Abdominal pathways and central origin of rat vagal fibers that stimulate gastric acid

The brainstem location and peripheral course of the vagal preganglionic fibers that stimulate gastric acid secretion were identified using electrical stimulation combined with retrograde (True Blue; Dr. K. G. Illing, Gross Umstadt, Germany) and anterograde (Dil; Molecular Probes) fluorescent neural tracers in rats with various selective vagotomies. Animals with only one or both gastric branch(es) spared had normal, large gastric acid responses to electrical stimulation of the ipsilateral cervical vagus and showed an abundance of Dil-labeled vagal fibers and terminals in the gastric myenteric plexus. Rats with only the unpaired hepatic branch spared had a much smaller but significant gastric acid response and a few labeled vagal profiles in the antral region of the stomach. In contrast, rats with only one or both celiac branch(es) intact had neither a gastric acid response, nor evidence for Dil transport to the stomach. Retrograde transport of True Blue through the spared vagal axons to the brainstem indicated that the cell bodies of the preganglionics that send their axons through the acid-positive gastric and hepatic branches occupy the medial longitudinal columnar subnuclei of the dorsal motor nucleus. It is concluded that besides the long-recognized gastric branches, which are the major access route to the parietal cells, the hepatic branch contains a small number of fibers that most likely reach the antrum through the right gastroepiploic artery along the greater curvature and/or the right gastric artery.

Topography of efferent vagal innervation of the rat gastrointestinal tract

The gastrointestinal territories innervated by the gastric, celiac, and hepatic abdominal vagi were identified in rats with selective branch vagotomies by means of 1) anterograde tracing with the carbocyanine dye DiI injected into the dorsal motor nucleus and 2) measurement of cervical vagal stimulation-induced motility responses throughout the gut axis. Presence of DiI-labeled vagal terminals in the myenteric plexus and evoked motility responses were well correlated across the sampled gastrointestinal (GI) sites. In animals with only the two gastric branches intact, the entire stomach and the most proximal duodenum showed significant motility responses and were densely innervated, having DiI-labeled vagal terminals in almost every ganglion. The hepatic branch was found to primarily innervate the duodenum, with minor projections to the distal antral stomach and the intestines. The two celiac branches were found to almost exclusively innervate the jejunum, ileum, cecum and entire colon, and, together with the other vagal branches, the duodenum. Therefore, while there is some degree of specific innervation by the abdominal vagal branches of the oral-to-anal gut axis, which could be called "viscerotopic," the considerably overlapping innervation of the duodenum does not satisfy a viscerotopy criterion and needs further functional analysis.

Simultaneous labeling of vagal innervation of the gut and afferent projections from the visceral forebrain with dil injected into the dorsal vagal complex in the rat

The vagal innervation of the different layers of the rat gastrointestinal wall was identified with the fluorescent carbocyanine dye Dil, injected into the dorsal motor nucleus of the vagus (dmnX). Multiple, bilateral injections were used to label all dmnX preganglionic motoneurons, and as a consequence, most of the vagal primary afferents that terminate in the adjacent nucleus of the solitary tract (nts) were also retrogradely and transganglionically labeled. With Fluorogold used to label the enteric nervous system completely and specifically, the Dil-labeled vagal profiles could be visualized and quantified in their anatomical relation to the neurons of the myenteric and submucous ganglia. In the myenteric plexus, vagal fibers and terminals were found throughout the gastrointestinal tract as far caudal as the descending colon, but there was a general decreasing proximodistal gradient in the density of vagal innervation. All parts of the gastric myenteric plexus (fundus, corpus, antrum), as well as the proximal duodenum, were extremely densely innervated, with vagal fibers and terminals in virtually every ganglion and connective. Further caudally, both the percentage of innervated myenteric ganglia and the average density of label within the ganglia rapidly decreased, with the exception of the cecum and proximal colon, where up to 65% of the ganglia were innervated. In the gastric and duodenal submucosa very few and in the mucosa no vagal fibers and terminals were found. With both normal epifluorescence and laser scanning confocal microscopy, highly varicose or beaded terminal structures of various size and geometry could be identified. The Dil injections, which impregnated the dmnX as well as the adjacent nts, resulted in retrograde and anterograde labeling of all the previously reported forebrain connections with the dorsal vagal complex. We conclude that the myenteric plexus is the primary target of vagal innervation throughout the gastrointestinal tract, and that its innervation is more complete than previously assumed. In contrast, vagal afferent (and efferent) innervation of mucosa and submucosa seems conspicuously sparse or absent. Furthermore, the use of more focal injections of Dil offers the prospect to simultaneously identify specific subsets of vagal preganglionics and their central nervous inputs.

Identification of vagal preganglionics that mediate cephalic phase insulin response

To define the vagal circuitry mediating the cephalic phase insulin response (CPIR), this reflex was measured in conscious, freely moving rats that had previously undergone selective abdominal vagotomies that spared different columnar subpopulations of dorsal motor nucleus of the vagus (dmnX) neurons. The CPIR was defined as an increase of plasma insulin from basal at 2 min after the start of ingestion. The CPIR measured in peripheral blood after chow ingestion was reliable and significant (P less than 0.05) in rats with all branches intact, +24.9 +/- 5.1 microU/ml (+130% increase from basal); rats with only the two gastric branches and the hepatic branch intact, +27.0 +/- 3.5 microU/ml (+153%); and rats with only the hepatic branch intact, +13.5 +/- 4.8 microU/ml (+188%). No significant response occurred in animals with only the two celiac branches intact, +1.8 +/- 1.8 microU/ml (+15%) or in those with none of the branches intact, +3.9 +/- 3.3 microU/ml (+21%). The CPIRs measured in portal vein blood were generally larger but showed the same pattern across groups. Plasma glucose measurements of portal vein blood indicated that with chow ingestion no significant absorption had occurred by 2 min, whereas with either milk or glucose intake absorption did occur. Subsequent bilateral electrical cervical vagal stimulation-induced insulin and glucagon responses in the same animals under anesthesia showed the same branch dependency. It is concluded that the CPIR is mediated by the two gastric and the hepatic branches but not the two celiac vagal branches. The perikarya of the preganglionics innervating the pancreatic B-cells are contained within a large pool occupying the two medial columns of the dmnX.

Localization of vagal preganglionics that stimulate insulin and glucagon secretion

Although it is generally acknowledged that pancreatic islets are under powerful vagal control, specifics of vagal pathways and their central representation in the brain stem are unclear. To define this circuitry, we combined a protocol measuring electrical vagal stimulation-induced insulin and glucagon secretion with a retrograde tracer strategy that delineated the pool of spared motoneurons in the dorsal motor nucleus of the vagus (dmnX) following selective abdominal branch vagotomies. Three of the five branches mediated both insulin and glucagon release: posterior gastric (+198 and +117% increase from basal for insulin and glucagon, respectively), anterior gastric (+177 and +104%), and hepatic branch (+103 and +60%). In contrast, unreliable and nonsignificant hormonal responses were produced by stimulation of fibers projecting through either the posterior celiac (+12% insulin and +12% glucagon) or accessory celiac (+15% insulin and +31% glucagon) branches. Since hexamethonium almost completely blocked both insulin and glucagon responses to stimulation, the effects are not likely to have resulted from inadvertent antidromic excitation of vagal afferents. Cell bodies of stimulated motoneurons, which were responsible for insulin and glucagon secretion, were found to occupy the medial two-thirds of the right (projecting through the posterior gastric branch) and left (projecting through the anterior gastric and hepatic branches) dmnX. These medial, longitudinal dmnX columns and their associated abdominal vagal branches are likely to play the predominant role in vagal control of the endocrine pancreas.

Retrograde tracer technique for assessment of selective and total subdiaphragmatic vagotomies

A new protocol that provides a sensitive, reliable, and practical test for completeness of selective as well as total subdiaphragmatic vagotomies is described. This protocol employs a microscopic inventory of retrogradely labeled neurons in topographically distinct regions of the dorsal motor nucleus to determine which vagal branches have been surgically destroyed. Physiological experiments for validation and observations on the use of the method with 243 rats indicate that the protocol described can assess total as well as at least 11 different types of selective subdiaphragmatic vagotomies, including surgeries for which no assays have existed. Furthermore, the technique can identify cases where a branch is only partially destroyed. Other strengths include the facts that the protocol provides a simultaneous inventory of the different branches in a single test, is not influenced by the general health of the animal, and does not interfere with concurrent behavioral or physiological tests. Limitations include the facts that the tracer inventory requires a minimal survival period, can only be done postmortem, and has low resolution for cuts of the vagal hepatic branch. Aspects of the protocol critical to its implementation, including specifics for using the fluorescent tracer true blue, are discussed. Other tracers with similar diffusion characteristics, such as fluoro-gold and fast blue, can be used with equal effectiveness with this protocol.

Characteristics of gastric and pancreatic responses to vagal stimulation with varied frequencies: evidence for different fiber calibers?

In order to explore the possibility that different-sized vagal motor fibers innervate different abdominal targets as well as to identify efficient stimulation parameters for future experimentation, response characteristics to various stimulus frequencies were determined for 6 visceral responses caused by vagal nerve stimulations. Gastric acid secretion, plasma levels of insulin, glucagon and glucose, as well as heart rate and mean arterial blood pressure were monitored in anesthetized rats subjected to 1-16 Hz square wave electrical stimulation of either the left or right cut cervical vagus. The frequency-response curves evidenced distinctly different profiles for the gastric, pancreatic, and cardiovascular responses respectively: acid secretion was half-maximal at less than 1 Hz, insulin and glucagon responses were half-maximal at approximately 3 Hz, and cardiovascular responses were shifted still more to the right (half maximal frequency approximately 15 Hz). These results suggest but do not prove that the gastric parietal cells may be innervated by small C-fiber caliber axons and the pancreatic islets by axons in the large C-fiber or small B-fiber range. Alternatively, these findings could reflect differences in neuro-effector couplings of the two organs. Furthermore, the present results provide an experimental technique that makes it feasible to select a frequency that will maximize one of the vagally mediated responses or minimize the activation of a second response.

Vagal stimulation-induced gastric acid secretion in the anesthetized rat

The dynamics of gastric acid secretion induced by electrical stimulation of the cervical vagus in the anesthetized rat were investigated using a continuous collection-titration system permitting high temporal resolution. Stimulation with pulse rates of 2 or 4 impulses/s (pps) produced maximal gastric acid responses with small cardiovascular effects. With continuous stimulation, secretion was sustained for at least 1 h. Frequency-response profiles suggested that the parietal cells are innervated predominantly by fine-caliber C-fibers. Continuous stimulation was 3 times as effective as stimulation in bursts of higher frequencies. The minimal latency for the onset of secretion was 2.6 min at 4 pps, however, one- and two-min stimulations still produced proportionate but delayed secretory responses. It is concluded that, with low frequency cervical vagus stimulation, the rat stomach preparation described and employed in the present experiment is a useful model for further studies on the interaction of neural and humoral factors on gastric acid secretion.

A method for large volume blood sampling and transfusion in rats

A new protocol that makes it feasible to withdraw large volumes of whole blood from an individual rat within 1 h or less is described. This method involves the use of indwelling catheters for withdrawal of blood from the inferior vena cava with concurrent isovolemic replacement of whole blood into the superior vena cava. Simultaneity of the transfusion and withdrawal, strict equality of volumes, and a smooth exchange of blood are assured by the use of separate channels of the same multiple-channel peristaltic pump for withdrawal and replacement. Validation experiments using both anesthetized and unanesthetized rats indicate that several responses remain essentially undisturbed during large volume blood sampling; these parameters include blood pressure, heart rate, hematocrit, plasma hormones including insulin and glucagon, plasma glucose levels, and feeding behavior. Considerations of technical and physiological limitations of the protocol, including choice of catheters and catheter placement, pump, sampling parameters, and obtaining donor blood, are discussed.

Diet and cephalic phase insulin responses

Cephalic phase digestive responses may be particularly critical in determining our various reactions to different diets, since these responses are the first physiological adjustments to food. The potential importance of the cephalic responses is also underscored by the fact that many of the most important food attributes for humans--color, appearance, flavor, aroma, and texture--can influence the individual's gastrointestinal physiology solely by affecting these early metabolic responses. The present survey examines in some detail the data available for one of the responses, the cephalic phase insulin response. Specific shortcomings of the existing analyses are discussed. In addition, given the possible significance of these reflexes, several suggestions for improvements of experimental protocols are considered, and a summary of major experimental questions is provided.

Altered plasma insulin and glucose after obesity-producing bipiperidyl brain lesions

A single systemic injection of bipiperidyl mustard (BPM) in the adult rat produces brain lesions and associated obesity without hyperphagia. To characterize some endocrine-metabolic aspects of the BPM preparation we measured plasma insulin and glucose dynamics as well as glucoprivic feeding. BPM-treated animals with verified lesions of the medial portion of the solitary tract nucleus (NTS) and the medial pole of the dorsal motor nucleus of the vagus (DMNX), as well as small lesions affecting the arcuate nucleus and basomedial portion of the ventromedial nucleus of the hypothalamus, showed the following characteristics: normal basal glycemia and insulinemia, exaggerated plasma insulin responses to oral or intravenous glucose and to oral saccharin, increased plasma glucose levels after oral glucose, unimpaired feeding to 2-deoxy-D-glucose challenge, decreased short-term intake of highly palatable food, and 36% more body fat at the end of the experiment. None of these changes occurred in rats that failed to develop lesions after BPM administration. These results suggest that BPM lesions (which appear to overlap distributions of central insulin binding sites) both affect a central mechanism controlling the pancreatic beta-cells and possibly influence gastric emptying and/or intestinal glucose absorption.

The relative contribution of the nervous system, hormones, and metabolites to the total insulin response during a meal in the rat

An attempt was made to quantitatively determine the relative contribution of the nervous system, hormonal factors, and metabolites to the total peripheral plasma insulin response (integrated incremental area) during a ten-minute liquid meal in conscious freely moving rats. The neurally mediated insulin response, as measured in the gastric-fistula bearing sham-feeding rat, amounted to at least 26%. The possible contribution of neural mechanisms triggered by the gastric, intestinal, and postabsorptive phases of the meal were, however, not determined. Hormonal factors were found to contribute at least 30% to the total insulin response on the basis of the insulin response to real feeding in atropinized rats, in the absence of any increases of plasma glucose and with only small elevations of plasma alpha-amino nitrogen. A possible atropine-suppressible hormonal factor was not isolated in the present study. Finally, the relative contribution of rising plasma glucose as determined by intravenous glucose infusions was found to amount to no more than 20%; however, the contribution of rising plasma amino acids was not determined. Thus, 23% of the total insulin response could not be segregated, but it is thought that a good part of it can be attributed to synergistic mechanisms. Because of such interactions, the sum of the effects of the isolated factors is less than the effect of the combined factors.

Vagal neurons and pathways to the rat's lower viscera: an electrophysiological study

The vagal pathways to the rat's pancreas are anatomically difficult to describe. A stimulation/recording technique has been used on various segments of the vagus to trace vagal pathways to the lower viscera, and a microelectrode recording technique to locate vagal neurons of origin in the brain stem's dorsal motor nucleus (DMX). The two main pathways (right cervical vagus to dorsal celiac branch and left cervical vagus to ventral celiac branch) are supplemented by two accessory ones where each cervical vagus gives some fibers to its contralateral homologue at the diaphragmatic level. These pathways consist almost exclusively of C-fibers. Neurons of origin of the dorsal vagal trunk fibers have been identified by the collision test and occupy the caudal half of the DMX; those of the dorsal celiac branch fibers originate from the medial part of that area.

Anatomical considerations for surgery of the rat abdominal vagus: distribution, paraganglia and regeneration

In order to provide a detailed surgical anatomy of the rat abdominal vagus, we examined pyridine silver-stained tissue from one group of normal animals and a second group that survived 9 months after vagotomy. In the normal sample, as has been established for man, there was considerable variability in the levels at which each of the vagal branches separated from the main trunks. Contrary to reports from dissection studies, most of the branches were not single fiber bundles but rather consisted of two or more separate bundles. At the extreme, the posterior gastric and coeliac branches each consisted of as many as 15 individual bundles. Even the main trunks of the subdiaphragmatic vagus were occasionally observed to have multiple components (anterior trunk, 13% of the cases; posterior, 25%). In addition to the classically recognized hepatic, anterior gastric, coeliac, and posterior gastric branches, we also observed an accessory coeliac branch of the anterior trunk in all animals. This accessory coeliac division originated just caudal to the hepatic branching and extended first laterally and then dorsally while running caudally to exit from the esophagus just before the separation of the coeliac branch from the posterior trunk. The vagi were observed to contain paraganglia consisting of islands of glomus cells, neurons, and extensive capillary beds, all situated within the perineurium. The paraganglia occurred in greatest frequency at the sites where the hepatic and coeliac branches divide from their respective trunks. Paraganglia were also observed peripherally within vagal branches; there they were most numerous within the coeliac branch and least numerous in the accessory coeliac. Other studies yielded evidence that regeneration had occurred after complete vagotomy. First, stumps of the branches distal to the resection scar contained axons. Central to the scar, axons grew out in all directions from the neuroma; some of them appeared to cross the scar and to reinnervate the distal stumps. Secondly, 30% of the animals in which regeneration was thought to be possible increased their insulin secretion in response to electrical stimulation of the cervical vagus. The implications of the above findings for experiments that involve manipulation or recording of the vagus are discussed.

A simple electrode for intact nerve stimulation and/or recording in semi-chronic rats

A cuff electrode for extracellular nerve stimulation and/or recording is described. It can be made from common laboratory material without the need of special equipment, and consists of a tubular silicone rubber holder enclosing the nerve and keeping it in position against two platinum wires. The assembly is sufficiently insulated to be kept amidst the surrounding tissue, hereby preventing the nerve to dry during recording periods. It can be attached to an adjoining structure, thus allowing further manipulation of the animal.

Increases in plasma insulin levels in response to electrical stimulation of the dorsal motor nucleus of the vagus nerve

In order to investigate the physiological counterpart of the anatomical finding showing that the dorsal motor nucleus of the vagus nerve (DMX) is a source of efferent vagal fibers innervating the pancreas, unilateral electrical stimulation using monopolar electrodes (50 microA, 30 Hz, 0.2 msec) at a glycemia of 150 mg/100 ml was performed in normal anesthetized rats. DMX stimulation resulted in rapid (within 1 min) rise in plasma insulin levels (greater than or equal to 200%). Stimulation of the nucleus of tractus solitarius, anatomically connected to DMX, also produced a 50% increase in insulinemia. The effect of DMX stimulation was almost completely abolished by atropine pretreatment or acute bilateral subdiaphragmatic vagotomy. The effect of DMX stimulation was not potentiated by the alpha-adrenergic blocker (infusion of phentolamine) indicating that no inhibitory fiber was recruited during DMX stimulation. It is concluded that DMX is connected to the endocrine pancreas exclusively via vagal fibers and has a role in neurally mediated insulin release.

Evidence for a role of the gastric, coeliac and hepatic branches in vagally stimulated insulin secretion in the rat

Either the left or right cervical vagus was electrically stimulated in anesthetized rats before and after selective transection of either the coeliac, gastric and hepatic abdominal branches in order to evaluate the contribution of these branches to vagal controlled insulin secretion. Changes of insulin secretion were estimated on the basis of insulin concentration in venous plasma, sampled by indwelling jugular catheters. Plasma glucose concentration in overnight food-deprived rats was clamped between 130 and 160 mg/dl by means of continuous i.v. glucose infusion, and surgical stress-induced sympathetic activity was blocked by concomitant i.v. infusion of phentolamine and propranolol. Before transection of any abdominal branch, both right and left cervical vagal stimulation induced a 3- to 4-fold increase of plasma insulin concentration and significant increases of plasma glucose concentration, while the heart rate decreased rapidly and significantly. The right cervical vagal stimulation-induced insulin response (integrated incremental area) was significantly decreased by either bilateral coeliac (-37%) or bilateral gastric (-57%), but not by hepatic (-5%) vagotomy. The left cervical vagal stimulation-induced insulin response was significantly decreased (-41%) by hepatic vagotomy. The concomitant rises of plasma glucose concentration may have contributed more than 50% to the vagal stimulation-induced insulin responses. However, calculating the purely neural components revealed that the right cervical vagal stimulation-induced insulin response was still decreased by coeliac (-48%) or gastric (-84%) and not decreased (+24%) by hepatic vagotomy, and the left cervical vagal stimulation-induced insulin response was decreased (-52%) by hepatic branch vagotomy. We conclude that cervical vagal stimulation-induced insulin-secreting activity reaches the pancreas via all 3 abdominal divisions of the vagus nerve, and suggest that pancreatic beta-cells are innervated through all 3 abdominal divisions.

Lack of gastric inhibitory polypeptide (GIP) response to vagal stimulation in the rat

The effect of sham feeding on the plasma concentration of gastric inhibitory polypeptide (GIP) was studied in unrestrained rats bearing chronic gastric fistulas and jugular catheters. While no increase of plasma GIP concentration could be detected during sham feeding (fistula open), during normal feeding (fistula closed), plasma GIP concentrations rose rapidly. In contrast to GIP, plasma insulin concentrations showed a rapid and phasic response during sham feeding in the absence of changes of glycemia. In anesthetized rats electrical stimulation of the vagus nerve was without any effect on plasma GIP concentration, while plasma insulin increased rapidly by as much as 150 percent. It is concluded that under the conditions used, the full gastric and/or intestinal phases of food ingestion are necessary to trigger GIP release, and that vagal activation alone is unable to stimulate GIP release in the rat.

Brain stem infusion of the gamma-aminobutyric acid antagonist bicuculline increases plasma insulin levels in the rat

Previous neuroanatomical and physiological studies have indicated that nucleus ambiguus (Amb) is one source of vagal motoneurons in the brain stem that innervates the pancreas and which, when stimulated, increases insulin release. To investigate one of the neurotransmitter inputs to Amb neurons and its relation to insulin secretion, bicuculline, a specific gamma-aminobutyric acid (GABA) antagonist, was infused bilaterally into the Amb region as well as into a neighboring area, the rostral level of the lateral nucleus tractus solitarius (nts) of anesthetized male rats. Experiments were carried out in the presence or absence of the alpha-adrenergic blocker phentolamine. In the absence of phentolamine, no increases in plasma insulin levels were seen after bicuculline or vehicle infusion into the Amb region or after bicuculline infusion into the nts, while plasma glucose levels were significantly increased. In the presence of phentolamine, bicuculline infusion into the Amb region led to a prompt and significant increase in plasma insulin levels that could not be accounted for by changes in glycemia. The infusion of vehicle into Amb or of bicuculline into nts produced small or insignificant increases in plasma insulin levels. These results suggest that Amb neurons capable of modulating plasma insulin levels are under tonic GABA inhibition, an effect that appears to be specific for Amb neurons, since bicuculline infusion into another brain stem nucleus (nts) had no effect on insulin release. The fact that phentolamine pretreatment was necessary to reveal the bicuculline-induced effects corroborates previous studies showing that in addition to a central nervous system inhibition of vagal motoneurons by GABA, there is a tonic sympathetic inhibitory input to the endocrine pancreas capable of masking any disinhibition of vagal motoneurons. The physiological role of GABA inhibition of Amb neurons that innervate the pancreatic beta-cells remains to be determined.

Sham feeding-induced cephalic phase insulin release in the rat

The effect of the cephalic phase of food ingestion on plasma insulin and glucagon concentration was assessed in the sham-feeding rat, bearing chronically implanted gastric drainage fistulas. It was found that continuous sham feeding produced a significant and phasic peripheral insulin response in the absence of any significant changes of glycemia. The response was almost completely blocked by prior intravenous administration of 2 mg/kg of atropine methyl nitrate and potentiated by prior intravenous administration of 1.0 or 2.5 mg/kg of phentolamine. In spite of the larger insulin response after phentolamine, there was no hypoglycemia detected. Furthermore, continuous sham feeding did not produce a significant glucagon response, whereas real feeling did. The results demonstrate that cholinergic insulin release is triggered phasically by continuous ingestion of familiar food and that this insulin response is inhibited by an alpha-adrenergic sympathetic tone. It is further concluded that the increased glucose disposal produced by the neurally released insulin is not counteracted by a concomitant glucagon response or by direct adrenergic stimulation of hepatic glucose production.

Importance of cholinergic innervation of the pancreas for glucose tolerance in the rat

Because cholinergic innervation of the pancreas is of importance in control of oral glucose tolerance, it seems important to determine whether transplanted pancreatic tissue becomes reinnervated with cholinergic fibers. Oral and intravenous glucose tolerance tests (ivGTT) were performed with and without atropine treatment on streptozotocin-diabetic rats treated by intraportal transplantation of isogenic islets 13-15 wk previously and on sham-operated nondiabetic controls. Atropine had no effect on the ivGTT of transplanted rats or controls. In controls atropine caused a deterioration of the oral glucose tolerance, abolished the preabsorptive insulin release, and also diminished the early part of the glucose-induced insulin release in these animals. In the absence of atropine, transplanted rats had pathological oral glucose tolerance, preabsorptive insulin release was absent, and glucose-induced insulin release was diminished compared to controls. Atropine had little effect on the oral GTT of transplanted rats. The present results underline the importance of the vagus nerve in the control of oral glucose tolerance and show that the vagus nerve in rats, at least under these experimental conditions, does not modulate the insulin response to intravenous glucose. The results suggest that intraportally transplanted islets remain functionally vagotomized.

Chorda tympani and vagus nerve convergence onto caudal brain stem neurons in the rat

Neurons responsive to chorda tympani (CT) and cervical vagus (CV) nerve stimulation were identified electrophysiologically in the caudal brain stem of the anesthetized rat. All identified dually responsive (CT/CV) neurons included in this study were orthodromically activated by both CT and CV stimulation (n = 80). These cells were located mainly in the lateral nucleus tractus solitarius (nts) or more ventrally in the region of nucleus ambiguus (amb). No CT/CV cells were found to lie clearly within the dorsal motor nucleus of the vagus nerve. Among CT/CV neurons in nts, there was no correlation of response latency to Ct stimulation versus response latency to CV stimulation, while CT/CV neurons found in the region of amb demonstrated a good correlation between the two stimuli and suggested that sensory convergence occurred prior to the level of amb. Conditioning pulses applied to CV were able to alter the neural response of characteristics to CT test pulses in a majority of the units tested. It is concluded that oropharyngeal afferents converge on brain stem neurons that are also responsive to vagal afferent input. These dually responsive CT/CV neurons are implicated in the integration of sensory information relevant for cephalic phase reflexes.

Nucleus ambiguus stimulation increases plasma insulin levels in the rat

The ventral lateral brainstem has been explored for sites that facilitate insulin release unilaterally, using electrical stimulation techniques in male rats anesthetized with alpha-chloralose/urethane. Stimulation in the region of nucleus ambiguus (amb) produced a rapid rise (by 1 min) in plasma insulin levels, whereas stimulation of brainstem regions further than 500 micrometers from amb had no consistent effect on insulin levels. The amb-induced rise in insulin was markedly exaggerated by the alpha-adrenergic antagonist, phentolamine, and was greatly diminished by bilateral cervical vagotomy or atropine pretreatment. These results strongly suggest that the amb is one source of vagal motoneurons that facilitate insulin secretion. However, amb electrical stimulation alone also activated an apparent sympathetic efferent output whose inhibitory effect on insulin release could be blocked by phentolamine.

Cephalic phase, reflex insulin secretion. Neuroanatomical and physiological characterization

Using chronically catheterized, freely moving male Wistar rats, we have shown that the sweet taste of a saccharin solution reliably triggers a rapid cephalic phase insulin response (CPIR), in the absence of any significant change of glycemia. To establish the neural mediation of this reflex response we used rats that were cured from streptozotocin diabetes by intrahepatic islet-transplantation as a denervated B-cell preparation. The complete lack of any saccharin-induced CPIR in these rats suggests that it is indeed mediated by the peripheral autonomic nervous system, and that the insulin-stimulating gastrointestinal hormones are not involved in this response. It was further found that this reflex insulin secretion is not easily extinguishable and thus might have an unconditioned component. To investigate the central neural pathways involved in this reflex response we used both electrophysiological methods in anesthetized and semi-micro CNS manipulations in freely moving rats. On the basis of our preliminary results, and several reports, using the decerebrate rat preparation for measuring behavioral or saliva secretory oral taste reactivity, it appears that the CPIR might be organized at the brain stem/midbrain level, receiving strong modulatory influences from the diencephalon. But much further work has to be done to establish the central nervous circuitry. Finally, in two experiments, aiming at the question of how important and physiologically relevant the CPIR might be, we found that, on one hand, its lack can result in pathological oral glucose tolerance and on the other hand its exaggeration might contribute to the behavioral reaction to highly palatable sweet food and the resulting development of dietary obesity.

Cephalic phase, reflex insulin secretion neuroanatomical and physiological characterization

Using chronically catheterized, freely moving male Wistar rats, we have shown that the sweet taste of a saccharin solution reliably triggers a rapid cephalic phase insulin response (CPIR), in the absence of any significant change of glycemia. To establish the neural mediation of this reflex response we used rats that were cured from streptozotocin diabetes by intrahepatic islet-transplantation as a denervated B-cell preparation. The complete lack of any saccharin-induced CPIR in these rats suggests that it is indeed mediated by the peripheral autonomic nervous system, and that the insulin-stimulating gastrointestinal hormones are not involved in this response. It was further found that this reflex insulin secretion is not easily extinguishable and thus might have an unconditioned component. To investigate the central neural pathways involved in this reflex response we used both electrophysiological methods in anesthetized and semi-micro CNS manipulations in freely moving rats. On the basis of our preliminary results, and several reports, using the decerebrate rat preparation for measuring behavioral or saliva secretory oral taste reactivity, it appears that CPIR might be organized at the brain stem/midbrain level, receiving strong modulatory influences from the diencephalon. But much further work has to be done to establish the central nervous circuitry. Finally, in two experiments, aiming at the question of how important and physiologically relevant the CPIR might be, we found that, on one hand, its lack can result in pathological oral glucose tolerance and on the other hand its exaggeration might contribute to the behavioral reaction to highly palatable sweet food and the resulting development of dietary obesity.

CNS modulation of pancreatic endocrine function. Multiple modes of expression

The involvement of the CNS in pancreatic hormone release has been studied. 1.) It has been shown that one source of vagal efferent fibers capable of facilitating insulin secretion originated in the rostral half of the nucleus ambiguus. 2.) Acute lesions of the ventromedial hypothalamus resulted in hyperinsulinaemia that could be abolished by acute vagotomy. 3.) Chronic lesions of the ventromedial hypothalamus increased secretion of insulin and glucagon, and decreased secretion of somatostatin when the pancreas was subsequently isolated and perfused. These changes were attributed to altered cholinergic activity related to previous ventromedial hypothalamic lesions as they could be reversed toward normal by atropine infusion or mimicked by the cholinergic agonist, methacholine. 4.) Electrical stimulation of the lateral hypothalamus in anaesthetized rats produced both an inhibitory component of insulin secretion, probably related to adrenergic stimulation, and a stimulatory component, probably due to the release into the blood of factor(s) that promote insulin secretion. 5.) The anatomical organization of brain of the genetically obese (ob/ob) mice is abnormal. These abnormalities could be involved in the endocrinological disturbances of these animals.

Importance of preabsorptive insulin release on oral glucose tolerance: studies in pancreatic islet transplanted rats

The role of preabsorptive (cephalic phase) insulin release in oral glucose tolerance was investigated using diabetic rats treated by intraportal transplantation of isogenic islets. This early neurally mediated phase of insulin release has been shown to be absent in such rats. When the body weight of transplanted rats was normalised, glucose tolerance tests (GTTs) were performed in the unstressed state using permanent cardiac catheters. Transplanted rats had a normalised intravenous GTT, whereas, as we have shown previously, their oral GTT remained clearly pathological. During both tests peripheral insulin levels were decreased compared with controls. While during intravenous GTT the onset of insulin release occurred as early in transplanted rats as in controls, during oral GTT there was a clear delay, probably because of the absence of the cephalic phase. Re-establishment of normal preabsorptive insulin levels was attempted by a small intravenous insulin injection during this period. This resulted in a transient increase in peripheral insulin levels, which, at two minutes after glucose ingestion, gave values similar to those found in controls at that time. This small insulin injection caused a marked improvement of the oral GTT which was most evident after exogenous insulin had disappeared from the blood. While the injection did not affect the 60 minute incremental insulin area, the glucose area was decreased by 50%, to a value not significantly different from that of control rats. The cephalic phase of insulin release appears, therefore, to be one important factor in the control of glycaemia during food intake. Its absence plays a major role in the pathological oral glucose tolerance of diabetic rats treated by intraportal islet transplantation.

[Modulation by the central nervous system (CNS) of the activity of the endocrine pancreas]

Electrical stimulation of Nucleus Ambiguus (NA) and of Lateral Hypothalamus (LH) produces a sympathetic response that partly inhibits insulin secretion in anesthetized rats in vivo. This inhibitory component is accompanied by a stimulatory one that can, in both cases, be revealed by the infusion of an alpha-blocker to the rats. The analogy between the stimulatory effect on insulin secretion due to NA or LH electrical stimulation is only partial. Indeed the NA-induced insulin secretion is abolished by vagotomy, while that induced by LH stimulation appears to be mediated by possible release of humoral factor(s). Acute bilateral lesions of the ventromedial hypothalamus (VMH) in anesthetized rats produce, within minutes, an hyperinsulinemia that is abolished by superimposed vagotomy. Semi-chronic (7 d.) lesions of the VMH in rats result in changes in the activity of their subsequently isolated perfused pancreases. Indeed, and compared to controls, pancreases from VMH-lesioned rats oversecrete insulin and glucagon while undersecreting somatostatin. Such alterations due to lesions of the VMH appear to be the result of increased vagal activity and (possibly via a trophic effect of the nerve) of increased cholinergic (muscarinic) receptors. CNS organisation of genetically obese (ob/ob) mice is abnormal when compared to controls. Abnormalities are mainly a relative increase of the size of neurons of the LH and the dorsomotor nucleus of the vagus (DMX), compared to that of other CNS neurons. These abnormalities could conceivably play a role in the genesis of hyperinsulinemia of these animals.

Role of the autonomic nervous system in the mediation of LHA electrical stimulation-induced effects on insulinemia and glycemia

The short-term effects of bilateral electrical stimulation of the lateral component of the lateral hypothalamic/medial forebrain bundle area (LHA) on peripheral plasma levels of glucose (G), immunoreactive insulin (IRI) and glucagon (IRG) were measured in food-derived, anesthetized rats in the presence of continuous low i.v. glucose infusion. In normal rats LHA stimulation induced a rapid but moderate rise in IRI in the presence of a simultaneous rapid rise in G. At the end of the 5 min stimulation period the glucose-induced IRI response was clearly attenuated. In rats receiving a continuous phentolamine (i.v.) infusion, the rapid IRI response was much larger in spite of only marginal rises of G. These findings suggest that electrical LHA stimulation activates two opposing mechanisms on IRI secretion, an excitatory component which is masked in normal animal by a sympathetic alpha-adrenergic inhibitory component. In an attempt to define the mediating pathway of this LHA stimulation-induced IRI excitatory component, both the parasympathetic and sympathetic divisions of the autonomic nervous system were pharmacologically and surgically manipulated, all in the presence of phentolamine infusion. Neither bilateral subdiaphragmatic vagotomy, atropine, propranolol infusion, nor spinal cord transection at T1 were able to block the IRI response. The findings are consistent with a humoral mediation of this IRI stimulatory response and support the existence of a lateral hypothalamic or hypophyseal factor which is released by LHA stimulation. The implications and physiological role of the LHA in the neural control of insulin secretion and ingestive behavior are discussed.

Cephalic-phase insulin secretion in normal and pancreatic islet-transplanted rats

The ability of saccharin, in comparison with glucose and tap water, to elicit glycemia-independent neurally mediated insulin secretion was investigated in chronically catheterized, freely moving rats. Plasma glucose and insulin concentrations were measured continuously from venous blood with a sampling resolution of one per minute. In normal rats, 1 ml of 0.15% saccharin caused a significant rapid rise in peripheral plasma insulin levels lasting up to 5 min, without significant changes in glycemia. Tap water alone also induced a transient elevation in insulinemia but was much smaller than the saccharin-induced response. In streptozotocin diabetic rats bearing intrahepatic, presumably denervated islet isografts, these rapid insulin responses to oral saccharin and tap water stimulation were completely abolished, whereas the early insulin response to intravenous glucose was decreased by only about 30%. These results are consistent with the concept of gustatory and other oral sensory signals acting as triggers for neurally mediated insulin release.

Hypothalamic input to brain stem neurons responsive to oropharyngeal stimulation

The aim of this study was to determine if electrical stimulation of diencephalic sites previously implicated in feeding behavior and metabolic homeostasis could influence the unit activity of caudal brain stem neurons receiving oropharyngeal input. Electrical stimulation of the chorda tympani nerve (CT) was used to first identify brain stem units. A significant number of these CT-responsive neurons could also be driven by lateral hypothalamic area (LHA) stimulation. Medially placed diencephalic stimulating electrodes were not effective in altering unit activity of CT-responsive brain stem neurons. Tests of temporal summation showed that LHA conditioning pulses could significantly alter brain stem unit responses to CT test pulses at short interpulse intervals. These results demonstrate the presence of descending input from the LHA-medial forebrain bundle (MFB) to brain stem neurons possibly involved in oropharyngeal sensation and gustatory-evoked phenomena.

Intraportal islet transplantation: functional assessment in conscious unrestrained rats

We have found that rats with transplanted pancreatic islets, when compared to normal rats, have a delayed onset of insulin release in response to orally, but not to iv administered glucose. Furthermore, while glucose tolerance of the rats with transplanted islets was similar to that of normal controls when the glucose was administered iv, the tolerance was markedly less when it was administered orally. These tests were carried out using permanently implanted cardiac catheters and chronic oral fistulae and were conducted at a time when the body weight of the transplanted animals had returned to levels similar to those of the controls. During the tests the rats were conscious and unrestrained. The difference in the fine control of insulin secretion in transplanted islets from that in the normal pancreas may be due to defective innervation of such islets. These results may have implications for the use of transplanted islets in the control of diabetes mellitus in man. The methods employed can be further used to define other areas in which the response of transplanted islets in rats differs from that of the normal pancreas.

Acute hyperinsulinemia and its reversal by vagotomy after lesions of the ventromedial hypothalamus in anesthetized rats

The acute effect of bilateral electrolytic ventromedial hypothalamic lesions (20-25-m Coulomb stainless steel electrodes) on plasma levels of insulin and glucose was studied in anesthetized rats to determine early effects that would occur before hyperphagia and obesity. In rats fed ad libitum, lesions in the ventromedial hypothalamus (VMH) but not in the cortex produced a marked increase in circulating insulin levels (starting at 20 min postlesion) and a small increase in glycemia which, however, was not significant and could therefore not be the cause of increased insulin secretion. Hyperinsulinemia after VMH lesions was more pronounced when glucose was infused iv at a rate of 7-8 mg/kg . min. Bilateral subdiaphragmatic vagotomy, performed 50 min after VMH lesions, immediately and completely reversed the observed hyperinsulinemia. With the exception of a tendency of lesions producing the highest degree of hyperinsulinemia to be slightly larger than the lesions not producing any hyperinsulinemia, no statement about the critical involvement of a specific hypothalamic locus can be made. It is concluded that electrolytic VMH destruction causes immediate hypersecretion of the pancreatic B cell, an effect that requires the integrity of the vagus nerves. Further localization of the central circuitry responsible for this mechanism, however, will require more specific methods than electrolytic lesions.

Ingestive behavior after intracerebral and intracerebroventricular infusions of glucose and 2-deoxy-D-glucose

Direct infusions of 2-DG into the lateral hypothalamic area (LHA), the ventromedial hypothalamus (VMH), the dorsal hippocampus, the amygdaloid complex or the caudate nucleus were all ineffective in eliciting drinking or feeding in satiated rats. However, 2-DG (but not D-glucose) infused into the lateral ventricles of satiated rats elicited feeding preceded by an initial burst of drinking, which could be blocked by prior intraventricular infusion of the alpha-adrenergic antagonist, phentolamine, by the serotonergic blocker, methysergide, but only slightly by the dopaminergic receptor blocker, spiroperidol. The feeding response was totally blocked by phentolamine but not by methysergide or spiroperidol. These results show that 1) intraventricular administration of 2-DG can induce feeding, as well as drinking, supporting the hypothesis of cerebral glucoreceptors for the initiation of feeding behavior; and 2) local cytoglucopenia in the LHA, the VMH, or other limbic structures is not sufficient to initiate feeding. It is hypothesized that a specific pattern of cerebral glucoprivation rather than local glucoprivation, or an action of 2-DG on the presynaptic membrane of noradrenergic "feeding neurons" could be the mechanisms of 2-DG-induced feeding.