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-HT2CR seems to play a pivotal role. Yet, the role of 5-HT2ARs 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-HT2CR and 5-HT2AR 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 \uce\ubcg/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-HT2A/CR agonists. SB242084 or MDL11939 (both at 200 \uce\ubcg/kg, iv) did not modify neuronal firing when injected alone, but reverted the bidirectional effects of Ro 60-0175 or TCB-2, respectively. 5-HT2CRs and 5-HT2ARs are expressed in less than the 20% of the LHb neurons, and they neither colocalize nor make heterodimers. Strikingly, only 5-HT2ARs are expressed by the majority of LHb astrocyte cells. Conclusions: Peripheral administration of 5-HT2AR agonist promotes a heterogeneous pattern of neuronal responses in the LHb, and these effects are more prominent than those induced by the 5-HT2CR activation

Targeting the Serotonin (5-HT) system to control seizures

Compelling animal and human evidence suggests that serotonin plays an important role in the pathophysiology of epilepsy as it is involved in iperexcitability, epileptogenesis, seizure generation, depression and psychiatric disorders comorbid with epilepsy. Serotonin involvement in epilepsy is complex; the reasons are twofold i) epilepsy is in reality a spectrum disorder, and ii) serotonin effects vary from one form of epilepsy to another, due also to the different serotonin receptors involved. Here, we will focus on the role of serotonin and its 5-HT2 receptors in absence epilepsy. Our recent pharmacological experimental evidence in GAERS will be reviewed together with our preliminary optogenetic results. 5-HT2C receptor agonists may represent a new approach to interfere with seizure generation and seizure management. Our optogenetic experiments also indicate that by modulating rhythmic cortical activity, optogenetic stimulation of the serotonergic system may provide seizure control without the adverse effects induced by pharmacological activation of 5-HT2C receptors. Thus, targeting the serotonergic system could provide novel insights into the pathophysiological mechanisms of seizure generation and lead to potentially novel treatments.peer-reviewe

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-HT2CR has emerged as a promising drug target for smoking cessation. The 5-HT2CRs 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-HT2CR 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-HT2CR-immunoreactive (IR) area while decreasing the number of 5-HT2CR-IR neurons in the LHb. On the other hand, chronic nicotine increased both the 5-HT2CR-IR area and 5-HT2CR-IR LHb neurons in the LHb. Western blot analysis confirmed these findings and further revealed an increase of 5-HT2CR 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-HT2CR function mainly in the LHb and this may be relevant in nicotine addiction and its treatment.peer-reviewe

An activator of voltage-gated K+ channels Kv1.1 as a therapeutic candidate for episodic ataxia type 1

Loss-of-function mutations in the KCNA1(Kv1.1) gene cause episodic ataxia type 1 (EA1), a neurological disease characterized by cerebellar dysfunction, ataxic attacks, persistent myokymia with painful cramps in skeletal muscles, and epilepsy. Precision medicine for EA1 treatment is currently unfeasible, as no drug that can enhance the activity of Kv1.1-containing channels and offset the functional defects caused by KCNA1 mutations has been clinically approved. Here, we uncovered that niflumic acid (NFA), a currently prescribed analgesic and anti-inflammatory drug with an excellent safety profile in the clinic, potentiates the activity of Kv1.1 channels. NFA increased Kv1.1 current amplitudes by enhancing the channel open probability, causing a hyperpolarizing shift in the voltage dependence of both channel opening and gating charge movement, slowing the OFF-gating current decay. NFA exerted similar actions on both homomeric Kv1.2 and heteromeric Kv1.1/Kv1.2 channels, which are formed in most brain structures. We show that through its potentiating action, NFA mitigated the EA1 mutation-induced functional defects in Kv1.1 and restored cerebellar synaptic transmission, Purkinje cell availability, and precision of firing. In addition, NFA ameliorated the motor performance of a knock-in mouse model of EA1 and restored the neuromuscular transmission and climbing ability in Shaker (Kv1.1) mutant Drosophila melanogaster flies (Sh5). By virtue of its multiple actions, NFA has strong potential as an efficacious single-molecule-based therapeutic agent for EA1 and serves as a valuable model for drug discovery

Systemic administration of ivabradine, a hyperpolarization‐activated cyclic nucleotide‐gated channel inhibitor, blocks spontaneous absence seizures

Objective: Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are known to be involved in the generation of absence seizures (ASs), and there is evidence that cortical and thalamic HCN channel dysfunctions may have a proabsence role. Many HCN channel blockers are available, but their role in ASs has been investigated only by localized brain injection or in in vitro model systems due to their limited brain availability. Here, we investigated the effect on ASs of orally administered ivabradine (an HCN channel blocker approved for the treatment of heart failure in humans) following injection of the P-glycoprotein inhibitor elacridar, which is known to increase penetration into the brain of drug substrates for this efflux transporter. The action of ivabradine was also tested following in vivo microinjection into the cortical initiation network (CIN) of the somatosensory cortex and in the thalamic ventrobasal nucleus (VB) as well as on cortical and thalamocortical neurons in brain slices. Methods: We used electroencephalographic recordings in freely moving Genetic Absence Epilepsy Rats From Strasbourg (GAERSs) to assess the action of oral administration of ivabradine, with and without elacridar, on ASs. Ivabradine was also microinjected into the CIN and VB of GAERSs in vivo and applied to Wistar CIN and GAERS VB slices while recording patch-clamped cortical Layer 5/6 and thalamocortical neurons, respectively. Results: Oral administration of ivabradine markedly and dose-dependently reduced ASs. Ivabradine injection into CIN abolished ASs and elicited small-amplitude 4–7-Hz waves (without spikes), whereas in the VB it was less potent. Moreover, ivabradine applied to GAERS VB and Wistar CIN slices selectively decreased HCN channel-dependent properties of cortical Layer 5/6 pyramidal and thalamocortical neurons, respectively. Significance: These results provide the first demonstration of the antiabsence action of a systemically administered HCN channel blocker, indicating the potential of this class of drugs as a novel therapeutic avenue for ASs

In vivo astrocyte calcium activity in the somatosensory cortex during locomotion and non-convulsive seizures

Astrocytes make up a significant portion of the total cells in the brain. Intracellular Ca2+ changes in astrocytes have been attributed to various physiological processes that directly influence neuronal activity and brain network synchrony. Moreover, altered Ca2+ patterns are indicative of astrocyte dysfunction and of diseased states. Astrocytes are active following sensory stimulation, such as visual cues and tactile whisker stimuli. Astrocytes participate in the cortical response to sensory input and synchronously respond to locomotion with an intracellular Ca2+ increase, predominantly modulated by widespread neuromodulator release and direct activation of astrocyte α1-adrenergic receptors. However, an in-depth study focussed on astrocyte activity in the primary somatosensory cortex (S1) during locomotion is lacking. To address this gap in the field, we recorded astrocyte Ca2+ changes in head-restrained, awake mice that were free to run on a wheel. Animals were chronically implanted with ECoG electrodes and a cranial window above S1. Ca2+ changes were visualised using a virally delivered genetically encoded Ca2+ indicator, GCaMP6f, driven by a GFAP promoter for selective expression in astrocytes. Semi-automated Ca2+ analysis protocols were used to measure Ca2+ fluorescence within the different sub-cellular compartments. We show that layer 2/3 astrocytes rapidly respond to locomotion onset, within a sub-second timescale. This was most evident in the cell periphery that activated most consistently to locomotion, even when soma activation was weak or absent. The Stargazer mouse, a model of ataxia, maintained a strong astrocyte response to locomotion, which was enhanced in view of its quicker astrocyte response time, especially in the cell periphery. Astrocytes regulate neuronal network excitability and modulate cortical state switching and network synchrony. Astrocytes are capable of modifying neuronal activity through gliotransmitter release and have been consequently shown to elicit epileptic discharges in slice models, as well as precede epileptic events in in vivo epilepsy models. During absence seizures (AS), an increased rhythmicity and synchrony among thalamic and cortical neurons is evidenced by generalised spike and wave discharges (SWD) throughout this network. fMRI studies have revealed potential astrocyte recruitment preceding seizure onset and astrocyte GABA dysfunction is known to contribute to SWD occurrence. However, astrocyte calcium activity during AS is not known. We imaged astrocyte Ca2+ changes to determine if astrocytes were indeed active prior to seizure onset and potentially drove the neuronal paroxysmal activity of AS. We showed for the first time, astrocyte Ca2+ activity in two genetic animal models of AS. Stargazer mice exhibited enhanced microdomain and endfoot activation during periods of prolonged ictal ECoG activity, but no clear widespread astrocyte activation preceded seizure onset. Astrocytes within the cortical initiation site of AS were imaged in the Genetic Absence Epilepsy Rat from Strasbourg, using the Inscopix miniature head-mounted microscope. Astrocytes within the cortical initiation site engaged in a synchronous Ca2+ pattern during a state of prolonged seizure activity. Moreover, astrocyte-specific chemogenetic activation or IP3 receptor type 2 knockout, did not influence seizure occurrence. In conclusion, we show a diverse and complex astrocyte response to locomotion which is driven by microdomain activation with potential summation of inputs that leads to widespread soma and endfoot activation. Furthermore, evidence we obtained from two well-established models of AS further strengthens past findings of dysfunctional astrocytes within the cortico-thalamic network involved in AS initiation, and we reveal novel astrocyte Ca2+ patterns associated with spontaneous generalised seizures

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 (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

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-HT2A receptors (5-HT2ARs) and the lateral habenula (LHb), a central area in nicotine addiction that we showed to be under a strong 5-HT2AR-modulation. Single-cell extracellular recording of LHb neurons was used to study the 5-HT2AR 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-HT2AR-immuno reactive (IR) neuron number and the 5-HT2AR 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-HT2AR 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-HT2AR function in different brain areas and this might be relevant in nicotine addiction and its treatment.peer-reviewe

Locus Coeruleus Neurons’ Firing Pattern Is Regulated by ERG Voltage-Gated K⁺ Channels

Locus coeruleus (LC) neurons, with their extensive innervations throughout the brain, control a broad range of physiological processes. Several ion channels have been characterized in LC neurons that control intrinsic membrane properties and excitability. However, ERG (ether-à-go-go–related gene) K+ channels that are particularly important in setting neuronal firing rhythms and automaticity have not as yet been discovered in the LC. Moreover, the neurophysiological and pathophysiological roles of ERG channels in the brain remain unclear despite their expression in several structures. By performing immunohistochemical investigations, we found that ERG-1A, ERG-1B, ERG-2 and ERG-3 are highly expressed in the LC neurons of mice. To examine the functional role of ERG channels, current-clamp recordings were performed on mouse LC neurons in brain slices under visual control. ERG channel blockade by WAY-123,398, a class III anti-arrhythmic agent, increased the spontaneous firing activity and discharge irregularity of LC neurons. Here, we have shown the presence of distinct ERG channel subunits in the LC which play an imperative role in modulating neuronal discharge patterns. Thus, we propose that ERG channels are important players behind the changes in, and/or maintenance of, LC firing patterns that are implicated in the generation of different behaviors and in several disorders

Cortical drive and thalamic feed-forward inhibition control thalamic output synchrony during absence seizures

International audienceBehaviorally and pathologically relevant cortico-thalamo-cortical oscillations are driven by diverse interacting cell-intrinsic and synaptic processes. However, the mechanism that gives rise to the paroxysmal oscillations of absence seizures (ASs) remains unknown. Here we report that during ASs in behaving animals, cortico-thalamic excitation drives thalamic firing by preferentially eliciting tonic rather than T-type Ca 2+ channels (T-channels)-dependent burst firing in thalamocortical (TC) neurons, and by temporally framing thalamic output via feed-forward reticular thalamic (NRT)-to-TC neuron inhibition. In TC neurons, overall ictal firing is markedly reduced and bursts rarely occur. Moreover, block of T-channels in cortical and NRT neurons suppresses ASs, but in TC neurons has no effect on seizures or on ictal thalamic output synchrony. These results demonstrate ictal bidirectional cortico-thalamic communications and provide th