Demonstration of habenular neurons which receive afferent fibers from the nucleus accumbens and send their axons to the midbrain periaqueductal gray

Dynamics of Lateral Habenula-Ventral Tegmental Area Microcircuit on Pain-Related Cognitive Dysfunctions

Chronic pain is a health problem that affects the ability to work and perform other activities, and it generally worsens over time. Understanding the complex pain interaction with brain circuits could help predict which patients are at risk of developing central dysfunctions. Increasing evidence from preclinical and clinical studies suggests that aberrant activity of the lateral habenula (LHb) is associated with depressive symptoms characterized by excessive negative focus, leading to high-level cognitive dysfunctions. The primary output region of the LHb is the ventral tegmental area (VTA), through a bidirectional connection. Recently, there has been growing interest in the complex interactions between the LHb and VTA, particularly regarding their crucial roles in behavior regulation and their potential involvement in the pathological impact of chronic pain on cognitive functions. In this review, we briefly discuss the structural and functional roles of the LHb-VTA microcircuit and their impact on cognition and mood disorders in order to support future studies addressing brain plasticity during chronic pain conditions.

Lateral Habenula Responses During Eye Contact in a Reward Conditioning Task

For many animals, social interaction may have intrinsic reward value over and above its utility as a means to the desired end. Eye contact is the starting point of interactions in many social animals, including primates, and abnormal patterns of eye contact are present in many mental disorders. Whereas abundant previous studies have shown that negative emotions such as fear strongly affect eye contact behavior, modulation of eye contact by reward has received scant attention. Here we recorded eye movement patterns and neural activity in lateral habenula while monkeys viewed faces in the context of Pavlovian and instrumental conditioning tasks. Faces associated with larger rewards spontaneously elicited longer periods of eye contact from the monkeys, even though this behavior was not required or advantaged in the task. Concurrently, lateral habenula neurons were suppressed by faces signaling high value and excited by faces signaling low value. These results suggest that the reward signaling of lateral habenula may contribute to social behavior and disorders, presumably through its connections with the basal ganglia.

Electroacupuncture Relieves Pain During Alcohol Withdrawal

Background: Alcohol use disorder (AUD) is one of the most prevalent chronic relapsing substance use disorders. The negative emotional state, including pain hypersensitivity that often occurs during abstinence, is believed to be a significant driving force for intensive seeking and relapse drinking. Studies have revealed that this may involve the inhibition of midbrain dopamine transmission and activation of the "antireward" system in the lateral habenula (LHb). Acupuncture has been proven effective in reducing pain and certain syndromes associated with AUD. There have been extensive studies conducted on acupuncture. However, the neuroanatomical basis behind acupuncture practice is still unclear. Objective: To briefly describe recent research about acupuncture on pain, particularly those related to AUD. Results: Preclinical studies found that electrostimulation of acupoints (electroacupuncture [EA]) effectively relieves hyperalgesia during withdrawal from chronic alcohol administration. This effect is mediated by the μ-opioid receptors in the LHb. Other studies revealed that the analgesic effect of EA could be mediated by mechanisms independent of the opioid system. Other evidence shows that acupuncture's strong anti-inflammatory effect also contributes to its analgesic effect. Conclusion: Acupuncture could alleviate pain, including the pain in alcoholics, through mechanisms either dependent or independent of the opioid system. Since alcohol abuse causes inflammation, which is also a significant cause of pain, the strong anti-inflammatory effect of acupuncture may also contribute to its analgesic effect. Thus, acupuncture is a nonaddictive therapeutic choice for pain related to substance use disorders, including alcohol.

Possible mechanism and potential application of anti-opioid effect of diazepam-binding inhibitor

AIMS

Our previous study has demonstrated that porcine diazepam-binding inhibitor (pDBI) and its active fragments, pDBI-16 and pDBI-19, have inhibition effect on morphine analgesia in mice. The present study aimed to investigate the underlying mechanism and potential application of this anti-opioid effect.

MATERIALS AND METHODS

Effect of DBI on morphine analgesia was examined by the tail electric stimulation vocalization test. Complementary peptides and antiserum were used to further confirm the effect of DBI in morphine tolerance and dependence. Pharmacological and microinjection methods were used to investigate the underlying mechanism.

KEY FINDINGS

Firstly, pDBI administered either intracerebroventricularly or intravenously dose-dependently inhibited morphine analgesia, while blocking DBI-16 or DBI-19 by the complementary peptides for DBI-16 (CP-DBI-16) or DBI-19 (CP-DBI-19) potentiated it in mice. Secondly, explicit immunoexpression of DBI in the lateral habenular (LHb) was observed in naive rats, and intra-LHb injection of pDBI dose-dependently abolished analgesic effect produced by intra-periaqueductal gray (PAG) injection of morphine in rats. Thirdly, pretreatment with N-Methyl-d-Aspartate receptor (NMDAR) antagonist MK-801 or nitric oxide (NO) synthase inhibitor L-NAME abolished the inhibition effect of pDBI, pDBI-16 or pDBI-19 on morphine analgesia in mice. Finally, antiserum against DBI dose-dependently reversed analgesic tolerance induced by increasing doses of morphine twice daily for 13 days in mice, while CP-DBI-16 or CP-DBI-19 significantly inhibited naloxone-precipitated morphine withdrawal jumping in mice.

SIGNIFICANCE

Taken together, our results demonstrated that NMDAR/NO signaling and LHb-PAG pathway are crucially involved in the anti-opioid effect of DBI, which could provide a potential biological target for opioid tolerance and dependence.

Electroacupuncture Attenuates Hyperalgesia in Rats Withdrawn from Chronic Alcohol Drinking via Habenular Mu Opioid Receptors

BACKGROUND

Hyperalgesia or increased sensitivity to pain is often found in alcoholics during alcohol withdrawal and may contribute to relapse drinking. Alternative therapies such as acupuncture and electroacupuncture (EA), through mechanisms involving opioid receptors, may reduce pain and substance dependence and withdrawal syndromes. The lateral habenula (LHb), an epithalamic structure rich in mu opioid receptors (MORs), is a critical target for both drugs of abuse and pain. We previously observed hyperalgesia in rats withdrawn from chronic ethanol (EtOH) drinking and found that EA at the acupoint Zusanli (ST36) reduced EtOH intake. This raised question of whether EA can alleviate hyperalgesia during alcohol withdrawal and, if so, whether the mechanism involves MORs in the LHb.

METHODS

We trained male rats to drink EtOH using the intermittent access 20% EtOH 2-bottle free-choice drinking paradigm for 8 weeks, after which the alcohol supply was discontinued. We measured pain sensitivity using radiant heat (a light beam directed at the hind paw of rats) and compared the paw withdrawal latencies (PWLs) with and without EA at ST36.

RESULTS

The PWLs were significantly shorter in rats at 24, 48, and 72 hours and 7 days after the discontinuation of EtOH when compared to EtOH-naïve rats. After a single administration of 2-Hz EA for 20 minutes at ST36, the PWLs at 24 hours after the withdrawal of EtOH were significantly greater than those of the sham group (2-Hz EA at the tail). Furthermore, the effect of EA on PWLs was significantly attenuated by bilateral intrahabenula infusion of the MOR antagonist naltrexone.

CONCLUSIONS

These results suggest that EA can alleviate hyperalgesia during EtOH withdrawal through a mechanism involving MORs in the habenula. Based on this, EA could be of potential value as a therapy for hyperalgesia in alcohol dependence.

Mu Opioid Receptor Actions in the Lateral Habenula

Increased activity of lateral habenula (LHb) neurons is correlated with aversive states including pain, opioid abstinence, rodent models of depression, and failure to receive a predicted reward. Agonists at the mu opioid receptor (MOR) are among the most powerful rewarding and pain relieving drugs. Injection of the MOR agonist morphine directly into the habenula produces analgesia, raising the possibility that MOR acts locally within the LHb. Consequently, we examined the synaptic actions of MOR agonists in the LHb using whole cell patch clamp recording. We found that the MOR selective agonist DAMGO inhibits a subset of LHb neurons both directly and by inhibiting glutamate release onto these cells. Paradoxically, DAMGO also presynaptically inhibited GABA release onto most LHb neurons. The behavioral effect of MOR activation will thus depend upon both the level of intrinsic neuronal activity in the LHb and the balance of activity in glutamate and GABA inputs to different LHb neuronal populations.

Direct projection from the lateral habenula to the trigeminal mesencephalic nucleus in rats

Trigeminal mesencephalic nucleus (Vmes) neurons are primary afferents conveying deep sensation from the masticatory muscle spindles or the periodontal mechanoreceptors, and are crucial for controlling jaw movements. Their cell bodies exist in the brain and receive descending commands from a variety of cortical and subcortical structures involved in limbic (emotional) systems. However, it remains unclear how the lateral habenula (LHb), a center of negative emotions (e.g., pain, stress and anxiety), can influence the control of jaw movements. To address this issue, we examined whether and how the LHb directly projects to the Vmes by means of neuronal tract tracing techniques in rats. After injections of a retrograde tracer Fluorogold in the rostral and caudal Vmes, a number of neurons were labeled in the lateral division of LHb (LHbl) bilaterally, whereas a few neurons were labeled in the medial division of LHb (LHbm) bilaterally. After injections of an anterograde tracer, biotinylated dextranamine (BDA) in the LHbl, a small number of labeled axons were distributed bilaterally in the rostral and caudal levels of Vmes, where some labeled axonal boutons contacted the cell body of rostral and caudal levels of Vmes neurons bilaterally. After the BDA injection into the LHbm, however, no axons were labeled bilaterally in the rostral and caudal levels of Vmes. Therefore, the present study for the first time demonstrated the direct projection from the LHbl to the Vmes and the detailed projection patterns, suggesting that jaw movements are modulated by negative emotions that are signaled by LHbl neurons.

Insights into the Role of the Habenular Circadian Clock in Addiction

Drug addiction is a brain disease involving alterations in anatomy and functional neural communication. Drug intake and toxicity show daily rhythms in both humans and rodents. Evidence concerning the role of clock genes in drug intake has been previously reported. However, the implication of a timekeeping brain locus is much less known. The epithalamic lateral habenula (LHb) is now emerging as a key nucleus in drug intake and addiction. This brain structure modulates the activity of dopaminergic neurons from the ventral tegmental area, a central part of the reward system. Moreover, the LHb has circadian properties: LHb cellular activity (i.e., firing rate and clock genes expression) oscillates in a 24-h range, and the nucleus is affected by photic stimulation and has anatomical connections with the main circadian pacemaker, the suprachiasmatic nucleus. Here, we describe the current insights on the role of the LHb as a circadian oscillator and its possible implications on the rhythmic regulation of the dopaminergic activity and drug intake. These data could inspire new strategies to treat drug addiction, considering circadian timing as a principal factor.

Altered formalin-induced pain and Fos induction in the periaqueductal grey of preadolescent rats following neonatal LPS exposure

Animal and human studies have demonstrated that early pain experiences can produce alterations in the nociceptive systems later in life including increased sensitivity to mechanical, thermal, and chemical stimuli. However, less is known about the impact of neonatal immune challenge on future responses to noxious stimuli and the reactivity of neural substrates involved in analgesia. Here we demonstrate that rats exposed to Lipopolysaccharide (LPS; 0.05 mg/kg IP, Salmonella enteritidis) during postnatal day (PND) 3 and 5 displayed enhanced formalin-induced flinching but not licking following formalin injection at PND 22. This LPS-induced hyperalgesia was accompanied by distinct recruitment of supra-spinal regions involved in analgesia as indicated by significantly attenuated Fos-protein induction in the rostral dorsal periaqueductal grey (DPAG) as well as rostral and caudal axes of the ventrolateral PAG (VLPAG). Formalin injections were associated with increased Fos-protein labelling in lateral habenula (LHb) as compared to medial habenula (MHb), however the intensity of this labelling did not differ as a result of neonatal immune challenge. These data highlight the importance of neonatal immune priming in programming inflammatory pain sensitivity later in development and highlight the PAG as a possible mediator of this process.

Habenula lesions decrease the responsiveness of dorsal raphe serotonin neurons to cocaine

The median and dorsal (MR and DR) raphe nuclei are the origin of serotonin (5-HT)-containing neurons that innervate the forebrain. Neurons originating in the medial and lateral habenula provide an extensive afferent input to the midbrain that could serve as a negative feedback circuit. The present study was undertaken to establish whether intact habenula nuclei are required to observe the depressant effects of cocaine on the neural activity of 5-HT somata in the DR. To this end, the spontaneous activity of DR 5-HT neurons was assessed in male rats that had previously received bilateral radiofrequency lesions of the habenula complex either 1-4 h (short term) or 7 days (long term) prior to extracellular recordings of single 5-HT neurons of the DR. In rats with short-term lesions, the inhibitory response to cocaine was significantly attenuated. The mean dose to inhibit activity by 50% (ID50) was increased from 0.68 mg/kg in controls to 2.5 mg/kg in lesioned rats. Short-term habenula lesions also significantly decreased the numbers (but not the firing rates) of 5-HT neurons encountered in the DR. In contrast, the dose-response to cocaine as well as the numbers and firing rates of 5-HT neurons found in rats with long-term habenula lesions did not differ from controls. These results suggest that the inhibitory effects of cocaine on DR 5-HT neuronal activity depend in part on the ability of cocaine to affect habenula control of raphe 5-HT function.