Early Hippocampal Stress Responses to Short-Term Cigarette Smoke Exposure in Mice

Cigarette smoke (CS) is a major environmental toxicant associated with cognitive impairment and neurodegenerative disease, yet the earliest hippocampal responses to short-term exposure remain poorly understood. Here, we investigated the effects of short-term CS exposure on hippocampal neuropeptide signalling, endoplasmic reticulum (ER) stress, mitochondrial homeostasis, neuroplasticity and astroglial activation. Mice were exposed to the equivalent of 0, 2, 4 or 6 cigarettes per exposure session, twice daily for four consecutive days, using a whole-body exposure paradigm. Hippocampal tissue was analysed by reverse transcription quantitative PCR, ELISA, immunofluorescence and histology. Short-term CS exposure disrupted hippocampal PACAP/VIP signalling, characterised by reduced Vip, Adcyap1r1 and Vipr1 expression, decreased VIP protein levels and increased Vipr2 expression across all smoke-exposed groups, while PACAP remained unaffected. CS also activated ER stress, evidenced by increased phosphorylated IRE1alpha immunoreactivity and increasing exposure-related trends in Ern1, Ppp1r15a and Eif2ak3 expression. Mitochondrial and stress-associated transcriptional responses included increased Dnm1l, Opa1 and Mmp9 and reduced Sod1 expression; however, associations among these genes did not remain significant after adjustment for exposure and false-discovery rate. Both Bdnf and Adnp expression showed a decreasing linear trend without significant individual treatment-versus-control comparisons. Neuronal BDNF immunoreactivity increased in CA1 and CA2/3 without detectable changes in CREB phosphorylation. CS further induced an increasing trend in Gfap expression and selective astroglial response within CA2/3. These molecular and cellular responses occurred without overt hippocampal histopathological abnormalities, indicating that brief CS exposure initiates hippocampal stress responses before structural damage is detectable.