Silent synapses in the developing hippocampus: lack of functional AMPA receptors or low probability of glutamate release? (25/267)

At early developmental stages, silent synapses have been commonly found in different brain areas. These synapses are called silent because they do not respond at rest but are functional at positive membrane potentials. A widely accepted interpretation is that N-methyl-d-aspartate (NMDA) but not alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors are functionally expressed on the subsynaptic membrane. Here we show that, in both CA3 and CA1 hippocampal regions, AMPA-mediated synaptic responses can be detected already at early stages of postnatal development. However, some synapses appear silent because of a very low probability of glutamate release. They can be converted into functional ones by factors that enhance release probability such as paired-pulse stimulation, increasing the temperature or cyclothiazide (CTZ), a drug that blocks AMPA receptor desensitization and increases transmitter release. Conversely, conducting synapses can be switched off by increasing the frequency of stimulation. Although we cannot exclude that "latent AMPA receptors" can become functional after activity-dependent processes, our results clearly indicate that, in the neonatal hippocampus, a proportion of glutamatergic synaptic connections are presynaptically rather than postsynaptically silent.  (+info)

Choosing a first-line drug in the management of elevated blood pressure: what is the evidence? 2: Beta-blockers. (26/267)

Elevated blood pressure is associated with an increased risk of cardiovascular illness and death. Efforts to reduce that risk have led to recommendations for a wide array of nondrug and drug therapies. Choosing the optimal first-line drug for hypertensive patients should address a hierarchy of treatment goals: decrease in morbidity and mortality associated with hypertension, decrease in blood pressure, good tolerance, dosing convenience and low cost. This article examines the evidence for beta-blockers as a class of first-line antihypertensive drugs in light of these treatment goals. The evidence indicates that beta-blockers are probably not as effective in reducing morbidity and mortality as low-dose thiazide diuretics and that there may be significant differences in effectiveness among various beta-blockers.  (+info)

Effects of the potent ampakine CX614 on hippocampal and recombinant AMPA receptors: interactions with cyclothiazide and GYKI 52466. (27/267)

R,S-alpha-Amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor up-modulators of the benzamide type ("ampakines") have previously been shown to enhance excitatory synaptic transmission in vivo and in vitro and AMPA receptor currents in excised patches. The present study analyzed the effects of an ampakine (CX614; 2H,3H, 6aH-pyrrolidino[2",1"-3',2']1,3-oxazino[6',5'-5,4]benz o[e]1, 4-dioxan-10-one) that belongs to a benzoxazine subgroup characterized by greater structural rigidity and higher potency. CX614 enhanced the size (amplitude and duration) of field excitatory postsynaptic potentials in hippocampal slices and autaptically evoked excitatory postsynaptic currents in neuronal cultures with EC(50) values of 20 to 40 microM. The compound blocked desensitization (EC(50) = 44 microM) and slowed deactivation of responses to glutamate by a factor of 8.4 in excised patches. Currents through homomeric, recombinant AMPA receptors were enhanced with EC(50) values that did not differ greatly across GluR1-3 flop subunits (19-37 microM) but revealed slightly lower potency at corresponding flip variants. Competition experiments using modulation of [(3)H]fluorowillardiine binding suggested that CX614 and cyclothiazide share a common binding site but cyclothiazide seems to bind to an additional site not recognized by the ampakine. CX614 did not reverse the effect of GYKI 52466 on responses to brief glutamate pulses, which indicates that they act through separate sites, a conclusion that was confirmed in binding experiments. In sum, these results extend prior evidence that ampakines are effective in enhancing synaptic responses, most likely by slowing deactivation, and that their effects are exerted through sites that are only in part shared with other modulators.  (+info)

AMPA receptor current density, not desensitization, predicts selective motoneuron vulnerability. (28/267)

Spinal motoneurons are more susceptible to AMPA receptor-mediated injury than are other spinal neurons, a property that has been implicated in their selective degeneration in amyotrophic lateral sclerosis (ALS). The aim of this study was to determine whether this difference in vulnerability between motoneurons and other spinal neurons can be attributed to a difference in AMPA receptor desensitization and/or to a difference in density of functional AMPA receptors. Spinal motoneurons and dorsal horn neurons were isolated from embryonic rats and cultured on spinal astrocytes. Single-cell RT-PCR quantification of the relative abundance of the flip and flop isoforms of the AMPA receptor subunits, which are known to affect receptor desensitization, did not reveal any difference between the two cell populations. Examination of AMPA receptor desensitization by patch-clamp electrophysiological measurements on nucleated and outside-out patches and in the whole-cell mode also yielded similar results for the two cell groups. However, AMPA receptor current density was two- to threefold higher in motoneurons than in dorsal horn neurons, suggesting a higher density of functional AMPA receptors in motoneuron membranes. Pharmacological reduction of AMPA receptor current density in motoneurons to the level found in dorsal horn neurons eliminated selective motoneuron vulnerability to AMPA receptor activation. These results suggest that the greater AMPA receptor current density of spinal motoneurons may be sufficient to account for their selective vulnerability to AMPA receptor agonists in vitro.  (+info)

Preferential potentiation of fast-releasing synaptic vesicles by cAMP at the calyx of Held. (29/267)

We have studied the effects of cAMP on synaptic transmission at the calyx of Held and found that forskolin (an activator of adenylate cyclase) and 8-Br-cAMP (a membrane-permeable analog of cAMP) potentiated excitatory postsynaptic currents (EPSCs). Direct sampling of miniature EPSCs (mEPSCs) and nonstationary fluctuation analysis showed that mEPSCs were not modulated by cAMP, suggesting that the locus of modulation is presynaptic. Deconvolution was used to examine effects of cAMP on quantal-release rates. By using this method, it was shown recently that release probabilities of readily releasable vesicles are heterogeneous. Here, we show that cAMP selectively increases the number of vesicles with higher release probabilities, whereas a slow component of the EPSC, representing vesicles that fuse more slowly, is unchanged. cAMP increases the apparent Ca2+ sensitivity for secretion, but this increase does not reflect an increase in release probability necessarily but rather an increase in the number of highly sensitive vesicles.  (+info)

Combining deconvolution and noise analysis for the estimation of transmitter release rates at the calyx of held. (30/267)

The deconvolution method has been used in the past to estimate release rates of synaptic vesicles, but it cannot be applied to synapses where nonlinear interactions of quanta occur. We have extended this method to take into account a nonlinear current component resulting from the delayed clearance of glutamate from the synaptic cleft. We applied it to the calyx of Held and verified the important assumption of constant miniature EPSC (mEPSC) size by combining deconvolution with a variant of nonstationary fluctuation analysis. We found that amplitudes of mEPSCs decreased strongly after extended synaptic activity. Cyclothiazide (CTZ), an inhibitor of glutamate receptor desensitization, eliminated this reduction, suggesting that postsynaptic receptor desensitization occurs during strong synaptic activity at the calyx of Held. Constant mEPSC sizes could be obtained in the presence of CTZ and kynurenic acid (Kyn), a low-affinity blocker of AMPA-receptor channels. CTZ and Kyn prevented postsynaptic receptor desensitization and saturation and also minimized voltage-clamp errors. Therefore, we conclude that in the presence of these drugs, release rates at the calyx of Held can be reliably estimated over a wide range of conditions. Moreover, the method presented should provide a convenient way to study the kinetics of transmitter release at other synapses.  (+info)

Quantitative relationship between transmitter release and calcium current at the calyx of held synapse. (31/267)

A newly developed deconvolution method (Neher and Sakaba, 2001) allowed us to resolve the time course of neurotransmitter release at the calyx of Held synapse and to quantify some basic aspects of transmitter release. First, we identified a readily releasable pool (RRP) of synaptic vesicles. We found that the size of the RRP, when tested with trains of strong stimuli, was constant regardless of the exact stimulus patterns, if stimuli were confined to a time interval of approximately 60 msec. For longer-lasting stimulus patterns, recruitment of new vesicles to the RRP made a substantial contribution to the total release. Second, the cooperativity of transmitter release as a function of Ca(2+) current was estimated to be 3-4, which confirmed previous results (Borst and Sakmann, 1999; Wu et al., 1999). Third, an initial small Ca(2+) influx increased the efficiency of Ca(2+) currents in subsequent transmitter release. This type of facilitation was blocked by a high concentration of EGTA (0.5 mm). Fourth, the release rates of synaptic vesicles at this synapse turned out to be heterogeneous: once a highly Ca(2+)-sensitive population of vesicles was consumed, the remaining vesicles released at lower rates. These components of release were more clearly separated in the presence of 0.5 mm EGTA, which prevented the buildup of residual Ca(2+). Conversely, raising the extracellular Ca(2+) concentration facilitated the slower population such that its release characteristics became more similar to those of the faster population under standard conditions. Heterogeneous release probabilities are expected to support the maintenance of synaptic transmission during high-frequency stimulation.  (+info)

Domain interactions regulating ampa receptor desensitization. (32/267)

Desensitization is a common property of glutamate and other ligand-gated ion channels, yet its molecular mechanism is unknown. For glutamate receptors, agonist binding involves interactions with identified amino acids from two lobes and may result in stabilizing the lobes in a closed "clamshell" conformation. The present studies demonstrate that two structures, beta-strands 7 and 8 and alpha-helices J and K, functionally interact with each other and likely form hinges between the two lobes, influencing the coupling between agonist binding and desensitization. Two amino acids identified within these regions form a solvent-exposed interface with a third amino acid, a mutation of which was shown previously to block receptor desensitization (L(507) in glutamate receptor 3). This interface may regulate a concerted conformational shift of the AMPA subtype of glutamate receptor subunits to the desensitized state.  (+info)