Aggressiveness in Mice and Thyroid Hormones

Behavioural patterns receiving the label aggressiveness are a consequence of subtle interactions between external and internal factors. It may be said that the individual is born with an internal potential for aggressiveness but that situational, and experiential factors determine whether this potential is developed 1 . Internal factors implicated in aggressiveness may include three components, namely: Genetic factors e.g. different levels of aggressiveness have been described in varied strains of laboratory mice 2 . Neural circuits and specific brain areas e.g. there are numerous examples of claimed associations between lesions of particular neural areas and the incidence and intensity of aggressiveness as a proof of the relationship between neurophysiological function and aggression. Natural lesions 3 , 4 , surgical lesions 5 , 6 , and electrostimulation 3 , 7 , 8 , 9 can modulate the expression of human aggression. There is considerable evidence of the involvement of a wide range of neurotransmitters in the control of aggressive behaviour. Some neurophysiological studies suggest that there are several forms of aggression which are probably mediated by different neurotransmitter systems. The three most commonly employed paradigms in psychopharmacological analysis of animal aggression are: isolation-induced fighting in mice, shock induced aggression in mice and rats and mouse-killing (predatory) behaviour. Experimental investigations have elucidated the neurochemical correlates of the different types of aggressive behaviours 1 . Hormones can modulate aggressiveness by: Early exposure having lasting organizational effects on the brain that alter the likelihood of expressing aggressive behaviour in adult life 10 , 11 . Inducing indirect activational effects in adult organisms by modifying the production or reception of intraspecific social cues (e.g. pheromones and olfactory perception) 12 , 13 . Producing direct activational effects in adult animals by neuromodulatory actions. The last of these possibilities needs some amplification.