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M Constantine-Paton

Publications and source records attributed to M Constantine-Paton.

At least 37 records · Page 2Linked to original sources
PubMed1994

Glutamate receptor binding in juvenile and adult Rana pipiens CNS.

Autoradiographic methods were used to map NMDA- and quisqualate-sensitive glutamate binding sites in the brain of mature and juvenile Rana pipiens frogs. NMDA- and quisqualate-sensitive sites were consistently co-localized in the CNS. The highest glutamate binding occurred in the telencephalon, hypothalamus, and cerebellum. Glutamate binding sites were also specifically localized in visual pathways, including the superficial neuropil of the optic tectum, consistent with glutamate being the retinal ganglion cell neurotransmitter. The distribution of glutamate binding sites in the brain of juvenile postmetamorphic frogs was similar to that in adults. In general, Quis binding increased about twofold in adults compared to juveniles, whereas NMDA binding did not show a comparable developmental increase. To test whether glutamate binding sites are located on retinal axon terminals or on tectal cell dendrites in the optic tectum, juvenile postmetamorphic frogs were enucleated unilaterally, and receptor binding was performed following 1, 3, 7, and 14 days survival. The denervated tectal neuropil showed a delayed decrease in NMDA- and quisqualate-sensitive binding, consistent with the receptors being located on postsynaptic tectal cell dendrites.

Afferent Pathways↗
PubMed1994

Regulation of N-methyl-D-aspartate (NMDA) receptor function during the rearrangement of developing neuronal connections.

There is evidence from a number of studies that the molecular and biophysical properties of NMDA receptors are altered during normal development. A temporal correlation with changes in NMDA receptor efficacy and periods of synaptic plasticity has been demonstrated in several systems, suggesting that NMDA receptors have a critical function in determining periods of synaptic plasticity. Data from our laboratory demonstrate reduced NMDA sensitivity of the tectal evoked potential following chronic application of NMDA to the tadpole tectum, a treatment that may mimic a naturally occurring mechanism for limiting neuronal plasticity to certain stages of development. Our analysis of the expression pattern of mRNA coding for various glutamate receptor subunits in the rat retinocollicular system establishes that differential regulation of NMDA receptor subunits at the mRNA level could be a molecular basis for changes in biophysical and pharmacological properties of the NMDA receptor complex. However, even though the NMDA receptor is the best studied candidate to function as a 'plasticity switch', there are large gaps in our understanding of the complete set of factors that control the ability of synapses to rearrange during development.

Animals↗
PubMed1994

Regulation of NMDA receptor mRNA during visual map formation and after receptor blockade.

The topographic refinement of the rat retinocollicular projection is dependent on normal NMDA receptor function. Here we examined the expression of NMDA and non-NMDA glutamate receptor mRNA in the rat superior colliculus (SC) during this postnatal refinement period. The temporal expression pattern of mRNA coding for the NMDA receptor subunit NR1 in the superficial SC followed the time course of collicular synaptogenesis. A pronounced increase of NR1 mRNA levels occurred during the late stages of retinocollicular map refinement. In cortex, the time course of the expression of NR1 mRNA in cortex was found to be similar to that observed in SC, with low levels during the first postnatal week, a maximum at P19, and a decrease thereafter. In SC, but not in cortex, there was a change in the ratio of the two NR1 transcripts during the second postnatal week that parallels a previously demonstrated developmental change in the mean open time of NMDA channels in collicular neurons. In contrast, the mRNA expression pattern of the non-NMDA receptor subunit GluR2 in the developing SC was not closely correlated with synaptic changes. Chronic treatment of the SC with the NMDA receptor antagonist 2-amino-5-phosphonovalerate (APV) for 12 or 19 days, which disrupts retinocollicular map formation, appears to block the developmental rise in NR1 mRNA levels. These findings support a specific role for the NMDA receptor subtype of glutamate receptors in the control of synaptogenesis and developmental plasticity in the SC.

2-Amino-5-phosphonovalerate↗
PubMed1994

Modulation of NMDA receptor function: implications for vertebrate neural development.

The NMDA subtype of glutamate receptor is hypothesized to mediate synaptic competition in the developing brain by stabilizing converging synapses that have correlated activity patterns. Disruption of NMDA receptor function during development interferes with synapse elimination and sensory map formation. Moreover, many studies indicate that NMDA receptor function is high during times of synaptic rearrangement. In this review, a corollary of the NMDA receptor hypothesis for activity-dependent synapse stabilization is proposed. As developing inputs increase in number and strength, the increasing excitatory synaptic activity in young neurons should lead to increases in postsynaptic Ca2+ influx through NMDA receptors. This Ca2+ flux is postulated to trigger a feedback system that changes the subunit composition of the NMDA receptor complex so that less Ca2+ enters postsynaptic cells upon NMDA receptor activation. Changes in NMDA receptor effectiveness resulting from manipulations of activity are consistent with the idea that NMDA receptor function is under the control of activity. This postulate of activity-dependent control of NMDA receptor expression has implications for the control of brain plasticity. If particular combinations of NMDA receptor subunits typically expressed in young animals are better than adult receptor types at maintaining synapses in regions where they are not well correlated with other inputs, then expression of these juvenile subunit combinations could facilitate synaptic rearrangements in the mature brain after the normal end of synaptic plasticity. Thus, understanding the regulation of NMDA receptor function during development could provide a novel approach to restructuring circuitry in the adult brain to compensate for damage produced by trauma or disease.

Animals↗
PubMed1994

Effects of NMDA receptor antagonists on the developing brain.

This overview describes two effects of N-methyl-D-aspartate (NMDA) receptor antagonists that are not strictly speaking toxic: There are no signs that cells are dying. Nevertheless, these antagonists, if applied for prolonged periods in young children, could permanently cripple normal brain function. Disturbing the function of the NMDA channel during development can severely disrupt the wiring of defined neural circuits. It also can disrupt the developmental upregulation of the receptor protein itself and possibly many other molecular components of the synapse.

Animals↗
PubMed1993

The development of non-retinal afferent projections to the frog optic tectum and the substance P immunoreactivity of tectal connections.

Accessibility in early development and the presence of a retinotopic map have made the amphibian optic tectum a popular system for exploration of the role of synaptic function in central map refinement. Although a great deal is known about the development of retinal innervation of the tectum, little information exists about the development of non-retinal tectal inputs. Since these other afferent systems may contribute to the synaptic drive of developing tectal cells and thereby be involved in the activity-dependent refinement of the retinotectal map, we sought to determine whether these inputs are present at the early tadpole stages when the first retinal axons refine their synaptic order within the tectal neuropil. Rhodamine-labelled latex beads, retrogradely transported from injection sites in the optic tecta, were used to identify tectal afferent projections. Projection patterns in very young tadpoles were identical to those found in juvenile frogs and heavily labelled regions included areas of the posteroventral tegmental field, the posterior tuberculum, the ventromedial thalamic nucleus, the ventral part of the ventrolateral thalamic nucleus, the suprachiasmatic nucleus and discrete regions within the central and anterior thalamic nuclei. Ipsilateral nucleus isthmi cells were also labelled, indicating the existence of an isthmo-tectal projection in even the youngest animals examined. Additionally, substance P-like immunoreactive tecto-isthmal fibers were traced from the optic tectum to the nucleus isthmi. The presence of these connections suggests that feedback from the nucleus isthmi and/or input from the other brain areas projecting to the tectum may play a role in modulating the cellular mechanisms that underlie the formation of the visual map.

Afferent Pathways↗
PubMed1993

The contributions of NMDA, non-NMDA, and GABA receptors to postsynaptic responses in neurons of the optic tectum.

Activation of the NMDA subtype of glutamate receptor has been implicated in activity-dependent development and plasticity in several systems, including the retinotectal system of amphibians. To gain a better understanding of the response properties of tectal neurons, with particular emphasis on the role of both non-NMDA and NMDA glutamate receptors, we have developed an in vitro slice preparation of the diencephalon and midbrain of frog (Rana pipiens) tadpoles. In these slices, we electrically stimulated the optic tract and recorded both mono- and polysynaptic responses in single tectal neurons using whole-cell voltage clamp or current clamp. By including biocytin in the recording electrode, we were also able to determine the location and morphology of many of these neurons. Using these techniques, we found that the current-voltage (I-V) relations for both mono- and polysynaptic responses of tectal neurons showed voltage dependence only in the presence of extracellular Mg2+. This dependence reflects the hyperpolarization-dependent block of the NMDA channel by Mg2+. Bath application of 6-cyano-7-nitroquinoxaline-2,3-dione, a non-NMDA glutamate receptor antagonist, reduced both mono- and polysynaptic responses of tectal neurons. Bath application of the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (DL-APV) strongly reduced polysynaptic responses. When neurons were depolarized by the voltage clamp, relieving the Mg(2+)-dependent block of the NMDA channel, DL-APV application also reduced monosynaptic responses. Application of the GABAA receptor antagonist (-)bicuculline methiodide significantly increased the polysynaptic responses of tectal neurons, reflecting block of inhibition. We further confirmed the presence of these three types of receptors by examining postsynaptic currents evoked by iontophoretic application of the three agonists, NMDA, (R,S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), and GABA. These results confirm that the dominant excitatory transmitter in the tectum appears to be glutamate. Furthermore, the retinotectal synapses (i.e., monosynaptic currents) express functional NMDA receptors that are voltage dependent and are not responsible for the bulk of normal excitatory transmission. Polysynaptic responses, however, are mediated by both non-NMDA and NMDA receptors, and inhibition plays a significant role in sculpting these polysynaptic responses.

2-Amino-5-phosphonovalerate↗
PubMed1993

Fine-structural alterations and clustering of developing synapses after chronic treatments with low levels of NMDA.

In the visual pathway of frogs it is possible to apply low levels of NMDA chronically to the optic tectum and study the mechanisms underlying the stabilization of synapses developing within the CNS. Earlier studies (Cline and Constantine-Paton, 1990) found that chronic NMDA treatment of tecta innervated by two retinas results in a reduction of branching within the terminal arbors of retinal ganglion cells (RGCs). We now report that this same chronic NMDA treatment produces fine-structural changes in synaptic morphology as well as local synaptic rearrangements within the retinotectal neuropil. Chronic NMDA treatment of doubly innervated tecta was associated with a thickening or darkening of both pre- and postsynaptic densities. These changes in synapse morphology were restricted to the superficial neuropil of tecta in regions where reductions in branches of RGC axonal arbors were observed at the light microscopic level. The fine-structural effects were absent from similarly treated tecta innervated by only one eye, where RGC axonal arbor pruning was not observed. Stereological analyses indicated that the incidence of two or more presynaptic profiles converging on the same postsynaptic process was significantly increased in the NMDA-treated, doubly innervated tecta. This observed increase in synaptic clustering was not associated with a larger synaptic active zone, or with an increase in the number of synapses per unit volume. These data are discussed in the context of the hypothesis that chronic NMDA treatment raises the threshold for synapse stabilization in tectal neurons, causing the selective loss of poorly correlated synapses of both retinal and non-retinal origin from tectal neuropil that is innervated by two retinas: increased pre- and postsynaptic thickening could reflect greater efficiency in the remaining synaptic contacts and their closer spatial proximity on the same postsynaptic process is consistent with greater cooperativity and less competition.

Animals↗
PubMed1992

N-methyl-D-aspartate receptor antagonists disrupt the formation of a mammalian neural map.

The topographic ordering of retinal connections in the rat superior colliculus emerges during early postnatal life from an initially diffuse projection. Disruption of N-methyl-D-aspartate (NMDA) receptor activity in the superior colliculus during this period interferes with map remodeling. In rats chronically treated with NMDA receptor antagonists during the first two postnatal weeks, aberrant axons remain and arborize at topographically incorrect sites. These results indicate that, at a stage preceding visually evoked activity, normal NMDA receptor function is important for the development of an ordered neural map in the mammalian brain.

2-Amino-5-phosphonovalerate↗
PubMed1992

Physiological effects of chronic and acute application of N-methyl-D-aspartate and 5-amino-phosphonovaleric acid to the optic tectum of Rana pipiens frogs.

Visually elicited activity contributes to the formation of orderly connections in the optic tectum of frogs. Glutamate receptors of the N-methyl-D-aspartate class participate in this process. Blocking those receptors interferes with activity-dependent refinement of maps in normal frogs and of ocular dominance bands in surgically produced animals with three eyes. Chronic application of N-methyl-D-aspartate sharpens the bands. The possibility that 5-amino-phosphonovaleric acid depresses tectal responsiveness was motivation for studying the effects of 5-amino-phosphonovaleric acid and N-methyl-D-aspartate applied both chronically and acutely. We evaluated tectal responsiveness to visual input by presenting flashes of light to one eye and recording responses in the ipsilateral tectal lobe. This method reveals the output of the tectal cells contralateral to the stimulated eye. These cells project via the nucleus isthmi to the opposite tectal lobe. We also mapped the receptive field dimensions of the crossed isthmotectal axons. Our results show that acute topical application of 500 microM or 1 mM N-methyl-D-aspartate dramatically increases spontaneous activity, while 100 microM N-methyl-D-aspartate causes little change. Chronic treatment with N-methyl-D-aspartate at a low dose (estimated to be in the micromolar range) shown to influence retinotectal mapping, reduces response latencies but produces no statistically significant changes in tectal cell firing rates or receptive field size. Acute application of 5-amino-phosphonovaleric acid produces complex results: 10 microM produces no changes in firing, 100 microM 5-amino-phosphonovaleric acid decreases firing, and doses of 500-100 microM increase the firing.(ABSTRACT TRUNCATED AT 250 WORDS)

2-Amino-5-phosphonovalerate↗
PubMed1991

Chronic application of NMDA decreases the NMDA sensitivity of the evoked tectal potential in the frog.

The activity-dependent mechanism that refines the topography of the retinotectal projection in frogs is mediated by the NMDA receptor. Earlier studies found that chronic treatment of the optic tectum with the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (DL-AP5) desegregated eye-specific stripes in three-eyed frogs, while chronic treatment with NMDA sharpened stripe borders (Cline et al., 1987; Cline and Constantine-Paton, 1990). We now report that this same chronic treatment with NMDA decreases the electro-physiologically measured sensitivity of the optic tectum to applied NMDA: acute application of a given concentration of NMDA decreased the evoked tectal potential to a lesser extent in animals chronically treated with NMDA than it did in normal and sham-treated animals. This is observed as a shift to the right in the NMDA dose-response curves for both the positive and negative postsynaptic components of the evoked tectal response. We believe that this decreased NMDA receptor effectiveness further restricts the intermingling of axon branches from the two eyes by limiting synapse stabilization to areas where afferent activity is most correlated. This would account for the anatomical sharpening of stripe borders (i.e., increased afferent segregation). Quantitative autoradiographic analysis of 3H-glutamate binding to NMDA receptors indicated that binding densities within the tectum did not differ between control groups and NMDA chronically treated groups. We suggest that in the experimental animals the response to NMDA may be decreased by a change in the effectiveness of individual NMDA receptors rather than by decreases in receptor number. This experimentally induced change may be analogous to naturally occurring decreases in receptor function that correlate with the end of some periods of visual plasticity in mammals.

2-Amino-5-phosphonovalerate↗
PubMed1990

Activity-dependent tuning and the NMDA receptor.

The refinement of the topographic map of visual space within the optic tectum of the frog is activity-dependent. The use of the three-eyed frog preparation to assay the operation of this fine-tuning mechanism indicates that this process is mediated by the NMDA receptor: Chronic in vivo treatment with APV, an NMDA antagonist, disrupts the segregation of retinal afferents into eye-specific zones while NMDA treatment sharpens this pattern. This latter effect is accompanied by a decreased sensitivity of the system to applied NMDA. Activation of the NMDA receptor may mediate the fine-tuning mechanism by initiating the stabilization of appropriate synapses. The requirements for NMDA receptor activation necessitate the convergence of terminals carrying correlated activity patterns. Such patterns of activity are provided by ganglion cells whose cell bodies lie near one another in the retina, and who should therefore, in an accurate visual map, terminate near one another in the tectum. Synapses from ganglion cells who do not neighbor one another in the retina have uncorrelated firing patterns and therefore do not activate the NMDA receptor. These synapses then would not be stabilized relative to one another. In addition to organizing the retinal projection, NMDA receptor activation may also modulate retinal ganglion cell arbor morphology, since chronic in vivo APV or NMDA treatments decrease arbor density. These results are discussed in terms of the effect of NMDA receptor activation on branch initiation and the rate of branch retraction.

Animals↗
PubMed1990

The role of the NMDA receptor in the development of the frog visual system.

The importance of patterned retinal activity in visual system development has been recognized since Hubel and Wiesels early experiments (1963). The NMDA receptor is one cellular mechanism which can recognize patterned retinal activity and convey an intracellular message of that activity. It will now be of considerable interest to elucidate the cellular events involved in synapse stabilization subsequent to NMDA receptor activation.

Animals↗
PubMed1990

Evoked pre- and post-synaptic activity in the optic tectum of the cannulated tadpole.

We describe the cannulated Rana pipiens, tadpole preparation that allows for stable recording in the tectum of the extracellular potential elicited by optic nerve stimulation. The largest components of the evoked tectal response consist of two previously identified waves and a major third, long-latency wave of long duration. These components were reversibly eliminated by perfusion of high magnesium/no calcium Ringer's solution or Ringer's solution containing cobalt chloride. In contrast, perfusion of high calcium/no magnesium Ringer's increased the amplitude and area of these components. We conclude that these components represent post-synaptic activity. Additionally, small, short-duration waves were identified as arising from the activity of retinal afferents. They consisted of a short-latency (3.1-7.6 ms) and a long-latency (12-23 ms) group. Waves belonging to both of these classes were still visible in both high magnesium/no calcium Ringer's solution or Ringer's solution containing cobalt chloride and were unaffected by high calcium/no magnesium Ringer's. The average conduction velocities of the short- and long-latency groups matched the conduction velocities of, respectively, edge and convexity detectors in the adult. This indicates that retinal afferent input may already be present in adult patterns at the time that tectal circuitry is developing.

Animals↗
PubMed1990

Granule cell induction of 9-O-acetyl gangliosides on cerebellar glia in microcultures.

In previous studies we have shown that the expression of acetylated gangliosides recognized by the JONES monoclonal antibody is correlated with regions of cell migration in the developing rat nervous system. In this study we have investigated the expression of these gangliosides in two different types of cultures prepared from dissociated postnatal rat cerebella. In the first type, cells are plated after dissociation under conditions where most of the glial cells develop a stellate morphology that anchors neurons but does not support their migration. In the second type of culture, cells are plated in a ratio of four neurons to one glial cell and under these conditions the predominant form of astroglia is an elongate form that supports the migration of granule neurons. Granule neurons express JONES antigens in dissociated cell suspensions and in cultures in which cells are plated either after dissociation or in a 4:1 neuron:glia ratio. On the other hand, glial cells grown in the absence of neurons are JONES negative. In addition, the expression of JONES gangliosides by glial cells is different in the two types of culture. In cultures where the astroglial cells display the stellate morphology only a small proportion show JONES staining. Cultures in which the glial cells assume the elongate morphology have a significantly higher number of JONES-positive astroglia.

Acetylation↗
PubMed1990

The differential influence of protein kinase inhibitors on retinal arbor morphology and eye-specific stripes in the frog retinotectal system.

We investigated retinal axon morphology and eye-specific afferent termination zones in the optic tectum of three-eyed tadpoles that were chronically treated with protein kinase inhibitors. The kinase inhibitors sphingosine, H-7, and phorbol ester, which down-regulates protein kinase C with chronic exposure, were applied to the tecta in a slow release plastic, Elvax. In vivo protein phosphorylation assays in drug-treated tadpoles indicated that the treatments decreased 32P incorporation into some protein bands by as much as 60%. Although the drugs did not cause a desegregation of the eye-specific stripes, treated retinal axon arbors covered about half the area covered by untreated arbors or arbors treated with inactive analogs of the drugs. We conclude that eye-specific segregation can be maintained under conditions that markedly alter retinal ganglion cell axon arbor size and that significantly perturb protein phosphorylation. Furthermore, we conclude that the protein kinase(s) that we blocked with these treatments is involved in the growth of axon arbors.

1-(5-Isoquinolinesulfonyl)-2-Methylpiperazine↗