HOROPITO: For Pain

Posted: May 2022
Author: Phil Rasmussen | M.Pharm., M.P.S., Dip. Herb. Med.; M.N.I.M.H.(UK),  F.N.Z.A.M.H. 

Horopito (Pseudowintera axillaris and Pseudowintera colorata) was used both internally and topically by Māori and early European settlers to Aotearoa New Zealand for a diverse number of applications. Decoctions of leaves or bark and other preparations were used as an antiseptic and digestive tonic, and conditions where painful symptoms often feature. The leaves were also chewed for toothache, and applications made from bruised leaves or inner bark were traditionally used to treat painful sprains and swellings and burns.^{1, 2, 3, 4}

Analgesic properties

A leaf decoction was used “to allay inward pain”, and was given the name ‘Maori painkiller’.⁵ Traditional usage also included applications for painful conditions such as toothache, stomach ache, headache and swollen joints.⁶

Polygodial, the sesquiterpene dialdehyde is considered a prominent active constituent of P. colorata⁷ and is found also in various other plants including Persicaria hydropiper (Japanese water pepper),⁸ Drimys winteri (Winters bark), and Tasmannia lanceolata (Tasmanian mountain pepper).⁹  Studies on polygodial or Horopito in recent years show anti-inflammatory, antifungal, antibacterial, and antinociceptive (analgesic) properties within its leaves.¹

Animal studies have shown pronounced and long-lasting antinociceptive (analgesic) activity for polygodial, effects related to possible influences on opiate, serotonergic and α-1-adrenoceptors, and modulation of glutamatergic neurotransmission.^{1, 10, 11, 12} 

Capsicum and other pungent plant compounds activate the expression of TRPV1 (transient receptor potential vanilloid 1) receptor channels, key transducers of thermal, chemical and other sensory cues and for pain transmission from sensory nerve terminals.^{13, 14} Increased expression of TRPV1 receptors is typically followed by a refractory period during which TRPV1 desensitisation and neuropeptide depletion occur. Anti-inflammatory and antihyperalgesic responses then occur resulting in a pronounced reduction of chemical and thermal inflammatory pain, thus contributing to the counterirritant or rubefacient effects of various warming or pungent plants used topically for symptomatic relief in inflammatory muscle or joint pain.

Topical Horopito applications seem to exhibit such counter-irritant properties, in a similar albeit weaker manner to those of other hot and pungent spices such as chilli, ginger and mustard.¹ Like capsaicin, TRPV1 agonist activity has been proposed as a mechanism for the persistent sensory changes apparent in primary afferent nerve fibres responsible for pain transmission, observed after administration of polygodial and other related terpenoids to animals.^{15, 16, 17} 

Polygodial also strongly activates another type of transient receptor channel, TRPA1 (transient receptor potential A1), a sensor for noxious and electrophilic compounds and pungent natural products such as allyl isothiocyanate from mustard oil.¹⁸ TRPA1 agonist activity of polygodial is almost ten times stronger than that of allyl isothiocyanate, which is a potent TRPA1 agonist and probably largely responsible for the counter-irritant and anti-inflammatory effects of topical mustard applications.

Apart from polygodial, other sesquiterpene compounds including those found in P axillaris, tannins and essential oil components such as eugenol, are likely to contribute to Horopito’s therapeutic activities. Eugenol is largely responsible for the local anaesthetic action of clove oil, although levels in Horopito are low, and so its contribution to analgesic actions is questionable.

Anti-inflammatory properties

In vitro anti-inflammatory activity for Horopito is evident through its inhibition of the formation of the inflammatory mediator nitric oxide (NO) from macrophage cells stimulated with bacterial lipopolysaccharide.¹⁹

Polygodial exhibits anti-inflammatory activity at micromolar and submicromolar concentrations in a mouse model of gout, through inhibition of monosodium urate-induced neutrophil infiltration and neutrophil superoxide production.²⁰ This suggests potential usefulness for Horopito in the treatment of neutrophil-driven inflammatory conditions such as gout and rheumatoid arthritis. Red regions of Horopito are enriched in anthocyanins, flavonols, dihydroflavonols and hydroxycinnamic acids, all of which have antioxidant activity.²¹

Stomach pain and diarrhoea

Infusions and decoctions of the bark or leaves were used by early bushmen and settlers as a quinine substitute for chronic diarrhoea and stomach pain.^{3, 22, 23, 24}

Various Horopito compounds and pharmacological properties may underlie these applications. The astringency of tannin constituents is potentially useful to help alleviate diarrhoea, and antibacterial and anti-inflammatory actions of polygodial and other constituents probably also contribute.

Potent gastroprotective effects have been shown for polygodial in rats,^{25, 26} and reduced colon permeability in malnourished mice.²⁷ Modulation of endogenous prostaglandins and nitric oxide, seem to be involved in these activities.

Relaxant properties on abdominal smooth muscle are implicated by animal studies showing polygodial produces a concentration-dependent antagonism of gut contraction induced by inflammatory mediators.^{28, 29} These effects as well as the analgesic properties of polygodial as previously discussed may contribute to the traditional reputation of Horopito to alleviate painful abdominal conditions.
 

References:

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