Peripheral GABA_A receptor activation modulates rat tongue afferent mechanical sensitivity

Abstract

Objectives

The expression of GABA_A receptors and the effects of GABAA receptor agonists on the response properties of tongue afferent fibres were investigated in female rats to determine if peripheral GABA receptors might be a target of topical benzodiazepines when used for pain relief in burning mouth syndrome patients.

Design

Nerve fibres in tongue sections from six female rats were identified using protein gene product 9.5, and the co-expression of the γ subunit of GABAA receptor and substance P assessed in the nerve fibres. In vivo extracellular recordings of trigeminal ganglion neurons that innervate the tongue were undertaken in 27 anesthetised female rats and their responses to mechanical and thermal stimulation characterised before and after topical application of GABA, the GABA_A receptor selective agonist muscimol or vehicle control.

Results

The vast majority of tongue nerve fibres examined (95%) expressed the γ subunit of GABA_A receptor. Bath application of muscimol, but not GABA, significantly increased the mechanical thresholds of tongue afferent fibres compared to vehicle, but only after the tongue had been heated with 60 °C water.

Conclusions

GABA_A receptors are present on tongue nerve fibres and their activation alters the mechanical sensitivity these fibres. These findings suggest that topical application of benzodiazepines to the oral mucosa may decrease pain in burning mouth syndrome through a local action on peripheral GABA_A receptors.

Introduction

Burning mouth syndrome is a chronic intraoral pain condition for which no medical or dental cause has yet been identified.1, 2, 3 The prevalence of this syndrome in the general population has been estimated to be around 1%.4, 5, 6, 7, 8, 9 Burning mouth syndrome is characterised by symptoms of “burning-like” pain most commonly experienced on the anterior of the tongue, as well as the anterior hard palate and lips, although other sites in the mucosa of the oral cavity and throat can also be involved.3, 10, 11, 12 Burning mouth syndrome may go into remission after a period of years,¹³ and thus conservative medical treatment aimed at symptom resolution is the normal therapeutic avenue. Unfortunately, burning mouth syndrome symptoms have proven quite resistant to conventional pharmacotherapy.

Recently, it was reported that a non-conventional approach that involved sucking a 1 mg tablet of the benzodiazepine clonazepam, without swallowing, three times a day for 14 days was associated with significant symptomatic improvement in pain scores.¹⁴ Benzodiazepines are anxiolytic drugs thought to mediate their effects in the central nervous system by increasing the ability of γ-amino-butyric acid (GABA) to open the GABA_A receptor; a ligand-gated chloride channel. The interaction between benzodiazepines and the GABA_A receptor occurs at the interface between the α and γ subunits.15, 16, 17, 18 Activation of peripheral GABA_A receptors in orofacial tissues is not excitatory and appears to exert anti-nociceptive actions in some animal models of acute craniofacial pain.19, 20, 21 These findings suggest that topical application of benzodiazepines to the oral mucosa may decrease pain in burning mouth syndrome through a local action on peripheral GABA_A receptors to decrease the excitability of nociceptors.¹⁴ However, evidence for a direct effect of GABA_A receptor activation on nociceptors in the oral cavity is lacking.

In the present study, the expression of GABA_A receptors in tongue nerve fibres and the effects of GABA and the GABA_A receptor selective agonist, muscimol, on the response properties of tongue afferent fibres were investigated in female rats. It was hypothesised that GABA_A receptors would be widely expressed by nerve fibres in the tongue and that activation of these receptors would alter the mechanical threshold of these nerve fibres.