Long term variations of the jaw opening reflex (J.O.R.) induced by tooth pulp stimulation (T.P.S.) have been investigated in the awake, freely moving rat. This study was specially centered on variations which could occur during the utilization of the J.O.R. as a test for analgesic drugs. Bipolar electrodes were implanted in the inferior incisor tooth pulp for stimulation and in digastric muscle for recording. J.O.R. recordings were subsequently integrated. Long term variations of J.O.R. were then observed during uninterrupted T.P.S. (1 Hz) alone or during subcutaneous (s.c.) injection, gavage or foot shock in addition to T.P.S.: --8 out of a total of 24 rats, observed during long uninterrupted T.P.S., showed a decrease of the J.O.R. The mean of reflex values, 15 min later, was 74 +/- 6% (n = 8) of the prestimulation level; --s.c. injection of saline, light handling, experimentator's brisk irruption, or sudden noise evoked a total disappearance of the reflex which completely returned within 30 sec; --gavage of arabic gum syrup gave either: . the same transient disappearance as for the s.c. injection in 6 rats; . a progressive, 28 min +/- 11 (n = 6), return to the initial value; . a progressive return to a subcontrol value of 52 +/- 4% (n = 7); --a 30 min foot shock delivered for 10 sec twice a minute at the infra-vocalization threshold evoked a marked decrease of the reflex reaching to 68 +/- 6% (n = 10) of the control (prestimulation level).
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