Surviving mossy cells enlarge and receive more excitatory synaptic input in a mouse model of temporal lobe epilepsy

Numerous hypotheses of temporal lobe epileptogenesis have been proposed, and several involve hippocampal mossy cells. Building on previous hypotheses we sought to test the possibility that after epileptogenic injuries surviving mossy cells develop into super‐connected seizure‐generating hub cells. If so, they might require more cellular machinery and consequently have larger somata, elongate their dendrites to receive more synaptic input, and display higher frequencies of miniature excitatory synaptic currents (mEPSCs). To test these possibilities pilocarpine‐treated mice were evaluated using GluR2‐immunocytochemistry, whole‐cell recording, and biocytin‐labeling. Epileptic pilocarpine‐treated mice displayed substantial loss of GluR2‐positive hilar neurons. Somata of surviving neurons were 1.4‐times larger than in controls. Biocytin‐labeled mossy cells also were larger in epileptic mice, but dendritic length per cell was not significantly different. The average frequency of mEPSCs of mossy cells recorded in the presence of tetrodotoxin and bicuculline was 3.2‐times higher in epileptic pilocarpine‐treated mice as compared to controls. Other parameters of mEPSCs were similar in both groups. Average input resistance of mossy cells in epileptic mice was reduced to 63% of controls, which is consistent with larger somata and would tend to make surviving mossy cells less excitable. Other intrinsic physiological characteristics examined were similar in both groups. Increased excitatory synaptic input is consistent with the hypothesis that surviving mossy cells develop into aberrantly super‐connected seizure‐generating hub cells, and soma hypertrophy is indirectly consistent with the possibility of axon sprouting. However, no obvious evidence of hyperexcitable intrinsic physiology was found. Furthermore, similar hypertrophy and hyper‐connectivity has been reported for other neuron types in the dentate gyrus, suggesting mossy cells are not unique in this regard. Thus, findings of the present study reveal epilepsy‐related changes in mossy cell anatomy and synaptic input but do not strongly support the hypothesis that mossy cells develop into seizure‐generating hub cells. © 2014 Wiley Periodicals, Inc.     

Gamma frequency oscillation in the hippocampus of the rat: intracellular analysis in vivo

Gamma frequency field oscillations reflect synchronized synaptic potentials in neuronal populations within the ≈ 10–40 ms range. The generation of gamma activity in the hippocampus was investigated by intracellular recording from principal cells and basket cells in urethane anaesthetized rats. The recorded neurones were verified by intracellular injection of biocytin. Gamma frequency field oscillations were nested within the slower theta waves. The phase and amplitude of intracellular gamma were voltage dependent with an almost complete phase reversal at Cl^– equilibrium potential in pyramidal cells. Basket cells fired at gamma frequency and were phase-locked to the same phase of the gamma oscillation as pyramidal cells. Current-induced depolarization coupled with synaptically induced inhibition resulted in gamma frequency discharge (30–80 Hz) of pyramidal cells without accommodation. These observations suggest that at least part of the gamma frequency field oscillation reflects rhythmic hyperpolarization of principal cells, brought about by the rhythmically discharging basket neurones. Resonant properties of pyramidal cells might facilitate network synchrony in the gamma frequency range.     

Laminar and cellular localization of cytochrome oxidase in the cat striate cortex

Cytochrome oxidase (C.O.) was histochemically localized in the cat striate cortex at the light and electron microscopic levels. The results indicate that the oxidative metabolic activity within the cat striate cortex may vary between (1) different laminae, (2) neurons and glia, (3) different neuron types, (4) dendrite and soma of the same cell, (5) different types of dendrites, (6) different segments of the same dendrite, and (7) different classes of symmetric and asymmetric axon terminals. Maximal laminar C.O. staining was localized within geniculoreceptive layer IV. Darkly reactive neurons include the large (presumed corticotectal) pyramids of layer V, and various classes of large and medium-sized presumed GABAergic nonpyramidal cells sparsely distributed throughout layers II-VI. The small and medium-sized pyramids of layers II, III, V, and VI, as well as many of the smaller presumed GABAergic neurons, were only lightly or moderately reactive. The darkly reactive neurons tended to be those that received convergent or proximally localized asymmetric axosomatic synapses, implying that they are strongly driven by excitatory synaptic input. The darkly reactive nonpyramids resembled those that form GAD+, symmetric axosomatic synapses with pyramidal cells. The dark reactivity of the symmetric synaptic terminals indicates that they mediate strong inhibition of neuronal discharge. The dark reactivity of a class of large asymmetric terminals in layer IV is likely to represent highly active geniculocortical terminals. The predominant distribution of elevated C.O. reactivity in dendrites is correlated with reported sites of (1) convergent excitatory synaptic input, (2) maximal field potentials, (3) highly active ion transport, and (4) Na+, K+-ATPase.     

Dendritic reorganization in the denervated dentate gyrus of the rat following entorhinal cortical lesions: A Golgi and electron microscopic analysis

The organization of the dendritic tree and the morphology of individual dendrites of the dentate granule cell were analyzed qualitatively and quantitatively by means of the rapid Golgi technique 2, 4, 8, 10, 14, 30, 60, and 250 days after unilateral lesions of the entorhinal cortex (EC). Three dendritic field parameters were analyzed from camera lucida drawings of de-nervated granule cells at each survival time: (1) dendritic field spread, (2) dendritic length, and (3) dendritic branching. Cells in the contralateral dentate gyrus served as controls. Spines were counted at each postlesion interval from material stained with a modification of the Golgi-Kopsch method. The amount of tissue occupied by dendritic shafts at different postlesion intervals was also evaluated in samples of the ventral leaf of the dentate gyrus prepared for electron microscopy. After unilateral lesions of the EC, dendrites of the granule cells undergo modifications which appear to represent deterioration and recovery. When the dendrites reach the denervated zone, they abruptly change their orientation and tend to follow a course parallel to the granule cell layer. In contrast to normal dendrites, those in the denervated neuropil only occasionally reach the outer boundaries of the molecular layer. At the time of maximal denervation there is often a sudden reduction in dendritic diameter as the dendrite enters the denervated zone. Varicosities are also prominent. The alterations in individual dendrites are not evident 2 days after the lesion, are fully developed 10 days after deafferentation; and disappear for the most part by 30 days postlesion. The quantitative analysis of Golgi-stained granule cells reveals that there is a 40% reduction in the total length of the granule cell dendritic tree. Electron microscopic analysis confirms the Golgi observations, indicating that the amount of neuropil occupied by dendritic shafts in the denervated zone is initially reduced and later recovers to values close to those observed in control animals. While the apparent dendritic loss is mainly restricted to the denervated zone, significant modifications occur in the inner npnde-nervated molecular layer; there is an increase in the length of primary dendrites as evidenced by an increase in the distance to the first branch point. There is also a polarized redistribution of 1st, 2nd, 3rd, and 4th branch points toward the middle and outer molecular layer that persists even 30 days after the lesion. The results are discussed in terms of the capabilities of the granule cells of the dentate gyrus to reorganize and remodel their dendritic surface after partial deafferentation.     

The Purkinje cell dendritic tree: a computer-aided study of its development in the cat and in culture

Golgi-prepared cerebella from 1, 10, 13 and 30-day-old kittens were analyzed and compared with 30-45 days in vitro (DIV) HRP-stained organotypic cultures of newborn kitten cerebella. Computer reconstructions and morphometric parameters allowed a quantitative analysis of the Purkinje cell (P-cell) dendritic trees. In intact animals the dendritic organization appeared monoplanar as early as one day after birth and biplanar in 85% of the cells at day 13; however, by day 30, 90% of the cells were monoplanar. During the first 4 postnatal weeks, the dendritic expansion was due mostly to an increase in the total number of segments and the total dendritic length, whereas the overall mean segment length remained almost unchanged. In culture, the 30-45 DIV P-cell dendritic trees always appeared reduced in size when compared to their in vivo counterparts due mostly to a reduction in the total number of segments. Nevertheless, these cells retained several primary dendritic trunks and their overall mean segment length was longer. These supposedly 'mature' cultured P-cells never reached full adult development: a discriminant analysis classified them as resembling those from intact animals of 13 days but often maintaining some properties of newborn animals. These results demonstrate that the presence of all normal inputs is required to achieve the full elaboration and the planar disposition of the P-cell dendrites.     

Apical dendritic spines of the visual cortex and light deprivation in the mouse

Summary The effects of light deprivation on the number of apical dendritic spines have been studied in the visual cortex of the mouse. In the portion of the apical dendrites of layer V-pyramidal cells traversing layer IV, dendritic segments of 50 in length from different cells were selected. The number of spines on each of 50 different segments per animal was counted. The countings were done in the areae striata and temporalis prima from mice raised in complete darkness since birth up to 22–25 days old. The observations were compared with the countings obtained in the areae striata and temporalis prima from mice raised under normal conditions. The results indicated that mice raised in darkness had a significant reduction in the number of spines per dendritic segment at the level of layer IV in area striata when compared with control animals. No significant difference was found in the number of spines per dendritic segment in layer IV between both groups of normal and dark raised mice in the area temporalis prima. The mean number of dendritic spines per consecutive segments along complete apical dendrites of layer V-pyramidal cells in area striata has been found to increase exponentially with the distance from the cell body. The same exponential relation, but with somewhat lower values, was obtained in the apical dendrites in area striata in mice raised in darkness. The significance of these findings were discussed. It was concluded that: First, visual sensory deprivation affect the fine structure of the central nervous system. Second, the results observed support the assumption that structural changes in the nerve cells occur as the result of experience.     

A unique morphological subtype of horizontal cell in the rabbit retina with orientation-sensitive response properties.

Intracellular recordings were obtained from horizontal cells in the rabbit retina to assess the orientation sensitivity of their visual responses to moving and stationary rectangular slits of light. Cells were subsequently labeled with horseradish peroxidase (HRP) for morphological identification. The responses of A-type horizontal cells and those of the somatic and axon terminal endings of B-type horizontal cells (with the exception of one cell) were found to be insensitive to the orientation of light stimuli. However, 20 horizontal cells encountered within or just superior to the visual streak displayed clear orientation-sensitive response properties. These cells were divided into two groups: the majority (70%) showed preference for light stimuli oriented parallel to the visual streak, whereas the remainder preferred stimuli oriented orthogonal to the visual streak. Analysis of the shape of the receptive fields of these cells by means of a narrow, displaced slit of light revealed an anisotropy with the major or elongated axis of the receptive field of each cell aligned along the same angle as its physiological preferred orientation. Morphologically, the orientation-sensitive horizontal cells formed a homogeneous group with an architecture corresponding to that of elongated A-type or Ae-type horizontal cells reported previously in the rabbit retina. These cells showed a marked elongation of their dendritic arbors with the major axes oriented either parallel or orthogonal to the visual streak. Furthermore, the orientation of the dendritic arbor of each cell matched that of its physiological preferred orientation. The present results, then, suggest strongly that the orientation sensitivity of Ae-type horizontal cells results directly from the asymmetry in their dendritic arbors. The spatial location and specialized physiology of Ae-type horizontal cells suggest that they play a role in the formation of orientation-sensitive properties exhibited by more proximal neurons in the rabbit retina.     

Axonal endings terminating on dendrites of identified large trigeminospinal projection neurons in rat trigeminal nucleus oralis

The retrograde horseradish peroxidase technique was used to: (1) identify and assess the overall morphology of large neurons in the ventrolateral portion (VL) of rat trigeminal nucleus oralis projecting to cervical, thoracic and lumbosacral levels of the spinal cord; and (2) characterize the synaptic endings terminating on their dendrites. The morphology of large VL neurons projecting to all spinal levels is similar. They have 25–50 μm pyramidal-shaped somata which emit 3–6 primary dendrites. These primary dendrites give rise to spherical to elliptical-shaped dendritic arbors measuring up to 700 μm in diameter. Labeled axons enter either a deep axon bundle or the medial portion of the spinal V tract. Dendrites of labeled neurons are contacted by axonal endings of 3 types. The most numerous endings are filled with clear, spherical synaptic vesicles and usually form a single asymmetrical contacts along the entire length of dendritic shafts. Synapsing less frequently on dendritic shafts are endings containing pleomorphic synaptic vesicles and forming single symmetrical synaptic contacts. The least frequently encountered synaptic terminal contains flattened synaptic vesicles and makes a single symmetrical synaptic contact with a dendritic shaft.     

The microtubular apparatus of cerebellar purkinje cell dendrites during postnatal development of the rat: The density and cold-stability of microtubules increase with age and are sensitive to thyroid hormone deficiency

A quantitative ultrastructural study of microtubules in Purkinje cell dendrites of normal and hypothyroid developing rats was performed after fixation either at room or at low temperature (4°C). In normal animals, the density of microtubules and their fold-stability increased with age, more especially during the period of intense dendritic growth. Thyroid deficiency delayed the appearance of microtubules and still more the acquisition of their fold-stability. These effects might explain the defects in Purkinje cell dendritic growth and branching observed in hypothyroid animals.