Activity-dependent metaplasticity of inhibitory and excitatory synaptic transmission in the lamprey spinal cord locomotor network.
Paired intracellular recordings have been used to examine the activity-dependent plasticity and neuromodulator-induced metaplasticity of synaptic inputs from identified inhibitory and excitatory interneurons in the lamprey spinal cord. Trains of spikes at 5-20 Hz were used to mimic the frequency of spiking that occurs in network interneurons during NMDA or brainstem-evoked locomotor activity. Inputs from inhibitory and excitatory interneurons exhibited similar activity-dependent changes, with synaptic depression developing during the spike train. The level of depression reached was greater with lower stimulation frequencies. Significant activity-dependent depression of inputs from excitatory interneurons and inhibitory crossed caudal interneurons, which are central elements in the patterning of network activity, usually developed between the fifth and tenth spikes in the train. Because these interneurons typically fire bursts of up to five spikes during locomotor activity, this activity-dependent plasticity will presumably not contribute to the patterning of network activity. However, in the presence of the neuromodulators substance P and 5-HT, significant activity-dependent metaplasticity of these inputs developed over the first five spikes in the train. Substance P induced significant activity-dependent depression of inhibitory but potentiation of excitatory interneuron inputs, whereas 5-HT induced significant activity-dependent potentiation of both inhibitory and excitatory interneuron inputs. Because these metaplastic effects are consistent with the substance P and 5-HT-induced modulation of the network output, activity-dependent metaplasticity could be a potential mechanism underlying the coordination and modulation of rhythmic network activity. (+info)
Otx expression during lamprey embryogenesis provides insights into the evolution of the vertebrate head and jaw.
Agnathan or jawless vertebrates, such as lampreys, occupy a critical phylogenetic position between the gnathostome or jawed vertebrates and the cephalochordates, represented by amphioxus. In order to gain insight into the evolution of the vertebrate head, we have cloned and characterized a homolog of the head-specific gene Otx from the lamprey Petromyzon marinus. This lamprey Otx gene is a clear phylogenetic outgroup to both the gnathostome Otx1 and Otx2 genes. Like its gnathostome counterparts, lamprey Otx is expressed throughout the presumptive forebrain and midbrain. Together, these results indicate that the divergence of Otx1 and Otx2 took place after the gnathostome/agnathan divergence and does not correlate with the origin of the vertebrate brain. Intriguingly, Otx is also expressed in the cephalic neural crest cells as well as mesenchymal and endodermal components of the first pharyngeal arch in lampreys, providing molecular evidence of homology with the gnathostome mandibular arch and insights into the evolution of the gnathostome jaw. (+info)
Development of cephalic neural crest cells in embryos of Lampetra japonica, with special reference to the evolution of the jaw.
Neural crest cells contribute extensively to vertebrate head morphogenesis and their origin is an important question to address in understanding the evolution of the craniate head. The distribution pattern of cephalic crest cells was examined in embryos of one of the living agnathan vertebrates, Lampetra japonica. The initial appearance of putative crest cells was observed on the dorsal aspect of the neural rod at stage 20.5 and ventral expansion of these cells was first seen at the level of rostral somites. As in gnathostomes, cephalic crest cells migrate beneath the surface ectoderm and form three major cell populations, each being separated at the levels of rhombomeres (r) 3 and r5. The neural crest seems initially to be produced at all neuraxial levels except for the rostral-most area, and cephalic crest cells are secondarily excluded from levels r3 and r5. Such a pattern of crest cell distribution prefigures the morphology of the cranial nerve anlage. The second or middle crest cell population passes medial to the otocyst, implying that the otocyst does not serve as a barrier to separate the crest cell populations. The three cephalic crest cell populations fill the pharyngeal arch ventrally, covering the pharyngeal mesoderm laterally with the rostral-most population covering the premandibular region and mandibular arch. The third cell population is equivalent to the circumpharyngeal crest cells in the chick, and its influx into the pharyngeal region precedes the formation of postotic pharyngeal arches. Focal injection of DiI revealed the existence of an anteroposterior organization in the neural crest at the neurular stage, destined for each pharyngeal region. The crest cells derived from the posterior midbrain that express the LjOtxA gene, the Otx2 cognate, were shown to migrate into the mandibular arch, a pattern which is identical to gnathostome embryos. It was concluded that the head region of the lamprey embryo shares a common set of morphological characters with gnathostome embryos and that the morphological deviation of the mandibular arch between the gnathostomes and the lamprey is not based on the early embryonic patterning. (+info)
Interactive effects of the GABABergic modulation of calcium channels and calcium-dependent potassium channels in lamprey.
The GABAB-mediated modulation of spinal neurons in the lamprey is investigated in this study. Activation of GABAB receptors reduces calcium currents through both low- (LVA) and high-voltage activated (HVA) calcium channels, which subsequently results in the reduction of the calcium-dependent potassium (KCa) current. This in turn will reduce the peak amplitude of the afterhyperpolarization (AHP). We used the modulatory effects of GABAB receptor activation on N-methyl-D-aspartate (NMDA)-induced, TTX-resistant membrane potential oscillations as an experimental model in which to separate the effects of GABAB receptor activation on LVA calcium channels from that on KCa channels. We show experimentally and by using simulations that a direct effect on LVA calcium channels can account for the effects of GABAB receptor activation on intrinsic membrane potential oscillations to a larger extent than indirect effects mediated via KCa channels. Furthermore, by conducting experiments and simulations on intrinsic membrane potential oscillations, we find that KCa channels may be activated by calcium entering through LVA calcium channels, providing that the decay kinetics of the calcium that enters through LVA calcium channels is not as slow as the calcium entering via NMDA receptors. A combined experimental and computational analysis revealed that the LVA calcium current also contributes to neuronal firing properties. (+info)
Calcium channels involved in synaptic transmission from reticulospinal axons in lamprey.
The pharmacology of calcium channels involved in glutamatergic synaptic transmission from reticulospinal axons in the lamprey spinal cord was analyzed with specific agonists and antagonists of different high-voltage activated calcium channels. The N-type calcium channel blocker omega-conotoxin GVIA (omega-CgTx) induced a large decrease of the amplitude of reticulospinal-evoked excitatory postsynaptic potentials (EPSPs). The P/Q-type calcium channel blocker omega-agatoxin IVA (omega-Aga) also reduced the amplitude of the reticulospinal EPSPs, but to a lesser extent than omega-CgTx. The dihydropyridine agonist Bay K and antagonist nimodipine had no effect on the amplitude of the reticulospinal EPSP. Combined application of omega-CgTx and omega-Aga strongly decreased the amplitude the EPSPs but was never able to completely block them, indicating that calcium channels insensitive to these toxins (R-type) are also involved in synaptic transmission from reticulospinal axons. We have previously shown that the group III metabotropic glutamate receptor agonist L(+)-2-amino-4-phosphonobutyric acid (L-AP4) mediates presynaptic inhibition at the reticulospinal synapse. To test if this presynaptic effect is mediated through inhibition of calcium influx, the effect of L-AP4 on reticulospinal transmission was tested before and after blockade of N-type channels, which contribute predominantly to transmitter release at this synapse. Blocking the N-type channels with omega-CgTx did not prevent inhibition of reticulospinal synaptic transmission by L-AP4. In addition, L-AP4 had no affect on the calcium current recorded in the somata of reticulospinal neurons or on the calcium component of action potentials in reticulospinal axons. These results show that synaptic transmission from reticulospinal axons in the lamprey is mediated by calcium influx through N-, P/Q- and R-type channels, with N-type channels playing the major role. Furthermore, presynaptic inhibition of reticulospinal transmission by L-AP4 appears not to be mediated through inhibition of presynaptic calcium channels. (+info)
Neuropeptide-mediated facilitation and inhibition of sensory inputs and spinal cord reflexes in the lamprey.
The effects of neuromodulators present in the dorsal horn [tachykinins, neuropeptide Y (NPY), bombesin, and GABAB agonists] were studied on reflex responses evoked by cutaneous stimulation in the lamprey. Reflex responses were elicited in an isolated spinal cord preparation by electrical stimulation of the attached tail fin. To be able to separate modulator-induced effects at the sensory level from that at the motor or premotor level, the spinal cord was separated into three pools with Vaseline barriers. The caudal pool contained the tail fin. Neuromodulators were added to this pool to modulate sensory inputs evoked by tail fin stimulation. The middle pool contained high divalent cation or low calcium Ringer to block polysynaptic transmission and thus limit the input to the rostral pool to that from ascending axons that project through the middle pool. Ascending inputs and reflex responses were monitored by making intracellular recordings from motor neurons and extracellular recordings from ventral roots in the rostral pool. The tachykinin neuropeptide substance P, which has previously been shown to potentiate sensory input at the cellular and synaptic levels, facilitated tail fin-evoked synaptic inputs to neurons in the rostral pool and concentration dependently facilitated rostral ventral root activity. Substance P also facilitated the modulatory effects of tail fin stimulation on ongoing locomotor activity in the rostral pool. In contrast, NPY and the GABAB receptor agonist baclofen, both of which have presynaptic inhibitory effects on sensory afferents, reduced the strength of ascending inputs and rostral ventral root responses. We also examined the effects of the neuropeptide bombesin, which is present in sensory axons, at the cellular, synaptic, and reflex levels. As with substance P, bombesin increased tail fin stimulation-evoked inputs and ventral root responses in the rostral pool. These effects were associated with the increased excitability of slowly adapting mechanosensory neurons and the potentiation of glutamatergic synaptic inputs to spinobulbar neurons. These results show the possible behavioral relevance of neuropeptide-mediated modulation of sensory inputs at the cellular and synaptic levels. Given that the types and locations of neuropeptides in the dorsal spinal cord of the lamprey show strong homologies to that of higher vertebrates, these results are presumably relevant to other vertebrate systems. (+info)
Presynaptic mitochondria and the temporal pattern of neurotransmitter release.
Mitochondria are critical for the function of nerve terminals as the cycling of synaptic vesicle membrane requires an efficient supply of ATP. In addition, the presynaptic mitochondria take part in functions such as Ca2+ buffering and neurotransmitter synthesis. To learn more about presynaptic mitochondria, we have examined their organization in two types of synapse in the lamprey, both of which are glutamatergic but are adapted to different temporal patterns of activity. The first is the giant lamprey reticulospinal synapse, which is specialized to transmit phasic signals (i.e. bursts of impulses). The second is the synapse established by sensory dorsal column axons, which is adapted to tonic activity. In both cases, the presynaptic axons were found to contain two distinct types of mitochondria; small 'synaptic' mitochondria, located near release sites, and larger mitochondria located in more central parts of the axon. The size of the synapse-associated mitochondria was similar in both types of synapse. However, their number differed considerably. Whereas the reticulospinal synapses contained only single mitochondria within 1 micron distance from the edge of the active zone (on average 1.2 per active zone, range of 1-3), the tonic dorsal column synapses were surrounded by clusters of mitochondria (4.5 per active zone, range of 3-6), with individual mitochondria sometimes apparently connected by intermitochondrial contacts. In conjunction with studies of crustacean neuromuscular junctions, these observations indicate that the temporal pattern of transmitter release is an important determinant of the organization of presynaptic mitochondria. (+info)
Commissural interneurons in rhythm generation and intersegmental coupling in the lamprey spinal cord.
Commissural interneurons in rhythm generation and intersegmental coupling in the lamprey spinal cord. To test the necessity of spinal commissural interneurons in the generation of the swim rhythm in lamprey, longitudinal midline cuts of the isolated spinal cord preparation were made. Fictive swimming was then induced by bath perfusion with an excitatory amino acid while recording ventral root activity. When the spinal cord preparation was cut completely along the midline into two lateral hemicords, the rhythmic activity of fictive swimming was lost, usually replaced with continuous ventral root spiking. The loss of the fictive swim rhythm was not due to nonspecific damage produced by the cut because rhythmic activity was present in split regions of spinal cord when the split region was still attached to intact cord. The quality of this persistent rhythmic activity, quantified with an autocorrelation method, declined with the distance of the split spinal segment from the remaining intact spinal cord. The deterioration of the rhythm was characterized by a lengthening of burst durations and a shortening of the interburst silent phases. This pattern of deterioration suggests a loss of rhythmic inhibitory inputs. The same pattern of rhythm deterioration was seen in preparations with the rostral end of the spinal cord cut compared with those with the caudal end cut. The results of this study indicate that commissural interneurons are necessary for the generation of the swimming rhythm in the lamprey spinal cord, and the characteristic loss of the silent interburst phases of the swimming rhythm is consistent with a loss of inhibitory commissural interneurons. The results also suggest that both descending and ascending commissural interneurons are important in the generation of the swimming rhythm. The swim rhythm that persists in the split cord while still attached to an intact portion of spinal cord is thus imposed by interneurons projecting from the intact region of cord into the split region. These projections are functionally short because rhythmic activity was lost within approximately five spinal segments from the intact region of spinal cord. (+info)