TTX-SENSITIVE DENDRITIC SODIUM-CHANNELS UNDERLIE OSCILLATORY DISCHARGE IN A VERTEBRATE SENSORY NEURON

Immunocytochemical and electrophysiological techniques were used to lo calize TTX-sensitive sodium channels (NaChs) over the soma-dendritic a xis of basilar and nonbasilar pyramidal cells of the electrosensory la teral line lobe (ELL) of weakly electric fish (Apteronotus leptorhynch us). Dense NaCh-like immunolabel was detected on the membranes of basi lar and nonbasilar pyramidal cell somata. Punctate regions of immunola bel (similar to 15 mu m) were separated by nonlabeled expanses of memb rane over the entire extent of basal dendrites. Similar punctate immun olabel was observed over the apical dendrites, and frequently on membr anes of afferent parallel fiber boutons in the distal apical dendritic region. Intracellular recordings from pyramidal cell somata or proxim al apical dendrites (75-200 mu m) were obtained using an in vitro ELL slice preparation. TTX-sensitive potentials were identified by focal p ressure ejection of TTX. Somatic recordings demonstrated both TTX-sens itive fast spike discharge and a slow prepotential; similar but lower amplitude potentials were recorded in apical dendrites. Dendritic spik es were composed of at least two active components triggered by a fast prepotential(FPP) generated by the somatic spike. TTX-sensitive spike s propagated in a retrograde fashion over at least the proximal 200 mu m of the apical dendrites, as determined by the conduction of an anti dromic population spike and focal TTX ejections. Somatic spikes were f ollowed by a depolarizing afterpotential (DAP) that was similar in dur ation and refractory period to that of proximal dendritic spikes. Duri ng repetitive spike discharge, the DAP could increase in amplitude and attain somatic spike threshold, generating a high-frequency spike dou blet and a subsequent hyperpolarization that terminated spike discharg e. Repetition of this process gave rise to an oscillatory burst discha rge (2-6 spikes/burst) with a frequency of 40-80 Hz. Both the DAP and oscillatory discharge were selectively blocked by TTX ejections restri cted to the proximal apical dendritic region. The present study demons trates an immunolocalization of NaChs over somatic and dendritic membr anes of a vertebrate sensory neuron that correlates with the distribut ion of TTX-sensitive potentials. The interaction of somatic and dendri tic action potentials is further shown to underlie an oscillatory disc harge believed to be important in electrosensory processing.