Intrinsic optical signals in respiratory brain stem regions of mice: Neurotransmitters, neuromodulators, and metabolic stress

In the rhythmic brain stem slice preparation, spontaneous respiratory activ ity is generated endogenously and can be recorded as output activity from h ypoglossal XII rootlets. Here we combine these recordings with measurements of the intrinsic optical signal (IOS) of cells in the regions of the peria mbigual region and nucleus hypoglossus of the rhythmic slice preparation. T he IOS, which reflects changes of infrared light transmittance and scatteri ng, has been previously employed as an indirect sensor for activity-related changes in cell metabolism. The IOS is believed to be primarily caused by cell volume changes, but it has also been associated with other morphologic al changes such as dendritic beading during prolonged neuronal excitation o r mitochondrial swelling. An increase of the extracellular K+ concentration from 3 to 9 mM, as well as superfusion with hypotonic solution induced a m arked increase of the IOS, whereas a decrease in extracellular K+ or superf usion with hypertonic solution had the opposite effect. During tissue anoxi a, elicited by superfusion of N-2-gassed solution, the biphasic response of the respiratory activity was accompanied by a continuous rise in the IOS. On reoxygenation, the IOS returned to control levels. Cells located at the surface of the slice were observed to swell during periods of anoxia. The r egion of the nucleus hypoglossus exhibited faster and larger IOS changes th an the periambigual region, which presumably reflects differences in sensit ivities of these neurons to metabolic stress. To analyze the components of the hypoxic IOS response, we investigated the IOS after application of neur otransmitters known to be released in increasing amounts during hypoxia. In deed, glutamate application induced an IOS increase, whereas adenosine slig htly reduced the IOS. The IOS response to hypoxia was diminished after appl ication of glutamate uptake blockers, indicating that glutamate contributes to the hypoxic IOS. Blockade of the Na+/K+-ATPase by ouabain did not provo ke a hypoxia-like IOS change. The influences of K-ATP channels were analyze d, because they contribute significantly to the modulation of neuronal exci tability during hypoxia. IOS responses obtained during manipulation of K-AT P channel activity could be explained only by implicating mitochondrial vol ume changes mediated by mitochondrial K-ATP channels. In conclusion, the hy poxic IOS response can be interpreted as a result of cell and mitochondrial swelling. Cell swelling can be attributed to hypoxic release of neurotrans mitters and neuromodulators and to inhibition of Na+/K+-pump activity.