Short Trains Of Action Potentials Research Articles

Do canonical transient receptor potential channels mediate cholinergic excitation of cortical pyramidal neurons?

Activation of M1-type muscarinic acetylcholine receptors excites neocortical pyramidal neurons, in part by gating a nonselective cation conductance that produces calcium-dependent 'afterdepolarizing potentials' (ADPs) following short trains of action potentials. Although the identity of the cation conductance mediating the ADP is not known, previous work has implicated canonical transient receptor potential (TRPC) channels, specifically the TRPC5 and TRPC6 subtypes. Using pharmacological and genetic approaches, we tested the role of TRPC channels in generating cholinergic ADPs in layer 5 pyramidal neurons in the mouse medial prefrontal cortex (mPFC). A variety of compounds that block TRPC channels, including 2-aminoethoxydiphenyl borate, flufenamic acid, lanthanum, SKF-96365, and Pyr-3, had little, if any, impact on cholinergic ADPs. Similarly, genetic deletion of several TRPC subunits, including TPRC1, TRPC5, and TRPC6 (single knockouts), or both TRPC5 and TRPC6 together (double knockout), failed to reduce the amplitude of cholinergic ADPs. These data suggest that TRPC5 and TRPC6 subunits are not required for cholinergic excitation of layer 5 pyramidal neurons in the mouse mPFC and that the focus of future work should be expanded to test the involvement of other potential ionic effectors.

Organization and function of transmitter release sites at the neuromuscular junction.

The neuromuscular junction is known as a strong and reliable synapse. It is strong because it releases an excess of chemical transmitter, beyond what is required to bring the postsynaptic muscle cell to threshold. Because the synapse can sustain suprathreshold muscle activation during short trains of action potentials, it is also reliable. The presynaptic mechanisms that lead to reliability during short trains of activity have only recently been elucidated. It appears that there are relatively few calcium channels in individual active zones, that channels open with a low probability during action potential stimulation and that even if channels open the resulting calcium flux only rarely triggers vesicle fusion. Thus, each synaptic vesicle may only associate with a small number of calcium channels, forming an unreliable single vesicle release site. Strength and reliability of the neuromuscular junction emerge as a result of its assembly from thousands of these unreliable single vesicle release sites. Hence, these synapses are strong while at the same time only releasing a small subset of available docked vesicles during each action potential, thus conserving transmitter release resources. This prevents significant depression during short trains of action potential activity and confers reliability.

Clustering of excess growth resources within leading growth cones underlies the recurrent “deposition” of varicosities along developing neurites

Varicosities (VRs) are ubiquitous neuronal structures that are considered to serve as presynaptic structures. The mechanisms of their assembly are unknown. Using cultured Aplysia neurons, we found that in the absence of postsynaptic targets, VRs form at the leading edge of extending neurites when anterogradely transported organelles accumulate within the palm of the growth cone (GC) at a rate that exceeds their utilization by the GC machinery. The aggregation of excess organelles at the palm of the GC leads to slowdown of the GC's advance. As the size of the organelle clusters increases, the rate of organelle sequestration diminishes and the supply of building blocks to the GC resumes. The GCs' advance is re-initiated, “leaving behind” an organelle-loaded nascent VR. These mechanisms account for the recurrent “deposition” of almost equally spaced VRs by advancing GCs. Consistent with the view that VRs serve as “ready-to-go” presynaptic terminals, we found that a short train of action potentials leads to exocytosis of labeled vesicles within the varicosities. We propose that the formation and spacing of VRs by advancing GCs is the default outcome of the balance between the rate of supply of growth-supporting resources and the usage of these resources by the GC's machinery at the leading edges of specific neurites.

Bridging scales: from cortical single-neuron bursting to macroscopic high-frequency EEG

One of the challenges of neuroscience is to understand the relation between multiple levels of neuronal organization: from ion channels to single cells to local neuronal networks to the entire brain. In particular, the relation between single neurons and population activity has recently gained much interest. Combining experimental and modeling work, we investigate here how these two levels are related in a phenomenon called 'population bursting'. This term refers to short trains of action potentials that are synchronous over a large population of neurons and thus rendered visible in local and even far field potentials (LFP, EEG). Such responses were identified in many brain areas, among others in hippocampus and cortex. Here, we focus on bursts elicited by neurons in primary somatosensory cortex after peripheral nerve stimulation. We show that these single-unit bursts are closely related to high-frequency EEG responses (hf-EEG >500 Hz) recorded from scalp or dura in both, human subjects and non-human primates [1,2]. In order to understand the mechanisms of burst generation, we developed a simple single-neuron model that reproduces burst activity and provides a means for estimating the contribution of a single neuron to the macroscopic EEG. The model consists of a linear neuron with spiking threshold (leaky integrate-and-fire) that receives conductance-based inputs through depressing synapses (short-term synaptic depression). The inputs to the neuron are provided by a population of Poisson neurons all firing at a constant rate. We fitted the model to spike trains recorded in the somatosensory cortex of non-human primates and found that the subthreshold potential produced by the neuron resembles the time-course of evoked hf-EEG oscillations (Figure 1). This suggests that the hf-EEG might give access to the microscopic properties of neural systems, such as synaptic and neuronal time constants, thus bridging the gap between single-neuron dynamics and macroscopic population response.

Electrical activity and exocytotic correlates of biphasic insulin secretion from β-cells of canine islets of Langerhans: Contribution of tuning two modes of Ca2+ entry-dependent exocytosis to two modes of glucose-induced electrical activity

Biphasic insulin secretion in response to glucose, consisting of a transient first phase followed by a progressive second phase, is a well described in pancreatic islets. Using single canine β-cells we have compared the time courses of electrical activity and insulin granule exocytosis to biphasic insulin secretion. Short trains of action potentials, similar those found during first phase insulin secretion, trigger phasic exocytosis from a small pool of insulin granules, likely an immediately releasable pool docked near voltage activated Ca2+ channels. In contrast, plateau depolarizations to between −35 and −20 mV resembling those during second phase insulin secretion, trigger tonic exocytosis from a larger pool of insulin granules, likely a highly Ca2+-sensitive pool farther from Ca2+ channels. Both phasic and tonic modes of exocytosis are enhanced by glucose, via its metabolism. Hence, in canine β-cells two distinct components of exocytosis, tuned to two components of electrical activity, may contribute significantly to biphasic insulin secretion.

Hippocalcin signaling via site-specific translocation in hippocampal neurons

Hippocalcin is a Ca2+-binding protein, which belongs to the family of neuronal Ca2+ sensors. It is highly expressed in the hippocampus but molecular mechanisms underlying its action in this part of the brain have not been investigated in detail. To study whether intrinsic neuronal activity could result in hippocalcin-mediated signal transduction we examined spontaneous and action potential (AP)-dependent changes in fluorescence of yellow fluorescent protein-tagged hippocalcin (HPCA-YFP) in transiently transfected hippocampal cultured neurons. In 6–12 DIV neurons HPCA-YFP spontaneously translocated longitudinally to specific sites within diffusionally confined domains of neuronal processes. The translocations to these sites were expressed as fast, reversible increases in HPCA-YFP fluorescence coincided with a decrease in adjacent sites indicating genuine protein translocation. Physiologically relevant neuronal stimulation with short trains of action potentials also resulted in fast, simultaneous, reversible, and [Ca2+]i-dependent translocations of HPCA-YFP to several sites synchronizing hippocalcin signaling in different parts of neuronal processes. The amount of translocated protein increased with the number of action potentials in a train decoding the number of APs into the amount of translocated protein. We conclude that hippocalcin may signal within diffusionally restricted domains of neuronal processes in which it might play a physiological role in Ca2+-dependent local activation of specific molecular targets.

Behavioural report of single neuron stimulation in somatosensory cortex

Understanding how neural activity in sensory cortices relates to perception is a central theme of neuroscience. Action potentials of sensory cortical neurons can be strongly correlated to properties of sensory stimuli and reflect the subjective judgements of an individual about stimuli. Microstimulation experiments have established a direct link from sensory activity to behaviour, suggesting that small neuronal populations can influence sensory decisions. However, microstimulation does not allow identification and quantification of the stimulated cellular elements. The sensory impact of individual cortical neurons therefore remains unknown. Here we show that stimulation of single neurons in somatosensory cortex affects behavioural responses in a detection task. We trained rats to respond to microstimulation of barrel cortex at low current intensities. We then initiated short trains of action potentials in single neurons by juxtacellular stimulation. Animals responded significantly more often in single-cell stimulation trials than in catch trials without stimulation. Stimulation effects varied greatly between cells, and on average in 5% of trials a response was induced. Whereas stimulation of putative excitatory neurons led to weak biases towards responding, stimulation of putative inhibitory neurons led to more variable and stronger sensory effects. Reaction times for single-cell stimulation were long and variable. Our results demonstrate that single neuron activity can cause a change in the animal's detection behaviour, suggesting a much sparser cortical code for sensations than previously anticipated.

Behavioural report of single neuron stimulation in somatosensory cortex

Understanding how neural activity in sensory cortices relates to perception is a central theme of neuroscience. Action potentials of sensory cortical neurons can be strongly correlated to properties of sensory stimuli and reflect the subjective judgements of an individual about stimuli. Microstimulation experiments have established a direct link from sensory activity to behaviour, suggesting that small neuronal populations can influence sensory decisions. However, microstimulation does not allow identification and quantification of the stimulated cellular elements. The sensory impact of individual cortical neurons therefore remains unknown. Here we show that stimulation of single neurons in somatosensory cortex affects behavioural responses in a detection task. We trained rats to respond to microstimulation of barrel cortex at low current intensities. We then initiated short trains of action potentials in single neurons by juxtacellular stimulation. Animals responded significantly more often in single-cell stimulation trials than in catch trials without stimulation. Stimulation effects varied greatly between cells, and on average in 5% of trials a response was induced. Whereas stimulation of putative excitatory neurons led to weak biases towards responding, stimulation of putative inhibitory neurons led to more variable and stronger sensory effects. Reaction times for single-cell stimulation were long and variable. Our results demonstrate that single neuron activity can cause a change in the animal's detection behaviour, suggesting a much sparser cortical code for sensations than previously anticipated.

Synaptic depression and short-term habituation are located in the sensory part of the mammalian startle pathway

BackgroundShort-term habituation of the startle response represents an elementary form of learning in mammals. The underlying mechanism is located within the primary startle pathway, presumably at sensory synapses on giant neurons in the caudal pontine reticular nucleus (PnC). Short trains of action potentials in sensory afferent fibers induce depression of synaptic responses in PnC giant neurons, a phenomenon that has been proposed to be the cellular correlate for short-term habituation. We address here the question whether both this synaptic depression and the short-term habituation of the startle response are localized at the presynaptic terminals of sensory afferents. If this is confirmed, it would imply that these processes take place prior to multimodal signal integration, rather than occurring at postsynaptic sites on PnC giant neurons that directly drive motor neurons.ResultsPatch-clamp recordings in vitro were combined with behavioral experiments; synaptic depression was specific for the input pathway stimulated and did not affect signals elicited by other sensory afferents. Concordant with this, short-term habituation of the acoustic startle response in behavioral experiments did not influence tactile startle response amplitudes and vice versa. Further electrophysiological analysis showed that the passive properties of the postsynaptic neuron were unchanged but revealed some alterations in short-term plasticity during depression. Moreover, depression was induced only by trains of presynaptic action potentials and not by single pulses. There was no evidence for transmitter receptor desensitization. In summary, the data indicates that the synaptic depression mechanism is located presynaptically.ConclusionOur electrophysiological and behavioral data strongly indicate that synaptic depression in the PnC as well as short-term habituation are located in the sensory part of the startle pathway, namely at the axon terminals of sensory afferents in the PnC. Our results further corroborate the link between synaptic depression and short-term habituation of the startle response.

Suprathreshold excitation of frog tectal neurons by short spike trains of single retinal ganglion cell.

It has been established that coincident inputs from multiple presynaptic axons are required to achieve a suprathreshold level of excitation for the most of central neurons. The present study, however, was designed to determine whether a train of spikes of an individual retinal ganglion cell (that is, input from a single presynaptic axon) targeting a frog tectum layer F could evoke suprathreshold excitation of tectal neurons. The lungs of immobilized frog were artificially ventilated during experiments. An individual ganglion cell was electrically stimulated in the retina through a multi-channel electrode. Responses evoked in the tectum by the stimulation were recorded extracellularly from a terminal arborization of the retinotectal fiber using the carbon-fiber microelectrode. Negative and negative-positive spikes (referred to as first type population responses) and polyphasic spikes followed by excitatory synaptic potentials (referred to as second type population responses) were observed in the recordings of retinotectal activity. Usually, the population responses have ensued after the frequency facilitated first and/or second testing individual retinotectal synaptic potential and disappeared in a threshold manner with a reduction of retinotectal transmission by an application of kynurenic acid. These observations have suggested that the population responses were a consequence of a suprathreshold excitation of tectal neurons and, therefore, could serve as the sign for such an excitation. Recordings have also demonstrated that sources of the first type population responses (likely, the hillocks of axons or somas of postsynaptic neurons) lie deeper than the optic fiber layer F of the tectum, whereas sources of the second type population responses (likely, axon terminal arborizations of these postsynaptic neurons) are scattered throughout the optic fiber layers. The findings have suggested: 1) a short train of action potentials of an individual retinal ganglion cell (likely darkness, also known as 5th, detector) can excite tectal neurons to suprathreshold level; 2) tectal and perhaps, nucleus isthmi neurons that make up recurrent connection circuits to the optic fiber layers of the tectum are also activated; 3) a suprathreshold level for an individual retinotectal input is achieved primarily due to the frequency facilitation of synaptic potentials; and 4) an artificial ventilation of the lungs of immobilized frog favors the eliciting of a suprathreshold excitation of tectal neurons, demonstrating that the ventilation certainly improves the physiological condition of a frog.

Differences in multiple forms of short-term plasticity between excitatory and inhibitory hippocampal neurons in culture.

Synaptic transmission is highly dynamic, especially during periods of repetitive activity. This short-term synaptic plasticity, elicited by either pairs or short trains of action potentials at moderate frequencies (1-10 Hz), may give rise to either depression or facilitation of synaptic transmission. We analyzed these processes in isolated, synaptically coupled pairs of inhibitory or excitatory neurons grown in low-density cultures of hippocampal neurons. Most inhibitory and excitatory synapses in these cultures displayed paired pulse depression, although the responses of excitatory synapses were more variable and occasionally facilitation was seen. With tetanic stimuli, inhibitory synapses showed depression, but excitatory synapses showed a much richer repertoire of behaviors, including depression and facilitation. While many inhibitory synapses showed posttetanic depression following short trains of action potentials, excitatory synapses instead showed posttetanic facilitation. This facilitation is accompanied by an increase in paired pulse ratio, suggesting that it is the result of presynaptic mechanisms. Finally, excitatory synapses often displayed paired pulse and tetanic facilitation of asynchronous release, a process not seen in inhibitory synapses in these cultures. These similarities and differences in short-term plasticity exhibited by inhibitory and excitatory cells are likely to be critical for information processing and the control of neuronal excitability, under both normal and pathological conditions, such as epilepsy.

Contribution of Presynaptic Na+ Channel Inactivation to Paired-Pulse Synaptic Depression in Cultured Hippocampal Neurons

Paired-pulse depression (PPD) of synaptic transmission is important for neuronal information processing. Historically, depletion of the readily releasable pool of synaptic vesicles has been proposed as the major component of PPD. Recent results suggest, however, that other mechanisms may be involved in PPD, including inactivation of presynaptic voltage-dependent sodium channels (NaChs), which may influence coupling of action potentials to transmitter release. In hippocampal cultures, we have examined the potential role and relative contribution of presynaptic NaCh inactivation in excitatory postsynaptic current (EPSC) PPD. Based on current- and voltage-clamp recordings from somas, our data suggest that NaCh inactivation could potentially participate in PPD. Paired stimulation of somatic action potentials (20- to 100-ms interval) results in subtle changes in action potential shape that are mimicked by low concentrations of tetrodotoxin (TTX) and that appear to be generated by a combination of fast and slow recovery from NaCh inactivation. Dilute concentrations of TTX dramatically depress glutamate release. However, we find evidence for only minimal contribution of NaCh inactivation to EPSC PPD under basal conditions. Hyperpolarization of presynaptic elements to speed recovery from inactivation or increasing the driving force on Na(+) ions through active NaChs had minimal effects on PPD while more robustly reversing the effects of pharmacological NaCh blockade. On the other hand, slight depolarization of the presynaptic membrane potential, by elevating extracellular [K(+)](o), significantly increased PPD and frequency-dependent depression of EPSCs during short trains of action potentials. The results suggest that NaCh inactivation is poised to modulate EPSC amplitude with small tonic depolarizations that likely occur with physiological or pathophysiological activity.

Chemoelectrical signal transduction in olfactory sensory neurons of air-breathing vertebrates.

When odorants bind to the sensory cilia of olfactory sensory neurons, the cells respond with an electrical output signal, typically a short train of action potentials. This review describes the present state of knowledge about the olfactory signal transduction process. In the last decade, a set of transduction molecules has been identified which help to explain many aspects of the sensory response. Odor-induced second-messenger production, activation of transduction channels, the central role of the ciliary Ca2+ concentration, as well as mechanisms that mediate adaptation, are all qualitatively understood on the basis of a consistent scheme for chemoelectrical transduction. This scheme, although necessarily incomplete, can serve as a working model for further experimentation which may reveal kinetical aspects of signal transduction processes in olfactory sensory neurons.

Cholinergic stimulation enhances cytosolic calcium ion accumulation in mouse hippocampal CA1 pyramidal neurones during short action potential trains.

Acetylcholine is a regulatory cofactor for numerous activity-dependent processes of central nervous system development and plasticity in which increases in cytosolic calcium ion concentration ([Ca(2+)](cyto) couple membrane excitation to cellular changes. We examined how cholinergic receptor activation affects temporal and spatial aspects of increases in [Ca(2+)](cyto) during short trains of action potentials in hippocampal CA1 pyramidal neurones. Membrane-impermeant Ca(2+)-sensitive dye was introduced into the cytosol during whole-cell recordings, and Ca(2+)-dependent fluorescence was recorded from somatic, nuclear and proximal dendrite regions with high temporal resolution. In all neuronal compartments, the cholinergic agonist carbachol (5 microM) increased resting [Ca(2+)](cyto) and the maximum [Ca(2+)](cyto) attained during a short action potential train. Carbachol also slowed the recovery of [Ca(2+)](cyto) towards resting levels. The largest increases in peak cytosolic Ca(2+) concentration (delta [Ca(2+)](cyto) were seen in the dendrite and apical cell body, while relaxations of the carbachol-induced increase in delta [Ca(2+)](cyto) showed greater prolongation in the nucleus and basal cell body. Most significantly, the difference between Ca(2+) signals recorded before and during exposure to carbachol consistently showed a monotonic rise and smooth fall in all cell compartments, suggesting that the increase in [Ca(2+)](cyto) associated with each action potential was not altered by carbachol. Consistent with this view, changes in Ca(2+) signalling were not accompanied by changes in action potential waveforms. The effects of carbachol were partially reversed by simultaneous exposure to atropine, or partially inhibited by inclusion of heparin in the intracellular solution, indicating the involvement of muscarinic acetylcholine receptors and InsP(3)-sensitive Ca(2+)-release channels. Our data indicate that carbachol-induced slowing of [Ca(2+)]cyto relaxations after each action potential results in enhanced accumulation of Ca(2+) in the cytosol in the absence of changes in action potential-driven Ca(2+) entry. By modulating the time course of Ca(2+) signals, cholinergic stimulation may regulate the activation of Ca(2+)-dependent intracellular processes dependent on patterns of [Ca(2+)](cyto) changes.

Postnatal Development of Membrane Properties and δ Oscillations in Thalamocortical Neurons of the Cat Dorsal Lateral Geniculate Nucleus

The development of membrane properties, firing patterns, and delta oscillations in neurons of the cat dorsal lateral geniculate nucleus (dLGN) was investigated in vitro during the first 7 postnatal weeks. Compared with adult neurons, the resting membrane potential was more depolarized at postnatal days 1-9 (P1-P9), the input resistance was higher at P1-P7, and action potentials had a higher threshold and a smaller amplitude at P1-P3 and a longer duration at P1-P9. At P1-P3 trains longer than 200 msec were rarely observed, and trains with more than three action potentials were only present in 41% of the neurons, whereas at P1-P7 the normalized slope of the instantaneous frequencies at the first five interspike intervals was smaller than in the adult. A long-lasting (up to 6 sec) afterhyperpolarization followed a short train of action potentials in 88 and 30% of neurons at P1-P3 and P30-P32, respectively, but it was rarely observed in the adult. The low-threshold Ca2+ potential could evoke a burst of action potentials since P1. However, at P1-P7 the number of action potentials per burst was smaller (range, one to five), and at P1-P9 their maximum instantaneous frequency was lower (<190 Hz) than in the adult (range, six to eight, and 344 Hz, respectively). No delta oscillations were observed until P17, and their frequency (0.36 Hz) was lower than that in the adult (1.8 Hz). The percentage of neurons displaying delta oscillations and their frequency reached adult values by the end of the seventh postnatal week, i.e., well after the maturation of the membrane properties and firing patterns (second postnatal week). In conclusion, the maturation of the electrophysiological properties of thalamocortical neurons in the cat dLGN is completed later than the retinogeniculate axon segregation (Shatz CJ, 1983), and the immaturity of the oscillatory, and not of the burst-firing, activity is a limiting factor in the development of delta waves.

Synaptic input from CA3 pyramidal cells to dentate basket cells in rat hippocampus.

1. Excitatory inputs from CA3 pyramidal cells to dentate basket cells were examined using the whole-cell recording technique in neonatal (10-16 days) rat hippocampal slices to characterize this unexpected feedback pathway. 2. Minimal electrical stimulation of the CA3 pyramidal layer evoked in basket cells short latency (5.2 +/- 0.4 ms) glutamate receptor-mediated excitatory postsynaptic currents (EPSCs) with fast rise times (at -70 mV, 0.9 +/- 0.2 ms), fast decay time constants (3.6 +/- 0.6 ms), and small amplitudes (-14 +/- 3.4 pA). Minimal electrical stimulation evoked monosynaptic EPSCs in only 48 +/- 9.2% of the trials suggesting that the CA3 pyramidal cell to basket cell pathway was unreliable. 3. CA3 pyramidal cell layer stimulation did not antidromically or synaptically activate granule cells but did evoke polysynaptic IPSCs in granule cells, suggesting that the net effect of CA3 pyramidal cell firing on the dentate gyrus was granule cell inhibition. 4. Stimulation of the CA3 pyramidal cell layer evoked both monosynaptic and polysynaptic EPSCs in basket cells, which were eliminated by a knife lesion separating CA3 from the dentate gyrus. The latencies of the EPSCs evoked in 0.6 mM extracellular calcium were the same as the earliest latencies of EPSCs in 1.5 mM calcium, suggesting that those EPSCs were monosynaptic. The polysynaptic input was more prominent in the presence of 10 microM bicuculline, implying that inhibitory GABAergic circuits normally limit this feedback from CA3 to basket cells. 5. In recordings from 103 pairs of CA3 pyramidal cells and dentate basket cells from 11 slices, two polysynaptic connections were found that were active only when the presynaptic CA3 pyramidal neuron fired in bursts. No monosynaptic connections between CA3 pyramidal cells and basket cells were identified indicating that connections between the two cell types may be sparse. 6. Raising the external potassium concentration from 3.5 to 8.5 mM, which elicited burst firing in CA3 pyramidal cells, resulted in a barrage of EPSCs and action potentials in basket cells. In contrast, granule cells neither fired action potentials nor exhibited increased EPSC frequency in elevated potassium but instead received a higher frequency of bicuculline-sensitive IPSCs, consistent with interneuron firing. The CA3 pyramidal cell to basket cell monosynaptic pathway exhibited paired-pulse facilitation as manifested by an increased probability of release, which supports the idea that basket cells were better activated by short trains of action potentials than by single inputs.(ABSTRACT TRUNCATED AT 400 WORDS)

Activity of rostral trigeminal sensory neurons in the cat during wakefulness and sleep.

1. Relatively little is known about the activity of trigeminal sensory neurons during naturally occurring behavioral states of sleep and wakefulness. Accordingly, experiments were performed in chronic unanesthetized behaving cats in which neuronal activity in the rostral trigeminal sensory nuclear complex (TSNC) was recorded extracellularly in response to low-intensity stimulation of mandibular and maxillary divisions of cranial V nerve. The peripheral responses of TSNC neurons were evaluated during naturally occurring episodes of wakefulness, quiet sleep, and active sleep. 2. The location of the rostral TSNC was confirmed by recording characteristic orthodromic field potentials generated in response to afferent volleys from tooth pulp and inferior alveolar (IAN) nerve stimuli. Antidromic fields from the trigeminal (MotV) and facial (MotVII) motor pools were used to demarcate the anterior and posterior limits of the rostral TSNC (i.e., main sensory nucleus and nucleus oralis pars gamma). 3. In the absence of peripherally applied stimuli, individual rostral TSNC neurons recorded in the chronic, unanesthetized cat during the behavioral state of wakefulness did not display ongoing spike activity. 4. The response characteristics of individual TSNC neurons to low-intensity stimuli delivered to V afferents emanating from the canine tooth pulps during the behavioral state of drowsy wakefulness consisted of a short train of action potentials characterized by a short latency-to-onset (7.2 +/- 0.4 ms, mean +/- SE, n = 51). TSNC neurons fell into two categories on the basis of their response to graded intensities of tooth pulp stimuli. "Stimulus intensity-dependent" neurons demonstrated evoked responses that had a response profile that increased with stimulus intensity. In contrast, the response profile of "stimulus intensity-independent" neurons remained stable irrespective of the stimulus intensity used. 5. During episodes of wakefulness and quiet sleep, IAN-evoked orthodromic fields did not differ in their amplitude or other waveform parameters. However, during active sleep, the IAN-evoked orthodromic field potential was suppressed by an average of 28% as compared with wakefulness. 6. The number of action potentials evoked by consecutive presentation of low-intensity tooth pulp stimuli were compared during sleep and wakefulness. The evoked responses were suppressed during active sleep (29%, n = 42). Suppression observed during active sleep occurred in both ("stimulus-dependent" and "stimulus-independent") groups of TSNC neurons. During the phasic rapid-eye-movement (REM) episodes of active sleep, both the orthodromic field potentials and unitary action potentials were further suppressed or abolished. 7. The conclusion is reached that synaptic transmission through the rostral trigeminal sensory nucleus is dependent on the behavioral state of the animal.(ABSTRACT TRUNCATED AT 400 WORDS)

Whole-cell analysis of ionic currents underlying the firing pattern of neurons in the gustatory zone of the nucleus tractus solitarii.

1. Previous work from this laboratory has shown that rostral nucleus tractus solitarii (rNTS) neurons can be separated into four different classes on the basis of responses to a current injection paradigm consisting of membrane hyperpolarization immediately followed by a depolarizing pulse. These classes have been termed Group I, II, III, and IV neurons. The regular repetitive firing discharge pattern of Group I cells is changed into an irregular spike train by membrane hyperpolarization. Hyperpolarization of Group II neurons delays the firing discharge induced by depolarization. Hyperpolarization had the least effect on the discharge pattern of Group III neurons. The discharge pattern of Group IV neurons consisted of a short burst of spikes. We used whole-cell recordings and pharmacological channel blockers in an in vitro brain stem slice preparation to determine the ionic basis for the repetitive firing properties of rNTS neurons. 2. Application of 4-aminopyridine (4-AP, 1 mM) decreased input resistance and increased action potential duration in all groups of neurons. However, the discharge pattern of Group I, III, and IV neurons was either unaltered or slightly modified by 4-AP. In contrast the delay in firing of Group II cells induced by hyperpolarization was strongly reduced and in some cases completely suppressed by application of 4-AP. This suggests that a 4-AP-sensitive conductance primarily underlies the firing pattern of Group II cells. 3. Voltage-clamp recordings revealed that the delay in Group II neurons is due to a transient outward potassium current that is partially inactivated around the resting membrane potential. Hyperpolarization removed this inactivation, causing a delay in the firing of the cell. The potassium current was blocked by 4-AP. A similar current was occasionally seen in neurons of the other groups. On the basis of its voltage and pharmacological dependence this current was presumed to be an A-current (IKA). 4. Blockade of calcium currents by a low-calcium (0.5 mM) saline containing 2 mM Co2+ depressed the excitability of rNTS cells. For Group II neurons the delay in firing activity was increased. In the other groups the repetitive firing pattern was suppressed. In addition the amplitude of the afterhyperpolarization occurring after a short train of action potentials was substantially reduced. This indicates that calcium currents (ICa) and calcium-activated potassium currents (IKCa) contribute to the repetitive firing properties of rNTS neurons. 5. In about half of Group I, III, and IV neurons an additional property was found.(ABSTRACT TRUNCATED AT 400 WORDS)

Gastric vagal-evoked and greater splanchnic-evoked unitary responses in the hypothalamus

Gastric vagal and greater splanchnic nerve fibers were electrically stimulated to localize and characterize neuronal interactions in the hypothalamus of anesthetized cats. Extracellular recordings from 635 hypothalamic units were identified that responded to electrical stimulation of the left greater splanchnic nerve or gastric vagal fibers serving the proximal stomach. A total of 504 hypothalamic units in this group received input from both gastric vagal and greater splanchnic nerves. The gastric vagal-evoked hypothalamic (GVeH) and greater splanchnic-evoked hypothalamic (SeH) responses were widely distributed in the medial, paraventricular, and dorsomedial nuclei and lateral hypothalamus. The conduction velocity of the SeH response was significantly greater than the GVeH response. The latency of the SeH response showed two peaks [58 +/- 15.7 (SD) ms and 136 +/- 18.3 (SD) ms] indicating that the splanchnic input terminated on two different groups or populations of hypothalamic neurons. It also suggested that different pathways or fiber diameters in the pathway may be involved in the transmission of splanchnic input to the hypothalamus. The majority of the GVeH and SeH unitary responses were multiple spikes or short trains of action potentials. Excitatory and inhibitory responses were observed in tonically active hypothalamic units that responded to gastric vagal or greater splanchnic input. The duration of decreased excitability to gastric vagal or greater splanchnic input was significantly greater than the period of increased excitability. The condition-test paradigm was used to determine the time course of convergent gastric vagal-greater splanchnic input on single hypothalamic neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ efflux mechanisms following depolarization evoked calcium transients in cultured rat sensory neurones.

1. We have used a combination of microfluorimetry and patch-clamp techniques to investigate cytoplasmic Ca2+ ([Ca2+]i) buffering in response to physiological Ca2+ loads in neurones cultured from the dorsal root ganglia of the rat. 2. In cells loaded with Indo-1 AM and using high resistance microelectrodes to initiate and record action potentials, single action potentials were associated with a measurable rise in [Ca2+]i. Short trains of action potentials evoked [Ca2+]i transients with monoexponential recovery rates with time constants of around 5 s. 3. Similar Ca2+ buffering properties were seen in cells perfused with patch-clamp pipettes in the whole-cell recording mode suggesting that the slow (seconds) Ca2+ buffering properties were not seriously perturbed by the recording technique. 4. In cells held under voltage clamp, reversal of the Na(+)-Ca2+ exchanger driving force had a small but significant effect on the rate of Ca2+ removal. 5. Increasing extracellular pH or adding vanadate (200 microM) to the internal solution dramatically slowed the rate of recovery. Addition of calmidazolium to the pipette solution also produced a significant but much less dramatic slowing of Ca2+ efflux. 6. The results demonstrate that the activity of a plasmalemmal Ca(2+)-ATPase is important for the removal of somatic Ca2+ loads of a similar amplitude to those generated by the firing of a few action potentials.