Correlation of anoxic neuronal responses and calbindin‐D 28k localization in stratum pyramidale of rat hippocampus

Immunohistochemical staining for the calcium‐binding protein calbindin‐D 28k (CaBP) was combined with Lucifer Yellow (LY) identification and intracellular recording of changes in membrane parameters of pyramidal neurons in CA2, CA1, and the sebiculum of rat hippocampal slices during brief exposure (4.0 ± 0.19 min) to N 2 . Anoxia evoked either a depolarization or hyperpolarization of membrane potential (V M ) (+21.5 ± 2.79 mV above V M = −70.5 ± 1.50 mV, n = 30 and −7.2 ± 0.72 mV below V M = −68.2 ± 1.34 mV, n = 24, respectively) and a fall in membrane resistance of =20%. Differences in the response could be correlated with the presence or absence of CaBP and the localization of neurons in different layers of stratum pyramidale and sectors of the hippocampus. For neurons immunopositive for calbindin (CaBP( + )), depolarization was observed more frequently (83%) than hyperpolarization (17%); in contrast, 44% of responses of calbindin‐negative (CaBP( − )) neurons were depolarizing and 56% were hyperpolarizing. Depolarizations of CaBP( + ) neurons were more gradual in slope, and more rapidly reached a plateau in comparison with those recorded in CaBP( − ) neurons. Responses of neurons in the superficial layer of stratum pyramidale (in which 79% of CaBP( + ) pyramidal neurons were situated) were mainly depolarizing (91%), while for those in the deep layer (which contained 89% of the CaBP( − ) cells) such responses were observed less often (45%). Depolarization was also more common than hyperpolarization for cells located in CA2/CA1c/CA1b (63%) than in the CA1a/subicular region (37%). The depolarizing response of the majority of pyramidal neurons which are CaBP( + ), superficial, and closer to CA3 may reflect an efficient buffering of intracellular Ca 2+ , which maintains a low [Ca 2+ ] i , steep gradient for Ca 2+ influx and may facilitate the movement of Ca 2+ away from points of entry. The neurons which are CaBP( − ), deep, and closer to subiculum and in which N 2 evokes hyperpolarization, on the other hand, may have a sustained elevation/accumulation of cytosolic Ca 2+ which could activate K + conductance, inhibit Ca 2+ influx, and stabilize the membrane potential. These experiments provide a functional correlate for CaBP and suggest that it may have a significant role in Ca 2+ homeostasis and the determination of selective neuronal vulnerability. © 1995 Wiley‐Liss, Inc.