We reveal a central role for chance neuronal events in the decision of a male fly to court, which can be modeled as a coin flip with odds set by motivational state. The decision is prompted by a tap of a female with the male's pheromone-receptor-containing foreleg. Each tap evokes competing excitation and inhibition onto P1 courtship command neurons. A motivating dopamine signal desensitizes P1 to the inhibition, increasing the fraction of taps that successfully initiate courtship. Once courtship has begun, the same dopamine tone potentiates recurrent excitation of P1, maintaining the courtship of highly motivated males for minutes and buffering against termination. Receptor diversity within P1 creates separate channels for tuning the propensities to initiate and sustain courtship toward appropriate targets. These findings establish a powerful invertebrate system for cue-triggered binary decisions and demonstrate that noise can be exploited by motivational systems to make behaviors scalable and flexible.

Newborns and hatchlings can perform incredibly sophisticated behaviors, but many animals abstain from sexual activity at the beginning of life. Hormonal changes have long been known to drive both physical and behavioral changes during adolescence, leading to the largely untested assumption that sexuality emerges from organizational changes to neuronal circuitry. We show that the transition to sexuality in male Drosophila is controlled by hormonal changes, but this regulation is functional rather than structural. In very young males, a broadly acting hormone directly inhibits the activity of three courtship-motivating circuit elements, ensuring the complete suppression of sexual motivation and behavior. Blocking or overriding these inhibitory mechanisms evokes immediate and robust sexual behavior from very young and otherwise asexual males. Similarities to mammalian adolescence suggest a general principle in which hormonal changes gate the transition to sexuality not by constructing new circuitry but by permitting activity in otherwise latent motivational circuit elements.

Despite decades of investigation, the neuronal and molecular bases of motivational states remain mysterious. We have recently developed a novel, reductionist, and scalable system for in-depth investigation of motivation using the mating drive of male Drosophila melanogaster (Drosophila), the methods for which we detail here. The behavioral paradigm centers on the finding that male mating drive decreases alongside fertility over the course of repeated copulations and recovers over ~3 d. In this system, the powerful neurogenetic tools available in the fly converge with the genetic accessibility and putative wiring diagram available for sexual behavior. This convergence allows rapid isolation and interrogation of small neuronal populations with specific motivational functions. Here we detail the design and execution of the satiety assay that is used to measure and alter courtship motivation in the male fly. Using this assay, we also demonstrate that low male mating drive can be overcome by stimulating dopaminergic neurons. The satiety assay is simple, affordable, and robust to influences of genetic background. We expect the satiety assay to generate many new insights into the neurobiology of motivational states.