Analysis of chemical signals by nervous systems

Olfactory processing: maps, time and codes

Natural odors are complex, multidimensional stimuli. Yet, they are learned and recognized by the brain with a great deal of specificity and accuracy. This implies that central olfactory circuits are optimized to encode these complex chemical patterns and to store and recognize their neural representations. What shape this optimization takes remains somewhat mysterious. Recent results from studies focusing on odor representation in the first olfactory relay (i.e. one synapse downstream of the receptor neurons) suggest a great deal of order and precision in the spatial and temporal features of odor representation. Whether these spatio-temporal features of neural activity are an essential part of the code for odors (i.e. whether these features are essential for the downstream decoding circuits) remains a central issue.

Coexpression of two odorant-binding protein homologs in Drosophila: implications for olfactory coding

Odorant-binding proteins (OBPs) are small soluble proteins present in the aqueous medium surrounding olfactory receptor neurons. Their function in olfaction is still unknown: they have been proposed to facilitate the transit of hydrophobic molecules to olfactory receptors, to deactivate the odorant stimulus, and/or to play a role in chemosensory coding. In this study we examine the genomic organization and expression patterns of two olfactory-specific genes (OS-E and OS-F) of Drosophila melanogaster, the products of which are members of a protein family in Drosophila sharing sequence similarity with moth OBPs. We show that the OS-E and OS-F transcription units are located <1 kb apart. They are oriented in the same direction and display a similar intron-exon organization. Expression of both OS-E and OS-F proteins is restricted spatially to the ventrolateral region of the Drosophila antenna. Within this region both OS-E and OS-F proteins are expressed within two different types of sensory hairs: in most, if not all, sensilla trichodea and in approximately 40% of the interspersed small sensilla basiconica. We consistently observe that OS-E and OS-F are coexpressed, indicating that an individual sensillum can contain more than one odorant-binding protein. The functional significance of the observed expression pattern and its implications for olfactory coding are discussed.

Coexpression of two odorant-binding protein homologs in Drosophila: implications for olfactory coding

Odorant-binding proteins (OBPs) are small soluble proteins present in the aqueous medium surrounding olfactory receptor neurons. Their function in olfaction is still unknown: they have been proposed to facilitate the transit of hydrophobic molecules to olfactory receptors, to deactivate the odorant stimulus, and/or to play a role in chemosensory coding. In this study we examine the genomic organization and expression patterns of two olfactory-specific genes (OS-E and OS-F) of Drosophila melanogaster, the products of which are members of a protein family in Drosophila sharing sequence similarity with moth OBPs. We show that the OS-E and OS-F transcription units are located <1 kb apart. They are oriented in the same direction and display a similar intron-exon organization. Expression of both OS-E and OS-F proteins is restricted spatially to the ventrolateral region of the Drosophila antenna. Within this region both OS-E and OS-F proteins are expressed within two different types of sensory hairs: in most, if not all, sensilla trichodea and in approximately 40% of the interspersed small sensilla basiconica. We consistently observe that OS-E and OS-F are coexpressed, indicating that an individual sensillum can contain more than one odorant-binding protein. The functional significance of the observed expression pattern and its implications for olfactory coding are discussed.

Voltage-activated and odor-modulated conductances in olfactory neurons of Drosophila melanogaster

Voltage-activated currents and odor-modulated conductances were studied in cells in semi-intact Drosophila third antennal segments (the main olfactory organ) using patch-clamp techniques. All neurons expressed outward currents, and most expressed labile fast transient inward currents with kinetics similar to Na+ currents in other systems. Action potentials were detected as bipolar capacitative current transients in cell-attached or loose patches from the soma of both odor-sensitive (97%) and insensitive neurons. A mixture of odorants from five chemical classes caused an increase (approximately 70%), decrease (approximately 10%), or no effect on firing frequency in pharate adult neurons. The development of chemosensitivity was examined and odor-induced changes in action potential firing frequency were recorded in pupal antennal neurons as early as P8, a stage after completion of sensillar development. The character of odor-induced responses was more profound and complex later in development; small, tonic increases in firing frequency were observed at pupal stages P8 through P11 (ii), while in older pupae and young adults approximately 25% of the increased responses were phasic-tonic. The apical dendrite was the site of odor modulation in approximately 90% and 100% of responsive adult and early pupal neurons, respectively. Whole-cell recordings revealed that apparent nonselective cation and chloride conductances were modulated by a mixture of odorants in separate antennal neurons.

Mechanisms of olfactory discrimination: converging evidence for common principles across phyla

Olfaction begins with the transduction of the information carried by odor molecules into electrical signals in sensory neurons. The activation of different subsets of sensory neurons to different degrees is the basis for neural encoding and further processing of the odor information by higher centers in the olfactory pathway. Recent evidence has converged on a set of transduction mechanisms, involving G-protein-coupled second-messenger systems, and neural processing mechanisms, involving modules called glomeruli, that appear to be adapted for the requirements of different species. The evidence is highlighted in this review by focusing on studies in selected vertebrates and in insects and crustaceans among invertebrates. The findings support the hypothesis that olfactory transduction and neural processing in the peripheral olfactory pathway involve basic mechanisms that are universal across most species in most phyla.

Dynamical representation of odors by oscillating and evolving neural assemblies

Although smells are some of the most evocative and emotionally charged sensory inputs known to us, we still understand relatively little about olfactory processing and odor representation in the brain. This review summarizes physiological results obtained from an insect olfactory system and presents a functional scheme for odor coding that is compatible with data from other animals, including mammals. This coding scheme consists of three main and concurrent odor-induced phenomena: 20-30 Hz oscillatory mass activity; patterned and odor-specific neuronal responses; and transient, dynamic synchronization of odor-specific neural assemblies. When these phenomena are considered together, odors appear to be represented combinatorially by dynamical neural assemblies, defined partly by the transient but stimulus-specific synchronization of their neuronal components.

A chemical-detecting system based on a cross-reactive optical sensor array

The vertebrate olfactory system has long been recognized for its extraordinary sensitivity and selectivity for odours. Chemical sensors have been developed recently that are based on analogous distributed sensing properties, but although an association between artificial devices and the olfactory system has been made explicit in some previous studies, none has incorporated comparable mechanisms into the mode of detection. Here we describe a multi-analyte fibre-optic sensor modelled directly on the olfactory system, in the sense that complex, time-dependent signals from an array of sensors provide a 'signature' of each analyte. In our system, polymer-immobilized dye molecules on the fibre tips give different fluorescent response patterns (including spectral shifts, intensity changes, spectral shape variations and temporal responses) on exposure to organic vapours, depending on the physical and chemical nature (for example, polarity, shape and size) of both the vapour and the polymer. We use video images of temporal responses of the multi-fibre tip as the input signals to train a neural network for vapour recognition. The system is able to identify individual vapours at different concentrations with great accuracy. 'Artificial noses' such as this should have wide potential application, most notably in environmental and medical monitoring.

Temporal representations of odors in an olfactory network

The responses of projection neurons in the antennal lobe of the locust brain (the functional analog of mitral-tufted cells in the vertebrate olfactory bulb) to natural blends and simple odors were studied with multiple intra- and extracellular recordings in vivo. Individual odors evoked complex temporal response patterns in many neurons. These patterns differed across odors for a given neuron and across neurons for a given odor, but were stable for each neuron over repeated presentations (separated by seconds to minutes) of the same odor. The response of individual neurons to an odor was superimposed on an odor-specific coherent oscillatory population activity. Each neuron usually participated in the coherent oscillations during one or more specific epochs of the ensemble activity. These epochs of phase locking were reliable for each neuron over tens of repeated presentations of one odor. The timing of these epochs of synchronization differed across neurons and odors. Correlated activity of specific pairs of neurons, hence, generally occurred transiently during the population response, at times that were specific to these pairs and to the odor smelled. The field potential oscillations, therefore, fail to reveal a progressive transformation of the synchronized ensemble as the response to the odor unfolds. We propose that (1) odors are represented by spatially and temporally distributed ensembles of coherently firing neurons, and (2) the field potential oscillations that characterize odor responses in the olfactory system occur, at least in this animal, in parallel with a slower dynamic odor representation.

Antennal lobe interneurons in the desert locust Schistocerca gregaria (Forskal): processing of aggregation pheromones in adult males and females

Physiological and morphological characteristics of antennal lobe interneurons in male and female Schistocerca gregaria were studied by using intracellular recording and staining techniques. For the first time, the responses of projection neurons to behaviourally active and potential aggregation pheromone compounds and plant odour compounds were recorded in young adult locusts. Excitatory, inhibitory, or combined excitatory/inhibitory responses to the presented odours were detected. The stained neurons had their cell bodies in the frontal cell group, arborized in 10 to 25 glomeruli at equal distances from the central fibre core, and sent their axons to the calyces of the mushroom body and to the lateral protocerebrum. The projection neurons responded to the set of different stimuli with varying specificity. In females, more neurons responded specifically to single plant or aggregation pheromone compounds than in males, where more generalist responses were found. "Blend specialist" neurons, responding only to mixtures of behaviourally active aggregation pheromone compounds, but not to the single compounds, were present in both males and females. Most neurons responded to the behaviourally active aggregation pheromone mixtures and to single compounds present in these mixtures, as well as to plant odours. Fewer neurons responded to the potential aggregation pheromone compounds tested. In several experiments, two spike sizes in the recording were correlated with two stained neurons in the antennal lobe, suggesting electrical coupling of the neurons. No response to any of the stimuli was found in antennal lobe interneurons in old adults. The morphological and physiological features of the projection neurons in S. gregaria are compared with projection neuron characteristics in other insects.

Synaptic connection between olfactory receptor cells and uniglomerular projection neurons in the antennal lobe of the American cockroach, Periplaneta americana

Both antennal receptor cell axons and uniglomerular projection neurons of the antennal lobe were specifically labeled, and their synaptic relationship was studied at the fine structural level. The labelings were applied in different combinations: i) Experimentally induced anterograde degeneration of sensory-afferent axons was combined with injection of horseradish peroxidase into uniglomerular projection neurons. ii) Lucifer Yellow was injected into uniglomerular projection neurons, and receptor cell axons were anterogradely labeled with the lipophilic dye DiI. The fluorescent dyes were transformed by immuno- or photochemical treatment into electron-dense markers. In both types of preparations, a considerable number of monosynaptic output synapses from antennal receptor neurons onto processes of uniglomerular projection neurons were identified within the glomeruli of the lobe. In most cases, the receptor axon was connected in a dyadic fashion firstly to a process of a projection neuron and secondly to a nonlabeled process. The results clearly demonstrate a direct connection between receptor cells and output neurons of the cockroach antennal lobe which exists in parallel to the already proposed and demonstrated polysynaptic connection via inhibitory local interneurons.

Temporal representations of odors in an olfactory network

The responses of projection neurons in the antennal lobe of the locust brain (the functional analog of mitral-tufted cells in the vertebrate olfactory bulb) to natural blends and simple odors were studied with multiple intra- and extracellular recordings in vivo. Individual odors evoked complex temporal response patterns in many neurons. These patterns differed across odors for a given neuron and across neurons for a given odor, but were stable for each neuron over repeated presentations (separated by seconds to minutes) of the same odor. The response of individual neurons to an odor was superimposed on an odor-specific coherent oscillatory population activity. Each neuron usually participated in the coherent oscillations during one or more specific epochs of the ensemble activity. These epochs of phase locking were reliable for each neuron over tens of repeated presentations of one odor. The timing of these epochs of synchronization differed across neurons and odors. Correlated activity of specific pairs of neurons, hence, generally occurred transiently during the population response, at times that were specific to these pairs and to the odor smelled. The field potential oscillations, therefore, fail to reveal a progressive transformation of the synchronized ensemble as the response to the odor unfolds. We propose that (1) odors are represented by spatially and temporally distributed ensembles of coherently firing neurons, and (2) the field potential oscillations that characterize odor responses in the olfactory system occur, at least in this animal, in parallel with a slower dynamic odor representation.

Olfactory information processing in the brain: encoding chemical and temporal features of odors

A fundamental problem in studying the neural mechanisms of odor recognition and discrimination in the olfactory system lies in determining the features or "primitives" of an odor stimulus that are analyzed by glomerular circuits at the first level of processing in the brain. Several recent studies support the idea that it is not simply the molecular features of odors that contain important information, but also the intermittent pattern of their presentation to the olfactory epithelium that helps determine the behavioral response to odor.

Information processing in mammalian olfactory system

In recent years, considerable progress has been made in understanding how the olfactory system uses neural space to encode sensory information. In this review, we focus on recent studies aimed at understanding the organizational strategies used by the mammalian olfactory system to encode information. The odorant receptor gene family is discussed in the context of its genomic organization as well as the specificity of olfactory sensory neurons. These data have important consequences for the mechanisms of odorant receptor gene choice by a given sensory neuron. Division of the olfactory epithelium into zones that express different sets of odorant receptors is the first level of input organization. The topographical relationship between periphery and olfactory bulb represents a further level of processing of information and results in the formation of a highly organized spatial map of information in the olfactory bulb. There, local circuitry refines the sensory input through various lateral interactions. Finally, the factors that may drive the development of such a spatial map are discussed. The onset of expression and the establishment of the zonal organization of odorant receptor genes in the epithelium are not dependent upon the presence of the olfactory bulb, suggesting that the functional identity of olfactory sensory neurons is determined independently of target selection.

Responses of olfactory receptor neurons in the spiny lobster to binary mixtures are predictable using a noncompetitive model that incorporates excitatory and inhibitory transduction pathways

Coding of binary mixtures by a population of olfactory receptor neurons in the spiny lobster (Panulirus argus) was examined. Extracellular single-unit responses of 50 neurons to seven compounds and their binary mixtures were recorded. The ability of a noncompetitive model with correction for binding inhibition to predict responses to mixtures based on responses to their components was compared with the predictive abilities of other models. This model assumes that different compounds activate different transduction processes in the same neuron leading to excitation or inhibition, and it includes a term quantifying the degree to which binding of an odorant to its receptor sites is inhibited by other compounds. The model accurately predicted the absolute response magnitude of the population of neurons for 13 of 15 mixtures assessed, which is superior to the predictive power of any of the other models. The model also accurately predicted the across neuron patterns generated by the binary mixtures, as evaluated by multidimensional scaling analysis. The results suggest that there is no emergence of unique qualities for binary mixtures relative to components of these mixtures.

Relation of chemical structure to specificity of response in olfactory glomeruli

Recent work on the mammalian olfactory system shows that sensory neurons expressing the same type of odor receptor converge their axons onto one or a few glomeruli. This nearly one-to-one correspondence between an odor receptor type and an olfactory glomerulus gives rise to the tuning specificity of bulbar mitral and tufted cells, such that individual cells are only activated by a range of odor molecules having a similar chemical structure. These findings now make it possible to analyze the molecular mechanisms involved in the functional differentiation of sensory neurons, as well as those involved in their specific connections with bulbar neurons.

Chemistry of sex attraction

The chemical communication system used to attract mates involves not only the overt chemical signals but also indirectly a great deal of chemistry in the emitter and receiver. As an example, in emitting female moths, this includes enzymes (and cofactors, mRNA, genes) of the pheromone biosynthetic pathways, hormones (and genes) involved in controlling pheromone production, receptors and second messengers for the hormones, and host plant cues that control release of the hormone. In receiving male moths, this includes the chemistry of pheromone transportation in antennal olfactory hairs (binding proteins and sensillar esterases) and the chemistry of signal transduction, which includes specific dendritic pheromone receptors and a rapid inositol triphosphate second messenger signal. A fluctuating plume structure is an integral part of the signal since the antennal receptors need intermittent stimulation to sustain upwind flight. Input from the hundreds of thousands of sensory cells is processed and integrated with other modalities in the central nervous system, but many unknown factors modulate the information before it is fed to motor neurons for behavioral responses. An unknown brain control center for pheromone perception is discussed relative to data from behavioral-threshold studies showing modulation by biogenic amines, such as octopamine and serotonin, from genetic studies on pheromone discrimination, and from behavioral and electrophysiological studies with behavioral antagonists.