M. Gustavo Murer

Substantia nigra pars reticulata single unit activity in normal and 60HDA-lesioned rats: Effects of intrastriatal apomorphine and subthalamic lesions

The spontaneous activity and the response to intrastriatal application of apomorphine of substantia nigra pars reticulata (SNpr) single units was studied in four experimental groups of rats: (1) normal rats; (2) subthalamic nucleus (STN) lesioned rats; (3) rats bearing a 6‐hydroxydopamine (6OHDA) lesion; and (4) 6OHDA‐lesioned animals with an additional STN lesion. Thirty‐eight percent of units from 6OHDA‐lesioned rats showed a bursting pattern of spontaneous activity, which was never found in normal rats. STN lesions had no effect on the spontaneous activity of SNpr units from normal rats, but reduced the percentage of burst units in 6OHDA‐lesioned animals. Intrastriatal apomorphine produced responses in 62% of SNpr units from normal rats and 85% of units from 6OHDA‐lesioned animals (P < 0.05). In addition, the modifications in the firing rate and in the coefficient of variation of the interspike intervals induced by intrastriatal apomorphine were significantly greater for the units isolated from 6OHDA‐lesioned rats. In particular, it was noted that all the burst units responded to apomorphine, showing the highest changes in firing rate and coefficient of variation. However, intrastriatal apomorphine did not always turn the activity of burst units into a more physiological pattern. STN lesions reduced the percentage of units responding to intrastriatal apomorphine in normal rats. In 6OHDA‐lesioned rats, STN lesions reduced the number of responsive units, and their change in mean firing rate and coefficient of variation. Our results show that the STN participates in the genesis of the bursting pattern of activity of SNpr units in 6OHDA‐lesioned rats, and that STN lesions can partially revert the abnormal spontaneous and apomorphine‐induced responses of SNpr units in these animals. Synapse 27:278–293, 1997.

Disruption of the two-state membrane potential of striatal neurones during cortical desynchronisation in anaesthetised rats

In anaesthetised animals, the very negative resting membrane potential of striatal spiny neurones (down state) is interrupted periodically by depolarising plateaux (up states) which are probably driven by excitatory input. In the absence of active synaptic input, as occurs in vitro, potassium currents hold the membrane potential of striatal spiny neurones in the down state. Because striatal spiny neurones fire action potentials only during the up state, these plateau depolarisations have been perceived as enabling events that allow information processing through cerebral cortex‐basal ganglia circuits. Recent studies have demonstrated that the robust membrane potential fluctuation of spiny neurones is strongly correlated to the slow electroencephalographic rhythms that are typical of slow wave sleep and anaesthesia. To further understand the impact of cortical activity states on striatal function, we studied the membrane potential of striatal neurones during cortical desynchronised states. Simultaneous in vivo recordings of striatal neurones and the electrocorticogram in urethane‐anaesthetised rats revealed that rhythmic alternation between up and down states was disrupted during episodes of spontaneous or induced cortical desynchronisation. Instead of showing robust two‐state fluctuations, the membrane potential of striatal neurones displayed a persisting depolarised state with fast, low‐amplitude modulations. Spiny neurones remained in this persistent up state until the cortex resumed ∼1 Hz synchronous activity. Most of the recorded neurones exhibited a low firing probability, irrespective of the cortical activity state. Time series analysis failed to reveal significant correlations between the membrane potential of striatal neurones and the desynchronised electrocorticogram. Our results suggest that during cortical desynchronisation continuous uncorrelated excitatory input sustains the membrane potential of striatal neurones in a persisting depolarised state, but that substantial additional input is necessary to impel the neurones to threshold. Our data support that the prevailing cortical activity state determines the duration of the enabling depolarising events that take place in striatal spiny neurones.

Spreading of slow cortical rhythms to the basal ganglia output nuclei in rats with nigrostriatal lesions

A high proportion of neurons in the basal ganglia display rhythmic burst firing after chronic nigrostriatal lesions. For instance, the periodic bursts exhibited by certain striatal and subthalamic nucleus neurons in 6‐hydroxydopamine‐lesioned rats seem to be driven by the ∼ 1 Hz high‐amplitude rhythm that is prevalent in the cerebral cortex of anaesthetized animals. Because the striatum and subthalamic nucleus are the main afferent structures of the substantia nigra pars reticulata, we examined the possibility that the low‐frequency modulations (periodic bursts) that are evident in approximately 50% nigral pars reticulata neurons in the parkinsonian condition were also coupled to this slow cortical rhythm. By recording the frontal cortex field potential simultaneously with single‐unit activity in the substantia nigra pars reticulata of anaesthetized rats, we proved the following. (i) The firing of nigral pars reticulata units from sham‐lesioned rats is not coupled to the ∼ 1 Hz frontal cortex slow oscillation. (ii) Approximately 50% nigral pars reticulata units from 6‐hydroxydopamine‐lesioned rats oscillate synchronously with the ∼ 1 Hz cortical rhythm, with the cortex leading the substantia nigra by ∼ 55 ms; the remaining ∼ 50% nigral pars reticulata units behave as the units recorded from sham‐lesioned rats. (iii) Periodic bursting in nigral pars reticulata units from 6‐hydroxydopamine‐lesioned rats is disrupted by episodes of desynchronization of cortical field potential activity. Our results strongly support that nigrostriatal lesions promote the spreading of low‐frequency cortical rhythms to the substantia nigra pars reticulata and may be of outstanding relevance for understanding the pathophysiology of Parkinsons disease.

Turning off cortical ensembles stops striatal Up states and elicits phase perturbations in cortical and striatal slow oscillations in rat in vivo

In vivo, cortical neurons and striatal medium spiny neurons (MSN) display robust subthreshold depolarizations (Up states) during which they are enabled to fire action potentials. In the cortex, Up states are believed to occur simultaneously in a neuronal ensemble and to be sustained by local network interactions. It is known that MSN are impelled into the Up state by extra‐striatal (primarily cortical) inputs, but the mechanisms that sustain and determine the end of striatal Up states are still debated. Furthermore, it has not been established if brisk perturbations of ongoing cortical oscillations alter rhythmic transitions between Up and Down states in striatal neurons. Here we report that MSN Up states terminate abruptly when persistent activity in cortical ensembles providing afferents to a given striatal region is turned off by local electrical stimulation or ends spontaneously. In addition, we found that phase perturbations in MSN membrane potential slow oscillations induced by cortical stimulation replicate the stimulus‐induced dynamics of spiking activity in cortical ensembles. Overall, these results suggest that striatal Up states are single‐cell subthreshold representations of episodes of persistent spiking in cortical ensembles. A precise spatial and temporal alignment between episodes of cortical persistent activity and striatal Up states would allow MSN to detect specific cortical inputs embedded within a more general cortical signal.

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 Parkinsons 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 Parkinsons 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 Parkinsons 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 Parkinsons disease.

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 Parkinsons 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.

Consequences of partial and severe dopaminergic lesion on basal ganglia oscillatory activity and akinesia.

Severe chronic dopamine (DA) depletion increases the proportion of neurons in the basal ganglia that fire rhythmic bursts of action potential (LFO units) synchronously with the cortical oscillations. Here we report on how different levels of mesencephalic DA denervation affect substantia nigra pars reticulata (SNpr) neuronal activity in the rat and its relationship to akinesia (stepping test). Chronic nigrostriatal lesion induced with 0 (control group), 4, 6 or 8 µg of 6‐hydroxydopamine (6‐OHDA) into the medial forebrain bundle resulted in a dose‐dependent decrease of tyrosine hydroxylase positive (TH+) neurons in the SN and ventral tegmental area (VTA). Although 4 µg of 6‐OHDA reduced the number of TH+ neurons in the SN by ∼60%, both stepping test performance and SNpr neuronal activity remained indistinguishable from control animals. By contrast, animals that received 6 µg of 6‐OHDA showed a marked reduction of TH+ cells in the SN (∼75%) and VTA (∼55%), a significant stepping test deficit and an increased proportion of LFO units. These changes were not dramatically enhanced with 8 µg 6‐OHDA, a dose that induced an extensive DA lesion (> 95%) in the SN and ∼70% reduction of DA neurons in the VTA. These results suggest a threshold level of DA denervation for both the appearance of motor deficits and LFO units. Thus, the presence of LFO activity in the SNpr is not related to a complete nigrostriatal DA neuron depletion (ultimate stage parkinsonism); instead, it may reflect a functional disruption of cortico‐basal ganglia dynamics associated with clinically relevant stages of the disease.

Differential gene expression induced by chronic levodopa treatment in the striatum of rats with lesions of the nigrostriatal system

Levodopa, the major treatment for patients with Parkinsons disease, has been shown to induce a variety of compensatory effects, including facilitation of sprouting by dopaminergic neurons, in experimental animals with lesions leading to denervation of the striatum. To better understand the cellular and molecular environment where most of these compensatory changes take place, in particular elements that might contribute to the recovery of dopaminergic innervation, we have constructed a differential expression library enriched in transcripts from the striata of rats with lesions of the medial forebrain bundle treated with levodopa for 6 months. We have used this library to screen an expression array of rat genes representing the major cell functions, and have identified several that are involved in neurotrophic mechanisms and plasticity. We have confirmed the differential expression of selected transcripts by non‐radioactive in situ hybridization, and report that the growth factor pleiotrophin, myelin basic protein and calmodulin are overexpressed in the denervated striatum of levodopa‐treated rats.

Nigrostriatal lesion induces D2-modulated phase-locked activity in the basal ganglia of rats.

There is a debate as to what modifications of neuronal activity underlie the clinical manifestations of Parkinsons disease and the efficacy of antiparkinsonian pharmacotherapy. Previous studies suggest that release of GABAergic striatopallidal neurons from D2 receptor‐mediated inhibition allows spreading of cortical rhythms to the globus pallidus (GP) in rats with 6‐hydroxydopamine‐induced nigrostriatal lesions. Here this abnormal spreading was thoroughly investigated. In control urethane‐anaesthetized rats most GP neurons were excited during the active part of cortical slow waves (‘direct‐phase’ neurons). Two neuronal populations having opposite phase relationships with cortical and striatal activity coexisted in the GP of 6‐hydroxydopamine‐lesioned rats. ‘Inverse‐phase’ GP units exhibited reduced firing coupled to striatal activation during slow waves, suggesting that this GP oscillation was driven by striatopallidal hyperactivity. Half of the pallidonigral neurons identified by antidromic stimulation exhibited inverse‐phase activity. Therefore, spreading of inverse‐phase oscillations through pallidonigral axons might contribute to the abnormal direct‐phase cortical entrainment of basal ganglia output described previously. Systemic administration of the D2 agonist quinpirole to 6‐hydroxydopamine‐lesioned rats reduced GP inverse‐phase coupling with slow waves, and this effect was reversed by the D2 antagonist eticlopride. Because striatopallidal hyperactivity was only slightly reduced by quinpirole, other mechanisms might have contributed to the effect of quinpirole on GP oscillations. These results suggest that antiparkinsonian efficacy may rely on other actions of D2 agonists on basal ganglia activity. However, abnormal slow rhythms may promote enduring changes in functional connectivity along the striatopallidal axis, contributing to D2 agonist‐resistant clinical signs of parkinsonism.

Functional integration across a gradient of corticostriatal channels controls UP state transitions in the dorsal striatum

Coordinated near-threshold depolarized states in cortical and striatal neurons may contribute to form functionally segregated channels of information processing. Recent anatomical studies have identified pathways that could support spiraling interactions across corticostriatal channels, but a functional outcome of such spiraling remains to be identified. Here, we examined whether plateau depolarizations (UP states) in striatal neurons relate better to active epochs in local field potentials recorded from closely related cortical areas than to those recorded in less-related cortical areas. Our results show that, in anesthetized rats, the coordination between cortical areas and striatal regions obeys a mediolateral gradient and keeps track of slow wave trajectory across the neocortex. Moreover, activity in one cortical area induced phase advances in UP state onset and phase delays in UP state termination in nonmatching striatal regions, reflecting the existence of functional connections that could encode large-scale interactions between corticostriatal channels as subthreshold influences on striatal projection neurons.