Biomedical Engineering Reference
In-Depth Information
for a signifi cant part of the session. These data are highly indicative that there is
higher, executive attenuation of dopamine release during contingent responses.
This phenomenon has been observed in both the core and shell of the nucleus
accumbens
(21)
and in the caudate-putamen
(22)
.
These results seem to be consistent with the proposed role of dopamine
in the prediction error theory for reward
(23)
in which only unanticipated
responses produce dopaminergic responses. Evidence for the source of the
attenuation may be provided by the work of Henriksen and co-workers, who
have recently shown that during intracranial self-stimulation session in trained
rats, a group of
-aminobutyric acid-ergic (GABAergic) neurons in the ventral
tegmental area increase their fi ring rates just prior to the lever-press
(24)
. These
neurons may interact with dopaminergic neurons to inhibit propagation of the
stimulus and/or dopamine release.
5.2. Natural Stimuli
As well as being suitable for the measurement of transient dopamine
concentrations produced by electrical stimulation of dopaminergic neurons,
fast-scan cyclic voltammetry can also be used to measure changes in dopamine
levels resulting from endogenous neuronal activity. Specifi cally, the burst
fi ring of dopaminergic neurons, synchronized by gap junctions
(25)
, is likely
to produce extracellular dopamine transients that achieve high concentrations
(>100 n
M
). Electrophysiological studies demonstrate that such burst fi ring
occurs during the presentation of rewards and associated stimuli
(3)
and various
alerting stimuli
(26)
.
The fi rst chemical measurement of a rapid dopaminergic response to a
natural stimulus was by Rebec et al.
(1)
using fast-scan cyclic voltammetry.
An increase in extracellular dopamine concentration (indicated by the cyclic
voltammogram), lasting <10 s, was measured in the shell of the nucleus
accumbens of rats at free-choice entry into a novel environment. Neither
subsequent exploratory behavior nor reentry to the environment was associated
with further increases in dopamine concentrations, emphasizing the role of
novelty as opposed to locomotion in eliciting the neurochemical response. The
response was also regionally specifi c, as it was not observed in the core of the
nucleus accumbens or the caudate-putamen.
We recently measured faster dopamine transients in the core of the nucleus
accumbens of male rats during sexual interaction
(2)
. In the fi rst experiment,
we monitored dopamine during introduction of a receptive female into the
test cage. Dopamine transients occurred during the initial interaction with the
female, that is, approach and sniffi ng behaviors, but not during copulatory
behaviors such as mounting, intromission, or ejaculation. Additional experi-
ments addressed whether this phasic dopamine response is specifi c for recogni-
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