Seasonal Changes in β-Endorphin-like Immunoreactivity in the Olfactory System of the Female Catfish, Clarias batrachus (Linn.)

Abstract

In the olfactory system of the catfish Clarias batrachus, β-endorphin-like immunoreactivity was seen in several olfactory receptor neurons (ORN) and their fiber projections extending caudally over the olfactory nerve to the olfactory bulb (OB). With β-endorphin-like immunoreactivity as a cellular marker, the olfactory system in the female fish was investigated at different stages of its annual reproductive cycle. The reproductive cycle of the fish is divisible into four distinct phases: preparatory (February–April), prespawning (May–June), spawning (July–August), and postspawning (September–January). The gonosomatic index and the immunocytochemical profile of β-endorphin-like immunoreactivity showed distinct changes as the fish progressed from one phase to another. In the preparatory phase, limited immunoreactivity was seen in the periphery of the bulb. However, the immunoreactivity showed a robust increase as the immunolabeled fibers extended progressively deeper into the bulb toward the mitral cell layer during the prespawning and spawning phases. Significant reduction in the immunoreactivity was noticed in the olfactory nerve layer of the fish in the postspawning phase. Several granule cells showed poor to moderate immunoreactivity during the spawning phase, although no immunoreactivity was seen in the inner cell layer during the rest of the year. The β-endorphin-like immunoreactivity in the ORN also showed season-related changes, although these were less distinct. Whereas weak immunoreactivity confined to a few ORN was noticed in the fish collected in the preparatory phase, those in the prespawning phase showed conspicuous augmentation in immunoreactivity. During the spawning phase, the sensory layer of the olfactory epithelium showed reduced, homogenous immunoreactivity. In the postspawning phase, several ORN revealed distinct granular immunoreactivity, suggesting possibilities of de novo synthesis. These annual cyclic changes in the β-endorphin-like immunoreactivity were consistently observed over a 30-month study period that spanned three consecutive spawning phases. The results suggest that the β-endorphin-containing ORN, their fiber projections to the OB, and several granule cells in the inner cell layer may be involved in the processing of reproduction/reproductive behavior-related signals.