It is well documented that dopamine(DA) and norepinephrine (NE) can regulate gonadotropin secretion in fishes. The presence of DA and NE in the brain of mature Amphioxus ［Branchiostoma belcheri Gray］ has been demonstrated by high performance liquid chromatography(HPLC), however, whether these two neurotransmitters are involved in the regulation of the secreton of epithelial cell (primitive gonadotropic cell)of Hatschek's pit (homologous to pituitary in vertebrates ) remains unknown. In the present study, the immunocytochemical localization of DA and NE in the nervous system and gonads of Amphioxus was investigated with (avidin-biotin complex ABC) method.
Amphioxus (18～52.4mm in total length) were collected from the Tong'an-Qiongtou sea area near Xiamen). Each animal was divided into three segments, rostal(from tentaculum peristomial to the first pair of gonads), medial(from the first pair of gonads to the anus) and caudal. They were fixed in Bouin's fixative without acetic acid, and embedded in paraplast. Longitudinal or cross sections of 6 μm thickness were cut. Sections with the structure of the brain, neural tube and gonads were identified under a light microscope and selected for immunostaining.
Medium and small types of DA and NE immunopositive nerve cells were found in the posterior part of telencephalon, anterior and middle part of midbrain, and the positive substance occurred in the cytoplasm of nerve cells. No immunoreactivity for DA and NE in this study was detected in the posterior of midbrain and hindbrain of Amphioxus. We found for the first time that DA immunopositive nerve cells and fibers also located in the infundibular part of Amphioxus, a region extending from the middle region of midbrain to the Hatschek's pit and corresponding to the fish infundibular which connected hypothalamus with pituitary. Large type of NE immunopositive nerve cells were showed in the posterior part of telencephalon. NE immunopositive nerve cells and fiber connecting with another nerve cells was observed in the middle part of midbrain. On the other hand, DA and NE immunopositive nerve cells were identified on the dorsal side and middle part of spinal cord, and the immunopositive nerve fibers could be found on the ventral side. The distribution pattern of DA and NE in the nervous system of adult Amphioxus was different from that in larvae. Additionally, we observed for the first time that the immunoreactivity of DA and NE also located in the ovary and testis of Amphioxus in various development stages except in the mature testis.
In conclusion, the results suggested a direct action of DA and an indirect action of NE on the secretion of Hatschek's pit through the brain GnRH neuronal system in Amphioxus, as has been shown in fishes.