A light and electron microscopic study of the effects of 3-acetylpyridine intoxication on the inferior olivary complex and cerebellar cortex

Spontaneous neuronal regeneration in the forebrain of the leopard gecko (Eublepharis macularius) following neurochemical lesioning

BACKGROUND

Neurogenesis is the ability to generate new neurons from resident stem/progenitor populations. Although often understood as a homeostatic process, several species of teleost fish, salamanders, and lacertid lizards are also capable of reactive neurogenesis, spontaneously replacing lost or damaged neurons. Here, we demonstrate that reactive neurogenesis also occurs in a distantly related lizard species, Eublepharis macularius, the leopard gecko.

RESULTS

To initiate reactive neurogenesis, the antimetabolite 3-acetylpyridine (3-AP) was administered. Four days following 3-AP administration there is a surge in neuronal cell death within a region of the forebrain known as the medial cortex (homolog of the mammalian hippocampal formation). Neuronal cell death is accompanied by a shift in resident microglial morphology and an increase neural stem/progenitor cell proliferation. By 30 days following 3-AP administration, the medial cortex was entirely repopulated by NeuN+ neurons. At the same time, local microglia have reverted to a resting state and cell proliferation by neural stem/progenitors has returned to levels comparable with uninjured controls.

CONCLUSIONS

Together, these data provide compelling evidence of reactive neurogenesis in leopard geckos, and indicate that the ability of lizards to spontaneously replace lost or damaged forebrain neurons is more taxonomically widespread and evolutionarily conserved than previously considered.

Two-Photon Laser Ablation and In Vivo Wide-Field Imaging of Inferior Olive Neurons Revealed the Recovery of Olivocerebellar Circuits in Zebrafish

The cerebellum, a brain region with a high degree of plasticity, is pivotal in motor control, learning, and cognition. The cerebellar reserve is the capacity of the cerebellum to respond and adapt to various disorders via resilience and reversibility. Although structural and functional recovery has been reported in mammals and has attracted attention regarding treatments for cerebellar dysfunction, such as spinocerebellar degeneration, the regulatory mechanisms of the cerebellar reserve are largely unidentified, particularly at the circuit level. Herein, we established an optical approach using zebrafish, an ideal vertebrate model in optical techniques, neuroscience, and developmental biology. By combining two-photon laser ablation of the inferior olive (IO) and long-term non-invasive imaging of "the whole brain" at a single-cell resolution, we succeeded in visualization of the morphological changes occurring in the IO neuron population and showed at a single-cell level that structural remodeling of the olivocerebellar circuit occurred in a relatively short period. This system, in combination with various functional analyses, represents a novel and powerful approach for uncovering the mechanisms of the cerebellar reserve, and highlights the potential of the zebrafish model to elucidate the organizing principles of neuronal circuits and their homeostasis in health and disease.

Histopathological evaluation of the nervous system in National Toxicology Program rodent studies: a modified approach

This article outlines the changes and underlying rationale for modifications to the histopathological evaluation of the nervous system during toxicology and carcinogenesis studies conducted by the National Toxicology Program (NTP). In the past, routine evaluation of the nervous system was mostly limited to three sections of brain, and occasionally the spinal cord and peripheral nerves. Factors such as the increasing occurrence of human neurological diseases and associated economical cost burden, the role of unidentified environmental stressors in neurodegenerative disorders, multiple therapeutic drug-induced neuropathies noted in human clinical trials, and the exponential use of environmental chemicals with unknown neurotoxic potential necessitate a more extensive evaluation of the nervous system. The NTP has modified its protocol to include examination of key anatomic subsites related to neurodegenerative diseases such as Parkinson's disease. Modifications include four additional sections of the brain. Increasing the number of brain sections permits examination of a greater number of specific anatomic subsites with unique vulnerability. In addition, the spinal cord, peripheral nerves, trigeminal ganglion, and intestinal autonomic ganglia will be evaluated as needed. It is expected that this modified approach will increase the sensitivity of detecting neurotoxicants and neurocarcinogens important in human neurologic and neurodegenerative disorders.

Reinnervation of late postnatal Purkinje cells by climbing fibers: neosynaptogenesis without transient multi-innervation

Synaptic partner selection and refinement of projections are important in the development of precise and functional neuronal connections. We investigated the formation of new synaptic connections in a relatively mature system to test whether developmental events can be recapitulated at later stages (i.e., after the mature synaptic organization has been established), using a model of postlesional reinnervation in the olivo-cerebellar pathway. During the development of this pathway, synaptic connections between climbing fibers (CFs) and Purkinje cells (PCs) are diffuse and redundant before synapse elimination refines the pattern. The regression of CFs during the first 2 postnatal weeks in the rat leads to mono-innervation of each PC. After unilateral transection of the rat olivo-cerebellar pathway and intracerebellar injection of BDNF 24 h after lesion, axons from the remaining inferior olive can sprout into the deafferented hemicerebellum and establish new contacts with denervated PCs at later developmental stages. We found that these contacts are first established on somatic thorns before the CFs translocate to the PC dendrites, recapitulating the morphological steps of normal CF-PC synaptogenesis, but on a relatively mature PC. However, electrophysiology of PC reinnervation by transcommissural CFs in these animals showed that each PC is reinnervated by only one CF. This mono-innervation contrasts with the reinnervation of grafted immature PCs in the same cerebellum. Our results provide evidence that relatively mature PCs do not receive several olivary afferents during late reinnervation, suggesting a critical role of the target cell state in the control of CF-PC synaptogenesis. Thus, synapse exuberance and subsequent elimination are not a prerequisite to reach a mature relationship between synaptic partners.

Episodic vestibular disruption following ablation of the inferior olive in rats: Behavioral correlates

The experiments herein investigate whether the behavioral responses to transient and episodic vestibular disruption and permanent ablation are distinct in the absence of climbing fiber input. Subjects in group 1 received an IP injection of PBS followed by an IP injection of niacinamide. Seven days later these rats received the first of 3 serial transtympanic injections of TTX on the same side with 7 days between each injection. Following each TTX injection rats displayed unilateral vestibular symptoms that persisted beyond 48h. Spontaneous barrel rolling behavior was not observed. Group 2 subjects received an IP injection of 3-acetylpyridine (3-AP)+niacinamide followed by the same TTX regimen as group 1. Following each TTX injection vestibular symptoms (severe body twisting and persistent spontaneous barrel rolling) emerged rapidly (<15min) and resolved by 72h. Group 3 subjects received an IP injection of 3-AP+niacinamide and 7 days later a single unilateral transtympanic injection of sodium arsanilate. Rats in group 3 developed vestibular symptoms similar to those observed in group 2 although there was no resolution of these symptoms. The results indicate that TTX has a rapid rate of infiltration and blockade of the VIIIth nerve that persists for >48h and then completely resolves. The contrast in vestibular symptoms between groups 1 and 2 suggest that climbing fibers are recruited soon after onset of vestibular disruption and play a role in attenuating the severity of vestibular symptoms associated with transient/episodic vestibular disruption.

Differential effects oftrans-crotononitrile and 3-acetylpyridine on inferior olive integrity and behavioural performance in the rat

The inferior olive climbing fibre projection is key to cerebellar contributions to motor control. Here we present evidence for a novel tool, trans-crotononitrile (TCN), to selectively inactivate the olive to study its functions. Anatomical, electrophysiological and behavioural techniques have been used in rats to assess the CNS effects of TCN, with a focus on the olivocerebellar projection. These findings were compared with those obtained with 3-acetylpyridine (plus nicotinamide administered 3.5 h later, 3AP + 3.5 h). Fluoro-Jade B cell labelling showed that TCN and 3AP + 3.5 h induce neurodegeneration primarily within the inferior olive, with no other targets in common. Recordings of evoked field potentials on the cerebellar cortical surface showed that both neurotoxins can reduce transmission in climbing fibre but not mossy fibre pathways. Both histological and electrophysiological differences suggest that TCN and 3AP have distinct mechanisms of action. Estimates of the numbers of surviving cells within individual subdivisions of the olive indicate that TCN and 3AP + 3.5 h cause different patterns of subtotal olivary lesion: most surviving neurons are present in the rostral (TCN) or caudal (3AP + 3.5 h) parts of the medial accessory olive, which are associated with two different cerebellar modules: the C2 and A modules, respectively. In behavioural studies, TCN and 3AP + 3.5 h produced differences in motor deficits consistent with the notion that these cerebellar modules have distinct functional responsibilities. Thus, studies using TCN as compared with 3AP + 3.5 h have the potential to shed light on the contributions of different cerebellar modules in motor control.

Comparative effects of lesions to the ponto-cerebellar and olivo-cerebellar pathways on motor and spatial learning in the rat

Emerging evidence supports the role of the cerebellum in motor learning and previous studies have also shown that olivary projections to the cerebellum are involved in motor learning. Since the pontine nuclei make up the other main relay centre in the cerebro-cerebellar pathway, the purpose of the present study was to verify the involvement of the ponto-cerebellar pathway in motor and spatial learning, by comparing these functions in intact animals and in rats with selective injury of the olivary or pontine neurons. Two groups of rats were used: the first was treated with 3-acetylpyridine to destroy the inferior olivary complex, the second received electrolytic lesions of the middle cerebellar peduncle to interrupt the ponto-cerebellar pathway. Control and lesioned rats were then submitted to three tasks: unrotated rod, rota-rod at 20 r.p.m., and Morris water maze. In the first task both 3-acetylpyridine-treated rats and rats with lesions of the middle cerebellar peduncle showed static equilibrium deficiencies. Through training, however, they reached the maximal score attained by the controls. The rats submitted to the rota-rod at 20 r.p.m. obtained scores significantly inferior to the controls. The Morris water maze results indicated that the lesion of inferior olivary complex and middle cerebellar peduncle both alter learning of the spatial task. These findings show that both the ponto- and olivo-cerebellar pathways are involved in learning complex motor sequences and spatial tasks. Since both projections converge onto Purkinje cells, our results suggest an integration of these two pathways in the cerebellar control of learning mechanism.

The olivocerebellar projection mediates ibogaine-induced degeneration of Purkinje cells: a model of indirect, trans-synaptic excitotoxicity

Ibogaine, an indole alkaloid that causes hallucinations, tremor, and ataxia, produces cerebellar neurotoxicity in rats, manifested by degeneration of Purkinje cells aligned in narrow parasagittal bands that are coextensive with activated glial cells. Harmaline, a closely related alkaloid that excites inferior olivary neurons, causes the same pattern of Purkinje cell degeneration, providing a clue to the mechanism of toxicity. We have proposed that ibogaine, like harmaline, excites neurons in the inferior olive, leading to sustained release of glutamate at climbing fiber synapses on Purkinje cells. The objective of this study was to test the hypothesis that increased climbing fiber activity induced by ibogaine mediates excitotoxic Purkinje cell degeneration. The inferior olive was pharmacologically ablated in rats by a neurotoxic drug regimen using 3-acetylpyridine, and cerebellar damage attributed to subsequent administration of ibogaine was analyzed using immunocytochemical markers for neurons and glial cells. The results show that ibogaine administered after inferior olive ablation produced little or no Purkinje cell degeneration or glial activation. That a lesion of the inferior olive almost completely prevents the neurotoxicity demonstrates that ibogaine is not directly toxic to Purkinje cells, but that the toxicity is indirect and dependent on integrity of the olivocerebellar projection. We postulate that ibogaine-induced activation of inferior olivary neurons leads to release of glutamate simultaneously at hundreds of climbing fiber terminals distributed widely over the surface of each Purkinje cell. The unique circuitry of the olivocerebellar projection provides this system with maximum synaptic security, a feature that confers on Purkinje cells a high degree of vulnerability to excitotoxic injury.

Evidence for an active type of cell death with ultrastructural features distinct from apoptosis: the effects of 3-acetylpyridine neurotoxicity

3-Acetylpyridine is a niacinamide antagonist with potent neurotoxic properties in vitro and in vivo. 3-Acetylpyridine neurotoxicity was associated with positive DNA end-labelling and displayed features of active cell death without the ultrastructural changes of apoptotic cell death. After systemic administration in rats (70 mg/kg), we detected labelled nuclei in the inferior olive using in situ DNA end-labelling. However, the conventional chromatin stain did not show chromatin condensation or fragmentation and electron microscopy studies failed to reveal features of apoptosis. Although areas of condensed chromatin were present in some nuclei, cytoplasmic damage with extensive organelle swelling was the most prominent finding. In vitro, 3-acetylpyridine (0.1-1 mM) induced degeneration of cerebellar granule neurons in a concentration- and time-dependent manner. The protein synthesis inhibitor cycloheximide (10 micrograms/ml) and the transcriptional inhibitor actinomycin D (10 microM) protected against 3-acetylpyridine toxicity. In contrast, neither the free radical scavenger alpha-phenyl-N-tertbutylnitron (100 microM), nor glutathione ethyl ester (10-100 microM), N-acetyl-cysteine (10-200 microM) or 3-aminobenzamide (0.1-4 mM), an inhibitor of poly(ADP-ribose) synthesis, were effective. 3-Acetylpyridine-induced neuronal death in vitro was associated with positive in situ DNA labelling. However, DNA fragmentation could not be demonstrated prior to neuronal cell loss and no DNA "laddering" was detected by DNA gel electrophoresis. Correspondingly, no apoptotic nuclei were revealed upon electron microscopy but organelle swelling and extensive vacuolization, changes similar to autophagocytosis. In conclusion, 3-acetylpyridine induces an active form of cell death that required de novo protein synthesis but is distinct from apoptosis. A loss of glutathione accompanies, but does not precede, cell death.

3-Acetylpyridine-induced degeneration and regeneration in the adult lizard brain: a qualitative and quantitative analysis

The neurotoxin 3-acetylpyridine (3AP) produces highly selective neuronal damage in specific areas of the lizard brain. Following 3AP intoxication, proliferation and migration of replacement neurons born in the ventricular walls lead to regeneration of the lesioned areas. Earlier studies established the time course of 3AP-induced degeneration and subsequent regeneration in the medial cerebral cortex of adult lizards (Font, E., García-Verdugo, J.M., Alcántara, S. and Lopez-García, C., Neuron regeneration reverses 3-acetylpyridine-induced cell loss in the cerebral cortex of adult lizards, Brain Res., 551 (1991) 230-235 [13]). Complementary to our previous studies, we now provide a qualitative and quantitative account of the extent and distribution of neurotoxic damage in the brain as a whole of lizards treated with 3AP using Nissl and Golgi stains, a degeneration-sensitive reduced-silver method, and electron microscopy. Additionally, [3H]thymidine autoradiography was used to assess changes in the rate of neurogenesis caused by the 3AP treatment. Single doses of 3AP caused degenerative changes in all the cortical areas, anterior dorsal ventricular ridge, deep layers of the lateral cortex, lateral amygdaloid nucleus, and nucleus sphericus, while sparing other brain areas. The most frequent neuropathic change after 3AP treatment was clumping of the nuclear chromatin with formation of pyknotic nuclei. Occasionally, a second type of injury was observed in neurons of the cell layer of the dorsomedial cortex (DMC). 3AP also caused a conspicuous loss of dendritic spines in bipyramidal neurons of the dorsomedial and dorsal cortices possibly representing transneuronal degeneration. Numbers of [3H]thymidine-labeled cells were higher in lizards previously treated with 3AP than in controls. These results demonstrate that the neurotoxic lesion is capable of inducing an increase in the normal rate of adult neurogenesis. Whereas regeneration in the remaining areas was morphologically and histologically complete, in some animals, cell proliferation in the DMC resulted in formation of an abnormal cell plate.

Olivopontocerebellar pathology in multiple system atrophy

Olivopontocerebellar atrophy (OPCA) is widely accepted as part of the neuropathological spectrum of multiple system atrophy (MSA). The distribution of affected sites in the olivopontocerebellar (OPC) system and their interrelationship remain poorly understood due to lack of quantitative studies. To further investigate the OPC pathology in MSA, we performed a morphometric analysis of 20 MSA cases and eight healthy controls. In the MSA cases, mean neuronal cell densities were significantly reduced in (medial and dorsal) accessory and principal inferior olives, pontine nuclei, cerebellar vermis (except nodulus), and hemispheres. Inferior olives and pontine nuclei were more severely affected than cerebellar Purkinje cells in most cases. Cerebellar Purkinje cells were more severely depleted in vermis rather than in hemisphere. There was a poor topographic correlation between neuronal cell loss in inferior olives and cerebellar cortex. These results suggest a primary degeneration of olivopontine nuclei and cerebellar Purkinje cells in OPCA. Inferior olives, pontine nuclei and cerebellar cortex were all significantly more severely affected in cases with a pure or predominating cerebellar syndrome (OPCA type, n = 4) compared to those with pure or predominating parkinsonism (SND type, n = 14). However, although cerebellar signs had been noted in life in only six cases, morphometry revealed OPCA in 17 of the 20 MSA brains.

Reduction of GABA inhibition in Purkinje and cerebellar nuclei neurons in climbing fibre deafferented cerebella of rat

GABA agonists were iontophoretically applied to Purkinje cells (PCs) of the cerebellar cortex and to neurons of the cerebellar nuclei (NCNs) in normal and in climbing fibre (CF) deafferented cerebella of rat. The experiments were performed one and three months after CF deafferentation obtained by total inferior olive destruction with 3-acetylpyridine. All control PCs were dose-dependently inhibited by GABA and muscimol and nearly all by baclofen. After CF deafferentation, the number of PCs sensitive to muscimol remained the same but the number sensitive to baclofen was greatly reduced one month later and almost absent after three months. The number of NCNs inhibited by GABA was slightly reduced one month after deafferentation compared to controls, but reduced to less than half three months after. Thus CF deafferentation of the PC leads to changes in postsynaptic sensitivity to GABA, the presumed inhibitory neurotransmitter, affecting GABAA receptors of the NCNs and GABAB receptors of the PCs.

3-Acetylpyridine-induced degeneration in the dorsal root ganglia: involvement of small diameter neurons and influence of axotomy

3-Acetylpyridine (3-AP), an analogue of nicotinamide, produces highly selective CNS lesions, the severity of which may be influenced by prior alterations in the metabolic activity of the affected neurons. The present study was undertaken to determine whether prior axotomy modified the response of dorsal root ganglia (DRG) and anterior horn (AH) neurons to 3-AP. A single administration (50 or 80 mg/kg i.p.) of 3-AP to adult rats resulted in degeneration of primarily small-dark DRG neurons by 24 h. The AH neurons were not affected by either dose of 3-AP. Light and electron microscopy of the DRG revealed a spectrum of damage ranging from loss of Nissl substance and cytoplasmic degradation to frank necrosis with neuronophagia. Frequently, injured neurons exhibited perinuclear aggregation of cytoplasmic organelles with dissolution of Nissl substance, clearing of the peripheral cytoplasm, and formation of large peripheral vacuoles. Occasionally, a second pattern of 3-AP injury was observed in which the nuclear chromatin of the neurons was condensed and there was formation of small vacuoles throughout the cytoplasm without peripheral clearing or perinuclear aggregation of cytoplasmic organelles. Axotomy induced typical axon reactions in both large-pale and small-dark DRG neurons. The combination of axotomy followed by 3-AP 4 days later produced morphological features characteristic of both axotomy and 3-AP exposure, but did not appear to alter the incidence of neuronal cell death. The almost exclusive vulnerability of the small dorsal root ganglion neurons to 3-AP neurotoxicity make this model potentially useful for the study of small fibre neuropathies.

Ibotenic acid-induced calcium deposits in rat substantia nigra. Ultrastructure of their time-dependent formation

The excitotoxin ibotenic acid (IBO) induces local calcium deposits upon injection into rat substantia nigra. Their formation has been investigated at the ultrastructural level in a time course study from 2 days to 8 weeks survival. Potassium bichromate stain was used to visualize pathological calcium accumulation. Two days after IBO application, reaction product for calcium was observed in mitochondria of degenerating perikarya and dendrites, but not in axons, boutons or glia. Four days after the lesion, calcium stain was found, in addition, in a seemingly free form in a few dendrites, especially those still contacted by intact boutons and not sequestrated by invading glia. Two days later, most of these calcium-accumulating dendrites were separated by astroglia from their synaptic partners. At the border between glia and dendrite a fibrillar matrix was formed which further accumulated calcium. During the following weeks this matrix enlarged stepwise and was infiltrated with calcium, thus giving a picture resembling the annual growth rings of trees. The evolving bodies incorporated smaller deposits in their vicinity, finally representing the large concretions seen at the light microscopic level from the 4th postoperative week onward. Similarities and dissimilarities of these observations with the results from other ultrastructural studies on excitotoxin lesions are detailed. It is suggested that the different outfit of neuronal subpopulations and of glia with ligand-gated and metabotropic glutamate receptors in the single brain region, as well as the local response repertoire of glial cells towards the excitotoxic injury with the subsequent formation of a calcium-accumulating matrix provide the molecular basis for the formation of calcium deposits.

Reinnervation of cerebellar Purkinje cells by climbing fibres surviving a subtotal lesion of the inferior olive in the adult rat. I. Development of new collateral branches and terminal plexuses

Cerebellar climbing fibres react by collateral sprouting after subtotal lesions of the inferior olive, and the newly formed branches are able to reinnervate neighbouring denervated Purkinje cells. In the present paper, we used the Phaseolus vulgaris leucoagglutinin (PHA-L) tracing technique to label the climbing fibres and study their plasticity in detail at the light microscopical level. The specific objectives were to study the time course and morphological aspects of their sprouting, to estimate their extent of growth, and to compare the newly formed terminal plexuses with normal climbing fibres. Intraperitoneal injection of 3-acetylpyridine induced degeneration of the majority of the olivary neurones, which terminate as climbing fibres in the cerebellar cortex. Regularly, small numbers of neurones survived in the inferior olive. In the cerebellar cortex scattered surviving climbing fibres were found, which were devoid of any sign of injury. Already 3 days after the lesion, surviving climbing fibres had emitted collateral branches, which elongated for some distance through the molecular layer and ended with a number of varicosities and very fine branchlets. By 7 days, it was possible to recognize new developing arbours which grew in the molecular layer with the same orientation as normal climbing fibres. At longer survival times, extensive terminal arbours had developed and double labelling experiments confirmed that they terminated around the proximal dendrites of Purkinje cells. The newly formed terminal plexuses resembled, in all essential aspects, normal climbing fibres. In addition, from 1 month onward, it was evident that every surviving climbing fibre was able to form several new terminal plexuses reinnervating a number of neighbouring Purkinje cells. The result of this process was the formation of large clusters of newly formed plexuses around the parental arborization. Quantitative estimates indicated that the domain of innervation of single surviving climbing fibres could be increased by more than six times. It is concluded that climbing fibres surviving a subtotal olivary lesion are capable of extensive sprouting, axonal growth, and formation of new terminal plexuses, which resemble normal climbing fibres. Previous electrophysiological evidence indicates that this reinnervation is functional. The high specificity with which sprouting olivary axons reinnervate the proximal Purkinje cell dendrites suggests the existence of precise interactions between the growing fibres and their target. This example of "homotypic" collateral sprouting and reinnervation may thus provide a useful model for the study of nerve-target interactions.

Neuron regeneration reverses 3-acetylpyridine-induced cell loss in the cerebral cortex of adult lizards

Systemic administration of the neurotoxin 3-acetylpyridine to adult lizards results in extensive loss of neurons in the medial cerebral cortex, other brain areas remaining largely unaffected. After the neurotoxic trauma, new cells are produced by mitotic division of cells in the ventricular wall. The new cells migrate along radial glial fibers and replace lost neurons in the medial cortex. Electron microscopic examination of cells labeled with [3H]thymidine confirms that the newly generated cells are neurons. Thus, neuron regeneration can occur in the cerebral cortex of adult lizards.

Effect of 3-acetylpyridine on serotonin uptake and release from rat cerebellar slices

The cerebellum receives indolaminergic fibers influencing Purkinje cell discharges. Data from our laboratories have demonstrated an endogenous release of serotonin (5-HT) and a Na(+)-dependent uptake and Ca(2+)-dependent release of [3H]5-HT from slices, homogenates and synaptosomal fractions of the rat cerebellar molecular layer. While the neurotransmitter produced by climbing fibers has been sought for in several studies and some of the classical transmitters have been ruled out, as yet this neurotransmitter is unknown. The aim of this work was to measure the 5-HT uptake and release from rat cerebellar slices, 6 h and 15 days after intraperitoneal injection of 3-acetylpyridine (3-AP) (75 mg/kg), harmaline (15 mg/kg) and nicotinamide (300 mg/kg). A histological study of medulla and cerebellar cortex in these animals showed destruction of neurons in the inferior olivary nuclei and changes in the granulation of the cortical molecular layer in the cerebellum. A significant reduction of the 5-HT content (100%), 5-HT uptake (60%) and its Vmax (60%) was seen on the 5th day, in cerebellar preparations obtained from rats injected with 3-AP. The Ca(2+)-dependent release of 5-HT from these preparations was found to be similar to the basal values, in spite of depolarizing stimuli with 53 mM KCl or veratrine (60 micrograms/ml). The results suggest that 5-HT could play an important role as neurotransmitter produced by some climbing fibers.

Decreased sensitivity of cerebellar nuclei neurons to GABA and taurine: effects of long-term inferior olive destruction in the rat

The effects of iontophoretically applying the presumed Purkinje cell inhibitory neurotransmitters, GABA and taurine, were tested on neurons of the cerebellar nuclei in normal and in climbing-fiber-deafferented cerebella. Rats treated with 3-acetylpyridine to totally destroy the inferior olive were used for acute experiments 105-185 days after treatment. In controls, nearly all neuronal firing was dose-dependently depressed by both inhibitory amino acids. The depression in firing for both were antagonized by bicuculline and picrotoxin but not by strychnine while TAG specifically antagonized only responses to taurine. At sufficient doses, bicuculline and TAG induced disinhibitory responses (significant release of neuron discharge) in the absence of applied antagonist. In deafferented animals, the inhibitory efficacy of GABA and taurine were drastically reduced; most of the neurons failed to respond to these amino acids at the same iontophoretic parameters as for the control rats. Moreover, high doses of bicuculline and TAG did not induce any disinhibitory response (no significant increase in discharge rate) in most of the neurons tested. These results clearly demonstrate that climbing fiber deafferentation reduces postsynaptic sensitivity of the cerebellar nuclei neurons for the presumed Purkinje cell inhibitory neurotransmitters.

Loss of N-methyl-D-aspartate sensitivity of cerebellar Purkinje cells after climbing fiber deafferentation. An in vivo study in the rat

The sensitivity of cerebellar Purkinje cells to brief iontophoretic applications of excitatory amino acids has been studied in vivo in rats treated from 15 days to 4 months beforehand with 3-acetylpyridine in order to destroy the inferior olive. Responses of Purkinje cells (PCs) chronically deprived of climbing fibers were thus investigated using extracellular microelectrodes and compared to those of a group of control rats. No changes in the relative efficiencies of L-glutamate, L-aspartate, quisqualate and kainate have been observed. In contrast, the excitations induced by N-methyl-D-aspartate (NMDA) on most PCs in control animals, were no longer present after climbing fiber deprivation. These results show that both NMDA and non-NMDA receptors are present on PCs of adult rodents and that the NMDA responses are strongly depressed when PCs are deafferented of the climbing fibers.

Persistent reduction of Purkinje cell inhibition on neurones of the cerebellar nuclei after climbing fibre deafferentation

The long lasting effects of inferior olive (IO) destruction were studied in rats treated with 3-acetylpyridine two years before. The activities of the Purkinje cells (PCs) and of their target neurones in the cerebellar nuclei (ECNs), were investigated and compared to those of a group of non-treated rats. Our results show that long-term deafferented PCs recovered a mean firing frequency similar to that of the controls while the ECNs discharge is enhanced. Furthermore, the cryodestruction of the cerebellar cortex produces a significant release of the ECNs firing in the control rats but not in the poisoned animals. Thus, it appears that IO destruction induces a permanent impairment of the inhibitory control exerted by the PCs on the activity of their target neurones.

Nerve cell atrophy and loss in the inferior olivary complex of "Purkinje cell degeneration" mutant mice

The genetically determined loss of cerebellar Purkinje cells (PCs) in "Purkinje cell degeneration" (pcd) mutant mice deprives inferior olivary (IO) neurons of their major postsynaptic target. The degeneration of PCs starts on postnatal day (P) 17 and loss of these neurons is virtually complete by P45. We examined the inferior olivary complex (IOC) of normal and pcd mutant mice by quantitative light microscopy to determine whether the degeneration of PCs is associated with atrophy and loss of their presynaptic neurons in the IOC. The number of IO neurons in 17-day-old mutants did not differ significantly from controls (P greater than .1). IO neurons in 23-day-old mutants were 23% (95% confidence limits: 12-34%) fewer than in age-matched controls, and in 300-day-old mutants they were 48% (95% confidence limits: 37-58%) fewer than in their controls (P less than .001 in both cases). The decline of the number of IO neurons in pcd mice between days 17 and 300 was 49% (P less than .0001; 95% confidence limits: 38-57%). The medial accessory olive (MAO) appeared less affected than the principal (PO) and the dorsal accessory olive (DAO). The mean neuronal diameter in control mice was 11.6 micron at 23 days and 10.8 micron at 300 days of age. The respective values in pcd mutants were 11.5 micron and 8.7 micron. Diameters in old mutants were significantly smaller than those in both age-matched controls and young mutants (P less than .001). These findings suggest that in the mature olivocerebellar system the stability of IO neurons depends on the state of their postsynaptic PCs.

A comparison of the effects of climbing fiber deafferentation in adult and weanling rats

The climbing fiber input to the cerebellar cortex was destroyed using both electrolytic and chemical (3-acetylpyridine) lesions. The long-term effects of climbing fiber deafferentation on the ansiform lobule of weanling and adult rats were examined at both the light and electron microscopic levels. Image analysis of Golgi-impregnated Purkinje cells indicated a significantly lower number of smooth branches and spiny branchlets following climbing fiber deafferentation of both adult and weanling rats. The results suggest that the lower number of smooth branches and spiny branchlets following climbing fiber deafferentation of the weanling rat is the result of a loss of postnatal growth rather than transneuronal degeneration. Ultrastructural evidence is provided in confirmation of these quantitative findings. Formation of ectopic dendritic spines was found following climbing fiber deafferentation of the weanling rat, but not the adult. It is shown that ectopic spines and the denervated dendritic thorns of these animals were synaptically innervated by the parallel fiber system and basket axons. The formation of ectopic spines on climbing fiber deafferentated Purkinje cells may represent a form of dendritic plasticity. Ultrastructurally, the dendritic arborizations of weanling deafferentated Purkinje cells showed no signs of transneuronal degeneration. However, the primary response to climbing fiber deafferentation in the adult rat was marked transneuronal degeneration of the Purkinje cell dendrites. It is suggested that the inability of the adult Purkinje cell to form ectopic spines and to replace the excitatory postsynaptic potential of the climbing fiber varicosity is directly related to the Purkinje cell's subsequent transneuronal degeneration.

Long term modification of rubral activity after inferior olive destruction

The long term effects of the inferior olive degeneration on red nucleus activity were studied in the rat. The animals were injected with 3-acetylpyridine to produce a pharmacological destruction of the inferior olive and were then used for acute experiments at 1-2, 5-7, 14-18, 29-37, 81-110 and 236-255 days later. After degeneration of the inferior olive, there was an 'initial period' lasting for a few days, characterized by a low discharge frequency of the red nucleus neurones. A 'period of adaptation' followed during the first month, characterized by a slow recovery towards the control firing rates of the rubral units. Nevertheless, the temporal distribution of the discharges was not recovered since the firing became organized in a bursting activity. From 1 up to 8 months, the normal unit activity was not restored. The hypothesis is advanced that the suppression of the inferior olive which increases the cerebellar inhibition, produces a consequent disfacilitation of red nucleus activity which persists for a few days. Then at increasing survival times, a progressive compensation takes place without a real restoration of the initial rubral activity.

Inferior olive destruction induces dysfacilitation of the red nucleus activity

The spontaneous discharge frequency of the red nucleus neurones was evaluated in rat before and after total destruction of the inferior olive with the 3-acetylpyridine. It was found that the sustained firing recorded in the control animals (33.7 +/- 23.7/s) drastically decreased to a low rate (4.6 +/- 7.1/s) after the intoxication. This effect took place between 2 h 15 min and 2 h 45 min after the injection. which is the critical period corresponding to the inferior olive degeneration and the consequent climbing fiber deafferentation. It lasted at least as long as the acute experiment. The conclusions are reached that the inferior olive destruction leading to an enhanced cerebellar inhibition, produces a dysfacilitatory effect on the red nucleus neurones.

The glial cells in the nerve fiber layer of the rat olfactory bulb

In mammals the olfactory receptor neurons are the only ones that are known to undergo continuous cell renewal in the adult animal. This means that the axon of each newly formed neuron must grow into the olfactory bulb to find its appropriate target cell. It is presumed that astrocytes ensheath the olfactory axons as they course through the nerve fiber layer of the bulb even though the cells in question differ ultrastructurally from typical astrocytes. The purpose of the present study was to examine the glial cells in the nerve fiber layer of the rat olfactory bulb in an effort to resolve this apparent discrepancy. Two morphologically distinct types of glial cell were found in the nerve fiber layer. One type, which resembled the typical astrocytes that are present in other areas of the central nervous system, contained bundles of filaments in an electron-lucent cytoplasm. These cells also formed endfeet on blood vessels and formed part of the external glial limiting membrane. They did not, however, ensheath the olfactory axons. The cytoplasm of the other type of glial cell was denser than that of typical astrocytes and contained fewer filaments, which were seldom grouped into bundles. These cells also formed part of the glial limiting membrane at the surface of the bulb and were the only ones that ensheathed the olfactory axons. It is concluded that the cell ensheathing the olfactory axons in the nerve fiber layer of the rat olfactory bulb is a morphological variant of the typical astrocyte.(ABSTRACT TRUNCATED AT 250 WORDS)

Sensitivity of rat inferior olivary neurons to 3-acetylpyridine

Climbing fiber deafferentation of the weanling rat was performed initially by means of intraperitoneal administration of 3-acetylpyridine (3-AP, 65 mg/kg). This dosage of 3-AP, which has been used routinely to achieve olivary ablations in the adult rat, is inappropriate for the total destruction of the inferior olivary complex in weanling rats (21-23 days). Examination of the inferior olivary complex at both the light and electron microscopic levels has revealed a consistent sparing of neurons within the nucleus beta, the genu of the principal nucleus and the caudal portion of the medial accessory olive following long-term (120 days) 3-AP treatment. Complete olivary ablations in weanling rats were obtained only after the injection of a higher dose of 3-AP (90 mg/kg). It would appear that immature inferior olivary neurons are less sensitive to the toxic effect of 3-AP but become increasingly susceptible to 3-AP with maturation. If the appropriate dosage of 3-AP is employed, however, it can be used reliably to produce a chemical ablation of the total climbing fiber input to the cerebellar cortex in weanling rats.

Studies on the possible sites of chlordecone-induced tremor in rats

A series of psychopharmacological agents were administered to adult male Fischer-344 rats pretreated with a tremorigenic dose of chlordecone in an attempt to elucidate the involvement of spinal and supraspinal processes in the mediation and/or expression of chlordecone-induced tremor. Agents effective in attenuating the frequency of tremor were chlordiazepoxide, muscimol, and mecamylamine; quipazine exacerbated the tremor. Catecholaminergic agents including yohimbine, clonidine, propranolol, and haloperidol did not affect the frequency of chlordecone-induced tremor. Disinhibition of postsynaptic inhibitory sites in the spinal cord with strychnine and antagonism of spinal and supraspinal polysynaptic pathways with mephenesin exacerbated and attenuated the effects of chlordecone, respectively. Destruction of the climbing fibers with 3-acetylpyridine effectively blocked harmine, but not chlordecone-induced tremor, suggesting that chlordecone does not act through this pathway.

Cerebellar output regulation by the climbing and mossy fibers with and without the inferior olive

The activity of the olivocerebellar complex and the structures related in series with it have been studied using the complementary action of harmaline and 3-acetylpyridine to isolate the two principal inputs to the cerebellar Purkinje cells. The activities of the various nuclei as well as the entire brain have been simultaneously monitored using the [14C]2-deoxy-glucose method under the various combined effects of the pharmacological agents. (1) Tremogenic doses of harmaline increased the frequency of discharge in selected parts of the olivocerebellar system, increasing climbing fiber input and reducing Purkinje cell simple spike discharges in corresponding parts of the cerebellar cortex. The metabolic activity increased in the inferior olive and in the red nucleus. The results are interpreted as a net reduction of Purkinje cell inhibition on their target neurons, leading to a facilitatory cerebellar output. (2) Systemic injection of neurotoxic doses of 3-acetylpyridine selectively produced total degeneration of the neurons in the inferior olive, resulting in the suppression of complex spikes and a net increase in simple spike output from the Purkinje cells. The metabolic consequences were a reduction or absence in the inferior olive, decrease in the red nucleus, and increases in the Purkinje cell target neuron regions, including the intracerebellar and vestibular nuclei. The study of long survival times following the neurotoxic treatment revealed a transient metabolic marking of the inferior olive during the active glial processes accompanying the degeneration. In other parts the radioautographic changes caused by the destruction of the inferior olive persisted for about 1 month after the administration of the drug. (3) Tremogenic doses of harmaline were given to rats at different times following treatment with 3-acetylpyridine. It was demonstrated that: (a) intoxication of the inferior olive started within the second hour after 3-acetylpyridine administration, corresponding to the time at which the metabolic response to harmaline was also abolished; and (b) the increased metabolic activity produced by harmaline in the olivocerebellar complex was a consequence of an increased activity of the neurons of the inferior olive rather than a direct pharmacological effect of the drug. (4) Partial lesions of the inferior olive led to increased metabolic activity of those parts of the intracerebellar nuclei topographically related to the destroyed parts of the inferior olive. (5) In 3-acetylpyridine-treated animals, local ablation as well as local inactivation of the cerebellar cortex produced localized suppression of the intense labeling in the intracerebellar nuclei obtained in these animals. Since these regions receive synapses which are normally inhibitory, suppression of labeling clearly supports the hypothesis that regional marking may very well be produced by the activity of the presynaptic terminals themselves...

Climbing fiber destruction affects dendrite and spine membrane organization in Purkinje cells

Destruction of climbing fibers, one of the presynaptic inputs to Purkinje cells, was achieved by intraperitoneal injection of 3-acetylpyridine in rats. Freeze-fracture morphology of the Purkinje cell membrane was studied under these conditions. Quantitative analysis reveals a decrease in the number of intramembrane particles (IMP) in the membrane E-face of dendrites and spines of large dendrites, both postsynaptic targets for climbing fibers. The membrane of the perikaryon and of the spines from the spiny branchlets were unaffected by climbing fiber destruction. These results suggest that under the conditions studied, the membrane organization in well-defined areas of the Purkinje cell may be influenced by presynaptic factors.

Relationship of inferior olive-climbing fibers to p,p'-DDT-induced myoclonus in rats

The effects of 3-acetylpyridine (3-AP), which destroys the inferior olive, and harmaline, which stimulates inferior olive-climbing fiber activity, on DDT-induced myoclonus, wee studied in rats. 3-AP shortened and harmaline delayed the time of onset of myoclonus after intragastric administration of DDT. 3-AP also counteracted the antimyoclonic action of L-5-hydroxytryptophan plus chlorimipramine, clonazepam and phenoxybenzamine in this animal model. The results suggest that these antimyoclonic agents require an intact olivocerebellar pathway for their action.