Burst-like Firing Research Articles

Highly Activated Neuronal Firings Monitored by Implantable Microelectrode Array in the Paraventricular Thalamus of Insomnia Rats

Insomnia is a common sleep disorder around the world, which is harmful to people’s health, daily life, and work. The paraventricular thalamus (PVT) plays an essential role in the sleep–wake transition. However, high temporal-spatial resolution microdevice technology is lacking for accurate detection and regulation of deep brain nuclei. The means for analyzing sleep–wake mechanisms and treating sleep disorders are limited. To detect the relationship between the PVT and insomnia, we designed and fabricated a special microelectrode array (MEA) to record electrophysiological signals of the PVT for insomnia and control rats. Platinum nanoparticles (PtNPs) were modified onto an MEA, which caused the impedance to decrease and improved the signal-to-noise ratio. We established the model of insomnia in rats and analyzed and compared the neural signals in detail before and after insomnia. In insomnia, the spike firing rate was increased from 5.48 ± 0.28 spike/s to 7.39 ± 0.65 spike/s, and the power of local field potential (LFP) decreased in the delta frequency band and increased in the beta frequency band. Furthermore, the synchronicity between PVT neurons declined, and burst-like firing was observed. Our study found neurons of the PVT were more activated in the insomnia state than in the control state. It also provided an effective MEA to detect the deep brain signals at the cellular level, which conformed with macroscopical LFP and insomnia symptoms. These results laid the foundation for studying PVT and the sleep–wake mechanism and were also helpful for treating sleep disorders.

Kv7 and Kir6 Channels Shape the Slow AHP in Mouse Dentate Gyrus Granule Cells and Control Burst-like Firing Behavior

The slow afterhyperpolarizing potential (sAHP) can silence a neuron for hundreds of milliseconds. Thereby, the sAHP determines the discharge behavior of many types of neurons. In dentate granule cells (DGCs), serving as a filter into the hippocampal network, mostly tonic or adapting discharge properties have been described. As under standard whole-cell recording conditions the sAHP is inhibited, we reevaluated the intrinsic functional phenotype of DGCs and the conductances underlying the sAHP, using gramicidine-perforated patch-clamp technique. We found that in 97/113 (86%) of the DGCs, a burst of action potentials (APs) to excitation ended by a large sAHP, despite continued depolarization. This result suggests that burst-like firing is the default functional phenotype of DGCs and that sAHPs are important for it. Indeed, burst-like firing DGCs showed a significantly higher sAHP-current (IsAHP) amplitude compared to spike-frequency adapting cells (16/113 = 14%). The IsAHP was mediated by Kv7 and Kir6 channels by pharmacological inhibition using XE991 and tolbutamide, although heterogeneously among DGCs. The percent inhibition of IsAHP by these compounds also correlated with the AP number and AP burst length. Application of 100 µM nickel after XE991 and tolbutamide detected a third conductance contributing to burst-like firing and the sAHP, most likely mediated by T-type calcium channels. Lastly, medial perforant path-dentate gyrus long-term potentiation was amplified by XE991 and tolbutamide. In conclusion, the sAHP shapes intrinsic burst-like firing which, under physiological circumstances, could be controlled via cholinergic afferents and ATP metabolism.

Hypothermia-induced dystonia and abnormal cerebellar activity in a mouse model with a single disease-mutation in the sodium-potassium pump.

Mutations in the neuron-specific α3 isoform of the Na+/K+-ATPase are found in patients suffering from Rapid onset Dystonia Parkinsonism and Alternating Hemiplegia of Childhood, two closely related movement disorders. We show that mice harboring a heterozygous hot spot disease mutation, D801Y (α3+/D801Y), suffer abrupt hypothermia-induced dystonia identified by electromyographic recordings. Single-neuron in vivo recordings in awake α3+/D801Y mice revealed irregular firing of Purkinje cells and their synaptic targets, the deep cerebellar nuclei neurons, which was further exacerbated during dystonia and evolved into abnormal high-frequency burst-like firing. Biophysically, we show that the D-to-Y mutation abolished pump-mediated Na+/K+ exchange, but allowed the pumps to bind Na+ and become phosphorylated. These findings implicate aberrant cerebellar activity in α3 isoform-related dystonia and add to the functional understanding of the scarce and severe mutations in the α3 isoform Na+/K+-ATPase.

Cell-Autonomous Excitation of Midbrain Dopamine Neurons by Endocannabinoid-Dependent Lipid Signaling

The major endocannabinoid in the mammalian brain is the bioactive lipid 2-arachidonoylglycerol (2-AG). The best-known effects of 2-AG are mediated by G-protein-coupled cannabinoid receptors. In principle, 2-AG could modify neuronal excitability by acting directly on ion channels, but such mechanisms are poorly understood. Using a preparation of dissociated mouse midbrain dopamine neurons to isolate effects on intrinsic excitability, we found that 100nM 2-AG accelerated pacemaking and steepened the frequency-current relationship for burst-like firing. In voltage-clamp experiments, 2-AG reduced A-type potassium current (IA) through a cannabinoid receptor-independent mechanism mimicked by arachidonic acid, which has no activityon cannabinoid receptors. Activation of orexin, neurotensin, and metabotropic glutamate Gq/11-linked receptors mimicked the effects of exogenous2-AG and their actions were prevented by inhibiting the 2-AG-synthesizing enzyme diacylglycerol lipaseα. The results show that 2-AG and related lipid signaling molecules can directly tune neuronal excitability in a cell-autonomous manner by modulating IA.

Optogenetic activation of VGLUT2-expressing excitatory neurons blocks epileptic seizure-like activity in the mouse entorhinal cortex

We investigated whether an anti-epileptic effect is obtained by selectively activating excitatory neurons expressing ChR2 under the promoter for the synaptic vesicular glutamate transporter 2 (VGLUT2). VGLUT2-expressing cells were optically stimulated while local field potential and whole-cell patch-clamp recordings were performed in mouse entorhinal cortical slices perfused with the proconvulsive compound 4-aminopyridine (4-AP). In control conditions, blue light flashes directly depolarized the majority of putative glutamatergic cells, which in turn synaptically excited GABAergic interneurons. During bath perfusion with 4-AP, photostimuli triggered a fast EPSP-IPSP sequence which was often followed by tonic-clonic seizure-like activity closely resembling spontaneous ictal discharges. The GABAA-receptor antagonist gabazine blocked the progression of both light-induced and spontaneous seizures. Surprisingly, prolonged photostimuli delivered during ongoing seizures caused a robust interruption of synchronous discharges. Such break was correlated with a membrane potential depolarization block in principal cells, while putative GABAergic interneurons changed their firing activity from a burst-like to an irregular single-spike pattern. These data suggest that photostimulation of glutamatergic neurons triggers seizure-like activity only in the presence of an intact GABAergic transmission and that selectively activating the same glutamatergic cells robustly interrupts ongoing seizures by inducing a strong depolarization block, resulting in the disruption of paroxysmal burst-like firing.

Ventral Tegmental Area Dopamine Neurons Firing Model Reveals Prenatal Nicotine Induced Alterations.

The dopamine (DA) neurons found in the ventral tegmental area (VTA) are widely involved in the addiction and natural reward circuitry of the brain. Their firing patterns were shown to be important modulators of dopamine release and repetitive burst-like firing activity was highlighted as a major firing pattern of DA neurons in the VTA. In the present study we use a state space model to characterize the DA neurons firing patterns, and trace transitions of neural activity through bursting and non-bursting states. The hidden semi-Markov model (HSMM) framework, which we use, offers a statistically principled inference of bursting states and considers VTA DA firing patterns to be generated according to a Gamma process. Additionally, the explicit Gamma-based modeling of state durations allows efficient decoding of underlying neural information. Consequently, we decode and segment our single unit recordings from DA neurons in VTA according to the sequence of statistically discriminated HSMM states. The segmentation is used to study bursting state characteristics in data recorded from rats prenatally exposed to nicotine (6 mg/kg/day starting with gestational day 3) and rats from saline treated dams. Our results indicate that prenatal nicotine exposure significantly alters burst firing patterns of a subset of DA neurons in adolescent rats, suggesting nicotine exposure during gestation may induce severe effects on the neural networks involved in addiction and reward.

A Persistent Na<sup>+</sup> Current and Its Contribution to Burst-Like Firing in Ventral Tegmental Area Dopamine Neurons

The ventral tegmental area dopamine (DA VTA) neurons have the spontaneous tonic activity and an alteration of firing pattern from tonic to burst accelerates dopamine transmission more effectively in the mesoaccumbal dopaminergic system, leading to the reinforcing process of drugs of abuse such as alcohol and nicotine. In the present study, we examined whether a persistent Na+ current would contribute to burst firing in DA VTA neuronsusing nystatin-perforated recording. Tetrodotoxin (TTX) (1 μM) or riluzole (10 μM) hyperpolarized the membrane potential and stopped spontaneous firing of DA VTA neurons. In voltage-clamp analysis, a TTX and riluzole-sensitive and persistent Na+ current was activated at ?60 mV and reached maximal amplitude at ?40 mV. This persistent Na+ current was potentiated by a negative shift of the voltage of activation by eliminating Ca2+ from the extracellular solution. The Ca2+-free extracellular solution depolarized the membrane potential and increased the firing frequency of DA VTA neurons. When a continuous hyperpolarizing current was injected, the firing pattern of the DA VTA neurons transformed into burst-like firing; with average spike number of 4.9, average inter-spike interval of 221 ms, and an average plateau potential, on which the train of spikes generated, was 11 mV. The burst-like firing of DA VTA neurons was abolished by 10 μM riluzole. The concurrent blockade of both T-type Ca2+ current and small conductance Ca2+-activated K+(SK) currents by 100 μM nickel did not induce burst-like firing with or without continuous hyperpolarizing current injection in DA VTA neurons. In conclusion, increases in a persistent Na+ current that mediates a depolarizing driving force by removing extracellular Ca2+ contributes to burst-like firing in DA VTA neurons.

Computational modeling of light activated ion channels

Channelrhodpsin-2 (ChR2) is a light sensitive ion channel protein currently investigated for millisecond time scale optogenetic control of neural activity [1]. Two competing mathematical models, a 3-state and a 4-state rate transition model are currently available to mimic the ChR2 photocurrent kinetics [2]. While both models are able to capture the key temporal features of ChR2 photocurrent in response to light stimulation pulses of different intensities and durations, little is known about their efficacy to model the neural response to light stimulation protocols of various frequencies and pulse width characteristics. Moreover, it is unclear to what degree the two models can capture the photocurrent kinetics of the recently engineered ChR2 mutants, designed to allow for more precise optical control of neural activity [3]. To address these issues, we investigate a 3-state and a 4-state transition rate model to mimic the photocurrent kinetic of wild type ChR2 (ChR2wt) and a newly validated ChR2 mutant with fast photocurrent kinetics (ChRETA) consistent with the experimental measurements in Gunaydin et al. [3]. We incorporate these models into a fast spiking hippocampal interneuron model [4] in order to examine to what degree the neural response to different experimental stimulation protocols can be successfully simulated. We find that the 3-state model can qualitatively reproduce the neural activity induced by periodic short time interval (2 ms) light pulse stimulation only for low frequencies (<20Hz). The model however fails to capture the experimentally observed features of neural response to higher frequencies (80 and 200 Hz) for both variants of ChR2 investigated. The 4-state model is able to reliably mimic the neural response to light pulse stimulation of all frequencies in both ChR2 variants. Furthermore, for ChR2wt, the 4-state model but not the 3-state model is able to reproduce both the burst-like neural firing activity induced by single, brief (2ms width) light pulses and the plateau potentials observed in neurons for high frequency (200 Hz) light stimulation. Both models were able to mimic the occurrence of missed spikes in ChR2wt when prolonged (60 light pulses) light stimulation is delivered, with better fidelity provided by the 4-state model. Finally, the 4-state model better captures the neural response to Poisson and Gaussian distributed light stimulation pulses. Motivated by the overall better performance of the 4-state model, we investigate the conditions under which a characteristic specific to the 3-state model, namely the mono-exponential decay of the ChR2 photocurrent kinetics (often reported in experimental literature) can occur in the 4-state model. Using analytical methods, we show that for each variant, the photocurrent component associated with the unreported decay constant is vanishingly small, which may explain the experimental difficulty in its empirical evaluation. In summary, we have systematically analyzed the 3-state and 4-state transition rate models for ChR2 photocurrent kinetics and demonstrated that independent of the variant, the 4-state model is able to better capture both qualitative and quantitative features of photocurrent kinetics and the neural response to several different light stimulation protocols. The results suggest that the 4-state transition rate model is a suitable candidate for a universal mathematical framework to model the photocurrent kinetic of ChR2 proteins.

Position-dependent patterning of spontaneous action potentials in immature cochlear inner hair cells.

Spontaneous action potential activity is crucial for mammalian sensory system development. In the auditory system, patterned firing activity has been observed in immature spiral ganglion cells and brain-stem neurons and is likely to depend on cochlear inner hair cell (IHC) action potentials. It remains uncertain whether spiking activity is intrinsic to developing IHCs and whether it shows patterning. We found that action potentials are intrinsically generated by immature IHCs of altricial rodents and that apical IHCs exhibit bursting activity as opposed to more sustained firing in basal cells. We show that the efferent neurotransmitter ACh, by fine-tuning the IHC’s resting membrane potential (Vm), is crucial for the bursting pattern in apical cells. Endogenous extracellular ATP also contributes to the Vm of apical and basal IHCs by activating SK2 channels. We hypothesize that the difference in firing pattern along the cochlea instructs the tonotopic differentiation of IHCs and auditory pathway.

Sharpening of directional selectivity from neural output of rabbit retina

The estimation of motion direction from time varying retinal images is a fundamental task of visual systems. Neurons that selectively respond to directional visual motion are found in almost all species. In many of them already in the retina direction selective neurons signal their preferred direction of movement. Scientific evidences suggest that direction selectivity is carried from the retina to higher brain areas. Here we adopt a simple integrate-and-fire neuron model, inspired by recent work of Casti et al. (2008), to investigate how directional selectivity changes in cells postsynaptic to directional selective retinal ganglion cells (DSRGC). Our model analysis shows that directional selectivity in the postsynaptic cells increases over a wide parameter range. The degree of directional selectivity positively correlates with the probability of burst-like firing of presynaptic DSRGCs. Postsynaptic potentials summation and spike threshold act together as a temporal filter upon the input spike train. Prior to the intricacy of neural circuitry between retina and higher brain areas, we suggest that sharpening is a straightforward result of the intrinsic spiking pattern of the DSRGCs combined with the summation of excitatory postsynaptic potentials and the spike threshold in postsynaptic neurons.

Oscillatory firing of dopamine neurons: Differences between cells in the substantia nigra and ventral tegmental area

Neuronal oscillations have been suggested to play an important role in information processing in the brain. Using spectral analysis, we have recently shown that the repetitive burst-like firing in many dopamine (DA) neurons in the ventral tegmental area (VTA) can be described as a slow oscillation (SO) in firing rate. In this study, we examined whether DA neurons in the adjacent substantia nigra (SN) also display a SO. DA neurons were recorded extracellularly using the cells/track technique in chloral hydrate-anesthetized rats. Spectral analysis showed that firing patterns of SN DA neurons exhibited a SO similar to that observed in VTA DA neurons. The amplitude of the SO, however, was much reduced in the SN compared with that in the VTA and so was the number of DA neurons qualified as high-SO cells. In high-SO DA neurons, the amplitude of the SO was strongly correlated with the degree of bursting, and this correlation was observed in both the VTA and SN. In low-SO cells, however, the SO was more significantly correlated with the variability of firing than with firing rate and bursting. Since the generation of the SO depends on afferent inputs to DA neurons, a better understanding of its difference between the SN and VTA may provide important insights into the neural networks that control DA neurons in the two areas.

Subregional changes in discharge rate, pattern, and drug sensitivity of putative GABAergic nigral neurons in the kindling model of epilepsy

The substantia nigra pars reticulata (SNr) is thought to act as a seizure-gating mechanism in kindling and other epilepsy models. We investigated whether the kindling process induces site-specific (anterior-posterior) and seizure-outlasting alterations in the activity of putative GABAergic SNr neurons and in their response to pharmacological manipulation. Female Wistar rats were kindled via the basolateral amygdala by daily stimulation. In vivo extracellular single unit recordings of SNr neurons were performed in kindled rats 1 day after a generalized seizure in order to examine activity changes that outlast the kindled seizures. Sham-kindled and naive rats served as controls. We found a significant and seizure-outlasting increase of discharge rates within the posterior but not within the anterior SNr of kindled rats when compared to controls. Furthermore, kindling resulted in seizure-outlasting burst-like firing pattern of SNr neurons. The antiepileptic drug valproic acid (VPA; 100 mg/kg i.v.) significantly reduced SNr discharge rates in all animal groups. Interestingly, neurons located in the anterior SNr of kindled rats were significantly less depressed by VPA compared to the reduction obtained in naive controls. The present data disclose kindling induced functional plasticity within basal ganglia regions. The findings are relevant for a better understanding of the mechanisms underlying the seizure-gating function of the SNr and might provide new targets for rational therapeutic manipulations, which aim to establish a remote control of epileptic seizures.

Altered spontaneous discharge rate and pattern of basal ganglia output neurons in the circling ( ci2) rat mutant

The circling rat is an autosomal recessive mutant (homozygous ci2/ ci2) characterized by lateralized rotational behavior, locomotor hyperactivity, ataxia, stereotypic head movements, and deafness. Previous neurochemical investigations showed that ci2 rats of both genders have a lower tissue content of dopamine in the striatum ipsilateral to the preferred direction of circling. For further evaluation as to whether this striatal imbalance has functional consequences within basal ganglia structures, the spontaneous extracellular single unit activity of GABAergic neurons located in the striatum and, downstream to the dopaminergic nigrostriatal system, the substantia nigra pars reticulata (SNr) was recorded bilaterally in anesthetized ci2 rats. Heterozygous ( ci2/+) littermates that display normal behavior, and rats from the background strain (LEW/Ztm) served as controls. No significant hemispheric imbalances in striatal discharge rate and firing pattern were evident in ci2 rats. Furthermore, there were no significant intergroup differences in striatal activity. However, the mean spontaneous discharge rate of SNr neurons was significantly increased in both brain sides, and there was a significant shift toward rhythmic burst-like firing in ci2 mutants. Again, no hemispheric differences were detected. The data substantiate previous findings of altered basal ganglia function in ci2 rats. The abnormal basal ganglia output activity, i.e. of the SNr, is likely to contribute to the complex behavioral disturbances seen in ci2 rats.

Altered Discharge Pattern of Basal Ganglia Output Neurons in an Animal Model of Idiopathic Dystonia

A decreased activity of basal ganglia output neurons is thought to underlie idiopathic dystonias and other hyperkinetic movement disorders. We found recently a reduced spontaneous discharge rate of entopeduncular neurons (internal globus pallidus in primates) in<i>dt</i>^<i>sz</i> hamsters, an unique model for idiopathic paroxysmal dystonia in which stress-inducible attacks show an age-dependent severity. Otherwise, it has been suggested that an altered discharge pattern may be more important for the occurrence of dystonia than a reduced discharge rate. Based on qualitative and computerized quantitative evaluations of interspike interval histograms and spike trains of extracellularly recorded single neurons, we investigated the spontaneous discharge pattern of GABAergic entopeduncular and nigral neurons in<i>dt^sz</i> hamsters at different ages. The discharge pattern of entopeduncular neurons was highly irregular and showed an altered burst-like firing in<i>dt^sz</i> hamsters at the age of the most marked expression of dystonia when compared with age-matched nondystonic controls. In line with a recently reported normalization of discharge rates after age-dependent disappearance of dystonia, we found an almost complete normalization of the discharge pattern of entopeduncular neurons after remission of dystonia in<i>dt^sz</i> hamsters. Investigations of GABAergic nigral neurons, reported recently to have the same spontaneous discharge rates in dystonic and nondystonic hamsters, did not show an altered firing pattern in<i>dt^sz</i> hamsters. The present data clearly indicate the fundamental importance of an altered discharge pattern of entopeduncular neurons for the expression of paroxysmal dystonia, and probably also for other dyskinesias, and may explain the improvements obtained by pallidotomy in dystonic patients despite an obviously reduced pallidal output.

Altered discharge pattern of basal ganglia output neurons in an animal model of idiopathic dystonia.

A decreased activity of basal ganglia output neurons is thought to underlie idiopathic dystonias and other hyperkinetic movement disorders. We found recently a reduced spontaneous discharge rate of entopeduncular neurons (internal globus pallidus in primates) in dt(sz) hamsters, an unique model for idiopathic paroxysmal dystonia in which stress-inducible attacks show an age-dependent severity. Otherwise, it has been suggested that an altered discharge pattern may be more important for the occurrence of dystonia than a reduced discharge rate. Based on qualitative and computerized quantitative evaluations of interspike interval histograms and spike trains of extracellularly recorded single neurons, we investigated the spontaneous discharge pattern of GABAergic entopeduncular and nigral neurons in dt(sz) hamsters at different ages. The discharge pattern of entopeduncular neurons was highly irregular and showed an altered burst-like firing in dt(sz) hamsters at the age of the most marked expression of dystonia when compared with age-matched nondystonic controls. In line with a recently reported normalization of discharge rates after age-dependent disappearance of dystonia, we found an almost complete normalization of the discharge pattern of entopeduncular neurons after remission of dystonia in dt(sz) hamsters. Investigations of GABAergic nigral neurons, reported recently to have the same spontaneous discharge rates in dystonic and nondystonic hamsters, did not show an altered firing pattern in dt(sz) hamsters. The present data clearly indicate the fundamental importance of an altered discharge pattern of entopeduncular neurons for the expression of paroxysmal dystonia, and probably also for other dyskinesias, and may explain the improvements obtained by pallidotomy in dystonic patients despite an obviously reduced pallidal output.

Voltage-sensitive calcium currents and their role in regulating phrenic motoneuron electrical excitability during the perinatal period.

This study examined the ontogeny of voltage-sensitive calcium conductances in rat phrenic motoneurons (PMNs) and their role in regulating electrical excitability during the perinatal period. Specifically, we studied the period spanning from embryonic day (E)16 through postnatal day (P)1, when PMNs undergo fundamental transformation in their morphology, passive properties, ionic channel composition, synaptic inputs, and electrical excitability. Low voltage-activated (LVA) and high voltage-activated (HVA) conductances were measured using whole cell patch recordings utilizing a cervical slice-phrenic nerve preparation from perinatal rats. Changes between E16 and P0-1 included the following: an approximately 2-fold increase in the density of total calcium conductances, an approximately 2-fold decrease in the density of LVA calcium conductances, and an approximately 3-fold increase in the density of HVA conductances. The elevated expression of T-type calcium channels during the embryonic period lengthened the action potential and enhanced electrical excitability as evidenced by a hyperpolarization-evoked rebound depolarization. The reduction of LVA current density coupled to the presence of a hyperpolarizing outward A-type potassium current had a critical effect in diminishing the rebound depolarization in neonatal PMNs. The increase in HVA current density was concomitant with the emergence of a calcium-dependent "hump-like" afterdepolarization (ADP) and burst-like firing. Neonatal PMNs develop a prominent medium-duration afterhyperpolarization (mAHP) as the result of coupling between N-type calcium channels and small conductance, calcium-activated potassium channels. These data demonstrate that changes in calcium channel expression contribute to the maturation of PMN electrophysiological properties during the time from the commencement of fetal inspiratory drive to the onset of continuous breathing at birth.

Dendritic modulation of burst-like firing in sensory neurons.

This report describes the variability of spontaneous firing characteristics of sensory neurons, electrosensory lateral line lobe (ELL) pyramidal cells, within the electrosensory lateral line lobe of weakly electric fish in vivo. We show that these cells' spontaneous firing frequency, measures of spike train regularity (interspike interval coefficient of variation), and the tendency of these cells to produce bursts of action potentials are correlated with the size of the cell's apical dendritic arbor. We also show that bursting behavior may be influenced or controlled by descending inputs from higher centers that provide excitatory and inhibitory inputs to the pyramidal cells' apical dendrites. Pyramidal cells were classified as "bursty" or "nonbursty" according to whether or not spike trains deviated significantly from the expected properties of random (Poisson) spike trains of the same average firing frequency, and, in the case of bursty cells, the maximum within-burst interspike interval characteristic of bursts was determined. Each cell's probability of producing bursts above the level expected for a Poisson spike train was determined and related to spontaneous firing frequency and dendritic morphology. Pyramidal cells with large apical dendritic arbors have lower rates of spontaneous activity and higher probabilities of producing bursts above the expected level, while cells with smaller apical dendrites fire at higher frequencies and are less bursty. The effect of blocking non-N-methyl-D-aspartate (non-NMDA) glutamatergic synaptic inputs to the apical dendrites of these cells, and to local inhibitory interneurons, significantly reduced the spontaneous occurrence of spike bursts and intracellular injection of hyperpolarizing current mimicked this effect. The results suggest that bursty firing of ELL pyramidal cells may be under descending control allowing activity in electrosensory feedback pathways to influence the firing properties of sensory neurons early in the processing hierarchy.

Discrete local application of corticotropin-releasing factor increases locus coeruleus discharge and extracellular norepinephrine in rat hippocampus

The most prominent afferents impinging upon the noradrenergic neurons of the locus coeruleus (LC) utilize GABA and glutamate. However, peptide neurotransmitters such as galanin, neuropeptide Y, and corticotropin-releasing factor (CRF) have also been localized to LC afferents. The evidence for CRF modulation of LC activity was examined in the present studies. Specifically, the impact of local CRF administration on both LC-NE discharge characteristics and release of norepinephrine (NE) in hippocampus was determined. First, the ability of CRF microinfused into the LC area to increase NE efflux in the dorsal hippocampus was determined using in vivo microdialysis techniques in awake rats. CRF into the LC dose-dependently increased extracellular NE in the ipsilateral hippocampus. Second, a more detailed analysis was performed in halothane-anesthetized rats by characterizing the electrophysiological activity of LC-NE neurons in response to local application of CRF. Changes in the firing rate and pattern of single LC-NE neurons were measured while simultaneously monitoring the extracellular level of NE in hippocampus. A dose of 30 ng CRF applied directly into LC via pressure ejection elicited an 88% increase in the discharge rate of LC-NE neurons and increased the incidence of burst firing from 14% to 33%. This manipulation simultaneously increased extracellular NE in hippocampus by 63%. The CRF-induced increases in discharge rate of LC-NE neurons and extracellular NE efflux in hippocampus were prevented by prior i.c.v. administration of the CRF antagonist, d-PheCRF(12-41 )(3 microg / 3 microl). The present findings demonstrate that CRF applied directly into the LC increases both the activity of LC-NE neurons and the release of NE in an LC terminal region. The shift in activity of LC-NE neurons to more burst-like firing in response to CRF may provide a means for enhanced release of NE in LC projection fields. This is the first report to demonstrate a dose-dependent increase in extracellular NE levels evoked by intra-LC infusion of CRF in unanesthetized animals.

Relations between cortical and thalamic cellular activities during absence seizures in rats.

In a rat model of generalized absence epilepsies (Genetic Absence Epilepsy Rats from Strasbourg, GAERS), multiunit activity was recorded simultaneously at different sites of the thalamocortical system under neurolept anaesthesia (fentanyl-droperidol). Under these conditions, bilaterally synchronized spike-and-wave-discharges (SWDs) occurred spontaneously on the electroencephalogram (EEG) that were in principle identical to those reported earlier from unanaesthetized preparations. The generation of SWDs on the EEG was associated with spike-concurrent, rhythmic burst-like activity in (mono-)synaptically connected regions of specific (somatosensory) thalamic regions and layers IVN of the somatosensory cortex, and the reticular thalamic nucleus. Precursor activity was typically recorded in cortical units, concomitant with 'embryonic' SW seizures on the EEG, before the paroxysm was evident on the gross EEG and in the thalamus. On average, SWD-correlated activity in layers IVN of the somatosensory cortex started significantly earlier than correlated burst-like firing in reticular and in ventrobasal thalamic neurons. Cellular peak firing in thalamus and cortex during bilaterally synchronized SWDs was related to the spike component on the gross EEG with the temporal rank order ventroposteromedial > ventrolateral > or = ventroposterolateral thalamic > > rostral reticular thalamic nuclei > or = cortex (layers IVN) = caudal reticular thalamic nucleus. A spike-related depression and wave-related increase in firing was recorded in anteroventral ventrolateral thalamic areas, presumably reflecting their peculiar anatomical arrangement within the thalamus. These results from an in vivo preparation with intact synaptic connections that spontaneously produces SWDs indicate that SWDs spread within the thalamocortical network, involving short and long delays. The order of concurrent rhythmic firing observed in thalamocortical circuits during SW seizures are supportive of the hypothesis that the processes of rhythmogenesis recruit local thalamic networks, while cortical mechanisms appear to synchronize rhythmic activities on a larger spatiotemporal scale, thereby providing an important contribution to the generalization of epileptiform activity and expression of SWDs on the EEG.

Electrophysiological and morphological heterogeneity of neurons in slices of rat suprachiasmatic nucleus

1. Whole cell patch clamp recordings of neurons in slices of the suprachiasmatic nucleus (SCN) were made in order to assess their electrophysiological and morphological heterogeneity. This assessment was accomplished by (i) quantification of intrinsic membrane properties recorded in current clamp mode, (ii) studying frequency distributions of these properties, (iii) grouping of cells based on visual inspection of data records, and (iv) use of cluster analysis methods. 2. Marked heterogeneity was found in the resting membrane potential, input resistance, time constant, rate of frequency adaptation, size of rebound depolarization (low-threshold Ca2+ potential) and regularity of firing. The frequency distribution of these membrane properties deviated significantly from a normal distribution. Other parameters, including spike amplitude and width, amplitude and rising slope of the spike after-hyperpolarization (AHP) and amplitude of the spike train AHP, showed considerable variability as well but generally obeyed a normal distribution. 3. Visual inspection of the data led to partitioning of cells into three clusters, viz. cluster I characterized by monophasic spike AHPs and irregular firing in the frequency range from 1.5 to 5.0 Hz; cluster II with biphasic spike AHPs and regular firing in the same range; and cluster III with large rebound depolarizations and biphasic spike AHPs. In a post hoc analysis, these clusters also appeared to differ in other membrane properties. This grouping was confirmed by hierarchical tree clustering and multidimensional scaling. 4. The light microscopic properties of recorded neurons were studied by biocytin labelling. Neurons had monopolar, bipolar or multipolar branching patterns and were often varicose. Axons sometimes originated from distal dendritic segments and usually branched into multiple collaterals. Many cells with extra-SCN projections also possessed intranuclear axon collaterals. We found no morphological differences between clusters except that cluster III neurons possessed more axon collaterals than cluster I or II cells. 5. These results suggest that SCN neurons are heterogeneous in some basic as well as active membrane properties and can be partitioned into at least three clusters. Cluster I and II cells fire spontaneously in a regular and irregular mode, respectively, and sustain prolonged spike trains. In contrast, cluster III cells have low firing rates but may adopt a burst-like firing mode when receiving appropriate input. While all clusters transmit output to target cells within and outside SCN, cluster III cells in particular are suggested to affect excitability of large numbers of SCN neurons by their extensive local network of axon collaterals.