PROPERTIES OF THE HYPERPOLARIZATION-ACTIVATED CURRENT IN RAT HIPPOCAMPAL CA1 PYRAMIDAL CELLS

1. Voltage and current clamp recordings were performed on CA 1 rat hip pocampal pyramidal cells using the patch clamp technique on ''in vitro '' slice preparations. 2. Hyperpolarizations from a holding potential of -35 mV elicited activation of the hyperpolarization-activated curre nt (I(h)) starting at voltages near -50 mV. 3. 1, recorded in voltage clamp conditions was blocked by external caesium (5 mM). 4. Raising th e external K concentration from 4.35 to 24.35 mM sensibly increased th e slope of the current-voltage (I/V) curve. Decreasing the external Na concentration from 133.5 to 33.5 mM depressed I(h) without grossly al tering the I/V slope. 5. The I(h) fully activated I/V relation measure d in the range -140 to -45 mV was linear with an extrapolated reversal at -17.0 +/- -1.6 (SE) mV. The current activation curve comprised the range between about -50 and -140 mV with a half-maximal activation at about -98 mV. 6. Perfusion of unclamped neurons with Cs (2 mM) hyperp olarized their resting potential by 3.8 +/- 0.2 mV and decreased the m embrane conductance, as expected if 1, were activated at rest. Firing caused by depolarizing current steps was prevented by Cs-induced hyper polarization, and could be restored by returning the membrane voltage to resting level by constant current injection. 7. The Cd-insensitive (medium-duration) afterhyperpolarization (AHP) elicited by a train of action potentials at -60 mV had an amplitude of 3.9 +/- 0.3 mV and was nearly fully abolished by 2 mM Cs (82.7 +/- 7.4%). Cs removed the dep olarizing part of the afterhyperpolarization as expected if I(h) activ ation was responsable for this phase. 8. These results indicate that i n CA1 hippocampal pyramidal neurons, the current Ih is tonically activ e at resting potentials and contributes to controlling afterhyperpolar izations and cell excitability.