Postnatal development of connectional specificity of corticospinal terminals in the cat.

The purpose of this study was to examine postnatal development of connectional specificity of corticospinal terminals. We labeled a small population of primary motor cortex neurons with the anterograde tracer biotinylated dextran amine. We reconstructed individual corticospinal segmental axon terminals in the spinal gray matter in cats of varying postnatal ages and adults. We found that at days 25 and 35 the segmental termination field of reconstructed axons was large, estimated to cover more than half of the contralateral gray matter. Branches and varicosities were sparse and had a relatively uniform distribution. When we examined the terminal fields of multiple axons, reconstructed over the same set of spinal sections (120-200 microm), we found that there was extensive overlap. By day 55, the morphology and termination fields had changed remarkably. There were many short branches, organized into discrete clusters, and varicosities were preferentially located within these clusters. The termination field of individual axons was substantially reduced compared with that of younger animals, and there was minimal overlap between the terminals of neighboring corticospinal neurons. In adults, a further reduction was seen in the spatial extent of terminals, branching, and varicosity density. Termination overlap was not substantially different from that in PD 55 animals. Development of spatially restricted clusters of short terminal branches and dense axonal varicosities occurred just prior to development of the motor map in primary motor cortex and may be necessary for ensuring that the corticospinal system can exert a dominant influence on skilled limb movement control in maturity.

A low-selenium diet increases thyroxine and decreases 3,5,3'triiodothyronine in the plasma of kittens.

The effect of a low-selenium diet on thyroid hormone metabolism was investigated in growing kittens. Twelve specific-pathogen-free kittens with ages ranging from 16 to 18 weeks were divided into two groups of equal number with equal sex distribution in each group. One group was fed a yeast-based low-selenium diet (0.02 mg Se/kg diet) while the other group was fed the same diet supplemented with Na2SeO3 at 0.4 mg Se/kg diet for 8 weeks. Food intake, body weight and body weight gain were not affected by the low-Se diet during the study period. However, kittens given the low-Se diet had significantly reduced plasma selenium concentration and glutathione peroxidase activity. Plasma total thyroxine (T4) increased and total 3,5,3'triiodothyronine (T3) decreased significantly in kittens fed the low-Se diet at the end of the study. These results suggest that type I deiodinase in cats is a selenoprotein- or a selenium-dependent enzyme.

Postnatal development of thalamocortical projections upon striate and extrastriate visual cortical areas in the cat.

The development of visual thalamocortical projections was analyzed quantitatively by comparing, in cresyl violet-stained brain sections of early postnatal (10-17 days) and adult cats, the cell body dimensions and total cell packing density (CPD) of neuronal populations in different laminae (A, A1 and C) of the dorsal lateral geniculate (dLGN), medial interlaminar nucleus (MIN), and in lateral (LPl), intermediate (LPi) and medial (LPm) subdivisions of the lateral posterior complex. Following injections of different fluorescent tracers (FB, NY, EB, RITC) into cortical visual areas 17/18, posterior medial (PMLS) and posterior lateral (PLLS) lateral suprasylvian and anterior ectosylvian (AEV), the thalamic distribution and densities of retrogradely labeled neurons were analyzed. Projection CPDs and ratios of projection/total CPDs were determined and compared within the different thalamic components in the kitten and adult cat. A significant decrease in total cell packing density was observed in the various thalamic components of the adult cat, varying between 43% and 65%, and a marked increase in mean cell body diameter in the A, A1 and C laminae and MIN from kitten to adult (8.4+/-1.8 and 11.8+/-2.8 microm respectively) compared to the LP subnuclei (9.0+/-1.3 and 9.1+/-1.5 microm). The ratios of projection/total CPDs decreased significantly for projections upon areas 17/18 stemming from layers A and A1 (20 and 25%, respectively) and from LPi upon both PMLS (34%) and AEV (16%). Thalamocortical projections observed in the kitten from LPi upon areas 17/18 and from the A-laminae upon PMLS were absent in the adult cat. The data indicate that, in comparison to the lateral posterior nucleus, the maturation of neurons within the dLGN and MIN is incomplete with respect to cell body size during the early postnatal period. In addition, the developmental changes observed involve both reductions in the total number of thalamic neurons and a differential loss of cortical projections. The selective elimination of early cortical connections stemming from dorsal lateral geniculate laminae A and A1 and from the intermediate division of the lateral posterior nucleus may occur through a process of axon collateral withdrawal from the expanded cortical sites, thereby giving rise to the adult pattern.

Growing kittens require less dietary calcium than current allowances.

We previously demonstrated that a purified diet containing 3.125 microg of cholecalciferol/kg was adequate to maintain plasma concentrations of 25-hydroxyvitamin D in growing kittens. With the use of this concentration of cholecalciferol, the response of growing kittens to varying levels of calcium in purified diets was measured. Five groups (treatments 1-5), each comprised of seven weaned kittens, were given diets containing 3.8, 5.0, 6.0, 7.2 or 8. 1 g calcium/kg diet (Ca:P ratio of 1:1.25) from 9 to 18 wk of age. Two further groups of kittens (treatments 6 and 7) received similar diets containing 6.0 g Ca/kg diet, with Ca:P ratios of 1:1.55 and 1:2.61, respectively. No clinical signs of calcium deficiency were observed, i.e., growth rate, energy intake and plasma total calcium were not affected by the treatments. However, ionized calcium was significantly lower in kittens in treatment 7. Plasma phosphorus was lower in kittens in treatment 7 than in kittens in treatments 1, 2, 3 and 4, and there was a negative relationship between dietary and plasma phosphorus concentrations. Kittens in treatment 7 had a significantly higher alkaline phosphatase concentration in plasma than kittens in treatments 1, 2, 3 and 5. Kittens in treatment 1 had a lower percentage of bone minerals measured by dual-energy X-ray absorptiometry than kittens in treatments 2-6. These results indicate that the calcium requirement of growing kittens is not >6.0 g/kg diet, (calculated metabolizable energy approximately 20 kJ/g) and that kittens are not very sensitive to inverse Ca:P ratios up to 1:1.55.

Developmental alterations in auditory arousal from sleep in healthy and virus-infected cats.

To investigate the impact of respiratory-tract infections on arousability from sleep, we evaluated the auditory arousal responses of healthy and virus-infected cats in two age groups. Arousability was positively associated with stimulus intensity but negatively associated with experimental day. Infected animals were more responsive to auditory stimuli than were uninfected ones, and younger cats were less responsive than older animals. Within the younger groups, infected cats demonstrated increased arousability across the entire range of stimulus volumes during quiet sleep but not during active sleep. Young infected cats also demonstrated lower EEG delta-wave amplitudes than did uninfected animals. These effects on arousability and EEG amplitudes were not observed in older cats. Thus, under the conditions studied, cats with mild viral infections show an age-related enhancement of auditory arousability from sleep, suggesting that they sleep less deeply than do healthy, age-matched animals. The generalization of these observations to other arousing stimuli, such as hypoxia or hypercapnia, and possible implications for the pathogenesis of sudden infant death syndrome remain to be determined.

The minimum sodium requirement of growing kittens defined on the basis of plasma aldosterone concentration.

The minimum sodium requirement of growing kittens was measured using a 6 x 6 Latin square design. Twelve specific-pathogen-free short-hair growing kittens (six males, six females) were fed casein and lactalbumin-based purified diets supplemented with various levels of sodium (NaCI). Using six growing kittens (four males, two females), a sodium depletion and repletion study was conducted to define the variables associated with sodium deficiency. Sodium-deficient kittens exhibited anorexia, impaired growth, polydypsia, polyuria, hemoconcentration, reduced urinary sodium output and specific gravity, and elevated aldosterone concentration in plasma and output in urine. Plasma sodium concentration was not affected by dietary sodium intake. Urinary sodium output was positively related to (r = 0.818, P < 0.001), but fecal sodium loss was independent of sodium intake. These results suggest that sodium balance in kittens is essentially regulated by renal excretion. The recommended minimum sodium requirement of kittens for growth is 1.6 g Na/kg diet (energy density, 22 kJ ME/g diet), or 0.07 mg Na/kJ ME, or 34 mg Na x kg body wt(-1) x d(-1). A sodium requirement of adult cats for maintenance was estimated to be 21 mg Na x kg body wt(-1) x d(-1). These requirements are considerably greater than those recommended by the National Research Council in 1986.

Effect of chronic high protein intake on magnesium, calcium, and phosphorus balance in growing cats.

The effects of high protein feeding on food and water intake, and the retention and urinary excretion of macrominerals (magnesium (Mg), phosphorus (P), and calcium (Ca)) were examined in growing cats. Seven female cats aged 4 months were fed diets containing 55% crude protein (n = 4) or 29% crude protein (n = 3) for 12 months on an ad libitum basis. Mineral balances were determined at 0.5, 2, 6, 10, and 12 months of feeding. The higher protein intake stimulated daily water intake and urine excretion throughout the study, although daily food intake was not affected by dietary protein levels. The urinary Mg concentration was decreased by the high protein intake, resulting from both increased urine volume and reduced excretion of urinary Mg. In contrast, the concentration and daily excretion of urinary P were increased by the high protein intake. The protein-induced increase in urinary P would not necessarily imply the increased excretion of PO4(3-), the anion responsible for struvite crystallization, because the dissociation of phosphate depends on urinary pH. Urinary Ca excretion was not affected by the dietary protein levels, but the high protein intake caused less retention of P and Ca as a result of enhanced urinary P excretion and lowered Ca absorption. The possibility of high protein feeding for the prevention of struvite crystallization in growing cats is discussed.

Topography of developing thalamic and cortical pathways in the visual system of the cat.

Adult patterns of connectivity could emerge during development by a process of selective elimination from an earlier, more widespread, connectivity. We have addressed this issue by examining the topography of developing projections to area 17 in the cat. At different postnatal ages, paired injections of the retrograde tracers diamidino yellow and fast blue were made in area 17. Interinjection separations were carefully controlled and the spatial distribution of the two populations of labelled neurones investigated. Projections to the striate cortex from the lateral geniculate nucleus, area 18, as well as connections intrinsic to area 17 were analysed quantitatively with a graphic method that uses a two-dimensional model of the projection. This allows two parameters of the projection to be calculated: the divergence (the spatial extent of area 17 contacted by an infinitely small region of an afferent structure) and the convergence (the extent of an afferent structure that projects to an infinitely small region of area 17). During postnatal development, the bulk of the connections making up the geniculostriate and corticocortical pathways showed no variation either in their convergence and divergence. However, the projection of area 18 to area 17 and the intrinsic area 17 connections (but not the geniculostriate projection) in the 3-15-day-old kittens were each found to contain a small subpopulation of widely scattered neurones with widespread axonal trajectories. These results, showing that many initially formed connections display a high degree of topographical order, are discussed in terms of the control mechanisms specifying axonal trajectories during development.

Tritiated uridine uptake in the retinal ganglion cell layer of the cat during normal development and after visual cortex ablation.

The short-term metabolic response of immature retinal ganglion cells to destruction of their target cells in the dorsal lateral geniculate nucleus (dLGN) was assessed in newborn cats. Retrograde degeneration of virtually all dLGN cells was induced by ablation of the 13 contiguous areas of visual cortex on the day of birth. The metabolic response of retinal ganglion cells to this loss of target cells in dLGN was determined by exposing the ganglion cell layer to tritiated uridine, a precursor of RNA. Control measurements were made from unoperated littermates. Following sectioning and processing of the retinae from both groups of kittens for autoradiography, silver grain densities overlying the cellular profiles in the ganglion cell layer were calculated. These calculations revealed levels of uridine incorporation at Postnatal Day 4 in both groups of kittens significantly higher than at either Postnatal Day 2 or 7, but no significant differences between the two groups on any day examined. These results show that the level of RNA synthesis in retinal ganglion cells increases temporarily during the first postnatal week and that this synthesis is unaffected by the death of target cells in the dLGN. The temporary increase may be related to the establishment of synaptic connections on retinal ganglion cells by their afferent bipolar and amacrine neurons in the inner nuclear layer.

[Acetylcholinesterase activity during the cortical development of the cat brain]. / Actividad de acetilcolinesterasa durante el desarrollo cortical del cerebro de gato.

Acetylcholinesterase (AChe) hydrolyses acetylcholine to choline and acetate, thereby inactivating the neurotransmitter. However, the possible non-cholinergic function of this enzyme has been recently suggested. In the present work, changes in AChe activity during the postnatal development of the cat brain are described, in order to show if the enzyme could play a part in the maturing processes of the CNS. The study was performed in the frontal and parietal cortices, area 17 and areas 18 and 19 as a whole in the 15 and 30 days postnatal stages. Significant increases with age were observed in all the brain areas under study. It is suggested that this enzyme activity play a part in the maturation of the cat brain cortices, possibly in the cholinergic development and/or as a cell growth regulatory factor.

Functional synaptic circuits in the subplate during fetal and early postnatal development of cat visual cortex.

Among the first postmitotic cells of the cerebral cortex is a special population located below the cortical plate: the subplate neurons. These neurons reach a high degree of morphological maturity during fetal life, well before the neurons of the cortical layers have matured, yet nearly all of these cells die after birth in the cat. Subplate neurons are also known to receive synaptic contacts. Here we have investigated whether these contacts are functional by making intracellular recordings from subplate neurons in cortical slices maintained in vitro. Subplate neurons were identified based on their location and morphology by injecting them with biocytin following the intracellular recordings. At all ages studied between embryonic day 50 and postnatal day 9, electrical stimulation of the optic radiations elicited EPSPs and synaptic and antidromic spikes in subplate neurons, indicating that some of the synapses seen at the ultrastructural level are indeed capable of synaptic transmission. The spiking patterns of 39 morphologically identified subplate neurons were examined by injecting depolarizing current, which revealed that a large majority gave only a single spike or a brief train of spikes in response to maintained depolarization, in contrast to the regular spiking pattern found in many neurons of adult cortex. Biocytin injections into subplate neurons revealed that they are a morphologically heterogeneous population with respect to their dendritic branching patterns; roughly half were inverted pyramids, the classic subplate neuron morphology. The axonal processes of subplate neurons were remarkable in that many not only arborized within the subplate, but also entered the cortical plate and terminated in the marginal zone. At early postnatal ages, these axons also gave off collaterals within cortical layer 4. The results of this study indicate that subplate neurons participate in synaptic microcircuits during development. While the presynaptic identity of the input to subplate neurons is not known conclusively, it is likely that geniculocortical axons, which wait in close proximity to subplate neurons, contribute significantly. The pattern of axonal branching of subplate neurons also implies that information conferred to subplate neurons may be relayed, in turn, to the neurons of cortical layer 4. Finally with the death of subplate neurons, the geniculocortical axons leave the subplate and invade the cortical plate to innervate directly the neurons of layer 4. Thus, subplate neurons may function as a crucial, but transient synaptic link between waiting geniculocortical axons and their ultimate target cells in the cortex.

Vitamin B-6 requirement of growing kittens.

Two experiments were conducted to determine the vitamin B-6 requirement for growing kittens. Ten kittens were divided into two groups and given a purified diet containing 8.0 mg pyridoxine (PN)/kg diet (+PN) or a PN-free diet (-PN) for 11 wk. Daily body weight gain, food intake, weekly plasma free amino acids, plasma B-6 vitamers, urinary oxalate excretion, hemoglobin (Hb) and hematocrit were measured. Kittens fed a -PN diet had depressed body weight gain, food intake, plasma pyridoxal phosphate (PLP) and pyridoxal (PL), Hb and hematocrit, and had elevated urinary oxalate, plasma tyrosine and plasma cystathionine. In a second experiment, 24 kittens were given a -PN diet for 45 d to deplete their body reserves. The kittens were then divided into six groups of four kittens each and given a purified diet containing either 0.5, 1.0, 2.0, 3.0, 4.0 or 8.0 mg PN/kg diet for 46 d. Following supplementation, positive responses in body weight gain, PLP, Hb and hematocrit, and decreased urinary oxalate excretion, plasma tyrosine and plasma cystathionine occurred in all groups except those fed 0.5 mg PN/kg diet. At the end of the repletion period, kittens fed 1.0 mg PN/kg diet had lower body weight gain, higher plasma tyrosine and cystathionine, slower rate of decrease in urinary oxalate, and lower values for Hb, hematocrit and PLP than did the kittens from groups fed 2.0-8.0 mg PN/kg diet. These findings indicate that the dietary requirement for PN is greater than 1.0 mg/kg diet, but 2.0 mg PN/kg diet is adequate for growing kittens given a 35% casein diet.

Limiting the range of polyenoic fatty acids available from purified diets affects the growth of domestic cats.

The cat is deficient in delta-6-desaturase, preventing its utilisation of the plant-derived essential fatty acids. The species circumvents this problem by eating non-felines capable of this process. Kittens were fed diets varying in the type of post delta-6-desaturation fatty acids (PDFA). Growth patterns and external signs monitored for a period of 270 days, after which metabolic rates were measured, blood samples taken and the animals killed for other analyses. Differences were demonstrated in growth and metabolic rates, and the liver fatty acid profiles, and these were dependent on both the nature and amount of the PDFA provided by the diets.

Effects of high dietary methionine on activities of selected enzymes in the liver of kittens (Felis domesticus).

1. Growing male kittens were fed an 18% casein diet supplemented with 2, 3, or 4% L-methionine (MET) for 6 weeks. 2. Free MET concentration in liver increased 30-fold and cystathionine two- to three-fold; the activity of adenosyl-MET transferase and cystathionase also increased but remained lower than previously found in rats. 3. Taurine concentration in liver decreased in cats fed excess MET and appeared to depend on taurine intake. 4. Alanine aminotransferase activity was high in all groups while serine dehydratase activity was very low. 5. Pyruvate kinase and malic enzyme activities which are normally low in cat liver increased after excess MET. Also, glucose 6-phosphate and 6-phosphogluconate dehydrogenases increased. 6. Cat liver metabolism showed limited adaptation to an excess dietary intake of methionine compared to that found in rats.