Suppression of sprouting: An early function of NMDA receptors in the absence of AMPA/kainate receptor activity

Electrophysiological studies have documented the existence of synapses showing only NMDA ionotropic glutamate receptor function that are therefore presumably "silent" at resting membrane potentials. Silent synapses are more prevalent in young than in older neurons, and NMDA receptor activity at such contacts may facilitate the appearance of functional AMPA receptors. However, it is uncertain whether such silent synapses actually have a function in young neurons independent of AMPA receptor induction. Using a newly characterized culture system for neurons from larval Xenopus tecta, we show that blocking NMDA receptors or preventing changes in intracellular free Ca2+ concentration with BAPTA AM significantly increases neurite sprouting and elongation in contacted but not in isolated neurons. Blocking AMPA/KA receptors or Na+-dependent action potentials does not mimic this effect. Moreover, in these young neurons, NMDA receptor-dependent Ca2+ responses to glutamate measured with confocal fluo-3 imaging are retained during AMPA/KA receptor blockade. The data suggest that many of the young contacts in these cultures are active even though they use only NMDA ionotropic glutamate receptors. Calcium influx through the NMDA receptor at these contacts seems to reduce neurite motility. This effect should lead to the accumulation of glutamatergic inputs on NMDA receptor-expressing dendrites, which could facilitate the onset of AMPA/KA receptor function and the action potential-dependent phase of synaptogenesis.

Experience-dependent modifications in MAP2 phosphorylation in rat olfactory bulb

Microtubule-associated protein 2 (MAP2) is a neuron-specific cytoskeletal protein, enriched in dendrites and cell bodies, that helps determine dendritic shape. MAP2 regulates microtubule stability in a phosphorylation-dependent manner. The present study used immunocytochemistry with phosphoepitope-specific and phosphorylation state-independent antibodies to examine experience-dependent changes in MAP2 expression during postnatal development of the olfactory bulb. Our results demonstrate that immunoreactivity reflecting total MAP2 expression reaches a maximal level by postnatal day 20 (P20). The degree of staining for phosphoindependent forms of MAP2 is relatively unaffected by blocking odorant passage to one half the nasal epithelium via unilateral naris closure, a manipulation that attenuates physiological activity in the bulb. However, olfactory restriction from P1 dramatically reduces immunoreactivity for antibody AP18, which recognizes MAP2 only when phosphorylated on Ser136. Quantification of staining in the granule cell layer indicates that the greatest difference (64%) between control and experimental bulbs occurs after occlusion from P1 to P30 compared with animals deprived from P1 to P10 or P1 to P20. The shift in MAP2 phosphorylation occurs even when deprivation is delayed until P30, after the sensitive period for experience-dependent changes in bulb volume. Thus, the degree of the phosphorylation shift depends on the duration but not the time of onset of naris closure. Because staining for phosphorylation-independent forms of MAP2 is unchanged by naris closure, the total amount of the protein per unit area is probably not significantly altered. However, the large reductions of AP18-immunoreactivity in the bulb after olfactory restriction suggest that there is an activity-dependent stimulation of MAP2 phosphorylation.

Activity-dependent regulation of NMDAR1 immunoreactivity in the developing visual cortex

NMDA receptors have been implicated in activity-dependent synaptic plasticity in the developing visual cortex. We examined the distribution of immunocytochemically detectable NMDAR1 in visual cortex of cats and ferrets from late embryonic ages to adulthood. Cortical neurons are initially highly immunostained. This level declines gradually over development, with the notable exception of cortical layers 2/3, where levels of NMDAR1 immunostaining remain high into adulthood. Within layer 4, the decline in NMDAR1 immunostaining to adult levels coincides with the completion of ocular dominance column formation and the end of the critical period for layer 4. To determine whether NMDAR1 immunoreactivity is regulated by retinal activity, animals were dark-reared or retinal activity was completely blocked in one eye with tetrodotoxin (TTX). Dark-rearing does not cause detectable changes in NMDAR1 immunoreactivity. However, 2 weeks of monocular TTX administration decreases NMDAR1 immunoreactivity in layer 4 of the columns of the blocked eye. Thus, high levels of NMDAR1 immunostaining within the visual cortex are temporally correlated with ocular dominance column formation and developmental plasticity; the persistence of staining in layers 2/3 also correlates with the physiological plasticity present in these layers in the adult. In addition, visual experience is not required for the developmental changes in the laminar pattern of NMDAR1 levels, but the presence of high levels of NMDAR1 in layer 4 during the critical period does require retinal activity. These observations are consistent with a central role for NMDA receptors in promoting and ultimately limiting synaptic rearrangements in the developing neocortex.

Temporal correlations between functional and molecular changes in NMDA receptors and GABA neurotransmission in the superior colliculus

Activation of the NMDA subtype of glutamate receptor is required for activity-dependent structural plasticity in many areas of the young brain. Previous work has shown that NMDA receptor currents decline approximately at the time that developmental synaptic plasticity ends, and in situ hybridization studies have suggested that receptor subunit changes may be occurring during the same developmental interval. To establish a system in which the relationship between these properties of developing synapses can be explored, we have combined patch-clamp recordings with mRNA- and protein-level biochemical analyses to study the developmental regulation of NMDA receptors in the superficial layers of the rat superior colliculus. These experiments document an abrupt decrease in the NMDA receptor contribution to synaptic currents that occurs before eye opening and is closely associated with changes in NR1 protein, rapidly rising levels of the NMDA receptor subunit NR2A, and decreasing levels of NR2B. The functional and molecular changes also are correlated with the developmental decline in structural plasticity in these layers. In addition, both physiological and biochemical methods show evidence of GABA-mediated inhibition in the superficial collicular layers beginning after eye opening. This may provide an additional heterosynaptic mechanism for controlling excitation and plasticity in this neuropil by pattern vision. Thus our findings lend support to the idea that high levels of NMDA receptor function are associated with the potential for structural rearrangement in CNS neuropil and that the functional downregulation of this molecule results, at least partially, from changes in its subunit composition.

Brain-derived neurotrophic factor mediates the activity-dependent regulation of inhibition in neocortical cultures

The excitability of cortical circuits is modulated by interneurons that release the inhibitory neurotransmitter GABA. In primate and rodent visual cortex, activity deprivation leads to a decrease in the expression of GABA. This suggests that activity is able to adjust the strength of cortical inhibition, but this has not been demonstrated directly. In addition, the nature of the signal linking activity to GABA expression has not been determined. Activity is known to regulate the expression of the neurotrophin brain-derived neurotrophic factor (BDNF), and BDNF has been shown to influence the phenotype of GABAergic interneurons. We use a culture system from postnatal rat visual cortex to test the hypothesis that activity is regulating the strength of cortical inhibition through the regulation of BDNF. Cultures were double-labeled against GABA and the neuronal marker MAP2, and the percentage of neurons that were GABA-positive was determined. Blocking spontaneous activity in these cultures reversibly decreased the number of GABA-positive neurons without affecting neuronal survival. Voltage-clamp analysis of inhibitory currents demonstrated that activity blockade also decreased GABA-mediated inhibition onto pyramidal neurons and raised pyramidal neuron firing rates. All of these effects were prevented by incubation with BDNF during activity blockade, but not by neurotrophin 3 or nerve growth factor. Additionally, blockade of neurotrophin signaling mimicked the effects of activity blockade on GABA expression. These data suggest that activity regulates cortical inhibition through a BDNF-dependent mechanism and that this neurotrophin plays an important role in the control of cortical excitability.

Activity-dependent expression of NT-3 in muscle cells in culture: implications in the development of neuromuscular junctions

Although activity-dependent expression of neurotrophins has been studied extensively in the CNS, its physiological role during synapse development is not well established. At the developing neuromuscular junction in culture, exogenous application of the neurotrophin BDNF or NT-3 has been shown to acutely potentiate synaptic transmission and chronically promote synapse maturation. Using the same cell culture model, we have investigated activity-dependent neurotrophin expression in muscle cells and its role in developing neuromuscular synapses. Membrane depolarization, elicited by either depolarizing agents or repetitive electric stimulation, rapidly and specifically increased the levels of NT-3 mRNA in developing Xenopus laevis muscle cells in culture. NT-3 gene expression also was enhanced by acetylcholine (ACh), the neurotransmitter that causes muscle membrane depolarization. The effects of depolarization were mediated by increasing intracellular calcium concentration. Moreover, factor(s) induced by membrane depolarization appeared to enhance synaptic transmission at the developing neuromuscular junction. The frequency of spontaneous synaptic currents (SSCs) recorded from neuromuscular synapses was increased significantly after treatment with conditioned medium from depolarized muscle cultures. The amplitude, rise time, and decay time of SSCs were not affected, indicating a presynaptic action of the conditioned medium. The effects of the conditioned medium were blocked, partially, by the NT-3 scavenger TrkC-IgG, suggesting that the potentiation of synaptic efficacy was attributable, at least in part, to elevated NT-3 as a consequence of muscle depolarization. Thus, activity-dependent expression of muscle NT-3 may contribute to the development of the neuromuscular synapse.