Morphological and electrophysiological characteristics of rat cerebellar slices maintained in vitro

In situ cell-type-specific cell-surface proteomic profiling in mice

Cell-surface proteins (CSPs) mediate intercellular communication throughout the lives of multicellular organisms. However, there are no generalizable methods for quantitative CSP profiling in specific cell types in vertebrate tissues. Here, we present in situ cell-surface proteome extraction by extracellular labeling (iPEEL), a proximity labeling method in mice that enables spatiotemporally precise labeling of cell-surface proteomes in a cell-type-specific environment in native tissues for discovery proteomics. Applying iPEEL to developing and mature cerebellar Purkinje cells revealed differential enrichment in CSPs with post-translational protein processing and synaptic functions in the developing and mature cell-surface proteomes, respectively. A proteome-instructed in vivo loss-of-function screen identified a critical, multifaceted role for Armh4 in Purkinje cell dendrite morphogenesis. Armh4 overexpression also disrupts dendrite morphogenesis; this effect requires its conserved cytoplasmic domain and is augmented by disrupting its endocytosis. Our results highlight the utility of CSP profiling in native mammalian tissues for identifying regulators of cell-surface signaling.

Receptor variants and the development of centrally acting medications

The progressive changes in research paradigms observed in the largest pharmaceutical companies and the burgeoning of biotechnology startups over the last 10 years have generated a need for outsourcing research facilities. In parallel, progress made in the fields of genomics, protein expression in recombinant systems, and electrophysiological recording methods have offered new possibilities for the development of contract research organizations (CROs). Successful partnering between pharmaceutical companies and CROs largely depends upon the competences and scientific quality on offer for the discovery of novel active molecules and targets. Thus, it is critical to review the knowledge in the field of neuroscience research, how genetic approaches are augmenting our knowledge, and how they can be applied in the translation from the identification of potential molecules up to the first clinical trials. Taking these together, it is apparent that CROs have an important role to play in the neuroscience of drug discovery.


The cerebellum, cerebellar disorders, and cerebellar research--two centuries of discoveries

Research on the cerebellum is evolving rapidly. The exquisiteness of the cerebellar circuitry with a unique geometric arrangement has fascinated researchers from numerous disciplines. The painstaking works of pioneers of these last two centuries, such as Rolando, Flourens, Luciani, Babinski, Holmes, Cajal, Larsell, or Eccles, still exert a strong influence in the way we approach cerebellar functions. Advances in genetic studies, detailed molecular and cellular analyses, profusion of brain imaging techniques, emergence of behavioral assessments, and reshaping of models of cerebellar function are generating an immense amount of knowledge. Simultaneously, a better definition of cerebellar disorders encountered in the clinic is emerging. The essentials of a trans-disciplinary blending are expanding. The analysis of the literature published these last two decades indicates that the gaps between domains of research are vanishing. The launch of the society for research on the cerebellum (SRC) illustrates how cerebellar research is burgeoning. This special issue gathers the contributions of the inaugural conference of the SRC dedicated to the mechanisms of cerebellar function. Contributions were brought together around five themes: (1) cerebellar development, death, and regeneration; (2) cerebellar circuitry: processing and function; (3) mechanisms of cerebellar plasticity and learning; (4) cerebellar function: timing, prediction, and/or coordination?; (5) anatomical and disease perspectives on cerebellar function.

The origin of the complex spike in cerebellar Purkinje cells

Activation of the climbing fiber input powerfully excites cerebellar Purkinje cells via hundreds of widespread dendritic synapses, triggering dendritic spikes as well as a characteristic high-frequency burst of somatic spikes known as the complex spike. To investigate the relationship between dendritic spikes and the spikelets within the somatic complex spike, and to evaluate the importance of the dendritic distribution of climbing fiber synapses, we made simultaneous somatic and dendritic patch-clamp recordings from Purkinje cells in cerebellar slices. Injection of large climbing fiber-like synaptic conductances at the soma using dynamic clamp was sufficient to reproduce the complex spike, independently of dendritic spikes, indicating that neither a dendritic synaptic distribution nor dendritic spikes are required. Furthermore, we found that dendritic spikes are not directly linked to spikelets in the complex spike, and that each dendritic spike is associated with only 0.24 +/- 0.09 extra somatic spikelets. Rather, we demonstrate that dendritic spikes regulate the pause in firing that follows the complex spike. Finally, using dual somatic and axonal recording, we show that all spikelets in the complex spike are axonally generated. Thus, complex spike generation proceeds relatively independently of dendritic spikes, reflecting the dual functional role of climbing fiber input: triggering plasticity at dendritic synapses and generating a distinct output signal in the axon. The encoding of dendritic spiking by the post-complex spike pause provides a novel computational function for dendritic spikes, which could serve to link these two roles at the level of the target neurons in the deep cerebellar nuclei.

mGluR1 agonists elicit a Ca 2+ signal and membrane hyperpolarization mediated by apamin-sensitive potassium channels in immature rat purkinje neurons

The type 1 metabotropic glutamate receptor (mGluR1) plays an import role in the synaptic physiology and development of cerebellar Purkinje neurons. mGluR1 expression occurs early in the developmental program of Purkinje neurons, at an age that precedes expression of the dendritic structure. Few studies have investigated the physiological response produced by mGluR1 activation in early-developing Purkinje neurons. To address this question, simultaneous recording of membrane potential and intracellular Ca(2+) was performed in immature cultured Purkinje neurons coupled with exogenous application of mGluR1 agonists. Membrane potential was measured using the perforated patch method of whole-cell recording, and intracellular Ca(2+) was measured using fura-2-based Ca(2+) imaging. Brief, 1-sec micropressure application of the group I mGluR-selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) evoked a prominent Ca(2+) signal and coincident fast hyperpolarization in the immature neurons. The mGluR1-selective antagonist 7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxylate ethyl ester blocked the Ca(2+) signal and fast hyperpolarization, confirming the involvement of mGluR1s. Amplitude of the fast hyperpolarization varied as a function of membrane potential and intracellular Ca(2+) and was blocked by apamin, an antagonist of the small-conductance Ca(2+)-activated K(+) channel (SK), identifying this K(+) channel as an underlying mechanism. In similar experiments with mature cultured Purkinje neurons, DHPG elicited a Ca(2+) signal, but fast membrane hyperpolarization was not evident. These results suggest that mGluR1 activation and the resulting release of Ca(2+) from intracellular stores and activation of SK channels may be a mechanism through which mGluR1 can modulate neuronal excitability of Purkinje neurons during early development.

Synaptic organization of the mouse cerebellar cortex in organotypic slice cultures

The cellular and synaptic organization of new born mouse cerebellum maintained in organotypic slice cultures was investigated using immunohistochemical and patch-clamp recording approaches. The histological organization of the cultures shared many features with that observed in situ. Purkinje cells were generally arranged in a monolayer surrounded by a molecular-like neuropil made of Purkinje cell dendritic arborizations. Purkinje cell axons ran between clusters of small round cells identified as granule cells by Kv3.1b potassium channel immunolabelling. The terminal varicosities of the Purkinje cells axons enwrapped presumptive neurons of the cerebellar nuclei whereas their recurrent collaterals were in contact with Purkinje cells and other neurons. Granule cell axons established contacts with Purkinje cell somata and dendrites. Parvalbumin and glutamine acid decarboxylase (GAD) immunohistochemistry revealed the presence of presumptive interneurons throughout the culture. The endings of granule cell axons were observed to be in contact with these interneurons. Similarly, interneurons endings were seen close to Purkinje cells and granule cells. Whole cell recordings from Purkinje cell somata showed AMPA receptor-mediated spontaneous excitatory post-synaptic currents (sEPSCs) and GABAA receptor-mediated spontaneous inhibitory post-synaptic currents (sIPSCs). Similar events were recorded from granule cell somata except that in this neuronal type EPSPs have both a NMDA component and an AMPA component. In addition, pharmacological experiments demonstrated a GABAergic control of granule cell activity and a glutamatergic control of GABAergic neurons by granule cells. This study shows that a functional neuronal network is established in such organotypic cultures even in the absence of the two normal excitatory afferents, the mossy fibers and the climbing fibers.

Feedforward inhibition controls the spread of granule cell-induced Purkinje cell activity in the cerebellar cortex

Synapses associated with the parallel fiber (pf) axons of cerebellar granule cells constitute the largest excitatory input onto Purkinje cells (PCs). Although most theories of cerebellar function assume these synapses produce an excitatory sequential "beamlike" activation of PCs, numerous physiological studies have failed to find such beams. Using a computer model of the cerebellar cortex we predicted that the lack of PCs beams is explained by the concomitant pf activation of feedforward molecular layer inhibition. This prediction was tested, in vivo, by recording PCs sharing a common set of pfs before and after pharmacologically blocking inhibitory inputs. As predicted by the model, pf-induced beams of excitatory PC responses were seen only when inhibition was blocked. Blocking inhibition did not have a significant effect in the excitability of the cerebellar cortex. We conclude that pfs work in concert with feedforward cortical inhibition to regulate the excitability of the PC dendrite without directly influencing PC spiking output. This conclusion requires a significant reassessment of classical interpretations of the functional organization of the cerebellar cortex.

No parallel fiber volleys in the cerebellar cortex: evidence from cross-correlation analysis between Purkinje cells in a computer model and in recordings from anesthetized rats

Purkinje cells aligned on the medio-lateral axis share a large proportion of their approximately 175,000 parallel fiber inputs. This arrangement has led to the hypothesis that movement timing is coded in the cerebellum by beams of synchronously active parallel fibers. In computer simulations I show that such synchronous activation leads to a narrow spike cross-correlation between pairs of Purkinje cells. This peak was completely absent when shared parallel fiber input was active in an asynchronous mode. To determine the presence of synchronous parallel fiber beams in vivo I recorded from pairs of Purkinje cells in crus IIa of anesthetized rats. I found a complete absence of precise spike synchronization, even when both cells were strongly modulated in their spike rate by trains of air-puff stimuli to the face. These results indicate that Purkinje cell spiking is not controlled by volleys of synchronous parallel fiber inputs in the conditions examined. Instead, the data support a model by which granule cells primarily control Purkinje cell spiking via dynamic population rate changes.

Morphological study of organotypic cerebellar cultures

Organotypic cerebellar cultures from 8-days-old (P8) mouse pups were studied following 11 days of in vitro (I IDIV) culturing. The cerebellar cytoarchitectonic structure was maintained in most parasagittal cerebellar cortical slice cultures (also containing the deep cerebellar nuclei). The two main extrinsic excitatory inputs (the climbing and the mossy fibers) seem to be replaced by other axonal types: in the molecular layer mostly by parallel fibers (for climbing fibers) and in the granular layer by intrinsic mossy fiber collaterals of local excitatory interneurons, the unipolar brush cells. However, in a few organotypic cultures, which (although preserving the trilaminar cerebellar cortical structure) were "granuloprival" but also contained some of the deep cerebellar nuclei, the participation of extracortical axons from the deep cerebellar nuclei in the replacement of the missing afferents is suggested.

Structure and function of dendritic spines

Spines are neuronal protrusions, each of which receives input typically from one excitatory synapse. They contain neurotransmitter receptors, organelles, and signaling systems essential for synaptic function and plasticity. Numerous brain disorders are associated with abnormal dendritic spines. Spine formation, plasticity, and maintenance depend on synaptic activity and can be modulated by sensory experience. Studies of compartmentalization have shown that spines serve primarily as biochemical, rather than electrical, compartments. In particular, recent work has highlighted that spines are highly specialized compartments for rapid large-amplitude Ca(2+) signals underlying the induction of synaptic plasticity.

Sequential stimulation of rat and guinea pig cerebellar granular cells in vitro leads to increasing population activity in parallel fibers

Sequential stimulation of the granular layer of the cerebellar cortex in vitro using 11 linearly aligned stimulating electrodes leads to massive population activity in the parallel fiber system and to spike activity in Purkinje cells (Heck, D., Neurosci. Lett., 157 (1993) 95-98; Heck, D., Naturwissenschaften, 82 (1995) 201-2030). The induced parallel fiber activity, however, might have been a result of direct stimulation of parallel fibers themselves and not of stimulation of granular cells or their ascending axons. We report here that using sequential 'moving' stimuli and varying the distance covered by the 'movement', parallel fiber population spike amplitude increases with distance and saturates for distances longer than 1.0 mm. This effect cannot be explained if parallel fibers are directly stimulated, but requires stimulation of the granular cells or their ascending axons. We conclude that the population spike activity and Purkinje cell responses induced by sequential stimulation of the granular layer of the cerebellar cortex slices in this and earlier experiments consists of orthodromic parallel fiber spikes.

Glutamate-immunoreactive climbing fibres in the cerebellar cortex of the rat

The climbing fibre system, one of the two main excitatory inputs to the cerebellar cortex, is anatomically and physiologically well characterized, while the nature of its neurotransmitter is still a matter of debate. We wished to determine whether glutamate-immunoreactive profiles with the morphological characteristics of climbing fibres could be found in the rat cerebellar cortex. For this purpose, a monoclonal 'anti-glutamate' antibody has been used in combination with a sensitive postembedding immunoperoxidase method on semi-thin sections or in combination with a postembedding immunogold method on ultrathin sections. At the light microscopic level, climbing fibres appeared as strongly stained fibrous profiles, chains of interconnected varicosities or heavily labelled dots of various sizes, often in close apposition to principal Purkinje cell dendrites. At the electron microscopic level, certain labelled varicosities or more elongated profiles resembling climbing fibre terminals were in synaptic contact with dendritic spines of Purkinje cells. Quantitative analysis of gold particle densities showed that such elements were about three to four times more heavily labelled than their postsynaptic partners. The results obtained in this study demonstrate that at least a subset of climbing fibres and their terminals contain relatively high levels of glutamate-like immunoreactivity and provide additional evidence for a role of glutamate as transmitter in these cerebellar afferents.

Rat cerebellar cortex in vitro responds specifically to moving stimuli

In spite of the detailed anatomical knowledge available, the functional significance of the cerebellar wiring diagram is still obscure. Since there are no variations in the anatomy throughout the whole cortical plane, it is plausible to assume that the basic operation is the same in all parts of the cerebellum. The 'wiring' suggests that local activity must depend on the spatio-temporal organization of the inputs. Neocortical input reaches the cerebellar cortex from nearly all cortical areas via mossy fibers terminating on the granular cells. We simulated such an input to the granular layer in acute slices of rat cerebellar cortex using an array of 11 stimulating electrodes. By successively switching the stimulus current from one electrode to the next, a 'moving' input to the granular layer is simulated. Our experiments show that the cerebellar cortex is specifically activated by 'moving' stimuli applied to the granular layer. The activation is a function of 'movement' direction and velocity. Thus, it enables the cerebellar cortex to act as a movement detector. Such behavior has previously been postulated on anatomical grounds [Braitenberg, J. Theoret. Neurobiol., 2 (1983) 237-241; Braitenberg, In Glickstein et al. (Eds.), Cerebellum and Neuronal Plasticity, Plenum, 1987, pp. 193-207].

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)

Background firing activity in guinea-pig neocortex in vitro

The background firing activity was recorded extracellularly in experiments on guinea-pig neocortical slices maintained in vitro. The following types of background firing activity were revealed: (i) high regular single spikes (48%), (ii) irregular single spikes (15%), (iii) bursts (7%), (iv) groups (7%), (v) mixed activity where single spikes alternated with bursts or groups (28%). The specific interspike interval distribution and the specific shape of autocorrelogram corresponded to each of these background firing activity types. Furie analysis of autocorrelograms showed periodic components in spike sequences with the maxima at 3, 12, and 28 Hz. When blocking synaptic transmission with 100 mM adenosine, about 70% of the background active cells "fell silent" and the remaining 30% of neurons continued to generate action potentials. The latter seem to be actual spontaneously active neurons, i.e. they were capable of autonomous spike generation. We failed to find any correlation between the type of neuronal firing and the ability of neurons to be spontaneously active. The selective blockade of inhibitory synapses with 100 mM picrotoxine did not practically change the character of background firing activity though the responses to stimulation became epileptic. An important conclusion to emerge from this study is that the background firing activity in cortical slices can include the actual spontaneous discharges related to intrinsic cell properties as well as those concerned with synaptic actions. Furthermore, a small number of spontaneously active neurons seem to be able to synaptically activate twice the number of cells. The inhibitory interneurons did not significantly influence the propagation of excitation with the absence of stimulation.

Synaptic- and agonist-induced excitatory currents of Purkinje cells in rat cerebellar slices

1. Postsynaptic currents originating from activation of the two major excitatory inputs to Purkinje cells were studied in thin slices of rat cerebellum, using the tight-seal whole-cell recording technique. Two types of excitatory postsynaptic currents were analysed: those evoked by stimulation of the granule cell-parallel fibre system (PF-EPSC) and those elicited by stimulation of the climbing fibres (CF-EPSC). 2. Both types of postsynaptic currents had a linear current-voltage relation, reversing at membrane potentials close to 0 mV. Their time course of activation was independent of the membrane potential. 3. For both types of postsynaptic currents, the time course of decay was well described by a single exponential function, with a time constant which increased as the membrane potential was made more positive. 4. Postsynaptic currents arising from stimulation of the climbing fibre generally had a slightly faster time course of onset and decay than those associated with stimulation of the granule cell-parallel fibre system. The average values of the 10-90% rise time were 1.8 +/- 0.4 ms (means +/- S.D., n = 7) for PF-EPSCs and 0.8 +/- 0.3 ms (n = 9) for CF-EPSCs. Time constants of decay, at a holding potential of -60 mV, had values of 8.3 +/- 1.6 ms (n = 7) and 6.4 +/- 1.1 ms (n = 9) for PF-EPSCs and CF-EPSCs respectively. 5. CF-EPSCs and PF-EPSCs had the characteristics described above in slices derived from animals aged 9-22 days old and 9-15 days old, respectively. The PF-EPSCs in animals older than 15 days had very slow time courses and positive apparent reversal potentials, suggesting that they originated from distal locations, not under accurate voltage control. 6. In order to assess the quality of the voltage clamp, responses to hyperpolarizing pulses from -70 mV were analysed. The capacitive currents could be fitted by the sum of two exponentials, and were interpreted with an equivalent electrical circuit comprising two main compartments (soma and proximal dendrites on one hand, distal dendrites on the other). Analysis of synaptic currents in terms of this model suggested that the recorded time course of decay was approximately correct. 7. CF-EPSCs as well as PF-EPSCs were insensitive to the NMDA receptor antagonist 3-3(2-carboxypiperazine-4-yl)propyl-1-phosphonate (CPP), but were blocked in a dose-dependent reversible manner by the non-NMDA antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX).(ABSTRACT TRUNCATED AT 400 WORDS)

Pairing of pre- and postsynaptic activities in cerebellar Purkinje cells induces long-term changes in synaptic efficacy in vitro

1. An in vitro slice preparation of rat cerebellar cortex was used to analyse long-lasting modifications of synaptic transmission at parallel fibre (PF)-Purkinje cell (PC) synapses. These use-dependent changes were induced by pairing PF-mediated EPSPs evoked at low frequency (1 Hz) with different levels of membrane polarization (or bioelectrical activities) of PCs for 15 min. 2. Experiments were performed on forty-eight PCs recorded intracellularly in a conventional perfused chamber, and in fifty other cells maintained in a static chamber either in the presence (n = 21) or in the absence (n = 29) of 400 nM-phorbol 12,13-dibutyrate (PDBu). 3. In these three experimental conditions, PF-mediated EPSPs were always measured on PCs maintained at a holding potential of -75 mV, and further hyperpolarized by constant hyperpolarizing pulses. This allowed us both to test the input resistance of PCs and to avoid their firing during PF-mediated EPSPs. 4. In all cells retained for the present study, latencies of PF-mediated EPSPs evoked at 0.2 Hz were stable during the pre-pairing period, and the same was true for their amplitude and time course. 5. In the perfused chamber, pairing of PF-mediated EPSPs with the same hyperpolarization of PCs as that used for measurements of synaptic responses had no effect on these EPSPs in 30% of PCs. It induced long-term depression (LTD) and long-term potentiation (LTP) in 23 and 47% of the tested cells respectively (n = 17). 6. In the perfused chamber, pairing of PF-mediated EPSPs with moderate depolarization of PCs (n = 19) giving rise to a sustained firing of sodium spikes significantly favoured the appearance of LTP as compared to the previous pairing protocol. However, there were still 27 and 15% of cells which showed no modification and LTD respectively. 7. In contrast, pairing of PF-mediated EPSPs with calcium (Ca2+) spikes evoked by strong depolarization of PCs (n = 12) led to LTD of synaptic transmission in nearly half of the tested cells, whereas LTP was now observed in less than 20% of them. 8. In the static chamber and in the absence of PDBu, LTD of PF-mediated EPSPs was observed in most cells, whatever the pairing protocol with sodium or Ca2+ spikes. 9. This shift towards LTD was significantly reversed by PDBu in the pairing protocol using firing of sodium spikes, but not in the case of pairings with Ca2+ spikes.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Fate of grafted embryonic Purkinje cells in the cerebellum of the adult "Purkinje cell degeneration" mutant mouse. II. Development of synaptic responses: an in vitro study

Solid pieces of cerebellar primordia from 12-day-old C57Bl embryos were implanted in the cerebellar vermis of 3-4-month-old "Purkinje cell degeneration" mutant mice. Ten to 22 days after grafting, mutant mice were sacrificed, and synaptic responses of grafted Purkinje cells were studied by intracellular recordings performed in 400 microns thick sagittal slices in vitro. As early as 10 days after transplantation, grafted Purkinje cells have already completed their migration from the implant into the host molecular layer. Accordingly, inhibitory as well as excitatory responses were already elicited in these cells by electrical stimulation of the host subcortical white matter. Furthermore, a transient stage of multiple innervation of Purkinje cells by climbing fibers exists between 10 and 15 days after grafting, as revealed by the stepwise variation in amplitude of the climbing fiber-mediated excitatory postsynaptic potentials recorded before 15 days after grafting. Thirteen days after transplantation, typical all-or-none climbing fiber-mediated responses, parallel fiber-mediated excitatory postsynaptic potentials, and inhibitory postsynaptic potentials were also already present. Finally, normal adult-type synaptic responses were observed in all tested cells 15 to 17 days after grafting. Together with the companion paper (Sotelo et al., 1990), these results demonstrate that grafted Purkinje cells are able to impose on host afferents a pattern of synaptogenesis which closely follows that occurring during normal development, in particular, the transient stage of multiple innervation of Purkinje cells by climbing fibers.

Synaptic currents in cerebellar Purkinje cells

Cerebellar Purkinje cells are known to receive strong excitatory input from two major pathways originating outside the cerebellum and inhibitory input from two types of neurons in the cerebellar cortex. The functions and synaptic strengths of these pathways are only partially known. We have used the patch-clamp technique applied to Purkinje cells in thin slices of rat cerebellum to measure directly the postsynaptic currents arising from the two major excitatory pathways and one of the inhibitory inputs. Inhibitory synaptic currents occur spontaneously with high frequency and are variable in amplitude, ranging, in our recording conditions with high internal Cl-, from less than 100 pA to more than 1 nA. These currents are blocked by the gamma-aminobutyrate type A antagonist bicuculline. One of the excitatory inputs is all or none. For threshold stimulation, the synaptic current is either full amplitude, when the presynaptic fiber is successfully stimulated, or completely absent. This synaptic current is often larger than 1 nA and is virtually eliminated by 2 microM 6-cyano-7-nitroquinoxaline-2,3-dione, a blocking agent thought to be specific for glutamate receptors that are not of the N-methyl-D-aspartate type. Its all-or-none character identifies it as arising from a climbing-fiber synapse. The other excitatory input produces a synaptic current that is smoothly graded as a function of stimulus intensity. This response we believe arises from the stimulation of mossy fibers or granule cells. The synaptic current associated with this input is also largely eliminated by 2 microM 6-cyano-7-nitroquinoxaline-2,3-dione.

Aspartate-like and glutamate-like immunoreactivities in the inferior olive and climbing fibre system: a light microscopic and semiquantitative electron microscopic study in rat and baboon (Papio anubis)

A post-embedding immunogold procedure was used to analyse, in a semiquantitative manner, the distributions of aspartate-like and glutamate-like immunoreactivities in the inferior olive and climbing fibre system in rats and baboons. The neurons in the inferior olive were uniformly labelled for aspartate as well as glutamate, indicating a 100% co-localization of these two amino acids in the cell bodies. The level of glutamate-like immunoreactivity in the climbing fibre terminals was similar to that in the parent cell bodies, as judged by a computer-assisted calculation of gold particle densities. In contrast, the level of aspartate-like immunoreactivity in the climbing fibre terminals was only one-seventh of that of the olivary neurons. No differences were found between the hemispheres and vermis. Nerve terminals in the inferior olive were generally moderately labelled with the aspartate antiserum, as were cell bodies of astrocytes. With a few exceptions, the results obtained in baboons were similar to those in rats. Notably, no evidence was found of an enrichment of aspartate-like immunoreactivity in climbing fibres. The present results do not support previous data suggesting that aspartate is the transmitter of the climbing fibres but indicate that glutamate or another excitatory compound should be considered as candidate for this role. Our findings show that the presence of aspartate-like immunoreactivity in cell bodies is an unreliable indicator of transmitter identity.

Excitatory amino acid receptors mediate slow synaptic transmission in turtle cerebellum

In the isolated turtle cerebellum intracellular recordings from Purkinje cell dendrites and somata reveal novel slow excitatory synaptic potentials evoked by activation of climbing fiber (CF) or parallel fiber (PF) inputs. Classical fast excitatory synaptic responses to CF and PF stimulation are followed by large, slow excitatory postsynaptic potentials (sEPSPs) which are associated with an increase in conductance and are enhanced by hyperpolarization. Both sEPSPs are blocked by the excitatory amino acid (EAA) antagonist kynurenate, but not by DL-2-amino-5-phosphonovalerate (AP-5). The EAA receptor antagonist L-amino-4-phosphonobutyric acid (L-AP-4) reversibly blocked the PF-sEPSP without affecting the CF-sEPSP. Two novel slow synaptic potentials mediated by excitatory amino acid receptors can therefore be observed in turtle cerebellum which may play an important role in synaptic integration.

Selective effects of serotonin upon excitatory amino acid-induced depolarizations of Purkinje cells in cerebellar slices from young rats

The effects of serotonin on responses induced in Purkinje cells (PCs) by microiontophoretic administration of excitatory amino acids (EAAs) in their dendritic fields were tested in vitro by extracellular recording and by single electrode voltage clamp methods in cerebellar slices from rats aged 16-22 days. Serotonin diminished excitations produced by glutamate (Glu) and quisqualate (Quis) selectively, those caused by N-methyl-D-aspartate (NMDA) being affected much less. These suppressions of Glu- and Quis-induced responses generally occurred without there being any effect on intrinsic membrane properties of PCs, although on occasion serotonin increased membrane conductance slightly and/or induced an outward current in the recorded cells. All these effects of serotonin were maintained in the presence of tetrodotoxin and reversed upon removal of the amine. On the few occasions when serotonin enhanced Quis-induced responses, the effect was mimicked by ejection from a control solution of saline, made up at the same pH as the drug solution of serotonin.

Synaptic control of excitability in turtle cerebellar Purkinje cells

1. In turtle Purkinje cells in vitro successive climbing fibre responses (CFRs) gradually induced a hyperpolarization that persisted with maintained stimulation and decayed over minutes after climbing fibre stimulation was terminated. 2. The rate of development and the amplitude of this long-lasting hyperpolarization (LHP) increased with the frequency of CFRs. 3. The LHP was also induced by Ca2+ spikes evoked by current injection but not by Na+ spikes. The LHP was blocked by Co2+ but not by tetrodotoxin and could not be explained solely by an increased K+ conductance. 4. Depolarizing current during a train of CFRs enhanced the regenerative component of CFRs and promoted the LHP. Hyperpolarizing current during the stimulus train reduced the regenerative component of CFRs and attenuated the resulting LHP. 5. In the range of membrane potentials attained at different levels of climbing fibre activity the regenerative component of CFRs varied from being dominant at very low stimulus frequency (0.1 s-1) to being inconspicuous at high stimulus frequency (10 s-1). 6. It is concluded that successive CFRs induce a Ca2+-dependent, long-lasting hyperpolarization. The magnitude of the hyperpolarization is regulated by the rate of CFRs and by the voltage- and frequency-dependent configuration of each individual CFR. 7. The active, non-synaptic properties of turtle Purkinje cells make the Ca2+ influx during climbing fibre responses prone to regulation by on-going synaptic activity and by the after-effects of synaptic activity on a time scale of minutes. We suggest that this arrangement may enhance the capacity and complexity of spatial and temporal synaptic integration in Purkinje cells.

Excitatory synaptic responses in turtle cerebellar Purkinje cells

1. Climbing fibre responses (CFRs) and parallel fibre responses (PFRs) in Purkinje cells have been analysed in intracellular recordings obtained at various levels from cell body to terminal dendrites in the turtle cerebellum in vitro. 2. With increasing stimulus intensity, the PFR recorded in distal dendrites displayed an early regenerative component which was graded at rest and at hyperpolarized membrane potentials, but was all-or-none at depolarized membrane potentials. 3. The all-or-none component had the same characteristics as Ca2+ spikes triggered by passing depolarizing current through the recording electrode. 4. The repolarizing phase of the PFR had a fast component enhanced by depolarization and diminished by hyperpolarization. 5. In the mid-molecular layer the PFR also included a plateau component which was increasingly prolonged by depolarization and abolished by hyperpolarization. 6. CFRs recorded in the soma had a plateau component, prolonged by local depolarization and abolished by local hyperpolarization. 7. The CFR in distal dendrites included a regenerative component. In some cells this component appeared in an all-or-none manner with local depolarization. In other cells it was smoothly graded with local polarization. 8. In mid-molecular records the CFR was prolonged by local depolarization and presumably electrotonically affected by the configuration of the response more distally and proximally in the cell. 9. It is concluded that excitatory synaptic responses in Purkinje cells include a regenerative Ca2+-mediated spike component in the spiny dendrites and a plateau component located in the proximal dendrites and/or the cell body. It is shown that both responses are modulated in configuration by the local membrane potential. In the spiny dendrites activation and inactivation of the transient hyperpolarizing potential appear to govern the Ca2+ influx during the CFR.

Development of ionic conductances in neurons of the inferior olive in the rat: an in vitro study

Neurons of the inferior olive of the rat were studied at different stages of their postnatal (PN) development by using the current clamp technique in slices maintained in vitro. Antidromic and synaptic activation of inferior olivary neurons could be achieved in preparations as young as PN day 2. Neurons at this age already exhibited a variety of ionic conductances which included fast sodium-dependent spikes, high-threshold and low-threshold calcium spikes, potassium-dependent currents, Ca-dependent after-hyperpolarizing potentials (AHPS), and both instantaneous and time-dependent inward rectification at hyperpolarized levels of membrane potential. The two types of Ca-dependent responses recorded in olivary neurons during the first postnatal week were graded with the magnitude of the depolarization imposed on the cells. Furthermore, the high-threshold Ca spikes were only clearly observed during this early period when K conductances were depressed by the injection of caesium into the cells or by bath application of 4-aminopyridine. In contrast, the high-threshold Ca spikes could be obtained without suppression of K currents and were all-or-none in character in some neurons after PN day 8 and in all neurons after PN day 11. The observations suggest that the balance between K and Ca currents changes throughout maturation and is largely in favour of the K current until about the end of the first PN week. At all ages studied, the low-threshold Ca spikes were much less sensitive to the Ca channel blocker cadmium than were the high-threshold Ca spikes. Finally, spontaneous, regular oscillations of the membrane potential were observed for the first time at PN day 16 and were only commonly observed after PN day 19, suggesting a late development of electrotonic coupling between olivary neurons.

Electrophysiological demonstration of a synaptic integration of transplanted Purkinje cells into the cerebellum of the adult Purkinje cell degeneration mutant mouse

After implantation of solid pieces of cerebellar primordia from 12-day-old C57BL embryos into the cerebellar parenchyma of 3- to 4-month-old "Purkinje cell degeneration" mutant mice, Purkinje cells from the donor leave the implant and differentiate while migrating into the host molecular layer. Electrophysiological studies were performed using in vitro cerebellar slice preparations from "Purkinje cell degeneration" mutants 1-2 months after grafting, when grafted Purkinje cells have reached their final location in the host molecular layer and have completed their morphological differentiation. Intracellular recordings obtained from 45 Purkinje cells in mutant mice demonstrated that such grafted neurons have normal bioelectrical properties including sodium and calcium conductances and inward rectification. Moreover, all grafted Purkinje cells responded to electrical white matter stimulation by a typical all-or-none climbing fiber response. Responses mediated through the activation of mossy and parallel fibers, as well as inhibitory postsynaptic potentials, were also recorded in a significant number of grafted Purkinje cells. On the whole, all these excitatory and inhibitory responses in grafted "Purkinje cell degeneration" mutant mice have characteristics comparable to those in control mice. After electrophysiological studies, Purkinje cells were further characterized by their positive staining by calbindin antibody. Neurons of this class were dispersed throughout the molecular layer of the host folia in which the electrophysiological recordings had been performed. The ectopic location of their perikarya, the presence of dendritic trees spanning most of the molecular layer (without entering the granular layer), and the occasional presence of axons emerging from the ectopic neurons and forming loose bundles at the white matter axis of the folia, corroborate the grafted nature of the Purkinje cells studied. Therefore, these experiments demonstrate that embryonic Purkinje cells from the graft can complete differentiation in the adult host cerebellum, and establish specific synaptic contacts with the presynaptic elements previously impinging on the missing neurons of "Purkinje cell degeneration" mutants. This process leads to a qualitative functional synaptic restoration of the cortical cerebellar network.

Optical recording of electrical activity from parallel fibres and other cell types in skate cerebellar slices in vitro

1. A reliable and simple fish brain slice preparation was obtained from the cerebellum of the skate, and its properties were described. 2. A potentiometric oxonol dye, RH-482, and multiple site optical recording of transmembrane voltage (MSORTV) were used to reveal the electrophysiological properties of the parallel fibre action potential and to measure its conduction (0.13 m/s). The parallel fibre action potential was blocked in the presence of tetrodotoxin (TTX) and prolonged by tetraethylammonium (TEA), suggesting that the upstroke depends upon sodium entry and the repolarization upon potassium efflux. An after-hyperpolarization results from a calcium-dependent potassium conductance. 3. A second potentiometric dye, RH-155, differing only slightly from RH-482, exhibited a high affinity for glial cell membrane, and could be used to monitor changes in extracellular potassium concentration by detecting changes in glial membrane potential. 4. Calcium channel blockers such as cadmium ions blocked the optical signal that reflected the extracellular accumulation of potassium. 5. Interventions that modified the extracellular volume, and thereby affected the accumulation of potassium, produced large changes in the optical signal that monitored glial depolarization. Hypertonic and hypotonic bathing solutions resulted in decreases and increases, respectively, in the magnitude of the extrinsic absorption change that tracked potassium accumulation. 6. Blocking sodium-potassium pump activity by means of ouabain prolonged the time course of the optical signal that was related to potassium accumulation in the extracellular space. 7. Extracellular potassium accumulation was revealed to be critically dependent upon intracellular calcium ions.

Synaptic modification of parallel fibre-Purkinje cell transmission in in vitro guinea-pig cerebellar slices

1. Synaptic transmission from parallel fibres to Purkinje cells and its modification by paired stimulation of parallel fibres and climbing fibres were studied in in vitro slices of the cerebellum obtained from guinea-pigs. 2. Intracellular recordings were made from Purkinje cells, mainly from dendrites in the middle third of the molecular layer, but also, in a few cases, from somata. Climbing fibres were activated by stimulation of the white matter, while parallel fibres were stimulated with an electrode placed near the pial surface of the molecular layer. 3. Stimulation of the white matter elicited antidromic spikes, all-or-none climbing fibre responses, disynaptic responses through mossy fibres and parallel fibres, and trisynaptic responses through inhibitory interneurones. Climbing fibre responses were often followed by a small plateau potential, usually less than 2-3 mV in amplitude and less than 100 ms in duration, followed by a slow hyperpolarization which reached its peak in several seconds. Inhibitory inputs to Purkinje cells were blocked with picrotoxin for the experiments described below. 4. Stimulation of the superficial molecular layer with currents less than 50 microA produced graded parallel fibre-mediated excitatory postsynaptic potentials (e.p.s.p.s) ranging from 4 to 8 mV in peak amplitude. 5. Conjunctive stimulation of climbing fibres and parallel fibres at 4 Hz for 25 s induced depression of parallel fibre-mediated e.p.s.p.s in Purkinje cells, both in the peak amplitudes and in the slopes. The depression was about 30% on average and lasted for more than 50 min. 6. No such depression occurred when the intensity of the white matter stimulation was set just subthreshold for the climbing fibre innervating the Purkinje cell under study. Instead, the parallel fibre-mediated e.p.s.p.s were moderately potentiated for a period ranging from 10 to 50 min. Repetitive stimulation of the climbing fibre alone did not affect parallel fibre-mediated e.p.s.p.s. 7. Immediately after the conjunctive stimulation or the repetitive stimulation of climbing fibres alone, a transient hyperpolarization which lasted for several minutes was seen. Its time course was similar to that of the hyperpolarization following a climbing fibre response. Except for this, there were no associated changes in the membrane potential, the input resistance, or the magnitudes of climbing fibre responses in any of the cases mentioned in 5 and 6 above.

Excitation of rat prefrontal cortical neurons by dopamine: an in vitro electrophysiological study

The effects of dopamine (DA) on prefrontal pyramidal neurons were studied in vitro on rat cerebral cortex slices using intracellular recordings. Pyramidal neurons were first identified by Lucifer yellow and some of their basic bioelectrical properties were analysed. At resting potential, white matter stimulation mainly evoked depolarizing inhibitory postsynaptic potentials (IPSPs) which reversed between -60 and -50 mV and were almost totally abolished by bicuculline. Furthermore, pyramidal cells often exhibited spontaneous depolarizing IPSPs abolished by bicuculline. Under tetrodotoxin (TTX) this synaptic noise was partly blocked suggesting that it was due both to the spontaneous firing of presynaptic gamma-aminobutyric acid (GABA)ergic neurons and to a spontaneous quantal release from these afferent fibers. In pyramidal cells, DA enhanced the number of spikes evoked by depolarizing current pulses, and the input resistance was increased by 10-20%. DA also clearly increased the inhibitory synaptic noise. This effect was blocked by fluphenazine. In contrast, evoked IPSPs were not consistently affected by DA. Taken altogether, these results suggest, that in the prefrontal cortex, dopamine has a mild excitatory effect on both pyramidal cells and GABAergic interneurons impinging on them.

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.

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

The sensitivity of intracerebellar nuclei neurones to pulse applications of L-aspartate, L-glutamate, N-methyl-D,L-aspartate and quisqualate was tested in rat cerebellar slices maintained in vitro. The responses of the nuclear neurones to the four agonists consisted of a transient and dose-dependent increase in their firing of simple spikes. When suprathreshold currents were used, quisqualate induced the highest increase in the spike discharge frequency of the cells. Quisqualate mediated responses were unaffected by steady applications of 2-amino-5-phosphonovalerate, whereas the sensitivity of the responses induced by the three other agonists was in the order N-methyl-D,L-aspartate, L-aspartate, L-glutamate. When the superfusing solution was devoid of Mg2+ ions, N-methyl-D,L-aspartate and L-aspartate mediated responses were much potentiated, while quisqualate induced responses were not enhanced. In such a medium, L-glutamate elicited responses were more or less potentiated depending on cells. These results suggest that rat intracerebellar nuclei neurones bear both N-methyl-D-aspartate and non-N-methyl-D-aspartate, probably quisqualate, receptors, and that L-aspartate and L-glutamate have a mixed action upon both types. L-Aspartate preferentially activates N-methyl-D-aspartate receptors, whereas L-glutamate predominantly acts via non-N-methyl-D-aspartate receptors. Furthermore, the potency of L-glutamate in activating N-methyl-D-aspartate receptors appears to vary as a function of the cells.

Inward rectification and low threshold calcium conductance in rat cerebellar Purkinje cells. An in vitro study

The bioelectrical properties of Purkinje cells were analysed in sagittal slices of adult rat cerebellum by the use of intracellular recordings performed at a somatic level in current or in voltage clamp. The passive electrical constants of Purkinje cells were determined by measuring the time course and the amplitude of the voltage responses induced by hyperpolarizing current pulses. The mean value of input resistance was 21 +/- 1 M omega. Mean values of the membrane time constant and of the total electrotonic length of Purkinje cells were 19.5 +/- 1.7 ms and 0.59 +/- 0.01 ms respectively. A time dependent inward rectification was present in all cells. In current-clamp experiments it appeared as a sag in hyperpolarizing voltage responses which were followed by well developed anodal breaks. In voltage-clamped cells, the inward relaxation induced by hyperpolarizing commands fitted to a single exponential. It was already present near resting potential and could reach an amplitude of up to 4 nA for jumps near to -120 mV. This relaxation was provisionally termed Ih. Tail current relaxations also fitted to a single exponential when they were recorded in the presence of tetrodotoxin (TTX) and of Co. The inward relaxation induced by hyperpolarizing commands was readily blocked by Cs, whereas it was unaffected when Ba replaced Ca in the bath, except near rest where it was strongly reduced. The Ca channel blockers Cd, Co and D600 also markedly depressed or even suppressed the inward rectification near resting potential, and up to about -85 mV, whereas this blocking effect was much less apparent or even absent at more negative potentials. Ih was clearly enhanced when the external K concentration was raised up to 20 mM. In the presence of TTX and Co in the bath, inward relaxations induced by hyperpolarizing jumps were unaffected in Na-free solution, whereas the amplitude of tail currents was reduced. Furthermore, the reversal potential of Ih which ranged between -45 and -56 mV in the Co plus TTX containing solution, shifted toward more negative values in the Na-free medium. In contrast, Ih remained unchanged in low Cl solution. From these experiments, it is likely that K and Na are the main charge carriers of Ih. Furthermore, this current seems to be contaminated near resting potential by a Ca-dependent K current. Anodal breaks following hyperpolarizing commands were slightly attenuated when Cd or TTX were added to the bath.(ABSTRACT TRUNCATED AT 400 WORDS)

Climbing and parallel fiber responses recorded intracellularly from Purkinje cell dendrites in guinea pig cerebellar slices

Climbing fiber responses (CFRs) evoked by white matter stimulation and parallel fiber responses (PFRs) elicited by the stimulation of the surface of the folium were compared using intracellular recording from the dendrites of Purkinje cells in the in vitro slice preparation of the guinea pig cerebellum. Intracellular injection of caesium ions allowed Purkinje cell dendrites to depolarize to a range of -20 to +30 mV, and to reverse both CFR and PFR. The mean reversal potential for the CFR was less negative than for the PFR. CF exhibited a depleting property of a neurotransmitter. The contribution of the Ca-component at the rising phase of CFR was discussed.

Pharmacological evidence for L-aspartate as the neurotransmitter of cerebellar climbing fibres in the guinea-pig

Climbing fibre responses (c.f.r.s) evoked by white matter stimulation and the depolarizations induced by iontophoretically applied L-glutamate and L-aspartate were recorded intracellularly from the proximal dendrites of Purkinje cells in in vitro slice preparations of the guinea-pig cerebellum. Short pulses of L-glutamate and L-aspartate dose-dependently depolarized the Purkinje cell dendrite. Even small doses of these amino acids reduced the input resistance. The maximum decrease in input resistance induced by L-glutamate was 36% and that by L-aspartate was 38%. Intracellular injection of Cs+ allowed Purkinje cell dendrites to be depolarized to a range of -15 to +30 mV. The mean reversal potential for the c.f.r. (Ec) was found to be +10.2 mV (n = 4). The mean reversal potentials obtained for L-glutamate (Eg) and for L-aspartate (Ea) were +7.3 mV (n = 7) and +5.6 mV (n = 7) respectively. When external Na+ concentration was reduced, Ec, Ea and Eg were linearly and similarly shifted in the negative direction, indicating that all these reversal potentials are determined primarily by a Na+ conductance. The effects of the glutamate antagonists 2-amino-5-phosphonovaleric acid (APV), gamma-D-glutamylglycine (gamma-DGG), N-methyl-DL-aspartic acid (NMDLA) and glutamic acid diethylester (GDEE) were compared as to the responses to L-glutamate and L-aspartate and Ca2+-activated focal climbing fibre responses (c.f.c.f.r.s) in order to investigate the receptor type at the synapses formed by the climbing fibres with Purkinje cell dendrites. The order of antagonistic potency to the c.f.c.f.r. was : APV (mean percentage blockade = 99%) greater than gamma-DGG (87%) greater than NMDLA (71%) greater than GDEE (28%). The order of antagonistic potency to the response to L-aspartate was: gamma-DGG (69%) greater than APV (66%) greater than NMDLA (60%) greater than GDEE (31%), and that to the response to L-glutamate was: GDEE (63%) greater than NMDLA (22%) greater than gamma-GDD (15%) greater than APV (14%). APV was found to be the most effective anatagonist of the c.f.c.f.r. Its action was reversible, selective for L-aspartate-induced depolarization and had no effect on the responses to L-glutamate. NMDLA, which has no activity as an agonist, was a greater suppressant of the responses to L-aspartate than those to L-glutamate. These electrophysiological and pharmacological findings suggest that the receptor for the transmitter at the synapses formed by climbing fibres with Purkinje cell dendrites is of the L-aspartate-preferring type, and are thus consistent with the bio-and histochemical findings that L-aspartate may be the endogenous transmitter at this synapse.

Electrophysiological studies on the postnatal development of intracerebellar nuclei neurons in rat cerebellar slices maintained in vitro. I. Postsynaptic potentials

The development of the synaptic responses of intracerebellar nuclei neurons was studied in the rat by the use of thick sagittal cerebellar slices maintained in vitro. It has been shown that functional excitatory synapses are present on these neurons from birth, probably due to climbing and/or mossy fiber collaterals; functional inhibitory synapses, due to monosynaptic projections of Purkinje cell axons onto intracerebellar nuclei, are present as early as postnatal day 2; and a more complex pattern of synaptic responses, including short latency excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs), longer latency IPSPs, and late depolarizing responses, can be elicited in nuclear neurons as early as postnatal day 3, indicating an early development of some complete functional cerebellar circuits involving the intracerebellar nuclei.

Membrane resistivity estimated for the Purkinje neuron by means of a passive computer model

A multicompartment passive electrotonic computer model is constructed for the cerebellar Purkinje cell of the guinea-pig. The model has 1089 coupled compartments to accurately represent the morphology of the Purkinje cell. In order that the calculated behavior of the model fit the published electrophysiological observations of somatic and dendritic input conductance, the neural membrane resistivity must be spatially non-uniform. The passive electrical parameter values for which the model best fits the observations of input conductances, pulse attenuation and current-clamp voltage transients are rm,dend = 45,740 omega cm2, rm,soma = 760 omega cm2, ri = 225 omega cm and cm = 1.16 microF/cm2 (the membrane and cytoplasm specific resistivities and membrane specific capacitance, respectively). The model with these parameter values is electrically compact, with electrotonic length X = 0.33 and dendritic dominance ratio p = 0.44. Analysis of the calculated voltage transient of the multicompartment model by the methods of equivalent-cylinder cable theory is shown to result in very different and unreliable conclusions. The significance for neuronal function of the estimated electrical parameter values is discussed. The possible effect of active conductances on these conclusions is assessed.

Electrophysiological properties of dentate granule cells in mouse hippocampal slices maintained in vitro

Intracellular recordings were obtained from granule cells of the dentate gyrus in mouse hippocampal slices maintained in vitro. All the spikes observed in standard Krebs solution had a short duration and were tetrodotoxin (TTX)-sensitive. When elicited by synaptic activation or by direct electrical stimulation of the cells, these fast sodium spikes were followed by a brief spike afterhyperpolarization. In contrast, antidromic spikes elicited by electrical stimulation of the hilum as well as spikes arising at the end of hyperpolarizing current pulses passed through the recording microelectrode were followed by depolarizing afterpotentials (DAPs). These DAPs were reversed into brief spike afterhyperpolarizations by depolarization of the cells. After substitution of calcium (Ca) by barium (Ba) or after introduction of tetraethylammonium (TEA) in the bath, the fast spike repolarization became slower and the brief spike afterhyperpolarizations were abolished, suggesting that they involved fast K conductances. Slow spikes and long-lasting depolarizations were also elicited in granule cells in the presence of Ba or TEA. Since these slow events were left unaffected by TTX and were selectively abolished by the Ca channel blockers cobalt or cadmium, they are likely to be mediated by voltage-dependent Ca conductances, unmasked by the reduction of the fast K conductances.

Norepinephrine elicits both excitatory and inhibitory responses from Purkinje cells in the in vitro rat cerebellar slice

Superfusion of Purkinje neurons in the in vitro rat cerebellar slice with norepinephrine caused increases and decreases of spontaneous Purkinje cell firing. Excitations were evoked by low concentrations of norepinephrine (0.5-10 microM) and by the beta receptor agonist isoproterenol (0.1-5 microM). These excitations were reduced by timolol (1-2 microM), a beta receptor antagonist. Perfusion with higher concentrations of norepinephrine (greater than 16 microM), caused a depression of Purkinje neuron spontaneous activity. This inhibitory response was blocked by the alpha receptor antagonist phentolamine. The alpha 1 selective agonist phenylephrine had no effect on spontaneous activity at concentrations up to 100 microM, but the alpha 2 selective agonist clonidine (1-50 microM) elicited decreases in firing rate. These responses appeared to be due to a direct action on Purkinje cells, because neither the excitation nor the depression of Purkinje neuron activity elicited by norepinephrine was substantially altered when tested in a medium which substantially blocked synaptic transmission within the slice. Under these in vitro conditions, norepinephrine appears to increase the firing rate of Purkinje neurons via an interaction with beta adrenergic receptors, while norepinephrine induced depressions may be linked to alpha adrenergic receptor interactions; both receptors appear to be located directly on the Purkinje neurons.

Selective absence of calcium spikes in Purkinje cells of staggerer mutant mice in cerebellar slices maintained in vitro

The bioelectrical properties of Purkinje cells (intrasomatic recordings) were studied in sagittal cerebellar slices of both adult staggerer and control mice. Mean input resistances of Purkinje cells were 25 +/- 4 M omega and 48 +/- 7 M omega in normal and staggerer mice respectively. In both groups, time-dependent inward rectifications were apparent in the hyperpolarizing voltage-responses. In normal mice, tetrodotoxin (TTX)-sensitive simple spikes and slower-rising multiphasic spikes, abolished when Ca was replaced by Cd in the bath, spontaneously occurred in Purkinje cells. These Na- and Ca-dependent spikes were also elicited by depolarizing current pulses. In the mutant, Ca spikes were never observed, even in strongly depolarized cells. On the contrary, TTX-sensitive simple spikes occurred spontaneously or were elicited by depolarizing current pulses. When Ca was replaced by Ba in the bath, the Ca spikes evoked in normal Purkinje cells by direct stimulation were first enhanced and then replaced by prolonged action potentials (1-6 s in duration) which were TTX-resistant and Cd-sensitive. These (Ba) action potentials were also triggered by climbing fibre activation of the cells. In staggerer mice, Ca spikes were never elicited by direct stimulation in Ba-containing medium, although in a few cells prolonged action potentials were occasionally elicited by depolarizing current pulses. However, this latter type of response was never evoked by climbing fibre activation of Purkinje cells. In the mutant, extracellular application of tetraethylammonium (TEA) generated prolonged action potentials, the plateaux of depolarization of which were less positive than those elicited by Ba in control mice. These plateaux were abolished by TTX and left unaffected by the substitution of Ca by Cd in the bath, suggesting that they were due to a non-inactivating Na conductance. On the whole, the present study strongly suggests that voltage-dependent Ca channels are missing in most staggerer Purkinje cells or at least that their characteristics and/or distribution are such that they cannot be activated. Na channels appear unaffected.

Potassium accumulation around individual purkinje cells in cerebellar slices from the guinea-pig

K+-selective micropipettes were used to measure external K+ concentration [( K+]o) in the immediate vicinity of Purkinje cells in slices from guinea-pig cerebellum. The cells were either spontaneously active or were polarized via a separate intracellular micro-electrode. The level of [K+]o rose by 1-3 mM around the soma and dendrites of Purkinje cells during spike activity. The increases in [K+]o were usually greater during Ca2+-mediated spikes than during Na+-mediated spikes. This was even true at the soma where the Ca2+ spike only invaded electrotonically from the dendrites, in contrast to the Na+ spikes which were generated at the soma. No [K+]o changes were seen in the vicinity of Purkinje cells when the cells were hyperpolarized with current passage nor was any [K+]o change seen during subthreshold depolarizations. In glial cells, however, a hyperpolarizing current reduced [K+]o while a depolarizing current increased [K+]o in a symmetrical manner. When Ba2+ was substituted for Ca2+ in the bathing Ringer solution, prolonged plateau-potential spikes could be evoked from Purkinje cells. These spikes were accompanied by large [K+]o elevations but the plateau potentials outlasted the [K+]o elevations. These experiments suggest that large [K+]o increases can occur in the absence of Ca2+-mediated K+ conductances. Substitution of Mn2+ for Ca2+ in the Ringer solution removed some of the [K+]o increases at the Purkinje cell soma. Addition of tetrodotoxin to normal Ringer solution also reduced, but did not abolish the [K+]o increases at the soma. These experiments confirmed that both Na+ and Ca2+ spikes were involved in the [K+]o change. The diffusion characteristics of the slices were determined by an ionophoretic method using tetramethylammonium and ion-selective micropipettes. The extracellular volume fraction of the slice averaged 0.28 while the tortuosity averaged 1.84. These values were close to those found previously in the intact rat cerebellum. These data were used to make quantitative estimates of the expected [K+]o accumulation in the vicinity of a single cell (see Appendix). Such estimates showed reasonable agreement with the measured values. Our data show that quite large increases in [K+]o may occur around single Purkinje cells. Such increases have previously only been evident during the activation of cell populations in mammalian preparations. The present results are probably due to the superior recording conditions of the slice. Implications for intercellular communication are discussed.

Differential sensitivity of cerebellar purkinje neurons to ethanol in selectively outbred lines of mice: maintenance in vitro independent of synaptic transmission

The effects of ethanol on spontaneous firing of cerebellar Purkinje neurons were examined in outbred lines of mice (short-sleep, SS; and long-sleep, LS) which exhibit differential behavioral sensitivity to ethanol. In order to determine whether the differences in Purkinje cell ethanol sensitivity which are observed in situ reflect differences in intrinsic properties of Purkinje neurons, we developed an isolated in vitro preparation of mouse cerebellum. Even when synaptic transmission was largely inhibited by elevating Mg2+ and decreasing Ca2+ concentrations, Purkinje cells demonstrated stable long-term firing rates quite similar to those observed in vivo. Purkinje cells responded to superfusion of ethanol with both increases and decreases in firing rate. Inhibition of rate was more commonly observed, and was the only response which was demonstrably dose-dependent. The differential sensitivity to ethanol which we have previously reported in vivo was maintained even under under these conditions, with the LS mice being approximately 5 times more sensitive to the depressant effects of ethanol. In addition, it was shown that ethanol, at the concentrations used in these experiments, decreased the amplitude and increased the duration of single action potentials. Thus, taken together, these results suggest that the differential sensitivity of outbred lines to the soporific effects of ethanol are paralleled by differences in the sensitivity of Purkinje neurons in vitro to superfusion with ethanol. Because these differences can be observed even when synaptic transmission is largely suppressed, it would appear that these differences are intrinsic to the purkinje neurons themselves.

Cultured cerebellar neurons: endogenous and exogenous components of Purkinje cell activity and membrane response to putative transmitters

Modified explant cultures of fetal rat cerebellum were developed for electrophysiological and pharmacological studies, at the membrane level, of Purkinje neurons. The goals of the present series of experiments were to identify possible endogenous and exogenous components to the electrical activity of Purkinje neurons, to assess the sensitivity of these neurons to putative excitatory and inhibitory neurotransmitters, and to characterize the membrane response to the transmitters. Intracellular recordings were made from Purkinje neurons, identified on a morphological basis, using conventional electrophysiological techniques. Virtually all Purkinje neurons displayed spontaneous activity. A contribution of both endogenous and exogenous components to the spontaneous activity was indicated by alterations in the pattern and amount of activity when the membrane potential was varied and by the characteristics of the individual potentials themselves. Several types of activity were considered to be endogenous: the most common type consisted of pacemaker-like potentials which generated a pattern of firing similar to that characterized as simple spike activity in previous in vivo studies; another type of endogenous activity consisted of large membrane depolarizations that evoked one or two spikes. These depolarizing responses were similar to the membrane response generated by climbing fiber input to Purkinje cells in vivo. The exogenous components to the spontaneous activity consisted of synaptic potentials including excitatory (EPSPs) and inhibitory (IPSPs) synaptic potentials and biphasic EPSP/IPSPs. Several putative transmitters thought to mediate these synaptic potentials were tested by focal micropressure application to determine if they could mimic the action of the endogenous transmitters. The putative transmitter glutamate depolarized the cultured Purkinje neurons and evoked action potentials, characteristics which were displayed by the excitatory synaptic potentials. The putative inhibitory transmitter GABA hyperpolarized the cultured Purkinje neurons and depressed activity, characteristics which were displayed by the inhibitory synaptic potentials. The putative inhibitory transmitters glycine and taurine were ineffective. Norepinephrine, the transmitter mediating the inhibitory input from the locus coeruleus to Purkinje neurons, was also tested. When applied in the microM range, NE effects were variable. When applied in the mM range, NE depressed the spontaneous activity in a manner suggestive of a presynaptic action.

The inhibitory effect of the olivocerebellar input on the cerebellar Purkinje cells in the rat

1. In rats under Nembutal anaesthesia the inferior olive region has been reversibly inactivated by applying a cooling probe to the ventral surface of the medulla. Simple and complex spike activity has been recorded from Purkinje cells of the cerebellar cortex.2. Following cooling of the inferior olive of one side we have observed a remarkable increase of the simple spike activity in all the twenty-two Purkinje cells, showing a disappearance of the complex spike activity.3. In some rats two Purkinje cells were recorded simultaneously from each side of the cerebellar cortex. Following cooling of the left inferior olive the effect on the Purkinje cell was observed only or predominantly on the contralateral cerebellar cortex.4. In a group of animals the inferior olive has been destroyed by 3-acetylpyridine 4-221 days before the recording session. Cooling of the inferior olive region was not accompanied by any significant and consistent increase in the spike activity of presumed Purkinje cells of the contralateral cerebellar cortex.5. These results indicate that the remarkable increase of the simple spike frequency following cooling of the inferior olive region is due specifically to the suppression of the activity of the olivocerebellar neurones.6. Only a small amount of the simple spike frequency increase is attributable to the removal of the post-climbing fibre pause.7. In some lesioned rats recording was made from Purkinje cells, which showed complex spikes due to the few surviving inferior olive cells. In these Purkinje cells cooling of the inferior olive region was accompanied by a disappearance of the complex spike and by a small increase of the simple spike frequency of discharge. Such an increase is mainly attributable to the removal of the post-climbing fibre pause.8. These results suggest that a given Purkinje cell is not only under the inhibitory influence of its own climbing fibre, but also of other olivocerebellar neurones, probably through climbing fibre collaterals to the cerebellar cortical interneurones.9. It is suggested that one role of the olivocerebellar system is to exert a powerful tonic inhibitory action on the Purkinje cells and consequently to exert a significant control on the excitability of the subcerebellar centres.

Kainic acid receptors and neurotoxicity in adult and immature rat cerebellar slices

The neurotoxic actions of kainate were examined in incubated slices of adult and immature rat cerebellum using light- and electron-microscopy. In the adult, Purkinje cells and inhibitory interneurones became selectively necrotic at concentrations between 5 micro M and 20 micro M. At 30 micro M, granule cells also became affected. In the immature cerebellum, at an age (8 days after birth) when the parallel fibres (thought to use glutamate as transmitter) are largely yet to be developed, selective toxicity was still evident but Purkinje cells and inhibitory interneurones were about 10-fold, and granule cells about 30-fold, less sensitive to kainate than in the adult. Kainate and other excitotoxins also increased cyclic GMP levels in cerebellar slices, apparently through the activation of excitatory amino acid receptors. In the adult tissue, the dose-cyclic GMP response curve to kainate was biphasic suggesting the presence of two components. The lower concentrations of kainate eliciting the first component mirrored those inducing selective necrosis of Purkinje cells and inhibitory interneurones while the second component correlated with necrosis of granule cells. Similar correlations applied to the immature cerebellum, but here kainate neurotoxicity appeared to be associated with the activation of receptor types different from those evident in the adult. It is suggested that kainate receptors, whose activation is associated with both neurotoxic damage and elevation of cyclic GMP levels, are located on all cell types in the adult cerebellum, with Purkinje cells and inhibitory interneurones displaying a higher sensitivity to kainate than granule cells. The lower sensitivity of immature cerebellum to the neurotoxic effect of kainate is probably due to lower levels of kainate receptors.

Effect of glutamate, aspartate and related derivatives on cerebellar purkinje cell dendrites in the rat: an in vitro study

1. The responses of Purkinje cells to short duration (pulse) ionophoretic applications of L-aspartate (L-asp), L-glutamate (L-glu), N-methyl DL-aspartate (NMDLA) and quisqualic acid in their dendritic fields were studied in vitro on sagittal slices of lobules IX and X of the adult rat cerebellum.2. Pulse application of L-asp or L-glu evoked transient and dose-dependent increases in the firing rate of the simple spikes recorded extracellularly as single units. When the ionophoretic electrode was positioned in the dendritic field of the Purkinje cells, the lowest thresholds for L-glu and L-asp mediated excitations of the cells were as low as 25 and 35 pC respectively, with a latency for maximal responses as brief as 7 ms.3. In intracellular recordings these excitatory responses consisted of depolarizations of up to 18 mV in amplitude and with depolarizing slopes up to 0.52 mV/ms. They were generally unaccompanied by changes in cell input resistance in contrast to the marked decrease which occurred in response to steady applications of large doses of L-asp and L-glu.4. The spatial distribution of the excitatory sites confirmed that the dendritic sensitivity to L-glu was greater than that of the soma and showed that the same was true for L-asp. In 34% of cells the sensitivity for L-asp declined markedly in the upper region of the molecular layer, whereas it remained high for L-glu; no such differential sensitivity was detected in the remaining 66% of cells.5. Inhibitory responses, antagonized by 10(-5) M-bicuculline in the bath, were also induced in Purkinje cells by L-glu and L-asp when the ionophoretic electrode was withdrawn from the excitatory sites by as little as 8 mum and up to 40 mum upward or downward along the track of parallel fibres or positioned as far as 250 mum laterally.6. Whenever it was applied in the molecular layer, the pulse application of NMDLA elicited no excitatory response in Purkinje cells recorded extra or intracellularly. However, slow depolarizations accompanied by a slight increase in cell input resistance were obtained with steady applications of 20-50 nA of the drug for 20-30 s.7. In contrast, pulse application of quisqualic acid appeared to have the same type of fast excitatory effect on Purkinje cells as L-asp and L-glu, but its potency was greater and its action more prolonged. Furthermore, its steady application led to an abrupt and marked decrease in cell membrane resistance.8. The excitatory effects of L-asp, L-glu and quisqualic acid were antagonized by L-glutamic acid diethyl ester more consistently than by D-alpha-aminoadipate, suggesting together with previous observations that L-asp and L-glu act on Purkinje cells via quisqualic acid rather than via NMDLA receptors.