Pentylenetetrazol and Morphine Interaction in a State-dependent Memory Model: Role of CREB Signaling

Abstract

Introduction: State-dependent (STD) memory is a process, in which the learned information can be optimally retrieved only when the subject is in the state similar to the encoding phase. This phenomenon has been widely studied with morphine. Several studies have reported that Pentylenetetrazole (PTZ) impairs memory in experimental animal models. Due to certain mechanistic interactions between morphine and PTZ, it is hypothesized that PTZ may interfere with the morphine-STD. The cyclic adenosine monophosphate Response Element-Binding (CREB) is considered as the main downstream marker for long-term memory. This study was designed to determine the possible interaction between PTZ and morphine STD and the presumable changes in CREB mRNA.

Methods: In an Inhibitory Avoidance (IA) model, posttraining morphine (2.5, 5, and 7.5 mg/ kg-i.p.) was used. The pre-test morphine was evaluated for morphine-induced STD memory. Moreover, the effect of a pre-test PTZ (60 mg/kg-i.p.) was studied along with morphine STD. Locomotion testing was carried out using open-field. Eventually, using real-time-PCR, the CREB mRNA changes in the hippocampus were evaluated.

Results: Posttraining MOR (7.5 mg/kg-i.p.) impaired IA memory (P<0.001). The pre-test injection of similar doses of morphine recovered the morphine-induced memory impairment (P<0.001). The pre-test PTZ impaired the IA memory recall (P<0.001); however, the pre-test PTZ along with morphine STD potentiated the morphine-induced STD (P<0.001). Alterations in CREB mRNA were observed in all groups. No difference was seen in the locomotor activity.

Conclusion: Presumably, the certain interactive effect of PTZ on morphine-induced STD is mediated through gamma-aminobutyric acid and opioid systems via CREB signaling.

Keywords: CREB; Inhibitory avoidance memory; Morphine; PTZ; State-dependent memory.

Figure 1.

The morphine effect on memory, cyclic adenosine monophosphate response element element-binding (CREB) mRNA expression, and locomotor activity. A: The effects of posttraining and pre-test saline on the step-through latencies. Each value represents the Mean±SEM for 7 rats. *** P<0.001 compared with the posttraining and pre-testing saline; B: The relative expression of CREB mRNA (A/U). Each bar shows the Mean±SEM for 4 rats. *P<0.05 compared with the naïve group. ⁺⁺⁺ P<0.001 compared with the saline/saline group; C: The locomotor activity as the total distance traveled (cm); The values are represented as the Mean±SEM for 7 rats.

Figure 2.

The morphine state-dependent memory, cyclic adenosine monophosphate response element element-binding (CREB) mRNA expression, and locomotor activity. A: The effect of morphine on state-dependent memory retrieval. Data are shown as Mean±SEM for seven animals per group. *P<0.05 compared with the posttraining saline/pre-test saline group; ^#P<0.05 compared with the posttraining morphine (7.5mg/kg)/pre-test saline group; B: The relative expression of CREB mRNA (A/U). Each bar shows the Mean±SEM for 4 rats. *P<0.05 compared with the naïve group; ⁺P <0.05 and ⁺⁺⁺P<0.001 compared with the saline/saline group, and ^###P<0.001 compared with the morphine/saline group; C: The locomotor activity as the total distance traveled (cm); The values are represented as the Mean±SEM for 7 rats.

Figure 3.

The interaction between morphine state-dependent memory and pentylenetetrazole (PTZ), cyclic adenosine monophosphate response element element-binding (CREB) mRNA expression, and locomotor activity A: The effects of pre-test PTZ in the presence or absence of morphine. Data are shown as Mean±SEM for seven animals per group, *P<0.05 and ***P<0.01 compared with the posttraining saline/pre-test saline group, ⁺⁺⁺P<0.001, ^§§§ P<0.001 compared with the post-training morphine/pre-test morphine group and ^### P<0.001 compared with the posttraining saline/pre-test PTZ group; B: The relative expression of CREB mRNA (A/U). Each bar shows the Mean±SEM for 4 rats, ***P<0.001 compared with the saline/saline group, ⁺⁺P<0.01 compared with the morphine/morphine group; C: The locomotor activity as the total distance traveled (cm). The values are represented as the Mean±SEM for 7 rats.