Biomedical subjects

C Giovanni Galizia

Publications and source records attributed to C Giovanni Galizia.

15 recordsLinked to original sources

PubMed2006

Role of histamine as a putative inhibitory transmitter in the honeybee antennal lobe.

BACKGROUND: Odors are represented by specific spatio-temporal activity patterns in the olfactory bulb of vertebrates and its insect analogue, the antennal lobe. In honeybees inhibitory circuits in the AL are involved in the processing of odors to shape afferent odor responses. GABA is known as an inhibitory transmitter in the antennal lobe, but not all interneurons are GABAergic. Therefore we sought to analyze the functional role of the inhibitory transmitter histamine for the processing of odors in the honeybee AL. RESULTS: We optically recorded the representation of odors before, during and after histamine application at the input level (estimated from a compound signal), and at the output level (by selectively measuring the projection neurons). For both, histamine led to a strong and reversible reduction of odor-evoked responses. CONCLUSION: We propose that histamine, in addition to GABA, acts as an inhibitory transmitter in the honeybee AL and is therefore likely to play a role in odor processing.

Journal Article↗

PubMed2006

A command chemical triggers an innate behavior by sequential activation of multiple peptidergic ensembles.

BACKGROUND: At the end of each molt, insects shed their old cuticle by performing the ecdysis sequence, an innate behavior consisting of three steps: pre-ecdysis, ecdysis, and postecdysis. Blood-borne ecdysis-triggering hormone (ETH) activates the behavioral sequence through direct actions on the central nervous system. RESULTS: To elucidate neural substrates underlying the ecdysis sequence, we identified neurons expressing ETH receptors (ETHRs) in Drosophila. Distinct ensembles of ETHR neurons express numerous neuropeptides including kinin, FMRFamides, eclosion hormone (EH), crustacean cardioactive peptide (CCAP), myoinhibitory peptides (MIP), and bursicon. Real-time imaging of intracellular calcium dynamics revealed sequential activation of these ensembles after ETH action. Specifically, FMRFamide neurons are activated during pre-ecdysis; EH, CCAP, and CCAP/MIP neurons are active prior to and during ecdysis; and activity of CCAP/MIP/bursicon neurons coincides with postecdysis. Targeted ablation of specific ETHR ensembles produces behavioral deficits consistent with their proposed roles in the behavioral sequence. CONCLUSIONS: Our findings offer novel insights into how a command chemical orchestrates an innate behavior by stepwise recruitment of central peptidergic ensembles.

Animals↗

PubMed2006

Temporal integrity of an airborne odor stimulus is greatly affected by physical aspects of the odor delivery system.

There is currently a debate about the role played by temporal patterns in neural activity in olfactory coding. An accurate analysis of this question, however, is only possible if the temporal properties of a stimulus itself are well defined. So far, no technique with sufficient temporal resolution has been available to accomplish this. Using a photoionization detector (PID), we show that the configuration of the odor delivery apparatus and the airflow settings greatly influence the integrity of a stimulus profile within an odor delivery apparatus. In a situation where pulsatile odor stimuli are applied to a stationary preparation, we tested the effect of 1) axial and off-center location within the airstream, 2) airflow of the odor delivery, 3) exit tube length, 4) exit tube diameter, 5) orientation of the odor delivery device in relation to the exhaust flow, and 6) exhaust tube air speed. This has important implications for the study of time in olfaction; significant planning must be incorporated into the design of the experiment to provide a well-defined odor delivery system.

Air Movements↗

PubMed2006

The molecular receptive range of an olfactory receptor in vivo (Drosophila melanogaster Or22a).

Understanding how odors are coded within an olfactory system requires knowledge about its input. This is constituted by the molecular receptive ranges (MRR) of olfactory sensory neurons that converge in the glomeruli of the olfactory bulb (vertebrates) or the antennal lobe (AL, insects). Aiming at a comprehensive characterization of MRRs in Drosophila melanogaster we measured odor-evoked calcium responses in olfactory sensory neurons that express the olfactory receptor Or22a. We used an automated stimulus application system to screen [Ca(2+)] responses to 104 odors both in the antenna (sensory transduction) and in the AL (neuronal transmission). At 10(-2) (vol/vol) dilution, 39 odors elicited at least a half-maximal response. For these odorants we established dose-response relationships over their entire dynamic range. We tested 15 additional chemicals that are structurally related to the most efficient odors. Ethyl hexanoate and methyl hexanoate were the best stimuli, eliciting consistent responses at dilutions as low as 10(-9). Two substances led to calcium decrease, suggesting that Or22a might be constitutively active, and that these substances might act as inverse agonists, reminiscent of G-protein coupled receptors. There was no difference between the antennal and the AL MRR. Furthermore we show that Or22a has a broad yet selective MRR, and must be functionally described both as a specialist and a generalist. Both these descriptions are ecologically relevant. Given that adult Drosophila use approximately 43 ORs, a complete description of all MRRs appears now in reach.

Animals↗

PubMed2006

Sensory memory for odors is encoded in spontaneous correlated activity between olfactory glomeruli.

Sensory memory is a short-lived persistence of a sensory stimulus in the nervous system, such as iconic memory in the visual system. However, little is known about the mechanisms underlying olfactory sensory memory. We have therefore analyzed the effect of odor stimuli on the first odor-processing network in the honeybee brain, the antennal lobe, which corresponds to the vertebrate olfactory bulb. We stained output neurons with a calcium-sensitive dye and measured across-glomerular patterns of spontaneous activity before and after a stimulus. Such a single-odor presentation changed the relative timing of spontaneous activity across glomeruli in accordance with Hebb's theory of learning. Moreover, during the first few minutes after odor presentation, correlations between the spontaneous activity fluctuations suffice to reconstruct the stimulus. As spontaneous activity is ubiquitous in the brain, modifiable fluctuations could provide an ideal substrate for Hebbian reverberations and sensory memory in other neural systems.

Animals↗

PubMed2005

Sparsening and temporal sharpening of olfactory representations in the honeybee mushroom bodies.

We explored the transformations accompanying the transmission of odor information from the first-order processing area, the antennal lobe, to the mushroom body, a higher-order integration center in the insect brain. Using Ca2+ imaging, we recorded activity in the dendrites of the projection neurons that connect the antennal lobe with the mushroom body. Next, we recorded the presynaptic terminals of these projection neurons. Finally, we characterized their postsynaptic partners, the intrinsic neurons of the mushroom body, the clawed Kenyon cells. We found fundamental differences in odor coding between the antennal lobe and the mushroom body. Odors evoked combinatorial activity patterns at all three processing stages, but the spatial patterns became progressively sparser along this path. Projection neuron dendrites and boutons showed similar response profiles, but the boutons were more narrowly tuned to odors. The transmission from projection neuron boutons to Kenyon cells was accompanied by a further sparsening of the population code. Activated Kenyon cells were highly odor specific. Furthermore, the onset of Kenyon cell responses to projection neurons occurred within the first 200 ms and complex temporal patterns were transformed into brief phasic responses. Thus two types of transformations occurred within the MB: sparsening of a combinatorial code, mediated by pre- and postsynaptic processing within the mushroom body microcircuits, and temporal sharpening of postsynaptic Kenyon cell responses, probably involving a broader loop of inhibitory recurrent neurons.

Action Potentials↗

PubMed2005

Computational modeling suggests that response properties rather than spatial position determine connectivity between olfactory glomeruli.

Olfactory responses require the representation of high-dimensional olfactory stimuli within the constraints of two-dimensional neural networks. We used a computational model of the honeybee antennal lobe to test how inhibitory interactions in the antennal lobe should be organized to best reproduce the experimentally measured input-output function in this structure. Our simulations show that a functionally organized inhibitory network, as opposed to an anatomically or all-to-all organized inhibitory network, best reproduces the input-output function of the antennal lobe observed with calcium imaging. In this network, inhibition between each pair of glomeruli was proportional to the similarity of their odor-response profiles. We conclude that contrast enhancement between odorants in the honeybee antennal lobe is best achieved when interglomerular inhibition is organized based on glomerular odor response profiles rather than on anatomical neighborhood relations.

Animals↗

PubMed2004

Odor-driven attractor dynamics in the antennal lobe allow for simple and rapid olfactory pattern classification.

The antennal lobe plays a central role for odor processing in insects, as demonstrated by electrophysiological and imaging experiments. Here we analyze the detailed temporal evolution of glomerular activity patterns in the antennal lobe of honeybees. We represent these spatiotemporal patterns as trajectories in a multidimensional space, where each dimension accounts for the activity of one glomerulus. Our data show that the trajectories reach odor-specific steady states (attractors) that correspond to stable activity patterns at about 1 second after stimulus onset. As revealed by a detailed mathematical investigation, the trajectories are characterized by different phases: response onset, steady-state plateau, response offset, and periods of spontaneous activity. An analysis based on support-vector machines quantifies the odor specificity of the attractors and the optimal time needed for odor discrimination. The results support the hypothesis of a spatial olfactory code in the antennal lobe and suggest a perceptron-like readout mechanism that is biologically implemented in a downstream network, such as the mushroom body.

Animals↗

PubMed2003

The coding of odour-intensity in the honeybee antennal lobe: local computation optimizes odour representation.

We investigated strategies involved in odour intensity coding by the primary olfactory centre of insects, the antennal lobe (AL), the structural and functional analogue of the olfactory bulb. Using calcium imaging in the honeybee, we simultaneously measured the projection neuron output responses and a compound signal dominated by receptor neuron input in identified olfactory glomeruli to odours spanning seven log units of concentration. A comparison of the two processing levels indicates that the intercellular computation within the AL modulates and contrast-enhances the primary olfactory signals. As a result the AL network optimizes the olfactory code: odour representation is improved at lower concentrations, the relative activity of olfactory glomeruli allows encoding odour quality over up to four log-unit concentrations, and odour-intensity is reliably represented in the overall excitation across AL.

Aldehydes↗

PubMed2003

Odor similarity does not influence the time needed for odor processing.

The brain's link between perception and action involves several steps, which include stimulus transduction, neuronal coding of the stimulus, comparison to a memory template and choice of an appropriate behavioral response. All of these need time, and many studies report that the time needed to compare two stimuli correlates inversely with the perceived distance between them. We developed a behavioral assay in which we tested the time that a honeybee needs to discriminate between odors consisting of mixtures of two components, and included both very similar and very different stimuli spanning four log-concentration ranges. Bees learned to discriminate all odors, including very similar odors and the same odor at different concentrations. Even though discriminating two very similar odors appears to be a more difficult task than discriminating two very distinct substances, we found that the time needed to make a choice for or against an odor was independent of odor similarity. Our data suggest that, irrespective of the nature of the olfactory code, the bee olfactory system evaluates odor quality after a constant interval. This may ensure that odors are only assessed after the olfactory network has optimized its representation.

Acyclic Monoterpenes↗

PubMed2002

Hydroxyurea-induced partial mushroom body ablation does not affect acquisition and retention of olfactory differential conditioning in honeybees.

The mushroom bodies (MBs), a paired structure in the insect brain, play a major role in storing and retrieving olfactory memories. We tested whether olfactory learning and odor processing is impaired in honeybees in which MB subunits were partially ablated. Using hydroxyurea (HU) to selectively kill proliferating cells, we created honeybees with varying degrees of MB lesions. Three-dimensional reconstructions of brains were generated to analyze the drug-induced morphological changes. These reconstructions show that, with few exceptions, only the MBs were affected by the drug, while other brain areas remained morphometrically intact. Typically, lesions affected only the MB in one hemisphere of the brain. To preclude HU-induced physiologic deficits in the antennal lobe (AL) affecting olfactory learning, we measured the responses to odors in the AL using an in vivo calcium imaging approach. The response patterns did not differ between the AL of intact versus ablated brain sides within respective specimens. We, therefore, carried out side-specific classical discriminative olfactory conditioning of the proboscis extension reflex (PER) with control bees and with HU-treated bees with or without MB ablations. All experimental groups learned equally to discriminate and respond to a rewarded (CS+) but not to an unrewarded (CS-) conditioned stimulus during acquisition and retention tests. Thus, our results indicate that partial MB lesions do not affect this form of elemental olfactory learning.

Animals↗

PubMed2002

Genetically expressed cameleon in Drosophila melanogaster is used to visualize olfactory information in projection neurons.

Complex external stimuli such as odorants are believed to be internally represented in the brain by spatiotemporal activity patterns of extensive neuronal ensembles. These activity patterns can be recorded by optical imaging techniques. However, optical imaging with conventional fluorescence dyes usually does not allow for resolving the activity of biologically defined groups of neurons. Therefore, specifically targeting reporter molecules to neuron populations of common genetic identity is an important goal. We report the use of the genetically encoded calcium-sensitive fluorescence protein cameleon 2.1 in the Drosophila brain. We visualized odorant-evoked intracellular calcium concentration changes in selectively labeled olfactory projection neurons both postsynaptically in the antennal lobe, the primary olfactory neuropil, and presynaptically in the mushroom body calyx, a structure involved in olfactory learning and memory. As a technical achievement, we show that calcium imaging with a genetically encoded fluorescence probe is feasible in a brain in vivo. This will allow one to combine Drosophila's advanced genetic tools with the physiological analysis of brain function. Moreover, we report for the first time optical imaging recordings in synaptic regions of the Drosophila mushroom body calyx and antennal lobe. This provides an important step for the use of Drosophila as a model system in olfaction.

Animals↗

PubMed2002

Digital atlases of the antennal lobe in two species of tobacco budworm moths, the Oriental Helicoverpa assulta (male) and the American Heliothis virescens (male and female).

The antennal lobe of the moth brain is the primary olfactory center processing information about pheromones and plant odors. We present here a digital atlas of the glomerular antennal lobe structures in the male of Helicoverpa assulta and the male and female of Heliothis virescens, based on synaptic antibody staining combined with confocal microscopy. The numbers of the glomeruli in the three specimens were similar, 65, 66, and 62, respectively. Whereas the male antennal lobe has a macroglomerular complex consisting of three and four units in the two species, the female lobe has two enlarged glomeruli at a corresponding position, near the entrance of the antennal nerve. Another large glomerulus, showing homology in the three specimens, is ventrally located. The small size of the heliothine moths is advantageous for confocal microscopy because the entire brain can be visualized as a single image stack. The maps are freely accessible on the internet, and the digital form of the data allows each atlas to be rotated and sectioned at any angle, providing for the identification of glomeruli in different preparations.

Animals↗

PubMed2002

Spatial representation of odours in the antennal lobe of the moth Spodoptera littoralis (Lepidoptera: Noctuidae).

Glomeruli within the antennal lobe (AL) of moths are convergence sites for a large number of olfactory receptor neurons (ORNs). The ORNs target single glomeruli. In the male-specific cluster of glomeruli, the macroglomerular complex (MGC), the input is chemotypic in that each glomerulus of the MGC receives information about a specific component of the conspecific female sex pheromone. Little is known about how neurons that detect other odorants arborize in and amongst glomeruli. The present study focuses on how sex pheromones and biologically relevant semiochemicals are represented in the ALs of both sexes of the moth Spodoptera littoralis. To assess this, we optically measured odour-evoked changes of calcium concentration in the ALs. Foci of calcium increase corresponded in size and shape with anatomical glomeruli. More than one glomerulus was normally activated by a specific non-pheromonal odorant and the same glomerulus was activated by several odorants. All odorants and pheromone components tested evoked unique patterns of glomerular activity that were highly reproducible at repeated stimulations within an individual. Odour-evoked patterns were similar between individuals for a given odorant, implicating a spatial olfactory code. In addition, we demonstrated that activity patterns evoked by host-plant related volatiles are similar between males and females.

Animals↗

PubMed2002

Role of inhibition for temporal and spatial odor representation in olfactory output neurons: a calcium imaging study.

The primary olfactory brain center, the antennal lobe (AL) in insects or the olfactory bulb in vertebrates, is a notable example of a neural network for sensory processing. While physiological properties of the input, the olfactory receptor neurons, have become clearer, the operation of the network itself remains cryptic. Therefore we measured spatio-temporal odor-response patterns in the output neurons of the olfactory glomeruli using optical imaging in the honeybee Apis mellifera. We mapped these responses to identified glomeruli, which are the structural and functional units of the AL. Each odor evoked a complex spatio-temporal activity pattern of excited and inhibited glomeruli. These properties were odor- and glomerulus-specific and were conserved across individuals. We compared the spatial pattern of excited glomeruli to previously published signals, which derived mainly from the receptor neurons, and found that they appeared more confined, showing that inhibitory connections enhance the contrast between glomeruli in the AL. To investigate the underlying mechanisms, we applied GABA and the GABA-receptor antagonist picrotoxin (PTX). The results show the presence of two separate inhibitory networks: one is GABAergic and modulates overall AL activity, the other is PTX-insensitive and glomerulus-specific. Inhibitory connections of the latter network selectively inhibit glomeruli with overlapping response profiles, in a way akin to "lateral" inhibition in other sensory systems. Selectively inhibited glomeruli need not be spatial neighbors. The net result is a globally modulated, contrast-enhanced and predictable representation of odors in the olfactory output neurons.

Animals↗