Antitussive activity of sigma-1 receptor agonists in the guinea-pig

Current antitussive medications have limited efficacy and often contain the opiate-like agent dextromethorphan (DEX). The mechanism whereby DEX inhibits cough is ill defined. DEX displays affinity at both NMDA and sigma receptors, suggesting that the antitussive activity may involve central or peripheral activity at either of these receptors. This study examined and compared the antitussive activity of DEX and various putative sigma receptor agonists in the guinea-pig citric-acid cough model. Intraperitoneal (i.p.) administration of DEX (30 mg kg−1) and the sigma-1 agonists SKF-10,047 (1–5 mg kg−1), Pre-084 (5 mg kg−1), and carbetapentane (1–5 mg kg−1) inhibited citric-acid-induced cough in guinea-pigs. Intraperitoneal administration of a sigma-1 antagonist, BD 1047 (1–5 mg kg−1), reversed the inhibition of cough elicited by SKF-10,047. In addition, two structurally dissimilar sigma agonists SKF-10,047 (1 mg ml−1) and Pre-084 (1 mg ml−1) inhibited cough when administered by aerosol. Aerosolized BD 1047 (1 mg ml−1, 30 min) prevented the antitussive action of SKF-10,047 (5 mg kg−1) or DEX (30 mg kg−1) given by i.p. administration and, likewise, i.p. administration of BD 1047 (5 mg kg−1) prevented the antitussive action of SKF-10,047 given by aerosol (1 mg ml−1). These results therefore support the argument that antitussive effects of DEX may be mediated via sigma receptors, since both systemic and aerosol administration of sigma-1 receptor agonists inhibit citric-acid-induced cough in guinea-pigs. While significant systemic exposure is possible with aerosol administration, the very low doses administered (estimated <0.3 mg kg−1) suggest that there may be a peripheral component to the antitussive effect. Keywords: Sigma-1 agonist, antitussive, cough, guinea-pig Introduction Chronic cough associated with either irritation and/or inflammation of the airways is a common symptom of many respiratory diseases such as COPD, chronic bronchitis and asthma (Higenbottam, 2002). The most common antitussives in use today include the opioid, codeine, and the opiate-like agent dextromethorphan (DEX) (Grattan et al., 1995; Chung, 2003). These antitussives have limited efficacy and appear to act via a central mechanism (through receptors close to the cough center in the brainstem) (Chou & Wang, 1975). As a consequence, they often display an undesirable side effect profile including sedation, respiratory depression, and altered gastrointestinal motility (Tortella et al., 1994; Tarkkila et al., 1997; Eckhardt et al., 1998; Walker & Zacny, 1998; Hoffmann et al., 2003). Thus, there is a clinical need for a more effective antitussive that exhibits efficacy with a reduction in side effects. The precise mechanism whereby DEX inhibits cough is still ill defined. DEX displays antagonist and agonist activity at the cationic NMDA receptor channel (Franklin & Murray, 1992) and sigma receptors (Chau et al., 1983), respectively, suggesting that the antitussive action of DEX may involve an action at either of these receptors. There is some additional evidence that DEX may also have some action at opioid receptors due to the structural similarities to opiates (Allen et al., 2002; Baker et al., 2002a, 2002b; Zhu et al., 2003). In mice, capsaicin-induced cough is attenuated by the NMDA receptor antagonists AP-5, AP-7, and MK801 (Kamei et al., 1989), at a similar dose as elicited by DEX. Sigma receptor agonists pentazocine and N-allylnormetazocine (SKF-10,047) also inhibit cough in rodents (Kamei et al., 1992a). The antitussive effect of DEX, given by intraperitoneal (i.p.) injection, is reversed by rimcazole, a nonselective sigma receptor antagonist (Kamei et al., 1993; Kotzer et al., 2000). Despite this evidence, the role of the sigma receptor in the modulation of cough as a novel therapeutic target for antitussives has been largely ignored. Sigma receptors are a ubiquitous type of receptor whose function has not been well defined. The sigma-1 receptor sequence, while highly conserved (>90%) across mammalian species/tissues (Barnes et al., 1992; Schuster et al., 1995), does not display homology to any other known receptors, although it does share 30% homology with a fungal sterol isomerase (Moebius et al., 1996). Sigma receptors have a single transmembrane region (Prasad et al., 1998; Yamamoto et al., 1999), are 18–26 kDa molecular weight proteins, and there are at least two subtypes (σ1 and σ2) (McCann et al., 1994; Vilner et al., 1995; Bergeron & Debonnel, 1997). To date, only the sigma-1 receptor has been cloned (Kekuda et al., 1996; Seth et al., 1998; Mei & Pasternak, 2001) with cloning of the sigma-2 receptor proving difficult. Initially, the sigma receptor was thought to be an opioid receptor due to high-affinity enantiomer selective binding of various opiates as well as steroids and psychoactive drugs to the sigma receptor (Martin et al., 1976). This misnomer still persists, despite the re-classification as a nonopioid receptor and the effects of these opiates being insensitive to the opioid antagonist naltrexone (Vaupel, 1983). Sigma receptors are found in various tissues throughout the body; however, the density is not uniform. The highest concentration of receptors is seen mainly in limbic and motor areas of the CNS, followed by the periphery (liver, spleen, endocrine, GI tract, and lung) (Roman et al., 1989; Wolfe et al., 1989; 1997; Whitlock et al., 1996; Alonso et al., 2000). The endogenous ligand for the sigma receptor is not yet known, but has been hypothesized to be progesterone (Meyer et al., 1998). While the specific function of the sigma receptor remains elusive, sigma receptors are present in high concentrations in areas of the CNS related to sensory processing such as the dorsal root ganglia and the nucleus tractus solitarus (NTS) (Walker et al., 1990; Alonso et al., 2000). The NTS is a site where the airway afferent fibers first synapse and an area very close to the cough center in the brainstem. This region, therefore, seems ideally placed to function as a ‘gate' for the cough reflex, and therefore antitussives acting through the sigma receptor could conceivably modulate afferent activity prior to reaching the cough center. The role of sigma receptors in mediating the antitussive effects of DEX has not been systematically examined. Existing literature, which indicates the possible involvement of sigma-1 receptors, has all been conducted in the rat (Kamei et al., 1992a), a species in which it is difficult to measure cough consistently. To our knowledge, no one has investigated the role of sigma receptors using the guinea-pig, which, owing to the higher propensity to cough than rats, is a much preferred species to conduct antitussive research. The objective of this study was to further investigate whether DEX, as well as other putative sigma receptor agonists, inhibit citric-acid-induced cough in guinea-pigs.