The Effects of Maternal Defense on Development of a Conditioned Place Preference and Tyrosine Hydroxylase Activation in Lactating Primiparous Mice

Maternal defense behavior is a behavior that is uniquely associated with the hormonal changes that accompany late pregnancy and lactation. The general onset and maintenance of maternal behavior in a lactating dam is dependent on the dam having a functional and intact mesolimbic system. Due to the role that the mesolimbic system plays in maternal behaviors, dams are wired to find pups and pup-related behaviors rewarding. These interactions result in a general increase in dopamine (DA) activity within the mesolimbic system and maternal brain regions. The role the mesolimbic system may play in maternal defense behavior, however, has not yet been investigated. Research on aggression in male rodents suggests that males find aggressive encounters with other male rodents to be rewarding and that they seek out opportunities to be aggressive to subordinate male rodents. This aggressive-seeking behavior has been linked to increases in DA activity within the mesolimbic system in males and the display of aggression in male rodents in general is dependent on an intact mesolimbic system. Due to the rewarding nature of pup-related interactions in mothers and because we know maternal behaviors are dependent on DA functioning, it is expected that maternal defense behavior can be rewarding to dams in a similar way it is rewarding to males. Therefore, the present study compared the total percentage of time spent in each chamber between one of three stimuli: an unobtainable male, a maternal defense test, or a control group. Additionally, on postnatal day 8, mothers were exposed to one of these stimuli in the conditioned place preference (CPP) apparatus, and cells expressing tyrosine hydroxylase (TH) activity, which is a marker of DA activity, were counted in brain regions related to reward (the ventral tegmental area and the nucleus accumbens) and regions related to maternal behaviors (the medial preoptic area and bed nucelus of the stria terminalis). Dams did not develop a preference for the maternal defense stimulus in the CPP paradigm, and no significant differences were found between any group in the total percentage of time spent in either the stimulus-paired or null chambers. Additionally, there were no significant differences in cells expressing tyrosine hydroxylase in maternal or mesolimbic brain structures. Finally, tyrosine hydroxylase activation had no relationship to aggressive behavior or the total time dams in the maternal defense group spent in the stimulus-paired chamber. These findings suggest that the maternal defense is a not a rewarding behavior for dams to perform, and DA neurotransmission is not related to maternal defense behavior. Changes in neuroendocrine mechanisms that alter arousal and maternal behaviors following a stressor during lactation specifically could block any rewarding qualities of maternal defense behavior in favor of pup-biased behaviors.