Neural systems controlling food intake and energy balance in the modern world

Rethinking the role of microglia in obesity

Microglia are the macrophages of the central nervous system (CNS), implying their role in maintaining brain homeostasis. To achieve this, these cells are sensitive to a plethora of endogenous and exogenous signals, such as neuronal activity, cellular debris, hormones, and pathological patterns, among many others. More recent research suggests that microglia are highly responsive to nutrients and dietary variations. In this context, numerous studies have demonstrated their significant role in the development of obesity under calorie surfeit. Because many reviews already exist on this topic, we have chosen to present the state of our reflections on various concepts put forth in the literature, bringing a new perspective whenever possible. Our literature review focuses on studies conducted in the arcuate nucleus of the hypothalamus, a key structure in the control of food intake. Specifically, we present the recent data available on the modifications of microglial energy metabolism following the consumption of an obesogenic diet and their consequences on hypothalamic neuron activity. We also highlight the studies unraveling the mechanisms underlying obesity-related sexual dimorphism. The review concludes with a list of questions that remain to be addressed in the field to achieve a comprehensive understanding of the role of microglia in the regulation of body energy metabolism. This article is part of the Special Issue on "Microglia".

Sports Diet and Oral Health in Athletes: A Comprehensive Review

Food and fluid supply is fundamental for optimal athletic performance but can also be a risk factor for caries, dental erosion, and periodontal diseases, which in turn can impair athletic performance. Many studies have reported a high prevalence of oral diseases in elite athletes, notably dental caries 20-84%, dental erosion 42-59%, gingivitis 58-77%, and periodontal disease 15-41%, caused by frequent consumption of sugars/carbohydrates, polyunsaturated fats, or deficient protein intake. There are three possible major reasons for poor oral health in athletes which are addressed in this review: oxidative stress, sports diet, and oral hygiene. This update particularly summarizes potential sports nutritional effects on athletes' dental health. Overall, sports diet appropriately applied to deliver benefits for performance associated with oral hygiene requirements is necessary to ensure athletes' health. The overall aim is to help athletes, dentists, and nutritionists understand the tangled connections between sports diet, oral health, and oral healthcare to develop mitigation strategies to reduce the risk of dental diseases due to nutrition.

Primary, Secondary, and Tertiary Effects of Carbohydrate Ingestion During Exercise

The purpose of this current opinion paper is to describe the journey of ingested carbohydrate from 'mouth to mitochondria' culminating in energy production in skeletal muscles during exercise. This journey is conveniently described as primary, secondary, and tertiary events. The primary stage is detection of ingested carbohydrate by receptors in the oral cavity and on the tongue that activate reward and other centers in the brain leading to insulin secretion. After digestion, the secondary stage is the transport of monosaccharides from the small intestine into the systemic circulation. The passage of these monosaccharides is facilitated by the presence of various transport proteins. The intestinal mucosa has carbohydrate sensors that stimulate the release of two 'incretin' hormones (GIP and GLP-1) whose actions range from the secretion of insulin to appetite regulation. Most of the ingested carbohydrate is taken up by the liver resulting in a transient inhibition of hepatic glucose release in a dose-dependent manner. Nonetheless, the subsequent increased hepatic glucose (and lactate) output can increase exogenous carbohydrate oxidation rates by 40-50%. The recognition and successful distribution of carbohydrate to the brain and skeletal muscles to maintain carbohydrate oxidation as well as prevent hypoglycaemia underpins the mechanisms to improve exercise performance.

Can taste be ergogenic?

Taste is a homeostatic function that conveys valuable information, such as energy density, readiness to eat, or toxicity of foodstuffs. Taste is not limited to the oral cavity but affects multiple physiological systems. In this review, we outline the ergogenic potential of substances that impart bitter, sweet, hot and cold tastes administered prior to and during exercise performance and whether the ergogenic benefits of taste are attributable to the placebo effect. Carbohydrate mouth rinsing seemingly improves endurance performance, along with a potentially ergogenic effect of oral exposure to both bitter tastants and caffeine although subsequent ingestion of bitter mouth rinses is likely required to enhance performance. Hot and cold tastes may prove beneficial in circumstances where athletes' thermal state may be challenged. Efficacy is not limited to taste, but extends to the stimulation of targeted receptors in the oral cavity and throughout the digestive tract, relaying signals pertaining to energy availability and temperature to appropriate neural centres. Dose, frequency and timing of tastant application likely require personalisation to be most effective, and can be enhanced or confounded by factors that relate to the placebo effect, highlighting taste as a critical factor in designing and administering applied sports science interventions.

Homeostatic and non-homeostatic controls of feeding behavior: Distinct vs. common neural systems

Understanding the neurobiological controls of feeding behavior is critical in light of the growing obesity pandemic, a phenomenon largely based on excessive caloric consumption. Feeding behavior and its underlying biological substrates are frequently divided in the literature into two separate categories: [1] homeostatic processes involving energy intake based on caloric and other metabolic deficits, and [2] non-homeostatic processes that involve feeding driven by environmental and cognitive factors. The present review summarizes both historic and recent research examining the homeostatic regulation of feeding with specific emphasis on hypothalamic and hindbrain circuitry that monitor and regulate various metabolic signals. Regarding non-homeostatic controls, we highlight higher-order brain structures that integrate feeding-relevant external, interoceptive, and cognitive factors, including sensory cortical processing, learned associations in the hippocampus, and reward-based processing in the nucleus accumbens and interconnected mesolimbic circuitry. Finally, the current review focuses on recent evidence that challenges the traditional view that distinct neural systems regulate homeostatic vs. non-homeostatic controls of feeding behavior. Specifically, we highlight several feeding-related endocrine systems that act on both lower- and higher-order substrates, present evidence for the modulation of learned and cognitive feeding-relevant behaviors by lower-order brain regions, and highlight data showing that apparent homeostatic-based feeding behavior is modulated by higher-order brain regions. Our concluding perspective is that the classic dissociation between homeostatic and non-homeostatic constructs in relation to feeding behavior is limited with regards to understanding the complex integrated neurobiological systems that control energy balance.

The CB1 Receptor as the Cornerstone of Exostasis

The type-1 cannabinoid receptor (CB1) is the main effector of the endocannabinoid system (ECS), which is involved in most brain and body functions. In this Perspective, we provide evidence indicating that CB1 receptor functions are key determinants of bodily coordinated exostatic processes. First, we will introduce the concepts of endostasis and exostasis as compensation or accumulation for immediate or future energy needs and discuss how exostasis has been necessary for the survival of species during evolution. Then, we will argue how different specific biological functions of the CB1 receptor in the body converge to provide physiological exostatic processes. Finally, we will introduce the concept of proactive evolution-induced diseases (PEIDs), which helps explain the seeming paradox that an evolutionary-selected physiological function can become the cause of epidemic pathological conditions, such as obesity. We propose here a possible unifying theory of CB1 receptor functions that can be tested by future experimental studies.

Effect of fish oil intake on glucose levels in rat prefrontal cortex, as measured by microdialysis

Background

Brain glucose sensing may contribute to energy homeostasis control. The prefrontal cortex (PFC) participates in the hedonic component of feeding control. As high-fat diets may disrupt energy homeostasis, we evaluated in male Wistar rats whether intake of high-fat fish-oil diet modified cortical glucose extracellular levels and the feeding induced by intracerebroventricular glucose or PFC glucoprivation.

Methods

Glucose levels in PFC microdialysates were measured before and after a 30-min meal. Food intake was measured in animals receiving intracerebroventricular glucose followed, 30-min. later, by 2-deoxy-D-glucose injected into the PFC.

Results

The fish-oil group showed normal body weight and serum insulin while fat pads weight and glucose levels were increased. Baseline PFC glucose and 30-min. carbohydrates intake were similar between the groups. Feeding-induced PFC glucose levels increased earlier and more pronouncedly in fish-oil than in control rats. Intracerebroventricular glucose inhibited feeding consistently in the control but not in the fish-oil group. Local PFC glucoprivation with 2-DG attenuated glucose-induced hypophagia.

Conclusions

The present experiments have shown that, following food intake, more glucose reached the prefrontal cortex of the rats fed the high-fat fish-oil diet than of the rats fed the control diet. However, when administered directly into the lateral cerebral ventricle, glucose was able to consistently inhibit feeding only in the control rats. The findings indicate that, an impairment of glucose transport into the brain does not contribute to the disturbances induced by the high-fat fish-oil feeding.

Intranasal insulin modulates intrinsic reward and prefrontal circuitry of the human brain in lean women

AIM

There is accumulating evidence that food consumption is controlled by a wide range of brain circuits outside of the homeostatic system. Activation in these brain circuits may override the homeostatic system and also contribute to the enormous increase of obesity. However, little is known about the influence of hormonal signals on the brain's non-homeostatic system. Thus, selective insulin action in the brain was investigated by using intranasal application.

METHODS

We performed 'resting-state' functional magnetic resonance imaging in 17 healthy lean female subjects to assess intrinsic brain activity by fractional amplitude of low-frequency fluctuations (fALFF) before, 30 and 90 min after application of intranasal insulin.

RESULTS

Here, we showed that insulin modulates intrinsic brain activity in the hypothalamus and orbitofrontal cortex. Furthermore, we could show that the prefrontal and anterior cingulate cortex response to insulin is associated with body mass index.

CONCLUSION

This demonstrates that hormonal signals as insulin may reduce food intake by modifying the reward and prefrontal circuitry of the human brain, thereby potentially decreasing the rewarding properties of food. Due to the alarming increase in obesity worldwide, it is of great importance to identify neural mechanisms of interaction between the homeostatic and non-homeostatic system to generate new targets for obesity therapy.

Depot naltrexone decreases rewarding properties of sugar in patients with opioid dependence

BACKGROUND

Opioid neurotransmission mediates hedonic value of sweet tastants; their intake may be exaggerated by the consumption of exogenous opioids (e.g., opioid dependence). Sweet Taste Test (STT) is a validated quantitative instrument assessing taste perception and hedonic features of sugar (sucrose) using a randomized and double-blind administration at five different sucrose concentrations ranging from 0.05 to 0.83 M.

METHODS

The STT and cue-induced craving procedure were administered to opioid-dependent patients (n = 15) before and 1 week after the injection of a long-acting depot naltrexone (XRNT) preparation.

RESULTS

Analyses of covariance, employing sucrose concentration and its perceived taste as covariates, showed that XRNT therapy significantly reduced the self-reported hedonic and motivational characteristics of sucrose. Greater reductions in both these characteristics were associated with more diminution in the cue-induced opioid craving.

CONCLUSIONS

Opioid antagonism in opioid-dependent subjects leads to a smaller sweet taste reward, which, in turn, may be proportional to decreased opioid craving. These pilot results support the heuristic value of the STT as a potential marker of the XRNT treatment response and call for further inquiry into potential clinical applications of the test.

Effect of mouth-rinsing carbohydrate solutions on endurance performance

Ingesting carbohydrate-electrolyte solutions during exercise has been reported to benefit self-paced time-trial performance. The mechanism responsible for this ergogenic effect is unclear. For example, during short duration (≤1 hour), intense (>70% maximal oxygen consumption) exercise, euglycaemia is rarely challenged and adequate muscle glycogen remains at the cessation of exercise. The absence of a clear metabolic explanation has led authors to speculate that ingesting carbohydrate solutions during exercise may have a 'non-metabolic' or 'central effect' on endurance performance. This hypothesis has been explored by studies investigating the performance responses of subjects when carbohydrate solutions are mouth rinsed during exercise. The solution is expectorated before ingestion, thus removing the provision of carbohydrate to the peripheral circulation. Studies using this method have reported that simply having carbohydrate in the mouth is associated with improvements in endurance performance. However, the performance response appears to be dependent upon the pre-exercise nutritional status of the subject. Furthermore, the ability to identify a central effect of a carbohydrate mouth rinse maybe affected by the protocol used to assess its impact on performance. Studies using functional MRI and transcranial stimulation have provided evidence that carbohydrate in the mouth stimulates reward centres in the brain and increases corticomotor excitability, respectively. However, further research is needed to determine whether the central effects of mouth-rinsing carbohydrates, which have been seen at rest and during fatiguing exercise, are responsible for improved endurance performance.

Contributions of the hippocampus and medial prefrontal cortex to energy and body weight regulation

The effects of selective ibotenate lesions of the complete hippocampus (CHip), the hippocampal ventral pole (VP), or the medial prefrontal cortex (mPFC) in male rats were assessed on several measures related to energy regulation (i.e., body weight gain, food intake, body adiposity, metabolic activity, general behavioral activity, conditioned appetitive responding). The testing conditions were designed to minimize the nonspecific debilitating effects of these surgeries on intake and body weight. Rats with CHip and VP lesions exhibited significantly greater weight gain and food intake compared with controls. Furthermore, CHip-lesioned rats, but not rats with VP lesions, showed elevated metabolic activity, general activity in the dark phase of the light-dark cycle, and greater conditioned appetitive behavior, compared with control rats without these brain lesions. In contrast, rats with mPFC lesions were not different from controls on any of these measures. These results indicate that hippocampal damage interferes with energy and body weight regulation, perhaps by disrupting higher-order learning and memory processes that contribute to the control of appetitive and consummatory behavior.

Differences in response to food stimuli in a rat model of obesity: in-vivo assessment of brain glucose metabolism

OBJECTIVE

Food intake is regulated by factors that modulate caloric requirements as well as food's reinforcing properties. In this study, we measured brain glucose utilization to an olfactory stimulus (bacon scent), and we examined the role of food restriction and genetic predisposition to obesity on such brain metabolic activity.

METHODS

Zucker obese (Ob) and lean (Le) rats were divided into four groups: (1) Ob ad-libitum fed, (2) Ob food restricted (70% of ad libitum), (3) Le ad-libitum fed and (4) Le food restricted. Rats were scanned using micro-positron emission tomography and 2-[(18)F]-fluoro-2-deoxy-D-glucose under two conditions: (1) baseline scan (no stimulation) and (2) challenge scan (food stimulation, FS).

RESULTS

FS resulted in deactivation of the right and left hippocampus. Ob rats showed greater changes with FS than Le rats (deactivation of hippocampus and activation of the medial thalamus) and Ob but not Le animals deactivated the frontal cortex and activated the superior colliculus. Access to food resulted in an opposite pattern of metabolic changes to the food stimuli in olfactory nucleus (deactivated in unrestricted and activated in restricted) and in right insular/parietal cortex (activated in unrestricted and deactivated in restricted). In addition, restricted but not unrestricted animals activated the medial thalamus.

CONCLUSIONS

The greater changes in the Ob rats suggest that leptin modulates the regional brain responses to a familiar food stimulus. Similarly, the differences in the pattern of responses with food restriction suggest that FS is influenced by access to food conditions. The main changes with FS occurred in the hippocampus, a region involved in memory, the insular cortex, a region involved with interoception (perception of internal sensations), the medial thalamus (region involved in alertness) and in regions involved with sensory perception (olfactory bulb, olfactory nucleus, occipital cortex, superior colliculus and parietal cortex), which corroborates their relevance in the perception of food.

Integrated physiology and pathophysiology of CB1-mediated effects of the endocannabinoid system

The discovery of the endocannabinoid system (ECS) has raised a large interest in the scientific community providing us with a strikingly long list of apparently independent multi organ effects. As a result, in most reviews on this issue the main function of the ECS is considered as modulatory. Unfortunately, this vision does not add much to our understanding of the specific biological function of the ECS. Thus, modulatory is what in general all biological systems are or should be. In this review we will show that the apparent inconsistent puzzle of the very different tissue specific effects of endocannabinoids (ECs) can be reconstructed in one unitary picture. This picture clearly shows that all the different CB1-mediated effects of ECs sub-serve one major physiological function: to facilitate and increase energy storage. We will also analyze the implications of this unitary vision of the ECS in different contexts. First, in the context of the systems that regulate energy balance, introducing a new systematization based on two homeostatic systems: an endostatic and an exostatic system. Second, in the context of evolution, showing how the function of the ECS has shifted from essential to survival to almost pathological in current times. Finally, in a pathophysiological context, introducing the new concept of "proactive evolution diseases", which can explain the current obesity epidemic and the role the ECS plays in it.

Neurogenesis and neural stem cells in the dorsal vagal complex of adult rat brain: New vistas about autonomic regulations—a review

The dorsal vagal complex (DVC) of the brainstem is the major reflex center of autonomic nervous system. Several neuroplasticity effectors have been identified in the DVC of adult rat, such as PSA-NCAM, GAP-43, BDNF and its receptor TrkB; moreover, acute vagal stimulation was found to induce c-fos and to down-regulate western-blot-assayed tissular concentration of PSA-NCAM. Adult neurogenesis was first shown in rat DVC by BrdU incorporation combined with phenotypic labelling in situ; new neurons are generated in equal proportions with new astrocytes and at a lower rate than in olfactory bulb or hippocampus. Intrinsic proliferative cells were then detected within the DVC of adult rat by means of Ki-67 immunohistochemistry and western-blot of D-cyclins. The presence of neural stem cells within DVC was directly demonstrated by applying the in vitro neurosphere assay on microdissected adult DVC explants; DVC-derived neurospheres display lower proliferation rate and neurogenic potential than forebrain ones. Vagotomy in adult promotes massive and transient increase of neurogenic and microglial proliferations within DVC, the kinetics and location of which were analyzed by Ki-67 immunohistochemistry and cyclin D western blot. These mechanisms shed light on so far unknown plasticity potential in DVC, which brings novel cues about physiological adaptations of autonomic reflexes in adult mammals.

Current knowledge in the neurophysiologic modulation of obesity

Obesity is today one of the commonest of life-threatening diseases in developed countries and generally results from an imbalance between energy intake and energy expenditure. Although there is increasing evidence for a genetic basis of obesity in some clinical syndromes, this seems to be the cause only in a limited number of patients and obesity is far from being considered as a gene-related disease. Eating is a complex and multifactorial process involving autonomous pathways that transfer sensory and motor information between the entire length of the digestive tract and the central nervous system. Modulation of the amount of energy that we take in as food involves several mechanisms and networks that connect the brain with the gut, this process being key to the regulation of body weight over time, as well as to the modification of long-term eating behaviors. Furthermore, this axis is closely coupled to other systems that are involved in energy homeostasis, namely, food preference, energy expenditure, and lifestyle. The identification of several neuropeptides that modulate eating behavior in various ways, along with studies performed in animal models, have focused attention on the role of these molecules and their clinical implications in the development of obesity in humans.

Obesity and the neuroendocrine control of energy homeostasis: the role of spontaneous locomotor activity

Obesity represents one of the most urgent global health threats as well as one of the leading causes of death throughout industrialized nations. Efficacious and safe therapies remain at large. Attempts to decrease fat mass via pharmacological reduction of energy intake have had limited potency or intolerable side effects. Increasingly widespread sedentary lifestyle is often cited as a major contributor to the increasing prevalence of obesity. Moreover, low levels of spontaneous physical activity (SPA) are a major predictor of fat mass accumulation during overfeeding in humans, pointing to a substantial role for SPA in the control of energy balance. Despite this, very little is known about the molecular mechanisms by which SPA is regulated. The overview will attempt to summarize available information on neuroendocrine factors regulating SPA.

Neuronal Representations of Stimuli in the Mouth: The Primate Insular Taste Cortex, Orbitofrontal Cortex and Amygdala

The responses of 3687 neurons in the macaque primary taste cortex in the insula/frontal operculum, orbitofrontal cortex (OFC) and amygdala to oral sensory stimuli reveals principles of representation in these areas. Information about the taste, texture of what is in the mouth (viscosity, fat texture and grittiness, which reflect somatosensory inputs), temperature and capsaicin is represented in all three areas. In the primary taste cortex, taste and viscosity are more likely to activate different neurons, with more convergence onto single neurons particularly in the OFC and amygdala. The different responses of different OFC neurons to different combinations of these oral sensory stimuli potentially provides a basis for different behavioral responses. Consistently, the mean correlations between the representations of the different stimuli provided by the population of OFC neurons were lower (0.71) than for the insula (0.81) and amygdala (0.89). Further, the encoding was more sparse in the OFC (0.67) than in the insula (0.74) and amygdala (0.79). The insular neurons did not respond to olfactory and visual stimuli, with convergence occurring in the OFC and amygdala. Human psychophysics showed that the sensory spaces revealed by multidimensional scaling were similar to those provided by the neurons.

Does perinatal omega-3 polyunsaturated fatty acid deficiency increase appetite signaling?

OBJECTIVE

To investigate the effect of maternal dietary omega-3 polyunsaturated fatty acid (PUFA) deficiency and repletion on food appetite signaling.

RESEARCH METHODS AND PROCEDURES

Sprague-Dawley rat dams were maintained on diets either supplemented with (CON) or deficient in (DEF) omega-3 PUFA. All offspring were raised on the maternal diet until weaning. After weaning, two groups remained on the respective maternal diet (CON and DEF groups), whereas a third group, born of dams fed the DEF diet, were switched to the CON diet (REC). Experiments on food intake began when the male rats reached 16 weeks of age. Food intake was stimulated either by a period of food restriction, by blocking glucose utilization (by 2-deoxyglucose injection), or by blocking beta-oxidation of fatty acids (by beta-mercaptoacetate injection).

RESULTS

DEF animals consumed more than CON animals in response to all stimuli, with the greatest difference (1.9-fold) demonstrated following administration of 2-deoxyglucose. REC animals also consumed more than CON animals in response to food restriction and 2-deoxyglucose but not to beta-mercaptoacetate.

DISCUSSION

These findings indicate that supply of omega-3 PUFA, particularly during the perinatal period, plays a role in the normal development of mechanisms controlling food intake, especially glucoprivic (i.e. reduced glucose availability) appetite signaling. Dietary repletion of omega-3 PUFA from 3 weeks of age restored intake responses to fatty acid metabolite signaling but did not reverse those in response to food restriction or glucoprivic stimuli.

The primate amygdala: Neuronal representations of the viscosity, fat texture, temperature, grittiness and taste of foods

The primate amygdala is implicated in the control of behavioral responses to foods and in stimulus-reinforcement learning, but only its taste representation of oral stimuli has been investigated previously. Of 1416 macaque amygdala neurons recorded, 44 (3.1%) responded to oral stimuli. Of the 44 orally responsive neurons, 17 (39%) represent the viscosity of oral stimuli, tested using carboxymethyl-cellulose in the range 1-10,000 cP. Two neurons (5%) responded to fat in the mouth by encoding its texture (shown by the responses of these neurons to a range of fats, and also to non-fat oils such as silicone oil ((Si(CH(3))(2)O)(n)) and mineral oil (pure hydrocarbon), but no or small responses to the cellulose viscosity series or to the fatty acids linoleic acid and lauric acid). Of the 44 neurons, three (7%) responded to gritty texture (produced by microspheres suspended in cellulose). Eighteen neurons (41%) responded to the temperature of liquid in the mouth. Some amygdala neurons responded to capsaicin, and some to fatty acids (but not to fats in the mouth). Some amygdala neurons respond to taste, texture and temperature unimodally, but others combine these inputs. These results provide fundamental evidence about the information channels used to represent the texture and flavor of food in a part of the brain important in appetitive responses to food and in learning associations to reinforcing oral stimuli, and are relevant to understanding the physiological and pathophysiological processes related to food intake, food selection, and the effects of variety of food texture in combination with taste and other inputs on food intake.

Food restriction and leptin impact brain reward circuitry in lean and obese Zucker rats

The rewarding effect produced by electrically stimulating certain sites in the lateral hypothalamus (LH) can be potentiated by food restriction and body weight loss in lean rats. Central leptin and insulin administration can suppress the rewarding impact of the stimulation. To determine whether there are additional peripheral signals that mediate the effect of weight loss on brain reward circuitry, we assessed changes in LH-self-stimulation following food restriction in the obese Zucker rat which develops resistance to circulating leptin and insulin. In addition, we examined the impact of acute food deprivation and leptin administration on LH self-stimulation in lean and obese Zucker rats. The number of brain stimulation rewards earned was measured over a range of LH stimulation frequencies that drove reward rates from zero to asymptotic levels. Restriction reduced frequency thresholds in a subset of lean and obese rats, whereas BSR was unaltered by acute food deprivation. Despite impairment in leptin signaling, intraventricular leptin (4 microg) increased thresholds in most lean and obese rats in which the rewarding effect was sensitive to restriction. These results show that brain reward circuitry in the obese Zucker rat is sensitive to weight loss and leptin.

Primate Insular/Opercular Taste Cortex: Neuronal Representations of the Viscosity, Fat Texture, Grittiness, Temperature, and Taste of Foods

It is shown that the primate primary taste cortex represents not only taste but also information about many nontaste properties of oral stimuli. Of 1,122 macaque anterior insular/frontal opercular neurons recorded, 62 (5.5%) responded to oral stimuli. Of the orally responsive neurons, some (53%) represented the viscosity, tested using carboxymethyl-cellulose in the range 1–10,000 cP. Other neurons (8%) responded to fat in the mouth by encoding its texture (as shown similar responses to nonfat oils), and 8% responded to gritty texture. Some neurons (35%) responded to the temperature of the liquid in the mouth. Some neurons responded to capsaicin, and others to fatty acids. Some neurons (56%) had taste responses. Some (50%) of these neurons were unimodal, responding to one of these types of stimulus, and the majority combined responsiveness to these types of stimulus, with 23% responding for example to both taste and temperature. Some neurons respond to taste, texture, and temperature unimodally, but others combine these inputs. None of these orally responsive neurons responded to odor or to the sight of food. These results provide fundamental evidence about the information channels used to represent the taste, texture, and temperature of food in the first cortical area involved in taste in the primate brain. The results are relevant to understanding the physiological and pathophysiological processes related to how the properties of oral stimuli are represented in the brain and thus to the control of food intake and food selection.