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Subsections

8 Lateral Habenula (LHb)

8.1 Afferents

The 2 strongest forebrain afferents to the LHb are the EP and LH and other connections include the lateral preoptic area and the VP (Parent et al., 1981).

Araki, 1984, states that the connection from EP to LHb is GABAergic (Araki et al., 1984) but Shabel 2012 qualifies it as excitatory and glutamatergic (Shabel et al., 2012).

Mok and Mogenson (1974) proposed that the actual connection was mainly excitatory. A view supported by Poller et al. (2013) who found a strong glutamatergic projection that targeted VTA and RMTg projecting neurons.

The LHb also receives DA input from the midbrain VTA and SNc (Kowski et al., 2009) and reciprocally its main, inhibitory projections are to the VTA, SNc and the DRN (Christoph et al., 1986; Rajakumar et al., 1993; Ji and Shepard, 2007).

8.2 Efferents

The main targets of the LHb are the VTA, Midbrain reticular nucleus (MRN), periaqueductal gray (PAG) and the superior central nucleus Raphe (CS) (Quina et al., 2015).

Slightly puzzling is that the effect on the VTA by the LHb is known to be inhibitory. Hong et al. (2011) theorise that as LHb neurons are largely glutamatergic their inhibitory function must be through an intermediary, the RMTg.

8.3 Function

Bromberg-Martin et al. (2010) view LHb as the most important source of reward memory in DA neurons. They cited three potential pathways for transmission; 1) prefrontal cortex to the striatum to the global pallidus to LHb 2) For rats, the mPFC to LHb 3) common sources project reward memory signals to LHb and DA neurons. They found that many LHb and DA neurons signalled past reward results in their tonic activity, this was surprising as previous studies had reported this was purely the case for phasic activity .

The LHb is considered to be the main source of the negative reward signal that facilitates the DA reward prediction error as LHb innervation inhibits midbrain DA (Shen et al., 2012; Barrot et al., 2012; Shabel et al., 2012; Matsumoto and Hikosaka, 2007).