Page 7 - Brain_morphology_2008

Original Paper

The results of this study have allowed the 2004). In many teleost fish studied, immunohisto-
immunolocalization of the distribution of GnRH- chemistry highlighted sGnRH immunoreactivity of
like structures in the brain and pituitary gland of the neurons of the anterior ventral brain (telen-
bluefin tuna. cephalon and preoptic region), whereas the large
Eight GnRH variants have been identified in the cell bodies of the midbrain tegmentum were found
brain of teleost fish,but (c) GnRH II,(s) GnRH and to react to anti cGnRH-II serum (see Lethimonier
(sb) GnRH have been found to coexist in some per- et al., 2004).
ciformes and other fishes (Lethimonier et al., The perikarya of the preopticus-periventricularis
2004). nucleus immunoreacted with anti sGnRH 1667,
Four different polyclonal antibodies against anti cGnRH-II 675 and anti cGnRH-II 6 sera.
sGnRH, cGnRH and sbGnRH were used. In this Differences in the distribution, % and size of the
study three antisera (sGnRH 1667, cGnRH-II 675, immunoreactive perikarya were observed. Anti
cGnRH-II 6) revealed immunoreactivity in the sGnRH- and anti cGnRH-positive cells were mainly
perikarya of the oculomotor nucleus, anterior mid- found in the preopticus and caudal part of periven-
brain tegmentum, preopticus-periventricular and tricularis nucleus. sGnRH-immunoreactive cells
lateralis tuberis nuclei in the diencephalon, the neu- comprised around 20-30% of the total whereas
rohypophysis and in the junction between olfactory cGnRH-like neurons comprised around 50%. Anti
bulbs and telencephalon. sGnRH-immunoreactive neurons were smaller than
Surprisingly no immunoreactive structures were the anti cGnRH ones. The presence of GnRH neu-
found with anti sbGnRH serum although sbGnRH is rons in the preopticus nucleus has been detected as
thought to be the most important regulator of a sGnRH variant in Cypryniformes (Yu et al., 1988;
gonadal function (Lethimonier et al., 2004; Pham Powel et al., 1996) and Salmoniformes (Okuzawa
et al., 2006), and sbGnRH-producing cell bodies et al., 1990; Amano et al., 1991), whereas the
project their axons mainly to the pituitary gland cGnRH-II form has been reported in midbrain
(White et al., 1995; Gonzalez-Martinez et al., tegmentum of most of the fish species studied (see
2001).This negative result is of difficult interpreta- Lethimonier et al., 2004 for review). In bluefin
tion despite the fact that different dilutions and tuna, we were able to find weakly immunostained
incubation times were used in the experiments car- nerve fibres in both the preopticus nucleus and the
ried out here. The lack of anti sbGnRH immunos- neurohypophysis.This is in line with the function of
taining likely depends on technical reasons, includ- the preopticus nucleus which is considered to be the
ing the non-specificity of the primary antibody main source of the innervation of the neurohypoph-
used, as well as likely antigen damage during the ysis (Yamamoto et al., 1998).
fixation and paraffin wax embedding. However, The nucleus the lateralis tuberis exhibited
since large differences in the levels of sbGnRH have immunostaining with anti sGnRH 1667 serum in
been shown among perciform fishes (Senthil- 40% of the neurons, whereas it did not react with
kumaran et al., 1999), a possible explanation is antisera against cGnRH and sbGnRH. The occur-
that the bluefin tuna contains such a low amount of rence of GnRH cells in this nucleus has been
the sbGnRH form that it becomes undetectable to observed in catfish Clarias batrachus (Subhedar
the immunostaining.Therefore, other types of stud- and Rama Krishna, 1988; Khan et al., 1999;
ies,including RIA and in situ hybridization might be Sarkar and Subhedar, 2000). GnRH cells have been
helpful in finding a solution to this still unanswered reported in the caudo-distal pars of neonatal platy-
question. fish (Schreibman et al., 1982).
cGnRH-II is expressed in all teleosts examined, In this study, we were able to find neurons and
including sarcopterigy and actinopterigy (review- nerve fibres immunoreactive to cGnRH and sGnRH
ered by Letimonier et al., 2004). With the excep- sera in the bulbs as well as in the olfactory nerve of
tions of the butterfly fish (O’neill et al., 1998), the bluefin tuna. According to several authors, the
European eel (King et al., 1990) and the catfish GnRH-immunoreactive cells detected in this tract
(Ngamvongchon et al., 1992), sGnRH has been of the brain are the cells most strongly subjected to
detected in all teleost species from primitive fish, the action of pheromones (Fujita et al., 1991;
osteoglossiformes (Okubo and Aida, 2001; O’neill Egorova et al., 2001) and are likely to be involved
et al., 1998), to acanthopterigii (Lethimonier et al., in fish sexual behaviour neuromodulation (Oka and

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