Modulation of antennal scanning in the honeybee by sucrose stimuli, serotonin, and octopamine: behavior and electrophysiology.

Antennal motor activity of the honeybee was used to test the effects of sucrose stimuli and of serotonin and octopamine microinjections into the brain. The antennal scanning behavior was analyzed in behavioral experiments. Activity of an antennal muscle, the "fast pedicellus flexor muscle" which dominates scanning behavior, was used as a physiological measure of modulatory effects. A single sucrose stimulus applied to both the antenna and the proboscis leads to significant increases of the frequency of antennal scanning compared to those of untreated controls and animals stimulated with water. A single sucrose stimulus applied only to the antenna or the proboscis has no significant behavioral effects. Injection of small volumes (approximately 500 pl) of serotonin (5HT) or octopamine (OA) at concentrations of 10(-5) M into the dorsal lobe, the sensory motor center of the antenna, leads to functionally antagonistic behavioral effects. While 5HT injection significantly reduces the antennal scanning frequency, OA significantly enhances it. The degree of behavioral modulation is significantly correlated with the activity of the animals. In animals which display low scanning activity, OA injection has an enhancing effect, while 5HT has no effect. In contrast, 5HT injection, but not OA injection, produces a behavioral effect in animals with high scanning activity. Behavioral changes and changes of activity of the fast pedicellus flexor muscle are closely correlated. Significant, functionally antagonistic effects of 5HT and OA on muscle activity were found after injections of the compounds into the dorsal lobe. 5HT leads to a reduction of the muscle potential frequency starting immediately after injection and lasting at least 15 min. OA injection results in an increase of frequency, which has its maximum 5 min after injection. The experiments demonstrate that sucrose, the reward stimulus during associative learning in the bee, also modulates motor activity under nonassociative conditions. The similar effects of sucrose stimulation and OA injection are consistent with the hypothesis that OA mediates the effects of sucrose stimuli.