HIPPOCAMPAL ACTIVITY DURING TRANSIENT RESPIRATORY EVENTS IN THE FREELY BEHAVING CAT

We measured dorsal hippocampal activity accompanying sighs and apnea u sing reflectance imaging and electrophysiologic measures in freely beh aving cats. Reflected 660-nm light from a 1-mm(2) area of CA1 was capt ured during sighs and apnea at 25 Hz through a coherent image conduit coupled to a charge coupled device camera. Sighs and apnea frequently coincided with state transitions. Thus, state transitions without apne a or sighs were separately assessed to control for state-related activ ity changes. All dorsal hippocampal sites showed discrete regions of a ctivation and inactivation during transient respiratory events. Imaged hippocampal activity increased 1-3 s before the enhanced inspiratory effort associated with sighs, and before resumption of breathing after apnea. State transitions lacking sighs and apnea did not elicit analo gous optical activity patterns. The suprasylvian cortex, a control for site, showed no significant overall reflectance changes during phasic respiratory events, and no discrete regions of activation or inactiva tion. Spectral estimates of hippocampal electroencephalographic activi ty From 0-12 Hz showed significantly increased power at 3-4 Hz rhythmi cal slow activity before sighs and apnea, and increased 5-6 Hz rhythmi cal slow activity power during apnea, before resumption of breathing. Imaged activity and broadband hippocampal electroencephalogram power d ecreased during sighs. We propose that increased hippocampal activity before sigh onset and apnea termination indicates a role for the hippo campus in initiating inspiratory effort during transient respiratory e vents.