Long-term potentiation and depression of synaptic transmission in rat posterior cingulate cortex

A Gradient of Hippocampal Inputs to the Medial Mesocortex

Memory-guided decisions depend on complex interactions between the hippocampus (HIPP) and medial mesocortical (MMC) regions, including the anterior cingulate (CG) and retrosplenial (RSC). The functional circuitry underlying these interactions is unclear. Using anatomy, electrophysiology, and optogenetics, we show that such circuitry is characterized by a functional-anatomical gradient. While the CG receives hippocampal excitatory projections originated in CA1 stratum pyramidale, the RSC additionally receives long-range inhibitory inputs from radiatum and lacunosum-moleculare. Such hippocampal projections establish bona fide synapses, with the RSC densely targeted on its superficial layers L1-L3 by a combination of inhibitory and excitatory synapses. We show that the MMC is targeted by dorsal-intermediate CA1 (diCA1) axons following a caudorostral gradient in which a dense, dual (excitatory/inhibitory), layer-specific projection is progressively converted in a sparse, excitatory, and diffuse projection. This gradient is reflected in higher oscillatory synchronicity between the HIPP and RSC in the awake-behaving animal, compatible with their known functional proximity and contrasting with that found in the CG.

Fast voltage-sensitive dye imaging of excitatory and inhibitory synaptic transmission in the rat granular retrosplenial cortex

Rodent granular retrosplenial cortex (GRS) has dense connections between the anterior thalamic nuclei (ATN) and hippocampal formation. GRS superficial pyramidal neurons exhibit distinctive late spiking (LS) firing property and form patchy clusters with prominent apical dendritic bundles. The aim of this study was to investigate spatiotemporal dynamics of signal transduction in the GRS induced by ATN afferent stimulation by using fast voltage-sensitive dye imaging in rat brain slices. In coronal slices, layer 1a stimulation, which presumably activated thalamic fibers, evoked propagation of excitatory synaptic signals from layers 2-4 to layers 5-6 in a direction perpendicular to the layer axis, followed by transverse signal propagation within each layer. In the presence of ionotropic glutamate receptor antagonists, inhibitory responses were observed in superficial layers, induced by direct activation of inhibitory interneurons in layer 1. In horizontal slices, excitatory signals in deep layers propagated transversely mainly from posterior to anterior via superficial layers. Cortical inhibitory responses upon layer 1a stimulation in horizontal slices were weaker than those in the coronal slices. Observed differences between coronal and horizontal planes suggest anisotropy of the intracortical circuitry. In conclusion, ATN inputs are processed differently in coronal and horizontal planes of the GRS and then conveyed to other cortical areas. In both planes, GRS superficial layers play an important role in signal propagation, which suggests that superficial neuronal cascade is crucial in the integration of multiple information sources.NEW & NOTEWORTHY Superficial neurons in the rat granular retrosplenial cortex (GRS) show distinctive late-spiking (LS) firing property. However, little is known about spatiotemporal dynamics of signal transduction in the GRS. We demonstrated LS neuron network relaying thalamic inputs to deep layers and anisotropic distribution of inhibition between coronal and horizontal planes. Since deep layers of the GRS receive inputs from the subiculum, GRS circuits may work as an integrator of multiple sources such as sensory and memory information.

The cannabinoid system in the retrosplenial cortex modulates fear memory consolidation, reconsolidation, and extinction

Despite the fact that the cannabinoid receptor type 1 (CB1R) plays a pivotal role in emotional memory processing in different regions of the brain, its function in the retrosplenial cortex (RSC) remains unknown. Here, using contextual fear conditioning in rats, we showed that a post-training intra-RSC infusion of the CB1R antagonist AM251 impaired, and the agonist CP55940 improved, long-term memory consolidation. Additionally, a post-reactivation infusion of AM251 enhanced memory reconsolidation, while CP55940 had the opposite effect. Finally, AM251 blocked extinction, whereas CP55940 facilitated it and maintained memory extinguished over time. Altogether, our data strongly suggest that the cannabinoid system of the RSC modulates emotional memory.

Facilitation of synaptic transmission in the anterior cingulate cortex in viscerally hypersensitive rats

Electrophysiological studies have shown the enhanced response of anterior cingulate cortex (ACC) to colorectal distension in viscerally hypersensitive (VH) rats, which can be observed up to 7 weeks following colonic anaphylaxis, independent of colon inflammation, suggesting a mechanism for learning and triggering of pain memories in the ACC neuronal circuitry. Activity-dependent plasticity in synaptic strength may serve as a key mechanism that reflects cortical plasticity. However, only a few reports have indicated the synaptic plasticity of ACC in vivo. In the present study, electrophysiological recording showed long-lasting potentiation of local field potential in the medial thalamus (MT)-ACC synapses in VH rats. Theta burst stimulation in the MT reliably induced long-term potentiation in the MT-ACC pathway in normal rats, but was occluded in the VH state. Further, repeated tetanization of MT increased ACC neuronal activity and visceral pain responses of normal rats, mimicking VH rats. In conclusion, we demonstrated for the first time that visceral hypersensitivity is associated with alterations of synaptic plasticity in the ACC. The ACC synaptic strengthening in chronic visceral pain may engage signal transduction pathways that are in common with those activated by electrical stimulation, and serves as an attractive cellular model of functional visceral pain.

The postsubiculum and spatial learning: the role of postsubicular synaptic activity and synaptic plasticity in hippocampal place cell, object, and object-location memory

Visual landmarks exert stimulus control over spatial behavior and the spatially tuned firing of place, head-direction, and grid cells in the rodent. However, the neural site of convergence for representations of landmarks and representations of space has yet to be identified. A potential site of plasticity underlying associations with landmarks is the postsubiculum. To test this, we blocked glutamatergic transmission in the rat postsubiculum with CNQX, or NMDA receptor-dependent plasticity with d-AP5. These infusions were sufficient to block evoked potentials from the lateral dorsal thalamus and long-term depression following tetanization of this input to the postsubiculum, respectively. In a second experiment, CNQX disrupted the stability of rat hippocampal place cell fields in a familiar environment. In a novel environment, blockade of plasticity with d-AP5 in the postsubiculum did not block the formation of a stable place field map following a 6 h delay. In a final behavioral experiment, postsubicular infusions of both compounds blocked object-location memory in the rat, but did not affect object recognition memory. These results suggest that the postsubiculum is necessary for the recognition of familiar environments, and that NMDA receptor-dependent plasticity in the postsubiculum is required for the formation of new object-place associations that support recognition memory. However, plasticity in the postsubiculum is not necessary for the formation of new spatial maps.

Short-term synaptic plasticity in the nociceptive thalamic-anterior cingulate pathway

Background

Although the mechanisms of short- and long-term potentiation of nociceptive-evoked responses are well known in the spinal cord, including central sensitization, there has been a growing body of information on such events in the cerebral cortex. In view of the importance of anterior cingulate cortex (ACC) in chronic pain conditions, this review considers neuronal plasticities in the thalamocingulate pathway that may be the earliest changes associated with such syndromes.

Results

A single nociceptive electrical stimulus to the sciatic nerve induced a prominent sink current in the layer II/III of the ACC in vivo, while high frequency stimulation potentiated the response of this current. Paired-pulse facilitation by electrical stimulation of midline, mediodorsal and intralaminar thalamic nuclei (MITN) suggesting that the MITN projection to ACC mediates the nociceptive short-term plasticity. The short-term synaptic plasticities were evaluated for different inputs in vitro where the medial thalamic and contralateral corpus callosum afferents were compared. Stimulation of the mediodorsal afferent evoked a stronger short-term synaptic plasticity and effectively transferred the bursting thalamic activity to cingulate cortex that was not true for contralateral stimulation. This short-term enhancement of synaptic transmission was mediated by polysynaptic pathways and NMDA receptors. Layer II/III neurons of the ACC express a short-term plasticity that involves glutamate and presynaptic calcium influx and is an important mechanism of the short-term plasticity.

Conclusion

The potentiation of ACC neuronal activity induced by thalamic bursting suggest that short-term synaptic plasticities enable the processing of nociceptive information from the medial thalamus and this temporal response variability is particularly important in pain because temporal maintenance of the response supports cortical integration and memory formation related to noxious events. Moreover, these modifications of cingulate synapses appear to regulate afferent signals that may be important to the transition from acute to chronic pain conditions associated with persistent peripheral noxious stimulation. Enhanced and maintained nociceptive activities in cingulate cortex, therefore, can become adverse and it will be important to learn how to regulate such changes in thalamic firing patterns that transmit nociceptive information to ACC in early stages of chronic pain.

Coupling of mesoscopic brain oscillations: recent advances in analytical and theoretical perspectives

Oscillatory brain activities have been traditionally studied in the context of how oscillations at a single frequency recorded from a single area could reveal functional insights. Recent advances in methodology used in signal analysis have revealed that cross-frequency coupling, within or between functional related areas, is more informative in determining the possible roles played by brain oscillations. In this review, we begin by describing the cellular basis of oscillatory field potentials and its theorized as well as demonstrated role in brain function. The recent development of mathematical tools that allow the investigation of cross-frequency and cross-area oscillation coupling will be presented and discussed in the context of recent advances in oscillation research based on animal data. Particularly, some pitfalls and caveats of methods currently available are discussed. Data generated from the application of examined techniques are integrated back into the theoretical framework regarding the functional role of brain oscillations. We suggest that the coupling of oscillatory activities at different frequencies between brain regions is crucial for understanding the brain from a functional ensemble perspective. Effort should be directed to elucidate how cross-frequency and area coupling are modulated and controlled. To achieve this, only the correct application of analytical tools may shed light on the intricacies of information representation, generation, binding, encoding, storage and retrieval in the brain.

Anterior thalamic lesions stop synaptic plasticity in retrosplenial cortex slices: expanding the pathology of diencephalic amnesia

Recent, convergent evidence places the anterior thalamic nuclei at the heart of diencephalic amnesia. However, the reasons for the severe memory loss in diencephalic amnesia remain unknown. A potential clue comes from the dense, reciprocal connections between the anterior thalamic nuclei and retrosplenial cortex, another region vital for memory. We now report a loss of synaptic plasticity [long-term depression (LTD)] in rat retrosplenial cortex slices months following an anterior thalamic lesion. The loss of LTD was lamina-specific, occurring only in superficial layers of the cortex and was associated with a decrease in GABA(A)-mediated inhibitory transmission. As retrosplenial cortex is itself vital for memory, this distal lesion effect will amplify the impact of anterior thalamic lesions. These findings not only provide novel insights into the functional pathology of diencephalic amnesia and have implications for the aetiology of the posterior cingulate hypoactivity in Alzheimer's disease, but also show how distal changes in plasticity could contribute to diaschisis.

Synaptic organization and input-specific short-term plasticity in anterior cingulate cortical neurons with intact thalamic inputs

The absence of a slice preparation with intact thalamocortical pathways has held back elucidation of the cellular and synaptic mechanisms by which thalamic signals are differentially transmitted to and processed in the anterior cingulate cortex (ACC). In this report we introduce an innovative mouse brain slice preparation in which it is possible to explore the electrophysiological properties of ACC neurons with intact long-distance inputs from medial thalamic (MT) nuclei by intracellular recordings; this MT-ACC neuronal pathway plays an integral role in information transmission. Biocytin-labeled fibers in a functional slice could be traced anterogradely or retrogradely from the MT via the reticular thalamic nuclei, striatum and corpus callosum to the cingulate cortical areas. Eighty-seven cells downstream of the thalamic projections in 49 slices were recorded intracellularly. Intracellular recordings in the ACC showed that thalamocingulate transmission involves both alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA)/kainate and N-methyl-D-aspartate (NMDA) subtypes of glutamate receptors. Thalamus-evoked responses recorded extracellularly in the ACC were activated and progressed along a deep-superficial-deep trajectory loop across the ACC layers. We observed enhanced paired-pulse facilitation and tetanic potentiation of thalamocingulate synapses, suggestive of input-specific ACC plasticity and selective processing of information relayed by thalamocingulate pathways. Furthermore, we observed differential responses of ACC neurons to thalamic burst stimulation, which underscores the importance of MT afferents in relaying sensory information to the ACC. This new slice preparation enables the contribution of MT-evoked ACC synaptic transmission to short-term plasticity in the neuronal circuitry underlying sensory information processing to be examined in detail.

Intracortical circuits in rat anterior cingulate cortex are activated by nociceptive inputs mediated by medial thalamus

We investigated the afferents and intracortical synaptic organization of the anterior cingulate cortex (ACC) during noxious electrical stimulation. Extracellular field potentials were recorded simultaneously from 16 electrodes spanning all layers of the ACC in male Sprague-Dawley rats anesthetized by halothane inhalation. Laminar-specific transmembrane currents were calculated with the current source density analysis method. Two major groups of evoked sink currents were identified: an early group (latency = 54.04 +/- 2.12 ms; 0.63 +/- 0.07 mV/mm(2)) in layers V-VI and a more intense late group (latency = 80.07 +/- 4.85 ms; 2.16 +/- 0.22 mV/mm(2)) in layer II/III and layer V. Multiunit activities were evoked mainly in layer V and deep layer II/III with latencies similar to that of the early and late sink groups. The evoked EPSP latencies of pyramidal neurons in layers II/III and V related closely with the sink currents. The sink currents were inhibited by intracortical injection of CNQX (1 mM, 1 microl), a glutaminergic receptor antagonist, and enhanced by intraperitoneal (5 mg/kg) and intracortical (10 microg/microl, 1 microl) injection of morphine, a mu-opioid receptor agonist. Paired-pulse depression was observed with interpulse intervals of 50 to 1,000 ms. High-frequency stimulation (100 Hz, 11 pulses) enhanced evoked responses in the ACC and evoked medial thalamic (MT) unit activities. MT lesions blocked evoked responses in the ACC. Our results demonstrated that two distinct synaptic circuits in the ACC were activated by noxious stimuli and that the MT is the major thalamic relay that transmits nociceptive information to the ACC.

Short-term facilitation in the anterior cingulate cortex following stimulation of the medial thalamus in the rat

The present study examined the distribution and localization of synaptic activities (field potentials, multiunit activities and sink source currents) evoked in the anterior cingulate cortex (ACC) by electrical paired pulse stimulation of the ipsilateral medial thalamus (MT). Male Sprague-Dawley rats were anesthetized with halothane (1.0-1.5%), and electrical paired pulses stimuli (100-300 microA, inter-pulse interval, 100 ms) were delivered to the MT. Tungsten microelectrodes and a multichannel Michigan probe were used to record the evoked field potentials and multiunit activities in the ACC. Paired pulse stimulation facilitated field potentials and multiunit activities elicited from several MT nuclei. The second component of the negative field potential (com2) was augmented to about 2.5 times that of the first component (com1), and the integrated multiunit activities were facilitated by about 1.6-fold. Paired stimulation produced an expansion of the maximal negative potential from layer II/III into the deeper layers of the cingulate cortex area 1 (Cg1). Furthermore, the potentiated activity spread into adjacent secondary motor cortex (M2) and prelimbic cortex (PrL). Meanwhile, the area covered by the maximal integrated multiunit activities expanded from layer V (com1) to layers II-V (com2) in M2, Cg1 and PrL. The current source density (CSD) analysis revealed that the short latency sinks were located in layer II/III and layer V/VI. The sink currents were potentiated and expanded to more superficial and to deeper layers when a second pulse was delivered with a 100-ms time delay. Sink currents and the paired pulse facilitation (PPF) were reduced by morphine treatment (5 mg/kg, i.v.), and this effect could be blocked by naloxone. Electrical stimulation at 10 Hz in the MT induced more pronounced c-fos immunolabeling of neurons in the medial prefrontal cortex than did 1-Hz stimulation. The short-term facilitation occurred in the middle layers and expanded to the deeper layers of the ACC. These changes may mediate the effective signal transference in the specific frequency associated with painful responses.

The effects of chronic administration of quetiapine on the phencyclidine-induced reference memory impairment and decrease of Bcl-XL/Bax ratio in the posterior cingulate cortex in rats

Quetiapine, a new atypical antipsychotic drug, effectively alleviates positive and negative symptoms, as well as cognitive impairment that may be caused by neurodegeneration, in schizophrenia patients. Earlier in vivo and in vitro studies have demonstrated that quetiapine may be a neuroprotectant. The present study was designed to examine the beneficial effects of quetiapine on the possible cognitive impairment and changes of brain apoptotic regulation proteins induced by phencyclidine (PCP) in rats. Rats were treated with quetiapine (10 mg/kg/day; intraperitoneal (i.p.)) or vehicle for 16 days. On day 14, 1 h after the administration of quetiapine, the rats were given PCP (50 mg/kg; subcutaneous (s.c.)) or vehicle. Then quetiapine was administrated for an additional 2 days. One day after the last quetiapine injection (3 days after the PCP injection), the rats were trained on a spatial memory task in a radial arm maze. After the behavioural test, the rats were decapitated for Western blot analysis. PCP induced reference memory impairment, and a decrease of the ratio of an anti-apoptotic Bcl-2 family member (Bcl-XL) to a pro-apoptotic analogue (Bax) in the posterior cingulate cortex. Chronic administration of quetiapine counteracted the PCP-induced reference memory impairment and decrease of Bcl-XL/Bax ratio in the posterior cingulate cortex. These results suggest that quetiapine may have ameliorating effects on the cognitive impairment and brain apoptotic processes induced by PCP.

Theta oscillation in the human anterior cingulate cortex during all-night sleep: an electrocorticographic study

Ten epileptic patients each with subdural electrodes surgically attached to the anterior cingulate cortex (ACC; two cases), the orbitofrontal cortex (OFC; seven cases), or both (one case) were included in this study. We recorded each patient's ACC or OFC electrocorticogram (ECoG) during the time period that the patient was awake and naturally asleep. We performed a Fast Fourier Transformation (FFT) power spectral analysis on each ECoG to examine its frequency component. We found that the ACC showed regular and continuous theta oscillation (5-7Hz) during wakefulness and rapid eye movement (REM) sleep, but not during slow wave sleep. Theta waves observed in REM sleep were not as distinct as those found in wakefulness. We also discovered that the orbitofrontal signals represented spectral peaks in the theta band only during wakefulness. This suggests the coexistence of theta oscillation in the ACC. Considering our previous observations of gamma and beta oscillations in the human hippocampus, we hypothesize that the human limbic system manifests two oscillatory activities. The results obtained in this study suggest that electrophysiological activity in the ACC could be related to particular psychological functions in wakefulness and in REM sleep. These results are useful in elucidating the human brain mechanism.

Long-term potentiation and evoked spike responses in the cingulate cortex of freely mobile rats

Long-term potentiation of synaptic efficiency is regarded as a major candidate for the role of the physiological mechanism of long-term memory. However, the limited development of concepts of the cellular and subcellular characteristics of the induction of long-term potentiation in animals in conditions of free behavior does not correspond to the importance of this question. The present study was undertaken to determine whether the characteristics of potentiation in the cingulate cortex in response to stimulation of fibers of the subiculo-cingulate tract are truly long-term, i.e., develop through all known phases and last at least 24 h, in freely moving animals. In addition, the study aims included identification of the effects of application of blockers of different types of glutamate receptors on the development of long-term potentiation and identification of the characteristics of spike responses of single cingulate cortex neurons to stimulation of the subiculo-cingulate tract. Long-term potentiation, lasting more than 24 h, was obtained in freely moving adult rats not treated with GABA blockers. Injection of glutamate NMDA synapse blockers led to significant decreases in evoked cingulate cortex potentials in response to test stimulation. Activatory short-latency spike responses were characterized by a low probability of spike generation, and this increased with increases in the stimulation current. These data demonstrated that it is methodologically possible to compare, in freely moving rats, the involvement of individual neurons in the mechanisms involved in learning one or another type of adaptive behavior and the dynamics of their evoked spike activity during the formation of long-term potentiation.

Potentiation of local field potentials in the anterior cingulate cortex evoked by the stimulation of the medial thalamic nuclei in rats

The limbic thalamus and cingulate cortex are essential components in mediating the affective component of pain responses. In the present study, we examined the excitatory properties of medial thalamus (MT)-evoked field potentials in the anterior cingulated cortex (ACC). We also examined the effects of paired pulses and brief tetanic stimuli of the MT. The aim of this study was to determine whether nociceptive inputs to medial thalamic afferents cause plastic changes in the ACC. In alpha-chloralose (50 mg/kg, i.v.) anaesthetized rats, tungsten microelectrodes were used to stimulate the MT and to record field potentials in the ACC. The locations of MT were identified by searching and examining their responses to peripheral noxious stimuli. Early negative (about 4.7 ms latency) and late positive (about 11.7 ms) potentials could be evoked in the ACC by MT stimuli. The evoked field potentials were potentiated by prepulse stimulation. Maximal paired pulse facilitation (509+/-51%) was produced in 80-150 ms interpulse intervals. Evoked field potentials were also potentiated (28.8+/-6.3% and 29.6+/-5.9%, respectively) by low (10 Hz/10 s) and high (100 Hz/2s/2x) frequency tetanic stimulation of the MT, with a duration maintained for about 90 s and 120 min, respectively. The potentiation of MT-evoked ACC potentials provides a neural basis for synaptic plasticity, which may be essential for the establishment of pain-initiated conditioning behavior and affective responses to noxious stimuli.

Effects of posttraining treatments in the posterior cingulate cortex on short- and long-term memory for inhibitory avoidance in rats

Adult male Wistar rats were bilaterally implanted with indwelling cannulae in the caudal region of the posterior cingulate cortex. After recovery, animals were trained in a step-down inhibitory avoidance task (3.0-s, 0.4-mA foot shock) and received, right after training, a 0.5-microl infusion of vehicle (phosphate-buffered saline, pH 7.4), of the GABA(A) receptor agonist muscimol (0.1 or 0.5 microg), of the cAMP-dependent protein kinase (PKA) stimulant Sp-cAMPS (0.1 or 0.5 microg), or of the PKA inhibitor Rp-cAMPS (0.1 or 0.5 microg). Animals were tested twice, 1.5 h and, again, 24 h after training, in order to examine the effects of these agents on short- and long-term memory, respectively. Muscimol (0.5 but not 0.1 microg) hindered retention for both short- and long-term memory (p <.05). Rp-cAMPS (0.1 or 0.5 microg) hindered retention for short-term memory (p <.05). In addition, these animals showed lower, but not significantly lower, latencies than controls in the test session for long-term memory (p >.10). A trend toward an amnesic effect on long-term memory was also observed after Sp-cAMPS infusion at 0.1 microg (p <.10). These results show that strong stimulation of GABAergic synapses in the caudal region of the rat posterior cingulate cortex right after training impairs short- and long-term memory (the latter less dramatically). The same occurs by inhibiting PKA activity with regard to STM and possibly to LTM.

Blockade of adenosine A1 receptors in the posterior cingulate cortex facilitates memory in rats

Male Wistar rats were bilaterally implanted with indwelling cannulae in the caudal region of the posterior cingulate cortex. After recovery, animals were trained in a step-down inhibitory avoidance task (3.0-s, 0.4-mA foot shock) and received, immediately after training, a 0.5-microl infusion of the adenosine A1 receptor agonist N6-cyclopentyladenosine (CPA; 1, 50 or 100 nM) or of the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 1, 25 or 50 nM). Animals were tested twice, 1.5 h and, again, 24 h after training, in order to examine the effects of these agents on short- and long-term memory, respectively. Only 50-nM DPCPX was effective in altering memory, promoting a facilitation. These results suggest that adenosine A1 receptors in the posterior cingulate cortex inhibit memory consolidation in a way that their blockade facilitates memory for inhibitory avoidance in rats.

Effects of inhibitory avoidance training and/or isolated foot-shock on ectonucleotidase activities in synaptosomes of the anterior and posterior cingulate cortex and the medial precentral area of adult rats

Compelling evidence has indicated the involvement of extracellular ATP and adenosine in the mechanisms of synaptic plasticity and memory formation. In the present study, adult rats were trained in a step-down inhibitory avoidance task (IA) or submitted to isolated foot-shock (IF) (0.4 mA) before measuring ectonucleotidase activities in the synaptosomes of the anterior and posterior cingulate cortex (AC and PC, respectively) and the medial precentral area (Fr2). IA increased ATP and ADP hydrolysis immediately after training in the synaptosomes of PC and AC, respectively, (P<0.05). Foot-shock (independent of occurring during IA or IF) increased ATP hydrolysis in synaptosomes of AC and Fr2 immediately after application and decreased AIP hydrolysis in AC 90 min after application (P<0.05). Foot-shock (independent of occurring during IA or IF) increased ATP hydrolysis in PC immediately and 90 min after application, and in Fr2, but only immediately after application (P<0.05). These results suggest that the ectonucleotidase pathway responds to a mild foot-shock in AC, PC and Fr2 and may be involved in memory consolidation of step-down inhibitory avoidance in the cingulate cortex.

Long-term depression: a cascade of induction and expression mechanisms

The aims of this paper are to provide a comprehensive and up to date review of the mechanisms of induction and expression of long-term depression (LTD) of synaptic transmission. The review will focus largely on homosynaptic LTD and other forms of LTD will be considered only where appropriate for a fuller understanding of LTD mechanisms. We shall concentrate on what are felt to be some of the most interesting recent findings concerning LTD in the central nervous system. Wherever possible we shall try to consider some of the disparities in results and possible reasons for these. Finally, we shall briefly consider some of the possible functional consequences of LTD for normal physiological function.

Laminar and temporal heterogeneity of NMDA/metabotropic glutamate receptor binding in posterior cingulate cortex

Both N-methyl-D-aspartate (NMDA) and quisqualate/AMPA-insensitive metabotropic glutamate (mGlu) receptors mediate plasticity induction in neocortex, but their interlaminar distribution in cortical microcircuits is largely unknown. We used (+)(3)H-MK801 and (3)H-glutamate binding plus saturating concentrations of NMDA, AMPA, and quisqualate to autoradiographically map NMDA and mGlu receptor sites by lamina in posterior cingulate cortex in adult male rats. Specific binding at NMDA receptor sites in laminae II/III and VI was significantly reduced in comparison to other laminae. Brains prepared from rats killed during dark phase of a 12h/12h light/dark cycle showed a mean 129% increase in overall (+)(3)H-MK801 binding versus light phase brains but retained reduced binding densities in laminae II/III and VI. In contrast to NMDA findings, specific binding at mGlu sites was consistently elevated during light phase in both laminae II/III and VI. Specific (3)H-glutamate binding in dark-phase brains showed an overall 147% increase versus light phase binding but did not retain significant interlaminar heterogeneity. Interpreted in accordance with our physiologically derived models of hippocampo-cortical microcircuitry, these results suggest that spatial and temporal variations in glutamate receptor distribution may play an important role in intracingulate neural processing of afferent input from hippocampus.

Differential effects of post-training muscimol and AP5 infusions into different regions of the cingulate cortex on retention for inhibitory avoidance in rats

Adult male Wistar rats were bilaterally implanted with indwelling cannulae in four different coordinates of the cingulate cortex: (1) the anterior cingulate (AC), (2) the rostral region of the posterior cingulate (RC), (3) the upper portion of the caudal region of the posterior cingulate (UC), and (4) the lower portion of the caudal region of the posterior cingulate (LC). After recovery, animals were trained in a step-down inhibitory avoidance task (3.0-s, 0.4-mA foot shock). Either immediately, or 90 or 180 min after training, animals received a 0.5-microl infusion of vehicle (phosphate buffer, pH 7.4), of muscimol (0.5 microg), or of AP5 (5.0 microg). Retention testing was carried out 24 h after training. Muscimol was amnestic when given into any of the three coordinates of the posterior cingulate cortex 90 min after training, and when given into LC immediately post-training. In addition, AP5 was amnestic when given into UC 90 min post-training, but not when given into any other region and/or at any other time. None of the treatments had any effect when given into AC. The results suggest that memory processing of the inhibitory avoidance task is regulated by the posterior but not by the anterior cingulate cortex, through muscimol-sensitive synapses, relatively late after training. AP5-sensitive synapses appear to play a very limited role in these processes, restricted to UC.

Input-and layer-dependent synaptic plasticity in the rat perirhinal cortex in vitro

The perirhinal cortex is crucially important in several forms of memory. Whilst it is important to understand the underlying mechanisms of this role in memory, little is known about the synaptic physiology or plasticity of this region of transitional cortex. In this study, we recorded evoked field potentials in superficial layers (approximately layer I) of the perirhinal cortex in vitro. One stimulating electrode was placed on the temporal side and the other on the entorhinal side of the rhinal sulcus in either the superficial or intermediate layers (approximately layers II/III). Paired stimuli resulted in depression of the second response. Paired-pulse depression was maximal at a 200-ms interpulse interval. Low-frequency stimulation resulted in synaptic depression, which returned to baseline within 60 min. The magnitude of both paired-pulse depression and low-frequency stimulation-induced depression was significantly greater at synapses activated from the temporal intermediate pathway than the other three pathways. Long-term potentiation, stable for at least 60 min, was induced by high-frequency stimulation of intermediate but not superficial pathways. Long-lasting depression (depotentiation) was induced by low-frequency stimulation following the induction of long-term potentiation. The induction of both long-term potentiation and depotentiation was N-methyl-D-aspartate receptor dependent. The group I/II metabotropic glutamate receptor antagonist (S)-alpha-methyl-4-carboxyphenylglycine was without effect on either of these forms of plasticity. Thus, both long- and short-lasting forms of synaptic plasticity exist at synapses in the perirhinal cortex, and these may mediate the changes in neuronal responses associated with visual recognition memory.

Multiple behavioral anomalies in GluR2 mutant mice exhibiting enhanced LTP

We have previously disrupted the ionotropic glutamate receptor type 2 gene (GluR2) using gene targeting in embryonic stem cells and generated mice which lacked the GluR2 gene product. Neurophysiological analyses of these mice showed a markedly enhanced long-term potentiation (LTP) and a 9-fold increase in kainate induced Ca2+ permeability in the hippocampus. Here, we analyze the behavioral and neuroanatomical consequences of GluR2 deficiency in homozygous null mutant and age-matched littermate control mice. We show that despite unaltered gross brain morphology, several aspects of behavior were abnormal in the mutants. Object exploration, rearing, grooming and locomotion were altered in the novel arena. Eye-closure reflex, motor performance on the rotating rod and spatial and non-spatial learning performance in the water maze were also abnormal in the mutants. These abnormalities together with the widespread expression pattern of GluR2 in most excitatory CNS pathways suggest that the absence of GluR2 leads to neurological phenotypes associated with not only the hippocampus but several other brain regions potentially including the cortex and cerebellum. We speculate that GluR2 mutant mice suffer from an overall non-specifically increased excitability that may alter cognitive functions ranging from stimulus processing to motivation and learning.

Potential for multiple mechanisms, phenomena and algorithms for synaptic plasticity at single synapses

Recent experimental evidence indicates that in the neocortex, the manner in which each synapse releases neurotransmitter in response to trains of presynaptic action potentials is potentially unique. These unique transmission characteristics arise because of a large heterogeneity in various synaptic properties that determine frequency dependence of transmission such as those governing the rates of synaptic depression and facilitation. A theoretical analysis was therefore undertaken to explore the phenomenologies of changes in the values of these synaptic parameters. The results illustrate how the change in any one of several synaptic parameters produces a distinctive effect on synaptic transmission and how these distinctive effects can point to the most likely biophysical mechanisms. These results could therefore be useful in studies of synaptic plasticity in order to obtain a full characterization of the phenomenologies of synaptic modifications and to isolate potential biophysical mechanisms. Based on this theoretical analysis and experimental data, it is proposed that there exists multiple mechanisms, phenomena and algorithms for synaptic plasticity at single synapses. Finally, it is shown that the impact of changing the values of synaptic parameters depends on the values of the other parameters. This may indicate that the various mechanisms, phenomena and algorithms are interlinked in a 'synaptic plasticity code'.

Long-term plasticity in cingulate cortex requires both NMDA and metabotropic glutamate receptor activation

We tested whether induction of homosynaptic long-term potentiation and long-term depression of synaptic strength in posterior cingulate cortex requires NMDA and/or metabotropic glutamate (mGlu) receptor activation. In in-vitro slices of rat posterior cingulate cortex, the NMDA receptor antagonist D-2-amino-5-phosphonopentanoic acid (D-AP5; 15-20 microM) blocked induction of both long-term potentiation and long-term depression of mono- and polysynaptic population potentials in deep laminae. In contrast, DL-2-amino-3-phosphonopropionic acid (DL-AP3; 15-25 microM), a selective mGlu receptor antagonist, blocked homosynaptic long-term potentiation and long-term depression of monosynaptic transmission, but was ineffective in blocking the induction of either type of plasticity at polysynaptically-driven sites. The selective mGlu receptor agonist, trans-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD), induced a marked depression of subicular-evoked monosynaptic potentials which reversed upon drug washout, but produced little depression of polysynaptic responses. We conclude that metabotropic glutamate receptor activation is necessary for the induction of long-term synaptic plasticity only at monosynaptic subiculo-cingulate terminals, while NMDA receptor activation is necessary for the induction of long-term potentiation/long-term depression of both mono- and polysynaptic pathways.

Repetitive applications of ATP potentiate potassium current by Ca2+/calmodulin kinase in cultured rat hippocampal neurons

ATP evoked whole-cell potassium currents in hippocampal neurons. The second application of ATP to the same cell potentiated the current amplitude to around 140% and the current potentiation was maintained by further applications. A calmodulin inhibitor, W-7, or a selective Ca2+/calmodulin-dependent protein kinase II (CaMKII) inhibitor, KN-62, inhibited the current potentiation, although a selective protein kinase C inhibitor, GF109203X, or a selective cAMP-dependent protein kinase inhibitor, H-89, had no effect. In addition, ATP enhanced intracellular free Ca2+ concentration, which may activate CaMKII, but this enhancement was blocked by repetitive applications. These results provide an indication that CaMKII may be involved in the current potentiation by repetitive applications of ATP.