Responses of amygdala single units to odors in the rat

Memory effects of visual and olfactory landmark information in human wayfinding

Non-human animals are exceptionally good at using smell to find their way through the environment. However, the use of olfactory cues for human navigation is often underestimated. Although the sense of smell is well-known for its distinct connection to memory and emotion, memory effects in human navigation using olfactory landmarks have not been studied yet. Therefore, this article compares wayfinding and recognition performance for visual and olfactory landmarks learned by 52 participants in a virtual maze. Furthermore, it is one of the first empirical studies investigating differences in memory effects on human navigation by using two separate test situations 1 month apart. The experimental task was to find the way through a maze-like virtual environment with either olfactory or visual cues at the intersections that served as decision points. Our descriptive results show that performance was above chance level for both conditions (visual and olfactory landmarks). Wayfinding performance did not decrease 1 month later when using olfactory landmarks. In contrast, when using visual landmarks wayfinding performance decreased significantly, while visual landmarks overall lead to better recognition than olfactory landmarks at both times of testing. The results demonstrate the unique character of human odor memory and support the conclusion that olfactory cues may be used in human spatial orientation. Furthermore, the present study expands the research field of human wayfinding by providing a study that investigates memory for landmark knowledge and route decisions for the visual and olfactory modality. However, more studies are required to put this important research strand forward.

Basolateral amygdala to posterior piriform cortex connectivity ensures precision in learned odor threat

Odor perception can both evoke emotional states and be shaped by emotional or hedonic states. The amygdala complex plays an important role in recognition of, and response to, hedonically valenced stimuli, and has strong, reciprocal connectivity with the primary olfactory (piriform) cortex. Here, we used differential odor-threat conditioning in rats to test the role of basolateral amygdala (BLA) input to the piriform cortex in acquisition and expression of learned olfactory threat responses. Using local field potential recordings, we demonstrated that functional connectivity (high gamma band coherence) between the BLA and posterior piriform cortex (pPCX) is enhanced after differential threat conditioning. Optogenetic suppression of activity within the BLA prevents learned threat acquisition, as do lesions of the pPCX prior to threat conditioning (without inducing anosmia), suggesting that both regions are critical for acquisition of learned odor threat responses. However, optogenetic BLA suppression during testing did not impair threat response to the CS+ , but did induce generalization to the CS-. A similar loss of stimulus control and threat generalization was induced by selective optogenetic suppression of BLA input to pPCX. These results suggest an important role for amygdala-sensory cortical connectivity in shaping responses to threatening stimuli.

Quantifying the lagged Poincaré plot geometry of ultrashort heart rate variability series: automatic recognition of odor hedonic tone

The application of Poincaré plot analysis to characterize inter-beat interval dynamics has been successfully proposed in the scientific literature for the assessment of humans' physiological states and related aberrations. In this study, we proposed novel descriptors to trace the evolution of Poincaré plot shape over the lags. Their reliability in ultra-short cardiovascular series analysis was validated on synthetic inter-beat series generated through a physiologically plausible integral pulse frequency modulation model. Furthermore, we used the proposed approach for the investigation of the direct relationship between autonomic nervous system (ANS) dynamics and hedonic olfactory elicitation, in a group of 30 healthy subjects. Participants with a similar olfactory threshold were selected, and were asked to score 5-s stimuli in terms of arousal and valence levels according to the Russell's circumflex model of affect. Their ANS response was investigated in 35-s windows after the elicitation. Experimental results showed a gender-specific, high discriminant power of the proposed approach, discerning between pleasant and unpleasant odorants with an accuracy of 83.33% and 73.33% for men and for women, respectively. Graphical Abstract Olfaction plays a crucial role in our life and is strictly related to the Autonomic Nervous System (ANS) activity, which can be monitored studying Heart Rate Variability. We used the Lagged Poincare Plot approach to recognize gender-specific ANS response in 35-second windows after the elicitation through pleasant/unpleasant odorants.

Central olfactory structures

Axons from the olfactory bulb (OB) project to multiple central structures of the brain, many of which, in turn, send axons back into the OB and/or to one another. These secondary sensory regions underlie many aspects of odor representation, valence, and learning, as well as serving some nonolfactory functions, though many details remain unclear. We here describe the connectivity and essential structural and functional properties of these postbulbar olfactory regions in the mammalian brain.

Basolateral amygdala circuitry in positive and negative valence

All organisms must solve the same fundamental problem: they must acquire rewards and avoid danger in order to survive. A key challenge for the nervous system is therefore to connect motivationally salient sensory stimuli to neural circuits that engage appropriate valence-specific behavioral responses. Anatomical, behavioral, and electrophysiological data have long suggested that the amygdala plays a central role in this process. Here we review experimental efforts leveraging recent technological advances to provide previously unattainable insights into the functional, anatomical, and genetic identity of neural populations within the amygdala that connect sensory stimuli to valence-specific behavioral responses.

In vivo electrophysiological recordings in amygdala subnuclei reveal selective and distinct responses to a behaviorally identified predator odor

Chemosensory cues signaling predators reliably stimulate innate defensive responses in rodents. Despite the well-documented role of the amygdala in predator odor-induced fear, evidence for the relative contribution of the specific nuclei that comprise this structurally heterogeneous structure is conflicting. In an effort to clarify this we examined neural activity, via electrophysiological recordings, in amygdala subnuclei to controlled and repeated presentations of a predator odor: cat urine. Defensive behaviors, characterized by avoidance, decreased exploration, and increased risk assessment, were observed in adult male hooded Wistar rats (n = 11) exposed to a cloth impregnated with cat urine. Electrophysiological recordings of the amygdala (777 multiunit clusters) were subsequently obtained in freely breathing anesthetized rats exposed to cat urine, distilled water, and eugenol via an air-dilution olfactometer. Recorded units selectively responded to cat urine, and frequencies of responses were distributed differently across amygdala nuclei; medial amygdala (MeA) demonstrated the greatest frequency of responses to cat urine (51.7%), followed by the basolateral and basomedial nuclei (18.8%) and finally the central amygdala (3.0%). Temporally, information transduction occurred primarily from the cortical amygdala and MeA (ventral divisions) to other amygdala nuclei. Interestingly, MeA subnuclei exhibited distinct firing patterns to predator urine, potentially revealing aspects of the underlying neurocircuitry of predator odor processing and defensiveness. These findings highlight the critical involvement of the MeA in processing olfactory cues signaling predator threat and converge with previous studies to indicate that amygdala regulation of predator odor-induced fear is restricted to a particular set of subnuclei that primarily include the MeA, particularly the ventral divisions.

Enhanced olfactory sensitivity in autism spectrum conditions

Background

People with autism spectrum conditions (ASC) report heightened olfaction. Previous sensory experiments in people with ASC have reported hypersensitivity across visual, tactile, and auditory domains, but not olfaction. The aims of the present study were to investigate olfactory sensitivity in ASC, and to test the association of sensitivity to autistic traits.

Methods

We recruited 17 adult males diagnosed with ASC and 17 typical adult male controls and tested their olfactory sensitivity using the Alcohol Sniff Test (AST), a standardised clinical evaluation of olfactory detection. The AST involves varying the distance between subject and stimulus until an odour is barely detected. Participants with ASC also completed the Autism Spectrum Quotient (AQ) as a measure of autism traits.

Results

The ASC group detected the odour at a mean distance of 24.1 cm (SD =11.5) from the nose, compared to the control group, who detected it at a significantly shorter mean distance of 14.4 cm (SD =5.9). Detection distance was independent of age and IQ for both groups, but showed a significant positive correlation with autistic traits in the ASC group (r =0.522).

Conclusions

This is the first experimental demonstration, as far as the authors are aware, of superior olfactory perception in ASC and showing that greater olfactory sensitivity is correlated with a higher number of autistic traits. This is consistent with results from previous findings showing hypersensitivity in other sensory domains and may help explain anecdotal and questionnaire accounts of heightened olfactory sensitivity in ASC. Results are discussed in terms of possible underlying neurophysiology.

Role of the basolateral amygdala in retrieval of conditioned flavors in the awake rat

Learned association between odor, taste and further post-ingestive consequence is known as flavor nutrient conditioned preference. Amygdala is supposed to be one of the areas involved in these associations. In the present study, one flavor was associated with a 16% glucose (CS(+)) whereas another flavor was paired with less reinforcing 4% glucose (CS(-)). We showed that CS(+) presentation after conditioning increased Fos expression in the basolateral nucleus of amygdala (BLA). Furthermore, we performed electrophysiological recordings in the BLA in free moving rats. After preference acquisition, rats were exposed to either the CS(+) or the CS(-). The proportion of neurons showing a decreased activity during the CS(-) presentation was significantly higher in conditioned rats compared to controls. Among this neuronal population recorded in conditioned rats, we noticed a significant proportion of neurons that also showed a decreased activity during the CS(+) presentation. Our data indicate an involvement of BLA during retrieval of learned flavors. It also suggests that both flavors might have acquired a biological value through conditioning.

Olfaction: anatomy, physiology, and disease

The olfactory system is an essential part of human physiology, with a rich evolutionary history. Although humans are less dependent on chemosensory input than are other mammals (Niimura 2009, Hum. Genomics 4:107-118), olfactory function still plays a critical role in health and behavior. The detection of hazards in the environment, generating feelings of pleasure, promoting adequate nutrition, influencing sexuality, and maintenance of mood are described roles of the olfactory system, while other novel functions are being elucidated. A growing body of evidence has implicated a role for olfaction in such diverse physiologic processes as kin recognition and mating (Jacob et al. 2002a, Nat. Genet. 30:175-179; Horth 2007, Genomics 90:159-175; Havlicek and Roberts 2009, Psychoneuroendocrinology 34:497-512), pheromone detection (Jacob et al. 200b, Horm. Behav. 42:274-283; Wyart et al. 2007, J. Neurosci. 27:1261-1265), mother-infant bonding (Doucet et al. 2009, PLoS One 4:e7579), food preferences (Mennella et al. 2001, Pediatrics 107:E88), central nervous system physiology (Welge-Lüssen 2009, B-ENT 5:129-132), and even longevity (Murphy 2009, JAMA 288:2307-2312). The olfactory system, although phylogenetically ancient, has historically received less attention than other special senses, perhaps due to challenges related to its study in humans. In this article, we review the anatomic pathways of olfaction, from peripheral nasal airflow leading to odorant detection, to epithelial recognition of these odorants and related signal transduction, and finally to central processing. Olfactory dysfunction, which can be defined as conductive, sensorineural, or central (typically related to neurodegenerative disorders), is a clinically significant problem, with a high burden on quality of life that is likely to grow in prevalence due to demographic shifts and increased environmental exposures.

Neurons in the human basolateral amygdala and hippocampal formation do not respond to odors

Extracellular action potentials were recorded from human hippocampal, hippocampal gyrus, and basolateral amygdala neurons during passive and active olfactory stimulation. Introduction of an odor into a continuous stream of air passing over the olfactory mucosa resulted in no detectable change in firing. Actively sniffing from an odorous flask, whether or not the odor was detected or recognized, also failed to elicit an immediate unit response. However, after a latency of 10-15 sec, the slight hyperventilation inherent in sniffing did induce a strong change in firing by many neurons. In contrast to these unit responses, a short-latency EEG spindle was recorded in the amygdala that appeared directly related to mechanical stimulation of the olfactory mucosa.

Neural substrates of cognitive inflexibility after chronic cocaine exposure

Cognitive changes in addicts and animals exposed to addictive drugs have been extensively investigated over the past decades. One advantage of studying addiction using cognitive paradigms is that neural processing in addicts or drug-exposed animals can be compared to that in normal subjects. Tests of cognitive flexibility that measure the ability to change responding to a previously rewarded or punished stimulus are of potential interest in the study of addiction, because addiction can itself be viewed as an inability to change responding to stimuli previously associated with drug reward. One such test is reversal learning, which is impaired in cocaine addicts and animals that have chronically self-administered or been exposed to cocaine. A circuit including orbitofrontal cortex, basolateral amygdala and striatum subserves reversal learning. In rats that have been previously exposed to cocaine, neurons in these regions show selective and distinct changes in how they encode information during reversal learning. These changes suggest that in these rats, orbitofrontal cortex loses the ability to signal expected outcomes, and basolateral amygdala becomes inflexible in its encoding of cue significance. These changes could explain cocaine-induced impairments to cognitive flexibility and may have theoretical importance in addiction.

Critical role of amygdala in flavor but not taste preference learning in rats

The role of the amygdala (AMY) in learning to associate complex flavor (taste + odor cues) with the oral and post-oral properties of nutrients was examined. Rats with excitotoxic lesions of the basolateral AMY learned to prefer flavors paired with intragastric (IG) infusions of maltodextrin or corn oil (Experiment 1), although the preference was slightly attenuated. However, rats with large AMY lesions failed to develop a preference for flavors paired with IG infusions of the same nutrients (Experiments 2 and 4) but were able to learn a preference for a taste mixture paired with IG maltodextrin infusions (Experiment 3). The rats with large AMY lesions also did not acquire a preference for a flavor cue paired with the sweet taste of fructose (Experiment 5). Collectively, these data provide evidence that AMY is essential for flavor- but not taste-nutrient preference learning.

Oxiracetam pre- but not post-treatment prevented social recognition deficits produced with trimethyltin in rats

The social recognition paradigm was used to investigate the effect of trimethyltin (TMT) in adult male rats. Consequently, the effect of chronic oxiracetam (OXI) treatment in TMT impaired animals was evaluated. In all experiments, a behavioural testing was performed 3 weeks after TMT administration. Experiment 1: A single TMT oral dose, 5 and 7.5 but not 2.5mg/kg, impaired the natural ability of the adults to recognize a juvenile conspecific that they encountered 30 min before. The dose of 5mg/kg TMT was chosen to be used in subsequent experiments. Experiment 2: Chronic OXI pre-+post-treatment, daily 3 or 30 mg/kg sc for 7 days before and 7 days after the insult, protected the adults against recognition deficit produced by TMT. Experiment 3: OXI pre- but not post-treatment (always 3 and 30 mg/kg) had beneficial effects on the social recognition. The findings suggest that social recognition ability of adult male rats pre-treated sufficiently long with OXI is resistant to the neurotoxicity effect of TMT.

Medial amygdala modulation of predator odor-induced unconditioned fear in the rat

This study examined the participation of the medial amygdala (MeA) in unconditioned fear. Rats received ibotenic acid lesions in the MeA or central amygdala (CeA) prior to cat-odor exposure. MeA-lesioned rats exhibited a significant reduction in freezing duration and made frequent contact with a cloth containing cat odor. In contrast, CeA lesions had no significant effects on unconditioned fear. The freezing reduction produced by MeA lesions was not due to a performance deficit because MeA-lesioned rats, unlike CeA-lesioned rats, were capable of freezing in postshock test intervals. Furthermore, MeA lesions did not alter olfactory function and general locomotor activity. Results demonstrate that the MeA plays a major role in modulating predator odor-induced unconditioned fear.

Efferent connections of the nucleus of the lateral olfactory tract in the rat

The efferent connections of the nucleus of the lateral olfactory tract (LOT) were examined in the rat with the Phaseolus vulgaris leucoagglutinin (PHA-L) technique. Our observations reveal that layers II and III of LOT have largely segregated outputs. Layer II projects chiefly ipsilaterally to the olfactory bulb and anterior olfactory nucleus, bilaterally to the anterior piriform cortex, dwarf cell cap regions of the olfactory tubercle and lateral shell of the accumbens, and contralaterally to the lateral part of the interstitial nucleus of the posterior limb of the anterior commissure. Layer III sends strong bilateral projections to the rostral basolateral amygdaloid complex, which are topographically organized, and provides bilateral inputs to the core of the accumbens, caudate-putamen, and agranular insular cortex (dorsal and posterior divisions). Layer II projects also to itself and to layers I and II of the contralateral LOT, whereas layer III projects to itself, to ipsilateral layer II, and to contralateral layer III of LOT. In double retrograde labeling experiments using Fluorogold and cholera toxin subunit b tracers, LOT neurons from layers II and III were found to provide collateral projections to homonymous structures on both sides of the brain. Unlike other parts of the olfactory amygdala, LOT neither projects directly to the extended amygdala nor to the hypothalamus. Thus, LOT seemingly influences nonpheromonal olfactory-guided behaviors, especially feeding, by acting on the olfactory bulb and on ventral striatal and basolateral amygdaloid districts that are tightly linked to lateral prefrontal cortical operations.

Modulation of memory consolidation for olfactory learning by reversible inactivation of the basolateral amygdala

The role of the basolateral amygdala (BLA) in the consolidation of an association between an olfactory stimulus and footshock was investigated with a reversible lesion technique of post-training intra-BLA infusions of tetrodotoxin. Rats receiving tetrodotoxin infusions following paired odor-shock presentations spent more time near the odor, and reacted differently on contact with the odor when tested 24 hr after training, than did rats receiving paired presentations and saline infusions, but they did not differ from rats receiving unpaired presentations and saline infusions. The results indicate that the BLA plays a similar role in influencing consolidation of olfactory-based memory as it does for memory based on other modalities. Thus, these findings strengthen the view that the BLA plays a general role in modulation of memory storage for emotionally arousing events.

Functional heterogeneity in human olfactory cortex: an event-related functional magnetic resonance imaging study

Studies of patients with focal brain injury indicate that smell perception involves caudal orbitofrontal and medial temporal cortices, but a more precise functional organization has not been characterized. In addition, although it is believed that odors are potent triggers of emotion, support for an anatomical association is scant. We sought to define the neural substrates of human olfactory information processing and determine how these are modulated by affective properties of odors. We used event-related functional magnetic resonance imaging (fMRI) in an olfactory version of a classical conditioning paradigm, whereby neutral faces were paired with pleasant, neutral, or unpleasant odors, under 50% reinforcement. By comparing paired (odor/face) and unpaired (face only) conditions, odor-evoked neural activations could be isolated specifically. In primary olfactory (piriform) cortex, spatially and temporally dissociable responses were identified along a rostrocaudal axis. A nonhabituating response in posterior piriform cortex was tuned to all odors, whereas activity in anterior piriform cortex reflected sensitivity to odor affect. Bilateral amygdala activation was elicited by all odors, regardless of valence. In posterior orbitofrontal cortex, neural responses evoked by pleasant and unpleasant odors were segregated within medial and lateral segments, respectively. The results indicate functional heterogeneity in areas critical to human olfaction. They also show that brain regions mediating emotional processing are differentially activated by odor valence, providing evidence for a close anatomical coupling between olfactory and emotional processes.

Multiple parallel memory systems in the brain of the rat

A theory of multiple parallel memory systems in the brain of the rat is described. Each system consists of a series of interconnected neural structures. The "central structures" of the three systems described are the hippocampus, the matrix compartment of the dorsal striatum (caudate-putamen), and the amygdala. Information, coded as neural signals, flows independently through each system. All systems have access to the same information from situations in which learning occurs, but each system is specialized to represent a different kind of relationship among the elements (stimulus events, responses, reinforcers) of the information that flows through it. The speed and accuracy with which a system forms a coherent representation of a learning situation depend on the correspondence between the specialization of the system and the relationship among the elements of the situation. The coherence of these stored representations determines the degree of control exerted by each system on behavior in the situation. Although they process information independently the systems interact in at least two ways: by simultaneous parallel influence on behavioral output and by directly influencing each other. These interactions can be cooperative (leading to similar behaviors) or competitive (leading to different behaviors). Experimental findings consistent with these ideas, mostly from experiments with rats, are reviewed.

Enhanced frontal cortex activation in rats by convergent amygdaloid and noxious sensory signals

The modulation of frontal cortical EEG activation to noxious somatosensory (tail pressure) and olfactory (acetone) stimulation by the basal amygdala was examined in urethane-anesthetized rats. Mild tail pressure produced no EEG activation, while acetone (sniffed by freely breathing rats or drawn across the olfactory epithelium in tracheotomized rats) produced a moderate suppression of large-amplitude synchronized EEG patterns. Concurrent, low-intensity 100 Hz stimulation of the basal amygdala permitted EEG activation to tail pressure to occur, and strongly enhanced olfactory-induced cortical activation. These results indicate that excitation of the basal amygdala potentiates frontal cortical responsiveness to aversive sensory events. This may provide a mechanism to facilitate cortical excitability and processing by amygdaloid neuronal activity.

Neuronal correlates of fear in the lateral amygdala: multiple extracellular recordings in conscious cats

Much data implicates the amygdala in the expression and learning of fear. Yet, few studies have examined the neuronal correlates of fear in the amygdala. This study aimed to determine whether fear is correlated to particular activity patterns in the lateral amygdaloid (LA) nucleus. Cats, chronically implanted with multiple microelectrodes in the LA and a catheter in the femoral artery, learned that a series of tones interrupted by a period of silence (5 sec) preceded the administration of a footshock. During the silent period, their blood pressure increased, indicating that they anticipated the noxious stimulus. In parallel, the firing rate of LA neurons doubled, and the discharges of simultaneously recorded cells became more synchronized. Moreover, cross-correlation of focal LA waves revealed a significant increase in synchrony restricted to the theta band. In keeping with this, perievent histograms of neuronal discharges revealed rhythmic changes in the firing probability of LA neurons in relation to focal theta waves. Finally, the responsiveness of LA cells to the stimuli predicting the footshock (the tones) increased during the trials, whereas responses to unrelated stimuli (perirhinal shocks) remained stable. Thus, during the anticipation of noxious stimuli, a state here defined anthropomorphically as fear, the firing rate of LA neurons increases, and their discharges become more synchronized through a modulation at the theta frequency. The presence of theta oscillations in the LA might facilitate cooperative interactions between the amygdala and cortical areas involved in memory.

Odor-induced fast waves in the dentate gyrus depend on a pathway through posterior cerebral cortex: effects of limbic lesions and trimethyltin

Previous research has shown that the odor of a variety of organic solvents and of components of the anal scent gland excretions of various predators will elicit a burst of fast waves of about 20 Hz in the olfactory bulb, pyriform cortex, and dentate gyrus of the rat. The present experiments show that large lesions of the caudal cerebral cortex, involving particularly the entorhinal and subicular cortices and the angular bundle, abolish the olfactory fast wave response of the dentate gyrus, but not the similar response of the olfactory bulb. In confirmation of previous work, such a lesion also abolishes an average evoked response elicited in the dentate gyrus by electrical stimulation of the olfactory bulb. Systemic treatment with the neurotoxin trimethyltin abolished the olfactory fast wave response in the olfactory bulb and both the olfactory fast wave and the olfactory evoked potential in the dentate gyrus. Large lesions of the amygdala or the septal nuclei did not eliminate either the dentate olfactory evoked potential or the odor-induced dentate fast wave response. However, the septal lesion reduced the amplitude of both spontaneous and odor-induced dentate fast wave activity. It is suggested that olfactory stimuli elicit 20 Hz dentate fast waves via a pathway from the olfactory bulb through the entorhinal cortex and, further, that cholinergic interneurons in the dentate gyrus may be essential to the dentate fast wave response.

Effects of amygdaloid lesions, hippocampal lesions, and buspirone on black-white exploration and food carrying in rats

Unlesioned rats exploring a black-white two compartment box spent most of the time in the covered, black half of the box and only little time in the uncovered, white half (67 s/5 min). Large radio-frequency lesions of the amygdala or hippocampus did not alter this pattern of exploration, but rats with hippocampus lesions were more active than the other two groups of rats. Treatment with the 5-HT1A receptor agonist buspirone (0.1 mg/kg, s.c.) increased the time that unlesioned rats spent in the uncovered compartment (103 s), an effect that was less pronounced in hippocampus-lesioned rats and completely abolished by amygdala lesions. In a food transport test, unlesioned rats that traveled from a home cage to an exposed food source consumed small and medium-sized pellets immediately at the food source. Larger pellets, however, were carried back to the home cage for consumption. Rats with amygdala lesions ate fewer pellets at the food source and tended to carry more pellets back to the home cage for consumption than unlesioned rats. Rats with hippocampus lesions carried fewer pellets back to the home cage and ate more pellets at the food source. Buspirone (0.5-1.5 mg/kg, s.c.) reduced the carrying of large food items to the home cage and increased consumption of these pellets at the food source in all groups of rats. These results suggest that neither the amygdala nor the hippocampus play an important role in controlling exploratory behavior in a black-white compartment box, but that the amygdala may have some role in mediating the effect of buspirone to increase exploration of the white/open compartment. Further, the amygdala and hippocampus have opposing influences on the transport of food items to a shelter, the amygdala suppressing food carrying, and the hippocampus enhancing it. Neither structure is essential for the effect of buspirone to reduce food carrying. The hypothesis that limbic structures mediate 'fear/anxiety' responses is discussed critically.

Cortical pathways to the mammalian amygdala

The amygdaloid nuclear complex is critical for producing appropriate emotional and behavioral responses to biologically relevant sensory stimuli. It constitutes an essential link between sensory and limbic areas of the cerebral cortex and subcortical brain regions, such as the hypothalamus, brainstem, and striatum, that are responsible for eliciting emotional and motivational responses. This review summarizes the anatomy and physiology of the cortical pathways to the amygdala in the rat, cat and monkey. Although the basic anatomy of these systems in the cat and monkey was largely delineated in studies conducted during the 1970s and 1980s, detailed information regarding the cortico-amygdalar pathways in the rat was only obtained in the past several years. The purpose of this review is to describe the results of recent studies in the rat and to compare the organization of cortico-amygdalar projections in this species with that seen in the cat and monkey. In all three species visual, auditory, and somatosensory information is transmitted to the amygdala by a series of modality-specific cortico-cortical pathways ("cascades") that originate in the primary sensory cortices and flow toward higher order association areas. The cortical areas in the more distal portions of these cascades have stronger and more extensive projections to the amygdala than the more proximal areas. In all three species olfactory and gustatory/visceral information has access to the amygdala at an earlier stage of cortical processing than visual, auditory and somatosensory information. There are also important polysensory cortical inputs to the mammalian amygdala from the prefrontal and hippocampal regions. Whereas the overall organization of cortical pathways is basically similar in all mammalian species, there is anatomical evidence which suggests that there are important differences in the extent of convergence of cortical projections in the primate versus the nonprimate amygdala.

Emotion, olfaction, and the human amygdala: amygdala activation during aversive olfactory stimulation

Electrophysiologic and lesion studies of animals increasingly implicate the amygdala in aspects of emotional processing. Yet, the functions of the human amygdala remain poorly understood. To examine the contributions of the amygdala and other limbic and paralimbic regions to emotional processing, we exposed healthy subjects to aversive olfactory stimuli while measuring regional cerebral blood flow (rCBF) with positron emission tomography. Exposure to a highly aversive odorant produced strong rCBF increases in both amygdalae and in the left orbitofrontal cortex. Exposure to less aversive odorants produced rCBF increases in the orbitofrontal cortex but not in the amygdala. Change of rCBF within the left amygdala and the left OFC was highly intercorrelated, indicating a strong functional interaction between these brain regions. Furthermore, the activity within the left amygdala was associated significantly with subjective ratings of perceived aversiveness. These findings provide evidence that the human amygdala participates in the hedonic or emotional processing of olfactory stimuli.

Odorant evoked magnetic fields in humans

We investigated the olfactory evoked magnetic fields (OEFs) in 14 normal subjects. Pulses of odorant air containing amyl acetate or phenethyl alcohol, and odorless air were administered to the subject through a nasal tube. A clear and consistent OEF component, 1M, was identified in all subjects, and a second component, 2M, was detected in seven subjects, but no consistent component was identified in response to the odorless air. The peak latencies of the 1M and 2M components were approximately 320 and 630 ms, respectively. The waveforms produced by the odorless air were subtracted from the waveforms produced by the odorant air to obtain the 'subtraction' waveform, which indicated the 1M and 2M component more clearly. Their equivalent current dipoles (ECDs) were estimated in the regions around the Sylvian fissure symmetrically in both hemispheres. Therefore, these areas are proposed to be involved in olfactory perception in humans.

Organization of projections from the basomedial nucleus of the amygdala: a PHAL study in the rat

The organization of axonal projections from the basomedial nucleus of the amygdala (BMA) was examined with the Phaseolus vulgaris leucoagglutinin (PHAL) method in adult male rats. The anterior and posterior parts of the BMA, recognized on cytoarchitectonic grounds, display very different projection patterns. Within the amygdala, the anterior basomedial nucleus (BMAa) heavily innervates the central, medial, and anterior cortical nuclei. In contrast, the posterior basomedial nucleus (BMAp) sends a dense projection to the lateral nucleus, and to restricted parts of the central and medial nuclei. Extra-amygdalar projections from the BMA are divided into ascending and descending components. The former end in the cerebral cortex, striatum, and septum. The BMAa mainly innervates olfactory (piriform, transitional) and insular areas, whereas the BMAp also innervates inferior temporal (perirhinal, ectorhinal) and medial prefrontal (infralimbic, prelimbic) areas and the hippocampal formation. Within the striatum, the BMAa densely innervates the striatal fundus, whereas the nucleus accumbens receives a heavy input from the BMAp. Both parts of the BMA send massive projections to distinct regions of the bed nuclei of the stria terminalis. Descending projections from the BMA end primarily in the hypothalamus. The BMAa sends a major input to the lateral hypothalamic area, whereas the BMAp innervates the ventromedial nucleus particularly heavily. Injections were also placed in the anterior cortical nucleus (COAa), a cell group superficially adjacent to the BMAa. PHAL-labeled axons from this cell group mainly ascend into the amygdala and olfactory areas, and descend into the thalamus and lateral hypothalamic area. Based on connections, the COAa and BMAa are part of the same functional system. The results suggest that cytoarchitectonically distinct anterior and posterior parts of the BMA are also hodologically distinct and form parts of distinct anatomical circuits probably involved in mediating different behaviors (for example, feeding and social behaviors vs. emotion-related learning, respectively).

Neuronal responsiveness to various sensory stimuli, and associative learning in the rat amygdala

Neuronal activities were recorded from the amygdala and amygdalostriatal transition area of behaving rats during discrimination of conditioned auditory, visual, olfactory, and somatosensory stimuli associated with positive and/or negative reinforcements. Neurons were also tested with taste solution and various sensory stimuli that were not associated with reinforcement. Of the 1195 neurons tested, 475 responded to one or more sensory stimuli. Of these, 256 neurons responded exclusively to a unimodal sensory stimulus, 128 to multimodal sensory stimuli, and the remaining 91 could not be classified. Distribution of unimodal neurons was correlated with anatomical projections to the amygdala from sensory thalamus or sensory cortices. Multimodal neurons were located mainly in the basolateral and central nuclei of the amgydala. Response latencies of neurons in the basolateral nucleus were longer than those in other nuclei and neurons in the central nucleus had both short and long latencies. Neurons responsive to a given stimulus were more frequently encountered in the amygdalas of the trained rats than in those of the rats not trained to associate that stimulus with a reinforcement. Multimodal neurons that responded to conditioned and/or unconditioned stimuli used in the associative learned tasks were concentrated in the basolateral and central nuclei. The results indicate that some amygdalar neurons receive exclusive single sensory information, and the others receive information from two or more sensory inputs. Considering the long latencies and multimodal responsiveness, the basolateral and central nuclei of the amygdala might be foci where various kinds of sensory information converge. It is also suggested that the basolateral and central nuclei of the amygdala have critical roles in associative learning to relate sensory information to reinforcement or affective significance.

Ultrastructural evidence for an olfactory-autonomic pathway through the rat central amygdaloid nucleus

The innervation of medullary projection neurons in the central amygdaloid nucleus (Ce) by afferents from the ventral taenia tecta (VTT) was investigated using combined lesion-induced axonal degeneration and retrograde transport of horseradish peroxidase-conjugated wheat germ agglutinin (HRP-WGA). Injections of HRP-WGA into the nucleus tractus solitarii resulted in retrograde labeling of neurons in the medial Ce. Ultrastructurally HRP-WGA reaction product was identifiable in the perikarya and proximal dendrites of Ce neurons. Degenerating terminals, probably due to damage at the HRP-WGA injection site, were few and confined to the ventral Ce. Electrolytic coagulation of the VTT resulted in approximately 9% of terminals in medial Ce showing signs of degeneration at 5 days post-lesion. Of the terminals sampled, slightly more than 40% were in contact with the dendrites of retrogradely labeled neurons. Where evident, these terminals formed exclusively symmetrical synaptic contacts. These data provide evidence for an oligosynaptic olfactory-autonomic pathway in the rat that may mediate olfactory influences on gastric and cardiovascular aspects of autonomic function.

Neural correlates of conditioned odor avoidance in infant rats

Newborn rat pups can learn to either approach or avoid odor cues through associative conditioning. The present results demonstrate that preference conditioning and avoidance conditioning both modify olfactory bulb responses (focal 2-deoxyglucose uptake and mitral-tufted cell single unit responses) to the conditioned odor. Despite opposing behavioral responses to the conditioned odor, however, olfactory bulb neural responses did not detectably differ between learned odor cues signaling approach and those signaling avoidance. Control pups exhibited neither the behavioral nor neural changes. Furthermore, both the behavioral and neural changes to these odor cues could be extinguished. These results suggest that the olfactory bulb in neonates may code learned odor importance, but specific information attached to that importance may require processing in other brain regions.

Effects of intracerebroventricular angiotensin II and olfactory stimuli on multiple unit activity in preoptic and anterior hypothalamic areas: medial-lateral comparison

Under urethane anesthesia multiple unit activity (MUA) recordings were taken from medial and lateral preoptic and anterior hypothalamic sites in 21 rats during multiple dose intracerebroventricular (i.vt.) injections of angiotensin II (AII), using artificial CSF as control. Olfactory stimuli were also presented. Whilst lateral sites on average were significantly less responsive to AII than were medial sites, some of the former were very responsive. None of the 14 lateral sites that yielded an MUA response to AII failed to yield an MUA response to olfactory stimulation. On the other hand, 11 of 12 medial sites that yielded an MUA response to AII failed to yield an MUA response to olfactory stimulation. On the basis of these data it is suggested that the medial and lateral regions of the basal forebrain serve different functions, the former more related to internal sensing and the latter more related to integration of internal and external sensing. The findings are discussed in relation to the dual olfactory system and to theories of motivation.

Feeding related lateral hypothalamic neuron responses to odors depend on food deprivation in rats

It has been investigated feeding related LHA neuronal activity and responses to odor stimulation in rats at various levels of satiation. Extracellular responses of 168 neurons to three odors, isoamylacetate (AA), cineole (CL), and isovaleric acid (VA), were recorded from 168 LHA neurons of Wistar-SPF male rats. Of 168 units, 107 (63.7%) responded to from one to three odors, but not to light or phonic stimulation. Of the responding units, 94.4% (101/107) were excited, and 5.6% were inhibited. In response to a single electrical stimulation (0.5 msec, 1-10 V) of the OB, 61 units were excited with latencies of 6-43 msec (19.8 +/- 12.0 msec, mean +/- S.D.) indicating compound OB-LHA relations--mono- and polysynaptic through myelinated and nonmyelinated fibers. The results suggest predominantly excitatory effects of both electrical stimulation of the OB and odor stimulation on the LHA. Firing frequency in response to AA or VA was significantly (p less than 0.05) greater for the long fasting group (38 hr, LF, n = 8) than for the NF (nonfasting, n = 12) group; differences between the LF and MF (24 hr, n = 6) groups were not significant. Glucose-sensitive neurons (GSN, n = 19) responded more to odors than non-GSNs (n = 86), and discharge frequency increase depended markedly on food deprivation. Food deprivation results suggest that responsiveness of feeding related LHA neurons to odors depends on the degree of satiation. In conclusion, it was confirmed that olfactory functions are important in the responses of hypothalamic feeding related neurons.

Topography of projections from the medial prefrontal cortex to the amygdala in the rat

The projections from the rat medial prefrontal cortex to the amygdaloid complex were investigated using retrograde transport of fluorescent dyes and anterograde transport of horseradish peroxidase-WGA. The ventral anterior cingulate, prelimbic, infralimbic and medial orbital areas and the taenia tecta were found to project to the amygdaloid complex. The projections from the prelimbic area arose bilaterally. The medial orbital, prelimbic and anterior cingulate areas send convergent projections to the basolateral nucleus. The prelimbic area has additional projections to the posterolateral cortical nucleus and amygdalo-hippocampal area. The infralimbic area does not project to the basolateral nucleus and cortico-amygdaloid projections from this area are focussed on the anterior cortical nucleus and the anterior amygdaloid area. Both prelimbic and infralimbic areas project to an area situated between the central, medial and basomedial nuclei. Based on similar projections, this area appears to be a caudal continuation of the anterior amygdaloid area. The results indicate that the medial prefrontal component of the "basolateral limbic circuit" is restricted to the anterior cingulate and prelimbic areas. No evidence was obtained to support the existence of a medial prefronto-amygdaloid component of the "visceral forebrain".

Studies on the olfactory nervous system of the Old World monkey

From the results of our electrophysiological and HRP studies in the old world monkey, multiple olfactory pathways have been clarified. The old world monkey has two neocortical olfactory areas, but no functional vomeronasal system. The response patterns to odors in various olfactory areas have also been studied. On the other hand, in the rabbit (Onoda and Iino, 1980) and dog (Onoda et al., 1981, 1982), which do have active vomeronasal systems, only one neocortical olfactory area was found. This important difference had already been indicated in three previous papers in which Takagi (1979, 1980, 1981) theorized that mammals can be divided into two groups according to their olfactory nervous mechanisms. One group includes old world monkeys, higher primates and man, and the other new world monkeys and lower mammals.

The principal pathway from the piriform cortex to the deep amygdaloid nuclei in the cat

Pathways relaying olfactory information to the deep amygdaloid nuclei (AMYGd) were analyzed with electrophysiological techniques in anesthetized cats. Stimulation of the piriform cortex (PC) produced orthodromic spikes in some AMYGd neurons with a mean latency of 12.2 ms and antidromic responses in other neurons with a mean latency of 9.3 ms. Stimulation of the AMYGd produced antidromic spikes in some PC neurons with a mean latency of 11.5 ms. Some neurons in the entorhinal area (EA) were activated orthodromically from the PC with a mean latency of 22 ms, and a proportion of these cells was also activated antidromically from the AMYGd. Some neurons in the agranular insular cortex were activated orthodromically from the PC, but none of them responded antidromically to AMYGd stimulation. From these observations, it is suggested that olfactory information reaches the AMYGd directly from the PC or indirectly via the EA, and that the direct path conveys the major olfactory input from the PC to the AMYGd.

The effects of stimulation of substantia innominata and sensory receiving areas of the forebrain upon the activity of neurons within the amygdala of the anesthetized cat

The present study investigated the response characteristics of individual neurons in the amygdala following stimulation of the substantia innominata (SI), and compared these responses with those elicited by stimulation of insular and temporal polar cortices and the lateral olfactory tract (LOT). Recordings were made from single units within the medial, central, basal, and lateral amygdaloid nuclei of anesthetized, male cats. Stimulating electrodes were located in the SI, LOT, and sylvian cortex (SG). Unit responses were classified as either excitation or inhibition. Excitatory responses were further divided into fixed latency excitation (FLE) and variable latency excitation (VLE) based on the variability of the onset latency of the response. The majority of responses to SI stimulation were of the FLE type, implying a direct orthodromic, monosynaptic activation of amygdaloid units. Proportionally more FLE responses were recorded laterally, especially in the magnocellular basal nucleus, compared to VLE responses which were more common in the medial and central nuclei. SI stimulation consistently affected the activity of many more units than did SG or LOT stimulation. The onset latencies of the population of cells exhibiting excitatory responses elicited by SI stimulation were distributed bimodally, and this may reflect a dual projection pathway of amygdaloid afferents from this basal forebrain region. This correlates with anatomical descriptions indicating that SI projections to amygdala pass via the ventral amygdalofugal pathway as well as in the stria terminalis. Excitatory onset latencies of responses to SI stimulation were the shortest in the lateral and magnocellular basal nuclei and the longest in the parvocellular basal nucleus. Amygdaloid units exhibited convergent input from the stimulus sites. A clear topographical distribution of units was not demonstrated. The data suggests that units receiving a convergent input were rarely driven monosynaptically by more than one stimulus site. The basal nucleus contained the smallest fraction of units exhibiting a convergent input. A small population of antidromic responses was recorded and indicates that within the system studied the caudal aspect of the basal nucleus is a major source of amygdaloid efferents. Antidromically driven units did not exhibit transynaptic responses following stimulation of any of the test sites.

Effect of odor quality and intensity on conditioned odor aversion learning in the rat

Odor quality and intensity were varied to test the ability of rats to associate odor with an induced illness. Rats were allowed 10 minutes access to water on each of nine days; deodorized air was directed towards each rat's nose while drinking at familiarization and recovery sessions (days 1-5 and 7-8, respectively) and odorized air at treatment and test sessions (days 6 and 9, respectively). Each rat was injected with LiCl following its drinking period on day 6. The difference between the amount of water consumed on day 6 and day 9 gave a measure of the conditioned aversion. Only mild or no aversion occurred with odors of n-butyric acid, benzylamine, cyclohexanone, and n-butanol. Strong conditioned aversions were obtained to odors of triethylamine, 1,4-cineole, and isoamyl acetate, and the degree of aversion increased linearly with the log of odor concentration. The effect of odor quality, intensity and presentation method, and the role of the different chemoreceptor systems in the acquisition of odor aversions are discussed.

Inputs to testosterone-sensitive stria terminalis neurones in the rat brain and the effects of castration

1. The inputs to cortico-medial amygdala neurones which project directly to the area of the medial preoptic/anterior hypothalamic junction were studied electrophysiologically in urethane anaesthetized male rats. 2. In experiments with fifteen male rats it was found that none of these neurones was responsive to electrical stimulation of the ipsilateral olfactory bulb or accessory olfactory bulb or odour stimulation. 3. Experiments with four rats showed that electrical stimulation of the lateral portion of the contralateral fimbria excited 81% of these cortico-medial amygdala neurones. Their typical response to stimulation of the contralateral fimbria was a single action potential followed by an inhibitory period (20-100 ms). 4. Analysis of the polarity of evoked waves in the amygdala suggested that the fimbria input terminated in the basolateral nucleus of the amygdala and that this nucleus subsequently projected to the cortico-medial amygdala. The fimbria input was found to be contralateral in origin, crossing the mid line in the anterior fornical commissure. 5. In a further experiment 118 identified cortico-medial amygdala neurones were recorded from twelve rats (six gonadally intact and six castrated). Castration significantly decreased the percentage of these neurones responding to stimulation of the ipsilateral fimbria (20 vs. 97%) and lengthened post-excitatory inhibitory periods. 6. Results are discussed with respect to the initial finding by Kendrick & Drewett (1979) of testosterone-sensitive absolute refractory periods in cortico-medial amygdala neurones.

Role of mediodorsal thalamic nucleus in olfactory discrimination learning in rats

Severe deficits in the acquisition of an olfactory learning-set task resulted from lesions of the central (olfactory) component of the mediodorsal thalamic nucleus but not from large lesions that destroyed olfactory projections to the amygdala. Complex olfactory learning may be mediated by the olfactory thalamocortical system and not by olfactory projections to the limbic system.

Effects of insulin injection on responses of olfactory bulb and amygdala single units to odors

The effect of insulin injection on transmission of neural activity within the olfactory system of the anesthetized male rat was investigated at the single unit level. It was found that insulin changed the response to odors of approximately 27% of olfactory bulb units and 21% of amygdala units tested. Many of the changes were in the direction of an increase in response magnitude, but there were some reversals in response direction and other complex changes. There was no evidence of a selective facilitation of responses to food odor as compared to non-food odors. Control observations of the response of thalamic somatosensory units to tactual stimulation showed no effects of insulin. These results suggest that hypothalamic hunger mechanisms may normally interact with olfactory mechanisms to augment and otherwise change the response of some olfactory system units to various odors.