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

78 S.I. SÁ AND M.D. MADEIRA
(Matsumoto and Arai, 1986b; Miller and Aoki, 1991), both
factors that can interfere with the end results and, thus,
with the conclusions regarding the existence of sex-related
differences in the number of synapses. In fact, the volume
of the VMNvl and of its neuropil is not constant at all
phases of the estrus cycle (Madeira et al., 2001), and areal
as well as numerical densities are influenced by variations
in the reference volume (for a review, see Oorschot, 1994).
Moreover, estimations of particle numbers from twodimensional
probes, that is, from single sections, are likely
to be biased by variations in the size of the particle under
study, and in the present case the postsynaptic densities
of axospinous synapses are larger in males than in females.
Finally, but not least important, is the fact that
small or tangentially cut synapses cannot be reliably recognized
in single sections (Curcio and Hinds, 1983;
deToledo-Morrell et al., 1988) and, consequently, synaptic
numerical densities estimated from single sections are
systematically underestimated by as much as 20% (Curcio
and Hinds, 1983). Since the size of the postsynaptic densities
is smaller in females than in males, it is possible
that the underestimation might be greater in females
than in males, thus complicating the global detection of
sex-related differences in the number of synapses.
The presence of sex differences that would favor females
was somehow expected on the grounds of the existence of
a higher spine density in females than in males (Madeira
et al., 2001). In fact, it is generally held that spines develop
as a response to a signal of the presynaptic component,
thus indicating that the sprouting of a spine is
associated with synapse formation and spine retraction
with synapse elimination. Data obtained in other hypothalamic
nuclei lend support to this statement. Specifically,
in the medial preoptic area, spine density is greater
in females than in males (Madeira et al., 1999) and sex
differences in the density of spine synapses favor females
(Raisman and Field, 1973), similar to what has been observed
in the arcuate nucleus where male–female differences
in the density of spine synapses (Matsumoto and
Arai, 1980) reflect similar differences in the density of
dendritic spines (Leal et al., 1998).
CONCLUSIONS
By using robust stereological approaches that generate
numbers of synapses per neuron, we have demonstrated
that the synaptic contacts established between VMNvl
neurons and their afferents are not stable in number over
the estrus cycle. These cyclical variations are due to the
formation and withdrawal of axodendritic, axospinous,
and axosomatic synapses, which are particularly numerous
at high estradiol levels. Consequently, proestrus rats
have approximately twice the number of dendritic synapses
as male rats, whereas diestrus rats have the same
number. Conversely, males have approximately twice as
much axosomatic synapses as diestrus rats and the same
number as proestrus rats. Finally, we have shown that the
postsynaptic densities do not vary in size over the estrus
cycle and that they are globally larger in males than in
females.
ACKNOWLEDGMENTS
The authors thank Mrs. M.M. Pacheco and Mr. A.
Pereira for technical assistance.
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