The drug discrimination procedure in animals has been extensively utilized to model the abuse related, subjective effects of drugs in humans, but it has seldom been used to examine abused volatile inhalants like toluene. The present study sought to characterize the temporal aspects of toluene's discriminative stimulus as well assess toluene blood concentrations under identical exposure conditions. B6SJLF1/J mice were trained to discriminate 10 min of exposure to 6000 ppm inhaled toluene vapor from air. Toluene vapor concentration dependently substituted for the training exposure condition with longer exposures to equivalent concentrations producing greater substitution than shorter exposures. Toluene's discriminative stimulus effects dissipated completely by 60 min after the cessation of exposure. Injected liquid toluene dose-dependently substituted for toluene vapor as well as augmenting the discriminative stimulus effects of inhaled toluene. Toluene blood concentrations measured under several exposure conditions which produced full substitution were all nearly identical suggesting that the concentration of toluene in the animals tissues at the time of testing determined discriminative performance. These results indicate that the discriminative stimulus effects of inhaled toluene vapor are likely mediated by CNS effects rather than by it's pronounced peripheral stimulus effects.

Rationale Despite widespread abuse there are few validated methods to study the rewarding effects of inhalants. One model that that may have utility for this purpose is intracranial self-stimulation (ICSS). Objectives We wished to compare and contrast the ICSS reward-facilitating effects of abused inhalants to other classes of abused drugs. Compounds were examined using two different ICSS procedures in mice to determine the generality of each drug's effects on ICSS and the sensitivity of the procedures. Methods Male C57BL/6J mice with electrodes implanted in the medial forebrain bundle were trained under a three component rate-frequency as well as a progressive ratio (PR) ICSS procedure. The effects of nitrous oxide, toluene vapor, cocaine and diazepam on ICSS were then examined. Results Concentrations of 1360-2900 ppm inhaled toluene vapor significantly facilitated ICSS in the rate frequency procedure and 1360 ppm increased PR breakpoint. A concentration of 40% nitrous oxide facilitated ICSS in the rate-frequency procedure but reduced PR breakpoint. Doses of 3-18 mg/kg cocaine facilitated ICSS in the rate frequency procedure and 10 and 18 mg/kg increased PR breakpoint. Doses of 1 and 3 mg/kg diazepam facilitated ICSS in the rate frequency procedure and 3 mg/kg increased PR breakpoint. Conclusions The reinforcement facilitating effect of toluene in ICSS is at least as great as diazepam. In contrast, nitrous oxide weakly enhances ICSS in only the rate frequency procedure. The data suggest that the rate frequency procedure may be more sensitive than the PR schedule to the reward facilitating effects of abused inhalants.

Despite both widespread clinical use and a long history of abuse, there is a limited understanding of the CNS actions of the anesthetic gas nitrous oxide (N2O). In vitro data has shown that N2O oxide alters the function of NMDA and GABAA receptors, amongst others. Our overarching goal was to use drug discrimination to assess the neurotransmitter systems responsible for producing the intoxicating, subjective stimulus effects of N2O. Sixteen male B6SJLF1/J mice were trained to discriminate 10 min of exposure to 60% inhaled N2O/40% O2 versus 100% O2 in daily 5‐min operant sessions. Subsequently substitution tests with other compounds were conducted. We hypothesized that if the discriminative stimulus effects of N2O are mediated by NMDA antagonism, drugs which attenuate NMDA receptor function would substitute for N2O. Thus far the NMDA channel blockers ketamine and MK‐801 have shown partial substitution for N2O. Of 8 mice tested with ketamine, 5 fully substituted at 7 or 10 mg/kg. Of 6 mice thus far tested with MK‐801, 4 fully substituted at 0.1– 0.3 mg/kg. The NMDA competitive antagonist CGS 19755 did not substitute for N2O, producing exclusively vehicle appropriate responding in 4 of 5 mice. These results suggest the subjective stimulus properties of N2O overlap with those of noncompetitive NMDA antagonists, suggesting common underlying mechanisms of action. Supported by NIDA grant RO1‐DA020553.

The present study utilized the drug discrimination paradigm to explore the neurotransmitter systems underlying isoflurane intoxication. Sixteen B6SJLF1/J mice were trained to discriminate 10 min of exposure to 6000 ppm isoflurane vapor from air using a standard 2‐lever operant task. The discriminative stimulus of the benzodiazepine midazolam and the barbiturate methohexital substantially overlapped with that of isoflurane. However, neither the benzodiazepine antagonist flumazenil nor the negative allosteric modulator Ro15‐4515 attenuated isoflurane's discriminative stimulus. The GABAA neurosteroid‐site positive modulator allopregnanolone and the extrasynaptic GABAA receptor agonist, gaboxadol, failed to substitute for isoflurane. Ethanol and the noncompetitive NMDA antagonist, ketamine partially substituted for isoflurane. These findings suggest that the discriminative stimulus effects of isoflurane are the result of GABAA positive modulation and to a lesser extent NMDA antagonism. However, they do not appear to be the result of a direct interaction with the benzodiazepine binding site on the GABAA receptor. Supported by NIDA grant RO1‐DA020553.