Susana Isabel Ferreira da Silva de Sá ESTROGÉNIOS E ...

Neuroscience 133 (2005) 919–924
NEURONAL ORGANELLES AND NUCLEAR PORES OF HYPOTHALAMIC
VENTROMEDIAL NEURONS ARE SEXUALLY DIMORPHIC AND CHANGE
DURING THE ESTRUS CYCLE IN THE RAT
S. I. SÁ AND M. D. MADEIRA*
Department of Anatomy, Porto Medical School, Alameda Prof. Hernâni
Monteiro, 4200-319 Porto, Portugal
Abstract—Neurons in the ventrolateral division of the hypothalamic
ventromedial nucleus (VMNvl) become hypertrophied
when exposed to high estrogen levels, an effect that
has been observed after estrogen treatment of ovariectomized
rats as well as during the proestrus stage of the ovarian
cycle. In an attempt to examine whether the neuronal
hypertrophy noticed in these conditions reflects metabolic
activation of the neurons we have examined, using quantitative
methods, the cytoplasmic organelles involved in protein
synthesis and the nuclear pores of VMNvl neurons from females
on proestrus, when estrogen levels are high, and on
diestrus, when estrogen levels are low. Because VMNvl neurons
are sexually dimorphic with respect to their size we have
performed, in parallel, similar analyses in neurons from agematched
male rats. Our results show that the volume and the
surface area of the rough endoplasmic reticulum (RER) and
Golgi apparatus are increased at proestrus. They also show
that the density of nuclear pores is greater in males than in
females whereas the volume and the surface area of the RER
and Golgi apparatus are sexually dimorphic only at specific
phases of the ovarian cycle: the male–female differences are
notorious in the RER when females are on diestrus and in the
Golgi apparatus when they are on proestrus. Given that the
size of the RER and of the Golgi apparatus correlates with the
level of neuronal protein synthesis, data obtained in this
study suggest that the sex-related differences and the estrus
cycle variations in neuronal size reflect corresponding differences
and fluctuations in the metabolic activity of VMNvl
neurons. © 2005 IBRO. Published by Elsevier Ltd. All rights
reserved.
Key words: hypothalamus, sex differences, estrus, rough
endoplasmic reticulum, Golgi apparatus, stereology.
The ventromedial nucleus of the hypothalamus (VMN) is
widely recognized as a sexually dimorphic cell group of the
brain that has been implicated in the modulation of a
variety of physiological mechanisms and behaviors, such
as the female reproductive behavior (Pfaff, 1980). The sex
differences noticed in its anatomy are particularly evident in
the ventrolateral division (VMNvl), where numerous neurons
that express receptors for gonadal steroids are concentrated
*Corresponding author. Tel: 351-22-509-6808; fax: 351-22-550-5640.
E-mail address: madeira@med.up.pt (M. D. Madeira).
Abbreviations: ANOVA, one-way analysis of variance; RER, rough
endoplasmic reticulum; S.D., standard deviation; S.E.M., standard
error of the mean; Sv, surface density; VMN, ventromedial nucleus;
VMNvl, ventrolateral division of the ventromedial nucleus; Vv, volume
density.
0306-4522/05$30.000.00 © 2005 IBRO. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.neuroscience.2005.02.033
919
(Pfaff and Keiner, 1973; Simerly et al., 1990; Shughrue et al.,
1997). Neurons located in the VMNvl display male–female
differences in their volume (Matsumoto and Arai, 1983;
Madeira et al., 2001), length of dendritic trees, dendritic
spine density (Madeira et al., 2001), neurochemistry (reviewed
in Madeira and Lieberman, 1995) as well as in the
density (Matsumoto and Arai, 1986; Miller and Aoki, 1991)
and total number (Sá and Madeira, in press) of the synapses
established upon their dendritic shafts and spines. It
is also known that some of these sex differences, namely
those noticed in the volume of the neuronal cell bodies and
in the length of the dendritic trees, are not constant across
the ovarian cycle, being evident when females are in
diestrus and disappearing when they are in proestrus
(Madeira et al., 2001).
In contrast to the wealth of information regarding the
influence of sex steroids in shaping the cytoarchitecture of
the VMNvl, relatively little is known about their effects on
the size of the cytoplasmic organelles of its constituent
neurons. Earlier studies have shown that the administration
of estrogen to ovariectomized rats leads to hypertrophy
of neuronal perikarya, condensation of nucleolar material,
increased stacking of the rough endoplasmic reticulum,
enlargement of the Nissl substance and Golgi
complexes, and emergence of pleomorphic mitochondria
in VMNvl neurons (Cohen and Pfaff, 1981; Carrer and
Aoki, 1982; Cohen et al., 1984; Meisel and Pfaff, 1988).
Even though these data suggest that exogenous estrogen
modifies the metabolic activity of VMNvl neurons, it is not
known whether identical changes occur in response to the
fluctuation of estrogen levels across the estrus cycle. In
addition, to our knowledge only one study has addressed
the possibility that the cytoplasmic organelles of VMNvl
neurons might display sexual dimorphic features (Ishunina
et al., 2001). This investigation, performed in human material,
showed that the size of the Golgi apparatus relative
to cell size is larger in young women than in young men.
In this study we sought to quantitatively analyze
whether the morphology of the cytoplasmic organelles of
VMNvl neurons differs between the sexes and, also, if in
females the size of these organelles varies over the estrus
cycle. To accomplish this, we have used electron microscopy
and unbiased stereological techniques to examine
the volume and the surface area of the rough endoplasmic
reticulum (RER) and of the Golgi apparatus of VMNvl
neurons in males and in females on proestrus, when the
circulating levels of estrogen are high, and on diestrus day
1, when the circulating levels of estrogen are low. Nuclear
pores are known to mediate the selective and energy-
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