Background calcium induced by subthreshold depolarization modifies homosynaptic facilitation at a synapse in Aplysia.

Some synapses show two forms of short-term plasticity, homosynaptic facilitation, and a plasticity in which the efficacy of transmission is modified by subthreshold changes in the holding potential of the presynaptic neuron. In a previous study we demonstrated a further interactive effect. We showed that depolarizing changes in the presynaptic holding potential can increase the rate at which facilitation occurs. These experiments studied synaptic transmission between an Aplysia sensory neuron (B21) and its postsynaptic follower, the motor neuron (B8). We have also shown that subthreshold depolarizations of B21 produce widespread increases in its [Ca2+]i via activation of a nifedipine-sensitive current. To determine whether it is this change in 'background' calcium that modifies synaptic transmission we compared the facilitation observed at the B21-B8 synapse under control conditions to the facilitation observed in nifedipine. Nifedipine had a depressing effect. Other investigators studying facilitation have focused on Cares (i.e., the calcium that remains in a neuron after spiking). Our results indicate that facilitation can also be impacted by calcium channels opened before spiking begins.

Activity-dependent increases in [Ca2+]i contribute to digital-analog plasticity at a molluscan synapse.

In a type of short-term plasticity that is observed in a number of systems, synaptic transmission is potentiated by depolarizing changes in the membrane potential of the presynaptic neuron before spike initiation. This digital-analog form of plasticity is graded. The more depolarized the neuron, the greater the increase in the efficacy of synaptic transmission. In a number of systems, including the system presently under investigation, this type of modulation is calcium dependent, and its graded nature is presumably a consequence of a direct relationship between the intracellular calcium concentration ([Ca2+]i) and the effect on synaptic transmission. It is therefore of interest to identify factors that determine the magnitude of this type of calcium signal. We studied a synapse in Aplysia and demonstrate that there can be a contribution from currents activated during spiking. When neurons spike, there are localized increases in [Ca2+]i that directly trigger neurotransmitter release. Additionally, spiking can lead to global increases in [Ca2+]i that are reminiscent of those induced by subthreshold depolarization. We demonstrate that these spike-induced increases in [Ca2+]i result from the activation of a current not activated by subthreshold depolarization. Importantly, they decay with a relatively slow time constant. Consequently, with repeated spiking, even at a low frequency, they readily summate to become larger than increases in [Ca2+]i induced by subthreshold depolarization alone. When this occurs, global increases in [Ca2+]i induced by spiking play the predominant role in determining the efficacy of synaptic transmission.NEW & NOTEWORTHY We demonstrate that spiking can induce global increases in the intracellular calcium concentration ([Ca2+]i) that decay with a relatively long time constant. Consequently, summation of the calcium signal occurs even at low firing frequencies. As a result there is significant, persistent potentiation of synaptic transmission.

A New Method to Assess Asymmetry in Fingerprints Could Be Used as an Early Indicator of Type 2 Diabetes Mellitus.

BACKGROUND: Inexpensive screening tools are needed to identify individuals predisposed to developing diabetes mellitus (DM). Such early identification coupled with an effective intervention could help many people avoid the substantial health costs of this disease. We investigated the hypothesis that fluctuating asymmetry (FA) in fingerprints is an indicator of type 2 diabetes mellitus (T2DM). METHODS: Participants with T2DM, with T1DM, and without any indication or known family history of diabetes were fingerprinted with a Crossmatch Verifier 320 LC scanner. Asymmetry scores for each finger pair were assessed using both pattern analysis (ridge counts), and a wavelet-based analysis. RESULTS: Both methods for scoring asymmetry predicted risk of T2DM for finger pair IV, controlling for gender and age. AUC scores were significantly greater than the null for pattern asymmetry scores (finger IV AUC = 0.74), and wavelet asymmetry scores for finger pair IV (AUC = 0.73) and finger pair V (AUC = 0.73), for predicting T2DM. In addition, wavelet asymmetry scores for finger pair IV (AUC = 0.80) and finger pair V (AUC = 0.85) significantly predicted risk of T1DM. CONCLUSIONS: A diagnostic tool based on FA in the fingerprints of finger pair IV, measured using a wavelet analysis could be developed for predicting risk prior to associated health problems for both T2DM and T1DM. In addition, given that that the prints for fingers IV and V develop during the 14-17 weeks of gestation, we predict that interventions during this time period of pregnancy will be most successful.

Monitoring changes in the intracellular calcium concentration and synaptic efficacy in the mollusc Aplysia.

It has been suggested that changes in intracellular calcium mediate the induction of a number of important forms of synaptic plasticity (e.g., homosynaptic facilitation). These hypotheses can be tested by simultaneously monitoring changes in intracellular calcium and alterations in synaptic efficacy. We demonstrate how this can be accomplished by combining calcium imaging with intracellular recording techniques. Our experiments are conducted in a buccal ganglion of the mollusc Aplysia californica. This preparation has a number of experimentally advantageous features: Ganglia can be easily removed from Aplysia and experiments use adult neurons that make normal synaptic connections and have a normal ion channel distribution. Due to the low metabolic rate of the animal and the relatively low temperatures (14-16 °C) that are natural for Aplysia, preparations are stable for long periods of time. To detect changes in intracellular free calcium we will use the cell impermeant version of Calcium Orange which is easily 'loaded' into a neuron via iontophoresis. When this long wavelength fluorescent dye binds to calcium, fluorescence intensity increases. Calcium Orange has fast kinetic properties and, unlike ratiometric dyes (e.g., Fura 2), requires no filter wheel for imaging. It is fairly photo stable and less phototoxic than other dyes (e.g., fluo-3). Like all non-ratiometric dyes, Calcium Orange indicates relative changes in calcium concentration. But, because it is not possible to account for changes in dye concentration due to loading and diffusion, it can not be calibrated to provide absolute calcium concentrations. An upright, fixed stage, compound microscope was used to image neurons with a CCD camera capable of recording around 30 frames per second. In Aplysia this temporal resolution is more than adequate to detect even a single spike induced alteration in the intracellular calcium concentration. Sharp electrodes are simultaneously used to induce and record synaptic transmission in identified pre- and postsynaptic neurons. At the conclusion of each trial, a custom script combines electrophysiology and imaging data. To ensure proper synchronization we use a light pulse from a LED mounted in the camera port of the microscope. Manipulation of presynaptic calcium levels (e.g. via intracellular EGTA injection) allows us to test specific hypotheses, concerning the role of intracellular calcium in mediating various forms of plasticity.

Effect of holding potential on the dynamics of homosynaptic facilitation.

We study a form of short-term synaptic plasticity that was originally described as a graded potentiating effect of holding potential on spike-mediated synaptic transmission (Shimahara and Tauc, 1975). This form of plasticity has recently generated considerable interest, as it has become apparent that it is present in the mammalian brain (Clark and Häusser, 2006; Marder, 2006). It has been suggested that it adds a previously unappreciated analog component to spike-mediated synaptic transmission (Alle and Geiger, 2006, 2008). A limitation of most previous research in this area is that effects of holding potential have been studied in relative isolation. Presynaptic neurons are stimulated at low frequencies so that a second form of plasticity (homosynaptic facilitation) is not induced. Under physiological conditions, however, both forms of plasticity are likely to be coinduced. In this report, we study the two types of plasticity together in an experimentally advantageous preparation (the mollusk Aplysia californica). Somewhat surprisingly, we find that effects of holding potential can be relatively modest when presynaptic neurons are activated at low frequencies. Interestingly, however, changes in membrane potential are highly effective when homosynaptic facilitation is induced. In this situation, PSPs facilitate at an increased rate. To summarize, our research suggests a novel view of the effect of holding potential on synaptic transmission. We propose that, under physiological conditions, it modifies the dynamics of homosynaptic facilitation.

Effect of presynaptic membrane potential on electrical vs. chemical synaptic transmission.

The growing realization that electrical coupling is present in the mammalian brain has sparked renewed interest in determining its functional significance and contrasting it with chemical transmission. One question of interest is whether the two types of transmission can be selectively regulated, e.g., if a cell makes both types of connections can electrical transmission occur in the absence of chemical transmission? We explore this issue in an experimentally advantageous preparation. B21, the neuron we study, is an Aplysia sensory neuron involved in feeding that makes electrical and chemical connections with other identified cells. Previously we demonstrated that chemical synaptic transmission is membrane potential dependent. It occurs when B21 is centrally depolarized prior to and during peripheral activation, but does not occur if B21 is peripherally activated at its resting membrane potential. In this article we study effects of membrane potential on electrical transmission. We demonstrate that maximal potentiation occurs in different voltage ranges for the two types of transmission, with potentiation of electrical transmission occurring at more hyperpolarized potentials (i.e., requiring less central depolarization). Furthermore, we describe a physiologically relevant type of stimulus that induces both spiking and an envelope of depolarization in the somatic region of B21. This depolarization does not induce functional chemical synaptic transmission but is comparable to the depolarization needed to maximally potentiate electrical transmission. In this study we therefore characterize a situation in which electrical and chemical transmission can be selectively controlled by membrane potential.

Motor outputs in a multitasking network: relative contributions of inputs and experience-dependent network states.

Network outputs elicited by a specific stimulus may differ radically depending on the momentary network state. One class of networks states-experience-dependent states-is known to operate in numerous networks, yet the fundamental question concerning the relative role that inputs and states play in determining the network outputs remains to be investigated in a behaviorally relevant manner. Because previous work indicated that in the isolated nervous system the motor outputs of the Aplysia feeding network are affected by experience-dependent states, we sought to establish the behavioral relevance of these outputs. We analyzed the phasing of firing of radula opening motoneurons (B44 and B48) relative to other previously characterized motoneurons. We found that the overall pattern of motoneuronal firing corresponds to the phasing of movements during feeding behavior, thus indicating a behavioral relevance of network outputs. Previous studies suggested that network inputs act to trigger a response rather than to shape its characteristics, with the latter function being fulfilled by network states. We show this is an oversimplification. In a rested state, different inputs elicited distinct responses, indicating that inputs not only trigger but also shape the responses. However, depending on the combination of inputs and states, responses were either dramatically altered by the network state or were indistinguishable from those observed in the rested state. We suggest that the relative contributions of inputs and states are dynamically regulated and, rather than being fixed, depend on the specifics of states and inputs.

Two distinct mechanisms mediate potentiating effects of depolarization on synaptic transmission.

Two distinct mechanisms mediate potentiating effects of depolarization on synaptic transmission. Recently there has been renewed interest in a type of plasticity in which a neuron's somatic membrane potential influences synaptic transmission. We study mechanisms that mediate this type of control at a synapse between a mechanoafferent, B21, and B8, a motor neuron that receives chemical synaptic input. Previously we demonstrated that the somatic membrane potential determines spike propagation within B21. Namely, B21 must be centrally depolarized if spikes are to propagate to an output process. We now demonstrate that this will occur with central depolarizations that are only a few millivolts. Depolarizations of this magnitude are not, however, sufficient to induce synaptic transmission to B8. B21-induced postsynaptic potentials (PSPs) are only observed if B21 is centrally depolarized by >or=10 mV. Larger depolarizations have a second impact on B21. They induce graded changes in the baseline intracellular calcium concentration that are virtually essential for the induction of chemical synaptic transmission. During motor programs, subthreshold depolarizations that increase calcium concentrations are observed during one of the two antagonistic phases of rhythmic activity. Chemical synaptic transmission from B21 to B8 is, therefore, likely to occur in a phase-dependent manner. Other neurons that receive mechanoafferent input are electrically coupled to B21. Differential control of spike propagation and chemical synaptic transmission may, therefore, represent a mechanism that permits selective control of afferent transmission to different types of neurons contacted by B21. Afferent transmission to neurons receiving chemical synaptic input will be phase specific, whereas transmission to electrically coupled followers will be phase independent.

Mechanoafferent neuron with an inexcitable somatic region: consequences for the regulation of spike propagation and afferent transmission.

In the Aplysia mechanoafferent B21, afferent transmission is in part regulated via the control of active spike propagation. When B21 is peripherally activated at its resting membrane potential, spikes fail to propagate to an output process, and afferent transmission does not occur. In this report, we show that the propagation failure is in part a result of the fact that the somatic region of B21 is relatively inexcitable. We isolate this region and demonstrate that net currents evoked by depolarizing pulses are outward. Furthermore, we show that all-or-none spikes are not triggered when current is injected. Previous reports have, however shown that spiking is triggered when current is somatically injected and cells are intact. We demonstrate that spikes evoked under these circumstances do not originate in the soma. Instead they originate in an adjacent part of the neuron that is excitable (the medial process). In summary, we show that the mechanoafferent B21 consists of excitable input and output processes separated by a relatively inexcitable somatic region. A potential advantage of this arrangement is that somatic depolarization can be used to modify spike propagation from the input to the output processes without altering the encoding of peripherally generated activity.