Brain-derived Neurotrophic Factor Reverses Experience-dependent Synaptic Modifications in Kitten Visual Cortex

During a critical period of early postnatal development the functional architecture of the visual cortex is shaped by experience-dependent circuit selection following a Hebbian mechanism. One consequence is that monocular deprivation (MD) leads to competitive repression of the input from the deprived eye. Recently it has been proposed that this process might involve activity-dependent competition for neurotrophic substances because the synthesis of brain-derived neurotrophic factor (BDNF) is regulated by visual input. Here we investigate the effects of intracortical infusion of BDNF and nerve growth factor (NGF) on MD effects in the visual cortex. Neuronal responses were monitored with optical and single-unit recording techniques in the visual cortex of kittens that had been infused intracortically either with BDNF, NGF or cytochrome C while subjected to MD for 1 week during the peak of the critical period. NGF or cytochrome C had no effect on the consequences of MD. After BDNF treatment, by contrast, ocular dominance (OD) shifted towards the deprived eye in a zone extending 2.5–3.5 mm from the infusion cannula, and neurons lost their orientation selectivity. At intermediate distances both eyes activated the cortex equally well and responses were again tuned for orientation; at still larger distances OD was shifted towards the normal eye. Thus, BDNF antagonizes the functional effects of MD and at high concentrations causes paradoxical disconnection of non-deprived afferents and a loss of orientation selectivity.     

Nogo-A and Nogo-A receptor expression in periventricular white matter of experimental autoimmune encephalomyelitis rats

BACKGROUND:Previous studies have focused on the correlation between Nogo-A expression and multiple sclerosis or between Nogo-A receptor (NgR) expression and multiple sclerosis in the central nervous system. Expression patterns of Nogo-A and NgR remain poorly understood in rat models of experimental autoimmune encephalomyelitis (EAE).OBJECTIVE:To observe dynamic changes in Nogo-A and NgR protein expression, and to verify the correlation between Nogo-A and NgR protein, as well as expression patterns at various time points, in periventricular tissue of EAE rats.DESIGN, TIME AND SETrlNG:A neuroimmunological, randomized, controlled experiment was performed at the Clinical Institute of Hunan People's Hospital of China from September to November 2008.MATERIALS:Immunohistochemistry (streptavidin-biotin-peroxidase complex method) kit was purchased from Boster, China.METHODS:A total of 60 female, Wistar rats, aged 6-8 weeks, ware randomly assigned to EAE and control groups (n = 30, respectively). Guinea pig spinal cord homogenate, self-made complete Freund's adjuvant (0.2 mL/100 g), and pertussis vaccine (0.2 mL) were subcutaneously injected into the hindlimb foot pad of rats from the EAE group to create rat models of EAE. Complete Freund's adjuvant (0.2 mL) was infused into rats from the control group.MAIN OUTCOME MEASURES:Nogo-A and NgR protein expression was determined in periventricular white matter using immunohistochemical methods. Neurological scores ware determined in all rats.RESULTS:Rats from the EAE group developed acute-onset EAE following immunization. The pathogenetic symptoms reached a peak on day 15, and neurological scores ware also greatest at this time point. Neurological scores decreased with recovery of the illness. Nogo-A was shown to be expressed in neuronal cells and oligodendrocytes, and expression increased 11 days after immunization (P < 0.01), decreased by day 13 (P < 0.01), and then increased again by day 15. Nogo-A expression remained greater in the EAE group compared with the control group at day 30 (P < 0.01). In the EAE group, NgR protein was primarily expressed on the surface of neuronal bodies and axons. NgR expression increased 13-18 days after immunization (P < 0.01 or P < 0.05).CONCLUSION:Nogo-A and NgR protein expression altered with disease course in periventdcular white matter of EAE rats. Results suggested that Nogo-A and NgR were involved in EAE occurrence.     

Brain-derived neurotrophic factor enhances expression of superior cervical ganglia clone 10 in lateral geniculate nucleus and visual cortex of developing kittens

Neuronal growth-associated proteins, including superior cervical ganglia clone 10 (SCG10) family molecules, play roles in neurite outgrowth and network formation as well as structural and functional plasticity. The present ontogenetic study revealed that the expression of neuronal growth-associated proteins in the visual cortex (VC) exhibited a sharp peak in the early postnatal period when growing lateral geniculate nucleus (LGN) axon terminals segregate into the ocular dominance columns depending on retinal activity. We then hypothesized that SCG10 family molecules, known for catastrophic factors of microtubules, play important roles in the formation of ocular dominance columns. To test this hypothesis, we studied whether: (i) monocular blockade of retinal activity changed the SCG10 expression in LGN and VC and (ii) brain-derived neurotrophic factor (BDNF) cortical infusion modified the expression of SCG10 family molecules and the number of excitatory/inhibitory cortical synapses. Using northern blot and in situ hybridization, we revealed that: (i) silencing retinal activity with tetrodotoxin eye injections dynamically reduced the expression of SCG10 mRNA and (ii) it was enhanced by BDNF in VC and LGN of kittens but not adult cats. These findings suggest that cortical infusion of BDNF and retinal activity up-regulate the expression of SCG10 in the LGN and VC and that up-regulated SCG10 in turn initiates marked reorganization of the microtuble network, eventually resulting in increase in synapse formation in the VC.     

Dark rearing prolongs physiological but not anatomical plasticity of the cat visual cortex

Recent studies (Cynader and Mitchell, '80; Mower et al., '81) have shown that total dark rearing prolongs susceptibility to the physiological effects of monocular deprivation (MD) in visual cortex beyond the normal age limits. The present study addressed whether this delayed physiological plasticity is accompanied by delayed anatomical plasticity in the geniculocortical pathway. Ocular dominance (OD) columns as defined by transsynaptic autoradiography following injection of 3H proline into one eye were studied both qualitatively and quantitatively in 17 cats. Compared to normal rearing (N-3), both binocular eyelid suture (N-2) and total dark rearing (N-3) resulted in incomplete segregation of OD columns in area 17. This apparent immaturity after binocular deprivation, however, did not reflect a delayed capacity for development and plasticity. Visual experience after dark rearing produced no marked changes. In cats who experienced MD after dark rearing, injection of either the nondeprived (N-2) or deprived eye (N-3) resulted in a nearly uniform distribution of label throughout layer IV of area 17. The same result occurred with binocular vision after dark rearing (N-1). MD from birth, however, produced expansion of columns from the nondeprived eye (N-1) and contraction of columns from the deprived eye (N-1). MD imposed after 4 months of normal vision resulted in normal OD columns (N-1). Electrophysiological studies revealed a high proportion of binocular cells within layer IV in cats who experienced monocular or binocular vision after dark rearing. Outside of layer IV there were clear environmental effects on OD of single cells in these cats. Measurements of cell sizes in the clateral geniculate nucleus showed shrinkage of cells innervated by the deprived eye when MD was initiated at birth (N-3). MD after dark rearing (N-4) produced no differences in cell sizes. It is concluded that visual input is necessary for the formation of normal OD columns, the critical period for formation and environmental modification of OD columns is limited to early life, and the physiological effects of visual experience after dark rearing reflect changes occurring beyond the geniculocortical pathway.     

Upregulation of Nogo receptor expression induces apoptosis of retinal ganglion cells in diabetic rats

The Nogo receptor is an essential factor for neuronal apoptosis, but the changes in Nogo receptor expression in the retina and the effects of the Nogo receptor on retinal ganglion cell apoptosis in diabetes mellitus remain unclear. We found that Nogo receptor expression was mainly visible in retinal ganglion cells of a rat model of diabetes mellitus induced by streptozotocin. At 12 weeks after onset of diabetes mellitus, Nogo receptor and Rho kinase expression signiifcantly increased in the retina, and retinal ganglion cell apoptosis was apparent. When RNA interference was used to suppress Nogo receptor expression in rat retina, Rho kinase expression was obviously inhibit-ed, and retinal ganglion cell apoptosis was evidently reduced in rats with diabetes mellitus. These results indicate that upregulation of Nogo receptor expression is an important mechanism of retinal ganglion cell apoptosis in rats with diabetes mellitus.     

Lack of binocularity in cells of area 19 of cat visual cortex following monocular deprivation

We have studied the visual receptive field properties of neurons in cortical area 19 of monocularly deprived cats. Almost all visually responsive units responded to stimulation of the non-deprived eye only. Receptive field properties assessed through the non-deprived eye were found to be normal. Monocular deprivation appears thus to have sharply reduced the normal binocularity of neurons in area 19. Since the W-cell component of the visual pathways provide the predominant input to area 19, our results suggest that W-cells are vulnerable to environmental manipulation.     

Reduced ocular dominance plasticity and long-term potentiation in the developing visual cortex of protein kinase A RII alpha mutant mice

The cAMP-dependent protein kinase (PKA) signalling pathway has been shown to play an important role in long-term potentiation (LTP) and depression (LTD), and ocular dominance plasticity in the visual cortex. In order to investigate further the involvement of individual PKA subunits in visual cortical plasticity, LTP and LTD in vitro and ocular dominance plasticity in vivo in the developing visual cortex were examined in mice lacking the RII alpha subunit of PKA. Here we show that LTP in layers II/III was decreased in RII alpha knockout mice, but LTD was almost unaffected, and the ocular dominance shift induced by monocular deprivation was also partially blocked. These data provide evidence that RII alpha is involved in LTP and ocular dominance plasticity, and further suggest that different afferent inputs could selectively activate particular subunits of PKA and thereby direct specific aspects of visual cortical plasticity.     

Effects of dark-rearing on the vascularization of the developmental rat visual cortex

Cerebral vascular density corresponds to metabolic demand, which increases in highly active areas. External inputs play an important role in the modeling and development of the visual cortex. Experience-mediated development is very active during the first postnatal month, when accurate simultaneous blood supply is needed to satisfy increased demand. We studied the development of visual cortex vascularization in relation to experience, comparing rats raised in darkness with rats raised in standard conditions. The parameters measured were cortical thickness, vascular density and number of perpendicular vessels, constituting the first stage of cortical vascular development. Vessels were stained using butyryl cholinesterase histochemistry, which labels some neurons and microvascularization (vessels from 5 to 50 μm). Animals from both groups were sampled at 0, 7, 14, 21 and 60 days postnatal. Vascularization of the brain starts with vertically oriented intracortical vascular trunks whose density decreases notably after birth in rats reared in standard laboratory conditions. The most striking finding of our work is the significantly lower decrease in the number of these vessels in dark-reared rats. Our results also show that cortex thickness and vessel density are significantly lower in dark-reared rats. These results suggest that the absence of visual stimuli retards the maturation of the visual cortex including its vascular bed.     

Involvement of T-type Ca2+ channels in the potentiation of synaptic and visual responses during the critical period in rat visual cortex

Neocortical neuronal circuits are refined by experience during the critical period of early postnatal life. The shift of ocular dominance in the visual cortex following monocular deprivation has been intensively studied to unravel the mechanisms underlying the experience-dependent modification. Synaptic plasticity is considered to be involved in this process. We previously showed in layer 2/3 pyramidal neurons of rat visual cortex that low-frequency stimulation-induced long-term potentiation (LTP) at excitatory synapses, which requires the activation of Ni²⁺-sensitive (R-type or T-type) voltage-gated Ca²⁺ channels (VGCCs) for induction, shared a similar age and experience dependence with ocular dominance plasticity. In this study, we examined whether this LTP is involved in ocular dominance plasticity. In visual cortical slices, LTP was blocked by mibefradil, kurtoxin and R -(−)-efonidipine, T-type VGCC blockers, but not by SNX-482, an R-type VGCC blocker, indicating that LTP induction requires T-type VGCC activation. Mibefradil did not affect synaptic transmission even at a dose about 30 times higher than that required for LTP blockade. Therefore, this drug was used to test the effect of T-type VGCC blockade on ocular dominance shift produced by 6 days of monocular deprivation during the critical period using visual evoked potentials (VEPs). Although this monocular deprivation commonly produced both depression of deprived eye responses and potentiation of nondeprived eye responses, only the former change occurred when mibefradil was infused into the visual cortex during monocular deprivation. Mibefradil infusion produced no acute effects on VEPs. These results suggest that T-type VGCC-dependent LTP contributes to the experience-dependent enhancement of visual responses.     

Expression of Nogo-A mRNA after injury of the rat central nervous system

BACKGROUND： Nogo protein has been identified as an inhibitor of axonal growth, which was highly expressed in central nervous system; however, there are only a few studies on changes of Nogo-A expression following central nervous system injury. OBJECTIVE： To investigate the dynamic expression of Nogo-A mRNA after rat central nervous system injury. DESIGN： Randomized controlled animal study. MATERIALS： Thirty-five rats were randomly divided into two groups, normal animal group （n = 5） and model group （n = 30）. The model group was then divided into six subgroups at six time points： 12, 24 hours and 3, 9, 15, and 21 days post-injury, with five rats in each subgroup. METHODS： The left parietal lobe of rats was contused by free-fall strike, and total RNA was extracted from the entire brain tissue. Semi-quantitative RT-PCR was used to detect Nogo-A mRNA expression, and the ratio between expression of the target gene and glyceraldehyde phosphate dehydrogenase was used to determine the relative expression level. MAIN OUTCOME MEASURES： To determine whether Nogo-A mRNA expression was higher than usual following brain injury. RESULTS： The level of Nogo-A mRNA started to increase 12 hours after injury （P 〈 0.05） and decreased slightly by 24 hours post-injury. Expression increased again on day 3 and reached a peak on day 9. Nogo-A mRNA expression started to decrease on day 15, and then decreased to normal levels at days 21 （P 〉 0.05）. CONCLUSION： After injury of the central nervous system, Nogo-A may play a pivotal role in obstructing regeneration of the nerve.     

Suppression of visual cortical evoked responses following deprivation of pattern vision in adult mice

The effect of visual loss on the adult neocortex can have significant impact on the success of a visual implant. Recent research has shown that the adult neocortex retains substantial plasticity following a disruption to its afferent input. The result of these changes may hamper the development of a visual prosthesis if visual sensation cannot be effectively restored by stimulation of the surviving elements of the visual pathway. In order to evaluate further the visual performance of the mammalian adult brain following visual loss, especially the dominant form of blindness in humans, namely loss of pattern vision, we examined the cortical evoked potential of adult mice following 7, 30 and 120 days of visual deprivation via bilateral eyelid suture. Cortical potentials were elicited with a flash visual stimulus or by electrical stimulation of the retina. We found that after 7 days deprivation there was a potentiation of the evoked response while at 30 and 120 days deprivation the visual evoked responses were significantly reduced. Increasing the visual stimulus intensity reduced the effects. The electrical evoked potential demonstrated a corresponding reduction in stimulus threshold at 7 days and a corresponding rise (40–50%) after 30 and 120 days. These findings suggest that the adult brain exhibited significant experience-dependent modifications following visual loss, and the impact depended on the duration of deprivation. Such reduction in visual responsiveness, especially with electrical activation, will need to be taken into account in the development of a visual implant.     

Activity regulates the expression of AMPA receptor subunit GluR4 in developing visual cortex

In the developing visual cortex, the expression of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) subunit GluR4 precedes that of the other AMPAR subunits GluR1-3, and then declines to become almost absent in adults. The current study shows that the neuronal activity regulates the expression of GluR4 by a culture system in vitro and a dark-rearing (DR) system in vivo. Membrane depolarization by treatment of cultured neurons of the visual cortex with a high concentration of KCl (35 mm; HK) promoted a decline in the expression of GluR4. This effect of HK on the expression of GluR4 was significantly blocked by the addition of an N-methyl-d-aspartate receptor (NMDAR) antagonist, (D)-2-amino-5-phosphonovaleric acid (APV), but not by the voltage-sensitive calcium channel antagonist nifedipine. Moreover, the Ca(2+)-calmodulin-dependent kinase (CaMKII) inhibitor KN62 and the cAMP-dependent protein kinase A (PKA) inhibitor H-89 blocked this effect, which suggests the involvement of Ca(2+) influx via NMDAR and the subsequent activation of CaMKII and PKA. Conversely, the MAP kinase inhibitor PD98059 promoted the effect of HK on the expression of GluR4. Significantly, APV, KN62, H-89 and PD98059 either promoted or inhibited the expression of GluR4 even in normal KCl (5 mm) conditions. The developmental change in the expression of GluR4 was significantly attenuated in DR in vivo, and the results suggest that neuronal activity such as visual experience may be involved in the mechanism of the expression of GluR4, which is mediated by NMDAR and tuned by certain protein kinases at an early developmental stage in the visual cortex.     

Comparative pharmacological effects on visual cortical neurons in monocularly deprived cats

Monocularly deprived (MD) cats show a loss of responsiveness to visual stimulation of the deprived eye among visual cortical neurons. Several lines of evidence suggest that this effect involves, at least in part, a suppression of deprived eye input, possibly mediated by GABA inhibition. In order to better understand the nature of this suppression we have evaluated the effectiveness of different types of disinhibitory and excitatory agents to reverse the effects of MD. We investigated bicuculline (a GABA antagonist); picrotoxin (a GABA antagonist with a different mechanism of action from bicuculline); strychnine (a glycine antagonist); ammonium ion (a blocker of membrane chloride channels); physostigmine (a cholinesterase inhibitor); and naloxone (an opiate antagonist and also a GABA antagonist). All drugs were given intravenously. Bicuculline restored binocularity to 50% of the visual cortical neurons tested and naloxone to 36%. With both drugs, receptive fields of the normal eye tended to lose specificity. The emergent deprived eye receptive fields were usually similar to those of the normal eye after drug administration. Ammonium ion produced binocular responses in 27% of neurons tested, but receptive fields were grossly abnormal; moreover, ammonium infusion tended to depress neuronal responsiveness. All other drugs tested failed to restore binocularity. These experiments lend further credence to the hypothesis that GABA inhibition contributes to the cortical effects of MD, since only drugs with GABA antagonistic action were effective in restoring neuronal responsiveness to the deprived eye.     

Effects of Fujian tablet on Nogo-A mRNA expression and plasticity of the corticospinal tract in a rat model of focal cerebral ischemia

The present study investigated the effects of Fujian tablet, a Chinese medicine compound that can nourish liver and kidney, on corticospinal tract plasticity and cervical cord microenvironment in rats with focal cerebral ischemia. Results showed that motor function of rats with right proximal middle cerebral artery occlusion was significantly improved following treatment with Fujian tablet, 9 g crude drug/kg. Anterograde tracing revealed significantly increased biotinylated dextran amine expression in the denervated (left) side of the cervical cord (C4-6) following Fujian tablet treatment, and significantly decreased Nogo-A mRNA expression was detected in the denervated side of the cervical cord (C4-6) using in situ hybridization. Pearson’s correlation analysis showed a negative correlation between biotinylated dextran amine and Nogo-A mRNA expression (r = -0.943, P < 0.01). Results demonstrated that Fujian tablet can promote corticospinal tract plasticity possibly through the inhibitory effect on Nogo-A mRNA expression in the cervical spinal cord, thereby improving motor dysfunction.     

Rapid visual stimulation increases extrasynaptic glutamate receptor expression but not visual‐evoked potentials in the adult rat primary visual cortex

The model most used to study synaptic plasticity, long‐term potentiation (LTP), typically employs electrical stimulation of afferent fibers to induce changes in synaptic strength. It would be beneficial for understanding the behavioral relevance of LTP if a model could be developed that used more naturalistic stimuli. Recent evidence suggests that the adult visual cortex, previously thought to have lost most of its plasticity once past the critical period, is in fact capable of LTP‐like changes in synaptic strength in response to sensory manipulations alone. In a preliminary study, we used a photic tetanus (PT; flashing checkerboard stimulus) to induce an enhancement of the visual‐evoked potential (VEP) in the primary visual cortex of anesthetised adult rats. In the present study, we sought to compare the mechanisms of this novel sensory LTP with those of traditional electrical LTP. Unexpectedly, we found that sensory LTP was not induced as reliably as we had observed previously, as manipulations of several parameters failed to lead to significant potentiation of the VEP. However, we did observe a significant increase in visual cortex glutamate receptor expression on the surface of isolated synapses following the PT. Both AMPA receptor expression and N‐methyl‐d ‐aspartate (NMDA) receptor subunit expression were increased, specifically in extrasynaptic regions of the membrane, in PT animals. These results provide biochemical confirmation of the lack of change in the VEP in response to PT, but suggest that PT may prime synapses for strengthening upon appropriate subsequent activation, through the trafficking of glutamate receptors to the cell surface.     

Development of MK-801, kainate,AMPA, and muscimol binding sites and the effect of dark rearing in rat visual cortex

We used quantitative autoradiography to determine whether the development of glutamate receptors correlates with the plastic period for monocular deprivation in rat visual cortex. To study glutamate receptors, we incubated sections of rat visual cortex with tritiated (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]-cyclohepten-5,10imine maleate (MK-801), tritiated kainate, and tritiated amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid (AMPA). [³H]MK-801 is a noncompetitive ligand for the N-methyl-D-aspartate (NMDA) receptor. [³H]kainate and [³H]AMPA are competitive ligands for non-NMDA receptors. To compare glutamate binding sites with a nonglutamate binding site, we studied [³H]muscimol, which binds to γ-aminobutyric acid (GABA)_A receptors. [³H]MK-801 binding was maximal at postnatal day 26 (P26) and decreased in adulthood. [³H]AMPA binding was maximal at P18. [³H]kainate binding and [³H]muscimol binding were not age dependent. Dark rearing partially prevented the age-dependent decrease in [³H]MK-801 binding but had no effect on [³H]kainate or [³H]AMPA binding. Dark rearing decreased muscimol binding in adult animals. These results suggest that NMDA receptors, but not other glutamate receptors or GABA_A receptors, are likely to be critical for developmental plasticity in rat visual cortex. J. Comp. Neurol. 383:73–81, 1997. © 1997 Wiley-Liss, Inc.