Altered Regulation of Potassium and Calcium Channels by GABA_Band Adenosine Receptors in Hippocampal Neurons From Mice Lacking Gα_o

Abstract

To examine the role of G(o) in modulation of ion channels by neurotransmitter receptors, we characterized modulation of ionic currents in hippocampal CA3 neurons from mice lacking both isoforms of Galpha(o). In CA3 neurons from Galpha(o)(-/-) mice, 2-chloro-adenosine and the GABA(B)-receptor agonist baclofen activated inwardly rectifying K(+) currents and inhibited voltage-dependent Ca(2+) currents just as effectively as in Galpha(o)(+/+) littermates. However, the kinetics of transmitter action were dramatically altered in Galpha(o)(-/-) mice in that recovery on washout of agonist was much slower. For example, recovery from 2-chloro-adenosine inhibition of calcium current was more than fourfold slower in neurons from Galpha(o)(-/-) mice [time constant of 12.0 +/- 0.8 (SE) s] than in neurons from Galpha(o)(+/+) mice (time constant of 2.6 +/- 0.2 s). Recovery from baclofen effects was affected similarly. In neurons from control mice, effects of both baclofen and 2-chloro-adenosine on Ca(2+) currents and K(+) currents were abolished by brief exposure to external N-ethyl-maleimide (NEM). In neurons lacking Galpha(o), some inhibition of Ca(2+) currents by baclofen remained after NEM treatment, whereas baclofen activation of K(+) currents and both effects of 2-chloro-adenosine were abolished. These results show that modulation of Ca(2+) and K(+) currents by G protein-coupled receptors in hippocampal neurons does not have an absolute requirement for Galpha(o). However, modulation is changed in the absence of Galpha(o) in having much slower recovery kinetics. A likely possibility is that the very abundant Galpha(o) is normally used but, when absent, can readily be replaced by G proteins with different properties.

Gβγ-activated Inwardly Rectifying K+ (GIRK) Channel Activation Kinetics via Gαi and Gαo-coupled Receptors Are Determined by Gα-specific Interdomain Interactions That Affect GDP Release Rates

“…G␣ i2 mutations designed to disrupt the domain-domain interaction accelerated GIRK activation rates consistent with the biochemical studies of mutant G␣ i1 subunits that increased basal GDP release rates (5). Thus our findings establish experimental conditions whereby the GPCR-catalyzed G␣ GDP release rate is the rate-limiting step in receptor-dependent GIRK activation as originally proposed by Breitwieser and Szabo (6) for I K,ACh in atrial myocytes, and importantly identifies molecular determinants that mediate functional differences in GIRK channels activated by GPCRs coupled to G␣ i and G␣ o proteins that may impact the kinetics and magnitude of inhibitory GIRK channel-mediated postsynaptic currents (7). FIG.…”

Modulation of Hippocampal Glutamatergic Transmission by ATP Is Dependent on Adenosine A1 Receptors

“…Nevertheless, presynaptic as well as postsynaptic effects of adenosine observed in the rat are recapitulated in the mouse (Luscher et al, 1997;Greif et al, 2000;Jarolimek et al, 2000). Herein, we report that fEPSP responses evoked by Schaffer collateral/commissural stimulation are potently inhibited by adenosine, although the EC 50 for this response was higher than we observed previously in the rat (Dunwiddie et al, 1986;Brundege et al, 1997).…”

Functional Specificity of Gαqand Gα11in the Cholinergic and Glutamatergic Modulation of Potassium Currents and Excitability in Hippocampal Neurons

“…A subtle, partial effect linked to G␣ 11 , unmasked in G␣ q knock-out mice, would be expected to be slower when compared with the G␣ q -mediated effect observed in their wild-type littermates, given the lower density of the G␣ 11 subtype in CA1 pyramidal neurons (Mailleux et al, 1992;Milligan, 1993;Tanaka et al, 2000). Indeed, a compensatory phenomenon with altered (slower) kinetics has been reported recently for the modulation of potassium and calcium channels by GABA B and adenosine receptors in hippocampal neurons from mice lacking G␣ o (Greif et al, 2000). This seems unlikely to occur in our case, because the time course of the residual muscarinic and glutamatergic effects on sI AHP was not different in G␣ q knock-out mice when compared with their wild-type littermates [compare Figs.…”