| Abstract |
Plasticity, a characteristic property of the brain, is defined as its ability to alter its structure and function in order to adapt to a constantly changing environment. Such an increased adaptability is particularly evident in teleosts, which among all vertebrates exhibit a remarkable variety in their reproductive strategies. This plasticity in sexual development ranges between complete gonochorism to sequential hermaphoditism. It is believed that the interaction of the brain with external queues such as an altered social environment determines the fate of the gonads, leading to the alteration of sexual phenotypes. Adult cell proliferation, migration and differentiation as well as neurotransmitter systems such as the noradrenergic and nitric oxide (NO.) are involved in the modificaton of widespread, multifunctional brain circuits related to reproductive behavior. The ability of the adult brain to produce new neuronal cells is one of the most important plasticity mechanisms. Noradrenaline is a well conserved neurotransmitter, involved in the regulation of a wide variety of systems and behaviors such as general awareness, learning, memory, stress responses, feeding and reproduction. Its action is mediated by membrane receptors coupled to G-proteins, several types of which have been characterized: α1, α2, β1, β2, β3 and localized pre- , post- and/or extra-synaptically. In contrast, ΝΟ. is a non-typical neurotransmitter: it is a small gaseous molecule, readily diffusible in aqueous media and through cellular membranes, exerting its actions, mainly interacting with guanylyl cyclase. We investigated the role of cell proliferation, migration and differentiation in addition to the involvement of α2 and β___ adrenergic receptors as well as ΝΟ., to determine neural mechanisms active during postembryonic development and sex reversal in the brain of sequentially hermaphrodite teleosts; protandrous Sparus aurata, protogynous Pagrus pagrus and their sterile hybrids, produced by crosses between male S. aurata and female P. pagrus and vice versa. The immunohistochemical method of bromodeoxyuridine (BrdU) was utilized to study cell proliferation and migration. The distribution of α2 and β___ adrenoceptors was determined by in vitro quantitative autoradiographic binding of radiolabeled ligands as well as the use of, specific to each subtype, antibodies. Finally, the activity of ΝΟ. synthase was determined by the histochemical method of NADPH-diaphorase. The observed impressive rate of cell proliferation is unique to the brain of adult teleost fish among all vertebrates and it is evident that the substantial increase in the size of the teleost brain is partially due to the generation and addition of new cells to the already existing populations. Postnatal neurogenesis was found in ventricular and subventricular areas of the brain, with the cerebellum exhibiting the highest mitotic activity. In addition, high mitotic activity was observed in the hypothalamic, thalamic and telencephalic ventricular areas, as well as the dorsal and ventral rim of the optic tectum. Among the brain areas studied, the dorsal hypothalamic area (dHA) showed higher mitotic activity in the female brain compared to that of the other sexual phases. Specifically the generation of new cells was three times higher in the female hypothalamus compared to the males, with the sex changing animals exhibiting intermediate levels. Comparison of the hybrids with the parental species did not provide evidence for differences regarding the distribution and number of proliferation zones however, significant differences in the hypothalamic rate of cell proliferation were detected, with the hybrids exhibiting minimal levels of cell birth. These differences persisted even after the migration and final differentiation of the newborn cells. The observed widespread distribution of α2 and β___ adrenoceptors is in agreement with previous reports on mammals and birds. High densities of α2 and β___ adrenoceptors were detected in most medial telencephalic, preglomelular, hypothalamic and posterior sensorymotor areas from the first year of life. Gradual decreases in receptor density with age were prominent in some pretectal, preoptic, ventral thalamic and hypothalamic brain regions. It is of great interest that sex reversal was accompanied by a transient increase in the expression of α2 adrenoceptors in dorsomedial telencephalic and diencephalic areas and β___ adrenoceptors in dorsal hypothalamic and preoptic areas. This specific transient increase in noradrenergic activity was particularly evident in dHA and the center of noradrenergic activity, locus coeruleus (LoC). The distribution and density of NO.-positive neurons and fibers was similar between all species, different age classes and sexual phases examined. However, sexually dimorphic regions like the hypothalamus and the preoptic area were rich in labeled neurons and highly innervated by NO.-positive fibers, always in close proximity with the zones of final differentiation of the newborn neurons. In conclusion, our findings suggest that a transient increase in the levels of α2 and β adrenergic receptors in limbic telencephalic (Dm), dorsal hypothalamic (dHA), preoptic (POA), thalamic input relay stations (PG) and locus coeruleus (LoC), areas that are highly innervated by NO fibers, triggers the onset of sexual inversion. In addition, masculinization or feminization of hermaphrodite teleosts also involves a significant increase in the rate of hypothalamic cell proliferation with the addition of newborn neurons and glia to the already existing cell populations. These findings highlight the important regulatory role of such mechanisms interacting to form complex brain circuits, which in turn determine development, sex reversal and reproductive behavior in hermaphrodite fish.
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Φυλετικοί και αναπτυξιακοί μηχανισμοί πλαστικότητας στον εγκέφαλο ερμαφρόδιτων τελεόστεων. Μελέτη της έκφρασης νοραδρενεργικών υποδοχέων και της γένεσης και μετανάστευσης κυττάρων
