Pharmacological Characterization of Glycine-Gated Chloride Currents Recorded in Rat Hippocampal Slices

Abstract

Chattipakorn, Siriporn C. and Lori L. McMahon. Pharmacological characterization of glycine-gated chloride currents recorded in rat hippocampal slices. J Neurophysiol 87: 1515-1525, 2002; 10.1152/jn. 00365.2001. An inhibitory role for strychnine-sensitive glycine-gated chloride channels (GlyRs) in mature hippocampus has been overlooked, largely due to the misconception that GlyR expression ceases early during development and to few functional studies demonstrating their presence. As a result, little is known regarding the physiological and pharmacological properties of native GlyRs expressed by hippocampal neurons. In this study, we used pharmacological tools and whole cell patch-clamp recordings of CA1 pyramidal cells and interneurons in acutely prepared hippocampal slices from 3-to 4-wk old rats to characterize these understudied receptors. We show that glycine application to recorded pyramidal cells and interneurons elicited strychnine-sensitive chloride-mediated currents (I gly ) that did not completely desensitize in the continued presence of agonist but reached a steady state at 45-60% of the peak amplitude. Additionally, the inhibitory amino acid, taurine, which has been shown to activate GlyRs in other systems, activated GlyRs expressed by both pyramidal cells and interneurons, although with much less potency than glycine, having an EC 50 10-fold higher. To examine the potential subunit composition of hippocampal GlyRs, we tested the effect of the GABA A receptor antagonist, picrotoxin, on I gly recorded from both cell types. At low micromolar concentrations of picrotoxin (Յ100 M), which selectively block ␣ homomeric GlyRs, I gly was partially attenuated in both cell types, indicating that ␣ homomeric receptors are expressed by pyramidal cells and interneurons. At picrotoxin concentrations Յ1 mM, ϳ10 -20% of the whole cell current remained, suggesting that ␣␤ heteromeric GlyRs are also expressed because this subtype of GlyR is relatively resistant to picrotoxin antagonism. Finally, we examined whether hippocampal GlyRs are modulated by zinc. Consistent with previous reports in other preparations, zinc elicited a bidirectional modulation of GlyRs, with physiological zinc concentrations (1-100 M) increasing whole cell currents and concentrations Ͼ100 M depressing them. Furthermore, the same concentration of zinc that potentiates I gly suppressed currents mediated by the N-methyl-D-aspartate subtype of the glutamate receptor. Thus we provide a pharmacological characterization of native GlyRs expressed by both major neuron types in hippocampus and show that these receptors can be activated by taurine, an amino acid that is highly concentrated in hippocampus. Furthermore, our data suggest that at least two GlyR subtypes are present in hippocampus and that GlyR-mediated currents can be potentiated by zinc at concentrations that suppress glutamate-mediated excitability.

Glycine Uptake Regulates Hippocampal Network Activity via Glycine Receptor-Mediated Tonic Inhibition

“…Indeed, we found that endogenous accumulation of glycine by GlyT1 antagonists induced an STN-sensitive tonic current in CA1 pyramidal neurons ( Figure 5). The reported existence of GlyRs containing homomeric a2 subunits in the hippocampus (Chattipakorn and McMahon, 2002;Mori et al, 2002;Thio et al, 2003;Yoon et al, 1998) may underlie this tonic current. In general, functional consequences of the tonic inhibition are not well understood.…”

“…The difference in half-maximal concentrations of glycine responses in hippocampus between single neuron and slice preparations suggests the existence of powerful reuptake mechanisms in slices (Chattipakorn and McMahon, 2002). To look for direct evidence of a role of GlyTs in tuning tonic activation of GlyRs in hippocampus, we studied the effect of sarcosine, a specific GlyT1 antagonist, on E-S coupling.…”

“…Electrophysiological (Thio et al, 2003;Chattipakorn and McMahon, 2003), immunocytochemical (Becker et al, 1993), and in situ hybridization studies (Malosio et al, 1991) have shown that functional GlyRs are also present in many regions of developing and mature brain, including the hippocampus, where they are expressed by CA1 pyramidal cells and interneurons. Although no glycinergic synaptic transmission has been found in the hippocampus (Mody et al, 1994), accumulating evidence shows the presence of non-synaptic GlyRs containing at least a2 subunit (Chattipakorn and McMahon, 2002;Mori et al, 2002;Thio et al, 2003;Yoon et al, 1998). Thus, GlyRs are likely to have profound effects on cortical function, and may play a fundamental role in modulating hippocampal excitability.…”

Glycine transporter-1 regulates the proliferation of neural stem/progenitor cells derived from the embryonic mouse hippocampus

“…Blocking of GLYT1 is known to enhance the activation of NMDA receptor-dependent synaptic transmission [17] and to reduce neuronal signaling [18,19]. Previous reports have suggested that Figure 1.…”

Molecular architecture of glycinergic synapses

“…Moreover, also in hippocampal tissue and cultured neurons GlyR 2 subunits are found in clusters, both synaptically and extrasynaptically (Brackmann et al 2004;Danglot et al 2004). Pharmacological and single channel analysis showed that in acutely prepared hippocampal slices GlyRs in interneurons and CA1 pyramidal cells are mostly homo-oligomeric, pictrotoxinin sensitive channels with larger single channel conduction states (>100 pS) whereas hetero-oligomeric, subunit containing GlyRs were found with lower frequency (10-20%) (Chattipakorn and McMahon 2002). Also in cultured hippocampal neurons 2 homo-olgomeric receptors, which were replaced by subunit containing GlyRs were observed (J. Kuhse, unpublished observation).…”