Rat Primary Visual Cortex Research Articles

Dark-rearing decreases NR2A N-methyl- d-aspartate receptor subunit in all visual cortical layers

Maturation of the visual cortex is a visual experience-dependent process. It has been shown that visual input triggers changes in N-methyl- d-aspartate receptor (NMDAR) subunit expression in the visual cortex. However, no data are available on the layer distribution of these molecular changes. Here we describe the laminar distribution of the cells expressing the NMDAR subunits NR2A and NR2B in the rat primary visual cortex at postnatal day (P) 21 and 37 using anti-NR2A and anti-NR2B antibodies and a stereological method to count labelled neurons. The percentage of neurons expressing the NR2A subunit in the layers II–VI remained unchanged between P21 and P37 with a slight decrease in layer V. Dark-rearing from P21 to P37 induced a pronounced decrease of the staining intensity and a significant decrease in the percentage of NR2A-expressing neurons. The changes in NR2A expression caused by dark rearing occur at similar levels in layers II–VI. The percentage of NR2B-positive cells in the different cortical layers remains unchanged from P21 to P37. The NR2B pattern was not significantly affected by dark-rearing. Thusly, the expression of NR2A depends upon visual experience after P21.

Neuronal activity regulates protein and gene expressions of GluR2 in postnatal rat visual cortical neurons in culture.

Ionotropic glutamate receptors, the principal excitatory neurotransmitter receptors in the CNS, are classified into NMDA and non-NMDA subtypes. Previously, we found a direct relationship between neuronal activity and NMDA receptor subunit 1 in rat primary neuronal cultures and monkey visual cortex. The present study focused on the relationship between neuronal activity and subunit 2 of AMPA glutamate receptor, GluR2. GluR2 controls Ca(2+) permeability of AMPA receptors, and the transcription of its gene is activated by nuclear respiratory factor 1, which also activates the transcription of a few subunit genes of cytochrome oxidase (CO). Primary neuronal cultures of postnatal rat visual cortex were subjected to impulse blockade with tetrodotoxin (TTX) for 6 days, or 20 mM KCl depolarizing treatment for 1, 2, 5, 10, 20, 30, and 40 hrs. After 20 hrs of KCl treatment, GluR2 immunoreactivity and CO activity were significantly increased above controls (P < 0.01), and both remained high at 30 and 40 hrs of treatment. However, GluR2 mRNA level as shown by in situ hybridization was already up-regulated above controls after 10 hrs of KCl treatment (P < 0.01) and remained elevated with longer periods of depolarization. TTX blockade, on the other hand, induced a significant down-regulation of GluR2 immunoreactivity, GluR2 gene expression as well as CO activity (P < 0.01 for all). Our results indicate that both protein and mRNA expressions of GluR2 in cultured visual cortical neurons are tightly controlled by neuronal activity.

Ovarian hormones after postnatal day 20 reduce neuron number in the rat primary visual cortex.

Previous work from our lab has documented a sex difference in neuron number in the binocular region of the adult rat primary visual cortex (Oc1B), with males having 19% more neurons than females. In the present study, the role of developmental steroid hormones in the formation of this difference was explored. Male and female rats underwent neonatal hormone manipulation (female + testosterone or dihydrotestosterone; male + flutamide) followed by gonadectomy on postnatal day 20. Animals that did not undergo hormone manipulation were either gonadectomized or sham operated at day 20. Neuron number was quantified in the monocular (Oc1M) and binocular (Oc1B) subfields of the adult rat primary visual cortex using the optical disector technique. As adults, day 20 gonadectomized females, as well as females + testosterone and females + dihydrotestosterone, had significantly more neurons than intact females. There was no difference in neuron number between postnatal day 20 gonadectomized males, males + flutamide, and intact males. Also, intact males had significantly more neurons than intact females in both in Oc1M and Oc1B. It appears that ovarian steroids after day 20 are the primary cause of the lower number of neurons in the primary visual cortex of the female rat.

Stimulus representation in rat primary visual cortex: multi-electrode recordings with micro-machined silicon probes and estimation theory

The study of neural population codes relies on massively parallel recordings in combination with theoretically motivated analysis tools. We applied two multi-site recording techniques to record from cells throughout cortical depth in a minimally invasive way. The feasibility of such experiments in area 17 of the anesthetized rat is demonstrated. Bayesian reconstruction and the interpretative framework of Fisher information are introduced. We demonstrate applicability and usefulness of Bayesian stimulus reconstruction and show that even small numbers of neurons can yield a high degree of representational accuracy under favorable conditions. Results are discussed and future lines of research outlined.

Axo-axonic synapses formed by somatostatin-expressing GABAergic neurons in rat and monkey visual cortex.

In cerebral cortex of rat and monkey, the neuropeptide somatostatin (SOM) marks a population of nonpyramidal cells (McDonald et al. [1982] J. Neurocytol. 11:809-824; Hendry et al. [1984] J. Neurosci. 4:2497:2517; Laemle and Feldman [1985] J. Comp. Neurol. 233:452-462; Meineke and Peters [1986] J. Neurocytol. 15:121-136; DeLima and Morrison [1989] J. Comp. Neurol. 283:212-227) that represent a distinct type of gamma-aminobutyric acid (GABA) -ergic neuron (Gonchar and Burkhalter [1997] Cereb. Cortex 7:347-358; Kawaguchi and Kubota [1997] Cereb. Cortex 7:476-486) whose synaptic connections are incompletely understood. The organization of inhibitory inputs to the axon initial segment are of particular interest because of their role in the suppression of action potentials (Miles et al. [1996] Neuron 16:815:823). Synapses on axon initial segments are morphologically heterogeneous (Peters and Harriman [1990] J. Neurocytol. 19:154-174), and some terminals lack parvalbumin (PV) and contain calbindin (Del Rio and DeFelipe [1997] J. Comp. Neurol. 342:389-408), that is also expressed by many SOM-immunoreactive neurons (Kubota et al. [1994] Brain Res. 649:159-173; Gonchar and Burkhalter [1997] Cereb. Cortex 7:347-358). We studied the innervation of pyramidal neurons by SOM neurons in rat and monkey visual cortex and examined putative contacts by confocal microscopy and determined synaptic connections in the electron microscope. Through the confocal microscope, SOM-positive boutons were observed to form close appositions with somata, dendrites, and spines of intracortically projecting pyramidal neurons of rat area 17 and pyramidal cells in monkey striate cortex. In addition, in rat and monkey, SOM boutons were found to be associated with axon initial segments of pyramidal neurons. SOM axon terminals that were apposed to axon initial segments of pyramidal neurons lacked PV, which was shown previously to label axo-axonic terminals provided by chandelier cells (DeFelipe et al. [1989] Proc. Natl. Acad. Sci. USA 86:2093-2097; Gonchar and Burkhalter [1999a] J. Comp. Neurol. 406:346:360). Electron microscopic examination directly demonstrated that SOM axon terminals form symmetric synapses with the initial segments of pyramidal cells in supragranular layers of rat and monkey primary visual cortex. These SOM synapses differed ultrastructurally from the more numerous unlabeled symmetric synapses found on initial segments. Postembedding immunostaining revealed that all SOM axon terminals contained GABA. Unlike PV-expressing chandelier cell axons that innervate exclusively initial segments of pyramidal cell axons, SOM-immunoreactive neurons innervate somata, dendrites, spines, and initial segments, that are just one of their targets. Thus, SOM neurons may influence synaptic excitation of pyramidal neurons at the level of synaptic inputs to dendrites as well as at the initiation site of action potential output.

Apical tuft input efficacy in layer 5 pyramidal cells from rat visual cortex.

1. The integration of synaptic inputs to the apical dendrite of layer 5 neocortical pyramidal cells was studied using compartment model simulations. The goal was to characterize the generation of regenerative responses to synaptic inputs under two conditions: (a) where there was an absence of background synaptic input, and (b) when the entire cell surface was subjected to a uniform blanket of synaptic background conductance such that somatic input resistance was reduced 5-fold. 2. Dendritic morphology corresponded to a layer 5 thick-trunked pyramidal cell from rat primary visual cortex at postnatal day 28 (P28), with distribution of dendritic active currents guided by the electrophysiological characteristics of the apical trunk reported in this cell type. Response characteristics for two dendritic channel distributions were compared, one of which supported Ca(2+) spikes in the apical dendrite. 3. In the absence of background, synaptic input to the apical tuft was surprisingly effective in eliciting somatic firing when compared with input to apical oblique branches. This result obtained even when the tuft membrane was the least excitable in the dendritic tree. 4. The special efficacy of tuft input arose because its electrotonic characteristics favour development of a sustained depolarization which charged the apex of the apical trunk to its firing threshold; once initiated in the distal trunk, firing propagated inward to the soma. This mechanism did not depend upon the presence of depolarizing channels in tuft membrane, but did require an excitable apical trunk. 5. Rather than disconnect the tuft, background synaptic conductance enhanced the efficacy advantage enjoyed by input arriving there. This counterintuitive result arose because background reduced the subthreshold spread of voltage, and so diminished the ability of the excitation of various individual oblique branches to combine to charge the relatively thick adjacent trunk. In contrast, drive from the depolarized tuft is exerted at a single critical point, the apex of the distal trunk, and so was relatively undiminished by the background. Further, once initiation at the apex occurred, background had little effect on inward propagation along the trunk. 6. We conclude that synaptic input to the apical tuft of layer 5 cells may be unexpectedly effective in triggering cell firing in vivo. The advantage in efficacy was not dependent upon the characteristics of tuft membrane excitability, but rather stemmed from the geometry of the tuft and its junction with the distal apical trunk. The efficacy of tuft input was, however, critically dependent upon inward propagation, suggesting that modulation of membrane currents which affect propagation in the apical trunk might sensitively control the efficacy of tuft input.

Intra- and inter-areal connections between the primary visual cortex V1 and the area immediately surrounding V1 in the rat

We have qualitatively and quantitatively analysed the anatomical connections within and between rat primary visual cortex (V1) and the rim region surrounding area V1, using both ortho- and retrograde anatomical tracers (biotinylated dextran amine, biocytin, cholera toxin b subunit). From the analysis of the projection patterns, and with the assumption that single points in the rat visual cortex, as in other species, have projection fields made up of multiple patches of terminals, we have concluded that just two V1 recipient areas occupy the entire rim region: an anterolateral area, probably homologous with V2 in other mammals, previously named Oc2L, and a medial area, corresponding to Oc2M. A non-reciprocal projection from the anterolateral area to the medial area was identified. Small injections (300–600 μm uptake zone diameter) of the anatomical tracers in area V1, or in the rim region, label orthograde intra-areal connections from each injection site to offset small patches. This is found in all regions of the rim and within at least the relatively expanded central dorsal field representation of V1. From the extent of these projections in V1 and the two rim regions, we have estimated that the neurons at the injection site send diverging laterally spreading projections to other neurons whose receptive fields share any part of the area included in the pooled receptive fields of the neurons at the injection site. Orthogradely labelled inter-areal feedforward projections from V1 to either rim region are estimated to diverge in their projections to neurons that share any part of the area of the pooled receptive fields of the V1 intra-areal connectional field of the same injection. The orthogradely labelled feedback projections to V1, from injection sites in either rim region, reach V1 neurons whose pooled receptive fields match those of the neurons in the rim injection site, i.e. with no divergence. Despite patchy anatomical connectional fields, our estimates indicate that visual space is represented continuously in the receptive fields of neurons postsynaptic to each intra- or inter-areal field of orthograde label. We suggest that, despite the absence of regularly mapped functions in rat V1 (e.g. regularly arranged orientation specificity), which in other species (e.g. primates and cats) relate to the patchy connectional patterns, the rat visual cortex intra- and inter-areal anatomical connections follow similar patterns and scaling factors to those in other species.

Androgens reduce cell death in the developing rat visual cortex

We have previously shown that males have more neurons than females in the primary visual cortex, and neonatal androgens play an important role in this difference. Also, we have found that females experience more cell death during development in this region than in males. Therefore, we hypothesized that the neonatal hormone environment directly influences the amount of cell death. In the present experiment, female rats were implanted with dihydrotestosterone (DHT) or estradiol at postnatal day 1. These animals, along with control males and females, were sacrificed on postnatal days 6, 11 and 25. Using unbiased stereology to quantify neuronal and pyknotic cell density, we observed that females implanted with DHT had a similar pattern and proportion of cells dying as control males. Additionally, developmental cell death in females implanted with estradiol was not significantly different than control females. Thus, neonatal androgens have an inhibitory effect on developmental cell death in the rat primary visual cortex.

Visual deprivation increases capability of layer II/III for epileptiform activity in the rat visual cortical slices

Effects of visual deprivation on the induction of epileptiform activity were studied in layer II/III of 29–39-day-old rat primary visual cortex. Field potentials were evoked by stimulation of layer IV in slices from control (CON) and dark-reared (DR) rats. Picrotoxin (PTX)-induced epileptiform activity was characterized by spontaneous and evoked epileptic field potentials (EFPs). The results showed that DR slices demonstrate greater susceptibility for induction of spontaneous EFP. PTX-induced changes in the characteristics of evoked field potentials also showed higher tendency of DR animals to generate epileptiform activity. In both groups, field potentials consisted of pEPSP 1 (population excitatory postsynaptic potential 1, i.e., first negativity) and pEPSP 2 (second negativity), respectively. There was no significant difference between the characteristics of field potentials in CON and DR slices. PTX significantly increased amplitude and duration of pEPSP 2, but it had no significant effect on pEPSP 1. Effects of PTX on pEPSP 2 were significantly higher in DR slices. It is concluded that visual deprivation results in a heightened potential in layer II/III of the rat visual cortex to generate PTX-induced epileptiform activity.

Connectivity of GABAergic calretinin-immunoreactive neurons in rat primary visual cortex.

In rat visual cortex neurons that are immunoreactive for the calcium-binding protein calretinin (CR+) constitute a distinct family which accounts for 17% of gamma-aminobutyric acid (GABA)-expressing cells. It is not clear, however, (i) whether CR is expressed exclusively in GABAergic neurons and (ii) how CR+ neurons are incorporated into neuronal circuits of rat visual cortex. To address these questions we studied synaptic relationships of CR+ neurons with GABA+ and GABA- elements in the neuropil of rat primary visual cortex (area 17). All CR+ neurons are nonpyramidal cells with smooth or sparsely spiny and often beaded dendrites. Of all CR+ neurons, 56% are located in layers 1 and 2/3. In layer 2/3, most CR+ neurons are bipolar-shaped and have vertically oriented dendrites. Many ascending dendritic branches reach layer 1 where they run parallel to pial surface. CR+ axons are thin, highly branched near the cell body and often send descending collaterals to layers 5 and 6. Double immunofluorescence labeling revealed GABA in 94% of CR+ cell bodies in layer 2/3. Electron microscopic analysis shows that all CR+ axon terminals contain elongated vesicles and form symmetric synapses. Postembedding staining shows that 98% of CR+ terminals are GABA+. GABA-immunoreactivity is also present in somata and thick dendrites of CR+ neurons but many thin dendrites are GABA-. CR+ somata, dendrites and axon terminals are enriched in mitochondria. Somata and thick CR+ dendrites are densely innervated. At least 68% of the targets of CR+ terminals in layer 2/3 are GABA+ and > or = 50% of these are other CR+ neurons. The remainder (32%) of targets of CR+ terminals are thin dendrites of GABA- cells. In contrast, in layers 5 and 6, 60% of CR+ terminals form synapses with GABA- somatic profiles. The preferential interactions of layer 2/3 CR+ neurons with GABAergic neurons, and with CR+ neurons in particular, suggests that these cells play a role in the inhibition of inhibitory neurons of the same layer. Through these interactions CR+ cells may reduce inhibition of pyramidal cells in layers 2/3, 5 and 6 and thus disinhibit a column of neurons.

Differential regulation of brain-derived neurotrophic factor messenger RNA cellular expression in the adult rat visual cortex

In this study, we report a comparative analysis of the distribution of brain-derived neurotrophic factor messenger RNA in the binocular primary visual cortex of rats analysed at the end of the critical period for monocular deprivation (postnatal day 35) and during adulthood (postnatal day 90). High-resolution non-isotopic in situ hybridization coupled with Nissl staining allowed to determine the relative number of neurons expressing brain-derived neurotrophic factor messenger RNA. In postnatal day 90 rats, the relative number of neurons positive for brain-derived neurotrophic factor messenger RNA significantly decreases in layer II/III with respect to postnatal day 35 animals, being constant in all the other cortical layers. Moreover, we demonstrate that dark rearing for 22 days, starting from postnatal day 90, determines: (i) a decrease of the overall level of brain-derived neurotrophic factor messenger RNA with a consequent reduction of labelling intensity in all cells throughout cortical layers II–VI; (ii) an increase of cell numbers expressing brain-derived neurotrophic factor messenger RNA in layers IV and V; and (iii) a decreased intensity of staining for brain-derived neurotrophic factor messenger RNA in dendrites after dark rearing. A re-exposure to light for 2 h after the period of darkness almost restores the number of brain-derived neurotrophic factor RNA-positive neurons. We conclude that the maturation of brain-derived neurotrophic factor messenger RNA in neurons of layer II/III goes beyond postnatal days 35–40, which can be considered the end of the critical period [Fagiolini M. et al. (1994) Vis. Res., 34, 709–720]. Moreover, we show that the cellular expression of brain-derived neurotrophic factor messenger RNA is regulated by light in adult rats as well as during development.

Receptive field properties of single neurons in rat primary visual cortex.

The rat is used widely to study various aspects of vision including developmental events and numerous pathologies, but surprisingly little is known about the functional properties of single neurons in the rat primary visual cortex (V1). These were investigated in the anesthetized (Hypnorm-Hypnovel), paralyzed animal by presenting gratings of different orientations, spatial and temporal frequencies, dimensions, and contrasts. Stimulus presentation and data collection were automated. Most neurons (190/205) showed sharply tuned (</=30 degrees bandwidth at half height) orientation selectivity with a bias for horizontal stimuli (31%). Analysis of response modulation of oriented cells showed a bimodal distribution consistent with the distinction between simple and complex cell types. Orientation specific interactions occurred between the center and the periphery of receptive fields, usually resulting in strong inhibition to center stimulation when both stimuli had the same orientation. There was no evidence for orientation columns nor for orderly change in optimal orientation with tangential tracks through V1. Responses were elicited by spatial frequencies ranging from zero (no grating) to 1.2 cycle/degree (c/ degrees ), peaking at 0.1 c/ degrees, and with a modal cutoff of 0.6 c/ degrees. Half of the neurons responded optimally to drifting gratings rather than flashing uniform field stimuli. Directional preference was seen for 59% of oriented units at all depths in the cortex. Optimal stimuli velocities varied from 10 to 250 degrees /s. Some units, mainly confined to layer 4, responded to velocities as high as 700 degrees /s. Response versus contrast curves (best fit with Naka-Rushton) varied from nearly linear to extremely steep (mean contrast semisaturation 50% and threshold 6%). There was a trend for cells from superficial layers to be more selective to different stimulus parameters than deeper layers cells. We conclude that neurons in rat V1 have complex and diverse visual properties, necessary for precise visual form perception with low spatial resolution.

A new form of synaptic plasticity is transiently expressed in the developing rat visual cortex: a modulatory role for visual experience and brain-derived neurotrophic factor

Synaptic plasticity has been implicated in the mechanisms contributing to the shaping of the cortical circuits responsible for the transmission of the visual input in the rat primary visual cortex. However, the degree of plasticity of the thalamocortical synapse may change during development, perhaps reflecting the degree of stabilization of the circuitry subserving it. We have chosen the ability of this synapse to be first depressed and then potentiated as a specific indicator of its plasticity. In this study we have investigated how this parameter changes during development and the factors controlling it. Extracellular field potentials in cortical layers 2/3 were evoked by stimulation of the white matter in rat primary visual cortex slices prepared at different postnatal ages. Low-frequency stimulation (900 pulses at 1 Hz) of the white matter was used to induce long-term depression of field potential amplitude, whereas long-term potentiation was evoked by high-frequency stimulation consisting of three trains at 100 Hz. We provide evidence that while it is possible to potentiate previously depressed synapses soon after eye opening (postnatal day 17) this synaptic characteristic decreases rapidly thereafter. The decrease in this form of cortical synaptic plasticity closely matches the stabilization of the cortical circuitry towards an adult pattern of connectivity and function. Depressed cortical synapses cannot be potentiated in normal rats at postnatal 23, but they can be potentiated in rats reared in the dark from postnatal days 17 to 29. Moreover, application of brain-derived neurotrophic factor, known to be expressed in an activity-dependent manner, was able to restore the ability of synapses to be potentiated after long-term depression, thus indicating its important modulatory role in brain development.

Co-expression of TrkB and the N-methyl- d-aspartate receptor subunits NR1-C1, NR2A and NR2B in the rat visual cortex

In the visual cortex, brain-derived neurotrophic factor expression is modulated through glutamate receptors, including the N-methyl- d-aspartate glutamate receptor. It has been proposed that the N-methyl- d-aspartate glutamate receptor subunit composition itself might be regulated by brain-derived neurotrophic factor. Here, we investigated the co-expression of the neurotrophin-4/brain-derived neurotrophic factor receptor TrkB with the N-methyl- d-aspartate glutamate receptor subunits NR1-C1, NR2A and NR2B, on postnatal days 10 and 22 and in the adult rat primary visual cortex. At both postnatal days 10 and 22, TrkB is co-expressed in all cortical layers with the studied N-methyl- d-aspartate glutamate receptor subunits. In the adult, in layers IV–V, co-expression is restricted to a subpopulation of neurons, while in layers II–III, VI nearly all neurons co-express TrkB with NR1-C1, NR2A and NR2B. We conclude that in layers IV–V, the co-expression of TrkB with subunits NR2B and NR2A is developmentally regulated.

Long term depression is expressed during postnatal development in rat visual cortex: a role for visual experience

Long term forms of synaptic plasticity and in particular LTD/LTP are both present in the mammalian visual cortex. However, while LTP is not inducible in adulthood LTD can be elicited in the mature brain, but its developmental pattern is unknown. Aim of this work was to investigate whether LTD is expressed during postnatal development and if it is modulated by visual experience. To investigate these points we have used rat primary visual cortex slices taken at different stages of functional maturation process, i.e., postnatal day 17 (P17), P23 and P30–35. LTD was assessed by measuring the amplitude of extracellular field potentials recorded in cortical layers 2/3 and elicited by low frequency stimulation to the white matter. LTD was expressed at all ages investigated without significant differences between age groups. These data indicate that LTD developmental expression is not temporally related with the period of functional maturation of rat visual cortex. Dark rearing from birth to P23 resulted in a reduction of LTD amplitude while light deprivation from P17 to P30 did not affect LTD expression in comparison to age matched control values. We suggest that light imprinting is essential for a normal LTD expression during postnatal development.

Dark rearing blocks the developmental down-regulation of brain-derived neurotrophic factor messenger RNA expression in layers IV and V of the rat visual cortex

In this study, we describe the distribution of brain-derived neurotrophic factor messenger RNA in the binocular primary visual cortex of the rat during postnatal development, starting at postnatal day (P) 13. High-resolution non-isotopic in situ hybridization combined with Nissl staining were used to quantify the number of cells expressing brain-derived neurotrophic factor messenger RNA. At P13, most of the cells express brain-derived neurotrophic factor messenger RNA. After eye opening (P14–P15), the relative number of brain-derived neurotrophic factor messenger RNA-positive cells decreases by a factor of two in layer IV, i.e. that receiving the visual input, and in layer V. To verify the hypothesis that light could trigger this decrease, pups were kept in complete darkness from birth. At P22, pups reared in the dark were killed and the visual cortex processed for in situ hybridization and northern blotting. The results obtained in dark-reared animals prove that light deprivation can: (i) decrease the general levels of brain-derived neurotrophic factor messenger RNA and (ii) increase the relative number of brain-derived neurotrophic factor messenger RNA-positive cells in layers IV and V with respect to control rats. Exposure to light for five days after the period of darkness restored the number of brain-derived neurotrophic factor messenger RNA-positive cells. We conclude that the expression of brain-derived neurotrophic factor messenger RNA in the rat primary visual cortex is regulated during development and that this process is under the control of visual input.

Serotonin inhibits the induction of long-term potentiation in rat primary visual cortex

1. 1. The effect of serotonin (5-hydroxytryptamine; 5-HT) on the induction of long-term potentiation (LTP) in rat visual cortex was investigated by using slice preparations in vitro. 2. 2. Bath application of 5-HT (0.1–10 μM) did not affect the baseline synaptic potentials evoked by single-pulse test stimulation, but inhibited the induction of LTP in a concentration-dependent manner. 3. 3. The effect of 5-HT was blocked by the 5-HT 1 receptor antagonist pindolol or the 5-HT 2, 7 receptor antagonist ritanserin, but not by the 5-HT 3, 4 receptor antagonist MDL72222. 4. 4. These results suggest that 5-HT plays a role in suppressing the induction of LTP in the rat visual cortex.

Three kinetically distinct Ca2+-independent depolarization-activated K+ currents in callosal-projecting rat visual cortical neurons.

Three kinetically distinct Ca2+-independent depolarization-activated K+ currents in callosal-projecting rat visual cortical neurons. J. Neurophysiol. 78: 2309-2320, 1997. Whole cell, Ca2+-independent, depolarization-activated K+ currents were characterized in identified callosal-projecting (CP) neurons isolated from postnatal day 7-16 rat primary visual cortex. CP neurons were identified in vitro after in vivo retrograde labeling with fluorescently tagged latex microbeads. During brief (160-ms) depolarizing voltage steps to potentials between -50 and +60 mV, outward K+ currents in these cells activate rapidly and inactivate to varying degrees. Three distinct K+ currents were separated based on differential sensitivity to 4-aminopyridine (4-AP); these are referred to here as IA, ID, and IK, because their properties are similar (but not identical) K+ currents termed IA, ID, and IK in other cells. The current sensitive to high (>/=100 mu M) concentrations of 4-AP (IA) activates and inactivates rapidly; the current blocked completely by low (</=50 mu M) 4-AP (ID) activates rapidly and inactivates slowly. A slowly activating, slowly inactivating current (IK) remains in the presence of 5 mM 4-AP. IA, ID, and IK also were separated and characterized in experiments that did not rely on the use of 4-AP. All CP cells express all three K+ current types, although the relative densities of IA, ID, and IK vary among cells. The experiments here also have revealed that IA, ID, and IK display similar voltage dependences of activation and steady state inactivation, whereas the kinetic properties of the currents are distinct. At +30 mV, for example, mean +/- SD activation taus are 0. 83 +/- 0.24 ms for IA, 1.74 +/- 0.49 ms for ID, and 14.7 +/- 4.0 ms for IK. Mean +/- SD inactivation taus for IA and ID are 26 +/- 7 ms and 569 +/- 143 ms, respectively. Inactivation of IK is biexponential with mean +/- SD inactivation time constants of 475 +/- 232 ms and 3,128 +/- 1,328 ms; approximately 20% of the 4-AP-insensitive current is noninactivating. For all three components, activation is voltage dependent, increasing with increasing depolarization, whereas inactivation is voltage independent. Both IA and IK recover rapidly from steady state inactivation with mean +/- SD recovery time constants of 38 +/- 7 ms and 79 +/- 26 ms, respectively; ID recovers an order of magnitude more slowly (588 +/- 274 ms). The properties of IA, ID, and IK in CP neurons are compared with those of similar currents described previously in other mammalian central neurons and, in the accompanying paper, the roles of these conductances in regulating the firing properties of CP neurons are explored.

Role of voltage-gated K+ currents in mediating the regular-spiking phenotype of callosal-projecting rat visual cortical neurons.

Role of voltage-gated K+ currents in mediating the regular-spiking phenotype of callosal-projecting rat visual cortical neurons. J. Neurophysiol. 78: 2321-2335, 1997. Whole cell current- and voltage-clamp recordings were combined to examine action potential waveforms, repetitive firing patterns, and the functional roles of voltage-gated K+ currents (IA, ID, and IK) in identified callosal-projecting (CP) neurons from postnatal (day 7-13) rat primary visual cortex. Brief (1 ms) depolarizing current injections evoke single action potentials in CP neurons with mean +/- SD (n = 60) durations at 50 and 90% repolarization of 1.9 +/- 0.5 and 5.5 +/- 2.0 ms, respectively; action potential durations in individual cells are correlated inversely with peak outward current density. During prolonged threshold depolarizing current injections, CP neurons fire repetitively, and two distinct, noninterconverting "regular-spiking" firing patterns are evident: weakly adapting CP cells fire continuously, whereas strongly adapting CP cells cease firing during maintained depolarizing current injections. Action potential repolarization is faster and afterhyperpolarizations are more pronounced in strongly than in weakly adapting CP cells. In addition, input resistances are lower and plateau K+ current densities are higher in strongly than in weakly adapting CP cells. Functional studies reveal that blockade of ID reduces the latency to firing an action potential, and increases action potential durations at 50 and 90% repolarization. Blockade of ID also increases firing rates in weakly adapting cells and results in continuous firing of strongly adapting cells. After applications of millimolar concentrations of 4-aminopyridine to suppress IA (as well as block ID), action potential durations at 50 and 90% repolarization are further increased, and firing rates are accelerated over those observed when only ID is blocked. Using VClamp/CClamp and the voltage-clamp data in the preceding paper, mathematical descriptions of IA, ID, and IK are generated and a model of the electrophysiological properties of rat visual cortical CP neurons is developed. The model is used to simulate the firing properties of strongly adapting and weakly adapting CP cells and to explore the functional roles of IA, ID, and IK in shaping the waveforms of individual action potentials and controlling the repetitive firing properties of these cells.

A Quantitative Description of Short-Term Plasticity at Excitatory Synapses in Layer 2/3 of Rat Primary Visual Cortex

Cortical synapses exhibit several forms of short-term plasticity, but the contribution of this plasticity to visual response dynamics is unknown. In part, this is because the simple patterns of stimulation used to probe plasticity in vitro do not correspond to patterns of activity that occur in vivo. We have developed a method of quantitatively characterizing short-term plasticity at cortical synapses that permits prediction of responses to arbitrary patterns of stimulation. Synaptic responses were recorded intracellularly as EPSCs and extracellularly as local field potentials in layer 2/3 of rat primary visual cortical slices during stimulation of layer 4 with trains of electrical stimuli containing random mixtures of frequencies. Responses exhibited complex dynamics that were well described by a simple three-component model consisting of facilitation and two forms of depression, a stronger form that decayed exponentially with a time constant of several hundred milliseconds and a weaker, but more persistent, form that decayed with a time constant of several seconds. Parameters obtained from fits to one train were used to predict accurately responses to other random and constant frequency trains. Control experiments revealed that depression was not caused by a decrease in the effectiveness of extracellular stimulation or by a buildup of inhibition. Pharmacological manipulations of transmitter release and postsynaptic sensitivity suggested that both forms of depression are mediated presynaptically. These results indicate that firing evoked by visual stimuli is likely to cause significant depression at cortical synapses. Hence synaptic depression may be an important determinant of the temporal features of visual cortical responses.