Four weeks of environmentally relevant second hand smoke exposure decreased neuronal excitability of cardiac vagal neurons in the dorsal motor nucleus of vagus

Abstract

BACKGROUNDSecond hand smoke (SHS), a main indoor air pollutant, is a significant risk factor for cardiovascular morbidity and mortality. We previously showed that exposure to SHS, for as short as four weeks, significantly reduced heart rate variability (HRV), suggesting a decreased vagal regulation of the heart. We further showed that, in cardiac vagal neurons in the nucleus ambiguus (NA), SHS exposure significantly reduced spiking response to depolarizing current injections and increased current‐ and voltage‐threshold for action potential generation. However, we have no information on the effects of SHS exposure on cardiac vagal neurons in the dorsal motor nucleus of vagus (DMNV). Here, we sought to test the hypothesis that SHS exposure decreases neuronal excitability of cardiac vagal neurons in the DMNV.METHODSSeven‐week old male C57BL/6J mice underwent surgeries to retrograde label cardiac vagal neurons by applying a fluorescent dye (DiI) to the sinoatrial node. Two weeks after surgery, mice were randomly assigned to either filtered air‐ (FA, n = 14) or SHS‐exposed (n = 16) group. The SHS exposure (6 hr/d, 5 d/wk) was set at a level that is similar to that of a smoky bar (3 mg/m3). After four weeks of FA or SHS exposure, whole‐cell current clamp recordings in brainstem slices were performed on anatomically identified cardiac vagal neurons in the DMNV with fluorescent labeling.RESULTSDMNV cardiac vagal neurons from the SHS‐exposed group were significantly more hyperpolarized than the FA control group (−53.3 ± 0.9 mV vs. −55.7 ± 0.7 mV, FA vs. SHS respectively, p < 0.05). Four weeks of SHS exposure also significantly decreased neuronal spontaneous spiking activity at a holding potential of −45 mV (1.6 ± 0.4 Hz vs. 0.8 ± 0.2 Hz, FA vs. SHS respectively, p < 0.05). The decreased spontaneous activity was not associated with the spiking response to depolarizing current injections from a holding potential of −60 mV, current and voltage threshold for action potential generation, action potential peak and afterhyperpolarization. Furthermore, there was no significant difference in the neuronal input‐output relationship of DMNV cardiac vagal neurons between FA control and SHS‐exposed groups, suggesting an unchanged dynamic responsiveness to stimuli.CONCLUSIONSFour weeks exposure to environmentally relevant level of SHS decreased neuronal excitability of cardiac vagal neurons in a region‐dependent manner – reduced resting membrane potential and spontaneous spiking activity in the DMNV and reduced spiking response to depolarizing current injections and increased threshold for action potential generation in the NA.Support or Funding InformationFunding: R01 ES025229