Electrophysiological studies on the postnatal development of intracerebellar nuclei neurons in rat cerebellar slices maintained in vitro. II. Membrane conductances

Cerebellum Lecture: the Cerebellar Nuclei-Core of the Cerebellum

The cerebellum is a key player in many brain functions and a major topic of neuroscience research. However, the cerebellar nuclei (CN), the main output structures of the cerebellum, are often overlooked. This neglect is because research on the cerebellum typically focuses on the cortex and tends to treat the CN as relatively simple output nuclei conveying an inverted signal from the cerebellar cortex to the rest of the brain. In this review, by adopting a nucleocentric perspective we aim to rectify this impression. First, we describe CN anatomy and modularity and comprehensively integrate CN architecture with its highly organized but complex afferent and efferent connectivity. This is followed by a novel classification of the specific neuronal classes the CN comprise and speculate on the implications of CN structure and physiology for our understanding of adult cerebellar function. Based on this thorough review of the adult literature we provide a comprehensive overview of CN embryonic development and, by comparing cerebellar structures in various chordate clades, propose an interpretation of CN evolution. Despite their critical importance in cerebellar function, from a clinical perspective intriguingly few, if any, neurological disorders appear to primarily affect the CN. To highlight this curious anomaly, and encourage future nucleocentric interpretations, we build on our review to provide a brief overview of the various syndromes in which the CN are currently implicated. Finally, we summarize the specific perspectives that a nucleocentric view of the cerebellum brings, move major outstanding issues in CN biology to the limelight, and provide a roadmap to the key questions that need to be answered in order to create a comprehensive integrated model of CN structure, function, development, and evolution.

Activity of Cerebellar Nuclei Neurons Correlates with ZebrinII Identity of Their Purkinje Cell Afferents

Purkinje cells (PCs) in the cerebellar cortex can be divided into at least two main subpopulations: one subpopulation that prominently expresses ZebrinII (Z+), and shows a relatively low simple spike firing rate, and another that hardly expresses ZebrinII (Z-) and shows higher baseline firing rates. Likewise, the complex spike responses of PCs, which are evoked by climbing fiber inputs and thus reflect the activity of the inferior olive (IO), show the same dichotomy. However, it is not known whether the target neurons of PCs in the cerebellar nuclei (CN) maintain this bimodal distribution. Electrophysiological recordings in awake adult mice show that the rate of action potential firing of CN neurons that receive input from Z+ PCs was consistently lower than that of CN neurons innervated by Z- PCs. Similar in vivo recordings in juvenile and adolescent mice indicated that the firing frequency of CN neurons correlates to the ZebrinII identity of the PC afferents in adult, but not postnatal stages. Finally, the spontaneous action potential firing pattern of adult CN neurons recorded in vitro revealed no significant differences in intrinsic pacemaking activity between ZebrinII identities. Our findings indicate that all three main components of the olivocerebellar loop, i.e., PCs, IO neurons and CN neurons, operate at a higher rate in the Z- modules.

Quantitative Model for Ion Transport and Cytoplasm Conductivity of Chinese Hamster Ovary Cells

In mammalian cells cytoplasm ion concentrations and hence cytoplasm conductivity is an important indicator of their physiological state. Changes in the cytoplasm conductivity has been associated with physiological changes such as progression of cancer and apoptosis. In this work, a model that predicts the effects of physiological changes in ion transport on the cytoplasm conductivity of Chinese hamster ovary (CHO) cells is demonstrated. We determined CHO-specific model parameters, Na⁺/K⁺ ATPase pumps and ion channels densities, using a flux assay approach. The obtained sodium (P_Na), potassium (P_K) and chloride (P_Cl) permeability and Na⁺/K⁺ ATPase pump density were estimated to be 5.6 × 10^-8 cm/s, 5.6 × 10^-8 cm/s, 3.2 × 10^-7 cm/s and 2.56 × 10^-11 mol/cm², respectively. The model was tested by comparing the model predictions with the experimentally determined temporal changes in the cytoplasm conductivity of Na⁺/K⁺ ATPase pump inhibited CHO cells. Cells' Na⁺/K⁺ ATPase pumps were inhibited using 5 mM Ouabain and the temporal behavior of their cytoplasm conductivity was measured using dielectrophoresis cytometry. The measured results are in close agreement with the model-calculated values. This model will provide insight on the effects of processes such as apoptosis or external media ion concentration on the cytoplasm conductivity of mammalian cells.

Integrated plasticity at inhibitory and excitatory synapses in the cerebellar circuit

The way long-term potentiation (LTP) and depression (LTD) are integrated within the different synapses of brain neuronal circuits is poorly understood. In order to progress beyond the identification of specific molecular mechanisms, a system in which multiple forms of plasticity can be correlated with large-scale neural processing is required. In this paper we take as an example the cerebellar network, in which extensive investigations have revealed LTP and LTD at several excitatory and inhibitory synapses. Cerebellar LTP and LTD occur in all three main cerebellar subcircuits (granular layer, molecular layer, deep cerebellar nuclei) and correspondingly regulate the function of their three main neurons: granule cells (GrCs), Purkinje cells (PCs) and deep cerebellar nuclear (DCN) cells. All these neurons, in addition to be excited, are reached by feed-forward and feed-back inhibitory connections, in which LTP and LTD may either operate synergistically or homeostatically in order to control information flow through the circuit. Although the investigation of individual synaptic plasticities in vitro is essential to prove their existence and mechanisms, it is insufficient to generate a coherent view of their impact on network functioning in vivo. Recent computational models and cell-specific genetic mutations in mice are shedding light on how plasticity at multiple excitatory and inhibitory synapses might regulate neuronal activities in the cerebellar circuit and contribute to learning and memory and behavioral control.

Adult-like action potential properties and abundant GABAergic synaptic responses in amygdala neurons from newborn marmosets

The amygdala plays an important role in the processing of emotional events. This information processing is altered by development, but little is known about the development of electrophysiological properties of neurons in the amygdala. We studied the postnatal development of electrophysiological properties of neurons in the basolateral amygdala (BLA) of the common marmoset (Callithrix jacchus). Whole-cell patch-clamp recordings were obtained from BLA pyramidal neurons in brain slices prepared from developing and adult marmosets, and electrophysiological properties known to change during development in rats were analysed. Two passive electrical properties of the neuronal membrane - the input resistance (R(in)) and the membrane time constant () - significantly decreased with postnatal development. In contrast, the action potential only showed a slight decrease in duration during the first month of life, whereas the amplitude did not change after birth. Passive electrical properties and action potentials in neurons of 4-week-old marmosets were similar to those in neurons of 4-year-old marmosets. The development of the action potential duration was not correlated with the development of R(in) or , whereas the development of R(in) and was correlated with each other. Abundant spontaneous and noradrenaline-induced GABAergic currents were present immediately after birth and did not change during postnatal development. These results suggest that newborn infant marmoset BLA pyramidal neurons possess relatively mature action potentials and receive vigorous GABAergic synaptic inputs, and that they acquire adult-like electrophysiological properties by the fourth week of life.

Control of cerebellar nuclear cells: a direct role for complex spikes?

The question of what modulates the firing of the cerebellar nuclei (CN) is one to which we presently have a surprisingly incomplete answer. Because most synaptic input to the CN originates from Purkinje cells (PCs), and simple spikes (SSs) are far more numerous than complex spikes (CSs), SSs are generally thought to be the dominant influence on the CN. However, evidence, reviewed here, suggests that this appears not to be the case in some physiologically important situations. As an alternative, we propose that CS activity may have at least as significant an effect on CN firing as do SSs. In particular, we suggest that CS activity has a role in controlling the bursts CN neurons show during various movements, during sleep states, and under ketamine-xylazine anesthesia. The ability to perform this role rests on the fact that CSs can be highly synchronized among PCs that project to the same CN neuron. Specifically, we suggest that synchronized CSs help determine the temporal course of the CN bursts, most often their offset, and that SSs and activity from cerebellar afferents may modulate the specific firing pattern within each burst. This joint control of CN activity may help explain anomalies present in the standard model for synaptic control of CN activity in which determination of CN firing patterns is attributed primarily to SSs.

STD-dependent and independent encoding of input irregularity as spike rate in a computational model of a cerebellar nucleus neuron

Neurons in the cerebellar nuclei (CN) receive inhibitory inputs from Purkinje cells in the cerebellar cortex and provide the major output from the cerebellum, but their computational function is not well understood. It has recently been shown that the spike activity of Purkinje cells is more regular than previously assumed and that this regularity can affect motor behaviour. We use a conductance-based model of a CN neuron to study the effect of the regularity of Purkinje cell spiking on CN neuron activity. We find that increasing the irregularity of Purkinje cell activity accelerates the CN neuron spike rate and that the mechanism of this recoding of input irregularity as output spike rate depends on the number of Purkinje cells converging onto a CN neuron. For high convergence ratios, the irregularity induced spike rate acceleration depends on short-term depression (STD) at the Purkinje cell synapses. At low convergence ratios, or for synchronised Purkinje cell input, the firing rate increase is independent of STD. The transformation of input irregularity into output spike rate occurs in response to artificial input spike trains as well as to spike trains recorded from Purkinje cells in tottering mice, which show highly irregular spiking patterns. Our results suggest that STD may contribute to the accelerated CN spike rate in tottering mice and they raise the possibility that the deficits in motor control in these mutants partly result as a pathological consequence of this natural form of plasticity.

Distinct roles for I(T) and I(H) in controlling the frequency and timing of rebound spike responses

The ability for neurons to generate rebound bursts following inhibitory synaptic input relies on ion channels that respond in a unique fashion to hyperpolarization. Inward currents provided by T-type calcium channels (I(T)) and hyperpolarization-activated HCN channels (I(H)) increase in availability upon hyperpolarization, allowing for a rebound depolarization after a period of inhibition. Although rebound responses have long been recognized in deep cerebellar nuclear (DCN) neurons, the actual extent to which I(T) and I(H) contribute to rebound spike output following physiological levels of membrane hyperpolarization has not been clearly established. The current study used recordings and simulations of large diameter cells of the in vitro rat DCN slice preparation to define the roles for I(T) and I(H) in a rebound response. We find that physiological levels of hyperpolarization make only small proportions of the total I(T) and I(H) available, but that these are sufficient to make substantial contributions to a rebound response. At least 50% of the early phase of the rebound spike frequency increase is generated by an I(T)-mediated depolarization. An additional frequency increase is provided by I(H) in reducing the time constant and thus the extent of I(T) inactivation as the membrane returns from a hyperpolarized state to the resting level. An I(H)-mediated depolarization creates an inverse voltage-first spike latency relationship and produces a 35% increase in the precision of the first spike latency of a rebound. I(T) and I(H) can thus be activated by physiologically relevant stimuli and have distinct roles in the frequency, timing and precision of rebound responses.

Rebound discharge in deep cerebellar nuclear neurons in vitro

Neurons of the deep cerebellar nuclei (DCN) play a critical role in defining the output of cerebellum in the course of encoding Purkinje cell inhibitory inputs. The earliest work performed with in vitro preparations established that DCN cells have the capacity to translate membrane hyperpolarizations into a rebound increase in firing frequency. The primary means of distinguishing between DCN neurons has been according to cell size and transmitter phenotype, but in some cases, differences in the firing properties of DCN cells maintained in vitro have been reported. In particular, it was shown that large diameter cells in the rat DCN exhibit two phenotypes of rebound discharge in vitro that may eventually help define their functional roles in cerebellar output. A transient burst and weak burst phenotype can be distinguished based on the frequency and pattern of rebound discharge immediately following a hyperpolarizing stimulus. Work to date indicates that the difference in excitability arises from at least the degree of activation of T-type Ca(2+) current during the immediate phase of rebound firing and Ca(2+)-dependent K(+) channels that underlie afterhyperpolarizations. Both phenotypes can be detected following stimulation of Purkinje cell inhibitory inputs under conditions that preserve resting membrane potential and natural ionic gradients. In this paper, we review the evidence supporting the existence of different rebound phenotypes in DCN cells and the ion channel expression patterns that underlie their generation.

Determinants of synaptic integration and heterogeneity in rebound firing explored with data-driven models of deep cerebellar nucleus cells

Significant inroads have been made to understand cerebellar cortical processing but neural coding at the output stage of the cerebellum in the deep cerebellar nuclei (DCN) remains poorly understood. The DCN are unlikely to just present a relay nucleus because Purkinje cell inhibition has to be turned into an excitatory output signal, and DCN neurons exhibit complex intrinsic properties. In particular, DCN neurons exhibit a range of rebound spiking properties following hyperpolarizing current injection, raising the question how this could contribute to signal processing in behaving animals. Computer modeling presents an ideal tool to investigate how intrinsic voltage-gated conductances in DCN neurons could generate the heterogeneous firing behavior observed, and what input conditions could result in rebound responses. To enable such an investigation we built a compartmental DCN neuron model with a full dendritic morphology and appropriate active conductances. We generated a good match of our simulations with DCN current clamp data we recorded in acute slices, including the heterogeneity in the rebound responses. We then examined how inhibitory and excitatory synaptic input interacted with these intrinsic conductances to control DCN firing. We found that the output spiking of the model reflected the ongoing balance of excitatory and inhibitory input rates and that changing the level of inhibition performed an additive operation. Rebound firing following strong Purkinje cell input bursts was also possible, but only if the chloride reversal potential was more negative than −70 mV to allow de-inactivation of rebound currents. Fast rebound bursts due to T-type calcium current and slow rebounds due to persistent sodium current could be differentially regulated by synaptic input, and the pattern of these rebounds was further influenced by HCN current. Our findings suggest that active properties of DCN neurons could play a crucial role for signal processing in the cerebellum.

Analysis of distinct short and prolonged components in rebound spiking of deep cerebellar nucleus neurons

Deep cerebellar nucleus (DCN) neurons show pronounced post-hyperpolarization rebound burst behavior, which may contribute significantly to responses to strong inhibitory inputs from cerebellar cortical Purkinje cells. Thus, rebound behavior could importantly shape the output from the cerebellum. We used whole-cell recordings in brain slices to characterize DCN rebound properties and their dependence on hyperpolarization duration and depth. We found that DCN rebounds showed distinct fast and prolonged components, with different stimulus dependence and different underlying currents. The initial depolarization leading into rebound spiking was carried by hyperpolarization-activated cyclic nucleotide-gated current, and variable expression of this current could lead to a control of rebound latency. The ensuing fast rebound burst was due to T-type calcium current, as previously described. It was highly variable between cells in strength, and could be expressed fully after short periods of hyperpolarization. In contrast, a subsequent prolonged rebound component required longer and deeper periods of hyperpolarization before it was fully established. We found using voltage-clamp and dynamic-clamp analyses that a slowly inactivating persistent sodium current fits the conductance underlying this prolonged rebound component, resulting in spike rate increases over several seconds. Overall, our results demonstrate that multiphasic DCN rebound properties could be elicited differentially by different levels of Purkinje cell activation, and thus create a rich repertoire of potential rebound dynamics in the cerebellar control of motor timing.

Interneurons of the cerebellar cortex toggle Purkinje cells between up and down states

We demonstrate that single interneurons can toggle the output neurons of the cerebellar cortex (the Purkinje cells) between their two states. The firing of Purkinje cells has previously been shown to alternate between an "up" state in which the cell fires spontaneous action potentials and a silent "down" state. We show here that small hyperpolarizing currents in Purkinje cells can bidirectionally toggle Purkinje cells between down and up states and that blockade of the hyperpolarization-activated cation channels (H channels) with the specific antagonist ZD7288 (10 microM) blocks the transitions from down to up states. Likewise, hyperpolarizing inhibitory postsnyaptic potentials (IPSPs) produced by small bursts of action potentials (10 action potentials at 50 Hz) in molecular-layer interneurons induce these bidirectional transitions in Purkinje cells. Furthermore, single interneurons in paired interneuron --> Purkinje cell recordings, produce bidirectional switches between the two states of Purkinje cells. The ability of molecular-layer interneurons to toggle Purkinje cells occurs when Purkinje cells are recorded under whole-cell patch-clamp conditions as well as when action potentials are recorded in an extracellular loose cell-attached configuration. The mode switch demonstrated here indicates that a single presynaptic interneuron can have opposite effects on the output of a given Purkinje cell, which introduces a unique type of synaptic interaction that may play an important role in cerebellar signaling.

Local insulin-like growth factor I expression is essential for Purkinje neuron survival at birth

IGF1, an anabolic and neuroprotective factor, promotes neuronal survival by blocking apoptosis. It is released into the bloodstream by the liver, or synthesized locally by muscles and neural cells, acting in an autocrine or paracrine fashion. Intriguingly, genetic studies conducted in invertebrate and murine models also suggest that an excess of IGF1 signaling may trigger neurodegeneration. This emphasizes the importance of gaining a better understanding of the mechanisms controlling IGF1 regulation and gene transcription. In the cerebellum, Igf1 expression is activated just before birth in a subset of Purkinje cells (PCs). Mice carrying a null mutation for HLH transcription factor EBF2 feature PC apoptosis at birth. We show that Igf1 is sharply downregulated in Ebf2 null PCs starting before the onset of PC death. In vitro, EBF2 binds a conserved distal Igf1 promoter region. The pro-survival PI3K signaling pathway is strongly inhibited in mutant cerebella. Finally, Ebf2 null organotypic cultures respond to IGF1 treatment by inhibiting PC apoptosis. Consistently, wild type slices treated with an IGF1 competitor feature a sharp increase in PC death. Our findings reveal that IGF1 is required for PC survival in the neonatal cerebellum, and identify a new mechanism regulating its local production in the CNS.

Homeostatic regulation of intrinsic excitability and synaptic transmission in a developing visual circuit

One of the major challenges faced by the developing visual system is how to stably process visual information, yet at the same time remain flexible enough to accommodate growth and plasticity induced by visual experience. We find that in the Xenopus retinotectal circuit, during a period in development when the retinotectal map undergoes activity-dependent refinement and visual inputs strengthen, tectal neurons adapt their intrinsic excitability such that a stable relationship between the total level of synaptic input and tectal neuron spike output is conserved. This homeostatic balance between synaptic and intrinsic properties is maintained, in part, via regulation of voltage-gated Na+ currents, resulting in a stable neuronal input-output function. We experimentally manipulated intrinsic excitability or synapse strengthening in developing tectal neurons in vivo by electroporation of a leak K+ channel gene or a peptide that interferes with normal AMPA receptor trafficking. Both manipulations resulted in a compensatory increase in voltage-gated Na+ currents. This suggests that intrinsic neuronal properties are actively regulated as a function of the total level of neuronal activity experienced during development. We conclude that the coordinated changes between synaptic and intrinsic properties allow developing optic tectal neurons to remain within a stable dynamic range, even as the pattern and strength of visual inputs changes over development, suggesting that homeostatic regulation of intrinsic properties plays a central role in the functional development of neural circuits.

Neuroprotective effect of mifepristone involves neuron depolarization

In several regions of the developing nervous system, neurons undergo programmed cell death. In the rat cerebellum, Purkinje cell apoptosis is exacerbated when cerebellar slices are cultured during the first postnatal week. To understand the mechanism of this developmental apoptosis, we took advantage of its inhibition by the steroid analog mifepristone. This effect did not involve the classical steroid nuclear receptors. Microarray analysis revealed that mifepristone down-regulated mRNA levels of the Na+/K+-ATPase alpha3 subunit more than three times. Consistent with the down-regulation of the Na+/K+-ATPase, mifepristone caused Purkinje cell membrane depolarization. Depolarizing agents like ouabain (1 microM), tetraethylammonium (2 mM), and veratridine (2 microM) protected Purkinje cells from apoptosis. These results suggest a role of excitatory inputs in Purkinje cell survival during early postnatal development. Indeed, coculturing cerebellar slices with glutamatergic inferior olivary neuron preparations allowed rescue of Purkinje cells. These findings reveal a new neuroprotective mechanism of mifepristone and support a pivotal role for excitatory inputs in the survival of Purkinje neurons. Mifepristone may be a useful lead compound in the development of novel therapeutic approaches for maintaining the resting potential of neurons at values favorable for their survival under neuropathological conditions.

Morphological correlates of intrinsic electrical excitability in neurons of the deep cerebellar nuclei

To what degree does neuronal morphology determine or correlate with intrinsic electrical properties within a particular class of neuron? This question has been examined using microelectrode recordings and subsequent neurobiotin filling and reconstruction of neurons in the deep cerebellar nuclei (DCN) of brain slices from young rats (P13-16). The neurons reconstructed from these recordings were mostly large and multipolar (17/21 cells) and were likely to represent glutamatergic projection neurons. Within this class, there was considerable variation in intrinsic electrical properties and cellular morphology. Remarkably, in a correlation matrix of 18 electrophysiological and 6 morphological measures, only one morphological characteristic was predictive of intrinsic excitability: neurons with more spines had a significantly slower basal firing rate. To address the possibility that neurons with fewer spines represented a slowly maturing subgroup, recordings and reconstructions were also made from neurons at a younger age (P6-9). While P6-9 neurons were morphologically indistinguishable from P13 to 16 neurons, they were considerably less excitable: P6-9 neurons had a lower spontaneous spiking rate, larger fast AHPs, higher resting membrane potentials, and smaller rebound depolarizations. Thus while the large projection neurons of the DCN are morphologically mature by P6-9, they continue to mature electrophysiologically through P13-16 in a way that renders them more responsive to the burst-and-pause pattern that characterizes Purkinje cell inhibitory synaptic drive.

Efficacy and short-term plasticity at GABAergic synapses between Purkinje and cerebellar nuclei neurons

Although the entire output of the cerebellar cortex is conveyed to the deep cerebellar nuclei neurons (DCNs) via the GABAergic synapses established by Purkinje cells (PCs), very little is known about the strength and dynamic properties of PC-DCN connections. Here we show that activation of PC-DCN unitary connections induced large conductance changes (11.7 nS) in DCNs recorded in whole cell patch configuration in acute slices, suggesting that activity of single PCs might significantly affect the output of its target neurons. Based on the large unitary quantal content (18) inferred from calculations of PC-DCN quantal size (0.65 nS) and the near absence of failures in synaptic transmission during control conditions, we conclude that PC-DCN connections are highly multi-sited. The analysis of dynamic properties of PC-DCN synapses demonstrated remarkable paired pulse depression (PPD), maximal at short intervals (paired pulse ratio of 0.15 at 7-ms interval). We provide evidence that PPD is presynaptic in origin and release-independent. In addition, multiple pulse stimulation revealed that PC-DCN synapses exhibited larger sensitivity to dynamic than to steady signals. We postulate that the, otherwise paradoxical, combination of marked short-term depression with strong multi-sited connections is optimal to transfer dynamic information at unitary level by performing spatial average of release probability across the numerous release sites. This feature could enable these synapses to encode presynaptic time-varying signals of single PCs as moment-to-moment changes in synaptic strength, a capacity well suited to the postulated role of cerebellum in control of temporal aspects of motor or cognitive behaviors.

Morphological classification of the rat lateral cerebellar nuclear neurons by principal component analysis

The deep cerebellar nuclei (DCN) constitute the major structures by which the cerebellum forwards its output to the rest of the brain. Although the connectivity of the DCN has been well studied, little is known about the interface-the neurons' soma and dendrites-between the DCN's inputs and outputs. We therefore decided to analyze the neurons' somatic and dendritic morphology by applying a multivariate approach (principal component analysis; PCA), in order to define morphological groups possibly related to distinct positions in the nuclear microcircuitry. The PCA was based on intracellularly stained neurons from the rat's lateral DCN and on 19 parameters that described the neurons' morphology. The PCA yielded two principal components that accounted for 46% of the variance. The first component, correlated with soma size, separated the majority of neurons (type I) from a population of small neurons (type II). The second component showed negative correlation with larger cells with more numerous primary dendrites and a more multipolar appearance (type Ia) and positive correlation with smaller neurons with asymmetric dendritic fields and tufted dendrites (type Ib). The preponderance of small somata in our type Ib neurons suggests that these neurons probably correspond to the inferior olive projection neurons. In summary, our results are in agreement with previous classifications, which distinguished projection neurons (type I) from local neurons (type II); furthermore, our results point to a hitherto undescribed dendritic morphological difference in the projection neurons. The latter may be important for understanding the phylogenetic changes seen in the mammalian lateral cerebellar nucleus.

Developmental changes in the electrophysiological properties and response characteristics of Manduca antennal-lobe neurons

Using whole cell patch-clamp recordings, we have examined changes in the electrophysiological properties and response characteristics of antennal lobe (AL) neurons associated with the metamorphic adult development of the sphinx moth, Manduca sexta. Whole cell current profiles and electrical excitability were examined in dispersed AL neurons in vitro, and in medial-group AL neurons in situ in semi-intact brain preparations. Around stages 2-4 of the 18 stages of metamorphic adult development, whole cell current profiles were dominated by large outward (K+) currents. Calcium-dependent action potentials could be elicited at this stage, but only a small percentage of cells exhibited sodium spikes. From stages 3 to 10, there was a rapid increase in the proportion of AL neurons exhibiting rapidly activating, transient sodium currents, and many cells in vitro exhibited spontaneous bursts of spike activity at this time. As development progressed, action-potential waveforms became shorter in duration and larger in amplitude. Cell-type-specific differences in the prevalence of spontaneous activity, and in the electrophysiological properties and response characteristics of AL neurons, were most apparent late in metamorphosis. While removal of antennal sensory input to the ALs early (stage 1-2) in metamorphosis had no detectable effect on the development of cell excitability, a significantly higher percentage of neurons in vitro from stage 4 pupae exhibited sodium-based action potentials following the addition of serotonin to the culture medium. Characteristic forms of electrical excitability in developing Manduca AL neurons, and their modulation by serotonin, seem likely to play a central role in the functional development of the ALs.

Two types of neurons in the rat cerebellar nuclei as distinguished by membrane potentials and intracellular fillings

Classically, three classes of neurons in the cerebellar nuclei (CN), defined by different projection targets and content of transmitters, have been distinguished. However, evidence for different types of neurons based on different intrinsic properties is lacking. The present study reports two types of neurons defined mainly by their intrinsic properties, as determined by whole-cell patch recordings. The majority of cells (type I, n = 63) showed cyclic burst firing whereas a small subset (type II, n = 7) did not. Burst firing was used to distinguish the two types of neurons because, as it turned out, pharmacological interference could not be used to convert the non-bursting cells to bursting ones. Some of the membrane potentials exclusively present in type I neurons, such as sodium and calcium plateau potentials, low-threshold calcium spikes, and a slow calcium-dependent afterhyperpolarization, were found to contribute to the generation of burst firing. Other membrane potentials of type I neurons were not obviously related to the generation of bursts. These were 1) the lower amplitude and width of the action potential during spontaneous activity, 2) a sequence of afterhyperpolarization-afterdepolarization-afterhyperpolarization following each spike, and 3) the high spontaneous firing rate. In contrast, type II neurons lacked slow plateau potentials and low threshold spikes. Their action potentials showed higher amplitude and width and were followed by a single deep afterhyperpolarization. Furthermore, they showed a lower firing rate at rest. In both types of neurons, a delayed inward rectification was present. Neurons filled with neurobiotin revealed that the sizes of the somata and dendritic fields of type I neurons comprised the whole range known from Golgi studies, whereas those of the few type II neurons recovered were found to be in the lowest range. In view of their size and scarcity, we propose that type II neurons may correspond to CN interneurons.

Ontogenetic changes in the neural mechanisms of eyeblink conditioning

The rodent eyeblink conditioning paradigm is an ideal model system for examining the relationship between neural maturation and the ontogeny of associative learning. Elucidation of the neural mechanisms underlying the ontogeny of learning is tractable using eyeblink conditioning because the necessary neural circuitry (cerebellum and interconnected brainstem nuclei) underlying the acquisition and retention of the conditioned response (CR) has been identified in adult organisms. Moreover, the cerebellum exhibits substantial postnatal anatomical and physiological maturation in rats. The eyeblink CR emerges developmentally between postnatal day (PND) 17 and 24 in rats. A series of experiments found that the ontogenetic emergence of eyeblink conditioning is related to the development of associative learning and not related to changes in performance. More recent studies have examined the relationship between the development of eyeblink conditioning and the physiological maturation of the cerebellum, a brain structure that is necessary for eyeblink conditioning in adult organisms. Disrupting cerebellar development with lesions or antimitotic treatments impairs the ontogeny of eyeblink conditioning. Studies of the development of physiological processes within the cerebellum have revealed striking ontogenetic changes in stimulus-elicited and learning-related neuronal activity. Neurons in the interpositus nucleus and Purkinje cells in the cortex exhibit developmental increases in neuronal discharges following the unconditioned stimulus (US) and in neuronal discharges that model the amplitude and time-course of the eyeblink CR. The developmental changes in CR-related neuronal activity in the cerebellum suggest that the ontogeny of eyeblink conditioning depends on the development of mechanisms that establish cerebellar plasticity. Learning and the induction of neural plasticity depend on the magnitude of the US input to the cerebellum. The role of developmental changes in the efficacy of the US pathway has been investigated by monitoring neuronal activity in the inferior olive and with stimulation techniques. The results of these experiments indicate that the development of the conditioned eyeblink response may depend on dynamic interactions between multiple developmental processes within the eyeblink neural circuitry.

Implication of Bcl-2 and Caspase-3 in age-related Purkinje cell death in murine organotypic culture: an in vitro model to study apoptosis

Neuronal cell death is an essential feature of nervous system development and neurodegenerative diseases. Most Purkinje cells in murine cerebellar organotypic culture die when taken from 1-5-day-old mice (P1-P5), whereas they survive when taken before or after these ages. Using DNA gel electrophoresis, terminal deoxynucleotidyl transferase-mediated dUTP nick-end labelling (TUNEL) and electron microscopic analyses, we were able to show that this massive Purkinje cell death is apoptotic in nature and reaches a peak at P3. From the several endogenous genes known to be involved in the apoptotic process, we have focused on two: the bcl-2 and the caspase-3 that encode for anti-apoptotic and pro-apoptotic proteins, respectively. Immunostaining for activated Caspase-3 correlated with Purkinje cell death. A better survival of Purkinje cells was observed in P3 slices taken from hu-bcl-2 transgenic mice, and in slices treated with z-DEVD.fmk (an inhibitor of numerous caspases). Thus, these two genes are implicated in the age-related Purkinje cell apoptosis in organotypic culture. As Purkinje cell death in vitro takes place at the same age as Purkinje cells engaged in intense synaptogenesis and dendritic remodeling in vivo, we propose that this apoptosis reflects a naturally occurring Purkinje cell death during this critical period.

The control of rate and timing of spikes in the deep cerebellar nuclei by inhibition

Cerebellar nucleus neurons were recorded in vitro, and dynamic clamping was used to simulate inhibitory synaptic input from Purkinje cells likely to occur in vivo. Inhibitory input patterns with varying synaptic amplitudes and synchronicity were applied to determine how spike rate and spike timing can be controlled by inhibition. The excitatory input conductance was held constant to isolate the effect of dynamic inhibitory inputs on spiking. We found that the timing of individual spikes was controlled precisely by short decreases in the inhibitory conductance that were the consequence of synchronization between many inputs. The spike rate of nucleus neurons was controlled in a linear way by the rate of inhibitory inputs. The spike rate, however, also depended strongly on the amount of synchronicity present in the inhibitory inputs. An irregular spike train similar to in vivo data resulted from applied synaptic conductances when the conductance was large enough to overcome intrinsic pacemaker currents. In this situation subthreshold fluctuations in membrane potential closely followed the time course of the combined reversal potential of excitation and inhibition. This indicates that the net synaptic driving force for realistic input levels in vivo may be small and that synaptic input may operate primarily by shunting. The accurate temporal control of output spiking by inhibitory input that can be achieved in this way in the deep cerebellar nuclei may be particularly important to allow fine temporal control of movement via inhibitory output from cerebellar cortex.

Developmental changes in eye-blink conditioning and neuronal activity in the cerebellar interpositus nucleus

Neuronal activity was recorded in the cerebellar interpositus nucleus in infant rats during classical conditioning of the eye-blink response. The percentage and amplitude of eye-blink conditioned responses increased as a function of postnatal age. Learning-specific neuronal activity in the cerebellum emerged ontogenetically in parallel with the eye-blink conditioned response. There were also age-specific changes in neuronal activity after the onset of the conditioned and unconditioned stimuli. The results indicate that the development of the eye-blink conditioned response may depend on the development of stimulus-evoked neuronal responses and learning-specific plasticity in the cerebellum. Functional immaturity in the afferent neural pathways may limit the induction of neural plasticity in the cerebellum and thereby limit the development of the eye-blink conditioned response.

Regulation of the rebound depolarization and spontaneous firing patterns of deep nuclear neurons in slices of rat cerebellum

Current-clamp recordings were made from the deep cerebellar nuclei (DCN) of 12- to 15-day-old rats to understand the factors that mediate intrinsic spontaneous firing patterns. All of the cells recorded were spontaneously active with spiking patterns ranging continuously from regular spiking to spontaneous bursting with the former predominating. A robust rebound depolarization (RD) leading to a Na(+) spike burst was elicited after the offset of hyperpolarizing current injection. The voltage and time dependence of the RD was consistent with mediation by low-threshold voltage-gated Ca(2+) channels. In addition, induction of a RD also may be affected by activation of a hyperpolarization-activated cation current, I(h). A RD could be evoked efficiently after brief high-frequency bursts of inhibitory postsynaptic potentials (IPSPs) induced by stimulation of Purkinje cell axons. IPSP-driven RDs were typically much larger and longer than those elicited by direct hyperpolarizing pulses of approximately matched amplitude and duration. Intracellular perfusion of the Ca(2+) buffer bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid (BAPTA) dramatically enhanced the RD and its associated spiking, sometimes leading to a plateau potential that lasted several hundred milliseconds. The effects of BAPTA could be mimicked partly by application of apamin, a blocker of small conductance Ca(2+)-gated K(+) channels, but not by paxilline, which blocks large conductance Ca(2+)-gated K(+) channels. Application of both BAPTA and apamin, but not paxilline, caused cells that were regularly spiking to burst spontaneously. Taken together, our data suggest that there is a strong relationship between the ability of DCN cells to elicit a RD and their tendency burst spontaneously. The RD can be triggered by the opening of T-type Ca(2+) channels with an additional contribution of hyperpolarization-activated current I(h). RD duration is regulated by small-conductance Ca(2+)-gated K(+) channels. The RD also is modulated tonically by inhibitory inputs. All of these factors are in turn subject to alteration by extrinsic modulatory neurotransmitters and are, at least in part, responsible for determining the firing modes of DCN neurons.

GABA-ergic transmission in deep cerebellar nuclei

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

Properties of developing lateral geniculate neurones in the mouse

This study describes the properties of neurones recorded in vitro from the dorsal lateral geniculate nucleus (dLGN) of the mouse between developmental stages E16 and P36 and represents the first systematic study of the development of rodent thalamic neurones. The results demonstrate that thalamo-cortical neurones in the mouse dLGN undergo a series of important changes as they mature. Prenatally recorded cells had low resting potentials and could not generate action potentials but as they mature, mouse dLGN neurones become more polarised and show an increase in membrane time constant and spike threshold, while action potentials increase in amplitude and decrease in width. The low-threshold spike (LTS) complex appears at the time of birth, but does not show properties typical of adult cells until at least the third postnatal week. Immature action potentials are primarily sodium-dependent but gain a significant calcium component in the second postnatal week, which is associated with a supra-threshold oscillation of the membrane potential. The electrical activity during this critical period is strongly influenced by the interaction of powerful inward and outward rectification with calcium conductances which determines the appearance of voltage responses to intracellular current injection. The membrane potential in recordings from neurones during the first postnatal week was dominated by intense TTX-sensitive depolarising synaptic-like events which attained amplitudes of 60 mV in several neurones at stages P5-P8. These changes are discussed in relationship to the formation of appropriate connections in the developing visual system.

Na+ channel beta 1 subunit mRNA expression in developing rat central nervous system

The sodium channel beta 1 subunit (Na beta 1) is a component of the rat brain voltage-dependent sodium channel. We have used nonradioactive in situ hybridization cytochemical techniques to demonstrate that transcript levels of Na beta 1 are differentially upregulated during postnatal development of several CNS regions, with selective labeling of specific neuronal populations. In the hippocampus, labeling of the pyramidal cell layer (particularly in the CA3 region) and dentate granule cells was initially observed at postnatal day 2 (P2) and P10, respectively, and became progressively more intense with maturation. Labeled cells were first observed in the hilus at P10. In the developing cerebellum, transient labeling was observed in the external granule cell layer beginning at P1 while label increased in the internal granule cell layer up to P21. Purkinje cells showed significant label beginning at P4 and increasing up to P21. Weak signal was seen in neurons of deep nuclei at P1 and increased up to P21. Na beta 1 labeling in the spinal cord was first observed in the ventral horn at P2, and the intensity of labeling in these large motoneurons gradually increased. In addition, there was a ventral-dorsal gradient in this region, with label appearing subsequently in neurons of Rexed laminae IX, VII and VIII, and in the dorsal horn (Rexed laminae I-VI). In these regions, the labeling reached a plateau within the first 2-3 weeks after birth and persisted into the adult rat. The time course and regional heterogeneity of Na beta 1 expression are consistent with the hypothesis that the expression of mature Na+ channels, including Na beta 1, contributes to the development of circuitry that supports complex patterns of electrogenesis.

Pyramidal neurons in layer 5 of the rat visual cortex. II. Development of electrophysiological properties

Two major classes of pyramidal neurons can be distinguished in layer 5 of the adult rat visual cortex. Cells of the "thick/tufted" type have stout apical dendrites with terminal tufts, and most of them project to the superior colliculus (Larkman and Mason: J Neurosci 10:407, '90; Kasper et al.: J Comp Neurol, this issue, 339:459-474). "Slender/untufted" cells have thinner apical trunks with no obvious terminal tufts, and a substantial proportion of them project to the contralateral visual cortex. These two types also differ in their intrinsic electrophysiological features. In this study we describe the postnatal maturation of the electrophysiological and synaptic properties of layer 5 pyramidal neurons and relate these findings to the morphological development and divergence of the two cell types. Living slices were prepared from the visual cortex of rats aged between postnatal day 3 (P3) and young adults and maintained in vitro. Stable intracellular impalements were obtained from a total of 63 pyramidal cells of layer 5 at various ages, which were injected with biocytin so that morphological and electrophysiological data could be obtained from the same cell. Before P15, injection of a single cell sometimes stained a cluster of neurons of similar morphology, probably as a result of dye coupling. The incidence of such clustering and the number of neurons within each cluster decreased with age. There was no obvious difference in electrophysiological properties between cells in clusters and age-matched, noncoupled neurons. From P5, the apical dendrites of neurons could easily be classified as "thick/tufted" or "slender/untufted." On average, the resting potential became more negative, and membrane time constant and input resistance decreased with age. Electrophysiological differences between the "thick/tufted" and "slender/untufted" cell types did not become apparent until the third postnatal week, after which the "thick/tufted" cells on average had lower input resistances and slightly faster time constants than "slender/untufted" cells. The current-voltage relations of the neurons became progressively more nonlinear during maturation, with both rapid inward rectification and time-dependent rectification or "sag" becoming more prominent. There were also changes in the amplitude and waveform of action potentials, which generally approached adult values by 3 weeks of age. Action potential threshold became more negative, both in absolute terms and relative to the resting membrane potential. Action potentials became larger in peak amplitude and of shorter duration, with both rise and fall times decreasing progressively during development.(ABSTRACT TRUNCATED AT 400 WORDS)

Afterpotential characteristics and firing patterns in maturing rat hippocampal CA1 neurones in in vitro slices

The postnatal evolution of depolarizing after-potentials (DAPs) and after-hyperpolarizations (AHPs) was studied in rat CA1 hippocampal neurones (5-68 days of age) using in vitro slices. Results were pooled into 4 age groups: P5-9, P10-16, P17-24 and P26-68. In P5-9 cells, DAPs were seen as passive signals, with a time constant similar to the time constant of the membrane. The evolution of the DAP was characterized by a decrease in amplitude, an increase in duration and a change in contour. In P10-16 and P17-24 cells, the DAPs often had a plateau or a hump-like shape which increased the probability of firing and the occurrence of spike doublets. The firing pattern and bursting behaviour of P10-16 CA1 neurones differed from the pattern typical of the adult. P5-9 and P10-16 cells had post-burst AHPs with a smaller amplitude and a more prolonged early phase than at late stages of development.

Neurophysiological maturation of cat caudate neurons: evidence from in vitro studies

The membrane properties and synaptic physiology of developing cat caudate (Cd) nucleus neurons were studied in in vitro slice preparations. Recordings were obtained from 98 cells in kittens from fetal day (F) 56 to postnatal day (P) 90. With increasing age, the following maturational changes occurred; resting membrane potentials became more negative, action potential rise times decreased, action potential amplitudes increased, and action potential durations and input resistances decreased. The frequency of occurrence of afterhyperpolarizations and of anomalous rectification increased with age. The primary response to local extracellular stimulation was a depolarization usually accompanied by an action potential. Evoked hyperpolarizing responses were seen after P28 but only occurred if the membrane was depolarized by intracellular current injection. Cells identified by intracellular injection of Lucifer yellow were primarily medium-sized spiny neurons although it was not always possible to determine the cell type in slices from animals less than P5. Somatic diameter, dendritic length, and spine density increased with age. Dye-coupling occurred in slices less than P20. Its frequency decreased with age. These results show that Cd neurons undergo significant maturation during late prenatal and early postnatal periods. In contrast, substantia nigra neurons mature more rapidly and should be capable of influencing the less mature Cd neurons during development.(ABSTRACT TRUNCATED AT 250 WORDS)

Neurophysiological maturation of cat substantia nigra neurons: evidence from in vitro studies

The membrane properties and synaptic physiology of developing cat substantia nigra (SN) neurons were studied in in vitro slice preparations. Stable intracellular recordings were obtained from 46 neurons in 20 kittens ranging in age from fetal day (F) 51 to postnatal day (P) 120. Only two of these properties changed with development. The percentage of cells displaying inward rectification and the percentage of cells that generated low-threshold Ca++ spikes increased with age. Properties that did not change included resting membrane potentials, action potential amplitudes and durations, and input resistances. At all ages locally evoked synaptic responses consisted of sequences of excitatory postsynaptic potentials followed by inhibitory postsynaptic potentials. Most of the cells recorded had the electrophysiological properties which have been attributed to SN dopamine-containing neurons. To identify neurons morphologically, and verify the recording site, cells were filled with Lucifer yellow at the end of each experiment. Somatic shapes varied widely from oval to fusiform to triangular. Somatic diameters and dendritic length increased with development. Filopodial processes and growth cones were present up to the first postnatal month. Dye-coupling occurred only in the fetal group. These results indicate that cat SN neurons have many mature physiological properties during late fetal and early postnatal development. This contrasts with the significant maturation that occurs in cat caudate neurons during the same developmental period.

Postnatal development of cat hind limb motoneurons. III: Changes in size of motoneurons supplying the triceps surae muscle

The postnatal changes of neuronal dimensions were studied in cat triceps surae motoneurons intracellularly labeled with horseradish peroxidase. Systematic correlations were observed in the analysis of single dendrites at each studied stage, from birth to 44-46 days post natum (d.p.n.) age, between size parameters intrinsic to the dendrites as the diameter of a 1st-order dendrite, the combined dendritic length, the dendritic membrane area, and the degree of branching. Some variability among samples was evident in each studied age group. The correlations were, however, sufficiently close to permit indirect estimations of both combined dendritic length and dendritic membrane area for larger samples of neurons from data on dendritic stem caliber. The total postnatal increase in dendritic membrane area was, on the average, 400%, i.e., from close to 100 X 10(3) microns2 to about 500 X 10(3) microns2. The corresponding increase in soma area amounted to 100%. Analysis revealed that there was a time lag between the increase in somatic and dendritic size. Thus, adult somatic dimensions were attained at age 44-46 d.p.n.; however, at this stage, the mean total dendritic membrane area was only about half of the adult value. The postnatal increase in size appeared to vary among neurons, yielding a wider neuronal size spectrum in the adult cat than that observed in kittens. The measured increase in size corresponded to a calculated average addition of dendritic membrane area of 3700 microns2/day from birth to 22-24 d.p.n. and from that stage to 44-46 d.p.n. of 2700 microns2 per day. Likewise, the increase in combined dendritic length could initially be as large as 1 mm/day down to 0.4 mm/day between 22-24 and 44-46 d.p.n., with a mean growth during the first 44-46 d.p.n. of 0.5 to 0.6 mm/day. The ratios of daughters to parent branch diameters (sigmadd1.5: dp1.5) and the dendritic trunk parameter (sigma d1.5) recorded along the proximodistal dendritic path distance revealed transient changes that might impact on the electrotonic properties of the dendrites during postnatal development. Computations from the measured changes in dendritic branch lengths and calibers indicated that if membrane and internal resistivity remain unaltered during postnatal development, the dendritic domain is electrotonically more compact in the newborn kitten than in the adult cat.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Post-natal development of electrophysiological properties of rat cerebral cortical pyramidal neurones

1. The post-natal development of the electrophysiological properties of cortical layer V pyramidal neurons was investigated with intracellular recordings from rat sensorimotor cortical slices, in vitro. 2. At all ages post-natally (post-natal day 1 to day 36; P1-P36) neurons were capable of generating a train of Na+-dependent action potentials in response to intracellular injection of sufficient depolarizing current. During the second and third week post-natally, these action potentials changed substantially, becoming faster in both their rising and falling phases, shorter in duration, and larger in amplitude. 3. Both mature (greater than P21) and immature (P2-P4) cortical neurones could generate Ca2+-dependent action potentials only if a substantial portion of K+ conductances were blocked. The maximum rate of rise of Ca2+ spikes also increased with age. 4. The apparent input resistance, specific membrane resistance, and membrane time constant all decreased with age from P1 to P30. Immature neurones had I-V relationships that were substantially more linear than those of adult cells, although rectification was often present in both the hyperpolarizing and depolarizing range. Inward rectification in the depolarizing range was Na+ dependent and was substantially larger in mature versus immature neurones. 5. Single, or trains of, action potentials in immature neurones were followed by short duration (10-50 ms) and long duration (1-5 s) after-hyperpolarizations (a.h.p.s) respectively. The duration of the latter appeared to decrease with age. The presence of large a.h.p.s indicates that Ca2+ entry occurs during the action potential of immature, as well as mature, neurones. 6. Responses to intracellular injection of depolarizing current pulses indicated that immature neurones have frequency versus injected current (f-I) relationships which are in general less steep than those for adult neurones and more limited in terms of the range of firing frequencies. 7. Our results are consistent with the hypothesis that there is a considerable increase in the density of voltage-dependent ionic channels underlying the electro-responsiveness of cortical pyramidal neurones during post-natal development.

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.

Amino acid neurotoxicity: relationship to neuronal depolarization in rat cerebellar slices

It has long been proposed that the excitatory and toxic properties of acidic amino acid receptor agonists are linked. To test this hypothesis, the depolarizing effects of quisqualate, kainate and N-methyl-D-aspartate in adult and immature rat cerebellar slices have been studied in relation to their neurotoxic effects in the same tissues (reported separately). A "grease-gap" method was used to measure the depolarizing responses of Purkinje cells and granule cells in lobule VI to the agonists. The depolarizing potencies of kainate and quisqualate were apparently similar on both cell types and at both ages studied although maximal responses to kainate were always larger. N-Methyl-D-aspartate was a very weak agonist in the adult slices but was much more effective in the immature tissues, apparently on both Purkinje cells and granule cells. Comparison of the depolarizing effects of the agonists with their neurotoxic effects on Purkinje cells and granule cells suggested that: (a) the ability to depolarize is a required condition for an agonist to be neurotoxic, (b) the magnitude of depolarization, rather than depolarizing potency, is the more pertinent determinant of neurotoxic potency and (c) resistance to the neurotoxicity of an agonist is not necessarily associated with resistance to its depolarizing actions. Histological studies indicated that the neurotoxicity of N-methyl-D-aspartate and kainate in immature cerebellar slices could largely not be replicated by veratridine (50 microM) or high extracellular K+ (124 mM) indicating that receptor-mediated ionic fluxes may be needed in addition to those caused by depolarization. Exposure of the slices to anoxia in the absence of glucose partially reproduced the toxicity of the receptor agonists. Application of ouabain for 30 min caused necrosis of all the cells which are vulnerable to the agonists but spared the cells which are not vulnerable. Profound ionic imbalance thus appears to be a sufficient explanation for amino acid neurotoxicity.

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.