Microcircuitry of posterior cingulate cortex in vitro: electrophysiology and laminar analysis using the current source density method

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.

Theta-activity in anterior cingulate cortex predicts task rules and their adjustments following errors

Accomplishing even simple tasks depend on neuronal circuits to configure how incoming sensory stimuli map onto responses. Controlling these stimulus-response (SR) mapping rules relies on a cognitive control network comprising the anterior cingulate cortex (ACC). Single neurons within the ACC convey information about currently relevant SR mapping rules and signal unexpected action outcomes, which can be used to optimize behavioral choices. However, its functional significance and the mechanistic means of interaction with other nodes of the cognitive control network remain elusive and poorly understood. Here, we report that core aspects of cognitive control are encoded by rhythmic theta-band activity within neuronal circuits in the ACC. Throughout task performance, theta-activity predicted which of two SR mapping rules will be established before processing visual target information. Task-selective theta-activity emerged particularly early during those trials, which required the adjustment of SR rules following an erroneous rule representation in the preceding trial. These findings demonstrate a functional correlation of cognitive control processes and oscillatory theta-band activity in macaque ACC. Moreover, we report that spike output of a subset of cells in ACC is synchronized to predictive theta-activity, suggesting that the theta-cycle could serve as a temporal reference for coordinating local task selective computations across a larger network of frontal areas and the hippocampus to optimize and adjust the processing routes of sensory and motor circuits to achieve efficient sensory-motor control.

Short-term synaptic plasticity in layer II/III of the rat anterior cingulate cortex

Recent in vivo electrophysiological studies in our laboratory demonstrated medial thalamus (MT) induced short-term facilitation in the middle layers of the anterior cingulate cortex (ACC). The aim of the present study was to investigate different forms of short-term plasticity (STP) in layer II/III of the ACC in an in vitro slice preparation. Extracellular field potentials in layer II/III consisting of an early component (fAP) and a late component (fPSP) were activated by electrical stimulation of the deep layers. The fPSP and intracellularly recorded excitatory post-synaptic potential (EPSP) could be facilitated by paired-pulse stimulation at a low frequency (0.033Hz, pulse interval 20-400ms). An initial facilitation and subsequent depression were obtained when high frequency (12.5, 25 and 50Hz) tetanus stimulations were applied to the ACC slice. A post-tetanic augmentation 30s in duration was also observed. The effects of tetanic stimulation were altered in the presence of an increased or a decreased calcium concentration. Application of omega-conotoxin GVIA (CTX) in normal calcium concentration conditions decreased overall responses during tetanic stimulation similar to reducing calcium exposure. However CTX application did not increase paired-pulse facilitation (PPF) as is seen under low calcium conditions. These results indicate that calcium is involved in the formation of certain features of STP in layer II/III of the ACC and that N-type calcium channels contribute to some, but not all, components of these plastic changes. Two-site electrical stimulation testing showed that two separate presynaptic inputs can produce short-term facilitation. Our findings implicate a post-synaptic mechanism in STP in layer II/III of the ACC.

Glutamate-mediated neuroplasticity in a limbic input to the hypothalamus

Emotionally-salient stressors are processed by cortical and limbic circuits that provide important regulatory input to the hypothalamic-pituitary-adrenal (HPA) axis. However, exposure to chronic or severe stress may cause disregulation of the axis and a variety of physiological and psychological symptoms. The mechanisms that underlie stress-induced alterations in HPA axis function are not well characterized, but one possibility is that severe stress causes plastic changes in limbic inputs to the hypothalamus. We examined plasticity within the bed nucleus of stria terminalis (BNST) and the hypothalamic paraventricular nucleus (PVN) with a stimulating electrode in the BNST and a recording electrode in the PVN. High-frequency BNST stimulation produced long-lasting suppression of evoked field potentials recorded from the PVN, and this effect was blocked by administration of MK-801. Accordingly, rapid glutamate-mediated neuroplasticity in the BNST to PVN neurocircuitry may contribute to plasticity in limbic regulation of the HPA axis.

Alzheimer's disease and proton magnetic resonance spectroscopy of limbic regions: a suggestion of a clinical-spectroscopic staging

OBJECTIVE

To compare magnetic resonance proton spectroscopic with clinical data and to propose a spectroscopic staging of Alzheimer's disease (AD).

METHOD

Subjects (n = 46), normals (12) and with AD (34), paired to age (CDR0-CDR3); AD diagnosis according to DSM-IV/NINCDS-ADRDA criteria; 1H-MRS with Signa Horizon LX-GE, 1.5T; single voxel at hippocampal region/HCR and posterior cingulate area/PCA.

RESULTS

Statistically significant decrease (p < 0.01) only of Naa/Cr--at HCR among the CDR0, CDR1+CDR2, and CDR3, and at PCA between CDR0 and CDR1+CDR2 in relation to CDR3.

CONCLUSION

The HCR is the first to show Naa reduction (CDR1). The PCA suffers later (CDR3). These values decline progressively according to the severity stages. Considering the disparities between the HCR and PCA it is possible to suggest a spectroscopic (metabolite) staging (MS) of AD, as follows: MS0 (-CDR0) = both normal HCR and PCA, MS1-2 (approximately CDR1-2) = abnormal HCR and normal PCA, and MS3 (approximately CDR3) = both abnormal HCR and PCA. These results make possible the early diagnosis, to follow the degenerative process throughout the course, and to suggest a spectroscopic staging related to the clinical stages of AD.

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.

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.

LTD, LTP, and the sliding threshold for long-term synaptic plasticity

LTD of synaptic transmission is a form of long-term synaptic plasticity with the potential to be as significant as LTP to both the activity-dependent development of neural circuitry and adult memory storage. In addition, interactions between LTP and LTD and the dynamic regulation of the gain of synaptic plasticity mechanisms are also very important. In particular, the computational ability of LTD to properly counterbalance LTP may be essential to maintaining synaptic strengths in the linear range, and to maximally sharpen the ability of synapses to compute and store frequency-based information about the phase relation between synapses. Experimental data confirm the presence of an activity-dependent "sliding threshold" with the expected properties. That is, when levels of neuronal activity are high, indicating circumstances increasing the likelihood of inducing LTP, compensatory changes cause the suppression of LTP and an enhanced likelihood of LTD. Conversely, we would predict that low levels of synaptic activity would shift the threshold in favor of greater LTP and less LTD, a hypothesis which has yet to be tested. The sliding threshold for LTP and LTD also has implications for underlying cellular mechanisms of both forms of long-term synaptic plasticity. If the thresholds for LTP and LTD are tightly and reciprocally co-regulated, that could imply that at least one component of LTD is a true depotentiation caused by reversal of a change mediating LTP. If so, the intuitively simplest hypothesis is that phosphorylation of AMPA glutamate receptors causes LTP of synaptic e.p.s.p.s, while dephosphorylation of the same site or sites causes depotentiation LTD. Of course, this hypothesis would refer only to a postsynaptic component of both LTP and LTD. There has been a recent report that, in neonatal rat hippocampus, a form of LTD that is expressed developmentally earlier than LTP appears to have a postsynaptic induction site, but is expressed as decreased presynaptic transmitter release (Bolshakov and Siegelbaum, 1994). Whether these properties will be retained as LTD matures is unknown, as is the likelihood that, if a component of LTP is expressed presynaptically, depotentiation of that presynaptic component can also occur. Equally unclear is the persistence of LTD relative to LTP. The few rigorous long-term anatomical studies available suggest that the latest phases of LTP may be expressed as changes in dendritic spine shapes and/or synaptic morphology. While heterosynaptic LTD has been reported to have a duration of weeks in vivo (Abraham et al., 1994), we do not know whether LTP-induced morphological changes that take many days to appear can be reversed in an activity-dependent manner. An important feature of the consolidation of memories may turn out to be the slow development of LTP that is resistant to reversal by LTD. While we still at an earlier stage in our understanding of the mechanisms underlying LTD compared to LTP, some things are becoming clear. LTD is induced by afferent neuronal activity that is relatively ineffective in exciting the postsynaptic cell--an "anti-hebbian" condition. This property, coupled with the hebbian properties of LTP and the dynamic nature of membrane conductances, necessarily confers upon synapses the ability to compute and store the results of a covariance function. However, the role of such a computation in processing and/or memory is unclear. In addition, LTD appears to require the activation of NMDA and metabotropic subtypes of glutamate receptors, release of Ca2+ from intracellular stores, and an increase in intracellular [Ca2+] that is lower than that necessary to induce LTP. The early evidence is consistent with some overlap of targets for modification by LTP and LTD, with some forms of LTD likely to be a reversal, or "depotentiation," of previous LTP, perhaps through dephosphorylation of AMPA receptors.

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

We used stimulation of corpus callosum (CAL) and the subiculo-cingulate tract (SCT), in an in vitro brain slice preparation, to study activity-dependent changes in synaptic efficacy in posterior cingulate cortex (PCC). SCT stimulation monosynaptically excites the apical dendrites of deep laminae (V-VI) pyramidal neurons, while CAL afferents drive these same cells via synapses on their basal dendrites. In contrast, most superficial laminae (II/III-IV) pyramids appear to be driven polysynaptically via ascending axonal collaterals of deep pyramids. In slices retaining these connectivities, we contrasted characteristics of synaptic plasticity in superficial vs deep laminae field and intracellular potentials evoked by conditioning stimuli given at frequencies of 100, 20, 8, 5 and 1 Hz. Tetanic stimulation (100 Hz) of SCT or CAL yielded homosynaptic long-term potentiation (LTP) of each pathway, while stimulus trains of 8-20 Hz did not. 1-5 Hz stimulation of SCT and CAL elicited homosynaptic long-term depression (LTD) of synaptic strength in each pathway. Associative LTD was induced by interleaving 5 Hz pulses to the SCT pathway with 100 Hz theta-burst stimulation of CAL, but was not induced when these stimulus loci were switched. Heterosynaptic non-associative LTD was also observed in the alternate pathway following tetanization of either SCT or CAL. In all cases, LTP and LTD were observed only in deep laminae recordings. In contrast, superficial records showed only paired-pulse facilitation and short-term post-tetanic potentiation. In in vivo experiments in anaesthetized rats, PCC responses to SCT stimulation were contrasted with responses to stimulation of anteroventral and anterodorsal thalamic nuclei (AV/AD). SCT-elicited field potentials closely resembled those evoked in the slice, with maximal amplitude tuned to the 4-8 Hz frequency band. AV/AD stimulation elicited field potentials which were not frequency tuned. Overall, these data suggest that the acute circuit properties of PCC superficial laminae, modulated by thalamic input and synaptic plasticity in deep laminae, can transform hippocampal synaptic inflow before relaying it to extracingulate targets.