Adaptation at synaptic connections to layer 2/3 pyramidal cells in rat visual cortex

Neocortical synapses express differential dynamic properties. When activated at high frequencies, the amplitudes of the subsequent postsynaptic responses may increase or decrease, depending on the stimulation frequency and on the properties of that particular synapse. Changes in the synaptic dynamics can dramatically affect the communication between nerve cells. Motivated by this question, we studied dynamic properties at synapses to layer 2/3 pyramidal cells with intracellular recordings in slices of rat visual cortex. Synaptic responses were evoked by trains of test stimuli, which consisted of 10 pulses at different frequencies (5-40 Hz). Test stimulation was applied either without any adaptation (control) or 2 s after an adaptation stimulus, which consisted of 4 s stimulation of these same synapses at 10, 25, or 40 Hz. The synaptic parameters were then assessed from fitting the data with a model of synaptic dynamics. Our estimates of the synaptic parameters in control, without adaptation are broadly consistent with previous studies. Adaptation led to pronounced changes of synaptic transmission. After adaptation, the amplitude of the response to the first pulse in the test train decreased for several seconds and then recovered back to the control level with a time constant of 2-18 s. Analysis of the data with extended models, which include interaction between different pools of synaptic vesicles, suggests that the decrease of the response amplitude was due to a synergistic action of two factors, decrease of the release probability and depletion of the available transmitter. After a weak (10 Hz) adaptation, the decrease of the response amplitude was accompanied by and correlated with the decrease of the release probability. After a strong adaptation (25 or 40 Hz), the depletion of synaptic resources was the main cause for the reduced response amplitude. Adaptation also led to pronounced changes of the time constants of facilitation and recovery, however, these changes were not uniform in all synapses, and on the population level, the only consistent and significant effect was an acceleration of the recovery after a strong adaptation. Taken together, our results suggest, that apart from decreasing the amplitude of postsynaptic responses, adaptation may produce synapse-specific effects, which could result in a kind of re-distribution of activity within neural networks.     

Morphology of identified projection neurons in layer 5 of rat visual cortex

We studied the morphology of neurons in layer 5 of rat visual cortex (area 17) projecting to the contralateral hemisphere and the superior colliculus. Double labelling with fluorescent tracers indicated that these projections arise from different populations of cells. To reveal the morphology of the cells we stained retrogradely labelled neurons intracellularly in living brain slices. Callosal projecting pyramidal cells have 3-6 basal dendrites and an apical dendrite which never reaches higher than layer 3. Corticotectal cells have 6-8 basal dendrites and a prominent apical dendrite which always forms a large tuft in layer 1. Thus, neurons in the same cortical layer that give rise to different projections also differ in their morphology. However, each population of neurons has a rather stereotyped dendritic branching pattern, despite a large variation in soma size.     

Cortical connections of the anteromedial extrastriate visual cortex in the rat

Summary Efferent and afferent connections of the visually responsive cortex (area anteromedial, AM) located in the anterior portion of area 18b were studied with degeneration and horseradish peroxidase (HRP) methods following small lesions and HRP injections into this area. Degenerating axons, terminals and retrogradely HRP-labeled neurons were observed in a broad region of the cortex including areas located lateral, medial and anterior to the striate cortex. The main finding of this study is that connections of area AM with area 18a are distributed in discrete patches whose arrangement is similar to that of the lateral extrastriate visual areas postulated in previous physiological and anatomical reports. These results thus suggest that visual area AM is reciprocally connected with visual areas in area 18a. Area AM is also connected with other regions within area 18b, thus supporting the notion advanced by recent studies that area 18b contains more than one visual area. A weak afferent connection to area AM from the dorsal lateral geniculate nucleus of the thalamus was noted. Previously described connections of area 18b with areas 8 and 29 as well as with the lateral and latero-posterior thalamic nuclei were confirmed in the present study.     

Cholinergic neurons and fibres in the rat visual cortex

Summary Choline acetyltransferase (ChAT), the acetylcholine synthesizing enzyme, was localized immunocytochemically in neurons and fibres in the rat visual cortex using a monoclonal antibody. ChAT-labelled cells were non-pyramidal neurons, primarily of the bipolar form, distributed in layers II through VI but concentrated in layers II & III. Their perikarya contained a large nucleus and a small amount of perinuclear cytoplasm. The somata and dendrites of all labelled cells received Gray's type I and type II synapses.ChAT-stained axons formed a dense and diffuse network throughout the visual cortex and particularly in layer V. Electron microscopy revealed that the great majority formed type II synaptic contacts with dendrites of various sizes, unlabelled non-pyramidal somata and, on a few occasions, with ChAT-labelled cells. However, a very small number of terminals appeared to form type I synaptic contacts. This study describes the morphological organization of the cholinergic system in the visual cortex, the function of which has been under extensive investigation.     

Vasoactive intestinal polypeptide-immunoreactive neurons in rat visual cortex

An antibody to vasoactive intestinal polypeptide (VIP) was used to examine the forms of VIP-positive neurons and the synapses made by VIP-positive axon terminals. Vasoactive intestinal polypeptide-positive cells are most common in layers II and III and the majority of them are typical bipolar neurons, with two primary dendrites which emanate from the upper and lower poles of the cell body. Their somata, which have only a few symmetric and asymmetric synapses, generally have a fusiform or "tear-drop" shape and contain nuclei with a vertically oriented cleft. The dendritic trees are arranged vertically and often extend through five cortical layers. The axons are thin and extend either from the soma or from one of the primary dendrites. The axons also follow a vertical trajectory. Other VIP-positive neurons are modified bipolar cells and a few of them are multipolar cells. The synapses formed by the VIP-positive axon terminals in the neuropil are symmetric in form, and although the synaptic clefts are narrow, the junctions are usually long and continuous, rather like those described for asymmetric synapses. Most of the VIP-positive axon terminals synpase with small dendritic shafts, but a few synapse with neuronal cell bodies. Since the majority of the VIP-positive neurons are bipolar cells it is concluded that these are the source of most of the VIP-positive axon terminals. If this is so, then the VIP-positive bipolar cells form symmetric synapses. This is in contrast to the observations of Peters and Kimerer (1981. J. Neurocytol. 10, 921-946) for the bipolar cells they examined in a Golgi-electron microscopic study had axon terminals forming asymmetric synapses. It is suggested that this disparity can be reconciled if it is assumed that the bipolar cell population consists of subgroups which have different biochemical characteristics and different synaptic relationships.     

Response properties of neurons in the visual cortex of the rat

Summary Response properties of neurons in the visual cortex, area 17, of Long Evans pigmented rats were investigated quantitatively with computer-controlled stimuli. Ninety percent of the cells recorded (296/327) were responsive to visual stimulation. The majority (95%, 281/296) responded to moving images and were classified as complex (44%), simple (27%), hypercomplex (13%) and non-oriented (16%) according to criteria previously established for cortical cells in the cat and monkey. The remaining 5% of the neurons responded only to stationary stimuli flashed on-off in their receptive field. Results of this study indicate that neurons of the rat visual cortex have properties similar to those of cells in the striate cortex of more visual mammals.Supported by grant EY02964, the Biological Humanics Foundation and the Bendix Corporation     

Muscarinic control of intracortical connections to layer II in rat entorhinal cortex slice.

The cholinergic system is critically involved in oscillatory network activity and synaptic plasticity in the entorhinal cortex (EC) hippocampal formation. Here we demonstrate robust inhibition of field potentials in layer II of the medial EC evoked by stimulation in the deep EC or in the lateral layer II by carbachol (CCh, 0.1-100 microM, K(D) approximately 1 microM). This effect appears not to be mediated by suppression of presynaptic Ca(2+)-signals since paired pulse facilitation was increased by CCh. Blockade of the effect by the muscarinic antagonists atropine and pirenzepine demonstrates mediation by muscarinic receptors, most likely of the M1 subtype. The effect is characterized by absence of desensitization and should be important for laminar shaping of oscillatory activity and synaptic plasticity during acetylcholine-dependent theta-rhythmic activity.     

Prosencephalic connections of striate and extrastriate areas of rat visual cortex

Summary Afferent connections of rat primary visual cortex (area 17 or V1 area) and the rostral and caudal parts of areas 18a and 18b were studied, by placing in each of the areas, small electrophoretic injections of enzyme horseradish peroxidase (HRP) or wheat germ agglutinated-HRP. The results indicate that: 1) each of the areas has a distinct pattern of distribution of afferent neurons in the ipsilateral visual thalamus — area 17 receives its principal thalamic input from the dorsal lateral geniculate nucleus, the caudal parts of areas 18a and 18b receive a major thalamic input from the lateral posterior nucleus and a minor input from the posterior nucleus, while the rostral parts of areas 18a and 18b receive a major input from the posterior nucleus, and a minor projection from the lateral posterior nucleus; 2) the rostral and caudal parts of areas 18a and 18b each receive an associational input from area 17; 3) the rostral parts of areas 18a and 18b each receive associational input from three different extrastriate regions, the caudal part of the same extrastriate area, and the rostral and caudal parts of the other extrastriate area, whereas the caudal parts of areas 18a and 18b receive associational inputs only from one or two extrastriate regions; 4) area 17, area 18b and rostral area 18a each receive a substantial associational input from lamina V of the caudal part of the frontal eye field (FEF) in the motor cortex; however the input from the FEF to caudal area 18a (if present) is very small; 5) The extrastriate areas studied receive associational input from the restrosplenial cingulate area 29d; however, the input from area 29d to area 17 appears to be very small. The distinct patterns of distribution of prosencephalic afferents suggest to us that multiple retinotopically organized areas described previously in the rat cortex (cf Montero 1981; Espinoza and Thomas 1983) represent functionally distinct areas.Abbreviations AL anterolateral area - AM anteromedial area - DLG dorsal lateral geniculate nucleus - FEF frontal eye field - HRP horseradish peroxidase - ILN intralaminar thalamic nucleus - LM lateromedial area - LP lateral posterior thalamic nucleus - PM posteromedial area - PO posterior thalamic nuclear complex - V1 primary visual cortex (area 17) - VM ventromedial thalamic nucleus - WGA-HRP HRP conjugated with wheatgerm agglutinin     

Conserved patterns of cortico-cortical connections define areal hierarchy in rat visual cortex

Summary The prevalence of reciprocal connections in the cerebral cortex indicates that they play a fundamental role in the processing of sensory information. We have investigated the laminar termination patterns of such paired connections between different visual cortical areas of the rat, and have found two basic projection types: one which includes layer 4 and a second which includes layer 1 and avoids layer 4. The projections from primary visual cortex (area 17) to extrastriate visual cortical targets in the cytoarchitectonical areas 18a and 18b, and from 18a to a site in 18b, are of the first type. In contrast, the return projections from 18a and 18b to area 17 and from 18b to 18a, are of the second type. Thus each pair of connections has one element of each type, giving every circuit a nearly identical asymmetric structure. These laminar patterns resemble those of forward and feedback connections in primate cortex, indicating that cortico-cortical connectivity patterns are highly conserved through evolution, and that, as in monkeys, these connections define a hierarchical organization of areas in rat visual cortex.     

Zinc-rich neurones in the rat visual cortex give rise to two laminar segregated systems of connections

Zinc-rich synaptic boutons in the neocortex arise from the neocortex itself. However, the precise organisation of these circuits is not known. Therefore, the laminar and areal pattern of zinc-rich cortico-cortical connections between visual areas was studied by retrograde tracing using intracerebral injections of sodium selenite. This tracer was injected in supragranular and infragranular layers in various cortical visual areas in order to precipitate zinc in the synaptic boutons, which was retrogradely transported to neuronal somata. Supragranular injections led to retrogradely labelled neurones in layer II-III, ipsilaterally and contralaterally. Neurones often appeared in groups or clusters. Infragranular injections labelled neurones in layers II-III, VI and, to a lesser extent, in layer V, both ipsilaterally and contralaterally. Neurones in layer VI formed a wide continuous band. Concerning the connections between visual (=occipital) areas, injections in occipital area 2, lateral part (Oc2L), rendered the largest number of retrogradely labelled neurones, which were located in occipital area 1 (Oc1), occipital area 2, medial part (Oc2M) and outside the visual cortex. Callosal zinc-rich projections were dense in the homotopic area but sparse in Oc1 and temporal cortex. Injections in Oc1 rendered moderate numbers of labelled neurones in occipital areas, in both hemispheres. Injections in Oc2M labelled moderate numbers of neurones in occipital areas in both hemispheres and in the frontal and cingulate cortices.These results indicate that zinc-rich cortico-cortical connections are organised into two segregated systems arising from either supragranular or infragranular neurones. In addition, in the visual cortex, zinc-rich systems appear to converge on Oc2L. Zinc-rich connections appear as an extensive, highly organised association system.     

The development of non-pyramidal neurons in the visual cortex of the rat

Summary We have studied the maturation of non-pyramidal cells in layers II–VI of the visual cortex of albino rats from birth to maturity, using Golgi-Cox and rapid Golgi preparations. At birth, non-pyramidall cells are sparse, immature and concentrated in the deep part of the cortical plate: their number increases towards the end of the first week but they remain sparse and immature in the upper part of the cortical plate. During the second postnatal week, the number, size and extent of dendritic and axonal branching of these cells undergo considerable increases and the cells become conspicuous in layer IV and apparent in the supragranular layers: this growth spurt occurs just after (and may be related to) the arrival and establishment in the cortex during the second half of the first postnatal week, of extrinsic afferents.During the third postnatal week, most of the cells complete their maturation. At the end of this week, the number of spinous cells is greater and the spine density of some cells is higher than in the adult, falling to adult values during the fourth postnatal week. It is noteworthy that the non-pyramidal cells appear to reach maturity at about the same time in all the layers studied, and at the same time as the pyramidal cells with which they are associated. These observations are not in accord with the prevalent view that non-pyramidal cells complete their differentiation much later than pyramidal cells.     

Inter- and intralaminar subcircuits of excitatory and inhibitory neurons in layer 6a of the rat barrel cortex

Approximately half the excitatory neurons in layer 6 (L6) of the rat barrel cortex project to the thalamus with axon collaterals ramifying in the granular L4; the remaining project within cortex with collaterals restricted to infragranular laminae. In analogy, L6 inhibitory neurons also include locally arborizing and inter-laminar projecting neurons. We examined whether L6 neurons participating in different laminar interactions were also morphologically and electrically distinct. Corticothalamic (CT) neurons were labeled by in vivo injections of a retrogradely transported fluorescent tracer into the primary thalamic nucleus. Whole cell current-clamp recordings were performed from labeled and unlabeled L6 neurons in brain slices of juvenile rats; the morphology of cells was subsequently recovered and reconstructed. Corticocortical (CC) neurons were distinguished from CT cells based on the absence of a subcortical projection and the predominantly infragranular arborization of their axon collaterals. Two morphological CC subtypes could be further distinguished based on the structure of their apical dendrite. Electrically, CT neurons had shorter membrane time-constants and action potential (AP) durations and higher rheobase currents. CC neurons fired high-frequency spike doublets or triplets on sustained depolarization; the burst frequency also distinguished the two morphological CC subtypes. Among inhibitory L6 cells, the L4-projecting (L6i_L4) and local (L6i_L6) inhibitory neurons also had contrasting firing properties; L6i_L4 neurons had broader APs and lower maximal firing rates. We propose that L6 excitatory and inhibitory neurons projecting to L4 constitute specialized subcircuits distinct from the infragranular network in their connectivity and firing patterns.     

Retinotopy of cortical connections between the striate cortex and extrastriate visual areas in the rat

Previous studies have determined that the striate cortex of the rat is reciprocally connected with multiple extrastriate cortical areas that are retinotopically organized. The objective of this study was to investigate the retinotopy of the striate-extrastriate connections in the rat, by placing triple or double injections of fluorescent tracers (fluorogold, fast blue, rhodamine dextran, or rhodamine-labeled microspheres) in different regions of the striate cortex (Oc1) and mapping the distribution of cells and fibers labeled with the different tracers in the lateral (Oc2L) and medial (Oc2M) extrastriate cortex. The tracer injection sites were visualized in tangential sections of the flattened cortex and correlated with the myelin layout of the striate cortex and with an electrophysiological map from previous studies. The results showed retinotopically organized Oc1 connections with ten different extrastriate cortical areas. The location of these extrastriate areas and the retinotopy of their striate connections remained mostly invariant despite changes of the injection sites in Oc1. Thus, the quadrantic retinotopy was obtained for striate connections to areas posterior, posterolateral, lateromedial, laterointermediate, laterolateral, anterolateral and rostrolateral in Oc2L; and to areas posteromedial, anteromedial, and anterior in Oc2M. The present anatomical map correlates well with electrophysiological maps of the rat extrastriate cortex from previous studies. Furthermore, they provide a definition of the retinotopy of some areas that have not been completely mapped before. These results reaffirm the existence of multiple extrastriate visual areas in the rat.     

Development of the ultrastructural features of somatostatin-immunoreactive neurons in the rat visual cortex

Summary The peroxidase-antiperoxidase immunocytochemical technique has been used to examine the development of the ultrastructural features of somatostatin (SRIF)-immunoreactive neurons in the visual cortex of the rat between embryonic day 17 and postnatal day 32. In the adult, stained neurons are distributed in layers II through VI and characterized by an abundance of cytoplasm containing a plethora of organelles, most conspicuous of which are cisternae of granular endoplasmic reticulum organized in parallel arrays.In embryonic tissue, SRIF-positive cells are present in the subplate and in the border between the cortical plate and marginal zone. These cells possess scanty cytoplasm containing a few organelles; synapses onto immunoreactive perikarya and dendrites are evident at this stage. At birth and in early postnatal life, labelled cells are confined to the subplate region. Already at this age a number of cells display signs of ultrastructural features which characterize them in adult life. At the end of the first postnatal week, SRIF-immunoreactive neurons span a considerable spectrum of maturity. At one extreme are a few cells with little cytoplasm surrounding a large nucleus and at the other are the majority of labelled neurons showing abundant cytoplasm including prominent arrays of granular endoplasmic reticulum.Labelled cells first appear in the more superficial layers at the beginning of the second postnatal week and attain a distribution similar to that observed in adult animals at the end of this week. At this time their ultrastructural features closely resemble those of their adult counterparts, and differences in cytoplasmic maturity between superficial and deep labelled cells are not evident. This suggests that the SRIF-producing neurons in the superficial layers begin to express this peptide after they complete their migration and have acquired their morphological features.Maturation proceeds during the third postnatal week; at this stage most cells acquire their mature nuclear and cytoplasmic features and an adult complement of synapses. However, a number of SRIF-immunoreactive cells contain a particularly prominent accumulation of cytoplasmic organelles and appear hypertrophic.     

The development of the thalamic projections to layer I of the visual cortex of the rat

Summary The development of the thalamic afferent projections to layer I of the visual cortex of the albino rat was studied using the retrograde transport of horseradish peroxidase. The results show that the projections to layer I which arise in the nonspecific thalamic nuclei (lateral posterior nucleus, posterior complex and ventromedial nucleus) develop earlier than the projection from the specific nucleus (dorsal lateral geniculate nucleus).     

The claustrum of the sheep and its connections to the visual cortex

The present study analyses the organization and selected neurochemical features of the claustrum and visual cortex of the sheep, based on the patterns of calcium-binding proteins expression. Connections of the claustrum with the visual cortex have been studied by tractography. Parvalbumin-immunoreactive (PV-ir) and Calbindin-immunoreactive (CB-ir) cell bodies increased along the rostro-caudal axis of the nucleus. Calretinin (CR)-labeled somata were few and evenly distributed along the rostro-caudal axis. PV and CB distribution in the visual cortex was characterized by larger round and multipolar cells for PV, and more bitufted neurons for CB. The staining pattern for PV was the opposite of that of CR, which showed densely stained but rare cell bodies. Tractography shows the existence of connections with the caudal visual cortex. However, we detected no contralateral projection in the visuo-claustral interconnections. Since sheep and goats have laterally placed eyes and a limited binocular vision, the absence of contralateral projections could be of prime importance if confirmed by other studies, to rule out the role of the claustrum in stereopsis.     

Tectocortical connections in the rat visual system

Electrical stimulation of the superior colliculus by square pulses evokes responses in the ipsilateral occipital region whose localization coincides with the zone of evoked potentials to photic stimulation. The short latent period of the visual cortical evoked potentials to stimulation of the superior colliculus (mean 3.2±1.1 msec; P<0.05) together with morphological data suggests that only one relay, in the structures of the thalamus, occurs along the tectocortical pathway in the rat visual system. In the poorly differentiated cortex of the rat no separate representation of retino-geniculo-cortical and tectocortical channels could be found.Translated from Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova, Vol. 62, No. 3, pp. 335–341, March, 1976.     

The development of excitatory transmitter amino acid-containing neurons in the rat visual cortex

The excitatory amino acids l-glutamate and l-aspartate are believed to be utilized as neurotransmitters by the pyramidal neurons in the mammalian cerebral cortex. These cells can be recognized early in development, while glutamate might play an important part in the maturation and plasticity of the cortex. Here, we used light and electron microscopic immunocytochemistry to study the time of appearance and maturation of glutamate and aspartate in neurons of the rat visual cortex. Glutamate- and aspartate-immunoreactive cells were first detected in deep cortical layers at postnatal day 3. During the next 3 weeks, labelled neurons were observed progressively in more superficial layers, but did not demonstrate their adult pattern of distribution until postnatal week 4. Electron microscopic analysis showed that glutamate- and aspartate-labelled neurons gradually develop their cytological and synaptic features during the first 4 postnatal weeks, with this process of differentiation originating in the deep cortical layers and progressively extending to the superficial layers. These findings suggest that cortical pyramidal neurons begin to express detectable levels of transmitter glutamate and/or aspartate after they have completed their migration. Their neurochemical differentiation follows an inside-out pattern similar to the pattern described for the genesis and morphological differentiation of this population of cortical neurons.