Brain Research zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Brllerin, Vol. 25, pp. 165-168. 0 Pergamon Press plc, 1990. Printed in the U.S.A 0361-9230190 $3.00 + .oO RAPID COMMUNICATION Estradiol Increases the Dendritic Length of Ventromedial Hypothalamic Neurons in Female Syrian Hamsters ROBERT L. MEISEL AND VICKIE R. LUTTRELL Department of Psychological Sciences, Purdue University, W est Lafayette, IN 47907 Received 26 March 1990 MEISEL, R. L. AND V. R. LUTTRELL. Estradiol increases the dendritic length ofventromedial hypothalamic neurons in female Sy rian hamsters. BRALN RES BULL 25(l) 165-168, 1990. -We examined the effects of ovarian hormones on dendritic architecture of neurons in the ventromedial nucleus of the hypothalamus in female Syrian hamsters. Treatment with 10 Fg of estradiol benzoate for two days, or estradiol benzoate for two days followed by an injection of 500 ug of progesterone, increased the total dendritic length of ventromedial nucleus neurons by almost 50% compared with neurons from the ventromedial nucleus of ovariectornized, oil-treated females. Neurons in a control region, the dorsomedial nucleus of the hypothalamus, were unaffected by these hormone treatments. These results demonstrate that steroids can induce changes in dendritic structure within 48 hr. suggesting that such morphological reconfiguration of hypothalamic neurons may underlie variations in behavior associated with the female’s 4-day estrous cycle. Estradiol Dendritic morphology Female Syrian hamster THERE is clear evidence that environmental factors, learning, and trophic agents (e.g., hormones) can alter neuronal connectivity in adult animals (7). For example, androgens can increase the size of the dendritic fields in the forebrain of songbirds or male hamsters, in the amphibian brainstem, and in the spinal cord of male rats (3, 5, 6, 9, 10). Since androgens normally exert their actions on the nervous system over long periods of time [e.g., (16)], these hormones must be applied for weeks or months for changes in dendritic arborization to be apparent. Similar studies have not been pursued in female rodents, such as mice, rats and hamsters, perhaps due to the brief duration of their ovarian cycles which are only 4-5 days long (4). For these animals, the actions of ovarian steroids on nerve cells have been traditionally ascribed to alterations in neurochemical or electro- physiological properties [see review by (15)]. Recent studies indicate that there are ultrastructural changes in ventromedial hypothalamic neurons occurring within only a few hours (8.13) or a few days (12) of estradiol or progesterone treatment, and that these ultrastructural patterns may occur only following behavior- ally effective steroid treatments (12,13). Given the literature cited above, we considered the possibility that there may be structural reconfigurations of the dendrites of ventromedial hypothalamic neurons following short-term steroid exposure. In the present study, we tested whether exogenous treatment with steroid hor- mones, designed to approximate the temporal pattern of exposure during the estrous cycle, could affect dendritic length in neurons of the ventromedial nucleus of the hypothalamus of female Syrian hamsters. METHOD Subjects were 15 adult female Syrian hamsters (Harlan Sprague-Dawley, Midland, MI facility), housed three per cage in plastic cages (5 1 X 41 x 20 cm) under conditions of controlled temperature (23°C) and lighting. Lights in the colony were off between 13:00 and 23:00 hr. Food and water were freely available. When between 83 and 155 days of age, the females were bilaterally ovariectomized while anesthetized with pentobarbital (7.5 mg/lOO g body weight). Five days after ovariectomy each female received one of three hormone treatments over three consecutive days: 1) three daily subcutaneous injections of 0.1 ml cottonseed oil (n = 4), 2) two injections of estradiol benzoate (10 pg/day in 0.1 ml oil) followed by an injection of 0.1 ml of oil on the third day (n = 8), or 3) two estradiol benzoate injections (10 &day) followed by progesterone (500 kg in 0.1 ml of oil) on the third day (n = 3). The Golgi procedures were adapted from those of Ramon- Moliner as modified by Gomez and Newman (5). Five hr after the last injection, the anesthetized animals were injected intracardially with heparin (0.25 ml; 1000 III/ml), and perfused with a solution of 0.9% saline and 0.1% sodium nitrate for 10 min, followed for 20 mitt by a solution of 2.4% potassium dichromate, 1.5% potassium chlorate, 6.0% chloral hydrate, and 4.0% formalde- hyde. A constant perfusion flow rate (20 ml/mm) was maintained by the use of a MasterFlex peristaltic pump. After the perfusion, the brains were removed and postfixed overnight in the formalde- hyde mixture in light-protected jars. 165