Stressful situations tend to reduce the desire to mate in many animal species, but the exact brain mechanisms behind this response remain difficult to pin down. Recent research published in journals iscience We provide evidence that male Drosophila melanogaster courtship behavior is suppressed when confined to a small space. The findings suggest that the brain’s dopamine system is required to maintain this low sexual motivation even after the stressful event is over.
When animals experience physical or psychological stress, changes occur in the brain that can alter basic physiological functions and behavior. These changes can last long after the stressful event has ended. In many mammals, including humans, stress often reduces the desire for sexual activity.
However, the specific cellular pathways linking stressful experiences and reduced sexual motivation are not well understood. Drosophila melanogaster, academically known as Drosophila melanogaster, serves as an excellent model for studying these brain pathways. These insects have highly mapped nervous systems and genetic tools that allow scientists to turn on or off specific brain cells.
Dopamine is a chemical messenger in the brain associated with reward and motivation, and is known to influence the desire to mate in fruit flies. Stress tends to alter dopamine levels in many species, so the researchers wanted to investigate whether dopamine plays a role in the process by which stress reduces sexual behavior. Previous research has shown stress-induced changes in dopamine function 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, and 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 a small chamber where they can move their legs and rotate their bodies, but are unable to walk freely. This setup mimics the psychological stress of confinement without completely paralyzing the animal.
To examine how small space stress affects courtship, scientists gathered groups of virgin male Drosophila flies that were 3 to 6 days old. They placed individual males in very small acrylic chambers. These small enclosures were only 3 millimeters in diameter and 2 millimeters deep. Control flies were placed in a standard chamber 15 mm in diameter and allowed to ambulate normally.
The men were held captive for 10, 30, or 60 minutes. Immediately after confinement, the researchers had the males cryopreserve the virgin females they had killed. They recorded courtship activity of male flies for 10 minutes in an observation room.
Courtship activity was quantified using an index called the courtship index. This index is defined as the percentage of time a male spends displaying mating behavior during a 10-min observation period. After 10 minutes of confinement, there was no change in male courtship behavior compared to the control group.
However, males trapped for 30 or 60 minutes showed a significant decrease in courtship index. The researchers noted that 60 minutes of confinement reduced courtship activity more strongly than 30 minutes of confinement. This provides evidence that the severity of behavioral inhibition depends on the length of the stressful experience.
The researchers then tracked how long this behavioral change lasted. They measured the males’ courtship index immediately, 1 hour, 2 hours, and 4 hours after 60 minutes of confinement. Although suppression of mating behavior was still present after 1 hour, males returned to normal levels of courtship after 2 and 4 hours.
To confirm that the flies were not simply reacting to a new environment, the scientists tested the males in a larger observation chamber, measuring 21 millimeters in diameter. It was only the small confinement cells that caused a decrease in courtship activity. The team also used custom tracking software to assess general movement and an automated feeding monitoring system to assess feeding behavior. Mobility was slightly reduced immediately after stress, but returned to normal within an hour, with no effect on feeding behavior.
Next, the authors investigated the effects of longer-term stress exposure. They locked the males in this small room for either 7 or 24 hours and fed them a diet of flies to prevent them from starving. These prolonged periods of stress resulted in a significant reduction in courtship activity, which lasted for at least 5 days.
The researchers then investigated the role of dopamine by giving a group of male flies a compound called 3-iodo-L-tyrosine for two days. This chemical inhibits the enzyme needed to produce dopamine, essentially reducing dopamine levels in the brain. Drug-treated flies showed reduced courtship activity even immediately after 60 minutes of confinement.
However, after 1 hour, drug-treated males returned to normal mating behavior, while untreated control males still showed low courtship activity. To test this finding, scientists used genetic tools to block dopamine synthesis throughout the nervous system. They used a technique called RNA interference to reduce the expression of dopamine-producing enzymes. Similar to drug-treated flies, these genetically modified males showed normal suppression immediately after stress, but were unable to maintain suppression 1 hour later.
The researchers also tested whether dopamine release is required during or after stress. They genetically engineered flies with temperature-sensitive mutations in dopamine neurons. At a tolerably low temperature of about 68 degrees Fahrenheit, neurons function normally. At a limited warm temperature of about 86 degrees Fahrenheit, neurons stop sending chemical signals.
By manipulating temperature, scientists specifically blocked dopamine transmission during a one-hour stress period. This did not stop the immediate decline in courtship behavior. However, when dopamine transmission was blocked during stress or during a 1-hour rest period after stress, the flies were unable to maintain low courtship activity. This suggests that dopamine release is not required to initiate behavioral change, but to maintain it.
Drosophila has four main types of dopamine receptors, which are cell surface proteins that detect dopamine. Scientists tested genetically modified flies lacking each of these four receptors. Flies lacking one of three specific receptors, Dop1R1, Dop1R2, and Dop2R, were unable to maintain low courtship activity one hour after stress.
The researchers then focused on a specific brain region called the mushroom body, a structure involved in learning, memory, and processing higher-order sensory information. They used genetic tools to specifically reduce the levels of dopamine receptors in cells of the mushroom bodies. Reducing the levels of Dop1R1 and Dop2R receptors in this region prevented sustained suppression of courtship behavior.
Finally, the team investigated which specific clusters of dopamine-producing neurons were sending signals that maintained this behavioral change. The adult Drosophila brain contains about 300 dopamine neurons divided into several clusters. By turning off communication within separate neuron clusters, they discovered that two specific groups of dopamine neurons were responsible. These clusters, known as PAM and PPL1, erased the lasting effects of confinement stress by sending direct connections to the mushroom body and blocking its signals.
There are some limitations to consider. The authors note that the specific chemical messengers that cause the immediate reduction in courtship behavior are still unknown. Future research will likely focus on identifying the non-dopamine pathways that drive this initial response to physical restriction.
Additionally, the current method did not include measuring the electrical activity of dopamine neurons in real time during confinement. Future studies may use fluorescence imaging techniques to observe how these specific brain cells fire when the animal’s movement is restricted. Scientists also need to pinpoint the downstream neurons that receive dopamine signals from the mushroom’s body to fully map the circuitry that controls this stress response.
The study, “The role of dopamine signaling in confinement stress-induced male courtship suppression in Drosophila,” was authored by Tomohito Sato, Ranna Toyama, Toshihiro Kitamoto, and Takaomi Sakai.
