Behavioral Neuroscience 1987. Vol 101. No 5,709-717 Copyright 1987 by the American Ps\chological Association, Inc 07.15-7044/87/SOO 7S Olfactory Mechanisms in the Control of Maternal Aggression, Appetite, and Fearfulness: Effects of Lesions to Olfactory Receptors, Mediodorsal Thalamic Nucleus, and Insular Prefrontal Cortex Annabel Ferreira, Lars-Gosta Dahlof, and Stefan Hansen University of Goteborg, Gbteborg, Sweden During lactation the female rat is hyperphagic, aggressive toward adult conspecifics, and less fearful than usual. In the first experiment the importance of olfactory receptors was investigated by surgically removing the olfactory epithelium of the nasal cavity. Mother rats subjected to this treatment consumed significantly less food and weighed less than sham-operated females. Moreover, experimental subjects displayed a dramatic decrease in maternal aggression. Fear behavior (sound-elicited freezing), on the other hand, was not affected by the lesions. The mediodorsal thalamic nucleus and the prefrontal insular cortex form part of the central olfactory system. The second experiment assessed the involvement of this olfactory-related thalamocortical system on the behavioral profile of mother rats. It was found that whereas the thalamic and cortical lesions left food intake and fear behavior unaffected, they significantly decreased the frequency with which the mother would attack an intruder male placed into her home cage. The sense of smell appears, according to the present experiments, to play a crucial role in maternal aggression. Motherhood in the rat is accompanied by a number of behavioral adaptations that serve to optimize the survival of the offspring. Thus, in addition to pup nursing, the postpar- tum rat displays hyperphagia (e.g., Fleming, 1976). heightened aggressiveness toward conspecifics (reviewed by Ostermeyer, 1983), and reduced Tearfulness (Fleming & Luebke, 1981: Hard & Hansen, 1985). All of these behavioral changes appear to be dependent on the offspring, because removal of the pups reduces food intake (e.g., Fleming, 1976) and maternal aggres- sion (Ferreira & Hansen, 1986; Gandelman & Simon, 1980). whereas Tearfulness increases (Hard & Hansen, 1985). It is clear, then, that the pups alter their mother's feeding habits and affective behavior, but the ways in which the young manipulate their mother's behavior are not well understood. The suckling stimulus appears to contribute to lactational hyperphagia and aggressiveness (e.g., Hansen & Ferreira, 1986b), but several experiments indicate that olfactory stimuli are important as well. Considering maternal aggression, Fer- reira and Hansen (1986) found that mother rats whose pups were enclosed in a nylon mesh bag remained aggressive to- ward an intruder, whereas mothers with pups placed in a glass jar did not. Because the mesh bag condition, in contrast to the glass jar condition, permitted uninterrupted olfactory stimulation from the pups, it was concluded that odor cues from the pups (who were not emitting distress calls) are crucial for maternal aggression (Ferreira & Hansen, 1986). This investigation was supported by the Bank of Sweden Tercen- tenary Foundation and by Swedish Medical Research Council Grant B85-2IX-06605-03B. The discussions with E. B. Keverne of the University of Cambridge, England, are deeply appreciated. Correspondence concerning this article should be addressed to Stefan Hansen. Department of Psychology, University of Goteborg, Box 14158, S^tOO 20 Goteborg, Sweden. If olfactory cues indeed contribute to the changes in eating and affective behavior observed during motherhood, then the integrity of the central olfactory system should be vital for this behavioral profile. For instance, on the basis of the Ferreira and Hansen (1986) study, one would expect that anosmia would reduce maternal aggression. In the first exper- iment of the present study we tested this idea by observing affective behavior and food intake of mothers in which the olfactory receptors of the nasal cavity had been removed. In a second experiment, we investigated the behavioral effects of lesions in the mediodorsal thalamic nucleus and the prefrontal insular cortex. Neuroanatomical (reviewed by Heimer. 1978: Kolb, 1984; Price. 1985: Reep, 1984). electrophysiological (e.g.. Price & Slotnick, 1983), and neuropsychological (Ei- chenbaum, Clegg, & Feeley, 1983; Eichenbaum, Shedlack. & Eckman, 1980; Sapolsky & Eichenbaum, 1980; Slotnick & Kaneko, 1981) studies suggest that this thalamocortical circuit forms a prominent part of the central olfactory system, sub- serving higher order processing of olfactory information. General Method Subjects Nullipareous Wistar rats, about 3 months of age and purchased from Mollegard Breeding Laboratories (Ejby. Denmark), were mated and housed individually a few days before expected deliveries in transparent cages (Experiment 1: 55 x 33 x 20 cm. Experiment 2: 45 x 28 x 26 cm). They were kept in an air-conditioned, temperature- controlled (24 °C) colony room in which the lights were off between 1000 and 2200 hours. Litter size was adjusted to 10 pups within 2 days after parturition. Shredded paper towels were provided as nesting material. 709 This document is copyrighted by the American Psychological Association or one of its allied publishers. This article is intended solely for the personal use of the individual user and is not to be disseminated broadly.