DENDRITIC ELECTROGENESIS IN RAT HIPPOCAMPAL CA1 PYRAMIDAL NEURONS - FUNCTIONAL-ASPECTS OF NA+ AND CA2+ CURRENTS IN APICAL DENDRITES

The regenerative properties of CA1 pyramidal neurons were studied thro ugh differential polarization with external electrical fields. Recordi ngs were obtained from somata and apical dendrites in the presence of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), DL-2-amino-5-phosphonoval eric acid (APV), and bicuculline. S+ fields hyperpolarized the distal apical dendrites and depolarized the rest of the cell, whereas S divid ed by fields reversed the polarization. During intradendritic recordin gs, S+ fields evoked either fast spikes or compound spiking. The thres hold response consisted of a low-amplitude fast spike and a slow depol arizing potential. At higher field intensities the slow depolarizing p otential increased in amplitude, and additional spikes of high amplitu de appeared. During intrasomatic recordings, S+ field evoked repetitiv e firing of fast spikes, whereas S divided by fields evoked a slow dep olarizing potential on top of which high- and low-amplitude spikes wer e evoked. Tetrodotoxin (TTX) blocked all types of responses in both de ndrites and somata. Perfusion with Ca2+-free, Co2+-containing medium i ncreased the frequency and amplitude of fast spikes evoked by S+ field and substantially reduced the slow depolarizing potential evoked by S divided by fields. Antidromic stimulation revealed that an all-or-non e dendritic component was activated in the distal apical dendrites by back-propagating somatic spikes. The dendritic component had an absolu te refractory period of about 4 ms and a relative refractory period of 10-12 ms. Ca2+-dependent spikes in the dendrites were followed by a l ong-lasting afterhyperpolarization (AHP) and a decrease in membrane in put resistance, during which dendritic excitability was selectively re duced. The data suggest that generation of fast Na+ currents and slow Ca2+ currents in the distal part of apical dendrites is highly sensiti ve to the dynamic state of the dendritic membrane. Depending on the mo de and frequency of activation these currents can exert a substantial influence on the input-output behavior of the pyramidal neurons. (C) 1 996 Wiley-Liss, Inc.