Functional development of the visual system in normal and protein-deprived rats. VI. Evoked responses in adult rats, protein deprived in early life and nutritionally rehabilitated from weaning

The effects of dietary protein restriction on chorda tympani nerve taste responses and terminal field organization

Prenatal dietary sodium restriction produces profound developmental effects on rat functional taste responses and formation of neural circuits in the brainstem. Converging evidence indicates that the underlying mechanisms for these effects are related to a compromised nutritional state and not to direct stimulus-receptor interactions. We explored whether early malnourishment produces similar functional and structural effects to those seen following dietary sodium restriction by using a protein deficient, sodium replete diet. To determine if early dietary protein-restriction affects the development of the peripheral gustatory system, multi-fiber neurophysiological recordings were made from the chorda tympani nerve and anterograde track tracing of the chorda tympani nerve into the nucleus of the solitary tract (NTS) was accomplished in rats fed a protein-restricted or a control diet (6% and 20%, respectively). The dietary regimens began on embryonic day 7 and continued until rats were used for neurophysiological recordings (postnatal days (P) 35-50) or for chorda tympani terminal field labeling (P40-50). Responses to a concentration series of NaCl, sodium acetate, KCl, and to 0.50 M sucrose, 0.03 M quinine-HCl, and 0.01 N HCl revealed attenuated responses (30-60%) to sodium-specific stimuli in rats fed the 6% protein diet compared with those fed the 20% protein diet. Responses to all other stimuli were similar between groups. Terminal field volumes were nearly twofold larger in protein-restricted rats compared with controls, with the differences located primarily in the dorsal-caudal zone of the terminal field. These results are similar to the results seen previously in rats fed a sodium-restricted diet throughout pre- and postnatal development, suggesting that dietary sodium- and protein-restriction share similar mechanisms in altering gustatory development.

Abnormal flash visual evoked potentials in malnourished infants: an evaluation using principal component analysis

OBJECTIVE

To characterize the morphology of flash visual evoked potentials (fVEPs) obtained from infants hospitalized with severe, chronic malnutrition (marasmus).

METHODS

A covariance-based principal component analysis with Promax factor rotation was applied to fVEPs obtained from malnourished infants and age-matched control subjects.

RESULTS

The analysis suggests the presence of a late positive complex in the fVEP, with at least one of its components being significantly diminished in marasmic infants. The N3 component was also diminished in marasmic infants. Following remediation, the marasmic group no longer differed with respect to these components. However, an abnormally large late, positive deflection was evident at discharge.

CONCLUSIONS

The fVEP morphology of infants hospitalized with severe malnutrition was found to be significantly different from age-matched controls. Moreover, although there was evidence of recovery following remediation, fVEPs continued to show abnormality at discharge, suggesting the possibility that nutritional rehabilitation did not fully eliminate the physiological deficit.

SIGNIFICANCE

Malnourishment during early infancy results in altered neurophysiological functioning, possibly in cortical areas responsible for higher order visual processing.

Functional development of the visual system in normal and protein-deprived rats. VIII. Post-natal development of optic nerve axons

Protein deprivation results in a persistent impairment of transmission of impulses from the specific cortical projection within the visual cortex. In order to evaluate changes in subcortical structures during pre-weaning development, a study was made on number, calibre and myelination of optic nerve axons in control (C) and protein-deprived (PD) rats 5, 12, 20 and 30 days of age. Protein deprivation was induced by giving rat mothers a diet containing 7% protein by weight (control diet 14%, during gestation and lactation. The cross-sectional area of the nerve was measured on a digitizer. Between 3000 and 4000 axons collected from 7-13 field areas sampled by a random, systematic procedure from a cross-shaped area of the nerve were counted and measured. Between 5 and 12 days after birth, the number of axons was reduced by 50% in C rats. The total number of optic nerve axons was not significantly different in PD compared to C rats, indicating that protein deprivation does not affect the formation or naturally occurring nerve cell death of retinal ganglion cells. At all ages examined there were significant reductions in the number of axons larger than 0.52 micron as well as the number of myelinated axons. The rates of growth/maturation and myelination of axons in C rats and PD rats suggested that the reductions seen in PD rats up to 20 days of age may represent a developmental delay of approximately 4 days. At 30 days, a delay or a distortion of development may present. The retarded development of optic nerve axons is discussed in relation to delays, distortions and deficits during visual system maturation in protein-deprived rats.

Functional development of the visual system in normal and protein-deprived rats. VII. Lamination of oxidative enzyme activity in the visual cortex during post-natal development

Lamination of activity of two mitochondrial enzymes-succinate dehydrogenase (SDH) and menadione-dependent alpha-glycerophosphate dehydrogenase (M-GPDH) - was examined in the visual cortex of control (C) and protein-deprived (PD) rats from 10 days old to adult. In C rats, lamination of enzyme activity was evident from 10 days by a band of higher activity in lamina IV. A band of higher SDH activity was seen in superficial lamina VI from 15 days. Lamination of M-GPDH activity became less pronounced with increasing age, whereas distinct bands of high SDH activity in lamina IV and VI remained also in adult rats. PD rats showed a developmental delay of approximately 5 days with regard to the general increase in neuropile activity. A band of higher enzyme activity in lamina IV was present from 10 days, but it was markedly less distinct in the PD rats compared to the C rats between 15 and 25 days. A band of higher SDH activity in superficial lamina VI was not seen until 25 days in PD rats. No apparent differences were seen between adult C and PD rats. The observed developmental alterations are discussed in relation to other delays, distortions and deficits found in the visual system of PD rats.

Functional development of the visual system in normal and protein-deprived rats. IX. Visual evoked response in young rats

Changes in latencies of the visual evoked response (VER) during early post-natal development were examined in protein-deprived (PD) rats. The evoked response to light-flash stimulation was recorded in the dorsal lateral geniculate nucleus (dLGN) and on the surface of the visual cortex. In control rats, latencies of the cortical VER decreased rapidly up to 20 days and slowly thereafter. In PD rats, the latencies of the cortical VER were increased by 10-15 ms at 17 days; the developmental decrease was delayed by approximately 3 days. After 20 days, PD rats also went into a phase with slow decrease of the latencies, and the onset latency of the cortical VER was still increased by some 10 ms at 26/27 days. At this age, PD rats showed an increase in the latencies of the VER in the dLGN which was of similar magnitude to that in the cortical VER, indicating that alterations were more marked in the peripheral parts of the visual system at this stage of development. The findings are in agreement with previous studies indicating that there is a delay of visual system development in PD rats before 20 days. A maturational event which turns rapid into slow development at approximately 20 days in both C and PD rats turns this delay into a distortion of development. The delays and distortions of visual system development may be one causative factor for the functional deficit present in the visual cortex of adult PD rats.

Functional development of the visual system in normal and protein-deprived rats. V. Specific cortical response and repetitive stimulation in adult rats

Recent studies on the specific visual system have indicated that early induced protein deprivation results in an impaired transmission from cell to cell in the cortex. To further investigate cortical mechanisms the response to repetitive light flashes and to electrical stimulation of the optic tract (OT) and the dorsal lateral geniculate nucleus (dLGN) were examined. The first potential of the surface visual evoked response (VER) and the monosynaptic response to OT and dLGN stimulation showed similar sensitivity to repetitive stimulation in C and PD rats. In PD rats later activity was significantly more sensitive to repetitive stimulation than in controls. By using paired stimuli with a short inter-stimulus interval, similar inhibitory mechanisms in PD and C rats were demonstrated. The results support the view that protein deprivation results in impaired cortical synaptic activation. The results are discussed in relation to previously described impairment at the cortical level in malnourished animals.