Hemispheric asymmetries in spontaneous firing characteristics of substantia nigra pars reticulata neurons following a unilateral 6-hydroxydopamine lesion of the rat nigrostriatal pathway

Targeting parvalbumin-expressing neurons in the substantia nigra pars reticulata restores motor function in parkinsonian mice

The activity of substantia nigra pars reticulata (SNr) neurons, the main output structure of basal ganglia, is altered in Parkinson's disease (PD). However, neither the underlying mechanisms nor the type of neurons responsible for PD-related motor dysfunctions have been elucidated yet. Here, we show that parvalbumin-expressing SNr neurons (SNr-PV+) occupy dorsolateral parts and possess specific electrophysiological properties compared with other SNr cells. We also report that only SNr-PV+ neurons' intrinsic excitability is reduced by downregulation of sodium leak channels in a PD mouse model. Interestingly, in anesthetized parkinsonian mice in vivo, SNr-PV+ neurons display a bursty pattern of activity dependent on glutamatergic tone. Finally, we demonstrate that chemogenetic inhibition of SNr-PV+ neurons is sufficient to alleviate motor impairments in parkinsonian mice. Overall, our findings establish cell-type-specific dysfunction in experimental parkinsonism in the SNr and provide a potential cellular therapeutic target to alleviate motor symptoms in PD.

Oxytocin-conjugated saporin injected into the substantia nigra of male rats alters the activity of the nigrostriatal dopaminergic system: A behavioral and neurochemical study

Saporin conjugated to oxytocin (OXY-SAP) destroys neurons expressing oxytocinergic receptors. When injected unilaterally in the substantia nigra of male rats, OXY-SAP causes a dose-dependent decrease up to 55 % in nigral Tyrosine Hydroxylase (TH)-immunoreactivity compared to control mock peptide BLANK-SAP- and PBS-treated rats or the contralateral substantia nigra. TH decrease was parallel to a dopamine content decrease in the ipsilateral striatum compared to BLANK-SAP- or PBS-treated rats or the contralateral striatum. OXY-SAP-treated rats showed a small but significant increase of locomotor activity 28 days after intranigral injection in the Open field test compared to BLANK-SAP- or PBS-treated rats, in line with an inhibitory role of nigral oxytocin on locomotor activity. OXY-SAP-, but not BLANK-SAP- or PBS-treated rats, also showed marked dose-dependent rotational turning ipsilateral to the injected substantia nigra when challenged with d-amphetamine, but not with apomorphine. Under isoflurane anesthesia OXY-SAP-treated rats showed levels of extracellular dopamine in the dialysate from the ipsilateral striatum only half those of BLANK-SAP- or PBS-treated rats or the contralateral striatum. When treated with d-amphetamine, OXY-SAP_60/120 rats showed increased extracellular dopamine levels in the dialysate from the ipsilateral striatum two third/one third only of those found in BLANK-SAP- or PBS-treated rats or the contralateral striatum, respectively. These results show that OXY-SAP destroys nigrostriatal dopaminergic neurons expressing oxytocin receptors leading to a reduced striatal dopamine function.

Metformin regulates astrocyte reactivity in Parkinson's disease and normal aging

Parkinson's disease (PD) is a neurodegenerative disease characterized by the progressive loss of dopaminergic neurons in the substantia nigra, leading to motor symptoms. Despite the remarkable improvements in the management of PD in recent decades, many patients remain significantly disabled. Metformin is a primary medication for the management of type 2 diabetes. We previously showed that co-treatment with metformin and 3,4-dihydroxyphenyl-l-alanine (l-DOPA) prevented the development of l-DOPA-induced dyskinesia in a 6-hydroxydopamine (6-OHDA)-lesioned animal model of PD. However, effects of metformin on PD- and aging-induced genes in reactive astrocytes remain unknown. In this study, we assessed the effect of metformin on motor function, neuroprotection, and reactive astrocytes in the 6-OHDA-induced PD animal model. In addition, the effects of metformin on the genes expressed by specific types of astrocytes were analyzed in PD model and aged mice. Here, we showed that metformin treatment effectively improves the motor symptoms in the 6-OHDA-induced PD mouse model, whereas metformin had no effect on tyrosine hydroxylase-positive neurons. The activation of AMPK and BDNF signaling pathways was induced by metformin treatment on the 6-OHDA-lesioned side of the striatum. Metformin treatment caused astrocytes to alter reactive genes in a PD animal model. Moreover, aging-induced genes in reactive astrocytes were effectively regulated or suppressed by metformin treatment. Taken together, these results suggest that metformin should be evaluated for the treatment of Parkinson's disease and related neurologic disorders characterized by astrocyte activation.

Proliferation of Inhibitory Input to the Substantia Nigra in Experimental Parkinsonism

The substantia nigra pars reticulata (SNr) is one of the output nuclei of the basal ganglia (BG) and plays a vital role in movement execution. Death of dopaminergic neurons in the neighboring nucleus, the substantia nigra pars compacta (SNc), leads to Parkinson's disease. The ensuing dopamine depletion affects all BG nuclei. However, the long-term effects of dopamine depletion on BG output are less characterized. In this in vitro study, we applied electrophysiological and immunohistochemical techniques to investigate the long-term effects of dopamine depletion on GABAergic transmission to the SNr. The findings showed a reduction in firing rate and regularity in SNr neurons after unilateral dopamine depletion with 6-OHDA, which we associate with homeostatic mechanisms. The strength of the GABAergic synapses between the globus pallidus (GP) and the SNr increased but not their short-term dynamics. Consistent with this observation, there was an increase in the frequency and amplitude of spontaneous inhibitory synaptic events to SNr neurons. Immunohistochemistry revealed an increase in the density of vGAT-labeled puncta in dopamine depleted animals. Overall, these results may suggest that synaptic proliferation can explain how dopamine depletion augments GABAergic transmission in the SNr.

Synchronized activation of striatal direct and indirect pathways underlies the behavior in unilateral dopamine-depleted mice

For more than three decades it has been known, that striatal neurons become hyperactive after the loss of dopamine input, but the involvement of dopamine (DA) D1‐ or D2‐receptor‐expressing neurons has only been demonstrated indirectly. By recording neuronal activity using fluorescent calcium indicators in D1 or D2 eGFP‐expressing mice, we showed that following dopamine depletion, both types of striatal output neurons are involved in the large increase in neuronal activity generating a characteristic cell assembly of particular neurons that dominate the pattern. When we expressed channelrhodopsin in all the output neurons, light activation in freely moving animals, caused turning like that following dopamine loss. However, if the light stimulation was patterned in pulses the animals circled in the other direction. To explore the neuronal participation during this stimulation we infected normal mice with channelrhodopsin and calcium indicator in striatal output neurons. In slices made from these animals, continuous light stimulation for 15 s induced many cells to be active together and a particular dominant group of neurons, whereas light in patterned pulses activated fewer cells in more variable groups. These results suggest that the simultaneous activity of a large dominant group of striatal output neurons is intimately associated with parkinsonian symptoms.

Acute dopamine receptor blockade in substantia nigra pars reticulata: a possible model for drug-induced Parkinsonism

Dopamine (DA) depletion modifies the firing pattern of neurons in the substantia nigra pars reticulata (SNr), shifting their mostly tonic firing toward irregularity and bursting, traits of pathological firing underlying rigidity and postural instability in Parkinson's disease (PD) patients and animal models of Parkinsonism (PS). Drug-induced Parkinsonism (DIP) represents 20-40% of clinical cases of PS, becoming a problem for differential diagnosis, and is still not well studied with physiological tools. It may co-occur with tardive dyskinesia. Here we use in vitro slice preparations including the SNr to observe drug-induced pathological firing by using drugs that most likely produce it, DA-receptor antagonists (SCH23390 plus sulpiride), to compare with firing patterns found in DA-depleted tissue. The hypothesis is that SNr firing would be similar under both conditions, a prerequisite to the proposal of a similar preparation to test other DIP-producing drugs. Firing was analyzed with three complementary metrics, showing similarities between DA depletion and acute DA-receptor blockade. Moreover, blockade of either nonselective cationic channels or Ca_v3 T-type calcium channels hyperpolarized the membrane and abolished bursting and irregular firing, silencing SNr neurons in both conditions. Therefore, currents generating firing in control conditions are in part responsible for pathological firing. Haloperidol, a DIP-producing drug, reproduced DA-receptor antagonist firing modifications. Since acute DA-receptor blockade induces SNr neuron firing similar to that found in the 6-hydroxydopamine model of PS, output basal ganglia neurons may play a role in generating DIP. Therefore, this study opens the way to test other DIP-producing drugs. NEW & NOTEWORTHY Dopamine (DA) depletion enhances substantia nigra pars reticulata (SNr) neuron bursting and irregular firing, hallmarks of Parkinsonism. Several drugs, including antipsychotics, antidepressants, and calcium channel antagonists, among others, produce drug-induced Parkinsonism. Here we show the first comparison between SNr neuron firing after DA depletion vs. firing found after acute blockade of DA receptors. It was found that firing in both conditions is similar, implying that pathological SNr neuron firing is also a physiological correlate of drug-induced Parkinsonism.

Short- and long-term unilateral 6-hydroxydopamine lesions in rats show different changes in characteristics of spontaneous firing of substantia nigra pars reticulata neurons

The unilateral 6-hydroxydopamine (6-OHDA) lesion of the medial forebrain bundle induces hemiparkinsonism in rats and is a well established animal model of Parkinson's disease. In this study, we assessed the spontaneous activity of substantia nigra pars reticulata (SNr) neurons in unilateral 6-OHDA- or sham-treated rats. Extracellular single cell recordings revealed a bilaterally decreased firing rate in short-term 6-OHDA-lesioned rats (8-10 weeks post lesion) while no rate differences were evident in long-term lesioned animals (5-8 months post lesion) in vivo under chloral hydrate anaesthesia. However, firing pattern of the SNr neurons (indicated by interspike interval (ISI) histogram parameters: coefficient of variation, skewness and kurtosis) was significantly altered only after long-term lesion: 53.8 % of the recorded cells in the ipsilateral 6-OHDA-lesioned SNr fired in a bursting pattern (compared to 5.9-16.7 % in contralateral SNr or sham controls). Additionally, behavioural effects of the lesion were assessed 4 weeks post lesion by the forelimb adjusting stepping test. A decreased number of adjusting steps with the contralateral forepaw, as well as an increased performance with the ipsilateral paw was found for the 6-OHDA-lesioned rats as compared to sham controls. Furthermore, stepping values were negatively correlated with the ISI parameters after long-term lesion, while there were no correlations with the short-term groups. Firing rate was not correlated regardless of the time frame. In conclusion, long-term changes in firing pattern may represent a neuronal correlate of the 6-OHDA-induced hemiparkinsonism and may be useful for the interpretation of 6-OHDA-induced motor deficits and compensatory mechanisms as well.

Abnormal burst patterns of single neurons recorded in the substantia nigra reticulata of behaving 140 CAG Huntington's disease mice

Huntington's disease (HD) is an inherited neurodegenerative disorder that causes neurological pathology in the basal ganglia and related circuitry. A key site of HD pathology is striatum, the principal basal ganglia input structure; striatal pathology likely changes basal ganglia output but no existing studies address this issue. In this report, we characterize single-neuron activity in the substantia nigra reticulata (SNr) of awake, freely behaving 140 CAG knock-in (KI) mice at 16-40 weeks. KI mice are a well characterized model of adult HD and are mildly symptomatic in this age range. As the primary basal ganglia output nucleus in rodents, the SNr receives direct innervation from striatum, as well as indirect influence via polysynaptic inputs. We analyzed 32 single neurons recorded from KI animals and 44 from wild-type (WT) controls. We found increased burst rates, without a concordant change in spike discharge rate, in KI animals relative to WTs. Furthermore, although metrics of burst structure, such as the inter-spike interval in bursts, do not differ between groups, burst rate increases with age in KI, but not WT, animals. Our findings suggest that altered basal ganglia output is a physiological feature of early HD pathology.

A quantitative comparison of basal ganglia neuronal activities of normal and Parkinson's disease model rats

The purpose of this study was to identify consistent characteristic changes of neuronal activity in basal ganglia (BG) nuclei associated with Parkinson's disease (PD) so that a reliable index of PD can be derived. A simple algorithm for automatic identification of firing patterns was devised as an essential tool to achieve this goal. A detailed quantitative analysis of firing patterns as well as firing rate was performed in three BG nuclei: the subthalamic nucleus (STN), the substantia nigra pars reticulate (SNpr), and the globus pallidus (GP). The results showed that the firing rate of STN neurons was not significantly altered in PD model rats. We also did not find a significant alteration in firing rates in the SNpr and GP between normal and PD model rats. In contrast, consistent changes of firing patterns were observed in all three BG nuclei in that the percentage of neurons with a regular firing pattern decreased whereas those with irregular, mixed, or burst patterns increased. This enables a simple algorithm based on burst detection and the shape of the interspike interval histogram to identify whether the neuronal activity is from normal or PD model rats.

The interhemispheric connections of the striatum: Implications for Parkinson's disease and drug-induced dyskinesias

Parkinson's disease (PD) is characterized by loss of nigrostriatal neurons and depletion of dopamine. This pathological feature leads to alterations to basal ganglia circuitry and subsequent motor disability. Pharmacological dopamine replacement therapy with medications such as levodopa ameliorates the symptoms of PD but can lead to motor complications known as drug-induced dyskinesias. We have recently shown that clinically hemiparkinsonian rhesus monkeys do not develop levodopa-induced dyskinesias despite chronic intermittent exposure and significant unilateral loss of nigrostriatal neurons and dopamine. It is currently unclear what mechanisms prevent the onset of dyskinesias in these animals. Based on our study and results from previous lesioning studies in both the rat and monkey models of PD, we hypothesize that one potential mechanism that may prevent the genesis of dyskinesias in these animals is interhemispheric neuromodulation. Two potential interhemispheric connections that may modulate dyskinesias are the interhemispheric nigrostriatal and corticostriatal pathways. Few investigators have examined the interhemispheric nigrostriatal and corticostriatal connections and the functional role they may play in drug-induced dyskinesias in PD. Therefore, in the following review, we assess the neuroanatomical, electrophysiological and behavioral properties of these interhemispheric connections. Future studies evaluating these interhemispheric striatal pathways and the pathophysiological changes that occur to these pathways in the dyskinetic state are warranted to further develop treatments that prevent or mitigate drug-induced dyskinesias in PD.

The effect of striatal dopaminergic grafts on the neuronal activity in the substantia nigra pars reticulata and subthalamic nucleus in hemiparkinsonian rats

The electrophysiological correlates of parkinsonism in the basal ganglia have been well studied in patients with Parkinson's disease and animal models. Separately, striatal dopaminergic cell transplantation has shown promise in ameliorating parkinsonian motor symptoms. However, the effect of dopaminergic grafts on basal ganglia electrophysiology has not thoroughly been investigated. In this study, we transplanted murine foetal ventral mesencephalic cells into rats rendered hemiparkinsonian by 6-hydroxydopamine injection. Three months after transplantation, extracellular and local field potential recordings were taken under urethane anaesthesia from the substantia nigra pars reticulata and subthalamic nucleus along with cortical electroencephalograms and were compared to recordings from normal and hemiparkinsonian controls. Recordings from cortical slow-wave activity and global activation states were analysed separately. Rats with histologically confirmed xenografts showed behavioural improvement measured by counting apomorphine-induced rotations and with the extended body axis test. Firing rates in both nuclei were not significantly different between control and grafted groups. However, burst firing patterns in both nuclei in the slow-wave activity state were significantly reduced (P < 0.05) in rats with large surviving grafts, compared to hemiparkinsonian controls. The neuronal firing entropies and oscillations in both nuclei were restored to normal levels in the large-graft group. Electroencephalogram spike-triggered averages also showed normalization in the slow-wave activity state (P < 0.05). These results suggest that local continuous dopaminergic stimulation exerts a normalizing effect on the downstream parkinsonian basal ganglia firing patterns. This novel finding is relevant to future preclinical and clinical investigations of cell transplantation and the development of next-generation therapies for Parkinson's disease that ameliorate pathophysiological neural activity and provide optimal recovery of function.

In-vivo deep brain recordings of intranigral grafted cells in a mouse model of Parkinson's disease

We recently showed that intranigral transplantation of embryonic neurons in a mouse model of Parkinson's disease led to anatomical and functional recovery of the nigrostriatal pathway. Here we report, in-vivo electrophysiological characteristics of these grafted neurons 2 months after transplantation. Extracellular activity was mapped within the transplant using microarray electrodes and exploration was done with antidromic and orthodromic striatal stimulation. Grafted neurons expressed spontaneous electrophysiological activity with dopaminergic-like characteristics, and antidromic and orthodromic responses suggest a functional recovery of the nigrostriatal loop.

Changes in firing rate and pattern of GABAergic neurons in subregions of the substantia nigra pars reticulata in rat models of Parkinson's disease

The substantia nigra pars reticulata (SNr) plays a key role in the pathophysiology of Parkinson's disease (PD). It has been well documented that the SNr is not a homogeneous structure, and the lateral and medial subregions of the SNr receive different projections from the sensorimotor and limbic striatum, respectively. However, specific changes in firing activity of SNr subregions in PD remain unclear. In the present study, the spontaneous firing activity of GABAergic neurons in the lateral and medial SNr of rats with unilateral 6-hydroxydopamine lesions of the substantia nigra pars compacta (SNc) or medial forebrain bundle (MFB) has been examined. Extracellular recordings indicated that the firing rate of lateral SNr neurons increased significantly and firing pattern of these neurons changed towards more irregular and bursty after SNc or MFB lesions compared to normal rats. In contrast, the firing rate and pattern of medial SNr neurons in rats with SNc lesions were unaltered when compared with that of normal rats. However, MFB lesions in rats decreased the firing rate of medial SNr neurons and firing pattern of these neurons changed towards more bursty. In addition, SNc lesions in rats increased the firing rate of the neurons with regular and irregular firing patterns within lateral but not in medial SNr, while the firing rate of the neurons within lateral and medial SNr with each firing pattern was not altered after MFB lesions. These results suggest that GABAergic neurons of SNr subregions have differential change of firing activity in the pathophysiology of PD.

Beta frequency synchronization in basal ganglia output during rest and walk in a hemiparkinsonian rat

Synchronized oscillatory neuronal activity in the beta frequency range has been observed in the basal ganglia of Parkinson's disease patients and hypothesized to be antikinetic. The unilaterally lesioned rat model of Parkinson's disease allows examination of this hypothesis by direct comparison of beta activity in basal ganglia output in non-lesioned and dopamine cell lesioned hemispheres during motor activity. Bilateral substantia nigra pars reticulata (SNpr) recordings of units and local field potentials (LFP) were obtained with EMG activity from the scapularis muscle in control and unilaterally nigrostriatal lesioned rats trained to walk on a rotary treadmill. After left hemispheric lesion, rats had difficulty walking contraversive on the treadmill but could walk in the ipsiversive direction. During inattentive rest, SNpr LFP power in the 12-25 Hz range (low beta) was significantly greater in the dopamine-depleted hemisphere than in non-lesioned and control hemispheres. During walking, low beta power was reduced in all hemispheres, while 25-40 Hz (high beta) activity was selectively increased in the lesioned hemisphere. High beta power increases were reduced by l-DOPA administration. SNpr spiking was significantly more synchronized with SNpr low beta LFP oscillations during rest and high beta LFP oscillations during walking in the dopamine-depleted hemispheres compared with non-lesioned hemispheres. Data show that dopamine loss is associated with opposing changes in low and high beta range SNpr activity during rest and walk and suggest that increased synchronization of high beta activity in SNpr output from the lesioned hemisphere during walking may contribute to gait impairment in the hemiparkinsonian rat.

Dissociable effects of dopamine on neuronal firing rate and synchrony in the dorsal striatum

Previous studies showed that dopamine depletion leads to both changes in firing rate and in neuronal synchrony in the basal ganglia. Since dopamine D1 and D2 receptors are preferentially expressed in striatonigral and striatopallidal medium spiny neurons, respectively, we investigated the relative contribution of lack of D1 and/or D2-type receptor activation to the changes in striatal firing rate and synchrony observed after dopamine depletion. Similar to what was observed after dopamine depletion, co-administration of D1 and D2 antagonists to mice chronically implanted with multielectrode arrays in the striatum caused significant changes in firing rate, power of the local field potential (LFP) oscillations, and synchrony measured by the entrainment of neurons to striatal local field potentials. However, although blockade of either D1 or D2 type receptors produced similarly severe akinesia, the effects on neural activity differed. Blockade of D2 receptors affected the firing rate of medium spiny neurons and the power of the LFP oscillations substantially, but it did not affect synchrony to the same extent. In contrast, D1 blockade affected synchrony dramatically, but had less substantial effects on firing rate and LFP power. Furthermore, there was no consistent relation between neurons changing firing rate and changing LFP entrainment after dopamine blockade. Our results suggest that the changes in rate and entrainment to the LFP observed in medium spiny neurons after dopamine depletion are somewhat dissociable, and that lack of D1- or D2-type receptor activation can exert independent yet interactive pathological effects during the progression of Parkinson's disease.

Cortical lesion-induced visual hemineglect is prevented by NMDA antagonist pretreatment

Large unilateral visual cortex lesions produce enduring contralesional visual orientation deficits. To examine whether glutamate excitotoxicity is involved in establishing these deficits, cats were pretreated with the NMDA receptor antagonist dizocilpine (MK-801) 30 min before unilateral visual cortex ablation. Pretreated MK-801 animals were trained first in an orientation task in which they were required to fixate directly ahead and then orient to stimuli introduced at various eccentricities throughout the visual field. They did not display the characteristic ipsilesional head and neck asymmetries and/or spontaneous ipsiversive rotational behaviors or show the profound contralesional visual neglect seen postoperatively in nonpretreated control animals. Rather, pretreated animals were able to orient to visual stimuli in the contralesional hemifield immediately following surgical recovery. Postmortem histology revealed severe retrograde degeneration of the ipsilesional lateral geniculate nucleus in both experimental groups, suggesting that postlesion visuomotor behavioral competencies in pretreated animals are attributable to preserved function in nongeniculocortical visual pathways. These observations are consistent with the hypothesis that visual cortex lesions normally induce secondary alterations via NMDA-mediated excitotoxicity in these other pathways that prevents them from supporting visuomotor behaviors. The similar behavioral competencies of MK-801-pretreated animals and those whose lesion-induced deficits are ameliorated by removing basal ganglia afferents to the ipsilesional superior colliculus are consistent with this hypothesis and highlight the normal functional capabilities of this circuit. It is likely that MK-801 pretreatment acts, at least in part, by preserving the normal interhemispheric control dynamics with which the basal ganglia influence superior colliculus-mediated orientation behaviors.

Parafascicular thalamic nucleus activity in a rat model of Parkinson's disease

Parkinson's disease is associated with increased oscillatory firing patterns in basal ganglia output, which are thought to disrupt thalamocortical activity. However, it is unclear how specific thalamic nuclei are affected by these changes in basal ganglia activity. The thalamic parafascicular nucleus (PFN) receives input from basal ganglia output nuclei and directly projects to the subthalamic nucleus (STN), striatum and cortex; thus basal ganglia-mediated changes on PFN activity may further impact basal ganglia and cortical functions. To investigate the impact of increased oscillatory activity in basal ganglia output on PFN activity after dopamine cell lesion, PFN single-unit and local field potential activities were recorded in neurologically intact (control) rats and in both non-lesioned and dopamine lesioned hemispheres of unilateral 6-hydroxydopamine lesioned rats anesthetized with urethane. Firing rates were unchanged 1-2 weeks after lesion; however, significantly fewer spontaneously active PFN neurons were evident. Firing pattern assessments after lesion showed that a larger proportion of PFN spike trains had 0.3-2.5 Hz oscillatory activity and significantly fewer spike trains exhibited low threshold calcium spike (LTS) bursts. In paired recordings, more PFN-STN spike oscillations were significantly correlated, but as these oscillations were in-phase, results are inconsistent with feedforward control of PFN activity by inhibitory oscillatory basal ganglia output. Furthermore, the decreased incidence of LTS bursts is incompatible with inhibitory basal ganglia output inducing rebound bursting in PFN after dopamine lesion. Together, results show that robust oscillatory activity observed in basal ganglia output nuclei after dopamine cell lesion does not directly drive changes in PFN oscillatory activity.

Mean-field modeling of the basal ganglia-thalamocortical system. I Firing rates in healthy and parkinsonian states

Parkinsonism leads to various electrophysiological changes in the basal ganglia-thalamocortical system (BGTCS), often including elevated discharge rates of the subthalamic nucleus (STN) and the output nuclei, and reduced activity of the globus pallidus external (GPe) segment. These rate changes have been explained qualitatively in terms of the direct/indirect pathway model, involving projections of distinct striatal populations to the output nuclei and GPe. Although these populations partly overlap, evidence suggests dopamine depletion differentially affects cortico-striato-pallidal connection strengths to the two pallidal segments. Dopamine loss may also decrease the striatal signal-to-noise ratio, reducing both corticostriatal coupling and striatal firing thresholds. Additionally, nigrostriatal degeneration may cause secondary changes including weakened lateral inhibition in the GPe, and mesocortical dopamine loss may decrease intracortical excitation and especially inhibition. Here a mean-field model of the BGTCS is presented with structure and parameter estimates closely based on physiology and anatomy. Changes in model rates due to the possible effects of dopamine loss listed above are compared with experiment. Our results suggest that a stronger indirect pathway, possibly combined with a weakened direct pathway, is compatible with empirical evidence. However, altered corticostriatal connection strengths are probably not solely responsible for substantially increased STN activity often found. A lower STN firing threshold, weaker intracortical inhibition, and stronger striato-GPe inhibition help explain the relatively large increase in STN rate. Reduced GPe-GPe inhibition and a lower GPe firing threshold can account for the comparatively small decrease in GPe rate frequently observed. Changes in cortex, GPe, and STN help normalize the cortical rate, also in accord with experiments. The model integrates the basal ganglia into a unified framework along with an existing thalamocortical model that already accounts for a wide range of electrophysiological phenomena. A companion paper discusses the dynamics and oscillations of this combined system.

Altered neuronal activity relationships between the pedunculopontine nucleus and motor cortex in a rodent model of Parkinson's disease

The pedunculopontine nucleus (PPN) is a new deep brain stimulation (DBS) target for Parkinson's disease (PD), but little is known about PPN firing pattern alterations in PD. The anesthetized rat is a useful model for investigating the effects of dopamine loss on the transmission of oscillatory cortical activity through basal ganglia structures. After dopamine loss, synchronous oscillatory activity emerges in the subthalamic nucleus and substantia nigra pars reticulata in phase with cortical slow oscillations. To investigate the impact of dopamine cell lesion-induced changes in basal ganglia output on activity in the PPN, this study examines PPN spike timing with reference to motor cortex (MCx) local field potential (LFP) activity in urethane- or ketamine-anesthetized rats. Seven to ten days after unilateral 6-hydroxydopamine lesion of the medial forebrain bundle, spectral power in PPN spike trains and coherence between PPN spiking and PPN LFP activity increased in the approximately 1 Hz range in urethane-anesthetized rats. PPN spike timing also changed from firing predominantly in phase with MCx slow oscillations in the intact urethane-anesthetized rat to firing predominantly antiphase to MCx oscillations in the hemi-parkinsonian rat. These changes were not observed in the ketamine-anesthetized preparation. These observations suggest that dopamine loss alters PPN spike timing by increasing inhibitory oscillatory input to the PPN from basal ganglia output nuclei, a phenomenon that may be relevant to motor dysfunction and PPN DBS efficacy in PD patients.

Synchronous oscillations and phase reorganization in the basal ganglia during akinesia induced by high-dose haloperidol

Movement disorders such as tremor and akinesia observed in Parkinson's disease have been attributed to dopamine (DA) depletion in the basal ganglia. The changes in subcortical neuronal discharge patterns that follow DA depletion have been a matter of much discussion. Here, we implanted rats with chronic recording electrodes bilaterally in the striatum (CPu) and external globus pallidus (GPe), and induced both acute and repeated DA blockade by administration of high-dose haloperidol. Recordings were made in baseline states, as well as before and after haloperidol injections, which rendered rats akinetic. The immediate physiological effect of pharmacological DA blockade was the development of prominent oscillatory firing in the 6-8 Hz range in both CPu and GPe. Importantly, this oscillatory pattern was not accompanied by consistent changes in the firing rate of either CPu or GPe neurons. Cross-correlation analysis further indicated that neurons within the CPu and GPe fired synchronously after DA blockade. Furthermore, although phase lags between neuronal discharges in the GPe and CPu were uniformly distributed prior to haloperidol administration, CPu significantly lagged GPe discharges after repeated DA blockade. Our results demonstrate that acute DA blockade is sufficient to produce synchronous oscillatory activity across basal ganglia neuron populations, and that prolonged DA blockade results in phase lag changes in pallidostriatal synchrony.

Lesion of subthalamic nucleus in parkinsonian rats : effects of dopamine d(1) and d(2) receptor agonists on the neuronal activities of the substantia nigra pars reticulata

OBJECTIVE

It was hypothesized that dopamine agonist administration and subthalamic nucleus (STN) lesion in the rat might have a synergistic effect on the neuronal activities of substantia nigra pars reticulata (SNpr) as observed in patients with Parkinson's disease. The effects of SKF38393 (a D(1) receptor agonist) and Quinpirole (a D(2) receptor agonist) were compared in parkinsonian rat models with 6- hydroxydopamine (6-OHDA) after STN lesion.

METHODS

SKF38393 and Quinpirole were consecutively injected intrastriatally. SNpr was microrecorded to ascertain the activity of the basal ganglia output structure. The effect of SKF38393 or Quinpirole injection on the firing rate and firing patterns of SNpr was investigated in medial forebrain bundle (MFB) lesioned rats and in MFB+STN lesioned rats.

RESULTS

The administration of SKF38393 decreased SNpr neuronal firing rates and the percentage of burst neurons in the MFB lesioned rats, but did not alter them in MFB+STN lesioned rats. The administration ofQuinpirole significantly decreased the spontaneous firing rate in the MFB lesioned rats. However, after an additional STN lesion, it increased the percentage of burst neurons.

CONCLUSION

This study demonstrated that dopamine agonists and STN lesion decreased the hyperactive firing rate and the percentage of burst neurons of SNpr neurons in 6-OHDA lesioned rats, respectively. Quinpirole with STN lesion increased a percentage of burst neurons. To clear the exact interactive mechanism of D(1) and D(2) agonist and the corresponding location, it should be followed a study using a nonselective dopamine agonist and D(1), D(2) selective antagonist.

Phase relationships support a role for coordinated activity in the indirect pathway in organizing slow oscillations in basal ganglia output after loss of dopamine

The goal of the present study was to determine the phase relationships of the slow oscillatory activity that emerges in basal ganglia nuclei in anesthetized rats after dopamine cell lesion in order to gain insight into the passage of this oscillatory activity through the basal ganglia network. Spike train recordings from striatum, subthalamic nucleus (STN), globus pallidus (GP), and substantia nigra pars reticulata (SNpr) were paired with simultaneous local field potential (LFP) recordings from SNpr or motor cortex ipsilateral to a unilateral lesion of substantia nigra dopamine neurons in urethane-anesthetized rats. Dopamine cell lesion induced a striking increase in incidence of slow oscillations (0.3-2.5 Hz) in firing rate in all nuclei. Phase relationships assessed through paired recordings using SNpr LFP as a temporal reference showed that slow oscillatory activity in GP spike trains is predominantly antiphase with oscillations in striatum, and slow oscillatory activity in STN spike trains is in-phase with oscillatory activity in cortex but predominantly antiphase with GP oscillatory activity. Taken together, these results imply that after dopamine cell lesion in urethane-anesthetized rats, increased oscillatory activity in GP spike trains is shaped more by increased phasic inhibitory input from the striatum than by phasic excitatory input from STN. In addition, results show that oscillatory activity in SNpr spike trains is typically antiphase with GP oscillatory activity and in-phase with STN oscillatory activity. While these observations do not rule out additional mechanisms contributing to the emergence of slow oscillations in the basal ganglia after dopamine cell lesion in the anesthetized preparation, they are compatible with 1) increased oscillatory activity in the GP facilitated by an effect of dopamine loss on striatal 'filtering' of slow components of oscillatory cortical input, 2) increased oscillatory activity in STN spike trains supported by convergent antiphase inhibitory and excitatory oscillatory input from GP and cortex, respectively, and 3) increased oscillatory activity in SNpr spike trains organized by convergent antiphase inhibitory and excitatory oscillatory input from GP and STN, respectively.

Neural responses in multiple basal ganglia regions following unilateral dopamine depletion in behaving rats performing a treadmill locomotion task

To investigate basal ganglia (BG) neural responses to dopamine (DA) depletion, multiple channel, single unit recording was carried out in freely moving rats performing a treadmill locomotion task. Single unit activity from 64 microelectrodes in the striatum (STR), globus pallidus (GP), subthalamic nucleus (STN) and substantia nigra pars reticulata (SNr) was recorded simultaneously before and after a unilateral DA lesion induced by microinjection of 6-hydroxydopamine (6-OHDA) into the medial forebrain bundle. The DA lesion resulted in an impairment of treadmill walking manifested by a significant decrease in swing time of both forelimbs. The stance time, however, increased significantly only in ipsilateral (good) forelimbs, reflecting compensatory changes in the good limb for motor deficits. Neural activity in the STR and GP ipsilateral to the lesion decreased during the 7-day period following the DA lesion. Conversely, an increase in spike discharges appeared in the ipsilateral SNr and STN several days after the DA lesion. Changes in the type of neural response associated with treadmill locomotion were also found in some neurons after DA depletion. Such changes were most prominent in the STR. Limb movement-related neural activity increased significantly mainly in the SNr. Additionally, neural responses to the tone cue associated with the onset of the treadmill diminished greatly in the lesioned side of the BG. Increased activity in SNr neurons is consistent with the concept that inhibition of thalamus contributes to hypokinesis in the absence of DA. Substantial decrease in striatal activity supports a concept that DA loss leads to a global suppression of recurrent cortical striatal thalamic activity that degrades normal information flow in Parkinson's diseases.

MK801 and amantadine exert different effects on subthalamic neuronal activity in a rodent model of Parkinson's disease

Efforts to develop adjuvant therapies for the treatment of Parkinson's disease (PD) have led to interest in drugs that could mimic the therapeutic effects of lesion or deep brain stimulation of the subthalamic nucleus (STN). Extracellular single unit recordings were conducted to determine whether noncompetitive NMDA receptor blockade, suggested to have potential as an adjuvant treatment in PD, attenuates rate increases and firing pattern changes observed in the STN in a rodent model of PD. Systemic administration of the noncompetitive NMDA antagonist MK801 to rats with unilateral dopamine cell lesions did not significantly alter burstiness or interspike interval coefficient of variation, although mean firing rate decreased by a modest 20% with 50% of neurons showing decreases in rate >15% and spike train power in the 3-8-Hz (theta) range was reduced. MK801, combined with the D1 dopamine agonist SKF 38393 in intact rats or administered alone in lesioned rats, also significantly reduced incidence of multisecond (2-60 s) periodic oscillatory activity. Amantadine, a drug currently used as an adjuvant agent in PD whose beneficial effects are commonly attributed to its noncompetitive NMDA antagonist properties, had effects that contrasted with those of MK801. In both intact and lesioned animals, amantadine significantly increased STN firing rates and total spike train power in the 8-50-Hz range and did not alter spike power in the 3-8-Hz range or multisecond oscillatory activity. These observations show that an effective noncompetitive NMDA antagonist such as MK801 induces modest change in STN activity in 6-hydroxydopamine (6-OHDA)-lesioned rats, with the most notable effect on multisecond periodicities in firing rate and theta frequency total spike power. Amantadine's effects differed from MK801's, raising questions about its primary mechanism of action and the role in PD pharmacotherapy of the STN rate increases induced by this drug.

Response of the GABAergic and dopaminergic mesostriatal projections to the lesion of the contralateral dopaminergic mesostriatal pathway in the rat

Although dopamine is the main neurotransmitter in the mesostriatal system, recent studies indicate the existence of two nigrostriatal GABAergic projections: one arising from neurons immunoreactive for GABA, glutamic acid decarboxylase (GAD67), and parvalbumin (PV) lying in the substantia nigra pars reticulata (nigrostriatal GABA cells) and the other arising from a subpopulation of dopaminergic neurons lying in the substantia nigra pars compacta and ventral tegmental area, which under normal conditions, contains mRNA for GAD65 (one of the two isoforms of glutamic acid decarboxylase), but which is not immunoreactive for GABA and GAD65 (nigrostriatal dopaminergic [DA]/GABA cells). With the aim of improving our knowledge about the interaction between the nigrostriatal system of both brain hemispheres, we have studied the response of these three components of the mesostriatal system (GABA, DA/GABA, and DA) to the lesion of the contralateral mesostriatal DA pathway, by using morphological and neurophysiological techniques. Our findings show that, in the side contralateral to the lesion, (1) the number of nigrostriatal GABA cells increases from 6% to 17% with respect to the total number of nigrostriatal cells, (2) the soma of DA/GABA cells becomes immunoreactive for GABA and GAD65, and (3) there is an increase in the firing rate and burst activity of DA-neurons, except in those projecting to the striatum, which may be under the action of the GABA hyperactivity. Taken together, our results suggest that the GABAergic components of the mesostriatal projection play a regulatory role on the DA component, activated or upregulated after contralateral DA lesion and are probably addressed to restoring the functional symmetry in basal ganglia and to slowing down the contralateral progression of DA-cell degeneration in Parkinson's disease.

Dopamine receptor supersensitivity in rat subthalamus after 6-hydroxydopamine lesions

The subthalamic nucleus (STN) receives direct dopaminergic innervation from the substantia nigra pars compacta, but the importance of this input in the pathophysiology of parkinsonism remains to be determined. We used whole-cell patch-clamp recordings in brain slices to study presynaptic dopaminergic modulation of synaptic inputs to the STN in unilateral 6-hydroxydopamine (6-OHDA)-lesioned rats. Here, we report that dopamine was more potent for inhibiting GABA IPSCs and glutamate EPSCs in the STN ipsilateral to the lesion, and was less potent for suppressing IPSCs and EPSCs in the STN contralateral to the lesion, compared with the effects of dopamine in control STN. Dopamine reduced IPSCs with an IC50 value of 20.9 +/- 3.6 microM in control STN, whereas IC50 values were 0.83 +/- 0.15 and 55.1 +/- 11.1 microM in STN ipsilateral and contralateral to 6-OHDA lesions, respectively. Dopamine also inhibited EPSCs with an IC50 value of 12.8 +/- 2.8 microM in control STN, whereas IC50 values were 4.5 +/- 0.9 and 41.6 +/- 9.8 microM in STN ipsilateral and contralateral to 6-OHDA lesions, respectively. Results with paired stimuli to evoke EPSCs and IPSCs suggest that endogenous dopamine acts presynaptically to inhibit transmitter release in the STN. These results show that chronic dopamine denervation significantly alters the regulation of synaptic input to the STN. Our results also suggest that the STN may be an important target for levodopa therapy in Parkinson's disease.

Dose and behavioral context dependent inhibition of movement and basal ganglia neural activity by ??9-tetrahydrocannabinol during spontaneous and treadmill locomotion tasks in rats

The effects of Delta-9-tetrahydrocannabinole (Delta-9-THC) on locomotor activities and related basal ganglia neural responses were investigated in rats. A multiple-channel, single unit recording method was used to record neuronal activity in the dorsal lateral striatum, the globus pallidus, the subthalamic nucleus, and the substantia nigra pars reticulata simultaneously during spontaneous movement and treadmill locomotion. Delta-9-THC treatment (0.05-2.0 mg/kg, i.p.) dose-dependently decreased spontaneous motor activity and altered walking patterns in treadmill locomotion in that stance time was increased and step number was decreased. In parallel with the behavioral effects, Delta-9-THC treatment inhibited neural activity across all four basal ganglia areas recorded during both motor tests. Further, this inhibition of basal ganglia neural activity was behavioral context-dependent. Greater inhibition was found during resting than during walking periods in the treadmill locomotion test. Delta-9-THC treatment also changed firing patterns in the striatum and globus pallidus. More neurons in these regions discharged in an oscillatory pattern during treadmill walking with Delta-9-THC, and the oscillatory frequency was similar to that of the step cycle. Synchronized firing patterns were found in few basal ganglia neurons in the control condition (approximately 1%). Synchronized firing patterns increased during the treadmill resting phase after Delta-9-THC treatment, but still represented a very small proportion of the total neural population (1.9%). The drug treatment did not change neural responses to the tone cue proceeding treadmill locomotion. This study demonstrates dose-dependent inhibitory effects of cannabinoid injection on motor activity. This effect may be related to the behavioral context-dependent inhibition observed in the basal ganglia system where CB1 receptors are densely distributed.

Effect of subthalamic lesion with kainic acid on the neuronal activities of the basal ganglia of rat parkinsonian models with 6-hydroxydopamine

The aim of the present study was to investigate the alteration of neuronal activities in the substantia nigra pars reticulata (SNpr) and globus pallidus (GP), after ipsilateral STN lesioning by kainic acid in the rat hemi-parkinsonian 6-hydroxydopamine (6-OHDA) model. In various rat Parkinson's disease (PD) models, an increase in the SNpr firing rate was observed, despite the occurrence of bursting patterns, and subthalamic lesion was found to reduce the mean firing rates and the percentage of bursting neurons in the SNpr. However, the relative proportion of bursting neurons, among all GP neurons, was slightly increased as a result of the subthalamic lesion. The significance of bursting activity in the SNpr and GP remains obscure. Further study is necessary to elucidate the pathophysiological mechanism behind Parkinson's disease.

Altered spontaneous discharge rate and pattern of basal ganglia output neurons in the circling (ci2) rat mutant

The circling rat is an autosomal recessive mutant (homozygous ci2/ci2) characterized by lateralized rotational behavior, locomotor hyperactivity, ataxia, stereotypic head movements, and deafness. Previous neurochemical investigations showed that ci2 rats of both genders have a lower tissue content of dopamine in the striatum ipsilateral to the preferred direction of circling. For further evaluation as to whether this striatal imbalance has functional consequences within basal ganglia structures, the spontaneous extracellular single unit activity of GABAergic neurons located in the striatum and, downstream to the dopaminergic nigrostriatal system, the substantia nigra pars reticulata (SNr) was recorded bilaterally in anesthetized ci2 rats. Heterozygous (ci2/+) littermates that display normal behavior, and rats from the background strain (LEW/Ztm) served as controls. No significant hemispheric imbalances in striatal discharge rate and firing pattern were evident in ci2 rats. Furthermore, there were no significant intergroup differences in striatal activity. However, the mean spontaneous discharge rate of SNr neurons was significantly increased in both brain sides, and there was a significant shift toward rhythmic burst-like firing in ci2 mutants. Again, no hemispheric differences were detected. The data substantiate previous findings of altered basal ganglia function in ci2 rats. The abnormal basal ganglia output activity, i.e. of the SNr, is likely to contribute to the complex behavioral disturbances seen in ci2 rats.

Effects of dopaminergic cell degeneration on electrophysiological characteristics and GAD65/GAD67 expression in the substantia nigra: different action on GABA cell subpopulations

The motor disturbances occurring in Parkinson's disease have been partially attributed to a hyperactivity of gamma-aminobutyric acid (GABA)-ergic nigral cells largely in the substantia nigra pars reticulata (SNr) secondary to the degeneration of dopaminergic nigrostriatal neurons. However, some aspects of this response remain unclear. In this work, different electrophysiological and neurochemical parameters were studied in GABAergic cells of the SN after unilateral nigrostriatal dopaminergic lesion using 6-hydroxydopamine injection in rats. Our data showed that 1) the SN under normal conditions contains different subsets of GABAergic cells according to their firing pattern and glutamic acid decarboxylase mRNA levels, and 2) the response of these GABAergic cell subgroups was different after the ipsi- and contralateral dopaminergic cell degeneration. These findings indicate a complex regulation of nigral GABAergic activity after nigrostriatal dopaminergic degeneration that probably involves local mechanisms, the nigro-striato-nigral loop, as well as interhemispheric mechanisms whose anatomical basis remains unstudied.

Response of GABAergic cells in the deep mesencephalic nucleus to dopaminergic cell degeneration: an electrophysiological and in situ hybridization study

The deep mesencephalic nucleus (DMN) is a large midbrain reticular region located between the substantia nigra compacta and the superior colliculus. It contains GABAergic cells that share striatal afferents, thalamic and collicular efferents, as well as neurochemical and electrophysiological similarities, with those of the substantia nigra reticulata. In the present paper we used electrophysiological (firing rate and firing pattern) and morphological (densitometric analysis of in situ hybridization histochemical labeling for glutamic acid decarboxylase (GAD)65 and GAD67 mRNA) techniques, to study the response of DMN GABAergic cells to the degeneration of nigral dopaminergic cells. Our results showed that unilateral dopaminergic cell loss (after injection of 6-hydroxydopamine in the medial forebrain bundle) induces a bilateral and symmetrical increase in both firing rate and GAD67 mRNA levels and a decrease in GAD65 mRNA levels. These findings support the involvement of DMN GABAergic cells in the basal ganglia modifications that follow dopaminergic cell loss, also suggesting its participation in the pathophysiology of Parkinson's disease. The symmetry of effects, together with its recently reported bilateral projections to the thalamus and superior colliculus, suggest that unlike substantia nigra reticulata, DMN is involved in the interhemispheric regulation of basal ganglia, probably keeping their functional symmetry even after asymmetric lesions.

Nigrostriatal lesion and dopamine agonists affect firing patterns of rodent entopeduncular nucleus neurons

Altered activity of the entopeduncular nucleus, the rodent homologue of the globus pallidus internal segment in primates, is thought to mediate behavioral consequences of midbrain dopamine depletion in rodents. Few studies, however, have examined dopaminergic modulation of spiking activity in this nucleus. This study characterizes changes in entopeduncular neuronal activity after nigrostriatal dopaminergic lesion and the effects of systemic treatment with selective D(1) (SKF 38393) and D(2) (quinpirole) agonists in lesioned rats. Extracellular single-unit recordings were performed in awake immobilized rats, either in neurologically intact animals (n = 42) or in animals that had received unilateral 6-hydroxydopamine infusion into the medial forebrain bundle several weeks previously (n = 35). Nigrostriatal lesion altered baseline activity of entopeduncular neurons in several ways. Interspike interval distributions had significantly decreased modes and significantly increased coefficient of variation, skewness and kurtosis; yet interspike interval mean (the inverse of firing rate) was not affected. Also, spectral analysis of autocorrelograms indicated that lesion significantly reduced the incidence of regular-spiking neurons and increased the incidence of neurons with 4-18 Hz oscillations. Dopamine agonist treatment reversed some lesion-induced effects: quinpirole reversed changes in interspike interval distribution mode and coefficient of variation, while combined quinpirole and SKF 38393 blocked the appearance of 4-18 Hz oscillations. However, no agonist treatment normalized all aspects of entopeduncular activity. Additionally, inhibition of firing rates by D(1) or combined D(1)/D(2) receptor activation indicated that dopamine agonists affected the overall level of entopeduncular activity in a manner similar to that found in the substantia nigra pars reticulata and globus pallidus internal segment after dopamine neuron lesion. These data demonstrate that lesion of the nigrostriatal tract leads to modifications of several aspects of firing pattern in the rodent entopeduncular nucleus and so expand on similar findings in the rodent substantia nigra pars reticulata and in the globus pallidus internal segment in humans and nonhuman primates. The results support the view that dysfunction in the basal ganglia after midbrain dopamine neuron loss relates more consistently to abnormal activity patterns than to net changes in firing rate in the basal ganglia output nuclei, while overall decreases in firing rate in these structures may play a more important role in adverse motor reactions to dopamine agonist treatments.

Alterations in responses of ventral pallidal neurons to excitatory amino acids after long-term dopamine depletion

The present study explored the possibility that excitatory amino acid (EAA) sensitivity within the ventral pallidum (VP) is altered by long-term removal of dopamine (DA). Electrophysiological experiments were conducted in chloral hydrate-anesthetized rats 21 to 28 days after they received unilateral substantia nigra injections of the dopaminergic toxin 6-hydroxydopamine (6-OHDA). VP neurons increased firing at low microiontophoretic ejection currents of the EAA agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA); however, high currents decreased action potential amplitude and rapidly caused cessation of neuronal firing. These responses likely reflected the induction of depolarization block for they were reversed by coiontophoresis of the hyperpolarizing transmitter gamma-aminobutyric acid (GABA) at ejection current levels that normally suppressed firing. The ability of NMDA and AMPA to induce such inactivation was greater in the VP of 6-OHDA-lesioned hemispheres, but unchanged in reserpinized rats, verifying that the alterations in responding to NMDA were the result of chronic, rather than acute, DA removal. The adaptations do not appear to be the consequence of a diminished GABAergic tone for the ability of bicuculline to increase firing (due to blocking a tonic GABAergic input) was not changed. However, low ejection currents of GABA that were insufficient to alter firing rate greatly attenuated the ability of NMDA to induce an apparent depolarization inactivation when coiontophoresed with NMDA onto VP neurons of the lesioned, but not the unlesioned, hemisphere. These studies show that chronic DA removal altered the EAA-induced amplitude-decreasing (i.e., the apparent depolarization inactivation) effects in VP neurons in the absence of a decrease in GABAergic tone.

Electrophysiological and behavioral output of the rat basal ganglia after intrastriatal infusion of d-amphetamine: lack of support for the basal ganglia model

Dopamine, by acting upon D1 and D2 dopamine receptors located on striatonigral and striatopallidal neurons, respectively, has been postulated to inhibit output from the substantia nigra pars reticulata (SNpr) and internal pallidal segment (GPi). The inhibition of the SNpr/GPi should, in turn, disinhibit the thalamus to facilitate movement. The present study tests this prediction in intact (unlesioned) rats by attempting to correlate changes in the single unit activities of SNpr neurons with motor (i.e. behavioral) responses in the 20-30 min after infusions of d-amphetamine into the striatum. Unilateral injections of amphetamine (20 microg/microl) into either the dorsal-rostral, central, or ventral-lateral striatum failed to appreciably alter behavior and, in parallel electrophysiological studies, failed to consistently or significantly alter the activities of SNpr neurons in either chloral hydrate-anesthetized rats or awake locally anesthetized rats. However, when amphetamine was infused bilaterally into the ventral-lateral striatum (VLS; 20 microg/microl per side), a robust behavioral activation ensued (increased locomotor activity, oral movements, and sniffing) with an onset ranging from immediate to 20 min post-infusion and persisting for at least 40 min. In parallel studies, bilateral amphetamine infusions into VLS also caused changes in the firing frequency of a majority of SNpr neurons. However, the changes in firing were extremely variable and, contrary to expectation, the net population response of SNpr neurons was an increase in firing which corresponded in time with the period of peak behavioral activation. These results show that (i) bilateral but not unilateral activation of striatal dopamine receptors is needed to elicit behavioral and electrophysiological output from the basal ganglia, and (ii) motor activation is apparently not signaled by a generalized inhibition of SNpr firing, as is predicted by the basal ganglia model.

Subthalamic nucleus lesions reduce low frequency oscillatory firing of substantia nigra pars reticulata neurons in a rat model of Parkinson's disease

Single unit recordings performed in animal models of Parkinson's disease revealed that output nuclei neurons display modifications in firing pattern and firing rate, which are supposed to give rise to the clinical manifestations of the illness. We examined the activity pattern of single units from the substantia nigra pars reticulata, the main output nuclei of the rodent basal ganglia, in urethane-anesthetized control and 6-hydroxydopamine-lesioned rats (a widespread model of Parkinson's disease). We further studied the effect of a subthalamic nucleus lesion in both experimental groups. Subthalamic nucleus lesion produces behavioral improvement in animal models of Parkinson's disease, and was expected to reverse the changes induced by 6-hydroxydopamine lesions. A meticulous statistical investigation, which included a non-biased classification of the recorded units by means of cluster analysis, allowed us to identify a low frequency oscillation of firing rate ( approximately 0.9 Hz) occurring in approximately 35% of the units recorded from 6-hydroxydopamine-lesioned rats, as the main feature differentiating 6-hydroxydopamine-lesioned and control rats. Subthalamic nucleus lesions significantly reduced the proportion of oscillatory units in 6-hydroxydopamine-lesioned rats. However, the population of nigral units recorded from rats bearing both lesions still differed significantly from control units. These results suggest that oscillatory activity in the basal ganglia output nuclei may be related to some clinical features of parkinsonism, and suggest a putative mechanism through which therapeutic interventions aimed at modifying subthalamic nucleus function produce clinical benefit in Parkinson's disease.

Kindling causes persistent in vivo changes in firing rates and glutamate sensitivity of central piriform cortex neurons in rats

The present experiments were undertaken to study whether amygdala kindling induces persistent alterations in the functional status of neurons of the central piriform cortex, a subregion of the piriform cortex identified previously as a site involved in the kindling process. Extracellular, single-unit recordings of piriform cortex neurons were made in anesthetized fully kindled rats at an interval of at least five weeks after the last seizure. Electrode implanted but not kindled rats served as sham controls. An additional group of non-implanted rats was used as naive controls. Spontaneously firing piriform cortex neurons were characterized in all groups by smooth, sharp, biphasic (i.e. positive/negative) action potentials with a duration of 0.8-1.8 ms, and were primarily located at the border between piriform cortex layers II and III. In kindled rats, neurons in the central piriform cortex exhibited a significantly higher firing rate compared to controls. Based on median group values, the increase in basal activity in kindled rats averaged about 90%. The responsiveness of piriform cortex neurons to neurotransmitters was tested by microiontophoretic application of glutamate, N-methyl-D-aspartate and GABA. Piriform cortex neurons of kindled rats exhibited a significantly lower responsiveness to the excitatory effect of glutamate than naive controls. A lowered glutamate responsiveness was also seen in sham controls. No significantly altered transmitter sensitivities of piriform cortex neurons from kindled rats were seen with N-methyl-D-aspartate or GABA. The data indicate that amygdala kindling causes persistent interictal changes in both basal activity and glutamate responsiveness of central piriform cortex neurons which could contribute to the abnormal hyperexcitability characteristic of kindling.

Substantia nigra pars reticulata units in 6-hydroxydopamine-lesioned rats: responses to striatal D2 dopamine receptor stimulation and subthalamic lesions

In order to increase our understanding of Parkinson's disease pathophysiology, we studied the effects of intrastriatally administered selective dopamine receptor agonists on single units from the substantia nigra pars reticulata of 6-hydroxydopamine (6-OHDA)-lesioned rats with or without an additional subthalamic nucleus lesion. Nigral pars reticulata units of 6-OHDA-lesioned rats were classified into two types, showing regular and bursting discharge patterns, respectively ('non-burst' and 'burst' units). Non-burst and burst units showed distinct responses to intrastriatal quinpirole (the former were excited and burst units inhibited). Furthermore, subthalamic nucleus lesions significantly decreased the number of nigral units showing a spontaneous bursting pattern, and reduced the proportion of units that responded to quinpirole. In contrast, subthalamic lesions did not alter the proportion of nigral units that responded to SKF38393, although the lesions changed some response features, e.g. response type and magnitude. Burst analysis showed that quinpirole did not modify the discharge pattern of burst units, whereas SKF38393 produced a shift to regular firing in 62% of the burst units tested. In conjunction, our results support that: (i) the subthalamic nucleus has an important influence on output nuclei firing pattern; (ii) striatal D2 receptors have a strong influence on nigral firing rate, and a less relevant role in controlling firing pattern; (iii) burst and non-burst units differ in their response to selective stimulation of striatal dopamine receptors; (iv) the effects of striatal D2 receptors on nigral units are mainly, though not exclusively, mediated by the subthalamic nucleus; and (v) nigral responses to SKF38393 involve the subthalamic nucleus.

Dopamine agonist-mediated rotation in rats with unilateral nigrostriatal lesions is not dependent on net inhibitions of rate in basal ganglia output nuclei

Current models of basal ganglia function predict that dopamine agonist-induced motor activation is mediated by decreases in basal ganglia output. This study examines the relationship between dopamine agonist effects on firing rate in basal ganglia output nuclei and rotational behavior in rats with nigrostriatal lesions. Extracellular single-unit activity ipsilateral to the lesion was recorded in awake, locally-anesthetized rats. Separate rats were used for behavioral experiments. Low i.v. doses of D1 agonists (SKF 38393, SKF 81297, SKF 82958) were effective in producing rotation, yet did not change average firing rate in the substantia nigra pars reticulata or entopeduncular nucleus. At these doses, firing rate effects differed from neuron to neuron, and included increases, decreases, and no change. Higher i.v. doses of D1 agonists were effective in causing both rotation and a net decrease in rate of substantia nigra pars reticulata neurons. A low s.c. dose of the D1/D2 agonist apomorphine (0.05 mg/kg) produced both rotation and a robust average decrease in firing rate in the substantia nigra pars reticulata, yet the onset of the net firing rate decrease (at 13-16 min) was greatly delayed compared to the onset of rotation (at 3 min). Immunostaining for the immediate-early gene Fos indicated that a low i.v. dose of SKF 38393 (that produced rotation but not a net decrease in firing rate in basal ganglia output nuclei) induced Fos-like immunoreactivity in the striatum and subthalamic nucleus, suggesting an activation of both inhibitory and excitatory afferents to the substantia nigra and entopeduncular nucleus. In addition, D1 agonist-induced Fos expression in the striatum and subthalamic nucleus was equivalent in freely-moving and awake, locally-anesthetized rats. The results show that decreases in firing rate in basal ganglia output nuclei are not necessary for dopamine agonist-induced motor activation. Motor-activating actions of dopamine agonists may be mediated by firing rate decreases in a small subpopulation of output nucleus neurons, or may be mediated by other features of firing activity besides rate in these nuclei such as oscillatory firing pattern or interneuronal firing synchrony. Also, the results suggest that dopamine receptors in both the striatum and at extrastriatal sites (especially the subthalamic nucleus) are likely to be involved in dopamine agonist influences on firing rates in the substantia nigra pars reticulata and entopeduncular nucleus.

Extracellular single-unit recordings of piriform cortex neurons in rats: influence of different types of anesthesia and characterization of neurons by pharmacological manipulation of serotonin receptors

In epilepsy research, there is a growing interest in the role of the piriform cortex (PC) in the development and maintenance of limbic kindling and other types of limbic epileptogenesis leading to complex partial seizures. Neurophysiological studies on PC or amygdala-PC slice preparations from kindled rats showed that kindling of the amygdala induces long-lasting changes in synaptic efficacy in the ipsilateral PC, including spontaneous discharges and enhanced susceptibility of PC neurons to evoked burst responses. These long-lasting electrophysiological changes in the PC during kindling appear to be due, at least in part, to impaired function of gamma-aminobutyric acid (GABA)ergic interneurons. The aim of the present study was to develop an anesthetic protocol allowing electrophysiological single-unit recordings from inhibitory, presumably GABAergic PC interneurons in vivo. In addition to recording of spontaneously active PC neurons, microiontophoretic application of glutamate was used to activate silent neurons. Anesthesia of rats with ketamine/xylazine was not suited for single-unit recordings in the PC because of marked cardiovascular depression. Anesthesia with chloral hydrate allowed recording of spontaneous or glutamate-driven single-unit activity in approximately 40% of all animals. A similar percentage was obtained when recordings were done with the narcotic opioid fentanyl (plus gallamine), after all surgical preparations were performed under anesthesia with repeated administration of the barbiturate methohexital. To avoid brain accumulation of methohexital by repeated applications, we modified the anesthetic protocol in that methohexital was only injected once for initiation of surgical anesthesia, followed by the short-acting anesthetic propofol which does not accumulate upon repeated application. Again, after surgical preparation, electrophysiological recordings were done under fentanyl (plus gallamine). By this procedure, spontaneous or glutamate-driven single-unit activity could be measured in all rats in either layer II or III of the PC. Based on shape and frequency of action potentials, two types of neurons were recorded. The predominant type was similar in its firing characteristics to GABAergic neurons in other brain regions, was mainly located in layer III, and could be suppressed by the serotonin2A receptor antagonist MDL 100,907, suggesting that this type of PC neuron represents inhibitory, putative GABAergic interneurons. This new in vivo preparation may be useful for evaluation of PC neurons in kindled rats.