SUMMARY1. The ionic mechanism underlying the fast, GABAA receptor-mediated inhibitory postsynaptic potential (IPSPA) was examined in rat neocortical neurones using intracellular recording techniques. Synaptic responses were evoked by orthodromic stimulation applied to the subcortical white matter or to the pial surface. All experiments were carried out at a constant extracellular Cl-concentration.2. The resting membrane potential was -76-2 + 1I0 mV (mean+s. E. M., n = 32) and in most cells IPSPA was depolarizing. The reversal potential of IPSPA (EIPSP-A) was -70-2 + 0 9 mV (n = 32) and that of a more slowly developing hyperpolarizing response (IPSPB) was -91 4 + 1 3 mV (n = 28).3. An examination of the temporal relationships between excitatory postsynaptic potentials (EPSPs) and IPSPAS in different cells suggested that, despite partial overlap of these responses, EPSPs had little influence on the measured values of DEIPSP-A, 4. Application of 20 mm trimethylamine (TriMA), a membrane-permeant weak base which is expected to produce a rise in pHi (and hence in intracellular HC03-), induced a reversible positive shift in EIPSP-A of up to + 9-0 mV (mean + 4-2 mV) at an extracellular pH (pH.) of 7 4. In some experiments, the shift in reversal potential was associated with a change in the polarity of IPSPA from hyperpolarizing to depolarizing.5. Application of 20 mm lactate (a membrane-permeant weak acid which is expected to produce a fall in pHi and hence in intracellular HCO3 ) at pH. 7 0 produced a hyperpolarizing shift in EIPSP-A of up to -75 mV (mean -56 mV). In some experiments, exposure to lactate changed the polarity of IPSPA from depolarizing to hyperpolarizing.6. Changes in pH. from 7-4 to 7-0 reduced the effect of TriMA and augmented that of lactate on EIPSP-A' as could be expected on the basis of the pHo-dependent change in the fraction of membrane permeable non-charged weak base or acid.
High-frequency stimulation (HFS) applied to stratum radiatum of a rat hippocampal slice in the presence of ionotropic glutamate receptor antagonists evokes a biphasic GABA(A) receptor-dependent response in CA1 pyramidal neurons, with a brief hyperpolarizing IPSP (hIPSP) followed by a long-lasting depolarization. We show now that it is possible to pharmacologically separate the hIPSP and late depolarization from one another. In neurons intracellularly perfused for 1-2 hr with F(-) as the major anion and no ATP, the hIPSP (and the corresponding current, hIPSC) evoked by HFS was blocked, whereas neither the late depolarization nor its underlying current was attenuated. In contrast, internal perfusion with a high concentration (5 mM) of the impermeant lidocaine derivative QX-314 selectively abolished the depolarizing component of the biphasic response and also strongly reduced depolarizations evoked by extracellular microinjection of K(+). Bath application of quinine (0. 2-0.5 mM) or quinidine (0.1 mM) resulted in a pronounced inhibition of the HFS-induced extracellular K(+) concentration ([K(+)](o)) transient but not of the bicarbonate-dependent alkaline shift in extracellular pH. The attenuation of the [K(+)](o) transient was closely paralleled by a suppression of the HFS-evoked depolarization but not of the hIPSP. Quini(di)ne did not affect depolarizations induced by exogenous K(+) either. These data provide direct pharmacological evidence for the view that the HFS-induced biphasic response of the pyramidal neuron is composed of mechanistically distinct components: a direct GABA(A) receptor-mediated phase, which is followed by a slow, nonsynaptic [K(+)](o)-mediated depolarization. The bicarbonate-dependent, activity-induced [K(+)](o) transient can be blocked by quini(di)ne, whereas its depolarizing action in the pyramidal neuron is inhibited by internal QX-314. The presence of fundamentally distinct components in GABA(A) receptor-mediated actions evoked by HFS calls for further investigations of their functional role(s) in standard experimental maneuvers, such as those used in studies of synaptic plasticity and induction of gamma oscillations.
1. The role of voltage-gated calcium channels in the generation of activity-induced alkaline shifts in extracellular pH (pHo) was studied in rat hippocampal slices (area CAI) by means of Ca(2+)-and H(+)-selective microlectrodes inserted into the stratum pyramidale and/or stratum radiatum. 2. After complete pharmacological blockade of ionotropic glutamate receptors and gamma-aminobutyric acid-A (GABAA) receptors, trains (5-10 Hz, 5-10s) of antidromic spikes in pyramidal neurons were associated with a fast alkaline transient of up to 0.17 pH units and a fall in the extracellular Ca2+ concentration ([Ca2+]o). The alkaline shift was strongly enhanced upon inhibition of extracellular carbonic anhydrase. 3. Application of 100 microM Ni2+ plus 100 microM Cd2+ inhibited both the fall in [Ca2+]o and the alkaline transient triggered by antidromic spikes. The alkaline shift was abolished in the absence of extracellular Ca2+. 4. In the absence of postsynaptic receptor antagonists, alkaline transients linked to a given level of synaptic excitation in s. radiatum were strongly suppressed after blockade of somatic (and, consequently, of dendritic "backpropagating") spikes by microdrop application of tetrodotoxin to the cell-body layer. 5. We have previously shown that activity-induced alkaline transients in the CAI region are due to an influx of Ca2+ into neurons, which triggers an influx of H+ ions probably caused by activation of a plasmalemmal Ca2+/H+ ATPase. The present results indicate that much (in s. pyramidale perhaps all) of the pH-changing influx of Ca2+ is mediated by voltage-gated Ca2+ channels.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.