Hypercapnia modulates synaptic interaction of cultured brainstem neurons

CO2 is an important metabolic product whose concentrations are constantly monitored by CO2 chemoreceptors. However, the high systemic CO2 sensitivity may not be achieved by the CO2 chemoreceptors without neuronal network processes. To show modulation of network properties during hypercapnia, we studied brainstem neurons dissociated from embryonic rats (P17-19) in multielectrode arrays (MEA) after initial period (3 weeks) of culture. Spike trains of 33,622 pairs of units were analyzed using peri-event histograms (PEH). The amplitude of peri-central peaks between two CO2-stimulated units increased and the peak latency decreased during hypercapnia. Similar enhancement of synaptic strength was observed in those sharing a common input. These phenomena were not seen in CO2-unresponsive neurons. The amplitude of peri-central peaks between two CO2 inhibited units also increased without changing latency. Over 60% CO2-stimulated neurons studied received mono-/oligosynaptic inputs from other CO2-stimulated cells, whereas only ∼10% CO2-unresponsive neurons had such synaptic inputs. A small number of brainstem neurons showed electrical couplings. The coupling efficiency of CO2-stimulated but not CO2-unresponsive units was suppressed by ∼50% with high PCO2. Inhibitory synaptic projections were also found, which was barely affected by hypercapnia. Consistent with the strengthening of excitatory synaptic connections, CO2 sensitivity of post-synaptic neurons was significantly higher than presynaptic neurons. The difference was eliminated with blockade of presynaptic input. Based on these indirect assessments of synaptic interaction, our PEH analysis suggests that hypercapnia appears to modulate excitatory synaptic transmissions, especially those between CO2-stimulated neurons.