The present study was focused on the morphology of the
diencephalic nuclei (likely involved in reproductive functions)
as well as on the distribution of GnRH (gonadotropin-releasing
hormone) in the rhinencephalon, telencephalon and the
diencephalon of the brain of bluefin tuna (Thunnus thynnus)
by means of immunohistochemistry. Bluefin tuna has an
encephalization quotient (QE) similar to that of other large
pelagic fish. Its brain exhibits well-developed optic tecta and
corpus cerebelli. The diencephalic neuron cell bodies
involved in reproductive functions are grouped in two main
nuclei: the nucleus preopticus-periventricularis and the
nucleus lateralis tuberis. The nucleus preopticus-periventricularis
consists of the nucleus periventricularis and the nucleus
preopticus consisting of a few sparse multipolar neurons
in the rostral part and numerous cells closely packed and
arranged in several layers in its aboral part. The nucleus lateralis
tuberis is located in the ventral-lateral area of the
diencephalon and is made up of a number of large multipolar
neurones.
Four different polyclonal primary antibodies against salmon
(s)GnRH, chicken (c)GnRH-II (cGnRH-II 675, cGnRH-II 6)
and sea bream (sb)GnRH were employed in the immunohistochemical
experiments. No immunoreactive structures were
found with anti sbGnRH serum. sGnRH and cGnRH-II antisera
revealed immunoreactivity in the perikarya of the olfactory
bulbs, preopticus-periventricular nucleus, oculomotor
nucleus and midbrain tegmentum. The nucleus lateralis
tuberis showed immunostaining only with anti-sGnRH serum.
Nerve fibres immunoreactive to cGnRH and sGnRH sera were
found in the olfactory bulbs, olfactory nerve and neurohypophysis.
The significance of the distribution of the GnRHimmunoreactive
neuronal structures is discussed
