Normal development of the ipsilateral retinocollicular pathway and its disruption in double endothelial and neuronal nitric oxide synthase gene knockout mice

Impaired clustered protocadherin-α leads to aggregated retinogeniculate terminals and impaired visual acuity in mice

Clustered protocadherins (cPcdhs) comprising cPcdh-α, -β, and -γ, encode a large family of cadherin-like cell-adhesion molecules specific to neurons. Impairment of cPcdh-α results in abnormal neuronal projection patterns in specific brain areas. To elucidate the role of cPcdh-α in retinogeniculate projections, we investigated the morphological patterns of retinogeniculate terminals in the lateral geniculate (LG) nucleus of mice with impaired cPcdh-α. We found huge aggregated retinogeniculate terminals in the dorsal LG nucleus, whereas no such aggregated terminals derived from the retina were observed in the olivary pretectal nucleus and the ventral LG nucleus. These aggregated terminals appeared between P10 and P14, just before eye opening and at the beginning of the refinement stage of the retinogeniculate projections. Reduced visual acuity was observed in adult mice with impaired cPcdh-α, whereas the orientation selectivity and direction selectivity of neurons in the primary visual cortex were apparently normal. These findings suggest that cPcdh-α is required for adequate spacing of retinogeniculate projections, which may be essential for normal development of visual acuity.

Activity dependent mechanisms of visual map formation--from retinal waves to molecular regulators

The refinement of neural connections requires activity-dependent mechanisms in addition to the genetic program initially establishing wiring diagrams. The well-understood organization of the visual system makes it an accessible model for analyzing the contribution of activity in the formation of connectivity. Prior to visual experience, patterned spontaneous activity in the form of retinal waves has an important role for the establishment of eye-specific and retinotopic maps by acting on the refinement of axon arborization. In the present review, which focuses on experimental data obtained in mice and ferrets, we highlight the features of retinal activity that are important for visual map formation and question whether synaptic release and Hebbian based competition rules apply to this system. Recent evidence using genetic tools that allowed the manipulation of different features of neural activity have clarified the controversy on whether activity is instructive or permissive for visual map formation. Furthermore, current evidence strongly suggests that different mechanisms are at play for different types of axons (ipsilateral vs. contralateral), maps (eye-specific vs. retinotopic) or targets. Many molecules that either modulate activity or are modulated by activity are important in the formation of the visual map, such as adenylate cyclase 1, serotonin, or molecules from the immune system. Finally, new players in the game include retrograde messengers signaling from the target cell to the retinal axons as well as microglia that could help to eliminate inappropriate synapses.

Ten-m3 is required for the development of topography in the ipsilateral retinocollicular pathway

Background

The alignment of ipsilaterally and contralaterally projecting retinal axons that view the same part of visual space is fundamental to binocular vision. While much progress has been made regarding the mechanisms which regulate contralateral topography, very little is known of the mechanisms which regulate the mapping of ipsilateral axons such that they align with their contralateral counterparts.

Results

Using the advantageous model provided by the mouse retinocollicular pathway, we have performed anterograde tracing experiments which demonstrate that ipsilateral retinal axons begin to form terminal zones (TZs) in the superior colliculus (SC), within the first few postnatal days. These appear mature by postnatal day 11. Importantly, TZs formed by ipsilaterally-projecting retinal axons are spatially offset from those of contralaterally-projecting axons arising from the same retinotopic location from the outset. This pattern is consistent with that required for adult visuotopy. We further demonstrate that a member of the Ten-m/Odz/Teneurin family of homophilic transmembrane glycoproteins, Ten-m3, is an essential regulator of ipsilateral retinocollicular topography. Ten-m3 mRNA is expressed in a high-medial to low-lateral gradient in the developing SC. This corresponds topographically with its high-ventral to low-dorsal retinal gradient. In Ten-m3 knockout mice, contralateral ventrotemporal axons appropriately target rostromedial SC, whereas ipsilateral axons exhibit dramatic targeting errors along both the mediolateral and rostrocaudal axes of the SC, with a caudal shift of the primary TZ, as well as the formation of secondary, caudolaterally displaced TZs. In addition to these dramatic ipsilateral-specific mapping errors, both contralateral and ipsilateral retinocollicular TZs exhibit more subtle changes in morphology.

Conclusions

We conclude that important aspects of adult visuotopy are established via the differential sensitivity of ipsilateral and contralateral axons to intrinsic guidance cues. Further, we show that Ten-m3 plays a critical role in this process and is particularly important for the mapping of the ipsilateral retinocollicular pathway.

Synapse maturation is enhanced in the binocular region of the retinocollicular map prior to eye opening

In the developing visual system of mammals, retinal axons from the two eyes compete for postsynaptic partners. After eye opening, this process is regulated in part by homeostatically constrained competition for synaptic connectivity with target neurons. However, prior to eye opening, the functional and synaptic basis of binocular map development is unclear. To examine the role of binocular interactions during early stages of visual map development, we performed in vitro patch-clamp recordings from the superior colliculus (SC) of neonatal mice. Using newly designed slice preparations, we compared retinocollicular synapse development in the medial SC, which receives binocular input, and the lateral SC, which is predominantly monocular. Surprisingly, we found that at P6-7, when eye-specific segregation has just emerged, retinocollicular synapses were stronger and more mature and dendritic arbors were more elaborate in the medial than the lateral SC. Furthermore, monocular enucleation of the ipsilateral eye at P0 selectively reduced synaptic strength and dendritic branching in the medial SC and abolished the differences normally observed between the two slices at P6-7. This specifically implicates binocular interactions in the development of retinocollicular connectivity prior to eye opening. Our findings contrast with the predictions of a constrained-connectivity model of binocular map development and suggest instead that binocular competition prior to eye opening enhances retinocollicular synaptic strength and the morphological development of retino-recipient neurons.

Nitric oxide synthase regulates morphogenesis of zebrafish spinal cord motoneurons

Nitric oxide (NO) is a signaling molecule that is synthesized in a range of tissues by the NO synthases (NOSs). In the immature nervous system, the neuronal isoform of NOS (NOS1) is often expressed during periods of axon outgrowth and elaboration. However, there is little direct molecular evidence to suggest that NOS1 influences these processes. Here we address the functional role of NOS1 during in vivo zebrafish locomotor circuit development. We show that NOS1 is expressed in a population of interneurons that lie close to nascent motoneurons of the spinal cord. To determine how this protein regulates spinal network assembly, we perturbed NOS1 expression in vivo with antisense morpholino oligonucleotides. This treatment dramatically increased the number of axon collaterals formed by motoneuron axons, an effect mimicked by pharmacological inhibition of the NO/cGMP signaling pathway. In contrast, exogenous elevation of NO/cGMP levels suppressed motor axon branching. These effects were not accompanied by a change in motoneuron number, suggesting that NOS1 does not regulate motoneuron differentiation. Finally we show that perturbation of NO signaling affects the ontogeny of locomotor performance. Our findings provide evidence that NOS1 is a key regulator of motor axon ontogeny in the developing vertebrate spinal cord.

Effects of trkB knockout on topography and ocular segregation of uncrossed retinal projections

TrkB is an important receptor for brain-derived neurotrophic factor and NT4, members of the neurotrophin family. TrkB signaling is crucial in many activity-dependent and activity-independent processes of neural development. Here, we investigate the role of trkB signaling in the development of two distinct, organizational features of retinal projections--the segregation of crossed and uncrossed retinal inputs along the "lines of projection" that represent a single point in the visual field and the "retinotopic" mapping of retinofugal axons within their cerebral targets. Using anterograde tracing, we obtained quantitative measures of the distribution of retinal projections in the dorsal nucleus of the lateral geniculate body (LGd) and superior colliculus (SC) of wild-type mice and mice homozygous for constitutive null mutation (knockout) of the full-length trkB receptor (trkB(FL)(-/-)). In trkB(FL)(-/-) mice, uncrossed retinal projections cluster normally but there is a topographic expansion in the distribution of these clusters across the SC. By contrast, the absence of trkB signaling has no significant effect on the segregation of crossed and uncrossed retinal projections along the lines of projection in LGd or SC. We conclude that the normal topographic organization of uncrossed retinal projections depends upon trkB signaling, whereas the segregation of crossed and uncrossed retinal projections is trkB-independent. We also found that in trkB(FL)(-/-) mice, neuronal number was reduced in the LGd and SC and in the caudate-putamen. Previous studies by ourselves and others have shown that the number of retinal ganglion cells (RGCs) is unchanged in trkB(FL)(-/-) mice. Together, these results demonstrate that there is no matching of the numbers of RGCs with neuronal numbers in the LGd or SC.

Injury to retinal ganglion cell axons increases polysialylated neural cell adhesion molecule (PSA-NCAM) in the adult rodent superior colliculus

The adult mammalian central nervous system (CNS) exhibits a limited regenerative response to injury. It is well established that polysialylated neural cell adhesion molecule (PSA-NCAM) contributes to nervous system plasticity. In the visual system, PSA-NCAM participates in retinal ganglion cell (RGC) axon growth during development and specifically influences RGC innervation of its principle target tissue, the superior colliculus (SC). The goals of this study were to determine whether PSA-NCAM is expressed in the normal adult mouse SC and to evaluate PSA-NCAM expression following RGC injury. In the normal rostral, but not caudal, SC we find that PSA-NCAM is present in the retinorecipient layers; however, PSA-NCAM and RGC axons do not co-localize. In the deeper collicular layers, PSA-NCAM is observed as several distinct patches that occur at the same depth along the medial-lateral axis throughout the colliculus. RGC axotomy denervates predominantly the contralateral colliculus, where increased PSA-NCAM levels are seen at 7 and 10 days after the injury. Further evaluation of the retinorecipient layers of the partially denervated SC reveals that some intact CTB-traced RGC axons (less than 5%) labeled from the ipsilateral eye do co-localize with PSA-NCAM. This study is the first characterization of PSA-NCAM expression in the normal and partially denervated adult SC and may indicate that PSA-NCAM is involved in attempted visual system remodeling after injury.

Nicotine exposure refines visual map topography through an NMDA receptor-mediated pathway

The precise mapping of one surface onto another is fundamental to visual system organization and depends upon adequate stimulation of postsynaptic targets to stabilize correctly placed synapses. As exogenous nicotine alters neuronal activity, we investigated whether it would affect the visual map created by retinal ganglion cell terminals in the frog optic tectum. Chronic exposure of the tectum to nicotine decreased the retinal area from which cells project to a given tectal site. This map refinement was also produced by exposure to either the alpha-bungarotoxin sensitive nicotinic receptor agonist, anatoxin-a or the alpha-bungarotoxin-insensitive nicotinic receptor agonist epiboxidine. Immunocytochemical studies using mAb306 and mAb22 demonstrated that alpha-bungarotoxin-sensitive and -insensitive nicotinic receptors, respectively, occupied different tectal sites. Choline acetyltransferase immunoreactivity overlapped with mAb306, but not mAb22, staining. The developing optic tectum was more sensitive to nicotine than the adult tectum and nicotine induced both map refinements and map disruptions in a concentration-dependent manner. Blockade of the N-methyl-D-aspartate (NMDA) receptor with D(-)-2-amino-5-phosphonopentanoic acid (D-APV) prevented nicotine from refining the map in the adult tectum. Exposure to the use-dependent NMDA antagonist MK801 alone had no effect on retinotectal topography but in combination with either NMDA or nicotine it disrupted the map. Exposure to NMDA alone produced refinement. We conclude that the map refinement induced by chronic nicotine treatment has as its basis an increase in the level of NMDA receptor activity. The data are consistent with a model whereby map topography can be bidirectionally affected by either increasing or decreasing NMDA receptor activity.

Changes in nNOS and NADPH diaphorase in frog retina and tectum after axotomy and FGF-2 application

We have shown previously that application of fibroblast growth factor-2 (FGF-2) to the cut optic nerve of the frog, Rana pipiens, augments the survival of retinal ganglion cells (RGCs). In this study, we examine the effects of axotomy and FGF-2 treatment upon the distribution of nitric oxide synthase (NOS) and NADPH diaphorase (NADPH-d) activity in the frog retina and tectum. We find that NOS and NADPH-d are largely absent from RGCs but present in amacrine neurons and in retinorecipient tectal layers. Axotomy alone has little effect on NOS expression or diaphorase activity, apart from slightly increasing the levels of expression in a subpopulation of amacrine cells that arborize in the On sublamina of the inner plexiform layer. FGF-2 application to the optic nerve down-regulates NOS expression and activity in the retina and up-regulates it in the tectum, particularly in retinorecipient layers. Electron microscopy of the optic nerve and neurofilament immunostaining of the tectum suggests that FGF-2 treatment increases the number of regenerating retinal axons arriving at the tectum. The effects in the retina and tectum are probably indirect, that in the retina being due to retrograde signaling from RGCs to amacrine neurons, and that in the tectum being due to re-induction of NOS expression in tectal neurons by the arrival of regenerating axons. At this stage, it appears unlikely that these changes in NOS play a role in the FGF-2's survival effect on RGCs.

Blockade of arachidonic acid pathway induces sprouting in the adult but not in the neonatal uncrossed retinotectal projection

The uncrossed retinotectal projection of rats undergoes extensive axonal elimination and subsequent growth of axonal arbors in topographically appropriate territories within the first two/three postnatal weeks. Nitric oxide has been implicated in development and stabilization of synapses in the retinotectal pathway since blockade of nitric oxide synthesis disrupts the normal pattern of retinal innervation in subcortical nuclei. The present work investigated the role of arachidonic acid pathway in the development and maintenance of ipsilateral retinotectal axons. We also investigated the role of this retrograde messenger in the modulation of plasticity that follows retinal lesions in the opposite eye. Pigmented rats received systemic treatment with quinacrine, a phospholipase A2 inhibitor, indomethacin, a cyclooxygenase inhibitor, nordihydroguaiaretic acid, a 5-lipoxygenase inhibitor or vehicle during 4-8 days at various postnatal ages. Rats given a unilateral temporal retinal lesion were treated with either quinacrine or vehicle during the same period. For anterograde tracing of ipsilateral retinal projections, animals received intraocular injections of horseradish peroxidase. Before the third postnatal week no difference was observed in the laminar or topographic organization of the ipsilateral retinotectal projection between vehicle and treated rats in either normal or lesion conditions. After the third postnatal week, however, systemic blockade of phospholipase A2 or 5-lipoxygenase, but not cyclooxygenase induced sprouting of uncrossed axons throughout the collicular visual layers in unoperated rats. In retinal lesion groups, phospholipase A2 blockade increased the sprouting of uncrossed intact axons to the collicular surface in the same period. The results suggest that arachidonic acid or lipoxygenase metabolites play a role in the maintenance of the retinotectal synapses after the critical period and that the blockade of the arachidonic acid pathway induces reactive sprouting of retinal axons late in development.

ECTO- AND EXO-PROTEIN KINASES IN SCHISTOSOMA MANSONI: REGULATION OF SURFACE PHOSPHORYLATION BY ACETYLCHOLINE AND IDENTIFICATION OF THE ALPHA SUBUNIT OF CKII AS A MAJOR SECRETED PROTEIN KINASE

The Schistosoma mansoni parasite life cycle involves complex developmental processes that enable it to cause severe hepatic damage. Protein phosphorylation has previously been implicated in the transformation of cercariae to schistosomula of S. mansoni. Here, we studied the possible involvement of surface (ecto) and shed (exo) protein kinases (PKs) in this developmental process. We found that ecto-PKs are indeed located on the surface of the schistosomula and can phosphorylate up to 5 distinct proteins at this location. Surface phosphorylation was sensitive to acetylcholine, which increased phosphorylation of 3 proteins and reduced phosphorylation of the other 2. The ecto-PKs can be shed from the surface into the incubation medium during parasite differentiation. The main exo-PK is CKII, as concluded from the substrate specificity of the PK, its inhibition by heparin, activation by spermin, and recognition by antibody directed to the anti--alpha-subunit of CKII in the incubation medium of the schistosomula. In spite of its similarity to the ecto-PKs, the activity of the exo-PK is not affected by addition of acetylcholine. These results indicate that ecto- and exo-PKs could be involved in the parasite's development or host-parasite interactions.

Rapid and long-term plasticity in the neonatal and adult retinotectal pathways following a retinal lesion

The uncrossed retinotectal projection restricts its terminal fields to the ventral boundary of the visual layers at the rostral tectum during early post natal development. During this critical period, temporal retinal lesions in one eye induce laminar rearrangements in the uncrossed pathway of the intact eye toward the collicular surface previously occupied, almost exclusively, by the crossed retinal axon population. We have compared, using anterograde tracing techniques, the time course and magnitude of the axonal sprouting resulting from retinal lesions in neonates and adults. Early retinal lesions (within the first two post natal weeks) induced extensive and rapid plasticity of the ipsilateral projection 48 h after the lesions. On the third post natal week, similar retinal lesions induced a small reorganization of the intact eye's uncrossed projection within a 3-week survival time. Nevertheless, giving the animals a long-term survival, resulted in an increased plastic capability, suggesting that even after the critical period, intact retinal axons can respond efficiently to injury. The results suggest two phases of axonal reorganization within this subcortical pathway: a rapid plasticity within the critical period and a slow, but continuous plasticity in adulthood.

Contribution of GABAergic inhibition to synaptic responses and LTD early in postnatal development in the rat superior colliculus

We studied the development of optic tract evoked field potentials (FP) in the rodent superior colliculus (SC) and the effect of GABA antagonists upon their development and upon induction of long-term depression (LTD). Brain slices were cut from Lister Hooded rats. The optic tract was stimulated while recording from the superficial grey layer. GABAergic inhibition was assessed by adding 100 microm picrotoxin and 3 microm CGP55845 antagonists to block GABA A,B,C receptors. LTD was induced with a 50 Hz, 20 s tetanus. At age P2, the FP consisted only of a presynaptic spike. The GABA antagonists had no effect. By P4, the FP consisted of a presynaptic spike, a longer latency population spike, and a field excitatory postsynaptic potential (fEPSP). The fEPSP was slightly prolonged by the GABA antagonists at this age. By P7-P14, a prominent FP with trailing fEPSP was recorded. The GABA antagonists usually had a large effect, with the fEPSP increasing in both amplitude and duration. A mature FP was usually recorded in P15-P23 slices where the GABA antagonist effect remained substantial. LTD could be induced in 17 of 30 control slices from rats aged P4-P26. The average fEPSP amplitude after tetanus was 77.9% of control. Pre-treatment with GABA antagonists produced a short-term potentiation (average 114.0%), rather than LTD, in 14 of 19 cases. This STP was followed by a more prolonged potentiation in 12 of the 14 cases. We conclude that GABAergic inhibitory circuits mature before eye opening and that GABA contributes to induction of LTD in the developing SC.

The effect of L-arginine, a nitric oxide synthase substrate, on retinal cell proliferation in the postnatal rat

The effects of L-arginine, a nitric oxide synthase (NOS) substrate, on cell proliferation in the developing postnatal rat retina were studied by immunocytochemistry using anti-bromodeoxyuridine (BrdU) antiserum, and by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL). Densitometric analysis by immunoblotting confirmed that neuronal NOS expression significantly increased in the L-arginine-treated retinas in comparison with the control retinas at postnatal day (P) 5 to P10. In the retinas of the control and L-arginine-treated rats, BrdU-labeled cells were only seen in the neuroblastic layer of the retinas up to P7. BrdU-labeled cells were significantly more numerous in the retinas of L-arginine-treated rats than in control retinas at P5 in the central retina and at P5 and P7 in the peripheral retina. In addition, TUNEL-positive apoptotic cells were more numerous in the retinas of L-arginine-treated rats at P5 and P7. Our results suggest that NO might play an important role in retinal maturation through regulating proliferative phases in the early stages of rat postnatal development.

Chemoarchitecture of GABAergic neurons in the ferret superior colliculus

gamma-Aminobutyric acid (GABA)ergic neurons are thought to play a key role both in visual processing and in the complex sensory-motor transformations that take place in the mammalian superior colliculus. To understand the organization of GABAergic neurons in the ferret superior colliculus, we applied antisera to several markers of GABAergic function, including GABA, two isoforms of its synthetic enzyme glutamic acid decarboxylase (GAD-65 and GAD-67), and the GABA transporter, GAT-1. We also applied antisera to several calcium binding proteins (calbindin [CB], calretinin [CR], and parvalbumin [PV]) and neuronal nitric oxide synthase (NOS), chemical markers that colocalize with GABA in some areas of the central nervous system. The distribution of GABAergic neurons in the ferret is similar to that of other mammalian species. GABAergic neurons in the ferret superior colliculus were small, morphologically diverse, and widely distributed throughout all layers of the colliculus. As has been shown in other mammalian species, neurons expressing PV, CB, CR, and NOS were differentially distributed in layers and patches throughout the ferret colliculus. None of these markers, however, showed a distribution that mirrored that of GABAergic neurons. Furthermore, few GABAergic neurons colocalized these neurochemical markers. Only 14% of GABAergic neurons in the superficial layers and 18% of neurons in the deeper layers colocalized PV, 14% of GABAergic neurons in the superficial layers and 10% in the deeper layers colocalized CB, and only 1% of GABAergic neurons in both the superficial and deep layers colocalized nitric oxide synthase. Thus, the arrangement of GABAergic neurons in the ferret superior colliculus is broadly distributed and is distinct from other recognized organizational patterns in the superior colliculus.

Expression of the L-type calcium channel in the developing mouse visual system by use of immunocytochemistry

Developmental refinement of the retinogeniculate and retinocollicular pathways is partially dependent upon Ca(2+) channel function [J. Comp. Neurol. 440 (2001) 177-191]. We have examined the development of the L-type voltage gated Ca(2+) channel to determine if the onset of expression matches this period of refinement. Labeling by an antibody directed against the alpha 1C subunit of this channel was examined in the superior colliculus (SC), lateral geniculate nucleus (LGN), visual cortex (CTX), hippocampus (HC) and cerebellum (CB) in mice aged P3-4, P8-9, P15, P21, P28, and adults. At P3-4, labeled cells within the SC were concentrated within a dense band in the retinorecipient zone of the superficial gray layer. More lightly labeled neurons were seen in other layers. This dense band was still seen at P15, while more labeled neurons were seen in other layers. By P21-P28, labeled neurons were fairly uniformly distributed throughout all layers of SC. Neuronal cell types appeared to be labeled at all ages examined within the LGN. Within CTX, putative layer V-VI pyramidal neurons were well labeled at P4 and later ages, and labeled layer II-III pyramids could be distinguished by P9 and later ages. The dendrites and cell bodies of pyramidal neurons within CA1-CA3 of HC, granule neurons in the dentate gyrus, and Purkinje neurons in CB were labeled at all ages examined. We conclude that the L-type Ca(2+) channel is expressed in many neurons within retinorecipient targets as well as in other brain regions during the developmental period in which pathway refinement and synaptic plasticity occurs.

Inhibition of nitric oxide synthase induces increased production of growth-associated protein 43 in the developing retina of the postnatal rat

We investigated the effects of N(G)-nitro-L-arginine methyl ester (L-NAME), a NOS inhibitor, on retinal development in the postnatal rat by immunocytochemistry and immunoblotting using antisera against neuronal nitric oxide synthase (nNOS) or growth-associated protein 43 (GAP-43). An nNOS-immunoreactive band of 155 kDa and a GAP-43-immunoreactive band of 48 kDa were present in the extracts of both control and L-NAME-treated rat retinas. The intensity of the nNOS-immunoreactive band was much weaker in the treated rats, whereas the intensity of the GAP-43-immunoreactive band of 48 kDa was much stronger in the treated rats. Much stronger GAP-43 immunoreactivity was visible in the inner plexiform layer (IPL) of the treated retinas at P10, P14 and P21. Our findings suggest that NO may play an important role in the maturation of the IPL in the developing rat retina.

Properties of LTD and LTP of retinocollicular synaptic transmission in the developing rat superior colliculus

The developing retinocollicular pathway undergoes synaptic refinement in order to form the precise retinotopic pattern seen in adults. To study the mechanisms which underlie refinement, we investigated long-term changes in retinocollicular transmission in rats aged P0-P25. Field potentials (FPs) in the superior colliculus (SC) were evoked by stimulation of optic tract fibers in an in vitro isolated brainstem preparation. High intensity stimulation induced long-term depression (LTD) in the SC after both low (1000 stimuli at 1 Hz) and higher (1000 stimuli at 50 Hz) frequency stimulation. The induction of LTD was independent of activation of NMDA and GABA(A) receptors, because D-APV (100 microM) and bicuculline (10 microM) did not block LTD. Induction of LTD was dependent upon activation of L-type Ca(2+) channels as 10 microM nitrendipine, an L-type Ca(2+) channel blocker, significantly decreased the magnitude of LTD. LTD was down-regulated during development. LTD magnitude was greatest in rats aged P0-P9 and significantly less in rats aged P10-P25. Long-term potentiation (LTP) was induced by low intensity stimulation and only after high frequency tetanus (1000 stimuli at 50 Hz). LTP was NMDA receptor dependent because d-APV (100 microM) completely abolished it. LTP induction was also blocked by the L-type Ca2+ channel blocker nitrendipine. The magnitude of LTP first increased with age, being significantly greater at P7-P13 than at P0-3 and then decreased at P23-25. In summary, both LTD and LTP are present during retinocollicular pathway refinement, but have different transmitter and ionic mechanisms and time courses of expression.

Sculpting the visual map: the distribution and function of serotonin-1A and serotonin-1B receptors in the optic tectum of the frog

Agonists of serotonin (5-HT)-1 receptors modulate the synaptic strength of the connection between retinal ganglion cells and neurons of the frog optic tectum in brain slices (Brain Res. 1998;781:167-181). We have now used autoradiographic receptor binding techniques to determine the location of 5-HT1A and 5-HT1B binding sites in the laminated optic tectum. 5-HT1A binding sites, as labeled with [3H]8-hydroxy-dipropylaminotetralin (8-OH-DPAT), were highest in the superficial, retinorecipient layers of the tectum, intermediate in layers 6 and 7 and low in the remaining layers. Binding densities in all of these layers were unaffected by optic nerve lesion. 5-HT1B binding sites were visualized using [125I]iodocyanopindolol (ICYP). Binding densities were highest in the plexiform layers 5 and 7 and intermediate in layers 6 and 8. Binding sites were present at low levels in layer 9; however, optic nerve lesion resulted in a strong upregulation of these sites in this layer. Pharmacological manipulation of receptor activation resulted in changes in the activity-dependent visual map that is created at the tectum by retinal ganglion cell terminals. Chronic treatment of the tectum with SB-224289, a selective antagonist of 5-HT1B receptors, disrupted the topographic map. In contrast, exposure to WAY-100635, a selective antagonist of 5-HT1A receptors, refined it. We conclude that both 5-HT1A and 5-HT1B receptors are present in the adult frog tectum and that changes in their activation levels can produce changes in retinotectal transmission levels that drive visual plasticity in opposite directions.

Peripheral vagal control of heart rate is impaired in neuronal NOS knockout mice

The role of nitric oxide (NO) in the vagal control of heart rate (HR) is controversial. We investigated the cholinergic regulation of HR in isolated atrial preparations with an intact right vagus nerve from wild-type (nNOS+/+, n = 81) and neuronal NO synthase (nNOS) knockout (nNOS-/-, n = 43) mice. nNOS was immunofluorescently colocalized within choline-acetyltransferase-positive neurons in nNOS+/+ atria. The rate of decline in HR during vagal nerve stimulation (VNS, 3 and 5 Hz) was slower in nNOS-/- compared with nNOS+/+ atria in vitro (P < 0.01). There was no difference between the HR responses to carbamylcholine in nNOS+/+ and nNOS-/- atria. Selective nNOS inhibitors, vinyl-L-niohydrochloride or 1-2-trifluoromethylphenyl imidazole, or the guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one significantly (P < 0.05) attenuated the decrease in HR with VNS at 3 Hz in nNOS+/+ atria. NOS inhibition had no effect in nNOS-/- atria during VNS. In all atria, the NO donor sodium nitroprusside significantly enhanced the magnitude of the vagal-induced bradycardia, showing the downstream intracellular pathways activated by NO were intact. These results suggest that neuronal NO facilitates vagally induced bradycardia via a presynaptic modulation of neurotransmission.

Development of the visual pathway is disrupted in mice with a targeted disruption of the calcium channel beta(3)-subunit gene

Refinement of the retinal pathways to the superior colliculus (SC) and dorsal lateral geniculate nucleus (dLGN) is mediated by nitric oxide (NO). Long-term depression (LTD) can also be induced in SC and LGN during the time at which these pathways are refined, and this LTD is partially dependent on NO and L-type Ca(2+) channel function. In an effort to determine whether NO-mediated pathway refinement is also mediated by Ca(2+) channel function, we have examined the refinement of the retinocollicular and retinogeniculate pathways in mice which lack the gene for the Ca(2+) channel beta(3) subunit (CCKO) and which have significantly reduced L-type Ca(2+) currents. Injections of the anterograde tracer cholera toxin subunit B/HRP were made into one eye of these knockout animals and in wild-type mice ages postnatal day (P) 13, P19, and P26. After 48 hours, mice were perfused and sections processed by using tetramethylbenzidine histochemistry. Labeling distribution in some animals was analyzed quantitatively. Obvious differences in the distribution of the ipsilateral retinocollicular pathway were observed at P15, with the pathway being more exuberant in CCKO mice. This difference was statistically significant. More subtle differences were seen at P21 and P28. Obvious differences were also seen in the contralateral retinogeniculate pathway which in CCKO mice filled most of the domain normally occupied by ipsilateral eye fibers. This difference was also statistically significant. We conclude that reduction in L-type Ca(2+) currents has an effect on axonal refinement similar to that which occurs in NO knockout mice, which supports the possibility that L-type Ca(2+) channel-dependent LTD mediates NO-dependent axonal refinement.

Nitric oxide, impulse activity, and neurotrophins in visual system development(1)

Topographic refinement of synaptic connections within the developing visual system involves a variety of molecules which interact with impulse activity in order to produce the precise retinotopic maps found in the adult brain. Nitric oxide (NO) has been implicated in this process, as have various growth factors. Within the subcortical visual system, we have recently shown that nitric oxide contributes to pathway refinement in the superior colliculus (SC). Long-term potentiation (LTP) and long-term depression (LTD) are also expressed in SC during the time that this pathway undergoes refinement. The role of NO has been demonstrated by showing that refinement of ipsilateral fibers in the retinocollicular pathway is significantly delayed in gene knockout mice in which both the endothelial and neuronal isoforms of nitric oxide synthase (NOS) have been disrupted. The effect also depends upon Ca(2+) channels because refinement of both the ipsilateral retinocollicular and retinogeniculate pathways is disrupted in genetic mutants in which the beta3 subunit of the Ca(2+) channel has been deleted. LTD may also be involved in this process, because the time course of its expression correlates with that of pathway refinement and LTD magnitude is depressed by nitrendipine, an L-type Ca(2+) channel blocker. LTP is also expressed during early postnatal development in the LGN and SC and may contribute to synaptic stabilization. The role of neurotrophins in pathway refinement in the visual system is also reviewed.