Ž . Developmental Brain Research 109 1998 99–108 Research report Development of sex differences in the bed nucleus of the accessory olfactory tract in the rat Paloma Collado, Santiago Segovia, Antonio Guillamon ) ´ Departamento de Psicobiologıa, UniÕersidad Nacional de Educacion a Distancia, Ciudad UniÕersitaria, s r n, PO Box 60.148. 28040-Madrid, Spain ´ ´ Accepted 7 April 1998 Abstract Ž . In the present study, we investigate the development of sex differences in the bed nucleus of the accessory olfactory tract BAOT , a sexually dimorphic structure in which masculinization of male rats depends on the level of postnatal estradiol. Male and female Wistar rats were sacrificed postnatally when they were 1, 3, 7, 15, 45 and 60 days old, and stereological methods were used to estimate the volume and number of neurons in the BAOT of the subjects. With respect to volume, sex differences were established from P15. In Ž . relation to the number of neurons, the males acquire their adult morphological parameter by P15, while females show a decrease P3 , Ž . Ž . increase from P3 to P15 and later decrease from P15 to P45 . Sex differences from P3 to P7 might be related to greater cell death in the BAOT of the females. These results support the hypothesis that both male and female rats reach their adult morphological pattern within this sexually dimorphic nucleus in an active manner. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Bed nucleus of the accessory olfactory tract; Sex difference; Development; Vomeronasal system; Apoptosis; Stereology 1. Introduction Ž . The bed nucleus of the accessory olfactory tract BAOT is a secondary olfactory structure in the vomeronasal sys- Ž . w x tem VNS 3,38 . The VNS is a complex neural network w x which is sexually dimorphic 16,17,39,40 and mediates the action of pheromones involved in eliciting and main- taining masculine and feminine reproductive behaviors, and in priming mechanisms that affect gestation, the es- w x trous cycle and puberty 22,44,45 . The BAOT is a forebrain cell group associated with the w x accessory olfactory tract 3,12,38 . Rostrally, it appears Ž . ventral to the nucleus of the lateral olfactory tract LOT , while its caudal portion is ventrolaterally located next to Ž . w x the anterior medial amygdala MeA 12,25,28,38 . When stained with Cresyl violet, the BAOT appears to be a well-delineated small cluster of medium-sized cells. It is reciprocally connected to the accessory olfactory bulb Ž . w x AOB 9,11,38 and the posteromedial cortical amyg- Ž . w x daloid nucleus PMCo 25,33 . Moreover, it receives af- Ž . w x ferents from the medial amygdala Me 25 and is one of ) Corresponding author. the main sources of vomeronasal input to the medial Ž . w x preoptic area MPA 12 . All of these connections and w x structures belong to the VNS 25,36,38,43 . The BAOT is involved in the control of parental behav- w x w x ior in both female 13 and male 23 rats, and based on studies of lesions, it has been suggested that the BAOT w x inhibits expression of parental behavior 13,23 . This hy- pothesis is supported by a study in which the levels of 14 C 2-deoxyglucose were lower in the BAOT of sensitized Ž i.e., virgin females that develop maternal behavior after . being exposed to pups and parturient females than in w x control groups 14 . In previous studies, we have shown the existence of sex differences in the BAOT. Specifically, males have a larger wx volume and number of cells than female rats 6 . These differences are related to the levels of androgens present shortly after birth since early postnatal orchidectomy of wx males and androgenization of females reverse them 6. Nevertheless, masculinization of the male BAOT seems to depend upon the aromatization of testosterone to estradiol, since orchidectomized males injected with a single dose of Ž . estradiol benzoate on the day of birth P1 show a BAOT volume and neuron number similar to those of control wx males 7. Moreover, dihydrotestosterone is unable to wx counteract the effects of P1 orchidectomy 7 . 0165-3806r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.