Neurotransmitter Release at Central Synapses

Are unreliable release mechanisms conserved from NMJ to CNS?

The frog neuromuscular junction (NMJ) is a strong and reliable synapse because, during activation, sufficient neurotransmitter is released to trigger a postsynaptic action potential (AP). Recent evidence supports the hypothesis that this reliability emerges from the assembly of thousands of unreliable single vesicle release sites. The mechanisms that govern this unreliability include a paucity of voltage-gated calcium channels, a low probability of calcium channel opening during an AP, and the rare triggering of synaptic vesicle fusion even when a calcium channel does open and allows calcium flux. Here, we discuss the evidence that these unreliable single vesicle release sites may be the fundamental building blocks of many types of synapses in both the peripheral and central nervous system (PNS and CNS, respectively).

Ca(2+) influx and neurotransmitter release at ribbon synapses

Ca(2+) influx through voltage-gated Ca(2+) channels triggers the release of neurotransmitters at presynaptic terminals. Some sensory receptor cells in the peripheral auditory and visual systems have specialized synapses that express an electron-dense organelle called a synaptic ribbon. Like conventional synapses, ribbon synapses exhibit SNARE-mediated exocytosis, clathrin-mediated endocytosis, and short-term plasticity. However, unlike non-ribbon synapses, voltage-gated L-type Ca(2+) channel opening at ribbon synapses triggers a form of multiquantal release that can be highly synchronous. Furthermore, ribbon synapses appear to be specialized for fast and high throughput exocytosis controlled by graded membrane potential changes. Here we will discuss some of the basic aspects of synaptic transmission at different types of ribbon synapses, and we will emphasize recent evidence that auditory and retinal ribbon synapses have marked differences. This will lead us to suggest that ribbon synapses are specialized for particular operating ranges and frequencies of stimulation. We propose that different types of ribbon synapses transfer diverse rates of sensory information by expressing a particular repertoire of critical components, and by placing them at precise and strategic locations, so that a continuous supply of primed vesicles and Ca(2+) influx leads to fast, accurate, and ongoing exocytosis.

Synaptotagmin 1 is necessary for the Ca2+ dependence of clathrin-mediated endocytosis

The role of Ca²⁺ in synaptic vesicle endocytosis remains uncertain due to the diversity in various preparations where several forms of endocytosis may contribute variably in different conditions. Although recent studies have demonstrated that Ca²⁺ is important for clathrin-mediated endocytosis (CME), the mechanistic role of Ca²⁺ in CME remains to be elucidated. By monitoring CME of single vesicles in mouse chromaffin cells with cell-attached capacitance measurements that offer millisecond time resolution, we demonstrate that the dynamics of vesicle fission during CME is Ca²⁺ dependent but becomes Ca²⁺ independent in synaptotagmin 1 (Syt1) knock-out cells. Our results thus suggest that Syt1 is necessary for the Ca²⁺ dependence of CME.

Not noisy, just wrong: the role of suboptimal inference in behavioral variability

Behavior varies from trial to trial even when the stimulus is maintained as constant as possible. In many models, this variability is attributed to noise in the brain. Here, we propose that there is another major source of variability: suboptimal inference. Importantly, we argue that in most tasks of interest, and particularly complex ones, suboptimal inference is likely to be the dominant component of behavioral variability. This perspective explains a variety of intriguing observations, including why variability appears to be larger on the sensory than on the motor side, and why our sensors are sometimes surprisingly unreliable.

Three functional facets of calbindin D-28k

Many neurons of the vertebrate central nervous system (CNS) express the Ca²⁺ binding protein calbindin D-28k (CB), including important projection neurons like cerebellar Purkinje cells but also neocortical interneurons. CB has moderate cytoplasmic mobility and comprises at least four EF-hands that function in Ca²⁺ binding with rapid to intermediate kinetics and affinity. Classically it was viewed as a pure Ca²⁺ buffer important for neuronal survival. This view was extended by showing that CB is a critical determinant in the control of synaptic Ca²⁺ dynamics, presumably with strong impact on plasticity and information processing. Already 30 years ago, in vitro studies suggested that CB could have an additional Ca²⁺ sensor function, like its prominent acquaintance calmodulin (CaM). More recent work substantiated this hypothesis, revealing direct CB interactions with several target proteins. Different from a classical sensor, however, CB appears to interact with its targets both, in its Ca²⁺-loaded and Ca²⁺-free forms. Finally, CB has been shown to be involved in buffered transport of Ca²⁺, in neurons but also in kidney. Thus, CB serves a threefold function as buffer, transporter and likely as a non-canonical sensor.

Estimation of the time course of neurotransmitter release at central synapses from the first latency of postsynaptic currents

Measurement of the release time course (RTC) and of the quantal content is important for quantifying synaptic precision and understanding the molecular basis of the release process at central synapses. In theory, the RTC can be determined directly from the histogram of first latencies of quantal events only if a maximum of one vesicle is released per trial, but at most synapses multiple vesicles are released. Traditionally, first latency histograms have been corrected for multiple releases using a simple correction, derived by Barrett and Stevens (BS; 1972b) for quantifying release at the neuromuscular junction. This correction has also been used to quantify release at central synapses. We show, by combining an analytical approach and numerical simulations of stochastic quantal release, that the BS correction gives a biased estimate for RTC and quantal content. The bias increases with release probability, and is therefore particularly problematic for central synapses. We show that this is due to assuming infinite availability of releasable vesicles and we derive a formula for estimating the RTC from first latencies without this assumption. The resulting ‘binomial correction’ requires knowledge of the maximum number of quanta that can be released following an action potential (N), which can be estimated with variance-mean analysis. We show with simulations that estimating RTC and quantal content from first latencies using the binomial correction is robust in the presence of noise and when release probability is non-uniform. We also provide an alternative method for estimating RTC from the first latencies when N cannot be determined.

The role of hydrogels with tethered acetylcholine functionality on the adhesion and viability of hippocampal neurons and glial cells

In neural tissue engineering, designing materials with the right chemical cues is crucial in providing a permissive microenvironment to encourage and guide neuronal cell attachment and differentiation. Modifying synthetic hydrogels with biologically active molecules has become an increasingly important route in this field to provide a successful biomaterial and cell interaction. This study presents a strategy of using the monomer 2-methacryloxyethyl trimethylammonium chloride (MAETAC) to provide tethered neurotransmitter acetylcholine-like functionality with a complete 2-acetoxy-N,N,N-trimethylethanaminium segment, thereby modifying the properties of commonly used, non-adhesive PEG-based hydrogels. The effect of the functional monomer concentration on the physical properties of the hydrogels was systematically studied, and the resulting hydrogels were also evaluated for mice hippocampal neural cell attachment and growth. Results from this study showed that MAETAC in the hydrogels promotes neuronal cell attachment and differentiation in a concentration-dependent manner, different proportions of MAETAC monomer in the reaction mixture produce hydrogels with different porous structures, swollen states, and mechanical strengths. Growth of mice hippocampal cells cultured on the hydrogels showed differences in number, length of processes and exhibited different survival rates. Our results indicate that chemical composition of the biomaterials is a key factor in neural cell attachment and growth, and integration of the appropriate amount of tethered neurotransmitter functionalities can be a simple and effective way to optimize existing biomaterials for neuronal tissue engineering applications.

Nanodomain coupling between Ca²⁺ channels and sensors of exocytosis at fast mammalian synapses

The physical distance between presynaptic Ca(2+) channels and the Ca(2+) sensors that trigger exocytosis of neurotransmitter-containing vesicles is a key determinant of the signalling properties of synapses in the nervous system. Recent functional analysis indicates that in some fast central synapses, transmitter release is triggered by a small number of Ca(2+) channels that are coupled to Ca(2+) sensors at the nanometre scale. Molecular analysis suggests that this tight coupling is generated by protein-protein interactions involving Ca(2+) channels, Ca(2+) sensors and various other synaptic proteins. Nanodomain coupling has several functional advantages, as it increases the efficacy, speed and energy efficiency of synaptic transmission.

Quantitative analysis of synaptic vesicle pool replenishment in cultured cerebellar granule neurons using FM dyes

After neurotransmitter release in central nerve terminals, SVs are rapidly retrieved by endocytosis. Retrieved SVs are then refilled with neurotransmitter and rejoin the recycling pool, defined as SVs that are available for exocytosis^1,2. The recycling pool can generally be subdivided into two distinct pools - the readily releasable pool (RRP) and the reserve pool (RP). As their names imply, the RRP consists of SVs that are immediately available for fusion while RP SVs are released only during intense stimulation^1,2. It is important to have a reliable assay that reports the differential replenishment of these SV pools in order to understand 1) how SVs traffic after different modes of endocytosis (such as clathrin-dependent endocytosis and activity-dependent bulk endocytosis) and 2) the mechanisms controlling the mobilisation of both the RRP and RP in response to different stimuli.

FM dyes are routinely employed to quantitatively report SV turnover in central nerve terminals^3-8. They have a hydrophobic hydrocarbon tail that allows reversible partitioning in the lipid bilayer, and a hydrophilic head group that blocks passage across membranes. The dyes have little fluorescence in aqueous solution, but their quantum yield increases dramatically when partitioned in membrane⁹. Thus FM dyes are ideal fluorescent probes for tracking actively recycling SVs. The standard protocol for use of FM dye is as follows. First they are applied to neurons and are taken up during endocytosis (Figure 1). After non-internalised dye is washed away from the plasma membrane, recycled SVs redistribute within the recycling pool. These SVs are then depleted using unloading stimuli (Figure 1). Since FM dye labelling of SVs is quantal¹⁰, the resulting fluorescence drop is proportional to the amount of vesicles released. Thus, the recycling and fusion of SVs generated from the previous round of endocytosis can be reliably quantified.

Here, we present a protocol that has been modified to obtain two additional elements of information. Firstly, sequential unloading stimuli are used to differentially unload the RRP and the RP, to allow quantification of the replenishment of specific SV pools. Secondly, each nerve terminal undergoes the protocol twice. Thus, the response of the same nerve terminal at S1 can be compared against the presence of a test substance at phase S2 (Figure 2), providing an internal control. This is important, since the extent of SV recycling across different nerve terminals is highly variable¹¹.

Any adherent primary neuronal cultures may be used for this protocol, however the plating density, solutions and stimulation conditions are optimised for cerebellar granule neurons (CGNs)^12,13.

Specific mechanism of use-dependent channel block of calcium-permeable AMPA receptors provides activity-dependent inhibition of glutamatergic neurotransmission

This study examined the blocking action of the selective channel blocker of calcium-permeable (CP) AMPA receptors, N1-(1-phenylcyclohexyl)pentane-1,5-diaminium bromide (IEM-1925), on excitatory postsynaptic currents in rat neostriatal and cortical neurons and in fly neuromuscular junctions. In both preparations, the blocking of CP-AMPA receptor currents increased along with the stimulation frequency. The continuous presence of kainate, which activates AMPA receptors, in the external solution also caused an enhanced blocking effect. Likewise, decrease of the synaptic release by lowering calcium concentration resulted in significant reduction of the blocking action. The activity dependence of the block is explained using the guarded receptor model. The drug molecule can only bind if the channel is open. After the channel has closed, the drug molecule remains trapped inside. However, the trapped molecule slowly egresses from closed channels to the cytoplasm. The total block effect is determined by the equilibrium between accumulation of the drug in the open channels and relief from the closed channels. Therefore, the conditions that favour the open state result in enhanced inhibition. This significant finding reveals a new way to modulate CP-AMPAR-mediated transmission using a physiologically relevant approach. Moreover, it allows the involvement of CP-AMPARs in the physiological and pathological processes – such as high-frequency synaptic activity or increase of the steady-state glutamate concentration – to be examined.

Sensory pathways and emotional context for action in primate prefrontal cortex

Connections of the primate prefrontal cortex are associated with action. Within the lateral prefrontal cortex, there are preferential targets of projections from visual, auditory, and somatosensory cortices associated with directing attention to relevant stimuli and monitoring responses for specific tasks. Return pathways from lateral prefrontal areas to sensory association cortices suggest a role in selecting relevant stimuli and suppressing distracters to accomplish specific tasks. Projections from sensory association cortices to orbitofrontal cortex are more global than to lateral prefrontal areas, especially for posterior orbitofrontal cortex (pOFC), which is connected with sensory association cortices representing each sensory modality and with structures associated with the internal, or emotional, environment. A specialized projection from pOFC to the intercalated masses of the amygdala is poised to flexibly affect autonomic responses in emotional arousal or return to homeostasis. The amygdala projects to the magnocellular mediodorsal thalamic nucleus, which projects most robustly to pOFC among prefrontal cortices, suggesting sequential processing for emotions. The specialized connections of pOFC distinguish it as a separate orbitofrontal region that may function as the primary sensor of information for emotions. Lateral prefrontal areas 46 and 9 and the pOFC send widespread projections to the inhibitory thalamic reticular nucleus, suggesting a role in gating sensory and motivationally salient signals and suppressing distracters at an early stage of processing. Intrinsic connections link prefrontal areas, enabling synthesis of sensory information and emotional context for selective attention and action, in processes that are disrupted in psychiatric disorders, including attention-deficit/hyperactivity disorder.

RIM proteins activate vesicle priming by reversing autoinhibitory homodimerization of Munc13

At a synapse, the presynaptic active zone mediates synaptic vesicle exocytosis. RIM proteins are active zone scaffolding molecules that--among others--mediate vesicle priming and directly or indirectly interact with most other essential presynaptic proteins. In particular, the Zn²+ finger domain of RIMs binds to the C₂A domain of the priming factor Munc13, which forms a homodimer in the absence of RIM but a heterodimer with it. Here, we show that RIMs mediate vesicle priming not by coupling Munc13 to other active zone proteins as thought but by directly activating Munc13. Specifically, we found that the isolated Zn²+ finger domain of RIMs autonomously promoted vesicle priming by binding to Munc13, thereby relieving Munc13 homodimerization. Strikingly, constitutively monomeric mutants of Munc13 rescued priming in RIM-deficient synapses, whereas wild-type Munc13 did not. Both mutant and wild-type Munc13, however, rescued priming in Munc13-deficient synapses. Thus, homodimerization of Munc13 inhibits its priming function, and RIMs activate priming by disrupting Munc13 homodimerization.

Prefrontal pathways target excitatory and inhibitory systems in memory-related medial temporal cortices

The anterior cingulate cortex (ACC), situated in the caudal part of the medial prefrontal cortex, is involved in monitoring on-going behavior pertaining to memory of previously learned outcomes. How ACC information interacts with the medial temporal lobe (MTL) memory system is not well understood. The present study used a multitiered approach to address two questions on the interactions between the ACC and the parahippocampal cortices in the rhesus monkey: (1) What are the presynaptic characteristics of ACC projections to the parahippocampal cortices? (2) What are the postsynaptic targets of the pathway and are there laminar differences in innervation of local excitatory and inhibitory systems? Labeled ACC terminations were quantified in parahippocampal areas TH and TF and a cluster analysis showed that boutons varied in size, with a population of small (≤0.97 μm) and large (>0.97 μm) terminations that were nearly evenly distributed in the upper and deep layers. Exhaustive sampling as well as unbiased stereological techniques independently showed that small and large boutons were about evenly distributed within cortical layers in the parahippocampal cortex. Synaptic analysis of the pathway, performed at the electron microscope (EM), showed that while most of the ACC projections formed synapses with excitatory neurons, a significant proportion (23%) targeted presumed inhibitory classes with a preference for parvalbumin (PV+) inhibitory neurons. These findings suggest synaptic mechanisms that may help integrate signals associated with attention and memory.

Enduring medial perforant path short-term synaptic depression at high pressure

The high pressure neurological syndrome develops during deep-diving (>1.1 MPa) involving impairment of cognitive functions, alteration of synaptic transmission and increased excitability in cortico-hippocampal areas. The medial perforant path (MPP), connecting entorhinal cortex with the hippocampal formation, displays synaptic frequency-dependent-depression (FDD) under normal conditions. Synaptic FDD is essential for specific functions of various neuronal networks. We used rat cortico-hippocampal slices and computer simulations for studying the effects of pressure and its interaction with extracellular Ca²⁺ ([Ca²⁺]_o) on FDD at the MPP synapses. At atmospheric pressure, high [Ca²⁺]_o (4–6 mM) saturated single MPP field EPSP (fEPSP) and increased FDD in response to short trains at 50 Hz. High pressure (HP; 10.1 MPa) depressed single fEPSPs by 50%. Increasing [Ca²⁺]_o to 4 mM at HP saturated synaptic response at a subnormal level (only 20% recovery of single fEPSPs), but generated a FDD similar to atmospheric pressure. Mathematical model analysis of the fractions of synaptic resources used by each fEPSP during trains (normalized to their maximum) and the total fraction utilized within a train indicate that HP depresses synaptic activity also by reducing synaptic resources. This data suggest that MPP synapses may be modulated, in addition to depression of single events, by reduction of synaptic resources and then may have the ability to conserve their dynamic properties under different conditions.

GABAergic mechanism in the rostral ventrolateral medulla contributes to the hypotension of moxonidine

AIMS

The depressor action of the centrally antihypertensive drug moxonidine has been attributed to activation of I(1)-imidazoline receptor in the rostral ventrolateral medulla (RVLM). The objective of this study was to determine the role of the γ-aminobutyric acid (GABA) mechanisms in the RVLM in mediating the effect of moxonidine in anaesthetized normotensive rats.

METHODS AND RESULTS

The relationship between the effects of microinjection or picoinjection of moxonidine and the functional state of GABA receptors at the level of the RVLM or pre-sympathetic neuron was determined. Microdialysis was performed to detect the effect of moxonidine on the release of GABA in the RVLM. Western blot analysis was carried out to test the effect of chronic intracerebroventricular injection of moxonidine on the protein expression of GABA receptors in the RVLM. Pre-treatment with the GABA(A) or GABA(B) receptor antagonist bicuculline (5 pmol) or CGP35348 (200 pmol), respectively, microinjected into the RVLM significantly attenuated the decrease in blood pressure and renal sympathetic nerve activity induced by moxonidine. In 22 moxonidine-sensitive pre-sympathetic neurons in the RVLM, picoinjection of bicuculline (100 fmol/5 nL) significantly attenuated the neuronal inhibition evoked by moxonidine (100 pmol/5 nL). The release of GABA in the RVLM was increased after intravenous moxonidine (50 μg/kg). Central infusion of moxonidine upregulated the protein expression of both GABA(A) and GABA(B) receptors in the RVLM.

CONCLUSION

The current data demonstrate that GABAergic mechanisms in the RVLM are responsible for the hypotension and sympathoinhibition of moxonidine.

GSK-3 beta inhibits presynaptic vesicle exocytosis by phosphorylating P/Q-type calcium channel and interrupting SNARE complex formation

Glycogen synthase kinase-3 (GSK-3), a Ser/Thr protein kinase abundantly expressed in neurons, plays diverse functions in physiological and neurodegenerative conditions. Our recent study shows that upregulation of GSK-3 suppresses long-term potentiation and presynaptic release of glutamate; however, the underlying mechanism is elusive. Here, we show that activation of GSK-3beta retards the synaptic vesicle exocytosis in response to membrane depolarization. Using calcium imaging, whole-cell patch-clamp, as well as specific Ca(2+) channel inhibitors, we demonstrate that GSK-3beta phosphorylates the intracellular loop-connecting domains II and III (L(II-III)) of P/Q-type Ca(2+) channels, which leads to a decrease of intracellular Ca(2+) rise through the P/Q-type voltage-dependent calcium channel. To further illustrate the mechanisms of GSK-3beta's action, we show that activation of GSK-3beta interferes with the formation of the soluble N-ethylmaleimide-sensitive factor attachment protein (SNAP) receptor (SNARE) complex through: (1) weakening the association of synaptobrevin with SNAP25 and syntaxin; (2) reducing the interactions among the phosphorylated L(II-III) and synaptotagmin, SNAP25, and syntaxin; and (3) inhibiting dissociation of synaptobrevin from synaptophysin I. These results indicate that GSK-3beta negatively regulates synaptic vesicle fusion events via interfering with Ca(2+)-dependent SNARE complex formation.

Ca2+ dependence of the binomial parameters p and n at the mouse neuromuscular junction

The Ca(2+) dependence of synaptic quantal release is generally thought to be restricted to probability of vesicular release. However, some studies have suggested that the number of release sites (n) at the neuromuscular junction (NMJ) is also Ca(2+) dependent. In this study, we recorded endplate currents over a wide range of extracellular Ca(2+) concentrations and found the expected Ca(2+) dependency of release. A graphical technique was used to estimate p (probability of release) and n using standard binomial assumptions. The results suggested n was Ca(2+) dependent. The data were simulated using compound binomial statistics with variable n (Ca(2+) dependent) or fixed n (Ca(2+) independent). With fixed n, successful simulation of increasing Ca(2+) required that p increase abruptly at some sites from very low to high values. Successful simulation with variable n required the introduction of previously silent release sites (p = 0) with high values of p. Thus the success of both simulations required abrupt, large increases of p at a subset of release sites with initially low or zero p. Estimates of the time course of release obtained by deconvolving evoked endplate currents with average miniature endplate currents decreased slightly as Ca(2+) increased, thus arguing against sequential release of multiple quanta at higher Ca(2+) levels. Our results suggest that the apparent Ca(2+) dependence of n at the NMJ can be explained by an underlying Ca(2+) dependence of a spatially variable p such that p increases abruptly at a subset of sites as Ca(2+) is increased.

Multiquantal release underlies the distribution of synaptic efficacies in the neocortex

Inter-pyramidal synaptic connections are characterized by a wide range of EPSP amplitudes. Although repeatedly observed at different brain regions and across layers, little is known about the synaptic characteristics that contribute to this wide range. In particular, the range could potentially be accounted for by differences in all three parameters of the quantal model of synaptic transmission, i.e. the number of release sites, release probability and quantal size. Here, we present a rigorous statistical analysis of the transmission properties of excitatory synaptic connections between layer-5 pyramidal neurons of the somato-sensory cortex. Our central finding is that the EPSP amplitude is strongly correlated with the number of estimated release sites, but not with the release probability or quantal size. In addition, we found that the number of release sites can be more than an order of magnitude higher than the typical number of synaptic contacts for this type of connection. Our findings indicate that transmission at stronger synaptic connections is mediated by multiquantal release from their synaptic contacts. We propose that modulating the number of release sites could be an important mechanism in regulating neocortical synaptic transmission.

Calcium dependence of exo- and endocytotic coupling at a glutamatergic synapse

The mechanism coupling exocytosis and endocytosis remains to be elucidated at central synapses. Here, we show that the mechanism linking these two processes is dependent on microdomain-[Ca2+](i) similar to that which triggers exocytosis, as well as the exocytotic protein synaptobrevin/VAMP. Furthermore, block of endocytosis has a limited, retrograde action on exocytosis, delaying recruitment of release-ready vesicles and enhancing short-term depression. This effect sets in so rapidly that it cannot be explained by the nonavailability of recycled vesicles. Rather, we postulate that perturbation of a step linking exocytosis and endocytosis temporarily prevents new vesicles from docking at specialized sites for exocytosis.

Alterations in corticostriatal synaptic plasticity in mice overexpressing human alpha-synuclein

Most forms of Parkinson's disease (PD) are sporadic in nature, but some have genetic causes as first described for the alpha-synuclein gene. The alpha-synuclein protein also accumulates as insoluble aggregates in Lewy bodies in sporadic PD as well as in most inherited forms of PD. The focus of the present study is the modulation of synaptic plasticity in the corticostriatal pathway of transgenic (Tg) mice that overexpress the human alpha-synuclein protein throughout the brain (ASOTg). Paired-pulse facilitation was detected in vitro by activation of corticostriatal afferents in ASOTg mice, consistent with a presynaptic effect of elevated human alpha-synuclein. However basal synaptic transmission was unchanged in ASOTg, suggesting that human alpha-synuclein could impact paired-pulse facilitation via a presynaptic mechanism not directly related to the probability of neurotransmitter release. Mice lacking alpha-synuclein or those expressing normal and A53T human alpha-synuclein in tyrosine hydroxylase-containing neurons showed, instead, paired-pulse depression. High-frequency stimulation induced a presynaptic form of long-term depression solely in ASOTg striatum. A presynaptic, N-methyl-d-aspartate receptor-independent form of chemical long-term potentiation induced by forskolin (FSK) was enhanced in ASOTg striatum, while FSK-induced cAMP levels were reduced in ASOTg synaptoneurosome fractions. Overall the results suggest that elevated human alpha-synuclein alters presynaptic plasticity in the corticostriatal pathway, possibly reflecting a reduction in glutamate at corticostriatal synapses by modulation of adenylyl cyclase signaling pathways. ASOTg mice may recapitulate an early stage in PD during which overexpressed alpha-synuclein dampens corticostriatal synaptic transmission and reduces movement.

Developmental hearing loss disrupts synaptic inhibition: implications for auditory processing

Hearing loss during development leads to central deficits that persist even after the restoration of peripheral function. One key class of deficits is due to changes in central inhibitory synapses, which play a fundamental role in all aspects of auditory processing. This review focuses on the anatomical and physiological alterations of inhibitory connections at several regions within the central auditory pathway following hearing loss. Such aberrant inhibitory synaptic function may be linked to deficits in encoding binaural and spectral cues. Understanding the cellular changes that occur at inhibitory synapses following hearing loss may provide specific loci that can be targeted to improve function.

Probing synaptic vesicle fusion by altering mechanical properties of the neuronal surface membrane

Because synaptic vesicle exocytosis is a nano-mechanical process, it should be influenced by the mechanical properties of the cell membrane to which the vesicle fuses. By dissolving surfactants at various concentrations in the neuronal membrane, we have perturbed mechanical properties of the membrane and have found that dissolved surfactants lower the probability that a synaptic vesicle will open its fusion pore when the fusion machinery of the vesicle is activated by binding calcium. By using standard theories from the physics and chemistry of surfaces, we can account for this decrease in fusion probability and can infer that a vesicle, when activated, opens its fusion pore approximately 3 times out of 4 and that the area of the fusion pore is approximately 4 nm(2).

Two Ca(2+)-dependent steps controlling synaptic vesicle fusion and replenishment at the cerebellar basket cell terminal

Cerebellar basket cells inhibit postsynaptic Purkinje cells in a rapid and precise manner. To investigate the mechanisms of transmitter release underlying this rapid inhibition, Ca(2+) uncaging was employed to measure the intracellular Ca(2+) dependence of transmitter release and the kinetics of synaptic vesicle pool transitions in immature basket cell synapses at room temperature. Vesicle release properties distinct from those previously observed at excitatory synapses were seen, including a relatively high intracellular Ca(2+) sensitivity of vesicle fusion, rapid vesicle pool mobilization with few reluctant vesicles, and vesicle replenishment driven by unusually high Ca(2+) levels from both local and residual Ca(2+) sources during action potential trains. These results suggest that inhibitory basket cell synapses are optimized for rapid and precise temporal and spatial Ca(2+) coordination of synaptic vesicle fusion and replenishment, which may contribute to the unique physiology of inhibitory synaptic transmission, including phasic release during action potential trains and tonic release by residual intracellular Ca(2+).

The coupling between synaptic vesicles and Ca2+ channels determines fast neurotransmitter release

In order to release neurotransmitter synchronously in response to a presynaptic action potential, synaptic vesicles must be both release competent and located close to presynaptic Ca2+ channels. It has not been shown, however, which of the two is the more decisive factor. We tested this issue at the calyx of Held synapse by combining Ca2+ uncaging and electrophysiological measurements of postsynaptic responses. After depletion of the synaptic vesicles that are responsible for synchronous release during action potentials, uniform elevation of intracellular Ca2+ by Ca2+ uncaging could still elicit rapid release. The Ca2+ sensitivity of remaining vesicles was reduced no more than 2-fold, which is insufficient to explain the slow-down of the kinetics of release (10-fold) observed during a depolarizing pulse. We conclude that recruitment of synaptic vesicles to sites where Ca2+ channels cluster, rather than fusion competence, is a limiting step for rapid neurotransmitter release in response to presynaptic action potentials.

Accessory proteins stabilize the acceptor complex for synaptobrevin, the 1:1 syntaxin/SNAP-25 complex

Syntaxin/SNAP-25 interactions precede assembly of the ternary SNARE complex that is essential for neurotransmitter release. This binary complex has been difficult to characterize by bulk methods because of the prevalence of a 2:1 dead-end species. Here, using single-molecule fluorescence, we find the structure of the 1:1 syntaxin/SNAP-25 binary complex is variable, with states changing on the second timescale. One state corresponds to a parallel three-helix bundle, whereas other states show one of the SNAP-25 SNARE domains dissociated. Adding synaptobrevin suppresses the dissociated helix states. Remarkably, upon addition of complexin, Munc13, Munc18, or synaptotagmin, a similar effect is observed. Thus, the 1:1 binary complex is a dynamic acceptor for synaptobrevin binding, and accessory proteins stabilize this acceptor. In the cellular environment the binary complex is actively maintained in a configuration where it can rapidly interact with synaptobrevin, so formation is not likely a limiting step for neurotransmitter release.

Fully automated microchip system for the detection of quantal exocytosis from single and small ensembles of cells

A lab-on-a-chip device that enables positioning of single or small ensembles of cells on an aperture in close proximity to a mercaptopropionic acid (MPA) modified sensing electrode has been developed and characterized. The microchip was used for the detection of Ca(2+)-dependent quantal catecholamine exocytosis from single as well as small assemblies of rat pheochromocytoma (PC12) cells. The frequency of events increased considerably upon depolarization of the PC12 cell membrane using a high extracelluar concentration of potassium. The number of recorded events could be correlated with the number of cells immobilized on the electrode. Quantal characteristics, such as the number of released molecules per recorded event, are equivalent to data obtained using conventional carbon fiber microelectrodes. The detection sensitivity of the device allows for the detection of less than 10 000 dopamine molecules in a quantal release. The distribution of peak rise-time and full width at half maximum was constant during measurement periods of several minutes demonstrating the stability of the MPA modified surface.

Making quantal analysis more convenient, fast, and accurate: user-friendly software QUANTAN

Quantal analysis of synaptic transmission is an important tool for understanding the mechanisms of synaptic plasticity and synaptic regulation. Although several custom-made and commercial algorithms have been created for the analysis of spontaneous synaptic activity, software for the analysis of action potential evoked release remains very limited. The present paper describes a user-friendly software package QUANTAN which has been created to analyze electrical recordings of postsynaptic responses. The program package is written using Borland C++ under Windows platform. QUANTAN employs and compares several algorithms to extract the average quantal content of synaptic responses, including direct quantal counts, the analysis of synaptic amplitudes, and the analysis of integrated current traces. The integration of several methods in one user-friendly program package makes quantal analysis of action potential evoked release more reliable and accurate. To evaluate the variability in quantal content, QUANTAN performs deconvolution of the distributions of amplitudes or areas of synaptic responses employing a ridge regression method. Other capabilities of QUANTAN include the analysis of the time-course and stationarity of quantal release. In summary, QUANTAN uses digital records of synaptic responses as an input and computes the distribution of quantal content and synaptic parameters. QUANTAN is freely available to other scholars over the internet.

Paired-pulse ratio of synaptically induced transporter currents at hippocampal CA1 synapses is not related to release probability

When a synapse is stimulated in rapid succession, the second post-synaptic response can be larger than the first and termed paired-pulse facilitation. It has been reported that the paired-pulse ratio (PPR), which is the ratio of the amplitude of the second response to that of the first, depends on the probability of vesicular release at the synapse, and PPR has been used as an easy measure of the release probability. To re-examine the relation of PPR with transmitter release probability, we made whole-cell recordings from astrocytes and pyramidal neurons in the CA1 area of rat hippocampal slices, and studied responses evoked by paired-pulse stimulus of the Schaffer collaterals. In a control condition in which blockers for ionotropic glutamate receptors were added to the artificial cerebrospinal fluid, synaptically induced transporter currents (STCs) recorded from astrocytes showed PPF with similar dependency on stimulus interval as the AMPA-receptor-mediated excitatory post-synaptic currents (AMPA-EPSCs) recorded from pyramidal neurons. When the transmitter release was enhanced by raising Ca2+ concentration in the bathing medium or by applying 8-CPT, an adenosine A1 receptor antagonist, the PPR of the neuronal AMPA-EPSCs decreased significantly. In the same condition, although the amplitude of STCs was significantly increased, the PPR of STCs did not show significant change. The PPR of AMPA-EPSCs, however, recovered by lowering the stimulus intensity or by applying low concentration of NBQX, a competitive antagonist for AMPA-receptor. These results imply that the PPR of transmitter release at Schaffer collateral synapses stays constant as the release probability was altered.

Neuronal somatic ATP release triggers neuron-satellite glial cell communication in dorsal root ganglia

It has been generally assumed that the cell body (soma) of a neuron, which contains the nucleus, is mainly responsible for synthesis of macromolecules and has a limited role in cell-to-cell communication. Using sniffer patch recordings, we show here that electrical stimulation of dorsal root ganglion (DRG) neurons elicits robust vesicular ATP release from their somata. The rate of release events increases with the frequency of nerve stimulation; external Ca(2+) entry is required for the release. FM1-43 photoconversion analysis further reveals that small clear vesicles participate in exocytosis. In addition, the released ATP activates P2X7 receptors in satellite cells that enwrap each DRG neuron and triggers the communication between neuronal somata and glial cells. Blocking L-type Ca(2+) channels completely eliminates the neuron-glia communication. We further show that activation of P2X7 receptors can lead to the release of tumor necrosis factor-alpha (TNFalpha) from satellite cells. TNFalpha in turn potentiates the P2X3 receptor-mediated responses and increases the excitability of DRG neurons. This study provides strong evidence that somata of DRG neurons actively release transmitters and play a crucial role in bidirectional communication between neurons and surrounding satellite glial cells. These results also suggest that, contrary to the conventional view, neuronal somata have a significant role in cell-cell signaling.

Ryanodine receptor-transmitter release site coupling increases quantal size in a synapse-specific manner

The mechanisms by which presynaptic neurones differentially regulate synaptic transmission with multiple postsynaptic targets in the brain are not fully understood. Using intracellular sharp electrode and whole-cell voltage-clamp recordings of soma-soma synapses between identified Lymnaea neurones, we provide direct evidence that quantal size is regulated presynaptically through the coupling of multiple release sites. This coupling effectively multiplies quantal size, thereby providing significant influence over parameters of synaptic transmission that are influenced by quantal size, such as the variance in transmitter release at stationary release probabilities. Variation in the degree of coupling is dependent on the identity of the postsynaptic cell, even though the variation in quantal size is of presynaptic origin. We have therefore demonstrated the presence of a novel mechanism by which presynaptic neurones may differentially regulate quantal size at select synaptic connections, in turn providing them with a means of regulating synaptic transmission with multiple postsynaptic cells.

A cell-semiconductor synapse: transistor recording of vesicle release in chromaffin cells

The release of dense-core vesicles in bovine chromaffin cells is a model for the presynaptic process in neurons. It is usually studied by microamperometry of catecholamines with carbon fibers. Here we introduce transistor recording as a tool to study vesicle release. When we stimulate a chromaffin cell placed on a field-effect transistor, the gate voltage exhibits peaks that correlate with a simultaneously performed amperometric recording. We attribute the transistor signal to a release of protons from the extruded matrix of vesicles that lowers the extracellular pH and changes the electrical surface potential of the gate oxide. The rise time of the transistor signals is similar to that of amperometric responses, whereas their duration is distinctly longer. In a model computation, the rise time is identified with the extrusion of vesicle matrix into the narrow extracellular space between cell and gate oxide, and the decay time is attributed to pH equilibration through slow diffusion in the extruded matrix. Because the transistor recording relies on protons, it can be applied to acidic vesicles with electrochemically inactive hormones or transmitters.

A neuroinductive biomaterial based on dopamine

Chemical messengers such as neurotransmitters play an important role in cell communication, differentiation, and survival. We have designed and synthesized a bioactive biomaterial that derived its biological activity from dopamine. The resultant biodegradable polymer, PCD, has pendent groups bearing dopamine functionalities. Image analysis demonstrated that nerve growth factor-primed rat pheochromocytoma cells (PC12) and explanted rat dorsal root ganglions attached well and displayed substantial neurite outgrowth on the polymer surface. Furthermore, PCD promoted more vigorous neurite outgrowth in PC12 cells than tissue culture polystyrene, laminin, and poly(d-lysine). The histogram of neurite length of PC12 cells showed distinctive patterns on PCD that were absent on the controls. A subset of PC12 cells displayed high filopodium density on PCD. The addition of dopamine in culture medium had little effect on the differentiation of PC12 cells on tissue culture polystyrene. Tyrosine, the precursor of dopamine, did not exhibit this ability to impart specific bioactivity to an analogous polymer. Thus, the dopamine functional group is likely the origin of the inductive effect. PCD did not cause nerve degeneration or fibrous encapsulation when implanted immediately adjacent to the rat sciatic nerves. This work is a step toward creating a diverse family of bioactive materials using small chemical messengers as monomers.

Bayesian inference with probabilistic population codes

Recent psychophysical experiments indicate that humans perform near-optimal Bayesian inference in a wide variety of tasks, ranging from cue integration to decision making to motor control. This implies that neurons both represent probability distributions and combine those distributions according to a close approximation to Bayes' rule. At first sight, it would seem that the high variability in the responses of cortical neurons would make it difficult to implement such optimal statistical inference in cortical circuits. We argue that, in fact, this variability implies that populations of neurons automatically represent probability distributions over the stimulus, a type of code we call probabilistic population codes. Moreover, we demonstrate that the Poisson-like variability observed in cortex reduces a broad class of Bayesian inference to simple linear combinations of populations of neural activity. These results hold for arbitrary probability distributions over the stimulus, for tuning curves of arbitrary shape and for realistic neuronal variability.

The role of extracellular calcium in exo- and endocytosis of synaptic vesicles at the frog motor nerve terminals

In the present study we combined FM 1-43 imaging and electrophysiological recording of miniature end-plate currents (MEPCs) to determine the role of extracellular calcium in synaptic vesicle exo- and endocytosis at the frog motor nerve terminals. We replaced extracellular Ca2+ ions with other bivalent cations (Sr2+, Ba2+, Cd2+, Mg2+) or used a calcium-free solution and monitored fluorescent staining of the nerve terminals in the presence of caffeine, which promotes the release of Ca2+ from intracellular stores. Caffeine has induced FM1-43 internalization only in the presence of bivalent cations in the external solution. The exposure of the neuromuscular junction to caffeine in a calcium-free solution caused a reversible failure of FM 1-43 loading and an increase in the nerve terminal width. This effect of a calcium-free solution was not due to a decrease in exocytosis, because caffeine-induced FM1-43 unloading from the previously loaded nerve terminals, as well as a degree of the MEPCs frequency increase, was unchanged. We conclude that the presence of Ca2+ or other bivalent cations in extracellular space is necessary for endocytosis but not for exocytosis of synaptic vesicles, while transmitter release is promoted by efflux of Ca2+ from intracellular stores. The effect of extracellular Ca2+ on endocytosis might be driven by the non-specific interactions with membrane lipids.

Synaptic vesicle protein 2 enhances release probability at quiescent synapses

We report a thorough analysis of neurotransmission in cultured hippocampal neurons lacking synaptic vesicle protein 2 (SV2), a membrane glycoprotein present in all vesicles that undergo regulated secretion. We found that SV2 selectively enhances low-frequency neurotransmission by priming morphologically docked vesicles. Loss of SV2 reduced initial release probability during a train of action potentials but had no effect on steady-state responses. The amount and decay rate of asynchronous release, two measures sensitive to presynaptic calcium concentrations, are not altered in SV2 knock-outs, suggesting that SV2 does not act by modulating presynaptic calcium. Normal neurotransmission could be temporarily recovered by delivering an exhaustive stimulus train. Our results indicate that SV2 primes vesicles in quiescent neurons and that SV2 function can be bypassed by an activity-dependent priming mechanism. We propose that SV2 action modulates synaptic networks by ensuring that low-frequency neurotransmission is faithfully conveyed.

Calcium sensitivity of neurotransmitter release differs at phasic and tonic synapses

The efficacy of synaptic transmission varies greatly among synaptic contacts. We have explored the origins of differences between phasic and tonic crustacean neuromuscular junctions. Synaptic boutons of a phasic motor neuron release three orders of magnitude more quanta to a single action potential and show strong depression to a train, whereas tonic synapses are nearly unresponsive to single action potentials and display an immense facilitation. Phasic and tonic synapses display a similar nonlinear dependence on extracellular [Ca2+]. We imposed similar spatially uniform intracellular [Ca2+] ([Ca2+]i) steps in phasic and tonic synapses by photolysis of presynaptic caged calcium. [Ca2+]i was measured fluorometrically while transmitter release was monitored electrophysiologically from single boutons in which the [Ca2+]i was elevated. Phasic synapses released the readily releasable pool (RRP) of vesicles at a much higher rate and with a shorter delay than did tonic synapses. Comparison of several kinetic models of molecular events showed that a difference in Ca2+-sensitive priming of vesicles in the RRP combined with a revision of the kinetic Ca2+-binding sequence to the secretory trigger produced the best fit to the markedly different responses to Ca2+ steps and action potentials and of the characteristic features of synaptic plasticity in phasic and tonic synapses. The results reveal processes underlying one aspect of synaptic diversity that may also regulate changes in synaptic strength during development and learning and memory formation.

Acrylamide inhibits dopamine uptake in rat striatal synaptic vesicles

Evidence suggests that acrylamide (ACR) neurotoxicity is mediated by decreased presynaptic neurotransmitter release. Defective release might involve disruption of neurotransmitter storage, and therefore, we determined the effects of in vivo and in vitro ACR exposure on 3H-dopamine (DA) transport into rat striatal synaptic vesicles. Results showed that vesicular DA uptake was decreased significantly in rats intoxicated at either 50 mg/kg/day x 5 days or 21 mg/kg/day x 21 days. ACR intoxication also was accompanied by a reduction in KCl-evoked synaptosomal DA release, although consistent changes in presynaptic membrane transport were not observed. Silver stain and immunoblot analyses suggested that reduced vesicular uptake was not due to active nerve terminal degeneration or to a reduction in the synaptic vesicle content of isolated striatal synaptosomes. Nor did the in vivo presynaptic effects of ACR involve changes in synaptosomal glutathione concentrations. In vitro exposure of striatal vesicles showed that both ACR and two sulfhydryl reagents, N-ethylmaleimide (NEM) and iodoacetic acid (IAA), produced concentration-dependent decreases in 3H-DA uptake. Although ACR was significantly less potent than either NEM or IAA, all three chemicals caused comparable maximal inhibitions of vesicular uptake. Kinetic analysis of DA uptake showed that in vitro exposure to either ACR or NEM decreased V(max) and increased K(m). Determination of radiolabel efflux from 3H-DA-loaded vesicles indicated that in vitro ACR did not affect neurotransmitter retention. These data suggest that ACR impaired neurotransmitter uptake into striatal synaptic vesicles, possibly by interacting with sulfhydryl groups on functionally relevant proteins. The resulting disruption of neurotransmitter storage might mediate defective presynaptic release.

The actin cytoskeleton: integrating form and function at the synapse

Synapses are highly specialized intercellular junctions that mediate the transmission of information between axons and target cells. A fundamental property of synapses is their ability to modify the efficacy of synaptic communication through various forms of synaptic plasticity. Recent developments in imaging techniques have revealed that synapses exhibit a high degree of morphological plasticity under basal conditions and also in response to neuronal activity that induces alterations in synaptic strength. The underlying molecular basis for this morphological plasticity has attracted much attention, yet its functional significance to the mechanisms of synaptic transmission and synaptic plasticity remains elusive. These morphological changes ultimately require the dynamic actin cytoskeleton, which is the major structural component of synapses. Delineating the physiological roles of the actin cytoskeleton in supporting synaptic transmission and synaptic plasticity, therefore, paves the way for gaining molecular insights into when and how synaptic machineries couple synapse form and function.

Transmission between type II hair cells and bouton afferents in the turtle posterior crista

Synaptic activity was recorded with sharp microelectrodes during rest and during 0.3-Hz sinusoidal stimulation from bouton afferents identified by their efferent-mediated inhibitory responses. A glutamate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) decreased quantal size (qsize) while lowering external Ca(2+) decreased quantal rate (qrate). Miniature excitatory postsynaptic potentials (mEPSPs) had effective durations (qdur) of 3.5-5 ms. Their timing was consistent with Poisson statistics. Mean qsizes ranged in different units from 0.25 to 0.73 mV and mean qrates from 200 to 1,500/s; there was an inverse relation across the afferent population between qrate and qsize. qsize distributions were consistent with the independent release of variable-sized quanta. Channel noise, measured during AMPA-induced depolarizations, was small compared with quantal noise. Excitatory responses were larger than inhibitory responses. Peak qrates, which could approach 3,000/s, led peak excitatory mechanical stimulation by 40 degrees . Quantal parameters varied with stimulation phase with qdur and qsize being maximal during inhibitory stimulation. Voltage modulation (vmod) was in phase with qrate and had a peak depolarization of 1.5-3 mV. On average, 80% of vmod was accounted for by quantal activity; the remaining 20% was a nonquantal component that persisted in the absence of quantal activity. The extracellular accumulation of glutamate and K(+) are potential sources of nonquantal transmission and may provide a basis for the inverse relation between qrate and qsize. Comparison of the phases of synaptic and spike activity suggests that both presynaptic and postsynaptic mechanisms contribute to variations across afferents in the timing of spikes during sinusoidal stimulation.

Synaptic distinction of laminar-specific prefrontal-temporal pathways in primates

Prefrontal pathways exert diverse effects in widespread cortical areas, issuing projections both to the middle layers and to layer I, which are anatomically and functionally distinct. Here we addressed the still unanswered question of whether cortical pathways that terminate in different layers are distinct at the synaptic level. We addressed this issue using as a model system the robust and functionally significant pathways from prefrontal areas 10 and 32 to superior temporal areas in rhesus monkeys. Boutons from prefrontal axons synapsing in the middle layers of superior temporal cortex were significantly larger than boutons synapsing in layer I. Most synapses were on spines in both layers, which are found on dendrites of excitatory neurons. The less prevalent synapses on smooth dendrites, characteristic of inhibitory interneurons, were more common in the middle cortical layers than in layer I. Bouton volume was linearly related to vesicular and mitochondrial content in both layers, though a subset of small boutons, found mostly in layer I, contained no mitochondria. The systematic laminar-specific presynaptic differences in stable cortical synapses in adult primates were independent of their origin in the functionally distinct prefrontal areas 10 and 32, or their destination in architectonically distinct superior temporal areas. This synaptic distinction suggests differences in efficacy of synaptic transmission and metabolic demands in laminar-specific pathways that may be selectively recruited in behavior.

Quantal size is independent of the release probability at hippocampal excitatory synapses

Short-term synaptic plasticity changes the reliability of transmission during repetitive activation and allows different neuronal ensembles to encode distinct features of action potential trains. Identifying the mechanisms and the locus of expression of such plasticity is essential for understanding neuronal information processing. To determine the quantal parameters and the locus of alterations during short-term plasticity of cortical glutamatergic synapses, EPSCs were evoked in hippocampal oriens-alveus interneurons by CA1 pyramidal cells. The robust short-term facilitation of this connection allowed us to examine the transmission under functionally relevant but widely different release probability (P(r)) conditions. Paired whole-cell recordings permitted the functional and post hoc morphological characterization of the synapses. To determine the quantal size (q), the P(r), and the number of functional release sites (N(F)), two independent quantal analysis methods were used. Light and electron microscopy were performed to identify the number of synaptic junctions (N(EM)) between the recorded cells. The mean number of functional release sites (N(F(f)) = 2.9 +/- 0.4; n = 8) as inferred from a simple binomial model with no quantal variance agreed well with the mean of N(EM) (2.8 +/- 0.8; n = 6), but N(F(f)) never matched N(EM) when they were compared in individual pairs; however, including quantal variance in the model improved the functional prediction of the structural data. Furthermore, an increased P(r) (4.8 +/- 0.8-fold) fully accounted for the marked short-term facilitation of EPSCs (5.0 +/- 0.7-fold), and q was independent of P(r). Our results are consistent with the "one-release site, one-vesicle" hypothesis.

Dynamic mobility of functional GABAA receptors at inhibitory synapses

Importing functional GABAA receptors into synapses is fundamental for establishing and maintaining inhibitory transmission and for controlling neuronal excitability. By introducing a binding site for an irreversible inhibitor into the GABAA receptor alpha1 subunit channel lining region that can be accessed only when the receptor is activated, we have determined the dynamics of receptor mobility between synaptic and extrasynaptic locations in hippocampal pyramidal neurons. We demonstrate that the cell surface GABAA receptor population shows no fast recovery after irreversible inhibition. In contrast, after selective inhibition, the synaptic receptor population rapidly recovers by the import of new functional entities within minutes. The trafficking pathways that promote rapid importation of synaptic receptors do not involve insertion from intracellular pools, but reflect receptor diffusion within the plane of the membrane. This process offers the synapse a rapid mechanism to replenish functional GABAA receptors at inhibitory synapses and a means to control synaptic efficacy.

Indeterminacy in brain and behavior

The central goal of modern science that evolved during the Enlightenment was the empirical reduction of uncertainty by experimental inquiry. Although there have been challenges to this view in the physical sciences, where profoundly indeterminate events have been identified at the quantum level, the presumption that physical phenomena are fundamentally determinate seems to have defined modern behavioral science. Programs like those of the classical behaviorists, for example, were explicitly anchored to a fully deterministic worldview, and this anchoring clearly influenced the experiments that those scientists chose to perform. Recent advances in the psychological, social, and neural sciences, however, have caused a number of scholars to begin to question the assumption that all of behavior can be regarded as fundamentally deterministic in character. Although it is not yet clear whether the generative mechanisms for human and animal behavior will require a philosophically indeterminate approach, it is clear that behavioral scientists of all kinds are beginning to engage the issues of indeterminacy that plagued physics at the beginning of the twentieth century.

Synaptic vesicle cycling at type-identified diaphragm neuromuscular junctions

Differences in neuromuscular transmission and neuromuscular junction morphology exist across muscle fiber types. We hypothesized that these fiber-type differences are reflected in the size of the cycling synaptic vesicle pool. Synaptic vesicle cycling at type-identified rat diaphragm neuromuscular junctions was examined by fluorescently labeling presynaptic vesicles with FM4-64. We found that FM4-64 fluorescence uptake was higher at presynaptic terminals of type I/IIa fibers than type IIx/IIb fibers. However, no fiber-type differences in the rate of FM4-64 destaining were found with repetitive nerve stimulation. Synaptic vesicle density at active zones was examined by transmission electron microscopy. In accordance with FM4-64 uptake, synaptic vesicle density was greater at type I/IIa than IIx/IIb fibers. These results demonstrate differences in synaptic vesicle cycling across diaphragm muscle fiber types, which may underlie previously observed differences in neuromuscular transmission across diaphragm muscle fiber types. In the diaphragm, motor units comprising type I and type IIa fibers are most frequently recruited with a duty cycle of approximately 40%. Motor units comprising IIx/IIb fibers are infrequently recruited and only for short durations. The capacity for synaptic vesicle release and cycling at different muscle fiber types matches the functional requirements of these motor units. If the demand for recruitment of motor units comprising IIx/IIb fibers increases, for example, with mechanical loading, there is an increased risk for neuromuscular transmission failure that my relate to the capacity for synaptic vesicle release and cycling. Muscle fiber type-specific adaptations should be considered when examining neuromuscular disorders.

The unique inhibitory potentials in motoneurons that occur during active sleep are comprised of minimal unitary potentials

Loss of muscle tone during active (rapid-eye-movement, REM) sleep is due to the inhibition of motoneurons. This inhibition is manifest in high-gain intracellular electrophysiological records as hyperpolarizing synaptic noise, which includes large amplitude active sleep-specific inhibitory postsynaptic potentials (IPSPs). We report here evidence that the large active sleep-specific IPSPs are comprised of a small number of minimal unitary potentials that are characterized by fast rise-times (10-90% rise-times < or = 0.75 ms); they are present in high-gain records during quiet sleep or during active sleep where they are intermingled with larger active sleep-specific IPSPs with 10-90% rise-times > or = 1.00 ms and amplitudes that are integer multiples of the minimal unitary potentials. In hypoglossal motoneurons, the amplitude of these minimal unitary potentials averaged 0.33 +/- 0.04 mV (mean +/- S.D., n = 6). It is concluded that the large IPSPs with slow rise-times that are observed in motoneurons during active sleep are due to the nearly simultaneous arrival of multiple (< or = 5) minimal unitary potentials. We hypothesize that the same inhibitory interneurons that produce small IPSPs with fast rise-times during quiet sleep are also responsible for the large amplitude active sleep-specific IPSPs.