.4rrh. oralEio/. Vo1.7. pp.413415, 1962. Pergamon Press Ltd. Printed in Gt. Britain.

CHEMICAL EXCITANTS OF PAIN IN HUMAN DENTINE AND DENTAL PULP

D. J. ANDERSON and M. N. NAYLOR

Physiological Laboratory and Department of Dental Medicine,

Guy’s Hospital Medical School, London S.E. 1, England

Abstract-Previous investigators have found that a variety of substances, including aeetylcholine, KCI, histamine and 5_hydroxytryptamine, can evoke pain when applied in dilute solution to the exposed base of a blister in the skin. In this investigation, the range of substances and concentrations found effective in the blister-base preparation were tested on the dentine and exposed pulp. In ten subjects none of the solutions evoked pain from dentine and in six out of seven subjects in whom the solutions were applied to the exposed pulp, all except hypotonic NaCl, tryptamine and water caused pain.

ANDERSON, CURWEN and HOWARD (1958) applied various substances in solution to human dentine exposed in cavity preparation, with the object of investigating the

mechanism of sensation in that tissue. For these experiments they employed acetyl-

choline and KCI, which ARMSTRONG et a/. (1953) found capable of evoking pain

when applied in solution to nerve endings in the exposed base of a cutaneous blister. The present report covers an extension of the experiments of ANDERSON et al. (1958:)

to follow more closely the range and concentrations of substances used by ARMSTRONG

et al. (1953) and to test when possible the direct action of these pain-producing

substances on the exposed dental pulp.

METHOD

Table 1 shows the list of substances and concentrations employed; except where

this was impossible, the solutions were made up with isotonic NaCl at pH 7.0-7.5. The subjects were ten adolescent’males between the ages 15-17 years, none of whom

had been subjects for experiments on sensation before. The teeth used were sound

premolars scheduled for extraction for orthodontic reasons. Dentine was exposed

by cutting an orthodox cavity from the occlusal surface, using a slow-running tungsten carbide bur under a stream of cold water. Cavity preparation ceased as soon as it was obvious that the amelo-dentinal junction had been passed.

The solutions were numbered and were applied in random order at 37°C by means of small syringes, the operator being unaware of the order in which the solutions were arranged. 3 min elapsed between every stimulation. After the period of stimulation the cavity was rinsed with warm saline and the subject was asked to report on any sensation experienced. In seven of the ten subjects this series of tests

413

414 D. J. ANDERWN AND M. N. NAYLOR

TABLE 1

Solute Concentration

1. 2. 3. 4. 5. 6. 7. 8. 9.

10. Il. 12. 13. 14. 15.

Sodium chloride Acetyl choline (cont. expressed as the chloride) Acetyl choline (cont. expressed as the chloride) Acetyl choline (cont. expressed as the chloride) Potassium chloride Potassium chloride Sodium chloride Distilled H,O

0.9 g/l00 ml 10-s g/ml 2.5 x 1O-5 g/ml 5 x 10-b g/ml 0.12 all00 ml O~OSS~g/lOO ml 0.2 g/l00 ml

Histamine (cont. expressed as the base) 1O-3 g/ml Histamine (cont. expressed as the base) lo-’ g/ml Histamine (cow. expressed as the base) lO-B g/ml 5-Hydroxytryptamine creatinine SO, (as salt) lO-5 g/ml 5-Hydroxytryptamine creatinine SO, (as salt) 1O-B g/ml 5-Hydroxytryptamine creatinine SO, (as salt) lo-’ g/ml Tryptamine (expressed as the salt) lo-’ g/ml

was followed by deepening of the cavity until the pulp tissue was exposed, usually

at one of the cornua. The solutions were then applied as before but to the exposed

Pulp. After this second group of tests, a local anaesthetic was injected, the teeth were

removed and prepared for histological sectioning to confirm that the pulp tissue

had been exposed.

RESULTS

In none of the subjects was any of the solutions successful in evoking pain when applied to the dentine. In all except one subject all the solutions except water, NaCl

and tryptamine caused pain of varying intensity when applied to the exposed pulp

and the pattern of pain experienced followed that described by ARMSTRONG et al. in the blister-base preparation. No explanation can be offered for the failure of water,

hypotonic NaCl and tryptamine, although they cause pain in the blister preparation. The absence of a painful response to every solution when applied to the pulp of

one subject was probably due to the fact, revealed by histological examination, that the pulp exposure was covered by tooth debris.

DISCUSSION

When human dentine is exposed in cavity preparation it is a common observation that stimulation by mechanical, thermal and chemical means can cause pain. The simplest hypothesis to explain this is that the dentine receives an innervation from the pulpal nerves by means of branches coursing through the dentinal tubules. Although there has been a dispute for many years concerning the histological support for this hypothesis and it has not yet been settled, it was expected that if such a receptor mechanism existed it would respond to stimulation by chemical substances capable of evoking pain elsewhere. The failure of pain-producing substances when

CHEMICALEXCITANTS OFPAIN IN HUMAN DENTINEAND DENTALPULP 415

applied to dentine suggests two possible explanations. First, there are no nervous elements in the dentine and when pain is evoked it is due to stimulation of receptor mechanisms in the pulp by a disturbance transmitted through the tubules by non- nervous means. Second, there are receptor mechanisms in the dentine which can be stimulated indirectly, but cannot be reached for direct stimulation by chemical agents because of some barrier to diffusion in the tubules. This question of diffusion is investigated in a subsequent paper.

Acknowledgements-The authors are very grateful to the subjects who have taken part in this investigation. This investigation was supported by U.S.P.H.S. Research Grant D-1037 from the National Institute of Dental Research, National Institutes of Health, Bethesda, Maryland, U.S.A.

REFERENCES

ANDERSON, D. J., CURWEN, M. P. and HOWARD, L. V. 1958. The sensitivity of human dentine. J. dent. Res. 31, 669-617.

ARMSTRONG, D., DRY, R. M. L., KEELE, C. A. and MARKHAM, J. W. 1953. Observations on chemical excitants of cutaneous pain in man. J. Physiol. 120, 326-351.