CYTOCHEMICAL-LOCALIZATION OF ADENYLYL-CYCLASE ACTIVITY WITHIN THE SENSORY EPITHELIUM OF THE TROUT SACCULE

Adenylyl cyclase, the enzyme of synthesis of cAMP, the second messenge r molecule mediating signal transduction in response to sensory, neuro transmitter and hormonal stimuli, has been localized in the sensory ep ithelium of the rainbow trout (Salmo gairdneri R.) saccule by cytochem ical detection of enzyme activity. In the sensory receptor cell, or ha ir cell, reaction product has been visualized in the stereocilia in cl ose association with the outer cell membrane and also at the apical su rface of the cuticular plate. A diffuse distribution of precipitate wa s observed within the cytoplasm of terminal endings of nerve fibers pr esumed to be efferent on the basis of characteristic synaptic speciali zations including presynaptic vesicles and a postsynaptic cistern lyin g within the hair cell. Occasionally, reaction product was observed to be associated with the external cell membrane of these nerve terminal s. There appeared to be little or no adenylyl cyclase activity associa ted with the plasma membrane at the base of the hair cell or in presum ptive afferent nerve endings. However, a subpopulation of nerve fiber endings which exhibited both efferent and afferent synaptic specializa tions contained precipitate. A concentration of adenylyl cyclase activ ity in hair cell stereocilia and efferent nerve terminals in the senso ry epithelium is suggestive of a role for cAMP in second messenger act ion at these sites, possibly related to mechanosensory transduction an d efferent neuromodulation, respectively.