Distinct populations of GABAergic interneurons located in the stratum oriens (SO) of the hippocampus are characterised by a horizontal somatodendritic architecture and axon that projects into the stratum radiatum (SR) or lacunosum moleculare (SLM). Such interneurons may act as feedback regulators of CA1 pyramidal cell activity receiving excitatory postsynaptic potentials (EPSPs) from pyramidal axon collateral branches in SO and in turn innervating the distal dendrites of pyramidal cells in the SLM and evoking inhibitory postsynaptic potentials (IPSPs) (1). Anatomical data, however, suggests that these interneurons have synaptic targets not only on the distal dendrites of pyramidal cells but also on GABAergic elements (2). The properties of these interneuron-interneuron synapses are not known. Therefore, electrophysiological recordings were performed on acute slices and slice cultures from juvenile rats. Recorded neurons were filled with biocytin and analysed post-hoc. Minimal stimulation of SO elicited unitary inhibitory postsynaptic currents (uIPSCs) in interneurons located in SR and SLM. The uIPSCs recorded with a K-gluconate-based patch solution at -50mV had an amplitude of 20.3 ± 5pA, rise time 2.1 ± 0.3ms, decay 21.8 ± 2.9ms and jitter 1.1 ± 0.3ms (average ± SEM, n=12). Interestingly, uIPSCs were sensitive to either the N-type calcium channel antagonist conotoxin MVIIA (1µM, n=4), or the P/Q-type calcium channel antagonist agatoxin IVA (200nM, n=4) or both (n=4). Paired recordings between interneurons in SO and SR or SLM showed uIPSCs with similar kinetic properties, namely amplitude of 15.6 ± 1.3pA, rise time 2.3 ± 0.5ms, decay 30.7 ± 8.2ms and jitter 1.3 ± 0.3ms (average ± SEM, n=11). Likewise, these uIPSCs were sensitive to conotoxin MVIIA (1µM, n=2) or agatoxin IVA (200nM, n=1) or both (n=3). The uIPSCs had a stable control amplitude for at least 1h and were abolished by 1.2µM gabazine (n=5). We are currently investigating whether there is a correlation between the postsynaptic interneuron type and the physiological and pharmacological properties of the synapse. Calcium transients were imaged at a single SLM varicosity of the axon of interneurons with the soma in SO (n=2) in order to study the action of calcium channel antagonists. We demonstrate the existence and properties of unitary connections between interneurons in SO and their interneuron target(s) in the SR and SLM, providing a direct link between CA1 feed-back and feed-forward inhibitory interneurons. Different calcium channels can be functional within an individual connection enriching the view that release at inhibitory synapses in the hippocampus is initiated by a unique class of calcium channel.