A neurophysiological analysis of the effect of kainic acid on nerve fibres and terminals in the cat spinal cord

Motor Neuron Susceptibility in ALS/FTD

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disease characterized by the death of both upper and lower motor neurons (MNs) in the brain, brainstem and spinal cord. The neurodegenerative mechanisms leading to MN loss in ALS are not fully understood. Importantly, the reasons why MNs are specifically targeted in this disorder are unclear, when the proteins associated genetically or pathologically with ALS are expressed ubiquitously. Furthermore, MNs themselves are not affected equally; specific MNs subpopulations are more susceptible than others in both animal models and human patients. Corticospinal MNs and lower somatic MNs, which innervate voluntary muscles, degenerate more readily than specific subgroups of lower MNs, which remain resistant to degeneration, reflecting the clinical manifestations of ALS. In this review, we discuss the possible factors intrinsic to MNs that render them uniquely susceptible to neurodegeneration in ALS. We also speculate why some MN subpopulations are more vulnerable than others, focusing on both their molecular and physiological properties. Finally, we review the anatomical network and neuronal microenvironment as determinants of MN subtype vulnerability and hence the progression of ALS.

Excitotoxicity and amyotrophic lateral sclerosis

Since its description by Charcot more than 130 years ago, the pathogenesis of selective motor neuron degeneration in amyotrophic lateral sclerosis (ALS) remains unsolved. Over the years, many pathogenic mechanisms have been proposed. Amongst others these include: oxidative stress, excitotoxicity, aggregate formation, inflammation, growth factor deficiency and neurofilament disorganization. This multitude of contributing factors indicates that ALS is a complex disease and also suggests that ALS is a multifactorial disorder. Excitotoxicity is not the newest and most spectacular hypothesis in the ALS field, but it is undoubtedly one of the most robust pathogenic mechanisms supported by an impressive amount of evidence. Moreover, the therapeutic efficacy of riluzole, the only drug proven to slow disease progression in ALS, is most likely related to its anti-excitotoxic properties. In this review, we will give an overview of the arguments in favor of the involvement of excitotoxicity in ALS and of the possible mechanisms leading to motor neuron death. We will also summarize the intrinsic properties of motor neurons that render these cells particularly vulnerable to excitotoxicity and could explain the selective vulnerability of motor neurons in ALS. All this information could help to develop new and better therapeutic strategies that could protect motor neurons from excitotoxicity.

The role of excitotoxicity in the pathogenesis of amyotrophic lateral sclerosis

Unfortunately and despite all efforts, amyotrophic lateral sclerosis (ALS) remains an incurable neurodegenerative disorder characterized by the progressive and selective death of motor neurons. The cause of this process is mostly unknown, but evidence is available that excitotoxicity plays an important role. In this review, we will give an overview of the arguments in favor of the involvement of excitotoxicity in ALS. The most important one is that the only drug proven to slow the disease process in humans, riluzole, has anti-excitotoxic properties. Moreover, consumption of excitotoxins can give rise to selective motor neuron death, indicating that motor neurons are extremely sensitive to excessive stimulation of glutamate receptors. We will summarize the intrinsic properties of motor neurons that could render these cells particularly sensitive to excitotoxicity. Most of these characteristics relate to the way motor neurons handle Ca(2+), as they combine two exceptional characteristics: a low Ca(2+)-buffering capacity and a high number of Ca(2+)-permeable AMPA receptors. These properties most likely are essential to perform their normal function, but under pathological conditions they could become responsible for the selective death of motor neurons. In order to achieve this worst-case scenario, additional factors/mechanisms could be required. In 1 to 2% of the ALS patients, mutations in the SOD1 gene could shift the balance from normal motor neuron excitation to excitotoxicity by decreasing glutamate uptake in the surrounding astrocytes and/or by interfering with mitochondrial function. We will discuss point by point these different pathogenic mechanisms that could give rise to classical and/or slow excitotoxicity leading to selective motor neuron death.

The organization and function of endogenous antinociceptive systems

Much progress has been made the understanding of endogenous pain-controlling systems. Recently, new concepts and ideas which are derived from neurobiology, chaos research and from research on learning and memory have been introduced into pain research and shed further light on the organization and function of endogenous antinociception. These most recent developments will be reviewed here. Three principles of endogenous antinociception have been identified, as follows. (1) Supraspinal descending inhibition: the patterns of neuronal activity in diencephalon, brainstem and spinal cord during antinociceptive stimulation in midbrain periaqueductal gray (PAG) or medullary nucleus raphe magnus have now been mapped on the cellular level, using the c-Fos technique. Results demonstrate that characteristic activity patterns result within and outside the PAG when stimulating at its various subdivisions. The descending systems may not only depress mean discharge rates of nociceptive spinal dorsal horn neurons, but also may modify harmonic oscillations and nonlinear dynamics (dimensionality) of discharges. (2) Propriospinal, heterosegmental inhibition: antinociceptive, heterosegmental interneurons exist which may be activated by noxious stimulation or by supraspinal descending pathways. (3) Segmental spinal inhibition: a robust long-term depression of primary afferent neurotransmission in A delta fibers has been identified in superficial spinal dorsal horn which may underlie long-lasting antinociception by afferent stimulation, e.g. by physical therapy or acupuncture.

Generators of the brainstem auditory evoked potential in cat. I. An experimental approach to their identification

This paper is the first in a series aimed at identifying the cellular generators of the brainstem auditory evoked potential (BAEP) in cats. The approach involves (1) developing experimental procedures for making small selective lesions and determining the corresponding changes in BAEP waveforms, (2) identifying brainstem regions involved in BAEP generation by examining the effects of lesions on the BAEP and (3) identifying specific cell populations involved by combining the lesion results with electrophysiological and anatomical information from other kinds of studies. We created lesions in the lower brainstem by injecting kainic acid which is generally toxic for neuronal cell bodies but not for axons and terminals. This first paper describes the justifications for using kainic acid, explains the associated problems, and develops a methodology that addresses the main difficulties. The issues and aspects of the specific methods are generally applicable to physiological and anatomical studies using any neurotoxin, as well as to the present BAEP study. The methods chosen involved (1) measuring the BAEP at regular intervals until it reached a post-injection steady state and perfusing the animals with fixative shortly after the last BAEP recordings were made, (2) using objective criteria to distinguish injection-related BAEP changes from unrelated ones, (3) making control injections to identify effects not due to kainic acid toxicity, (4) verifying the anatomical and functional integrity of axons in lesioned regions, and (5) examining injected brainstems microscopically for cell loss and cellular abnormalities indicating dysfunction. This combination of methods enabled us to identify BAEP changes which are clearly correlated with lesion locations.

Effects of kainic acid administered to the caudal ventrolateral medulla on arterial blood pressure in the spontaneously hypertensive and normotensive Wistar-Kyoto rats

To compare the inhibitory influence of the caudal ventrolateral medulla (CVLM) on rostral ventrolateral medulla (RVLM) between spontaneously hypertensive (SH) and normotensive Wistar-Kyoto (WKY) rats, we studied the blood pressure responses to administration of kainic acid, an excitotoxic agent, to CVLM. Unilateral or bilateral administration of kainic acid led to an initial reduction in blood pressure in both types of rats. In SH rats, this was followed by a gradual and sustained pressor response. This biphasic response in arterial blood pressure was observed in only one-third of the WKY rats. The elevation in blood pressure was significantly greater in SH than in WKY rats. The finding suggests that the tonic inhibitory influence from CVLM to RVLM may be greater in SH than in WKY rats.

A quantitative autoradiographic study of [3H]kainate binding sites in the normal human spinal cord, brainstem and motor cortex

The quantitative autoradiographic distribution of the kainate subtype of non-NMDA receptor in the normal human motor cortex, brainstem and spinal cord has been investigated using [3H]kainate. In the motor cortex specific [3H]kainate binding was present in all cortical laminae with the highest density in laminae and II and the upper part of III and lower densities in the middle and deep laminae. In the premotor cortex a band of high density was found in laminae V and VI as well as in the superficial laminae. In the normal brainstem kainate receptors had a heterogeneous distribution. Brainstem motor nuclei which tend to be affected in motor neuron disease (MND) had very low densities of binding sites, whereas the oculomotor nucleus had a higher density. Specific [3H]kainate binding was found throughout the spinal grey matter, the greatest density being found in the substantia gelatinosa and much lower densities in the rest of the grey matter including the ventral horns. Excitotoxicity at non-NMDA receptors has been implicated in the pathogenesis of MND. This study shows that the motor neuron groups vulnerable in MND express a low density of [3H]kainate binding sites and suggests that the density of kainate receptors does not account for selective vulnerability in this disorder.

Cell culture evidence for neuronal degeneration in amyotrophic lateral sclerosis being linked to glutamate AMPA/kainate receptors

Amyotrophic lateral sclerosis (ALS) is a neurodegenerative disorder affecting motor neurons. Glutamate, a potent central-nervous-system toxin, has been proposed as one possible factor in this motoneuron disease. Serum from patients with ALS is known to be toxic when added to neurons in culture. We report on the toxicity to rat neurons in culture of cerebrospinal fluid (CSF) from patients with ALS. CSF were obtained from 10 ALS patients, 10 neurological controls, and 10 other controls. ALS CSF was added at dilutions of 50%, 20%, or 10% and neuron survival was assessed after 24 h. The neuroprotective effects of antagonists to two glutamate receptors were also assessed. ALS CSF was significantly neurotoxic, with a neuronal survival rate of only 47% compared with 80% or so for control CSF. This neurotoxicity was blocked by CNQX, an antagonist to the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptor but not by two N-methyl-D-aspartate (NMDA) antagonists. ALS CSF contains a specific neurotoxic factor which is AMPA/kainate-like which could have a role in the neuronal degeneration of this disease.

Selectivity of kainic acid as a neurotoxin within the dorsal lateral geniculate nucleus of the cat: a model for transneuronal retrograde degeneration

In situ injections of the cytotoxin kainic acid were used to make localized lesions of the dorsal lateral geniculate nucleus in the adult cat to produce a model for studying the effects of postsynaptic target loss. Kainic acid has been used extensively to produce lesions of neuronal cell bodies within the central nervous system. However, the selectivity of kainic acid has been questioned, as it may also affect afferent terminals or axons of passage. Retinal projections to degenerated geniculate nuclei were visualized 1 week after kainate injection using anterograde labelling with horseradish peroxidase and electron microscopy. The results demonstrate the presence of afferent terminals within regions of neuronal loss, and hence the selectivity of kainic acid for intrinsic geniculate neurons.

Kainic acid on the rat ventral medullary surface depresses hypoxic and hypercapnic ventilatory responses

Kainic acid, topically applied to the ventral surface of the medulla immediately caudal to the trapezoid body in the urethane/chloralose anaesthetised rat, led to a depression of ventilation and a sustained rise in blood pressure; ventilatory responses to hypercapnia (10% carbon dioxide) and hypoxia (11% oxygen) were slightly depressed. Widespread application of kainic acid to an area at and slightly rostral to the rootlets of the hypoglossal nerve produced a stimulation of ventilation and an unsustained rise in blood pressure. Apnea ensued 12-28 min after application. Ventilatory responses to hypercapnia and hypoxia were markedly attenuated; more discrete bilateral application revealed two regions, one immediately rostral and lateral to the hypoglossal rootlets and the other over the point of exit of the hypoglossal nerve rootlets, which specifically contributed to the diminution of the chemosensory responses. These results raise questions about the medullary circuitry which mediates the chemoreflex regulation of breathing.

Kainic acid induces early and delayed degenerative neuronal changes in rat spinal cord

Intrathecal injections of kainic acid and sodium chloride were performed in rats to study the cellular modifications observed in spinal cord. Early neuronal changes (2 and 24 h) associated dark and shrunken or swollen and vacuolated cytoplasms. Delayed (3, 6, 14 days) changes, mainly consisted in degenerative aspect of motoneurons with eccentric and indented nucleus, swollen cytoplasms with proximal neurite enlargements, the presence of 'lipofuscin-like' pigments, disorganized intracytoplasmic organelles and filament accumulations.

Effects of kainic acid lesions in rat ventral lateral geniculate nucleus upon field potentials of the superior colliculus: correlation between morphological and physiological observations

Morphological and physiological effects of kainic acid (KA) lesions in rat ventral lateral geniculate nucleus (LGV) were studied 1.5 and 6 h after KA injection. Morphological changes were examined mainly by electron micrographs. At 1.5 h after KA injection dendrites were dilated and some vacuolations occurred in both dendrites and perikarya including geniculotectal relay neurons while axons were completely intact and cell organelles almost remained intact. Six h after KA injection dendrites and cell bodies were massively dilated with degeneration of cell organelles accompanied by sparse cytoplasm and deformed chromatin in the nucleus. However, almost all presynaptic axons, mainly retinogeniculate fibers, still remained intact. The electron micrographs demonstrate that destruction occurred first in dendrites, next in cell bodies and finally axons were likely to be affected. These morphological changes induced by KA are compatible with physiological effects which were assessed by the field response of the superior colliculus (SC) evoked by stimulation of the optic chiasm. During 1.5-2 h after KA injection all components of the SC response, the presynaptic and postsynaptic negative-positive waves were enhanced. The enhancement of the SC response may be correlated with morphological changes in terms of excitatory action of KA resulting in facilitation of geniculotectal transmission. Six h after KA injection postsynaptic negative-positive waves gradually declined in amplitude while the presynaptic wave returned to control level. The late suppression of postsynaptic components of the SC response may be attributable to a marked loss of geniculotectal transmission resulting from destruction of geniculotectal relay neurons by KA.

Kainic acid-induced terminal degeneration in the dorsal lateral geniculate of tree shrew

The dorsal lateral geniculate nucleus of tree shrews is very susceptible to the neurotoxic effects of kainic acid. In addition to neuronal loss, there is a profound loss of retinal terminals that is manifested through a disruption of anterograde transport of WGA/HRP from the retina to the kainic acid-lesioned area of the geniculate nucleus. The actions of kainic acid upon both the presynaptic terminals and geniculate neurons may be mediated by a glutamatergic pathway and questions the hypothesis that kainic acid is solely neuron-specific in its toxic action.

Cardiovascular role of A1 catecholaminergic neurons in the rabbit. Effect of chronic lesions on responses to methyldopa, clonidine and 6-OHDA induced transmitter release

We confirmed the findings of previous investigators that bilateral anodal lesions of the A1 region were associated with hypertension, bradycardia, pulmonary edema and a high mortality. All these sequelae (except the bradycardia) no longer occurred after cathodal lesions and these were therefore used to investigate the role of the catecholaminergic (CA) neurons of the A1 region in circulatory regulation. Conscious rabbits were studied 2-4 weeks after A1 lesions or sham-operation, when resting mean arterial pressure (MAP) and heart rate (HR) were closely similar in both groups. We tested for differences in MAP and HR responses between lesioned and sham-operated groups: to intracisternal (i.c.) alpha-methyldopa (MD) and to clonidine; and to the acute effects of i.c. 6-hydroxydopamine (6-OHDA) which elicits central CA release. Since these tests depend on the integrity of the central CA neurons, response differences between lesioned and sham-operated groups denote participation by the CA neurons of the A1 region in the central circulatory pathways. The bradycardia responses in the above tests were all smaller in lesioned than sham-operated rabbits, but there were no differences in MAP responses. Electrical stimulation of the region under alfathesin anaesthesia produced depressor responses at low frequencies and pressor responses at high frequencies. From the results in conscious rabbits CA neurons of the A1 region mainly influence the pathways regulating HR, rather than blood pressure. The changes in MAP during electrical stimulation are thus probably mediated through non-CA neurons.