The fourth member of the endogenous neuropeptide opioid family is the 17-amino acid peptide called nociceptin or orphanin FQ, discovered in 1995 (Meunier et al., 1995, Reinscheid et al., 1995). Henceforth referred to as N/OFQ, this neuropeptide acts through a Gi/o coupled receptor called nociceptin opioid peptide receptor (NOP), the fourth opioid receptor subtype, but has no measurable affinity for classical opioid receptors mu, delta and kappa (MOP, DOP, KOP) (Meunier et al., 2000). Since its discovery, the NOP-N/OFQ system has been shown to regulate many physiological functions including pain, inflammation, reward, food intake, stress, and memory (Lambert, 2008), which made the NOP receptor a promising pharmacological target. Preclinical studies have suggested in particular that NOP agonists may be beneficial for the treatment of pain, substance abuse, and anxiety (Toll et al., 2021, Zaveri, 2016). The neuropharmacology of the NOP-N/OFQ system provides compelling rationale that NOP receptor agonism is a viable approach to attenuate addiction-related behaviors and for medication development. Several small-molecule NOP agonists have been investigated in preclinical rodent models of drug reinforcement and drug reward such as self-administration, conditioned place preference (CPP), and reinstatement to drug seeking (Witkin et al., 2014, Zaveri, 2011). We recently showed that the NOP-selective agonist AT-312, from our laboratory, decreased cocaine CPP in mice (Zaveri et al., 2018), decreased cocaine self-administration in rats and reduced conditioned reinstatement of cocaine seeking without modifying food self-administration (Cippitelli et al., 2020). However, while NOP agonists are devoid of the usual opioid-induced side effects such as respiratory depression and abuse liability, the effects of chronic repeated administration of NOP agonists, as would be required for clinical use, has not been studied extensively. One concern is the putative amnesic effect of NOP agonists since N/OFQ has been shown to negatively affect learning and memory in rodents (Mouledous, 2019).

Intracerebroventricular (icv) administration of high doses of N/OFQ in mice or rats compromises learning and memory in the Morris water maze (MWM) (Kuzmin et al., 2009), the fear conditioning (FC) and inhibitory avoidance (IA) paradigms (Fornari et al., 2008, Hiramatsu and Inoue, 1999, Mamiya et al., 2003), and the object recognition (OR) task (Goeldner et al., 2008, Han et al., 2019). Similarly, systemic administration of the NOP agonist Ro 64-6198 (Wichmann et al., 2000) impairs learning and/or memory in the MWM (Higgins et al., 2002, Kuzmin et al., 2009), contextual FC (Goeldner et al., 2009), IA (Adem et al., 2017), and OR (Goeldner et al., 2008). In the FC paradigm, amnesic effects have been described for two other small-molecule NOP agonists, SR-8993 (Andero et al., 2013) and Ro 65-6570 (Rekik et al., 2017). However, the interpretation of the effects of these compounds are complicated by the existence of off-target effects due to their moderate selectivity for the NOP receptor. Indeed, the inhibition of IA learning by Ro 64-6198 was not blocked by the NOP antagonist [Nphe1]N/OFQ(1–13)-NH2 (Adem et al., 2017). Therefore, from a translational perspective, further evaluation of the amnesic effect of NOP agonists is essential.

Based on the above, this study was designed to assess the potential risks to memory from the use of NOP agonists. For these studies we used two different small-molecule NOP agonists having varying levels of selectivity, namely AT-202 (previously known as SR16835) which displays moderate selectivity for NOP over MOP receptor (Toll et al., 2009) and AT-524, a recently discovered highly selective NOP agonist. We first compared the effects of a single injection of these two NOP agonists on long-term memory using the object location (OL) memory task, an episodic-like memory task highly relevant for daily-life functioning in humans (Postma et al., 2008). We next assessed the consequences of repeated administration of high doses of these two NOP agonists on OL memory. Given the evidence of sex differences in the N/OFQ system regulation and functions (Claiborne et al., 2006, Sinchak et al., 2006, Zhang et al., 2018), the experiments were conducted in both male and female mice.