Takashi Kondoh

Mechanisms of Neural Response to Gastrointestinal Nutritive Stimuli: The Gut-Brain Axis

BACKGROUND & AIMS The gut-brain axis, which transmits nutrient information from the gastrointestinal tract to the brain, is important for the detection of dietary nutrients. We used functional magnetic resonance imaging of the rat forebrain to investigate how this pathway conveys nutrient information from the gastrointestinal tract to the brain. METHODS We investigated the contribution of the vagus nerve by comparing changes of blood oxygenation level-dependent signals between 24 control rats and 22 rats that had undergone subdiaphragmatic vagotomy. Functional data were collected under alpha-chloralose anesthesia continuously 30 minutes before and 60 minutes after the start of intragastric infusion of L-glutamate or glucose. Plasma insulin, L-glutamate, and blood glucose levels were measured and compared with blood oxygenation level-dependent signals. RESULTS Intragastric administration of L-glutamate or glucose induced activation in distinct forebrain regions, including the cortex, hypothalamus, and limbic areas, at different time points. Vagotomy strongly suppressed L-glutamate-induced activation in most parts of the forebrain. In contrast, vagotomy did not significantly affect brain activation induced by glucose. Instead, blood oxygenation level-dependent signals in the nucleus accumbens and amygdala, in response to gastrointestinal glucose, varied along with fluctuations of plasma insulin levels. CONCLUSIONS These results indicate that the vagus nerve and insulin are important for signaling the presence of gastrointestinal nutrients to the rat forebrain. These signal pathways depend on the ingested nutrients.

Activation of the gut-brain axis by dietary glutamate and physiologic significance in energy homeostasis

l-Glutamate is a multifunctional amino acid involved in taste perception, intermediary metabolism, and excitatory neurotransmission. In addition, recent studies have uncovered new roles for l-glutamate in gut-brain axis activation and energy homeostasis. l-Glutamate receptors and their cellular transduction molecules have recently been identified in gut epithelial cells. Stimulation of such l-glutamate receptors by luminal l-glutamate activates vagal afferent nerve fibers and then parts of the brain that are targeted directly or indirectly by these vagal inputs. Notably, 3 areas of the brain-the medial preoptic area, the hypothalamic dorsomedial nucleus, and the habenular nucleus-are activated by intragastric l-glutamate but not by glucose or sodium chloride. Furthermore, the chronic, ad libitum ingestion of a palatable solution of monosodium l-glutamate (1% wt:vol) by rats has also been found to reduce weight gain, fat deposition, and plasma leptin concentrations compared with rats that ingest water alone. No difference in food intake was observed. Such effects may also be vagally mediated. Together, such findings contribute to the growing knowledge base that indicates that l-glutamate signaling via taste and gut l-glutamate receptors may influence multiple physiologic functions, such as thermoregulation and energy homeostasis.

MSG intake suppresses weight gain, fat deposition, and plasma leptin levels in male Sprague-Dawley rats.

Monosodium l-glutamate (MSG), an umami taste substance, may be a key molecule coupled to a food intake signaling pathway, possibly mediated through a specific l-glutamate (GLU) sensing mechanism in the gastrointestinal tract. Here we investigated the effect of the spontaneous ingestion of a 1% MSG solution and water on food intake and body weight in male Sprague-Dawley rats fed diets of varying caloric density, fat and carbohydrate contents. Fat mass and lean mass in the abdomen, blood pressure, and several blood metabolic markers were also measured. Rats given free access to MSG and water showed a high preference (93-97%) for the MSG solution, regardless of the diet they consumed. Rats ingesting MSG had a significantly smaller weight gain, reduced abdominal fat mass, and lower plasma leptin levels, compared to rats ingesting water alone. Naso-anal length, lean mass, food and energy intakes, blood pressure, blood glucose, and plasma levels of insulin, triglyceride, total cholesterol, albumin, and GLU were not influenced by the ingestion of the MSG solution. These same effects were observed in a study of adult rats. Together, these results suggest that MSG ingestion reduces weight gain, body fat mass, and plasma leptin levels. Moreover, these changes are likely to be mediated by increased energy expenditure, not reduced energy intake or delayed development. Conceivably, these effects of MSG might be mediated via gut GLU receptors functionally linked to afferent branches of the vagus nerve in the gut, or the afferent sensory nerves in the oral cavity.

Melatonin reduces cerebral edema formation caused by transient forebrain ischemia in rats.

Reduction of cerebral edema, an early symptom of ischemia, is one of the most important remedies for reducing subsequent chronic neural damage in stroke. Melatonin, a metabolite of tryptophan released from the pineal gland, has been found to be effective against neurotoxicity in vitro. The present study was aimed to demonstrate the effectiveness of melatonin in vivo in reducing ischemia-induced edema using magnetic resonance imaging (MRI). Rats were subjected to middle cerebral artery (MCA) occlusion/reperfusion surgery. Melatonin was administered twice (6.0 mg/kg, p.o.): just prior to 1 h MCA occlusion and 1 day after the surgery. T2-weighted multislice spin-echo images were acquired 1 day after the surgery. Increases in T2-weighted signals in ischemic sites of the brain were clearly observed after MCA occlusion. The signal increase was found mainly in the striatum and in the cerebral cortex in saline-treated control rats. In the melatonin-treated group, the total volume of cerebral edema was reduced by 45.3% compared to control group (P < 0.01). The protective effect of melatonin against cerebral edema was more clearly observed in the cerebral cortex (reduced by 56.1%, P < 0.01), while the reduction of edema volume in the striatum was weak (reduced by 23.0%). The present MRI study clearly demonstrated that melatonin is effective in reducing edema formation in ischemic animals in vivo, especially in the cerebral cortex. Melatonin may be highly useful in preventing cortical dysfunctions such as motor, sensory, memory, and psychological impairments.

Magnetic resonance imaging and histologic evidence of postoperative back muscle injury in rats.

Study Design. Postoperative back muscle injury was evaluated in rats by magnetic resonance imaging and histologic analyses. Objective. To compare the magnetic resonance imaging manifestation of back muscle injury with the histologic findings in rats and to subsequently clarify the histopathologic appearance of the high intensity regions on T2-weighted images in human postoperative back muscles. Summary of Background Data. In a previous study, it was found that the signal intensity on T2-weighted images of the postoperative back muscles was increased in patients who had postsurgical lumbar muscle impairment, especially in those with a prolonged surgery duration. However, the specific histopathologic changes that cause the high signal intensity on T2-weighted images remain unclear. Methods. Rats were divided into three groups: sham operation group, 1-hour retraction group, and 2-hour retraction group. Magnetic resonance imaging and histology of the multifidus muscles were examined before surgery and at 2, 7, and 21 days after surgery. Results. T2-weighted imaging was more useful than T1-weighted imaging to estimate back muscle injury. The high signal intensity of the multifidus muscles on T2-weighted images remained 21 days after surgery only in the 2-hour retraction group. Histologically, the regeneration of the multifidus muscles was complete at 21 days after surgery in the 1-hour retraction group, but the regenerated muscle fibers in the 2-hour retraction group had a small diameter, and the extracellular fluid space remained large. Conclusion. The high signal intensity on T2-weighted images of the postoperative multifidus muscles in the regenerative phase may be due to an increased extracellular space and incomplete muscle fiber regeneration.

Melatonin suppresses cerebral edema caused by middle cerebral artery occlusion/reperfusion in rats assessed by magnetic resonance imaging

Abstract: Melatonin, a pineal secretory product synthesized from tryptophan, has been found to be effective against neurotoxicity. The present study was aimed at demonstrating the effectiveness of melatonin in vivo in reducing ischemia‐induced cerebral edema using magnetic resonance imaging (MRI). Rats were subjected to middle cerebral artery (MCA) occlusion/reperfusion surgery. Melatonin was administered twice (6.0 mg/kg, p.o.) just prior to 1 hr of MCA occlusion and 1 day after the surgery. T2‐weighted multislice spin‐echo images were acquired 1 day after the surgery. In the saline‐treated control rats, increases in T2‐weighted signals (water content) were clearly observed in the striatum and in the cerebral cortex. In the melatonin‐treated group, total volume of edema was reduced by 51.6% compared with control group (P < 0.01). The protective effect of melatonin against edema was more clearly observed in the cerebral cortex (reduced by 59.8%, P < 0.01) than in the striatum (reduced by 34.2%, P < 0.05). Edema volume in a coronal slice was the greatest at the level of the bregma. Suppression of cerebral edema by melatonin was more effective posterior than anterior to the bregma. Melatonin appeared to reduce the volume of the edematous sites rather than to shift the signal intensity distribution. The present MRI study clearly demonstrates the effectiveness of melatonin against cerebral edema formation in ischemic animals in vivo, especially in the cerebral cortex. Melatonin may be highly useful in preventing cortical dysfunctions such as motor, sensory, memory, and psychological impairments associated with ischemic stroke.

Forebrain activation induced by postoral nutritive substances in rats.

Recent studies have shown the nutrient-sensing systems transmitting nutritive information from the gut to the brain. However neural activity evoked by ingested dietary nutrients has not beeninvestigated adequately. Using functional magnetic resonance imaging, we demonstrated that rat forebrain responded to intragastric administration of glucose, L-glutamate, and NaCl. These dietary nutrients led to a significant activation in the forebrain regions including nucleus accumbens, hypothalamic area, and limbic system with different timings. These data indicate that several forebrain regions have important roles on perception and process of postingestive nutrient information.

Conditioned flavor preference learning by intragastric administration of L-glutamate in rats

The preference for foods or fluids in rats is partly dependent on its postingestive consequences. Many studies have investigated postingestive effect of high caloric substances, such as carbohydrate or fat. In this study, we examined postingestive effect of L-glutamate at the preferable concentration using conditioned flavor preference paradigm. Adult male rats with chronic intragastric (IG) cannula were trained to drink a flavored solution (conditioned stimulus; CS+) paired with IG infusion of nutrient solution and another flavored solution (CS-) with IG distilled water infusion on alternate days. The nutrient solution was 60mM monosodium L-glutamate, sodium chloride or glucose. Before and after conditioning, rats received 30min two-bottle choice tests for CS+ and CS- solution. All groups exhibited no significant preference for CS+ in pre-test period. By the last half of conditioning period, intake of CS+ solution was significantly higher than that of CS- in MSG group, but not in NaCl and glucose groups. After conditioned, the MSG group showed significantly higher intake and preference for CS+ solution (69.9%), while the NaCl and glucose group did not show any significant intake and preference for CS+ solution (50.9%, 43.5%, respectively). These results indicate that the amino acid L-glutamate at a preferable concentration has a positive postingestive effect as demonstrated by its ability to condition a flavor preference. The mechanism(s) for this positive effect could be through a direct effect on gut Glu receptors rather than the provision of calories or glucose from metabolized Glu; Further studies are needed to test these hypotheses.

Effects of D2 dopamine receptor agonist and antagonist on brain activity in the rat assessed by functional magnetic resonance imaging

The effects of D2 dopamine receptor agonist, bromocriptine (BROMO), and antagonist, haloperidol (HPD), on brain activity were investigated in rats by functional magnetic resonance imaging. T2*-weighted signal intensity was increased in the hypothalamus at 120 min after acute administration of BROMO, and in the ventral posterior and dorsomedial nuclei of the thalamus from 30 to 120 min. In contrast, the signal intensity was decreased in the caudate-putamen at 30 min after acute administration of HPD, in the hypothalamus from 30 to 60 min, and in the perirhinal cortex at 30 min. After chronic (2 weeks) HPD treatment, acute administration of HPD decreased signal intensity in the caudate-putamen at 60 min, in the hypothalamus at 30 min, the perirhinal cortex from 2 to 120 min, the dorsomedial and ventral posterior nuclei of the thalamus from 2 to 120 min, and the medial nucleus of the amygdala from 60 to 120 min. These results suggest that (1) the D2 receptor agonist increased the activity of the thalamic nuclei and the hypothalamus, while the D2 receptor antagonist suppressed brain activity in the regions where D2 receptors were present, (2) the suppression of brain activity in the thalamic nuclei and the perirhinal cortex by acute HPD administration was enhanced by chronic HPD treatment, and (3) the effects of antipsychotic drugs on the thalamus, amygdala, and perirhinal cortex may be related to their therapeutic efficacy, since clinical improvement in schizophrenic patients appears several days after the start of HPD treatment.

Spatio–temporal dynamics of brain activated regions during drinking behavior in rats

Spatio-temporal dynamics of activated brain areas induced by drinking were investigated and visualized in behaving rats using functional magnetic resonance imaging (fMRI). The rats were trained to drink in the magnet bore, and the images were taken during and after drinking glucose and distilled water. During glucose ingestion, the signal intensity was increased continuously and maximally in the lateral hypothalamic area (LHA) and the ventromedial hypothalamus (VMH). Somewhat less intense activation in the central nucleus of the amygdala (AMc), and transient activation in the piriform cortex and the mediodorsal nucleus of the thalamus were observed. The signal intensities of other regions measured were largely unchanged. During post-ingestive periods, the signals re-increased in the hypothalamic areas and AMc. When water was given, LHA and VMH were similarly activated, however, the signal intensity in the amygdala was not significantly increased. The results indicate that these brain regions are activated differentially during drinking behavior, and that LHA and VMH play a central role in the control of not only feeding but also drinking. The regional activities in LHA and VMH are not principally related to the gustatory sensation, and the reactivation after drinking may be related to satisfaction or post-ingestive nutritional information. Also, the responses of AMc are probably due to reward value difference. To the best of our knowledge, this is the first report of mapping of brain areas using fMRI in behaving rats. The improved method described in this study for collecting fMRI data in behaving animals will be useful for studying functional network during animal behavior.