Effects of interruption of the nigrostriatal pathway and of dopaminergic agents on the spontaneous activity of globus pallidus neurons in the awake monkey

Motor impairments and neurochemical changes after unilateral 6-hydroxydopamine lesion of the nigrostriatal dopaminergic system in monkeys

Unilateral lesions of the nigrostriatal dopaminergic system were induced in five monkeys by intranigral injections of the neurotoxin 6-hydroxydopamine. Following the lesion, all monkeys showed a transient reluctance in using the contralateral forelimb, accompanied, in two monkeys by semi-flexed posture of the disabled forelimb. Three of the monkeys that had been conditioned to perform a visually triggered goal-directed arm movement, showed an increase in latency and duration of contralateral arm movements. Task performance recovered spontaneously to preoperative levels within four months in two monkeys despite significant reductions of endogenous dopamine and dihydroxyphenylacetic acid contents in the caudate nucleus, putamen and globus pallidus ipsilateral to the neurotoxic nigral injection. The third monkey exhibited a persistent increase in movement latency associated with a near complete loss of dopamine in both the putamen and the caudate nucleus. In all cases, an increase the dihydroxyphenyl-acetic acid to dopamine ratio was detected in the striatum and pallidum suggesting a compensatory increase in dopamine turnover in remaining intact dopaminergic nerve terminals. The level of serotonin was changed in all monkeys consisting of either a decrease or an increase, depending on the striatopallidal regions studied. Changes in choline acetyltransferase and glutamic acid decarboxylase activities in the same regions were only seen in some cases. The present results show that 6-hydroxydopamine-induced partial unilateral lesion of nigral dopaminergic neurons produced predominantly contralateral hypokinesia, accompanied by reductions of dopamine content in the ipsilateral striatum and pallidum. The use of this locally applied neurotoxin appears to be a suitable method for investigating neurophysiological mechanisms underlying hypokinesia since deficits in both initiating and executing movements can be expressed independently of other behavioral symptoms. The results show more persistent deficits in starting movements than in their execution and thus suggest that motor initiation is more dependent upon the functional integrity of the nigrostriatal dopamine system than movement completion.

Modifications of precentral cortex discharge and EMG activity in monkeys with MPTP-induced lesions of DA nigral neurons

1. Individual neurons were recorded extracellularly in the precentral forelimb area of two monkeys trained to perform rapid, large amplitude flexion and extension movements of the contralateral forearm in response to auditory signals. Electromyographic (EMG) activity in the biceps/triceps muscles was recorded separately under the same conditions. The dopaminergic (DA) neurons of the substantia nigra (SN) were destroyed selectively by repeated series of intravenous injections of MPTP. The lesion was verified on serial slices using both tyrosine hydroxylase immunocytochemistry and classical staining methods. 2. In normal monkeys, the frequency of firing of precentral neurons shows rapid changes shortly before the onset of displacement. In our sample (n = 102), most of the neurons (49%) tested during movement in both directions (flexion, extension) showed a reciprocal pattern of activity for the two directions of movement, a small percentage (19%) exhibited a change for only one direction (unidirectional neurons), and the remaining 32% displayed a similar change for both directions of movement (bidirectional neurons). 3. In MPTP-treated monkeys, movement-related modification of neuronal activity was more gradual, beginning earlier and lasting longer relative to the onset of movement. The cellular reaction time (the time between the auditory cue and a significant change in neuronal activity) was not significantly altered. Spontaneous firing of precentral neurons (n = 124) did not increase significantly, and the dynamic discharge rate was unchanged after the nigral lesion. However, only 18% of cortical neurons still presented a reciprocal pattern of discharge for the two directions of movement, while the percentage of unidirectional neurons increased (50%), and the percentage of bidirectional neurons remained the same (32%). 4. After MPTP treatment, alterations in movement parameters and EMG activity were observed. Mean reaction time and movement duration increased by 20-25% and 25-30% respectively. The movements were slower and were associated with a generalized depression in the shape and the amplitude of EMG activity in the agonist muscle. 5. The neuronal basis for the observed central and peripheral disturbance in the MPTP-treated monkeys is discussed. We conclude that SN lesion leads to two main disturbances of cortical activity: i) the loss of the reciprocal pattern of response of movement-related cortical cells, and ii) an inability of the motor cortex to modify its activity in response to peripheral input.

Electrophysiological properties of nigrothalamic neurons after 6-hydroxydopamine lesions in the rat

Extracellular recordings were made from electrophysiologically identified nigrothalamic cells in the substantia nigra pars reticulata of anaesthetized rats. The firing rate, firing pattern and responses to striatal stimulation were investigated in normal animals and in animals in which dopamine concentration in the ipsilateral striatum was reduced by more than 90%. At relatively short times after the lesion (less than 10 days) the mean firing rate of the spontaneously active cells in the population was significantly reduced and there was an increase in the occurrence of bursting activity. There was also a significant increase in the number of silent cells, located by antidromic stimulation from the thalamus. In spite of this reduction in mean firing rate the responses of neurons to stimulation of either the ipsilateral striatum or ventromedial thalamus was much larger in cells from lesioned animals. At longer times after the lesion (more than six months) the average firing rate of the neurons had returned to normal but there was still a prevalence of bursting activity and a consequent reduction in mean inter-spike intervals. There was little evidence of the previous hyper-responsiveness to thalamic stimulation but the responsiveness to striatal stimulation was still significantly elevated.

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

Striatal projections to the globus pallidus and entopeduncular nucleus are thought to be GABAergic and inhibitory. Thus, striatal lesions might be expected to increase the spontaneous discharge rate of neurons in these nuclei. To test this prediction, we recorded spontaneous single unit activity from awake cats sitting quietly before and 7-160 days after striatal lesions. Striatal lesions were produced by injecting ibotenic acid into the caudate nucleus and putamen. Median, standard deviation, mean, and coefficient of variation of the interspike intervals were calculated for each unit. In globus pallidus the striatal lesion resulted in a significant decrease in median interval length, i.e. an increase in the discharge rate. The prelesion median of 36 ms (S.E.M. = 2.3) decreased 11% to a postlesion value of 32 ms (S.E.M. = 2.1). The lesion also resulted in a significant decrease in the variability of interspike intervals. The coefficient of variation, 1.31 (S.E.M. = 0.08) before the lesion, decreased 25% to 0.97 (S.E.M. = 0.06) after the lesion. In entopeduncular nucleus, the lesion had no statistically significant effect on the rate of activity, but a significant decrease in the variability of activity occurred. The median interval was 33 ms (S.E.M. = 3.3) before the lesion and decreased 2% to 32 ms (S.E.M. = 2.4). The coefficient of variation decreased 48% from 1.44 (S.E.M. = 0.1) to 0.73 (S.E.M. = 0.03). These observations support the hypothesis that loss of GABAergic inputs to the globus pallidus results in disinhibition. The discharge rate in entopeduncular nucleus was not affected by the striatal lesion, suggesting that striatal substance P or subthalamic excitatory inputs may have a role in regulating discharge rate in the entopeduncular nucleus.

Dopaminergic innervation of the basal ganglia in the squirrel monkey as revealed by tyrosine hydroxylase immunohistochemistry

The organization of the dopaminergic mesostriatal fibers and their patterns of innervation of the basal ganglia in the squirrel monkey (Saimiri sciureus) were studied immunohistochemically with an antiserum raised against tyrosine hydroxylase (TH). Numerous fibers arose from midbrain TH-positive cell bodies of the substantia nigra pars compacta (group A9), the retrorubral area (group A8), and the lateral portion of the ventral tegmental area (group A10). These fibers accumulated dorsomedially to the rostral pole of the substantia nigra where they formed a massive bundle that coursed through the prerubral field and ascended along the laterodorsal aspect of the medial fore-brain bundle in the lateral hypothalamus. Some ventrally located fibers ran throughout the rostrocaudal extent of the lateral preopticohypothalamic area and could be followed up to the olfactory tubercle, whereas other fibers turned laterodorsally to invade the head of the caudate nucleus. At more dorsal levels in the lateral hypothalamus, many fiber fascicles detached themselves from the main bundle and swept laterally to reach the globus pallidus, the putamen, and the amygdala. Several TH-positive fibers coursed along the dorsal surface of the subthalamic nucleus, and some invaded the dorsomedial third of this structure. The remaining portion of the subthalamic nucleus contained relatively few TH-positive elements. In contrast, the globus pallidus received a dense dopaminergic innervation deriving mostly from two fascicles that coursed backward along the two major output pathways of the pallidum: the lenticular fasciculus caudodorsally and the ansa lenticularis rostroventrally. At the pallidal level, the labeled fibres merged within the medullary laminae and arborized profusely in the internal pallidal segment and less abundantly in the external pallidal segment. However, the caudoventral portion of the external pallidum displayed a dense field of TH-positive axonal varicosities. Other fibers ran through the dorsal two-thirds of the external pallidum en route to the putamen. The striatum contained a multitude of thin axonal varicosities among which a few long and varicosed fibers were scattered. These immunoreactive neuronal profiles were rather uniformly distributed along the rostrocaudal extent of the striatum but appeared slightly more numerous in the ventral striatum than in the dorsal striatum. The pattern of distribution of the TH-positive axonal varicosities in the dorsal striatum was markedly heterogeneous: it consisted of typical zones of poor TH immunoreactivity lying within a matrix of dense terminal labeling.(ABSTRACT TRUNCATED AT 400 WORDS)

Similar time course changes in striatal levels of glutamic acid decarboxylase and proenkephalin mRNA following dopaminergic deafferentation in the rat

The time course changes in levels of mRNA encoding glutamic acid decarboxylase (GAD) and proenkephalin (PPE) was analyzed in the rat striatum following unilateral lesion of substantia nigra with 6-hydroxydopamine. The levels of both GAD and PPE mRNAs increased after the dopaminergic deafferentation, reaching concomitantly a maximal twofold increase on day 25. Thereafter, the mRNA levels declined; at 4 months, the amount of PPE mRNA remained slightly elevated whereas GAD mRNA had returned to the control value, suggesting the action of a compensatory mechanism. We also observed a rise of glial fibrillary acidic protein mRNA level which reflects a reactive astrocytosis. In contrast, alpha-tubulin mRNA level remained unchanged, indicating that no significant synaptogenesis occurs in this experimental situation. No obvious modification in mRNA levels was detected in the striatum contralateral to the lesion. These results highlight the role of the modulation of gene expression in adaptive processes to dopamine deficiency in striatal efferent pathways. Its relevance to the pathophysiology of Parkinson's disease is discussed.

Abnormal influences of passive limb movement on the activity of globus pallidus neurons in parkinsonian monkeys

Extracellular single unit activity was recorded in the globus pallidus of waking Macaca fascicularis during passive limb movement. The main upper and lower limb joints were investigated bilaterally. The animals were either intact or rendered parkinsonian by the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Cell counts showed that at least 90% of nigral neurons of the compacta-type were degenerated in the parkinsonian animals. In the intact animals, only 17% of the pallidal neurons responded to the natural stimulus. As already reported by others, the responses were typically related to movement about a single contralateral joint and in only one direction. In the parkinsonian animals, however, more neurons responded, often more vigorously, to the same stimulation. In many of these neurons the responses were elicited by movement about more than one joint of both upper and lower limbs or ipsi-and contralateral sides and in more than one direction. The increase in number and magnitude and loss of specificity of responses were much greater in the internal pallidal segment, where the number of responding neurons quadrupled. These results suggest that dopaminergic mechanisms regulate gain and selectivity in the basal ganglia. In animals with decreased dopaminergic functions, the excessive and unselective motor responses may explain all 3 major signs of parkinsonism: rigidity, tremor and akinesia.

Distribution of D1 and D2 dopamine receptors in the basal ganglia of the cat determined by quantitative autoradiography

The patterns of dopamine D1 and D2 receptors were examined in the corpus striatum and related structures in the cat brain by quantitative autoradiography after in vitro radioligand binding with [3H]SCH23390 (D1 antagonist) and [3H]spiperone (D2 antagonist). Highly specific binding for both radioligands occurs in striatal structures known to receive dopaminergic input: the caudate nucleus, putamen, nucleus accumbens, and olfactory tubercle. However, the density of binding varies from one structure to another, and the density distribution within striatal nuclei is heterogeneous. In all but one portion of the striatum, the concentration of bound D1 radioligand ranges from 46 to 230% more than that of the D2 radioligand. The exception to this difference occurs at caudal putamenal levels where the two radioligands bind in equal concentrations (approximately equal to 220 fmol/mg tissue wet-weight). The highest density of both D1 and D2 radioligand binding occurs in irregular zones in the head and body of the caudate nucleus. Such high-density zones of D2 radioligand binding appear mainly in the dorsolateral part of the caudate's head. For the D1 radioligand, the high-density zones are more widespread throughout the caudate nucleus, nucleus accumbens, and putamen. The D2 radioligand binding (but not the D1) also exhibits low-density zones at more caudal levels of the caudate nucleus, and these are often in register with the high-density zones of D1 radioligand binding. In the putamen, inverted concentration gradients exist for the two radioligands: the [3H]SCH23390 gradient runs from higher levels rostrally to lower levels caudally. The lowest levels of bound [3H]spiperone in the striatum occur in the nucleus accumbens-olfactory tubercle area, whereas the lowest binding of [3H]SCH23390 occurs in the caudal putamen. Pallidal and nigral structures show marked disparities in binding of the two different radioligands. The D2 radioligand binding in the globus pallidus (80 +/- 8 fmol/mg tissue wet-weight) is about twice that in the entopedunuclear nucleus and pars reticulata of the substantia nigra, the latter two having equal levels (35 +/- 3 fmol/mg). No specific binding of the D2 radioligand occurs in the ventral pallidum. In contrast, D1 radioligand binding is highest in the entopeduncular nucleus (217 +/- 6 fmol/mg) and in the pars reticulata of the substantia nigra (198 +/- 2 fmol/mg) and moderate in the ventral pallidum (135 +/- 15 fmol/mg). In the globus pallidus, no detectable D1 radioligand binding occurs.(ABSTRACT TRUNCATED AT 400 WORDS)

Unilateral lesion of the nigrostriatal pathway decreases the firing rate and alters the firing pattern of globus pallidus neurons in the rat

Activities of spontaneously firing neurons in the globus pallidus of intact rats and rats that survived unilateral lesions of the nigrostriatal pathway for 3 days, 1 week, or 6-11 weeks were compared. No significant differences in neuronal firing rate, firing pattern, and number of cells per pass were observed between chloral hydrate-anesthetized control and lesioned animals. However, in locally anesthetized animals, pallidal cells fired significantly faster than in chloral hydrate-anesthetized animals, and the lesion caused a decrease in the firing rates of pallidal cells 1 week and 6-9 weeks postlesion. In addition, significant differences in the firing pattern of pallidal cells, as determined by the ratio of the mean to median interspike intervals, were seen between locally anesthetized controls and animals surviving 3 days, 1 week, and 6-9 weeks postlesion. This altered firing pattern tended to return to normal with time. The number of cells per pass was not significantly altered by the lesion. Data from this study suggest that, in locally anesthetized animals, the removal of the tonic dopaminergic input to the basal ganglia causes pallidal cells to decrease their firing rates in a time-dependent fashion and causes reversable firing pattern changes. This suggests that tonically active dopamine neurons, probably acting through the striatopallidal pathway, regulate the firing rate and mechanisms controlling the temporal ordering of spontaneous discharges of globus pallidus neurons.

The effects of L-DOPA on regional cerebral glucose utilization in rats with unilateral lesions of the substantia nigra

Using [14C]2-deoxyglucose autoradiography, we have studied the effects of systemically administered L-DOPA (10, 25 and 50 mg/kg s.c.) on regional cerebral glucose utilization (RCGU) in rats with unilateral substantia nigra lesions. In comparison with lesioned rats treated with saline, the lesioned-DOPA treated rats demonstrated contralateral turning and RCGU changes in both ipsilateral and contralateral brain regions. L-DOPA treatment markedly increased RCGU in the ipsilateral entopeduncular nucleus (EP) and substantia nigra pars reticulata (SNr), cell groups that receive direct striatal input and function as major outflow pathways of corpus striatal activity. In contrast, L-DOPA did not alter RCGU in the globus pallidus (GP), supporting the thesis that dopamine (DA) has different effects on striatal outflow to the GP compared with outflow to both the EP and SNr. Moderate RCGU increases were observed in the ipsilateral subthalamic nucleus (STN), lateral midbrain reticular formation (LMRF), and deep layers of the superior colliculus (DLSC), all regions which receive direct projections from the GP, EP or SNr. L-DOPA decreased RCGU in the ipsilateral lateral habenular nucleus (LHN) and increased RCGU in the contralateral LHN, changes that we suggest are mediated via altered neuronal activity in the striatum and EP. The results suggest that systemically administered L-DOPA, after conversion to DA in the brain, interacts with supersensitive DA receptors in the DA-depleted striatum to selectively activate efferent pathways. Furthermore, the data suggest that the LMRF and DLSC are functionally activated during L-DOPA induced turning and support the hypothesis that nigroreticular and nigrocollicular projections are of physiologic significance in the expression of striatal activity.

Differential time-course of reaction time recovery depending on variations in the amplitude of a goal-directed movement after nigrostriatal lesion in monkeys

Unilateral striatal dopamine depletion induced by 6-hydroxydopamine injections into the substantia nigra of behaving monkeys was found to increase the latency of a visually guided pointing movement performed by the contralateral forelimb. The time-course of recovery of the movement latency was faster in small displacements of the limb than in movements with larger amplitudes. The efficiency of putative compensatory mechanisms that may develop progressively in response to the striatal dopamine deficit depends on the amplitude of the movement to be initiated.

Ultrastructural evidence of dopaminergic input to enkephalinergic neurons in rat neostriatum

The synaptic relationship between neuronal structures reacting with antibodies to tyrosine hydroxylase (TH) and Leu- or Met-enkephalin (ENK) was studied by the 'mirror technique' in adjacent sections of rat neostriatum. TH-immunoreactive (TH-IR) axonal boutons surrounding the neural perikarya and proximal dendrites of ENK-immunoreactive (ENK-IR) neurons were very thin (0.1-0.4 micrograms). They contained many small clear vesicles and sometimes had symmetrical membrane specializations. This provides morphological evidence for catecholaminergic, presumably dopaminergic inputs to rat striatal enkephalin neurons.

Responses of rat pallidum cells to cortex stimulation and effects of altered dopaminergic activity

The aim of the study was to investigate the influences of dopamine on oligosynaptic corticopallidal neurotransmission. Different cortical areas were electrically stimulated and the responses in the pallidum were recorded by single-cell electrophysiology. Out of 377 pallidal neurons, 192 (51%) responded to stimulation of at least one of the cortical areas investigated. Convergence between frontal cortex and at least one of the other cortical areas was seen in 59 of 110 (54%) pallidal neurons responding to frontal cortex stimulation. Nearly three-quarters (73%) of all responsive pallidal neurons showed a short latency reduction of activity following the stimulus, the rest responded with pure activation or an activation-depression sequence. The dopaminergic influences on this corticopallidal impulse transmission were assessed by the systemic administration of the dopamine receptor-blocking neuroleptics, haloperidol and fluphenazine, as well as by conditioning electrical stimulation of the substantia nigra. Neuroleptic administration augmented the responses to cortical stimulation in 12 of 34 pallidal neurons. Stimulation of the substantia nigra diminished the responses in 24 and augmented them in 6 of 63 of the tested neurons. We propose from the present results, and in agreement with data from conceptually different studies done by others, that dopaminergic influences reduce the flow of information from the cortex to the pallidum. This may constitute a focussing mechanism by which only information form the strongest cortical inputs would pass to the pallidum while less prominent activity would be lost.

Low doses of apomorphine elicit two opposing influences on dopamine cell electrophysiology

Dopamine (DA) cells are known to be very sensitive to direct acting DA agonists, and inhibition of DA cell firing by low doses of DA agonists is generally considered to be an action of these agonists on DA cell autoreceptors. During intracellular recording from spontaneously discharging DA cells in vivo, intravenous administration of apomorphine (20 micrograms/kg i.v.) elicited a hyperpolarization and an increase in input resistance. The calculated reversal potential of the apomorphine effect was approximately -40 mV. However, in non-firing DA cells the reversal potential of these effects was significantly different (P less than 0.01), being close to the reversal potential of responses induced by stimulation of striatonigral pathways (i.e. -67 mV). In addition, haloperidol (0.01 mg/kg i.v.) reversed the hyperpolarization produced by apomorphine but not the increase in input resistance. Transection of striatonigral pathways eliminated most of the increase in input resistance accompanying apomorphine administration, and shifted the apomorphine reversal potential to a value positive to 0 mV. Low doses of apomorphine were also found to affect a class of zona reticulata (ZR) interneurons. Apomorphine caused decreases in ZR cell firing rate, which were abolished by striatonigral pathway transection. Thus, the following mechanism is proposed for the electrophysiological actions of autoreceptor-selective doses of apomorphine on DA cells: (1) apomorphine directly inhibits spontaneous DA cell discharge by inhibiting the slow depolarization preceding action potentials and thereby hyperpolarizes the DA cell, (2) decreased DA cell firing disinhibits GABAergic striatal cells, whose increased firing preferentially (3) inhibits GABAergic ZR interneurons, and thus (4) removes an inhibitory input to DA cells, resulting in an increase in input resistance.

Tyrosine hydroxylase-immunoreactive boutons in synaptic contact with identified striatonigral neurons, with particular reference to dendritic spines

Tyrosine hydroxylase-immunoreactive fibres in the rat neostriatum were studied in the electron microscope in order to determine the nature of the contacts they make with other neural elements. The larger varicose parts of such fibres contained relatively few vesicles and rarely displayed synaptic membrane specializations; however, thinner parts of axons (0.1-0.4 micron) contained many vesicles and had symmetrical membrane specializations, indicative of en passant type synapses. By far the most common postsynaptic targets of tyrosine hydroxylase-immunoreactive boutons were dendritic spines and shafts, although neuronal cell bodies and axon initial segments also received such input. Six striatonigral neurons in the ventral striatum were identified by retrograde labelling with horseradish peroxidase and their dendritic processes were revealed by Golgi impregnation using the section-Golgi procedure. The same sections were also developed to reveal tyrosine hydroxylase immunoreactivity and so we were able to study immunoreactive boutons in contact with the Golgi-impregnated striatonigral neurons. Each of the 280 immunoreactive boutons examined in the electron microscope displayed symmetrical synaptic membrane specializations: 59% of the boutons were in synaptic contact with the dendritic spines, 35% with the dendritic shafts and 6% with the cell bodies of striatonigral neurons. The dendritic spines of striatonigral neurons that received input from immunoreactive boutons invariably also received input, usually more distally, from unstained boutons that formed asymmetrical synaptic specializations. A study of 87 spines along the dendrites of an identified striatonigral neuron showed that the most common type of synaptic input was from an individual unstained bouton making asymmetrical synaptic contact (53%), while 39% of the spines received one asymmetrical synapse and one symmetrical immunoreactive synapse. It is proposed that the spatial distribution of presumed dopaminergic terminals in synaptic contact with different parts of striatonigral neurons has important functional implications. Those synapses on the cell body and proximal dendritic shafts might mediate a relatively non-selective inhibition. In contrast, the major dopaminergic input that occurs on the necks of dendritic spines is likely to be highly selective since it could prevent the excitatory input to the same spines from reaching the dendritic shaft. One of the main functions of dopamine released from nigrostriatal fibres might thus be to alter the pattern of firing of striatal output neurons by regulating their input.

Recent physiological and pathophysiological aspects of Parkinsonian movement disorders

Deficits in the neural control of limb movements constitute a major part of Parkinsonian symptoms and are linked to a decay of dopaminergic neurotransmission. In animal models, Parkinsonian-like hypokinesia is consistently reproduced with large nigrostriatal dopamine depletions, while tremor and rigidity are less readily obtained. Lesions leading to a less than 70% striatal dopamine depletion are largely compensated by an increased activity of dopamine terminals. With more important lesions, supersensitivity of striatal non-adenylate cyclase-linked dopamine receptors occurs. Electrophysiological studies in Parkinsonian patients demonstrate increased reaction times and a reduced build-up of movement-related muscular activity underlying hypokinesia and provide circumstantial evidence for a central origin of tremor and rigidity. Single cell activity in unlesioned, behaving monkeys shows an increasingly direct relationship to movements when following the neural connections from mid-brain dopamine cells via striatum, globus pallidus, thalamus to pyramidal tract neurons of motor cortex. These data corroborate experimentally the concept that Parkinsonian hypokinesia is due to a failure of basic behavioral activating mechanisms.

Characteristic alterations in responses to imposed wrist displacements in parkinsonian rigidity and dystonia musculorum deformans

The amplitude and temporal modulation of the segmented EMG activity in flexor carpi radialis, evoked by imposed angular wrist extension, was studied with respect to the level of pre-existing background activity in rigid parkinsonian (PK) and dystonia musculorum deformans (DMD) patients. The interdependence of the evoked M1 and M2-3 segments on pre-existing background EMG activity and initial velocity of imposed displacement was established previously for a normal population. Individual responses of 21 parkinsonian and 12 dystonic patients were compared to the established normal "response volume". The augmented magnitude of the M2-3 segment in rigid PK patients, which correlates to the measure of rigidity, could not be accounted for by the low level of pre-existing EMG activity. Therefore, increased descending facilitation does not impinge directly on alpha motoneurons. Paradoxical excitation in the shortened muscle and resetting of tonic tremor of the stretched muscle by the imposed wrist extension are two other demonstrated abnormalities which may also contribute to PK rigidity. In contrast, DMD patients demonstrated normal amplitude modulation of the M1 and M2-3 segments, but exhibited a disturbance of normal temporal mechanisms that result in constant duration of the M1 and M2-3 responses with imposed force step loads.

Behaviour correlates of neurotransmitter activity

Most disorders of motor activity including disturbances of muscle tone and of locomotor activity observed in patients with neurological disorders have been reproduced experimentally in animals. Most motor disorders of the extrapyramidal type including those associated with Parkinson's disease and choreiform and athetoid involuntary movements, have been reproduced exclusively in primates. This is most likely related to the highly complex organization of the extrapyramidal and related nervous mechanisms subserving the corresponding peculiar type of motor control in the primate brain. Other types of motor disturbances including cervical and trunkal dystonias, ataxia, hypotonicity, spasticity and intention tremor, however, have been successfully induced in various mammalian species. The latter types of motor disorders are related to disturbances of central nervous mechanisms which show similar patterns in the brains of different animal species. Histopathological and neurochemical changes associated with extrapyramidal disorders have been discovered and more precisely determined as a consequence of the development of new technical approaches. Therefore numerous morphological, physiological and neurochemical data concerning the extrapyramidal system are now available but a better knowledge of their precise and subtle interrelationship is greatly needed in order to develop more efficient therapeutic procedures.

Defective utilization of sensory input as the basis for bradykinesia, rigidity and decreased movement repertoire in Parkinson's disease: a hypothesis

From a review of the anatomical relationships and single unit activity in the components of the basal ganglia related to limb movement, it is concluded that the major outflow from basal ganglia circuits is via the motor cortex (area 4). Recent results of recording from area 4 neurons revealed that they preferentially "encode" the higher derivatives of movement, i.e. acceleration and jerk. In the parkinsonian (PK) patient and in the monkeys treated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), EMG responses to imposed loads show a markedly increased gain of the "M2" component which depends upon the integrity of area 4 and which correlates with the severity of PK rigidity. The above observations are considered, along with those of others (demonstrating prolonged movement times, a decreased "repertoire" of voluntary movements fractionation of voluntary movements, inability in tracking movements without visual input, and failure to improve performance in PK's) in relation to a model of the interactions between sensory input and motor programs. Using this model, it is hypothesized that the above PK movement deficits, as well as rigidity, can be accounted for by abnormal processing of the mechanoreceptor sensory input utilized in the generation and execution of movements. The MPTP treated monkey is suggested as a model in which to directly test the hypothesis.

A Golgi and ultrastructural study of the monkey globus pallidus

Golgi preparations reveal that the most frequent type of pallidal neuron (principal cell), which has been recognized in all previous reports, is large (20-50 microns), fusiform, with dendrites up to 700 microns long. Large neurons of globular shape are less frequently impregnated. The morphology of dendrites varies considerably within the same neuron. Some exhibit numerous spines and protrusions and are seen to terminate in elaborate arborizations. A small interneuron (12 microns), with relatively short dendrites, up to 150 microns, and a short sparsely branching axon is observed less frequently. At least two types of afferent axons are present. A small-diameter fiber from the neostriatum enters the pallidum in bundles and gives rise to numerous thin branching processes with varicosities about 1 micron in size. The axon collaterals are oriented orthogonal to the main axon and parallel to the dendrites of principal cells. A large-caliber fiber with clusters of 2-3 microns swellings can also be seen in close proximity to large pallidal dendrites. Ultrastructurally, principal cell dendrites (trunks, spines, and protrusions) are totally covered by synapsing axon terminals. In contrast, some small dentrites, presumed to belong to interneurons, form very few synapses. At least six categories of profiles containing vesicles are observed. One group has cytologic features of dendrites and participates in serial and triadic synapses with other profiles in the pallidal neuropil. Results suggest that the synaptic organization of the globus pallidus may be viewed as a repetitive, geometric arrangement of striatal and other afferent axons ensheathing and synapsing with the dendrites of principal cells. This pattern is interrupted by the presence of presynaptic dendrites, probably belonging to interneurons, which participate in complex synaptic arrangements.

An intracellular HRP study of the rat globus pallidus. I. Responses and light microscopic analysis

A study of the intracellularly recorded responses of rat globus pallidus neurons to activation of striopallidal fibers was combined with light microscopic examination of the morphology of these same neurons using intracellular horseradish peroxidase. The response to stimulation of caudate-putamen is an inhibitory postsynaptic potential with observed latencies ranging from 5.1 to 9.8 msec. These values correspond to conduction velocities of 0.4 to 0.8 m/second for striopallidal fibers. Comparison with extracellular controls shows no excitatory component to the response. All recovered and analyzed neurons (n = 11) were of the large type of pallidal neuron known from Golgi studies but the addition that two subtypes could be recognized. Large neurons located medially in the nucleus had dendritic fields with large dorsoventral extent (ca. 1 mm) when compared to their mediolateral and rostrocaudal dimensions (ca. 0.4 mm) and these neurons emitted no axon collaterals. Large neurons located laterally in the nucleus had disklike dendritic fields with both dorsoventral and rostrocaudal dimensions being on the order of 1 mm but with a minor axis of approximately 100 micrometers. The axons of these neurons possessed collaterals. As a consequence of their disk-shaped dendritic field, neurons belonging to the laterally placed subgroup and occupying the narrow (ca. 100 micrometers thick) striopallidal border zone known to receive a distinct input from neostriatum have dendrites restricted to that zone.

Contextual organization of unitary information processes in the cortex by the thalamus and basal ganglia and the central control of attention

Short-term memory experiments suggest the presence within the neocortex of a basic information unit (a data structure plus an algorithm) able to store and return a relatively fixed number of items. Units of a similar format may be involved in all cortical motor and sensory activities of an integrated nature, and may interact to yield higher-order-information units dealing with interrelationships among groups of units. If individual units can reference other similar units, a hierarchical organization of computing is possible that could form the basis for contextual information processing and the hierarchical structuring characteristic of many specifically human forms of behavior. Vertical columns of neurons in the cortex may provide the physical basis for information units of the type suggested. The selection and organizing of activity in groups of columns may be regulated by a neural loop involving the basal ganglia and the thalamus, which would thus implement the central adjustment of attention.