Evaluation of Neuropharmacological Activity and Determination of Chemical Composition of the Essential Oil of Plectranthus Aegyptiacus Fresh Leaf in Mice

Idris Oyemitan


Background: Plectranthus   aegyptiacus is an ethnomedicinal plant found in South-west Nigeria where it is used to manage fever, sensory diseases and cough among other ailments. The objective of this study was to evaluate the neuropharmacological activities of the essential oil of P. aegyptiacus fresh leaf in mice in addition to determine the oil’s chemical composition. Method: Essential oil of P. aegyptiacus (EOPA) was extracted from fresh leaves of the plant by hydro-distillation and analyzed to determine its chemical composition.  The LD50 of the oil was determined orally and intraperitoneally.  The EOPA (50-200 mg/kg, i.p., n=6) was tested for novelty-induced behavioural (NIBs), anxiolytic, sedative, anticonvulsant and analgesic activities using standard protocols. The probable mechanism(s) of the effect of the EOPA on the various neural pathways were studied using various antagonists. Results obtained for the oil were statistically analyzed and compared to negative and positive controls. Results: The  LD50  values  obtained for  the  EOPA were  2154  and 490  mg/kg  for  the  oral  and intraperitoneal routes respectively. The EOPA significantly (p<0.05–0.01) inhibited   all   behavioural   display, significantly (p<0.05-0.01) increased the time spent on the open arms of the elevated plus maze, blocked the hind limb tonic extension on the maximal electric shock and protected the mice against   PTZ–induced mortality, significantly (p<0.05-0.001) shortened sleep latency and prolonged total sleeping time induced by ketamine (100 mg/kg), significantly (p<0.05)) reduced writhings caused by acetic acid (1% v/v) and significantly (p<0.05) increased the reaction time on the hot plate. Flumazenil (2 mg/kg) significantly (p<0.05) reversed the effect of the oil on NIBs; atropine, naloxone and cyproheptadine significantly (p<0.01-0.001) potentiated the inhibitory effect of the oil,   while   yohimbine did   not alter the effect of the oil   on   NIBs.   Major compounds identified in the oil were antioxine, germacrene-D and p-cimene. Conclusion: The major effect of the oil was depression of the CNS and it demonstrated significant anxiolytic, sedative, anticonvulsant and analgesic activities in mice.  The mechanism   of   action   of   the   oil is suggested   to   be   mainly   augmentation   of GABAergic   neurotransmission. Furthermore, this research inferentially validated the pharmacological basis for the folkloric use of the plant   Keywords: Plectranthus aegyptiacus, chemical composition, central nervous system activities, antioxine                      

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