Irregular Firing Research Articles

Ceramide inhibits the inwardly rectifying potassium current in GH(3) lactotrophs.

The effects of ceramide on ion currents in rat pituitary GH(3) cells were investigated. Hyperpolarization-elicited K(+) currents present in GH(3) cells were studied to determine the effect of ceramide and other related compounds on the inwardly rectifying K(+) current (I(K(IR))). Ceramide (C(2)-ceramide) suppressed the amplitude of I(K(IR)) in a concentration-dependent manner, with an IC(50) value of 5 microM. Ceramide caused a rightward shift in the midpoint for the activation curve of I(K(IR)). Pretreatment with PD-98059 (30 microM) or U-0126 (30 microM) did not prevent ceramide-mediated inhibition of I(K(IR)). However, the magnitude of ceramide-induced inhibition of I(K(IR)) was attenuated in GH(3) cells preincubated with dithiothreitol (10 microM). TNF alpha (100 ng/g) also suppressed I(K(IR)). In the inside-out configuration, application of ceramide (30 microM) to the bath slightly suppressed the activity of large conductance Ca(2+)-activated K(+) channels. Under the current clamp mode, ceramide (10 microM) increased the firing of action potentials. Cells that exhibited an irregular firing pattern were converted to those displaying a regular firing pattern after application of ceramide (10 microM). Ceramide also suppressed I(K(IR)) in neuroblastoma IMR-32 cells. Therefore, ceramide can produce a depressant effect on I(K(IR)). The blockade of this current by ceramide may affect cell function.

Electrophysiology of interneurons in the glomerular layer of the rat olfactory bulb.

In the mammalian olfactory bulb, glomeruli are surrounded by a heterogeneous population of interneurons called juxtaglomerular neurons. As they receive direct input from olfactory receptor neurons and connect with mitral cells, they are involved in the initial stages of olfactory information processing, but little is known about their detailed physiological properties. Using whole cell patch-clamp techniques, we recorded from juxtaglomerular neurons in rat olfactory bulb slices. Based on their response to depolarizing pulses, juxtaglomerular neurons could be divided into two physiological classes: bursting and standard firing. When depolarized, the standard firing neurons exhibited a range of responses: accommodating, nonaccommodating, irregular firing, and delayed to firing patterns of action potentials. Although the firing pattern was not rigorously predictive of a particular neuronal morphology, most short axon cells fired accommodating trains of action potentials, while most delayed to firing cells were external tufted cells. In contrast to the standard firing neurons, bursting neurons produced a calcium-channel-dependent low-threshold spike when depolarized either by current injection or by spontaneous or evoked postsynaptic potentials. Bursting neurons also could oscillate spontaneously. Most bursting cells were either periglomerular cells or external tufted cells. Based on their mode of firing and placement in the bulb circuit, these bursting cells are well situated to drive synchronous oscillations in the olfactory bulb.

Structure and Function of South-east Australian Estuaries

An attempt is made to synthesize the geological properties, water quality attributes and aspects of the ecology of south-east Australian estuaries so as to provide a framework for addressing coastal management issues. The approach is based on the underlying causal factors of geology and morphology and more immediate environmental factors (e.g. salinity and sediments) which are associated with ecological distributions, species richness and fisheries catch. This ‘broad brush’ approach seeks to maximize reality and generality, albeit at the expense of precision and local variability in individual circumstances. It disregards small-scale ecological patterns as noise. Unlike in the Northern Hemisphere, conditions in temperate Australia are characterized by irregular flood and fire regimes that strongly influence estuary hydrology and nutrient inputs. Three main types of estuary (tide-dominated, wave-dominated and intermittently closed) are recognized based on geological criteria and having particular entrance conditions that control tidal exchange. Four zones (marine flood-tidal delta, central mud basin, fluvial delta and riverine channel/alluvial plain) are also recognized common to each type of estuary. These zones correspond to mappable sedimentary environments in all estuaries and have characteristic water quality, nutrient cycling/primary productivity signatures and ecosystems. The ecology of a zone is modified by (a) estuary type which determines the salinity regime; (b) stage of sediment filling (evolutionary maturity) which controls the spatial distribution/size of the zones; and (c) impacts of various forms of development. By using the zones/habitats as a common currency among all estuaries, it is possible to link ecological aspects such as species richness and commercial fisheries production so as to compare different estuaries or within-estuary zones.

Apamin-induced irregular firing in vitro and irregular single-spike firing observed in vivo in dopamine neurons is chaotic.

Dopamine neurons of the substantia nigra often fire action potentials irregularly in vivo, but in vitro fire in a regular, pacemaker-like firing pattern. Bath application of apamin, a blocker of calcium-activated potassium channels, can shift a dopamine neuron from pacemaker-like to irregular firing. To determine whether the irregular firing was caused by intrinsic cellular mechanisms rather than random synaptic input or some other form of noise, spike density functions of interspike interval records were analyzed using non-linear forecasting methods to quantify any non-linear (non-periodic) structure. Intrinsic cellular mechanisms are capable of producing chaotic firing, which is deterministic, non-linear, and loses predictability exponentially with increasing forecast time.To determine whether forecasting spike density functions could reliably measure predictability, forecasting was first applied to spike density functions produced by computer simulations of pacemaker-like, chaotic, and random firing, as well as pacemaker-like and chaotic firing that were randomly synaptically driven. Exponential loss of predictability was successfully detected in both chaotic and randomly driven chaotic firing. Predictability scaled faster than exponentially for random spiking, and linearly (slower than exponentially) for randomly driven pacemaker firing. The method was then applied to experimental records of apamin-induced irregular firing of rat dopaminergic neurons of the substantia nigra in vitro and in vivo. Exponential loss of predictability was detected in both cases, consistent with chaotic firing. Experimental records of pacemaker-like firing in vitro showed linear scaling, consistent with a randomly driven pacemaker. Several schemes for neural encoding of synaptic inputs have been suggested, such as rate codes or temporal codes. However, our results suggest that under some conditions, the irregular firing of dopamine neurons does not reflect the random temporal dynamics of its inputs, but rather the intrinsic, deterministic dynamics of dopamine cells themselves, under the tonic neuromodulatory influence of apamin in vitro and possibly that of an unidentified endogenous modulatory substance in vivo.

Coefficient of variation vs. mean interspike interval curves: What do they tell us about the brain?

A number of models have been produced recently to explain the high variability of natural spike trains (Softky and Koch, J. Neurosci. 13 (1) (1993) 334). These models use a range of different biological mechanisms including partial somatic reset, concurrent inhibition and excitation, correlated inputs and network dynamics effects. In this paper we examine which model is more likely to reflect the mechanisms used in the brain and we evaluate the ability of each model to reproduce the experimental coefficient of variation (CV) vs. mean interspike interval (ISI) curves (CV=standard deviation/mean ISI). The results show that the partial somatic reset mechanism is the most likely candidate to reflect the mechanism used in the brain for reproducing irregular firing.

Dispersal strategies, spatial heterogeneity and colonization success in fire-managed grasslands

Interactions between fire regime, dispersal strategies and patch structure were examined as key issues for the management of floristic composition of grasslands, through a model that simulates the population dynamics of two competing fire-cued and non-sprouting species. The model describes a heterogeneous environment composed by several patches of grassland, only related by seed dispersal. The last burn date at each patch determines the accumulation level of fuel-biomass provided by a third, dominant species, which in turn controls for the exclusion rate of both colonizer species. The population dynamics of both species was approached following density-dependent models and parameterized for two opposite dispersal strategies: low spatial and high temporal dispersion of seeds (type 1), high spatial and low temporal dispersion of seeds (type 2). Only under the most variable scenarios (when non-synchronous and irregular fire regimes were combined with a proportion of patches ( p) with initially depleted seed banks) did the relative success of dispersal strategies vary with the length of the fire-free period. Irrespective of p, smaller interval lengths favored the postburn density of the strategy 1. Strategy 2 was favored over strategy 1 when the fire-free interval increased, such difference being maximum for intermediate p-values. These general tendencies agree with those observed from a reference system: the Flooding Pampa grasslands dominated by Paspalum quadrifarium where short no-fire intervals promote the postburn abundance of a type 1 species ( Lotus tenuis) over two type 2 species ( Carduus acanthoides and Cirsium vulgare) while for long fire-free intervals the opposite is true. This results suggest that frequency, time since last burn, and burning synchrony are useful components of a fire regime to take advantage of variation in dispersal strategies.

Differential neurosensory responses of adult Colorado potato beetle, Leptinotarsa decemlineata, to glycoalkaloids.

Neurons from chemosensory hairs on the galeae of adult Colorado potato beetle (CPB), Leptinotarsa decemlineata (Say), were investigated for responses to glycoalkaloids of the family Solanaceae. While solanine and tomatine elicited irregular firing by multiple neurons and bursting activity at 1 mM concentration in most sensory hairs, stimulation with leptine I resulted in consistently high-frequency, slowly adapting responses with a dose-dependent effect between 0.03 and 0.3 mM concentrations. Responses to a mixture of solanine and leptine I suggested possible modification of the leptine I response by other glycoalkaloids, resulting in reduced neural activity relative to leptine I alone. These results establish a method for specifically evaluating leptine I and other glycoalkaloids for effects on feeding behavior of CPB and provide a sensory component for incorporating deterrent chemistry into biorational control methods for the CPB.

Characterization of Inhibition by Risperidone of the Inwardly Rectifying K+ Current in Pituitary GH3 Cells

The effects of risperidone on ionic currents in rat pituitary GH3 cells were investigated with the aid of the patch-clamp technique. Hyperpolarization-activated K+ currents in GH3 cells bathed in high-K+ Ca2+-free solution were studied to determine the effect of risperidone and other related compounds on the inwardly rectifying K+ current (IK(IR)). Risperidone (0.1–10 μM) suppressed the amplitude of IK(IR) in a concentration-dependent manner. The IC50 value for the risperidone-induced inhibition of IK(IR) was 1 μM. Risperidone (3 μM) was found to slow the rate of activation. An increase in current deactivation by the presence of risperidone was also observed. Haloperidol (10 μM) and thioridazine (10 μM) inhibited the amplitude of IK(IR) effectively, and clozapine slightly suppressed it; however, metoclopramide (10 μM) had no effect on it. Risperidone (10 μM) had no effect on voltage-dependent K+ and L-type Ca2+ currents. However, in the inside-out configuration, risperidone (10 μM) did not alter the single-channel conductance, but reduced the activity of large-conductance Ca2+-activated K+ (BKCa) channels. Under the current-clamp mode, risperidone (3 μM) depolarized the membrane potential and increased the firing rate. With the aid of the spectral analysis, cells that exhibited an irregular firing pattern were also converted to those displaying a regular firing pattern after addition of risperidone (3 μM). The present study provides evidence that risperidone, in addition to the blockade of dopamine receptors, can produce a depressant effect on IK(IR) and BKCa channels, and implies that the blockade of these ionic currents by risperidone may affect membrane excitability and prolactin secretion in GH3 cells.

Detection of determinism within time series of irregular burst firing from the injured sensory neuron.

Irregular burst firing is spontaneously generated by rat dorsal root ganglion neuron after chronic compression. To investigate the time series of burst firing, we recorded interspike intervals (ISIs) of single-unit firing in vitro and processed the ISIs to obtain interevent intervals (IEIs). Then, two non-linear methods were applied to detect deterministic dynamic behaviors in IEI series. No evidence for the existence of determinism was found with non-linear prediction method. Using unstable periodic orbit identification method, significant period-1 orbits were identified in all 10 data, period-2 orbits in eight, and period-3 orbits in six. The results indicate that there exist significant deterministic behaviors in the time series of irregular burst firing from the injured sensory neuron.

Electrophysiological characteristics of substantia nigra neurons in organotypic cultures: spontaneous and evoked activities

Morphological and electrophysiological characteristics of dopaminergic and non-dopaminergic neurons in the substantia nigra and their postsynaptic responses to stimulation of the tegmental pedunculopontine nucleus were studied in rat organotypic triple cultures. These cultures consisted of the subthalamic nucleus explant, ventral mesencephalic explant, inclusive of the substantia nigra and the mesopontine tegmentum explant, inclusive of the tegmental pedunculopontine nucleus, prepared from one- to two-day-old rats. Intracellular sharp and whole-cell recordings were obtained from three- to eight-week-old organotypic cultures. Recorded neurons were identified as dopaminergic and non-dopaminergic neurons with tyrosine hydroxylase immunohistochemistry. Dopaminergic neurons had long duration action potentials, prominent afterhyperpolarization, time-dependent inward and outward rectification and strong frequency adaptation. Spontaneous firing patterns varied from regular, irregular to burst firing. Non-dopaminergic neurons had short duration action potentials, in general no rectifying currents, and maintained high firing frequencies. Spontaneous firing patterns in these neurons were irregular or burst firing. Morphological analysis of the recorded neurons labeled with neurobiotin revealed that non-dopaminergic neurons had more extensive arborization of higher-order dendrites than dopaminergic neurons. Dopaminergic and non-dopaminergic neurons receive glutamatergic and cholinergic excitatory inputs from the tegmental pedunculopontine nucleus. These results indicate that morphological and electrophysiological characteristics of substantia nigra neurons in the organotypic culture are generally similar to those reported in in vitro slice and in vivo studies. However, spontaneous activities of dopamine neurons observed in the organotypic culture preparation more closely resemble those in in vivo preparation compared to in vitro preparation.

Coupled oscillator model of the dopaminergic neuron of the substantia nigra.

Calcium imaging using fura-2 and whole cell recording revealed the effective location of the oscillator mechanism on dopaminergic neurons of the substantia nigra, pars compacta, in slices from rats aged 15-20 days. As previously reported, dopaminergic neurons fired in a slow rhythmic single spiking pattern. The underlying membrane potential oscillation survived blockade of sodium currents with TTX and was enhanced by blockade of voltage-sensitive potassium currents with TEA. Calcium levels increased during the subthreshold depolarizing phase of the membrane potential oscillation and peaked at the onset of the hyperpolarizing phase as expected if the pacemaker potential were due to a low-threshold calcium current and the hyperpolarizing phase to calcium-dependent potassium current. Calcium oscillations were synchronous in the dendrites and soma and were greater in the dendrites than in the soma. Average calcium levels in the dendrites overshot steady-state levels and decayed over the course of seconds after the oscillation was resumed after having been halted by hyperpolarizing currents. Average calcium levels in the soma increased slowly, taking many cycles to achieve steady state. Voltage clamp with calcium imaging revealed the voltage dependence of the somatic calcium current without the artifacts of incomplete spatial voltage control. This showed that the calcium current had little or no inactivation and was half-maximal at -40 to -30 mV. The time constant of calcium removal was measured by the return of calcium to resting levels and depended on diameter. The calcium sensitivity of the calcium-dependent potassium current was estimated by plotting the slow tail current against calcium concentration during the decay of calcium to resting levels at -60 mV. A single compartment model of the dopaminergic neuron consisting of a noninactivating low-threshold calcium current, a calcium-dependent potassium current, and a small leak current reproduced most features of the membrane potential oscillations. The same currents much more accurately reproduced the calcium transients when distributed uniformly along a tapering cable in a multicompartment model. This model represented the dopaminergic neuron as a set of electrically coupled oscillators with different natural frequencies. Each frequency was determined by the surface area to volume ratio of the compartment. This model could account for additional features of the dopaminergic neurons seen in slices, such as slow adaptation of oscillation frequency and may produce irregular firing under different coupling conditions.

Impairment Tutorial: Rating Sensory and Motor Deficits of the Upper Extremity

Abstract The three components of electrodiagnosis useful in evaluation of the peripheral nervous system and spinal cord include electromyography (EMG), electroneurography (nerve conduction studies), and somatosensory evoked potentials. EMG examination involves introduction of a special recording needle into a muscle belly. Electrical potentials located within a few millimeters of the needle are picked up by an electrode and are transmitted from the muscle to amplifiers that filter and display results visually for the electromyographer. Three types of spontaneous activity in electrical potentials are of the greatest relevance: positive sharp waves, fibrillation potentials, and fasciculations (fasciculation potentials on the EMG result from irregular firing of motor units). Electromyography can help assess the status of nerve fibers indirectly, but the integrity of large myelinated sensory and motor neurons can be evaluated directly by nerve conduction studies (NCS), also known as electroneurography. NCS can assess motor neurons, sensory neurons, or mixed nerve trunks. Sensory nerve conduction velocity can be studied in a manner analogous to motor conduction velocity: sensory fibers can be directly stimulated, and the evoked response can be measured at the wrist and elbow. Somatosensory evoked potentials occasionally are useful as an adjunct to EMG and NCS in the diagnosis of peripheral nervous system pathology. These tests also are useful when it is unclear whether an individual has a true radiculopathy.

Interacting biological and electronic neurons generate realistic oscillatory rhythms.

Small assemblies of neurons such as central pattern generators (CPG) are known to express regular oscillatory firing patterns comprising bursts of action potentials. In contrast, individual CPG neurons isolated from the remainder of the network can generate irregular firing patterns. In our study of cooperative behavior in CPGs we developed an analog electronic neuron (EN) that reproduces firing patterns observed in lobster pyloric CPG neurons. Using a tuneable artificial synapse we connected the EN bidirectionally to neurons of this CPG. We found that the periodic bursting oscillation of this mixed assembly depends on the strength and sign of the electrical coupling. Working with identified, isolated pyloric CPG neurons whose network rhythms were impaired, the EN/biological network restored the characteristic CPG rhythm both when the EN oscillations are regular and when they are irregular.

Ultradian rhythmic neuronal oscillation in the intergeniculate leaflet.

Our paper is the first to describe ultradian rhythmic neuronal oscillation in the intergeniculate leaflet (IGL) of the rat. We recorded a multiple-unit neuronal activity (MUA) from dorsal to ventral parts of the lateral geniculate nucleus (LGN) in anaesthetized rats. In all the subdivisions of the lateral geniculate complex we observed spontaneous irregular firing rates of cells. However only at the anatomical localisation of the IGL, after the light was on, those responses exhibited burst firing with a constant interburst interval, which lasted several hours until the light was off. The duration of that rhythmic oscillation obtained by means of Fourier's analysis was approximately 124 s. To date we have not had sufficient data to discuss possible mechanisms of this neuronal rhythmicity. We can only conclude that light is the most important stimulus not only for suprachiasmatic nuclei (SCN), but also for the IGL. On the other hand, we can neither exclude nor confirm that in order to evoke ultradian rhythmical oscillation in the IGL, in addition to light also non-photic information is necessary.

Properties of solitary tract neurons receiving inputs from the sub-diaphragmatic vagus nerve

Vagal afferents ascending from the gastrointestinal tract synapse on neurons in the nucleus of the solitary tract. Although these neurons constitute a significant proportion of solitary tract cells their firing behaviour and synaptic properties are not documented. Since gastrointestinal tract afferent termination sites overlap with regions mediating cardiorespiratory reflexes the possibility of convergence with afferents mediating cardiovascular and respiratory reflexes was proposed. Here we describe some electrophysiological and morphological properties of solitary tract neurons orthodromically driven from the sub-diaphragmatic vagus nerves and assess possible convergent inputs from cardiorespiratory afferents. Whole-cell recordings of solitary tract neurons responding to electrical stimulation of the sub-diaphragmatic vagus nerves (0.1–1 ms; 1–10 V; 2–20 Hz) were made in a working heart–brainstem preparation of rat. Baroreceptors were stimulated by raising pressure in the aorta or carotid sinus, whereas aortic injection of sodium cyanide (0.05% solution 25–50 μl) was used to activate peripheral chemoreceptors. Phrenic nerve activity and heart rate were monitored continuously. Of 88 solitary tract neurons tested, 39 responded with an evoked excitatory synaptic potential following stimulation of the sub-diaphragmatic vagus nerves. Resting membrane potential and input resistance of sub-diaphragmatic vagus nerve driven solitary tract neurons were 53.2±0.5 mV and 291+17 MΩ, respectively (mean±S.E.M.). Response latencies to sub-diaphragmatic vagus nerve stimulation were divided into two groups: <20 ms (16.0±2 ms, n=7; mean±S.E.M.) and >20 ms (77.3±5 ms, n=32). One additional neuron displayed an evoked inhibitory postsynaptic potential (latency 175 ms). Nineteen neurons showed ongoing activity which consisted of either irregular single action potential firing (0.5–10 Hz; n=12) or burst discharge ( n=7). Of 33 neurons tested, 17 showed spike frequency adaptation during injection of positive current, whereas 19 of 38 cells displayed rebound excitation following release from hyperpolarized potentials. There was no correlation between these properties and synaptic latencies. Ninety-one per cent of neurons tested displayed synaptic depression following paired pulse stimulation of the sub-diaphragmatic vagus nerve over intervals up to 500 ms. Stimulation of either baroreceptors ( n=31) or chemoreceptors ( n=36) failed to elicit a synaptic response in all sub-diaphragmatic vagus nerve-driven solitary tract neurons. Neurobiotin-labelled solitary tract neurons ( n=10) were from both latency groups and were located medial to the solitary tract at the level of area postrema, −0.3 mm to +1 mm from the obex. One cell was located in commissural subnucleus at midline, seven cells dorsal to the tractus solitarius and three ventral and medial to it. Soma sizes were 23±9.6×14±4.9 μm (range: 50×16 μm to 15×7 μm). The number of primary dendrites varied from three to five, secondary from one to eight and tertiary zero to four. Labelled axons were found in seven cells which ramified extensively in the solitary tract nucleus ( n=3) and/or branched extensively in the dorsal vagal motonucleus ( n=3) and/or projected towards the ventrolateral medulla ( n=3). We conclude that solitary tract neurons receiving signals from the sub-diaphragmatic vagus nerves (most likely from gastrointestinal tract structures) appear to be a distinct pool of neurons. There was a heterogeneity in terms of both their ongoing activity and projection targets but despite this, there were three consistent properties. First, sub-diaphragmatic vagus nerve evoked predominantly excitatory synaptic responses in solitary tract neurons; second, neurons exhibited lasting paired pulse depression following activation of sub-diaphragmatic vagus nerves; and third, sub-diaphragmatic vagus nerve-driven solitary tract neurons were not responsive to either baroreceptor or chemoreceptor stimulation.

Behavior-related changes in the activity of substantia nigra pars reticulata neurons in freely moving rats

As one of the primary targets of the striatum, the substantia nigra pars reticulata (SNr) has been hypothesized to play a role in normal motor behavior. Specifically, inhibition of usually high, tonic SNr output is predicted to correlate with motor activation. While support for this has come primarily from electrophysiological studies in primates performing goal-directed movements, we tested this hypothesis in rats behaving in an open-field arena. SNr single-unit activity was recorded during spontaneous bouts of open-field behavior (e.g., head and body movements, locomotion) and after rats were given d-amphetamine (1.0 mg/kg, s.c.), which reliably increases motor activity and elevates the firing of motor-related striatal neurons. Prior to drug administration, SNr neurons had either regular, slightly irregular or irregular firing patterns when animals rested quietly. During movement, some inhibitions were observed, but the majority (∼79%) of analyzed units increased firing by as much as 38%. Regardless of the predrug behavioral response of the cell, amphetamine strongly inhibited firing rate (∼90% below nonmovement baseline) and changed firing pattern such that all cells fired irregularly. Subsequent injection with the dopamine antagonist haloperidol (1.0 mg/kg, s.c.) reversed amphetamine-induced inhibitions in all tested cells, which supports a role for dopamine in this effect. These results suggest that the pattern of striatal activity established by amphetamine, which may be critical for determining the drug-induced behavioral pattern, is represented in the SNr regardless of the predrug behavioral response of the cell.

Spontaneous Activity of Neostriatal Cholinergic InterneuronsIn Vitro

Neostriatal cholinergic interneurons fire irregularly but tonically in vivo. The summation of relatively few depolarizing potentials and their temporal sequence are thought to underlie spike triggering and the irregularity of action potential timing, respectively. In these experiments we used whole-cell, cell-attached, and extracellular recording techniques to investigate the role of spontaneous synaptic inputs in the generation and patterning of action potentials in cholinergic interneurons in vitro. Cholinergic cells were spontaneously active in vitro at 25 +/- 1 degrees C during whole-cell recording from 2 to 3 week postnatal slices and at 35 +/- 2 degrees C during cell-attached and extracellular recording from 3 to 4 week postnatal slices. A range of firing frequencies and patterns was observed including regular, irregular, and burst firing. Blockade of AMPA and NMDA receptors altered neither the firing rate nor the pattern, and accordingly, voltage-clamp data revealed a very low incidence of spontaneous EPSCs. GABAA receptor antagonists were also ineffective in altering the spiking frequency or pattern owing to minimal inhibitory input in vitro. Functional excitatory and inhibitory inputs to cholinergic cells were disclosed after application of 4-aminopyridine (100 microM), indicating that these synapses are present but not active in vitro. Blockade of D1 or D2 dopamine receptors or muscarinic receptors also failed to influence tonic activity in cholinergic cells. Together these data indicate that cholinergic interneurons are endogenously active and generate action potentials in the absence of any synaptic input. Interspike interval histograms and autocorrelograms generated from unit recordings of cholinergic cells in vitro were indistinguishable from those of tonically active neurons recorded in vivo. Irregular spiking is therefore embedded in the mechanism responsible for endogenous activity.

Using realistic models to study synaptic integration in cerebellar Purkinje cells.

This review presents an approach to modeling which we call "experiments in computo". The use of realistic models makes it possible to generate new predictions that can be confirmed experimentally. Several examples are given of how this approach has improved our understanding of synaptic integration by the Purkinje cell active dendrite. The computer model was constructed to replicate neuronal behavior which has no direct relevance to synaptic integration: it was tuned to reproduce the response of Purkinje cells to current injection in vitro, which consists of a high frequency, regular rhythm of somatic Na+ spikes, interrupted by spontaneous dendritic Ca2+ spikes. The in vivo firing behavior of Purkinje cells is quite different as it consists of highly irregular simple spike firing only, without spontaneous dendritic Ca2+ spikes. The computer model predicted-that the Purkinje cell needs to receive a continuous background inhibitory synaptic drive in addition to the excitation by parallel fibers to obtain this typical in vivo firing. This prediction was confirmed by blocking inhibition during in vivo intracellular recordings. More recently, we demonstrated that the net inhibitory drive to the Purkinje cell dendrite has to be larger than the excitatory synaptic drive. Inhibition hyperpolarizes the dendrite compared to the soma, making it act as a current sink during most of the spiking cycle. These predictions have been confirmed with the dynamic clamp method in the cerebellar slice preparation. Synchronous focal excitatory input by parallel fiber leads in the model to activation of voltage-gated Ca2+ channels which amplify the somatic response by a variable amount. The variability of this graded amplification is due both to position of the input, effectively canceling the cable attenuation, and to the effect of preceding background input. Differences between the model and experimental results in this aspect can be explained by the relative hyperpolarized state of Purkinje cells in the in vitro experimental preparation. These studies led to a new theory about the function of long-term depression in the cerebellum which can explain recent experimental results. In conclusion, our modeling approach generated predictions which contradicted prevalent ideas on how the cerebellum, or neurons in general, works and led to experiments which otherwise would not have been carried out.

Distinct responses of osphradial neurons to chemical stimuli and neurotransmitters in Lymnaea stagnalis L.

1. In Lymnaea stagnalis L. (Pulmonata, Basommatophora) the neurons in the osphradium were visualized by staining through the inner right parietal nerve by 5,6-carboxyfluorescein (5,6-CF). Three types of neurons were identified: three large ganglionic cells (GC1-3; 80-100 microns), the small putative sensory neurons (SC; 20 microns) and very small sensory cells (3-5 microns). 2. The ganglionic and putative sensory neurons were investigated by whole cell patch-clamp method in current-clamp condition. The three giant ganglionic neurons (GC1-3) located closely to the root of osphradial nerve, had a membrane potential (MP) between -30 and -70 mV and showed tonic or bursting activities. The small putative sensory cells (SCs) scattered throughout the osphradial ganglion, possessed a MP between -25 and -55 mV and showed an irregular firing pattern with membrane oscillations. At resting MP the GC1-3 cells were depolarized and increased the frequency of their firing, while the SCs were hyperpolarized and inhibited by NaCl (10(-2) M) and L-aspartate (10(-5) M) applied to the osphradium. 3. 5-Hydroxytryptamine (5HT, 10(-6) M), gamma-aminobutyric acid (GABA; 10(-6) M) and the GABAB agonist baclofen (10(-6) M) depolarized the neurons GC1-3 and increased their firing frequency. In contrast, on the GC1-3 neurons, acetylcholine (Ach; 10(-6) M) and FMRFamide (10(-6) M) caused hyperpolarization and cessation of the firing activity. The 5HT effect was blocked by mianserin (10(-6) M) but picrotoxin (10(-5) M) failed to block the GABA-induced effect on the GC1-3 cells. 4. The small putative sensory neurons (SCs) were excited by Ach (10(-6) M) and 5HT (10(-6) M) but were inhibited by GABA (10(-6) M). FMRFamide (10(-6) M) had a biphasic response. The Ach effect was blocked by hexamethonium (10(-6) M) and tetraethylammonium (10(-6) M), indicating the involvement of nicotinic cholinergic receptors. 5. The distinct responses of the two populations of osphradial neurons to chemical stimuli and neurotransmitters suggest that they can differently perceive signals from environment and hemolymph.

Neurophysiological identification of the subthalamic nucleus in surgery for Parkinson's disease.

Microelectrode recording methods for stereotactic localization of the subthalamic nucleus (STN) and surrounding structures are described. These methods accurately define targets for chronic deep brain stimulation in the treatment of Parkinson's disease. Mean firing rates and a burst index were determined for all recorded neurons, and responses to active and passive limb and orofacial movements were tested. STN neurons had a mean firing rate of 37+/-17 Hz (n = 248) and an irregular firing pattern (median burst index, 3.3). Movement-related activity and tremor cells were identified in the STN. Ventral to the STN, substantia nigra pars reticulata neurons had a mean rate of 71+/-23 Hz (n = 56) and a more regular firing pattern (median burst index, 1.7). Short trains (1-2 seconds) of electrical microstimulation of STN could produce tremor arrest but were not found to be useful for localization. Compared with data from normal monkeys our findings suggest that STN neuronal activity is elevated in Parkinson's disease.