In the ovary, DA and OA were detected in the cytoplasm of
previtellogenic oocytes, vitellogenic oocytes and mature oocytes,
whereas immunoreactivities were not detected in the oogonia and
two types of follicular cells. However, DA- and OA-ir were more
intense in the early steps of oocyte differentiation (Meeratana and
Sobhon, 2007), implying that both neurotransmitters may play
some key roles in controlling early oocyte development in this
species. This finding is in agreement with our earlier report using
HPLC analysis that the concentration of DA in the ovary was significantly
higher in early ovarian stages (stages I–II) than in the late
and mature stages (ovarian stages III–IV) (Tinikul et al., 2008). The
precise actions of DA and OA in oocyte development needed to be
clarified by further studies.