THE EXCITABILITY OF CA1 PYRAMIDAL CELL DENDRITES IS MODULATED BY A LOCAL CA2-DEPENDENT K+-CONDUCTANCE()

Intracellular recordings were made from distal apical dendrites of CA1 pyramidal neurones in the rat hippocampal slice preparation. Injectio n of a threshold current evoked two predominant firing patterns: fast spiking and compound spiking. Suprathreshold current injection evoked high frequency dendritic spiking followed by a pronounced slow afterhy perpolarization (sAHP((dend))) lasting for several hundred millisecond s, during which spiking was inhibited for a variable period. In fast s piking dendrites, the size of the sAHP((dend)) depended on the number and frequency of preceding spikes, whereas, in compound spiking dendri tes, it was more closely related to the size and duration of preceding Ca2+-spikes. During the peak of the sAHP((dend)), the membrane conduc tance was increased by 56%. The sAHP((dend)) was blocked by perfusion with Co2+ and by intradendritic injection of ethyleneglycol-bis-(beta- aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA; 0.01 or 0.02 M), i ndicating that the activation of the sAHP((dend)) depends on a rise in intradendritic Ca2+. The sAHP((dend)) was also blocked by low concent rations (0.5-1 mu M) of carbachol. The data presented here therefore, provide strong evidence that the sAHP((dend)) is due to the activation of a local Ca2+-dependent K+-conductance. Possible implications of a dendritic Ca2+-dependent K+-conductance for the integration of synapti c potentials are discussed.