Physostigmine enhances performance on an odor mixture discrimination test

Cholinergic and behavior-dependent beta and gamma waves are coupled between olfactory bulb and hippocampus

Olfactory oscillations may enhance cognitive processing through coupling with beta (β, 15-30 Hz) and gamma (γ, 30-160 Hz) activity in the hippocampus (HPC). We hypothesize that coupling between olfactory bulb (OB) and HPC oscillations is increased by cholinergic activation in control rats and is reduced in kainic-acid-treated epileptic rats, a model of temporal lobe epilepsy. OB γ2 (63-100 Hz) power was higher during walking and immobility-awake (IMM) compared to sleep, while γ1 (30-57 Hz) power was higher during grooming than other behavioral states. Muscarinic cholinergic agonist pilocarpine (25 mg/kg ip) with peripheral muscarinic blockade increased OB power and OB-HPC coherence at β and γ1 frequency bands. A similar effect was found after physostigmine (0.5 mg/kg ip) but not scopolamine (10 mg/kg ip). Pilocarpine increased bicoherence and cross-frequency coherence (CFC) between OB slow waves (SW, 1-5 Hz) and hippocampal β, γ1 and γ2 waves, with stronger coherence at CA1 alveus and CA3c than CA1 stratum radiatum. Bicoherence further revealed a nonlinear interaction of β waves in OB with β waves at the CA1-alveus. Beta and γ1 waves in OB or HPC were segregated at one phase of the OB-SW, opposite to the phase of γ2 and γ3 (100-160 Hz) waves, suggesting independent temporal processing of β/γ1 versus γ2/γ3 waves. At CA1 radiatum, kainic-acid-treated epileptic rats compared to control rats showed decreased theta power, theta-β and theta-γ2 CFC during baseline walking, decreased CFC of HPC SW with γ2 and γ3 waves during baseline IMM, and decreased coupling of OB SW with β and γ2 waves at CA1 alveus after pilocarpine. It is concluded that β and γ waves in the OB and HPC are modulated by a slow respiratory rhythm, in a cholinergic and behavior-dependent manner, and OB-HPC functional connectivity at β and γ frequencies may enhance cognitive functions.

Olfactory function and discrimination ability in the elderly: a pilot study

We recently reported that subjects with a higher olfactory identification threshold for rose odor declined more in attentional ability in the elderly. This study focuses on discrimination ability and olfactory identification threshold in twelve elderly subjects living in a community (age: 80.9 ± 1.6). Olfactory function was assessed by the rose odor identification threshold. We assessed the discrimination ability by distinguishing 5 similar odor pairs. Our results showed that the subjects with a higher olfactory identification threshold (≥ 5) declined more in discrimination ability (14% ± 14%, p = 0.03) compared to those with a lower threshold (≤ 4) (averaged value set at 100%). As discrimination ability is related to the basal forebrain cholinergic system, our results suggest that olfactory impairment links to the decline in cognitive function relating the cholinergic system.

Target-specific control of olfactory bulb periglomerular cells by GABAergic and cholinergic basal forebrain inputs

The olfactory bulb (OB), the first relay for odor processing in the brain, receives dense GABAergic and cholinergic long-range projections from basal forebrain (BF) nuclei that provide information about the internal state and behavioral context of the animal. However, the targets, impact, and dynamic of these afferents are still unclear. How BF synaptic inputs modulate activity in diverse subtypes of periglomerular (PG) interneurons using optogenetic stimulation and loose cell-attached or whole-cell patch-clamp recording in OB slices from adult mice were studied in this article. GABAergic BF inputs potently blocked PG cells firing except in a minority of calretinin-expressing cells in which GABA release elicited spiking. Parallel cholinergic projections excited a previously overlooked PG cell subtype via synaptic activation of M1 muscarinic receptors. Low-frequency stimulation of the cholinergic axons drove persistent firing in these PG cells, thereby increasing tonic inhibition in principal neurons. Taken together, these findings suggest that modality-specific BF inputs can orchestrate synaptic inhibition in OB glomeruli using multiple, potentially independent, inhibitory or excitatory target-specific pathways.

Olfaction, cholinergic basal forebrain degeneration, and cognition in early Parkinson disease

INTRODUCTION

Impaired olfaction and reduced cholinergic nucleus 4 (Ch4) volume both predict greater cognitive decline in Parkinson's disease (PD). We examined the relationship between olfaction, longitudinal change in cholinergic basal forebrain nuclei and their target regions, and cognition in early PD.

METHODS

We analyzed a cohort of 97 PD participants from the Parkinson's Progression Markers Initiative with brain MRIs at baseline, 1 year, 2 years, and 4 years. Using probabilistic maps, regional grey matter density (GMD) was calculated for Ch4, cholinergic nuclei 1, 2, and 3 (Ch123), and their target regions.

RESULTS

Baseline University of Pennsylvania Smell Identification Test score correlated with change in GMD of all regions of interest (all p < 0.05). Rate of change of Ch4 GMD was correlated with rate of change of Ch123 (p = 0.034), cortex (p = 0.001), and amygdala GMD (p < 0.001), but not hippocampus GMD (p = 0.38). Rate of change of Ch123 GMD was correlated with rate of change of cortex (p = 0.001) and hippocampus (p < 0.001), but not amygdala GMD (p = 0.133). In a linear regression model including change in GMD of all regions of interest and age as predictors, change in cortex GMD (βˆ_slope= 38.2; 95 % CI: [0.47, 75.9]) and change in hippocampus GMD (βˆ_slope= 24.8; 95 % CI: [0.80, 48.8]) were significant predictors of Montreal Cognitive Assessment score change over time.

CONCLUSION

Impaired olfaction is associated with degeneration of the cholinergic basal forebrain and bilateral cortex, amygdala, and hippocampus in PD. The relationship between impaired olfaction and cognitive decline may be mediated by greater atrophy of the cortex and hippocampus.

Modulation of Neural Microcircuits That Control Complex Dynamics in Olfactory Networks

Neuromodulation influences neuronal processing, conferring neuronal circuits the flexibility to integrate sensory inputs with behavioral states and the ability to adapt to a continuously changing environment. In this original research report, we broadly discuss the basis of neuromodulation that is known to regulate intrinsic firing activity, synaptic communication, and voltage-dependent channels in the olfactory bulb. Because the olfactory system is positioned to integrate sensory inputs with information regarding the internal chemical and behavioral state of an animal, how olfactory information is modulated provides flexibility in coding and behavioral output. Herein we discuss how neuronal microcircuits control complex dynamics of the olfactory networks by homing in on a special class of local interneurons as an example. While receptors for neuromodulation and metabolic peptides are widely expressed in the olfactory circuitry, centrifugal serotonergic and cholinergic inputs modulate glomerular activity and are involved in odor investigation and odor-dependent learning. Little is known about how metabolic peptides and neuromodulators control specific neuronal subpopulations. There is a microcircuit between mitral cells and interneurons that is comprised of deep-short-axon cells in the granule cell layer. These local interneurons express pre-pro-glucagon (PPG) and regulate mitral cell activity, but it is unknown what initiates this type of regulation. Our study investigates the means by which PPG neurons could be recruited by classical neuromodulators and hormonal peptides. We found that two gut hormones, leptin and cholecystokinin, differentially modulate PPG neurons. Cholecystokinin reduces or increases spike frequency, suggesting a heterogeneous signaling pathway in different PPG neurons, while leptin does not affect PPG neuronal firing. Acetylcholine modulates PPG neurons by increasing the spike frequency and eliciting bursts of action potentials, while serotonin does not affect PPG neuron excitability. The mechanisms behind this diverse modulation are not known, however, these results clearly indicate a complex interplay of metabolic signaling molecules and neuromodulators that may fine-tune neuronal microcircuits.

Molecular and Genetic Factors Involved in Olfactory and Gustatory Deficits and Associations with Microbiota in Parkinson's Disease

Deficits in olfaction and taste are among the most frequent non-motor manifestations in Parkinson’s disease (PD) that start very early and frequently precede the PD motor symptoms. The limited data available suggest that the basis of the olfactory and gustatory dysfunction related to PD are likely multifactorial and may include the same determinants responsible for other non-motor symptoms of PD. This review describes the most relevant molecular and genetic factors involved in the PD-related smell and taste impairments, and their associations with the microbiota, which also may represent risk factors associated with the disease.

Extrinsic neuromodulation in the rodent olfactory bulb

Evolutionarily, olfaction is one of the oldest senses and pivotal for an individual's health and survival. The olfactory bulb (OB), as the first olfactory relay station in the brain, is known to heavily process sensory information. To adapt to an animal's needs, OB activity can be influenced by many factors either from within (intrinsic neuromodulation) or outside (extrinsic neuromodulation) the OB which include neurotransmitters, neuromodulators, hormones, and neuropeptides. Extrinsic sources seem to be of special importance as the OB receives massive efferent input from numerous brain centers even outweighing the sensory input from the nose. Here, we review neuromodulatory processes in the rodent OB from such extrinsic sources. We will discuss extrinsic neuromodulation according to points of origin, receptors involved, affected circuits, and changes in behavior. In the end, we give a brief outlook on potential future directions in research on neuromodulation in the OB.

Expression Profiling of Cytokine, Cholinergic Markers, and Amyloid-β Deposition in the APPSWE/PS1dE9 Mouse Model of Alzheimer's Disease Pathology

Background:

Alzheimer’s disease (AD), a neurodegenerative disease, is associated with dysfunction of the olfactory and the entorhinal cortex of the brain that control memory and cognitive functions and other daily activities. Pro-inflammatory cytokines, amyloid-β (Aβ), and the cholinergic system play vital roles in the pathophysiology of AD. However, the role of changes in cholinergic system components, Aβ accumulation, and cytokines in both the olfactory and entorhinal cortex is not known clearly.

Objective:

The present study is aimed to evaluate the changes of cholinergic system components, Aβ accumulation, and cytokines in both the olfactory bulb (OB) and entorhinal cortex (EC) of young and aged APP_SWE/PS1dE9 transgenic (Tg) mice.

Methods:

We have explored the changes of cholinergic system components, Aβ accumulation, and expression profiling of cytokines in the OB and EC of aged APPswe transgenic mice and age-matched wild type mice using quantitative Real-Time PCR assays and immunohistochemistry techniques.

Results:

In aged Tg mice, a significant increase of expression of interleukin (IL)-1β, tumor necrosis factor (TNF)-α, and chemokine MCP1 (p < 0.001, p < 0.001, and p = 0.001, respectively) and a significant reduction of nAChRα4 (p = 0.048) and AChE (p = 0.023) was observed when compared with age-matched wild type mice. Higher levels of AChE and BuChE are expressed in OB and EC of the APP_SWE/PS1dE9 of Tg mice. Aβ accumulation was observed in OB and EC of the APP_SWE/PS1dE9 of Tg mice.

Conclusion:

The study demonstrates the expression profiling of pro-inflammatory cytokines and cholinergic markers as well as Aβ accumulation in OB and EC of the APP_SWE/PS1dE9 Tg mice. Moreover, the study also demonstrated that the APP_SWE/PS1dE9 Tg mice can be useful as a mouse model to understand the role of pro-inflammatory cytokines and cholinergic markers in pathophysiology of AD.

Aversive learning-induced plasticity throughout the adult mammalian olfactory system: insights across development

Experiences, such as sensory learning, are known to induce plasticity in mammalian sensory systems. In recent years aversive olfactory learning-induced plasticity has been identified at all stages of the adult olfactory pathway; however, the underlying mechanisms have yet to be identified. Much of the work regarding mechanisms of olfactory associative learning comes from neonates, a time point before which the brain or olfactory system is fully developed. In addition, pups and adults often express different behavioral outcomes when subjected to the same olfactory aversive conditioning paradigm, making it difficult to directly attribute pup mechanisms of plasticity to adults. Despite the differences, there is evidence of similarities between pups and adults in terms of learning-induced changes in the olfactory system, suggesting at least some conserved mechanisms. Identifying these conserved mechanisms of plasticity would dramatically increase our understanding of how the brain is able to alter encoding and consolidation of salient olfactory information even at the earliest stages following aversive learning. The focus of this review is to systematically examine literature regarding olfactory associative learning across developmental stages and search for similarities in order to build testable hypotheses that will inform future studies of aversive learning-induced sensory plasticity in adults.

Olfactory Dysfunction in Neurodegenerative Diseases

PURPOSE OF REVIEW

The sense of smell is today one of the focuses of interest in aging and neurodegenerative disease research. In several neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease, the olfactory dysfunction is one of the initial symptoms appearing years before motor symptoms and cognitive decline, being considered a clinical marker of these diseases' early stages and a marker of disease progression and cognitive decline. Overall and under the umbrella of precision medicine, attention to olfactory function may help to improve chances of success for neuroprotective and disease-modifying therapeutic strategies.

RECENT FINDINGS

The use of olfaction, as clinical marker for neurodegenerative diseases is helpful in the characterization of prodromal stages of these diseases, early diagnostic strategies, differential diagnosis, and potentially prediction of treatment success. Understanding the mechanisms underlying olfactory dysfunction is central to determine its association with neurodegenerative disorders. Several anatomical systems and environmental factors may underlie or contribute to olfactory loss associated with neurological diseases, although the direct biological link to each disorder remains unclear and, thus, requires further investigation. In this review, we describe the neurobiology of olfaction, and the most common olfactory function measurements in neurodegenerative diseases. We also highlight the evidence for the presence of olfactory dysfunction in several neurodegenerative diseases, its value as a clinical marker for early stages of the diseases when combined with other clinical, biological, and neuroimage markers, and its role as a useful symptom for the differential diagnosis and follow-up of disease. The neuropathological correlations and the changes in neurotransmitter systems related with olfactory dysfunction in the neurodegenerative diseases are also described.

Impaired cholinergic transmission in patients with Parkinson's disease and olfactory dysfunction

Olfactory dysfunction represents a frequent and disturbing non-motor manifestation of Parkinson's disease (PD). The pathophysiology of olfactory dysfunction in PD is still poorly understood. Experimental evidence suggests that olfactory impairment could be related to central cholinergic dysfunction. Short latency afferent inhibition (SAI) technique gives the opportunity to test an inhibitory cholinergic circuit in the human cerebral motor cortex. The objective of the study was to assess the cholinergic function, as measured by SAI, in PD patients with different degrees of olfactory dysfunction. We applied SAI technique in 31 patients with PD. These patients also underwent Olfactory Event-Related Potentials (OERPs) studies to objectively evaluate the olfactory system and a battery of neuropsychological tests to assess the cognitive functions. Absent OERPs indicated a severe olfactory dysfunction in 13 subjects. The presence of OERPs with an alteration in latency and/or amplitude can be considered as a borderline condition of slight alteration of smell and was found in other 15 patients. Only 3 patients showed normal OERPs. SAI was significantly reduced in the PD patients with absent OERPs compared with those with present but abnormal OERPs. Neuropsychological examination showed a mild cognitive impairment in 12 out of 13 PD patients with severe olfactory dysfunction, and in 3 out of the 15 patients with borderline olfactory dysfunction. SAI abnormalities and presence of severe olfactory impairment strongly support the hypothesis of cholinergic dysfunction in some patients with PD, who will probably develop a dementia. Longitudinal studies are required to verify whether SAI abnormalities in PD patients with olfactory dysfunction can predict a future severe cognitive decline.

Mice Lacking M1 and M3 Muscarinic Acetylcholine Receptors Have Impaired Odor Discrimination and Learning

The cholinergic system has extensive projections to the olfactory bulb (OB) where it produces a state-dependent regulation of sensory gating. Previous work has shown a prominent role of muscarinic acetylcholine (ACh) receptors (mAChRs) in regulating the excitability of OB neurons, in particular the M1 receptor. Here, we examined the contribution of M1 and M3 mAChR subtypes to olfactory processing using mice with a genetic deletion of these receptors, the M1^−/− and the M1/M3^−/− knockout (KO) mice. Genetic ablation of the M1 and M3 mAChRs resulted in a significant deficit in odor discrimination of closely related molecules, including stereoisomers. However, the discrimination of dissimilar molecules, social odors (e.g., urine) and novel object recognition was not affected. In addition the KO mice showed impaired learning in an associative odor-learning task, learning to discriminate odors at a slower rate, indicating that both short and long-term memory is disrupted by mAChR dysfunction. Interestingly, the KO mice exhibited decreased olfactory neurogenesis at younger ages, a deficit that was not maintained in older animals. In older animals, the olfactory deficit could be restored by increasing the number of new born neurons integrated into the OB after exposing them to an olfactory enriched environment, suggesting that muscarinic modulation and adult neurogenesis could be two different mechanism used by the olfactory system to improve olfactory processing.

Increased olfactory bulb acetylcholine bi-directionally modulates glomerular odor sensitivity

The glomerular layer of the olfactory bulb (OB) receives heavy cholinergic input from the horizontal limb of the diagonal band of Broca (HDB) and expresses both muscarinic and nicotinic acetylcholine (ACh) receptors. However, the effects of ACh on OB glomerular odor responses remain unknown. Using calcium imaging in transgenic mice expressing the calcium indicator GCaMP2 in the mitral/tufted cells, we investigated the effect of ACh on the glomerular responses to increasing odor concentrations. Using HDB electrical stimulation and in vivo pharmacology, we find that increased OB ACh leads to dynamic, activity-dependent bi-directional modulation of glomerular odor response due to the combinatorial effects of both muscarinic and nicotinic activation. Using pharmacological manipulation to reveal the individual receptor type contributions, we find that m2 muscarinic receptor activation increases glomerular sensitivity to weak odor input whereas nicotinic receptor activation decreases sensitivity to strong input. Overall, we found that ACh in the OB increases glomerular sensitivity to odors and decreases activation thresholds. This effect, along with the decreased responses to strong odor input, reduces the response intensity range of individual glomeruli to increasing concentration making them more similar across the entire concentration range. As a result, odor representations are more similar as concentration increases.

Differential Muscarinic Modulation in the Olfactory Bulb

Neuromodulation of olfactory circuits by acetylcholine (ACh) plays an important role in odor discrimination and learning. Early processing of chemosensory signals occurs in two functionally and anatomically distinct regions, the main and accessory olfactory bulbs (MOB and AOB), which receive extensive cholinergic input from the basal forebrain. Here, we explore the regulation of AOB and MOB circuits by ACh, and how cholinergic modulation influences olfactory-mediated behaviors in mice. Surprisingly, despite the presence of a conserved circuit, activation of muscarinic ACh receptors revealed marked differences in cholinergic modulation of output neurons: excitation in the AOB and inhibition in the MOB. Granule cells (GCs), the most abundant intrinsic neuron in the OB, also exhibited a complex muscarinic response. While GCs in the AOB were excited, MOB GCs exhibited a dual muscarinic action in the form of a hyperpolarization and an increase in excitability uncovered by cell depolarization. Furthermore, ACh influenced the input-output relationship of mitral cells in the AOB and MOB differently showing a net effect on gain in mitral cells of the MOB, but not in the AOB. Interestingly, despite the striking differences in neuromodulatory actions on output neurons, chemogenetic inhibition of cholinergic neurons produced similar perturbations in olfactory behaviors mediated by these two regions. Decreasing ACh in the OB disrupted the natural discrimination of molecularly related odors and the natural investigation of odors associated with social behaviors. Thus, the distinct neuromodulation by ACh in these circuits could underlie different solutions to the processing of general odors and semiochemicals, and the diverse olfactory behaviors they trigger.

SIGNIFICANCE STATEMENT

State-dependent cholinergic modulation of brain circuits is critical for several high-level cognitive functions, including attention and memory. Here, we provide new evidence that cholinergic modulation differentially regulates two parallel circuits that process chemosensory information, the accessory and main olfactory bulb (AOB and MOB, respectively). These circuits consist of remarkably similar synaptic arrangement and neuronal types, yet cholinergic regulation produced strikingly opposing effects in output and intrinsic neurons. Despite these differences, the chemogenetic reduction of cholinergic activity in freely behaving animals disrupted odor discrimination of simple odors, and the investigation of social odors associated with behaviors signaled by the Vomeronasal system.

Muscarinic receptors modulate dendrodendritic inhibitory synapses to sculpt glomerular output

Cholinergic [acetylcholine (ACh)] axons from the basal forebrain innervate olfactory bulb glomeruli, the initial site of synaptic integration in the olfactory system. Both nicotinic acetylcholine receptors (nAChRs) and muscarinic acetylcholine receptors (mAChRs) are expressed in glomeruli. The activation of nAChRs directly excites both mitral/tufted cells (MTCs) and external tufted cells (ETCs), the two major excitatory neurons that transmit glomerular output. The functional roles of mAChRs in glomerular circuits are unknown. We show that the restricted glomerular application of ACh causes rapid, brief nAChR-mediated excitation of both MTCs and ETCs in the mouse olfactory bulb. This excitation is followed by mAChR-mediated inhibition, which is blocked by GABAA receptor antagonists, indicating the engagement of periglomerular cells (PGCs) and/or short axon cells (SACs), the two major glomerular inhibitory neurons. Indeed, selective activation of glomerular mAChRs, with ionotropic GluRs and nAChRs blocked, increased IPSCs in MTCs and ETCs, indicating that mAChRs recruit glomerular inhibitory circuits. Selective activation of glomerular mAChRs in the presence of tetrodotoxin increased IPSCs in all glomerular neurons, indicating action potential-independent enhancement of GABA release from PGC and/or SAC dendrodendritic synapses. mAChR-mediated enhancement of GABA release also presynaptically suppressed the first synapse of the olfactory system via GABAB receptors on sensory terminals. Together, these results indicate that cholinergic modulation of glomerular circuits is biphasic, involving an initial excitation of MTC/ETCs mediated by nAChRs followed by inhibition mediated directly by mAChRs on PGCs/SACs. This may phasically enhance the sensitivity of glomerular outputs to odorants, an action that is consistent with recent in vivo findings.

Olfactory dysfunction in Parkinson's disease: Positive effect of cigarette smoking

BACKGROUND

There is compelling evidence from over 60 epidemiological studies that smoking significantly reduces the risk of Parkinson's disease (PD). In general, those who currently smoke cigarettes, as well as those with a past history of such smoking, have a reduced risk of PD compared to those who have never smoked. Recently it has been suggested that a cardinal nonmotor sensory symptom of PD, olfactory dysfunction, may be less severe in PD patients who smoke than in PD patients who do not, in contrast to the negative effect of smoking on olfaction described in the general population.

METHODS

We evaluated University of Pennsylvania Smell Identification Test (UPSIT) scores from 323 PD patients and 323 controls closely matched individually on age, sex, and smoking history (never, past, or current).

RESULTS

Patients exhibited much lower UPSIT scores than did the controls (P < 0.0001). The relative decline in dysfunction of the current PD smokers was less than that of the never- and past-PD smokers (respective Ps = 0.0005 and 0.0019). Female PD patients outperformed their male counterparts by a larger margin than did the female controls (3.66 vs. 1.07 UPSIT points; respective Ps < 0.0001 and 0.06). Age-related declines in UPSIT scores were generally present (P < 0.0001). No association between the olfactory measure and smoking dose, as indexed by pack-years, was evident.

CONCLUSIONS

PD patients who currently smoke do not exhibit the smoking-related decline in olfaction observed in non-PD control subjects who currently smoke. The physiological basis of this phenomenon is yet to be defined.

Olfactory dysfunction and neurotransmitter disturbance in olfactory bulb of transgenic mice expressing human A53T mutant α-synuclein

Parkinson disease is a multi-system neurodegenerative disease characterized by both motor and non-motor symptoms. Hyposmia is one of the early non-motor symptoms occurring in more than 90% of Parkinson disease cases, which can precede motor symptoms even several years. Up to now, the relationship between hyposmia and Parkinson disease remains elusive. Lack of proper animal models of hyposmia restricts the investigation. In this study we assessed olfactory function in Prp-A53T-α-synuclein transgenic (αSynA53T) mice which had been reported to show age-dependent motor impairments and intracytoplasmic inclusions. We also examined cholinergic and dopaminergic systems in olfactory bulb of αSynA53T mice by immunofluorescent staining, enzyme linked immunosorbent assay and western blot. We found that compared to wild type littermates, αSynA53T mice at 6 months or older displayed a deficit of odor discrimination and odor detection. No significant changes were found in olfactory memory and odor habituation. Furthermore compared to wildtype littermates, in olfactory bulb of αSynA53T mice at 10 months old we detected a marked decrease of cholinergic neurons in mitral cell layer and a decrease of acetylcholinesterase activity, while dopaminergic neurons were found increased in glomerular layer, accompanied with an increase of tyrosine hydroxylase protein. Our studies indicate that αSynA53T mice have olfactory dysfunction before motor deficits occur, and the cholinergic and dopaminergic disturbance might be responsible for the Parkinson disease-related olfactory dysfunction.

Cholinergic inputs from Basal forebrain add an excitatory bias to odor coding in the olfactory bulb

Cholinergic modulation of central circuits is associated with active sensation, attention, and learning, yet the neural circuits and temporal dynamics underlying cholinergic effects on sensory processing remain unclear. Understanding the effects of cholinergic modulation on particular circuits is complicated by the widespread projections of cholinergic neurons to telencephalic structures that themselves are highly interconnected. Here we examined how cholinergic projections from basal forebrain to the olfactory bulb (OB) modulate output from the first stage of sensory processing in the mouse olfactory system. By optogenetically activating their axons directly in the OB, we found that cholinergic projections from basal forebrain regulate OB output by increasing the spike output of presumptive mitral/tufted cells. Cholinergic stimulation increased mitral/tufted cell spiking in the absence of inhalation-driven sensory input and further increased spiking responses to inhalation of odorless air and to odorants. This modulation was rapid and transient, was dependent on local cholinergic signaling in the OB, and differed from modulation by optogenetic activation of cholinergic neurons in basal forebrain, which led to a mixture of mitral/tufted cell excitation and suppression. Finally, bulbar cholinergic enhancement of mitral/tufted cell odorant responses was robust and occurred independent of the strength or even polarity of the odorant-evoked response, indicating that cholinergic modulation adds an excitatory bias to mitral/tufted cells as opposed to increasing response gain or sharpening response spectra. These results are consistent with a role for the basal forebrain cholinergic system in dynamically regulating the sensitivity to or salience of odors during active sensing of the olfactory environment.

A quantitative comparison of the efferent projections of the anterior and posterior subdivisions of the medial amygdala in female mice

In rodents, many aspects of sociosexual behavior are mediated by chemosignals released by opposite-sex conspecifics. These chemosignals are relayed via the main (MOS) and accessory olfactory systems (AOS) to the medial amygdala (Me). The Me is subdivided into anterior (MeA) and posterior (MeP) subnuclei, and lesions targeting these regions have different effects on proceptive courtship behaviors in female mice. Differential behavioral effects of MeA vs. MeP lesions could reflect a difference in the projections of neurons located in these Me subnuclei. To examine this question, we injected female mice with the anterograde tracer, Fluoro-Ruby into either the MeA or MeP and quantified labeled puncta in 11 forebrain target sites implicated in courtship behaviors using confocal fluorescence microscopy. We found that the MeP more densely innervates the medial and intermediate regions of the posterior bed nucleus of the stria terminalis (pBNST) and the posteromedial cortical amygdala (PMCo), while the MeA more densely innervates the horizontal diagonal band of Broca (HDB) and the medial olfactory tubercle (mOT), a region that may be a component of the circuitry responsible for olfactory-mediated motivated behaviors.

Dysfunctional chemosensation in myasthenia gravis: a systematic review

OBJECTIVES

Myasthenia gravis has traditionally been viewed as a disorder that solely affects the neuromuscular junction within the peripheral nervous system. However, there is now evidence that the cholinergic dysfunction of this disorder may be more widespread than previously believed. This article provides a systematic review of the studies that examined smell and taste function in myasthenia gravis.

METHODS

We analyzed studies that reported chemosensory function alterations in patients with myasthenia gravis. PubMed, MEDLINE, Web of Science, EMBASE, and SciELO, searched to identify articles published from January 1950 through December 2012, were supplemented by relevant articles. The following information was identified from each article: the number of patients, number of controls (if any), clinical stage of patients, neurological involvement, serological state, taste or smell involvement, chemosensory test used, and country of publication.

RESULTS

Ten studies reporting smell and taste function and dysfunction in patients with myasthenia gravis were identified, most of which were case reports commenting on apparent abnormalities in the taste system. The sole empirical study that investigated taste function, however, was negative, suggesting that some reports of taste loss may reflect olfactory loss. One study clearly documented olfactory dysfunction in patients with myasthenia gravis, dysfunction most likely attributable to altered central nervous system cholinergic function.

CONCLUSIONS

Chemosensory dysfunction has been reported in a number of patients with myasthenia gravis. Given the close association between complaints of taste dysfunction and loss of flavor sensations secondary to olfactory system damage, quantitative testing should be used to accurately assess the nature and degree of the dysfunction present in this debilitating disorder.

A two-layer biophysical model of cholinergic neuromodulation in olfactory bulb

Cholinergic inputs from the basal forebrain regulate multiple olfactory bulb (OB) functions, including odor discrimination, perceptual learning, and short-term memory. Previous studies have shown that nicotinic cholinergic receptor activation sharpens mitral cell chemoreceptive fields, likely via intraglomerular circuitry. Muscarinic cholinergic activation is less well understood, though muscarinic receptors are implicated in olfactory learning and in the regulation of synchronized oscillatory dynamics in hippocampus and cortex. To understand the mechanisms underlying cholinergic neuromodulation in OB, we developed a biophysical model of the OB neuronal network including both glomerular layer and external plexiform layer (EPL) computations and incorporating both nicotinic and muscarinic neuromodulatory effects. Our simulations show how nicotinic activation within glomerular circuits sharpens mitral cell chemoreceptive fields, even in the absence of EPL circuitry, but does not facilitate intrinsic oscillations or spike synchronization. In contrast, muscarinic receptor activation increases mitral cell spike synchronization and field oscillatory power by potentiating granule cell excitability and lateral inhibitory interactions within the EPL, but it has little effect on mitral cell firing rates and hence does not sharpen olfactory representations under a rate metric. These results are consistent with the theory that EPL interactions regulate the timing, rather than the existence, of mitral cell action potentials and perform their computations with respect to a spike timing-based metric. This general model suggests that the roles of nicotinic and muscarinic receptors in olfactory bulb are both distinct and complementary to one another, together regulating the effects of ascending cholinergic inputs on olfactory bulb transformations.

Optogenetic activation of basal forebrain cholinergic neurons modulates neuronal excitability and sensory responses in the main olfactory bulb

The main olfactory bulb (MOB) in mammals receives massive centrifugal input from cholinergic neurons in the horizontal limb of the diagonal band of Broca (HDB) in the basal forebrain, the activity of which is thought to be correlated with animal behaving states, such as attention. Cholinergic signals in the bulb facilitate olfactory discrimination and learning, but it has remained controversial how the activity of HDB cholinergic neurons modulates neuronal excitability and olfactory responses in the MOB. In this study, we used an optogenetic approach to selectively activate HDB cholinergic neurons and recorded the effect of this activation on the spontaneous firing activity and odor-evoked responses of mouse MOB neurons. Cells were juxtacellularly labeled and their specific types were morphologically determined. We find that light stimulation of HDB cholinergic neurons inhibits the spontaneous firing activity of all major cell types, including mitral/tufted (M/T) cells, periglomerular (PG) cells, and GABAergic granule cells. Inhibitions are significantly produced by stimulation at 10 Hz and further enhanced at higher frequencies. In addition, cholinergic activation sharpens the olfactory tuning curves of a majority of M/T cells but broadly potentiates odor-evoked responses of PG cells and granule cells. These results demonstrate strong effects of the basal forebrain cholinergic system on modulating neuronal excitability in the MOB and support the hypothesis that cholinergic activity increases olfactory discrimination capability.

Profound olfactory dysfunction in myasthenia gravis

In this study we demonstrate that myasthenia gravis, an autoimmune disease strongly identified with deficient acetylcholine receptor transmission at the post-synaptic neuromuscular junction, is accompanied by a profound loss of olfactory function. Twenty-seven MG patients, 27 matched healthy controls, and 11 patients with polymiositis, a disease with peripheral neuromuscular symptoms analogous to myasthenia gravis with no known central nervous system involvement, were tested. All were administered the University of Pennsylvania Smell Identification Test (UPSIT) and the Picture Identification Test (PIT), a test analogous in content and form to the UPSIT designed to control for non-olfactory cognitive confounds. The UPSIT scores of the myasthenia gravis patients were markedly lower than those of the age- and sex-matched normal controls [respective means (SDs) = 20.15 (6.40) & 35.67 (4.95); p<0.0001], as well as those of the polymiositis patients who scored slightly below the normal range [33.30 (1.42); p<0.0001]. The latter finding, along with direct monitoring of the inhalation of the patients during testing, implies that the MG-related olfactory deficit is unlikely due to difficulties sniffing, per se. All PIT scores were within or near the normal range, although subtle deficits were apparent in both the MG and PM patients, conceivably reflecting influences of mild cognitive impairment. No relationships between performance on the UPSIT and thymectomy, time since diagnosis, type of treatment regimen, or the presence or absence of serum anti-nicotinic or muscarinic antibodies were apparent. Our findings suggest that MG influences olfactory function to the same degree as observed in a number of neurodegenerative diseases in which central nervous system cholinergic dysfunction has been documented.

Olfaction in Parkinson's disease and related disorders

Olfactory dysfunction is an early 'pre-clinical' sign of Parkinson's disease (PD). The present review is a comprehensive and up-to-date assessment of such dysfunction in PD and related disorders. The olfactory bulb is implicated in the dysfunction, since only those syndromes with olfactory bulb pathology exhibit significant smell loss. The role of dopamine in the production of olfactory system pathology is enigmatic, as overexpression of dopaminergic cells within the bulb's glomerular layer is a common feature of PD and most animal models of PD. Damage to cholinergic, serotonergic, and noradrenergic systems is likely involved, since such damage is most marked in those diseases with the most smell loss. When compromised, these systems, which regulate microglial activity, can influence the induction of localized brain inflammation, oxidative damage, and cytosolic disruption of cellular processes. In monogenetic forms of PD, olfactory dysfunction is rarely observed in asymptomatic gene carriers, but is present in many of those that exhibit the motor phenotype. This suggests that such gene-related influences on olfaction, when present, take time to develop and depend upon additional factors, such as those from aging, other genes, formation of α-synuclein- and tau-related pathology, or lowered thresholds to oxidative stress from toxic insults. The limited data available suggest that the physiological determinants of the early changes in PD-related olfactory function are likely multifactorial and may include the same determinants as those responsible for a number of other non-motor symptoms of PD, such as dysautonomia and sleep disturbances.

Diverse populations of intrinsic cholinergic interneurons in the mouse olfactory bulb

Cholinergic activities affect olfactory bulb (OB) information processing and associated learning and memory. However, the presence of intrinsic cholinergic interneurons in the OB remains controversial. As a result, morphological and functional properties of these cells are largely undetermined. We characterized cholinergic interneurons using transgenic mice that selectively mark choline acetyltransferase (ChAT)-expressing cells and immunolabeling. We found a significant number of intrinsic cholinergic interneurons in the OB. These interneurons reside primarily in the glomerular layer (GL) and external plexiform layer (EPL) and exhibit diverse distribution patterns of nerve processes, indicating functional heterogeneity. Further, we found these neurons express ChAT and vesicular acetylcholine transporter (VAChT), but do not immunoreact to glutamatergic, GABAergic or dopaminergic markers and are distinct from calretinin-expressing interneurons. Interestingly, the cholinergic population partially overlaps with the calbindin D28K-expressing interneuron population, revealing the neurotransmitter identity of this sub-population. Additionally, we quantitatively determined the density of VAChT labeled cholinergic nerve fibers in various layers of the OB, as well as the intensity of VAChT immunoreactivity within the GL, suggesting primary sites of cholinergic actions. Taken together, our results provide clear evidence showing the presence of a significant number of cholinergic interneurons and that these morphologically and distributionally diverse interneurons make up complex local cholinergic networks in the OB. Thus, our results suggest that olfactory information processing is modulated by dual cholinergic systems of local interneuron networks and centrifugal projections.

Neural correlates of olfactory learning: Critical role of centrifugal neuromodulation

The mammalian olfactory system is well established for its remarkable capability of undergoing experience-dependent plasticity. Although this process involves changes at multiple stages throughout the central olfactory pathway, even the early stages of processing, such as the olfactory bulb and piriform cortex, can display a high degree of plasticity. As in other sensory systems, this plasticity can be controlled by centrifugal inputs from brain regions known to be involved in attention and learning processes. Specifically, both the bulb and cortex receive heavy inputs from cholinergic, noradrenergic, and serotonergic modulatory systems. These neuromodulators are shown to have profound effects on both odor processing and odor memory by acting on both inhibitory local interneurons and output neurons in both regions.

Olfactory discrimination varies in mice with different levels of α7-nicotinic acetylcholine receptor expression

Previous studies have shown that schizophrenics have decreased expression of α7-nicotinic acetylcholine (α7) receptors in the hippocampus and other brain regions, paranoid delusions, disorganized speech, deficits in auditory gating (i.e., inability to inhibit neuronal responses to repetitive auditory stimuli), and difficulties in odor discrimination and detection. Here we use mice with decreased α7 expression that also show a deficit in auditory gating to determine if these mice have similar deficits in olfaction. In the adult mouse olfactory bulb (OB), α7 expression localizes in the glomerular layer; however, the functional role of α7 is unknown. We show that inbred mouse strains (i.e., C3H and C57) with varying α7 expressions (e.g., α7 wild-type [α7+/+], α7 heterozygous knock-out [α7+/-] and α7 homozygous knock-out mice [α7-/-]) significantly differ in odor discrimination and detection of chemically-related odorant pairs. Using [(125)I] α-bungarotoxin (α-BGT) autoradiography, α7 expression was measured in the OB. As previously demonstrated, α-BGT binding was localized to the glomerular layer. Significantly more expression of α7 was observed in C57 α7+/+ mice compared to C3H α7+/+ mice. Furthermore, C57 α7+/+ mice were able to detect a significantly lower concentration of an odor in a mixture compared to C3H α7+/+ mice. Both C57 and C3H α7+/+ mice discriminated between chemically-related odorants sooner than α7+/- or α7-/- mice. These data suggest that α7-nicotinic-receptors contribute strongly to olfactory discrimination and detection in mice and may be one of the mechanisms producing olfactory dysfunction in schizophrenics.

Olfactory dysfunction, central cholinergic integrity and cognitive impairment in Parkinson's disease

Olfactory dysfunction is common in subjects with Parkinson’s disease. The pathophysiology of such dysfunction, however, remains poorly understood. Neurodegeneration within central regions involved in odour perception may contribute to olfactory dysfunction in Parkinson’s disease. Central cholinergic deficits occur in Parkinson’s disease and cholinergic neurons innervate regions, such as the limbic archicortex, involved in odour perception. We investigated the relationship between performance on an odour identification task and forebrain cholinergic denervation in Parkinson’s disease subjects without dementia. Fifty-eight patients with Parkinson’s disease (mean Hoehn and Yahr stage 2.5 ± 0.5) without dementia (mean Mini-Mental State Examination, 29.0 ± 1.4) underwent a clinical assessment, [¹¹C]methyl-4-piperidinyl propionate acetylcholinesterase brain positron emission tomography and olfactory testing with the University of Pennsylvania Smell Identification Test. The diagnosis of Parkinson’s disease was confirmed by [¹¹C]dihydrotetrabenazine vesicular monoamine transporter type 2 positron emission tomography. We found that odour identification test scores correlated positively with acetylcholinesterase activity in the hippocampal formation (r = 0.56, P < 0.0001), amygdala (r = 0.50, P < 0.0001) and neocortex (r = 0.46, P = 0.0003). Striatal monoaminergic activity correlated positively with odour identification scores (r = 0.30, P < 0.05). Multiple regression analysis including limbic (hippocampal and amygdala) and neocortical acetylcholinesterase activity as well as striatal monoaminergic activity, using odour identification scores as the dependent variable, demonstrated a significant regressor effect for limbic acetylcholinesterase activity (F = 10.1, P < 0.0001), borderline for striatal monoaminergic activity (F = 1.6, P = 0.13), but not significant for cortical acetylcholinesterase activity (F = 0.3, P = 0.75). Odour identification scores correlated positively with scores on cognitive measures of episodic verbal learning (r = 0.30, P < 0.05). These findings indicate that cholinergic denervation of the limbic archicortex is a more robust determinant of hyposmia than nigrostriatal dopaminergic denervation in subjects with moderately severe Parkinson's disease. Greater deficits in odour identification may identify patients with Parkinson's disease at risk for clinically significant cognitive impairment.

Compensatory responses to age-related decline in odor quality acuity: cholinergic neuromodulation and olfactory enrichment

The perceptual differentiation of odors can be measured behaviorally using generalization gradients. The steepness of these gradients defines a form of olfactory acuity for odor quality that depends on neural circuitry within the olfactory bulb and is regulated by cholinergic activity therein as well as by associative learning. Using this system as a reduced model for age-related cognitive decline, we show that aged mice, while maintaining almost the same baseline behavioral performance as younger mice, are insensitive to the effects of acutely elevated acetylcholine, which sharpens generalization gradients in young adult mice. Moreover, older mice exhibit evidence of chronically elevated acetylcholine levels in the olfactory bulb, suggesting that their insensitivity to further elevated levels of acetylcholine may arise because the maximum capacity of the system to respond to acetylcholine has already been reached. We propose a model in which an underlying, age-related, progressive deficit is mitigated by a compensatory cholinergic feedback loop that acts to retard the behavioral effects of what would otherwise be a substantial age-related decline in olfactory plasticity. We also treated mice with 10-day regimens of olfactory environmental enrichment and/or repeated systemic injections of the acetylcholinesterase inhibitor physostigmine. Each treatment alone sharpened odor quality acuity, but administering both treatments together had no greater effect than either alone. Age was not a significant main effect in this study, suggesting that some capacity for acetylcholine-dependent plasticity is still present in aged mice despite their sharply reduced ability to respond to acute increases in acetylcholine levels. These results suggest a dynamical framework for understanding age-related decline in neural circuit processing in which the direct effects of aging can be mitigated, at least temporarily, by systemic compensatory responses. In particular, a decline in cholinergic efficacy can precede any breakdown in cholinergic production, which may help explain the limited effectiveness of cholinergic replacement therapies in combating cognitive decline.

Smell identification test as a treatment response marker in patients with Alzheimer disease receiving donepezil

BACKGROUND

Olfactory dysfunction, impaired odor identification in particular, is known to occur in Alzheimer disease (AD). The entorhinal cortex and the olfactory bulb, critical areas for olfactory function, are rich in acetylcholine, the neurotransmitter implicated in AD pathology and treatment. In view of the common anatomical substrate, we aimed to determine whether performance on an olfaction test can be used as a clinical marker for monitoring the efficacy of donepezil in elderly people with AD.

METHODS

Twenty-five participants with mild to moderate AD, planned for donepezil treatment, were recruited from mental health services for older adults in this open-labeled study. Assessments before commencing donepezil included Mini-Mental State Examination, Neuropsychiatric Inventory, Bristol Activities of Daily Living, and the University of Pennsylvania Smell Identification Test. After 3 months of treatment, the primary outcome measure, the Clinician Interview Based Impression of Change plus caregiver input (CIBIC-plus), was completed, and the baseline assessments were repeated.

RESULTS

Eighteen patients continued to receive donepezil at follow-up. The CIBIC-plus outcome correlated with changes in University of Pennsylvania Smell Identification Test and Bristol Activities of Daily Living scores from the baseline (r = 0.7, P < 0.01). In addition, it was the change in smell identification function after treatment that best predicted CIBIC-plus outcome (P < 0.05) on ordinal regression analysis.

CONCLUSIONS

Smell identification function could be useful as a clinical measure to assess treatment response with donepezil in AD. This is a nonblind uncontrolled study, and the outcome indicates the need for a controlled study.

Bulbar acetylcholine enhances neural and perceptual odor discrimination

Experimental and modeling data suggest that the circuitry of the main olfactory bulb (OB) plays a critical role in olfactory discrimination. Processing of such information arises from the interaction between OB output neurons local interneurons, as well as interactions between the OB network and centrifugal inputs. Cholinergic input to the OB in particular has been hypothesized to regulate mitral cell odorants receptive fields (ORFs) and behavioral discrimination of similar odorants. We recorded from individual mitral cells in the OB in anesthetized rats to determine the degree of overlap in ORFs of individual mitral cells after exposure to odorant stimuli. Increasing the efficacy of the cholinergic neurotransmission in the OB by addition of the anticholinesterase drug neostigmine (20 mM) sharpened the ORF responses of mitral cells. Furthermore, coaddition of either the nicotinic antagonist methyllycaconitine citrate hydrate (MLA) (20 mM) or muscarinic antagonist scopolamine (40 mM) together with neostigmine (20 mM) attenuated the neostigmine-dependent sharpening of ORFs. These electrophysiological findings are predictive of accompanying behavioral experiments in which cholinergic modulation was manipulated by direct infusion of neostigmine, MLA, and scopolamine into the OB during olfactory behavioral tasks. Increasing the efficacy of cholinergic action in the OB increased perceptual discrimination of odorants in these experiments, whereas blockade of nicotinic or muscarinic receptors decreased perceptual discrimination. These experiments show that behavioral discrimination is modulated in a manner predicted by the changes in mitral cell ORFs by cholinergic drugs. These results together present a first direct comparison between neural and perceptual effects of a bulbar neuromodulator.

Presynaptic Muscarinic Receptors Enhance Glutamate Release at the Mitral/Tufted to Granule Cell Dendrodendritic Synapse in the Rat Main Olfactory Bulb

The mammalian olfactory bulb receives multiple modulatory inputs, including a cholinergic input from the basal forebrain. Understanding the functional roles played by the cholinergic input requires an understanding of the cellular mechanisms it modulates. In an in vitro olfactory bulb slice preparation we demonstrate cholinergic muscarinic modulation of glutamate release onto granule cells that results in γ-aminobutyric acid (GABA) release onto mitral/tufted cells. We demonstrate that the broad-spectrum cholinergic agonist carbachol triggers glutamate release from mitral/tufted cells that activates both AMPA and NMDA receptors on granule cells. Activation of the granule cell glutamate receptors leads to calcium influx through voltage-gated calcium channels, resulting in spike-independent, asynchronous GABA release at reciprocal dendrodendritic synapses that granule cells form with mitral/tufted cells. This cholinergic modulation of glutamate release persists through much of postnatal bulbar development, suggesting a functional role for cholinergic inputs from the basal forebrain in bulbar processing of olfactory inputs and possibly in postnatal development of the olfactory bulb.

Criteria for validating mouse models of psychiatric diseases

Animal models of human diseases are in widespread use for biomedical research. Mouse models with a mutation in a single gene or multiple genes are excellent research tools for understanding the role of a specific gene in the etiology of a human genetic disease. Ideally, the mouse phenotypes will recapitulate the human phenotypes exactly. However, exact matches are rare, particularly in mouse models of neuropsychiatric disorders. This article summarizes the current strategies for optimizing the validity of a mouse model of a human brain dysfunction. We address the common question raised by molecular geneticists and clinical researchers in psychiatry, "what is a 'good enough' mouse model"?

Muscarinic receptor activation modulates granule cell excitability and potentiates inhibition onto mitral cells in the rat olfactory bulb

The olfactory bulb is a second-order brain region that connects sensory neurons with cortical areas. However, the olfactory bulb does not appear to play a simple relay role and is subject instead to extensive local and extrinsic synaptic influences. Prime among the external, or centrifugal, inputs is the dense cholinergic innervation from the basal forebrain, which terminates in both the granule cell and plexiform layers. Cholinergic inputs to the bulb have been implicated in olfactory working memory tasks in rodents and may be related to olfactory deficits reported in people with neurodegenerative disorders that involve basal forebrain neurons. In this study, we use whole-cell recordings from acute rat slices to demonstrate that one function of this input is to potentiate the excitability of GABAergic granule cells and thereby modulate inhibitory drive onto mitral cells. This increase in granule cell excitability is mediated by a concomitant decrease in the normal afterhyperpolarization response and augmentation of an afterdepolarization, both triggered by pirenzepine-sensitive M1 receptors. The afterdepolarization was dependent on elevations in intracellular calcium and appeared to be mediated by a calcium-activated nonselective cation current (I(CAN)). Near firing threshold, depolarizing inputs could evoke quasipersistent firing characterized by irregular discharges that lasted, on average, for 2 min. In addition to regulating the excitability of the primary interneuronal subtype in the bulb, M1 receptors regulate the degree of adaptation that occurs during repetitive sniffing-like inputs and may therefore play a critical role in regulating short-term plasticity in the olfactory system.

Cholinergic modulation in the olfactory bulb influences spontaneous olfactory discrimination in adult rats

Cholinergic neuromodulation in the olfactory bulb has been hypothesized to regulate mitral cell molecular receptive ranges and the behavioral discrimination of similar odorants. We tested the effects of cholinergic modulation in the olfactory bulb of cannulated rats by bilaterally infusing cholinergic agents into the olfactory bulbs and measuring the rats' performances on separate spontaneous and motivated odor-discrimination tasks. Specifically, 6 microL/bulb infusions of vehicle (0.9% saline), the muscarinic antagonist scopolamine (7.6 mM and 38 mM), the nicotinic antagonist mecamylamine hydrochloride (3.8 mM and 19 mM), a combination of both antagonists, or the acetylcholinesterase inhibitor neostigmine (8.7 mM) were made 20 min prior to testing on an olfactory cross-habituation task or a rewarded, forced-choice odor-discrimination task. Spontaneous discrimination between chemically related odorants was abolished when nicotinic receptors were blocked in the olfactory bulb, and enhanced when the efficacy of cholinergic inputs was increased with neostigmine. Blocking muscarinic receptors reduced but did not abolish odor discrimination. Interestingly, no behavioral effects of modulating either nicotinic or muscarinic receptors were observed when rats were trained on a reward-motivated odor-discrimination task. Computational modeling of glomerular circuitry demonstrates that known nicotinic cholinergic effects on bulbar neurons suffice to explain these results.

Postnatal exposure to diisopropylfluorophosphate enhances discrimination learning in adult mice

Visual discrimination and reversal learning were tested in adult C57Bl/6 mice that had been treated on postnatal days (PND) 4-10 with diisopropylfluorophosphate (DFP), an acetylcholinesterase inhibitor. DFP-treated mice attained the learning criterion in the Y maze significantly earlier than saline-treated mice. Female mice treated with DFP showed a more rapid decline in errors in the initial discrimination task, compared to female mice treated with saline. There was no effect of DFP treatment on learning the reverse discrimination. The data suggest that long-lasting effects of treatment with an acetylcholinesterase inhibitor can improve discrimination learning, similarly to the improvement reported by acute administration in adults.

Type 4 phosphodiesterase inhibition impairs detection of low odor concentrations in mice

The cAMP-specific phosphodiesterase PDE4A is abundant in the dendrites, soma and axons of olfactory receptor neurons of the mouse, but it is not present in the cilia, where olfactory transduction initiates. Although the function of PDE4A in mammalian olfaction is unknown, patch clamp studies on deciliated olfactory receptor cells in the newt have shown that adrenaline or cAMP analogs can increase the contrast sensitivity to current injection. We used mice to ask whether increasing the levels of cAMP in sensory neurons by inhibiting PDE4A activity with rolipram could lead to changes in the perception of odorants that correspond to the in vitro cellular responses seen in newts. In an automated olfactometer, rolipram treatment (1mg/kg, i.p.) significantly impaired the detection accuracy of 1-propanol at relatively high dilutions but did not affect detection at lower dilutions. Meanwhile, the ability to discriminate amyl acetate alone from a mixture of amyl acetate+citronellal was not affected by rolipram at any odor dilution. In a different task in which mice were trained to discriminate between cups of scented versus unscented sand, rolipram treatment resulted in poorer discrimination at high and better discrimination at low, odor dilutions. In sum, PDE4 inhibition resulted in a consistent decrement in the ability of mice to detect low concentrations of odorants, but the effects of rolipram on detection of higher concentrations were task-dependent.

Acetylcholine and olfactory perceptual learning

Olfactory perceptual learning is a relatively long-term, learned increase in perceptual acuity, and has been described in both humans and animals. Data from recent electrophysiological studies have indicated that olfactory perceptual learning may be correlated with changes in odorant receptive fields of neurons in the olfactory bulb and piriform cortex. These changes include enhanced representation of the molecular features of familiar odors by mitral cells in the olfactory bulb, and synthetic coding of multiple coincident odorant features into odor objects by cortical neurons. In this paper, data are reviewed that show the critical role of acetylcholine (Ach) in olfactory system function and plasticity, and cholinergic modulation of olfactory perceptual learning at both the behavioral and cortical level.

Intensity modulation of olfactory acuity

Acuity is fundamental to sensory systems, establishing the foundation for detectable differences in stimulus quality and consequently shaping animals' sensory capacities. In the olfactory system, which samples intrinsically high-dimensional chemical information, acuity for odor quality is measurable by means of ad hoc dimensions based on behaviorally confirmed sets of sequentially similar odorants. The authors measure olfactory acuity in mice using a rewarded forced-choice odor generalization task and show that mice exhibit greater olfactory acuity in response to higher concentration (1,0 Pa) odorants than to lower concentration (0.01 Pa) odorants. Results suggest that the dynamic modulation of sensory acuity--not necessarily its maximization--is an important component of olfactory processing and reflects the salience of odorant stimuli.

The fundamental role of memory in olfactory perception

Current emphasis on odorant physiochemical features as the basis for perception largely ignores the synthetic and experience-dependent nature of olfaction. Olfaction is synthetic, as mammals have only limited ability to identify elements within even simple odor mixtures. Furthermore, olfaction is experience-bound, as exposure alone can significantly affect the extent to which stimuli can be discriminated. We propose that early analytical processing of odors is inaccessible at the behavioral level and that all odors are initially encoded as 'objects' in the piriform cortex. Moreover, we suggest that odor perception is wholly dependent on the integrity of this memory system and that its loss severely impairs normal perception.

Behavioral models of odor similarity

Carbon chain length in several classes of straight-chain aliphatic odorants has been proposed as a model axis of similarity for olfactory research, on the basis of successes of studies in insect and vertebrate species. To assess the influence of task on measured perceptual similarities among odorants and to demonstrate that the systematic similarities observed within homologous odorant series are not task specific, the authors compare 3 different behavioral paradigms for rats (olfactory habituation, generalization, and discrimination). Although overall patterns of odorant similarity are consistent across all 3 of these paradigms, both quantitative measurements of perceptual similarity and comparability with 2-deoxyglucose imaging data from the olfactory bulb are dependent on the specific behavioral tasks used. Thus, behavioral indices of perceptual similarity are affected by task parameters such as learning and reward associations.

Extensive immunolesions of basal forebrain cholinergic system impair offspring recognition in sheep

The involvement of the basal forebrain cholinergic system has been extensively investigated in instrumental learning but little is known of its participation in social memory, especially in the memorization of individual traits of a conspecific. The present study tested in sheep its contribution to both instrumental learning and individual offspring recognition. Six weeks before parturition, ewes received injections of a specific cholinergic immunotoxin (ME20.4 IgG-saporin) into the lateral ventricles (150 microg) and in some cases additional immunotoxin injections into the nucleus basalis (11 microg/side). After 3 weeks of recovery, ewes were trained on a classical instrumental visual discrimination task known to be sensitive to cholinergic deficits. The formation of memory of offspring was assessed through both olfactory and visual/auditory recognition tasks. Olfactory recognition was tested by presenting at suckling successively an alien and the familiar lamb at 2 and 4 h after parturition. Visual/auditory recognition of the lamb was performed using a non-olfactory discrimination test between the familiar and an alien lamb after 12 h of mother-young contact. The lesion extent was assessed by counting choline acetyltransferase-immunopositive neurons in the basal forebrain and measuring the density of acetylcholinesterase fibers in different target areas. Results showed that immunotoxic lesions delayed acquisition of the instrumental visual discrimination. Moreover, olfactory recognition of the lamb was severely impaired while visual/auditory lamb recognition was marginally altered. There was no evidence for sensorimotor or motivational deficits. Importantly, impairment was observed in animals for which loss of basal forebrain cholinergic neurons and their efferent fibers was higher than 75%, while striatal cholinergic neurons and Purkinje cells were unaffected. This study provides evidence that the basal forebrain cholinergic system contributes not only to instrumental but also to social learning. In addition, the cholinergic modulation seems of importance for processing visual and olfactory modalities. However, since only extensive lesions affect performance, this indicates that the basal forebrain cholinergic system possesses substantial reserve capacity to sustain cognitive functions.

The combined effects of pyridostigmine and chronic stress on brain cortical and blood acetylcholinesterase, corticosterone, prolactin and alternation performance in rats

Thousands of soldiers who served in the Gulf War have symptoms that have been collectively termed Gulf War Illness (GWI). It has been suggested that a combination of operational stress and pyridostigmine, a drug given as a pretreatment to protect soldiers against the effects of exposure to nerve agents, might have had unexpected adverse health effects causing these symptoms. Our laboratory has previously modeled operational stress in rats using a paradigm of around-the-clock intermittent signalled footshock. In the present studies, this model was used to investigate the potential synergistic effects of chronic stress and pyridostigmine on physiology and behavior. Seventy-two rats were trained to perform an alternation lever pressing task to earn their entire daily food intake. The rats were then implanted with osmotic minipumps containing vehicle, pyridostigmine (25 mg/ml pyridostigmine bromide) or physostigmine (20 mg/ml eserine hemisulfate). The pumps delivered 1 microl/h, which resulted in a cumulative dosing of approximately 1.5 mg/kg/day of pyridostigmine or 1.2 mg/kg/day of physostigmine, equimolar doses of the two drugs. The rats were then returned to their home cages where performance continued to be measured 24 h/day. After 4 days, 24 of the 72 rats were trained to escape signalled footshock (avoidance-escape group) and 24 other rats (yoked-stressed group) were each paired to a rat in the avoidance-escape group. The remaining 24 rats were not subjected to footshock (unstressed group). Shock trials were intermittently presented in the home cage 24 h/day for 3 days, while alternation performance continued to be measured. Since only 12 test cages were available, each condition was repeated to achieve a final n of six rats per group. Pyridostigmine and physostigmine each decreased blood acetylcholinesterase levels by approximately 50%. Physostigmine also decreased brain cortical acetylcholinesterase levels by approximately 50%, while pyridostigmine had no effect on cortical acetylcholinesterase activity. Alternation performance was impaired on the first day of stress and then recovered. Neither pyridostigmine nor physostigmine affected performance in the absence of stress or increased the effects of stress alone. Corticosterone was significantly increased in the yoked stress group compared to unstressed controls. These data suggest that pyridostigmine does not exacerbate the effects of stress on performance or levels of stress hormones. Furthermore, these data do not suggest that stress enables pyridostigmine to cross the blood brain barrier.

Selective loss of cholinergic neurons projecting to the olfactory system increases perceptual generalization between similar, but not dissimilar, odorants

The neuromodulator acetylcholine is thought to modulate information processing in the olfactory system. The authors used 192 IgG-saporin, a lesioning agent selective for basal forebrain cholinergic neurons, to determine whether selective lesions of cholinergic neurons projecting to the olfactory bulb and cortex affect odor perception in rats. Lesioned and sham-operated rats were tested in an olfactory generalization paradigm with sets of chemically related odorants (n-aliphatic aldehydes, acids, and alcohols). Lesioned rats generalized more between chemically similar odorants but did not differ from controls in their response to chemically unrelated odorants or in acquisition of the conditioned odor. Results show that cholinergic inputs to the olfactory system influence perceptual qualities of odorants and confirm predictions made by computational models of this system.