Effects of excitotoxic striatal lesions on single unit activity in globus pallidus and entopeduncular nucleus of the cat

Altered membrane properties and firing patterns of external globus pallidus neurons in the R6/2 mouse model of Huntington's disease

In mouse models of Huntington's disease (HD), striatal neuron properties are significantly altered. These alterations predict changes in striatal output regions. However, little is known about alterations in those regions. The present study examines changes in passive and active membrane properties of neurons in the external globus pallidus (GPe), the first relay station of the indirect pathway, in the R6/2 mouse model of juvenile HD at presymptomatic (1 month) and symptomatic (2 month) stages. In GPe, two principal types of neurons can be distinguished based on firing properties and the presence (type A) or absence (type B) of I_h currents. In symptomatic animals (2 month), cell membrane capacitance and input resistance of type A neurons were increased compared with controls. In addition, action potential afterhyperpolarization amplitude was reduced. Although the spontaneous firing rate of GPe neurons was not different between control and R6/2 mice, the number of spikes evoked by depolarizing current pulses was significantly reduced in symptomatic R6/2 animals. In addition, these changes were accompanied by altered firing patterns evidenced by increased interspike interval variation and increased number of bursts. Blockade of GABA_A receptors facilitated bursting activity in R6/2 mice but not in control littermates. Thus, alterations in firing patterns could be caused by changes in intrinsic membrane conductances and modulated by synaptic inputs. © 2016 Wiley Periodicals, Inc.

Distinct developmental origins manifest in the specialized encoding of movement by adult neurons of the external globus pallidus

Transcriptional codes initiated during brain development are ultimately realized in adulthood as distinct cell types performing specialized roles in behavior. Focusing on the mouse external globus pallidus (GPe), we demonstrate that the potential contributions of two GABAergic GPe cell types to voluntary action are fated from early life to be distinct. Prototypic GPe neurons derive from the medial ganglionic eminence of the embryonic subpallium and express the transcription factor Nkx2-1. These neurons fire at high rates during alert rest, and encode movements through heterogeneous firing rate changes, with many neurons decreasing their activity. In contrast, arkypallidal GPe neurons originate from lateral/caudal ganglionic eminences, express the transcription factor FoxP2, fire at low rates during rest, and encode movements with robust increases in firing. We conclude that developmental diversity positions prototypic and arkypallidal neurons to fulfil distinct roles in behavior via their disparate regulation of GABA release onto different basal ganglia targets.

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Arkypallidal and prototypic GPe cells have distinct origins and transcriptional codes

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Arkypallidal neurons rapidly and robustly increase firing rate at movement onset

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Movement is accurately encoded by single arkypallidal or prototypic neurons

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Two GPe cell types are fated to affect different targets in distinct ways in behavior

Synaptic localization of GABA(A) receptor subunits in the substantia nigra of the rat: effects of quinolinic acid lesions of the striatum

The inhibitory amino acid, gamma-aminobutyric acid (GABA), plays a critical role in the substantia nigra (SN) in health and disease. GABA transmission is controlled in part by the type(s) of GABA receptor expressed, their subunit composition and their location in relation to GABA release sites. In order to define the subcellular localization of GABA(A) receptors in the SN in normal and pathological conditions, sections of SN from control rats and rats that had received quinolinic acid lesions of the striatum were immunolabelled using the postembedding immunogold technique with antibodies against subunits of the GABA(A) receptor. Immunolabelling for alpha1, beta2/3 and gamma2 subunits was primarily located at symmetrical synapses. Double-labelling revealed that beta2/3 subunit-positive synapses were formed by terminals that were enriched in GABA. Colocalization of alpha1, beta2/3 and gamma2 subunits occurred at individual symmetrical synapses, some of which were identified as degenerating terminals derived from the striatum. In the SN ipsilateral to the striatal lesion there was a significant elevation of immunolabelling for beta2/3 subunits of the GABA(A) receptor at symmetrical synapses, but not of GluR2/3 subunits of the AMPA receptor at asymmetrical synapses. It was concluded that fast GABA(A)-mediated transmission occurs primarily at symmetrical synapses within the SN, that different receptor subunits coexist at individual synapses and that the upregulation of GABA(A) receptors following striatal lesions is expressed as increased receptor density at synapses. The upregulation of GABA(A) receptors in Huntington's disease and its models is thus likely to lead to an increased efficiency of transmission at intact GABAergic synapses in the SN and may partly underlie the motor abnormalities of this disorder.

Age-related changes in striatal function of freely-moving F344 rats

Multi-wire electrode arrays were used to record extracellular electrophysiological activity in striatal medium spiny-like neurons of freely-moving young (6-8 months) and aged (24-26 months) Fischer 344 rats. While overall basal firing rates did not differ between the two groups, d-amphetamine (5.0 mg/kg) increased firing rates more in the young rats. D-Amphetamine had heterogeneous effects on firing rates, however, exciting 63% of the neurons while inhibiting 37%. Neurons were classified according to their response to d-amphetamine (excited vs. inhibited) to examine age-related differences in firing rates and bursting activity. In the d-amphetamine-excited neurons, pre-drug intraburst firing rates were higher in the old rats. This effect was reversed by d-amphetamine. D-Amphetamine increased the percentage of spikes within bursts to a greater extent in the aged animals and decreased burst durations greater in the young group. In d-amphetamine-inhibited neurons, firing rates were diminished in the old rats more than they were in the young rats. These results demonstrate age-related alterations in striatal electrophysiological activity that may help explain motor deficits seen in senescence.

Fetal tissue transplants in animal models of Huntington's disease: the effects on damaged neuronal circuitry and behavioral deficits

Accumulating evidence indicates that grafts of embryonic neurons achieve the anatomical and functional reconstruction of damaged neuronal circuitry. The restorative capacity of grafted embryonic neural tissue is most illustrated by studies with striatal tissue transplantation in animals with striatal lesions. Striatal neurons implanted into the lesioned striatum receive some of the major striatal afferents such as the nigrostriatal dopaminergic inputs and the gluatmatergic afferents from the neocortex and thalamus. The grafted neurons also send efferents to the primary striatal targets, including the globus pallidus (GP, the rodent homologue of the external segment of the globus pallidus) and the entopeduncular nucleus (EP, the rodent homologue of the internal segment of the globus pallidus). These anatomical connections provide the reversal of the lesion-induced alterations in neuronal activities of primary and secondary striatal targets. Furthermore, intrastriatal striatal grafts improve motor and cognitive deficits seen in animals with striatal lesions. Since the grafts affect motor and cognitive behaviors that are critically dependent on the integrity of neuronal circuits of the basal ganglia, the graft-mediated recovery in these behavioral deficits is most likely attributable to the functional reconstruction of the damaged neuronal circuits. The fact that the extent of the behavioral recovery is positively correlated to the amount of grafted neurons surviving in the striatum encourages this view. Based on the animal studies, embryonic striatal tissue grafting could be a viable strategy to alleviate motor and cognitive disorders seen in patients with Huntington's disease where massive degeneration of striatal neurons occurs.

Embryonic striatal grafts restore neuronal activity of the globus pallidus in a rodent model of Huntington's disease

It has been demonstrated in rats that embryonic striatal grafts placed in the excitotoxically lesioned striatum establish neuronal connections with the host globus pallidus. In order to determine whether the morphologically verified connections between the grafts and host are functional, the present study investigated the effects of embryonic striatal grafts on changes in the neuronal activity of the globus pallidus in rats with quinolinic acid-induced striatal lesions. The activity of pallidal neurons was determined by use of quantitative cytochrome oxidase histochemistry and an electrophysiological technique. Striatal lesions induced an increase in both the cytochrome oxidase activity and the spontaneous firing rate of the globus pallidus ipsilateral to the lesions. Grafts derived from the lateral ganglionic eminence, but not the medial ganglionic eminence, reversed the lesion-induced increase in the cytochrome oxidase activity of the globus pallidus with concomitant reduction of apomorphine-induced rotational asymmetry. The lateral ganglionic eminence grafts also attenuate the increase in the firing rate of pallidal neurons in rats with striatal lesions. The present results provide evidence that striatal lesions lead to the loss of a tonic inhibitory input to the globus pallidus with consequent increase in the activity of pallidal neurons, and that intrastriatal striatal grafts reverse the altered activity of pallidal neurons. The findings strongly suggest that embryonic striatal grafts functionally repair the damaged striatopallidal pathway.

Changes in the mitochondrial enzyme activity in striatal projection areas after unilateral excitotoxic striatal lesions: partial restoration by embryonic striatal transplants

It is well established that the activity of cytochrome oxidase (CO), a mitochondrial enzyme, reflects the long-term, steady-state levels of neuronal activity. The present study investigated the long-term effects of unilateral striatal lesions induced by quinolinic acid on CO activity in primary striatal targets, including the globus pallidus (GP), entopeduncular nucleus (EP), and substantia nigra pars reticulata (SNR) and a secondary striatal projection area, such as subthalamic nucleus (STN), in rats. The activity of CO was determined by measuring staining intensity on brain sections processed for CO histochemistry. We also examined whether intrastriatal transplants of embryonic striatal tissue could affect the lesion-induced changes in the CO activity of those brain structures. Unilateral striatal lesions were found to lead to increases in the CO activity of the GP, EP, and SNR ipsilateral to the lesions. By contrast, the activity of the ipsilateral STN was decreased following striatal lesions, probably due to the increased inhibitory effect of the GP on the STN. Intrastriatal implantation of the lateral ganglionic eminence (LGN), but not the medial ganglionic eminence (MGE), reversed the lesion-induced changes in the CO activity of the GP and STN with concomitant attenuation of apomorphine-induced rotational asymmetry. The grafts failed to affect the activity of either the EP or SNR. The present results indicate that striatal lesions induce changes in the functional activity of basal ganglia nuclei and that the LGE grafts placed in the damaged striatum partly reverse the alterations in the functional state of the basal ganglia circuitry.

Increased subthalamic neuronal activity after nigral dopaminergic lesion independent of disinhibition via the globus pallidus

Electrophysiological records of unit activity were used to compare the effects of excitotoxic pallidal lesions and 6-hydroxydopamine-induced damage to the midbrain dopaminergic neurons on the discharge rates and patterns of the subthalamic neurons. Removal of the pallidal input induced a slight, but statistically significant, increase (19.5%) in the discharge rate and no change in the firing pattern when compared to control animals. The rats with a dopaminergic lesion showed greater increase (105.7%) while the firing pattern activity of the subthalamic neurons became more irregular, with burst. These results indicate that the increased activity of the subthalamic neurons following a midbrain dopaminergic lesion cannot be due solely to inhibition-disinhibition involving the striato-pallido-subthalamic pathway and induced by the striatal dopaminergic depletion.

Alterations in pallidal neuronal responses to peripheral sensory and striatal stimulation in symptomatic and recovered parkinsonian cats

The spontaneous activity, responses to peripheral sensory and ipsilateral caudate nucleus stimulation of globus pallidus (GP) and entopeduncular nucleus (ENTO) neurons were studied in cats while normal, symptomatic for 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) induced parkinsonism, and when spontaneously recovered from gross parkinsonian motor deficits. Administration of MPTP resulted in parkinsonian motor symptoms that spontaneously recovered approximately 4-6 weeks after the MPTP administration. Post-mortem dopamine levels in recovered animals was approximately 95% below levels previously measured in normal animals. In symptomatic animals, the mean spontaneous firing rate for GP units was decreased by 50% and increased by 55% for ENTO units recorded. Spontaneous firing rates for GP and ENTO units in recovered cats were not significantly different from those observed in normal cats. In normal cats, 31.4% of GP and 29% of ENTO units tested responded to tactile stimulation of the face. Only 12.2% of GP and 13% of ENTO units responded to such stimulation in parkinsonian animals while the responses were generally less specific (larger receptive fields, more bilateral receptive fields, and more responses to multiple stimulation types) than normal. In recovered cats GP and ENTO responses resembled those observed in normal cats. There was no difference in the overall percentage of pallidal units responding to striatal stimulation across the 3 experimental conditions. There was, however, an increase in the percentage of units responding with complex response sequences (i.e. decrease in activity followed by an increase in activity) in symptomatic animals as compared to normal and recovered animals. The results suggest that loss of striatal dopamine in parkinsonian animals has profound effects on the sensory responsiveness of GP and ENTO neurons and that these effects coincide with the appearance of and recovery from parkinsonian motor deficits. These data further support the notion that sensory information processing by the basal ganglia may play an important role in influencing motor output.

N-methyl-D-aspartate acutely increases proenkephalin mRNA in the rat striatum

Studies in which glutamate (GLU) neurotransmission has been reduced at striatal synapses have shown that GLU influences the biosynthesis of certain peptide cotransmitters by striatal neurons. The present experiment was designed to test the effects of direct activation of the NMDA or AMPA types of GLU receptor on the levels of two mRNAs that encode the peptide cotransmitters met5-enkephalin (ME) and substance P (SP). In situ hybridization histochemistry of forebrain tissue sections from rats 8 h after a single intracerebroventricular infusion of NMDA or AMPA revealed a significant and dose-dependent elevation (to a maximum of almost 50%) of striatal ME mRNA when compared to vehicle-injected controls. SP mRNA was not significantly affected. NMDA was more effective than AMPA over the dose range used. Pretreatment with a potent and highly specific AMPA antagonist (NBQX) predictably blocked the AMPA-mediated elevation, and was only slightly effective against the NMDA-induced response. In striking contrast, pretreatment with a potent and highly selective NMDA antagonist (CGP37849) fully opposed both the NMDA- and the AMPA-mediated elevation of ME mRNA. These data further implicate the NMDA receptor in the regulation of peptide cotransmitter gene transcription. They suggest also that the AMPA receptor may play an indirect, synergistic role in the genetic responses of striatal neurons to GLU transmission.

Differential synaptic innervation of neurons in the internal and external segments of the globus pallidus by the GABA- and glutamate-containing terminals in the squirrel monkey

The present study aimed at comparing the pattern of synaptic innervation of neurons in the external (GPe) and internal (GPi) pallidum by gamma-aminobutyric acid (GABA)- and glutamate-immunoreactive terminals in the squirrel monkey. Four major populations of terminals were encountered in GPe and GPi. Our findings combined with those obtained in previous tract-tracing studies reveal that the synaptic innervation of perikarya in GPe is strikingly different from that in GPi. Although the GABA-positive type I boutons (from the striatum) represent 85% of the terminals in contact with somata in GPe, only 32% of the axosomatic synapses involve this type of terminal in GPi. However, the type II terminals (from GPe), which display a moderate level of GABA and glutamate immunoreactivities, account for 48% of the boutons in contact with perikarya in GPi but only 10% in GPe. In both pallidal segments, less than 10% of the axosomatic synapses involve the glutamate-immunoreactive type III terminals (from the subthalamic nucleus). Finally, the type IIa boutons (unknown source), which show levels of immunoreactivities similar to the type II terminals, account for 12% of the boutons in contact with perikarya in GPi but only 4% in GPe. In contrast to perikarya, the innervation of dendritic shafts is similar in both GPe and GPi; more than 80% of the axodendritic synapses involve the type I terminals, 10-15% involve the type III terminals, less than 5% are formed by the type II boutons, and less than 1% involve the type IIa terminals. Three other categories of boutons (types IV, V, VI) account for less than 1% of the total population of terminals in GPe and GPi. In conclusion, our findings demonstrate a differential synaptic innervation of neuronal perikarya in GPe and GPi in primates. These data suggest that the two pallidal segments are separate functional entities of which the neuronal activity is largely controlled by extrinsic inputs that are differentially distributed at the level of single cells.

Bursting properties of units in cat globus pallidus and entopeduncular nucleus: the effect of excitotoxic striatal lesions

The bursting properties of units recorded in globus pallidus and entopeduncular nucleus were studied in awake cats sitting quietly before and after ipsilateral excitotoxic striatal lesions. A computerized statistical procedure was used to identify and evaluate bursts in the recorded spike trains. Bursts were assigned a quantitative statistical measure of burst 'strength' (or improbability) - the surprise value. Before the lesion, 34% of units in the globus pallidus and 60% of units in the entopeduncular nucleus exhibited bursts. Burst units had a significantly slower discharge rate and a significantly greater variability of discharge than non-burst units. The mean length of the interspike intervals immediately preceding the bursts was significantly longer than the overall median intervals in burst units. After the lesion, 21% of units in the globus pallidus and 11% of the units in the entopeduncular nucleus exhibited bursts. Burst units had significantly higher discharge rates and lower discharge variability after the lesion. In contrast, the lesion had no significant effect on the rate or variability of non-burst units. The differences between bursting and non-bursting units in discharge rate and variability disappeared after the lesion. In globus pallidus, the lesion resulted in a significant reduction in the mean number of bursts per unit, surprise value per burst, mean length of bursts, and number of spikes per burst, and a significant increase in the mean discharge rate of burst units. In entopeduncular nucleus, the small number of bursts recorded after the lesion precluded a useful statistical comparison of the effect of striatal lesions on the properties of the bursts. This study demonstrates that removing striatal projections to globus pallidus and entopeduncular nucleus decreases bursting in these nuclei, indicating that intact striatal projections are necessary for the normal production of bursts in these regions.

The temporal structure of spike trains in the primate basal ganglia: afferent regulation of bursting demonstrated with precentral cerebral cortical ablation

We studied the temporal pattern of discharge of single units in the basal ganglia of awake primates sitting quietly. Bursting was studied with a procedure that identified individual bursts in a spike train, quantifying burst intensity (surprise), bursts per 1000 spikes, spikes per burst and burst length. Autocorrelation techniques were used to assess the dependencies of spike trains on the temporal order of intervals. Striatal units had a greater tendency to burst (79% of units) than pallidal units (50%). The caudate nucleus and putamen had nearly identical burst properties on all measures. In the pallidum, bursting was more prevalent in the external segment and bursts were more intense and more frequent than in the internal segment. The autocorrelation analysis revealed that the temporal structure of the spike train was more dependent on the order of intervals in the striatum than in the pallidum. Bursting units had an increased probability of discharge after each spike and the relative refractory period was shorter in bursting units than units without bursts. Very few units exhibited cyclic discharge properties. Ablations of areas 4 and 6 in the precentral cortex demonstrated that striatal bursting was under afferent control. The putamen, which receives more cortical afferents from areas 4 and 6 than the caudate nucleus, had fewer and less intense bursts after the afferents were lesioned. Burst intensity did not change in the pallidum after the lesion. The findings indicate that bursting properties contribute to discharge variability in the basal ganglia and suggest that information transfer in the striatum may utilize bursts. In contrast, rate coding may be a more important mechanism for units in the pallidum.

Spontaneous neuronal unit activity in the primate basal ganglia and the effects of precentral cerebral cortical ablations

The discharge properties of single neuronal units in the putamen, caudate nucleus, and globus pallidus were studied in awake primates. The effects of restricted deafferentation of the striatum were determined by recording single unit activity in animals with unilateral ablation of areas 4 and 6 of Brodmann. The most striking change was on the regularity of unit firing in the putamen. Units in the normal putamen exhibited a wide range of firing rates and variability. In many units discharge rate was very slow. After the lesion, putaminal units discharged in steady spike trains with highly regular patterns of interspike intervals having on average a 63% reduction in the coefficient of variation. Contrary to expectations, average firing rates actually increased slightly (22%) from a median value of 4.88 Hz in controls to 5.95 Hz in lesioned animals. Although the rates and variability observed in lesioned animals completely overlapped the range of the sample observed in controls, the distributions were shifted such that there were more units with regular discharge patterns and slightly faster firing rates. The caudate nucleus showed no significant change in firing rate or variability. In the globus pallidus, firing rate decreased significantly in the internal segment, and both segments showed an increase in discharge variability. The findings demonstrate that the cerebral cortex strongly influences the spontaneous discharge properties in the basal ganglia. The effects on the variability of spontaneous activity are greater than on the maintenance of tonic firing.