Role of intracellular Ca2+ signaling in the antinociceptive and discriminative stimulus effects of the imidazoline I2 receptor agonist 2-BFI in rats

Abstract

Rationale Recent research has established the imidazoline I2 receptor as a promising target for the development of novel analgesics. However, despite an increasing understanding of imidazoline I2 receptor-mediated behavioral effects, little is known about post-I2-receptor signaling mechanisms. Objective This study examined the effects of several inhibitors of Ca2+ signaling mechanisms on two behavioral effects of the prototypical imidazoline I2 receptor ligand 2-BFI. Methods The von Frey filament test was used to examine the antinociceptive effects of 2-BFI in complete Freund’s adjuvant (CFA)-induced inflammatory pain in rats. A two-lever drug discrimination paradigm in which rats were trained to discriminate 5.6 mg/kg (i.p.) 2-BFI from its vehicle was used to examine the discriminative stimulus effects of 2-BFI. Results The L-type Ca2+ channel blockers verapamil and nimodipine, the calmodulin antagonist W-7, and the internal Ca2+ release inhibitor ryanodine all attenuated the antinociceptive effects of 2-BFI. Oxycodone- and acetaminophen-induced antinociception were unaffected by pretreatment with the Ca2+ channel blockers. Rats learned to reliably discriminate 5.6 mg/kg 2-BFI from saline. The I2 receptor agonists BU224, RS45041, tracizoline, and CR4056 all fully substituted for 5.6 mg/kg 2-BFI while idazoxan, S22687, 2,5-Dimethoxy-4-methylamphetamine (DOM), and phenyzoline produced partial or no substitution. Verapamil, nimodipine, and W-7 did not alter the discriminative stimulus effects of 2-BFI. Conclusion These results indicate that the antinociceptive effects of 2-BFI involve intracellular Ca2+ elevation and/or downstream Ca2+/calmodulin signaling, whereas the discriminative stimulus effects of 2-BFI are mediated by a distinct, independent mechanism.

Mechanisms of imidazoline I2 receptor agonist‐induced antinociception in rats: involvement of monoaminergic neurotransmission

“…These results support previous findings regarding dissociation between I 2 receptor-mediated behavioural effects (Siemian et al, 2017) and provide further in vivo evidence that I 2 receptor populations and/or post-receptor signalling mechanisms are heterogeneous. In contrast, the hypothermic and discriminative stimulus effects of 2-BFI appear to be mediated by distinct, unrelated mechanisms.…”

“…We finally tested the role of monoaminergic mechanisms in mediating the discriminative stimulus effects of 2-BFI. Support for this also comes from a recent finding that the 5-HT 2A receptor agonist DOM did not substitute for 2-BFI (Siemian et al, 2017). Interestingly, MacInnes and Handley (2002; reported that serotonergic and noradrenergic components were involved in the discriminative stimulus effects of 2-BFI.…”

“…Several I 2 receptor agonists have been previously studied as discriminative stimuli including 2-BFI (Jordan et al, 1996;Qiu et al, 2014;Qiu et al, 2015;Siemian et al, 2017). Similar divergence was found in a recent study, which showed that I 2 receptor-mediated antinociception and discriminative stimulus effects were differentially sensitive to inhibitors of calcium influx or signalling mechanisms (Siemian et al, 2017), which also lends credence to the present findings, as calcium influx is necessary for vesicular monoamine release (Matthews, 1996). For example, monoamine releasers (d-amphetamine, fenfluramine), MAO inhibitors (moclobemide, pargyline), as well as 5-HT (clomipramine, citalopram) and noradrenaline (desipramine, reboxetine) reuptake inhibitors all at least partly substituted for 2-BFI.…”

“…Drug discrimination studies were performed using previously described chambers (Coulbourn Instruments Inc., Allentown, PA, USA) and software (Graphic State 3.03, Coulbourn Instruments Inc., Whitehall, PA, USA) Qiu et al, 2015). A modification of previously described training protocols was used (Siemian et al, 2017). Seven rats were trained to discriminate 5.6 mg·kg À1 2-BFI injected i.p.…”

“…We finally tested the role of monoaminergic mechanisms in mediating the discriminative stimulus effects of 2-BFI. Several I 2 receptor agonists have been previously studied as discriminative stimuli including 2-BFI (Jordan et al, 1996;Qiu et al, 2014;Qiu et al, 2015;Siemian et al, 2017). Interestingly, MacInnes and Handley (2002; reported that serotonergic and noradrenergic components were involved in the discriminative stimulus effects of 2-BFI.…”