ARTICLE doi:10.1038/nature10776 Conditional modulation of spike-timing- dependent plasticity for olfactory learning Stijn Cassenaer 1,2 & Gilles Laurent 1,3 Mushroom bodies are a well-known site for associative learning in insects. Yet the precise mechanisms that underlie plasticity there and ensure their specificity remain elusive. In locusts, the synapses between the intrinsic mushroom body neurons and their postsynaptic targets obey a Hebbian spike-timing-dependent plasticity (STDP) rule. Although this property homeostatically regulates the timing of mushroom body output, its potential role in associative learning is unknown. Here we show in vivo that pre–post pairing causing STDP can, when followed by the local delivery of a reinforcement-mediating neuromodulator, specify the synapses that will undergo an associative change. At these synapses, and there only, the change is a transformation of the STDP rule itself. These results illustrate the multiple actions of STDP, including a role in associative learning, despite potential temporal dissociation between the pairings that specify synaptic modification and the delivery of reinforcement-mediating neuromodulator signals. Behavioural and genetic experiments in Drosophila and honeybees have revealed that the mushroom body, a brain area containing up to hundreds of thousands of neurons called Kenyon cells, is critical for associative learning of odours 1–11 but not for the expression of innate odour-driven behaviours 5 (Fig. 1a). Recent electrophysiological experiments in locusts and other insects show that the responses of Kenyon cells to odours are highly selective and, thus, rare 12–14 . By contrast, antennal lobe neurons, the source of the olfactory input to Kenyon cells, are few and promiscuous 12 . Odour codes are thus ‘compact’ in the antennal lobe—that is, the representation of each odour engages many neurons in a small population—but ‘sparse’ in the mushroom bodies (Fig. 1a). Although sparse codes require larger neuron populations, they are beneficial for memory because they can reduce interference between memory traces 12,15,16 . Kenyon cells project to two regions, called a- and b-lobes, where they synapse onto small populations of ‘extrinsic’ neurons. In locusts, the synapses between Kenyon cells and b-lobe neurons (bLNs) are modifiable by a Hebbian STDP rule 17–19 (Fig. 1a), but nothing so far implicates STDP in associative learning there; rather, STDP causes the homeostatic regulation of bLN spike timing 17 . Recent experimental results in moths 14 show that Kenyon cell responses to odours recorded during behavioural learning generally occur and end well before reward delivery, indicating that STDP alone cannot support associative con- ditioning 20,21 . Neuromodulation has been proposed recently as a potential solution 22,23 . We address this issue with in vivo electro- physiology in locusts and discover a complex interplay between STDP, reinforcer signals and odour codes in mushroom bodies. Dense odour representations in b-lobes We first examine odour representations in the b-lobes. Using intradendritic recordings, we sampled the responses of 55 bLNs to up to 16 odours (Methods and Fig. 1b). Each bLN responded to nearly every odour, with responses that differed in intensity, patterning, delay and duration across neuron–odour pairs. The probability that a given bLN responded to an odour was 0.97. On average, a bLN fired action potentials in approximately half of the local field potential (LFP) oscil- lation cycles during the odour presentation (0.502 6 0.219, n 5 126 neuron–odour pairs, 25 LFP cycles per response): on average, half of the population was active in any given cycle. Hence, these representa- tions resemble those in the antennal lobes 12 (and exceed them in promiscuity) rather than the sparse representations by Kenyon cells, to which bLNs are directly connected. We tested whether the broad tuning of bLNs might be explained by known features of mushroom body circuits. We implemented a simple model (Methods and ref. 17) constrained by Kenyon cell response statistics and timing 12 , by the properties of STDP at Kenyon cell synapses 17 and by Kenyon cell/bLN (KC–bLN) connectivity ratios estimated from experiments 17 . With such a model, we could reproduce the bLN firing phase observed experimentally 17 (Fig. 2a, left) but not the odour-response intensity or probability (Fig. 2a, right, and Sup- plementary Fig. 1.1a). Rather, activity across the model bLN (mbLN) population saturated rapidly when STDP was turned on. This beha- viour is a known property of rate-based Hebbian learning in model networks and can be counteracted by imposing synaptic weight bounds or renormalization rules 24 . Inhibition limits STDP and saturation In examining our experimental data on bLN responses to odours, we observed that the periodic excitatory input originating from Kenyon cells was often curtailed by phasic inhibitory postsynaptic potentials (Fig. 2b, arrowheads), with onsets at the typical phase of bLN action potentials. We hypothesized that these inhibitory post- synaptic potentials originate from lateral inhibition among bLNs (as put forward in a theoretical exploration 25 ). This was supported by extracellular stimulation of Kenyon cells (Supplementary Fig. 2) and confirmed in paired bLN recordings. Beta-lobe neurons inhibit each other (Fig. 2c) with an estimated connection probability of 28% (n 5 32 connections, 2 reciprocally connected pairs). Although unknown so far, other interneuron populations may also contribute to bLN inhibition. In each oscillation cycle, lateral inhibition reduces the likelihood of late bLN spikes; thus, it should limit the ability of STDP to potentiate KC–bLN synapses. This in turn should curb bLN population activity (see also ref. 26). As predicted, implementing lateral inhibition 1 Division of Biology, California Institute of Technology, Pasadena, California 91125, USA. 2 Broad Fellows Program in Brain Circuitry, California Institute of Technology, Pasadena, California 91125, USA. 3 Max Planck Institute for Brain Research, 60528 Frankfurt am Main, Germany. 2 FEBRUARY 2012 | VOL 482 | NATURE | 47 Macmillan Publishers Limited. All rights reserved ©2012