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Journal of Psychopharmacology 14(3) (supplement): A68 (abstract PH20).
Effects of lesions of the nucleus accumbens core and shell on response-specific
Cardinal RN, Parkinson JA*, Robbins TW, Dickinson A, Everitt BJ
Departments of Experimental Psychology and *Anatomy, University of Cambridge, Downing Street, Cambridge CB2 3EB, UK.
When an initially neutral stimulus has been paired in Pavlovian fashion
with an appetitive outcome such as food, noncontingent presentation of
this conditioned stimulus (CS) can enhance ongoing instrumental responding,
a phenomenon termed Pavlovian–instrumental transfer (PIT). In its simplest
form, PIT is assessed by presenting a CS for food while the subject is
responding in extinction on a single lever for the same food. It has previously
been shown that the nucleus accumbens, and particularly the core subdivision,
is critical for this form of PIT (Hall et al., 1999, Soc. Neuro.
Abstr. 25, P41.2). However, behavioural studies have shown that PIT
can be subdivided into a general, motivating effect of the Pavlovian CS,
and a response-specific PIT effect, seen as a further enhancement when
the Pavlovian and instrumental outcomes are the same (see Dickinson &
Balleine, 1994, Anim. L. Behav., 22, 1–18). In the present study,
rats received lesions to the core or shell of the nucleus accumbens before
being tested on a response-specific PIT task. In the Pavlovian phase, stimulus
CS(pel) was paired with pellet delivery, while stimulus CS(suc) was paired
with sucrose solution. The subjects were then trained to respond on two
levers, with one lever producing pellets and the other producing sucrose.
On test, lever-pressing was recorded in extinction while the stimuli were
presented noncontingently. Sham-operated subjects (n = 6) showed
a selective enhancement of lever-pressing on the lever paired with the
same outcome as the Pavlovian CS; this is the response-specific PIT effect.
Core-lesioned subjects (n = 4) showed a general enhancement of responding
during the CS, but this was not specific to one response. Shell-lesioned
animals (n = 4) showed no Pavlovian–instrumental transfer. It is
suggested that in this task, the shell is required for the ‘vigour’ and
the core for the ‘direction’ of the potentiation of responding by a noncontingent,
appetitive stimulus. This pattern closely resembles that previously observed
for the potentiation of responding for conditioned reinforcement by psychostimulant
drugs injected into the nucleus accumbens (Parkinson et al., 1999,
Neurosci., 19, 2401–11).