Hippocampal circuit dysfunction in the Tc1 mouse model of Down syndrome.

Hippocampal pathology is likely to contribute to cognitive disability in Down syndrome, yet the neural network basis of this pathology and its contributions to different facets of cognitive impairment remain unclear. Here we report dysfunctional connectivity between dentate gyrus and CA3 networks in the transchromosomic Tc1 mouse model of Down syndrome, demonstrating that ultrastructural abnormalities and impaired short-term plasticity at dentate gyrus-CA3 excitatory synapses culminate in impaired coding of new spatial information in CA3 and CA1 and disrupted behavior in vivo. These results highlight the vulnerability of dentate gyrus-CA3 networks to aberrant human chromosome 21 gene expression and delineate hippocampal circuit abnormalities likely to contribute to distinct cognitive phenotypes in Down syndrome.

Does logging and forest conversion to oil palm agriculture alter functional diversity in a biodiversity hotspot?

Forests in Southeast Asia are rapidly being logged and converted to oil palm. These changes in land-use are known to affect species diversity but consequences for the functional diversity of species assemblages are poorly understood. Environmental filtering of species with similar traits could lead to disproportionate reductions in trait diversity in degraded habitats. Here, we focus on dung beetles, which play a key role in ecosystem processes such as nutrient recycling and seed dispersal. We use morphological and behavioural traits to calculate a variety of functional diversity measures across a gradient of disturbance from primary forest through intensively logged forest to oil palm. Logging caused significant shifts in community composition but had very little effect on functional diversity, even after a repeated timber harvest. These data provide evidence for functional redundancy of dung beetles within primary forest and emphasize the high value of logged forests as refugia for biodiversity. In contrast, conversion of forest to oil palm greatly reduced taxonomic and functional diversity, with a marked decrease in the abundance of nocturnal foragers, a higher proportion of species with small body sizes and the complete loss of telecoprid species (dung-rollers), all indicating a decrease in the functional capacity of dung beetles within plantations. These changes also highlight the vulnerability of community functioning within logged forests in the event of further environmental degradation.

Synaptic P2X receptors.

Over the past two years, ATP has clearly been shown to act as a co-transmitter with GABA, glycine and probably glutamate in the central nervous system. Our understanding of the ATP-gated P2X receptors is progressing rapidly, and the pharmacology, stoichiometry and subunit combinations of heteropolymeric P2X channels has been substantially elucidated.

Effects of a naturally occurring neurosteroid on GABAA IPSCs during development in rat hippocampal or cerebellar slices.

1. The effects of the naturally occurring neurosteroid tetrahydrodeoxycorticosterone (THDOC) on GABAA receptor-mediated miniature, spontaneous and evoked IPSCs was tested using patch-clamp techniques in slices of hippocampus and cerebellum from rats at two developmental stages ( approximately 10 and approximately 20 days postnatal). The cells studied were hippocampal granule cells and cerebellar Purkinje and granule cells. 2. Most miniature GABAergic currents (mIPSCs) decayed with two exponentials and neurosteroids caused a approximately 4-fold increase in the decay time constant of the second exponential at the highest concentration used (2 microM). Similar effects were seen at high concentrations of THDOC (1-2 microM) in all cell groups tested. No effects were seen on amplitude or rise time of mIPSCs. 3. The effects of THDOC (1 microM) were shown to be stereoselective and rapidly reversible, indicating that the neurosteroid binds to the GABAA receptor, rather than acting genomically. 4. At concentrations of THDOC likely to occur physiologically (50-100 nM), the decay time of IPSCs was also enhanced (25-50 %) in all cerebellar cell groups tested. In contrast, at 100 nM THDOC, seven of 11 hippocampal granule cells were sensitive from the 10 day group but the 20 day hippocampal granule cells showed no significant enhancement in the presence of these lower concentrations of THDOC. 5. The differences in sensitivity of hippocampal and cerebellar cells to THDOC are compared to data reported in the literature on regional development of expression of different receptor subunits in the brain and it is suggested that the progressive relative insensitivity of the 20 day hippocampal cells may depend on increasing expression of the delta subunit of the GABAA receptor and possibly an increase in the alpha4 subunit.

Ca2+ permeability and kinetics of glutamate receptors in rat medial habenula neurones: implications for purinergic transmission in this nucleus.

1. We have previously investigated P2X receptor-mediated synaptic currents in medial habenula neurones and shown that they can be calcium permeable. We now investigate the receptor properties of glutamate, the other, more abundant excitatory transmitter, to determine its receptor subtypes and their relative calcium permeability. This may have implications for the physiological role of the P2X receptors which mediate synaptic currents. 2. Using fast application of ATP, L-glutamate or kainate to nucleated patches, glutamate receptors were determined to be of the AMPA subtype but no functional P2X receptors were detected. 3. The deactivation and desensitization rates of the AMPA channel were determined to have time constants of 1.77 +/- 0.21 ms (n = 10) and 4.01 +/- 0.85 ms (n = 9) at -60 mV, respectively. AMPA receptors recovered from desensitization with two exponential components with time constants of 21.08 +/- 2.95 and 233.60 +/- 51.1 ms (n = 3). None of the deactivation or desensitization properties of the GluR channels depended on membrane potential. 4. The current-voltage relationship under different ionic conditions revealed that the GluR channel was equally permeable to Cs+ and Na+ but relatively impermeable to Ca2+ (PCa/PCs = 0.13, n = 6). 5. For both synaptic currents and somatic currents activated by fast application of L-glutamate to nucleated patches, decay time constants were similar at +/-60 mV in the presence of Mg2+ ions. Thus GluR channels appear to be of the AMPA subtype and not the NMDA subtype. 6. Thus, under the conditions of this study, neurones of the medial habenula lack functional NMDA receptors and possess AMPA receptors that have low permeability to Ca2+. We conclude that the P2X receptor-mediated synaptic currents are the only calcium-permeable fast-transmitter gated currents in these neurones which may be important for their physiological function.

ATP and glutamate are released from separate neurones in the rat medial habenula nucleus: frequency dependence and adenosine-mediated inhibition of release.

1. ATP and glutamatergic synaptic currents were compared in slices of rat medial habenula nucleus using whole-cell patch-clamp techniques. 2. In most cells low voltage stimulation resulted in glutamatergic responses and not purinergic responses. In five cells where ATP currents could be stimulated with low voltages, wash out of glutamate antagonists did not reveal evoked glutamate currents. Spontaneous glutamate currents confirmed washout of antagonist. 3. Modulation of release probability of glutamate and ATP, assessed by changes in failure rate of synaptic currents, was compared under conditions of different stimulation frequencies and in the presence of adenosine agonists and antagonists. 4. ATP release, but not glutamate release, was shown to be modulated by increased stimulation frequency which resulted in inhibition of ATP release via A2-like adenosine receptors. A1 receptors caused inhibition of both ATP and glutamate release. 5. Endogenous adenosine inhibited glutamate release via A1 receptors but only inhibited ATP release via A2-like receptors. 6. Attempts to inhibit the degradation of ATP to adenosine did not alter the frequency dependence of the failure rate. 7. We conclude, from the direct demonstration and from the differences in pharmacology and frequency dependence of the modulation of release, that ATP and glutamate responses are due to release from separate neurones.

Properties of ATP receptor-mediated synaptic transmission in the rat medial habenula.

The properties of central ATP-mediated synaptic currents were studied using whole-cell patch-clamp recording in rat medial habenula slices. Release was shown to be calcium dependent with a Hill coefficient of approximately 2. The voltage dependence of synaptic current amplitudes was approximately linear. Some reduction of the synaptic current amplitudes was observed at 10 mM extracellular calcium, suggesting calcium block/permeability of the channels. This was confirmed by observation of current-voltage reversal potentials in different calcium concentrations. We estimate that the channels underlying half the synapses showed a negligible calcium permeability. In the other four out of eight synapses the results suggest a very high calcium permeability with an estimated PCa/PCs of > 10. Thus, at -70 mV, in 1 mM calcium, more than 15% of the ATP-mediated synaptic current is estimated to be carried by calcium, but only at synapses with calcium-permeable channels. Net current through these synaptic channels is also controlled by the voltage dependence of synaptic current decay time constants (increasing e-fold for 158 mV depolarization) and by a strong dependence of transmitter release on the frequency of stimulation of the presynaptic neurone, with failure rates increasing 3-fold as stimulation rates were increased from 1 to 10 Hz.

Features of P2X receptor-mediated synapses in the rat brain: why doesn't ATP kill the postsynaptic cell?

In addition to the widespread excitatory transmission mediated by glutamate receptors, P2X receptors also mediate fast excitatory synaptic transmission in the brain. The receptors in the brain have some features which are different from the more extensively characterized peripheral P2X receptors, possibly suggesting a difference in the subunits making up the protein. Perhaps the most notable feature of the central receptors is a higher Ca2+ permeability than seen in other areas, with a linear current voltage relation. The potential danger to the postsynaptic cell of the high Ca2+ permeability of neuronal P2X receptors is discussed and various forms of inbuilt features of the synapse and receptors are outlined which would combine to protect the neurons from excessive Ca2+ influx and consequent danger of cell death. These features include a rapid breakdown of transmitter in the cleft, which not only removes the ATP from the cleft quickly, but also results in the production of adenosine. Evidence for the interaction of ATP-mediated transmission and stimulation of purinoceptors by this adenosine is presented from experiments using patch-clamp recording in brain slices. This demonstrates an elegant negative feedback mechanism by which a fast excitatory transmitter can be inactivated by breakdown to a product which acts as a slow inhibitory modulator, controlling release of the original transmitter.

Anatomy and electrophysiology of fast central synapses lead to a structural model for long-term potentiation.

Detailed knowledge of the anatomy of central synapses is essential to the interpretation of the vast quantity of electrophysiological findings that have been published in recent years. When their function is considered, it is not surprising that, in both anatomy and electrophysiology, fast central synapses show important differences to the neuromuscular junction. This review concentrates on the detailed anatomy of the common excitatory synapses that impinge on dendritic spines, but also refers to other glutamatergic and GABAergic synapses. This information is brought together with present knowledge of the electrophysiology of fast neurotransmission in the brain. Various types of evidence are outlined, explaining why it is now widely accepted that release of transmitter from a single vesicle virtually saturates the small number of receptors available on the postsynaptic membrane of central synapses. Finally, the anatomic literature suggests that a particular type of spine synapse, which electron microscopy reveals to have a perforated active zone, may represent a synapse with high efficacy. This suggestion is shown to be completely compatible with the electrophysiological data, and a model is presented that shows that all the apparently conflicting data in the field of long-term potentiation could be compatible. This stresses the need for cooperative collaboration between laboratories that have apparently conflicting findings.

LTP--a structural model to explain the inconsistencies.

One of the major controversies in neuroscience concerns whether the expression of long-term potentiation (LTP) is a pre- or postsynaptic phenomenon, with apparently contradictory data being the norm. The model that is outlined in this article combines anatomical and electrophysiological evidence to allow apparently contradictory data to be compatible. Development of LTP involves both influx of Ca2+ through NMDA receptors, and activation of another factor, perhaps the metabotropic glutamate receptor. These two processes might result, respectively, in the insertion of activation of additional postsynaptic receptors, and the growth of microfilaments that could split simple synapses into perforated synapses, consisting of multiple active zones. Whether the latter occurred, and at what rate, would be likely to depend on multiple factors, such as temperature, the metabolic state of the cell, buffering of Ca2+, and the concentration of factors such as nitric oxide. These subtle experimental variables would thus determine whether the dominant effect observed was pre- or postsynaptic.

Patch-clamping in brain slices: synaptic transmission from ATP to long-term potentiation.

Application of patch-clamp techniques to brain slices has resulted in an enormous increase in the resolution of synaptic currents in mammalian central neurones. This improved resolution has allowed direct observation of miniature and evoked synaptic currents, leading, in agreement with other findings, to the conclusion that the quantal size of synaptic currents in the brain is limited by the number of postsynaptic receptors. Possible explanations for the skewed miniature distribution, observed at all fast central synapses are discussed, with reference to anatomical observations. As a result, a model is proposed which is consistent with much of the apparently contradictory data on the induction and maintenance of long-term potentiation. In addition to the study of mechanisms of synaptic transmission, improved resolution provided by the patch-clamp technique has allowed resolution of synaptic currents, in the brain, mediated by ATP. The role of ATP as a central neurotransmitter is discussed in terms of the above findings.

ATP receptors.

ATP receptors continue to be found in an ever wider range of tissues in the brain and the periphery. Much of the recent research on these receptors focuses on features that would distinguish ATP-mediated excitatory synaptic transmission from that mediated through glutamate or acetylcholine receptors. These features include kinetics of the synaptic currents, voltage-independent calcium permeability of the underlying channels, interactions with adenosine, and various direct modulators of the ATP-gated ion channel.

ATP--a fast neurotransmitter.

ATP receptor-mediated responses in peripheral and central neurons have many characteristics which suggest that ATP may act as a fast neurotransmitter. While the receptors underlying these responses have properties which are similar to other ligand-gated ion channels which mediate fast neurotransmission, the nature of their calcium permeability and the rapid breakdown of ATP to adenosine may confer unique properties on ATP mediated synaptic transmission. The evidence that ATP acts as a fast neurotransmitter is reviewed and the properties of ATP and its receptor channels are discussed in terms of synaptic transmission.

ATP receptor-mediated synaptic currents in the central nervous system.

Until now, the only well documented, fast excitatory neurotransmitter in the brain has been glutamate. Although there is evidence for adenosine 5'-triphosphate (ATP) acting as a transmitter in the peripheral nervous system, suggestions for such a role in the central nervous system have so far not been supported by any direct evidence. Here we report the recording of evoked and miniature synaptic currents in the rat medial habenula. The fast rise time of the currents showed that they were mediated by a ligand-activated ion channel rather than a second messenger system, thus limiting the known transmitter candidates. Evidence was found for the presence on the cells of glutamate, gamma-aminobutyric acid, acetylcholine and ATP receptors, but not for 5-hydroxytryptamine (5HT3) or glycine receptors. The evoked currents were unaffected by blockers of glutamate, gamma-aminobutyric acid or acetylcholine receptors but were blocked by the ATP receptor-blocker, suramin and the desensitizing ATP receptor-agonist alpha,beta-methylene-ATP. Our evidence identifies for the first time synaptic currents in the brain, mediated directly by ATP receptors.

Fast and slow components of unitary EPSCs on stellate cells elicited by focal stimulation in slices of rat visual cortex.

1. Voltage and current recordings were made from visually identified non-pyramidal neurones in slices of layer IV of rat primary visual cortex using the whole-cell configuration of the patch clamp technique. These neurones are characterized by a high input resistance (0.5-2 G omega) and a non-adaptive behaviour of action potential frequency following depolarizing current injection, which suggests that they are stellate cells. 2. Excitatory postsynaptic currents (EPSCs) were recorded from these neurones during focal stimulation of neighbouring cells by a second patch pipette, the tip of which was placed on the soma of the stimulated cell. The response amplitude as a function of stimulus strength showed a sharp increase at a critical stimulus strength suggesting that stimulus-evoked currents represent unitary EPSCs. 3. In most cases the latencies of stimulus-evoked EPSCs were unimodally distributed with means in the range of 2.1-3.6 ms. In some experiments two peaks were seen in the distribution of latencies. The EPSC rise times, measured as the time from 20 to 80% peak amplitude, fell into a distribution ranging from 0.1 to 0.8 ms with a peak at 0.2 ms. The EPSC decay time course at -70 mV membrane potential was fitted by a single exponential with a time constant of 2.39 +/- 0.99 ms (mean +/- S.D.). The rise and decay times were independent of EPSC peak amplitudes. 4. The peak amplitude of successive unitary EPSCs, elicited by a constant stimulus, fluctuated at random. At a holding potential of -70 mV the peak amplitudes varied between 5 and 90 pA. In two out of ten cells the histogram of peak amplitudes could be well fitted by the sum of several equidistant Gaussians with a peak distance of around 10 pA. This suggests that the quantal conductance change underlying the peak current fluctuations is of the order of 100 pS. 5. At membrane potentials more positive than -70 mV the decay of stimulus-evoked EPSCs showed two components with very different time courses. In standard extracellular solution the current-voltage (I-V) relation for the fast component was almost linear whereas the slow component showed a J-shaped I-V relation with a region of negative slope conductance between -30 and -70 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

[The use of thin slices of myocardium for recording the currents across single ion channels]. / Primenenie tonkikh srezov miokarda dlia registratsii tokov cherez odinochnye ionnye kanaly.

Thin cardiac slices (100-200 microns) from newborn (1-14 days old) rat heart ventricles were used for patch clamp recordings. High resistance seals (10-50 GOhms) between patch-clamp pipettes and the membrane of cardiac cells as well as classical patch-clamp configurations can be achieved on this preparation without any enzymatic treatment of tissue. Resisting potential for cardiac cells measured in whole-cell configuration ranged between -30 and -65 mV. Averaged sodium currents and single inward rectifying potassium elementary currents recorded in cell-attached mode displayed basic features similar to those previously reported for isolated rat ventricular cells. Application of the method described here in cardiac electrophysiology will allow patch-clamp studies on heart cells without the complicated procedures of cell isolation. In addition, the uncertainty associated with enzyme treatment can be avoided. In future, this technique could be a new tool for studying electrophysiological properties of heart cells in situ.