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

Where do we stand on fMRI in awake mice?

Imaging awake animals is quickly gaining traction in neuroscience as it offers a means to eliminate the confounding effects of anesthesia, difficulties of inter-species translation (when humans are typically imaged while awake), and the inability to investigate the full range of brain and behavioral states in unconscious animals. In this systematic review, we focus on the development of awake mouse blood oxygen level dependent functional magnetic resonance imaging (fMRI). Mice are widely used in research due to their fast-breeding cycle, genetic malleability, and low cost. Functional MRI yields whole-brain coverage and can be performed on both humans and animal models making it an ideal modality for comparing study findings across species. We provide an analysis of 30 articles (years 2011-2022) identified through a systematic literature search. Our conclusions include that head-posts are favorable, acclimation training for 10-14 d is likely ample under certain conditions, stress has been poorly characterized, and more standardization is needed to accelerate progress. For context, an overview of awake rat fMRI studies is also included. We make recommendations that will benefit a wide range of neuroscience applications.

Connecting the dots between cell populations, whole-brain activity, and behavior

Simultaneously manipulating and monitoring both microscopic and macroscopic brain activity in vivo and identifying the linkage to behavior are powerful tools in neuroscience research. These capabilities have been realized with the recent technical advances of optogenetics and its combination with fMRI, here termed "opto-fMRI." Opto-fMRI allows for targeted brain region-, cell-type-, or projection-specific manipulation and targeted Ca 2 + activity measurement to be linked with global brain signaling and behavior. We cover the history, technical advances, applications, and important considerations of opto-fMRI in anesthetized and awake rodents and the future directions of the combined techniques in neuroscience and neuroimaging.

HPA Axis Interactions with Behavioral Systems

Perhaps the most salient behaviors that individuals engage in involve the avoidance of aversive experiences and the pursuit of pleasurable experiences. Engagement in these behaviors is regulated to a significant extent by an individual's hormonal milieu. For example, glucocorticoid hormones are produced by the hypothalamic-pituitary-adrenocortical (HPA) axis, and influence most aspects of behavior. In turn, many behaviors can influence HPA axis activity. These bidirectional interactions not only coordinate an individual's physiological and behavioral states to each other, but can also tune them to environmental conditions thereby optimizing survival. The present review details the influence of the HPA axis on many types of behavior, including appetitively-motivated behaviors (e.g., food intake and drug use), aversively-motivated behaviors (e.g., anxiety-related and depressive-like) and cognitive behaviors (e.g., learning and memory). Conversely, the manuscript also describes how engaging in various behaviors influences HPA axis activity. Our current understanding of the neuronal and/or hormonal mechanisms that underlie these interactions is also summarized. © 2016 American Physiological Society. Compr Physiol 6:1897-1934, 2016.

Distinct Types of Feeding Related Neurons in Mouse Hypothalamus

The last two decades of research provided evidence for a substantial heterogeneity among feeding-related neurons (FRNs) in the hypothalamus. However, it remains unclear how FRNs differ in their firing patterns during food intake. Here, we investigated the relationship between the activity of neurons in mouse hypothalamus and their feeding behavior. Using tetrode-based in vivo recording technique, we identified various firing patterns of hypothalamic FRNs, which, after the initiation of food intake, can be sorted into four types: sharp increase (type I), slow increase (type II), sharp decrease (type III), and sustained decrease (type IV) of firing rates. The feeding-related firing response of FRNs was rigidly related to the duration of food intake and, to a less extent, associated with the type of food. The majority of these FRNs responded to glucose and leptin and exhibited electrophysiological characteristics of putative GABAergic neurons. In conclusion, our study demonstrated the diversity of neurons in the complex hypothalamic network coordinating food intake.

Decoding neural circuits that control compulsive sucrose seeking

The lateral hypothalamic (LH) projection to the ventral tegmental area (VTA) has been linked to reward processing, but the computations within the LH-VTA loop that give rise to specific aspects of behavior have been difficult to isolate. We show that LH-VTA neurons encode the learned action of seeking a reward, independent of reward availability. In contrast, LH neurons downstream of VTA encode reward-predictive cues and unexpected reward omission. We show that inhibiting the LH-VTA pathway reduces "compulsive" sucrose seeking but not food consumption in hungry mice. We reveal that the LH sends excitatory and inhibitory input onto VTA dopamine (DA) and GABA neurons, and that the GABAergic projection drives feeding-related behavior. Our study overlays information about the type, function, and connectivity of LH neurons and identifies a neural circuit that selectively controls compulsive sugar consumption, without preventing feeding necessary for survival, providing a potential target for therapeutic interventions for compulsive-overeating disorder.

In vivo high-resolution localized (1) H MR spectroscopy in the awake rat brain at 7 T

In vivo localized high-resolution (1) H MR spectroscopy was performed in multiple brain regions without the use of anesthetic or paralytic agents in awake head-restrained rats that were previously trained in a simulated MRI environment using a 7T MR system. Spectra were obtained using a short echo time single-voxel point-resolved spectroscopy technique with voxel size ranging from 27 to 32.4 mm(3) in the regions of anterior cingulate cortex, somatosensory cortex, hippocampus, and thalamus. Quantifiable spectra, without the need for any additional postprocessing to correct for possible motion, were reliably detected including the metabolites of interest such as γ-aminobutyric acid, glutamine, glutamate, myo-inositol, N-acetylaspartate, taurine, glycerophosphorylcholine/phosphorylcholine, creatine/phosphocreatine, and N-acetylaspartate/N-acetylaspartylglutamate. The spectral quality was comparable to spectra from anesthetized animals with sufficient spectral dispersion to separate metabolites such as glutamine and glutamate. Results from this study suggest that reliable information on major metabolites can be obtained without the confounding effects of anesthesia or paralytic agents in rodents.

Changes in differential functional magnetic resonance signals in the rodent brain elicited by mixed-nutrient or protein-enriched meals

BACKGROUND & AIMS

The hypothalamus and brain stem have important roles in regulating food intake; the roles of other nonhomeostatic centers in detecting nutrient content of ingested food have been poorly characterized. We used blood oxygen level-dependent functional magnetic resonance imaging (BOLD fMRI) to map brain regions that are responsive to intragastric infusion of isocaloric amounts of a mixed nutrient or protein, and assessed the role of blood glucose in the observed BOLD signal changes.

METHODS

Brain images were acquired, using a 9.4 T MRI system, from anesthetized rats during intragastric infusion of saline (n = 7), or 12 kcal of a mixed nutrient (n = 13) or protein (n = 6). Nutrient-induced changes in blood parameters and the effects of intravenous infusion of saline or glucose (n = 5/treatment) on BOLD fMRI signal changes were also evaluated. Intragastric nutrient infusion reduced the BOLD fMRI signal intensity in homeostatic (hypothalamus, nucleus tractus solitarius) and nonhomeostatic (thalamus, hippocampus, caudate putamen, cerebral cortex, cerebellum) centers; these effects were mimicked qualitatively by intravenous glucose. In contrast to a mixed meal, protein load reduced the BOLD fMRI signal in the amygdala. BOLD fMRI signal changes were inversely correlated with circulating concentrations of amylin, insulin, peptide YY, and glucagon-like peptide-1.

CONCLUSIONS

The caloric content of a meal is signaled from the gut to the brain and affects activity in homeostatic and non-homeostatic centers; blood glucose concentrations have an important role. The satiety effects of protein are associated with activity changes specifically in the amygdala.

Pleasurable behaviors reduce stress via brain reward pathways

Individuals often eat calorically dense, highly palatable "comfort" foods during stress for stress relief. This article demonstrates that palatable food intake (limited intake of sucrose drink) reduces neuroendocrine, cardiovascular, and behavioral responses to stress in rats. Artificially sweetened (saccharin) drink reproduces the stress dampening, whereas oral intragastric gavage of sucrose is without effect. Together, these results suggest that the palatable/rewarding properties of sucrose are necessary and sufficient for stress dampening. In support of this finding, another type of natural reward (sexual activity) similarly reduces stress responses. Ibotenate lesions of the basolateral amygdala (BLA) prevent stress dampening by sucrose, suggesting that neural activity in the BLA is necessary for the effect. Moreover, sucrose intake increases mRNA and protein expression in the BLA for numerous genes linked with functional and/or structural plasticity. Lastly, stress dampening by sucrose is persistent, which is consistent with long-term changes in neural activity after synaptic remodeling. Thus, natural rewards, such as palatable foods, provide a general means of stress reduction, likely via structural and/or functional plasticity in the BLA. These findings provide a clearer understanding of the motivation for consuming palatable foods during times of stress and influence therapeutic strategies for the prevention and/or treatment of obesity and other stress-related disorders.

Effects of isoflurane and alpha-chloralose anesthesia on BOLD fMRI responses to ingested L-glutamate in rats

It is important to investigate the effect of anesthesia on blood oxygenation level-dependent (BOLD) signals in an animal model. Many researchers have investigated the BOLD response to visual, sensory, and chemical stimuli in anesthetized rats. There are no reports, however, comparing the differences in the BOLD signal change between anesthetized and conscious rats when a visceral nutrient signal arises. Here, using functional magnetic resonance imaging (fMRI), we investigated the differences in the BOLD signal changes after intragastric administration of l-glutamate (Glu) under three anesthesia conditions: conscious, alpha-chloralose-anesthetized, and isoflurane-anesthetized condition. Under the conscious and alpha-chloralose condition, we observed the significant BOLD signal increase in the medial prefrontal cortex (mPFC), insular cortex (IC), hippocampus, and several hypothalamic regions including the lateral and ventromedial nucleus. In chloralose group, however, gut Glu stimulation induced BOLD signal increase in the prelimbic cortex and orbital cortex, which did not activate in conscious condition. Meanwhile, under isoflurane-anesthetized condition, we did not observe the BOLD signal increase in these areas. BOLD signal intensity in the nucleus of the solitary tract (NTS), to which vagus nerve transmits the visceral information from the gastrointestinal tract, increased in all conditions. Importantly, under conscious condition, we observed increased BOLD signal intensity in several regions related to the metabolic state (i.e. hunger or satiety), such as the mPFC, ventromedial and lateral hypothalamus (LH). Our results suggest that alpha-chloralose and isoflurane anesthesia caused distinct effects on BOLD response to the gut l-Glu stimulation in several brain regions.

Feeding specific glutamate surge in the rat lateral hypothalamus revealed by low-flow push-pull perfusion

Substantial evidence implicates the lateral hypothalamus (LH) in the control of ingestive behavior and previous studies have found that glutamate release within the LH increases during meals. It is not known, however, whether this effect is selective for feeding, or whether similar changes are also seen during drinking. In this work, we examined this question using low-flow push-pull perfusion which allows sampling from small tissue volumes. Presentation of highly palatable solid or liquid foods to food-deprived rats resulted in an immediate increase in glutamate output of more than 200% over baseline. The response was maximal immediately after food presentation. In contrast, significant changes in glutamate output were not seen when water was presented to water-deprived animals, despite the occurrence of vigorous drinking. These findings confirm reports of feeding related glutamate release in the LH and demonstrate that this effect is specific to feeding, rather than being a general concomitant of all ingestive behaviors. The push-pull technique described here may allow the relevant region of the LH to be identified with greater precision than other methods.

Methodological considerations in rat brain BOLD contrast pharmacological MRI

RATIONALE AND OBJECTIVES

Blood oxygen level dependent (BOLD) contrast pharmacological magnetic resonance imaging (phMRI) is an increasingly popular technique that allows the non-invasive investigation of spatial and temporal changes in rat brain function in response to pharmacological stimulation in vivo. Rat brain BOLD contrast phMRI is, at present, established in few neuropharmacological laboratories, and various issues associated with the technique require attention. The present review is primarily aimed at psychopharmacologists with no previous experience of phMRI, who are interested in the practical aspects that phMRI studies entail.

RESULTS AND DISCUSSION

Experimental and analytical considerations, including anaesthesia, physiological monitoring, drug dose and delivery, scanning protocols, statistical approaches and the interpretation of phMRI data, are discussed.

Functional MRI shows activation of the medial preoptic area during sleep

Changes in the activity of the basal forebrain sleep regulating areas were studied noninvasively in conscious rats by employing functional magnetic resonance imaging (fMRI). Sleep-wakefulness (S-W) stages were identified with the help of electrophysiological recordings carried out simultaneously. An increase in the signal intensity was observed in the medial preoptic area (mPOA) during sleep indicating a heightened activity of neurons in this area. In some rats, there was a decrease in the activity of the fronto-parietal cortex. The sleep-induced increase in activity in the mPOA and decrease in the fronto-parietal cortex are in relation to their levels in the awake state. The findings helped to localize the critical area for the maintenance of slow wave sleep at the mPOA. These results further corroborate some of the previous suggestions based on neurotoxic lesion, chemical stimulation and electrophysiological recordings.