Callosal responses of fast-rhythmic-bursting neurons during slow oscillation in cats

The cortically generated slow oscillation consists of long-lasting hyperpolarizations associated with depth-positive electroencephalogram (EEG) waves and neuronal depolarizations accompanied by firing during the depth-negative EEG waves. It has previously been shown that, during the prolonged hyperpolarizations, the transfer of information from prethalamic pathways to neocortex is impaired, whereas the intracortical dialogue is maintained. To study some of the factors that may account for the maintenance of the intracortical information transfer during the hyperpolarization, intracellular recordings from association areas 5 and 7 were performed in anesthetized cats, and the synaptic responsiveness of fast-rhythmic-bursting, regular-spiking and fast-spiking neurons was tested using single pulses to the homotopic sites in the contralateral areas. During the long-lasting hyperpolarizations callosal volleys elicited in fast-rhythmic-bursting neurons, but not in regular-spiking or fast-spiking neurons, large-amplitude excitatory post-synaptic potentials crowned by single action potentials or spike clusters. Our data show that callosal volleys excite and lead to spiking in fast-rhythmic-bursting neurons during prolonged hyperpolarizations, thus enabling them to transmit information within intracortical networks during slow-wave sleep.     

Intracellular recordings of pontine medial gigantocellular tegmental field neurons in the naturally sleeping cat: behavioral state-related activity and soma size difference in order of recruitment

Intracellular recordings and neurobiotin labeling of medial pontine gigantocellular tegmental field (m-PFTG) neurons in the undrugged, naturally sleeping cat were performed to establish the relationship between soma size and membrane potential (MP) activity before and during the onset of the rapid eye movement (REM) phase of sleep. Initial recordings without labeling revealed that recorded neurons in the m-PFTG had a tonic, sustained membrane depolarization in REM sleep as compared with more polarized MP levels in slow-wave sleep (S) and phasic depolarizations in wakefulness (W) on a more polarized MP level. In neurobiotin-labeled neurons, there was a strong correlation between the soma size of m-PFTG neurons and the 'lead time', the time of onset relative to the beginning of REM, of a sustained increase in membrane depolarization. Thirty-nine m-PFTG neurons with soma cross-sectional areas ranging from 2098 microm(2) to 5958 microm(2) (mean value 3833.8 microm(2)) were analyzed. A majority of these m-PFTG neurons showed an increase in membrane depolarization associated with depolarizing postsynaptic potentials (PSPs) and spike generation that occurred before electrographic signs of REM sleep onset, while the rest of the neurons depolarized at the beginning of or just after REM sleep onset. Our previous work had suggested that many of these m-PFTG neurons were output neurons to the spinal cord. Analysis of the onset time of sustained membrane depolarization (Leadtime(MP)) revealed that larger cells had a longer lead time, while analysis of the lead times for onset of sustained PSPs and action potentials (Leadtime(AP)) showed this measure not to be dependent on soma size, but to be rather uniform, occurring just before the onset of REM sleep. Hence recruitment time, defined as the difference between Leadtime(AP) and Leadtime(MP), was dependent on cell soma size, implying that larger neurons may take longer to depolarize to an MP level critical for generating sustained action potentials, while smaller neurons may require less time.     

The activity of thalamus and cerebral cortex neurons in rabbits during “slow wave-spindle” EEG complexes

“Slow wave-spindle” complexes were studied during slow wave sleep in rabbits at the thalamic (medial thalamus) and cortical (upper and lower layers of the sensorimotor cortex) levels. Slow wave complexes are biphasic positive-negative complexes or triphasic complexes with a predominantly negative component. Spindles have characteristics close to those of spontaneous sleep spindles. Complexes arise singly, as though inserted into the rhythm of spontaneous sleep spindles, or in series with periods similar to the spindle rhythm. Medial thalamus neurons and some cortical neurons had the same activity during waves as during spindles: if the neuron decreased (increased) its spike frequency in a spindle, then decreases (increases) in frequency were also seen in slow waves; if the neuron produced trains of discharges during spindles, then trains of activity were also seen from the slow-wave part of “slow wave-spindle” complexes. The membrane potential changed in a similar fashion: on a background of hyperpolarization which started at the slow wave, individual depolarization oscillations appeared in the EEG wave rhythm; these oscillations were not always accompanied by spike trains. The slow wave mechanisms, the rhythms of isolated complexes and simultaneous complexes and spontaneous sleep spindles may share a common underlying mechanism: slow, cyclical variations in excitability in thalamocortical neuronal networks, which have previously been demonstrated for spindle-likes activity. The possibility that there are common mechanisms for slow waves in complexes and other EEG slow waves, particularly δ activity, remains hypothetical. Translated from Rossiiskii Fiziologicheskii Zhurnal imeni I. M. Sechenova, Vol. 84, No. 3, pp. 182–190, March, 1998.     

Intracellular study of excitability in the seizure-prone neocortex in vivo

The excitability of neocortical neurons from cat association areas 5-7 was investigated during spontaneously occurring seizures with spike-wave (SW) complexes at 2-3 Hz. We tested the antidromic and orthodromic responsiveness of neocortical neurons during the "spike" and "wave" components of SW complexes, and we placed emphasis on the dynamics of excitability changes from sleeplike patterns to seizures. At the resting membrane potential, an overwhelming majority of neurons displayed seizures over a depolarizing envelope. Cortical as well as thalamic stimuli triggered isolated paroxysmal depolarizing shifts (PDSs) that eventually developed into SW seizures. PDSs could also be elicited by cortical or thalamic volleys during the wave-related hyperpolarization of neurons, but not during the spike-related depolarization. The latencies of evoked excitatory postsynaptic potentials (EPSPs) progressively decreased, and their slope and depolarization surface increased, from the control period preceding the seizure to the climax of paroxysm. Before the occurrence of full-blown seizures, thalamic stimuli evoked PDSs arising from the postinhibitory rebound excitation, whereas cortical stimuli triggered PDSs immediately after the early EPSP. These data shed light on the differential excitability of cortical neurons during the spike and wave components of SW seizures, and on the differential effects of cortical and thalamic volleys leading to such paroxysms. We conclude that the wave-related hyperpolarization does not represent GABA-mediated inhibitory postsynaptic potentials (IPSPs), and we suggest that it is a mixture of disfacilitation and Ca(2+)-dependent K(+) currents, similar to the prolonged hyperpolarization of the slow sleep oscillation.     

Synaptic and intrinsic control of membrane excitability of neostriatal neurons. I. An in vivo analysis

1. The relationship between membrane properties of neostriatal neurons and spontaneous and evoked synaptic potentials was studied with the use of intracellular recordings from anesthetized rats. Most of these neurons showed regular or irregular spontaneous depolarizing potentials that only in a few cases triggered action potentials at resting level. 2. The stimulation of the ipsilateral substantia nigra or of the sensorimotor cortex produced a relatively fast depolarizing post-synaptic potential (EPSP). In some cells this potential was followed by an inhibitory period that appeared as an hyperpolarization when the cell was depolarized from the resting level (inhibitory postsynaptic potential, IPSP). A late and long-lasting depolarization (LD) followed the EPSP or the EPSP-IPSP sequence. 3. Repetitive discharge with little adaptation was observed during direct depolarization. Most of the neurons tested for current-voltage (I-V) relationship showed nonlinearity of the input resistance in the hyperpolarizing direction. Spontaneous and evoked EPSPs were decreased in their amplitude and duration when the membrane potential was held at levels more hyperpolarized than -85 mV because of the strong rectification at these levels of hyperpolarization. 4. Local microiontophoretic application of bicuculline (BIC) or systemic administration of BIC and pentylenetetrazole (PTZ) produced a reduction of the IPSPs. The reduction of the inhibitory transmission caused a strong increase of the LD. The current-evoked firing pattern was not greatly altered. 5. The intracellular application of cesium increased the amplitude and the duration of the spontaneous depolarizations that triggered bursts of action potentials under this condition. Spikes were broadened and the rectification in the hyperpolarization direction was reduced. 6. Iontophoretically applied cadmium strongly depressed the amplitude of the spontaneous and evoked postsynaptic potentials. During cadmium application, nigral stimulation produced constant latency, all-or-none spikes in the absence of any synaptic potential. 7. Repetitive stimulation of the ipsilateral substantia nigra by electrical shocks (5 Hz, 25 s) produced a progressive and reversible decrease of the spontaneous depolarizing potentials (SDPs) and a decrease of the firing rate. In the same cells, when the train of stimulation was delivered in the ipsilateral cortex, a membrane depolarization coupled with an increase of the firing rate was observed. 8. We conclude that although synaptic circuits mediate a phasic inhibition in neostriatum, the low level of spontaneous firing of most neostriatal neurons is mainly because of the effects that membrane properties exert on the spontaneous and the evoked synaptic depolarizations in the striatum.(ABSTRACT TRUNCATED AT 400 WORDS)     

Focal synchronization of ripples (80-200 Hz) in neocortex and their neuronal correlates

Field potentials from different neocortical areas and intracellular recordings from areas 5 and 7 in acutely prepared cats under ketamine-xylazine anesthesia and during natural states of vigilance in chronic experiments, revealed the presence of fast oscillations (80-200 Hz), termed ripples. During anesthesia and slow-wave sleep, these oscillations were selectively related to the depth-negative (depolarizing) component of the field slow oscillation (0.5-1 Hz) and could be synchronized over ~10 mm. The dependence of ripples on neuronal depolarization was also shown by their increased amplitude in field potentials in parallel with progressively more depolarized values of the membrane potential of neurons. The origin of ripples was intracortical as they were also detected in small isolated slabs from the suprasylvian gyrus. Of all types of electrophysiologically identified neocortical neurons, fast-rhythmic-bursting and fast-spiking cells displayed the highest firing rates during ripples. Although linked with neuronal excitation, ripples also comprised an important inhibitory component. Indeed, when regular-spiking neurons were recorded with chloride-filled pipettes, their firing rates increased and their phase relation with ripples was modified. Thus besides excitatory connections, inhibitory processes probably play a major role in the generation of ripples. During natural states of vigilance, ripples were generally more prominent during the depolarizing component of the slow oscillation in slow-wave sleep than during the states of waking and rapid-eye movement (REM) sleep. The mechanisms of generation and synchronization, and the possible functions of neocortical ripples in plasticity processes are discussed.     

Attenuation of glutamate-action,excitatory postsynaptic potentials,and spikes by intracellular QX 222 in hippocampal neurons

The effects of intracellular applications of QX 222, a quaternary analogue of lidocaine, were investigated in CA1 neurons of in vitro hippocampal slices of guinea-pig brain. QX 222 produced a strong depression of spontaneous, electrically- (by current injection) or orthodromically-evoked action potentials. These dose-dependent effects were characterized by a reduction in the rate of rise and amplitude of spikes, presumed to be mediated by a Na⁺-conductance. Although resting membrane conductance tended to diminish with prolonged applications of QX 222, marked changes in resting potential generally were not observed. The threshold for eliciting spikes by intracellular injection of depolarizing current was increased greatly by QX 222, reflecting the impairment of Na⁺ -electrogenesis of spikes. The reduction of action potential amplitude by QX 222 may be partly attributable to enhanced inactivation of Na⁺-channels because brief depolarizing pulses preceded by strong tonic hyperpolarization, elicited spikes at a lower threshold and of considerably larger amplitude than in the absence of such tonic hyperpolarization. These observations on recovery are compatible with a removal of sodium inactivation. However, this experimental paradigm of current injection also might be expected to remove QX 222 molecules from their blocking sites at the inner end of Na⁺-channels. When spikes were abolished by QX 222, the depolarization evoked with application of S-glutamate by pressure ejection from an extracellular micropipette positioned close to the neuron was attenuated. This reversible blockade was reproducible in the 14 neurons where the interactions of QX 222 and glutamate were examined systematically. Excitatory postsynaptic potentials, evoked by stimulation of strata oriens or radiatum, were reduced in a similar manner by internal QX 222.These data confirm previous observations that voltage-dependent Na⁺-channels mediating spike genesis in CA1 neurons can be blocked by internal QX 222. However, QX 222 also apparently interferes with the functions of Na⁺ -channels activated by glutamate-receptor interaction or by receptor interactions with neurotransmitter(s) associated with certain excitatory postsynaptic potentials in CA1 neurons.     

Membrane potential of spinal motoneurons during natural sleep in cats.

The membrane potential of spinal motoneurons was recorded during wakefulness, NREM sleep, and REM sleep in minimally restrained, behaving cats. At the onset of sleep, the membrane potential generally increased in polarization in rough proportion to time spent asleep. During the postural atonia of REM sleep, the membrane potential of all motoneurons was tonically hyperpolarized. Antecedents of NREM sleep electromyographic suppressions, and REM sleep myoclonic twitches were seen as transient hyperpolarizations and depolarizations, respectively.     

Phasic changes in motoneuron membrane potential during REM periods of active sleep

Most of the phasically occurring periods of rapid eye movements (REMs) of active sleep are accompanied by enhanced suppression of somatomotor activity; however, during some of the REM episodes there are muscular twitches and jerks. The membrane potential changes underlying these motor processes were examined by recording intracellularly from lumbar motoneurons in cats that were undrugged, unanesthetized and normally respiring. Summated hyperpolarizing potentials were evident during REM episodes in conjunction with a decrease in motoneuron excitability. During other episodes of REMs there occurred summated depolarizing potentials which occasionally produced action potentials. These depolarizing events were in most cases preceded by a brief period of hyperpolarization. Thus, it appears that there is inhibitory input to lumbar motoneurons during all REM periods of active sleep; in some episodes the simulataneous coactivation of excitatory input leads to depolarization of the membrane and action potential generation.     

Calcium-dependent hyperpolarizations in bullfrog sympathetic neurons

Intracellular recordings were made from neurons in bullfrog sympathetic ganglia. Fast B and slow B neurons were identified and selected for the analysis [Dodd and Horn (1983) J. Physiol., Lond. 334, 255-269]. A single soma action potential was followed by a prolonged afterhyperpolarization lasting for several hundred ms up to 2 s. Part of the spike afterhyperpolarization was due to potassium conductance activation triggered by calcium entry during an action potential. Acetylcholine was directly applied onto the soma membrane by iontophoresis. A rapid nicotinic depolarization was followed by a slow muscarinic depolarization. The nicotinic depolarization was followed by a hyperpolarization when the muscarinic depolarization was blocked by scopolamine. This hyperpolarization was several mV in amplitude and from 1 to 10 s in duration. It disappeared when the preceding nicotinic depolarization was blocked by (+)-tubocurarine. A single fast excitatory postsynaptic potential was also followed by a hyperpolarization in the presence of scopolamine. The acetylcholine-induced hyperpolarization was due to potassium conductance activation triggered by calcium entry during the nicotinic depolarization. The present findings show that non-synaptic autoinhibition is operating in sympathetic neurons. In other words, a rapid nicotinic transmission leads to prolonged hyperpolarizations which are not mediated by any transmitters but are mediated by calcium.     

Ionic currents and firing patterns of mammalian vagal motoneurons in vitro

The electrophysiological properties of guinea-pig dorsal vagal motoneurons were studied in an in vitro slice preparation. Antidromic, orthodromic and direct stimulation of the neurons demonstrated that the action potential is comprised of several distinct components: a fast initial spike followed by afterdepolarization and an early and a late afterhyperpolarizations. The fast initial spike and the early afterhyperpolarization were blocked by tetrodotoxin and tetraethylammonium ions, respectively. The afterdepolarization (present on the falling phase of the spike) and the late afterhyperpolarization were blocked by the addition of ions known to block calcium conductance (CdCl2, CoCl2 or MnCl2), indicating close association between these two potentials. Prolonged outward current injection through the recording electrode produced two different firing patterns, depending on the initial level of the membrane potential. From resting potential (usually -60 mV) the firing pattern was characterized by a short train of action potentials appearing shortly after the onset of the depolarization step. By contrast, when the depolarization was delivered from a hyperpolarized membrane potential level, a short train of repetitive firing appeared after an initial delay of 300-400 ms. The membrane current responsible for this initial reduction in excitability was studied by means of a single-electrode voltage-clamp technique. The magnitude, direction and kinetics of such current flow are consistent with the presence of early potassium current (IA), partly inactive at the resting potential. Synaptic activation of vagal motoneurons could be obtained by electrical stimulation of the tissue surrounding the vagal nucleus or by direct activation of the vagal nerve. Perivagal stimulation generated excitatory and inhibitory synaptic potentials which could be reversed by shifting the membrane potential. Vagal nerve stimulation, in addition to the antidromic activation of the cells, generated depolarizing responses which were unitary in nature and did not show much sensitivity to shifts in membrane potential. Perivagal and vagal nerve-evoked depolarizations could generate action potentials as well as partial dendritic spikes. We conclude that spike electroresponsiveness in vagal motoneurons is generated by voltage-dependent Na+ and Ca2+ conductances. In addition, the Ca2+-dependent current triggers a K+ conductance which is responsible for modulating the firing frequency obtained from the normal resting level.(ABSTRACT TRUNCATED AT 400 WORDS)     

An intracellular analysis of some intrinsic factors controlling neural output from inferior mesenteric ganglion of guinea pigs

1. In vitro studies were conducted on neurons within the inferior mesenteric ganglion (IMG) of guinea pigs to investigate how intrinsic features of the spike-generating process interact with preganglionic inputs to produce the output firing patterns of these neurons. Intracellular-electrode techniques were used to monitor and control electrical activity of IMG neurons. Preganglionic inputs were activated either synchronously by stimulating an attached nerve trunk or asynchronously by leaving the ganglion attached to a segment of terminal colon and activating the colonic-IMG mechanosensory system. 2. Ninety-seven percent of the neurons studied demonstrated an afterspike hyperpolarization (ASH). The ASH process was activated only by the occurrence of a spike and did not have a synaptically induced component. Further activation of this process was produced by two or more spikes having interspike intervals less than the duration of an ASH following a single spike. An aftertrain hyperpolarization (ATH) resulted from this progressive activation. The amplitude of both the ASH and the ATH decreased when the resting membrane potential was hyperpolarized by current injection or by increasing the external potassium ion concentration. 3. Neuronal excitability was reduced during the ASH. From this observation it was concluded that when IMG neurons operate in the occasional-firing mode, the ASH process prevents output frequency from greatly exceeding the reciprocal of the ASH duration produced by a single spike. 4. Two types of synaptically induced slow depolarizations were observed: a slow, long-latency depolarization and a short-latency depolarization (SLD). These depolarizations differed in their latency, onset, and duration. Both were capable of converting synchronous, preganglionic input from subthreshold (non-spike-activating) to threshold (spike-activating) activity. 5. Neurons having resting potentials more positive than -60 mV were capable of firing in the rhythmic-firing mode; 40% of these neurons demonstrated tonic- and 60% phasic-firing behavior. Frequency-current relations for tonic-discharging neurons were linear from the rhythmic-firing threshold to current levels approximately 2.5 times the threshold value. Minimal frequency for tonic firing and the slope of the linear portion of the frequency-current relation were indirectly related to the duration of the ASH. 6. This study suggests that sympathetic, noradrenergic neurons of the IMG can operate in either the occasional- or rhythmic-firing mode. In the physiologic state in vivo, most IMG neurons probably do not produce action potentials in excess of 10-15 Hz because of their intrinsic properties which regulate firing in both modes of operation.     

Slow oscillations in human non-rapid eye movement sleep electroencephalogram: effects of increased sleep pressure

Slow oscillations (<1 Hz) in the non-rapid eye movement (NREM) sleep electroencephalogram (EEG) result from slow membrane potential fluctuations of cortical neurones, alternating between a depolarized up-state and a hyperpolarized down-state. They are thought to underlie the restorative function of sleep. We investigated the behaviour of slow oscillations in humans under increased sleep pressure to assess their contribution to sleep homeostasis. EEG recordings (C3A2) of baseline and recovery sleep after sleep deprivation (eight healthy males, mean age 23 years; 40 h of prolonged wakefulness) were analysed. Half-waves were defined as positive or negative deflections between consecutive zero crossings in the 0.5–2 Hz range of the band-pass filtered EEG. Increased sleep pressure resulted in a redistribution of half-waves between 0.5 and 2 Hz: the number of half-waves per minute was reduced below 0.9 Hz while it was increased above 1.2 Hz. EEG power was increased above 1 Hz. The increase in frequency was accompanied by increased slope of the half-waves and decreased number of multi-peak waves. In both baseline and recovery sleep, amplitude and slope were correlated highly over a broad frequency range and positive half-waves were characterized by a lower frequency than the negative ones, pointing to a longer duration of up- than down-states. We hypothesize that the higher frequency of slow oscillatory activity after prolonged wakefulness may relate to faster alternations between up- and down-states at the cellular level under increased sleep pressure. This study does not question slow-wave activity as a marker of sleep homeostasis, as the observed changes occurred within the same frequency range.     

Depolarization of cortical glial cells in response to electrical stimulation of the cortical surface

The responses of 155 neurones and 91 glial cells to the electrical stimulation of the cortex were recorded in the suprasylvian gyrus of 20 cats under pentobarbital anaesthesia. Glial cells were identified by electrophysiological criteria: absence of action potentials and postsynaptic potentials; high membrane potential; slow depolarization during the electrical stimulation of the cortex. 50 glial cells showed membrane potentials between 80 and 100 mV. Stimuli of low intensity which evoked only excitatory postsynaptic potentials of apical dendrites, the so-called dendritic potentials, failed to evoke glial depolarization. However, glial depolarization could be elicited at high-frequency stimulation. Stimuli, which evoked not only the dendritic potential but also subsequent slow negativity, could usually bring about glial depolarization too. The amplitude of glial depolarization in response to one stimulus did not exceed 2 mV, the latency being 3–5 ms. A phenomenon of decrementai summation of glial depolarization was observed. The stronger and more frequent the stimulation, the larger was glial depolarization. However, at frequencies over 50/s glial depolarization decay was observed already during the stimulation and in some cases, membrane potential was drastically reduced to zero. After cessation of stimulation, glial depolarization decayed exponentially in 3–4 s; in some cases the decay was prolonged up to 10s and slow irregular fluctuations of the membrane potential were recorded; at the same time, spikes of the neighbouring neurone could be recorded from the glial cell. With a decrease of the membrane potential glial depolarization was attenuated, but it could be elicited even at membrane potential below 20 mV.The results are interpreted in relation to the potassium ion hypothesis. It is suggested that glial depolarization is determined by release of K⁺, which is associated with excitation of non-myelinated fibres and with excitatory postsynaptic potentials generated in the cortical neuropile. Significant increases in the concentration of extracellular potassium ions could provoke actual movement of glial cells. It is supposed that glial depolarization of small magnitude which is recorded occasionally at the membrane potential below 30 mV is the result of electronic spread of glial depolarization from the neighbouring glial cells.     

Fast and slow depolarizations produced by substance P and other tachykinins in sympathetic neurons of rat prevertebral ganglia.

Using intracellular recording, we examined the effects of three mammalian tachykinins, substance P (SP), neurokinin A (NKA), and neurokinin B (NKB), on sympathetic neurons of isolated rat coeliac-superior mesenteric ganglia (C-SMG). The 3 tachykinins elicited two distinct depolarizing responses in ganglion cells: fast depolarization with time-to-peak of 1-2 sec and duration of 5-10 sec, and slow depolarization with time-to-peak of about 20 sec and duration of 120-140 sec. Both fast and slow responses persisted in a solution containing low Ca2+ and high Mg2+ or tetrodotoxin, which indicates that the tachykinins directly act on ganglion cells to produce fast and slow depolarizations. The two types of tachykinin-induced responses exhibited clearly distinguishable properties. The membrane conductance was increased during the fast response, but not significantly changed, slightly decreased or sometimes increased during the slow response. Within certain range of membrane potential, the amplitude of fast response increased upon membrane hyperpolarization and decreased upon depolarization of ganglion cells. In contrast, the amplitude of slow response associated with membrane conductance decrease was increased with membrane depolarization and decreased with hyperpolarization. The fast response was markedly suppressed in a Na(+)-deficient solution, a solution containing nominally zero Ca2+ (plus 0.1 mM EGTA in some cases), and in a solution containing Cd2+ or Mn2+, whereas the slow response was not affected in these solutions and was augmented in some cells in K(+)-free solution. Thus it seems that the increase in Ca(2+)-dependent cationic conductance underlies the fast response and that the slow response is produced at least in part by suppression of certain K+ channels. The fast response progressively decreased in amplitude upon repeated application of the peptides with short intervals, whereas the slow response was rather augmented by repeated application. Lowering the temperature markedly depressed the slow response, while the fast response remained almost unaffected. It is therefore likely that the fast and slow depolarizations are mediated by two different subtypes of tachykinin receptors or a single class of receptors linked with two different intracellular mechanisms. Measurement of tachykinins in several sympathetic ganglia by combined use of HPLC and radioimmunoassay revealed that the highest amount of SP occurs in the C-SMG where the content of SP (136.0 pmol/g protein) was higher than those of NKA (44.3) and NKB (18.7). SP thus appears to function as a major tachykinin in rat C-SMG.     

Synaptic and intrinsic responses of medical entorhinal cortical cells in normal and magnesium-free medium in vitro

1. Extracellular recordings were made from slices of hippocampus plus parahippocampal regions maintained in vitro. Field potentials, recorded in the entorhinal cortex after stimulation in the subiculum, resembled those observed in vivo. 2. Washout of magnesium from the slices resulted in paroxysmal events which resembled those occurring during sustained seizures in vivo. These events were greatest in amplitude and duration in layers IV/V of the medial entorhinal cortex and could occur both spontaneously and in response to subicular stimulation. Spontaneous seizure-like events were not prevented by severing the connections between the hippocampus and entorhinal cortex, but much smaller and shorter events occurring in the dentate gyrus were stopped by this manipulation. Both spontaneous and evoked paroxysmal events were blocked by perfusion with the N-methyl-D-aspartate (NMDA) receptor antagonist, DL-2-amino-5-phosphonovalerate (2-AP5). 3. Neurons in layers IV/V were characterized by intracellular recording. Injection of depolarizing current in most cells evoked a train of nondecrementing action potentials with only weak spike frequency accommodation and little or no posttrain after hyperpolarization. 4. A small number of cells displayed burst response when depolarized by positive current. The burst consisted of a slow depolarization with superimposed action potentials which decreased in amplitude and increased in duration during the discharge. The burst was terminated by a strong after hyperpolarization and thereafter, during prolonged current pulses a train of nondecrementing spikes occurred. The burst response remained if the cell was held at hyperpolarized levels but was inactivated by holding the cell at a depolarized level. 5. Depolarizing synaptic potentials could be evoked by stimulation in the subiculum. A delayed and prolonged depolarization clearly decremented with membrane hyperpolarization and, occasionally, increased with depolarization. 6. Washout of magnesium from the slices resulted in an enhancement of the late depolarization and a reversal of its voltage dependence. Eventually a single shock to the subiculum evoked a large all-or-none paroxysmal depolarization associated with a massive increase in membrane conductance. Similar events occurred spontaneously in all cells tested. The paroxysmal depolarizations, both spontaneous and evoked, were rapidly blocked by 2-AP5. 7. It is concluded that medial entorhinal cortical cells possess several intrinsic and synaptic properties which confer an extreme susceptibility to generation of sustained seizure activity.(ABSTRACT TRUNCATED AT 400 WORDS)     

Dynamics of Neuron Spike Activity in the Oral Nucleus of the Pons During the Sleep-Waking Cycle in Cats

Chronic experiments on five cats were performed to study the spike activity of neurons in the oral nucleus of the pons during waking, slow-wave sleep, and paradoxical sleep. Groups of neurons with different dynamics of spike frequency were identified. Cells with rare discharges during waking and slow-wave sleep and maximum spike frequencies in the phasic stage of paradoxical sleep were regarded as PS-on neurons. Cells discharging at maximum frequency during waking, with decreases in frequency during slow-wave sleep and further decreases in paradoxical sleep, were regarded as PS-off neurons. Cells showing decreases in discharge frequency on the transition from waking to slow-wave sleep and increases in discharge frequency in paradoxical sleep, with grouped spikes during oculomotor activity, appeared to be responsible for generating the phasic phenomena of paradoxical sleep. The question of the involvement of neurons of different populations in the mechanisms of paradoxical sleep is discussed.     

Changes in neuronal conductance during different components of cortically generated spike-wave seizures

Neuronal conductance was studied in anesthetized cats during cortically generated spike-wave seizures arising from slow sleep oscillation. Single and dual intracellular recordings from neocortical neurons were used. The changes were similar whether the seizures occurred spontaneously, or were evoked by electrical stimulation or induced by bicuculline. In all seizures, the conductance increased from the very onset of the seizure and returned to control values only at the end of the postictal depression. Simultaneous intracellular recordings from two neurons showed that the neuron leading the other neuron displayed the largest increase in membrane conductance. The changes in neuronal conductance during the two phases of the slow sleep oscillation, i.e. highest during depolarizations and lowest during hyperpolarizations, were similar to those occurring during the "spike" and "wave" components of seizures. (1) Maximal conductance was found during the paroxysmal depolarizing shift corresponding to the electroencephalogram "spike" (median: 252 nS; range: 90 to more than 400 nS). It was highest at the onset of the depolarized plateau and decreased thereafter. (2) During the hyperpolarization corresponding to the electroencephalogram "wave", the conductance was significantly lower (median: 71 nS; range: 41 to 140 nS). (3) The conductance was elevated during the fast runs (median: 230 nS; range: 92 to 350 nS) which occurred in two-thirds of the seizures. (4) The conductance values during postictal depression were situated between those measured during the seizure hyperpolarizations and during sleep hyperpolarizations. The conductance decreased exponentially back to the values of the slow sleep oscillation over the total duration of the postictal depression.The data suggest that the major mechanism underlying the "wave"-related hyperpolarizing component of spike-wave seizures relies mainly not on active inhibition, but on a mixture of disfacilitation and potassium currents.     

A model of the T-type calcium current and the low-threshold spike in thalamic neurons.

1. A model of the transient, low-threshold voltage-dependent (T-type) Ca2+ current is constructed using recent whole-cell voltage-clamp data from enzymatically isolated rat thalamocortical relay neurons. The T-type Ca2+ current is described according to the Hodgkin-Huxley scheme, using the m3h format, with rate constants determined from the experimental data (22-24 degrees C; extracellular Ca2+ concentration [Ca2+]o = 3 mM). 2. The T-type Ca2+ current inactivates rapidly during maintained depolarization (time constant, Tau h approximately 20 ms at -20 mV), yet recovery from inactivation is slow (time constant, Tau r approximately 270 ms at -80 mV). To reconcile these observations, a two-step kinetic scheme is proposed for the inactivation gate. Each of the time constants in this scheme is voltage dependent, with a maximum at about -85 mV (45 ms for one and 275 ms for the other). 3. Numerical simulations of recovery in a two-pulse, voltage-clamp protocol compare favorably with experimental results obtained by Coulter et al. as well as those obtained in an independent series of experiments with guinea pig thalamic neurons ([Ca2+]o = 10 mM). 4. For current-clamp simulations, a leakage current gL (V-VL) is included; with VL = -65 mV, the calculated resting membrane potential is -63 mV. 5. It is shown that the T-type Ca2+ current together with the leakage current suffices to describe the low-threshold spike (LTS), a slow, triangular-shaped depolarizing event that can be evoked only from relatively hyperpolarized membrane potentials and that underlies the burst firing of Na(+)-dependent action potentials in thalamic neurons. Outward currents are not required to reproduce the basic shape of the LTS. 6. The LTS can be activated with either a depolarizing current step from a sufficiently hyperpolarized level or on termination of a hyperpolarizing current step. In either case, the amplitude of the LTS is a monotonically increasing, sigmoid-shape function of the hyperpolarizing current step intensity. 7. Because of the slower kinetic step of the channel's inactivation gate, our model predicts that recovery of the LTS to greater than one-half amplitude would require a prolonged hyperpolarization of greater than 100 ms (at body temperature). This imposes an upper limit (approximately 10 Hz) on the frequency of repetitive hyperpolarization that can elicit a train of LTSs and hence on the frequency of any rhythm that requires LTS-mediated bursting of thalamic neurons.(ABSTRACT TRUNCATED AT 400 WORDS)     

Nimodipine increases excitability of rabbit CA1 pyramidal neurons in an age- and concentration-dependent manner.

1. Cellular properties were studied before and after bath application of the dihydropyridine L-type calcium channel antagonist nimodipine in aging and young rabbit hippocampal CA1 pyramidal cells in vitro. Various concentrations of nimodipine, ranging from 10 nM to 10 microM, were tested to investigate age- and concentration-dependent effects on cellular excitability. Drug studies were performed on a population of neurons at similar holding potentials to equate voltage-dependent effects. The properties studied under current-clamp conditions included steady-state current-voltage relations (I-V), the amplitude and integrated area of the postburst afterhyperpolarization (AHP), accommodation to a prolonged depolarizing current pulse (spike frequency adaptation), and single action-potential waveform characteristics following synaptic activation. 2. Numerous aging-related differences in cellular properties were noted. Aging hippocampal CA1 neurons exhibited significantly larger postburst AHPs (both the amplitude and the integrated area were enhanced). Aging CA1 neurons also exhibited more hyperpolarized resting membrane potentials with a concomitant decrease in input resistance. When cells were grouped to equate resting potentials, no differences in input resistance were noted, but the AHPs were still significantly larger in aging neurons. Aging CA1 neurons also fired fewer action potentials during a prolonged depolarizing current injection than young CA1 neurons. 3. Nimodipine decreased both the peak amplitude and the integrated area of the AHP in an age- and concentration-dependent manner. At concentrations as low as 100 nM, nimodipine significantly reduced the AHP in aging CA1 neurons. In young CA1 neurons, nimodipine decreased the AHP only at 10 microM. No effects on input resistance or action-potential characteristics were seen. 4. Nimodipine increased excitability in an age- and concentration-dependent manner by decreasing spike frequency accommodation (increasing the number of action potentials during prolonged depolarizing current injection). In aging CA1 neurons, this effect was significant at concentrations as low as 10 nM. In young CA1 neurons, nimodipine decreased accommodation only at higher concentrations (> or = 1.0 microM). 5. We conclude that aging CA1 neurons were less excitable than young neurons. In aging hippocampus, nimodipine restores excitability, as measured by size of the AHP and degree of accommodation, to levels closely resembling those of young adult CA1 neurons. These actions of nimodipine on aging CA1 hippocampal neurons may partly underlie the drug's notable ability to improve associative learning in aging rabbits and other mammals. Reversal of inhibitory postsynaptic potentials (IPSPs) by chloride ion and/or current injections into six motoneurons revealed the presence of inhibition during the period between phrenic bursts during fictive vomiting and also during the final phase of expulsion when phrenic discharge ceased by abdominal discharge continued. 3. Fictive coughing, evoked by repetitive electrical stimulation of superior laryngeal nerve afferents, was characterized by a large phrenic discharge followed immediately by a large abdominal nerve discharge. During fictive coughing, phrenic motoneurons retained their ramplike depolarizations throughout phrenic discharge; however, the amplitude of depolarization was greater than during inspiration. During the subsequent abdominal nerve discharge, the phrenic membrane potential usually underwent an initial rapid, transient hyperpolarization followed by a gradual repolarization associated with increased synaptic noise.(ABSTRACT TRUNCATED AT 400 WORDS)