"There is only one drive of which it can be said that it generally subjugates all others-the escape drive-but even this one sometimes meets its master." -Konrad Lorenz, On Aggression (1963)
"While we are enjoying a thing which we longed for, the body, from the act of enjoyment, acquires a new disposition, whereby it is determined in another way, other images of things are aroused in it, and the mind begins to conceive and desire something fresh." -Spinoza, Ethics (1677)
"....the small gilded fly does lecher in my sight. Let copulation thrive...." -Shakespeare, King Lear (1605)
"Is a vigorous young man, madly in love...free to tame his passion? Undoubtedly not...When is it that this young man can refrain despite the violence of his passion? When a stronger idea determines in a contrary sense the activity of his body and his soul. But what! the other animals will have the same liberty, then, the same power? Why not? They have senses, memory, feeling, perceptions, as we have. They act with spontaneity as we act. They must have also, as we have, the power of acting by virtue of their perceptions" -Voltaire, The Philosophical Dictionary (1764)
Interval timing and motivation are two mysterious outputs of the nervous system. We study both using the duration of copulation in Drosophila.
After successful courtship, male flies assume and maintain a stereotyped copulation posture for ~25 minutes with a standard deviation of ~4 minutes.
In the first 5 minutes of copulation the male is willing to sacrifice his life to sustain the mating, but 10 minutes later even minor disturbances cause him to truncate the mating. This makes sense because the benefits of mating decrease over time. We use this simple system to understand the neuronal mechanisms that control the decline in motivation over time.
What we've found so far is that small populations of dopaminergic and GABAergic neurons work in opposition to set the persistence state as it declines throughout mating. Stimulating the dopaminergic neurons increases the male's persistence and thereby increases the duration of the mating. Stimulating the GABAergic neurons has the opposite effect, decreasing persistence and shortening mating duration.
These initial results are exciting because of the obvious parallels to work on motivation in mammals where dopamine is often found to promote motivation, and to be counteracted by GABAergic neurons.
The duration of copulation is a quantitative readout of the dynamics of several neuronal populations, and the molecular machinery working within these neurons. Fortunately, assaying copulation duration is easy and amenable to large-scale screening. We believe that a detailed understanding of the molecular and neuronal processes that give rise to the 25 minutes of mating in the fly will lead to general principles that nervous systems use to keep time, control dopamine levels, and regulate motivation.