MEMBRANE-PROPERTIES AND SYNAPTIC CURRENTS EVOKED IN CA1 INTERNEURON SUBTYPES IN RAT HIPPOCAMPAL SLICES

1. Intrinsic membrane properties and pharmacologically isolated excita tory and inhibitory postsynaptic currents (EPSCs and IPSCs, respective ly) were characterized with the use of whole cell current- and voltage -clamp recordings, in combination with biocytin labeling, in different subtypes of CA1 interneurons and pyramidal cells in rat hippocampal s lices. 2. Three classes of interneurons were selected on the basis of their soma location in the CA1 region: 1) in stratum (str.) oriens nea r the alveus (O/A), 2) near str. pyramidale, and 3) near the border of str. radiatum and lacunosum-moleculare. Each class of biocytin-labele d cells demonstrated specific cellular morphology. The somata of all i nterneurons were nonpyramidal in shape and usually multipolar. However , the pattern of dendritic and axonal arborizations of labeled interne urons differed in each class. 3. In current-clamp recordings, all inte rneuron subtypes had shorter-duration and smaller-amplitude action pot entials than pyramidal cells. Fast- and medium-duration afterhyperpola rizations were larger in amplitude in interneurons. Cell input resista nce was greater and membrane time constant was faster in all interneur on subtypes than in pyramidal cells. 4. Depolarizing current pulses ev oked regular firing in all classes of interneurons, whereas burst firi ng was observed in 50% of pyramidal cells. With hyperpolarizing curren t pulses, all nonpyramidal and pyramidal cell types displayed inward r ectification followed by anodal break excitation. 5. Electrical stimul ation of nearby afferents evoked excitatory postsynaptic potentials (E PSPs) in all cells. EPSPs were of short duration and usually followed by inhibitory postsynaptic potentials (IPSPs). EPSPs were mediated by glutamate, because they were blocked by non-N-methyl-D-aspartate (non- NMDA) and NMDA antagonists [6-cyano-7-nitroquinoxaline-2,3-dione (CNQX ) and (+/-)-2-amino-5-phosphonopentanoic acid (AP5), respectively]. In the presence of these antagonists, IPSPs were evoked in isolation and reversed near -72 mV. 6. In voltage-clamp recordings, non-NMDA EPSCs were isolated pharmacologically in the presence of AP5 and the gamma-a minobutyric acid-A (GABA(A)) antagonist bicuculline (BIC). Their prope rties were similar in all interneuron subtypes and pyramidal cells. Cu rrent-voltage (I-V) relations were linear, and mean reversal potential s were near 5 mV. Non-NMDA EPSCs were reversibly antagonized by CNQX. 7. NMDA EPSCs were pharmacologically isolated during CNQX and BIC appl ication and were observed in all cell types. I-V relations of NMDA EPS Cs demonstrated a region of negative slope at membrane potentials betw een -80 and -20 mV and their reversal potential was near 7 mV. The ris e time of NMDA EPSCs was significantly slower in O/A interneurons than in other cell types. NMDA EPSCs were reversibly antagonized by AP5. 8 . GABA(A) IPSCs were pharmacologically isolated in AP5 and CNQX and th eir properties were similar in all cell types. I-V relations of GABA(A ) IPSCs were linear with mean reversal potentials near -32 mV. GABA(A) IPSCs were reversibly blocked by BIC. 9. In conclusion, morphological ly different subtypes of interneurons located in O/A, near str. pyrami dale, and near the str. radiatum/lacunosum-moleculare border displayed intrinsic membrane properties that were distinct from pyramidal cells , but were similar among them. In contrast, the properties of non-NMDA , NMDA, and GABA(A) postsynaptic currents were similar between interne urons and pyramidal cells, except for NMDA EPSCs, which had slower ris e times in O/A interneurons.