Chronic Effects of Haloperidol and SCH23390 on Operant and Licking Behaviors in the Rat

Abstract

Behavioral effects under chronic treatment of haloperidol and SCH23390 were examined for 18 days. Water deprived rats were trained either to perform an operant response on a fixed ratio 20 (FR20) schedule of reinforcement or to lick from a tube in a separate test. Both drugs completely impaired operant responding over all chronic administration days. Whether the lick performance was significantly affected differed depending upon the variable measured in that task. In contrast to the operant results, analysis of the microstructure of licking revealed very distinctive profiles of licking for each drug. Although the lick volume was consistantly reduced by both drugs across days, the decreased numbers of licks first observed were subsequently reversed back to the control level. However, the time courses and the reversal patterns were different for each drug. Haloperidol persistantly reduced the burst size for licking, whereas SCH23390 gradually enlarged it. For interlick interval (ILI) data, haloperidol had more impact on licks with longer ILI without influencing licks of shorter ILI. Contrarily, SCH23390 tended to lengthen the licks with shorter ILI without prolonging them enough to be classified as longer ILI licks. The dissociation of behavioral effects induced by chronic treatment of haloperidol and SCH23390 can be attributed to the drugs` blockade of different DA receptor subtypes. Also, the present study illustrates that using multiple behavioral tasks can be helpful to differentiate the subtle drug actions induced by DA receptor antagonists.