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

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synapse formation we extended our observations to estrogen-primed
rats treated with the PR antagonist RU486.
Again, we found no differences in the number of either type
of synapse relative to estrogen-primed rats treated or not
with progesterone.
Thus, our results show that despite the recognized role
of progesterone in the induction of behaviors that are
regulated by the VMNvl, this hormone does not seem to be
implicated in the changes of the connectivity pattern that its
neurons undergo over the estrous cycle (Sá and Madeira,
2005a).
Role of ERs in synaptic plasticity
In an attempt to identify the ERs involved in mediating the
synaptogenic effects of estrogens in the VMNvl, we have
independently estimated the number of synapses per neuron
in rats treated with either PPT, the agonist of the ER,
or DPN, the agonist of the ER. Our results revealed that
estrogens promote synaptogenesis by activating the ER
and the ER, as both agonists significantly increased the
number of spine and dendritic synapses received by each
VMNvl neuron. In addition, they also significantly decreased
the N V of VMNvl neurons, which suggests that,
similarly to estradiol (Carrer and Aoki, 1982; Jones et al.,
1985), PPT and DPN lead to hypertrophy of the neuronal
cell bodies and enlarge the neuropil in the VMNvl. The
similarity of the effects of PPT and DPN in the VMNvl,
where the expression of ER clearly exceeds that of ER,
strongly indicates that estrogens act upon VMNvl neurons
not only by activating classical signaling pathways in ERpositive
VMNvl neurons but also by enhancing the inputs
conveyed by afferent fibers to the VMNvl. In fact, VMNvl
neurons receive massive projections from brain regions
whose neurons, although expressing both subtypes of
ERs, are particularly rich in ER, as is the case of the
preoptic area and bed nucleus of the stria terminalis
(Shughrue et al., 1997b; Gréco et al., 2001; Shughrue and
Merchenthaler, 2001). However, the administration of PPT
induced the formation of significantly more spine and dendritic
synapses than the administration of DPN.
Studies in the CNS and other non-neural organs have
demonstrated that, depending on the tissue and variable
analyzed, ER and ER may mediate either divergent
(Frasor et al., 2003; Gréco et al., 2003; Lund et al., 2005)
or similar effects and, in this case, that ER activation has
weaker effects than ER activation (Frasor et al., 2003;
Lindberg et al., 2003). There is also evidence indicating
that when both receptors are activated, the activation of
the ER reduces the magnitude of the stimulation induced
by the ER (Lindberg et al., 2003; Gonzales et al., 2008;
Mazzucco et al., 2008). We cannot clearly conclude for the
same type of interaction between ER and ER in mediating
estrogen effects on synaptic plasticity in the VMNvl
because we did not include in our studies rats treated
simultaneously with PPT and DPN. Still, our results show
that the selective activation of ER in the absence of ER
activation did not consistently induce the formation of more
synapses than estradiol alone. In fact, our results show
that the number of synapses induced by PPT administra-
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S. I. Sá et al. / Neuroscience 162 (2009) 307–316
tion was higher than in EB-treated rats, but it did not differ
from that observed in 2 EB-treated rats. In addition, even
though there are reports showing dose-dependent effects
of PPT, namely, in body and uterine weights (Harris et al.,
2002; Frasor et al., 2003; Santollo et al., 2007), PR mRNA
levels (Harris et al., 2002) and proceptive and receptive
behaviors (Mazzucco et al., 2008), we have reasons to
assume that the exuberant effects of PPT on synapse
numbers are not attributable to the dosage used because
the administration of PPT produced an increase in uterine
weight that was similar to that provoked by estradiol.
CONCLUSION
Data presented herein reveal that despite the relevant
physiological role of progesterone in the facilitation of female
sexual behavior, this sex steroid plays no role in the
determination of the synaptic pattern required for the activation
of VMNvl neurons. In addition, they indicate that
although the expression of sexual behavior requires the
activation of the ER, synaptogenesis in the VMNvl is
mediated by activation of both subtypes of ERs.
Acknowledgments—The authors wish to thank Professor M. M.
Paula-Barbosa for helpful comments on the manuscript and Dr. M.
João Santos-Marques for helping with hormonal determinations.
This work was supported by a grant from Centro de Morfologia
Experimental (Unit 121/94) from a Fundação para Ciência e a
Tecnologia.
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