Characterisation of a novel class of GABAergic interneurone, the ivy cell, found in the CA1 region of the hippocampus

The hippocampus contains a diverse group of non-pyramidal cells called GABAergic interneurones. In the CA1 region of the hippocampus interneurones have been classified according to the shape of their axonal and dendritic arbours, the cells firing properties, and also the expression of neuropeptides or calcium binding proteins. Dual recordings using conventional sharp or whole cell recordings were used to study and compare electrophysiological parameters, axons, dendrites and inhibitory and excitatory connections in CA1. A novel GABAergic interneurone was identified in CA1 stratum pyramidale. These interneurones have been named "Ivy" cells. They form an abundant class of slow spiking, nitric oxide synthase-expressing GABAergic cells. The synaptic inputs and outputs of these cells were analysed. Ivy cells receive slow depressing excitatory postsynaptic potentials (EPSPs) from pyramidal cells, and in turn the pyramidal cells receive slowly rising inhibitory postsynaptic potentials (IPSPs) from Ivy cells that were significantly slower than IPSPs activated by bistratified or basket cells. Although the spike duration of Ivy cells was significantly longer than that of basket or bistratified cells, it was not long enough to account for the difference in rise times. Further analysis demonstrated that the slow rise times of IPSPs activated by Ivy cells could not be explained by jitter in the onset latencies of quantal components of the IPSPs, dendritic filtering or the involvement of different postsynaptic GABAA receptors. It remains to be determined whether it could be due to longer diffusion distances of GABA to its target site, since synaptic vesicles within the axons of Ivy cells may not be confined to synaptic boutons