Temporal dynamics of Na/K pump mediated memory traces: insights from conductance-based models of Drosophila neurons

Sodium potassium ATPases (Na/K pumps) mediate long-lasting, dynamic cellular memories that can last tens of seconds. The mechanisms controlling the dynamics of this type of cellular memory are not well understood and can be counterintuitive. Here, we use computational modeling to examine how Na/K pumps and the ion concentration dynamics they influence shape cellular excitability. In a Drosophila larval motor neuron model, we incorporate a Na/K pump, a dynamic intracellular Na+ concentration, and a dynamic Na+ reversal potential. We probe neuronal excitability with a variety of stimuli, including step currents, ramp currents, and zap currents, then monitor the sub- and suprathreshold voltage responses on a range of time scales. We find that the interactions of a Na+-dependent pump current with a dynamic Na+ concentration and reversal potential endow the neuron with rich response properties that are absent when the role of the pump is reduced to the maintenance of constant ion concentration gradients. In particular, these dynamic pump-Na+ interactions contribute to spike rate adaptation and result in long-lasting excitability changes after spiking and even after sub-threshold voltage fluctuations on multiple time scales. We further show that modulation of pump properties can profoundly alter a neuron’s spontaneous activity and response to stimuli by providing a mechanism for bursting oscillations. Our work has implications for experimental studies and computational modeling of the role of Na/K pumps in neuronal activity, information processing in neural circuits, and the neural control of animal behavior

Synaptic Plasticity and Hebbian Cell Assemblies

Synaptic dynamics are critical to the function of neuronal circuits on multiple timescales. In the first part of this dissertation, I tested the roles of action potential timing and NMDA receptor composition in long-term modifications to synaptic efficacy. In a computational model I showed that the dynamics of the postsynaptic [Ca2+] time course can be used to map the timing of pre- and postsynaptic action potentials onto experimentally observed changes in synaptic strength. Using dual patch-clamp recordings from cultured hippocampal neurons, I found that NMDAR subtypes can map combinations of pre- and postsynaptic action potentials onto either long-term potentiation (LTP) or depression (LTD). LTP and LTD could even be evoked by the same stimuli, and in such cases the plasticity outcome was determined by the availability of NMDAR subtypes. The expression of LTD was increasingly presynaptic as synaptic connections became more developed. Finally, I found that spike-timing-dependent potentiability is history-dependent, with a non-linear relationship to the number of pre- and postsynaptic action potentials. After LTP induction, subsequent potentiability recovered on a timescale of minutes, and was dependent on the duration of the previous induction. While activity-dependent plasticity is putatively involved in circuit development, I found that it was not required to produce small networks capable of exhibiting rhythmic persistent activity patterns called reverberations. However, positive synaptic scaling produced by network inactivity yielded increased quantal synaptic amplitudes, connectivity, and potentiability, all favoring reverberation. These data suggest that chronic inactivity upregulates synaptic efficacy by both quantal amplification and by the addition of silent synapses, the latter of which are rapidly activated by reverberation. Reverberation in previously inactivated networks also resulted in activity-dependent outbreaks of spontaneous network activity. Applying a model of short-term synaptic dynamics to the network level, I argue that these experimental observations can be explained by the interaction between presynaptic calcium dynamics and short-term synaptic depression on multiple timescales. Together, the experiments and modeling indicate that ongoing activity, synaptic scaling and metaplasticity are required to endow networks with a level of synaptic connectivity and potentiability that supports stimulus-evoked persistent activity patterns but avoids spontaneous activity

Modulation of Neuropeptide Release via Voltage-Dependent and -Independent Signaling in Isolated Neurohypophysial Terminals: a Dissertation

This thesis details my examination of several mechanisms for modulation of neuropeptide release via voltage-dependent and voltage-independent intraterminal signaling in isolated neurohypophysial terminals. The first part of this work characterizes depolarization-induced neuropeptide release in the absence of extracellular calcium. The goal of this project was to examine the relationship between depolarization-induced release of intracellular calcium stores and depolarization-secretion coupling of neuropeptides. We demonstrate that depolarization in the absence of extracellular calcium induced by either High K+ or electrical stimulation induces a rise in [Ca2+]i and subsequent neuropeptide release from Hypothalamic Neurohypophysial System (HNS) terminals. A portion of extracellular calcium-independent neuropeptide release is due to intraterminal calcium, but the remaining depolarization-induced release may be due to calcium-independent voltage-dependent (CIVD) release (Zhang and Zhou, 2002; Zhang et al., 2004; Yang et al., 2005). Nevertheless, our results clearly show that extracellular calcium is notnecessary for depolarization-induced neuropeptide secretion from these CNS terminals. In addition, I investigated the role of internal calcium stores in mediating μ-opioid inhibition of voltage-gated calcium channels (VGCCs). Inhibition of VGCCs via μ-opioid agonists has been shown to reduce neuropeptide release in response to High K+ stimulation of isolated terminals (Bicknell et al., 1985b; Russell et al., 1993; van Wimersma Greidanus and van de Heijning, 1993; Munro et al., 1994; Ortiz-Miranda et al., 2003; Russell et al., 2003; Ortiz-Miranda et al., 2005). My findings show μ-opioid inhibition, of VGCC and High K+-mediated rise in [Ca2+]i, are via a voltage-independent diffusible second-messenger targeting release of calcium from ryanodine-sensitive stores, possibly mediated via the cyclic ADP ribose signaling pathway. Furthermore, I detail a different intracellular messenger pathway mediating the κ-opioid inhibition of VGCC and High K+-mediated rise in [Ca2+]ii. In contrast to the μ-opioid inhibition, κ-receptor activation is coupled to a voltage-dependent membrane-delimited pathway. Inhibition of neuropeptide release via both endogenous and exogenous κ-opioid agonists has been extensively studied (Bicknell et al., 1985a; Nordmann et al., 1986a; Wammack and Racke, 1988; Munro et al., 1994; Ingram et al., 1996; Rusin et al., 1997a). My investigation shows that the κ-inhibition of VGCC is voltage-dependent and is furthermore, relieved within the context of a physiological burst of action potentials (APs). This physiologically-evoked, activity-dependent modulation of VGCC and subsequent release, represents an important mechanism for short-term synaptic plasticity at the level of the terminals. Given the ubiquitous nature of voltage-dependent G-protein signaling in the CNS, our results may prove important in understanding modulatory effects of specific bursting patterns throughout the CNS. In the last 30 years the neurohypophysial system has proven to be an excellent system to study the complexities of depolarization-secretion coupling (DSC). There have been many advances in our understanding of the underlying mechanisms involved and their physiological implications. The current work focuses on two important features of DSC; voltage and calcium. Although in many ways these two are intrinsically linked through VGCC activation, we have found that in isolated HNS terminals that is not always the case. We have further found that when voltage and calcium influx are linked during DSC, modulation by opioids may or may not be linked to activity-dependent relief depending on the opioid receptor activated. This finding has important implications in neuropeptide release during patterned stimulation in vivo. As I will discuss further, many factors play into the complexities of the regulatory mechanisms involving release. As investigations into this remarkable field continue, I hope to have contributed a valuable piece to the puzzle

Investigating the role of fast-spiking interneurons in neocortical dynamics

PhD ThesisFast-spiking interneurons are the largest interneuronal population in neocortex. It is well documented that this population is crucial in many functions of the neocortex by subserving all aspects of neural computation, like gain control, and by enabling dynamic phenomena, like the generation of high frequency oscillations. Fast-spiking interneurons, which represent mainly the parvalbumin-expressing, soma-targeting basket cells, are also implicated in pathological dynamics, like the propagation of seizures or the impaired coordination of activity in schizophrenia. In the present thesis, I investigate the role of fast-spiking interneurons in such dynamic phenomena by using computational and experimental techniques. First, I introduce a neural mass model of the neocortical microcircuit featuring divisive inhibition, a gain control mechanism, which is thought to be delivered mainly by the soma-targeting interneurons. Its dynamics were analysed at the onset of chaos and during the phenomena of entrainment and long-range synchronization. It is demonstrated that the mechanism of divisive inhibition reduces the sensitivity of the network to parameter changes and enhances the stability and exibility of oscillations. Next, in vitro electrophysiology was used to investigate the propagation of activity in the network of electrically coupled fast-spiking interneurons. Experimental evidence suggests that these interneurons and their gap junctions are involved in the propagation of seizures. Using multi-electrode array recordings and optogenetics, I investigated the possibility of such propagating activity under the conditions of raised extracellular K+ concentration which applies during seizures. Propagated activity was recorded and the involvement of gap junctions was con rmed by pharmacological manipulations. Finally, the interaction between two oscillations was investigated. Two oscillations with di erent frequencies were induced in cortical slices by directly activating the pyramidal cells using optogenetics. Their interaction suggested the possibility of a coincidence detection mechanism at the circuit level. Pharmacological manipulations were used to explore the role of the inhibitory interneurons during this phenomenon. The results, however, showed that the observed phenomenon was not a result of synaptic activity. Nevertheless, the experiments provided some insights about the excitability of the tissue through scattered light while using optogenetics. This investigation provides new insights into the role of fast-spiking interneurons in the neocortex. In particular, it is suggested that the gain control mechanism is important for the physiological oscillatory dynamics of the network and that the gap junctions between these interneurons can potentially contribute to the inhibitory restraint during a seizure.Wellcome Trust

Ion Channels of Nociception

The Special Issue “Ion Channels of Nociception” contains 13 articles united by a focus on the peripheral mechanisms of pain. The content covers the mechanisms of neuropathic, inflammatory, and dental pain as well as pain in migraine and diabetes; nociceptive roles of P2X3, ASIC, Piezo and TRP channels; pain control through GPCRs and pharmacological agents; and nonpharmacological treatment with electroacupuncture

Neuroglia

This book is a compiled version of the journal Neuroglia. It was a peer-review Open Access journal by MDPI that investigated a wide range of glia related topics. Now the journal is published as a section of the journal Brain Sciences, with a new section Editor-in-Chief Prof. Sergey Kasparov

Functional integration in the cortical neuronal network of conscious and anesthetized animals

General anesthesia consists of amnesia, analgesia, areflexia and unconsciousness. How anesthetics suppress consciousness has been a mystery for more than one and a half centuries. The overall goal of my research has been to determine the neural correlates of anesthetic-induced loss of consciousness. I hypothesized that anesthetics induce unconsciousness by interfering with the functional connectivity of neuronal networks of the brain and consequently, reducing the brain\u27s capacity for information processing. To test this hypothesis, I performed experiments in which neuronal spiking activity was measured with chronically implanted microelectrode arrays in the visual cortex of freely-moving rats during wakefulness and at graded levels of anesthesia produced by the inhalational anesthetic agent desflurane. I then applied linear and non-parametric information-theoretic analyses to quantify the concentration-dependent effect of general anesthetics on spontaneous and visually evoked spike firing activity in rat primary visual cortex. Results suggest that desflurane anesthesia disrupts cortical neuronal integration as measured by monosynaptic connectivity, spike burst coherence and information capacity. This research furthers our understanding of the mechanisms involved with the anesthetic-induced LOC which may facilitate in the development of better anesthetic monitoring devices and the creation of effective anesthetic agents that will be free of unwanted side effects

Modulating microcircuits in depression

Major depressive disorder (MDD) is globally the leading cause of disability with a worldwide prevalence of 4.4 %, affecting 322 million people in 2015. The treatment of MDD includes antidepressant medication and psychological therapies. However, approximately one-third of treated patients do not respond adequately to these treatments. These patients suffer from treatment-resistant depression (TRD). Deep brain stimulation (DBS) is a therapy modality widely researched for TRD, however, study outcomes show inconsistent results. This thesis focuses on DBS in TRD and researches i) if it is possible to disentangle TRD into different microcircuits, ii) how clinical DBS outcomes can be improved and iii) if DBS can be refined with a non-invasive technique called magnetothermal DBS (mDBS) introducing nanomaterial-mediated neuromodulation. MDBS is researched in collaboration with the research group of prof. dr. P. Anikeeva at the research laboratory of electronics (rle) at the Massachusetts Institute of Technology (MIT) (Boston, USA)