A PARAMETRIC ANALYSIS OF THE EFFECTS OF CARDIOPULMONARY VAGAL ELECTROSTIMULATION ON THE DIGASTRIC REFLEX IN CATS

The thoracic trunk and the cardiac branch of the vagus were stimulated electrically in chloralose-anesthetized cats. The experiments were conducted to determine the parameters of vagal afferent stimulation (VAS) capable of producing an inhibition of the digastric reflex (DGR), to assess the duration of this inhibition, and to test whether endogenous opioids mediate the inhibitory effects. In experiments using intermittent trains of pulses, the effects of pulse number (1, 2, 7 or 35 pulses), frequency (13, 66 or 333 Hz), intensity (0.1, 0.5, 1, 2, 3, 4 or 5 mA), and duration (1 or 3 ms) were evaluated. A 7 pulse train (3 mA) was sufficient to produce maximal inhibition (77 +/- 7%) of the tooth-pulp stimulation-evoked DGR regardless of the pulse duration or frequency. These effects were mediated by vagal afferents since stimulation of the central end produced as much inhibition as stimulation of the intact nerve. VAS also significantly reduced the DGR when elicited by tooth-pulp intensities at Ix -5x threshold. In experiments using 90 s of continuous VAS, 16 combinations of frequency and intensity yielded a threshold intensity for DGR inhibition between 0.1 and 0.5 mA and a threshold frequency at 2 Hz. Maximal DGR inhibition was produced at 5 Hz-0.5 mA by VAS. Reflex inhibition occurred within 10 s and outlasted VAS for longer than 60 s. Opiate-receptor blockade did not alter VAS inhibition of the DGR and, thus, opioids are not likely to mediate VAS-induced digastric inhibition. These results demonstrate that vagal afferent stimulation is capable of producing a profound suppression of the DGR and further support a role of vagal afferents in nociceptive modulation.