MEMBRANE-PROPERTIES AND SYNAPTIC RESPONSES OF INTERNEURONS LOCATED NEAR THE STRATUM LACUNOSUM-MOLECULARE RADIATUM BORDER OF AREA CA1 IN WHOLE-CELL RECORDINGS FROM RAT HIPPOCAMPAL SLICES

1. The membrane properties and synaptic inputs of interneurons, locate d at the stratum (s.) lacunosum-moleculare and radiatum border (L-M) o f the CA1 region, were examined with the use of current-clamp whole-ce ll recordings in rat hippocampal slices. 2. Biocytin-labeled GM intern eurons had nonpyramidal somata and aspinous, often beaded, dendrites t hat arborized in s. lacunosum-moleculare and radiatum, sometimes as fa r as a. moleculare of the dentate gyrus. Their axon coursed and branch ed in s. lacunosum-moleculare and radiatum. Axon collaterals were also observed traversing the hippocampal fissure and arborizing in s. mole culare of the dentate gyrus and s. radiatum of the CA3 region. 3. Seve ral membrane properties of interneurons were typically nonpyramidal: t hey had large input resistances, short-duration action potentials foll owed by prominent fast afterhyperpolarizations, and responded to hyper polarizing current pulses with little membrane rectification. L-M inte rneurons showed significant anodal break responses, and their mean mem brane time constant was 33 ms. After-depolarization elicited by subthr eshold depolarizing current pulses were larger in amplitude and decaye d more slowly at depolarized than hyperpolarized membrane potentials. 4. The majority of L-M interneurons (35 of 49 cells) were silent at re sting membrane potentials, whereas others displayed either spontaneous single action potentials (n = 12) or rhythmic bursts (n = 2). The rhy thmic bursts were insensitive to the N-methyl-D-aspartate (NMDA) and n on-NMDA excitatory amino acid receptor antagonists, 2-amino-5-phosphon opentanoic acid (AP-5; 50 mu M) and 6-cyano-7-nitroquinoxaline-2,3-dio ne (CNQX; 20 mu M), respectively. Both spontaneous single action poten tials and burst firing were blocked by membrane hyperpolarization, sug gesting that they were intrinsically rather than synaptically generate d. 5. L-M interneurons responded with regular sustained firing to depo larizing current pulses at resting membrane potential. However, at mor e hyperpolarized membrane potentials (near -75 mV), depolarizing curre nt pulses elicited action-potential firing with a delayed onset. This suggests that voltage-sensitive, transient outward currents may be act ivated in L-M interneurons from hyperpolarized membrane potentials. 6. Electrical stimulation of s. radiatum or lacunosum-moleculare elicite d predominantly long-duration excitatory postsynaptic potentials (EPSP s; n = 20 cells), or both EPSPs and inhibitory postsynaptic potentials (IPSPs; n = 17 cells). In most L-M interneurons (35/37), with increas ing intensities, up to two action potentials were elicited. Occasional ly, larger bursts (3-5 action potentials) were observed (n = 2). 7. Th e multiphasic components of the synaptic responses became more evident when stimulations were repeated at different membrane potentials. The EPSP consisted of a fast component that increased in amplitude, and o f a slow component that decreased, with membrane hyperpolarization. IP SPs sometimes appeared biphasic with a pronounced early phase and a sm aller late component, but both phases reversed at similar membrane pot entials (-54 and -64 mV).