Biomedical subjects

C F Stevens

Publications and source records attributed to C F Stevens.

At least 19 recordsLinked to original sources

PubMed2001

Inactivity produces increases in neurotransmitter release and synapse size.

When hippocampal synapses in culture are pharmacologically silenced for several days, synaptic strength increases. The structural correlate of this change in strength is an increase in the size of the synapses, with all synaptic components--active zone, postsynaptic density, and bouton--becoming larger. Further, the number of docked vesicles and the total number of vesicles per synapse increases, although the number of docked vesicles per area of active zone is unchanged. In parallel with these anatomical changes, the physiologically measured size of the readily releasable pool (RRP) and the release probability are increased. Ultrastructural analysis of individual synapses in which the RRP was previously measured reveals that, within measurement error, the same number of vesicles are docked as are estimated to be in the RRP.

Animals↗

PubMed2001

Vitamin A deprivation results in reversible loss of hippocampal long-term synaptic plasticity.

Despite its long history, the central effects of progressive depletion of vitamin A in adult mice has not been previously described. An examination of vitamin-deprived animals revealed a progressive and ultimately profound impairment of hippocampal CA1 long-term potentiation and a virtual abolishment of long-term depression. Importantly, these losses are fully reversible by dietary vitamin A replenishment in vivo or direct application of all trans-retinoic acid to acute hippocampal slices. We find retinoid responsive transgenes to be highly active in the hippocampus, and by using dissected explants, we show the hippocampus to be a site of robust synthesis of bioactive retinoids. In aggregate, these results demonstrate that vitamin A and its active derivatives function as essential competence factors for long-term synaptic plasticity within the adult brain, and suggest that key genes required for long-term potentiation and long-term depression are retinoid dependent. These data suggest a major mental consequence for the hundreds of millions of adults and children who are vitamin A deficient.

Adult↗

PubMed2001

An evolutionary scaling law for the primate visual system and its basis in cortical function.

A hallmark of mammalian brain evolution is the disproportionate increase in neocortical size as compared with subcortical structures. Because primary visual cortex (V1) is the most thoroughly understood cortical region, the visual system provides an excellent model in which to investigate the evolutionary expansion of neocortex. I have compared the numbers of neurons in the visual thalamus (lateral geniculate nucleus; LGN) and area V1 across primate species. Here I find that the number of V1 neurons increases as the 3/2 power of the number of LGN neurons. As a consequence of this scaling law, the human, for example, uses four times as many V1 neurons per LGN neuron (356) to process visual information as does a tarsier (87). I argue that the 3/2 power relationship is a natural consequence of the organization of V1, together with the requirement that spatial resolution in V1 should parallel the maximum resolution provided by the LGN. The additional observation that thalamus/neocortex follows the same evolutionary scaling law as LGN/V1 may suggest that neocortex generally conforms to the same organizational principle as V1.

Animals↗

PubMed2001

Synaptotagmin I functions as a calcium regulator of release probability.

In all synapses, Ca2+ triggers neurotransmitter release to initiate signal transmission. Ca2+ presumably acts by activating synaptic Ca2+ sensors, but the nature of these sensors--which are the gatekeepers to neurotransmission--remains unclear. One of the candidate Ca2+ sensors in release is the synaptic Ca2+-binding protein synaptotagmin I. Here we have studied a point mutation in synaptotagmin I that causes a twofold decrease in overall Ca2+ affinity without inducing structural or conformational changes. When introduced by homologous recombination into the endogenous synaptotagmin I gene in mice, this point mutation decreases the Ca2+ sensitivity of neurotransmitter release twofold, but does not alter spontaneous release or the size of the readily releasable pool of neurotransmitters. Therefore, Ca2+ binding to synaptotagmin I participates in triggering neurotransmitter release at the synapse.

Animals↗

PubMed2001

Dopamine D1/D5 receptor modulation of excitatory synaptic inputs to layer V prefrontal cortex neurons.

Dopamine acts mainly through the D1/D5 receptor in the prefrontal cortex (PFC) to modulate neural activity and behaviors associated with working memory. To understand the mechanism of this effect, we examined the modulation of excitatory synaptic inputs onto layer V PFC pyramidal neurons by D1/D5 receptor stimulation. D1/D5 agonists increased the size of N-methyl-d-aspartate (NMDA) component of excitatory postsynaptic currents (EPSCs) through a postsynaptic mechanism. In contrast, D1/D5 agonists caused a slight reduction in the size of the non-NMDA component of EPSCs through a small decrease in release probability. With 20 Hz synaptic trains, we found that the D1/D5 agonists increased depolarization of summating the NMDA component of excitatory postsynaptic potential (EPSP). By increasing the NMDA component of EPSCs, yet slightly reducing release, D1/D5 receptor activation selectively enhanced sustained synaptic inputs and equalized the sizes of EPSPs in a 20-Hz train.

Animals↗

PubMed2001

Morphological correlates of functionally defined synaptic vesicle populations.

By combining photoconversion of FM1-43-stained vesicles and electron microscopy of hippocampal synapses, we find evidence that the population of morphologically docked synaptic vesicles corresponds to the release-ready neurotransmitter quanta. Furthermore, those synaptic vesicles that are participating in cycles of exo- and endocytosis tend to be closer to the active zone than vesicles that are being held in reserve.

Animals↗

PubMed2000

"Kiss and run" exocytosis at hippocampal synapses.

We have combined electrophysiology and imaging to measure the release of neurotransmitter and fluorescent dye at synapses of cultured hippocampal neurons. These experiments have revealed a "kiss and run" mode of exocytosis in which synaptic vesicles release glutamate normally but do not permit dye to enter or escape from the vesicle. During "kiss and run," the vesicle interior may be exposed very transiently (<6 ms), or a special configuration of the fusion pore may prevent dye exchange. We estimate that about 20% of the vesicles normally use this "kiss and run" pathway, and that the fraction of "kiss and run" events can be increased to over 80% by superfusing the synapses with hypertonic solution.

Action Potentials↗

PubMed2000

Nonsaturation of AMPA and NMDA receptors at hippocampal synapses.

An important issue in synaptic physiology is the extent to which postsynaptic receptors are saturated by the neurotransmitter released from a single synaptic vesicle. Although the bulk of evidence supports receptor saturation, recent studies have started to reveal that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) and N-methyl-D-aspartate (NMDA) receptors may not be saturated by a single vesicle of glutamate. Here, we address this question through a study of putative single synapses, made by hippocampal neurons in culture, that are identified by FM1-43 staining. An analysis of the sources of variability in the amplitudes of miniature excitatory postsynaptic currents at single synapses reveals that this variability must arise presynaptically, from variations in the quantity of agonist released. Thus, glutamate receptors at hippocampal synapses are not generally saturated by quantal release.

6-Cyano-7-nitroquinoxaline-2,3-dione↗

PubMed2000

Dynamics of dendritic calcium transients evoked by quantal release at excitatory hippocampal synapses.

Synaptic N-methyl-D-aspartate (NMDA) receptors detect coincident pre- and postsynaptic activity and play a critical role in triggering changes in synaptic strength at central synapses. Despite intensive study of synaptic plasticity, relatively little is known about the magnitude and duration of calcium accumulation caused by unitary events at individual synapses. We used fluorescence imaging to detect NMDA receptor-mediated miniature synaptic calcium transients (MSCTs) caused by spontaneous release of synaptic vesicles in dendrites of cultured hippocampal neurons. MSCTs originated focally in dendritic regions <1 microm in length and decayed with a time constant of 0.35 +/- 0.03 s. Multiple occurrences of MSCTs recorded at single sites had fluctuating amplitudes, with a coefficient of variation of 0.34. From the reduction in the spatial spread of MSCTs with decreasing concentration of indicator dye, we estimated that the dominant endogenous calcium buffer in dendrites is relatively immobile (diffusion coefficient between 10 and 50 microm(2)/s). We conclude that calcium rise caused by spontaneous quantal synaptic NMDA receptor activation (i) is variable, (ii) lasts for a time period briefer than previous measurements indicate, and (iii) is confined by endogenous calcium buffers to local dendritic regions even when synapses are not on spines.

Aniline Compounds↗

PubMed2000

Signalling mechanisms: a decade of signalling.

Knowledge of signaling mechanisms has increased dramatically during the past decade, particularly in the areas of development, biochemical signaling cascades, synaptic transmission and ion channel biophysics.

Animals↗

PubMed1999

Quantitative fine-structural analysis of olfactory cortical synapses.

To determine the extent to which hippocampal synapses are typical of those found in other cortical regions, we have carried out a quantitative analysis of olfactory cortical excitatory synapses, reconstructed from serial electron micrograph sections of mouse brain, and have compared these new observations with previously obtained data from hippocampus. Both superficial and deep layer I olfactory cortical synapses were studied. Although individual synapses in each of the areas-CA1 hippocampus, olfactory cortical layer Ia, olfactory cortical area Ib-might plausibly have been found in any of the other areas, the average characteristics of the three synapse populations are distinct. Olfactory cortical synapses in both layers are, on average, about 2.5 times larger than their hippocampal counterparts. The layer Ia olfactory cortical synapses have fewer synaptic vesicles than do the layer Ib synapses, but the absolute number of vesicles docked to the active zone in the layer Ia olfactory cortical synapses is about equal to the docked vesicle number in the smaller hippocampal synapses. As would be predicted from studies on hippocampus that relate paired-pulse facilitation to the number of docked vesicles, the synapses in layer 1a exhibit facilitation, whereas the ones in layer 1b do not. Although hippocampal synapses provide as a good model system for central synapses in general, we conclude that significant differences in the average structure of synapses from one cortical region to another exist, and this means that generalizations based on a single synapse type must be made with caution.

Animals↗

PubMed1999

Augmentation is a potentiation of the exocytotic process.

Short-term synaptic enhancement is caused by an increase in the probability with which synaptic terminals release transmitter in response to presynaptic action potentials. Since exocytosed vesicles are drawn from a readily releasable pool of packaged transmitter, enhancement must result either from an increase in the size of the pool or an elevation in the fraction of releasable vesicles that undergoes exocytosis with each action potential. We show here that at least one major component of enhancement, augmentation, is not caused by an increase in the size of the readily releasable pool but is instead associated with an increase in the efficiency with which action potentials induce the exocytosis of readily releasable vesicles.

Action Potentials↗

PubMed1999

Response of hippocampal synapses to natural stimulation patterns.

We have studied the synaptic responses in hippocampal slices to stimulus patterns derived from in vivo recordings of place cell firing in a behaving rodent. We find that synaptic strength is strongly modulated during the presentation of these natural stimulus trains, varying 2-fold or more because of short-term plasticity. This modulation of synaptic strength is precise and deterministic, because the pattern of synaptic response amplitudes is nearly identical from one presentation of the train to the next. The mechanism of synaptic modulation is primarily a change in release probability rather than a change in the size of the elementary postsynaptic response. In addition, natural stimulus trains are effective in inducing long-term potentiation (LTP). We conclude that short-term synaptic plasticity--facilitation, augmentation, and depression--plays a prominent role in normal synaptic function.

Action Potentials↗

PubMed1999

Identification of a novel process limiting the rate of synaptic vesicle cycling at hippocampal synapses.

During intense presynaptic activity, the readily releasable pool (RRP) of synaptic vesicles empties more quickly than it can be refilled, and short-term depression results. Ordinarily, the pool refills within 20 s, but long, high-frequency trains of action potentials often induce a form of short-term depression that persists for a much longer time. Here, we report that replenishment of the RRP is governed by two simple processes: the previously identified mechanism termed refilling, and another process that appears after extensive exocytosis and produces a transient decrease in the capacity of the pool, lasting for several minutes. The data presented here place stringent constraints on the types of kinetic models that can be used to describe synaptic vesicular cycling and are inconsistent with the traditional multipool models of vesicular mobilization.

Action Potentials↗

PubMed1999

Reversal of synaptic vesicle docking at central synapses.

We used quantitative fluorescence imaging of vesicles labeled with membrane-soluble dyes to determine rates of undocking and spontaneous exocytosis of vesicles docked to the active zone of hippocampal synapses in culture. Individual vesicles undock about once per two minutes and spontaneously exocytose about once per eight minutes. Thus, not only does undocking occur, but it is over threefold faster than spontaneous fusion.

Cells, Cultured↗

PubMed1998

Neuronal diversity: too many cell types for comfort?

Recent studies indicate that there are many more different types of neuron in the brain than previously thought. This richness will complicate life for those aiming to understand how the brain works - particularly for the neural modellers.

Animals↗

PubMed1998

Comparison of hippocampal dendritic spines in culture and in brain.

We have quantified hippocampal spine structure at the light and ultrastructural levels in cell cultures approximately 1- 3 weeks old and in the brains of rodents 5 and 21 d old. The number of spines bearing synapses increases with age in cultures and in brain, but the structures are similar in both. In culture, about half of the synapses are formed on spines and the remainder are formed on dendritic shafts. In the 5-d-old brain, about half of the synapses occur on dendritic shafts, by 3 weeks of age only approximately 20% of synapses are found on dendritic shafts, and in the adult shaft synapses are very rare.

Animals↗

PubMed1998

The tetrameric structure of a glutamate receptor channel.

The subunit stoichiometry of several ligand-gated ion channel receptors is still unknown. A counting method was developed to determine the number of subunits in one family of brain glutamate receptors. Successful application of this method in an HEK cell line provides evidence that ionotropic glutamate receptors share a tetrameric structure with the voltage-gated potassium channels. The average conductance of these channels depends on how many subunits are occupied by an agonist.

Binding Sites↗