Differences in Ca2+ buffering properties between excitatory and inhibitoryhippocampal neurons from the rat

1. Endogenous calcium binding ratios (kappa(s)) in dendrites of cultured hi ppocampal neurons were estimated according to the single compartment model for transients in intracellular Ca2+ concentration ([Ca2+]). In addition, t he electrophysiological characteristics of neurons were classified by their autaptic currents and intrinsic firing patterns. These data were analysed in order to determine whether a correlation between Ca2+ buffers and electr ophysiological type exists. 2. Ca2+ binding ratios of endogenous buffers were estimated by eliciting [C a2+] transients with short depolarizations, while cells were loaded with fu ra-2. Two types of estimates could be obtained: one termed kappa(s)(tau), b ased on analysing time constants (tau) of [Ca2+] transients, and another te rmed kappa(s)(dCa), derived from an analysis of initial amplitudes of [Ca2] transients. 3. Values for kappa(s)(tau) and kappa(s)(dCa) were estimated as 57 +/- 10 ( mean +/- S.D., n = 10) and 60 +/- 14 (n = 10), respectively, in excitatory neurons, and 130 +/- 50 (n = 11) and 150 +/- 70 (n = 11), respectively, in inhibitory neurons. The kappa(s) values of excitatory and inhibitory cells were significantly different from each other, regardless of the measurement method (Student's t test, P<0.01). However, there was no significant diffe rence in kappa(s) between the groups classified according to firing pattern s. 4. Although kappa(s)(tau) values were well matched to those of kappa(s)(dCa ) in most excitatory cells, the two values did not agree in three out of th e fourteen inhibitory cells investigated. In these cells, the first few [Ca 2+] transients after obtaining the whole cell configuration displayed a dou ble exponential decay, suggesting that buffers with slow binding kinetics, such as parvalbumin, are involved. This hypothesis is further explored in a n accompanying paper.