Activity-Dependent Plasticity in an Olfactory Circuit

Gamma-aminobutyric acid (GABA)-mediated neural connections in the Drosophila antennal lobe

Inhibitory synaptic connections mediated by gamma-aminobutyric acid (GABA) play important roles in the neural computation of the brain. To obtain a detailed overview of the neural connections mediated by GABA signals, we analyzed the distribution of the cells that produce and receive GABA in the Drosophila adult brain. Relatively small numbers of the cells, which form clusters in several areas of the brain, express the GABA synthesis enzyme Gad1. On the other hand, many cells scattered across the brain express ionotropic GABA(A) receptor subunits (Lcch3 and Rdl) and metabotropic GABA(B) receptor subtypes (GABA-B-R1, -2, and -3). To analyze the expression of these genes in distinct identified cell types, we focused on the antennal lobe, where GABAergic neurons play important roles in odor coding. By combining fluorescent in situ hybridization and immunolabeling against GFP expressed with cell-type-specific GAL4 driver strains, we quantified the percentage of the cells that produce or receive GABA for each cell type. GABA was synthesized in the middle antennocerebral tract (mACT) projection neurons and two types of local neurons. Among them, mACT neurons had few presynaptic sites in the antennal lobe, making the local neurons essentially the sole provider of GABA signals there. On the other hand, not only these local neurons but also all types of projection neurons expressed both ionotropic and metabotropic GABA receptors. Thus, even though inhibitory signals are released from only a few, specific types of local neurons, the signals are read by most of the neurons in the antennal lobe neural circuitry.

A circuit supporting concentration-invariant odor perception in Drosophila

BACKGROUND

Most odors are perceived to have the same quality over a large concentration range, but the neural mechanisms that permit concentration-invariant olfactory perception are unknown. In larvae of the vinegar fly Drosophila melanogaster, odors are sensed by an array of 25 odorant receptors expressed in 21 olfactory sensory neurons (OSNs). We investigated how subsets of larval OSNs with overlapping but distinct response properties cooperate to mediate perception of a given odorant across a range of concentrations.

RESULTS

Using calcium imaging, we found that ethyl butyrate, an ester perceived by humans as fruity, activated three OSNs with response thresholds that varied across three orders of magnitude. Whereas wild-type larvae were strongly attracted by this odor across a 500-fold range of concentration, individuals with only a single functional OSN showed attraction across a narrower concentration range corresponding to the sensitivity of each ethyl butyrate-tuned OSN. To clarify how the information carried by different OSNs is integrated by the olfactory system, we characterized the response properties of local inhibitory interneurons and projection neurons in the antennal lobe. Local interneurons only responded to high ethyl butyrate concentrations upon summed activation of at least two OSNs. Projection neurons showed a reduced response to odors when summed input from two OSNs impinged on the circuit compared to when there was only a single functional OSN.

CONCLUSIONS

Our results show that increasing odor concentrations induce progressive activation of concentration-tuned olfactory sensory neurons and concomitant recruitment of inhibitory local interneurons. We propose that the interplay of combinatorial OSN input and local interneuron activation allows animals to remain sensitive to odors across a large range of stimulus intensities.

Learning-dependent structural plasticity in the adult olfactory pathway

Olfactory learning in humans leads to enhanced perceptual discrimination of odor cues. Examining mouse models of both aversive and appetitive conditioning, we demonstrate a mechanism which may underlie this adult learning phenomenon. Topographically unique spatial wiring of the olfactory system allowed us to demonstrate that emotional learning of odor cues alters the primary sensory representation within the nose and brain of adult mice. Transgenic mice labeled at the M71 odorant receptor (specifically activated by the odorant acetophenone) were behaviorally trained with olfactory-dependent fear conditioning or conditioned place preference using acetophenone. Odor-trained mice had larger M71-specific glomeruli and an increase in M71-specific sensory neurons within the nose compared with mice that were untrained, trained to a non-M71 activating odorant, or had nonassociative pairings of acetophenone. These data indicate that the primary sensory neuron population and its projections may remain plastic in adults, providing a structural mechanism for learning-enhanced olfactory sensitivity and discrimination.

Olfactory information processing in the Drosophila antennal lobe: anything goes?

When an animal smells an odor, olfactory sensory neurons generate an activity pattern across olfactory glomeruli of the first sensory neuropil, the insect antennal lobe or the vertebrate olfactory bulb. Here, several networks of local neurons interact with sensory neurons and with output neurons--insect projection neurons, or vertebrate mitral/tufted cells. The extent and form of information processing taking place in these local networks has been subject of controversy. To investigate the role of local neurons in odor information processing we have used the calcium sensor G-CaMP to perform in vivo recordings of odor-evoked spatiotemporal activity patterns in five genetically defined neuron populations of the antennal lobe of Drosophila melanogaster: three distinct populations of local neurons (two GABAergic and one cholinergic), as well as sensory neurons and projection neurons. Odor-specific and concentration dependent spatiotemporal response patterns varied among neuron populations. Activity transfer differed along the olfactory pathway for different glomerulus-odor combinations: we found cases of profile broadening and of linear and complex transfer. Moreover, the discriminability between the odors also varied across neuron populations and was maximal in projection neurons. Discriminatory power increased with higher odor concentrations over a wide dynamic range, but decreased at the highest concentration. These results show the complexity and diversity of odor information processing mechanisms across olfactory glomeruli in the fly antennal lobe.

Smelling, tasting, learning: Drosophila as a study case

Understanding brain function is to account for how the sensory system is integrated with the organism's needs to organize behaviour. We review what is known about these processes with regard to chemosensation and chemosensory learning in Drosophila. We stress that taste and olfaction are organized rather differently. Given that, e.g., sugars are nutrients and should be eaten (irrespective of the kind of sugar) and that toxic substances should be avoided (regardless of the kind of death they eventually cause), tastants are classified into relatively few behavioural matters of concern. In contrast, what needs to be done in response to odours is less evolutionarily determined. Thus, discrimination ability is warranted between different kinds of olfactory input, as any difference between odours may potentially be or become important. Therefore, the olfactory system has a higher dimensionality than gustation, and allows for more sensory-motor flexibility to attach acquired behavioural 'meaning' to odours. We argue that, by and large, larval and adult Drosophila are similar in these kinds of architecture, and that additionally there are a number of similarities to vertebrates, in particular regarding the cellular architecture of the olfactory pathway, the functional slant of the taste and smell systems towards classification versus discrimination, respectively, and the higher plasticity of the olfactory sensory-motor system. From our point of view, the greatest gap in understanding smell and taste systems to date is not on the sensory side, where indeed impressive advances have been achieved; also, a satisfying account of associative odour-taste memory trace formation seems within reach. Rather, we lack an understanding as to how sensory and motor formats of processing are centrally integrated, and how adaptive motor patterns actually are selected. Such an understanding, we believe, will allow the analysis to be extended to the motivating factors of behaviour, eventually leading to a comprehensive account of those systems which make Drosophila do what Drosophila's got to do.

Neuronal programmed cell death induces glial cell division in the adult Drosophila brain

Although mechanisms that lead to programmed cell death (PCD) in neurons have been analysed extensively, little is known about how surrounding cells coordinate with it. Here we show that neuronal PCD in the Drosophila brain induces glial cell division. We identified PCD in neurons and cell division in glia occurring in a consistent spatiotemporal manner in adult flies shortly after eclosion. Glial division was suppressed when neuronal PCD was inhibited by ectopic expression of the caspase inhibitor gene p35, indicating their causal relationship. Glia also responded to neural injury in a similar manner: both stab injury and degeneration of sensory axons in the brain caused by antennal ablation induced glial division. Eiger, a tumour necrosis factor superfamily ligand, appears to be a link between developmental PCD/neural injury and glial division, as glial division was attenuated in eiger mutant flies. Whereas PCD soon after eclosion occurred in eiger mutants as in the wild type, we observed excess neuronal PCD 2 days later, suggesting a protective function for Eiger or the resulting glial division against the endogenous PCD. In older flies, between 6 and 50 days after adult eclosion, glial division was scarcely observed in the intact brain. Moreover, 8 days after adult eclosion, glial cells no longer responded to brain injury. These results suggest that the life of an adult fly can be divided into two phases: the first week, as a critical period for neuronal cell death-associated glial division, and the remainder.

Cracking neural circuits in a tiny brain: new approaches for understanding the neural circuitry of Drosophila

Genetic screens in Drosophila have identified many genes involved in neural development and function. However, until recently, it has been impossible to monitor neural signals in Drosophila central neurons, and it has been difficult to make specific perturbations to central neural circuits. This has changed in the past few years with the development of new tools for measuring and manipulating neural activity in the fly. Here we review how these new tools enable novel conceptual approaches to 'cracking circuits' in this important model organism. We discuss recent studies aimed at defining the cognitive demands on the fly brain, identifying the cellular components of specific neural circuits, mapping functional connectivity in those circuits and defining causal relationships between neural activity and behavior.

A model of stimulus-specific neural assemblies in the insect antennal lobe

It has been proposed that synchronized neural assemblies in the antennal lobe of insects encode the identity of olfactory stimuli. In response to an odor, some projection neurons exhibit synchronous firing, phase-locked to the oscillations of the field potential, whereas others do not. Experimental data indicate that neural synchronization and field oscillations are induced by fast GABA(A)-type inhibition, but it remains unclear how desynchronization occurs. We hypothesize that slow inhibition plays a key role in desynchronizing projection neurons. Because synaptic noise is believed to be the dominant factor that limits neuronal reliability, we consider a computational model of the antennal lobe in which a population of oscillatory neurons interact through unreliable GABA(A) and GABA(B) inhibitory synapses. From theoretical analysis and extensive computer simulations, we show that transmission failures at slow GABA(B) synapses make the neural response unpredictable. Depending on the balance between GABA(A) and GABA(B) inputs, particular neurons may either synchronize or desynchronize. These findings suggest a wiring scheme that triggers stimulus-specific synchronized assemblies. Inhibitory connections are set by Hebbian learning and selectively activated by stimulus patterns to form a spiking associative memory whose storage capacity is comparable to that of classical binary-coded models. We conclude that fast inhibition acts in concert with slow inhibition to reformat the glomerular input into odor-specific synchronized neural assemblies.

Clonal analysis of Drosophila antennal lobe neurons: diverse neuronal architectures in the lateral neuroblast lineage

The antennal lobe (AL) is the primary structure in the Drosophila brain that relays odor information from the antennae to higher brain centers. The characterization of uniglomerular projection neurons (PNs) and some local interneurons has facilitated our understanding of olfaction; however, many other AL neurons remain unidentified. Because neuron types are mostly specified by lineage and temporal origins, we use the MARCM techniques with a set of enhancer-trap GAL4 lines to perform systematical lineage analysis to characterize neuron morphologies, lineage origin and birth timing in the three AL neuron lineages that contain GAL4-GH146-positive PNs: anterodorsal, lateral and ventral lineages. The results show that the anterodorsal lineage is composed of pure uniglomerular PNs that project through the inner antennocerebral tract. The ventral lineage produces uniglomerular and multiglomerular PNs that project through the middle antennocerebral tract. The lateral lineage generates multiple types of neurons, including uniglomeurlar PNs, diverse atypical PNs, various types of AL local interneurons and the neurons that make no connection within the ALs. Specific neuron types in all three lineages are produced in specific time windows, although multiple neuron types in the lateral lineage are made simultaneously. These systematic cell lineage analyses have not only filled gaps in the olfactory map, but have also exemplified additional strategies used in the brain to increase neuronal diversity.