Contributions of Identifiable Neurons and Neuron Classes to Lamprey Vertebrate Neurobiology

Among the advantages offered by the lamprey brainstem and spinal cord for studies of the structure and function of the nervous system is the unique identifiability of several pairs of reticulospinal neurons in the brainstem. These neurons have been exploited in investigations of the patterns of sensory input to these cells and the patterns of their outputs to spinal neurons, but no doubt these cells could be used much more effectively in exploring their roles in descending control of the spinal cord. The variability of cell positions of neurons in the spinal cord has precluded the recognition of unique spinal neurons. However, classes of nerve cells can be readily defined and characterized within the lamprey spinal cord and this has led to progress in understanding the cellular and synaptic mechanisms of locomotor activity. In addition, both the identifiable reticulospinal cells and the various spinal nerve cell classes and their known synaptic interactions have been used to demonstrate the degree and specificity of regeneration within the lamprey nervous system. The lack of uniquely identifiable cells within the lamprey spinal cord has hampered progress in these areas, especially in gaining a full understanding of the locomotor network and how neuromodulation of the network is accomplished

Segmental Distribution of Common Synaptic Inputs to Spinal Motoneurons During Fictive Swimming in the Lamprey

These experiments were designed to measure the degree of shared synaptic inputs coming to pairs of myotomal motoneurons during swimming activity in the isolated spinal cord of the lamprey. In addition, the experiments measured the decrease in the degree of shared synaptic inputs with the distance between the motoneurons to assess the segmental distribution of these shared inputs. Intracellular microelectrode recordings of membrane potential were made simultaneously on pairs of myotomal motoneurons during swimming activity induced with an excitatory amino acid. The swim cycle oscillations of motoneuron membrane potentials were removed with a digital notch filter, thus leaving the fast synaptic activities that underlie these slower oscillations. Cross-correlations of the fast synaptic activities in two simultaneously recorded motoneurons were made to measure the degree of shared inputs. The cross-correlation was done on time windows restricted to one swim cycle or to part of a swim cycle, and 50 consecutive swim cycle cross-correlograms then were averaged. The peak coefficients of the cross-correlations exhibited a wide range, even for pairs of motoneurons located near one another (range = 0.06–0.74, for pairs located within 2 segments). This observation suggests that there may be different functional classes of myotomal motoneurons with inputs originating from different sets of premotor interneurons. In spite of this variability, the mean peak correlation coefficients of motoneuron pairs clearly decreased with the distance between them. With separations of more than five segments, there was little or no clear correlation between the motoneurons (range = 0.04–0.10). These results suggest that common synaptic inputs to motoneurons during fictive swimming originate from local premotor interneurons and that beyond five spinal segments, common premotor inputs are rare or weak to motoneurons. Thus the premotor signals originating from the locomotor network have relatively short distribution lengths, on the order of 5 segments of 120 total spinal segments

Cellular and Synaptic Actions of Acetylcholine in the Lamprey Spinal Cord

This study investigated cellular and synaptic mechanisms of cholinergic neuromodulation in the in vitro lamprey spinal cord. Most spinal neurons tested responded to local application of acetylcholine (ACh) with depolarization and decreased input resistance. The depolarization persisted in the presence of either tetrodotoxin or muscarinic antagonist scopolamine and was abolished with nicotinic antagonist mecamylamine, indicating a direct depolarization through nicotinic ACh receptors. Local application of muscarinic ACh agonists modulated synaptic strength in the spinal cord by decreasing the amplitude of unitary excitatory and inhibitory postsynaptic potentials. The postsynaptic response to direct application of glutamate was unchanged by muscarinic agonists, suggesting a presynaptic mechanism. Cholinergic feedback from motoneurons was assessed using stimulation of a ventral root in the quiescent spinal cord while recording intracellularly from spinal motoneurons or interneurons. Mainly depolarizing potentials were observed, a portion of which was insensitive to removal of extracellular Ca2+, indicating electrotonic coupling. Hyperpolarizing potentials were also observed and were attenuated by the glycinergic antagonist strychnine, whereas depolarizing responses were potentiated by strychnine. Mecamylamine also reduced hyperpolarizing responses. The pharmacology of these responses suggests a Renshaw-like feedback pathway in lamprey. Immunohistochemistry for choline acetyltransferase, performed in combination with retrograde filling of motoneurons, demonstrated a population of nonmotoneuron cholinergic cells in the lamprey spinal cord. Thus endogenous cholinergic modulation of the lamprey spinal locomotor network is likely produced by both motoneurons and cholinergic interneurons acting via combined postsynaptic and presynaptic actions

Excitatory motor neurons are local oscillators for backward locomotion

Cell- or network-driven oscillators underlie motor rhythmicity. The identity of C. elegans oscillators remains unknown. Through cell ablation, electrophysiology, and calcium imaging, we show: (1) forward and backward locomotion is driven by different oscillators; (2) the cholinergic and excitatory A-class motor neurons exhibit intrinsic and oscillatory activity that is sufficient to drive backward locomotion in the absence of premotor interneurons; (3) the UNC-2 P/Q/N high-voltage-activated calcium current underlies A motor neuron\u27s oscillation; (4) descending premotor interneurons AVA, via an evolutionarily conserved, mixed gap junction and chemical synapse configuration, exert state-dependent inhibition and potentiation of A motor neuron\u27s intrinsic activity to regulate backward locomotion. Thus, motor neurons themselves derive rhythms, which are dually regulated by the descending interneurons to control the reversal motor state. These and previous findings exemplify compression: essential circuit properties are conserved but executed by fewer numbers and layers of neurons in a small locomotor network

INCF Lithuanian Workshop on Neuroscience and Information Technology

The aim of this workshop was to give a current overview of neuroscience and informatics research in Lithuania, and to discuss the strategies for forming the Lithuanian Neuroinformatics Node and becoming a member of INCF. The workshop was organized by Dr. Aušra Saudargiene (Department of Informatics, Vytautas Magnus University, Kaunas, and Faculty of Natural Sciences, Vilnius University, Lithuania) and INCF.
The workshop was attended by 15 invited speakers, among them 4 guests and 11 Lithuanian neuroscientists, and over 20 participants. The workshop was organized into three main sessions: overview of the INCF activities including the Swedish and UK nodes of INCF; presentations on Neuroscience research carried out in Lithuania; discussion about the strategies for forming an INCF national node, and the benefits of having such a node in Lithuania (Appendix A: Program; Appendix B: Abstracts)

Ion homeostasis in rhythmogenesis : the interplay between neurons and astroglia

Proper function of all excitable cells depends on ion homeostasis. Nowhere is this more critical than in the brain where the extracellular concentration of some ions determines neurons' firing pattern and ability to encode information. Several neuronal functions depend on the ability of neurons to change their firing pattern to a rhythmic bursting pattern, whereas, in some circuits, rhythmic firing is, on the contrary, associated to pathologies like epilepsy or Parkinson's disease. In this review, we focus on the four main ions known to fluctuate during rhythmic firing: calcium, potassium, sodium, and chloride. We discuss the synergistic interactions between these elements to promote an oscillatory activity. We also review evidence supporting an important role for astrocytes in the homeostasis of each of these ions and describe mechanisms by which astrocytes may regulate neuronal firing by altering their extracellular concentrations. A particular emphasis is put on the mechanisms underlying rhythmogenesis in the circuit forming the central pattern generator (CPG) for mastication and other CPG systems. Finally, we discuss how an impairment in the ability of glial cells to maintain such homeostasis may result in pathologies like epilepsy and Parkinson's disease

Membrane mechanisms during exo- and endocytosis in excitable cells

Excitable cells, like endocrine cells and neurons release hormones and transmitters through exocytosis to mediate many important functions, such as stress responses, immune responses, control of blood glucose, and synaptic transmission that mediates communication between neurons in the brain. This process relies on delicate endocytic mechanisms, to retrieve vesicular membranes and proteins previously fused at the plasma membrane and thus maintains the ability of exocytosis for excitable cells. Upon exocytosis, vesicle fusion with the plasma membrane generates an Ω-shape membrane profile with a narrow fusion pore (< ~5 nm), through which the vesicular content is released. For decades, it has been thought that the narrow pore of the Ω-profile either dilates till the Ω-profile flattens to facilitate content release (full-collapse) or closes to limit content release (kiss-and-run). Using a combination of confocal, super-resolution STED and electron microscopy, we found that this classical view needs to be modified significantly. We observed that the fusion pore may expand, constrict and/or close at different rates to regulate content release and that its size may vary between 0 and 490 nm. The cytoskeletal F-actin facilitates fusion pore opening. The merge between fused vesicles and the plasma membrane is not via the full-collapse fusion mode, but via Ω-profile shrinking while maintaining the Ω-shape (Ω-shrink fusion mode). F-actin provides sufficient membrane tension to facilitate Ω-shrink fusion in neuroendocrine chromaffin cells, and to facilitate vesicle merging at lamprey giant synapses. Thus, facilitation of vesicle merging by F-actin may be applicable to many secretory cells and synapses. Unlike the classical view that full-collapse and kiss-and-run facilitate and limit release, respectively, facilitation is mediated by shrink-fusion with a large non-dilating pore, whereas during inhibition, by enlarge-fusion that enlarges the Ω-profile, a small pore is maintained. Shrink and enlarge-fusion may account for diverse hormone and transmitter release kinetics observed in secretory cells. Clathrin-mediated endocytosis (CME), the most common form of endocytosis, is considered to retrieve vesicles from the flat plasma membrane. With conventional and platinum replica electron microscopy (EM), we found that clathrin-coated pits prefer to be localized at the bulk invagination of the plasma membrane instead of the flat plasma membrane. Using super-resolution STED microscopy, we observed for the first-time direct budding of vesicles from bulk membrane invagination. These results suggest that bulk membrane invaginations are a major platform for mediating clathrin-dependent endocytosis, calling for modification of the current view that clathrin-mediated endocytosis mostly originated from the flat plasma membrane. In summary, the studies described here improve our understanding of the regulation of hormone and transmitter release as well as endocytosis

Ion Channel Clustering at the Axon Initial Segment and Node of Ranvier Evolved Sequentially in Early Chordates

In many mammalian neurons, dense clusters of ion channels at the axonal initial segment and nodes of Ranvier underlie action potential generation and rapid conduction. Axonal clustering of mammalian voltage-gated sodium and KCNQ (Kv7) potassium channels is based on linkage to the actin–spectrin cytoskeleton, which is mediated by the adaptor protein ankyrin-G. We identified key steps in the evolution of this axonal channel clustering. The anchor motif for sodium channel clustering evolved early in the chordate lineage before the divergence of the wormlike cephalochordate, amphioxus. Axons of the lamprey, a very primitive vertebrate, exhibited some invertebrate features (lack of myelin, use of giant diameter to hasten conduction), but possessed narrow initial segments bearing sodium channel clusters like in more recently evolved vertebrates. The KCNQ potassium channel anchor motif evolved after the divergence of lampreys from other vertebrates, in a common ancestor of shark and humans. Thus, clustering of voltage-gated sodium channels was a pivotal early innovation of the chordates. Sodium channel clusters at the axon initial segment serving the generation of action potentials evolved long before the node of Ranvier. KCNQ channels acquired anchors allowing their integration into pre-existing sodium channel complexes at about the same time that ancient vertebrates acquired myelin, saltatory conduction, and hinged jaws. The early chordate refinements in action potential mechanisms we have elucidated appear essential to the complex neural signaling, active behavior, and evolutionary success of vertebrates

Excitatory Motor Neurons are Local Central Pattern Generators in an Anatomically Compressed Motor Circuit for Reverse Locomotion [preprint]

Central pattern generators are cell- or network-driven oscillators that underlie motor rhythmicity. The existence and identity of C. elegans CPGs remain unknown. Through cell ablation, electrophysiology, and calcium imaging, we identified oscillators for reverse locomotion. We show that the cholinergic and excitatory class A motor neurons exhibit intrinsic and oscillatory activity, and such an activity can drive reverse locomotion without premotor interneurons. Regulation of their oscillatory activity, either through effecting an endogenous constituent of oscillation, the P/Q/N high voltage-activated calcium channel UNC-2, or, via dual regulation, inhibition and activation, by the descending premotor interneurons AVA, determines the propensity, velocity, and sustention of reverse locomotion. Thus, the reversal motor executors themselves serve as oscillators; regulation of their intrinsic activity controls the reversal motor state. These findings exemplify anatomic and functional compression: motor executors integrate the role of rhythm generation in a locomotor network that is constrained by small cell numbers