All animals constantly negotiate external with
internal demands before and during action selection. Energy homeostasis is a
major internal factor biasing action selection. For instance, in addition to
physiologically regulating carbohydrate mobilization, starvation-induced sugar
shortage also biases action selection towards food-seeking and food consumption
behaviors (the counter-regulatory response). Biogenic amines are often involved when such widespread behavioral
biases need to be orchestrated. In mammals, norepinephrine (noradrenalin) is
involved in the counterregulatory response to starvation-induced drops in
glucose levels. The invertebrate homologue of noradrenalin, octopamine (OA) and
its precursor tyramine (TA) are neuromodulators operating in many different
neuronal and physiological processes. We investigated the role of those two
transmitters in Drosophila sugar responsiveness.
Tyrosine-ß-hydroxylase (tßh) mutants
are unable to convert TA into OA. Starved mutants show a reduced sugar response
and their hemolymph sugar concentration is elevated compared to control flies.
When starved, they survive longer. Temporally controlled rescue
experiments revealed an action of the OA/TA-system during the sugar response,
while spatially controlled rescue experiments suggest actions also outside of
the nervous system. Additionally, the analysis of two OA- and four TA-receptor
mutants suggests an involvement of both receptor types in the animals'
physiological and neuronal response to starvation. These results complement the
investigations in Apis mellifera described
in our companion paper (Buckemüller et al.).