title: Clozapine enhances breakpoint in common marmosets responding on a progressive ratio schedule.
abstract: BACKGROUND
  Motivational effects of psychotropic drugs may contribute to their therapeutic profile and progressive ratio (PR) schedules provide a method of measuring these effects in animals.
  
  
  OBJECTIVE
  Determine effects of selected antipsychotic, psychotomimetic, anxiolytic and antidepressant drugs on PR performance in common marmosets.
  
  
  METHODS
  Marmosets were trained to lever press for banana milkshake reinforcement using a PR schedule, in which the number of lever presses to achieve successive reinforcements increased by one until responding ceased (breakpoint).
  
  
  RESULTS
  Clozapine administered intramuscularly (0.01-2 mg/kg IM; 30 min pretreatment time (ptt) or by oral gavage (0.1-4 mg/kg PO; 60 min ptt) significantly increased the breakpoint. Independent tests of fluid consumption failed to show enhanced fluid intake after clozapine pretreatment, suggesting this effect was not due to drug induced polydipsia. Neither haloperidol (0.005-0.1 mg/kg PO; 60 min ptt) nor risperidone (0.0025-0.05 mg/kg PO; 60 min ptt) altered breakpoint. Olanzapine (0.01-1 mg/kg PO; 60 min ptt) significantly enhanced the breakpoint at 0.05 mg/kg PO, but the effect was not robust. Amphetamine (0.2-2 mg/kg SC; 30 min ptt) significantly reduced the breakpoint at 2 mg/kg and fluoxetine (0.1-1 mg/kg PO; 60 min ptt) was without effect. Diazepam significantly increased the breakpoint at 0.5 mg/kg PO. Drug-induced polydipsia might play a role in this response as independent tests showed increased fluid consumption following diazepam.
  
  
  CONCLUSIONS
  These results demonstrate that, unlike other antipsychotics, clozapine over a wide dose range increased the motivational state of marmosets to respond for banana milkshake. This motivational aspect of clozapine's actions may contribute to its unique clinical profile and the PR procedure may provide a method for detecting novel antipsychotics with a clozapine-like profile.
authors.affiliation: Neuroscience Research, GlaxoSmithKline, Harlow, Essex, UK.
authors.forename: J
authors.initials: J
authors.lastname: Cilia
authors.forename: D C
authors.initials: DC
authors.lastname: Piper
authors.forename: N
authors.initials: N
authors.lastname: Upton
authors.forename: J J
authors.initials: JJ
authors.lastname: Hagan
mesh_terms: D000661:Amphetamine / Q000494:pharmacology; D000818:Animals; D014151:Anti-Anxiety Agents / Q000494:pharmacology; D000928:Antidepressive Agents / Q000494:pharmacology; D014150:Antipsychotic Agents / Q000008:administration & dosage / Q000627:therapeutic use*; D002144:Callithrix; D000697:Central Nervous System Stimulants / Q000494:pharmacology; D003024:Clozapine / Q000008:administration & dosage / Q000627:therapeutic use*; D003216:Conditioning, Operant / Q000187:drug effects*; D003975:Diazepam / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004435:Eating / Q000187:drug effects; D005260:Female; D007273:Injections, Intramuscular; D008297:Male; D012055:Reinforcement Schedule
chemical_list: D014151:Anti-Anxiety Agents; D000928:Antidepressive Agents; D014150:Antipsychotic Agents; D000697:Central Nervous System Stimulants; D000661:Amphetamine; D003024:Clozapine; D003975:Diazepam