local gastric effects of substance P.

Single-unit activity that responded to phasic gastric distension was recorded extracellularly from neurons in brain stem of anesthetized cats during local substance P chemically induced changes in wall tension. Local gastric intra-arterial administration of substance P via splenic artery often produced a triphasic gastric response. Gastric changes were characterized by 1) an initial brief increase in distension of corpus immediately after peptide and 2) a subsequent contraction that gave way to 3) a prolonged late increase in distension exceeding control levels. The contraction phase was atropine sensitive, suggesting that one mechanism of action during this phase was linked to cholinergic enteric nerves. Distension phases were unaffected by atropine, suggesting a different mechanism of action. Increase in gastric wall tension after peptide resulted in 1) onset of or enhanced activity of brain stem unit during nondistending phase and 2) greater spike discharge per unit change in volume during distending phase in many neurons. Some neurons showed a brief flurry of tonic activity during distending and nondistending phases of the cycle after local gastric injection of peptide with no observable change in wall tension. This suggests that the peptide may also act on chemoreceptors served by vagal primary afferents, which impinge on this neuronal population in the brain stem. Less than 10% of the neurons showed no change in discharge rate after a significant increase in wall tension, which occurred after local gastric injection of substance P, suggesting a mechanism of action involving input from primary vagal afferent fibers serving mucosal receptors unaffected by this level of change in wall tension.(ABSTRACT TRUNCATED AT 250 WORDS)

Tachykinins: local gastric effects and brain stem responses.

The gastric motor or mechanical effects of a group of peptides, the tachykinins, were evaluated in anesthetized cats to determine the relationship between local motor events and brain stem neurons that regulate gastric activity. The peptides evaluated were substance P, physalaemin, and eledoisin. The tachykinin-induced gastric changes were dose related and were characterized by initial distention-sustained contraction-late distention phases. At lower doses distention was the dominant effect with a sustained contraction-late distention response appearing as the dose increased. The sustained contraction-late distention phases were frequently accompanied by phasic contractions with a frequency of 2-4/min. Atropine had a significant effect on the sustained contraction phase but no effect on the phasic contractions or distention phases. Bilateral cervical vagotomy had a significant effect on the early distention phase, suggesting a link with brain stem mechanisms. The activity of brain stem units that responded to phasic distention of the stomach reflected the tachykinin-induced changes in gastric distention. Although the gastric effects of these tachykinins shared distinct similarities, certain differences in the time sequence of the distention-contraction interactions suggests the possibility that dissimilar receptor types may be involved in the mechanisms of action. Their mechanisms of action may also involve a direct effect on the effector organ.

Regulation of gastric emptying.

Studies carried out in the years since William Beaumont's direct observations of gastric motility have provided increased understanding of the physiological roles of the stomach and of the mechanisms for the regulation of gastric motility. Tonic contractions of the proximal stomach are of primary importance for transfer of liquids from the stomach to the duodenum. Peristaltic contractions of the distal stomach are of primary importance for reducing the size of solid food particles and for transfer of solids to the duodenum. Because gastric emptying requires a net antral-duodenal pressure gradient, contractions of the duodenum also influence the rate of gastric emptying. Gastrointestinal hormones, including gastrin, cholecystokinin, secretin, somatostatin, and others, are released by contact of chyme with the intestinal mucosa, and affect contractions of the proximal stomach, distal stomach, and duodenum. Neural reflexes that arise from the stomach act through autonomic motor nerves to allow regulation by the central nervous system of gastric motility. gamma-Aminobutyric acid, opioids, and bombesin may serve as central neurochemical regulators of gastric motility.

Brain stem response to phasic gastric distension.

To explore the physiological characteristics of central projections of gastric afferent fibers, single-unit activity was recorded extracellularly from neurons in the brain stem during phasic distension of the stomach in cats anesthetized by halothane and nitrous oxide. The brain stem units were identified on the basis of discharge patterns that were phase locked to distension or relaxation of the stomach. Based on stereotaxic coordinates, the units responding to the distension stimulus were located in the region of nucleus and tractus solitarius and extended ventrally toward nucleus ambiguus. Both phasically and tonically discharging units were identified, which exhibited either an excitatory or inhibitory response during distension of the stomach at pressures of 6-8 cmH2O. Units excited by distension increased their firing rate or began to discharge near the peak of distension at rates of 4-6 Hz. Other units responded only during the phase when the distending pressure was decreasing. Sectioning the vagi abolished the response of these brain stem units to gastric distension. The origin of the stimulus that initiated the unit response was localized in the fundus and body of the stomach in many instances. The results indicate that neuronal activity in discrete areas of the brain stem of the cat is altered in response to phasic gastric distension and that this response is vagally mediated.