Faculty Bibliography

This article reviews the electroresponsive properties of single neurons in the mammalian central nervous system (CNS). In some of these cells the ionic conductances responsible for their excitability also endow them with autorhythmic electrical oscillatory properties. Chemical or electrical synaptic contacts between these neurons often result in network oscillations. In such networks, autorhythmic neurons may act as true oscillators (as pacemakers) or as resonators (responding preferentially to certain firing frequencies). Oscillations and resonance in the CNS are proposed to have diverse functional roles, such as (i) determining global functional states (for example, sleep-wakefulness or attention), (ii) timing in motor coordination, and (iii) specifying connectivity during development. Also, oscillation, especially in the thalamo-cortical circuits, may be related to certain neurological and psychiatric disorders. This review proposes that the autorhythmic electrical properties of central neurons and their connectivity form the basis for an intrinsic functional coordinate system that provides internal context to sensory input

Microfluorometric imaging was used to study the correlation of intracellular calcium concentration with voltage-dependent electrical activity in guinea pig cerebellar Purkinje cells. The spatiotemporal dynamics of intracellular calcium concentration are demonstrated during spontaneous and evoked activity. The results are in agreement with hypotheses of dendritic segregation of calcium conductances suggested by electrophysiological experiments. These in vitro slice fluorescence imaging methods are applicable to a wide range of problems in central nervous system biochemical and electrophysiological functions

The performance of a cryogenic system that monitors the extracranial magnetic field simultaneously at 14 positions over the scalp has been evaluated to determine the accuracy with which neuronal activity can be located within the human brain. Initially, measurements were implemented on two model systems, a lucite sphere filled with saline and a model skull. With a magnetic field strength similar to that of a human brain, the measurement and analysis procedures demonstrated a position accuracy better than 3 mm, for a current dipole 3 cm beneath the surface. Subsequently, measurements of the magnetic field pattern appearing 100 ms after the onset of an auditory tone-burst stimulus were obtained in three human subjects. The location of the current dipole representing intracellular ionic current in active neurons of the brain was determined, with 3-mm accuracy, to be within the cortex forming the floor of the Sylvian fissure of the individual subjects, corresponding closely to the Heschl gyrus as determined from magnetic resonance images. With the sensors placed at appropriate positions, the locations of neuronal sources for different tone frequencies could be obtained without moving the recording instrument. Adaptation of activity in human auditory cortex was shown to reveal long-term features with a paradigm that compared response amplitudes for three tones randomly presented

1. We describe here a technique which allows the long-term in vitro survival of the perfused isolated brain stem-cerebellum of adult guinea-pig. The viability of this preparation was assessed by comparing the electrophysiological properties of individual neurones and of neuronal pools to those obtained in vivo or in brain slices. The areas investigated included the cerebellar cortex, the inferior olive and the pontine nuclei. 2. Cerebellar field potential and intra- and extracellular single-cell recordings could be obtained for as long as 15 h after the preparation was initially isolated. The waveforms of field potentials recorded at various depths in the cerebellar cortex following surface folial stimulation were similar to those recorded in vivo. Extracellular recordings from single Purkinje cells following white matter stimulation demonstrated antidromic as well as mossy- and climbing fibre-mediated excitation. Stimulation of the cerebellar surface elicited orthodromic parallel fibre excitation of Purkinje cells and basket-stellate and Golgi cell inhibition. 3. Intrasomatic and intradendritic recordings from Purkinje cells reproduced all the phenomenology described earlier under in vivo conditions and in vitro slice preparations. In addition, spontaneous excitatory synaptic potentials generating simple spikes (mossy fibre-parallel fibre-mediated activity) and complex spikes (climbing fibre-mediated activity) were consistently observed. 4. Extracellular field potentials and extra- and intracellular recordings from inferior olive neurones were similar to those previously shown for the mammalian inferior olive. 5. Intracellular recordings were also obtained from pontine nuclei neurones, a major source of mossy fibre afferents to the cerebellum. Stimulation of the contralateral superior cerebellar peduncle produced antidromic invasion of these neurones whereas stimulation of the ipsilateral inferior cerebral peduncle resulted in their orthodromic activation. 6. The preparation responded to pharmacological challenge in a manner which demonstrated a sequential activation of sets of synaptic links in a given pathway. Thus, harmaline generated oscillations of inferior olivary neurones which were similar to those observed in vivo and which produced climbing fibre EPSPs in Purkinje cells at the same frequency as the inferior olivary oscillations. Climbing fibre activation of the Purkinje cells generated powerful inhibitory potentials in the cerebellar nuclear neurones at the same frequency.(ABSTRACT TRUNCATED AT 400 WORDS)

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

1. The long-term adaptation of repetitive firing in guinea pig superior colliculus neurons was studied in a mesencephalic slice preparation using intracellular recording techniques. 2. This long-term adaptation was characterized by a decrease in the number of action potentials generated by a depolarizing pulse of constant amplitude applied at frequencies of 0.5-2 Hz. Long-term adaptation appeared in all cells tested regardless of whether they showed short-term spike frequency adaptation during each pulse. 3. Long-term adaptation had a close-to-exponential time course with a time constant of 4.085 +/- 0.675 s (mean +/- SD, n = 8). This phenomenon developed more rapidly as the stimulus frequency increased and was paralleled by a progressive hyperpolarization of the membrane potential which, at the termination of the train of stimuli, remained 6-10 mV more negative than the resting value. 4. The hyperpolarization and the spike frequency adaptation recovered spontaneously in approximately 60 s. The time constant of recovery was 14.66 +/- 1.189 s (n = 4). 5. The afterhyperpolarization (AHP) was also paralleled by a decrease in the input resistance of the cells. This response and the adaptation disappeared after removal of Ca2+ or after addition of Cd2+ to the external solution. This suggests that Ca2+ entry during trains of action potentials activates a Ca2+-dependent K+ conductance with an unusually slow kinetics. 6. This conductance appears to differ from other Ca2+-dependent K+ conductances in that it was blocked by 4-aminopyridine. 7. The properties of this long-term adaptation are remarkably similar to those reported for visual habituation; thus this newly described K+ conductance may be pertinent to the understanding of this behavioral phenomenon

1. The electrophysiologic properties and ionic conductances of neurons located in the stratum griseum medium (SGM) of the guinea pig superior colliculus (SC) were studied by intracellular techniques in an in vitro mesencephalic slice preparation. 2. Cells were stained with Lucifer yellow and demonstrated a uniform appearance. They had an ovoid soma with dendrites directed toward the dorsal surface. These dendrites crossed the stratum opticum, and their fine ramifications reached the stratum zonale. 3. SGM cells had a mean resting potential of 59.4 +/- 5.1 (SE) mV (n = 30), a mean slope input resistance of 26.6 +/- 10 M omega (n = 30), and a mean time constant of 4.13 +/- 1.3 ms (n = 27). 4. Direct depolarization of SC neurons produced tonic repetitive firing. These Na+-dependent action potentials showed spike-frequency adaptation. After addition of tetrodotoxin (TTX) and replacement of Ca2+ by Ba2+, slow, high-threshold spikes were also generated. The trains of Ba2+ spikes did not show adaptation. 5. In about half of the cells direct hyperpolarization elicited a slow return of the membrane potential to base line at the termination of the pulse (probably due to activation of an A-type conductance) and no anomalous rectification. The remaining cells did not have an A-type conductance but demonstrated anomolous rectification which was reversibly abolished by Cs+ but unaffected by Ba2+. 6. Some cells could be anti- and/or orthodromically activated by a stimulating electrode placed at the intercollicular commissure. These, and action potentials elicited by direct activation, had a shoulder on their falling phase. The shoulder disappeared after removal of external Ca2+ or addition of Cd2+ to the bath. 7. During repetitive firing in those cells that demonstrated an A-type conductance, the shoulder became progressively more accentuated during the train of spikes, due to inactivation of this A-type conductance. This resulted in an increase in spike duration. 8. The electrophysiological properties of these cells and their morphological characteristics suggest that they may serve as the element integrating visual and nonvisual information at the superior colliculus

Roller tube cultures of parasagittal cerebellar slices were taken from young rats aged 9-11 days, and maintained in vitro for 1-2 weeks. Morphological aspects of cell types and synaptic relationships in such organ cultures were examined at light and electron microscopic levels. Some neurons were marked by intracellular injections of horseradish peroxidase for subsequent identification of their connection patterns. Cytoarchitecture of the cerebellar cortex was largely preserved in the organ cultures. Dendritic trees of Purkinje cells exhibited isoplanar organizations that often resembled their orientation at the time of explanation. Other cerebellar neurons, namely granule cells, Golgi cells, basket cells, stellate cells, all differentiated within the organ cultures. In addition, some neurons of the deep cerebellar nuclei remained viable during the period of culture. Mossy fibers most probably of cerebellar nuclear origin were found terminating on the dendrites of granule cells and Golgi cells. Quite unexpected were certain types of direct synapses of afferent fibers on short necked spines arising from Purkinje cell smooth dendrites and somata. Such terminals resembled climbing fibers. They were most likely modified mossy fiber afferents, since the organ cultures did not include neurons of the inferior olive which are well spearated from the cerebellar mass at postnatal stages. These 'ascending' mossy fibers presumably occupied postsynaptic surfaces that were either vacated by deafferentation or induced by the afferent fibers themselves. Intracellularly labeled Purkinje cells had widely distributed axonal collateral branches. Labeled axons were distributed within the Purkinje cell layer. Several recurrent Purkinje cell axon collaterals stained with reaction products of horseradish peroxidase tracer were followed at the ultrastructural level. In one case, labeled terminals were examined in an area of approximately 2 mm2. Terminals of Purkinje cell collaterals formed symmetric synapses with somata of basket cells and dendrites of Golgi cells, but not Purkinje cell somata. Some large boutons of serially traced Purkinje cell axon collaterals formed asymmetric contacts with profiles interpreted as Golgi cell dendrites. In contrast to the apparent axonal sprouting in cerebellar organ cultures, maturation of dendritic processes remained static. Astroglia cells of diverse shapes were observed following immunocytochemical staining with antisera to glia filament proteins. The distribution patterns of immunoreactive astrocytes changed dramatically in cerebellar slice cultures maintained for 3-6 weeks in vitro

A direct comparison was made between the electrical properties of rat Purkinje cells in cerebellar organotype cultures and those in acute slices from age-matched animals. Cultures were prepared from 9-11-day-old animals. Intracellular recordings were made 5-12 days later, at which time the folia architecture of the cerebellum was still well preserved. The resting membrane potentials and input resistances of Purkinje cells in cultured and acute slice preparations from young animals were comparable to those of mature Purkinje cells in slices. Neurons from animals younger than 14 days differed from mature Purkinje cells in that they fired at low frequencies in response to outward current pulses. The latter property was found in all cultured neurons studied, independent of their time in culture. These action potentials were generated by Na+ and Ca2+ conductances as shown by the application of selective channel blockers. Cultured or acute slice preparations from animals younger than 11 days shared other immature electroresponsive features. In both groups, Na+-dependent plateau depolarizations were observed in less than 10% of Purkinje cells unless K-conductances were blocked, and considerable membrane depolarization was often required to elicit Ca2+-dependent action potentials. These findings are compatible with the relative prominence of voltage-dependent outward currents in immature Purkinje cells, a property which may be enhanced in culture. The injection of hyperpolarizing current pulses revealed a marked time-dependent anomalous rectification in all Purkinje cells. At the breaks of such pulses, several events were observed. In all cells, a rebound conductance was identified which could generate post-anodal spike bursts. In cultured neurons, however, hyperpolarizing pulses were also followed by a slow return to resting potential. This membrane potential profile was similar to that produced by the activation of an A conductance. Experiments on acute slices from animals of different ages (P9-P17) showed that this A-like conductance was expressed only during a brief period in Purkinje cell development. A higher level of spontaneous synaptic activity was observed in cultured than in acute slice preparations. Both unitary excitatory postsynaptic potentials and inhibitory postsynaptic potentials could be elicited in the former by parallel fiber stimulation, and could be fully reversed by outward or inward transmembrane current injections, respectively.(ABSTRACT TRUNCATED AT 400 WORDS)