Effects of Pharmacological Block of GABA(A) Receptors on Pallidal Neurons in Normal and Parkinsonian State

Cortical and thalamic connections of the human globus pallidus: Implications for disorders of consciousness

A dominant framework for understanding loss and recovery of consciousness in the context of severe brain injury, the mesocircuit hypothesis, focuses on the role of cortico-subcortical recurrent interactions, with a strong emphasis on excitatory thalamofugal projections. According to this view, excess inhibition from the internal globus pallidus (GPi) on central thalamic nuclei is key to understanding prolonged disorders of consciousness (DOC) and their characteristic, brain-wide metabolic depression. Recent work in healthy volunteers and patients, however, suggests a previously unappreciated role for the external globus pallidus (GPe) in maintaining a state of consciousness. This view is consistent with empirical findings demonstrating the existence of “direct” (i.e., not mediated by GPi/substantia nigra pars reticulata) GPe connections with cortex and thalamus in animal models, as well as their involvement in modulating arousal and sleep, and with theoretical work underscoring the role of GABA dysfunction in prolonged DOC. Leveraging 50 healthy subjects' high angular resolution diffusion imaging (HARDI) dataset from the Human Connectome Project, which provides a more accurate representation of intravoxel water diffusion than conventional diffusion tensor imaging approaches, we ran probabilistic tractography using extensive a priori exclusion criteria to limit the influence of indirect connections in order to better characterize “direct” pallidal connections. We report the first in vivo evidence of highly probable “direct” GPe connections with prefrontal cortex (PFC) and central thalamic nuclei. Conversely, we find direct connections between the GPi and PFC to be sparse (i.e., less likely indicative of true “direct” connectivity) and restricted to the posterior border of PFC, thus reflecting an extension from the cortical motor zones (i.e., motor association areas). Consistent with GPi's preferential connections with sensorimotor cortices, the GPi appears to predominantly connect with the sensorimotor subregions of the thalamus. These findings are validated against existing animal tracer studies. These findings suggest that contemporary mechanistic models of loss and recovery of consciousness following brain injury must be updated to include the GPe and reflect the actual patterns of GPe and GPi connectivity within large-scale cortico-thalamo-cortical circuits.

Orexins alleviate motor deficits via increasing firing activity of pallidal neurons in a mouse model of Parkinson's disease

Orexin is a peptide neurotransmitter released in the globus pallidus. Morphological evidence reveals that both orexin 1 receptor (OX₁R) and orexin 2 receptor (OX₂R) exist in the globus pallidus. Here we showed that bilateral microinjection of both orexin-A and orexin-B into the globus pallidus alleviated motor deficits in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced parkinsonian mice. Further in vivo extracellular single-unit recording revealed that the basal spontaneous firing rate of the globus pallidus neurons in MPTP parkinsonian mice was slower than that of normal mice. Application of orexin-A or orexin-B significantly increased the spontaneous firing rate of pallidal neurons. The influx of Ca²⁺ through the L-type Ca²⁺ channel is the major mechanism involved in orexin-induced excitation in the globus pallidus. Orexin-A-induced increase in firing rate of pallidal neurons in MPTP parkinsonian mice was stronger than that of normal mice. Orexin-A exerted both electrophysiological and behavioral effects mainly via OX₁R, and orexin-B exerted the effects via OX₂R. Endogenous orexins modulated the excitability of globus pallidus neurons mainly through OX₁R. The present behavioral and electrophysiological results suggest that orexins ameliorate parkinsonian motor deficits through increasing the spontaneous firing of globus pallidus neurons.

Effects of GABAa receptor antagonists on motor behavior in pharmacological Parkinson's disease model in mice

The aim of this study was to evaluate the effects of two gamma‐amino butyric acid (GABA)a receptor antagonists on motor behavioral tasks in a pharmacological model of Parkinson disease (PD) in rodents. Ninety‐six Swiss mice received intraperitoneal injection of Haloperidol (1 mg/kg) to block dopaminergic receptors. GABAa receptors antagonists Bicuculline (1 and 5 mg/kg) and Flumazenil (3 and 6 mg/kg) were used for the assessment of the interaction among these neurotransmitters, in this PD model. The motor behavior of the animals was evaluated in the catalepsy test (30, 60, and 90 min after drugs application), through open field test (after 60 min) and trough functional gait assessment (after 60 min). Both Bicuculline and Flumazenil were able to partially reverse catalepsy induced by Haloperidol. In the open field test, Haloperidol reduced the number of horizontal and vertical exploration of the animals, which was not reversed trough application of GABAa antagonists. Furthermore, the functional gait assessment was not sensitive enough to detect motor changes in this animal model of PD. There is an interaction between dopamine and GABA in the basal ganglia and the blocking GABAa receptors may have therapeutic potential in the treatment of PD.

Adenosine A2A Receptor Modulates the Activity of Globus Pallidus Neurons in Rats

The globus pallidus is a central nucleus in the basal ganglia motor control circuit. Morphological studies have revealed the expression of adenosine A_2A receptors in the globus pallidus. To determine the modulation of adenosine A_2A receptors on the activity of pallidal neurons in both normal and parkinsonian rats, in vivo electrophysiological and behavioral tests were performed in the present study. The extracellular single unit recordings showed that micro-pressure administration of adenosine A_2A receptor agonist, CGS21680, regulated the pallidal firing activity. GABAergic neurotransmission was involved in CGS21680-induced modulation of pallidal neurons via a PKA pathway. Furthermore, application of two adenosine A_2A receptor antagonists, KW6002 or SCH442416, mainly increased the spontaneous firing of pallidal neurons, suggesting that endogenous adenosine system modulates the activity of pallidal neurons through adenosine A_2A receptors. Finally, elevated body swing test (EBST) showed that intrapallidal microinjection of adenosine A_2A receptor agonist/antagonist induced ipsilateral/contralateral-biased swing, respectively. In addition, the electrophysiological and behavioral findings also revealed that activation of dopamine D₂ receptors by quinpirole strengthened KW6002/SCH442416-induced excitation of pallidal activity. Co-application of quinpirole with KW6002 or SCH442416 alleviated biased swing in hemi-parkinsonian rats. Based on the present findings, we concluded that pallidal adenosine A_2A receptors may be potentially useful in the treatment of Parkinson's disease.

Striatal input- and rate-dependent effects of muscarinic receptors on pallidal firing

The globus pallidus (GP) plays a key role in the overall basal ganglia (BG) activity. Despite evidence of cholinergic inputs to GP, their role in the spiking activity of GP neurons has not received attention. We examine the effect of local activation and blockade of muscarinic receptors (MRs) in the spontaneous firing of GP neurons both in normal and ipsilateral striatum-lesioned rats. We found that activation of MRs produces heterogeneous responses in both normal and ipsilateral striatum-lesioned rats: in normal rats the response evoked by MRs depends on the predrug basal firing rate; the inhibition evoked by MRs is higher in normal rats than in striatum-lesioned rats; the number of neurons that undergo inhibition is lower in striatum-lesioned rats than in normal rats. Our data suggest that modulation of MRs in the GP depends on the firing rate before their activation and on the integrity of the striato-pallidal pathway.