Mapping quantitative trait loci associated with resistance to coccidiosis and growth

To map QTL associated with disease resistance to avian coccidiosis and growth, two commercial broiler lines with different degrees of resistance to the disease were crossed to generate an F1 generation that was intercrossed to produce 314 F2 generation offspring. The F2 offspring were inoculated with sporulated oocysts of Eimeria maxima. Five disease-associated phenotypes were measured after the infection. Intertrait comparisons revealed that oocyst shedding was a good parameter for evaluating disease resistance or susceptibility. One hundred nineteen microsatellite markers, covering 80% of the chicken genome with an average marker interval of 25 cM, were used for genotyping of F1 parents and F2 offspring. Statistical analysis based on the data of four families revealed a locus on chromosome 1 associated with oocyst shedding (logarithm of odds = 3.46). The genetic mechanism of this locus appeared additive. The genomic scan also identified three potential growth QTL on chromosomes 1, 6, and 8. These results provide the foundation for further investigation to validate the QTL.

Adjuvant effects of IL-1beta, IL-2, IL-8, IL-15, IFN-alpha, IFN-gamma TGF-beta4 and lymphotactin on DNA vaccination against Eimeria acervulina

Eight chicken cytokine genes (IL-1beta, IL-2, IL-8, IL-15, IFN-alpha, IFN-gamma, TGF-beta4, lymphotactin) were evaluated for their adjuvant effect on a suboptimal dose of an Eimeria DNA vaccine carrying the 3-1E parasite gene (pcDNA3-1E). Chickens were given two subcutaneous injections with 50 microg of the pcDNA3-1E vaccine plus a cytokine expression plasmid 2 weeks apart and challenged with Eimeria acervulina 1 week later. IFN-alpha (1 microg) or 10 microg of lymphotactin expressing plasmids, when given simultaneously with the pcDNA3-1E vaccine, significantly protected against body weight loss induced by E. acervulina. Parasite replication was significantly reduced in chickens given the pcDNA3-1E vaccine along with 10 microg of the IL-8, lymphotactin, IFN-gamma, IL-15, TGF-beta4, or IL-1beta plasmids compared with chickens given the pcDNA3-1E vaccine alone. Flow cytometric analysis of duodenum intraepithelial lymphocytes showed chickens that received the pcDNA3-1E vaccine simultaneously with the IL-8 or IL-15 genes had significantly increased CD3+ cells compared with vaccination using pcDNA3-1E alone or in combination with the other cytokine genes tested. These results indicate that the type and the dose of cytokine genes injected into chickens influence the quality of the local immune response to DNA vaccination against coccidiosis.

Dendritic mechanisms underlying the coupling of the dendritic with the axonal action potential initiation zone of adult rat layer 5 pyramidal neurons

1. Double, triple and quadruple whole-cell voltage recordings were made simultaneously from different parts of the apical dendritic arbor and the soma of adult layer 5 (L5) pyramidal neurons. We investigated the membrane mechanisms that support the conduction of dendritic action potentials (APs) between the dendritic and axonal AP initiation zones and their influence on the subsequent AP pattern. 2. The duration of the current injection to the distal dendritic initiation zone controlled the degree of coupling with the axonal initiation zone and the AP pattern. 3. Two components of the distally evoked regenerative potential were pharmacologically distinguished: a rapidly rising peak potential that was TTX sensitive and a slowly rising plateau-like potential that was Cd(2+) and Ni(2+) sensitive and present only with longer-duration current injection. 4. The amplitude of the faster forward-propagating Na(+)-dependent component and the amplitude of the back-propagating AP fell into two classes (more distinctly in the forward-propagating case). Current injection into the dendrite altered propagation in both directions. 5. Somatic current injections that elicited single Na(+) APs evoked bursts of Na(+) APs when current was injected simultaneously into the proximal apical dendrite. The mechanism did not depend on dendritic Na(+)-Ca(2+) APs. 6. A three-compartment model of a L5 pyramidal neuron is proposed. It comprises the distal dendritic and axonal AP initiation zones and the proximal apical dendrite. Each compartment contributes to the initiation and to the pattern of AP discharge in a distinct manner. Input to the three main dendritic arbors (tuft dendrites, apical oblique dendrites and basal dendrites) has a dominant influence on only one of these compartments. Thus, the AP pattern of L5 pyramids reflects the laminar distribution of synaptic activity in a cortical column.

Dendritic mechanisms underlying the coupling of the dendritic with the axonal action potential initiation zone of adult rat layer 5 pyramidal neurons

1. Double, triple and quadruple whole-cell voltage recordings were made simultaneously from different parts of the apical dendritic arbor and the soma of adult layer 5 (L5) pyramidal neurons. We investigated the membrane mechanisms that support the conduction of dendritic action potentials (APs) between the dendritic and axonal AP initiation zones and their influence on the subsequent AP pattern. 2. The duration of the current injection to the distal dendritic initiation zone controlled the degree of coupling with the axonal initiation zone and the AP pattern. 3. Two components of the distally evoked regenerative potential were pharmacologically distinguished: a rapidly rising peak potential that was TTX sensitive and a slowly rising plateau-like potential that was Cd(2+) and Ni(2+) sensitive and present only with longer-duration current injection. 4. The amplitude of the faster forward-propagating Na(+)-dependent component and the amplitude of the back-propagating AP fell into two classes (more distinctly in the forward-propagating case). Current injection into the dendrite altered propagation in both directions. 5. Somatic current injections that elicited single Na(+) APs evoked bursts of Na(+) APs when current was injected simultaneously into the proximal apical dendrite. The mechanism did not depend on dendritic Na(+)-Ca(2+) APs. 6. A three-compartment model of a L5 pyramidal neuron is proposed. It comprises the distal dendritic and axonal AP initiation zones and the proximal apical dendrite. Each compartment contributes to the initiation and to the pattern of AP discharge in a distinct manner. Input to the three main dendritic arbors (tuft dendrites, apical oblique dendrites and basal dendrites) has a dominant influence on only one of these compartments. Thus, the AP pattern of L5 pyramids reflects the laminar distribution of synaptic activity in a cortical column.

Screening for highly heterozygous chickens in outbred commercial broiler lines to increase detection power for mapping quantitative trait loci

Two commercial broiler lines were used in an experiment to map quantitative trait loci (QTL) affecting disease resistance. Chickens from these lines were genotyped with 27 microsatellite markers to estimate heterozygosity and polymorphism information content (PIC), the probability that one parent is heterozygous at a marker locus and the other has a different genotype. Heterozygosity estimated from allelic frequencies was 0.52 for the two lines; however, heterozygosity calculated from actual counts of heterozygous loci was much lower (0.36) than the estimated heterozygosity. The PIC was 0.45 in these lines, and average allele number per marker locus was about 3.5. Twenty-five males produced from a cross between these two lines were screened with the DNA markers to select birds with high heterozygosity at marker loci. Mating simulation showed that uninformative matings could be reduced by about 5% if 12 pairs of males and females with the highest heterozygosity at marker loci were selected, which was about a 25% reduction in total uninformative matings. This experiment demonstrated that the heterozygosity and PIC in commercial broiler lines were low and selection for the birds with high heterozygosity at marker loci could increase informative content in chickens used in the experiments of QTL mapping, thus increasing detection power for QTL mapping.

Postnatal synaptic potentiation: delivery of GluR4-containing AMPA receptors by spontaneous activity

To examine how functional circuits are established in the brain, we studied excitatory transmission in early postnatal hippocampus. Spontaneous neural activity was sufficient to selectively deliver GluR4-containing AMPA receptors (AMPA-Rs) into synapses. This delivery allowed non-functional connections to transmit at resting potentials and required NMDA receptors (NMDA-Rs) but not CaMKII activation. Subsequently, these delivered receptors were exchanged with non-synaptic GluR2-containing AMPA-Rs in a manner requiring little neuronal activity. The enhanced transmission resulting from this delivery and subsequent exchange was maintained for at least several days and required an interaction between GluR2 and NSF. Thus, this sequence of subunit-specific trafficking events triggered by spontaneous activity in early postnatal development may be crucial for initial establishment of long-lasting functional circuitry.

Solute balance of a maize (Zea mays L.) source leaf as affected by salt treatment with special emphasis on phloem retranslocation and ion leaching

Strategies for avoiding ion accumulation in leaves of plants grown at high concentration of NaCl (100 mol m(-3)) in the rooting media, i.e. retranslocation via the phloem and leaching from the leaf surface, were quantified for fully developed leaves of maize plants cultivated hydroponically with or without salt, and with or without sprinkling (to induce leaching). Phloem sap, apoplastic fluid, xylem sap, solutes from leaf and root tissues, and the leachate were analysed for carbohydrates, amino acids, malate, and inorganic ions. In spite of a reduced growth rate Na(+) and Cl(-) concentrations in the leaf apoplast remained relatively low (about 4-5 mol m(-3)) under salt treatment. Concentrations of Na(+) and Cl(-) in the phloem sap of salt-treated maize did not exceed 12 and 32 mol m(-3), respectively, and thus remained lower than described for other species. However, phloem transport rates of these ions were higher than reported for other species. The relatively high translocation rate of ions found in maize may be due to the higher carbon translocation rate observed for C(4) plants as opposed to C(3) plants. Approximately 13-36% of the Na(+) and Cl(-) imported into the leaves through the xylem were exported by the phloem. It is concluded that phloem transport plays an important role in controlling the NaCl content of the leaf in maize. Surprisingly, leaching by artificial rain did not affect plant growth. Ion concentrations in the leachate were lower than reported for other plants but increased with NaCl treatment.

Maturation of layer 5 neocortical pyramidal neurons: amplifying salient layer 1 and layer 4 inputs by Ca2+ action potentials in adult rat tuft dendrites

Changes in the arborization and electrical excitability of the apical dendritic tufts of pyramidal cells of cortical layer 5 were examined during the first 2 months (postnatal days (P)2-56) of postnatal development in rats. Reconstructions of biocytin-filled neurons showed that the apical dendritic trunk was continually growing, becoming longer and thicker and that the distance between the tuft and soma increased more than 5-fold. In P2 animals, both the tuft and soma had a high input resistance (> 500 MOmega) and the tuft was electrotonically close to the soma. In contrast, the apical tuft and soma of P56 neurons had a low input resistance (< 50 MOmega) and they were electrotonically isolated from each other. Depolarizing current pulses injected into the tuft of P2 cells generated mostly Na+-dependent regenerative dendritic potentials of short duration ( approximately 15 ms) while in the tuft of P56 animals, complex regenerative potentials were generated which had a longer duration ( approximately 55 ms) and were Na+ and Ca2+ dependent. In young and juvenile animals (P14-28) dendritic regenerative potentials could be restricted to the apical dendritic tuft whereas in adult animals (> P42), the complex regenerative potentials frequently occurred simultaneously with somatic action potentials. The main developmental change in layer 5 pyramidal neurons, as assayed with square pulse current injections and synaptic stimulations, is the progressive electrotonic isolation of the dendritic tuft from the soma. This change is concomitant with the appearance of complex, mostly Na+- and Ca2+-dependent, regenerative dendritic potentials initiated partly in the tuft and partly in the axon. The coupling of the dendritic tuft and axonal initiation zones for regenerative potentials by active dendritic Na+ and Ca2+ conductances enables mature layer 5 pyramidal neurons to detect selectively the salient distal synaptic inputs and coincident synaptic inputs arriving at different cortical layers.

[Clinical analysis of 35 patients with orbital maxillary zygoma complex fractures]

OBJECTIVE

To assess the treatment result of reposition and fixation to orbital maxillary zygoma complex fractures.

METHOD

The records of 35 patients, who were treated in 1996-1998 at our hospital, were analyzed retrospectively. The patients of open fracture operated debridement, at the same time, operated reposition and fixation. The patients of closed fracture operated hairline approach and sublabial approach or use two approach for reposition and fixation.

RESULT

We had 35 patients of operation. After operation, two of them had malocclusion, one of them had facial deformity, two of them had restriction of mouth opening. The treatment results of the rest appeared to be reasonably satisfactory.

CONCLUSION

To know topographic anatomy, earlier reposition and sturdy fixation is the key for curing orbital maxillary zygoma complex fractures.

Analysis of disease resistance-associated parameters in broiler chickens challenged with Eimeria maxima

To determine an optimal dose for coccidial inoculation and to evaluate genetic resistance or susceptibility in individual chickens, broilers were inoculated with four different doses of Eimeria maxima oocysts. Body weight gain, fecal oocyst shedding, concentrations of plasma NO2- + NO3-, carotenoid, and interferon-gamma were measured at two different time periods postinfection. The results showed significant dose and sex effects on most parameters and interaction between dose and sex in some parameters. The dose effects were generally linear; however, some significant quadratic effects were also observed. The measurements from chickens inoculated with 10(4) oocysts displayed the highest correlation coefficients among oocyst shedding, body weight gain, and concentrations of carotenoid and NO2- + NO3-. An infection index, calculated from the correlated parameters, displayed high correlation coefficients with the parameters. The infection index may be a better parameter for evaluating individual genetic resistance against coccidial infection.

Recurrent inhibitory circuitry in the deep layers of the rabbit superior colliculus

1. Local inhibition in the deep layers of the superior colliculus plays a crucial role in sensorimotor integration. Using intracellular and extracellular recording techniques, we studied the organization of inhibitory circuits in the deep layers of the superior colliculus in anaesthetized rabbits. 2. We identified a new cell type in the deep superior colliculus that showed a characteristic burst response to stimulation of both the predorsal bundle and optic chiasm. The response had a jittering latency and failed to follow high frequency stimuli, indicating trans-synaptic (orthodromic) events. Moreover, the predorsal bundle stimulation-evoked orthodromic response could be made to collide with the response to a preceding stimulation of the optic chiasm, suggesting that burst-firing cells received excitatory inputs from the axonal collaterals of predorsal bundle-projecting cells. 3. Stimulation of the predorsal bundle could evoke an IPSP in predorsal bundle-projecting cells. The latency of the IPSP was 0.5-1.0 ms longer than the orthodromic response in burst-firing cells. Simultaneous recordings showed that the IPSP in predorsal bundle-projecting cells was preceded by a burst of extracellular spikes from burst-firing cells with short latency ( approximately 0.9 ms), indicating an inhibitory monosynaptic connection from burst-firing cells to predorsal bundle-projecting cells. 4. Burst-firing cells exhibited a prolonged depression after the predorsal bundle or optic chiasm stimulation due to an inhibitory postsynaptic potential. Latency analysis implies that burst-firing cells may form mutual inhibitory connections. 5. Together our results suggest that burst-firing cells and predorsal bundle-projecting cells form reciprocal excitatory and inhibitory connections and burst-firing cells may function as the recurrent inhibitory interneurons in the deep layers of the rabbit superior colliculus.

Diurnal changes in xylem pressure of the hydrated resurrection plant Myrothamnus flabellifolia: evidence for lipid bodies in conducting xylem vessels

The resurrection plant Myrothamnus flabellifolia has the ability to recover from repeated prolonged and extreme desiccation cycles. During the dry state the inner walls of the xylem vessels seemed to be covered, at least partly, by a lipid film as shown by Sudan III and Nile Red staining. The lipid film apparently functioned as an 'internal cuticle' which prevented the adjacent parenchyma ray cells from complete water loss. The hydrophobic nature of the inner xylem walls was supported by the finding that benzene ascended as rapidly as water in the xylem of dry Myrothamnus branches. On watering, numerous lipid bodies were found in the water-conducting vessels, presumably formed from the lipid film and/or from lipids excreted from the adjacent living cells into the vessels. The presence of lipid bodies within the vessels, as well as the hydrophobic properties of the inner xylem walls, could explain the finding that the xylem pressure of hydrated, well watered plants (measured both under laboratory and field conditions with the xylem pressure probe) never dropped below c. -0.3 MPa and that cavitation occurred frequently at low negative xylem pressure values (-0.05 to -0.15 MPa). The xylem pressure of M. flabellifolia responded rapidly and strongly to changes in relative humidity and temperature, but less obviously to changes in irradiance (which varied between 10 and c. 4000 μmol m m^-2 s^-1 ). The morphological position of the stomata in the leaves could explain the extremely weak and slow response of the xylem pressure of this resurrection plant to illumination changes. Stomata were most abundant in the furrows, and were thus protected from direct sunlight. Simultaneous measurements of the cell turgor pressure in the leaf epidermal cells (made by using the cell turgor pressure probe) revealed that the xylem and the cell turgor pressure dropped in a ratio of 1:0.7 on changes in the environmental parameters, indicating a quite close hydraulic connection and, thus, water equilibrium between the xylem and cellular compartments. An increase in irradiance of c. 700 μmol m^-2 s^-1 resulted in a turgor pressure decrease from 0.63 to 0.48 MPa. Correspondingly, the cell osmotic pressure increased from 1.03 to 1.22 MPa. From these values and by assuming water equilibrium, the osmotic pressure of the xylem sap was estimated to be 0.25-0.4 MPa. This value seems to be fairly high but may, however, be explained by the reduction of the water volume within the vessels due to the floating lipid bodies.

Clonal analysis of a case of multiple meningiomas using multiple molecular genetic approaches: pathology case report

OBJECTIVE

Multiple meningiomas are uncommon brain tumors occurring concurrently in several intracranial locations in the same patient. In the present study, we determined the clonality, methylation status of deoxyribonucleic acid, and relationship of genetic alterations in eight meningiomas from one female patient.

METHODS

Six molecular genetic techniques, including two methylation-based clonality assays and one transcription-based clonality assay, methylation analysis of CpG islands by methylation-specific polymerase chain reaction, loss of heterozygosity, microsatellite instability, and mutational analysis of the NF2 gene on chromosome 22, were used in comparative investigations on clonality and genetic alterations.

RESULTS

The presence of clonal tumor cells was demonstrated by 1) loss of the same copy of chromosome 22 in all eight tumors; 2) transcription of the human AR gene from the same allele in six of eight tumors; 3) a common unmethylated allele at the AR locus in all eight tumors; and 4) the identical single-basepair insertion mutation in exon 9 of the NF2 gene in six of eight tumors. In addition, loss of a copy of the X chromosome in one tumor nodule and microsatellite instability in another nodule were observed.

CONCLUSION

Taken together, this case of multiple meningiomas was most likely monoclonal in origin. Loss of chromosome 22 was an early event during the development of multiple meningiomas and was followed by mutations at the NF2 locus. Later events, including loss of the X chromosome, variation of AR gene expression, or microsatellite instability, may also have played a role in the development of multiple meningiomas in this patient.

Different frequencies of p53 codon-249 hot-spot mutations in hepatocellular carcinomas in Jiang-su province of China

Environmental carcinogens often induce specific mutations in the p53 gene, apparent in tumors. The relation between aflatoxin B1 (AFB1 )-related hepatocellular carcinomas (HCCs) and hot spot at codon 249 of the p53 gene has received a great deal of attention, but its significance is still controversial. To clarify this problem, we analyzed the p53-mutational status of HCCs in Jiang-su province in China, where AFB1 contamination of the staple food significantly differs between the northern and southern parts (prominent only in the latter), while other conditions are quite similar. Background liver status and mutations in exons 5 to 8 of p53 in a total of 31 cases were divided approximately equally between the 2 areas. In all, 15 tumors exhibited a total of 17 mutations in the p53 gene; 9 cases from the southern part of the province had the hot-spot mutation at codon 249 (9/16, 56%), but only one case from the northern part (1/15, 8%). These results suggest that AFB1 contamination may correlate with codon-249 mutations in HCC.

Different frequencies of p53 codon-249 hot-spot mutations in hepatocellular carcinomas in Jiang-su province of China

Environmental carcinogens often induce specific mutations in the p53 gene, apparent in tumors. The relation between aflatoxin B1 (AFB1 )-related hepatocellular carcinomas (HCCs) and hot spot at codon 249 of the p53 gene has received a great deal of attention, but its significance is still controversial. To clarify this problem, we analyzed the p53-mutational status of HCCs in Jiang-su province in China, where AFB1 contamination of the staple food significantly differs between the northern and southern parts (prominent only in the latter), while other conditions are quite similar. Background liver status and mutations in exons 5 to 8 of p53 in a total of 31 cases were divided approximately equally between the 2 areas. In all, 15 tumors exhibited a total of 17 mutations in the p53 gene; 9 cases from the southern part of the province had the hot-spot mutation at codon 249 (9/16, 56%), but only one case from the northern part (1/15, 8%). These results suggest that AFB1 contamination may correlate with codon-249 mutations in HCC.

Properties of a hyperpolarization-activated cation current in interneurons in the rat lateral geniculate nucleus

A hyperpolarization-activated cation conductance contributes to the membrane properties of a variety of cell types. In the thalamus, a prominent hyperpolarization-activated cation conductance exists in thalamocortical cells, and this current is implicated in the neuromodulation of complex firing behaviors. In contrast, the GABAergic cells in the reticular nucleus in the thalamus appear to lack this conductance. The presence and role of this cation conductance in the other type of thalamic GABAergic cells, local interneurons, is still unclear. To resolve this issue, we studied 54 physiologically and morphologically identified local interneurons in the rat dorsal lateral geniculate nucleus using an in vitro whole-cell patch recording technique. We found that hyperpolarizing current injections induced depolarizing voltage sags in these geniculate interneurons. The I-V relationship revealed an inward rectification. Voltage-clamp study indicated that a slow, hyperpolarization-activated cation conductance was responsible for the inward rectification. We then confirmed that this slow conductance had properties of the hyperpolarization-activated cation conductance described in other cell types. The slow conductance was insensitive to 10 mM tetraethylammonium and 0.5 mM 4-aminopyridine, but was largely blocked by 1-1.5 mM Cs+. It was permeable to both K+ and Na+ ions and had a reversal potential of -44 mV. The voltage dependence of the hyperpolarization-activated cation conductance in interneurons was also studied: the activation threshold was about -55 mV, half-activation potential was about -80 mV and maximal conductance was about 1 nS. The activation and deactivation time constants of the conductance ranged from 100 to 1000 ms, depending on membrane potential. The depolarizing voltage sags and I-V relationship were further simulated in a model interneuron, using the parameters of the hyperpolarization-activated cation conductance obtained from the voltage-clamp study. The time-course and voltage dependence of the depolarizing voltage sags and I-V relationship in the model cell were very similar to those found in geniculate interneurons in current clamp. Taken together, the results of the present study suggest that thalamic local interneurons possess a prominent hyperpolarization-activated cation conductance, which may play important roles in determining basic membrane properties and in modulating firing patterns.

Three GABA receptor-mediated postsynaptic potentials in interneurons in the rat lateral geniculate nucleus

Inhibition is crucial for the thalamus to relay sensory information from the periphery to the cortex and to participate in thalamocortical oscillations. However, the properties of inhibitory synaptic events in interneurons are poorly defined because in part of the technical difficulty of obtaining stable recording from these small cells. With the whole-cell recording technique, we obtained stable recordings from local interneurons in the lateral geniculate nucleus and studied their inhibitory synaptic properties. We found that interneurons expressed three different types of GABA receptors: bicuculline-sensitive GABA(A) receptors, bicuculline-insensitive GABA(A) receptors, and GABA(B) receptors. The reversal potentials of GABA responses were estimated by polarizing the membrane potential. The GABA(A) receptor-mediated responses had a reversal potential of approximately -82 mV, consistent with mediation via Cl(-) channels. The reversal potential for the GABA(B) response was -97 mV, consistent with it being a K(+) conductance. The roles of these GABA receptors in postsynaptic responses were also examined in interneurons. Optic tract stimulation evoked a disynaptic IPSP that was mediated by all three types of GABA receptors and depended on activation of geniculate interneurons. Stimulation of the thalamic reticular nucleus evoked an IPSP, which appeared to be mediated exclusively by bicuculline-sensitive GABA(A) receptors and depended on the activation of reticular cells. The results indicate that geniculate interneurons form a complex neuronal circuitry with thalamocortical and reticular cells via feed-forward and feedback circuits, suggesting that they play a more important role in thalamic function than thought previously.

Burst firing in identified rat geniculate interneurons

We used whole-cell patch recording to study 102 local interneurons in the rat dorsal lateral geniculate nucleus in vitro. Input impedance with this technique (607.0+/-222.4 MOhm) was far larger than that measured with sharp electrode techniques, suggesting that interneurons may be more electrotonically compact than previously believed. Consistent and robust burst firing was observed in all interneurons when a slight depolarizing boost was given from a potential at, or slightly hyperpolarized from, resting membrane potential. These bursts had some similarities to the low-threshold spike described previously in other thalamic neuron types. The bursting responses were blocked by Ni+, suggesting that the low-threshold calcium current I(T), responsible for the low-threshold spike, was also involved in interneuron burst firing. Compared to the low-threshold spike of thalamocortical cells, however, the interneuron bursts were of relatively long duration and low intraburst frequency. The requirement for a depolarizing boost to elicit the burst is consistent with previous reports of a depolarizing shift of the I(T) activation curve of interneurons relative to thalamocortical cells, a finding we confirmed using voltage-clamp. Voltage-clamp study also revealed an additional long-lasting current that could be tentatively identified as the calcium activated non-selective cation current, I(CAN), based on reversal potential and on pharmacological characteristics. Computer simulation of the interneuron burst demonstrated that its particular morphology is likely due to the interaction of I(T) and I(CAN). In the slice, bursts could also be elicited by stimulation of the optic tract, suggesting that they may occur in response to natural stimulation. Synaptically triggered bursts were only partially blocked by Ni+, but could then be completely blocked by further addition of (+/-)-2-amino-5-phosphonopentanoic acid. The existence of robust bursts in this cell type suggests an additional role for interneurons in sculpting sensory responses by feedforward inhibition of thalamocortical cells. The low-threshold spike is a mechanism whereby activity in a neuron is dependent on a prior lack of activity in that same neuron. Understanding of the low-threshold spike in the other major neuron types of the thalamus has brought many new insights into how thalamic oscillations might be involved in sleep and epilepsy. Our description of this phenomenon in the interneurons of the thalamus suggests that these network oscillations might be even more complicated than previously believed.

A new cellular mechanism for coupling inputs arriving at different cortical layers

Pyramidal neurons in layer 5 of the neocortex of the brain extend their axons and dendrites into all layers. They are also unusual in having both an axonal and a dendritic zone for the initiation of action potentials. Distal dendritic inputs, which normally appear greatly attenuated at the axon, must cross a high threshold at the dendritic initiation zone to evoke calcium action potentials but can then generate bursts of axonal action potentials. Here we show that a single back-propagating sodium action potential generated in the axon facilitates the initiation of these calcium action potentials when it coincides with distal dendritic input within a time window of several milliseconds. Inhibitory dendritic input can selectively block the initiation of dendritic calcium action potentials, preventing bursts of axonal action potentials. Thus, excitatory and inhibitory postsynaptic potentials arising in the distal dendrites can exert significantly greater control over action potential initiation in the axon than would be expected from their electrotonically isolated locations. The coincidence of a single back-propagating action potential with a subthreshold distal excitatory postsynaptic potential to evoke a burst of axonal action potentials represents a new mechanism by which the main cortical output neurons can associate inputs arriving at different cortical layers.

Intrinsic firing patterns and whisker-evoked synaptic responses of neurons in the rat barrel cortex

We have used whole cell recording in the anesthetized rat to study whisker-evoked synaptic and spiking responses of single neurons in the barrel cortex. On the basis of their intrinsic firing patterns, neurons could be classified as either regular-spiking (RS) cells, intrinsically burst-spiking (IB) cells, or fast-spiking (FS) cells. Some recordings responded to current injection with a complex spike pattern characteristic of apical dendrites. All cell types had high rates of spontaneous postsynaptic potentials, both excitatory (EPSPs) and inhibitory (IPSPs). Some spontaneous EPSPs reached threshold, and these typically elicited only single action potentials in RS cells, bursts of action potentials in FS cells and IB cells, and a small, fast spike or a complex spike in dendrites. Deflection of single whiskers evoked a fast initial EPSP, a prolonged IPSP, and delayed EPSPs in all cell types. The intrinsic firing pattern of cells predicted their short-latency whisker-evoked spiking patterns. All cell types responded best to one or, occasionally, two primary whiskers, but typically 6-15 surrounding whiskers also generated significant synaptic responses. The initial EPSP had a relatively fixed amplitude and latency, and its amplitude in response to first-order surrounding whiskers was approximately 55% of that induced by the primary whisker. Second- and third-order surrounding whiskers evoked responses of approximately 27 and 12%, respectively. The latency of the initial EPSP was shortest for the primary whiskers, longer for surrounding whiskers, and varied with the neurons' depth below the pia. EPSP latency was shortest in the granular layer, longer in supragranular layers, and longest in infragranular layers. The receptive field size, defined as the total number of fast EPSP-inducing whiskers, was independent of each cell's intrinsic firing type, its subpial depth, or the whisker stimulus parameters. On average, receptive fields included >10 whiskers. Our results show that single neurons integrate rapid synaptic responses from a large proportion of the mystacial vibrissae, and suggest that the whisker-evoked responses of barrel neurons are a function of both synaptic inputs and intrinsic membrane properties.

An intrinsic oscillation in interneurons of the rat lateral geniculate nucleus

By using the whole cell patch recording technique in vitro, we examined the voltage-dependent firing patterns of 69 interneurons in the rat dorsal lateral geniculate nucleus (LGN). When held at a hyperpolarized membrane potential, all interneurons responded with a burst of action potentials. In 48 interneurons, larger current pulses produced a bursting oscillation. When relatively depolarized, some interneurons produced a tonic train of action potentials in response to a depolarizing current pulse. However, most interneurons produced only oscillations, regardless of polarization level. The oscillation was insensitive to the bath application of a combination of blockers to excitatory and inhibitory synaptic transmission, including 30 microM 6,7-dinitroquinoxaline-2,3-dione, 100 microM (+/-)-2-amino-5-phosphonopentanoic acid, 20 microM bicuculline, and 2 mM saclofen, suggesting an intrinsic event. The frequency of the oscillation in interneurons was dependent on the intensity of the injection current. Increasing current intensity increased the oscillation frequency. The maximal frequency of the oscillation was 5-15 Hz for most cells, with some ambiguity caused by the difficulty of precisely defining a transition from oscillatory to regular firing behavior. In contrast, the interneuron oscillation was little affected by preceding depolarizing and hyperpolarizing pulses. In addition to being elicited by depolarizing current injections, the oscillation could also be initiated by electrical stimulation of the optic tract when the interneurons were held at a depolarized membrane potential. This suggests that interneurons may be recruited into thalamic oscillations by synaptic inputs. These results indicate that interneurons may play a larger role in thalamic oscillations than was previously thought.

Cellular mechanisms underlying two muscarinic receptor-mediated depolarizing responses in relay cells of the rat lateral geniculate nucleus

We used the whole-cell recording technique in an in vitro preparation to examine the electrophysiological actions of the muscarinic receptors on relay cells in the rat lateral geniculate nucleus. Drop application of the muscarinic agonist acetyl-beta-methylcholine resulted in a slow depolarization that persisted for several minutes. The response was insensitive to the nicotinic antagonist hexamethonium, but was blocked by atropine, a muscarinic antagonist. The response was also insensitive to blockade of synaptic transmission by tetrodotoxin, indicating a direct muscarinic effect. The muscarinic depolarization consisted of two components that were somewhat separated in time. The early portion of the muscarinic response was mediated by a large inward current with little change in input resistance, while the later portion was mediated by a small inward current associated with a large increase in input resistance. Pharmacological agents were used to distinguish the two components. Drop application of McN-A-343, an ml receptor agonist, could only mimic the later component of the muscarinic response. This was supported by the result that the later component was blocked by low concentrations of pirenzepine. These data suggest that the ml receptor only mediates the late component of the muscarinic response, while the early component is mainly mediated by the m3 receptor. The idea that both ml and m3 receptors were involved in the muscarinic depolarization was further supported by voltage-clamp analysis. This revealed that activation of the ml receptor was associated with a decrease in an inward potassium current, IKleak, while activation of the m3 receptor was likely associated with both a decrease in IKleak and an increase in the hyperpolarization-activated cation current Ih. In summary, our data suggest that muscarinic responses in geniculate relay cells result from the activation of two receptors, which modulate IKleak and Ih. Given the fact that the ascending aminergic systems also depolarize geniculate relay cells via two receptors acting on IKleak and Ih, we concluded that ascending activating systems use common mechanisms to enact the depolarizing form of arousal in relay neurons.

Control of recurrent inhibition of the lateral posterior-pulvinar complex by afferents from the deep layers of the superior colliculus of the rabbit

We investigated the effect of stimulation of the deep layers of the superior colliculus (SC) on the recurrent inhibition of the lateral posterior-pulvinar complex (LP) in anesthetized rabbits. Intracellular recordings from 23 relay cells in LP showed that they responded to SC stimulation with a long-lasting (140.2 +/- 19.6 ms; mean +/- SD) inhibitory postsynaptic potential (IPSP), which sometimes was followed by a rebound burst of spikes. The same SC stimulation evoked a burst of spikes in extracellular recordings from 31 recurrent inhibitory interneurons in the LP-cortical pathway, which were located in the ventral part of the visual sector of the thalamic reticular nucleus. The mean latency of the burst in reticular cells was 1.6 ms shorter than that of the IPSP in LP relay cells, suggesting that the IPSP in LP cells was mediated by these reticular cells. Intracellular recordings from nine reticular cells showed that the burst of spikes evoked by SC stimulation resulted from an excitatory postsynaptic potential that was always followed by a long-lasting (143.3 +/- 24.0 ms) IPSP. Stimulation of the contralateral predorsal bundle, the main output pathway of deep SC neurons, elicited similar responses in LP cells or reticular neurons with latencies longer than those from SC stimulation. The latency difference between the responses to predorsal bundle and SC stimulation is equal to the antidromic conduction time of predorsal bundle fibers, suggesting that the inhibition in LP originates from the activation of predorsal bundle-projecting neurons. The response characteristics of the inhibitory circuit of LP and of the lateral geniculate nucleus to SC stimulation are strikingly similar, implying that a similar circuit is used by predorsal bundle-projecting neurons to control the recurrent inhibition in both lateral geniculate nucleus and LP. Because the predorsal bundle-projecting neurons are believed to be involved in the initiation of saccadic eye movements, we suggest that the inhibitory circuits may play an important role in modulating ascending visual information during saccadic eye movements.