Developmental Brain Research, 52 (1990) 11-15 I 1 Elsevier BRESD 51015 Sexual dimorphism in the parastrial nucleus of the rat preoptic area Agueda del Abril, Santiago Segovia and Antonio Guillam6n Departamento de Psicobiologia, Universidad Nacional de Educaci6n a Distancia, Ciudad Universitaria, Madrid (Spain) (Accepted 15 August 1989) Key words: Sexual dimorphism; Parastrial nucleus; Sex steroid; Preoptic area This work investigates the possible existence of sex differences in the volume of the parastrial nucleus (PSN), a component of the preoptic area in the rat. The effects of postnatal (on day 1 after birth) male orchidectomy and female androgenization on this nucleus were studied. The volume of the PSN was greater in the control females than in the control males and postnatal treatments reversed this sexual dimorphism. INTRODUCTION There are works showing the existence of sexual dimorphism in the preoptic area ( P O A ) 2-7'11"13'14'16' 21,23,25,28,29,31. However, sparse data are available in relation to sex differences in the preoptic dorsal region 23, which includes the striatal part of the preoptic area (STPOA) and the parastrial nucleus (PSN). The PSN is a cell group located beneath the anterior commissure, near the crossing of this tract of fibers, and it is composed of a rostral cell group, previously referred to as 'round nucleus '23, and a caudally located ovoid group of fusiform cells 25. The PSN is limited (see Fig. 1) medially by the STPOA, laterally by the bed nucleus of the stria terminalis (BNST) and ventrally by the lateral and medial POA (MPOA). In the latter the medial preoptic nucleus (MPN) can be distinguished (Fig. 1). The PSN has been considered either included into 22 or separated s'9"23 from the BNST. The exclusion of the PSN from the BNST is supported by the fact that the former does not receive either aminergic fibers or synapses from the strial axons 23. Other authors 25 tend to include the PSN within the MPOA. Raisman and Field 23 found in the female STPOA a greater number of non-amygdaloid origin synapses on dendritic spines than in male rats. Early postnatal treatments (male orchidectomy and female androgeniza- tion) alter the expression of this sexually differentiated pattern: orchidectomized males did not differ from control females, and a low number of spine synapses was observed in androgenized females with respect to control female rats. Morphological sex differences were found in the MPN. This nucleus and its divisions, lateral (MPNI) and medial (MPNm), are sexually dimorphic25. The sdn-POA that is distributed within the MPNm divisions [central (MPNc), anteroventral (MPNav) and medial exclusive of the MPNc and the MPNav (MPNm-excl)] also presents sexual dimorphism 4'5'13'14. There are also several comunications in the literature 8'9'17 showing the existence of sex differences in the BNST of the rat. The female presents a larger volume 9 and a greater neuronal number 9 in the medial anterior BNST (BNSTMa) than the male rat, while the male shows greater volume and a greater number of neurons in the medial posterior BNST (BNSTMp) than the female rat 8,9'17. However, in the ventral division of the BNST, which laterally borders the PSN, no sexual dimorphism was found 8,9. The aim of the present work was to study the possible existence of volumetric sex differences in the PSN and to determine the effects of the early postnata ! gonadal environment. In addition, the possible existence of sexual dimorphism in the PSN Would show that a preoptic region (STPOA-PSN) dorsally placed to the MPOA is also sexually dimorphic. MATERIALS AND METHODS Twenty Wistar rat pups (10 males and 10 females) (Alfn, Madrid, Spain) were divided into 4 groups immediately after birth: (1) 5 females were androgenized on the day of birth (D1) with a single injection of 1.25 mg of testosterone propionate (TP); (2) 5 males were surgically orchidectomized on D1; (3) 5 females received a single injection of the TP vehicle (sesame oil); and (4) 5 males were sham-operated (abdominal incision). For these treatments, all subjects were anesthetized by cooling at -10 °C for approximately 10 min. Three months later, the animals were deeply anesthetized Correspondence: A. del Abril, Departamento de Psicobiologfa, Universidad Nacional de Educaci6n a Distancia, Ciudad Universitaria, P.O. Box 50487, 28040 Madrid, Spain. 0165-3806/90/$03.50 © 1990 Elsevier Science Publishers B.V. (Biomedical Division)