Activation of nociceptin opioid peptide (NOP) receptor impairs contextual fear learning in mice through glutamatergic mechanisms

Abstract

The present study investigated whether the selective nociceptin opioid peptide (NOP) receptor agonist, Ro64-6198, impairs acquisition of fear conditioning through glutamatergic mechanisms. Systemic administration of Ro64-6198 (0.3 and 1mg/kg) or the non-competitive NMDA receptor antagonist, MK-801 (0.03 and 0.1mg/kg) prior to conditioning severely impaired contextual but not cued fear learning in C57BL/6N mice. When administered together at sub-effective doses, Ro64-6198 (0.5mg/kg) and MK-801 (0.05mg/kg), synergistically impaired contextual fear learning, but left cued fear learning intact. We next used the immediate shock deficit paradigm (ISD) to examine the effects of Ro64-6198 and MK-801 on contextual memory formation in the absence of the foot-shock. As expected, naive mice that were shocked briefly after being placed in the training chamber displayed no contextual fear conditioning. This learning deficit was elevated by prior exposure of mice to the training context. Furthermore, administration of Ro64-6198 and MK-801, either separately at amnesic doses (1mg/kg and 0.1mg/kg, respectively) or concomitantly at sub-effective doses (0.5mg/kg and 0.05mg/kg, respectively) significantly reduced the facilitating effects of context preexposure. These findings demonstrate the existence of functional antagonism between NOP and NMDA receptors that predominantly contributes to modulation of conditioned fear learning which involves spatial-processing demands.

Fig. 1

Effects of Ro64-6198 or morphine on acquisition of contextual and cued fear conditioning. Top panels, (a), (b) and (c): effects of Ro64-6198 (0, 0.3 and 1 mg/kg, n = 11–12 per dose) on immobility scores during conditioning, context and auditory cue testing, respectively. Mice were treated with Ro64-6198 injections before conditioning, and contextual and cued fear memories were assessed the following day in separate testing sessions. (d) Lack of state-dependant effects of Ro64-6198 on contextual fear conditioning. Mice received an injection of Ro64-6198 (0 or 1 mg/kg, n = 8–10 per dose) before both training and context testing sessions. Lower panels, (a′), (b′) and (c′): Effects of morphine (0, 2 and 4 mg/kg, n = 8–9 per dose) on immobility scores during conditioning, context and auditory cue testing, respectively. Mice were treated with morphine injections before conditioning and tested the following day drug free. (d′): State-dependant effects of morphine on contextual fear conditioning. Mice received an injection of morphine (0 or 4 mg/kg, n = 9 per dose) before both training and context testing sessions. Data are expressed as mean ± SEM percentage immobility. *p < 0.05, significantly different from corresponding control group (Tukey–Kramer post-hoc test). ⁺p < 0.05, significantly different from corresponding control group (Student's t tests).

Fig. 2

Effects of Ro64-6198 and MK-801 on acquisition of contextual and cued fear conditioning. Left panels, (a) and (b): effects of MK-801 (0, 0.03 and 0.1 mg/kg, n = 7–8 per dose) on immobility scores during context and auditory cue testing, respectively. Mice received injections of MK-801 before conditioning and submitted the following day to context and auditory cue testing drug free. Right panels, (c) and (d): effects of Ro64-6198 and MK-801 on immobility scores during context and auditory cue testing, respectively. Mice (n = 7–8 per treatment) received separate or conjoint injections of Ro64-6198 (0.5 mg/kg) and MK-801 (0.05 mg/kg) before conditioning, contextual and cued fear memories were then assessed the following day in separate testing sessions. Data are expressed as mean ± SEM percentage immobility. *p < 0.05, significantly different from vehicle-treated group (Tukey–Kramer post-hoc test). ^#p < 0.05, significantly different from all other groups (Tukey–Kramer post-hoc test).

Fig. 3

Effects of Ro64-6198 and MK-801 on contextual memory formation. (a) Effects of preexposure to training context on post-shock freezing behavior during the immediate shock deficit conditioning session. Preexposed groups (PE): mice were preexposed to the training context for 4 or 8 min and submitted the following day to the immediate shock deficit (ISD) conditioning session (n = 8 per duration). NPE: Non-preexposed control group (n = 17). (b) Effects of Ro64-6198 (1 mg/kg) and MK-801 (0.1 mg/kg) on the facilitation effects of context preexposure. Preexposed groups (PE): mice received an injection of vehicle (n = 9), Ro64-6198 (n = 8) or MK-801 (n = 7) before preexposure to training context (8 min duration) and submitted the following day to ISD conditioning session. NPE: non-preexposed mice received vehicle injection in their home cage in the first day and submitted the following day to the ISD conditioning session (n = 8). Naïve: non-preexposed mice that did not experience the foot-shock during the ISD conditioning session. (c) Effects of Ro64-6198 (0.5 mg/kg) and MK-801 (0.05 mg/kg) administered alone (n = 6–8 per treatment) or conjointly (n = 7) on the facilitation effects of context preexposure. Preexposed groups (PE): mice received drug treatments before preexposure to the training chamber and submitted to the ISD conditioning session the following day. Data are expressed as mean ± SEM percentage of immobility. *p < 0.05, significantly different from NPE control group (Tukey–Kramer post-hoc test). ^φp < 0.05, significantly different from naïve and NPE control groups (Tukey–Kramer post-hoc test). ⁺p < 0.05, significantly different from vehicle-treated counterparts (Tukey–Kramer post-hoc test).