Neurotransmitter Activation of Inwardly Rectifying Potassium Current in Dissociated Hippocampal CA3 Neurons: Interactions among Multiple Receptors

Abstract

We characterized potassium current activated by G-proteincoupled receptors in acutely dissociated hippocampal CA3 neurons. Agonists for serotonin, adenosine, and somatostatin receptors reliably activated a potassium-selective conductance that was inwardly rectifying and that was blocked by 1 mM external Ba 2ϩ . The conductance had identical properties to that activated by GABA B receptors in the same cells. In onehalf of the CA3 neurons that were tested, the metabotropic glutamate agonist 1S,3R-ACPD also activated inwardly rectifying Ba 2ϩ -sensitive potassium current. Activation of the current by serotonin and adenosine agonists occurred with a time constant of 200-700 msec after a lag of 50-100 msec; on removal of agonist the current deactivated with a time constant of 1-2 sec after a lag of 200-400 msec. These kinetics are similar to GABA B -activated current and consistent with a direct action of G-protein on the channels. For somatostatin, both activation and deactivation were approximately fourfold slower, probably limited by agonist binding and unbinding. The halfmaximally effective agonist concentrations were ϳ75 nM for somatostatin, ϳ100 nM for serotonin, and ϳ400 nM for 2-chloroadenosine. Dose-response relationships had Hill coefficients of 1.2-1.9, suggesting cooperativity in the receptor-tochannel coupling mechanism. At saturating concentrations of agonists, the combined application of baclofen and either somatostatin, serotonin, or 2-chloroadenosine produced effects that were subadditive and often completely occlusive. However, at subsaturating concentrations the effects of baclofen and 2-chloroadenosine were supra-additive. Thus, low levels of different transmitters can act synergistically in activating inwardly rectifying potassium current.

Agonist unbinding from receptor dictates the nature of deactivation kinetics of G protein-gated K + channels

“…Indeed the nature and numerical details of channel-activation and -deactivation kinetics we observe in our heterologous expression system are comparable to those observed with the native channel in hippocampal neurones. For example, baclofen-mediated responses in hippocampal neurones deactivate in Ϸ1 s after relatively prolonged agonist exposure, which is similar to our observations especially with RGS8 overexpression (12,41). With this much broader choice of heptahelical receptors, we question whether channel-deactivation kinetics are determined solely by the GTP hydrolysis rate of the G protein ␣ subunit.…”

“…Channel activation occurs because of direct binding of G␤␥ dimers, released from G i/o ␣-containing heterotrimers, to domains on the channel (7)(8)(9). G protein-gated Kir channels are also expressed in many central neurones, where they can be activated by a large variety of neurotransmitters acting at G i/o -coupled receptors (10) including ␥-aminobutyric acid (GABA) at the GABA type B (GABA B ) receptor complex and adenosine at A 1 receptors, and they mediate postsynaptic inhibitory events (9,11,12). The molecular counterparts of these currents have now been identified by cloning techniques (13)(14)(15)(16): the channel is a heterotetramer of members of the Kir3.0 family of K ϩ channels.…”

Multiple Peptides Converge to Activate the Same Voltage-Dependent Current in a Central Pattern-Generating Circuit

“…Other examples of modulator convergence are abundant in both invertebrates (Brezina, 1988;Bolshakov et al, 1993;van Tol-Steye et al, 1997 and vertebrates (Andrade and Aghajanian, 1985;Jones, 1985;Andrade et al, 1986;Christie and North, 1988;Bley and Tsien, 1990;Nicoll et al, 1990;Cox and Dunlap, 1992;Sodickson and Bean, 1998). It is simplest to assume that the convergence of the different modulators onto the same current in this study occurs subsequent to receptor binding.…”

“…First, it is surprising that so many substances converge on the same current. Examples of this large degree of convergence are relatively rare (Sodickson and Bean, 1998). Second, it is surprising that so many of the actions of these peptides on the motor patterns generated by the STG can be attributed to modulation of a single current, as demonstrated with dynamic clamp applications of a model proctolin current, which mimic well the actions of proctolin (Sharp et al, 1993a,b;Abbott and Marder, 1998).…”

“…In principle, the large number of different intrinsic and synaptic currents subject to modulation could provide the potential substrate for virtually infinite modifications of circuit behavior under different modulatory conditions. Nonetheless, there are numerous examples of convergent actions of modulators, in which several substances act on the same current (Dunlap and Fischbach, 1978;Jones, 1985;Christie and North, 1988;Nicoll et al, 1990;Bolshakov et al, 1993; Brezina et al, 1994a,b;van Tol-Steye et al, 1997;Sodickson and Bean, 1998). Convergent actions of modulators may appear to be redundant at the level of the single neuron but may not be redundant at the level of network behavior, if different substances activate different receptors on separate populations of target neurons.…”