Cerebellar macroneurons in microexplant cell culture. Postsynaptic amino acid pharmacology

Electrophysiology of the mammalian cerebellar cortex in organ culture

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)

Optical monitoring of excitatory synaptic activity between cultured hippocampal neurons by a multi-site Ca2+ fluorometry

The fluorescence intensities from cultured hippocampal neurons loaded with a Ca2+-indicator (fura-2), showed spontaneous, periodical oscillations, which were synchronized among the cells. Tetrodotoxin or 2-aminophosphonovalerate, a glutamate-receptor antagonist, blocked the oscillation. The rising phase of the fluorescence was accompanied by a burst of inward currents, as monitored by a patch electrode. It is thus suggested that the fluorescence elevation represents an increase in intracellular Ca2+ concentration accompanied by excitation of neurons which formed synaptic connections between each other. This method for detection of neuronal activity including synaptic excitation escapes from the limitations of the conventional technique employing microelectrodes.

Glutamate and aspartate activated channels and inhibitory synaptic currents in large cerebellar neurons grown in culture

Patch-clamp methods were used to examine large (greater than 30 microns) cerebellar neurons of the rat, maintained in cell culture. Cells possessed voltage-activated transient inward Na+ currents which were sensitive to tetrodotoxin. Spontaneous synaptic currents, present in whole-cell recordings, were abolished by bicuculline and picrotoxin and were carried by Cl-. Cells produced inward currents in response to the transmitter candidates glutamate and aspartate and also to the glutamate agonists kainate, quisqualate, N-methyl-D-aspartate and ibotenate. Analysis of glutamate and aspartate-current noise has been used to derive characteristics of the excitatory channels. Single channel currents have also been observed directly in whole-cell and outside-out patches. Both glutamate and aspartate are able to activate channels which were blocked by Mg2+ and had a maximum conductance of 50 pS.

Cerebellar granule cells in culture: monosynaptic connections with Purkinje cells and ionic currents

Electrophysiological properties of cerebellar granule cells and synapses between granule and Purkinje cells were studied in dissociated cultures. Electrophysiological properties of neurons and synapses in the mammalian central nervous system are best studied in dissociated cell cultures because of good target cell visibility, control over the contents of the extracellular solution, and the feasibility of whole-cell patch electrode recording, which has been a powerful technique in analyzing biophysical properties of ionic channels in small cells. We have applied this whole-cell recording technique to cultured cerebellar granule cells whose electrophysiological properties have been almost unknown because of their small cell size. In this study, simultaneous intracellular recordings from presynaptic granule and postsynaptic Purkinje cells demonstrated that granule cells made functional monosynaptic connections with Purkinje cells in dissociated cell cultures. Further, the existence of Na, Ca, and K channels in granule cells is demonstrated by external ion substitutions.

Patch-clamp studies of chloride channels activated by gamma-aminobutyric acid in cultured hippocampal neurones of the rat

The properties of the GABA-activated ion channel in hippocampal neurones prepared from 17- to 19-day-old fetal rats in dispersed cell cultures were studied with the patch-clamp techniques. These neurones had chemosensitivity to gamma-aminobutyric acid (GABA) in the absence of synaptic inputs. GABA activated chloride ion channels selectively in these neurones. The GABA-induced Cl- current was detectable with greater than 1 microM GABA. The amplitude of the current increased with higher concentrations of GABA and apparently saturated at 100 microM. The effective single-channel conductance (gamma) was estimated to be 19 pS from the mean and variance of the steady-state GABA-induced current fluctuation in the whole-cell recordings. The power density spectra for GABA-induced current fluctuations in the whole-cell as well as cell-free outside-out membrane patch recordings had more than a single Lorentzian component. The application of GABA induced discrete pulse-like current flows through the cell-free outside-out membrane patch, after the number of channels activated by GABA had decreased due to the rundown. The single-channel conductance estimated from the amplitude of the current pulse was 29 pS when the intra- and extracellular Cl- concentrations were 150 and 120 mM, respectively. In addition to the above conducting state, the GABA channel had several open states with lower conductances. The apparent discrepancy between the effective single-channel conductance estimated from noise analysis and the single-channel conductances directly measured with individual channels may be due, at least in part, to the presence of multiple conducting states.

Cerebellar macroneurons in serum-free cultures: evidence for intrinsic neuronotrophic and neuronotoxic activities

Cerebellar macroneurones survive and differentiate for at least 10 days in Eagle's minimum essential medium with insulin as the only supplement when cultured either as microexplants or in high-density dissociated cultures, while they do not survive if cultured in low density. The survival is related to the extracellular release of neuronotrophic factor(s). Using a quantitative bioassay of the neuronotrophic effect, it is possible to demonstrate that the effect is concentration dependent. The analysis of the dose-response curves suggests that the neuronotrophic activity is associated with a neuronotoxic activity. The two activities can be segregated using a simple physical method, allowing direct demonstration of the neuronotoxic activity.

Cerebellar macroneurons in microexplant cell culture: ultrastructural morphology

Microexplant cell cultures of fetal rat cerebellum contain essentially monolayer networks of Purkinje cells, occasional granule cells and neurons from the deep nuclei. The neurons and occasional filament-packed glial cells develop on top of a sheet of flattened, non-neuronal cells. In the absence of extrinsic input to the cerebellum and greatly reduced numbers of granule cells, the Purkinje cells develop a stunted and non-oriented dendritic arbor similar to that observed in agranular cerebella. The Purkinje cell dendritic branches, however, are spine-covered. Although the spines are not enveloped by glia and are only rarely contacted by a presynaptic bouton, most spines display a patch of electron-dense material resembling a postsynaptic membrane specialization. The Purkinje cells develop synaptic interactions among themselves and with granule cells. The ultrastructural morphology of boutons derived from both Purkinje cells and large neurons of the deep nuclei, identified after intracellular injection of horseradish peroxidase, is consistent with that observed in vivo. The present study indicates that cerebellar Purkinje cells survive and differentiate in a culture system in which individual neurons are accessible for electrophysiological and morphological analyses.

Kainic acid sensitivity of mammalian Purkinje cells in monolayer cultures

Purkinje cells from fetal mouse cerebellum were studied in a monolayer culture system. These neurons, labeled by intracellular iontophoresis of HRP, showed one or more thick, spine-covered dendrites and an axon whose swellings contacted other neuronal processes and Purkinje cell somata. In short-term experiments, kainic acid was cytotoxic to Purkinje neurons and not to granule cells in monolayer cultures of fetal rat cerebellum.