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The antennal lobe (AL) is the first olfactory center of the insect brain and is constituted of different functional units, the glomeruli. In the AL, odors are coded as spatiotemporal patterns of glomerular activity. In honeybees, olfactory learning during early adulthood modifies neural activity in the AL on a long-term scale and also enhances later memory retention. By means of behavioral experiments, we first verified that olfactory learning between the fifth and eighth day of adulthood induces better retention performances at a late adult stage than the same experience acquired before or after this period. We checked that the specificity of memory for the odorants used was improved. We then studied whether such early olfactory learning also induces long-term structural changes in the AL consistent with the formation of long-term olfactory memories. We also measured the volume of 15 identified glomeruli in the ALs of 17-day-old honeybees that either experienced an odor associated with sucrose solution between the fifth and eighth day of adulthood or were left untreated. We found that early olfactory experience induces glomerulus-selective increases in volume that were specific to the learned odor. By comparing our volumetric measures with calcium-imaging recordings from a previous study, performed in 17-day-old bees subjected to the same treatment and experimental conditions, we found that glomeruli that showed structural changes after early learning were those that exhibited a significant increase in neural activity. Our results make evident a correlation between structural and functional changes in the AL following early olfactory learning., (© 2012 The Authors. European Journal of Neuroscience © 2012 Federation of European Neuroscience Societies and Blackwell Publishing Ltd.)

The formation of a stable olfactory memory requires activity within several brain regions. The honeybee provides a valuable model to map complex olfactory learning tasks onto certain brain areas. To this end, we used injections of the local anaesthetic procaine to reversibly block spike activity in a specific brain region, the mushroom body (MB). We first investigated the physiological effects of procaine on cultured MB neurons from adult honeybee brains. Using the whole-cell configuration of the patch-clamp technique, we show that procaine blocks voltage-gated Na+ and K+ currents in a dose-dependent manner between 0.1 and 10 mm. The effects are reversible within a few minutes of wash. Lidocaine acts similarly, but is less effective at the tested concentrations. We then studied the role of the MBs during reversal learning by blocking the neural activity within these structures by injecting procaine. During reversal learning bees learn to revert their responses to two odorants, one rewarded (A+) and one unrewarded (B-), if their contingencies are changed (A- vs B+). Injecting procaine into the MBs impaired reversal learning. Procaine treatment during acquisition left the later retention of the initial learning (A+ vs B-) intact. Similarly, a differential conditioning task involving novel odorants (C+ vs D-) was intact under procaine treatment. Our experiments show that local injections of procaine can be used to map learning tasks onto specific regions of the insect brain. We conclude that intact MB activity is required for the acquisition of reversal learning, but not for simple differential learning tasks.