Histological changes in intervertebral discs after smoking and cessation: experimental study using a rat passive smoking model

Background. Passive smoking has been reported to induce intervertebral disc degeneration in rats, and the objective of the present study was to histologically investigate changes in smoking-induced intervertebral disc degeneration after cessation of smoking. Methods. Four-week-old rats were subjected to passive smoking for 8 weeks in a smoking box [20 cigarettes a day: one cigarette an hour (inhaled over 3 minutes and followed by ventilation with room air for 5 minutes)] to induce intervertebral disc degeneration. Smoke-free periods of different lengths were then established, and intervertebral discs were histologically analyzed. Results. Immediately after 8 weeks of passive smoking, intervertebral discs exhibited cracks, tears, and misalignment of the annulus fibrosus, and increased fibrous tissue was seen in the nucleus pulposus. In addition, the level of interleukin-1 beta in intervertebral discs was higher in the smoking group than in the non-smoking group. After cessation, progression of degeneration ceased, and the matrix of the nucleus pulposus and annulus fibrosus exhibited increased fibrous connective tissue and proteoglycan. However, there were no changes in annulus fibrosus misalignment. Interleukin-1 beta levels also remained significantly elevated after 8 weeks of cessation. Conclusions. While the annulus fibrosus degeneration caused by smoking was partially irreversible after cessation of smoking, the amount of mucin (proteoglycan) in the nucleus pulposus and annulus fibrosus tended to increase after cessation, thus suggesting the possibility that smoking-induced intervertebral disc degeneration can be repaired to some degree by cessation of smoking.