Habituation of an appetitive reflex in the honeybee.

1. The proboscis extension reflex is an appetitive component of the bee's feeding behavior that is elicited by touching one antenna with a droplet of sugar water. Repetitive stimulation leads to a decrement and finally to the disappearance of the response, which can be restored by stimulating the contralateral antenna. This behavioral plasticity conforms to essential parametric characteristics for habituation. 2. The response was quantified by recording extracellularly from a muscle involved in proboscis movement, by measuring the duration of the proboscis extension, or by determining the number of trials necessary to abolish any visible response. 3. Because habituation was restricted to the repetitively stimulated antenna and did not generalize to the contralateral hemisphere, the neural circuits mediating habituation may be confined to one hemisphere. 4. State dependence of habituation could be demonstrated by showing that hungry animals exhibited a smaller response decrement and required more trials until disappearance of the response compared with satiated animals. The initial response and the subsequent response decrement are separate components determined by satiation level and stimulus strength. 5. Depleting the nervous system of monoamines by the use of reserpine abolished the reflex in 30% of the animals and reduced responsiveness in the remainder. Injection of octopamine or its metabolic precursor tyramine restored the reflex in reserpinized unresponsive animals, and tyramine also enhanced the muscle-spike discharge of reserpinized, responsive animals. In undepleted animals, tyramine application also accelerated the rate of habituation of the reflex. We therefore propose that octopaminergic neurons participate in mediating food arousal and the state dependence of habituation and, in a separate process, influence the response decrement during habituation. 6. Application of an acetylcholine esterase (AChE) inhibitor and a cholinergic receptor blocker confirmed histochemical data that implicate cholinergic transmission in the reflex pathways. 7. The combined pharmacological dissection of the reflex and immunocytochemical investigations of its chemical architecture provide evidence that the proboscis extension reflex is mediated by nonaminergic and monoaminergic pathways operating in parallel.