Genetic deletion or pharmacological blockade of nociceptin/orphanin FQ receptors in the ventral tegmental area attenuates nicotine-motivated behaviour

Abstract

Background and purpose: The nociceptin/orphanin FQ (N/OFQ)-nociceptin opioid-like peptide (NOP) receptor system is widely distributed in the brain and pharmacological activation of this system revealed therapeutic potential in animal models of substance use disorder. Studies also showed that genetic deletion or pharmacological blockade of NOP receptors confer resistance to the development of alcohol abuse. Here, we have used a genetic and pharmacological approach to evaluate the therapeutic potential of NOP antagonism in smoking cessation.

Experimental approach: Constitutive NOP receptor knockout rats (NOP^-/- ) and their wild-type counterparts (NOP^+/+ ) were tested over a range of behaviours to characterize their motivation for nicotine. We next explored the effects of systemic administration of the NOP receptor antagonist LY2817412 (1.0 & 3.0 mg·kg^-1 ) on nicotine self-administration. NOP receptor blockade was further evaluated at the brain circuitry level, by microinjecting LY2817412 (3.0 & 6.0 μg·μl^-1 ) into the ventral tegmental area (VTA), nucleus accumbens (NAc) and central amygdala (CeA).

Key results: Genetic NOP receptor deletion resulted in decreased nicotine intake, decreased motivation to self-administer and attenuation of cue-induced nicotine reinstatement. LY2817412 reduced nicotine intake in NOP^+/+ but not in NOP^-/- rats, confirming that its effect is mediated by inhibition of NOP transmission. Finally, injection of LY2817412 into the VTA but not into the NAc or CeA decreased nicotine self-administration.

Conclusions and implications: These findings indicate that inhibition of NOP transmission attenuates the motivation for nicotine through mechanisms involving the VTA and suggest that NOP receptor antagonism may represent a potential treatment for smoking cessation.

Keywords: NOP; VTA; nicotine; reinforcement; relapse; reward.

Figure 1.. Nicotine addictive-behaviours in NOP ^(+/+) and NOP ^(−/−) rats.

(A) Acquisition pattern of nicotine self-administration in NOP ^(+/+) (n = 11) and NOP ^(−/−) (n = 12) rats. The number of nicotine infusions and total number of active and inactive lever presses are shown for fixed ratio-1 (FR-1; day 1–9) and fixed ratio-3 (FR-3; day 10 –25) conditions. ***p < 0.001 vs. NOP ^(+/+) and NOP ^(−/−) rats nicotine infusions on days 23, 24 and 25. *p < 0.05 vs. NOP ^(+/+) and NOP ^(−/−) rats active lever presses in FR3. (B) Motivation for nicotine as measured by the break point on a progressive-ratio (PR) schedule of reinforcement and corresponding nicotine infusions earned in NOP ^(+/+) (n = 11) and NOP ^(−/−) (n = 12) rats. **p < 0.01 vs. NOP ^(+/+) and NOP ^(−/−) rats. (C) Cue-induced reinstatement of nicotine seeking after 21 days of withdrawal in NOP ^(+/+) (n = 8) and NOP ^(−/−) (n = 9) rats compared to their respective baseline (average of active lever presses and inactive lever presses during the last 4 days of training). ***p < 0.001 vs. NOP ^(+/+) and NOP ^(−/−) rats. ***p < 0.001 vs. NOP ^(+/+) baseline responding and NOP ^(+/+) reinstatement responding. Values represent mean (± SEM).

Figure 2.. Systemic effects of the NOP antagonist LY2817412 on nicotine self-administration in NOP ^(+/+) and NOP ^(−/−) rats.

(A) Number of responses (infusions and total number of active and inactive lever presses) following systemic LY2817412 administration (0.0, 1.0 and 3.0 mg/kg) in NOP ^(+/+) (n = 11) and (B) NOP ^(−/−) (n = 12) rats. ***p < 0.001 vs. 0.0 mg/kg vs 1.0 and 3.0 mg/kg. Values represent mean (± SEM).

Figure 3.. Central effects of the NOP antagonist LY2817412 on nicotine self-administration in VTA, NAC and CeA in NOP ^(+/+) rats.

(A) Number of responses (infusions and total number of active and inactive lever presses) following central LY2817412 injection (0.0, 3.0 and 6.0 μg/μl) in VTA (n = 9), (B) NAc (n = 8) and (C) CeA (n = 11) in NOP ^(+/+) rats. ***p < 0.001 vs. 0.0 μg/μl vs 3.0 μg/μl and **p < 0.01 vs. 0.0 μg/μl vs 6.0 μg/μl. Values represent mean (± SEM).

Figure 4.. Schematic drawing of N/OFQ-NOP system in the ventral tegmental area (VTA) – Nucleus accumbens (Alexander et al.)

Within the VTA, NOP positive neurons are expressed in DAergic, GABAergic and Glutamatergic cells. The figure illustrates the possible mechanism through which NOP signalling influences the cellular components affecting the mesolimbic DA transmission. N/OFQ exerts an inhibitory role in the activity of both GABAergic interneurons, and DAergic neurons within the VTA. At a presynaptic lever, activation of the NOP receptors inhibits GABA release onto dopamine neurons, with subsequent disinhibition and increase in DA release. However simultaneous activation of NOP receptors that are located in DA cells prevent such disinhibition resulting in a final attenuation of the DA neurotransmission.