OPERANT PERFORMANCE AND CORTICAL ACETYLCHOLINE-RELEASE - ROLE OF RESPONSE RATE, REWARD DENSITY, AND NONCONTINGENT STIMULI

The relationship between acetylcholine (ACh) efflux in medial prefront al cortex (mPFC) and performance in a visual discrimination task and a variable interval (VI) schedule of reinforcement was studied in rats. Animals were pretrained in one of the two tasks and then unilaterally implanted with microdialysis guide cannula into the mPFC. Animals wer e then dialyzed, during 12 min collection intervals, in the operant ch ambers prior to task onset and during and after task performance. Each animal was dialyzed for a total of four sessions: two standard task s essions, one session in which a houselight was flashed at 0.5 Hz durin g the third 12 min block, and an extinction session (always the last s ession) in which reinforcement was withheld during the final three blo cks. Response accuracy in the discrimination task was very high (> 95% correct) and stable across the four blocks with a progressive increas e in omissions. The flashing houselight did not affect performance whe reas the loss of reinforcement led to an increase in omissions. VI per formance was associated with a high number of lever presses and a high reward rate that declined over the four blocks. Again, the flashing h ouselight did not affect VI performance whereas lever pressing decline d markedly during the extinction session. ACh efflux did not change, r elative to baseline, during performance in either task, or with the pr esentation of the flashing houselight or the loss of reinforcement. Th ese data contrast with the changes in cortical ACh efflux observed in situations characterized by the presentation of novel stimuli or chang ing demands on attentional processing and, therefore, assist in the sp ecification of hypotheses on the cognitive functions of cortical ACh. (C) 1997 Elsevier Science B.V.