When Minnie met Mickey: is rodent romance all in the mind?

Friday February 13 2015

With Valentine’s Day fast approaching, the Mail Online thoughtfully cushions its readers against possible rejection ahead of time: “You may have lit the candles, opened the wine and dimmed the lights. But, inexplicably, your partner still doesn't want to have sex ... Don't worry, it's not you – it’s your partner's hormones”.

Unless Mail Online readers are amorous hairy-backed rodents with groundbreaking wine opening and fire making skills, these statements are wide of the mark. The study it is reporting on didn’t involve people at all, only mice. 

The study found a link between activity in a specific area of the brain, sexually receptive state and social behaviour in female mice. The brain region involved was the ventrolateral region of the ventromedial hypothalamus (VMHvl), an area implicated in rodent sociosexual behaviour, aggression and mating. A plausible biological mechanism for the finding was that hormones stimulate the VMHvl. This was put forward by the researchers, but is unproven.

While mice and people have similar biology, studying sexual behaviour in female mice can only give you limited insight into sexual behaviour in humans.

Ultimately, this information is mainly useful for other research scientists. The average person on the street should take this research with a pinch of salt, or perhaps a slice of cheese.

Where did the story come from?

The study was carried out by researchers from the mysteriously named Champalimaud Centre for the Unknown in Portugal, and was funded by a Marie Curie Reintegration grant, Fundação para a Ciência e a Tecnologia postdoctoral fellowship, Uehara postdoctoral fellowship, and a Fundação Bial research grant.

The study was published in the peer-reviewed science journal Current Biology.

The Mail Online's report read as if the research was carried out with people and was directly applicable to human-to-human sexual interactions. This is a mistake, given the likely differences between the sexual behaviour and decision-making processes in female mice, compared with women. While similarities may exist, there are likely to be crucial differences.

What kind of research was this?

This was an animal study looking at how social behaviour, brain activity and reproductive state were linked and related to the sexual behaviour of mice.

The researchers explained that: “Social encounters often start with routine investigatory behaviors [sic] before developing into distinct outcomes, such as affiliative or aggressive actions. For example, a female mouse will initially engage in investigatory behavior with a male, but will then show copulation or rejection, depending on her reproductive state. To promote adaptive social behavior, her brain must combine internal ovarian signals and external social stimuli, but little is known about how socially evoked neural activity is modulated across the reproductive cycle.”

Their research investigated a specific region of the mice's brains called the ventrolateral region of the ventromedial hypothalamus (VMHvl). The VMHvl has been implicated in rodent sociosexual behaviour, it has access to social sensory stimuli, and is involved in aggression and mating. Furthermore, many VMHvl neurons express ovarian hormone receptors (they respond to the effects of hormones), which play a central role in female sociosexual behaviour.

What did the research involve?

The research involved recording activity in the VMHvl brain region of freely behaving, naturally cycling, female mice while they were interacting with potential mates of both genders.

Subject animals had regular estrous cycles (reproductive cycles), and were categorised into two different reproductive states:

  • sexually receptive (estrous)
  • not receptive (non-estrous)

Because the team were interested in the investigatory phase of social behaviour, copulation was not allowed during the chronic single-unit recording experiments to avoid pregnancy or pseudo-pregnancy. (Pseudo-pregnancy in mice is when a female experiences the hormonal changes associated with pregnancy, but does not actually conceive any offspring).

They said both events would lead to profound neuro-endocrine changes and cause the female to be in a different physiological state.

What were the basic results?

They found that a large fraction of VMHvl neurons were activated in female mice in the presence of other mice, with a clear increase in activity specifically in the presence of males. The activity of most VMHvl neurons was modulated throughout social interactions, rather than in response to specific social events.

Furthermore, VMHvl neuronal responses to male, but not female, mice were greater during the sexually receptive state. Thus, male-evoked VMHvl responses are modulated by the reproductive state.

How did the researchers interpret the results?

The results, they say: “suggest the existence of gender-specific inputs to VMHvl neurons and the ability of these inputs to be differentially modulated by ovarian hormones”.

They added that they were the first group to their knowledge to show: “electrophysiological evidence that the activity of hypothalamic neurons is modulated during social encounters, in a gender-specific and reproductive state-dependent manner”.

Conclusion

This research suggests a link between activity in a specific area of the brain, sexually receptive state, and social behaviour in female mice. The brain region involved was the ventrolateral region of the ventromedial hypothalamus (VMHvl). The VMHvl has been implicated in rodent sociosexual behaviour, aggression and mating, and has ovarian hormone receptors. This implies a plausible biological mechanism by which hormonal state influences brain activity, which influences sexual behaviour.

However, these links weren’t proven by this study. They did not, for example, look at the effect of blocking specific hormone receptors in the VMHvl to pinpoint which ones were important and behind the behaviour. This would have confirmed the likely role of hormones more directly and more precisely.

As interesting as this is research is, it has limited applicability to people at the moment. This is because we can’t be sure that similar processes are happening in women. Similarly, there are a myriad of other cultural, social and individual personality factors at play in person-to-person sexual behaviour interactions that differ from those in mice.

If you are having trouble wooing the woman of your dreams, we suspect offering to expose her hypothalamus to hormones isn’t going to do you any favours.

Analysis by Bazian
Edited by NHS Choices