Medicinal and aromatic plants – industrial profiles

Smooth muscle preparation
The preparation of the smooth muscle of the guinea-pig ileum will remain viable for several
hours after removal from the animal and will respond to electrical field stimulation with repro-
ducible contractions which are due to the stimulation of the parasympathetic nerve with the
release of acetylcholine.
Essential oils which stimulate smooth muscle contraction can be recognised immediately
while the site of action of those which reduce the size of the electrically-induced contraction can
be determined. Possible sites of action include inhibition of the release of acetylcholine, or relax-
ation of the tissue via stimulation of adrenoceptors, action on secondary messengers or on 
calcium or potassium channels. This preparation thus allows us to recognise spasmogenic 
and spasmolytic activity, to determine whether or not the activity is dose-related, to measure
duration of action and also attempt to determine the mechanism of action.
The question arises whether the knowledge of the activity of essential oils on smooth muscle
gives us any clues about the likely actions of these compounds if and when they enter the central
nervous system (CNS).
A famous English pharmacologist suggested that the intestine could be considered a para-
digm of the CNS, but it still remained almost impossible to infer action in the CNS from 
activity on isolated smooth muscle.
The reason for this is simply the complexity of the CNS, with the interaction between excita-
tory and inhibitory fibres being such that reduced activity in one neurone can lead either to

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sedation or excitation. Thus, alcohol appears to stimulate some behaviour although it is a CNS
depressant, the explanation being that the inhibition of inhibitory pathways removes a normal
break and behaviour therefore changes.
Another aspect of the complexity of the CNS is that any one particular behaviour is controlled
by several neurotransmitters, each of which is likely to be able to bind to different sub-groups of
receptors.
If one, for example, considers pain, this involves neurotransmitters in the afferent pathway
such as Substance P, glutamate and nitric oxide and this afferent pathway can be modulated by
neuronal pathways releasing a range of neurotransmitters including opioid peptides, acetyl-
choline, histamine, 5-hydroxytryptamine and cholecystokinin.
With so many neurotransmitters involved in the pain pathway it is not surprising that the
experience of pain can be influenced by many different compounds. For example, monoterpenes
like menthone and 

-terpineol (administered by the subcutaneous route) showed activity simi-
lar to that of accepted analgesics, for example, indomethacin and naproxen in reducing the
behavioural activity of the mouse to a noxious stimulus (Hart 
et al
., 1994).
In experiments studying the motor activity of mice after exposure to the aroma of various
essential oils (Buchbauer, 1991; 1992; Jager 
et al
., 1992) rosemary, jasmine and ylang ylang
increased activity while lavender, neroli, lime-blossom, passiflora, citronellol and linalool
decrease motor activity. The presence of components in the blood when applied by inhalation has
also been demonstrated ( Jager 
et al
., 1992). The effect on the motor activity has been shown to
be similar to that when the essential oil was injected. It has been assumed that changes in motor
activity are a central effect but the possible action on neuromuscular transmission has not been
investigated.
However, recent experiments on the motor-nerve skeletal muscle preparation (rat phrenic-
nerve diaphragm) by the authors has shown that lavender and tea tree oils cause a reduction in the
size of the twitch of the skeletal muscle in response to electrical stimulation of the motor nerve.
Linalool, which was shown to reduce motor activity in the mouse has also been shown to have
an action within the brain itself: using membranes from rat cerebral cortex, linalool exhibited 
a dose-related inhibition of the binding of glutamate, a main excitatory neurotransmitter of the
CNS (Elisabetsky 
et al
., 1995).
The effect of essential oils in man has been studied in several different ways including 
measuring the alertness and reaction times (Manley, 1993) and human brain activity (Torii 
et al
.,
1988; Kubota 
et al
., 1992) using Contingent Negative Variation (CNV). The latter is the brain
potential which occurs between a warning stimulus and an imperative stimulus, that is, when
the subject is expecting something to happen. The CNV amplitude is increased by caffeine, jas-
mine and peppermint and decreased by chlorpromazine, lavender and marjoram. There is some
discrepancy between results from different groups regarding many oils.
The present experiments were designed to see whether the effect of stress on man could be
mimicked using isolated segments of small intestine and by monitoring their spasmogenic or
spasmolytic effects, it could be possible to assess their relaxant or stimulant nature.