Stephen C. Bondy

Putative neurotransmitters of the avian visual pathway

The ability of homogenates of the chick optic lobe to accumulate a series of possible neurotransmitters has been studied. High affinity uptake of several possible neurotransmitters was examined in optic lobes of 21-day-old embryos that had a single eye removed on the third day of incubation and in 23-day-old chicks that had an eye removed at hatch. Embryonic enucleation resulted in severe reduction of development of the ability of the contralateral optic lobe to take up tritiated GABA, dopamine, choline, serotonin and glutamate from solutions around 10(-8)M. Unilateral eye removal of new-hatched chicks caused failure of the denervated optic lobe to grow, but only the uptake capacity for glutamate was significantly recuced. This deficit was apparent as early as 4 days after enucleation. The transport of other compounds was unimpaired. The uptake of glutamate by homogenates of the optic tract was 43% of that or the optic lobe. This was a much greater fraction than the corresponding value for other postulated neurotransmitters. These data suggest that glutamate may be the primary neurotransmitter of the fibers of the optic tract originating in the retinal ganglion cells.

Development of neurotransmitter uptake in regions of the chick brain

The uptake of postulated neurotransmitters of their precursors into regions of the developing chick brain and retina has been examined. The transport of low concentrations (around 10(-8) M) of GABA, glutamic acid, choline, dopamine and serotonin into homogenates was sodium and energy dependent and inhibited by a variety of pharmacological agents that are thought to act presynaptically. After morphological fractionation, the high affinity transport mechanism was concentrated in the nerve ending fraction. Compounds were poorly accumulated by the cerebral regions of the 6 day incubated chick embryo. After this time, the uptake capacity of each brain region studied exhibited a characteristic development profile. Mechanisms for GABA transport appeared early in development, while catecholamine and choline systems matured later. Homogenates of the cerebral hemispheres and optic lobes took up all compounds studied, while the retina and cerebellum of the young chick were able to take up only GABA to a significant extent.

Axonal transport of macromolecules. I. Protein migration in the central nervous system

SummaryThe avian visual system has been used to study the transport of proteins and their precursors along the optic tract. Various labeled compounds were injected into a single eye of new hatched chicks. The radioactivity of components in the optic lobe that was contralateral to, and innervated by, the injected eye was compared to radioactivity in the ipsilateral lobe, not innervated by the treated eye. Proteins migrating from the ganglion cells of the retina to the optic tectum seemed to be relatively stable and may be rich in proline and glycine. Microtubular protein migrated at a rate similar to nonmicrotubule soluble protein, and slower than particulate protein. With the exception of γ-aminobutyric acid, transport of free amino acids occurred to only a minor extent. Following monocular injection of tritiated fucose, a rapid asymmetry in the specific activities of protein from contralateral and ipsilateral lobes, was established. Thus the more rapidly migrating proteins may be attached to glycosidic residues. The carbohydrate moeity of these glycoproteins is attached in the nerve cell body, prior to their axonal transport to the optic tectum. There was no evidence for transneuronal transfer of protein as in no case was a differential in specific activity observed in labeled protein from paired cerebral hemispheres.

Axonal migration of various ribonucleic acid species along the optic tract of the chick.

—The avian visual system has been used to study the axonal transport of RNA and protein. After monocular injection of radioactive uridine into 1‐day‐old chicks, a considerable amount of labelled RNA migrated along the optic tract to the optic tectum contralateral to the injected eye. This RNA was largely ribosomal, although it was contained in several subcellular fractions. The migration of RNA appeared to be a slow process. However, following monocular injection of radioactive proline, the migration of ribosomal protein was rapid. This discrepancy was resolved by examination of the kinetics of labelling of RNA and protein within the retina after intraocular injection of a mixture of labelled uridine and proline. Cytoplasmic RNA was labelled much more slowly than cytoplasmic protein. This lag in labelling of RNA could account for the delayed arrival of RNA at the contralateral optic lobe and suggests that ribosomes may travel rapidly along the axon.

Stimulus deprivation and cerebral blood flow

The effect of unilateral visual deprivation on the vascular system of brain regions has been studied in the chick where the visual pathways are totally crossed. We injected 125I-iodantipyrine or 125I-iodinated albumin into the hearts of chicks at various times after monocular eyelid suture or unilateral eye extirpation. Radioactivity within optic lobes and cerebral hemispheres was assayed, to estimate the velocity of blood flow or the plasma volume. As early as 1 hr after monocular deprivation by eyelid suture or by enucleation, significant deficits in the velocity of the circulation through the contralateral brain regions were observed. These effects were maintained for at least 6 days and were of similar magnitude in contralateral optic lobes (which are directly innervated by the optic nerve) and the contralateral cerebral hemispheres (which receive no primary innervation from the optic nerve). After 1 hr of either form of monocular deprivation, a reduced plasma volume was found in contralateral optic lobes but not in cerebral hemispheres. These data suggest that maintenance of optimal cerebral vascular supply may be dependent on a patterned sensory input to the brain. A deficiency in the complexity of cerebral afferentation in the young animal may impair adequate vascularity and thus retard maturation.

Axoplasmic transport of a brain-specific soluble protein

The rate and extent of axoplasmic transport of the brain-specific soluble protein (14-3-2 protein) has been investigated in the avian visual system. 1-day-old chicks were injected monocularly with tritiated proline, Incorporation of the isotope into the 14-3-2 protein synthesized within the retina of the injected eye, as well as the appearance of the labeled protein in the optic lobes was determined at 6 h and 6 days. These time periods were chosen to distinguish between the rapid and slow phases of axophlasmic flowmfollowing preparation of high-speed supernatant fractions, dialysis, chromatography on Sephadex G-150 and immunoprecipitation with specific antiserum, identification of the labeled 14-3-2 protein was carried out by sodium dodecylsulfate-polyacrylamide gel analysis of the radioactive immunoprecipitates; 6 days after isotope administration, approxo% of the 14-3-2 protein synthesized in the chick retina had been transported to the contralateral optic lobe. By contrast, at 6 h no labeled 14-3-2 protein was detectablemthus, transport of this neuronal protein appears to be relatively slow process with little or no rapid component.

Selective regulation of the blood-brain barrier by sensory input

A University of California author or department has made this article openly available. Thanks to the Academic Senates Open Access Policy, a great many UC-authored scholarly publications will now be freely available on this site. Let us know how this access is important for you. We want to hear your story!

Migration of ribosomes along the axons of the chick visual pathway.

The axoplasmic migration of ribosomes has been detected in the visual system of the chick. Monocular injection of radioactive uridine or an amino acid mixture was followed by sedimentation analysis in sucrose or cesium sulfate density gradients, of ribosomes prepared from the retinae of injected eyes and the left and right optic lobes. By this means both RNA and protein components of ribosomes were found to migrate from the retina to the innervated contralateral optic lobe. Following denervation of the distal nerve segment by eye removal, the stability of the transported RNA was reduced, suggesting its presynaptic location. The transport of RNA was not significantly imparied by intraocular injection of inhibitors of informational RNA or mitochondrial RNA synthesis prior to injection of radioactive uridine but was depressed by a low dose of actinomycin D.

The regulation of regional blood flow in the brain by visual input

Abstract The blood supply to several regions of the chick brain is rapidly decreased following reduction of visual input by monocular eyelid Suture. This decrease is not confined to the primary visual area (the optic lobe) contralateral to and innervated by the sutured eye but is also apparent in the indirectly innervated cerebral hemispheres contralateral to the treated eye. After periods of bilateral eyelid suture, exposure of a single eye to patterned light by suture removal results in a rapid increase of blood flow through contralateral cerebral regions directly or secondarily innervated by the exposed eye. This response appears to be more closely related to the intensity rather than to the information content of incident light. The ability to regulate regional cerebral blood flow is lost following reduction of brain catecholamine levels with reserpine. However, cerebral blood flow is still responsive to afferent sensory input, in the presence of a concentration of carbon dioxide sufficient to cause near maximal dilation of cerebral blood vessels. This suggests that variations of vascular supply to specific brain regions may in part be effected by direct autonomic nervous mechanisms regulating the diameter of blood vessels within the brain.

The effect of low concentrations of an organic lead compound on the transport and release of putative neurotransmitters

Abstract Tri- n -butyl lead acetate at concentrations of 5.10 −5 -10 −7 M was effective in inhibiting high affinity uptake and in stimulating release of accumulated radioactive putative neurotransmitters by mouse brain homogenates. Mercurycontaining compounds and lead acetate were much less potent in inhibiting such transport and stimulating release. It is suggested that the neurotoxicity of organic lead compounds is, in part, related to effects on neurotransmission and that dopaminergic synapses are especially susceptible to such toxicity.