A low threshold calcium spike mediates firing pattern alterations in pontine reticular neurons

Scaling of recovery rates influences T-type Ca2+ channel availability following IPSPs

The excitability of neuronal membranes is crucially modulated by T-type Ca²⁺ channels (I_CaT) due to their low threshold of activation. I_CaT inactivates steeply at potentials close to the resting membrane potential. Therefore, the availability of I_CaT following changes in membrane potential depends on the time course of the onset of inactivation as well as on the time course of recovery from inactivation.

It was previously shown that the time course of recovery from inactivation depends on the duration of the conditioning pulse in cloned T-type Ca²⁺ channel subunits (Ca_v3.1-Ca_v3.3(Uebachs et al., 2006)). This provides a potential mechanism for an intrinsic form of short term plasticity. Here, we address the question, whether this mechanism results in altered availability of I_CaT following physiological changes in membrane potential. We found that the recovery of I_CaT during an IPSP depends on the duration of a preceding depolarized period.

Graded boosting of synaptic signals by low-threshold voltage-activated calcium conductance

Low-threshold voltage-activated calcium conductances (LT-VACCs) play a substantial role in shaping the electrophysiological attributes of neurites. We have investigated how these conductances affect synaptic integration in a premotor nonspiking (NS) neuron of the leech nervous system. These cells exhibit an extensive neuritic tree, do not fire Na(+)-dependent spikes, but express an LT-VACC that was sensitive to 250 μM Ni(2+) and 100 μM NNC 55-0396 (NNC). NS neurons responded to excitation of mechanosensory pressure neurons with depolarizing responses for which amplitude was a linear function of the presynaptic firing frequency. NNC decreased these synaptic responses and abolished the concomitant widespread Ca(2+) signals. Coherent with the interpretation that the LT-VACC amplified signals at the postsynaptic level, this conductance also amplified the responses of NS neurons to direct injection of sinusoidal current. Synaptic amplification thus is achieved via a positive feedback in which depolarizing signals activate an LT-VACC that, in turn, boosts these signals. The wide distribution of LT-VACC could support the active propagation of depolarizing signals, turning the complex NS neuritic tree into a relatively compact electrical compartment.

T-type Ca2+ channels in normal and abnormal brain functions

Low-voltage-activated T-type Ca(2+) channels are widely expressed in various types of neurons. Once deinactivated by hyperpolarization, T-type channels are ready to be activated by a small depolarization near the resting membrane potential and, therefore, are optimal for regulating the excitability and electroresponsiveness of neurons under physiological conditions near resting states. Ca(2+) influx through T-type channels engenders low-threshold Ca(2+) spikes, which in turn trigger a burst of action potentials. Low-threshold burst firing has been implicated in the synchronization of the thalamocortical circuit during sleep and in absence seizures. It also has been suggested that T-type channels play an important role in pain signal transmission, based on their abundant expression in pain-processing pathways in peripheral and central neurons. In this review, we will describe studies on the role of T-type Ca(2+) channels in the physiological as well as pathological generation of brain rhythms in sleep, absence epilepsy, and pain signal transmission. Recent advances in studies of T-type channels in the control of cognition will also be briefly discussed.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Leptin modulates spike coding in the rat suprachiasmatic nucleus

The mammalian circadian pacemaker, the suprachiasmatic nucleus (SCN), contains receptors to the adipose tissue hormone leptin. In the present study, the effects of leptin on the electrophysiological activity of the SCN cells were characterised in vitro in rat brain slices. During extracellular recording, application of 20 nm leptin (n = 36) decreased mean spike frequency (Wilcoxon signed rank test, z = -3.390, P < 0.001) and increased the irregularity of firing measured by the entropy of the log interspike interval distribution (Student's paired t-test, t = 2.377, P = 0.023), but had no consistent effect on spike patterning as measured by the mutual information between adjacent log interspike intervals (z = 0.745, P = 0.456). Intracellular current-clamp recordings (n = 25) revealed a hyperpolarising effect of 20 nm leptin on SCN neurones (z = -2.290, P = 0.022). The hyperpolarisation largely resulted from the effect of leptin on the subgroup of cells (n = 13) that generated 'rebound' spikes upon termination of a hyperpolarising current pulse (z = -2.697, P = 0.007). Leptin application also increased the group mean duration of the afterhyperpolarisation (n = 25, t = 2.512, P = 0.023). The effects of leptin on extracellularly recorded spike activity were consistent with the changes in membrane potential and spike shape. They suggest that leptin can directly modulate the electrical properties of SCN neurones and, in this way, contribute to the mechanism by which metabolic processes influence the circadian clock.

Voltage-dependent calcium signaling in rat cerebellar unipolar brush cells

Unipolar brush cells (UBCs) are a class of excitatory interneuron found in the granule cell layer of the vestibulocerebellum. Mossy fibers form excitatory inputs on to the paint brush shaped dendrioles in the form of giant, glutamatergic synapses, activation of which results in prolonged bursts of action potentials in the postsynaptic UBC. The axons of UBCs themselves form mossy fiber contacts with other UBCs and granule cells, forming an excitatory, intrinsic cerebellar network that has the capacity to synchronize and amplify mossy fiber inputs to potentially large populations of granule cells. In this paper, we demonstrate that UBCs in rat cerebellar slices express low voltage activated (LVA) fast-inactivating and high voltage activated (HVA) slowly inactivating calcium channels. LVA calcium currents are mediated by T-type calcium channels and they are associated with calcium increases in the dendrites and to a lesser extent the cell soma. HVA currents, mediated by L-type calcium channels, are slowly inactivating and they produce larger overall increases in intracellular calcium but with a similar distribution pattern. We review these observations alongside several recent papers that examine how intrinsic membrane properties influence UBCs firing patterns and we discuss how UBC signaling may affect downstream cerebellar processing.

Postinhibitory rebound spikes are modulated by the history of membrane hyperpolarization in the SCN

The suprachiasmatic nucleus (SCN) of the hypothalamus regulates biological circadian time thereby directly impacting numerous physiological processes. The SCN is composed almost exclusively of gamma-aminobutyric acid (GABA)ergic neurons, many of which synapse with other GABAergic cells in the SCN to exert an inhibitory influence on their postsynaptic targets for most, if not all, phases of the circadian cycle. The overwhelmingly GABAergic nature of the SCN, along with its internal connectivity properties, provide a strong model to examine how inhibitory neurotransmission generates output signals. In the present work we show that hyperpolarizations that range from 5 to 1000 ms elicit rebound spikes in 63% of all SCN neurons tested in voltage-clamp in the SCN of adult rats and hamsters. In current-clamp recordings, hyperpolarizations led to rebound spike formation in all cells; however, low-amplitude or short-duration current injections failed to consistently activate rebound spikes. Increasing the duration of hyperpolarization from 5 to 1000 ms is strongly and positively correlated with enhanced spike probability. Additionally, the magnitude of hyperpolarization exerts a strong influence on both the amplitude of the spike, as revealed by voltage-clamp recordings, and the latency to peak current obtained in either voltage- or current-clamp mode. Our results suggest that SCN neurons may use rebound spikes as one means of producing output signals from a largely interconnected network of GABAergic neurons.

Mechanism underlying rebound excitation in retinal ganglion cells

Retinal ganglion cells (RGCs) display the phenomenon of rebound excitation, which is observed as rebound sodium action potential firing initiated at the termination of a sustained hyperpolarization below the resting membrane potential (RMP). Rebound impulse firing, in contrast to corresponding firing elicited from rest, displayed a lower net voltage threshold, shorter latency and was invariably observed as a phasic burst-like doublet of spikes. The preceding hyperpolarization leads to the recruitment of a Tetrodotoxin-insensitive depolarizing voltage overshoot, termed as the net depolarizing overshoot (NDO). Based on pharmacological sensitivities, we provide evidence that the NDO is composed of two independent but interacting components, including (1) a regenerative low threshold calcium spike (LTCS) and (2) a non-regenerative overshoot (NRO). Using voltage and current clamp recordings, we demonstrate that amphibian RGCs possess the hyperpolarization activated mixed cation channels/current, Ih, and low voltage activated (LVA) calcium channels, which underlie the generation of the NRO and LTCS respectively. At the RMP, the Ih channels are closed and the LVA calcium channels are inactivated. A hyperpolarization of sufficient magnitude and duration activates Ih and removes the inactivation of the LVA calcium channels. On termination of the hyperpolarizing influence, Ih adds an immediate depolarizing influence that boosts the generation of the LTCS. The concerted action of both conductances results in a larger amplitude and shorter latency NDO than either mechanism could achieve on its own. The NDO boosts the generation of conventional sodium spikes which are triggered on its upstroke and crest, thus eliciting rebound excitation.

Melatonin modulates spike coding in the rat suprachiasmatic nucleus

The effects of the application of melatonin in vitro on the electrophysiological activity of suprachiasmatic neurones were characterised using novel measures of coding based on the analysis of interspike intervals. Perfusion of 1 nM melatonin in vitro (n = 53) had no consistent effect on mean spike frequency (Wilcoxon's sign rank, z = -0.01, P = 0.989), but increased the irregularity of firing (Student's paired t-test, t = -3.02, P = 0.004), as measured by the log interval entropy, and spike patterning (z = -3.43, P < 0.001), as measured by the mutual information between adjacent log intervals. Intracellular recordings in vitro in current clamp mode showed that 1 nM melatonin significantly hyperpolarised (n = 11, z = -2.35, P = 0.019) those cells that showed 'rebound' spikes upon termination of a hyperpolarising current pulse. Grouping all cells together (n = 27), melatonin application decreased the duration of the afterhyperpolarisation (z = -2.49, P = 0.013) and increased the amplitude of the depolarising afterpotential (z = -2.71, P = 0.007). The effects of melatonin seen in vitro from extracellular recordings on interspike interval coding were consistent with the changes in spike shape seen from intracellular recordings. A melatonin-induced increase in the size of the depolarising afterpotential of suprachiasmatic cells might underlie the increased irregularity of spike firing seen during the subjective night time. The method of analysis demonstrated a difference in spike firing that is not revealed by frequency alone and is consistent with the presence of a melatonin-induced depolarising current.

Normal and rebound impulse firing in retinal ganglion cells

Given that the action potential output of retinal ganglion cells (RGCs) determines the nature of the visual information that is transmitted from the retina, an understanding of their intrinsic impulse firing characteristics is critical for an appreciation of the overall processing of visual information. Recordings from RGCs within an isolated whole-mount retina preparation showed that their normal impulse firing from the resting membrane potential (RMP) was linearly correlated in its frequency with the stimulus intensity. In addition to describing the relationship between the magnitude of the current injection and the resulting impulse frequency (F/I relationship), we have characterized the properties of individual action potentials when they are elicited from the RMP. In contrast, hyperpolarizing below the RMP revealed that RGCs displayed a time dependent anomalous rectification, manifested by the appearance of a depolarizing sag in their voltage response. When an adequate period of hyperpolarization was terminated, a fast phasic period of “rebound excitation” was observed, characterized by a brief phasic burst of impulse activity. When compared to equivalent action potential firing evoked by depolarizing from the RMP, rebound spiking was associated with a lower threshold and shorter latency for impulse activation as well as a prominent, phasic, burst-like doublet, or triplet of impulses. The rebound action potential had a more positive voltage overshoot and displayed a higher peak rate of rise in its upstroke than those correspondingly generated by depolarizing current pulses from the RMP. Blocking sodium spikes with TTX confirmed that the preceding hyperpolarization led to the recruitment and subsequent generation of a transient depolarizing voltage overshoot, which we have termed the net depolarizing overshoot (NDO). We propose that the NDO boosts the generation of sodium spikes by triggering rebound spikes on its upstroke and crest, thus accounting for the observed voltage dependent change in the firing pattern of RGCs.

Electrophysiological characterization of neurons in the dorsolateral pontine rapid-eye-movement sleep induction zone of the rat: Intrinsic membrane properties and responses to carbachol and orexins

Pharmacological, lesion and single-unit recording techniques in several animal species have identified a region of the pontine reticular formation (subcoeruleus, SubC) just ventral to the locus coeruleus as critically involved in the generation of rapid-eye-movement (REM) sleep. However, the intrinsic membrane properties and responses of SubC neurons to neurotransmitters important in REM sleep control, such as acetylcholine and orexins/hypocretins, have not previously been examined in any animal species and thus were targeted in this study. We obtained whole-cell patch-clamp recordings from visually identified SubC neurons in rat brain slices in vitro. Two groups of large neurons (mean diameter 30 and 27 mum) were tentatively identified as cholinergic (rostral SubC) and noradrenergic (caudal SubC) neurons. SubC reticular neurons (non-cholinergic, non-noradrenergic) showed a medium-sized depolarizing sag during hyperpolarizing current pulses and often had a rebound depolarization (low-threshold spike, LTS). During depolarizing current pulses they exhibited little adaptation and fired maximally at 30-90 Hz. Those SubC reticular neurons excited by carbachol (n=27) fired spontaneously at 6 Hz, often exhibited a moderately sized LTS, and varied widely in size (17-42 mum). Carbachol-inhibited SubC reticular neurons were medium-sized (15-25 mum) and constituted two groups. The larger group (n=22) was silent at rest and possessed a prominent LTS and associated one to four action potentials. The second, smaller group (n=8) had a delayed return to baseline at the offset of hyperpolarizing pulses. Orexins excited both carbachol excited and carbachol inhibited SubC reticular neurons. SubC reticular neurons had intrinsic membrane properties and responses to carbachol similar to those described for other reticular neurons but a larger number of carbachol inhibited neurons were found (>50%), the majority of which demonstrated a prominent LTS and may correspond to pontine-geniculate-occipital burst neurons. Some or all carbachol-excited neurons are presumably REM-on neurons.

T-type Ca2+ channels encode prior neuronal activity as modulated recovery rates

T-type Ca2+ channels give rise to low-threshold inward currents that are central determinants of neuronal excitability. The availability of T-type Ca2+ channels is strongly influenced by voltage-dependent inactivation and recovery from inactivation. Here, we show that native and cloned T-type Ca2+ channel subunits selectively encode specific aspects of prior membrane potential changes via a powerful modulation of the rates with which these channels recover from inactivation. Increasing the duration of subthreshold (-70 to -55 mV) conditioning depolarizations caused a pronounced slowing of subsequent recovery from inactivation of both cloned (Ca(v)3.1-3.3) and native T-type channels (thalamic neurones). The scaling of recovery rates with increasing duration of conditioning depolarizations could be well described by a power law function. Different T-type channel isoforms exhibited overlapping but complementary ranges of recovery rates. Intriguingly, scaling of recovery rates was dramatically reduced in Ca(v)3.2 and Ca(v)3.3, but not Ca(v)3.1 subunits, when mock action potentials were superimposed on conditioning depolarizations. Our results suggest that different T-type channel subunits exhibit dramatic differences in scaling relationships, in addition to well-described differences in other biophysical properties. Furthermore, the availability of T-type channels is powerfully modulated over time, depending on the patterns of prior activity that these channels have encountered. These data provide a novel mechanism for cellular short-term plasticity on the millisecond to second time scale that relies on biophysical properties of specific T-type Ca2+ channel subunits.

Paradoxical potentiation of neuronal T-type Ca2+ current by ATP at resting membrane potential

Despite the marked influence on neuronal physiology of the low-voltage activated T-type Ca(2+) currents, little is known about the intracellular pathways and neurotransmitters involved in their regulations. Here, we report that in thalamocortical neurons a phosphorylation mechanism induces an increase both in the current amplitude (1.5 +/- 0.27-fold in the ventrobasal nucleus) and its inactivation kinetics. Dialysis of the neuron with an ATP-free solution suppresses the T-current potentiation, whereas it becomes irreversible in the presence of ATPgammaS. Phosphorylation occurs when the channels are inactivated and is slowly removed when they recover from inactivation and remain in closed states (time constants of the induction and removal of the potentiation: 579 +/- 143 msec and 4.9 +/- 1.1 sec, respectively, at 25 degrees C). The resulting apparent voltage sensitivity of this regulation follows the voltage dependence of the current steady-state inactivation. Thus, the current is paradoxically inhibited when the preceding hyperpolarization is lengthened, and maximal currents are generated after transient hyperpolarizations with a duration (0.7-1.5 sec) that is defined by the balance between the kinetics of the dephosphorylation and deinactivation. In addition, the phosphorylation will facilitate the generation of T current at resting membrane potential. This potentiation, which is specific to sensory thalamocortical neurons, would markedly influence the electroresponsiveness of these neurons and represent the first evidence of a regulation of native Cav3.1 channels.

Molecular physiology of low-voltage-activated t-type calcium channels

T-type Ca2+ channels were originally called low-voltage-activated (LVA) channels because they can be activated by small depolarizations of the plasma membrane. In many neurons Ca2+ influx through LVA channels triggers low-threshold spikes, which in turn triggers a burst of action potentials mediated by Na+ channels. Burst firing is thought to play an important role in the synchronized activity of the thalamus observed in absence epilepsy, but may also underlie a wider range of thalamocortical dysrhythmias. In addition to a pacemaker role, Ca2+ entry via T-type channels can directly regulate intracellular Ca2+ concentrations, which is an important second messenger for a variety of cellular processes. Molecular cloning revealed the existence of three T-type channel genes. The deduced amino acid sequence shows a similar four-repeat structure to that found in high-voltage-activated (HVA) Ca2+ channels, and Na+ channels, indicating that they are evolutionarily related. Hence, the alpha1-subunits of T-type channels are now designated Cav3. Although mRNAs for all three Cav3 subtypes are expressed in brain, they vary in terms of their peripheral expression, with Cav3.2 showing the widest expression. The electrophysiological activities of recombinant Cav3 channels are very similar to native T-type currents and can be differentiated from HVA channels by their activation at lower voltages, faster inactivation, slower deactivation, and smaller conductance of Ba2+. The Cav3 subtypes can be differentiated by their kinetics and sensitivity to block by Ni2+. The goal of this review is to provide a comprehensive description of T-type currents, their distribution, regulation, pharmacology, and cloning.

A Quantitative Description of Low- and High-threshold Ca2+ Spikes in Rat Sensory Neurons: A Perforated-patch Study

Action potentials generated by voltage-dependent Ca2+ conductances were studied at 25 degrees C with the perforated-patch technique, in freshly dispersed adult rat sensory neurons perfused with Na-free solutions containing tetraethylammonium. Brief depolarizing currents from membrane potentials negative to - 75 mV always elicited long (> 100 ms) plateau spikes which had different thresholds in different neurons: a low threshold around - 60/- 50 mV and a high-threshold at - 30/- 20 mV. Stimulations from potentials positive to - 55 mV, on the contrary, elicited spikes originating only in the high threshold region and sensitive to 25 microM Cd2+, designated high-threshold spikes. In neurons which showed spikes with low threshold, addition of 25 microM Cd2+ disclosed a smaller and shorter regenerative response, the low-threshold spike. Moreover, the classical 'anode-break' stimulation from - 50/- 60 mV uncovered isolated low-threshold spikes, indicating a time- and voltage-dependent de-inactivating process. From the properties of the low (LVA) and high (HVA) voltage-activated Ca2+ currents, recorded under the same extracellular conditions, a Hodgkin - Huxley model was derived and used to reconstruct all the features of the recorded spikes. The model was also able to simulate experimental blocking of LVA channels by amiloride, modulation of HVA channels by baclofen and induced oscillatory firing. This agreement between the behaviour of recorded spikes and their mathematical description led us to conclude that the LVA and HVA Ca2+ currents underlie the low- and high-threshold Ca2+ spikes, respectively. Furthermore, our data suggest that complex behaviour known to be typical of central nervous system neurons is also present in sensory peripheral neurons.

Pedunculopontine stimulation induces prolonged activation of pontine reticular neurons

Extracellular and intracellular recordings were carried out from neurons in the region of the pontine reticular formation at the transition between the nucleus reticularis pontis oralis and caudalis, and in the pontis caudalis. Responses were studied after stimulation of the mesopontine cholinergic pedunculopontine nucleus in precollicular-postmammillary transected, paralyzed preparations. Recordings of neurographic activity in hindlimb flexor and extensor nerves served to detect changes in fictive locomotion and muscle tone induced by pedunculopontine nucleus stimulation or occurring spontaneously. Short duration trains of pedunculopontine nucleus stimulation induced long lasting responses, on average over 12s in duration, in one-third of pontine reticular neurons. These prolonged responses were stimulation frequency-dependent such that the longest durations were induced by stimulation at 20-60Hz. In some cells, stimulation at lower (10Hz) or higher (100Hz) frequencies induced responses of shorter duration or were absent, while in others, higher frequencies prolonged the excitatory effects of pedunculopontine nucleus stimulation. We conclude that these stimulation frequency-dependent effects may be related to the modulation of postural muscle tone and locomotion by the pedunculopontine nucleus.

Dendro-somatic distribution of calcium-mediated electrogenesis in purkinje cells from rat cerebellar slice cultures

The role of Ca2+ entry in determining the electrical properties of cerebellar Purkinje cell (PC) dendrites and somata was investigated in cerebellar slice cultures. Immunohistofluorescence demonstrated the presence of at least three distinct types of Ca2+ channel proteins in PCs: the alpha1A subunit (P/Q type Ca2+ channel), the alpha1G subunit (T type) and the alpha1E subunit (R type). In PC dendrites, the response started in 66 % of cases with a slow depolarization (50 +/- 15 ms) triggering one or two fast (approximately 1 ms) action potentials (APs). The slow depolarization was identified as a low-threshold non-P/Q Ca2+ AP initiated, most probably, in the dendrites. In 16 % of cases, this response propagated to the soma to elicit an initial burst of fast APs. Somatic recordings revealed three modes of discharge. In mode 1, PCs display a single or a short burst of fast APs. In contrast, PCs fire repetitively in mode 2 and 3, with a sustained discharge of APs in mode 2, and bursts of APs in mode 3. Removal of external Ca2+ or bath applications of a membrane-permeable Ca2+ chelator abolished repetitive firing. Tetraethylammonium (TEA) prolonged dendritic and somatic fast APs by a depolarizing plateau sensitive to Cd2+ and to omega-conotoxin MVII C or omega-agatoxin TK. Therefore, the role of Ca2+ channels in determining somatic PC firing has been investigated. Cd2+ or P/Q type Ca2+ channel-specific toxins reduced the duration of the discharge and occasionallyinduced the appearance of oscillations in the membrane potential associated with bursts of APs. In summary, we demonstrate that Ca2+ entry through low-voltage gated Ca2+ channels, not yet identified, underlies a dendritic AP rarelyeliciting a somatic burst of APs whereas Ca2+ entry through P/Q type Ca2+ channels allowed a repetitive firing mainly by inducing a Ca2+-dependent hyperpolarization.

Association between the low threshold calcium spike and activation of NMDA receptors in guinea-pig substantia nigra pars compacta neurons

The aim of this study was to examine the interaction between N-methyl-D-aspartate (NMDA) receptor activation and the low threshold calcium spike (LTS) of phasically firing neurons in the rostral part of the substantia nigra pars compacta (SNpc) in mid-brain slices. Bath perfusion of 10 microM NMDA gradually increased the LTS area and the effect reached a maximum after 6 min of perfusion. This enhancement of the LTS by NMDA was blocked both by a competitive and non-competitive NMDA receptor antagonist, 50 microM D-AP5 and 10 microM MK801, respectively, demonstrating that this effect of NMDA was mediated through NMDA receptors. Prolonged exposure to increasing concentrations of NMDA (0.1-100 microM) progressively decreased the LTS area. The higher doses led to an irreversible marked depolarization and decrease of the membrane resistance. These results suggest that the LTS of SNpc neurons can trigger a NMDA receptor-dependent response which may have physiological and pathological roles.

Membrane properties of giant neurons in the caudal pontine reticular formation in vitro

The intrinsic membrane properties of giant neurons of the caudal pontine reticular nucleus (PnC) were evaluated in a rat brain slice preparation. These neurons show low RP levels, long membrane time constants, low membrane resistance, tonic AP firing, and a lack of rectification behavior. The results are compared to those measured in adjacent areas of the reticular formation and discussed for their impact on triggering and modulating the acoustic startle response.

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

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

Electrophysiological properties and cholinergic responses of rat ventral oral pontine reticular neurons in vitro

In order to characterize the electrophysiological properties of morphologically identified neurons of the ventral part of the oral pontine reticular (vRPO) nucleus and the effects of cholinergic agonists on them, intracellular recordings were obtained from 45 cells in a rat brain-slice preparation. Intracellular staining was performed with 2% biocytin in potassium acetate (1 M)-filled micropipettes. Results demonstrated the presence of two types of vRPO neurons. Type I cells (n = 12, 24%) were characterized by a break with a decrease of the depolarizing slope following hyperpolarizing pulses which delayed the return to the resting Vm and subsequent spike-firing. The delay was antagonized by 4-AP (200-500 microM) which specifically blocks the transient outward K+-mediated current I(A). Type II neurons (n = 38, 76%) displayed a typical depolarizing sag during hyperpolarizing current pulses which was blocked by Cs+. This behavior is characteristic of the hyperpolarization-activated current I(Q). These two neuronal types displayed different morphological features. Most type I and II cells (100 and 73.7%, respectively) were depolarized by acetylcholine (1-15 microM), carbachol (0.5-1 microM) and muscarine (1-10 microM) through the activation of post-synaptic muscarinic receptors. The remaining type II cells (26.3%) were hyperpolarized (1-10 min, 3-15 mV) through the activation of post-synaptic muscarinic receptors. Results are consistent with the hypothesis that the vRPO could be a neuronal target of Cch in eliciting paradoxical sleep because most of its neurons are activated by muscarinic agonists.

The vestibular system as a model of sensorimotor transformations. A combined in vivo and in vitro approach to study the cellular mechanisms of gaze and posture stabilization in mammals

To understand the cellular mechanisms underlying behaviours in mammals, the respective contributions of the individual properties characterizing each neuron, as opposed to the properties emerging from the organization of these neurons in functional networks, have to be evaluated. This requires the use, in the same species, of various in vivo and in vitro experimental preparations. The present review is meant to illustrate how such a combined in vivo in vitro approach can be used to investigate the vestibular-related neuronal networks involved in gaze and posture stabilization, together with their plasticity, in the adult guinea-pig. Following first a general introduction on the vestibular system, the second section describes various in vivo experiments aimed at characterizing gaze and posture stabilization in that species. The third and fourth parts of the review deal with the combined in vivo-in vitro investigations undertaken to unravel the physiological and pharmacological properties of vestibulo-ocular and vestibulo-spinal networks, together with their functional implications. In particular, we have tried to use the central vestibular neurons as examples to illustrate how the preparation of isolated whole brain can be used to bridge the gap between the results obtained through in vitro, intracellular recordings on slices and those collected in vivo, in the behaving animal.

Electrophysiological study of nucleus gigantocellularis neurons in guinea-pig brainstem slices

Gigantocellular neurons of the medullary nucleus gigantocellularis represent a major source of reticulospinal pathways. Among other roles, they have been involved in the processing of vestibular information. The aim of the present study was to describe the major intrinsic membrane properties of these cells in guinea-pig brainstem slices. We found nucleus gigantocellularis neurons to be segregated in two cell types. Type A nucleus gigantocellularis neurons were characterized by the presence of a single large afterhyperpolarization and a potent transient 4-aminopyridine-sensitive rectification likely due to the presence of a transient outward potassium current. In contrast, type B nucleus gigantocellularis neurons had a narrower and faster rising action potential followed by an early fast and a delayed slower after-hyperpolarization. In contrast to type A neurons, type B neurons were, in addition, endowed with subthreshold tetrodotoxin-sensitive sodium-dependent plateau potentials. Whereas both cell types were endowed with high-threshold calcium-dependent action potentials, only type B nucleus gigantocellularis neurons also displayed long-lasting calcium-dependent plateau potentials. These results show that nucleus gigantocellularis neurons can be segregated by their intrinsic membrane properties it two cell types which are very similar to those that we have previously described in the medial vestibular nucleus. The possibility that these differences between type A and B neurons might play a role in the segregation between tonic and kinetic cells is discussed.

The pharmacology and function of central GABAB receptors

In conclusion, GABAB receptors enable GABA to modulate neuronal function in a manner not possible through GABAA receptors alone. These receptors are present at both pre- and postsynaptic sites and can exert both inhibitory and disinhibitory effects. In particular, GABAB receptors are important in regulating NMDA receptor-mediated responses, including the induction of LTP. They also can regulate the filtering properties of neural networks, allowing peak transmission in the frequency range of theta rhythm. Finally, GABAB receptors are G protein-coupled to a variety of intracellular effector systems, and thereby have the potential to produce long-term changes in the state of neuronal activity, through actions such as protein phosphorylation. Although the majority of the effects of GABAB receptors have been reported in vitro, recent studies have also demonstrated that GABAB receptors exert electrophysiological actions in vivo. For example, GABAB receptor antagonists reduce the late IPSP in vivo and consequently can decrease inhibition of spontaneous neuronal firing following a stimulus (Lingenhöhl and Olpe, 1993). In addition, blockade of GABAB receptors can increase spontaneous activity of central neurons, suggesting the presence of GABAB receptor-mediated tonic inhibition (Andre et al., 1992; Lingenhöhl and Olpe, 1993). Despite these electrophysiological effects, antagonism of GABAB receptors has generally been reported to produce few behavioral actions. This lack of overt behavioral effects most likely reflects the modulatory nature of the receptor action. Nevertheless, two separate behavioral studies have recently reported an enhancement of cognitive performance in several different animal species following blockade of GABAB receptors (Mondadori et al., 1992; Carletti et al., 1993). Because of their small number of side effects, GABAB receptor antagonists may represent effective therapeutic tools for modulation of cognition. Alternatively, the lack of overt behavioral effects of GABAB receptors may indicate that these receptors are more important in pathologic rather than normal physiological states (Wojcik et al., 1989). For example, a change in receptor affinity or receptor number brought on by the pathology could enhance the effectiveness of GABAB receptors. Of significance, CGP 35348 has been shown to block absence seizures in genetically seizure prone animals, while inducing no seizures in control animals (Hosford et al., 1992; Liu et al., 1992). Thus, GABAB receptors may represent effective sites for pharmacological regulation of absence seizures. Perhaps further behavioral effects of these receptors will become apparent only after additional studies have been performed using the highly potent antagonists that have been recently introduced.(ABSTRACT TRUNCATED AT 400 WORDS)

Inward rectifier and low-threshold calcium currents contribute to the spontaneous firing mechanism in neurons of the rat suprachiasmatic nucleus

Intracellular and voltage-clamp studies were carried out to clarify the mechanism for spontaneous firing activity in neurons of the suprachiasmatic nucleus (SCN) of rat hypothalamic brain slices in vitro. SCN neurons displayed spontaneously firing action potentials that were preceded by a depolarizing pre-potential and followed by a short spike after-hyperpolarization (AHP). Injection of inward current with a duration longer than 50 ms resulted in a depolarizing voltage "sag" on hyperpolarizing electrotonic potentials. The inward rectification was depressed by bath application of caesium (1 mM) but not by barium (500 microM). SCN neurons also showed a rebound depolarization associated with spike discharge (anodal break) in response to relaxation of hyper polarizing current injection. The rebound depolarization was reduced by nominally zero calcium. Cadmium (500 microM), cobalt (1 mM) or caesium (1 mM) but not nicardipine also depressed the rebound depolarization. Under voltage-clamp conditions, hyperpolarizing steps to membrane potentials negative to approximately -60 mV caused an inward rectifier current, probably H current (IH), which showed no inactivation with time. Bath application of caesium (1-2 mM) suppressed IH. Caesium (2 mM) depressed the slope of the depolarizing spike pre-potential, resulting in a prolongation of the interspike interval of tonic firing neurons. We conclude that both the inward rectifier current, IH, and the low-threshold calcium current contribute to the spike prepotential of spontaneous action potentials in firing neurons of the rat SCN.

Serotonin1 and serotonin2 receptors hyperpolarize and depolarize separate populations of medial pontine reticular formation neurons in vitro

The action of serotonin on medial pontine reticular formation neurons was examined using intracellular electrophysiological methods in rat brainstem slices in vitro. A hyperpolarization associated with a decrease in input resistance was elicited by serotonin in 34% of the neurons, and a depolarization associated with an increase in input resistance was produced in 56% of the neurons. Both responses persisted in the presence of tetrodotoxin. The hyperpolarization resulted from a steady-state increase in outward current which varied with the external potassium concentration in a manner consistent with a conductance increase primarily to this ion. This response was mimicked by the serotonin1 agonist, 5-carboxamidotryptamine, as well as by the serotonin1a agonist, 8-hydroxy-dipropyl aminotetralin hydrobromide, and was blocked by spiperone, an antagonist of serotonin1 sites. The depolarization resulted from a steady-state decrease in outward current which varied with external potassium. The depolarization was mimicked by the serotonin2 agonist, alpha-methyl-5-hydroxytryptamine, and was blocked by the serotonin2 antagonist, ketanserin. Neither of these agents had any effect upon serotonin-induced hyperpolarizations. In conclusion, the excitability of medial pontine reticular formation neurons is influenced by serotonin acting to increase or decrease potassium conductance(s). These opposing effects reflect actions on distinct serotonin receptor subtypes that are segregated to distinct populations of medial pontine reticular formation neurons.

Serotonin hyperpolarizes cholinergic low-threshold burst neurons in the rat laterodorsal tegmental nucleus in vitro

Serotonergic suppression of cholinergic neuronal activity implicated in the regulation of rapid eye movement sleep and its associated phenomenon, pontogeniculooccipital waves, has long been postulated, but no direct proof has been available. In this study, intracellular and whole-cell patch-clamp recording techniques were combined with enzyme histochemistry to examine the intrinsic electrophysiological properties and response to serotonin (5-HT) of identified cholinergic rat laterodorsal tegmental nucleus neurons in vitro. Sixty-five percent of the recorded neurons demonstrated a prominent low-threshold burst, and of these, 83% were cholinergic. In current-clamp recordings 64% of the bursting cholinergic neurons tested responded to the application of 5-HT with a membrane hyperpolarization and decrease in input resistance. This effect was mimicked by application of the selective 5-HT type 1 receptor agonist carboxamidotryptamine maleate. Whole-cell patch-clamp recordings revealed that the hyperpolarizing response was mediated by an inwardly rectifying K+ current. Application of 5-HT decreased excitability and markedly modulated the discharge pattern of cholinergic bursting neurons: during a 5-HT-induced hyperpolarization these neurons exhibited no rebound burst after hyperpolarizing current input and a burst in response to depolarizing current input. In the absence of 5-HT, the relatively depolarized cholinergic bursting neurons responded to an identical hyperpolarizing current input with a burst and did not produce a burst after depolarizing current input. These data provide a cellular and molecular basis for the hypothesis that 5-HT modulates rapid eye movement sleep phenomenology by altering the firing pattern of bursting cholinergic neurons.

Electrophysiology of dopaminergic and non-dopaminergic neurones of the guinea-pig substantia nigra pars compacta in vitro

1. The membrane properties of substantia nigra pars compacta neurones were studied using an in vitro slice preparation of guinea-pig midbrain. 2. Neurones were divided into two classes based on their electrophysiological properties: bursting neurones displayed a burst of several rapidly accommodating action potentials in response to relaxation of hyperpolarizing current injected through the microelectrode, while non-bursting neurones produced regularly spaced action potentials. These neuronal types were found to be electrophysiologically distinct from those recorded in the substantia nigra pars reticulata and the subthalamic nucleus. 3. Non-bursting neurones, which comprised ca 85% of the sampled cells, were characterized by a slow, pacemaker pattern of firing at rest, broad action potentials, a pronounced spike after-hyperpolarization, long membrane time constants, and strong transient outward and time-dependent inward rectification. 4. Bursting neurones, comprising ca 15% of the sample, displayed rapid firing rates at rest, fast action potentials, a shallow spike after-hyperpolarization and briefer membrane time constants. All of these parameters were significantly different from those of the non-bursting type. Bursting neurones lacked transient outward or time-dependent inward rectification. 5. Both types of cells were capable of generating pronounced calcium-dependent, low-threshold spikes in the presence of tetrodotoxin (TTX). However, only the non-bursting type displayed calcium-dependent rhythmic oscillations in membrane potential near resting potential in the presence of TTX. The firing rate, action potential shape and after-hyperpolarization of non-bursting neurones were strongly influenced by calcium-dependent currents. 6. The majority of cells were injected with biocytin, which allowed morphological reconstruction of the neurones and confirmation of their location within the pars compacta. Non-bursting neurones had variable soma shapes and their dendrites were mostly directed in a medio-lateral direction. Many cells extended some of their dendrites into the pars reticulata. Bursting neurones were mainly fusiform in shape with their dendrites oriented in a medio-lateral direction; a few had dendrites extending into the pars reticulata. 7. Thirty-six neurones were also double labelled using a combination of biocytin or Lucifer Yellow injection with tyrosine hydroxylase (TH) immunohistochemistry. Non-bursting neurones all displayed TH immunofluorescence, while none of the bursting neurones were TH positive.(ABSTRACT TRUNCATED AT 400 WORDS)

Two patterns of firing in human neocortical neurons

Intracellular recordings were made in slices of human neocortex removed for surgical treatment of epilepsy. In response to prolonged suprathreshold current injection, regular spiking neurons (67.5% of sample) responded by repetitive firing throughout the stimulus from all membrane potentials. Bursting neurons (32.5% of sample) responded with a burst of 2-3 spikes which rode upon a voltage-dependent slow depolarization. Bursting behavior was only observed from membrane potentials more negative than -65 mV.

A cholinoceptive desynchronized sleep induction zone in the anterodorsal pontine tegmentum: spontaneous and drug-induced neuronal activity

The effect of carbachol microapplication (4 micrograms/250 nl per 90 s) on the discharge of neurons in the anterodorsal pons of four cats was studied using a newly devised microinjector-microelectrode assembly. Neurons were classified according to the magnitude of their discharge rate increases (or decreases) in physiological desynchronized sleep as desynchronized-on (or desynchronized-off) before injecting carbachol. When carbachol produced a desynchronized sleep-like state only half (15 out of 30) of the desynchronized-on cells were activated (desynchronized-on/desynchronized sleep-like state-on) while the other half were not (desynchronized-on/desynchronized sleep-like state-not on). Compared with the non-activated cells, the desynchronized-on/desynchronized sleep-like state-on cells had three features consistent with playing an active role in desynchronized sleep generation: these cells had a higher mean discharge frequency in desynchronized sleep and higher ratio of discharge frequency in desynchronized sleep compared with wakefulness; they did not fire in phase with electromyogram excitation of neck muscles; and they were concentrated in the short latency desynchronized sleep-like state induction zone described in the companion paper. The three-way correlation between the optimal anatomical site for short latency desynchronized sleep-like state induction, the selective neuronal discharge pattern in desynchronized sleep and the cholinergic activation pattern in the desynchronized sleep-like state suggest that we may have identified a neuronal population that is cholinoceptively activated as part of the physiological mechanism of desynchronized sleep generation.

Calcium currents in rat thalamocortical relay neurones: kinetic properties of the transient, low-threshold current

1. Calcium currents were recorded with whole-cell voltage-clamp procedures in relay neurones of the rat thalamus which had been acutely isolated by an enzymatic dissociation procedure. 2. Low-threshold and high-threshold Ca2+ currents were elicited by depolarizing voltage steps from holding potentials more negative than -60 mV. A transient current, analogous to the T-current in sensory neurones, was activated at low threshold near -65 mV and was completely inactivating at command steps up to -35 mV. Voltage steps to more depolarized levels activated a high-threshold current that inactivated slowly and incompletely during a 200 ms step depolarization. 3. The high-threshold current contained both non-inactivating and slowly inactivating components which were insensitive and sensitive to holding potential, respectively. 4. A 'T-type' current was prominent in relay neurones, in both absolute terms (350 pA peak current average) and in relation to high-threshold currents. The average ratio of maximum transient to maximum sustained current was greater than 2. 5. T-current could be modelled in a manner analogous to that employed for the fast Na+ current underlying action potential generation, using the m3h format. The rate of activation of T-current was voltage dependent, with a time constant (tau m) varying between 8 and 2 ms at command potentials of -60 to -10 mV at 23 degrees C. The rate of inactivation was also voltage dependent, and the time constant tau h varied between 50 and 20 ms over the same voltage range. With command potentials more positive than -35 mV, the inactivation of Ca2+ current could no longer be fitted by a single exponential. 6. Steady-state inactivation of T-current could be well fitted by a Boltzman equation with slope factor of 6.3 and half-inactivated voltage of -83.5 mV. 7. Recovery from inactivation of T-current was not exponential. The major component of recovery (70-80% of total) was not very voltage sensitive at potentials more negative than -90 mV, with tau r of 251 ms at -92 mV and 23 degrees C, compared to 225 ms at -112 mV. A smaller, voltage-sensitive component accounted for the remainder of recovery. 8. All kinetic properties, including rates of activation, inactivation, and recovery from inactivation, as well as the amplitude of T-current, were temperature sensitive with Q10 (temperature coefficient) values of greater than 2.5.(ABSTRACT TRUNCATED AT 400 WORDS)

Repetitive firing properties of medial pontine reticular formation neurones of the rat recorded in vitro

1. Intracellularly recorded neurones in nucleus reticularis pontis caudalis of the medial pontine reticular formation (mPRF) in the in vitro slice preparation were analysed for repetitive firing properties in response to intracellularly applied constant-current pulses. 2. Three neuronal classes were defined by this procedure: (1) non-burst neurones, which had only a non-burst firing pattern; (2) low-threshold burst neurones, which had either a low-threshold burst pattern or a non-burst pattern; (3) high-threshold burst neurones, which had either a high-threshold burst pattern or a non-burst pattern. 3. Histological characterization of electrophysiologically identified mPRF neurones with carboxyfluorescein showed no definite morphological difference between the first two classes. There was a trend for low-threshold burst neurones to have larger somata. 4. The low-threshold burst was generated by a slow calcium-dependent low-threshold spike, revealed in the presence of tetrodotoxin. The size of the low-threshold spike and thus the number of fast action potentials in the low-threshold burst was controlled by at least five factors including: activation; inactivation; amplitude of low-threshold conductance available to be activated; delayed outward conductance; and early transient outward conductance. 5. The non-burst pattern examined in both non-burst and low-threshold burst neurones appeared to be controlled primarily by one or more calcium-dependent potassium conductances sensitive to the removal of calcium and tetraethyl-ammonium. In the presence of tetrodotoxin (TTX), the addition of antagonists to calcium-dependent potassium current revealed a slow high-threshold calcium spike which was distinguished from the low-threshold spike by its threshold, lack of inactivation (at potentials negative to -40 mV) and insensitivity to Mg2+. A long-duration after-hyperpolarization (greater than 0.5 s) was not observed in any of these cells. 6. An early transient outward rectification sensitive to 4-aminopyridine and probably mediated by A-current was apparent in low-threshold burst and non-burst neurones and affected both the low-threshold burst and non-burst firing patterns. 7. Alteration of resting membrane potential, such as occurs in vivo during the depolarization associated with desynchronized sleep, may inactivate the low-threshold spike and the transient outward conductance responsible for the distinctive responses observed from more hyperpolarized membrane potentials and produce a more homogeneous non-burst response pattern. Membrane potential effects on intrinsic conductances thus may furnish an important mechanism for changes in mPRF neuronal responsivene

Cholinergic activation of medial pontine reticular formation neurons in vitro

Direct microinjection of cholinergic compounds into pontine reticular formation furnishes an excellent phenomenological model of the rapid eye movement phase of sleep (REM), but the mechanisms underlying this effect and whether they mimic the cellular events of natural REM remain unknown. Data presented here from intracellular recordings in vitro in the rat demonstrate that two-thirds of medial pontine reticular formation neurons respond to application of 0.5-1.0 microM carbachol with a depolarization characterized by a decreased conductance and a linear I/V curve. The resultant mimicry of REM cellular events by carbachol extends to membrane potential depolarization, increased cellular excitability, enhancement of PSPs from reticular stimulation, and the absence of a burst discharge pattern. The presence of these effects with tetrodotoxin and their blockade by atropine imply a direct, muscarinic cholinergic mediation. Other neurons tested responded with either a biphasic hyperpolarization-depolarization or a hyperpolarization. The hyperpolarization was associated with an increased conductance which exhibited pronounced inward rectification, an effect novel for cholinergic agonists in vertebrate CNS but described in heart cells.

Cholinergic activation of medial pontine reticular formation neurons in vitro

In vivo microinjections of cholinergic compounds into the medial pontine reticular formation have produced some or depending on the injection site, all of the phenomena of REM, thus providing the only adequate pharmacological model of this behavioral state. The necessary anatomical substrate, a cholinergic projection to the mPRF was recently demonstrated, however the direct effect of cholinergic agonists on mPRF neurons is unknown. We have examined the effects of carbachol on mPRF neurons recorded in vitro from brainstem slices of Sprague-Dawley rats (8-10 days old). Three kinds of response to the application of carbachol (0.5-1 microM) were observed (n = 15) as follows: a depolarizing response (67%), a hyperpolarizing response (20%) and a biphasic response consisting of a hyperpolarizing response followed by a depolarizing response (13%). Under voltage clamp control, the depolarizing response was observed as an inward current resulting from a decrease in conductance which was constant over the membrane potential range of -100 to -50 mV. Reversal potential was negative to -80 mV. An increase in the excitability of neurons (as measured by responses to identical intracellularly applied depolarizing current pulses) during the depolarizing responses was due to the increase in steady state inward current. When intracellular DC current of equal amplitude but opposite polarity was applied, no increase in excitability was observed. This response was always blocked by the addition of atropine (0.5-1 microM) to the perfusate. The hyperpolarizing response was observed as an increase in outward current due to an increase in conductance with marked voltage sensitivity (over the range of -100 to -50 mV) characteristic of the anomalous rectifier. Preliminary data indicated that the hyperpolarizing response was more sensitive to pirenzepine (complete blockade at 1.0 microM) than the depolarizing response (complete blockade at 2 microM) but neither response was affected by pirenzepine concentrations of 200 nM or less. Cholinergic effects on evoked depolarizing PSPs were examined on neurons with depolarizing (n = 3) and biphasic (n = 1) responses and in all cases, the PSPs were enhanced. This enhancement was blocked by atropine. In conclusion, it is suggested that activation of two different muscarinic receptors (neither of which is the M1 receptor) on mPRF neurons results in two different responses, a decrease in a voltage-insensitive potassium conductance and an increase in the anomalous rectifier.

The intrinsic electrophysiological properties of mammalian neurons: insights into central nervous system function

This article reviews the electroresponsive properties of single neurons in the mammalian central nervous system (CNS). In some of these cells the ionic conductances responsible for their excitability also endow them with autorhythmic electrical oscillatory properties. Chemical or electrical synaptic contacts between these neurons often result in network oscillations. In such networks, autorhythmic neurons may act as true oscillators (as pacemakers) or as resonators (responding preferentially to certain firing frequencies). Oscillations and resonance in the CNS are proposed to have diverse functional roles, such as (i) determining global functional states (for example, sleep-wakefulness or attention), (ii) timing in motor coordination, and (iii) specifying connectivity during development. Also, oscillation, especially in the thalamo-cortical circuits, may be related to certain neurological and psychiatric disorders. This review proposes that the autorhythmic electrical properties of central neurons and their connectivity form the basis for an intrinsic functional coordinate system that provides internal context to sensory input.

Actions of serotonin recorded intracellularly in rat dorsal lateral septal neurons

The actions of serotonin (5HT) on passive and active membrane properties of neurons in the rat dorsal lateral septal nucleus (LSN) were studied by using intracellular recordings in transverse, septal slices. Superfusion with 10 microM 5HT induced a hyperpolarization of the membrane in almost all neurons tested in the dorsolateral part of the LSN. The hyperpolarization was accompanied by a decrease in membrane resistance. These effects of 5HT persisted in a low-Ca2+/high-Mg2+-containing medium or medium with tetrodotoxin, indicating a post-synaptic site of action for 5HT. The reversal potential for the hyperpolarizing effect was ca. -95 mV. If the extracellular K+-concentration was raised, the reversal potential became less negative. These data suggest that 5HT hyperpolarizes LSN neurons by increasing a K+-conductance. Spontaneous, synaptically evoked action potentials and action potentials induced in LSN neurons by a depolarizing current step typically display a fast Na+-spike with a subsequent K+-afterhyperpolarization, followed by a much slower Ca2+-dependent afterdepolarization. The amplitude of the K+-afterhyperpolarization was decreased by 5HT, while at the same time the afterdepolarization became more pronounced. The Ca2+-spike of LSN neurons was not affected by 5HT. Synaptic responses that were evoked in LSN neurons by stimulation of the dorsal part of the LSN consisted of a fast EPSP or spike, followed by a Cl(-)-dependent fast IPSP and a K+-dependent late IPSP. Of these synaptic responses, 5HT suppressed particularly the late IPSP. The present data indicate that 5HT affects the conductance for active and passive K+-channels in LSN neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Low-threshold calcium spikes in hypothalamic neurons recorded near the paraventricular nucleus in vitro

From guinea-pig hypothalamic slices, intracellular studies demonstrate the existence of neurons responding to depolarizing current pulses by bursts of fast spikes riding on slow depolarizing potentials, when activated at the resting potential or from hyperpolarized levels (44 cells). Slow depolarizing potentials have a mean amplitude of 17.6 mV and a mean duration of 65.2 msec. They are also produced at the termination of hyperpolarizing current pulses. The ionic basis for these slow potentials have been investigated. Fast spikes constituting the burst discharge are blocked by TTX but the slow component is unaffected, being blocked by Co++ and enhanced by Ba++. Taken together, these results show that the slow depolarizing potentials are generated by a low-threshold Ca++ conductance which is de-inactivated by membrane hyperpolarization. When the neurons are spontaneously active, they exhibit bursts arising from slow depolarizing potentials reminiscent of those evoked by direct stimulation. They also show longer episodes of repetitive firing. Twelve neurons were intracellularly stained and were found in the periphery of the paraventricular nucleus (PVN), in close proximity to the groups of neurophysin-positive neuroendocrine neurons present in the lateral part of this nucleus. Injected neurons have the morphology of reticular cells, judging by their few multipolar, rectilinear and sparsely branched dendrites. Some of their processes are directed towards PVN. Because of their intrinsic electrophysiological properties and their possible relationships with PVN, the population of cells described in the present study may play a role in functions relating to the PVN.

Pacemaker potentials of serotonergic dorsal raphe neurons: contribution of a low-threshold Ca2+ conductance

Neurons of the dorsal raphe nucleus exhibit intrinsic pacemaker potentials (gradual interspike depolarizing ramps) enabling them to sustain spontaneous rhythmic activity in the absence of synaptic interactions. A depolarizing prepotential (PP) has been observed in these cells, which appears to trigger the spike toward the end of the pacemaker cycle. The purposes of this study, carried out in the rat dorsal raphe nucleus brain slice preparation, were to 1) determine the ionic nature of the PP, 2) investigate its time- and voltage-dependent properties, and 3) investigate the possible modulation of the underlying conductance by the alpha 1-agonist phenylephrine and by serotonin (5-HT), agents that modify dorsal raphe pacemaker activity. During intracellular recording under current clamp, PPs were completely and reversibly blocked by divalent cations indicating that Ca2+ carries a significant portion of the current causing the PPs. Ni2+ specifically inhibited the PP with no effect on high-threshold (-40 mV) Ca2+ spikes or the Ca2+-activated K+ conductance in these neurons. Activation threshold for the PP was found to be approximately -60 mV. Priming by hyperpolarization allowed removal of inactivation (de-inactivation) of the PP in a time- and voltage-dependent manner, with maximal PPs accompanying hyperpolarizing pulses to -90 mV and the de-inactivation beginning to occur between -65 and -75 mV. Single-electrode voltage-clamp experiments demonstrated a region of negative-slope conductance between -60 and -50 mV, which corresponds to the range of PP activation.(ABSTRACT TRUNCATED AT 250 WORDS)