. Sparsening and temporal sharpening of olfactory representations in the honeybee mushroom bodies. J Neurophysiol 94: [3303][3304][3305][3306][3307][3308][3309][3310][3311][3312][3313] 2005. First published July 13, 2005; doi:10.1152/jn.00397.2005. We explored the transformations accompanying the transmission of odor information from the first-order processing area, the antennal lobe, to the mushroom body, a higherorder integration center in the insect brain. Using Ca 2ϩ 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.

Memory is created by several interlinked processes in the brain, some of which require long-term gene regulation. Epigenetic mechanisms are likely candidates for regulating memory-related genes. Among these, DNA methylation is known to be a long lasting genomic mark and may be involved in the establishment of long-term memory. Here we demonstrate that DNA methyltransferases, which induce and maintain DNA methylation, are involved in a particular aspect of associative long-term memory formation in honeybees, but are not required for short-term memory formation. While long-term memory strength itself was not affected by blocking DNA methyltransferases, odor specificity of the memory (memory discriminatory power) was. Conversely, perceptual discriminatory power was normal. These results suggest that different genetic pathways are involved in mediating the strength and discriminatory power of associative odor memories and provide, to our knowledge, the first indication that DNA methyltransferases are involved in stimulus-specific associative long-term memory formation.

We investigated the effect of associative learning on early sensory processing, by combining classical conditioning with in vivo calcium-imaging of secondary olfactory neurons, the projection neurons (PNs) in the honey bee antennal lobe (AL). We trained bees in a differential conditioning paradigm in which one odour (A+) was paired with a reward, while another odour (B-) was presented without a reward. Two to five hours after differential conditioning, the two odour-response patterns became more different in bees that learned to discriminate between A and B, but not in bees that did not discriminate. This learning-related change in neural odour representations can be traced back to glomerulus-specific neural plasticity, which depended on the response profile of the glomerulus before training. (i) Glomeruli responding to A but not to B generally increased in response strength. (ii) Glomeruli responding to B but not to A did not change in response strength. (iii) Glomeruli responding to A and B decreased in response strength. (iv) Glomeruli not responding to A or B increased in response strength. The data are consistent with a neural network model of the AL, which we based on two plastic synapse types and two well-known learning rules: associative, reinforcer-dependent Hebbian plasticity at synapses between olfactory receptor neurons (ORNs) and PNs; and reinforcer-independent Hebbian plasticity at synapses between local interneurons and ORNs. The observed changes strengthen the idea that odour learning optimizes odour representations, and facilitates the detection and discrimination of learned odours.

The insect mushroom bodies are higher-order brain centers and critical for odor learning. We investigated experience dependent plasticity of their intrinsic neurons, the Kenyon cells (KCs). Using calcium imaging, we recorded KC responses and investigated nonassociative plasticity by applying repeated odor stimuli. Associative plasticity was examined by performing appetitive odor learning experiments. Olfactory, gustatory and tactile antennal stimuli evoked phasic calcium transients in sparse ensembles of responding KCs. Repeated stimulation with an odor led to a decrease in KCs' response strength. The pairing of an odor (conditioned stimulus, CS) with a sucrose reward (unconditioned stimulus) induced a prolongation of KC responses. After conditioning, KC responses to a rewarded odor (CS+) recovered from repetition-induced decrease, while the responses to a non-rewarded odor (CS−) decreased further. The spatiotemporal pattern of activated KCs changed for both odors when compared with the response before conditioning but the change was stronger for the CS−. These results demonstrate that KC responses are subject to non-associative plasticity during odor repetition and undergo associative plasticity after appetitive odor learning.

An important component in understanding central olfactory processing and coding in the insect brain relates to the characterization of the functional divisions between morphologically distinct types of projection neurons (PN). Using calcium imaging, we investigated how the identity, concentration and mixtures of odors are represented in axon terminals (boutons) of two types of PNs – lPN and mPN. In lPN boutons we found less concentration dependence, narrow tuning profiles at a high concentration, which may be optimized for fine, concentration-invariant odor discrimination. In mPN boutons, however, we found clear rising concentration dependence, broader tuning profiles at a high concentration, which may be optimized for concentration coding. In addition, we found more mixture suppression in lPNs than in mPNs, indicating lPNs better adaptation for synthetic mixture processing. These results suggest a functional division of odor processing in both PN types.