ELECTROCHEMICAL MONITORING OF EXTRACELLULAR DOPAMINE IN NUCLEUS-ACCUMBENS OF RATS LEVER-PRESSING FOR FOOD

High-speed chronoamperometry and monoamine-selective electrochemical p robes were used to monitor, during each of 5-6 consecutive daily sessi ons, changes in dopamine (DA)-related oxidation current in nucleus acc umbens (NAcc) of rats lever-pressing for a food reward. In trained ani mals, the first lever-press of each session was preceded by a gradual increase in the electrochemical signal and was followed, during the pe riod the animals retrieved and consumed the food pellet, by a further increase that peaked momentarily when the animal pressed the lever aga in. This pattern of increases accompanied the initial 1-3 lever-presse s of each session after which biphasic changes in electrochemical sign al began to emerge. In these cases, each lever-press was preceded by a gradual increase in signal that peaked at the moment the animals pres sed the lever and was followed by an abrupt decrease as the animals re trieved and consumed the food pellet. The signal would then start to i ncrease gradually again to peak at a slighly higher level at the momen t of the next lever-press. Thus, during the initial part of the sessio n there was a net increment in signal with each lever-press which resu lted in a gradual overall elevation of the signal. The increments, how ever, became progressively smaller as the decrease in signal that foll owed each lever-press became more pronounced; this slowed the overall rate of increase of the signal until it eventually reached a plateau a nd remained relatively stable at that level as long as the animals ate earned food pellets. On several occasions, lever-presses were reinfor ced by twice the usual amount of food, In these cases, lever-presses w ere preceded by similar increases but were followed by more pronounced and longer-lasting decreases in electrochemical signal. In contrast, non-reinforced lever-presses were followed by less pronounced decrease s in signal which then increased more rapidly than was observed after reinforced lever-presses. Toward the end of the session, animals would often ignore earned pellets of food. In these cases, no increases pre ceded and gradual, rather than abrupt, decreases in signal followed ea ch lever-press. Eventually, the animals ceased to lever-press entirely and this period was accompanied by a gradual return of the electroche mical signal toward baseline values. The tonic elevation of DA levels suggested by the present electrochemical results is in general agreeme nt with previous reports of increased DA efflux in NAcc of animals eng aged in feeding and feeding-related behaviors. However, the present re sults suggest that there are additional phasic changes in DA efflux th at are time-locked to each operant response. It would appear that incr eased DA transmission in NAcc occurs in response to stimuli closely as sociated with food presentation or in conjunction with behaviors direc ted toward food, while a transient cessation of DA transmission is ass ociated with the period generally thought to be critical for positive reinforcement of learned behaviors.