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

Taste and tactile fibers in the facial nerve of southern catfish (Silurus soldatovi meridionalis Chen)innervate extroraltaste buds terminate somatotopically in the facial lobe (FL)-a medullary structure crucial for gustatory-mediated food search.Thepresent study is performed to determine the taste responses of peripheral innervated regions and central receptive domains in thefacial lobe of maxillary ramus (max)and mandibular ramus (mand)(primary facial nerve branches)when their skin sensory domainswere stimulated by 6 amino acids and quinine hydrochloride.Dye racing, electrophysiology and histology were used in thisstudy.Facial lobe is divided into two lobules:the big one that occupies (733)%and the small one that occupies (58 4)%.Themax innervates upper lip and maxillary barbel, but the peripheral innervated regions of the mand are the lower lip andmandibular barbel.The former ends in the back portion of big lobule,whereas the latter terminates both in dorsal-internal and alsoin 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 thenumber of the amino acid receptors in the external innervated skin surface of the max is much more.And the reciprocal crossadaptationexperiments with L-arginine and L-alanine show the insensitivity and adaptation.Through synchronous recording ofbranches and projection regions,it is found that the taste neurons are distributed generally in the more dorsal regions of the tactilesensitive areas and respond with the highest frequency and broadest amplitude to L-argnine,L-alanine in max receptive regions orto quinine hydrochloride and histidine in mand receptive regions in FL.The present study reveals that the FLs of the southern catfishare organized in a somatotopic manner.