Tuesday, 3 April 2012

Sexual Behaviour in Male Mice





Males and females exhibit significantly different sexual behaviours during intercourse. Historically, these differences have been primarily attributed to exposure to different hormones at birth (Bonthius et. al 2012). In the foetus, males are exposed to testosterone 4 weeks earlier than females, giving rise to the belief that this is what caused differences in sexual behaviour between the sexes. It is also believed that responses by adults to hormones originating in the gonads can influence sexual behaviour (Hamezelou 2012). 
Recently, a study by scientists working at the University of Virginia’s Department of Biology and Molecular Genetics revealed that some of these differences may be attributable to the chromosomes in males, and asked whether or not the presence of certain genes influenced male sexual behaviour. The scientists were interested in studying while males exhibited certain sexual behaviours, such as frequency of ejaculation, thrusting, mounting and aggression towards other males within a breeding environment. 
To discover if chromosomes played a role in sexual behaviour, the scientists used a mouse model where the sex determining gene on the y-chromosome has jumped to a different chromosome, meaning that two X-chromosomes are present (Hamezelou 2012). Against expectations, the researchers found that male mice with two X chromosomes had more frequent ejaculations, and both mounted and thrusted more frequently during intercourse (Bonthius et. al 2012).
In order to show that the cause for the difference in sexual behaviour was a result of the X chromosome and not the lack of a Y chromosome, the researchers also used mice with Klinefelter’s Syndrome (XXY) (Hamezelou 2012). They found that these mice also exhibited more sexual behaviours than mice with just one X chromosome.  Other aspects associated with male sexual behaviour, such as aggression were also found to be heightened on mice with both X-chromosomes.
From the research conducted, the researchers came to the conclusion that the reason for this difference may lie in a gene that determines sexual behaviour that lies on the X-chromosome, and that the presence of two of these in males enhances the frequency of male sexual behaviours such as ejaculation, thrusts, mounting and aggression towards other males (Hamezelou 2012). It is believed that this gene perhaps codes for certain proteins that control sexual behaviour in mice. One particular gene is known to code for the production of nitric oxide, a neurotransmitter that may be involved with social behaviours (Nelson & Snyder 1995). A previous study by Nelson and Snyder (1995) found that mice lacking the gene that coded for nitric oxide exhibited significantly raised aggression during intercourse. Another study by Dominguez and Hull (2007) found that certain hormones could be used to enhance certain behaviours, such as frequency of ejaculation, thrusts and mounts. However, this study was associated with the injection of certain hormones, as opposed to internally produced hormones coded for be genes present on the X-chromosome. 
It is not yet known whether the gene present on the X-chromosome in mice codes for a particular neurotransmitter; protein involved with sexual behaviour or otherwise. It has also been acknowledged that there is potentially more than one gene involved with the behaviours exhibited (Nelson & Snyder 1995). 
It is hoped that once the location of this gene is found and mapped, that it can be used for further studies to find libido-enhancing drugs for use in humans (Hamezelou 2012). 

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