The glutamate receptor subtype mediating parallel fibre-Purkinje cell transmission in rabbit cerebellar cortex

The importance of N-methyl-d-aspartate (NMDA) receptors in subtraction of electrosensory reafference in the dorsal nucleus of skates

The dorsal nucleus of the little skate is a cerebellum-like sensory structure that adaptively filters out predictable electrosensory inputs. The filter's plasticity is mediated by anti-Hebbian associative depression at the synapses between parallel fibers and ascending efferent neurons (AENs). Changes in synaptic strength are indicated by the formation of a cancellation signal which is initiated by co-activation of parallel fibers and AENs, and can be reversed by parallel fiber activity in the absence of AEN activation. In other cerebellum-like sensory structures, the formation of the cancellation signal requires activation of postsynaptic NMDA receptors on the principal neurons. We demonstrate here by immunohistochemistry that the somas and the initial portion of both apical and basal dendrites of the AENs are labeled with antibodies raised against the NR1 subunit of NMDA receptors from a South American electric fish. In in vivo physiological experiments, we show that the formation of the cancellation signal induced by coupling an electrosensory stimulus to ventilatory movements or direct parallel fiber stimulation is blocked when either of the NMDA receptor antagonists 2-amino-5-phosphonovaleric acid (APV) or MK801 is injected into the molecular layer above the recorded AEN. Blocking NMDA receptors prevented formation of a cancellation signal in 79% (15/19; APV) and 60% (3/5; MK801) of the AENs. This blockage was reversible in 40% (6/15) of the AENs after APV removal. Thus, in the dorsal nucleus, the activity-dependent, long-lasting but reversible change in synaptic strength of the parallel fiber–AEN synapses appears to be an NMDA receptor-dependent process.

Reversible associative depression and nonassociative potentiation at a parallel fiber synapse

The electrosensory lobe (ELL) of mormyrid electric fish is one of several cerebellum-like sensory structures in fish that remove predictable features of the sensory inflow. This adaptive process obeys anti-Hebbian rules and appears to be mediated by associative depression at the synapses between parallel fibers and Purkinje-like cells of ELL. We show here that there is also a nonassociative potentiation at this synapse that depends only on the repeated occurrence of the EPSP. The depression can be reversed by the potentiation and vice versa. Finally, we show that the associative depression requires NMDA receptor activation, changes in postsynaptic calcium, and the occurrence of a postsynaptic dendritic spike within a few milliseconds following EPSP onset.

Synaptic plasticity in the mormyrid electrosensory lobe

The mormyrid electrosensory lateral line lobe (ELL) is one of several different sensory structures in fish that behave as adaptive sensory processors. These structures generate negative images of predictable features in the sensory inflow which are added to the actual inflow to minimize the effects of predictable sensory features. The negative images are generated through a process of association between centrally originating predictive signals and sensory inputs from the periphery. In vitro studies in the mormyrid ELL show that pairing of parallel fiber input with Na+ spikes in postsynaptic cells results in synaptic depression at the parallel fiber synapses. The synaptic plasticity observed at the cellular level and the associative process of generating negative images of predicted sensory input at the systems level share a number of properties. Both are rapidly established, anti-Hebbian, reversible, input-specific and tightly restricted in time. These common properties argue strongly that associative depression at the parallel fiber synapse contributes to the adaptive generation of negative images in the mormyrid ELL.

Impaired motor coordination in mice lacking prion protein

1. Prion protein (PrPC) is a host-encoded glycoprotein constitutively expressed on the neuronal cell surface. Accumulation of its protease-resistant isoform is closely related to pathologic changes and prion propagation in the brain tissue of a series of prion diseases. However, the physiological role of PrPC remains to be elucidated. 2. After long-term observation, we noted impaired motor coordination and loss of cerebellar Purkinje cells in the aged mice homozygous for a disrupted PrP gene, a finding which strongly suggests that PrPC plays a role in the long-term survival of Purkinje cells. 3. We also describe the resistance of the PrP null mice to the prion, indicating the requirement of PrPC for both the development of prion diseases and the prion propagation.

Selective changes in AMPA receptors in rabbit cerebellum following classical conditioning of the eyelid-nictitating membrane response

alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors are critically involved in several forms of synaptic plasticity proposed to be neural substrates for learning and memory, e.g., long-term potentiation and long-term depression (LTD). The present study was designed to determine changes in cerebellar AMPA receptors following classical conditioning of the eyeblink-nictitating membrane response (NMR) in the rabbit. Quantitative autoradiography was used to assess changes in ligand binding properties of cerebellar AMPA receptors following NMR conditioning elicited by pairing electrical stimulation of the pontine nuclei with an airpuff to the eye. [3H]AMPA and [3H]-6-cyano-7-nitroquinoxaline-2,3-dion (CNQX) binding were determined following preincubation of frozen-thawed brain tissue sections at 0 or 35 degreesC. With 0 degreesC preincubation, no significant differences in [3H]AMPA binding to cerebellar AMPA receptors were seen between any of the experimental groups tested. In contrast, preincubation at 35 degreesC revealed significant decreases in [3H]AMPA binding to the trained side of the cerebellar cortex resulting from paired presentations of the conditioned and the unconditioned stimuli, while unpaired presentations of the stimuli resulted in no significant effect. With 35 degreesC preincubation, there were no significant differences in [3H]CNQX binding between any of the experimental groups and no significant differences in [3H]AMPA binding in the untrained side of the cerebellum. These results indicate that NMR conditioning is associated with a selective modification of AMPA-receptor properties in brain structures involved in the storage of the associative memory. Furthermore, they support the hypothesis that cerebellar LTD, resulting from decreased synaptic efficacy at parallel fiber-Purkinje cell synapses mediated by a change in AMPA-receptor properties, is a form of synaptic plasticity that supports this type of learning.

The mormyrid electrosensory lobe in vitro: physiology and pharmacology of cells and circuits

This paper is concerned with the electrosensory lobe (ELL) of mormyrid electric fish as examined in in vitro slices. Intracellular recordings from morphologically identified cells and field potential recordings were used to characterize the physiology and pharmacology of ELL cells. Most intracellular recordings were from the Purkinje-like interneurons that are known as medium ganglion cells and from the two types of efferent neurons, large ganglion and large fusiform cells. Stimulation of primary afferent fibers elicits both excitatory and inhibitory effects in these cells, with the excitatory effects being mediated by both the AMPA and NMDA types of glutamate receptors and the inhibitory effects being mediated by both GABAA and glycine receptors. Parallel-fiber stimulation evokes an EPSP-IPSP sequence, with the EPSPs being mediated by both AMPA and NMDA receptors and the IPSPs being mediated by GABAA receptors only. The parallel fiber-evoked EPSPs and IPSPs show marked paired-pulse facilitation. A large and unusually broad spike is recorded inside medium ganglion cells, and field potential responses suggest that this spike is propagated into the apical dendrites. The results provide essential information for understanding how peripheral and central inputs are integrated in ELL.

Systemic administration of kainate induces marked increases of endogenous kynurenic acid in various brain regions and plasma of rats

The endogenous neuroinhibitory and neuroprotective excitatory amino acid receptor antagonist kynurenic acid has been hypothetically linked to the pathogenesis of epilepsy and several other brain disorders. In the present study, alterations in kynurenic acid levels were examined in the kainate model of temporal lobe epilepsy. Kainate was systemically injected in rats at a dose (10 mg/kg s.c.) which induces a characteristic behavioural syndrome with stereotypies and focal (limbic) and generalized seizures, eventually progressing into severe status epilepticus. Kynurenic acid was determined 3 h after kainate injection in various brain regions (olfactory bulb, frontal cortex, piriform cortex, amygdala, hippocampus, nucleus accumbens, caudate/putamen, thalamus, superior and inferior colliculus, pons and medulla, and cerebellar cortex) and in plasma, using a sensitive high-performance liquid chromatographic method. When data were analysed irrespective of individual seizure severity, significant increases in kynurenic acid were determined in all brain regions examined except the hippocampus, nucleus accumbens and pons/medulla. The most marked (200-500%) increases above controls were seen in the piriform cortex, amygdala, and cerebellar cortex. Furthermore, a significant kynurenic acid increase of about 200% above control was determined in plasma. When kynurenic acid levels were determined in subgroups of rats with different behavioural alterations in response to kainate, the most marked kynurenic acid increases were seen in subgroups with status epilepticus. Rats which only developed mild (focal) seizures or stereotyped behaviours (wet dog shakes) also exhibited significantly increased kynurenic acid levels, thus indicating that the increase in kynurenic acid in response to kainate was not solely due to sustained convulsive seizure activity. Whereas it was previously proposed that kynurenic acid is involved only in later stages of seizure disorders, the present data demonstrate that marked increases in central and peripheral kynurenic acid levels occur early after the onset of neuroexcitation, at least in the kainate model.

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)

Frequency dependent activation of a slow N-methyl-D-aspartate-dependent excitatory postsynaptic potential in turtle cerebellum by mossy fibre afferents

The synaptic responses of turtle cerebellar Purkinje cells to stimulation of localized mossy fibre systems have been studied by use of intrasomatic and intradendritic recordings in a brainstem-cerebellum preparation in vitro. Activation of mossy fibre inputs from the spinocerebellar pathway evoked fast, disynaptic postsynaptic potentials which were graded in amplitude with stimulus intensity and elicited at latencies consistent with those reported for peripheral nerve stimulation. Repetitive activation (50-100 Hz, 2-10 stimuli) of both spinocerebellar and trigeminocerebellar pathways evoked a slow, long-lasting excitatory postsynaptic potential regardless of whether single stimuli resulted in excitatory, inhibitory, or no postsynaptic responses. This slow potential was capable of triggering dendritic pacemaker discharges in recorded Purkinje cells in addition to volleys of simple spikes when activated at or near resting membrane potential. The fast excitatory synaptic potentials evoked by spinocerebellar stimulation were blocked by the glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione, consistent with the hypothesis that they are mediated by activation of ionotropic glutamate receptors of the alpha-amino-3-hydroxy-5-methylisox-azole-4-proprionic acid subtype at the mossy fibre-granule cell synapse and the subsequent parallel fibre-Purkinje cell synapse. The slow excitatory synaptic potential evoked by repetitive stimulation of either the spinocerebellar tract or trigeminal nerve was blocked by DL-2-amino-5-phosphonvalerate, indicating that this potential is primarily dependent upon N-methyl-D-aspartate receptors at the mossy fibre-granule cell synapse for its expression. This slow potential was reversibly potentiated by L-2-amino-4-phosphonobutyrate and bicuculline; the metabotropic glutamate antagonist (+)-alpha-methyl-4-carboxyphenylglycine did not block this potentiation. The ability of mossy fibre inputs to drive long, slow excitatory events in Purkinje cells adds another dimension to the mechanisms by which various sensory modalities can be processed interactively in the cerebellar cortex. The ability of incoming systems to access a second, longer duration response of the cerebellar output neuron may be of significant consequence to our understanding of the manner in which this neural centre integrates sensory information from multiple sources.

Long-term depression: a learning-related type of synaptic plasticity in the mammalian central nervous system

Studies of various forms of synaptic plasticity in the central nervous system have provided insights into the cellular and molecular mechanisms for certain types of learning and memory. Activity-induced decreases and increases in synaptic efficacy can be elicited in mammalian neurons. Long-term depression (LTD) and long-term potentiation (LTP) are two major forms of activity-dependent synaptic plasticity in the brain. LTD of excitatory synaptic transmission in the cerebellum in the most well studied form of synaptic depression. The induction of cerebellar LTD requires conjunctive activation of alpha-amino-3-hydroxy-5-methyl-4-isoxalepropionate (AMPA) receptors, metabotropic glutamate receptors (mGluRs) and L-type voltage-dependent Ca2+ channels. Several intracellular second messengers and protein kinases are critical for cerebellar LTD, including cGMP, cGMP-dependent protein kinase and protein kinase C (PKC). A novel intercellular messenger, nitric oxide (NO), is found in the cerebellum, is released durinng synaptic stimulation, and may contribute to cerebellar LTD. The expression of cerebellar LTD is mediated by postsynaptic desensitization of AMPA receptors. Recently, a form of homosynaptic LTD has been described in the CA1 region of the hippocampus. The induction of hippocampal LTD is postsynaptic. N-Methyl-D-aspartate receptors and mGluRs are important for induction of hippocampal LTD. Other intracellular and intercellular messengers, such as NO, cGMP and cAMP, might act downstream from glutamate receptors during hippocampal LTD. The expression of hippocampal LTD is likely to be in part presynaptic. While cerebellar LTD may be important for motor learning, the behavioral role of hippocampal LTD remains to be explored.

Optical imaging of parallel fiber activation in the rat cerebellar cortex: spatial effects of excitatory amino acids

Using optical imaging, the effects of excitatory amino acids and their antagonists on the spatial pattern of activity evoked by the stimulation of parallel fibers in the cerebellar cortex of the anesthetized rat were examined. A vermal folium was stained with the stryrl voltage-sensitive dye RH-795 and imaged with a cooled charge-coupled service system during the activation of the parallel fibers with surface stimulation. Stimulation of the cerebellar cortex produced discrete "beams" of optical activity consistent with extracellular field recordings. The signal-to-noise ratio was excellent (> 10:1), reducing the number of stimuli, exposure time, and acquisition time needed to produce images. Extracellular field potential recordings were used to assay neuronal activity as well as the effects of the various excitatory amino acid agonists. Application of several glutamate receptor antagonists reversibly blocked the optical signals as well as the synaptic components of the extracellular field potentials. Neither N-methyl-D-aspartate nor its antagonist, (+/-)-2-amino-7-phosphonoheptanoic acid, had any affect on the optical signals or field potentials. These results indicate that the optical signal is not due to the evoked parallel fiber activity, but is generated mainly by postsynaptic targets and the parallel fiber synaptic action is primarily mediated by non-N-methyl-D-aspartate receptors. Other excitatory amino acid agonists had a differential effect on the optical response. Glutamate and kainate increased the "on beam" optical signal evoked by parallel fiber stimulation, in amplitude and width. In contrast, quisqualate always decreased the amplitude and width of the optical beam. Also, quisqualate produced an increase in fluorescence lateral to the optical beam, possibly due to an increase in "off beam" inhibitory activity. The changes in the extracellular field potentials were in agreement with the effects on the optical signals. Two possible mechanisms are proposed to account for the inhibitory effect of quisqualate. One is that quisqualate desensitizes Purkinje cell receptors, the other is that inhibitory interneurons in the cerebellar cortex are more preferentially excited with quisqualate application which in turn inhibits Purkinje cells both "on beam" and "off beam". In conclusion, voltage-sensitive dye optical signals evoked by stimulation of the cerebellar surface were imaged at high signal-to-noise levels using a cooled charge-coupled device system. Use of excitatory amino acid agonists and antagonists demonstrated that the optical signal was dependent on postsynaptic activity and confirmed that the parallel fiber postsynaptic action is primarily mediated by non-N-methyl-D-aspartate receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Novel synaptic potentials in cerebellar Purkinje cells: probable mediation by metabotropic glutamate receptors

Glutamate receptors of both the ionotropic (ion channel-linked) and metabotropic (enzyme-linked) categories are abundantly expressed by Purkinje cells in the cerebellum but the functional significance of the latter receptors is unknown. We have tested the possibility that they are activated by the parallel fibre input by recording from Purkinje cells within a biplanar cerebellar slice preparation using the grease-gap technique. Under conditions where ionotropic (NMDA and non-NMDA) glutamate and GABA receptors were blocked pharmacologically, electrical stimulation of parallel fibres gave rise to two very slow potentials. The first peaked about 400 msec from the start of stimulation and was depolarising. It was not evident with single stimuli but reached maximum amplitude after 6 shocks delivered at 50 Hz. The wave was abolished when the slices were perfused with Ca(2+)-free solution or with drugs that inhibit synaptic transmission, but it was resistant to blockade of GABAB receptors, acetylcholine receptors and adrenergic receptors. Next came a slow hyperpolarising potential that peaked about 30 sec after stimulation and which was also Ca(2+)-dependent. The sequence of potentials was replicated by perfusion of an exogenous agonist acting selectively on metabotropic glutamate receptors. We conclude that parallel fibre-to-Purkinje cell synaptic transmission involves not only fast signals generated through ionotropic non-NMDA receptors but also much slower potentials that are likely to be mediated by metabotropic glutamate receptors. These potentials are likely to be significant both for shorter-term (seconds to minutes) Purkinje cell excitability as well as for the induction of longer-term synaptic plasticity.

Differential blocking action of Joro spider toxin analog on parallel fiber and climbing fiber synapses in cerebellar Purkinje cells

Synaptic potentials were recorded intracellularly from Purkinje cells in guinea pig cerebellar slices. EPSPs evoked by stimulation of parallel fibers were effectively blocked by perfusion of a slice with the synthetic analog of Joro spider toxin, 1-naphthylacetyl-spermine (NAS) at 250 microM. However, it did not influence those responses evoked by stimulation of climbing fibers. This action of NAS is in contrast to other commonly used glutamate antagonists, CNQX or APV: CNQX (5 microM) blocked both parallel fiber- and climbing fiber-induced responses, while APV (up to 1 mM) did not influence either except for a weak reduction observed in climbing fiber responses. NAS thus provides a useful tool for pharmacologically distinguishing parallel fiber and climbing fiber synapses.

Receptor sub-types involved in responses of Purkinje cell to exogenous excitatory amino acids and local electrical stimulation in cerebellar slices in the rat

The effects of the NMDA receptor antagonist, 2-amino-5-phosphonovalerate (APV) and non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) on responses of Purkinje cells to exogenous excitatory amino acids and to electrical stimulation of the parallel fibres, were investigated in slices of the cerebellum of the rat. Glutamate, aspartate, kainate and quisqualate all induced excitation of Purkinje cells. Responses to kainate and quisqualate were blocked by CNQX (10 microM) but not by APV (10 microM). N-Methyl-D-aspartate induced biphasic excitatory-inhibitory responses, both components of which were blocked by APV but not by CNQX. The inhibitory component was less sensitive to blockade by APV but was totally blocked by bicuculline, the GABAA receptor antagonist. Parallel fibre stimulation most commonly induced inhibition of Purkinje cells, with or without preceding excitation. This inhibition was blocked by APV and excitatory responses were often revealed. A less commonly-observed predominantly excitatory response was blocked by CNQX but not by APV and inhibition tended to be revealed. These data suggest that parallel fibre-Purkinje cell synapses possess non-NMDA postsynaptic receptors, while the parallel fibre-inhibitory interneuron synapses possess functional NMDA receptors.

A long-term depression of AMPA currents in cultured cerebellar Purkinje neurons

Cerebellar long-term depression (LTD) is a model of synaptic plasticity in which conjunctive stimulation of parallel fiber and climbing fiber inputs to a Purkinje neuron induces a persistent depression of the parallel fiber-Purkinje neuron synapse. We report that an analogous phenomenon may be elicited in the cultured mouse Purkinje neuron when iontophoretic glutamate application and depolarization of the Purkinje neurons are substituted for parallel fiber and climbing fiber stimulation, respectively. The induction of LTD in these cerebellar cultures requires activation of both ionotropic (AMPA) and metabotropic quisqualate receptors, together with depolarization in the presence of external Ca2+. This postsynaptic alteration is manifest as a depression of glutamate or AMPA currents, but not aspartate or NMDA currents. These results strengthen the contention that the expression of cerebellar LTD is at least in part postsynaptic and provide evidence that activation of both ionotropic and metabotropic quisqualate receptors are necessary for LTD induction.

Excitatory amino acid receptors coupled to the nitric oxide/cyclic GMP pathway in rat cerebellum during development

The coupling of excitatory amino acid receptors to the formation of nitric oxide (NO) from arginine during the postnatal development of rat cerebellum was assayed in slice preparations by measuring cyclic GMP accumulation. In the immature tissue, N-methyl-D-aspartate (NMDA) and glutamate were highly efficacious agonists, whereas alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and quisqualate evoked only small responses. The effect of glutamate at all concentrations tested (up to 10 mM) was abolished by the NMDA antagonist, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801). In adult slices, AMPA and quisqualate were much more effective and their effects were inhibited by 6-cyano-7-nitroquinoxaline-2,3-dione, an antagonist for ionotropic non-NMDA receptors, whereas the apparent efficacy of NMDA was greatly reduced. The major changes took place between 8 and 14 days postnatum and, in the case of NMDA, part of the loss of sensitivity appeared to reflect a decline in the ambient levels of glycine with age. Moreover, a component of the response to glutamate in the adult was resistant to MK-801. Cyclic GMP accumulations induced by NMDA and non-NMDA agonists alike were Ca(2+)-dependent and could be antagonized by competitive NO synthase inhibitors in an arginine-sensitive manner, indicating that they are all mediated by NO formation. With one of the inhibitors, L-NG-nitroarginine, a highly potent component (IC50 = 6 nM) evident in slices from rats of up to 8 days old was lost during maturation, indicating that there may be a NO synthase isoform which is prominent only in the immature tissue. Cyclic GMP levels in adult slices under "basal" conditions were reduced markedly by blocking NMDA receptors, by inhibiting action potentials with tetrodotoxin, or by NO synthase inhibition, suggesting that the endogenous transmitter released during spontaneous synaptic activity acts mainly through NMDA receptors to trigger NO formation.

Cerebellar excitatory amino acid binding sites in normal, granuloprival, and Purkinje cell-deficient mice

Using quantitative autoradiography, the cellular localization and characterization of cerebellar excitatory amino acid binding sites in normal, Purkinje cell-deficient and granuloprival (granule cell-deficient) mouse cerebella were investigated. In the molecular layer of normal mouse cerebellum, the quisqualate subtype of excitatory amino acid receptor (assayed by [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, quisqualate-sensitive L-[3H]glutamate, and [3H]6-cyano-7-nitroquinoxaline-2,3-dione binding) predominated. In the granule cell layer of the cerebellum, N-methyl-D-aspartate-sensitive L-[3H]glutamate and [3H]glycine binding sites were predominant. In the molecular layer of Purkinje cell-deficient mutant mice, [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding sites and [3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding were reduced to 24% (P less than 0.01) and 36% (P less than 0.001) of control, respectively, while quisqualate-sensitive [3H]glutamate binding sites were reduced to 54% of control (P less than 0.01). N-Methyl-D-aspartate-sensitive [3H]glutamate and [3H]glycine binding were unchanged. In the granule cell layer of these mouse cerebella, there was no change in excitatory amino acid receptor binding. In the molecular layer of granuloprival mouse cerebella, [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate binding was increased to 205% of control (P less than 0.01), [3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding was increased to 136% of control (P less than 0.02), and quisqualate-sensitive [3H]glutamate binding was increased to 152% of control (P less than 0.01). N-Methyl-D-aspartate-sensitive [3H]glutamate and [3H]glycine binding were unchanged. In areas of granule cell depletion N-methyl-D-aspartate-sensitive [3H]glutamate and [3H]glycine binding were reduced to 68% (P less than 0.01) and 59% (P less than 0.01) of control, respectively. In the granule cell layer, binding to quisqualate receptors was not significantly different from binding in controls with any of the ligands tested. These results suggest that three different receptor assays: [3H](RS)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate, quisqualate-sensitive L-[3H]glutamate, and [3H]6-cyano-7-nitro-quinoxaline-2,3-dione binding can be used to demonstrate that quisqualate receptor specific binding sites are located on Purkinje cell dendrites in the molecular layer of cerebellum, and that these binding sites apparently up-regulate in response to granule cell ablation and Purkinje cell deafferentation.

Synaptic receptors and intracellular signal transduction in the cerebellum

Glutamate receptor subtypes mediating excitatory synaptic neurotransmission in the cerebellar cortex are briefly reviewed from molecular biological, electrophysiological and pharmacological points of view. In particular, molecular biological findings of a novel family of AMPA-selective glutamate receptors are introduced, and the pharmacological and electrophysiological properties and the identity of cerebellar N-methyl-D-aspartate-sensitive receptors probably existing on Purkinje cells are discussed in comparison with well-established cerebral NMDA receptors. As possible intracellular mechanisms of the long-term depression of parallel fiber-Purkinje cell neurotransmission, the perspective of the roles of novel messengers, nitric oxide and arachidonic acid, is particularly commented based on recent information about cerebral long-term events. The specificity and possible independence of cerebellar excitatory amino acid receptors and linked intracellular second messengers are also suggested, taking the highly active guanylate cyclase system in Purkinje cells and other cerebellum-specific proteins into consideration.

Quisqualate- and NMDA-sensitive [3H]glutamate binding in primate brain

Excitatory amino acids (EAA) such as glutamate and aspartate are probably the neurotransmitters of a majority of mammalian neurons. Only a few previous studies have been concerned with the distribution of the subtypes of EAA receptor binding in the primate brain. We examined NMDA- and quisqualate-sensitive [3H]glutamate binding using quantitative autoradiography in monkey brain (Macaca fascicularis). The two types of binding were differentially distributed. NMDA-sensitive binding was most dense in dentate gyrus of hippocampus, stratum pyramidale of hippocampus, and outer layers of cerebral cortex. Quisqualate-sensitive binding was most dense in dentate gyrus of hippocampus, inner and outer layers of cerebral cortex, and molecular layer of cerebellum. In caudate nucleus and putamen, quisqualate- and NMDA-sensitive binding sites were nearly equal in density. However, in globus pallidus, substantia nigra, and subthalamic nucleus, quisqualate-sensitive binding was several-fold greater than NMDA-sensitive binding. In thalamus, [3H]glutamate binding was generally low for both subtypes of binding except for the anterior ventral, lateral dorsal, and pulvinar nuclei. In the brainstem, low levels of binding were found, and strikingly the red nucleus and pons, which are thought to receive glutamatergic projections, had approximately 1/20 the binding observed in cerebral cortex. These results demonstrate that NMDA- and quisqualate-sensitive [3H]glutamate binding are observed in all regions of primate brain, but that in some regions one subtype predominates over the other. In addition, certain areas thought to receive glutamatergic projections have low levels of both types of binding.

The effects of inferior olive lesion on strychnine seizure

Bilateral inferior olive lesions, produced by systemic administration of the neurotoxin 3-acetylpyridine (3AP) produce a proconvulsant state specific for strychnine-induced seizures and myoclonus. We have proposed that these phenomena are mediated through increased excitation of cerebellar Purkinje cells, through activation of glutamate receptors, in response to climbing fiber deafferentation. An increase in quisqualic acid (QA)-displaceable [3H]AMPA [(RS)-alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid] binding in cerebella from inferior olive-lesioned rats was observed, but no difference in [3H]AMPA binding displaced by glutamate, kainic acid (KA) or glutamate diethylester (GDEE) was seen. The excitatory amino acid antagonists GDEE and MK-801 [(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclo-hepten-5,10 imine] were tested as anticonvulsants for strychnine-induced seizures in 3AP inferior olive-lesioned and control rats. Neither drug effected seizures in control rats, however, both GDEE and MK-801 produced a leftward shift in the strychnine-seizure dose-response curve in 3AP inferior olive-lesioned rats. GDEE also inhibited strychnine-induced myoclonus in the lesioned group, while MK-801 had no effect on myoclonus. The decreased threshold for strychnine-induced seizures and myoclonus in the 3AP-inferior olive-lesioned rats may be due to an increase in glutamate receptors as suggested by the [3H]AMPA binding data.

Two types of quisqualate receptors are decreased in human olivopontocerebellar atrophy cerebellar cortex

We used receptor autoradiography to study the distribution of ionotropic and metabotropic quisqualate (QA) receptors in normal human cerebellar cortex and cerebellar cortex from 7 cases of olivopontocerebellar atrophy (OPCA). In normal human cerebellar cortex, both types of QA receptors were densest in the molecular layer. Both ionotropic and metabotropic QA receptors were significantly diminished in the molecular layer of OPCA specimens. These results suggest that both ionotropic and metabotropic QA receptors are localized on Purkinje cell dendrites.

Subcellular localization of a putative kainate receptor in Bergmann glial cells using a monoclonal antibody in the chick and fish cerebellar cortex

A monoclonal antibody, IX-50, that was raised against a kainate binding protein (Mr = 49,000) from chicken cerebellum, was used in light and electron microscopic immunocytochemical studies to localize putative kainate receptors. Pre- and postembedding immunoperoxidase and immunogold methods were used in the cerebellar cortices of one to 26-day old chickens and adult rainbow trout. Immunoreactivity was detected only in association with Golgi epithelial/Bergmann glial cells. Intracellular immunoreactivity was present in the granular and agranular endoplasmic reticulum, Golgi apparatus and in lysosomes, representing the sites of synthesis, glycosylation and degradation of the protein. In the fish the granular endoplasmic reticulum was not immunoreactive. Extracellular immunoreactivity was associated with the plasma membrane. In the fish it was established that the epitope is on the outer surface of the membrane. The protein seems to be uniformly distributed along the membrane including the somata, the radial stem processes and the leafy lamellae surrounding Purkinje cell dendrites. Areas of the glial membrane in contact with other glial cells were also immunopositive. High-resolution light microscopy demonstrated all the Bergmann glial plasma membrane in the cortex, providing a "negative" image of Purkinje cell dendrites. It is apparent that Bergmann glial processes selectively outline the dendrites of the Purkinje cells by surrounding the parallel fibre terminal/Purkinje cell spine synaptic complexes. The parallel fiber terminals were highly immunoreactive for glutamate, as shown by an immunogold procedure. The association of Bergmann glial processes, carrying the Mr = 49,000 kainate binding protein, with the Purkinje cell dendrites and spine synapses could provide a basis for neuronal signalling to the Bergmann glia, possibly by glutamate.

Excitatory amino acid receptors in the parallel fibre pathway in rat cerebellar slices

The grease-gap technique was used on young rat cerebellar slices to study the synaptic pharmacology of the parallel fibre pathway. Electrical stimulation of the parallel fibres produced a characteristic response in Purkinje cells: a sharp negative (N) potential, representing the population action potential and underlying parallel fibre EPSP, followed by a slow positive (P) wave, the population inhibitory postsynaptic potential (IPSP). In the presence of 1.2 mM Mg2+, D-2-amino-5-phosphonovalerate (APV, 30 microM) had no effect but both potentials could be inhibited by 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (CNQX, 10 microM). Removal of Mg2+ had no effect on the N-potential but enhanced the P-wave in an APV-sensitive fashion, particularly when CNQX was present. The results provide further evidence that glutamate is the parallel fibre transmitter and suggest that its acts only on non-NMDA (non-N-methyl-D-aspartate) receptors at synapses with Purkinje cells but on both NMDA and non-NMDA receptors at synapses with inhibitory interneurones. At the latter synapses, the NMDA system is likely to be brought into operation in an activity-dependent manner.

Inositolphospholipid-linked glutamate receptors mediate cerebellar parallel-fiber-Purkinje-cell synaptic transmission

In slices of adult rat cerebellum inositolphospholipid turnover is stimulated markedly by glutamate and its rigid analogues quisqualate and ibotenate. The drug and amino acid specificity of the response reflects a quisqualate-preferring excitatory amino acid receptor. The absence of glutamate-enhanced inositolphospholipid turnover in mice with Purkinje-cell degeneration indicates that the inositolphospholipid-linked quisqualate receptor mediates parallel fiber-Purkinje cell synaptic transmission. The quantitative prominence of this synapse accounts for the massive enrichment of elements of the inositolphospholipid system in cerebellar Purkinje cells.

Mode of induction of long-term depression at parallel fibre--Purkinje cell synapses in rabbit cerebellar cortex

In a superficial folium of the dorsal paraflocculus of high decerebrate rabbits, extracellular unitary spikes were recorded from a Purkinje cell, while two parallel fibre beams impinging onto that Purkinje cell were separately stimulated in the molecular layer. Climbing fibre afferents were stimulated at the contralateral inferior olive. Quisqualate was ionophoretically applied to the dendrite of the Purkinje cell intersecting one of the stimulated parallel fibre beams (test beam). Long-term depression (longer than 45 min) occurred in Purkinje cell responsiveness to the test beam, but not to the other beam (control beam), when quisqualate was applied for 4 min in conjunction with 2 Hz stimulation of climbing fibres. This effect was completely abolished by simultaneous application of a glutamate blocker, kynurenate, during conjunctive quisqualate-climbing fibre stimulation. Application of quisqualate alone caused a small degree of depression in parallel fibre-Purkinje cell transmission. This effect was abolished when spontaneous activity of climbing fibres was blocked by injection of tetrodotoxin or lidocaine to the contralateral inferior olive, and therefore was due to conjunction of quisqualate with spontaneous climbing fibre inputs that normally occurred at 0.5-1.2 Hz. These findings suggest that the occurrence of long-term depression is strictly dependent on conjunction of climbing fibre activity with quisqualate receptor activation.