c-Fos induction in response to saccharin after taste aversion learning depends on conditioning method

At the heart of the interoception network: Influence of the parasubthalamic nucleus on autonomic functions and motivated behaviors

The parasubthalamic nucleus (PSTN), a small nucleus located on the lateral edge of the posterior hypothalamus, has emerged in recent years as a highly interconnected node within the network of brain regions sensing and regulating autonomic function and homeostatic needs. Furthermore, the strong integration of the PSTN with extended amygdala circuits makes it ideally positioned to serve as an interface between interoception and emotions. While PSTN neurons are mostly glutamatergic, some of them also express neuropeptides that have been associated with stress-related affective and motivational dysfunction, including substance P, corticotropin-releasing factor, and pituitary adenylate-cyclase activating polypeptide. PSTN neurons respond to food ingestion and anorectic signals, as well as to arousing and distressing stimuli. Functional manipulation of defined pathways demonstrated that the PSTN serves as a central hub in multiple physiologically relevant networks and is notably implicated in appetite suppression, conditioned taste aversion, place avoidance, impulsive action, and fear-induced thermoregulation. We also discuss the putative role of the PSTN in interoceptive dysfunction and negative urgency. This review aims to synthesize the burgeoning preclinical literature dedicated to the PSTN and to stimulate interest in further investigating its influence on physiology and behavior.

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.

The role of the gustatory cortex in incidental experience-evoked enhancement of later taste learning

The strength of learned associations between pairs of stimuli is affected by multiple factors, the most extensively studied of which is prior experience with the stimuli themselves. In contrast, little data is available regarding how experience with "incidental" stimuli (independent of any conditioning situation) impacts later learning. This lack of research is striking given the importance of incidental experience to survival. We have recently begun to fill this void using conditioned taste aversion (CTA), wherein an animal learns to avoid a taste that has been associated with malaise. We previously demonstrated that incidental exposure to salty and sour tastes (taste preexposure-TPE) enhances aversions learned later to sucrose. Here, we investigate the neurobiology underlying this phenomenon. First, we use immediate early gene (c-Fos) expression to identify gustatory cortex (GC) as a site at which TPE specifically increases the neural activation caused by taste-malaise pairing (i.e., TPE did not change c-Fos induced by either stimulus in isolation). Next, we use site-specific infection with the optical silencer Archaerhodopsin-T to show that GC inactivation during TPE inhibits the expected enhancements of both learning and CTA-related c-Fos expression, a full day later. Thus, we conclude that GC is almost certainly a vital part of the circuit that integrates incidental experience into later associative learning.

MSG-Evoked c-Fos Activity in the Nucleus of the Solitary Tract Is Dependent upon Fluid Delivery and Stimulation Parameters

The marker of neuronal activation, c-Fos, can be used to visualize spatial patterns of neural activity in response to taste stimulation. Because animals will not voluntarily consume aversive tastes, these stimuli are infused directly into the oral cavity via intraoral cannulae, whereas appetitive stimuli are given in drinking bottles. Differences in these 2 methods make comparison of taste-evoked brain activity between results that utilize these methods problematic. Surprisingly, the intraoral cannulae experimental conditions that produce a similar pattern of c-Fos activity in response to taste stimulation remain unexplored. Stimulation pattern (e.g., constant/intermittent) and hydration state (e.g., water-restricted/hydrated) are the 2 primary differences between delivering tastes via bottles versus intraoral cannulae. Thus, we quantified monosodium glutamate (MSG)-evoked brain activity, as measured by c-Fos, in the nucleus of the solitary tract (nTS; primary taste nucleus) across several conditions. The number and pattern of c-Fos neurons in the nTS of animals that were water-restricted and received a constant infusion of MSG via intraoral cannula most closely mimicked animals that consumed MSG from a bottle. Therefore, in order to compare c-Fos activity between cannulae-stimulated and bottle-stimulated animals, cannulated animals should be water restricted prior to stimulation, and receive taste stimuli at a constant flow.

Latent inhibition of conditioned disgust reactions in rats

The present experiments, using the latent inhibition (LI) paradigm, evaluated the effect of nonreinforced exposure to saccharin on the acquisition of an LiCl-induced saccharin aversion as measured by conditioned disgust reactions in the taste reactivity test and conditioned taste avoidance in a consumption test. When rats were preexposed to saccharin by bottle exposure (Experiments 1 and 3), LI was evidenced only by conditioned taste avoidance (bottle testing), but not by conditioned disgust reactions (intraoral [IO] testing). On the other hand, when rats were preexposed to saccharin by IO infusion (Experiments 2 and 3), LI was evidenced only by conditioned disgust reactions, but not by conditioned taste avoidance. Experiment 4 showed that LI of conditioned disgust reactions does not appear to be affected by a context shift from preexposure to testing phases. These results show that the expression of LI of both conditioned taste avoidance and conditioned disgust reactions depends critically on a common method of flavor exposure during preexposure and testing.

Amount of training and cue-evoked taste-reactivity responding in reinforcer devaluation

In two experiments, rats received minimal (16) pairings of one auditory conditioned stimulus (CS) cue with a sucrose reinforcer, and extensive (112) pairings of another auditory CS with that reinforcer. After sucrose was devalued by pairing it with lithium chloride in some rats (Devalue groups) but not others (Maintain groups), taste reactivity (TR) and other responses to unflavored water were assessed in the presence of the auditory CSs alone. The minimally trained CS controlled substantially more evaluative TR responses than the extensively trained CS. Those TR responses were hedonic (positive) in the Maintain groups, but aversive (negative) in the Devalue groups. By contrast, food cup entry and other responses thought not to reflect evaluative taste processing were controlled more by the extensively trained cue. These responses were reduced by sucrose devaluation comparably, regardless of the amount of training. The results suggest rapid changes in the content of learning as conditioning proceeds. Early in training, CSs may be capable of activating preevaluative processing of an absent food reinforcer that includes information about its palatability, but that capability is lost as training proceeds.

Taste aversion learning induced c-fos expression in the nucleus of the solitary tract after spontaneous flavor intake: role of the inter-stimulus interval

Taste aversion learning (TAL) can be induced by associating a flavor intake with the immediate or delayed (30 min) intragastric administration of a noxious substance, e.g., hypertonic NaCl. The objective of this study was to analyze the induction of c-Fos immunoreactivity in the intermediate nucleus of the solitary nucleus (iNST) after acquisition of a contiguous or delayed TAL, offering the flavor for voluntary consumption in both cases. The behavioral results obtained indicate that, although the learning was established under both experimental conditions, an increase in c-Fos induction was only produced in the group that learned by means of a non-delayed TAL. Immunohistochemical analyses revealed the participation of different brain structures in these two TAL modalities. Thus, the nucleus of the solitary tract may be involved in the TAL procedure in which voluntary flavor intake and intragastric administration of the noxious visceral stimulus are contiguous but not in delayed TAL, which would depend on other anatomical circuits that do not include the iNST.

Effect of conditioning method and testing method on strength of lithium-induced taste aversion learning

Here the authors evaluated the effect of the method of conditioning (bottle or intraoral [IO] infusion) on the strength of a flavor-drug association when measured in a standard 1-bottle consumption test or when measured by IO infusion in a taste reactivity test. When tested with the bottle test in Experiment 1, rats conditioned by bottle displayed stronger taste avoidance than those conditioned by IO infusion. When tested for rejection reactions with the taste reactivity test in Experiment 2, rats conditioned by infusion displayed a stronger aversion than did rats conditioned by bottle. The results suggest that when the contextual cues of conditioning are similar at conditioning and testing, a stronger association is evident regardless of the individual specifics of each method. These results may shed light on recent reports that different neural mechanisms are involved in conditioning by active consumption and passive infusion.

Memory-dependent c-Fos expression in the nucleus accumbens and extended amygdala following the expression of a conditioned taste aversive in the rat

Retrieving the memory of a conditioned taste aversion involves multiple forebrain areas. Although the amygdala clearly plays a role in the expression of a conditioned taste aversion, critical regions, downstream from the amygdala remain to be defined. To this end, Fos immunoreactivity was used in the rat to explore forebrain structures associated with retrieval that have an anatomical relationship with the amygdala. The results showed that expression of a conditioned taste aversion to 0.5 M sucrose elicited neuronal activation in the nucleus accumbens and in a complex of structures collectively referred to as the extended amygdala. The posterior hypothalamus and parasubthalamic nucleus, which receive inputs from the extended amygdala, were also activated upon re-exposure to the sucrose conditioned stimulus. Fos immunoreactivity did not increase in these regions in response to an innately aversive tastant, quinine hydrochloride (conditioned stimulus control), nor to LiCl-induced visceral stimulation in unconditioned animals (unconditioned stimulus control). In addition, these regions did not respond to the sucrose conditioned stimulus in sham-conditioned animals. These results suggest that conditioned and innately aversive tastes are differentially processed in the forebrain circuitry that includes the nucleus accumbens and extended amygdala.

Fine structural survey of the intermediate subnucleus of the nucleus tractus solitarii and its glossopharyngeal afferent terminals

The intermediate subnucleus of the nucleus tractus solitarii (imNTS) receives somatosensory inputs from the soft palate and pharynx, and projects onto the nucleus ambiguus, thus serving as a relay nucleus for swallowing. The ultrastructure and synaptology of the rat imNTS, and its glossopharyngeal afferent terminals, have been examined with cholera toxin-conjugated horseradish peroxidase (CT-HRP) as an anterograde tracer. The imNTS contained oval or ellipsoid-shaped, small to medium-sized neurons (18.2 x 11.4 microm) with little cytoplasm, few cell organelles and an irregularly shaped nucleus. The cytoplasm often contained one or two nucleolus-like stigmoid bodies. The average number of axosomatic terminals was 1.8 per profile. About 83% of them contained round vesicles and formed asymmetric synaptic contacts (Gray's type I), while about 17% contained pleomorphic vesicles and formed symmetric synaptic contacts (Gray's type II). The neuropil contained small or large axodendritic terminals, and about 92% of them were Gray's type I. When CT-HRP was injected into the nodose ganglion, many labeled terminals were found in the imNTS. All anterogradely labeled terminals contacted dendrites but not somata. The labeled terminals were usually large (2.69+/-0.09 mum) and exclusively of Gray's type I. They often contacted more than two dendrites, were covered with glial processes, and formed synaptic glomeruli. A small unlabeled terminal occasionally made an asymmetric synaptic contact with a large labeled terminal. The large glossopharyngeal afferent terminals and the neurons containing stigmoid bodies characterized the imNTS neurons that received pharyngeal afferents.

Afferent and efferent connections of the parvicellular subdivision of iNTS: defining a circuit involved in taste aversion learning

Conditioned taste aversion (CTA) expression is associated with strong increases in Fos-like immunoreactivity (FLI) in a region of the brainstem identified as the parvicellular subdivision of the intermediate nucleus of the solitary tract (iNTSpc). To identify the projections to and from cells in iNTSpc which display strong FLI in response to expression of a CTA, anterograde and retrograde tract tracing was used. When appropriate, tract tracing was combined with double labeling for FLI in animals which received CTA training as well as tracer injections and were re-exposed to the CS taste. With respect to afferent projections, iNTSpc receives a strong, direct, ipsilateral projection from amygdala and the distribution of the fiber terminals yields a striking match to that of cells expressing FLI after CTA expression. As for efferent projections, these cells in iNTSpc are characterized by a mixed, rather than homogeneous, projection pattern. Targets of these cells include pons and forebrain as well as local medullary sites, all of which are known to be involved in gastrointestinal function. Thus, activation of these cells may provide a circuit through which gastrointestinal/visceral responses are coordinated as a component of the conditioned aversion.

Immune activation paired with intraoral sucrose conditions oral rejection

The effects of lipopolysaccharide (LPS) and LiCl on conditioned taste aversion acquisition using intraoral infusions as the method of taste delivery was examined. Rats received two pairings of an intraorally delivered sucrose (5 ml) taste with the effects of a systemic injection of LPS, LiCl or NaCl. The magnitude of conditioning was quantified by scoring taste reactivity responses to a brief intraoral infusion of sucrose in the absence of any drug injection. Rats previously conditioned with LiCl or LPS displayed clear evidence of conditioned aversion with increased oral rejection responses relative to saline controls. Our results suggest activation of the immune system with LPS can condition consummatory aspects of ingestion when this conditioning involves intraoral fluid presentation.

Comparing immune activation (lipopolysaccharide) and toxin (lithium chloride)-induced gustatory conditioning: lipopolysaccharide produces conditioned taste avoidance but not aversion

Feeding and drinking typically involve both appetitive and consummatory behaviors. Appetitive behaviors include those behaviors produced by an animal prior to the actual consumption, such as approach movements, whereas consummatory behaviors (such as licking and chewing) are involved in the actual consumption of food. The present research compared the gustatory conditioning effects of bacterial lipopolysaccharide (LPS) and lithium chloride (LiCl) in two different paradigms, conditioned taste avoidance and conditioned taste aversion which differentially affect the appetitive and consummatory components of feeding. Male rats were implanted with intraoral cannulae and habituated to a water deprivation schedule and afterwards received two conditioning days (Days 1 and 4). Each conditioning day consisted of 1 h access to a novel sucrose solution (0.3 M) immediately followed by a systemic injection of LPS (200 microg/kg), LiCl (0.15 M, 3 meq) or NaCl vehicle. Conditioned taste aversion was assessed using the taste reactivity test on Day 7, where orofacial and somatic responses were videotaped and analyzed during 3 brief (1 min) exposures to the sucrose solution. Conditioned taste avoidance was assessed on Days 8 and 9 using a two-bottle preference test (sucrose versus water). Animals conditioned with LiCl displayed typical aversive-like responses in the taste reactivity paradigm evidenced by significant reductions in positive ingestive responses (P<0.05) and an increase in active aversive responses (P<0.05) relative to controls. Furthermore, LiCl treatment resulted in conditioned avoidance of sucrose in the two-bottle preference test characterized by a decreased sucrose preference (P<0.05). Conditioning with LPS produced a reduced sucrose preference (P<0.05) relative to controls, comparable to the avoidance seen in LiCl-treated rats. In contrast, conditioning with LPS resulted in similar positive ingestive responses to intraorally infused sucrose as seen in controls. The present results demonstrate that LPS treatment produces conditioned avoidance but not aversion and suggest that LPS can selectively condition the appetitive aspects of feeding whereas the consummatory behaviors remain unaffected.

Differential pattern of cAMP response element-binding protein activation in the rat brain after conditioned aversion as a function of the associative process engaged: taste versus context association

Ample data indicate that cAMP-response element-binding protein (CREB) is essential for the formation of long-term memory in various species and learning systems. This implies that activated CREB could delineate neuronal circuits that subserve items in memory, while leaving open the possibility that the specifics of CREB activation itself contribute to the specificity of the internal representation encoded by the relevant circuit. We describe here the differential activation of CREB in the rat brain as a function of two related yet distinct forms of aversive conditioning: conditioned taste aversion (CTA) and conditioned context aversion (CCA). We found that CTA induces strong CREB activation in the insular cortex (IC) and the lateral septum (LS), but not in the parietal cortex (PC) and the medial septum (MS). In contrast, CCA results in strong activation in the PC and MS, but not in the IC and LS. These findings are congruent with a model that links differential pattern of activity within the LS and the MS with the acquisition of elemental versus contextual conditioning and, more generally, with the notion that CREB activation delineates learning-dependent circuits as a function of the type of cognitive process engaged.

MPEP, a selective metabotropic glutamate receptor 5 antagonist, attenuates conditioned taste aversion in rats

Metabotropic glutamate receptors (mGluRs) have been implicated in several types of cognitive and associative learning. Although recent evidence indicates an influence of mGluRs in conditioned taste aversion (CTA), the subtype-specific involvement of mGluRs in this learning paradigm remained to be determined. The aim of this study was to examine the role of Group I mGluR subtypes in CTA using a selective mGluR5 antagonist (2-methyl-6-(phenylethynyl)-pyridine, MPEP) and a selective mGluR1 antagonist (1-aminoindan-1,5-dicarboxylic acid, AIDA). Male, water-deprived, Sprague-Dawley rats were injected i.p. with 6 or 12 mg/kg MPEP or saline. Twenty-five minutes later, all rats received 15-min access to a 0.1% saccharin solution (Sac) immediately followed by an injection of 0.15M LiCl at 1.33% body weight. The animals were tested with 15-min access to Sac on each of four test days. MPEP-treated animals consumed more Sac on the test trials than saline-treated rats. In another experiment, controlled access to Sac was used by infusing the solution on the conditioning trial. Consistent with the above results, MPEP attenuated the degree of CTA. Similar experiments using the mGluR1 antagonist AIDA, have found no effect on CTA learning. These results suggest that the two subtypes of Group I mGluRs are differentially involved in taste aversion learning.

Novel Tastes Elevate c-fos Expression in the Central Amygdala and Insular Cortex: Implication for Taste Aversion Learning

Taste novelty strongly modulates the speed and strength of taste aversion conditioning. To identify molecular signals responsive to novel tastes, immunostaining for c-fos protein (Fos-like immunoreactivity [FLI]) was used to mark neurons that responded differentially to taste novelty. Novel saccharin induced larger increases in FLI than familiar saccharin. This pattern was seen in central amygdala and insular cortex, but not in basolateral amygdala, parabrachial nucleus, or nucleus of the solitary tract. Other parameters known to influence aversion learning were tested for effects on FLI. Manipulations known to reduce the strength of learning blunted the FLI response, supporting the idea that FLI marks neural pathways critical to taste processing during acquisition, and that c-fos expression is a key transcriptional event underlying this plasticity.

Effects of conditioned food aversions on nutritional behavior in humans

Conditioned food aversion (CFA) and taste aversion (CTA) are widely occurring phenomena mediating rejection of solids or liquids, the ingestion of which has induced the onset of post-ingestional malaise. It is a powerful and durable imprint learning that may influence food choice and intake in all animals, including humans. For ethical reasons, CTA has been extensively investigated in a wide variety of laboratory animal's species but only incidentally in humans. Nevertheless, convincing evidence has been provided that CFA and CTA learning are possible in a wide range of human subjects. The results in humans may have some limitations in accuracy since data are sparse, sometimes indirect, and poorly controlled. There is only limited information on the extent of CFA in the elderly since most studies have employed questionnaire and/or interview methods on young people (i.e. college students). The present review evaluates the literature derived both from laboratory animals and humans. In the first instance, the salient features of food and taste aversion learning and the neural mechanisms involved in this learning behavior will be examined. Then, the problems encountered when trying to assess the role of learned food and taste aversions in the nutritional status of healthy as well as sick young or elderly people will be considered. In particular, the importance of CFA on the nutritional status of cancer patients and treatment of alcoholism will be examined. It is concluded that the data are compelling enough to warrant further research and, some indications and recommendations are suggested.

Conditioned taste aversion as a learning and memory paradigm

Conditioned taste aversion (CTA) is a well established learning and memory paradigm in rats and mice that is considered to be a special form of classical conditioning. Rodents--as well as many other species including man--learn to associate a novel taste (CS) with nausea (US), and as a consequence avoid drinking fluid with this specific taste. In contrast to other types of classical conditioning, even CS-US intervals lasting several hours lead to an aversion to the gustatory CS. With increasing CS-US delay duration, however, the aversion against the CS gradually decreases. Mice differ from rats in their reaction to the CS as well as the US. They tolerate a much higher concentration of saccharin and they do not show any clear signs of nausea when injected with the US. Advantages of this task are its relative independence of motor behavior, well described pathways for the CS and partly the US, and the wealth of available anatomical and pharmacological data implying several brain structures (e.g. parabrachial nucleus, amygdala, insular cortex), neurotransmitters and their receptors (e.g. cholinergic system, NMDA-receptors), and cellular processes (e.g. expression of immediate early genes, Ras-MAP kinase signaling pathway, CREB phosphorilation, protein tyrosine phosphorilation, protein synthesis) in CTA. The CTA paradigm has also been successfully used to phenotype mouse mutants.

Glutamate in the parabrachial nucleus of rats during conditioned taste aversion

Brain microdialysis combined with HPLC and spectroscopic detection was used to monitor extracellular glutamate in the parabrachial nucleus (PBN) of rats during acquisition of a conditioned taste aversion (CTA). Microdialysis fractions taken every 20 min were used to assess the effects of presentation of the conditioned stimulus alone (CS, consumption of 0.1% saccharin), the unconditioned stimulus alone (US, intraperitoneal injection of 0.15 M LiCl, 2% b.w. induced malaise after water drinking) as well as that of CS-US pairing. After 15 min of saccharin drinking, the glutamate concentration in the eluate (20 microl/20 min) reached 80% above the baseline but returned to the basal value in the next fraction. LiCl alone (applied 1 h after 15 min drinking of water) increased glutamate only following some delay, i.e. in the second and third post-lithium fraction by 90 and 67%, respectively. However, when LiCl was injected 1 h after the onset of saccharin intake, the glutamate concentration rose significantly (by 95%) already in the first post-LiCl fraction and by 120% in the second one. It appears, therefore, that the 'saccharin trace' facilitates the effect of lithium on extracellular concentration of glutamate in PBN during acquisition of CTA.