Encoding predicted outcome and acquired value in orbitofrontal cortex during cue sampling depends upon input from basolateral amygdala

Certain goal-directed behaviors depend critically upon interactions between orbitofrontal cortex (OFC) and basolateral amygdala (ABL). Here we describe direct neurophysiological evidence of this cooperative function. We recorded from OFC in intact and ABL-lesioned rats learning odor discrimination problems. As rats learned these problems, we found that lesioned rats exhibited marked changes in the information represented in OFC during odor cue sampling. Lesioned rats had fewer cue-selective neurons in OFC after learning; the cue-selective population in lesioned rats did not include neurons that were also responsive in anticipation of the predicted outcome; and the cue-activated representations that remained in lesioned rats were less associative and more often bound to cue identity. The results provide a neural substrate for representing acquired value and features of the predicted outcome during cue sampling, disruption of which could account for deficits in goal-directed behavior after damage to this system.     

Neural encoding in ventral striatum during olfactory discrimination learning

A growing body of evidence implicates the ventral striatum in using information acquired through associative learning. The present study examined the activity of ventral striatal neurons in awake, behaving rats during go/no-go odor discrimination learning and reversal. Many neurons fired selectively to odor cues predictive of either appetitive (sucrose) or aversive (quinine) outcomes. Few neurons were selective when first exposed to the odors, but many acquired this differential activity as rats learned the significance of the cues. A substantial proportion of these neurons encoded the cues' learned motivational significance, and these neurons tended to reverse their firing selectivity after reversal of odor-outcome contingencies. Other neurons that became selectively activated during learning did not reverse, but instead appeared to encode specific combinations of cues and associated motor responses. The results support a role for ventral striatum in using the learned significance, both appetitive and aversive, of predictive cues to guide behavior.     

Basolateral amygdala lesions abolish orbitofrontal-dependent reversal impairments

Damage to orbitofrontal cortex (OFC) has long been associated with deficits in reversal learning. OFC damage also causes inflexible associative encoding in basolateral amygdala (ABL) during reversal learning. Here we provide a critical test of the hypothesis that the reversal deficit in OFC-lesioned rats is caused by this inflexible encoding in ABL. Rats with bilateral neurotoxic lesions of OFC, ABL, or both areas were tested on a series of two-odor go/no-go discrimination problems, followed by two serial reversals of the final problem. As expected, all groups acquired the initial problems at the same rate, and rats with OFC lesions were slower to acquire the reversals than sham controls. This impairment was abolished by accompanying ABL lesions, while ABL lesions alone had no effect on reversal learning. These results are consistent with the hypothesis that OFC facilitates cognitive flexibility by promoting updating of associative encoding in downstream brain areas.     

Adolescent Changes in Dopamine D1 Receptor Expression in Orbitofrontal Cortex and Piriform Cortex Accompany an Associative Learning Deficit

The orbitofrontal cortex (OFC) and piriform cortex are involved in encoding the predictive value of olfactory stimuli in rats, and neural responses to olfactory stimuli in these areas change as associations are learned. This experience-dependent plasticity mirrors task-related changes previously observed in mesocortical dopamine neurons, which have been implicated in learning the predictive value of cues. Although forms of associative learning can be found at all ages, cortical dopamine projections do not mature until after postnatal day 35 in the rat. We hypothesized that these changes in dopamine circuitry during the juvenile and adolescent periods would result in age-dependent differences in learning the predictive value of environmental cues. Using an odor-guided associative learning task, we found that adolescent rats learn the association between an odor and a palatable reward significantly more slowly than either juvenile or adult rats. Further, adolescent rats displayed greater distractibility during the task than either juvenile or adult rats. Using real-time quantitative PCR and immunohistochemical methods, we observed that the behavioral deficit in adolescence coincides with a significant increase in D1 dopamine receptor expression compared to juvenile rats in both the OFC and piriform cortex. Further, we found that both the slower learning and increased distractibility exhibited in adolescence could be alleviated by experience with the association task as a juvenile, or by an acute administration of a low dose of either the dopamine D1 receptor agonist SKF-38393 or the D2 receptor antagonist eticlopride. These results suggest that dopaminergic modulation of cortical function may be important for learning the predictive value of environmental stimuli, and that developmental changes in cortical dopaminergic circuitry may underlie age-related differences in associative learning.     

Learning to smell the roses: experience-dependent neural plasticity in human piriform and orbitofrontal cortices

It is widely presumed that odor quality is a direct outcome of odorant structure, but human studies indicate that molecular knowledge of an odorant is not always sufficient to predict odor quality. Indeed, the same olfactory input may generate different odor percepts depending on prior learning and experience. Combining functional magnetic resonance imaging with an olfactory paradigm of perceptual learning, we examined how sensory experience modifies odor perception and odor quality coding in the human brain. Prolonged exposure to a target odorant enhanced perceptual differentiation for odorants related in odor quality or functional group, an effect that was paralleled by learning-induced response increases in piriform cortex and orbitofrontal cortex (OFC). Critically, the magnitude of OFC activation predicted subsequent improvement in behavioral differentiation. Our findings suggest that neural representations of odor quality can be rapidly updated through mere perceptual experience, a mechanism that may underlie the development of odor perception.     

Different time courses for learning-related changes in amygdala and orbitofrontal cortex

The orbitofrontal cortex (OFC) and amygdala are thought to participate in reversal learning, a process in which cue-outcome associations are switched. However, current theories disagree on whether OFC directs reversal learning in the amygdala. Here, we show that during reversal of cues' associations with rewarding and aversive outcomes, neurons that respond preferentially to stimuli predicting aversive events update more quickly in amygdala than OFC; meanwhile, OFC neurons that respond preferentially to reward-predicting stimuli update more quickly than those in the amygdala. After learning, however, OFC consistently differentiates between impending reinforcements with?a shorter latency than the amygdala. Finally, analysis of local field potentials (LFPs) reveals a disproportionate influence of OFC on amygdala that emerges after learning. We propose that reversal learning is supported by complex interactions between neural circuits spanning the amygdala and OFC, rather than directed by any single structure.     

Neurogenesis in a rat model of age-related cognitive decline

Age-related decrements in hippocampal neurogenesis have been suggested as a basis for learning impairment during aging. In the current study, a rodent model of age-related cognitive decline was used to evaluate neurogenesis in relation to hippocampal function. New hippocampal cell survival was assessed approximately 1 month after a series of intraperitoneal injections of 5-bromo-2'-deoxyuridine (BrdU). Correlational analyses between individual measures of BrdU-positive cells and performance on the Morris water maze task provided no indication that this measure of neurogenesis was more preserved in aged rats with intact cognitive abilities. On the contrary, among aged rats, higher numbers of BrdU-positive cells in the granule cell layer were associated with a greater degree of impairment on the learning task. Double-labelling studies confirmed that the majority of the BrdU+ cells were of the neuronal phenotype; the proportion of differentiated neurons was not different across a broad range of cognitive abilities. These data demonstrate that aged rats that maintain cognitive function do so despite pronounced reductions in hippocampal neurogenesis. In addition, these findings suggest the interesting possibility that impaired hippocampal function is associated with greater survival of newly generated hippocampal neurons at advanced ages.     

Regulation of c-Fos expression in the rat olfactory bulbs during olfactory learning

The distribution of c-Fos-immunopositive neurons was examined in the mitral/tufted and granular cell layers in the medium part of the main olfactory bulbs of 18-day-old rats after they had been trained for propionic acid vapour-guided search for dam in the Y-maze. On the next day these pups exhibited a strong preference for the propionic acid odor as compared to the control pups trained for this task without the odor cue and odor-familiarized pups exposed to propionic acid as a novel neutral stimulus. Exposure to propionic acid produced a moderate activation of c-Fos expression, mainly in the granular layer of the dorsomedial part of the bulb. Training in the Y-maze devoid of odor cues resulted in diffuse increase in the number of c-Fos-positive neurons both in the mitral and granular cell layers in all parts of the olfactory bulb. Maze training with the odor cue produced activation of c-Fos expression (which significantly exceeded the non-odor Y-maze group) in the dorsomedial olfactory bulb. These data suggest that associative olfactory conditioning results in activation of c-Fos expression that combines the effect of diffuse motivational excitation and specific olfactory input to the neurons which process odor cues.     

Physiological properties and response modulations of mushroom body feedback neurons during olfactory learning in the honeybee, Apis mellifera

Mushroom bodies are central brain structures and essentially involved in insect olfactory learning. Within the mushroom bodies γ-aminobutyric acid (GABA)-immunoreactive feedback neurons are the most prominent neuron group. The plasticity of inhibitory neural activity within the mushroom body was investigated by analyzing modulations of odor responses of feedback neurons during olfactory learning in vivo. In the honeybee, Apis mellifera, feedback neurons were intracellularly recorded at their neurites. They produced complex patterns of action potentials without experimental stimulation. Summating postsynaptic potentials indicate that their synaptic input region lies within the lobes. Odor and antennal sucrose stimuli evoked excitatory phasic-tonic responses. Individual neurons responded to various odors; responses of different neurons to the same odor were highly variable. Response modulations were determined by comparing odor responses of feedback neurons before and after one-trial olfactory conditioning or sensitisation. Shortly after pairing an odor stimulus with a sucrose reward, odor-induced spike activity of feedback neurons decreased. Repeated odor stimulations alone, equally spaced as in the conditioning experiment, did not affect the odor-induced excitation. A single sensitisation trial also did not alter odor responses. These findings indicate that the level of odor-induced inhibition within the mushroom bodies is specifically modulated by experience. Accepted: 9 September 1999     

Crossmodal associative memory representations in rodent orbitofrontal cortex

Firing patterns of neurons in the orbitofrontal cortex (OF) were analyzed in rats trained to perform a task that encouraged incidental associations between distinct odors and the places where their occurrence was detected. Many of the neurons fired differentially when the animals were at a particular location or sampled particular odors. Furthermore, a substantial fraction of the cells exhibited odor-specific firing patterns prior to odor presentation, when the animal arrived at a location associated with that odor. These findings suggest that neurons in the OF encode cross-modal associations between odors and locations within long-term memory.     

Coordinated activity of ventral tegmental neurons adapts to appetitive and aversive learning

Our understanding of how value-related information is encoded in the ventral tegmental area (VTA) is based mainly on the responses of individual putative dopamine neurons. In contrast to cortical areas, the nature of coordinated interactions between groups of VTA neurons during motivated behavior is largely unknown. These interactions can strongly affect information processing, highlighting the importance of investigating network level activity. We recorded the activity of multiple single units and local field potentials (LFP) in the VTA during a task in which rats learned to associate novel stimuli with different outcomes. We found that coordinated activity of VTA units with either putative dopamine or GABA waveforms was influenced differently by rewarding versus aversive outcomes. Specifically, after learning, stimuli paired with a rewarding outcome increased the correlation in activity levels between unit pairs whereas stimuli paired with an aversive outcome decreased the correlation. Paired single unit responses also became more redundant after learning. These response patterns flexibly tracked the reversal of contingencies, suggesting that learning is associated with changing correlations and enhanced functional connectivity between VTA neurons. Analysis of LFP recorded simultaneously with unit activity showed an increase in the power of theta oscillations when stimuli predicted reward but not an aversive outcome. With learning, a higher proportion of putative GABA units were phase locked to the theta oscillations than putative dopamine units. These patterns also adapted when task contingencies were changed. Taken together, these data demonstrate that VTA neurons organize flexibly as functional networks to support appetitive and aversive learning.     

Flavor Preference Learning Increases Olfactory and Gustatory Convergence onto Single Neurons in the Basolateral Amygdala but Not in the Insular Cortex in Rats

The basolateral amygdala (BLA) and the insular cortex (IC) represent two major areas for odor-taste associations, i.e. flavor integration. This learning may require the development of convergent odor and taste neuronal activation allowing the memory representation of such association. Yet identification of neurons that respond to such coincident input and the effect of flavor experience on odor-taste convergence remain unclear. In the present study we used the compartmental analysis of temporal activity using fluorescence in situ hybridization for Arc (catFISH) to visualize odor-taste convergence onto single neurons in the BLA and in the IC to assess the number of cells that were co-activated by both stimuli after odor-taste association. We used a sucrose conditioned odor preference as a flavor experience in rats, in which 9 odor-sucrose pairings induce a reliable odor-taste association. The results show that flavor experience induced a four-fold increase in the percentage of cells activated by both taste and odor stimulations in the BLA, but not in the IC. Because conditioned odor preference did not modify the number of cells responding selectively to one stimulus, this greater odor-taste convergence into individual BLA neurons suggests the recruitment of a neuronal population that can be activated by both odor and taste only after the association. We conclude that the development of convergent activation in amygdala neurons after odor-taste associative learning may provide a cellular basis of flavor memory.     

Rats habituated to chronic feeding restriction show a smaller increase in olfactory bulb reactivity compared to newly fasted rats

During the 1970s, the multiunit reactivity of the olfactory bulb to food odor was extensively shown to increase before their usual meal in rats habituated to having a single 2 h daily meal compared to the same rats recorded after their usual meal. More recently, we reported dramatic modifications of mitral cell single-unit reactivity in adult rats following a simple a manipulation of the olfactory environment--exposure to an odor. The present study aimed at testing the hypothesis that a simple behavioral change such as habituation to chronic food restriction may induce profound changes in olfactory bulb responsiveness compared to occasional fasting. We compared mitral cell reactivity in non-fasted rats, in rats fasted during 22 h for the very first time, and in rats habituated during 15 days to a chronic 22 h food restriction. Mitral cell single-unit reactivity was found to increase less in rats habituated to fasting than in newly fasted rats. Indeed, the proportion of mitral cell responses to food and non-food odors was significantly higher in rats habituated to fasting than in non-fasted rats, but lower than in newly fasted rats. The proportion of simple unsynchronized and synchronized responses of 1b and 2b types was also lower in habituated rats whereas the proportion of complex synchronized responses of 4b type increased. This decreased responsiveness in habituated rats, similar to that observed in rats repeatedly exposed for 20 min per day to an odor during six consecutive days in our previous studies, is discussed with respect to olfactory bulb plasticity.     

EOG responses in anesthetized freely breathing rats

In mammals, access of odor molecules to the olfactory receptor neurons is controlled by respiratory activity. Thus, anesthetized, freely breathing rats were used to record from the olfactory mucosa in the intact nasal cavity (electroolfactogram or EOG) so as to study global response characteristics to odor stimuli. During alternation of the inspiratory phases of odor sampling and expiratory phases, the response was a succession of individual EOG events synchronized with respiration. These were characterized by a steep decrease that started approximately 100-150 ms after the beginning of inhalation, reached its maximum at the transition between inspiration and expiration and was followed by a slower rise until the next inhalation. They were greater during the first respiratory cycles following odor stimulation onset. Thereafter their amplitudes decreased throughout odor delivery, but a significant EOG signal was still present at the end of short (10 s) and long (60 s) odor presentations. Amplitude increased with odor concentration, but much less than expected from concentration changes. Lastly, for some odors EOG responses persisted well beyond the end of stimulation. These results are in agreement with the respiratory synchronization of mitral cell activities observed during short odor presentations and long duration odor exposures. They underline again the importance of taking into account the respiratory activity in studies on the functioning of the olfactory system.     

Ventilatory response to sustained hypoxia in carotid body denervated rats

Hypoxia stimulates ventilation, but when it is sustained, a decline in the ventilatory response is seen. The mechanism responsible for this decline lies within the CNS, but still remains unknown. In this study, we attempted to elucidate the possible role of hypoxia-induced depression of respiratory neurons by comparing the ventilatory response to hypoxia in intact rats and those with denervated carotid bodies. A whole-body plethysmograph was used to measure tidal volume, frequency of breathing and minute ventilation (VE) in awake and anesthetized intact rats and rats after carotid body denervation during exposure to hypoxia (FIO2 0.1). Fifteen-minute hypoxia induced an initial increase of VE in intact rats (to 248% of control ventilation in awake and to 227% in anesthetized rats) followed by a consistent decline (to 207% and 196% of control VE, respectively). Rats with denervated carotid bodies responded with a smaller increase in VE (to 134% in awake and 114% in anesthetized animals), but without a secondary decline (145% and 129% of control VE in the 15th min of hypoxia). These results suggest that afferentation from the carotid bodies and/or the substantial increase in ventilation are crucial for the biphasicity of the ventilatory response to sustained hypoxia and that a central hypoxic depression cannot fully explain the secondary decline in VE.     

Olfactory predictive codes and stimulus templates in piriform cortex

Neuroscientific models of sensory perception suggest that the brain utilizes predictive codes in advance of a stimulus encounter, enabling organisms to infer forthcoming sensory events. However, it is poorly understood how such mechanisms are implemented in the olfactory system. Combining high-resolution functional magnetic resonance imaging with multivariate (pattern-based) analyses, we examined the spatiotemporal evolution of odor perception in the human brain during an olfactory search task. Ensemble activity patterns in anterior piriform cortex (APC) and orbitofrontal cortex (OFC) reflected the attended odor target both before and after stimulus onset. In contrast, prestimulus ensemble representations of the odor target in posterior piriform cortex (PPC) gave way to poststimulus representations of the odor itself. Critically, the robustness of target-related patterns in PPC predicted subsequent behavioral performance. Our findings directly show that the brain generates predictive templates or "search images" in PPC, with physical correspondence to odor-specific pattern representations, to augment olfactory perception.     

大鼠眶额叶GABA及其B型受体在应激性抑郁行为发生中的作用及其影响机制

为了探讨眶额叶(orbital frontal cortex,OFC)GABA及其B型受体在应激性抑郁行为发生中的作用及其影响机制,实验采用强迫游泳方法建立急性应激抑郁模型。在OFC区微量注射γ-氨基丁酸(γ-aminobutyric acid,GABA)及其B型受体阻断剂,通过开场实验、强迫游泳方式检测动物行为学表现,用免疫组织化学染色和Western blotting方法检测OFC区Kalirin表达,用高尔基染色法观察锥体细胞树突和树突棘。结果显示:强迫游泳应激引起动物抑郁样行为表现,同时,OFC区Kalirin阳性颗粒数及表达量显著减少,且锥体细胞树突棘密度下降;OFC区微量注射GABA具有抗抑郁效应,使OFC区Kalirin表达显著升高,锥体细胞树突棘密度增加;GABA-B型受体阻断剂CGP35348可以抑制GABA的这种效应。由此可见,通过强迫游泳应激诱发的抑郁样的行为变化与OFC区Kalirin表达减少和神经元树突棘密度降低有关,GABA可能通过GABA-B型受体增加OFC区Kalirin表达,以防止神经元退行性变化而产生抗抑郁作用。     

Drosophila fasciclinII is required for the formation of odor memories and for normal sensitivity to alcohol.

Drosophila fasciclinII (fasII) mutants perform poorly after olfactory conditioning due to a defect in encoding, stabilizing, or retrieving short-term memories. Performance was rescued by inducing the expression of a normal transgene just before training and immediate testing. Induction after training but before testing failed to rescue performance, showing that Fas II does not have an exclusive role in memory retrieval processes. The stability of odor memories in fasII mutants are indistinguishable from control animals when initial performance is normalized. Like several other mutants deficient in odor learning, fasII mutants exhibit a heightened sensitivity to ethanol vapors. A combination of behavioral and genetic strategies have therefore revealed a role for Fas II in the molecular operations of encoding short-term odor memories and conferring alcohol sensitivity. The preferential expression of Fas II in the axons of mushroom body neurons furthermore suggests that short-term odor memories are formed in these neurites.     

Acquisition of enhanced natural killer cell activity under anesthesia

An increase in natural killer (NK) cell activity can be conditioned with a one trial learning paradigm to demonstrate the interaction between the central nervous system (CNS) and the immune system. In order to demonstrate learning possibilities during ‘non-conscious’ state, mice were anesthetized with a ketamin/rompun mixture and underwent one trial learning with odor cue as the conditioned stimulus (CS) preceding the unconditioned stimulus (US). The results indicated that mice that were exposed to camphor odor cue under the influence of anesthesia can associate the signal with the poly I:C unconditioned stimulus and were able to recall the conditioned response upon reexposure to the CS. Secondly, the conditioned association made in a conscious state can be recalled by exposure to the same olfactory odor cue in a ‘non-conscious’ state. The increase in the conditioned change in NK cell activity of both situations was significantly higher than the control group. The results demonstrate that learning can take place and the learned response can be recalled under the reduced awareness caused by anesthesia. The findings we report are unusual and novel in that they demonstrate that the CNS can learn new associations under conditions where the host is apparently unaware of the signals being linked. Anesthesia combined with the long interstimulus interval indicates that certain neuronal pathways in the CNS are receptive to second signals (elicited by the US) even when the second signal is separated by one day. This means the conditioned learning of a physiological response can take place unconsciously at a separate level and under situations where the host is totally unaware of the events which the brain is processing and linking as incoming information.     

The Dorsolateral Periaqueductal Gray and Its Role in Mediating Fear Learning to Life Threatening Events

The dorsolateral column of the periaqueductal gray (dlPAG) integrates aversive emotional experiences and represents an important site responding to life threatening situations, such as hypoxia, cardiac pain and predator threats. Previous studies have shown that the dorsal PAG also supports fear learning; and we have currently explored how the dlPAG influences associative learning. We have first shown that N-methyl-D-aspartate (NMDA) 100 pmol injection in the dlPAG works as a valuable unconditioned stimulus (US) for the acquisition of olfactory fear conditioning (OFC) using amyl acetate odor as conditioned stimulus (CS). Next, we revisited the ascending projections of the dlPAG to the thalamus and hypothalamus to reveal potential paths that could mediate associative learning during OFC. Accordingly, the most important ascending target of the dlPAG is the hypothalamic defensive circuit, and we were able to show that pharmacological inactivation using β-adrenoceptor blockade of the dorsal premammillary nucleus, the main exit way for the hypothalamic defensive circuit to thalamo-cortical circuits involved in fear learning, impaired the acquisition of the OFC promoted by NMDA stimulation of the dlPAG. Moreover, our tracing study revealed multiple parallel paths from the dlPAG to several thalamic targets linked to cortical-hippocampal-amygdalar circuits involved in fear learning. Overall, the results point to a major role of the dlPAG in the mediation of aversive associative learning via ascending projections to the medial hypothalamic defensive circuit, and perhaps, to other thalamic targets, as well. These results provide interesting perspectives to understand how life threatening events impact on fear learning, and should be useful to understand pathological fear memory encoding in anxiety disorders.