GABA, THIP and baclofen inhibition of Purkinje cells and cerebellar nuclei neurons

The sensitivity of Purkinje cells (PCs) and neurons of the cerebellar nuclei (NCNs) to iontophoretic application of gamma-aminobutyric acid (GABA), 4,5,6,7-tetrahydroisoxazolo(5,4-c)pyridin-3-ol (THIP) and baclofen, i.e., GABAA and GABAB agonists respectively, have been studied in anesthetized rats. All the agonists produced dose-dependent firing rate depression of the PCs but with different potencies. The inhibitory actions of both GABA and THIP were specifically antagonized by bicuculline (Bic) and the baclofen-induced responses by 2-hydroxysaclofen. GABA and THIP also depressed the spontaneous activity of NCNs while baclofen was ineffective. The present results therefore suggest that GABAA receptors are involved in the GABA-induced inhibition in the cerebellar cortex and in the cerebellar nuclei and GABAB receptors are involved only in the cerebellar cortex.

Avian photogenic epilepsy and embryonic brain chimeras: neuronal activity of the adult prosencephalon and mesencephalon

Photogenic genetic epilepsy was studied in an avian model, using either the Fayoumi epileptic chicken (Fepi) or neural chimeras obtained by replacement of embryonic brain vesicles in normal chickens with those of Fepi embryos. In these two kinds of animals motor seizures accompanied by electroencephalographic (EEG) desynchronization and flattening (DF) were evoked by intermittent light stimulation (ILS). In chimeras with only the prosencephalon grafted, motor seizures were less severe but DF remained. ILS-induced DF persisted under paralysis by gallamine triethiodide (Flaxedil). Extracellular recordings were made in the prosencephalon (wulst) and in the mesencephalon (optic tectum) of paralysed animals. Units recorded in the prosencephalon of Fepi and chimeras showed abnormal interictal bursting activity, distinctly different from the non-epileptic Fayoumi heterozygotes (Fhtz) and normal chickens. The mesencephalic units of Fepi and chimeras having both prosencephalon and mesencephalon grafted showed two types of abnormal activities during ILS-induced DF, which were distinct from the non-epileptic chickens: type I neurons displaying early, high sensitivity to ILS followed by a prolonged suppression of activity; type II neurons displaying an early and prolonged suppression of activity. The results are discussed with respect to the brain structures generating ictal and interictal EEG activities and motor seizures.

Pattern of electroencephalographic activity during light induced seizures in genetic epileptic chicken and brain chimeras

Genetic epilepsy was studied in Fayoumi epileptic (F.Epi) chickens and in neural chimeras obtained by selective substitution of embryonic brain vesicles of F.Epi donors in normal recipient chickens. Typical motor seizures accompanied by convulsions were evoked by intermittent light stimulation in F.Epi and in chimeras having embryonic substitution of the prosencephalon and the mesencephalon. The motor seizure was less severe in chimeras receiving only the prosencephalon. In the F.Epi, as well as in all the chimeras, the EEG during seizures was characterized by a desynchronized (or a flattening) pattern of activity. F.Epi and chimeras had a lower threshold to Metrazol induced seizures than control chickens. The experimental animals show that, in this model, large prosencephalic and mesencephalic areas are involved in the epileptic disease. The epileptic character of this genetic dysfunction is discussed.

Cerebellar nuclei and the nucleocortical projections in the rat: Retrograde tracing coupled to GABA and glutamate immunohistochemistry

The amino acids GABA and glutamate (Glu) are thought to be the principal substances in the central nervous system responsible for neuronal inhibition and excitation. Their distributions among the different neurons in a defined pathway may thus be indicative of the contributions of the cells to pathway function. Examples of such neurons are those of the cerebellar nuclei which, while regulating output from the Purkinje cells of the cerebellar cortex, are also found to project back to the cerebellar cortex. Immunohistochemical experiments were done to identify GABA and glutamate (Glu) containing cells in the adult rat cerebellar nuclei. Consecutive semithin and serial vibratome sections were incubated with antisera raised in rabbit against GABA and Glu. In semithin sections, only small neurons were intensely GABA immunoreactive (GABA-IR) (31.7%), and the majority (80.5%) were Glu immunoreactive (Glu-IR) of different sizes. Consistent with Glu being a metabolic precursor for GABA, 75.4% of the GABA-IR population colocalized Glu. In vibratome sections GABA-IR neurons showed some local differences in number, whereas the Glu-IR were uniformly distributed in the three nuclei studied. Measured mean diameters for these neurons showed a distinct size difference for the GABA- and Glu-IR with little overlap. Cerebellar nuclei neurons projecting to the cortex (nucleocortical neurons, NCN) were identified by locally preinjecting the retrograde transported WGA-apoHRP-colloidal gold complex in the cerebellar cortex. Vibratome sections of these cerebellar were silver intensified for the retrograde tracer and double labeled for GABA and Glu. Of the total number of identified NCN, 8.7% were GABA-IR (10 animals) and 47.7% Glu-IR (5 animals). Many retrograde labeled NCN in the core of the thick sections were immunonegative for both amino acids due to poor antibody penetration, thus underestimating the proportions of cells containing GABA and Glu. The size distributions for the GABA-IR and Glu-IR NCN were similar to those measured in non-retrograde labeled nuclei in thick sections. The conclusions reached are that GABA-IR neurons of the cerebellar nuclei, including the NCN, use GABA as the presumed inhibitory neurotransmitter and that Glu-IR neurons may use Glu or another excitatory neurotransmitter.

The immunocytochemical distribution of calbindin-D28k and parvalbumin in identified neurons of the pulvinar-lateralis posterior complex of the cat

The calbindin-D28k and parvalbumin immunoreactivities of the neurons of the pulvinar-lateral posterior complex (Pul-LP) were studied in the cat. The neurons of the Pul-LP projecting to the cerebral cortex were identified by a retrogradely transported tracer injected in the suprasylvian gyrus. Two populations of cells were found, a calbindin-D28k-immunoreactive, large-diameter population and a parvalbumin-immunoreactive, small-diameter group. The two kinds of cells are closely intermingled. The former includes the neurons retrogradely marked, and therefore projecting to the suprasylvian gyrus. The latter includes neurons which were not retrogradely marked, and therefore presumably intrinsic elements.

Tetrodotoxin induced calcium spikes: in vitro and in vivo studies of normal and deafferented Purkinje cells

Tetrodotoxin (TTX) is widely used to block the sodium dependent action potential in excitable cells to study their other ionic properties. TTX applied outside, selectively blocks voltage dependent sodium channels and is thought to have no other effects. We report here that TTX, applied to slices of rat cerebellum, suppressed sodium spikes of the Purkinje cells and induced firing in bursts of slower spikes. This activity was blocked by cobalt (2 mM) or cadmium (0.2 mM) in the medium as well as by hyperpolarizing currents showing that the slow spikes were due to voltage dependent calcium channels. The membrane potential was not significantly changed by TTX and the spikes during the bursts had the same threshold potentials and peak spike amplitudes as the voltage and Ca2+ dependent dendritic spikes evoked by injected current before adding TTX. This indicated that no marked changes in the membrane conductances were produced by the TTX. Unlike the burst firing induced by removing extracellular sodium, the TTX induced bursts were not followed by a large hyperpolarization. The same kind of results were obtained with extracellular recording in the in-vivo preparation with TTX applied topically or by pressure near the recording sites. TTX induced burst firing was not due to blocking afferent inhibitory input to the PC, since bicuculline (10(-6) M) applied without TTX, produced only increased firing of fast action potentials and no bursts. The bursts could be arrested within 1 to 2 min by intravenously administering 2 mg/kg sodium pentobarbital, the blockage lasted from 5 to 15 min.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Cerebellar localization and colocalization of GABA and calcium binding protein-D28K

Immunocytochemical studies using antibodies raised against the inhibitory neurotransmitter, gamma-aminobutyric acid (GABA) and against the 28 Kd vitamin D dependent calcium binding protein (calbindin) in the cerebellum, are reviewed. The GABA immunoreactive neurones found in the cerebellar cortex were the Purkinje cell (PC), the three classes of intrinsic inhibitory interneurones, stellate, basket and Golgi cells and the cells of Lugaro. Some of the neurons of the cerebellar nuclei were also found to be GABA immunoreactive. A part of these could be identified as extrinsic neurones projecting either back to the cerebellar cortex, or to the inferior olive, both these pathways being topographically highly organized but arising from independent parent neurons. The presumed inhibitory function of these two pathways are discussed. Calbindin immunoreactivity in the cerebellum was confined to the PCs, staining concerned the whole cell including soma, branching dendrites, axons and axons terminals. The antibody, which appears to be tightly bound to the PC in vivo, failed to stain some of the PC when cerebellar slices maintained in vitro were studied. The stability of the antigen-antibody binding and the use of calbindin as a marker specific for the PC in the cerebellum, is discussed. Co-localization of GABA with calbindin as well as with other calcium binding proteins are reported to be found in the PCs. While these co-localizations have led to much speculation, conclusive functional roles for them have not been identified at present.

Colocalization of calbindin and GABA in medial nucleus of the trapezoid body of the rat

Using immunocytochemical methods, both calbindin and GABA were found to be colocalized in the somas of all the cells of the medial nucleus of the trapezoid body (NMTB) of the rat auditory system. In the lateral superior olive (LSO), calbindin was also found in the terminals but not in the cells. Some terminal labelling was found in the medial superior olive (MSO). GABA was also found in the somas of some cells in both LSO and MSO, but most of the labelling was in terminals. In the rat, calbindin appears to be more involved in a pathway that detects interaural intensity differences.

The GABAergic neurones of the cerebellar nuclei in the rat: projections to the cerebellar cortex

The presence of gamma-aminobutyric acid (GABA) in the neurones of the cerebellar nucleocortical pathway is here reported. The pathway was identified by retrograde tracer and the GABA content was revealed immunohistochemically. It was found that most of the neurones giving rise to the reciprocal, non-reciprocal and symmetrical projections are indeed GABA-immunoreactive. They were observed in all the subdivisions of the nucleus medialis, of the nucleus interpositus and of the nucleus lateralis sending axons respectively to the sagittal zones A, C1-3 and D of the cerebellar cortex. The nucleus vestibularis lateralis and the related sagittal zone B were devoid of such projections.

The red nucleus activity in rats deprived of the inferior olivary complex

The long-term effects of the inferior olive destruction on the red nucleus activity, were studied in the rat following injection of 3-acetylpyridine. As soon as the olivary activity was suppressed, the discharge of the rubral units drastically decreased. Then, they progressively recovered the control frequency during the first month, although a normal rubral activity was not restored up to 8 months. The hypothesis is advanced that the olivocerebellar system is essential to shape the activity of the rubrospinal pathway.

Topographic organization of the interpositorubral connections in the rat. A WGA-HRP study

The projections from the anterior (NIA) and posterior (NIP) interposed nuclei to the magnocellular red nucleus (RNm) have been investigated in the rat, using the retrograde transport of horseradish peroxidase-wheatgerm agglutinin conjugate. Projections from the NIA extend throughout the RNm, whereas those from the NIP only reach its medial aspect. In addition, a topographical organization of the NIA-RNm pathway was found, such that the medial NIA projects ventrally, the lateral NIA projects dorsally.

The interposito-rubrospinal system. Anatomical tracing of a motor control pathway in the rat

The cerebello-rubromotor pathway, impinging on both spinal and facial motor nuclei, has been traced in the rat, using the bidirectional transport of horseradish peroxidase-wheat germ agglutinin conjugate. After injection of the tracer in the red nucleus (NR), retrograde labelling shows a topical arrangement of the cerebellorubral connection. The nucleus lateralis projects to the parvocellular NR (NRp) and the nucleus interpositus to the magnocellular NR (NRm). The nucleus interpositus anterior (NIA) reaches the entire NRm and this projection is topographically arranged: the medial NIA sends fibres ventrally, the lateral NIA dorsally. The medial two-thirds of the nucleus interpositus posterior (NIP) project only to the medial aspect of the NRm, with no apparent organization. No connection has been found between the lateral third of NIP and the NRm. After injection of the tracer in the spinal cord or the nucleus of the facial nerve, retrograde labelling is observed almost throughout the entire caudorostral extent of the NR, although labelling is more scant in NRp than in NRm. Rubrospinal and rubrofacial projections are somatotopically arranged in the dorsoventral direction: ventrolateral regions of NR reach the lumbar cord, medioventral regions the lower cervical levels, intermediary regions the upper cervical levels and finally the dorsalmost part of the NR projects to the nucleus of the facial nerve. After injection of the tracer in the cerebellar nuclei, anterograde labelling in the NR shows that interpositorubral connections determine two subregions in the NR: a lateral one under the exclusive control of the NIA, and a medial one under the control of both NIA-NIP afferents. It confirms in addition the topography of the NIA-NRm projection and shows the preponderant participation of the NIA afferents to the interpositorubral connection. Thus, it appears from our results that the cerebellorubral arrangement matches, to a great extent, the "rubromotor" efferent organization.

Release of cerebellar inhibitory activity by partial destruction of the inferior olive with kainic acid in rat

Partial destruction of the neurones of the inferior olive was obtained by local ionophoretic injection of kainic acid. Complex spike discharges of the Purkinje cells are suppressed 2-3 h after application of the drug. The metabolic activity increases in the region of the cerebellar nuclei within 2 h and persists for 3 days following the kainic acid injection. The increase is only observed in those parts of the cerebellar nuclei receiving terminals from the Purkinje cells deafferented of the climbing fibres. No changes were detected when the injection affected only the underlying reticular formation.

Brain stem nuclei giving fibers to lobules VI and VII of the cerebellar vermis

The HRP method has been used to identify all the brain stem nuclei, which may project to lobule VI and/or VII of the posterior cerebellar vermis. Three tentative degrees of labeling of the different structures have been assigned: 'massive', 'clear' and 'discrete'. (1) Massive projections have been found to reach lobule VI and VII from the inferior olive and lobule VII only from the nucleus reticularis tegmenti pontis. (2) Clear projections have been found to reach lobule VI only from the pontine nuclei, the nucleus reticularis tegmenti pontis, the nucleus reticularis lateralis and the reticularis paramedianus; lobule VII only from the raphe nuclei, and both VI and VII from the perihypoglossal and vestibular nuclei. (3) Discrete projections have been found to reach lobule VI and VII from the deep cerebellar nuclei; lobule VI only from the nucleus tracti solitarii and nucleus cuneatus externus; lobule VII only from the nucleus lemnisci lateralis pars ventralis, the nuclei parabrachiales and the nucleus subcoeruleus.

The cerebellorubral projection in the rat: retrograde anatomical study

The cerebellorubral projections have been studied in the rat using the retrograde transport of horseradish peroxidase-wheat germ agglutinin conjugate. The lateral cerebellar nucleus projects to the parvocellular red nucleus (RN), the anterior (NIA) and posterior (NIP) interposed nuclei project to the magnocellular RN. Whereas the projections from the NIP are limited to the medial aspect of the RN, those from the NIA extend throughout the magnocellular RN. NIA-RN projections are topographically arranged: the medial NIA projects ventrally, the lateral NIA projects dorsally. Functionally, this differential distribution seems to fit the hindlimb-forelimb areas of origin of the rubrospinal tract.

Long term modification of cerebellar inhibition after inferior olive degeneration

The long term effects of inferior olive destruction on the activities of the Purkinje cells and their target neurones in the cerebellar nuclei were studied in the rat. Careful observations were also made of motor behaviour throughout the study. Albino rats were injected with 3-acetylpyridine to produce a neurotoxic destruction of the inferior olive and then were used for acute recording experiments at 1-2 days, 5-7 days, 12-18 days, 35-38 days, 75-97 days and 230-252 days. After degeneration of the inferior olive, there was an initial period lasting for a few days, characterized by a high firing frequency of Purkinje cells associated with a very low level of activity of the neurones in the cerebellar nuclei. During this period, there was a deep depression of motor activity. A period of adaptation follows during the first month, characterized by a slow recovery of the initial firing frequency of the cerebellar units and a gradual recovery of spontaneous locomotion; nevertheless the firing pattern and motor behaviour remain abnormal. From one month on the unit activities disturbances and the motor deficiencies stabilize. The hypothesis is advanced that Purkinje cell inhibition on their target neurones, which increases during the initial period, gradually diminishes during the adaptation time, and then stabilizes to a subnormal state.

Release of cerebellar inhibition by climbing fiber deafferentation

Cerebellar units were recorded extracellularly in rats before and after an intraveinous injection of 3-acetylpyridine destroying selectively the IO. All the Purkinje cells show a loss of the complex discharge between 2 h 15 min and 2 h 45 min after treatment. This time, called the "critical period" corresponds to the degeneration of the neurons of the inferior olive as revealed by the decrease of their metabolic activity. The simple spikes of the Purkinje cells increase their discharge frequency soon after the climbing fibers cease firing. On the contrary the firing frequency of the inhibitory interneurons does not show significant changes after degeneration of the inferior olive. The efferent cerebellar neurons, including cells of the cerebellar and vestibular nuclei receiving the axon terminals of the Purkinje cells, decrease their discharge rate up to thirty times during and after the critical period. It is demonstrated that this effect is due to the increased inhibitory activity of the Purkinje cells deafferented from the climbing fibers, whereas the deafferentation of the efferent cerebellar neurones from the collaterals of the olivary cells has little impact.

Metabolic activity of intracerebellar nuclei in the rat: effects of inferior olive inactivation

Metabolic activity of the intracerebellar nuclei during cryoinactivation of the inferior olive was studied in the anaesthetized rat by using the 14C-2-deoxyglucose method. Single unit recording of Purkinje cells was simultaneously monitored in the cerebellar cortex. Local inactivation in the inferior olive resulted in regional suppression of complex spike discharges in the cerebellar cortex. An increased metabolic activity was observed in the cerebellar nuclei contralateral to the cryoinactivation site correlating the somatotopically arranged olivo-cerebello-nuclear circuit. This increase was shown to be due specifically to inactivation of the inferior olive, since it was not obtained in a rat in which the inferior olive was previously destroyed by neurotoxic doses of 3-acetylpyridine. The results are interpreted as being due to an increased presynaptic activity of the terminals of the Purkinje cells which fire simple spikes at high rates after climbing fibre deafferentation.

[Depression of the efferent activity of the cerebellar nuclei following destruction of the inferior olive]

The spontaneous discharge frequency of the fastigial and interpositus nuclei was evaluated in three experimental conditions in Rat: (a) in the "intact" animal; (b) in animals with total and selective destruction of the inferior olive, depriving the Purkinje cells of their afferent climbing fiber; (c) in animals having inferior olive destruction and cryocoagulation of the cerebellar cortex, destroying Purkinje cells innervating the neurones of the fastigial and interpositus nuclei. Unit activity was high in group (a) (32.9 +/- 22.9/s); it was markedly reduced in group (b) (1.1 +/- 1.3/s); it was higher in group (c) than in group (a) (43.7 +/- 25.5/s). Suppression of the inferior olive thus increases the Purkinje cell inhibitory action upon neurones of the cerebellar nuclei.