‘‘…when the gathering desire is sated, then for a while comes a little respite in their furious passion. Then the same madness returns, the old frenzy is back upon them.’’ -Lucretius (99-55 BCE) De rerum natura
We have identified several circuit elements controlling the increase in courtship drive with abstinence, and the decrease in courtship drive that occurs after mating. Assembling these elements into a wiring diagram and studying their interrelationships and molecular properties reveals some interesting features of motivational control that are likely to be conserved across animals and behaviors.
The central determinant of courtship drive is a dopaminergic signal that biases the probability that encounters with a female will activate P1 command neurons. This dopaminergic signal is elevated by the neuropeptide NPF, which engages in a recurrent excitation loop with a group of neurons called pCd. Activity in this loop is decreased by matings through the activity of Copulation Reporting Neurons (CRNs), decreasing the dopamine tone and making subsequent courtship less likely. Activity in the pCd/NPF loop has a tendency to increase over time due to mutual excitation, but this tendency is at first prevented and then slowed by the molecular composition of the loop neurons. During periods of high motivation, the high activity in the loop activates the activity-dependent transcription factor CREB, which makes leak potassium channels. Since activity in the loop is high, network activity is sustained even in this inhibitory environment. However, when the male is mating and the CRNs are activated, and the activity in the loop is decremented, the inhibitory environment remains and slows the reaccumulation of loop activity, sustaining satiety for several days.
The most interesting idea to emerge from all of this is that prior motivation is a direct determinant of the degree of satiety after goal achievement. That’s fun to think about!