MEASUREMENT OF ARTHRITIC PAIN AND EFFECTS OF ANALGESICS IN THE ADJUVANT-TREATED RAT

Katsumi HIROSE and Hirokuni JYOYAMA Shionogi Research Laboratory, Shionogi &Co., Ltd., Fukushima-ku, Osaka

Received for publication May 21, 1971

Adjuvant-induced arthritis in rat is widely used in laboratories for evaluating of the antirheumatic activity of drugs, since the pathophysiological syndromes in arthritic rats are analogous to those in rheumatoid arthritis patients in many respects, and since the method is simple and gives satisfactory reproducibility of results (1-3). Many reports have dealt with the anti-inflammatory activity of drugs on adjuvant-induced arthritis, but very little study has been done on the analgesic activity of a single dose of drug in arthritic rat. The great reduction of the reaction threshold to a mechanical stimulus in the arthritic foot of the rat prompted us to utilize this hyperesthesiafor testing analgesic activity of drug, especially non-narcotic analgesics.
In the present report a simple method for the determination of the pain threshold in arthritic foot is described and the effects of several non-narcotic analgesics,hydrocortisone and morphine are studied.

METHODS Female Sprague-Dawleyrats weighing about 200 g were used. Each group consisted of 8 animals. The adjuvant was a 0.6 % suspension of dead tubercle bacilli (human strains, Aoyama B) in liquid paraffin. The animals were given a single 0.1 ml intradermal injection of the adjuvant in the distal region of the tail. Animal exhibiting a typical arthritis in the foot by the 15th to 18th day after injection were chosen for the test. The pain threshold of the arthritic foot was determined by compressing the ankle joint with intestinal forceps. The force of the tip of the forceps was gradually increased until the animal reacted to the

FIG. 1. Arrangement of apparatus for measuring pain threshold. stimulus with squeaking or struggling. A strain gauge attached to one blade of the forceps showed a distortion proportional to the applied force. The measurement of the gauge was recorded on the Y-axis of a pen-writing recorder (Fig. 1). The value thus obtained was calibrated and expressed as the pain threshold (PT) in grams. The drugs tested were orally administered on a mg/kg body weight basis.

RESULTS

The pain threshold was 290±27.5 g (standard error) in normal rats and 53±3.8 g in arthritis rats. Changes of the PT induced by repeated pinching of the same foot could be observed only within a statistically insignificant range.

Aminopyrine, aspirin, salicylainide and benzydamine The PT increased to about twice the value of untreated animals I hour after 100 mg

TABLE 1. Increase of pain threshold in arthritic foot following administration of drugs.

* significant increase at p<0 .05. **at p<0.01. a) subcutaneous administration. of arninopyrine, whereas no significantincrease of the PT was observed after the same dose of aspirin, benzydamine and salicylamide. With a dose of 250 mg of these three drugs, a significantanalgesic activity with the PT of 2 to 3 times the control could be observed. The analgesic effect usually lasted for over 5 hours in all cases except salicylamide whose effect was short-acting and disappeared within 3 hours after injection. Phenylbutazone,indomethacin and inefenainic acid The PT was raised to 2 to 3 times the control value following 5 to 10 mg of phenylbu tazone, and about 4 times following 25 mg. The duration of action was over 5 hours. A small dose of indornethacin,0.5 to 1 mg, was enough to increase the PT to 2 to 3 times the control value and 2.5 mg produced a 5 to 6 fold increase of the PT. Following 10 to 25 mg of mefenamic acid a 2.5 to 3 fold increase of the PT was observed. Hydrocortisone and morphine A significanteffect could be seen when 25 mg (po) or 10 mg (sc) of hydrocortisone was administered but no effect was detected by 5 mg sc. Morphine significantlyincreased the PT at a dose of 1 mg, but its action had disappeared by 4 hours after injection. These results are summarized in Table 1. In all experiments,changes of edematous volume in the arthritic foot was simultaneously measured and it was found that changes in the PT did not parallel changes in foot volume. DISCUSSION The present results revealed that the so-called antirheumatoid agents were very effective in alleviating pain in the arthritic foot of rat, whereas the so-called antipyretics showed a weak or moderate analgesic acitivity. Such a clear difference of analgesic potency exist ing between antirheumatoids and antipyretics could not be detected by other methods for testing non-narcotic anglesics; Collier et al. (4) reported that phenylbutazone and mefenamic acid were less effective than aminopyrine in suppressing acetylcholineinduced writhing in mice, Randall and Selitto (5) observed almost the same analgesic potency for aminopyrine and phenylbutazonein yeast-inducedinflamed foot of rats, Winter and Flataker (6) showed that mefenamic acid was slightly stronger than aspirin in the test using yeast induced inflamed foot of rats, and Silvestrini et al. (7) reported that benzydamine, phenyl butazone and cortisone acetate displayed anaglesic activity in silver nitrate arthritis test, the activity of the latter two drugs being weaker and very inconstant. From these phar macological data, it is conceivable that the pain elicited in local inflammatory tissues is qualititatively or quantitatively different from adjuvant-induced arthritic pain, although the reason underlying this pathophysiological difference remains to be clarified. In the present study indomethacinwas more potent than phenylbutazone and mefenamic acid is slightly less potent. This order of activity is the same as that reported by Winter and Nuss (2) who tested the anti-inflammatory activity of these drugs in adjuvant-induced arthritis rats. However, the following facts suggest that the analgesic action of antirheu matoids tested here is independent to their anti-inflammatory action. First, these drugs showed a significant analgesic effect on the arthritic foot 1 hour after a single injection, whereas no decrease of edematous volume of the foot was detected throughout the experi ment. Second, hydrocortisone is known to be a most potent antirheumatoid agent but its analgesic action was least among the antirheurnatoids tested. Therefore, it is reasonable to consider that observed analgesic action is not subsequent to the anti-inflammatory action of the drugs.

SUMMARY A new method for testing analgesic activity of drugs using hyperesthesia in the arthritic foot of adjuvant-treated rats was explored and the effects of several non-narcotic analgesics, hydrocortisone, and morphine were studied. Antirheumatoids (indomethacin, phenylbutazone, mefenamic acid) are more effective in alleviatingpain in the arthritic foot of rat than antipyretics (arninopyrine, aspirin, benzy damine, salicylamide). The order of potency of the drugsas wsa follows; indomethacin> phenylbutazone>mefenamic acid>aminopyrine>aspirin, benzydamine>.salicylamide. Morphine was also effective but hydrocortisone was less effective than non-steroidal antirheumatoids.

REFERENCES 1) PEARSON,C.M.: Proc. Soc. exp. Biol. Wed. 91, 95 (1956) 2) WINTER,C.A. ANDNUSS, G.W.: ArthritisRheum. 9, 394 (1966) 3) GRAEME,M.L., FABRY,E. ANDSIGG, E.B.: J. Pharmac.exp. Ther.153, 373 (1966) 4) COLLIER,H.O.J., DINNEEN,L.C., JOHNSON,C.A. ANDSCHNEIDER, C.: Br. J. Pharmac. Chemother.32, 295 (1968) 5) RANDALL,L.O. ANDSELITTO, J.J.: Archs int. Pharmacodyn.Thér. 111, 409 (1957) 6) WINTER,C.A. ANDFLATAKFR, L.: J. Pharmac.exp. Ther.148, 373 (1965) 7) SILVESTRINI,B., GARAU, A., POZZATTI,C., ANDCIOLI, V.: ArzneimittelForsch. 16, 59 (1966)