An investigation into the function of single-neuron activity in the mesoaccumbens dopamine system of the rat
Abstract
The mesoaccumbens dopamine system has been implicated in many basic
psychological processes (e.g. "wanting" and "liking") and illnesses (e.g. addiction,
depression, schizophrenia). However, the precise computational functions of
nucleus accumbens and dopamine neurons within the system remain unknown.
In this thesis, we test some of the current hypotheses regarding the function of
this system using a behavioural neurophysiology approach in the rat. The first
question we wanted to answer was whether nucleus accumbens neurons
process reward-predictive stimuli (e.g. conditioned reinforcers) and reward
delivery differently, since previous studies report equivocal findings. To do so, we
trained thirsty rats to bar-press on a second-order schedule of saccharin
reinforcement, within which the temporal pattern of rats' bar-pressing was
reinforced by presentations of a conditioned reinforcer and primary reinforcer
(reward). We found that nucleus accumbens neurons typically responded to
these conditioned and primary reinforcers with opposite sign, which suggests
they were processed differently. We were not sure whether responses to
conditioned reinforcers encoded reward-prediction or facilitated a behavioural
switch in the rat's behaviour. Indeed, since studies using a variety of
experimental techniques have implicated the mesoaccumbens dopamine system
in both reward prediction and behavioural switching, we sought to test whether
neurons in the nucleus accumbens and dopamine-rich areas of the midbrain
respond to outcome-associated stimuli to predict reward or switch behaviour. We
found both sets of neurons predominantly did the former. Finally, to understand
more about reward consummatory responses from both sets of neurons, we
developed a rat behavioural task providing measures of reward "wanting" and
"liking". In conclusion, on the basis of our data, the most parsimonious
explanation for the function of the mesoaccumbens dopamine system is that it
acts to modulate goal-seeking behaviour. Further research is required to identify
the function of the interactions between nucleus accumbens and dopamine
neurons during goal-seeking and goal consumption.
Type
Thesis, PhD Doctor of Philosophy
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