The Journal of Neuroscience, November 1, 1998, 18(21):9130-9138
Changes in Medial Prefrontal Cortical Dopamine Levels Associated
with Response-Contingent Food Reward: An Electrochemical Study in
Rat
Nicole R.
Richardson and
Alain
Gratton
Douglas Hospital Research Centre, Department of Psychiatry, McGill
University, Verdun, Québec, Canada, H4H 1R3
Voltammetry was used to monitor in rats changes in medial
prefrontal cortex (PFC) dopamine (DA) levels associated with
response-contingent presentation of a condensed milk reward. During two
initial training sessions, minor DA signal fluctuations were seen when
animals consumed a standard 30 sec (0.2 ml) meal earned on a continuous reinforcement schedule. There was no evidence of experience-dependent changes in these fluctuations. Under delayed reinforcement conditions, lever-presses were followed by DA signal increases that were
time-locked to the delay duration, and these were followed by signal
decreases when animals eventually received the reward. Such decreases
became more pronounced when the standard rate of milk delivery was
tripled, but were attenuated when milk delivery was reduced to half the usual rate. Withholding earned milk resulted in signal increases. In
contrast, DA signal increases were observed during milk consumption when the standard meal duration was unexpectedly shortened to 15 sec or
lengthened to 60 or 90 sec. Orderly changes in DA signal were also
observed under partial reinforcement conditions. Unreinforced responses
were associated with DA signal decreases, whereas transient increases
were seen during the 30 sec meal that followed reinforced responses.
These findings indicate that response-contingent reward presentation
elicits synchronous changes in PFC DA transmission. They also suggest
that the DA input to PFC is activated when rewards are presented under
conditions that deviate from those that the animals had come to expect,
particularly so when the temporal structure of learned associations is
altered.
Key words:
voltammetry; mesocortical dopamine; expectancy; working
memory; instrumental conditioning; incentive motivation; reward
Copyright © 1998 Society for Neuroscience 0270-6474/98/18219130-09$05.00/0
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