Modulation of outer hair cell compliance and force by agents that affect hearing

Electrical stimulation of area infraradiata in the rat evokes transient changes in arterial pressure, but the locations that evoke these responses have not been mapped by neurochemical methods. To localize more specifically the regions of area infraradiata that modify cardiovascular activity, the present study measured cardiovascular responses to localized chemical stimulation of neurons in area infraradiata of urethane-anesthetized rats (n = 21). Microinjections (50-200 nl) of the glutamate agonist D,L-homocysteic acid into area infraradiata evoked both increases and decreases in arterial pressure and heart rate. Injections in the ventral subdivisions of rostral area infraradiata (IRa alpha and IRb alpha) produced cardiovascular responses with the highest probability and greatest magnitude. Of 53 injections in this area, 18 decreased arterial pressure and heart rate, whereas 4 increased arterial pressure and heart rate. In contrast to the results from the ventral subdivision of rostral infraradiata cortex, injections of D,L-homocysteic acid in the dorsal subdivision of rostral infraradiata cortex (IRc alpha) or any of the caudal subdivisions of area infraradiata (IR beta) produced less consistent changes in arterial pressure. To demonstrate that the general anesthesia did not significantly alter the evoked responses in this study, similar injections of D,L-homocysteic acid were made into area infraradiata of unrestrained, conscious rats (n = 10) and the responses were similar to the responses evoked in urethane-anesthetized rats. These results indicate that the ventral subdivisions of rostral area infraradiata (IRa alpha and IRb alpha) are more involved in cardiovascular regulation than other areas of infraradiata cortex (IRc alpha and IR beta), and that both pressor and depressor sites are present in both areas.