Dynamic and Integrated Synaptic Processing ... 135(Garcia-Segura et al., 1994). Accordingly, “...the emergence of perisomatic
axospine synapses provides an additional amount of perikaryal surface to
establish dynamically regulated contacts, in a place closer to the axon hillock,
that can strategically influence the neuronal output (Peters et al., 1991). This
was evident in late proestrus females, with a higher density of spines along the
perikaryal perimeter, which could represent the type of labile spines with
plastic capacities for local information processing and behavior modulation
along the estrous cycle... The shape diversity increased in the night of
proestrus with 33% stubby/wide spines; 22% thin spines; and 36% with a
ramified, transitional, or atypical aspect...The existence of spinules is also
suggestive of plastic properties for the somatic spines in the MePD of
proestrus females. Spinules on dendritic spines were associated with an
enhanced cellular activity because they are rapidly formed after synaptic
stimulation (Applegate and Landfield, 1988; Schuster et al., 1990; Tao-Cheng
et al., 2009; Brusco et al., 2014; Stewart et al., 2014), serve for transferring
large molecules between pre- and postsynaptic elements (Tarrant and
Routtenberg, 1977), and serve in intercellular signaling between active spines
and the presynaptic element (Spacek and Harris, 2004)” (Zancan et al., 2015).
These somatic spines would provide additional modulatory possibilities to the
synaptic activity than contacts made directly on the perykarion.
The complex somatic and dendritic synaptic processing in the female
MePD along the estrous cycle may occur with a concomitant switch in the
local neurotransmission (Micevych et al., 1988; Oro et al., 1988; Polston and
Erskine, 2001; Lehmann and Erskine, 2005; Lehmann et al., 2005; de
Castilhos et al., 2008). It is likely that, in the proestrus phase, the impact of
putative excitatory inputs in the cell body can be altered by the higher amount
of somatic spines, whereas, on dendrites, excitatory input decreases due to a
reduction in the density of proximal dendritic spines. We hypothesized
(Zancan et al., 2015) “that the morphological and synaptic rearrangement in
the left MePD can change the inhibitory impact of output projections on
hypothalamic circuits during the proestrus phase to disinhibit both the activity
of the ventromedial nucleus, involved in lordosis behavior, and in the medial
preoptic area, for the display of proreceptive behavior (Pfaus and Heeb, 1997;
for additional data and comments see Rasia-Filho et al., 2012a; Brusco et al.,
2014). One possible reason for this hemispheric lateralization is that the left
MeA would be specialized for chemosensory and/or steroid negative feedback
regulation of the secretion of hypothalamic luteinizing hormone (discussed in
Cooke and Woolley, 2005). These changes in the MePD occur during the
proestrus phase and, therefore, some hours prior to those reported in the