Examination Of The Roles Of Dopamine D2 Receptors And Serotonin 2A Receptors Olanzapine Sensitization In The Conditioned Avoidance Response Model In Rats

Olanzapine,as an atypical antipsychotic drug,is widely used in clinic,mainly for the treatment of schizophrenia,bipolar disorder and other psychological disorders.Preclinical studies indicate that acute olanzapine treatment selectively inhibits conditioned avoidance response（CAR）in rodents.With repeated treatment,this inhibitory effect gradually increases,a behavioral pattern termed olanzapine sensitization.Olanzapine sensitization is demonstrated in the conditioned avoidance response paradigm,a valid behavioral test of antipsychotic efficacy of antipsychotic drugs.A typical testing procedure includes an initial induction phase and a late expression phase.In the induction,well-trained rats are treated with olanzapine or vehicle for 3-5 days,and the number of daily avoidance response is recorded.After several drug-free retraining days,all rats are challenged with olanzapine in the expression phase.Olanzapine-treated rats typically make fewer avoidance responses than vehicle rats under the olanzapine challenge.In this study,we used CAR paradigm to explore the receptor mechanisms underlying the acute and repeated（sensitized）olanzapine treatment effects on CAR.Previous studies have shown that dopamine D₂ receptor and serotonin（5-HT）2A receptors may be involved in the sensitization of olanzapine on avoidance response,but the neural basis is not clear.Given the high expression of both receptors in the medial prefrontal cortex（mPFC） and nucleus accumbens shell（NAs）,we hypothesized that D₂ and 5-HT_2A in both regions play a role in the regulation of olanzapine sensitization effect on CAR.This project was designed to test this hypothesis using central drug microinjection methods.In Study 1,we investigated how dopamine D₂ receptors in the mPFC mediated olanzapine sensitization in CAR.In Experiment 1,we investigated how activation of dopamine D₂ receptors in mPFC in the induction phase alters olanzapine sensitization.Rats were trained in the CAR for 10 days to achieve a high level of CAR（>70% avoidance rates）,then they were implanted with bilateral stainless-steel guide cannulas into the mPFC（AP:+3.2mm,ML:±0.75mm,DV:-2.5mm）.All rats were allowed 7 days of recovery time before being used in the subsequent drug tests.On each of the drug test days（three times in total）,rats were first injected with olanzapine（0.0 or 2.0 mg/kg,sc,every two days）,45 min later,centrally injected with a dopamine D₂receptors agonist quinpirole（0.0,5.0,10.0 ug/0.5 ul/side）into mPFC.Ten min later,they were being placed in the CAR training boxes and avoidance responses were recorded in 30 trials.After two days of drug-free retraining,the final challenge test was conducted during which all rats were injected with olanzapine（1.0 mg/kg,sc）.Experiment 2 was similar to Experiment 1 with the only exception that central quinpirole microinjection（0.0,10.0 ug/0.5 ul/side）was administered on the challenge test day to test how activation of D₂ receptors affected the expression of olanzapine sensitization.Experiment 3 tested the effect of blocking D₂ receptors in the mPFC during the expression phase on olanzapine sensitization.The basic procedure was identical to that of Experiment 2 with the exception that the dopamine D₂ receptor antagonist raclopride（0.0,10.0ug/0.5 ul/side）was microinjected into the mPFC on the olanzapine challenge test day.Results show that activation of D₂ receptors in the mPFC during the induction phase did not affect the development of olanzapine sensitization and its expression.Blockade of D₂ receptors in the mPFC during the expression phase also failed to affect the expression of olanzapine sensitization.In Study 2,we investigated how activation of dopamine D₂ receptors in the NAs altered olanzapine sensitization.Experiment 1 examined how activation of NAs D₂ receptors in the induction phase affected the development of olanzapine sensitization.Experiment 2 examined its effects in the expression phase.The basic procedures for both experiments were similar to those used in Experiment 1 and 2 of Study 1 with the exception that rats were implanted with bilateral stainless-steel guide cannulas into the NAs（AP:+1.5mm,ML:±1.0mm,DV:-5.5mm）.Results showed that quinpirole microinjected into the NAs in the induction phase reduced the acute inhibition of olanzapine on CAR,but did not affect the expression of olanzapine sensitization.However,quinpirole microinjection in the expression phase reduced the expression of olanzapine sensitization.In Study 3,we investigated how activation of 5-HT_2A receptors in the mPFC or NAs altered olanzapine sensitization in CAR.Experiment 1 was similar to Experiment 1 of Study 1 with the only exception that TCB-2(a 5-HT_2A agonist 0.0,0.4,4.0 ug/0.5 ul/side),instead of quinpirole,was microinjected into the mPFC.Experiment 2 was similar to Experiment 1 with the only exception that TCB-2（0.0,0.4,4.0 ug/0.5 ul/side） was microinjected into the NAs.Results show that activation of 5-HT_2A receptors in either the mPFC or NAs during the induction phase did not affect the development of olanzapine sensitization and its expression.In summary,the three studies from this project found that（1）quinpirole microinjected into the NAs in the induction phase reduced the acute inhibition of olanzapine on CAR,and quinpirole microinjection in the expression phase reduced the expression of olanzapine sensitization;（2）activation of D₂ receptors in the mPFC during the induction phase did not affect the development of olanzapine sensitization and its expression,and blockade of D₂ receptors in the mPFC during the expression phase also did not affect the expression of olanzapine sensitization;（3）activation of 5-HT_2A receptors in the mPFC or NAs during the induction phase did not affect the development of olanzapine sensitization and its expression.Our study is significant as it reveals that D₂ receptors in the NAs is critical for the expression of olanzapine sensitization.This work suggests that targeting this receptor type in the NAs may be useful to modulate the expression of long-term olanzapine treatment effect.