GABAB receptors modulate an omega-conotoxin-sensitive calcium current that is required for synaptic transmission in the Xenopus embryo spinal cord

Abstract

Activation of GABAB receptors in the Xenopus embryo, a simple vertebrate, causes presynaptic inhibition of transmitter release from glycinergic spinal neurons and an increase in action potential threshold. To investigate the underlying mechanisms of GABAB receptor action, we have made whole-cell voltage-clamp recordings from acutely isolated Xenopus embryo spinal neurons. The GABAB receptor agonist baclofen caused a reversible reduction in the amplitude of Ca2+ currents. This reduction of Ca2+ currents appeared to be voltage dependent as it was removed at very positive potentials. Since the specific GABAB antagonists CGP35348, phaclofen, and 2-hydroxysaclofen all blocked the reduction in Ca2+ currents, we concluded that the modulation of the Ca2+ current was mediated by GABAB receptors. We have investigated the pharmacological identity of the Ca2+ current modulated by baclofen using the selective blocker omega-conotoxin, fraction GVIA (omega-CgTX). omega-CgTX selectively blocked voltage-gated Ca2+ currents without affecting the voltage-gated Na+ current. omega-CgTX substantially occluded the action of baclofen, suggesting that GABAB receptors modulate an omega-CgTX-sensitive Ca2+ current. Since GABAB receptors mediate presynaptic inhibition, we have studied the involvement of the omega-CgTX-sensitive Ca2+ current in synaptic transmission in the intact spinal cord. Inhibitory interneuron axons were stimulated to evoke monosynaptic IPSPs in motoneurons, and recorded intracellularly. Since omega-CgTX blocked inhibitory transmission, we concluded that the omega-CgTX-sensitive Ca2+ current plays an essential role in transmitter release. If modulation of this current were to occur in nerve terminals, it could contribute to the GABAB receptor-mediated presynaptic inhibition of transmitter release.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution and Function of GABABReceptors in Spider Peripheral Mechanosensilla

“…Whereas activation of an inwardly rectifying K ϩ current is the most commonly found modulatory effect of GABA B receptors on postsynaptic sites, other types of K ϩ currents are also often involved (Bowery et al 2002). For example, in Xenopus embryonic spinal cord baclofen caused a reversible enhancement of outward I K as well as reduction of I Ca (Wall and Dale 1994), closely corresponding to the findings here.…”

Experimentally derived model for the locomotor pattern generator in the Xenopus embryo.

“…A reduction of both the slow and fast K+ currents by 25% (ineffective at disrupting the motor pattern in the absence of recurrent inhibition) gave all three types of aberrant activity described previously (Fig. lOB;Wall & Dale, 1994b). Introduction of recurrent inhibition may therefore make the circuit slightly more realistic.…”

“…Repetitive firing could be evoked in the absence of a Ca2P current, but only by increasing the Na current to a value outside the range observed in the isolated neurons. Experimental evidence supports the notion that the Ca2P current is important for spike initiation: w-conotoxin, which blocks roughly half the Ca2P current, raises spike thresholds in Xenopus motoneurons (Wall & Dale, 1994b (Soffe, 1990;Dale, 1991). To test the contribution of the fast and slow K+ currents to the control of repetitive firing, their amplitudes in the modelled neurons were systematically varied.…”

Serotonergic Inhibition of the T-Type and High Voltage-Activated Ca2+Currents in the Primary Sensory Neurons ofXenopusLarvae

“…N-and P/Q-type calcium channels are involved in supporting synaptic transmission in the C NS and peripheral nervous systems (Luebke et al, 1993;L uebke and Dunlap, 1994;Wall and Dale, 1994;Wheeler et al, 1994) (for review, see Dunlap, 1997). Therefore, the reduction of N-and P-type currents could contribute to the previously reported presynaptic inhibition of transmitter release from R-B neurons (Sillar and Simmers, 1994).…”

“…In other CNS neurons N-type or P/Q-type high voltage-activated (HVA) calcium channels can be modulated by serotonin, mostly via 5-HT 1A receptor and membrane-delimited G-protein pathways (Pennington et al, 1991;Koike et al, 1994;Bayliss et al, 1995;Foehring et al, 1996). These calcium channels are known to be involved in triggering synaptic transmission (Luebke et al, 1993;Luebke and Dunlap, 1994;Wall and Dale, 1994;Wheeler et al, 1994) (for review, see Dunlap, 1997). By contrast, T-type channels are not involved in transmitter release but, instead, influence the firing properties of CNS neurons (Llinás and Yarom, 1981;Crunelli et al, 1989;Suzuki and Rogawski, 1989;White et al, 1989;Zhang et al, 1993).…”