Lesions of the lateral parabrachial nuclei disrupt aversion learning induced by electrical stimulation of the area postrema

Efferent projections of CGRP/Calca-expressing parabrachial neurons in mice

The parabrachial nucleus (PB) is composed of glutamatergic neurons at the midbrain-hindbrain junction. These neurons form many subpopulations, one of which expresses Calca, which encodes the neuropeptide calcitonin gene-related peptide (CGRP). This Calca-expressing subpopulation has been implicated in a variety of homeostatic functions, but the overall distribution of Calca-expressing neurons in this region remains unclear. Also, while previous studies in rats and mice have identified output projections from CGRP-immunoreactive or Calca-expressing neurons, we lack a comprehensive understanding of their efferent projections. We began by identifying neurons with Calca mRNA and CGRP immunoreactivity in and around the PB, including populations in the locus coeruleus and motor trigeminal nucleus. Calca-expressing neurons in the PB prominently express the mu opioid receptor (Oprm1) and are distinct from neighboring neurons that express Foxp2 and Pdyn. Next, we used Cre-dependent anterograde tracing with synaptophysin-mCherry to map the efferent projections of these neurons. Calca-expressing PB neurons heavily target subregions of the amygdala, bed nucleus of the stria terminalis, basal forebrain, thalamic intralaminar and ventral posterior parvicellular nuclei, and hindbrain, in different patterns depending on the injection site location within the PB region. Retrograde axonal tracing revealed that the previously unreported hindbrain projections arise from a rostral-ventral subset of CGRP/Calca neurons. Finally, we show that these efferent projections of Calca-expressing neurons are distinct from those of neighboring PB neurons that express Pdyn. This information provides a detailed neuroanatomical framework for interpreting experimental work involving CGRP/Calca-expressing neurons and opioid action in the PB region.

Differential effects of naloxone on rewarding electrical stimulation of the central nucleus of the amygdala and parabrachial complex in a place preference study

The central nucleus of the amygdala (CeA) is considered to be involved in different affective, sensory, regulatory, and acquisition processes. This study analyzed whether electrical stimulation of the PB-CeA system induces preferences in a concurrent place preference (cPP) task, as observed after stimulation of the parabrachial-insular cortex (PB-IC) axis. It also examined whether the rewarding effects are naloxone-dependent. The results show that electrical stimulation of the CeA and external lateral parabrachial subnucleus (LPBe) induces consistent preference behaviors in a cPP task. However, subcutaneous administration of an opiate antagonist (naloxone; 4mg/ml/kg) blocked the rewarding effect of the parabrachial stimulation but not that of the amygdala stimulation. These results are interpreted in the context of multiple brain reward systems that appear to differ both anatomically and neurochemically, notably with respect to the opiate system.

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.

Lesions of the lateral parabrachial area block the aversive component and induced-flavor preference for the delayed intragastric administration of nutrients in rats: Effects on subsequent food and water intake

The aim of this study was to examine the function of the lateral parabrachial area (LPB) in relation to the intragastric administration of nutrients. The consumption of flavors associated with intragastric nutrient administration and the subsequent food and water intake were measured in rats with lesions in the LPB. The results showed that bilateral LPB lesions prevented development of aversions and induced flavor preference when there was a delay between the presentation of a flavor and the intragastric administration of nutrients. However, these lesions did not disrupt development of the aversive process when there was no delay between the presentations. Likewise, the LPB lesions increased subsequent food intake when there was a delay but not when there was no delay between the presentations. In contrast, the water intake was reduced in both situations. These results are interpreted in terms of a dual visceral system for processing the intragastric effects of foods.

Central Fos expression and conditioned flavor avoidance in rats following intragastric administration of bitter taste receptor ligands

G protein-coupled receptors that signal bitter taste (T2Rs) are expressed in the mucosal lining of the oral cavity and gastrointestinal (GI) tract. In mice, intragastric infusion of T2R ligands activates Fos expression within the caudal viscerosensory portion of the nucleus of the solitary tract (NTS) through a vagal pathway (Hao S, Sternini C, Raybould HE. Am J Physiol Regul Integr Comp Physiol 294: R33-R38, 2008). The present study was performed in rats to further characterize the distribution and chemical phenotypes of brain stem and forebrain neurons activated to express Fos after intragastric gavage of T2R ligands, and to determine a potential behavioral correlate of this central neural activation. Compared with relatively low brain stem and forebrain Fos expression in control rats gavaged intragastrically with water, rats gavaged intragastrically with T2R ligands displayed significantly increased activation of neurons within the caudal medial (visceral) NTS and caudal ventrolateral medulla, including noradrenergic neurons, and within the lateral parabrachial nucleus, central nucleus of the amygdala, and paraventricular nucleus of the hypothalamus. A behavioral correlate of this Fos activation was evidenced when rats avoided consuming flavors that previously were paired with intragastric gavage of T2R ligands. While unconditioned aversive responses to bitter tastants in the oral cavity are often sufficient to inhibit further consumption, a second line of defense may be provided postingestively by ligand-induced signaling at GI T2Rs that signal the brain via vagal sensory inputs to the caudal medulla.

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 preferences induced by electrical stimulation of a food-related area of the parabrachial complex: Effects of naloxone

Electrical stimulation of the External Lateral Parabrachial Subnucleus (LPBe), a food-related area, induced behavioral preferences for associated stimuli in a taste discrimination learning task. Although this stimulation appeared to be ineffective to elicit standard lever press self-stimulation, it induced place preference for one of two training compartments of a rectangular maze in which animals (adult male Wistar rats) received concurrent electrical brain stimulation. In subjects that consistently showed a preference behavior in different trials, administration of the opioid antagonist naloxone (4 mg/ml/kg) blocked concurrent learning when the test was made in a new maze but not in the same maze in which animals had learned the task. These results are discussed in terms of the possible participation of the LPBe subnucleus in different natural and artificial brain reward systems.

Strain differences in the acquisition of nicotine-induced conditioned taste aversion

Lewis (LEW) and Fischer (F344) rat strains differ on a variety of physiological and behavioral endpoints, including reactivity to drugs of abuse. Although they differ in drug reactivity, such assessments are generally limited to morphine and cocaine. To determine if these differences generalize to other drugs, the present study examined these strains for their reactivity to the affective properties of nicotine, specifically their sensitivity to nicotine in the conditioned taste aversion preparation. For four or five conditioning cycles given every other day, rats from both strains were allowed access to saccharin and injected with nicotine (0.1, 0.4, 0.8 mg/kg) or vehicle. On intervening days, all rats were given access to water and injected with vehicle. Under this one-bottle training and testing procedure, neither strain displayed aversions at the lowest dose of nicotine (0.1 mg/kg). Aversions were evident for both strains at 0.4 and 0.8 mg/kg, although the F344 rats acquired the aversions at 0.4 mg/kg faster and displayed a significantly greater aversion at 0.8 mg/kg than subjects from the LEW strain. For both strains, aversions were evident at all doses (and in a dose-dependent manner) when subjects were given access to saccharin and water in a two-bottle test. There were, however, no strain differences on this test. Differences between the two strains in their acquisition of nicotine-induced taste aversions were discussed in the context of aversion assessments with other compounds as well as in relation to differences in the self-administration of nicotine in the two strains.

Lateral parabrachial lesions impair lithium chloride-induced aversive responses but not saccharin-induced flavor preference

Behavioral taste-guided experiments, as well as molecular studies employing c-FLI expression in response to aversive/appetitive unconditioned stimulus, have strongly suggested a visceral role for the lateral parabrachial subnuclei (lPB). The main objective in the present study was to further evaluate the functional role of the lPB in lithium chloride-induced behavioral/physiological responses. We employed a lesion/behavioral experimental strategy combining a lithium chloride-induced place aversion procedure together with the simultaneous evaluation of behavioral ("Lying on Belly", "LOB") and physiological (body temperature) responses elicited by the toxin. Data showed that lPB-lesioned animals failed to avoid the chamber previously paired with lithium chloride. Moreover, "LOB", and not hypothermia, in response to lithium chloride was impaired in parabrachial lesioned animals. Finally, all the animals were tested in a free discriminative flavor-preference task induced by saccharin, a non-caloric reinforcer, which precludes visceral feedback as essential in acquiring the learned response. As expected, both control and lesioned animals developed a clear flavor-preference to the flavor previously paired with saccharin, which shows normal gustatory and associative processing in lPB-lesioned animals. This study extends previous results on the functional visceral role of lPB subnuclei by providing alternative behavioral evidence other than taste-guided behavior, that the lPB is pivotal in visceral processing. Present data are discussed in the context of the visceral hypothesis that holds that the lPB is critically involved in processing post-oral visceral feedback.

The decade 1989-1998 in Spanish psychology: an analysis of research in psychobiology

In this paper, we present an analysis of the research published during the 1989-1998 decade by tenured Spanish faculty members from the area of psychobiology. Database search and direct correspondence with the 110 faculty members rendered a list of 904 psychobiological papers. Classification and analysis of these papers led to the definition of at least 70 different research trends. Topics are grouped into several specific research areas: Learning and Memory; Development and Neural Plasticity; Emotion and Stress; Ethology; Neuropsychology; Sensory Processing; and Psychopharmacology. The international dissemination of this research, published in journals of high impact index, and the increasing number of papers are two noteworthy features.

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.

Electrical stimulation of the insular cortex induces flavor-preferences in rats

The present study examined the role of the Insular cortex (IC) in flavor-guided behavior. For that purpose, a flavored stimulus was paired with delayed electrical stimulation of this region. In addition, a standard operant task explored the involvement of the IC in a prefrontal self-stimulation reward-circuit. The results showed strong preferences for the flavored stimulus previously paired to the Insular stimulation, in a discriminative free choice test. However, the operant task revealed a failure to induce IC self-stimulation, suggesting that flavor preferences elicited by electrical stimulation of the IC are not due to activation of the prefrontal-stimulation reward circuit. These results are discussed in terms of the Insular Cortex as critical in processing visceral stimulus, hedonic valence and/or food-reward incentive learning.

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.

Involvement of the spinoparabrachial pathway in inflammatory nociceptive processes: a c-Fos protein study in the awake rat

The effect of graded inflammatory stimuli (intraplantar-carrageenan, 0.2, 1, and 6 mg/150 microl) on paw edema and c-Fos protein expression at two levels of the spinoparabrachial pathway, the spinal cord and parabrachial area (PB), were studied. The present study, in awake rats, is an extension of previous study (Bester et al. [1997] J. Comp. Neurol. 383:439-458) which evaluated, in anesthetized rats, the effect of graded cutaneous heat stimulation on c-Fos-expression at the same levels. At the spinal level, the c-Fos-protein-like-immunoreactive (c-Fos-LI) neurons were located primarily in superficial laminae ipsilateral to intraplantar carrageenan. The number of c-Fos-LI neurons increased dose dependently (r = 0.973, n = 24) for carrageenan, from a number close to zero for the saline injection. At the PB level, c-Fos was predominantly expressed contralateral to intraplantar carrageenan. c-Fos-LI neurons were located primarily around the pontomesencephalic junction in (i) a restricted pontine area, centered in the lateral crescent, and including an adjacent part of the outer portion of the external lateral subnucleus, and (ii) the mesencephalic superior lateral subnuclei. The number of c-Fos-LI neurons in the PB area was correlated with that in the superficial laminae (r = 0.935, n = 24) and with the paw edema (r = 0.931, n = 24). No significant changes in c-Fos expression were observed in the nucleus of the solitary tract and ventrolateral medulla. The close correlation between c-Fos expression at both the spinal and PB levels and inflammatory edema provides further evidence for the involvement of spinoparabrachial pathway in inflammatory nociceptive processes. The present results are congruent with the existence of electrophysiologically demonstrated spinoparabrachio-amygdaloid and -hypothalamic nociceptive pathways.

Chronic food restriction alters mu and kappa opioid receptor binding in the parabrachial nucleus of the rat: a quantitative autoradiographic study

Using quantitative autoradiography, it was previously observed that chronic food restriction alters mu and kappa receptor binding in several regions of the rat forebrain. The present autoradiographic study was designed to investigate whether food restriction affects regional mu and kappa binding in the brainstem. [3H]DAGO (mu) and-mu/delta blocked [3H]BMZ (kappa) binding were analyzed in 21 brainstem regions. A significant decrease in mu binding was observed in the external lateral and external medial subnuclei of the parabrachial nucleus while a significant increase in kappa binding was observed in the external lateral subnucleus. The possible functional significance of these changes is discussed.

Organization of the efferent projections from the spinal cervical enlargement to the parabrachial area and periaqueductal gray: a PHA-L study in the rat

The organization of efferent projections from the spinal cervical enlargement to the parabrachial (PB) area and the periaqueductal gray (PAG) was studied in the rat by using microinjections of Phaseolus vulgaris-leucoagglutinin (PHA-L) into different laminae around the C7 level. The results demonstrated two areas of cervical enlargement which project in different ways to the PB area and PAG. First, the superficial laminae (I, II) showed a very dense projection, with a clear contralateral dominance at the coronal level where the inferior colliculus merges with the pons, to a restricted "superficial" portion of the PB area, namely the lateral crescent area, the dorsal lateral, the superior lateral (PBsl), and the outer portion of the external lateral PB subnuclei. Less dense projections were observed in the Kölliker-Fuse nucleus (KF) and in the ventrolateral/lateral quadrant of the caudal and mid PAG. By contrast, the labeling was weak or absent in the other PB subnuclei and the outer adjacent regions; in particular, no, or very little, labeling was found in the cuneiform nucleus. The PB area appeared to be the supraspinal target that received the densest projection from laminae I and II. Projections were less dense in the PAG and the thalamus and markedly less in other sites such as the ventrolateral medulla, the subnucleus reticularis dorsalis, and the nucleus of the solitary tract. Second, the reticular portion of lamina V, the medial portion of laminae IV-VI up to X and lamina VIII, showed bilateral projections with a weak ipsilateral dominance and a high to medium density on a very restricted portion of the PB area, namely the internal lateral PB subnucleus. A lesser projection was also observed in the adjacent portion of the PBsl, the KF, and the lateral quadrant of the PAG. These results suggest that signals carried by neurons from lamina I-II converge on a restricted superficial portion of the PB area and the ventral part of the lateral quadrant of the PAG. These results are discussed in the context of the role of the spino-PB and spino-PAG pathways in nociception.

Neural substrates for conditioned taste aversion in the rat

Conditioned taste aversions (CTAs) are well known to be robust and long-lasting instances of learning induced by a single CS (taste)-US (malaise) pairing. CTA can be taken as a general model to search for neural mechanisms of learning and memory. In spite of extensive research on CTAs using a variety of approaches during the last three decades, the neural mechanisms of taste aversion learning still remain unsolved. In this article we propose a model of neural substrates of CTAs on the basis of our recent studies incorporating previous findings by other workers. Our studies mainly included experiments using ibotenic acid injections into various parts of the rat brain as a lesion technique, and c-fos immunohistochemistry in naive and CTA trained rats. CTAs were established by pairing the ingestion of saccharin (CS) with an ip injection of LiCl (US). Behavioral studies have shown that the parabrachial nucleus (PBN), medial thalamus, and basolateral nucleus of the amygdala are essential for both acquisition and retention of CTAs. C-fos studies suggested that association between gustatory CS and visceral US takes place in the PBN. The gustatory cortex (GC) may modify the strength of this association depending on the nature of the CS, viz., novel or familiar. The amygdala is indispensable for the expressions of CTAs. Tastes with hedonic values are stored in the GC in a long-term manner.

Effects of gastric distension and electrical stimulation of dorsomedial medulla on neurons in parabrachial nucleus of rats

The effects of gastric distension and electrical stimulation of the dorsomedial medulla on neurons within the parabrachial nucleus (PB) were investigated electrophysiologically in urethane-chloralose anesthetized rats. Among 74 neurons tested, electrical stimulation of the nucleus of the solitary tract (NTS) excited 30 neurons (excitatory neurons) and inhibited 14 neurons (inhibitory neurons). Fourteen neurons increased and 12 neurons decreased their discharge rates in response to gastric distension. Twenty-two neurons responded to both electrical stimulation of the NTS and gastric distension. Both excitatory and inhibitory neurons showed either an increase or a decrease in discharge rate responding to gastric distension. Furthermore, three neurons that decreased their discharge rates and two neurons that increased their discharge rates during gastric distension also responded to intravenous administration of metaraminol indicating some effect of baroreceptor activation on the neural activity. The responses of another 49 neurons in the PB to electrical stimulation of area postrema and gastric distension were analyzed. Electrical stimulation of the AP excited 14 neurons and inhibited only one neuron. Five neurons increased and seven neurons decreased their discharge rates in response to gastric distension. Only one inhibitory neuron responded to gastric distension. These observations suggested that the PB neurons received gastric mechanoreceptive inputs from the NTS.

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).