Cell-cell coupling between CO₂-excited neurons in the dorsal medulla oblongata

Anatomically coupled neurons (17 of 137) and non-coupled neurons (120 of 137), in and near the nucleus tractus solitarius and dorsal motor nucleus (i.e. solitary complex), were studied by rapid perforated patch recording in slices (rat, 150-350 μm thick, postnatal day 0-21) before, during and after exposure to hypercapnic acidosis. Anatomical coupling refers to the intercellular transfer of Lucifer Yellow and Biocytin into adjoining neurons, presumably via gap junctions [see Dean et al. (1997) Neuroscience 80, 21- 40]. Eighty-six per cent of the anatomically coupled neurons (12 of 14) were depolarized by hypercapnic acidosis, a response referred to as CO₂ excitation or CO₂ chemosensitivity. In all, 28% (12 of 43) of the CO₂- excited neurons were anatomically coupled to at least one other neuron. None (0 of 39) of the CO₂-inhibited neurons were anatomically coupled, and only 4% (two of 46) of the CO₂-insensitive neurons were anatomically coupled. Increasing the fractional concentration of CO₂ from five to 10 and 15% in constant bicarbonate (26 mM) decreased intracellular pH (control 7.3-7.4, 22- 25°C) by ~ 1.0 and 1.5 pH units, respectively, as measured using the pH- sensitive fluorescent dye, 2',7'-bis (2-carboxyethyl)-5,6-carboxyfluorescein. Nine of the anatomically coupled neurons (six CO₂-excited, one CO₂- insensitive and two unidentified) exhibited spontaneous electrotonic postsynaptic potential-like activity, suggesting that they were also electrotonically coupled. During hypercapnic acidosis, the amplitudes of electrotonic postsynaptic potentials were unchanged, concomitant with small changes in input resistance. The frequency of electrotonic postsynaptic potentials increased during hypercapnic acidosis in many anatomically coupled neurons (eight of nine), indicating that both neurons of the coupled pair were stimulated. Cell-cell coupling occurred preferentially in and between CO₂-excited neurons of the solitary complex. Further, CO₂-excited neurons were not electrotonically uncoupled during intracellular acidosis, in contrast to the effect that decreased intracellular pH has on many other types of coupled cells. It was not determined whether anatomical coupling was affected by hypercapnic acidosis since dye mixture was always administered under normocapnic conditions. The high correlation between anatomical coupling, electrotonic coupling activity and CO₂-induced depolarization suggests that cell-cell coupling is an important electroanatomical feature in CO₂-excited neurons of the solitary complex. CO₂-excited neurons have been hypothesized to function in central chemoreception for the cardiorespiratory control systems, suggesting that cell-cell coupling may contribute in part to central chemoreception of CO₂ and H⁺.