Comparative analysis of GnRH neuronal systems in the amphibian brain

We have investigated the GnRH-ir neuronal systems in the brain of the oviparous urodele, Triturus vulgaris, ovoviviparous urodele, Salamandra salamandra, and viviparous caecilian, Typhlonectes compressicauda, and have reexamined Xenopus laevis, Ambystoma mexicanum, and Rana esculenta. Results showed that mGnRH neuronal system was diffused along the medioventral telencephalon and diencephalon with the numerical preponderance of GnRH cell bodies in the rostral mediobasal telencephalon in T. vulgaris and S. salamandra and in medial septal area and preoptic area respectively in Typhlonectes compressicauda and X. laevis. The cGnRH-II-ir perikarya were restricted to the midbrain tegmentum in X. laevis and T. compressicauda. In T. vulgaris, two distinct groups of cGnRH-II neurons were distinguished, one in the midbrain tegmentum and another in the paraventricular organ. The former was composed of comparatively bigger perikarya than the latter. In X. laevis brain, besides those in the rostralmost dorsomedial and ventromedial telencephalon and septopreoptic area, mGnRH neurons were also found in the habenulae and habenular commissure as well the infundibular hypothalamus. In A. mexicanum, reexamined, the preoptic area-located mGnRH neurons were distributed in the ependymal lining of the preoptic recess. In this neotenic urodele, furthermore, cGnRH-II neurons were also present in the rhombencephalon, as well as in the infundibular hypothalamus. It is thus clear that while GnRH-ir cell bodies are distributed in the fore-, mid- and hindbrain, their precise neuroanatomical localization varies somewhat within and among groups. Altogether, it is evident that mGnRH neuronal system is confined mainly to the forebrain, whereas cGnRH-II system is commonly found in the mid- and hindbrain. Additional morphological investigations are required to eventually define the functional neuroanatomy of GnRH in the amphibian brain.