UNITARY IPSPS EVOKED BY INTERNEURONS AT THE STRATUM-RADIATUM STRATUM LACUNOSUM-MOLECULARE BORDER IN THE CA1 AREA OF THE RAT HIPPOCAMPUS IN-VITRO

1. Hippocampal non-principal neurons at the stratum radiatum-stratum l acunosum-moleculare border (R-LM interneurons) of the CA1 area may con stitute several cell classes and have been implicated in the generatio n of GABAergic unitary IPSPs. Using biocytin filled electrodes we reco rded R-LM interneurons intracellularly in vitro and determined their p ostsynaptic effects in concomitantly recorded pyramidal cells. 2. Ligh t microscopic analysis revealed four populations of R-LM interneurons with distinct axons: (1) basket cells (n = 4) with axons predominantly ramifying in the pyramidal cell layer; (2) Schaffer collateral/commis sural pathway-associated interneurons (n = 10) stratifying in stratum radiatum and, to a lesser extent, stratum oriens; (3) perforant pathwa y-associated interneurons (n = 6) innervating the perforant path termi nation zone in stratum lacunosum-moleculare of the CA1 area as well as equivalent portions of the dentate gyrus and subiculum; and (4) neuro gliaform interneurons (n = 2) characterized by their dense, compact ax onal and dendritic arbour. 3. Random electron microscopic sampling of synaptic targets revealed a preponderance of pyramidal neurons as post synaptic elements. Basket cells had a synaptic target preference for s omata and proximal dendrites, whereas the remainder of R-LM interneuro ns innervated dendritic shafts and spines. The axon of dendrite-target ing cells formed up to six putative contacts with individual postsynap tic pyramidal cells. 4. Anatomically recovered R-LM interneurons (n = 22) had a mean resting membrane potential of -56.7 +/- 3.6 mV, a membr ane time constant of 12.9 +/- 7.7 ms and an input resistance of 86.4 /- 29.2 M Ohm. Depolarizing current pulses generally elicited overshoo ting action potentials (70.8 +/- 6.9 mV) which had a mean duration, wh en measured at half-amplitude, of 0.7 +/- 0.1 ms. In response to prolo nged (> 200 ms) depolarizing current pulses all R-LM interneurons disp layed (a varying degree of) spike frequency adaptation. 5. Basket cell s, Schaffer-associated and neurogliaform interneurons elicited small-a mplitude (< 2 mV), short-latency IPSPs in postsynaptic pyramids (n = 5 , 13 and 1, respectively). Those interactions in which an effect was e licited with the repetitive activation of the presynaptic neuron (n = 13) showed a substantial degree of postsynaptic response summation. Un itary IPSPs had fast kinetics and, whenever tested (n = 5; 1 basket ce ll and 4 Schaffer-associated interneurons), were abolished by the GABA (A) receptor antagonist bicuculline. 6. Thus, R-LM interneurons compri se several distinct populations which evoke fast GABA(A) receptor-medi ated IPSPs. The domain-specific innervation of postsynaptic pyramidal cells suggests functionally diverse effects on the integration of affe rent information in functionally non-equivalent compartments of pyrami dal cells.