Fig. 6.

In bicarbonate-buffered solution astrocytes contribute to chemosensitivity by a purinergic mechanism. A: trace of firing rate shows the response of an RTN neuron to increases in CO₂ from 5 to 15% in bicarbonate buffer alone or in the presence of PPADS. An initial exposure to 15% CO₂ increased the firing rate about 2.8 Hz. After returning to 5% CO₂, exposure to PPADS (100 μM) decreased baseline firing rate, suggesting that in bicarbonate buffer RTN chemoreceptors receive tonic excitatory purinergic input. A depolarizing current (∼1 nA) was delivered to the neuron to increase firing rate to near control levels. In the continued presence of PPADS (with baseline activity adjusted by DC current injection to near control levels), a second exposure to 15% CO₂ increased the firing rate about 1.5 Hz. After washing, PPADS CO₂ sensitivity returned to initial levels. Bar graph on the right summarizes the firing rate response of RTN neurons (n = 9) to 15% CO₂ in control and PPADS (*P < 0.05). B: firing rate trace shows the response of a RTN chemoreceptor to 15% CO₂ in bicarbonate-buffered solution alone and in the presence of suramin (100 μM). An initial exposure to 15% CO₂ increased the firing rate about 2.7 Hz. As was observed with PPADS, exposure to suramin (100 μM) in 5% CO₂ decreased baseline firing rate. A depolarizing current (∼1 nA) was delivered to the neuron to increase firing rate to near control levels. In the continued presence of suramin (with baseline activity adjusted by DC current injection to near control levels), a second exposure to 15% CO₂ increased firing rate about 1.7 Hz. After washing, suramin CO₂ sensitivity returned to initial levels. The asterisk designates a 5-min break. Bar graph on the right summarizes the firing rate response of RTN neurons (n = 5) to 15% CO₂ in control and suramin (*P < 0.05).