Processing of olfactory information at three neuronal levels in the spiny lobster

Chemically stimulated feeding behavior in marine animals : Importance of chemical mixtures and involvement of mixture interactions

A review is provided of the chemical components in tissue extracts that elicit feeding behavior in marine fish and crustaceans. For most species, the major stimulants of feeding behavior in excitatory extracts are an assemblage of common metabolites of low molecular weight including amino acids, quaternary ammonium compounds, nucleosides and nucleotides, and organic acids. It is often mixtures of substances rather than individual components that account for the stimulatory capacity of a natural extract. Recent studies using a shrimp,Palaemonetes pugio, are described in which behavioral bioassays were conducted with complex synthetic mixtures formulated on the basis of the composition of four tissue extracts. These results indicate that synergistic interactions occur among the mixture components. The neural mechanisms whereby marine crustaceans receive and code information about chemical mixtures are also reviewed. Narrowly tuned receptor cells, excited only by particular components of food extracts such as specific amino acids, nucleotides, quaternary ammonium compounds, and ammonium ions, are common in lobsters and could transmit information about mixtures as a labeled-line code. However, since physiological recordings indicate that most higher-level neurons in the brain each transmit information about many components of mixtures, rather than about a single component, it is suggested that information about a complex food odor is transmitted as an across-fiber pattern, instead of a labeled-line code. Electrophysiological recordings of responses of peripheral and central neurons of lobsters to odor mixtures and their components reveal that suppressive interactions occur, rather than the synergistic interactions noted earlier in the behavioral studies. Possible reasons for these differences are discussed. Evidence from the behavioral study indicates that the "direction" of a mixture interaction can be concentration-dependent and the synergism may occur at low mixture concentrations, while suppression may occur at high concentrations.

Mixture and odorant processing in the olfactory systems of insects: a comparative perspective

Natural olfactory stimuli are often complex mixtures of volatiles, of which the identities and ratios of constituents are important for odor-mediated behaviors. Despite this importance, the mechanism by which the olfactory system processes this complex information remains an area of active study. In this review, we describe recent progress in how odorants and mixtures are processed in the brain of insects. We use a comparative approach toward contrasting olfactory coding and the behavioral efficacy of mixtures in different insect species, and organize these topics around four sections: (1) Examples of the behavioral efficacy of odor mixtures and the olfactory environment; (2) mixture processing in the periphery; (3) mixture coding in the antennal lobe; and (4) evolutionary implications and adaptations for olfactory processing. We also include pertinent background information about the processing of individual odorants and comparative differences in wiring and anatomy, as these topics have been richly investigated and inform the processing of mixtures in the insect olfactory system. Finally, we describe exciting studies that have begun to elucidate the role of the processing of complex olfactory information in evolution and speciation.

Heterogeneity of voltage- and chemosignal-activated response profiles in vomeronasal sensory neurons

Liolaemus lizards were explored to ascertain whether they would make an amenable model to study single-cell electrophysiology of neurons in the vomeronasal organ (VNO). Despite a rich array of chemosensory-related behaviors chronicled for this genus, no anatomical or functional data exist for the VNO, the organ mediating these types of behaviors. Two Liolaemus species (L. bellii and L. nigroviridis) were collected in Central Chile in the Farellones Mountains and transported to the United States. Lizards were subjected to hypothermia and then a lethal injection of sodium pentabarbitol prior to all experiments described in the following text. Retrograde dye perfusion combined with histological techniques demonstrated a compartmentalization of the proportionally large VNO from the main olfactory epithelium (MOE) in cryosections of L. bellii. SDS-PAGE analysis of the VNO of both species demonstrated the expression of three G protein subunits, namely, G(alphao), G(alphai2), and G(beta), and the absence of G(alphaolf), G(alpha11), and G(q), the latter of which are traditionally found in the MOE. Vomeronasal (VN) neurons were enzymatically isolated for whole cell voltage-clamp electrophysiology of single neurons. Both species demonstrated a tetrodotoxin (TTX)-sensitive, rapidly inactivating sodium current and a tetraethylammonium (TEA)-sensitive potassium current that had a transient and sustained component. VN neurons were classified into two types dependent on the ratio of sodium over sustained potassium current. VN neurons exhibited outward and inward chemosignal-evoked currents when stimulated with pheromone-containing secretions taken from the feces, skin, and precloacal pores. Fifty-nine percent of the neurons were responsive to at least one compound when presented with a battery of five different secretions. The breadth of responsiveness (H metric) demonstrated a heterogeneous population of tuning with a mean of 0.29.

Patch-clamp analysis of voltage-activated and chemically activated currents in the vomeronasal organ of Sternotherus odoratus (stinkpot/musk turtle)

The electrophysiological basis of chemical communication in the specialized olfactory division of the vomeronasal (VN) organ is poorly understood. In total, 198 patch-clamp recordings were made from 42 animals (Sternotherus odoratus, the stinkpot/musk turtle) to study the electrically and chemically activated properties of VN neurons. The introduction of tetramethylrhodamine-conjugated dextran into the VN orifice permitted good visualization of the vomeronasal neural epithelium prior to dissociating it into single neurons. Basic electrical properties of the neurons were measured (resting potential, -54.5 +/- 2.7 mV, N=11; input resistance, 6.7 +/- 1.4 G Omega, N=25; capacitance, 4.2 +/- 0.3 pF, N=22; means +/- S.E.M.). The voltage-gated K(+) current inactivation rate was significantly slower in VN neurons from males than in those from females, and K(+) currents in males were less sensitive (greater K(i)) to tetraethylammonium. Vomeronasal neurons were held at a holding potential of -60 mV and tested for their response to five natural chemicals, female urine, male urine, female musk, male musk and catfish extract. Of the 90 VN neurons tested, 33 (34 %) responded to at least one of the five compounds. The peak amplitude of chemically evoked currents ranged from 4 to 180 pA, with two-thirds of responses less than 25 pA. Urine-evoked currents were of either polarity, whereas musk and catfish extract always elicited only inward currents. Urine applied to neurons harvested from female animals evoked currents that were 2-3 times larger than those elicited from male neurons. Musk-evoked inward currents were three times the magnitude of urine- or catfish-extract-evoked inward currents. The calculated breadth of responsiveness for neurons presented with this array of five chemicals indicated that the mean response spectrum of the VN neurons is narrow (H metric 0.11). This patch-clamp study indicates that VN neurons exhibit sexual dimorphism in function and specificity in response to complex natural chemicals.iol

The evolution of neural coding ideas in the chemical senses

A brief review of the evolution of hypotheses about neural coding in the chemical senses provides some perspective on the current status of these fields, and implications for further development.

Olfactory development in invertebrates. On the scent of central developmental issues

Invertebrate olfactory systems offer many advantages for cellular and molecular studies of development and for functional studies of developmental plasticity. For example, nematodes have chemical senses that can be studied using genetic approaches. Arthropods, which include insects and crustacea, have the advantages that certain neurons can be reliably identified from one individual to another, and that olfactory receptor neurons are located on peripheral appendages and thus can be manipulated independently of their brain targets even very early in development. Among the insects, olfactory learning can be displayed and used as a basis for studying olfactory plasticity in bees; genes are especially tractable in flies; individual growth cones can be visualized in the grasshopper embryo; and receptor neurons and glomeruli of known olfactory specificity and behavioral significance can be followed during early development in moths. In addition, many insect nervous systems are amenable to organ culture and dissociated-cell culture, opening the door to experimental studies of cellular interactions that can not be performed in situ. Recent research in the moth Manduca sexta attempts to identify the nature of the interactions between olfactory sensory axons, olfactory neurons of the brain, and glial cells in the creation of the array of glomeruli that underlie olfaction in the adult. Results indicate that timing of the ingrowth of olfactory receptor axons is critical for normal glomerulus development, that a subset of axons expresses a fasciclin II-like molecule that may play a role in guidance of their growth, and that glial cells must surround developing glomeruli in order to stabilize the 'protoglomerular' template made by receptor axon terminals. Moreover, glial cells are dye-coupled to each other early in glomerulus development and gradually become uncoupled. Electrical activity in neurons is not necessary for glomerulus formation; and some intercellular interactions, perhaps involving soluble factors, appear to involve tyrosine phosphorylation. In sum, a detailed picture is emerging of the cellular interactions that lead to the formation of glomeruli.

Cellular organization and growth-related plasticity of the crayfish olfactory midbrain

Little knowledge is available concerning the detailed anatomy of the crusctacean central olfactory pathway. We are using radiolabeling, Golgi and biocytin/neurobiotin tracer methodologies, at the correlated light and electron microscopical levels, to study the olfactory midbrain of the freshwater crayfish. We have found that primary afferent fibers from the antennular olfactory receptor cells branch extensively throughout the length of the glomerular columns within the olfactory lobes in the midbrain. Globuli cells of the lateral cell clusters ramify as dendritic arborizations within both the olfactory and accessory lobes; their axons project out the olfactory-globular tracts to the lateral protocerebrum, often branching to both sides. Developmental plasticity involving the connections made by afferent fibers within the olfactory lobes may permit detailed examination of organizational changes within the midbrain as the animal grows and adds new afferent input from the periphery.

Peripheral mechanisms of olfactory discrimination of complex mixtures by the spiny lobster: no cell types for mixtures but different contributions of the cells to the across neuron patterns

Toward understanding mechanisms of olfactory discrimination, we have examined the existence of cell types and the role of cells in the coding of odorant quality in the olfactory organ of the spiny lobster. The results consisted of responses of 30 antennular chemoreceptor cells to 8 behaviorally discriminable complex stimuli--4 natural extracts and 4 artificial mixtures, each at 3 concentrations. Multidimensional scaling and cluster analysis failed to identify unequivocal cell types, but rather suggested a continuum of cellular response profiles. The lack of cell types suggests that the code for the quality of natural odorants in this system is a population code. The distribution of cells along the response continuum was based on any of many features of their response profiles. The most effective stimulus (= best stimulus) and the least effective stimulus (= least stimulus), two features of the response profiles, could only partially explain the differences in response profiles of cells. Nonetheless, cells with different response profiles were shown to have different functions in odorant coding. Most cells contribute to some degree to the discrimination of any two stimuli, but a cell's contribution to the discrimination of two stimuli is usually disproportionally robust when those two stimuli produce very different responses in that cell.

Morphology and physiological properties of interneurons in the olfactory midbrain of the crayfish

1. Intracellular recording and staining was used to characterize neurons in the crayfish (Procambarus clarkii) brain that respond to chemical stimuli applied to the major olfactory organs, the antennules. 2. Two distinct morphological types of neurons that have major projections in the olfactory lobes (OLs) of the brain were characterized anatomically (Figs. 1, 2, 3; Table 2) and physiologically (Figs. 4, 5, 6; Table 3). 3. Different individual neurons of one type, with similar 'tree-like' projections in the OLs, have somata distributed in at least 5 different cell body clusters of the brain (Fig. 3) and link different subsets of neuropilar lobes through their distributed arbors (Fig. 1, Table 2). 4. Excitatory, inhibitory and mixed responses were recorded in different neurons when odorant mixtures or individual components of these mixtures were applied to the antennules. Response spectra to individual components were broad and overlapping, but not identical in the neurons tested (Fig. 4; Table 3). Mixture interactions appear to be additive in most of the neurons that we tested, but evidence was obtained for mixture suppression in several cases (Fig. 6). 5. Most of the neurons recorded in this study responded only to stimulation of the ipsilateral antennule (Fig. 5), although subthreshold activity to stimuli applied contralaterally was recorded in several neurons that were strongly excited by ipsilateral stimuli. 6. Chemoresponsive neurons without projections in OL's that have all of their branches confined to the brain, or that project an axon in the circumesophageal connective, are described (Fig. 7).

Morphological and physiological characterization of individual olfactory interneurons connecting the brain and eyestalk ganglia of the crayfish

1. In order to understand the functional organization of the crustacean olfactory system, we are using intracellular recording and staining techniques to correlate the structure and function of single, odorant-sensitive interneurons in the brain of the crayfish Procambarus clarkii. We describe here the anatomy and physiology of interneurons that connect the brain with the medullae terminales or other eyestalk ganglia. 2. All of the interneurons in our study are at least third-order olfactory neurons (second-order olfactory interneurons) because they respond to chemostimulation of the olfactory organ (the antennules) but do not branch in the olfactory lobe (the neuropil to which primary olfactory receptor cells of the antennules project). 3. Much of the central nervous system, including the three main divisions of the brain (protocerebrum, deuterocerebrum, tritocerebrum) and the medullae terminales, are involved in integrating olfactory or multimodal (including olfactory) information, since these areas contain neurites of olfactory interneurons. Previous studies have indicated that regions involved in such processing include the olfactory lobes and accessory lobes of the deuterocerebrum, and regions I, II, IV, and VII (in some species) of the medullae terminales. Our results show that also prominent among regions involved in olfactory or multimodal (including olfactory) integration are the anterior and posterior optic neuropils of the protocerebrum, the lateral and medial antennular neuropils of the deuterocerebrum, the tegumentary neuropils and the antennal neuropils of the tritocerebrum, and neuropils III, VI, XII of the medullae terminales. 4. These olfactory interneurons were sensitive to chemostimulation (unimodal), chemo- and mechanostimulation (bimodal), or chemo-, mechano-, and photostimulation (trimodal). Responses could be excitatory or inhibitory, even for a given neuron. Morphologically complex interneurons (those having bilateral branching) were more likely to have complex response characteristics (trimodal sensitivity) than were morphologically simpler interneurons (those having unilateral branching). Olfactory interneurons with a soma in the medulla terminalis showed the most complex response profiles: they were trimodal, and were exicted by odorants but were inhibited by touch and/or light. This finding suggests that these are complex, high order interneurons. 5. Our studies reveal that olfactory and other sensory information is transmitted between the brain and the medullae terminales (and possibly other eyestalk ganglia) by a coactivated, parallel array of structurally and functionally diverse neurons.

Specificity-related suppression of responses to binary mixtures in olfactory receptors of the Colorado potato beetle

Responses of antennal olfactory receptors of the Colorado potato beetle (Leptinotarsa decemlineata Say) to stimulation with 5 general green odour components, i.e. cis-3-hexen-1-ol, trans-2-hexenal, cis-3-hexenyl acetate, trans-2-hexen-1-ol and 1-hexanol, were recorded extracellularly. Response spectra derived from these recordings cannot be classified into distinct reaction types. The spectra overlap in their sensitivity to individual stimuli, but there are differences in their degree of specialization with a gradual conversion from generalist to specialist receptors. Moreover, specialization is found to different stimuli. Receptor reactions to stimulation with binary mixtures of 3 of these compounds indicated that suppression of the response to one chemical by another is very common in olfactory receptor cells. The more a receptor is specialized, the stronger is this suppression. Suppression in narrowly tuned olfactory receptor neurones, therefore, is expected to play a fundamental role in the recognition of natural odour blends.

Integration of olfactory information in the Colorado potato beetle brain

The processing of olfactory information in the Colorado potato beetle, Leptinotarsa decemlineata Say, was studied by recording responses of olfactory neurones intracellularly in the deutocerebrum. Response characteristics of neurones in this first relay station of the olfactory pathway were measured when the antennae were stimulated with five general green leaf volatiles, i.e. cis-3-hexen-1-ol, trans-2-hexenal, cis-3-hexenyl acetate, trans-2-hexen-1-ol and 1-hexanol. These compounds are part of the so-called green odour of potato, whose defined composition is essential for the beetle's host plant finding. The response spectra of deutocerebral neurones can be divided roughly into two classes: one class containing neurones which are not very specific for the tested compounds, and another class with highly specialized neurones. Their different responses to a potato leaf extract suggest two channels for the processing of olfactory information in the antennal lobe: one channel for the detection of the presence of green leaf odour components, and another one for an evaluation of the component ratios.

Atlas of serotonin-containing neurons in the optic lobes and brain of the crayfish, Cherax destructor

An atlas of neurons in the brain of the crayfish Cherax destructor that are immunoreactive to antibodies raised against serotonin has been compiled from whole mount preparations. Neuronal networks of serotonin-containing cells are identified in the optic lobes and protocerebrum, in the deutocerebrum, and in the tritocerebrum. The consistency of the whole-mount technique allows 50 out of a total of about 100 immunoreactive cells to be individually identified according to their neuronal architecture or the location of their cell somata or axons. Apart from six neurons with axons in the oesophageal connectives, all the immunoreactive cells are intrinsic to the optic lobes and brain.

Physiology of chemoreceptor cells in the legs of the freshwater prawn, Macrobrachium rosenbergii

1. Chemoreceptor cells in the first pereiopods (legs) of the freshwater prawn, Macrobrachium rosenbergii, were investigated using single-unit, extracellular electrophysiological recording techniques on an isolated, perfused leg preparation. 2. The cells were responsive to aqueous extracts of food (shrimp, mullet, trout chow), a salt mixture (artificial sea-water), amino acids (L-arginine HCl, taurine), a quaternary ammonium compound (betaine HCl) and ammonium chloride. 3. The response specificity of individual cells ranged from narrow to broad, but on average was broad, being more similar to chemoreceptor cells of freshwater crayfish than of marine spiny or clawed lobsters. 4. Responses were generally excitatory. However, some responses were inhibitory, the first such demonstration in aquatic crustaceans. 5. These electrophysiological results highly correlate with results of feeding behavioral assays carried out on M. rosenbergii.