Cardiovascular effects of gastric intubation and distension in healthy humans

Stomach-brain synchrony reveals a novel, delayed-connectivity resting-state network in humans

Resting-state networks offer a unique window into the brain’s functional architecture, but their characterization remains limited to instantaneous connectivity thus far. Here, we describe a novel resting-state network based on the delayed connectivity between the brain and the slow electrical rhythm (0.05 Hz) generated in the stomach. The gastric network cuts across classical resting-state networks with partial overlap with autonomic regulation areas. This network is composed of regions with convergent functional properties involved in mapping bodily space through touch, action or vision, as well as mapping external space in bodily coordinates. The network is characterized by a precise temporal sequence of activations within a gastric cycle, beginning with somato-motor cortices and ending with the extrastriate body area and dorsal precuneus. Our results demonstrate that canonical resting-state networks based on instantaneous connectivity represent only one of the possible partitions of the brain into coherent networks based on temporal dynamics.

The brain is always active. Even when it is not receiving sensory input, it generates its own spontaneous activity. This activity shapes how we interpret future sensory signals and creates our inner mental world. Moreover, this spontaneous activity is not random. When a healthy volunteer lies inside a brain scanner without performing any task, his or her brain shows predictable patterns of activity. Specific groups of brain regions – often with related roles – become active at the same time as one another. Each set of regions is referred to as a resting state network.

Of course, the brain does not operate in isolation from the rest of the body. Our internal organs continuously send signals to the brain via the spinal cord and cranial nerves. Specialized cells in the stomach wall in particular produce a slow rhythmic pattern of electrical activity. Known as the gastric rhythm, this activity helps ensure that the stomach muscles contract at the correct speed for digestion. But the stomach also produces this rhythm even when empty, suggesting that it has other roles too.

To find out what these might be, Rebollo et al. placed electrodes on the abdomen of healthy volunteers lying inside brain scanners. By examining the volunteers’ spontaneous brain activity, Rebollo et al. identified a new resting state network that is active in synchrony with the gastric rhythm. The regions within this so-called gastric network are not active at the same time as each other, but instead become active in a specific sequence that is repeated at each gastric cycle. Many of the regions within the gastric network belong to other resting state networks too. Some of the regions help regulate automatic bodily functions such as heart rate, while others process information about the body’s position in space.

The existence of the gastric network suggests a link between the automatic regulation of processes such as digestion, and spontaneous brain activity. Future studies could examine whether this link impacts perception and cognition, and whether this link plays a role in disorders where the connection between the digestive system and the brain appears to be altered.

Analysis of cardiorespiratory phase coupling and cardiovascular autonomic responses during food ingestion

The present study analyzed whether the phase coherency (λ) of respiratory sinus arrhythmia (RSA) is altered by food ingestion in healthy young subjects. After 5min of resting control, 13 healthy volunteers were asked to eat a solid meal with access to water at their own pace, followed by 5min of the postprandial state. The R-R interval (RRI), beat-to-beat blood pressure (BP), and respiratory activity were recorded using electrocardiography, a Finapres device, and inductance plethysmography, respectively. The stroke volume was calculated by the pulse-contour method from continuous BP measurement, and the cardiac output (CO) was obtained by multiplying the stroke volume by the heart rate. From the oscillatory signals of RSA and respiration, λ was computed; additionally, frequency domain indexes of the heart rate variability (HRV) were calculated using a short-time Fourier transform. A steady-state 3-min resting period (R), food ingestion period (FOOD), and the first 2-min and the last 3-min of the post prandial period were analyzed separately. We also compared the responses to gum chewing (GUM) and water intake (WATER) using the same protocol on separate days. A shortening of RRI and increases in BP and CO were observed in FOOD compared to R, suggesting a shift of sympathovagal balance toward sympathetic activation. Similar responses but smaller magnitudes were observed in the GUM condition, whereas only transient shortening of RRI was observed in the WATER condition. The HRV indexes did not show any significant changes in response to GUM and WATER but sympathovagal balance was shifted in favor of sympathetic dominance in FOOD. λ decreased during all of the conditions. There was a significant negative correlation between λ and the indirect measure of sympathovagal balance. These results suggest that ingestion of food induces enhanced cardiac sympathetic activity and that a phase coherence of RSA could provide a sensitive measure for evaluating the cardiac autonomic profile.

The sum of its parts--effects of gastric distention, nutrient content and sensory stimulation on brain activation

During food consumption the brain integrates multiple interrelated neural and hormonal signals involved in the regulation of food intake. Factors influencing the decision to stop eating include the foods' sensory properties, macronutrient content, and volume, which in turn affect gastric distention and appetite hormone responses. So far, the contributions of gastric distention and oral stimulation by food on brain activation have not been studied. The primary objective of this study was to assess the effect of gastric distention with an intra-gastric load and the additional effect of oral stimulation on brain activity after food administration. Our secondary objective was to study the correlations between hormone responses and appetite-related ratings and brain activation. Fourteen men completed three functional magnetic resonance imaging sessions during which they either received a naso-gastric infusion of water (stomach distention), naso-gastric infusion of chocolate milk (stomach distention + nutrients), or ingested chocolate-milk (stomach distention + nutrients + oral exposure). Appetite ratings and blood parameters were measured at several time points. During gastric infusion, brain activation was observed in the midbrain, amygdala, hypothalamus, and hippocampus for both chocolate milk and water, i.e., irrespective of nutrient content. The thalamus, amygdala, putamen and precuneus were activated more after ingestion than after gastric infusion of chocolate milk, whereas infusion evoked greater activation in the hippocampus and anterior cingulate. Moreover, areas involved in gustation and reward were activated more after oral stimulation. Only insulin responses following naso-gastric infusion of chocolate milk correlated with brain activation, namely in the putamen and insula. In conclusion, we show that normal (oral) food ingestion evokes greater activation than gastric infusion in stomach distention and food intake-related brain areas. This provides neural evidence for the importance of sensory stimulation in the process of satiation.

Trial Registration

ClinicalTrials.gov NCT01644539.

Gastric distention induced functional magnetic resonance signal changes in the rodent brain

Investigating the localization of gastric sensation within the brain is important for understanding the neural correlates of satiety. Previous rodent studies have identified the brain-stem and hypothalamus as key mediators of gastric distention-induced satiation. Although, recent blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) studies in humans have identified a role for higher cortico-limbic structures in mediating the satiation effects of gastric distention, the role of these regions in rodents remains to be characterized. We determined the effects of gastric distention on global spatio-temporal BOLD fMRI signal changes in the rodent brain. Brain images were acquired with a high resolution 9.4 T magnet during gastric distention with continuous monitoring of blood pressure in adult male Sprague Dawley rats (n=8-10). Distention of the stomach with an intragastric balloon, at rates which mimicked the rate of consumption and emptying of a mixed nutrient liquid meal, resulted in robust reduction in food intake and increase in blood pressure. Gastric distention increased BOLD fMRI activity within homeostatic regions such as the hypothalamus and nucleus tractus solitarius, as well as non homeostatic regions including the hippocampus, amygdala, thalamus, cerebellum and the cortex (cingulate, insular, motor and sensory cortices). Further, the increase in BOLD fMRI activity following distention was strongly correlated to an increase in blood pressure. These results indicate that gastric distention, mimicking the rate of intake and emptying of a liquid meal, increases BOLD fMRI activity in both homeostatic and non homeostatic brain circuits which regulate food intake, and that these BOLD fMRI signal changes may in part be attributable to transient increases in blood pressure.

Comparative effects of oral and intraduodenal glucose on blood pressure, heart rate, and splanchnic blood flow in healthy older subjects

Postprandial hypotension occurs frequently, particularly in the elderly. The magnitude of the fall in blood pressure (BP) and rise in heart rate (HR) in response to enteral glucose are greater when gastric emptying (GE) or small intestinal infusion are more rapid. Meal ingestion is associated with an increase in splanchnic blood flow. In contrast, gastric distension may attenuate the postprandial fall in BP. The aims of this study were to evaluate, in older subjects, the comparative effects of intraduodenal glucose infusion, at a rate similar to GE of oral glucose, on BP, HR, superior mesenteric artery (SMA) flow, and blood glucose. Eight healthy subjects (5 men, 3 women, age 66-75 yr) were studied on two occasions. On day 1, each subject ingested 300 ml of water containing 75 g glucose. GE was quantified by three-dimensional ultrasonography between time t = 0-120 min, and the rate of emptying (kcal/min) was calculated. On day 2, glucose was infused intraduodenally at the same rate as that on day 1. On both days, BP, HR, SMA flow, and blood glucose were measured. The mean GE of oral glucose was 1.3 +/- 0.1 kcal/min. Systolic BP (P < 0.01), SMA flow (P < 0.05), and blood glucose (P < 0.01) were greater and HR less (P < 0.01) after oral, compared with intraduodenal, glucose. There were comparable falls in diastolic BP during the study days (P < 0.01 for both). We conclude that the magnitude of the fall in systolic BP and rise in HR are less after oral, compared with intraduodenal, glucose, presumably reflecting the "protective" effect of gastric distension.