Understanding the habenula: A major node in circuits regulating emotion and motivation

Abstract

Over the last decade, the understanding of the habenula has rapidly advanced from being an understudied brain area with the Latin name 'habena" meaning "little rein", to being considered a "major rein" in the control of key monoaminergic brain centers. This ancient brain structure is a strategic node in the information flow from fronto-limbic brain areas to brainstem nuclei. As such, it plays a crucial role in regulating emotional, motivational, and cognitive behaviors and has been implicated in several neuropsychiatric disorders, including depression and addiction. This review will summarize recent findings on the medial (MHb) and lateral (LHb) habenula, their topographical projections, cell types, and functions. Additionally, we will discuss contemporary efforts that have uncovered novel molecular pathways and synaptic mechanisms with a focus on MHb-Interpeduncular nucleus (IPN) synapses. Finally, we will explore the potential interplay between the habenula's cholinergic and non-cholinergic components in coordinating related emotional and motivational behaviors, raising the possibility that these two pathways work together to provide balanced roles in reward prediction and aversion, rather than functioning independently.

Keywords: Cholinergic receptors; Depression; Drug addiction; Habenular therapies; Neurocircuitry; Synaptic transmission.

Figure 1.. Cell types and connectivity:

(A) Subnuclei within the habenula: The medial habenula (MHb) can be subdivided into 5 subnuclei, while the lateral habenula (LHb) can be subdivided into 4 subnuclei, with an area near the stria medularis along the border between the lateral and medial habenula designated HbX. (B) Topographic projection of neurons within the habenula: The MHb sends projections to the IPN that are arranged topographically. Dorsal MHb nuclei project bilaterally to the lateral subnuclei of the IPN, while ventral MHb subnuclei project to the rostral, intermediate, and central nuclei. Within the ventral MHb nuclei, a medial to lateral gradient of projections exists that correlates to a dorsal to ventral gradient within the IPN. Other relevant inputs into the LHb, MHb, and IPN are noted. (C) Cell marker expression in the habenula: Each subnucleus within the habenular complex is identified by a combination of markers. Here we have displayed a select list of relevant marker expression, based on transcript expression, fluorescent-tagged protein expression and proteomics, and confirmed by in situ hybridization using the Allen Brain Atlas. The level of expression correlates with the size of the circle, with the smallest circle indicating consistent, low-level detection, the mid-size circle indicating consistent moderate expression or scattered cells in the subnuclei with intense expression, and the large circle indicating intense expression and dense coverage across the subnucleus.

Figure 2.. Habenula-related behaviors:

Behavioral tests employed to assess the motivational and emotional behaviors regulated by the habenula in mice and rats. (A-C) Drug addiction is a sign of dysregulated motivation, evidenced by intense drug craving and compulsive drug-seeking behavior, and can be measured using the (A) voluntary two-bottle drinking paradigm and the (B) intravenous or intracranial self-administration. (C) The conditioned place preference test is used to evaluate whether the mouse has developed preference for the chamber where it has received nicotine after 4-5 days of conditioning. (E-H) Anxiety-like symptoms in rodents can be evaluated using the (D) light-dark room (preference for the dark compartment), (E) elevated plus maze (preference for the enclosed arms), and (H) open field (preference for the periphery). Depressive-like behaviors can be monitored in the (F) forced swim test (FST) and the (G) sucrose preference test (SPT). (F) The FST is based on the assumption that immobility reflects a measure of behavioral despair (time not swimming). This test involves the exposure of the animals to stress, which was shown to have a role in the tendency for major depression. (G) The SPT is based on a two-bottle choice paradigm. A reduction in the sucrose preference ratio in experimental relative to control mice is indicative of anhedonia. Anhedonia is the inability to experience pleasure from rewarding or enjoyable activities and is a core symptom of depression in humans. (I-J) Fear can be measured in the (I) contextual fear conditioning chamber where rodents learn to predict aversive events by pairing of an initially neutral stimulus (light signal and sound cue) with an aversive fear eliciting stimulus (electric foot shock). (J) The fear-potentiated startle reflex test is a paradigm in which amplitude of a simple reflex is increased when presented with a cue that has been previously paired with an aversive stimulus. A greater general startle reflex is also associated with greater anxiety levels. (K-L) Social and novelty exploration behaviors can be measured in the three-chamber test. This test measures (K) “sociability” as the preference to spend time with another mouse, as compared to time spent alone or with an object in an identical but empty chamber, and (L) “social novelty”, or “object novelty”, as the propensity to spend time with a previously novel (never-before-met) mouse/object rather than with a familiar mouse/object. (H-N) Locomotor activity tests include the (H) open field test which assess general locomotor activity levels, anxiety, and willingness to explore in rodents; (M) the rotarod that measures motor coordination skills; and (N) the running wheel that measures general activity, energy balance, and reward in rodents.

Figure 3.. Molecular mechanisms in habenula-IPN synapses

(A) Glutamatergic transmission: During low frequency synaptic transmission, Mg²⁺ blocks the NMDA receptor. High frequency transmission expels Mg²⁺ from the NMDA receptor, allowing Na⁺ and Ca²⁺ influx. (B) Glutamate and ACh corelease and vesicular synergy: Acetylcholine is coreleased with glutamate by habenular cholinergic neurons. Glutamate transmission follows the “wired transmission” mode. Acetylcholine transmission follows the “volume transmission” mode (Frahm et al., 2015; Ren et al., 2011). (C) Presynaptic facilitation by nAChRs: nAChRs located on cholinergic terminals facilitate the release of acetylcholine (ACh) constituting a fail-safe mechanism at the neuromuscular junction (Mandl and Kiss, 2007) but also in habenular cholinergic terminals (Frahm et al., 2015; Girod and Role, 2001). (D) GABA_B coupling to R-type Ca_v2.3 channels :GABA_B mediates excitation by amplifying presynaptic Ca²⁺ entry through Ca_v2.3 channels and potentiating co-release of glutamate, acetylcholine, and neurokinin B to excite interpeduncular neurons (Zhang et al., 2016). (E) Nitric oxide retrograde signaling: Chronic nicotine treatment in mice dramatically increases the levels of Nitric oxide synthetase (NOS1) in a specific a5-nAChR expressing IPN neuronal population. Activation of soluble guanylate cyclase (sGC) by nitric oxide (NO) in habenular terminals leads to formation of the second messenger cGMP, which inhibits synaptic transmission (Ables et al., 2017). (F) Somatostatin feedback inhibition: Chronic nicotine treatment in mice increases somatostatin (SST) levels in in a specific a5-nAChR- expressing IPN neuronal population. SST released from the IPN activates habenular SSTR2/4, reduces cAMP, and inhibits presynaptic neurotransmission (Ables et al., 2017).