Area postrema lesions attenuate LiCl-induced c-Fos expression correlated with conditioned taste aversion learning

Neuromolecular Basis of Impaired Conditioned Taste Aversion Acquisition in Valproate-Induced Rat Model of Autism Spectrum Disorder

BACKGROUND/OBJECTIVES

Autism spectrum disorder (ASD), defined by social, behavioral, and cognitive anomalies, is also associated with dysregulated appetite. ASD individuals, often described as "picky eaters", exhibit restricted dietary preferences and a pronounced avoidance of novel foods. This suggests that the perceived safety of specific tastants may be a crucial determinant of dietary acceptance in ASD. Here, we explore the hypothesis that conditioned taste aversion (CTA), a learned avoidance of foods whose intake promotes sickness, is exacerbated in ASD.

METHODS

We assessed the magnitude of a lithium chloride (LiCl)-induced CTA in the valproic acid (VPA) rat model of autism versus in healthy control rats. We also examined the effect of a standard 3 mEq LiCl dose on transcript and neuronal activation changes in brain circuits mediating feeding behavior and associative learning.

RESULTS

Surprisingly, we found that while 3 mEq LiCl induced CTA in healthy controls, even the 6 mEq dose was ineffective in generating aversion in VPA rats. LiCl at 3 mEq affected c-Fos immunoreactivity in the hypothalamus and amygdala in controls, whereas in VPA rats it did not produce any c-Fos changes. Gene expression analysis of feeding-related genes (AgRP, NPY, OXT) and those involved in regulating stress and anxiety (DOR and MC3R) were differentially regulated in the VPA rats. Interestingly, transcripts for COMT1, AgRP, OXT, and MC3R were downregulated in saline-treated VPA rats compared to saline-treated controls.

CONCLUSIONS

We conclude that VPA rats show blunted CTA responsiveness, which is reflected by a differential impact of LiCl on circuits that promote the acquisition of CTA in healthy versus autistic individuals.

Patterns of ingestion of rats during chronic oral administration of lithium chloride

Chronic lithium administration to rodents is used to explore the potential neural mechanisms of mood stabilization, as well as to model the side effects of chronic lithium on multiple organ systems. Oral administration of lithium in the maintenance diet or drinking water is convenient, but lithium can acutely affect intake and it can mediate acquisition of conditioned taste aversions (CTA). We compared ad libitum food and fluid intake by male rats with LiCl or NaCl solutions as their sole source of fluid across 20 days, with a commonly used dosage of LiCl (24 mM: 1 g / L LiCl). To quantify the pattern of intake, rats were housed in cages equipped with lickometers to detect licks and infrared photobeams to detect food access with 6-s resolution. To determine if rats formed a CTA to LiCl, they were subsequently tested with access to NaCl. Rats showed an immediate avoidance of the LiCl solution, as seen on the first day of access by an increased latency to initiate drinking and a decreased size of drinking bouts. Rats showed a differential response to LiCl vs. NaCl after as few as 5 licks. Chronic consumption of LiCl solution led to significantly decreased food and fluid intake compared to baseline, with concomitant weight loss. The decreased intake was realized by marked changes in the pattern of drinking and feeding bouts: a decrease in per-lick volume and a decrease in licks per drinking bout, and an increase in feeding bout duration resulting in an overall decrease in eating rate. Conversely, chronic NaCl access led to an increase in drinking bout number and licks/bout. The avoidance of LiCl was likely a combination of toxic effects of ingested LiCl and rapid acquisition of a learned aversion to the taste of LiCl, as shown by an extinguishable generalized aversion to NaCl solution during subsequent NaCl test days. The marked effect of chronic oral LiCl on ingestion may impact the oral dosing of lithium as well as the rat's metabolic status.

Behavioral and neural responses to high-strength magnetic fields are reduced in otolith mutant mice

Static high magnetic fields (MFs) interact with the vestibular system of humans and rodents. In rats and mice, exposure to MFs causes perturbations such as head movements, circular locomotion, suppressed rearing, nystagmus, and conditioned taste aversion acquisition. To test the role of otoconia, two mutant mouse models were examined, head-tilt Nox3het (het) and tilted Otop1 (tlt), with mutations, respectively, in Nox3, encoding the NADPH oxidase 3 enzyme, and Otop1, encoding the otopetrin 1 proton channel, which are normally expressed in the otolith organs, and are critical for otoconia formation. Consequently, both mutants show a near complete loss of otoconia in the utricle and saccule, and are nonresponsive to linear acceleration. Mice were exposed to a 14.1 Tesla MF for 30 min. After exposure, locomotor activity, conditioned taste aversion and c-Fos (in het) were assessed. Wild-type mice exposed to the MF showed suppressed rearing, increased latency to rear, locomotor circling, and c-Fos in brainstem nuclei related to vestibular processing (prepositus, spinal vestibular, and supragenual nuclei). Mutant het mice showed no response to the magnet and were similar to sham animals in all assays. Unlike het, tlt mutants exposed to the MF showed significant locomotor circling and suppressed rearing compared with sham controls, although they failed to acquire a taste aversion. The residual responsiveness of tlt versus het mice might reflect a greater semicircular deficit in het mice. These results demonstrate the necessity of the otoconia for the full effect of exposure to high MFs, but also suggest a semicircular contribution.

Conditioned nausea induced by cisplatin and emetine identified by a taste reactivity test in rats

No prior studies have shown that gaping reactions are produced with the avoidance of conditioned taste caused by cisplatin and emetine. Therefore, we tried to demonstrate it using a taste reactivity test in rats and found the gaping reactions induced when saccharin is readministered after gustatory conditioning that paired saccharin with cisplatin or emetine. Since conditioned gaping reactions indicate the aversion to saccharin taste and conditioned nausea, the present study suggest that the taste aversion is induced by cisplatin and emetine. It was also found that with intraperitoneal injections of emetine alone, gaping almost never occurs.

LiCl-induced sickness modulates rat gustatory cortical responses

Gustatory cortex (GC), a structure deeply involved in the making of consumption decisions, presumably performs this function by integrating information about taste, experiences, and internal states related to the animal's health, such as illness. Here, we investigated this assertion, examining whether illness is represented in GC activity, and how this representation impacts taste responses and behavior. We recorded GC single-neuron activity and local field potentials (LFPs) from healthy rats and rats made ill (via LiCl injection). We show (consistent with the extant literature) that the onset of illness-related behaviors arises contemporaneously with alterations in 7 to 12 Hz LFP power at approximately 12 min following injection. This process was accompanied by reductions in single-neuron taste response magnitudes and discriminability, and with enhancements in palatability-relatedness-a result reflecting the collapse of responses toward a simple "good-bad" code visible in the entire sample, but focused on a specific subset of GC neurons. Overall, our data show that a state (illness) that profoundly reduces consumption changes basic properties of the sensory cortical response to tastes, in a manner that can easily explain illness' impact on consumption.

Male and female rats exhibit comparable gaping behavior but activate brain regions differently during expression of conditioned nausea

Twenty-five to fifty percent of patients undergoing chemotherapy will develop anticipatory nausea and vomiting (ANV), in which symptoms occur in anticipation of treatment. ANV is triggered by environmental cues and shows little response to traditional antiemetic therapy, suggesting that unique neural pathways mediate this response. Understanding the underlying neural mechanisms of this disorder is critical to the development of novel therapeutic interventions. The purpose of the present study was to identify brain areas activated during ANV and characterize sex differences in both the behavior and the brain areas activated during ANV. We used a rat model of ANV by pairing a novel context with the emetic drug lithium chloride (LiCl) to produce conditioned nausea behaviors in the LiCl-paired environment. We quantitated gaping, an analog of human vomiting, after acute or repeated LiCl in a unique environment. To identify brain regions associated with gaping, we measured c-fos activation by immunochemical staining after these same treatments. We found that acute LiCl activated multiple brain regions including the supraoptic nucleus of the hypothalamus, central nucleus of the amygdala, nucleus of the solitary tract and area postrema, none of which were activated during ANV. ANV activated c-fos expression in the frontal cortex, insula and paraventricular nucleus of the hypothalamus of males but not females. These data suggest that therapies such as ondansetron which target the area postrema are not effective in ANV because it is not activated during the ANV response. Further studies aimed at characterizing the neural circuits and cell types that are activated in the conditioned nausea response will help identify novel therapeutic targets for the treatment of this condition, improving both quality of life and outcomes for patients undergoing chemotherapy.

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.

Effects of area postrema lesions and bilateral subdiaphragmatic afferent vagotomy on emetine-induced conditioned taste avoidance in rats

We investigated the effect of area postrema lesions and selective vagotomy of afferent fibers on emetine-induced nausea in rats. We evaluated the acquisition of the conditioned taste avoidance (CTA) to 0.1% saccharin solution after conditioning with emetine dihydrochloride (5.54 mg/kg, i.p., 1% BW). The CTA was measured in three groups of rats: a bilateral subdiaphragmatic afferent vagotomy group, an area postrema lesion group, and a sham lesion group. The bilateral vagotomy and sham groups of rats showed acquisition of CTA within 2 days of the test date. Taste avoidance was never conditioned in the area postrema lesion group. These results indicate that the area postrema plays a crucial role in the induction of emetine-induced nausea.

Antiemetic Effects of Cannabinoid Agonists in Nonhuman Primates

Attenuating emesis elicited by both disease and medical treatments of disease remains a critical public health challenge. Although cannabinergic medications have been used in certain treatment-resistant populations, Food and Drug Administration-approved cannabinoid antiemetics are associated with undesirable side effects, including cognitive disruption, that limit their prescription. Previous studies have shown that a metabolically stable analog of the endocannabinoid anandamide, methanandamide (mAEA), may produce lesser cognitive disruption than that associated with the primary psychoactive constituent in cannabis, Δ9-tetrahydrocannabinol (Δ9-THC), raising the possibility that endocannabinoids may offer a therapeutic advantage over currently used medications. The present studies were conducted to evaluate this possibility by comparing the antiemetic effects of Δ9-THC (0.032-0.1 mg/kg) and mAEA (3.2-10.0 mg/kg) against nicotine- and lithium chloride (LiCl)-induced emesis and prodromal hypersalivation in squirrel monkeys. Pretreatment with 0.1 mg/kg Δ9-THC blocked nicotine-induced emesis and reduced hypersalivation in all subjects and blocked LiCl-induced emesis and reduced hypersalivation in three of four subjects. Pretreatment with 10 mg/kg mAEA blocked nicotine-induced emesis in three of four subjects and LiCl-induced emesis in one of four subjects and reduced both nicotine- and LiCl-induced hypersalivation. Antiemetic effects of Δ9-THC and mAEA were reversed by rimonabant pretreatment, providing verification of cannabinoid receptor type 1 mediation. These studies systematically demonstrate for the first time the antiemetic effects of cannabinoid agonists in nonhuman primates. Importantly, although Δ9-THC produced superior antiemetic effects, the milder cognitive effects of mAEA demonstrated in previous studies suggest that it may provide a favorable treatment option under clinical circumstances in which antiemetic efficacy must be balanced against side effect liability. SIGNIFICANCE STATEMENT: Emesis has significant evolutionary value as a defense mechanism against ingested toxins; however, it is also one of the most common adverse symptoms associated with both disease and medical treatments of disease. The development of improved antiemetic pharmacotherapies has been impeded by a paucity of animal models. The present studies systematically demonstrate for the first time the antiemetic effects of the phytocannabinoid Δ9-tetrahydrocannabinol and endocannabinoid analog methanandamide in nonhuman primates.

GDF15 Is an Inflammation-Induced Central Mediator of Tissue Tolerance

Growth and differentiation factor 15 (GDF15) is an inflammation-associated hormone with poorly defined biology. Here, we investigated the role of GDF15 in bacterial and viral infections. We found that inflammation induced GDF15, and that GDF15 was necessary for surviving both bacterial and viral infections, as well as sepsis. The protective effects of GDF15 were largely independent of pathogen control or the magnitude of inflammatory response, suggesting a role in disease tolerance. Indeed, we found that GDF15 was required for hepatic sympathetic outflow and triglyceride metabolism. Failure to defend the lower limit of plasma triglyceride levels was associated with impaired cardiac function and maintenance of body temperature, effects that could be rescued by exogenous administration of lipids. Together, we show that GDF15 coordinates tolerance to inflammatory damage through regulation of triglyceride metabolism.

An assay of drug-induced emesis in the squirrel monkey (Saimiri sciureus)

BACKGROUND

Emesis has significant evolutionary value as a defense mechanism against ingested toxins; however, it is also one of the most common adverse symptoms associated with both disease and medical treatments of disease. The development of improved antiemetic pharmacotherapies has been impeded by a shortage of animal models.

METHODS

The present studies characterized the responses of the squirrel monkey to pharmacologically diverse emetic drugs. Subjects were administered nicotine (0.032-0.56 mg/kg), lithium chloride (150-250 mg/kg), arecoline (0.01-0.32 mg/kg), or apomorphine (0.032-0.32 mg/kg) and observed for emesis and prodromal hypersalivation.

RESULTS

Nicotine rapidly produced emesis and hypersalivation. Lithium chloride produced emesis with a longer time course without dose-dependent hypersalivation. Arecoline produced hypersalivation but not emesis. Apomorphine failed to produce emesis or hypersalivation.

CONCLUSIONS

The squirrel monkey is sensitive to drug-induced emesis by a variety of pharmacological mechanisms and is well-positioned to examine antiemetic efficacy and clinically important side effects of candidate antiemetic pharmacotherapies.

Salicylate induces anxiety-like behavior and slow theta oscillation and abolishes the relationship between running speed and fast theta oscillation frequency

Salicylate intoxication is a cause of tinnitus in humans and it is often used to produce tinnitus-like perception in animal models. Here, we assess whether salicylate induces anxiety-like electrophysiological and behavioral signs. Using microwire electrode arrays, we recorded local field potential in the ventral and, in some experiments dorsal hippocampus, in an open field arena 1 hr after salicylate (300 mg/kg) injection. We found that animals treated with salicylate moved dramatically less than saline treated animals. Salicylate-treated animals showed a strong 4-6 Hz (type 2) oscillation in the ventral hippocampus (with smaller peaks in dorsal hippocampus electrodes). Coherence in the 4-6 Hz-theta band was low in the ventral and dorsal hippocampus when compared to movement-related theta coherence (7-10 Hz). Moreover, movement related theta oscillation frequency decreased and its dependency on running speed was abolished. Our results suggest that salicylate-induced theta is mostly restricted to the ventral hippocampus. Slow theta has been classically associated to anxiety-like behaviors. Here, we show that salicylate application can consistently generate low frequency theta in the ventral hippocampus. Tinnitus and anxiety show strong comorbidity and the increase in ventral hippocampus low frequency theta could be part of this association.

Glial Endozepines Inhibit Feeding-Related Autonomic Functions by Acting at the Brainstem Level

Endozepines are endogenous ligands for the benzodiazepine receptors and also target a still unidentified GPCR. The endozepine octadecaneuropeptide (ODN), an endoproteolytic processing product of the diazepam-binding inhibitor (DBI) was recently shown to be involved in food intake control as an anorexigenic factor through ODN-GPCR signaling and mobilization of the melanocortinergic signaling pathway. Within the hypothalamus, the DBI gene is mainly expressed by non-neuronal cells such as ependymocytes, tanycytes, and protoplasmic astrocytes, at levels depending on the nutritional status. Administration of ODN C-terminal octapeptide (OP) in the arcuate nucleus strongly reduces food intake. Up to now, the relevance of extrahypothalamic targets for endozepine signaling-mediated anorexia has been largely ignored. We focused our study on the dorsal vagal complex located in the caudal brainstem. This structure is strongly involved in the homeostatic control of food intake and comprises structural similarities with the hypothalamus. In particular, a circumventricular organ, the area postrema (AP) and a tanycyte-like cells forming barrier between the AP and the adjacent nucleus tractus solitarius (NTS) are present. We show here that DBI is highly expressed by ependymocytes lining the fourth ventricle, tanycytes-like cells, as well as by proteoplasmic astrocytes located in the vicinity of AP/NTS interface. ODN staining observed at the electron microscopic level reveals that ODN-expressing tanycyte-like cells and protoplasmic astrocytes are sometimes found in close apposition to neuronal elements such as dendritic profiles or axon terminals. Intracerebroventricular injection of ODN or OP in the fourth ventricle triggers c-Fos activation in the dorsal vagal complex and strongly reduces food intake. We also show that, similarly to leptin, ODN inhibits the swallowing reflex when microinjected into the swallowing pattern generator located in the NTS. In conclusion, we hypothesized that ODN expressing cells located at the AP/NTS interface could release ODN and modify excitability of NTS neurocircuitries involved in food intake control.

Tolerance to repeated rewarding electrical stimulation of the parabrachial complex

The parabrachial complex has been related to various rewarding behavioral processes. As previously shown, electrical stimulation of the lateral parabrachial external (LPBe) subnucleus induces opiate-dependent concurrent place preference. In this study, two groups of animals (and their respective controls) were subjected to sessions of rewarding brain stimulation daily or on alternate days. The rats stimulated every other day maintained a consistent preference for the place associated with the brain stimulation. However, as also found in the Insular Cortex, there was a progressive decay in the initial place preference of animals receiving daily stimulation. These data suggest that the rewarding effects induced by electrical stimulation of LPBe subnucleus may be subject to tolerance. These findings are discussed with respect to other anatomical areas showing reward decay and to the reinforcing effects induced by various electrical and chemical rewarding agents.

Activation of the hypothalamic-pituitary-adrenal axis in lithium-induced conditioned taste aversion learning

Intraperitoneal injections (ip) of lithium chloride at large doses induce c-Fos expression in the brain regions implicated in conditioned taste aversion (CTA) learning, and also activate the hypothalamic-pituitary-adrenal (HPA) axis and increase the plasma corticosterone levels in rats. A pharmacologic treatment blunting the lithium-induced c-Fos expression in the brain regions, but not the HPA axis activation, induced CTA formation. Synthetic glucocorticoids at conditioning, but not glucocorticoid antagonist, attenuated the lithium-induced CTA acquisition. The CTA acquisition by ip lithium was not affected by adrenalectomy regardless of basal corticosterone supplement, but the extinction was delayed in the absence of basal corticosterone. Glucocorticoids overloading delayed the extinction memory formation of lithium-induced CTA. ip lithium consistently induced the brain c-Fos expression, the HPA activation and CTA formation regardless of the circadian activation of the HPA axis. Intracerebroventricular (icv) injections of lithium at day time also increased the brain c-Fos expression, activated the HPA axis and induced CTA acquisition. However, icv lithium at night, when the HPA axis shows its circadian activation, did not induce CTA acquisition nor activate the HPA axis, although it increased the brain c-Fos expression. These results suggest that the circadian activation of the HPA axis may affect central, but not peripheral, effect of lithium in CTA learning in rats, and the HPA axis activation may be necessary for the central effect of lithium in CTA formation. Also, glucocorticoids may be required for a better extinction; however, increased glucocorticoids hinder both the acquisition and the extinction of lithium-induced CTA.

Parabrachial calcitonin gene-related peptide neurons mediate conditioned taste aversion

Conditioned taste aversion (CTA) is a phenomenon in which an individual forms an association between a novel tastant and toxin-induced gastrointestinal malaise. Previous studies showed that the parabrachial nucleus (PBN) contains neurons that are necessary for the acquisition of CTA, but the specific neuronal populations involved are unknown. Previously, we identified calcitonin gene-related peptide (CGRP)-expressing neurons in the external lateral subdivision of the PBN (PBel) as being sufficient to suppress appetite and necessary for the anorexigenic effects of appetite-suppressing substances including lithium chloride (LiCl), a compound often used to induce CTA. Here, we test the hypothesis that PBel CGRP neurons are sufficient and necessary for CTA acquisition in mice. We show that optogenetic activation of these neurons is sufficient to induce CTA in the absence of anorexigenic substances, whereas genetically induced silencing of these neurons attenuates acquisition of CTA upon exposure to LiCl. Together, these results demonstrate that PBel CGRP neurons mediate a gastrointestinal distress signal required to establish CTA.

Neuroanatomical and functional characterization of CRF neurons of the amygdala using a novel transgenic mouse model

The corticotropin-releasing factor (CRF)-producing neurons of the amygdala have been implicated in behavioral and physiological responses associated with fear, anxiety, stress, food intake and reward. To overcome the difficulties in identifying CRF neurons within the amygdala, a novel transgenic mouse line, in which the humanized recombinant Renilla reniformis green fluorescent protein (hrGFP) is under the control of the CRF promoter (CRF-hrGFP mice), was developed. First, the CRF-hrGFP mouse model was validated and the localization of CRF neurons within the amygdala was systematically mapped. Amygdalar hrGFP-expressing neurons were located primarily in the interstitial nucleus of the posterior limb of the anterior commissure, but also present in the central amygdala. Secondly, the marker of neuronal activation c-Fos was used to explore the response of amygdalar CRF neurons in CRF-hrGFP mice under different experimental paradigms. C-Fos induction was observed in CRF neurons of CRF-hrGFP mice exposed to an acute social defeat stress event, a fasting/refeeding paradigm or lipopolysaccharide (LPS) administration. In contrast, no c-Fos induction was detected in CRF neurons of CRF-hrGFP mice exposed to restraint stress, forced swimming test, 48-h fasting, acute high-fat diet (HFD) consumption, intermittent HFD consumption, ad libitum HFD consumption, HFD withdrawal, conditioned HFD aversion, ghrelin administration or melanocortin 4 receptor agonist administration. Thus, this study fully characterizes the distribution of amygdala CRF neurons in mice and suggests that they are involved in some, but not all, stress or food intake-related behaviors recruiting the amygdala.

Oxytocin receptor blockade reduces acquisition but not retrieval of taste aversion and blunts responsiveness of amygdala neurons to an aversive stimulus

When gastrointestinal sickness induced by toxin injection is associated with exposure to novel food, the animal acquires a conditioned taste aversion (CTA). Malaise is accompanied by a surge in oxytocin release and in oxytocin neuronal activity; however, it is unclear whether oxytocin is a key facilitator of aversion or merely its marker. Herein we investigated whether blockade of the oxytocin receptor with the blood-brain barrier penetrant oxytocin receptor antagonist L-368,899 is detrimental for the acquisition and/or retrieval of lithium chloride (LiCl)-dependent CTA to a saccharin solution in mice. We also examined whether L-368,899 given prior to LiCl affects neuronal activity defined through c-Fos immunohistochemistry in select brain sites facilitating CTA acquisition. L-368,899 given prior to LiCl caused a 30% increase in saccharin solution intake in a two-bottle test, but when the antagonist was administered before the two-bottle test, it failed to diminish the retrieval of an existing CTA. LiCl administration increased c-Fos expression in the hypothalamic paraventricular and supraoptic nuclei, area postrema, nucleus of the solitary tract and basolateral and central (CNA) nuclei of the amygdala. L-368,899 injected before LiCl reduced the number of c-Fos positive CNA neurons and brought it down to levels similar to those observed in mice treated only with L-368,899. We conclude that oxytocin is one of the key components in acquisition of LiCl-induced CTA and the aversive response can be alleviated by the oxytocin receptor blockade. Oxytocin receptor antagonism blunts responsiveness of CNA to peripherally injected LiCl.

Fischer rats are more sensitive than Lewis rats to the suppressive effects of morphine and the aversive kappa-opioid agonist spiradoline

Data have suggested that rats avoid intake of an otherwise palatable saccharin cue when paired with a drug of abuse, at least, in part, because the value of the taste cue pales in anticipation of the availability of the highly rewarding drug. Earlier support for this hypothesis was provided by the finding that, relative to the less sensitive Fischer rats, Lewis rats exhibit greater avoidance of a saccharin cue when paired with a rewarding sucrose or cocaine unconditioned stimulus (US), but not when paired with the aversive agent, lithium chloride. More recent data, however, have shown that Fischer rats actually exhibit greater, not less, avoidance of the same saccharin cue when morphine serves as the US. Therefore, Experiment 1 evaluated morphine-induced suppression of intake of the taste cue in Lewis and Fischer rats when the morphine US was administered subcutaneously, rather than intraperitoneally. Experiment 2 examined the effect of strain on the suppression of intake of the saccharin cue when paired with spiradoline, a selective kappa-opioid receptor agonist. The results confirmed that Fischer rats are more responsive to the suppressive effects of morphine than Lewis rats, and that Fischer rats also exhibit greater avoidance of the saccharin cue when paired with spiradoline, despite the fact that spiradoline is devoid of reinforcing properties. Taken together, the data suggest that the facilitated morphine-induced suppression observed in Fischer rats, compared with Lewis rats, may reflect an increased sensitivity to the aversive, kappa-mediated properties of opiates.

Amylin and GLP-1 target different populations of area postrema neurons that are both modulated by nutrient stimuli

The area postrema mediates the hypophagic effect of the pancreatic hormone amylin and is also sensitive to glucagon-like peptide 1 (GLP-1). Protein seems to modulate amylin responsiveness because amylin seems to produce a stronger hypophagic effect and a stronger c-Fos expression when protein is absent from the diet. Accordingly, amylin induces a stronger c-Fos expression in the AP when injected in fasted compared to ad libitum fed rats. Here we tested the hypothesis that diet-derived protein attenuates the amylin dependent suppression of feeding and AP activation using isocaloric diets that differed in their protein content. Moreover, we investigated whether peripheral amino acid injection attenuates amylin-induced c-Fos expression in fasted rats. Since recent evidence suggests that GLP-1 may also reduce eating via the AP we tested whether 24 h fasting also increases neuronal AP responsiveness to GLP-1 similar to the fasting-induced increase in amylin responsiveness. Finally, we used the calcitonin receptor (CTR) as an immunohistochemical marker for amylin-receptive AP neurons to investigate whether amylin's target neurons differ from GLP-1 responsive AP neurons. We also dissociated amylin responsive cells from neurons implicated in other AP-mediated functions such as aversion or blood pressure regulation. For this purpose, we conducted c-Fos/CTR double staining after LiCl or angiotensin II treatment, respectively. Amylin (5 μg/kg s.c.) was more effective to reduce the intake of a 1% vs. an 8% or 18% protein diet and to induce c-Fos expression in the AP in rats receiving 1% vs. 18% protein diet. Increased protein intake was associated with increased blood amino acid levels. Peripheral injection of amino acids (1 g/kg i.p.) attenuated the amylin-induced AP activation in 24 h fasted rats. Similar to amylin, GLP-1 (100 μg/kg i.p.) elicited a significant c-Fos response only in fasted but not in ad libitum fed rats. However, in contrast to a high co-localization of amylin-induced c-Fos and CTR (68%), no c-Fos/CTR co-localization occurred after treatment with GLP-1 or the GLP-1R agonist exendin 4 (2 μg/kg ip). Similarly, LiCl (76 mg/kg ip) or AngII (50 μg/kg sc) led to c-Fos expression only in CTR negative AP neurons. In conclusion, our findings support a protein-dependent modulation of behavioral and neuronal amylin responsiveness under equicaloric feeding conditions. Amino acids might contribute to the inhibitory effect of diet-derived protein to reduce amylin-induced neuronal AP activation. Neuronal AP responsiveness to GLP-1 is also increased in the fasted state suggesting that diet-derived nutrients may also interfere with AP-mediated GLP-1 effects. Nevertheless, the primary target neurons for amylin appear to be distinct from cells targeted by GLP-1 and by stimuli producing aversion (LiCl) or contributing to blood pressure regulation (AngII) via the AP. Since amylin and GLP-1 analogs are targets for the treatment of obesity, the nutrient-dependent modulation of AP responsiveness might entail implications for such therapeutic approaches.

Palatable food avoidance and acceptance learning with different stressors in female rats

Stress activates the hypothalamus-pituitary-adrenal (HPA) axis leading to the release of glucocorticoids (GC). Increased activity of the HPA axis and GC exposure has been suggested to facilitate the development of obesity and metabolic syndrome. Nonetheless, different stressors can produce distinct effects on food intake and may support different directions of food learning e.g. avoidance or acceptance. This study examined whether interoceptive (LiCl and exendin-4) and restraint stress (RS) support similar or distinct food learning. Female rats were exposed to different stressors after their consumption of a palatable food (butter icing). After four palatable food-stress pairings, distinct intakes of the butter icing were observed in rats treated with different stressors. Rats that received butter icing followed by intraperitoneal injections of LiCl (42.3mg/kg) and exendin-4 (10μg/kg) completely avoided the palatable food with subsequent presentations. In contrast, rats experiencing RS paired with the palatable food increased their consumption of butter icing across trials and did so to a greater degree than rats receiving saline injections. These data indicate that interoceptive and psychosocial stressors support conditioned food avoidance and acceptance, respectively. Examination of c-Fos immunoreactivity revealed distinct neural activation by interoceptive and psychosocial stressors that could provide the neural basis underlying opposite direction of food acceptance learning.

Mitogen-activated protein kinase in the amygdala plays a critical role in lithium chloride-induced taste aversion learning

The intracellular mitogen-activated protein kinase (MAPK) pathway in the brain is necessary for the formation of a variety of memories including conditioned taste aversion (CTA) learning. However, the functional role of MAPK activation in the amygdala during lithium chloride (LiCl)-induced CTA learning has not been established. In the present study, we investigated if local microinjection of SL327, a MAPK kinase inhibitor, into the rat amygdala could alleviate LiCl-induced CTA learning. Our results revealed that acute administration of a high dose of LiCl (0.15M, 12 ml/kg, i.p.) rapidly increased the level of phosphorylated MAPK (pMAPK)-positive cells in the central nucleus of the amygdala (CeA) and nucleus of the solitary tract (NTS) of rats as measured by immunohistochemistry. Local microinjection of SL327 (1 μg/0.5 μl/hemisphere) into the CeA 10 min before LiCl administration decreased both the strength of LiCl-induced CTA paired with 0.125% saccharin and the level of LiCl-induced pMAPK-positive cells in the CeA, but not in the NTS. Our data suggest that the intracellular signaling cascade of the MAPK pathway in the CeA plays a critical role in the processing of visceral information induced by LiCl for CTA learning.