GABAB modulation of GABAA and glycine receptor-mediated synaptic currents in hypoglossal motoneurons

https://doi.org/10.1016/j.resp.2004.03.009Get rights and content

Abstract

We studied the effects of GABAB receptor activation on either glycine or GABAA receptor-mediated synaptic transmission to hypoglossal motoneurons (HMs, P8-13) using a rat brainstem slice preparation. Activation of GABAB receptors with baclofen, a GABAB receptor agonist, inhibited the amplitude of evoked glycine and GABAA receptor-mediated inhibitory postsynaptic currents. Additionally, with blockade of postsynaptic GABAB receptors baclofen decreased the frequency of both glycine and GABAA receptor-mediated spontaneous miniature inhibitory postsynaptic currents (mIPSCs), indicating a presynaptic site of action. Conversely, the GABAB receptor antagonist CGP 35348 increased the frequency of glycine receptor-mediated mIPSCs. Application of the GABA transport blocker SKF 89976A decreased the frequency of glycinergic mIPSCs. Lastly, we compared the effects of baclofen on the frequency of glycine and GABAA receptor-mediated mIPSC during HM development. At increased postnatal ages (P8-13 versus P1-3) mIPSC frequency was more strongly reduced by baclofen. These results show that presynaptic GABAB receptors inhibits glycinergic and GABAergic synaptic transmission to HMs, and the presynaptic sensitivity to baclofen is increased in P8-13 versus P1-3 HMs. Further, endogenous GABA is capable of modulating inhibitory synaptic transmission to HMs.

Introduction

GABA and glycine are the two main inhibitory neurotransmitters in the central nervous system. GABA activates two main classes of GABA receptors, ionotropic receptors namely GABAA and the less common GABAC and metabotropic GABAB receptors (Hill and Bowery, 1981, Bormann, 2000). GABAA and glycine receptors are ligand-gated ion channels permeable to chloride ions. Activation of these receptors normally results in rapid membrane hyperpolarization. GABAB receptors are GTP-binding protein receptors that are linked to cell membrane Ca2+ and K+ channels (Kerr and Ong, 1995, Takahashi et al., 1998). Effects of GABAB receptor activation include inhibition of neurotransmitter release from presynaptic terminals and hyperpolarization of postsynaptic membranes (Kerr and Ong, 1995).

Both GABAA and glycine ligand-gated ion channels are present on brainstem hypoglossal motoneurons (HMs) and both channel types are activated due to spontaneous release of GABA and glycine onto HMs in brainstem slices (O’Brien and Berger, 1999). Although in many regions of the CNS, either GABA or glycine is the dominant inhibitory neurotransmitter, in the hypoglossal nucleus, both GABA and glycine are co-released by synaptic terminals on HMs (O’Brien and Berger, 1999). Presynaptic neurons containing GABA and glycine that project to the hypoglossal nucleus have been found in the lateral medullary reticular formation (Li et al., 1997).

Previous anatomical and electrophysiological evidence indicates that GABAB receptors are present in the hypoglossal nucleus (Okabe et al., 1994, Margeta-Mitrovic et al., 1999). Immunohistochemical localization of GABAB receptors in the adult rat CNS has shown intense staining for GABAB receptors in the hypoglossal nucleus (Margeta-Mitrovic et al., 1999). Injection of baclofen, a GABAB receptor agonist, into the hypoglossal nucleus of anesthetized rat caused a marked reduction in the inspiratory-phase related respiratory activity recorded from HMs, suggesting a functional role for GABAB receptors in the hypoglossal nucleus (Okabe et al., 1994). Mechanisms for the action of baclofen have not been previously investigated in the hypoglossal motor nucleus. In the present study, we wanted to determine whether activation of GABAB receptors modulates both glycine and GABAA receptor-mediated synaptic transmission to HMs. We also wanted to determine whether endogenous GABA present in the brainstem slice can modulate inhibitory synaptic transmission to this important class of respiratory motoneuron.

Significant changes occur in both glycinergic and GABAergic synaptic transmission to HMs during the postnatal period (O’Brien and Berger, 2001, Singer et al., 1998). Previous studies of the rat brain have shown that properties of the GABAB receptor system also change during the postnatal period (Fritschy et al., 1999, Malitschek et al., 1998, Turgeon and Albin, 1994). Therefore, we also investigated whether there are developmental changes occurring in GABAB receptor-mediated modulation of inhibitory synaptic transmission to HMs.

Section snippets

Methods

Sprague–Dawley rats, separated into two age groups [neonatal (P1-3) and juvenile (P8-13)], were anesthetized either by intramuscular injection of a ketamine–xylazine mixture (200 and 14 mg/kg, respectively) or by inhalation of halothane. Following decapitation, the brainstem was isolated and removed. Transverse slices (250–300 μm) of the medulla were prepared using a vibratome (Pella). During slicing, incubation (1 h at 37 °C), and recording, brainstem slices were bathed in a Ringer’s solution

The effects of GABAB receptor activation on inhibitory synaptic transmission

To test whether activation of GABAB receptors could influence inhibitory synaptic transmission to HMs, we studied the effects of baclofen (a GABAB receptor agonist) on glycine and GABAA receptor-mediated eIPSCs in HMs of P8-13 rats. We stimulated the lateral reticular formation, which has been shown to contain GABAergic and glycinergic neurons that project to the hypoglossal nucleus (Li et al., 1997), and we pharmacologically isolated GABAA and glycine receptor-mediated eIPSCs. In most cases,

Discussion

Our results have shown that activation of presynaptic GABAB receptors at glycinergic and GABAergic synapses of rat HMs reduces these two forms of fast inhibitory synaptic transmission. We observed that as HMs mature during the postnatal period the inhibitory effect of activation of presynaptic GABAB receptors becomes greater at a given dose of the GABAB receptor agonist baclofen. Further, based on our experimental results using both a GABAB receptor antagonist and a GABA transport blocker, we

Acknowledgements

This research was supported by NIH grant HL 49657 to A.J.B. and PHS NRSA T32 GM07270 from NIGMS to J.S. We thank Dr. W. Satterthwaite and P. Huynh for technical assistance.

References (31)

  • S.M Turgeon et al.

    Postnatal ontogeny of GABAB binding in rat brain

    Neuroscience

    (1994)
  • H Altmann et al.

    Differential strength of action of glycine and GABA in hypoglossus nucleus

    Pflügers Arch.

    (1972)
  • N.G Bowery et al.

    (–)Baclofen decreases neurotransmitter release in the mammalian CNS by an action at a novel GABA receptor

    Nature

    (1980)
  • N Chéry et al.

    GABAB receptors are the first target of released GABA at lamina I inhibitory synapses in the adult rat spinal cord

    J. Neurophysiol.

    (2000)
  • V.A Doze et al.

    Calcium channel involvement in GABAB receptor-mediated inhibition of GABA release in area CA1 of the rat hippocampus

    J. Neurophysiol.

    (1995)
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