Male libido drops from the stress of confined spaces

While it is well known in mammals that exposure to stress causes a drop in male libido, the underlying mechanism within the brain has not yet been fully clarified. A collaborative research team including Professor Takaomi Sakai of the Graduate School of Science at Tokyo Metropolitan University and Associate Professor Toshihiro Kitamoto of the University of Iowa discovered that exposing male fruit flies (Drosophila melanogaster) to "confinement stress" by locking them in a tiny space for a certain period significantly reduces their courtship behavior due to a subsequent decline in libido.

Focusing on how stress alters the function of dopamine in the brain, their detailed analysis revealed that dopamine synthesis, release, and reception are required for this reduction in courtship behavior to persist. These findings are expected to contribute to the understanding of the neurobiological basis of stress-related sexual dysfunction. The study was published in iScience.

First, mature male flies were exposed to stress by being confined for a set duration in tiny acrylic containers (3 mm in diameter, 2 mm in depth), and their subsequent courtship behavior was quantified. Because a fruit fly's total body length is about 2 to 3 mm, the flies could move their legs to shift their body axes, groom themselves, and feed inside this container, but they were unable to walk around freely. While no change in courtship behavior was observed after 10 minutes of stress, courtship behavior decreased significantly after 30 and 60 minutes of stress exposure. Furthermore, 60 minutes of stress caused a stronger suppression than 30 minutes, demonstrating that courtship inhibition intensifies with an increase in the duration of the stress. Courtship inhibition following 1 hour of confinement stress persisted for up to 1 hour afterward but recovered after 2 to 4 hours.

On the other hand, when flies were subjected to much longer periods of stress (7 hours or 24 hours), the courtship inhibition persisted for at least 5 days, suggesting that the duration of stress is a critical factor determining the persistence of behavioral changes. Additionally, this courtship inhibition was not simply due to a decline in motor ability or loss of appetite. Although the flies' activity levels decreased immediately after stress, their locomotion recovered 1 hour later, and no particular effect was observed on their feeding behavior.

In other words, the reason the male flies' courtship behavior remains suppressed after experiencing stress is likely that they are maintained in a state of low sexual motivation toward females.

Next, to verify the role of dopamine in this courtship inhibition phenomenon, the team conducted experiments where male flies were fed a drug (3IY) that inhibits dopamine synthesis, as well as experiments using flies in which tyrosine hydroxylase (TH), an enzyme involved in dopamine synthesis, was knocked down. The results revealed that while courtship inhibition immediately after stress occurs independently of dopamine, dopamine is essential for maintaining this suppressed state after the stress has ended. Furthermore, experiments blocking dopamine release showed that stopping dopamine release during or after stress caused the courtship inhibition seen 1 hour later to disappear.

This demonstrated that dopamine is crucial not for the "initiation of courtship inhibition" but for the "maintenance of courtship inhibition." An analysis of dopamine receptors was also conducted, revealing that three types of dopamine receptors, namely Dop1R1, Dop1R2, and Dop2R, are required for the persistence of stress-induced courtship inhibition.

The fly brain contains a region called the mushroom body, where information from various sensory neurons is integrated, playing a vital role in learning and memory. It became clear that the dopamine receptors expressed in the neurons making up this mushroom body are required to maintain courtship inhibition after the experience of confinement stress.

Furthermore, conducting an analysis at the neural circuit level showed that inputs to the mushroom body from dopamine-releasing neuron clusters called PAM and PPL1 are necessary for the persistence of low libido after stress. This signifies that dopamine regulates libido through a specific circuit.

Based on these results, the research team proposed a model in which confinement stress first lowers libido through a dopamine-independent pathway and dopaminergic neurons are activated, causing plastic changes in the neural circuits that maintain the low-libido state for a certain duration.

This achievement presents a clear neurological basis, the "maintenance mechanism by dopamine," for the phenomenon of stress-induced sexual dysfunction and suggests the possibility that different neural circuits are involved depending on the type and duration of stress. The mechanism discovered in flies, where "stress activates dopaminergic neurons, causing a decline in male libido," may be shared across a diverse range of animal species, including humans. Therefore, it is expected to contribute to clarifying the mechanisms behind stress-induced loss of motivation and behavioral changes.

Journal Information
Publication: iScience
Title: Role of dopamine signaling in male courtship suppression induced by confinement stress in Drosophila
DOI: 10.1016/j.isci.2026.115906