Pavlovian conditioning of emotional responses to olfactory and contextual stimuli: a potential model for the development and expression of chemical intolerance

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.

Perirhinal damage produces modality-dependent deficits in fear learning

The perirhinal cortex (PER) receives multimodal and unimodal sensory information from all modalities. In addition, the PER is anatomically connected with several brain regions that support fear learning. Several studies suggest that the PER is involved in fear conditioning to discontinuous auditory cues but not to continuous auditory cues. To date, studies examining the role of the PER in fear conditioning has largely focused on auditory and contextual stimuli. The present study assessed whether the role of the PER in fear conditioning would extend to visual modalities. Rodents were randomly assigned to one of four conditioned stimuli, which consisted of either a tone or a light stimulus that was either continuous or discontinuous. Pre-training excitotoxic lesions to the PER significantly reduced freezing to auditory and visual cues during the acquisition phase regardless of stimulus continuity. During subsequent testing, perirhinal lesions produced significant decreases in freezing levels to both continuous and discontinuous tones but not to either of the light CS groups. These results suggest that the PER is involved in the acquisition of fear across multiple cue modalities. However, the PER may have a more limited role in the retrieval of the fear memory dependent upon the cue modality.

Pavlovian Olfactory Fear Conditioning: Its Neural Circuity and Importance for Understanding Clinical Fear-Based Disorders

Odors have proven to be the most resilient trigger for memories of high emotional saliency. Fear associated olfactory memories pose a detrimental threat of potentially transforming into severe mental illness such as fear and anxiety-related disorders. Many studies have deliberated on auditory, visual and general contextual fear memory (CFC) processes; however, fewer studies have investigated mechanisms of olfactory fear memory. Evidence strongly suggests that the neuroanatomical representation of olfactory fear memory differs from that of auditory and visual fear memory. The aim of this review article is to revisit the literature regarding the understanding of the neurobiological process of fear conditioning and to illustrate the circuitry of olfactory fear memory.

[Pathophysiology of multiple chemical sensitivity]

INTRODUCTION AND BACKGROUND

Multiple chemical sensitivity (MCS) is a complex clinical entity that includes a large number of non-specific symptoms, associated in a univocal manner in each patient and triggered by exposure to various chemicals at low concentrations, well below those known to cause toxic effects. However, no objective test exists currently to diagnose this syndrome. One of the main reasons is that the pathophysiology is poorly understood. However, many explanatory hypotheses have been proposed.

VIEWPOINTS AND CONCLUSIONS

Patients with symptoms of MCS are often encountered by pulmonologists. Their suffering is undeniable but, unfortunately, the lack of understanding of the pathophysiological mechanisms makes treatment difficult and empirical.

Functional emergence of the hippocampus in context fear learning in infant rats

The hippocampus is a part of the limbic system and is important for the formation of associative memories, such as acquiring information about the context (e.g., the place where an experience occurred) during emotional learning (e.g., fear conditioning). Here, we assess whether the hippocampus is responsible for pups' newly emerging context learning. In all experiments, postnatal day (PN) 21 and PN24 rat pups received 10 pairings of odor-0.5 mA shock or control unpaired odor-shock, odor only, or shock only. Some pups were used for context, cue or odor avoidance tests, while the remaining pups were used for c-Fos immunohistochemistry to assess hippocampal activity during acquisition. Our results show that cue and odor avoidance learning were similar at both ages, while contextual fear learning and learning-associated hippocampal (CA1, CA3, and dentate gyrus) activity (c-Fos) only occurred in PN24 paired pups. To assess a causal relationship between the hippocampus and context conditioning, we infused muscimol into the hippocampus, which blocked acquisition of context fear learning in the PN24 pups. Muscimol or vehicle infusions did not affect cue learning or aversion to the odor at PN21 or PN24. The results suggest that the newly emerging contextual learning exhibited by PN24 pups is supported by the hippocampus.

Enduring neurobehavioral effects of early life trauma mediated through learning and corticosterone suppression

Early life trauma alters later life emotions, including fear. To better understand mediating mechanisms, we subjected pups to either predictable or unpredictable trauma, in the form of paired or unpaired odor-0.5 mA shock conditioning which, during a sensitive period, produces an odor preference and no learning respectively. Fear conditioning and its neural correlates were then assessed after the sensitive period at postnatal day (PN)13 or in adulthood, ages when amygdala-dependent fear occurs. Our results revealed that paired odor-shock conditioning starting during the sensitive period (PN8–12) blocked fear conditioning in older infants (PN13) and pups continued to express olfactory bulb-dependent odor preference learning. This PN13 fear learning inhibition was also associated with suppression of shock-induced corticosterone, although the age appropriate amygdala-dependent fear learning was reinstated with systemic corticosterone (3 mg/kg) during conditioning. On the other hand, sensitive period odor-shock conditioning did not prevent adult fear conditioning, although freezing, amygdala and hippocampal 2-DG uptake and corticosterone levels were attenuated compared to adult conditioning without infant conditioning. Normal levels of freezing, amygdala and hippocampal 2-DG uptake were induced with systemic corticosterone (5 mg/kg) during adult conditioning. These results suggest that the contingency of early life trauma mediates at least some effects of early life stress through learning and suppression of corticosterone levels. However, developmental differences between infants and adults are expressed with PN13 infants' learning consistent with the original learned preference, while adult conditioning overrides the original learned preference with attenuated amygdala-dependent fear learning.

Enduring effects of infant memories: infant odor-shock conditioning attenuates amygdala activity and adult fear conditioning

BACKGROUND

Early life adverse experience alters adult emotional and cognitive development. Here we assess early life learning about adverse experience and its consequences on adult fear conditioning and amygdala activity.

METHODS

Neonatal rats were conditioned daily from 8-12 days-old with paired odor (conditioned stimulus, CS) .5mA shock, unpaired, odor-only, or naive (no infant conditioning). In adulthood, each infant training group was divided into three adult training groups: paired, unpaired or odor-only, using either the same infant CS odor, or a novel adult CS odor without or with the infant CS present as context. Adults were cue tested for freezing (odor in novel environment), with amygdala (14)C 2-DG autoradiography and electrophysiology assessment.

RESULTS

Infant paired odor-shock conditioning attenuated adult fear conditioning, but only if the same infant CS odor was used. The (14)C 2-DG activity correlated with infant paired odor-shock conditioning produced attenuated amygdala but heightened olfactory bulb activity. Electrophysiological amygdala assessment further suggests early experience causes changes in amygdala processing as revealed by increased paired-pulse facilitation in adulthood.

CONCLUSIONS

This suggests some enduring effects of early life adversity (shock) are under CS control and dependent upon learning for their impact on later adult fear learning.

Pesticide-initiated idiopathic environmental intolerance in South Korean farmers

This study was designed to study patients with intolerance to pesticide smells. Ten subjects chosen were complaining of vague symptoms such as headache, dizziness, fatigue, nausea, vomiting, abdominal pain, myalgia, flu-like symptoms, etc., whenever exposed to the pesticide smells even at low intensity. To determine whether the etiology of this kind of pesticide hypersensitivity was of organic or psychiatric nature, all the subjects underwent tests as follows: complete blood cell count, urinalysis, and blood chemistry as routine tests; esophogastroduodenoscopy and abdomen ultrasonography for the gastrointestinal symptoms; chest x-ray, pulmonary function tests, and electrocardiography for the respiratory and/or cardiac symptoms; nerve conduction velocity and brain magnetic resonance imaging (MRI) for peripheral and central nerve system symptoms; and K-WAIS, Rey-Kim memory test, Rorschach, Mini Mental State Examination (MMSE), and Minnesota Multiphasic Personality Inventory (MMPI) for psychoanalysis. Of the 10 cases in which the chief complaint was headache, symptoms of two cases were caused by maxillary sinusitis. Another two showed typical multiple chemical sensitivity (MCS) or idiopathic environmental intolerance (IEI). Six out of the 10 cases, whose symptoms closely resembled the others, did not conclusively meet the criteria of classic MCS or IEI. The subjects of this case shared vague fears, both fear of pesticides and hypochondriasis. Some subjects faced financial insecurity and social uncertainty; others felt uneasy about the future of their farming life. Thus, to help verify the causes of MCS or IEI, which is strongly suggestive of pesticide smells, diagnosis needs a dual approach: on the anima and soma. Psychoanalysis can delve into the mental status of the patients to see whether the patients are aware of their symptoms. Clinical tests can see through the physical structure and functions of the organs on which patients' complaints are centered.

The physical context of previous stress exposure modifies hypothalamic-pituitary-adrenal responses to a subsequent homotypic stress

The hypothalamic-pituitary-adrenal (HPA) axis becomes less responsive to some types of repeated stress over time, a process termed habituation. Many facets of the stressful stimulus can modify such HPA responses to stressors, such as predictability and controllability. However, the physical context in which the stressor occurred may also provide a discriminative stimulus that can affect the HPA response to that stressor. In the present study, we examined whether a change in the context in which stress exposure occurs can alter HPA responses to a subsequent [corrected] homotypic stressor. Three separate contexts were produced by manipulating odor cues. Rats housed in the 3 context rooms exhibited similar HPA responses to acute 30-min restraint or repeated (8th) 30-min restraint in their home environments. However, rats that were restrained for 30 min per day for 7 days in a room in one context and then restrained on day 8 in a novel context exhibited attenuated habituation compared to rats restrained on day 8 in the familiar context. These results provide evidence that repeated stress-induced HPA activity depends, in part, on the context in which the stress is experienced.

Olfactory loss as a result of toxic exposure

Olfactory loss can occur through accidental exposure, poor industrial hygiene, or exposure to low levels of toxins in the ambient air over long periods. This loss can lead to transient olfactory disorders, irreversible anosmia, temporary olfactory fatigue, or industrial anosmia. Inevitably, a practicing otolaryngologist will encounter a patient with complaints of decreased smell and taste that initially may be difficult to diagnose and treat. Much of the challenge in evaluating a patient with disturbances of olfaction is in obtaining adequate quantitative measurements of sensory dysfunction and identifying a source for the olfactory loss. Although there is no particular test for environmental toxins as a source of olfactory loss, an accurate cause can be determined by obtaining a careful, detailed history. A significant exposure history and lack of more common causes of olfactory loss strengthens an argument for environmental toxins as an etiology. Unfortunately, no available treatments can reverse permanent damage caused by toxic exposure, but removal from the source of toxins may allow for repair of the olfactory system and return of normal function, especially in acute exposures. Despite the increasing number of studies investigating toxic exposure on olfactory function, these effects are understood poorly. With continued study of human exposure to these substances and the use of animal models, the mechanisms by which damage occurs will be understood better and new approaches for diagnosis and treatment will be developed. Furthermore, with increasing regulations of occupational environments and stricter policies on industrial air pollution, olfactory dysfunction secondary to toxicity should become less prevalent.

Olfactory learning in the rat pup: A model that may permit visualization of a mammalian memory trace

Over the past 10 years considerable insight into intracellular interactions leading to long-term memory formation have been gleaned from various neural circuits within invertebrate and vertebrate species. This review suggests that, while certain intracellular signaling pathways are commonly involved across species, it is important to analyze specific neural systems because critical differences among systems appear to exist. The olfactory bulb has been used by our group to estimate the influence of neuromodulatory systems (serotonin and norepinephrine) on intracellular processes leading to learning. We describe here how activation of noradrenergic input to mitral cells increases cAMP leading to CREB phosphorylation when paired with a conditioning stimulus, odor. CREB phosphorylation is causal in odor preference learning leading to long-term memory for the odor. However, the relationship between cAMP activation and CREB phosphorylation is not straight forward; overstimulation of cAMP pathways impedes learning and prevents CREB phosphorylation. Excessive CREB phosphorylation also interferes with learning.

Long-term exposure to low levels of formaldehyde increases the number of tyrosine hydroxylase-immunopositive periglomerular cells in mouse main olfactory bulb

Multiple chemical sensitivity (MCS) in response to a long-term low-level chemical exposure is as yet an unclarified disorder. To determine the role of olfactory function in the induction of MCS, immunocytochemical analysis of the main olfactory bulb (MOB) was performed after exposure of mice to low levels of formaldehyde. A long-term exposure resulted in an increase in the number of tyrosine hydroxylase-immunopositive periglomerular cells and may affect the neuronal function of the MOB.

Repeated low level formaldehyde exposure produces enhanced fear conditioning to odor in male, but not female, rats

Multiple chemical sensitivity (MCS) is an ill-defined disorder in humans attributed to exposure to volatile organic compounds. This study draws on apparent parallels between individuals with posttraumatic stress disorder (PTSD) and panic disorder and a subset of those reporting MCS, using a conditioned fear task in rats. Male and female Sprague-Dawley rats were given repeated exposure to 2 ppm formaldehyde (Form) (1 h/day x 5 days/week x 4 week) or air, and after 2-3 weeks, rats were trained on the conditioned fear task. One half of Air and Form rats were given odor (orange oil, the conditioned stimulus, CS) paired with footshock (PRD) and the other half was given the same stimuli in an unpaired manner (UNP). After 24 h, rats were placed into the same context without the CS or footshock. Male and female PRD groups demonstrated contextual freezing 5-15% of the time, while the UNP groups showed freezing 30-50% of the time, with no effect of Air or Form pretreatment. For the next 5 days, rats were placed into a novel context and tested for freezing in the absence or presence of the CS. In male rats, Form pretreatment produced a significantly greater freezing response in both UNP and PRD groups in the presence of the CS, with no differences in freezing in the absence of the CS. In female rats, no significant differences between Form pretreated rats and Air controls were observed in either the PRD or UNP groups. The increase in conditioned fear responding to the CS after Form exposure in males suggests that repeated low-level Form may act as a stressor to produce sensitized responding within olfactory/limbic pathways, and may help explain the panic-like responses observed in a subset of individuals reporting MCS. Furthermore, the male/female differences suggest a gonadal hormonal contribution to this behavior.

Olfactory processing in a changing brain

The perception of odorant molecules provides the essential information that allows animals to explore their surrounding. We describe here how the external world of scents may sculpt the activity of the first central relay of the olfactory system, i.e., the olfactory bulb. This structure is one of the few brain areas to continuously replace one of its neuronal populations: the local GABAergic interneurons. How the newly generated neurons integrate into a pre-existing neural network and how basic olfactory functions are maintained when a large percentage of neurons are subjected to continuous renewal, are important questions that have recently received new insights. Furthermore, we shall see how the adult neurogenesis is specifically subjected to experience-dependent modulation. In particular, we shall describe the sensitivity of the bulbar neurogenesis to the activity level of sensory inputs from the olfactory epithelium and, in turn, how this neurogenesis may adjust the neural network functioning to optimize odor information processing. Finally, we shall discuss the behavioral consequences of the bulbar neurogenesis and how it may be appropriate for the sense of smell. By maintaining a constitutive turnover of bulbar interneurons subjected to modulation by environmental cues, we propose that adult ongoing neurogenesis in the olfactory bulb is associated with improved olfactory memory. These recent findings not only provide new fuel for the molecular and cellular bases of sensory perception but should also shed light onto cellular bases of learning and memory.