Long term potentiation varies with layer in rat visual cortex

Advances in the Electrophysiological Recordings of Long-Term Potentiation

Understanding neuronal firing patterns and long-term potentiation (LTP) induction in studying learning, memory, and neurological diseases is critical. However, recently, despite the rapid advancement in neuroscience, we are still constrained by the experimental design, detection tools for exploring the mechanisms and pathways involved in LTP induction, and detection ability of neuronal action potentiation signals. This review will reiterate LTP-related electrophysiological recordings in the mammalian brain for nearly 50 years and explain how excitatory and inhibitory neural LTP results have been detected and described by field- and single-cell potentials, respectively. Furthermore, we focus on describing the classic model of LTP of inhibition and discuss the inhibitory neuron activity when excitatory neurons are activated to induce LTP. Finally, we propose recording excitatory and inhibitory neurons under the same experimental conditions by combining various electrophysiological technologies and novel design suggestions for future research. We discussed different types of synaptic plasticity, and the potential of astrocytes to induce LTP also deserves to be explored in the future.

NMDA receptor-independent LTP in mammalian nervous system

Long-term potentiation (LTP) of synaptic transmission is a form of activity-dependent synaptic plasticity that exists at most synapses in the nervous system. In the central nervous system (CNS), LTP has been recorded at numerous synapses and is a prime candidate mechanism associating activity-dependent plasticity with learning and memory. LTP involves long-lasting increase in synaptic strength with various underlying mechanisms. In the CNS, the predominant type of LTP is believed to be dependent on activation of the ionotropic glutamate N-methyl-D-aspartate receptor (NMDAR), which is highly calcium-permeable. However, various forms of NMDAR-independent LTP have been identified in diverse areas of the nervous system. The NMDAR-independent LTP may require activation of glutamate metabotropic receptors (mGluR) or ionotropic receptors other than NMDAR such as nicotinic acetylcholine receptor (α7-nAChR), serotonin 5-HT3 receptor or calcium-permeable AMPA receptor (CP-AMPAR). In this review, NMDAR-independent LTP of various areas of the central and peripheral nervous systems are discussed.

Distinct Laminar Requirements for NMDA Receptors in Experience-Dependent Visual Cortical Plasticity

Primary visual cortex (V1) is the locus of numerous forms of experience-dependent plasticity. Restricting visual stimulation to one eye at a time has revealed that many such forms of plasticity are eye-specific, indicating that synaptic modification occurs prior to binocular integration of thalamocortical inputs. A common feature of these forms of plasticity is the requirement for NMDA receptor (NMDAR) activation in V1. We therefore hypothesized that NMDARs in cortical layer 4 (L4), which receives the densest thalamocortical input, would be necessary for all forms of NMDAR-dependent and input-specific V1 plasticity. We tested this hypothesis in awake mice using a genetic approach to selectively delete NMDARs from L4 principal cells. We found, unexpectedly, that both stimulus-selective response potentiation and potentiation of open-eye responses following monocular deprivation (MD) persist in the absence of L4 NMDARs. In contrast, MD-driven depression of deprived-eye responses was impaired in mice lacking L4 NMDARs, as was L4 long-term depression in V1 slices. Our findings reveal a crucial requirement for L4 NMDARs in visual cortical synaptic depression, and a surprisingly negligible role for them in cortical response potentiation. These results demonstrate that NMDARs within distinct cellular subpopulations support different forms of experience-dependent plasticity.

Layer-specific cholinergic modulation of synaptic transmission in layer 2/3 pyramidal neurons of rat visual cortex

It is known that top-down associative inputs terminate on distal apical dendrites in layer 1 while bottom-up sensory inputs terminate on perisomatic dendrites of layer 2/3 pyramidal neurons (L2/3 PyNs) in primary sensory cortex. Since studies on synaptic transmission in layer 1 are sparse, we investigated the basic properties and cholinergic modulation of synaptic transmission in layer 1 and compared them to those in perisomatic dendrites of L2/3 PyNs of rat primary visual cortex. Using extracellular stimulations of layer 1 and layer 4, we evoked excitatory postsynaptic current/potential in synapses in distal apical dendrites (L1-EPSC/L1-EPSP) and those in perisomatic dendrites (L4-EPSC/L4-EPSP), respectively. Kinetics of L1-EPSC was slower than that of L4-EPSC. L1-EPSC showed presynaptic depression while L4-EPSC was facilitating. In contrast, inhibitory postsynaptic currents showed similar paired-pulse ratio between layer 1 and layer 4 stimulations with depression only at 100 Hz. Cholinergic stimulation induced presynaptic depression by activating muscarinic receptors in excitatory and inhibitory synapses to similar extents in both inputs. However, nicotinic stimulation enhanced excitatory synaptic transmission by ~20% in L4-EPSC. Rectification index of AMPA receptors and AMPA/NMDA ratio were similar between synapses in distal apical and perisomatic dendrites. These results provide basic properties and cholinergic modulation of synaptic transmission between distal apical and perisomatic dendrites in L2/3 PyNs of the visual cortex, which might be important for controlling information processing balance depending on attentional state.

Brief Novel Visual Experience Fundamentally Changes Synaptic Plasticity in the Mouse Visual Cortex

LTP has been known to be a mechanism by which experience modifies synaptic responses in the neocortex. Visual deprivation in the form of dark exposure or dark rearing from birth enhances NMDAR-dependent LTP in layer 2/3 of visual cortex, a process often termed metaplasticity, which may involve changes in NMDAR subunit composition and function. However, the effects of reexposure to light after dark rearing from birth on LTP induction have not been explored. Here, we showed that the light exposure after dark rearing revealed a novel NMDAR independent form of LTP in the layer 2/3 pyramidal cells in visual cortex of mice of both sexes, which is dependent on mGluR5 activation and is associated with intracellular Ca²⁺ rise, CaMKII activity, PKC activity, and intact protein synthesis. Moreover, the capacity to induce mGluR-dependent LTP is transient: it only occurs when mice of both sexes reared in the dark from birth are exposed to light for 10-12 h, and it does not occur in vision-experienced, male mice, even after prolonged exposure to dark. Thus, the mGluR5-LTP unmasked by short visual experience can only be observed after dark rearing but not after dark exposure. These results suggested that, as in hippocampus, in layer 2/3 of visual cortex, there is coexistence of two distinct activity-dependent systems of synaptic plasticity, NMDAR-LTP, and mGluR5-LTP. The mGluR5-LTP unmasked by short visual experience may play a critical role in the faster establishment of normal receptive field properties.SIGNIFICANCE STATEMENT LTP has been known to be a mechanism by which experience modifies synaptic responses in the neocortex. Visual deprivation in the form of dark exposure or dark rearing from birth enhances NMDAR-dependent LTP in layer 2/3 of visual cortex, a process often termed metaplasticity. NMDAR-dependent form of LTP in visual cortex has been well characterized. Here, we report that an NMDAR-independent form of LTP can be promoted by novel visual experience on dark-reared mice, characterized as dependent on intracellular Ca²⁺ rise, PKC activity, and intact protein synthesis and also requires the activation of mGluR5. These findings suggest that, in layer 2/3 of visual cortex, as in hippocampus, there is coexistence of two distinct activity-dependent systems of synaptic plasticity.

Distinct Eligibility Traces for LTP and LTD in Cortical Synapses

In reward-based learning, synaptic modifications depend on a brief stimulus and a temporally delayed reward, which poses the question of how synaptic activity patterns associate with a delayed reward. A theoretical solution to this so-called distal reward problem has been the notion of activity-generated "synaptic eligibility traces," silent and transient synaptic tags that can be converted into long-term changes in synaptic strength by reward-linked neuromodulators. Here we report the first experimental demonstration of eligibility traces in cortical synapses. We demonstrate the Hebbian induction of distinct traces for LTP and LTD and their subsequent timing-dependent transformation into lasting changes by specific monoaminergic receptors anchored to postsynaptic proteins. Notably, the temporal properties of these transient traces allow stable learning in a recurrent neural network that accurately predicts the timing of the reward, further validating the induction and transformation of eligibility traces for LTP and LTD as a plausible synaptic substrate for reward-based learning.

Layer-specific endocannabinoid-mediated long-term depression of GABAergic neurotransmission onto principal neurons in mouse visual cortex

Visually induced endocannabinoid-mediated long-term depression of GABAergic neurotransmission (iLTD) mediates the maturation of GABAergic release in layer 2/3 of visual cortex. Here we examined whether the maturation of GABAergic transmission in other layers of visual cortex also requires endocannabinoids. The developmental plasticity of GABAergic neurotransmission onto the principal neurons in different layers of mouse visual cortex was examined in cortical slices by whole-cell recordings of inhibitory postsynaptic currents evoked by presynaptic inhibitory inputs. Theta burst stimulation of GABAergic inputs induced an endocannabinoid-mediated long-term depression of GABAergic neurotransmission onto pyramidal cells in layer 2/3 from postnatal day (P)10 to 30 and in layer 5 from P10 to 40, whereas that of GABAergic inputs did not induce iLTD onto star pyramidal neurons in layer 4 at any time postnatally, indicating that this plasticity is laminar-specific. The developmental loss of iLTD paralleled the maturation of GABAergic inhibition in both layer 2/3 and layer 5. Visual deprivation delayed the developmental loss of iLTD in layers 3 and 5 during a critical period, while 2 days of light exposure eliminated iLTD in both layers. Furthermore, the GABAergic synapses in layers 2/3 and 5 did not normally mature in the type 1 cannabinoid receptor knock-out mice, whereas those in layer 4 did not require endocannabinoid receptor for maturation. These results suggest that visually induced endocannabinoid-dependent iLTD mediates the maturation of GABAergic release in extragranular layer rather than in granular layer of mouse visual cortex.

Impaired critical period plasticity in primary auditory cortex of fragile X model mice

Fragile X syndrome, the most common form of heritable mental retardation, is a developmental disorder with known effects within sensory systems. Altered developmental plasticity has been reported in the visual and somatosensory systems in Fmr1 knock-out (KO) mice. Behavioral studies have revealed maladaptive auditory responses in fragile X syndrome patients and Fmr1 KO mice, suggesting that adaptive plasticity may also be impaired in the auditory system. Here we show that, whereas tonotopic frequency representation develops normally in Fmr1 KO mice, developmental plasticity in primary auditory cortex is grossly impaired. This deficit can be rescued by pharmacological blockade of mGluR5 receptors. These results support the mGluR hypothesis of fragile X mental retardation and suggest that deficient developmental plasticity may contribute to maladaptive auditory processing in fragile X syndrome.

Reciprocal Homosynaptic and heterosynaptic long-term plasticity of corticogeniculate projection neurons in layer VI of the mouse visual cortex

Most neurons in layer VI of the visual cortex project to the dorsal lateral geniculate nucleus (dLGN). These corticogeniculate projection neurons (CG cells) receive top-down synaptic inputs from upper layers (ULs) and bottom-up inputs from the underlying white matter (WM). Use-dependent plasticity of these synapses in layer VI of the cortex has received less attention than in other layers. In the present study, we used a retrograde tracer injected into dLGN to identify CG cells, and, by analyzing EPSPs evoked by electrical stimulation of the UL or WM site, examined whether these synapses show long-term synaptic plasticity. Theta-burst stimulation induced long-term potentiation (LTP) of activated synapses (hom-LTP) and long-term depression (LTD) of nonactivated synapses (het-LTD) in either pathway. The paired-pulse stimulation protocol and the analysis of coefficient variation of EPSPs suggested postsynaptic induction of these changes except UL-induced het-LTD, which may be presynaptic in origin. Intracellular injection of a Ca(2+)-chelator suggested an involvement of postsynaptic Ca(2+) rise in all types of long-term plasticity. Pharmacological analysis indicated that NMDA receptors and type-5 metabotropic glutamate receptors are involved in WM-induced and UL-induced plasticity, respectively. Analysis with inhibitors and/or in transgenic mice suggested an involvement of cannabinoid type 1 receptors and calcineurin in UL-induced and WM-induced het-LTD, respectively. These results suggest that hom-LTP and het-LTD may play a role in switching the top-down or bottom-up regulation of CG cell function and/or in maintaining stability of synaptic transmission efficacy through different molecular mechanisms.

Thalamocortical long-term potentiation becomes gated after the early critical period in the auditory cortex

Cortical maps in sensory cortices are plastic, changing in response to sensory experience. The cellular site of such plasticity is currently debated. Thalamocortical (TC) projections deliver sensory information to sensory cortices. TC synapses are currently dismissed as a locus of cortical map plasticity because TC synaptic plasticity is thought to be limited to neonates, whereas cortical map plasticity can be induced in both neonates and adults. However, in the auditory cortex (ACx) of adults, cortical map plasticity can be induced if animals attend to a sound or receive sounds paired with activation of cholinergic inputs from the nucleus basalis. We now show that, in the ACx, long-term potentiation (LTP), a major form of synaptic plasticity, is expressed at TC synapses in both young and mature mice but becomes gated with age. Using single-cell electrophysiology, two-photon glutamate uncaging, and optogenetics in TC slices containing the auditory thalamus and ACx, we show that TC LTP is expressed postsynaptically and depends on group I metabotropic glutamate receptors. TC LTP in mature ACx can be unmasked by cortical disinhibition combined with activation of cholinergic inputs from the nucleus basalis. Cholinergic inputs passing through the thalamic radiation activate M1 muscarinic receptors on TC projections and sustain glutamate release at TC synapses via negative regulation of presynaptic adenosine signaling through A1 adenosine receptors. These data indicate that TC LTP in the ACx persists throughout life and therefore can potentially contribute to experience-dependent cortical map plasticity in the ACx in both young and adult animals.

Cell type-specific, presynaptic LTP of inhibitory synapses on fast-spiking GABAergic neurons in the mouse visual cortex

Properties and plasticity of inhibitory synapses on fast-spiking (FS) GABAergic (FS-GABA) interneurons in layer II/III of the mouse visual cortex were examined in cortical slices by whole-cell recordings of IPSCs or IPSPs evoked by activation of presynaptic FS or non-FS GABAergic interneurons. Unitary IPSCs (uIPSCs) evoked by action potentials of FS-GABA neurons have shorter onset latency, faster rising slope, higher peak amplitude, and faster decay time than those evoked by action potentials of non-FS-GABA neurons. Tetanic activation of presynaptic FS-GABA neurons induced long-term potentiation (LTP) of uIPSCs, whereas that of presynaptic non-FS-GABA neurons did not induce LTP, indicating that long-term plasticity of inhibitory synapses on FS-GABA neurons is pathway specific. For further analysis of inhibitory synaptic plasticity, IPSPs evoked by electrical stimulation of an adjacent site in the cortex were recorded from FS-GABA neurons. Theta burst stimulation induced LTP of IPSPs in 12 of 14 FS-GABA neurons. The paired-pulse stimulation protocol and coefficient of variation analysis indicated that this form of LTP may be presynaptic in origin. Filling postsynaptic cells with a Ca(2+) chelator did not block the induction of LTP, suggesting no involvement of postsynaptic Ca(2+) rise. Also, this form of LTP was dependent neither on metabotropic glutamate receptors nor voltage-gated Ca(2+) channels of the L and T types. Further pharmacological analysis indicated that voltage-gated Ca(2+) channels other than the P/Q type, such as N and R types, were not involved in LTP, suggesting that P/Q-type channels are a candidate for factors inducing LTP of inhibitory synapses between FS-GABA neurons.

Experience-dependent switch in sign and mechanisms for plasticity in layer 4 of primary visual cortex

Neural circuits are extensively refined by sensory experience during postnatal development. How the maturation of recurrent cortical synapses may contribute to events regulating the postnatal refinement of neocortical microcircuits remains controversial. Here we show that, in the main input layer of rat primary visual cortex, layer 4 (L4), recurrent excitatory synapses are endowed with multiple, developmentally regulated mechanisms for induction and expression of excitatory synaptic plasticity. Maturation of L4 synapses and visual experience lead to a sharp switch in sign and mechanisms for plasticity at recurrent excitatory synapses in L4 at the onset of the critical period for visual cortical plasticity. The state of maturation of excitatory pyramidal neurons allows neurons to engage different mechanisms for plasticity in response to the same induction paradigm. Experience is determinant for the maturation of L4 synapses, as well as for the transition between forms of plasticity and the mechanisms they may engage. These results indicate a tight correlation between the effects of sensory drive and maturation on cortical neurons and provide a new set of cellular mechanisms engaged in the postnatal refinement of cortical circuits.

New insights into the regulation of synaptic plasticity from an unexpected place: hippocampal area CA2

The search for molecules that restrict synaptic plasticity in the brain has focused primarily on sensory systems during early postnatal development, as critical periods for inducing plasticity in sensory regions are easily defined. The recent discovery that Schaffer collateral inputs to hippocampal area CA2 do not readily support canonical activity-dependent long-term potentiation (LTP) serves as a reminder that the capacity for synaptic modification is also regulated anatomically across different brain regions. Hippocampal CA2 shares features with other similarly "LTP-resistant" brain areas in that many of the genes linked to synaptic function and the associated proteins known to restrict synaptic plasticity are expressed there. Add to this a rich complement of receptors and signaling molecules permissive for induction of atypical forms of synaptic potentiation, and area CA2 becomes an ideal model system for studying specific modulators of brain plasticity. Additionally, recent evidence suggests that hippocampal CA2 is instrumental for certain forms of learning, memory, and social behavior, but the links between CA2-enriched molecules and putative CA2-dependent behaviors are only just beginning to be made. In this review, we offer a detailed look at what is currently known about the synaptic plasticity in this important, yet largely overlooked component of the hippocampus and consider how the study of CA2 may provide clues to understanding the molecular signals critical to the modulation of synaptic function in different brain regions and across different stages of development.

Multiple patterns of spatiotemporal changes in layer-specific gene expression in the developing visual cortex of higher mammals

The mammalian cerebral cortex, which is stratified into six layers, has functional domains that vertically span the six layers, thereby requiring tight interlaminar connectivity within a domain. The synaptic connections in individual layers are first broadly formed under predetermined programs and later reinforced between neurons which reside in the same functional domain via experience-dependent reorganization during the critical period. However, the molecular mechanisms that control these two processes within each layer are still unclear. Therefore, we performed a differential screen for candidates and found seven genes with layer-specific expression during postnatal development of cat visual cortex. APLP1, a transmembrane protein mediating synaptogenesis, started dual-layer expression in layers 2/3 and 5 before the critical period, suggesting that it might execute coarse synapse formation of these layers. STMN2 (SCG10), which promotes microtubule turnover, was unique, as it dramatically shifted its dual-layer distribution from layers 2/3 and 5 to the deeper layers 4 and 6 at the onset of the critical period; it lost this new expression pattern in the adult. Surprisingly, brief dark rearing disturbed the shift in its dual-layer distribution around the onset of the critical period. Thus, by accelerating structural remodeling, STMN2 (SCG10) might launch experience-dependent reorganization of particular layers.

The influences of dark rearing on the transmission characteristics of layer II/III pyramidal cells during the critical period

The characteristics of synaptic plasticity on layer II/III pyramidal cells in different ages of rats have been studied extensively, and dark rearing is one of the important impact factors. To systematically analyze the influence of dark rearing on synaptic plasticity during the critical period of visual development, we studied the characteristics of short-term and long-term synaptic plasticities of layer II/III pyramidal cells of rats in three rearing conditions during P14 to P37. The paired-pulse ratio (PPR) of inhibitory postsynaptic currents (IPSCs) of layer II/III pyramidal cells was effected by both ages and rearing conditions, but the PPR of excitatory postsynaptic currents (EPSCs) did not change obviously. Moreover, long-term synaptic plasticity of rats in the dark rearing condition did not significantly change with age, while it was elevated during P16 and P21 for rats in the normal rearing condition. These results suggest that visual experience can affect the characteristics of short-term and long-term synaptic plasticities. The IPSC/EPSC ratio increased gradually with aging for NR rats, but the ratio slightly decreased for DR rats, which indicates the relative increase of inhibitory components during the critical period of visual development. The characteristics during P35 and P37 of the 30-day dark-reared (30D×N) group had similar trends with the normal-reared rats during P16 and P21, which emphasizes that dark rearing can postpone the timing of the critical period.

Brain-derived neurotrophic factor (BDNF) reverses the effects of rapid eye movement sleep deprivation (REMSD) on developmentally regulated, long-term potentiation (LTP) in visual cortex slices

Work in this laboratory demonstrated a role for rapid eye movement sleep (REMS) in critical period (CP), postnatal days (P) 17-30, synaptic plasticity in visual cortex. Studies in adolescent rats showed that REMS deprivation (REMSD) reinitiates a developmentally regulated form of synaptic plasticity that otherwise is observed only in CP animals. Subsequent work added that REMSD affects inhibitory mechanisms that are thought to be involved in terminating the CP. Neurotrophins are implicated in the synaptic plasticity that underlies CP maturation and also final closure of the CP in visual cortex. Expression of brain-derived neurotrophic factor (BDNF) is dependent upon neuronal activity, and REMSD may block BDNF expression. We propose that REMS contributes to the maturation of visual cortex through regulation of BDNF expression and consequent, downstream increase in cortical inhibitory tone. In this study, osmotic minipumps delivered BDNF into visual cortex on one side of brain. The opposite hemisphere was not implanted and served as an internal control. We tested the hypothesis that BDNF is blocked by REMSD in late-adolescent rats and investigated whether replacing BDNF prevents induction of LTPWM-III by theta burst stimulation (TBS). We also assessed relative inhibitory tone in visual cortex with paired-pulse stimulation (PPS) in animals that were similarly REMSD- and BDNF-infused. After REMSD, both hemispheres were prepared in parallel for in vitro synaptic plasticity studies (LTPWM-III or PPS). In visual cortex of REMSD rats on the side receiving BDNF infusions (8 of 8 animals), TBS consistently failed to induce LTPWM-III. In contrast, LTPWM-III was obtained (5 of 5 animals) in the matched, non-infused hemisphere, as expected in rats of this age. REMSD animals that were unilaterally infused with saline produced LTPWM-III in both hemispheres. PPS studies in another group of REMSD animals that were unilaterally BDNF-infused displayed age-appropriate inhibition of the second response on the BDNF-infused side (5/5), whereas on the non-infused side facilitation was observed (3/3). Intracortical infusion of BDNF in REMSD adolescent rats appears to restore neurochemical processes necessary for termination of the CP for developmentally regulated synaptic plasticity in visual cortex. The results suggest that REMSD blocks BDNF expression and also maturation of inhibitory processes in adolescent visual cortex. These data support REMS' function in brain development.

Regular spiking and intrinsic bursting pyramidal cells show orthogonal forms of experience-dependent plasticity in layer V of barrel cortex

Most functional plasticity studies in the cortex have focused on layers (L) II/III and IV, whereas relatively little is known of LV. Structural measurements of dendritic spines in vivo suggest some specialization among LV cell subtypes. We therefore studied experience-dependent plasticity in the barrel cortex using intracellular recordings to distinguish regular spiking (RS) and intrinsic bursting (IB) subtypes. Postsynaptic potentials and suprathreshold responses in vivo revealed a remarkable dichotomy in RS and IB cell plasticity; spared whisker potentiation occurred in IB but not RS cells while deprived whisker depression occurred in RS but not IB cells. Similar RS/IB differences were found in the LII/III to V connections in brain slices. Modeling studies showed that subthreshold changes predicted the suprathreshold changes. These studies demonstrate the major functional partition of plasticity within a single cortical layer and reveal the LII/III to LV connection as a major excitatory locus of cortical plasticity.

Recovery from chronic monocular deprivation following reactivation of thalamocortical plasticity by dark exposure

Chronic monocular deprivation induces severe amblyopia that is resistant to spontaneous reversal. However, dark exposure initiated in adulthood reactivates synaptic plasticity in the visual cortex and promotes recovery from chronic monocular deprivation in Long Evans rats. Here we show that chronic monocular deprivation induces a significant decrease in the density of dendritic spines on principal neurons throughout the deprived visual cortex. Nevertheless, dark exposure followed by reverse deprivation promotes the recovery of dendritic spine density of neurons in all laminae. Importantly, the ocular dominance of neurons in thalamo-recipient laminae of the cortex, and the amplitude of the thalamocortical visually evoked potential recover following dark exposure and reverse deprivation. Thus, dark exposure reactivates widespread synaptic plasticity in the adult visual cortex, including thalamocortical synapses, during the recovery from chronic monocular deprivation.

Visual experience induces long-term potentiation in the primary visual cortex

Stimulus-specific response potentiation (SRP) is a robust form of experience-dependent plasticity that occurs in primary visual cortex. In awake mice, visual evoked potentials (VEPs) recorded in layer 4 of binocular visual cortex undergo increases in amplitude with repeated presentation of a sinusoidal grating stimulus over days. This effect is highly specific to the experienced stimulus. Here, we test whether the mechanisms of thalamocortical long-term potentiation (LTP), induced with a theta burst electrical stimulation (TBS) of the dorsal lateral geniculate nucleus, are sufficient to account for SRP. First, we demonstrate that LTP similarly enhances the amplitude of VEPs, but in a way that generalizes across multiple stimuli, spatial frequencies, and contrasts. Second, we show that LTP occludes the subsequent expression of SRP. Third, we reveal that previous SRP occludes TBS-induced LTP of the VEP evoked by the experienced stimulus, but not by unfamiliar stimuli. Finally, we show that SRP is rapidly and selectively reversed by local cortical infusion of a peptide that inhibits PKMζ, a constitutively active kinase known to maintain NMDA receptor-dependent LTP and memory. Thus, SRP is expressed by the same core mechanisms as LTP. SRP therefore provides a simple assay to assess the integrity of LTP in the intact nervous system. Moreover, the results suggest that LTP of visual cortex, like SRP, can potentially be exploited to improve vision.

Metabotropic glutamate mGlu1 receptor stimulation and blockade: therapeutic opportunities in psychiatric illness

Metabotropic glutamate mGlu(1) receptors play a modulatory role in the nervous system. They enhance cell excitability, modulate synaptic neurotransmission and are involved in synaptic plasticity. During the last 10 years, several selective metabotropic glutamate mGlu(1) receptor competitive antagonists and potentiators have been discovered. These pharmacological tools, together with early and later work in metabotropic glutamate mGlu(1) receptor mutant mice have allowed studying the role of the receptor in various aspects of psychiatric illnesses such as anxiety, depression and schizophrenia. We here review the data on selective metabotropic glutamate mGlu(1) receptor antagonists in support of their potential as anxiolytic and antidepressant treatments. We propose a rationale for the development of metabotropic glutamate mGlu(1) receptor positive allosteric modulators for the treatment of schizophrenia. Potential side effects of blockade and activation of metabotropic glutamate mGlu(1) receptors are addressed, with special focus on the differential effects of metabotropic glutamate mGlu(1) receptor antagonists in cognition models with positive reinforcement versus those that use aversive learning procedures. Further development of negative allosteric modulators and more drug-like positive allosteric modulators will be required in order to decipher the therapeutic efficacy and safety margin of these compounds in the clinic.

Group 5 metabotropic glutamate receptors: role in modulating cortical activity and relevance to cognition

Group 5 metabotropic glutamate (mGlu(5)) receptors are abundant in forebrain and limbic regions and provide a novel pharmacological target for modulation of cognition. Here, we review recent advances in understanding the electrophysiology of these receptors which reveal a role for mGlu(5) receptors in the regulation of tonic and bursting modes of neuronal firing, maintenance of distinct forms of synaptic plasticity, and reversal of detrimental effects of NMDA receptor antagonism on cortical neuronal activity. Furthermore, recordings using recently developed positive allosteric modulators of the mGlu(5) receptor suggest that these agents have an electrophysiological profile comparable to the antipsychotic agent clozapine. These findings, in conjunction with behavioral evidence from preclinical studies of cognition, suggest a potential precognitive profile for the mGlu(5) receptor potentiators.

Metabotropic glutamate receptor-dependent long-term potentiation

The induction of the most common form of LTP is well known to involve activation of N-methyl-D-aspartate receptors. However, considerable evidence has also shown that certain forms of LTP induction at excitatory synapses onto both principle cells and interneurons are dependent on activation of metabotropic glutamate receptors (mGluRs). mGluR-dependent LTP occurs in widespread areas of the brain including the neocortex, hippocampus, striatum and nucleus accumbens. mGluR-dependent forms of LTP have been found to be diverse, involving activation of mGluR1 or mGluR5 and can be of AMPAR-mediated transmission or of NMDAR-mediated transmission. Furthermore, the mGluR-dependent LTP may involve activation of other receptors, in particular, activation of NMDAR, dopamine and adenosine receptors. mGluR-dependent LTP can be expressed presynaptically or postsynaptically, and can involve a range of intracellular mediators including protein kinase C (PKC) and protein kinase A (PKA), tyrosine kinase Src and nitric oxide (NO).

Bidirectional synaptic mechanisms of ocular dominance plasticity in visual cortex

As in other mammals with binocular vision, monocular lid suture in mice induces bidirectional plasticity: rapid weakening of responses evoked through the deprived eye followed by delayed strengthening of responses through the open eye. It has been proposed that these bidirectional changes occur through three distinct processes: first, deprived-eye responses rapidly weaken through homosynaptic long-term depression (LTD); second, as the period of deprivation progresses, the modification threshold determining the boundary between synaptic depression and synaptic potentiation becomes lower, favouring potentiation; and third, facilitated by the decreased modification threshold, open-eye responses are strengthened via homosynaptic long-term potentiation (LTP). Of these processes, deprived-eye depression has received the greatest attention, and although several alternative hypotheses are also supported by current research, evidence suggests that α-amino-3- hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor endocytosis through LTD is a key mechanism. The change in modification threshold appears to occur partly through changes in N-methyl-d-aspartate (NMDA) receptor subunit composition, with decreases in the ratio of NR2A to NR2B facilitating potentiation. Although limited research has directly addressed the question of open-eye potentiation, several studies suggest that LTP could account for observed changes in vivo. This review will discuss evidence supporting this three-stage model, along with outstanding issues in the field.

Molecular mechanisms of experience-dependent plasticity in visual cortex

A remarkable amount of our current knowledge of mechanisms underlying experience-dependent plasticity during cortical development comes from study of the mammalian visual cortex. Recent advances in high-resolution cellular imaging, combined with genetic manipulations in mice, novel fluorescent recombinant probes, and large-scale screens of gene expression, have revealed multiple molecular mechanisms that underlie structural and functional plasticity in visual cortex. We situate these mechanisms in the context of a new conceptual framework of feed-forward and feedback regulation for understanding how neurons of the visual cortex reorganize their connections in response to changes in sensory inputs. Such conceptual advances have important implications for understanding not only normal development but also pathological conditions that afflict the central nervous system.

Transcranial magnetic stimulation, synaptic plasticity and network oscillations

Transcranial magnetic stimulation (TMS) has quickly progressed from a technical curiosity to a bona-fide tool for neurological research. The impetus has been due to the promising results obtained when using TMS to uncover neural processes in normal human subjects, as well as in the treatment of intractable neurological conditions, such as stroke, chronic depression and epilepsy. The basic principle of TMS is that most neuronal axons that fall within the volume of magnetic stimulation become electrically excited, trigger action potentials and release neurotransmitter into the postsynaptic neurons. What happens afterwards remains elusive, especially in the case of repeated stimulation. Here we discuss the likelihood that certain TMS protocols produce long-term changes in cortical synapses akin to long-term potentiation and long-term depression of synaptic transmission. Beyond the synaptic effects, TMS might have consequences on other neuronal processes, such as genetic and protein regulation, and circuit-level patterns, such as network oscillations. Furthermore, TMS might have non-neuronal effects, such as changes in blood flow, which are still poorly understood.

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.

Metabotropic glutamate receptor type 5-dependent long-term potentiation of excitatory synapses on fast-spiking GABAergic neurons in mouse visual cortex

Long-term potentiation (LTP) of excitatory synapses on GABAergic neurons in layer II/III of visual cortical slices was examined in GAD67-GFP knock-in mice by whole-cell recordings of EPSPs evoked by layer IV stimulation. Theta burst stimulation (TBS) paired with postsynaptic depolarization induced LTP in 14 of 19 fast-spiking GABAergic (FS-GABA) neurons, whereas only in 6 of 17 non-FS GABAergic neurons. The mean magnitude of LTP in the former cell group was larger than that in the latter. The paired-pulse stimulation protocol and coefficient of variation analysis indicated that LTP of excitatory synapses on FS-GABA neurons may be postsynaptic in origin. Filling postsynaptic cells with a Ca2+-chelator blocked the induction of LTP, suggesting an involvement of postsynaptic Ca2+ rise. The developmental analysis of LTP indicated that almost the same magnitude of LTP was induced after postnatal day 17 to the young adulthood, suggesting no age dependence after eye opening. This form of LTP was dependent neither on NMDA receptors nor voltage-gated Ca2+ channels (L and T types). An antagonist for type 5 metabotropic glutamate receptors (mGluR5) blocked this form of LTP, whereas an antagonist for mGluR1 was not effective. An agonist for mGluR1/5 induced potentiation of EPSPs of FS-GABA neurons in concentration- and use-dependent manners. This potentiation and TBS-induced LTP occluded each other. Further pharmacological analyses suggested that this form of LTP at FS-GABA neurons is induced through an activation of mGluR5, which triggers Ca2+ release from internal stores via activations of phospholipase C and inositol triphosphate.

Homer 1a suppresses neocortex long-term depression in a cortical layer-specific manner

Homer1a/Vesl-1S is an activity-dependently induced member of the scaffold protein family Homer/Vesl, which is known to link group I metabotropic glutamate receptors (mGluRs) to endoplasmic calcium release channels and to regulate them. Here we studied roles of Homer 1a in inducing long-term depression (LTD) in rat visual cortex slices. Homer 1a protein was injected by diffusion from whole cell patch pipettes. In layer VI pyramidal cells, LTD was reduced in magnitude with Homer 1a. LTD in layer VI was suppressed by applying antagonists of mGluR5, a subtype of group I mGluRs expressed with higher density than mGluR1 in neocortex pyramidal cells, or inositol-1,4,5-triphosphate receptors (IP3Rs) but not that against N-methyl-d-aspartate receptors (NMDARs). In layer II/III or layer V, Homer 1a injection was unable to affect LTD, which is mostly dependent on NMDARs and not on group I mGluRs in these layers. To examine the effects of endogenous Homer 1a, electroconvulsive shock (ECS) was applied. Homer 1a thereby induced, as did Homer 1a injection, reduced LTD magnitude in layer VI pyramidal cells and failed to do so in layer II/III or layer V pyramidal cells. These results indicate that both exo- and endogenous Homer 1a suppressed LTD in a cortical layer-specific manner, and its layer-specificity may be explained by the high affinity of Homer 1a to group I mGluRs.

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.

Absence of metabotropic glutamate receptor-mediated plasticity in the neocortex of fragile X mice

Fragile X syndrome is a common heritable form of mental retardation in humans. Recent neuroanatomical studies indicate an apparent immature appearance of neurons in fragile X syndrome patients and fragile X mental retardation protein (FMRP)-knockout mice, an animal model of this condition. In this work, we investigated possible alterations in synaptic plasticity in the neocortex of FMRP-knockout mice. Extracellular field potentials were recorded from the deep-layer visual neocortex. Long-term potentiation (LTP) was severely attenuated in brain slices from knockout mice relative to that observed in slices from wild-type mice. Considering that neocortical LTP can involve both NMDA receptor-dependent and -independent mechanisms, we attempted to distinguish the nature of LTP attenuated in the knockout condition. In slices from wild-type mice, LTP was partially attenuated by the NMDA receptor antagonist 3-[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphate (CPP); however, the general metabotropic glutamate receptor (mGluR) antagonist alpha-methyl-4-carboxyphenylglycine (MCPG) strongly attenuated LTP, resulting in a response indistinguishable from that observed in slices from knockout mice. The selective mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) attenuated LTP to a similar degree as did MCPG in wild-type slices, but MPEP did not alter the reduced potentiation in knockout slices. Our results suggest that LTP in layer V visual neocortex depends primarily on mGluR5 activation. Our data also indicate that mGluR5-mediated synaptic plasticity is absent in the neocortex of FMRP-knockout mice. Such an alteration may contribute to the cognitive and learning deficits exhibited in these mice as well as in fragile X syndrome.

Dopamine D1-like receptor modulates layer- and frequency-specific short-term synaptic plasticity in rat prefrontal cortical neurons

The mesocortical dopamine (DA) input to the prefrontal cortex (PFC) is crucial for processing short-term working memory (STWM) to guide forthcoming behavior. Short-term plasticity-like post-tetanic potentiation (PTP, < 3 min) and short-term potentiation (STP, < 10 min) may underlie STWM. Using whole-cell voltage-clamp recordings, mixed glutamatergic excitatory postsynaptic currents (EPSCs) evoked by layer III or layer V stimulation (0.5 or 0.067 Hz) were recorded from layer V pyramidal neurons. With 0.5 Hz basal stimulation of layer III, brief tetani (2 x 50 Hz) induced a homosynaptic PTP (decayed: approximately 1 min). The D1-like antagonist SCH23390 (1 microm) increased the PTP amplitude and decay time without inducing changes to the tetanic response. The tetani may evoke endogenous DA release, which activates a presynaptic D1-like receptor to inhibit glutamate release to modulate PTP. With a slower (0.067 Hz) basal stimulation, the same tetani induced STP (lasting approximately 4 min, but only at 2x intensity only) that was insignificantly suppressed by SCH23390. With stimulation of layer-V-->V inputs at 0.5 Hz, layer V tetani yielded inconsisitent responses. However, at 0.067 Hz, tetani at double the intensity resulted in an STP (lasting approximately 6 min), but a long-term depression after SCH23390 application. Endogenous DA released by tetanic stimulation can interact with a D1-like receptor to induce STP in layer V-->V synapses that receive slower (0.067 Hz) frequency inputs, but suppresses PTP at layer III-->V synapses that receive higher (0.5 Hz) frequency inputs. This D1-like modulation of layer- and frequency-specific synaptic responses in the PFC may contribute to STWM processing.

LTP and LTD vary with layer in rodent visual cortex

Mechanisms of plasticity in the visual cortex have been studied with long-term potentiation (LTP), long-term depression (LTD) and ocular dominance plasticity (ODP). It is now possible to compare results from these three forms of plasticity using knockout mice, and also by pharmacological manipulations. A review of the literature shows that if both LTP and LTD are completely abolished, then ODP will also be abolished. In other situations, there is little correlation. We hypothesize that this lack of correlation occurs because the mechanisms for LTP and LTD vary with layer in the visual cortex, and results show that they do.

Layer variations of long-term depression in rat visual cortex

In vitro long-term depression (LTD) is thought to be a model for the loss of cortical responsiveness to an eye deprived of vision during the critical period. Using whole cell recording, the present study investigates the mechanisms of LTD in vitro across layers in developing rat visual cortex. LTD was induced in layers II/III, V, and VI but not layer IV with 10-min 1-Hz stimulation paired with postsynaptic depolarization. LTD in layers II/III and V could be blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist D-aminophosphonovaleric acid (D-AP5) but not by 100 microM (2S)-amino-2-[(1S,2S)-2-carboxycycloprop-1-yl]-3-(xanth-9-yl) propanoic acid (LY341495), a metabotropic glutamate receptor inhibitor. In contrast, LTD in layer VI was blocked by 100 microM LY341495 and (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA) but not D-AP5 and partially blocked by application of guanosine 5'-O-(2-thiodiphosphate) thilothium salt (GDP-beta-S) in patch pipette, suggesting an involvement of postsynaptic group I metabotropic glutamate receptors (mGluRs). These results indicate that LTD in developing rat visual cortex varies with layer: LTD was absent in layer IV, suggesting a unique plasticity mechanism at geniculocortical synapses; LTD in layers II/III and V depends on NMDA receptors but not mGluRs, and LTD in layer VI requires mGluRs but not NMDA receptors.