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Subsections

13 Ventral Tegmental Area (VTA)

Dopamine neurons make up 60-65% of VTA neurons and GABA 30-35% (Sesack and Grace, 2010). More recently also Glutamate Neurons have been identified but little is known (Morales and Margolis, 2017).

13.1 Afferents

Excitatory afferents to the VTA include the LH, PFC and pedunculopontine nucleus (PPTg). Inhibitory, modulatory projections include the NAcc and the VP (Sesack and Grace, 2010).

Electrophysiology suggest that the NAcc projects mainly on the GABAergic neurons in the VTA.

13.2 Efferents

The VTA projects to numerous targets which include (Beckstead et al., 1979):

Although it is principally cited for its DA output GABA also plays a major role in the activity of the VTA. GABA projections from the VTA to the NAcc are reciprocated with GABA projections to the VTA. There is also a large projection of GABA neurons from the VTA to the PFC (Carr and Sesack, 2000). Local GABA neurons can also inhibit their neighbouring dopamine neurons.(Sesack and Grace, 2010) and are a strong candidate to calculate the reward prediction error (Eshel et al., 2015).

13.3 Function

It is very well known that rats self stimulate the VTA indefinitely (Stuber and Wise, 2016) and that phasic optogenetic activation of the VTA drives behavioural conditioning (Tsai et al., 2009).

In particular the pathway from the VTA to the NAcc core (NAcc) and NAcc shell (NAcSh) is instrumental here. VTA dopamine release in response to a rewarding stimulus induces goal-directed behaviour to acquire and consume it (Morales and Margolis, 2017).

In an experiment where DA release was artificially triggered via an optogenetic stimulation caused robust reward seeking behaviour (Steinberg et al., 2013).

13.3.1 Phasic activity

Looking at single cell recordings some neurons were excited by the reward (US), some by the reward predicting CS and some reacted to both stimuli (Cohen et al., 2012). The response to the CS became gradually stronger and the ones to the US smaller.

DA in the VTA signals a reward prediction error resembling that of TD learning which has been first suggested by (Schultz et al., 1997) and then matched quantitatively by (Bayer and Glimcher, 2005).

DA VTA neurons react strongly to unexpected rewards, these responses diminish after repeated presentation of the reward but then rather spike when a CS is presented which predicts the reward.

During omission of the reward the DA activity supposed to experience a 'dip' in activity (Takahashi et al., 2017). However, except of a few examples it is usually a reduction of the DA response after omission.

Also the DA activity won't vanish completely after a reward is expected but is diminished. This behaviour can still be matched on TD learning when using long-lasting eligibility traces (Pan et al., 2005).

However, Sadacca et al. (2016) has recently challenged this view that DA neurons code simply a reward prediction error about an experienced reward but that they also respond to putative cached values of cues which have been previously paired with a reward.

13.3.2 Tonic activity (DA)

On the other hand the VTA generates tonic activity which can be seen as a motivational value signal which is principally sent to the NAcc (Bromberg-Martin et al., 2010; Sesack and Grace, 2010) .

13.3.3 Tonic activity (GABA)

Cohen et al. (2012) found that VTA GABA neurons signalled expected reward so that this can be used to calculate the reward prediction error locally in the VTA.