Electrophysiological evidence for reciprocal insulo-insular connectivity of baroreceptor-related neurons

The recent indications of specialized lateralization of cardiovascular regulation within the right and left posterior insular cortex of the rat, suggest the possibility of transcallosal connectivity between these regions. This has not been previously demonstrated using physiological techniques. Extracellular neural recordings in 34 urethane anesthetized male Sprague-Dawley rats demonstrated reciprocal interinsular antidromic and orthodromic activation, elicited with similar median onset latencies (18 ms). The corresponding conduction velocity of these fibers (0.6 m/s) suggests that they may be unmyelinated. Many of the cells showing interhemispheric connectivity also responded to baroreceptor activation, further emphasizing the connectivity pattern in baroreceptor-related units. Both 1 and 25 Hz microstimulation of the contralateral insula indicated that the most frequent orthodromic response was inhibitory, either alone or as part of a biphasic pattern including activation. Chemical stimulation of the insula using L-glutamate was associated with both excitatory and inhibitory orthodromic activation of the contralateral posterior insula, confirming that the orthodromic electrical stimulation was not solely due to activation of fibers of passage. These data suggest that the two insulae may communicate with each other to integrate and balance cardiovascular function between hemispheres.