Stereotypic and random patterns of connectivity in the larval mushroom body calyx of Drosophila

Abstract

The larval brain of Drosophila is a useful model to study olfactory processing because of its cellular simplicity. The early stages of central olfactory processing involve the detection of odor features, but the coding mechanisms that transform them into a representation in higher brain centers is not clear. Here we examine the pattern of connectivity of the main neurons that process olfactory information in the calyx (dendritic region) of the mushroom bodies, a higher brain center essential for associative olfactory learning. The larval calyx has a glomerular organization. We generated a map of calyx glomeruli, using both anatomical criteria and the pattern of innervation by subsets of its input neurons (projection neurons), molecularly identified by GAL4 markers. Thus, we show that projection neurons innervate calyx glomeruli in a stereotypic manner. By contrast, subsets of mushroom body neurons (Kenyon cells) that are labeled by GAL4 markers show no clear preference for specific glomeruli. Clonal subsets of Kenyon cells show some preference for subregions of the calyx, implying that they receive distinct input. However, at the level of individual glomeruli, dendritic terminals of larval-born Kenyon cells innervate about six glomeruli, apparently randomly. These results are consistent with a model in which Kenyon cells process olfactory information by integrating different inputs from several calyx glomeruli in a combinatorial manner.Kenyon cells ͉ MARCM ͉ olfactory learning ͉ projection neurons ͉ antennal lobe M uch is known about the early steps in detection of olfactory signals and about the circuitry that processes them in the first olfactory relay center of the brain, the antennal lobe (AL) in Drosophila or the olfactory bulb in mammals (1). From these centers, information is carried by second-order olfactory neurons to the secondary olfactory centers: by projection neurons (PNs) to the mushroom body (MB) and lateral horn in Drosophila (2-4) and by mitral͞tufted cells to a number of areas of olfactory cortex in mammals (5). Second-order neurons have stereotypic target areas within at least some of these secondary centers (2-4, 6, 7), and therefore carry a spatial representation of the olfactory map to there. However, the organization of the circuitry at secondary centers, which underlies information processing in both systems, is not well understood. Work in Drosophila and locusts suggests that, in contrast to the broad odor response tuning of PNs, the responses of MB neurons (Kenyon cells, KCs) to the same odors are usually rare and selective (8-10), and electrophysiological studies (9) suggest a model in which KCs act as coincidence detectors of odor input from PNs. In the mammalian piriform cortex, both anatomical and physiological data suggest that olfactory cortical neurons also integrate olfactory information in a combinatorial manner (reviewed in ref. 5).To understand connectivity in the olfactory circuitry, Drosophila offers a simpler system with fewer cells than in mammals. In the larva, single o...

Localized olfactory representation in mushroom bodies of Drosophila larvae

“…In total, we identified 23 calyx glomeruli receiving input by PNs from the 19 AL glomeruli, which matches the previous estimate of 23 GH146-positive calyx glomeruli (20). Although the general positions of calyx glomeruli connected to the same AL glomerulus were conserved, they appeared more variable than AL glomeruli (Fig.…”

“…The proximity of AL glomeruli dealing with similar odor qualities may facilitate cross-talk by local interneurons between glomeruli, broadening olfactory representations during their passage through the AL, but preferentially among subsets of PNs that deal with similar odor qualities. The criterion of AL glomerular arborization to identify PNs supports most features of the previous calyx map (20) and refines it by providing a potential molecular criterion for identifying calyx glomeruli. We thus identified 23 calyx glomeruli that receive input from 19 AL glomeruli.…”

“…2B and 3, Movie S5, and Movie S6). Because most KCs can integrate inputs from widely dispersed and apparently random subsets of calyx glomeruli (20), spatial proximity of glomeruli may be less important for processing olfactory representations in the calyx than in the AL, where the spatial organization of OSN input and PN output is very stereotypic.…”

“…positions relative to KC axon tracts and a few prominent calyx glomeruli at recognizable positions (20), and a new criterion, the AL glomerulus in which each of the 19 classes of PN arborized (see above). For the vast majority of PNs, we found consistent patterns of connectivity between specific AL glomeruli and specific calyx glomeruli ( Fig.…”

“…Each AL glomerulus appears to be connected to the MB calyx by a single PN (19). Whereas the adult calyx consists of hundreds of microglomeruli that cannot be individually identified, the larval calyx is organized in Ϸ34 glomeruli, in each of which a single identifiable PN contacts hundreds of KC dendrites (20). Therefore, the larval olfactory system comprises a small number of identifiable odor quality channels, each carried by a single neuron between successive layers of the olfactory pathway.…”