Laterodorsal tegmentum-ventral tegmental area projections encode positive reinforcement signals

The mesopontine tegmentum in reward and aversion: From cellular heterogeneity to behaviour

The mesopontine tegmentum, comprising the pedunculopontine tegmentum (PPN) and the laterodorsal tegmentum (LDT), is intricately connected to various regions of the basal ganglia, motor systems, and limbic systems. The PPN and LDT can regulate the activity of different brain regions of these target systems, and in this way are in a privileged position to modulate motivated behaviours. Despite recent findings, the PPN and LDT have been largely overlooked in discussions about the neural circuits associated with reward and aversion. This review aims to provide a timely and comprehensive resource on past and current research, highlighting the PPN and LDT's connectivity and influence on basal ganglia and limbic, and motor systems. Seminal studies, including lesion, pharmacological, and optogenetic/chemogenetic approaches, demonstrate their critical roles in modulating reward/aversive behaviours. The review emphasizes the need for further investigation into the associated cellular mechanisms, in order to clarify their role in behaviour and contribution for different neuropsychiatric disorders.

Dopamine release and negative valence gated by inhibitory neurons in the laterodorsal tegmental nucleus

GABAergic neurons in the laterodorsal tegmental nucleus (LDTGABA) encode aversion by directly inhibiting mesolimbic dopamine (DA). Yet, the detailed cellular and circuit mechanisms by which these cells relay unpleasant stimuli to DA neurons and regulate behavioral output remain largely unclear. Here, we show that LDTGABA neurons bidirectionally respond to rewarding and aversive stimuli in mice. Activation of LDTGABA neurons promotes aversion and reduces DA release in the lateral nucleus accumbens. Furthermore, we identified two molecularly distinct LDTGABA cell populations. Somatostatin-expressing (Sst+) LDTGABA neurons indirectly regulate the mesolimbic DA system by disinhibiting excitatory hypothalamic neurons. In contrast, Reelin-expressing LDTGABA neurons directly inhibit downstream DA neurons. The identification of separate GABAergic subpopulations in a single brainstem nucleus that relay unpleasant stimuli to the mesolimbic DA system through direct and indirect projections is critical for establishing a circuit-level understanding of how negative valence is encoded in the mammalian brain.

Striatal dopamine signals are region specific and temporally stable across action-sequence habit formation

Habits are automatic, inflexible behaviors that develop slowly with repeated performance. Striatal dopamine signaling instantiates this habit-formation process, presumably region specifically and via ventral-to-dorsal and medial-to-lateral signal shifts. Here, we quantify dopamine release in regions implicated in these presumed shifts (ventromedial striatum [VMS], dorsomedial striatum [DMS], and dorsolateral striatum [DLS]) in rats performing an action-sequence task and characterize habit development throughout a 10-week training. Surprisingly, all regions exhibited stable dopamine dynamics throughout habit development. VMS and DLS signals did not differ between habitual and non-habitual animals, but DMS dopamine release increased during action-sequence initiation and decreased during action-sequence completion in habitual rats, whereas non-habitual rats showed opposite effects. Consistently, optogenetic stimulation of DMS dopamine release accelerated habit formation. Thus, we demonstrate that dopamine signals do not shift regionally during habit formation and that dopamine in DMS, but not VMS or DLS, determines habit bias, attributing “habit functions” to a region previously associated exclusively with non-habitual behavior.

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Validation of a novel test that monitors habit development individually across time

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Dopamine release during habit development is stable across relevant striatal regions

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Only dopamine release in dorsomedial striatum correlates with habit development

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Optogenetic stimulation of dorsomedial striatal dopamine accelerates habit formation