The effect of immobilization induced stress on the lung of the adult male albino rat and the possible protective role of melatonin: A light, electron microscopic and immunohistochemical study

Background: Stress is known as one of the most important reasons of many diseases. Immobilization is one of the most common performed stresses on animals. immobilization stress may induce the formation of reactive oxygen species and can lead to suppression of immune system. Melatonin as an antioxidant, has an important role in the immune function. The lung has a large surface that is constantly in contact with air oxygen and pollutants. It is one of the organs commonly affected by reactive oxygen species generation which induces oxidative damage. It is a site of major reactive oxygen species production.
Aim of work: The present study aimed to investigate the effect of immobilization induced stress on the lung in the adult male albino rat and the possible protective role of melatonin.
Material and Methods: 45 adult male albino rats weighing 200–250 g were used in this study. The rats were randomly divided into three equal groups (15 rats each). Group (I): the rats kept undisturbed and served as non-stress control group. They were sacrificed at the end of the experiment. Group (II): the rats subjected to stress and placed on a wooden plate with their trunks wrapped in a confining harness for 90 min 5 days/week for 6 weeks. The animal was able to move its limbs and head but not its trunk. Group (III): the rats were exposed to stress as previously described and concurrently injected melatonin intraperitoneally in a dose of 10 mg/kg/day at 4:00 pm. All animals were sacrificed by decapitation under anaesthesia at the same time, then lungs dissected out. The specimens of each group were randomly divided into three subgroups; the first was processed for the light microscopic study (H & E), the second for the transmission electron microscopic study and the third for the immunohistochemical study (Caspase-3 and iNOS). Morphometric studies and statistical analysis were performed.
Results: The light microscopic and ultrastructural examination of the rat lungs after immobilization showed severe alveolar damage in the form of collapsed alveolar saccule and alveoli, markedly thickened interalveolar septa encroaching on alveoli, heavy inflammatory cellular infiltration and exudation. Pneumocyte type I showed indentation of its nuclear membrane, presence of chromatin clumps inside the nucleus and swollen mitochondria with disrupted cristae. Melatonin treated group revealed an evident reduction of all alveolar changes with nearly normal structure of the alveolar ducts, alveolar saccules and alveoli. Immunohistochemically stained lung sections for caspase-3 and iNOS after immobilization showed an intense brownish immune reactions. Melatonin treated group revealed a noticeable reduction in the immune reactions.
Conclusion: Immobilization stress has a damaging effect on the histological structure of the lung and a concomitant treatment with an antioxidant (melatonin) effectively protected the lung tissue.