APICAL DENDRITIC LOCATION OF SLOW AFTERHYPERPOLARIZATION CURRENT IN HIPPOCAMPAL PYRAMIDAL NEURONS - IMPLICATIONS FOR THE INTEGRATION OF LONG-TERM POTENTIATION

Trains of action potentials in hippocampal pyramidal neurons are follo wed by a prolonged afterhyperpolarization (AHP) lasting several second s, which is attributable to the activation of a slow calcium-activated potassium current (sl(AHP)). Here we examine the location of sl(AHP) on CAI pyramidal neurons by comparing it with two GABAergic inhibitory postsynaptic currents (IPSCs) with known somatic and dendritic locati ons. Whole-cell patch-clamp recordings were made from CA1 pyramidal ne urons in acute hippocampal slices. Stepping the membrane potential at the peak of sl(AHP) produced a relaxation (''switchoff'') of the AHP c urrent with a time constant of 7.4 +/- 0.4 msec (mean +/- SEM). The sw itchoff time constants for somatic and dendritic GABA, IPSCs were 3.5 +/- 0.5 msec and 8.8 +/- 0.3 msec, respectively. This data, together w ith cable modeling, indicates that active sl(AHP) channels are distrib uted over the proximal dendrites within similar to 200 mu m of the som a. Excitatory postsynaptic potentials (EPSPs) evoked in stratum (s.) r adiatum had their amplitudes shunted more by the AHP than did EPSPs ev oked in s. oriens, suggesting that active AHP channels are restricted to the apical dendritic tree. Blockade of the AHP during a tetanus, wh ich in control conditions elicited a decremental short-term potentiati on (STP), converted STP to long-term potentiation (LTP). Thus, activat ion of the AHP increases the threshold for induction of LTP. These res ults suggest that in addition to its established role in spike frequen cy adaptation, the AHP works as an adjustable gain control, variably h yperpolarizing and shunting synaptic potentials arising in the apical dendrites.