Long term depression is expressed during postnatal development in rat visual cortex: a role for visual experience

Light deprivation improves melatonin related suppression of hippocampal plasticity

In early postnatal life, sensory inputs deeply influence development as well as function of the brain. Plasticity of synaptic transmission including its experimentally induced form, long-term potentiation (LTP), is affected by sensory deprivation in neocortex. This study is devoted to assess if dark rearing and a dark phase synthesized hormone melatonin influence LTP in the hippocampus, an area of brain involved in learning and memory. In vivo experiments were carried out on two groups of 45-days-old male Wistar rats kept in standard 12-h light/dark condition [light reared (LR) tested during the light phase] or in complete darkness [dark reared (DR)] since birth to testing. Each group, in turn, was divided to two, vehicle- and melatonin-treated, groups. Stimulating the Schaffer collaterals of CA3 area of hippocampus extracellular postsynaptic potentials (EPSPs) were recorded in the CA1 area. Having the stable baseline responses to the test pulses, the hippocampus was perfused by either vehicle or 2 microg melatonin and EPSPs were recorded for 30 min. Then, for induction of LTP, the tetanus was applied to the Schaffer collaterals and the field potentials were pooled for 120-min post-tetanus. The light deprivation resulted in a significant augmentation in the amplitude of baseline responses. Also, we observed a melatonin-induced increase in amplitude of the baseline recordings in either LR or DR animals. Tetanic stimulation elicited LTP of EPSPs in both LR and DR groups, robustly in the former where it lasted for about 90 min. Generally, melatonin inhibited the production of LTP in the two groups especially in the LR animals leading to a noticeable depression. We concluded that higher level of neuronal activity in the DR rats gives rise to a lower level of LTP. Weaker effect of melatonin on blocking the potentiation of post-tetanus EPSPs in the DR rats may be the result of a desensitization of melatonin receptors due to chronically increased levels of this hormone in the visually deprived rats.

[Early hearing experience and sensitive developmental periods]

This article reviews the studies on functional deficits in the auditory cortex of congenitally deaf animals. It compares their results with psychophysical and imaging data obtained from prelingually deaf humans. The studies demonstrate that the development of the auditory cortex is affected by the absence of hearing experience. In humans, the restoration of hearing after congenital deafness shows a sensitive period of 4 years, whereas even within this sensitive period cortical plasticity is already decreasing with increasing age. The reasons for the sensitive period are developmental changes of synaptic plasticity, developmentally modified synaptogenesis and synaptic pruning as well as changes in connectivity of the auditory cortex. Absence of top-down interactions from higher order auditory areas is another cardinal reason for the sensitive period. All these mechanisms contribute to the decreasing capacity for cortical plasticity during postnatal development. From the developmental and neurophysiological point of view, an early identification of hearing loss is an important prerequisite for effective therapy.

Age-dependent decline in supragranular long-term synaptic plasticity by increased inhibition during the critical period in the rat primary visual cortex

Supragranular long-term potentiation (LTP) and depression (LTD) are continuously induced in the pathway from layer 4 during the critical period in the rodent primary visual cortex, which limits the use of supragranular long-term synaptic plasticity as a synaptic model for the mechanism of ocular dominance (OD) plasticity. The results of the present study demonstrate that the pulse duration of extracellular stimulation to evoke a field potential (FP) is critical to induction of LTP and LTD in this pathway. LTP and LTD were induced in the pathway from layer 4 to layer 2/3 in slices from 3-wk-old rats when FPs were evoked by 0.1- and 0.2-ms pulses. LTP and LTD were induced in slices from 5-wk-old rats when evoked by stimulation with a 0.2-ms pulse but not by stimulation with a 0.1-ms pulse. Both the inhibitory component of FP and the inhibitory/excitatory postsynaptic potential amplitude ratio evoked by stimulation with a 0.1-ms pulse were greater than the values elicited by a 0.2-ms pulse. Stimulation with a 0.1-ms pulse at various intensities that showed the similar inhibitory FP component with the 0.2-ms pulse induced both LTD and LTP in 5-wk-old rats. Thus extracellular stimulation with shorter-duration pulses at higher intensity resulted in greater inhibition than that observed with longer-duration pulses at low intensity. This increased inhibition might be involved in the age-dependent decline of synaptic plasticity during the critical period. These results provide an alternative synaptic model for the mechanism of OD plasticity.

Sequential development of long-term potentiation and depression in different layers of the mouse visual cortex

Visual deprivation affects the responses of layer IV cells more prominently during early postnatal development, whereas responses in layer II/III remain modifiable until later ages. We examined whether these laminar differences correlate with changes in long-term potentiation (LTP) and long-term depression (LTD) of the ascending pathways to layers IV and II/III in the mouse visual cortex. Our analysis revealed that LTP and LTD in layer IV principal cells is lost shortly after the eyes open, but persists in layers II/III beyond puberty. These results suggest that plasticity proceeds sequentially through cortical layers in a manner that parallels the flow of information during sensory processing.

Cochlear implants: cortical plasticity in congenital deprivation

Congenital auditory deprivation (deafness) leads to a dysfunctional intrinsic cortical microcircuitry. This chapter reviews these deficits with a particular emphasis on layer-specific activity within the primary auditory cortex. Evidence for a delay in activation of supragranular layers and reduction in activity in infragranular layers is discussed. Such deficits indicate the incompetence of the primary auditory cortex to not only properly process thalamic input and generate output within the infragranular layers, but also incorporate top-down modulations from higher order auditory cortex into the processing within primary auditory cortex. Such deficits are the consequence of a misguided postnatal development. Maturation of primary auditory cortex in deaf animals shows evidence of a developmental delay and further alterations in gross synaptic currents, spread of activation, and morphology of local field potentials recorded at the cortical surface. Additionally, degenerative changes can be observed. When hearing is initiated early in life (e.g., by chronic cochlear-implant stimulation), many of these deficits are counterbalanced. However, plasticity of the auditory cortex decreases with increasing age, so that a sensitive period for plastic adaptation can be demonstrated within the second to sixth months of life in the deaf cat. Potential molecular mechanisms of the existence of sensitive period are discussed. Data from animal research may be compared to electroencephalographic data obtained from cochlear-implanted congenitally deaf children. After cochlear implantation in humans, three phases of plastic adaptation can be observed: a fast one, taking place within the first few weeks after implantation, showing no sensitive period; a slower one, taking place within the first months after implantation (a sensitive period up to 4 years of age); and possibly a third, and the longest one, related to increasing activation of higher order cortical areas.

Lack of experience-mediated differences in the immunohistochemical expression of blood-brain barrier markers (EBA and GluT-1) during the postnatal development of the rat visual cortex

The development of the cortical vascular tree depends on functional development. External inputs are an essential requirement in the modeling of the visual cortex, mainly during the critical period, when congruous blood supply is needed. The blood brain barrier (BBB) function regulates the passage of substances between the blood and the brain parenchyma, which is one of the main differential features of central nervous system (CNS) microvessels. The endothelial barrier antigen (EBA) has been reported as a specific marker for the BBB physiological function in rats. We studied the postnatal development of EBA expression in the visual cortex of rats reared under opposite paradigms of visual experience, e.g., standard laboratory conditions, dark rearing, and enriched environment at 14, 21, 28, 35, 42, 49, 56, and 63 days postnatal (dpn). Parallel sections were immunohistochemically processed for endothelial barrier antigen (EBA) and glucose transporter-1 (GluT-1). Total vasculature was quantified by Lycopersicon esculentum (LEA) lectin histochemistry. No differences in EBA expression were found between groups, although quantitative differences were recorded paralleling differences in vascular density. Paradoxically, there was no expression in certain cortical vessels which were GluT-1 immunopositive and positivity was consistent in non-barrier areas such as the pineal gland. These findings were completely independent of age or experimental conditions. Therefore, the role of the EBA antigen in the BBB remains unclear: it has been undeniably linked to vascular permeability, but its presence in non-barrier vessels suggests another vascular function. Although visual experience modifies vascular density in the visual cortex, it has not been shown to have an influence on the maturation of the BBB function.

Long-term depression is not induced by low-frequency stimulation in rat visual cortex in vivo: a possible preventing role of endogenous brain-derived neurotrophic factor

Low-frequency stimulation (LFS) at 1 Hz for 15 min is an effective protocol to induce homosynaptic long-term depression (LTD) in visual cortical slices. It is reported that LFS becomes ineffective when brain-derived neurotrophic factor (BDNF) is applied to slices. It is not known, however, whether such a protocol induces LTD in visual cortex in vivo, and whether endogenous BDNF has the same or similar action. To address these questions, we recorded field potentials of rat visual cortex evoked by stimulation of lateral geniculate nucleus, white matter, or cortical layer IV. We found that LFS did not induce LTD of cortical responses in vivo. To test the possibility that spontaneous activity from retinas would interfere with the induction of LTD, both eyes were removed or inactivated by tetrodotoxin. LTD was not induced in these conditions either. To test whether the difference in temperature between the two preparations is a factor for the discrepancy, the temperature of slices was increased from 31 to 37 degrees C. LTD was induced in slices at either temperature. Then, we hypothesized that endogenous BNDF and its receptors, TrkB, prevent the induction of LTD. To test this, we infused the cortex with an inhibitor of Trk receptor tyrosine kinases, anti-TrkB IgG1, anti-BDNF, and anti-neurotrophin 4/5 antibodies. LTD was induced when the BDNF-TrkB system was blocked. In slices, the level of phosphorylation of Trks was found to decrease with time. These results indicate that activation of TrkB signal pathway prevents LFS from inducing synaptic depression in visual cortex in vivo.

Visual deprivation effects on the s100beta positive astrocytic population in the developing rat visual cortex: a quantitative study

After birth, exposure to visual inputs modulates cortical development, inducing numerous changes of all components of the visual cortex. Most of the cortical changes thus induced occur during what is called the critical period. Astrocytes play an important role in the development, maintenance and plasticity of the cortex, as well as in the structure and function of the vascular network. Dark-reared Sprague-Dawley rats and age-matched controls sampled at 14, 21, 28, 35, 42, 49, 56 and 63 days postnatal (dpn) were studied in order to elucidate quantitative differences in the number of positive cells in the striate cortex. The astrocytic population was estimated by immunohistochemistry for S-100beta protein. The same quantification was also performed in a nonsensory area, the retrosplenial granular cortex. S-100beta positive cells had adult morphology in the visual cortex at 14 dpn and their numbers were not significantly different in light-exposed and nonexposed rats up to 35 dpn, and were even higher in dark-reared rats at 21 dpn. However, significant quantitative changes were recorded after the beginning of the critical period. The main finding of the present study was the significantly lower astroglial density estimated in the visual cortex of dark-reared rats over 35 dpn as well as the lack of difference at previous ages. Our results also showed that there were no differences when comparing the measurements from a nonsensory area between both groups. This led us to postulate that the astrocytic population in the visual cortex is downregulated by the lack of visual experience.

Differential dependence of LTD on glutamate receptors in the auditory cortical synapses of cortical and thalamic inputs

Pyramidal neurons in the auditory cortex (AC) receive glutamatergic inputs from the medial geniculate body (MGB inputs) and other pyramidal neurons (pyramidal inputs). We found that the induction of long-term depression (LTD) in supragranular layers was only partially suppressed by 50 microM D-(-)-2-amino-5-phosphonovalerate (APV), an antagonist of N-methyl-D-aspartate (NMDA) receptors (NMDARs), and 500 microM (+)-alpha-methyl-4-carboxyphenylglycine (MCPG), an antagonist of metabotropic glutamate receptors (mGluRs). However, LTD was not observed in the mixture of APV and MCPG. We hypothesized that the mixed dependence of LTD on glutamate receptors could be attributed to the heterogeneity of MGB inputs and pyramidal inputs. To test this hypothesis, the angle of slicing and other recording conditions were adjusted so that postsynaptic potentials were recorded in normal slices, but not in the slices prepared from the rats with MGB lesion. In these experiments, LTD was suppressed by MCPG alone. The conditions were adjusted to minimize the contribution of MGB inputs in field potentials. In these experiments, the induction of LTD was suppressed by APV alone. Interestingly, the induction of LTD was partially suppressed by 20 microM nifedipine, a blocker of L-type Ca(2+) channels, in the slices prepared from the rats with MGB lesions, but not in normal slices. These findings suggest that the induction of LTD requires activation of mGluRs in the synapses of MGB inputs and of NMDARs in the synapses of pyramidal inputs.

Pairing-induced changes of orientation maps in cat visual cortex

We have studied the precise temporal requirements for plasticity of orientation preference maps in kitten visual cortex. Pairing a brief visual stimulus with electrical stimulation in the cortex, we found that the relative timing determines the direction of plasticity: a shift in orientation preference toward the paired orientation occurs if the cortex is activated first visually and then electrically; the cortical response to the paired orientation is diminished if the sequence of visual and electrical activation is reversed. We furthermore show that pinwheel centers are less affected by the pairing than the pinwheel surround. Thus, plasticity is not uniformly distributed across the cortex, and, most importantly, the same spike time-dependent learning rules that have been found in single-cell in vitro studies are also valid on the level of cortical maps.

Trk B signalling controls LTP but not LTD expression in the developing rat visual cortex

Neurotrophins have been suggested to act as liaison molecules between activity-dependent synaptic plasticity and the establishment of patterns of synaptic connectivity during postnatal developmental in different brain areas, including the visual cortex. In particular, recent studies have shown that Trk B ligands are involved in the formation of the ocular dominance columns during postnatal development. Here, we examined the contribution of endogenous Trk B activation to the regulation of different forms of synaptic plasticity including long-term potentiation (LTP), long-term depression (LTD) and LTP after LTD in the developing visual cortex. Rat cortical slices were incubated with a soluble form of Trk B receptor (TrkB IgG) preventing Trk B activation by endogenous ligands. LTP expression was also studied at P23 (postnatal), when the expression of brain-derived neurotrophic factor (BDNF) reaches a peak and the LTP expression is normally downregulated. The present results demonstrate that Trk B activation is required for the long-term maintenance, > 30 min, of both LTP and LTP after LTD at P17. At P23, a higher concentration of TrkB IgG was necessary to impair LTP. In contrast, neither amplitude nor duration of LTD were affected by Trk B ligands blockade. Taken together, these results indicate that endogenous Trk B ligands are necessary for the expression of LTP but not LTD at a critical time during postnatal cortical development.

Blocking the NGF-TrkA interaction rescues the developmental loss of LTP in the rat visual cortex: role of the cholinergic system

Although nerve growth factor (NGF) is a crucial factor in the activity-dependent development and plasticity of visual cortex, its role in synaptic efficacy changes is largely undefined. We demonstrate that the maintenance phase of long-term potentiation (LTP) is blocked by local application of exogenous NGF in rat visual cortex at an early stage of postnatal development. Long-term depression (LTD) and bidirectional plasticity are unaffected. At later postnatal ages, blockade of either endogenous NGF by immunoadhesin (TrkA-IgG) or TrkA receptors by monoclonal antibody rescues LTP. Muscarinic receptor activation/inhibition suggests that LTP dependence on NGF is mediated by the cholinergic system. These results indicate that NGF regulates synaptic strength in well-characterized cortical circuitries.

Synaptic plasticity of feedback connections in rat visual cortex

The issue we want to address in the present paper is to establish whether electrical stimulation of latero medial (LM) area, a secondary visual area in the rat, is able to induce Long Term Potentiation (LTP) and Long Term Depression (LTD) in primary visual cortex (V1). To this aim rat slices containing area V1 and LM were prepared at P23 and P40 and field potentials in layers 2/3 of area V1 were recorded stimulating LM. We showed that it was never possible to induce LTP in area V1, unless bicuculline, a gamma-aminobutyric acid (GABA) receptors blocker, was applied to the slice. In contrast, LTD was normally inducible. Thus, cortical gabaergic circuitry in area V1 controls LTP but not LTD elicited by stimulation of feedback connections from LM.

Primed-burst potentiation in adult rat visual cortex in vitro

The effectiveness of θ pattern primed-bursts (PBs) on development of PB potentiation was investigated in layer II/III of the adult rat visual cortex in vitro. Experiments were carried out in the visual cortical slices. Population excitatory postsynaptic potentials (pEPSPs) were evoked in layer II/III by stimulation of either white matter or layer IV. To induce long-term potentiation (LTP), eight episodes of PBs were delivered at 0.1 Hz. Regardless of stimulation site, field potential recorded in layer II/III consisted of two components: a short latency and high amplitude response called pEPSP1, and a long latency and low amplitude response called pEPSP2. The incidence of LTP produced by PBs of layer IV was higher than that of the white matter tetanization. In contrast, PBs of both layer IV and white matter reliably produced LTP of pEPSP2 in layer II/III. It is concluded that PBs, as a type of activity pattern, of either white matter or layer IV can gain access to the modifiable synapses that are related to pEPSP2 in layer II/III, but accessibility of the modifiable synapses that are related to pEPSP1 depends on tetanization site. Relevancy of the results to the plasticity gate hypothesis is also discussed.

Molecular basis for induction of ocular dominance plasticity

The most dramatic example of experience-dependent cortical plasticity is the shift in ocular dominance that occurs in visual cortex as a consequence of monocular deprivation during early postnatal life. Many of the basic properties of this type of synaptic plasticity have been described in detail. The important challenge that remains is to understand the molecular basis for these properties. By combining theoretical analysis with experiments in vivo and in vitro, some of the elementary molecular mechanisms for visual cortical plasticity have now been uncovered.

A new form of synaptic plasticity is transiently expressed in the developing rat visual cortex: a modulatory role for visual experience and brain-derived neurotrophic factor

Synaptic plasticity has been implicated in the mechanisms contributing to the shaping of the cortical circuits responsible for the transmission of the visual input in the rat primary visual cortex. However, the degree of plasticity of the thalamocortical synapse may change during development, perhaps reflecting the degree of stabilization of the circuitry subserving it. We have chosen the ability of this synapse to be first depressed and then potentiated as a specific indicator of its plasticity. In this study we have investigated how this parameter changes during development and the factors controlling it. Extracellular field potentials in cortical layers 2/3 were evoked by stimulation of the white matter in rat primary visual cortex slices prepared at different postnatal ages. Low-frequency stimulation (900 pulses at 1 Hz) of the white matter was used to induce long-term depression of field potential amplitude, whereas long-term potentiation was evoked by high-frequency stimulation consisting of three trains at 100 Hz. We provide evidence that while it is possible to potentiate previously depressed synapses soon after eye opening (postnatal day 17) this synaptic characteristic decreases rapidly thereafter. The decrease in this form of cortical synaptic plasticity closely matches the stabilization of the cortical circuitry towards an adult pattern of connectivity and function. Depressed cortical synapses cannot be potentiated in normal rats at postnatal 23, but they can be potentiated in rats reared in the dark from postnatal days 17 to 29. Moreover, application of brain-derived neurotrophic factor, known to be expressed in an activity-dependent manner, was able to restore the ability of synapses to be potentiated after long-term depression, thus indicating its important modulatory role in brain development.