|
1. An offense motivational mechanism; inhibited by estrogen/progestin and ACTH and facilitated by prolactin. I have previously proposed that there is a set of homogeneous neurons, called the offense motivational mechanism, that organizes the offense behaviors of muroid rodents. This has been proposed on the basis of sequence analyses of' interlrla1e fighting in rats (Adams, 1976;. Lehman and Adams, 1977), effects of brain lesions on intermale fighting (Adams, 1971), a comparative analysis of aggressive behavior in muroid rodents (Adams, submitted for publication), and an analysis of brain mechanisms of aggression in rats and cats (Adams, BBB 2(4) 1979). It may be activated by any of three types of motivating stimuli: testosterone-dependent pheromones (in males only); unfamiliar conspecific odors (in both males and females), and the effects associated with competition over food or potential mates (competitive fighting in both males and females). The motor patterns activated by offense include approach locomotion, full aggressive posture, bite-and-kick attack, and sideways or upright postures in all muroid rodent species. In some species, the motor patterns also include flank rub, offense pheromone secretion, piloerection, teeth-chattering and tail-rattling (Adams, submitted for publication). The location of the hypothetical offense motivational mechanism is probably in the midbrain, perhaps in the midbrain central gray (Adams, BBB 2(4) 1979). Offense behavior of females is generally reduced during behavioral estrus, an effect that may be due to direct inhibition of the offense motivational mechanism by estrogen and progestin (figure 1, site 1). The most convincing data have been obtained from female golden hamsters which normally attack both male and female opponents, but which do not attack during estrus. Following ovariectomy, the hamster continues to attack, but just as there is no estrus, so there is no periodic decrease in aggressiveness towards males; however, offense may be inhibited by experimental injection of estrogen and progestin, hormones that would norma1ly reach their peak around estrus (F1oody and Pfaff, 1974, 1977). Other data are available from Mus musculus, although investigations in this species are hampered by the fact that laboratory females do not usually show offense except during competitive fighting and lactation, perhaps because the olfactory stimuli in a laboratory are so uniform that all opponents are somewhat "familiar." In wild Mus musculus, however, offense may be obtained against unfamiliar opponents, and this offense is reduced during behavioral estrus {Hyde and Sawyer, 1977). Also, the offense which accompanies lactation in laboratory mice may be reduced by estrogen injections (Hansult and Kessler, 1977). In the laboratory rat, females also show offense only during lactation and competitive fighting; therefore, there are not many data on the question. An undergraduate working in my laboratory, Jeanette Talavera, found a reduction in competitive fighting by females during estrus, although the data were complicated by frequent pseudo-pregnancies due to the method of vaginal smear analysis. Since there is uptake of estrogen by neurons in the caudal portion of the midbrain central gray (Stumpf and Sar, 1976), it is possible that this indicates the location of neurons that mediate this effect. Adrenocorticotrophic hormone (ACTH) has been implicated in the reduced offense of animals that have been exposed to previous defeat. The most careful study on the effects of ACTH, recently completed by Leshner et a1 (1973), showed that ACTH reduced the probability of intermale fighting of mice despite manipulations which hold constant other hormones including testosterone and corticosteroids. This suggests that there may be a direct inhibitory effect upon the offense motivational mechanisms rather than simply indirect effects by way of other hormones. Some data from our laboratory suggest that the effect is most likely upon the motivational mechanism itself, rather than upon the motivating stimulus inputs: Zook and Adams (1975) found that smaller animals were less likely to initiate competitive fighting and attributed it to effects of previous defeat. Such an effect is very likely to have been mediated by ACTH which is known to be increased following defeat; if so, then not only intermale fighting, but also competitive fighting is inhibited by ACTH. Prolactin, it has been suggested, may be responsible for the increased female aggressiveness of virtually every muroid rodent which has been thoroughly studied. The time course of maternal aggression in the mouse parallels that of prolactin, peaking soon after parturition and then declining by the end of the second week of normal lactation (Gandelman, 1972; Svare and Gandelman, 1973). Like prolactin secretion, maternal aggression may be induced by stimulation from suckling of the nipples (Svare and Gandelman, 1970a), disappearing within five hours but not within one hour following removal of pups (Svare and Gandelroan, 1976b), and reappearing after reintroduction of pups (Svare and Gandelman, 1973). Like lactation (and presumably prolactin), maternal aggression can be induced by hysterectomy of a gestating female (Gandelman and Svare, 1974), although under normal circumstances these effects would not occur until the young were born. Whereas estrogen injection depresses maternal aggression (Svare and Gande1man 1975; Hansult and Kessler, 1977) and progestin injection has no effect (Hansult and Kessler, 1977), prolactin injection increases maternal aggression (Hansu1t and Kessler, 1977). Maternal aggression is specifically suppressed by ergocornine, which is an inhibitor of prolactin (Wise and Pryor, 1977) It appears likely that one effect of prolactin is a direct facilitation of the offense motivational mechanism. Practically all of the various motor patterns activated by offense have been reported during maternal aggression, including the offensive sideways posture (called "sideways displaying" in mice by Lynds, 1976, and "broadside" or "hip throw" displays in rats by Price and Belanger, 1977); the bite-and-kick attack, and ventral rub (the latter reported in Mongolian gerbils by Wallace et al, 1973). We have observed these motor patterns as well as piloerection and intensive grooming of the flank following maternal aggression in the rat; the latter may reflect increased secretion of offense pheromones from sebaceous glands of the flank (Lehman and Adams, 1977). Prolactin probably has other effects on aggression as well. Observations of maternal nest defense in our laboratory have shown that the lactating female exhibits not only the motor patterns of offense, but also the lunge-and-bite attack which is characteristic of defense. Therefore, as described below, I have hypothesized that prolactin has at least two different influences on aggression, one that facilitates offense, and another that facilitates defense by suppression of the consociate modulator. The sites of action of ACTH and prolactin upon the offense motivational mechanism are not known. As far as I know there have been no studies on differential uptake of prolactin in specific brain loci (see Moltz, 1974), and I am also unaware of studies which show the sites of ACTH uptake in the brain.
|