Chronic intermittent hypoxia and hypercapnia inhibit the hypothalamic paraventricular nucleus neurotransmission to parasympathetic cardiac neurons in the brain stem

Abstract

Obstructive sleep apnea is associated with chronic intermittent hypoxia/hypercapnia (CIHH) episodes during sleep that heighten sympathetic and diminish parasympathetic activity to the heart. Although one population of neurons in the paraventricular nucleus of the hypothalamus strongly influences sympathetic tone and has increased activity after CIHH, little is known about the role of this pathway to parasympathetic neurons and how this network is altered in CIHH. We hypothesized that CIHH inhibits the excitatory pathway from the paraventricular nucleus of the hypothalamus to parasympathetic cardiac vagal neurons in the brain stem. To test this hypothesis, channelrhodopsin was selectively expressed, using viral vectors, in neurons in the paraventricular nucleus of the hypothalamus and channelrhodopsin-expressing fibers were photoactivated to evoke postsynaptic currents in cardiac vagal neurons in brain stem slices. Excitatory postsynaptic currents were diminished in animals exposed to CIHH. The paired-pulse and prolonged facilitation of the postsynaptic current amplitudes and frequencies evoked by paired and bursts of photoactivation of channelrhodopsin fibers, respectively, occurred in unexposed rats but were blunted in CIHH animals. In response to an acute challenge of hypoxia/hypercapnia, the amplitude of postsynaptic events was unchanged during, but increased after hypoxia/hypercapnia in unexposed animals. In contrast, postsynaptic currents were inhibited during hypoxia/hypercapnia in rats exposed to CIHH. In conclusion, the excitatory pathway to cardiac vagal neurons is diminished in response to both acute and chronic exposures to hypoxia/hypercapnia. This could elicit a reduced cardioprotective parasympathetic activity and an enhanced risk of adverse cardiovascular events in episodes of apnea and chronic obstructive sleep apnea.

Keywords: anoxia; hypercapnia; hypothalamus.

Figure 1

Single photostimulation (3ms, 1 Hz) of PVN fibers expressing ChR2 resulted in EPSC in CVNs in the DMV. The peak amplitude of the EPSC was significantly attenuated in animals exposed to CIHH when compared to unexposed rats. Experiments from single neurons are shown in A (traces represent average from 60 sweeps), and the summary data are demonstrated in B. Arrow indicates laser photostimulation in this and all subsequent figures. Asterisk indicates statistically significant differences, * P < 0.05.

Figure 2

Paired-pulse photostimulation (paired 3 ms pulses at 10 Hz) elicited paired-pulse facilitation response in unexposed animals: the peak amplitude of the second EPSC was significantly enhanced when compared to the first EPSC. However, in animals exposed to CIHH the paired pulse facilitation was blunted. Traces in A represent average of the recording from single neurons (9 sweeps) and the summary data are shown in B. Asterisk indicates statistically significant differences, * P < 0.05.

Figure 3

In both unexposed and CIHH animals multiple photostimulations (4 light pulses, 3ms duration, at 1 Hz) elicited EPSCs in which the amplitude of third and second EPSCs were significantly increased when compared to the first EPSC. Traces in A represent average of the recording from single neurons (9 sweeps) and the summary data are demonstrated in B. Asterisks indicate statistically significant differences, * P < 0.05, and *** P < 0.001.

Figure 4

In unexposed animals, multiple photostimulations (4 light pulses, 3ms duration, at 1 Hz) resulted in the increased frequency of EPSCs in CVNs 2 s post stimulation and also in increased amplitude of the EPSCs 4 s post stimulation. However, no significant changes were detected in either EPSC frequency or amplitude post stimulation in CIHH animals. Typical experiments are shown in A and the summary data are demonstrated in B. Asterisks indicate statistically significant differences, * P < 0.05, and *** P < 0.001.

Figure 5

As shown in typical experiments (A) and in the summary data (B), acute H/H elicited a significant increase in the peak amplitude of EPSC evoked by single photostimulation of PVN fibers 15 min post acute H/H with no significant changes detected during acute H/H. In CIHH animals, in contrast, acute H/H resulted in significant inhibition of the EPSC amplitude 5 min after the onset of acute H/H, however, no significant alteration in the EPSC amplitude occurred post acute H/H. Asterisks indicate statistically significant differences, * P < 0.05, and *** P < 0.001.