RESUMO
In the orofacial area, noxious stimulation of the orofacial structure in the trigeminal region evokes parasympathetic reflex vasodilatation, which occurs via the trigeminal spinal nucleus (Vsp) and the inferior/superior salivatory nucleus (ISN/SSN). However, the neurotransmitter involved in the inhibitory synaptic inputs within these nuclei has never been described. This parasympathetic reflex vasodilatation is suppressed by GABAergic action of volatile anesthetics, such as isoflurane, sevoflurane, and halothane, suggesting that medullary GABAergic mechanism exerts its inhibitory effect on the parasympathetic reflex via an activation of GABA receptors. The aim of the present study was to determine the role of GABA(A) and GABA(B) receptors in the Vsp and the ISN in regulating the lingual nerve (LN)-evoked parasympathetic reflex vasodilatation in the lower lip. Under urethane anesthesia (1g/kg), change in lower lip blood flow elicited by electrical stimulation of the LN was recorded in cervically vago-sympathectomized rats. Microinjection of GABA (10 µM; 0.3 µl/site) into the Vsp or the ISN significantly and reversibly attenuated the LN-evoked parasympathetic reflex vasodilatation. Microinjection of the GABA(A) receptor-selective agonist muscimol (100 µM; 0.3 µl/site) or the GABA(B) receptor-selective agonist baclofen (100 µM; 0.3 µl/site) into the Vsp or the ISN significantly and irreversibly reduced this reflex vasodilatation, and these effects were attenuated by pretreatment with microinjection of each receptor-selective antagonists [GABA(A) receptor selective antagonist bicuculline methiodide (1mM; 0.3 µl/site) or GABA(B) receptor selective antagonist CGP-35348 (1mM; 0.3 µl/site)] into the Vsp or the ISN. Microinjection of these antagonists alone into the Vsp or the ISN had no significant effect on this reflex vasodilatation. In addition, microinjection (0.3 µl/site) of the mixture of muscimol (100 µM) and baclofen (100 µM) into the Vsp or the ISN also significantly reduced this reflex vasodilatation. These results suggest that medullary GABA signal transduction inhibits the parasympathetic reflex vasodilatation in the rat lower lip via GABA(A) and GABA(B) receptors in the Vsp and the ISN.
