PROPERTIES OF UNITARY IPSCS IN HIPPOCAMPAL PYRAMIDAL CELLS ORIGINATING FROM DIFFERENT TYPES OF INTERNEURONS IN YOUNG-RATS

Whole cell recordings were used in hippocampal slices of young rats to examine unitary inhibitory postsynaptic currents (uIPSCs) evoked in C A1 pyramidal cells at room temperature. Loose cell-attached stimulatio n was applied to activate single interneurons of different subtypes lo cated in stratum oriens (OR), near stratum pyramidale (PYR), and at th e border of stratum radiatum and lacunosum-moleculare (LM). uIPSCs evo ked by stimulation of PYR and OR interneurons had similar onset latenc y, rise time, peak amplitude, and decay. In contrast, uIPSCs elicited by activation of LM interneurons were significantly smaller in amplitu de and had a slower time course. The mean reversal potential of uIPSCs was -53.1 +/- 2.1 (SE) mV during recordings with intracellular soluti on containing potassium gluconate. With the use of recording solution containing the potassium channel blocker cesium, the reversal potentia l of uIPSCs was not significantly different (-58.5 +/- 2.6 mV), sugges ting that these synaptic currents were not mediated by potassium condu ctances. Bath application of the gamma-aminobutyric acid-A (GABA(A)) r eceptor antagonist bicuculline (25 mu M) reversibly blocked uIPSCs evo ked by stimulation of all interneuron subtypes. In bicuculline, the me an peak amplitude of uIPSCs recorded with potassium gluconate was redu ced to 3.5 +/- 4.4% of control (n = 7). Similarly, with cesium methane sulfonate, the mean amplitude in bicuculline was 2.9 +/- 3.1% of contr ol (n = 13). Application of the GABA(B) receptor antagonist CGP 55845A (5 mu M) resulted in a significant and reversible increase in the mea n amplitude of uIPSCs recorded with cesium-containing intracellular so lution. Thus uIPSCs from all cell types appeared under tonic presynapt ic inhibition by GABA(B) receptors. Paired stimulation of individual i nterneurons at 100- to 200-ms intervals did not result in paired pulse depression of uIPSCs. For individual responses, a significant negativ e correlation was observed between the amplitude of the first and seco nd uIPSCs. A significant paired pulse facilitation (154.0 +/- 8.0%) wa s observed when the first uIPSC was smaller than the mean of all first uIPSCs. A small, but not significant, paired pulse depression (90.8 /- 4.0%) was found when the first uIPSC was larger than the mean of al l first uIPSCs. Our results indicate that these different subtypes of hippocampal interneurons generate Cl--mediated GABA(A) uIPSCs. uIPSCs originating from different types of interneurons may have heterogeneou s properties and may be subject to tonic presynaptic inhibition via he terosynaptic GABA(B) receptors. These results suggest a specialization of function for inhibitory interneurons and point to complex presynap tic modulation of interneurons function.