Journal of Environmental Biology

Chronic low-dose natural radiation exposure and testicular response in wild Rattus rattus from high background radiation areas in Kerala, India

Y. Kinoshita¹, J. Shibata¹, K. Annaka¹, H. Ikema¹, J. Goto², K. Sasidharan¹, M.S. Kumar¹, A. Madhavan³, R. Ravinesh⁴, A.M. Hatha³, A. Nakata⁵, T. Suzuki⁶, H. Shinoda⁶, M.G. Palmerini⁷, S. Zaccardi⁷, D. Cocciolone⁷, G. Macchiarelli⁷, M. Abe⁸, M. Fukumoto⁹, T. Miura¹⁰ and H. Yamashiro¹¹*     

¹Graduate School of Science and Technology, Niigata University, Niigata-959 2181, Japan

²Institute for Research Administration, Niigata University, Niigata-951 8510, Japan

³Department of Marine Biology, Microbiology, Biochemistry, Cochin University of Science and Technology, Cochin-682 016, India

⁴Centre for Marine Living Resources and Ecology, Ministry of Earth Sciences, Kochi-682 508, India

⁵Faculty of Pharmaceutical Sciences, Hokkaido University of Science, Hokkaido-006 8585, Japan

⁶Graduate School of Dentistry, Tohoku University, Miyagi-980 8575, Japan

⁷Department of Life, Health and Environmental Sciences, University of L’Aquila, L’Aquila-67100, Italy

⁸Department of Animal Model Development, Brain Research Institute, Niigata University, Niigata-951 8510, Japan

⁹International Institute of Disaster Science, Tohoku University, Miyagi-980 8572, Japan

¹⁰Department of Risk Analysis and Biodosimetry, Institute of Radiation Emergency Medicine, Hirosaki University, Aomori-036 8564, Japan

¹¹Field Centre for Sustainable Agriculture, Faculty of Agriculture, Niigata University, Niigata-959 1701, Japan

Received: 24 December 2025                   Revised: 24 February 2026                   Accepted: 15 April 2026

*Corresponding Author Email : hyamashiro@agr.niigata-u.ac.jp                    *ORCiD: https://orcid.org/0000-0002-0133-2459

Abstract

Aim: Natural background radiation levels vary considerably worldwide; Chavara, located in Kerala, India, is known as a high-background radiation area (HBRA), where thorium-rich monazite sands produce radiation levels far exceeding the global average. The aim of this study was to evaluate the testicular histology and selected radiation-responsive gene expressionin wild male rats (Rattus rattus). Individuals captured from the Chavara HBRA and a nearby low-radiation area around the Cochin University of Science and Technology (CUSAT) were compared.

Methodology: Testicular tissues were examined by histological observations, immunofluorescence staining, and qPCR analysis of radiation-responsive genes (Lsp1, Ptprk) to evaluate spermatogenesis, apoptosis, and gene expression.

Results: Histological observations revealed preserved seminiferous tubule architecture and active spermatogenesis in HBRA individuals. PCNA expression indicated sustained proliferative activity, and no differences were detected in Lsp1 or Ptprk expression between groups. Apoptotic cells were less frequent in HBRA individuals; however, the overall testicular structure remained comparable between the regions.

Interpretation: Within the limitations of this field-based study, chronic natural radiation exposure was not associated with detectable impairment of spermatogenic parameters in wild R. rattus. However, subtle molecular or epigenetic effects cannot be excluded. Further investigations incorporating broader molecular analyses and larger sample sizes are warranted to clarify long-term biological consequences.

Key words: Environmental radiation, Low-dose exposure, Rattus rattus, Spermatogenesis

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