Activity coregulates quantal AMPA and NMDA currents at neocortical synapses.
(65/1058)
AMPA and NMDA receptors are coexpressed at many central synapses, but the factors that control the ratio of these two receptors are not well understood. We recorded mixed miniature or evoked synaptic currents arising from coactivation of AMPA and NMDA receptors and found that long-lasting changes in activity scaled both currents up and down proportionally through changes in the number of postsynaptic receptors. The ratio of NMDA to AMPA current was similar at different synapses onto the same neuron, and this relationship was preserved following activity-dependent synaptic scaling. These data show that AMPA and NMDA receptors are tightly coregulated by activity at synapses at which they are both expressed and suggest that a mechanism exists to actively maintain a constant receptor ratio across a neuron's synapses. (+info)
The calcineurin inhibitor cyclosporine A (CsA) has emerged as a major cause of secondary hypertension in humans, but the underlying pathogenetic mechanisms have remained enigmatic. Synapsins are a family of synaptic vesicle phosphoproteins that are essential for normal regulation of neurotransmitter release at synapses. In addition to synaptic vesicles, synapsins and other vesicle proteins are found on microvesicles in sensory nerve endings in peripheral tissues. However, the functions of the sensory microvesicles in general, and of synapsins in particular, are unknown. We now demonstrate in a mouse model that CsA raises blood pressure by stimulating renal sensory nerve endings that contain synapsin-positive microvesicles. In knockout mice lacking synapsin I and II, sensory nerve endings are normally developed but not stimulated by CsA whereas a control stimulus, capsaicin, is fully active. The reflex activation of efferent sympathetic nerve activity and the increase in blood pressure by CsA seen in control are greatly attenuated in synapsin-deficient mice. These results provide a mechanistic explanation for CsA-induced acute hypertension and suggest that synapsins could serve as a drug target in this refractory condition. Furthermore, these data establish evidence that synapsin-containing sensory microvesicles perform an essential role in sensory transduction and suggest a role for synapsin phosphorylation in this process. (+info)
Responses of nerve fibres of the rat saphenous nerve neuroma to mechanical and chemical stimulation: an in vitro study.
(67/1058)
The response of neuroma nerve endings to different stimuli was studied in a saphenous nerve neuroma preparation in vitro. Electrical activity was recorded from 141 single fibres dissected of saphenous nerve. One-third (27 %) displayed spontaneous activity. Based on their response to mechanical and chemical stimuli, neuroma nerve fibres were classified as mechanosensory fibres (47.5 %), mechanically insensitive chemosensory fibres (17.0 %), polymodal nociceptor fibres (28.4 %) and unresponsive fibres (7.1 %). Mechanosensory and polymodal neuroma endings responded to von Frey hair stimulation either with a few impulses (phasic units) or a sustained discharge (tonic units). Polymodal units were additionally activated by at least one of the following stimuli: acidic solutions; a combination of bradykinin, prostaglandin E2, serotonin, substance P and histamine (all at 1 microM) plus 7 mM KCl (inflammatory soup); 600 mM NaCl and capsaicin. Low pH solutions increased the firing discharge of polymodal endings proportionally to the proton concentration. The 'inflammatory soup' evoked a firing response characterized by the absence of tachyphylaxis, which appeared when its components were applied separately. Both stimuli sensitized polymodal fibres to mechanical stimulation. Hypertonic NaCl (600 mM) and capsaicin (3.3 mM) induced a prolonged discharge that outlasted the stimulus duration. Mechanically insensitive chemosensory neuroma fibres exhibited responses to chemical stimuli analogous to polymodal fibres. They became mechanically sensitive after chemical stimulation. These findings show that neuroma nerve endings in the rat saphenous nerve neuroma in vitro are functionally heterogeneous and exhibit properties reminiscent of those in intact mechanosensory, polymodal and 'silent' nociceptor sensory afferents, including their sensitization by algesic chemicals. (+info)
Myotendinous nerve endings in human infant and adult extraocular muscles.
(68/1058)
Myotendinous nerve endings in extraocular muscles of some mammals consist exclusively of palisade nerve endings incorporated in myotendinous cylinders. There is evidence for a similar form in man, some doubt remains. The objectives of the present study were to examine the structure and distribution of nerve endings in extraocular muscles of infant and adult human material. Muscles from five infants and six adults aged 3 days to 90 years were prepared for light and electron microscopy. Nerve endings were sparse in a 4-year-old and none were present in the muscles of younger donors. They were present in all adult samples. One group of nerve endings branched from single recurrent nerve fibers and were distributed in the encapsulated tendon of single Felderstruktur muscle fibers. Terminals were varicose and shared certain characteristics of known sensory endings and were similar to those of myotendinous cylinders except that none formed neuromuscular junctions. In other myotendinous complexes capsules were fragmented and nerve endings were dispersed in tendon common to two or more muscle fibers. In the myotendon of two adult donors, a further group of endings issuing from non-recurrent nerves were unencapsulated and distributed randomly in tendon. The frequency of nerve endings varied across myotendon and in some instances, most marked in one case, large areas lacked nerve endings. Golgi tendon organs were not present. The terminals having features characteristic of sensory endings suggest a proprioceptive function, which is apparently unavailable in infancy. In mature muscles, the irregular distribution and variety of terminal form cannot be equated with those found in extraocular muscles of animals. We suggest that these features reflect an aberrant development and conclude that their capacity to fulfil an effective proprioceptive role is open to question. (+info)
Early morphological remodeling of neuromuscular junction in a murine model of diabetes.
(69/1058)
Although skeletal muscle weakness is documented in diabetes, the time course for its development is not established. The present study examined the dorsiflexor muscle from animals that had been diabetic for 2 wk. Adult male c57BL mice were injected once with streptozotocin (STZ) to induce diabetes (60 mg/kg ip). Two weeks later, resting membrane potential and miniature end-plate potentials were recorded, and electron microscopy was utilized for ultrastructural evaluations. After STZ-induced diabetes, both resting membrane potential and miniature end-plate potentials were reduced. Nerve terminals showed less synaptic vesicles and had degenerated mitochondria. Furthermore, in the intramuscular nerves, disorganization of microtubules and neurofilaments was evidenced. Myelin-like figures were present in intramuscular nerves, neuromuscular junctions, and muscle fibers. At the muscle level, mitochondria were swollen, with disorganization of their cristae, disruption of T tubules, and myofibers with more deposition of glycogen granules. The present results revealed early STZ-induced nerve and muscle alterations. Observed ultrastructural modifications resemble those of motoneuron disorders and aging processes. These changes are possibly related to alterations in Ca(2+) mobilization across muscle membrane. Other mechanisms such as free radical-mediated actions may also be implicated in STZ-induced effects on skeletal muscle. (+info)
Calcium dynamics associated with action potentials in single nerve terminals of pyramidal cells in layer 2/3 of the young rat neocortex.
(70/1058)
Calcium dynamics associated with a single action potential (AP) were studied in single boutons of the axonal arbor of layer 2/3 pyramidal cells in the neocortex of young (P14-16) rats. We used fluorescence imaging with two-photon excitation and Ca2+-selective fluorescence indicators to measure volume-averaged Ca2+ signals. These rapidly reached a peak (in about 1 ms) and then decayed more slowly (tens to hundreds of milliseconds). Single APs and trains of APs reliably evoked Ca2+ transients in en passant boutons located on axon collaterals in cortical layers 2/3, 4 and 5, indicating that APs propagate actively and reliably throughout the axonal arbor. Branch point failures are unlikely to contribute to differences in synaptic efficacy and reliability in the connections made by layer 2/3 pyramidal cells. AP-evoked Ca2+ transients in boutons were mediated by voltage-dependent Ca2+ channels (VDCCs), predominantly by the P/Q- and N-subtypes. Ca2+ transients were, on average, of significantly larger amplitude in boutons than in the flanking segments of the axon collateral. Large amplitude Ca2+ transients in boutons were spatially restricted to within <= 3 m of axonal length. Single AP-evoked Ca2+ transients varied up to 10-fold across different boutons even if they were located on the same axon collateral. In contrast, variation of Ca2+ transients evoked by successive APs in a given single bouton was small (coefficient of variation, c.v. <= 0.21). Amplitudes of AP-evoked Ca2+ signals did not correlate with the distance of boutons from the soma. In contrast, AP-evoked Ca2+ signals in spines of basal dendrites decreased slightly (correlation coefficient, r2 = -0.27) with distance from the soma. Measurements with the low-affinity Ca2+ indicator Magnesium Green suggest that the volume-averaged residual free [Ca2+]i in a bouton increases on average by 500 nM following a single AP. Higher concentrations of indicator caused, on average, a decrease in the amplitude and an increase in the decay time constant of Ca2+ transients. Assuming a single-compartment model the concentration dependence of decay time constants suggests a low endogenous Ca2+ binding ratio close to 140, indicating that of the total Ca2+ influx ( approximately 2 fC) less than 1% remained free. The indicator concentration dependence of decay time constants further suggests that the residual free Delta[Ca2+]i associated with an AP decays with a time constant of about 60 ms (35 C) reflecting a high Ca2+ extrusion rate of about 2600 s(-1). The results show that AP-evoked volume-averaged Ca2+ transients in single boutons are evoked reliably and, on average, have larger amplitudes than Ca2+ transients in other subcellular compartments of layer 2/3 pyramidal cells. A major functional signature is the large variation in the amplitude of Ca2+ transients between different boutons. This could indicate that local interactions between boutons and different target cells modify the spatiotemporal Ca2+ dynamics in boutons and cause target cell-specific differences in their transmitter release properties. (+info)
Re-evaluation of phorbol ester-induced potentiation of transmitter release from mossy fibre terminals of the mouse hippocampus.
(71/1058)
To investigate the mechanisms by which phorbol esters potentiate transmitter release from mossy fibre terminals we used fura dextran to measure the intraterminal Ca2+ concentration in mouse hippocampal slices. A phorbol ester, phorbol 12,13-diacetate (PDAc), potentiated the field excitatory postsynaptic potential (fEPSP) slope. PDAc also enhanced the stimulation-dependent increase of [Ca2+]i in the mossy fibre terminal (Delta[Ca2+]pre). The magnitude of the PDAc-induced fEPSP potentiation (463+/-57% at 10 microM) was larger than that expected from the enhancement of Delta[Ca2+]pre (153+/-5%). The Delta[Ca2+]pre was suppressed by omega-agatoxin IVA (omega-AgTxIVA, 200 nM), a P/Q-type Ca2+ channel-specific blocker, by 31%. The effect of PDAc did not select between omega-AgTxIVA-sensitive and -resistant components. The PDAc-induced potentiation of the fEPSP slope was partially antagonized by the protein kinase C (PKC) inhibitor bisindolylmaleimide I (BIS-I, 10 microM), whereas the Delta[Ca2+]pre was completely blocked by BIS-I. Although the BIS-I-sensitive fEPSP potentiation was accompanied by a reduction of the paired-pulse ratio (PPR), the BIS-I-resistant component was not. Whole-cell patch clamp recording from a CA3 pyramidal neuron in a BIS-I-treated slice demonstrated that PDAc (10 microM) increased the frequency of miniature excitatory postsynaptic currents (mEPSCs, 259+/-33% of control) without a noticeable change in their amplitude (102+/-5% of control). These results suggest that PKC potentiates transmitter release by at least two distinct mechanisms, one Delta[Ca2+]pre dependent and the other Delta[Ca2+]pre independent. In addition, some phorbol ester-mediated potentiation of synaptic transmission appears to occur without activating PKC. (+info)
Reproducibility of repeated measures of cholinergic terminal density using.
(72/1058)
[18F](+)-4-fluorobenzyltrozamicol (FBT), which selectively binds to the vesicular acetylcholine transporter in the presynaptic cholinergic neuron, has previously been shown to be a useful ligand for the study of cholinergic terminal density in the basal ganglia with PET. The goal of this study was to assess the test-retest variability of [18F]FBT and PET measurements under baseline conditions in the basal ganglia. METHODS: After approval from the Animal Care and Use Committee, 6 rhesus monkeys underwent a series of 2 [18F]FBT PET scans (time between scans, 32-301 d) under isoflurane anesthesia. Each scan was initiated on the bolus injection of the radiotracer and consisted of 26 frames acquired during 180 min. Arterial blood samples were collected over the course of each scan to determine the metabolite-corrected arterial input function. Tissue time-activity curves were obtained from the scan data by drawing regions of interest over the basal ganglia and cerebellum. The distribution volume ratio for the basal ganglia was then determined for each scan by taking the ratio of the basal ganglia (specific binding) to cerebellum (nonspecific binding) distribution volume. Distribution volumes were derived using the Logan graphic analysis technique as well as a standard 3-compartment model. Additionally, the radioactivity concentration ratio was calculated as the ratio of the average [18F]FBT concentration in the basal ganglia to that in the cerebellum during the last half of the study (85-170 min). The constant K1, determined using the standard 3-compartment model, was used as an index of blood flow changes between studies. RESULTS: For all subjects, the test-retest variability was less than 15% for the distribution volume ratio and 12% for the radioactivity concentration ratio. Good agreement was found between the distribution volume ratio calculated using the graphic technique and the standard 3-compartment model. Using K1 as an index, the variability in blood flow seen in both the basal ganglia and the cerebellum was significantly reduced in their ratio. CONCLUSION: These results show the reproducibility of [18F]FBT and PET measurements in the basal ganglia. (+info)