Inhibitory influence of sympathetic nerves on afferent nerve-induced extravasation in the rat incisor pulp upon direct electrical stimulation of the tooth

Previous studies have shown that sympathetic nerve stimulation reduces afferent nerve-induced vasodilation by mechanisms unrelated to vasoconstriction in the rat incisor pulp. The present investigation concerned whether similar modulatory mechanisms might also influence neurogenic plasma extravasation in dental pulp. Rat mandibular incisors were electrically stimulated and blood flow reactions in the pulp were recorded by laser Doppler flowmetry. Plasma extravasation in the incisor pulp, gingiva and lip were indirectly assessed by the Evans-blue method. Stimulation of teeth with 50 μA (5 min) did not cause increased dye accumulation in the stimulated pulps whereas stimulation with 100 μA significantly increased the dye content in ipsilateral pulps by 32% as compared to controls; 100 μA stimulation was without effect in unilaterally denervated animals. Tooth stimulation with 50 μA (5 min), in the presence of either the α-adrenergic blocker phenoxybenzamine (3mg/kg), or the α 1-adrenergic antagonist prazosin (50 μg/kg), as well as in chronically sympathectomized animals, significantly increased the Evans-blue content in the stimulated pulps by 47, 83 and 86%, respectively. Application of short trains of impulses (same number of impulses as for the continuous stimulation but producing minimal vasoconstriction) resulted in some dye accumulation, which was enhanced in the ipsilateral pulps in the presence of prazosin (100 μg/kg) or after acute resection of the superior cervical sympathetic ganglion by 70 and 64%, respectively. The Evans-blue content in the lip and gingiva was uninfluenced by the tooth stimulation. The results indicate that activation of sympathetic nerves inhibits the afferent nerve-induced plasma extravasation in rat incisor pulp and this effect is mediated by α-adrenoceptors not associated with vasoconstriction.