AG‐3–5: a chemical producing sensations of cold

Menthol (Fig. 1) is widely used because it produces sensations of coolness and because it has a pleasing mint flavour and odour (USDHEW Publication 1979). The cold sensations elicited by menthol are a fact of human experience. In animal studies, it has been shown that menthol exerts a selective stimulation of peripheral cold receptors in the tongue, as measured by action potentials in the lingual nerve of the cat (Hensel& Zotterman 1951). We describe here a chemical, AG-3-5, which, like menthol, has the unusual property of producing cold sensations. AG-3-5 represents a class of compounds known as tetrahydropyrimidine-2-one derivatives (Fig. 1). In 1972, Podesva & DoNascimento of Delmar Chemicals, Ltd discovered that such compounds affect behaviour in animals. Processes for making these compounds were patented by these investigators. The effects of AG-3-5 were first described by Burford & Chappel (1972). Pharmacologists working at Delmar Chemicals, Ltd stated that AG-3-5 produced in rats and monkeys, among other effects, a behavioural change known as ‘wet shakes’ which are rotational body movements of animals similar to those made by a dog when wet. We were interested in AG-3-5 because we had been studying narcotic withdrawal and one prominent feature of withdrawal in rats is repetitive shaking movements of the body (Wei 1981). A sample of AG-3-5 was obtained from Delmar Chemicals, Ltd and, when tested in rodents, the shaking behaviour was observed (Wei 1976). Low doses of AG-3-5, usually less than 1 mg kg-1 , were sufficient to produce a high frequency of shaking. For example, 1 mg k g l of AG-3-5 administered intrapentoneally to rats elicited over 300 shakes in a 2 h observation period. AG-3-5 was effective in all laboratory species examined, namely, mice, rats, hamsters, gerbils, guinea-pigs, rabbits, cats and dogs (Wei 1976). The reasons for the pharmacological activity of AG-3-5 gradually became apparent as these experiments were conducted. In order to administer AG-3-5 to animals, this compound was dissolved in propylene glycol, a relatively non-toxic solvent frequently used in drug formulations. When the drug solution accidentally came into contact with our mucous membranes-nostrils, lips and eyelids-we noticed sensations of coolness which lasted for approximately 15 min. These sensations of coolness were also experienced by six other individuals in our laboratory who have had occasion to work with AG-3-5. Propylene glycol by itself was not active. On the basis of these preliminary observations, we decided to compare AG-3-5 to menthol, to examine its toxicity, and to taste and ingest a small quantity of the drug so that the sensations produced by this chemical might be characterized. Experiments were conducted on male albino rats, 250-450 g. In confirmation of previous results (Wei 1976), AG-3-5, dissolved in propylene glycol 1 mg k g l , produced in rats numerous and vigorous ‘wet shakes’ within 2 min of intraperitoneal (i.p.) injection. The median effective dose (ED50) for shaking, with a positive response defined as an animal which shook more than 10 times in the 10 min interval after injection, was estimated (Weill952) to be 0-18 mg kg1 (Table 1). Injections of menthol, dissolved 125 mg ml-l in 50% v/v ethanol-water, also produced shaking, the ED50 being 35 mg k g l i.p. The duration of shaking observed with menthol was shorter than for AG-3-5 possibly because at menthol doses >125 mg kg-1, animals became ataxic and lost their righting reflex. These high menthol doses also produced stretching movements of the abdomen (writhing) which may be indicative of local irritation. To determine the short-term toxic effects of AG-3-5, doses of up to 1.5 g k g l of Ag-3-5 were administered either orally or injected i.p. Surprisingly, none of the animals tested at these doses died or showed signs of debilitation or body weight loss in the one-month observation period following drug administration. This low acute toxicity of AG-3-5 was also mentioned in the patent disclosure for AG-3-5 but no data were presented (Podesva & DoNascimento 1974). By contrast, we confirmed the value for the median lethal dose (LD50)