Glial cells of the oligodendrocyte lineage express proton-activated Na+ channels

Neurons and oligodendrocytes, but not type I astrocytes and Schwann cells, generate large Na+ currents in response to a step increase of [H+]. Proton-activated Na+ channels are the first cationic channels expressed in neuronal precursor cells from the mammalian brain. Glial precursor cells cultured from mouse brain are also capable of generating Na+ currents in response to step acidification (INa(H]. With further development along the oligodendrocyte lineage, this property is retained, whereas voltage-activated Na+ and K+ currents disappear. Comparing INa(H) of oligodendrocytes with INa(H) of their precursor cells did not reveal a difference in current amplitude, suggesting a higher density of INa(H) channels on the (smaller) precursor cells. The properties of INa(H) in glial precursor cells and oligodendrocytes are similar to those of neurons, with respect to activation conditions, time course, and the effect of extracellular Ca2+ concentrations. The results are consistent with previous observations which showed that oligodendrocytes partially preserve their chemically activated, but completely lose their voltage-activated, ion channels.     

Involvement of alpha7- and alpha4beta2-type postsynaptic nicotinic acetylcholine receptors in nicotine-induced excitation of dopaminergic neurons in the substantia nigra: a patch clamp and single-cell PCR study using acutely dissociated nigral neurons

The receptor subtypes, which mediate nicotine-induced excitation of dopaminergic neurons in the substantia nigra, were investigated by whole-cell patch clamp studies and single-cell RT-PCR using acutely dissociated nigral neurons. Three types of current were observed when acetylcholine (1 mM) was applied to the neurons in the presence of atropine (1 microM) by the U-tube system, which allowed the rapid application of drugs. In 50% of neurons examined, acetylcholine (1 mM) plus atropine (1 microM) evoked a current with a rapidly desensitizing decay phase (designated as type Ia current). In 14% of neurons tested, the current induced by acetylcholine plus atropine had a decay phase with slow desensitization (designated as type II current). The third type of response, which had both characteristics of type Ia and II currents, was evoked in 36% of neurons tested (designated as type Ib currents). Nicotine (1 mM) also induced three types of inward currents which were similar to those induced by acetylcholine (1 mM) plus atropine (1 microM). In all three types of current, nicotine (0.1 microM-1 mM)-evoked inward currents were dose-dependent. Type Ia and II currents were inhibited by methyllycaconitine (MLA, 0.01 microM), a selective nicotinic alpha7 receptor antagonist, and dihydro-beta-erythroidine (DHbetaE, 0.1 microM), an antagonist for alpha4beta2 receptor, respectively. In type Ib currents, a fast-decaying phase was inhibited by MLA (0.01 microM), while a slow-decaying phase was blocked by DHbetaE (0.1 microM). After recording the type Ib current, single-cell RT-PCR analysis was performed using aspirated cytoplasm as total RNA templates. The results revealed that mRNAs for alpha7 nicotinic receptor subunit and tyrosine hydroxylase were detected in the same single neuron tested, which confirms the existence of alpha7-type nicotinic acetylcholine receptor in dopaminergic neurons of this area. These results suggest that nicotine directly acts on postsynaptic alpha7- and alpha4beta2-type nicotinic acetylcholine receptors and induces inward current, which result in the excitation of dopaminergic neurons in the substantia nigra.     

“Tripartite Synapses” in Taste Buds: A Role for Type I Glial-like Taste Cells

In mammalian taste buds, Type I cells comprise half of all cells. These are termed “glial-like” based on morphologic and molecular features, but there are limited studies describing their function. We tested whether Type I cells sense chemosensory activation of adjacent chemosensory (i.e., Types II and III) taste bud cells, similar to synaptic glia. Using Gad2;;GCaMP3 mice of both sexes, we confirmed by immunostaining that, within taste buds, GCaMP expression is predominantly in Type I cells (with no Type II and ≈28% Type III cells expressing weakly). In dissociated taste buds, GCaMP+ Type I cells responded to bath-applied ATP (10-100 μm) but not to 5-HT (transmitters released by Type II or III cells, respectively). Type I cells also did not respond to taste stimuli (5 μm cycloheximide, 1 mm denatonium). In lingual slice preparations also, Type I cells responded to bath-applied ATP (10-100 μm). However, when taste buds in the slice were stimulated with bitter tastants (cycloheximide, denatonium, quinine), Type I cells responded robustly. Taste-evoked responses of Type I cells in the slice preparation were significantly reduced by desensitizing purinoceptors or by purinoceptor antagonists (suramin, PPADS), and were essentially eliminated by blocking synaptic ATP release (carbenoxolone) or degrading extracellular ATP (apyrase). Thus, taste-evoked release of afferent ATP from type II chemosensory cells, in addition to exciting gustatory afferent fibers, also activates glial-like Type I taste cells. We speculate that Type I cells sense chemosensory activation and that they participate in synaptic signaling, similarly to glial cells at CNS tripartite synapses.SIGNIFICANCE STATEMENT Most studies of taste buds view the chemosensitive excitable cells that express taste receptors as the sole mediators of taste detection and transmission to the CNS. Type I “glial-like” cells, with their ensheathing morphology, are mostly viewed as responsible for clearing neurotransmitters and as the “glue” holding the taste bud together. In the present study, we demonstrate that, when intact taste buds respond to their natural stimuli, Type I cells sense the activation of the chemosensory cells by detecting the afferent transmitter. Because Type I cells synthesize GABA, a known gliotransmitter, and cognate receptors are present on both presynaptic and postsynaptic elements, Type I cells may participate in GABAergic synaptic transmission in the manner of astrocytes at tripartite synapses.     

Neurons of the periaqueductal gray matter as revealed by Golgi study

Neurons of the periaqueductal gray matter of the normal adult cat are classified into seven general types, Ia, Ib, II, IIIa, IIIb, IIIc, and IIId, based on the Golgi-Cox impregnated materials. Types Ia and Ib are spindle shaped bipolar neurons with one straight dendritic process forming varicosities and short stemmed spines. Type Ia is small in size and Ib is larger. The axon of each neuron emerges from another pole and projects beyond the PAG region. Occasionally it may share the origin with a dendrite or a tuft of dendrites. Type II, triangular shaped, had an apical dendrite that traverses a long distance within the PAG and an axon emerging from the basal portion and projecting beyond the PAG. Type IIIa, b, c, and d are pleomorphic multipolar neurons. Type IIIa has a rhomboid-shaped soma and dichotomically branching dendrites. Type IIIb has a spheroidal soma and short axons that terminate within the PAG. Type IIIc has a piriform soma and spiny dendrites that ramify perfusely and an axon which terminates within the PAG. Type IIId has the largest soma of all these neurons and the structure resembles an undifferentiated motor neuron of the CNS. Axons of the types IIIa and IIId are projecting in nature. Type Ia is found exclusively in the area immediately surrounding the aqueduct, the nucleus medialis. Types IIIc and IIId are found exclusively in the lateral region of the PAG which corresponds to the nucleus lateralis while the remaining cell types are found mostly in nuclei lateralis and dorsalis.     

Subtype-dependent postnatal development of taste receptor cells in mouse fungiform taste buds

Taste buds contain two types of taste receptor cells, inositol 1,4,5-triphosphate receptor type 3-immunoreactive cells (type II cells) and synaptosomal-associating protein-25-immunoreactive cells (type III cells). We investigated their postnatal development in mouse fungiform taste buds immunohistochemically and electrophysiologically. The cell density, i.e. the number of cells per taste bud divided by the maximal area of the horizontal cross-section of the taste bud, of type II cells increased by postnatal day (PD)49, where as that of type III cells was unchanged throughout the postnatal observation period and was equal to that of the adult cells at PD1. The immunoreactivity of taste bud cell subtypes was the same as that of their respective subtypes in adult mice throughout the postnatal observation period. Almost all type II cells were immunoreactive to gustducin at PD1, and then the ratio of gustducin-immunoreactive type II cells to all type II cells decreased to a saturation level, ～60% of all type II cells, by PD15. Type II and III cells generated voltage-gated currents similar to their respective adult cells even at PD3. These results show that infant taste receptor cells are as excitable as those of adults and propagate in a subtype-dependent manner. The relationship between the ratio of each taste receptor cell subtype to all cells and taste nerve responses are discussed.     

Survey of inward ionic currents acquired by the cochleovestibular ganglion of the early-aged embryonic chick

The acquisition of ion channels is critical to the formation of neuronal pathways in the peripheral and central nervous systems. This study describes the different types of inward currents (Ii) recorded from the soma of isolated cochleovestibular ganglion (CVG) cells of the embryonic chicken, Gallus gallus. Cells were isolated for whole-cell tight-seal recording from embryonic day (ED) 3, an age when the CVG is a cell cluster, to ED 9, an age when the cochlear and vestibular ganglia (CG, VG) are distinct structures. Results show Na+ and Ca2+ currents (INa and ICa) are acquired by ED 3, although INa dominates with greater density levels that peak by ED 6-7 in VG neurons. In the CG, INa acquisition is slower, reaching peak values by ED 8-9. Isolation of ICa, using Ba2+ as the charge carrier, showed both transient (IBaT)- and sustained (IBaL)-type currents on ED 3. Unlike INa, IBa density varied with age and ganglion. Total IBa increased steadily, showing a decline only in CG cells on ED 8-9 as a result of a decrease in IBaT. IBaL density increased over time, reaching a maximum on ED 6-7 in VG cells, followed by a decline on ED 8-9. In comparison, IBaL in CG neurons, did not increase significantly beyond mean values measured on ED 5. The early onset of these currents and the variations in Ca2+ channel expression between the ganglia suggests that intracellular signals relevant to phenotypic differentiation begin within these early time frames.     

Antithrombin Oslo: type Ib classification of the first reported antithrombin-deficient family, with a review of hereditary antithrombin variants

Patients with classical antithrombin deficiency (Type I) from seven unrelated kindreds were studied by crossed immunoelectrophoresis of plasma in the presence and absence of heparin. The only abnormal pattern was found in the kindred first reported by Egeberg in 1965. An abnormal cathodal peak of antithrombin antigen was found in the presence, but not the absence, of heparin in the first dimension gel. We have named this variant antithrombin Oslo. Such evidence of an abnormal protein, despite equivalent low levels of antithrombin antigen and activity, has been denoted previously by Sas as Type Ib deficiency. In the context of this new report, we review the literature to date on 33 other variants of the Types Ib, II and III subclassifications with a discussion of the value of the classification scheme.     

Alanine to valine substitutions in the pore helix IIIP1 and linker-helix IIIL45 confer cockroach sodium channel resistance to DDT and pyrethroids

Pyrethroid insecticides exert toxic effects by prolonging the opening of voltage-gated sodium channels. More than 20 sodium channel mutations from arthropod pests and disease vectors have been confirmed to confer pyrethroid resistance. These mutations have been valuable in elucidating the molecular interaction between pyrethroids and sodium channels, including identification of two pyrethroid receptor sites. Previously, two alanine to valine substitutions, one in the pore helix IIIP1 and the other in the linker-helix connecting S4 and S5 in domain III (IIIL45), were found in Drosophila melanogaster mutants that are resistant to DDT and deltamethrin (a type II pyrethroid with an α-cyano group at the phenylbenzyl alcohol position, which is lacking in type I pyrethroids), but their role in target-site-mediated insecticide resistance has not been functionally confirmed. In this study, we functionally examined the two mutations in cockroach sodium channels expressed in Xenopus laevis oocytes. Both mutations caused depolarizing shifts in the voltage dependence of activation, conferred DDT resistance and also resistance to two Type I pyrethroids by almost abolishing the tail currents induced by Type I pyrethroids. In contrast, neither mutation reduced the amplitude of tail currents induced by the Type II pyrethroids, deltamethrin or cypermethrin. However, both mutations accelerated the decay of Type II pyrethroid-induced tail currents, which normally decay extremely slowly. These results provided new insight into the molecular basis of different actions of Type I and Type II pyrethroids on sodium channels. Computer modeling predicts that both mutations may allosterically affect pyrethroid binding.     

Astrocytic cell clones derived from established cultures of 8-day postnatal mouse cerebella

Clonal permanent cell lines with astrocytic properties have been established from explant cultures of 8-day postnatal mouse cerebella after in vitro spontaneous transformation, i.e. without the addition of carcinogens or oncogenic viruses. The cell lines were derived in a multistage process. Slowly proliferating foci with several morphologies appeared 4 months after initiation of the cultures and became progressively enriched by cells with a homogeneous appearance. These cells could be established into permanent cell lines from which many clones were obtained. Some of these cloned cell lines bound anti-GFAP sera and therefore appeared to be astrocytic. According to their morphology, 3 separate types of these GFAP-positive clones could be distinguished. Type I and II cells had small somata; type I had several short processes, while type II had two processes, one of which was very thin and long ( 200 μm). Type III cells had large flat somata and no processes. The three types of clonal cell lines were labeled by monoclonal antibodies which bind to astrocytes in vivo. In particular, three monoclonal antibodies (BSP-3, M2 and M3) bound only to type II cells in a distinct pattern. Type I and II astrocytes are pseudodiploid and type III, heteroploid. The properties of these different clonal cell lines are very stable. We have thus obtained permanently established clonal cultures of mouse cerebellum astrocyte-like cells, which might be the in vitro counterparts of fibrous (type I), or velamentous (type III) astrocytes and of Golgi epithelial cells (type II).     

Cytoarchitecture and visual receptive neurons in the Wulst of the Japanese quail (Coturnix coturnix japonica)

The cellular organization of the Wulst was studied in Nissl- and Golgi-stained brain sections in order to identify the visual receptive neurons. Golgi-impregnated neurons were divided into four types according to their soma size, dendritic configuration, and density of spine distribution. Type I neurons, the largest cells in the Wulst, have long, straight dendrites with many spines. Type II neurons are medium-sized cells with long, straight dendrites. These dendrites have numerous spines. Type III neurons are medium-sized or small cells with spine-free dendrites. Type IV neurons, the smallest cells in the Wulst, have short dendrites with sparse spines. The projections of the nucleus dorsolateralis anterior thalami pars lateralis (DLL) to the Wulst were determined by the Fink-Heimer method. After lesions of the DLL, degenerating terminals are seen in a dorsolateral portion of the nucleus intercalatus hyperstriatum accessorium where the types II, III, and IV neurons are distributed. Postsynaptic elements to the DLL axons were identified by reconstruction of electron microscopic serial sections. Most of the postsynaptic elements were dendritic spines of the type II and IV neurons and a few were dendritic shafts of the type III neurons.     

Intracellular recording and staining of neurons in the pigeon nucleus lentiformis mesencephali

The pretectal nucleus lentiformis mesencephali in pigeons is involved in optokinetic nystagmus and consists of lateral (nLMl) and medial (nLMm) subnuclei. The present study using intracellular recordings and brain slices shows that pretectal cells respond to depolarizing current injection in different ways. Type I cells (32%) fire spontaneously and have regular spikes. Type II cells (20%) discharge regular spikes, whose frequency increases as current intensity increases. Type III cells (8%) produce a series of bursts, each of which consists of 2-5 spikes depending on current intensities. Type IV cells (39%) fire several spikes in a cluster at the onset of current injection and are then rapidly adapted. One cell of type V (1%) shows spontaneous firing and is inactivated by depolarizing currents. Cells of types III and V are only found in nLMm, and other types of cells exist in both subnuclei. This physiological difference might be a bias due to the small sampling of cells. Twenty-six cells are labeled with dye and they could be categorized into fusiform (23.1%), piriform (7.7%), or multipolar (69.2%) cells. Some correlation seems to exist between the physiological and morphological properties of pretectal neurons. Statistically, the somatic size of nLMm cells is significantly larger than that of nLMl cells, indicating that the nucleus could be divided cytoarchitecturally into magnocellular and parvocellular components as suggested previously.     

Firing properties and dye coupling of neurons in the pigeon nucleus semilunaris

Our previous study indicated that the nucleus semilunaris in birds is a visual center. The present study using pigeon brain slices shows that 84 semilunar cells examined could be grouped into five types according to responses to depolarizing current injections. Type I cells (early bursting, 44%) fire a single burst followed by regular spiking. Type II cells (regular spiking, 13%) regularly produce spikes, the rates of which are enhanced as currents are increased. Type III cells (bursting, 17%) discharge a series of bursts each consisting of 2-4 spikes. Type IV cells (dual spiking, 15%) evoke both spikes and spikelets. Type V cells (inhibition-following, 11%) are characterized by regular spiking followed by an inhibitory period after current cessation. Morphologically, semilunar neurons have piriform, round, or fusiform somata of 12-23 mum in diameter, which give rise to 2-4 primary dendrites with sparse branches. Dual spiking activity is invariably correlated with dye coupling, and bursting cells have a tendency to be fusiform in shape. Other types of semilunar cells do not show a correlation between their firing patterns and morphological features.     

Cytoskeleton mediates inhibition of the fast Na+ current in respiratory brainstem neurons during hypoxia

Whole-cell Na+ currents (INa) were recorded in inspiratory neurons in a medullary slice preparation from neonatal mouse that contains the functional respiratory network. Hypoxia and metabolic poisoning with KCN rapidly inhibited INa by reducing the number of Na+ channels available for opening during depolarization. Application of agents specific for G-proteins, protein kinase C and A, intracellular Ca2+ and pH did not prevent the hypoxic inhibition of INa. The effects of hypo-osmolarity and hypoxia were additive, whereas hyperosmolarity partially prevented a subsequent hypoxic inhibition of INa. Cytochalasin B and colchicine decreased, and taxol or phalloidin increased INa and reduced its hypoxic inhibition. We conclude that cytoskeleton rearrangements during hypoxia are responsible for suppression of a fast INa in brainstem respiratory neurons, which could be mediated by the uncoupling of channel inactivation gates from cytoskeletal elements.     

Three-dimensional reconstructions of mouse circumvallate taste buds using serial blockface scanning electron microscopy: I. Cell types and the apical region of the taste bud

Taste buds comprise four types of taste cells: three mature, elongate types, Types I–III; and basally situated, immature postmitotic type, Type IV cells. We employed serial blockface scanning electron microscopy to delineate the characteristics and interrelationships of the taste cells in the circumvallate papillae of adult mice. Type I cells have an indented, elongate nucleus with invaginations, folded plasma membrane, and multiple apical microvilli in the taste pore. Type I microvilli may be either restricted to the bottom of the pore or extend outward reaching midway up into the taste pore. Type II cells (aka receptor cells) possess a large round or oval nucleus, a single apical microvillus extending through the taste pore, and specialized “atypical” mitochondria at functional points of contact with nerve fibers. Type III cells (aka “synaptic cells”) are elongate with an indented nucleus, possess a single, apical microvillus extending through the taste pore, and are characterized by a small accumulation of synaptic vesicles at points of contact with nerve fibers. About one-quarter of Type III cells also exhibit an atypical mitochondrion near the presynaptic vesicle clusters at the synapse. Type IV cells (nonproliferative “basal cells”) have a nucleus in the lower quarter of the taste bud and a foot process extending to the basement membrane often contacting nerve processes along the way. In murine circumvallate taste buds, Type I cells represent just over 50% of the population, whereas Types II, III, and IV (basal cells) represent 19, 15, and 14%, respectively.     

Serotonin differentially modulates the electrical properties of different subsets of taste receptor cells in bullfrog

Serotonin (5-hydroxytryptamin, 5-HT) is localized in taste bud cells of vertebrates. Effects of the external application of 5-HT on the membrane currents of frog taste receptor cells (TRCs) were investigated using patch-clamp technique in whole-cell configuration. The 5-HT (0.1-1 micro m) and 5-HT1A receptor agonist (+/-)-8-OH-2-(D1-n-propyl-amino)tetralin (8-OH-DPAT) (1-20 micro m) inhibited both voltage-gated sodium current (INa) and voltage-gated potassium current (IK) in 50% of TRCs, but potentiated IK without any significant effect on INa in another subset of 18% of TRCs. Voltage-gated currents in the residual TRCs were not affected by 5-HT or 8-OH-DPAT. External application of 10 micro m forskolin and 300 micro m 8-cpt cAMP [8-(4-chlorophenylthio)adenosine 3':5'-cyclic monophosphate] mimicked the inhibitory effect of 5-HT and 8-OH-DPAT on IK and INa while internal dialysis with 50 micro m protein kinase A inhibitor prevented the 5-HT-mediated inhibitory effects on IK and INa in TRCs. Internal dialysis of TRCs with high Ca2+-pipette solution (1 micro m) increased the IK in 58% of TRCs. The 5-HT reversibly increased the [Ca2+]i in 17% of TRCs when measured by Ca2+-imaging using a Ca2+-sensitive dye (fura-2 AM). These results suggest that 5-HT differentially modulates the voltage-gated membrane currents in different subsets of TRCs.     

GABA sensitivity of neurons of the visual layer in the rat superior colliculus

Inhibitory action of iontophoretically applied GABA was examined on neurons in the visual layer of the rat superior colliculus (SC). Spontaneous discharges of all neurons tested were readily abolished by GABA ejected with currents less than 25 nA. In some cells the discharges evoked by near threshold electrical stimulation of the optic nerve or those evoked by a spot of light moving across receptive fields were suppressed by the same dose of GABA as that required to abolish the spontaneous discharge. However, in other cells the evoked discharges were much more resistant to GABA than the spontaneous activity.GABA sensitivity of the evoked activities was examined on various classes of SC cells which were identified by their recording depth, response latency to electrical stimulation of the optic chiasm and other properties. SC cells of the visual layer were classified into 8 types: classes Ia and Ib in the most superficial layer (N3 zone), class II in the thin layer below the N3 (N2 zone) and classes IIIa., IIIb, IVb and IVc in the deepest layer below the N2 (N1 zone). Effects of GABA upon these cell classes are summarized as follows; (1) Ia and IVb cells were readily suppressed by GABA, (2) Ib and II and most of IIIa IVc cells were GABA-insensitive, and (3) GABA sensitivity varied from cell to cell in classes IIIb and IVa.     

Neuropathological work-up of focal cortical dysplasias using the new ILAE consensus classification system - practical guideline article invited by the Euro-CNS Research Committee

FCDs are increasingly recognized in patients with drug-resistant epilepsies, and many patients benefit from tailored resection strategies. Yet, postsurgical seizure control cannot be sufficiently predicted and specification of FCD variants remains difficult during presurgical monitoring. The International League against Epilepsy (ILAE) has published a new consensus classification system for focal cortical dysplasias (FCDs). Based on a review of imaging data, electroclinical features and postsurgical seizure control correlation with neuropathological findings specify three clinico-pathological FCD subtypes: FCD Type I is characterized by aberrant radial (FCD Type Ia) or tangential lamination of the neocortex (FCD Type Ib) affecting one or multiple lobes. FCD Type II is characterized by cortical dyslamination and dysmorphic neurons without (Type IIa) or with balloon cells (Type IIb). It is important to note, that these types should not be associated with any other structural brain lesion (isolated FCD). In contrast, a new FCD Type III is introduced, which occurs in combination with hippocampal sclerosis (FCD Type IIIa), or with epilepsy-associated tumors (FCD Type IIIb). FCD Type IIIc is found adjacent to vascular malformations, whereas FCD Type IIId can be diagnosed in association with other epileptogenic lesions obtained in early life (i.e., traumatic injury, ischemic injury or encephalitis). Histopathological features are very similar to those observed in FCD Type I, but likely present postnatal development and maturation failures acquired by the principal lesion. This first international consensus classification may encourage neuropathologists to focus their attention onto this important histopathological group. Addressing more precisely defined clinico-pathological entities will also help to clarify underlying pathomechanisms and, thereby, improve treatment strategies for patients with difficult-to-treat epilepsies.     

Electron microscopic analysis of the synaptic organization of the globus pallidus in the cat

The synaptic organization of the feline globus pallidus (GP) was studied electron microscopically. The axon terminals were classified into five types on the basis of the size and shape of synaptic vesicles and the type of postsynaptic differentiations. Type I and II axon terminals were characterized by large, pleomorphic vesicles and by a symmetric and an asymmetric synaptic contact, respectively. Type III and IV axon terminals were characterized by small, pleomorphic vesicles and by a symmetric and an asymmetric synaptic contact, respectively. Type V axon terminals were characterized by elongated and large round vesicles and by a symmetric synaptic contact. The origins of these terminals were determined by a combined degeneration and HRP tracing technique. Following injections of HRP into the caudate nucleus or electrolytic lesions in this nucleus, type I terminals were anterogradely labeled with HRP or degenerated, respectively. Although type III, IV, and V terminals were labeled with HRP after HRP injections into the subthalamic nuclear region, only type IV and V terminals degenerated after lesions in that area. Type II terminals did not show any alterations following such treatment. These results suggest that type I terminals originate from the caudate nucleus, that type IV and V terminals come from the subthalamic nucleus or caudal to it, and that type III terminals are the terminals of intrinsic axon collaterals of GP neurons which send axons to the subthalamic nucleus. Occasionally convergence of different kinds of axon terminals on the same GP neuron was also observed. These terminals originated from the caudate nucleus and the subthalamic nucleus or caudal to it.(ABSTRACT TRUNCATED AT 250 WORDS)     

Functional characteristics of skate connexin35, a member of the gamma subfamily of connexins expressed in the vertebrate retina.

Retinal neurons are coupled by electrical synapses that have been studied extensively in situ and in isolated cell pairs. Although many unique gating properties have been identified, the connexin composition of retinal gap junctions is not well defined. We have functionally characterized connexin35 (Cx35), a recently cloned connexin belonging to the gamma subgroup expressed in the skate retina, and compared its biophysical properties with those obtained from electrically coupled retinal cells. Injection of Cx35 RNA into pairs of Xenopus oocytes induced intercellular conductances that were voltage-gated at transjunctional potentials >/= 60 mV, and that were also closed by intracellular acidification. In contrast, Cx35 was unable to functionally interact with rodent connexins from the alpha or beta subfamilies. Voltage-activated hemichannel currents were also observed in single oocytes expressing Cx35, and superfusing these oocytes with medium containing 100 microm quinine resulted in a 1.8-fold increase in the magnitude of the outward currents, but did not change the threshold of voltage activation (membrane potential = +20 mV). Cx35 intercellular channels between paired oocytes were insensitive to quinine treatment. Both hemichannel activity and its modulation by quinine were seen previously in recordings from isolated skate horizontal cells. Voltage-activated currents of Cx46 hemichannels were also enhanced 1. 6-fold following quinine treatment, whereas Cx43-injected oocytes showed no hemichannel activity in the presence, or absence, of quinine. Although the cellular localization of Cx35 is unknown, the functional characteristics of Cx35 in Xenopus oocytes are consistent with the hemichannel and intercellular channel properties of skate horizontal cells.     

Morphological and functional types of neurons in cat ventral posterior thalamic nucleus

Neurons in the thalamic ventral posterior (VB) nucleus of the cat were investigated by extracellular and intracellular recording and by anatomical methods involving either the retrograde transport of horseradish peroxidase (HRP) or the intracellular injection of HRP. Two morphological types of neurons could be detected by retrograde labeling from small injections of HRP in the internal capsule adjacent to VB. These two and one other type, judged to be an interneuron, could also be identified by intracellular staining. Type I cells are large, have thick proximal dendrites which branch in a tuft-like manner, and thick, rapidly conducting axons. They possess few or no dendritic appendages. Type II cells are smaller and have slender proximal dendrites which branch dichotomously and thin, slower conducting axons. Those injected intracellularly are covered in fine, hair-like dendritic appendages. Type III cells are small and have thin processes that give rise to many bulbous dilatations and no obvious axon. Type I and type II cells give off slender axon collaterals in the thalamic reticular nucleus but not in VB. Examples of both types of cell could be antidromically activated from the somatic sensory cortex. Type I and type II cells recovered histologically after intracellular recording included examples of most types of receptive field, including several forms of cutaneous and deep fields, as classified by us in a parallel intra- and extracellular study of unit responses. All but one type I cell, however, responded in a transient manner to peripheral stimulation. The remaining type I cell and all members of an admittedly small sample of type II cells responded in a sustained manner. The sample of recovered interneurons and of units that could not be driven antidromically from the cerebral cortex suggested that they, too, included all receptive field types. We conclude that submodality specificity in VB is not represented by morphological specificity in thalamocortical relay cells or interneurons. Some other functional parameter, such as tonic or phasic responsiveness, may be more obviously correlated with relay cell morphology.