Effects, in the rat, of various stressing procedures on the jaw-opening reflex induced by tooth-pulp stimulation

Jaw-opening reflex (JOR) induced by a repetitive tooth-pulp stimulation delivered at 1 Hz was observed in awake, freely-moving rats. Modifications of the reflex responses of the digastric muscle were produced by foot-shock stimulation, by an oesophageal administration of placebo and a subcutaneous injection of saline. Subcutaneous injection of saline evoked a total disappearance of the reflex which returned within 30 s. Foot shock delivered for 10 s and repeated twice a minute for 30 min of intensity, just below the vocalization threshold, evoked a marked decrease of the reflex to 60 +/- 6 per cent of the control value (n = 10). Administration of arabic gum by oesophageal tube gave either: the same transient disappearance as for the subcutaneous 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 per cent (n = 7). The application of two successive stimuli resulted in a more pronounced JOR decrease. The response to electrical foot shock can be explained by a central analgesia induced by stress.