Blockade of GABA transporter-1 and GABA transporter-3 in the lateral habenula improves depressive-like behaviors in a rat model of Parkinson's disease

Effects of astrocytes in the dorsal hippocampus on anxiety-like and depressive-like behaviors in hemiparkinsonian rats

Anxiety and depression are the most common neuropsychiatric manifestations of Parkinson's disease (PD) patients. Growing evidence have shown that the dorsal hippocampus (dHIPP) and astrocytes (AS) may be involved in regulating depression and anxiety, but the role and mechanism are still unclear, especially in PD-related depression and anxiety. Unilateral 6-hydroxydopamine lesions of the substantia nigra pars compacta (SNc) were used to establish the rat model of PD. Behavioral tests and measurement of monoamine levels in the depression and anxiety related brain regions were performed to investigate the effects of chemogenetic activation or inhibition of dHIPP AS on PD-related anxiety and depression. The present results showed that unilateral lesions of the SNc induced anxiety-like and depressive-like behaviors, decreased dopamine (DA) levels in some related brain regions, but did not change the density of glial fibrillary acidic protein-positive AS in the CA1, CA3 and dentate gyrus in rats. Chemogenetic inhibition of dHIPP AS significantly improved anxiety-like and depressive-like behaviors only in the lesioned rats, while chemogenetic activation of dHIPP AS had no effects on anxiety-like and depressive-like behaviors in sham-operated and the lesioned rats. Chemogenetic activation of dHIPP AS only decreased DA level in the ventral hippocampus (vHIPP) in sham-operated rats, while inhibition of dHIPP AS increased 5-hydroxytryptamine (5-HT) levels in the medial prefrontal cortex (mPFC) and vHIPP in sham-operated rats and also in the amygdala, mPFC, lateral habenula, dHIPP and vHIPP in the lesioned rats. These results indicate that chemogenetic inhibition of dHIPP AS improves the anxiety-like and depressive-like behaviors in the lesioned rats through the changes in monoamine in some brain regions.

Involvement of a central amygdaloid nucleus-lateral habenular nucleus pathway in the processing of formalin-induced pain

The central amygdaloid nucleus (CeA) and the lateral habenular nucleus (LHb) are essential nuclei playing modulatory roles in encoding noxious stimuli. Their interaction has recently been demonstrated in chronic pain-induced depression. However, little is known about the CeA-LHb pathway in a formalin-induced pain model. In the present study, we aimed to clarify whether the CeA-LHb pathway modulates the formalin-induced pain model using a neuroanatomical tracing method combined with a designer receptor exclusively activated by a designer drugs strategy (DREADD). The results revealed that the CeA predominantly sends projections to vesicular glutamate transporter-2 (VGluT2)-expressing neurons of the LHb, and inhibition of LHb function exhibits an analgesic effect in the formalin-induced pain model. Furthermore, activating the CeA-LHb pathway significantly attenuates pain sensation only in phase 2 of formalin-induced pain in mice. The present results indicate the participation of the LHb in inflammatory pain sensation and reveal a CeA-LHbVGluT2 pathway that displays analgesic effects in a formalin pain model.

GABA transporter 1 is a promising drug target for CUL4B mutation-associated epilepsy

Cullin 4B (CUL4B) is the scaffold protein in the CUL4B-RING E3 ubiquitin ligase (CRL4B) complex. Loss-of-function mutations in the human CUL4B gene result in syndromic X-linked intellectual disability (XLID). In addition to intellectual disability, patients with CUL4B mutations exhibit epilepsy. To date, the mechanism underlying epilepsy associated with CUL4B mutation has not been elucidated. Here, we show that male mice with Cul4b deleted in the nervous system are more susceptible to both pentylenetetrazole (PTZ)- and kainic acid (KA)-induced epilepsy and exhibit spontaneous epilepsy without any chemical inducers. We identify the CRL4B complex as an E3 ubiquitin ligase that targets GABA transporter 1 (GAT1). CUL4B deletion in male mice results in GAT1 accumulation and increased GABA reuptake, leading to impaired GABA-mediated inhibitory synaptic transmission. Treating CUL4B-deficient mice with the GAT1 inhibitor tiagabine effectively reverses the increased susceptibility to chemical-induced epilepsy and attenuates spontaneous epilepsy without the use of chemical inducers. We further confirm the role of CUL4B in the regulation of GAT1 levels and GABA uptake in neurons and astrocytes differentiated from induced pluripotent stem cells (iPSCs) derived from patients with CUL4B loss-of-function mutations. Our work reveals a novel mechanism underlying the pathogenesis of epilepsy and identifies a promising drug target for treating CUL4B mutation-associated epilepsy.

11-Plex DiLeu Isobaric Labeling Enables Quantitative Assessment of Brain Region Protein Association Networks Impacted by the Gut Microbiome

Gut microbiota can regulate host brain functions and influence various physiological and pathological processes through the brain-gut axis. To systematically elucidate the intervention of different gut environments on different brain regions, we implemented an integrated approach that combines 11-plex DiLeu isobaric tags with a "BRIDGE" normalization strategy to comparatively analyze the proteome of six brain regions in germ-free (GF)- and conventionally raised (ConvR)-mice. A total of 5945 proteins were identified and 5656 were quantifiable, while 1906 of them were significantly changed between GF- and ConvR-mice; 281 proteins were filtered with FC greater than 1.2 in at least one brain region, of which heatmap analysis showed clear protein profile disparities, both between brain regions and gut microbiome conditions. Gut microbiome impact is most overt in the hypothalamus and the least in the thalamus region. Collectively, this approach allows an in-depth investigation of the induced protein changes by multiple gut microbiome environments in a brain region-specific manner. This comprehensive proteomic work improves the understanding of the brain region protein association networks impacted by the gut microbiome and highlights the critical roles of the brain-gut axis.

Astrocytic GABAergic Regulation in Alcohol Use and Major Depressive Disorders

Gamma-aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the central nervous system (CNS). Most GABAergic neurons synthesize GABA from glutamate and release it in the synaptic cleft in the CNS. However, astrocytes can also synthesize and release GABA, activating GABA receptors in the neighboring neurons in physiological and pathological conditions. As the primary homeostatic glial cells in the brain, astrocytes play a crucial role in regulating GABA homeostasis and synaptic neurotransmission. Accumulating evidence demonstrates that astrocytic GABA dysregulation is implicated in psychiatric disorders, including alcohol use disorder (AUD) and major depressive disorder (MDD), the most prevalent co-occurring psychiatric disorders. Several current medications and emerging pharmacological agents targeting GABA levels are in clinical trials for treating AUD and MDD. This review offers a concise summary of the role of astrocytic GABA regulation in AUD and MDD. We also provide an overview of the current understanding and areas of debate regarding the mechanisms by which astrocytes regulate GABA in the CNS and their potential significance in the molecular basis of AUD and MDD, paving the way toward future research directions and potential therapeutic target areas within this field.

Molecular basis for substrate recognition and transport of human GABA transporter GAT1

γ-Aminobutyric acid (GABA), an important inhibitory neurotransmitter in the central nervous system, is recycled through specific GABA transporters (GATs). GAT1, which is mainly expressed in the presynaptic terminals of axons, is a potential drug target of neurological disorders due to its essential role in GABA transport. Here we report four cryogenic electron microscopy structures of human GAT1, at resolutions of 2.2-3.2 Å. GAT1 in substrate-free form or in complex with the antiepileptic drug tiagabine exhibits an inward-open conformation. In the presence of GABA or nipecotic acid, inward-occluded structures are captured. The GABA-bound structure reveals an interaction network bridged by hydrogen bonds and ion coordination for GABA recognition. The substrate-free structure unwinds the last helical turn of transmembrane helix TM1a to release sodium ions and substrate. Complemented by structure-guided biochemical analyses, our studies reveal detailed mechanism of GABA recognition and transport, and elucidate mode of action of the inhibitors, nipecotic acid and tiagabine.

5-HT1B receptor-AC-PKA signal pathway in the lateral habenula is involved in the regulation of depressive-like behaviors in 6-hydroxydopamine-induced Parkinson's rats

OBJECTIVE

The aim of the present study was to investigate whether serotonin_1B (5-HT_1B) receptor-adenylate cyclase (AC)-protein kinase A (PKA) signal pathway in the lateral habenula (LHb) is involved in Parkinson's disease-related depression in sham-lesioned and substantia nigra pars compacta (SNc)-lesioned rats.

METHODS

The sucrose preference and forced swim tests were used to measure depressive-like behaviors. In vivo electrophysiology and microdialysis were performed to observe the firing activity of LHb neurons and GABA and glutamate release in the LHb, respectively. Western blotting was used to analyze protein expression of 5-HT_1B receptors, AC and phosphorylated PKA at threonine 197 site (p-PKA-Thr197) in the LHb.

RESULTS

Unilateral 6-hydroxydopamine lesions of the SNc in rats induced depressive-like behaviors. Intra-LHb injection of 5-HT_1B receptor agonist CP93129 produced antidepressant-like effects and the antagonist SB216641 induced depressive-like behaviors in sham-lesioned and SNc-lesioned rats. Further, pretreatment with AC inhibitor SQ22536 and PKA inhibitor KT5720 blocked the behavioral effects of CP93129 in the two groups of rats, respectively. CP93129 decreased the firing rate of LHb neurons and release of GABA and glutamate, but increased the GABA/glutamate ratio, while SB216641 induced the opposite effects. Compared with sham-lesioned rats, effects of CP93129 and SB216641 on the depressive-like behaviors, electrophysiology, and microdialysis were decreased in SNc-lesioned rats, which were associated with decreased expression of 5-HT_1B receptors, AC and p-PKA-Thr197 in the LHb.

CONCLUSION

5-HT_1B receptor-AC-PKA signal pathway in the LHb is involved in the regulation of depressive-like behaviors, and depletion of DA reduces activity of 5-HT_1B receptor-AC-PKA signal pathway.

Structural basis of GABA reuptake inhibition

γ-Aminobutyric acid (GABA) transporter 1 (GAT1)¹ regulates neuronal excitation of the central nervous system by clearing the synaptic cleft of the inhibitory neurotransmitter GABA upon its release from synaptic vesicles. Elevating the levels of GABA in the synaptic cleft, by inhibiting GABA reuptake transporters, is an established strategy to treat neurological disorders, such as epilepsy². Here we determined the cryo-electron microscopy structure of full-length, wild-type human GAT1 in complex with its clinically used inhibitor tiagabine³, with an ordered part of only 60 kDa. Our structure reveals that tiagabine locks GAT1 in the inward-open conformation, by blocking the intracellular gate of the GABA release pathway, and thus suppresses neurotransmitter uptake. Our results provide insights into the mixed-type inhibition of GAT1 by tiagabine, which is an important anticonvulsant medication. Its pharmacodynamic profile, confirmed by our experimental data, suggests initial binding of tiagabine to the substrate-binding site in the outward-open conformation, whereas our structure presents the drug stalling the transporter in the inward-open conformation, consistent with a two-step mechanism of inhibition⁴. The presented structure of GAT1 gives crucial insights into the biology and pharmacology of this important neurotransmitter transporter and provides blueprints for the rational design of neuromodulators, as well as moving the boundaries of what is considered possible in single-particle cryo-electron microscopy of challenging membrane proteins.

Restoration and targeting of aberrant neurotransmitters in Parkinson's disease therapeutics

Neurotransmitters are considered as a fundamental regulator in the process of neuronal growth, differentiation and survival. Parkinson's Disease (PD) occurs due to extensive damage of dopamine-producing neurons; this causes dopamine deficits in the midbrain, followed by the alternation of various other neurotransmitters (glutamate, GABA, serotonin, etc.). It has been observed that fluctuation of neurotransmission in the basal ganglia exhibits a great impact on the pathophysiology of PD. Dopamine replacement therapy, such as the use of L-DOPA, can increase the dopamine level, but it majorly ameliorates the motor symptoms and is also associated with long-term complications (for e.g., LID). While the non-dopaminergic system can efficiently target non-motor symptoms, for instance, the noradrenergic system regulates the synthesis of BDNF via the MAPK pathway, which is important in learning and memory. Herein, we briefly discuss the role of different neurotransmitters, implementation of neurotransmitter receptors in PD. We also illustrate the recent advances of neurotransmitter-based drugs, which are currently under in vivo and clinical studies. Reinstating normal neurotransmitter levels has been believed to be advantageous in the treatment of PD. Thus, there is an increasing demand for drugs that can specifically target the neurotransmission system and reinstate the normal levels of neurotransmitters, which might prevent or delay neurodegeneration in PD.

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.

Blockade of pre-synaptic and post-synaptic GABAB receptors in the lateral habenula produces different effects on anxiety-like behaviors in 6-hydroxydopamine hemiparkinsonian rats

Although the output of the lateral habenula (LHb) controls the activity of midbrain dopaminergic and serotonergic systems, which are implicated in the pathophysiology of anxiety, it is not known how blockade of GABA_B receptors in the region affects anxiety-like behaviors, particularly in Parkinson's disease-related anxiety. In this study, unilateral 6-hydroxydopamine lesions of the substantia nigra pars compacta in rats induced anxiety-like behaviors, led to hyperactivity of LHb neurons and decreased the level of extracellular dopamine (DA) in the basolateral amygdala (BLA) compared to sham-lesioned rats. Intra-LHb injection of pre-synaptic GABA_B receptor antagonist CGP36216 produced anxiolytic-like effects, while the injection of post-synaptic GABA_B receptor antagonist CGP35348 induced anxiety-like responses in both groups. Further, intra-LHb injection of CGP36216 decreased the firing rate of the neurons, and increased the GABA/glutamate ratio in the LHb and release of DA and serotonin (5-HT) in the BLA; conversely, CGP35348 increased the firing rate of the neurons and decreased the GABA/glutamate ratio and release of DA and 5-HT in sham-lesioned and the lesioned rats. However, the doses of the antagonists producing these behavioral effects in the lesioned rats were lower than those in sham-lesioned rats, and the duration of action of the antagonists on the firing rate of the neurons and release of the neurotransmitters was prolonged in the lesioned rats. Collectively, these findings suggest that pre-synaptic and post-synaptic GABA_B receptors in the LHb are involved in the regulation of anxiety-like behaviors, and degeneration of the nigrostriatal pathway up-regulates function and/or expression of these receptors.

Downregulation of astroglial glutamate transporter GLT-1 in the lateral habenula is associated with depressive-like behaviors in a rat model of Parkinson's disease

Recent studies show that neuron-glial communication plays an important role in neurological diseases. Particularly, dysfunction of astroglial glutamate transporter GLT-1 has been involved in various neuropsychiatric disorders, including Parkinson's disease (PD) and depression. Our previous studies indicated hyperactivity of neurons in the lateral habenula (LHb) of hemiparkinsonian rats with depressive-like behaviors. Thus, we hypothesized that impaired expression or function of GLT-1 in the LHb might be a potential contributor to LHb hyperactivity, which consequently induces PD-related depression. In the study, unilateral lesions of the substantia nigra pars compacta (SNc) by 6-hydroxydopamine in rats induced depressive-like behaviors and resulted in neuronal hyperactivity as well as increased glutamate levels in the LHb compared to sham-lesioned rats. Intra-LHb injection of GLT-1 inhibitor WAY-213613 induced the depressive-like behaviors in both groups, but the dose producing behavioral effects in the lesioned rats was lower than that of sham-lesioned rats. In the two groups of rats, WAY-213613 increased the firing rate of LHb neurons and extracellular levels of glutamate, and these excitatory effects in the lesioned rats lasted longer than those in sham-lesioned rats. The functional changes of the GLT-1 which primarily expresses in astrocytes in the LHb may attribute to its downregulation after degeneration of the nigrostriatal pathway. Bioinformatics analysis showed that GLT-1 is correlated with various biomarkers of PD and depression risks. Collectively, our study suggests that astroglial GLT-1 in the LHb regulates the firing activity of the neurons, whereupon its downregulation and dysfunction are closely associated with PD-related depression.