Does taste or odor activate the same brain networks after retrieval of taste potentiated odor aversion?

Modulation of the magnitude of conditioned taste aversion in rats with excitotoxic lesions of the basolateral amygdala

The amygdala is one of the structures involved in the acquisition of conditioned taste aversion (CTA). Nevertheless, the specific roles that the nuclei of this structure play in CTA learning are controversial. Electrolytic lesions applied to the basolateral nucleus of the amygdala can eliminate or reduce the acquisition of this learning. This effect has been attributed to the involvement of fibers that pass through this nucleus and connect with other structures that are critical for CTA. Excitotoxic lesions may allow a clearer insight as to the potential involvement of this nucleus in the acquisition of CTA. The few studies to date that have used this paradigm have shown effects on taste aversion learning after applying extensive lesions to the amygdala. Thus, the aim of the present study was to determine the effect of selective excitotoxic lesions of the basolateral amygdala on the acquisition of CTA. The effects of these lesions on learning were compared with the effects observed in animals with sham lesions and animals with lesions of the hippocampus, which is a structure apparently not involved in CTA. The results revealed a decreased aversion in animals with basolateral lesions compared with both the sham and hippocampus-lesioned groups. Based on these findings, the role of this specific nucleus of the amygdala in the acquisition of taste aversion is briefly discussed.

The Origins and Organization of Vertebrate Pavlovian Conditioning

Pavlovian conditioning is the process by which we learn relationships between stimuli and thus constitutes a basic building block for how the brain constructs representations of the world. We first review the major concepts of Pavlovian conditioning and point out many of the pervasive misunderstandings about just what conditioning is. This brings us to a modern redefinition of conditioning as the process whereby experience with a conditional relationship between stimuli bestows these stimuli with the ability to promote adaptive behavior patterns that did not occur before the experience. Working from this framework, we provide an in-depth analysis of two examples, fear conditioning and food-based appetitive conditioning, which include a description of the only partially overlapping neural circuitry of each. We also describe how these circuits promote the basic characteristics that define Pavlovian conditioning, such as error-correction-driven regulation of learning.

Common brain activations for painful and non-painful aversive stimuli

BACKGROUND

Identification of potentially harmful stimuli is necessary for the well-being and self-preservation of all organisms. However, the neural substrates involved in the processing of aversive stimuli are not well understood. For instance, painful and non-painful aversive stimuli are largely thought to activate different neural networks. However, it is presently unclear whether there is a common aversion-related network of brain regions responsible for the basic processing of aversive stimuli. To help clarify this issue, this report used a cross-species translational approach in humans (i.e. meta-analysis) and rodents (i.e. systematic review of functional neuroanatomy).

RESULTS

Animal and human data combined to show a core aversion-related network, consisting of similar cortical (i.e. MCC, PCC, AI, DMPFC, RTG, SMA, VLOFC; see results section or abbreviation section for full names) and subcortical (i.e. Amyg, BNST, DS, Hab, Hipp/Parahipp, Hyp, NAc, NTS, PAG, PBN, raphe, septal nuclei, Thal, LC, midbrain) regions. In addition, a number of regions appeared to be more involved in pain-related (e.g. sensory cortex) or non-pain-related (e.g. amygdala) aversive processing.

CONCLUSIONS

This investigation suggests that aversive processing, at the most basic level, relies on similar neural substrates, and that differential responses may be due, in part, to the recruitment of additional structures as well as the spatio-temporal dynamic activity of the network. This network perspective may provide a clearer understanding of why components of this circuit appear dysfunctional in some psychiatric and pain-related disorders.

Involvement of newborn neurons in olfactory associative learning? The operant or non-operant component of the task makes all the difference

New neurons are continuously generated in the adult mammalian olfactory bulb. The role of these newborn neurons in olfactory learning has been debated. Blocking the addition of neurons has been reported either to result in memory alteration or to have no effect at all (Imayoshi et al., 2008; Breton-Provencher et al., 2009; Lazarini et al., 2009; Sultan et al., 2010). These discrepancies may have arisen from differences in the behavioral paradigms used: operant procedures indicated that neurogenesis blockade had substantial effects on long-term memory (Lazarini et al., 2009; Sultan et al., 2010) whereas other methods had little effect (Imayoshi et al., 2008; Breton-Provencher et al., 2009). Surprisingly, while operant learning is known to modulate the survival of new neurons, the effect of non-operant learning on newborn cells is unknown. Here we use mice to show that compared with operant learning, non-operant learning does not affect cell survival, perhaps explaining the current controversy. In addition, we provide evidence that distinct neural substrates at least partly underlie these two forms of learning. We conclude that the involvement of newborn neurons in learning is subtly dependent on the nature of the behavioral task.

Identifying a network of brain regions involved in aversion-related processing: a cross-species translational investigation

The ability to detect and respond appropriately to aversive stimuli is essential for all organisms, from fruit flies to humans. This suggests the existence of a core neural network which mediates aversion-related processing. Human imaging studies on aversion have highlighted the involvement of various cortical regions, such as the prefrontal cortex, while animal studies have focused largely on subcortical regions like the periaqueductal gray and hypothalamus. However, whether and how these regions form a core neural network of aversion remains unclear. To help determine this, a translational cross-species investigation in humans (i.e., meta-analysis) and other animals (i.e., systematic review of functional neuroanatomy) was performed. Our results highlighted the recruitment of the anterior cingulate cortex, the anterior insula, and the amygdala as well as other subcortical (e.g., thalamus, midbrain) and cortical (e.g., orbitofrontal) regions in both animals and humans. Importantly, involvement of these regions remained independent of sensory modality. This study provides evidence for a core neural network mediating aversion in both animals and humans. This not only contributes to our understanding of the trans-species neural correlates of aversion but may also carry important implications for psychiatric disorders where abnormal aversive behavior can often be observed.

Associatively learned representations of taste outcomes activate taste-encoding neural ensembles in gustatory cortex

Through learning processes, cues associated with emotionally salient reinforcing outcomes can come to act as substitutes for the reinforcer itself. According to one account of this phenomenon, the predictive cue associatively elicits a representation of the expected outcome by reactivating cells responsible for encoding features of the primary reinforcer. We tested this hypothesis by examining the role of neural ensembles in gustatory cortex (GC) during receipt of gustatory stimuli (sucrose and water) and cues associated with those stimuli using the immediate early genes (IEGs) Arc and Homer1a. Because these plasticity-related IEGs are expressed in the neuronal nucleus 5 and 30 min, respectively, after salient events, we examined how individual neurons encoded these stimuli in two separate behavioral epochs. In experiment 1, we showed that tasting identical sucrose solutions, but not tasteless water, in the two epochs increased both IEG activity and the degree of overlap between neural ensembles in GC. In experiment 2, odor cues associated with sucrose, but not water, evoked potentiation of IEG activity in GC similar to sucrose itself. Surprisingly, lesions of the basolateral amygdala had minimal effects on associative encoding in GC. Finally, these associatively driven representations of sucrose appeared to be outcome specific, as neural ensembles that were activated by the sucrose-associated cue were also activated by sucrose itself. This degree of overlap between associative and primary taste activity at the ensemble level suggests that GC neurons encode important information about anticipated outcomes. Such representations may provide outcome-specific information for guiding goal-directed behavior.

Does the olfactory cue activate the same brain network during aging in the rat after taste potentiated odor aversion retrieval?

Depending on the brain networks involved, aging is not accompanied by a general decrease in learning and memory capabilities. We demonstrated previously that learning and retrieval of taste potentiated odor aversion (TPOA) is preserved, and even slightly improved, in senescent rats showing some memory deficiencies in cognitive tasks (Dardou, Datiche, & Cattarelli, 2008). TPOA is a particular behavior in which the simultaneous presentation of odor and taste cues followed by a delayed visceral illness leads to a robust aversion towards both conditioned stimuli, which permits diet selection and animal survival. The present experiment was performed in order to investigate the stability or the evolution of the brain network underlying TPOA retrieval during aging. By using immunocytochemical detection of Fos and Egr1 proteins we mapped the cerebral activation induced by TPOA retrieval elicited by the odor presentation in the young, the adult and the senescent rats. The pattern of brain activation changed and the number of activated areas decreased with age. Nevertheless, the piriform cortex and the basolateral amygdala nucleus were always activated and seemed essential for TPOA retrieval. The hippocampus and the neocortical areas could have different implications in TPOA memory in relation to age. The patterns of expression of Fos and Egr1 were different, suggesting their differential involvement in TPOA retrieval. Data are discussed according to the possible roles of the brain areas studied and a model of schematic brain network subtending TPOA retrieval induced by the odor cue is proposed.

The way an odor is experienced during aversive conditioning determines the extent of the network recruited during retrieval: a multisite electrophysiological study in rats

Recent findings have revealed the importance of orthonasal and retronasal olfaction in food memory, especially in conditioned odor aversion (COA); however, little is known about the dynamics of the cerebral circuit involved in the recognition of an odor as a toxic food signal and whether the activated network depends on the way (orthonasal vs retronasal) the odor was first experienced. In this study, we mapped the modulations of odor-induced oscillatory activities through COA learning using multisite recordings of local field potentials in behaving rats. During conditioning, orthonasal odor alone or associated with ingested odor was paired with immediate illness. For all animals, COA retrieval was assessed by orthonasal smelling only. Both types of conditioning induced similarly strong COA. Results pointed out (1) a predictive correlation between the emergence of powerful beta (15-40 Hz) activity and the behavioral expression of COA and (2) a differential network distribution of this beta activity according to the way the animals were exposed to the odor during conditioning. Indeed, for both types of conditioning, the aversive behavior was predicted by the emergence of a strong beta oscillatory activity in response to the odor in the olfactory bulb, piriform cortex, orbitofrontal cortex, and basolateral amygdala. This network was selectively extended to the infralimbic and insular cortices when the odor was ingested during acquisition. These differential networks could participate in different food odor memory; these results are discussed in line with recent behavioral results that indicate that COA can be formed over long odor-illness delays only if the odor is ingested.

Inactivation of the basolateral amygdala impairs the retrieval of recent and remote taste-potentiated odor aversion memory

Memory reorganization as a time-dependent process can be investigated using various learning tasks such as the taste-potentiated odor aversion (TPOA). In this paradigm rats acquire a strong aversion to an olfactory cue presented simultaneously with a gustatory cue. Together these cues are paired with a delayed visceral illness. The basolateral amygdaloid nucleus (BLA) plays a key role in TPOA acquisition but its involvement in retrieval remains unclear. We investigated the involvement of the BLA in either recent or remote retrieval of TPOA. In each case, the number of licks observed in response to the presentation of either the odor or the taste was used to assess retrieval. Before the retrieval test, rats received a bilateral infusion of lidocaine to inactivate the BLA. We observed that both recent and remote TPOA retrieval tests induced by the odor presentation were disrupted in the lidocaine-injected rats. By contrast, the BLA inactivation had no effect upon the aversion towards the taste cue regardless of the time of retrieval. The present study provides evidence that BLA functioning is necessary for retrieval of aversive odor memory, even with a long post-acquisition delay.