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Subsections
3 Nucleus Accumbens (NAcc)
The Nacc is the ventral extension of the striatum, and therefore also
called the ventral striatum. The ventral striatum contains neurons
known as medium spiny neurons (MSN's).
The accumbens has two major subterritories (Heimer et al., 1991): the shell
(section 3.5) and the core (section refcore)
where the shell can be further subdivided
(Usuda et al., 1998) in lateral and medial parts and the core rather
depending on its D1 or D2 receptors.
The Nacc core and shell have distinct roles controlling reward based learning:
In reversal learning (Dalton et al., 2014) the shell seems to control
switching contingencies (i.e. the reward is moved from one site to
another) whereas the core controls the actual approach behaviour
towards the rewarding site.
Impulsivity is altered by 5HT antagonists which point towards an
important role of 5HT in the retrieval of delayed rewards. 5-HT(2A)
antagonists reduced decreased impulsive responding and the 5-HT(2C)
antagonist increased impulsivity (Robinson et al., 2008).
By measuring the DA concentration in both the core and shell
(
Saddoris et al., 2015) it turns out that Core DA follows the classical
prediction error signals were it spikes most to the predicting cue
whereas the shell responds to all reward related events during the
experiment.
It's well known the bursts of dopamine cause LTP in conjunction with
pre- and (possible) postsynaptic activity (so called 3 factor Hebbian
rule) or heterosynaptic LTP. However
Goto and Grace (2005) showed that D1
receptors cause LTP on hippocampal fibres whereas D2 receptors control
the cortical inputs in the opposite way. It appears to be that 5HT
causes retrograde cannabinoid (CB) release which inhibits pre-synaptic
GLU release (
Burattini et al., 2014). However, also postsynaptic HFS
causes CB1 mediated depression.
Mathur et al. (2011) tested this more
thoroughly in that they conclude that 5HT actually causes presynaptic
inhibition via the 5HT1B receptor, with that LTD and that HFS can have
a similar effect by releasing CB.
3.4 Nucleus Accumbens Core
The NAcc core receives inputs from the dorsal-medial prefrontal cortex
and the hippocampus (
Brog et al., 1993a).
There are two distinct output pathways from the NAcc core which have
its origins from the two sub-populations of neurons in the NAcc
core. The one sub-population carries mainly D1 receptors and the other
one carries mainly D2 receptors (
Humphries and Prescott, 2010;
Kelley, 2004).
The
D1 receptor carrying neurons feed directly into the SNr and are
able to inhibit tonically active SNr neurons, thus the NAcc core is
able to disinhibit motor programs.
There is an indirect pathway via the VP to the SNr originating from
the NAcc core. In contrast to the direct pathway these neurons in the
indirect pathway carry mainly D2 receptors which are inhibitory in
nature. D2 receptors are very sensitive to low DA concentrations and
will react to the tonic DA concentrations.
3.5 Nucleus Accumbens Shell
The shell can be further divided into the medial and lateral shell
(
Ikemoto, 2007;
Usuda et al., 1998) used in the model by
(
Humphries and Prescott, 2010).
The medial Shell projects to the medial Ventral Pallidum (VP)
(
Ikemoto, 2007).
The lateral shall projects to the ventrolateral Ventral Pallium (VP)
(
Ikemoto, 2007).
The shell seems to be responsible for behavioural flexibility meaning
that it controls the animal's ability to shift to another target when
the reward is lost (
Aquili et al., 2014)
Overall the Shell seems to learn the stimuli which are associated with
a reward, and thus enhances the future salience of those stimuli
(
Cassidy and Tong, 2017).
In this context it is also interesting that shell DA also tracks
rather the inventive value than the reward prediction error
(Sackett et al., 2017) which probably means that the VTA has regions
which work with the shell and that they are distinct from the core.
