Metabotropic glutamate receptors are involved in long-term potentiation in isolated slices of rat medial frontal cortex

Serotonergic neuromodulation of synaptic plasticity

Synaptic plasticity constitutes a fundamental process in the reorganization of neural networks that underlie memory, cognition, emotional responses, and behavioral planning. At the core of this phenomenon lie Hebbian mechanisms, wherein frequent synaptic stimulation induces long-term potentiation (LTP), while less activation leads to long-term depression (LTD). The synaptic reorganization of neuronal networks is regulated by serotonin (5-HT), a neuromodulator capable of modify synaptic plasticity to appropriately respond to mental and behavioral states, such as alertness, attention, concentration, motivation, and mood. Lately, understanding the serotonergic Neuromodulation of synaptic plasticity has become imperative for unraveling its impact on cognitive, emotional, and behavioral functions. Through a comparative analysis across three main forebrain structures-the hippocampus, amygdala, and prefrontal cortex, this review discusses the actions of 5-HT on synaptic plasticity, offering insights into its role as a neuromodulator involved in emotional and cognitive functions. By distinguishing between plastic and metaplastic effects, we provide a comprehensive overview about the mechanisms of 5-HT neuromodulation of synaptic plasticity and associated functions across different brain regions.

Long-term memory, synaptic plasticity and dopamine in rodent medial prefrontal cortex: Role in executive functions

Mnemonic functions, supporting rodent behavior in complex tasks, include both long-term and (short-term) working memory components. While working memory is thought to rely on persistent activity states in an active neural network, long-term memory and synaptic plasticity contribute to the formation of the underlying synaptic structure, determining the range of possible states. Whereas, the implication of working memory in executive functions, mediated by the prefrontal cortex (PFC) in primates and rodents, has been extensively studied, the contribution of long-term memory component to these tasks received little attention. This review summarizes available experimental data and theoretical work concerning cellular mechanisms of synaptic plasticity in the medial region of rodent PFC and the link between plasticity, memory and behavior in PFC-dependent tasks. A special attention is devoted to unique properties of dopaminergic modulation of prefrontal synaptic plasticity and its contribution to executive functions.

Neurochemical profiles of the anterior temporal lobe predict response of repetitive transcranial magnetic stimulation on semantic processing

Repetitive transcranial magnetic stimulation (rTMS) is a non-invasive technique used to modulate cortical excitability in the human brain. However, one major challenge with rTMS is that the responses to stimulation are highly variable across individuals. The underlying reasons why responses to rTMS are highly variable between individuals still remain unclear. Here, we investigated whether the response to continuous theta-burst stimulation (cTBS) - an effective rTMS protocol for decreasing cortical excitability - is related to individual differences in glutamate and GABA neurotransmission. We acquired resting-state magnetic resonance spectroscopy (MRS) and functional magnetic resonance imaging (fMRI) during semantic processing. Then, we applied cTBS over the anterior temporal lobe (ATL), a hub for semantic representation, to explore the relationship between the baseline neurochemical profiles in this region and the response to cTBS. We found that the baseline excitation-inhibition balance (glutamate + glutamine/GABA ratio) in the ATL was associated with individual cTBS responsiveness during semantic processing. Specifically, individuals with lower excitation-inhibition balance showed stronger inhibitory effect - poorer semantic performance. Our results revealed that non-responders (subjects who did not show an inhibitory effect of cTBS on subsequent semantic performance) had higher excitatory-inhibitory balance in the ATL, which led to up-regulated task-induced regional activity as well as increased ATL-connectivity with other semantic regions compared to responders. These results disclose that the baseline neurochemical state of a cortical region can be a significant factor in predicting responses to cTBS.

Sustained NMDA receptor hypofunction impairs brain-derived neurotropic factor signalling in the PFC, but not in the hippocampus, and disturbs PFC-dependent cognition in mice

BACKGROUND

Cognitive deficits profoundly impact on the quality of life of patients with schizophrenia. Alterations in brain derived neurotrophic factor (BDNF) signalling, which regulates synaptic function through the activation of full-length tropomyosin-related kinase B receptors (TrkB-FL), are implicated in the aetiology of schizophrenia, as is N-methyl-D-aspartate receptor (NMDA-R) hypofunction. However, whether NMDA-R hypofunction contributes to the disrupted BDNF signalling seen in patients remains unknown.

AIMS

The purpose of this study was to characterise BDNF signalling and function in a preclinical rodent model relevant to schizophrenia induced by prolonged NMDA-R hypofunction.

METHODS

Using the subchronic phencyclidine (PCP) model, we performed electrophysiology approaches, molecular characterisation and behavioural analysis.

RESULTS

The data showed that prolonged NMDA-R antagonism, induced by subchronic PCP treatment, impairs long-term potentiation (LTP) and the facilitatory effect of BDNF upon LTP in the medial prefrontal cortex (PFC) of adult mice. Additionally, TrkB-FL receptor expression is decreased in the PFC of these animals. By contrast, these changes were not present in the hippocampus of PCP-treated mice. Moreover, BDNF levels were not altered in the hippocampus or PFC of PCP-treated mice. Interestingly, these observations are paralleled by impaired performance in PFC-dependent cognitive tests in mice treated with PCP.

CONCLUSIONS

Overall, these data suggest that NMDA-R hypofunction induces dysfunctional BDNF signalling in the PFC, but not in the hippocampus, which may contribute to the PFC-dependent cognitive deficits seen in the subchronic PCP model. Additionally, these data suggest that targeting BDNF signalling may be a mechanism to improve PFC-dependent cognitive dysfunction in schizophrenia.

N-methyl-d-aspartate receptor dysfunction in the prefrontal cortex of stroke-prone spontaneously hypertensive rat/Ezo as a rat model of attention deficit/hyperactivity disorder

AIM

We previously reported that stroke-prone spontaneously hypertensive rat/Ezo (SHRSP/Ezo) has high validity as an attention deficit/hyperactivity disorder (AD/HD) animal model, based on its behavioral phenotypes, such as inattention, hyperactivity, and impulsivity. Fronto-cortical dysfunction is implicated in the pathogenesis of AD/HD. In this study, we investigated prefrontal cortex (PFC) function in SHRSP/Ezo rats by electrophysiological methods and radioreceptor assay.

METHODS

We recorded excitatory postsynaptic potential in layer V pyramidal neurons in the PFC by intracellular recording method to assess synaptic plasticity in the form of long-term potentiation (LTP). We also performed N-methyl-d-aspartate acid (NMDA) receptor binding assay in the PFC and hippocampus using radiolabeled NMDA receptor antagonist [³ H]MK-801.

RESULTS

Theta-burst stimulation induced LTP in the PFC of genetic control, WKY/Ezo, whereas failed to induce LTP in that of SHRSP/Ezo. The K_d value of [³ H]MK-801 binding for NMDA receptors in the PFC of SHRSP/Ezo was higher than in the WKY/Ezo. Neither the B_max nor K_d of [³ H]MK-801 binding in the SHRSP/Ezo hippocampus was significantly different to WKY/Ezo.

CONCLUSION

These results suggest that the AD/HD animal model SHRSP/Ezo has NMDA receptor dysfunction in the PFC.

Cholinesterase inhibitors, donepezil and rivastigmine, attenuate spatial memory and cognitive flexibility impairment induced by acute ethanol in the Barnes maze task in rats

Central cholinergic dysfunction contributes to acute spatial memory deficits produced by ethanol administration. Donepezil and rivastigmine elevate acetylcholine levels in the synaptic cleft through the inhibition of cholinesterases—enzymes involved in acetylcholine degradation. The aim of our study was to reveal whether donepezil (acetylcholinesterase inhibitor) and rivastigmine (also butyrylcholinesterase inhibitor) attenuate spatial memory impairment as induced by acute ethanol administration in the Barnes maze task (primary latency and number of errors in finding the escape box) in rats. Additionally, we compared the influence of these drugs on ethanol-disturbed memory. In the first experiment, the dose of ethanol (1.75 g/kg, i.p.) was selected that impaired spatial memory, but did not induce motor impairment. Next, we studied the influence of donepezil (1 and 3 mg/kg, i.p.), as well as rivastigmine (0.5 and 1 mg/kg, i.p.), given either before the probe trial or the reversal learning on ethanol-induced memory impairment. Our study demonstrated that these drugs, when given before the probe trial, were equally effective in attenuating ethanol-induced impairment in both test situations, whereas rivastigmine, at both doses (0.5 and 1 mg/kg, i.p.), and donepezil only at a higher dose (3 mg/kg, i.p.) given prior the reversal learning, attenuated the ethanol-induced impairment in cognitive flexibility. Thus, rivastigmine appears to exert more beneficial effect than donepezil in reversing ethanol-induced cognitive impairments—probably due to its wider spectrum of activity. In conclusion, the ethanol-induced spatial memory impairment may be attenuated by pharmacological manipulation of central cholinergic neurotransmission.

The Influence of Na(+), K(+)-ATPase on Glutamate Signaling in Neurodegenerative Diseases and Senescence

Decreased Na(+), K(+)-ATPase (NKA) activity causes energy deficiency, which is commonly observed in neurodegenerative diseases. The NKA is constituted of three subunits: α, β, and γ, with four distinct isoforms of the catalytic α subunit (α1-4). Genetic mutations in the ATP1A2 gene and ATP1A3 gene, encoding the α2 and α3 subunit isoforms, respectively can cause distinct neurological disorders, concurrent to impaired NKA activity. Within the central nervous system (CNS), the α2 isoform is expressed mostly in glial cells and the α3 isoform is neuron-specific. Mutations in ATP1A2 gene can result in familial hemiplegic migraine (FHM2), while mutations in the ATP1A3 gene can cause Rapid-onset dystonia-Parkinsonism (RDP) and alternating hemiplegia of childhood (AHC), as well as the cerebellar ataxia, areflexia, pescavus, optic atrophy and sensorineural hearing loss (CAPOS) syndrome. Data indicates that the central glutamatergic system is affected by mutations in the α2 isoform, however further investigations are required to establish a connection to mutations in the α3 isoform, especially given the diagnostic confusion and overlap with glutamate transporter disease. The age-related decline in brain α2∕3 activity may arise from changes in the cyclic guanosine monophosphate (cGMP) and cGMP-dependent protein kinase (PKG) pathway. Glutamate, through nitric oxide synthase (NOS), cGMP and PKG, stimulates brain α2∕3 activity, with the glutamatergic N-methyl-D-aspartate (NMDA) receptor cascade able to drive an adaptive, neuroprotective response to inflammatory and challenging stimuli, including amyloid-β. Here we review the NKA, both as an ion pump as well as a receptor that interacts with NMDA, including the role of NKA subunits mutations. Failure of the NKA-associated adaptive response mechanisms may render neurons more susceptible to degeneration over the course of aging.

The potential for non-invasive brain stimulation to improve function after amputation

PURPOSE

Lower limb amputee rehabilitation has traditionally focussed on restoration of gait and balance through use of prosthetic limbs and mobility aids. Despite these efforts, some amputees continue to experience difficulties with mastering prosthetic mobility. Emerging techniques in rehabilitation, such as non-invasive brain stimulation (NIBS), may be an appropriate tool to enhance prosthetic rehabilitation outcomes by promoting "normal" brain reorganisation and function. The purpose of this review is to highlight the potential of NIBS to improve functional outcomes for lower limb amputees.

METHODS

To demonstrate the rationale for applying NIBS to amputees, this study will first review literature regarding human motor control of gait, followed by neurophysiological reorganisation of the motor system after amputation and the relationship between brain reorganisation and gait function. We will conclude by reviewing literature demonstrating application of NIBS to lower limb muscle representations and evidence supportive of subsequent functional improvements.

RESULTS

Imaging, brain stimulation and behavioural evidence indicate that the cortex contributes to locomotion in humans. Following amputation both hemispheres reorganise with evidence suggesting brain reorganisation is related to functional outcomes in amputees. Previous studies indicate that brain stimulation techniques can be used to selectively promote neuroplasticity of lower limb cortical representations with improvements in function.

CONCLUSIONS

We suggest NIBS has the potential to transform lower limb amputee rehabilitation and should be further investigated. Implications for Rehabilitation Despite extensive rehabilitation some amputees continue to experience difficulty with prosthetic mobility Brain reorganisation following amputation has been related to functional outcomes and may be an appropriate target for novel interventions Non-invasive brain stimulation is a promising tool which has potential to improve functional outcomes for lower limb amputees.

Theta-burst LTP

This review covers the spatial and temporal rules governing induction of hippocampal long-term potentiation (LTP) by theta-burst stimulation. Induction of LTP in field CA1 by high frequency stimulation bursts that resemble the burst discharges (complex-spikes) of hippocampal pyramidal neurons involves a multiple-step mechanism. A single burst is insufficient for LTP induction because it evokes both excitatory and inhibitory currents that partially cancel and limit postsynaptic depolarization. Bursts repeated at the frequency (~5 Hz) of the endogenous theta rhythm induce maximal LTP, primarily because this frequency disables feed-forward inhibition and allows sufficient postsynaptic depolarization to activate voltage-sensitive NMDA receptors. The disinhibitory process, referred to as "priming", involves presynaptic GABA autoreceptors that inhibit GABA release. Activation of NMDA receptors allows a calcium flux into dendritic spines that serves as the proximal trigger for LTP. We include new data showing that theta-burst stimulation is more efficient than other forms of stimulation for LTP induction. In addition, we demonstrate that associative interactions between synapses activated during theta-bursts are limited to major dendritic domains since such interactions occur within apical or basal dendritic trees but not between them. We review evidence that recordings of electrophysiological responses during theta burst stimulation can help to determine if experimental manipulations that affect LTP do so by affecting events antecedent to the induction process, such as NMDA receptor activation, or downstream signaling cascades that result from postsynaptic calcium fluxes. Finally, we argue that theta-burst LTP represents a minimal model for stable, non-decremental LTP that is more sensitive to a variety of experimental manipulations than is LTP induced by other stimulation paradigms. This article is part of a Special Issue entitled SI: Brain and Memory.

Recognition memory and synaptic plasticity in the perirhinal and prefrontal cortices

Work is reviewed that relates recognition memory to studies of synaptic plasticity mechanisms in perirhinal and prefrontal cortices. The aim is to consider evidence that perirhinal cortex and medial prefrontal cortex store rather than merely transmit information necessary for recognition memory and, if so, to consider what mechanisms are potentially available within these cortices for producing such storage through synaptic change. Interventions with known actions on plasticity mechanisms are reviewed in relation to their effects on recognition memory processes. These interventions importantly include those involving antagonism of glutamatergic and cholinergic receptors but also inhibition of plasticity consolidation and expression mechanisms. It is concluded that there is strong evidence that perirhinal cortex is involved in information storage necessary for object recognition memory and, moreover, that such storage involves synaptic weakening mechanisms including the removal of AMPA glutamate receptors from synapses. There is good evidence that medial prefrontal cortex is necessary for associative and temporal order recognition memory and that this cortex expresses plasticity mechanisms that potentially allow the storage of information. However, the case for medial prefrontal cortex acting as a store requires further support.

Investigations into the involvement of NMDA mechanisms in recognition memory

This review will focus on evidence showing that NMDA receptor neurotransmission is critical for synaptic plasticity processes within brain regions known to be necessary for the formation of object recognition memories. The aim will be to provide evidence concerning NMDA mechanisms related to recognition memory processes and show that recognition memory for objects, places or associations between objects and places depends on NMDA neurotransmission within the perirhinal cortex, temporal association cortex medial prefrontal cortex and hippocampus. Administration of the NMDA antagonist AP5, selectively into each of these brain regions has revealed that the extent of the involvement NMDA receptors appears dependent on the type of information required to solve the recognition memory task; thus NMDA receptors in the perirhinal cortex are crucial for the encoding of long-term recognition memory for objects, and object-in-place associations, but not for short-term recognition memory or for retrieval. In contrast the hippocampus and medial prefrontal cortex are required for both long-term and short-term recognition memory for places or associations between objects and places, or for recognition memory tasks that have a temporal component. Such studies have therefore confirmed that the multiple brain regions make distinct contributions to recognition memory but in addition that more than one synaptic plasticity process must be involved.

This article is part of the Special Issue entitled ‘Glutamate Receptor-Dependent Synaptic Plasticity’.

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NMDAR blockade in PRH, mPFC and HPC produces different patterns of memory deficits.

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NMDARs within these brain regions make distinct contributions to recognition memory.

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NMDARs are also critical for synaptic plasticity in the same brain regions.

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More than one synaptic plasticity process must be involved in recognition memory.

Plasticity-inducing TMS protocols to investigate somatosensory control of hand function

Hand function depends on sensory feedback to direct an appropriate motor response. There is clear evidence that somatosensory cortices modulate motor behaviour and physiology within primary motor cortex. However, this information is mainly from research in animals and the bridge to human hand control is needed. Emerging evidence in humans supports the notion that somatosensory cortices modulate motor behaviour, physiology and sensory perception. Transcranial magnetic stimulation (TMS) allows for the investigation of primary and higher-order somatosensory cortices and their role in control of hand movement in humans. This review provides a summary of several TMS protocols in the investigation of hand control via the somatosensory cortices. TMS plasticity inducing protocols reviewed include paired associative stimulation, repetitive TMS, theta-burst stimulation as well as other techniques that aim to modulate cortical excitability in sensorimotor cortices. Although the discussed techniques may modulate cortical excitability, careful consideration of experimental design is needed to isolate factors that may interfere with desired results of the plasticity-inducing protocol, specifically events that may lead to metaplasticity within the targeted cortex.

Amphetamine modulation of long-term potentiation in the prefrontal cortex: dose dependency, monoaminergic contributions, and paradoxical rescue in hyperdopaminergic mutant

Amphetamine can improve cognition in healthy subjects and patients with schizophrenia, attention-deficit hyperactivity disorder, and other neuropsychiatric diseases; higher doses, however, can impair cognitive function, especially those mediated by the prefrontal cortex. We investigated how amphetamine affects prefrontal cortex long-term potentiation (LTP), a cellular correlate of learning and memory, in normal and hyperdopaminergic mice lacking the dopamine transporter. Acute amphetamine treatment in wild-type mice produced a biphasic dose-response modulation of LTP, with a low dose enhancing LTP and a high dose impairing it. Amphetamine-induced LTP enhancement was prevented by pharmacological blockade of D(1) - (but not D(2)-) class dopamine receptors, by blockade of β-adrenergic receptors, or by inhibition of cAMP-PKA signaling. In contrast, amphetamine-induced LTP impairment was prevented by inhibition of post-synaptic protein phosphatase-1, a downstream target of PKA signaling, or by blockade of either D(1) - or D(2)-class dopamine, but not noradrenergic, receptors. Thus, amphetamine biphasically modulates LTP via cAMP-PKA signaling orchestrated mainly through dopamine receptors. Unexpectedly, amphetamine restored the loss of LTP in dopamine transporter-knockout mice primarily by activation of the noradrenergic system. Our results mirror the biphasic effectiveness of amphetamine in humans and provide new mechanistic insights into its effects on cognition under normal and hyperdopaminergic conditions.

Hyperdopaminergic tone erodes prefrontal long-term potential via a D2 receptor-operated protein phosphatase gate

Dopamine (DA) plays crucial roles in the cognitive functioning of the prefrontal cortex (PFC), which, to a large degree, depends on lasting neural traces formed in prefrontal networks. The establishment of these permanent traces requires changes in cortical synaptic efficacy. DA, via the D(1)-class receptors, is thought to gate or facilitate synaptic plasticity in the PFC, with little role recognized for the D(2)-class receptors. Here we show that, when significantly elevated, DA erodes, rather than facilitates, the induction of long-term potentiation (LTP) in the PFC by acting at the far less abundant cortical D(2)-class receptors through a dominant coupling to the protein phosphatase 1 (PP1) activity in postsynaptic neurons. In mice with persistently elevated extracellular DA, resulting from inactivation of the DA transporter (DAT) gene, LTP in layer V PFC pyramidal neurons cannot be established, regardless of induction protocols. Acute increase of dopaminergic transmission by DAT blockers or overstimulation of D(2) receptors in normal mice have similar LTP shutoff effects. LTP in mutant mice can be rescued by a single in vivo administration of D(2)-class antagonists. Suppression of postsynaptic PP1 mimics and occludes the D(2)-mediated rescue of LTP in mutant mice and prevents the acute erosion of LTP by D(2) agonists in normal mice. Our studies reveal a mechanistically unique heterosynaptic PP1 gate that is constitutively driven by background DA to influence LTP induction. By blocking prefrontal synaptic plasticity, excessive DA may prevent storage of lasting memory traces in PFC networks and impair executive functions.

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).

Neurochemical effects of theta burst stimulation as assessed by magnetic resonance spectroscopy

Continuous theta burst stimulation (cTBS) is a novel transcranial stimulation technique that causes significant inhibition of synaptic transmission for Collapse

Key Words Collapse

MESH Headings

• Adult
• Aspartic Acid/analogs & derivatives
• Aspartic Acid/metabolism
• Brain Chemistry/physiology
• Brain Mapping
• Electric Stimulation/methods
• Female
• Glutamic Acid/metabolism
• Glutamine/metabolism
• Humans
• Magnetic Resonance Spectroscopy
• Male
• Motor Cortex/chemistry
• Neural Inhibition/physiology
• Statistics, Nonparametric
• Theta Rhythm
• Transcranial Magnetic Stimulation
• Young Adult
• gamma-Aminobutyric Acid/metabolism

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Grants

• 078204 Wellcome Trust
• G9409531 Medical Research Council
• G9409634 Medical Research Council
• 076358/Z/05Z Wellcome Trust

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Affiliation(s)

C J Stagg Centre for Functional Resonance Imaging of the Brain, University of Oxford, Oxford, United Kingdom.
M Wylezinska
P M Matthews
H Johansen-Berg
P Jezzard
J C Rothwell
S Bestmann

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Neuroactive steroid pregnenolone sulphate inhibits long-term potentiation via activation of alpha2-adrenoreceptors at excitatory synapses in rat medial prefrontal cortex

Pregnenolone sulphate (PREGS) is one of the most important neuroactive steroids. Previous study showed that PREGS enhanced long-term potentiation (LTP) via activation of post-synaptic NMDA receptors at excitatory synapses in the hippocampus. The present paper studied the effect of PREGS on LTP at excitatory synapses in the pyramidal cells of layers V-VI of the medial prefrontal cortex (mPFC) using whole-cell patch-clamp in slices and made a comparison with that in the hippocampus. We also studied the mechanism of the effect of PREGS in the mPFC. We found that PREGS inhibited induction of LTP in the mPFC and had no influence on NMDA currents, which was different from its effect in the hippocampus. Moreover, the effect of PREGS on LTP in the mPFC was cancelled by alpha2-adrenoreceptor antagonist, alpha2A-adrenoreceptor antagonist, Gi protein inhibitor, adenylate cyclase inhibitor and protein kinase A inhibitor. These results suggest that PREGS inhibits LTP via activation of the alpha2-adrenoreceptor-Gi protein-adenylate cyclase-protein kinase A signalling pathway in the mPFC.

N-methyl-D-aspartate-dependent long-term potentiation of excitatory transmission in trigeminal subnucleus oralis

This study for the first time demonstrates early developmental changes of passive/active membrane properties, and long-term potentiation (LTP) of excitatory synaptic transmission at spinal trigeminal subnucleus caudalis (Vc)-to-oralis (Vo) synapses. During postnatal development, the probability of Vo neurons with monosynaptic excitatory postsynaptic currents (EPSCs) upon Vc stimulation significantly increased, whereas the input resistances of Vo neurons and the latencies of monosynaptic EPSCs significantly decreased. Application of a 'pairing' protocol that comprises 2 Hz-conditioning stimulation of Vc with postsynaptic depolarization of Vo neuron to +30 mV generated LTP of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor-mediated monosynaptic EPSC amplitude in more than 70% of Vo neurons. The induction of LTP required the activation of N-methyl-D-aspartate receptor, but its magnitudes had correlation neither with postnatal ages nor with baseline EPSC amplitudes.

Dopamine D1/5 receptor-mediated long-term potentiation of intrinsic excitability in rat prefrontal cortical neurons: Ca2+-dependent intracellular signaling

Prefrontal cortex (PFC) dopamine D1/5 receptors modulate long- and short-term neuronal plasticity that may contribute to cognitive functions. Synergistic to synaptic strength modulation, direct postsynaptic D1/5 receptor activation also modulates voltage-dependent ionic currents that regulate spike firing, thus altering the neuronal input-output relationships in a process called long-term potentiation of intrinsic excitability (LTP-IE). Here, the intracellular signals that mediate this D1/5 receptor-dependent LTP-IE were determined using whole cell current-clamp recordings in layer V/VI rat pyramidal neurons from PFC slices. After blockade of all major amino acid receptors (V(hold) = -65 mV) brief tetanic stimulation (20 Hz) of local afferents or application of the D1 agonist SKF81297 (0.2-50 microM) induced LTP-IE, as shown by a prolonged (>40 min) increase in depolarizing pulse-evoked spike firing. Pretreatment with the D1/5 antagonist SCH23390 (1 microM) blocked both the tetani- and D1/5 agonist-induced LTP-IE, suggesting a D1/5 receptor-mediated mechanism. The SKF81297-induced LTP-IE was significantly attenuated by Cd(2+), [Ca(2+)](i) chelation, by inhibition of phospholipase C, protein kinase-C, and Ca(2+)/calmodulin kinase-II, but not by inhibition of adenylate cyclase, protein kinase-A, MAP kinase, or L-type Ca(2+) channels. Thus this form of D1/5 receptor-mediated LTP-IE relied on Ca(2+) influx via non-L-type Ca(2+) channels, activation of PLC, intracellular Ca(2+) elevation, activation of Ca(2+)-dependent CaMKII, and PKC to mediate modulation of voltage-dependent ion channel(s). This D1/5 receptor-mediated modulation by PKC coexists with the previously described PKA-dependent modulation of K(+) and Ca(2+) currents to dynamically regulate overall excitability of PFC neurons.

Excitatory actions of the ventral midline thalamus (rhomboid/reuniens) on the medial prefrontal cortex in the rat

The medial prefrontal cortex (mPFC) has been associated with diverse functions including attentional processes, visceromotor activity, decision making, goal directed behavior, and working memory. The present report examined the effects of stimulation of the midline thalamus, concentrating on ventral nuclei of the midline thalamus, on evoked activity at the mPFC. The nucleus reuniens (RE) of the ventral midline thalamus is a major source of projections to the hippocampus and to the mPFC, and has been shown to exert pronounced excitatory effects on the hippocampus. No previous study has systematically examined the actions of the ventral midline thalamus on the mPFC. We showed that stimulation of the dorsal and ventral midline thalamus, but not of an intermediate region lying between them (null zone), produced short latency, large amplitude evoked potentials throughout the dorsoventral extent of the medial PFC. The largest effects were elicited with ventral midline stimulation (rhomboid/reuniens nuclei) at the ventral mPFC--the prelimbic (PL) and infralimbic (IL) cortices. Specifically, stimulation of RE produced evoked potentials (early negative component, N2) at the PL cortex at a mean latency of 22.6 msec and mean amplitude of 0.85 mV, indicative of monosynaptic effects. In addition, we showed that paired pulse stimulation of RH/RE produced strong facilitatory actions (paired pulse facilitation) at IL (83%) and PL (75%). These findings indicate that RE exerts strong direct excitatory effects on the mPFC, and coupled with the demonstration that RE produces similar actions on the hippocampus, indicates that RE is in a position to influence and possibly coordinate the activity of these two forebrain structures subserving memory.

Synaptic Plasticity in Local Cortical Network In Vivo and Its Modulation by the Level of Neuronal Activity

Neocortical neurons maintain high firing rates across all behavioral states of vigilance but the discharge patterns vary during different types of brain oscillations, which are assumed to play an important role in information processing and memory consolidation. In the present study, we report that trains of stimuli applied to local neocortical networks of cats, at frequencies that mimic endogenous brain rhythms, produced depression or potentiation of postsynaptic potentials, which lasted for several minutes. This form of synaptic plasticity was not mediated through NMDA receptors since it persisted after blockade of these receptors, but was strongly modulated by the level of background neuronal activity. Using different preparations in vivo, we found that increased background neuronal activity decreased the probability of plastic changes but enhanced the probability of potentiation over depression. Conversely, when the level of background neuronal activity was low, plasticity was observed in all neurons, but mainly depression was induced. Our results demonstrate that high levels of neuronal activity in the cortical network promote potentiation and insure the stability of synaptic connections.

Neurobiology of mood, anxiety, and emotions as revealed by studies of a unique antidepressant: tianeptine

Recent studies have provided evidence that structural remodeling of certain brain regions is a feature of depressive illness, and the postulated underlying mechanisms contribute to the idea that there is more to antidepressant actions that can be explained exclusively by a monoaminergic hypothesis. This review summarizes recent neurobiological studies on the antidepressant, tianeptine (S-1574, [3-chloro-6-methyl-5,5-dioxo-6,11-dihydro-(c,f)-dibenzo-(1,2-thiazepine)-11-yl) amino]-7 heptanoic acid, sodium salt), a compound with structural similarities to the tricyclic antidepressant agents, the efficacy and good tolerance of which have been clearly established. These studies have revealed that the neurobiological properties of tianeptine involve the dynamic interplay between numerous neurotransmitter systems, as well as a critical role of structural and functional plasticity in the brain regions that permit the full expression of emotional learning. Although the story is far from complete, the schema underlying the effect of tianeptine on central plasticity is the most thoroughly studied of any antidepressants. Effects of tianeptine on neuronal excitability, neuroprotection, anxiety, and memory have also been found. Together with clinical data on the efficacy of tianeptine as an antidepressant, these actions offer insights into how compounds like tianeptine may be useful in the treatment of neurobiological features of depressive disorders.

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.

Synaptic plasticity in the absence of backpropagating spikes of layer II inputs to layer V pyramidal cells in rat visual cortex

Use-dependent changes of synaptic transmission are thought to depend on the evaluation of temporal correlations between pre- and postsynaptic excitatory activity. Previous studies have demonstrated long-term potentiation (LTP) in layer V pyramidal cells after coincident pairing of presynaptic excitatory input with postsynaptic depolarizations, evoking backpropagating action potentials (BAPs). Here we paired excitatory layer II input with somatic hyperpolarization, which blocked BAPs in layer V pyramidal cells of rat visual cortex and induced reliable long-term depression (LTD). Upon cholinergic receptor activation, this BAP-independent protocol also induced LTP, which was not dependent on N-methyl-d-aspartate receptor activation, but blocked by metabotropic glutamate receptor (mGluR) antagonists.

Theta burst stimulation of the human motor cortex

It has been 30 years since the discovery that repeated electrical stimulation of neural pathways can lead to long-term potentiation in hippocampal slices. With its relevance to processes such as learning and memory, the technique has produced a vast literature on mechanisms of synaptic plasticity in animal models. To date, the most promising method for transferring these methods to humans is repetitive transcranial magnetic stimulation (rTMS), a noninvasive method of stimulating neural pathways in the brain of conscious subjects through the intact scalp. However, effects on synaptic plasticity reported are often weak, highly variable between individuals, and rarely last longer than 30 min. Here we describe a very rapid method of conditioning the human motor cortex using rTMS that produces a controllable, consistent, long-lasting, and powerful effect on motor cortex physiology and behavior after an application period of only 20-190 s.

The principal features and mechanisms of dopamine modulation in the prefrontal cortex

Mesocortical [corrected] dopamine (DA) inputs to the prefrontal cortex (PFC) play a critical role in normal cognitive process and neuropsychiatic pathologies. This DA input regulates aspects of working memory function, planning and attention, and its dysfunctions may underlie positive and negative symptoms and cognitive deficits associated with schizophrenia. Despite intense research, there is still a lack of clear understanding of the basic principles of actions of DA in the PFC. In recent years, there has been considerable efforts by many groups to understand the cellular mechanisms of DA modulation of PFC neurons. However, the results of these efforts often lead to contradictions and controversies. One principal feature of DA that is agreed by most researchers is that DA is a neuromodulator and is clearly not an excitatory or inhibitory neurotransmitter. The present article aims to identify certain principles of DA mechanisms by drawing on published, as well as unpublished data from PFC and other CNS sites to shed light on aspects of DA neuromodulation and address some of the existing controversies. Eighteen key features about DA modulation have been identified. These points directly impact on the end result of DA neuromodulation, and in some cases explain why DA does not yield identical effects under all experimental conditions. It will become apparent that DA's actions in PFC are subtle and depend on a variety of factors that can no longer be ignored. Some of these key factors include distinct bell-shaped dose-response profiles of postsynaptic DA effects, different postsynaptic responses that are contingent on the duration of DA receptor stimulation, prolonged duration effects, bidirectional effects following activation of D1 and D2 classes of receptors and membrane potential state and history dependence of subsequent DA actions. It is hoped that these factors will be borne in mind in future research and as a result a more consistent picture of DA neuromodulation in the PFC will emerge. Based on these factors, a theory is proposed for DA's action in PFC. This theory suggests that DA acts to expand or contract the breadth of information held in working memory buffers in PFC networks.

Prefrontal cortex function, quasi-physiological stimuli, and synaptic plasticity

The prelimbic area of rat medial frontal cortex may be functionally analogous to human/primate dorsolateral prefrontal cortex. This area may be involved in selective attention to the external stimuli and the coupling of the attention to a repertory of actions. It was suggested that this function may rely on a form of long-term memory [Biol. Rev. 77 (2002) 563]. Indeed, during learning of this type of behavior, a portion of prelimbic neurons persistently change their firing characteristics [Prog. Brain Res. 126 (2000) 287]. It is therefore important to study long-term potentiation (LTP) and depression (LTD) in rat prelimbic neurons. In this article, the author first briefly reviews recent findings on the prefrontal cortex function and discusses that the prefrontal cortex may be involved in long-term memory. Second, the author will show some new results which indicate that quasi-physiological patterns of stimuli mimicking prelimbic neuronal activity during behavior can induce LTP in prelimbic pyramidal neuron synapses. These results suggest that prelimbic neuronal activity during behavior may lastingly modify prelimbic synaptic efficacy.

Effects of subchronic clozapine treatment on long-term potentiation in rat prefrontal cortex

Several studies postulated an interaction of clozapine with N-methyl-D-aspartate (NMDA) receptor-mediated transmission. We previously showed that acute clozapine application on rat prefrontal cortex (PFC) slices increased NMDA receptor-dependent long-term potentiation (LTP) in the prelimbic (PL) area. The present study explores the effects of subchronic clozapine treatment on LTP in the same brain area. After 21 days of treatment (30 mg/kg per day, via drinking water), rats were sacrificed and slices from the PFC were prepared for electrophysiological investigations. To this end, extracellular field potentials in the layer II-V pathway were recorded. In contrast to our previous study with acute application on the slice, subchronic clozapine treatment attenuated LTP as compared to non-treated animals. We interpret these findings to suggest that prolonged treatment with clozapine might result in a compensatory response to the acute facilitating action of clozapine on LTP-mediating processes.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Genetic evidence for the bidirectional modulation of synaptic plasticity in the prefrontal cortex by D1 receptors

To address the role of D1 receptors in the medial prefrontal cortex, we combined pharmacological and genetic manipulations to examine long-term synaptic potentiation (LTP)/long-term synaptic depression (LTD) in brain slices of rats and mice. We found that the D1 antagonist SCH23390 selectively blocked the maintenance but not the induction of LTP in the prefrontal cortex. Conversely, activation of D1 receptors facilitated the maintenance of LTP, and this effect is impaired in heterozygous D1 receptor knockout mice. Low-frequency stimulation induced a transient depression in the medial prefrontal cortex. This depression could be transformed into LTD by coapplication of dopamine. Coapplication of dopamine, however, shows no facilitating effect on LTD in heterozygous D1 receptor knockout mice. These results provide pharmacological and genetic evidence for a role of D1 receptors in the bidirectional modulation of synaptic plasticity in the medial prefrontal cortex. The absence of this modulation in heterozygous knockout mice shows that a dysregulation of dopamine receptor expression levels can have dramatic effects on synaptic plasticity in the prefrontal cortex.

Substrates for coincidence detection and calcium signaling for induction of synaptic potentiation in the neonatal visual cortex

Regulation of the efficacy of synaptic transmission by activity-dependent processes has been implicated in learning and memory as well as in developmental processes. We previously described transient potentiation of excitatory synapses onto layer 2/3 pyramidal neurons in the visual cortex that is induced by coincident presynaptic stimulation and postsynaptic depolarization. In the adult visual cortex, activation of N-methyl-d-aspartate (NMDA) glutamate receptors is necessary to induce this plasticity. These receptors act as coincidence detectors, sensing presynaptic glutamate release and postsynaptic depolarization, and cause an influx of Ca(2+) that is necessary for the potentiation. In the neurons of the neonatal visual cortex, on the other hand, coincident presynaptic stimulation and postsynaptic depolarization induce stable long-term potentiation (LTP). In addition, reduced but significant LTP can be induced in many neurons in the presence of the NMDA receptor (NMDAR) antagonist, 2-amino-5-phosphonovaleric acid despite the Ca(2+) requirement. Therefore there must be an alternative postsynaptic Ca(2+) source and coincidence detection mechanism linked to the LTP induction mechanism in the neonatal cortex operating in addition to NMDARs. In this study, we find that in layer 2/3 pyramidal neurons, release of Ca(2+) from inositol trisphosphate (InsP(3)) receptor-mediated intracellular stores and influx through voltage-gated Ca(2+) channels (VGCCs) provide alternative postsynaptic Ca(2+) sources. We hypothesize that InsP(3)Rs are coincidence detectors, sensing presynaptic glutamate release through linkage with group I metabotropic glutamate receptors (mGluRs), and depolarization, through VGCCs. We also find that the downstream protein kinases, PKA and PKC, have a role in potentiation in layer 2/3 pyramidal neurons of the neonatal visual cortex.

Metabotropic glutamate receptors (mGluRs) are involved in early phase of memory formation: possible role of modulation of glutamate release

Metabotropic glutamate receptors (mGluRs) groups I and II are involved in the cellular processes of long-term potentiation (LTP) and learning and memory formation. I.c.v. injection of the mGluRs agonist 1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) can impair memory formation in some types of learning task. The role of mGluRs in neurotransmitters release and production of second messengers has been suggested. The aim of the present study was to determine the effect of i.c.v. administration of the new potent mGluRs agonist ABHxD-I and compare its effect with that of ACPD. We studied the effect of both agonists on acquisition and memory for a one-trial passive avoidance learning task in day-old chicks and on the training related glutamate (Glu) release. ACPD or ABHxD-I (50 nmole per chick, i.c.v. injection) were administered at different times before or after training and chicks were tested at various times after training. Chicks injected with ABHxD-I 30 min before training showed amnesia when tested 30 min or 3h after training. The amnestic effect of ACPD was significant only 30 min after training. Glu release evoked by 70 mM KCl was measured in slices prepared from the IMHV of chick brain isolated from animals injected with either ACPD or ABHxD-I 30 min before training and tested 30 min after training. Glu concentration was measured using HPLC. Both ACPD and ABHxD-I significantly increased Glu release in slices isolated from untrained chicks (30 and 48% compare to control, respectively, P<0.05). Training itself increased Glu release (41% compared to control, P<0.01) and no additional effect of either ACPD or ABHxD-I was observed. These results suggest that mGluRs groups I and II are involved in the early stages of memory formation and that application of either of the studied mGluRs agonists may interfere with that process. The amnestic effect of ABHxD-I seems to be stronger and longer lasting. Although the mechanism of this effect still remains unclear, our results suggest that disregulation of Glu release by mGluR agonists may participate in this process.

Effects of clozapine, haloperidol and iloperidone on neurotransmission and synaptic plasticity in prefrontal cortex and their accumulation in brain tissue: an in vitro study

The mode of action of the antipsychotic drugs clozapine, haloperidol and iloperidone was investigated in layer V of prefrontal cortex slices using extracellular field potential, intracellular sharp-electrode as well as whole-cell voltage clamp recording techniques. Intracellular investigations on a broad range of concentrations revealed that the typical neuroleptic haloperidol at higher concentrations significantly depressed the excitatory postsynaptic component induced by electrical stimulation of layer II. This was not seen with the atypical neuroleptics clozapine and iloperidone. None of the three compounds had any effect on the resting membrane potential, spike amplitude or input resistance at relevant concentrations. Synaptic plasticity was assessed by means of extracellular field potential recordings. Clozapine significantly facilitated the potentiation of synaptic transmission, whereas haloperidol and iloperidone showed no effects. In line with its facilitating effect on synaptic plasticity, it could be demonstrated by whole-cell voltage clamp recordings that clozapine increased N-methyl-D-aspartic acid receptor-mediated excitatory postsynaptic currents in the majority of prefrontal cortical neurones. These investigations were made with neuroleptic drugs applied to the bath in the micromolar concentration range in order to approach clinical brain concentrations that are reached after administration of therapeutic doses. The drug concentrations reached in the slices after the experiments were assessed by means of high-pressure liquid chromatography coupled with mass-spectrometric detection. Surprisingly, drug accumulation in the in vitro preparation was of similar degree as reported in vivo. In conclusion, the typical neuroleptic haloperidol significantly depressed excitatory synaptic transmission in layer V neurones of the prefrontal cortex. In contrast, the two atypical neuroleptics iloperidone and clozapine revealed no depressing effects. This feature of the atypical neuroleptics might be beneficial since a hypofunctionality of this brain area is thought to be linked with the pathophysiology of schizophrenia. Additionally, clozapine facilitated long-term potentiation, which might be linked with the clinically observed beneficial effects on certain cognitive parameters. The clozapine-induced increase of N-methyl-D-aspartic acid receptor-mediated currents suggests that clozapine facilitates the induction of long-term potentiation. Furthermore, the present study points to the importance of considering the significant accumulation of neuroleptic drugs in in vitro studies.

Potentiation of prelimbic field potentials during and seconds after trains of excitations in the rat hippocampo-prefrontal pathway

Field potentials were recorded in the prelimbic cortex of anaesthetized rats after excitations of the hippocampo-prefrontal pathway. Stimuli were delivered to the hippocampal CA1 region and short-term changes of field potential amplitudes were observed in two situations. (1) Amplitudes were monitored during trains of stimulations given at frequencies between 1 and 20 Hz. Within trains, potentiation was followed by depression. Both types of changes were frequency dependent. (2) The time course of recovery from within-train plasticity was obtained from field potentials evoked at varying intervals after trains. This revealed a post-train potentiation having a maximum after 2-4 s and lasting for approximately 10 s. The maximal post-train potentiation was nearly independent of the frequency of the preceding train.

Memory trace in prefrontal cortex: theory for the cognitive switch

The dorsolateral prefrontal cortex in human and non-human primates functions as the highest-order executor for the perception-action cycle. According to this view, when perceptual stimuli from the environment are novel or complex, the dorsolateral prefrontal cortex serves to set consciously a goal-directed scheme which broadly determines an action repertory to meet the particular demand from the environment. In this respect, the dorsolateral prefrontal cortex is a short-term activation device with the properties of a cognitive switch', because it couples a particular set of perceptual stimuli to a particular set of actions. Here, I suggest that, in order for the organism to react systematically to the environment, neural traces for the switch function must be stored in the brain. Thus, the highest-order, perception-action interface function of the dorsolateral prefrontal cortex per se depends on permanently stored neural traces in the dorsolateral prefrontal cortex and related structures. Such a memory system may be located functionally between two of the well-documented memory systems in the brain: the declarative memory system and the procedural memory system. Finally, based on available neurophysiological data, the possible mechanisms underlying the formation of cognitive switch traces are proposed.

Long-term depression induced by postsynaptic group II metabotropic glutamate receptors linked to phospholipase C and intracellular calcium rises in rat prefrontal cortex

We have previously shown (Otani et al., 1999b) that bath application of (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV), the agonist of group II metabotropic glutamate receptors (mGluRs), induces postsynaptic Ca2+-dependent long-term depression (LTD) of layer I-II to layer V pyramidal neuron glutamatergic synapses of rat medial prefrontal cortex. In the present study, we examined detailed mechanisms of this DCG IV-induced LTD. First, the group II mGluR antagonist (RS)-alpha-methylserine-O-phosphate monophenyl ester blocked DCG IV-induced LTD, and another group II agonist (2S,3S,4S)-CCG/(2S,1'S,2'S)-2-(carboxycyclopropyl)glycine-induced LTD, suggesting that LTD is indeed mediated by the activation of group II mGluRs. Second, DCG IV-induced LTD was blocked by the NMDA receptor antagonist AP-5, whereas DCG IV did not potentiate NMDA receptor-mediated synaptic responses. Interruption of single test stimuli during DCG IV application blocked DCG IV-induced LTD. These results suggest that small NMDA receptor-mediated responses evoked by single synaptic stimuli contribute to DCG IV-induced LTD. Third, DCG IV-induced LTD was blocked or reduced by the following drugs: phospholipase C inhibitor U-73122 (bath-applied or postsynaptically injected), postsynaptically injected IP3 receptor blocker heparin, phospholipase D-linked mGluR blocker PCCG-13, PKC inhibitor RO318220, postsynaptically injected PKC inhibitor PKC(19-36), and PKA inhibitor KT-5720. Fourth, fluorescent Ca2+ analysis techniques revealed that DCG IV increases Ca2+ concentration in prefrontal layer V pyramidal neurons. These Ca2+ rises and the LTD were both blocked by postsynaptic heparin in the same cells. Taken together, these results suggest that postsynaptic group II mGluRs, linked to phospholipase C and probably also phospholipase D, induce LTD through postsynaptic PKC activation and IP3 receptor-mediated postsynaptic increases of Ca2+ concentration.

Metabotropic glutamate receptors mediate expression of LTP in slices of rat visual cortex

Long-term-potentiation (LTP) can be induced by application of a standard theta-burst stimulation protocol in slice preparations of the neocortex. This type of LTP is known to be dependent on the activation of NMDA receptors. The present study used specific experimental conditions to evoke a non-NMDA receptor mediated type of LTP. By use of weak theta-burst stimulation (wTBS) we describe a non-NMDA receptor dependent LTP in rat visual cortex in vitro, which is sensitive to an antagonist of metabotropic glutamate receptors (mGluR). In slices of the visual cortex we stimulated ascending inputs in cortical layer IV and recorded extracellular field potentials (FPs) from cortical layers II/III. In disinhibited slices (with 1 microm picrotoxin), a wTBS induced LTP to 138% of control. The expression of this potentiation was insensitive to the NMDA-receptor antagonist, D-AP5, but could be abolished by application of the mGluR antagonist MCPG. These data suggest an NMDA-independent mechanism for LTP induction in the visual cortex which can be observed in layer II/III neurons.

Synaptic plasticity in the medial vestibular nuclei: role of glutamate receptors and retrograde messengers in rat brainstem slices

The analysis of cellular-molecular events mediating synaptic plasticity within vestibular nuclei is an attempt to explain the mechanisms underlying vestibular plasticity phenomena. The present review is meant to illustrate the main results, obtained in vitro, on the mechanisms underlying long-term changes in synaptic strength within the medial vestibular nuclei. The synaptic plasticity phenomena taking place at the level of vestibular nuclei could be useful for adapting and consolidating the efficacy of vestibular neuron responsiveness to environmental requirements, as during visuo-vestibular recalibration and vestibular compensation. Following a general introduction on the most salient features of vestibular compensation and visuo-vestibular adaptation, which are two plastic events involving neuronal circuitry within the medial vestibular nuclei, the second and third sections describe the results from rat brainstem slice studies, demonstrating the possibility to induce long-term potentiation and depression in the medial vestibular nuclei, following high frequency stimulation of the primary vestibular afferents. In particular the mechanisms sustaining the induction and expression of vestibular long-term potentiation and depression, such as the role of various glutamate receptors and retrograde messengers have been described. The relevant role of the interaction between the platelet-activating factor, acting as a retrograde messenger, and the presynaptic metabotropic glutamate receptors, in determining the full expression of vestibular long-term potentiation is also underlined. In addition, the mechanisms involved in vestibular long-term potentiation have been compared with those leading to long-term potentiation in the hippocampus to emphasize the most significant differences emerging from vestibular studies. The fourth part, describes recent results demonstrating the essential role of nitric oxide, another retrograde messenger, in the induction of vestibular potentiation. Finally the fifth part suggests the possible functional significance of different action times of the two retrograde messengers and metabotropic glutamate receptors, which are involved in mediating the presynaptic mechanism sustaining vestibular long-term potentiation.

Characterization of spontaneous excitatory synaptic currents in pyramidal cells of rat prelimbic cortex

Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded with the whole-cell patch-clamp technique from 41 pyramidal cells in the layers V-VI of the prelimbic (PL) cortex. The sEPSCs occurred randomly and the averaged frequency in 41 cells was 1.81+/-0.27 Hz. The amplitude distribution was skewed toward larger events and could be adequately fitted by a sum of two or three Gaussian distributions, but they could not be fitted by a sum of Gaussian distributions with equidistant separation in all cells studied (n=24). In eight of 24 cells, after the transformation of the amplitudes into logarithms, the skewed histogram became bell-shaped and could be adequately fitted by a single Gaussian distribution, whereas in the other 16 cells, after the transformation the histograms were still skewed. However, for those latter cells, when the logarithms were transformed into difference, the distribution of the differences in 15 of 16 cells became bell-shaped and could be adequately fitted by a single Gaussian distribution. The pie distribution of different rise times within one cell in 1 ms bin showed that there were four different patterns of the rise time distribution. The amplitude distribution of the sEPSCs was unchanged in 10 of 22 cells after TTX, but in the other 12 cells, it was changed significantly. However, for these cells although TTX had a marked effect, it could not change the skewed distribution into a single Gaussian distribution in case of both original and transformed data.

How antidepressants work: new perspectives on the pathophysiology of depressive disorder

BACKGROUND

New research in animals is beginning to change radically our understanding of the biology of stress and the effects of antidepressant agents.

AIMS

To relate recent findings from the basic neurosciences to the pathophysiology of depressive disorder.

METHOD

Drawing together findings from molecular and physiological studies in rats, social studies in primates and neuropsychological studies in humans, we review the neurotrophic and neuroplastic effects of antidepressants and stress.

RESULTS

Stress and antidepressants have reciprocal actions on neuronal growth and vulnerability (mediated by the expression of neurotrophins) and synaptic plasticity (mediated by excitatory amino acid neurotransmission) in the hippocampus and other brain structures. Stressors have the capacity to progressively disrupt both the activities of individual cells and the operating characteristics of networks of neurons throughout the life cycle, while antidepressant treatments act to reverse such injurious effects.

CONCLUSIONS

We propose a central role for the regulation of synaptic connectivity in the pathophysiology of depressive disorder.

Plasticity at hippocampal to prefrontal cortex synapses: dual roles in working memory and consolidation

The involvement of the hippocampus and the prefrontal cortex in cognitive processes and particularly in learning and memory has been known for a long time. However, the specific role of the projection which connects these two structures has remained elusive. The existence of a direct monosynaptic pathway from the ventral CA1 region of the hippocampus and subiculum to specific areas of the prefrontal cortex provides a useful model for conceptualizing the functional operations of hippocampal-prefrontal cortex communication in learning and memory. It is known now that hippocampal to prefrontal cortex synapses are modifiable synapses and can express different forms of plasticity, including long-term potentiation, long-term depression, and depotentiation. Here we review these findings and focus on recent studies that start to relate synaptic plasticity in the hippocampo-prefrontal cortex pathway to two specific aspects of learning and memory, i.e., the consolidation of information and working memory. The available evidence suggests that functional interactions between the hippocampus and prefrontal cortex in cognition and memory are more complex than previously anticipated, with the possibility for bidirectional regulation of synaptic strength as a function of the specific demands of tasks.

Efferent projections of infralimbic and prelimbic areas of the medial prefrontal cortex in the Japanese monkey, Macaca fuscata

The infralimbic area (IL) and prelimbic area (PL) have been postulated as an autonomic motor region in the medial prefrontal cortex. The present study was conducted to reveal the projection sites of IL and PL of the monkey, Macaca fuscata, using biotinylated dextran amine as an anterograde tracer. IL and PL projected densely to the ventromedial caudate nucleus, the core and shell of the nucleus accumbens (Acb), parvicellular lateral basal and magnocellular accessory basal nuclei of the amygdala, lateral preoptic area, ventromedial hypothalamic nucleus, tubero-mammillary nucleus (TM), medial part of the magnocellular and dorsal part of the parvicellular (MDpc) dorsomedial thalamic nuclei, reunience and medial part of the medial pulvinar nucleus, and dorso-lateral part of the periaqueductal gray (PAGdl) in the mesencephalon. Moderately to weakly projected areas were the intermediate and lateral parts of the agranular insular cortex, orbital part of area 12, agranular and dysgranular part of the temporal pole cortex (TPa-g), auditory temporal cortex, lateral and medial (MS) septal nuclei, bed nucleus of the stria terminalis, diagonal band of Broca, substantia innominata, and medial preoptic area, dorsomedial, lateral, and posterior hypothalamic nuclei, magnocellular lateral basal and lateral amygdaloid nuclei, paratenial, paraventricular (PV), inter-antero-medial (IAM), reticular, central medial (CeM), parafascicular (PF) and limitans nuclei of the thalamus, lateral habenular nucleus, pedunculo-pontine nucleus, dorsal part of the lateral lemniscal nucleus, ventral tegmental area (VTA), dorsal raphe, superior central nucleus, medial and lateral parabrachial nuclei (PBl) and nucleus locus coeruleus (LC). A few scattered terminals were observed in the perifornical nucleus of the hypothalamus and substantia nigra pars compacta. PL and area 24 were characterized by projections to the entorhinal (Ent) and piriform (Pir) cortex as well as to the magnocellular part of the ventral anterior thalamic nucleus (VAmc). The morphology of the terminal arborization in each nuclei was different in appearance, perhaps reflecting the synaptic interaction between the nerve terminals and postsynaptic dendrites. PL projected uniquely to Ent, Pir and VAmc and IL projected uniquely to TPa-g, MS, IAM, CeM, MDpc, PF, PBl and LC. IL projected more strongly than PL to the shell of Acb, amygdaloid nuclei, PV, TM, VTA and PAGdl. The present results support the hypothesis that IL is a major cortical autonomic motor area and PL integrates limbic and autonomic inputs in the primate.

Is heterosynaptic modulation essential for stabilizing Hebbian plasticity and memory?

In 1894, Ramón y Cajal first proposed that memory is stored as an anatomical change in the strength of neuronal connections. For the following 60 years, little evidence was recruited in support of this idea. This situation changed in the middle of the twentieth century with the development of cellular techniques for the study of synaptic connections and the emergence of new formulations of synaptic plasticity that redefined Ramón y Cajal's idea, making it more suitable for testing. These formulations defined two categories of plasticity, referred to as homosynaptic or Hebbian activity-dependent, and heterosynaptic or modulatory input-dependent. Here we suggest that Hebbian mechanisms are used primarily for learning and for short-term memory but often cannot, by themselves, recruit the events required to maintain a long-term memory. In contrast, heterosynaptic plasticity commonly recruits long-term memory mechanisms that lead to transcription and to synpatic growth. When jointly recruited, homosynaptic mechanisms assure that learning is effectively established and heterosynaptic mechanisms ensure that memory is maintained.

Integrity of the mesocortical dopaminergic system is necessary for complete expression of in vivo hippocampal-prefrontal cortex long-term potentiation

The prefrontal cortex receives dopaminergic inputs from the ventral tegmental area and excitatory inputs from the hippocampus. Both afferent pathways target in close proximity dendritic spines of pyramidal cells in layer V-VI of the prefrontal cortex. In view of the prominent role of dopamine in cognitive functions we examined the effects of ventral tegmental area stimulation on the induction of long-term potentiation in the hippocampal-prefrontal cortex pathway of anesthetized rats. Stimulation of the ventral tegmental area at a frequency known to evoke dopamine overflow in the prefrontal cortex produces a long-lasting enhancement of the magnitude of the hippocampal-prefrontal cortex long-term potentiation. The role of dopamine was further examined by investigating the effects of prefrontocortical dopamine depletion induced by an electrolytic ventral tegmental area lesion. A significant correlation (r = 0.8; P < 0.001; n = 14) was obtained between cortical dopamine levels and cortical long-term potentiation amplitude, a depletion of more than 50% of cortical levels corresponding to a dramatic decrease in hippocampal-prefrontal cortex long-term potentiation. However, a recovery to normal long-term potentiation was observed 1 h after tetanic stimulation. In contrast to the effects on long-term potentiation, ventral tegmental area stimulation, when applied at low or high frequency, decreases the amplitude of the hippocampal-prefrontal cortex postsynaptic synaptic response. The present study demonstrates the importance of the integrity of the mesocortical dopaminergic system for long-term potentiation to occur in the hippocampal-prefrontal cortex pathway and suggests a frequency-dependent effect of dopamine on hippocampal-prefrontal cortex transmission.

Dopamine receptors and groups I and II mGluRs cooperate for long-term depression induction in rat prefrontal cortex through converging postsynaptic activation of MAP kinases

Tetanic stimuli to layer I-II afferents in rat prefrontal cortex induced long-term depression (LTD) of layer I-II to layer V pyramidal neuron glutamatergic synapses when tetani were coupled to bath application of dopamine. This LTD was blocked by the following metabotropic glutamate receptor (mGluR) antagonists coapplied with dopamine: (S)-alpha-methyl-4-carboxyphenylglycine (MCPG; group I and II antagonist), (RS)-1-aminoindan-1,5-dicarboxylic acid (AIDA; group I antagonist), or (RS)-alpha-methylserine-O-phosphate monophenyl ester (MSOPPE; group II antagonist). This suggests that the dopamine-facilitated LTD requires synaptic activation of groups I and II mGluRs during tetanus. LTD could also be induced by coupling tetani to bath application of groups I and II mGluR agonist (1S, 3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD). In the next series of experiments, coapplication of dopamine and 1S,3R-ACPD, but not application of either drug alone, consistently induced LTD without tetani or even single test stimuli during drug application, suggesting that coactivation of dopamine receptors and the mGluRs is sufficient for LTD induction. Immunoblot analyses with anti-active mitogen-activated protein kinases (MAP-Ks) revealed that D1 receptors, D2 receptors, group I mGluRs, and group II mGluRs all contribute to MAP-K activation in prefrontal cortex, and that combined activation of dopamine receptors and mGluRs synergistically or additively activate MAP-Ks. Consistently, LTD by dopamine + 1S, 3R-ACPD coapplication, as well as the two other forms of LTD (LTD by dopamine + tetani and LTD by 1S,3R-ACPD + tetani), was blocked by bath application of MAP-K kinase inhibitor PD98059. LTD by dopamine + 1S,3R-ACPD coapplication was also blocked by postsynaptic injection of synthetic MAP-K substrate peptide. Our results suggest that dopamine receptors and groups I and II mGluRs cooperate to induce LTD through converging postsynaptic activation of MAP-Ks.