A comparison of amygdaloid lesion effects in male and female rats

Abstract

Amygdaloid lesions decreased the latency of male rats to initiate consumption of a novel food but increased latencies among females. Lesions also depressed the rate of postoperative weight gain of male but not female subjects. Female rats showed shorter latencies on three other ingestional and grooming measures and more rapid acquisition of a passive avoidance task. There were no lesion effects or lesion by sex interactions on these measures. There were also no group differences or interactions in the acquisition of a simple active avoidance task. Further investigations of sex differences in amygdaloid function are suggested.Recent evidence indicates that the amygdaloid complex is sexually dimorphic in a number of its features. Sex differences in cell body volume of the central and medial amygdala which are abolished by neonatal castration have been noted by Staudt and Dorner (1976). Dimorphism in amygdaloid uptake of methionine in adult mice (MacKinnon, 1973) and puberal rats (Ter Haar & MacKinnon, 1975) and estradiol uptake in neonatal rats (Westley & Salam an , 1977) has also been demonstrated. Baum and Goldfoot (1975) also found that amygdaloid lesions in perpuberal male ferrets reduced blood and testis levels of testosterone and retarded body growth. Such lesions in prepuberal females hastened the onset of first estrous and had no effect on body growth. Raisman (1974) has shown that amygdaloid efferents in the stria terminalis, while not themselves dimorphic, synapse on sexually differentiated neurons within the preoptic area. Kling (1974) has also reported increased aggression in female, but not male, monkeys following amygdaloid lesions. Little attention, however, has been directed toward a systematic comparison of lesion effects on behavior in the two sexes. The present experiment explored this possibility. METHOD SubjectsSubjects were 16 male and 16 female rats supplied by the Holtzman Company (Madison, Wisconsin). Male rats weighed 437-515 g and females 250-295 g at the time of surgery. All subjects were of equivalent age. Eight rats of each sex sustained amygdaloid lesions; two received control operations and six sustained scalp incisions. Three female rats (one amygdaloid and two control) died during surgery. Except where noted, rats received ad-lib access to Purina Lab Chow and water throughout the experiment. Surgery and HistologyUsing pentobarbital anesthesia (45 mg/kg), amygdaloid lesions were produced by passing 2.0-mA anodal dc through At the conclusion of testing, 24-micron coronal sections were prepared through the lesion area of each experimental rat and stained with thionin. ProceduresReactivity to novel foods. Latency to consume a single piece of presweetened cereal (Froot Loops), a small (10 x lOx 5 mm, ±2 mm) piece of apple (Washington Delicious), and a single piece of lettuce leaf (approximately 2 cm') was measured in the home cage. The novel food was placed in a 3.8-cm-diam dish in the rat's cage, and latency to begin and complete consumption of the novel food was recorded by stopwatc...