Gordon W. Arbuthnott

Selective elimination of glutamatergic synapses on striatopallidal neurons in Parkinson disease models

Parkinson disease is a common neurodegenerative disorder that leads to difficulty in effectively translating thought into action. Although it is known that dopaminergic neurons that innervate the striatum die in Parkinson disease, it is not clear how this loss leads to symptoms. Recent work has implicated striatopallidal medium spiny neurons (MSNs) in this process, but how and precisely why these neurons change is not clear. Using multiphoton imaging, we show that dopamine depletion leads to a rapid and profound loss of spines and glutamatergic synapses on striatopallidal MSNs but not on neighboring striatonigral MSNs. This loss of connectivity is triggered by a new mechanism—dysregulation of intraspine Cav1.3 L-type Ca2+ channels. The disconnection of striatopallidal neurons from motor command structures is likely to be a key step in the emergence of pathological activity that is responsible for symptoms in Parkinson disease.

Amphetamine‐Induced Dopamine Release in the Rat Striatum: An In Vivo Microdialysis Study

The effects of a number of biochemical and pharmacological manipulations on amphetamine (AMPH)‐induced alterations in dopamine (DA) release and metabolism were examined in the rat striatum using the in vivo brain microdialysis method. Basal striatal dialysate concentrations were: DA, 7 nM; dihydroxyphenylacetic acid (DOPAC), 850 nM; homovanillic acid (HVA), 500 nM; 5‐hydroxyindoleacetic acid (5‐HIAA), 300 nM; and 3‐methoxytyramine (3‐MT), 3 nM. Intraperitoneal injection of AMPH (4 mg/kg) induced a substantial increase in DA efflux, which attained its maximum response 20–40 min after drug injection. On the other hand, DOPAC and HVA efflux declined following AMPH. The DA response, but not those of DOPAC and HVA, was dose dependent within the range of AMPH tested (2–16 mg/kg). High doses of AMPH (>8 mg/kg) also decreased 5‐HIAA and increased 3‐MT efflux. Depletion of vesicular stores of DA using reserpine did not affect significantly AMPH‐induced dopamine efflux. In contrast, prior inhibition of catecholamine synthesis, using α‐methyl‐p‐tyrosine, proved to be an effective inhibitor of AMPH‐evoked DA release (<35% of control). Moreover, the DA releasing action of AMPH was facilitated in pargyline‐pretreated animals (220% of control). These data suggest that AMPH releases preferentially a newly synthesised pool of DA. Nomifensine, a DA uptake inhibitor, was an effective inhibitor of AMPH‐induced DA efflux (18% of control). On the other hand, this action of AMPH was facilitated by veratrine and ouabain (200–210% of control). These results suggest that the membrane DA carrier may be involved in the actions of AMPH on DA efflux.

Dopamine reverses the depression of rat corticostriatal synapses which normally follows high-frequency stimulation of cortex In vitro

Learning deficits resulting from dopamine depletion suggest that striatal dopamine release is crucial for reinforcement. Recently described firing patterns of dopamine neurons in behaving monkeys show that transient increases in dopamine release are brought about by reinforcement. We describe an enduring change in the strength of synaptic transmission following pulsatile application of dopamine intended to mimic the transient increases associated with reinforcement. Intracellular records were made from neurons in slices of the rat corticostriatal system. Neurons having the properties of the medium-sized spiny neurons responded to cortical stimulation with depolarizing potentials (peak amplitude 12.0 +/- 1.3 mV; latency 9.2 +/- 0.1 ms; mean +/- S.D., n = 19), which had the properties of monosynaptic excitatory postsynaptic potentials. After trains of stimuli to the cortex had been applied in conjunction with intracellular depolarizing current, the size of these excitatory postsynaptic potentials was reduced (-27% at 20 min). Application of dopamine (approximately 30 microM) in a solution containing KCl concomitant with depolarization and presynaptic activation increased the subsequent excitatory postsynaptic potentials (+21% at 20 min) without significant lasting change in the membrane properties of the postsynaptic cell. This suggests that dopamine has an enduring, activity-dependent action on the efficacy of corticostriatal transmission, which may be a cellular basis for the learning-related effects of the nigrostriatal system.

Space, time and dopamine.

In recent years, dopamine has emerged as a key neurotransmitter that is crucially involved in incentive motivation and reinforcement learning. Dopamine release is evoked by rewards. The extensive divergence of outputs from a small number of dopaminergic neurons suggests a spatially nonselective action of dopamine, but it reinforces the specific actions that led to reward. How is this achieved? We propose that the selectivity of dopamine effects is achieved by the timing of dopamine release in relation to the activity of glutamatergic synapses, rather than by spatial localization of the dopamine signal to specific synaptic contacts. The synaptic mechanisms of these actions are unknown but reduced levels of dopamine, for example in Parkinsons disease, leads to a paucity of behavioural output, whereas its excess production has been associated with psychiatric problems. Clearly, there are therapeutic imperatives that require a better understanding of how dopamine functions at a synaptic level.

Evidence of a breakdown of corticostriatal connections in Parkinson’s disease

Dendritic spines are important structures which receive synaptic inputs in many regions of the CNS. The goal of this study was to test the hypothesis that numbers of dendritic spines are significantly reduced on spiny neurones in basal ganglia regions in Parkinsons disease as we had shown them to be in a rat model of the disease [Exp Brain Res 93 (1993) 17]. Postmortem tissue from the caudate and putamen of patients suffering from Parkinsons disease was compared with that from people of a similar age who had no neurological damage. The morphology of Golgi-impregnated projection neurones (medium-sized spiny neurones) was examined quantitatively. The numerical density of dendritic spines on dendrites was reduced by about 27% in both nuclei. The size of the dendritic trees of these neurones was also significantly reduced in the caudate nucleus from the brains of PD cases and their complexity was changed in both the caudate nucleus and the putamen. Dendritic spines receive crucial excitatory input from the cerebral cortex. Reduction in both the density of spines and the total length of the remaining dendrites is likely to have a grave impact on the ability of these neurones to function normally and may partly explain the symptoms of the disorder.

Spine density on neostriatal neurones changes with 6-hydroxydopamine lesions and with age ☆

Golgi-impregnated medium-size spiny neurones were studied in the rat neostriatum at various times after a unilateral 6-hydroxydopamine lesion of the medial forebrain bundle. At both short (12-26 days) and long (7-12 months) survival times, the density of spines was significantly lower (13%) on neostriatal neurones ipsilateral to the 6-hydroxydopamine injection. At longer times the density of spines was lower on neurones both ipsilateral and contralateral to the lesion when compared to neurones taken from short-term animals (23%).

Dopamine and synaptic plasticity in the neostriatum

After the unilateral destruction of the dopamine input to the neostriatum there are enduring changes in rat behaviour. These have been ascribed to the loss of dopamine and the animals are often referred to as ‘hemiparkinsonian’. In the denervated neostriatum, we have shown that not only are the tyrosine hydroxylase positive boutons missing, but also the medium sized densely spiny output cells have fewer spines. Spines usually have asymmetric synapses on their heads. In a recent stereological study we were able to show that there is a loss of approximately 20% of asymmetric synapses in the lesioned neostriatum by 1 mo after the lesion. Current experiments are trying to establish the specificity of this loss. So far we have evidence suggesting that there is no obvious preferential loss of synapses from either D1 or D2 receptor immunostained dendrites in the neostriatum with damaged dopamine innervation. These experiments suggest that dopamine is somehow necessary for the maintenance of corticostriatal synapses in the neostriatum. In a different series of experiments slices of cortex and neostriatum were maintained in vitro in such a way as to preserve at least some of the corticostriatal connections. In this preparation we have been able to show that cortical stimulation results in robust excitatory postsynaptic potentials (EPSPs) recorded from inside striatal neurons. Using stimulation protocols derived from the experiments on hippocampal synaptic plasticity we have shown that the usual consequence of trains of high frequency stimulation of the cortex is the depression of the size of EPSPs in the striatal cell. In agreement with similar experiments by others, the effect seems to be influenced by NMDA receptors since the unblocking of these receptors with low Mg++ concentrations in the perfusate uncovers a potentiation of the EPSPs after trains of stimulation. Dopamine applied in the perfusion fluid round the slices has no effect but pulsatile application of dopamine, close to the striatal cell being recorded from, and in temporal association with the cortical trains, leads to a similar LTP like effect. The reduction of K+ channel conductance in the bath with TEA also has the effect of making cortical trains induce potentiation of corticostriatal transmission. TEA applied only to the cell being recorded from has no similar effect; the cortical stimulation again depresses the EPSP amplitude, so the site of action of TEA may well be presynaptic to the striatal cell. The morphological and physiological experiments may not necessarily be related but it is tempting to suggest that dopamine protects some corticostriatal synapses by potentiating them but that in the absence of dopamine others simply disconnect and are no longer detectable on electron microscopy.

Therapeutic Deep Brain Stimulation in Parkinsonian Rats Directly Influences Motor Cortex

Much recent discussion about the origin of Parkinsonian symptoms has centered around the idea that they arise with the increase of beta frequency waves in the EEG. This activity may be closely related to an oscillation between subthalamic nucleus (STN) and globus pallidus. Since STN is the target of deep brain stimulation, it had been assumed that its action is on the nucleus itself. By means of simultaneous recordings of the firing activities from populations of neurons and the local field potentials in the motor cortex of freely moving Parkinsonian rats, this study casts doubt on this assumption. Instead, we found evidence that the corrective action is upon the cortex, where stochastic antidromic spikes originating from the STN directly modify the firing probability of the corticofugal projection neurons, destroy the dominance of beta rhythm, and thus restore motor control to the subjects, be they patients or rodents.

Effects of Selective Monoamine Oxidase Inhibitors on the In Vivo Release and Metabolism of Dopamine in the Rat Striatum

Abstract: Brain microdialysis was used to examine the in vivo efflux and metabolism of dopamine (DA) in the rat striatum following monoamine oxidase (MAO) inhibition. Relevant catecholamines and indoleamines were quantified by HPLC coupled with a electrochemical detection system. The MAO‐B inhibitor selegiline only affected DA deamination at a dose shown to inhibit partially type A MAO. Alterations in DA and metabolite efflux were not observed when using the MAO‐B‐selective dose of 1 mg/kg of selegiline. At 10 mg/kg, selegiline reduced the efflux of DA metabolites to ∼70% of basal values without affecting DA efflux. K+‐and veratrine‐stimulated DA efflux was not affected by selegiline. Experiments using amphetamine and the DA uptake inhibitor nomifensine demonstrated that the effect of selegiline on DA metabolism was unlikely to be mediated either by inhibition of DA uptake or by an indirect effect of its metabolite amphetamine. The possibility that the effect of selegiline is mediated via a nonspecific inhibition of MAO is discussed. In contrast, the MAO‐A inhibitor clorgyline inhibited basal DA metabolism and increased basal and de‐polarisation‐induced DA efflux. A 1 mg/kg dose of clorgyline reduced basal DA metabolite efflux (40–60% of control values) without affecting DA efflux. At 10 mg/kg of clorgyline, DA efflux increased to 253 ± 19% of basal values, whereas efflux of DA metabolites was reduced to between 15 and 26% of control values. The release of DA induced by K+ and vera‐trine was not affected by 1 mg/kg of clorgyline but was increased by ∼200% following pretreatment with 10 mg/kg of clorgyline. The nonselective MAO inhibitor pargyline caused similar but more pronounced alterations in these parameters. For example, using a 75 mg/kg dose of pargyline, efflux of DA increased maximally to 310 ± 20% of basal values, and the release of DA induced by K+ and veratrine was increased by ∼300% following pargyline pretreatment. These data suggest that DA metabolism is mediated principally by MAO‐A in the rat striatum. However, under conditions of MAO‐A inhibition, a component of metabolism mediated by the type B enzyme becomes apparent.

Morphological changes in the rat neostriatum after unilateral 6-hydroxydopamine injections into the nigrostriatal pathway.

Destruction of the dopamine-containing neurons in the rat substantia nigra results in morphological changes in the striatum which have been characterized at both the light and electron microscopic levels. After a unilateral 6-hydroxydopamine injection into the medial forebrain bundle, Golgi-impregnated medium-sized spiny neurons in the neostriatum ipsilateral to the injection had a lower density of spines on their dendrites than those on the contralateral side. A similar decrease in spine density was apparent from 12 days until at least 13.5 months after the lesion. A bilateral loss of spines occurred with increasing age regardless of the presence or absence of the nigrostriatal dopaminergic pathway. At the ultrastructural level, the general pattern of synaptic input to the Golgi-impregnated medium-sized spiny neurons was similar on both sides of the brain. The most obvious class of afferent boutons contacting these spiny neurons formed prominent asymmetrical synaptic specializations with the heads of the spines. The numbers of asymmetric synaptic profiles counted in random electron micrographs from the striata ipsilateral and contralateral to the lesion were not significantly different from each other. A small but significant increase in the length of asymmetric synaptic specialization profiles was, however, detected in the striata lacking a dopamine input.