THE TONIC SYMPATHETIC INPUT TO THE COCHLEAR VASCULATURE IN GUINEA-PIG

Vascular tone is an essential component in maintaining steady regional blood flow and dynamic responsiveness of a vascular bed. Sympathetic innervation can contribute to vascular tone. Although certain studies have reported evoked changes in cochlear blood flow (CBF) with activat ion of the sympathetic fibers to the cochlear vasculature, other studi es have failed to show evidence of sympathetic contribution to CBF reg ulation when the cervical sympathetic fibers were unilaterally section ed. We hypothesized that the bilateral 'sympathectomy of the stellate ganglia' would remove sufficient sympathetic input to the cochlea to y ield a change in CBF resting level. To test this hypothesis a new tech nique was used to expose the stellate ganglia (SG) bilaterally and ind uce a chemical sympathectomy. We observed that unilateral SG blockade with 2 mu l of 4 mM lidocaine hydrochloride on either side produced a 5-10% increase in CBF, which recovered to baseline during the followin g 2 min. A subsequent blockade of the contralateral SG produced a rapi d 25-35% increase, which then recovered partially during the following 3-4 min, remaining 5-15% above the baseline over a 20 min measurement period. Superior cervical ganglion transection did not affect CBF. Ou r results provide evidence for the existence of a tonic sympathetic co mponent in the control of vascular tone in guinea pig cochlea. This ne ural effect is derived bilaterally from SG. This result is consistent with previous anatomical studies showing the bilateral innervation of the cochlea by the SG sympathetic fibers and with previous physiologic al studies on the bilaterality of evoked changes in CBF due to electri c stimulation of SG.