Somatic amplification of distally generated subthreshold EPSPs in rat hippocampal pyramidal neurones

1. Intracellular recordings from hippocampal CA1 pyramidal neurones reveale d that EPSPs evoked by selective stimulation of the isolated afferent input to the distal third of the apical dendrites were relatively insensitive to changes in dendritic membrane potential (V-m) but amplified by depolarizat ions of the somatic V-m. The amplification was present at potentials depola rized from resting membrane potential (RMP) but was most marked when the EP SPs were close to threshold for action potential generation. The amplificat ion consisted of a uniform component and a variable component which was onl y present when the EPSPs were threshold straddling. 2. The somatic amplification was caused br an intrinsic membrane current wh ich was blocked by somatic application of tetrodotoxin (TTX, 10 mu M), but, was insensitive to bath application of NiCl2, (100-200 mu M). We therefore suggest that the amplification of the subthreshold EPSP is due primarily t o the activation of a non-inactivating Na+ current (I-NaP). 3. Injection of 4-aminopyridine (4-AP, 25-50 mM) during intradendritic reco rdings resulted in amplification of the EPSPs in 37 % of the dendrites, whi ch was similar to that observed in somatic recordings. However, in the one case in which somatic application of TTX was tested, dendritic amplificatio n was blocked, suggesting that it is a reflection of the somatic amplificat ion. 4. Because the shift to variable amplification was very abrupt and it is pr esent in only a very narrow voltage range close to threshold, we suggest th at the variable component is caused by the regenerative activation of I-NaP . The variability itself is probably due to the simultaneous activation of different outward K+ currents. 5. The present results indicate that the somatic region of CA1 pyramidal ne urones can function as a voltage-dependent amplifier of distally evoked EPS Ps and that this is due to the activation of a somatic I-NaP. The presence of this amplifying mechanism will have important functional consequences fo r the way in which distally generated EPSPs are integrated.