Ontogeny of GnRH-like immunoreactive neuronal systems in the forebrain of the Indian major carp, Cirrhinus mrigala.

GnRH immunoreactivity appeared in the medial olfactory placode very early in the development of Cirrhinus mrigala. The immunoreactive elements were divisible into distinct migratory and non-migratory components. The migratory component appeared as a patch of intensely immunoreactive cells located close to the olfactory epithelium in day 6 post-fertilization larvae. Subsequently, these neurons migrate caudally along the ventromedial aspect of the developing forebrain and enroute give rise to GnRH immunoreactive neurons in the (1) nervus terminalis located in ventral and caudal part of the olfactory bulb (day 8), and (2) basal telencephalon (day 9). The non-migratory GnRH immunoreactive component appeared in the olfactory placode of day 1 post-fertilization larvae. It consisted of few olfactory receptor neuron (ORN)-like cells with distinct flask-shaped somata, dendrites that communicate with the periphery and a single axon on the basal side; GnRH immunoreactivity was seen throughout the neuron. Considerable increase in the number of immunoreactive ORNs was encountered in day 2 post-fertilization larvae. On day 3, the dendrites of ORNs sprout bunches of apical cilia, while on the basal side the axonal outgrowths can be traced to the olfactory bulb. GnRH immunoreactive fibers were distributed in the olfactory nerve layer in the periphery of the bulb and glomeruli-like innervation was clearly established in 5 days old larvae. The innervation to the olfactory bulb showed a considerable increase in GnRH immunoreactivity in 9 and 19 days old larvae. However, GnRH immunoreactivity in non-migratory as well as migratory components gradually diminished and disappeared altogether by the age of 68 days. Results of the present study suggest that GnRH may serve a neurotransmitter role in the ORNs during early stages of development in C. mrigala.