Progressive postnatal increases in Fos immunoreactivity in the forebrain and brain stem of rats after viscerosensory stimulation with lithium chloride

Behavior and Fos activation reveal that male and female rats differentially assess affective valence during CTA learning and expression

Females are more affected by psychiatric illnesses including eating disorders, depression, and post-traumatic stress disorder than males. However, the neural mechanisms mediating these sex differences are poorly understood. Animal models can be useful in exploring such neural mechanisms. Conditioned taste aversion (CTA) is a behavioral task that assesses how animals process the competition between associated reinforcing and aversive stimuli in subsequent task performance, a process critical to healthy behavior in many domains. The purpose of the present study was to identify sex differences in this behavior and associated neural responses. We hypothesized that females would value the rewarding stimulus (Boost®) relative to the aversive stimulus (LiCl) more than males in performing CTA. We evaluated behavior (Boost® intake, LiCl-induced behaviors, ultrasonic vocalizations (USVs), CTA performance) and Fos activation in relevant brain regions after the acute stimuli [acute Boost® (AB), acute LiCl (AL)] and the context-only task control (COT), Boost® only task (BOT) and Boost®-LiCl task (BLT). Acutely, females drank more Boost® than males but showed similar aversive behaviors after LiCl. Females and males performed CTA similarly. Both sexes produced 55 kHz USVs anticipating BOT and inhibited these calls in the BLT. However, more females emitted both 22 kHz and 55 kHz USVs in the BLT than males: the latter correlated with less CTA. Estrous cycle stage also influenced 55 kHz USVs. Fos responses were similar in males and females after AB or AL. Females engaged the gustatory cortex and ventral tegmental area (VTA) more than males during the BOT and males engaged the amygdala more than females in both the BOT and BLT. Network analysis of correlated Fos responses across brain regions identified two unique networks characterizing the BOT and BLT, in both of which the VTA played a central role. In situ hybridization with RNAscope identified a population of D1-receptor expressing cells in the CeA that responded to Boost® and D2 receptor-expressing cells that responded to LiCl. The present study suggests that males and females differentially process the affective valence of a stimulus to produce the same goal-directed behavior.

Feeding circuit development and early-life influences on future feeding behaviour

A wide range of maternal exposures - undernutrition, obesity, diabetes, stress and infection - are associated with an increased risk of metabolic disease in offspring. Developmental influences can cause persistent structural changes in hypothalamic circuits regulating food intake in the service of energy balance. The physiological relevance of these alterations has been called into question because maternal impacts on daily caloric intake do not persist to adulthood. Recent behavioural and epidemiological studies in humans provide evidence that the relative contribution of appetitive traits related to satiety, reward and the emotional aspects of food intake regulation changes across the lifespan. This Opinion article outlines a neurodevelopmental framework to explore the possibility that crosstalk between developing circuits regulating different modalities of food intake shapes future behavioural responses to environmental challenges.

Vagal Interoceptive Modulation of Motivated Behavior

In addition to regulating the ingestion and digestion of food, sensory feedback from gut to brain modifies emotional state and motivated behavior by subconsciously shaping cognitive and affective responses to events that bias behavioral choice. This focused review highlights evidence that gut-derived signals impact motivated behavior by engaging vagal afferents and central neural circuits that generally serve to limit or terminate goal-directed approach behaviors, and to initiate or maintain behavioral avoidance.

Impaired Interoception in a Preclinical Model of Functional Dyspepsia

INTRODUCTION

The etiologies of functional dyspepsia symptoms, including postprandial distress syndrome, remain unknown. We tested the hypothesis that neonatal colon inflammation induces postprandial distress syndrome-like symptoms in adult life that associate with increased activation of vagal afferent pathways and forebrain limbic regions.

RESULTS

These rats showed a significant decrease in nutrient meal consumption to satiety after an overnight fast, decrease in gastric emptying, decrease in total distance traveled, and decrease in percent distance traveled in midfield versus control rats in open field test, indicating postprandial anxiety- and depression-like behaviors. Adult naïve rats treated with oral iodoacetamide to induce H. pylori-like mild gastritis demonstrated similar postprandial effects as the above rats.

CONCLUSIONS

We concluded that neonatal colon inflammation is a risk factor for the development of postprandial distress syndrome-like symptoms. While mild gastritis can induce symptoms similar to those of neonatal colon inflammation, gastritis in these rats does not worsen the symptoms.

Developmental specification of metabolic circuitry

The hypothalamus contains a core circuitry that communicates with the brainstem and spinal cord to regulate energy balance. Because metabolic phenotype is influenced by environmental variables during perinatal development, it is important to understand how these neural pathways form in order to identify key signaling pathways that are responsible for metabolic programming. Recent progress in defining gene expression events that direct early patterning and cellular specification of the hypothalamus, as well as advances in our understanding of hormonal control of central neuroendocrine pathways, suggest several key regulatory nodes that may represent targets for metabolic programming of brain structure and function. This review focuses on components of central circuitry known to regulate various aspects of energy balance and summarizes what is known about their developmental neurobiology within the context of metabolic programming.

Simplified CLARITY for visualizing immunofluorescence labeling in the developing rat brain

CLARITY is an innovative technological advance in which intact biological tissue is transformed into a "nanoporous hydrogel-hybridized form" (Chung et al. 2013; Chung and Deisseroth 2013) with markedly improved chemical and optical accessibility, permitting fluorescent visualization and extraction of high-resolution structural data from mm-thick blocks of tissue. CLARITY affords an excellent but as yet unexploited opportunity to visualize the growth and maturation of phenotypically identified neurons and axonal processes in the developing brain. This brief report describes a moderately revised, simplified, and less expensive CLARITY protocol that effectively reveals the structure of chemically identified neurons in whole neonatal/juvenile rat brains and tissue slabs. Rats [postnatal day (P)0-24] were transcardially perfused with one of two fixative/hydrogel solutions, followed by hydrogel polymerization to generate brain hybrids. Whole brain hybrids or 2.0-mm-thick coronal slabs were passively cleared of lipid and then processed for dual immunofluorescence labeling, including labeling using tyramide signal amplification. After refractive index matching using 2,20-Thiodiethanol (60 % solution), a Leica confocal microscope equipped with a CLARITY objective was used to view the hypothalamus in whole brain hybrids or slabs. Collected image stacks revealed the distribution and three-dimensional structure of hypothalamic pro-oxyphysin (oxytocin)-, neuropeptide Y-, glucagon-like peptide-1-, and tyrosine hydroxylase-immunopositive neurons and processes within large tissue volumes. Outstanding structural preservation and immunolabeling quality demonstrates the efficacy of this approach for interrogating chemically defined neural circuits as they develop in postnatal rodent brain.

Early life experience shapes the functional organization of stress-responsive visceral circuits

Emotions are closely tied to changes in autonomic (i.e., visceral motor) function, and interoceptive sensory feedback from body to brain exerts powerful modulatory control over motivation, affect, and stress responsiveness. This manuscript reviews evidence that early life experience can shape the structure and function of central visceral circuits that underlie behavioral and physiological responses to emotive and stressful events. The review begins with a general discussion of descending autonomic and ascending visceral sensory pathways within the brain, and then summarizes what is known about the postnatal development of these central visceral circuits in rats. Evidence is then presented to support the view that early life experience, particularly maternal care, can modify the developmental assembly and structure of these circuits in a way that impacts later stress responsiveness and emotional behavior. The review concludes by presenting a working hypothesis that endogenous cholecystokinin signaling and subsequent recruitment of gastric vagal sensory inputs to the caudal brainstem may be an important mechanism by which maternal care influences visceral circuit development in rat pups. Early life experience may contribute to meaningful individual differences in emotionality and stress responsiveness by shaping the postnatal developmental trajectory of central visceral circuits.

Functional emergence of the hippocampus in context fear learning in infant rats

The hippocampus is a part of the limbic system and is important for the formation of associative memories, such as acquiring information about the context (e.g., the place where an experience occurred) during emotional learning (e.g., fear conditioning). Here, we assess whether the hippocampus is responsible for pups' newly emerging context learning. In all experiments, postnatal day (PN) 21 and PN24 rat pups received 10 pairings of odor-0.5 mA shock or control unpaired odor-shock, odor only, or shock only. Some pups were used for context, cue or odor avoidance tests, while the remaining pups were used for c-Fos immunohistochemistry to assess hippocampal activity during acquisition. Our results show that cue and odor avoidance learning were similar at both ages, while contextual fear learning and learning-associated hippocampal (CA1, CA3, and dentate gyrus) activity (c-Fos) only occurred in PN24 paired pups. To assess a causal relationship between the hippocampus and context conditioning, we infused muscimol into the hippocampus, which blocked acquisition of context fear learning in the PN24 pups. Muscimol or vehicle infusions did not affect cue learning or aversion to the odor at PN21 or PN24. The results suggest that the newly emerging contextual learning exhibited by PN24 pups is supported by the hippocampus.

Early experience alters limbic forebrain Fos responses to a stressful interoceptive stimulus in young adult rats

The present study examined whether manipulation of the early life experience of rat pups might alter the later ability of an interoceptive challenge to recruit central neural circuits that receive visceral sensory signals and generate stress responses. For this purpose, litters were exposed to daily maternal separation for either 15min (MS-15) or 180min (MS-180) from postnatal days (P)1 to P10. Pups in control litters were raised under standard conditions (i.e., no separations). Similar to previous reports in adult rats, adolescent rats (P35-45) with a developmental history of MS-15 displayed less anxiety-like behavior on the elevated plus maze compared to control and MS-180 rats. As young adults (P50-60), rats were anesthetized and perfused with fixative 90min after viscerosensory stimulation via lithium chloride (LiCl, 0.15M, 1% BW, i.p.) or saline control. In all three rearing groups, Fos activation within brainstem and forebrain regions of interest was significantly enhanced after LiCl vs. saline. MS-15 rats tended to display fewer LiCl-activated neurons in most brain regions compared with rats in the other two rearing groups. This trend reached significance within the dorsal bed nucleus of the stria terminalis. The ability of MS-15 to alter limbic forebrain activation in rats after an interoceptive challenge may contribute to the effect of early life experience to modulate physiological and behavioral stress responses more generally.

D-Cycloserine enhances conditioned taste aversion learning in rats

Conditioned taste aversion (CTA) is a form of associative learning in which the pairing of a taste with a toxin causes an animal to avoid the taste. NMDA receptor mediated neurotransmission has been implicated in CTA, but the role of the NMDA receptor glycine-binding site has not been examined. To examine the effects on CTA of the glycinergic NMDA receptor agonist D-cycloserine, rats received D-cycloserine (15 mg/kg, i.p.) or vehicle 15 min before 10-min access to 0.125% saccharin, followed by a low dose of LiCl (19 mg/kg, i.p.). CTA was measured with 24-h, 2-bottle preference tests between water and saccharin. Vehicle-treated rats formed a mild CTA that rapidly extinguished, while d-cycloserine-treated rats formed a stronger CTA that extinguished slowly. The effect of d-cycloserine was specific to the NMDA receptor glycine-binding site, because pretreatment with HA-966 (6 mg/kg), a partial glycinergic agonist, blocked enhancement by D-cycloserine. Three follow-up experiments suggest that the enhancement of CTA was not due to an aversive effect of D-cycloserine. First, saccharin paired with D-cycloserine (15 mg/kg) alone did not induce a CTA, although a higher dose (30 mg/kg) did significantly lower saccharin preference. Second, pretreatment with D-cycloserine did not increase the duration of "lying-on-belly" behavior induced by LiCl. Third, pretreatment with D-cycloserine did not increase c-Fos induction by either LiCl or vehicle injection in central visceral relays (the nucleus of the solitary tract, the parabrachial nucleus, the central nucleus of the amygdala, the supraoptic nucleus, and the paraventricular nucleus). These results confirm the participation of NMDA receptor, and specifically the glycine-binding site of NMDA receptor, in CTA learning.

Visceral sensory inputs to the endocrine hypothalamus

Interoceptive feedback signals from the body are transmitted to hypothalamic neurons that control pituitary hormone release. This review article describes the organization of central neural pathways that convey ascending visceral sensory signals to endocrine neurons in the paraventricular (PVN) and supraoptic nuclei (SON) of the hypothalamus in rats. A special emphasis is placed on viscerosensory inputs to corticotropin releasing factor (CRF)-containing PVN neurons that drive the hypothalamic-pituitary-adrenal axis, and on inputs to magnocellular PVN and SON neurons that release vasopressin (AVP) or oxytocin (OT) from the posterior pituitary. The postnatal development of these ascending pathways also is considered.