Chemoresponsiveness of intracellular nuclei neurones to L-aspartate, L-glutamate and related derivatives in rat cerebellar slices maintained in vitro

GABA-ergic transmission in deep cerebellar nuclei

gamma-Aminobutyric acid (GABA) is the inhibitory transmitter released at Purkinje cell axon terminals in deep cerebellar nuclei (DCN). Neurons in DCN also receive excitatory glutamatergic inputs from the inferior olive. The output of DCN neurons, which depends on the balance between excitation and inhibition on these cells, is involved in cerebellar control of motor coordination. Plasticity of synaptic transmission observed in other areas of the mammalian central nervous system (CNS) has received wide attention. If GABA-ergic and/or glutamatergic synapses in DCN also undergo plasticity, it would have major implications for cerebellar function. In this review, literature evidence for GABA-ergic synaptic transmission in DCN as well as its plasticity are discussed. Studies indicate that fast inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) in neurons of DCN are mediated by GABAA receptors. While GABAB receptors are present in DCN, they do not appear to be activated by Purkinje cell axons. The IPSPs undergo paired-pulse, as well as frequency-dependent, depressions. In addition, tetanic stimulation of inputs can induce a long-term depression (LTD) of the IPSPs and IPSCs. Excitatory synapses do not appear to undergo long-term potentiation or LTD. The LTD of the IPSP is not input-specific, as it can be induced heterosynaptically and is associated with a reduced response of DCN neurons to a GABAA receptor agonist. Postsynaptic Ca2+ and protein phosphatases appear to contribute to the LTD. The N-methyl-D-aspartate receptor-gated, as well as the voltage-gated Ca2+ channels are proposed to be sources of the Ca2+. It is suggested that LTD of GABA-ergic transmission, by regulating DCN output, can modulate cerebellar function.

The physiological effects of serotonin on spontaneous and amino acid-induced activation of cerebellar nuclear cells: an in vivo study in the cat

It is well established that cerebellar efferents originate from neurons located within the cerebellar nuclei. Neurons within these nuclei receive excitatory inputs derived from the axons that arise from cells in several different regions of the brainstem and spinal cord, some of which continue on to terminate as mossy fibers and climbing fibers in the cerebellar cortex. GABA-induced inhibition in the nuclei is derived primarily from Purkinje cells located in the overlying cortex and possibly from axonal collaterals of a population of small, GABAergic nuclear neurons. In addition, a third chemically defined system of afferents that contain the monoamine serotonin forms a dense plexus of fibers throughout the cat's cerebellar nuclei. The intent of this study is to determine the physiological effects of serotonin on the spontaneous activity of cerebellar nuclear cells as well as that induced by application of the excitatory amino acids glutamate and aspartate in an adult in vivo preparation. Iontophoretic application of serotonin in anesthetized preparations suppresses both spontaneous and excitatory amino acid induced activity. In addition, interactions between serotonin and the amino acid analogs quisqualate and NMDA were analyzed; 5HT suppresses the excitatory responses of neurons to both analogs. However, there is a stronger suppressive effect on quisqualate-induced excitation as compared to that elicited by NMDA. In addition to modulating the effects of the excitatory amino acids, serotonin also potentiates the inhibitory effects of GABA. However, the effect was greatest if the neuron was initially preconditioned with GABA. In summary, serotonin acts to suppress amino acid induced activity in cerebellar nuclear neurons and to enhance gABA-mediated inhibition. The net effect is a decrease in nuclear cell activity and consequently in cerebellar output.

Low nanomolar serotonin inhibits the glutamate receptor/nitric oxide/cyclic GMP pathway in slices from adult rat cerebellum

The function of serotonin afferents to the cerebellum has been investigated by monitoring the effects of serotoninergic drugs on the production of cyclic GMP elicited in cerebellar slices by activation of ionotropic glutamate receptors. Exposure of adult rat cerebellar slices to N-methyl-D-aspartate (1 nM to 1 microM) or to (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA; 1 nM to 10 microM) elicited concentration-dependent and saturable rises in the levels of cyclic GMP. These responses were blocked by selective antagonists at the N-methyl-D-aspartate or AMPA receptors and by inhibiting nitric oxide synthase, but were insensitive to tetrodotoxin. When tested between 0.1 and 10 nM, serotonin, the serotonin1A receptor agonist (+/-)-8-hydroxy-2-(di-n-propylamino)tetralin and the serotonin2 receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane inhibited, concentration-dependently, the cyclic GMP responses evoked by near-maximal (0.1 microM) concentrations of N-methyl-D-aspartate or AMPA. The EC50 values (concentrations causing half-maximal effect) ranged between 0.7 and 2.1 nM. The actions of serotonin were totally abolished by methiothepin, a mixed-type serotonin receptor antagonist. Thus, the serotonergic cerebellar afferents may exert a potent inhibitory control on the excitatory transmission mediated by N-methyl-D-aspartate and AMPA receptors; the inhibition occurs through both serotonin1A and serotonin2 receptors. As the glutamate receptor-dependent cyclic GMP responses involve production of nitric oxide, a diffusible activator of guanylate cyclase, the above inhibitory serotonin receptors may have multiple localization.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional distribution of transient [3H]kainic acid-binding sites in the central nervous system of the developing mouse: an autoradiographic study

[3H]kainate-binding site distribution in mouse brain was studied by in vitro autoradiography during postnatal development. Sites, highly concentrated at early postnatal ages and undetectable at adult ages, were observed in deep cerebellar nuclei, inferior olive, pontine nuclei, inferior colliculus and stratum lacunosum moleculare of the area CA1 in the hippocampus as well as in previously described rat brain areas. It is suggested that the molecules carrying these sites play a role in the development of the regions where they are transiently expressed.

Excitatory synaptic potentials in neurons of the deep nuclei in olivo-cerebellar slice cultures

Excitatory postsynaptic potentials evoked in neurons of the deep cerebellar nuclei, either by electrical stimulation within the nuclei in cerebellar slice cultures or by electrical stimulation of olivary explants in olivo-cerebellar co-cultures, were investigated in the rat by means of intracellular recordings. In neurons of the deep cerebellar nuclei, stimulation of the nuclear tissue, as well as stimulation of the olivary tissue, induced a fast rising excitatory postsynaptic potential, followed by an inhibitory postsynaptic potential and a long-lasting excitation. The fast rising excitatory postsynaptic potential and the following inhibitory postsynaptic potential were blocked by 6-cyano-7-nitroquinoxaline-2,3-dione. The remaining depolarization was abolished by D-(-)-2-amino-5-phosphonovalerate, suggesting that this potential was mediated by N-methyl-D-aspartate receptors. With only D-(-)-2-amino-5-phosphonovalerate added to the bath, the slow excitation was depressed, whereas the fast excitatory and inhibitory postsynaptic potentials were not affected. In the presence of bicuculline, the 6-cyano-7-nitroquinoxaline-2,3-dione- and the D-(-)-2-amino-5-phosphonovalerate-sensitive excitatory postsynaptic potentials elicited by stimulation of the olivary tissue had the same latency, and were both graded with stimulation strength. The time-to-peak and the duration of the D-(-)-2-amino-5-phosphonovalerate-sensitive excitatory postsynaptic potentials were considerably longer than those of the 6-cyano-7-nitroquinoxaline-2,3-dione-sensitive excitatory postsynaptic potentials.(ABSTRACT TRUNCATED AT 250 WORDS)

Excitatory and inhibitory amino acid binding sites in human dentate nucleus

Autoradiography of excitatory and inhibitory amino acid binding sites in human dentate nuclei indicated virtually no binding to N-methyl-D-aspartate (NMDA) or gamma-aminobutyric acidB (GABAB) binding sites, and a low density of kainate binding sites. alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid, metabotropic-quisqualate, benzodiazepine, and gamma-aminobutyric acidA (GABAA) binding sites were present in moderate abundance. Our NMDA results differ from those found previously in rodents. GABAA receptors are probably the primary mediators of inhibitory neurotransmission and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and metabotropic-quisqualate receptors are probably the primary mediators of excitatory neurotransmission within the human deep cerebellar nuclei.

Receptor types mediating the rise in the cytosolic free calcium concentration by L-aspartate and L-glutamate in immature cerebellar neurons with N-methyl-D-aspartate receptors

L-Aspartate (Asp) and L-glutamate (Glu) increased the cytosolic free calcium concentration ([Ca]in) in medium-sized cerebellar neurons having N-methyl-D-aspartate (NMDA) receptors. The sustained rise in [Ca]in induced by Asp, but not by Glu, was reduced by the addition of NMDA antagonists, which consistently suppressed the rise in [Ca]in induced by a low concentration of Asp or Glu in combination with glycine. The results suggest that Asp is a more preferential agonist of NMDA receptors than Glu, although Glu can also be an agonist in the presence of glycine.

Responses to excitatory amino acids of Purkinje cells' and neurones of the deep nuclei in cerebellar slice cultures

1. The actions of the endogenous excitatory amino acids (EAAS) glutamate (Glu), aspartate (Asp) and homocysteate (HCA) on Purkinje cells and neurones of the deep nuclei in cerebellar slice cultures were investigated using intracellular recordings in the single-electrode voltage-clamp mode and the whole-cell configuration of the patch-clamp technique. 2. Purkinje cells and neurones of deep cerebellar nuclei were identified according to their localization in the living cultures, their morphology as revealed by intracellular injections of Lucifer Yellow and their immunoreactivity to antibodies to the 28 kDa Ca2(+)-binding protein. 3. When Purkinje cells were voltage-clamped near their resting membrane potential in a TTX-containing salt solution, Glu, Asp and HCA induced inward currents which were abolished by 6-cyano-7-nitroxaline-2,3-dione (CNQX), a selective antagonist of the non-N-methyl-D-aspartate (NMDA) subtype of EAA receptors. The selective antagonist of NMDA receptors, D-(-)-2-amino-5-phosphonovaleric acid (D-APV), was ineffective in blocking the responses induced by these three amino acids. NMDA, even at high concentrations and in magnesium-free bathing solution, had no detectable effect on membrane properties of Purkinje cells grown in culture during 11-34 days. 4. In magnesium-containing saline, the amplitude of the responses induced by Glu, Asp and HCA was a linear function of the membrane potential. 5. In contrast, neurones of the deep cerebellar nuclei were responsive to NMDA and the inward currents induced by Glu, Asp and HCA were partially blocked both by CNQX and by D-APV. 6. In magnesium-containing saline, the amplitude of the currents induced by NMDA as well as by the three endogenous EAAs decreased at hyperpolarizing holding potentials whereas the current-voltage relation of the responses induced by quisqualate (QA) was strictly linear. 7. It is concluded that Purkinje cells in cerebellar slice cultures do not express NMDA receptors and that excitation of these neurones by the endogenous amino acids Glu, Asp and HCA is mediated exclusively through the activation of non-NMDA receptors. In the same preparation, neurones of the deep cerebellar nuclei possess NMDA and non-NMDA receptors which can be both activated by the three endogenous excitatory amino acids.

Regulation of the cytosolic free calcium concentration by Na+ spikes in immature cerebellar neurons with N-methyl-D-aspartate receptors

N-Methyl-D-aspartate and glycine increased the cytosolic free calcium concentration ([Ca]in) in medium-sized cerebellar neurons. Spontaneous changes in [Ca]in were occasionally observed in NMDA-responsive cells, but large increases in [Ca]in were triggered only through depolarizations by adding veratridine or K+ channel blockers in every cell examined. The [Ca]in increase was suppressed by voltage-dependent Na+ and Ca2+ channel blockers and by an inhibitory transmitter (GABA), suggesting that the generation of Na+ spikes is involved in the increase in [Ca]in.

Taurine in deep cerebellar nuclei of the rat. In vivo comparison to GABA inhibitory effect

The sensitivity of neurons of the deep cerebellar nuclei to iontophoretic application of taurine (TAU) and gamma-aminobutyric acid (GABA) has been studied in vivo in rats. Both TAU and GABA produced a dose-dependent depression of the firing rate. Both the inhibitory actions of TAU and GABA were antagonized by bicuculline (BIC) and picrotoxin (PIC) but not by strychnine (STRY) meaning that TAU had a 'GABA-like' effect in the deep cerebellar nuclei. Simultaneous application of TAU and GABA induced not an additive but a synergistic inhibitory effect on the discharge of the neurons of the interposed and dentate nuclei. These results indicate the involvement of TAU in the inhibitory processes acting on neurons of the deep cerebellar nuclei.

Developmentally regulated changes of glutamate binding sites in mouse deep cerebellar nuclei

The expression of L-[3H]glutamate binding sites of different ionic and pharmacological sensitivities was studied in mouse deep cerebellar nuclei during early postnatal development by means of in vitro autoradiography. Ca2+/Cl(-)-dependent, quisqualate/AMPA/ibotenate-sensitive, and APB-insensitive binding sites are present at high density in the deep cerebellar nuclei of young animals, but greatly decrease between the 10th and 25th postnatal day and remain low in the adult. The density of Ca2+/Cl(-)-independent binding sites remains low and constant during the whole of postnatal development. The possible involvement of the Ca2+/Cl(-)-dependent binding sites in brain development is discussed.

Whole-cell current noise produced by excitatory and inhibitory amino acids in large cerebellar neurones of the rat

1. Membrane noise and current changes produced by glutamate and related excitatory amino acids have been examined in cultured large cerebellar neurones (including Purkinje cells), with whole-cell patch-clamp methods. The sensitivity of these neurones to the inhibitory amino acids gamma-aminobutyric acid (GABA) and glycine has also been studied. 2. The neurones formed inhibitory synapses in culture, and displayed spontaneous synaptic currents. Reducing the pipette Cl- concentration (i.e. intracellular synaptic currents. Reducing the pipette Cl- concentration (i.e. intracellular concentration) caused a negative shift in their reversal potential, and the currents could be blocked with bicuculline (10 microM), suggesting that they were mediated by GABAA receptors. Spontaneous synaptic activity was also considerably reduced in the presence of 3 microM-tetrodotoxin. 3. Analysis of the increase in whole-cell current noise produced by the application of GABA (3 microM) gave noise spectra that were fitted by two Lorentzian components with slow and fast time constants of 23.6 and 1.9 ms at a membrane potential (Vm) of -110 mV. The mean single-channel conductance estimated from GABA noise was gamma noise = 12 pS. Glycine (10 microM) whole-cell current responses were Cl(-)-mediated and reversibly abolished by 1 microM-strychnine. 4. Bath application of excitatory amino acids gave whole-cell current changes accompanied by an increase in synaptic activity. Postsynaptic responses to the excitatory amino acids were more readily seen after the inhibitory synaptic currents had been abolished by bicuculline. Membrane current changes were obtained in response to the putative transmitters glutamate and aspartate, and the agonists NMDA (N-methyl-D-aspartate), ibotenate, quisqualate and kainate. Their reversal potential was approximately -5 mV. 5. A majority of noise spectra produced by the various glutamate receptor agonists were fitted by two Lorentzian components; the rest were fitted with a single Lorentzian component. The noise time constants were apparently not dependent on the type of glutamate agonist used to activate the receptor channels. Pooling data for all agonists gave a mean time constant for single-component spectra of tau noise = 4.8 +/- 0.3 ms; for two-component spectra the time constants were tau 1 = 22.7 +/- 1.8 ms and tau 2 = 2.2 +/- 0.12 ms (Vm = -110 to -50 mV). It is likely that the two components present in whole-cell noise spectra reflect complex kinetics of glutamate receptor channels. 6. The mean single-channel conductance was estimated from whole-cell noise for the various excitatory amino acids.(ABSTRACT TRUNCATED AT 400 WORDS)

Characterization of excitatory amino acid receptors in cultured chick cerebellar neurons

In order to characterize excitatory amino acid receptors in cultured chick cerebellar neurons, the effects of amino acid agonists on the input resistance and the antagonist specificity of the depolarization induced by each agonist were intracellularly studied. In Mg-containing medium, glutamate (especially at low doses), aspartate and N-methyl-D-aspartate not only decreased the input resistance at depolarized membrane potentials but also increased it at around the resting potential. In Mg-free medium, glutamate (high and low doses) and all other agonists simply decreased the input resistance. The effects of antagonists on amino acid-induced depolarizations in Mg-free medium were as follows: Mg and alpha-aminoadipate blocked N-methyl-D-aspartate and aspartate most strongly, glutamate and kainate moderately, and quisqualate only slightly; 2-amino-4-phosphonobutyrate antagonized N-methyl-D-aspartate most strongly, aspartate moderately and others mildly; 2-amino-5-phosphonovalerate blocked aspartate most strongly and others mildly; gamma-D-glutamylglycine blocked kainate most strongly and others moderately; and kynurenate was rather nonselective but most strongly antagonized N-methyl-D-aspartate and aspartate. These results suggest that all receptor subtypes (N-methyl-D-aspartate-, quisqualate- and kainate-types) are present in cultured chick cerebellar neurons, but their antagonist specificities are different from those in other central neurons, and also that glutamate at a low dose activates N-methyl-D-aspartate receptors, while it acts on quisqualate receptors at a high dose.

Electrophysiology of guinea-pig cerebellar nuclear cells in the in vitro brain stem-cerebellar preparation

1. Intracellular recordings were obtained from cerebellar nuclear neurones in the isolated brain stem-cerebellar preparation of guinea-pigs in vitro. The electrical properties of the cells were quite similar to those reported in in vitro slice studies. They had an average resting potential of -56.7 +/- 1.8 mV, an input resistance of 23.8 +/- 4.9 M omega, and a time constant of 12.5 +/- 2.7 ms. The action potentials had an average amplitude of 57.3 +/- 5.28 mV (n = 20). 2. In addition to the ionic mechanisms required for the generation of the fast action potential, cerebellar nuclear neurones displayed a low-threshold Ca2+-dependent spike which produced a powerful rebound excitation following anodal break. This type of electroresponsiveness was absent in the slice preparation. 3. The anodal break response was further enhanced by the presence of a non-inactivating Na+ conductance similar to that described in Purkinje cells. 4. Following electrical stimulation of the cerebellar cortex or the underlying white matter, excitatory and inhibitory synaptic potentials (EPSP-IPSP sequences) could be recorded in cerebellar nuclear neurones. The EPSPs were elicited by direct activation of collaterals of mossy or climbing fibre afferents. The IPSPs followed direct or orthodromic Purkinje cell activation. 5. The integrity of the olivo-cerebellar system was tested by the administration of harmaline which produced powerful EPSP-IPSP sequences or pure IPSPs in cerebellar nuclear neurones. These IPSPs were often followed by a rebound firing of the cells. 6. These results indicate that the olivo-cerebellar pathway, in addition to its activation of the cerebellar cortex, exerts a powerful and complex set of synaptic events on cerebellar nuclear cells. As such it is a true afferent system, having a distinct role in cerebellar physiology.

Cardiovascular and respiratory responses evoked from the posterior cerebellar cortex and fastigial nucleus in the cat

1. In both anaesthetized and decerebrate cats the cerebellar cortex (lobules VI, VII, VIII, IX and X) and the fastigial nucleus (f.n.) have been stimulated electrically, and chemically, while recording changes in phrenic nerve discharge, heart rate, arterial blood pressure and renal and femoral blood flow. 2. Stimulation of lobules VI, VII, VIII and Xb failed to elicit any cardiovascular or respiratory changes. Activation of lobule IX (the uvula), and in some preparations sub-lobule Xa, evoked cardiovascular and respiratory responses consistently. In the anaesthetized cat, electrical stimulation of the uvula evoked apnoea, a small bradycardia and a depressor response associated with vasodilatation in the hindlimb vascular bed. In contrast, stimulation in an equivalent region in a decerebrate preparation elicited an apneustic discharge, a pronounced tachycardia and a rise in arterial pressure with vasoconstriction in both renal and femoral vascular beds. In both the anaesthetized and decerebrate animals the pattern of response elicited by chemical activation was identical to that seen with electrical stimulation. 3. Electrical, or chemical, stimulation after administration of anaesthetic to the decerebrate cat then evoked an identical pattern of response to that seen in the 'intact' anaesthetized animal. This evidence suggests that the reversal in the pattern of the response in an effect of the anaesthetic agent and not the decerebration itself. 4. The only area of the f.n. to produce cardiovascular effects was the rostral region. Electrical stimulation of the rostral f.n. in both anaesthetized and decerebrate preparations inhibited central inspiratory activity and evoked tachycardia, along with a pressor response associated with vasoconstriction in both renal and femoral vascular beds. In contrast, chemical excitation of those sites in the rostral f.n. shown previously to produce pronounced cardiovascular and respiratory changes failed to elicit any changes in the recorded variables. 5. The present evidence suggests that there are two areas in the cat cerebellum which can exert pronounced cardiovascular and respiratory effects. The patterns of response elicited by electrical stimulation of the posterior cortex and rostral f.n. are mediated by two separate cerebellar-brainstem pathways as judged by the two different effects of anaesthesia on the evoked responses. We suggest that the f.n. may not play a role in the control of the cardiovascular system since chemical excitation of cell bodies of the rostral f.n. failed to elicit the so-called 'fastigial pressor response'.

Differential effects of serotonin on the spontaneous discharge and on the excitatory amino acid-induced responses of deep cerebellar nuclei neurons in rat cerebellar slices

The effects of steady iontophoretic applications of serotonin on the spontaneous discharge and on the excitatory responses induced in deep cerebellar nuclei neurons by iontophoretic pulse applications of L-glutamate, L-aspartate, N-methyl-D,L-aspartate and quisqualate were studied in rat cerebellar slices maintained in vitro. Serotonin increased the spontaneous firing rate of deep cerebellar nuclei neurons in 91% of the tested cells by 109% on the average and had no effect on the remaining recorded neurons. Conversely, the monoamine induced a depression of the excitatory responses induced by four agonists tested and the depressant potency of serotonin was in the order quisqualate, glutamate, aspartate, N-methyl-D,L-aspartate. These effects persisted in low calcium high magnesium solution, suggesting that the serotonin receptors involved in these phenomena were, at least partially, postsynaptically located. The serotonin-induced increase in the cell firing rate appeared to be methysergide-resistant whereas the serotonin-induced decrease in the responses elicited by excitatory amino acids was depressed by this antagonist, which could indicate that these differential effects of serotonin are mediated via different mechanisms and/or serotonin receptor subtypes.

Postnatal development of the chemosensitivity of rat cerebellar Purkinje cells to excitatory amino acids. An in vitro study

In vitro sagittal slices of immature rat cerebellum were used to study the development of the sensitivity of Purkinje cells (PCs) to L-aspartate (L-Asp), L-glutamate (L-Glu) and related derivatives. As early as postnatal day 0 all PCs already displayed clear excitatory responses to short iontophoretic applications of L-Asp, L-Glu and quisqualate while in the same conditions no effect of N-methyl-D,L-aspartate (NMDLA) was detected. By postnatal day 5, i.e. after the onset of the synaptogenesis, the sensitivity of PCs to L-Asp, L-Glu and quisqualate significantly increased up to values similar to those recorded in adult rat cerebellum and surprisingly nearly all (87%) the recorded cells now also displayed excitatory responses to NMDLA. Although this sensitivity of PCs to NMDLA was significantly lower than that observed with the other drugs, it persisted until the end of the first postnatal month when the adult type of connectivity is already well established but at this stage only 30 per cent of the tested cells were still sensitive to the agonist. During this period, excitatory responses elicited by NMDLA were selectively antagonized by 2-amino-5-phosphonovalerate (2-APV), suggesting that during postnatal development, NMDA receptor types are transiently expressed on PCs membranes since in the adult, NMDLA no longer had an excitatory effect. Instead, this drug now exerted a preferential antagonistic action on the excitatory response elicited by L-Asp. Also in the adult, no major changes occurred in the sensitivity of PCs to L-Asp, L-Glu and quisqualate when these drugs were ejected at a dendritic site whereas, when ejected at the somatic level, the sensitivity of the cell appeared 2-3 times lower.