**4. Discussion**

## **4.1. Open field test**

**Figure 2.** Relationship between NE concentration and noise exposure duration.

**Figure 3.** Neuronal and synaptic morphologies of the temporal lobe of rats.

clear, and mitochondria and synaptic vesicles.

We observed the neuronal and synaptic morphologies of the temporal lobes from TEM and the representative pictures are shown in **Figure 3**. In our experiments, neuronal and synaptic damages were observed in the temporal lobe of EG-II rats, but no damage was seen in the CG.

(a) The nuclei (N) in the neurons of the temporal lobe of the CG were oval and their membrane structure was clear, yet mitochondria (M), rough endoplasmic reticula, and other organelles could be seen in cytoplasm, and their distributions were uniform and morphologically normal. (b) The nuclei (N) of neurons in the temporal lobe of EG-II were irregular-shaped, the nuclear membrane was deep-stained and its structure vague, chromatin accumulated along the edge, and cytoplasm was condensed. (c) The synaptic cleft of the temporal lobe area of the CG was

**3.3. Temporal lobe cell morphology**

168 Recent Progress in Some Aircraft Technologies

First line indent crossing is defined as that rats breach the lattices at the bottom of the open field and the number of line crossing reflects the animals' horizontal mobility, exploration, and anxiety [13]. First and foremost, previous studies have shown that motor behaviors of rats in OFT increased after 1 h of acute stress by white noise of 95 dB [4]. However, in our test, rats of EG-I and EG-II showed no significant difference in line crossing compared with CG after 1 d of noise exposure, which means acute effects of noise exposure were not evident in our experiment. It may be that noise intensity below *L*WECPN of 80 dB is "moderate" to rats. In addition, after suffering airport noise exposure for 8 d, rats of EG-II showed that the line crossing number decreased, which means horizontal mobility and exploration ability of the rat decreased and some anxiety appeared [13]. However, Pan *et al*. found that two weeks' noise stress (2 h/d, 85 dB) increased square crossing and vertical movement, which is in contrast to our results [14]. There are a number of possible explanations. First, the intensity of noise is an important factor. Second, the duration of daily noise exposure cannot be ignored because, in our test aircraft, noise was exposed throughout the day. Third, the neurobehavioral effect of continuous white noise is different from that of intermittent aircraft noise [15]. Last but not least, from **Figure 1a**, we also know that line crossing of rats was not significantly different between CG and EG-II, except on the 8th day. The reason is that the behaviors of rats manifest itself differently depending on the duration of stress [16]. Conrad *et al*. have also shown that behaviors of rats turned from an excited state to inhibitory state under prolonged stress [17]. Another reason may be that the mechanisms of resistance and/or adaptability are generated after longer term noise stress.

In our study, center area duration is defined as latency of the rats before leaving the center area, which is measured by anxiety-like behavior. As is known, high center area duration indicates high anxiety levels [12]. Thus, the emotions of rats in EG-II altered to anxiety after 8 d of noise exposure. The results of center area duration on other days suggest that the effect of aircraft noise on anxiety is not permanent. This conclusion is also in line with the line crossing results.

## **4.2. Levels of plasma NE**

Epinephrine, NE, and cortisol are stress hormones that are used as indicators of body stress upon noise exposure [6, 18, 19]. NE plays an important role as a stress hormone in conducting and adapting to stress [20, 21]. Therefore, plasma NE level of EG-II increased after long-term noise exposure (29 d) in our experiment, possibly due to the cumulative effect of high-intensity noise. Unfortunately, we do not know the plasma NE level of rats after 29 d of noise exposure, because it is difficult to collect blood from more fierce rats.

Plasma NE is primarily secreted by the sympathetic nerve endings of the heart, blood vessels, and adrenal medulla, and is controlled by the sympathetic nervous system [22]. Plasma NE levels reflect the excitability of the peripheral sympathetic system [23, 24]. Due to aircraft noise exposure in our experiments, the levels of NE in rat plasma increased, indicating that aircraft noise stimulates sympathetic excitement of the adrenal medulla system.

Several studies have shown that trait anxiety is significantly associated with increased NE concentration in blood plasma. Individuals with higher plasma NE concentrations also have more severe anxiety symptoms and the concentrations of plasma NE of patients with anxiety disorders are drastically higher than those of healthy individuals [25]. We conclude that highintensity aircraft noise exposure may similarly induce anxiety symptoms in rats. Further, patients with hypertension have also been found to have higher plasma NE concentrations [26]. In addition, epidemiological investigations have pointed out that the incidences of heart disease and hypertension are directly related to aircraft noise exposure [1, 27]. Our results partly provide pathological evidence supporting this epidemiological research.

## **4.3. Temporal lobe cell morphology**

Previous studies have shown that necrosis and apoptosis of neurons occur when the body is subjected to physical, chemical, or severe pathological stimulation [28]. For this reason, we consider that the long-term noise stress in rats resulted in the lesions in temporal lobe neurons and synapses of EG-II.

First line indent lobe areas are closely related to perception and memory [29]. Therefore, when neurons of the temporal lobe are damaged, a variety of mental disorders are likely to occur, such as cognitive decline, memory reduction, or subjective emotional instability [30]. When synaptic morphology changes, the related functions of the brain and CNS change accordingly, further leading to changes in behavior [31]. The results of our experiments are consistent with changes in rat behavior due to long-term exposure of aircraft noise.
