Quantitative analysis of the effects of some "atypical" and "conventional" antipsychotics on progressive ratio schedule performance.
BACKGROUND
Performance on progressive ratio schedules has been proposed as a means of assessing the effects of drugs on the value or "efficacy" of reinforcers. A mathematical model affords a basis for quantifying the effects of drugs on progressive ratio schedule performance. According to this model, the relation between response rate and ratio size is described by a bitonic (inverted-U) function. One parameter of the function, alpha, expresses the motivational or "activating" effect of the reinforcer (duration of activation of responding produced by the reinforcer), whereas another parameter, delta, expresses the minimum time needed to execute a response, and is regarded as an index of "motor capacity". In a previous experiment we found that the "atypical" antipsychotic clozapine increased alpha, indicating an increase in the efficacy of a food reinforcer.


OBJECTIVE
We examined the effects of four "atypical" and four "conventional" antipsychotics on progressive ratio schedule performance.


METHODS
Rats responded for a sucrose reinforcer (0.6 M, 50 microl) on a time-constrained progressive ratio schedule (50-min sessions). After 90 preliminary training sessions, they received acute doses of antipsychotics (doses in mg kg(-1)): atypical: clozapine (2, 4, 8, IP; n=15), quetiapine (1.25, 2.5, 5, 10, SC; n=23), olanzapine (0.25, 0.5, 1, IP; n=15), ziprasidone (0.625, 1.25, 2.5, IP, n=15); conventional: haloperidol (0.025, 0.05, 0.1, IP, n=15), pimozide (0.125, 0.25, 0.5, IP; n=15), raclopride (0.25, 0.5, 1, SC; n=12), cis-flupenthixol (0.2, 0.4, 0.8, SC; n=15). Values of a and delta were estimated from the response rate functions obtained under each treatment condition, and were compared between drug and vehicle-alone treatments.


RESULTS
The atypical antipsychotics significantly increased alpha (indicating enhancement of reinforcer efficacy), and also increased delta (indicating reduction of motor capacity). Haloperidol, pimozide and raclopride significantly increased delta; none of the conventional antipsychotics significantly altered alpha.


CONCLUSIONS
The results extend previous findings with clozapine to other atypical antipsychotics and suggest that enhancement of the efficacy of reinforcers may be a common feature of atypical antipsychotics not shared by conventional antipsychotics.
Zhang
Z
Z
Psychopharmacology Section, Division of Psychiatry, Medical School, Queen's Medical Centre, University of Nottingham, Nottingham, NG7 2UH, UK.
Rickard
J F
JF
Asgari
K
K
Body
S
S
Bradshaw
C M
CM
Szabadi
E
E
D000818:Animals; D014150:Antipsychotic Agents / Q000494:pharmacology*; D003216:Conditioning, Operant / Q000187:drug effects*; D004305:Dose-Response Relationship, Drug; D005260:Female; D005508:Food Deprivation; D051381:Rats; D017208:Rats, Wistar; D011930:Reaction Time / Q000187:drug effects; D012055:Reinforcement Schedule; D013395:Sucrose
D014150:Antipsychotic Agents; D013395:Sucrose