The Blockade of D1/D2-Like Dopamine Receptors within the Dentate Gyrus of Hippocampus Decreased the Reinstatement of Morphine-Extinguished Conditioned Place Preference in Rats

Abstract

Introduction: The hippocampus (HIP), the primary brain structure related to learning and memory, receives sparse but comprehensive dopamine innervations and contains dopamine D1/D2-like receptors. It is demonstrated that dopamine receptors in dentate gyrus (DG) region of HIP have a remarkable function in spatial reward processing. Much less is known about the involvement of HIP and its D1/D2 dopamine receptors in drug-seeking behaviors, more particularly, in the morphine extinguished conditioned place preference (CPP).

Methods: To find out the role of D1/D2-like receptors within the DG in morphine-seeking behaviors, forty adult male albino Wistar rats weighing 220-280g were unilaterally implanted by a cannula into the DG. The CPP paradigm was done; conditioning score and locomotors activity were recorded by Ethovision software. All drugs/vehicles were microinjected one day after extinction (just before the CPP test) into the DG as reinstatement day.

Results: The results showed that intra-DG administration of different dose of SCH23390 (0.25, 1 and 4μg/0.5μl saline), as a selective D1-like receptor antagonist and sulpiride (0.25, 1 and 4μg/0.5μl DMSO), as a selective D2-like receptor antagonist dose-dependently attenuated the morphine-extinguished CPP reinstated by priming injection of morphine (1 mg/kg;sc).

Discussion: It can be concluded that D1/D2-like receptors within this region have an important role in morphine-seeking behaviors in extinguished rats.

Keywords: D1-like receptor; D2-like receptor; Dentate gyrus; Morphine; Reinstatement; Reward.

Figure 1

The control and experimental groups, used in the present study, received their treatments (vehicles or D1/D2-like receptor antagonists) during the reinstatement phase. (A) Schematic time-line for showing the conditioned place preference (CPP) protocol and its phases. Animals in all control and experimental groups received the effective dose of subcutaneous (sc) morphine (5 mg/kg) during 3-day conditioning phase for induction of reward-related behaviors in the CPP paradigm. Then, in the extinction phase (eight days), animals had access to all compartments in the CPP apparatus without any drug injection for 30 min each day. The day after extinction was reinstatement phase. In the reinstatement phase, the preference to drug-paired chamber can be reinstated by using (B) priming/ineffective dose of morphine (5 mg/kg; sc), (C) different doses of SCH-23390/vehicle, and (D) different doses of sulpiride/vehicle in the morphine-extinguished rats.

Figure 2

Three coronal schematic microinjection sites in the dentate gyrus (○ = saline or DMSO microinjection and • = SCH23390 or Sulpride microinjection and ▴ = misplacement). All microinjections were performed bilaterally. cc, corpus callosum; CA1, field CA1 of hippocampus; CA2, field CA2 of hippocampus; CA3, field CA3 of hippocampus; 3V, 3rd ventricle; D3V, dorsal 3rd ventricle; LV, lateral ventricle; and DG, dentate gyrus; SLu, stratum lucidum of the hippocampus; GrDG, granular layer of the dentate gyrus; MoDG, molecular layer of dentate gyrus; PoDG, polymorph layer of dentate gyrus, Po; posterior thalamic nuclear group; ic, internal capsule; mt, mammillothalamic tract; f, fornix. Scale bare is 1mm.

Figure 3

The effects of administration of (A) saline as a vehicle and different doses of SCH23390 as selective D1-like receptor antagonist, (B) 0.25μg (C) 1μg and (D) 4μg per 0.5μl saline into the dentate gyrus (DG), 5 min before injection of priming dose of morphine (1 mg/kg;sc) in reinstatement day (24h after the last extinction day) dose-dependently decreased the conditioning score as compared to that in post-test day. Each column represents the mean ±SEM of 5–6 rats. *P<0.05 and ***P<0.001 compared with pre-test day ††P<0.01 and †††P<0.001 compared with post-test day +P<0.05, ++P<0.01 and +++P<0.001 compared with the last day of extinction period

Figure 4

The effects of administration of different doses of SCH23390 as selective D1-like receptor antagonist into the dentate gyrus (DG) on the percentage of CPP scores calculated in reinstatement day to pre-test day. *P<0.05 and ***P<0.001 compared with saline-treated group †P<0.05 and ††P<0.01

Figure 5

The effects of administration of (A) 12% DMSO as a vehicle and different doses of sulpiride as selective D2-like receptor antagonist, (B) 0.25μg (C) 1μg and (D) 4μg per 0.5μl DMSO into the dentate gyrus (DG), 5 min before injection of priming dose of morphine (1 mg/kg;sc) in reinstatement day (24h after the last extinction day) decreased the conditioning score as compared to that in post-test day. Each column represents the mean ±SEM of 5–6 rats. **P<0.01 and ***P<0.001 compared with pre-test day †††P<0.001 compared with post-test day +P<0.05 and +++P<0.001 compared with the last day of extinction period

Figure 6

The effects of administration of different doses of sulpiride as selective D2-like receptor antagonist into the dentate gyrus (DG) on the percentage of CPP scores calculated in reinstatement day to pre-test day. *P<0.05 and **P<0.01 compared with DMSO-treated group †P<0.05 and ††P<0.01