Respiratory responses to olfactory stimuli in Parkinson's disease

There are many reports of olfactory impairment in patients with Parkinson's disease (PD) and the impairment can be observed before the appearance of typical PD symptom. Accordingly, olfactory screening tests may predict disease onset and indicates a need for early treatment before classic signs of the disease. Olfaction is dependent on inspiration, and activation of olfactory limbic areas are synchronized with the natural breathing cycle in animals and humans. Subconscious changes in respiratory pattern occur in response to odor stimulation. The use of olfactory stimuli to investigate respiratory pattern could be used to assess olfactory perception and serve as an index for olfactory limbic activation. In this study, we tested olfactory acuity in normal subjects and in patients with PD and recorded V(O2) and respiratory variables during pleasant and unpleasant odor presentation. All subjects were able to detect the odorants; however patients with PD were assigned to one of two groups, group that could recognize odors or the group with impaired odor recognition. Respiratory response toward unpleasant and pleasant odor recognition were weak in PD groups who could recognize odors than normal subject as well as emotional response to odor stimuli. PD group with impaired odor recognition showed no respiratory response toward odor stimuli. PD may experience difficulty in feeling positive emotions toward pleasant odors prior to the unpleasant odor because respiratory responses to pleasant odors may also be related to higher processes including intentional control of breathing pattern as a result of olfactory cortex processing and perceptions or emotions.     

The human brain representation of odor identification

Odor identification (OI) tests are increasingly used clinically as biomarkers for Alzheimer's disease and schizophrenia. The aim of this study was to directly compare the neuronal correlates to identified odors vs. nonidentified odors. Seventeen females with normal olfactory function underwent a functional magnetic resonance imaging (fMRI) experiment with postscanning assessment of spontaneous uncued OI. An event-related analysis was performed to compare within-subject activity to spontaneously identified vs. nonidentified odors at the whole brain level, and in anatomic and functional regions of interest (ROIs) in the medial temporal lobe (MTL). Parameter estimate values and blood oxygenated level-dependent (BOLD) signal curves for correctly identified and nonidentified odors were derived from functional ROIs in hippocampus, entorhinal, piriform, and orbitofrontal cortices. Number of activated voxels and max parameter estimate values were obtained from anatomic ROIs in the hippocampus and the entorhinal cortex. At the whole brain level the correct OI gave rise to increased activity in the left entorhinal cortex and secondary olfactory structures, including the orbitofrontal cortex. Increased activation was also observed in fusiform, primary visual, and auditory cortices, inferior frontal plus inferior temporal gyri. The anatomic MTL ROI analysis showed increased activation in the left entorhinal cortex, right hippocampus, and posterior parahippocampal gyri in correct OI. In the entorhinal cortex and hippocampus the BOLD signal increased specifically in response to identified odors and decreased for nonidentified odors. In orbitofrontal and piriform cortices both identified and nonidentified odors gave rise to an increased BOLD signal, but the response to identified odors was significantly greater than that for nonidentified odors. These results support a specific role for entorhinal cortex and hippocampus in OI, whereas piriform and orbitofrontal cortices are active in both smelling and OI. Moreover, episodic as well as semantic memory systems appeared to support OI.     

Hippocampus and olfactory discrimination learning: effects of entorhinal cortex lesions on olfactory learning and memory in a successive-cue, go-no-go task

Three experiments assessed the effect of entorhinal cortex lesions on olfactory learning and memory using a successive-cue olfactory discrimination paradigm. In contrast to the results of other studies that used a simultaneous-cue paradigm, lesions of the entorhinal cortex facilitated rats' acquisition of individual odor discrimination problems, with no impairment in memory for the individual odors across both short (24-hr) and long (65-day) retention intervals and despite limited training. When considered together with previous observations of facilitation or impairment in learning after damage to the hippocampal system, the present data suggest that the hippocampus is preferentially involved in encoding relations among multiple stimuli. By this account, facilitation of performance is due to an interaction between hippocampal system dysfunction and task conditions that hinder direct comparisons among cues.     

Neural correlates of social odor recognition and the representation of individual distinctive social odors within entorhinal cortex and ventral subiculum

Recognition of individual conspecifics is important for social behavior and requires the formation of memories for individually distinctive social signals. Individual recognition is often mediated by olfactory cues in mammals, especially nocturnal rodents such as golden hamsters. In hamsters, this form of recognition requires main olfactory system input to the lateral entorhinal cortex (LEnt). Here, we tested whether neurons in LEnt and the nearby ventral subiculum (VS) would show cellular correlates of this natural form of recognition memory. Two hundred ninety single neurons were recorded from both superficial (SE) and deep layers of LEnt (DE) and VS while male hamsters investigated volatile odorants from female vaginal secretions. Many neurons encoded differences between female's odors with many discriminating between odors from different individual females but not between different odor samples from the same female. Other neurons discriminated between odor samples from one female and generalized across collections from other females. LEnt and VS neurons showed enhanced or suppressed cellular activity during investigation of previously presented odors and in response to novel odors. A majority of SE neurons decreased firing to odor repetition and increased activity to novel odors. In contrast, DE neurons often showed suppressed activity in response to novel odors. Thus, neurons in LEnt and VS of male hamsters encode information that is critical for the identification and recognition of individual females by odor cues. This study reveals cellular mechanisms in LEnt and VS that may mediate a natural form of recognition memory in hamsters. These neuronal responses were similar to those observed in rats and monkeys during performance in standard recognition memory tasks. Consequently, the present data extend our understanding of the cellular basis for recognition memory and suggest that individual recognition requires similar neural mechanisms as those employed in laboratory tests of recognition memory.     

Remembering the past with slow breathing associated with activity in the parahippocampus and amygdala

Breathing plays an important role in perception of odors and the experience of emotions. We used the dipole tracing method to analyze brain areas related to odor-induced autobiographical memory and emotions estimated from averaged electroencephalograms triggered by inspiration onset during odor presentation. Odor stimuli were perfumes subjects named that elicited a specific, pleasant and personal memory as well as two pleasant odors for controls. The perfumes induced specific emotional responses during memory retrieval, arousal level of the memory, feelings of pleasantness and a sense of familiarity with the odor. Respiration measurement indicated that tidal volume increased and respiratory frequency decreased during presentation of perfume stimuli, showing a deep and slow breathing pattern. Throughout the olfactory stimulation, electroencephalograms and respiration were simultaneously recorded. In the averaged potentials, low frequency oscillation was phase-locked to inspiration. Dipole analysis showed that perfumes activated more widespread areas of the right parahippocampal cortex and converged in the right amygdala compared to control odors. Slow breathing synchronized with odor-induced autobiographical memory and emotions may be subconsciously stored in the parahippocampal cortex and amygdala.     

Discrimination of social odors and their locations: role of lateral entorhinal area

Discrimination of individual conspecifics by their odors has been reported for many mammalian species, but little information is available on the brain mechanisms underlying such discrimination. A previous study reported that large parahippocampal lesions, centered on entorhinal cortex but extending into adjacent areas of the brain, eliminated female hamsters' ability to discriminate the flank gland odors of different individuals, as tested with habituation-dishabituation methods. The current study examined the effects of lesions restricted to the lateral entorhinal area on such discriminations. Female hamsters were tested in several types of habituation procedure that differed across a sequence of trials in the locations of familiar and novel social odors. Discrimination of two individuals' odors depended on the sequences of locations of the odors, indicating that odor identity and location were simultaneously salient to female hamsters. Lesions of lateral entorhinal area interfered with this spatial-olfactory discrimination. When confounding spatial cues were eliminated, hamsters did discriminate between novel and familiar odors, and lesions in the entorhinal area did not eliminate this ability. Thus, although the lateral entorhinal area is not necessary for individual odor discrimination, it is involved in processing odor-place combinations.     

Regional differences in the severity of Lewy body pathology across the olfactory cortex

We studied α-synuclein pathology in the rhinencephalon of ten cases of Parkinson's disease (PD) and twelve neurologically normal controls, of which seven had incidental Lewy bodies in the substantia nigra at autopsy and five had no pathological evidence of neurological disease. In all PD and incidental Lewy bodies cases, α-synuclein pathology was found in all five subregions of the primary olfactory cortex that were sampled, and amongst them the pathology was significantly more severe in the temporal division of the piriform cortex than in the frontal division of the piriform cortex, olfactory tubercle or anterior portions of the entorhinal cortex. The orbitofrontal cortex, which is an area of projection from the primary olfactory cortex, was affected in some cases but overall the α-synuclein pathology was less severe in this area than in the primary olfactory cortex. Because different areas of the rhinencephalon are likely to play different roles in olfaction and our data indicate a differential involvement by α-synuclein deposition of structures implicated in smell, future prospective studies investigating the pathophysiological basis of hyposmia in PD should consider to examine the areas of primary olfactory cortex separately.     

Architecture of odor information processing in the olfactory system

Since the discovery of the superfamily of approximately 1000 odorant receptor genes in rodents, the structural simplicity as well as the complexity of the olfactory system have been revealed. The simple aspects include the one neuron-one receptor rule and the exclusive convergence of projections from receptor neurons expressing the same receptors to one or two glomeruli in the olfactory bulb. Odor decoding in the olfactory cortex or higher cortical areas is likely to be a complicated process that depends on the sequence of signal activation and the relative signal intensities of receptors overlapping for similar but different odors. The aim of the present study was to investigate odor information processing both in receptors and in the olfactory cortex. At the receptor level, the similarity and difference in receptor codes between a pair of chiral odorants were examined using the tissue-printing method for sampling all the epithelial zones. In order to dissect odor-driven signal processing in the olfactory cortex by reducing cross-talk with the non-olfactory activities, such as cyclic respiration or other sensory inputs, an in vitro preparation of isolated whole brain with an attached nose was developed, and the methodologies and resulting hypothesis of receptor-sensitivity-dependent hierarchical odor information coding were reviewed.     

Olfactory-evoked regional cerebral blood flow in Alzheimer's disease

Olfaction is impaired in Alzheimer's disease (AD). It was hypothesized that AD would reduce olfactory-evoked perfusion in mesial temporal olfactory (piriform) cortex, where neuropathology begins. Seven AD patients and 8 elderly controls (ECs) underwent olfactory threshold and identification tests and olfactory stimulation during positron emission tomography. Odor identification was impaired in AD, but threshold was not. Olfactory stimulation in ECs activated right and left piriform areas and right anterior ventral temporal cortex. AD patients had less activation in right piriform and anterior ventral temporal cortex but not in the left piriform area. Although orbital cortex did not activate in ECs, there was a significant between-groups difference in this area. Right piriform activation correlated with odor identification. Impaired odor identification likely reflects sensory cortex dysfunction rather than cognitive impairment. Given olfactory bulb projections to the mesial temporal lobe, olfactory stimulation during functional imaging might detect early dysfunction in this region.     

Neuroanatomical correlates of olfactory performance

We investigated associations between olfactory function and gray matter thickness in 46 healthy young subjects by means of an automated technique for measuring cortical thickness. We used an extended version of the Sniffin’ Sticks test to assess olfactory function, including odor threshold, concentration discrimination, quality discrimination, and odor identification. We observed a correlation between olfactory performance and cortical thickness of structures involved in earlier and later stages of chemosensory processing such as right medial orbitofrontal cortex, right insula, and adjacent cortex. Furthermore, we found significant bilateral correlations of olfactory performance with cortical thickness of areas around the central sulcus bilaterally, structures responsible for voluntary respiration and sniffing. In addition to expected general sex effects on cortical thickness, we observed areas, such as the entorhinal cortex, occipital cortex, intraparietal sulcus and insula (all in the right hemisphere), where the correlation between higher order olfactory functions and cortical thickness differed between women and men. These data demonstrate, for some neuroanatomical structures, a link between cortical thickness and olfactory function, in that thicker cortex is usually associated with better performance, but not always. This association between anatomy and olfactory performance suggests a possible biological explanation for the high degree of individual differences and sex effects observed in higher order olfactory tasks.     

Functional mapping of human brain in olfactory processing: a PET study

This study describes the functional anatomy of olfactory and visual naming and matching in humans, using positron emission tomography (PET). One baseline control task without olfactory or visual stimulation, one control task with simple olfactory and visual stimulation without cognition, one set of olfactory and visual naming tasks, and one set of olfactory and visual matching tasks were administered to eight normal volunteers. In the olfactory naming task (ON), odors from familiar items, associated with some verbal label, were to be named. Hence, it required long-term olfactory memory retrieval for stimulus recognition. The olfactory matching task (OM) involved differentiating a recently encoded unfamiliar odor from a sequentially presented group of unfamiliar odors. This required short-term olfactory memory retrieval for stimulus differentiation. The simple olfactory and visual stimulation resulted in activation of the left orbitofrontal region, the right piriform cortex, and the bilateral occipital cortex. During olfactory naming, activation was detected in the left cuneus, the right anterior cingulate gyrus, the left insula, and the cerebellum bilaterally. It appears that the effort to identify the origin of an odor involved semantic analysis and some degree of mental imagery. During olfactory matching, activation was observed in the left cuneus and the cerebellum bilaterally. This identified the brain areas activated during differentiation of one unlabeled odor from the others. In cross-task analysis, the region found to be specific for olfactory naming was the left cuneus. Our results show definite recruitment of the visual cortex in ON and OM tasks, most likely related to imagery component of these tasks. The cerebellar role in cognitive tasks has been recognized, but this is the first PET study that suggests that the human cerebellum may have a role in cognitive olfactory processing as well.     

Estrous odors and sexually conditioned neutral odors activate separate neural pathways in the male rat

Olfactory stimuli play important roles in sexual behavior. Previous studies have demonstrated that both estrous odors and initially neutral odors paired with copulation influence the sexual behavior of male rats. The present study examines the pattern of neural activation as revealed by Fos immunoreactivity (Fos-IR) following exposure to bedding scented with either a neutral odor (almond) paired previously with copulation, estrous odors or no odor. Following exposure to estrous odors Fos-IR increased in the accessory olfactory bulb, medial amygdala, medial bed nucleus of the stria terminalis, medial preoptic area, ventromedial hypothalamus, ventral tegmental area, and both the nucleus accumbens core and shell. Conversely, following exposure to the sexually conditioned odor Fos-IR increased in the piriform cortex, basolateral amygdala, nucleus accumbens core, and the anterior portion of the lateral hypothalamic area. In addition, following exposure to almond odor Fos-IR increased in the main olfactory bulb independent of its pairing with copulation. These patterns of Fos-IR following exposure to estrous or sexually conditioned odors were not influenced by either the addition or omission of the other type of odor. These findings demonstrate that estrous and sexually conditioned odors are processed by distinct neural pathways and converge in the nucleus accumbens core, suggesting that this structure has a unique role in processing sexual stimuli of both pheromonal and olfactory natures.     

Olfactory tests as possible probes for detecting and monitoring Alzheimer's disease

One of the characteristics of Alzheimer's disease is the early loss of neurons in pathways involved in processing olfactory information. Olfactory function was assessed in subjects with Alzheimer's disease using a conventional Smell Identification Test and a simple three odor match-to-sample problem. The patients exhibited a diminished capacity to identify common odors but were severely impaired in their ability to use novel odors in a match-to-sample task. Subjects with Parkinson's disease had a severe deficit for identifying common odors with the majority scoring as anosmic. Multiple sclerosis was not accompanied by detectable changes in olfactory functioning. The results of the Alzheimer's group are similar to recent animal studies that have shown lesions of the piriform-entorhinal cortex produce a variety of memory deficits that are particularly acute in tasks involving novel odors.     

Identification of single dissimilar odors is achieved by humans with a single sniff

The duration that a single odor needs to be sniffed for identification was determined for 18 humans. A hot wire anemometer and an oscilloscope were used to monitor the duration, volume and inhalation rate of sniffs. In Experiment 1 subjects used 1, 3 or 5 natural sniffs, or an unlimited number of natural sniffs to sample seven dissimilar single odors of moderate perceived intensity, and demonstrated that each odor could be identified with a single sniff. In Experiment 2 subjects demonstrated that each of the odors could be identified with the shortest sniff (0.42 sec) they could physically achieve. In Experiment 3 tests with two of the odorants at several concentrations showed that sniff duration influences identification over a narrow range of concentrations that is just above the recognition threshold. These results together with earlier data that described the optimum conditions for the detection of an odor and the perception of odor intensity, provide information that is necessary for the development of a standard olfactometer and standard methods for human olfactory measurements.     

Discrimination among odorants by single neurons of the rat olfactory bulb

1. Intracellular and extracellular recordings were made from rat olfactory bulb mitral and tufted cells during odor stimulation and during electrical stimulation of the olfactory nerve. Neurons were identified by horseradish peroxidase injections and/or antidromic activation. The presentation of multiple concentrations of at least one odorant in a cyclic artificial sniff paradigm, as reported previously (10), allowed the study of odor responses. This approach was extended to multiple odorants to compare their concentration-response profiles. This procedure avoids the problems of interpretation resulting from nonequivalence of the effective concentrations of different odorants used as stimuli that have characterized previous studies of odor quality effects. Comparisons of intracellular events and responses to electrical stimulation with the odor-induced spike train activity allow us to begin to delineate the local circuitry involved in generating odor-induced responses. 2. The concentration-response profiles of the 72 cells in the present study are comparable to those previously reported for output neurons of the olfactory bulb, showing ordered changes in the temporal patterning of spike activity with step changes in odor concentration. However, eight of the neurons exhibited inhibitory responses to lower concentrations, but excitation, at similar latency, to higher concentrations of the same odorant. These data emphasize that to study pattern changes induced by changing odor quality the influence of stimulus intensity must also be carefully examined. The data also provide evidence that the temporal pattern evoked by an odorant is probably not in itself the code for odor quality recognition. 3. Complete concentration-response profiles, including subthreshold concentrations, to more than one odorant show that, although responses to the different odorant can evolve systematically with concentration, the responses to different odorants can evolve through very different patterns. For example, in some cells, the response patterns to different odors were complementary in form. These results demonstrate that the patterned responses of olfactory bulb neurons can reflect changes in odor quality as well as intensity. 4. Intracellular recording was employed to compare the temporal patterning of spikes during odor stimulation with membrane potential changes. In some cases, the spike pattern was closely correlated with apparent postsynaptic potentials. However, there were several clear exceptions. In five cells, a prominent hyperpolarization, seen in the first sniff of a series of 10 consecutive sniffs, was associated with pauses in spike activity. In the following     

Inspiratory phase-locked alpha oscillation in human olfaction: source generators estimated by a dipole tracing method

Olfactory perception and related emotions are largely dependent on inspiration. We acquired simultaneous respiration and electroencephalographic recordings during pleasant odour and unpleasant odour stimulation. We sought to identify changes in respiratory pattern, inspiratory-related potentials and location of dipoles estimated from the potentials. Electroencephalographic recording was triggered by inspiration onset. Respiratory frequency decreased at pleasant odour recognition, and it increased at unpleasant odour detection and recognition. O₂ consumption records showed that these changes were not due to metabolic demand. During olfactory stimulation, inspiratory phase-locked alpha oscillation (I-α) was found in the averaged potential triggered by inspiration onset. I-α was observed at both pleasant odour and unpleasant odour detection and recognition, but it was not seen in the inspiration-triggered potentials of normal air breathing. Electroencephalographic dipole tracing identified the location of dipoles from the I-α in the limbic area and the cortex; the entorhinal cortex, hippocampus, amygdala, premotor area and centroposterior orbitofrontal cortex subserve odour detection, and the rostromedial orbitofrontal cortex subserves odour recognition. We suggest that the I-α in our study originated from the olfactory cortex in the forebrain and was phase-locked to inspiration.     

Odor specificity of habituation in the rat anterior piriform cortex

Exposure to odorants results in a rapid (<10 s) reduction in odor-evoked activity in the rat piriform cortex despite relatively maintained afferent input from olfactory bulb mitral cells. To further understand this form of cortical plasticity, a detailed analysis of its odor specificity was performed. Habituation of odor responses in anterior piriform cortex single units was examined in anesthetized, freely breathing rats. The magnitude of single-unit responses of layer II/III neurons to 2-s odor pulses were examined before and after a 50-s habituating stimulus of either the same or different odor. The results demonstrated that odor habituation was odor specific, with no significant cross-habituation between either markedly different single odors or between odors within a series of straight chain alkanes. Furthermore, habituation to binary 1:1 mixtures produced minimal cross-habituation to the components of that mixture. These latter results may suggest synthetic odor processing in the olfactory system, with novel odor mixtures processed as unique stimuli. Potential mechanisms of odor habituation in the piriform cortex must be able to account for the high degree of specificity of this effect.     

Behavioral and neural correlates of postnatal olfactory conditioning: I. Effect of respiration on conditioned neural responses

Following olfactory classical conditioning, infant rats exhibit a preference for the conditioned odor and exhibit enhanced uptake of focal 14C 2-deoxyglucose (2-DG) within the olfactory bulb. The present experiments assessed the role of respiration on the expression of the enhanced 2-DG uptake response. Pups were conditioned from postnatal day (PN) 1-18 with an olfactory stimulus paired with a reinforcing tactile stimulus which mimics maternal contact (Odor-Stroke). Control pups received odor only or tactile stimulation only. On PN 19, pups received 1 of 3 tests: 1) a two-odor choice test, 2) an odor/2-DG test with normal respiration allowed, or 3) an odor/2-DG test with respiration experimentally controlled. The results indicated that: 1) Odor-Stroke pups learned the conditioned odor preference, 2) Odor-Stroke, normally respiring pups exhibited enhanced olfactory bulb 2-DG uptake when compared to control pups. No difference in respiration rate was detected between groups in normally respiring pups. 3) Odor Stroke pups whose breathing was experimentally controlled exhibited enhanced olfactory bulb 2-DG uptake when compared to control pups with an identical number of respirations. Together, these results demonstrate that modified respiration during testing is not required for the expression of a modified olfactory bulb response to learned attractive odors. Therefore, the data suggest that the olfactory system itself is modified by early learning.     

Inputs from the olfactory bulb and olfactory cortex to the entorhinal cortex in the cat

Summary The spatial organization and laminar distribution of projections from the olfactory bulb and the anterior (PPCa) and posterior (PPCp) divisions of the prepiriform cortex to the entorhinal cortex were studied with anterograde (³H-leucine) and retrograde (WGA-HRP) tracing techniques. After ³H-leucine injections into the olfactory bulb transported labeling was seen over the lateral entorhinal area, except its most medial part, and over the rostral part of the medial entorhinal area. The labeling covers exclusively layer Ia. The lateral and medial entorhinal areas are also reached by fibers from the prepiriform cortex. The projection to the medial entorhinal area has not been described previously. Following injections of ³H-leucine into the PPCa transported labeling is present over the entire expanse of the entorhinal cortex and is located over layer Ib with the greatest density in its superficial part. Injections of ³H-leucine into the PPCp give rise to transported labeling over much of the entorhinal cortex. No labeling was found over the most medial parts of the medial subdivision (VMEA) of the lateral entorhinal area and the medial entorhinal area. Labeling occupies layer Ib, especially its middle part, and layers II and III. Both PPCa and PPCp appear to project most heavily to the dorsal (DLEA) and ventral (VLEA) subdivisions of the lateral entorhinal area. From the retrograde experiments it can be inferred that cells of layers II and III of the PPCa project predominantly to the DLEA, whereas those of the PPCp project predominantly to the VLEA. The MEA receives its heaviest projection from layer II of both PPCa and PPCp. In control experiments with ³H-leucine injections into the endopiriform nucleus it was found that this nucleus projects to the entire expanse of the entorhinal cortex. The fibers distribute to all layers with the exception of layer Ia.Abbreviations AI agranular insular cortex - AL lateral nucleus of the amygdala - BL basolateral nucleus of the amygdala - BM basomedial nucleus of the amygdala - C claustrum - CoA cortical nucleus of the amygdala - DLEA dorsal division of the lateral entorhinal cortex - END endopiriform nucleus - H hippocampus - I granular insular cortex - lot lateral olfactory tractus - MCL mitral cell layer of the olfactory bulb - MEA medial entorhinal area - OB olfactory bulb - PPCa anterior part of the prepiriform nucleus - PPCp posterior part of the prepiriform nucleus - VLEA ventral division of the lateral entorhinal cortex - VMEA ventromedial division of the lateral entorhinal cortex - 35 area 35 of the perirhinal cortex - 36 area 36 of the perirhinal cortex