Origin of the pituitary innervation in the goldfish

Despite the large number of studies devoted to the pituitary of teleosts, the origin of the direct pituitary innervation is still largely unknown. Although such a model is ideal for applying retrograde transport techniques, these methods involve the difficult in vivo injection of tracers into the pituitary and have never been applied. Recently, a lipophilic fluorescent dye (1-1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanin; DiI) has been introduced and shown to have the capacity of being transported by the membranes of paraformaldehyde-fixed tissues. Microcrystals of DiI were implanted via a ventral approach into the pituitary of goldfish previously fixed by intracardiac perfusion of paraformaldehyde. The goldfish heads were kept immersed in paraformaldehyde for various periods of time (2-6 weeks). The brains were then dissected and cut transversally using a Vibratome. The results demonstrate that hypophysiotrophic areas are essentially restricted to the preoptic region, the mediobasal hypothalamus and the nucleus dorsolateralis thalami. In addition, cell bodies probably containing gonadotrophin releasing-hormone were also retrogradely stained along a pathway that can be traced up to the olfactory bulbs. The results also confirm that cell bodies, located around the ventral aspect of the preoptic recess and probably corresponding to dopaminergic neurons, project to the pituitary. Large neurons have also been observed in the rostral dorsal midbrain tegmentum just caudal to the posterior commissure. Most neurons of the so-called paraventricular organ remain unstained. Finally, a fiber tract originating from an undetermined territory of the posterior brain has been observed. The results are discussed in relation to the possible chemical nature of the hypophysiotropic neurons.