Testosterone rapidly increases the volume of the medial preoptic nucleus in male Japanese quail

In many vertebrate species, testosterone (T) influences the
volume and cellular characteristics of a variety
steroid-dependent nuclei, including the medial preoptic
nucleus (POM) of male Japanese quail. The quail POM is a
key nucleus in the control of male sexual behavior and
exhibits a sex difference (larger in males than in females).
In castrated quail, the POM volume is markedly increased
by T but previous studies always assessed this effect after a
period of 8-14 days and its specific time-course is
unknown. We recently reported that antisense treatments
suppressing steroid receptor coactivator-1 expression block
T effects on POM volume but when treatment was
discontinued, a significant increase of POM size occurred
within two days. We therefore asked whether this rapid
neuroanatomical change was specifically linked to the
SCR-1 modulation or reflected the normal rate of T-induced
effects. We collected brains from castrated male quail after
1, 2, 8 and 15 days of T treatment (CX+T) while in untreated
castrates (CX) brains were collected after 1 or 15 days. The
POM volume defined by Nissl staining increased in a
time-dependent fashion in CX+T to reach a 40% increase
after 15 days while no change was observed in CX. An
increase in the average POM volume was detected on day 1
(13%) and this increase was statistically significant (25%)
after only 2 days of exposure to T. No volume change was
observed in the steroid-insensitive nucleus rotundus. The
activation of male sexual behavior was positively
correlated with the increase in POM volume. Because new
neurons are not incorporated in the adult avian
hypothalamus, these rapid volumetric changes must
reflect increases in soma size, neuropile or extracellular
space. The mechanisms underlying this unexpectedly
rapid neural plasticity are currently under study,
focusing in particular on the potential modifications of the
steroid sensitive aromatase expressing cells.