COMPARING RESEARCH ON THE TASTE RESPONSES OF TWO FACIAL NERVE BRANCH’S SENSORY DOMAINS AND CENTRAL PROJECTIONS OF SOUTHERN CATFISH

Abstract

Taste and tactile fibers in the facial nerve of southern catfish (Silurus soldatovi meridionalis Chen) innervate extroral taste buds terminate somatotopically in the facial lobe (FL)-a medullary structure crucial for gustatory-mediated food search.The present study is performed to determine the taste responses of peripheral innervated regions and central receptive domains in the facial lobe of maxillary ramus (max) and mandibular ramus (mand) (primary facial nerve branches) when their skin sensory domains were stimulated by 6 amino acids and quinine hydrochloride.Dye racing,electrophysiology and histology were used in this study.Facial lobe is divided into two lobules: the big one that occupies (73±3)% and the small one that occupies (58±4)%.The max innervates upper lip and maxillary barbel,but the peripheral innervated regions of the mand are the lower lip and mandibular barbel.The former ends in the back portion of big lobule,whereas the latter terminates both in dorsal-internal and also in the ventral-internal portion of the small lobule.In the projection regions of the two branches,there are three types of neurons.Among the 6 testing amino acids (L-alanine,L-arginine,L-histidine,L-glycin,L-methionine and L-lysine) and quinine hydrochloride,the most stimulatory for two nerve branches are L-alanine and L-arginine.The response of max is more sensitive and the number of the amino acid receptors in the external innervated skin surface of the max is much more.And the reciprocal cross-adaptation experiments with L-arginine and L-alanine show the insensitivity and adaptation.Through synchronous recording of branches and projection regions,it is found that the taste neurons are distributed generally in the more dorsal regions of the tactile sensitive areas and respond with the highest frequency and broadest amplitude to L-argnine,L-alanine in max receptive regions or to quinine hydrochloride and histidine in mand receptive regions in FL.The present study reveals that the FLs of the southern catfish are organized in a somatotopic manner.