Fast pre-potential generation in rat hippocampal CA1 pyramidal neurons.

Small all-or-none pre-potentials have been shown under some conditions to underlie antidromic and orthodromic spike discharge in somatic recordings of hippocampal pyramidal neurons [Andersen P. and Lomo T. (1966) Expl Brain Res. 2, 247-260; Kandel E. R. et al. (1961) J. Neurophysiol. 24, 225-242; Schwartzkroin P. A. (1977) Brain Res. 128, 53-68; Spencer W. A. and Kandel E. R. (1961) J. Neurophysiol. 24, 272-285]. These potentials are taken to reflect spike discharge in distant regions of the cell (axonal or dendritic) [Andersen P. and Lomo T. (1966) Expl Brain Res. 2, 247-260; Kandel E. R. and Spencer W. A. (1961) Ann. N. Y. Acad. Sci. 94, 570-603; Schwartzkroin P. A. (1977) Brain Res. 128, 53-68] or electronic spike conduction across a gap junction between neighboring pyramidal cells [Dudek F. E. et al. (1983) In Basic Mechanisms of Neural Hyperexcitability, pp. 31-73]. The present study compared pre-potentials recorded at the somatic and dendritic levels and used restricted applications of tetrodotoxin to examine the relationship between pre-potentials and Na+ spike discharge. Intrasomatic and intradendritic recordings were obtained from CA1 pyramidal neurons of rat hippocampal slices maintained in vitro. Orthodromic and antidromic spike discharge was evoked by stimulation of afferent fibers in stratum radiatum and pyramidal cell axons in the alveus, respectively. Focal pressure application of tetrodotoxin in the immediate vicinity of somatic or dendritic recordings uncovered pre-potentials following blockade of antidromic spike discharge. Blockade of these pre-potentials required the diffusion of tetrodotoxin to a location remote from the recording site. Focal application of tetrodotoxin in the cell body layer reliably uncovered orthodromic pre-potentials at the soma only when stimulus intensity was raised beyond threshold for somatic spike discharge; e.g. to intensities shown to initiate spike discharge in apical dendritic locations [Turner R. W. et al. (1991) J. Neurosci. 11, 2270-2280]. These data provide evidence that propagation of a Na+ spike over the pyramidal cell axis is preceded by a depolarization in the form of a pre-potential. The uncovering of orthodromic somatic pre-potentials by tetrodotoxin during suprathreshold activation further supports the proposal [Spencer W. A. and Kandel E. R. (1961) J. Neurophysiol. 24, 272-285] that dendritic spike discharge [Turner R. W. et al. (1991) J. Neurosci. 11, 2270-2280] can underlie fast pre-potential generation in pyramidal cell somata.