Pharmacology Biochemistry and Behavior, Vol. 61, No. 3, pp. 271–280, 1998 © 1998 Elsevier Science Inc. Printed in the USA. All rights reserved 0091-3057/98 $19.00 + .00 PII S0091-3057(98)00108-7 271 The Effects of Inhaled Isoparaffins on Locomotor Activity and Operant Performance in Mice SCOTT E. BOWEN 1 AND ROBERT L. BALSTER Department of Pharmacology and Toxicology, Medical College of Virginia, Virginia Commonwealth University, Richmond, VA 23298-0310 Received 6 October 1997; Revised 5 March 1998; Accepted 25 March 1998 BOWEN, S. E. AND R. L. BALSTER. The effects of inhaled isoparaffins on locomotor activity and operant performance in mice. PHARMACOL BIOCHEM BEHAV 61(3) 271–280, 1998.—Very little is known qualitatively or quantitatively about the acute central nervous system effects of isoparaffin solvents that are widely used in household and commercial appli- cations. Four isoparaffinic hydrocarbon solvent products differing in predominant carbon number and volatility (ISOPAR- C™, -E™ -G™, -H™) were tested for their acute effects on locomotor activity and operant performance after inhalation ex- posure in mice. For both measures, concentration–effect curves were obtained for 30-min exposures using a within-subject design. The more volatile products, ISOPAR-C™ and -E™, were as easily vaporized under our conditions as vapors such as toluene and TCE, which have acute effects on human behavior and are abused. ISOPAR-G™ was slowly volatilized and ISOPAR-H™ could not be completely volatilized within our 30-min exposures, suggesting that acute human exposures may be less likely and that it may be more difficult to abuse them. ISOPAR-C™, -E™, and -G™ produced reversible increases in locomotor activity of mice at 4000 and 6000 ppm while ISOPAR-C™ and -E™ produced reversible concentration-dependent decreases in rates of schedule-controlled operant behavior in approximately the same concentration range as they affected lo- comotor activity. The fact that only locomotor activity increases were observed with these isoparaffins provides evidence that they produce a different pattern of effects than those reported for abused solvents such as toluene and TCE. Further research will be needed to determine if this different pattern of effects on animal behavior between isoparaffins and abused solvents is correlated with a different pattern of acute intoxication and abuse potential in humans. © 1998 Elsevier Science Inc. Solvents Inhalant abuse Isoparaffins Operant behavior Locomotor activity Toxicology HUMANS have always experimented with ways to achieve intoxication (28). A relatively simple means of altering con- sciousness is achieved through the inhalation of vapors from selected volatile compounds. Because these compounds are highly lipophilic and volatile at room temperature, their inha- lation results in a rapid absorption. This rapid uptake into the brain results in central nervous system (CNS) effects that can include euphoria and intoxication (42). This has resulted in the widespread abuse and sometimes lethal experimentation with chemical solvents used in many industrial and household products (1, 7, 25, 29, 36). Isoparaffins are branched aliphatic hydrocarbons used in the manufacture of many products, including paints and enamels, polishes, stain removers, inks, adhesives, charcoal lighter fluids, and typewriter correction fluids. They can also be found in some lotions intended for human use. Part of the basis for their increased use stems from the move to replace halogenated hydrocarbon solvents that damage the ozone with less environmentally toxic materials. In general, isoparaf- fins do not have a strong odor and are considered to have low systemic toxicity in humans (35). Isoparaffin solvent products are typically blends of isomers of branched hydrocarbons dif- fering in carbon number. This composition affects their vola- tility and hence suitability for various products. For example, the Exxon Corporation manufactures a variety of isoparaffins, named ISOPARs™ (18) including the more volatile ISO- PAR-C™ and ISOPAR-E™ and the less volatile ISOPAR- G™ and ISOPAR-H™ (see Table 1). Very little is known 1 Present address: Department of Psychology, Wayne State University, 71 W. Warren Ave., Detroit, MI 48202. Requests for reprints should be addressed to Robert L. Balster, Department of Pharmacology and Toxicology, Medical College of Virginia, Virginia Commonwealth University, Richmond, VA 23298-0310.