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

Neuroscience 162 (2009) 307–316
EFFECTS OF ESTROGENS AND PROGESTERONE ON THE SYNAPTIC
ORGANIZATION OF THE HYPOTHALAMIC VENTROMEDIAL NUCLEUS
S. I. SÁ, E. LUKOYANOVA AND M. D. MADEIRA*
Department of Anatomy, Porto Medical School, University of Porto,
Alameda Hernâni Monteiro, 4200-319, Porto, Portugal
Abstract—The majority of the studies on the actions of estrogens
in the ventrolateral part of the hypothalamic ventromedial
nucleus (VMNvl) concern the factors that modulate
the receptive component of the feminine sexual behavior and
the expression of molecular markers of neuronal activation.
To further our understanding of the factors that regulate
synaptic plasticity in the female VMNvl, we have examined
the effects of estradiol and progesterone, and of estrogen
receptor (ER) subtype selective ligands on the number of
dendritic and spine synapses established by individual
VMNvl neurons and on sexual behavior. In contrast to earlier
studies that analyzed synapse densities, our results show
that exogenous estradiol increases the number of spine as
well as of dendritic synapses, irrespective of the dose and
regimen of administration. They also reveal that an effective
dose of estradiol administered as one single pulse induces
the formation of more synapses than the same dose administered
as two pulses on consecutive days. Our results
further show that both ER subtypes are involved in the mediation
of the synaptogenic effects of estrogens on VMNvl
neurons since the administration of the selective ER, propyl-pyrazole-triol
(PPT), and ER, diarylpropionitrile (DPN),
agonists induced a significant increase in the number of
synapses that, however, was more exuberant for PPT. Despite
its relevant role in feminine sexual behavior, progesterone
had no synaptogenic effect in the VMNvl as no changes
in synapse numbers were noticed in rats treated with progesterone
alone, with estradiol followed by progesterone or with
the antiprogestin mifepristone (RU486). Except for the sequential
administration of estradiol and progesterone, none
of the regimens was associated with lordosis response to
vaginocervical stimulation. Therefore, from the sex steroids
that undergo cyclic variations over the estrous cycle, only
estrogens, acting through both ER and ER, play a key role
in the activation of the neural circuits involving the ventromedial
nucleus of the hypothalamus. © 2009 IBRO. Published
by Elsevier Ltd. All rights reserved.
Key words: ventromedial nucleus, synapses, estrogen receptor,
propyl-pyrazole-triol, diarylpropionitrile, mifepristone.
The ventromedial nucleus of the hypothalamus (VMN) has
been implicated in a wide variety of functions, such as
*Corresponding author. Tel: 351-22-5513616; fax: 351-22-5513617.
E-mail address: madeira@med.up.pt (M. D. Madeira).
Abbreviations: ANOVA, analysis of variance; DPN, diarylpropionitrile;
EB, estradiol benzoate; ER, estrogen receptor; ER, estrogen receptor-;
ER, estrogen receptor-; N V, numerical density; O, oil; P,
progesterone; PPT, propyl-pyrazole-triol; PR, progestin receptors;
RU486, mifepristone; VCS, vaginocervical stimulation; VMN, ventromedial
nucleus of the hypothalamus; VMNvl, ventrolateral part of the
ventromedial nucleus of the hypothalamus.
0306-4522/09 $ - see front matter © 2009 IBRO. Published by Elsevier Ltd. All rights reserved.
doi:10.1016/j.neuroscience.2009.04.066
307
feeding and defensive behaviors, regulation of the autonomic
responses and hormonal production by the anterior
pituitary, antinociceptive mechanisms and somatomotor
control (for a review see, Canteras et al., 1994). However,
the VMN is particularly known for the important role it plays
in the control of the feminine sexual behavior, namely the
lordosis reflex (Pfaff and Sakuma, 1979a,b). In normally
cycling rats, this behavior occurs typically at proestrus
when females become sexually receptive as a response to
the sequential secretion of estrogens and progesterone by
the ovaries. Estrogens greatly increase the expression
of progestin receptors (PR) in the VMN (MacLusky and
McEwen, 1980; Parsons et al., 1982; Brown et al., 1987;
Shughrue et al., 1997a), as they do in other regions of the
brain (Gréco et al., 2001), and progesterone, by acting on
estrogen-primed VMN neurons, facilitates the lordosis behavior
(Rubin and Barfield, 1983a,b; Mani et al., 1994).
Notably, estrogens appear to be essential for this response,
as opposed to progesterone whose role can be
fulfilled by other hormones, neurotransmitters and signaling
molecules (Kow and Pfaff, 1998; Auger, 2001, 2004;
Blaustein, 2003; Wu et al., 2006).
Cells in the ventrolateral division of the ventromedial
nucleus of the hypothalamus (VMNvl) are essential for the
development of the lordosis reflex. Neurons in this division
express nuclear and extranuclear estrogen receptors
(ERs; Pfaff and Keiner, 1973; Simerly et al., 1990; Milner et
al., 2008) and PRs (MacLusky and McEwen, 1980; Parsons
et al., 1982) and exhibit cyclic changes in their morphology
over the estrous cycle in response to the natural
fluctuations in the circulating levels of sex steroids (Madeira
et al., 2001). Studies carried out in rats at different
phases of the estrous cycle have shown that during
proestrus VMNvl neurons have larger cell bodies, have
longer dendritic trees with more dendritic spines, and establish
more synapses than neurons from rats in diestrus
(Frankfurt et al., 1990; Madeira et al., 2001; Sá and Madeira,
2005a,b). Because changes of the same type have
been noticed in response to the administration of estradiol
to ovariectomized rats (Carrer and Aoki, 1982; Jones et al.,
1985; Frankfurt et al., 1990; Frankfurt and McEwen,
1991a,b; Calizo and Flanagan-Cato, 2000), the cyclic variations
that VMNvl neurons undergo over the estrous cycle
have been tentatively ascribed to the trophic actions of
estrogens. However, it is not known if, and how, progesterone
contributes to these alterations, namely at proestrus
when the endogenous levels of estradiol and progesterone
are both elevated (Butcher et al., 1974). In this study, we
address this issue by analyzing the number of synapses
established by each VMNvl neuron in rats treated either with
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