Peptidergic Counter-Regulation of Ca2+- and Na+-Dependent K+Currents Modulates the Shape of Action Potentials in Neurosecretory Insect Neurons

COVID-19-induced neurological symptoms: focus on the role of metal ions

Neurological symptoms are prevalent in both the acute and post-acute phases of coronavirus disease 2019 (COVID-19), and they are becoming a major concern for the prognosis of COVID-19 patients. Accumulation evidence has suggested that metal ion disorders occur in the central nervous system (CNS) of COVID-19 patients. Metal ions participate in the development, metabolism, redox and neurotransmitter transmission in the CNS and are tightly regulated by metal ion channels. COVID-19 infection causes neurological metal disorders and metal ion channels abnormal switching, subsequently resulting in neuroinflammation, oxidative stress, excitotoxicity, neuronal cell death, and eventually eliciting a series of COVID-19-induced neurological symptoms. Therefore, metal homeostasis-related signaling pathways are emerging as promising therapeutic targets for mitigating COVID-19-induced neurological symptoms. This review provides a summary for the latest advances in research related to the physiological and pathophysiological functions of metal ions and metal ion channels, as well as their role in COVID-19-induced neurological symptoms. In addition, currently available modulators of metal ions and their channels are also discussed. Collectively, the current work offers a few recommendations according to published reports and in-depth reflections to ameliorate COVID-19-induced neurological symptoms. Further studies need to focus on the crosstalk and interactions between different metal ions and their channels. Simultaneous pharmacological intervention of two or more metal signaling pathway disorders may provide clinical advantages in treating COVID-19-induced neurological symptoms.

Endocrine signals fine-tune daily activity patterns in Drosophila

Animals need to balance competitive behaviors to maintain internal homeostasis. The underlying mechanisms are complex but typically involve neuroendocrine signaling. Using Drosophila, we systematically manipulated signaling between energy-mobilizing endocrine cells producing adipokinetic hormone (AKH), octopaminergic neurons, and the energy-storing fat body to assess whether this neuroendocrine axis involved in starvation-induced hyperactivity also balances activity levels under ad libitum access to food. Our results suggest that AKH signals via two divergent pathways that are mutually competitive in terms of activity and rest. AKH increases activity via the octopaminergic system during the day, while it prevents high activity levels during the night by signaling to the fat body. This regulation involves feedback signaling from octopaminergic neurons to AKH-producing cells (APCs). APCs are known to integrate a multitude of metabolic and endocrine signals. Our results add a new facet to the versatile regulatory functions of APCs by showing that their output contributes to shape the daily activity pattern under ad libitum access to food.

In-situ recording of ionic currents in projection neurons and Kenyon cells in the olfactory pathway of the honeybee

The honeybee olfactory pathway comprises an intriguing pattern of convergence and divergence: ~60.000 olfactory sensory neurons (OSN) convey olfactory information on ~900 projection neurons (PN) in the antennal lobe (AL). To transmit this information reliably, PNs employ relatively high spiking frequencies with complex patterns. PNs project via a dual olfactory pathway to the mushroom bodies (MB). This pathway comprises the medial (m-ALT) and the lateral antennal lobe tract (l-ALT). PNs from both tracts transmit information from a wide range of similar odors, but with distinct differences in coding properties. In the MBs, PNs form synapses with many Kenyon cells (KC) that encode odors in a spatially and temporally sparse way. The transformation from complex information coding to sparse coding is a well-known phenomenon in insect olfactory coding. Intrinsic neuronal properties as well as GABAergic inhibition are thought to contribute to this change in odor representation. In the present study, we identified intrinsic neuronal properties promoting coding differences between PNs and KCs using in-situ patch-clamp recordings in the intact brain. We found very prominent K⁺ currents in KCs clearly differing from the PN currents. This suggests that odor coding differences between PNs and KCs may be caused by differences in their specific ion channel properties. Comparison of ionic currents of m- and l-ALT PNs did not reveal any differences at a qualitative level.

Hands-on parameter search for neural simulations by a MIDI-controller

Computational neuroscientists frequently encounter the challenge of parameter fitting – exploring a usually high dimensional variable space to find a parameter set that reproduces an experimental data set. One common approach is using automated search algorithms such as gradient descent or genetic algorithms. However, these approaches suffer several shortcomings related to their lack of understanding the underlying question, such as defining a suitable error function or getting stuck in local minima. Another widespread approach is manual parameter fitting using a keyboard or a mouse, evaluating different parameter sets following the users intuition. However, this process is often cumbersome and time-intensive. Here, we present a new method for manual parameter fitting. A MIDI controller provides input to the simulation software, where model parameters are then tuned according to the knob and slider positions on the device. The model is immediately updated on every parameter change, continuously plotting the latest results. Given reasonably short simulation times of less than one second, we find this method to be highly efficient in quickly determining good parameter sets. Our approach bears a close resemblance to tuning the sound of an analog synthesizer, giving the user a very good intuition of the problem at hand, such as immediate feedback if and how results are affected by specific parameter changes. In addition to be used in research, our approach should be an ideal teaching tool, allowing students to interactively explore complex models such as Hodgkin-Huxley or dynamical systems.

Beyond faithful conduction: short-term dynamics, neuromodulation, and long-term regulation of spike propagation in the axon

Most spiking neurons are divided into functional compartments: a dendritic input region, a soma, a site of action potential initiation, an axon trunk and its collaterals for propagation of action potentials, and distal arborizations and terminals carrying the output synapses. The axon trunk and lower order branches are probably the most neglected and are often assumed to do nothing more than faithfully conducting action potentials. Nevertheless, there are numerous reports of complex membrane properties in non-synaptic axonal regions, owing to the presence of a multitude of different ion channels. Many different types of sodium and potassium channels have been described in axons, as well as calcium transients and hyperpolarization-activated inward currents. The complex time- and voltage-dependence resulting from the properties of ion channels can lead to activity-dependent changes in spike shape and resting potential, affecting the temporal fidelity of spike conduction. Neural coding can be altered by activity-dependent changes in conduction velocity, spike failures, and ectopic spike initiation. This is true under normal physiological conditions, and relevant for a number of neuropathies that lead to abnormal excitability. In addition, a growing number of studies show that the axon trunk can express receptors to glutamate, GABA, acetylcholine or biogenic amines, changing the relative contribution of some channels to axonal excitability and therefore rendering the contribution of this compartment to neural coding conditional on the presence of neuromodulators. Long-term regulatory processes, both during development and in the context of activity-dependent plasticity may also affect axonal properties to an underappreciated extent.

Hormonal regulation of energy metabolism in insects as a driving force for performance

Since all life processes depend on energy, the endocrine control of energy metabolism is one of the driving forces for the performance of an individual. Here, we review the literature on the key players in the endocrine regulation of energy homeostasis in insects, the adipokinetic hormones. These pleiotropic peptides not only control dynamic performance traits (flight, swimming, walking) but also regulatory performance traits (egg production, larval growth, and molting). Adipokinetic hormone is released into the hemolymph during intense muscular activity (flight) and also during apparently less energy-demanding locomotory activities, such as swimming and even walking, and, finally, activates the catabolic enzymes phosphorylase and/or triacylglycerol lipase that mobilize carbohydrates and/or lipids and proline, respectively. At the same time, anabolic processes such as the synthesis of protein, lipid, and glycogen are inhibited. Furthermore, adipokinetic hormones affect locomotory activity via neuromodulatory mechanisms that apparently employ biogenic amines. During oogenesis, it is thought that adipokinetic hormone performs similar tasks, because energetic substrates have to be mobilized and transported from the fat body to the ovaries in order to support oocyte growth. Inhibition of anabolic processes by exogenous adipokinetic hormone results in females that lay fewer and smaller eggs. Much less is known about the role of adipokinetic hormones during larval development and during molting but in this case energy homeostasis has to be tightly regulated as well: in general, during the early phase of a larval instar intake of food prevails and the energy stores of the fat body are established, whereas, prior to the molt, insects stop feeding and mobilize energy stores in the fat body, thereby fueling energy-demanding processes such as the formation of the new cuticle and the emergence from the old one. From the few data available to date, it is clear that adipokinetic hormones are involved in the regulation of these events in larvae.

The omega-atracotoxins: selective blockers of insect M-LVA and HVA calcium channels

The omega-atracotoxins (omega-ACTX) are a family of arthropod-selective peptide neurotoxins from Australian funnel-web spider venoms (Hexathelidae: Atracinae) that are candidates for development as biopesticides. We isolated a 37-residue insect-selective neurotoxin, omega-ACTX-Ar1a, from the venom of the Sydney funnel-web spider Atrax robustus, with high homology to several previously characterized members of the omega-ACTX-1 family. The peptide induced potent excitatory symptoms, followed by flaccid paralysis leading to death, in acute toxicity tests in house crickets. Using isolated smooth and skeletal nerve-muscle preparations, the toxin was shown to lack overt vertebrate toxicity at concentrations up to 1 microM. To further characterize the target of the omega-ACTXs, voltage-clamp analysis using the whole-cell patch-clamp technique was undertaken using cockroach dorsal unpaired median neurons. It is shown here for the first time that omega-ACTX-Ar1a, and its homolog omega-ACTX-Hv1a from Hadronyche versuta, reversibly block both mid-low- (M-LVA) and high-voltage-activated (HVA) insect calcium channel (Ca(v)) currents. This block occurred in the absence of alterations in the voltage-dependence of Ca(v) channel activation, and was voltage-independent, suggesting that omega-ACTX-1 family toxins are pore blockers rather than gating modifiers. At a concentration of 1 microM omega-ACTX-Ar1a failed to significantly affect global K(v) channel currents. However, 1 microM omega-ACTX-Ar1a caused a modest 18% block of insect Na(v) channel currents, similar to the minor block of Na(v) channels reported for other insect Ca(v) channel blockers such as omega-agatoxin IVA. These findings validate both M-LVA and HVA Ca(v) channels as potential targets for insecticides.

Differential regulation of two distinct voltage-dependent sodium currents by group III metabotropic glutamate receptor activation in insect pacemaker neurons

Using whole cell patch-clamp technique and immunocytochemistry on adult dorsal unpaired median (DUM) neurons isolated from the cockroach Periplaneta americana CNS, we reported the characterization of a native mGluR, sharing pharmacological properties with vertebrate metabotropic glutamate receptor III (mGluRIII) that regulated voltage-dependent sodium current (I(Na)). The global I(Na) was dissociated by means of l-glutamate sensitivity, deactivation time constant, voltage dependence of activation and inactivation, recovery from inactivation, and intracellular regulation process. These two currents were respectively designated I(Na1) and I(Na2) for l-glutamate-sensitive and -insensitive sodium currents. l-glutamate selectively reduced I(Na1) by an increase of intracellular cAMP level. Using different activators and/or inhibitors of G proteins and cAMP/PKA cascade, together with St-Ht31 (an inhibitor of PKA binding to AKAP) and AKAP-79 antibodies, we established that mGluRIII was linked to I(Na1) by a Gi/o and a suspected Gs protein. According to the activated signaling pathway, l-glutamate elevated the cAMP level, which thereby activated cytosolic PKA and released PKA bound to AKAP. As expected from both biophysical and pharmacological studies, we showed that, through an inhibition of I(Na1), l-glutamate increased DUM neuron spontaneous electrical activity. These results indicated that such mGluRIII-activated dual processes provided a new physiological control of pacemaker neuronal firing.

Differential Receptor Activation by Cockroach Adipokinetic Hormones Produces Differential Effects on Ion Currents, Neuronal Activity, and Locomotion

Adipokinetic hormone (AKH) peptides in insects serve the endocrine control of energy supply. They also produce, however, neuronal, vegetative, and motor effects, suggesting that AKHs orchestrate adaptive behavior by multiple actions. We have cloned, for Periplaneta americana, the AKH receptor to determine its localization and, based on current measurements in neurons and heterologous expression systems, the mechanisms of AKH actions. Apart from fat body, various neurons express the AKH receptor, among them abdominal dorsal unpaired median (DUM) neurons, which release the biogenic amine octopamine. They are part of the arousal system and are involved in the control of circulation and respiration. Both the two Periplaneta AKHs activate the Gspathway, and AKH I also potently activates Gq. AKH I and—with much less efficacy—AKH II accelerate spiking of DUM neurons through an increase of the pacemaking Ca2+current. Because the AKHs are released from the corpora cardiaca into the hemolymph, they must penetrate the blood-brain barrier for acting on neurons. That this happens was shown electrophysiologically by applying AKH I to an intact ganglion. Systemically injected AKH I stimulates locomotion potently in striking contrast to AKH II. This behavioral difference can be traced back conclusively to the different effectiveness of the AKHs on the level of G proteins. Our findings also show that AKHs act through the same basic mechanisms on neuronal and nonneuronal cells, and they support an integration of metabolic and neuronal effects in homoeostatic mechanisms.

TRPgamma channels are inhibited by cAMP and contribute to pacemaking in neurosecretory insect neurons

From a neuronal cDNA library of the cockroach Periplaneta americana we isolated a 3585-bp cDNA sequence encoding Periplaneta transient receptor potential gamma (pTRPgamma), a protein of 1194 amino acids showing 65% identity to the orthologous Drosophila channel protein dTRPgamma. Heterologous expression of pTRPgamma in HEK293 cells produced a constitutively active, non-selective cation channel with a Ca2+:Na+ permeability ratio of 2. In contrast to dTRPgamma-mediated currents, pTRPgamma currents were partially inhibited by 8-bromo-cAMP, and this effect was not mediated by protein kinase A (PKA) activation. pTRPgammab, a truncated pTRPgamma splice variant missing most of the C terminus, was insensitive to 8-bromo-cAMP. Thus, the critical cAMP-binding site seems to be located in the C-terminal part of pTRPgamma, although there is no common cAMP-binding consensus sequence. While dTRPgamma is only expressed in the photoreceptors, pTRPgamma is expressed throughout the nervous system. In particular it is expressed in dorsal unpaired median (DUM) neurons. In these octopamine-releasing, neurosecretory cells a Ca2+ background current contributing to pacemaker activity was found to be up-regulated by the reduction of cAMP level. In addition, the Ca2+ background current was inhibited by LOE-908, 2-APB, and La3+, which similarly affected the pTRPgamma current. We thus propose that the pTRPgamma protein is involved in forming the channel passing the Ca2+ pakemaking background current in DUM neurons.