A combination of social withdrawal and increased aggression is characteristic of several mental disorders. Most previous studies have investigated the neurochemical bases of social behavior and aggression independently, as opposed to how these behaviors are regulated in concert. Neuronal nitric oxide synthase (nNOS) produces gaseous nitric oxide, which functions as a neurotransmitter and is known to affect several types of behavior including mating and aggression. Compared with wild-type mice, we observed that nNOS knockout mice showed reduced behavioral responses to an intruder behind a wire barrier. Similar results were observed in mice treated with the selective nNOS inhibitor 3-bromo-7-nitroindazole (3BrN). In habituation-dishabituation tests, treatment with 3BrN did not block recognition of male urine but did attenuate investigation time compared with oil-treated animals. Finally, nNOS knockout mice and 3BrN treated mice were significantly more aggressive than wild-type and oil-treated males, respectively. In general, these behavioral effects are less pronounced in pair-housed males compared with singly-housed males. Thus, nNOS inhibition results in a phenotype that displays reduced social investigation and increased aggression. These data suggest that further study of nNOS signaling is warranted in mental disorders characterized by social withdrawal and increased aggression.
Keywordsaggression; autism; nitric oxide synthase; serotonin; social investigationThe neurobiological substrates underlying social behavior have received renewed attention in efforts to gain insights into the underpinnings of mental disorders that involve deficits in social behavior (Choleris et al., 2003;DiCicco-Bloom et al., 2006;Insel & Fernald, 2004). In some cases, deficits in social interactions are associated with increased aggression (Anckarsater, 2006;Soderstrom, Sjodin, Carlstedt, & Forsman, 2004). The relationships between mechanisms of social investigation and aggression are poorly understood. The most common test used to study the neurobiological bases of aggression is the rodent resident-intruder test, in which an unfamiliar intruder is introduced into a resident's' home cage. Used successfully to identify neural circuits (Newman, 1999) and chemical systems ) that mediate aggression, the resident-intruder test eliminates the period of social assessment that precedes aggression in naturalistic contexts (Anderson & Hill, 1965).Mechanisms of social investigation generally have been examined outside of the context of aggressive behavior. A notable exception is a study of the vasopressin receptor 1b (AVPr1b), as selective deletion of AVPr1b results in decreased aggression and decreased investigation of social stimuli (Wersinger, Ginns, O'Carroll, Lolait, & Young, 2002
NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript knockout mice retained the ability to detect both male and female urine (Wersinger et al., 2004). Reduced olfactory investigation and aggression is also observed after treatment wi...