Slow dorsal-ventral rhythm generator in the lamprey spinal cord.
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In the isolated lamprey spinal cord, a very slow rhythm (0.03-0.11 Hz), superimposed on fast N-methyl-D-aspartate (NMDA)-induced locomotor activity (0.26-2.98 Hz), could be induced by a blockade of GABA(A) or glycine receptors or by administration of (1 s, 3 s)-l-aminocyclopentane-1,3-dicarboxylic acid a metabotropic glutamate receptor agonist. Ventral root branches supplying dorsal and ventral myotomes were exposed bilaterally to study the motor pattern in detail. The slow rhythm was expressed in two main forms: 1) a dorsal-ventral reciprocal pattern was the most common (18 of 24 preparations), in which bilateral dorsal branches were synchronous and alternated with the ventral branches, in two additional cases a diagonal dorsal-ventral reciprocal pattern with alternation between the left (or right) dorsal and the right (or left) ventral branches was observed; 2) synchronous bursting in all branches was encountered in four cases. In contrast, the fast locomotor rhythm occurred always in a left-right reciprocal pattern. Thus when the slow rhythm appeared in a dorsal-ventral reciprocal pattern, fast rhythms would simultaneously display left-right alternation. A longitudinal midline section of the spinal cord during ongoing slow bursting abolished the reciprocal pattern between ipsilateral dorsal and ventral branches but a synchronous burst activity could still remain. The fast swimming rhythm did not recover after the midline section. These results suggest that in addition to the network generating the swimming rhythm in the lamprey spinal cord, there is also a network providing slow reciprocal alternation between dorsal and ventral parts of the myotome. During steering, a selective activation of dorsal and ventral myotomes is required and the neural network generating the slow rhythm may represent activity in the spinal machinery used for steering. (+info)
Lamprey Dlx genes and early vertebrate evolution.
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Gnathostome vertebrates have multiple members of the Dlx family of transcription factors that are expressed during the development of several tissues considered to be vertebrate synapomorphies, including the forebrain, cranial neural crest, placodes, and pharyngeal arches. The Dlx gene family thus presents an ideal system in which to examine the relationship between gene duplication and morphological innovation during vertebrate evolution. Toward this end, we have cloned Dlx genes from the lamprey Petromyzon marinus, an agnathan vertebrate that occupies a critical phylogenetic position between cephalochordates and gnathostomes. We have identified four Dlx genes in P. marinus, whose orthology with gnathostome Dlx genes provides a model for how this gene family evolved in the vertebrate lineage. Differential expression of these lamprey Dlx genes in the forebrain, cranial neural crest, pharyngeal arches, and sensory placodes of lamprey embryos provides insight into the developmental evolution of these structures as well as a model of regulatory evolution after Dlx gene duplication events. (+info)
G protein betagamma subunit-mediated presynaptic inhibition: regulation of exocytotic fusion downstream of Ca2+ entry.
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The nervous system can modulate neurotransmitter release by neurotransmitter activation of heterotrimeric GTP-binding protein (G protein)-coupled receptors. We found that microinjection of G protein betagamma subunits (Gbetagamma) mimics serotonin's inhibitory effect on neurotransmission. Release of free Gbetagamma was critical for this effect because a Gbetagamma scavenger blocked serotonin's effect. Gbetagamma had no effect on fast, action potential-evoked intracellular Ca2+ release that triggered neurotransmission. Inhibition of neurotransmitter release by serotonin was still seen after blockade of all classical Gbetagamma effector pathways. Thus, Gbetagamma blocked neurotransmitter release downstream of Ca2+ entry and may directly target the exocytotic fusion machinery at the presynaptic terminal. (+info)
Calcium influx-independent depression of transmitter release by 5-HT at lamprey spinal cord synapses.
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1. The mechanisms by which 5-hydroxytryptamine (5-HT) depresses transmitter release from lamprey reticulospinal axons were investigated. These axons make glutamatergic synapses onto spinal ventral horn neurons. 5-HT reduces release at these synapses, yet the mechanisms remain unclear. 2. Excitatory postsynaptic currents (EPSCs) evoked by stimulation of reticulospinal axons were recorded in ventral horn neurons. 5-HT depressed the EPSCs in a dose-dependent manner with an apparent Km of 2.3 microM. 3. To examine the presynaptic effect of 5-HT, electrophysiological and optical recordings were made from presynaptic axons. Action potentials evoked Ca(2+) transients in the axons loaded with a Ca(2+)-sensitive dye. 5-HT slightly reduced the Ca(2+) transient. 4. A third-power relationship between Ca(2+) entry and transmitter release was determined. However, presynaptic Ca(2+) currents were unaffected by 5-HT. 5. Further, in the presence of a K(+) channel blocker, 4-aminopyridine (4-AP), 5-HT left unaltered the presynaptic Ca(2+) transient, ruling out the possibility of its direct action on presynaptic Ca(2+) current. 5-HT activated a 4-AP-sensitive current with a reversal potential of -95 mV in these axons. 6. The basal Ca(2+) concentration did not affect 5-HT-mediated inhibition of release. Although 5-HT caused a subtle reduction in resting axonal [Ca(2+)]i, synaptic responses recorded during enhanced resting [Ca(2+)]i, by giving stimulus trains, were equally depressed by 5-HT. 7. 5-HT reduced the frequency of TTX-insensitive spontaneous EPSCs at these synapses, but had no effect on their amplitude. We propose a mechanism of inhibition for transmitter release by 5-HT that is independent of presynaptic Ca(2+) entry. (+info)
Characterization of a high-voltage-activated IA current with a role in spike timing and locomotor pattern generation.
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Transient A-type K+ channels (I(A)) in neurons have been implicated in the delay of the spike onset and the decrease in the firing frequency. Here we have characterized biophysically and pharmacologically an I(A) current in lamprey locomotor network neurons that is activated by suprathreshold depolarization and is specifically blocked by catechol at 100 microM. The biophysical properties of this current are similar to the mammalian Kv3.4 channel. The role of the I(A) current both in single neuron firing and in locomotor pattern generation was analyzed. The I(A) current facilitates Na+ channel recovery from inactivation and thus sustains repetitive firing. The role of the I(A) current in motor pattern generation was examined by applying catechol during fictive locomotion induced by N-methyl-d-aspartate. Blockade of this current increased the locomotor burst frequency and decreased the firing of motoneurons. Although an alternating motor pattern could still be generated, the cycle duration was less regular, with ventral roots bursts failing on some cycles. Our results thus provide insights into the contribution of a high-voltage-activated I(A) current to the regulation of firing properties and motor coordination in the lamprey spinal cord. (+info)
Evolution of vertebrate steroid receptors from an ancestral estrogen receptor by ligand exploitation and serial genome expansions.
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The evolution of novelty in tightly integrated biological systems, such as hormones and their receptors, seems to challenge the theory of natural selection: it has not been clear how a new function for any one part (such as a ligand) can be selected for unless the other members of the system (e.g., a receptor) are already present. Here I show-based on identification and phylogenetic analysis of steroid receptors in basal vertebrates and reconstruction of the sequences and functional attributes of ancestral proteins-that the first steroid receptor was an estrogen receptor, followed by a progesterone receptor. Genome mapping and phylogenetic analyses indicate that the full complement of mammalian steroid receptors evolved from these ancient receptors by two large-scale genome expansions, one before the advent of jawed vertebrates and one after. Specific regulation of physiological processes by androgens and corticoids are relatively recent innovations that emerged after these duplications. These findings support a model of ligand exploitation in which the terminal ligand in a biosynthetic pathway is the first for which a receptor evolves; selection for this hormone also selects for the synthesis of intermediates despite the absence of receptors, and duplicated receptors then evolve affinity for these substances. In this way, novel hormone-receptor pairs are created, and an integrated system of increasing complexity elaborated. This model suggests that ligands for some "orphan" receptors may be found among intermediates in the synthesis of ligands for phylogenetically related receptors. (+info)
Crystalline ligand transitions in lamprey hemoglobin. Structural evidence for the regulation of oxygen affinity.
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The hemoglobins of the Sea Lamprey (Petromyzon marinus) exist in an equilibrium between low affinity oligomers, stabilized by proton binding, and higher affinity monomers, stabilized by oxygen binding. Recent crystallographic analysis revealed that dimerization is coupled with key changes at the ligand binding site with the distal histidine sterically restricting ligand binding in the deoxy dimer but with no significant structural rearrangements on the proximal side. These structural insights led to the hypothesis that oxygen affinity of lamprey hemoglobin is distally regulated. Here we present the 2.9-A crystal structure of deoxygenated lamprey hemoglobin in an orthorhombic crystal form along with the structure of these crystals exposed to carbon monoxide. The hexameric assemblage in this crystal form is very similar to those observed in the previous deoxy structure. Whereas the hydrogen bonding network and packing contacts formed in the dimeric interface of lamprey hemoglobin are largely unaffected by ligand binding, the binding of carbon monoxide induces the distal histidine to swing to positions that would preclude the formation of a stabilizing hydrogen bond with the bound ligand. These results suggest a dual role for the distal histidine and strongly support the hypothesis that ligand affinity in lamprey hemoglobin is distally regulated. (+info)
Ion channels of importance for the locomotor pattern generation in the lamprey brainstem-spinal cord.
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The intrinsic function of the spinal network that generates locomotion can be studied in the isolated brainstem-spinal cord of the lamprey, a lower vertebrate. The motor pattern underlying locomotion can be elicited in the isolated spinal cord. The network consists of excitatory glutamatergic and inhibitory glycinergic interneurones with known connectivity. The current review addresses the different subtypes of ion channels that are present in the cell types that constitute the network. In particular the roles of the different subtypes of Ca2+ channels and potassium channels that regulate integrated neuronal functions, like frequency regulation, spike frequency adaptation and properties that are important for generating features of the motor pattern (e.g. burst termination), are reviewed. By knowing the role of an ion channel at the cellular level, we also, based on previous knowledge of network connectivity, can understand which effect a given ion channel may exert at the different levels from molecule and cell to network and behaviour. (+info)