GABAergic Input onto CA3 Hippocampal Interneurons Remains Shunting throughout Development

Abstract

In hippocampus, the net flow of excitability is controlled by inhibitory input provided by the many populations of local circuit inhibitory interneurons. In principal cells, GABA A receptor-mediated synaptic input undergoes a highly coordinated shift from depolarizing early in life to a more conventional hyperpolarizing inhibition on maturation. This switch in inhibitory input polarity is controlled by the developmental regulation of two chloride cotransporters (NKCC1 and KCC2) that results in a net shift from high to low intracellular Cl Ϫ . Whether inhibitory input onto inhibitory interneurons demonstrates a similar developmental shift in intracellular ClϪ is unexplored. Using the gramicidin perforated-patch configuration, we recorded from CA3 hippocampal stratum lucidum interneurons and pyramidal cells to monitor inhibitory input across a broad developmental range. GABA A receptor-mediated synaptic input onto stratum lucidum inhibitory interneurons was shunting in nature across the entire developmental age range tested, as resting membrane potential and the IPSC reversal potential remained within a few millivolts (1-4 mV) between postnatal day 5 (P5) and P31. Furthermore, sensitivity to block of the two chloride cotransporters KCC2 and NKCC1 did not differ across the same age range, suggesting that their relative expression is fixed across development. In contrast, pyramidal cell synaptic inhibition demonstrated the well described switch from depolarizing to hyperpolarizing over the same age range. Thus, in contrast to principal cells, inhibitory synaptic input onto CA3 interneurons remains shunting throughout development.

Interneurons Provide Circuit-Specific Depolarization and Hyperpolarization

“…Likewise, uField recordings in the PCL of CA3 revealed hyperpolarizing signals generated by single CA3 interneurons including PIIs (Bazelot et al, 2010). Moreover, these data are consistent with perforated-patch recordings from CA3 PCs that identified an E GABA of perisomatically evoked IPSPs of ϳϪ73 mV, which is ϳ10 mV more negative than the average resting membrane potential (Banke and McBain, 2006). The combination of uField and perforated-patch recordings in this study was a necessary step to examine the effect of identified interneurons on their postsynaptic target cells without perturbing the membrane potential or changing the Cl Ϫ gradient by strong extracellular stimulation (Staley et al, 1995) and to directly quantify E GABA .…”

“…Perforatedpatch recordings from DG interneurons during PCL stimulation revealed a mean E GABA of Ϫ53.0 Ϯ 1.8 mV and an average resting potential of Ϫ69.1 Ϯ 1.3 mV (10 cells; data not shown). Furthermore, CA1 interneurons were characterized by a mean perisomatic E GABA of Ϫ64.1 Ϯ 3.2 and resting potential of Ϫ64.5 Ϯ 1.5 mV (12 cells; data not shown), comparable to data obtained from interneuron recordings in CA3 (Banke and McBain, 2006). Thus, depolarizing or shunting soma-near inhibition on GABAergic cells seems to be a general principle applying to the entire hippocampus.…”

“…Depo-larizing inhibition in the DG was observed at juvenile (ϳ3 weeks) and mature (Ͼ6 weeks; Ϫ2.4 Ϯ 0.4 V, 3 cells) stages, excluding a late developmental change to hyperpolarization in this circuit (Fig. 2G) (Banke and McBain, 2006). Thus, perisomatic inhibition is depolarizing in the DG but hyperpolarizing in the CA1 PC population.…”

“…During the course of the second to third postnatal week, however, E GABA changes to more negative values, resulting in hyperpolarizing inhibition (Banke and McBain, 2006). This change can be largely explained by developmentally regulated expression of the Cl Ϫ transporters KCC2 and NKCC1 (Rivera et al, 1999;Yamada et al, 2004).…”

“…Although gramicidin perforated-patch recordings showed that GABA A receptor-mediated signaling is shunting in various postsynaptic interneuron types (Martina et al, 2001;Chavas and Marty, 2003;Vida et al, 2006;Sauer and Bartos, 2010) and remains shunting throughout postnatal development (Banke and McBain, 2006), one of the most basic questions, namely whether interneurons hyperpolarize or depolarize PCs, is still open. Gramicidin perforated-patch recordings have uncovered depolarizing actions of GABAergic input synapses on the dendrites and axon initial segment of neocortical PCs (Gulledge and Stuart, 2003;Szabadics et al, 2006), suggesting that they may recruit instead of inhibit PCs.…”