Differential control of three after-hyperpolarizations in rat hippocampal neurones by intracellular calcium buffering

1. The whole-cell recording technique, combined with internal perfusion, wa s used to study the effects of intracellular Ca2+ buffering on fast, medium and slow after-hyperpolarizations (fAHP, mAHP and sAHP) in hippocampal CA1 pyramidal neurones in rat brain slices at room temperature. 2. The action potentials and the fAHP were unaffected by 100 mu M to 3 mM c oncentrations of the internally applied fast Ca2+ chelator BAPTA. At higher (10-15 mM) concentrations, BAPTA inhibited the fAHP and prolonged the deca y of the action potential, suggesting that the corresponding large-conducta nce Ca2+-activated K+ channels are located close to the sites of Ca2+ entry during an action potential. Addition of Ca2+ to the BAPTA-containing solut ion (at it ratio of 4.5 [Ca2+] : 10 [BAPTA]) to maintain the control level of [Ca2+](i) did not prevent the effects of high concentrations of BAPTA. 3. The mAHP, activated by a train of action potentials, was inhibited by in ternally applied BAPTA within the range of concentrations used (100 mu M to 15 mM), and this effect could not be reversed or prevented by addition of Ca2+ to the BAPTA-containing solution. The inhibition of the mAHP by BAPTA could also be observed after blockade of the hyperpolarization-activated I- Q type mixed Na+-K+ current (also known as I-h) component of the mAHP by ba th-applied 3-5 mM Cs+, suggesting that the inhibition of the mAHP by BAPTA is due to inhibition of the depolarization-activated I-M (muscarinic) type K+ current. 4. The sAHP, activated by a train of action potentials, was potentiated by 100-300 mu M internally applied BAPTA, both with and without added Ca2+. At 1-2 mM or higher concentrations, the potentiation of the sAHP by BAPTA wit hout added Ca2+ was transient and a as followed by a fast decrease. With ad ded Ca2+, however, BAPTA caused a persistent potentiation of the sAHP with more than a 10-fold increase in duration for periods exceeding 1 h even at concentrations of the buffer as high as 10-15 mM. Earlier reports showing a blockade of the sAHP by BAPTA, based on experiments without added Ca2+, we re apparently due to a sharp reduction in intracellular free [Ca2+] and to a high intracellular concentration of the free buffer. 5. Internally applied BAPTA caused a prolongation of the spike discharge du ring an 800 ms-long depolarizing current step. At 100-300 mu M BAPTA, but n ot at 1-2 mM or higher concentrations, this effect could be reversed by add ition of Ca2+. The effects of BAPTA on the spike discharge occurred in para llel with the changes in the sAHP time course, which was more prolonged at higher concentrations of the buffer. 6. The concentration-dependent differential control of the three types of A HP in hippocampal neurones by BAPTA is related to modulation of intracellul ar Ca2+ diffusion by a fast acting mobile Ca2+ buffer.