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

SYNAPTIC PLASTICITY IN THE VM NUCLEUS
and axospinous synapses rather than a shift in synapse
location between dendritic shafts and dendritic spines. In
addition, and in line with observations carried out in other
regions of the brain, such as the hippocampal formation
(Woolley and McEwen, 1992) and the cerebral cortex (Trachtenberg
et al., 2002), our results show that in the VM-
Nvl of the adult rat changes in dendritic spine density
correlate positively with changes in the number of axospinous
synapses.
Data obtained in this study also show that estrogen does
not interfere with the size of individual synaptic contacts.
In fact, the surface area of the individual postsynaptic
densities of axodendritic and axospinous synapses did not
differ between proestrus and diestrus rats, a finding that
is in agreement with the lack of variations in the length of
the synaptic contacts in the VMNvl after the administration
of estrogen to ovariectomized rats (Carrer and Aoki,
1982). However, because the number of axodendritic and
axospinous synapses is higher in proestrus than in
diestrus rats, the total area of dendritic membrane occupied
by synaptic contacts is significantly increased when
estrogen levels are high. The finding that the percentage
of plasmalemma of dendritic spines occupied by postsynaptic
densities does not vary across the estrus cycle indicates
that spines that are newly formed in each proestrus,
as hormone levels rise, do not differ with respect to their
size from the spines that persist during diestrus, when
hormone levels decline. There is increasing evidence that
spines with large heads are stable and contribute to strong
synaptic connections, contrary to spines with small heads
that are motile and unstable, and contribute to weak or
silent synaptic connections (Kasai et al., 2003). Therefore,
our data indicate that the synapses that are added to, and
removed from, VMNvl neurons during each estrus cycle do
not differ with respect to their stability and function from
the pool of synapses received by these neurons at low
estrogen levels.
We have also found that sex steroids influence not only
the dendritic synapses, but also the smaller population of
synapses established upon the cell bodies of VMNvl neurons.
In effect, although somatic synapses did not change
in size over the estrus cycle, they were more numerous at
high estrogen levels, that is, during proestrus. This observation
lends support to the view that the plastic changes
displayed by hypothalamic synapses over the estrus cycle
are region-specific. Actually, in the nearby located arcuate
nucleus, the number of axosomatic synapses does not increase
in response to estrogen, as happens in the VMNvl,
but declines when estrogen levels are low, i.e., during
estrus (Olmos et al., 1989), as opposed to what occurs in
the anteroventral periventricular nucleus, where low estrogen
levels lead to increases in the number of axosomatic
synapses (Langub et al., 1994). It is known that
somatic synapses are mostly symmetrical and that most of
these synapses are inhibitory (Uchizono, 1965; Colonnier,
1968), contrary to asymmetrical synapses, which are presumed
excitatory (Westrum and Blackstad, 1962; Harris
and Stevens, 1989). Our data indicate that, in each
proestrus, individual VMNvl neurons receive more than
3,000 excitatory synapses and more than 40 inhibitory
synapses than in the preceding or following diestrus,
which suggests that estrogen enhances the activity of
VMNvl neurons by promoting a proportionally greater
increase in the excitatory drive than in the inhibitory
inputs they receive.
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Sex differences in the synaptic organization
of the VMN
The VMN has been classically regarded as a sexually
dimorphic nucleus, a concept corroborated by data obtained
in this study. Actually, we have found that females
in proestrus have more synapses than males, a sex-related
difference no longer apparent when females are in
diestrus. The existence of more dendritic synapses in
proestrus rats than in males was accounted for by sex
differences in the number of axospinous (54%) and axodendritic
(24%) synapses that favor females. Interestingly,
opposite sex differences were noticed in the number
of somatic synapses, which were 46% more numerous in
males than in diestrus rats. These data show that, by
comparison with males, VMNvl neurons from proestrus
females receive more excitatory inputs and the same
amount of inhibitory inputs, whereas neurons from
diestrus females receive a similar amount of excitatory,
but a smaller amount of inhibitory inputs. This sexual
dimorphic pattern appears to be specific to the VMNvl, as
it does not mirror the sex differences in the synaptic
organization of other steroid-sensitive regions of the
brain. For example, in the arcuate nucleus, which, similar
to the VMNvl, displays sex differences in its anatomy
(Leal et al., 1998) and exhibits phasic synaptic changes
across the estrus cycle (Olmos et al., 1989), the density of
axodendritic synapses does not differ between the sexes
contrary to the density of axospinous and axosomatic synapses,
which is greater in females (Matsumoto and Arai,
1980; Pérez et al., 1990).
Interestingly, the sole parameter that consistently
showed sex-related differences was the surface area of the
postsynaptic densities of individual axospinous and axosomatic
synapses, which was 30% smaller in females
than in males. It seems clear that the size of postsynaptic
densities is determined by the organizational effects of sex
steroids during the perinatal period and is not sensitive to
the activational effects of these hormones during adulthood,
as no correlation with hormonal levels was noticed
in females. Because the postsynaptic densities of axospinous
synapses are larger in males than in females and the
percentage of plasmalemma occupied by postsynaptic densities
does not differ between the sexes, there are reasons
to assume that the volume of dendritic spines is greater in
males than in females. This was not an unexpected finding,
because earlier studies have shown that the size of the
postsynaptic densities positively correlates with the size of
spines (Harris and Stevens, 1989; Sorra and Harris, 2000;
Kasai et al., 2003). Given that spines with larger heads
and postsynaptic densities have greater sensitivity to glutamate
(Harris and Stevens, 1989; Schikorski and
Stevens, 1997; Kasai et al., 2003), there are reasons to
assume that, in addition to differences in the number of
synapses, males and females also differ with respect to the
efficacy of synaptic transmission in the VMNvl.
Previous studies have shown that in the VMNvl there
are sex-related differences that favor males in the density
of axodendritic and axosomatic synapses (Matsumoto and
Arai, 1986b; Miller and Aoki, 1991). Discrepancies between
these and our own data are difficult to understand
because in all cases, including the present study, analyses
were centered on the VMNvl. However, in those studies
the phase of the estrus cycle was not identified and, as
already mentioned, the estimates report synapse densities
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