Background Manganese is an essential trace element for the human body and maintains normal development of many organs including the brain. However, long-term exposure to a high manganese environment or excessive manganese intake will lead to manganese poisoning and result in neurological diseases, and currently no effective treatment plan is available.

Objective To develop an animal model for subchronic manganese exposure and assess the impact of Dendrobium nobile Lindl. alkaloids (DNLA) on manganese associated behavioral and hippocampal effects in rats.

Methods Fifty male SPF SD rats were randomly allocated into a control group (0.9% normal saline by intraperitoneal injection), two experimental groups 7.5 mg·kg⁻¹ (low) or 15 mg·kg⁻¹ (high) of MnCl₂·4H₂O by intraperitoneal injection, and two DNLA antagonistic groups 15 mg·kg⁻¹ MnCl₂·4H₂O by intraperitoneal injection then either 20 mg·kg⁻¹ (low) or 40 mg·kg⁻¹ (high) DNLA by oral administration. All groups of rats were adminaistered 5 d per wek, once a day, for consecutive 13 weeks. Following modeling, neurobehavioral assessments were conducted using open field, Morris water maze, and Y maze. Inductively coupled plasma mass spectrometry (ICP-MS) was utilized to measure manganese levels in the blood and brain tissues of the rats, and hematoxylin-eosin (HE) staining was employed to examine neuronal morphological changes in the hippocampal tissues of the rats.

Results The neurobehavioral tests revealed that the manganese-exposed rats exhibited decreased total movement distance, prolonged central zone dwelling time, and reduced motor activity in the open field test, indicating tendencies toward depression and anxiety (P<0.05). In the Y-maze test, the mean exploration distance in the novel arm, the number of entries into the novel arm, and the time spent in the novel arm of the managanses-exposed rats were all reduced, while the latency period increased, suggesting impaired spatial exploration and learning-memory functions (P<0.05). In the Morris water maze navigation test, the escape latency was significantly longer in the manganese-exposed rats compared to the control group, and the number of platform crossings decreased in the spatial probe test, indicating a significant decline in spatial learning and memory (P<0.05). The ICP-MS analysis showed elevated manganese concentrations in the blood and hippocampus of the exposed rats (P<0.05), and the histopathological observation revealed hippocampal damage. Following the DNLA intervention, the manganese-exposed rats showed increased total movement distance and reduced central zone dwelling time in the open field test (P<0.05). In the Y-maze test, the mean exploration distance in the novel arm, the number of entries into the novel arm, and the time spent in the novel arm increased, while the latency period decreased, suggesting alleviation of anxiety and improved exploratory behavior (P<0.05). In the Morris water maze test, the escape latency gradually shortened, and both the number of platform crossings and the percentage of time spent in the target quadrant increased, indicating improved spatial learning and memory (P<0.05). Additionally, the manganese levels in the blood and hippocampus decreased (P<0.05), and the hippocampal pathological changes were partially restored.

Conclusion DNLA demonstrates the ability to counteract multiple neurotoxic effects following the elevation of manganese levels in the blood and hippocampal tissues of rats induced by subchronic manganese exposure. Specifically, DNLA is shown to ameliorate the behavioral alterations observed in rats after manganese exposure, and mitigate the hippocampal damage in manganese-exposed rats.