Differential effects of subdiaphragmatic vagotomy on NaCl-induced aversion learning

Disruption of re-intake after partial withdrawal of gastric food contents in rats lesioned in the gelatinous part of the nucleus of the solitary tract

Sensory information from the upper gastrointestinal tract is critical in food intake regulation. Signals from different levels of the digestive system are processed to the brain, among other systems, via the vagus nerve, which mainly projects towards the nucleus of the solitary tract (NST). The objective of this study was to analyze the participation of the gelatinous part (SolG) of the NST in short-term food intake. One-third of the stomach food content was withdrawn at 5 min after the end of a meal, and food was then available ad libitum for different time periods. SolG-lesioned and control animals ingested a similar amount of the initial liquid meal, but the former consumed significantly smaller amounts and failed to compensate for the food deficit, whereas the controls re-ingested virtually the same amount as extracted. These data suggest that the SolG, as in the case of related anatomical structures such as the vagus nerve or external lateral parabrachial subnucleus, may be relevant in particular circumstances that require the rapid processing of vagal-related food intake adjustment associated to the upper gastrointestinal tract.

The role of the dorsal-most part of the lateral parabrachial nucleus in the processing of hypertonic NaCl using different conditioned flavor avoidance paradigms

The parabrachial nucleus (PBN) has been strongly associated with taste aversion learning (TAL) acquisition. Independent of its suggested associative functions, this brain stem centre plays a key role in the sensorial processing of both gustatory and visceral information. The sensory visceral functions have been attributed to the lateral area of the PBN (PBNl) but, recently, it has been proposed that within this area a form of anatomical and functional segregation may also exist, determined by factors such as, the learning paradigm used, the nature of aversive agent used, or the route chosen for the administration of this agent. This study used a lesion approach in rats to address the question of whether the dorsal most portion of the PBNl plays a key role in the acquisition of a conditioned avoidance to flavored stimuli induced by hypertonic sodium chloride (intra gastric), and whether this role is dependent on the flavor avoidance learning (FAL) paradigm used, concurrent (experiment 1) or delayed-sequential FAL (experiment 2). Results showed a clear disruptive effect of the PBNl electrolytic lesion on the acquisition of the concurrent FAL, but hardly any attenuation of the delayed-sequential FAL. This finding is discussed in the context of the hypothesis that two separate and apparently non-redundant routes exist for the processing of the visceral information.

Learned flavor preferences induced by intragastric administration of rewarding nutrients: role of capsaicin-sensitive vagal afferent fibers

Learned flavor preferences can be established after intragastric nutrient administration by two different behavioral procedures, concurrent and sequential. In a concurrent procedure, two flavored stimuli are offered separately but at the same time on a daily basis: one stimulus is paired with the simultaneous intragastric administration of partially digested food and the other with physiological saline. In sequential learning, the two stimuli are presented during alternate sessions. Neural mechanisms underlying these learning modalities have yet to be fully elucidated. The aim of this study was to examine the role of vagal afferent fibers in the visceral processing of rewarding nutrients during concurrent (experiment 1) and sequential (experiment 2) flavor preference learning in Wistar rats. For this purpose, capsaicin, a neurotoxin that destroys slightly myelinated or unmyelinated sensory axons, was applied to the subdiaphragmatic region of the esophagus to selectively damage most of the vagal afferent pathways that originate in the gastrointestinal system. Results showed that capsaicin [1 mg of capsaicin dissolved in 1 ml of vehicle (10% Tween 80 in oil)] blocked acquisition of concurrent but not sequential flavor preference learning. These results are interpreted in terms of a dual neurobiological system involved in processing the rewarding effects of intragastrically administered nutrients. The vagus nerve, specifically capsaicin-sensitive vagal afferent fibers, would only be essential in concurrent flavor preference learning, which requires rapid processing of visceral information.

Effects of perivagal administration of capsaicin on post-surgical food intake

The vagus nerve has been related to the short-term control of food intake. This involvement has previously been explored by examining the food intake of animals after recovery from a vagotomy or immediately after the intervention, among other methods. In the present work, a study was conducted on the impact of the perivagal application of capsaicin (a specific neurotoxic treatment that destroys most of the vagal afferent pathways) on the intake of water and solid (experiment 1) or liquid (experiment 2) food presented after the surgery The results of experiment 1 showed that lesioned animals consume significantly larger amounts of food and water compared with controls at 6, 12, and 24 h (but not at 48 or 72 h) after the surgical intervention. Likewise, experiment 2 revealed a greater intake of liquid food by capsaicin-treated animals at the first post-surgical sessions. These data are discussed in terms of the role played by vagal afferent fibers in the control of short-term food intake.

Learning about cues in their absence: evidence from flavour preferences and aversions

A simple behavioural task may involve the presentation of two or more stimuli. Any learning that takes place in such a situation may be analysed in terms of the formation of an association between the central representations of those stimuli. Presumably performance based on this learning can occur because presentation of one stimulus will then activate the representations of other stimuli that were previously presented with it. To examine the role that these representations play in learning in and of themselves requires that the stimuli themselves are absent. A review of a number of flavour preference and aversion studies indicates that an associatively activated stimulus representation can support learning that is both similar to and the opposite of that maintained by the presentation of the stimulus itself. Which occurs is dependent upon the nature of the reinforcer and the temporal relationships between the training events. Although this pattern of results appears suggestive of separate learning rules, a reanalysis raises the possibility of an explanation in terms of a single associative system.

The role of the external lateral parabrachial subnucleus in flavor preferences induced by predigested food administered intragastrically

A study was undertaken of the role of the external lateral parabrachial subnucleus (PBNLe) in flavor preferences induced by the intragastric administration of predigested/cephalic food. These preferences were developed using two different learning procedures, concurrent and sequential. In the concurrent procedure, two different-flavored stimuli were presented at the same time: one stimulus was paired with the simultaneous intragastric administration of partially digested food and the other with physiological saline. In the sequential learning procedure, the two stimuli were presented at alternate sessions. The results showed that PBNLe lesions blocked acquisition of concurrent learning but had no effect on the sequential procedure. In the latter case, both lesioned and control animals showed a strong preference for the gustatory stimulus paired with partially digested food. These results are interpreted in terms of a dual neurobiological system involved in the rewarding effects of visceral signals.

Effects of a flavor-placement reversal test after different modalities of taste aversion learning

Taste aversion learning is induced through two different behavioral procedures: a short-term or concurrent (two-daily flavors) and a long-term or sequential (one-daily flavor) procedure. For the concurrent group of animals, two gustatory/olfactory stimuli are presented separately but at the same time on a daily basis. One is paired with simultaneous intragastric administration of hypertonic NaCl and the other with physiological saline. For the sequential group, the two stimuli are presented on alternate days, one of them followed by intragastric injection of the aversive stimulus and the other by saline, both after a delay of 15 min. The two groups learned the task, but when they were subjected to a flavor-placement reversal test only the sequential group was successful in achieving it. In a second experiment, three groups of animals had to learn concurrent or sequential discrimination tasks (with either simultaneous or delayed administration of the visceral stimulus) using only spatial/proprioceptive cues. The data show that none of the groups learned them under these conditions. The results are discussed in terms of the different modalities of learning. Short-term and long-term taste aversion learning are different in the anatomical structures involved, the number of trials required for acquisition and, as shown in this paper, flexibility.

Lateral parabrachial lesions impair taste aversion learning induced by blood-borne visceral stimuli

The lateral parabrachial area (LPB), main relay from the area postrema (AP), plays a role in processing visceral information and is thus of potential importance in taste aversion learning (TAL). This study used a lesion approach to address whether LPB functional relevance depends upon the features of toxins that serves as visceral stimuli in TAL. In addition, we explored whether LPB involvement in TAL is restricted to those toxic events detected by the AP or whether it has a more general role. Results showed that LPB-lesioned animals were disrupted in acquiring a TAL induced by blood-borne AP-dependent aversive stimuli (intraperitoneal methylscopolamine) and by AP-independent stimulus (intraperitoneal ethanol), but still, clearly developed strong aversions when intragastric hypertonic sodium chloride, a vagally processed aversive stimulus, served as the aversive stimulus. These findings suggest that the LPB plays a critical role in TAL induced by blood-borne toxins, such as methylscopolamine or ethanol, but is not necessary for vagally mediated stimulus, such as sodium chloride. The present results are discussed in the context of the hypothesis holding separable and independent neural systems underlying TAL.

Electrolytic lesions of the pedunculopontine nucleus disrupt concurrent learned aversion induced by NaCl

Bilateral electrolytic lesions in the pedunculopontine nucleus (PPN) impair acquisition of short-term, or concurrent, Taste Aversion Learning (TAL) in rats. This type of TAL is characterized by the daily presentation of two different flavor stimuli at the same time, one associated with simultaneous intragastric administration of an aversive product (hypertonic NaCl) and the other with physiological saline. Sham-lesioned control animals learn this taste discrimination task, but both lesioned animals and control animals learn a long-term, or delayed, TAL task in which each gustatory stimulus is presented individually every other day and the intragastric products, LiCl (0.15 M) and physiological saline, are administered after a 15-min delay. These results are analyzed in the context of the cerebellar circuits involved in learning and in relation to the two TAL modalities described above.

Lateral parabrachial lesions impair intraperitoneal but not intraventricular methylscopolamine-induced taste aversion learning

The role of the lateral parabrachial area (lPB) in the acquisition of a delayed taste aversion learning task (TAL) was examined by delivering the peripherally acting aversive compound, methylscopolamine (MSP), through two different routes, intraperitoneal and intraventricular. Consistent with previous anatomical, behavioral and molecular work, electrolytic lesions centered at the lPB did impair TAL when the MSP was injected intraperitoneally. However, lPB-lesioned animals exhibited intact learning capacities when MSP was administered intraventricularly. These results are interpreted in terms of the lPB as a critical anatomical relay involved in bottom-up visceral processing of aversive stimuli and also in relation to the relevance of forebrain structures in TAL.

Retention of concurrent taste aversion learning after electrolytic lesioning of the interpositus-dentate region of the cerebellum

Lesions in the interpositus-dentate region of the cerebellum impair short-term, or concurrent, TAL. In this type of learning, animals must discriminate between two flavor stimuli presented at the same time, one of which is associated with an aversive product. The task is learned by the control animals, and within this group the animals that acquire it adequately enough (15/22, 70% criterion) retain the learned taste discrimination when they are subjected to it again after being lesioned in the interpositus-dentate region. These results suggest that the deep nuclei are essential in the concurrent TAL acquisition process, but not in its retention.

Inferior olive lesions impair concurrent taste aversion learning in rats

Taste aversion learning can be established according to two different procedures, concurrent and sequential. For the concurrent task, two different taste stimuli are offered at the same time, one associated with simultaneous intragastric administration of an aversive stimulus and the other associated with physiological saline. This discrimination is learned by sham-lesioned control animals and by animals with lesions in the cerebellar cortex but not by rats lesioned in the inferior olive. At the same time, animals with lesions in the inferior olive and sham-lesioned animals achieve sequential learning when the gustatory stimuli are offered individually during each daily session. The results obtained show that electrolytic lesions in the inferior olive impair acquisition of concurrent learning and are analyzed in terms of an anatomical system consisting of the vagus nerve, inferior olive, and cerebellum, which differentiates between the two modalities of taste aversion learning, concurrent and sequential.

The parabrachial nucleus and conditioned taste aversion

The parabrachial nucleus (PBN) surrounds the brachium conjunctivum in the dorsolateral pons. Although composed of numerous subnuclei, the PBN is typically organized into medial and lateral subdivisions according to their location relative to the brachium. In rodents, the medial PBN is part of the central gustatory system, whereas the lateral PBN is a component of the visceral sensory system. Lesions of the PBN disrupt conditioned taste aversion, a critically important learning mechanism that prevents the repeated ingestion of toxic food. Relevant neurobehavioral literature is reviewed to elucidate the role of the PBN in taste aversion learning.

Role of the vagus nerves in neophobia and conditioned-reflex taste aversion

Combination of ingestion of water with discomfort in rats with intact vagus nerves on selection between water and saccharine solution (an unknown taste) produced aversion not to water but to saccharine, with sharp increases in water consumption. In vagotomized rats, this combination led to a significant increase in saccharine consumption with no change in water intake. After extinction of neophobia to saccharine, combination of ingestion of water with rotation induced aversion to water in both groups (this being delayed in vagotomized rats). Vagus nerve signaling activity in selection conditions appears to determine the choice of behavior strategy.

Bilateral lesions in the cerebellar interpositus-dentate region impair taste aversion learning in rats

Taste Aversion Learning (TAL) has been induced through two different behavioral procedures: a short-term o concurrent (two-daily flavors) and a long-term (one-daily flavor) procedure. For the first, two gustatory/olfactory stimuli are presented separately but at the same time on a daily basis. One of the flavors is paired with simultaneous intragastric administration of hypertonic NaCl and the other is paired with physiological saline. In the long-term procedure, the two stimuli are presented on alternate days, one of them followed by intragastric injection of the aversive stimulus, and the other by saline. The subjects for both types of tests were animals that had been lesioned in the interpositus-dentate region of the cerebellum. The experiments show that the lesions disrupt short-term TAL, but have no effect on long-term TAL. The results are discussed in terms of the role of the cerebellum in relation to TAL and the different anatomical substrates of both learning modalities.

Separate populations of neurons within the paraventricular hypothalamic nucleus of the rat project to vagal and thoracic autonomic preganglionic levels and express c-Fos protein induced by lithium chloride

The role of different hypothalamic nuclei, particularly the paraventricular nucleus (PVN), in the control of food intake and feeding behaviour is well known. It is also well established that lithium chloride (LiCl) causes various disorders in feeding behaviour. In this study, we analyzed the precise distribution of hypothalamic neurons activated by i.p. LiCl administration (LCA neurons) and compared it to that of hypothalamic neurons which project to autonomic preganglionic levels (HAP neurons). We also analysed the possibility that some neurons belong to both populations of nerve cells. To this end, a multiple-labelling technique, using two retrograde fluorescent tracers together with c-Fos-like immunohistochemistry, was performed. Fast Blue was injected in the dorsal motor nucleus of the vagus and Fluorogold (FG) in the thoracic intermedial-lateral cell column, to trace parasympathetic and sympathetic pathways, respectively. LiCl was used as stimulus for c-Fos-like immunohistochemistry. HAP neurons were located mainly in the dorsal, ventral and lateral regions of the parvocellular PVN, while LCA neurons were observed predominantly in the magnocellular region of the PVN rostrally to HAP neurons. A significant number of FG/Fos double-labelled neurons were located in the dorsal parvocellular subnucleus of the PVN (dp) in the LiCl-stimulated rats. We concluded that there is a clear segregation of LCA neurons from HAP neurons within the PVN. The presence of FG/Fos double-labelled neurons in the dp suggests that this nucleus could mediate a sympathetic response after LiCl administration.

The functional relevance of medial parabrachial nucleus in intragastric sodium chloride-induced short-term (concurrent) aversion learning

The functional meaning of the visceral information processing in the medial parabrachial nucleus (PBNm) was analyzed in this study through a short-term aversion learning task. In this short-term task the animals (Wistar rats) had to learn to discriminate between two different gustatory-olfactory stimuli presented simultaneously (two graduated burettes); one of the stimuli was associated with the concurrent intragastric administration of an aversive chemical agent (hypertonic NaCl) and the other stimulus was paired with no injection. In the first experiment, the PBNm-lesioned animals are unable to learn the task using gustatory stimuli (saccharin and quinine) that surpassed the detection threshold of parabrachial-lesioned rats. Moreover, in a second experiment, the PBNm-lesioned animals were unable to learn the task when there was no initial preference for either of the gustatory-olfactory stimuli presented (strawberry or coconut). However, this short-term task is learned by lateral parabrachial nucleus (PBNl) lesioned animals. The possibility that the PBNm and the PBNl are involved in distinct mechanisms of visceral processing is discussed.

The functional relevance of the lateral parabrachial nucleus in lithium chloride-induced aversion learning

Lesions to the lateral parabrachial nucleus (PBN), one of the subnuclei that make up the pontine parabrachial complex, impairs the acquisition of taste aversion learning (TAL) with LiCl as the toxic stimulus. In this experiment, PBNl-lesioned and control rats were trained to learn a delayed task with a 15-min interval between presentation of the gustatory and the aversive stimulus. The impairment in learning observed after lesions of the PBNl is discussed in terms of disruption of the transmission of toxic stimuli (LiCl) processed by the humoral pathway and the area postrema (AP).

Medullary afferent vagal axotomy disrupts NaCl-induced short-term taste aversion learning

The effect of medullary afferent vagal axotomy on NaCl-induced short-term and long-term taste aversion learning (TAL) was examined to assess the relevance of the vagus nerve in drug-induced TAL. The results show that medullary afferent vagal axotomy disrupts NaCl-induced short-term (nondelayed) TAL, while having no effect on learning acquired with the same product in long-term (delayed) TAL protocols. Acquisition of learning in delayed discrimination tasks may be mediated by alternative mechanisms of nonvagal nature, e.g., the humoral system. The possibility that short-term and long-term TAL may be mediated by different neurobiological substrates is discussed.