LETTER TO THE EDITOR On the Mechanism of Nitriles Toxicity

This is apropos to some issues raised by Llorens et al. (2009) on our recent publication unraveling the mode of toxicities of acrylonitrile (ACN) and iminodipropionitrile (IDPN) in rats (Khan et al., 2009). In the following text, we have attempted to provide point-wise explanations to the queries of Llorens et al. in the light of relevant literature. The transient behavioral effects appeared after ACN injections were evaluated by visual observation and the grading scores of (absent), þ (mild), þþ (moderate), and þþþ (severe) were collectively assigned to each treatment group (Khan et al., 2009). In fact, our main concentration was to assess the behavioral abnormalities associated with the excitation, chorea, and circling (ECC) syndrome, for which IDPN was used as a positive control and a well-defined behavioral testing battery was applied (Khan et al., 2009). The use of subjective observation of behavioral and morphological signs is a common practice in pharmacological and toxicological studies. Llorens et al. themselves have used similar subjective grading of loss of vestibular function, rated from 0 (normal) to 4 (extreme alteration) using tail hang test (Seoane et al., 1999, 2003). Moreover, they have also evaluated the nitrile-induced corneal opacity by simple observation and subjective rating of the animals from 0 (no evidence of corneal opacity) to 4 (maximal corneal opacity) (Balbuena and Llorens, 2001, 2003; Seoane et al., 1999). In their Letter to Editor, Llorens et al have objected the overlooking of certain references by us while discussing the behavioral effects of ACN (Khan et al., 2009). In fact, we preferred to cite the work of Gagnaire et al. (1998) being more recent and relevant to the dose regimen of ACN used by us. We have also discussed the findings of Rongzhu et al. (2007) on neurobehavioral alterations related to locomotor activities, motor coordination, learning, and memory in ACN-treated rats to portrait a wider picture on this topic. Nevertheless, we feel that the selection of references be solely left on authors’ discretion. Of course the learned Reviewers of the manuscript are the most competent authorities to suggest any modification in the cited literature. Llorens et al. (1993) have suggested an identity between the IDPN-induced ECC syndrome and the behavioral deficits resulting from bilateral labyrinthectomy; they justified their claim on the basis of only one rat subjected to bilabyrinthectomy. We agree that bilabyrinthectomy can produce behavioral deficits akin to those seen in IDPN exposed rats. However, the pathogenesis of both the conditions may not be identical unless evidences prove that bilabyrinthectomy can also cause axonal swelling similar to IDPN (Chou and Hartmann, 1964; Clark et al., 1980) as well as the drugs that modify IDPN-induced ECC syndrome also exert similar effects on the behavior of bilabyrinthectomized rats. Regarding the structure-function relationship, ACN (CH21⁄4CH–CN) is apparently more similar to allylnitrile (CH21⁄4CH–CH2–CN) than crotonitrile (CH3– CH1⁄4CH–CN) as the later compound possesses the peculiar CH1⁄4CH group rendering it to conceive cis-trans isomerism. Our findings on intense tyrosine hydroxylase (TH) immunostaining of dopaminergic neurons in the anterior striatum (but not the medial region) of IDPN-treated rats (Khan et al., 2009) are somewhat different from the observations of Llorens group who found no change in striatal dopamine (DA) concentrations following IDPN exposure in rats (data not shown in their paper) (Seoane et al., 1999). This discrepancy may be attributed to the differences in animal strain (Wistar vs. Long-Evans), dose of IDPN (8 3 100 vs. 3 3 400 mg/kg), analysis time post dosing (2 days vs. 4 weeks) used in our and their studies respectively. It is also important to note that DA turnover and not the DA content alone is a reliable predictor of dopaminergic status (Ogawa et al., 1990, 1991; Tariq et al., 1999). Moreover the role of dopaminergic neurotransmission in IDPN toxicity is supported by several studies demonstrating significant inhibition of IDPN-induced behavioral syndrome by DA antagonists (Cadet et al., 1987; Khan et al., 2004; Ogawa et al., 1991). Another point raised by Llorens et al. is the use of a single rat per group for the histopathology of vestibular sensory 1 To whom correspondence should be addressed: Prince Sultan Research Chair for Environment & Wildlife, College of Science, Bld 5, Room 2B42, King Saud University, P.O. Box 2455, Riyadh 11451, Saudi Arabia. E-mail: [email protected].