Plasticity in the visual system: role of neurotrophins and electrical activity

We report recent results concerning the action of neurotrophins on the development and plasticity of the visual system of mammals and in particular of their visual cortex. It has been demonstrated that NGF prevents all the effects of monocular deprivation during the critical period. BDNF, that in part also prevents the effects of monocular deprivation, has the interesting additional property of accelerating the development of inhibitory processes. In transgenic mice overexpressing BDNF only in the cortex, the critical period for plasticity initiates a week earlier and presents a precocious closure. Visual acuity also develops much before than in normal animals. These phenomenological observations are paralleled by a precocious increase of inhibitory synapses and inhibitory currents in pyramidal neurons. LTP, tested by stimulation of the white matter, recording in layers 2 and 3 of the visual cortex, presents modifications correlated with the alterations observed in the critical period. Last we report the finding from in vitro and in vivo experiments that MAPkase (Erg 1 and 2) is the molecular chain of events driven both by light and neurotrophins, likely at the bases of the phenomena of plasticity observed during the critical period.

Cardiovascular involvement in thalassaemic patients with pseudoxanthoma elasticum-like skin lesions: a long-term follow-up study

BACKGROUND

Congenital haemolytic anaemia may be associated with pseudoxanthoma elasticum (PXE)-like clinical manifestations.

METHODS

The cardiovascular system of 14 homozygous and double heterozygous beta-thalassaemia patients with skin and retinal vessel alterations similar to those in genetic PXE was analysed over a period of 12 years and compared with that of 13 relatives (five sets of parents, one single parent, two thalassaemic brothers), and that of the control group composed of 16, age- and sex-matched, thalassaemic patients.

RESULTS

All patients with clinical PXE-like skin lesions exhibited, by light and electron microscopy, dermal alterations and mineralization of elastic fibres identical to those typical of inherited PXE. None of the relatives and none of the control group showed clinical or structural findings of PXE. The follow-up started in 1988. After 12 years of clinical observation, six patients showed dramatic progression of skin involvement, angioid streaks had progressed in two subjects. One patient had recurrent gastrointestinal bleeding and underwent partial stomach removal for gastric artery aneurysm, one underwent colon resection for intestinal infarct, one patient had a transitory ischaemic attack, one died after an intracranial haemorrhage, two patients died from cardiovascular disease and one from neoplasia.

CONCLUSIONS

Thalassaemic patients with PXE-like skin lesions also manifest PXE-like vessel alterations that progress with time. Considering the severe outcome of these lesions, accurate monitoring should be routinely performed on the cardiovascular system of thalassaemic patients with PXE-like skin manifestations.

Requirement of ERK activation for visual cortical plasticity

Experience-dependent plasticity in the developing visual cortex depends on electrical activity and molecular signals involved in stabilization or removal of inputs. Extracellular signal-regulated kinase 1,2 (also called p42/44 mitogen-activated protein kinase) activation in the cortex is regulated by both factors. We show that two different inhibitors of the ERK pathway suppress the induction of two forms of long-term potentiation (LTP) in rat cortical slices and that their intracortical administration to monocularly deprived rats prevents the shift in ocular dominance towards the nondeprived eye. These results demonstrate that the ERK pathway is necessary for experience-dependent plasticity and for LTP of synaptic transmission in the developing visual cortex.

Requirement of the nicotinic acetylcholine receptor beta 2 subunit for the anatomical and functional development of the visual system

In the mammalian visual system the formation of eye-specific layers at the thalamic level depends on retinal waves of spontaneous activity, which rely on nicotinic acetylcholine receptor activation. We found that in mutant mice lacking the beta2 subunit of the neuronal nicotinic receptor, but not in mice lacking the alpha4 subunit, retinofugal projections do not segregate into eye-specific areas, both in the dorso-lateral geniculate nucleus and in the superior colliculus. Moreover, beta2-/- mice show an expansion of the binocular subfield of the primary visual cortex and a decrease in visual acuity at the cortical level but not in the retina. We conclude that the beta2 subunit of the nicotinic acetylcholine receptor is necessary for the anatomical and functional development of the visual system.

Selective Intra-arterial Thrombolysis for Embolic Complications of Cerebral Endovascular Therapy

SUMMARY

Cerebral thrombo-embolic complications seldom occur during intracranial endovascular therapeutic procedures. Yet, despite a systemic preventive regimen of pre-operative intravenous acetylsalicylic acid and effective systemic heparinisation, we encountered seven complications of this type over a period of five years, a 3.7 per thousand incidence (7/1858). Local intra-arterial thrombolysis performed immediately after the thrombo-embolic complication resulted in complete recanalization in five cases and partial recanalization in two. CT scans performed 24 hours after thrombolysis were normal in six of the seven patients. Only one female patient with partial recanalization showed a limited cortical infarct in the parietal distribution of the middle cerebral artery territory. In all seven cases recanalization of the occluded vessel resulted in a return to the baseline initial neurological status. This short series demonstrates the usefulness of emergent thrombolysis dealing with the thrombo-embolic complications of angiography or endovascular therapy. These results are in agreement with these experimental data showing that early recanalization can prevent macroscopically visible infarcts.

A comparative morphometric analysis of the optic nerve in two cetacean species, the striped dolphin (Stenella coeruleoalba) and fin whale (Balaenoptera physalus)

A comparative study was made on one Mysticete (the fin whale, Balaenoptera physalus) and one Odontocete species (the striped dolphin, Stenella coeruleoalba) by measuring several morphological characteristics seen in cross sections of the optic nerve. We found that the two cetacean nerves share a number of specializations that distinguish them from the optic nerve of terrestrial mammals. Fiber density is approximately two-fold lower than in land mammals. A corresponding increase in the cross-sectional area occupied by astrocytes is observed. A population of "giant" (up to 15 microm in diameter) optic axons is present in both the B. physalus and the S. coeruleoalba nerve. It is argued that these features probably reflect common adaptations to the constraints imposed by the aquatic environment. "Giant" optic axons might ensure short-latency detection of preys and other targets during navigation while the increased astroglial content might be related to the maintenance of neuronal function during periods of anaerobic metabolism under water.

Bcl-2 overexpression per se does not promote regeneration of neonatal crushed optic fibers

We have explored whether overexpression of the bcl-2 gene 'per se' can promote regeneration of retinal ganglion cells (RGCs) after optic nerve axotomy in developing transgenic mice. We have used newborn mice (postnatal day 5) because at this age the central nervous system environment is more permissive for regeneration than in adults, thus, maximizing the probability to detect a regeneration-promoting role of bcl-2. Thirty days postsurgery we found that in mice overexpressing bcl-2, a high proportion of retinal ganglion cells survived and also that some fibers in the proximal stump of the optic nerve were preserved. However, the optic nerve of transgenic mice does not show signs of regeneration. On the contrary, in the presence of Schwann cell transplants, there are signs of fiber regrowth. Indeed, many axonal terminals cross the crush site and reach the chiasm in both wild type and transgenic mice nerves. These results suggest that bcl-2 overexpression is not sufficient 'per se' to increase the regenerative potentiality of axotomized RGCs.

Brain-derived neurotrophic factor is an anterograde survival factor in the rat visual system

BACKGROUND

The neurotrophins, which include nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), NT-4/5 and NT-6, are a family of proteins that play fundamental roles in the differentiation, survival and maintenance of peripheral and central neurons. Much research has focused on the role of neurotrophins as target-derived, retrogradely transported trophic molecules. Although there is recent evidence that BDNF and NT-3 can be transported in an anterograde direction along peripheral and central axons, there is as yet no conclusive evidence that these anterograde factors have direct post-synaptic actions.

RESULTS

We report that BDNF travels in an anterograde direction along the optic nerve. The anterogradely transported BDNF had rapid effects on retinal target neurons in the superior colliculus and lateral geniculate nucleus of the brain. When endogenous BDNF within the developing superior colliculus was neutralised, the rate of programmed neuronal death increased. Conversely, provision of an afferent supply of BDNF prevented the degeneration of geniculate neurons after removal of their cortical target.

CONCLUSIONS

BDNF released from retinal ganglion cells acts as a survival factor for post-synaptic neurons in retinal target fields.

Brain-derived neurotrophic factor causes cAMP response element-binding protein phosphorylation in absence of calcium increases in slices and cultured neurons from rat visual cortex

Neurotrophins play a crucial role in the developmental plasticity of the visual cortex, but very little is known about the cellular mechanisms involved in their action. In many models of synaptic plasticity, increases in cytosolic calcium concentration and activation of the transcription factor cAMP response element-binding protein (CREB) are crucial factors for the induction and maintenance of long-lasting changes of synaptic efficacy. Whether BDNF modulates intracellular calcium levels in visual cortical neurons and the significance of this action for BDNF signal transduction is still controversial. We investigated whether CREB phosphorylation and calcium changes are elicited by acute BDNF presentation in postnatal visual cortical slices and cultures. We found that BDNF did not cause any calcium increase, but it induced robust CREB phosphorylation in neurons from both preparations. We further analyzed signal transduction and its dependency on calcium changes in cultured neurons. CREB phosphorylation required trkB activation because treatment with the trk inhibitor k252a completely blocked CREB phosphorylation. In agreement with the imaging experiments, we verified that calcium changes were not necessary for CREB activation because preincubation with BAPTA-AM did not diminish the level of CREB phosphorylation induced by BDNF stimulation. CREB phosphorylation was accompanied by gene expression, because we observed the upregulation of c-fos expression, which was also not affected by preincubation with BAPTA-AM. Finally, BDNF caused phosphorylation of mitogen-activated protein kinase (MAPK), and because the treatment with the MAPK inhibitor U0126 completely abolished CREB activation and c-fos upregulation, it is likely that both processes depend mainly on the MAP kinase pathway. These results indicate that MAPK and CREB, but not intracellular calcium, are important mediators of neurotrophin actions in the visual cortex.

Effects of neurotrophins on cortical plasticity: same or different?

Neurotrophins are important regulators of visual cortical plasticity. It is still unclear, however, whether they play similar or different roles and which are their effects on the electrical activity of cortical neurons in vivo. We therefore compared the effects of all neurotrophins, nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-4 (NT-4), and neurotrophin-3 (NT-3) on visual cortical plasticity and on cell spontaneous and visually evoked activity. Rats were monocularly deprived for 1 week at the peak of the critical period, and neurotrophins were infused intracortically. The main finding is that, with the exception of NT-3, all neurotrophins affect the outcome of monocular deprivation, but there are clear differences in their mechanisms of action. In particular, NT-4 and NGF counteract monocular deprivation effects without causing detectable alterations either in spontaneous or visually evoked neuronal activity. BDNF is less effective on ocular dominance plasticity and, in addition, strongly affects spontaneous and visually evoked activity in cortical neurons.

Role of neurotrophins in the development and plasticity of the visual system: experiments on dark rearing

An extensive series of studies, beginning with the pioneering experiments of Wiesel and Hubel, have shown that correct visual experience is crucial for the development of the visual system. Several years ago, we put forward the hypothesis that neurotrophic factors of the neurotrophin family (NGF, BDNF, NT-3, NT-4) have a role in mediating the effects of visual experience in the developing visual system. This theory is based on the following experimental results: (a) exogenous supply of neurotrophins during the critical period prevents the effects of monocular deprivation; and (b) transplant of cells releasing NGF allows a normal development of the functional properties of visual cortical neurons in dark-reared rats.

Critical periods during sensory development

Recent studies have made progress in characterizing the determinants of critical periods for experience-dependent plasticity. They highlight the role of neurotrophins, NMDA receptors and GABAergic inhibition. In particular, genetic manipulation of a single molecule, brain-derived neurotrophic factor (BDNF), has been shown to alter the timing of the critical period of plasticity in mouse visual cortex, establishing a causal relation between neurotrophin action, the development of visual function, and the duration of the critical period.

Optic nerve crush: axonal responses in wild-type and bcl-2 transgenic mice

Retinal ganglion cells of transgenic mice overexpressing the anti-apoptotic protein Bcl-2 in neurons show a dramatic increase of survival rate after axotomy. We used this experimental system to test the regenerative potentials of central neurons after reduction of nonpermissive environmental factors. Survival of retinal ganglion cells 1 month after intracranial crush of the optic nerve was found to be 100% in adult bcl-2 mice and 44% in matched wild-type (wt) mice. In the optic nerve, and particularly at the crush site, fibers regrowing spontaneously or simply sprouting were absent in both wt and bcl-2 mice. We attempted to stimulate regeneration implanting in the crushed nerves hybridoma cells secreting antibodies that neutralize central myelin proteins, shown to inhibit regeneration (IN-1 antibodies) (Caroni and Schwab, 1988). Again, we found that regeneration of fibers beyond the site of crush was virtually absent in the optic nerves of both wt and bcl-2 mice. However, in bcl-2 animals treated with IN-1 antibodies, fibers showed sprouting in the proximity of the hybridoma implant. These results suggest that neurons overexpressing bcl-2 are capable of surviving axotomy and sprout when faced with an environment in which inhibition of regeneration has been reduced. Nevertheless, extensive regeneration does not occur, possibly because other factors act by preventing it.

From visual experience to visual function: roles of neurotrophins

Recently, a role for neurotrophins in regulating cortical developmental plasticity has clearly emerged. We present in this review a summary of the early data on the action of nerve growth factor (NGF) in visual cortical development and plasticity in the rat and of other neurotrophins in the visual cortex of other mammals. In addition, to clarify the differences in the results obtained with the various neurotrophins in different animal preparations, we also report new data on the action of NGF, brain-derived neurotrophic factor (BDNF), neurotrophin (NT)3, and NT4 in the same preparation-namely, the visual cortex of the rat. We discuss old and new results in a physiological model in which different neurotrophins play different roles in regulating visual cortical development and plasticity by acting on different neural targets, such as lateral geniculate nucleus (LGN) afferents, intracortical circuitry, and subcortical afferents, and propose a tentative scheme summarizing these actions.

BDNF regulates the maturation of inhibition and the critical period of plasticity in mouse visual cortex

Maturation of the visual cortex is influenced by visual experience during an early postnatal period. The factors that regulate such a critical period remain unclear. We examined the maturation and plasticity of the visual cortex in transgenic mice in which the postnatal rise of brain-derived neurotrophic factor (BDNF) was accelerated. In these mice, the maturation of GABAergic innervation and inhibition was accelerated. Furthermore, the age-dependent decline of cortical long-term potentiation induced by white matter stimulation, a form of synaptic plasticity sensitive to cortical inhibition, occurred earlier. Finally, transgenic mice showed a precocious development of visual acuity and an earlier termination of the critical period for ocular dominance plasticity. We propose that BDNF promotes the maturation of cortical inhibition during early postnatal life, thereby regulating the critical period for visual cortical plasticity.

The visual physiology of the wild type mouse determined with pattern VEPs

Genetically manipulated mice are important tools for studies on plasticity and degeneration/regeneration in the visual system. However, a description of the basic properties of the visual performance of the wild type mouse is still lacking. To characterize the visual physiology of the wild type (C57BL/6J) mouse we recorded Visual Evoked Potentials (VEPs) from the primary visual cortex. As compared to behavioral methods, VEPs may have the advantage that different aspects of vision can be screened readily and simultaneously in the same animals, including those with poor visual behavior due to motor or learning deficits. Local VEP responses to patterned visual stimuli have been recorded from the binocular visual cortex of anesthetized mice. Spatial (visual acuity, contrast threshold) and temporal (temporal function, response latency, motion sensitivity) aspects of VEPs were evaluated. The mouse VEP acuity was 0.6 c/deg, which is comparable to the behavioral visual acuity. The VEP peak contrast threshold was 5% (no behavioral data are available). Cortical representation of visual coordinates and cortical magnification factor corresponded to those previously reported using single cell recordings. Laminar analysis of VEPs indicated a dipole source in the supragranular layers of the visual cortex as a major response generator. VEPs showed contribution from both eyes, although biased strongly towards the eye contralateral to the recorded cortex. Results provide a comprehensive framework for characterizing visual phenotypes of a variety of transgenic mice.

Effects of nerve growth factor on visual cortical plasticity require afferent electrical activity

It is known that administration of nerve growth factor (NGF) prevents the ocular dominance shift induced by monocular deprivation in the rat. To determine whether electrical activity in the visual afferent pathway is required for NGF effects on ocular dominance, we infused NGF into the cortex of animals subjected to complete monocular blockade of retinal discharges. Rats at the peak of the critical period received intravitreal tetrodotoxin (TTX) injections to silence activity in one eye for a period of 6-7 days; NGF was concurrently delivered into the visual cortex by means of osmotic minipumps. At the end of the treatment period, the ocular dominance distribution of cortical neurons was assessed by single-cell recordings. The results demonstrate that while infusion of NGF is effective in preventing the ocular dominance shift in lid-sutured rats, virtually no rescue can be observed in TTX-injected animals. Identical results were obtained when a specific agonist of the NGF receptor TrkA, the bivalent anti-rat TrkA IgG (RTA), was infused into the cortex in place of NGF. We conclude that NGF signalling via the TrkA receptor must be coupled to afferent electrical activity to produce its effects on the eye preference of cortical neurons. This suggests a generalized mechanism in which high-affinity neurotrophin receptor activation and afferent discharge interact to modulate neuronal plasticity in the developing visual cortex.

Expression of the transcription factor Zif268 in the visual cortex of monocularly deprived rats: effects of nerve growth factor

Neurotrophins are known to be involved in experience-dependent plasticity of the visual cortex. Here, we have characterized in detail the effects of intraventricular nerve growth factor infusion in monocularly deprived rats by using immunostaining for the immediate-early gene product Zif268 as a marker of functional activity with cellular resolution. We have taken advantage of the rapid regulation of Zif268 by visual input to reveal the cortical units that are responsive to the deprived eye after a period of monocular deprivation. We found that responses to the deprived eye were significantly preserved in the cortex of monocularly deprived rats infused with nerve growth factor. The effects of nerve growth factor were greater for cortical cells located in deep layers and with more peripheral receptive fields. Results from Zif268 staining correlated very well with those obtained by single-cell recordings from the visual cortex. Our results demonstrate that exogenous nerve growth factor preserves the functional input from the deprived eye, enabling cortical neurons to activate immediate-early gene expression in response to stimulation of the deprived eye. Furthermore, we show that the intraventricular infusion of nerve growth factor differentially affects the ocular dominance of cells at various depths and eccentricities in the developing cortex.

Vision in mice with neuronal redundancy due to inhibition of developmental cell death

Transgenic mice overexpressing bcl-2, due to inhibition of naturally occurring cell death, have much larger brains and optic nerves as compared to wild-type mice. Since developmental cell death is believed to exert a crucial role in establishing the mature neural circuitry and function, we asked the question of whether basic aspects of vision were altered in bcl-2 mice. Local visually evoked potentials (VEPs) in response to patterned stimuli were recorded from the primary visual cortex. The representation of the vertical meridian was displaced by about 15% in the bcl-2 mouse, accounting for brain expansion. However, visual acuity, contrast threshold, and response latency were normal, indicating that compensatory mechanisms can ensure normal basic properties of vision in spite of marked neuronal redundancy.

Monocular deprivation decreases brain-derived neurotrophic factor immunoreactivity in the rat visual cortex

Neurotrophins play a crucial role in the development and activity-dependent plasticity of the visual cortex [Berardi N. et al. (1994) Proc. natn. Acad. Sci. U.S.A. 91, 684-688; Bonhoeffer T. (1996) Curr. Opin. Neurobiol. 6, 119-126; Cellerino A. and Maffei L. (1996) Prog. Neurobiol. 49, 53-71; Domenici L. et al. (1994) NeuroReport 5, 2041-2044; Galuske R. A. W. et al (1996) Eur. J. Neurosci. 8, 1554-1559; Katz L. C. and Shatz C. J. (1996) Science 274, 1133-1138; Maffei L. et al. (1992) J. Neurosci. 12, 4651-4662; Pizzorusso T. and Maffei L. (1996) Curr. Opin. Neurol. 9, 122-125; Thoenen H. (1995) Science 270, 593-598]. As a possible mechanism of action, it has been postulated that the activity-dependent expression of neurotrophins by cortical cells could regulate synapse stabilization during the first period of postnatal life (critical period). Indeed, brain-derived neurotrophic factor messenger RNA expression in the visual cortex is regulated by neuronal activity as well as during development [Castrén E. et al. (1992) Proc. natn. Acad. Sci. U.S.A. 89, 9444-9448]. Moreover, we showed that monocular deprivation decreases brain-derived neurotrophic factor messenger RNA levels in the visual cortex receiving input from the deprived eye [Bozzi Y. et al. (1995) Neuroscience 69, 1133-1144]. What is missing, however, is the demonstration that brain-derived neurotrophic factor protein expression follows that of brain-derived neurotrophic factor messenger RNA. The aim of the present study is to fill this important gap in order to support the hypothesis that brain-derived neurotrophic factor is fundamental in the plasticity of the visual cortex. We found that brain-derived neurotrophic factor immunoreactivity peaks during the critical period and that it is preferentially localized in layers II-III and V-VI. We also demonstrated that monocular deprivation determines a decrease of brain-derived neurotrophic factor immunoreactivity exclusively in the visual cortex contralateral to the deprived eye. Our results support the proposed role for brain-derived neurotrophic factor in the development and activity-dependent plasticity of the visual cortex [Cabelli R. J. et al. (1995) Science 267, 1662-1666].

Behavioural visual acuity of wild type and bcl2 transgenic mouse

Since the advent of gene manipulating techniques, it has become increasingly important to study the neural functional properties of the mouse. The bcl2 gene has a powerful inhibitory action on naturally occurring cell death. As a consequence the brain of bcl2 overexpressing mouse is 1.5 times bigger than the brain of a wild type animal and the retina has more than twice the ganglion cells than normal (Martinou, Dubois-Dauphin, Staple, Rodriguez, Frankowski, Missotten, Albertini, Talabot, Catsicas, Pietra, & Huarte (1994). Neuron, 13: 1017-1030). Since in most mammals the upper limit of behavioural visual acuity is imposed by ganglion cells density, the visual acuity should be higher in bcl2 mice than in wild type mice. We measured behavioural visual acuity in wild type and transgenic mice and, contrary to the expectation, we found it to be of the same order (0.5-0.6 c/deg) in the two groups of animals, indicating that an increase in ganglion cells density is not effective in improving visual resolution.

TrkA activation in the rat visual cortex by antirat trkA IgG prevents the effect of monocular deprivation

It has been recently shown that intraventricular injections of nerve growth factor (NGF) prevent the effects of monocular deprivation in the rat. We have tested the localization and the molecular nature of the NGF receptor(s) responsible for this effect by activating cortical trkA receptors in monocularly deprived rats by cortical infusion of a specific agonist of NGF on trkA, the bivalent antirat trkA IgG (RTA-IgG). TrkA protein was detected by immunoblot in the rat visual cortex during the critical period. Rats were monocularly deprived for 1 week (P21-28) and RTA-IgG or control rabbit IgG were delivered by osmotic minipumps. The effects of monocular deprivation on the ocular dominance of visual cortical neurons were assessed by extracellular single cell recordings. We found that the shift towards the ipsilateral, non-deprived eye was largely prevented by RTA-IgG. Infusion of RTA-IgG combined with antibody that blocks p75NTR (REX), slightly reduced RTA-IgG effectiveness in preventing monocular deprivation effects. These results suggest that NGF action in visual cortical plasticity is mediated by cortical TrkA receptors with p75NTR exerting a facilitatory role.

Nerve growth factor and brain-derived neurotrophic factor increase neurotransmitter release in the rat visual cortex

A number of experiments have shown that neurotrophins are involved in the development and plasticity of the visual cortex (Bonhoeffer, T., Curr. Op. Neurobiol., 6, 119, 1996). A possible mechanism underlying these effects is the neurotrophin modulation of synaptic transmission. We investigated whether nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) can modulate the release of neurotransmitter in the rat visual cortex at the peak of the critical period for plasticity (P23). The release of glutamate, acetylcholine and gamma-aminobutyric acid (GABA) from visual cortical synaptosomes was analysed in continuous perfusion conditions. We found that NGF enhances the depolarization-evoked release of glutamate (approximately 90%) and acetylcholine (approximately 35%) but not that of GABA. By contrast, BDNF enhances the depolarization-evoked release of all three neurotransmitters investigated (approximately 30%). BDNF and NGF were ineffective on basal release of neurotransmitters. The effect of NGF was not blocked by cholinergic antagonists atropine and mecamylamine. NGF and BDNF potentiation of transmitter release was strongly but not completely blocked by K252a, a tyrosine kinase inhibitor. The role of TrkA and p75NTR receptors was investigated in NGF-induced potentiation of glutamate release. Block of NGF binding to p75NTR using specific blocking antibodies (REX-IgG) slightly but significantly reduced the effect of NGF. Activation of TrkA in isolation by RTA-IgG, an antibody that specifically activates TrkA, was less effective than activation of both receptors by NGF. These results show that neurotrophin action on neurotransmitter release was mostly mediated by Trk receptors with p75NTR having a little but significant positive role. Antigen blot analysis showed the presence of TrkA, TrkB and p75NTR receptors in the visual cortex.

Protection of retinal ganglion cells and preservation of function after optic nerve lesion in bcl-2 transgenic mice

Multicellular organisms face the necessity of removing superfluous or injured cells during normal development, tissue turn-over and in response to damaging conditions. These finalised killings occur throughout a process, commonly called programmed cell death (PCD), which is placed under strict cellular control. PCD is regulated by the products of the expression of a number of genes. This fact raises the intriguing possibility of inhibiting such degenerative processes by operating on some of the controlling genes. Central neurons of transgenic mice overexpressing bcl-2, a powerful inhibitor of PCD, are remarkably resistant to degeneration induced by noxious stimuli. We have explored the ate of retinal ganglion cells and of their axons, when such transgenic animals have been challenged by a lesion of the optic nerve. These results have direct bearing on the possibility of attaining functional restoration of the injured pathway.