Background: Nucleus accumbens dopamine has been shown to play a role in the
processing of behaviorally relevant stimuli and to mediate ethanol-reinfor
ced responding. Previous research that used a fixed-ratio schedule of respo
nding maintained by the presentation of small dippers (0.1 ml) of ethanol d
emonstrated that the dopamine D-2 antagonist, raclopride, decreased total r
esponding for ethanol by both delaying the onset of responding and causing
the early termination of lever-press behavior. Because these studies requir
ed animals to continuously respond to obtain access to small amounts of eth
anol over a period of self-administration, this procedure assessed a combin
ation of appetitive (seeking) and consummatory (drinking) behavior. The par
adigm used in the present study separated the appetitive or seeking respons
e from the consummatory response to assess the effects of raclopride on bot
h types of ethanol-related behaviors.
Methods: Male Long-Evans rats were trained to emit a fixed number of lever-
press responses that resulted in access to a drinking tube that contained 1
0% ethanol for 20 min, once each day. We measured the effects of microinjec
tions of raclopride (1.0, 3.0, and 10.0 mug/subject) into the nucleus accum
bens, before the sessions, on appetitive and consummatory responding.
Results: Raclopride delayed the onset of ethanol-seeking (appetitive respon
ding) at all doses and decreased the number of responses made at the low an
d high doses. The rate of responding, however, was unaffected. Raclopride h
ad no effect on the latency to begin consuming ethanol or on any of the cha
racteristics of the initial bout of ethanol intake at all doses tested. Tot
al ethanol intake was decreased, after an initially "normal" pattern of sel
f-administration, following only the highest dose of raclopride. Mean ethan
ol intake (g/kg) was 0.54 (+/-0.03) after no injection, 0.51 (+/-0.04) afte
r sham treatment, and 0.38 (+/-0.05) after 10 mug of raclopride.
Conclusions: The procedural separation of the seeking and intake responses
used in this experiment allowed us to assess the effects of dopamine recept
or antagonism in the nucleus accumbens on these two different behaviors. Ov
erall, appetitive responding that preceded the delivery of an ethanol solut
ion was more sensitive to raclopride treatment than was consummatory respon
ding. These findings are consistent with a stimulus-processing function of
the mesolimbic dopamine system.