Activation of a nonselective cationic conductance by metabotropic glutamatergic and muscarinic agonists in CA3 pyramidal neurons of the rat hippocampus

Abstract

We have characterized a cationic membrane conductance activated by metabotropic glutamatergic and muscarinic cholinergic agonists in CA3 neurons in hippocampal slice cultures using the patch-clamp technique. When the potassium concentration in the superfusing fluid was raised above 5 mM, a biphasic current was observed in cells held at -60 mV in response to stimulation of postsynaptic metabotropic glutamate receptors (mGluRs) with 1S,3R-ACPD (50 microM) or muscarinic receptors with methacholine (MCh, 5 microM). The initial inward component was due to an increase in a cationic membrane conductance as determined by its reversal potential and its sensitivity to changes in extracellular K+ or Na+. The conductance underlying this current displayed no apparent voltage sensitivity over the range -120 to -50 mV. The response was reduced by extracellular application of Ba2+, Cd2+, Mg2+, or TEA, whereas extracellular Cs+ or loading cells with BAPTA or Cs+ did not affect the current. The effects of 1S,3R-ACPD were reversibly inhibited by bath-applied MCPG, an antagonist at mGluRs. Experiments with atropine and pirenzepine indicated that non-M1 muscarinic receptors mediated the MCh-induced current. A decrease in a resting leak potassium conductance (IK,leak) was responsible for the late component of the 1S,3R-ACPD- and MCh-induced response, seen as an outward current in the bathing solution with high K+ concentration. Loading cells with GDP beta S, GTP gamma S, or GTP did not alter the cationic current, while, in the same cells, the reduction in IKleak was abolished or irreversibly activated. Single-channel recordings of cationic channel activity in the cell-attached configuration provided evidence for the requirement of second messengers in coupling these receptors to the cationic channels. The data indicate that in addition to the previously described reduction of IK,leak, IM, and IAHP, both 1S,3R-ACPD and MCh activate a nonselective cationic conductance that is clearly revealed upon elevating external K+ concentration. This current is mediated by activation of metabotropic receptors, although no evidence could be obtained to show an involvement of G-proteins.

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

“…The underlying current has been ascribed to the inhibition of at least two types of K ϩ currents: the voltage-dependent M current (Brown and Adams, 1980;Halliwell and Adams, 1982;McCormick and Williamson, 1989) and a voltage-independent leak current (Madison et al, 1987;Guerineau et al, 1994). More recently, a cation nonselective current has been proposed to be mainly responsible for the depolarization generated by muscarinic and mGluR agonists in cortical and hippocampal neurons (Crepel et al, 1994;Greene et al, 1994;Guerineau et al, 1995;HajDahmane andAndrade, 1997, 1999). Interestingly, mGluR 1 receptors have been shown recently to elicit an inward, depolarizing current associated with a slow excitatory postsynaptic response in a G-protein-independent manner in CA3 pyramidal neurons (Heuss et al, 1999).…”

“…Acetylcholine and glutamate, beside mediating fast excitatory synaptic transmission in the CNS, modulate neuronal metabolic responses by acting on metabotropic receptors (muscarinic and metabotropic glutamate receptors, mGluRs). Previous studies have focussed on the molecular and cellular mechanisms by which muscarinic and mGluR agonists regulate membrane excitability in central and peripheral neurons and have revealed their effects on several ionic currents, such as the following: (1) the M current (I M ), a voltage-dependent K ϩ current (Brown and Adams, 1980;Halliwell and Adams, 1982); (2) a voltage-independent "leak" K ϩ current [I K(leak) ] (Madison et al, 1987;McCormick and von Krosigk, 1992;Guerineau et al, 1994); (3) the slowly inactivating K ϩ current I D (Wu and Barish, 1999); (4) the delayed rectifier K ϩ current I K (Zhang et al, 1992); (5) the unspecific cationic pacemaker current (I h ) (Colino and Halliwell, 1993); (6) a Ca 2ϩ -dependent, cation-nonspecific current (Crepel et al, 1994;Greene et al, 1994;Guerineau et al, 1995;Andrade, 1997, 1999); and (7) the Ca 2ϩ -activated K ϩ current responsible for the slow afterhyperpolarization (sI AHP ) (Benardo and Prince, 1982;Nicoll, 1983, 1984;Charpak et al, 1990).…”

Group I Metabotropic Glutamate Receptors on Second-Order Baroreceptor Neurons Are Tonically Activated and Induce a Na+–Ca2+Exchange Current

“…Other mechanisms mediating mGluR-induced cationic currents include Ca 2ϩ -dependent or -independent selective cation currents (Congar et al 1997;Crepel et al 1994;Guerineau et al 1995;Zheng et al 1995), voltage-dependent Ca 2ϩ channels, and Ca 2ϩ -sensitive transient receptor potential (TRP)-like currents (Gee et al 2003). Other studies have linked Group I mGluR activation to inhibition of K ϩ currents, including the delayed outward rectifier (I K ) (Charpak et al 1990;Hay and Lindsley 1995;Lüthi et al 1996;Schrader and Tasker 1997), resting K ϩ current (Guerineau et al1994;Schrader and Tasker 1997), and slow Ca 2ϩ -dependent K ϩ current (I AHP ) (Abdul-Ghani et al 1996a,b; Gerber et al 1992; Schrader and Tasker 1997).…”

Control of neuronal excitability by Group I metabotropic glutamate receptors

“…Surprisingly, activation of this current was not blocked by GDPβS or GTPγS, indicating that G-proteins were not required to couple activation of the mGlu receptor to its effector (Guérineau et al 1995). Rather, activation of this Group I mGlu receptor current depends on Src family protein tyrosine Kinases (Heuss et al 1999).…”

“…Despite this apparent preference for protein G q/11 , pertussis toxin, a highly selective inhibitor of Gα i/o , inhibits responses of Group I mGlu receptors overexpressed in Xenopus oocytes (Houamed et al 1991;Masu et al 1991;Sugiyama et al 1987) and natively expressed in neurons (Bertaso et al 2010;Holohean et al 1999;Kreibich et al 2004;Linn 2000). Moreover, some effects mediated by Group I mGlu receptors persist even after disabling G-protein function with GDPβS (e.g., Guérineau et al 1995). It is thus clear that activated Group I mGlu receptors can engage multiple parallel Gprotein-dependent and -independent signaling pathways.…”