Stratum lacunosum-moleculare interneurons of hippocampal CA1 region. II. Intrasomatic and intradendritic recordings of local circuit synaptic interactions

Abstract

Simultaneous intracellular recordings were obtained from stratum lacunosum-moleculare (L-M) interneurons and CA1 cells, and their local circuit synaptic interactions were examined. Synaptic interactions with pyramidal cells were evaluated in both intrasomatic and intradendritic pyramidal cell recordings. Stimulation of L-M interneurons evoked small- amplitude IPSPs in 21% of intrasomatic (9/42 cell pairs) and in 26% of intradendritic (11/43) pyramidal cell recordings. The IPSP mean peak amplitude was 0.91 mV for intrasomatic and 0.67 mV for intradendritic recordings. IPSPs had slow onset and decay (approximately 80–90 msec), decreased in amplitude with membrane hyperpolarization, and were not associated with any apparent change in input resistance. No physiologic evidence of synaptic connections was found from pyramidal cells to L-M interneurons. Inhibitory synaptic interactions were also seen between L- M interneurons and stratum pyramidale interneurons (2 of 4 cell pairs). The IPSPs recorded in pyramidale interneurons were similar to the IPSPs recorded in pyramidal cells. During simultaneous recordings, L-M interneurons were activated at a shorter latency, i.e., in a feedforward manner with respect to pyramidal cells. Thus, L-M interneurons may mediate feedforward inhibition of CA1 pyramidal cells. The L-M interneuron-evoked IPSPs in pyramidal cells share some characteristics of the late IPSP recorded in CA1 pyramidal cells and may therefore contribute to this component of the IPSP.

Networks of interneurons with fast and slow γ-aminobutyric acid type A (GABA A ) kinetics provide substrate for mixed gamma-theta rhythm

“…The precise identity of the GABA A,slow interneurons is not known, but they may correspond to the SL-M interneurons reported by Lacaille and colleagues (17,21,27,30,31). These interneurons are conditional oscillators, with slow voltage-gated membrane currents that constrain them to fire at theta frequencies under some experimental conditions.…”

Interaction of Calcineurin and Type-A GABA Receptor γ2Subunits Produces Long-Term Depression at CA1 Inhibitory Synapses

“…Much of the difficulty in assigning changes at the level of individual inhibitory synapses is that the IPSCs, originating from feed-forward (Alger and Nicoll, 1982) as well as feed-back activation of the CA1 interneurons (Lacaille and Schwartzkroin, 1988) by the Schaffer-collateral stimulation, are polysynaptic and thus obscure any link between properties of IPSCs and the efficacy of individual inhibitory synapses. Conventionally, the GABA A R-IPSCs in CA1 pyramidal neurons are studied under conditions in which excitatory glutamate receptors are blocked.…”

Nucleus Reuniens Thalami Modulates Activity in Hippocampal Field CA1 through Excitatory and Inhibitory Mechanisms

“…These interneurons have an extensively arborizing axon that is largely confined to the stratum pyramidale, forming symmetrical (inhibitory) synapses on the somata and primary dendrites of numerous CA1 cells. Functionally, they are assumed to mediate both feedforward and feedback inhibition of pyramidal cells (Lacaille et al, 1987;Lacaille and Schwarzkroin, 1988;Samulack et al, 1993). Interneurons located in the distal radiatum that f ulfill the criterion to receive RE input are likely the ones containing GABA as well as the putative excitatory transmitters cholecystokinin (CCK) and /or vasoactive intestinal polypeptide (V I P) (Kosaka et al, 1985;Gulyás et al, 1991;Haas and Gáhwiler, 1992;Acsády et al, 1996a,b).…”