While it is generally believed that sex is always on mind of men, researchers have shown scientifically that it is actually the case thanks to two male-specific neurons.
Researchers at University College London (UK) collaborated with those fro Albert Einstein College of Medicine (USA) and showed that there is a direct link between ontrasting behaviour of male and female worms and differences in brain development and structure in areas involved in higher order processing.
Scientists reveled through their study of male nematode worms that there are two male-specific neurons in the brain that hint that preference for sex over anything else can be genetically hardwired. Researchers say that these findings are important to understand how brains vary to give the two sexes different preferences, aptitudes and judgments.
Senior author of the study Dr Arantza Barrios, UCL Cell & Developmental Biology, says that the findings of the study clearly suggest that there are genetic and developmental differences between the two sexes and this lead to structural changes in the brain of male worms during sexual maturation. The changes that have been brought about make male brains work differently, such that they remember previous sexual encounters and prioritise sex in future situations.
The findings of the study are surprising as the team has effectively identified previously unidentified cells that are responsible for the behavioural change despite the fact that worms are an extremely well studied model organism. They were able to show that the cells from which these male brain neurons are born share common characteristics to the cells that give rise to human brain neurons. They are glial cells – companion and support cells of neurons.
Co-senior author Dr Richard Poole, UCL Cell & Developmental Biology, said: “This is the first well-described example of glia making neurons outside vertebrates and is particularly exciting as we find that the glial cells in question are fully differentiated cells, an issue that has been tricky to address in higher organisms.
“We can now exploit this system to understand how fully differentiated glia can re-enter the cell cycle and generate neurons. This could have important therapeutic implications in the future”.
The newly identified pair of neurons – called ‘mystery cells of the male’ or ‘MCMs’ – create behavioural differences between the sexes by changing a brain circuit common to both. Whether the neurons are born or not depends on the genetic sex of the glial cells from which they arise and not on the sex of the animal or on hormones. The MCM neurons are only made from glial cells that have male chromosomes.
Dr Barrios added: “Our findings suggest that differences in learning and perception depend not just on the sex of the animal but also on the sex of the individual neural progenitor cells. This means that different aspects of an animal’s behavior may well develop independently of each other in some circumstances, instead of through the co-ordinated action of hormones. Of course not all behavioural differences are genetically hardwired, environment can also play an important role.”
The worm species used in the study, Caenorhabditis elegans, has two sexes: males and hermaphrodites. These hermaphrodites are essentially modified females that carry their own sperm and do not need to have sex in order to reproduce. The MCMs were identified using fluorescently tagged markers and their function could be probed by surgically removing them using a laser microbeam.
The effect of the cells on the worms’ behaviour was tested using classic conditioning behavioural assays in which worms learn to associate aversive or pleasant experiences (such as starvation or mates) with another stimulus (salt) and change their behavioural responses to that stimulus. Worms that were previously starved in the presence of salt, learned to move away from areas with high concentrations of salt when placed in a new environment that had various different salt concentrations. This indicated that worms had learned to perceive salt as a sign for the absence of food.
Both males and hermaphrodites perform this type of learning. In contrast, when males were starved in the presence of salt and mates (i.e. sexual partners), and then placed in a new environment that had different salt concentrations, males sought areas of high concentrations of salt. This indicated that the association of salt with sex was stronger than and preferred over the association of salt with lack of food. This change in behaviour does not occur in hermaphrodites. Importantly, it also does not occur in males whose MCM neurons were surgically removed – demonstrating that these neurons are required for sex-based differences in learning.
The team at Albert Einstein College of Medicine used electron micrographs of serial sections to reconstruct and analyse the connections made by the MCMs with other neurons in the male brain. They found that the MCMs connected with neurons that are present in both sexes and that the presence of MCMs only in males remodeled these circuits to change the way information is processed.
Co-author Professor Scott Emmons, from the Departments of Genetics and Neuroscience at Albert Einstein College of Medicine, said: “Only in C. elegans, at the moment, is it possible to identify every synapse in a neural circuit in the way we have done here. Though the work is carried out in a small worm, it nevertheless gives us a perspective that helps us appreciate and possibly understand the variety of human sexuality, sexual orientation, and gender identification.”
The scientists hope to discover how glial cells make neurons, as it is a promising avenue for repairing damaged areas of the brain. They also want to determine what specific properties of brain circuits regulate the acquisition and retention of information in order to understand how learning occurs.