Reflex changes in sympathetic activity affect the tooth tap-digastric reflex in rabbits.

We have previously shown that, in the anaesthetized rabbit, electrical stimulation of the sympathetic trunks inhibits the digastric opening reflex, as elicited by standardized tooth taps and monitored by integrated EMG. The site of action was most likely in the periodontal ligament, since the digastric reflex evoked by electrical stimulation of the inferior alveolar nerve was unaffected. In the present investigation we have studied the effects on the reflex of more naturally occurring changes in sympathetic activity, as induced by the baroreceptor reflex. The sympathetic discharge was increased by withdrawal of blood from the femoral vein, and reduced by electrical stimulation of the aortic baroreceptor nerve. Preliminary results in five rabbits showed that haemorrhage reduced the digastric reflex by more than 10% in 7 out of 13 tests (median 19%), while aortic nerve stimulation increased the reflex in 11 out of 13 tests (median 160%). It is therefore concluded that the tooth tap-digastric reflex may be affected also by more naturally occurring changes in sympathetic discharge.