Fos imaging reveals differential neuronal activation of areas of rat temporal cortex by novel and familiar sounds

The medial entorhinal cortex encodes multisensory spatial information

Animals employ spatial information in multisensory modalities to navigate their natural environments. However, it is unclear whether the brain encodes such information in separate cognitive maps or integrates all into a single, universal map. We addressed this question in the microcircuit of the medial entorhinal cortex (MEC), a cognitive map of space. Using cellular-resolution calcium imaging, we examined the MEC of mice navigating virtual reality tracks, where visual and auditory cues provided comparable spatial information. We uncovered two cell types: "unimodality cells" and "multimodality cells". The unimodality cells specifically represent either auditory or visual spatial information. They are anatomically intermingled and maintain sensory preferences across multiple tracks and behavioral states. The multimodality cells respond to both sensory modalities with their responses shaped differentially by auditory and visual information. Thus, the MEC enables accurate spatial encoding during multisensory navigation by computing spatial information in different sensory modalities and generating distinct maps.

Intranasal Nanotransferosomal Gel for Quercetin Brain Targeting: II. Antidepressant Effect in an Experimental Animal Model

Depression is a serious mental disorder and the most prevalent cause of disability and suicide worldwide. Quercetin (QER) demonstrated antidepressant effects in rats exhibiting anxiety and depressive-like behaviors. In an attempt to improve QER's antidepressant activity, a QER-loaded transferosome (QER-TFS) thermosensitive gel for intranasal administration was formulated and optimized. The therapeutic effectiveness of the optimized formulation was assessed in a depressed rat model by conducting a behavioral analysis. Behavioral study criteria such as immobility, swimming, climbing, sucrose intake, number of crossed lines, rearing, active interaction, and latency to feed were all considerably enhanced by intranasal treatment with the QER-TFS in situ gel in contrast to other formulations. A nasal histopathological study indicated that the QER-TFS thermosensitive gel was safe for the nasal mucosa. An immunohistochemical analysis showed that the animals treated with the QER-TFS thermosensitive gel had the lowest levels of c-fos protein expression, and brain histopathological changes in the depressed rats were alleviated. According to pharmacodynamic, immunohistochemical, and histopathological experiments, the intranasal administration of the QER-TFS thermosensitive gel substantially alleviated depressive symptoms in rats. However, extensive preclinical investigations in higher animal models are needed to anticipate its effectiveness in humans.

Object Recognition Memory: Distinct Yet Complementary Roles of the Mouse CA1 and Perirhinal Cortex

While the essential contribution of the hippocampus to spatial memory is well established, object recognition memory has been traditionally attributed to the perirhinal cortex (PRh). However, the results of several studies indicate that under specific procedural conditions, temporary or permanent lesions of the hippocampus affect object memory processes as measured in the Spontaneous Object Recognition (SOR) task. The PRh and hippocampus are considered to contribute distinctly to object recognition memory based on memory strength. Allowing mice more, or less, exploration of novel objects during the encoding phase of the task (i.e., sample session), yields stronger, or weaker, object memory, respectively. The current studies employed temporary local inactivation and immunohistochemistry to determine the differential contributions of neuronal activity in PRh and the CA1 region of the hippocampus to strong and weak object memory. Temporary inactivation of the CA1 immediately after the SOR sample session impaired strong object memory but spared weak object memory; while temporary inactivation of PRh post-sample impaired weak object memory but spared strong object memory. Furthermore, mRNA transcription and de novo protein synthesis are required for the consolidation of episodic memory, and activation patterns of immediate early genes (IEGs), such as c-Fos and Arc, are linked to behaviorally triggered neuronal activation and synaptic plasticity. Analyses of c-Fos and Arc protein expression in PRh and CA1 neurons by immunohistochemistry, and of Arc mRNA by qPCR after distinct stages of SOR, provide additional support that strong object memory is dependent on CA1 neuronal activity, while weak object memory is dependent on PRh neuronal activity. Taken together, the results support the view that both PRh and CA1 are required for object memory under distinct conditions. Specifically, our results are consistent with a model that as the mouse begins to explore a novel object, information about it accumulates within PRh, and a weak memory of the object is encoded. If object exploration continues beyond some threshold, strong memory for the event of object exploration is encoded; the consolidation of which is CA1-dependent. These data serve to reconcile the dissension in the literature by demonstrating functional and complementary roles for CA1 and PRh neurons in rodent object memory.

Familiarity with social sounds alters c-Fos expression in auditory cortex and interacts with estradiol in locus coeruleus

When a social sound category initially gains behavioral significance to an animal, plasticity events presumably enhance the ability to recognize that sound category in the future. In the context of learning natural social stimuli, neuromodulators such as norepinephrine and estrogen have been associated with experience-dependent plasticity and processing of newly salient social cues, yet continued plasticity once stimuli are familiar could disrupt the stability of sensorineural representations. Here we employed a maternal mouse model of natural sensory cortical plasticity for infant vocalizations to ask whether the engagement of the noradrenergic locus coeruleus (LC) by the playback of pup-calls is affected by either prior experience with the sounds or estrogen availability, using a well-studied cellular activity and plasticity marker, the immediate early gene c-Fos. We counted call-induced c-Fos immunoreactive (cFos-IR) cells in both LC and physiologically validated fields within the auditory cortex (AC) of estradiol or blank-implanted virgin female mice with either 0 or 5-days prior experience caring for vocalizing pups. Estradiol and pup experience interacted both in the induction of c-Fos-IR in the LC, as well as in behavioral measures of locomotion during playback, consistent with the neuromodulatory center’s activity being an online reflection of both hormonal and experience-dependent influences on arousal. Throughout core AC, as well as in a high frequency sub-region of AC and in secondary AC, a main effect of pup experience was to reduce call-induced c-Fos-IR, irrespective of estradiol availability. This is consistent with the hypothesis that sound familiarity leads to less c-Fos-mediated plasticity, and less disrupted sensory representations of a meaningful call category. Taken together, our data support the view that any coupling between these sensory and neuromodulatory areas is situationally dependent, and their engagement depends differentially on both internal state factors like hormones and external state factors like prior experience.

Region- and Layer-Specific Activation of the Higher Order Auditory Cortex Te2 after Remote Retrieval of Fear or Appetitive Memories

The auditory cortex is involved in encoding sounds which have acquired an emotional-motivational charge. However, the neural circuitry engaged by emotional memory processes in the auditory cortex is poorly understood. In this study, we investigated the layers and regions that are recruited in the higher order auditory cortex Te2 by a tone previously paired to either fear or appetitive stimuli in rats. By tracking the protein coded by the immediate early gene zif268, we found that fear memory retrieval engages layers II-III in most regions of Te2. These results were neither due to an enhanced fear state nor to fear-evoked motor responses, as they were absent in animals retrieving an olfactory fear memory. These layers were also activated by appetitive auditory memory retrieval. Strikingly, layer IV was recruited by fear, but not appetitive memories, whereas layer V activity was related to the behavioral responses displayed to the CS. In addition to revealing the layers and regions that are recruited in the Te2 by either fear or appetitive remote memories, our study also shows that the neural circuitry within the Te2 that processes and stores emotional memories varies on the basis of the affective motivational charge of tones.

A Non-canonical Reticular-Limbic Central Auditory Pathway via Medial Septum Contributes to Fear Conditioning

In the mammalian brain, auditory information is known to be processed along a central ascending pathway leading to auditory cortex (AC). Whether there exist any major pathways beyond this canonical auditory neuraxis remains unclear. In awake mice, we found that auditory responses in entorhinal cortex (EC) cannot be explained by a previously proposed relay from AC based on response properties. By combining anatomical tracing and optogenetic/pharmacological manipulations, we discovered that EC received auditory input primarily from the medial septum (MS), rather than AC. A previously uncharacterized auditory pathway was then revealed: it branched from the cochlear nucleus, and via caudal pontine reticular nucleus, pontine central gray, and MS, reached EC. Neurons along this non-canonical auditory pathway responded selectively to high-intensity broadband noise, but not pure tones. Disruption of the pathway resulted in an impairment of specifically noise-cued fear conditioning. This reticular-limbic pathway may thus function in processing aversive acoustic signals.

Balanced Noise-Evoked Excitation and Inhibition in Awake Mice CA3

The hippocampus is known as a neuronal structure involved in learning, memory and spatial navigation using multi-sensory cues. However, the basic features of its response to acoustic stimuli without any behavioral tasks (conditioning) remains poorly studied. Here, we investigated the CA3 response to auditory stimuli using in vivo loose-patch recordings in awake and anesthetized C57 mice. Different acoustic stimuli in addition to broadband noise such as click, FM sound and pure tone were applied to test the response of CA3 in awake animals. It was found that the wakefulness of the animal is important for the recorded neurons to respond. The CA3 neurons showed a stronger response to broadband noise rather than the other type of stimuli which suggested that auditory information arrived at CA3 via broadband pathways. Finally, we investigated the excitatory and inhibitory inputs to CA3 neurons by using in vivo whole-cell voltage-clamp techniques with the membrane potential holding at −70 and 0 mV, respectively. In awake animals, the excitatory and inhibitory inputs CA3 neurons receive induced by noise are balanced by showing stable intervals and proportional changes of their latencies and peak amplitudes as a function of the stimulation intensities.

Knowledge About Sounds-Context-Specific Meaning Differently Activates Cortical Hemispheres, Auditory Cortical Fields, and Layers in House Mice

Activation of the auditory cortex (AC) by a given sound pattern is plastic, depending, in largely unknown ways, on the physiological state and the behavioral context of the receiving animal and on the receiver's experience with the sounds. Such plasticity can be inferred when house mouse mothers respond maternally to pup ultrasounds right after parturition and naïve females have to learn to respond. Here we use c-FOS immunocytochemistry to quantify highly activated neurons in the AC fields and layers of seven groups of mothers and naïve females who have different knowledge about and are differently motivated to respond to acoustic models of pup ultrasounds of different behavioral significance. Profiles of FOS-positive cells in the AC primary fields (AI, AAF), the ultrasonic field (UF), the secondary field (AII), and the dorsoposterior field (DP) suggest that activation reflects in AI, AAF, and UF the integration of sound properties with animal state-dependent factors, in the higher-order field AII the news value of a given sound in the behavioral context, and in the higher-order field DP the level of maternal motivation and, by left-hemisphere activation advantage, the recognition of the meaning of sounds in the given context. Anesthesia reduced activation in all fields, especially in cortical layers 2/3. Thus, plasticity in the AC is field-specific preparing different output of AC fields in the process of perception, recognition and responding to communication sounds. Further, the activation profiles of the auditory cortical fields suggest the differentiation between brains hormonally primed to know (mothers) and brains which acquired knowledge via implicit learning (naïve females). In this way, auditory cortical activation discriminates between instinctive (mothers) and learned (naïve females) cognition.

Topographic Distribution of Stimulus-Specific Adaptation across Auditory Cortical Fields in the Anesthetized Rat

Stimulus-specific adaptation (SSA) in single neurons of the auditory cortex was suggested to be a potential neural correlate of the mismatch negativity (MMN), a widely studied component of the auditory event-related potentials (ERP) that is elicited by changes in the auditory environment. However, several aspects on this SSA/MMN relation remain unresolved. SSA occurs in the primary auditory cortex (A1), but detailed studies on SSA beyond A1 are lacking. To study the topographic organization of SSA, we mapped the whole rat auditory cortex with multiunit activity recordings, using an oddball paradigm. We demonstrate that SSA occurs outside A1 and differs between primary and nonprimary cortical fields. In particular, SSA is much stronger and develops faster in the nonprimary than in the primary fields, paralleling the organization of subcortical SSA. Importantly, strong SSA is present in the nonprimary auditory cortex within the latency range of the MMN in the rat and correlates with an MMN-like difference wave in the simultaneously recorded local field potentials (LFP). We present new and strong evidence linking SSA at the cellular level to the MMN, a central tool in cognitive and clinical neuroscience.

This study of higher-order auditory cortex strengthens the case for long-latency stimulus-specific adaptation as a genuine neural correlate of the mismatch negativity, which flags salient stimuli.

Sensory systems automatically detect salient events in a monotonous ambient background. In humans, this change detection process is indexed by the mismatch negativity (MMN), a mid-late component of the auditory-evoked potentials that has become a central tool in cognitive and clinical neuroscience over the last 40 years. However, the neuronal correlate of MMN remains controversial. Stimulus-specific adaptation (SSA) is a special type of adaptation recorded at the neuronal level in the auditory pathway. Attenuating the response only to repetitive, background stimuli is a very efficient mechanism to enhance the saliency of any upcoming deviant or novel stimulus. Thus, SSA was originally proposed as a neural correlate of the MMN, but previous studies in the auditory cortex reported SSA only at very early latencies (circa 20–30 ms) and only within the primary auditory cortex (A1), whereas MMN analogs in the rat occur later, between 50 and 100 ms after change onset, and are generated mainly within nonprimary fields. Here, we report very strong SSA in nonprimary fields within the latency range of the MMN in the rat, providing empirical evidence of the missing link between single neuron response studies in animal models and the human MMN.

The higher order auditory cortex is involved in the assignment of affective value to sensory stimuli

The sensory cortex participates in emotional memory but its role is poorly understood. Here we show that inactivation of the higher order auditory cortex Te2 in rats during early memory consolidation impairs remote first- and second-order fear memories but not the association between two neutral cues. Furthermore, Te2 inactivation prevents changes in the valence of such information. Following the presentation of two auditory cues previously paired with either pleasant or painful stimuli, a large percentage of cells responds to both experiences but also a small fraction of neurons responds exclusively to one of them. The latter type of neurons signals the valence rather than the salience or the motor responses associated with the stimuli, and reflects selective associative processes. Pharmacogenetic silencing of memory-activated neurons causes amnesia. Thus, Te2 represents a crucial node for the assignment of the affective value to sensory stimuli and for the storage of such information.

The auditory cortex Te2 represents a key node for the assignment of the affective value to sensory stimuli in rodents. Using pharmacogenetic manipulations, this study shows that in Te2 there are neurons which respond to the emotional valence of sounds and their inactivation impairs emotional memories retrieval.

Decreased approach behavior and nucleus accumbens immediate early gene expression in response to Parkinsonian ultrasonic vocalizations in rats

Many individuals with Parkinson disease (PD) have difficulty producing normal speech and voice, resulting in problems with interpersonal communication and reduced quality of life. Translational animal models of communicative dysfunction have been developed to assess disease pathology. However, it is unknown whether acoustic feature changes associated with vocal production deficits in these animal models lead to compromised communication. In rodents, male ultrasonic vocalizations (USVs) have a well-established role in functional inter-sexual communication. To test whether acoustic deficits in USVs observed in a PTEN-induced putative kinase 1 (PINK1) knockout (KO) PD rat model compromise communication, we presented recordings of male PINK1 KO USVs and normal wild-type (WT) USVs to female rat listeners. We measured approached behavior and immediate early gene expression (c-Fos) in brain regions implicated in auditory processing and sexual motivation. Our results suggest that females show reduced approach in response to PINK1 KO USVs compared with WT. Moreover, females exposed to PINK1 KO USVs had lower c-Fos immunolabeling in the nucleus accumbens, a region implicated in sexual motivation. These results are the first to demonstrate that vocalization deficits in a rat PD model result in compromised communication. Thus, the PINK1 KO PD model may be valuable for assessing treatments aimed at restoring vocal communicative function.

Auditory cortex involvement in emotional learning and memory

Emotional memories represent the core of human and animal life and drive future choices and behaviors. Early research involving brain lesion studies in animals lead to the idea that the auditory cortex participates in emotional learning by processing the sensory features of auditory stimuli paired with emotional consequences and by transmitting this information to the amygdala. Nevertheless, electrophysiological and imaging studies revealed that, following emotional experiences, the auditory cortex undergoes learning-induced changes that are highly specific, associative and long lasting. These studies suggested that the role played by the auditory cortex goes beyond stimulus elaboration and transmission. Here, we discuss three major perspectives created by these data. In particular, we analyze the possible roles of the auditory cortex in emotional learning, we examine the recruitment of the auditory cortex during early and late memory trace encoding, and finally we consider the functional interplay between the auditory cortex and subcortical nuclei, such as the amygdala, that process affective information. We conclude that, starting from the early phase of memory encoding, the auditory cortex has a more prominent role in emotional learning, through its connections with subcortical nuclei, than is typically acknowledged.

Echoes of the spoken past: how auditory cortex hears context during speech perception

What do we hear when someone speaks and what does auditory cortex (AC) do with that sound? Given how meaningful speech is, it might be hypothesized that AC is most active when other people talk so that their productions get decoded. Here, neuroimaging meta-analyses show the opposite: AC is least active and sometimes deactivated when participants listened to meaningful speech compared to less meaningful sounds. Results are explained by an active hypothesis-and-test mechanism where speech production (SP) regions are neurally re-used to predict auditory objects associated with available context. By this model, more AC activity for less meaningful sounds occurs because predictions are less successful from context, requiring further hypotheses be tested. This also explains the large overlap of AC co-activity for less meaningful sounds with meta-analyses of SP. An experiment showed a similar pattern of results for non-verbal context. Specifically, words produced less activity in AC and SP regions when preceded by co-speech gestures that visually described those words compared to those words without gestures. Results collectively suggest that what we ‘hear’ during real-world speech perception may come more from the brain than our ears and that the function of AC is to confirm or deny internal predictions about the identity of sounds.

In search of a recognition memory engram

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The role of the perirhinal cortex in familiarity discrimination is reviewed.

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Behavioural, pharmacological and electrophysiological evidence is considered.

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The cortex is found to be essential for memory acquisition, retrieval and storage.

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The evidence indicates that perirhinal synaptic weakening is critically involved.

A large body of data from human and animal studies using psychological, recording, imaging, and lesion techniques indicates that recognition memory involves at least two separable processes: familiarity discrimination and recollection. Familiarity discrimination for individual visual stimuli seems to be effected by a system centred on the perirhinal cortex of the temporal lobe. The fundamental change that encodes prior occurrence within the perirhinal cortex is a reduction in the responses of neurones when a stimulus is repeated. Neuronal network modelling indicates that a system based on such a change in responsiveness is potentially highly efficient in information theoretic terms. A review is given of findings indicating that perirhinal cortex acts as a storage site for recognition memory of objects and that such storage depends upon processes producing synaptic weakening.

Contrasting networks for recognition memory and recency memory revealed by immediate-early gene imaging in the rat

The expression of the immediate-early gene c-fos was used to compare networks of activity associated with recency memory (temporal order memory) and recognition memory. In Experiment 1, rats were first familiarized with sets of objects and then given pairs of different, familiar objects to explore. For the recency test group, each object in a pair was separated by 110 min in the time between their previous presentations. For the recency control test, each object in a pair was separated by less than a 1 min between their prior presentations. Temporal discrimination of the objects correlated with c-fos activity in the recency test group in several sites, including area Te2, the perirhinal cortex, lateral entorhinal cortex, as well as the dentate gyrus, hippocampal fields CA3 and CA1. For both the test and control conditions, network models were derived using structural equation modeling. The recency test model emphasized serial connections from the perirhinal cortex to lateral entorhinal cortex and then to the CA1 subfield. The recency control condition involved more parallel pathways, but again highlighted CA1 within the hippocampus. Both models contrasted with those derived from tests of object recognition (Experiment 2), because stimulus novelty was associated with pathways from the perirhinal cortex to lateral entorhinal cortex that then involved both the dentate gyrus (and CA3) and CA1 in parallel. The present findings implicate CA1 for the processing of familiar stimuli, including recency discriminations, while the dentate gyrus and CA3 pathways are recruited when the perirhinal cortex signals novel stimuli.

Differential Cortical c-Fos and Zif-268 Expression after Object and Spatial Memory Processing in a Standard or Episodic-Like Object Recognition Task

Episodic memory reflects the capacity to recollect what, where, and when a specific event happened in an integrative manner. Animal studies have suggested that the medial temporal lobe and the medial pre-frontal cortex are important for episodic-like memory (ELM) formation. The goal of present study was to evaluate whether there are different patterns of expression of the immediate early genes c-Fos and Zif-268 in these cortical areas after rats are exposed to object recognition (OR) tasks with different cognitive demands. Male rats were randomly assigned to five groups: home cage control, empty open field (CTR-OF), open field with one object (CTR-OF + Obj), novel OR task, and ELM task and were killed 1 h after the last behavioral procedure. Rats were able to discriminate the objects in the OR task. In the ELM task, rats showed spatial (but not temporal) discrimination of the objects. We found an increase in the c-Fos expression in the dorsal dentate gyrus (DG) and in the perirhinal cortex (PRh) in the OR and ELM groups. The OR group also presented an increase of c-Fos expression in the medial prefrontal cortex (mPFC). Additionally, the OR and ELM groups had increased expression of Zif-268 in the mPFC. Moreover, Zif-268 was increased in the dorsal CA1 and PRh only in the ELM group. In conclusion, the pattern of activation was different in tasks with different cognitive demands. Accordingly, correlation tests suggest the engagement of different neural networks in the tasks used. Specifically, perirhinal-DG co-activation was detected after the what-where memory retrieval, but not after the novel OR task. Both regions correlated with the respective behavioral outcome. These findings can be helpful in the understanding of the neural networks underlying memory tasks with different cognitive demands.

What pharmacological interventions indicate concerning the role of the perirhinal cortex in recognition memory

Findings of pharmacological studies that have investigated the involvement of specific regions of the brain in recognition memory are reviewed. The particular emphasis of the review concerns what such studies indicate concerning the role of the perirhinal cortex in recognition memory. Most of the studies involve rats and most have investigated recognition memory for objects. Pharmacological studies provide a large body of evidence supporting the essential role of the perirhinal cortex in the acquisition, consolidation and retrieval of object recognition memory. Such studies provide increasingly detailed evidence concerning both the neurotransmitter systems and the underlying intracellular mechanisms involved in recognition memory processes. They have provided evidence in support of synaptic weakening as a major synaptic plastic process within perirhinal cortex underlying object recognition memory. They have also supplied confirmatory evidence that that there is more than one synaptic plastic process involved. The demonstrated necessity to long-term recognition memory of intracellular signalling mechanisms related to synaptic modification within perirhinal cortex establishes a central role for the region in the information storage underlying such memory. Perirhinal cortex is thereby established as an information storage site rather than solely a processing station. Pharmacological studies have also supplied new evidence concerning the detailed roles of other regions, including the hippocampus and the medial prefrontal cortex in different types of recognition memory tasks that include a spatial or temporal component. In so doing, they have also further defined the contribution of perirhinal cortex to such tasks. To date it appears that the contribution of perirhinal cortex to associative and temporal order memory reflects that in simple object recognition memory, namely that perirhinal cortex provides information concerning objects and their prior occurrence (novelty/familiarity).

Contrasting brain activity patterns for item recognition memory and associative recognition memory: insights from immediate-early gene functional imaging

Recognition memory, the discrimination of a novel from a familiar event, can be classified into item recognition and associative recognition. Item recognition concerns the identification of novel individual stimuli, while associative recognition concerns the detection of novelty that arises when familiar items are reconfigured in a novel manner. Experiments in rodents that have mapped the expression of immediate-early genes, e.g., c-fos, highlight key differences between these two forms of recognition memory. Visual item novelty is consistently linked to increased c-fos activity in just two brain sites, the perirhinal cortex and the adjacent visual association area Te2. Typically there are no hippocampal c-fos changes. In contrast, visual associative recognition is consistently linked to c-fos activity changes in the hippocampus, but not the perirhinal cortex. The lack of a c-fos perirhinal change with associative recognition presumably reflects the fact that the individual items in an array remain familiar, even though their combinations are unique. Those exceptions, when item recognition is associated with hippocampal c-fos changes, occur when rats actively explore novel objects. The increased engagement with objects will involve multisensory stimulus processing and potentially create conditions in which rats can readily learn stimulus attributes such as object location or object order, i.e., attributes involved in associative recognition. Correlations based on levels of immediate-early gene expression in the temporal lobe indicate that actively exploring novel stimuli switches patterns of entorhinal-hippocampal functional connectivity to emphasise direct entorhinal-dentate gyrus processing. These gene activity findings help to distinguish models of medial temporal lobe function.

Perirhinal and parahippocampal cortices differentially contribute to later recollection of object- and scene-related event details

How the different elements of our experiences are encoded into episodic memories has remained one of the major questions in memory research. Although the pivotal role of the medial temporal lobe as a whole for memory formation is well established, much controversy surrounds the precise contributions of the subregions in the medial temporal lobe cortex (MTLC), most notably the perirhinal cortex (PrC) and the parahippocampal cortex (PhC). Although one prominent view links PrC processes with familiarity-based memory and PhC with recollection, an alternative organizing principle is the representational domain critical for successful memory performance (e.g., object- versus scene-related information). In this functional magnetic resonance imaging study, we directly compared successful source encoding during object versus scene imagery, holding perceptual input constant across the two representational domains. Although the hippocampus contributed to associative encoding of both object and scene information, our results revealed a clear double dissociation between PrC and PhC for object- versus scene-related source encoding. In particular, PrC, but not PhC, encoding activation predicted later source memory for the object imagery task, whereas PhC, but not PrC, encoding activation predicted later source memory for the scene imagery task. Interestingly, the transitional zone between PrC and posterior PhC contributed to both object and scene source encoding, possibly reflecting a gradient in domain preference along MTLC. In sum, these results strongly point to representational domain as a key factor determining the involvement of different MTLC subregions during successful episodic memory formation.

Separate but interacting recognition memory systems for different senses: the role of the rat perirhinal cortex

Two different models (convergent and parallel) potentially describe how recognition memory, the ability to detect the re-occurrence of a stimulus, is organized across different senses. To contrast these two models, rats with or without perirhinal cortex lesions were compared across various conditions that controlled available information from specific sensory modalities. Intact rats not only showed visual, tactile, and olfactory recognition, but also overcame changes in the types of sensory information available between object sampling and subsequent object recognition, e.g., between sampling in the light and recognition in the dark, or vice versa. Perirhinal lesions severely impaired object recognition whenever visual cues were available, but spared olfactory recognition and tactile-based object recognition when tested in the dark. The perirhinal lesions also blocked the ability to recognize an object sampled in the light and then tested for recognition in the dark, or vice versa. The findings reveal parallel recognition systems for different senses reliant on distinct brain areas, e.g., perirhinal cortex for vision, but also show that: (1) recognition memory for multisensory stimuli involves competition between sensory systems and (2) perirhinal cortex lesions produce a bias to rely on vision, despite the presence of intact recognition memory systems serving other senses.

Arc/Arg3.1 mRNA expression reveals a subcellular trace of prior sound exposure in adult primary auditory cortex

Acquiring the behavioral significance of sound has repeatedly been shown to correlate with long term changes in response properties of neurons in the adult primary auditory cortex. However, the molecular and cellular basis for such changes is still poorly understood. To address this, we have begun examining the auditory cortical expression of an activity-dependent effector immediate early gene (IEG) with documented roles in synaptic plasticity and memory consolidation in the hippocampus: Arc/Arg3.1. For initial characterization, we applied a repeated 10 min (24 h separation) sound exposure paradigm to determine the strength and consistency of sound-evoked Arc/Arg3.1 mRNA expression in the absence of explicit behavioral contingencies for the sound. We used 3D surface reconstruction methods in conjunction with fluorescent in situ hybridization (FISH) to assess the layer-specific subcellular compartmental expression of Arc/Arg3.1 mRNA. We unexpectedly found that both the intranuclear and cytoplasmic patterns of expression depended on the prior history of sound stimulation. Specifically, the percentage of neurons with expression only in the cytoplasm increased for repeated versus singular sound exposure, while intranuclear expression decreased. In contrast, the total cellular expression did not differ, consistent with prior IEG studies of primary auditory cortex. Our results were specific for cortical layers 3-6, as there was virtually no sound driven Arc/Arg3.1 mRNA in layers 1-2 immediately after stimulation. Our results are consistent with the kinetics and/or detectability of cortical subcellular Arc/Arg3.1 mRNA expression being altered by the initial exposure to the sound, suggesting exposure-induced modifications in the cytoplasmic Arc/Arg3.1 mRNA pool.

Recognition memory: material, processes, and substrates

The proposal that a system centering on the perirhinal cortex is responsible for familiarity discrimination, particularly for single items, whereas a system centering on the hippocampus is responsible for recollective and more complex associational aspects of recognition memory is reviewed in the light of recent findings. In particular, the proposal is reviewed in relation to recent animal work with rats and results from human clinical studies. Notably, progress has been made in determining potential neural memory substrate mechanisms within the perirhinal cortex in rats. Recent findings have emphasized the importance of specifying the type of material, the type of test, and the strategy used by subjects to solve recognition memory tests if substrates are to be accurately inferred. It is to be expected that the default condition is that both the hippocampal and perirhinal systems will contribute to recognition memory performance. Indeed, rat lesion experiments provide examples of where cooperation between both systems is essential. Nevertheless, there remain examples of the independent operation of the hippocampal and perirhinal systems. Overall, it is concluded that most, though not all, of the recent findings are in support of the proposal. However, there is also evidence that the systems involved in recognition memory need to include structures outside the medial temporal lobe: there are significant but as yet only partially defined roles for the prefrontal cortex and sensory association cortices in recognition memory processes.

Role of secondary sensory cortices in emotional memory storage and retrieval in rats

Visual, acoustic, and olfactory stimuli associated with a highly charged emotional situation take on the affective qualities of that situation. Where the emotional meaning of a given sensory experience is stored is a matter of debate. We found that excitotoxic lesions of auditory, visual, or olfactory secondary sensory cortices impaired remote, but not recent, fear memories in rats. Amnesia was modality-specific and not due to an interference with sensory or emotional processes. In these sites, memory persistence was dependent on ongoing protein kinase Mzeta activity and was associated with an increased activity of layers II-IV, thus suggesting a synaptic strengthening of corticocortical connections. Lesions of the same areas left intact the memory of sensory stimuli not associated with any emotional charge. We propose that secondary sensory cortices support memory storage and retrieval of sensory stimuli that have acquired a behavioral salience with the experience.

Medial prefrontal cortex activity can disrupt the expression of stress response habituation

Recent findings suggest that the expression of hypothalamic-pituitary-adrenal (HPA) axis stress response adaptation in rats depends on top-down neural control. We therefore examined whether the medial prefrontal cortex (mPFC) modulates expression of stress response habituation. We transiently suppressed (muscimol microinfusion) or stimulated (picrotoxin microinfusion) mPFC neural activity in rats and studied the consequence on the first time response to psychological stress (restraint) or separately on the development and expression of habituation to repeated restraint. We monitored both the hormonal (corticosterone) and neural (forebrain c-fos mRNA) response to stress. Inactivation of the mPFC had no effect on the HPA-axis response to first time restraint, however increased mPFC activity attenuated stress-induced HPA-axis activity. In a three day repeated restraint stress regimen, inactivation of the mPFC on days 1 and 2, but not day 3, prevented the expression of HPA-axis hormone response habituation. In these same rats, the mPFC activity on day 3 interfered with the expression of c-fos mRNA habituation selectively within the mPFC, lateral septum and hypothalamic paraventricular nucleus. In contrast, inactivation of the mPFC only on day 3, or on all 3 days did not interfere with the expression of habituation. We conclude that the mPFC can permit or disrupt expression of HPA-axis stress response habituation, and this control depends on alteration of neural activity within select brain regions. A possible implication of these findings is that the dysregulation of PFC activity associated with depression and post-traumatic stress disorder may contribute to impaired expression of stress-response adaptation and consequently exacerbation of those disorders.

Category-specificity in the human medial temporal lobe cortex

The medial temporal lobe cortex (MTLC) occupies a pivotal position at the interface between neocortical association areas and the hippocampus. It has been suggested that the MTLC contains functionally distinct regions, with perirhinal cortex (PRc) preferentially supporting object processing and posterior parahippocampal cortex (PHc) preferentially supporting encoding of spatial information. Measuring differential BOLD responsiveness to objects, scenes, and other stimulus categories, we find a double dissociation between an anterior PRc response to objects and a posterior PHc response to scene stimuli. Furthermore, an anatomical ROI based approach was undertaken in an effort to understand the response profile underlying this double dissociation. We did not see any evidence for a sharp border between putatively distinct scene-preferential and object-preferential MTLC regions. Instead, scene-preferential responsiveness was noted to drop off in a graded, linear fashion in successively anterior MTLC regions until object-preferential responsiveness emerged in anterior PRc, although objects produced above baseline responses across the anterior-posterior extent of the parahippocampal gyrus. Other stimulus categories, such as faces and words, led to above baseline activation in either a few confined regions (faces) or none at all (words). Thus, what differentiated regions along the parahippocampal gryus was the relative response to objects and scenes, not simply above baseline responses to either category. This pattern raises the possibility that posterior PHc, and anterior PRc are situated at the ends of a single organizational continuum supported by the entire length of MTLC.

Morphological correlates of emotional and cognitive behaviour: insights from studies on inbred and outbred rodent strains and their crosses

Every study in rodents is also a behavioural genetic study even if only a single strain is used. Outbred strains are genetically heterogeneous populations with a high intrastrain variation, whereas inbred strains are based on the multiplication of a unique individual. The aim of the present review is to summarize findings on brain regions involved in three major components of rodent behaviour, locomotion, anxiety-related behaviour and cognition, by paying particular attention to the genetic context, genetic models used and interstrain comparisons. Recent trends correlating gene expression in inbred strains with behavioural data in databases, morpho-behavioural-haplotype analyses and problems arising from large-scale multivariate analyses are discussed. Morpho-behavioural correlations in multiple strains are presented, including correlations with projection neurons, interneurons and fibre systems in the striatum, midbrain, amygdala, medial septum and hippocampus, by relating them to relevant transmitter systems. In addition, brain areas differentially activated in different strains are described (hippocampus, prefrontal cortex, nucleus accumbens, locus ceruleus). Direct interstrain comparisons indicate that strain differences in behavioural variables and neuronal markers are much more common than usually thought. The choice of the appropriate genetic model can therefore contribute to an interpretation of positive results in a wider context, and help to avoid misleading interpretations of negative results.

Tool-use training in a species of rodent: the emergence of an optimal motor strategy and functional understanding

Background

Tool use is defined as the manipulation of an inanimate object to change the position or form of a separate object. The expansion of cognitive niches and tool-use capabilities probably stimulated each other in hominid evolution. To understand the causes of cognitive expansion in humans, we need to know the behavioral and neural basis of tool use. Although a wide range of animals exhibit tool use in nature, most studies have focused on primates and birds on behavioral or psychological levels and did not directly address questions of which neural modifications contributed to the emergence of tool use. To investigate such questions, an animal model suitable for cellular and molecular manipulations is needed.

Methodology/Principal Findings

We demonstrated for the first time that rodents can be trained to use tools. Through a step-by-step training procedure, we trained degus (Octodon degus) to use a rake-like tool with their forelimbs to retrieve otherwise out-of-reach rewards. Eventually, they mastered effective use of the tool, moving it in an elegant trajectory. After the degus were well trained, probe tests that examined whether they showed functional understanding of the tool were performed. Degus did not hesitate to use tools of different size, colors, and shapes, but were reluctant to use the tool with a raised nonfunctional blade. Thus, degus understood the functional and physical properties of the tool after extensive training.

Conclusions/Significance

Our findings suggest that tool use is not a specific faculty resulting from higher intelligence, but is a specific combination of more general cognitive faculties. Studying the brains and behaviors of trained rodents can provide insights into how higher cognitive functions might be broken down into more general faculties, and also what cellular and molecular mechanisms are involved in the emergence of such cognitive functions.

Changes in neural activity associated with a surprising change in the predictive validity of a conditioned stimulus

Changes in how well a conditioned stimulus (CS) predicts future events can alter the amount of attention paid to that cue. For example, the unexpected violation of a previously established relationship between a CS and another stimulus can increase attentional processing and subsequent conditioning to that cue [J.M. Pearce & G. Hall (1980)Psych. Rev., 106, 532-552]. Previous lesion studies have implicated the central nucleus of the amygdala (CN) and basal forebrain corticopetal cholinergic system in mediating surprise-induced changes in attention. Here, expression of the immediate-early gene c-fos was used to determine which cortical targets of the basal forebrain cholinergic system are activated during an increase in attentional processing. Consistent with previous studies, increased Fos expression was observed in the posterior parietal cortex (PPC) when a visual stimulus no longer reliably predicted occurrence of a tone. Similar results were observed in the secondary auditory cortex; however, there were no significant changes in Fos expression in other auditory or visual cortices or in other cortical association areas that have been implicated in attentional function (frontal, cingulate or retrosplenial cortex). These findings support the notion that the PPC is the primary cortical component of a neural system mediating incremental changes in attention. In addition, an increase in Fos-positive cells was detected in the substantia innominata/nucleus basalis and the CN at the time of surprise. An opposite pattern of results was observed in the basal lateral nucleus of the amygdala, providing evidence for different stimulus-processing mechanisms in regions of the amygdala.

Organization and origin of the connection from the inferior to the superior colliculi in the rat

The inferior colliculus (IC) is the main ascending auditory relay station prior to the superior colliculus (SC). The morphology and origin of the connection from inferior to superior colliculus (I-SC) was analyzed both by anterograde and retrograde tracing. Irrespective of the subregion of the IC in which they originate, the terminal fields of these connections formed two main tiers in the SC. While the dorsal one primarily involved the stratum opticum and the stratum griseum intermediale, the ventral one innervated the deep strata, although some fibers did connect these tiers. While the dorsal tier occupied almost the whole extension of the SC, the ventral one was mostly confined to its caudomedial quadrant. The fiber density in these tiers decreased gradually in a rostral gradient and the terminal fields became denser as the anterograde tracer at the injection site was distributed more externally in the cortex of the IC. Retrograde tracing confirmed this result, although it did not reveal any topographic ordering for the I-SC pathway. Most presynaptic boutons of the I-SC terminal field were located either inside or close to the patches of acetylcholinesterase activity. Together with previous anatomical and physiological studies, our results indicate that the I-SC connection relays behaviorally relevant information for sensory-motor processing. Our observation that this pathway terminates in regions of the superior colliculus, where neurons involved in fear-like responses are located, reinforce previous suggestions of a role for the IC in generating motor stereotypes that occur during audiogenic seizures.

Efferent connections of the ventral auditory area in the rat cortex: implications for auditory processing related to emotion

In the rat auditory cortex, ventral (VA) and posterodorsal (PD) areas are the two major auditory fields that receive thalamic afferents from the dorsal division of the medial geniculate body (MGD). VA and PD are presumed to serve distinct functions in tandem as the pair of major cortical recipients of extralemniscal thalamic inputs. To deduce the functional significance of VA, efferent connections of VA were examined with the anterograde tracer biocytin. VA lies primarily in the ventral margin of area Te1 and represents frequencies primarily < 15 kHz [Donishi, T., Kimura, A., Okamoto, K. & Tamai, Y. (2006) Neuroscience, 141, 1553-1567.] Biocytin was iontophoretically injected into cortical regions which were defined as VA based on histological location, auditory response and thalamocortical connectivity. Anterograde labelling revealed two important aspects of cortical projections. First, VA sent a projection to a well-confined region in the caudal end of the insular cortex (Ins) pivotal for fear memory formation during aversive conditioning. Second, VA sent parallel projections to cortical regions that probably comprise the other nonprimary auditory fields, including PD. The results suggest that VA relays auditory input from the MGD to the Ins for affective memory formation and at the same time dispatches the auditory signal, which may represent emotional content, to the remaining nonprimary auditory fields. PD is assumed to play a pivotal role in auditory spatial processing for directed attention (Kimura et al., 2004). As the counterpart of PD, VA is assumed to give rise to another major stream of cortical information processing, most probably related to emotion.

Birdsong memory: a neural dissociation between song recognition and production

Songbirds learn their song from an adult conspecific tutor when they are young, much like the acquisition of speech in human infants. When an adult zebra finch is re-exposed to its tutor's song, there is increased neuronal activation in the caudomedial nidopallium (NCM), the songbird equivalent of the auditory association cortex. This neuronal activation is related to the fidelity of song imitation, suggesting that the NCM may contain the neural representation of song memory. We found that bilateral neurotoxic lesions to the NCM of adult male zebra finches impaired tutor-song recognition but did not affect the males' song production or their ability to discriminate calls. These findings demonstrate that the NCM performs an essential role in the representation of tutor-song memory. In addition, our results show that tutor-song memory and a motor program for the bird's own song have separate neural representations in the songbird brain. Thus, in both humans and songbirds, the cognitive systems of vocal production and auditory recognition memory are subserved by distinct brain regions.

Differential activation of anterior and midline thalamic nuclei following retrieval of aversively motivated learning tasks

Two thalamic nuclear groups, the anterior thalamic nuclei (ATN) and midline and intralaminar thalamic complex (MITC) have connections to the prefrontal cortex, amygdala, hippocampus and accumbens that are important for learning and memory. However, the anatomical proximity between the ATN and MITC makes it difficult to reveal their roles in memory retrieval of aversive conditioned behavior. To address the issue, we explored the activation of the ATN and MITC, as represented by the expression of the immediate early gene c-fos, following either the retrieval of a conditioned taste aversion (CTA) induced by taste-LiCl pairing (visceral aversion) or of inhibitory avoidance (IA) induced by context-foot shock pairing (somatic aversion) in rats. The anterodorsal (AD) nucleus in the ATN was activated by foot shock and the recall of IA, but not by i.p. injection of LiCl or the recall of CTA. No significant elevation was observed in the other ATN following these treatments. Among nuclei of the MITC, the paraventricular thalamic nucleus (PVT) was activated by the delivery of shock or LiCl and by the recall of both CTA and IA, while the mediodorsal thalamus (MD) and central medial and intermediate thalamus (CM/IMD) were not. The innately aversive taste of quinine did not elevate c-fos expression in either the ATN or MITC. These results suggest that the PVT in the MITC is involved in the processing and retrieval of both taste-malaise and context-shock association tasks, while the AD in the ATN is involved in those of context-shock association only. The difference of the activity between the ATN and MITC demonstrates their functional and anatomical heterogeneity in neural substrates for aversive learning tasks.

Auditory responses in the visual cortex of neonatally enucleated rats

A number of studies on humans and animals have demonstrated better auditory abilities in blind with respect to sighted subjects and have tried to define the mechanisms through which this compensation occurs. The aim of the present study, therefore, was to examine the participation of primary visual cortex (V1) to auditory processing in early enucleated rats. Here we show, using gaussian noise bursts, that about a third of the cells in V1 responded to auditory stimulation in blind rats and most of these (78%) had ON-type responses and low spontaneous activity. Moreover, they were distributed throughout visual cortex without any apparent tonotopic organization. Optimal frequencies determined using pure tones were rather high but comparable to those found in auditory cortex of blind and sighted rats. On the other hand, sensory thresholds determined at these frequencies were higher and bandwidths were wider in V1 of the blind animals. Blind and sighted rats were also stimulated for 60 min with gaussian noise, their brains removed and processed for c-Fos immunohistochemistry. Results revealed that c-Fos positive cells were not only present in auditory cortex of both groups of rats but there was a 10-fold increase in labeled cells in V1 and a fivefold increase in secondary visual cortex (V2) of early enucleated rats in comparisons to sighted ones. Also, the pattern of distribution of these labeled cells across layers suggests that the recruitment of V1 could originate at least in part through inputs arising from the thalamus. The ensemble of results appears to indicate that cross-modal compensation leading to improved performance in the blind depends on cell recruitment in V1 but probably also plastic changes in lower- and higher-order visual structures and possibly in the auditory system.

Different effects of lesions to auditory core and belt cortex on auditory recognition in dogs

Auditory recognition memory, in contrast to memory in other modalities, is not affected by damage to the perihinal cortex, and its neural basis remains unknown. In an attempt to elucidate this problem, we investigated the role of canine auditory core and belt areas in auditory recognition. Either core or posterior belt areas were surgically removed. The core and belt regions were defined on the basis of response properties and thalamocortical connectivity established in previous studies. The animals were tested on auditory delayed matching to sample (DMS, a recognition memory task) using complex, trial-unique auditory stimuli. Both core and belt lesions impaired auditory recognition, however, the underlying deficit was different. Lesions to the core areas impaired auditory localization abilities. Lesions to the posterior belt areas did not affect this component of the recognition task, but affected auditory quality discrimination and/or recognition. The deficit following the posterior belt lesion did not increase with retention delay, suggesting that auditory belt areas do not constitute a substrate for auditory recognition memory. Their main function appears to be processing of complex sound patterns, including immediate recognition.

Functional local connections with differential activity-dependence and critical periods surrounding the primary auditory cortex in rat cerebral slices

Sensory information is processed in neural networks connecting the primary sensory cortices with surrounding higher areas. Here, we investigated the properties of local connections between the primary auditory cortex (area 41) and surrounding areas (areas 20, 36, 18a and 39) in rat cerebral slices. Neural activities elicited by repetitive electrical stimulation were visualized using the activity-dependent changes in endogenous fluorescence derived from mitochondrial flavoproteins, which mostly reflect activities produced by polysynaptic glutamatergic transmission. Polysynaptic feedforward propagation was dominant compared with the corresponding polysynaptic feedback propagation between the primary (area 41) and secondary (areas 20 and 36) auditory cortices, while such a tendency was less clear in other pathways. Long inter-areal (>1 mm) propagation with the same dominancy was observed after layer V stimulation between areas 41 and 20, and was not affected by cutting the underlying white matter. Activity-dependent changes in neural activities induced by low-frequency stimulation in the presence of 1 microM bicuculline were investigated using Ca2+ imaging. Significant potentiation of the polysynaptic Ca2+ activities was only observed in polysynaptic feedforward pathways from the primary to secondary auditory cortices. Experience-dependence of the connections between areas 41 and 20 was investigated using flavoprotein fluorescence imaging. The activities from areas 41 to 20 were reduced by cochlear lesions produced at P12 but not at P28, while the activities from areas 20 to 41 were reduced by the lesions at P28, suggesting the critical period for the polysynaptic feedforward connection was before P28, while for the polysynaptic feedback connection was after P28.

Neural mechanisms of birdsong memory

The process through which young male songbirds learn the characteristics of the songs of an adult male of their own species has strong similarities with speech acquisition in human infants. Both involve two phases: a period of auditory memorization followed by a period during which the individual develops its own vocalizations. The avian 'song system', a network of brain nuclei, is the probable neural substrate for the second phase of sensorimotor learning. By contrast, the neural representation of song memory acquired in the first phase is localized outside the song system, in different regions of the avian equivalent of the human auditory association cortex.

“Ventral” area in the rat auditory cortex: A major auditory field connected with the dorsal division of the medial geniculate body

The rat auditory cortex is made up of multiple auditory fields. A precise correlation between anatomical and physiological areal extents of auditory fields, however, is not yet fully established, mainly because non-primary auditory fields remain undetermined. In the present study, based on thalamocortical connection, electrical stimulation and auditory response, we delineated a non-primary auditory field in the cortical region ventral to the primary auditory area and anterior auditory field. We designated it as "ventral" area after its relative location. At first, based on anterograde labeling of thalamocortical projection with biocytin, ventral auditory area was delineated as a main cortical terminal field of thalamic afferents that arise from the dorsal division of the medial geniculate body. Cortical terminal field (ventral auditory area) extended into the ventral margin of temporal cortex area 1 (Te1) and the dorsal part of temporal cortex area 3, ventral (Te3V), from 3.2-4.6 mm posterior to bregma. Electrical stimulation of the dorsal division of the medial geniculate body; evoked epicortical field potentials confined to the comparable cortical region. On the basis of epicortical field potentials evoked by pure tones, best frequencies were further estimated at and around the cortical region where electrical stimulation of the dorsal division of the medial geniculate body evoked field potentials. Ventral auditory area was found to represent frequencies primarily below 15 kHz, which contrasts with our previous finding that the posterodorsal area, the other major recipient of the dorsal division of the medial geniculate body; projection, represents primarily high frequencies (>15 kHz). The posterodorsal area is thought to play a pivotal role in auditory spatial processing [Kimura A, Donishi T, Okamoto K, Tamai Y (2004) Efferent connections of "posterodorsal" auditory area in the rat cortex: implications for auditory spatial processing. Neuroscience 128:399-419]. The ventral auditory area, as the other main cortical region that would relay auditory input from the dorsal division of the medial geniculate body to higher cortical information processing, could serve an important extralemniscal function in tandem with the posterodorsal area. The results provide insight into structural and functional organization of the rat auditory cortex.

Memory-dependent c-Fos expression in the nucleus accumbens and extended amygdala following the expression of a conditioned taste aversive in the rat

Retrieving the memory of a conditioned taste aversion involves multiple forebrain areas. Although the amygdala clearly plays a role in the expression of a conditioned taste aversion, critical regions, downstream from the amygdala remain to be defined. To this end, Fos immunoreactivity was used in the rat to explore forebrain structures associated with retrieval that have an anatomical relationship with the amygdala. The results showed that expression of a conditioned taste aversion to 0.5 M sucrose elicited neuronal activation in the nucleus accumbens and in a complex of structures collectively referred to as the extended amygdala. The posterior hypothalamus and parasubthalamic nucleus, which receive inputs from the extended amygdala, were also activated upon re-exposure to the sucrose conditioned stimulus. Fos immunoreactivity did not increase in these regions in response to an innately aversive tastant, quinine hydrochloride (conditioned stimulus control), nor to LiCl-induced visceral stimulation in unconditioned animals (unconditioned stimulus control). In addition, these regions did not respond to the sucrose conditioned stimulus in sham-conditioned animals. These results suggest that conditioned and innately aversive tastes are differentially processed in the forebrain circuitry that includes the nucleus accumbens and extended amygdala.

Contrasting hippocampal and perirhinal cortex function using immediate early gene imaging

The perirhinal cortex and hippocampus have close anatomical links, and it might, therefore, be predicted that they have close, interlinked roles in memory. Lesion studies have, however, often failed to support this prediction, providing dissociations and double dissociations between the two regions on tests of object recognition and spatial memory. In a series of rat studies we have compared these two regions using the expression of the immediate early gene c-fos as a marker of neuronal activity. This gene imaging approach makes it possible to assess the relative involvement of different brain regions and avoids many of the limitations of the lesion approach. A very consistent pattern of results was found as the various hippocampal subfields but not the perirhinal cortex show increased c-fos activity following tests of spatial learning. In contrast, the perirhinal cortex but none of the hippocampal subfields show increased c-fos activity when presented with novel rather than familiar visual objects. When novel scenes are created by the spatial rearrangement of familiar objects it is the hippocampus and not the perirhinal cortex that shows c-fos changes. This double dissociation for gene expression accords with that found from lesion studies and highlights the different contributions of the perirhinal cortex and hippocampus to memory.

Hippocampal dopamine receptors modulate cFos expression in the rat nucleus accumbens evoked by chemical stimulation of the ventral hippocampus

Recently, we have shown that D1 and D2 receptors in the ventral hippocampus (VH) modulate both the locomotor activation and the increase in dopamine (DA) levels in the rat nucleus accumbens (NAc) induced by NMDA stimulation of the VH. In the present study we analyze the possible role of VH D1 and D2 receptors in the modulation of the cFos expression in NAc (core and shell subregions) and in dorsal striatum. This was assessed by immunohistochemical analysis of cFos expression in the rat brains after retro-dialysis application of NMDA (50mM, 10 min) into VH, in absence and in presence of either the D1/D5 receptor antagonist SCH 23390 (100 and 250 microM, 60 min) or the D2 receptor antagonist raclopride (100 and 250 microM, 60 min). NMDA induced a robust increase in the cFos expression in the NAc shell, both in the ipsilateral and contralateral side. No statistically significant increases were observed in the NAc core and in the dorsal striatum. Simultaneous application of SCH 23390 and NMDA into the VH attenuated the NMDA-evoked cFos expression in NAc shell. In contrast, raclopride had no significant effect. Our present results show that the NMDA receptor mediated effects in the VH require D1 receptors and suggest that DA in VH strongly modulates the excitatory outputs from this brain area.

Attraction to sexual pheromones and associated odorants in female mice involves activation of the reward system and basolateral amygdala

Adult female mice are innately attracted to non-volatile pheromones contained in male-soiled bedding. In contrast, male-derived volatiles become attractive if associated with non-volatile attractive pheromones, which act as unconditioned stimulus in a case of Pavlovian associative learning. In this work, we study the chemoinvestigatory behaviour of female mice towards volatile and non-volatile chemicals contained in male-soiled bedding, in combination with the analysis of c-fos expression induced by such a behaviour to clarify: (i) which chemosensory systems are involved in the detection of the primary attractive non-volatile pheromone and of the secondarily attractive volatiles; (ii) where in the brain male-derived non-volatile and volatile stimuli are associated to induce conditioned attraction for the latter; and (iii) whether investigation of these stimuli activates the cerebral reward system (mesocorticolimbic system including the prefrontal cortex and amygdala), which would support the view that sexual pheromones are reinforcing. The results indicate that non-volatile pheromones stimulate the vomeronasal system, whereas air-borne volatiles activate only the olfactory system. Thus, the acquired preference for male-derived volatiles reveals an olfactory-vomeronasal associative learning. Moreover, the reward system is differentially activated by the primary pheromones and secondarily attractive odorants. Exploring the primary attractive pheromone activates the basolateral amygdala and the shell of nucleus accumbens but neither the ventral tegmental area nor the orbitofrontal cortex. In contrast, exploring the secondarily attractive male-derived odorants involves activation of a circuit that includes the basolateral amygdala, prefrontal cortex and ventral tegmental area. Therefore, the basolateral amygdala stands out as the key centre for vomeronasal-olfactory associative learning.

Environmental enrichment enhances auditory takeover of the occipital cortex in anophthalmic mice

Enrichment of the environment is an effective means of enhancing neuronal development and plasticity but its effect on the cross-modal compensation resulting from sensory deprivation has never been investigated. The present study used c-Fos immunohistochemistry and dextran-biotin neuronal tracing to examine the reorganization of sensory modalities in the brain of anophthalmic mutant mice (ZRDCT/An) raised in either enriched or standard environments. Auditory stimulation was found to elicit strong neuronal activation in thalamic and cortical structures that are normally visual. An important finding was that the latter auditory-evoked cortical activity was considerably enhanced in blind mice raised in the enriched environment. The axonal tracing study demonstrated auditory inputs from the inferior colliculus to the visual thalamus. This animal model will be useful for understanding neuronal mechanisms underlying some cross-modal sensory phenomena observed in blind or deaf humans.

Auditory perception vs. recognition: representation of complex communication sounds in the mouse auditory cortical fields

Details of brain areas for acoustical Gestalt perception and the recognition of species-specific vocalizations are not known. Here we show how spectral properties and the recognition of the acoustical Gestalt of wriggling calls of mouse pups based on a temporal property are represented in auditory cortical fields and an association area (dorsal field) of the pups' mothers. We stimulated either with a call model releasing maternal behaviour at a high rate (call recognition) or with two models of low behavioural significance (perception without recognition). Brain activation was quantified using c-Fos immunocytochemistry, counting Fos-positive cells in electrophysiologically mapped auditory cortical fields and the dorsal field. A frequency-specific labelling in two primary auditory fields is related to call perception but not to the discrimination of the biological significance of the call models used. Labelling related to call recognition is present in the second auditory field (AII). A left hemisphere advantage of labelling in the dorsoposterior field seems to reflect an integration of call recognition with maternal responsiveness. The dorsal field is activated only in the left hemisphere. The spatial extent of Fos-positive cells within the auditory cortex and its fields is larger in the left than in the right hemisphere. Our data show that a left hemisphere advantage in processing of a species-specific vocalization up to recognition is present in mice. The differential representation of vocalizations of high vs. low biological significance, as seen only in higher-order and not in primary fields of the auditory cortex, is discussed in the context of perceptual strategies.

Hippocampal mGluR1 and consolidation of contextual fear conditioning

The effects of post-training intra-hippocampal injections of group I mGluR agonists and antagonists, were examined in the contextual fear test, in rats. It was found that (S)-3,5-dihydroxyphenylglycine (DHPG) (a mGluR1-5 agonist) decreased, and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) (a mGluR1 antagonist) increased fear conditioning (a freezing reaction), examined 24h after conditioning session. (RS)-2-Chloro-5-hydroxyphenylglycine (CHPG) (a mGluR5 agonist), and 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) (a mGluR5 antagonist) did not cause any effect. In the immunocytochemical study, the post-conditioning administration of AIDA decreased the c-Fos induction in the dentate gyrus and CA1 layer of the hippocampus proper, 2h after exposure of animals to the aversive context, and 24h after conditioning session. It is suggested that overactivation of glutamatergic transmission in the critical for memory trace formation structure and period of time, may result in an attenuation of memory consolidation. On the other hand, reduction of an exaggerated glutamatergic tone can facilitate learning and memory processes. The immunocytochemical study and factor analysis of experimental data revealed that hippocampal mGlu1 receptors significantly influence the memory consolidation in a way dependant on the level of glutamatergic activity. Furthermore, they indicate that changes of glutamatergic activity within brain limbic structures can affect the threshold for the induction of the long-term neuronal plasticity, involved in some forms of learning and memory.

Posttraining Sleep Enhances Automaticity in Perceptual Discrimination

Perceptual learning can develop over extended periods, with slow, at times sleep-dependent, improvement seen several days after training. As a result, performance can become more automatic, that is, less dependent on voluntary attention. This study investigates whether the brain correlates of this enhancement of automaticity are sleep-dependent. Event-related potentials produced in response to complex auditory stimuli were recorded while subjects' attention was focused elsewhere. We report here that following training on an auditory discrimination task, performance continued to improve, without significant further training, for 72 hr. At the same time, several event-related potential components became evident 48–72 hr after training. Posttraining sleep deprivation prevented neither the continued performance improvement nor the slow development of cortical dynamics related to an enhanced familiarity with the task. However, those brain responses associated with the automatic shift of attention to unexpected stimuli failed to develop. Thus, in this auditory learning paradigm, posttraining sleep appears to reduce the voluntary attentional effort required for successful perceptual discrimination by facilitating the intrusion of a potentially meaningful stimulus into one's focus of attention for further evaluation.