Vago-vagal reflexes to the colon of the anaesthetized ferret

Electrical stimulation of the central end of the vagal communicating branch in the thorax at frequencies between 2 and 20 Hz elicited, after a latency of 7.2 +/- 0.8 s, large-amplitude colonic contractions. 5 Hz stimulation gave near maximal contractions and, because vomiting was more likely to occur at higher stimulus frequencies, was used as the standard stimulus for subsequent experiments. At this frequency the peak colonic contraction was 6.5 +/- 0.9 kPa. Following atropine the characteristics of the response to central vagal stimulation differed from that seen before atropinization. The latency was longer (45.7 +/- 8.2 s) and the amplitude greatly attenuated (0.7 +/- 0.2 kPa). Cooling the vagus nerves to 2 degrees C at a level either above or below the site of stimulation completely abolished both the cholinergic and the atropine-resistant colonic responses to central vagal stimulation. These results are consistent with the vagus containing two motor pathways to the colon which are reflexly stimulated by a vagal afferent input. The functional significance of these reflexes is discussed.

Vagal control of colonic motility in the anaesthetized ferret: evidence for a non-cholinergic excitatory innervation

Spontaneous colonic motility in the urethane-anaesthetized ferret consists of two distinct types of contraction which correspond to the patterns recorded myoelectrically in conscious animals. This motility was abolished or greatly reduced when nervous conduction was prevented in the cervical vagi by cooling to below 4 degrees C. On rewarming the nerves the colonic motility returned, after a short latency, to the pre-cool level. Atropine transiently abolished colonic motility. On its return the motility was significantly reduced but still sensitive to vagal integrity. Thus the atropine-resistant colonic motility was also abolished or markedly reduced by cooling the cervical vagi to below 4 degrees C. On rewarming there was a longer latency for the return of motility compared to that before atropinization. Electrical vagal stimulation produced, after a short latency, large-amplitude colonic contractions. Following atropine, the short-latency response to electrical vagal stimulation was replaced in the majority of animals by a long-latency response whose characteristics were quite different from those of the cholinergic response. These results are consistent with the vagus containing two functional motor pathways to the colon, one to cholinergic post-ganglionic neurones and the other operating via a non-cholinergic mechanism.

Vagal influences on the jejunal 'minute rhythm' in the anaesthetized ferret

Spontaneous jejunal motility in the urethane-anaesthetized ferret shows a cyclical pattern of contraction bursts alternating with quiescent periods described as 'minute rhythm' in conscious animals. Cooling the cervical vagi to below 4 degrees C or acute vagotomy abolished this pattern of motility. On re-warming the vagi there was a return to cyclical motility after a latency which depended upon the contractile state at the time vagal conduction was restored. Electrical vagal stimulation produced bursts of contractions at the same frequency as the spontaneous motility. Longer periods of stimulation gave rise to bursts of contractions interrupted by periods of relative quiescence, mimicking the spontaneous motility, despite the continuous stimulation. Following atropinization all spontaneous motility was abolished, but electrical stimulation of the vagi revealed a non-cholinergic, non-adrenergic response whose characteristics differed from that of the cholinergic response. It is concluded that the vagus plays a permissive role in regulating the jejunal 'minute rhythm' via a cholinergic pathway and that there is a second excitatory vagal pathway which innervates non-cholinergic post-ganglionic neurones whose functional significance and transmitter mechanism is unknown.

The response of the pancreas of the anaesthetized cat to secretin before, during and after reversible vagal blockade

Cooling the cervical vagi of the anaesthetized splanchnectomized cat to 2 degrees C caused a 54.4 +/- 8.8% inhibition of pancreatic electrolyte secretion stimulated submaximally with pure secretin. On rewarming the vagi there was a prolonged increase in secretion rate over and above the control rate which existed before cooling. The increase lasted about 90 min. There were no changes in acid/base status due to interference of the lung inflation reflex which could account for the inhibition of secretion and the subsequent rebound. Cold block of the cervical vagi increased the transpancreatic electrical conductance, indicating that vasodilation had occurred and therefore eliminated a vasomotor cause for the inhibition. Electrolyte secretion was also inhibited by bilateral vagal section. Atropine only partially prevented the inhibitory response to vagal cooling. A cholinergic mechanism, therefore, accounted for some but not all of the response to vagal cooling. It is concluded that even in the fasted, anaesthetized animal vagal impulses facilitate the action of secretin on the pancreas. This facilitation is only partially cholinergic; the major part of the response is due to some non-cholinergic transmitter substance. Such a mechanism may be necessary to potentiate the action of the very small amounts of secretin which appear to be released during a meal.

Non-atropinic anticholinergic action of quinidine on the ferret stomach

1 The effect of quinidine on the cholinergic response of the ferret gastric corpus was investigated in vivo and in vitro. 2 In vivo, the atropine sensitive contractions of the gastric corpus evoked by stimulation of the cervical vagus in the anaesthetized ferret were antagonized by quinidine (10-30 mg/kg). 3 In vitro, quinidine (10 micrograms/ml) antagonized the contractile response elicited by transmural stimulation (T.M.) of strips of corpus. At this dose of quinidine, the response to acetylcholine at doses adjusted to match the response to T.M. stimulation were unaffected. At higher doses of quinidine (50 micrograms/ml) the response to stimulation of the non-cholinergic, non-adrenergic inhibitory neurones was also antagonized. 4 These results indicate that, at low doses, quinidine has an inhibitory effect on the cholinergic neurones.

Effect of stimulation of the vagus nerve in bursts on gastric acid secretion and motility in the anaesthetized ferret

1. The effect of electrical stimulation of the vagus nerve with different patterns of impulses (the total number of stimuli remaining constant) on gastric acid secretion and gastric motility were investigated in the anaesthetized ferret. Three stimulus patterns were used: continuous, bursts at ten times the continuous frequency but for a tenth of the time, and a natural burst pattern obtained from a recording of vagal efferent fibre discharge. 2. The natural burst pattern gave rise to gastric contractions of larger amplitude than either the artificial burst or continuous stimulation, while continuous stimulation gave rise to larger changes in tonus. Acid secretion, however, was reduced by the natural pattern of stimulation as compared to the output during continuous stimulation, but to a lesser extent than that due to artificial burst stimulation. 3. Burst stimulation at 10 impulses/sec resulted in a larger output of acid and greater amplitude of gastric contraction than the equivalent continuous stimulation. This potentiation was lost at stimulation frequencies above 30 impulses/sec. 4. At burst frequencies of 60 and 120 impulses/sec there was a marked reduction in acid output and amplitude of contraction as compared with continuous stimulation at 6 and 12 impulses/sec respectively. 5. These results are discussed in relation to the functional significance of the different patterns of vagal discharge seen in the anaesthetized ferret.

Cardiovascular changes in elicited by vagal gastric afferents in the rat

Cardiovascular changes elicited by gastric distension in the urethane anaesthetized rat are described. Heart rate changes were mediated via a vago-vagal pathway with 'in-series' tension receptors in the stomach wall providing the afferent input and cholinergic cardio-inhibitory fibres forming the efferent pathway. Blood pressure changes occurred independently of the heart rate changes; the main afferent pathway was again vagal but the efferent outflow was non-cholinergic and probably sympathetic.

Modulation of vagal efferent fibre discharge by mechanoreceptors in the stomach, duodenum and colon of the ferret

1. A single-fibre-dissection technique was used to investigate the reflex modulation of vagal efferent fibre discharge by afferent fibres from various parts of the gastrointestinal tract of the urethane-anaesthetized ferret. 2. All but four of the 168 efferent fibres isolated in this study were spontaneously active. The majority of these had discharge frequencies of less than 6 spikes/sec. 3. All the efferent units received an afferent input from mechanoreceptors in the stomach. Two main types of response to gastric distension were seen: (i) an increase in efferent discharge and (ii) a decrease or complete suppression of efferent discharge. 4. The vagal efferent discharge was also modulated by duodenal and colonic distension, with the major effect being one of inhibition. 5. Bilateral vagotomy completely abolished the response to gastric distension in 68% of the units tested. The response to colonic and duodenal distension, however, was relatively unaffected by vagotomy. Thus the vagus provides the major afferent pathway from the stomach to these vagal efferent fibres, whilst the major input from the duodenum and colon is via a non-vagal pathway. Both vagal and splanchnic afferents therefore converge on to the vagal nucleus. 6. The destinations of these vagal efferent fibres and their possible functions are discussed.

The role of the vagus and splanchnic nerves in the regulation of intragastric pressure in the ferret

1. The role of the vagus and splanchnic nerves in the regulation of intragastric pressure was investigated by quantifying the effect of nerve section on the response to a standard fluid inflation of the stomach. An opportunity was also taken to compare the responses to a step and physiological ramp inflation of the same volume (50 ml). 2. The peak pressure at the end ot the step inflation (13.6 +/- 1.0 cmH2O) was twice that for a ramp inflation (6.2 +/- 0.4 cmH2O). 5 min after the peak the pressures were similar (4.8 +/- 0.3 cmH2O; 4.7 +/- 0.3 cmH2O) for both. 3. Vagotomy produced a significant increase in the intragastric pressure response for both the peak and plateau levels of the step and ramp inflations. 4. Atropine produced a statistically significant decrease in the pressure response to both step and ramp inflation. 5. Splanchnic nerve section produced an increase in the intragastric pressure response to both step and ramp inflations although the values failed to reach the 5% significance level. However, following vagotomy or atropine, section of the splanchnic nerves produced a statistically significant increase in the pressure response to inflation. 6. These results indicate that the vagal cholinergic excitatory and vagal non-cholinergic, non-adrenergic inhibitory fibres are activated at physiological levels of gastric inflation and play a role in the regulation of intragastric pressure. The splanchnic nerves also play a role although in the intact animal they are suppressed by the action of the vagus nerve.

Vagal afferent discharge from mechanoreceptors in different regions of the ferret stomach

1. The rate and volume of gastric filling was estimated in conscious ferrets by measuring the amount of milk they would drink after an overnight fast. The mean volume was 94.5 +/- 7.5 ml. at a rate of 13.0 +/- 0.74 ml./min. An intragastric infusion rate of 10 ml./min to a total of 50 ml. was selected as a standard distension stimulus. 2. Action potentials were recorded from single gastric afferent fibres in the cervical vagus. All but two of thirty-six afferent units were tonically active when the stomach was deflated. 3. Afferent fibres arising from receptors in the antrum showed modulation in phase with spontaneously occurring antral contractions. Afferent fibres from the corpus and fundus, however, discharged at irregular rates between 0.35 and 7.5 Hz with no correlation with the intragastric pressure rises associated with the antral contractions. 4. Inflation of the stomach with 50 ml. 0.9% NaCl at a rate of 10 ml./min stimulated antral motility and the rhythmic afferent discharge from the antrum was enhanced Receptors in the corpus and fundus increased their rate of discharge with increasing gastric volumes. Receptors in the region between the antrum and the corpus had the property of both types so that they responded to both distension and contractions. 5. On distension with 0.9% NaCl, fluid is distributed unevenly in the stomach. 80% was accommodated in the corpus and fundus, the remainder of the fluid entering the antrum. 6. The tension was measured in strips of stomach wall taken from corpus and antrum. For equal increments of stretch the development of tension was greater in the antral than in corpus strips. This physical property together with neurally mediated receptive relaxation of the corpus is the reason for the fluid distribution described above. 7. It is concluded that the properties of the tension receptor are determined by their site in the stomach. Those in the body and fundus signal the degree of distension and those in the antrum signal information concerning the amplitude, rate and duration of antral contractions.

Reflex excitation of antral motility induced by gastric distension in the ferret

1. In the urethane anaesthetized ferret gastric inflation with 50 ml. 0.9% NaCl at a rate of 10 ml./min evoked large gastric contractions. The threshold of intragastric pressure required to elicit the reflex was 3.1 +/- 0.5 cmH2O. 2. The response was enhanced by splanchnic nerve section and markedly reduced by bilateral cervical vagotomy, indicating that both afferent and efferent limbs of the reflex were in the vagus nerves. 3. By using a transected stomach it was shown that distension of the corpus evoked contractions of the antrum that were abolished by vagotomy. 4. The reflex activation of antral motility by vagal afferents with receptive fields in the corpus may play a physiological role in gastric emptying.

Modulation of single vagal efferent fibre discharge by gastrointestinal afferents in the rat

1. A single fibre dissection technique was used to record activity from efferent fibres in the left cervical vagus nerve of urethane anaesthetized rats. 2. The efferent discharge, in all units, was modulated by gastric inflation, gastric contractions or compression of the stomach wall. The receptors mediating these effects are the slowly adapting 'in-series' tension receptors in the gastric musculature with afferent fibres in the vagus nerves. 3. Efferent units were classified according to their response to passive gastric distension and active gastric contractions. 4. Four distinct types of efferent unit were isolated. Type I units were excited and Type II units were inhibited by gastric distension and contraction. Type III units were excited at low or moderate levels of inflation and inhibited at high levels of inflation or during gastric contractions. Type IV units were inhibited by low levels of inflation but excited at higher levels. 5. Since there is clearly a reciprocal organization at least of some neurones in the vagal nucleus the possibility of reciprocal control of antagonist, cholinergic and 'purinergic' vagal pathways is discussed.