Updated Review-12

All three motivational mechanisms are affected by motivating stimuli based on recognition or lack of recognition of the other animal, which may be considered as a kind of learning, and which depends primarily on the olfactory and vomeronasal systems in muroid rodents,. Although considerable research has been carried out recently on the systems involved in individual odor recognition (Brennan et al., 1999; Haberly, 2001; Petrulis and Eichenbaum, 2003), the particular neural mechanisms of motivating stimuli have not been identified. One may assume that the various regions of the amygdala and pyriform cortex, with their rich inputs from the olfactory and vomeronasal systems, are involved to a great degree.

Behavioral data indicate that both patrol/marking and offense are motivated by unfamiliar conspecific odors, but the exact neural mechanism is unknown. This is indicated in the figure as learning point 3. As noted above, competitive fighting may also take place between familiar animals.

Contradictory results concerning the effects of septal or medial preoptic and amygdala lesions on offense may be related to the fact that there are several different analyzers of motivating stimuli involved, including the androgen-dependent pheromones in males (discussed above) and an analyzer tuned to unfamiliarity of a conspecific opponent. It was argued in the previous review (Adams, 1979a) that laboratory rats and mice share a colony odor and most fighting is between males; hence offense depends more on pheromones and less on unfamiliarity and hence greater importance of the septum or medial preoptic hypothalamus than the amygdala. Other species with more female-female offense may depend less on pheromones and more on unfamiliary of the opponent, hence greater importance of the amygdala. This was cited to explain a series of contradictory data between laboratory rats and mice, on the one hand, and hamsters on the other. Offense in mice and rats was found to be reduced by septal and preoptic lesions but not amygdala lesions, while in hamsters offense was reduced by amygdala lesions but not by septal lesions. More recent studies have tended to confirm the earlier findings. In line with earlier studies, the offense behavior of rats is reduced after lesions involving the medial preoptic hypothalamus (Albert et al., 1987; Edwards et al., 1993), although one study did not concur (Bergvall and Hansen, 1990). In rats offense is not reduced after lesions of the medial amygdala (McGregor and Herbert, 1992; Oakes and Coover, 1997). In contradiction, the latter study obtained some reduction (about 50%) in the offense of rats following basolateral amygdala lesions, and another study found a reduction of offense only in experienced rats, but not in inexperienced (see Vochteloo and Koolhaas , 1987, above on priming effects). Also in line with earlier studies, the offense behavior of hamsters is affected in the opposite direction by these lesions. Offense in hamsters is reduced by medial amygdala lesions (Takahashi and Gladstone, 1988), but, if anything, increased by septal lesions (Potegal et al., 1981).