Optical responses evoked by white matter stimulation in rat visual cortical slices and their relation to neural activities

Spatiotemporal properties of the action potential propagation in the mouse visual cortical slice analyzed by calcium imaging

The calcium ion (Ca(2+)) is an important messenger for signal transduction, and the intracellular Ca(2+) concentration ([Ca(2+)](i)) changes in response to an excitation of the cell. To reveal the spatiotemporal properties of the propagation of an excitatory signal with action potentials in the primary visual cortical circuit, we conducted a Ca(2+) imaging study on slices of the mouse visual cortex. Electrical stimulation of layer 4 evoked [Ca(2+)](i) transients around the stimulus electrode. Subsequently, the high [Ca(2+)](i) region mainly propagated perpendicular to the cortical layer (vertical propagation), with horizontal propagation being restricted. When the excitatory synaptic transmission was blocked, only weak and concentric [Ca(2+)](i) transients were observed. When the action potential was blocked, the [Ca(2+)](i) transients disappeared almost completely. These results suggested that the action potential contributed to the induction of the [Ca(2+)](i) transients, and that excitatory synaptic connections were involved in the propagation of the high [Ca(2+)](i) region in the primary visual cortical circuit. To elucidate the involvement of inhibitory synaptic connections in signal propagation in the primary visual cortex, the GABA(A) receptor inhibitor bicuculline was applied. In this case, the evoked signal propagated from layer 4 to the entire field of view, and the prolonged [Ca(2+)](i) transients were observed compared with the control condition. Our results suggest that excitatory neurons are widely connected to each other over the entire primary visual cortex with recurrent synapses, and inhibitory neurons play a fundamental role in the organization of functional sub-networks by restricting the propagation of excitation signals.

VSDI: a new era in functional imaging of cortical dynamics

During the last few decades, neuroscientists have benefited from the emergence of many powerful functional imaging techniques that cover broad spatial and temporal scales. We can now image single molecules controlling cell differentiation, growth and death; single cells and their neurites processing electrical inputs and sending outputs; neuronal circuits performing neural computations in vitro; and the intact brain. At present, imaging based on voltage-sensitive dyes (VSDI) offers the highest spatial and temporal resolution for imaging neocortical functions in the living brain, and has paved the way for a new era in the functional imaging of cortical dynamics. It has facilitated the exploration of fundamental mechanisms that underlie neocortical development, function and plasticity at the fundamental level of the cortical column.

Monocarboxylate transporters contribute to the adaptation of neuronal activity to repeated glucose deprivation in the rat lateral septal nucleus

Using optical recording methods in the rat lateral septal nucleus (LSN) slice, we examined the question of whether antecedent hypoglycemia protects neurons from the adverse effects of subsequent hypoglycemic stimuli. The first exposure of LSN neurons to glucose deprivation for 15 min produced a marked depression of neuronal activity. The neuronal activity recovered by reapplication of glucose to the neurons. In neurons previously exposed to glucose deprivation, subsequent applications of glucose-free artificial cerebrospinal fluid (ACSF) produced only weak depression of the neuronal activity. The contribution of monocarboxylate transporters to this adaptation of neuronal activity to repeated glucose deprivation was examined in LSN neurons. alpha-Cyano-4-hydroxycinnamate (4-CIN, 100 microM), an inhibitor of the monocarboxylate transporters, did not significantly affect the depression of the neuronal activity induced by the first glucose deprivation. However, in the presence of 4-CIN (100 microM), a second glucose deprivation produced a typical depression of the neuronal activity, indicating that 4-CIN had nullified the adaptation of neuronal activity to a second glucose deprivation. Cytochalasin B (CCB, 20 microM), an inhibitor of glucose transporters, depressed the neuronal activity in the presence of 11 mM glucose. Pyruvate (11 mM) partially restored the neuronal activity depressed by pretreatment with CCB (20 microM) for 30-40 min. These results suggest that antecedent glucose deprivation stimulates monocarboxylate-transporters to supply energy substrates to LSN neurons, thus protecting the neurons against subsequent glucose deprivation. .

Anatomical pathways and molecular mechanisms for plasticity in the barrel cortex

The barrel cortex has yielded a wealth of information about cortical plasticity in recent years. Barrel cortex is one of the few cortical areas studied so far where plasticity can be examined from birth through to adulthood. This review looks at plasticity mechanisms in three periods of life: early post-natal development, adolescence and adulthood. Separate consideration is given to depression and potentiation mechanisms. Plasticity can be induced in barrel cortex by whisker deprivation. Single whisker experience leads to expansion of the area of cortex responding to the spared whisker. In early post-natal life, plasticity occurs in thalamocortical pathways, while later in adolescence, intracortical pathways become more important. Ablation of the spared whisker's barrel prevents expression of plasticity in the cortex. A row of lesions between the spared and an adjacent barrel prevents expression of plasticity in the adjacent barrel. This evidence, together with latency of response data and an analysis of pathways capable of inducing long-term potentiation (LTP) within barrel cortex, leads to the view that horizontal and/or diagonal pathways between barrels are responsible for plasticity expression. The mouse has become the most commonly mutated mammalian species and has a well-developed barrel cortex. Therefore, mutations can be used to study the role of particular molecules in experience-dependent plasticity of barrel cortex. Through this work, it has become clear that the major post-synaptic density protein, alpha-CaMKII, and its T286 autophosphorylation site are essential for experience-dependent plasticity. This points to a major role for excitatory transmission in cortical plasticity and raises the possibility that LTP like mechanisms are involved. Furthermore, transgenic mice carrying a reporter gene for CRE have provided evidence that CRE-mediated gene expression is also involved in barrel cortex plasticity. This view is supported by studies on alpha/delta CREB knockouts, and provides a starting point for studying the role of gene expression in experience-dependent cortical plasticity.

Effects of temperature increase on the propagation of presynaptic action potentials in the pathway between the Schaffer collaterals and hippocampal CA1 neurons

Effects of temperature increase on the neuronal activity of hippocampal CA2-CA1 regions were examined by using optical and electrophysiological recording techniques. Stimulation of the Schaffer collaterals at the CA2 region evoked depolarizing optical signals that spread toward the CA1 region at 32 degrees C. The optical signal recorded by 49 pixels was characterized by fast and slow components that were closely related to presynaptic action potentials and excitatory postsynaptic responses, respectively. The optical signal was depressed by temperature increase to 38-40 degrees C. The temperature increase to 38 degrees C produced a hyperpolarization and a depression of the excitatory postsynaptic potential (EPSP) in single hippocampal CA1 pyramidal neurons. The depression of the neuronal activity induced by temperature increase was attenuated by application of glucose (22 mM) or pyruvate (22 mM). Adenosine (200 microM) did not block the presynaptic action potential but strongly depressed the excitatory postsynaptic response. 8-Cyclopentyl-1,3-dimethylxanthine (8-CPT) (10 microM), an antagonist for adenosine A(1) receptors, attenuated the depression of the excitatory postsynaptic response but not the inhibition of the presynaptic action potential at 38 degrees C. These results suggest that adenosine mediates the high-temperature-induced depression of the excitatory synaptic transmission but not that of action potential propagation in rat CA1 neurons.

Improved data processing for optical imaging of developing neuronal connectivity in the neonatal mouse barrel cortex

Optical recording methods using voltage-sensitive dyes have proven valuable for the analysis of neuronal networks both in vivo and in vitro. This technique detects membrane potential changes as changes in the absorption or fluorescence of voltage-sensitive dyes incorporated into the cellular plasma membranes. The reliability of the optical recording technique is dependent on the dye-related response being fast enough to follow the electrical activity and of the response being more or less proportional to the amplitude of the membrane potential change. A high spatial resolution can be achieved using an appropriate imaging system and a dye with a response of sufficiently high signal-to-noise ratio. Thus, it is now anticipated that this method will be able to shed more light on the spatio-temporal information processing of neocortical circuitry. While the FUJIX HR Deltaron 1700 optical imaging system offers a reasonably high time (0.6 ms) and space-resolution (7 microm at 10x magnification), one drawback of this system, however, is its relatively poor data processing capabilities. We have therefore developed a protocol to improve the signal-to-noise ratio by modifying the calculation algorithm of the optical data. Consequently, we characterized optical responses in thalamocortical slices to find developmental landmarks of thalamocortical and intracortical connectivity in the neonatal mouse barrel cortex. Successful application of this method has been published on the analysis of thalamocortical glutamatergic connectivity [8].

5-Hydroxytryptamine facilitates spatiotemporal propagation of optical signals in the hippocampal-septal pathway

The role of 5-hydroxytryptamine (5-HT) on the propagation of neuronal excitation in the hippocampal-septal pathway was examined in a brain slice by optical and electrophysiological recording techniques. After electrical stimulation of the fimbrial pathway, optical signals first occurred at the caudal region of lateral septal nucleus (LSN), then propagated toward the rostral region of LSN. All of the evoked optical signals were blocked by tetrodotoxin (TTX). The optical signal that propagated to the LSN was blocked by either the removal of external Ca(2+) or bath-application of 6-cyano-7-nitroquinoxaline-2,3-(1H,4H)-dione (CNQX). Bath-application of 5-HT (1-50 microM) to the LSN for 10 min produced an increase in the propagation area of the optical signal and prolonged the falling phase of the optical signal. Bicuculline blocked the 5-HT-induced facilitation of the optical signal. 8-Hydroxy-di-n-propylamino tetralin (8-OH-DPAT), a selective 5-HT(1A) agonist, mimicked the facilitation of 5-HT. 1-(2-Methoxyphenyl)-4-(4-phthalimidobutyl)piperazine (NAN-190), a 5-HT(1A) antagonist, blocked the facilitation induced by 5-HT. 5-HT enhanced the amplitude of the field potential in septal slices, where the optical signals had been enhanced. These results indicate that 5-HT increases the efficacy of excitatory synaptic transmission in the hippocampal-septal circuit via 5-HT(1A) receptors of LSN neurons.

Cortex-restricted disruption of NMDAR1 impairs neuronal patterns in the barrel cortex

In the rodent primary somatosensory cortex, the configuration of whiskers and sinus hairs on the snout and of receptor-dense zones on the paws is topographically represented as discrete modules of layer IV granule cells (barrels) and thalamocortical afferent terminals. The role of neural activity, particularly activity mediated by NMDARs (N-methyl-D-aspartate receptors), in patterning of the somatosensory cortex has been a subject of debate. We have generated mice in which deletion of the NMDAR1 (NR1) gene is restricted to excitatory cortical neurons, and here we show that sensory periphery-related patterns develop normally in the brainstem and thalamic somatosensory relay stations of these mice. In the somatosensory cortex, thalamocortical afferents corresponding to large whiskers form patterns and display critical period plasticity, but their patterning is not as distinct as that seen in the cortex of normal mice. Other thalamocortical patterns corresponding to sinus hairs and digits are mostly absent. The cellular aggregates known as barrels and barrel boundaries do not develop even at sites where thalamocortical afferents cluster. Our findings indicate that cortical NMDARs are essential for the aggregation of layer IV cells into barrels and for development of the full complement of thalamocortical patterns.

Brain-derived neurotrophic factor requirement for activity-dependent maturation of glutamatergic synapse in developing mouse somatosensory cortex

The maturation of cortical circuitry critically depends on experience. Recently, a model of silent synapse has been proposed as a mechanism of activity-mediated transition of immature synapse to mature synapse. It is not clear, however, how activity could regulate this transition. Here, we show the evidence that endogenous brain-derived neurotrophic factor (BDNF) is required for the maturation of glutamatergic synapse in developing mouse somatosensory cortex. Field potential recordings of thalamocortical glutamatergic synaptic activity with brain slices from the BDNF mutant mice showed that AMPA receptor responses are low, but NMDA receptor responses remain high in layer 4, thus, the relative contribution of AMPA receptor response is significantly lower compared to the age-matched wild-type mouse. Furthermore, optical images of development of thalamocortical connectivity with a voltage-sensitive dye showed that NMDA receptor-dominant synapse is established first in layer 4 and layer 5/6 then AMPA receptor response appears later in concomitant with reduction of NMDA receptor response in layer 4 and that the maturation of the silent synapse is impaired in the BDNF mutant mice. In layer 5/6, NMDA receptor response was suppressed without upregulation of AMPA receptor response. This process also required BDNF function. Interestingly, whisker-trimming of the wild-type mouse from just after birth showed quite similar results with the homozygous mutant of their whiskers left intact. Therefore, we would propose that BDNF is a critical mediator for the maturation of glutamatergic synapse in developing mouse somatosensory cortex.

Acetylcholine suppresses the spread of excitation in the visual cortex revealed by optical recording: possible differential effect depending on the source of input

Optical recording with a voltage-sensitive dye was performed in visual cortical slices of the rat to determine the effect of acetylcholine (ACh) on the spread of excitation. In the presence of ACh, the spread of excitation initiated by stimulation at the white matter/layer VI (WM/VI) was greatly suppressed throughout the cortex, with less suppression in the middle layers. By comparing the effect of ACh with that of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), the fraction of the synaptic component that was sensitive to ACh was evaluated. ACh suppressed approximately 40-50% (maximum 55.8%, n = 11) of the initial synaptic component in the superficial and deep layers. In the middle, however, the effect was weakest and only approximately 20-30% (minimum 20.9%, n = 11) of the initial synaptic component was suppressed. On the basis of histological analysis, the region with the weakest ACh effect extended from upper V to lower II/III. To identify the site of ACh action in terms of pre- versus postsynaptic localization, exogenous glutamate was applied. Because ACh did not suppress the excitation induced by glutamate, the site of the ACh action was indicated to be presynaptic. When layer II/III was stimulated instead of WM/VI, the suppression was uniform throughout the cortex. A muscarinic receptor antagonist, atropine, blocked the suppression by ACh. In conclusion, our results indicate the following two points. First, ACh strongly suppresses intracortical connectivity through presynaptic muscarinic receptors. Secondly, in contrast to the intracortical connection, some group(s) of fibres, possibly thalamocortical afferents that arise from white matter and terminate in the middle cortical layers are suppressed much less by ACh. While ACh has been reported to have confusingly diverse effects, e.g. direct depolarization and hyperpolarization as well as synaptic facilitation and suppression, its effect on the propagation of excitation is very clear; suppression on intracortical connection, leaving thalamocortical inputs rather intact. We postulate that cholinergic innervation enables the afferent input to have a relatively dominant effect in the cortex.

Altered spatial patterns of functional thalamocortical connections in the barrel cortex after neonatal infraorbital nerve cut revealed by optical recording

In rodents, the somatosensory cortex has a cell aggregation cluster termed the barrel, reflecting a whisker vibrissa, and this barrel formation is disrupted by infraorbital nerve cut at birth. In the present study, we prepared thalamocortical slice preparations from rats that received infraorbital nerve cut either at birth or at postnatal day (P) 7 and those from normal rats, recorded the optical response reflecting neural excitation in the somatosensory cortex with a voltage-sensitive dye (RH482) and compared the optical responses from lesioned rats with those from normal rats. In normal rats at P10, the optical response elicited electrically by thalamic stimulation propagated to the cortex, and then several patchy clusters appeared in layer IV. The size and location of these patchy responses precisely matched either barrels identified by cytochrome oxidase staining or terminal arbors of thalamocortial axons stained with biotinylated dextran amine. In contrast, at P10 in P0-lesioned rats, clusters having a wider horizontal width but smaller amplitude than those seen in normal rats appeared in layer IV. Correspondingly, neither cytochrome oxidase staining nor biotinylated dextran amine labeling of thalamocortical axons showed any barrel-like clusters or glomerular axon terminals. Likewise, at P5-P6, the tangential width of clusters in layer IV were larger than that in normal rats. At P10 in P7-lesioned rats, small cluster-matched barrels were seen in the optical response as well as in normal rats. These results suggest that P0 infraorbital nerve cut interrupted segregation of functional synapses into the barrels and retarded the maturation of thalamocortical transmission.

Role of NMDA receptors in the propagation of excitation in rat visual cortex as studied by optical imaging

To examine the role of the N-methyl-D-aspartate (NMDA) type of glutamate receptors in the propagation of information in visual cortex, optical imaging with high spatial and temporal resolution of neuronal activity was used in cortical slices of rats. Single-shock stimulation of the white matter elicited a vertical propagation of excitation toward the cortical surface simultaneously with a horizontal spread of excitation in lower layers. The horizontal spread in upper layers occurred subsequent to the vertical spread reaching these layers. The results from perfusion of Ca2+-free medium and application of an antagonist of non-NMDA receptors indicated that this intracortical propagation of signals is due mostly, if not exclusively, to the postsynaptic excitation of cortical neurons. Blockade of NMDA receptors attenuated the rising and peak phases of the upper horizontal spread, but did not affect those of the lower horizontal or vertical propagation of excitation. Perfusion with Mg2+-free solution enhanced the upper horizontal spread, but in most cases did not significantly change the spread of excitation in the other pathways. These results indicate that NMDA receptors are involved in the flow of information in the upper layers of visual cortex, and further suggest that this propagation of activity is mediated mainly by horizontal connections intrinsic to the upper layers.

Modulation of intrinsic circuits by serotonin 5-HT3 receptors in developing ferret visual cortex

Serotonergic projections are widespread in the developing neocortex, but their functions are obscure. The effects of 5-HT3 receptor agonists on cortical circuit response properties were studied in slices of ferret primary visual cortex using high-speed optical imaging of voltage-sensitive dye signals and whole-cell patch-clamp recording. Activation of the 5-HT3 receptor decreased the amplitude and lateral extent of excitation throughout postnatal development. This effect peaks after eye opening, which indicates a function for serotonergic modulation of circuit responses during the period of refinement of cortical connections. Whole-cell patch-clamp recordings from single neurons revealed that synaptic responses evoked by white matter stimulation were reduced by 5-HT3 receptor agonists, whereas the frequency of spontaneous GABAergic synaptic currents was enhanced dramatically. This indicates that the modulation of spontaneous synaptic activity by fast-acting serotonin receptors is reflected in an inhibition of the circuit response, in line with the notion of background synaptic activity altering the spatiotemporal integration properties of cortical cells by changing their membrane potential and their electrotonic structure. These mechanisms may regulate the response properties of intrinsic circuits in both the adult and developing neocortex.

Delayed signal propagation via CA2 in rat hippocampal slices revealed by optical recording

Signal propagation from mossy fibers to CA1 neurons was investigated in rat hippocampal slices by a combination of electrical and optical recordings. The slices were prepared by oblique sectioning of the middle part of the hippocampus to preserve fiber connections. The mossy fibers were stimulated to induce population spikes (PSs) and excitatory postsynaptic potentials in the middle part of the CA1 region. Latencies of maximal PSs in CA1 varied widely among slices; they ranged from 7 to 13.5 ms, with two maxima at 9 and 11.5 ms. The fastest PSs probably are evoked by the Schaffer collaterals that connect the CA3 and CA1 regions in the well-known trisynaptic circuit. However, the slower PSs suggest the existence of additional delayed inputs. To determine the source of the delayed input, slices were stained with a voltage-sensitive dye, RH482, and the optical signals relevant to membrane potential changes were detected by a high-resolution optical imaging system. Optical recording of responses to mossy fiber stimulation indicated two distinct types of signal propagation from CA3 to CA1. In preparations evincing the fast type of propagation, signals spread to CA1 within 7.2 ms after the mossy fiber stimulation. During such propagation, activity flowed directly from CA3 to the stratum radiatum of CA1. Other preparations illustrated slow signal propagation, in which optical signals were generated in CA2 before spreading to CA1. During such slow signal transmission, activity persisted in CA2 and its surrounding area for 3 ms before propagating to the strata radiatum and oriens in CA1. In such cases, CA1 activity was detected within 10.8 ms of mossy fiber stimulation. In some slices, a mixture of the fast and slow propagation patterns was observed, indicating that these two transmission modes can coexist. Our data reveal that CA2 neurons can transmit delayed excitatory signals to CA1 neurons. We therefore conclude that consideration of electrical signal propagation through the hippocampus should include flow through the CA2 region in addition to the traditional dentate gyrus-CA3-CA1 trisynaptic circuit.