AN IDENTIFIED NEURON MEDIATES THE UNCONDITIONED STIMULUS IN ASSOCIATIVE OLFACTORY LEARNING IN HONEYBEES

DURING classical conditioning, animals learn to associate a neutral st imulus with a meaningful, or unconditioned, stimulus. The unconditione d 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 representa tion of the unconditioned stimulus is therefore required for understan ding both basic and higher-order features of conditioning. In honeybee s, conditioning of the proboscis extension reflex occurs after a singl e 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 th e unconditioned stimulus in this associative learning. Its physiology is also compatible with a function in complex forms of associative lea rning. This neuron provides the first direct access to the cellular me chanisms underlying the reinforcing properties of the unconditioned st imulus pathway.