Effects of central activation of serotonin 5-HT2A/2C or dopamine D 2/3 receptors on the acute and repeated effects of clozapine in the conditioned avoidance response test

Abstract

Rationale: Acute administration of clozapine (a gold standard of atypical antipsychotics) disrupts avoidance response in rodents, while repeated administration often causes a tolerance effect.

Objective: The present study investigated the neuroanatomical basis and receptor mechanisms of acute and repeated effects of clozapine treatment in the conditioned avoidance response test in male Sprague-Dawley rats.

Methods: 2,5-dimethoxy-4-iodo-amphetamine (DOI, a preferential 5-HT2A/2C agonist) or quinpirole (a preferential dopamine D2/3 agonist) was microinjected into the medial prefrontal cortex (mPFC) or nucleus accumbens shell (NAs), and their effects on the acute and long-term avoidance disruptive effect of clozapine were tested.

Results: Intra-mPFC microinjection of quinpirole enhanced the acute avoidance disruptive effect of clozapine (10 mg/kg, sc), while DOI microinjections reduced it marginally. Repeated administration of clozapine (10 mg/kg, sc) daily for 5 days caused a progressive decrease in its inhibition of avoidance responding, indicating tolerance development. Intra-mPFC microinjection of DOI at 25.0 (but not 5.0) μg/side during this period completely abolished the expression of clozapine tolerance. This was indicated by the finding that clozapine-treated rats centrally infused with 25.0 μg/side DOI did not show higher levels of avoidance responses than the vehicle-treated rats in the clozapine challenge test. Microinjection of DOI into the mPFC immediately before the challenge test also decreased the expression of clozapine tolerance.

Conclusions: Acute behavioral effect of clozapine can be enhanced by activation of the D2/3 receptors in the mPFC. Clozapine tolerance expression relies on the neuroplasticity initiated by its antagonist action against 5-HT2A/2C receptors in the mPFC.

Fig. 1

Histological representations of infusion sites and schematic diagrams showing the location of the injector tips in the nucleus accumbens shell (Experiment 1) and medial prefrontal cortex (Experiment 1, 2 & 3). Data are reconstructed from Paxinos and Watson (2004). Numbers to the left of the sections indicate anteroposterior distance from bregma in millimeters. The arrow denotes the infusion placement.

Fig. 2

Effects of microinjection of quinpirole (QUI) at 0, 1, 5 or 10 μg/side or DOI at 0, 1, 5 or 25 μg/side into the nucleus accumbens shell (NAs; a, b) or medial prefrontal cortex (mPFC; c, d) on the acute CLZ (10.0 mg/kg, sc)-induced avoidance disruption. n = 9, 8 for NAs − QUI or DOI, and n = 11, 9 for mPFC − QUI or DOI, respectively. Each data point represents the percentage avoidance response (Mean + SEM, number of avoidance responses divided by the total number of trials) made by rats during the pre-drug and the drug test days . Rats received a central infusion 30 min after systematic CLZ injection (10.0 mg/kg, sc), and were tested at 45 min and 95 min after CLZ injection (10 min and 60 min after the central infusion). ^*p < 0.05 in comparison to the VEH group during each test.

Fig. 3

Number of intertrial crossings made by rats on the pre-drug and 5 drug test days. On each drug test day, rats received a central infusion of quinpirole (QUI) at 0, 1, 5 or 10 μg/side or DOI at 0, 1, 5 or 25 μg/side into the nucleus accumbens shell (NAs; a, b) or medial prefrontal cortex (mPFC; c, d) 30 min after systematic CLZ injection (10.0 mg/kg, sc), and were tested at 45 min and 95 min after CLZ injection (10 min and 60 min after the central infusion).

Fig. 4

Effects of microinjection of DOI into the medial prefrontal cortex during the induction phase of CLZ tolerance. The data represent the percentage avoidance response and number of intertrial crossings (Mean + SEM) made by rats during the pre-drug and 5 drug test days (a, b), and on the retraining day and the challenge test day (c, d). Rats received either systemic vehicle or CLZ (10.0 mg/kg, sc) treatment in combination with central DOI treatment (0, 5 or 25 μg/side; n= 6, 8, 8 for VEH; n = 7, 8, 7 for CLZ) for 5 test days and were challenged with CLZ (10.0 mg/kg) after two drug-free retraining days. ^***p < 0.001, ^**p < 0.01 in comparison to the corresponding group (c, d).

Fig. 5

Effects of microinjection of DOI into the medial prefrontal cortex on the challenge day on the expression of clozapine tolerance. The data represent the percentage avoidance response and number of intertrial crossings (Mean + SEM) made by rats during the pre-drug and 5 drug test days (a, b), and on the retraining day and the challenge test day (c, d), respectively. Rats received systemic vehicle or CLZ (10.0 mg/kg, sc) injection for 5 test days and were challenged with CLZ (10.0 mg/kg) after two drug-free retraining days. On the challenge test day, rats received a central infusion of saline (VEH-SAL and CLZ-SAL, n = 8/group) or DOI 25 μg/side (CLZ-DOI 25, n = 8) 15 min before the test. ^**p < 0.01, ^*p < 0.05 in comparison to the corresponding group (c, d).