Abstract
The lateral habenula (LHb) is emerging as a crucial structure capable of conveying rewarding and aversive information. Recent evidence indicates that a rapid increase in the activity of LHb neurons drives negative states and avoidance. Furthermore, the hyperexcitability of neurons in the LHb, especially those projecting to the midbrain, may represent an important cellular correlate for neuropsychiatric disorders like depression and drug addiction. Despite the recent insights regarding the implications of the LHb in the context of reward and aversion, the exact nature of the synaptic and cellular players regulating LHb neuronal functions remains largely unknown. Here we focus on the synaptic and cellular physiology of LHb neurons. First, we discuss the properties of excitatory transmission and the implications of glutamate receptors for long-term synaptic plasticity; second, we review the features of GABAergic transmission onto LHb neurons; and finally, we describe the contribution that neuromodulators such as dopamine (DA) and serotonin may have for LHb neuronal physiology. We relate these findings to the role that the LHb can play in processing aversive and rewarding stimuli, both in health and disease states.
Introduction
Significant evidence is converging to the idea that the lateral habenula (LHb) strongly controls midbrain targets including the ventral tegmental area (VTA), substantia nigra pars compacta and the raphe nuclei (Christoph et al., ; Hikosaka, ). Functionally, the LHb contributes to the encoding of aversion and reward, and also plays a role in associated pathological conditions such as mood disorders and drug addiction (Hikosaka, ). A comprehensive understanding of LHb functions and their implications in neuropsychiatric disorders requires the dissection of the cellular and synaptic properties of neurons in this nucleus. Here, we describe how LHb neurons activity can be influenced by excitatory and inhibitory synapses and by neuromodulators, and we discuss the significance of this in relation to the potential role of the LHb in encoding rewarding and aversive stimuli, and in associated psychiatric disorders.
The lateral habenula (LHb): a highway to the midbrain for processing aversion and reward
Anatomical and morphological organization
The LHb, together with the medial habenula (MHb), is part of the epithalamus, and located above the thalamus at its posterior end close to the midline. The LHb comprises a medial and a lateral division (Andres et al., ; Hikosaka, ). LHb neurons present a dendritic arborization with postsynaptic spines (Figure 1; Weiss and Veh et al., 2011; Maroteaux and Mameli, 2012). Morphological and immunohistochemical studies revealed heterogeneous populations randomly distributed throughout the LHb based on somatodendritic organization and receptor expression (Weiss and Veh et al., 2011; Aizawa et al., ).
Figure 1
The main output of the LHb is glutamatergic (Li et al.,
LHb in aversion and reward
The LHb is pivotal in processing aversive and rewarding information. Delivery of an unexpected airpuff, a cue that predicts its onset, or even the omission of an expected reward, leads to a strong increase in the activity of LHb neurons in monkeys. Conversely, unexpected delivery of rewards and cues predicting a reward decrease LHb neuron firing. The LHb is inhibited more strongly as expected reward probability or magnitude increase (Matsumoto and Hikosaka, 2007, 2009). Importantly, the activity in LHb neurons is the inverse of that of dopamine (DA) neurons in the midbrain during aversive and rewarding states. The punishment-driven increase in excitation of LHb occurs earlier than that of DA neurons, suggesting the upstream control of the LHb on the DA system. This may not be reflected in the case of reward delivery, leaving the causality of this relationship still as an open question (Matsumoto and Hikosaka, 2007). Together these findings suggest that LHb neurons code for discrepancies between reward or punishment-related expectation and outcome.
Recent advances have shed light on the implication of LHb in driving aversive behaviors. Indeed, optogenetic activation of excitatory projection to the LHb, and LHb terminals in the midbrain drives aversive behaviors (Lammel et al.,
Synaptic transmission in the lateral habenula (LHb)
Excitatory transmission in the lateral habenula (LHb)
LHb neurons receive glutamatergic projections from various structures that include the MGP, as well as the lateral hypothalamus, the cortex and likely the VTA (Weiss and Veh et al., 2011; Hnasko et al.,
Glutamate receptors
The predominant receptor in the LHb mediating fast excitatory transmission is the AMPA-type glutamate receptor (AMPAR): a heterotetrameric complex highly sensitive to experience-driven changes (Hollmann et al.,
Metabotropic glutamate receptors (mGluRs) generally strongly modulate CP-AMPAR expression. Indeed, mGluRs trigger long-term depression (LTD) of AMPAR currents specifically when CP-AMPARs are present (Luscher and Huber, 2010). In several structures including the VTA, the nucleus accumbens and the cerebellum, mGluR activation triggers a switch from CP-AMPARs to calcium impermeable AMPARs. mGluRs evoke CP-AMPAR internalization, regulate experience-induced synaptic plasticity, and in some cases restore experience-driven adaptations (Bellone and Luscher,
Interestingly, the NMDA-mediated component at LHb synapses at positive potentials is small compared to the one mediated by AMPA receptors, indicative for a low expression of synaptic N-methyl-d-aspartate receptors (NMDAR; Li et al.,
Synaptic plasticity in the LHb
CP-AMPARs regulate the induction of several forms of long-term synaptic plasticity (Kullmann and Lamsa,
Inhibitory transmission in the lateral habenula (LHb)
The LHb receives strong GABAergic innervation (Araki et al.,
GABAA receptors
GABAARs are pentameric ionotropic receptors, assembled out of a larger available pool of 19 subunits, which determine the conduction kinetics of the receptor as well as its affinity for GABA (Farrant and Nusser,
GABAergic signaling within the LHb has relevance in the context of drug intake. GABA (but not glutamate) immunolabeling decreased in the LHb in rats withdrawn (5 days) from chronic treatment with cocaine (Meshul et al., 1998). Prolonged cocaine withdrawal (21 days) increased binding of a radiolabeled benzodiazepine (which binds to GABAARs) in the whole LHb, whereas acute withdrawal from this treatment slightly decreased binding (Keys and Ellison,
GABAB receptors
GABA transmission also relies on Gi/o protein-coupled GABABRs, which are responsible for slower and late inhibitory conductance (Hill,
GABABR activation inhibits adenylyl cyclase and mediates the hyperpolarization of post-synaptic membranes by activation of inwardly rectifying potassium (GIRK) channels (Luscher et al., 1997). Interestingly, a moderate expression of Kir3.2 potassium channel subunit has been detected in the LHb (Geisler et al.,
Neuromodulatory systems in the lateral habenula (LHb)
Dopamine (DA) modulation in the LHb
Dopaminergic nuclei such as the VTA and substantia nigra pars compacta receive input from the LHb, and also provide feedback projections, suggesting that DA may modulate LHb activity (Phillipson and Pycock, 1982; Gruber et al.,
Fibers expressing tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of DA, have been demonstrated in the LHb (Geisler et al.,
5HT modulation in the LHb
Analogously to the DA system, there is evidence that LHb neurons powerfully modulate raphe serotonin neurons, but also receive a serotoninergic feedback signal (Figure 1; Wang and Aghajanian, 1977; Mengod et al., 1990; Aizawa et al.,
Concluding remarks
Initial evidence collected in the early 1980’s pointed to the potential importance of the LHb in controlling midbrain structures. However, only much more recently it is starting to become clear that this structure participates in the encoding of aversive and rewarding stimuli, influences motivational states, and contributes to pathologies such as mood disorders and addiction. These findings highlight the importance to understand how LHb neuronal activity levels are influenced by the integration of local synaptic and neuromodulatory signals.
While we have discussed the presumably key input signals and their receptors in this review, tentative evidence suggests that other potentially relevant molecules may also act in the LHb, by mechanisms that remain largely unknown. Among these potential modulatory signals are orexins (Peyron et al., 1998), acetylcholine (Geisler et al.,
The big challenge in this field is now to identify which synaptic and intrinsic properties adapt during or after exposure to aversion or reward; and to understand the downstream implications of such changes on monoamines like the DA and serotonin systems. Such a framework may represent the basis to understand the maladaptive mechanisms taking place in neuropsychiatric disorders where the activity of the LHb is altered, and may offer a window to further develop therapeutic strategies.
Statements
Acknowledgments
We thank C. Bellone for helpful comments on the manuscript. This work is supported by the Ecole de Neuroscience de Paris, Inserm Atip-Avenir and City of Paris (Manuel Mameli). Frank J. Meye is supported by the Fyssen Postdoctoral Fellowship.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that may represent a potential conflict of interest.
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Summary
Keywords
lateral habenula, synaptic transmission, AMPA receptors, GABA receptors, neuromodulators
Citation
Meye FJ, Lecca S, Valentinova K and Mameli M (2013) Synaptic and cellular profile of neurons in the lateral habenula. Front. Hum. Neurosci. 7:860. doi: 10.3389/fnhum.2013.00860
Received
25 September 2013
Accepted
25 November 2013
Published
16 December 2013
Volume
7 - 2013
Edited by
Ramiro Salas, Baylor College of Medicine, USA
Reviewed by
Hidenori Aizawa, Tokyo Medical and Dental University, Japan; Carl R. Lupica, National Institutes of Health, National Institute on Drug Abuse, USA
Copyright
© 2013 Meye, Lecca, Valentinova and Mameli.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Manuel Mameli, Institut du Fer à Moulin, UMRS-S 839, INSERM, 17 Rue du Fer à Moulin, 75005 Paris, France e-mail: manuel.mameli@inserm.fr
This article was submitted to the journal Frontiers in Human Neuroscience.
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