Dynamic and Integrated Synaptic Processing ... 133according to the synaptic demand in the integrated neural networks.
Specifically, “... the density of proximal dendritic spines in the MePD of adult
Wistar rats is sexually dimorphic (higher in males than in proestrous, estrous
or metaestrous females) and is affected by the normal fluctuations in plasma
ovarian steroids along the estrous cycle (near 35% of reduction during the
transition from diestrous to proestrous)... In the sampled MePD neurons,
diestrous females had approximately 51% of thin spines, 31% of stubby/wide,
12% of mushroom-like, and 6% belonging to the other spine shapes.
Proestrous females showed near 53% of thin spines, 28% of stubby/wide, 10%
of mushroom-like, and 9% belonging to the other spine shapes. Estrous
females showed around 47% of thin spines, 34% of stubby/wide, 14% of
mushroom-like, and 5% belonging to the other spine shapes...” (Rasia-Filho et
al., 2012a).
The structure of dendritic spines, with a continuum of shapes and sizes,
would impact on the strength of synapses and, ultimately, the neuronal
function (Kasai et al., 2010; Segal, 2010; Yuste, 2013; Tønnesen and Nägerl,
2016). Considering the morphological features of stubby/wide spines, thin
spines with long necks or mushroom-like spines, it is likely that each spine
type have: (1) a variable area for receptor trafficking and the presence of
postsynaptic density; (2) local electrical resistance; (3) degree of biochemical
compartmentalization or coupling with parent dendrites, and, then, (4) capacity
to alter synaptic strength, integration, and cellular enduring effects (Arellano et
al., 2007; Yuste, 2013; Tonnesen et al., 2014). Synaptic processing might also
occur in microdomains in ramified spines or in more complex and atypical
forms (Verzi and Noris, 2009). This can occur at the level of each single spine
and depending on the brain region, the neuron type, and the functional
implication for the synaptic demand (Chen et al., 2011; Chen and Sabatini,
2012; Dalpian et al., 2015).
Further data indicated the modulation of dendritic spines by ovarian
steroids. That is, “In females, the dendritic spine density observed after 1 week
following OVX is comparable to the lower values obtained in virgin rats from
estrus to metaestrus... This suggests that MePD neurons can display a
minimum basal number of dendritic spines that cannot be affected by the
actions of ovarian steroids or be even reduced after OVX. On the other hand,
the increase in the number of spines detected in OVX+estrogen-treated rats
resembled that one found in normal females during the afternoon of diestrus...
This effect became supra-physiological following progesterone injection,
when the highest spine data were observed. Progesterone can induce complex
actions (Guerra-Araiza et al., 2003; Villamar-Cruz et al., 2006) and, notably,