Differential role of GABAA and GABAB receptors in two distinct output stations of the rat striatum: studies on the substantia nigra pars reticulata and the globus pallidus

Optogenetic fMRI reveals therapeutic circuits of subthalamic nucleus deep brain stimulation

While deep brain stimulation (DBS) is widely employed for managing motor symptoms in Parkinson's disease (PD), its exact circuit mechanisms remain controversial. To identify the neural targets affected by therapeutic DBS in PD, we analyzed DBS-evoked whole brain activity in female hemi-parkinsonian rats using functional magnetic resonance imaging (fMRI). We delivered subthalamic nucleus (STN) DBS at various stimulation pulse repetition rates using optogenetics, allowing unbiased examination of cell-type specific STN feedforward neural activity. Unilateral optogenetic STN DBS elicited pulse repetition rate-dependent alterations of blood-oxygenation-level-dependent (BOLD) signals in SNr (substantia nigra pars reticulata), GP (globus pallidus), and CPu (caudate putamen). Notably, this modulation effectively ameliorated pathological circling behavior in animals expressing the kinetically faster Chronos opsin, but not in animals expressing ChR2. Furthermore, mediation analysis revealed that the pulse repetition rate-dependent behavioral rescue was significantly mediated by optogenetic DBS induced activity changes in GP and CPu, but not in SNr. This suggests that the activation of GP and CPu are critically involved in the therapeutic mechanisms of STN DBS.

Optogenetic fMRI reveals therapeutic circuits of subthalamic nucleus deep brain stimulation

While deep brain stimulation (DBS) is widely employed for managing motor symptoms in Parkinson's disease (PD), its exact circuit mechanisms remain controversial. To identify the neural targets affected by therapeutic DBS in PD, we analyzed DBS-evoked whole brain activity in female hemi-parkinsonian rats using function magnetic resonance imaging (fMRI). We delivered subthalamic nucleus (STN) DBS at various stimulation pulse repetition rates using optogenetics, allowing unbiased examinations of cell-type specific STN feed-forward neural activity. Unilateral STN optogenetic stimulation elicited pulse repetition rate-dependent alterations of blood-oxygenation-level-dependent (BOLD) signals in SNr (substantia nigra pars reticulata), GP (globus pallidus), and CPu (caudate putamen). Notably, these manipulations effectively ameliorated pathological circling behavior in animals expressing the kinetically faster Chronos opsin, but not in animals expressing ChR2. Furthermore, mediation analysis revealed that the pulse repetition rate-dependent behavioral rescue was significantly mediated by optogenetically induced activity changes in GP and CPu, but not in SNr. This suggests that the activation of GP and CPu are critically involved in the therapeutic mechanisms of STN DBS.

Pallidal GABA B receptors: involvement in cortex beta dynamics and thalamic reticular nucleus activity

The external globus pallidus (GP) firing rate synchronizes the basal ganglia-thalamus-cortex network controlling GABAergic output to different nuclei. In this context, two findings are significant: the activity and GABAergic transmission of the GP modulated by GABA B receptors and the presence of the GP-thalamic reticular nucleus (RTn) pathway, the functionality of which is unknown. The functional participation of GABA B receptors through this network in cortical dynamics is feasible because the RTn controls transmission between the thalamus and cortex. To analyze this hypothesis, we used single-unit recordings of RTn neurons and electroencephalograms of the motor cortex (MCx) before and after GP injection of the GABA B agonist baclofen and the antagonist saclofen in anesthetized rats. We found that GABA B agonists increase the spiking rate of the RTn and that this response decreases the spectral density of beta frequency bands in the MCx. Additionally, injections of GABA B antagonists decreased the firing activity of the RTn and reversed the effects in the power spectra of beta frequency bands in the MCx. Our results proved that the GP modulates cortical oscillation dynamics through the GP-RTn network via tonic modulation of RTn activity.

Optogenetic Globus Pallidus Stimulation Improves Motor Deficits in 6-Hydroxydopamine-Lesioned Mouse Model of Parkinson's Disease

Excessive inhibition of the external globus pallidus (GPe) by striatal GABAergic neurons is considered a central mechanism contributing to motor symptoms of Parkinson's disease (PD). While electrophysiological findings support this view, behavioral studies assessing the beneficial effects of global GPe activations are scarce and the reported results are controversial. We used an optogenetic approach and the standard unilateral 6-hydroxydopamine nigrostriatal dopamine (DA) lesion model of PD to explore the effects of GPe photostimulation on motor deficits in mice. Global optogenetic GPe inhibition was used in normal mice to verify whether it reproduced the typical motor impairment induced by DA lesions. GPe activation improved ipsilateral circling, contralateral forelimb akinesia, locomotor hypoactivity, and bradykinesia in 6-OHDA-lesioned mice at ineffective photostimulation parameters (532 nm, 5 Hz, 3 mW) in normal mice. GPe photoinhibition (450 nm, 12 mW) had no effect on locomotor activity and forelimb use in normal mice. Bilateral photoinhibition (450 nm, 6 mW/side) reduced directed exploration and improved working memory performances indicating that recruitment of GPe in physiological conditions may depend on the behavioral task involved. Collectively, these findings shed new light on the functional role of GPe and suggest that it is a promising target for neuromodulatory restoration of motor deficits in PD.

Substantia Nigra Abnormalities Provide New Insight on the Neural Mechanisms Underlying the Sleep-Arousal Phase Dysfunctions in Sudden Infant Death Syndrome

The purpose of this study was to research possible developmental alterations of the substantia nigra (SN) in sudden infant death syndrome (SIDS), a syndrome frequently attributed to arousal failure from sleep. Brain stems of 46 victims of sudden infant death, aged from 1 to about 7 months (4 to 30 postnatal weeks), were investigated. Twenty-six of these cases were diagnosed as SIDS, due to the lack of any pathological finding, while the remaining 20 cases in which the cause of death was determined at autopsy served as controls. Maternal smoking was reported in 77% of SIDS and 10% of controls. Histopathological examination of the SN was done on 5-µm-thick sections of caudal midbrain stained with both hematoxylin-eosin and Klüver-Barrera. Densitometry, immunohistochemistry and histochemistry were applied to highlight the neuronal concentration, the tyrosine hydroxylase (TH) expression, and the presence of neuromelanin (NM) in this structure. Hypoplasia of the pars compacta portion of the SN was observed in 69% of SIDS but never in controls; TH expression was significantly higher in controls than in SIDS; and NM was observed only in 4 infants of the control group but not in SIDS. A significant correlation was found between SIDS, hypoplasia/low neuronal density, low TH expression in the pars compacta, and maternal smoking. Because the SN pars compacta, being the major dopamine brain center, controls many functions, including the sleep-arousal phase, its alterations, especially concurrently with smoking exposure, may contribute to explain the pathogenesis of SIDS that occur in the great part of cases at awakening from sleep.

GABAergic function in the lateral hypothalamus regulates feeding behavior: Possible mediation via orexin

Aim

The lateral hypothalamus (LH) is known as the hunger center, but the mechanisms through which the LH regulates food intake are unclear. Since GABA neurons are reported to project to the LH, the present study investigated the role of GABAergic function in the LH in the regulation of feeding behavior.

Methods

GABA levels in the LH were measured by in vivo microdialysis. Food intake after drug injection into the LH was measured every 1 hour for 4 hours. The mRNA levels were measured using RT‐PCR.

Results

Food intake significantly increased GABA levels in the LH, suggesting that food intake stimulates GABAergic function in the LH. Injection of the GABA_A receptor agonist muscimol into the LH significantly inhibited food intake, whereas injection of the GABA_A receptor antagonist bicuculline into the LH did not significantly affect food intake. The inhibitory effect of muscimol injected into the LH was blocked by co‐administration of bicuculline. These results indicate that the stimulation of GABA_A receptors in the LH inhibits food intake. We next examined whether the stimulation of GABA_A receptors affects hypothalamic neuropeptides that are known to regulate feeding behavior. The injection of muscimol significantly decreased preproorexin mRNA in the hypothalamus.

Conclusion

These results indicate that food intake activates GABAergic function in the LH, which terminates feeding behavior by stimulating GABA_A receptors. Moreover, it is suggested that the stimulation of GABA_A receptors in the LH reduces food intake through inhibition of orexin neurons.

We investigated whether GABAergic function in the lateral hypothalamus (LH) regulates feeding behavior. We showed that food intake increased GABA levels in the LH in in vivo microdialysis and that the GABA_A receptor agonist muscimol injected into the LH decreased food intake. These results suggest that food intake activates GABA neurons projecting to the LH, and it terminates feeding behavior through GABA_A receptors.

Nucleus Accumbens Microcircuit Underlying D2-MSN-Driven Increase in Motivation

The nucleus accumbens (NAc) plays a central role in reinforcement and motivation. Around 95% of the NAc neurons are medium spiny neurons (MSNs), divided into those expressing dopamine receptor D1 (D1R) or dopamine receptor D2 (D2R). Optogenetic activation of D2-MSNs increased motivation, whereas inhibition of these neurons produced the opposite effect. Yet, it is still unclear how activation of D2-MSNs affects other local neurons/interneurons or input terminals and how this contributes for motivation enhancement. To answer this question, in this work we combined optogenetic modulation of D2-MSNs with in loco pharmacological delivery of specific neurotransmitter antagonists in rats. First, we showed that optogenetic activation of D2-MSNs increases motivation in a progressive ratio (PR) task. We demonstrated that this behavioral effect relies on cholinergic-dependent modulation of dopaminergic signalling of ventral tegmental area (VTA) terminals, which requires D1R and D2R signalling in the NAc. D2-MSN optogenetic activation decreased ventral pallidum (VP) activity, reducing the inhibitory tone to VTA, leading to increased dopaminergic activity. Importantly, optogenetic activation of D2-MSN terminals in the VP was sufficient to recapitulate the motivation enhancement. In summary, our data suggests that optogenetic stimulation of NAc D2-MSNs indirectly modulates VTA dopaminergic activity, contributing for increased motivation. Moreover, both types of dopamine receptors signalling in the NAc are required in order to produce the positive behavioral effects.

The external globus pallidus: progress and perspectives

The external globus pallidus (GPe) of the basal ganglia is in a unique and powerful position to influence processing of motor information by virtue of its widespread projections to all basal ganglia nuclei. Despite the clinical importance of the GPe in common motor disorders such as Parkinson's disease, there is only limited information about its cellular composition and organizational principles. In this review, recent advances in the understanding of the diversity in the molecular profile, anatomy, physiology and corresponding behaviour during movement of GPe neurons are described. Importantly, this study attempts to build consensus and highlight commonalities of the cellular classification based on existing but contentious literature. Additionally, an analysis of the literature concerning the intricate reciprocal loops formed between the GPe and major synaptic partners, including both the striatum and the subthalamic nucleus, is provided. In conclusion, the GPe has emerged as a crucial node in the basal ganglia macrocircuit. While subtleties in the cellular makeup and synaptic connection of the GPe create new challenges, modern research tools have shown promise in untangling such complexity, and will provide better understanding of the roles of the GPe in encoding movements and their associated pathologies.

Investigating complex basal ganglia circuitry in the regulation of motor behaviour, with particular focus on orofacial movement

Current concepts of basal ganglia function have evolved from the essentially motoric, to include a range of extramotoric functions that involve not only dopaminergic but also cholinergic, γ-aminobutyric acid (GABA)ergic and glutamatergic mechanisms. We consider these mechanisms and their efferent systems, including spiralling, feed-forward striato-nigro-striatal circuitry, involving the dorsal and ventral striatum and the nucleus accumbens (NAc) core and shell. These processes are illustrated using three behavioural models: turning-pivoting, orofacial movements in rats and orofacial movements in genetically modified mice. Turning-pivoting indicates that dopamine-dependent behaviour elicited from the NAc shell is funnelled through the NAc-nigro-striato-nigro-pedunculopontine pathway, whereas acetylcholine-dependent behaviour elicited from the NAc shell is funnelled through the NAc-ventral pallidum-mediodorsal thalamus pathway. Cooperative/synergistic interactions between striatal D1-like and D2-like dopamine receptors regulate individual topographies of orofacial movements that are funnelled through striatal projection pathways and involve interactions with GABAergic and glutamatergic receptor subtypes. This application of concerted behavioural, neurochemical and neurophysiological techniques implicates a network that is yet broader and interacts with other neurotransmitters and neuropeptides within subcortical, cortical and brainstem regions to 'sculpt' aspects of behaviour into its topographical collective.

Motor tics evoked by striatal disinhibition in the rat

Motor tics are sudden, brief, repetitive movements that constitute the main symptom of Tourette syndrome (TS). Multiple lines of evidence suggest the involvement of the cortico-basal ganglia system, and in particular the basal ganglia input structure-the striatum in tic formation. The striatum receives somatotopically organized cortical projections and contains an internal GABAergic network of interneurons and projection neurons' collaterals. Disruption of local striatal GABAergic connectivity has been associated with TS and was found to induce abnormal movements in model animals. We have previously described the behavioral and neurophysiological characteristics of motor tics induced in monkeys by local striatal microinjections of the GABAA antagonist bicuculline. In the current study we explored the abnormal movements induced by a similar manipulation in freely moving rats. We targeted microinjections to different parts of the dorsal striatum, and examined the effects of this manipulation on the induced tic properties, such as latency, duration, and somatic localization. Tics induced by striatal disinhibition in monkeys and rats shared multiple properties: tics began within several minutes after microinjection, were expressed solely in the contralateral side, and waxed and waned around a mean inter-tic interval of 1-4 s. A clear somatotopic organization was observed only in rats, where injections to the anterior or posterior striatum led to tics in the forelimb or hindlimb areas, respectively. These results suggest that striatal disinhibition in the rat may be used to model motor tics such as observed in TS. Establishing this reliable and accessible animal model could facilitate the study of the neural mechanisms underlying motor tics, and the testing of potential therapies for tic disorders.

Spiraling dopaminergic circuitry from the ventral striatum to dorsal striatum is an effective feed-forward loop

Central dopamine systems are key players in the cerebral organization of behavior and in various neurological and psychiatric diseases. We demonstrate the presence of a neurochemical feed-forward loop characterized by region-specific changes in dopamine efflux in serially connected striatal regions, providing evidence in favor of the existence of so-called spiraling striato-nigro-striatal connections. Using in vivo microdialysis of rats, we show that simultaneous stimulation of dopamine D1 and D2 receptors in the accumbal shell decreased dorsal striatal dopamine efflux via a direct or indirect feed-forward loop involving shell, core, ventrolateral and dorsal part of the striatum: simultaneous stimulation of dopamine D1 and D2 receptors in the shell decreased dopamine efflux in the core; flupenthixol-induced inhibition of dopamine D1 and D2 receptors in the core increased dopamine efflux in the ventrolateral part of the striatum, and simultaneous stimulation of dopamine D1 and D2 receptors in the ventrolateral part of the striatum decreased dopamine efflux in the dorsal part of the striatum. Finally, simultaneous stimulation of dopamine D1 and D2 receptors in the shell decreased dopamine efflux in the dorsal part of the striatum. Thus, distinct striatal regions act also in series, providing a better understanding of the neural mechanisms underlying dopamine-dependent behaviors and the progression of dopamine-dependent disorders such as depression, schizophrenia, attention deficit hyperactivity disorder (ADHD), and addiction.

Lateral hypothalamic area mediated the aggravated effect of microinjection of Baclofen into cerebellar fastigial nucleus on stress gastric mucosal damage in rats

Cerebellum, primarily believed as a subcortical somatic motor center, is increasingly considered to be implicated in visceral activities. However, little is known about its regulation on gastrointestinal organs. In this research, we investigated the aggravated effect of microinjection of gamma-aminobutyric acid receptor subtype B (GABA(B)R) agonist, Baclofen into cerebellar fastigial nucleus (FN) on stress gastric mucosal damage (SGMD) and its possible regulatory mechanism. The gastric mucosal damage index was chosen to indicate the severity of gastric mucosal injure. Immunohistochemistry and transferase-mediated dUTP-biotin nick-endlabeling (TUNEL) methods were used to detect the variations of lateral hypothalamic area (LHA) and gastric mucosa. It had been demonstrated that FN participates in regulation of SGMD via its GABA(B)R and GABA neural pathway, which passes through the decussation of superior cerebellar peduncle and projects to the GABA receptors in LHA. Meanwhile, celiac sympathetic nerve involves in this process via mediating neural discharge, which results in the decrease of gastric mucosal blood flow. Additionally, apoptosis, proliferation and oxidation in gastric mucosa, and gastric acid contribute in the mechanism. It could be expected that these results might suggest insights to the cerebellar and hypothalamic function, and the treatment of gastrointestinal diseases.

Prolonged effects of intracerebroventricular angiotensin II on drinking, eating and locomotor behavior in mice

The effects of centrally administered Angiotensin II (Ang II) on water and food intake in rodent models are well known. However, most studies have focused on the acute effects of intracranial Ang II. In the current study, we evaluated the effects of intracerebroventricular Ang II on food and water intake as well as locomotor activity over the entire dark phase of the murine diurnal cycle. Consistent with the previous reports, centrally administered Ang II rapidly stimulated water intake over the initial 1-hour period following treatment. However, this acute increase was immediately followed by a marked reduction in water intake resulting in decreased cumulative water intake approximately 7h after Ang II treatment. Pretreating animals with an Ang II type 1 receptor blocker, Losartan, completely antagonized the acute effect of Ang II and abolished initial water intake. In contrast, application of an Ang II type 2 receptor blocker, PD123319, abrogated the prolonged inhibitory effect of Ang II on drinking behavior and partially suppressed the initial increases in water intake. The suppressive effects of Ang II on cumulative food intake and spontaneous physical activity were also evident throughout the entire dark phase of diurnal cycle. These experiments are the first to suggest that the stimulatory effect of central Ang II treatment on water consumption is very temporary and that it causes a sustained suppressive effect on voluntary locomotion and food intake behavior in mice.