Mariana Zancan and Alberto A. Rasia-Filho134
results after progesterone injection contrast with those observed in the
proestrus (when endogenous estrogen+progesterone peak in circulation). The
density of dendritic spines in proestrus females was comparatively low in the
MePD... In this sense, current estrogen+progesterone substitutive treatments
might not mimic exactly the daily physiological variations in the plasma
concentrations of gonadal steroids (and also prolactin) across the different
phases of the estrous cycle” (de Castilhos et al., 2008).
The MePD cellular and synaptic organization changes once more when
females have the experience of motherhood. Different from the other MeA
subnuclei, there is a decrease in the density of proximal dendritic spines after
motherhood in the MePD. That is, dendritic spines are reduced approximately
24% in postpartum diestrus females when compared to age-matched virgin
females also in the diestrus phase (Rasia-Filho et al., 2004). In these rats, the
lasting reduced density of dendritic spines is likely involved with both the
decrease of avoidance behavior toward pups and the development of adaptive
skills in the mother for the new demands from the environment, for nursing
behavior, and memory formation about this life event (Kinsley et al., 1999;
Rasia-Filho et al., 2004).
Furthermore, there is a complex interaction between sex steroid-
responsive neurons and glial cells in the MePD. According to Zancan et al.
(2015), “glial processes can dynamically change their extension, possibly
restructuring the astrocytic cytoskeleton and folding properties to accompany
the changeable available surface of perikaryal membrane. In proestrus, a
decreased surface would obligate glial processes to partially retract and
condense, possibly contributing to increase the glial fibrillary acidic protein-
immunoreactivity in the MePD neuropil. This effect cyclically changes along
hours to days and starts to recover from estrus to diestrus. In addition, the
MePD neuronal activity in the late proestrus phase would involve the intrinsic
biophysical properties of the perikaryon for computing synaptic inputs and
generate somatic action potentials associated with the modulatory function of
new somatic spines with increased density and different shapes. In estrus,
when ovarian hormones drop in circulation, these effects are no longer found.”
Given the number of synapses that are made upon the available cell
membrane surface, it could be concluded that the perikaryon is not the
principal site of synaptic input to nerve cell (Peters et al., 1991). This does not
mean that axo-somatic synapses cannot generate important consequences for
the neuronal activity. It is well demonstrated the functional role of engulfing
glial layers isolating axons from the perykarial membrane in the arcuate
nucleus of the hypothalamus for the feedback control of the GnRH release