In molluscs, the neurotransmitter serotonin (5-HT) has been linked to a variety of biological roles including gamete maturation and spawning. The possible involvement of 5-HT in abalone gamete release was demonstrated by a dose-dependent increase in Haliotis rubra gonad contractile bioactivity following 5-HT stimulation. Physiological functions associated with 5-HT, are mediated through binding to 5-HT receptors. A cDNA encoding a putative 5-HT receptor consisting of 359 amino acids was isolated from the tropical abalone H. asinina, termed 5-HT(1 ha). The 5-HT(1 ha) shares G-protein-coupled receptor motifs with metazoan 5-HT receptors, including predicted transmembrane domains, active sites for protein kinase action, and N-linked glycosylation sites. However, the third intracellular loop of 5-HT(1 ha) is relatively short, and only six transmembrane domains are predicted, implying a truncated receptor. Phylogenetic analysis with known 5-HT receptor genes suggests that 5-HT(1 ha) belongs to the type 1 5-HT receptor family. Expression analysis by RT-PCR showed that 5-HT(1 ha) mRNA was present in all tissues examined, including the neural ganglia and gonad tissues. Immunocytochemistry revealed the presence of 5-HT(1 ha) specifically within the soma of neuronal cells located in the outer cortex of both cerebral and pleuropedal ganglia. In ovarian and testicular tissues, 5-HT(1 ha) immunoreactivity was observed in epithelial cells of the outer capsule and connective tissue of the trabeculae to which the gamete follicles adhere. Whether this receptor transcript is translated to a functional protein needs to be verified, but if so, it could play a role in reproduction.
Germ cells in the ovary of H. asinina are divided into six stages: oogonia and five stages of oocytes. The oogonium is a scallop-shaped cell 8-10 pm in diameter, closely adhered to a trabecula. Its nucleus exhibits small blocks of heterochromatin along the nuclear envelope and a small nucleolus. The cytoplasm contains abundant ribosomes. The stage I oocyte is a round cell 12-25 pm in diameter. Its nucleus contains numerous lampbrush chromosomes consisting of chromatin fibers with three sizes, i.e., 100-200,4040 and 7-1 2 nrn in diameter. The cytoplasm has numerous mitochondria, few rough endoplasmic reticulum, and abundant ribosomes. The stage I1 oocyte is a round cell 25-35 pm in diameter. Its nucleus exhibits increasingly decondensed chromatin and a nucleolus, and the nuclear envelope exhibits numerous nuclear pores. The cytoplasm contains numerous and well-developed Golgi bodies, rough endoplasmic recticulum and abundant ribosomes. There are two types of secretory granules: both have a spherical shape, 350-450 nrn in diameter, with an electron-dense and electron-lucent matrix, respectively. The stage I11 oocyte is a pear-shaped cell about 35x70 pm in size. Lampbrush chromosomes are almost completely unraveled. The two types of secretory granules are greater in number and cluster around the Golgi bodies. Larger and more electron-dense ovoid-shaped yolk granules start to appear. The stage IV oocyte is a flask-shaped cell about 50x80 pm in size. Its nucleus contains completely decondensed chromatin and a highly enlarged nucleolus. The cytoplasm is filled with lipid droplets (1.5-3 pm in diameter) and yolk granules (1.5-2.5 pm in diameter). The vitelline-cum-jelly coat starts to develop, and could be derived from the fmt type of secretory granules which are translocated to be exocytosed at the plasma membrane. The stage V oocyte is similar to the stage IV oocyte except its vitelline-jelly coat achieves maximum thickness and appears fibrous in comparison to the amorphous appearance at stage IV.
Immunohistochemistry was used to identify the distribution of both APGWamide-like and RFamide-like peptides in the central nervous system (CNS) and ovary of the mature female giant freshwater prawn, Macrobrachium rosenbergii. APGWamide-like immunoreactivity (ALP-ir) was found only within the sinus gland (SG) of the eyestalk, in small- and medium-sized neurons of cluster 4, as well as their varicosed axons. RFamide-like immunoreactivity (RF-ir) was detected in neurons of all neuronal clusters of the eyestalk and CNS, except clusters 1 and 5 of the eyestalk, and dorsal clusters of the subesophageal, thoracic, and abdominal ganglia. The RF-ir was also found in all neuropils of the CNS and SG, except the lamina ganglionaris. These immunohistochemical locations of the APGWamide-like and RF-like peptides in the eyestalk indicate that these neuropeptides could modulate the release of the neurohormones in the sinus gland. The presence of RFamide-like peptides in the thoracic and abdominal ganglia suggests that it may act as a neurotransmitter which controls muscular contractions. In the ovary, RF-ir was found predominantly in late previtellogenic and early vitellogenic oocytes, and to a lesser degree in late vitellogenic oocytes. These RFs may be involved with oocyte development, but may also act with other neurohormones and/or neurotransmitters within the oocyte in an autocrine or paracrine manner.
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