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PubMed2005

Modulation of synaptic transmission in neocortex by network activities.

Neocortical neurons integrate inputs from thousands of presynaptic neurons that fire in vivo with frequencies that can reach 20 Hz. An important issue in understanding cortical integration is to determine the actual impact of presynaptic firing on postsynaptic neuron in the context of an active network. We used dual intracellular recordings from synaptically connected neurons or microstimulation to study the properties of spontaneous and evoked single-axon excitatory postsynaptic potentials (EPSPs) in vivo, in barbiturate or ketamine-xylazine anaesthetized cats. We found that active states of the cortical network were associated with higher variability and decrease in amplitude and duration of the EPSPs owing to a shunting effect. Moreover, the number of apparent failures markedly increased during active states as compared with silent states. Single-axon EPSPs in vivo showed mainly paired-pulse facilitation, and the paired-pulse ratio increased during active states as compare to silent states, suggesting a decrease in release probability during active states. Raising extracellular Ca(2+) concentration to 2.5-3.0 mm by reverse microdialysis reduced the number of apparent failures and significantly increased the mean amplitude of individual synaptic potentials. Quantitative analysis of spontaneous synaptic activity suggested that the proportion of presynaptic activity that impact at the soma of a cortical neuron in vivo was low because of a high failure rate, a shunting effect and probably dendritic filtering. We conclude that during active states of cortical network, the efficacy of synaptic transmission in individual synapses is low, thus safe transmission of information requires synchronized activity of a large population of presynaptic neurons.

Action Potentials↗
PubMed1982

Generation and maintenance of the respiratory rhythm.

Activities of the phrenic and internal intercostal nerves show that the central nervous rhythm of respiration consists of 3 phases: inspiratory, postinspiratory and expiratory. The discharge patterns of medullary respiratory neurones of the anaesthetized, paralysed cat can be correlated with these phases of the central respiratory cycle, and the postsynaptic activity of individual cells can be analysed to obtain information about the populations of neurones converging upon them. Inferences are drawn about respiratory neurone connectivity and a theory is developed that the respiratory network primarily employs inspiratory-related neurones and that medullary expiratory neurones are less important for the rhythmogenesis of respiration. It is suggested that the inspiratory network consists of a ramp generating excitatory loop network of interneurones whose discharge is brought to an end ('off-switched') by inhibitory late-inspiratory interneurones. The discharge pattern of the latter type of neurone is explained by inhibition arriving from early-inspiratory interneurones. Subsequent to 'off-switching' the ramp generator is assumed to be immediately gated by a very powerful postinspiratory inhibition whereas expiratory activity seems to be disfacilitated at this time. This is the period when 'passive' (stage 1) expiration occurs. Following this interposed postinspiratory phase 'active' (stage 2) expiration may begin, depending on the amount of excitatory inflow to the inspiratory ramp generator. When expiratory neurones are activated the inspiratory system is again synaptically inhibited and the frequency of ventilation is markedly slowed.

Animals↗
PubMed1988

Anatomy of antenno-cerebral pathways in the brain of the sphinx moth Manduca sexta.

In the moth Manduca sexta, the number and morphology of neuronal connections between the antennal lobes and the protocerebrum were examined. Cobalt injections revealed eight morphological types of neurons with somata adjacent to the AL neuropil that project in the inner, middle, and outer antenno-cerebral tracts to the proto-cerebrum. Neurons innervating the macroglomerular complex and many neurons with fibers in the inner antenno-cerebral tract have uniglomerular antennal-lobe arborizations. Most neurons in the middle and outer antenno-cerebral tracts, on the other hand, seem to innervate more than one glomerulus. Protocerebral areas receiving direct input from the antennal lobe include the calyces of the mushroom bodies, and circumscribed areas termed "olfactory foci" in the lateral horn of the protocerebrum and several other regions, especially areas in close proximity to the mushroom bodies. Fibers in the inner antenno-cerebral tract that innervate the male-specific macroglomerular complex have arborizations in the protocerebrum that are distinct from the projections of sexually non-specific neurons. Protocerebral neurons projecting into the antennal lobe are much less numerous than antennal-lobe output cells. Most of these protocerebral fibers enter the antennal lobe in small fiber tracts that are different from those described above. In the protocerebrum, these centrifugal cells arborize in olfactory foci and also in the inferior median protocerebrum and the lateral accessory lobes. The morphological diversity of connections between the antennal lobes and the protocerebrum, described here for the first time on a single-cell level, suggests a much greater physiological complexity of the olfactory system than has been assumed so far.

Animals↗
PubMed1994

The effects of undernutrition on connectivity in the cerebellar cortex of adult rats.

The effects of a 30 d period of undernutrition, followed in some animals by nutritional rehabilitation, on neuronal connectivity in adult rat cerebellum were investigated using the disector method. There was no significant difference between well fed (719 +/- 74, mean +/- S.E.) and undernourished (709 +/- 53) synapse-to-neuron ratios in 134-d-old rat cerebellar cortex, nor was there a significant difference in synapse-to-neuron ratios between control animals (941 +/- 71) and previously undernourished rats (813 +/- 42) at 175 d of age. However, the age-related changes were significant (P < 0.05) in the controls, but not in the experimental group. It may be that the period of undernutrition caused subtle changes in the rehabilitating group which reduced the capacity for growth seen in well fed, matched control animals.

Animals↗
PubMed2002

Apoptosis and age-related disorders: role of caspase-dependent and caspase-independent pathways.

The execution of the apoptotic program involves a relatively limited number of pathways that converge on the activation of the caspase family of proteases. However, there is increasing evidence that other protease families may contribute to produce apoptotic-like features. This has posed the question as to whether caspase inhibitors may then be used to treat diseases characterised by an excess apoptosis. In several neurodegenerative diseases including acute neuronal loss as in stroke or slowly developing diseases at least two major events contribute to neurodegeneration: the loss of neuronal connectivity and cell loss. In many of these conditions, mitochondrial dysfunction and the resulting ATP depletion may preclude caspase activation, and consequently switch execution of cell death towards necrosis. A block or partial inhibition of the typical apoptotic demise may have profound implications in vivo, as persistence within the nervous system of damaged, but 'undead' cells, followed by delayed lysis may favour neuroinflammatory reactions. Furthermore, caspases may be involved in loss of neurons, but not in the loss of connectivity that seems to initiate degenerative processes in the nervous system. Some recent findings, which suggest that degenerating neurons may use multiple execution pathways will be discussed.

Age Factors↗
PubMed2006

Selective spread of neurotropic herpesviruses in the rat hippocampus.

Transsynaptically spreading viruses are widely used for tracing neuronal circuits in both the central and peripheral nervous systems. However, viruses are capricious tools with selective spreading properties that can produce false-negative results. Using herpes simplex virus type 1 and two pseudorabies virus strains, we aimed at mapping quantitatively neuronal connections in the rat hippocampus. We found that none of the tested viruses infected CA3 pyramidal neurons across synapses following inoculation into the CA1 area. Combined injections of the viruses with the retrograde tracer cholera toxin B (CTB) resulted in CTB, but not virus labeling of CA3 pyramidal neurons. In contrast, other brain regions known to send inputs to the CA1 (the entorhinal cortex, medial septum and diagonal band of Broca, raphe nuclei) were transsynaptically infected. Our results indicate that Schaffer collaterals of CA3 pyramidal cells lack the appropriate cellular machinery for successful neurotropic herpesvirus infection. After injections of viruses into the dentate gyrus/CA3 area, we found labeling in commissurally projecting mossy cells and their afferent granule cells but not in contralateral CA3 pyramidal cells. Using this unique spreading property, we estimated that single mossy cells receive input from a compact cluster of 30-40 granule cells.

Animals↗
PubMed2006

Improved neuronal tract tracing using manganese enhanced magnetic resonance imaging with fast T(1) mapping.

There has been growing interest in using manganese-enhanced MRI (MEMRI) to detect neuronal activation, neural architecture, and neuronal connections. Usually Mn(2+) produces a very wide range of T(1) change. In particular, in neuronal tract tracing experiments the site of Mn(2+) injection can have very short T(1) while distant regions have small T(1) reductions, primarily due to dilution of Mn(2+). Most MEMRI studies use T(1)-weighted sequences, which can only give optimal contrast for a narrow range of T(1) changes. To improve sensitivity to the full extent of Mn(2+) concentrations and to optimize detection of low concentrations of Mn(2+), a fast T(1) mapping sequence based on the Look and Locker technique was implemented. Phantom studies demonstrated less than 6.5% error in T(1) compared to more conventional T(1) measurements. Using center-out segmented EPI, whole-brain 3D T(1) maps with 200-microm isotropic resolution were obtained in 2 h from rat brain. Mn(2+) transport from the rat olfactory bulb through appropriate brain structures could be detected to the amygdala in individual animals. The method reliably detected less than 7% reductions in T(1). With this quantitative imaging it should be possible to study more extensive pathways using MEMRI and decrease the dose of Mn(2+) used.

Animals↗
PubMed2005

Central autonomic control of the bone marrow: multisynaptic tract tracing by recombinant pseudorabies virus.

Bone marrow is the primary place of hematopoiesis, where the development, survival and release of multipotent stem cells, progenitors, precursors and mature cells are under continuous humoral and neural control. Dense network of nerve fibers, containing various neurotransmitters is found in the bone marrow, however, the central neuronal circuit that regulates the activities of the bone marrow through these fibers remained unexplored. Transsynaptically connected neurons were mapped by virus-based transneuronal tracing technique using two isogenic, genetically engineered pseudorabies viruses, Bartha-DupGreen and Ba-DupLac expressing green fluorescent protein and beta-galactosidase, respectively. Bartha-DupGreen was injected into the femoral bone marrow of male rats and the progression of infection was followed 4-7 days post-inoculation. Virus-labeled cells were revealed in ganglia of the paravertebral chain and in the intermediolateral cell column of the lower thoracic spinal cord. Neurons were retrogradely labeled in the C1, A5, A7 catecholaminergic cell groups and several other nuclei of the ventrolateral and ventromedial medulla, the periaqueductal gray matter, the paraventricular and other hypothalamic nuclei, and in the insular and piriform cortex. Nerve transections and double-virus tracing from the bone marrow and the surrounding muscles were used to confirm the specific spreading of the virus. These results provide anatomical evidence for the CNS control of the bone marrow and identify putative brain areas, which are involved in autonomic regulation of the hematopoiesis, the release of progenitor cells, the blood supply and the immune cell function in the bone marrow.

Animals↗
PubMed2002

Olfactory maps and odor images.

The understanding of neuronal processing of olfactory stimuli has been furthered by genetic studies and specialized imaging of particular neuronal populations. Selective optical imaging of odor-induced presynaptic and postsynaptic glomerular activity in the olfactory bulb/antennal lobe has visualized odorant-responsive receptor repertoires and shown a more confined odor image at the level of projection neurons compared to their olfactory receptor neuron input. Genetic tracing of projection neurons connected to particular glomeruli has revealed a somewhat dispersed spatial map of termination areas for these neurons both in insects and in vertebrates. Modifications of the glomerular odor map have resulted in altered percepts of the corresponding odors.

Animals↗
PubMedDate not supplied

Examining neocortical circuits: some background and facts.

The first definitive studies of where afferents to cerebral cortex terminate were made possible by the finding that as they degenerate axon terminals become electron dense. Gold toning of Golgi impregnated neurons allowed the postsynaptic targets of these afferents to be identified by electron microscopy and also allowed the termination sites of axons from a variety of types of cortical neurons to be ascertained, while the development of antibodies to GAD and to GABA made it possible to determine which types of cortical neurons are inhibitory. Subsequently the use of gold toned, Golgi impregnated material to examine neuronal connectivity was made redundant by the development of techniques that allowed the physiological properties of cortical neurons to be evaluated in neurons filled intracellularly with markers. Intracellular filling showed the axonal trees of cortical neurons are much more widespread than had been revealed by Golgi impregnations. As a result of numerous studies of the axons of identified neurons, we know a great deal about where most of the different types of neurons in cerebral cortex form their synapses, but on the other side of the picture there is a dearth of information about the origins of the inputs that specific types of cortical neurons receive. However, it is evident that each cortical neuron is the focus of input from many other neurons, and on the basis of the available data it is estimated that a single pyramidal cell in cortex receives its input from as many as 1,000 other excitatory neurons and as many as 75 inhibitory neurons.

Animals↗
PubMed1995

Paradoxical phase response at late expiration by superior laryngeal nerve stimulation.

Superior laryngeal nerve (SLN) stimulation during expiration prolongs the respiratory cycle in decerebrate, vagotomized and paralysed cats. In a few animals, however, the cycle can be terminated prematurely by the same stimulus. We developed a mathematical model of the respiratory neural network to stimulate these responses. The model contained inspiratory decrementing (I-DEC), and augmenting (I-AUG) and expiratory decrementing (E-DEC), and augmenting (E-AUG) neurones. Connections were based on published findings. SLN stimulation during late expiration prolonged the cycle when it was assumed to excite principally E-DEC neurones, whereas it terminated the cycle prematurely when it was assumed to excite both I-DEC and E-DEC neurones. Therefore, phase-resetting depends on the differential strength of afferent connections on the network's elements.

Animals↗
PubMed1987

Full-wave rectification from a mixed electrical-chemical synapse.

Electrical and chemical synapses usually reinforce one another, but the pyloric late-to-lateral pyloric (PL-to-LP) neuronal connections in lobster stomatogastric ganglia create an inverted U-shaped transfer function between the two neurons: regardless of whether the PL membrane voltage swings positive or negative, the postsynaptic LP voltage will go negative. When the presynaptic cell voltage goes negative, the effect on the LP voltage is due to electrical coupling. During positive presynaptic voltages, the strong contribution of graded chemical inhibition from the PL to the LP neuron overrides the positive electrical coupling to produce net negativity.

Animals↗
PubMed2006

Orexin A induces GnRH gene expression and secretion from GT1-7 hypothalamic GnRH neurons.

Orexin A, a recently discovered hypothalamic peptide, has been shown to have a stimulatory effect on release of gonadotropin-releasing hormone (GnRH) from rat hypothalamic explants in vitro. However, it is presently unclear whether in vivo this effect is mediated directly at the level of the GnRH neuron, or via multiple afferent neuronal connections. Therefore, in the present study, we investigated the direct action of orexin A on GnRH neurons using the immortalized GnRH-secreting GT1-7 hypothalamic cells. Orexin-1 receptor (OX1R) expression was detected in GT1-7 cells by RT-PCR and Western blot. Results showed that 0.1-1 nM orexin A, when administered in culture media for 4 h, can significantly stimulate GnRH mRNA expression in GT1-7 cells (p < 0.05). Administration of 1 microM OX1R antagonist, SB-334867, completely blocked the observed orexin A responses in these cells, indicating that orexin A stimulation of GnRH neurons is specifically through OX1R. Moreover, 0.1 nM orexin A stimulated GnRH release after 30-45 min. To examine possible signal transduction pathways involved in mediating these effects, a MEK inhibitor (UO-126), PKC inhibitor (calphostin C), and PKA inhibitor (H-89), were used, with each blocking orexin A-induced GnRH transcription and release from immortalized cells. Collectively, our results show that orexin A is capable of directly stimulating GnRH transcription and neuropeptide release from these immortalized hypothalamic neurons, and that the effects of orexin A appear to be mediated via the OX1R, coupled with activation of the PKC-, MAPK- and PKA-signaling pathways. It is suggested that the stimulatory effect of orexin A on GnRH transcription and release may also occur directly at the level of GnRH neurons in vivo.

Animals↗
PubMed2000

Structure of the principal olfactory tract.

Although the purpose and importance of the sense of smell in human beings has not been totally clarified, it is one of the principal information channels in macrosmatic animals. It was the first long-distance information system to have appeared in phylogenetic evolution. The objective of this article is to deepen the knowledge of the pathways that join the olfactory epithelium with the cortical olfaction areas, to better understand olfactory dysfunction in human beings. Differential staining and marking techniques were applied to histologic sections obtained from 155 animals of different species, to study the different connections existing among olfactory tract components. Our study of the connections between the olfactory mucosa and the principal olfactory bulb deserves special mention. The distribution of second neuron connections of the olfactory tract with the central nervous system is quite complex and diffuse. This indicates an interrelation between the sense of smell and a multitude of functions. These connections seem to be of different quantitative importance according to species, but qualitatively they exist in both human beings and other macrosmatic animals.

Animals↗
PubMed1989

CNS connections with the median raphe nucleus: retrograde tracing with WGA-apoHRP-Gold complex in the rat.

In this work we examined the neuronal input to one of the serotoninergic centers in the brain, median raphe nucleus (MR). Special consideration is given to projections of the hypothalamus. To describe the afferents to MR, a retrograde transport technique was used after microinjection of WGA-apoHRP-Gold complex under pressure and subsequent gold-silver intensification on formaldehyde-fixed rat brain sections. Optimal conditions were obtained when the coordinates of the injection site were A +/- 1.5, L +/- 0.15, and H +/- 2.7 according to Paxinos and Watson (The Rat Brain in Stereotaxic Coordinates. New York: Academic Press, '82). Results obtained under these conditions show a heterogeneous distribution of labeled neurons throughout the brain, including a large proportion (+/- 65%) of hypothalamic neurons. Extra-hypothalamic neurons projecting to MR were from the prefrontal cortex, lateral and medial habenular nuclei, the pontine area of the central grey, interpeduncular nucleus, dorsal raphe nucleus, oculomotor and trochlear nuclei, dorsal and laterodorsal tegmental nuclei, parabrachial nuclei, and lateral and interpositus cerebellar nuclei. Hypothalamic neurons connected to MR were found to be from medial and lateral preoptic areas, lateral hypothalamus, dorsomedian nucleus, the perifornical area, and the complex of mammillary bodies. Many other discrete regions contained different densities of labeled perikarya: the medial preoptic nucleus, paraventricular nucleus, retrochiasmatic area, arcuate nucleus, lateral magnocellular nucleus, and the posterior area. The MR appears as an integrative center receiving many neuroanatomically and functionally heterogeneous inputs from the whole brain.

Animals↗
PubMed2006

Canadian Association of Neurosciences review: postnatal development of the mammalian neocortex: role of activity revisited.

The mammalian neocortex is the largest structure in the brain, and plays a key role in brain function. A critical period for the development of the neocortex is the early postnatal life, when the majority of synapses are formed and when much of synaptic remodeling takes place. Early studies suggest that initial synaptic connections lack precision, and this rudimentary wiring pattern is refined by experience-related activity through selective elimination and consolidation. This view has been challenged by recent studies revealing the presence of a relatively precise pattern of connections before the onset of sensory experience. The recent data support a model in which specificity of neuronal connections is largely determined by genetic factors. Spontaneous activity is required for the formation of neural circuits, but whether it plays an instructive role is still controversial. Neurotransmitters including acetylcholine, serotonin, and gamma-Aminobutyric acid (GABA) may have key roles in the regulation of spontaneous activity, and in the maturation of synapses in the developing brain.

Action Potentials↗
PubMed1988

A Golgi-electron microscopic study of fusiform neurons in the hilar region of the dentate gyrus.

The fusiform cells of the dentate gyrus are located in a portion of the hilus within 100 micron of the granule cell layer. They have ovoid somata and bipolar dendrites that generally run parallel to the granule cell layer. The dendrites of these cells are either spiny or sparsely spiny. The spiny fusiform cell has numerous spines along its dendrites, which are contacted by terminals with the features of granule cell axon collaterals. This cell type also displays somal spines that are contacted by similar terminals. In contrast, the sparsely spiny fusiform cell displays only a few spines, which are contacted by multiple small axon terminals that synapse with both the stalk and end bulb of the spine. Most synaptic input for this cell type is made with the smooth surfaces of the soma and dendrites. A variety of terminals form synapses with the sparsely spiny fusiform cell, including terminals that resemble the fine axon collaterals of mossy fibers. The somata of these two cell types also display differences in the amount of Nissl bodies and the degree of nuclear infolding. The results indicate that spiny fusiform cells are similar to mossy cells, another hilar cell type that receives its major synaptic input from axon collaterals of mossy fibers from granule cells. The distribution of the dendrites of spiny fusiform cells and the pattern of granule cell axon collaterals suggest a high degree of convergence from granule cells. In contrast, the variety of axodendritic synapses for sparsely spiny fusiform cells suggests that more diverse inputs affect this cell's activity. Therefore, the structure and circuitry of these two hilar cell types are probably different. This study adds further evidence to indicate that the hilus contains a large variety of cell types with different neuronal connections.

Animals↗
PubMed1994

Cortical columns as devices for maximizing neuronal diversity.

Columns are a fundamental feature of cerebral cortex organization, yet the function served by this architecture remains elusive. Here it is proposed that the columnar organization of the cortex serves to maximize diversity of neuronal connections in supragranular cortical layers.

Animals↗