An identified neuron mediates the unconditioned stimulus in associative olfactory learning in honeybees

DURING classical conditioning, animals learn to associate a neutral stimulus with a meaningful, or unconditioned, stimulus. The unconditioned stimulus is essential for forming associations, and modifications in the processing of the unconditioned stimulus are thought to underlie more complex learning forms1–4. Information on the neuronal representation of the unconditioned stimulus is therefore required for understanding both basic and higher-order features of conditioning. In honeybees, conditioning of the proboscis extension reflex occurs after a single pairing of an odour (conditioned stimulus) with food (unconditioned stimulus)5,6 and shows several higher-order features of conditioning6–8. I report here the identification of an interneuron that mediates the unconditioned stimulus in this associative learning. Its physiology is also compatible with a function in complex forms of associative learning. This neuron provides the first direct access to the cellular mechanisms underlying the reinforcing properties of the unconditioned stimulus pathway.

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