Similar properties of transient, persistent, and resurgent Na currents in GABAergic and non-GABAergic vestibular nucleus neurons

Sodium currents in fast firing neurons are tuned to support sustained firing rates >50-60 Hz. This is typically accomplished with fast channel kinetics and the ability to minimize the accumulation of Na channels into inactivated states. Neurons in the medial vestibular nuclei (MVN) can fire at exceptionally high rates, but their Na currents have never been characterized. In this study, Na current kinetics and voltage-dependent properties were compared in two classes of MVN neurons with distinct firing properties. Non-GABAergic neurons (fluorescently labeled in YFP-16 transgenic mice) have action potentials with faster rise and fall kinetics and sustain higher firing rates than GABAergic neurons (fluorescently labeled in GIN transgenic mice). A previous study showed that these neurons express a differential balance of K currents. To determine whether the Na currents in these two populations were different, their kinetics and voltage-dependent properties were measured in acutely dissociated neurons from 24- to 40-day-old mice. All neurons expressed persistent Na currents and large transient Na currents with resurgent kinetics tuned for fast firing. No differences were found between the Na currents expressed in GABAergic and non-GABAergic MVN neurons, suggesting that differences in properties of these neurons are tuned by their K currents.     

Firing properties and dendrotoxin-sensitive sustained potassium current in vestibular nuclei neurons of the hatchling chick

To understand the emergence of excitability in vestibular nuclei neurons, we performed patch-clamp recordings on brain slices to characterize the firing pattern on depolarization and the underlying currents in principal cells of the chick tangential nucleus. This study, on 0- to 3-day-old hatchlings, distinguishes electrophysiologically one main group of principal cells based on their response to depolarizing current pulses (300-400 ms) in current-clamp recordings. This group (90%; n=29) displayed nonaccommodating, repetitive firing on depolarization. The remaining cells fired one action potential at the beginning of the current pulse and then accommodated. In voltage-clamp recordings, a low-threshold, sustained, dendrotoxin-sensitive (DTX; 200 nM) potassium current, I(DS), was identified in both cell groups. In the repetitively firing principal cells, the mean proportion of the DTX-sensitive sustained current contributing to the total outward current was less than 20%. This percentage is significantly less than that reported (45%) in a previous study performed in late chick embryos (E16), in which most of the cells (83%; n=89) were accommodating neurons. Tonic firing is an important electrophysiological feature characterizing most mature, second-order vestibular neurons, since it allows the neurons to process signals from behaviorally relevant inputs. Accordingly, this study contributes toward defining the emergence of the mature pattern of neuronal excitability and the ionic currents involved.     

Firing behaviour of squirrel monkey eye movement-related vestibular nucleus neurons during gaze saccades

The firing behaviour of vestibular nucleus neurons putatively involved in producing the vestibulo-ocular reflex (VOR) was studied during active and passive head movements in squirrel monkeys. Single unit recordings were obtained from 14 position-vestibular (PV) neurons, 30 position-vestibular-pause (PVP) neurons and 9 eye-head-vestibular (EHV) neurons. Neurons were sub-classified as type I or II based on whether they were excited or inhibited during ipsilateral head rotation. Different classes of cell exhibited distinctive responses during active head movements produced during and after gaze saccades. Type I PV cells were nearly as sensitive to active head movements as they were to passive head movements during saccades. Type II PV neurons were insensitive to active head movements both during and after gaze saccades. PVP and EHV neurons were insensitive to active head movements during saccadic gaze shifts, and exhibited asymmetric sensitivity to active head movements following the gaze shift. PVP neurons were less sensitive to ondirection head movements during the VOR after gaze saccades, while EHV neurons exhibited an enhanced sensitivity to head movements in their on direction. Vestibular signals related to the passive head movement were faithfully encoded by vestibular nucleus neurons. We conclude that central VOR pathway neurons are differentially sensitive to active and passive head movements both during and after gaze saccades due primarily to an input related to head movement motor commands. The convergence of motor and sensory reafferent inputs on VOR pathways provides a mechanism for separate control of eye and head movements during and after saccadic gaze shifts.     

Three types of depolarization-activated potassium currents in acutely isolated mouse vestibular neurons

The nature and electrophysiological properties of Ca(2+)-independent depolarization-activated potassium currents were investigated in vestibular primary neurons acutely isolated from postnatal mice using the whole cell configuration of the patch-clamp technique. Three types of currents were identified. The first current, sensitive to TEA (I(TEA)) and insensitive to 4-aminopyridine (4-AP), activated at -40 mV and exhibited slow activation (tau(ac), 38.4 +/- 7.8 ms at -30 mV, mean +/- SD). I(TEA) had a half activation potential [V(ac(1/2))] of -14.5 +/- 2.6 mV and was inactivated by up to 84.5 +/- 5.7% by 10-s conditioning prepulses with a half inactivation potential [V(inac(1/2))] of -62.4 +/- 0.2 mV. The second current, sensitive to 4-AP (maximum block around 0.5 mM) and to alpha-dendrotoxin (I(DTX)) appeared at -60 mV. Complete block of I(DTX) was achieved using either 20 nM alpha-DTX or 50 nM margatoxin. This current activated 10 times faster than I(TEA) (tau(ac), 3.5 +/- 0.8 ms at -50 mV) with V(ac(1/2)) of -51.2 +/- 0.6 mV, and inactivated only slightly compared with I(TEA) (maximum inactivation, 19.7 +/- 3.2%). The third current, also sensitive to 4-AP (maximum block at 2 mM), was selectively blocked by application of blood depressing substance (BDS-I; maximum block at 250 nM). The BDS-I-sensitive current (I(BDS-I)) activated around -60 mV. It displayed fast activation (tau(ac), 2.3 +/- 0.4 ms at -50 mV) and fast and complete voltage-dependent inactivation. I(BDS-I) had a V(ac(1/2)) of -31.3 +/- 0.4 mV and V(inac(1/2)) of -65.8 +/- 0.3 mV. It displayed faster time-dependent inactivation and recovery from inactivation than I(TEA). The three types of current were found in all the neurons investigated. Although I(TEA) was the major current, the proportion of I(DTX) and I(BDS-I) varied considerably between neurons. The ratio of the density of I(BDS-I) to that of I(DTX) ranged from 0.02 to 2.90 without correlation with the cell capacitances. In conclusion, vestibular primary neurons differ by the proportion rather than the type of the depolarization-activated potassium currents they express.     

Spontaneous synaptic activity is primarily GABAergic in vestibular nucleus neurons of the chick embryo

The principal cells of the chick tangential nucleus are vestibular nucleus neurons participating in the vestibular reflexes. In 16-day embryos, the application of glutamate receptor antagonists abolished the postsynaptic responses generated on vestibular-nerve stimulation, but spontaneous synaptic activity was largely unaffected. Here, spontaneous synaptic activity was characterized in principal cells from brain slices at E16 using whole cell voltage-clamp recordings. With KCl electrodes, the frequency of spontaneous inward currents was 3.1 Hz at -60 mV, and the reversal potential was +4 mV. Cs-gluconate pipette solution allowed the discrimination of glycine/GABA(A) versus glutamate receptor-mediated events according to their different reversal potentials. The ratio for spontaneous excitatory to inhibitory events was about 1:4. Seventy-four percent of the outward events were GABA(A), whereas 26% were glycine receptor-mediated events. Both pre- and postsynaptic GABA(B) receptor effects were shown, with presynaptic GABA(B) receptors inhibiting 40% of spontaneous excitatory postsynaptic currents (sEPSCs) and 53% of spontaneous inhibitory postsynaptic currents (sIPSCs). With TTX, the frequency decreased approximately 50% for EPSCs and 23% for IPSCs. These data indicate that the spontaneous synaptic activity recorded in the principal cells at E16 is primarily inhibitory, action potential-independent, and based on the activation of GABA(A) receptors that can be modulated by presynaptic GABA(B) receptors.     

Ultrastructural features of non-commissural GABAergic neurons in the medial vestibular nucleus of the monkey.

The ultrastructural characteristics of non-degenerating GABAergic neurons in rostrolateral medial vestibular nucleus were identified in monkeys following midline transection of vestibular commissural fibers. In the previous papers, we reported that most degenerated cells and terminals in this tissue were located in rostrolateral medial vestibular nucleus, and that many of these neurons were GABA-immunoreactive. In the present study, we examined the ultrastructural features of the remaining neuronal elements in this medial vestibular nucleus region, in order to identify and characterize the GABAergic cells that are not directly involved in the vestibular commissural pathway related to the velocity storage mechanism. Such cells are primarily small, with centrally-placed nuclei. Axosomatic synapses are concentrated on polar regions of the somata. The proximal dendrites of GABAergic cells are surrounded by boutons, although distal dendrites receive only occasional synaptic contacts. Two types of non-degenerated GABAergic boutons are distinguished. Type A terminals are large, with very densely-packed spherical synaptic vesicles and clusters of large, irregularly-shaped mitochondria with wide matrix spaces. Such boutons form symmetric synapses, primarily with small GABAergic and non-GABAergic dendrites. Type B terminals are smaller and contain a moderate density of round/pleomorphic vesicles, numerous small round or tubular mitochondria, cisterns and vacuoles. These boutons serve both pre- and postsynaptic roles in symmetric contacts with non-GABAergic axon terminals. On the basis of ultrastructural observations of immunostained tissue, we conclude that at least two types of GABAergic neurons are present in the rostrolateral portion of the monkey medial vestibular nucleus: neurons related to the velocity storage pathway, and a class of vestibular interneurons. A multiplicity of GABAergic bouton types are also observed, and categorized on the basis of subcellular morphology. We hypothesize that "Type A" boutons correspond to Purkinje cell afferents in rostrolateral medial vestibular nucleus, "Type B" terminals represent the axons of GABAergic medial vestibular nucleus interneurons, and "Type C" boutons take origin from vestibular commissural neurons of the velocity storage pathway.     

Firing characteristics of neurons mediating optokinetic responses to rat's vestibular neurons

1. The responses of single units in the pretectum (Pt) and in the n. reticularis tegmenti pontis (NRTP) to constant velocity horizontal rotation (0.25–60 deg/s) of a large-field visual pattern were studied in immobilized, non-anesthetized DA-HAN rats. In addition, responses of Pt and NRTP neurons to pure vestibular stimuli (rotation in the dark) were studied.
2. Pt neurons showed seven response types to optokinetic stimulation (Table 1). The most frequent response (48%) consisted of a very rapid increase in firing to steady state on temporonasal motion stimulation of the contralateral eye; nasotemporal stimuli yielded no change in resting rate as did stimulation of the ipsilateral eye. The response maximum occurred at a retinal slip velocity of 1 deg/s. None of the Pt units tested responded to pure vestibular stimuli.
3. NRTP neurons — as Pt units — most frequently (43%) increased their discharge rate on temporonasal stimulation of the contralateral eye and maintained a constant resting rate during nasotemporal motion. Peak response amplitudes also occurred with retinal slip velocities of 1 deg/s. Contrary to the fast time-to-peak of the responses of Pt neurons NRTP units showed a slow rise in frequency of firing to peak response levels.
4. NRTP neurons responded to pure vestibular stimuli (horizontal angular acceleration in the dark). The vestibular responses were synergistic with those evoked in the same neurons by optokinetic stimuli. Thus, the most frequently encountered type of optokinetic response (s. above) showed a type II vestibular response.
5. Comparison of OKN and Vn optokinetic responses with those of Pt and NRTP suggests that the unidirectional-selective Pt and NRTP neurons are important links in the central optokinetic path. In addition, the NRTP may represent the site at which the retinal slip signal and the eye velocity signal converge. This convergence has been postulated in models of the system [12].

     

Quantitative analysis of the feline dorsal column nuclei and their GABAergic and non-GABAergic neurons

Summary The gracile and internal and external cuneate nuclei of four adult cats were studied, using recently developed stereological techniques. The length, volume and position of the nuclei in relation to the level of obex were calculated, as well as the number of neurones, the neuronal density and volume of the three nuclei and different regions in the gracile and internal cuneate nucleus. Material processed for GABA immunocytochemistry was used in order to compare GABAergic and non-GABAergic neurones. The results demonstrate variations in the same nucleus in different animals, and in the nucleus of the left and right sides of the same animal. The same nucleus can vary up to 4 mm in its rostrocaudal position in relation to the obex. The mean sizes of the gracile, internal and external cuneate nuclei are 4.2, 8.4 and 5.6 mm', respectively and their mean neuronal numbers are about 52000, 76 000 and 33000, respectively. The neuronal density was highest (12907 cells/mm³) in the gracile, and lowest in the external cuneate nucleus (5987 cells/mm³). The external cuneate nucleus had a larger relative volume (7.9%) occupied by nerve cell bodies compared with the two medial nuclei (5.1% and 5.8%). In the gracile and internal cuneate nuclei, the GABAergic neurones constituted 28% and 25% of the whole population, respectively, while the external cuneate nucleus was devoid of such cells. All the nuclei contained GABA-positive boutons, however. The mean volume of the GABA-stained neurones in the gracile nucleus was 2319, and internal cuneate 3065 m³, while the corresponding volume of unlabelled neurones in the gracile, internal and external cuneate nuclei was 3745, 8147 and 13 318 m³, respectively. When cyto-fibro-architectonic characteristics were used to subdivide the gracile and cuneate nuclei into rostral, middle and caudal regions, and the data of the three compartments compared, it was found that in both nuclei the middle region had the highest neuronal packing density, and the caudal region the largest mean nerve cell volume.This paper is dedicated to Professor Fred Walberg on the occasion of his 70th birthday.     

Physiological and anatomical properties of mouse medial vestibular nucleus neurons projecting to the oculomotor nucleus

Neurons in the medial vestibular nucleus (MVN) vary in their projection patterns, responses to head movement, and intrinsic firing properties. To establish whether neurons that participate in the vestibulo-ocular reflex (VOR) have distinct intrinsic physiological properties, oculomotor nucleus (OMN)-projecting neurons were identified in mouse brainstem slices by fluorescent retrograde labeling from the oculomotor complex and targeted for patch-clamp recordings. Such neurons were located in the magnocellular portion of the MVN contralateral to tracer injection, were mostly multipolar, and had soma diameters of around 20 mum. They fired spontaneous action potentials at rates higher than those of other MVN neurons and their spikes were of unusually short duration. OMN-projecting neurons responded to 1-s intracellular current injection with exceptionally high firing rates of >500 spikes/s. Their current-firing relationship was highly linear, with weak firing response adaptation during steady depolarization and little postinhibitory rebound firing after membrane hyperpolarization. Their firing responses were approximately in phase with sinusoidal current injection. The response dynamics of OMN-projecting neurons could be simulated with a simple integrate-and-fire model modified with the addition of small adaptation and rebound conductances. These findings indicate that the membrane properties of OMN-projecting neurons allow them to respond to head movements reliably and with high sensitivity but without substantially altering input dynamics.     

Adaptation of chicken vestibular nucleus neurons to unilateral vestibular ganglionectomy

Vestibular compensation refers to the behavioral recovery after a unilateral peripheral vestibular lesion. In chickens, posture and balance deficits are present immediately following unilateral vestibular ganglionectomy (UVG). After three days, most operated chickens begin to recover, but severe deficits persist in others. The tangential nucleus is a major avian vestibular nucleus whose principal cells are vestibular reflex projection neurons. From patch-clamp recordings on brain slices, the percentage of spontaneous spike firing principal cells, spike discharge rate, ionic conductances, and spontaneous excitatory postsynaptic currents (sEPSCs) were investigated one and three days after UVG. Already by one day after UVG, sEPSC frequency increased significantly on the lesion side, although no differences were detected in the percentage of spontaneous spike firing cells or discharge rate. In compensated chickens three days after UVG, the percentage of spontaneous spike firing cells increased on the lesion side and the discharge rate increased bilaterally. In uncompensated chickens three days after UVG, principal cells on the lesion side showed increased discharge rate and increased sEPSC frequency, whereas principal cells on the intact side were silent. Typically, silent principal cells exhibited smaller persistent sodium conductances and higher activation thresholds for the fast sodium channel than spiking cells. In addition, silent principal cells on the intact side of uncompensated chickens had larger dendrotoxin-sensitive potassium conductance, with a higher ratio of Kv1.1 surface/cytoplasmic expression. Increased sEPSC frequency in principal cells on the lesion side of uncompensated chickens was accompanied by decreased Kv1.2 immunolabeling of presynaptic terminals on principal cell bodies. Thus, both intrinsic ionic conductances and excitatory synaptic inputs play crucial roles at early stages after lesions. Unlike the principal cells in compensated chickens which showed similar percentages of spontaneous spike firing cells, discharge rates, and sEPSC frequencies bilaterally, principal cells in uncompensated chickens displayed gross asymmetry in these properties bilaterally.     

Organization and properties of GABAergic neurons in solitary tract nucleus (NTS)

Cranial visceral afferents enter the brain at the solitary tract nucleus (NTS). GABAergic neurons are scattered throughout the NTS, but their relation to solitary tract (ST) afferent pathways is imprecisely known. We hypothesized that most GABAergic NTS neurons would be connected only indirectly to the ST. We identified GABAergic neurons in brain stem horizontal slices using transgenic mice in which enhanced green fluorescent protein (EGFP) expression was linked to glutamic acid decarboxylase expression (GAD(+)). Finely graded electrical shocks to ST recruit ST-synchronized synaptic events with all-or-none thresholds and individual waveforms did not change with greater suprathreshold intensities--evidence consistent with initiation by single afferent axons. Most (approximately 70%) GAD(+) neurons received ST-evoked excitatory postsynaptic currents (EPSCs) that had minimally variant latencies (jitter, SD of latency <200 micros) and waveforms consistent with single, direct ST connections (i.e., monosynaptic). Increasing stimulus intensity evoked additional ST-synchronized synaptic responses with jitters >200 micros including inhibitory postsynaptic currents (IPSCs), indicating indirect connections (polysynaptic). Shocks of suprathreshold intensity delivered adjacent (50-300 microm) to the ST failed to excite non-ST inputs to second-order neurons, suggesting a paucity of axons passing near to ST that connected to these neurons. Despite expectations, we found similar ST synaptic patterns in GAD(+) and unlabeled neurons. Generally, ST information that arrived indirectly had small amplitudes (EPSCs and IPSCs) and frequency-dependent failures that reached >50% for IPSCs to bursts of stimuli. This ST afferent pathway organization is strongly use-dependent--a property that may tune signal propagation within and beyond NTS.     

Birth-date dependent alignment of GABAergic neurons occurs in a different pattern from that of non-GABAergic neurons in the developing mouse visual cortex

In the developing mouse cerebral cortex, gamma-aminobutyric acid (GABA)ergic neurons and non-GABAergic neurons arise in distinct places and migrate into the cortical plate (CP) via different pathways. Although the "inside-out" alignment of projection neurons in the cortex has been thoroughly analyzed, the pattern of interneuron alignment is not well understood. Herein, we show that in the postnatal day (P) 9.5 mouse visual cortex, GABAergic neurons born on embryonic day (E) 12.5 were distributed around two peak locations, mainly around layer V and also around layer II/III, while non-GABAergic neurons born on E12.5 were distributed around only one peak in layer VI. Both cell populations born on E15.5 exhibited only one common peak distribution in layer II/III. The two peak locations of GABAergic neurons born on E12.5 still existed at P30. When the subtypes of GABAergic neurons were analyzed, calretinin-positive cells born on E12.5 were distributed in the cortex around one peak location near layer II/III, whereas somatostatin-positive E12.5 cells were distributed in the cortex around one peak location near layer V. These results suggest that the alignment of interneurons is regulated differently according to subtypes and from that of projection neurons having the same embryonic day of origin.     

The roles potassium currents play in regulating the electrical activity of ventral cochlear nucleus neurons

Using kinetic data from three different K+ currents in acutely isolated neurons, a single electrical compartment representing the soma of a ventral cochlear nucleus (VCN) neuron was created. The K+ currents include a fast transient current (IA), a slow-inactivating low-threshold current (ILT), and a noninactivating high-threshold current (IHT). The model also includes a fast-inactivating Na+ current, a hyperpolarization-activated cation current (Ih), and 1-50 auditory nerve synapses. With this model, the role IA, ILT, and IHT play in shaping the discharge patterns of VCN cells is explored. Simulation results indicate that IHT mainly functions to repolarize the membrane during an action potential, and IA functions to modulate the rate of repetitive firing. ILT is found to be responsible for the phasic discharge pattern observed in Type II cells (bushy cells). However, by adjusting the strength of ILT, both phasic and regular discharge patterns are observed, demonstrating that a critical level of ILT is necessary to produce the Type II response. Simulated Type II cells have a significantly faster membrane time constant in comparison to Type I cells (stellate cells) and are therefore better suited to preserve temporal information in their auditory nerve inputs by acting as precise coincidence detectors and having a short refractory period. Finally, we demonstrate that modulation of Ih, which changes the resting membrane potential, is a more effective means of modulating the activation level of ILT than simply modulating ILT itself. This result may explain why ILT and Ih are often coexpressed throughout the nervous system.     

Firing characteristics of vestibular nuclei neurons in the alert monkey after bilateral vestibular neurectomy

Summary After destruction of the peripheral vestibular system which is not activated by moving large-field visual stimulation, not only labyrinthine-ocular reflexes but also optokinetic-ocular responses related to the velocity storage mechanism are abolished. In the normal monkey optokinetic-ocular responses are reflected in sustained activity changes of central vestibular neurons within the vestibular nuclei. To account for the loss of optokinetic responses after labyrinthectomy, inactivation of central vestibular neurons consequent on the loss of primary vestibular activity is assumed to be of major importance. To test this hypothesis we recorded the neural activity within the vestibular nuclear complex in two chronically prepared Rhesus monkeys during a period from one up to 9 and 12 months after both vestibular nerves had been cut. The discharge characteristics of 829 cells were studied in relation to eye fixation, and to a moving small and large (optokinetic) visual stimulus producing smooth pursuit (SP) eye movements and optokinetic nystagmus (OKN). Units were grouped into different subclasses.After chronic bilateral vestibular neurectomy (BVN) we have found: (1) a rich variety of spontaneously active cells within the vestibular nuclear complex, which — as far as comparison before and after BVN is possible — belong to all subclasses of neurons functionally defined in normal monkey; and (2) no sustained activity changes which are related to the activation of the velocity storage mechanism; this is especially true for pure-vestibular, vestibular-pause and tonic-vestibular-pause cells in normal monkey which show a pure, pause and tonic-pause firing pattern after BVN. Neurons which are modulated by eye position are, however, modulated with the velocity of slow eye movements with comparable sensitivity during SP and OKN. Retinal slip is extremely rarely encoded. The results of the present study do not directly answer the question why the velocity storage mechanism is abolished after BVN but they suggest that only a small number of central vestibular cells may be inactivated by neurectomy.Supported by SNF grant no. 3.510-0.86     

Morphological and electrophysiological properties of GABAergic and non-GABAergic cells in the deep cerebellar nuclei

The deep cerebellar nuclei (DCN) integrate inputs from the brain stem, the inferior olive, and the spinal cord with Purkinje cell output from cerebellar cortex and provide the major output of the cerebellum. Despite their crucial function in motor control and learning, the various populations of neurons in the DCN are poorly defined and characterized. Importantly, differences in electrophysiological properties between glutamatergic and GABAergic cells of the DCN have been largely elusive. Here, we used glutamate decarboxylase (GAD) 67-green fluorescent protein (GFP) knock-in mice to unambiguously identify GABAergic (GAD-positive) and non-GABAergic (GAD-negative, most likely glutamatergic) neurons of the DCN. Morphological analysis of DCN neurons patch-clamped with biocytin-containing electrodes revealed a significant overlap in the distributions of the soma sizes of GAD-positive and GAD-negative cells. Compared with GAD-negative DCN neurons, GAD-positive DCN neurons fire broader action potentials, display stronger frequency accommodation, and do not reach as high firing frequencies during depolarizing current injections. Furthermore, GAD-positive cells display slower spontaneous firing rates and have a more shallow frequency-to-current relationship than the GAD-negative cells but exhibit a longer-lasting rebound depolarization and associated spiking after a transient hyperpolarization. In contrast to the rather homogeneous population of GAD-positive cells, the GAD-negative cells were found to consist of two distinct populations as defined by cell size and electrophysiological features. We conclude that GABAergic DCN neurons are specialized to convey phasic spike rate information, whereas tonic spike rate is more faithfully relayed by the large non-GABAergic cells.     

The magnitudes of hyperpolarization-activated and low-voltage-activated potassium currents co-vary in neurons of the ventral cochlear nucleus

In the ventral cochlear nucleus (VCN), neurons have hyperpolarization-activated conductances, which in some cells are enormous, that contribute to the ability of neurons to convey acoustic information in the timing of their firing by decreasing the input resistance and speeding-up voltage changes. Comparisons of the electrophysiological properties of neurons in the VCN of mutant mice that lack the hyperpolarization-activated cyclic nucleotide-gated channel α subunit 1 (HCN1(-/-)) (Nolan et al. 2003) with wild-type controls (HCN1(+/+)) and with outbred ICR mice reveal that octopus, T stellate, and bushy cells maintain their electrophysiological distinctions in all strains. Hyperpolarization-activated (I(h)) currents were smaller and slower, input resistances were higher, and membrane time constants were longer in HCN1(-/-) than in HCN1(+/+) in octopus, bushy, and T stellate cells. There were significant differences in the average magnitudes of I(h), input resistances, and time constants between HCN1(+/+) and ICR mice, but the resting potentials did not differ between strains. I(h) is opposed by a low-voltage-activated potassium (I(KL)) current in bushy and octopus cells, whose magnitudes varied widely between neuronal types and between strains. The magnitudes of I(h) and I(KL) were correlated across neuronal types and across mouse strains. Furthermore, these currents balanced one another at the resting potential in individual cells. The magnitude of I(h) and I(KL) is linked in bushy and octopus cells and varies not only between HCN1(-/-) and HCN1(+/+) but also between "wild-type" strains of mice, raising the question to what extent the wild-type strains reflect normal mice.     

Firing activity and postsynaptic properties of morphologically identified neurons of ventral oral pontine reticular nucleus

The ventral part of the oral pontine reticular nucleus (vRPO) is an important region for the generation and maintenance of REM sleep. Firing activity and synaptic response properties of morphologically identified vRPO neurons have been investigated in urethane-anaesthetized cats. Extracellular recordings were performed through recording micropipettes and neurons were extracellularly stained with biocytin. Two types of neurons were identified under spontaneous conditions: type I neurons (77%) are characterized by non-rhythmic firing; type II neurons (23%) display single spikes firing rhythmically at between 7 and 22 Hz. Type I neurons displayed ellipsoid somata with thick dendritic trunks and axons that arose from either the soma or the initial dendritic segment; these axons could not be clearly followed. Type II neurons showed polygonal somata with radial dendrites; their axons branched at a small distance from the soma. Electrical stimulation of the contralateral vRPO elicited responses in both neuron types (57% and 31%, respectively); this effect was blocked by the non-NMDA glutamatergic receptor antagonist CNQX. Electrical stimulation of the PpT evoked orthodromic responses in type I neurons (41%) and inhibited the firing rate of all type II neurons for 50-100 ms. Both effects were blocked by the muscarinic receptor antagonist atropine. The cholinergic agonist, carbachol, increased the firing rate in most type I neurons and inhibited most type II neurons in these animals.The results demonstrated that the activity of vRPO neurons is modulated through the postsynaptic activation exerted by extrinsic afferents on cholinergic and glutamatergic receptors.