Morphology of layer V pyramidal neurons in the cat somatosensory cortex: an intracellular HRP study

Telencephalon: Neocortex

The neocortex is an ultracomplex, six-layered structure that develops from the dorsal palliai sector of the telencephalic hemispheres (Figs. 2.24, 2.25, 11.1). All mammals, including monotremes and marsupials, possess a neocortex, but in reptiles, i.e. the ancestors of mammals, only a three-layered neocortical primordium is present [509, 511]. The term neocortex refers to its late phylogenetic appearance, in comparison to the “palaeocortical” olfactory cortex and the “archicortical” hippocampal cortex, both of which are present in all amniotes [509].

High-energy and -protein diet increases brain and corticospinal tract growth in term and preterm infants after perinatal brain injury

OBJECTIVE

Our hypothesis was that infants with perinatal brain injury fail to thrive in the first postnatal year because of increased energy and protein requirements from deficits that accumulated during neonatal intensive care. Our aim was to assess whether dietary energy and protein input was a rate-limiting factor in brain and body growth in the first year after birth.

METHODS

We conducted a prospective, double-blind and randomized, 2-stage group sequential study and controlled for gestation, gender, and brain lesion. Neonates with perinatal brain damage were randomly allocated to receive either a high- (120% recommended average intake) or average (100% recommended average intake) energy and protein diet. The study began at term and continued for 12 months. Three-day dietary diaries estimated energy and protein intake. The primary outcome measure was growth of occipitofrontal circumference. Other measures were growth of axonal diameters in the corticospinal tract, which were estimated by using transcranial magnetic stimulation, weight gain, and length.

RESULTS

The study was terminated at the first analysis when the 16 subjects had completed the protocol, because the predetermined stopping criterion of >1 SD difference in occipitofrontal circumference at 12 months' corrected age in those receiving the higher-energy and -protein diet had been demonstrated. Axonal diameters in the corticospinal tract, length, and weight were also significantly increased.

CONCLUSIONS

These data support our hypothesis that infants with significant perinatal brain damage have increased nutritional requirements in the first postnatal year and suggest that decreased postnatal brain growth may exacerbate their impairment. There are no measures of cognitive ability at 12 months of age, and whether there will be any improvement in the status of these children, therefore, remains to be shown.

Target-specific differences in somatodendritic morphology of layer V pyramidal neurons in rat motor cortex

Dendritic geometry has been shown to be a critical determinant of information processing and neuronal computation. However, it is not known whether cortical projection neurons that target different subcortical nuclei have distinct dendritic morphologies. In this study, fast blue retrograde tracing in combination with intracellular Lucifer yellow injection and diaminobenzidine (DAB) photoconversion in fixed slices was used to study the morphological features of corticospinal, corticostriatal, and corticothalamic neurons in layer V of rat motor cortex. Marked differences in the distribution of soma, somal size, and dendritic profiles were found among the three groups of pyramidal neurons. Corticospinal neurons were large, were located in deep layer V, and had the most expansive dendritic fields. The apical dendrites of corticospinal pyramidal neurons were thick, spiny, and branched. In contrast, nearly all corticostriatal neurons were small cells located in superficial layer V. Their apical dendritic shafts were significantly more slender, though spiny like those of corticospinal neurons. Corticothalamic neurons, which were located in superficial layer V and in layer VI, had small or medium-sized soma, slender apical dendritic shafts, and dendrites that were largely spine free. This study indicates that, in layer V of rat motor cortex, each population of projection neurons has a unique somatodendritic morphology and suggests that distinct modes of cortical information processing are operative in corticospinal, corticostriatal, and corticothalamic neurons.

Spatial arrangement of cerebro-pontine terminals

Understanding the interaction of the cerebral cortex and cerebellum requires knowledge of the highly complex spatial characteristics of cerebro-cerebellar signal transfer. Cerebro-pontine fibers from one neocortical site terminate in several sharply demarcated patches across large parts of the pontine nuclei (PN), and fibers from different neocortical areas terminate in the same pontine region. To determine whether projections from segregated neocortical sites overlap in the PN, we studied double anterograde tracing of cerebro-pontine terminals from large parts of rat neocortex. In none of these experiments, including double injection into two functionally related areas, were we able to demonstrate overlapping patches, although close spatial relationships were always detected. This non-overlapping distribution is consistent with a compartmentalized organization of the cerebro-pontine projection and may be the basis of the fractured type of maps found in the cerebellar granular layer. The critical distance between two sites on the neocortical surface that project to non-overlapping patches in the PN was found to be 600 microm, by using double injection within the whisker representation of the primary somatosensory area. This matches the diameter of dendritic trees of layer 5 projection neurons, indicating that non-overlapping populations of neocortical projection neurons possess non-overlapping patches of pontine terminals. Estimations based on this critical distance and the pontine volume anterogradely labeled by one injection site indicate that the size of the PN may be well suited to accommodate a complete set of non-overlapping pontine patches from all possible neocortical sites.

GABAergic characteristics of transcallosal activity of cat motor cortical neurons

GABAergic characteristics of transcallosal activity of cat pyramidal tract neurons (PTNs) and non-PTNs (nPTNs) were studied with stressing on GABAB receptors. PTNs and nPTNs were further classified into group 1 (< 10 ms) and group 2 (> 10 ms) based on the latency upon transcallosal stimulation. However, mainly the results of group 1 neurons were presented here, due to the small number of group 2 neurons. GABA, bicuculline, CGP 35348 and phaclofen were iontophoretically applied. The spike number to 20 trials of transcallosal stimulation was 8.9 +/- 4.3 (mean +/- S.D.) for group 1 PTNs (n = 14) and 10.4 +/- 4.5 for group 1 nPTNs (n = 38) under the control conditions. CGP 35348, phaclofen and bicuculline significantly increased the spike numbers in both cases. The increase was greater for nPTNs than for PTNs. GABA decreased them. The transcallosal latency was 3.9 +/- 1.1 ms for PTNs under the control conditions. CGP 35348, phaclofen and bicuculline significantly shortened the latency, and GABA elongated it. The transcallosal latency for nPTNs under the control conditions was 2.7 +/- 1.2 ms. This was significantly shortened by application of CGP 35348, phaclofen or bicuculline. GABA restored it. In conclusion, CGP 35348, phaclofen and bicuculline increased spike discharge and shortened the latency upon transcallosal stimulation for both group 1 PTNs and nPTNs.

The mode of cerebellar activation of pyramidal neurons in the cat parietal cortex (areas 5 and 7): an intracellular HRP study

Response properties of 180 pyramidal neurons (11 layer II, 66 layer III, 7 layer IV, 76 layer V and 20 layer VI neurons) in the cat parietal cortex (areas 5 and 7) were examined intracellularly with stimulations of the cerebellar nucleus (CN), the thalamic ventroanterior nucleus (VA) and the lateral posterior nucleus (LP) under pentobarbital anesthesia. Pyramidal neurons were identified morphologically or electrophysiologically with antidromic responses by pontine nuclei (PN) stimulation. CN and VA stimulations evoked EPSPs in layer II, III, V and VI pyramidal neurons, but scarcely induced EPSPs in layer IV pyramidal ones. The mean latencies of CN-EPSPs were nearly the same through layers II to V, giving values of around 4.5 ms, but that of layer VI pyramidal neurons was much longer (5.5 +/- 0.8 ms: mean +/- SD). Those of VA-EPSPs were also nearly the same through all layers (around 2.2-2.8 ms). In case of LP stimulation, EPSPs were scarcely recorded in layer II and IV pyramidal neurons. The mean latency of LP-EPSPs was much shorter in layer III (2.6 +/- 1.0 ms) than those in layers V and VI (3.8-3.9 ms). It was noted that pyramidal neurons in layers II and III responded more frequently to CN and VA stimulations than those in deeper layers, and that the amplitudes of CN- and VA-EPSPs were large and conspicuous in layer III. In layer V, a statistical difference in the latency distributions of CN-, VA-, and LP-EPSPs between fast- and slow-type pyramidal neurons was not detected. The present results suggest that there exist the different modes of thalamocortical (T-C) innervations in the cat parietal cortex; the superficial T-C innervation of CN-VA inputs and the deep T-C innervation of LP inputs.

Post-inhibitory excitation and inhibition in layer V pyramidal neurones from cat sensorimotor cortex

1. The effect of conditioning pre-pulses on repetitive firing evoked by intracellular current injection was studied in layer V pyramidal neurones in a brain slice preparation of cat sensorimotor cortex. Most cells displayed spike frequency adaptation (monotonic decline of firing rate to a tonic value) for several hundred milliseconds when depolarized from resting potential, but the cells differed in their response when pre-pulses to other potentials were employed. In one group of cells, the initial firing rate increased as the pre-pulse potential was made more negative (post-hyperpolarization excitation). Adaptation was abolished by depolarizing prepulses. In a second group, the initial firing rate decreased as the pre-pulse potential was made more negative (post-hyperpolarization inhibition). Hyperpolarizing pre-pulses caused the initial firing to fall below and accelerate to the tonic rate over a period of several seconds. A third group displayed a mixture of these two responses: the first three to seven interspike intervals became progressively shorter and subsequent intervals became progressively longer as the conditioning pre-pulse was made more negative (post-hyperpolarization mixed response). 2. Cells were filled with horseradish peroxidase or biocytin after the effect of pre-pulses was determined. All cells whose firing patterns were altered by pre-pulses were large layer V pyramidal neurones. Cells showing post-hyperpolarization excitation or a mixed response had tap root dendrites, fewer spines on the apical dendrite and larger soma diameters than cells showing post-hyperpolarization inhibition. 3. Other electrophysiological parameters varied systematically with the response to conditioning pre-pulses. Both the mean action potential duration and the input resistance of cells showing post-hyperpolarization excitation were about half the values measured in cells showing post-hyperpolarization inhibition. Values were intermediate in cells showing a post-hyperpolarization mixed response. The after-hyperpolarization following a single evoked action potential was 20% briefer in cells showing post-hyperpolarization excitation compared to those showing inhibition. 4. Membrane current measured during voltage clamp suggested that two ionic mechanisms accounted for the three response patterns. Post-hyperpolarization excitation was caused by deactivation of the inward rectifier current (Ih). Selective reduction of Ih with extracellular caesium diminished post-hyperpolarization excitation, whereas blockade of calcium influx had no effect. Post-hyperpolarization inhibition was caused by enhanced activation of a slowly inactivating potassium current. Selective reduction of this current with 4-aminopyridine diminished the post-hyperpolarization inhibition. 5. Chord conductances underlying both Ih and the slow-transient potassium current were measured and divided by leakage conductance to control for differences in cell size.(ABSTRACT TRUNCATED AT 400 WORDS)

Morphological types of projection neurons in layer 5 of cat visual cortex

Pyramidal cells in layer 5 of the visual cortex have multiple cortical and subcortical projection sites. Previous studies found that many cells possess bifurcating axons and innervate more than one cortical or subcortical target, but cells projecting to both cortical and subcortical targets were not observed. The present study examines the morphology of cells in cat visual cortex projecting to the superior colliculus, the main subcortical target of layer 5, and cells in layer 5 projecting to cortical areas 18 and 19. The neurons that give rise to these different projections were retrogradely labelled and intracellularly stained in living brain slices. Our results show that cells within each projection group have several morphological features in common. All corticotectal cells have a long apical dendrite forming a large terminal tuft in layer 1. Their cell bodies are medium sized to large, and their basal dendrites form a dense and symmetrical dendritic field. Corticocortical cells in layer 5 have a very different morphology: their apical dendrites are short and they never reach higher than layers 2/3. Their cells bodies are small to medium sized and they have fewer basal dendrites than corticotectal cells. Thus there are two morphologically distinct projection systems in layer 5, one projecting to cortical and the other one to subcortical targets, suggesting that these two systems transmit different information from the visual cortex. Among the corticotectal cells with the largest cell bodies we found some cells whose basal and apical dendrites were almost devoid of spines. Spiny and spinefree corticotectal cells also have different intrinsic axon collaterals and therefore play different roles in the cortical circuitry. While many spiny corticotectal cells have axon collaterals that project to layer 6, spinefree corticotectal cells have fewer axon collaterals and these do not arborize in layer 6. We suggest that the two morphological types of corticotectal cells might be related to functional differences known to exist among these cells. We discuss how the presence or absence of spines affects the integration of the synaptic input and how this might be related to the cells' functional properties.

Local intra- and interlaminar connections in mouse barrel cortex

Focal injections of horseradish peroxidase (HRP) in dimethylsulfoxide (DMSO) were targeted into mouse somatosensory cortex, in vitro, with a template. Injections were made at different depths and in different locations in the whisker-barrel-defined somatosensory map in order to determine quantitative connectivity patterns within and between barrel-defined cortical columns. Cortices were sectioned in a plane parallel to the pia at 75 microns. Data were collected directly from microscope slides by computer. Data are presented as: 1) Plots of computer-mapped HRP reaction product density in neurons and cell locations for each section in relation to barrel boundaries; 2) histograms of label in cortical layers related to individual barrel-defined columns; 3) polar plots of relative amounts of label within individual barrel columns in sections through each barrel column; 4) vectors which represent HRP reaction product density as a function of direction and distance from the injection site; 5) statistical analysis of the shape of the label distribution pattern in the plane of the cortex as a function of injection site depth; and 6) probability of labeling of any other barrel column given a labeled barrel column. The principal findings are: 1) The pattern of label distribution, after an injection directly above or directly below an individual barrel, is hour-glass shaped with the waist of the hour-glass in layer IV. 2) Connections within barrel cortex are asymmetrical. Barrel-related columns within a row are more strongly interconnected than those in different rows. 3) Connections of the small barrels associated with whiskers on the upper lip are strongest with other small barrels, but strong connections also exist between these small barrels and the larger barrels. 4) The pattern of intracortical connections in SII is not asymmetrical; interlaminar connections in SII are fundamentally different from those in barrel cortex. 5) Quantitative intracortical projection patterns are highly consistent with functional data on intracortical processing of whisker information. As such, the quantitative data clearly indicate the spatial extent and relative magnitude of populations of neurons involved in intracortical processing of sensory information. The spatial arrangements of these intracortical connections, in conjunction with known developmental events, make it highly likely that the distribution of intracortical axons in mouse barrel cortex is sculpted in part by experience.