Neuronal activity in the medial orbitofrontal cortex of the behaving monkey: modulation by glucose and satiety

A Method for Evaluating Hunger and Thirst in Monkeys by Measuring Blood Ghrelin and Osmolality Levels

Hunger and thirst drive animals' consumption behavior and regulate their decision-making concerning rewards. We previously assessed the thirst states of monkeys by measuring blood osmolality under controlled water access and examined how these thirst states influenced their risk-taking behavior in decisions involving fluid rewards. However, hunger assessment in monkeys remains poorly performed. Moreover, the lack of precise measures for hunger states leads to another issue regarding how hunger and thirst states interact with each other in each individual. Thus, when controlling food access to motivate performance, it remains unclear how these two physiological needs are satisfied in captive monkeys. Here, we measured blood ghrelin and osmolality levels to respectively assess hunger and thirst in four captive macaques. Using an enzyme-linked immunosorbent assay, we identified that the levels of blood ghrelin, a widely measured hunger-related peptide hormone in humans, were high after 20 h of no food access (with ad libitum water). This reflects a typical controlled food access condition. One hour after consuming a regular dry meal, the blood ghrelin levels in three out of four monkeys decreased to within their baseline range. Additionally, blood osmolality measured from the same blood sample, the standard hematological index of hydration status, increased after consuming the regular dry meal with no water access. Thus, ghrelin and osmolality may reflect the physiological states of individual monkeys regarding hunger and thirst, suggesting that these indices can be used as tools for monitoring hunger and thirst levels that mediate an animal's decision to consume rewards.

Preferences reveal dissociable encoding across prefrontal-limbic circuits

Individual preferences for the flavor of different foods and fluids exert a strong influence on behavior. Most current theories posit that preferences are integrated with other state variables in the orbitofrontal cortex (OFC), which is thought to derive the relative subjective value of available options to guide choice behavior. Here, we report that instead of a single integrated valuation system in the OFC, another complementary one is centered in the ventrolateral prefrontal cortex (vlPFC) in macaques. Specifically, we found that the OFC and vlPFC preferentially represent outcome flavor and outcome probability, respectively, and that preferences are separately integrated into value representations in these areas. In addition, the vlPFC, but not the OFC, represented the probability of receiving the available outcome flavors separately, with the difference between these representations reflecting the degree of preference for each flavor. Thus, both the vlPFC and OFC exhibit dissociable but complementary representations of subjective value, both of which are necessary for decision-making.

Preferences reveal separable valuation systems in prefrontal-limbic circuits

Individual preferences for the flavor of different foods and fluids exert a strong influence on behavior. Most current theories posit that preferences are integrated with other state variables in orbitofrontal cortex (OFC), which is thought to derive the relative subjective value of available options to drive choice behavior. Here we report that instead of a single integrated valuation system in OFC, another separate one is centered in ventrolateral prefrontal cortex (vlPFC) in macaque monkeys. Specifically, we found that OFC and vlPFC preferentially represent outcome flavor and outcome probability, respectively, and that preferences are separately integrated into these two aspects of subjective valuation. In addition, vlPFC, but not OFC, represented the outcome probability for the two options separately, with the difference between these representations reflecting the degree of preference. Thus, there are at least two separable valuation systems that work in concert to guide choices and that both are biased by preferences.

Higher-Order Inputs Involved in Appetite Control

The understanding of the neural control of appetite sheds light on the pathogenesis of eating disorders such as anorexia nervosa and obesity. Both diseases are a result of maladaptive eating behaviors (overeating or undereating) and are associated with life-threatening health problems. The fine regulation of appetite involves genetic, physiological, and environmental factors, which are detected and integrated in the brain by specific neuronal populations. For centuries, the hypothalamus has been the center of attention in the scientific community as a key regulator of appetite. The hypothalamus receives and sends axonal projections to several other brain regions that are important for the integration of sensory and emotional information. These connections ensure that appropriate behavioral decisions are made depending on the individual's emotional state and environment. Thus, the mechanisms by which higher-order brain regions integrate exteroceptive information to coordinate feeding is of great importance. In this review, we will focus on the functional and anatomical projections connecting the hypothalamus to the limbic system and higher-order brain centers in the cortex. We will also address the mechanisms by which specific neuronal populations located in higher-order centers regulate appetite and how maladaptive eating behaviors might arise from altered connections among cortical and subcortical areas with the hypothalamus.

Recent Advances in Neural Circuits for Taste Perception in Hunger

Feeding is essential for survival and taste greatly influences our feeding behaviors. Palatable tastes such as sweet trigger feeding as a symbol of a calorie-rich diet containing sugar or proteins, while unpalatable tastes such as bitter terminate further consumption as a warning against ingestion of harmful substances. Therefore, taste is considered a criterion to distinguish whether food is edible. However, perception of taste is also modulated by physiological changes associated with internal states such as hunger or satiety. Empirically, during hunger state, humans find ordinary food more attractive and feel less aversion to food they usually dislike. Although functional magnetic resonance imaging studies performed in primates and in humans have indicated that some brain areas show state-dependent response to tastes, the mechanisms of how the brain senses tastes during different internal states are poorly understood. Recently, using newly developed molecular and genetic tools as well as in vivo imaging, researchers have identified many specific neuronal populations or neural circuits regulating feeding behaviors and taste perception process in the central nervous system. These studies could help us understand the interplay between homeostatic regulation of energy and taste perception to guide proper feeding behaviors.

Thirst regulates motivated behavior through modulation of brainwide neural population dynamics

Physiological needs produce motivational drives, such as thirst and hunger, that regulate behaviors essential to survival. Hypothalamic neurons sense these needs and must coordinate relevant brainwide neuronal activity to produce the appropriate behavior. We studied dynamics from ~24,000 neurons in 34 brain regions during thirst-motivated choice behavior in 21 mice as they consumed water and became sated. Water-predicting sensory cues elicited activity that rapidly spread throughout the brain of thirsty animals. These dynamics were gated by a brainwide mode of population activity that encoded motivational state. After satiation, focal optogenetic activation of hypothalamic thirst-sensing neurons returned global activity to the pre-satiation state. Thus, motivational states specify initial conditions that determine how a brainwide dynamical system transforms sensory input into behavioral output.

Decision making and reward in frontal cortex: complementary evidence from neurophysiological and neuropsychological studies

Patients with damage to the prefrontal cortex (PFC)—especially the ventral and medial parts of PFC—often show a marked inability to make choices that meet their needs and goals. These decision-making impairments often reflect both a deficit in learning concerning the consequences of a choice, as well as deficits in the ability to adapt future choices based on experienced value of the current choice. Thus, areas of PFC must support some value computations that are necessary for optimal choice. However, recent frameworks of decision making have highlighted that optimal and adaptive decision making does not simply rest on a single computation, but a number of different value computations may be necessary. Using this framework as a guide, we summarize evidence from both lesion studies and single-neuron physiology for the representation of different value computations across PFC areas.

Neural Ensemble Coding of Satiety States

The motivation to start or terminate a meal involves the continual updating of information on current body status by central gustatory and reward systems. Previous electrophysiological and neuroimaging investigations revealed region-specific decreases in activity as the subject's state transitions from hunger to satiety. By implanting bundles of microelectrodes in the lateral hypothalamus, orbitofrontal cortex, insular cortex, and amygdala of hungry rats that voluntarily eat to satiety, we have measured the behavior of neuronal populations through the different phases of a complete feeding cycle (hunger-satiety-hunger). Our data show that while most satiety-sensitive units preferentially responded to a unique hunger phase within a cycle, neuronal populations integrated single-unit information in order to reflect the animal's motivational state across the entire cycle, with higher activity levels during the hunger phases. This distributed population code might constitute a neural mechanism underlying meal initiation under different metabolic states.

Understanding Obsessive–Compulsive Disorder: Focus on Decision Making

Current approaches to obsessive-compulsive disorder (OCD) have suggested that neurobiological abnormalities play a crucial role in the etiology and course of this psychiatric illness. In particular, a fronto-subcortical circuit, including the orbitofrontal cortex, basal ganglia and thalamus appears to be involved in the expression of OCD symptoms. Neuropsychological studies have also shown that patients with OCD show deficits in cognitive abilities that are strictly linked to the functioning of the frontal lobe and its related fronto-subcortical structures, such as executive functioning deficits and insufficient cognitive-behavioral flexibility. This article focuses on decision making, an executive ability that plays a crucial role in many real-life situations, whereby individuals choose between pursuing strategies of action that involve only immediate reward and others based on long-term reward. Although the role of decision-making deficits in the evolution of OCD requires further research, the collected findings have significant implications for understanding the clinical and behavioral heterogeneity that characterizes individuals with OCD.

Neuronal activities in the monkey primary and higher-order gustatory cortices during a taste discrimination delayed GO/NOGO task and after reversal

The correlation between different gustatory areas in the frontal operculum, orbitofrontal area, and insula and the representation of different aspects of cues during a salt-water discrimination delayed GO/NOGO task was studied in a Japanese monkey. Four groups were identified among 169 neurons responding to cues before/after task reversal. Group I (n=78) responded to the physicochemical nature of the cue, Group II (n=8) responded to both the physicochemical nature of the cue and the subsequent behavior, Group III (n=51) (three subgroups) produced discharges related to the subsequent behavior, and Group IV (n=32) produced non-differential responses probably related to attention. The primary gustatory areas (area G and the oral part of area 3) almost exclusively contained Group I neurons, whereas the so-called secondary gustatory areas (the PrCO and area 12) contained most of the Group III neurons. Group IIIc showed discharges accelerating to the LED onset, probably representing preparation for subsequent behavior, and the response differed between the PrCO and area 12. The PrCO also contained Group IV neurons. The primary gustatory areas process pure gustatory signals, whereas the PrCO and area 12 may be involved in gustatory perception, attention, or behavior.

Gustatory cortex of primates: anatomy and physiology

Clinical and physiological studies of patients with ageusia or gustatory hallucination suggest that the primary gustatory area (area G) lies at the anterior insula or at the base of the central sulcus. However, physiological and anatomical studies in subhuman primates, e.g. squirrel monkeys or macaque monkeys, locate area G at the buried frontal operculum (Fop) and dorsal insula. The presence of secondary or higher gustatory areas are claimed because taste neurons are found in the precentral opercular area (PrCO) or orbitofrontal cortex in alert monkeys. Part of the anterior insula is suggested to subserve the interface between area G and the amygdala. Many physiological studies have been conducted lacking knowledge of the histological boundaries of the primary and secondary gustatory areas. Some difference has been found in the physiological properties of taste neurons in the primary and secondary gustatory areas: the primary gustatory area contains various categories of taste neurons, whereas most of the taste neurons in the secondary gustatory areas (e.g., PrCO, area 1-2) are specifically sensitive to one of the four basic tastes, and taste neurons in the orbitofrontal opercular area (OFO), another secondary gustatory area, show sensory-specific hunger as well.

Laminar origin of striatal and thalamic projections of the prefrontal cortex in rhesus monkeys

Prefrontostriatal and prefrontothalamic connections in rhesus monkeys have been shown to be organized in a topographic manner. These projections originate largely from infragranular layers V and VI. To examine whether the striatal and thalamic connections from the prefrontal cortex arise from separate neuronal populations or are collateralized, two different fluorescent retrograde tracers (diamidino yellow and fast blue) were injected into topographically similar regions of the head of the caudate nucleus and the mediodorsal nucleus in the same animal. The results show that although prefrontostriatal and prefrontothalamic projections arise from similar topographic regions, their laminar origins are distinctive. The connections to the head of the caudate nucleus originate mainly from layer Va, to a lesser extent from layer Vb, with a minor contribution from layers III and VI. In contrast, the projections to the mediodorsal nucleus emanate largely from layer VI, and also from layer Vb. Only occasional double-labeled neurons were observed, indicating that prefrontostriatal and prefrontothalamic connections originate from separate neuronal populations. The differential laminar distributions of neurons projecting to the head of the caudate nucleus and the mediodorsal nucleus suggest that these structures may receive independent types of information from the prefrontal cortex.

Prefrontostriatal connections in relation to cortical architectonic organization in rhesus monkeys

Prefrontostriatal connections were investigated in rhesus monkeys using the autoradiographic technique to examine whether there are systematic relationships with regard to the architectonic organization of the prefrontal cortex. On the basis of progressive laminar elaboration, the different regions of the prefrontal cortex can be grouped into two architectonic trends. The dorsal trend, which begins in the medial proisocortical areas, can be followed through the dorsolateral prefrontal cortex, culminating in the dorsal arcuate region. The ventral trend, which originates in the orbital proisocortex, can be traced through the inferior prefrontal convexity to the ventral arcuate region. The results show that the main connections from the prefrontal cortex to the striatum are to the head and body of the caudate nucleus. These connections are topographically organized. Medial and dorsal prefrontal areas project predominantly to the dorsal and central portion of the head and body of the caudate nucleus, whereas orbital and inferior prefrontal areas are related mainly to the ventral and central portion. Moreover, prefrontostriatal connections have a medial-lateral topography. Medial and orbital prefrontal areas project medially in the head and body of the caudate nucleus, whereas the dorsal and ventral arcuate regions project laterally, adjacent to the internal capsule. The prefrontal regions above and below the principal sulcus project mainly to the intermediate sector of the head and body of the nucleus. However, there appears to be some degree of overlap of corticostriatal projections from the dorsal and ventral prefrontal regions, as well as within each trend. Relatively minor projections are directed to the putamen as well as to the tail of the caudate nucleus from certain subregions of the prefrontal cortex. Thus the distribution of prefrontostriatal connections seems to reflect the architectonic organization of the prefrontal cortex. Possible functional aspects of prefrontostriatal connectivity are considered in the light of behavioral and physiological studies.

Prefrontal cortex alpha 2 adrenoceptors and energy balance

The sulcal prefrontal cortex (SPC) influences thermogenesis, energy substrate utilization and feeding behaviour. The present study examined the role of SPC alpha noradrenergic receptors in these effects. Fifty nmol norepinephrine (NE) injected into the SPC produced a large and long-lasting increase in respiratory quotient (RQ), indicating enhanced carbohydrate utilization and fat synthesis. This dose also reduced energy expenditure without corresponding decreases in locomotor activity, suggesting an inhibition of thermogenesis. Neither a lower dose of NE (25 nmol) injected into the SPC, nor injections of NE (50 nmol) into a variety of sites adjacent to the SPC affected energy balance. The alpha 2 agonist clonidine (20 nmol) injected into the SPC produced similar effects to 50 nmol NE, with a large increase in RQ and a decrease in thermogenesis. Forty nmol clonidine, however, decreased RQ and reduced both energy expenditure and activity. The alpha 1 agonist L-phenylephrine (20 and 40 nmol) injected into the SPC had no clear effect on energy balance. Finally, it was shown that clonidine or NE injected into the SPC promotes food intake. These results implicate alpha 2 adrenoceptors in the sulcal prefrontal cortex in the control of food intake, thermogenesis and metabolic substrate utilization.

Feeding-related activity of glucose- and morphine-sensitive neurons in the monkey amygdala

Feeding-related neuronal activity of monkey amygdalar glucose-sensitive and morphine-sensitive cells was investigated during a task that required bar-pressing to obtain food. Both glucose-sensitive and morphine-sensitive cells, located mostly in the centromedial part of the amygdala, decreased firing during the bar-press period more often than insensitive cells. Naloxone attenuated the decrease in activity during the bar press period. The results suggest involvement of these glucose- and morphine-sensitive cells in the control of food acquisition behavior.

Topography of projections from the medial prefrontal cortex to the amygdala in the rat

The projections from the rat medial prefrontal cortex to the amygdaloid complex were investigated using retrograde transport of fluorescent dyes and anterograde transport of horseradish peroxidase-WGA. The ventral anterior cingulate, prelimbic, infralimbic and medial orbital areas and the taenia tecta were found to project to the amygdaloid complex. The projections from the prelimbic area arose bilaterally. The medial orbital, prelimbic and anterior cingulate areas send convergent projections to the basolateral nucleus. The prelimbic area has additional projections to the posterolateral cortical nucleus and amygdalo-hippocampal area. The infralimbic area does not project to the basolateral nucleus and cortico-amygdaloid projections from this area are focussed on the anterior cortical nucleus and the anterior amygdaloid area. Both prelimbic and infralimbic areas project to an area situated between the central, medial and basomedial nuclei. Based on similar projections, this area appears to be a caudal continuation of the anterior amygdaloid area. The results indicate that the medial prefrontal component of the "basolateral limbic circuit" is restricted to the anterior cingulate and prelimbic areas. No evidence was obtained to support the existence of a medial prefronto-amygdaloid component of the "visceral forebrain".

A computerized control system for a bar-press feeding task initiated by monkey vocalization

Computerized control of a bar-press feeding task initiated by a monkey's vocalization was designed to study the motivational roles of brain regions in feeding behavior. In this paradigm, the monkey was required to vocalize in order to initiate the task and then press the bar 30 times to obtain the reward. A microcomputer was used to sense the vocalization and to control the task. Using this system, changes in single neuron activity in the orbitofrontal cortex and the lateral hypothalamic area preceding the time of the vocalization were observed. The entire system including interface and control program is described.