Mariana Zancan and Alberto A. Rasia-Filho132
participates in the secretion of gonadotrophin releasing hormone (GnRH), by
its connection with the hypothalamic anteroventral periventricular nucleus,
during a short period along the estrous cycle (Simerly, 2002; Rasia-Filho et al.,
2004 and references therein). In addition, from de Castilhos et al. (2008),
“...in normally cycling females, the activation of glutamate NMDA receptors
in the MePD is relevant to initiate long-term changes in prolactin secretion
needed for pregnancy/pseudopregnancy or mnemonic events at the time of
mating (Polston and Erskine, 2001; Lehmann et al., 2005). It is possible that
these local NMDA receptors can show main effects when there are two
concomitant physiological conditions: the variation in the hormonal milieu in
the proestrus phase and the occurrence of repeated and temporally patterned
overthreshold vaginocervical stimulation coming to specific MePD neurons
(Polston et al., 2001; Lehmann and Erskine, 2005; Lehmann et al., 2005).
Interestingly, small amounts of vaginocervical stimulation facilitate lordosis,
moderate ones stimulate pacing behavior and neuroendocrine secretion, and
large amounts induce the termination of estrus behavior (Pfaus and Heeb,
1997)”.
It is remarkable that ovarian steroids have region-specific effects in the
female brain (Hansberg-Pastor et al., 2015) and the MePD is sexually
dimorphic and/or affected by sex steroids in rodents (Rasia-Filho et al.,
2012a,b and references therein). For example, male and female differ in the
spatial orientation of dendrites within the MePD neuropil, a finding likely
involved with different axonal input properties (Dall ́Oglio et al., 2008a). It is
also evident that sex steroids can remarkably alter the function of the MePD
cells by concomitantly change the number of dendritic spines (reviewed in
Rasia-Filho et al., 2012a) and the synaptic inputs in both dendritic shafts and
spines (Brusco et al., 2014). Dendritic shafts receive most synaptic contacts
(~76%) in the MePD of males and cycling females (Brusco et al., 2014). The
complexity of dendritic contacts involves the proportion of inhibitory synapses
on dendritic shafts in the right MePD of proestrus females, higher than in the
left MePD, and higher than in the right MePD of males or diestrus and estrus
females (Brusco et al., 2014). Synapses made directly on dendritic shaft
synapses can be stronger and make larger synaptic currents recorded at the
soma than axo-spiny synapses, with the exception for those contacts on stubby
spines (Segal, 2010). Dendritic spines form usually asymmetric contacts
assumed as excitatory ones, although some receive inhibitory terminals and
others are multisynaptic spines (Brusco et al., 2014). Then, most dendritic
spines are specialized units with excitatory postsynaptic properties that
modulate the occurrence, strength, and plasticity of signal transmission