The role of β-Nicotyrine in E-Cigarette abuse liability I: Drug Discrimination

Abstract

Background: β-Nicotyrine (β-Nic) is a unique minor alkaloid constituent in electronic nicotine delivery systems (ENDS) that is derived from nicotine (Nic) degradation and can reach 25% of Nic concentrations in ENDS aerosol. β-Nic slows Nic metabolism and prolongs systemic Nic exposure, which may alter the discriminability of Nic. The present study sought to examine β-Nic has interoceptive effects itself, and if it alters the subjective effects ENDS products within a drug-discrimination paradigm.

Methods: The pharmacodynamics of β-Nic were examined in vitro, and a nicotine discrimination paradigm was used to determine if β-Nic (0 - 5.0 mg/kg) shares discriminative stimulus properties with Nic (0.2 mg/kg) in male (n = 13) and female (n = 14) rats after 10- & 60-min β-Nic pretreatment delays. A second group of rats was trained to discriminate β-Nic and Nornicotine (Nornic) from saline to determine if β-Nic alone has interoceptive properties and whether they are similar to Nornic.

Results: β-Nic had similar binding affinity and efficacy at the α4β2 nicotinic receptor subtype as Nornic, ~50% of Nic efficacy. However, β-Nic only weakly substituted for Nic during substitution testing in female rats, but not males, whereas Nornic fully substituted for Nic. Combination testing at the 10 and 60-min pretreatment intervals showed that β-Nic dose-dependently increased the duration of nicotine's discriminative stimulus effects, especially at the 60-min delay. Drug naïve rats could reliably discriminate Nornic, but not β-Nic, from Sal.

Conclusion: β-Nic increased and prolonged the interoceptive stimulus properties of Nic, suggesting it may alter to the abuse liability of ENDS through its ability to slow Nic metabolism.

Figure 1.

(A) Inhibition curve showing total nicotine radioligand binding (DPM) across a range of Nicotine, β-Nic and Nornic doses (−5 = 10μM, −6 = 1 μM, etc.). (B) Concentration-effect curve for stimulation of ⁸⁶Rb⁺ efflux across a range of Nicotine, β-Nic and Nornic doses in Mols (M) as a percentage of 100 uM of nicotine. (−4 = 100μM, −5 = 10μM, etc.). (C) Concentration-effect curves for β-Nic and Nornic showing the percent inhibition of 100 uM nicotine on ⁸⁶Rb⁺ efflux.

Figure 2.

Percent nicotine lever responding and response rate (r/s) during the 2-min nicotine substation probes in male and female rats administered Nic, Nornic or β-Nic 10-min prior to the start of the session. † - indicates a main effect of sex. & - indicats interaction of sex × dose and at what dose post-hoc tests revealed differences. * and # - indicates a significant Sidak post-hoc difference from saline in females and males, respectively.

Figure 3.

Percent nicotine lever responding during the 2-min test session as a function of Nic dose (note log axis) during Nic alone and Nic + 0.05/0.25/1.0 β-Nic under a 10-minute pretreatment schedule. Note that the Nic alone cure is the same in each panel within each sex to facilitate comparisons to combination treatments. # denotes a significant main effect of β-Nic. & denotes a significant interaction with β-Nic. * denotes a significant difference from Nic alone during post-hoc tests.

Figure 4.

Percent nicotine lever responding during the 2-min test session as a function of Nic dose (note log axis) during Nic alone and Nic + 0.05/0.25/1.0 β-Nic under a 60-minute pretreatment schedule. # denotes a significant main effect of, or an interaction with β-Nic. * denotes a significant difference from Nic alone during post-hoc tests. † - indicates a significant difference between sexes during post-hoc tests of the Nic alone condition.

Figure 5.

Response rates (responses/second; r/s) during the 2-min test session across β-Nic doses under the 10- and 60-min pretreatment intervals in male and female rats. *&#$ each denote a significant difference from Sal from Nic alone and Nic + 0.025/0.05/1.0 β-Nic combination doses, respectively, during post-hoc tests. † - indicates a significant difference between sexes across the bracketed Nic doses collapsing across β-Nic dose.

Figure 6.

The left panel shows the percent drug appropriate lever responding across training sessions with 2 and 5 mg/kg training dose of β-Nic and Nornic and the right panel shows the individual averages of the last three sessions in each Nornic training session in male and female rats. * denotes significant difference between Nornic and β-Nic during the final 3 training sessions.