Submitted on May 1, 2009
Revised on June 15, 2009
Accepted on June 18, 2009
Chemosensory responses to CO2 in multiple brainstem nuclei determined using a voltage-sensitive dye in brain slices from rats
Joseph S Erlichman1, Andrew C. Boyer1, Patrick Reagan1, Robert W. Putnam2, Nick A. Ritucci2, and J. C. Leiter3*
1 St. Lawrence University
2 Wright State University Boonshoft School of Medicine
3 Dartmouth Medical School
* To whom correspondence should be addressed. E-mail: james.c.leiter{at}dartmouth.edu.
We used epifluorescence microscopy and a voltage sensitive dye, di-8-ANEPPS, to study changes in membrane potential during hypercapnia with or without synaptic blockade in chemosensory brainstem nuclei: the locus coeruleus (LC), the nucleus of the solitary tract (NTS), lateral paragigantocellularis nucleus (PGCL), raphé pallidus and raphé obscurus, and in putative non-chemosensitive nuclei, the gigantocellularis reticular nucleus (GCR) and the spinotrigeminal nucleus (STG). We studied the response to hypercapnia in LC cells to evaluate the performance characteristics of the voltage-sensitive dye. Hypercapnia depolarized many LC cells, and the voltage responses to hypercapnia were diminished, but not eradicated by synaptic blockade (there were intrinsically CO2 sensitive cells in the LC). The voltage response to hypercapnia was substantially diminished after inhibiting fast Na+ channels with tetrodotoxin (TTX). Thus, action potential related activity was responsible for most of the optical signal that we detected. We systematically examined CO2 sensitivity among cells in brainstem nuclei to test the hypothesis that CO2 sensitivity is a ubiquitous phenomenon, not restricted to nominally CO2 chemosensory nuclei. We found intrinsically CO2 sensitive neurons in all the nuclei that we examined; even the non-chemosensory nuclei had small numbers of intrinsically CO2 sensitive neurons. However, synaptic blockade significantly altered the distribution of CO2 sensitive cells in all of the nuclei so that the cellular response to CO2 in more intact preparations may be difficult to predict based on studies of intrinsic neuronal activity. Thus, CO2 sensitive neurons are widely distributed in chemosensory and non-chemosensory nuclei, and CO2 sensitivity is dependent on inhibitory and excitatory synaptic activity even within brain slices. Neuronal CO2 sensitivity important for the behavioral response to CO2 in intact animals will, therefore, be determined as much by synaptic mechanisms and patterns of connectivity throughout the brain as by intrinsic CO2 sensitivity.
