Temporary basolateral amygdala lesions disrupt acquisition of socially transmitted food preferences in rats

Lesions of the basolateral amygdala (BLA) have long been associated with abnormalities of taste-related behaviors and with failure in a variety of taste- and odor-related learning paradigms, including taste-potentiated odor aversion, conditioned taste preference, and conditioned taste aversion. Still, the general role of the amygdala in chemosensory learning remains somewhat controversial. In particular, it has been suggested that the amygdala may not be involved in a form of chemosensory learning that has recently received a substantial amount of study-socially transmitted food preference (STFP). Here, we provide evidence for this involvement by pharmacologically inactivating the basolateral amygdala bilaterally during STFP training. The same inactivation sites that impaired taste aversion learning eliminated the normally conditioned preference for a food smelled on a conspecific's breath. Impairments of learned preference persisted even in testing sessions in which BLA was not inactivated, and learning was normal when the BLA was inactivated only during testing sessions; thus, the impairment was a true acquisition deficit. In conjunction with previous results from other paradigms, therefore, our data suggest that the amygdala is vital for learning procedures involving pairings of potent and arbitrary chemosensory stimuli.     

Acetylcholine in the orbitofrontal cortex is necessary for the acquisition of a socially transmitted food preference

The social transmission of food preference task (STFP) has been used to examine the involvement of the hippocampus in learning and memory for a natural odor-odor association. However, cortical involvement in STFP has not been extensively studied. The orbitofrontal cortex (OFC) is important in odor-guided learning, and cholinergic depletion of the entire neocortex results in impairments in STFP. Here we examined the specific role of cholinergic modulation in the OFC by assessing the effect of 192 immunoglobulin G-saporin infusion directly into OFC prior to training on STFP. Cholinergic depletion in the OFC impaired expression of the socially transmitted odor association measured 2 d after training, indicating that cholinergic function in the OFC is essential for this form of associative learning.     

Muscarinic cholinergic receptor blockade in the rat prelimbic cortex impairs the social transmission of food preference

Previous findings demonstrate the involvement of the cholinergic NBM in the acquisition of the social transmission of food preference (STFP), a relational associative odor-guided learning task. There is also evidence that muscarinic receptors in the medial prefrontal cortex, an important NBM target area, may modulate olfactory associative memory. The present experiment determined the consequences of blocking muscarinic cholinergic receptors in a component of the medial prefrontal region (the prelimbic cortex) on the STFP task. Adult male Wistar rats were bilaterally infused with scopolamine (20 microg/site) prior to training and showed a severe impairment in the expression of the task measured in two retention sessions, both immediately and 24h after training. Local scopolamine injections in the prelimbic cortex did not affect other behavioral measures such as olfactory perception, social interaction, motivation to eat, neophobia, or exploration. Results suggest that muscarinic transmission in the prelimbic cortex is essential for the STFP, supporting the hypothesis that ACh in a specific prefrontal area is important for this naturalistic form of olfactory relational memory. Current data are discussed in the context of disruption of learning as a result of interferences in PLC functions such as behavioral flexibility, attention, and strategic planning.     

Excitotoxic lesions of the parafascicular nucleus produce deficits in a socially transmitted food preference

The parafascicular (PF) nucleus, a posterior component of the intralaminar nuclei of the thalamus, is considered to be an essential structure in the feedback circuits of basal ganglia-thalamo-cortical systems that critically participate in cognitive processes. To study the PF contribution to processing of behaviorally significant information during specific episodes of learning, we investigated the effects of damaging the PF nucleus in the acquisition of a natural form of social olfactory learning, the socially transmitted food preference (STFP) task. This task is a non-spatial paradigm that exhibits some of the characteristics of relational memory because it requires that animals use information obtained in one episode to guide later behavior in different circumstances. Adult male Wistar rats were submitted to pretraining bilateral N-methyl-D-aspartate (0.15 M, pH 7.4) lesions of the PF (0.4 microl/side, 0.2 microl/min). The behavioral effects of PF lesions were compared to vehicle- and sham-operated control groups and two retention delays were considered in separate groups: immediately (Lesion-I, Vehicle-I, and Sham-I groups) and 24h after training (Lesion-24, Vehicle-24, and Sham-24 groups). PF lesions produced delay-independent impairments in the STFP suggesting that this nucleus might modulate the acquisition of this odor-odor association task. Results are discussed in the context of medial prefrontal cortex deafferentation induced by PF damage.     

Hippocampal c-fos is necessary for long-term memory of a socially transmitted food preference

The present article examined the requirement of hippocampal c-Fos for learning a socially transmitted food preference (STFP). We reported previously that expression of the c-Fos protein is increased in the dorsal and ventral hippocampus of rats trained on the STFP (Countryman, Orlowski, Brightwell, Oskowitz, & Colombo, 2005). Pretraining intrahippocampal antisense to the immediate early gene c-fos was administered to adult male Long-Evans rats to determine if c-fos expression is necessary for either short- or long-term memory for STFP. Guide cannulae were implanted bilaterally into the dorsal hippocampus. Antisense oligodeoxynucleotides (ODNs) were administered unilaterally either 6.5, 8.5, 10.5, or 12.5 h prior to STFP training while either sense ODNs or saline were infused into the opposite hemisphere. Immunocytochemistry was performed, and cells showing c-Fos immunoreactivity (ir) were counted from the antisense-treated hemisphere and compared to cell counts from the control hemisphere. The results indicated significant suppression of learning-induced c-Fos protein at the 8.5 and 10.5 infusion-train intervals. Additional rats were implanted with cannulae into the dorsal and ventral hippocampus, and antisense ODNs, sense ODNs, or saline were administered bilaterally 8.5h prior to training. Rats were tested immediately and 14 days after training. Rats in all groups showed a significant preference for the demonstrated food at the short-term memory test. At the long-term memory test, however, rats infused with c-fos antisense showed no preference for the demonstrated food whereas rats infused with either sense or saline maintained their preference. The present findings suggest that c-fos is necessary for consolidation of non-spatial hippocampal-dependent memory.     

Effects of anesthetic agents on socially transmitted olfactory memories in mice

Mice can learn a food preference from odor cues transmitted on the breath of a conspecific, even if the “demonstrator” is anesthetized. To our knowledge there are no studies examining the effect of anesthetizing the “observer” on development of memory for socially transmitted food preferences (STFP). In Experiment 1 we found that 2–4 month-old F2 C57Bl/6x129sv male and female mice demonstrated a STFP after a 5 min exposure to an anesthetized demonstrator mouse when tested 24 h later. In Experiment 2, observer mice anesthetized with Sagatal (60 mg/kg) prior to the “social interaction” preferentially avoided the cued food when tested 24 h later. This aversion was not due to any overt aversive effects of this dose of Sagatal because mice that ate the food and were then anesthetized, or could only smell the food for 5 min while anesthetized, showed no preference or aversion. In a third experiment we found that the Sagatal-induced aversion was not a general property of anesthesia because there were varied results produced by observer mice treated with anesthetic drugs with different mechanisms of action. Vetalar (200 mg/kg) and Rompun (10 mg/kg) treated animals ate similar amounts of cued and non-cued food at test, indicating an absence of learning. Hypnorm (0.5 ml/kg) treated animals showed a preference for the cued food whereas those treated with Hypnovel (2.5 ml/kg) showed an aversion to the cued food. These results show that the food aversion observed with Sagatal is not a general property of anesthetic agents, but appears to be restricted to those acting primarily on the GABAergic system. Thus, we have shown that under certain conditions it is possible for an anesthetized observer mouse to learn a preference or aversion of a socially-linked olfactory cue.     

Acetylcholine release in the hippocampus and prelimbic cortex during acquisition of a socially transmitted food preference

Interference with cholinergic functions in hippocampus and prefrontal cortex impairs learning and memory for social transmission of food preference, suggesting that acetylcholine (ACh) release in the two brain regions may be important for acquiring the food preference. This experiment examined release of ACh in the hippocampus and prefrontal cortex of rats during training for social transmission of food preference. After demonstrator rats ate a food with novel flavor and odor, a social transmission of food preference group of rats was allowed to interact with the demonstrators for 30 min, while in vivo microdialysis collected samples for later measurement of ACh release with HPLC methods. A social control group observed a demonstrator that had eaten food without novel flavor and odor. An odor control group was allowed to smell but not ingest food with novel odor. Rats in the social transmission but not control groups preferred the novel food on a trial 48 h later. ACh release in prefrontal cortex, with probes that primarily sampled prelimbic cortex, did not increase during acquisition of the social transmission of food preference, suggesting that training-initiated release of ACh in prelimbic cortex is not necessary for acquisition of the food preference. In contrast, ACh release in the hippocampus increased substantially (200%) upon exposure to a rat that had eaten the novel food. Release in the hippocampus increased significantly less (25%) upon exposure to a rat that had eaten normal food and did not increase significantly in the rats exposed to the novel odor; ACh release in the social transmission group was significantly greater than that of the either of the control groups. Thus, ACh release in the hippocampus but not prelimbic cortex distinguished well the social transmission vs. control conditions, suggesting that cholinergic mechanisms in the hippocampus but not prelimbic cortex are important for acquiring a socially transmitted food preference.     

Time-courses of Fos expression in rat hippocampus and neocortex following acquisition and recall of a socially transmitted food preference

The present study examined expression of the immediate-early gene, c-Fos, following acquisition, 48-h (recent) recall, and 1-week (remote) recall of a socially transmitted food preference (STFP) in multiple brain regions implicated in learning and memory. In comparisons with controls, trained Long-Evans rats had increased Fos immunoreactivity in the ventral hippocampus following acquisition and recent recall. In the parahippocampal cortices, Fos was increased in the lateral entorhinal cortex after acquisition. In the orbitofrontal cortex, increased Fos immunoreactivity was observed in the lateral orbital cortex following both recent and remote recall and in the ventral orbital cortex following remote recall, indicating a role for the orbitofrontal cortex in the remote recall of STFP memory. In contrast, in the medial prefrontal cortex, increased Fos-ir was found following acquisition in the prelimbic cortex and following recent recall in the prelimbic and infralimbic cortices. No differences in Fos expression were found between trained rats and controls in the dorsal hippocampus, posterior parietal cortex, or amygdala. The present findings support a time-limited role of the hippocampus in the acquisition and recall of STFP memory and implicate neocortical regions involved in STFP acquisition, recent, and remote recall.     

Rigidity in social and emotional memory in the R6/2 mouse model of Huntington's disease

Four experiments were conducted to examine social and emotional memory in the R6/2 transgenic mouse model of Huntington’s disease. First, R6/2 mice were tested in a social transmission of food preference task where they had to acquire a preference for a flavoured food (acquisition) and subsequently to learn a preference for a different flavour (shifted reinforcement). R6/2 mice performed well in the acquisition trial. However, they were impaired in the shifted reinforcement trial and perseverated on the first preference learned. Second, mice were trained in an inhibitory avoidance paradigm, with either one or two footshocks delivered during the training. WT mice given one footshock showed retention levels lower than those of mice trained with two footshocks. By contrast, there was no difference in retention levels of R6/2 mice given either one or two footshocks. Third, mice were tested in an active avoidance task that paired a mild footshock with a warning light. R6/2 mice had a strong age-dependent deficit in this task. Finally, mice were tested in a conditioned taste aversion task that paired a saccharine solution with a nausea-inducing agent (LiCl). R6/2 mice displayed normal aversion, however this was not extinguished following repeated exposure to saccharine solution alone. Our data show that while R6/2 mice have functional hippocampus-based memory, they have deficits in striatum-based memory skills. Further, social and emotional memories appear to be encoded in a rigid way that is not influenced by subsequent learning or by arousal levels.     

Social transmission and memory of food preferences in pine voles (Microtus pinetorum)

The social transmission of food preferences is affected by factors including the length of time a preference is maintained. The authors investigated the social transmission of food preferences in pine voles (Microtus pinetorum) and whether food items had to be present for memory to persist. A demonstrator vole was fed cocoa-flavored food, a nonpreferred food. After interacting with a demonstrator vole, observer voles preferred cocoa-flavored food as compared with either naive voles that had not interacted with demonstrators or voles that had been exposed to odors. Observers retained this preference for 72 hr after interacting with an observer whether or not flavored foods were present. The ability to learn which food items are palatable and safe may benefit a generalist herbivore that has faced selection to avoid toxic baits.     

Neuronal representation of conditioned taste in the basolateral amygdala of rats

Animals develop robust learning and long lasting taste aversion memory once they experience a new taste that is followed by visceral discomfort. A large body of literature has supported the hypothesis that basolateral amygdala (BLA) plays a critical role in the acquisition and extinction of such conditioned taste aversions (CTA). Despite the evidence that BLA is crucially engaged during CTA training, it is unclear how BLA neural activity represents the conditioned tastes. Here, we incorporated a modified behavioral paradigm suitable for single unit study, one which utilizes a sequence of pulsed saccharin and water infusion via intraoral cannulae. After conditioning, we investigated BLA unit activity while animals experience the conditioned taste (saccharin). Behavioral tests of taste reactivity confirmed that the utilized training procedure produced reliable acquisition and expression of the aversion throughout test sessions. When neural activity was compared between saccharin and water trials, half of the recorded BLA units (77/149) showed differential activity according to the types of solution. 76% of those cells (29/38) in the conditioned group showed suppressed activity, while only 44% of taste reactive cells (17/39) in controls showed suppressed activity during saccharin trials (relative to water trials). In addition, the overall excitability of BLA units was increased as shown by altered characteristics of burst activity after conditioning. The changes in BLA activity as a consequence of CTA were maintained throughout test sessions, consistent with the behavioral study. The current study suggests that the neuronal activity evoked by a sweet taste is altered as a consequence of CTA learning, and that the overall change might be related to the learning induced negative affect.     

Social learning of food types in zebra finches (<Emphasis Type="Italic">Taenopygia guttata</Emphasis>) is directed by demonstrator sex and feeding activity

In this study we examined how social learning of feeding preferences by zebra finches was affected by the identity of different demonstrators. We presented adult zebra finches with two demonstrators, one male and one female, that exhibited different food choices, and we recorded their subsequent preference when given a choice between the two food types. Previously it was found that young zebra finches' patterns of social learning are affected by the sex of the individual demonstrating a feeding behaviour. This result could be explained by the lack of exposure these animals had to the opposite sex, or by their mating status. Therefore, we investigated the social learning preferences of adult mated zebra finches. We found the same pattern of directed social learning of a different type of feeding behaviour (food colour): female zebra finches preferred the colour of food eaten by male demonstrators, whereas male zebra finches showed little evidence of any preference for the colour of food eaten by female demonstrators. Furthermore, we found that female observers' preferences were biased by demonstrators' relative feeding activity: the female demonstrator was only ever preferred if it ate less than its male counterpart. Electronic Publication     

Blockade of NMDA receptors in the amygdala prevents latent inhibition of fear-conditioning

The association between a conditioned stimulus (CS) and an unconditioned stimulus (US) in fear-conditioning depends on N-methyl-D-aspartate (NMDA) receptors in the basolateral amygdala complex (BLA). Latent inhibition (LI) is the retardation in learning due to nonreinforced presentation of the prospective CS before conditioning. Disruption of LI in rats is an animal model of schizophrenia, reflecting the deficits of schizophrenic patients in neglecting irrelevant information. We investigated whether the BLA is involved in LI of fear-potentiated startle. Infusions of the NMDA receptor antagonist D,L-2-amino-5-phosphonopentanoic acid (AP-5; 12.5 nmoles) into the BLA before preexposure of rats to the neutral stimulus prevent LI of fear-conditioning. We also demonstrated by the same method that a complex of thalamic nuclei, comprising the medial part of the medial geniculate nucleus, the posterior intralaminar nucleus, and the suprageniculate nucleus, is involved in fear-conditioning, but not in LI. This suggests that the presentation of an innocuous stimulus during preexposure leads to an NMDA receptor-dependent change of neurotransmission in the BLA, but not in the thalamus. Our data show that the BLA but not the thalamus regulates in LI of fear-potentiated startle. Furthermore, it supports the hypothesis that the inability of schizophrenic patients to ignore irrelevant stimuli may be caused by hypofunction of the glutamatergic transmission in the brain and suggests an involvement of the amygdala in the neuropathology of schizophrenia.     

Differential effects of damage within the hippocampal region on memory for a natural, nonspatial Odor-Odor Association

Debate continues on whether the role of rodent hippocampus in memory is limited to the spatial domain. Recently, this controversy has been addressed with studies on the social transmission of food preference, an odor-odor association task with no spatial requirements. Multiple reports have concluded that damage to the hippocampal region impairs memory in this task, but there remain questions about the extent of damage essential to produce an impairment. Furthermore, a recent study () found no effect of hippocampal lesions on memory in this task. We tested animals with complete lesions of the hippocampus (H) lesions of the hippocampus plus subiculum (HS), and lesions of the adjacent, anatomically related cortices of the parahippocampal region (PHR). H lesions produced an impairment on spatial delayed alternation, but not on memory for the social transmission of food preference, whereas HS and PHR lesions produced severe and equivalent impairments on memory for the socially acquired food preference. We discuss possible explanations for the discrepancy with the results of and conclude that the hippocampus and subiculum together play a critical role in the formation of this form of nonspatial, relational memory.     

Enhancement of extinction memory consolidation: the role of the noradrenergic and GABAergic systems within the basolateral amygdala

Evidence from previous studies indicates that the noradrenergic and GABAergic influences within the basolateral amygdala (BLA) modulate the consolidation of memory for fear conditioning. The present experiments investigated whether the same modulatory influences are involved in regulating the extinction of fear-based learning. To investigate this issue, male Sprague Dawley rats implanted with unilateral or bilateral cannula aimed at the BLA were trained on a contextual fear conditioning (CFC) task and 24 and 48 h later were given extinction training. Immediately following each extinction session they received intra-BLA infusions of the GABAergic antagonist bicuculline (50 ng), the beta-adrenocepter antagonist propranolol (500 ng), bicuculline with propranolol, norepinephrine (NE) (0.3, 1.0, and 3.0 microg), the GABAergic agonist muscimol (125 ng), NE with muscimol or a control solution. To investigate the involvement of the dorsal hippocampus (DH) as a possible target of BLA activation during extinction, other animals were given infusions of muscimol (500 ng) via an ipsilateral cannula implanted in the DH. Bilateral BLA infusions of bicuculline significantly enhanced extinction, as did infusions into the right, but not left BLA. Propranolol infused into the right BLA together with bicuculline blocked the bicuculline-induced memory enhancement. Norepinephrine infused into the right BLA also enhanced extinction, and this effect was not blocked by co-infusions of muscimol. Additionally, muscimol infused into the DH did not attenuate the memory enhancing effects of norepinephrine infused into the BLA. These findings provide evidence that, as with original CFC learning, noradrenergic activation within the BLA modulates the consolidation of CFC extinction. The findings also suggest that the BLA influence on extinction is not mediated by an interaction with the dorsal hippocampus.     

Pre-training anandamide infusion within the basolateral amygdala impairs plus-maze discriminative avoidance task in rats

Endocannabinoids (eCBs) modulate a variety of brain functions via activation of the widely expressed CB1 receptor. One site of high density of this receptor is the basolateral amygdala (BLA), a structure involved in the formation of aversive memories. The activation and blockade of CB1 receptors by systemic or hippocampal drug administrations have been shown to modify memory processing. However, little is known about the role of the BLA endocannabinoid system in aversive memories. Additionally, BLA endocannabinoid transmission seems to be related to emotional states, but the relevance of these effects to memory formation is still unknown. In this study we investigated the effects of the eCB anandamide (AEA) and the CB1 antagonist/inverse agonist AM251 infused into the BLA on the acquisition of an aversive memory task, concomitantly evaluating basal anxiety levels in rats. Male rats received pre-training micro-injection of AEA, AM251 or vehicle bilaterally into the BLA, and were studied with the plus-maze discriminative avoidance task (a paradigm that allows concomitant and independent evaluation of anxiety-like behavior and the memory of an aversive task). Our results showed that AEA into the BLA before training prevented memory retrieval 24 h later, as evaluated by exploration of the aversive arm of the maze, while AM251 into the BLA did not interfere with animals' performance. In addition, AEA had no effect on anxiety-like behavior (as evaluated by open arm exploration and risk assessment), while AM251 induced an anxiogenic effect. Our data indicate an important role of BLA CB1 receptors in aversive memory formation, and suggest that this involvement is not necessarily related to a possible modulation of anxiety states.     

Phthalic acid amygdalopetal lesion of the nucleus basalis magnocellularis induces reversible memory deficits in rats

The basolateral amygdala (BLA) is extensively implicated in emotional learning and memory. The current study investigated the contribution of cholinergic afferents to the BLA from the nucleus basalis magnocellularis in influencing aversive learning and memory. Sprague-Dawley rats were given permanent unilateral phthalic acid (300 ng) lesions of the nucleus basalis magnocellularis and were chronically implanted with cannulas aimed at the ipsilateral BLA. Lesioned rats showed a pronounced inhibitory avoidance task retention deficit that was attenuated by acute posttraining infusions of the muscarinic cholinergic agonist oxotremorine (4 ng) or the indirect agonist physostigmine (1 microg) into the BLA. Continuous multiple-trial inhibitory avoidance training and testing revealed that lesioned rats have a mild acquisition deficit, requiring approximately 1 additional shock to reach the criterion, and a pronounced consolidation deficit as indicated by a shorter latency to enter the shock compartment on the retention test. Because lesioned rats did not differ from sham-operated controls in performance on a spatial water maze task or in shock sensitivity, it is not likely that the memory impairments produced by the phthalic acid lesions are due to any general sensory or motor deficits. These findings suggest that the dense cholinergic projection from the nucleus basalis magnocellularis to the BLA is involved in both the acquisition and the consolidation of the aversive inhibitory avoidance task.     

Intracranial self-stimulation recovers learning and memory capacity in basolateral amygdala-damaged rats

We studied the capacity of post-training intracranial self-stimulation (SS) to reverse or ameliorate learning and memory impairments caused by amygdala damage in rats. A first experiment showed that lesions of the basolateral amygdala (BLA) slow down acquisition of two-way active avoidance conditioning (2wAA). In a second experiment we observed that a post-training SS treatment administered immediately after each 2wAA conditioning session is able to completely reverse the disruptive effects of the BLA lesions, and the facilitative effect lasts for 10 days. A third experiment allowed us to differentiate the strong recuperative effects of the SS treatment from the slight effect caused by overtraining the same conditioning response. We concluded that SS is able to counteract the behavioral deficit induced by BLA damage, probably by activating alternative undamaged brain structures related to learning and memory, such as the hippocampus.