GABAergic characteristics of transcallosal activity of cat motor cortical neurons

Is bilateral corticospinal connectivity impaired in patients with chronic obstructive pulmonary disease?

KEY POINTS

During moderate and high levels of quadriceps force production, the ipsilateral motor cortex is concomitantly activated with the contralateral motor cortex throughout the corpus callosum to generate the motor command. Chronic obstructive pulmonary disease (COPD) patients display a structurally impaired corpus callosum that may explain the reduced motor command in this population, which in turn contributes to COPD-related muscle weakness of the knee extensors. The study aimed to determine whether bilateral connectivity was impaired and ipsilateral activation was lowered during unilateral strength production of the knee extensors. Our results indicate impaired bilateral connectivity but preserved ipsilateral activation in patients during unilateral isometric contractions of 50% of maximum voluntary strength. The preservation of ipsilateral activation during force production despite impaired bilateral connectivity is consistent with a reorganization of bilateral motor network function that drives unilateral strength production.

ABSTRACT

The contralateral primary motor cortex (M1) is not the only brain area implicated in motor command generation. During moderate and high levels of quadriceps force production, the ipsilateral M1 is concomitantly activated. Such activation is mediated by the corpus callosum, the main component of bilateral connectivity. Structural damage to the corpus callosum has been observed in chronic obstructive pulmonary disease (COPD) patients, which might reduce ipsilateral activation and contribute to the lower motor command associated with COPD muscle weakness. We thus aimed to determine whether bilateral connectivity and ipsilateral activation were impaired in COPD. Twenty-two COPD patients and 21 healthy age-matched controls were evaluated by transcranial magnetic stimulation, at rest and during 50% of maximal voluntary isometric contraction (MVIC) of the dominant vastus lateralis muscle. Bilateral connectivity was determined by the ipsilateral silent period (iSP) during 50% MVIC. Ipsilateral activation was determined as the increase in ipsilateral excitability from rest to 50% MVIC. As expected, COPD patients had significantly lower MVIC (-25%, p = 0.03). These patients also showed a significantly lower iSP (-53%, p < 0.001) compared to controls. The ipsilateral excitability was increased in patients and controls (×2.5 and ×3.5, respectively, p < 0.001) but not differently between groups (p = 0.84). Despite impaired bilateral connectivity in COPD, ipsilateral activation was not increased. Reorganization in the patients' interhemispheric pathways could explain the preserved ipsilateral activation.

Implication of the ipsilateral motor network in unilateral voluntary muscle contraction: the cross-activation phenomenon

Voluntary force production requires that the brain produces and transmits a motor command to the muscles. It is widely acknowledged that motor commands are executed from the primary motor cortex (M1) located in the contralateral hemisphere. However, involvement of M1 located in the ipsilateral hemisphere during moderate to high levels of unilateral muscle contractions (>30% of the maximum) has been disclosed in recent years. This phenomenon has been termed cross-activation. The activation of the ipsilateral M1 relies on complex inhibitory and excitatory interhemispheric interactions mediated via the corpus callosum and modulated according to the contraction level. The regulatory mechanisms underlying these interhemispheric interactions, especially excitatory ones, remain vague, and contradictions exist in the literature. In addition, very little is known regarding the possibility that other pathways could also mediate the cross-activation. In the present review, we will therefore summarize the concept of cross-activation during unilateral voluntary muscle contraction and explore the associated mechanisms and other nervous system pathways underpinning this response. A broader knowledge of these mechanisms would consequently allow a better comprehension of the motor system as a whole, as distant brain networks working together to produce the motor command.

Pharmacological mechanisms of interhemispheric signal propagation: a TMS-EEG study

Interhemispheric connections across the corpus callosum have a predominantly inhibitory effect. Previous electrophysiology studies imply that local inhibitory circuits are responsible for inducing transcallosal inhibition, likely through inhibitory GABA_B-mediated neurotransmission. We investigated the neurochemical mechanisms involved in interhemispheric connectivity by measuring transcranial magnetic stimulation (TMS)-induced interhemispheric signal propagation (ISP) in the motor cortex and dorsolateral prefrontal cortex (DLPFC) with electroencephalography (EEG) recordings under the pharmacological effects of baclofen, L-DOPA, dextromethorphan, and rivastigmine. We hypothesized that for both stimulated regions, GABA_B receptor agonist baclofen would decrease ISP when compared against baseline while drugs that target other neurotransmitter systems (dopaminergic, acetylcholinergic, and glutamatergic systems) would have no effect on ISP. Twelve right-handed healthy volunteers completed this study and underwent TMS across five sessions in a randomized order. In the motor cortex, participants showed a significant decrease in ISP under baclofen, but not in the other drug conditions. There were no drug-induced changes in ISP in the DLPFC and baseline ISP did not differ across experimental sessions for both brain regions. Together, our results suggest that the inhibitory effects observed with interhemispheric signal transmission are mediated by a population of interneurons involving GABA_B receptor neurotransmission. Inhibitory mechanisms of ISP may be more salient for motor-related functions in the motor cortex than for cognitive control in the DLPFC. These findings are a fundamental step in advancing our understanding of interhemispheric connectivity and may be used to identify treatments for disorders in which transcallosal transmission is dysfunctional.

Age-Related Changes in Frontal Network Structural and Functional Connectivity in Relation to Bimanual Movement Control

Changes in both brain structure and neurophysiological function regulating homotopic as well as heterotopic interhemispheric interactions (IHIs) are assumed to be responsible for the bimanual performance deficits in older adults. However, how the structural and functional networks regulating bimanual performance decline in older adults, as well as the interplay between brain structure and function remain largely unclear. Using a dual-site transcranial magnetic stimulation paradigm, we examined the age-related changes in the interhemispheric effects from the dorsolateral prefrontal cortex and dorsal premotor cortex onto the contralateral primary motor cortex (M1) during the preparation of a complex bimanual coordination task in human. Structural properties of these interactions were assessed with diffusion-based fiber tractography. Compared with young adults, older adults showed performance declines in the more difficult bimanual conditions, less optimal brain white matter (WM) microstructure, and a decreased ability to regulate the interaction between dorsolateral prefrontal cortex and M1. Importantly, we found that WM microstructure, neurophysiological function, and bimanual performance were interrelated in older adults, whereas only the task-related changes in IHI predicted bimanual performance in young adults. These results reflect unique interactions between structure and function in the aging brain, such that declines in WM microstructural organization likely lead to dysfunctional regulation of IHI, ultimately accounting for bimanual performance deficits.

SIGNIFICANCE STATEMENT

The structural and functional changes in the aging brain are associated with a decline in movement control, compromising functional independence. We used MRI and noninvasive brain stimulation techniques to investigate white matter microstructural organization and neurophysiological function in the aging brain, in relation to bimanual movement control. We found that less optimal brain microstructural organization and task-related modulations in neurophysiological function resulted in poor bimanual performance in older adults. By interrelating brain structure, neurophysiological function, and behavior, the current study provides a comprehensive picture of biological alterations in the aging brain that underlie declines in bimanual performance.

Functional difference in short- and long-latency interhemispheric inhibitions from active to resting hemisphere during a unilateral muscle contraction

The aim of the present study was to investigate whether there is a functional difference in short-latency (SIHI) and long-latency (LIHI) interhemispheric inhibition from the active to the resting primary motor cortex (M1) with paired-pulse transcranial magnetic stimulation during a unilateral muscle contraction. In nine healthy right-handed participants, IHI was tested from the dominant to the nondominant M1 and vice versa under resting conditions or during performance of a sustained unilateral muscle contraction with the right or left first dorsal interosseous muscle at 10% and 30% maximum voluntary contraction. To obtain measurements of SIHI and LIHI, a conditioning stimulus (CS) was applied over the M1 contralateral to the muscle contraction, followed by a test stimulus over the M1 ipsilateral to the muscle contraction at short (10 ms) and long (40 ms) interstimulus intervals. We used four CS intensities to investigate SIHI and LIHI from the active to the resting M1 systematically. The amount of IHI during the unilateral muscle contractions showed a significant difference between SIHI and LIHI, but the amount of IHI during the resting condition did not. In particular, SIHI during the muscle contractions, but not LIHI, significantly increased with increase in CS intensity compared with the resting condition. Laterality of IHI was not detected in any of the experimental conditions. The present study provides novel evidence that a functional difference between SIHI and LIHI from the active to the resting M1 exists during unilateral muscle contractions.

The origin of neocortical nitric oxide synthase-expressing inhibitory neurons

Inhibitory neurons are critical for regulating effective transfer of sensory information and network stability. The precision of inhibitory function likely derives from the existence of a variety of interneuron subtypes. Their specification is largely dependent on the locale of origin of interneuron progenitors. Neocortical and hippocampal inhibitory neurons originate the subpallium, namely in the medial and caudal ganglionic eminences (MGE and CGE), and in the preoptic area (POA). In the hippocampus, neuronal nitric oxide synthase (nNOS)-expressing cells constitute a numerically large GABAergic interneuron population. On the contrary, nNOS-expressing inhibitory neurons constitute the smallest of the known neocortical GABAergic neuronal subtypes. The origins of most neocortical GABAergic neuron subtypes have been thoroughly investigated, however, very little is known about the origin of, or the genetic programs underlying the development of nNOS neurons. Here, we show that the vast majority of neocortical nNOS-expressing neurons arise from the MGE rather than the CGE. Regarding their molecular signature, virtually all neocortical nNOS neurons co-express the neuropeptides somatostatin (SST) and neuropeptide Y (NPY), and about half of them express the calcium-binding protein calretinin (CR). nNOS neurons thus constitute a small cohort of the MGE-derived SST-expressing population of cortical inhibitory neurons. Finally, we show that conditional removal of the transcription factor Sox6 in MGE-derived GABAergic cortical neurons results in an absence of SST and CR expression, as well as reduced expression of nNOS in neocortical nNOS neurons. Based on their respective abundance, origin and molecular signature, our results suggest that neocortical and hippocampal nNOS GABAergic neurons likely subserve different functions and have very different physiological relevance in these two cortical structures.

Effects of GABAA and GABAB agonists on interhemispheric inhibition in man

OBJECTIVE

Animal studies on neurotransmitter systems that mediate interhemispheric inhibition (IHI) suggest that, (i) callosal transmission is regulated by presynaptic GABA(B) receptors, and (ii) GABA(A)-ergic neurones mediate early IHI, whereas GABA(B)-ergic neurones mediate later IHI. In humans the mechanism is unclear. Interactions between cortical inhibitory circuits suggest a postsynaptic GABA(B)-ergic mechanism. We will here test this hypothesis.

METHODS

Short-latency IHI (s-IHI) and long-latency IHI (l-IHI) were evaluated using the paired pulse paradigm before and under medication with (i) a GABA(B)-agonist (baclofen) in 17 subjects, and (ii) a GABA(A)-agonist (midazolam) in 10 subjects participating twice.

RESULTS

Baclofen did not significantly enhance s-IHI. L-IHI between 20 and 50ms was significantly strengthened, and obtained also at ISIs between 100 and 200ms. Midazolam had no effect on s-IHI, whereas l-IHI was attenuated.

CONCLUSIONS

Our results support the hypothesis, that l-IHI in humans is mediated by postsynaptic GABA(B) receptors. GABA(A)-ergic medication resulted in attenuation of l-IHI. Regarding s-IHI, our results are inconclusive and require further investigation.

SIGNIFICANCE

This is the first human study evaluating the effect of baclofen on IHI, indicating that l-IHI is mediated by GABA(B)-ergic neurones. Because interhemispheric interaction is now also been used as a therapeutic approach, understanding the underlying neurotransmitter systems will be increasingly relevant.

Glutamic acid decarboxylase immunoreactivity in callosal projecting neurons of cat and rat somatic sensory areas

The distribution of GABAergic callosally projecting neurons was analysed in the somatic sensory areas of cat and rat cerebral cortex by combining retrograde tracing of nerve cell bodies and glutamic acid decarboxylase (GAD) immunocytochemistry. A retrograde tracer (colloidal gold- labelled wheat germ agglutinin conjugated to enzymatically inactive horseradish peroxidase) was injected in the first or second somatic sensory area. Brain sections were processed for the simultaneous visualisation of the retrograde tracer and GAD immunoreactivity. In all animals, double-labelled neurons were found in the hemisphere contralateral to the injection site (double-labelled callosal neurons). Their proportion was similar in both species (0.8% of all retrogradely-labelled neurons in cat, 0.7% in rat). These results: 1) confirm the existence of a small proportion of GABAergic callosally projecting neurons in rat somatic sensory cortices; 2) indicate the presence of a small but significant proportion of GAD-positive callosally projecting neurons in cat somatic sensory cortices; and 3) show that the proportion of GAD-positive callosal neurons is similar in the two species.

GABA-B-related activity in processing of transcallosal response in cat motor cortex

GABA-ergic characteristics of transcallosal (TC) responses were studied with specific antagonists of both GABA-A and GABA-B receptor subtypes. We used a paired-pulse paradigm to get insight into the role of GABA in interhemispheric interactions between motor cortices. 3-Amino-2-(4-chlorophenyl)-propylphosphonic acid (phaclofen) and 3-aminopropyl-diethoxymethyl-phosphinic acid (CGP 35348) were used as GABA-B antagonists and bicuculline methiodide (BMI) was used to block the GABA-A receptor. Although both GABA-A and GABA-B antagonists increased spike discharge upon transcallosal stimulation, in both pyramidal tract and non-pyramidal tract neurons, they had different effects on the responses to the first and second stimuli of paired-pulse stimulation (200 msec interval). Although the inhibition seen with the second stimulus was greatly attenuated by antagonists of the GABA-B receptor, it was maintained in the presence of the GABA-A antagonist. These finding support a presynaptic regulation of callosal transmission by GABA-B receptors in the callosal synapse of the cat motor cortex.

Dopaminergic modulation of transcallosal activity of cat motor cortical neurons

The effects of dopamine (DA) and its antagonists on the transcallosal activity of pyramidal tract neurons (PTNs) and non-PTNs in the anesthetized cat motor cortex were studied with iontophoretic applications; dopamine, SCH 23390 (D1 antagonist), sulpiride (D2 antagonist) and haloperidol. Neuronal activity was recorded with a multi-barreled glass microelectrode. Transcallosal neuronal activity was evoked by stimulation of the contralateral motor cortex. The number of spikes thus activated was counted for the control and test conditions after application of each drug: (1) dopamine application decreased the number of spikes evoked by transcallosal stimulation; (2) application of SCH 23390, sulpiride and haloperidol restored these decreased spike numbers to the control level; (3) latency of neuronal response to transcallosal stimulation was not affected by the application of either DA, SCH 23390, sulpiride or haloperidol; and (4) there was no significant difference between PTNs and non-PTNs in the manner of response to DA and its antagonist applications. Our conclusion is that dopamine modulated the transcallosal neuronal response in the cat motor cortex in a suppressive manner. This fact suggested that interhemispheric neuronal communications could be subjected to suppressive modification by the dopaminergic system.