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 pyramida l neurons in CA2, CA1, and the subiculum of rat hippocampal slices dur ing brief exposure (4.0 +/- 0.19 min) to N-2. Anoxia evoked either a d epolarization 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 mem brane resistance of approximate to 20%. Differences in the response co uld be correlated with the presence or absence of CaBP and the localiz ation 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 hyperpolar ization (17%); in contrast, 44% of responses of calbindin-negative (Ca BP((-))) neurons were depolarizing and 56% were hyperpolarizing. Depol arizations of CaBP((+)) neurons were more gradual in slope, and more r apidly reached a plateau in comparison with those recorded in CaBP((-) ) neurons. Responses of neurons in the superficial layer of stratum py ramidale (in which 79% of CaBP((+)) pyramidal neurons were situated) w ere mainly depolarizing (91%), while for those in the deep layer (whic h contained 89% of the CaBP((-)) cells) such responses were observed l ess often (45%). Depolarization was also more common than hyperpolariz ation for cells located in CA2/CA1c/CA1b (63%) than in the CA1a/subicu lar region (37%). The depolarizing response of the majority of pyramid al neurons which are CaBP(+), superficial, and closer to CA3 may refle ct an efficient buffering of intracellular Ca2+, which maintains a low [Ca2+](i), steep gradient for Ca-2+ influx and may facilitate the mov ement of Ca2+ away from points of entry. The neurons which are CaBP((- )), deep, and closer to subiculum and in which Nz evokes hyperpolariza tion, on the other hand, may have a sustained elevation/accumulation o f cytosolic Ca2+ which could activate K+ conductance, inhibit Ca2+ inf lux, and stabilize the membrane potential. These experiments provide a functional correlate for CaBP and suggest that it may have a signific ant role in Ca2+ homeostasis and the determination of selective neuron al vulnerability. (C) 1995 Wiley-Liss, Inc.