Prophylactic Inhalation of Bergamot Essential Oil Ameliorates Hormone‐Simulated Pseudopregnancy Withdrawal‐Induced Postpartum Depression by Modulating Hippocampal Synaptic Plasticity

ABSTRACT

Postpartum depression (PPD) affects approximately 17.2% of women giving birth globally, but current pharmacotherapies face significant limitations including invasive intravenous delivery (brexanolone) and high costs Bergamot essential oil (BEO) has demonstrated efficacy in aromatherapy for depression, yet its role in PPD remains unexplored. This study investigated whether prophylactic or therapeutic inhalation of nebulized BEO ameliorates PPD‐like phenotypes in a rat model of hormone‐simulated pseudopregnancy (HSP) with abrupt withdrawal, and explored potential mechanisms involving hippocampal synaptic plasticity and neuroinflammation. The main components of BEO were characterized using gas chromatography–mass spectrometry (GC–MS). A rat model of PPD was constructed using the HSP post‐withdrawal method, and animals were randomly divided into control group (Ctrl), sham surgery group (Sham), model group (PPD), fluoxetine group (Flu; 10 mg/kg, i.p., post‐modelling) as a positive control representing first‐line SSRI therapy (brexanolone, though FDA‐approved for PPD, requires costly intravenous administration and specialized medical supervision, and was therefore not used as a comparator), therapeutic BEO group (BEO; 2.5% v/v inhalation post‐modelling) and prophylactic BEO group (P‐BEO; 2.5% v/v inhalation during HSP modelling). The sucrose preference test (SPT), open field test (OFT) and elevated plus maze (EPM) were used to evaluate the depressive‐like behaviours, and HE staining was used to observe the effects of BEO on the thymus and spleen of immune organs. Nissl staining was utilized to quantify the Nissl bodies in the hippocampus (HIPP) of PPD model rats. Additionally, immunohistochemistry and Western blotting were conducted to detect alterations in postsynaptic density protein‐95 (PSD‐95) and synaptophysin (SYP) within the hippocampus of the rats. Enzyme‐linked immunosorbent assay (ELISA) was employed to further analyse the levels of insulin‐like growth factor‐1 (IGF‐1, a neurotrophic factor linked to synaptic plasticity), interleukin‐1β (IL‐1β, a pro‐inflammatory cytokine implicated in PPD pathogenesis) and interleukin‐10 (IL‐10, an anti‐inflammatory cytokine implicated in PPD pathogenesis) in serum, cerebrospinal fluid (CSF) and HIPP. Molecular docking was performed to predict interactions between BEO's core constituents (limonene, γ‐terpinene, α‐pinene, β‐pinene, α‐phellandrene) and target proteins (PSD‐95, SYP, IL‐1β, IGF‐1, IL‐10). Both therapeutic and prophylactic BEO inhalation significantly increased sucrose preference (from 78.78% ± 17.25% to 96.44% ± 2.60% [BEO] and 97.33% ± 0.87% [P‐BEO], p  < 0.05), total distance travelled in the OFT (from 900.07 ± 523.07 to 1932.64 ± 665.88 cm [BEO] and 1671.51 ± 544.36 cm [P‐BEO]; p  < 0.05) and open arm time in the EPM (from 16.34 ± 15.44 to 49.91 ± 24.99 s [BEO] and 50.81 ± 16.12 s [P‐BEO]; p  < 0.05). Both BEO groups exhibited increased Nissl body counts in hippocampal CA1 (from 7.44 ± 1.59 to 18.44 ± 2.60 [BEO] and 19.56 ± 2.07 [P‐BEO]; p  < 0.05), CA2, CA3 and DG subregions ( p  < 0.05 vs. PPD), as well as increased PSD‐95/SYP protein abundance. Both BEO regimens increased IGF‐1 levels in serum and CSF ( p  < 0.05 vs. PPD), with the P‐BEO group showing a significantly higher serum IGF‐1 levels compared to the Flu group ( p  < 0.05). Additionally, both P‐BEO and BEO groups decreased IL‐1β levels in serum and HIPP, while increasing IL‐10 levels in the CSF ( p  < 0.05 vs. PPD for all). Molecular docking revealed favourable binding affinities (binding energy < −5.0 kcal/mol) between BEO constituents and all five target proteins. No significant histopathological alterations were observed in thymus or spleen following BEO exposure. Collectively, these findings suggest that BEO inhalation may attenuate PPD‐like phenotypes in this rat model, potentially through modulation of hippocampal synaptic plasticity and neuroinflammation via regulation of IGF‐1, IL‐1β and IL‐10. While these results are preliminary, the non‐invasive delivery and observed dual‐phase efficacy suggest that BEO inhalation may represent a more accessible approach worthy of further investigation for PPD management, particularly in resource‐limited settings where current FDA‐approved treatments are unavailable.