Intracellular signal pathways controlling respiratory neurons1

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Abstract

Medullary respiratory neurons are influenced by a variety of neuromodulators, but there is a lack of information about the specific intracellular signal pathways involved. In this report we describe the modulatory effects of the cyclic adenosine-triphosphate (cAMP)-dependent protein kinase and of protein kinase C pathways on voltage- and ligand-controlled ionic conductances and demonstrate their functional significance in regulating the excitability of medullary respiratory neurons of the vivo cat. Evidence is presented that PKA and PKC pathways are persistently activated. PKA regulates current flow through persistently activated and GABAB receptor-controlled potassium channels as well as GABAA receptor-controlled chloride channels. PKC also depresses persistent potassium currents but it potentiates excitatory and inhibitory synaptic currents. The clinical significance of these intracellular signal pathways is demonstrated in a case of a child suffering from apneustic breathing, who was successfully treated with a 5HT-1A receptor agonist.

Introduction

The rhythmic discharge of medullary respiratory neurons is determined by synaptic network interactions that are dependent on the activation of glutamate-, glycine- and GABAA-regulated receptors (Bianchi et al., 1995, Bonham, 1995, Richter, 1996). All synaptic processes are influenced by neuromodulators, such as serotonin, norepinephrine, dopamine, adenosine, acetylcholine, opioids and GABA acting on GABAB receptors (Champagnat et al., 1979, Pierrefiche and Foutz, 1993, Lalley et al., 1994a, Lalley et al., 1995, Bianchi et al., 1995, Schmidt et al., 1995). These neuromodulators influence neuronal excitability by activating G protein-controlled intracellular signal pathways altering ionotropic channel conductances through two general transduction mechanisms: (i) directly through binding of their β,γ subunits to channel proteins, or (ii) indirectly via alterations of adenylyl cyclase, cyclic adenosine-triphosphate (cAMP), cAMP-dependent protein kinase (PKA) or inositol-trisphosphate (IP3) and Ca2+ or diacylglycerol and protein kinase C (PKC) (Wickman and Clapman, 1995). In this report, we show that alterations in the activities of the PKA and PKC signal pathways produce substantial changes in voltage- and ligand-controlled conductances that determine ongoing oscillations of the membrane potential and rhythmic burst discharges of medullary respiratory neurons in vivo. We also provide evidence that these intracellular signal pathways mediate the responses to serotonin 5HT-1A and 5HT-2A receptor activation. Finally, we describe how activation of these modulatory processes were used to successfully treat life-threatening apneustic respiratory disturbances in a child after tumor resection in the brainstem.

Section snippets

Methods

The data presented here are derived from experiments on in total 59 adult cats (2.5–4.5 kg) of either sex. Care and use of animals were in accordance with the guiding principles of the German Physiological Society. Animals were anesthetized with pentobarbital sodium (40 mg kg−1 i.p. initially, followed by 4–12 mg h−1, i.v.). Neuromuscular paralysis was produced by gallamine triethiodide (4–8 mg kg−1 initially, followed by 4–8 mg h−1) and animals were mechanically ventilated with oxygen-enriched

PKA alters respiratory neuronal activity

Intracellular injection of PKA-selective chemicals produce significant changes in the spontaneous activity of medullary respiratory neurons. For a detailed description see Lalley et al. (1997). Injection of the activator Sp-cAMPS into expiratory neurons reduced inhibitory synaptic drive potentials by 4.8±1.0 mV (mean±SE; n=13) and augmented by 2.3±0.8 mV (n=13) excitatory synaptic drive potentials. In SEVC, inhibitory synaptic drive currents were depressed by 0.43±0.1 nA (n=6), whereas

Discussion

Using the methods of intra- and extracellular ionophoresis of drugs together with single electrode current and voltage clamp, we described neuromodulatory changes of a variety of postsynaptic membrane conductances that lead to significant changes of synaptic currents and potentials. For intracellularly ionophoresed substances with limited membrane permeabilities, we conclude that the effects originated from the individual postsynaptic neurons studied. Only with extracellular ionophoresis of

Conclusion

The studies have shown that the intracellular PKA, PKC and Ca+2 signalling pathways have functional significance in modulating the behavior of medullary respiratory neurons. Persistant activation of these pathways produces significant increases of excitability in medulary respiratory neurons. Activation of 5HT-1A or 5HT-2A receptors alters PKA or PKC activities that seem to contribute to behavioral changes in medullary respiratory neurons. It is probable that the PKA, PKC and Ca+2 signal

Acknowledgements

This work was supported by the Deutsche Forschungsgemeinschaft (SFB 406).

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    Paper presented at the conference on Neural Control of Breathing: Molecular to Organismal Perspectives, Madison, WI, 21–25 July 1996.

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