APL synapses distribution on PN and KC meshes, primary data, calcium imaging macros and python scripts from Prisco et al.

To identify and memorize discrete but similar environmental inputs, the
brain needs to distinguish between subtle differences of activity patterns
in defined neuronal populations. The Kenyon cells of the Drosophila adult
mushroom body (MB) respond sparsely to complex olfactory input, a property
that is thought to support stimuli discrimination in the MB. To understand
how this property emerges, we investigated the role of the inhibitory
anterior paired lateral neuron (APL) in the input circuit of the MB, the
calyx. Within the calyx, presynaptic boutons of projection neurons (PNs)
form large synaptic microglomeruli (MGs) with dendrites of postsynaptic
Kenyon cells (KCs). Combining EM data analysis and in vivo calcium
imaging, we show that APL, via inhibitory and reciprocal synapses
targeting both PN boutons and KC dendrites, normalizes odour-evoked
representations in MGs of the calyx. APL response scales with the PN input
strength and is regionalized around PN input distribution. Our data
indicate that the formation of a sparse code by the Kenyon cells requires
APL-driven normalization of their MG postsynaptic responses. This work
provides experimental insights on how inhibition shapes sensory
information representation in a higher brain centre, thereby supporting
stimuli discrimination and allowing for efficient associative memory
formation.