Using local anaesthetics to block neuronal activity and map specific learning tasks to the mushroom bodies of an insect brain

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Abstract
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.

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