Increased Kv1 Channel Expression May Contribute to Decreased sIPSC Frequency Following Chronic Inhibition of NR2B-Containing NMDAR

Abstract

Numerous studies have documented the effects of chronic N-methyl-D-aspartate receptor (NMDAR) blockade on excitatory circuits, but the effects on inhibitory circuitry are not well studied. NR2A- and NR2B-containing NMDARs play differential roles in physiological processes, but the consequences of chronic NR2A- or NR2B-containing NMDAR inhibition on glutamatergic and GABAergic neurotransmission are unknown. We investigated altered GABAergic neurotransmission in dentate granule cells and interneurons following chronic treatment with the NR2B-selective antagonist, Ro25,6981, the NR2A-prefering antagonist, NVP-AAM077, or the non-subunit-selective NMDAR antagonist, D-APV, in organotypic hippocampal slice cultures. Electrophysiological recordings revealed large reductions in spontaneous inhibitory postsynaptic current (sIPSC) frequency in both granule cells and interneurons following chronic Ro25,6981 treatment, which was associated with minimally altered sIPSC amplitude, miniature inhibitory postsynaptic current (mIPSC) frequency, and mIPSC amplitude, suggesting diminished action potential-dependent GABA release. Chronic NVP-AAM077 or D-APV treatment had little effect on these measures. Reduced sIPSC frequency did not arise from downregulated GABA(A)R, altered excitatory or inhibitory drive to interneurons, altered interneuron membrane properties, increased failure rate, decreased action potential-dependent release probability, or mGluR/GABA(B) receptor modulation of GABA release. However, chronic Ro25,6981-mediated reductions in sIPSC frequency were occluded by the K+ channel blockers, dendrotoxin, margatoxin, and agitoxin, but not dendrotoxin-K or XE991. Immunohistochemistry also showed increased Kv1.2, Kv1.3, and Kv1.6 in the dentate molecular layer following chronic Ro25,6981 treatment. Our findings suggest that increased Kv1 channel expression/function contributed to diminished action potential-dependent GABA release following chronic NR2B-containing NMDAR inhibition and that these Kv1 channels may be heteromeric complexes containing Kv1.2, Kv1.3, and Kv1.6.

Synaptic plasticity in glutamatergic and GABAergic neurotransmission following chronic memantine treatment in an in vitro model of limbic epileptogenesis

“…Organotypic hippocampal slice cultures were used as a model of deafferentation-induced epileptogenesis (for review see Bausch, 2009) and were prepared from postnatal day 10–11 Sprague-Dawley rat pups (Taconic, Germantown, NY) using the Stoppini method (Stoppini et al, 1991) as described previously (Bausch et al, 2006; He et al, 2012, 2013; Wang and Bausch, 2004). All treatment of animals complied with National Institutes of Health, Department of Defense and institutional guidelines and was approved by the Uniformed Services University Institutional Animal Care and Use Committee.…”

“…Concentration-response analyses were not conducted due to the small chronic memantine-induced effects at 1 μM (see Results) and questionable clinical relevance at higher concentrations. Data presented herein for vehicle-treated hippocampal slice cultures were published previously (He et al, 2012, 2013) and constitute a shared control group to reduce animal numbers. Despite disparate publication dates, all cultures treated with vehicle and memantine were always studied concurrently under identical experimental conditions.…”

Synaptic and extrasynaptic plasticity in glutamatergic circuits involving dentate granule cells following chronic N-methyl-d-aspartate receptor inhibition

“…However, although low to moderate GluN2D is expressed in glutamatergic hippocampal principal neurons during early postnatal development , after postnatal day 7 GluN2C and GluN2D are expressed only at very low levels in subsets of hippocampal formation interneurons Scherzer et al 1998;Standaert et al 1996). We showed previously that chronic Ro25-6981, but not NVP-AAM077, altered GABAergic transmission (He et al 2012).…”

“…Furthermore, chronic Ro25-6981-mediated effects on circuit excitability may depend on the mechanisms underlying changes in sEPSC and mEPSC AMPAR frequency (i.e., increased synapse number vs. potential increased release probability). Finally, excitatory glutamatergic transmission is subject to GABAergic modulation, and we showed previously (He et al 2012) that action potential-dependent GABA release was altered following chronic Ro25-6981, but not D-APV or NVP-AAM077, treatment. Any or all of these possibilities may contribute to the discrepancy between EPSC frequency measures and circuit excitability.…”

“…Representative NMDAR antagonists were chosen on the basis of their pharmacological properties as described previously (He et al 2012;Wang and Bausch 2004). Concentrations were selected to 1) elicit maximal antagonism while maintaining selectivity and 2) allow direct comparison with our previous studies (Bausch et al 2006;Wang and Bausch 2004).…”