Ablation of striatal interneurons influences activities of entopeduncular neurons

Temporal coupling with cortex distinguishes spontaneous neuronal activities in identified basal ganglia-recipient and cerebellar-recipient zones of the motor thalamus

Neurons of the motor thalamus mediate basal ganglia and cerebellar influences on cortical activity. To elucidate the net result of γ-aminobutyric acid-releasing or glutamatergic bombardment of the motor thalamus by basal ganglia or cerebellar afferents, respectively, we recorded the spontaneous activities of thalamocortical neurons in distinct identified “input zones” in anesthetized rats during defined cortical activity states. Unexpectedly, the mean rates and brain state dependencies of the firing of neurons in basal ganglia-recipient zone (BZ) and cerebellar-recipient zone (CZ) were matched during slow-wave activity (SWA) and cortical activation. However, neurons were distinguished during SWA by their firing regularities, low-threshold spike bursts and, more strikingly, by the temporal coupling of their activities to ongoing cortical oscillations. The firing of neurons across the BZ was stronger and more precisely phase-locked to cortical slow (∼1 Hz) oscillations, although both neuron groups preferentially fired at the same phase. In contrast, neurons in BZ and CZ fired at different phases of cortical spindles (7–12 Hz), but with similar strengths of coupled firing. Thus, firing rates do not reflect the predicted inhibitory–excitatory imbalance across the motor thalamus, and input zone-specific temporal coding through oscillatory synchronization with the cortex could partly mediate the different roles of basal ganglia and cerebellum in behavior.

Reduced pallidal output causes dystonia

Dystonia is a neurological disorder characterized by sustained or repetitive involuntary muscle contractions and abnormal postures. In the present article, we will introduce our recent electrophysiological studies in hyperkinetic transgenic mice generated as a model of DYT1 dystonia and in a human cervical dystonia patient, and discuss the pathophysiology of dystonia on the basis of these electrophysiological findings. Recording of neuronal activity in the awake state of DYT1 dystonia model mice revealed reduced spontaneous activity with bursts and pauses in both internal (GPi) and external (GPe) segments of the globus pallidus. Electrical stimulation of the primary motor cortex evoked responses composed of excitation and subsequent long-lasting inhibition, the latter of which was never observed in normal mice. In addition, somatotopic arrangements were disorganized in the GPi and GPe of dystonia model mice. In a human cervical dystonia patient, electrical stimulation of the primary motor cortex evoked similar long-lasting inhibition in the GPi and GPe. Thus, reduced GPi output may cause increased thalamic and cortical activity, resulting in the involuntary movements observed in dystonia.

Cortically evoked long-lasting inhibition of pallidal neurons in a transgenic mouse model of dystonia

Dystonia is a neurological disorder characterized by sustained or repetitive involuntary muscle contractions and abnormal postures. To understand the pathophysiology of dystonia, neurophysiological analyses were performed on hyperkinetic transgenic mice generated as a model of DYT1 dystonia. Abnormal muscle activity, such as coactivation of agonist and antagonist muscles and sustained muscle activation, was frequently observed in these mice. Recording of neuronal activity in the awake state revealed reduced spontaneous activity with bursts and pauses in both the external and internal segments of the globus pallidus. Motor cortical stimulation evoked responses composed of excitation and subsequent long-lasting inhibition in both pallidal segments, which were never observed in the normal mice. In addition, the somatotopic arrangements in both pallidal segments were disorganized. Long-lasting inhibition induced by cortical inputs in the internal pallidal segment may disinhibit thalamic and cortical activity, resulting in the motor hyperactivity observed in the transgenic mice.

Possible Mechanisms of the Involvement of Dopaminergic Cells and Cholinergic Interneurons in the Striatum in the Conditioned-Reflex Selection of Motor Activity

A possible mechanism for the involvement of cholinergic interneurons in the striatum and dopaminergic cells in the substantia nigra in the selection from among several types of motor activity during learning is proposed. Selection is triggered by simultaneous increases in the activity of dopaminergic neurons and a pause in the activity of cholinergic interneurons in response to the conditioned signal. The appearance of the pause may facilitate activation of GABAergic interneurons in the striatum and the action of dopamine on D2 receptors on cholinergic interneurons. Differently directed changes in dopamine and acetylcholine levels synergistically modulate the efficiency of corticostriatal inputs, such that the rules for modulation of the "strong" and "weak" inputs are opposite in sign. The subsequent reorganization of neuron activity in the cortex-basal ganglia-thalamus-cortex circuit leads to increased activity in those cortical neurons providing "strong" innervation to the striatum with simultaneous decreases in the activity of neurons providing "weak" innervation to the striatum, which may underlie the selection of the movement reaction, in which the neocortex is involved. It follows from this model that if the delay between the conditioned and unconditioned stimuli is not longer than the latent period of the reactions of dopaminergic and cholinergic cells (about 100 msec), selection of movement activity in response to the conditioned signal and learning is hindered.

Impairment of skilled forelimb use after ablation of striatal interneurons expressing substance P receptors in rats: an analysis using a pasta matrix reaching task

Local injection of substance P (SP)-saporin can cause selective ablation of striatal interneurons expressing SP receptors (SPR). In this study, we evaluated quantitatively the impairment of skilled forelimb use after unilateral ablation of the striatal interneurons using a pasta matrix reaching task in rats. We found a significant decrease of the number of the pasta pieces (uncooked spaghetti) retrieved using the paw of the experimental side contralateral to the ablation, whereas the number of the pasta pieces retrieved using the paw of the intact side increased significantly. These findings, with our previous reports, suggest that the modulation of the cortico-striato-entopeduncular direct pathway by striatal interneurons is important for maintaining normal basal-ganglia control for skilled forelimb movements.

Cytochrome oxidase activity in the monkey globus pallidus and subthalamic nucleus after ablation of striatal interneurons expressing substance P receptors

To understand functional roles of striatal interneurons in primate basal ganglia circuitry, we ablated interneurons expressing substance P (SP) receptors (SPR) in the putamen with SP-saporin, a SPR selective neurotoxin. The effect of SP-saporin injection into the putamen was evaluated by examining the loss of cholinergic interneurons and NADPHd-positive (nicotinamide adenine dinucleotide phosphate diaphorase positive) interneurons. We then analyzed regional metabolic changes using cytochrome oxidase (CO) histochemistry. CO activity in some regions of the internal and external segments of the globus pallidus (GP) in the lesioned hemisphere was lower than that in the contralateral or surrounding GP regions. CO activity in the subthalamic nucleus, however, showed no significant change. The present findings suggest that striatopallidal projection neurons exert enhanced inhibitory influence on the GP without modulatory control by the striatal SPR-expressing interneurons.