Serotonin in rat incisor pulp after sympathetic and afferent nerve lesions

Pergamon

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SEROTONIN SYMPATHETIC N. P. KEREZOUDIS, Departments

Archs oral Bid. Vol. 39, Suppl., 137S, 1994 Copyright 6 1994 Elsevier Science Ltd Printed in Great Britain. All rights reserved 0003.9969194 $7 00 + 0.00

of

IN RAT INCISOR AND AFFERENT G. G. NOMIKOS,

PULP AFTER NERVE LESIONS

T. H. SVENSSON

and L. OLGART

Physiology and Pharmacology, Division of Pharmacology and Endodontics, Karolinska Institute, 171 77 Stockholm, Sweden Key words:

5hydroxytryptamine,

Serotonin (5hydroxytryptamine; 5-HT) is a wellknown inflammatory mediator and pain-producing substance. Local application of serotonin in deep cavities prepared in the canine teeth of cats and dogs excited intradental nerves (Olgart, 1974) and caused vasoconstriction (Liu et al., 1990). Serotonin also sensitized both A6 and A/j’ nerve fibres to hydrodynamic stimuli in dog teeth (Ngassapa, NBrhi and Hirvonen, 1992). Autoradiographic studies have revealed serotonin receptors on pulpal nerves (Kim et al., 1992). Platelets are considered as the main source of serotonin in the pulp. However, Liu et al. (1991) have recently reported the existence of serotoninpositive neurones in pulps of dog teeth. We have now sought to investigate further the origin and the physiological importance of serotonin-containing nerves in the incisor pulp of the rat. In one group of male Sprague-Dawley rats (n = 21) (weight 350-450 g), the cervical sympathetic nerve (n = 10) or the inferior alveolar nerve (n = 11) was cut unilaterally at 10 and 7 days, respectively, before tissue collection. The heads of the animals were perfused through the common carotid arteries with 100 ml of cold saline (4°C). The content of serotonin, its oxidative metabolite 5hydroxyindolacetic acid and the sympathetic neurotransmitter dopamine were biochemically assessed in the incisor pulp and gingiva by high-performance liquid chromatography coupled with electrochemical detection. In another group of anaesthetized animals (n = lo), the effects of serotonin antagonists on blood-flow reactions in the pulp and gingiva produced by electrical stimulation of the cervical sympathetic nerve (4 V, 1 ms, 4 and 16 Hz) for 1 min were studied by laser Doppler flowmetry (Periflux PFZb, Perimed KB). Data were statistically evaluated by Student’s t-test. Biochemical analysis revealed the presence of serotonin, 5-hydroxyindolacetic acid and dopamine in both pulp and gingiva. The serotonin content in gingiva was higher than that of the pulp (6820 k 123 1 (SEM) vs 296 f44 (SEM) pmol/g, respectively; p < 0.001, n = 21) whereas the levels of 5-hydroxyindolacetic acid were similar in both tissues (about 500 pmol/g). Chronic surgical sympathectomy failed to affect the levels of serotonin in both tissues but it reduced the content of 5-hydroxyindolacetic acid in sympathectomized pulps by 26% (p = 0.061, n = 10). Following sympathectomy, the dopamine content in pulp and gingiva was diminished by 79% (p < 0.05) and 65% @ < 0.01) respectively. The dopamine content in the inferior alveolar denervated pulps was 137s

dopamine,

Department

of

pulp.

reduced by 50% as compared to intact pulps (p = 0.054, n = 11) but was unchanged in the gingiva. Inferior alveolar denervation failed to influence serotonin and 5-hydroxyindolacetic acid levels in the pulp and gingiva. Infusion (i.v.) of the non-selective serotonin-receptor blocker methysergide (0.5 mg/kg), the 5-HTz-receptor antagonist ritanserin (0.5 mg/kg) or the 5-HT,-receptor blocker ICS 205-930 (100 pg/kg) failed to affect the sympathetic vasoconstriction observed at both 4 and 16 Hz both in the pulp and gingiva (n = 4). In contrast, cc,-adrenergic blockade (prazosin, 50 pg/kg) greatly attenuated the vasoconstriction (n = 6). Our findings indicate that serotonin in the rat incisor pulp and gingiva is mainly of extraneuronal origin, coming from blood platelets or mast cells for example (Benditt et al., 1955). That the serotonin content of gingiva was 30-fold larger than that of pulp supports its probable localization in mast cells, as numerous mast cells have been demonstrated in rat gingiva (Carranza and Cabrini, 1959) but very few in the pulp (Radden, 1959). Our finding that levels of 5-hydroxyindolacetic acid in the pulp were reduced after sympathectomy indicates that pulpal sympathetic nerves may play a part in serotonin metabolism. Such metabolic activity of sympathetic nerves may be of importance in early stages of pulp inflammation where more mast cells will be expected to appear (Zachrisson, 1971). The finding that levels of dopamine in the pulp were reduced after sympathectomy but only moderately diminished after section of the inferior alveolar nerve indicates that the inferior alveolar nerve carries some but not all the sympathetic nerves supplying rat incisor pulps. REFERENCES

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