Inhibition of gastric mechanoreceptor discharge by cholecystokinin in the rat

Spinal Afferent Innervation of the Colon and Rectum

Despite their seemingly elementary roles, the colon and rectum undertake a variety of key processes to ensure our overall wellbeing. Such processes are coordinated by the transmission of sensory signals from the periphery to the central nervous system, allowing communication from the gut to the brain via the "gut-brain axis". These signals are transmitted from the peripheral terminals of extrinsic sensory nerve fibers, located within the wall of the colon or rectum, and via their axons within the spinal splanchnic and pelvic nerves to the spinal cord. Recent studies utilizing electrophysiological, anatomical and gene expression techniques indicate a surprisingly diverse set of distinct afferent subclasses, which innervate all layers of the colon and rectum. Combined these afferent sub-types allow the detection of luminal contents, low- and high-intensity stretch or contraction, in addition to the detection of inflammatory, immune, and microbial mediators. To add further complexity, the proportions of these afferents vary within splanchnic and pelvic pathways, whilst the density of the splanchnic and pelvic innervation also varies along the colon and rectum. In this review we traverse this complicated landscape to elucidate afferent function, structure, and nomenclature to provide insights into how the extrinsic sensory afferent innervation of the colon and rectum gives rise to physiological defecatory reflexes and sensations of discomfort, bloating, urgency, and pain.

Cholecystokinin induces esophageal longitudinal muscle contraction and transient lower esophageal sphincter relaxation in healthy humans

Cholecystokinin (CCK) is known to cause lower esophageal sphincter (LES) relaxation through the activation of inhibitory motor neurons. CCK receptor agonists increase the frequency of transient LES relaxation through a peripheral mechanism. Recent studies show that the longitudinal muscle contraction (LMC)-related axial stretch might play a role in the LES relaxation by activating the mechanosensitive inhibitory motor neurons. The aim of our study was to determine whether the CCK-induced LES relaxation and the characteristics of LMC resemble those seen with spontaneous transient LES relaxation in humans. Nine healthy volunteers (5 Fr, 40 ± 12 yr) received escalating doses of CCK-octapeptide (CCK-8) (5, 10, 20, and 40 ng/kg). All subjects demonstrated a monophasic response to 5 ng/kg of CCK-8. In the majority of subjects, this response consisted of partial LES relaxation. All subjects showed a biphasic response to 40 ng/kg of CCK-8. The latter in most subjects consisted of 1) a period of partial relaxation followed by 2) complete LES relaxation along with crural diaphragm inhibition. The length of the esophagus decreased by 0.9 ± 0.4 cm, and muscle thickness increased by 40 ± 14% to 1.4 ± 0.2 mm ( P < 0.05) during initial partial LES relaxation. During complete LES relaxation there was greater LMC, as demonstrated by an esophageal shortening of 1.9 ± 0.5 cm and an increase in muscle thickness of 100 ± 16% ( P < 0.01). The complete phase 2 LES relaxation typically terminated with a robust after-contraction. Atropine significantly attenuated the CCK-induced esophageal LMC, prevented crural diaphragm inhibition, and abolished the phase 2 complete LES relaxation. NEW & NOTEWORTHY The phenotypic features of CCK-induced longitudinal muscle contraction (LMC), complete lower esophageal sphincter (LES) relaxation, and crural diaphragm inhibition, followed by a robust after-contraction, resemble those seen during spontaneous transient LES relaxation. A strong temporal relationship between the LMC and complete transient LES relaxation supports our hypothesis that the LMC plays an important role in the LES relaxation and crural diaphragmatic inhibition.

Enteroendocrine cells: neglected players in gastrointestinal disorders?

Enteroendocrine cells (EEC) form the basis of the largest endocrine system in the body. They secrete multiple regulatory molecules which control physiological and homeostatic functions, particularly postprandial secretion and motility. Their key purpose is to act as sensors of luminal contents, either in a classical endocrine fashion, or by a paracrine effect on proximate cells, notably vagal afferent fibres. They also play a pivotal role in the control of food intake, and emerging data add roles in mucosal immunity and repair. We propose that EEC are fundamental in several gastrointestinal pathologies, notably Post-infectious Irritable Bowel Syndrome, infectious enteritis, and possibly inflammatory bowel disease. Further work is needed to fully illustrate the importance, detailed biology and therapeutic potential of these frequently overlooked cells.

Electrophysiological evidence for distinct vagal pathways mediating CCK-evoked motor effects in the proximal versus distal stomach

Intravenous cholecystokinin octapeptide (CCK-8) elicits vago-vagal reflexes that inhibit phasic gastric contractions and reduce gastric tone in urethane-anaesthetized rats. A discrete proximal subdivision of the ventral gastric vagus nerve (pVGV) innervates the proximal stomach, but the fibre populations within it have not been characterized previously.We hypothesized that I.V. CCK-8 injection would excite inhibitory efferent outflow in the pVGV, in contrast to its inhibitory effect on excitatory efferent outflow in the distal subdivision (dVGV), which supplies the distal stomach. In each VGV subdivision, a dual-recording technique was used to record afferent and efferent activity simultaneously, while also monitoring intragastric pressure (IGP). CCK-8 dose dependently (100-1000 pmol kg(-1), I.V.) reduced gastric tone, gastric contractile activity and multi-unit dVGV efferent discharge, but increased pVGV efferent firing. Single-unit analysis revealed a minority of efferent fibres in each branch whose response differed in direction from the bulk response. Unexpectedly, efferent excitation in the pVGV was significantly shorter lived and had a significantly shorter decay half-time than did efferent inhibition in the dVGV, indicating that distinct pathways drive CCK-evoked outflow to the proximal vs. the distal stomach. Efferent inhibition in the dVGV began several seconds before, and persisted significantly longer than, simultaneously recorded dVGV afferent excitation.Thus, dVGV afferent excitation could not account for the pattern of dVGV efferent inhibition. However, the time course of dVGV afferent excitation paralleled that of pVGV efferent excitation. Similarly, the duration of CCK-8-evoked afferent responses recorded in the accessory celiac branch of the vagus (ACV) matched the duration of dVGV efferent responses. The observed temporal relationships suggest that postprandial effects on gastric complicance of CCK released from intestinal endocrine cells may require circulating concentrations to rise to levels capable of exciting distal gastric afferent fibres, in contrast to more immediate effects on distal gastric contractile activity mediated via vago-vagal reflexes initiated by paracrine excitation of intestinal afferents.

Ghrelin selectively reduces mechanosensitivity of upper gastrointestinal vagal afferents

Ghrelin is a peptide released from gastric endocrine cells that has an orexigenic effect via a vagal pathway. Here we determine the effect of ghrelin on mechanosensitivity of upper-intestinal vagal afferent fibers in ferret and mouse. The responses of gastroesophageal vagal afferents to graded mechanical stimulation were determined in vitro before and during application of ghrelin to their peripheral endings. Three types of vagal afferent were tested: tension receptors responding to circumferential tension, mucosal receptors responding only to mucosal stroking, and tension/mucosal (TM) receptors in ferret esophagus that responded to both stimuli. In the mouse, ghrelin did not significantly affect the response of mucosal receptors to mucosal stroking with calibrated von Frey hairs. However, it significantly reduced responses of tension receptors to circumferential tension (P < 0.005; two-way ANOVA) by up to 40%. This inhibition was reversed by the ghrelin receptor antagonist [d-Lys-3]-growth hormone-releasing peptide (GHRP)-6. In the ferret, ghrelin significantly reduced the response of mucosal and TM receptors to mucosal stroking with calibrated von Frey hairs. Surprisingly, ghrelin did not significantly alter the response to circumferential tension in either tension or TM receptors. RT-PCR analysis indicated that both ghrelin and its receptor are expressed in vagal afferent cell bodies in mouse nodose ganglia. In conclusion, ghrelin selectively inhibits subpopulations of mechanically sensitive gastroesophageal vagal afferents; there is also potential for ghrelin release from vagal afferents. However, the subpopulation of afferents inhibited differs between species. These data have broad implications for ghrelin's role in food intake regulation and reflex control of gastrointestinal function.

Characterization of mechanosensitive splanchnic nerve afferent fibers innervating the rat stomach

Splanchnic nerve fibers innervating the stomach were studied in anesthetized rats; 997 fibers in the T(9) or T(10) dorsal roots were identified by electrical stimulation of the splanchnic nerve. Thirty-one fibers responded to gastric distension. Extrapolated response thresholds ranged between 0 and 53 mmHg; seven fibers had thresholds for response > or =30 mmHg. Thermo- and/or chemosensitivity was tested in 18 of the 31 fibers. Four of twelve fibers responded to intragastric perfusion of heated saline; none of eight fibers tested responded to perfusion of cold saline. Infusion of glucose, L-arginine, or potassium oleate produced no change in resting activity. Intragastric instillation of 12% glycerol or an inflammatory soup (bradykinin 10(-5) M, PGE(2) 10(-5) M, serotonin 10(-5) M, histamine 10(-5) M, and KCl 10(-3) M) and prior heat stimulation sensitized responses to distension. The results reveal the presence of low- and high-threshold mechanosensitive fibers in the splanchnic innervation of the stomach. These fibers have the ability to sensitize, and they likely contribute to pain and altered sensations that can arise from the stomach.

Parametric analysis of gastric distension responses in the parabrachial nucleus

The parabrachial nucleus (PBN) is regarded as an important locus for the processing and integration of sensory inputs from oral, gastrointestinal, and postabsorptive receptor sites and is thus thought to play an important role in regulating food intake. Gastric distension is an important satiation cue; however, such responses have been qualitatively characterized only over a limited area of the PBN. To more fully characterize gastric distension responses throughout the PBN, the responses of single units to gastric distension were tested using computer-controlled balloon inflation (3-18 ml air) in pentobarbital sodium- and/or urethan-anesthetized male rats. Distension-responsive neurons were indeed distributed throughout the nucleus from rostral areas typically considered to be visceral to more caudal areas associated with gustatory function, providing further anatomical support for the hypothesis that the PBN integrates taste and visceral signals that control feeding. Most PBN neurons had thresholds of 6 ml or less, similar to vagal afferent fibers. However, in contrast to the periphery, there were both excitatory and inhibitory responses. Increases in volume were associated with two distinct effects. First, as volume increased, the response rate increased; second, the duration of the response increased. In fact, in a subset of cells, responses to gastric distension lasted well beyond the stimulation period, particularly at larger volumes. Prolonged gastric distension responses are not common in the periphery and may constitute a central mechanism that contributes to satiation processes.

Effect of CCK pretreatment on the CCK sensitivity of rat polymodal gastric vagal afferent in vitro

To prevent the blood-borne interference and reflex actions via neighboring organs and the central nervous system, the study was conducted in an in vitro isolated stomach-gastric vagus nerve preparation obtained from overnight-fasted, urethan-anesthetized rats. Afferent unit action potentials were recorded from the gastric branch of the vagus nerve. The left gastric artery was catheterized for intra-arterial injection. In vitro we found that 1) 55/70 gastric vagal afferents (GVAs) were polymodal, responding to CCK-8 and mechanical stimuli, 13 were mechanoreceptive, and 2 were CCK-responsive; 2) sequential or randomized intra-arterial injections of CCK-8 (0.1-200 pmol) dose-dependently increased firing rate and reached the peak rate at 100 pmol; 3) the action was suppressed by CCK-A (Devazepide) but not by CCK-B (L-365,260) receptor antagonist; 4) neither antagonist blocked the mechanosensitivity of GVA fibers. These results are consistent with corresponding in vivo well-documented findings. Histological data indicate that the layered structure of the stomach wall was preserved in vitro for 6-8 h. Based on these results, it seems reasonable to use the in vitro preparation for conducting a study that is usually difficult to be performed in vivo. For instance, because there was no blood supply in vitro, the composition of the interstitial fluid, i.e., the ambient nerve terminals, can be better controlled and influenced by intra-arterial injection of a defined solution. Here we report that acutely changing the ambient CCK level by a conditioning stimulus (a preceding intra-arterial injection of increasing doses of CCK-8) reduced the CCK sensitivity of GVA terminals to a subsequent test stimulus (a constant dose of CCK-8 intra-arterial injection).

Exogenous cholecystokinin-8 reduces vagal efferent nerve activity in rats through CCK(A) receptors

It has been proposed that the vagus nerve plays a role in mediating cholecystokinin-8 (CCK-8) effect on such gastric functions as motility, emptying and gastric acid secretion. To examine the contribution of the efferent pathways in realizing these effects, efferent mass activity in the ventral gastric vagal nerve in Sprague-Dawley rats was recorded. Intravenous infusion of CCK-8 (0.1-1 nmol) suppressed the efferent activity. The effect of CCK-8 was significantly reduced in animals with total subdiaphragmatic vagotomy in comparison to those with partial vagotomy. Intravenous infusion of CCK(A) receptor antagonist L-364,718 (1-100x10(-6) g) blocked the response of vagal efferent activity to 0.1 nmol CCK-8, but the CCK(B) receptor antagonist L-365,260 (1-100x10(-6) g) did not in the conditions of either partial or total vagotomy. Intracisternal infusion of L-364,718 (1x10(-6) g) blocked the response of vagal efferent activity to 0.1 nmol CCK-8 i.v. Infusion of exogenous CCK-8 did not affect the activity of supradiaphragmatic vagal afferents. The results suggest that the effect of systemically administered CCK-8 on vagal efferent activity is mediated by both peripherally (subdiaphragmatically) and centrally localized CCK(A) receptors.

Sensitivity of vagal mucosal afferents to cholecystokinin and its role in afferent signal transduction in the rat

1. Extracellular recordings from rat mesenteric paravascular nerve bundles were made in order to characterize the responses of different populations of afferents supplying the small intestine to intravenous cholecystokinin (CCK; in the form of sulphated CCK8). 2. Approximately 70% of mesenteric nerve bundles contained CCK-sensitive afferent fibres. Responsive afferents had low spontaneous discharge (1.6 +/- 0.3 impulses s-1) and showed a 14-fold increase in firing at the peak of the response to 50 pmol CCK with the overall response lasting several minutes. The onset of the response occurred after a latency of (3.9 +/- 0.1 s) following i.v. administration of CCK, which corresponds largely to the circulation delay in these animals. The threshold dose of CCK was < 5 pmol. 3. The response to 100 pmol CCK was completely abolished by devazepide (0.5 mg kg-1) and by chronic subdiaphragmatic vagotomy performed 10-14 days prior to experimentation, indicating that CCK sensitivity was via CCKA receptors and exclusively mediated via vagal afferents rather than splanchnic or enteric afferents. 4. Evidence that CCK-sensitive afferents had mucosal receptive fields was indicated by the lack of any response to luminal distension and the sensitivity of the CCK response to luminal anaesthesia. Furthermore, CCK-sensitive afferents responded to luminal hydrochloric acid (50 mM) in a slowly adapting manner. The response to acid was significantly reduced (P < 0.005), but not abolished, by devazepide at a time when the response to exogenous CCK had been completely eliminated. 5. The exquisite sensitivity of some vagal mucosal afferents to CCK suggests that they may play a physiological role in the reflex and behavioural consequences of CCK release from the small intestine, possibly acting in a paracrine fashion. However, this sensitivity to CCK represents only one aspect of the broad chemosensitivity of these mucosal afferents and is not an obligatory component of the signal transduction pathway.