Syzygium oleosum (F.Muell.) B.Hyland (Myrtaceae), an Unexplored Australian Species: Anatomical and Micromorphological Study of Leafy Twigs, and Characterization and Biological Activity of Their Essential Oil

Syzygium oleosum (F.Muell.) B.Hyland is an Australian native species whose essential oil (EO), known commercially as “Mango Myrtle,” is gaining popularity in aromatherapy, yet remains poorly studied. This work provides the first comprehensive pharmacognostic investigation of S. oleosum. Anatomical and micromorphological analyses revealed numerous secretory cavities and calcium oxalate druses in both leaves and twigs. GC-MS analysis identified 16 components in the EO, predominantly hydrocarbon monoterpenes, with terpinolene (30.79%), β-pinene (26.79%), α-pinene (10.69%), and γ-terpinene (9.86%) as major constituents. In vitro assays showed moderate antioxidants (IC50 ≤ 4.95 mg/mL) and anti-inflammatory effects (IC50 ≤ 5.93 mg/mL), with specific monoterpenes contributing differentially to each activity. The EO displayed weak inhibitory activity against acetylcholinesterase (IC50 19.4 mg/mL) and butyrylcholinesterase (IC50 15.9 mg/mL), and no effect on GABA transaminase. Microelectrode array recordings on primary cortical neurons demonstrated a concentration-dependent inhibition of network activity (0.059–1.19 mg/mL) without affecting cell viability, indicating a neuromodulatory property. These results provide new insights into the pharmacological potential of S. oleosum EO and support its further evaluation as a neuroactive and anti-inflammatory agent.