Chronic hypoxia suppresses the CO2 response of solitary complex (SC) neurons from rats

Abstract

We studied the effect of chronic hypobaric hypoxia (CHx; 10-11% O(2)) on the response to hypercapnia (15% CO(2)) of individual solitary complex (SC) neurons from adult rats. We simultaneously measured the intracellular pH and firing rate responses to hypercapnia of SC neurons in superfused medullary slices from control and CHx-adapted adult rats using the blind whole cell patch clamp technique and fluorescence imaging microscopy. We found that CHx caused the percentage of SC neurons inhibited by hypercapnia to significantly increase from about 10% up to about 30%, but did not significantly alter the percentage of SC neurons activated by hypercapnia (50% in control vs. 35% in CHx). Further, the magnitudes of the responses of SC neurons from control rats (chemosensitivity index for activated neurons of 166+/-11% and for inhibited neurons of 45+/-15%) were the same in SC neurons from CHx-adapted rats. This plasticity induced in chemosensitive SC neurons by CHx appears to involve intrinsic changes in neuronal properties since they were the same in synaptic blockade medium.

Figure 1

The average increase in hematocrit for each of the 16 groups of rats exposed to CHx. Hematocrit was recorded in both chamber control adult rats and adult rats adapted to CHx to verify that hypoxia had been achieved in the adult rats adapted to CHx. The hematocrit was 47.2 ± 0.2% for chamber control animals (n=44). The average change in hematocrit (18.7 ± 0.8% for all CHx animals (n=79) (P<0.0001)) was significantly increased in all sixteen groups of adult rats adapted to CHx. The height of each bar represents the mean change in hematocrit for each group adapted to CHx and the error bars represent 1 SEM (n=4-6 for each group).

Figure 2

A: The pH_i and firing rate responses of an individual SC neuron that was activated by hypercapnic acidosis from a chamber control adult rat. B: The pH_i and firing rate responses of an individual SC neuron that was activated by hypercapnic acidosis from an adult rat adapted to CHx. The top panel shows the experimental protocol used. The second panel shows the pH_i response of the SC neuron to hypercapnic acidosis over time, which was a maintained acidification with a lack of pH_i recovery. Notice that once the hypercapnic solution was removed, pH_i returned back towards initial pH_i. The bottom panel shows the firing rate response of the SC neuron to hypercapnic acidosis over time, which was a reversible increase in firing rate in response to hypercapnic acidosis.

Figure 3

A: The pH_i and firing rate responses of an individual SC neuron that was inhibited by hypercapnic acidosis from a chamber control adult rat. B: The pH_i and firing rate responses of an individual SC neuron that was inhibited by hypercapnic acidosis in an adult rat adapted to CHx. The top panel shows the experimental protocol used. The second panel shows the pH_i response of the SC neuron to hypercapnic acidosis over time, which was a maintained acidification with a lack of pH_i recovery. Notice that once the hypercapnic solution was removed, pH_i returned back towards initial pH_i. The bottom panel shows the firing rate response of the SC neuron to hypercapnic acidosis over time, which was a reversible decrease in response to hypercapnic acidosis.

Figure 4

A: The percentage of SC neurons from control adult rats (18 neurons from 9 rats) (white bar) and adult rats adapted to CHx (30 neurons from 20 rats) (black bar) that were activated by hypercapnic acidosis. The N values are denoted on each bar (39 neurons from 10 control rats and 87 neurons from 27 CHx rats). There is no significant change in the percentage of SC neurons activated by hypercapnia in CHx-adapted rats. B: The percentage of SC neurons from control adult rats (3 neurons from 3 rats) (white bar) and adult rats adapted to CHx (24 neurons from 18 rats) (black bar) that were inhibited by hypercapnic acidosis. The N values are denoted on each bar. The remaining, non-chemosensitive, neurons amounted to 18 neurons from 10 control rats and 33 neurons from 17 CHx rats. Notice that the percentage inhibited by hypercapnic acidosis is significantly increased by CHx (* indicates P = 0.017). C: The chemosensitivity index of SC neurons from control adult rats and adult rats adapted to CHx that were activated by hypercapnic acidosis. D: The chemosensitivity index of SC neurons from control adult rats and adult rats adapted to CHx that were inhibited by hypercapnic acidosis. Notice that CHx does not affect the chemosensitivity index of SC neurons activated (C) or inhibited (D) by hypercapnic acidosis. The height of each bar represents the mean chemosensitivity index for that group and the error bars represent 1 SEM.

Figure 5

A: Effect of synaptic blockade medium (SNB—11.4 mM Mg²⁺ and 0.2 mM Ca²⁺) on basal firing rate of an SC neuron from a control adult rat. Exposure to SNB solution is denoted by the arrow. Notice that SNB causes an increase in basal firing rate. B: Effect of SNB on basal firing rate of an SC neuron from an adult rat adapted to CHx. Exposure to SNB solution is denoted by the arrow. Notice that SNB causes an increase in basal firing rate. C: The average basal firing rate for SC neurons from control animals (white bar) significantly increases in the presence of SNB (22 neurons from 7 rats) (gray bar) (* indicates P = 0.0035). The average basal firing rate for SC neurons from adult rats adapted to CHx (black bar) also significantly increases in the presence of SNB (22 neurons from 9 rats) (gray bar) (* indicates P = 0.0004). The height of each bar represents the mean firing rate for that group and the error bars represent 1 SEM.

Figure 6

A: The percentage of SC neurons from control adult rats that were activated by hypercapnic acidosis in the absence (7 neurons from 4 rats) (white bar) and presence of synaptic blockage medium (SNB—11.4 mM Mg²⁺ and 0.2 mM Ca²⁺) (8 neurons from 6 rats) (gray bar) and SC neurons from adult rats adapted to CHx that were activated by hypercapnic acidosis in the absence (6 neurons from 6 rats) (black bar) and the presence (gray bar) of SNB (6 neurons from 6 rats). The N values are denoted on each bar (14 neurons from 6 control rats and 20 neurons from 9 CHx rats). B: The percentage of SC neurons from control adult rats that were inhibited by hypercapnic acidosis in the absence (2 neurons from 2 rats) (white bar) and presence (2 neurons from 2 rats) of SNB (gray bar) and of SC neurons from adult rats adapted to CHx that were inhibited by hypercapnic acidosis in the absence (5 neurons from 4 rats) (black bar) and the presence (6 neurons from 3 rats) (gray bar) of SNB. The N values are denoted on each bar. The remaining, non-chemosensitive, neurons amounted to 5 neurons from 4 control rats and 7 neurons from 5 CHx rats. Notice that SNB does not affect the percentage of neurons that respond within control or CHx animals. C: The chemosensitivity index of SC neurons from control adult rats that were activated by hypercapnic acidosis in the absence (white bar) and presence (gray bar) of SNB and of SC neurons from adult rats adapted to CHx that were activated by hypercapnic acidosis in the absence (black bar) and presence (gray bar) of SNB. D: The chemosensitivity index of SC neurons from control adult rats that were inhibited by hypercapnic acidosis in the absence (white bar) and presence (gray bar) of SNB and of SC neurons from adult rats adapted to CHx that were inhibited by hypercapnic acidosis in the absence (black bar) and presence (gray bar) of SNB. The height of each bar represents the mean chemosensitivity index for that group and the error bars represent 1 SEM. There are no error bars for the CI of inhibited neurons from control animals in the absence and presence of SNB because the two inhibited neurons in the absence and presence of SNB had the same CI. Notice that CHx does not affect the chemosensitivity index of SC neurons activated or inhibited by hypercapnic acidosis.