An intracranial small cell lung cancer inoculation model in BALB/c-nu mice reveals tumor-associated neuroinflammation and synaptic injury.

BACKGROUND: Small cell lung cancer (SCLC) exhibits the highest incidence of brain metastasis among lung cancer subtypes, often leading to severe neurological dysfunction and poor prognosis. However, preclinical orthotopic brain parenchymal SCLC xenograft models that recapitulate relevant neurotoxic injuries are still scarce. This study aimed to establish a SCLC brain parenchymal model using stereotaxic-guided intracranial injection in BALB/c-nu and systematically characterize the resulting neurological injury. METHODS: Mice in model group received an intracranial injection of 1 × 106 SCLC-1 cells in 5 μL PBS. Neurobehavioral evaluations including the open field test and Morris water maze (MWM) were performed at days 14 and 21 post-injection. Following behavioral assessment, all mice were euthanized, brain tissues were harvested for hematoxylin and eosin (HE) staining. Immunofluorescence (IF) staining was utilized to detect synaptic markers, Synaptophysin/PSD95, neuroinflammation/neuron markers, CK19/Iba-1/NeuN, as well as tight junction proteins, Occludin, ZO1. Immunohistochemistry (IHC) was applied to examine CD56 and Chromogranin A expression. RESULTS: Distinct tumor foci developed at the intracranial injection site in model mice, accompanied by robust inflammatory cell infiltration. IHC revealed strong CD56 and Chromogranin A expression within tumor lesions, confirming the neuroendocrine characteristics of intracranial SCLC lesions. Neurobehavioral assays uncovered evident neurological dysfunction in tumor-bearing animals. Specifically, model mice exhibited significantly prolonged escape latency in the MWM, indicative of defective spatial learning and memory capacity. Meanwhile, shorter total movement distance and longer immobility time were shown in the model group in the open field test, which reflects reduced spontaneous activity. IF further demonstrated multiple pathological alterations: decreased expression and fragmented distribution of tight junction proteins Occludin and ZO-1; a prominent decline in the colocalization coefficient of Synaptophysin and PSD-95, suggestive of synaptic structural disruption, massive accumulation of Iba-1-labeled microglia surrounding CK19-positive tumor lesions; and a pronounced loss of NeuN-expressing mature neurons. CONCLUSIONS: Stereotaxic-guided intraparenchymal injection can be used to establish a stable intracranial orthotopic implantation model of SCLC in BALB/c-nu nude mice. This model can mimic partial typical pathological changes of clinical SCLC brain metastasis, and pathological features including tumor lesions accompanied by inflammatory infiltration, synaptic structural damage, microglial activation and neuronal loss can be observed in the model. Among existing relevant studies, reports on constructing intracranial lesion models of SCLC via intracranial injection remain scarce. However, this model has certain limitations and cannot fully reproduce the complete pathological process of clinical brain metastasis triggered by spontaneous tumor migration and hematogenous dissemination.