Effects of nicotine in combination with drugs described as positive allosteric nicotinic acetylcholine receptor modulators in vitro: discriminative stimulus and hypothermic effects in mice

Abstract

Some drugs that are positive allosteric nAChR modulators in vitro, desformylflustrabromine (dFBr), PNU-120596 and LY 2087101, have not been fully characterized in vivo. These drugs were examined for their capacity to share or modify the hypothermic and discriminative stimulus effects of nicotine (1mg/kg s.c.) in male C57Bl/6J mice. Nicotine, dFBr, and PNU-120596 produced significant hypothermia, whereas LY 2087101 (up to 100mg/kg) did not. Nicotine dose-dependently increased nicotine-appropriate responding and decreased response rate; the respective ED50 values were 0.56mg/kg and 0.91mg/kg. The modulators produced no more than 38% nicotine-appropriate responding up to doses that disrupted operant responding. Rank order potency was the same for hypothermia and rate-decreasing effects: nicotine>dFBr>PNU-120596=LY 2087101. Mecamylamine and the α4β2 nAChR antagonist dihydro-β-erythroidine, but not the α7 antagonist methyllycaconitine, antagonized the hypothermic effects of nicotine. In contrast, mecamylamine did not antagonize the hypothermic effects of the modulators. The combined discriminative stimulus effects of DFBr and nicotine were synergistic, whereas the combined hypothermic effects of nicotine with either dFBr or PNU-120596 were infra-additive. PNU-120596 did not modify the nicotine discriminative stimulus, and LY 2087101 did not significantly modify either effect of nicotine. Positive modulation of nicotine at nAChRs by PNU-120596 and LY 2087101 in vitro does not appear to confer enhancement of the nAChR-mediated hypothermic or discriminative stimulus effects of nicotine. However, dFBr appears to be a positive allosteric modulator of some behavioral effects of nicotine at doses of dFBr smaller than the doses producing unwanted effects (e.g. hypothermia) through non-nAChR mechanisms.

Keywords: Additivity; Drug discrimination; Hypothermia; Nicotinic acetylcholine receptor; Positive allosteric modulator; Synergy.

Fig. 1

Change in rectal temperature following nicotine, dFBr, PNU-120596, or LY 2087101 as a function of dose. Abscissa: Saline (Sal), vehicle consisting of propylene glycol, Tween 80, and saline (PGT) and dose in milligram per kilogram body weight. Ordinate: change in rectal temperature from pre-injection baseline expressed as °C.

Fig. 2

Change in rectal temperature over time following nicotine (1 mg/kg), dFBr (32 mg/kg) or PNU-120596 (100 mg/kg) alone and in combination with either 1 mg/kg mecamylamine (top) or 3.2 mg/kg mecamylamine (bottom). Abscissae: time in min following administration of agonist or vehicle. Ordinates: change in rectal temperature from pre-injection baseline (i.e. immediately before time 0) expressed as °C. Data for nicotine alone (open circles, left), dFBr alone (open triangles, middle) and PNU-120596 alone (open inverted triangles, right) are re-plotted in the top and bottom panels.

Fig. 3

Change in rectal temperature over time following nicotine (1 mg/kg) or dFBr (32 mg/kg) alone and in combination with either 3.2 mg/kg DHβE (top) or 10 mg/kg MLA (bottom). Abscissae: time in min following administration of agonist or vehicle. Ordinates: change in rectal temperature from pre-injection baseline (i.e. immediately before time 0) expressed as °C. Data for nicotine alone (open circles, left) and dFBr alone (open triangles, right) are re-plotted in the top and bottom panels.

Fig. 4

Change in rectal temperature as a function of nicotine dose administered alone and in combination with dFBr (10 and 32 mg/kg; left) or PNU-120596 (100 mg/kg; right). Abscissae: dose of positive allosteric modulator alone or vehicle (Veh) and dose of nicotine in milligram per kilogram bodyweight. Ordinates: change in rectal temperature from pre-injection baseline (i.e. immediately before time 0) expressed as °C. The same control nicotine dose-response function (circles) is plotted in the left and right panels.

Fig. 5

The expected effects (open symbols) and observed effects (filled symbols) of nicotine in combination with either 32 mg/kg dFBr (left) or 100 mg/kg PNU-120596 (right). The expected effects were calculated as described in section 2.6 Data analyses. Abscissae: nicotine dose expressed as the sum of the experimentally administered dose and the dose of nicotine corresponding to the change in rectal temperature produced by 32 mg/kg dFBr (left) or 100 mg/kg PNU-120596 alone (right), as determined from non-linear regression of the experimentally derived nicotine dose-response function. Ordinates: change in rectal temperature from pre-injection baseline (i.e. immediately before time 0) expressed as °C.

Fig. 6

Dose-effect functions for the discriminative stimulus effects (top) and rate effects (bottom) of nicotine, dFBr, PNU-120596 and LY 2087101. Abscissae: Saline (Sal), vehicle consisting of propylene glycol, Tween 80, and saline (PGT) and dose in milligram per kilogram body weight. Ordinates: the percentage of nicotine-appropriate responses (top) and rate of responding in responses per s calculated as a percent of the control response rate (bottom).

Fig. 7

Dose-effect functions for the discriminative stimulus effects (top) and rate effects (bottom) of nicotine alone and in combination with 3.2 mg/kg dFBr (left), 17.8 mg/kg PNU-120596 (middle) or 56 mg/kg LY 2087101 (right). Abscissae: doses of positive allosteric modulator alone or vehicle (Veh) and dose of nicotine in milligram per kilogram bodyweight. Ordinates: the percentage of nicotine-appropriate responses (top) and rate of responding in responses per s calculated as a percent of the control response rate (bottom). The same control nicotine dose-response function (circles) are plotted in the top panels for discriminative stimulus effects and the bottom panels for rate-decreasing effects.