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Received October 30, 2002
Accepted after revision November 22, 2002
1 Department of Physiology, Royal Free and UCL Medical School, Rowland Hill Street, London NW3 2PF, UK
2 Department of Physiology, Royal Free and UCL Medical School, Rowland Hill Street, London NW3 2PF, UK
* To whom correspondence should be addressed. E-mail: h.pawelzik{at}rfc.ucl.ac.uk.
To determine whether autaptic inhibition plays a functional role in the adult hippocampus, the action potential afterhyperpolarisations (spike AHPs) of CA1 interneurones were investigated in 25 basket, three bistratified and eight axo-axonic cells. The spike AHPs showed two minima in all regular-spiking (5), burst-firing (3) and in many fast-spiking cells (17:28). The fast component had a time-to-peak (TTP) of 1.2 ± 0.5 ms, the slower TTP was very variable (range of 3.3-103 ms). The AHP width at half-amplitude (HW) was 12.5 ± 5.7 ms in fast-spiking, 29.3 ± 18 ms in regular-spiking and 99.7 ± 42 ms in burst-firing cells. Axo-axonic cells never establish autapses, and the fast-spiking variety showed narrow (HW: 3.9 ± 0.7 ms) spike AHPs with only one AHP minimum (TTP: 0.9 ± 0.1 ms). When challenged with GABAA receptor modulators, spike AHPs in basket and bistratified cells were enhanced by zolpidem (HW by 18.4 ± 6.2 % in 10:15 cells tested), diazepam (45.2 ± 0.5 %, 6:7), etomidate (43.9 ± 36 %, 6:8) and pentobarbitone sodium (41 %, 1:1), and were depressed by bicuculline (-41 ± 5.7 %, 5:8) and picrotoxin (-54 %, 1:1), and the enhancement produced by zolpidem was reduced by flumazenil (-31 ± 13 %, relative to the AHP HW during exposure to zolpidem, 3:4). Neuronal excitability was modulated in parallel. The spike AHPs of three axo-axonic cells tested showed no sensitivity to etomidate, pentobarbitone or diazepam. Interneurone-to-interneurone inhibitory postsynaptic potentials (IPSPs), studied with dual intracellular recordings, had time courses resembling those of the spike AHPs. The IPSP HW was 13.4 ± 2.8 ms in fast-spiking (n = 16) and 28.7 ± 5.8 ms in regular-spiking/burst-firing cells (n = 6), and the benzodiazepine1-selective modulator zolpidem strongly enhanced these IPSPs (45 ± 28 %, n = 5). Interneurones with spike AHPs affected by the GABAA receptor ligands exhibited 3.8 ± 1.9 close autaptic appositions. In three basket cells studied at the ultrastructural level 6 of 6, 1 of 2 and 1 of 2 close appositions were confirmed as autapses. Therefore, in the hippocampus autaptic connections contribute to spike AHPs in many interneurones. These autapses influence neuronal firing and responses to GABAA receptor ligands.
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