||Offense Motivational Mechanism||Page 8|
At least two types of offense must be distinguished which have similar motor patterns but different motivating stimuli and hormonal influences. As demonstrated by Adams et al. (1994) and Zook and Adams (1975), the motivating stimuli of competitive fighting include hunger or thirst and frustration of ongoing activity. Although competitive fighting is facilitated by unfamiliarity of the opponent, unlike territorial fighting, competitive fighting may also take place between familiar animals. Whereas territorial fighting, at least in rats, is stronger in males, competitive fighting is stronger in females. These differences are reflected in the changes introduced into the model of the offense motivational mechanism.
Despite considerable research, the neural basis of the offense motivational mechanism is not much better understood since the 1979 review (Adams, 1979a) when it was reported that "The neural basis of offense has not been investigated in the cat (see Flynn, 1976), and only occasionally in the rat; therefore, there are few data available on the question of the neuroanatomical locus of the hypothetical offense motivational mechanism."
Studies in our laboratory during the ensuing years concentrated on two potential locations for an offense motivational mechanism: the ventromedial tegmentum of the midbrain and the anterior hypothalamus. Following up an earlier study in mice (Kostowski and Valzelli, 1974), it was possible to completely abolish territorial fighting in the rat with lesions in the ventromedial tegmentum while leaving defense and predation behaviors intact (Adams, 1986). This clearly distinguishes offense from defense since lesions of the midbrain central gray abolish defense (Edwards and Adams, 1974) but do not abolish offense (Mos et al., 1983). Subsequently, we were able to abolish or greatly reduce competitive fighting by ventromedial tegmental lesions in female rats and we found that unilateral lesions were as effective as bilateral lesions in males (unpublished observations). However, neither electrical nor chemical stimulation could produce offense behavior in this region (Adams et al., 1993), which suggests that lesions were destroying descending fibers of passage rather than cell bodies related to offense.
Investigating the anterior hypothalamus, we found that offense behavior including both offensive sideways posture and bite-and-kick attack could be produced by chemical stimulation, involving cell bodies that could, in principle, correspond to an offense motivational mechanism (Adams et al., 1993). Lesions of the anterior hypothalamus abolish or reduce competitive fighting in both male and female rats, according to unpublished data from our laboratory previously reported in the 1979 review (Adams, 1979a) and confirmed in subsequent experiments. Other investigators have also found that anterior hypothalamic lesions abolish or reduce offense, although the lesions have usually involved other adjacent hypothalamic regions as well (Albert et al., 1986; Albert et al., 1987; Bermond, 1982; Edwards et al., 1993; Grossman and Grossman, 1970; Olivier et al., 1983). One study failed to abolish offense in female hamsters by anterior hypothalamic lesions, but the representative lesion shown in the publication was dorsal to the region implicated by other studies (Hammond and Rowe, 1976). Enhanced c-Fos activity has been found in the anterior hypothalamus during offense by Halasz et al. (2002). In contradiction, however, the activity of the anterior hypothalamus was found to increase less in the offensive animal than in the paired defensive animal (Kollack-Walker et al., 1997), and in another study, there was no increased activity as measured by c-Fos in the anterior hypothalamus following attack as compared to controls (Delville et al., 2000). The ambiguity of these data may be related to the close inter-mingling of cells responsible for offense and other cells or fibers involved in defense; see discussion below on the relation of offense and defense in the hypothalamus.
Many studies have obtained attack behavior in the rat by electrical stimulation of the lateral hypothalamus at the level of the anterior hypothalamus and immediately posterior (e.g. Kruk et al., 1998; Lammers et al., 1988; Roeling et al., 1994). However, the bite-and-kick attack (offense) is usually mixed with a lunge-and-bite attack (defense), suggesting that both motivational systems are excited simultaneously. Also, electrical stimulation fails to elicit the full range of offense motor patterns since offensive sideways posture is not seen (Kruk et al., 1998).
If the offense motivational mechanism is located in the anterior hypothalamus, then its descending outputs to motor patterning mechanisms probably pass by way of the lateral hypothalamus and ventromedial midbrain tegmentum, the two areas where lesions have been shown to abolish all offense motor patterns (Adams, 1971, 1986).