Long-term potentiation during the activity-dependent sharpening of the retinotopic map in goldfish

A family of photoswitchable NMDA receptors

NMDA receptors, which regulate synaptic strength and are implicated in learning and memory, consist of several subtypes with distinct subunit compositions and functional properties. To enable spatiotemporally defined, rapid and reproducible manipulation of function of specific subtypes, we engineered a set of photoswitchable GluN subunits ('LiGluNs'). Photo-agonism of GluN2A or GluN2B elicits an excitatory drive to hippocampal neurons that can be shaped in time to mimic synaptic activation. Photo-agonism of GluN2A at single dendritic spines evokes spine-specific calcium elevation and expansion, the morphological correlate of LTP. Photo-antagonism of GluN2A alone, or in combination with photo-antagonism of GluN1a, reversibly blocks excitatory synaptic currents, prevents the induction of long-term potentiation and prevents spine expansion. In addition, photo-antagonism in vivo disrupts synaptic pruning of developing retino-tectal projections in larval zebrafish. By providing precise and rapidly reversible optical control of NMDA receptor subtypes, LiGluNs should help unravel the contribution of specific NMDA receptors to synaptic transmission, integration and plasticity.

N-methyl-D-aspartate receptors strongly regulate postsynaptic activity levels during optic nerve regeneration

During development, neuronal activity is used as a cue to guide synaptic rearrangements to refine connections. Many studies, especially in the visual system, have shown that the N-methyl-D-aspartate receptor (NMDAr) plays a key role in mediating activity-dependent refinement through long-term potentiation (LTP)-like processes. Adult goldfish can regenerate their optic nerve and utilize neuronal activity to generate precise topography in their projection onto tectum. Although the NMDAr has been implicated in this process, its precise role in regeneration has not been extensively studied. In examining NMDAr function during regeneration, we found salient differences compared with development. By using field excitatory postsynaptic potential (fEPSP) recordings, the contribution of the NMDAr at the primary optic synapse was measured. In contrast to development, no increase in NMDAr function was detectable during synaptic refinement. Unlike development, LTP could not be reliably elicited during regeneration. Unexpectedly, we found that NMDAr exerted a major effect on regulating ongoing tectal (postsynaptic) activity levels during regeneration. Blocking NMDAr strongly suppressed spontaneous activity during regeneration but had no significant effect in the normal projection. This difference could be attributed to an occlusion effect of strong optic drive in the normal projection, which dominated ongoing tectal activity. During regeneration, this optic drive is largely absent. Optic nerve stimulation further indicated that the NMDAr had little effect on the ability of optic fibers to evoke early postsynaptic impulse activity but was important for late network activity. These results indicate that, during regeneration, the NMDAr may play a critical role in the homeostatic regulation of ongoing activity and network excitability.

Rapid homeostatic plasticity in the intact adult visual system

Neurons may possess activity-dependent homeostatic mechanisms that permit them to globally alter synaptic strength as activity varies. We used the retinotectal projection of goldfish to test this idea in the intact adult CNS. We first altered tectal neuron activity by selectively manipulating excitatory input. When excitatory synaptic drive to tectal neurons was eliminated by blocking optic fibers, current evoked at optic synapses increased by 183% within 90 min. With partial activity blockade, the increase in synaptic strength scaled with the magnitude of activity depression. This silence-induced potentiation was also rapidly reversible. Conversely, an increase in optic input was followed by a decrease in evoked synaptic current. When optic drive was not altered and tectal neuronal activity was instead increased or decreased pharmacologically via GABA(A) receptors, synaptic strength again changed inversely with activity, indicating that synaptic strength changed in response to neuronal activity and not excitatory drive. Furthermore, altered synaptic strength tended to return ongoing activity to baseline. Changes in synaptic strength could also be detected in heterosynaptic pathways, indicating a global response. Finally, changes in synaptic strength were associated with corresponding changes in ongoing and evoked firing rates, indicating that the responsivity of tectal neurons was altered. Thus, tectal neurons exhibit archetypical homeostasis, one of the first robust examples in the intact adult CNS.

Melatonin decreases calcium levels in retinotectal axons of Xenopus laevis by indirect activation of group III metabotropic glutamate receptors

Melatonin is a neuromodulator that binds to receptors in the retinotectal laminae of the amphibian optic tectum. The effect of melatonin on calcium dynamics in Xenopus retinotectal axons was investigated by imaging retinotectal axons labeled with the fluorescent indicator Fluo-4. Melatonin exerted an inhibitory influence on depolarization-evoked calcium increases, and the melatonin receptor antagonist 4-P-PDOT blocked this effect. Blockade of group III metabotropic receptors (mGluRs) counteracted the effect of melatonin on retinotectal axons. Application of the group II/group III mGluR antagonist MSPG or the group III-selective antagonist MSOP abolished the effect of melatonin. Conversely, this effect was not significantly affected by the group I mGluR antagonist LY367385 nor by EGLU or LY341495 at concentrations that specifically inhibit group II mGluRs. Furthermore, a higher concentration of LY341495 that affects group III mGluRs inhibited the effect of melatonin. The data therefore support the hypothesis that, in retinotectal axons, melatonin reduces cAMP levels, thereby relieving PKA-induced inhibition of group III mGluRs; the newly activated mGluRs in turn inhibit voltage-sensitive calcium channels, leading to a decrease in Ca2+ concentrations. The role of GABA(C) receptors in retinotectal responses was also evaluated. GABA(C) receptor antagonists did not block the effects of melatonin but instead were additive. Moreover, while other studies have shown that in Xenopus tectal cells, GABA(C) receptors mediate inhibition, in retinotectal axons, the opposite appears to occur since depolarization-evoked calcium rises in retinotectal axons were inhibited by GABA(C) receptor blockade. This result suggests that activation of GABA(C) receptors produces an increase in the synaptic excitability of retinotectal axon terminals.

Failure to form a stable topographic map during optic nerve regeneration: abnormal activity-dependent mechanisms

Visually evoked responses in the optic tectum are mediated by glutamate receptors. During development, there is a switch from N-methyl-d-aspartate (NMDA)- to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-mediated activity as the retinotectal map refines and visual function ensues. A similar pattern is seen in goldfish as the map refines during optic nerve regeneration. Here we examined glutamate receptors during optic nerve regeneration in the lizard, Ctenophorus ornatus, in which an imprecise retinotopic map forms transiently but degrades, leaving animals blind via the experimental eye. Receptor function was examined using NMDA and AMPA/kainate antagonists during in vitro tectal recording of visually evoked post-synaptic extracellular responses. Expression of NR1 (NMDA) and GluR2 (AMPA) receptor subtypes was examined immunohistochemically. In unoperated control animals, responses were robust and AMPA/kainate receptor-mediated. When the imprecise map was present, responses were difficult to evoke and insecure; periods of spontaneous activity as well as inactivity were also noted. Although AMPA/kainate-mediated activity persisted and GluR2 immunoreactivity increased transiently, NMDA receptor-mediated activity was also consistently detected and NR1 expression increased. In the long term, when the map had degraded, responses were readily evoked and predominantly AMPA/kainate receptor-mediated although some NMDA-mediated activity and NR1 expression remained. We suggest that the asynchronous activity reaching the optic tectum results in an inability to recapitulate the appropriate functional sequences of expression of NMDA and AMPA/kainate receptors necessary to refine the retinotectal map.

Adenosine A1 and class II metabotropic glutamate receptors mediate shared presynaptic inhibition of retinotectal transmission

Presynaptic inhibition is one of the major control mechanisms in the CNS. Previously we reported that adenosine A1 receptors mediate presynaptic inhibition at the retinotectal synapse of goldfish. Here we extend these findings to metabotropic glutamate receptors (mGluRs) and report that presynaptic inhibition produced by both A1 adenosine receptors and group II mGluRs is due to G(i) protein coupling to inhibition of N-type calcium channels in the retinal ganglion cells. Adenosine (100 microM) and an A1 (but not A2) receptor agonist reduced calcium current (I(Ca2+)) by 16-19% in cultured retinal ganglion cells, consistent with their inhibition of retinotectal synaptic transmission (-30% amplitude of field potentials). The general metabotropic glutamate receptor (mGluR) agonist 1S,3R-1-amino-cyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD, 50 microM) and the selective group II mGluR receptor agonist (2S, 2'R,3'R)-2-(2',3'-dicarboxy-cyclopropyl)glycine (DCG-IV, 300 nM) inhibited both synaptic transmission and I(Ca2+), whereas the group III mGluR agonist L-2-amino-4-phosphono-butyrate (L-AP4) inhibited neither synaptic transmission nor I(Ca2+). When the N-type calcium channels were blocked with omega-conotoxin GVIA, both adenosine and DCG-IV had much smaller percentage effects on the residual 20% of I(Ca2+), suggesting effects mainly on the N-type calcium channels. The inhibitory effects of A1 adenosine receptors and mGluRs were both blocked by pertussis toxin, indicating that they are mediated by either G(i) or G(o). They were also inhibited by activation of protein kinase C (PKC), which is known to phosphorylate and inhibit G(i). Finally, when applied sequentially, inhibition by adenosine and DCG-IV were not additive but occluded each other. Together these results suggest that adenosine A1 receptors and group II mGluRs mediate presynaptic inhibition of retinotectal synaptic transmission by sharing a pertussis toxin (PTX)-sensitive, PKC-regulated G(i) protein coupled to N-type calcium channels.

The modulatory cholinergic system in goldfish tectum may be necessary for retinotopic sharpening

The cholinergic circuit within the tectum and the cholinergic input from the nucleus isthmi mediate a presynaptic augmentation of retinotectal transmitter release via nicotinic receptors. In this study, the cholinergic systems were either eliminated using the cholinergic neurotoxin AF64A or blocked using nicotinic antagonists to test for effects on the activity-driven sharpening of the regenerating retinotectal projection. The effectiveness of the AF64A was verified by recording field potentials elicited by optic tract stimulation and by immunohistochemical staining for choline acetyltransferase (ChAT). At 1 week after intracranial (IC) injection of AF64A (12 to 144 nmoles) into the fluid above the tectum, field potentials showed a selective dose-dependent decrement of the cholinergic polysynaptic component with no effect on the amplitude of the glutamatergic monosynaptic component. The decrement was only partially recovered in recordings at 2 and 6 weeks. In normal fish, the ChAT antibody stains a population of periventricular neurons, their apical dendrites, and a dense plexus within the optic terminal lamina that consists of their local axons and fine dendrites and of input fibers from the nucleus isthmi. One week after IC AF64A injection (48-72 nmoles), most immunostaining in superficial tectum was lost but most neuronal somas in the deep tectum could still be seen, and staining in the tegmentum below the tectum was completely intact. At 2 weeks and later, the staining of neuronal somata largely recovered, but staining of the superficial plexus did not. AF64A treatment at 18 days after nerve crush, when regenerating retinal fibers are beginning to form synapses, prevented retinotopic sharpening of the projection. Recordings showed a rough retinotopic map on the tectum but the multiunit receptive fields (MURFs) at each tectal point averaged 34 deg vs. 11 deg in vehicle-injected control regenerates. AF64A treatment before nerve crush also blocked sharpening, ruling out a direct effect on retinal growth cones or retinal fibers, as AF64A rapidly decomposes, whereas its effect on the cholinergic fibers is long-lasting. IC injection or minipump infusion of the nicotine antagonists alpha-bungarotoxin (alpha BTX), neuronal bungarotoxin (nBTX), and pancuronium during regeneration also prevented sharpening (MURFs averaging 29.4 deg, 33.0 deg, and 31.4 deg, respectively). Control Ringer's solution infusions or injections over the same period (19-37 days postcrush) had no effect on regenerated MURF size (11.7 deg). The results show that the cholinergic innervation, which modulates transmitter release, is required for activity-driven retinotopic sharpening, thought to be triggered by NMDA receptor activation.

The return of phosphorylated and nonphosphorylated epitopes of neurofilament proteins to the regenerating optic nerve of Xenopus laevis

Neurofilament proteins of mammalian axotomized peripheral axons, which regenerate effectively, resemble those of embryonic axons. However, injured centrally projecting mammalian axons, which fail to regenerate, have very different neurofilament compositions than during development. If changes in neurofilament composition after injury reflect the ability of axotomized neurons to regenerate effectively, then the neurofilaments of centrally projecting axons that can regenerate should more closely resemble those of developing axons. In this study, the neurofilament compositions of injured optic axons of the frog, Xenopus laevis, were examined, since these axons can regenerate a fully functional projection. Antibodies to phosphorylated and nonphosphorylated forms of neurofilament proteins that had been used previously to study the neurofilament composition of newly developing X. laevis optic axons were used in immunocytochemical studies to examine the return of neurofilaments to the optic nerve after an intraorbital nerve crush. Intraocularly injected wheat germ agglutinin conjugated to horseradish peroxidase was used to label the regenerating axons independently of their neurofilaments. Neurofilament immunoreactivities disappeared rapidly from crushed axons during the first week after surgery. By nine days after surgery, antibodies to nonphosphorylated forms of middle (NF-M) and low molecular weight (NF-L) neurofilament proteins and the Xenopus neuronal intermediate filament protein (XNIF) began to stain the nerve just beyond the lesion. By this time, however, growing axonal terminals had reached the optic chiasm. Antibodies to phosphorylated epitopes of NF-M began to stain axons at 15 days, just as growing axons began to arrive at the optic tectum. Nonphosphorylated high molecular weight neurofilament protein (NF-H) began to appear in axons between 18 and 21 days after surgery. Thus, the reappearance of neurofilaments during optic axon regeneration resembled the general pattern seen during development. The chief difference between development and regeneration was that neurofilament epitopes took longer to emerge during regeneration. One possibility is that cues encountered along the optic pathway influence the neurofilament composition of retinal ganglion cell axons. Then, the greater distances travelled by regenerating axons could account for the longer time taken for their neurofilament compositions to mature.

C-kinase manipulations disrupt activity-driven retinotopic sharpening in regenerating goldfish retinotectal projection

Regenerating optic axons initially branch over a wide area in tectum to form a crude retinotopic map. The map is sharpened, and retinotopically appropriate synapses are stabilized via NMDA receptors that detect, via summation of EPSPs, the coincident activity of neighboring ganglion cells that make synapses onto common tectal cells. Sharpening shares a number of properties with long-term potentiation (LTP) in hippocampus. This study tested whether protein kinase C (PKC) activation is necessary for sharpening as it is for LTP. Intraocular (IO) or intracranial (IC) injections of kinase inhibitors or activators were made every other day from 19 to 37 days postcrush (sensitive period), and the projections formed were later recorded. Retinotopic sharpening was prevented by IC injection of the following agents: (1) general kinase inhibitors sphingosine and H7 (100-200 microM in fluid above brain), (2) active but not inactive phorbols (TPA, 1 microM), and (3) calphostin C (1 microM), a specific and irreversible PKC inhibitor. The mature projection on the opposite tectum, however, when examined was not unsharpened. Lack of sharpening was reflected in multiunit fields at each tectal point that averaged 27 degrees-30 degrees versus 11 degrees in Ringers and inactive phorbol control regenerates. Intraocular injections of either TPA (1 microM), or calphostin C (1 microM) also prevented sharpening (26 degrees and 32 degrees multiunit fields), suggesting action on PKC axonally transported to the presynaptic terminals. Calphostin C had no noticeable effect on the firing patterns of retinal ganglion cells. The endogenous activator of PKC, arachidonic acid (AA), disrupted sharpening at 20 microM or higher (IC injection, 32 degrees multiunit fields), while a control fatty acid, elaidic acid, had no effect. Although AA at 5 microM showed no effect, and diacylglycerol at 5 microM exhibited only small effects, together they produced a large synergistic effect (32 degrees multiunit fields). Such synergy mirrors the synergy in the activation of several isoforms of PKC. Actual concentrations in the extradural fluid around the brain were assayed via injections of 3H-AA. Levels fell about sixfold after a day and by an additional fivefold the second day before the next injection. The results confirm that activity-driven retinotopic sharpening is very sensitive to manipulations of kinases, especially PKC.

Nucleus isthmi in goldfish: in vitro recordings and fiber connections revealed by HRP injections

Recordings of field potentials in nucleus isthmi (NI) were obtained in an in vitro preparation of goldfish brain using a lateral approach. Horseradish peroxidase (HRP) was injected from recording electrodes to verify recordings within the nucleus and to label axonal pathways and cell bodies. Activity in NI was repetitive and could be elicited by stimulation of the optic nerve, tectum, pretectum, or tectobulbar tract. Spontaneous activity was present in some preparations and consisted of bursts with intervening silent periods. Anatomical and electrophysiological evidence indicated that the primary isthmotectal pathway is composed of fine fibers that exit NI rostrally and pass through pretectum to enter tectum rostrally. An afferent pathway consisting of both fine- and large-diameter fibers entered NI ventromedially; the large diameter axons have been previously reported in percomorph fishes, but were not thought to be present in cyprinids such as goldfish. The large diameter axons arise from labeled cell bodies in the region of the lateral thalamic nucleus. No labeled cell bodies were seen in ipsilateral nucleus pretectalis superficialis, pars magnocellularis, where they are seen in percomorphs. The fine axons, which have not been reported in percomorph fishes, were shown to arise from tectal bipolar (type VI) neurons. As in percomorphs, tectal type XIV neurons were also labeled. This and corroborating recordings from nucleus isthmi constitute the fist demonstration of a tectoisthmic projection in a cyprinid fish.