Functional characterization and modulation of feedback inhibitory circuits in area CA3 of hippocampal slice cultures

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Functional characterization and modulation of feedback inhibitory circuits in area CA3 of hippocampal slice cultures C Fortunato,

D Debanne,

M Scanziani,

Pyramidal neurons of the hippocampus, especially in the CA3 region, are interconnected by recurrent excitatory axon collaterals. The spread of excitation is controlled by feedback and feedforward inhibitory circuits. Intracellular recordings in current clamp mode were used to study feedback inhibition. Single action potentials (APs) were evoked by brief depolarizing current pulses in CA3 pyramidal cells. In 27% of pyramidal cells, the AP was followed by an inhibitory postsynaptic potential (IPSP) within 3-5 ms, which was blocked by bicuculline, hence mediated by GABAA receptors. This short latency and the high probability of transmission (78-100% of AP followed by IPSPs) were consistent with a disynaptic circuit generating these IPSPs, as could be confirmed with paired recordings of reciprocally connected pyramidal neurons and intemeurons. The probability of activation of the inhibitory cell was decreased by reducing the probability of its excitation with adenosine (0.1-5 PM). In most cases (IZ= l8/2 I ), adenosine increased the number of failures of transmission from < 10%

BH Gghwiler,

SM Thompson

to > 50% of APs (indicating that the EPSPs in the interneurons were indeed reduced) and decreased the amplitude of nonfailure IPSPs. The comparison of the IPSP amplitude distribution before and after adenosine application suggests that the simultaneous activation of multiple interneurons was decreased by adenosine. In some experiments (n = 3/21), adenosine only increased the failure rate. without changing the IPSP amplitude, suggesting that feedback inhibition was mediated by only one intemeuron in these circuits. We conclude that more than one inhibitory intemeuron is usually involved in feedback inhibition in area CA3 of hippocampal cultures. Repetitive stimulation at 2Z4 Hz decreased the amplitude of non-failure IPSPs in circuits involving only one or more interneurons, and usually increased the number of failures of transmission. These effects were reversible and insensitive to the GABAa antagonist CGP 35 348. We conclude that both the excitation of the intemeurons and the release of GABA from intemeurons are modulated by repetitive stimulation. This work is currently in press in the Eur J Nrurosci.

Pre- and post-synaptic regulation of synapsin I and syntaxin 1B expression in neuronal networks: Differential effects of neuronal activity and long&m potentiation in viva A Hicksb,

S Davisa,

J Rodgerb,

A Helme-Guizonb.

S Larochea,

J Malletb

“L,ahoratoire de Neumbiologie de I’Apprmtissage et de la Mhnoire, CNRS URA 1491, Lhiversit~ Paris Sud, 91405 Orsay; bL.ahoratoire de Ghzktique Mol&dairr de la Nnrrotrar~smissio~~ et des Pmcussus Neurad&fntratifs. CNRS UMR 9923, H6pital dr iu Pit2 SalpBtrkre. 75013 Paris, Frame

Although advances have been made into understanding the different steps that occur during normal exocytosis and the proteins involved in this process, little is known about the regulatory effects that synaptic plasticity in viva or learning (but see Davis etd. this issue) may have on this process. Therefore, we investigated possible changes in the mRNA encoding several vesicle-associated proteins involved in exocytosis, 2 and 5 h following induction of long-term potentiation (LTP) in the dorsal dentate gyms by stimulation of the perforant path (R = I I). In addition, to differentiate between plasticity-induced modifications and cellular excitability, we matched the output level of discharge of the dentate granule cell induced by LTP by raising the intensity of stimulation in another group of rats (n = I I). These two groups were compared with groups of rats receiving low-frequency stimulation only (12= 18). Induction of LTP induced 30% potentiation of the EPSP and increased stimulation induced a comparable increase in the EPSP Low frequency stimulation induced no change. Of the proteins investigated (synapsins I and II, syntaxins I A and lB, synaptophysin, synaptotagmin I, synaptobrevins I and II, SNAP25aand b, nSEC1 andRab3aand b), only those encoding synapsin I and syntaxin 1B were specifically increased.

Two hours following the induction of LTP, the mRNA for syntaxin I B was significantly increased over and above that of the low-frequency stimulation controls (59.7 + 12.8%). Interestingly. high-intensity stimulation also resulted in a significant increase in syntaxin IB mRNA levels at this time point (94.1 f 26.3%). Synapsin I was increased, although not significantly (34.8 + 20%) and there was no change in the mRNA if the intensity of stimulation was increased. The increase in syntaxin I B mRNA induced by high-intensity stimulation at 2 h had greatly diminished by 5 h and showed no difference from controls. In contrast, by 5 h following induction of LTP, both syntaxin 1B and synapsin 1 mRNA levels were significantly increased compared with controls (I I9 f 26.4% and 98.7 + 13.6%. respectively). In addition to the increase in syntaxin IB and synapsin I expression at 5 h following the induction of LTI? there were also significant increases in the contralateral. non-stimulated dentate gyrus compared with controls (97 f 33% and 52 f 150/o,respectively). No change was observed in the group where the stimulation intensity was increased. These data show that the expression of two of the key proteins involved in exocytosis display both temporally