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PubMed1990

The membrane properties and firing characteristics of rat jaw-elevator motoneurones.

1. We have determined the membrane and firing properties of fifty-six jaw-elevator motoneurones in rats that were anaesthetized with pentobarbitone, paralysed and artificially ventilated. 2. Forty-two neurones were identified as masseter motoneurones and fourteen as masseter synergist motoneurones. The membrane potentials for the sample ranged from -60 to -86 (mean = -68; S.D. = 7.3; n = 56), and spike amplitudes from 50 to 95 mV. The duration of the after-hyperpolarization following antidromic spikes in masseter motoneurones ranged from 15 to 50 ms (mean = 30; S.D. = 12.8) and their amplitudes from 1.0 to 4.5 mV (mean = 2.7; S.D. = 2.2; n = 42). 3. The mean input resistance for the total sample was 2.3 M omega (S.D. = 0.9; n = 56), membrane time constant 3.9 ms (S.D. = 0.9; n = 48) and rheobase 4.2 nA (S.D. = 2.6; n = 56). The distribution of these parameters was independent of membrane potential. We found no significant interrelationships between the membrane properties and one interpretation of this is that our sample may be drawn from a homogenous population of motoneurones. We also suggest that elevator motoneurones may have a lower Rm (specific membrane resistivity) value than cat hindlimb motoneurones because they have a similar range of input resistance values but only half the total surface area. 4. Forty-six out of forty-nine neurones fired repetitively to a depolarizing current pulse at a mean threshold of 1.6 x rheobase. Current-frequency plots were constructed for thirteen neurones and all but one showed a primary and secondary range in the firing of the first interspike interval. The mean slope in the primary range was 31 impulses s-1 nA-1 and 77 impulses s-1 nA-1 for the secondary range. The mean minimal firing frequency for steady firing was 26 impulses s-1 and, in response to an increase of stimulation, the rate increased monotonically with a slope of 11 impulses s-1 nA-1. 5. The dynamic sensitivity of twelve neurones was assessed from their response to ramp waveforms of current of constant amplitude but varying frequencies (0.2-2 Hz). Firing initially increased along a steep slope up to a frequency of between 40 and 60 impulses s-1 and then increased along a much shallower slope. Both the threshold for eliciting firing and the firing at the transition point of the two slopes remained constant with changes in ramp frequency.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗
PubMed1999

Positive feedbacks in the fire dynamic of closed canopy tropical forests

The incidence and importance of fire in the Amazon have increased substantially during the past decade, but the effects of this disturbance force are still poorly understood. The forest fire dynamics in two regions of the eastern Amazon were studied. Accidental fires have affected nearly 50 percent of the remaining forests and have caused more deforestation than has intentional clearing in recent years. Forest fires create positive feedbacks in future fire susceptibility, fuel loading, and fire intensity. Unless current land use and fire use practices are changed, fire has the potential to transform large areas of tropical forest into scrub or savanna.

Journal Article↗
PubMed2004

Threshold firing frequency-current relationships of neurons in rat somatosensory cortex: type 1 and type 2 dynamics.

Neurons and dynamical models of spike generation display two different types of threshold behavior, with steady current stimulation: type 1 [the firing frequency vs. current (f-I) relationship is continuous at threshold) and type 2 (discontinuous f-I)]. The dynamics at threshold can have profound effects on the encoding of input as spikes, the sensitivity of spike generation to input noise, and the coherence of population firing. We have examined the f-I and frequency-conductance (f-g) relationships of cells in layer 2/3 of slices of young (15-21 DIV) rat somatosensory cortex, focusing in detail on the nature of the threshold. Using white-noise stimulation, we also measured firing frequency and interspike interval variability as a function of noise amplitude. Regular-spiking (RS) pyramidal neurons show a type 1 threshold, consistent with their well-known ability to fire regularly at very low frequencies. In fast-spiking (FS) inhibitory interneurons, although regular firing is supported over a wide range of frequencies, there is a clear discontinuity in their f-I relationship at threshold (type 2), which has not previously been highlighted. FS neurons are unable to support maintained periodic firing below a critical frequency fc, in the range of 10 to 30 Hz. Very close to threshold, FS cells switch irregularly between bursts of periodic firing and subthreshold oscillations. These characteristics mean that the dynamics of RS neurons are well suited to encoding inputs into low-frequency firing rates, whereas the dynamics of FS neurons are suited to maintaining and quickly synchronizing to gamma and higher-frequency input.

Action Potentials↗
PubMed2004

Mechanism of irregular firing of suprachiasmatic nucleus neurons in rat hypothalamic slices.

The mechanisms of irregular firing of spontaneous action potentials in neurons from the rat suprachiasmatic nucleus (SCN) were studied in hypothalamic slices using cell-attached and whole cell recording. The firing pattern of spontaneous action potentials could be divided into regular and irregular, based on the interspike interval (ISI) histogram and the membrane potential trajectory between action potentials. Similar to previous studies, regular neurons had a firing rate about >3.5 Hz and irregular neurons typically fired about <3.5 Hz. The ISI of irregular-firing neurons was a linear function of the sum of inhibitory postsynaptic potentials (IPSPs) between action potentials. Bicuculline (10-30 microM) suppressed IPSPs and converted an irregular pattern to a more regular firing. Bicuculline also depolarized SCN neurons and induced bursting-like activity in some SCN neurons. Gabazine (20 microM), however, suppressed IPSPs without depolarization, and also converted irregular activity to regular firing. Thus GABAA receptor-mediated IPSPs appear responsible for irregular firing of SCN neurons in hypothalamic slices.

Action Potentials↗
PubMed2005

Dynamics of excitatory synaptic components in sustained firing at low rates.

Sustained firing is necessary for the persistent activity associated with working memory. The relative contributions of the reverberation of excitation and of the temporal dynamics of the excitatory postsynaptic potential (EPSP) to the maintenance of activity are difficult to evaluate in classical preparations. We used simplified models of synchronous excitatory networks, hippocampal autapses and pairs, to study the synaptic mechanisms underlying firing at low rates. Calcium imaging and cell attached recordings showed that these neurons spontaneously fired bursts of action potentials that lasted for seconds over a wide range of frequencies. In 2-wk-old cells, the median firing frequency was low (11 +/- 8.8 Hz), whereas in 3- to 4-wk-old cells, it decreased to a very low value (2 +/- 1.3 Hz). In both cases, we have shown that the slowest synaptic component supported firing. In 2-wk-old autapses, antagonists of N-methyl-d-aspartate receptors (NMDARs) induced rare isolated spikes showing that the NMDA component of the EPSP was essential for bursts at low frequency. In 3- to 4-wk-old neurons, the very low frequency firing was maintained without the NMDAR activation. However EGTA-AM or alpha-methyl-4-carboxyphenylglycine (MCPG) removed the very slow depolarizing component of the EPSP and prevented the sustained firing at very low rate. A metabotropic glutamate receptor (mGluR)-activated calcium sensitive conductance is therefore responsible for a very slow synaptic component associated with firing at very low rate. In addition, our observations suggested that the asynchronous release of glutamate might participate also in the recurring bursting.

Action Potentials↗
PubMed2005

Regional and laminar differences in in vivo firing patterns of primate cortical neurons.

The firing rates of cortical neurons change in time; yet, some aspects of their in vivo firing characteristics remain unchanged and are specific to individual neurons. A recent study has shown that neurons in the monkey medial motor areas can be grouped into 2 firing types, "likely random" and "quasi-regular," according to a measure of local variation of interspike intervals. In the present study, we extended this analysis to area TE of the inferior temporal cortex and addressed whether this classification applies generally to different cortical areas and whether different types of neurons show different laminar distribution. We found that area TE did consist of 2 groups of neurons with different firing characteristics, one similar to the "likely random" type in the medial motor cortical areas, and the other exhibiting a "clumpy-bursty" firing pattern unique to TE. The quasi-regular type was rarely observed in area TE. The likely random firing type of neuron was more frequently found in layers V-VI than in layers II-III, whereas the opposite was true for the clumpy-bursty firing type. These results show that neocortical areas consist of heterogeneous neurons that differ from one area to another in their basic firing characteristics. Moreover, we show that spike trains obtained from a single cortical neuron can provide a clue that helps to identify its layer localization.

Action Potentials↗
PubMed2006

Differential contributions of Shaker and Shab K+ currents to neuronal firing patterns in Drosophila.

Different K(+) currents participate in generating neuronal firing patterns. The Drosophila embryonic "giant" neuron culture system has facilitated current- and voltage-clamp recordings to correlate distinct excitability patterns with the underlying K(+) currents and to delineate the mutational effects of identified K(+) channels. Mutations of Sh and Shab K(+) channels removed part of inactivating I(A) and sustained I(K), respectively, and the remaining I(A) and I(K) revealed the properties of their counterparts, e.g., Shal and Shaw channels. Neuronal subsets displaying the delayed, tonic, adaptive, and damping spike patterns were characterized by different profiles of K(+) current voltage dependence and kinetics and by differential mutational effects. Shab channels regulated membrane repolarization and repetitive firing over hundreds of milliseconds, and Shab neurons showed a gradual decline in repolarization during current injection and their spike activities became limited to high-frequency, damping firing. In contrast, Sh channels acted on events within tens of milliseconds, and Sh mutations broadened spikes and reduced firing rates without eliminating any categories of firing patterns. However, removing both Sh and Shal I(A) by 4-aminopyridine converted the delayed to damping firing pattern, demonstrating their actions in regulating spike initiation. Specific blockade of Shab I(K) by quinidine mimicked the Shab phenotypes and converted tonic firing to a damping pattern. These conversions suggest a hierarchy of complexity in K(+) current interactions underlying different firing patterns. Different lineage-defined neuronal subsets, identifiable by employing the GAL4-UAS system, displayed different profiles of spike properties and K(+) current compositions, providing opportunities for mutational analysis in functionally specialized neurons.

Action Potentials↗
PubMed1988

Spread of synchronous firing in longitudinal slices from the CA3 region of the hippocampus.

1. Mechanisms underlying the propagation of synchronous epileptiform activity in disinhibited hippocampal slices were examined in experimental and computer simulation studies. 2. Experiments were performed with longitudinal slices of the CA3 region. Synchronous firing was initiated by stimulating stratum radiatum fibers in the presence of picrotoxin. It propagated smoothly and without decrement at velocities close to 0.15 m/s over distances up to 10 mm. 3. In elevated extracellular calcium, neuronal firing threshold was increased and synchronous burst firing did not spread. Monophasic excitatory postsynaptic potentials (EPSPs) were recorded in cells at limited distances from a stimulus in the presence of 10 mM Ca and picrotoxin. Axonal conduction velocity, estimated from EPSP latencies, was several times faster than the spread of synchronous firing. 4. EPSPs recorded in 5-7 mM Ca and picrotoxin could consist of two components. The properties of the first component were similar to those of synaptic events recorded in 10 mM Ca. The second component was of longer latency and unlike the first component was suppressed in responses to paired stimuli at interval 50-300 ms. Recordings from cells at different distances from a stimulus suggested that the second component spread further and more slowly than the first component. 5. In computer simulations the CA3 region was represented by a spatially distributed network of 9,000 excitatory neurons and 900 inhibitory cells. Individual cells and synapses had properties based on experimental data. The effects of varying synaptic strength and connectivity on the spread of activity in the model was examined. 6. When synaptic inhibition was functional in simulations, firing was restricted to a single action potential in model cells close to the stimulus, as in experiments. Synchronous burst firing spread throughout the neuronal array when fast synaptic inhibition was absent. The velocity of propagation was slower than conduction in simulated axons when synaptic contacts made by excitatory cells were spatially limited. Propagation velocity increased with increases in the spatial extent of excitatory connectivity. 7. Increasing the threshold of neurons in a region of the model network reduced the speed at which synchronous firing spread. In experiments focal application of gamma-aminobutyric acid (GABA) elevated neuronal firing threshold and slowed the propagation of synchrony in a local region. 8. As the strength of synaptic inhibition was gradually reduced, neuronal activity spread further and faster through the simulated neuronal network.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1997

Electrophysiological properties of rat pontine nuclei neurons In vitro. I. Membrane potentials and firing patterns.

We used a new slice preparation of rat brain stem to establish the basic membrane properties of neurons in the pontine nuclei (PN). Using standard intracellular recordings, we found that pontine cells displayed a resting membrane potential of -63 +/- 6 mV (mean +/- SD), an input resistance of 53 +/- 21 MOmega, a membrane time constant of 5.3 +/- 2.4 ms and were not spontaneously active. The current-voltage relationship of most of the PN neurons showed the characteristics of inward rectification in both depolarizing and hyperpolarizing directions. A prominent feature of the firing of pontine neurons was a marked firing rate adaptation, which eventually caused the cells to cease firing. Several types of membrane conductances possibly contribute to this feature. For one, a medium and a slow type of afterhyperpolarization (AHP) control the pattern of firing. The medium AHP was partly susceptible to blockade of calcium influx, whereas it was abolished completely by blockade of potassium channels with tetraethylammonium, indicating that it is based on at least two conductances: a calcium-dependent and a calcium-independent one. The slow AHP was carried by potassium ions and could be blocked effectively by preventing calcium influx into the cell. It was present after single spikes but was strongest after a high-frequency spike train. Calcium entry into the cell was mediated by high-threshold calcium channels that were detected by the generation of calcium spikes under blockade of potassium channels. Furthermore, the early phase of the firing rate adaptation was shown to be related to the time course of a slow, tetrodotoxin (TTX)-sensitive, persistent sodium potential, which was activated already in the subthreshold range of membrane potentials. This potential was time dependent and imposed as a depolarizing "hump" with a maximum occurring in most cases between 50 and 100 ms after stimulus onset. In the suprathreshold range, it generated plateau potentials following fast spikes, if potassium channels were blocked. After the complete adaptation of the firing rate, PN neurons were observed to display irregular fluctuations of the membrane potential, which sometimes reached firing threshold thereby eliciting an irregular low-frequency spike train. As these fluctuations could be blocked with TTX, they probably are based on the persistent sodium currents. The opposing drive in hyperpolarizing direction may be provided by strong outward currents that generated a marked outward rectification in the current-voltage relationship under TTX. In conclusion, PN neurons show complex membrane properties that are reminiscent in many ways to cerebrocortical "regular firing" neurons.

Animals↗
PubMed1999

Correlated firing in rabbit retinal ganglion cells.

A ganglion cell's receptive field is defined as that region on the retinal surface in which a light stimulus will produce a response. While neighboring ganglion cells may respond to the same stimulus in a region where their receptive fields overlap, it generally has been assumed that each cell makes an independent decision about whether to fire. Recent recordings from cat and salamander retina using multiple electrodes have challenged this view of independent firing by showing that neighboring ganglion cells have an increased tendency to fire together within +/-5 ms. However, there is still uncertainty about which types of ganglion cells fire together, the mechanisms that produce coordinated spikes, and the overall function of coordinated firing. To address these issues, the responses of up to 80 rabbit retinal ganglion cells were recorded simultaneously using a multielectrode array. Of the 11 classes of rabbit ganglion cells previously identified, coordinated firing was observed in five. Plots of the spike train cross-correlation function suggested that coordinated firing occurred through two mechanisms. In the first mechanism, a spike in an interneuron diverged to produce simultaneous spikes in two ganglion cells. This mechanism predominated in four of the five classes including the ON brisk transient cells. In the second mechanism, ganglion cells appeared to activate each other reciprocally. This was the predominant pattern of correlated firing in OFF brisk transient cells. By comparing the receptive field profiles of ON and OFF brisk transient cells, a peripheral extension of the OFF brisk transient cell receptive field was identified that might be produced by lateral spike spread. Thus an individual OFF brisk transient cell can respond both to a light stimulus directed at the center of its receptive field and to stimuli that activate neighboring OFF brisk transient cells through their receptive field centers.

Action Potentials↗
PubMed1999

Rhythmically firing neostriatal neurons in monkey: activity patterns during reaction-time hand movements.

While previous studies have identified rhythmically firing neurons (RFNs) in monkey neostriatum and these rhythmic firing patterns have been shown to evolve in neostriatal tonically active neurons (TANs) after dopamine input depletion, the activity patterns of RFNs during motor behavior are still far from completely understood. We examined the single-unit activity patterns of neostriatal neurons, recorded in awake behaving monkeys during a wrist movement task, for evidence of rhythmic activity. Monkeys made ballistic wrist flexion and extension movements in response to vibrotactile cues. Animals held a steady wrist position for 0.5 to 2.0 s while awaiting the onset of the go-cues (hold period). Although the majority of neostriatal neurons (274/306) did not fire rhythmically, approximately 10% of the neurons (32/306) fired rhythmically at 10-50 Hz during the hold period. Most RFNs (28/32) showed significant activity changes during the time between go-cue presentation and movement onset (premovement activity). One-half of RFNs exhibited premovement activity that differed as a function of movement direction. Only one RFN may have responded to the delivery of a fruit juice reward. Neuronal firing was analyzed using interspike interval distributions, autocorrelations, and serial correlation techniques. These analyses showed that the activity patterns of most RFNs were consistent with an integrate-and-fire model of neuronal rhythm generation. Changes in RFN activity patterns during the premovement interval and intertrial variations in firing frequency could be explained by changes in the general level of excitatory input. These observations are consistent with the firing properties reported for neostriatal cholinergic interneurons. It has been suggested that tonically active neurons may be cholinergic interneurons and that these neurons show changes in activity related to specific aspects of behavioral paradigms, such as rewards. RFNs may constitute a special class of TANs. The results presented here suggest that RFNs may have a role in movement initiation. We speculate that RFNs may modulate the propagation of cortical oscillations via basal ganglia-thalamic-cortical loops.

Animals↗
PubMed2001

Ionic mechanisms underlying burst firing of layer III sensorimotor cortical neurons of the cat: an in vitro slice study.

We examined the ionic mechanisms underlying burst firing in layer III neurons from cat sensorimotor cortex by intracellular recording in a brain slice. Regular spiking was observed in 77.4% of 137 neurons in response to constant intracellular current pulses of 0.5- to 1-s duration. The rest of the neurons showed burst firing. An initial burst followed by regular-spike firing was seen in 71.0% of 31 bursting neurons. The rest of the bursting neurons (n = 9) exhibited repetitive bursting. In the bursting neurons, spikes comprising the burst were triggered from the afterdepolarization (ADP) of the first spike of the burst. We examined the ionic mechanisms underlying the ADP by applying channel-blocking agents. The ADP was enhanced (rather than blocked) by Ca2+ channel blockade. This enhancement of the ADP by Ca2+ channel blockade was apparent even after blockade of the afterhyperpolarization by apamin or intracellular Ca2+ chelation by EGTA. The firing rate of the regular-spiking cells was increased by apamin, intracellular EGTA or Ca2+ channel blockers. In 17.9% of the neurons examined (n = 56), these agents switched the regular-spiking pattern into a bursting one. Burst firing could not be changed to regular spiking by these agents. Four neurons that responded with a single initial burst in control solution responded with repetitive bursting after application of these agents. We conclude that the main function of Ca2+ influx in layer III neurons is to activate Ca2+-dependent K+ conductance, which prevents or limits burst firing. At a time when spike amplitude was unchanged, the ADP was blocked and the burst firing changed to regular spiking by extracellularly applied tetrodotoxin (TTX) or intracellularly applied N-(2,6-dimethylphenylcarbamoylmethyl) triethyl ammonium bromide (QX314). We concluded that a TTX- and QX314-sensitive Na+ current underlies the ADP and therefore contributes to the burst firing of layer III neurons from the cat cortex.

Action Potentials↗
PubMed2001

A comparison of the firing properties of putative excitatory and inhibitory neurons from CA1 and the entorhinal cortex.

The superficial layers of the entorhinal cortex (EC) provide the majority of the neocortical input to the hippocampus, and the deep layers of the EC receive the majority of neocortically bound hippocampal outputs. To characterize information transmission through the hippocampal and EC circuitry, we recorded simultaneously from neurons in the superficial EC, the CA1 region of hippocampus, and the deep EC while rodents ran for food reward in two environments. Spike waveform analysis allowed us to classify units as fast-spiking (FS) putative inhibitory cells or putative excitatory (PE) cells. PE and FS units' firing were often strongly correlated at short time scales, suggesting the presence a monosynaptic connection from the PE to FS units. EC PE units, unlike those found in CA1, showed little or no tendency to fire in bursts. We also found that the firing of FS and PE units from all regions was modulated by the approximately 8 Hz theta rhythm, although the firing of deep EC FS units tended to be less strongly modulated than that of the other types of units. When we examined the spatial specificity of FS units, we determined that FS units in all three regions showed low specificity. At the same time, retrospective coding, in which firing rates were related to past position, was present in FS units from all three regions and deep EC FS units often fired in a "path equivalent" manner in that they were active in physically different, but behaviorally related positions both within and across environments. Our results suggest that while the firing of FS units from CA1 and the EC show similarly low levels of position specificity, FS units from each region differ from one another in that they mirrored the associated PE units in terms of their tendency to show more complex positional firing properties like retrospective coding and path equivalence.

Action Potentials↗
PubMed1993

Sleep deprivation is a less potent agent than clomipramine in increasing firing rate in lateral septal neurons in the rat.

Acute injection of clomipramine or a 24-hour period of sleep deprivation produces an increased firing rate in lateral septal neurons of the rat. However, it is unknown whether changes in firing rate in lateral septal neurons also appear after repetitive treatments. This study explored and compared the effects of long-term clomipramine with those of sleep deprivation from 1 to 4 days in the firing rate of lateral septal neurons. The firing rate increased after both treatments; however, maximal effects occurred on different days. Clomipramine (1.25 mg/kg, i.p., twice a day) produced a stable, increased firing rate after 20 days of treatment. Twenty-four hours of total sleep deprivation produced changes in firing rate comparable to 10 days of clomipramine treatment. Changes in firing rate did not appear when animals were exposed to 12 h of sleep deprivation alternated with 12 h in housing cages. Therefore, it is concluded that 24 h of total sleep deprivation is a less effective treatment than clomipramine for producing changes in firing rate in lateral septal neurons.

Animals↗
PubMed1999

Firing properties of single muscle vasoconstrictor neurons in the sympathoexcitation associated with congestive heart failure.

BACKGROUND: Congestive heart failure (CHF) in humans is associated with a marked sympathoexcitation, including an augmented muscle sympathetic nerve activity (MSNA) in intraneural multiunit recordings. In the present study, single-unit recording was used to evaluate whether the firing properties of individual muscle vasoconstrictor neurons can reveal underlying mechanisms for this increase in MSNA. METHODS AND RESULTS: Eight patients with CHF (NYHA class II to IV; left ventricular ejection fraction, 29+/-5%, mean+/-SEM) were studied. In standard multiunit recordings, MSNA burst incidence (bursts/100 heartbeats) ranged from 65% to 100% (88+/-5%). Using selective tungsten microelectrodes, we made recordings from 16 single muscle vasoconstrictor axons. Mean unit firing probability (ie, the percentage of cardiac intervals in which a single axon fired) was 54.5+/-5.2% (range, 21 to 89%), and mean firing frequency was 0.98+/-0.22 Hz (0.14 to 3.86 Hz), both of which were higher than seen previously in healthy subjects (P<0.001). Although single neurons occasionally generated multiple spikes per sympathetic burst, such multiple firing was rare and was not different from that seen in healthy subjects. CONCLUSIONS: An increased firing frequency of individual vasoconstrictor neurons is one mechanism for the increased number of multiunit MSNA bursts at rest in CHF. The neurons discharge in more diastoles than in healthy subjects (ie, firing probability is increased), but the likelihood of discharging >1 impulse per sympathetic burst is not increased. Despite the intense multiunit activity at rest, the firing characteristics of individual vasoconstrictor axons indicate a remaining capacity for transient increases of MSNA in CHF.

Adult↗
PubMed2000

Firing rate of nucleus accumbens neurons is dopamine-dependent and reflects the timing of cocaine-seeking behavior in rats on a progressive ratio schedule of reinforcement.

The progressive ratio (PR) schedule of reinforcement is used to determine the reinforcing properties of rewards such as drugs of abuse. In this schedule, the animal is required to press a lever a progressively increasing number of times to receive a reward; the highest ratio obtained before the animal ceases responding is termed "breakpoint." We recorded neuronal spike activity from cells in the nucleus accumbens (NAc) of rats responding on a PR schedule for cocaine reinforcement. A common subtype of NAc cells demonstrated firing rates that varied according to the time between cocaine deliveries. The firing rate was inversely related to the NAc cocaine level predicted by a pharmacokinetic model. At higher response-to-reward ratios, inter-reward intervals were increased, resulting in a decrease in modeled cocaine level and a concomitant increase in firing rate over the session. The final increase in firing rate above a threshold value suggests a neural correlate of breakpoint. The effects of preadministration of dopamine D1 or D2 antagonists on the animals' behavior were similar in that both reduced breakpoint; however, each antagonist had markedly different effects on NAc cell firing. The D1 antagonist SCH23390 reduced firing rates, even at low cocaine levels, whereas the D2 antagonist eticlopride induced a rightward shift in the dose dependence of NAc cell firing relative to modeled cocaine level. Our results suggest that the firing of NAc cells reflects changes in cocaine levels and thereby contributes to the temporal spacing of self-administration and to the cessation of responding at breakpoint.

Animals↗
PubMed2002

Intrinsic firing dynamics of vestibular nucleus neurons.

Individual brainstem neurons involved in vestibular reflexes respond to identical head movements with a wide range of firing responses. This diversity of firing dynamics has been commonly assumed to arise from differences in the types of vestibular nerve inputs to vestibular nucleus neurons. In this study we show that, independent of the nature of inputs, the intrinsic membrane properties of neurons in the medial vestibular nucleus substantially influence firing response dynamics. Hyperpolarizing and depolarizing inputs evoked a markedly heterogenous range of firing responses. Strong postinhibitory rebound firing (PRF) was associated with strong firing rate adaptation (FRA) and occurred preferentially in large multipolar neurons. In response to sinusoidally modulated input current, these neurons showed a pronounced phase lead with respect to neurons lacking strong PRF and FRA. A combination of the hyperpolarization-activated H current and slow potassium currents contributed to PRF, whereas FRA was predominantly mediated by slow potassium currents. An integrate-and-fire-type model, which simulated FRA and PRF, reproduced the phase lead observed in large neurons and showed that adaptation currents were primarily responsible for variations in response phase. We conclude that the heterogeneity of firing dynamics observed in response to head movements in intact animals reflects intrinsic as well as circuit properties.

Adaptation, Physiological↗
PubMed2000

[Relationship between the sensitivity to tetraethylammonium and firing patterns of injured dorsal root ganglion neurons].

In order to study the relationship between the firing patterns of injured dorsal root ganglion (DRG) neurons of rat and their sensitivity to tetraethylammonium (TEA), spontaneous activities from single fibers of chronically compressed DRG neurons were recorded. Two patterns of spontaneous activity were found: periodic pattern and non-periodic pattern. These two types of neurons showed different sensitivity to TEA with 27.3% in periodic and 93.2% in non-periodic firing neurons responding to TEA (2 mmol/L) (P<0.01). The responses to TEA of different concentrations were greater in the non-periodic firing neurons than in the periodic firing ones (P<0.01). The responses to TEA became apparent in 5 neurons after their periodic firing pattern was transformed into non-periodic firing pattern through an increase in Ca(2+). The non-periodic interburst interval of the bursting discharge decreased significantly while the periodic burst interspike interval had no response to TEA. The unstable period orbits were found in bursting discharge, indicating deterministic chaos during bursting discharge. It is concluded that non-periodic firing neurons with chaos are more sensitive to TEA than periodic firing neurons.

Animals↗