Department of Pharmacology, School of Pharmacy, University of London, London,WC1N 1AX,
UK
To date, 17, classes of inhibitory interneurones, including the newly characterised Ivy cell (Fuentealba et al, in press), have been described in the CA1 region of the hippocampus.
Dual sharp electrode, or whole cell recordings were used to study and compare the inhibitory postsynaptic potentials (IPSPs) elicited by 3 different subclasses of interneurones in postsynaptic hippocampal CA1 pyramidal cells.
The 10-90% rise times of IPSPs elicited by Ivy cells were significantly slower than those of bistratified or basket cell IPSPs. Although the spike duration of Ivy cells was significantly longer than those of basket or bistratified cells, the spike duration was not long enough to account for this difference in rise times. Nor could it be explained by dendritic filtering, as bistratified and Ivy cells both target dendritic regions of pyramidal cells, or by a significant jitter in the onset latencies of quantal components of the IPSPs.
To determine whether the slow rise times could be due to the involvement of different postsynaptic GABAA receptors, IPSPs elicited by proximally targeting basket cells and dendrite-preferring bistratified and Ivy cells were challenged with benzodiazepine site ligands. The amplitudes of basket and Ivy cell IPSPs, unlike bistratified cell IPSPs, were significantly enhanced by zolpidem and by Etomidate, suggesting these synapses utilise GABAA receptors containing alpha 1, 2 or 3 and beta 2 or beta3 subunits respectively. These data suggest that the slow rise times of Ivy cell IPSPs are not due to GABAA receptors with unusually slow kinetics, to presynaptic spike duration or conduction jitter, or to dendritic filtering. It remains to be determined whether it could be due to longer diffusion distances of GABA.