PubMed · 15552372
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Marya Ostrowski. 2004. Stop it, now.. https://pubmed.ncbi.nlm.nih.gov/15552372/
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Frequency-dependent selection is an important process in the maintenance of genetic variation in fitness. In humans, it has been proposed that the polymorphism of handedness is maintained by negative frequency-dependent selection, through a strategic advantage of left-handers in fighting interactions. Using simple mathematical models, we explore: (1) whether it is possible to predict the range of left-handedness frequencies observed in human populations by the frequency and the violence of fighting interactions; (2) the consequences of the sex differences in the probability of transmission of hand preference to offspring. We show that a wide range of values of the frequency of left-handers can be obtained with realistic changes of the parameters values. Our models reinforce the idea that negative frequency-dependence may have played a role in maintaining left-handedness in human populations, and provide further support for the importance of fighting interactions in the evolution of hand preference. Moreover, they suggest an explanation for the occurrence of left-handedness among women in this context, namely an indirect selective advantage through their male offspring.
Recently, this laboratory provided evidence that interleukin-1beta (IL-1beta), an immune and brain-derived cytokine, microinjected into the medial hypothalamus, potentiates defensive rage behavior in the cat elicited from the midbrain periaqueductal gray (PAG), and that such effects are blocked by a 5-HT2 receptor antagonist. Since this finding represents the first time that a brain cytokine has been shown to affect defensive rage behavior, the present study replicated and extended these findings by documenting the specific potentiating role played by IL-1beta Type 1 receptor (IL-1RI), and the anatomical relationship between IL-1beta and 5-HT2 receptors in the medial hypothalamus. IL-1beta (10 ng) microinjected into the medial hypothalamus induced two separate phases of facilitation, one at 60 min and another at 180 min, post-injection. In turn, these effects were blocked with pretreatment of the selective IL-1 Type I receptor antagonist (IL-1ra) (10 ng), demonstrating the selectivity of the effects of IL-1beta on medial hypothalamic neurons upon PAG-elicited defensive rage behavior. The next stage of the study utilized immunohistochemical methods to demonstrate that IL-1beta and 5-HT2 receptors were present on the same neurons within regions of the medial hypothalamus where IL-1beta and the IL-1beta receptor antagonists were administered. This provided anatomical evidence suggesting a relationship between IL-1RI and 5-HT2 receptors in the medial hypothalamus that is consistent with the previous pharmacological observations in our laboratory. The overall findings show that activation of IL-1RI in the medial hypothalamus potentiates defensive rage behavior in the cat and that these effects may also be linked to the presence of 5-HT2 receptors on the same groups of neurons in this region of hypothalamus.
Maternal defense behavior (or maternal aggression) is a highly conserved behavior for protecting offspring that is normally associated with decreased fear and anxiety in rodent dams. Environmental enrichment can decrease indices of fearfulness and anxiety (and elevate novelty seeking) in laboratory animals. This study examined whether enrichment could alter levels of maternal aggression and elevated plus maze performance in lactating mice. One group of female mice was exposed to a series of novel objects for 1.5 months while the other group was not. As expected, mice that had been exposed to an enriched environment showed significantly more entries to and time on the open arms of an elevated plus maze while they were lactating. Enriched mice showed similar maze performances when tested 3 months later in a non-lactating state, even though enrichment had been removed for 3 months. Further, offspring of enriched dams exhibited maze performance similar to that of their dams, suggesting possible epigenetic influences on maze performance. In contrast, no differences in measures of maternal aggression or pup retrieval were detected between enriched and control lactating mice. Together, these results indicate that environmental enrichment has long lasting effects on plus maze performance, but does not alter maternal aggression. The findings also suggest that whatever neural pathways are altered by enrichment do not strongly overlap or regulate those governing maternal aggression.