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expert reaction to Nature paper on serotonergic sexual discrimination

The paper demonstrated a method of studying the molecular and genetic basis of sexual preference in mice.


Dr William Davies, RCUK Fellow in Translational Research in Experimental Medicine, MRC Centre for Neuropsychiatric Genetics and Genomics, Cardiff University, said:

“The paper by Liu and colleagues uses a combination of elegant genetic, pharmacological, and behavioural techniques to provide novel and important insights into the behaviour of male laboratory mice. Specifically, the results strongly support the conclusion that the serotonergic system is extremely important in mediating sexual preference in such mice. However, it needs to be recognised that the data were obtained in an artificial, controlled environment using just one sex of an inbred strain of one species; as such, generalisations about the conclusions to female mice, to mice in the wild, and to other mammalian species (including humans) must be made extremely cautiously.

Specific caveats:

“In the laboratory, following weaning, male and female mice are usually housed in sex-specific groups. This situation does not happen in the wild where the sexes are continuously free to interact in colonies typically consisting of a single dominant male, several females and their offspring. Hence, interactions between male and female laboratory mice (and indeed male and male laboratory mice) may not be equivalent to those seen in nature. Examination of the behaviour of mice with serotonergic manipulations under experimental conditions more closely resembling those seen in the wild would be worthwhile.

“Mice have very different social structures and social behavioural repertoires relative to humans. In particular, they produce large litters, may mate with closely-related individuals and rely heavily upon tactile stimulation. Moreover, mice are extremely olfactory-based creatures and sexual behaviour in mice is thought to be largely driven by pheromones detected by the relatively large vomeronasal organ. In humans, the existence and utility of a vomeronasal organ is debatable and, as such, human reliance on pheromones as signalling cues is thought to be limited. In humans, higher decision-making processes (and brain structures underlying these) are thought to contribute towards ensuring successful reproductive and social strategies to a greater extent than in mice [1]. Thus, if serotonin is acting via influencing vomeronasal organ function and/or higher cognitive processes, the results may not necessarily apply to both mice and men.

“Finally, there seem to be key species differences between the serotonergic systems of mice, rats, monkeys and humans, not least in concentration of the neurotransmitter [2]. Hence, the effects of serotonergic manipulations in one species may not be recapitulated in another.”

[1] Curley & Keverne (2005) Trends in Ecology & Evolution 20(10):561-7 [2] Fitzgerald (2009) Neuroscience and Biobehavioral Reviews 33:1037-1041
Professor Margaret McCarthy, University of Maryland School of Medicine, listed the following strengths and weaknesses of the study: Strengths: “The authors are correct that while serotonin has long been implicated as a major regulator of male sexual behavior, there has been little attention to the possibility that it may also regulate sexual preference. “Moreover, the biological basis of sexual preference is largely unknown from a neurochemical point of view, although there are certain key brain areas which if ablated will result in a loss of sexual preference. Anatomical changes in the same regions have been associated with naturally occurring same-sex preference in males of two species, sheep and humans. “The results here are largely unambiguous in that the same behavioral phenotypic change is seen in two genetically altered mice strains in which serotonin neurons are lost, and the phenotype can be rescued by exogenous treatment with a precursor to the missing serotonin (Note: There appears to be a major mistake in Figure 6 which is purported to show a rescue of the behavioral deficits in serotonergic mice by injections of a serotonin precursor. In panels B and C the animals that did not receive the precursor show the recovery in mount latency and frequency whereas those that did receive the precursor do not.) “The authors are careful to restrict the interpretation of their results to a loss of sexual preference and not an induction of same sex preference, which is critical as these are very different things. We do not know if the same biological principles apply to a preference for the opposite sex as mediate a preference for same sex. This is often assumed but there is not data to support it and the data here do not achieve that goal.” Weaknesses: “There are important limitations and qualifications that must be considered when interpreting these findings. “First, it is notable that the heterozygote mice of both genotypes in which serotonin neurons were reduced showed no behavioral phenotype. In other words, there had to be a complete depletion of serotonin neurons in the brain for there to be any loss of sexual preference. This means that serotonin is necessary for sexual preference, but it is not sufficient. Moreover, there are no naturally occurring conditions, human or otherwise, where there is a complete loss of serotonin neurons in the brain. Therefore it is unclear whether serotonergic neurotransmission really explains anything about the basis of sexual preference. “Second, transgenic or genetically modified mice are notorious for showing lots of unusual changes in behavior in response to single gene deletions which are later found to be non-specific. Two cautionary tales illustrate the point. There was a great deal of attention to a study of a mouse missing a transcription factor called Fos-B. The females missing this gene did not take care of their pups when they were born, showing no interest in being mothers. The study was featured on the cover of Cell and Fos-B was declared the ‘nurturing gene’, with all sorts of outrageous claims made about the genetic basis of maternal care and why some women kill their children. Within a year it was apparent that a large number of genetically modified mice do not care for their young, and that maternal care by the so-called wild-type, the background genetic strain, is relatively weak to begin with. The spectacular initial finding faded away and has taught us nothing about the biological basis of maternal behavior. Similarly, there was a great deal of excitement when a prominent scientific group reported that knocking out a gene called e-nos made mice hyperaggressive, so much so that they killed any cage mates. There were again outrageous claims made about the genetic basis of murder and aggression. Within a year it became clear that large numbers of genetically modified mice are hyper aggressive, so much so that some scientists took to pointing out the rare exceptions when a gene could be deleted and the animals did NOT become hyperaggressive. Again, nothing was learned about the biological basis of aggression. Whether or not the current finding will suffer the same fate remains to be seen. “Lastly, in light of the above, it is unfortunate that the authors did not try to extend their findings beyond mice. There is an easily used chemical treatment to deplete serotonin in the brain temporarily and this could be readily applied to rats, hamsters, dogs, guinea pigs, sheep, even primates. In the absence of any evidence that their findings hold beyond genetically modified mice, the conclusions should be both tempered and restricted.”

Dr Keith Kendrick of the BBSRC’s Babraham Institute, Cambridge, said:

“Obviously any animal model research showing that interference with a specific neurochemical system profoundly impairs the normal male preference for females and results in sexual interest in males is likely to provoke considerable interest.

“This study in male mice using genetic approaches to target the brain serotonin system is both elegant and comprehensive. The paper provides compelling evidence that, in mice at least, serotonin signalling is important for controlling preference for female over male odours and mating only with females. It also does a pretty good job in showing that this is not simply a problem of not being able to discriminate between male and female odours per se.

“An important remaining question, as the authors concede, is whether reduced serotonin is in some way altering basic olfactory processing system to produce its effects on sexual preferences rather than on hypothalamic and other brain regions controlling sexual motivation and behaviour. A number of previous studies have shown that there are circuits in early olfactory processing structures in mice which have innate responses to odours of great biological importance and while in the current study responses to odours of predators, such as foxes, are apparently unaffected it is still possible that serotonin has a particularly important role in regulating innate responses to sexual odours. Indeed, there is good evidence showing that serotonergic projections to the primary brain processing region for smell, the olfactory bulb, do influence its responses to odours.

“In terms of having potential relevance to understanding human sexual preferences/orientation, we are of course far less influenced by odour cues in this context than mice are. There is some very limited evidence for altered responses to selective serotonin uptake inhibitors (SSRIs) in the brains of homosexuals, but we have been using psychoactive drugs which either increase or decrease serotonin function for quite some time now, and while effects on sexual arousal, impulsivity and aggression have often been reported, no effects on sexual preference/orientation have. At this time therefore any potential links between serotonin and human sexual preferences must be considered somewhat tenuous. For male mice on the other hand which are highly dependent on smell for control of sexual preferences and behaviour, this research shows very convincingly that serotonin signalling is of great importance.”

Molecular regulation of sexual preference revealed by genetic studies of 5-HT in the brains of male mice, Yan Liu et al., published online in Nature, Wednesday 23rd March 2011.

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