Initial Segment Spikes Research Articles

Developmental changes in the electrical activity of locus coeruleus neurons during cortical spreading depression

A previous study has demonstrated that during cortical spreading depression (CoSD), locus coeruleus (LC) neurons in adult rats reveal antidromic burst activity consisting of multiple initial segment (IS) spikes and IS-somatodendritic (SD) spikes with a distinct IS–SD break. In addition, the spontaneous firing rate of the neurons was reduced during CoSD. In the present experiments, we studied developmental changes in the electrical activity of LC neurons during CoSD. Since stable and repetitive DC shifts occurred in rats older than postnatal day 13 (PD13), the electrical activity of LC neurons at developmental stages later than PD13 was examined. The CoSD-related burst activity similar to that observed in adults was recorded at all stages of development, and the proportion of LC neurons showing the burst activity was nearly the same through all developmental stages and in adults. The frequency of IS spikes in the burst activity at PD13–P15 was not different from that in adults. However, the spontaneous firing rate of LC neurons at early developmental stages remained unchanged during CoSD. Based on these findings, it is conceivable that the mechanism of the generation of multiple IS spikes during CoSD at early developmental stages is different from that at later developmental stages and in adults.

Antidromic burst activity of locus coeruleus neurons during cortical spreading depression

The electrical activity of locus coeruleus neurons was investigated during cortical spreading depression in urethane-anaesthetized rats. Cortical spreading depression was induced by a direct application of 1–3 M KCl solution to the surface of the cerebral cortex. The occurrence of cortical spreading depression was assessed by recording negative d.c. shifts and in some experiments by monitoring the extracellular potassium concentrations. The mean spontaneous firing rate of locus coeruleus neurons was significantly reduced during cortical spreading depression. Approximately 60% of locus coeruleus neurons recorded during cortical spreading depression revealed anomalous burst activity consisting of multiple initial segment spikes as well as full initial segment-somatodendritic spikes with a marked initial segment-somatodendritic break. Each spike of the cortical spreading depression-related burst activity occurred at intervals ranging from 15.0 ms to 90.1 ms (34.9±0.5 ms). The burst activity appeared unpredictably at variable intervals in a phasic or tonic manner during cortical spreading depression. The cortical spreading depression-related burst activity of locus coeruleus neurons mimicked antidromic spikes induced by train stimulation of the cerebral cortex at short interspike intervals during iontophoretic application of GABA to locus coeruleus neurons, whereas it was totally different from synaptically-activated burst activity induced by tail pinch. The full spikes and initial segment spikes in the cortical spreading depression-related burst activity failed to collide with cortically elicited antidromic spikes, even when they appeared within the collision interval. The proportion of initial segment spikes in the cortical spreading depression-related burst activity was reduced following an increase in membrane excitability by iontophoretic application of glutamate, and increased during a decreased membrane excitability by GABA application. The antidromic burst activity of locus coeruleus neurons also appeared for a short time during cortical spreading depression prior to the occurrence of seizure waves induced by GABA antagonists, while the burst activity could not be observed during seizure activity. These results indicate that the cortical spreading depression-related burst activity was of antidromic origin and that the marked initial segment-somatodendritic break in spontaneous spikes of locus coeruleus neurons during cortical spreading depression was due to reduced excitability of the somatodendritic membrane. The cortical spreading depression-related burst activity may cause release of a large amount of noradrenaline in vast regions of locus coeruleus terminal fields through the numerous axon collaterals, thereby playing a role in functional changes of brain neurons related to cortical spreading depression.

The control of jaw-opener motoneurons during active sleep

Brainstem and spinal cord motoneurons that innervate somatic musculature serving antigravity functions are postsynaptically inhibited during active sleep [3,12,22,23]. However, it has been reported that hypoglossal motoneurons (which do not innervate antigravity muscles) are not postsynaptically inhibited during active sleep, but are disfacilitated [16,17]. In the present report we describe changes, during active sleep, in the excitability and membrane potential of digastric and synergistic motoneurons of the trigeminal motor pool; these neurons do not perform antigravity functions [21]. The experiments described in the present report were performed in chronic cats that were prepared for intracellular recording. The motoneurons hyperpolarized an average of 11 mV (S.D.± 1.29, n = 8, P < 0.005) during active sleep compared to quiet sleep. Hyperpolarization was accompanied by a reduction in the excitability of the somadendritic regions of the neurons, as indicated by an increase in the delay of propagation of antidromic spikes from the initial segment to the somadendritic portion of the cell. High gain membrane potential recordings from these motoneurons revealed the occurrence of a remarkably large number of hyperpolarizing potentials during active sleep. When K-chloride-filled microelectrodes were utilized and chloride ions were injected intracellularly, the polarity of these potentials was reversed. During phasic episodes of active sleep, there was a clear increase in hyperpolarizing potential activity, a blockade of somadendritic spikes and phasic reductions in the amplitude of the initial segment spikes. Hyperpolarizing potentials occurred in conjunction with ponto-geniculo-occipital waves. These potentials were reversed in polarity when chloride ions were injected into the cells, indicating that motoneurons of the trigeminal motor pool that do not perform antigravity functions are postsynaptically inhibited during active sleep. The similarities between the inhibitory phenomena in these neurons and those observed in jaw-closer and spinal cord motoneurons lead us to conclude that the excitability of trigeminal motoneurons that innervate muscles with reciprocal functions is suppressed by a postsynaptic inhibitory process during active sleep, as are most other somatic motoneurons [3,12,22,23].

Evidence for the functional compartmentalization of spike generating regions of rat mdbrain dopamine neurons recorded in vitro

The regulation of spike generation in rat midbrain dopamine (DA) neurons was investigated using in vitro intracellular recordings. DA neurons fired long (greater than 1.8 ms) action potentials that exhibited comparatively depolarized spike thresholds (approx. −35 to −45 mV). Depolarization of the DA neuron increased the duration and the threshold of subsequent action potentials. The action potential was composed of two distinct components, a fast (0.8–1.5 ms duration) initial segment (IS) spike which triggered a slow (1.5–3 ms duration) somatodendritic (SD) component. Cobalt application (2 mM) blocked the SD spike component and revealed fast TTX-sensitive spikes. These fast spikes were also observed in untreated neurons following large hyperpolarizing pulses, and showed consistent changes in threshold and amplitude during membrane depolarization. Administration of 4-aminopyridine decreased the threshold of this TTX-sensitive spike, whereas tetraethylammonium (TEA) had no effect. When the fast spike was blocked by TTX, depolarization was ineffective in triggering further spike activity. However, after the administration of TEA (but not 4-AP), high threshold cobalt-sensitive spike activity could be triggered by relatively small depolarizations. TEA increased the duration of the SD portion of the action potential without altering the action potential threshold. The effect of 4-AP on spike threshold and the increase in SD spike duration caused by TEA were similar in nature to the changes in action-potential waveforms produced by polarizing the DA neuron membrane. Drawing from evidence gathered here and in previous in vivo studies, the properties of the TTX-sensitive fast spike are consistent with those of the IS spike component of the action potential, whereas the SD component is similar in nature to the high threshold calcium spike. One hypothesis that can be drawn from these studies is that dendritic and axonal spiking regions may exist in different functional subcompartments of the DA neuron, and may be independently modulated by pharmacologically distinct conductances. Under these conditions, synaptic influences could exist to modulate dendritic excitability and thus regulate putative dendritic spike-dependent functions, such as neuronal activity state, electrical coupling, and dendritic DA synthesis and release.

Electrophysiological and morphological properties of type C vagal neurons in the nodose ganglion of the cat

Some passive and active electrical properties of type C neurons were studied intracellularly, in situ, in the nodose ganglia of adult cats. From the neuronal responses to hyperpolarizing and depolarizing rectangular current pulses it was possible to determine the input resistance (34.4 Mω) and specific membrane resistance (2373ω · cm 2). Significant changes in magnitude and duration of the action potential evoked by vagal stimulation result from changes in the resting potential caused by the passage of steady polarizing currents across the cell membrane. The action potentials evoked by infranodose vagal stimulation had a long duration, a long latency and comprised several components. The fast main spike was followed by a long post-hyperpolarization. The double shock technique showed that the fast main potential was composed of an initial segment spike (‘A spike’) and a somatic spike (‘S spike’), and made it possible to determine the somatic refractory periods. After electrical identification of the cells, horseradish peroxidase was injected ionophoretically into the soma, and it was shown that the central processes were about four times smaller in diameter than the peripheral processes.

Mesocortical dopaminergic neurons. 1. Electrophysiological properties and evidence for soma-dendritic autoreceptors

Mesencephalic dopaminergic neurons were electrophysiologically identified by a variety of criteria, including antidromic activation from prefrontal or cingulate cortex, neostriatum, or nucleus accumbens in urethane-anesthetized rats. The mean firing rate of 98 mesocortical dopaminergic neurons was 2.9 ± 0.3 spikes/sec and did not differ from the mean firing rate found for nigrostriatal or nucleus accumbens dopaminergic neurons. Spontaneously active mesocortical dopaminergic neurons were inhibited by intravenous administration of either apomorphine (6 μg/kg) or amphetamine (0.25 mg/kg). Whereas most antidromic responses of nigrostriatal and mesoaccumbens neurons consisted of the initial segment spike only, cortically-elicited antidromic responses typically consisted of a full initial segment-soma-dendritic spike. These findings are discussed with regard to the presence of soma-dendritic autoreceptors on mesocortical dopaminergic neurons.

Electrical activity of rat ocular motoneurons recorded In vitro

The electrophysiological properties of rat oculomotor neurons were studied in an in vitro slice preparation. Motoneurons were identified by their antidromic response to third nerve rootlet stimulation, and by their orthodromic responses to medial longitudinal fasciculus and reticular stimulations. Passive membrane properties showed the existence of an inward rectification mechanism in all the recorded motoneurons. The action potential is comprised of several distinct components. The fast initial spike, composed of an initial segment spike and a somatodendritic spike, is followed by a delayed depolarization, an afterhyperpolarization and a late afterdepolarization. The afterhyperpolarization has a maximum duration of 55 ms. The late afterdepolarization is a voltage-dependent mechanism that produces an oscillatory behavior in depolarized cells. Two types of motoneurons were distinguished on the basis of their response to long-lasting depolarizing current pulses. The intensity-frequency curves show the existence of a primary and secondary range of discharge and the study of the interspike intervals points to specific properties of the conductance underlying the afterhyperpolarization. It is concluded that large, stellate motoneurons of the brainstem maintained in vitro retain specific electrophysiological properties, comparable to those described in vivo and which differentiate the ocular motoneurons from spinal motoneurons.

Multiple sites for the initiation of action potentials in neurons of the inferior mesenteric ganglion of the guinea-pig

Responses to nerve stimulation were recorded with intracellular microelectrodes from neurons in the inferior mesenteric ganglion of the guinea-pig. An event was recorded which was subthreshold for a somatic action potential but which had a short rise time and a rapid initial repolarization that gave the event a small but well-defined peak. This event was termed a “partial spike”. During repeated stimulation of the same nerve trunk, the fluctuations in the amplitude of the partial spike were small compared to those of evoked synaptic potentials. Stimulation of different nerve trunks evoked partial spikes of different amplitudes. When different nerve trunks were stimulated at short intervals between stimuli, one partial spike could occlude another partial spike. Antidromic responses could not be blocked by a preceding partial spike. This suggests that partial spikes are not initial segment spikes. Tubocurarine reversibly abolished partial spikes which is indicative of a synaptic origin. It is concluded that partial spikes result from action potentials initiated by synaptic potentials in the dendrites and which fail to generate somatic action potentials.

Antidromic discharge property of meso-accumbens dopaminergic VTA neurons in rats

Three groups of meso-accumbens (Acc) neurons in the ventral tegmental area were differentiated by their antidromic discharge property; dopaminergic type 1 (n = 10), non-dopaminergic type 2 (n = 2) and unclassified (n = 2) neurons. During repetitive activation at 10 Hz, the latency of the initial segment (IS) spike, which was often not followed by the somadendritic (SD) spike, was gradually prolonged in type 1, but not in type 2 and unclassified neurons. The latency prolongation of type 1 neurons was reduced to about a half of the normal in rats treated with kainic acid plus haloperidol, but only slightly when treated with kainic acid or picrotoxin. The rate of SD invasion tended to increase after all kinds of chemical treatment. Stimulation of the medial forebrain bundle in type 1 neurons gave responses comparable to Acc stimulation. It is suggested that the latency prolongation of IS spike is produced mainly by axonal mechanism. But additional somatic mechanisms such as dopaminergic self-inhibition and GABAergic and non-GABAergic inputs from the Acc would make some contribution, and at the same time produce frequent suppression of the antidromic SD spike.

On re‐excitation of feline motoneurones: its mechanism and consequences.

Conditions required for re-excitation of lumbosacral motoneurones, i.e. for double impulses in the motor axons associated with a single soma-dendritic action potential, were examined in cats anaesthetized with pentobarbitone and paralysed with gallamine triethiodide. Simultaneous recording from a motoneurone (intracellular, and in some experiments also extracellular), and from its axon in a ventral root, was used to assess the relations between the soma and the double axonal action potentials. Action potentials (greater than 70 mV) evoked by brief depolarizing current pulses applied intracellularly were never observed to cause re-excitation. Re-excitation could, however, be regularly induced by procedures which increased the delay between the initial segment and soma-dendritic components of these potentials. Re-excitation could be evoked (i) when brief hyperpolarizing pulses were applied before the onset of the soma-dendritic spikes, (ii) when the depolarizing pulses were applied on a background of long hyperpolarizing pulses or (iii) when two action potentials were evoked in a quick succession (by two brief depolarizing pulses). No relationship was found between the presence of re-excitation of motor axons and the presence of the delayed depolarization which follows the soma-dendritic spikes. Neither re-excitation nor delayed depolarization were found to be dependent upon re-excitation of the initial segment. These observations are thus at variance with previous suggestions that the initial segment spikes induce the re-excitation of motor axons and that the initial segment spikes cause the delayed depolarization following soma-dendritic spikes. Since re-excitation of a motor axon occurred without any signs of a second initial segment spike, it is concluded that it is initiated at the level of the axon, most likely at the first node of Ranvier. Re-excitation of motor axons was also observed during repetitive firing induced by intracellular current injection. However, it occurred then only occasionally, and only under strong depolarizing drive. It is thus not expected to be a common phenomenon under natural conditions of repetitive firing.

Intracellular and extracellular electrophysiology of nigral dopaminergic neurons—2. Action potential generating mechanisms and morphological correlates

Intracellular recordings from identified nigral dopamine neurons in the rat revealed that their potentials are composed of four components: (1) a slow depolarization, (2) an initial segment spike, (3) a somatodendritic spike, and (4) an afterhyperpolarization. By combining intracellular and extracellular recording techniques with anatomical studies using intracellular injections of Lucifer yellow, an attempt was made to localize each of these potentials to various neuronal compartments. Lucifer yellow injections demonstrated that the dopamine neurons recorded have a pyramidal or polygonal shaped soma, 12–30 μm in diameter, with 3–6 thick major dendrites which extend 10–50 μm from the soma before bifurcating. The axon appears to rise from a major dendrite 15–30 μm from the soma. Based on this anatomical configuration, results from the electrophysiological studies suggest that: (1) the slow depolarization is a pacemaker-like conductance most likely localized to the somatic region, (2) the initial segment spike is a low-threshold spike probably located at the axon hillock, (3) the somatodendritic spikes are long duration spikes that rapidly inactivate with depolarization, have a high threshold, and are localized to the dendritic regions. The action potential is then terminated by a long duration afterhyperpolarization. Our data further suggest that spike generation may be initiated by a slow depolarization at the soma triggering a spike in the low-threshold axon hillock which then spreads across the already-depolarized soma to trigger the dendritic spike. Based on the above findings, dopamine neurons can be compartmentalized electrophysiologically and morphologically into subcomponents, each associated with spikes and specific ionic currents. The high threshold dendritic component of the action potential demonstrates rapid inactivation with depolarization, and thus occurs over a rather narrow range of membrane polarization. This limited range of action potential generation may be important in control of dendritic dopamine release and/or modulation of electrical coupling between dopaminergic neurons.

Effect of gamma-aminobutyric acid on excitability of tubero-infundibular neurons in rat hypothalamic slices.

The effect of gamma-aminobutyric acid (GABA) on antidromically identified tubero-infundibular (TI) neurons was examined in hypothalamic slices of ovariectomized female rats. Twenty antidromically evoked spikes were obtained in the medial basal hypothalamus, including the arcuate and ventromedial nuclei, by electrical stimulation of the median eminence. Sixteen of them had a notch in the rising phase and fractionation of the initial segment (IS)- and somatodendritic (SD)-spikes was elicited by repeated stimulation at frequencies higher than 10 Hz. The application of 0.5-1.5 mM GABA to the incubation medium inhibited SD spikes in 7 of these 16 neurons. The latency, amplitude and threshold of IS spikes were not affected by GABA except for one spike whose latency fluctuated. On the remaining 9 neurons having the notch, no effect of 5-10 mM GABA was discernible. Four of 20 antidromically evoked spikes, which had a smooth rising phase and a shorter duration, were not inhibited by 5-10 mM GABA, but a fluctuation of the latency was observed in one neuron. Fifteen neurons having spontaneous unit activity were also obtained in the arcuate nucleus and its adjacent area and tested with GABA. In 10 of the 15 neurons, spontaneous unit activity disappeared following 0.1-1.5 mM GABA perfusion, while the firing rate in the remaining 5 neurons was not affected by 5-10 mM GABA. These results provide evidence for a direct inhibitory effect of GABA on TI neurons and support the involvement of GABAergic neurons in regulating neuroendocrine functions.

Discharge rate and excitability of cortically projecting intralaminar thalamic neurons during waking and sleep states

Spontaneous firing and antidromically or synaptically evoked discharges of 89 single neurons in centralis lateralis-paracentralis (CL-Pc) intralaminar thalamic nuclei were examined during waking and sleep states in behaving cats with chronic pontine lesions. Twenty-four neurons were activated synaptically at short latencies from the midbrain reticular formation (MRF) after anterograde degeneration of passing fibers. Sixty-five neurons were identified antidromically as projecting to motor or parietal association cortical areas; of them, 23 also could be excited synaptically from the MRF. These neurons were regarded as possibly being involved in the transfer toward the neocortex of the tonic excitation from the MRF during EEG-desynchronized behavioral states. Rates of spontaneous discharge in CL-Pc neurons doubled from synchronized sleep (S) to either wakefulness (W) or desynchronized sleep (D). First order measures of discharge patterns indicated that interval modes in both W and D states (greater than 10 msec) are significantly different from those in S (2.5 msec). During S, the intervals found in the less than 5-msec class indicated the intraburst frequencies; a later minor mode (200 to 350 msec) reflected the interburst silent periods. All neurons tested for antidromic activation from cortical areas had enhanced responsiveness in both W and D states as compared to S sleep. In some cases, the enhanced antidromic excitability was observed in conjunction with a transformation from initial segment spikes during S to full spikes in EEG-desynchronized states. During both W and D states, compared to S sleep, the probability of monosynaptically elicited single discharges to MRF stimulation was increased, and the latency and duration of high frequency bursts evoked by MRF volleys were shortened. We conclude that the features of cortically projecting intralaminar neurons that relay MRF activity fit in well with their hypothesized role in the tonic activation processes that characterize both W and D states. Several lines of evidence suggest that sustained hyperpolarization prevails in intralaminar neurons during S sleep. This is the basic prerequisite for thalamic bursting. The effect of long lasting inhibitory potentials in thalamic neurons provides a mechanism for closing sensory channels during S sleep.