Pulse stimulation with odors or IBMX/forskolin potentiates responses in isolated olfactory neurons

Many odor responses are mediated by the adenosine 3',5'-cyclic monophosphate (cAMP) pathway in which the cAMP-gated current is amplified by Ca2+-dependent Cl- current. In olfactory neurons, prolonged exposure to odors decreases the odor response and is an adaptive effect. Several studies suggest that odor adaptation is linked to elevated intracellular Ca2+. In the present study, using the perforated configuration of the patch clamp technique, we found that repetitive odor stimulation elicits a potentiation of the subsequent responses in olfactory neurons. This potentiation is mimicked by stimulating the cAMP pathway and does not appear to be related to phosphorylation of ion channels since protein kinase inhibitors could not block it. Our data suggest that local increases in [Ca2+]i via activation of the cAMP pathway mediate the pulse-elicited potentiation. In the first odor application, entry of Ca2+ through cyclic nucleotide-gated channels appears to be buffered. Repetitive stimulation allows local increases in [Ca2+]i, recruiting more Ca2+-dependent Cl- channels with each subsequent odor pulse.     

Failure to implicate cAMP in transduction in lobster olfactory receptor cells

The possible role of adenosine 3',5'-cyclic monophosphate (cAMP)in olfactory transduction in the spiny lobster was investigatedusing radioimmunoassay of cAMP and intracellular recording.Application of forskolin or 1-isobutyl-3-methylxanthine increasedcAMP levels in intact sensilla containing the chemoreceptiveouter dendritic segments of the lobster olfactory receptor cell,thereby demonstrating adenylate cyclase and phosphodiesteraseactivity in the sensilla. A complex odor mixture and identifiedexcitatory odor molecules failed to stimulate the productionof cAMP, however In intracellular recordings, superfusion ofthe outer dendritic segments with forskolin, 1-isobutyl-3-methylxanthineand cyclic nucleotide analogs had no direct effect on odor-responsivecells. These compounds did cause infrequent enhancements (sixof 42 cells) of odor-evoked receptor potentials, but processesother than transduction are the most likely causes of this effect.We conclude that cAMP-dependent transduction mechanisms areunlikely to mediate most odor responses in lobsters, in contrastto transduction mechanisms in amphibians and rats.     

Appetitive odor learning does not change olfactory coding in a subpopulation of honeybee antennal lobe neurons

Odors elicit spatio-temporal patterns of activity in the olfactory bulb of vertebrates and the antennal lobe of insects. There have been several reports of changes in these patterns following olfactory learning. These studies pose a conundrum: how can an animal learn to efficiently respond to a particular odor with an adequate response, if its primary representation already changes during this process? In this study, we offer a possible solution for this problem. We measured odor-evoked calcium responses in a subpopulation of uniglomerular AL output neurons in honeybees. We show that their responses to odors are remarkably resistant to plasticity following a variety of appetitive olfactory learning paradigms. There was no significant difference in the changes of odor-evoked activity between single and multiple trial forward or backward conditioning, differential conditioning, or unrewarded successive odor stimulation. In a behavioral learning experiment we show that these neurons are necessary for conditioned odor responses. We conclude that these uniglomerular projection neurons are necessary for reliable odor coding and are not modified by learning in this paradigm. The role that other projection neurons play in olfactory learning remains to be investigated.     

Punishment prediction by dopaminergic neurons in Drosophila

The temporal pairing of a neutral stimulus with a reinforcer (reward or punishment) can lead to classical conditioning, a simple form of learning in which the animal assigns a value (positive or negative) to the formerly neutral stimulus. Olfactory classical conditioning in Drosophila is a prime model for the analysis of the molecular and neuronal substrate of this type of learning and memory. Neuronal correlates of associative plasticity have been identified in several regions of the insect brain. In particular, the mushroom bodies have been shown to be necessary for aversive olfactory memory formation. However, little is known about which neurons mediate the reinforcing stimulus. Using functional optical imaging, we now show that dopaminergic projections to the mushroom-body lobes are weakly activated by odor stimuli but respond strongly to electric shocks. However, after one of two odors is paired several times with an electric shock, odor-evoked activity is significantly prolonged only for the "punished" odor. Whereas dopaminergic neurons mediate rewarding reinforcement in mammals, our data suggest a role for aversive reinforcement in Drosophila. However, the dopaminergic neurons' capability of mediating and predicting a reinforcing stimulus appears to be conserved between Drosophila and mammals.     

Parallel odor processing by two anatomically distinct olfactory bulb target structures

The olfactory cortex encompasses several anatomically distinct regions each hypothesized to provide differential representation and processing of specific odors. Studies exploring whether or not the diversity of olfactory bulb input to olfactory cortices has functional meaning, however, are lacking. Here we tested whether two anatomically major olfactory cortical structures, the olfactory tubercle (OT) and piriform cortex (PCX), differ in their neural representation and processing dynamics of a small set of diverse odors by performing in vivo extracellular recordings from the OT and PCX of anesthetized mice. We found a wealth of similarities between structures, including odor-evoked response magnitudes, breadth of odor tuning, and odor-evoked firing latencies. In contrast, only few differences between structures were found, including spontaneous activity rates and odor signal-to-noise ratios. These results suggest that despite major anatomical differences in innervation by olfactory bulb mitral/tufted cells, the basic features of odor representation and processing, at least within this limited odor set, are similar within the OT and PCX. We predict that the olfactory code follows a distributed processing stream in transmitting behaviorally and perceptually-relevant information from low-level stations.     

Spatio-temporal dynamics of odor representations in the mammalian olfactory bulb

We explored the spatio-temporal dynamics of odor-evoked activity in the rat and mouse main olfactory bulb (MOB) using voltage-sensitive dye imaging (VSDI) with a new probe. The high temporal resolution of VSDI revealed odor-specific sequences of glomerular activation. Increasing odor concentrations reduced response latencies, increased response amplitudes, and recruited new glomerular units. However, the sequence of glomerular activation was maintained. Furthermore, we found distributed MOB activity locked to the nasal respiration cycle. The spatial distribution of its amplitude and phase was heterogeneous and changed by sensory input in an odor-specific manner. Our data show that in the mammalian olfactory bulb, odor identity and concentration are represented by spatio-temporal patterns, rather than spatial patterns alone.     

The effect of chronic fatty acid treatment on lipolysis in 3T3-L1 adipocytes.

Various saturated and unsaturated fatty acids were included in the culture medium to test their effects on lipolysis in 3T3-L1 adipocytes. Following prolonged incubation, only oleate was found to exert enhancing effect on basal and isoproterenol-stimulated lipolysis. The effect of oleate was concentration-dependent and was accompanied with increased intracellular cAMP content. Furthermore, the lipolytic response induced by isobutyl-methylxanthine, forskolin or dibutyryl cAMP was also increased in adipocytes treated with oleate. Thus, it appears that in addition to an increased cAMP accumulation, a step distal to cAMP production in the cells may be involved in inducing enhanced lipolysis in 3T3-L1 adipocytes by prolonged exposure to oleate.     

Two-photon calcium imaging reveals an odor-evoked map of activity in the fly brain

An understanding of the logic of odor perception requires a functional analysis of odor-evoked patterns of activity in neural assemblies in the brain. We have developed a sensitive imaging system in the Drosophila brain that couples two-photon microscopy with the specific expression of the calcium-sensitive fluorescent protein, G-CaMP. At natural odor concentration, each odor elicits a distinct and sparse spatial pattern of activity in the antennal lobe that is conserved in different flies. Patterns of glomerular activity are similar upon imaging of sensory and projection neurons, suggesting the faithful transmission of sensory input to higher brain centers. Finally, we demonstrate that the response pattern of a given glomerulus is a function of the specificity of a single odorant receptor. The development of this imaging system affords an opportunity to monitor activity in defined neurons throughout the fly brain with high sensitivity and excellent spatial resolution.     

Reshaping of bulbar odor response by nasal flow rate in the rat

Background

The impact of respiratory dynamics on odor response has been poorly studied at the olfactory bulb level. However, it has been shown that sniffing in the behaving rodent is highly dynamic and varies both in frequency and flow rate. Bulbar odor response could vary with these sniffing parameter variations. Consequently, it is necessary to understand how nasal airflow can modify and shape odor response at the olfactory bulb level.

Methodology and Principal Findings

To assess this question, we used a double cannulation and simulated nasal airflow protocol on anesthetized rats to uncouple nasal airflow from animal respiration. Both mitral/tufted cell extracellular unit activity and local field potentials (LFPs) were recorded. We found that airflow changes in the normal range were sufficient to substantially reorganize the response of the olfactory bulb. In particular, cellular odor-evoked activities, LFP oscillations and spike phase-locking to LFPs were strongly modified by nasal flow rate.

Conclusion

Our results indicate the importance of reconsidering the notion of odor coding as odor response at the bulbar level is ceaselessly modified by respiratory dynamics.     

Maturation of odor representation in the honeybee antennal lobe

The antennal lobe (AL) is the first center for processing odors in the insect brain, as is the olfactory bulb (OB) in vertebrates. Both the AL and the OB have a characteristic glomerular structure; odors sensed by olfactory receptor neurons are represented by patterns of glomerular activity. Little is known about when and how an odor begins to be perceived in a developing brain. We address this question by using calcium imaging to monitor odor-evoked neural activity in the ALs of bees of different ages. We find that odor-evoked neural activity already occurs in the ALs of bees as young as 1 or 2 days. In young bees, the responses to odors are relatively weak and restricted to a small number of glomeruli. However, different odors already evoke responses in different combinations of glomeruli. In mature bees, the responses are stronger and are evident in more glomeruli, but continue to have distinct odor-dependent signatures. Our findings indicate that the specific glomerular patterns for odors are conserved during the development, and that odor representations are fully developed in the AL during the first 2 weeks following emergence.     

Cortical processing of odor objects

Natural odors, generally composed of many monomolecular components, are analyzed by peripheral receptors into component features and translated into spatiotemporal patterns of neural activity in the olfactory bulb. Here, we will discuss the role of the olfactory cortex in the recognition, separation and completion of those odor-evoked patterns, and how these processes contribute to odor perception. Recent findings regarding the neural architecture, physiology, and plasticity of the olfactory cortex, principally the piriform cortex, will be described in the context of how this paleocortical structure creates odor objects.     

The effect of some drugs on the mitral cell odor-evoked responses in the gecko olfactory bulb

The activity of odor-evoked olfactory mitral cell response of the gecko was recorded extracellularly by glass microelectrodes. The activities of the mitral cell observed during the presentation of the odor (n-amyl acetate) could be described as excitation, suppression or zero. The present experiments were undertaken to study the neural activities of the mitral cell in the olfactory bulb by perfusion application of some drugs (cobalt chloride, carnosine, norepinephrine, GABA and D-L-homocysteate) on the olfactory bulb surface or iontophoretic application of some drugs (carnosine, norepinephrine, GABA and D-L-homocysteate) to the glomerulus and the external plexiform layer to change the physiological environment. The effect of the drugs suggested that the synaptic neurons on the mitral cell have different chemical characteristics.     

Clustering of cyclic-nucleotide-gated channels in olfactory cilia

Olfactory cilia contain the known components of olfactory signal transduction, including a high density of cyclic-nucleotide-gated (CNG) channels. CNG channels play an important role in mediating odor detection. The channels are activated by cAMP, which is formed by a G-protein-coupled transduction cascade. Frog olfactory cilia are 25-200 microm in length, so the spatial distribution of CNG channels along the length should be important in determining the sensitivity of odor detection. We have recorded from excised cilia and modeled diffusion of cAMP into a cilium to determine the spatial distribution of the CNG channels along the ciliary length. The proximal segment, which in frog is the first 20% of the cilium, appears to express a small fraction of the CNG channels, whereas the distal segment contains the majority, mostly clustered in one region.     

Brain–Immune Interaction Accompanying Odor-Evoked Autobiographic Memory

The phenomenon in which a certain smell evokes a specific memory is known as the Proust phenomenon. Odor-evoked autobiographic memories are more emotional than those elicited by other sensory stimuli. The results of our previous study indicated that odor-evoked autobiographic memory accompanied by positive emotions has remarkable effects on various psychological and physiological activities, including the secretion of cytokines, which are immune-signaling molecules that modulate systemic inflammation. In this study, we aimed to clarify the neural substrates associated with the interaction between odor-evoked autobiographic memory and peripheral circulating cytokines. We recruited healthy male and female volunteers and investigated the association between brain responses and the concentration of several cytokines in the plasma by using positron emission tomography (PET) recordings when an autographic memory was evoked in participants by asking them to smell an odor that was nostalgic to them. Participants experienced positive emotions and autobiographic memories when nostalgic odors were presented to them. The levels of peripheral proinflammatory cytokines, such as the tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ), were significantly reduced after experiencing odor-evoked autobiographic memory. Subtraction analysis of PET images indicated that the medial orbitofrontal cortex (mOFC) and precuneus/posterior cingulate cortex (PCC) were significantly activated during experiences of odor-evoked autobiographic memory. Furthermore, a correlation analysis indicated that activities of the mOFC and precuneus/PCC were negatively correlated with IFN-γ concentration. These results indicate that the neural networks including the precuneus/PCC and mOFC might regulate the secretion of peripheral proinflammatory cytokines during the experience of odor-evoked autobiographic memories accompanied with positive emotions.     

Drosophila DPM neurons form a delayed and branch-specific memory trace after olfactory classical conditioning

Formation of normal olfactory memory requires the expression of the wild-type amnesiac gene in the dorsal paired medial (DPM) neurons. Imaging the activity in the processes of DPM neurons revealed that the neurons respond when the fly is stimulated with electric shock or with any odor that was tested. Pairing odor and electric-shock stimulation increases odor-evoked calcium signals and synaptic release from DPM neurons. These memory traces form in only one of the two branches of the DPM neuron process. Moreover, trace formation requires the expression of the wild-type amnesiac gene in the DPM neurons. The cellular memory traces first appear at 30 min after conditioning and persist for at least 1 hr, a time window during which DPM neuron synaptic transmission is required for normal memory. DPM neurons are therefore "odor generalists" and form a delayed, branch-specific, and amnesiac-dependent memory trace that may guide behavior after acquisition.     

Excitatory interactions between olfactory processing channels in the Drosophila antennal lobe

Each odorant receptor gene defines a unique type of olfactory receptor neuron (ORN) and a corresponding type of second-order neuron. Because each odor can activate multiple ORN types, information must ultimately be integrated across these processing channels to form a unified percept. Here, we show that, in Drosophila, integration begins at the level of second-order projection neurons (PNs). We genetically silence all the ORNs that normally express a particular odorant receptor and find that PNs postsynaptic to the silent glomerulus receive substantial lateral excitatory input from other glomeruli. Genetically confining odor-evoked ORN input to just one glomerulus reveals that most PNs postsynaptic to other glomeruli receive indirect excitatory input from the single ORN type that is active. Lateral connections between identified glomeruli vary in strength, and this pattern of connections is stereotyped across flies. Thus, a dense network of lateral connections distributes odor-evoked excitation between channels in the first brain region of the olfactory processing stream.     

A behavioral switch: cGMP and PKC signaling in olfactory neurons reverses odor preference in C. elegans

Innate chemosensory preferences are often encoded by sensory neurons that are specialized for attractive or avoidance behaviors. Here, we show that one olfactory neuron in Caenorhabditis elegans, AWC(ON), has the potential to direct both attraction and repulsion. Attraction, the typical AWC(ON) behavior, requires a receptor-like guanylate cyclase GCY-28 that acts in adults and localizes to AWC(ON) axons. gcy-28 mutants avoid AWC(ON)-sensed odors; they have normal odor-evoked calcium responses in AWC(ON) but reversed turning biases in odor gradients. In addition to gcy-28, a diacylglycerol/protein kinase C pathway that regulates neurotransmission switches AWC(ON) odor preferences. A behavioral switch in AWC(ON) may be part of normal olfactory plasticity, as odor conditioning can induce odor avoidance in wild-type animals. Genetic interactions, acute rescue, and calcium imaging suggest that the behavioral reversal results from presynaptic changes in AWC(ON). These results suggest that alternative modes of neurotransmission can couple one sensory neuron to opposite behavioral outputs.     

Origin of basal activity in mammalian olfactory receptor neurons

Mammalian odorant receptors form a large, diverse group of G protein-coupled receptors that determine the sensitivity and response profile of olfactory receptor neurons. But little is known if odorant receptors control basal and also stimulus-induced cellular properties of olfactory receptor neurons other than ligand specificity. This study demonstrates that different odorant receptors have varying degrees of basal activity, which drives concomitant receptor current fluctuations and basal action potential firing. This basal activity can be suppressed by odorants functioning as inverse agonists. Furthermore, odorant-stimulated olfactory receptor neurons expressing different odorant receptors can have strikingly different response patterns in the later phases of prolonged stimulation. Thus, the influence of odorant receptor choice on response characteristics is much more complex than previously thought, which has important consequences on odor coding and odor information transfer to the brain.