Spontaneous bioelectric activity of neurons in cerebellar cultures: evidence for synaptic interactions

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Maturation of cerebellar neuronal elements in a tissue culture

The process of differentiation of neurons was traced on an organotypic cerebellar culture of newborn mice. Cerebellar cells reach morphological maturity by the 18-21st day of culturing. An increase of the membrane potential begins on the 6-7th day of culturing. It reaches values characteristic for the definitive stage of the neuron (65-75 mV) by the 9-10th day of culturing. Spontaneous action potentials begin to be recorded on the 10-12th day of culturing. At this time they markedly differ in their characteristics from the action potential of the mature neuron. The differences become less noticeable by the 16-18th day of culturing. However, final maturation of the action potential occurs at later times. The formation of specific sensitivity of cerebellar neurons to acetylcholine correlates with the time of formation of the action potential.

Postnatal lead exposure alters spontaneous cerebellar Purkinje neuron discharge

The effects of early postnatal lead administration on the spontaneous activity of cerebellar Purkinje neurons were studied electrophysiologically in adult male Sprague-Dawley rats. Newborn rat litters were divided into three groups and injected daily from Day 1 to Day 20 after birth with 8 mg NaAc/kg, 1 mg PbAc/kg, or 8 mg PbAc/kg intraperitoneally. Purkinje neurons in all three groups showed regular and sustained discharge. However, the mean spontaneous firing rate was significantly lower in the 8-mg PbAc/kg group (26.13 +/- 1.28 Hz) compared to the NaAc controls (32.39 +/- 1.93, P greater than 0.01). Furthermore, the distribution of the firing rates of the Purkinje cells were different in the two groups, with an obvious loss of faster firing cells in the 8-mg PbAc/kg group. No differences were seen between the 1-mg PbAc/kg group and the NaAc controls. As the 8-mg PbAc/kg group weighed significantly less than the NaAc group, malnourished animals were produced by using oversized litters, to control for any changes caused by the lower body weight. However, when compared to concomitantly raised controls, no changes could be seen in Purkinje cell discharge in the malnourished animals. It is concluded that neonatal exposure to lead can cause permanent changes in spontaneous Purkinje cell discharge.

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

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

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

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

Cerebellar macroneurons in microexplant cell culture. Methodology, basic electrophysiology, and morphology after horseradish peroxidase injection

Cerebellar macroneurons, including Purkinje cells, survive and differentiate in long-term monolayer cultures, which are prepared by a partial dissociation procedure we refer to as a microexplant technique. Intracellular recording demonstrated that these neurons were functional, showing spontaneous spiking activity and electrical excitability, and both spontaneous and evoked synaptic activity. In order to further characterize cell types, light and electron microscopic studies were performed after intracellular iontophoresis of horseradish peroxidase. Purkinje neurons were identified by their form of dendritic arborization and numerous dendritic spines. Cortical granule cells and macroneurons derived from the deep nuclei could also be demonstrated.

Organotypic monolayer cultures of nervous tissue

Cultivation of nervous tissue by means of the roller-tube technique yields thin organotypic cultures. Explants or slices prepared from 1- to 20-day-old rats are embedded in a plasma clot on flying coverslips and cultivated for weeks in roller-tubes. Due to the flattening of the tissue, individual nerve cells are often arranged in monolayer thickness and can, therefore, be viewed with phase-contrast microscopy. This technique is utilized to culture and co-culture nervous tissue derived from various brain regions. The degree of organotypic organization depends on the age of the animals used for culturing. Stable intracellular recordings arae obtained from nerve cells which are impaled under visual control. In view of the accessibility of individual living cells, this approach seems to be particularly well-suited for physiological and pharmacological studies on morphologically identified nerve cells.

Laser microirradiation of cerebellar neurons in culture. Electrophysiological and morphological effects

Electrophysiological and ultrastructural effects of focused laser radiation on neurons from neonatal rat cerebellum in tissue culture are reported. Action potentials were elicited by an extracellular current pulse train. The stimulator voltage required for half-maximum response frequency was measured as a function of the energy delivered by a single laser pulse. Above a "threshold" laser energy, the cell response to stimulation became negligible for all stimulator voltages. Electron micrographs of cells revealed that the mitochondria are preferentially damaged at an energy comparable to the electrophysiological threshold. The damaged mitochondria showed swollen matrix space and disrupted cristae membranes. Higher laser energies resulted in damage to other cytoplasmic structures. The results are consistent with a model that assumes that light interaction with the nerve cells proceeds by local heating of the mitochondria and nearby structures and leads to an increased conductance of the membrane to some ionic species.

Neurones with synchronous bursting discharges in organ cultures of the hypothalamic supraoptic nucleus area

Explants of the hypothalamic supraoptic nucleus area from newborn rats were cultured. Ultrastructural studies revealed the existence of typical neurosecretory granules in neuronal perikarya as well as in axons. Large nerve cells that were spontaneously active discharged at an average firing rate of 7.2 +/- 4.4 (S.D., n = 98) spikes/sec and 42% of these neurones displayed a phasic firing pattern as shown by the existence of peaks in their autocorrelograms. The firing of 59% of the neurones was synchronous with the activity of other nerve cells. In some neurones, only the onsets of bursts were correlated, whereas in others periods of high correlation alternated with periods of no correlation. The relative proportion of rhythmically or synchronously firing hypothalamic neurones was not altered when a neurohypophysial explant was co-cultured. Field stimulation in the cultures resulted in a short-latency excitation followed by an inhibition which was found to be bicuculline-sensitive. The existence of functional synapses was furthermore demonstrated by intracellular recordings of postsynaptic potentials.

The effects of putative neurotransmitters on the resting membrane potential of dissociated brain neurones in culture

Cultures established from mechanically dissociated neonatal mouse brains were found to be suitable for electrophysiological investigation of drug action. During culture most cells were aggregated into either monolayer regions or thick cords joining monolayer regions. A few cells remained isolated. The neurones in the monolayer regions were distinguished from glial cells by differential staining, and were found to be the best subject for intracellular recording. Frequency of resting membrane potentials of these cells proved to be reproducible in cultures of the same age, and were a useful index of sensitivity to bath applied drugs. Acetylcholine, dopamine, histamine, serotonin and noradrenaline depolarized various neurones; GABA caused hyperpolarization, while glutamate and glycine had no significant effect. Antagonism of the responses to acetylcholine, dopamine, serotonin and GABA was seen using atropine, pimozide, methysergide and bicuculline respectively. It is concluded that dissociated brain neurones in culture show chemosensitivity and may be useful in further pharmacological studies.

Spontaneous bioelectric activity of cultured Purkinje cells during exposure to glutamate, glycine, and strychnine

The addition of glutamate to the bathing medium increased the average firing rate of cerebellar rat Purkinje cells in vitro. At concentrations lower than 10(-6) M, there was no deviation from controls in the firing pattern or rate that was detectable. At 10(-3) M glutamate, the amplitude of the action potentials was gradually decreased until all activity was abolished. The action of glutamate was rapid in onset and reversible. Glycine produced sustained depression of firing at concentrations higher than 10(-3) M. This inhibition was strychnine-insensitive and considered nonspecific. Strychnine, on the other hand, exerted an excitatory influence on Purkinje cells when applied at low concentrations (10(-8) TO 10(-6) M). The firing became more irregular and complex discharges appeared. Higher concentrations of strychnine (greater than 10(-5) M) inhibited the spontaneous activity. The effect of strychnine was partly reversible. The data suggest that low concentrations of strychnine lower the threshold for inputs at excitatory as well as inhibitory synapses.

The effects of GABA, Picrotoxin and bicuculline on the spontaneous bioelectric activity of cultured cerebellar Purkinje cells

Addition of GABA to the bathing medium in concentrations of 10(-6) to 10(-5) M abolished the spontaneous bioelectric activity of rat Purkinje cells in vitro. Lower concentrations of GABA (less than or equal to 10(-6) M) influenced the firing pattern of spontaneous discharges in such a way that the relative number of long interspike intervals increased. The inhibitory effect of GABA was reversible; the cells resumed normal activity after being transferred to a normal balanced salt solution. The action of GABA was antagonized by bicuculline and picrotoxin, whereas strychnine was unable to overcome the inhibition induced by GABA. Both bicuculline (10(-10) to 10(-6) M) and picrotoxin (10(-7) to 10(-4) M) exerted an excitatory effect on cultured Purkinje cells when added to the medium. This excitatory action suggests the existence of inhibitory synapses in cultures of rat cerebellum in which endogenous GABA is used as a transmitter. The ratio between the dose of picrotoxin and that of bicuculline necessary to produce maximal excitation was in the order of 10(3). Both substances showed an inhibitory action at high concentrations.

Formation of a functional adrenergic input to intraocular cerebellar grafts: ingrowth of inhibitory sympathetic fibers

The intraocular transplantation technique was used to study the ingrowth of peripheral sympathetic adrenergic nerves from the iris into transplants of fetal rat cerebellum, and the possible function of these nerves. The transplants, grown in oculo for one-half to eight months, were analyzed by fluorescence histochemistry and electrophysiological techniques. Peripheral sympathetic adrenergic fibers from the iris were able to grow into the cerebellar transplants and arborize in a pattern similar to that in situ, appearing in all three cortical layers and the noncortical areas of the transplants. The density of visible nerves without pretreatment and after preincubation in 10(-6) or 10(-5) M alpha-methylnorepinephrine was comparable to mature rat cerebellum. The spontaneous discharge of the Purkinje cells in oculo was inhibited by microiontophoresis of norepinephrine (NE) and amphetamine in sympathetically innervated, as well as sympathectomized transplants denervated by ganglionectomy. The NE response was blocked by the adrenergic beta-receptor blocker MJ-1999. GABA also inhibited the Purkinje cell activity while glutamate accelerated the discharge. Parenteral amphetamine inhibited Purkinje cell activity in sympathetically innervated transplants, but was ineffective in denervated transplants. The Purkinje cell spontaneous activity was inhibited by electrical stimulation of the NE fiber input through the cervical sympathetic trunk. This inhibition could be antagonized by parenteral reserpine or the beta-adrenergic antagonist propranolol. The responses of the Purkinje cells within the transplants to drugs and transmitters mimic those of the adult rat in situ. In view of the fluorescence histochemical evidence for an ingrowth of peripheral sympathetic adrenergic fibers into the cerebellar transplants, and the results of stimulating the sympathetic trunk, it is suggested that peripheral adrenergic fibers may be able to establish functional connections with the Purkinje cells similar to the cerebellar adrenergic synapses normally formed in situ by fibers from the locus coeruleus.

Anti-synaptic antibody in allergic encephalomyelitis. II. The synapse-blocking effects in tissue culture of demyelinating sera from experimental allergic encephalomyelitis

It has been suggested that demyelination cannot account for all of the observed clinical symptoms of multiple sclerosis (MS), in particular the rapidity of onset and remission of the disease, and attention has been focussed on the role of the synapse in 'demyelinating diseases'. In the present paper we have attempted to resolve the fundamental question of the site of action of a demyelinating disease, experimental allergic encephalomyelitis (EAE), by the use of cultures of neonatal rat cerebellum. Electrophysiological and morphological development in these cultures run hand-in-hand, and in the first few days in vitro there is a 4-5 day period when synapses are both seen ultrastructurally and known to be functioning but before the onset of myelination. The serum from guinea pigs with EAE was added to these cultures at different stages during their development and the morphological and electrophysiological effects observed. An abolitionary effect on the bioelectric activity of the culture was only observed when the serum was added to mature, myelinated cultures. Also the same active sera had no effect on synaptic activity before myelination had occurred. We conclude that the synaptic blocking effect occurs only when myelin is destroyed.