Mu Opioid Receptor-Expressing Neurons in the Dorsal Raphe Nucleus Are Involved in Reward Processing and Affective Behaviors

BACKGROUND

Mu opioid receptors (MORs) are key for reward processing, mostly studied in dopaminergic pathways. MORs are also expressed in the dorsal raphe nucleus (DRN), which is central for the modulation of reward and mood, but MOR function in the DRN remains underexplored. Here, we investigated whether MOR-expressing neurons of the DRN (DRN-MOR neurons) participate in reward and emotional responses.

METHODS

We characterized DRN-MOR neurons anatomically using immunohistochemistry and functionally using fiber photometry in responses to morphine and rewarding/aversive stimuli. We tested the effect of opioid uncaging on the DRN on place conditioning. We examined the effect of DRN-MOR neuron optostimulation on positive reinforcement and mood-related behaviors. We mapped their projections and selected DRN-MOR neurons projecting to the lateral hypothalamus for a similar optogenetic experimentation.

RESULTS

DRN-MOR neurons form a heterogeneous neuronal population essentially composed of GABAergic (gamma-aminobutyric acidergic) and glutamatergic neurons. Calcium activity of DRN-MOR neurons was inhibited by rewarding stimuli and morphine. Local photo-uncaging of oxymorphone in the DRN produced conditioned place preference. DRN-MOR neuron optostimulation triggered real-time place preference and was self-administered, promoted social preference, and reduced anxiety and passive coping. Finally, specific optostimulation of DRN-MOR neurons projecting to the lateral hypothalamus recapitulated the reinforcing effects of total DRN-MOR neuron stimulation.

CONCLUSIONS

Our data show that DRN-MOR neurons respond to rewarding stimuli and that their optoactivation has reinforcing effects and promotes positive emotional responses, an activity which is partially mediated by their projections to the lateral hypothalamus. Our study also suggests a complex regulation of DRN activity by MOR opioids, involving mixed inhibition/activation mechanisms that fine-tune DRN function.

Alteration of Lateral Habenula Function Prevents the Proper Exploration of a Novel Environment

The lateral habenula (LHb) is an epithalamic brain region viewed as a converging hub, integrating information from a large connectome and then projecting to few critical midbrain monoaminergic systems. Numerous studies have explored the roles of the LHb, notably in aversion and avoidance. An important recurring finding when manipulating the LHb is the induction of anxiety-related behaviours. However, its exact role in such behaviours remains poorly understood. In the present study, we used two pharmacological approaches altering LHb activity, intra-LHb infusion of either the GABA-A receptor agonist, Muscimol, or the glutamatergic AMPA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and exposed rats to three consecutive open field (OF) sessions. We found that both pharmacological treatments prevented rats to explore the centre of the OF, considered as the most anxiogenic part of the apparatus, across the three OF sessions. In addition, during the first, but not the two consecutive sessions, both treatments prevented a thorough exploration of the OF. Altogether, these results confirm the crucial role played by the LHb in anxiety-related behaviours and further suggest its implication in the exploration of new anxiogenic environments.

Lateral Habenula Beyond Avoidance: Roles in Stress, Memory, and Decision-Making With Implications for Psychiatric Disorders

In this Perspective review, we highlight some of the less explored aspects of lateral habenula (LHb) function in contextual memory, sleep, and behavioral flexibility. We provide evidence that LHb is well-situated to integrate different internal state and multimodal sensory information from memory-, stress-, motivational-, and reward-related circuits essential for both survival and decision making. We further discuss the impact of early life stress (ELS) on LHb function as an example of stress-induced hyperactivity and dysregulation of neuromodulatory systems within the LHb that promote anhedonia and motivational deficits following ELS. We acknowledge that recent technological advancements in manipulation and recording of neural circuits in simplified and well-controlled behavioral paradigms have been invaluable in our understanding of the critical role of LHb in motivation and emotional regulation as well as the involvement of LHb dysfunction in stress-induced psychopathology. However, we also argue that the use of ethologically-relevant behaviors with consideration of complex aspects of decision-making is warranted for future studies of LHb contributions in a wide range of psychiatric illnesses. We conclude this Perspective with some of the outstanding issues for the field to consider where a multi-systems approach is needed to investigate the complex nature of LHb circuitry interactions with environmental stimuli that predisposes psychiatric disorders.

From controlled to compulsive drug-taking: The role of the habenula in addiction

Addiction is now recognized as a neurobiological and cognitive brain disorder and is generally viewed as a switch from recreational or voluntary to compulsive substance use despite aversive consequences. The habenula, composed of medial (MHb) and lateral (LHb) domains, has been implicated in regulating behavioral flexibility and anxiety-related behaviors and is considered a core component of the brain "anti-reward" system. These functions position the habenula to influence voluntary behaviors. Consistent with this view, emerging evidence points to alterations in habenula activity as important factors to contributing the loss of control over the use of drugs of abuse and the emergence of compulsive drug seeking behaviors. In this review, we will discuss the general functions of the MHb and LHb and describe how these functional properties allow this brain region to promote or suppress volitional behaviors. Then, we highlight mechanisms by which drugs of abuse may alter habenular activity, precipitating the emergence of addiction-relevant behavioral abnormalities.

Neuroscience: Brain Mechanisms of Blushing

The dorsal hypothalamic area regulates increases in body temperature in response to stress, but precise mechanisms are unclear. A new study suggests that glutamatergic neurons in this brain area regulate this action and, surprisingly, may also be involved in blushing.

The Lateral Habenula as a Relay of Cortical Information to Process Working Memory

Working memory is a cognitive ability allowing the temporary storage of information to solve problems or adjust behavior. While working memory is known to mainly depend on the medial prefrontal cortex (mPFC), very few is known about how cortical information are relayed subcortically. By its connectivity, the lateral habenula (lHb) might act as a subcortical relay for cortical information. Indeed, the lHb receives inputs from several mPFC subregions, and recent findings suggest a role for the lHb in online processing of spatial information, a fundamental aspect of working memory. In rats, in a delayed non-matching to position paradigm, using focal microinjections of the GABAA agonist muscimol we showed that inactivation of the lHb (16 ng in 0.2 µL per side), as well as disconnection between the prelimbic region of the mPFC (mPFC/PrL, 32 ng in 0.4 µL in one hemisphere) and the lHb (16 ng in 0.2 µL in the lHb in the contralateral hemisphere) impaired working memory. The deficits were unlikely to result from motivational or motor deficits as muscimol did not affect reward collection or cue responding latencies, and did not increase the number of omissions. These results show for the first time the implication of the lHb in mPFC-dependent memory processes, likely as a relay of mPFC/PrL information. They also open new perspectives in the understanding of the top-down processing of high-level cognitive functions.

[Sutureless transconjunctival 20-gauge vitrectomy]

PURPOSE

To evaluate the safety and efficacy of 20-gauge transconjunctival sutureless vitrectomy for a variety of vitreoretinal conditions.

PATIENTS AND METHODS

This study was a retrospective clinical case series examining 29 eyes of 29 patients treated with 20-gauge transconjunctival sutureless vitrectomy. The main outcome measures included intraocular pressure, visual acuity, surgical time, and intraoperative and postoperative complications.

RESULTS

The mean follow-up duration was 5.7 months. There were only two cases of postoperative ocular hypotony (6.9%) and one suspect case of endophthalmitis. Mean overall acuity improved from 0.495 logMar at baseline to 0.29 logMar (p=0.002) and mean total operative time was 19 min. We observed no cases of retinal detachment.

CONCLUSION

Twenty-gauge transconjunctival sutureless vitrectomy is an effective surgical technique, but other studies are needed to confirm its safety and its value in other indications.