Preferential modulation of the lateral habenula activity by serotonin-2A rather than -2C receptors: Electrophysiological and neuroanatomical evidence

Preferential Modulatory Action of 5-HT2A Receptors on the Dynamic Regulation of Basal Ganglia Circuits

In rodents, cortical information is transferred to the substantia nigra pars reticulata (SNr) through motor and medial prefrontal (mPF) basal ganglia (BG) circuits implicated in motor and cognitive/motivational behaviors, respectively. The serotonergic 5-HT_2A receptors are located in both of these neuronal networks, displaying topographical differences with a high expression in the associative/limbic territories, and a very low expression in the subthalamic nucleus. This study investigated whether the stimulation of 5-HT_2A receptors could have a specific signature on the dynamic regulation of BG circuits, preferentially modulating the mPF information processing through trans-striatal pathways. We performed in vivo single-unit extracellular recordings to assess the effect of the 5-HT_2A agonist TCB-2 on the spontaneous and cortically evoked activity of lateral and medial SNr neurons in male rats (involved in motor and mPF circuits, respectively). TCB-2 (50-200 µg/kg, i.v.) increased the basal firing rate and enhanced the cortically evoked inhibitory response of medial SNr neurons (transmission through the direct striato-nigral pathway). A prior administration of the preferential 5-HT_2A receptor antagonist MDL11939 (200 µg/kg, i.v.) did not modify any electrophysiological parameter, but occluded TCB-2-induced effects. In animals treated with the 5-HT synthesis inhibitor pCPA (4-chloro-dl-phenylalanine methyl ester hydrochloride), TCB-2 failed to induce the above-mentioned effects, thus suggesting the contribution of endogenous 5-HT. However, the mobilization of 5-HT induced by the acute administration of fluoxetine (10 mg/kg, i.p.) did not mimic the effects triggered by TCB-2. Overall, these data suggest that 5-HT_2A receptors have a preferential modulatory action on the dynamic regulation of BG circuitry.SIGNIFICANCE STATEMENT Motor and medial prefrontal (mPF) basal ganglia (BG) circuits play an important role in integrative brain functions like movement control or cognitive/motivational behavior, respectively. Although these neuronal networks express 5-HT_2A receptors, the expression is higher in associative/limbic structures than in the motor ones. We show a topographical-dependent dissociation in the effects triggered by the 5HT_2A agonist TCB-2, which specifically increases the medial substantia nigra pars reticulata neuron activity and has a preferential action on mPF information processing through the striato-nigral direct pathway. These are very likely to be 5-HT_2A receptor-mediated effects that require mobilization of the endogenous 5-HT system. These findings provide evidence about the specific signature of 5-HT_2A receptors on the dynamic regulation of BG circuits.

Multi-level variations of lateral habenula in depression: A comprehensive review of current evidence

Despite extensive research in recent decades, knowledge of the pathophysiology of depression in neural circuits remains limited. Recently, the lateral habenula (LHb) has been extensively reported to undergo a series of adaptive changes at multiple levels during the depression state. As a crucial relay in brain networks associated with emotion regulation, LHb receives excitatory or inhibitory projections from upstream brain regions related to stress and cognition and interacts with brain regions involved in emotion regulation. A series of pathological alterations induced by aberrant inputs cause abnormal function of the LHb, resulting in dysregulation of mood and motivation, which present with depressive-like phenotypes in rodents. Herein, we systematically combed advances from rodents, summarized changes in the LHb and related neural circuits in depression, and attempted to analyze the intrinsic logical relationship among these pathological alterations. We expect that this summary will greatly enhance our understanding of the pathological processes of depression. This is advantageous for fostering the understanding and screening of potential antidepressant targets against LHb.

Nicotine modulation of the lateral habenula/ventral tegmental area circuit dynamics: An electrophysiological study in rats

Nicotine, the addictive component of tobacco, has bivalent rewarding and aversive properties. Recently, the lateral habenula (LHb), a structure that controls ventral tegmental area (VTA) dopamine (DA) function, has attracted attention as it is potentially involved in the aversive properties of drugs of abuse. Hitherto, the LHb-modulation of nicotine-induced VTA neuronal activity in vivo is unknown. Using standard single-extracellular recording in anesthetized rats, we observed that intravenous administration of nicotine hydrogen tartrate (25-800 μg/kg i.v.) caused a dose-dependent increase in the basal firing rate of the LHb neurons of nicotine-naïve rats. This effect underwent complete desensitization in chronic nicotine (6 mg/kg/day for 14 days)-treated animals. As previously reported, acute nicotine induced an increase in the VTA DA neuronal firing rate. Interestingly, only neurons located medially (mVTA) but not laterally (latVTA) within the VTA were responsive to acute nicotine. This pattern of activation was reversed by chronic nicotine exposure which produced the selective increase of latVTA neuronal activity. Acute lesion of the LHb, similarly to chronic nicotine treatment, reversed the pattern of DA cell activation induced by acute nicotine increasing latVTA but not mVTA neuronal activity. Our evidence indicates that LHb plays an important role in mediating the effects of acute and chronic nicotine within the VTA by activating distinct subregional responses of DA neurons. The LHb/VTA modulation might be part of the neural substrate of nicotine aversive properties. By silencing the LHb chronic nicotine could shift the balance of motivational states toward the reward.

Lateral Habenula 5-HT2C Receptor Function Is Altered by Acute and Chronic Nicotine Exposures

Serotonin (5-HT) is important in some nicotine actions in the CNS. Among all the 5-HT receptors (5-HTRs), the 5-HT_2CR has emerged as a promising drug target for smoking cessation. The 5-HT_2CRs within the lateral habenula (LHb) may be crucial for nicotine addiction. Here we showed that after acute nicotine tartrate (2 mg/kg, i.p.) exposure, the 5-HT_2CR agonist Ro 60-0175 (5–640 µg/kg, i.v.) increased the electrical activity of 42% of the LHb recorded neurons in vivo in rats. Conversely, after chronic nicotine treatment (6 mg/kg/day, i.p., for 14 days), Ro 60-0175 was incapable of affecting the LHb neuronal discharge. Moreover, acute nicotine exposure increased the 5-HT_2CR-immunoreactive (IR) area while decreasing the number of 5-HT_2CR-IR neurons in the LHb. On the other hand, chronic nicotine increased both the 5-HT_2CR-IR area and 5-HT_2CR-IR LHb neurons in the LHb. Western blot analysis confirmed these findings and further revealed an increase of 5-HT_2CR expression in the medial prefrontal cortex after chronic nicotine exposure not detected by the immunohistochemistry. Altogether, these data show that acute and chronic nicotine exposure differentially affect the central 5-HT_2CR function mainly in the LHb and this may be relevant in nicotine addiction and its treatment.

Multiple facets of serotonergic modulation

The serotonergic system of the central nervous system (CNS) has been implicated in a broad range of physiological functions and behaviors, such as cognition, mood, social interaction, sexual behavior, feeding behavior, sleep-wake cycle and thermoregulation. Serotonin (5-hydroxytryptamine, 5-HT) establishes a plethora of interactions with neurochemical systems in the CNS via its numerous 5-HT receptors and autoreceptors. The facets of this control are multiple if we consider the molecular actors playing a role in the autoregulation of 5-HT neuron activity including the 5-HT_1A, 5-HT_1B, 5-HT_1D, 5-HT_2B, 5-HT₇ receptors as well as the serotonin transporter. Moreover, extrinsic loops involving other neurotransmitters giving the other 5-HT receptors the possibility to impact 5-HT neuron activity. Grasping the complexity of these interactions is essential for the development of a variety of therapeutic strategies for cognitive defects and mood disorders. Presently we can illustrate the plurality of the mechanisms and only conceive that these 5-HT controls are likely not uniform in terms of regional and neuronal distribution. Our understanding of the specific expression patterns of these receptors on specific circuits and neuronal populations are progressing and will expand our comprehension of the function and interaction of these receptors with other chemical systems. Thus, the development of new approaches profiling the expression of 5-HT receptors and autoreceptors should reveal additional facets of the 5-HT controls of neurochemical systems in the CNS.

Serotonin/dopamine interaction: Electrophysiological and neurochemical evidence

The interaction between serotonin (5-HT) and dopamine (DA) in the central nervous system (CNS) plays an important role in the adaptive properties of living animals to their environment. These are two modulatory, divergent systems shaping and regulating in a widespread manner the activity of neurobiological networks and their interaction. The concept of one interaction linking these two systems is rather elusive when looking at the mechanisms triggered by these two systems across the CNS. The great variety of their interacting mechanisms is in part due to the diversity of their neuronal origin, the density of their fibers in a given CNS region, the distinct expression of their numerous receptors in the CNS, the heterogeneity of their intracellular signaling pathway that depend on the cellular type expressing their receptors, and the state of activity of neurobiological networks, conditioning the outcome of their mutual influences. Thus, originally conceptualized as inhibition of 5-HT on DA neuron activity and DA neurotransmission, this interaction is nowadays considered as a multifaceted, mutual influence of these two systems in the regulation of CNS functions. These new ways of understanding this interaction are of utmost importance to envision the consequences of their dysfunctions underlined in several CNS diseases. It is also essential to conceive the mechanism of action of psychotropic drugs directly acting on their function including antipsychotic, antidepressant, antiparkinsonian, and drug of abuse together with the development of therapeutic strategies of Alzheimer's diseases, epilepsy, obsessional compulsive disorders. The 5-HT/DA interaction has a long history from the serendipitous discovery of antidepressants and antipsychotics to the future, rationalized treatments of CNS disorders.

The impact of chronic daily nicotine exposure and its overnight withdrawal on the structure of anxiety-related behaviors in rats: Role of the lateral habenula

Tobacco smoking is a serious health problem worldwide and a leading cause of mortality. Nicotine, the addictive component of tobacco, affects a range of emotional responses, including anxiety-related behaviors. Although perceived by smokers to be anxiolytic, evidence suggests that smoking increases anxiety and that mood fluctuates with nicotine intake. Thus, nicotine addiction may depend on easing the psychobiological distress caused by its abuse. The lateral habenula (LHb) has been implicated as a neural substrate for acute nicotine-induced anxiety, but its role in anxiety-like behaviors associated with chronic nicotine exposure has not been explored. Here, we assessed the effect of chronic nicotine exposure and its subsequent overnight withdrawal on anxiety-like behavior using both quantitative and multivariate T-pattern analysis in rats tested using the hole-board apparatus. Additionally, we explored the role of the LHb by comparing the behavioral effects of short-term nicotine withdrawal in chronically treated LHb-lesioned rats. Quantitative analysis revealed increased anxiety-like behavior in chronically treated overnight nicotine-deprived rats, as manifested in reduced general and focused exploratory behaviors, which was eased in animals that received nicotine. Quantitative analysis failed to reveal a role of the LHb in overnight nicotine deprivation-induced anxiety. Conversely, T-pattern analysis of behavioral outcomes revealed that chronic nicotine-treated rats still show anxiety-like behavior following nicotine challenge. Moreover, it demonstrated that the LHb lesion induced a stronger anxiolytic-like response to the acute challenge of nicotine in chronically nicotine-exposed animals, implicating the LHb in the anxiogenic effect of chronic nicotine exposure. These data further highlight the LHb as a promising target for smoking cessation therapies and support the importance of T-pattern analysis for behavioral analysis.

Serotonergic neurons in the treatment of mood disorders: The dialogue with astrocytes

Astrocytes were traditionally regarded as cells important to neuronal activity, providing both metabolic and structural supports. Recent evidence suggests that they may also play a crucial role in the control of higher brain functions. In keeping with this hypothesis, it is now well accepted that astrocytes contribute to stress but also react to antidepressant drugs as they express serotonergic transporters and receptors. However, the downstream mechanisms leading to the fine-tuned regulation of mood are still unknown. This chapter pays attention to the role of astrocytes in the regulation of emotional behavior and related serotonergic neurotransmission. In particular, it gives a current state of the clinical and preclinical evidence showing that astrocytes respond to environmental conditions and antidepressant drugs through the release of gliotransmitters and neurotrophic factors which in turn, influence serotonergic tone in discrete brain areas. This state-of-the-art review aims at demonstrating the remarkable potential for novel therapeutic antidepressant strategies targeting these glial cells.

The Emerging Role of LHb CaMKII in the Comorbidity of Depressive and Alcohol Use Disorders

Depressive disorders and alcohol use disorders are widespread among the general population and are significant public health and economic burdens. Alcohol use disorders often co-occur with other psychiatric conditions and this dual diagnosis is called comorbidity. Depressive disorders invariably contribute to the development and worsening of alcohol use disorders, and vice versa. The mechanisms underlying these disorders and their comorbidities remain unclear. Recently, interest in the lateral habenula, a small epithalamic brain structure, has increased because it becomes hyperactive in depression and alcohol use disorders, and can inhibit dopamine and serotonin neurons in the midbrain reward center, the hypofunction of which is believed to be a critical contributor to the etiology of depressive disorders and alcohol use disorders as well as their comorbidities. Additionally, calcium/calmodulin-dependent protein kinase II (CaMKII) in the lateral habenula has emerged as a critical player in the etiology of these comorbidities. This review analyzes the interplay of CaMKII signaling in the lateral habenula associated with depressive disorders and alcohol use disorders, in addition to the often-comorbid nature of these disorders. Although most of the CaMKII signaling pathway's core components have been discovered, much remains to be learned about the biochemical events that propagate and link between depression and alcohol abuse. As the field rapidly advances, it is expected that further understanding of the pathology involved will allow for targeted treatments.

Expression of serotonin 1A and 2A receptors in molecular- and projection-defined neurons of the mouse insular cortex

The serotonin (5-HT) system is the target of multiple anxiolytics, including Buspirone, which is a partial agonist of the serotonin 1A receptor (5-HT1A). Similarly, ligands of the serotonin 2A receptor (5-HT2A) were shown to alter anxiety level. The 5-HT1A and 2A receptors are widely expressed across the brain, but the target region(s) underlying the influence of those receptors on anxiety remain unknown. Interestingly, recent studies in human and non-human primates have shown that the 5-HT1A and 5-HT2A binding potentials within the insular cortex (insula) are correlated to anxiety. As an initial step to define the function of 5-HT transmission in the insula, we quantified the proportion of specific neuronal populations of the insula expressing 5-HT1A or 5-HT2A. We analyzed seven neural populations, including three defined by a molecular marker (putative glutamate, GABA or parvalbumin), and four defined by their projections to different downstream targets. First, we found that more than 70% of putative glutamatergic neurons, and only 30% of GABAergic neurons express the 5-HT1A. Second, within insular projection neurons, 5-HT1A is highly expressed (75-80%) in the populations targeting one sub-nuclei of the amygdala (central or basolateral), or targeting the rostral or caudal sections of the lateral hypothalamus (LH). Similarly, 70% of putative glutamatergic neurons and only 30% of insular GABAergic neurons contain 5-HT2A. Finally, the 5-HT2A is present in a majority of insula-amygdala and insula-LH projection neurons (73-82%). These observations suggest that most glutamatergic neurons can respond to 5-HT through 5-HT1A or 5-HT2A in the insula, and that 5-HT directly affects a limited number of GABAergic neurons. This study defines a molecular and neuroanatomical map of the 5-HT system within the insular cortex, providing ground knowledge to identify the potential role of serotonergic modulation of selective insular populations in anxiety.

Acute and Chronic Nicotine Exposures Differentially Affect Central Serotonin 2A Receptor Function: Focus on the Lateral Habenula

Nicotine addiction is a serious public health problem causing millions of deaths worldwide. Serotonin (5-hydroxytryptamine; 5-HT) is involved in central nervous system (CNS) nicotine effects, and it has been suggested as a promising pharmacological target for smoking cessation. In this regard, what is particularly interesting are the 5-HT_2A receptors (5-HT_2ARs) and the lateral habenula (LHb), a central area in nicotine addiction that we showed to be under a strong 5-HT_2AR-modulation. Single-cell extracellular recording of LHb neurons was used to study the 5-HT_2AR function by intravenously administrating the potent agonist TCB-2. Acute nicotine (2 mg/kg, intraperitoneal, i.p.) and chronic nicotine (6 mg/kg/day for 14 days) differently affected both the 5-HT_2AR-immuno reactive (IR) neuron number and the 5-HT_2AR immunostaining area in the different brain areas studied. After acute nicotine, TCB-2 cumulative doses (5–640 µg/kg, intravenous, i.v.) bidirectionally affected the activity of 74% of LHb recorded neurons. After chronic nicotine treatment, TCB-2 was only capable of decreasing the LHb firing rate. The expression of 5-HT_2AR under acute and chronic nicotine exposure was studied in the LHb and in other brain areas involved in nicotine effects in rats by using immunohistochemistry. These data reveal that acute and chronic nicotine differentially affect the 5-HT_2AR function in different brain areas and this might be relevant in nicotine addiction and its treatment.

Lorcaserin bidirectionally regulates dopaminergic function site-dependently and disrupts dopamine brain area correlations in rats

Lorcaserin, which is a selective agonist of serotonin2C receptors (5-HT_2CRs), is a new FDA-approved anti-obesity drug that has also shown therapeutic promise in other brain disorders, such as addiction and epilepsy. The modulation of dopaminergic function might be critical in the therapeutic effect of lorcaserin, but its exact effect is unknown. Here, we studied the effect of the peripheral administration of lorcaserin on the ventral tegmental area (VTA), the substantia nigra pars compacta (SNc) dopaminergic neural activity, dopamine (DA) dialysis levels in the nucleus accumbens and striatum and on DA tissue levels in 29 different rat brain regions. Lorcaserin (5-640 μg/kg, i.v.) moderately inhibited only a subpopulation of VTA DA neurons, but had no effect on the SNc neurons. Lorcaserin (0.3, 3 mg/kg, i.p.) did not change VTA and SNc DA population neural activity but slightly decreased the firing rate and burst firing of the spontaneously active VTA neurons, without altering DA extracellular dialysate levels in both the nucleus accumbens and the striatum. Quantitative analysis of DA and metabolites tissue contents of the 29 areas studied revealed that lorcaserin (0.3 or 3 mg/kg, i.p.) only affected a few brain regions, i.e., increased DA in the central amygdala, ventral hypothalamus and nucleus accumbens core and decreased it in the ventromedial striatum. On the other hand, lorcaserin dramatically changed the direction and reduced the number of correlations of DA tissue content among several brain areas. These effects on DA terminal networks might be significant in the therapeutic mechanism of lorcaserin. This article is part of the special issue entitled 'Serotonin Research: Crossing Scales and Boundaries'.

Anxiety during alcohol withdrawal involves 5-HT2C receptors and M-channels in the lateral habenula

Anxiety disorders often co-occur with alcohol use disorders, but the mechanisms underlying this comorbidity remain elusive. Previously, we reported that rats withdrawn from chronic alcohol consumption (Post-EtOH rats) exhibited robust anxiety-like behaviors (AB), which were accompanied by neuronal hyperexcitability, and the downregulation of M-type potassium channels (M-channels) in the lateral habenula (LHb); and that serotonin (5-HT) stimulated LHb neurons via type 2C receptors (5-HT_2CRs). Also, 5-HT_2CR activation is known to inhibit M-current in mouse hypothalamic neurons. The present study investigated whether LHb 5-HT_2CRs and M-channels contribute to AB in adult male Long-Evans rats. We used the intermittent-access to 20% ethanol two-bottle free-choice drinking paradigm to induce dependence. We measured AB with the elevated plus-maze, open-field, and marble-burying tests at 24 h withdrawal. We found that intra-LHb infusion of SB242084, a selective 5-HT_2CR antagonist alleviated AB and reduced the elevated c-Fos expression in the LHb of Post-EtOH rats. By contrast, intra-LHb infusion of the selective 5-HT_2CR agonist WAY161503 induced AB and increased c-Fos expression in the LHb in alcohol-naive but not Post-EtOH rats. Also, intra-LHb SB242084 significantly reduced self-administration of alcohol intake in the operant chambers. Furthermore, both 5-HT_2CR protein levels and 5-HIAA/5-HT ratio was increased in the LHb of Post-EtOH rats. Finally, intra-LHb SB242084 increased LHb KCNQ2/3 membrane protein expression in Post-EtOH rats. Collectively, these results suggest that enhanced LHb 5-HT_2CR signaling that interacted with M-channels triggers AB in Post-EtOH rats and that 5-HT_2CRs may be a promising target for treating comorbid anxiety disorders in alcoholics.

Adaptation in 5-HT2 receptors-CaMKII signaling in lateral habenula underlies increased nociceptive-sensitivity in ethanol-withdrawn rats

Alcoholics often experience hyperalgesia, especially during abstinence, yet the underlying cellular and molecular bases are unclear. Recent evidence suggests that 5-HT type 2 receptors (5-HT₂Rs) at glutamatergic synapses on lateral habenula (LHb) neurons may play a critical role. We, therefore, measured paw withdrawal responses to thermal and mechanical stimuli, and alcohol intake in a rat model of intermittent drinking paradigm, as well as spontaneous glutamatergic transmission (sEPSCs), and firing of LHb neurons in brain slices. Here, we report that nociceptive sensitivity was higher in rats at 24 h withdrawal from chronic alcohol consumption than that of alcohol-naive counterparts. The basal frequency of sEPSCs and firings was higher in slices of withdrawn rats than that of Naïve rats, and 5-HT2R antagonists attenuated the enhancement. Also, an acute ethanol-induced increase of sEPSCs and firings was smaller in withdrawal than in Naïve rats; it was attenuated by 5-HT₂R antagonists but mimicked by 5-HT₂R agonists. Importantly, intra-LHb infusion of 5-HT₂R agonists increased nociceptive sensitivity in Naïve rats, while antagonists or 5-HT reuptake blocker decreased nociceptive sensitivity and alcohol intake in withdrawn rats. Additionally, KN-62, a CaMKII inhibitor, attenuated the enhancement of EPSCs and firing induced by acute alcohol and by 5-HT₂R agonist. Furthermore, intra-LHb KN-62 reduced nociceptive sensitivity and alcohol intake. Quantitative real-time PCR assay detected mRNA of 5-HT2A and 2C in the LHb. Thus adaptation in 5-HT₂R-CaMKII signaling pathway contributes to the hyper-glutamatergic state, the hyperactivity of LHb neurons as well as the higher nociceptive sensitivity in rats withdrawn from chronic alcohol consumption.

Preferential modulation of the lateral habenula activity by serotonin-2A rather than -2C receptors: Electrophysiological and neuroanatomical evidence

AIMS

Serotonergic (5-HT) modulation of the lateral habenula (LHb) activity is central in normal and pathologic conditions such as mood disorders. Among the multiple 5-HT receptors (5-HTRs) involved, the 5-HT_2C R seems to play a pivotal role. Yet, the role of 5-HT_2A Rs in the control of the LHb neuronal activity is completely unknown.

METHODS

Single-cell extracellular recording of the LHb neurons was used in rats to study the effect of the general activation and blockade of the 5-HT_2C R and 5-HT_2A R with Ro 60-0175 and SB242084, TCB-2 and MDL11939, respectively. The expression of both receptors in the LHb was confirmed using immunohistochemistry.

RESULTS

Cumulative doses (5-640 μg/kg, iv) of Ro 60-0175 and TCB-2 affected the activity of 34% and 63% of the LHb recorded neurons, respectively. LHb neurons were either inhibited at low doses or excited at higher doses of the 5-HT_2A/C R agonists. SB242084 or MDL11939 (both at 200 μg/kg, iv) did not modify neuronal firing when injected alone, but reverted the bidirectional effects of Ro 60-0175 or TCB-2, respectively. 5-HT_2C Rs and 5-HT_2A Rs are expressed in less than the 20% of the LHb neurons, and they neither colocalize nor make heterodimers. Strikingly, only 5-HT_2A Rs are expressed by the majority of LHb astrocyte cells.

CONCLUSIONS

Peripheral administration of 5-HT_2A R agonist promotes a heterogeneous pattern of neuronal responses in the LHb, and these effects are more prominent than those induced by the 5-HT_2C R activation.