Distribution and properties of GABA(B) antagonist [3H]CGP 62349 binding in the rhesus monkey thalamus and basal ganglia and the influence of lesions in the reticular thalamic nucleus

A theoretical framework for the site-specific and frequency-dependent neuronal effects of deep brain stimulation

BACKGROUND

Deep brain stimulation is an established therapy for several neurological disorders; however, its effects on neuronal activity vary across brain regions and depend on stimulation settings. Understanding these variable responses can aid in the development of physiologically-informed stimulation paradigms in existing or prospective indications.

OBJECTIVE

Provide experimental and computational insights into the brain-region-specific and frequency-dependent effects of extracellular stimulation on neuronal activity.

METHODS

In patients with movement disorders, single-neuron recordings were acquired from the subthalamic nucleus, substantia nigra pars reticulata, ventral intermediate nucleus, or reticular thalamus during microstimulation across various frequencies (1-100 Hz) to assess single-pulse and frequency-response functions. Moreover, a biophysically-realistic computational framework was developed which generated postsynaptic responses under the assumption that electrical stimuli simultaneously activated all convergent presynaptic inputs to stimulation target neurons. The framework took into consideration the relative distributions of excitatory/inhibitory afferent inputs to model site-specific responses, which were in turn embedded within a model of short-term synaptic plasticity to account for stimulation frequency-dependence.

RESULTS

We demonstrated microstimulation-evoked excitatory neuronal responses in thalamic structures (which have predominantly excitatory inputs) and inhibitory responses in basal ganglia structures (predominantly inhibitory inputs); however, higher stimulation frequencies led to a loss of site-specificity and convergence towards neuronal suppression. The model confirmed that site-specific responses could be simulated by accounting for local neuroanatomical/microcircuit properties, while suppression of neuronal activity during high-frequency stimulation was mediated by short-term synaptic depression.

CONCLUSIONS

Brain-region-specific and frequency-dependant neuronal responses could be simulated by considering neuroanatomical (local microcircuitry) and neurophysiological (short-term plasticity) properties.

Effects of intrathecal baclofen therapy in subjects with disorders of consciousness: a reappraisal

Baclofen is a structural analogue of gamma-amino-butyric acid (GABA), which reduces spastic hypertonia of striated muscle due to a mechanism of GABA_B-ergic inhibition of mono- and polysynaptic reflexes at the spinal level. There are reports of patients with severe disorders of consciousness that presented a substantial improvement following intrathecal baclofen (ITB) administration for severe spasticity. The neural mechanisms underlying the clinical recovery after ITB have not yet been clarified. Baclofen could modulate sleep-wake cycles that may be dysregulated and thus interfere with alertness and awareness. The diminished proprioceptive and nociceptive sensory inputs may relieve thalamo-cortical neural networks involved in maintaining the consciousness of the self and the world. ITB treatment might also promote the recovery of an impaired GABAergic cortical tone, restoring the balance between excitatory and inhibitory cortical activity. Furthermore, glutamatergic synapses are directly or indirectly modulated by GABA_B-ergic receptors. Neurophysiological techniques (such as transcranial magnetic stimulation, electroencephalography, or the combination of both) can be helpful to explore the effects of intrathecal or oral baclofen on the modulation of neural cortical circuits in humans with disorders of consciousness.

Physiological mechanisms of thalamic ventral intermediate nucleus stimulation for tremor suppression

Ventral intermediate thalamic deep brain stimulation is a standard therapy for the treatment of medically refractory essential tremor and tremor-dominant Parkinson's disease. Despite the therapeutic benefits, the mechanisms of action are varied and complex, and the pathophysiology and genesis of tremor remain unsubstantiated. This intraoperative study investigated the effects of high frequency microstimulation on both neuronal firing and tremor suppression simultaneously. In each of nine essential tremor and two Parkinson's disease patients who underwent stereotactic neurosurgery, two closely spaced (600 µm) microelectrodes were advanced into the ventral intermediate nucleus. One microelectrode recorded action potential firing while the adjacent electrode delivered stimulation trains at 100 Hz and 200 Hz (2-5 s, 100 µA, 150 µs). A triaxial accelerometer was used to measure postural tremor of the contralateral hand. At 200 Hz, stimulation led to 68 ± 8% (P < 0.001) inhibition of neuronal firing and a 53 ± 5% (P < 0.001) reduction in tremor, while 100 Hz reduced firing by 26 ± 12% (not significant) with a 17 ± 6% (P < 0.05) tremor reduction. The degree of cell inhibition and tremor suppression were significantly correlated (P < 0.001). We also found that the most ventroposterior stimulation sites, closest to the border of the ventral caudal nucleus, had the best effect on tremor. Finally, prior to the inhibition of neuronal firing, microstimulation caused a transient driving of neuronal activity at stimulus onset (61% of sites), which gave rise to a tremor phase reset (73% of these sites). This was likely due to activation of the excitatory glutamatergic cortical and cerebellar afferents to the ventral intermediate nucleus. Temporal characteristics of the driving responses (duration, number of spikes, and onset latency) significantly differed between 100 Hz and 200 Hz stimulation trains. The subsequent inhibition of neuronal activity was likely due to synaptic fatigue. Thalamic neuronal inhibition seems necessary for tremor reduction and may function in effect as a thalamic filter to uncouple thalamo-cortical from cortico-spinal reflex loops. Additionally, our findings shed light on the gating properties of the ventral intermediate nucleus within the cerebello-thalamo-cortical tremor network, provide insight for the optimization of deep brain stimulation technologies, and may inform controlled clinical studies for assessing optimal target locations for the treatment of tremor.

Synthesis and Evaluation of a New 18F-Labeled Radiotracer for Studying the GABAB Receptor in the Mouse Brain

New GABA_B agonists, fluoropyridyl ether analogues of baclofen, have been synthesized as potential PET radiotracers. The compound with highest inhibition binding affinity as well as greatest agonist response, ( R)-4-amino-3-(4-chloro-3-((2-fluoropyridin-4-yl)methoxy)phenyl)butanoic acid (1b), was radiolabeled with ¹⁸F with good radiochemical yield, high radiochemical purity, and high molar radioactivity. The regional brain distribution of the radiolabeled ( R)-4-amino-3-(4-chloro-3-((2-[¹⁸F]fluoropyridin-4-yl)methoxy)phenyl)butanoic acid, [¹⁸F]1b, was studied in CD-1 male mice. The study demonstrated that [¹⁸F]1b enters the mouse brain (1% ID/g tissue). The accumulation of [¹⁸F]1b in the mouse brain was inhibited (35%) by preinjection of GABA_B agonist 1a, suggesting that the radiotracer brain uptake is partially mediated by GABA_B receptors. The presented data demonstrate a feasibility of imaging of GABA_B receptors in rodents and justify further development of GABA_B PET tracers with improved specific binding and greater blood-brain barrier permeability.

Intrathecal baclofen associated with improvement of consciousness disorders in spasticity patients

OBJECTIVE

Intrathecal baclofen (ITB) pump is a therapeutic option for persistent vegetative state and minimal conscious state patients that have associated spasticity. We investigated whether this treatment modality can affect their level of consciousness.

METHOD

In this prospective, open label, observational study, we implanted ITB pumps for the treatment of spasticity in eight patients with disorders of consciousness (vegetative state and minimally conscious state) and we followed them with the Coma Recovery Scale-Revised, the Eastern Cooperative Oncology Group (ECOG) performance scale, and the Modified Ashworth spasticity scale. Baclofen dose and complications also were noted.

RESULTS

The offending pathologies were traumatic brain injury in six, anoxia due to cardiac arrest in one, acute obstructive hydrocephalus in one. Two of the patients showed a marked, persistent improvement that fulfilled the criteria of emergence from minimally conscious state. Two of patients had their ITB pumps prematurely removed because of complications. The ECOG score was 4 for all patients and did not change during the study.

CONCLUSION

ITB might be associated with a significant improvement in the disorder of consciousness of two patients from a total of six that had a chronic ITB treatment.

GABA(B) receptors in the centromedian/parafascicular thalamic nuclear complex: an ultrastructural analysis of GABA(B)R1 and GABA(B)R2 in the monkey thalamus

Strong gamma-aminobutyric acid type B (GABA(B)) receptor binding has been shown throughout the thalamus, but the distribution of the two GABA(B) receptor subunits, GABA(B) receptor subunit 1 (GABA(B)R1) and GABA(B) receptor subunit 2 (GABA(B)R2), remains poorly characterized. In primates, the caudal intralaminar nuclei, centromedian and parafascicular (CM/PF), are an integral part of basal ganglia circuits and a main source of inputs to the striatum. In this study, we analyzed the subcellular and subsynaptic distribution of GABA(B) receptor subunits by using light and electron microscopic immunocytochemical techniques. Quantitative immunoperoxidase and immunogold analysis showed that both subunits display a similar pattern of distribution in CM/PF, being expressed largely at extrasynaptic and perisynaptic sites in neuronal cell bodies, dendrites, and axon-like processes and less abundantly in axon terminals. Postsynaptic GABA(B)R1 labeling was found mostly on the plasma membrane (70-80%), whereas GABA(B)R2 was more evenly distributed between the plasma membrane and intracellular compartments of CM/PF neurons. A few axon terminals forming symmetric and asymmetric synapses were also labeled for GABA(B)R1 and GABA(B)R2, but the bulk of presynaptic labeling was expressed in small axon-like processes. About 20% of presynaptic vesicle-containing dendrites of local circuit neurons displayed GABA(B)R1/R2 immunoreactivity. Vesicular glutamate transporters (vGluT1)-containing terminals forming asymmetric synapses expressed GABA(B)R1 and/or displayed postsynaptic GABA(B)R1 at the edges of their asymmetric specialization. Overall, these findings provide evidence for multiple sites where GABA(B) receptors could modulate GABAergic and glutamatergic transmission in the primate CM/PF complex.

Deep brain stimulation

Deep brain stimulation (DBS) has provided remarkable benefits for people with a variety of neurologic conditions. Stimulation of the ventral intermediate nucleus of the thalamus can dramatically relieve tremor associated with essential tremor or Parkinson disease (PD). Similarly, stimulation of the subthalamic nucleus or the internal segment of the globus pallidus can substantially reduce bradykinesia, rigidity, tremor, and gait difficulties in people with PD. Multiple groups are attempting to extend this mode of treatment to other conditions. Yet, the precise mechanism of action of DBS remains uncertain. Such studies have importance that extends beyond clinical therapeutics. Investigations of the mechanisms of action of DBS have the potential to clarify fundamental issues such as the functional anatomy of selected brain circuits and the relationship between activity in those circuits and behavior. Although we review relevant clinical issues, we emphasize the importance of current and future investigations on these topics.

GABA(B) receptor agonists reverse akinesia following intranigral or intracerebroventricular injection in the reserpine-treated rat

1. This study examined whether GABA(B) receptor agonists injected directly into the substantia nigra pars reticulata (SNr) and globus pallidus (GP), or given intracerebroventricularly, could reverse reserpine-induced akinesia in the rat. 2. Male Sprague-Dawley rats, stereotaxically cannulated above the SNr, GP or third ventricle, were rendered akinetic by injection of reserpine (5 mg kg(-1) s.c.). After 18 h, the locomotor effects of the GABA(B) receptor agonists, baclofen or SKF 97541 were examined. 3. Unilateral injection of baclofen (1-5 micro g in 0.5 micro l) into the GP failed to evoke any locomotor response (n=6). In contrast, unilateral intranigral injection of baclofen (0.08-1.6 micro g in 0.5 micro l) produced a dose-dependent increase in net contraversive rotations reaching a maximum of 162+/-24 turns 90 min(-1) (n=6-8). Pretreatment with the selective GABA(B) receptor antagonist, CGP 46381 (2.4 micro g in 0.5 micro l), inhibited the effects of baclofen (0.8 micro g) by 68+/-9% (n=6). 4. Following intracerebroventricular injection, baclofen (0.8-4 micro g in 2 micro l) produced a dose-dependent increase in net arbitrary locomotor units (ALUs), reaching a maximum of 447+/-154 ALUs in 35 min (n=6-7). SKF 97541 (4-32 micro g in 2 micro l) similarly reversed akinesia, reaching 129+/-69 ALUs in 15 min (n=6). 5. These data show that activation of GABA(B) receptors within the SNr, but not the GP, reverses reserpine-induced akinesia. The success of intracerebroventricular injection of baclofen suggests a potential for systemically active GABA(B) receptor agonists in the treatment of akinesia in Parkinson's disease.

Distribution and binding parameters of GABAA receptors in the thalamic nuclei of Macaca mulatta and changes caused by lesioning in the globus pallidus and reticular thalamic nucleus

Ascending output from the basal ganglia to the primate motor thalamus is carried by GABAergic nigro- and pallido-thalamic pathways, which interact with intrinsic thalamic GABAergic systems represented in primates by local circuit neurons and axons of the reticular thalamic nucleus. Disease-triggered pathological processes in the basal ganglia can compromise any of these pathways either directly or indirectly, yet the effects of basal ganglia lesioning on its thalamic afferent-receiving territories has not been studied in primates. Two GABA(A) receptor ligands, [(3)H]muscimol and [(3)H]flunitrazepam, were used to study the distribution and binding properties of the receptor in intact monkeys, those with kainic acid lesions in the globus pallidus, and those with ibotenic acid lesions in the reticular nucleus using quantitative autoradiographic technique on cryostat sections of fresh frozen brain tissue. In control monkeys the binding affinities for [(3)H]muscimol averaged 50 nM in the thalamic nuclei and 86 nM in the basal ganglia while the binding densities varied (maximum density of binding sites [Bmax] range of 99.4-1000.1 fmol/mg of tissue). Binding affinities and Bmax values for [(3)H]flunitrazepam averaged 2.02 nM and 81-113 fmol/mg of tissue, respectively. Addition of 100-microM GABA increased average affinity to 1.35 nM whereas Bmax values increased anywhere from 1-50% in different nuclei. Zolpidem (100 nM) decreased binding by 68-80%. Bmax values for both ligands were decreased at the two survival times in both medial and lateral globus pallidus implying involvement of both nuclei in the lesion. Statistically significant, 40% decrease (P=0.055) of Bmax for [(3)H]muscimol was observed in the ventral anterior nucleus pars densicellularis (VAdc, the main pallidal projection territory in the thalamus) 1 week after globus pallidus lesioning and a 36% decrease (P=0.017) 4 months post-lesioning. In contrast, [(3)H]flunitrazepam Bmax values in the VAdc of the same animals were increased by 23% (P=0.021) at 1 week and 28% (P=0.005) 4 months postlesion, respectively. One week after the reticular nucleus lesioning, the binding densities of [(3)H]muscimol and [(3)H]flunitrazepam were decreased in the thalamic nuclei receiving projections from the lesioned reticular nucleus sector by approximately 50% (P<0.05) and 10-33% (P<0.05), respectively. The results suggest that different GABA(A) receptor subtypes are associated with different GABAergic systems in the thalamus which react differently to deafferentation.

Ligands for expression cloning and isolation of GABA(B) receptors

The scope of the plenary lecture at the occasion of the Xth Meeting on Heterocyclic Structures in Medicinal Chemistry, Palermo 2002, is considerably larger than that of the main lecture at the XVIth International Symposium on Medicinal Chemistry, Bologna 2000, described by Froestl et al. in Farmaco 56 (2001) 101. Additional information is presented, in particular, on the reaction conditions for the 31 step synthesis of the combined affinity chromatography and photoaffinity radioligand [125I]CGP84963 and on the recent developments of the molecular biology of GABA(B) receptors.

Ionotropic and metabotropic GABA and glutamate receptors in primate basal ganglia

The functions of glutamate and GABA in the CNS are mediated by ionotropic and metabotropic, G protein-coupled, receptors. Both receptor families are widely expressed in basal ganglia structures in primates and nonprimates. The recent development of highly specific antibodies and/or cDNA probes allowed the better characterization of the cellular localization of various GABA and glutamate receptor subtypes in the primate basal ganglia. Furthermore, the use of high resolution immunogold techniques at the electron microscopic level led to major breakthroughs in our understanding of the subsynaptic and subcellular localization of these receptors in primates. In this review, we will provide a detailed account of the current knowledge of the localization of these receptors in the basal ganglia of humans and monkeys.