Dopamine dictates how long sexual drive remains suppressed after stress

Stressful situations tend to reduce the drive to mate across many animal species, but the exact brain mechanisms behind this reaction remain difficult to pin down. A recent study published in the journal iScience provides evidence that being confined to a small space suppresses the courtship behavior of male fruit flies. The findings suggest that the brain’s dopamine system is required to maintain this low sexual motivation after the stressful event has ended.

When an animal experiences physical or psychological stress, the brain undergoes changes that can alter basic physiological functions and behaviors. These shifts can persist long after the stressful event stops. In many mammals, including humans, stress often reduces the desire to engage in sexual behavior.

However, the specific cellular pathways that link a stressful experience to a drop in sexual motivation are not well understood. Fruit flies, scientifically known as Drosophila melanogaster, serve as an excellent model for studying these brain pathways. These insects have a highly mapped nervous system and genetic tools that allow scientists to turn specific brain cells on or off.

Dopamine is a chemical messenger in the brain associated with reward and motivation, and it is known to influence the mating drive in fruit flies. Because stress tends to alter dopamine levels across many species, researchers wanted to investigate if dopamine plays a role in how stress reduces sexual behavior. Previous studies note that stress induced changes in dopamine function are seen in species ranging from rodents to oysters and ants.

The research team consisted of Tomohito Sato, Rana Toyama, and Takaomi Sakai from Tokyo Metropolitan University, along with Toshihiro Kitamoto from the University of Iowa. They designed a specific type of stress test called small space stress. In this setup, male flies are confined to tiny chambers where they can move their legs and rotate their bodies, but they cannot walk freely. This setup mimics the psychological stress of confinement without completely paralyzing the animal.

To examine how small space stress affects courtship, the scientists gathered groups of three to six day old virgin male fruit flies. They placed the individual males into extremely small acrylic chambers. These tiny enclosures measured just three millimeters in diameter and two millimeters in depth. Control flies were placed in standard chambers that measured fifteen millimeters in diameter, allowing them to walk normally.

The males were confined for ten, thirty, or sixty minutes. Immediately after the confinement, the researchers introduced the males to freeze killed virgin females. They recorded the courtship activity of the male flies in observation chambers for ten minutes.

Courtship activity was quantified using a metric called the courtship index. This index is defined as the percentage of time the male spends displaying mating behaviors during the ten minute observation period. A ten minute confinement did not alter the males’ courtship behavior compared to the control group.

However, males confined for thirty or sixty minutes showed a significant reduction in their courtship index. The researchers noted that the sixty minute confinement reduced courtship activity more intensely than the thirty minute confinement. This provides evidence that the severity of the behavioral suppression depends on the length of the stressful experience.

The researchers then tracked how long this behavioral change lasted. They measured the courtship index of males immediately, one hour, two hours, and four hours after a sixty minute confinement. The suppression of mating behavior was still present one hour later, but the males returned to their normal courtship levels after two and four hours.

To ensure the flies were not just reacting to a new environment, the scientists tested males in larger observation chambers measuring twenty one millimeters in diameter. Only the tiny confinement chambers caused a drop in courtship activity. The team also evaluated general movement using custom tracking software and assessed feeding behavior using an automated feeding monitor system. While movement was slightly lower immediately after the stress, it returned to normal within an hour, and feeding behavior was completely unaffected.

The authors then investigated the effects of much longer stress exposures. They confined males to the tiny chambers for either seven hours or twenty four hours, providing fly food to prevent starvation. These prolonged stress periods caused a severe drop in courtship activity that lasted for at least five days.

Next, the research team explored the role of dopamine by feeding groups of male flies a chemical compound called 3-iodo-L-tyrosine for two days. This chemical inhibits an enzyme required to create dopamine, essentially lowering the dopamine levels in the brain. The drug treated flies still showed reduced courtship activity immediately after the sixty minute confinement.

However, one hour later, the drug treated males returned to normal mating behaviors, while the untreated control males still exhibited low courtship activity. To verify this finding, the scientists used genetic tools to block dopamine synthesis across the entire nervous system. They utilized a technique called RNA interference to reduce the expression of the dopamine producing enzyme. Like the drug treated flies, these genetically altered males showed normal suppression immediately after the stress but failed to maintain the suppression one hour later.

The researchers also tested whether the release of dopamine during or after the stress is necessary. They genetically engineered flies with a temperature sensitive mutation in their dopamine neurons. At permissive cooler temperatures of around 68 degrees Fahrenheit, the neurons function normally. At restrictive warmer temperatures of around 86 degrees Fahrenheit, the neurons stop transmitting chemical signals.

By manipulating the temperature, the scientists blocked dopamine transmission specifically during the one hour stress period. This did not stop the immediate drop in courtship behavior. However, when they blocked dopamine transmission either during the stress or during the one hour resting period afterward, the flies failed to maintain the low courtship activity. This suggests that dopamine release is not needed to start the behavioral change but is required to sustain it.

Fruit flies have four main types of dopamine receptors, which are proteins on the surface of cells that detect dopamine. The scientists tested genetically modified flies lacking each of these four receptors. Flies missing any of three specific receptors named Dop1R1, Dop1R2, and Dop2R failed to maintain the low courtship drive one hour after the stress.

The researchers then focused on a specific brain region called the mushroom body, which is a structure involved in learning, memory, and processing higher order sensory information. They used genetic tools to lower the levels of the dopamine receptors specifically within the mushroom body cells. Lowering the levels of the Dop1R1 and Dop2R receptors in this region prevented the lasting suppression of courtship behavior.

Finally, the team investigated which specific clusters of dopamine producing neurons send the signals that maintain this behavioral change. The adult fruit fly brain contains around 300 dopamine neurons divided into several clusters. By turning off communication in distinct neuron clusters, they found that two specific groups of dopamine neurons are responsible. These clusters, known as PAM and PPL1, send connections directly into the mushroom body, and blocking their signals erased the persistent effects of the confinement stress.

There are some limitations to consider. The authors note that the specific chemical messengers that initiate the immediate drop in courtship behavior remain unknown. Future research tends to focus on identifying the non dopamine pathways that trigger this initial reaction to physical restriction.

Additionally, the current methods did not involve measuring the real time electrical activity of the dopamine neurons during the confinement. Future studies could use fluorescent imaging techniques to watch how these specific brain cells fire while the animal’s movement is restricted. Scientists also need to identify the exact downstream neurons that receive the dopamine signals from the mushroom body to fully map out the circuits governing this stress response.

The study, “Role of dopamine signaling in male courtship suppression induced by confinement stress in Drosophila,” was authored by Tomohito Sato, Rana Toyama, Toshihiro Kitamoto, and Takaomi Sakai.