Nigella sativa seed extract and its bioactive compound thymoquinone: the new melanogens causing hyperpigmentation in the wall lizard melanophores Sharique A. Ali and Keisham V. Meitei Department of Biotechnology, Saifia College of Science and Education, Bhopal, India Abstract Objective The effects of the lyophilized seed extract of Nigella sativa and its active ingredient, thymoquinone, were studied on the isolated melanophores of the wall lizard to find the mechanism of skin darkening at the cellular level. Methods The integumental melanophores of the wall lizard, Hemidactylus flaviviridis, were assayed using the mean melanophore size index and their responses were recorded in the presence of various concentrations of the plant extract, thymoquinone, specific antago- nists and potentiator. Key findings Significant skin darkening activity of the extract of N. sativa and thymo- quinone was observed on the isolated melanophores of the wall lizard. The pigment cells responded by distinct dispersion leading to skin darkening. The effect was physiologically significant as re-immersion in physiological saline made the melanophores return to their normal intermediate state. These melanin dispersal effects were antagonized by atropine as well as hyoscine and were also found to be highly potentiated by neostigmine, an anticho- linesterase agent. Conclusions These findings suggest that the extract of N. sativa, as well as its active principle, mimic the action of acetylcholine in melanin dispersion leading to skin darkening via stimulation of cholinergic receptors of muscarinic nature within the melanophores of wall lizard. This study opens new vistas for the use of N. sativa active ingredient, thymo- quinone, as a novel melanogen for its clinical application in skin disorders such as hypop- igmentation or vitiligo. Keywords cholinergic receptors; extract; Nigella sativa; thymoquinone; wall lizard melanophores-dispersion Introduction Vertebrate melanophores are melanin-containing dark pigment cells of neural crest origin and interestingly have been designated as a disguised type of smooth muscles, which due to their intracellular movement of melanin granules, regulate skin colour leading to either darkening or lightening of the skin. [1] It is well known that the pigment cells are controlled by either nerves alone or by hormones or by a combination of both. [2,3] Involvement of cellular receptors of different types, such as adrenergic, cholinergic and histaminergic, leading to darkening or lightening of vertebrate skin has been suggested by several workers. [3–5] It has been well documented that some cellular receptors, such as adrenergic or cholinergic, present on the melanophore-membrane, via elevation of intracellular cyclic adenosine monophosphate (cAMP), cause melanophore dispersion leading to skin darken- ing. The lowering of adenyl cyclase via stimulation of a-adrenergic or histaminergic recep- tors causes the opposite responses, thereby making the skin appear pale. [1,2] It has been shown that melanophores of lower vertebrates, particularly those of amphibians and reptiles, are affected by cholinergic agents such as acetylcholine leading to skin darkening. [6,7] Since thymoquinone is a known cholinergic stimulant in some muscle systems, such as the respiratory tract of guinea-pigs, [8] it would be interesting to explore its role as a neuromodu- lator in reptilian pigment cells. Further, there are very few studies on the effects of phar- maceutical agents from natural plant extracts that have melanogenic action on vertebrate pigment cells, the melanophores, which offer excellent opportunities to study the cellular reactions in response to externally applied stimuli. Short Communication JPP 2011, 63: 741–746 © 2011 The Authors JPP © 2011 Royal Pharmaceutical Society Received August 12, 2010 Accepted February 21, 2011 DOI 10.1111/j.2042-7158.2011.01271.x ISSN 0022-3573 Correspondence: Sharique A. Ali, Department of Biotechnology, Saifia College of Science and Education, Bhopal 462001, India. E-mail: drshariqueali@yahoo.co.in 741 Downloaded from https://academic.oup.com/jpp/article/63/5/741/6135566 by guest on 14 June 2022